Associative Mechanism of the ORR

The ORR proceeds in four or five steps. Four of these elementary steps involve proton and electron transfer. The main ORR mechanisms are known as dissociative and associative mechanism. In the dissociative mechanism, the oxygen molecule first adsorbs onto the metal and then dissociates by the breaking of the 0-0 bond. Dissociation of O2 is followed by the transfer of two electrons and two protons to form two adsorbed OYiad- Another coupled proton-electron transfer process transforms each of the OH into water. However, DFT studies have shown that direct dissociation of O2 has an activation barrier of 0.5 eV, rendering this process an unlikely reaction step (Hyman and Medlin, 2005). 

The subsequent treatment focuses on the associative mechanism of the ORR. In this mechanism, O2 first adsorbs on the metal surface, followed immediately by the first proton-electron transfer to form an adsorbed superoxide intermediate. 

OORad is reduced further via two alternative pathways. In the first pathway, two subsequent steps transform OORad into Oad and ORad through a chemical step. 

Oflrf + H+ + OHorf (step 3). The second pathway involves a step 

Continuing with the first pathway through step 2 and step 3, two identical reactions involving the last two concerted proton-electron transfers transform adsorbed 

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