Molecular structure and catalytic activity of V2O5/TiO2 catalysts for the SCR of NO by NH3: In situ Raman spectra in the presence of O2, NH3, NO, H2, H2O, and SO2

In situ Raman spectroscopy has been used at temperatures up to 400 °C under O2, NH3/N2, H2/N2, NH3/NO/O2/N2, O2/H2O/N2, and SO2/O2/N2 for studying the influence of these gases on the molecular structure of V2O5/TiO2 catalysts with V surface density, nsns, in the range 2.5–18.7 VOx/nm2. The catalyst activities for the SCR of NO by NH3 have been determined to derive structure–activity relationships in combination with the Raman data. Isolated monovanadates and polyvanadates are formed at various proportions (depending on the loading) on the catalyst surface under dehydrated conditions. The band positions and characteristics are discussed in terms of possible configurations for the dispersed VOx species. The bands observed, the surface composition, and the bond conservation rule allow to propose a small size for the VOV chains of polyvanadates (i.e., 2, 3). The reducing action of NH3 is favored in the presence of adjacent V sites; at low loadings, the presence of NH3 has no effect on the structural properties of surface VOx. The reducibility in H2 follows an opposite trend and is favored at low nsns, as indicated by both in situ Raman and H2-TPR. The SO2 presence affects only the molecular structure of catalysts with low nsns, for which a significant part of surface TiO2 sites are vacant; the effect (judged from the in situ Raman data) is merely one of driving the dispersed vanadia species in a state of “virtually” high surface density by crowding them together, thereby providing more adjacent V sites for activation of NH3 in SCR reaction conditions. The NO TOF values initially increase with increasing nsns, suggesting that the number of active sites per V atom increases with increasing nsns below monolayer. The formation of adjacent VOTi sites is favored either at increasing nsns or at conditions of “virtually” high nsns (in the presence of SO2). The increase in the number of such centers per V atom correlates with the increase in TOF values with increasing nsns.