Co-reporter:Kairat Sabyrov, Juncong Jiang, Omar. M. Yaghi, and Gabor A. Somorjai
Journal of the American Chemical Society September 13, 2017 Volume 139(Issue 36) pp:12382-12382
Publication Date(Web):August 25, 2017
DOI:10.1021/jacs.7b06629
Exceptionally high surface area and ordered nanopores of a metal–organic framework (MOF) are exploited to encapsulate and homogeneously disperse a considerable amount of phosphotungstic acid (PTA). When combined with platinum nanoparticles positioned on the external surface of the MOF, the construct shows a high catalytic activity for hydroisomerization of n-hexane, a reaction requiring hydrogenation/dehydrogenation and moderate to strong Brønsted acid sites. Characterization of the catalytic activity and acidic sites as a function of PTA loading demonstrates that both the concentration and strength of acidic sites are highest for the catalyst with the largest amount of PTA. The MOF construct containing 60% PTA by weight produces isoalkanes with 100% selectivity and 9-fold increased mass activity as compared to a more traditional aluminosilicate catalyst, further demonstrating the capacity of the MOF to contain a high concentration of active sites necessary for the isomerization reaction.
Co-reporter:Gabor A. Somorjai, Heinz Frei and Jeong Y. Park
Journal of the American Chemical Society November 25, 2009 Volume 131(Issue 46) pp:16589-16605
Publication Date(Web):November 4, 2009
DOI:10.1021/ja9061954
The challenge of chemistry in the 21st century is to achieve 100% selectivity of the desired product molecule in multipath reactions (“green chemistry”) and develop renewable energy based processes. Surface chemistry and catalysis play key roles in this enterprise. Development of in situ surface techniques such as high-pressure scanning tunneling microscopy, sum frequency generation (SFG) vibrational spectroscopy, time-resolved Fourier transform infrared methods, and ambient pressure X-ray photoelectron spectroscopy enabled the rapid advancement of three fields: nanocatalysts, biointerfaces, and renewable energy conversion chemistry. In materials nanoscience, synthetic methods have been developed to produce monodisperse metal and oxide nanoparticles (NPs) in the 0.8−10 nm range with controlled shape, oxidation states, and composition; these NPs can be used as selective catalysts since chemical selectivity appears to be dependent on all of these experimental parameters. New spectroscopic and microscopic techniques have been developed that operate under reaction conditions and reveal the dynamic change of molecular structure of catalysts and adsorbed molecules as the reactions proceed with changes in reaction intermediates, catalyst composition, and oxidation states. SFG vibrational spectroscopy detects amino acids, peptides, and proteins adsorbed at hydrophobic and hydrophilic interfaces and monitors the change of surface structure and interactions with coadsorbed water. Exothermic reactions and photons generate hot electrons in metal NPs that may be utilized in chemical energy conversion. The photosplitting of water and carbon dioxide, an important research direction in renewable energy conversion, is discussed.
Co-reporter:Chenlu Xie, Chen Chen, Yi Yu, Ji Su, Yifan Li, Gabor A. Somorjai, and Peidong Yang
Nano Letters June 14, 2017 Volume 17(Issue 6) pp:3798-3798
Publication Date(Web):May 11, 2017
DOI:10.1021/acs.nanolett.7b01139
Conversion of carbon dioxide to C2–C4 hydrocarbons is a major pursuit in clean energy research. Despite tremendous efforts, the lack of well-defined catalysts in which the spatial arrangement of interfaces is precisely controlled hinders the development of more efficient catalysts and in-depth understanding of reaction mechanisms. Herein, we utilized the strategy of tandem catalysis to develop a well-defined nanostructured catalyst CeO2–Pt@mSiO2–Co for converting CO2 to C2–C4 hydrocarbons using two metal-oxide interfaces. C2–C4 hydrocarbons are found to be produced with high (60%) selectivity, which is speculated to be the result of the two-step tandem process uniquely allowed by this catalyst. Namely, the Pt/CeO2 interface converts CO2 and H2 to CO, and on the neighboring Co/mSiO2 interface yields C2–C4 hydrocarbons through a subsequent Fischer–Tropsch process. In addition, the catalysts show no obvious deactivation over 40 h. The successful production of C2–C4 hydrocarbons via a tandem process on a rationally designed, structurally well-defined catalyst demonstrates the power of sophisticated structure control in designing nanostructured catalysts for multiple-step chemical conversions.Keywords: C2−C4 hydrocarbons; CO2 hydrogenation; interfaces; tandem catalysis;
Co-reporter:Feifei Shi, Philip N. Ross, Gabor A. Somorjai, and Kyriakos Komvopoulos
The Journal of Physical Chemistry C July 13, 2017 Volume 121(Issue 27) pp:14476-14476
Publication Date(Web):June 12, 2017
DOI:10.1021/acs.jpcc.7b04132
While silicon is the most promising next-generation anode material for lithium-ion batteries (LIBs), silicon electrodes exhibit significant capacity fade with cycling. A common hypothesis is that the capacity loss is due to the solid electrolyte interphase (SEI) forming in the first cycle and becoming destabilized by large cyclic volume changes. A cell for in situ attenuated total reflection-Fourier transform infrared spectroscopy with controllable penetration depth was used to study the chemistry at the electrode–electrolyte interface. The SEI product precursors at the interface were successfully identified and differentiated from free or solvated solvent molecules in the bulk electrolyte. Intriguingly, for the most common electrolyte consisting of ethylene carbonate and diethyl carbonate, ethylene carbonate was found to directly reduce to lithium ethylene dicarbonate on the lithiated silicon surface and diethyl carbonate to selectively reduce to diethyl 2,5-dioxahexane dicarboxylate on the surface of the native silicon-oxide film. Understanding this surface dependence of the SEI composition is critical to tuning the silicon electrode surface condition and, ultimately, enhancing the performance of future LIBs.
Co-reporter:Jie Zhao;Bing Yuan;Rong Ye;Wen-Chi Liu;Matthew Chang;Franco F. Faucher;Joyce Rodrigues De Araujo;Christophe V. Deraedt;F. Dean Toste
Nano Letters January 11, 2017 Volume 17(Issue 1) pp:584-589
Publication Date(Web):December 14, 2016
DOI:10.1021/acs.nanolett.6b04827
The Hayashi–Ito aldol reaction of methyl isocyanoacetate (MI) and benzaldehydes, a classic homogeneous Au(I)-catalyzed reaction, was studied with heterogenized homogeneous catalysts. Among dendrimer encapsulated nanoparticles (NPs) of Au, Pd, Rh, or Pt loaded in mesoporous supports and the homogeneous analogues, the Au NPs led to the highest yield and highest diastereoselectivity of products in toluene at room temperature. The Au catalyst was stable and was recycled for at least six runs without substantial deactivation. Moreover, larger pore sizes of the support and the use of a hydrophobic solvent led to a high selectivity for the trans diastereomer of the product. The activation energy is sensitive to neither the size of Au NPs nor the support. A linear Hammett plot was obtained with a positive slope, suggesting an increased electron density on the carbonyl carbon atom in the rate-limiting step. IR studies revealed a strong interaction between MI and the gold catalyst, supporting the proposed mechanism, in which rate-limiting step involves an electrophilic attack of the aldehyde on the enolate formed from the deprotonated MI.Keywords: aldol reaction; gold catalyst; Heterogenized homogeneous catalyst; mechanism; selectivity; support effect;
Co-reporter:Jeong Y. Park;Yusuke Yamada;Hyun Sook Lee;Cesar Aliaga;Chia-Kuang Tsung;Peidong Yang
The Journal of Physical Chemistry C April 16, 2009 Volume 113(Issue 15) pp:6150-6155
Publication Date(Web):2017-2-22
DOI:10.1021/jp8108946
We report the structure of the organic capping layers of platinum colloid nanoparticles and their removal by UV−ozone exposure. Sum frequency generation vibrational spectroscopy (SFGVS) studies identify the carbon−hydrogen stretching modes on poly(vinylpyrrolidone) (PVP) and tetradecyl tributylammonium bromide (TTAB)-capped platinum nanoparticles. We found that the UV−ozone treatment technique effectively removes the capping layer on the basis of several analytical measurements including SFGVS, X-ray photoelectron spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The overall shape of the nanoparticles was preserved after the removal of capping layers, as confirmed by transmission electron microscopy (TEM). SFGVS of ethylene hydrogenation on the clean platinum nanoparticles demonstrates the existence of ethylidyne and di-σ-bonded species, indicating the similarity between single-crystal and nanoparticle systems.
Co-reporter:Dr. Walter T. Ralston;Dr. Gérôme Melaet;Tommy Saephan; Gabor A. Somorjai
Angewandte Chemie International Edition 2017 Volume 56(Issue 26) pp:7415-7419
Publication Date(Web):2017/06/19
DOI:10.1002/anie.201701186
AbstractThe Fischer–Tropsch process, or the catalytic hydrogenation of carbon monoxide (CO), produces long chain hydrocarbons and offers an alternative to the use of crude oil for chemical feedstocks. The observed size dependence of cobalt (Co) catalysts for the Fischer–Tropsch reaction was studied with colloidally prepared Co nanoparticles and a chemical transient kinetics reactor capable of measurements under non-steady-state conditions. Co nanoparticles of 4.3 nm and 9.5 nm diameters were synthesized and tested under atmospheric pressure conditions and H2/CO=2. Large differences in carbon coverage (ΘC) were observed for the two catalysts: the 4.3 nm Co catalyst has a ΘC less than one while the 9.5 nm Co catalyst supports a ΘC greater than two. The monomer units present on the surface during reaction are identified as single carbon species for both sizes of Co nanoparticles, and the major CO dissociation site is identified as the B5-B geometry. The difference in activity of Co nanoparticles was found to be a result of the structure sensitivity caused by the loss of these specific types of sites at smaller nanoparticle sizes.
Co-reporter:Yonatan Horowitz;Hui-Ling Han;Walter T. Ralston;Joyce Rodrigues de Araujo;Eric Kreidler;Chris Brooks
Advanced Energy Materials 2017 Volume 7(Issue 17) pp:
Publication Date(Web):2017/09/01
DOI:10.1002/aenm.201602060
Fluorine-based additives have a tremendously beneficial effect on the performance of lithium-ion batteries, yet the origin of this phenomenon is unclear. This paper shows that the formation of a solid-electrolyte interphase (SEI) on the anode surface in the first five charge/discharge cycles is affected by the stereochemistry of the electrolyte molecules on the anode surface starting at open-circuit potential (OCP). This study shows an anode-specific model system, the reduction of 1,2-diethoxy ethane with lithium bis(trifluoromethane)sulfonimide, as a salt on an amorphous silicon anode, and compares the electrochemical response and SEI formation to its fluorinated version, bis(2,2,2-trifluoroethoxy) ethane (BTFEOE), by sum frequency generation (SFG) vibrational spectroscopy under reaction conditions. The SFG results suggest that the CF3 end-groups of the linear ether BTFEOE change their adsorption orientation on the a-Si surface at OCP, leading to a better protective layer. Supporting evidence from ex situ scanning electron microscopy and X-ray photoelectron spectroscopy depth profiling measurements shows that the fluorinated ether, BTFEOE, yields a smooth SEI on the a-Si surface and enables lithium ions to intercalate deeper into the a-Si bulk.
Co-reporter:Kairat Sabyrov;Nathan Musselwhite;Gérôme Melaet
Catalysis Science & Technology (2011-Present) 2017 vol. 7(Issue 8) pp:1756-1765
Publication Date(Web):2017/04/21
DOI:10.1039/C7CY00203C
As the impact of acids on catalytically driven chemical transformations is tremendous, fundamental understanding of catalytically relevant factors is essential for the design of more efficient solid acid catalysts. In this work, we employed a post-synthetic doping method to synthesize a highly selective hydroisomerization catalyst and to demonstrate the effect of acid strength and density, catalyst microstructure, and platinum nanoparticle size on the reaction rate and selectivity. Aluminum doped mesoporous silica catalyzed gas-phase n-hexadecane isomerization with remarkably high selectivity to monobranched isomers (∼95%), producing a substantially higher amount of isomers than traditional zeolite catalysts. Mildly acidic sites generated by post-synthetic aluminum grafting were found to be the main reason for its high selectivity. The flexibility of the post-synthetic doping method enabled us to systematically explore the effect of the acid site density on the reaction rate and selectivity, which has been extremely difficult to achieve with zeolite catalysts. We found that a higher density of Brønsted acid sites leads to higher cracking of n-hexadecane presumably due to an increased surface residence time. Furthermore, regardless of pore size and microstructure, hydroisomerization turnover frequency linearly increased as a function of Brønsted acid site density. In addition to strength and density of acid sites, platinum nanoparticle size affected catalytic activity and selectivity. The smallest platinum nanoparticles produced the most effective bifunctional catalyst presumably because of higher percolation into aluminum doped mesoporous silica, generating more ‘intimate’ metallic and acidic sites. Finally, the aluminum doped silica catalyst was shown to retain its remarkable selectivity towards isomers even at increased reaction conversions.
Co-reporter:Yongju Yun;Joyce R. Araujo;Gerome Melaet;Jayeon Baek
Catalysis Letters 2017 Volume 147( Issue 3) pp:622-632
Publication Date(Web):2017 March
DOI:10.1007/s10562-016-1915-2
Dehydrogenation of propane to propene is one of the important reactions for the production of higher-value chemical intermediates. In the commercial processes, platinum- or chromium oxide-based catalysts have been used for catalytic propane dehydrogenation. Herein, we first report that bulk tungsten oxide can serve as the catalyst for propane dehydrogenation. Tungsten oxide is activated by hydrogen pretreatment and/or co-feeding of hydrogen. Its catalytic activity strongly depends on hydrogen pretreatment time and partial pressure of hydrogen in the feed gas. The activation of tungsten oxide by hydrogen is attributed to reduction of the metal oxide and presence of multivalent oxidation states. Comparison of the catalytic performance of partially reduced WO3−x to other highly active metal oxides shows that WO3−x exhibits superior catalytic activity and selectivity than Cr2O3 and Ga2O3. The findings of this work provide the possibility for activation of metal oxides for catalytic reactions and the opportunity for the development of new type of catalytic systems utilizing partially reduced metal oxides.
Co-reporter:Gabor A. Somorjai;Hans-Joachim Freund
Catalysis Letters 2017 Volume 147( Issue 1) pp:1
Publication Date(Web):2017 January
DOI:10.1007/s10562-016-1966-4
Co-reporter:Christophe DeraedtGérôme Melaet, Walter T. Ralston, Rong Ye, Gabor A. Somorjai
Nano Letters 2017 Volume 17(Issue 3) pp:
Publication Date(Web):February 2, 2017
DOI:10.1021/acs.nanolett.6b05156
Pt, Rh, and Pd nanoclusters stabilized by PAMAM dendrimer are used for the first time in a gas flow reactor at high temperature (150–250 °C). Pt nanoclusters show a very high activity for the hydrogenation of the methylcyclopentane (MCP) at 200–225 °C with turnover freqency (TOF) up to 334 h–1 and selectivity up to 99.6% for the ring opening isomerization at very high conversion (94%). Rh nanoclusters show different selectivity for the reaction, that is, ring opening isomerization at 175 °C and cracking at higher temperature whereas Pd nanoclusters perform ring enlargement plus dehydrogenation, while maintaining a high activity. The difference in these results as compared to unsupported/uncapped nanoparticles, demonstrates the crucial role of dendrimer. The tunability of the selectivity of the reaction as well as the very high activity of the metal nanoclusters stabilized by dendrimer under heterogeneous conditions open a new application for dendrimer catalysts.Keywords: C—C breaking; dendrimer; heterogeneous catalysis; MCP; Nanoparticle;
Co-reporter:Bunyarat Rungtaweevoranit, Jayeon Baek, Joyce R. Araujo, Braulio S. Archanjo, Kyung Min Choi, Omar M. Yaghi, and Gabor A. Somorjai
Nano Letters 2016 Volume 16(Issue 12) pp:7645-7649
Publication Date(Web):November 2, 2016
DOI:10.1021/acs.nanolett.6b03637
We show that the activity and selectivity of Cu catalyst can be promoted by a Zr-based metal–organic framework (MOF), Zr6O4(OH)4(BDC)6 (BDC = 1,4-benzenedicarboxylate), UiO-66, to have a strong interaction with Zr oxide [Zr6O4(OH)4(−CO2)12] secondary building units (SBUs) of the MOF for CO2 hydrogenation to methanol. These interesting features are achieved by a catalyst composed of 18 nm single Cu nanocrystal (NC) encapsulated within single crystal UiO-66 (Cu⊂UiO-66). The performance of this catalyst construct exceeds the benchmark Cu/ZnO/Al2O3 catalyst and gives a steady 8-fold enhanced yield and 100% selectivity for methanol. The X-ray photoelectron spectroscopy data obtained on the surface of the catalyst show that Zr 3d binding energy is shifted toward lower oxidation state in the presence of Cu NC, suggesting that there is a strong interaction between Cu NC and Zr oxide SBUs of the MOF to make a highly active Cu catalyst.Keywords: CO2 hydrogenation; heterogeneous catalyst; metal nanocrystal; Metal−organic framework; methanol; strong metal−support interaction;
Co-reporter:Ji Su, Chenlu Xie, Chen Chen, Yi Yu, Griffin Kennedy, Gabor A. Somorjai, and Peidong Yang
Journal of the American Chemical Society 2016 Volume 138(Issue 36) pp:11568-11574
Publication Date(Web):September 2, 2016
DOI:10.1021/jacs.6b03915
The concept of tandem catalysis, where sequential reactions catalyzed by different interfaces in single nanostructure give desirable product selectively, has previously been applied effectively in the production of propanal from methanol (via carbon monoxide and hydrogen) and ethylene via tandem hydroformylation. However, the underlying mechanism leading to enhanced product selectivity has remained elusive due to the lack of stable, well-defined catalyst suitable for in-depth comprehensive study. Accordingly, we present the design and synthesis of a three-dimensional (3D) catalyst CeO2–Pt@mSiO2 with well-defined metal–oxide interfaces and stable architecture and investigate the selective conversion of ethylene to propanal via tandem hydroformylation. The effective production of aldehyde through the tandem hydroformylation was also observed on propylene and 1-butene. A thorough study of the CeO2–Pt@mSiO2 under different reaction and control conditions reveals that the ethylene present for the hydroformylation step slows down initial methanol decomposition, preventing the accumulation of hydrogen (H2) and favoring propanal formation to achieve up to 80% selectivity. The selectivity is also promoted by the fact that the reaction intermediates produced from methanol decomposition are poised to directly undergo hydroformylation upon migration from one catalytic interface to another. This synergistic effect between the two sequential reactions and the corresponding altered reaction pathway, compared to the single-step reaction, constitute the key advantages of this tandem catalysis. Ultimately, this in-depth study unravels the principles of tandem catalysis related to hydroformylation and represents a key step toward the rational design of new heterogeneous catalysts.
Co-reporter:Rong Ye; Bing Yuan; Jie Zhao; Walter T. Ralston; Chung-Yeh Wu; Ebru Unel Barin; F. Dean Toste
Journal of the American Chemical Society 2016 Volume 138(Issue 27) pp:8533-8537
Publication Date(Web):June 20, 2016
DOI:10.1021/jacs.6b03977
Understanding the C–C bond activation mechanism is essential for developing the selective production of hydrocarbons in the petroleum industry and for selective polymer decomposition. In this work, ring-opening reactions of cyclopropane derivatives under hydrogen catalyzed by metal nanoparticles (NPs) in the liquid phase were studied. 40-atom rhodium (Rh) NPs, encapsulated by dendrimer molecules and supported in mesoporous silica, catalyzed the ring opening of cyclopropylbenzene at room temperature under hydrogen in benzene, and the turnover frequency (TOF) was higher than other metals or the Rh homogeneous catalyst counterparts. Comparison of reactants with various substitution groups showed that electron donation on the three-membered ring boosted the TOF of ring opening. The linear products formed with 100% selectivity for ring opening of all reactants catalyzed by the Rh NP. Surface Rh(0) acted as the active site in the NP. The capping agent played an important role in the ring-opening reaction kinetics. Larger particle size tended to show higher TOF and smaller reaction activation energy for Rh NPs encapsulated in either dendrimer or poly(vinylpyrrolidone). The generation/size of dendrimer and surface group also affected the reaction rate and activation energy.
Co-reporter:Griffin Kennedy, Gérôme Melaet, Hui-Ling Han, Walter T. Ralston, and Gabor A. Somorjai
ACS Catalysis 2016 Volume 6(Issue 10) pp:7140
Publication Date(Web):September 17, 2016
DOI:10.1021/acscatal.6b01640
The hydrogenation of crotonaldehyde by platinum nanoparticles supported on cobalt oxide was used as a reaction to probe the effect of the interface between the two materials on the activity and selectivity of the catalyst. Four potential products can be formed by this reaction: propylene, butyraldehyde, crotyl alcohol, and butanol. When Pt nanoparticles are supported on SiO2, an inert support, only propylene and butyraldehyde are formed. However, when Pt is supported on cobalt oxide, the alcohols make up roughly 40% of the total activity, indicating that cobalt oxide plays a pivotal role in the reaction, much like other active supports such as TiO2. To elucidate the mechanism of alcohol formation, in situ sum frequency generation vibrational spectroscopy (SFG) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) were utilized to probe the reactant adsorption and intermediate formation and the chemical state of the materials under working catalytic conditions. The SFG data indicate that crotonaldehyde adsorbs on the oxide surface, likely through the aldehyde oxygen as well as on the Pt surface through the alkene group. AP-XPS results show that the surface of the Co3O4 support becomes partially reduced under the reaction conditions and Pt exists in its metallic state. Taking these results together, we propose that the crotonaldehyde adsorbs at reduced oxide surface sites and that this adsorption mode is responsible for the production of alcohol products. A platinum nanoparticle density dependence study was also undertaken to change the abundance of interface sites and study their effect on the reaction. The selectivity between the two alcohol products was altered as a function of the Pt nanoparticle density: higher selectivity toward butanol and lower selectivity toward crotyl alcohol was obtained with increasing density, while propylene and butyraldehyde selectivities were constant with respect to density. On the basis of the data presented, we propose that butanol is preferentially formed at the metal–oxide interface, while crotyl alcohol is formed at oxide surface sites by reaction with spillover hydrogen.Keywords: ambient-pressure XPS; cobalt oxide; hydrogenation; in situ characterization; sum frequency generation; support effects
Co-reporter:Jeong Young Park
Catalysis Letters 2016 Volume 146( Issue 1) pp:1-11
Publication Date(Web):2016 January
DOI:10.1007/s10562-015-1657-6
The development of catalytic nanodiodes to measure the flow of hot electrons generated at metal–oxide interfaces has proven that exothermic catalytic reactions on platinum induce a steady flux of hot electrons. Based on the simultaneous measurement of hot electrons and chemical reactions, it was found that chemicurrent is correlated with turnover frequency. It was shown that charge transport between the metal and oxide interfaces also influences the catalytic activity and product distribution of multipath reactions. Metal–oxide interfaces appear to produce ions that carry out reactions; these reactions have long been called “acid-base catalysis” by the organic chemistry community. A typical catalytic structure is a mesoporous oxide that is produced to hold metal nanoparticles. The structures provide high-surface-area oxide–metal interfaces that create the catalytic architecture for acid-base catalysis. Studies where the transition metal oxide is changed and only a single metal (i.e., platinum) is used for the nanoparticles show a tremendous amplification effect of the oxide–metal interfaces in the reactions (e.g., carbon monoxide oxidation). In this Perspective, we address the role of metal–oxide interfaces in generating a flow of charge carriers, thus implying a link between acid-base catalysis, the spillover process, and hot electron chemistry. We highlight recent studies on the amplification of catalytic activity when using Pt nanoparticles and various oxides (e.g., cobalt oxide, nickel oxide, manganese oxide, iron oxide) under CO oxidation, n-hexane isomerization, and cyclisation reactions, which imply that charge transfer between the metal and the oxide plays a key role in catalytic activity and selectivity. We suggest that catalytic nanodiodes can be used to detect hot electron flow, spillover, and charged reactive intermediates, which can improve a fundamental understanding of electronic excitation and charge flow in chemical reactions.
