Comparison of the reduction of metal oxide surfaces: TiO2-anatase, TiO2-rutile and SnO2-rutile

Reduction of stoichiometric metal oxide can be reached by two processes: oxygen vacancies or hydrogenation. We present DFT-GGA periodic calculations for the O vacancies in the bulk and selected slabs of TiO2-rutile, TiO2-anatase, and SnO2-rutile as well as their hydrogenation. We focus on the comparison between these structures. Anatase is found more difficult to reduce than rutile. Contrary to the reduced rutile structure which has a high spin state, all the electrons of the reduced anatase are paired. SnO2 is more easily reduced than TiO2. Strongly reduced (1 1 0) surfaces undergo reconstructions. Hydrogenated structures of rutile and anatase show also different patterns. While on the rutile (1 1 0) face, all the H atoms are adsorbed on the bridging O atoms in rows and form bridging OH groups, in the most stable hydrogenated anatase (1 0 0) structure only half of them are located while the other half is bound to the fivefold coordinated Ti surface atoms.