Wei Xia

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Organization: Ruhr-University Bochum , Germany

Department: Laboratory of Industrial Chemistry

Title: (PhD)

TOPICS

Material Chemistry

Carbon Nanotubes

Chemicals
References

Influence of Water on the Initial Growth Rate of Carbon Nanotubes from Ethylene over a Cobalt-Based Catalyst

Co-reporter:Martin Muhler;Kunpeng Xie

Industrial & Engineering Chemistry Research October 2, 2013 Volume 52(Issue 39) pp:14081-14088

Publication Date(Web):Publication Date (Web): August 30, 2013

DOI:10.1021/ie401829e

Water-assisted growth of multiwalled carbon nanotubes (CNTs) was studied over a Co-based catalyst under plug-flow conditions. The influence of water concentration and temperature on the growth kinetics within the first 300 s was analyzed by measuring the conversion of ethylene. Feeding 200 ppm H2O vapor at 650 °C accelerated the initial growth rate and extended the mean lifetime of the catalytically active sites. Higher water concentrations of up to 500 ppm led to lower growth rates and lower CNT yields. Water of 200 ppm showed a promoting effect at 650 °C, but an inhibiting effect at 550 °C. The CO generated by steam gasification of deposited carbon was monitored online indicating coking of the catalyst. The results demonstrate that water plays a dual role: the removal of amorphous carbon on the catalyst by gasification and partial oxidation of the metallic Co catalyst. Water also influenced the diameter distribution of the CNTs.

Very low amount of TiO2 on N-doped carbon nanotubes significantly improves oxygen reduction activity and stability of supported Pt nanoparticles

Co-reporter:Anqi Zhao, Justus Masa and Wei Xia

Physical Chemistry Chemical Physics 2015 vol. 17(Issue 16) pp:10767-10773

Publication Date(Web):20 Mar 2015

DOI:10.1039/C5CP00369E

Electrochemical corrosion is a major problem for carbon materials used in electrocatalysis. Highly dispersed TiO2 was deposited on O-functionalized and N-doped carbon nanotubes by chemical vapour deposition to tackle the carbon corrosion problem. Very low Ti loadings of about 1 wt% were applied to minimize the negative influence of TiO2 as a semiconductor on the high conductivity of carbon materials. Both N doping and TiO2 coating facilitate strong metal–support interactions and favour the formation of small Pt particles. N doping improved the intrinsic catalytic activity of the carbon support and enhanced the conductivity due to the removal of surface oxygen groups, while the negative effect of TiO2 on conductivity is counterbalanced by its promoting effect on metal–support interactions leading to enhanced overall catalytic performance. Pt/TiO2/NCNTs showed the highest ORR activity, and significantly outperformed Pt/NCNTs in electrochemical stability tests.

Co3O4MnO2CNT Hybrids Synthesized by HNO3 Vapor Oxidation of Catalytically Grown CNTs as OER Electrocatalysts

Co-reporter:Dr. Kunpeng Xie;Dr. Justus Masa;Dr. Edyta Madej;Fengkai Yang;Philipp Weide;Weiwen Dong;Dr. Martin Muhler;Dr. Wolfgang Schuhmann;Dr. Wei Xia

ChemCatChem 2015 Volume 7( Issue 18) pp:3027-3035

Publication Date(Web):

DOI:10.1002/cctc.201500469

Abstract

An efficient two-step gas-phase method was developed for the synthesis of Co₃O₄–MnO₂–CNT hybrids used as electrocatalysts in the oxygen evolution reaction (OER). Spinel Co–Mn oxide was used for the catalytic growth of multiwalled carbon nanotubes (CNTs) and the amount of metal species remaining in the CNTs was adjusted by varying the growth time. Gas-phase treatment in HNO₃ vapor at 200 °C was performed to 1) open the CNTs, 2) oxidize encapsulated Co nanoparticles to Co₃O₄ as well as MnO nanoparticles to MnO₂, and 3) to create oxygen functional groups on carbon. The hybrid demonstrated excellent OER activity and stability up to 37.5 h under alkaline conditions, with longer exposure to HNO₃ vapor up to 72 h beneficial for improved electrocatalytic properties. The excellent OER performance can be assigned to the high oxidation states of the oxide nanoparticles, the strong electrical coupling between these oxides and the CNTs as well as favorable surface properties rendering the hybrids a promising alternative to noble metal based OER catalysts.