Co-reporter:James M. Krier
The Journal of Physical Chemistry C 2016 Volume 120(Issue 15) pp:8246-8250
Publication Date(Web):March 31, 2016
DOI:10.1021/acs.jpcc.6b01615
Platinum nanoparticles (NPs) capped with polyvinylpyrrolidone (PVP) were studied with sum frequency generation (SFG) vibrational spectroscopy under reaction conditions during cyclohexene (CH) and 1,4-cyclohexadiene (1,4-CHD) hydrogenation at 295 K. Despite similar vibrational features observed during reaction, CH and 1,4-CHD proceed through mutually exclusive pathways on 1.7 and 4.6 nm Pt-PVP NPs unlike Pt(111) studied previously. The intense red-shifted C–H stretch of adsorbed 1,4-CHD at 2770 cm–1 was monitored for both reactions. SFG and kinetic experiments show CH hydrogenation is active and reversible, while 1,4-CHD hydrogenation poisons the surface.
Co-reporter:Selim Alayoglu;Rong Ye;Tyler J. Hurlburt;Kairat Sabyrov
PNAS 2016 Volume 113 (Issue 19 ) pp:5159-5166
Publication Date(Web):2016-05-10
DOI:10.1073/pnas.1601766113
Colloidal chemistry is used to control the size, shape, morphology, and composition of metal nanoparticles. Model catalysts
as such are applied to catalytic transformations in the three types of catalysts: heterogeneous, homogeneous, and enzymatic.
Real-time dynamics of oxidation state, coordination, and bonding of nanoparticle catalysts are put under the microscope using
surface techniques such as sum-frequency generation vibrational spectroscopy and ambient pressure X-ray photoelectron spectroscopy
under catalytically relevant conditions. It was demonstrated that catalytic behavior and trends are strongly tied to oxidation
state, the coordination number and crystallographic orientation of metal sites, and bonding and orientation of surface adsorbates.
It was also found that catalytic performance can be tuned by carefully designing and fabricating catalysts from the bottom
up. Homogeneous and heterogeneous catalysts, and likely enzymes, behave similarly at the molecular level. Unifying the fields
of catalysis is the key to achieving the goal of 100% selectivity in catalysis.
Co-reporter:Jeong Young Park, L. Robert Baker, and Gabor A. Somorjai
Chemical Reviews 2015 Volume 115(Issue 8) pp:2781
Publication Date(Web):March 20, 2015
DOI:10.1021/cr400311p
Co-reporter:James M. Krier, William D. Michalak, Xiaojun Cai, Lindsay Carl, Kyriakos Komvopoulos, and Gabor A. Somorjai
Nano Letters 2015 Volume 15(Issue 1) pp:39-44
Publication Date(Web):October 1, 2014
DOI:10.1021/nl502566b
1,3-Butadiene (1,3-BD) hydrogenation was performed on 4 nm Pt, Pd, and Rh nanoparticles (NPs) encapsulated in SiO2 shells at 20, 60, and 100 °C. The core–shells were grown around polyvinylpyrrolidone (PVP) coated NPs (Stöber encapsulation) prepared by colloidal synthesis. Sum frequency generation (SFG) vibrational spectroscopy was performed to correlate surface intermediates observed in situ with reaction selectivity. It is shown that calcination is effective in removing PVP, and the SFG signal can be generated from the metal surface. Using SFG, it is possible to compare the surface vibrational spectrum of Pt@SiO2 (1,3-BD is hydrogenated through multiple paths and produces butane, 1-butene, and cis/trans-2-butene) to Pd@SiO2 (1,3-BD favors one path and produces 1-butene and cis/trans-2-butene). In contrast to Pt@SiO2 and Pd@SiO2, SFG and kinetic experiments of Rh@SiO2 show a permanent accumulation of organic material.
Co-reporter:Xing-Zhong Shu; Son C. Nguyen; Ying He; Fadekemi Oba; Qiao Zhang; Christian Canlas; Gabor A. Somorjai; A. Paul Alivisatos;F. Dean Toste
Journal of the American Chemical Society 2015 Volume 137(Issue 22) pp:7083-7086
Publication Date(Web):May 29, 2015
DOI:10.1021/jacs.5b04294
An efficient method for the synthesis of heterogeneous gold catalysts has been developed. These catalysts were easily assembled from readily available silica materials and gold complexes. The heterogeneous catalysts exhibited superior reactivity in various reactions where protodeauration is the rate-limiting step. Dramatic enhancement in regio- and enantioselectivity was observed when compared to the homogeneous unsupported gold catalyst. The catalysts are easily recovered and recycled up to 11 times without loss of enantioselectivity.
Co-reporter:Kyung Min Choi; Kyungsu Na; Gabor A. Somorjai;Omar M. Yaghi
Journal of the American Chemical Society 2015 Volume 137(Issue 24) pp:7810-7816
Publication Date(Web):May 29, 2015
DOI:10.1021/jacs.5b03540
Chemical environment control of the metal nanoparticles (NPs) embedded in nanocrystalline metal–organic frameworks (nMOFs) is useful in controlling the activity and selectivity of catalytic reactions. In this report, organic linkers with two functional groups, sulfonic acid (−SO3H, S) and ammonium (−NH3+, N), are chosen as strong and weak acidic functionalities, respectively, and then incorporated into a MOF [Zr6O4(OH)4(BDC)6 (BDC = 1,4-benzenedicarboxylate), termed UiO-66] separately or together in the presence of 2.5 nm Pt NPs to build a series of Pt NPs-embedded in UiO-66 (Pt⊂nUiO-66). We find that these chemical functionalities play a critical role in product selectivity and activity in the gas-phase conversion of methylcyclopentane (MCP) to acyclic isomer, olefins, cyclohexane, and benzene. Pt⊂nUiO-66-S gives the highest selectivity to C6-cyclic products (62.4% and 28.6% for cyclohexane and benzene, respectively) without acyclic isomers products. Moreover, its catalytic activity was doubled relative to the nonfunctionalized Pt⊂nUiO-66. In contrast, Pt⊂nUiO-66-N decreases selectivity for C6-cyclic products to <50% while increases the acyclic isomer selectivity to 38.6%. Interestingly, the Pt⊂nUiO-66-SN containing both functional groups gave different product selectivity than their constituents; no cyclohexane was produced, while benzene was the dominant product with olefins and acyclic isomers as minor products. All Pt⊂nUiO-66 catalysts with different functionalities remain intact and maintain their crystal structure, morphology, and chemical functionalities without catalytic deactivation after reactions over 8 h.
Co-reporter:Nathan Musselwhite; Kyungsu Na; Kairat Sabyrov; Selim Alayoglu
Journal of the American Chemical Society 2015 Volume 137(Issue 32) pp:10231-10237
Publication Date(Web):July 13, 2015
DOI:10.1021/jacs.5b04808
Several types of mesoporous aluminosilicates were synthesized and evaluated in the catalytic isomerization of n-hexane, both with and without Pt nanoparticles loaded into the mesopores. The materials investigated included mesoporous MFI and BEA type zeolites, MCF-17 mesoporous silica, and an aluminum modified MCF-17. The acidity of the materials was investigated through pyridine adsorption and Fourier Transform-Infrared Spectroscopy (FT-IR). It was found that the strong Brönsted acid sites in the micropores of the zeolite catalysts facilitated the cracking of hexane. However, the medium strength acid sites on the Al modified MCF-17 mesoporous silica greatly enhanced the isomerization reaction. Through the loading of different amounts of Pt into the mesopores of the Al modified MCF-17, the relationship between the metal nanoparticles and acidic sites on the support was revealed.
Co-reporter:Feifei Shi; Philip N. Ross; Hui Zhao; Gao Liu; Gabor A. Somorjai;Kyriakos Komvopoulos
Journal of the American Chemical Society 2015 Volume 137(Issue 9) pp:3181-3184
Publication Date(Web):February 17, 2015
DOI:10.1021/ja5128456
Although controlling the interfacial chemistry of electrodes in Li-ion batteries (LIBs) is crucial for maintaining the reversibility, electrolyte decomposition has not been fully understood. In this study, electrolyte decomposition on model electrode surfaces (Au and Sn) was investigated by in situ attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. Simultaneously obtained ATR-FTIR spectra and cyclic voltammetry measurements show that lithium ethylene dicarbonate and lithium propionate form on the Au electrode at 0.6 V, whereas diethyl 2,5-dioxahexane dicarboxylate and lithium propionate form on the Sn electrode surface at 1.25 V. A noncatalytic reduction path on the Au surface and a catalytic reduction path on the Sn surface are introduced to explain the surface dependence of the overpotential and product selectivity. This represents a new concept for explaining electrolyte reactions on the anode of LIBs. The present investigation shows that catalysis plays a dominant role in the electrolyte decomposition process and has important implications in electrode surface modification and electrolyte recipe selection, which are critical factors for enhancing the efficiency, durability, and reliability of LIBs.
Co-reporter:Yonatan Horowitz; Hui-Ling Han; Philip N. Ross
Journal of the American Chemical Society 2015 Volume 138(Issue 3) pp:726-729
Publication Date(Web):December 11, 2015
DOI:10.1021/jacs.5b10333
The key factor in long-term use of batteries is the formation of an electrically insulating solid layer that allows lithium ion transport but stops further electrolyte redox reactions on the electrode surface, hence solid electrolyte interphase (SEI). We have studied a common electrolyte, 1.0 M LiPF6/ethylene carbonate (EC)/diethyl carbonate (DEC), reduction products on crystalline silicon (Si) electrodes in a lithium (Li) half-cell system under reaction conditions. We employed in situ sum frequency generation vibrational spectroscopy (SFG-VS) with interface sensitivity in order to probe the molecular composition of the SEI surface species under various applied potentials where electrolyte reduction is expected. We found that, with a Si(100)-hydrogen terminated wafer, a Si-ethoxy (Si-OC2H5) surface intermediate forms due to DEC decomposition. Our results suggest that the SEI surface composition varies depending on the termination of Si surface, i.e., the acidity of the Si surface. We provide the evidence of specific chemical composition of the SEI on the anode surface under reaction conditions. This supports an electrochemical electrolyte reduction mechanism in which the reduction of the DEC molecule to an ethoxy moiety plays a key role. These findings shed new light on the formation mechanism of SEI on Si anodes in particular and on SEI formation in general.
Co-reporter:Hyosun Lee;Ievgen I. Nedrygailov;Changhwan Lee; Gabor A. Somorjai; Jeong Young Park
Angewandte Chemie 2015 Volume 127( Issue 8) pp:2370-2374
Publication Date(Web):
DOI:10.1002/ange.201410951
Abstract
Generation of hot electron flows and the catalytic activity of Pt nanoparticles (NPs) with different sizes were investigated using catalytic nanodiodes. We show that smaller Pt NPs lead to higher chemicurrent yield, which is associated with the shorter travel length for the hot electrons, compared with their inelastic mean free path. We also show the impact of capping on charge carrier transfer between Pt NPs and their support.
Co-reporter:Hyosun Lee;Ievgen I. Nedrygailov;Changhwan Lee; Gabor A. Somorjai; Jeong Young Park
Angewandte Chemie International Edition 2015 Volume 54( Issue 8) pp:2340-2344
Publication Date(Web):
DOI:10.1002/anie.201410951
Abstract
Generation of hot electron flows and the catalytic activity of Pt nanoparticles (NPs) with different sizes were investigated using catalytic nanodiodes. We show that smaller Pt NPs lead to higher chemicurrent yield, which is associated with the shorter travel length for the hot electrons, compared with their inelastic mean free path. We also show the impact of capping on charge carrier transfer between Pt NPs and their support.
Co-reporter:Hyosun Lee;Ievgen I. Nedrygailov;Changhwan Lee; Gabor A. Somorjai; Jeong Young Park
Angewandte Chemie International Edition 2015 Volume 54( Issue 8) pp:
Publication Date(Web):
DOI:10.1002/anie.201580861
Co-reporter:Hyosun Lee;Ievgen I. Nedrygailov;Changhwan Lee; Gabor A. Somorjai; Jeong Young Park
Angewandte Chemie 2015 Volume 127( Issue 8) pp:
Publication Date(Web):
DOI:10.1002/ange.201580861
Co-reporter:G. A. Somorjai;S. K. Beaumont
Topics in Catalysis 2015 Volume 58( Issue 10-11) pp:560-572
Publication Date(Web):2015 August
DOI:10.1007/s11244-015-0398-5
Most heterogeneous, homogeneous and enzyme catalysts are nanoparticles. Conquering the complexity of such materials’ mode of operation at the atomic and molecular level necessitates being able to elucidate their structure under operational conditions. Here, we show examples of the crucial interplay of atomic or molecular resolution in situ techniques with atomically and molecularly well-defined nanoparticle catalysts to achieve this goal. In particular we focus on mono-dispersed metal nanoparticles in the 0.8–10 nm range with precise size distribution provided by modern colloidal synthetic techniques. These have been used in conjunction with a range of in situ techniques for understanding the complexity of a number of catalytic phenomena. Drawing on the nanoparticle size discrimination afforded by this approach, most metal nanoparticle catalysed covalent bond making/breaking reactions are identified as being structure sensitive, even when that was previously not thought to be the case. Small nanoparticles, below 2 nm, have been found to have changes of electronic structure that give rise to high oxidation state clusters under reaction conditions. These have been utilized to heterogenize typically homogeneous catalytic reactions using metal nanoclusters in the range of 40 atoms or less to carry out reactions on their heterogenized surfaces that would typically be expected only to occur at the higher oxidation state metal centre of a homogeneous organometallic catalyst. The combination of in situ techniques and highly controlled metal nanoparticle structure also allows valuable insights to be achieved in understanding the mechanisms of multicomponent catalysts, catalysis occurring in different fluid phases and phenomena occurring at the metal–oxide interface.
Co-reporter:Dr. Hui-Ling Han;Dr. Gérôme Melaet;Dr. Selim Alayoglu;Dr. Gabor A. Somorjai
ChemCatChem 2015 Volume 7( Issue 22) pp:3625-3638
Publication Date(Web):
DOI:10.1002/cctc.201500642
Abstract
The present review discusses the current state of the art microscopic and spectroscopic characterization techniques available to study surfaces and interfaces under working conditions. Microscopic techniques such as environmental transmission electron microscopy and in situ transmission electron microscopy are first discussed showing their applications in the field of nanomaterials and catalysis. Next sum frequency generation vibrational spectroscopy is discussed, giving probing examples of surface studies in gaseous conditions. Synchrotron based X-ray techniques are also examined with a specific focus on ambient pressure X-ray photoelectron and absorption techniques such as near and extended X-ray absorption fine structure. Each of the techniques is evaluated, whilst the pros and cons are discussed in term of surface sensitivity, spatial resolution and/or time resolution. The second part of the articles is articulated around the future of in situ characterization, giving examples of the probable development of the discussed techniques as well as an introduction of emerging tools such as scanning transmission X-ray microscopy, ptychography, and X-ray photon correlation spectroscopy.
Co-reporter:Nathan Musselwhite
Topics in Catalysis 2015 Volume 58( Issue 2-3) pp:184-189
Publication Date(Web):2015 March
DOI:10.1007/s11244-014-0357-6
Modern industrial catalysts are highly engineered multi-component materials, which are optimized to be highly efficient for the given reaction. The key components of every catalyst are: (1) the active metal and (2) the support. The former is active through the formation of covalent bonds with surface species, activating the bonds in the targeted molecule. In many systems the support is a metal oxide, which can promote charged intermediates in acid–base type catalytic chemistry. When the covalent chemistry of the metal catalyst and the acid–base chemistry of the support work together, the overall catalytic productivity can be much greater than the sum of the parts. This synergic interaction also works in favor of changes in the selectivity of complex reactions. This intent of this article is to analyze covalent metal catalysis both alone and in tandem with acid–base heterogeneous catalysis.
Co-reporter:Selim Alayoglu
Catalysis Letters 2015 Volume 145( Issue 1) pp:249-271
Publication Date(Web):2015 January
DOI:10.1007/s10562-014-1398-y
Model nano-catalysts with monodisperse particle sizes and architectures are essential for a fundamental understanding of surface property dynamics during catalytic reactions. Surface tools and techniques, when conducted under catalytically relevant temperature and pressure conditions, render possible measurements of dynamic surface properties such as oxidation state, composition, coordination, and bonding. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy with purposely built in situ reaction cells and ambient pressure X-ray photoelectron spectroscopy (APXPS) provide (near) surface sensitive and chemical specific information on the oxidation states of metal and oxide (co-)catalysts as well as adsorbent functional elements such C, O and N under reactive gas atmospheres and even liquid environments. Likewise, sum frequency generation (SFG) vibrational spectroscopy with in situ reaction cells helps uncover the bonding geometry and configuration of the topmost surface again under conditions pertinent to catalysis. Furthermore, the local dynamics in the nanoscale and on the single particle level are revealed by environmental transmission electron microscopy (ETEM) and the spectro-microscopy techniques equipped within. A correlative approach, where an array of these in situ tools and techniques were conducted in parallel with catalytic measurements, was employed to gain molecular insight into some of the modern scientific challenges in heterogeneous catalysis. Several case examples of this correlative approach are presented here. The CO oxidation reaction over hybrid nano-catalysts of Pt nanoparticles (NPs) with various mesoporous metal oxides such as Co3O4, MnO2 and CeO2 was explored in relation to bifunctional catalysis and interfacial charge transfer chemistry by using in situ NEXAFS spectroscopy. Likewise, bimetallic CoPt and PtSn nanoparticle catalysts supported on silica were investigated by using a combination of in situ NEXAFS spectroscopy and APXPS. Next, CO2 hydrogenation was carried out over bimetallic CoPt/SiO2 and Co/TiO2 hybrid nano-catalysts. In this case, in situ NEXAFS spectroscopy, APXPS, and ETEM indicated severe, yet reversible, surface restructuring that involved hydrogen atom spillover. Finally, ~2 nm Pt NPs were investigated using in situ SFG to study hydrogenation and hydrogenative isomerization reactions. Specifically, SFG indicated that the hydrogenation of furfural and crotonaldehyde proceed by interfacial hydrogen atom spillover from TiO2, while the hydrogenative isomerization of methylcyclopentane (MCP) proceeds by spillover and surface diffusion of cyclohexene over mesoporous zeolites. These studies unequivocally indicated the presence of a particular reaction channel that involved one way flow of charged (i.e. electrons or protons) or neutral species (i.e. reactants) at a broadly defined interface between metals and oxides. In addition to these case studies, experimental approaches employing capillary flow micro-reactors are discussed in relation toward the goal of short time resolutions that could help isolate such charged or neutral intermediates in the future.
Co-reporter:Kyungsu Na
Catalysis Letters 2015 Volume 145( Issue 1) pp:193-213
Publication Date(Web):2015 January
DOI:10.1007/s10562-014-1411-5
The research field of hierarchically nanoporous zeolites has been growing at an enormous pace over the past decades. Hierarchically nanoporous zeolites have versatile structural properties such as high surface area and large pore volume that can alleviate diffusional limitations of conventional zeolites with solely microporous framework. In this review, various synthesis strategies to hierarchically nanoporous zeolites and their structural advantages in catalytic reactions will be reviewed. In the first part, many novel synthetic approaches for hierarchically nanoporous zeolites such as post-demetallation, soft-templating, hard-templating, and dual-pore-generating surfactant-directed methods will be introduced. In the second part, catalytic applications of hierarchically nanoporous zeolites on various chemical reactions involving isomerization, cracking, alkylation and oxidation will be discussed. The present comprehensive review will provide future opportunities and perspectives on the research of hierarchically nanoporous zeolites including their applications to catalytic reactions.
Co-reporter:Kwangjin An
Catalysis Letters 2015 Volume 145( Issue 1) pp:233-248
Publication Date(Web):2015 January
DOI:10.1007/s10562-014-1399-x
In recent heterogeneous catalysis, much effort has been made in understanding how the size, shape, and composition of nanoparticles and oxide-metal interfaces affect catalytic performance at the molecular level. Recent advances in colloidal synthetic techniques enable preparing diverse metallic or bimetallic nanoparticles with well-defined size, shape, and composition and porous oxides as a high surface support. As nanoparticles become smaller, new chemical, physical, and catalytic properties emerge. Geometrically, as the smaller the nanoparticle the greater the relative number of edge and corner sites per unit surface of the nanoparticle. When the nanoparticles are smaller than a critical size (2.7 nm), finite-size effects such as a change of adsorption strength or oxidation state are revealed by changes in their electronic structures. By alloying two metals, the formation of heteroatom bonds and geometric effects such as strain due to the change of metal–metal bond lengths cause new electronic structures to appear in bimetallic nanoparticles. Ceaseless catalytic reaction studies have been discovered that the highest reaction yields, product selectivity, and process stability were achieved by determining the critical size, shape, and composition of nanoparticles and by choosing the appropriate oxide support. Depending on the pore size, various kinds of micro-, meso-, and macro-porous materials are fabricated by the aid of structure-directing agents or hard-templates. Recent achievements for the preparation of versatile core/shell nanostructures composing mesoporous oxides, zeolites, and metal organic frameworks provide new insights toward nanocatalysis with novel ideas.
Co-reporter:Kwangjin An, Qiao Zhang, Selim Alayoglu, Nathan Musselwhite, Jae-Youn Shin, and Gabor A. Somorjai
Nano Letters 2014 Volume 14(Issue 8) pp:4907-4912
Publication Date(Web):July 31, 2014
DOI:10.1021/nl502434m
Designing catalysts with high thermal stability and resistance to deactivation while simultaneously maintaining their catalytic activity and selectivity is of key importance in high-temperature reforming reactions. We prepared Pt nanoparticle catalysts supported on either mesoporous SiO2 or TiO2. Sandwich-type Pt core@shell catalysts (SiO2@Pt@SiO2 and SiO2@Pt@TiO2) were also synthesized from Pt nanoparticles deposited on SiO2 spheres, which were encapsulated by either mesoporous SiO2 or TiO2 shells. n-Hexane reforming was carried out over these four catalysts at 240–500 °C with a hexane/H2 ratio of 1:5 to investigate thermal stability and the role of the support. For the production of high-octane gasoline, branched C6 isomers are more highly desired than other cyclic, aromatic, and cracking products. Over Pt/TiO2 catalyst, production of 2-methylpentane and 3-methylpentane via isomerization was increased selectively up to 420 °C by charge transfer at Pt–TiO2 interfaces, as compared to Pt/SiO2. When thermal stability was compared between supported catalysts and sandwich-type core@shell catalysts, the Pt/SiO2 catalyst suffered sintering above 400 °C, whereas the SiO2@Pt@SiO2 catalyst preserved the Pt nanoparticle size and shape up to 500 °C. The SiO2@Pt@TiO2 catalyst led to Pt nanoparticle sintering due to incomplete protection of the TiO2 shells during the reaction at 500 °C. Interestingly, over the Pt/TiO2 catalyst, the average size of Pt nanoparticles was maintained even after 500 °C without sintering. In situ ambient pressure X-ray photoelectron spectroscopy demonstrated that the Pt/TiO2 catalyst did not exhibit TiO2 overgrowth on the Pt surface or deactivation by Pt sintering up to 600 °C. The extraordinarily high stability of the Pt/TiO2 catalyst promoted high reaction rates (2.0 μmol·g–1·s–1), which was 8 times greater than other catalysts and high isomer selectivity (53.0% of C6 isomers at 440 °C). By the strong metal–support interaction, the Pt/TiO2 was turned out as the best catalyst with great thermal stability as well as high reaction rate and product selectivity in high-temperature reforming reaction.
Co-reporter:Andras Sapi, Fudong Liu, Xiaojun Cai, Christopher M. Thompson, Hailiang Wang, Kwangjin An, James M. Krier, and Gabor A. Somorjai
Nano Letters 2014 Volume 14(Issue 11) pp:6727-6730
Publication Date(Web):October 22, 2014
DOI:10.1021/nl5035545
Pt nanoparticles with controlled size (2, 4, and 6 nm) are synthesized and tested in ethanol oxidation by molecular oxygen at 60 °C to acetaldehyde and carbon dioxide both in the gas and liquid phases. The turnover frequency of the reaction is ∼80 times faster, and the activation energy is ∼5 times higher at the gas–solid interface compared to the liquid–solid interface. The catalytic activity is highly dependent on the size of the Pt nanoparticles; however, the selectivity is not size sensitive. Acetaldehyde is the main product in both media, while twice as much carbon dioxide was observed in the gas phase compared to the liquid phase. Added water boosts the reaction in the liquid phase; however, it acts as an inhibitor in the gas phase. The more water vapor was added, the more carbon dioxide was formed in the gas phase, while the selectivity was not affected by the concentration of the water in the liquid phase. The differences in the reaction kinetics of the solid–gas and solid–liquid interfaces can be attributed to the molecular orientation deviation of the ethanol molecules on the Pt surface in the gas and liquid phases as evidenced by sum frequency generation vibrational spectroscopy.
