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Jingguang G Chen - One of the best experts on this subject based on the ideXlab platform.
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Tungsten Carbides as alternative electrocatalysts from surface science studies to fuel cell evaluation
Industrial & Engineering Chemistry Research, 2011Co-Authors: Alan L Stottlemyer, Erich C Weigert, Jingguang G ChenAbstract:In the current paper we will provide a review of our recent efforts in experimental studies of Tungsten Carbides as alternative electrocatalysts for methanol electro-oxidation. We will first discuss ultrahigh vacuum (UHV) studies on single crystal surfaces to demonstrate the feasibility of using Tungsten Carbides for methanol decomposition. We then discuss UHV studies on polycrystalline thin films and foils to approximate commercially relevant catalysts, thus bridging the “materials gap” and demonstrating that the fundamental chemistry observed in UHV over single crystal surfaces is applicable to morphologically complex surfaces. Electrochemical studies of thin films will be discussed to bridge the “pressure gap” and to verify that Tungsten Carbides are both active and stable in an electrochemical environment. Finally, we will provide performance data from direct methanol fuel cell (DMFC) testing that incorporates Tungsten Carbides as the anode electrocatalysts.
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a combined surface science and electrochemical study of Tungsten Carbides as anode electrocatalysts
Topics in Catalysis, 2007Co-Authors: Erich C Weigert, Alan L Stottlemyer, Michael B Zellner, Jingguang G ChenAbstract:An effective anode electrocatalyst in direct methanol fuel cell (DMFC) should have high activity for the oxidation of methanol and the decomposition of water, while remaining stable under the relatively harsh anode environment. Although the Pt/Ru bimetallic alloy is currently the most effective anode electrocatalyst, both Pt and Ru are expensive due to limited supplies and both are susceptible to CO poisoning. Consequently, the discovery of less expensive and more CO tolerant alternatives to the Pt/Ru catalysts would help facilitate the commercialization of DMFC. In this paper we will discuss the possibility of using Tungsten Carbides (WC) and Pt-modified WC as potential anode electrocatalysts in DMFC. We will provide an overview of our recent work, using a combined approach of fundamental surface science studies and in-situ electrochemical evaluation of the activity and stability of Tungsten Carbides. We will demonstrate the feasibility to bridge fundamental surface science studies on single crystals with the electrochemical evaluation on polycrystalline WC films. We will also discuss the synergistic effect by supporting low coverages of Pt on the WC substrate to further enhance the electrochemical performance of WC.
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potential application of Tungsten Carbides as electrocatalysts synergistic effect by supporting pt on c w 110 for the reactions of methanol water and co
Journal of The Electrochemical Society, 2005Co-Authors: Michael B Zellner, Jingguang G ChenAbstract:In this paper we report a surface science study aimed at fundamental understanding of supporting Pt on the surfaces of Tungsten Carbides. The reaction pathways of methanol, water, and carbon monoxide over platinum-modified C/W( 110) surfaces are studied using temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy (AES). The decomposition of methanol occurs readily on submonolayer Pt-modified C/W( 110) surfaces. The presence of low-coverage Pt on C/W(110) effectively inhibits the undesirable pathway leading to production of CH 4 on the unmodified C/W(110) surface. In addition, Pt-modified C/W(110) surfaces are active toward the dissociation of water. Furthermore, their surfaces display a relatively low desorption temperature for carbon monoxide at ∼329 K. The results on the Pt-modified C/W(110) surfaces are also compared to our earlier studies on the Pt-modified C/W(111) and PVD (physical vapor deposition) synthesized WC surfaces. Overall, the comparison of Pt/C/W(110), Pt/C/W(111), and Pt/WC surfaces confirms a synergistic effect by supporting submonolayer Pt on Tungsten Carbides, leading to higher activity toward the dissociation of methanol and water and a weaker bonding of carbon monoxide. Such a synergistic effect should be advantageous for the potential application of Pt-modified Carbides as fuel cell electrocatalysts.
