Density functional theory calculations and free energy simulations show a spontaneous charge separation of hydrogen atoms at metal-water interfaces, where a proton is solvated in the water structure, and an electron is donated to the metal surface. Moreover, the presence of water gives rise to a significant metal-to-O₂ charge transfer, increasing the adsorption strength as compared to the adsorption in the absence of water.

Abstract

Reactions at the metal-water interface are essential in a range of fundamental and technological processes. Using Density Functional Theory calculations, we demonstrate that water substantially affects the adsorption of H and O₂ on Cu(111), Ag(111), Au(111), Pd(111) and Pt(111). In water, H is found to undergo a spontaneous charge separation, where a proton desorbs to the water solution while an electron is donated to the surface. The reaction is exothermic over Au and Pt and associated with low barriers. The process is facile also over Pd, albeit slightly endothermic. For O₂, water is found to increase the metal-to-adsorbate charge transfer, enhancing the adsorption energy and O−O bond length as compared to the adsorption in the absence of water. The magnitudes of the effects are system dependent, which implies that calculations should treat water explicitly. The results elucidate previous experimental results and highlights the importance of charge-transfer effects at the metal-water interface; both to describe the potential energy landscape, and to account for alternative reaction routes in the presence of water.