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

The proton migration on a cubic ZrO 2 (110) surface is investigated by ab initio molecular dynamics simulation. H 2 O molecules form a hydrated multilayer on a ZrO 2 surface consisting of terminating H 2 O adsorbates and hierarchically hydrogen-bonded H 2 O layers. A portion of H 2 O molecules chemisorbed on zirconium atoms (Zr–OH 2 ) dissociates into H + and OH –, forming polydentate and monodentate hydroxyls (>OH + and Zr–OH – ). The coexistence of acid and base sites (Zr–OH 2 and Zr–OH – ) in the equilibrium state is confirmed by analyses of both forward and reverse reactions of H 2 O dissociation on the ZrO 2 surface. Proton hopping from Zr–OH 2 to Zr–OH – occurs by both a direct proton transfer and a chain protonation reaction via surrounding H 2 O molecules. During these processes, Zr–OH 2 donates an extra proton to Zr–OH – directly or via H 2 O molecules in the multilayers, indicating that the coexistence of Zr–OH 2 and Zr–OH – is a necessary condition for the proton conduction on the oxide surface with various basicities.