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potential application of Tungsten Carbides as electrocatalysts
Journal of Vacuum Science and Technology, 2003Co-Authors: Henry H Hwu, Jingguang G ChenAbstract:The reactions of methanol, water, and carbon monoxide over clean and modified Tungsten carbide surfaces are studied by using temperature programmed desorption, high-resolution electron energy loss spectroscopy, and Auger electron spectroscopy. The carbide-modified W(111) surface is highly active toward the decomposition of methanol, with 55% going to complete decomposition, 31% to CO, and 14% to CH4. Additionally, the C/W(111) surface exhibits strong activity toward the dissociation of water. Furthermore, the desorption of CO from C/W(111) occurs at a relatively low temperature of ∼330 K. When modified by the presence of submonolayer Pt, the decomposition pathways of methanol are significantly altered. The presence of low-coverage Pt onto C/W(111) effectively inhibits the production of CH4, an undesirable side product in direct methanol fuel cells. The Pt-modified C/W(111) surface also remains active toward the dissociative of water. When C/W(111) is modified by oxygen, the surface retains significant act...
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potential application of Tungsten Carbides as electrocatalysts iii reactions of methanol water and hydrogen on pt modified c w 111 surfaces
Journal of Catalysis, 2003Co-Authors: Ning Liu, Kostantinos Kourtakis, Juan C Figueroa, Jingguang G ChenAbstract:Abstract The bonding and dissociation of methanol, water, and hydrogen on Pt-modified C/W(111) surfaces have been studied using high-resolution electron energy loss spectroscopy and temperature-programmed desorption. The decomposition pathways of methanol on C/W(111) are significantly modified by the presence of submonolayer coverages of Pt. For example, on the unmodified C/W(111) surface, the decomposition of methanol occurs via three pathways: (i) partial decomposition to produce carbon monoxide and hydrogen (∼31% selectivity), (ii) partial decomposition to form methane and atomic oxygen (∼14% selectivity), and (iii) complete decomposition to produce hydrogen, atomic carbon, and atomic oxygen (∼55% selectivity). In contrast, on the 0.6 monolayer (ML) Pt-modified C/W(111) surface, ∼49% of methanol partially dissociates to carbon monoxide and hydrogen, and the other ∼51% of methanol molecules undergo complete decomposition to hydrogen, atomic carbon, and atomic oxygen. The presence of submonolayer coverages of Pt prohibits the production of methane, which is an undesirable side product in direct methanol fuel cells. Furthermore, both the C/W(111) and the Pt-modified C/W(111) surfaces are active toward the dissociation of water. However, the amount of adsorbed water that undergoes dissociation is reduced from 0.18 H 2 O per W atom on C/W(111) to 0.056 H 2 O per W on the 0.6 ML Pt-modified C/W(111) surface. Finally, both C/W(111) and 0.6 ML Pt/C/W(111) surfaces are active toward the dissociation of hydrogen. These results demonstrated the possible synergistic effects by supporting low loading of Pt onto Tungsten Carbides for potential applications in methanol or hydrogen fuel cells.
M Boudart - One of the best experts on this subject based on the ideXlab platform.
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synthesis characterization and catalytic properties of clean and oxygen modified Tungsten Carbides
Catalysis Today, 1992Co-Authors: Enrique Iglesia, F H Ribeiro, M Boudart, Joseph E BaumgartnerAbstract:Abstract High surface area WC and β-W 2 C powders (30–100 m 2 g −1 ) were prepared by direct isothermal carburization of WO 3 and W 2 N in CH 4 -H 2 mixtures. After surface cleaning with H 2 , their surfaces are equilibrated with bulk stoichiometric Carbides and free of polymeric carbon; they chemisorb 0.2–0.4 monolayers of CO and H. These Carbides catalyze neopentane hydrogenolysis with high selectivity. Chemisorbed oxygen also inhibits hydrogenolysis rates and introduces surface sites for neopentane isomerization, a reaction that occurs only on Pt, Ir, and Au metals. Chemisorbed oxygen also inhibits hydrogenolysis of n-hexane and n-heptane on Tungsten Carbides and introduces surface sites that lead to high isomerization selectivity (70–99%). Kinetic and isotopic tracer studies of n-heptane, 3,3 dimethylpentane, methylcyclohexane, propylene, and methanol reactions show that dehydrogenation reactions and methyl-shifts of unsaturated intermediates occur on oxygen-modified WC powders. Carbidic sites (WC x ) catalyze C-H activation reactions; chemisorbed oxygen titrates such WC x sites and introduces Bronsted acid surface sites (WO x ). Thus, these materials catalyze both dehydrogenation and carbenium-ion reactions, reflecting the bifunctional nature of oxygen-modified transition metal carbide surfaces.
