In water-promoted CO oxidation, water was thought not to directly participate in CO2 production. Here we report that via a water-mediated Mars–van Krevelen (MvK) mechanism, water can directly contribute to about 50% of CO2 production on a single-atom Pt1/CeO2 catalyst. The origin is the facile reaction of CO with the hydroxyl from dissociated water to yield the carboxyl intermediate, which dehydrogenates subsequently with the help of a lattice hydroxyl to generate CO2 and water. The water-mediated MvK type reaction found here provides new insights in the promotion role of water in heterogeneous catalysis.Keywords: CO oxidation; Mars−van Krevelen; Pt1/CeO2; single atom catalyst; water promotion;

AbstractKnowledge of the photocatalytic H2 evolution mechanism is of great importance for designing active catalysts toward a sustainable energy supply. An atomic-level insight, design, and fabrication of single-site Co1-N4 composite as a prototypical photocatalyst for efficient H2 production is reported. Correlated atomic characterizations verify that atomically dispersed Co atoms are successfully grafted by covalently forming a Co1-N4 structure on g-C3N4 nanosheets by atomic layer deposition. Different from the conventional homolytic or heterolytic pathway, theoretical investigations reveal that the coordinated donor nitrogen increases the electron density and lowers the formation barrier of key Co hydride intermediate, thereby accelerating H–H coupling to facilitate H2 generation. As a result, the composite photocatalyst exhibits a robust H2 production activity up to 10.8 μmol h−1, 11 times higher than that of pristine counterpart.

AbstractKnowledge of the photocatalytic H2 evolution mechanism is of great importance for designing active catalysts toward a sustainable energy supply. An atomic-level insight, design, and fabrication of single-site Co1-N4 composite as a prototypical photocatalyst for efficient H2 production is reported. Correlated atomic characterizations verify that atomically dispersed Co atoms are successfully grafted by covalently forming a Co1-N4 structure on g-C3N4 nanosheets by atomic layer deposition. Different from the conventional homolytic or heterolytic pathway, theoretical investigations reveal that the coordinated donor nitrogen increases the electron density and lowers the formation barrier of key Co hydride intermediate, thereby accelerating H–H coupling to facilitate H2 generation. As a result, the composite photocatalyst exhibits a robust H2 production activity up to 10.8 μmol h−1, 11 times higher than that of pristine counterpart.

Selective hydrogenation is an important industrial catalytic process in chemical upgrading, where Pd-based catalysts are widely used because of their high hydrogenation activities. However, poor selectivity and short catalyst lifetime because of heavy coke formation have been major concerns. In this work, atomically dispersed Pd atoms were successfully synthesized on graphitic carbon nitride (g-C3N4) using atomic layer deposition. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) confirmed the dominant presence of isolated Pd atoms without Pd nanoparticle (NP) formation. During selective hydrogenation of acetylene in excess ethylene, the g-C3N4-supported Pd NP catalysts had strikingly higher ethylene selectivities than the conventional Pd/Al2O3 and Pd/SiO2 catalysts. In-situ X-ray photoemission spectroscopy revealed that the considerable charge transfer from the Pd NPs to g-C3N4 likely plays an important role in the catalytic performance enhancement. More impressively, the single-atom Pd1/C3N4 catalyst exhibited both higher ethylene selectivity and higher coking resistance. Our work demonstrates that the single-atom Pd catalyst is a promising candidate for improving both selectivity and coking-resistance in hydrogenation reactions.

