Potential–pH diagrams for hydroxyl and hydrogen adsorbed on a copper surface

Abstract

The principle of equilibrium potential–pH diagrams for two-dimensional layers adsorbed on metal surfaces in aqueous solution is applied to the case of hydrogen and oxygen species electroadsorbed on copper. The conditions of adsorption of species on copper surfaces in water may be predicted from thermodynamic calculations of the equilibrium potentials of the dissolved species/adsorbed species couples and drawing of potential–pH diagrams. Standard chemical potentials for hydroxyl chemisorbed on Cu(1 1 1) are derived from recent electrochemical data, while those for hydrogen chemisorbed on Cu(1 1 1) are derived from gas phase adsorption data. The E–pH relations associated with the equilibria between water and adsorbed hydroxyl, and between solvated protons and adsorbed hydrogen are given. The possible diagrams showing the E–pH domains of thermodynamic stability of the species adsorbed on a copper (1 1 1) surface (H 2O ads, OH ads and H ads) in water are calculated at 25 °C and superimposed to the usual Pourbaix diagram for the Cu–H 2O system.