Jordan K. Lampert

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

Abstract As little as 1 ppm SOx present in the exhaust of a lean burn natural gas engine strongly inhibits the oxidation of CH4 over a Pd containing catalyst. Non-methane emissions oxidation, such as C2H6, C3H8 and CO, are also inhibited by low SOx concentrations, but to a lesser extent than CH4 emissions. The mechanism for SOx inhibition indicates a 1 : 1 selective adsorption of SOx on PdO for palladium on a non-sulfating support such as SiO2. Deactivation is therefore very rapid. In contrast, palladium on sulfating supports, that is γ-Al2O3, deactivate more slowly and can tolerate more SOx because the SOx is also adsorbed onto the carrier. The activation energy for methane oxidation is dramatically increased after SOx 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 SOx 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 SOx. 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 SOx is sufficiently active.

Thermal decomposition and reformation of PdO catalysts; support effects

The thermal decomposition of PdO and the reformation of Pd to PdO is dependent on the support upon which they are dispersed indicating significant metal (oxide) support interactions. The study defines temperature regions of different stabilities for Pd:O species. The nature of the Pd:O structure is of great importance for a large number of environmental catalytic applications including the gasoline automobile catalytic converter, ozone decomposition catalysts, abating emissions from natural gas fueled vehicles, primary combustion catalysts, etc. The ZrO2 support shows the largest hysteresis effect between the temperature of decomposition and reformation. In contrast both TiO2 and CeO2 have small hysteresis effects because of a significant increase in the temperature for reformation of the PdO. There exists a large region of temperature stability of the PdO when dispersed on these two carriers. This is quite favorable for those catalytic reactions where the oxide state is important such as the primary catalytic combustion of natural gas. Further evidence of the importance of PdO for methane oxidation is presented.