Mechanistic effects of Water on Carbon monoxide and propylene oxidation on platinum and palladium bimetallic catalysts

Abstract

Low temperature combustion (LTC) diesel engines are more fuel efficient and have coincident lower temperature exhaust gas and higher CO and hydrocarbon exhaust gas concentrations than today’s standard diesel engine. To meet regulations, diesel oxidation catalysts (DOCs) will need to be improved and optimized to handle these higher CO and hydrocarbon concentrations emitted and the simultaneous lower exhaust temperatures. Pt-Pd bimetallic catalysts are often used as oxidation catalysts. Here, the mechanistic effects of water on CO and C3H6 oxidation were studied over model monometallic Pt and Pd catalysts, and a bimetallic 1:1 Pt-Pd/ γ-Al2O3 catalyst. Water in the reaction mixture improved CO oxidation light-off for the monometallic Pd and Pt-Pd catalysts and had a negative impact on light-off over the monometallic Pt catalyst. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that more CO adsorbed on the Pd and Pt-Pd catalysts when water was present, particularly at the particle/support interface, while the opposite occurred on the Pt catalyst. For C3H6 oxidation, water in the reaction showed a negative impact on the light-off temperature for the Pd catalyst and a positive effect for the Pt catalyst. The Pt-Pd catalyst showed intermediate light-off behavior between the two monometallic catalysts. In terms of selectivity, slightly more CO and acetic acid were formed over the Pd and Pt-Pd catalysts when water was in the reaction mixture, while less CO was formed over the Pt catalyst, but overall, the selectivity towards certain partial oxidation products was not greatly affected with the addition of water to the reaction mixture.