The Effect of Water on the CO Oxidation on Ag(111) and Au(111) Surfaces: A First-Principle Study

Density functional theory has been used to study the effect of water on the molecular (02, 0, and CO) adsorption, O-2 dissociation, and CO oxidation on the Ag(111) and Au(111) surfaces. It is found that, though CO oxidation with atomic O is facile on both surfaces with barriers less than 0.29 eV, considerable barriers for O-2 dissociation (1.03 eV for Ag(111) and 1.97 eV for Au(111)) and weak adsorption of reactants limit their overall reactivity. Our calculations show that the reactants can be stabilized by coadsorption of water via the formation of an H bond and/or the interaction mediated through the substrates. The stabilization induced by coadsorbed water affects not only adsorption of reactants but also the transition states and intermediates, which enhances overall reactivity for CO oxidation, correspondingly. H bonds facilitate the dissociation of O-2 with reduction of barrier by 0.36 eV on Ag(111). Moreover, a highly active reaction pathway for CO oxidation via molecular assistant by water with overall barrier of 0.15 eV is identified. Atomic oxygen formed may either react with adsorbed CO or react with adsorbed water to form hydroxyls, which oxidizes CO subsequently, to complete the catalytic cycle.