Electrocatalysis of dioxygen reduction

Bettelheim, A., B.A. White, and R.W. Murray (1987). Electrocatalysis of dioxygen reduction in aqueous acid and base by multimolecular layer films of electropolymerized cobalt tetra(ortho-aminophenyl)porphyrin. J. Electroanal. Chenu 217, 271-286. 

Bettelheim A, Reed RA, Hendricks NH, Collman JP, Murray RW (1988) Application of a polymersolid electrol34e for the vapor phase electrocatalysis of dioxygen reduction by some cobalt porphyrins. J Electroanal Chem, 259-264... 

Electrocatalytic Reduction of Dioxygen and Hydrogen Peroxide These two processes must be emphasized because reduction of dioxygen, and eventually hydrogen peroxide, features the usually claimed pathway for reoxidation of reduced POMs after the participation of the latter in oxidation processes. As a consequence, electrocatalysis of dioxygen and hydrogen peroxide reduction is a valuable catalytic test with most new POMs [154, 156,161]. 

The first accounts that seemed to give direct enzyme electrochemistry were the reports concerning a soluble blue Cu oxidase, laccase, which catalyzed the rapid four-electron reduction of dioxygen to water. An efficient electrocatalysis of O2 reduction by adsorbed fungal laccase on pyrolytic graphite, glassy carbon, and C02-treated carbon black electrodes was first described by Tarasevich and co-workers (48). Several control experiments were carried out to verify direct electron transfer from the electrode to the Cu sites of the enzyme. 

Matsufuji, A., S. Nakazawa, and K. Yamamoto (2001). Electrocatalysis of the reduction of dioxygen by 7t-conjugated polymer complexes with dinuclear cobalt porphyrin. J. Inorg. Organomet. Polym. 11, 47-61. 

Liu H, Weaver M, Wang C, Chang C. Dependence of electrocatalysis for dioxygen reduction by adsorbed cofacial dicobalt porphyrins upon catal5 st structure. J Electroanal Chem 1983 145 439-47. 

Modification of PANI-coated electrodes with metal tetrasul-fonated phthalocyanines (MeTSPc), where the metal is cobalt or iron, resulted in significant changes in the electrocatalysis of the reduction of diojygen [504]. Obviously, insertion of the MeTsPc into the polymer occurs [505]. This was supported in an investigation by Coutanceau et al. by the results of in situ UV-vis spectroscopy. The role of the polymer in the mechanism and the kinetics of dioxygen electroreduction seemed to be somewhat difficult to elucidate. Insertion of CoTsPc resulted in a positive shift of the onset of dioxygen reduction. The two-electron pathway that results in hydrogen peroxide as a reduction product remains. 

Anson FC, Ni CL, Saveant JM. 1985. Electrocatalysis at redox polymer electrodes with separation of the catalytic and charge propagation roles. Reduction of dioxygen to hydrogen peroxide as catalyzed by cobalt(II) tetrakis(4-A-methylpyridyl)porphyrin. J Am Chem Soc 107 3442. 

Buttry DA, Anson FC. 1984. New strategies for electrocatalysis at polymer-coated electrodes. Reduction of dioxygen by cobalt porphyrins immobilized in Nalion coatings on graphite electrodes. J Am Chem Soc 106 59. 

Electrocatalysis employing Co complexes as catalysts may have the complex in solution, adsorbed onto the electrode surface, or covalently bound to the electrode surface. This is exemplified with some selected examples. Cobalt(I) coordinatively unsaturated complexes of 2,2 -dipyridine promote the electrochemical oxidation of organic halides, the apparent rate constant showing a first order dependence on substrate concentration.1398,1399 Catalytic reduction of dioxygen has been observed on a glassy carbon electrode to which a cobalt(III) macrocycle tetraamine complex has been adsorbed.1400,1401... 

The molecule is very stable and can be sublimed [i]. Numerous metal phthalocyanines can reversibly bind molecules like, e.g., dioxygen at the metal ion. This can result in activation of internal bonds and subsequent facilitation of chemical reaction, in this case of dioxygen -> electroreduction. Thus these molecules have attracted attention as catalysts for various reactions, in particular dioxygen reduction in, e.g., fuel cells [ii], in general -> electrocatalysis [iii] and in -> sensors [iv]. Their strong coloration, which can be modified electrochemically by reduction/oxidation, suggests use in -> electrochromic devices [v]. 

This chapter provides a critical review of transition metal macrocycles, both in intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. Fundamental aspects of electrocatalysis, oxygen reduction and transition metal macrocycles will be highlighted in this brief introduction, which should serve as background material for the subsequent more specialized sections. 

The macrocycles Co (111) (cyclam) and Fe( 111)TM PyP display high activity for dioxygen reduction and negligible affinity for carbonaceous surfaces providing close to ideal conditions to warrant analyses of electrochemical data within the strict homogeneous electrocatalysis framework. Their most salient features are summarized in the two sub-sections to follow. 

Gouerec, P, A. Biloul, O. Contamin, G. Scarbeck, M. Savy, J. M. Barbe, and R. Guilard (1995). Dioxygen reduction electrocatalysis in acid media Effect of peripheral ligand substitution on cobalt tetraphenylporphyrin, J. Electroanal. Chem. 398, 67-75. 

Appleby AJ. Electrocatalysis of aqueous dioxygen reduction. J Electroanal Chem 1993 357 117-79. 

In recent years, electrocatalysis has been widely employed to reductively activate dioxygen [22b, 146,147]. The reduction of proceeds through two pathways, which are mainly determined by the electrocatalyst and electrode potential two-electron reduction into (Eq. 14.34) and four-electron reduction into H O (Eq. 14.35). 

Figure 2.13. Garten and Weiss s mechanism for reduction on carbon surface [2]. (Reprinted from Journal of Molecular Catalysis, 38(1-2), Yeager Ernest, Dioxygen electrocatalysis mechanisms in relation to catalyst structure, 5-25, 1986, with permission from Elsevier.)...

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