Methane and Nitric Oxide Conversion Over a Catalyst Dedicated for Natural Gas Vehicles

Methane and nitric oxide conversion was studied over a Pd-based catalyst at steady state conditions. The gas mixture contained methane (0.125 %), Nitric oxide (0.125 %), carbon monoxide (0.7 %), oxygen and argon as carrier gas. The experiments were performed in a well-stirred reactor (Berty reactor) which provided constant gas composition over the catalyst. Lambda scans from λ=1.01 to 0.99 and back performed by varying the oxygen content, revealed a hysteresis in both the methane conversion and the nitric oxide conversion. The temperature and presence of nitric oxide affected the hysteresis.

Complementary experiments in a synthetic exhaust gas rig revealed a more pronounced hysteresis in the presence of carbon dioxide and water.

An attempt to model the hysteresis effect as a function of the palladium and palladium-oxide transformations was made.

The effects are expected to have a strong influence on the light-off and low temperature performance on the emission control from natural gas fuelled vehicles.