Interaction of Cobalt Nanoparticles with Oxygen- and Nitrogen-Functionalized Carbon Nanotubes and Impact on Nitrobenzene Hydrogenation Catalysis

Co-reporter:Peirong Chen, Fengkai Yang, Aleksander Kostka, and Wei Xia

ACS Catalysis 2014 Volume 4(Issue 5) pp:1478

Publication Date(Web):March 31, 2014

DOI:10.1021/cs500173t

The type and the amount of functional groups on the surface of carbon nanotubes (CNTs) were tuned to improve the activity of supported Co nanoparticles in hydrogenation catalysis. Surface nitrogen species on CNTs significantly promoted the decomposition of the cobalt precursor and the reduction of cobalt oxide, and improved the resistance of metallic Co against oxidation in ambient atmosphere. In the selective hydrogenation of nitrobenzene in the gas phase, Co supported on CNTs with the highest surface nitrogen content showed the highest activity, which is ascribed to the higher reducibility and the lower oxidation state of the Co nanoparticles under reaction conditions. For Co nanoparticles supported on CNTs with a smaller amount of surface nitrogen groups, a repeated reduction at 350 °C was essential to achieve a comparable high catalytic activity reaching 90% conversion at 250 °C, pointing to the importance of nitrogen species for the supported Co nanoparticles in nitrobenzene hydrogenation.Keywords: carbon nanotubes; cobalt nanoparticles; nitrobenzene; nitrogen doping; selective hydrogenation; surface

Separating the initial growth rate from the rate of deactivation in the growth kinetics of multi-walled carbon nanotubes from ethene over a cobalt-based bulk catalyst in a fixed-bed reactor

Co-reporter:Michael J. Becker, Wei Xia, Kunpeng Xie, Arne Dittmer, Kristian Voelskow, Thomas Turek, Martin Muhler

Carbon 2013 Volume 58() pp:107-115

Publication Date(Web):July 2013

DOI:10.1016/j.carbon.2013.02.038

The initial growth kinetics of multi-walled carbon nanotubes (CNTs) was investigated using a highly active Co-based mixed-oxide catalyst in a tubular fixed-bed reactor under plug-flow conditions with ethene as carbon source. The growth temperature and the ethene concentration were systematically varied in the range from 758 to 923 K and from 5 to 45 vol.%, respectively. The carbon mass accumulation was derived from the ethene conversion and analyzed by a kinetic model, from which the initial CNT growth rate and the mean lifetime of the active sites were derived permitting the prediction of the maximum theoretical CNT yield. With increasing growth temperatures up to 923 K both the initial growth rate and the mean lifetime of active sites increased strongly with a significantly prolonged lifetime above 848 K. The initial growth rate was slow at lower ethene concentrations, but the mean life time was very high. Increasing the ethene concentration up to 45 vol.% led to a much higher initial growth rate, but shortened the mean lifetime strongly. Due to the fast deactivation at high ethene concentrations, the predicted maximum yield decreased considerably approaching the yield obtained after 5 min of time on stream.

N-doped carbon synthesized from N-containing polymers as metal-free catalysts for the oxygen reduction under alkaline conditions

