Effects of tungsten oxide on soot oxidation activity and sulfur poisoning resistance of Pt/Al2O3catalyst

Abstract

Tungsten oxide was impregnated into the alumina-supported platinum catalyst in order to improve the soot oxidation activity and the resistance to sulfur dioxide. The catalysts were characterized by X-ray diffraction (XRD), Raman, UV-vis spectroscopy, Brunauer–Emmett–Teller (BET), Fourier transform infrared (FT-IR) spectroscopy, temperature-programmed desulfation, NO_x temperature-programmed desorption (NO_x-TPD), NO temperature-programmed oxidation (NO-TPO) and soot temperature-programmed oxidation (soot-TPO). The deposition of WO_x was found to reduce the availability of Pt active sites on the fresh catalyst. This, as well as the acidic property of tungsten oxide, decreases the NO oxidation and the NO_x adsorption abilities. However, a higher soot oxidation activity was achieved on the WO_x-modified catalyst in the presence of NO and O₂, which is associated with the presence of more platinum in the metallic state by interacting with the electronegative tungsten oxide. The acidity of tungsten oxide and the improved oxidation-resistance of platinum may be critical to the NO ↔ NO₂ recycling efficiency and decomposition of surface oxygen complexes. After the sulfur poisoning treatment, fewer sulfates are formed on the WO_x-modified catalyst and decompose at lower temperatures. The IR spectra of CO adsorption indicate that less platinum active sites on this catalyst are affected by sulfates and a higher level of Pt dispersion is obtained, which is responsible for the high NO oxidation and soot oxidation activities.