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

Abstract

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 VO x 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.

Vanadium Oxide Oligomers and Ordered Monolayers Supported on CeO2(111): Structure and Stability Studied by Density Functional Theory

“…Two approaches to construct models of supported vanadium oxide ML structures are reported in the literature: (i) substitution of metal atoms of the support with vanadium atoms, for example in anatase TiO2(001), 37 α-Al2O3(0001), 38 κ-Al2O3(001) 39 and Al2O3/NiAl(110), 40 or (ii) deposition of vanadium oxide layers cut from bulk phases on the support, e.g. V2O3 on…”

Assessing thermochemical properties of materials through ab initio quantum-mechanical methods: the case of α-Al2O3

“…1 Corundum (α-Al 2 O 3 , the most stable form of aluminum oxide, whose atomic structure is sketched in Figure 1) and its polymorphs (the so-called transitions aluminas) also exhibit unique surface properties, which make them suitable both as catalysts for several reactions [2][3][4][5][6][7][8][9] and as support for other catalysts. [10][11][12][13][14] Furthermore, the thermodynamic properties of corundum are of particular interest in calorimetric studies in that it constitutes a standard reference material (SRM-720) for the calibration of some calorimeters. 15 First-principles techniques based on the density functional theory (DFT) are becoming an important complementary tool in the investigation of mineral structure and thermodynamics, as well as surface properties and reactivity, as they allow to analyse the nature of the fundamental interactions giving rise to the observed phenomena.…”

The structure of epitaxial V2O3 films and their surfaces: A medium energy ion scattering study

“…1). Detailed theoretical totalenergy calculations based on density functional theory (DFT) have explored the surface relaxations to be expected for each of these terminations [11,29,30,31] and the relative energies have been evaluated as a function of the equilibrium oxygen chemical potential in the gas phase [11,32,33], generally favouring either a partial or zero vanadyl coverage of an otherwise oxygen-terminated surface under typical UHV surface preparation conditions. Experimental support for the presence of surface vanadyl species comes from vibrational spectroscopy; both HREELS (high-resolution electron-energy loss spectroscopy) [22] and infrared spectroscopy [12] have shown an adsorption band at ~130 meV that was attributed to a vanadyl V-O stretching mode.…”