O2-reduction mechanism

All of these considerations of O2 reduction mechanisms suggest that the four-electron reduction of O2, with its very favorable potential, overestimates the oxidative power of O2 in chemical systems. This should be kept in mind in the following discussions of redox reactions if O2 is the oxidant. 

Jaouen F, Dodelet JP (2009) O2 reduction mechanism noble metal catalysts for PEM fuel cells. Part I Experimental rates for O2 electroreduction, H2O2 electroreduction, and H2O2 disproportionation. J Phys Chem C 113 15422-15432... 

Figure 16 represents a generally agreed upon O2 reduction mechanism (Fig. 16) (41,156,159). 

Damjanovic A, Hudson PG. 1988. On the kinetics and mechanism of O2 reduction at oxide film covered Pt electrodes. I. Effect of oxide film thickness on kinetics. J Electrochem Soc 135 2269-2273. 

Gattrell G, MacDougall B. 2003. Reaction mechanisms of the O2 reduction/evolution reaction. In Vielstich W, Lamm A, Gasteiger HA. editors. Handbook of Euel Cells— Eundamentals, Technology and Applications. Volume 2. Chichester Wiley. 

The major conclusion from the smdies of the mechanism of O2 reduction by CcO is that formation of a peroxo-level intermediate bridging two metal ions is not a prerequisite for four-electron reduction, at least in molecular complexes. 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

Litde is known about the stability of these porphyrins in O2 reduction, how this peripheral substitution affects O2 affinity of the metalloporphyrin, how the peripheral metal complexes perturb the energetics of various intermediates, and/or the kinetics of various steps or the mechanisms of O2 reduction by these porphyrins. At present, it remains to be seen if the strategy of coordinating metal complexes on the periphery of a metalloporphyrin can be exploited in the rational design of new ORR catalysts. 

The lower the slope, the lower the activation loss for the O2 reduction reaction in acid media, the observed slope is 0.060 V/ decade and it can be obtained theoretically by assuming the oxygen discharge step as the slowest one (the rate determining step, the r.d.s. ( 7)). For hydrogen the mechanism is different and the r.d.s. is the atomic dissociation and in this case a Tafel slope of 0.030... 

Pt is, of course, not a good electrocatalyst for the O2 evolution reaction, although it is the best for the O2 reduction reaction. However, also with especially active oxides of extended surface area, the theoretical value of E° has never been observed. For this reason, the search for new or optimized materials is a scientific challenge but also an industrial need. A theoretical approach to O2 electrocatalysis can only be more empirical than in the case of hydrogen in view of the complexity of the mechanisms. However, a chemical concept that can be derived from scrutiny of the mechanisms mentioned above is that oxygen evolution on an oxide can be schematized as follows [59] ... 

To darify better the role of Pt in the reduction mechanism, a physical mixture of the binary Pt/Y-Al2O3 (1 100 w/w) and Ba/Y-Al2O3 (20 100 w/w) samples was prepared and tested [117]. Also in this case NO was stored at 350 °C upon adsorption of NO in the presence of excess O2 [102], then the stability/reactivity of stored nitrates was investigated by means of TPD and H2 TPSR. The TPD data showed that decomposition of adsorbed nitrates occurs at temperatures very dose to that of adsorption the presence of H2 (TPSR run) does not decrease significantly the temperature threshold for nitrate decomposition (which is still observed near 350 °C), but instead provokes the reduction of the evolved NO to NH3 and N2. Hence the previously invoked Pt-catalyzed route is active only when Pt and Ba are dispersed over the same particle of the support. 

From these various observations it can be concluded that H2O2 in neutral media and H02 in basic solutions are the major products of the reduction of O2, the further 2e reduction to water being limited by a slow step. The disproportionation or reduction of the peroxide is then time dependent. It can thus be understood why the apparent number of electrons for the reduction of O2 in these media will differ with the surface state of the Pt electrode, the timescale of the experiment or the rotation rate of the electrode. There is a general agreement about the fact that H2O2 is an intermediate in O2 reduction on platinum, in a basic environment however, various reduction schemes have been proposed, with very different adsorbed intermediates and mechanisms... 

The reversible O2 potential has not been observed on palladium, due to the existence of a local cell, but O2 evolves from water oxidation on a Pd02 covered surface and the reduction mechanism is supposed to be the same as on Pt, the rate determining step being the first electron addition [31]. In basic media, the reduction on Pd proceeds mainly through the 4e mechanism, which has been attributed to a high catalytic activity for the peroxide reduction, or disproportionation [83, 84]. 

The two well-separated waves for the reduction of oxygen on mercury were reported by Heyrovsky [91]. The first wave corresponds to the 2e reduction of O2 to H2O2 in acidic or neutral solutions and to H02 in basic media, species that are reduced to water or OH at lower potentials. Jacq and Bloch have developed the square-scheme concept for the discussion of the mechanism of O2 reduction on Hg and on carbon [28]. 

Cyanide is a very weak acid and has a pAi near 10. The mechanisms of HCN and CN reacting with the metal site are completely different from each one another. Determination of the reactive form of cyanide is not trivial for understanding the function of the O2 reduction site. The report in 1948 by Stannard and Horecker should not be ignored. In the report, the inhibitory effect of cyanide on cytochrome c oxidase activity was carefully examined at various pH values and it was concluded that HCN was the reactive form. 

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