Compressed natural gas (CNG) has been regarded as an alternative fuel for current fossil fuels such as gasoline and diesel. Recently the increasing interest in shale gas is drawing more attention to CNG vehicles of which number is expected to increase.
Exhaust gas from CNG engines with lean combustion contains relatively low nitrogen oxides and particulate matters compared to conventional fossil fuel based engines. However, high amount of unburned methane, which has much higher greenhouse warming potential than CO2, limits the wide use of CNG for many applications. Even though Pd-based catalysts have been popularly studied in order to convert methane, their activity and durability have not been sufficient for practical applications to aftertreatment of lean burn CNG engines and the formation of a new Pd containing.
In the present study, we developed an improved Pd-based catalyst for CNG engines by introducing Pt and promoters to enhance methane oxidation activity at low temperature and long term durability. Pt was more active to oxidize paraffinic hydrocarbons that comprise ∼10% of total hydrocarbons in exhaust gas from lean burn CNG engines. Exotherm resulting from the paraffinic hydrocarbon oxidation by Pt could promote the remaining methane oxidation by Pd. Furthermore, potential causes for the deactivation of Pd-based catalyst were investigated from the various points of Pd-sintering by high temperature exposure, PdO decomposition, sulfation and coking, and the formation of new Pd containing compound. Optimal use of support material for Pd/Pt and promoters to keep Pd stable were identified as key factors for the design of more active and durable catalyst.
