Effect of alkali doping on a V2O5/TiO2 catalyst from periodic DFT calculations

The role of alkali (Li, Na, K) and H in a vanadia/titania catalyst model is discussed in terms of geometry, electronic structure, and reactivity. Neutral alkali interact with several oxygen sites involving active phase and support. The vanadyl VO groups are affected by the presence of such dopants and elongate with respect to the undoped systems. Neutral alkali adsorption takes place through an electron transfer from the adatom to the surface vanadium atoms, pushing the Fermi level to higher energies. Alkali-doped slabs show a state in the gap lying 0.25 eV below the conduction band. This state appears for H, Li, and K only after using the GGA+U method. Undoped slabs are more active (higher adsorption energies) than the doped ones for electrostatic (methanol adsorption) and redox (hydrogenation) processes. Methanol adsorbs dissociatively, and the adsorption energy becomes less exothermic in the series H-Li-Na-K which corresponds to an increase in the basicity of the slabs and to a decrease of the hardness of the alkali dopant. Hydrogenation takes place by electron transfer to the vanadium sites and becomes less exothermic in the series H-Li-Na-K. Bridging V-O-Ti sites are preferred to vanadyl VO sites in both methanol and hydrogen adsorption final structures.