Oxygen reduction mechanisms, alternatives

Research on alternative catalysts for the ORR for use in PEM fuel cell cathodes is an exciting and growing field of research. Several classes of materials show potential for replacing precious metal cathodes, especially for automotive power applications and direct methanol systems. Increasing the understanding of active sites in alternative catalysts, the mechanisms for oxygen reduction, and optimization of full fuel cell preparation using alternative materials, will lead to further improvements in performance. 

Regarding the mechanism, several authors have proposed a redox route for soot oxidation, which utilizes some form of oxygen from the support in a typical reduction/oxidation mechanism where the catalyst undergoes a partial reduction. An alternative redox route involves oxygen from the support and gas phase, which reacts with soot to form adsorbed GOg in the form of surface carbon oxygen complexes (like adsorbed carbonates). Decomposition of carbonate is then stimulated by gas-phase oxygen, which also reoxidizes the support, as illustrated below ... 

Initially, two plausible mechanisms were considered, as depicted in Scheme 5.16. The first was a hydroboration route (a), where the B-H bond was added across the olefin from the same face of S-0 and upon aqueous work-up, the resulting C-B bond was replaced with a C-H bond. The tis B-H addition to the olefin led to the cis-stereochemistry of the two adjacent aryl substituents. The reduction of the sulfoxide oxygen occurs in the next step. The alternative mechanism was the borane reduction route (h), which was similar to 1,4-addition of hydride,... 

Wacker (1) A general process for oxidizing aliphatic hydrocarbons to aldehydes or ketones by the use of oxygen, catalyzed by an aqueous solution of mixed palladium and copper chlorides. Ethylene is thus oxidized to acetaldehyde. If the reaction is conducted in acetic acid, the product is vinyl acetate. The process can be operated with the catalyst in solution, or with the catalyst deposited on a support such as activated caibon. There has been a considerable amount of fundamental research on the reaction mechanism, which is believed to proceed by alternate oxidation and reduction of the palladium ... 

In addition to the intercalation mechanism described, the anthracycline ring of doxorubicin can undergo a one-electron reduction to form free radicals and participate in further electron transfer. These highly active substances can then react with tissue macromolecules. This type of interaction suggests an alternative mechanism of cytotoxicity for the anthracyclines. In particular, the cardiac toxicity of anthracyclines may result from the generation of free radicals of oxygen. 

A second mechanism by which impurities present in the solution interfere with measurements in electrode kinetics is associated less with their providing alternative paths to reactions and more with their adsorption on the electrode surface. The presence of adsorbed impurity material on the electrode surface may affect the rate of an intended reaction, particularly where there is an intermediate in the mechanism of the intended reaction.25 Thus, the adsorbed impurity may block reactive sites on the electrode. Since the intended reaction (say, the reduction of oxygen) cannot now use the blocked active sites on the electrode surface, its kinetics are slowed down, particularly when the impurity adsorbs on the most active catalytic sites on the electrode. Again,... 

NOx reduction conversions met in the DOC are quite low. Excess of air in burned lean fuel mixture results in excess of oxygen in the exhaust. Under such conditions, the reducing components naturally present in diesel exhaust (CO, H2 and HC) are readily oxidized by the excessive oxygen and NOx remains unreduced. However, the unburned hydrocarbons still exhibit a certain activity for NO reduction on NM/y-Al203 and NM/zeolites catalysts under lean conditions (HC-SCR). Many efforts have been put into the investigation of different NM-based or alternative catalysts tailored for the HC-SCR reaction and the development of reliable reaction mechanisms—cf., e.g., Joubert et al. 

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