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A matrix of pelleted catalysts composed of Pt, Pd, Pt co-impregnated with Pd, and Pt physically mixed with Pd supported on A l2O3 were compared with the same noble metal formulations supported on CeO2/Al2O3 for lightoff and warmed-up performance in net lean exhaust. These catalysts were tested as prepared (fresh) and following a relatively severe thermal aging treatment (cycled between net lean and net rich environment at 1000°C for 4 h). Pd showed better lightoff performance than Pt for catalyzing the oxidation of propylene, while Pt showed better lightoff and warmed-up performance than Pd for catalyzing the oxidation of propane. Having both Pt and Pd present as a result of co-impregnation or physical mixture results in good lightoff and warmed-up performance for the conversion of both types of hydrocarbons. The presence of CeO2 generally decreases lightoff performance for most of these catalysts.

Pelleted and monolith three-way catalysts were treated in a laboratory reactor under conditions closely simulating automobile exhaust and then characterized using x-ray photoemission and temperature programmed desorption techniques to determine relative amounts and oxidation states of sulfur stored by the catalyst. Sulfur originating as SO2 in the feed was stored on the support component of both catalysts in the form of adsorbed sulfates and sulfites and on the noble metals in the form of elemental sulfur. The monolith catalyst stored a greater amount of sulfur and equilibrated more rapidly with the sulfur content in the feed than did the pelleted catalyst. Operation in a rich environment removes sulfur from the support components, while operation in a lean environment removes sulfur from the noble metal surfaces. This behavior is consistent with the observation that sulfur inhibits three-way activity in a rich environment.