Metal oxides carbon-supported

As a solution to provide a long-term solution to Pt cost and scarcity, a variety of non-noble metal-based catalysts has been explored as promising cathode catalysts for fuel cells. These ORR catalysts include heat-treated metal-nitrogen-carbon complexes (M-Nx/C, M = Fe or Co), carbon-supported chalcogen-ides, and carbon-supported metal oxides. These catalysts have been synthesized and showed considerable ORR activity and stability when compared to those of Pt/C catalyst. In the exploration, RDE/RRDE techniques are the most commonly employed tools in evaluating the catalysts activity and stability toward ORR and its associated mechanism. 

In basic solution, some carbon-supported metal oxides, such as Ti407/C, Lao.6Cao.4Co03/C, Carbon-Lao.6Cao.4Co03/C, MnOx/C, and Me-MnOx/C (Me = Ni, Mg), have been explored for ORR catalysts, evaluated by RDE/RRDE techniques. ... 

In general, there are two possibilities to prepare nanocarbon-supported metal(oxide) catalysts. The in situ approach grows the catalyst nanoparticles directly on the carbon surface. The ex situ strategy utilizes pre-formed catalyst particles, which are deposited on the latter by adsorption [94]. Besides such solution-based methods, there is also the possibility of gas phase metal (oxide) loading, e.g., by sputtering [95], which is used for preparation of highly loaded systems required for electrochemical applications not considered here. 

As for hydrogenation, heterogeneous catalytic oxidation of carbohydrates was essentially performed in the presence of carbon-supported metal catalysts, namely Pt, Pd or Bi-doped Pd.[57] Oxidation of glucose into gluconic acid, the worldwide production of which is around 60000 tons year 1,[52] is used in the food and pharmaceutical industry, and is produced today by enzymatic oxidation of D-glucose with a selectivity in gluconic acid close to 100%. 

Although hydrotalcites are relahvely stable (up to circa 500 °C), they are also of potential applicahon as precursors of mixed metal oxide catalysts, for example Reference [66]. Dehydrahon-rehydration equilibria account for the switching between hydrotalcites and mixed/supported metal oxides, which is somehmes termed the memory effect [67-69]. Recent advances have seen attempts to prepare highly dispersed LDH systems, such as those dispersed within mesoporous carbon [70]. Owing to widespread interest in their application, hydrotalcite catalysts have been the subject of a number of reviews, for example References [71-75]. Other layered-based systems have also attracted attention for application in catalysis, for example Reference [76]. 

Adsorptive and Catalytic Properties of Carbon Monoxide and Carbon Dioxide Over Supported Metal Oxides... 

Homogeneous catalysis is now widely used across the organic chemicals industry. It offers a number of advantages over traditional carbon or metal oxide supported catalysts. For example, the design of ligands with particular steric or electronic... 

X-ray diffraction analysis is used routinely by every catalyst manufacturer to determine the phase composition of the catalysts produced and the size of coherently scattering domains, and hardly needs a detailed description. An aspect that we would like to emphasize concerns the influence of the enviromnent on the oxidation state of carbon-supported metal nanoparticles. Quite often, authors try to correlate electrochemical performance with the phase composition of as-prepared samples. It has, however, been demonstrated convincingly in a number of publications by both x-ray diffraction [155] and x-ray absorption spectroscopy [156] that as-prepared fuel cell catalysts and samples stored under ambient conditions are often in the form of metal oxides but are reduced under the conditions of PEMFC or DMFC operation. The most dramatic changes are observed for samples with high metal dispersions, while larger particles are affected only marginally [17]. One should keep in mind, however, that the extent of the particle oxidation depends critically on the preparation procedure. 

The development of catalysts based on carbon supports is related to the challenge that solid properties determining the catalytic properties are not easily accessible. Regardless of the fact that catalysts do not show obvious differences with respect to solid properties (e.g., morphological and smface properties of the carbon support, metal particle size, particle dispersion or solid phase and oxidation state of the active metal), they often reveal differences in their catalytic behavior. For industrial application of catalysts in fine chemistry, these circumstances are serious obstacles for a straightforward rational development and the identification of suitable catalysts for conversion of certain substrates. 

P-08 - Formation of carbon nanotubes on various molecular sieves supported metal oxides... 

Palladium species immobilized on various supports have also been applied as catalysts for Suzuki cross-coupling reactions of aryl bromides and chlorides with phenylboronic acids. Polymers, dendrimers, micro- and meso-porous materials, carbon and metal oxides have been used as carriers for Pd particles or complexes for these reactions. Polymers as supports were applied by Lee and Valiyaveettil et al. (using a particular capillary microreactor) [173] and by Bedford et al. (very efficient activation of aryl chlorides by polymer bound palladacycles) [174]. Buch-meiser et al. reported on the use of bispyrimidine-based Pd catalysts which were anchored onto a polymer support for Suzuki couplings of several aryl bromides [171]. Investigations of Corma et al. [130] and Plenio and coworkers [175] focused on the separation and reusability of Pd catalysts supported on soluble polymers. Astruc and Heuze et al. efficiently converted aryl chlorides using diphosphino Pd(II)-complexes on dendrimers [176]. 

