Vanadium Oxides on Aluminum Oxide Supports. 3. Metastable κ-Al₂O₃(001) Compared to α-Al₂O₃(0001)

Low-coverage vanadia species (monomers, dimers, trimers, and one-dimensional vanadia rows) as well as vanadium oxide films of varying thickness supported on the metastable κ-Al2O3(001) surface are investigated by density functional theory in combination with statistical thermodynamics. At low-vanadium chemical potentials and typical reducing conditions, species with V−O(3)−Al interface bonds are stable. These aggregates are partially reduced with vanadium in the VIII oxidation state. This correlates with defect formation energy values for the initial removal of lattice oxygen in the range of 1.3−2.7 eV. As the length of the polymeric species increases, the reduction energy decreases. We demonstrate that the support structure does affect the structure of the model catalyst and the lattice oxygen bond strength. On the α-Al2O3(0001) surface, the only stable low-coverage VOx species are dimers with V−O(2)−Al interface bonds and a defect formation energy of 2.8 eV. Reduction remains more facile for vanadia films on κ-Al2O3 than on α-Al2O3. The systematic lower values relate to the presence of tetrahedral sites that allow for significant lattice relaxation upon reduction. Using the oxygen defect formation energy as a reactivity descriptor, we discuss possible effects of the support structure and vanadia loading in Mars-van Krevelen-type oxidation reactions. We also analyze the influence of the support structure on the interface vibrational modes.