Promotional effect of niobium substitution on the low-temperature activity of a WO3/CeZrOx monolithic catalyst for the selective catalytic reduction of NOx with NH3

Abstract

A series of Nb-substituted WO₃/CeZrO_x catalysts were prepared by the co-impregnation method and applied in the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). NH₃ oxidation, N₂ sorption, XRD, Raman, UV-vis, XPS, H₂-TPR, O₂/NH₃-TPD and in situ DRIFTS were performed to correlate the redox property and surface acidity to NH₃-SCR performance of Nb-substituted catalysts. The catalyst with 5 wt% substitution amount of Nb₂O₅ presented excellent deNO_x activity and N₂ selectivity in a broad reaction temperature window of 190–434 °C at a gas space velocity of 30 000 h⁻¹. The characterization results demonstrated that the partial substitution of WO₃ by Nb₂O₅ not only led to strong redox properties arising from abundant surface active oxygen species, but also promoted the adsorption of NH₃ and the redistribution of acid sites due to Nb–OH related to Brønsted acid sites and NbO bonded to strong Lewis acid sites. The enhancement of surface active oxygen species and Brønsted acid sites promoted the low-temperature (below 250 °C) deNO_x activity. However, the preoxidation of NH₃ at high temperatures slightly suppressed the NO_x conversion of the catalyst with more strong Lewis acid sites at above 400 °C. Moreover, the catalyst also showed excellent sulfur tolerance and could be a promising candidate for practical applications in NO_x abatement.