A close-up to the promoting effect of tungsten in Ce/TiO2 catalysts for the selective catalytic reduction of NO with NH3

Abstract

The effect of tungsten on the activity of CeOx/TiO2 catalysts for the SCR of NO by NH3 was studied with catalysts of similar composition prepared via different impregnation routes from precursors used in industry. Reaction rates were related to structural and reactivity data collected by nitrogen physisorption, XRD, TPR, TPD of NH3, FT-Raman spectroscopy, DRIFTS of adsorbed NH3, and XPS. It was observed that proper promotion of Ce-Ti oxide catalysts requires previous formation of W-O-Ti linkages: while co-impregnation resulted in the segregation of bulk W oxide, probably because of a preference of the Ce-O-Ti over the Ce-O-W interaction, interaction of tungstate with anatase appears to induce Brønsted sites associated with W. These may interact with ceria species in subsequent impregnation steps thus favoring the subsequent formation of Ce-O-W bridges. High activity requires high abundance of Ce-O-Ti and Ce-O-W bridging sites accessible to the gas phase, avoiding bulk oxide phases as CeO2, and probably also TiO2 polymorphs. On the other hand, O 1s XPS signal intensity above 530.5eV often assigned to reactive oxygen (“Oα”) and surface Ce3+ concentration could not be correlated with SCR activity, neither could be the abundance of Brønsted sites, though Brønsted acidity may favor the SCR without being integrated in the active sites.