Palladium catalyst performance for methane emissions abatement from lean burn natural gas vehicles

Abstract

As little as 1 ppm SO x present in the exhaust of a lean burn natural gas engine strongly inhibits the oxidation of CH 4 over a Pd containing catalyst. Non-methane emissions oxidation, such as C 2H 6, C 3H 8 and CO, are also inhibited by low SO x concentrations, but to a lesser extent than CH 4 emissions. The mechanism for SO x inhibition indicates a 1 : 1 selective adsorption of SO x on PdO for palladium on a non-sulfating support such as SiO 2. Deactivation is therefore very rapid. In contrast, palladium on sulfating supports, that is γ-Al 2O 3, deactivate more slowly and can tolerate more SO x because the SO x is also adsorbed onto the carrier. The activation energy for methane oxidation is dramatically increased after SO x poisoning for all Pd catalysts, while the Arrhenius pre-exponential term is relatively constant, indicating a transformation from very active PdO sites to less active PdO SO x sites. Platinum catalysts are considerably less active than Pd as evidenced by a much lower pre-exponential term, but are more resistant to deactivation by SO x . Non-methane hydrocarbon and particulate emissions standards for lean burn natural gas engines for the United States can be met with Pd catalysts. However, the non-enforced methane emissions standards are not met. For the European truck test cycle, methane emissions standards are met since the test cycle heavily weights the hotter modes where Pd SO x is sufficiently active.