Co-reporter:Anqi Zhao, Justus Masa, Martin Muhler, Wolfgang Schuhmann, Wei Xia

Electrochimica Acta 2013 Volume 98() pp:139-145

Publication Date(Web):30 May 2013

DOI:10.1016/j.electacta.2013.03.043

•N-doped carbon synthesized by mixing carbon and N-containing polymers and thermal treatment.•N-doped carbon used as metal-free electrocatalysts for ORR under alkaline conditions.•ORR activity not correlated to the total nitrogen amount.•A linear relation between onset potential and (Npyridinic + Nquaternary)/Ntotal ratio.Nitrogen-doped carbon materials were synthesized and used as metal-free electrocatalysts for the oxygen reduction reaction (ORR) under alkaline conditions. The synthesis was achieved by thermal treatment of nitrogen-containing polymers diluted in different carbon materials. Polypyrrole, polyaniline and polyacrylonitrile were used as N precursors. Carbon black and two types of commercial carbon nanotubes were used as carbon matrices. The obtained N contents were in the range of 1–1.8 wt.%. Different N species including pyridinic, pyrrolic and quaternary N were quantitatively determined by X-ray photoelectron spectroscopy. The ORR activities were evaluated in 0.1 M KOH. Rotating disc electrode studies revealed the presence of multiple active centers in all the samples. The sample obtained using polypyrrole and small diameter nanotubes (ca. 15 nm) had the highest onset potential at −0.07 V vs. Ag/AgCl/3 M KCl, which also showed a significantly higher electrochemical stability than the sample from carbon black and polypyrrole. The ORR activity was not correlated to the total nitrogen amount, but to the amount of pyridinic and quaternary N species. For the onset potential and the (Npyridinic + Nquaternary)/Ntotal ratio a quasi-linear relation was found, which points to the substantial role of pyridinic- and quaternary-N species in ORR catalysis.

The structural and electronic promoting effect of nitrogen-doped carbon nanotubes on supported Pd nanoparticles for selective olefin hydrogenation

Co-reporter:Peirong Chen, Ly May Chew, Aleksander Kostka, Martin Muhler and Wei Xia

Catalysis Science & Technology 2013 vol. 3(Issue 8) pp:1964-1971

Publication Date(Web):08 Apr 2013

DOI:10.1039/C3CY00097D

A high-performance Pd catalyst for selective olefin hydrogenation was synthesized by supporting Pd nanoparticles on nitrogen-doped carbon nanotubes (NCNTs). X-ray diffraction, hydrogen chemisorption, transmission electron microscopy and X-ray photoelectron spectroscopy (XPS) were used to characterize Pd supported on NCNTs and nitrogen-free oxygen-functionalized CNTs (OCNTs). The Pd nanoparticles were stabilized on NCNTs with narrower size distribution compared with OCNTs. The XPS analysis revealed that the nitrogen functional groups favor the reduction of Pd on CNTs suggesting an electronic promoter effect. The Pd/NCNT catalyst showed extraordinary catalytic performance in terms of activity, selectivity and stability in the selective hydrogenation of cyclooctadiene, which is related to the structural and electronic promoting effect of the NCNT support.

Activation and Stabilization of Nitrogen-Doped Carbon Nanotubes as Electrocatalysts in the Oxygen Reduction Reaction at Strongly Alkaline Conditions

Co-reporter:Anqi Zhao, Justus Masa, Wolfgang Schuhmann, and Wei Xia

The Journal of Physical Chemistry C 2013 Volume 117(Issue 46) pp:24283-24291

Publication Date(Web):October 22, 2013

DOI:10.1021/jp4059438

Nitrogen-doped carbon nanotubes (NCNTs) are highly active electrocatalysts in the oxygen reduction reaction (ORR) at alkaline conditions. However, the initial activation and stabilization of NCNTs have rarely been investigated at industrially relevant conditions. Three types of NCNTs were synthesized by catalytic growth (NCNT-growth) or posttreatment of oxygen-functionalized CNTs with NH3 (NCNT-NH3) or aniline (NCNT-aniline). The obtained NCNTs were treated in 10 M KOH at 80 °C for 5 h, and the formation of oxygen groups by alkaline treatment and their interaction with existing nitrogen groups was analyzed. X-ray photoelectron spectroscopy showed that the concentrations of pyridinic and quaternary nitrogen increased in NCNT-growth due to the KOH treatment accompanied by the decrease of pyrrolic nitrogen, whereas the nitrogen groups changed differently in NCNT-NH3 and NCNT-aniline. NCNT-NH3 showed the highest ORR activity before alkaline treatment. After the treatment, the activity of NCNT-growth was higher, whereas those of NCNT-NH3 and NCNT-aniline were lower. These results were found to be correlated with changes in the nitrogen groups caused by alkaline treatment. Furthermore, NCNTs showed different C═O/C–O ratios after alkaline treatment as compared to a strong increase of C–O in CNTs, indicating that the presence of nitrogen in NCNTs influences the formation of oxygen groups on carbon and surface oxidation.

