Carbon-supported catalysts and

As shown in Figure 1.6, the optimized cathode and anode structures in PEMFCs include carbon paper or carbon cloth coated with a carbon-PTFE (polytetrafluoroethylene) sub-layer (or diffusion layer) and a catalyst layer containing carbon-supported catalyst and Nafion ionomer. The two electrodes are hot pressed with the Nafion membrane in between to form a membrane electrode assembly (MEA), which is the core of the PEMFC. Other methods, such as catalyst coated membranes, have also been used in the preparation of MEAs. 

Under certain approximations, using the concepts of percolation theory, the basic parameters can be related to the volume portions of the components of the layer. This offers a relationship between the structure of the porous composite catalyst layer and its performance. An optimum composition (in terms of volume fractions of electrolyte material, carbon and carbon-supported catalyst, and pore space) is a Holy Grail here. Albeit this goal can still be far away in view of the simplified character of the models used, these models give at least some rational scheme for... 

In the following sections issues such as vanadium-based catalysts, noble metal catalysts, zeolites, carbon-supported catalysts, and promoters are discussed. 

The DMFC based on carbon supported catalyst with low catalyst loading (1.3 mg/cm ) has been successfully tested in a methanol/air environment. The cell shows better performance in comparison to the cell based on unsupported catalyst with twice the Pt-black loading. These results are explained by the higher surface area of Pt carbon supported catalyst and are in good correlation with CV and BET data. The results show that carbon supported catalyst can be successfully used as the electrode material for the fabrication of relatively cheap cathode catalyst layers in DMFC. Further work is needed to estimate the lower concentration limit of the catalyst, which is sufficient to maintain stable performance and long-term endurance. 

Several methods of applying the catalyst layer to the gas diffusion electrode have been reported. These methods are spreading, spraying, and catalyst power deposition. For the spreading method, a mixture of carbon support catalyst and electrolyte is spread on the GDL smface by rolling a metal cylinder on its surface [22]. In the spraying method, the catalyst and electrolyte mixture is repeatedly sprayed onto the GDL surface until a desired thickness is achieved. 

The main function of the GDL is to diffuse the gas. The porous nature of the backing material facilitates the effective diffusion of each reactant gas to the catalyst on the MEA. The GDL is also an electrical coimection between the carbon-supported catalyst and the bipolar plate or other current collectors. In addition, the GDL also helps in managing water in the fuel cell as it carries the product water away from the electrolyte surface [32]. 

Fig. 16.1 Schematic diagram of the binder presence in the catalyst layer of HT-PEMFC MEAs with the carbon-supported catalyst and H3P04-doped membrane, (a) The...

In a carbon-supported metal electrocatalyst, the electronic interaction between metal and carbon support has a significant effect on its electrochemical performance [4], For carbon-supported Pt electrocatalyst, carbon could accelerate the electron transfer at the electrode-electrolyte interface, leading to an accelerated electrode process. Typically, the electrons are transferred from platinum clusters to the oxygen species on the surfece of a carbon support material and the chemical bond formation or the charge transfer process occurs at the contacting phase, which is considered to be beneficial to the enhancement of the catalytic properties in terms of activity and stability of the electrocatalysts. Experimentally, the investigation into the electron interaction between metal catalyst and support materials could be realized by various physical, spectroscopic, and electrochemical approaches. The electron donation behavior of Pt to carbon support materials has been demonstrated by the electron spin resonance (ESR) X-ray photoelectron spectroscopy (XPS) studies, with the conclusion that the electron interaction between Pt and carbon support depends on their Fermi level of electrons. It is considered that the electronic structure change of Pt on carbon support induced by the electron interaction has positive effect toward the enhancement of the catalytic properties and the improvement of the stability of the electrocatalyst system. However, the exact quantitative relationship between electronic interaction of carbon-supported catalyst and its electrocatalytic performance is still not yet fully established [4]. 

Hydrogenation. Hydrogenation is one of the oldest and most widely used appHcations for supported catalysts, and much has been written in this field (55—57). Metals useflil in hydrogenation include cobalt, copper, nickel, palladium, platinum, rhenium, rhodium, mthenium, and silver, and there are numerous catalysts available for various specific appHcations. Most hydrogenation catalysts rely on extremely fine dispersions of the active metal on activated carbon, alumina, siHca-alumina, 2eoHtes, kieselguhr, or inert salts, such as barium sulfate. 

Rapoport s findings have been confirmed in the authors laboratory where the actions of carbon-supported catalysts (5% metal) derived from ruthenium, rhodium, palladium, osmium, iridium, and platinum, on pyridine, have been examined. At atmospheric pressure, at the boiling point of pyridine, and at a pyridine-to-catalyst ratio of 8 1, only palladium was active in bringing about the formation of 2,2 -bipyridine. It w as also found that different preparations of palladium-on-carbon varied widely in efficiency (yield 0.05-0.39 gm of 2,2 -bipyridine per gram of catalyst), but the factors responsible for this variation are not knowm. Palladium-on-alumina was found to be inferior to the carbon-supported preparations and gave only traces of bipyridine,... 

