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

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, O-2 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 O-2 + H+ + e -> HO2,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 O-2 render the ORR observable only at potentials negative of the equilibrium potential (E-eq) of the redox of H2O2/O-2 in acidic media or of HO2-/O-2 in an alkaline environment. The apparent activation energy (E-a,E-app) for O-2 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 O-2 reduction to H2O2 on Au(100) is attributed to the small pre-exponential factor.