Co-reporter:Kyungsu Na, Kyung Min Choi, Omar M. Yaghi, and Gabor A. Somorjai
Nano Letters 2014 Volume 14(Issue 10) pp:5979-5983
Publication Date(Web):September 8, 2014
DOI:10.1021/nl503007h
The growth of nanocrystalline metal–organic frameworks (nMOFs) around metal nanocrystals (NCs) is useful in controlling the chemistry and metric of metal NCs. In this Letter, we show rare examples of nMOFs grown in monocrystalline form around metal NCs. Specifically, Pt NCs were subjected to reactions yielding Zr(IV) nMOFs [Zr6O4(OH)4(fumarate)6, MOF-801; Zr6O4(OH)4(BDC)6 (BDC = 1,4-benzenedicarboxylate), UiO-66; Zr6O4(OH)4(BPDC)6 (BPDC = 4,4′-biphenyldicarboxylate), UiO-67] as a single crystal within which the Pt NCs are embedded. These constructs (Pt⊂nMOF)nanocrystal are found to be active in gas-phase hydrogenative conversion of methylcyclopentane (MCP) and give unusual product selectivity. The Pt⊂nUiO-66 shows selectivity to C6-cyclic hydrocarbons such as cyclohexane and benzene that takes place with 100 °C lower temperature than the standard reaction (Pt-on-SiO2). We observe a pore size effect in the nMOF series where the small pore of Pt⊂nMOF-801 does not produce the same products, while the larger pore Pt⊂nUiO-67 catalyst provides the same products but with different selectivity. The (Pt⊂nMOF)nanocrystal spent catalyst is found to maintain the original crystallinity, and be recyclable without any byproduct residues.
Co-reporter:Simon K. Beaumont, Selim Alayoglu, Colin Specht, Norbert Kruse, and Gabor A. Somorjai
Nano Letters 2014 Volume 14(Issue 8) pp:4792-4796
Publication Date(Web):July 15, 2014
DOI:10.1021/nl501969k
Hydrogen spillover is of great importance to understanding many phenomena in heterogeneous catalysis and has long been controversial. Here we exploit well-defined nanoparticles to demonstrate its occurrence through evaluation of CO2 methanation kinetics. Combining platinum and cobalt nanoparticles causes a substantial increase in reaction rate, but increasing the spatial separation between discrete cobalt and platinum entities results in a dramatic ∼50% drop in apparent activation energy, symptomatic of H atom surface diffusion limiting the reaction rate.
Co-reporter:Kyungsu Na ; Selim Alayoglu ; Rong Ye
Journal of the American Chemical Society 2014 Volume 136(Issue 49) pp:17207-17212
Publication Date(Web):November 14, 2014
DOI:10.1021/ja509273h
The effect of acidic properties of mesoporous zeolites on the control of product selectivity during the hydrogenative isomerization of methylcyclopentane has been investigated. A series of mesoporous zeolites with controlled acidic properties were prepared by postdealumination process with hydrochloric acid under hydrothermal conditions, and the resultant zeolites used for supporting colloidal Pt nanoparticles (NPs) with a mean size of 2.5 nm (±0.6 nm). As compared to the pure Pt NPs supported on catalytically inert mesoporous silica (MCF-17) as the reference catalyst that can produce isomers most selectively (∼80%), the Pt NPs supported on mesoporous zeolites produced C6-cyclic hydrocarbons (i.e., cyclohexane and benzene) most dominantly. The type and strength of the Brönsted (B) and Lewis (L) acid sites of those zeolites with a controlled Al amount are analyzed by using FT-IR after the adsorption of pyridine and NH3 temperature-programmed desorption measurements, and they are correlated with the selectivity change between cyclohexane and benzene. From this investigation, we found a linear relationship between the number of Brönsted acid sites and the formation rate for cyclohexane. In addition, we revealed that more Lewis acidic zeolite having relatively smaller B/L ratio is effective for the cyclohexane formation, whereas more Brönsted acidic zeolite having relatively larger B/L ratio is effective for the benzene formation.
Co-reporter:Gérôme Melaet ; Walter T. Ralston ; Cheng-Shiuan Li ; Selim Alayoglu ; Kwangjin An ; Nathan Musselwhite ; Bora Kalkan
Journal of the American Chemical Society 2014 Volume 136(Issue 6) pp:2260-2263
Publication Date(Web):January 24, 2014
DOI:10.1021/ja412447q
Hydrogenations of CO or CO2 are important catalytic reactions as they are interesting alternatives to produce fine chemical feedstock hence avoiding the use of fossil sources. Using monodisperse nanoparticle (NP) catalysts, we have studied the CO/H2 (i.e., Fischer–Tropsch synthesis) and CO2/H2 reactions. Exploiting synchrotron based in situ characterization techniques such as XANES and XPS, we were able to demonstrate that 10 nm Co NPs cannot be reduced at 250 °C while supported on TiO2 or SiO2 and that the complete reduction of cobalt can only be achieved at 450 °C. Interestingly, cobalt oxide performs better than fully reduced cobalt when supported on TiO2. In fact, the catalytic results indicate an enhancement of 10-fold for the CO2/H2 reaction rate and 2-fold for the CO/H2 reaction rate for the Co/TiO2 treated at 250 °C in H2 versus Co/TiO2 treated at 450 °C. Inversely, the activity of cobalt supported on SiO2 has a higher turnover frequency when cobalt is metallic. The product distributions could be tuned depending on the support and the oxidation state of cobalt. For oxidized cobalt on TiO2, we observed an increase of methane production for the CO2/H2 reaction whereas it is more selective to unsaturated products for the CO/H2 reaction. In situ investigation of the catalysts indicated wetting of the TiO2 support by CoOx and partial encapsulation of metallic Co by TiO2–x.
Co-reporter:Elad Gross ; Xing-Zhong Shu ; Selim Alayoglu ; Hans A. Bechtel ; Michael C. Martin ; F. Dean Toste
Journal of the American Chemical Society 2014 Volume 136(Issue 9) pp:3624-3629
Publication Date(Web):February 5, 2014
DOI:10.1021/ja412740p
Analysis of catalytic organic transformations in flow reactors and detection of short-lived intermediates are essential for optimization of these complex reactions. In this study, spectral mapping of a multistep catalytic reaction in a flow microreactor was performed with a spatial resolution of 15 μm, employing micrometer-sized synchrotron-based IR and X-ray beams. Two nanometer sized Au nanoclusters were supported on mesoporous SiO2, packed in a flow microreactor, and activated toward the cascade reaction of pyran formation. High catalytic conversion and tunable products selectivity were achieved under continuous flow conditions. In situ synchrotron-sourced IR microspectroscopy detected the evolution of the reactant, vinyl ether, into the primary product, allenic aldehyde, which then catalytically transformed into acetal, the secondary product. By tuning the residence time of the reactants in a flow microreactor a detailed analysis of the reaction kinetics was performed. An in situ micrometer X-ray absorption spectroscopy scan along the flow reactor correlated locally enhanced catalytic conversion, as detected by IR microspectroscopy, to areas with high concentration of Au(III), the catalytically active species. These results demonstrate the fundamental understanding of the mechanism of catalytic reactions which can be achieved by the detailed mapping of organic transformations in flow reactors .
Co-reporter:Nathan Musselwhite, Kyungsu Na, Selim Alayoglu, and Gabor A. Somorjai
Journal of the American Chemical Society 2014 Volume 136(Issue 47) pp:16661-16665
Publication Date(Web):October 31, 2014
DOI:10.1021/ja509638w
When pure mesoporous silica (MCF-17) was modified with aluminum (Al modified MCF-17), Lewis acid sites were created, but this material was inactive for the catalytic conversion (reforming) of n-hexane to isomers. When colloidally synthesized platinum nanoparticles were loaded onto traditional MCF-17, the catalyst showed very low activity toward isomer production. However, when Pt nanoparticles were loaded onto Al modified MCF-17, isomerization became the dominant catalytic pathway, with extremely high activity and selectivity (>90%), even at high temperatures (240–360 °C). This highly efficient catalytic chemistry was credited to the tandem effect between the acidic Al modified MCF-17 and the Pt metal.
Co-reporter:Simon K. Beaumont, Selim Alayoglu, Colin Specht, William D. Michalak, Vladimir V. Pushkarev, Jinghua Guo, Norbert Kruse, and Gabor A. Somorjai
Journal of the American Chemical Society 2014 Volume 136(Issue 28) pp:9898-9901
Publication Date(Web):June 30, 2014
DOI:10.1021/ja505286j
The mechanistic role of platinum and precious metals in promoting cobalt hydrogenation catalysts of the type used in reactions such as Fischer–Tropsch synthesis is highly debated. Here we use well-defined monometallic Pt and Co nanoparticles (NPs) and CO2 methanation as a probe reaction to show that Pt NPs deposited near Co NPs can enhance the CO2 methanation rate by up to a factor of 6 per Co surface atom. In situ NEXAFS spectroscopy of these same Pt NP plus Co NP systems in hydrogen shows that the presence of nearby Pt NPs is able to significantly enhance reduction of the Co at temperatures relevant to Fischer–Tropsch synthesis and CO2 methanation. The mechanistic role of Pt in these reactions is discussed in light of these findings.
Co-reporter:Hailiang Wang, Andras Sapi, Christopher M. Thompson, Fudong Liu, Danylo Zherebetskyy, James M. Krier, Lindsay M. Carl, Xiaojun Cai, Lin-Wang Wang, and Gabor A. Somorjai
Journal of the American Chemical Society 2014 Volume 136(Issue 29) pp:10515-10520
Publication Date(Web):July 3, 2014
DOI:10.1021/ja505641r
We synthesize platinum nanoparticles with controlled average sizes of 2, 4, 6, and 8 nm and use them as model catalysts to study isopropanol oxidation to acetone in both the liquid and gas phases at 60 °C. The reaction at the solid/liquid interface is 2 orders of magnitude slower than that at the solid/gas interface, while catalytic activity increases with the size of platinum nanoparticles for both the liquid-phase and gas-phase reactions. The activation energy of the gas-phase reaction decreases with the platinum nanoparticle size and is in general much higher than that of the liquid-phase reaction which is largely insensitive to the size of catalyst nanoparticles. Water substantially promotes isopropanol oxidation in the liquid phase. However, it inhibits the reaction in the gas phase. The kinetic results suggest different mechanisms between the liquid-phase and gas-phase reactions, correlating well with different orientations of IPA species at the solid/liquid interface vs the solid/gas interface as probed by sum frequency generation vibrational spectroscopy under reaction conditions and simulated by computational calculations.
Co-reporter:Hailiang Wang;Kwangjin An;Andras Sapi;Fudong Liu
Catalysis Letters 2014 Volume 144( Issue 11) pp:1930-1938
Publication Date(Web):2014 November
DOI:10.1007/s10562-014-1347-9
We compare catalytic methanol oxidation reactions in the gas and liquid phases by focusing on the kinetic effects of platinum nanoparticle size and metal/support interactions. Under the reaction conditions at 60 °C, methanol can be oxidized to multiple products including carbon dioxide (full-oxidation product), formaldehyde (partial-oxidation product) and methyl formate (partial-oxidation and coupling product). We use 2, 4, 6 and 8 nm platinum nanoparticles supported on mesoporous silica as catalysts to study the size effect, and 2.5 nm platinum nanoparticles supported on mesoporous SiO2, Co3O4, MnO2, Fe2O3, NiO and CeO2 to study the metal/oxide interface effect. We find that all three products are formed with comparable selectivities in the gas phase, but in the liquid phase formaldehyde is the dominant product. While the influence of size on activity is not substantial in the gas phase, the liquid-phase reaction rates monotonically increase by a factor of 6 in the size range of 2–8 nm. The reaction rates in the gas phase are dramatically affected by the strong interactions between the platinum nanoparticles and transition metal oxide supports. While the Pt/MnO2 is 135 times less active, the Pt/CeO2 is 12 times more active, both compared to the Pt/SiO2. However in the liquid phase, the support effect is less significant, with the most active catalyst Pt/MnO2 exhibiting an enhancement factor of 2.5 compared to the Pt/SiO2. Our results suggest that the kinetic effects of platinum nanoparticle size and metal/support interactions can be totally different between the solid/gas and solid/liquid interfaces even for the same chemical reaction.
Co-reporter:Gérôme Melaet;Avery E. Lindeman
Topics in Catalysis 2014 Volume 57( Issue 6-9) pp:500-507
Publication Date(Web):2014 April
DOI:10.1007/s11244-013-0206-z
We have investigated the effect of cobalt nanoparticle size in Fischer–Tropsch synthesis (CO/H2) and have compared it to data obtained for carbon dioxide hydrogenation (CO2/H2) using model catalysts produced by colloidal methods. Both reactions demonstrated size dependence, in which we observed an increase of the turnover frequency with increasing average particle size. In both case, a maximum activity was found for cobalt particles around 10–11 nm in size. Regarding the selectivity, no size-dependent effect has been observed for the CO2 hydrogenation, whereas CO hydrogenation selectivity depends both on the temperature and on the size of the particles. The hydrogenation of CO2 produces mainly methane and carbon monoxide for all sizes and temperatures. The Fischer–Tropsch reaction exhibited small changes in the selectivity at low temperature (below 250 °C) while at high temperatures we observed an increase in chain growth with the increase of the size of cobalt particles. At 250 °C, large crystallites exhibit a higher selectivity to olefin than to the paraffin equivalents, indicating a decrease in the hydrogenation activity.
Co-reporter:Zhongwei Zhu, Cédric Barroo, Leonid Lichtenstein, Baran Eren, Cheng Hao Wu, Baohua Mao, Thierry Visart de Bocarmé, Zhi Liu, Norbert Kruse, Miquel Salmeron, and Gabor A Somorjai
The Journal of Physical Chemistry Letters 2014 Volume 5(Issue 15) pp:2626-2631
Publication Date(Web):July 18, 2014
DOI:10.1021/jz501341r
We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C2H4 in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the clusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C2H4 partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidyne—a strong adsorbate formed from ethylene—that does not form on the 4-fold (100)-step sites on Pt(557).Keywords: ambient-pressure X-ray photoelectron spectroscopy; high-pressure scanning tunneling microscopy; platinum; step orientation; surface reconstruction;
Co-reporter:Kyungsu Na, Nathan Musselwhite, Xiaojun Cai, Selim Alayoglu, and Gabor A. Somorjai
The Journal of Physical Chemistry A 2014 Volume 118(Issue 37) pp:8446-8452
Publication Date(Web):April 28, 2014
DOI:10.1021/jp501775q
Selective C–C and C–H bond activations are an important catalytic process to produce various value-added hydrocarbons via reforming processes. For producing desired product with a high yield, control of reaction pathway through the design of catalyst and fundamental understanding and clarification of reaction mechanism are prerequisite. In this work, we designed heterogeneous catalysts by combining Pt nanoparticles and two different mesoporous zeolites with microporous frameworks of BEA and MFI for the hydrogenative model reforming reaction of hydrocarbon (i.e., methylcyclopentane). Depending on the catalyst combination, the reaction pathways of (i) dehydrogenation, (ii) ring-opening with isomerization, and ring-enlargement with (iii) hydrogenation and (iv) dehydrogenation of C5-cyclic ring to C6-cyclic ring (i.e., cyclohexane and benzene) can be controlled to produce various products with high yields. Furthermore, we revealed a reaction intermediate formed at the interface of Pt and zeolite by real-time surface vibrational sum-frequency generation spectroscopic studies. This study would provide practical and fundamental insights for design of heterogeneous catalyst for controlling reaction pathways.
Co-reporter:Elad Gross
Topics in Catalysis 2014 Volume 57( Issue 10-13) pp:812-821
Publication Date(Web):2014 June
DOI:10.1007/s11244-014-0243-2
The environment that surrounds catalytically active metallic nanoclusters has an important role in tuning their catalytic reactivity and selectivity and can initiate novel reaction routes. In this review we will demonstrate two different approaches for utilization of the environment in bi-functional, mesoscale catalysts. In these catalytic systems, the molecules that surround the metallic nanoclusters have an active role as co-catalysts. In the first bi-functional system, the steric effects of metal-adsorbed ligands have been exploited for regulating the adsorption orientation of reactants. By favoring specific orientation of the reactants, the products selectivity was widely tuned. In the second bi-functional catalytic system, the catalytic properties of the metallic nanoclusters were controlled by their encapsulation within a polymeric matrix. The oxidation state, catalytic reactivity and stability of metallic nanoclusters were tuned by their encapsulation in polyamidoamine dendrimer molecules. Oxidation of dendrimer-encapsulated nanoclusters into highly oxidized metal-ions activated the catalyst toward a variety of reactions which were previously catalyzed by homogeneous catalysts. Moreover, by modifying the properties of the polymeric matrix, enhanced chemo-, diastereo- and enantio-selectivity were obtained. These two examples of mesoscale catalysts indicate the important role of the surrounding environment in tuning the catalytic reactivity and selectivity. In addition, it is demonstrated that these catalysts can function as a bridge between homogeneous and heterogeneous catalysis.
Co-reporter:Kyungsu Na;Qiao Zhang
Journal of Cluster Science 2014 Volume 25( Issue 1) pp:83-114
Publication Date(Web):2014 January
DOI:10.1007/s10876-013-0636-6
Metal nanoparticles are key materials in heterogeneous catalysis due to their high catalytic activity and selectivity to the desired product. Accordingly, they are playing a pivotal role in most heterogeneous catalytic reactions that are steeply growing with the development of a colloidal synthetic protocol that enables fine control of size, shape, morphology and composition of metal nanoparticles at an atomic level. These colloidal metal nanoparticles can be dispersed on a rigid support such as mesoporous silica, metal oxide and zeolite, which utilizes metal nanoparticles as model heterogeneous catalysts in industrially important processes involving hydrogenation/dehydrogenation, isomerization and cracking. In this review article, we highlight the recent progress on general colloidal synthetic routes with technological advances in characterization tools that enable the atomic-scale observation of metal nanoparticles. Structure-dependent contributions on the control of product selectivity and turnover rate are also discussed by combining advanced ex situ and in situ surface characterization tools that can monitor the structural change of metal nanocatalysts as well as the evolution of reaction intermediates under the reaction conditions.
Co-reporter:Griffin Kennedy;Dr. L. Robert Baker; Gabor A. Somorjai
Angewandte Chemie 2014 Volume 126( Issue 13) pp:3473-3476
Publication Date(Web):
DOI:10.1002/ange.201400081
Abstract
The hydrogenation of crotonaldehyde in the presence of supported platinum nanoparticles was used to determine how the interaction between the metal particles and their support can control catalytic performance. Using gas-phase catalytic reaction studies and in situ sum-frequency generation vibrational spectroscopy (SFG) to study Pt/TiO2 and Pt/SiO2 catalysts, a unique reaction pathway was identified for Pt/TiO2, which selectively produces alcohol products. The catalytic and spectroscopic data obtained for the Pt/SiO2 catalyst shows that SiO2 has no active role in this reaction. SFG spectra obtained for the Pt/TiO2 catalyst indicate the presence of a crotyl-oxy surface intermediate. By adsorption through the aldehyde oxygen atom to an O-vacancy site on the TiO2 surface, the CO bond of crotonaldehyde is activated, by charge transfer, for hydrogenation. This intermediate reacts with spillover H provided by the Pt to produce crotyl alcohol.
Co-reporter:Griffin Kennedy;Dr. L. Robert Baker; Gabor A. Somorjai
Angewandte Chemie International Edition 2014 Volume 53( Issue 13) pp:3405-3408
Publication Date(Web):
DOI:10.1002/anie.201400081
Abstract
The hydrogenation of crotonaldehyde in the presence of supported platinum nanoparticles was used to determine how the interaction between the metal particles and their support can control catalytic performance. Using gas-phase catalytic reaction studies and in situ sum-frequency generation vibrational spectroscopy (SFG) to study Pt/TiO2 and Pt/SiO2 catalysts, a unique reaction pathway was identified for Pt/TiO2, which selectively produces alcohol products. The catalytic and spectroscopic data obtained for the Pt/SiO2 catalyst shows that SiO2 has no active role in this reaction. SFG spectra obtained for the Pt/TiO2 catalyst indicate the presence of a crotyl-oxy surface intermediate. By adsorption through the aldehyde oxygen atom to an O-vacancy site on the TiO2 surface, the CO bond of crotonaldehyde is activated, by charge transfer, for hydrogenation. This intermediate reacts with spillover H provided by the Pt to produce crotyl alcohol.
Co-reporter:Gérôme Melaet ; Walter T. Ralston ; Wen-Chi Liu
The Journal of Physical Chemistry C 2014 Volume 118(Issue 46) pp:26921-26925
Publication Date(Web):October 28, 2014
DOI:10.1021/jp5095917
The catalytic hydrogenation of carbon monoxide, known as the Fischer–Tropsch process, is a technologically important, complex multipath reaction which produces long-chain hydrocarbons. In order to access the initial kinetics and the mechanism, we developed a reactor that provides information under nonsteady state conditions. We tested a CoMgO catalyst and monitored the initial product formation within 2 s of exposure to CO as well as the time dependence of high molecular weight products (in a 60 s window) and found drastic changes in the product selectivity. The probability for forming branched isomer (C4 and C5) peaks in the first 25 s, and within that time frame no unsaturated products were detected. The subsequent decline (at ∼35 to 40 s) of branched isomers coincides with the detection of olefins (from C2 to C5), indicating a change in the reaction path.
Co-reporter:Feifei Shi ; Hui Zhao ; Gao Liu ; Philip N. Ross ; Gabor A. Somorjai ;Kyriakos Komvopoulos
The Journal of Physical Chemistry C 2014 Volume 118(Issue 27) pp:14732-14738
Publication Date(Web):June 16, 2014
DOI:10.1021/jp500558x
The formation of passive films on electrodes due to electrolyte decomposition significantly affects the reversibility of Li-ion batteries (LIBs); however, understanding of the electrolyte decomposition process is still lacking. The decomposition products of ethylene carbonate (EC)-based electrolytes on Sn and Ni electrodes are investigated in this study by Fourier transform infrared (FTIR) spectroscopy. The reference compounds, diethyl 2,5-dioxahexane dicarboxylate (DEDOHC) and polyethylene carbonate (poly-EC), were synthesized, and their chemical structures were characterized by FTIR spectroscopy and nuclear magnetic resonance (NMR). Assignment of the vibration frequencies of these compounds was assisted by quantum chemical (Hartree–Fock) calculations. The effect of Li-ion solvation on the FTIR spectra was studied by introducing the synthesized reference compounds into the electrolyte. EC decomposition products formed on Sn and Ni electrodes were identified as DEDOHC and poly-EC by matching the features of surface species formed on the electrodes with reference spectra. The results of this study demonstrate the importance of accounting for the solvation effect in FTIR analysis of the decomposition products forming on LIB electrodes.
Co-reporter:Andras Sapi;Chris Thompson;Hailiang Wang;William D. Michalak
Catalysis Letters 2014 Volume 144( Issue 7) pp:1151-1158
Publication Date(Web):2014 July
DOI:10.1007/s10562-014-1272-y
The effect of pretreatment (O2 or H2) and catalyst history was investigated through room temperature ethylene hydrogenation reaction over several types of platinum based nanoparticle systems: 1.6 nm Pt/TTAB, 4.1 nm Pt/PVP (with and without UV treatment), 4.1 nm Pt with a silica shell, and e-beam evaporated Pt thin films were tested. The H2 pretreatment resulted in the absence of activity. However, Pt active sites for the ethylene hydrogenation reaction were recovered after an O2 pretreatment irrespective of the catalyst history, regardless of the particle size nor the presence, absence or type of capping agent. The calculation of the average TOF resulted in 10.13 ± 3.27. This value correlates well with data from the literature. Thus, the ethylene hydrogenation reaction can be used to determine available sites of Pt catalysts if the reaction is following an O2 pretreatment.
