Hydrogen peroxide and oxygen reduction studies on Pt stepped surfaces: Surface charge effects and mechanistic consequences

Abstract

The hydrogen peroxide reduction reaction (HPRR) is investigated on Pt(111) as well as on Pt(S)[(n-1)(111) × (110)] and Pt(S)[n(111) × (100)] stepped surfaces in 0.1 M HClO4 by means of voltammetric studies using the hanging meniscus rotating disk electrode (HMRDE) configuration. Results point out that there is a direct correlation between the potential value (Einhibition) at which the inhibition of the HPRR at low potential starts and the local potential of maximum entropy (pme) for the terraces. In addition, there is also a correspondence between the potential of the peak (Epeak) that appears at E < 0.3 V vs. RHE for stepped surfaces and the local pme for the steps. Additional experiments for stepped surfaces from acidic to neutral pH confirm this correlation since Einhibition shifts 0.059 V per pH units towards more positive potentials, which is the same observed behavior for the pme from laser-induced temperature-jump experiments. However, adsorbed OH can influence this trend when the pme values are near the region of adsorption of these species. The effect of surface charge on the structure of interfacial water can also influence the current inhibition as inferred from measurements in alkaline media. Finally, ORR measurements and the differences observed with the HPRR results in the same conditions suggest that the formation of H2O2 intermediate is less favored as the pH is increased, and therefore a previous bifurcation point in the mechanism should exist.