Platinum anode catalysis

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

Anodic oxidation of JV,N-dimethyl-co-hydroxyamides (57) in CH3OH-Bu4NBF4 at a platinum anode leads to formation of N-methoxy-JV-methyl-co-hydroxyamides (58) in high yield.116 The latter could in some cases (formation of five-, six-, and seven-membered rings) easily be transformed to l,3-oxaza-4-oxo heterocyclic systems (59) by acid catalysis [Eq. (49)]. No direct formation of the 1,3-oxazaheterocycles was observed, e.g., 57++59. An intramolecular addition of the hydroxy group to the intermediate acylam-monium ion is believed to be hindered by adsorption phenomena at the anode surface. 

Fletcher D, Solis V. A further investigation of the catalysis by lead ad-atoms of formic acid oxidation at a platinum anode. J Electroanal Chem 1982 131 309-23. 

The additivity principle was well obeyed on adding the voltammograms of the two redox couples involved even though the initially reduced platinum surface had become covered by a small number of underpotential-deposited mercury monolayers. With an initially anodized platinum disk the catalytic rates were much smaller, although the decrease was less if the Hg(I) solution had been added to the reaction vessel before the Ce(lV) solution. The reason was partial reduction by Hg(l) of the ox-ide/hydroxide layer, so partly converting the surface to the reduced state on which catalysis was greater. 

Ralph TR, Hogarth MP. 2002b. Catalysis for low temperature fuel cells. Part II The anode challenges. Platinum Metals Rev 46 117-135. 

The ideal performance of a fuel cell depends on the electrochemical reactions that occur with different fuels and oxygen as summarized in Table 2-1. Low-temperature fuel cells (PEFC, AFC, and PAFC) require noble metal electrocatalysts to achieve practical reaction rates at the anode and cathode, and H2 is the only acceptable fuel. With high-temperature fuel cells (MCFC, ITSOFC, and SOFC), the requirements for catalysis are relaxed, and the number of potential fuels expands. Carbon monoxide "poisons" a noble metal anode catalyst such as platinum (Pt) in low-temperature... 

Ralph T R ah, 2002, Catalysis for Low Temperature Fuel Cells, Part 2, The Anode Challenges. Platinum Metals Review, 46(1). 

Ralph, T.R. and Hogarth, M.P., Catalysis for low temperature fuel cell. Part II. The anode challenges. Platinum Metal Rev., 46, 117, 2002. 

Dithianes and gemdithioacetals could be alternatively oxidized indirectly by means of the redox catalysis method. The technique appeared to be particularly mild and mainly avoided inhibition and adsorption phenomena relative to the anode platinum interface. Thus aromatic hydrocarbons (e.g. 9,10-diphenylanthracene) [83] and judiciously substituted triphenylamines [84] afford quite stable cation radicals used homogeneously as oxidants. Their standard potential, E°x, will determine the rate of electron exchange with the concerned sulfur compound. The cleavage of a C—S bond in any dithiane can be regarded as fast enough to draw the redox catalysis process to the indirect oxidation. 

Table 16.2. Physical characterization of PtRu alloy electrocatalysts [43]. (Reprinted from Ralph TR, Hogarth MP. Catalysis for low temperature fuel cells, part II the anode challenges. Plat Met Rev 2002 46(3) 117-35, 2002. With permission from Platinum Metals Review.)...

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