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.