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

摘要

A series of Nb-substituted WO3/CeZrOx catalysts were prepared by the co-impregnation method and applied in the selective catalytic reduction of NOx with NH3 (NH3-SCR). NH3 oxidation, N-2 sorption, XRD, Raman, UV-vis, XPS, H-2-TPR, O-2/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 deNOx activity and N-2 selectivity in a broad reaction temperature window of 190-434 degrees 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 Bronsted acid sites and Nb=O bonded to strong Lewis acid sites. The enhancement of surface active oxygen species and Bronsted acid sites promoted the low-temperature (below 250 degrees C) deNO(x) activity. However, the preoxidation of NH3 at high temperatures slightly suppressed the NOx conversion of the catalyst with more strong Lewis acid sites at above 400 degrees C. Moreover, the catalyst also showed excellent sulfur tolerance and could be a promising candidate for practical applications in NOx abatement.