Design Considerations for Natural Gas Vehicle Catalytic Converters

Bench reactor experiments were carried out to investigate the effects of operating temperature, precious metal loading, space velocity, and air-fuel (A/F) ratio on the performance of palladium (Pd) catalysts under simulated natural gas vehicle (NGV) exhaust conditions. The performance of these catalysts under simulated gasoline vehicle (GV) conditions was also investigated. In the case of simulated NGV exhaust, where methane was used as the prototypical hydrocarbon (HC) species, peak three-way conversion was obtained under richer conditions than required with simulated GV exhaust (propane and propene HC species). Moreover, the hydrocarbon efficiency of the catalyst under simulated NGV exhaust conditions was more sensitive to both A/F ratio and perturbations in A/F ratio than the HC efficiency under GV exhaust conditions. Although emissions of unburned methane are currently unregulated, these data show that unburned methane is very unreactive compared to typical hydrocarbons in gasoline vehicle exhaust. Currently, NGVs are attractive candidates for meeting federal tier II and California ultra-low emissions standards because of their low non-methane organic (NMOG) emissions. Results of this study show, however, that this situation could change dramatically if a total HC standard were enacted. In that event, high exhaust temperatures (i.e. close-coupled converters), high catalyst loadings and large converter volumes might all be required to effect significant conversion of unburned methane. Aging studies conducted on a pulse flame combustor show that the activity of the Pd catalyst for CH, removal deteriorated sharply with aging.