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The purification performances of some kinds of catalytic converters ( Pt, Rh, Pd, Pd/Rh, Pd/Pt, Pt/Rh/Pd, Pt_Pd and Pd_Cu) were investigated to select suitable catalytic converters for natural gas fueled automotive engines. Pd series catalysts showed better performance among the noble metal catalysts for oxidation of unburned methane in exhaust gas. The optimum loading of Pd catalyst is the range of 1.6 to 3.2 g/L. The dual-bed catalyst, Pt_Pd, consisting of a Pt catalyst in the front and a Pd catalyst in the rear, showed a performance better than Pd series catalysts. When aged to an accumulated running distance of 50000 miles, the catalytic activity of the Rh catalysts is much reduced, but those of Pd and Pt catalyst are affected little by aging. The aged Pd/Rh catalyst showed superior emissions' purification performance at the stoichiometric condition, but poor at lean mixture conditions.

Experiments have been conducted to investigate the inhibitory effect of the catalytic reaction of methane and the formation of formaldehyde in the presence of NO. With precious metal catalysts, the presence of NO showed an inhibitory effect on the methane oxidation and caused the formation of formaldehyde. In the absence of oxygen, however, formaldehyde was not produced in the catalytic reaction of methane and NO. Maximum formation of formaldehyde was about 0.6 % of NO with the Pt and Rh series catalysts. N2O formation is similar to the formaldehyde formation from the catalytic oxidation of methane in the presence of NO.