Conversion of various hydrocarbons over supported Pd during simulated cold-start conditions

This paper reports measured oxidation rates of synthetic automotive exhausts over an aged palladium (Pd) catalyst during simulated cold-start conditions. Surface reaction rates and species concentration profiles at points along a catalyst of Pd on Al2O3 with ceria are reported for A/F ratios from 10 to 16. The synthetic exhaust streams contain levels of carbon monoxide and hydrocarbons like those in auto exhausts during a cold-start, and include representative amounts of CO, CO2, O-2, H-2, NO, SO2 and H2O in N-2. The hydrocarbon compounds are represented by 1-butene as a surrogate for the most reactive hydrocarbons, and propane and methane as surrogates for species with intermediate and very low reactivities, respectively. The data reveal important features of multicomponent oxidation under fuel-rich conditions. First, as expected, hydrogen is the most reactive species, reaching diffusion-limited oxidation at the inlet of the reactor for a temperature of 320 degrees C, even for exhaust streams with 500 ppm NO. Second, the oxidation of carbon monoxide and 1-butene over an aged palladium catalyst follows similar conversion histories for excessively fuel rich and near-stoichiometric conditions. Third, the hydrocarbons oxidize simultaneously, but at rates that follow the ranking of reactivities seen in tests with individual fuel species. Whereas the magnitude of the 1-butene conversion rate is much lower than carbon monoxide's because of its much lower inlet level, this hydrocarbon is eliminated on nearly the same time scale as carbon monoxide. But propane and methane are converted on much longer time scales. Fourth, the oxidation of all the fuel species is inhibited by the presence of nitric oxide in the feedstream, especially propane.