Quantum chemical elucidation of the mechanism for hydrogenation of TiO2 anatase crystals

Hydrogenation of TiO2 is relevant to hydrogen storage and water splitting. We have carried out a detailed mechanistic study on TiO2 hydrogenation through H and/or H-2 diffusion from the surface into subsurface layers of anatase TiO2 (101) by periodic density functional theory calculations implementing on-site Coulomb interactions (DFT + U). Both H atoms and H-2 molecules can migrate from the crystal surface into TiO2 near subsurface layer with 27.8 and 46.2 kcal/mol energy barriers, respectively. The controlling step for the former process is the dissociative adsorption of H-2 on the surface which requires 47.8 kcal/mol of energy barrier. Both hydrogen incorporation processes are expected to be equally favorable. The barrier energy for H-2 migration from the first layer of the subsurface O-sub1 to the 2nd layer of the subsurface oxygen O-sub2 requires only 6.6 kcal. The presence of H atoms on the surface and inside the subsurface layer tends to promote both H and H-2 penetration into the subsurface layer by reducing their energy barriers, as well as to prevent the escape of the H-2 from the cage by increasing its escaping barrier energy. The H-2 molecule inside a cage can readily dissociate and form 2HO-species exothermically (Delta H = -31.0 kcal/mol) with only 26.2 kcal/mol barrier. The 2HO-species within the cage may further transform into H2O with a 22.0 kcal/mol barrier and 19.3 kcal/mol exothermicity relative to the caged H-2 molecule. H2O formation following the breaking of Ti-O bonds within the cage may result in the formation of O-vacancies and surface disordering as observed experimentally under a high pressure and moderately high temperature condition. According to density of states analysis, the projected density of states of the interstitial H, H-2, and H2O appear prominently within the TiO2 band gap; in addition, the former induces a shift of the band gap position notably towards the conduction band. The thermochemistry for formation of the most stable sub-surface species (2HO and H2O) has been predicted. These results satisfactorily account for the photo-catalytic activity enhancement observed experimentally by hydrogenation at high temperatures and high pressures. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799800]