•Decreasing the Pd particle size increases the catalytic activity of Pd/C in FA dehydrogenation.•The 2.1 nm Pd/C catalyst showed a TOF of 835 h−1 at 25 °C among the highest activities for monometallic Pd catalysts.•Both low- and high-coordination Pd surface atoms are the active sites in FA dehydrogenation.•The improved catalytic activity on smaller Pd nanoparticles is likely attributed to the positive charged Pd species.Hydrogen generation from formic acid (FA) under mild conditions has received significant attention, where Pd-based catalysts have been widely employed due to their superior activities. However, the Pd particle size effect has been much less systematically investigated. In this study, carbon-supported Pd nanoparticles (NPs) with five different Pd particle sizes, ranging from 2.1 to 4.5 nm were synthesized using sodium citrate as the stabilizing agent. The Pd particle sizes were determined by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The FA dehydrogenation reaction was conducted in a FA-sodium formate (SF) aqueous solution in a batch reactor at room temperature. We found that decreasing the Pd particle size from 4.5 ± 0.5 to 2.1 ± 0.3 nm remarkably boosted the catalytic activity by about 3.6 times, resulting in a turnover frequency of 835 h−1, which is among the highest values for supported monometallic Pd catalysts in the literature. Our results suggest that both low- and high-coordination Pd surface atoms participated in the reaction. The remarkably higher activity of smaller Pd NPs was attributed to both higher Pd dispersion and the presence of a larger proportion of Pd species with positive charge, through which the Coulomb interaction between the positive Pd species and negative charged formate ions, the key reaction intermediate, is enhanced. Finally, the deactivation and regeneration of Pd/C catalysts were also discussed.Download high-res image (80KB)Download full-size image

Bifunctional catalysts that contain both metal and acidic functions have been widely used in renewable biomass conversions. The bifunctionality closely depends on the distance between the metal and acid sites. However, the metal–acid proximity effect has rarely investigated in biomass conversions. In this work, we precisely deposited a porous Al2O3 overcoat onto a Pt/Al2O3 catalyst using atomic layer deposition to improve the proximity between the Pt metal and the alumina acid sites by increasing the area of the metal–acid interface. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of pyridine chemisorption confirmed that the overall catalyst acidity did not change considerably after applying the alumina overcoat. In the aqueous-phase, hydrogenolysis of glycerol was used to demonstrate that the alumina overcoat significantly improved the activity approximately 2.8-fold, as well as the selectivity to 1,2-propanediol (1,2-PD) at high conversions. DRIFTS measurements of CO chemisorption indicated that the Pt-alumina interface had greater area for alumina coated Pt/Al2O3 than for the uncoated analog. Moreover, we used the hydrogenation of acetol, the key reaction intermediate in glycerol hydrogenolysis, as a control experiment to confirm that the observed activity improvement in the hydrogenolysis of glycerol could be attributed to the enhancement of the dehydration reaction step, which requires acidic function. In brief, our work provides solid evidence that close metal–acid proximity enhances bifunctionality, thus improving the catalytic activity.Deposition of Al2O3 on Pt/Al2O3 catalyst using ALD can significantly enhance the activity and selectivity to 1,2-propanediol in the hydrogenolysis of glycerol through the metal–acid proximity effect.Download high-res image (102KB)Download full-size image

Pd catalysts are industrially used in the selective hydrogenation of acetylene to ethylene. Terrace Pd atoms of the closely packed {111} facets on supported Pd particles are generally considered to be the catalytically active sites. We herein report that deposition of an appropriate amount of Ga2O3 adlayers on Pd particles supported on alumina by the atomic layer deposition (ALD) technique substantially enhanced the catalytic activity, selectivity, and stability in the selective hydrogenation of acetylene to ethylene. Structural characterization results demonstrate that Ga2O3 is preferentially deposited at the edges and open facets of Pd particles with the ALD technique. This transforms the poisoning edge sites of the {111} facets into the catalytically active terrace-like sites, leading to an increase in the number of active sites and subsequently the enhancement of the catalytic activity; this also suppresses the formation of poisoning carbonaceous deposits on the open facets and blocks the migration of carbonaceous deposits from the open facets to the neighboring active {111} facets, leading to a significant improvement in catalytic stability. These results demonstrate a concept of selective oxide decoration to comprehensively improve the performance of supported metal catalysts and provide a practical strategy.Keywords: activity; atomic layer deposition; density functional theory; selectivity; stability; subsurface carbon

