Rapid Deactivation of Lean-Burn Natural Gas Engine Exhaust Oxidation Catalysts

Methane emissions from lean-burn natural gas engines can be relatively high. As natural gas fueled vehicles become more prevalent, future regulations may restrict these emissions. Preliminary reports indicated that conventional, precious metal oxidation catalysts rapidly deactivate (in less than 50 hours) in lean-burn natural gas engine exhaust. This investigation is directed at quantifying this catalyst deactivation and understanding its cause. The results may also be relevant to oxidation of lean-burn propane and gasoline engine exhaust. A platinum/palladium on alumina catalyst and a palladium on alumina catalyst were aged in the exhaust of a lean-burn natural gas engine (Cummins B5.9G). The engine was fueled with compressed natural gas. Catalyst aging was accomplished through a series of steady state cycles and heavy-duty transient tests (CFR 40 Part 86 Subpart N) lasting 10 hours. Hydrocarbons in the exhaust were speciated by gas chromatography. Catalysts were compared on the basis of conversion and first order rate constants for conversion of specific hydrocarbon species. CO and ethylene conversions were nearly 100% at all conditions and did not decrease over time. Steady state and transient data show significant catalyst deactivation for methane oxidation over both catalysts tested. The global rate constant for methane conversions declined by more than 60% over the 10 hour experiment. Rate constants for ethane and propane oxidation declined by roughly 40 and 20%, respectively. No decrease in activity was observed for heavier hydrocarbons but their concentration in the fuel and in the exhaust was very low. Preliminary characterization of both the fresh and used catalysts was performed by x-ray diffraction, Auger electron spectroscopy, and x-ray photoelectron spectroscopy. XRD indicated no bulk structural changes in the catalysts. Auger electron spectra indicate the presence of only PdO in the fresh catalyst but two distinct chemical forms of Pd in the used catalyst samples. Auger also indicated the presence of a phosphate coating on the monolith external surface. XPS confirmed the presence of reduced pd in the aged samples.