First-principles studies of vanadia-titania catalysts: Beyond the monolayer

Periodic density functional calculations have been used to investigate the structure and stability of epitaxial vanadium oxide films grown on the TiO ₂(001) anatase surface. The formation energy of films of V ₂O ₅ stoichiometry, initially low, is found to rapidly increase with the film thickness, at variance to what is obtained for reduced pseudomorphic VO ₂ films. This is in tune with results of oxygen-assisted molecular beam epitaxy. The oxidation of thick, viz. > 2 monolayers (ML), VO ₂ films yields a c(2 × 2) reconstructed surface, in agreement with low energy electron diffraction. These films are composed by partially reduced inner V atoms in a distorted-octahedral environment, and by isolated surface dioxovanadium centers exhibiting a distorted trigonal-bipyramidal coordination, Single scattering simulations of X-ray photoelectron diffraction patterns have also been performed, taking both 2- and 3-ML surface surface-oxidized films as models. Results are in fair agreement with experiments referring to films grown in oxidizing conditions, which suggests that coherent vanadia ultrathin films could be formed in vanadia-titania catalysts. The electronic structure of the films has been finally studied, finding that the terminal oxygens carried by the surface dioxovanadium species have a strong nucleophilic character, which makes them potential active centers for selective oxidation catalysis.