Impacts of niobia loading on active sites and surface acidity in NbOx/CeO2–ZrO2 NH3–SCR catalysts

Abstract

A series of NbOx/Ce0.75Zr0.25O2 catalysts for the selective catalytic reduction of NO with ammonia (NH3–SCR) were synthesized using a wetness impregnation method. The effect of niobia loading was studied in relation to the active sites and surface acidity. NH3/NO oxidation, X-ray diffraction, Infrared spectroscopy, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, O2/NH3 temperature-programmed desorption, and diffuse reflectance infrared Fourier transformed spectroscopy experiments were performed to correlate the catalyst structure and surface properties to catalytic performance after Nb2O5 modification. The catalyst with 15wt.% Nb2O5 loading showed high NH3–SCR activity and nearly 100% N2 selectivity within a broad operation temperature window (190–460°C) at a high space velocity (300,000h−1). On this catalyst, Nbn+ was mainly distributed in the form of typical monomeric and polymeric NbOx species, and was partially incorporated into the Ce0.75Zr0.25O2 lattice at the Nbn+OCen+ (Zrn+) interface. The electron redistribution effect arising from the occupation of cerium sites by Nbn+ ions promoted the formation of Ce3+ ions, oxygen vacancies and active oxygen species. This interaction was closely associated with the distribution of NbOx species which varied with niobia loading. NbOx themselves were acid sites and by attracting electrons they enhanced Lewis acid sites on CZ surface, which promoted the adsorption of NH3 and inhibited the unselective oxidation of NH3 to NOx. The increased amounts of active oxygen species over NbCZ catalysts promoted the adsorptive oxidation of NH3 to NH2 and NO to NO3− at low temperatures, and thus facilitated the reaction of ads-NH3 and ads-NO3−/NO2− species. This effect as well as the increased amount of acid sites led to good NH3–SCR performance of Nb15CZ in a wide temperature range.