THE EFFECT OF SULFUR ON THREE-WAY CATALYSTS

Abstract Both pelleted and monolithic catalysts have been tested in a laboratory reactor under conditions designed to simulate the operation of the catalyst in a vehicle. The warmed-up and light-off performances of both types of catalyst were deteriorated by the sulfur. The monolithic catalyst rapidly recovered all of the lost activity when sulfur was removed from the feed gas, but the pelleted catalyst only recovered a portion of the lost activity. Examination of the effect of sulfur on Pt, Pd, and Rh indicated that both Pt and Rh rapidly recovered all of its lost activity when sulfur was removed from the feed, but Pd did not. Additionally, a pelleted Pt/Rh catalyst which did not contain any Ce also rapidly recovered all lost activity when sulfur was removed from the feed, but a similar catalyst which contained Ce did not. Hence, the slow recovery from sulfur poisoning is related both to the the high Ce surface area seen in pelleted catalysts and to the presence of Pd. The impact of the sulfur decreased when the cycling frequency was increased and the cycling amplitude decreased, indicating that sulfur will have less of an effect on cars with better A/F control. This result was confirmed in tests which used engine exhaust.