Co-reporter:Hailiang Wang, Yihai Wang, Zhongwei Zhu, Andras Sapi, Kwangjin An, Griffin Kennedy, William D. Michalak, and Gabor A. Somorjai
Nano Letters 2013 Volume 13(Issue 6) pp:2976-2979
Publication Date(Web):May 23, 2013
DOI:10.1021/nl401568x
Pt nanoparticles with various sizes of 1, 2, 4, and 6 nm were synthesized and studied as catalysts for gas-phase methanol oxidation reaction toward formaldehyde and carbon dioxide under ambient pressure (10 Torr of methanol, 50 Torr of oxygen, and 710 Torr of helium) at a low temperature of 60 °C. While the 2, 4, and 6 nm nanoparticles exhibited similar catalytic activity and selectivity, the 1 nm nanoparticles showed a significantly higher selectivity toward partial oxidation of methanol to formaldehyde, but a lower total turnover frequency. The observed size effect in catalysis was correlated to the size-dependent structure and oxidation state of the Pt nanoparticles. X-ray photoelectron spectroscopy and infrared vibrational spectroscopy using adsorbed CO as molecular probes revealed that the 1 nm nanoparticles were predominantly oxidized while the 2, 4, and 6 nm nanoparticles were largely metallic. Transmission electron microscopy imaging witnessed the transition from crystalline to quasicrystalline structure as the size of the Pt nanoparticles was reduced to 1 nm. The results highlighted the important impact of size-induced oxidation state of Pt nanoparticles on catalytic selectivity as well as activity in gas-phase methanol oxidation reactions.
Co-reporter:Feifei Shi, L. Robert Baker, Antoine Hervier, Gabor A. Somorjai, and Kyriakos Komvopoulos
Nano Letters 2013 Volume 13(Issue 9) pp:4469-4474
Publication Date(Web):August 7, 2013
DOI:10.1021/nl402392u
Two times higher activity and three times higher stability in methanol oxidation reaction, a 0.12 V negative shift of the CO oxidation peak potential, and a 0.07 V positive shift of the oxygen reaction potential compared to Pt nanoparticles on pristine TiO2 support were achieved by tuning the electronic structure of the titanium oxide support of Pt nanoparticle catalysts. This was accomplished by adding oxygen vacancies or doping with fluorine. Experimental trends are interpreted in the context of an electronic structure model, showing an improvement in electrochemical activity when the Fermi level of the support material in Pt/TiOx systems is close to the Pt Fermi level and the redox potential of the reaction. The present approach provides guidance for the selection of the support material of Pt/TiOx systems and may be applied to other metal-oxide support materials, thus having direct implications in the design and optimization of fuel cell catalyst supports.
Co-reporter:Sun Mi Kim, Seon Joo Lee, Seung Hyun Kim, Sangku Kwon, Ki Ju Yee, Hyunjoon Song, Gabor A. Somorjai, and Jeong Young Park
Nano Letters 2013 Volume 13(Issue 3) pp:1352-1358
Publication Date(Web):February 21, 2013
DOI:10.1021/nl400367m
Hybrid nanocatalysts consisting of metal nanoparticle–semiconductor junctions offer an interesting platform to study the role of metal–oxide interfaces and hot electron flows in heterogeneous catalysis. Here, we report that hot carriers generated upon photon absorption significantly impact the catalytic activity of CO oxidation. We found that Pt–CdSe–Pt nanodumbbells exhibit a higher turnover frequency by a factor of 2 during irradiation by light with energy higher than the bandgap of CdSe, while the turnover rate on bare Pt nanoparticles did not depend on light irradiation. We found that Pt nanoparticles deposited on a GaN substrate under light irradiation exhibit changes in catalytic activity of CO oxidation that depends on the type of doping of the GaN. We suppose that hot electrons are generated upon the absorption of photons by the semiconducting nanorods or substrates, whereafter the hot electrons are injected into the Pt nanoparticles, resulting in the change in catalytic activity. The results imply that hot carrier flows generated during light irradiation significantly influence the catalytic activity of CO oxidation, leading to potential applications as a hot electron-based catalytic actuator.
Co-reporter:Elad Gross ; Jack H. Liu ; Selim Alayoglu ; Matthew A. Marcus ; Sirine C. Fakra ; F. Dean Toste
Journal of the American Chemical Society 2013 Volume 135(Issue 10) pp:3881-3886
Publication Date(Web):February 13, 2013
DOI:10.1021/ja310640b
Research to develop highly versatile, chiral, heterogeneous catalysts for asymmetric organic transformations, without quenching the catalytic reactivity, has met with limited success. While chiral supramolecular structures, connected by weak bonds, are highly active for homogeneous asymmetric catalysis, their application in heterogeneous catalysis is rare. In this work, asymmetric catalyst was prepared by encapsulating metallic nanoclusters in chiral self-assembled monolayer (SAM), immobilized on mesoporous SiO2 support. Using olefin cyclopropanation as an example, it was demonstrated that by controlling the SAM properties, asymmetric reactions can be catalyzed by Au clusters embedded in chiral SAM. Up to 50% enantioselectivity with high diastereoselectivity were obtained while employing Au nanoclusters coated with SAM peptides as heterogeneous catalyst for the formation of cyclopropane-containing products. Spectroscopic measurements correlated the improved enantioselectivity with the formation of a hydrogen-bonding network in the chiral SAM. These results demonstrate the synergetic effect of the catalytically active metallic sites and the surrounding chiral SAM for the formation of a mesoscale enantioselective catalyst.
Co-reporter:Zhongwei Zhu ; Gérôme Melaet ; Stephanus Axnanda ; Selim Alayoglu ; Zhi Liu ; Miquel Salmeron ;Gabor A Somorjai
Journal of the American Chemical Society 2013 Volume 135(Issue 34) pp:12560-12563
Publication Date(Web):August 16, 2013
DOI:10.1021/ja406497s
The surface structure of Pt(557) during the catalytic oxidation of hydrogen was studied with in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. At 298 K, the surface Pt oxide formed after exposing Pt(557) to approximately 1 Torr of O2 can be readily removed by H2, at H2 partial pressures below 50 mTorr. Water is detected as the product in the gas phase, which also coadsorbs with hydroxyl groups on the Pt(557) surface.
Co-reporter:Kwangjin An ; Selim Alayoglu ; Nathan Musselwhite ; Sheba Plamthottam ; Gérôme Melaet ; Avery E. Lindeman
Journal of the American Chemical Society 2013 Volume 135(Issue 44) pp:16689-16696
Publication Date(Web):October 3, 2013
DOI:10.1021/ja4088743
The interaction of the metal and support in oxide-supported transition-metal catalysts has been proven to have extremely favorable effects on catalytic performance. Herein, mesoporous Co3O4, NiO, MnO2, Fe2O3, and CeO2 were synthesized and utilized in CO oxidation reactions to compare the catalytic activities before and after loading of 2.5 nm Pt nanoparticles. Turnover frequencies (TOFs) of pure mesoporous oxides were 0.0002–0.015 s–1, while mesoporous silica was catalytically inactive in CO oxidation. When Pt nanoparticles were loaded onto the oxides, the TOFs of the Pt/metal oxide systems (0.1–500 s–1) were orders of magnitude greater than those of the pure oxides or the silica-supported Pt nanoparticles. The catalytic activities of various Pt/oxide systems were further influenced by varying the ratio of CO and O2 in the reactant gas feed, which provided insight into the mechanism of the observed support effect. In situ characterization using near-edge X-ray absorption fine structure (NEXAFS) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) under catalytically relevant reaction conditions demonstrated a strong correlation between the oxidation state of the oxide support and the catalytic activity at the oxide–metal interface. Through catalytic activity measurements and in situ X-ray spectroscopic probes, CoO, Mn3O4, and CeO2 have been identified as the active surface phases of the oxide at the interface with Pt nanoparticles.
Co-reporter:Hailiang Wang, James M. Krier, Zhongwei Zhu, Gérôme Melaet, Yihai Wang, Griffin Kennedy, Selim Alayoglu, Kwangjin An, and Gabor A. Somorjai
ACS Catalysis 2013 Volume 3(Issue 10) pp:2371
Publication Date(Web):September 18, 2013
DOI:10.1021/cs400579j
We characterize the surface chemical states of reactants and catalysts under reaction conditions to elucidate the composition effect of platinum–iron bimetallic nanoparticles on catalytic hydrogenation of organic molecules. The catalytic hydrogenation of ethylene is drastically accelerated on the surface of 2 nm PtFe bimetallic nanoparticles as compared to pure Pt. Sum frequency generation (SFG) vibrational spectroscopy indicates that incorporation of Fe into Pt nanoparticle catalysts weakens the adsorption of ethylidyne, an inactive spectator species, on the catalyst surface. Similarly, the turnover frequency of cyclohexene hydrogenation is also significantly enhanced by incorporating Fe into Pt nanoparticle catalysts. Ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) reveals the surface composition and oxidation states of the PtFe nanoparticles under reaction conditions. The oxidation state distribution of Fe responded to the gas atmosphere and the probing depth, whereas the Pt remained largely metallic in all probing conditions. This work represents a molecular level correlation between catalyst structure and catalytic performance.Keywords: AP-XPS; cyclohexene hydrogenation; ethylene hydrogenation; Pt−Fe bimetallic nanoparticles; SFG
Co-reporter:Derek R. Butcher, Zhongwei Zhu, Baohua Mao, Hailiang Wang, Zhi Liu, Miquel Salmeron and Gabor A. Somorjai
Chemical Communications 2013 vol. 49(Issue 61) pp:6903-6905
Publication Date(Web):12 Jun 2013
DOI:10.1039/C3CC42312C
By using high-pressure scanning tunneling microscopy and ambient-pressure X-ray photoelectron spectroscopy, we studied the mobility along with composition, structure and reactivity on the Pt(100)-hex surface. Adsorbates are mobile under 1 Torr of C2H4 and C2H4–H2 mixtures, but adding 3 mTorr of CO quenches the mobility. Ethylene-related adsorbates can also weaken Pt–Pt bonds and thus facilitate displacements in the hexagonal layer.
Co-reporter:William D. Michalak, James M. Krier, Kyriakos Komvopoulos, and Gabor A. Somorjai
The Journal of Physical Chemistry C 2013 Volume 117(Issue 4) pp:1809-1817
Publication Date(Web):December 27, 2012
DOI:10.1021/jp311772p
The product selectivity during 1,3-butadiene hydrogenation on monodisperse, colloidally synthesized, Pt nanoparticles was studied under reaction conditions with kinetic measurements and in situ sum frequency generation (SFG) vibrational spectroscopy. SFG was performed with the capping ligands intact in order to maintain nanoparticle size by reduced sintering. Four products are formed at 75 °C: 1-butene, cis-2-butene, trans-2-butene, and n-butane. Ensembles of Pt nanoparticles with average diameters of 0.9 and 1.8 nm exhibit a ∼30% and ∼20% increase in the full hydrogenation products, respectively, as compared to Pt nanoparticles with average diameters of 4.6 and 6.7 nm. Methyl and methylene vibrational stretches of reaction intermediates observed under working conditions using SFG were used to correlate the stable reaction intermediates with the product distribution. Kinetic and SFG results correlate with previous DFT predictions for two parallel reaction pathways of 1,3-butadiene hydrogenation. Hydrogenation of 1,3-butadiene can initiate with H-addition at internal or terminal carbons leading to the formation of 1-buten-4-yl radical (metallocycle) and 2-buten-1-yl radical intermediates, respectively. Small (0.9 and 1.8 nm) nanoparticles exhibited vibrational resonances originating from both intermediates, while the large (4.6 and 6.7 nm) particles exhibited vibrational resonances originating predominately from the 2-buten-1-yl radical. This suggests each reaction pathway competes for partial and full hydrogenation and the nanoparticle size affects the kinetic preference for the two pathways. The reaction pathway through the metallocycle intermediate on the small nanoparticles is likely due to the presence of low-coordinated sites.
Co-reporter:Nathan Musselwhite;Selim Alayoglu;Gérôme Melaet
Catalysis Letters 2013 Volume 143( Issue 9) pp:907-911
Publication Date(Web):2013 September
DOI:10.1007/s10562-013-1068-5
Composition and size of PtxRh1−x bimetallic nanoparticles were varied in order to study the effects in the catalytic reforming of n-hexane. Hexane isomerization, an analogue to the important industrial process of hydrocarbon reforming is a reaction in which we aim to investigate the molecular level details of catalysis. It is known, that in hydrocarbon isomerization, Pt atoms act to isomerize the reactants, while small amounts of “promoter metal” atoms (such as Rh, Ir, Re and Sn) provide C–C and C–H bond breaking activity. Herein, we report on the effect of composition and size in model bimetallic PtxRh1−x nanoparticle catalysts utilized in n-hexane reforming. Both nanoparticle composition and size were shown to influence catalytic turnover frequency and product selectivity. It was found, through ambient pressure X-ray photoelectron spectroscopy, that the surface of these nanoparticles is both dynamic, and Rh rich under relevant reaction conditions. The findings suggest that an ensemble effect exists, in which the highest isomer production occurs when Rh atoms are surrounded by Pt atoms on the metal surface.
Co-reporter:Selim Alayoglu ; Simon K. Beaumont ; Gérôme Melaet ; Avery E. Lindeman ; Nathan Musselwhite ; Christopher J. Brooks ; Matthew A. Marcus ; Jingua Guo ; Zhi Liu ; Norbert Kruse
The Journal of Physical Chemistry C 2013 Volume 117(Issue 42) pp:21803-21809
Publication Date(Web):September 25, 2013
DOI:10.1021/jp405745n
In this paper, we report the colloidal synthesis and detailed characterization of 11 nm bimetallic CoCu nanoparticle catalysts. Presently Co and Cu is an attractive combination because of their respective properties for industrially important Fischer–Tropsch and methanol synthesis reactions of CO (and CO2) with H2. We report the preparation of catalysts by deposition of bimetallic metal nanoparticles, both within mesoporous silica (MCF-17) and on the native oxide surface of a silicon wafer. Subsequent phase separation into phase-segregated (i.e., dimer) particles is found to occur upon redox treatment. These nanoparticle catalysts have then been investigated using an array of techniques including synchrotron-based ambient pressure X-ray photoelectron spectroscopy (APXPS) and in situ near edge and extended X-ray absorption fine structure (NEXAFS/EXAFS) spectroscopies. CO2 hydrogenation is used as a probe reaction. All three techniques combine to show that an oxygen atmosphere segregates copper to the surface. In doing so the oxygen produces oxides of both Co and Cu metals. Significant hydrogen pressure and temperature are required to fully rereduce both metals to a metallic state as demonstrated by NEXAFS spectroscopy. Under the conditions of the CO2/H2 reaction monitored in situ using NEXAFS spectroscopy, both metals exist in a fully reduced state at 2.7 bar, 1:3 CO2:H2, and 260 °C.
Co-reporter:Yuri Borodko, Peter Ercius, Danylo Zherebetskyy, Yihai Wang, Yintao Sun, and Gabor Somorjai
The Journal of Physical Chemistry C 2013 Volume 117(Issue 50) pp:26667-26674
Publication Date(Web):November 26, 2013
DOI:10.1021/jp409960p
Structured platinum nanoclusters Ptn (n = 5–30) capped by poly(N-vinylpyrrolidone) (PVP) have unique and highly attractive properties as potential selective catalysts. We show that the assembly of Pt mononuclear compounds in aqueous and tetrahydrofuran (THF) solutions under UV irradiation proceed via several steps: formation of linear Ptn clusters (n = 2–8), coalescence into mesocrystals, and transformation into Pt nanocrystals. The “quantum” size range of Ptn (n = 5–100) clusters is intermediate between those clusters with molecular properties and those with metallic properties. The PVP “cage” acts as a nano reactor and can hinder diffusion of photoexcited Pt atoms. The diffusion of the Pt from the polymer cage is strongly affected by the hydrophobic or hydrophilic property of the solution. An aqueous solution of [PtCl6]2– + PVP transforms into noncrystalline aggregates of molecules of less than 1.5–2 nm in diameter, whereas in THF solution Pt nanocrystals increase proportional to the UV irradiation time up to 10 nm in diameter. Dynamic imaging by high-resolution transmission electron microscopy and low-frequency UV Raman spectra show the initial stages of Pt atoms assembled into Ptn clusters. The assignment of the Raman bands is supported by density functional theory calculations. The proposed scheme of photoinduced reactions suggests the coupling of coordinatively unsaturated Pt ions inside the amidate-rich polymeric stabilizer.
Co-reporter:Zhongwei Zhu, Derek R. Butcher, Baohua Mao, Zhi Liu, Miquel Salmeron, and Gabor A. Somorjai
The Journal of Physical Chemistry C 2013 Volume 117(Issue 6) pp:2799-2804
Publication Date(Web):January 27, 2013
DOI:10.1021/jp3101893
We have studied the structures of the Pt(100) surface in the presence of gas-phase ethylene at room temperature. High-pressure scanning tunneling microscopy shows that the hexagonal reconstruction on the clean Pt(100) surface is preserved under 1 Torr of C2H4, which produces an ethylidyne and di-σ-bonded ethylene saturated surface. At 5 × 10–6 Torr of C2H4, coadsorbed CO from the background gases lifts the reconstruction, with the excess Pt atoms from the hexagonal surface forming islands on the surface. The chemisorption of CO from background gases in the vacuum system, in the nominally pure C2H4, is revealed by ambient-pressure X-ray photoelectron spectroscopy.
Co-reporter:Elad Gross
Topics in Catalysis 2013 Volume 56( Issue 12) pp:1049-1058
Publication Date(Web):2013 August
DOI:10.1007/s11244-013-0069-3
The catalytic reactivity and selectivity of metallic nanoclusters supported on a metal-oxide can be tuned by electronic charge. In this review, different approaches for controlling the electronic properties of metallic nanoclusters and its impact on catalytic reactions are discussed. Electronic charge can transfer from the metal-oxide support to the metallic catalyst and change the metal–reactants interaction and as a consequence modify as-well the catalytic reactivity and selectivity. In other cases, the electronic properties of the metal-oxide have an active role in the catalytic process and the metal oxide can be used as a co-catalyst. Another approach is to directly change the electronic properties of the metallic catalyst. It is demonstrated that dendrimer-encapsulated metallic nanoparticles can be directly oxidized by the addition of an inorganic oxidizer to the solution phase. In this case, even while supported on inert oxides, novel catalytic reactivity and selectivity can be gained by the formation of highly oxidized metal ions.
Co-reporter:Nathan Musselwhite
Topics in Catalysis 2013 Volume 56( Issue 15-17) pp:1277-1283
Publication Date(Web):2013 November
DOI:10.1007/s11244-013-0150-y
Metal surface structure is often a crucial component in determining the activity and selectivity of heterogeneous catalytic reactions. Many important industrial reactions, such as ammonia synthesis, catalytic combustion, Fischer–Tropsch synthesis, and hydrocarbon reforming have been labeled as structure-sensitive. Metal single crystal studies utilizing ultra high vacuum techniques have repeatedly shown the importance of surface structure in reaction kinetics. Recent advances in the field of colloidal synthesis allow for fine control of the size and shape of metal nanoparticles, which permits catalytic studies of structure sensitivity to be performed on nanometer sized catalysts. It is clear that in order to optimize the performance of a catalyst, a complete molecular level understanding of the role of surface structure in the reaction of interest is essential. This article aims to review the importance of surface structure in heterogeneous catalysts, ranging from single crystals to size and shape controlled nanocatalysts.
Co-reporter:William D. Michalak
Topics in Catalysis 2013 Volume 56( Issue 18-20) pp:1611-1622
Publication Date(Web):2013 December
DOI:10.1007/s11244-013-0096-0
Catalysts are essential for the generation of energy carriers like hydrocarbon fuels, hydrogen, and electrical current. The performance of catalysts can be related to their nanostructure (i.e., size and shape) and composition. To rationally design catalysts by tuning these properties, they should be measured in a meaningful way using surface-sensitive spectroscopic tools under reaction conditions. In this perspective, we provide case histories of recently published research aimed at understanding these properties using a spectroscopic strategy under reaction conditions. We limit this perspective to studies whose main focus was to understand how the nanostructure and composition impact the active phase and/or efficiency of catalysts for the generation and conversion of energy carriers. We discuss studies of a Pd/Ga2O3 catalyst for the generation of hydrogen fuel from methanol and water, a PtMo catalyst for the generation of hydrogen fuel from biomass and water, Pt/Rh catalysts for the conversion of hydrogen into electrical current, a CeOx catalyst for the conversion of hydrogen into electrical current, and Fe and Co/CoPt catalysts for the generation of hydrocarbon fuel from carbon monoxide and hydrogen. Each study emphasizes how the use of spectroscopic tools under reactive conditions is beneficial for making rational decisions for improving catalysts. The studies demonstrate how different synthesis methods dictate the nanostructure and distribution of alloy components in the catalyst, certain pretreatment conditions create the active surface phase, while reactions and post-treatments can destroy it, and the nanostructure and composition change the electronic structure and alter the selectivity and activity.
Co-reporter:Viacheslav Iablokov, Simon K. Beaumont, Selim Alayoglu, Vladimir V. Pushkarev, Colin Specht, Jinghua Gao, A. Paul Alivisatos, Norbert Kruse, and Gabor A. Somorjai
Nano Letters 2012 Volume 12(Issue 6) pp:3091-3096
Publication Date(Web):May 2, 2012
DOI:10.1021/nl300973b
Model cobalt catalysts for CO2 hydrogenation were prepared using colloidal chemistry. The turnover frequency at 6 bar and at 200–300 °C increased with cobalt nanoparticle size from 3 to 10 nm. It was demonstrated that near monodisperse nanoparticles in the size range of 3–10 nm could be generated without using trioctylphosphine oxide, a capping ligand that we demonstrate results in phosphorus being present on the metal surface and poisoning catalyst activity in our application.
Co-reporter:Vladimir V. Pushkarev, Nathan Musselwhite, Kwangjin An, Selim Alayoglu, and Gabor A. Somorjai
Nano Letters 2012 Volume 12(Issue 10) pp:5196-5201
Publication Date(Web):August 31, 2012
DOI:10.1021/nl3023127
Vapor-phase transformations of furfural in H2 over a series of Pt nanoparticles (NPs) with various particle sizes (1.5–7.1 nm size range) and shapes (rounded, cubes, octahedra) encapsulated in poly(vinylpyrrolidone) (PVP) and dispersed on MCF-17 mesoporous silica were investigated at ambient pressure in the 443–513 K temperature range. Furan and furfuryl alcohol (FFA) were two primary products as a result of furfural decarbonylation and hydrogenation reactions, respectively. Under conditions of the study both reactions exhibited structure sensitivity evidenced by changes in product selectivities, turnover rates (TORs), and apparent activation energies (EA's) with Pt particle size and shape. For instance, upon an increase in Pt particle size from 1.5 to 7.1 nm, the selectivity toward FFA increases from 1% to 66%, the TOR of FFA production increases from 1 × 10–3 s–1 to 7.6 × 10–2 s–1, and EA decreases from 104 kJ mol–1 to 15 kJ mol–1 (9.3 kPa furfural, 93 kPa H2, 473 K). Conversely, under the same experimental conditions the decarbonylation reaction path is enhanced over smaller nanoparticles. The smallest NPs (1.5 nm) produced the highest selectivity (96%) and highest TOR values (8.8 × 10–2 s–1) toward furan formation. The EA values for decarbonylation (∼62 kJ mol–1) was Pt particle size independent. Furan was further converted to propylene via a decarbonylation reaction, but also to dihydrofuran, tetrahydrofuran, and n-butanol in secondary reactions. Furfuryl alcohol was converted to mostly to 2-methylfuran.
Co-reporter:Kamran Qadir, Sang Hoon Joo, Bongjin S. Mun, Derek R. Butcher, J. Russell Renzas, Funda Aksoy, Zhi Liu, Gabor A. Somorjai, and Jeong Young Park
Nano Letters 2012 Volume 12(Issue 11) pp:5761-5768
Publication Date(Web):October 15, 2012
DOI:10.1021/nl303072d
Recent progress in colloidal synthesis of nanoparticles with well-controlled size, shape, and composition, together with development of in situ surface science characterization tools, such as ambient pressure X-ray photoelectron spectroscopy (APXPS), has generated new opportunities to unravel the surface structure of working catalysts. We report an APXPS study of Ru nanoparticles to investigate catalytically active species on Ru nanoparticles under oxidizing, reducing, and CO oxidation reaction conditions. The 2.8 and 6 nm Ru nanoparticle model catalysts were synthesized in the presence of poly(vinyl pyrrolidone) polymer capping agent and deposited onto a flat Si support as two-dimensional arrays using the Langmuir–Blodgett deposition technique. Mild oxidative and reductive characteristics indicate the formation of surface oxide on the Ru nanoparticles, the thickness of which is found to be dependent on nanoparticle size. The larger 6 nm Ru nanoparticles were oxidized to a smaller extent than the smaller Ru 2.8 nm nanoparticles within the temperature range of 50–200 °C under reaction conditions, which appears to be correlated with the higher catalytic activity of the bigger nanoparticles. We found that the smaller Ru nanoparticles form bulk RuO2 on their surfaces, causing the lower catalytic activity. As the size of the nanoparticle increases, the core–shell type RuO2 becomes stable. Such in situ observations of Ru nanoparticles are useful in identifying the active state of the catalysts during use and, hence, may allow for rational catalyst designs for practical applications.
