The combined experimental and theoretical study of intrinsic hydrogen diffusion on bridge-bonded oxygen rows of TiO2(110) was presented. Sequences of isothermal scanning tunnelling microscopy images demonstrated a complex behavior of hydrogen formed by water dissociation on bridge-bonded oxygen vacancies. Different diffusion rates were observed for the two hydrogens in the original geminate OH pair suggesting the presence of a long-lived polaronic state. For the case of separated hydroxyls, both theory and experiment yield comparable temperature-dependent diffusion rates. Density functional theory calculations showed that there were two comparable low energy diffusion pathways for hydrogen motion along the bridge-bonded oxygen from one bridge-bonded oxygen to its neighbor, one by a direct hop and the other by an intermediate minimum at a terrace O. The values of kinetic parameters (prefactors and diffusion barriers) determined experimentally and theoretically were significantly different and indicate the presence of a more complex diffusion mechanism. It was speculated that the hydrogen diffusion proceeded via a two-step mechanism: the initial diffusion of localized charge, followed by the diffusion of hydrogen. Both experiment and theory indicated the presence of repulsive OH-OH interactions.

Intrinsic Diffusion of Hydrogen on Rutile TiO2(110). Li, S.C., Zhang, Z., Sheppard, D., Kay, B.D., White, J.M., Du, Y., Lyubinetsky, I., Henkelman, G., Dohnálek, Z.: Journal of the American Chemical Society, 2008, 130[28], 9080-8