Evaluation of Mn and Sn-Modified Pd-Ce-Based Catalysts for Low-Temperature Diesel Exhaust Oxidation

摘要

Pd-impregnated Ce-based catalysts were tested for carbon monoxide (CO) and hydrocarbon (HC) oxidation under challenging low-temperature diesel combustion conditions. The results indicate that the light-off temperatures for CO over Pd/CeO_2, Pd/MnO_x-CeO_2 (Pd/MC), and Pd/SnO_2-MnO_x-CeO_2 (Pd/SMC) catalysts shift to higher temperatures in the presence of simulated diesel exhaust gas. The lowest T_(50) for CO is observed over Pd/MC at 173 °C, whereas Pd/CeO_2 is shown to oxidize most of the HCs at temperatures below 400 °C. In all catalysts, the oxidation of HCs starts right after the onset of CO oxidation, revealing that the competitive adsorption of CO, NO, and alkenes controls the catalytic activity. Further evaluation of the catalytic activity in the presence of only CO and C_3H_6 reveals the immediate inhibiting effect of C_3H_6 at catalyst temperatures below 150 °C. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments performed over Pd/CeO_2, Pd/MC, and Pd/SMC show that C_3H_6 inhibits the formation of carbonyl species on Pd~(n+) sites, which limits the catalytic activity for CO. Such inhibition is observed on all supports, implying that the activity is independent of oxygen storage capacity (OSC) or lattice oxygen reducibility of the supports in the presence of C_3H_6.