Reduction of the (001) surface of γ-V2O5 compared to α-V2O5

The defect-free gamma-V2O5(001) surface and ordered structures of oxygen vacancies have been studied for a wide range of defect concentrations, Theta ((1)/(6) monolayer (ML) less than or equal to Theta less than or equal to 1 ML), combining density functional theory and statistical thermodynamics. The gamma polymorph Of V2O5 is characterized by two structurally different vanadium sites, V-A and V-B. The V-A sites having a weaker bond to an adjacent crystal layer are easier to reduce. Up to (1)/(2) ML, the V-A defect structures with defects aligned along the [010] direction are increasingly more stable as in alpha-V2O5(001). At higher defect concentrations, the different coordination of the V-B vanadium atoms at the gamma-V2O5 surface causes an increase in the vacancy formation energy of similar to0.8 eV/atom at Theta = 1.0 compared to Theta = (1)/(2). For alpha-V2O5, this increase amounts to 0.2 eV/atom only. Under conditions (low oxygen partial pressures and high temperatures) at which the alpha-V2O5(001) surface would be fully reduced, the gamma-V2O5-(001) surface is only partially reduced. The presence of surface vanadyl oxygen groups at VB sites may change the surface reactivity compared to that of alpha-V2O5(001).