Simple Fe Porphyrins

Electrocatalytic reduction of both O2 and H2O2 starts at potentials close to the Fe / potential in the absence of a substrate (which for most porphyrins is 0.2-0V vs. NHE at pH 6 the exception being Fe(TMPyP), E 0.5 V). Catalytic reduction of H2O2 by simple ferric porphyrins is too slow to be detectable in typical electrocatalytic experiments. 

Depending on pH, increasing the acidity of the solution either makes the potential required to yield a fixed turnover frequency more oxidizing by 

60 mV/pH or does not affect it. This pH dependence is in most cases the same as that of the Fe / couple in the absence of a substrate. These identical pH dependencies suggest a pre-equilibrium between the ferric and ferrous forms of the catalyst followed by a kinetically irreversible step that does not involve proton or electron transfer, e.g., O2 binding. 

Usually simple Fe porphyrins degrade rapidly during catalytic O2 reduction. 

This reaction pathway dominates because the kinetics of 0-0 bond heterolysis is unfavorable relative to the other two competing pathways (reactions rxn8 and rxnlO). At potentials comparable to, or more reducing than, that of [X(por)Fe / (a = 1,2) the ferric-hydroperoxo intermediate is consumed in a le reduction, followed by irreversible 0-0 bond heterolysis to yield the oxoferryl (Fe ) intermediate. The flux of H2O2 decreases. 

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Hydroxyl radical is a strong indiscriminate outer-sphere oxidant (generating OH ) and H-atom abstractor (generating H2O) [Huie and Neta, 1999]. Simple Fe porphyrins are known to promote 0-0 bond homolysis in reaction with H2O2 [Watanabe, 2000]. Because of its high reactivity, once generated, "OH probably reacts with the... 

Simple Fe porphyrins whose catalytic behavior in the ORR has been smdied fairly extensively are shown in Fig. 18.9. Literature reports disagree substantially in quantitative characterization of the catalytic behavior overpotential, stability of the catalysts, pH dependence, etc.). It seems plausible that in different studies the same Fe porphyrin possesses different axial hgation, which depends on the electrolyte and possibly specific residues on the electrode surface the thicknesses and morphologies of catalytic films may also differ among studies. AU of these factors may contribute to the variabUity of quantitative characteristics. The effect of the supporting surface on... 

Figure 18.9 Chemical structures of simple Fe porphyrins whose catal3ftic properties in the ORR have heen studied extensively. These properties are tabulated in CoUman et al. [2004a].

Anaerobic cyclic voltammetry suggests that simple Fe porphyrins deposited on an electrode in contact with an aqueous buffer contain two axial water molecules (or an... 

Figure 18.11 Plausible catalytic cycle for the ORR by simple Fe porphyrins adsorbed on the electrode surface and side Reactions (18.15)-(18.18). At pH < 3, the resting state of the catalyst is assumed to be ferric-aqua.

The second mechanism often invoked to explain the increase in n y of simple Fe porphyrins at potentials more reducing than that of the Fe couple (under anaerobic conditions) is based on the fact that at such potentials the fraction of the catalyst in the 5 -coordinate ferrous state is maximal because (i) the equilibrium (18.9) is shifted completely to the ferrous form and (ii) the concentration of O2 in the catalytic film is low owing to mass transport limitations. The higher the concentration of the 5-coor-dinate ferrous porphyrin in the catalytic film, the greater the probability that any released H2O2 will re-enter the catalytic cycle by coordinating to a molecule of ferrous porphyrin and decay according to (18.13b) instead of (18.17). 

Within the mechanism in Fig. 18.11, it seems implausible that simple Fe porphyrins can be effective ORR catalysts, since large overpotentials are required to access intermediates in which 0-0 bond heterolysis is facile. The only strategy discovered so far to facilitate this 0-0 bond heterolysis in the ferric-hydroperoxo intermediate is to control both the distal and the proximal environments of Fe porphyrins. In those cases, the overpotential of ORR reduction appears to be controlled by the potential of the (por)Fe / couple (see Section 18.6). 

Figure 1.11. Typical current-potential curves for O2 reduction by simple Fe porphyrins immobilized on a graphite electrode. The simulated traces are based on data in ref. [40] for (A) Fe(TPP) (B) Fe(PPIX) (C) Fe(TPyP) (see Figure 1.10 for chemical structures) in a pH 0 electrolyte. Qualitatively similar voltammograms are observed at other pH.

Related compounds with the tris-quinoUne superstructure with and without Cu (4bFeCu and 4bFe-only) manifest electrocatalytic properties (Table 1.2) comparable to those of simple Fe porphyrins. ... 

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