Oxygen Reduction and Hydrogen Oxidation Reactions on Pt(111) and Pt(100) in Solutions Containing Copper Ions

The effects of underpotentially deposited copper (Cu upd ) on the kinetics of the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) have been studied at Pt(111) and Pt(100) surfaces in 0.05 M H 2 SO 4 containing either Cu 2+ or mixtures of Cu 2+ and Cl - ions. The results show that Cu upd has a strong inhibiting effect on both the ORR and the HOR. The deactivation is linked to an enhanced anion adsorption on platinum atoms adjacent to Cu upd atoms. This mechanism explains the anomalously large inhibition by a very small amount of Cu upd . The ORR on Cu upd -modified electrodes proceeds mostly as a 4e - reaction pathway with negligible solution phase peroxide formation, ca. 3%. The physical model that appears to realize this result is one in which the active sites for the adsorption of molecular O 2 are a small number of platinum islands which are created when desorption of anions takes place from Cu upd -unmodified platinum sites. The same sites are also active centers for the HOR.