In the first step, O2 in gas phase captures a proton and is reduced to OOH*. This is followed by the non-electrochemical dissociation of OOH* into O* and OH*. Then, O* interacts with a transferred proton-electron (H + + e -) pair to give OH*. During the final (H + + e -) transfer steps, the remaining two OH* molecules are further hydrogenated to H2O.