Co-reporter:L. Robert Baker, Antoine Hervier, Griffin Kennedy, and Gabor A. Somorjai
Nano Letters 2012 Volume 12(Issue 5) pp:2554-2558
Publication Date(Web):March 30, 2012
DOI:10.1021/nl3007787
Using a Pt/Si catalytic nanodiode, we externally control the rate of CO oxidation on a Pt nanofilm. The catalytic reaction can be turned on and off by alternating between bias states of the device. Additionally, the reaction rate is sensitive to photocurrent induced by visible light. The effects of both bias and light show that negative charge on the Pt increases catalytic activity, while positive charge on the Pt decreases catalytic activity for CO oxidation.
Co-reporter:Zhongwei Zhu, Franklin (Feng) Tao, Fan Zheng, Rui Chang, Yimin Li, Lars Heinke, Zhi Liu, Miquel Salmeron, and Gabor A. Somorjai
Nano Letters 2012 Volume 12(Issue 3) pp:1491-1497
Publication Date(Web):February 2, 2012
DOI:10.1021/nl204242s
We studied the oxygen-induced restructuring process on a stepped Pt(557) single crystal surface using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at O2 pressures up to 1 Torr. HP-STM has revealed that nanometer-sized clusters are created on Pt(557) at 1 Torr of O2 and at room temperature. These clusters are identified as surface Pt oxide by AP-XPS. The appearance of clusters is preceded by the formation of 1D chain structures at the step edges. By using a Pt(111) surface as a reference, it was found that the step sites are the nucleation centers for the formation of surface oxide clusters. These surface oxide clusters disappear and the stepped structure is restored on Pt(557) after evacuating O2 to 10–8 Torr. Changes in the surface oxide concentration in response to variations in the O2 gas pressure are repeatable for several cycles. Our results that small clusters are initiated at step sites at high pressures demonstrate the importance of performing in situ characterization of stepped Pt catalysts under reaction conditions.
Co-reporter:L. Robert Baker ; Griffin Kennedy ; Matthijs Van Spronsen ; Antoine Hervier ; Xiaojun Cai ; Shiyou Chen ; Lin-Wang Wang
Journal of the American Chemical Society 2012 Volume 134(Issue 34) pp:14208-14216
Publication Date(Web):August 7, 2012
DOI:10.1021/ja306079h
This work describes a molecular-level investigation of strong metal–support interactions (SMSI) in Pt/TiO2 catalysts using sum frequency generation (SFG) vibrational spectroscopy. This is the first time that SFG has been used to probe the highly selective oxide–metal interface during catalytic reaction, and the results demonstrate that charge transfer from TiO2 on a Pt/TiO2 catalyst controls the product distribution of furfuraldehyde hydrogenation by an acid–base mechanism. Pt nanoparticles supported on TiO2 and SiO2 are used as catalysts for furfuraldehyde hydrogenation. As synthesized, the Pt nanoparticles are encapsulated in a layer of poly(vinylpyrrolidone) (PVP). The presence of PVP prevents interaction of the Pt nanoparticles with their support, so identical turnover rates and reaction selectivity is observed regardless of the supporting oxide. However, removal of the PVP with UV light results in a 50-fold enhancement in the formation of furfuryl alcohol by Pt supported on TiO2, while no change is observed for the kinetics of Pt supported on SiO2. SFG vibrational spectroscopy reveals that a furfuryl-oxy intermediate forms on TiO2 as a result of a charge transfer interaction. This furfuryl-oxy intermediate is a highly active and selective precursor to furfuryl alcohol, and spectral analysis shows that the Pt/TiO2 interface is required primarily for H spillover. Density functional calculations predict that O-vacancies on the TiO2 surface activate the formation of the furfuryl-oxy intermediate via an electron transfer to furfuraldehyde, drawing a strong analogy between SMSI and acid–base catalysis.
Co-reporter:S. J. Kweskin, R. M. Rioux, H. Song, K. Komvopoulos, P. Yang, and G. A. Somorjai
ACS Catalysis 2012 Volume 2(Issue 11) pp:2377
Publication Date(Web):October 2, 2012
DOI:10.1021/cs3005067
A model catalytic system of a monolayer consisting of 9-nm average size, cubic, single-crystal Pt nanoparticles and poly(vinylpyrrolidone) (PVP) polymer capping agent deposited on a sapphire prism was investigated by sum-frequency generation (SFG) vibrational spectroscopy in total internal reflection (TIR) geometry. Exposure of a clean nanoparticle monolayer after removal of PVP by cyclic oxidation–reduction treatment to high-pressure ethylene at room temperature led to the formation of ethylidyne and di-σ bonded ethylene. Low-pressure ethylene adsorption on a pseudohexagonal reconstructed Pt(100) single crystal resulted only in the formation of di-σ bonded ethylene. High-pressure adsorption of ethylene on Pt nanoparticle monolayers and Pt(100) led to the formation of both ethylidyne and di-σ bonded ethylene and stabilized the pseudohexagonal reconstruction of Pt(100) on both the single crystal and the surface of clean cubic nanoparticles. Restructuring of the PVP layer caused by CO adsorption indicated a small fraction of the Pt surface was available for adsorption. The stretching frequency of linear-bound CO red-shifted relative to CO adsorption on a clean Pt nanoparticle monolayer. PVP reversibly restructured upon the removal of CO by oxidation at room temperature. After the near complete removal of PVP by a cyclic low-temperature oxidation–reduction process, the peak position of the linear-bound CO blue-shifted to a frequency consistent with the adsorption of CO on a clean Pt surface. The successful application of TIR-SFG to catalytically relevant surfaces under high-pressure conditions demonstrated in this study is a significant advance in the detection of surface intermediates.Keywords: adsorbate-induced reconstruction; carbon monoxide; ethylene; nanoparticles; platinum; poly(vinylpyrrolidone); single crystals; sum-frequency generation (SFG) vibrational spectroscopy
Co-reporter:Selim Alayoglu, James M. Krier, William D. Michalak, Zhongwei Zhu, Elad Gross, and Gabor A. Somorjai
ACS Catalysis 2012 Volume 2(Issue 11) pp:2250
Publication Date(Web):September 21, 2012
DOI:10.1021/cs3004903
This review paper discusses the in situ surface characterization and catalytic measurements of colloidally synthesized model metal nanoparticle (NP) catalysts studied in the Somorjai lab. Sum Frequency Generation (SFG) vibrational spectroscopy technique revealed the vibrational signatures of binding geometry and surface orientation of adsorbate molecules by probing the immediate surface structure during the catalytic reactions. Metal surfaces were studied by Synchrotron-based spectroscopic techniques at the Advanced Light Source in the Lawrence Berkeley National Laboratory. Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) was employed to measure chemical and elemental structure of bimetallic NP catalysts under the catalytically relevant pressures in the Torr range. Surface chemical structure (i.e., oxidation states) of metals was obtained by X-ray Absorption Fine Structure Spectroscopy by constructing a gas flow cell that operates under atmospheric pressures as the reaction occurs. Environmental Transmission Electron Microscopy (E-TEM) supplemented the bimetallic structure that was obtained by X-ray spectroscopies. The morphology and chemistry induced by gas reactants on the stepped single crystal surfaces as determined by high-pressure in situ Scanning Tunneling Microscopy (HPSTM) and APXPS were also described.Keywords: ambient pressure x-ray photoelectron spectroscopy; catalytic reactivity measurements; colloidal nanoparticle catalysts; elemental composition; in situ surface probes; near edge x-ray absorption fine structure; oxidation states; surface sum frequency generation vibrational spectroscopy;
Co-reporter:Selim Alayoglu;Vladimir V. Pushkarev;Nathan Musselwhite
Topics in Catalysis 2012 Volume 55( Issue 11-13) pp:723-730
Publication Date(Web):2012 August
DOI:10.1007/s11244-012-9873-4
Size-controlled model Pt nanoparticle catalysts, synthesized by colloidal chemistry, were used to study the hydrogenative reforming of three C6 hydrocarbons in mixtures with 5:1 excess of H2: methylcyclopentane, n-hexane and 2-methylpentane. We found a strong particle size dependence on the distribution of different reaction products for the hydrogenolysis of methylcyclopentane. The reactions of 50 Torr methylcyclopentane in 250 Torr H2 at 320 °C, using 1.5 and 3.0 nm Pt nanoparticles produced predominantly C6 isomers, especially 2-methylpentane, whereas 5.2 and 11.3 nm Pt nanoparticles were more selective for the formation of benzene. For the hydrogenolysis of n-hexane and 2-methylpentane, strong particle size effects on the turnover rates were observed. Hexane and 2-methylpentane reacted up to an order of magnitude slower over 3.0 nm Pt than over the other particle sizes. At 360 °C the isomerization reactions were more selective than the other reaction pathways over 3.0 nm Pt, which also yielded relatively less benzene.
Co-reporter:Elad Gross;James M. Krier;Lars Heinke
Topics in Catalysis 2012 Volume 55( Issue 1-2) pp:13-23
Publication Date(Web):2012 March
DOI:10.1007/s11244-012-9780-8
The reactivity of small (<1.5 nm), highly oxidized metallic nanoparticles and the utilization of Sum Frequency Generation spectroscopy (SFG) and Scanning Tunneling Microscopy (STM) for investigations of catalysts under reaction conditions are discussed in this review paper. Batch and flow reactor studies were carried out using highly oxidized 40 atom clusters (Pt, Pd and Rh) to measure reaction rate and product distribution of electrophilic reactions, using toluene as a solvent. These heterogeneous catalysts show reactivity which is similar and sometimes even higher than the homogeneous catalysts. The combination of an in situ SFG and STM measurements facilitate a detection of the surface structure and reaction intermediates under reaction conditions. While the STM detects the surface reconstruction and the mobility of products and reactants molecules, the SFG can correlate the reactivity and more importantly the selectivity, to the active surface intermediates. The recent developments in these two research areas are detailed in this review paper.
Co-reporter:Vladimir V. Pushkarev;Zhongwei Zhu;Kwangjin An;Antoine Hervier
Topics in Catalysis 2012 Volume 55( Issue 19-20) pp:1257-1275
Publication Date(Web):2012 December
DOI:10.1007/s11244-012-9915-y
We aim to develop novel catalysts that exhibit high activity, selectivity and stability under real catalytic conditions. In the recent decades, the fast development of nanoscience and nanotechnology has allowed synthesis of nanoparticles with well-defined size, shape and composition using colloidal methods. Utilization of mesoporous oxide supports effectively prevents the nanoparticles from aggregating at high temperatures and high pressures. Nanoparticles of less than 2 nm sizes were found to show unique activity and selectivity during reactions, which was due to the special surface electronic structure and atomic arrangements that are present at small particle surfaces. While oxide support materials are employed to stabilize metal nanoparticles under working conditions, the supports are also known to strongly interact with the metals through encapsulation, adsorbate spillover, and charge transfer. These factors change the catalytic performance of the metal catalysts as well as the conductivity of oxides. The employment of new in situ techniques, mainly high-pressure scanning tunneling microscopy (HPSTM) and ambient-pressure X-ray photoelectron spectroscopy (APXPS) allows the determination of the surface structure and chemical states under reaction conditions. HPSTM has identified the importance of both adsorbate mobility to catalytic turnovers and the metal substrate reconstruction driven by gaseous reactants such as CO and O2. APXPS is able to monitor both reacting species at catalyst surfaces and the oxidation state of the catalyst while it is being exposed to gases. The surface composition of bimetallic nanoparticles depends on whether the catalysts are under oxidizing or reducing conditions, which is further correlated with the catalysis by the bimetallic catalytic systems. The product selectivity in multipath reactions correlates with the size and shape of monodisperse metal nanoparticle catalysts in structure sensitive reactions.
Co-reporter:Yuri Borodko, Peter Ercius, Vladimir Pushkarev, Chris Thompson, and Gabor Somorjai
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 2) pp:236-241
Publication Date(Web):January 4, 2012
DOI:10.1021/jz201599u
The dynamics of structural transformations of Pt aggregates in the “quantum size” range where their molecular structure transforms into crystalline nanoparticles with metallic properties is an important issue in nanoscience. Here, we show high-resolution transmission electron microscopy (HRTEM) and spectroscopic observations of the polyamidoamine (PAMAM) dendrimer-mediated system, Pt–PAMAM, after UV irradiation that reveal the formation of small Ptδ+n clusters (n = 2–8) with linear chains of −Pt–Pt–, which are the building blocks of stable nanocrystals. Dynamic imaging in an aberration corrected TEM at atomic resolution shows intermediate molecular and crystalline states and coalescence of Pt clusters into stable nanocrystals via an oriented attachment assembly process. We propose that the structural transformation from Pt aggregates to nanocrystals occurs between 1.5 and 2 nm and that a phase transition of the type “disordered-to-crystalline” exists depending on the number of atoms in the cluster.Keywords: aberration corrected TEM; nanocrystal formation; polymer microreactor; Pt blue intermediates; Pt mesocrystal;
Co-reporter:L. Robert Baker;Griffin Kennedy;James M. Krier
Catalysis Letters 2012 Volume 142( Issue 11) pp:1286-1294
Publication Date(Web):2012 November
DOI:10.1007/s10562-012-0904-3
Inherent in the colloidal synthesis of nanoparticle catalysts is the presence of an organic capping agent that encapsulates the nanoparticles to prevent aggregation. However, this capping agent often remains present on the nanoparticles during catalytic reaction, and the effect of this coating on catalysis is an important question that will influence the future applications of colloidal nanoparticles. In this study, the structure of poly(vinylpyrrolidone) (PVP) ligands on Pt nanoparticles is probed using sum frequency generation vibrational spectroscopy before and after cap removal by UV light. When the PVP is removed, carbonaceous fragments remain on the surface that dynamically restructure in H2 and O2. These fragments form a porous coating around the Pt in H2 but collapse to a tightly closed shell in O2. Using ethylene hydrogenation and methanol oxidation as a probe for the catalytic activity of the nanoparticles in H2 and O2, respectively, it is shown that the structure of these carbonaceous fragments controls the catalytic activity of the nanoparticles across several orders of magnitude by opening in H2 and collapsing to block Pt sites in O2. Kinetic experiments on thermally-cleaned PVP-capped and oleic acid-capped nanoparticles show that these findings apply to multiple capping agents and cleaning methods. This work highlights the dominant role of an organic cap to mediate nanoparticle catalysis and provides one example where capped nanoparticles are dramatically better catalysts than their uncapped analogues.Open image in new window
Co-reporter:Dr. Kwangjin An ; Gabor A. Somorjai
ChemCatChem 2012 Volume 4( Issue 10) pp:1512-1524
Publication Date(Web):
DOI:10.1002/cctc.201200229
Abstract
A nanoparticle with well-defined surfaces, prepared through colloidal chemistry, enables it to be studied as a model heterogeneous catalyst. The colloidal synthetic approach provides versatile tools to control the size and shape of nanoparticles. Traditional nucleation and growth mechanisms have been utilized to understand how nanoparticles can be uniformly synthesized and unprecedented shapes can be controlled. Now, the size of metal particles can be controlled to cluster regimes by using dendrimers. By using seeds and foreign atoms, specific synthetic environments such as seeded growth and crystal overgrowth can be induced to generate various shaped mono- or bi-metallic, core/shell, or branched nanostructures. For green chemistry, catalysis in 21st century is aiming for 100 % selectivity to produce only one desired product at high turnover rates. Recent studies on nanoparticle catalysts clearly demonstrate size and shape dependent selectivity in many catalytic reactions. By combining in situ surface characterization techniques, real-time monitoring of nanoparticles can be performed under reaction environments, thus identifying several molecular factors affecting catalytic activity and selectivity.
Co-reporter:James M. Krier ; William D. Michalak ; L. Robert Baker ; Kwangjin An ; Kyriakos Komvopoulos
The Journal of Physical Chemistry C 2012 Volume 116(Issue 33) pp:17540-17546
Publication Date(Web):June 17, 2012
DOI:10.1021/jp303363m
Recent work with nanoparticle catalysts shows that size and shape control on the nanometer scale influences reaction rate and selectivity. Sum frequency generation (SFG) vibrational spectroscopy is a powerful tool for studying heterogeneous catalysis because it enables the observation of surface intermediates during catalytic reactions. To control the size and shape of catalytic nanoparticles, an organic ligand was used as a capping agent to stabilize nanoparticles during synthesis. However, the presence of an organic capping agent presents two major challenges in SFG and catalytic reaction studies: it blocks a significant fraction of active surface sites and produces a strong signal that prevents the detection of reaction intermediates with SFG. Two methods for cleaning Pt nanoparticles capped with poly(vinylpyrrolidone) (PVP) are examined in this study: solvent cleaning and UV cleaning. Solvent cleaning leaves more PVP intact and relies on disordering with hydrogen gas to reduce the SFG signal of PVP. In contrast, UV cleaning depends on nearly complete removal of PVP to reduce SFG signal. Both UV and solvent cleaning enable the detection of reaction intermediates by SFG. However, solvent cleaning also yields nanoparticles that are stable under reaction conditions, whereas UV cleaning results in aggregation during reaction. The results of this study indicate that solvent cleaning is more advantageous for studying the effects of nanoparticle size and shape on catalytic selectivity by SFG vibrational spectroscopy.
Co-reporter:Robert M. Onorato, Alfred P. Yoon, James T. Lin, and Gabor A. Somorjai
The Journal of Physical Chemistry C 2012 Volume 116(Issue 18) pp:9947-9954
Publication Date(Web):April 23, 2012
DOI:10.1021/jp210879p
Aqueous solutions of the amino acids l-phenylalanine, l-lysine, and l-glycine and the homo- and heterodipeptides comprising these amino acids were studied by vibrational sum frequency generation (SFG) and quartz crystal microbalance (QCM) at the hydrophobic polystyrene interface. The phenyl ring of the phenylalanine side chain was determined to adsorb preferentially in a nearly flat geometry relative to the hydrophobic surface based on the concentration dependence of the SFG spectra and symmetry arguments. The amount of adsorbed dipeptide follows a hydrophobic series at concentrations well below monolayer formation, as determined by QCM. However, at higher concentrations, adsorbate–adsorbate interactions play a significant role in the adsorption, and adsorption no longer follows a hydrophobic series. These changes in the quantitative adsorption from QCM correlate with changes in the SFG spectra for phenylalanine, lysyl-phenylalanine, and glycyl-phenylalanine, but not for lysyl-lysine, which shows the most striking adsorbate interaction effect.
Co-reporter:Dr. Kwangjin An ; Gabor A. Somorjai
ChemCatChem 2012 Volume 4( Issue 10) pp:
Publication Date(Web):
DOI:10.1002/cctc.201290035
Co-reporter:Hyungtak Seo, L. Robert Baker, Antoine Hervier, Jinwoo Kim, J. L. Whitten, and Gabor A. Somorjai
Nano Letters 2011 Volume 11(Issue 2) pp:751-756
Publication Date(Web):December 22, 2010
DOI:10.1021/nl1039378
True n-type doping of titanium oxide without formation of midgap states would expand the use of metal oxides for charge-based devices. We demonstrate that plasma-assisted fluorine insertion passivates defect states and that fluorine acts as an n-type donor in titanium oxide. This enabled us to modify the Fermi level and transport properties of titanium oxide outside the limits of O vacancy doping. The origin of the electronic structure modification is explained by ab initio calculation.
Co-reporter:Fan Zheng, Selim Alayoglu, Jinghua Guo, Vladimir Pushkarev, Yimin Li, Per-Anders Glans, Jeng-lung Chen, and Gabor Somorjai
Nano Letters 2011 Volume 11(Issue 2) pp:847-853
Publication Date(Web):January 19, 2011
DOI:10.1021/nl104209c
In-situ near edge X-ray absorption fine structure spectroscopy was performed to monitor the oxidation states of Co and CoPt nanoparticles (NPs) of 4 nm size in the presence of H2 and O2 in the pressure range of 1 bar and 36 Torr respectively. Platinum helps the rapid reduction of cobalt oxides in hydrogen at a rather low temperature (38 °C). In addition, reversible changes of the oxidation states of cobalt in the Co and CoPt NPs as a function of cycling oxygen pressure (in the range of millitorr to 36 Torr) are quantified and compared. The role of Pt in the process of Co reducing and oxidizing was explored. Our findings permit the prediction of the cobalt oxidation states as the reaction conditions are altered. The experimental results also suggest the presence of tetrahedral structure of Cobalt oxide that differs from the Co3O4 spinel structure.
Co-reporter:Young Keun Lee, Chan Ho Jung, Jonghyurk Park, Hyungtak Seo, Gabor A. Somorjai, and Jeong Young Park
Nano Letters 2011 Volume 11(Issue 10) pp:4251-4255
Publication Date(Web):September 14, 2011
DOI:10.1021/nl2022459
A continuous flow of hot electrons that are not at thermal equilibrium with the surrounding metal atoms is generated by the absorption of photons. Here we show that hot electron flow generated on a gold thin film by photon absorption (or internal photoemission) is amplified by localized surface plasmon resonance. This was achieved by direct measurement of photocurrent on a chemically modified gold thin film of metal-semiconductor (TiO2) Schottky diodes. The short-circuit photocurrent obtained with low-energy photons is consistent with Fowler’s law, confirming the presence of hot electron flows. The morphology of the metal thin film was modified to a connected gold island structure after heating such that it exhibits surface plasmon. Photocurrent and optical measurements on the connected island structures revealed the presence of a localized surface plasmon at 550 ± 20 nm. The results indicate an intrinsic correlation between the hot electron flow generated by internal photoemission and localized surface plasmon resonance.
Co-reporter:George J. Holinga ; Roger L. York ; Robert M. Onorato ; Christopher M. Thompson ; Nic E. Webb ; Alfred P. Yoon
Journal of the American Chemical Society 2011 Volume 133(Issue 16) pp:6243-6253
Publication Date(Web):March 31, 2011
DOI:10.1021/ja1101954
Sum frequency generation (SFG) vibrational spectroscopy was employed to characterize the interfacial structure of eight individual amino acids—l-phenylalanine, l-leucine, glycine, l-lysine, l-arginine, l-cysteine, l-alanine, and l-proline—in aqueous solution adsorbed at model hydrophilic and hydrophobic surfaces. Specifically, SFG vibrational spectra were obtained for the amino acids at the solid−liquid interface between both hydrophobic d8-polystyrene (d8-PS) and SiO2 model surfaces and phosphate buffered saline (PBS) at pH 7.4. At the hydrophobic d8-PS surface, seven of the amino acids solutions investigated showed clear and identifiable C−H vibrational modes, with the exception being l-alanine. In the SFG spectra obtained at the hydrophilic SiO2 surface, no C−H vibrational modes were observed from any of the amino acids studied. However, it was confirmed by quartz crystal microbalance that amino acids do adsorb to the SiO2 interface, and the amino acid solutions were found to have a detectable and widely varying influence on the magnitude of SFG signal from water at the SiO2/PBS interface. This study provides the first known SFG spectra of several individual amino acids in aqueous solution at the solid−liquid interface and under physiological conditions.
