Electronic Structure Investigation of NO₂ and NO Binding on Vanadium Oxides

NO₂ and NO, which are generated in combustion processes, binding to vanadium oxide clusters including TiO₂-supported catalyst models in the selective catalytic reduction (SCR) of NO has been studied by density functional theory and coupled cluster methods. NO_x binding on vanadium oxides is predicted to depend on several factors, including the excitation energy of the oxide, ionization energies of both the unbound oxide and the deoxygenated reduced oxide, and the strength of the molecular V-O bonds. NO₂ chemisorption occurs either through covalent bond formation in a HONO-like pattern or through abstraction of a metal oxide oxygen leading to the formation of NO₃^-. Nitrate formation is more favorable than what was predicted for group IVB or group VIB oxides [except (CrO₃)] and is either the lowest energy binding mode or within a few kcal/mol of the lowest mode in all clusters, likely due to the stability of V in the +4 oxidation state. Physisorption on V oxides is very weak. V with 2 oxo groups have a lower excitation energy and a more sterically open geometry which results in strong chemisorption as predicted for group IVB oxides. Tetrahedrally coordinated vanadia with a single oxo group and 3 V-O single bonds are predicted to have significantly higher excitation energies and behave like group VIB oxides such that chemisorption is unlikely and weak physisorption dominates the interaction. In larger clusters, including SCR catalyst models, only tetrahedrally coordinated vanadia are present and NO₂ binding is not expected to occur. NO adsorption is weaker overall than NO₂ binding and occurs either as physisorption or as chemisorption through the formation of NO₂^- analogous to nitrate formation in NO₂ binding. The ability of NO to bind reflects the patterns predicted for NO₂, such that NO is strongly bound vanadia with two V═O groups and only weakly physisorbed when there is a single V═O or none at all.