Single-electron reduction pathway, process

Since only one electron is involved in this reduction process, this mechanism is called single-electron reduction pathway. Apparently, in this single-electron... 

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]. 

The concept of the one-electron transfer scheme provides a common feature to the seemingly unrelated reductive transformation pathways presented in Section 3.B. Each of these processes occurs initially by transfer of a single electron in the ratedetermining step. In each case, a radical anion is formed that is more susceptible to reduction than the parent compound. 

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. 

While a wide variety of reduction mechanisms have been proposed, it is unanimously accepted that, whatever the pathway of the process is, the first step consists of a single electronation of the CO2 molecule, forming an anion radical (Eq. 1) [8,9] ... 

Figure 4. Energy map for various possible pathways for a complex process (dioxygen reduction in acid solution), as a function of 0-0 and H-O bond coordinates, admitting only single sequential electron transfer, electron-atom transfer, or atom transfer steps.

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