Oxygen Adsorption at the Electrochemical Metal/Water Interface: Au(111) vs Pt(111)

Reaction mechanisms in heterogeneous electrocatalysis have been known to change through the local field that arises at the electrified metal/liquid interface. First-principles simulations specifically predict that such a field can significantly modify the surface binding of dipolar or polarizable intermediates along a catalytic reaction path. This dependence can then lead to a corresponding dependence upon applied potential even for so-called ‘chemical’ reaction steps that do not involve an explicit proton-coupled-electron-transfer such as, e.g., O2 adsorption during the oxygen reduction reaction (ORR). And yet, such effects are only now starting to be systematically explored at the atomic level. In this study, we directly compare the potential dependence of O2 adsorption as the first ORR step on Au(111) vs Pt(111). Using ab initio molecular dynamics and free energy sampling, we find that the strong dependence previously predicted at Au(111) is specifically lost at Pt(111). By decoupling the O2 potential response into contributions of different physical origin, we explain this contrast through the inherently different reactivity of the two metals. We finally discuss an indirect effect of potential through the surface coverage that can promote a dissociative-type mechanism at Pt(111) by breaking the O-O bond already during the first step of the ORR mechanism. Our results thus overall suggest a more complex mechanistic picture than hitherto anticipated; one where the local field may manifest in different, yet potentially equally important, ways over different systems.