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bifunctional reactions of alkanes on Tungsten Carbides modified by chemisorbed oxygen
Journal of Catalysis, 1991Co-Authors: Enrique Iglesia, F H Ribeiro, Joseph E Baumgartner, M BoudartAbstract:Abstract Tungsten Carbides modified by chemisorbed oxygen catalyze n-heptane isomerization with high selectivity. Kinetic, isotopic tracer, and deuterium-exchange measurements show that the reaction proceeds via sequential n-heptane dehydrogenation and heptene isomerization steps. At low temperatures, isomerization rates are limited by heptene rearrangements but dehydrogenation steps become increasingly rate-limiting as temperature increases. The isomer distribution in n-heptane and 3,3-dimethylpentane reaction products and the 13C distribution in isoheptanes formed from n-heptane-1-13C show that isomerization occurs predominantly by methyl migration steps typical of carbenium-ion rearrangements on acid sites. WOx species on carbide surfaces appear to introduce acid sites similar to those present in supported Tungsten oxides. n-Heptane dehydrocyclization and hydrogenolysis reactions also require heptene intermediates. Dehydrocyclization occurs predominantly by (1, 6) ring closure while hydrogenolysis leads to random cleavage of carbon-carbon bonds in n-heptane.
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Reactivity of Tungsten Carbides I. Catalytic and temperature-programmed reactions of methanol
Catalysis Letters, 1991Co-Authors: M BoudartAbstract:When methanol reacts over Tungsten carbide in a steady-state catalytic mode, methyl formate is formed with a selectivity higher than 90%. On the other hand, temperature-programmed decomposition of methanol preadsorbed on the same surface produces mostly carbon monoxide. The difference in selectivity in both modes of reaction is discussed. By contrast, platinum catalyzes the transformation of methanol to dimethyl ether with high selectivity.
G. Maire - One of the best experts on this subject based on the ideXlab platform.
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obtaining Tungsten Carbides from Tungsten bipyridine complexes via low temperature thermal treatment
Applied Catalysis A-general, 2000Co-Authors: S Wanner, L Hilaire, P Wehrer, J P Hindermann, G. MaireAbstract:Tungsten Carbides were prepared by thermal treatment under nitrogen of a Tungsten bipyridine complex. The catalysts obtained have been characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), temperature programmed reduction (TPR) and elemental analysis. The reaction with 2-methylpentane was used as a chemical probe for the formation of Tungsten carbide.
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catalytic activity of bulk Tungsten Carbides for alkane reforming ii catalytic activity of Tungsten Carbides modified by oxygen
Journal of Catalysis, 1997Co-Authors: Valerie Keller, R. Ducros, P Wehrer, F Garin, G. MaireAbstract:The influence of oxygen on the reforming activity of bulk Tungsten carbide (WC) has been studied for the reaction of pentanes, hexanes, heptanes, and two olefins (2-methyl-2-pentene and 4-methyl-1-pentene). Depending on the air treatment, at low (−78°C), moderate (350°C), or high (700°C) temperature, these alkanes lead to different reaction products as a result of different reaction mechanisms. Whatever the oxygen treatment, heptanes react faster than hexanes, which are more reactive than pentanes. Furthermore, cyclanes (methylcyclopentane or ethylcyclopentane) are less reactive than linear alkanes (n-pentane,n-hexane, orn-heptane), which react more slowly than the branched ones (isopentane, 2-methylpentane, 3-methylhexane). Whatever the oxygen treatment, no cyclic mechanism is involved and isomerization proceeds only through two kinds of bond-shift mechanisms. In order to obtain more information about the possible mechanisms, i.e., a bifunctional mechanism with dehydrogenation/hydrogenation on metallic sites and carbenium ion rearrangement on acidic sites, two unsaturated reactants (2-methyl-2-pentene and 4-methyl-1-pentene) have been tested. The reaction mechanisms and a kinetic model are discussed in detail in a forthcoming paper.