The reaction of bromobenzene and styrene (points A to C) is complete within a few minutes at 140 °C (without microwave heating ) with the best supported Pd-catalysts (e.g. palladium on carbon, on metal oxides or in zeolites Fig. 10.16). The dissolution of palladium from the support (point A) starts (only ) at reaction temperature (140 C for bromobenzene) and depends on a variety of parameters (substrate, catalyst, solvent, temperature, base, additives, atmosphere). The maximum palladium concentration in solution correlates well with the highest reaction rate (turning point B of conversion curve). Palladium concentration in solution decreases with continued conversion (point C decrease of aryl halide concentration) leading to (nearly) complete re-precipitation (point D) of the originally dissolved palladium (fortunately) onto the support. Increased temperature and/or excess of a reducing agent (e.g. sodium formate) added at the end of the... 

Other diols, polyols [77], amino alcohols [77] and glucose [77, 80,120] have also been oxidized with remarkable selectivity to hydroxy- and amino- acids by oxygen in strong basic aqueous media using gold nanoparticles supported on carbon and metal oxides as catalysts (Schemes 12.14 and 12.15). 

On the other hand, the oxidative coupling reaction of CH4 in the presence of Og, even when performed in membrane-type reactors, is mainly catalysed by metal oxide catalysts. Also oligomerisation, aromatisation and the partial oxidation to methanol or formaldehyde apply non-metallic heterogeneous catalysts (i.e. zeolites, supported metal oxides or heterogenized metalcarbon nanofibers or nanotubes from methane, these being catalysed by metal nanoparticles, but at the moment this is not considered as a Cl chemistry reaction. Again we direct the attention of the reader to some reviews on this type of process. ... 

In the previous example the supported metal oxide onto which the metal precursor was adsorbed did not reduce which will be the case for many promoted systems. In many systems, however, the supported metal oxide will reduce, especially through hydrogen spillover, and a bimetallic catalyst can be synthesized. The idea is illustrated in Figure 3.11a for the Pd/Co/C electrocatalyst system. The idea will be to adsorb Pd complexes onto a well-dispersed, carbon-supported C03O4 phase, and reduce to get bimetallic Pd/Co particles that are perhaps core-shell in morphology. 

Catalyst support (<50nm) (carbon or metal oxide)... 

Figure 3.4 Schematics of ORR electrocatalyst s morphologies. (A) Metal catalyst such as Pt and metal alloy catalyst such as PtPd supported on a conductive material such as carbon or metal oxide (B) core—shell catalyst such as Au Pt supported on conductive material such as carbon or metal oxide (C) metal catalyst such as Pt and metal alloy catalyst such as PtPd supported on a nanofibre such as carbon or metal-oxide nanofibre and (D) core—shell catalyst such as Au Pt supported on conductive nanotubings such as carbon nanotubings. (For color version of this figure, the reader is referred to the online version of this book.)...

Studies of methanol oxidation with polyaniline supported catalysts have focused on the use of bimetallic catalysts. As for the bulk metals and carbon supported metals, Pt-Ru and Pt-Sn are both more effective than Pt for methanol oxidation vAien supported on polyaniline (26, 27). Pt-Ru also provides more complete oxidation of methanol to CO2 (27). 

Wang et al. (2004) and Zhang (2008). Systematic modifications at the fabrication stage include (i) the selection of materials (e.g., carbon or metal-oxide-based support materials, Pt or Pt-alloy catalyst materials, perfluorinated or alternative ionomer materials), (ii) size dispersion of catalyst and support particles, (iii) gravimetric composition of the ink (amounts of carbon, ionomer, and Pt) and solvent properties, (iv) thickness of the CL, and (v) fabrication conditions (temperature, solvent evaporation rate, pressure, and tempering procedures). 

Electro-catalyst supports play a vital role in ascertaining the performance, durability, and cost of PEMFC and DMFC systems. A myriad of nano-structured materials including carbon nanostructures, metal oxides, conducting polymers, transition metals nitrides and carbides, and many hybrid conjugates, have been exhaustively researched to improve the existing support and also to develop novel PEMFC/DMFC catalyst support. One of the main challenges in the immediate future is to develop new catalyst supports that improve the durability of the catalyst layer and, in a best-case scenario, also impact the electronic properties of the active phase to leapfrog to improve catalyst kinetics. 

However, this process often leads to the loss of ordered mesoporous structures during the crystallization at a high temperature. The Wiesner group developed a novel thermalization process, the combined assembly by soft and hard (CASH) chemistries, to retain the mesostructure while crystallizing [43], CASH is a two-step thermal process (see Fig. 5), which includes thermal treatment of BCP + metal oxide hybrids under an inert gas (e.g., Ar, N2) to generate a sturdy in-situ carbon scaffold that supports metal oxide nanopores as crystallization occurs. The carbon scaffold can subsequently be oxidized away by letting air into the sample chamber, resulting in the final fully crystallized oxide. 

Moreover, a general strategy for the synthesis of carbon-supported metal or metal oxide nanoparticles starting from MOFs was reported by Poddar s group. The authors found a correlation between the redox potential of the metal" /metal redox couple and the ability of a metal" -based MOF to form metal nanoparticles on pyrolysis. Cu and Co have redox potentials of -1-0.34 and -0.28 V versus SHE, whereas Zn , Mn , and Mg have much lower potentials of -0.76, -1.18, and -2.37 V versus SHE, respectively It was concluded from experimental results that metal cations with a potential above ca. -0.27 V SHE could react with the organic ligands of the MOFs to be reduced to zero oxidation state during pyrolysis in inert atmosphere, whereas others resulted in metal oxide nanoparticles after pyrolysis. 

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