Co-reporter:Yimin Li ; Jack Hung-Chang Liu ; Cole A. Witham ; Wenyu Huang ; Matthew A. Marcus ; Sirine C. Fakra ; Pinar Alayoglu ; Zhongwei Zhu ; Christopher M. Thompson ; Arpana Arjun ; Kihong Lee ; Elad Gross ; F. Dean Toste
Journal of the American Chemical Society 2011 Volume 133(Issue 34) pp:13527-13533
Publication Date(Web):July 1, 2011
DOI:10.1021/ja204191t
The design and development of metal-cluster-based heterogeneous catalysts with high activity, selectivity, and stability under solution-phase reaction conditions will enable their applications as recyclable catalysts in large-scale fine chemicals production. To achieve these required catalytic properties, a heterogeneous catalyst must contain specific catalytically active species in high concentration, and the active species must be stabilized on a solid catalyst support under solution-phase reaction conditions. These requirements pose a great challenge for catalysis research to design metal-cluster-based catalysts for solution-phase catalytic processes. Here, we focus on a silica-supported, polymer-encapsulated Pt catalyst for an electrophilic hydroalkoxylation reaction in toluene, which exhibits superior selectivity and stability against leaching under mild reaction conditions. We unveil the key factors leading to the observed superior catalytic performance by combining X-ray absorption spectroscopy (XAS) and reaction kinetic studies. On the basis of the mechanistic understandings obtained in this work, we also provide useful guidelines for designing metal-cluster-based catalyst for a broader range of reactions in the solution phase.
Co-reporter:James Russell Renzas, Wenyu Huang, Yawen Zhang, Michael E. Grass, Dat Tien Hoang, Selim Alayoglu, Derek R. Butcher, Franklin (Feng) Tao, Zhi Liu and Gabor A. Somorjai
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 7) pp:2556-2562
Publication Date(Web):24 Dec 2010
DOI:10.1039/C0CP01858A
Bimetallic 15 nm Rh1−xPdxnanoparticle catalysts of five different compositions and supported on Si wafers have been synthesized, characterized using TEM, SEM, and XPS, and studied in CO oxidation by O2 in two pressure regimes: atmospheric pressure and 100–200 mTorr. The RhPd bimetallic nanocrystals exhibited similar synergetic effect of increased reaction activity at both atmospheric (760 Torr) and moderate (100–200 mTorr) pressures compared with pure Pd or Rh. The magnitude of the effect depends on the relative pressures of the CO and O2 reactant gases and the reaction temperature. The catalytic activity of the nanocrystals measured at moderate pressure is directly correlated to the APXPS studies, which were carried out in the same pressure. The APXPS studies suggest that the Pd–Rh interfaces are important for the enhanced activity of the bimetallic nanoparticles.
Co-reporter:Antoine Hervier ; L. Robert Baker ; Kyriakos Komvopoulos
The Journal of Physical Chemistry C 2011 Volume 115(Issue 46) pp:22960-22964
Publication Date(Web):October 13, 2011
DOI:10.1021/jp2066327
Platinum films of 1 nm thickness were deposited by electron beam evaporation onto 100 nm thick titanium oxide films (TiOx) with variable oxygen vacancy concentrations and fluorine (F) doping. Methanol oxidation on the platinum films produced formaldehyde, methyl formate, and carbon dioxide. F-doped samples demonstrated significantly higher activity for methanol oxidation when the TiOx was stoichiometric (TiO2), but lower activity when it was nonstoichiometric (TiO1.7 and TiO1.9). These results correlate with the chemical behavior of the same types of catalysts in CO oxidation. Fluorine doping of stoichiometric TiO2 also increased selectivity toward partial oxidation of methanol to formaldehyde and methyl formate, but had an opposite effect in the case of nonstoichiometric TiOx. Introduction of oxygen vacancies and fluorine doping both increased the conductivity of the TiOx film. For oxygen vacancies, this occurred by the formation of a conduction channel in the band gap, whereas in the case of fluorine doping, F acted as an n-type donor, forming a conduction channel at the bottom of the conduction band, about 0.5–1.0 eV higher in energy. The higher energy electrons in F-doped stoichiometric TiOx led to higher turnover rates and increased selectivity toward partial oxidation of methanol. This correlation between electronic structure and turnover rate and selectivity indicates that the ability of the support to transfer charges to surface species controls in part the activity and selectivity of the reaction.
Co-reporter:L. Robert Baker ; Antoine Hervier ; Hyungtak Seo ; Griffin Kennedy ; Kyriakos Komvopoulos
The Journal of Physical Chemistry C 2011 Volume 115(Issue 32) pp:16006-16011
Publication Date(Web):July 11, 2011
DOI:10.1021/jp203151y
The role of the oxide-metal interface in determining the activity and selectivity of chemical reactions catalyzed by metal particles on an oxide support is an important topic in science and industry. A proposed mechanism for this strong metal–support interaction is electronic activation of surface adsorbates by charge carriers. Motivated by the goal of using electronic activation to drive nonthermal chemistry, we investigated the ability of the oxide support to mediate charge transfer. We report an approximately 2-fold increase in the turnover rate of catalytic carbon monoxide oxidation on platinum nanoparticles supported on stoichiometric titanium dioxide (TiO2) when the TiO2 is made highly n-type by fluorine (F) doping. However, for nonstoichiometric titanium oxide (TiOX<2) the effect of F on the turnover rate is negligible. Studies of the titanium oxide electronic structure show that the energy of free electrons in the oxide determines the rate of reaction. These results suggest that highly n-type TiO2 electronically activates adsorbed oxygen (O) by electron spillover to form an active O– intermediate.
Co-reporter: Gabor A. Somorjai;Simon K. Beaumont;Selim Alayoglu
Angewandte Chemie International Edition 2011 Volume 50( Issue 43) pp:10116-10129
Publication Date(Web):
DOI:10.1002/anie.201008214
Abstract
In the last two decades, surface-science experiments and techniques have been developed to focus on obtaining molecular information under reaction conditions at high pressures (near or above 1 bar) and liquid interfaces. This Minireview describes the results of these studies obtained by surface-sensitive laser spectroscopies, scanning tunneling microscopy, and X-ray spectroscopies usually practiced at a synchrotron light source. The use of model surfaces, single crystals, and monodisperse nanoparticles with variable size (1–10 nm) and shape facilitates meaningful interpretation of the experimental data. These methods allow evaluation of the molecular structures of intermediates, oxidation states of metals, and mobility of adsorbants. New techniques that are likely to make major contributions to the investigation of surfaces under reaction conditions are also discussed.
Co-reporter:S. Alayoglu;C. Aliaga;C. Sprung;G. A. Somorjai
Catalysis Letters 2011 Volume 141( Issue 7) pp:914-924
Publication Date(Web):2011 July
DOI:10.1007/s10562-011-0647-6
Monodisperse Pt nanoparticles (NPs) with well-controlled sizes in the range between 1.5 and 10.8 nm, and shapes of octahedron, cube, truncated octahedron and spheres (~6 nm) were synthesized employing the polyol reduction strategy with polyvinylpyrrolidone (PVP) as the capping agent. We characterized the as-synthesized Pt nanoparticles using transmission electron microscopy (TEM), high resolution TEM, sum frequency generation vibrational spectroscopy (SFGVS) using ethylene/H2 reaction as the surface probe, and the catalytic ethylene/H2 reaction by means of measuring surface concentration of Pt. The nanoparticles were supported in mesoporous silica (SBA-15 or MCF-17), and their catalytic reactivity was evaluated for the methylcyclopentane (MCP)/H2 ring opening/ring enlargement reaction using 10 torr MCP and 50 torr H2 at temperatures between 160 and 300 °C. We found a strong correlation between the particle shape and the catalytic activity and product distribution for the MCP/H2 reaction on Pt. At temperatures below 240 °C, 6.3 nm Pt octahedra yielded hexane, 6.2 nm Pt truncated octahedra and 5.2 nm Pt spheres produced 2-methylpentane. In contrast, 6.8 nm Pt cubes led to the formation of cracking products (i.e. C1–C5) under similar conditions. We also detected a weak size dependence of the catalytic activity and selectivity for the MCP/H2 reaction on Pt. 1.5 nm Pt particles produced 2-methylpentane for the whole temperature range studied and the larger Pt NPs produced mainly benzene at temperatures above 240 °C.
Co-reporter:Selim Alayoglu;Franklin Tao;Virginia Altoe;Colin Specht
Catalysis Letters 2011 Volume 141( Issue 5) pp:633-640
Publication Date(Web):2011 May
DOI:10.1007/s10562-011-0565-7
AuxPd1−x (x = 0, 0.25, 0.5, 0.75, 1) nanoparticle (NP) catalysts (8–11 nm) were synthesized by a one-pot reaction strategy using colloidal chemistry. XPS depth profiles with variable X-ray energies and scanning transmission electron microscopy (STEM) analyses show that the as-synthesized AuxPd1−x (x = 0.25 and 0.5) bimetallic NPs have gradient alloy structures with Au-rich cores and Pd-rich shells. The evolution of composition and structure in the surface region corresponding to a mean free path of 0.6–0.8 nm (i.e., 2–3 layers to the bulk from the particle surface) was studied with ambient pressure X-ray photoelectron spectroscopy (AP-XPS) under CO/O2 reaction in the Torr pressure regime. Under the reaction conditions of 80 mTorr CO and 200 mTorr O2 at 200 °C, the surface region of Au0.75Pd0.25 NP is Au-rich (~70% by Au). All AuxPd1−x (x = 0.25, 0.5, 0.75) NP catalysts have higher turnover rates for the model CO/O2 reaction than pure Pd and pure Au NPs. The Pd-rich Au0.25Pd0.75 NPs show the highest turnover rates and the Pd-rich Au0.5Pd0.5 NPs the lowest turnover rates at 200 °C. Interestingly, the Au-rich Au0.75Pd0.25 NPs exhibit steady-state turnover rates which are intermediate to those of the Pd-rich bimetallic nanoparticles.
Co-reporter:James Russell Renzas;Wenyu Huang;Yawen Zhang;Michael E. Grass
Catalysis Letters 2011 Volume 141( Issue 2) pp:235-241
Publication Date(Web):2011 February
DOI:10.1007/s10562-010-0462-5
Bimetallic 15 nm Pd-core Rh-shell Rh1−xPdx nanoparticle catalysts have been synthesized and studied in CO oxidation by NO. The catalysts exhibited composition-dependent activity enhancement (synergy) in CO oxidation in high NO pressures. The observed synergetic effect is attributed to the favorable adsorption of CO on Pd in NO-rich conditions. The Pd-rich bimetallic catalysts deactivated after many hours of oxidation of CO by NO. After catalyst deactivation, product formation was proportional to the Rh molar fraction within the bimetallic nanoparticles. The deactivated catalysts were regenerated by heating the sample in UHV. This regeneration suggests that the deactivation was caused by the adsorption of nitrogen atoms on Pd sites.
Co-reporter: Gabor A. Somorjai;Simon K. Beaumont;Selim Alayoglu
Angewandte Chemie 2011 Volume 123( Issue 43) pp:10298-10311
Publication Date(Web):
DOI:10.1002/ange.201008214
Abstract
In den letzten zwanzig Jahren wurden Experimente und Methoden zur Erforschung von Oberflächenphänomenen entwickelt, die Informationen auf molekularer Ebene bei Reaktionsbedingungen unter hohen Drücken (d. h. ungefähr 1 bar) an Fest-flüssig-Grenzflächen liefern. Dieser Kurzaufsatz beschreibt die Ergebnisse dieser Studien, die durch oberflächenempfindliche Laser-Spektroskopie, Rastertunnelmikroskopie und Röntgenspektroskopie erhalten wurden. Die Verwendung von Modelloberflächen, Einkristallen sowie monodispersen Nanopartikeln mit variabler Größe (1–10 nm) und Form erleichtert die Interpretation der experimentellen Daten. Die verwendeten Methoden ermöglichen die Untersuchung der Molekularstruktur von Zwischenprodukten, des Oxidationszustandes von Metallen und der Beweglichkeit adsorbierter Moleküle. Auch werden neue Techniken diskutiert, die bedeutende Beiträge zur Untersuchung von Oberflächen unter Reaktionsbedingungen erwarten lassen.
Co-reporter:Gabor A. Somorjai;Yimin Li
PNAS 2011 Volume 108 (Issue 3 ) pp:917-924
Publication Date(Web):2011-01-18
DOI:10.1073/pnas.1006669107
The applications of molecular surface chemistry in heterogeneous catalyst technology, semiconductor-based technology, medical
technology, anticorrosion and lubricant technology, and nanotechnology are highlighted in this perspective. The evolution
of surface chemistry at the molecular level is reviewed, and the key roles of surface instrumentation developments for in
situ studies of the gas–solid, liquid–solid, and solid–solid interfaces under reaction conditions are emphasized.
Co-reporter:Sang Hoon Joo, Jeong Y. Park, J. Russell Renzas, Derek R. Butcher, Wenyu Huang and Gabor A. Somorjai
Nano Letters 2010 Volume 10(Issue 7) pp:2709-2713
Publication Date(Web):June 22, 2010
DOI:10.1021/nl101700j
Carbon monoxide oxidation over ruthenium catalysts has shown an unusual catalytic behavior. Here we report a particle size effect on CO oxidation over Ru nanoparticle (NP) catalysts. Uniform Ru NPs with a tunable particle size from 2 to 6 nm were synthesized by a polyol reduction of Ru(acac)3 precursor in the presence of poly(vinylpyrrolidone) stabilizer. The measurement of catalytic activity of CO oxidation over two-dimensional Ru NPs arrays under oxidizing reaction conditions (40 Torr CO and 100 Torr O2) showed an activity dependence on the Ru NP size. The CO oxidation activity increases with NP size, and the 6 nm Ru NP catalyst shows 8-fold higher activity than the 2 nm catalysts. The results gained from this study will provide the scientific basis for future design of Ru-based oxidation catalysts.
Co-reporter:Yimin Li and Gabor A. Somorjai
Nano Letters 2010 Volume 10(Issue 7) pp:2289-2295
Publication Date(Web):June 4, 2010
DOI:10.1021/nl101807g
In this perspective, we present an overview of nanoscience applications in catalysis, energy conversion, and energy conservation technologies. We discuss how novel physical and chemical properties of nanomaterials can be applied and engineered to meet the advanced material requirements in the new generation of chemical and energy conversion devices. We highlight some of the latest advances in these nanotechnologies and provide an outlook at the major challenges for further developments.
Co-reporter:Feng Tao ; Michael E. Grass ; Yawen Zhang ; Derek R. Butcher ; Funda Aksoy ; Shaul Aloni ; Virginia Altoe ; Selim Alayoglu ; James R. Renzas ; Chia-Kuang Tsung ; Zhongwei Zhu ; Zhi Liu ; Miquel Salmeron
Journal of the American Chemical Society 2010 Volume 132(Issue 25) pp:8697-8703
Publication Date(Web):June 3, 2010
DOI:10.1021/ja101502t
Three series of bimetallic nanoparticle catalysts (RhxPd1−x, RhxPt1−x, and PdxPt1−x, x = 0.2, 0.5, 0.8) were synthesized using one-step colloidal chemistry. X-ray photoelectron spectroscopy (XPS) depth profiles using different X-ray energies and scanning transmission electron microscopy showed that the as-synthesized RhxPd1−x and PdxPt1−x nanoparticles have a core−shell structure whereas the RhxPt1−x alloys are more homogeneous in structure. The evolution of their structures and chemistry under oxidizing and reducing conditions was studied with ambient-pressure XPS (AP-XPS) in the Torr pressure range. The RhxPd1−x and RhxPt1−x nanoparticles undergo reversible changes of surface composition and chemical state when the reactant gases change from oxidizing (NO or O2 at 300 °C) to reducing (H2 or CO at 300 °C) or catalytic (mixture of NO and CO at 300 °C). In contrast, no significant change in the distribution of the Pd and Pt atoms in the PdxPt1−x nanoparticles was observed. The difference in restructuring behavior under these reaction conditions in the three series of bimetallic nanoparticle catalysts is correlated with the surface free energy of the metals and the heat of formation of the metallic oxides. The observation of structural evolution of bimetallic nanoparticles under different reaction conditions suggests the importance of in situ studies of surface structures of nanoparticle catalysts.
Co-reporter:Wenyu Huang ; Jack Hung-Chang Liu ; Pinar Alayoglu ; Yimin Li ; Cole A. Witham ; Chia-Kuang Tsung ; F. Dean Toste
Journal of the American Chemical Society 2010 Volume 132(Issue 47) pp:16771-16773
Publication Date(Web):November 9, 2010
DOI:10.1021/ja108898t
A highly active heterogeneous Pd-nanoparticle catalyst for the intramolecular addition of phenols to alkynes was developed and employed in a continuous flow reaction system. Running the reaction in flow mode revealed reaction kinetics, such as the activation energy and catalyst deactivation, and provides many potential practical advantages.
Co-reporter:Yawen Zhang, Michael E. Grass, Wenyu Huang, and Gabor A. Somorjai
Langmuir 2010 Volume 26(Issue 21) pp:16463-16468
Publication Date(Web):May 5, 2010
DOI:10.1021/la101213q
Monodisperse sub-10 nm (6.5 nm) sized Rh nanocrystals with (111) and (100) surface structures were synthesized by a seedless polyol reduction in ethylene glycol, with poly(vinylpyrrolidone) as a capping ligand. When using [Rh(Ac)2]2 as the metal precursor, (111)-oriented Rh nanopolyhedra containing 76% (111)-twinned hexagons (in 2D projection) were obtained; whereas, when employing RhCl3 as the metal precursor in the presence of alkylammonium bromide, such as tetramethylammonium bromide and trimethyl(tetradecyl)ammonium bromide, (100)-oriented Rh nanocubes were obtained with 85% selectivity. The {100} faces of the Rh nanocrystals are stabilized by chemically adsorbed Br− ions from alkylammonium bromides, which led to (100)-oriented nanocubes. Monolayer films of the (111)-oriented Rh nanopolyhedra and (100)-oriented Rh nanocubes were deposited on silicon wafers in a Langmuir−Blodgett trough to make model 2D nanoarray catalysts. These nanocatalysts were active for CO oxidation by O2, and the turnover frequency was independent of nanoparticle shape, consistent with that previously observed for Rh(111) and Rh(100) single crystals.
Co-reporter:James Russell Renzas
The Journal of Physical Chemistry C 2010 Volume 114(Issue 41) pp:17660-17664
Publication Date(Web):September 22, 2010
DOI:10.1021/jp104793p
Ultra-thin-film 5 nm Rh/TiOx and Rh/GaN catalytic metal−semiconductor Schottky nanodiodes were fabricated using dc magnetron reactive sputtering, rapid thermal annealing, and electron beam evaporation. The diode barrier heights were characterized in different gas mixtures using current−voltage measurements. Barrier heights were found to vary with the local gas composition for Rh/TiOx nanodiodes but did not vary with gas composition for Rh/GaN nanodiodes. The nanodiodes were also studied during the oxidation of CO by O2 and during the oxidation of CO by NO using chemicurrent measurements and gas chromatography. The hot electron chemicurrent was determined from the current in situ during reaction and the thermoelectric current measured in oxidizing conditions. Similar activation energies were measured from both the chemicurrent and gas chromatography, which indicates a correlation between hot electron chemicurrent and catalytic activity.
Co-reporter:Yuri Borodko, Hyun Sook Lee, Sang Hoon Joo, Yawen Zhang and Gabor Somorjai
The Journal of Physical Chemistry C 2010 Volume 114(Issue 2) pp:1117-1126
Publication Date(Web):December 22, 2009
DOI:10.1021/jp909008z
Poly(N-vinylpyrrolidone (PVP)-capped platinum and rhodium nanoparticles (7−12 nm) have been studied with UV−vis, FTIR, and Raman spectroscopy. The absorption bands in the region of 190−900 nm are shown to be sensitive to the electronic structure of surface Rh and Pt atoms as well as to the aggregation of the nanoparticles. In-situ FTIR-DRIFT spectroscopy of the thermal decay of PVP-stabilized Rh and Pt nanoparticles in H2 and O2 atmospheres in temperatures ranging from 30 to 350 °C reveals the decomposition of PVP above 200 °C; PVP transforms into a “polyamide−polyene”-like material that is, in turn, converted into a thin layer of amorphous carbon above 300 °C. Adsorbed carbon monoxide was used as a probing molecule to monitor changes of the electronic structure of surface Rh and Pt atoms and accessible surface area. The behavior of surface Rh and Pt atoms with ligated CO and amide groups of pyrrolidones resembles that of surface coordination compounds.
Co-reporter:Norbert Kruse;Gabor Somorjai
Catalysis Letters 2010 Volume 134( Issue 1-2) pp:1
Publication Date(Web):2010 January
DOI:10.1007/s10562-009-0253-z
Co-reporter:Gabor A. Somorjai;Yimin Li
Topics in Catalysis 2010 Volume 53( Issue 13-14) pp:832-847
Publication Date(Web):2010 August
DOI:10.1007/s11244-010-9511-y
Monodispersed transition metal (Pt, Rh, Pd) nanoparticles (NP) in the 0.8–15 nm range have been synthesized and are being used to probe catalytic selectivity in multipath organic transformation reactions. For NP systems, the turnover rates and product distributions depend on their size, shape, oxidation states, and their composition in case of bimetallic NP systems. Dendrimer-supported platinum and rhodium NPs of less than 2 nm diameter usually have high oxidation states and can be utilized for catalytic cyclization and hydroformylation reactions which previously were produced only by homogeneous catalysis. Transition metal nanoparticles in metal core (Pt, Co)––inorganic shell (SiO2) structure exhibit exceptional thermal stability and are well-suited to perform catalytic reactions at high temperatures (>400 °C). Instruments developed in our laboratory permit the atomic and molecular level study of NPs under reaction conditions (SFG, ambient pressure XPS and high pressure STM). These studies indicate continuous restructuring of the metal substrate and the adsorbate molecules, changes of oxidation states with NP size and surface composition variations of bimetallic NPs with changes of reactant molecules. The facile rearrangement of NP catalysts required for catalytic turnover makes nanoparticle systems (heterogeneous, homogeneous and enzyme) excellent catalysts and provides opportunities to develop hybrid heterogeneous-homogeneous, heterogeneous-enzyme and homogeneous-enzyme catalyst systems.
Co-reporter:Zhi Liu;Derek R. Butcher;Lin-Wang Wang;Hendrik Bluhm;Miquel Salmeron;Sefa Dag;Feng Tao
Science 2010 Volume 327(Issue 5967) pp:
Publication Date(Web):
DOI:10.1126/science.1182122
From Steps to Clusters
When a flat surface of a single crystal is formed by cutting or cleavage, the atoms may move little from their bulk positions, or the surface may reconstruct as the atoms move to more energetically favorable positions. The adsorption of molecules can also change the energetic landscape and cause reconstruction. Tao et al. (p. 850; see the Perspective by Altman) examined “stepped” platinum surfaces, the (557) and (332) surfaces in which flat terraces are connected by atomic steps. Scanning tunneling microscopy and x-ray photoelectron spectroscopy revealed a reversible breakup into nanometer-scale clusters when CO surface coverages were very high. Density functional theory calculations suggest that this new morphology increases the number of edge sites for adsorption and relieves unfavorable CO-CO repulsions.
Co-reporter:Antoine Hervier, J. Russell Renzas, Jeong Y. Park and Gabor A. Somorjai
Nano Letters 2009 Volume 9(Issue 11) pp:3930-3933
Publication Date(Web):September 4, 2009
DOI:10.1021/nl9023275
Hydrogen oxidation on platinum is shown to be a surface catalytic chemical reaction that generates a steady state flux of hot (>1 eV) conduction electrons. These hot electrons are detected as a steady-state chemicurrent across Pt/TiO2 Schottky diodes whose Pt surface is exposed to hydrogen and oxygen. Kinetic studies establish that the chemicurrent is proportional to turnover frequency for temperatures ranging from 298 to 373 K for PH2 between 1 and 8 Torr and PO2 at 760 Torr. Both chemicurrent and turnover frequency exhibit a first order dependence on PH2.
Co-reporter:Feng Tao, Sefa Dag, Lin-Wang Wang, Zhi Liu, Derek R. Butcher, Miquel Salmeron and Gabor A. Somorjai
Nano Letters 2009 Volume 9(Issue 5) pp:2167-2171
Publication Date(Web):April 24, 2009
DOI:10.1021/nl900809u
The atomic-scale restructuring of hex-Pt(100) induced by carbon monoxide with a wide pressure range was studied with a newly designed chamber-in-chamber high-pressure STM and theoretical calculations. Both experimental and DFT calculation results show that CO molecules are bound to Pt nanoclusters through a tilted on-top configuration with a separation of ∼3.7−4.1 Å. The phenomenon of restructuring of metal catalyst surfaces induced by adsorption and, in particular, the formation of small metallic clusters suggests the importance of studying structures of catalyst surfaces under high-pressure conditions for understanding catalytic mechanisms.
