Periodic ab initio study of the hydrogenated rutile TiO2(110) surface

We have performed periodic ab initio calculations to study the adsorption of hydrogen atoms on TiO2 (110) rutile surface. Two adsorption modes have been considered on slabs. Even if the presence of atomic hydrogen on the surface does not arise from the H-2 dissociation. we will refer to the H-H cleavage for the analysis. Hence, in the H mode, the heterolytic cleavage of H-2 yields to H+ and H. The proton interacts with the most basic oxygen atoms of the surface, while H- adsorbs on a metal cation which is the Lewis acidic site. In the R mode, two H fix on top of the outer surface anions and two Ti(IV) are reduced to Ti(111). Our calculated adsorption energies values show that the R mode is the most favorable. The surface reduction is only apparent through the spin polarization and does not lead to a full electron charge transfer from the hydrogen atoms to the surface. The change of the electronic State and of the adsorption strength with the slab thickness is presented. Using NH3 as a probe molecule to test the surface acidity, we show that the Ti reduction modifies its reactivity. Surface relaxation significantly contributes to increase the adsorption strengths; however it does not modify the qualitative conclusions. (C) 2001 Elsevier Science B.V. All rights reserved.