Effects of Zr addition on the performance of the Pd-Pt/Al2O3 catalyst for lean-burn natural gas vehicle exhaust purification

Abstract

The catalytic activity, hydrothermal aging resistance, and sulfur tolerance of a Pd-Pt-based methane oxidation catalyst were evaluated in a fixed fluidized bed reactor containing simulated lean-burn natural gas vehicle exhaust gases. Zirconium-doped Pd-Pt/Al2O3 (Pd-Pt/ZrxAl(1-x)O(3+x)/2) was found to significantly improve the catalytic activity, hydrothermal aging resistance, and sulfur tolerance. Zr-modified alumina supports were prepared by co-precipitation with molar ratios of Zr to Al of 0 : 1, 0.25 : 0.75, 0.5 : 0.5, 0.75 : 0.25, and 1 : 0. The Pd-Pt bimetallic catalysts containing 1.5% (w, mass fraction) Pd and 0.3% (w) Pt supported on the above-modified composite supports were prepared by the co-impregnating method. The catalysts were characterized by N2 adsorption/desorption, X-ray diffraction(XRD), H2 temperatureprogrammed reduction (H2-TPR), O2 temperature-programmed desorption, and X-ray photoelectron spectroscopy (XPS). The results show that the crystallinity of the samples, dispersion of the active component, number of Pd2+ species, and electron density around Pd2+ species increase after addition of ZrO2 to Al2O3 supports. Compared with the activity results of Pd-Pt/Al2O3 and Pd-Pt/ZrO2 catalysts after different pretreatment conditions, the performance of the catalyst is greatly enhanced by adding ZrO2 in the Al2O3 supports, and Pd-Pt/Zr0.5Al0.5O1.75 shows the best catalytic activity, strongest hydrothermal aging resistance, and highest sulfur tolerance among the investigated catalysts.