Rhodium-on-carbon has also been found to bring about the formation of 2,2 -biquinoline from quinoline, the yield and the percentage conversion being similar to that obtained with palladium-on-carbon. On the other hand, rhodium-on-carbon failed to produce 2,2 -bipyridine from pyridine, and it has not yet been tried with other bases. Experiments with carbon-supported catalysts prepared from ruthenium, osmium, iridium, and platinum have shown that none of these metals is capable of bringing about the formation of 2,2 -biquinoline from quinoline under the conditions used with palladium and rhodium. ... 

The commercial process for the production of vinyl acetate monomer (VAM) has evolved over the years. In the 1930s, Wacker developed a process based upon the gas-phase conversion of acetylene and acetic acid over a zinc acetate carbon-supported catalyst. This chemistry and process eventually gave way in the late 1960s to a more economically favorable gas-phase conversion of ethylene and acetic acid over a palladium-based silica-supported catalyst. Today, most of the world s vinyl acetate is derived from the ethylene-based process. The end uses of vinyl acetate are diverse and range from die protective laminate film used in automotive safety glass to polymer-based paints and adhesives. 

Kauranen, R, E. Skou, and J. Munk, Kinetics of methanol oxidation on carbon-supported Pt and Pt + Ru catalysts, J. Electroanal. Chem., 404, 1 (1996). 

Carbons and Carbon Supported Catalysts in Hydroprocessing 2 Chiral Sulfur Ligands Asymmetric Catalysis... 

A significant volume of literature relates to our work. Concerning choice of support, Montassier et al. have examined silica-supported catalysts with Pt, Co, Rh Ru and Ir catalysts.However, these systems are not stable to hydrothermal conditions. Carbon offers a stable support option. However, the prior art with respect to carbon-supported catalysts has generally focused on Ru and Pt as metals.Additionally, unsupported catalysts have also been reported effective including Raney metals (metal sponges).Although the bulk of the literature is based on mono-metallic systems, Maris et al. recently reported on bimetallic carbon-supported catalysts with Pt/Ru and Au/Ru. In contrast, our work focuses primarily on the development of a class of rhenium-based carbon supported catalysts that have demonstrated performance equal to or better than much of the prior art. A proposed reaction mechartism is shown in Figure 34.2 °l... 

The object of the present study was to use in the above mentioned hydrogenations improved carbon supported catalysts, which could compete with the Pd black catalyst. Carbon materials are common supports, their surface properties can be modified easily and it is possible to prepare carbons with different proportion of micro-, meso- and macropores, which can be key factors influencing their performances. A highly mesoporous carbon was synthesised and used as support of Pd catalysts in the enantioselective hydrogenations. To our knowledge this is the first report on the use of highly mesoporous carbon for the preparation of Pd catalysts for liquid-phase hydrogenation. 

The anchoring and the reduction methods of precious metal precursors influence the particle size, the dispersion and the chemical composition of the catalyst. The results of SEM and H2 chemisorption measurements are summarised in Table 3. The XPS measurements indicate that the catalysts have only metallic Pd phase on their surface. The reduction of catalyst precursor with sodium formate resulted in a catalyst with lower dispersion than the one prepared by hydrogen reduction. The mesoporous carbon supported catalysts were prepared without anchoring agent, this explains why they have much lower dispersion than the commercial catalyst which was prepared in the presence of a spacing and anchoring agent (15). 

The reaction was performed over a series of Pt/Al203, Ru/Al203, and carbon-supported catalysts under the action of pulsed microwave radiation conversions exceeded 90 % and acetonitrile was formed as the byproduct. 

Figure 9.22 Left Mo K-edge EXAFS Fourier transforms of MoS2 and sulfided, carbon supported Mo and Co-Mo catalysts, showing the reduced S and Mo coordination in the first shells around molybdenum in the catalyst (from Bouwens et at. [68]). Right Co K-edge Fourier transforms of the same catalysts and of a Co9S8 reference. Note the presence of a contribution from Mo neighbors in the Fourier transform of the Co-Mo-S phase (from Bouwens et at. [76]).

The EXAFS signal from the Co K edge gives information on the surroundings of cobalt. As an active sulfided Co-Mo/AI2O3 catalyst contains at least two cobalt species, namely ions inside the A1203 lattice and in the Co-Mo-S phase, it is better to investigate the Co-Mo-S phase in carbon-supported catalysts. The latter can be... 

S. Ozkara, and A. E. Aksoylu, Selective low temperamre carbon monoxide oxidation in H2-rich gas streams over activated carbon supported catalysts, Appl. Catal. 251(1), 75—83 (2003). 

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