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Tungsten Carbides as substitutes of platinoids in heterogeneous catalysis I. The effect of surface composition on the reactivity of methylcyclopentane on Tungsten Carbides
Catalysis Letters, 1991Co-Authors: V. Keller, M. Cheval, M. Vayer, R. Ducros, G. MaireAbstract:Reactions of methylcyclopentane on Tungsten Carbides show that in presence of oxygen the extensive hydrogenolysis is inhibited in favour of ring enlargement and ring opening via a bifunctional mechanism due to the coexistence of surface acid-rearrangement and hydrogenation-dehydrogenation sites. The oxycarbidic materials are very active and selective in skeletal rearrangement of hydrocarbons.
F H Ribeiro - One of the best experts on this subject based on the ideXlab platform.
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synthesis characterization and catalytic properties of clean and oxygen modified Tungsten Carbides
Catalysis Today, 1992Co-Authors: Enrique Iglesia, F H Ribeiro, M Boudart, Joseph E BaumgartnerAbstract:Abstract High surface area WC and β-W 2 C powders (30–100 m 2 g −1 ) were prepared by direct isothermal carburization of WO 3 and W 2 N in CH 4 -H 2 mixtures. After surface cleaning with H 2 , their surfaces are equilibrated with bulk stoichiometric Carbides and free of polymeric carbon; they chemisorb 0.2–0.4 monolayers of CO and H. These Carbides catalyze neopentane hydrogenolysis with high selectivity. Chemisorbed oxygen also inhibits hydrogenolysis rates and introduces surface sites for neopentane isomerization, a reaction that occurs only on Pt, Ir, and Au metals. Chemisorbed oxygen also inhibits hydrogenolysis of n-hexane and n-heptane on Tungsten Carbides and introduces surface sites that lead to high isomerization selectivity (70–99%). Kinetic and isotopic tracer studies of n-heptane, 3,3 dimethylpentane, methylcyclohexane, propylene, and methanol reactions show that dehydrogenation reactions and methyl-shifts of unsaturated intermediates occur on oxygen-modified WC powders. Carbidic sites (WC x ) catalyze C-H activation reactions; chemisorbed oxygen titrates such WC x sites and introduces Bronsted acid surface sites (WO x ). Thus, these materials catalyze both dehydrogenation and carbenium-ion reactions, reflecting the bifunctional nature of oxygen-modified transition metal carbide surfaces.
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bifunctional reactions of alkanes on Tungsten Carbides modified by chemisorbed oxygen
Journal of Catalysis, 1991Co-Authors: Enrique Iglesia, F H Ribeiro, Joseph E Baumgartner, M BoudartAbstract:Abstract Tungsten Carbides modified by chemisorbed oxygen catalyze n-heptane isomerization with high selectivity. Kinetic, isotopic tracer, and deuterium-exchange measurements show that the reaction proceeds via sequential n-heptane dehydrogenation and heptene isomerization steps. At low temperatures, isomerization rates are limited by heptene rearrangements but dehydrogenation steps become increasingly rate-limiting as temperature increases. The isomer distribution in n-heptane and 3,3-dimethylpentane reaction products and the 13C distribution in isoheptanes formed from n-heptane-1-13C show that isomerization occurs predominantly by methyl migration steps typical of carbenium-ion rearrangements on acid sites. WOx species on carbide surfaces appear to introduce acid sites similar to those present in supported Tungsten oxides. n-Heptane dehydrocyclization and hydrogenolysis reactions also require heptene intermediates. Dehydrocyclization occurs predominantly by (1, 6) ring closure while hydrogenolysis leads to random cleavage of carbon-carbon bonds in n-heptane.
