Proposed ORR Mechanism

FIGURE 15.17 Comparison of uMET Afi= (0.TV, O2) - fiiO.SV, Ar) (upper line) with simulated for the structure shown. Afit curve (lower line) was offset by (ApX4) + 0.2 and shifted -5 eV for visualization. (Reproduced with permission from Ref. [74]. Copyright 2014, Wiley-VCH.) 

FIGURE 15.18 Cu k-edge XANES of laccase and (a) RPA and (b) RPB plotted as a function of electtol3tte saturation gas at the indicated fixed potential, (c) Comparison plots of A / spectra calculated using the XANES in (a) and (h), A /=- / (Ar), with the A//t simulations of the clusters depicted on the right. Visualization offsets used in (c) were Ap -1-0.13 and - 1.5eV (onefold O-l-Onat) (top curves A /J, A /-1-0.09 (RPB, bottom curves), and A /-1-0.09 and E—5.5 eV for O2 parallel simulation. (Reproduced with permission from Ref [74]. Copyright 2014, Wiley-VCH.) 

FIGURE 15.19 Diagram of laccase T. versicolor) illustrating the close proximity of the T3 coppers to the larger electrolyte channel. 

If XAS were employed to track the reaction progress (assuming A contains the element of interest for XAS characterization), AB will not be detected as XAS is bulk 

FIGURE 15.20 Proposed ORR mechanism. (Reproduced with permission from Ref. [74]. Copyright 2014, Wiley-VCH.) 

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Proposed ORR Mechanisms and Their Analysis Based on the RRDE Data 216... 

Figure 7.5 (A) Oxygen reduction and hydrogen peroxide oxidation on a bare pyrolytic graphite disk-Pt ring electrode (B) Oxygen reduction and hydrogen peroxide oxidation on an oxidized pyrolytic graphite disk-Pt ring electrode in Oa-saturated 0.1 M KOH and (C) Proposed ORR mechanism. Potential scan rate 5 mV s ring potential 1.34 V vs 1 M NaOH//Hg/HgO. Reprinted with permission from Ref. 24.

Figure 7.7 (A) Oxygen reduction on a bare GC disk electrode, and (B) peroxide oxidation on an Au ring electrode of RRDE at different electrode rotating rates in Oa-saturated 0.1 M KOH potential scan rate 10mVs ring potential 0.05 V vs SCE and (C) proposed ORR mechanism. ...

Fig. 20.2 Left, proposed ORR mechanism on bimetallic surfaces, where Ma atoms red) favor OOH formation, and Mb atoms (blue) favor O and OH reduction. Right simplified model of the ORR on the heme/Cu site of cytochrome-c oxidase metalloenzyme (adapted from [80])...

F. 5 Proposed ORR mechanism on a dicobalt cofacial dipoiphyrin. Adapted with permission from [106]. Copyright 2004 American Chemical Society... 

Figure 15.6. Proposed ORR mechanism for M02N/C catalyst [81]. (Reprinted from Electrochemistry Communications, 8(5), Zhong Hexiang, Zhang Huamin, Liu Gang, Liang Yongmin, Hu Jingwei and Yi Baolian, A novel non-noble electrocatalyst for PEM fuel cell based on molybdenum nitride, 707-12, 32006, with permission from Elsevier.)...

In attempts to simulate the experimentally observed oxygen reduction reaction, several kinetic models have been reported in the literature, based on various proposed ORR mechanisms [38—41]. These models employed either the associative or the dissociative model. The former was used by Antoine et al. [38] in their simulation of a polarization curve and impedance spectra, and by Du et al. [40] in a model-based electrochemical impedance spectroscopy study. Antoine et al. considered the following reaction steps ... 

The proposed ORR mechanism for Li+-containing aprotic solvents breaks in two parts, depending on the strength of catalyst-oxygen surface bonds, as shown in Figure 4.16. For catalysts with a weak catalyst-oxygen bond, Li202 is expected to be the major reaction... 

FIGURE 4.16 Proposed ORR mechanism for Li -containing aprotic solvents. Reprinted from Ref. [172], with permission from the Electrochemical Society. 

Let us consider the oxygen reduction reaction (ORR) that occurs in the cathode of the polymer electrolyte membrane fuel cell (PEMFC), in an acidic environment. Although a variety of ORR mechanisms have been proposed, the four-electron pathway is primarily used to characterize the behavior of this reaction at a platinum electrode or a glassy carbon electrode coated with a platinum-based catalyst. The overall reaction is given by... 

Generally, the ORR mechanism on carbon materials is complicated, involving several elementary steps. The first step was thought to involve adsorption and electron transfer to form superoxide (OJ). Following this, either further reduction or disproportionation of superoxide would happen. A step involving a transformation between two different forms of adsorbed superoxide was also proposed. In alkaline solutions... 

ORR is probably one of the most widely studied processes due to its important applications. In literature, the ORR mechanisms have been proposed and extensively studied using RRDE technique. Here, we take the RRDE measurements of ORR as examples to illustrate the RRDE data analysis methods. 

An ORR mechanism was proposed as shown in Figure 7.5(C), based on which and the approach discussed in Chapter 6 of this book, the relationships of Af vs and N vs were formulated and the corresponding plots, as examples, are shown in Figure 7.6 (note that the expression of current densities in this figure are different from the expressions we used in Chapter 6). From the slopes and the intercepts of the plots in Figure 7.6, the ORR constants can be obtained. For example, the obtained values for kz at different potentials were in the range of 1.0 x 10 to 1.1 X 10 cm s when potential was changed from 0.2 to 0.7 V. 

As discussed above, the ORR mechanism catalyzed by the Co-N/C catalyst was a mixed process of 2- and 4-electron-transfer pathways, dominated by the 4-electron pathway. An ORR mechanism was proposed in this work as follows ... 

The ORR mechanism on Pt has also been investigated by theoretical calculation based on the electronic structure, using density functional theory [43 5]. The dissociative mechanism and the associative mechanism are proposed for a low current density range and a high current density range, respectively [43,44] ... 

Three new criteria were proposed [Collman et al., 2003a] to establish that the four-electron reduction of O2 by a catalytic film represents the selectivity of an individual molecule of the catalyst (i) independence of the i /i ratio on the catalyst surface coverage (ii) much higher stabUity of the catalyst in the ORR compared with reduction of H2O2 under conditions that reproduce the concentration of H2O2 in the film that would be generated if ORR proceeded by a step-wise mechanism and (iii) the nature of the turnover-determining step. 

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 ... 

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