CHARACTERIZATION OF THE NATURE OF SURFACE SITES ON VANADIA TITANIA CATALYSTS BY FTIR

With the aim of understanding the vanadia-titania catalysts used for selective catalytic reduction of NO by NH3 in the SCR DeNOx, process, the surface structure of vanadia, titania, and vanadia supported on titania catalysts were studied by FTIR. The interactions of NO, NH3, and NO + NH3 with the catalysts in different states of oxidation/reduction have also been investigated. Vanadia was found to be highly dispersed on the TiO2 as a result of interaction with TiOH. This interaction phase also exhibited new VOH bands not observed on V2O5. The nature of the titania and vanadia surface hydroxyl groups was observed to be very sensitive to sample pretreatment in that oxidation enhanced their concentration whereas reduction removed these OH groups. Extensive reduction in H2 of the vanadia-titania sample broke up the surface vanadia structure, eliminated the VOH species, and reexposed the surface TiOH groups. Adsorption of NH3 on the surface demonstrated the presence of both Brønsted and Lewis acid sites on the vanadia-titania catalysts, while Brønsted and Lewis acid sites were observed to dominate on V2O5 and TiO5, respectively. The ratio of Brønsted to Lewis acid sites on the vanadia-titania catalysts was found to depend on the number of VOH and VO groups, the vanadia loading, or the vanadia coverage, as well as the oxidation state of vanadia. No adsorption of NO was evidenced on the oxidized or the NH3-reduced surface of vanadia-titania and adsorption occurred only on the H2-reduced samples. On a partially reduced catalyst with preadsorbed NH3, NO was observed to oxidize the surface at room temperature accompanied by a transformation of Lewis to Brønsted acid sites. The redox properties of these catalysts were found to play an essential role in the surface adsorption/reaction process. Evidence of interaction of NO with surface NH4+ species further suggested the importance of Brønsted acid sites and thus also the VOH groups in the SCR DeNOx reaction mechanism. © 1991.