Theoretical Insight into the Nature of Ammonia Adsorption on Vanadia-Based Catalysts for SCR Reaction

Density functional theory (DFT) calculations were carried out on monomeric and oligomeric vanadium oxide clusters to probe the factors leading to the formation of NH4 species from the adsorption of ammonia. The interaction of ammonia with monomeric vanadium oxide clusters leads to the formation of hydrogen-bonded NH3 species, with energy changes for ammonia adsorption near -50 kJ/mol. The interaction of ammonia with oligomeric vanadium oxide clusters leads to the formation of bidentate NH_4 species, where the ammonium cation is coordinated between two V=O groups on adjacent vanadium cations. The energy change for ammonia adsorption in this mode is near -100 kJ/mol. Adsorption of ammonia as NH4 species was not observed when the oligomeric vanadium oxide clusters were reduced by addition of hydrogen atoms, i.e., in clusters where the formal oxidation state of the vanadium cations was 4+. Based on our findings, a model for the generation of Brönsted acidity through the interaction of vanadium oxide oligomers with the titanium oxide support is proposed.