Structure-relationships in electrocatalysis: oxygen reduction and hydrogen oxidation reactions on Pt(111) and Pt(100) in solutions containing chloride ions

Kinetics of the oxygen reduction reaction (orr) and the hydrogen evolution-oxidation reactions (her/hor) were studied on the Pt(111) and Pt(100) surfaces in 0.05 M H2SO4 containing Cl-. The orr is strongly inhibited on the (100) surface modified by adsorbed Cl- (Cl-ad), and it occurs as a 3.5e(-) reduction via, solution phase peroxide formation. In the hydrogen adsorption (H-upd) potential region, the orr is even more inhibited, and corresponds only to a 2 e(-) reduction at the negative potential limit where the electrode is covered by one monolayer of H-upd and some (unknown) amount of Cl-ad. On the Pt(111)-Cl-ad surface, the orr is inhibited relatively little tin addition to that caused by strong bisulfate anion adsorption on this surface), and the reaction pathway is the same as in Cl- free solution. The kinetics of the hor on Pt(111) are the same in pure solution and in a solution containing Cl-, since Cl-ad does not affect platinum sites required for the breaking of the H-H bond. A relatively large inhibition of the hor is observed on the (100) surface, implying that strongly adsorbed Cl-ad is present on the surface even near 0 V. (C) 2001 Elsevier Science B.V. All rights reserved.