Co-reporter:Gabor A. Somorjai, Jeong Y. Park
Surface Science 2009 Volume 603(10–12) pp:1293-1300
Publication Date(Web):1 June 2009
DOI:10.1016/j.susc.2008.08.030
Over the past forty years, surface science has evolved to become both an atomic scale and a molecular scale science. Gerhard Ertl’s group has made major contributions in the field of molecular scale surface science, focusing on vacuum studies of adsorption chemistry on single crystal surfaces. In this review, we outline three important aspects which have led to recent advances in surface chemistry: the development of new concepts, in situ instruments for molecular scale surface studies at buried interfaces (solid–gas and solid–liquid), and new model nanoparticle surface systems, in addition to single crystals. Combined molecular beam surface scattering and low energy electron diffraction (LEED)- surface structure studies on metal single crystal surfaces revealed concepts, including adsorbate-induced surface restructuring and the unique activity of defects, atomic steps, and kinks on metal surfaces. We have combined high pressure catalytic reaction studies with ultra high vacuum (UHV) surface characterization techniques using a UHV chamber equipped with a high pressure reaction cell. New instruments, such as high pressure sum frequency generation (SFG) vibrational spectroscopy and scanning tunneling microscopy (STM) which permit molecular-level surface studies have been developed. Tools that access broad ranges of pressures can be used for both the in situ characterization of solid–gas and solid–liquid buried interfaces and the study of catalytic reaction intermediates. The model systems for the study of molecular surface chemistry have evolved from single crystals to nanoparticles in the 1–10 nm size range, which are currently the preferred media in catalytic reaction studies.
Co-reporter:Michael E. Grass, Sang Hoon Joo, Yawen Zhang and Gabor A. Somorjai
The Journal of Physical Chemistry C 2009 Volume 113(Issue 20) pp:8616-8623
Publication Date(Web):April 28, 2009
DOI:10.1021/jp901288m
A particle size dependence for CO oxidation over rhodium nanoparticles of 1.9−11.3 nm has been investigated and determined to be modified by the existence of the capping agent poly(vinylpyrrolidone) (PVP). The particles were prepared using a polyol reduction procedure with PVP as the capping agent. The Rh nanoparticles were subsequently supported on SBA-15 during hydrothermal synthesis to produce Rh/SBA-15 supported catalysts for size-dependent catalytic studies. CO oxidation by O2 at 40 Torr CO and 100 Torr O2 was investigated over two series of Rh/SBA-15 catalysts: as-synthesized Rh/SBA-15 covering the full range of Rh sizes and the same set of catalysts after high temperature calcination and reduction. The turnover frequency at 443 K increases from 0.4 to 1.7 s−1 as the particle size decreases from 11.3 to 1.9 nm for the as-synthesized catalysts. After calcination and reduction, the turnover frequency is between 0.1 and 0.4 s−1 with no particle size dependence. The apparent activation energy for all catalysts is ∼30 kcal mol−1 and is independent of particle size and thermal treatment. Infrared spectroscopy of CO on the Rh nanoparticles indicates that the heat treatments used influence the mode of CO adsorption. As a result, the particle size dependence for CO oxidation is altered after calcination and reduction of the catalysts. CO adsorbs at two distinct bridge sites on as-synthesized Rh/SBA-15, attributable to metallic Rh(0) and oxidized Rh(I) bridge sites. After calcination and reduction, however, CO adsorbs only at Rh(0) atop sites. The change in adsorption geometry and oxidation activity may be attributable to the interaction between PVP and the Rh surface. This capping agent affect may open new possibilities for the tailoring of metal catalysts using solution nanoparticle synthesis methods.
Co-reporter:Jeong Y. Park;Cesar Aliaga;J. Russell Renzas;Hyunjoo Lee
Catalysis Letters 2009 Volume 129( Issue 1-2) pp:1-6
Publication Date(Web):2009 April
DOI:10.1007/s10562-009-9871-8
We report the catalytic activity of colloid platinum nanoparticles synthesized with different organic capping layers. On the molecular scale, the porous organic layers have open spaces that permit the reactant and product molecules to reach the metal surface. We carried out CO oxidation on several platinum nanoparticle systems capped with various organic molecules to investigate the role of the capping agent on catalytic activity. Platinum colloid nanoparticles with four types of capping layer have been used: TTAB (Tetradecyltrimethylammonium Bromide), HDA (hexadecylamine), HDT (hexadecylthiol), and PVP (poly(vinylpyrrolidone)). The reactivity of the Pt nanoparticles varied by 30%, with higher activity on TTAB coated nanoparticles and lower activity on HDT, while the activation energy remained between 27 and 28 kcal/mol. In separate experiments, the organic capping layers were partially removed using ultraviolet light-ozone generation techniques, which resulted in increased catalytic activity due to the removal of some of the organic layers. These results indicate that the nature of chemical bonding between organic capping layers and nanoparticle surfaces plays a role in determining the catalytic activity of platinum colloid nanoparticles for carbon monoxide oxidation.
Co-reporter:Roger L. York, George J. Holinga and Gabor A. Somorjai
Langmuir 2009 Volume 25(Issue 16) pp:9369-9374
Publication Date(Web):April 24, 2009
DOI:10.1021/la900654m
Sum frequency generation vibrational spectroscopy (SFG) and quartz crystal microbalance with dissipation monitoring (QCM-D) are employed to study the interfacial structure and adsorbed amount of the amino acids l-lysine and l-proline and their corresponding homopeptides, poly-l-lysine and poly-l-proline, at two liquid−solid interfaces. SFG and QCM-D experiments of these molecules are carried out at the interface between phosphate buffered saline at pH 7.4 (PBS) and the hydrophobic deuterated polystyrene (d8-PS) surface as well as the interface between PBS and hydrophilic fused silica (SiO2). The SFG spectra of the amino acids studied here are qualitatively similar to their corresponding homopeptides; however, the SFG signal from amino acids at the solid/PBS interface is smaller in magnitude relative to their more massive homopeptides at the concentrations studied here. Substantial differences are observed in SFG spectra for each species between the hydrophobic d8-PS and the hydrophilic SiO2 liquid−solid interfaces, suggesting surface-dependent interfacial ordering of the biomolecules. Over the range of concentrations used in this study, QCM-D measurements also indicate that on both surfaces poly-l-lysine adsorbs to a greater extent than its constituent amino acid l-lysine. The opposite trend is demonstrated by poly-l-proline which sticks to both surfaces less extensively than its corresponding amino acid, l-proline. Lastly, we find that the adsorption of the molecules studied here can have a strong influence on interfacial water structure as detected in the SFG spectra.
Co-reporter:Gabor A. Somorjai
Catalysis Letters 2009 Volume 133( Issue 1-2) pp:
Publication Date(Web):2009 November
DOI:10.1007/s10562-009-0152-3
Heinz Heineman came to Berkeley in 1978 and stayed there for 15 years. This was the time of the energy crisis and we did not have anybody like him who had such a tremendous industrial experience with oil and coal conversion technology and science. He was interested in the conversion of coal to gaseous molecules and our studies with model catalysts appealed to him and attracted him. In a way, Heinz Heineman was bigger than life, since he played such a seminal role in the history of American catalysis science.
Co-reporter:Roger L. York, Yimin Li, George J. Holinga and Gabor A. Somorjai
The Journal of Physical Chemistry A 2009 Volume 113(Issue 12) pp:2768-2774
Publication Date(Web):February 24, 2009
DOI:10.1021/jp808629r
The influence of experimental geometry on infrared total internal reflection surface sum frequency generation (SFG) vibrational spectra at the water/solid interface has been examined. A detailed analysis of the experimental geometry revealed that the enhancement of SFG signal for the “critical angle” can be much weaker than previously thought if the index of refraction of the transmitted or reflected medium is treated as a complex value (i.e., the imaginary part of the index of refraction is not zero and not neglected). The theoretical analysis outlined here agreed well with the experimental results of the SFG spectra of the silica/water interface in two different geometries. This paper deals with the SSP polarization combination.
Co-reporter:Michael E. Grass;Robert M. Rioux
Catalysis Letters 2009 Volume 128( Issue 1-2) pp:1-8
Publication Date(Web):2009 March
DOI:10.1007/s10562-008-9754-4
The selectivity and activity for the hydrogenation of crotonaldehyde to crotyl alcohol and butyraldehyde was studied over a series of Pt nanoparticles (diameter of 1.7, 2.9, 3.6, and 7.1 nm). The nanoparticles were synthesized by alcohol reduction of a Pt salt in the presence of poly(vinylpyrrolidone) (PVP), followed by incorporation into mesoporous SBA-15 silica. The rate of crotonaldehyde hydrogenation and selectivity towards crotyl alcohol both increase with increasing particle size. With an increase in particle size from 1.7 nm to 7.1 nm, the selectivity towards crotyl alcohol increases from 13.7% to 33.9% (8 Torr crotonaldehyde, 160 Torr H2 and 353 K). The turnover frequency increases from 2.1 × 10−2 s−1 to 4.8 × 10−2 s−1 with increasing particle size. Additionally, the decarbonylation pathway to form propene and CO is enhanced over smaller nanoparticles. The apparent activation energy remains constant (~16 kcal mol−1 for the formation of butyraldehyde and ~8 kcal mol−1 for the formation of crotyl alcohol) as a function of particle size as does the reaction order in H2, which is unity. In the presence of 130–260 mTorr CO, the reaction rate decreases for all products with a CO reaction order of −1 to −1.4 for crotyl alcohol and butyraldehyde. Hydrogen reduction at 673–723 K results in increased activity and selectivity relative to reduction at either higher or lower temperature; this is discussed with respect to the organic capping agent, PVP.
Co-reporter:Gabor A. Somorjai and Jeong Y. Park
Chemical Society Reviews 2008 vol. 37(Issue 10) pp:2155-2162
Publication Date(Web):30 Jul 2008
DOI:10.1039/B719148K
Model systems for studying molecular surface chemistry have evolved from single crystal surfaces at low pressure to colloidal nanoparticles at high pressure. Low pressure surface structure studies of platinum single crystals using molecular beam surface scattering and low energy electron diffraction techniques probe the unique activity of defects, steps and kinks at the surface for dissociation reactions (H–H, C–H, C–C, OO bonds). High-pressure investigations of platinum single crystals using sum frequency generation vibrational spectroscopy have revealed the presence and the nature of reaction intermediates. High pressure scanning tunneling microscopy of platinum single crystal surfaces showed adsorbate mobility during a catalytic reaction. Nanoparticle systems are used to determine the role of metal–oxide interfaces, site blocking and the role of surface structures in reactive surface chemistry. The size, shape and composition of nanoparticles play important roles in determining reaction activity and selectivity and is covered in this tutorial review.
Co-reporter:Wenyu Huang, John N. Kuhn, Chia-Kuang Tsung, Yawen Zhang, Susan E. Habas, Peidong Yang and Gabor A. Somorjai
Nano Letters 2008 Volume 8(Issue 7) pp:2027-2034
Publication Date(Web):June 11, 2008
DOI:10.1021/nl801325m
Monodisperse rhodium (Rh) and platinum (Pt) nanoparticles as small as ∼1 nm were synthesized within a fourth generation polyaminoamide (PAMAM) dendrimer, a hyperbranched polymer, in aqueous solution and immobilized by depositing onto a high-surface-area SBA-15 mesoporous support. X-ray photoelectron spectroscopy indicated that the as-synthesized Rh and Pt nanoparticles were mostly oxidized. Catalytic activity of the SBA-15 supported Rh and Pt nanoparticles was studied with ethylene hydrogenation at 273 and 293 K in 10 torr of ethylene and 100 torr of H2 after reduction (76 torr of H2 mixed with 690 torr of He) at different temperatures. Catalysts were active without removing the dendrimer capping but reached their highest activity after hydrogen reduction at a moderate temperature (423 K). When treated at a higher temperature (473, 573, and 673 K) in hydrogen, catalytic activity decreased. By using the same treatment that led to maximum ethylene hydrogenation activity, catalytic activity was also evaluated for pyrrole hydrogenation.
Co-reporter:GaborA. Somorjai ;JeongY. Park Dr.
Angewandte Chemie 2008 Volume 120( Issue 48) pp:9352-9368
Publication Date(Web):
DOI:10.1002/ange.200803181
Abstract
Selektivität – die Bildung eines bestimmten Produkts aus einer Auswahl von thermodynamisch möglichen molekularen Produkten – ist der Schlüssel zur Entwicklung sauberer Prozesse, bei denen keinerlei Nebenprodukte entstehen (“grüne Chemie”). Kleine Unterschiede in den Aktivierungsbarrieren für einzelne Schritte bestimmen, über welchen Reaktionsweg das gewünschte molekulare Produkt gebildet wird (Selektivität), wohingegen die Aktivierungsbarriere der Gesamtreaktion die Umsatzgeschwindigkeiten vorgibt (Aktivität). Kürzlich wurde gezeigt, wie Parameter auf atomarer oder molekularer Ebene abgestimmt werden können – beispielsweise hängen Umsatzgeschwindigkeiten und Selektivitäten von der Größe und Form von Nanopartikeln ab, weil sich die aktiven Zentren auf ihren Oberflächen unterscheiden. Hier rücken wir sieben molekulare Faktoren in den Blickpunkt, die die Selektivität heterogenkatalytischer Reaktionen an Metalleinkristall-Modelloberflächen und an Kolloidnanopartikeln beeinflussen: die Oberflächenstruktur, Strukturveränderungen durch Adsorbatbindung, die Beweglichkeit des Adsorbats, Reaktionszwischenstufen, die Oberflächenzusammensetzung, der Ladungstransport sowie der Oxidationszustand. Wir stellen die Bedeutung der einzelnen Faktoren anhand von Beispielen heraus und beschreiben, welche Methoden uns zur In-situ-Untersuchung ihrer Rolle bei Oberflächenreaktionen zu Gebote stehen.
Co-reporter:Zhi Liu;Miquel Salmeron;Derek R. Butcher;Michael E. Grass;Yawen Zhang;James R. Renzas;Feng Tao;Bongjin S. Mun;Jen Y. Chung
Science 2008 Volume 322(Issue 5903) pp:932-934
Publication Date(Web):07 Nov 2008
DOI:10.1126/science.1164170
Abstract
Heterogeneous catalysts that contain bimetallic nanoparticles may undergo segregation of the metals, driven by oxidizing and reducing environments. The structure and composition of core-shell Rh0.5Pd0.5 and Pt0.5Pd0.5 nanoparticle catalysts were studied in situ, during oxidizing, reducing, and catalytic reactions involving NO, O2, CO, and H2 by x-ray photoelectron spectroscopy at near-ambient pressure. The Rh0.5Pd0.5 nanoparticles underwent dramatic and reversible changes in composition and chemical state in response to oxidizing or reducing conditions. In contrast, no substantial segregation of Pd or Pt atoms was found in Pt0.5Pd0.5 nanoparticles. The different behaviors in restructuring and chemical response of Rh0.5Pd0.5 and Pt0.5Pd0.5 nanoparticle catalysts under the same reaction conditions illustrates the flexibility and tunability of the structure of bimetallic nanoparticle catalysts during catalytic reactions.
Co-reporter:Yawen Zhang ; Wenyu Huang ; Susan E. Habas ; John N. Kuhn ; Michael E. Grass ; Yusuke Yamada ; Peidong Yang
The Journal of Physical Chemistry C 2008 Volume 112(Issue 32) pp:12092-12095
Publication Date(Web):July 22, 2008
DOI:10.1021/jp805788x
Near-monodisperse Ni1−xCux (x = 0.2−0.8) bimetallic nanocrystals were synthesized by a one-pot thermolysis approach in oleylamine/1-octadecene, using metal acetylacetonates as precursors. The nanocrystals form large-area 2D superlattices, and display a catalytic synergistic effect in the hydrolysis of NaBH4 to generate H2 at x = 0.5 in a strongly basic medium. The Ni0.5Cu0.5 nanocrystals show the lowest activation energy, and also exhibit the highest H2 generation rate at 298 K.
Co-reporter:Gabor A. Somorjai
Topics in Catalysis 2008 Volume 48( Issue 1-4) pp:1-7
Publication Date(Web):2008 May
DOI:10.1007/s11244-008-9039-6
Over 40 years, there have been major efforts to aim at understanding the properties of surfaces, structure, composition, dynamics on the molecular level and at developing the surface science of heterogeneous and homogeneous catalysis. Since most catalysts (heterogeneous, enzyme and homogeneous) are nanoparticles, colloid synthesis methods were developed to produce monodispersed metal nanoparticles in the 1–10 nm range and controlled shapes to use them as new model catalyst systems in two-dimensional thin film form or deposited in mezoporous three-dimensional oxides. Studies of reaction selectivity in multipath reactions (hydrogenation of benzene, cyclohexene and crotonaldehyde) showed that reaction selectivity depends on both nanoparticle size and shape. The oxide-metal nanoparticle interface was found to be an important catalytic site because of the hot electron flow induced by exothermic reactions like carbon monoxide oxidation.
Co-reporter:Matthias M. Koebel;Louis C. Jones
Journal of Nanoparticle Research 2008 Volume 10( Issue 6) pp:1063-1069
Publication Date(Web):2008 August
DOI:10.1007/s11051-008-9370-7
We demonstrate a preparative method which produces highly monodisperse Pt-nanoparticles of tunable size without the external addition of seed particles. Hexachloroplatinic acid is dosed slowly to an ethylene glycol solution at 120 °C and reduced in the presence of a stabilizing polymer poly-N-vinylpyrrolidone (PVP). Slow addition of the Pt-salt will first lead to the formation of nuclei (seeds) which then grow further to produce larger particles of any desired size between 3 and 8 nm. The amount of added hexachloroplatinic acid precursor controls the size of the final nanoparticle product. TEM was used to determine size and morphology and to confirm the crystalline nature of the nanoparticles. Good reproducibility of the technique was demonstrated. Above 7 nm, the particle shape and morphology changes suddenly indicating a change in the deposition selectivity of the Pt-precursor from (100) towards (111) crystal faces and breaking up of larger particles into smaller entities.
Co-reporter:Kaitlin M. Bratlie ; Kyriakos Komvopoulos
The Journal of Physical Chemistry C 2008 Volume 112(Issue 31) pp:11865-11868
Publication Date(Web):July 10, 2008
DOI:10.1021/jp801583q
Pyridine hydrogenation in the presence of a surface monolayer consisting of cubic Pt nanoparticles stabilized by tetradecyltrimethylammonium bromide (TTAB) was investigated by sum frequency generation (SFG) vibrational spectroscopy using total internal reflection (TIR) geometry. TIR-SFG spectra analysis revealed that a pyridinium cation (C5H5NH+) forms during pyridine hydrogenation on the Pt nanoparticle surface, and the NH group in the C5H5NH+ cation becomes more hydrogen bound with the increase of the temperature. In addition, the surface coverage of the cation decreases with the increase of the temperature. An important contribution of this study is the in situ identification of reaction intermediates adsorbed on the Pt nanoparticle monolayer during pyridine hydrogenation.
Co-reporter:GaborA. Somorjai ;JeongY. Park Dr.
Angewandte Chemie 2008 Volume 120( Issue 48) pp:
Publication Date(Web):
DOI:10.1002/ange.200890294
Co-reporter:MichaelE. Grass;Yawen Zhang ;DerekR. Butcher;JeongY. Park Dr.;Yimin Li Dr.;Hendrik Bluhm Dr.;KaitlinM. Bratlie Dr.;Tianfu Zhang Dr.;GaborA. Somorjai
Angewandte Chemie 2008 Volume 120( Issue 46) pp:9025-9028
Publication Date(Web):
DOI:10.1002/ange.200803574
Co-reporter:GaborA. Somorjai ;JeongY. Park Dr.
Angewandte Chemie International Edition 2008 Volume 47( Issue 48) pp:
Publication Date(Web):
DOI:10.1002/anie.200890245
Co-reporter:GaborA. Somorjai ;JeongY. Park Dr.
Angewandte Chemie International Edition 2008 Volume 47( Issue 48) pp:9212-9228
Publication Date(Web):
DOI:10.1002/anie.200803181
Abstract
Selectivity—the production of one molecule out of many other thermodynamically feasible product molecules—is the key concept in developing clean processes that do not produce by-products (green chemistry). Small differences in the potential-energy barriers of single reaction steps control which reaction channel is more likely to yield the desired product molecule (selectivity), while the overall activation energy of the reaction controls the turnover rates (activity). Recent studies have demonstrated that tailoring parameters at the atomic or molecular level—such as the surface structures of active sites—gives turnover rates and reaction selectivities that depend on the nanoparticle size and shape. Here, we highlight seven molecular components that influence the selectivity of heterogeneous catalyst reactions on single-crystal model surfaces and colloid nanoparticles: surface structure, adsorbate-induced restructuring, adsorbate mobility, reaction intermediates, surface composition, charge transport, and oxidation states. We show the importance of the single factors by means of examples and describe in situ analyses that permit their roles in surface reactions to be investigated.
Co-reporter:MichaelE. Grass;Yawen Zhang ;DerekR. Butcher;JeongY. Park Dr.;Yimin Li Dr.;Hendrik Bluhm Dr.;KaitlinM. Bratlie Dr.;Tianfu Zhang Dr.;GaborA. Somorjai
Angewandte Chemie International Edition 2008 Volume 47( Issue 46) pp:8893-8896
Publication Date(Web):
DOI:10.1002/anie.200803574
Co-reporter:R. M. Rioux;B. B. Hsu;M. E. Grass;H. Song;G. A. Somorjai
Catalysis Letters 2008 Volume 126( Issue 1-2) pp:
Publication Date(Web):2008 November
DOI:10.1007/s10562-008-9637-8
The role of particle size during the hydrogenation/dehydrogenation of cyclohexene (10 Torr C6H10, 200–600 Torr H2, and 273–650 K) was studied over a series of monodisperse Pt/SBA-15 catalysts. The conversion of cyclohexene in the presence of excess H2 (H2: C6H10 ratio = 20:60) is characterized by three regimes: hydrogenation of cyclohexene to cyclohexane at low temperature (<423 K), an intermediate temperature range in which both hydrogenation and dehydrogenation occur; and a high temperature regime in which the dehydrogenation of cyclohexene dominates (>573 K). The rate of both reactions demonstrated maxima with temperature, regardless of Pt particle size. For the hydrogenation of cyclohexene, a non-Arrhenius temperature dependence (apparent negative activation energy) was observed. Hydrogenation is structure insensitive at low temperatures, and apparently structure sensitive in the non-Arrhenius regime; the origin of the particle-size dependent reactivity with temperature is attributed to a change in the coverage of reactive hydrogen. Small particles were more active for dehydrogenation and had lower apparent activation energies than large particles. The selectivity can be controlled by changing the particle size, which is attributed to the structure sensitivity of both reactions in the temperature regime where hydrogenation and dehydrogenation are catalyzed simultaneously.
Co-reporter:Gabor A. Somorjai;Jeong Y. Park
Topics in Catalysis 2008 Volume 49( Issue 3-4) pp:126-135
Publication Date(Web):2008 August
DOI:10.1007/s11244-008-9077-0
Recent breakthroughs in the synthesis of nanosciences have achieved the control of size and shape of nanoparticles that are relevant for catalyst design. In this article, we review advances in the synthesis of nanoparticles, fabrication of two- and three-dimensional model catalyst systems, characterization, and studies of activity and selectivity. The ability to synthesize monodispersed platinum and rhodium nanoparticles 1–10 nm in size permitted us to study the influence of composition, structure, and dynamic properties of monodispersed metal nanoparticles on chemical reactivity and selectivity. We review the importance of the size and shape of nanoparticles to determine reaction selectivity in multi-path reactions. The influence of metal–support interaction has been studied by probing the hot electron flows through the metal–oxide interface in catalytic nanodiodes. Novel designs of nanoparticle catalytic systems are also discussed.
