The effects of structure on the catalytic activity and selectivity of V 2O 5/TiO 2 for the reduction of NO by NH 3

Abstract

An investigation of the effect of catalyst structure on the activity and selectivity of TiO 2 (anatase)-supported V 2O 5 for the selective catalytic reduction of NO by NH3 has been carried out. The structure of the catalyst and the adsorbed species present on the surface was characterized by in situ laser Raman spectroscopy (LRS), and the interaction of NH 3 was investigated using temperature-programmed desorption (TPD). At vanadia loadings corresponding to less than a theoretical monolayer, the vanadia is present in the form of monomeric vanadyl and polymeric vanadate species. When the vanadia coverage exceeds a monolayer, crystallites of V 2O 5 form at the expense of the polymeric species. Analysis of the catalytic activity shows that the specific activity of the polymeric vanadates species is about 10 times greater than that of the monomeric vanadyl species. Monomeric species produce N 2 as the principle reaction product, independent of the presence or absence of O 2 in the feed, whereas polymeric vanadates species produce both N 2 and N 2O, with the selectivity to N 2 decreasing with increasing concentrations of O 2 in the feed. LRS experiments reveal that in the absence of O 2 in the feed stream, the catalyst undergoes reduction but that in the presence of O 2, the catalyst remains in a nearly fully oxidized state. TPD experiments indicate that a crucial step in the catalytic reduction of NO is the activation of adsorbed NH 3 to produce NH x ( x = 0 − 2) species. The removal of H atoms from adsorbed NH 3 occurs via reaction with V O groups present in clusters of monomeric vanadyl species and in polymeric vanadate species, the latter being more reactive than the former. The observations reported in this study are interpreted in terms of a reaction mechanism, which accounts for the effects of catalyst structure and oxidation state on the observed properties.