Catalyst synthesis with precise control over the structure of catalytic active sites at the atomic level is of essential importance for the scientific understanding of reaction mechanisms and for rational design of advanced catalysts with high performance. Such precise control is achievable using atomic layer deposition (ALD). ALD is similar to chemical vapor deposition (CVD), except that the deposition is split into a sequence of two self-limiting surface reactions between gaseous precursor molecules and a substrate. The unique self-limiting feature of ALD allows conformal deposition of catalytic materials on a high surface area catalyst support at the atomic level. The deposited catalytic materials can be precisely constructed on the support by varying the number and type of ALD cycles. As an alternative to the wet-chemistry based conventional methods, ALD provides a cycle-by-cycle “bottom-up” approach for nanostructuring supported catalysts with near atomic precision.In this review, we summarize recent attempts to synthesize supported catalysts with ALD. Nucleation and growth of metals by ALD on oxides and carbon materials for precise synthesis of supported monometallic catalyst are reviewed. The capability of achieving precise control over the particle size of monometallic nanoparticles by ALD is emphasized. The resulting metal catalysts with high dispersions and uniformity often show comparable or remarkably higher activity than those prepared by conventional methods. For supported bimetallic catalyst synthesis, we summarize the strategies for controlling the deposition of the secondary metal selectively on the primary metal nanoparticle but not on the support to exclude monometallic formation. As a review of the surface chemistry and growth behavior of metal ALD on metal surfaces, we demonstrate the ways to precisely tune size, composition and structure of bimetallic metal nanoparticles. The cycle-by-cycle “bottom up” construction of bimetallic (or multiple components) nanoparticles with near atomic precision on supports by ALD is illustrated. Applying metal oxide ALD over metal nanoparticles can be used to precisely synthesize nanostructured metal catalysts. In this part, the surface chemistry of Al2O3 ALD on metals is specifically reviewed. Next, we discuss the methods of tailoring the catalytic performance of metal catalysts including activity, selectivity and stability, through selective blocking of the low-coordination sites of metal nanoparticles, the confinement effect, and the formation of new metal-oxide interfaces. Synthesis of supported metal oxide catalysts with high dispersions and “bottom up” nanostructured photocatalytic architectures are also included. Therein, the surface chemistry and morphology of oxide ALD on oxides and carbon materials as well as their catalytic performance are summarized.

•Si nanospheres are precisely coated with TiO2 by ALD method.•The Si@TiO2 composite with a 3 nm TiO2 layer exhibits the best cycling stability.•TiO2 thickness is critical to balance structural stability and conductivity of Si.•The difference in coating electrode and active material by ALD is also revealed.Silicon (Si) has been regarded as next-generation anode for high-energy lithium-ion batteries (LIBs) due to its high Li storage capacity (4200 mA h g−1). However, the mechanical degradation and resultant capacity fade critically hinder its practical application. In this regard, we demonstrate that nanocoating of Si spheres with a 3 nm titanium dioxide (TiO2) layer via atomic layer deposition (ALD) can utmostly balance the high conductivity and the good structural stability to improve the cycling stability of Si core material. The resultant sample, Si@TiO2-3 nm core–shell nanospheres, exhibits the best electrochemical performance of all with a highest initial Coulombic efficiency and specific charge capacity retention after 50 cycles at 0.1C (82.39% and 1580.3 mA h g−1). In addition to making full advantage of the ALD technique, we believe that our strategy and comprehension in coating the electrode and the active material could provide a useful pathway towards enhancing Si anode material itself and community of LIBs.

For TiO2 supported Au catalysts, the Au particle size and the interfacial perimeter sites between Au particles and the TiO2 support both play important roles in CO oxidation reaction. However, changing the Au particle size inevitably accompanied by the change of the perimeter length makes it extremely difficult to identify their individual roles. Here we reported a new strategy to isolate them by applying TiO2 overcoat to Au/Al2O3 and Au/SiO2 catalysts using atomic layer deposition (ALD) where the new Au–TiO2 interfacial length was precisely tuned to different degrees while preserving the particle size. High resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements of CO chemisorption all confirmed that the TiO2 overcoat preferentially decorates the low-coordinated sites of Au nanoparticles and generates Au–TiO2 interfaces. In CO oxidation, we demonstrated a remarkable improvement of the catalytic activities of Au/Al2O3 and Au/SiO2 catalysts by the ALD TiO2 overcoat. More interestingly, the activity as a function of TiO2 ALD cycles obviously showed a volcano-like behavior, providing direct evidence that the catalytic activities of TiO2 overcoated Au catalysts strongly correlate with the total length of perimeter sites. Finally, our work suggests that this strategy might be a new method for atomic level understanding the reaction mechanism and high performance catalyst design.