Synthesis of an improved hierarchical carbon-fiber composite as a catalyst support for platinum and its application in electrocatalysis

Co-reporter:Shankhamala Kundu, Tharamani Chikka Nagaiah, XingXing Chen, Wei Xia, Michael Bron, Wolfgang Schuhmann, Martin Muhler

Carbon 2012 Volume 50(Issue 12) pp:4534-4542

Publication Date(Web):October 2012

DOI:10.1016/j.carbon.2012.05.037

A hierarchical carbon-fiber composite was synthesized based on carbon cloth (CC) modified with primary carbon microfibers (CMF) and subsequently secondary carbon nanotubes (CNT), thus forming a three-dimensional hierarchical structure with high BET surface area. The primary CMFs and the secondary CNTs are grown with electrodeposited iron nanoparticles as catalysts from methane and ethylene, respectively. After deposition of Pt nanoparticles by chemical vapor deposition from (trimethyl)cyclopentadienylplatinum, the resulting hierarchical composite was used as catalyst in the electrocatalytic oxygen reduction (oxygen reduction reaction, ORR) as specific test reaction. The modification of the CC with CMFs and CNTs improved the electrochemical properties of the carbon composite as revealed by electrochemical impedance measurements evidencing a low charge transfer resistance for redox mediators at the modified CC. X-ray photoelectron spectroscopy measurements were carried out to identify the chemical state and the surface atomic concentration of the Pt catalysts deposited on the hierarchical carbon composites. The ORR activity of Pt supported on different composites was investigated using rotating disk electrode measurements and scanning electrochemical microscopy. These electrochemical studies revealed that the obtained structured catalyst support is very promising for electrochemical applications, e.g. fuel cells.

Tailoring of CNT surface oxygen groups by gas-phase oxidation and its implications for lithium ion batteries

Co-reporter:Stefan Klink, Edgar Ventosa, Wei Xia, Fabio La Mantia, Martin Muhler, Wolfgang Schuhmann

Electrochemistry Communications 2012 Volume 15(Issue 1) pp:10-13

Publication Date(Web):February 2012

DOI:10.1016/j.elecom.2011.11.012

Multi-walled CNT were oxidised with nitric acid in liquid and gas-phase. By splitting the capacity and initial charge loss during lithium intercalation into different potential regions, it was possible to relate these values to the CNT surface oxygen groups as determined by XPS. Gas-phase oxidised CNT show a significantly lower amount of initial charge loss (172 mAh/g) compared to liquid-phase oxidised CNT (283 mAh/g). This decrease originates from less pronounced exfoliation likely caused by an increase of surface carbonyl groups.Highlights► Multi-walled CNT exhibit high initial charge losses when used as lithium ion battery anodes. ► Charge loss reactions vary depending on the reduction potential. ► Gas-phase oxidation using nitric acid changes surface oxygen chemistry. ► Carbonyl surface groups prevent exfoliation better than single-bonded oxygen.

Quantitative Studies on the Oxygen and Nitrogen Functionalization of Carbon Nanotubes Performed in the Gas Phase

Co-reporter:Chuang Li ; Anqi Zhao ; Wei Xia ; Changhai Liang ;Martin Muhler

The Journal of Physical Chemistry C 2012 Volume 116(Issue 39) pp:20930-20936

Publication Date(Web):September 6, 2012

DOI:10.1021/jp306866q

Gas-phase methods were applied for the oxygen and nitrogen functionalization of multiwalled carbon nanotubes (CNTs). The oxygen functionalization was performed by HNO3 vapor treatment at temperatures from 200 to 250 °C for 12 h up to 120 h. The oxygen-functionalized CNTs were used as the starting material for nitrogen functionalization through thermal treatment under NH3. The BET surface area increased after the treatment in HNO3 vapor, which also caused the weight loss due to carbon corrosion. The oxygen content increased with increasing treatment time but decreased with increasing temperature, as disclosed by elemental analysis, X-ray photoelectron spectroscopy, and temperature-programmed desorption (TPD) results. The surface acidity increased with increasing treatment time as shown by TPD using NH3 as a probe molecule. As to nitrogen functionalization, the amount of nitrogen was correlated with the oxygen amount in the starting CNTs. A higher NH3 concentration caused a lower BET surface area due to carbon corrosion. The incorporation of both oxygen and nitrogen lowered the thermal resistance of CNTs. The nitrogen-functionalized CNTs showed only a slight decrease, in contrast to a significant decrease observed for O-functionalized CNTs. The formation or removal of coordinatively unsaturated carbon like amorphous carbon or defects was found to be involved in all of the functionalization, desorption, and oxidation processes.

