Promoting effect exerted by WOx on Pt/ZrO2 catalysts during NOx reduction using H2 in the excess of O2

Abstract

This work addresses the reduction of NOx by H2 under O2-rich conditions below 200 °C using Pt/W/ZrO2 catalysts with tungsten loadings from 0 to 14 wt%. Samples were thoroughly characterized by N2 physisorption, temperature-programmed desorption of CO, X-ray diffraction, laser Raman spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as probe molecule. Catalytic investigations showed the highest efficiency for the sample with 5 wt% W providing NOx conversion of 94 % and N2O selectivity of 15 %. The effect of W promoter is mostly attributed to structural promotion by isolated WO6 entities stabilizing the ZrO2 surface and leading to a high number of active Pt sites. By contrast, negligible electronic interaction between the promoter and Pt sites appears. Moreover, DRIFTS studies suggest that, in addition to the predominate NOx reduction by H2 on Pt sites, a side reaction occurs including NHx species formed at the interface of Pt and WOx/ZrO2.