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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.

Three vehicle and dynamometer-aged converters were evaluated for performance using the warmed-up portion (termed the “hot-18” cycles) of the Federal Test Procedure (FTP) and during a steady-state cruise using low and high sulfur content fuel. During the steady-state tests the catalyst performance was also monitored using two oxygen sensors designed to satisfy Phase II On-Board Diagnostics (OBD-II) requirements for vehicles sold in California in MY 1994 and thereafter. The performance of two of the converters decreased significantly upon switching to the high sulfur content fuel, and one of the converters which was diagnosed as passing with low sulfur fuel was diagnosed as failing with high sulfur fuel.

Catalysts taken from the lightoff and underfloor converters of two 1986 Corvette exhaust systems which had been vehicle-aged for 100,000+ miles were cut into 1″ thick sections along their axis and characterized for lean lightoff and warmed-up performance using a laboratory reactor. Sections were then treated to remove the poisons, and the characterization was repeated. An axial gradient in both lightoff and warmed-up conversion efficiency for HC and CO was detected within the first 2″ of both lightoff converters for both vehicles. This activity gradient is in agreement with the gradient in the phosphorus and zinc concentrations found in the front 2-3″ sections of the lightoff converters.

The performance of the OBD-II catalyst monitor of a 2.2 Corsica TLEV was investigated while operating on fuels with differing sulfur levels. Testing was performed using a driving schedule producing operating conditions which were acceptable to the production OBD-II system for active diagnosis of catalyst performance. During each test, catalyst diagnostic data and emissions performance data were collected simultaneously. Tests were performed at 22° C using California Phase II (35 ppm sulfur content) and Indolene clear (470 ppm sulfur content) fuels, and at 4°C using a commercial oxygenated Winter fuel (320 ppm sulfur content). Four dynamometer-aged converters were using in the testing. Converter performance in the form of HC conversion efficiency was found to generally decrease when the sulfur content in the fuel was increased.