Oxygen reduction on electrode surfaces modified by foreign metal ad-atoms: Lead ad-atoms on gold

Abstract

Oxygen reduction on gold is considerably catalysed by foreign metal ad-atoms. The catalytic effects of lead have been studied in more detail as most illustrative. The two-electron reduction of O 2 to HO 2 − on Au changes into a four-electron process on Au modified by lead. In the potential region where AuOH constitutes the surface, the interaction of Pb ions with AuOH causes catalytic effects. At more negative potentials, on bare Au surface, the underpotential deposition of Pb ad-atoms gives rise to the catalytic effects. At AuOH surface modified by Pb ions the O 2 reduction involves a “series” mechanism, with only minute quantities of HO 2 − leaving the electrode surface. The reduction of HO 2 − is considerably catalysed. The mechanism of this reaction is changed from the rate-determining chemical step into the charge-transfer rate-determining step. The rate-determining step for O 2 reduction involves the first charge transfer: O 2+e→O 2 −(ads) The mechanism of HO 2 − formation is uncertain, while its reduction most probably involves a direct process. There are indications that on Au surface with Pb ad-atoms a “parallel” mechanism may be operative. The catalytic effect originates in the interaction of Pb 2+ with AuOH surface, which considerably reduces a partial negative charge on OH. Such a surfaces, as well as that of Au covered by Pb ad-atoms, are more suitable for adsorption of O 2, O 2 − and HO 2 − which considerably alters the free energy of adsorption of these species.