Four electron pathway

The direct four-electron pathway in acid solutions is... 

Figure 6.15. Simplified schematic of the most important reaction pathways of the oxygen reduction reaction. The four-electron pathway results in the formation of water. The two-electron pathway forms hydrogen peroxide. Adsorption of molecular oxygen can form atomic oxygen (dissociative pathway) or form a superoxide species (associative pathway). The formation of Pt—OH and Pt— from water molecules represents the backward reactions of the later portion of the four-electron reduction pathway.

In this scheme the k values represent rate constants, which are generally potential dependent. It must be emphasized that the four-electron pathway does not imply the transfer of four electrons in a single step rather, it underscores the fact that all intermediate species, such as, but not restricted to peroxide, remain bound to the electrode surface yielding, upon further reduction, water as the sole product. Also depicted in Scheme 3.1 are mass transport processes (diff) responsible for the replenishment of 02 and removal of solution phase peroxide next to the interface, and the adsorption and desorption of the peroxide intermediate, for which the rate constants are labeled as ks and k6, respectively. Not shown, for simplicity, is the one-electron reduction of dioxygen to superoxide, a radical species that exhibits moderate lifetime in strongly alkaline electrolytes [15]. 

Two-electron peroxide pathway Four-electron pathway ... 

Rather surprisingly, the monocobalt 1,8-anthryldiporphyrin material yielded values for tj k and iring (see solid lines, Figure 3.62B) very similar to those found for the dicobalt counterpart (see solid lines Figure 3.62A), and consistent with the reaction proceeding mostly through a four-electron pathway however, the onset potential for the reduction was about 0.2 V more negative for the mono-, compared with the dicobalt derivative. 

The OERR is usually considered to proceed via two reaction pathways, namely, the peroxide and the direct four-electron pathways. 

The direct four-electron pathway involves no hydrogen peroxide formation in the solution. This fact, however, does not preclude the participation of an adsorbed peroxide intermediate in the course of the reaction. The distinction between both reaction pathways is usually investigated by the rotating ring-disc electrode technique [55]. From the rotation speed and potential dependence of the disc electrode to ring electrode current ratio, it is possible to determine the relative contribution of each reaction pathway to the overall reaction [56]. 

The kinetics of the OERR on carbon [27] and graphite [27] in alkahne solution has been explained in terms of the dominant contribution of the peroxide reaction pathway. On the other hand, the direct four-electron pathway predominates on graphite electrodes modified by adsorbed tetrasul-fonated phtalocyanine [57] and attached face-to-face di-cobalt-porphyrin complexes [58]. In principle, when both pathways operate simultaneously on a given surface, the kinetics is referred to as involving a parallel mechanism [59]. 

Bimetallic pathways were implicated in the formation of both RuIV=0 and Ruvi(0)2. There is no doubt that rich chemistry of ruthenium and osmium in their reactions with O2 is ripe for additional exploration, and will provide new insights into the mechanisms of 0-0 bond breaking with concomitant generation of relatively stable high-valent metal-oxo intermediates. Furthermore, these metals provide the best opportunity for true monometallic activation of dioxygen via a formally four-electron pathway affording the M" + 4(0)2 metal-oxo species. This opportunity has to be pursued. 

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... 

The electrocatalytic reduction of oxygen according to the overall equations following a direct four-electron pathway (Eqs. 9-6 and 9-7) ... 

The reduction of oxygen is itself a complex electrochemical process. As described in Section 9.2, O2 reduction is considered to proceed along two parallel pathways, the direct four-electron pathway and the peroxide pathway (Eq. 9-9). Both pathways... 

In alkaline solution the peroxide pathway is dominant and relatively fast. Organic or inorganic impurities at the surface favor this pathway. Generally, Pt is the most active catalyst. The reduction kinetics are faster in concentrated KOH or NaOH than in concentrated H3PO4 or H2SO4. At highly dispersed Pt on carbon support, the reduction occurs predominantly via the four-electron pathway. 

The reduction of oxygen is one of the most studied reactions in electrochemistry. On a first glance there are two possible pathways for oxygen reduction in acid electrolytes the so called four electron pathway leading to the formatiOTi of water. 

In spite of the considerable effort expended in trying to unravel the fundamental aspects of the O2 electroreduction reaction, many details about the mechanism are not fully understood. The electrochemical reduction of oxygen is a multielectron reaction that occurs via two main pathways one involving the transfer of two electrons to give peroxide, and the so-called direct four-electron pathway to give water. The latter involves the rupture of the 0-0 bond. The nature of the electrode strongly influences the preferred pathway. Most electrode materials catalyze the reaction via two electrons to give peroxide Peroxide pathway in acid... 

Electrodes modified with films obtained from polymerization of CoTAPc (cobalt tetraamino phthalocyanine) show two waves for the reduction of O2. At low polarizations two-electron reduction waves are observed with the formation of peroxide and a four-electron reduction to form water is observed at higher polarizations It is not clear why these polymeric films can promote the four-electron pathway. It can be argued that within the polymeric films Co atoms belonging to different MN4 units become separated by the ideal distance for O2 to bind to two cobalt centers forming a bridge. However there are no detailed studies of the structures of these polymeric films to clarify this point. A similar behavior has been observed with adsorbed layers of vitamin B12 where a four-electron reduction mechanism is observed at high polarizations " . In all these cases, it is likely that O2 interacts with one single site at the time and vitamin B12 is still able to promote four-electron reduction process. 

The ORR mechanism expressed by Reactions (4-V)—(4-X) may be too complicated to be quantitatively treated to obtain the reaction rate expression. In order to obtain the relationship between the current and the electrode potential, some reasonable assumptions may be made to simplify this mechanism. We may make two assumptions, one is to assume x = 0 so that the whole mechanism only goes through a four-electron pathway to produce water (this may be true for ORR catalyzed by Pt), and the other is to combine the fast Reactions (4-DQ and (4-X) together into one reaction. In this way, the ORR mechanism may be simplified as the following three reactions ... 

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. 

The ORR is a multi-electron reactiOTi that includes a number of elementary reactions. Yeager proposed two pathways for the ORR in acidic medium [36] (1) a direct four-electron pathway where O2 is reduced directly to water without involvement of hydrogen peroxide (H2O2), O2 + 4H" + 4e 2H2O and (2) a series pathway in which O2 is reduced to H2O2, O2 + + 2e —> H2O2,... 

Liu et al. prepared carbon-supported zirconium oxynitride (ZrO jNy/C) by ammonolysis of carbon-supported zirconia (Zr02/C) at 950°C [81]. The onset potential of the TxO ylC for the ORR was 0.7 V vs. RHE, and the four-electron pathway for the ORR was achieved on the surface of the ZrO cN3,/C. The maximum power density of the single cell using the ZrO N C as a cathode at 80°C was 50 mW cm, which was much lower than that of a single ceU using commercial Pt/C as a cathode. The enhancement of the catalytic activity is required to obtain a superior single cell performance. 

ORR kinetic currents at a given potential than those on the individual components, namely, CoPc/C, CoPcFig/C, or Ag/C. The RDE and RRDE measurements indicate that the ORR on these new catalysts occurs almost entirely via a four-electron pathway and shows 50 mV lower overpotentials for CoPc Ag/C and 82 mV for CoPcFi6 Ag/C as compared to that found on Ag/C catalyst. The current densities are higher, and Tafel slopes are lower for ORR on CoPc Ag/C and CoPcFi6 Ag/C than on Ag/C catalysts in the high overpotential region. 

---