Proton Migration on Hydrated Surface of Cubic ZrO<sub>2</sub>: <i>Ab initio</i> Molecular Dynamics Simulation

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