The Subsurface Diffusion of Hydrogen on Rutile TiO₂ Surfaces: A Periodic DFT Study

Titanium oxide (TiO₂) is an oxide with a wide range of technological and industrial applications. The interaction of hydrogen with TiO₂ surface is an important step in many catalytic processes, such as spillover or water photosplitting. In the last years, the role of the subsurface hydrogen has been invoked many times to explain different phenomena, from black titania to reversible storage. In this work, we systematically investigate the paths for hydrogen diffusion from surface into subsurface focusing on the surface topology, the thermal and isotopic effects, and the degree of reduction of the substrate, by means of state of the art periodic DFT calculations. We find differences in the behavior of the rutile TiO₂ surfaces (001), (100) and (110). Reaction energies are lightly exothermic, from − 0.02 to − 0.15 eV, and activation energies range from 0.39 to 1.00 eV. Deuterium diffusion from rutile (110) surface to the bulk, as well as thermal effects, were found to affect less than 0.04 eV the energy profile. On the contrary, the degree of reduction of the rutile (110) was found to noticeably decrease the activation barrier down to 0.8 eV for the (110) slab. We analyze structural, electronic and reactivity parameters that affect the kinetic barriers for the surface to subsurface H diffusion, and conclude that topology and reduction degree are valid strategies to tune the surface-to-bulk migration process.