The Role of Oxygen- and Nitrogen-containing Surface Groups on the Sintering of Iron Nanoparticles on Carbon Nanotubes in Different Atmospheres

Co-reporter:Dr. Miguel D. Sánchez;Peirong Chen;Dr. Thomas Reinecke;Dr. Martin Muhler;Dr. Wei Xia

ChemCatChem 2012 Volume 4( Issue 12) pp:1997-2004

Publication Date(Web):

DOI:10.1002/cctc.201200286

Abstract

The sintering of iron nanoparticles on carbon nanotubes (CNTs) under different atmospheres was investigated. CNTs were first treated with HNO₃ vapor at 200 °C to obtain O-functionalized CNTs (OCNTs). The OCNTs were treated in ammonia at 400 °C to obtain N-doped CNTs (NCNTs). Highly dispersed FeO_x nanoparticles were subsequently deposited by chemical vapor deposition from ferrocene under oxidizing conditions. The obtained FeO_x/OCNT and FeO_x/NCNT samples were allowed to sinter at 500 °C under flowing helium, hydrogen, or ammonia. The samples were studied by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. A significant increase in particle size and a decrease in Fe surface atomic concentration were observed in all the sintered samples. The sintering on OCNTs was more severe than on NCNTs, which can be attributed to stronger metal-substrate interactions and a higher amount of surface defects on NCNTs. The applied gas atmosphere had a substantial influence on the sintering behavior of the nanoparticles: treatment in helium led to the growth of particles and a significant widening of particle size distributions, whereas treatment in hydrogen or ammonia resulted in the growth of particles, but not in the widening of particle size distributions.

Enhanced Electrocatalytic Stability of Platinum Nanoparticles Supported on a Nitrogen-Doped Composite of Carbon Nanotubes and Mesoporous Titania under Oxygen Reduction Conditions

Co-reporter:Justus Masa;Dr. Ankur Bordoloi;Dr. Martin Muhler;Dr. Wolfgang Schuhmann;Dr. Wei Xia

ChemSusChem 2012 Volume 5( Issue 3) pp:523-525

Publication Date(Web):

DOI:10.1002/cssc.201100643

Visualization and functions of surface defects on carbon nanotubes created by catalytic etching

Co-reporter:Wei Xia, Xiuli Yin, Shankhamala Kundu, Miguel Sánchez, Alexander Birkner, Christof Wöll, Martin Muhler

Carbon 2011 Volume 49(Issue 1) pp:299-305

Publication Date(Web):January 2011

DOI:10.1016/j.carbon.2010.09.025

Surface defects were created on carbon nanotubes (CNTs) by catalytic steam gasification or catalytic etching with iron as catalysts. The structure and morphology of the etched CNTs were studied by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). The electronic structure of the etched CNTs was investigated by ultraviolet photoelectron spectroscopy (UPS). The etched CNTs were treated by nitric acid to obtain oxygen-containing functional groups. The amount and the thermal stability of these groups were studied by temperature-resolved X-ray photoelectron spectroscopy (XPS). Temperature-programmed desorption with ammonia as a probe molecule (NH3-TPD) was employed to investigate the interaction of the surface defects with foreign molecules in gas phase. TEM and STM studies disclosed the presence of surface defects especially edge planes on the etched CNTs. Etching of CNTs led to a less pronounced p-π band than the as-is CNTs, as evidenced by UPS studies. The XPS and NH3-TPD studies demonstrated that the defects on the CNTs enhanced the reactivity of the exposed surfaces allowing obtaining a higher degree of oxygen functionalization and more active adsorption sites.Graphical abstractResearch highlights► Created surface defects on carbon nanotubes visualized by STM. ► Oxygen anchored on etched CNTs more stable as indicated by temperature-resolved XPS. ► Etching led to a less pronounced p-π band as disclosed by UPS. ► Defects active for adsorption of ammonia as evidenced by TPD.