Co-reporter:Gabor A. Somorjai, Roger L. York, Derek Butcher and Jeong Y. Park
Physical Chemistry Chemical Physics 2007 vol. 9(Issue 27) pp:3500-3513
Publication Date(Web):22 Mar 2007
DOI:10.1039/B618805B
The material and pressure gap has been a long standing challenge in the field of heterogeneous catalysis and have transformed surface science and biointerfacial research. In heterogeneous catalysis, the material gap refers to the discontinuity between well-characterized model systems and industrially relevant catalysts. Single crystal metal surfaces have been useful model systems to elucidate the role of surface defects and the mobility of reaction intermediates in catalytic reactivity and selectivity. As nanoscience advances, we have developed nanoparticle catalysts with lithographic techniques and colloidal syntheses. Nanoparticle catalysts on oxide supports allow us to investigate several important ingredients of heterogeneous catalysis such as the metal-oxide interface and the influence of noble metal particle size and surface structure on catalytic selectivity. Monodispersed nanoparticle and nanowire arrays were fabricated for use as model catalysts by lithographic techniques. Platinum and rhodium nanoparticles in the 1–10 nm range were synthesized in colloidal solutions in the presence of polymer capping agents. The most catalytically active systems are employed at high pressure or at solid–liquid interfaces. In order to study the high pressure and liquid interfaces on the molecular level, experimental techniques with which we bridged the pressure gap in catalysis have been developed. These techniques include the ultrahigh vacuum system equipped with high pressure reaction cell, high pressure Sum Frequency Generation (SFG) vibration spectroscopy, High Pressure Scanning Tunneling Microscopy (HP-STM), and High Pressure X-ray Photoemission Spectroscopy (HP-XPS), and Quartz Crystal Microbalance (QCM). In this article, we overview the development of experimental techniques and evolution of the model systems for the research of heterogeneous catalysis and biointerfacial studies that can shed light on the long-standing issues of materials and pressure gaps.
Co-reporter:Roger L. York;Phillip L. Geissler;William K. Browne
Israel Journal of Chemistry 2007 Volume 47(Issue 1) pp:51-58
Publication Date(Web):10 MAR 2010
DOI:10.1560/IJC.47.1.51
Sum frequency generation (SFG) vibrational spectroscopy has been used to characterize the interfacial structure of a series of model peptides at the hydrophobic polystyrene-buffer interface. The peptides contain two types of amino acids, one hydrophobic (X) and one hydrophilic (Y). Their sequences are Ac-XYYXXYXXYYXXYX-NH2 (XY14) and Ac-XYXYXYX-NH2 (XY7), respectively, where the X and Y combinations are: leucine (L) and lysine (K); alanine (A) and lysine (K); alanine (A) and arginine (R); and phenylalanine (F) and arginine (R), respectively. One additional peptide was synthesized and characterized, Ac-LKKLLKL-NH2, referred to as LK7 α. The XY14 peptides showed SFG spectra that were characteristic of the hydrophobic (X) amino acid of the peptide. Comparison with the 7-amino acid peptides shows that the molecular orientation of alanine is more sensitive to changes in sequence and chain length than leucine or phenylalanine. The hydrophilic amino acids are not observed in the SFG spectra of these peptides at the hydrophobic polystyrene interface (with the possible exception of the AR7 peptide), suggesting the hydrophilic amino acids studied here have a random orientation at this interface. The results of these studies are put into the context of recent SFG studies of proteins adsorbed onto hydrophobic surfaces. Furthermore, our approach to theoretical understanding of interfacial peptide structure is outlined. The results of a molecular dynamics simulation of the LK14 peptide on a hydrophobic interface are presented and discussed.
Co-reporter:Gabor A. Somorjai;Jeong Y. Park
Catalysis Letters 2007 Volume 115( Issue 3-4) pp:87-98
Publication Date(Web):2007 June
DOI:10.1007/s10562-007-9137-2
Surface science developed instruments for atomic- and molecular-scale studies of catalyst surfaces, their composition and structure, both in a vacuum and at high pressures, under reaction conditions (bridging the pressure gap). Surfaces ranging from single crystals, nanoparticles and thin films to porous high surface area catalytic materials have been studied. Classes of surface structure sensitive and insensitive reactions have been identified by surface science studies, including ammonia synthesis, hydrodesulfurization, reforming, combustion and hydrogenation. Rates of reactions often vary by orders of magnitude between using the right and the wrong surface structures. The roles of many promoters that modify the catalyst surface structures and bonding of adsorbates have been verified. Surface reaction intermediates could be identified and the mobility of adsorbates and the adsorbate induced reconstruction of the catalysts attest to the dynamic nature of the catalytic systems during the reaction turnover. The important active sites for catalysis include the low coordination surface step, kink, oxygen and chloride ion vacancies sites and sites at oxide-metal interfaces. Uncovering the molecular ingredients of heterogeneous catalysts will have a major impact on the understanding of reaction selectivity to help the evolution of green chemistry and selective reaction of many types.
Co-reporter:Vladimir V. Pushkarev, Kwangjin An, Selim Alayoglu, Simon K. Beaumont, Gabor A. Somorjai
Journal of Catalysis (August 2012) Volume 292() pp:64-72
Publication Date(Web):1 August 2012
DOI:10.1016/j.jcat.2012.04.022
Vapor-phase hydrogenation of benzene and toluene over a size-dependent (1.5–5.2 nm) series of Pt nanoparticles (NPs) encapsulated in polyvinylpyrrolidone (PVP) or polyamidoamine (PAMAM) dendrimer and supported in SBA-15 mesoporous silica was investigated at ambient pressure between 343 K and 398 K. Under these conditions, both reactions exhibit a moderate structure sensitivity evidenced by changes in their turnover rate (TOR), apparent activation energy (EA), pre-exponential factor ln(r0), and deactivation rate parameters with Pt particle size. For instance, the 2.4–3.1 nm Pt NPs showed the highest TOR for both reactions as compared to NPs of either smaller or larger sizes. Also, the EA and ln(r0) factor values were larger for smaller Pt particle sizes, and the rates of catalyst deactivation were more pronounced for catalysts with larger Pt particle sizes. However, the reaction orders on H2 and hydrocarbon were independent of particle size.Graphical abstractVapor-phase hydrogenation of benzene and toluene over a size-dependent (1.5–5.2 nm) series of monodispersed Pt/SBA-15 catalysts were investigated at ambient pressure between 343 K and 398 K. Under these conditions, both reactions exhibit a moderate structure sensitivity evidenced by changes in their turnover frequency, EA and ln(r0) factor values, and deactivation rate parameters with Pt particle size.Download high-res image (91KB)Download full-size imageHighlights► We studied vapor-phase benzene and toluene hydrogenation over 1.5–5.2 nm Pt/SBA-15. ► The catalysts are highly selective toward complete hydrogenation reaction paths. ► TORs, EA, and ln(r0) were Pt particle size dependent between 334 K and 398 K. ► Reaction orders on H2 and hydrocarbon were independent of Pt particle size.
Co-reporter:William D. Michalak, James M. Krier, Selim Alayoglu, Jae-Yoon Shin, Kwangjin An, Kyriakos Komvopoulos, Zhi Liu, Gabor A. Somorjai
Journal of Catalysis (April 2014) Volume 312() pp:17-25
Publication Date(Web):1 April 2014
DOI:10.1016/j.jcat.2014.01.005
•Pt and Sn remain in an intermixed phase under H2 atmospheres.•Pt and Sn atoms segregate to form metallic Pt and Sn oxide domains during CO oxidation.•Sn domains provide adsorption sites for O atoms.•CO and O interact at the interface of Pt and Sn oxides to form CO2.The barrier to CO oxidation on Pt catalysts is the strongly bound adsorbed CO, which inhibits O2 adsorption and hinders CO2 formation. Using reaction studies and in situ X-ray spectroscopy with colloidally prepared, monodisperse ∼2 nm Pt and PtSn nanoparticle catalysts, we show that the addition of Sn to Pt provides distinctly different reaction sites and a more efficient reaction mechanism for CO oxidation compared to pure Pt catalysts. To probe the influence of Sn, we intentionally poisoned the Pt component of the nanoparticle catalysts using a CO-rich atmosphere. With a reaction environment comprised of 100 Torr CO and 40 Torr O2 and a temperature range between 200 and 300 °C, Pt and PtSn catalysts exhibited activation barriers for CO2 formation of 133 kJ/mol and 35 kJ/mol, respectively. While pure Sn is readily oxidized and is not active for CO oxidation, the addition of Sn to Pt provides an active site for O2 adsorption that is important when Pt is covered with CO. Sn oxide was identified as the active Sn species under reaction conditions by in situ ambient pressure X-ray photoelectron spectroscopy measurements. While chemical signatures of Pt and Sn indicated intermixed metallic components under reducing conditions, Pt and Sn were found to reversibly separate into isolated domains of Pt and oxidic Sn on the nanoparticle surface under reaction conditions of 100 mTorr CO and 40 mTorr O2 between temperatures of 200–275 °C. Under these conditions, PtSn catalysts exhibited apparent reaction orders in O2 for CO2 production that were 0.5 and lower with increasing partial pressures. These reaction orders contrast the first-order dependence in O2 known for pure Pt. The differences in activation barriers, non-first-order dependence in O2, and the presence of a partially oxidized Sn indicate that the enhanced activity is due to a reaction mechanism that occurs at a Pt/Sn oxide interface present at the nanoparticle surface.Graphical abstractDownload high-res image (153KB)Download full-size image
Co-reporter:John N. Kuhn, Chia-Kuang Tsung, Wenyu Huang, Gabor A. Somorjai
Journal of Catalysis (25 July 2009) Volume 265(Issue 2) pp:209-215
Publication Date(Web):25 July 2009
DOI:10.1016/j.jcat.2009.05.001
The influence of oleylamine (OA), trimethyl tetradecyl ammonium bromide (TTAB), and polyvinlypyrrolidone (PVP) capping agents upon the catalytic properties of Pt/silica catalysts was evaluated. Pt nanoparticles that were 1.5 nm in size were synthesized by the same procedure (ethylene glycol reduction under basic conditions) with the various capping agents added afterward for stabilization. Before examining catalytic properties for ethylene hydrogenation and CO oxidation, the Pt NPs were deposited onto mesoporous silica (SBA-15) supports and characterized by transmission electron microscopy (TEM), H2 chemisorption, and elemental analysis (ICP-MS). PVP- and TTAB-capped Pt yielded mass-normalized reaction rates that decreased with increasing pretreatment temperature, and this trend was attributed to the partial coverage of the Pt surface with decomposition products from the organic capping agent. Once normalized to the Pt surface area, similar intrinsic activities were obtained regardless of the pretreatment temperature, which indicated no influence on the nature of the active sites. Consequently, a chemical probe technique using intrinsic activity for ethylene hydrogenation was demonstrated as an acceptable method for estimating the metallic surface areas of Pt. Amine (OA) capping exhibited a detrimental influence on the catalytic properties as severe deactivation and low activity were observed for ethylene hydrogenation and CO oxidation, respectively. These results were consistent with amine groups being strong poisons for Pt surfaces, and revealed the need to consider the effects of capping agents on the catalytic properties.Catalytic differences are demonstrated over Pt for different capping agents. These results revealed the need to consider the application while developing synthetic approaches for morphology control.Download high-res image (54KB)Download full-size image
Co-reporter:Robert M. Rioux, Russell Komor, Hyunjoon Song, James D. Hoefelmeyer, Michael Grass, Krisztian Niesz, Peidong Yang, Gabor A. Somorjai
Journal of Catalysis (15 February 2008) Volume 254(Issue 1) pp:1-11
Publication Date(Web):15 February 2008
DOI:10.1016/j.jcat.2007.10.015
The influence of particle size on the poisoning of ethylene hydrogenation by CO was studied over a series of catalysts composed of nearly monodisperse Pt nanoparticles (1.7–7.1 nm) encapsulated in mesoporous silica (SBA-15). The turnover frequency at 403 K in the presence of 0.5 Torr CO was ∼2 × 10−2 s−1 (compared with ∼102 s−1 in the absence of CO). The apparent activation energy in the absence and presence of 0.2 Torr CO was ∼10 and 20 kcal mol−1, respectively. The pressure dependency changes significantly in the presence of CO; reaction orders in hydrogen were 1/2 in the presence of CO at 403 K and noncompetitive with regard to co-adsorption with C2H4. In the absence of CO at similar temperatures, H2 adsorption was primarily irreversible (first-order dependence), and H2 and C2H4 compete for the same sites. Ethylene orders at 403 K were first order in the presence of 0.2 Torr CO and remained unity with increasing CO pressure. At similar reaction conditions in the absence of CO, ethylene had an inhibitory effect (negative reaction order) on the overall hydrogenation reaction. The change in C2H4 and H2 kinetics suggests strong competitive adsorption between C2H4 and CO for the same type of site, whereas H2 apparently adsorbs on distinct surface sites due either to steric hindrance or H2-induced CO desorption. Incorporation of a quasi-equilibrated CO adsorption step into a noncompetitive Langmuir–Hinshelwood mechanism predicts the experimentally observed pressure dependencies and a doubling of the apparent activation energy. Hydrogenation of ethylene in the presence of 1 Torr CO was examined under reaction conditions at 403 K by infrared spectroscopy; the only surface species identified under reaction conditions was linear-bound CO. The hydrogenation of ethylene on clean Pt catalysts was structure-insensitive and remains insensitive in the presence of CO; rates decreased only by a factor of two with increasing particle size.
Co-reporter:Jeong Y. Park ; Hyunjoo Lee ; J. Russell Renzas ; Yawen Zhang
Nano Letter () pp:
Publication Date(Web):June 24, 2008
DOI:10.1021/nl8012456
Hot electron flow generated on colloid platinum nanoparticles during exothermic catalytic carbon monoxide oxidation was directly detected with Au/TiO2 diodes. Although Au/TiO2 diodes are not catalytically active, platinum nanoparticles on Au/TiO2 exhibit both chemicurrent and catalytic turnover rate. Hot electrons are generated on the surface of the metal nanoparticles and go over the Schottky energy barrier between Au and TiO2. The continuous Au layer ensures that the metal nanoparticles are electrically connected to the device. The overall thickness of the metal assembly (nanoparticles and Au thin film) is comparable to the mean free path of hot electrons, resulting in ballistic transport through the metal. The chemicurrent and chemical reactivity of nanoparticles with citrate, hexadecylamine, hexadecylthiol, and TTAB (tetradecyltrimethylammonium bromide) capping agents were measured during catalytic CO oxidation at pressures of 100 Torr O2 and 40 Torr CO at 373∼513 K. We found that chemicurrent yield varies with each capping agent but always decreases with increasing temperature. We suggest that this inverse temperature dependence is associated with the influence of charging effects due to the organic capping layer during hot electron transport through the metal-oxide interface.
Co-reporter:Kwangjin An ; Selim Alayoglu ; Nathan Musselwhite ; Kyungsu Na
Journal of the American Chemical Society () pp:
Publication Date(Web):April 28, 2014
DOI:10.1021/ja5018656
Selective isomerization toward branched hydrocarbons is an important catalytic process in oil refining to obtain high-octane gasoline with minimal content of aromatic compounds. Colloidal Pt nanoparticles with controlled sizes of 1.7, 2.7, and 5.5 nm were deposited onto ordered macroporous oxides of SiO2, Al2O3, TiO2, Nb2O5, Ta2O5, and ZrO2 to investigate Pt size- and support-dependent catalytic selectivity in n-hexane isomerization. Among the macroporous oxides, Nb2O5 and Ta2O5 exhibited the highest product selectivity, yielding predominantly branched C6 isomers, including 2- or 3-methylpentane, as desired products of n-hexane isomerization (140 Torr n-hexane and 620 Torr H2 at 360 °C). In situ characterizations including X-ray diffraction and ambient-pressure X-ray photoelectron spectroscopy showed that the crystal structures of the oxides in Pt/oxide catalysts were not changed during the reaction and oxidation states of Nb2O5 were maintained under both H2 and O2 conditions. Fourier transform infrared spectra of pyridine adsorbed on the oxides showed that Lewis sites were the dominant acidic site of the oxides. Macroporous Nb2O5 and Ta2O5 were identified to play key roles in the selective isomerization by charge transfer at Pt–oxide interfaces. The selectivity was revealed to be Pt size-dependent, with improved isomer production as Pt sizes increased from 1.7 to 5.5 nm. When 5.5 nm Pt nanoparticles were supported on Nb2O5 or Ta2O5, the selectivity toward branched C6 isomers was further increased, reaching ca. 97% with a minimum content of benzene, due to the combined effects of the Pt size and the strong metal–support interaction.
Co-reporter:Gabor A. Somorjai ; Heinz Frei ;Jeong Y. Park
Journal of the American Chemical Society () pp:
Publication Date(Web):November 4, 2009
DOI:10.1021/ja9061954
The challenge of chemistry in the 21st century is to achieve 100% selectivity of the desired product molecule in multipath reactions (“green chemistry”) and develop renewable energy based processes. Surface chemistry and catalysis play key roles in this enterprise. Development of in situ surface techniques such as high-pressure scanning tunneling microscopy, sum frequency generation (SFG) vibrational spectroscopy, time-resolved Fourier transform infrared methods, and ambient pressure X-ray photoelectron spectroscopy enabled the rapid advancement of three fields: nanocatalysts, biointerfaces, and renewable energy conversion chemistry. In materials nanoscience, synthetic methods have been developed to produce monodisperse metal and oxide nanoparticles (NPs) in the 0.8−10 nm range with controlled shape, oxidation states, and composition; these NPs can be used as selective catalysts since chemical selectivity appears to be dependent on all of these experimental parameters. New spectroscopic and microscopic techniques have been developed that operate under reaction conditions and reveal the dynamic change of molecular structure of catalysts and adsorbed molecules as the reactions proceed with changes in reaction intermediates, catalyst composition, and oxidation states. SFG vibrational spectroscopy detects amino acids, peptides, and proteins adsorbed at hydrophobic and hydrophilic interfaces and monitors the change of surface structure and interactions with coadsorbed water. Exothermic reactions and photons generate hot electrons in metal NPs that may be utilized in chemical energy conversion. The photosplitting of water and carbon dioxide, an important research direction in renewable energy conversion, is discussed.
Co-reporter:Kairat Sabyrov, Nathan Musselwhite, Gérôme Melaet and Gabor A. Somorjai
Catalysis Science & Technology (2011-Present) 2017 - vol. 7(Issue 8) pp:NaN1765-1765
Publication Date(Web):2017/03/21
DOI:10.1039/C7CY00203C
As the impact of acids on catalytically driven chemical transformations is tremendous, fundamental understanding of catalytically relevant factors is essential for the design of more efficient solid acid catalysts. In this work, we employed a post-synthetic doping method to synthesize a highly selective hydroisomerization catalyst and to demonstrate the effect of acid strength and density, catalyst microstructure, and platinum nanoparticle size on the reaction rate and selectivity. Aluminum doped mesoporous silica catalyzed gas-phase n-hexadecane isomerization with remarkably high selectivity to monobranched isomers (∼95%), producing a substantially higher amount of isomers than traditional zeolite catalysts. Mildly acidic sites generated by post-synthetic aluminum grafting were found to be the main reason for its high selectivity. The flexibility of the post-synthetic doping method enabled us to systematically explore the effect of the acid site density on the reaction rate and selectivity, which has been extremely difficult to achieve with zeolite catalysts. We found that a higher density of Brønsted acid sites leads to higher cracking of n-hexadecane presumably due to an increased surface residence time. Furthermore, regardless of pore size and microstructure, hydroisomerization turnover frequency linearly increased as a function of Brønsted acid site density. In addition to strength and density of acid sites, platinum nanoparticle size affected catalytic activity and selectivity. The smallest platinum nanoparticles produced the most effective bifunctional catalyst presumably because of higher percolation into aluminum doped mesoporous silica, generating more ‘intimate’ metallic and acidic sites. Finally, the aluminum doped silica catalyst was shown to retain its remarkable selectivity towards isomers even at increased reaction conversions.
Co-reporter:Derek R. Butcher, Zhongwei Zhu, Baohua Mao, Hailiang Wang, Zhi Liu, Miquel Salmeron and Gabor A. Somorjai
Chemical Communications 2013 - vol. 49(Issue 61) pp:NaN6905-6905
Publication Date(Web):2013/06/12
DOI:10.1039/C3CC42312C
By using high-pressure scanning tunneling microscopy and ambient-pressure X-ray photoelectron spectroscopy, we studied the mobility along with composition, structure and reactivity on the Pt(100)-hex surface. Adsorbates are mobile under 1 Torr of C2H4 and C2H4–H2 mixtures, but adding 3 mTorr of CO quenches the mobility. Ethylene-related adsorbates can also weaken Pt–Pt bonds and thus facilitate displacements in the hexagonal layer.
Co-reporter:Gabor A. Somorjai and Jeong Y. Park
Chemical Society Reviews 2008 - vol. 37(Issue 10) pp:NaN2162-2162
Publication Date(Web):2008/07/30
DOI:10.1039/B719148K
Model systems for studying molecular surface chemistry have evolved from single crystal surfaces at low pressure to colloidal nanoparticles at high pressure. Low pressure surface structure studies of platinum single crystals using molecular beam surface scattering and low energy electron diffraction techniques probe the unique activity of defects, steps and kinks at the surface for dissociation reactions (H–H, C–H, C–C, OO bonds). High-pressure investigations of platinum single crystals using sum frequency generation vibrational spectroscopy have revealed the presence and the nature of reaction intermediates. High pressure scanning tunneling microscopy of platinum single crystal surfaces showed adsorbate mobility during a catalytic reaction. Nanoparticle systems are used to determine the role of metal–oxide interfaces, site blocking and the role of surface structures in reactive surface chemistry. The size, shape and composition of nanoparticles play important roles in determining reaction activity and selectivity and is covered in this tutorial review.
Co-reporter:James Russell Renzas, Wenyu Huang, Yawen Zhang, Michael E. Grass, Dat Tien Hoang, Selim Alayoglu, Derek R. Butcher, Franklin (Feng) Tao, Zhi Liu and Gabor A. Somorjai
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 7) pp:NaN2562-2562
Publication Date(Web):2010/12/24
DOI:10.1039/C0CP01858A
Bimetallic 15 nm Rh1−xPdxnanoparticle catalysts of five different compositions and supported on Si wafers have been synthesized, characterized using TEM, SEM, and XPS, and studied in CO oxidation by O2 in two pressure regimes: atmospheric pressure and 100–200 mTorr. The RhPd bimetallic nanocrystals exhibited similar synergetic effect of increased reaction activity at both atmospheric (760 Torr) and moderate (100–200 mTorr) pressures compared with pure Pd or Rh. The magnitude of the effect depends on the relative pressures of the CO and O2 reactant gases and the reaction temperature. The catalytic activity of the nanocrystals measured at moderate pressure is directly correlated to the APXPS studies, which were carried out in the same pressure. The APXPS studies suggest that the Pd–Rh interfaces are important for the enhanced activity of the bimetallic nanoparticles.
Co-reporter:Gabor A. Somorjai, Roger L. York, Derek Butcher and Jeong Y. Park
Physical Chemistry Chemical Physics 2007 - vol. 9(Issue 27) pp:NaN3513-3513
Publication Date(Web):2007/03/22
DOI:10.1039/B618805B
The material and pressure gap has been a long standing challenge in the field of heterogeneous catalysis and have transformed surface science and biointerfacial research. In heterogeneous catalysis, the material gap refers to the discontinuity between well-characterized model systems and industrially relevant catalysts. Single crystal metal surfaces have been useful model systems to elucidate the role of surface defects and the mobility of reaction intermediates in catalytic reactivity and selectivity. As nanoscience advances, we have developed nanoparticle catalysts with lithographic techniques and colloidal syntheses. Nanoparticle catalysts on oxide supports allow us to investigate several important ingredients of heterogeneous catalysis such as the metal-oxide interface and the influence of noble metal particle size and surface structure on catalytic selectivity. Monodispersed nanoparticle and nanowire arrays were fabricated for use as model catalysts by lithographic techniques. Platinum and rhodium nanoparticles in the 1–10 nm range were synthesized in colloidal solutions in the presence of polymer capping agents. The most catalytically active systems are employed at high pressure or at solid–liquid interfaces. In order to study the high pressure and liquid interfaces on the molecular level, experimental techniques with which we bridged the pressure gap in catalysis have been developed. These techniques include the ultrahigh vacuum system equipped with high pressure reaction cell, high pressure Sum Frequency Generation (SFG) vibration spectroscopy, High Pressure Scanning Tunneling Microscopy (HP-STM), and High Pressure X-ray Photoemission Spectroscopy (HP-XPS), and Quartz Crystal Microbalance (QCM). In this article, we overview the development of experimental techniques and evolution of the model systems for the research of heterogeneous catalysis and biointerfacial studies that can shed light on the long-standing issues of materials and pressure gaps.
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