Ab initio study of surface acid-base reactions. The case of molecular and dissociative adsorption of ammonia on the (011) surface of rutile TiO2

The interaction of ammonia molecules with Lewis acid centers (Ti4+ metal ions) of the (011) surface of rutile TiO2 is investigated by density functional theory in order to understand, from first principle, the nature of acid-base reactions on solid surfaces. Unlike the rutile (110) surface that contains alternating rows of 5-fold and 6-fold Ti atoms, all Ti atoms of the (011) surface are 5-fold coordinated. This surface has shown considerable activity for numerous chemical reactions and is thus an ideal prototype. At 1/2 monolayer coverage, with respect to surface Ti atoms, the adsorption energy is found to be equal to 100 kJ mol-1, and drops to 58 kJ mol-1 at one monolayer coverage. Analysis of the electronic density of states (DOS) revealed information regarding the mode of adsorption. In particular, the nitrogen 3a1 and 2a1 orbitals appear to undergo significant changes upon adsorption, in agreement with photoelectron spectroscopy studies. Dissociative adsorption was also investigated on the same surface. Both NH2(Tis) + H(Os) and NH(Tis) + 2H(Os) modes of dissociative adsorption, where s stands for surface, are found to be less stable than the molecular (non dissociated) adsorption.