Mechanistic and kinetic implications on the ORR on a Au(100) electrode: pH, temperature and H–D kinetic isotope effects

Abstract

pH, temperature and H–D kinetic isotope effects (KIEs) on the ORR on Au(100) have been examined systematically using a hanging meniscus rotating disk electrode system. We found that for the cases with pH > 7, the ORR mainly goes through a 4-electron reduction to OH⁻ at E > pzc (potential of zero charge) without any pH and H–D KIEs. When the pH at the electrode/electrolyte interface (pH^s) is below 7, O₂ only reduces to H₂O₂, its activity increases with pH^s, and a H–D KIE of above 2 is observed in 0.1 M HClO₄. According to the experimental results in acid solution, a mechanism with O₂ + H⁺ + e → HO_2,ad as the rate determining step followed by decoupled electron and proton transfer steps is proposed. The high activation barrier for O–O bond breaking and the fast oxidation of H₂O₂ or HO₂⁻ to O₂ render the ORR observable only at potentials negative of the equilibrium potential (E_eq) of the redox of H₂O₂/O₂ in acidic media or of HO₂⁻/O₂ in an alkaline environment. The apparent activation energy (E_a,app) for O₂ reduction to H₂O₂ is ca. 35 ± 3 kJ mol⁻¹ and to OH⁻ is 60 ± 6 kJ mol⁻¹, while the pre-exponential factor (A) for the former is ca. 3–6 orders of magnitude smaller than that of the latter. The lower activity for O₂ reduction to H₂O₂ on Au(100) is attributed to the small pre-exponential factor.