Serial reaction pathway

The ORR is a multielectron reaction that may include a number of elementary steps involving different reaction intermediates. The detailed mechanism is still not known, since neither ex situ nor in situ techniques are capable of identifying all reaction intermediates formed under genuine reaction conditions exist. Of the various possibilities [13], it has been proposed that on metal surfaces the most plausible reaction pathway for the ORR in both alkaline and acidic electrolytes can be described by the so-called serial reaction pathway (Scheme 3.1) where after the transfer of two electrons and (simultaneous) fast protonation of superoxo/peroxo radicals (not included in the reaction scheme), O is reduced to H O (with rate constant k. ... 

Scheme 3.1 Serial reaction pathway for oxygen reduction reaction...

The catalyst performance depends on the H2 to CCI2F2 feed ratio. The selectivities to CH2F2 and CHCIF2 are influenced by the H2 to CCI2F2 feed ratio, while the selectivity to methane is independent of this ratio. We have previously proposed a reaction mechanism with serial reactions on the catalyst surface and minor readsorption of the intermediate products, which is depicted in figure 8 [4,5]. Thus the kinetics of the reaction follows mainly parallel reaction pathways, in which the selectivities are not influenced by the conversion, and a... 

In many practical cases, several reaction pathways are possible, and we may even have a complicated network of parallel and serial reactions. Here we only treat the simple cases of two parallel and two serial first-order reactions. 

Thus, this work challenges the validity of some earlier concepts on the mechanism of the ORR, and suggests that the serial pathway may be equally important, if not dominant. This shows how little, in fact, we understand about this practically important reaction. 

The key step in the reduction of oxygen at a catalytic surfece is the breaking of the 0—0 bond that requires four coupled proton and electron transfers, opening up the possibility of many side reactions and products (see Figure 2.4) [6]. The complexity of the ORR and its numerous potential side products means that it is still relatively poorly understood, although the consensus is that it proceeds either via a direct four-electron reduction pathway or via a peroxide intermediate in a 2 + 2 serial four-electron pathway [16-18]. 

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