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.