Vanadia and Water Coadsorption on Tetragonal Zirconia Surfaces

Abstract

The surface structure of vanadia supported on zirconia is examined by density functional theory with periodic boundary conditions. Adsorption of V2O5 and H2O on the (101) and (001) surfaces of tetragonal zirconia is studied for different loadings up to (V2O5)4 or V2O5·6H2O per eight Zr surface sites (monolayer coverage). The considered surface species include vanadia monomers, dimers, and polymers on water-free surfaces as well as different combinations of coadsorbed vanadia, hydroxyls, and water. Calculated vibrational spectra do not show unique features that could be used for identifying certain surface species. Statistical thermodynamics is used to evaluate the relative stability of different surface structures as function of vanadia loading, water partial pressure, and temperature (surface phase diagrams). The presence of water has a strong influence on the surface species. Under water free conditions and for Θ(V2O5) = 0.25 vanadia is present as a monomeric species on the (001) surface and as dimeric species on the (101) surface. For water partial pressures typical of oxidative dehydrogenation of propane (10−2 bar, 775 K), hydrolysis of the dimeric species on the (101) surface is observed. The presence of water also affects the reactivity in partial oxidations as characterized by oxygen defect formation or hydrogen attachment energies. At Θ(V2O5) = 0.25 the presence of water stabilizes less reactive species, whereas at high vanadia coverage more reactive species are present.