It is well-known that the particle size of Au nanoparticles enormously affects the catalytic activity of supported gold catalysts in many reactions. The origin of the Au particle size effect is still widely debated. In this work, we precisely deposited different thicknesses of ultrathin TiO2 overcoats onto three Au/TiO2 catalysts with Au particle sizes of 2.9 ± 0.6 (Au/TiO2-S), 5.0 ± 0.8 nm (Au/TiO2-M), and 10.2 ± 1.6 nm (Au/TiO2-L) using atomic layer deposition (ALD). High-resolution transmission electron microscopy illustrated the Au nanoparticles on these three samples were encapsulated by the TiO2 overcoat. X-ray photoelectron spectroscopy measurements showed that the Au nanoparticles remained at metallic state after applying TiO2 ALD overcoat. Diffuse reflectance infrared Fourier transform spectroscopy measurements of CO chemisorption further revealed that the TiO2 overcoat preferentially decorated at the low-coordination sites of Au nanoparticles and broadly tuned the population of these sites accessible for participating in reactions. In CO oxidation reaction, the TiO2 coated Au/TiO2-S catalysts strikingly demonstrated considerably higher activities than the uncoated Au/TiO2-M and Au/TiO2-L catalysts, even though the former ones contained significantly less amount of CO adsorption sites due to the TiO2 overcoating. Our work shows direct evidence that CO adsorption on the low-coordination Au sites is not the rate-determining step, and the Au particle size effect in CO oxidation is NOT related with either the number of the low-coordination Au sites or the changes in oxidation states. Size-related change in the length of perimeter sites at the Au–TiO2 interface could certainly play a role in the Au particle size effect.

Pd typically exhibits relatively low catalytic activity in CO oxidation, as CO is apt to be adsorbed on Pd to poison the surface for O2 activation. In this Letter, we report that this limitation can be overcome by integrating Pd with TiO2. The TiO2 was coated on Pd nanocubes with a controllable thickness using atomic layer deposition (ALD) method. Given the different work functions of TiO2 and Pd, the electrons in TiO2 semiconductor will flow toward Pd. With the electron density increased on Pd, the adsorption of CO to Pd will be weakened while the oxygen activation can be facilitated. Meanwhile, the interface-confined sites at Pd-TiO2 may further enhance the oxygen activation. As the species adsorption and activation are strongly correlated with electron density, the performance of Pd-TiO2 in CO oxidation turns out to depend on the TiO2 thickness, which determines the number of transferred electrons, within a certain range (<1.8 nm). This work provides a new strategy for enhancing catalytic performance through tailoring charge densities in hybrid catalysts.

We reported that atomically dispersed Pd on graphene can be fabricated using the atomic layer deposition technique. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure spectroscopy both confirmed that isolated Pd single atoms dominantly existed on the graphene support. In selective hydrogenation of 1,3-butadiene, the single-atom Pd1/graphene catalyst showed about 100% butenes selectivity at 95% conversion at a mild reaction condition of about 50 °C, which is likely due to the changes of 1,3-butadiene adsorption mode and enhanced steric effect on the isolated Pd atoms. More importantly, excellent durability against deactivation via either aggregation of metal atoms or carbonaceous deposits during a total 100 h of reaction time on stream was achieved. Therefore, the single-atom catalysts may open up more opportunities to optimize the activity, selectivity, and durability in selective hydrogenation reactions.

Metal catalyst in selective hydrogenation reactions often suffers from low selectivity and especially poor durability due to heavy coke formation. Here we report that precisely controlled porous alumina overcoating on a Pd catalyst using atomic layer deposition (ALD) not only remarkably enhances the selectivity to butenes, especially to 1-butene, but also achieves the best ever durability against deactivation in selective hydrogenation of 1,3-butadiene in the absence (or presence) of propene. Therein no visible activity declines or selectivity changes were observed during a total 124 h of reaction time on stream.Keywords: Al2O3 overcoating; atomic layer deposition; durability; hydrogenation of 1,3-butadiene; Pd catalyst; selectivity