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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 dynamics of the desulfation of a Ba-containing and a K-containing NOx storage catalyst have been investigated. When both catalysts were desulfated using a temperature ramp in exhaust that simulated gasoline exhaust with a 13:1 A/F, the maximum desulfation rate for the Ba-containing catalyst was seen at 620°C, while the maximum for the K-containing catalyst was at 760°C. This is consistent with the widely known fact that K2SO4 is more stable than BaSO4. The BaSO4 decomposed when either hydrogen or water was in the feed, but not when both were absent. The decomposition, therefore, requires hydrogen to be present and the water can provide sufficient hydrogen for the decomposition via the water-gas shift reaction. With either water or hydrogen in the uncycled feed, the primary sulfur compound formed from the decomposition was H2S for both the Ba and K-containing catalysts.

A Cu/ZSM-5 catalyst has been tested in both the laboratory and in a gasoline-fueled, lean-burn car. This catalyst is generally believed to be the current “state of the art” for removing NOx from lean engine exhaust. The laboratory tests showed that the HC species was the exhaust component which reduced the NOx. This catalyst actually produced CO, but the CO had no impact on the NOx reduction. The NOx conversion was inhibited by oxygen at oxygen levels below 4%, but was also modestly inhibited by oxygen levels above 4%. The NOx, conversion of these catalysts was less affected by SO2 than were the HC and CO conversions. These catalysts showed very poor thermal durability, losing much of their activity after aging under conditions milder than those typically used for three-way catalysts. The vehicle tests of the fresh Cu/ZSM-5 catalyst showed between 30 and 40% average NOx conversion.

Vehicle exhaust emissions tests, using the Federal Test Procedure, were conducted to determine the effect of gasoline sulfur content on the performance of three-way catalysts. The test fuels had sulfur concentrations of 0.01, 0.03, and 0.09 percent. An increase in the fuel sulfur content from 0.01 to 0.09 percent reduced the conversion of hydrocarbons, carbon monoxide, and oxides of nitrogen, resulting in higher tailpipe emissions. The effects were generally small, but statistically significant. The lower conversion was due to poisoning of the catalyst by sulfur species in the exhaust. The poisoning was reversible.

A variety of lightoff and sweep tests were developed to characterize the performance of three-way catalysts using a computer-controlled laboratory reactor. Both fresh and thermally aged pelleted catalysts were characterized using these lightoff and sweep tests. For all of the lightoff and sweep tests, the aged catalyst showed a loss of performance when compared to the fresh catalyst. Although the tests which use a cycled environment show better agreement with engine dynamometer tests, the non-cycled tests were generally better able to distinguish between fresh and thermally aged catalysts. These results imply that while the laboratory reactor can perform tests which closely simulate the dynamic behavior of engine exhaust under closed-loop control, non-cycled tests may be more useful in ranking catalysts for durability.

A three-way catalyst containing only Rh and another containing Pt, Pd, and Rh were aged in engine exhaust. The engine was operated on fuel containing phosphorus or lead additives at concentrations high enough to accelerate the poisoning of the catalysts. Lead did not noticeably affect either catalyst's CO conversion. It did, however, reduce both catalysts' HC and NO conversions. The poisoning effect of phosphorus increased as the amount of phosphorus on the catalyst increased. While the amounts of phosphorus in the fuel consumed by the engine were similar in each test, the phosphorus collected by the catalysts varied greatly, depending on the nature of the fuel additives.

The activities of three-way catalysts containing various Pt and Rh loadings have been tested in a laboratory reactor. These tests were performed on the catalysts both when they were fresh and following aging in engine exhaust. Increasing the Pt and Rh loadings from 0.1 wt % and 0.01 wt %, respectively, to 0.2 wt % and 0.02 wt %, respectively, resulted in improved catalyst lightoff and improved warmed-up performances during A/F ratio cycling for both the fresh and the aged catalysts. Increasing only the Rh loading of these bimetallic catalysts resulted in a greater improvement in the lightoff and warmed-up NO conversion efficiencies than resulted from increasing only the Pt loading. Increasing the Pt loading, however, resulted in a greater improvement in the warmed-up CO conversion efficiencies than did increasing the Rh loading. Increasing either the Pt or the Rh loading resulted in similar improvements in the warmed-up HC conversion efficiencies.

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.

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.