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reactions of neopentane methylcyclohexane and 3 3 dimethylpentane on Tungsten Carbides the effect of surface oxygen on reaction pathways
Journal of Catalysis, 1991Co-Authors: F H RibeiroAbstract:Abstract High surface area Tungsten Carbides with WC and β-W 2 C structure were prepared by direct carburization of W0 3 in CH 4 H 2 mixtures. Their surfaces appear devoid of excess polymeric carbon and adsorb between 0.2 and 0.4 monolayers of CO and H. These materials are very active in neopentane hydrogenolysis. Chemisorbed oxygen inhibits hydrogenolysis reactions and leads to the appearance of isopentane among the reaction products. Neopentane isomerization to isopentane occurs only on Pt, Ir, and An surfaces. Thus, oxygen-exposed Tungsten Carbides catalyze reactions characteristic of noble metal catalysts. 3,3-Dimethylpentane isomerizes much faster than neopentane on oxygen-exposed Carbides; the isomer distribution suggests that isomerization proceeds via a methyl shift mechanism rather than through the C 5 -ring hydrogenolysis pathways characteristic of highly dispersed Pt. The apparent involvement of 3,3-dimethyl-l-pentene reactive intermediates is consistent with carbenium-type methyl shift pathways. Secondary carbon atoms, capable of forming stable carbenium ions, are present in 3,3-dimethylpentane but not in neopentane; they account for the high 3,3-dimethylpentane isomerization rate and selectivity on oxygen-exposed Tungsten carbide powders. Both dehydrogenation and isomerization reactions of methylcyclohexane occur on these carbide powders. These results suggest the presence of a bifunctional surface that catalyzes dehydrogenation and carbenium ion reactions typically occurring on reforming catalysts.
Enrique Iglesia - One of the best experts on this subject based on the ideXlab platform.
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synthesis characterization and catalytic properties of clean and oxygen modified Tungsten Carbides
Catalysis Today, 1992Co-Authors: Enrique Iglesia, F H Ribeiro, M Boudart, Joseph E BaumgartnerAbstract:Abstract High surface area WC and β-W 2 C powders (30–100 m 2 g −1 ) were prepared by direct isothermal carburization of WO 3 and W 2 N in CH 4 -H 2 mixtures. After surface cleaning with H 2 , their surfaces are equilibrated with bulk stoichiometric Carbides and free of polymeric carbon; they chemisorb 0.2–0.4 monolayers of CO and H. These Carbides catalyze neopentane hydrogenolysis with high selectivity. Chemisorbed oxygen also inhibits hydrogenolysis rates and introduces surface sites for neopentane isomerization, a reaction that occurs only on Pt, Ir, and Au metals. Chemisorbed oxygen also inhibits hydrogenolysis of n-hexane and n-heptane on Tungsten Carbides and introduces surface sites that lead to high isomerization selectivity (70–99%). Kinetic and isotopic tracer studies of n-heptane, 3,3 dimethylpentane, methylcyclohexane, propylene, and methanol reactions show that dehydrogenation reactions and methyl-shifts of unsaturated intermediates occur on oxygen-modified WC powders. Carbidic sites (WC x ) catalyze C-H activation reactions; chemisorbed oxygen titrates such WC x sites and introduces Bronsted acid surface sites (WO x ). Thus, these materials catalyze both dehydrogenation and carbenium-ion reactions, reflecting the bifunctional nature of oxygen-modified transition metal carbide surfaces.
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bifunctional reactions of alkanes on Tungsten Carbides modified by chemisorbed oxygen
Journal of Catalysis, 1991Co-Authors: Enrique Iglesia, F H Ribeiro, Joseph E Baumgartner, M BoudartAbstract:Abstract Tungsten Carbides modified by chemisorbed oxygen catalyze n-heptane isomerization with high selectivity. Kinetic, isotopic tracer, and deuterium-exchange measurements show that the reaction proceeds via sequential n-heptane dehydrogenation and heptene isomerization steps. At low temperatures, isomerization rates are limited by heptene rearrangements but dehydrogenation steps become increasingly rate-limiting as temperature increases. The isomer distribution in n-heptane and 3,3-dimethylpentane reaction products and the 13C distribution in isoheptanes formed from n-heptane-1-13C show that isomerization occurs predominantly by methyl migration steps typical of carbenium-ion rearrangements on acid sites. WOx species on carbide surfaces appear to introduce acid sites similar to those present in supported Tungsten oxides. n-Heptane dehydrocyclization and hydrogenolysis reactions also require heptene intermediates. Dehydrocyclization occurs predominantly by (1, 6) ring closure while hydrogenolysis leads to random cleavage of carbon-carbon bonds in n-heptane.
