Proton Migration on Hydrated Surface of Cubic ZrO₂: Ab initio Molecular Dynamics Simulation

The proton migration on a cubic ZrO₂ (110) surface is investigated by ab initio molecular dynamics simulation. H₂O molecules form a hydrated multilayer on a ZrO₂ surface consisting of terminating H₂O adsorbates and hierarchically hydrogen-bonded H₂O layers. A portion of H₂O molecules chemisorbed on zirconium atoms (Zr–OH₂) dissociates into H⁺ and OH^–, forming polydentate and monodentate hydroxyls (>OH⁺ and Zr–OH^–). The coexistence of acid and base sites (Zr–OH₂ and Zr–OH^–) in the equilibrium state is confirmed by analyses of both forward and reverse reactions of H₂O dissociation on the ZrO₂ surface. Proton hopping from Zr–OH₂ to Zr–OH^– occurs by both a direct proton transfer and a chain protonation reaction via surrounding H₂O molecules. During these processes, Zr–OH₂ donates an extra proton to Zr–OH^– directly or via H₂O molecules in the multilayers, indicating that the coexistence of Zr–OH₂ and Zr–OH^– is a necessary condition for the proton conduction on the oxide surface with various basicities.