Electronic Structure of Unsaturated V2O5(001) and (100) Surfaces: Ab Initio Density Functional Theory Studies

Vanadium oxides based materials are well known to play an active role as catalysts in many chemical processes of technological importance like for example hydrocarbon oxidation reactions or selective catalytic reduction of NO _x in the presence of ammonia. Usually the (010) surface is pointed out as the most important, however one has to underline that other low-indices surfaces are by far less studied. In the present study the electronic structure of V₂O₅(001) and (100) surfaces are determined by ab initio DFT methods using gradient-corrected RPBE exchange-correlation functional. As models of surface sections different embedded V₁₄O₄₅H₂₀, V₁₄O₄₄H₁₈, and V₂₁O₆₅H₂₅ clusters are considered for the (001) surface and V₁₂O₄₀H₂₀, V₁₄O₄₆H₂₂, V₁₆O₅₂H₂₄ for the (100) surface. Detailed analyses of the electronic structure of each cluster are performed using charge density distributions, Mayer bond orders, electrostatic potential maps, character of frontier orbitals, and density of states (total as well as partial, atom projected). Results of the calculations show that overall negative charge of the surface oxygen sites scales with their coordination independent of the surface orientation. Terminal oxygen O(1) is charged the least negatively while doubly coordinated atoms –O(2) and O^e(2) have charge twice as large. This indicates that bridging (for (001) and (100) netplanes) and edging (only for (001) netplane) oxygen sites are more nucleophilic than terminal vanadyl sites, which becomes important in view of the reactivity of the different sites for surface chemical reactions. Vanadium atoms present at these surfaces are positively charged (electrophilic) and may play a role of electron acceptors. The unsaturated surfaces show a strong tendency to surface relaxation that manifest by large relaxation energies.