A highly efficient gas-phase route for the oxygen functionalization of carbon nanotubes based on nitric acid vapor

Co-reporter:Wei Xia, Chen Jin, Shankhamala Kundu, Martin Muhler

Carbon 2009 Volume 47(Issue 3) pp:919-922

Publication Date(Web):March 2009

DOI:10.1016/j.carbon.2008.12.026

A simple, highly effective method for the functionalization of CNTs with HNO3 vapor is developed, thus eliminating separation by filtration. A significantly higher amount of oxygen species compared to conventional wet HNO3 treatment was detected by X-ray photoelectron spectroscopy, and the morphology and the degree of agglomeration did not deteriorate because of the treatment.

Oxygen-deficient titania as alternative support for Pt catalysts for the oxygen reduction reaction

Co-reporter:Anqi Zhao, Justus Masa, Wei Xia

Journal of Energy Chemistry (November 2014) Volume 23(Issue 6) pp:701-707

Publication Date(Web):1 November 2014

DOI:10.1016/S2095-4956(14)60202-3

Insufficient electrochemical stability is a major challenge for carbon materials in oxygen reduction reaction (ORR) due to carbon corrosion and insufficient metal-support interactions. In this work, titania is explored as an alternative support for Pt catalysts. Oxygen deficient titania samples including TiO2–x and TiO2–xNy were obtained by thermal treatment of anatase TiO2 under flowing H2 and NH3, respectively. Pt nanoparticles were deposited on the titania by a modified ethylene glycol method. The samples were characterized by N2-physisorption, X-ray diffraction and X-ray photoelectron spectroscopy. The ORR activity and long-term stability of supported Pt catalysts were evaluated using linear sweep voltammetry and chronoamperometry in 0.1 mol/L HClO4. Pt/TiO2–x and Pt/TiO2–xNy showed higher ORR activities than Pt/TiO2 as indicated by higher onset potentials. Oxygen deficiency in TiO2–x and TiO2–xNy contributed to the high ORR activity due to enhanced charge transfer, as disclosed by electrochemical impedance spectroscopy studies. Electrochemical stability studies revealed that Pt/TiO2–x exhibited a higher stability with a lower current decay rate than commercial Pt/C, which can be attributed to the stable oxide support and strong interaction between Pt nanoparticles and the oxygen-deficient TiO2–x support.Titania was used as support for Pt nanoparticles used as electrocatalysts for oxygen reduction reactions. The effect of oxygen deficiency and nitrogen-doping was investigated. The samples showed a moderate activity but a high electrocatalytic stability.Download full-size image

The influence of the residual growth catalyst in functionalized carbon nanotubes on supported Pt nanoparticles applied in selective olefin hydrogenation

Co-reporter:Peirong Chen, Ly May Chew, Wei Xia

Journal of Catalysis (November 2013) Volume 307() pp:84-93

Publication Date(Web):1 November 2013

DOI:10.1016/j.jcat.2013.06.030

•Residual growth catalyst strongly influences the surface functionalization of CNTs.•Removing the residual catalyst in CNTs is essential for Pt catalyst in hydrogenation.•Pt on NCNTs is stabilized in a smaller particle size and a higher oxidation state.•Pt on NCNTs is less active but more selective in olefin hydrogenation than on OCNTs.The influence of the residual growth catalyst on the reducibility and catalytic activity of Pt nanoparticles supported on oxygen- and nitrogen-functionalized CNTs (OCNTs and NCNTs) was systematically investigated. It was found that the presence of the residual growth catalyst significantly influenced the oxygen and nitrogen functionalization of CNTs, which consequently altered the reducibility of the supported Pt nanoparticles. Pt nanoparticles on NCNTs showed a higher stability against sintering in reducing atmosphere at 200 °C and 400 °C than those on OCNTs. On NCNTs, Pt was in a higher oxidation state and was not as easily reducible as on OCNTs. In hydrogenation catalysis, removing the residual growth catalyst is essential for the supported Pt catalyst to achieve a better performance. Compared with Pt on OCNTs, Pt on NCNTs was less active, but more selective in olefin hydrogenation due to the poisoning effect of the surface nitrogen species.Graphical abstractIn olefin hydrogenation, residual growth catalysts in CNTs have negative influence on the activity of supported Pt, whereas surface nitrogen groups are favorable for a high selectivity.Download high-res image (83KB)Download full-size image

