Mechanism of ORR

Figure 1.2 Free energy map forthe associative mechanism of ORR at Pt metal and the steps considered in this mechanism [Nprskov et al., 2004].

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. 

Although impressive progress has been made in unraveling the mechanism of ORR catalysis by cofacial porphyrins, much remains to be learned before we can understand how this mechanism relates to those in heme enzymes and simple metalloporphyrins and use our mechanistic knowledge to rationally design improved metalloporphyrin catalysts for the ORR. 

A detailed mechanism of ORR catalysis for electrode-adsorbed complexes was proposed (Fig. 18.20) [Boulatov et al., 2002 Boulatov, 2004], based on determination of the following ... 

Oxygen binding to a metal can be represented by models sketched on Scheme 5, and it is understandable that the two atoms of the oxygen molecule linked to two metal atoms of the surface (bridge) is the most favorable position for weakening the 0—0 bond, leading to its dissociation. Recent ab initio calculations (DFT) have shown that the (111), (100), and (110) surfaces of Pt have different properties for the adsorption of O2, O, OH, OOH, and H2O2, which should influence differently the mechanism of ORR [39]. 

In 2001, Tarasevich and his collaborators reported a comparison between electrocatalysts for oxygen reduction prepared using a disperse synthetic diamond powder promoted with CoTMPP and its pyropolymers . Two types of diamond powders with specific area of 5.8 and 170 m /g were used as catalyst supports and the activity of the catalysts obtained with the diamond supports was compared to that obtained with the same CoTMPP precursor loaded on acetylene black. In all cases, the loading was one monolayer of CoTMPP. These authors found a much lower activity for the electrocatalysts prepared on synthetic diamonds than for that catalyst prepared on acetylene black. The kinetic mechanisms of ORR was, however, the same for both supports. 

In Chapter 2, the fundamentals of both chemical and electrochemical kinetics have been presented, which will be used as a basis for our discussion about the electrochemical kinetics and mechanism of ORR in this chapter. 

In addition to Pt—Au/C catalysts, several other Pt-based alloy catalysts, such as PtCo/C, PtNi/C, PtV/C, and PtPd/C were also reported, and the mechanism of enhancing ORR activity was investigated using both RDE and RRDE techniques. The mechanism of ORR improvement by alloying is ascribed to (1) increase in the catalyst surface roughness,(2) decrease in the coverage of surface oxides and an enrichment of the Pt-active sites of the catalyst surface,(3) increase in the d-orbital vacancy, which strengthened the Pt—O2 interaction, and (4) decrease in the Pt—Pt distance and the Pt—Pt coordination numbers. Table 7.4 lists the properties of PtCo/C and PtNi/C catalysts and their ORR performance parameters measured by RDE techniques for comparison. 

While the detailed mechanism of ORR still remains elusive [7], it is widely accepted that the ORR on platinum surfaces is dominantly a multistep four-electron reduction process with H2O being the final product. The overall four-electron reduction of O2 in acid aqueous solutions is... 

FIGURE 9.30 Schematic of the purposed mechanism of ORR at CNTs and N-CNTs. CVs display the respective steps of the ORR at nondoped CNTs, which presents two electrochemical reductions, while only one is observed at N-CNTs. (Reprinted with permission from Wiggins-Camacho, J.D. and Stevenson, K.J., Mechanistic discussion of the oxygen reduction reaction at nitrogen-doped carbon nanotubes, J. Phys. Chem. C, 115, 20002-20010. Copyright 2011, American Chemical Society.)... 

The mechanism presented above represents a series of stiuctures based on prior knowledge of the laccase system from the hterature and the XAS/FEFF8 analysis presented in this chapter. It is to some extent still unclear in areas (particularly structures II and V), but the aim was to use in situ XAS to elucidate the mechanism of ORR in laccase as it occurs on a BFC cathode. As a result, the mechanism in Figure 15.20 describes the behavior of laccase under the constraints of the experimental conditions for which the measurements were made. It is important to note that many other mechanisms have been proposed. In Particular, the Solomon and Atanassov research groups have been instmmental in providing important and detailed information on the active sites of a variety of blue copper oxidases [42,45,47-50,62,65,68-73]. 

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