Four-electron transfer pathway

Normally, ORR catalyzed by Pt catalyst occurs predominately through a four-electron transfer pathway to water (or to... 

Recently, Kadish et al. synthesized three series of Co corroles (shown in Figure 4.11(D)) and investigated their catalytic activity toward the O2 reduction reaction.The mixed valent Co(II)/ Co(III) complexes, (PCY)Co2, and the biscorrole complexes, (BCY)Co2, both contain two Co(III) ions in their air-stable forms. It was foimd that all these complexes could catalyze the direct four-electron pathway for O2 reduction to H2O in aqueous acidic electrolyte. The most efficient catalysis process was observed when the complex had an anthracene spacer. The four-electron transfer pathway was further confirmed by RRDE measurement, in which only a relatively small amount of hydrogen peroxide was detected at the ring electrode in the vicinity of E1/2 0.47 V vs SCE for (PCA)Co2 and 0.39 V for (BCA)Co2. The cobalt(III) mono-corrole, (Me4Ph5Cor)Co, could also catalyze ORR at En2 = 0.38 V, with the final products being an approximate 50% mixture of H2O2 and H2O. 

Unfortunately, Ir is one of the rarest and most expensive metals which would render its use very costly. Nonetheless, the complex should serve as a suitable model, such that theoretical and experimental knowledge gained from its study may serve to tailor the synthesis of improved catalysts. Table 7.3 lists some metallomacrocyclic complexes which accomplish the reduction of oxygen via the four-electron transfer pathway in acidic electrolytes. 

Some studies have shown that certain modification procedures can be used to transform two-electron reduction metalloN4-macrocyclic complexes into hybrid materials with the capability to reduce oxygen to water, either via the direct four-electron transfer pathway or in the series two-electron transfer pathway. Carbon nanomaterials, carbon nanotubes in particular [58-65], have been reported to significantly increase the catalytic oxygen reduction current, with a substantial reduction of the overpotential for ORR reported in some cases, as shown by the examples in Table 7.4. 

FIGURE 17. Electron transfer pathways in FCSD. Through-space jumps are indicated hy dotted lines and paths along hydrogen bonds are indicated by dashed lines. Four paths (ln4) with decreasing electronic coupling are indicated. Fp and Cy indicate residues in the flavo-protein and the cytochrome subunits, respectively. 

FIGURE 18. Electron transfer pathways from the 2Fe-2S center to the flavin ring in fumarate reductase. Dotted lines represent through-space jumps. The four best paths (ln4) are indicated, all of which involve an initial transfer of an electron from the 2Fe-2S center to the SG atom of Cys57 which forms a ligand to an iron atom. 

It is of note that Kaminskaya, Konstantinov and Shuvalov also examined in detail the kinetics of low-temperature photooxidation of all four cytochromes in Rp. viridis reaction centers and established the complete heme sequence as HI, LI, H2, L2, as shown in Fig. 7, thus putting into doubt the earlier model involving two parallel H/L electron-transfer pathways, as suggested by the model in Fig. 2. 

Four crystal structures of Ni CODHs have been determined from the following organisms C. hydrogenoformans [87,88], Rhodospirillum (R.) rubrum [89] and the bifunctional CODH/ACS from M. thermoacetica [90,91]. In each case CODH has a very similar homodimeric quaternary structure with a diameter of about 100 A in the largest dimension and a total of five FeS clusters (Fig. 3A). An initially unexpected [4Fe - 4S] center, now called the D-cluster, is coordinated by the two subimits very close to the molecular surface. Each subunit also binds an additional [4Fe-4S] center, called the B-cluster, as well as the catalytic Ni-containing C-cluster. There is an electron transfer pathway between the physiological redox partner, the exposed D-cluster, then the B-cluster of one subunit and finally the C-cluster of the other subunit. The electron flow direction will depend on whether the enzyme reduces CO2 or oxidizes CO (Eq. 2). 

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