Temperature and pressure effects on surface processes at noble metal electrodes. Part 2.—Volume of adsorbed H and oxygen species at Pt and Au

Abstract

The multiple states of electrosorbed H and oxygen species at Pt, which arise below monolayer coverage, are of current interest in electrochemical surface science.

The results of experiments on the effects of high hydraulic pressure on H chemisorption and surface oxide formation at Pt electrodes are described. “Clean” electrochemical surface studies can be performed in a shrinkable Teflon vessel, under oil, up to several thousand bars. The equilibrium volume changes for electrodeposition of H and surface oxidation of Pt are evaluated allowing for the effect of pressure on the potential of the reversible reference electrode used. The volume of electrodeposited H is found to be ≈ 5.1 cm³ mol^{– 1} at Pt, a value similar to that for H in methylene groups.

The pressure effects on surface oxidation arise from (a) the volume change in the surface oxidation reaction itself and (b) the volume change in desorption of specifically adsorbed anions which controls, in part, the potential for onset of surface oxidation of Pt and Au electrodes. Semi-quantitative interpretations of the observed behaviour are offered.