Catalysts for natural gas emission control applications

Several parameters that affect catalytic methane oxidation on a natural gas vehicle were investigated with laboratory aged noble metal catalysts and a simulated vehicle exhaust. These include the air/fuel control strategy, the noble metal loading, the role of base metals and the exhaust hydrocarbon composition. The catalytic performance of several formulations was compared both slightly fuel-rich of the stoichiometric point and under extreme lean conditions. Catalyst formulations different from those used near the stoichiometric point appear to be required if the vehicle is run under extreme lean conditions, such as those found with diesel engines. Under diesel conditions low exhaust temperatures may pose a challenge, since methane requires relatively high temperatures for catalytic oxidation. Spark; ignition lean burn engines might offer a higher temperature exhaust, but NOx conversion becomes more problematic in this environment. Increasing the Pd load improves methane and NOx conversions near the stoichiometric point and methane activity under lean conditions. Ce addition was found to be beneficial for aged catalyst performance and increasing the Ce loading resulted in improved methane activity over Pd near the stoichiometric point. For natural gas vehicles run under closed loop control near the stoichiometric point, the composition of the controlled exhaust will modulate around the set point. The effect of the amplitude and frequency of these modulations on methane oxidation was explored. Increasing the frequency or decreasing the amplitude of exhaust modulations results in improved methane conversions, especially near the maximum methane conversion point. The effect of natural gas fuel composition was investigated with feedstreams containing various hydrocarbon mixtures. Small amounts of propane in the feedstream led to increased hydrocarbon conversions lean of the stoichiometric point. This improvement may be due to propane reacting with and removing surface oxygen species, which otherwise block methane adsorption on the catalyst surface. Larger amounts led to improvements in hydrocarbon conversion rich of the stoichiometric point as well, due to the easier oxidation of propane. The information gained from the above studies was used to design a catalyst for a natural gas vehicle tested by the U. S. EPA. With this catalyst, the vehicle achieved California Ultra Low Emmission Vehicule standards.