Oxygen Adsorption\nat the Electrochemical Metal/Water\nInterface: Au(111) vs Pt(111)

Reaction mechanisms in heterogeneous electrocatalysis\nhave been\nknown to change through the local field that arises at the electrified\nmetal/liquid interface. First-principles simulations specifically\npredict that such a field can significantly modify the surface binding\nof dipolar or polarizable intermediates along a catalytic reaction\npath. This dependence can then lead to a corresponding dependence\nupon applied potential even for so-called “chemical”\nreaction steps that do not involve an explicit proton-coupled electron\ntransfer such as, e.g., O<sub>2</sub> adsorption during the oxygen\nreduction reaction (ORR). And yet, such effects are only now starting\nto be systematically explored at the atomic level. In this study,\nwe directly compare the potential dependence of O<sub>2</sub> adsorption\nas the first ORR step on Au(111) vs Pt(111). Using ab initio molecular\ndynamics and free energy sampling, we find that the strong dependence\npreviously predicted at Au(111) is specifically lost at Pt(111). By\ndecoupling the O<sub>2</sub> potential response into contributions\nof different physical origin, we explain this contrast through the\ninherently different reactivity of the two metals. We finally discuss\nan indirect effect of potential through the surface coverage that\ncan promote a dissociative-type mechanism at Pt(111) by breaking the\nO–O bond already during the first step of the ORR mechanism.\nOur results thus overall suggest a more complex mechanistic picture\nthan hitherto anticipated; one where the local field may manifest\nin different, yet potentially equally important, ways over different\nsystems.