Theoretical Investigation of the Hydrogenation of (TiO2)N Clusters (N=1-10)

We present density functional calculations of H-2 adsorption on small-sized titania clusters for (TiO2)(N), N = 1-10. Initial hydrogen atoms easily bind with adsorption energies larger than 10 kcal/mol only to terminal oxygen sites, resulting in a reduction of the titanium sites. The adsorption of a second hydrogen atom is poorly exothermic except on clusters of sizes N = 1, 4, and 8. The most stable hydrogenated cluster structures do not always correspond to those of the most stable naked clusters. The structures, energetics, and spin states of the most stable reduced clusters are explained as a balance of local geometry, orbital energy levels, and ionicity. On a single cluster, a step by step mechanism for H2 dissociation is investigated, proceeding through a hydride intermediate and consecutive migration. The barriers depend on the local environment of the adsorption site and are relatively high compared with the limit of atomic desorption, so other dissociation mechanisms cannot be ruled out. Contrary to rutile single crystals of titania, H migration to bulk positions is not found to be favorable for gas-phase clusters of size N = 8-10.