Purified oxygen- and nitrogen-modified multi-walled carbon nanotubes as metal-free catalysts for selective olefin hydrogenation

Co-reporter:Peirong Chen, Ly May Chew, Aleksander Kostka, Kunpeng Xie, ... Wei Xia

Journal of Energy Chemistry (March 2013) Volume 22(Issue 2) pp:312-320

Publication Date(Web):1 March 2013

DOI:10.1016/S2095-4956(13)60038-8

Oxygen- and nitrogen-functionalized carbon nanotubes (OCNTs and NCNTs) were applied as metal-free catalysts in selective olefin hydrogenation. A series of NCNTs was synthesized by NH3 post-treatment of OCNTs. Temperature-programmed desorption, N2 physisorption, Raman spectroscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were employed to characterize the surface properties of OCNTs and NCNTs, aiming at a detailed analysis of the type and amount of oxygen- and nitrogen-containing groups as well as surface defects. The gas-phase treatments applied for oxygen and nitrogen functionalization at elevated temperatures up to 600 °C led to the increase of surface defects, but did not cause structural damages in the bulk. NCNTs showed a clearly higher activity than the pristine CNTs and OCNTs in the hydrogenation of 1,5-cyclooctadiene, and also the selectivity to cyclooctene was higher. The favorable catalytic properties are ascribed to the nitrogen-containing surface functional groups as well as surface defects related to nitrogen species. In contrast, oxygen-containing surface groups and the surface defects caused by oxygen species did not show clear contribution to the hydrogenation catalysis.

A reevaluation of the correlation between the synthesis parameters and structure and properties of nitrogen-doped carbon nanotubes

Co-reporter:Kunpeng Xie, Fengkai Yang, Petra Ebbinghaus, Andreas Erbe, ... Wei Xia

Journal of Energy Chemistry (July 2015) Volume 24(Issue 4) pp:407-415

Publication Date(Web):1 July 2015

DOI:10.1016/j.jechem.2015.06.016

Nitrogen-doped carbon nanotubes (NCNTs) were synthesized by chemical vapor deposition using cobalt-based oxides as catalyst and ethylenediamine (EDA) as carbon/nitrogen precursor. The influence of growth time, EDA concentration and growth temperature on the morphology, yield, composition, graphitization and oxidation resistance of the NCNTs was systematically investigated by using Raman spectroscopy, temperature-programmed oxidation and other techniques. The NCNT growth from ethylenediamine with a high N/C ratio involves several processes including mainly (1) catalytic growth of NCNTs, (2) homogeneous gas-phase decomposition of EDA, (3) non-catalytic deposition of pyrolytic carbon/nitrogen species and (4) surface etching of amorphous carbon or carbon at defect sites through gasification. At a later growth stage the etching process appears to be dominating, leading to the thinning of nanotubes and the decrease of yield. Moreover, the surface etching through carbon gasification strongly influences the structure and degree of graphitization of NCNTs.The NCNT growth from ethylenediamine involves thickening by catalytic growth and homogeneous deposition of pyrolytic carbon/nitrogen, and thinning by surface etching of carbon through gasification.Download high-res image (134KB)Download full-size image

The structural and electronic promoting effect of nitrogen-doped carbon nanotubes on supported Pd nanoparticles for selective olefin hydrogenation

Co-reporter:Peirong Chen, Ly May Chew, Aleksander Kostka, Martin Muhler and Wei Xia

Catalysis Science & Technology (2011-Present) 2013 - vol. 3(Issue 8) pp:NaN1971-1971

Publication Date(Web):2013/04/08

DOI:10.1039/C3CY00097D

Very low amount of TiO2 on N-doped carbon nanotubes significantly improves oxygen reduction activity and stability of supported Pt nanoparticles

Co-reporter:Anqi Zhao, Justus Masa and Wei Xia

Physical Chemistry Chemical Physics 2015 - vol. 17(Issue 16) pp:NaN10773-10773

Publication Date(Web):2015/03/20

DOI:10.1039/C5CP00369E