Three catalysts consisting of palladium supported on Al2O3, 5% CeO2/Al2O3, and 8.6% La2O3/Al2O3 were characterized for three-way catalytic performance using a laboratory reactor designed to simulate the operation of the catalyst in a vehicle. Different concentrations of SO, were used in the feedstream to evaluate the magnitude and reversibility of the impact of sulfur on three-way activity. Both lightoff and warmed-up activities of all of the catalyst formulations were significantly decreased when sulfur was present in a cycled, stoichiometric feedstream. The magnitude of the impact on activity increased with increasing sulfur content. The lightoff activity also decreased with increasing sulfur content, following a similar trend as with the warmed-up activity. For all three catalyst formulations, the increase in the lightoff temperature due to the addition of 30 PPM SO2 to the feedstream was about the same as the increase in the lightoff temperature due to the effect of relatively severe thermal aging. Although the addition of base metal oxides to the catalyst resulted in an apparent improvement in activity compared to Pd/Al2O3 alone, the improvement attributable to cerium was completely canceled when SO, was added to the feedstream, while the improvement attributable to lanthanum was partially canceled when SO2 was added. During isothermal tests at 500-degrees-C, the aged catalysts recovered only part of their lost activity when sulfur was removed from the feedstream. Exposure to a rich or cycled stoichiometric environment at or above 700-degrees-C was required to obtain the original activity of these aged catalysts.
