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The performance of three way and NOx catalysts was evaluated on vehicles utilizing non-feedback fuel control and electronic feedback fuel control. The vehicles accumulated 80,450 km (50,000 miles) using fuels representing the extremes in hydrogen-carbon ratio available for commercial use. Feedback carburetion compared to non-feedback carburetion improved highway fuel economy by about 0.4 km/l (1 mpg) and reduced deterioration of NOx with mileage accumulation. NOx emissions were higher with the low H/C fuel in the three way catalyst system; feedback reduced the fuel effect on NOx in these cars by improving conversion efficiency with the low H/C fuel. Feedback had no measureable effect on HC and CO catalyst efficiency. Hydrocarbon emissions were lower with the low H/C fuel in all cars. Unleaded gasoline octane improver, MMT, at 0.015g Mn/l (0.06 g/gal) increased tailpipe hydrocarbon emissions by 0.05 g/km (0.08 g/mile).

Poisoning of a typical platinum-rhodium (Pt-Rh) automotive three-way catalyst (TWC) was determined as a function of lead (Pb), sulfur (S) and phosphorus (P) fuel levels, thermal aging and sulfur dioxide (SO2) content in the evaluation fuel. In laboratory studies catalysts were durability tested in pulse-flame reactors followed by flow-reactor activity measurements. Engine dynamometer-aged catalysts were evaluated on a slave vehicle. For Pt-Rh TWCs the activities for nitric oxide (NO), carbon monoxide (CO) and hydrocarbon (HC) conversions were poisoned by trace levels of 1-6 mg Pb/gal (0.3 - 1.6 g/m3). When the peak temperature in the aging cycle was increased from 730 to 870°C (1346 to 1598°F), the activities improved significantly. In an attempt to mimic the effect on TWCs of misfueling with Pb levels typical of commercially available leaded fuels, TWC activities were severely poisoned.

The durability of automotive three-way catalysts (TWCs) for European applications were investigated as a function of higher temperatures encountered in autobahn driving modes over extended periods of time, potentially higher residual lead (Pb) levels anticipated in European marketed unleaded fuels, and occasional misfueling with leaded fuels. In laboratory durability and dynamometer aging studies, platinum-rhodium (Pt-Rh) TWCs at higher loadings than currently used in US applications maintained substantial three-way conversions when aged under rich conditions (λ ∼ 0.9) at maximum temperatures of ∼ 900 to 1000°C with 3 mg Pb/L fuel levels. Increasing maximum catalyst aging temperatures from 730°C to 1000°C resulted in ∼50% reduction in BET surface area which increased stoichiometric hydrocarbon light-off temperatures, but improved net NO and HC conversions after light-off due to lower Pb retention on the TWC.

This is a continuation of our earlier paper on the laboratory evaluation of three-way catalysts, SAE 76201. A number of recent 3-way catalyst formulations were evaluated in a laboratory flow-reactor when fresh, after 25,000 simulated miles on a pulse-flame reactor and after 100 or 200 hours of accelerated AMA dynamometer durability. A comparison was made of the effects of contaminant levels on the performance of pulsator - and dynamometer-aged selected catalysts. The 4-fold decrease in contaminant (lead and phosphorus) levels in 76/77 certification fuel compared with the 75/76 fuel significantly improved the durability of 3-way catalysts. The problems of increased NH3 formation on pulsator - and dynamometer-aged catalysts which contain base-metal oxides as oxygen-storage or water-gas shift components is attributed to S-poisoning. An inverse relationship between NH3 formation and the amount of rhodium on aged 3-way catalysts was noted.

Prototype metallic and electrically heated metallic catalysts (EHC) are being evaluated on a gasoline fueled vehicle. The vehicle used for this evaluation is a 5.0L Mustang equipped with an emissions control system which includes mass air, and sequential electronic fuel injection (SEFI). FTP tests are performed to evaluate non-heated metallic and electrically heated metallic catalysts in both production and close-coupled configurations. The objective of the evaluation is to determine light-off characteristics of both the non-heated metallic and the EHCs and their effect on cold-start emissions (Bag 1). FTP results are compared to those obtained from conventional ceramic catalysts in the same configurations. Initial data show some emissions benefits for the prototype EHC converter during the first 60 seconds or so of the FTP. From 60 seconds on in the FTP cycle, the ceramic catalyst was slightly more efficient, leading to overall FTP emissions about the same for the two systems.

This is the third and final communication in this series of laboratory evaluation of three-way catalysts. The effect of inlet NO concentration and temperature on the NH3 formation over fresh, pulsator-aged and dynamometer-aged three-way catalysts of the current generation has been investigated under temperatures and exhaust compositions of practical interest. In spite of differences in aging procedures employed, both the pulsator and dynamometer-aged catalysts show similar selectivity behavior. The effect of SO2 in feed-gas on gross NO conversion and NH3 formation was studied over Pt-Rh and Pt-Rh-Ru types of three-way catalysts. A strong dependence of the gross NO conversion on the SO2 concentration in exhaust gas mixtures was noted. A simultaneous suppression of gross NO conversion and NH3 formation, in presence of SO2 in feed-gas, is attributed to the poisoning of Pt sites on aged three-way catalysts.

A comprehensive laboratory evaluation was carried out on recent three-way catalyst formulations. The evaluation of selectivity characteristics was made in a synthetic exhaust mixture where “window” widths and positions for three-way conversion and their change after durability runs were determined. The durability runs were made in combusted gases from laboratory pulse-flame exhaust generators using both contaminant-free fuel and fuels with 1975 levels of Pb, P and S. A thorough evaluation of the “oxygen-storage” capability of the catalysts was performed and the results correlated with engine dynamometer experiments designed to utilize this property of three-way catalysts which allows a wider A/F ratio tolerance. A new technique which involves intentional modulation of the A/F ratio was found to extend the usefulness of such catalysts.

On occasions automotive fuels have been contaminated by adventitious admixtures of silicon (Si)-containing compounds which have deleterious effects on automotive catalysts and oxygen sensors. The deactivation of monolithic automotive catalysts by fuel-derived silicon is due to deposition of crystalline silica (∝-SiO2) on the catalyst surface which causes mass transfer limitations and may ultimately result in plugging of the monolith. Stoichiometric conversions efficiency of three-way catalysts (TWCs) from various low-mileage vehicles were significantly deteriorated; e.g., from typical three-way efficiencies of −95% conversion to <50% conversion at 550°C after only 1500 mi of vehicle use. Laboratory aging of a TWC exposed to combustion products of isooctane fuel containing 20 ppm Si resulted in a continual decline in three-way conversions to <40% after 15,000 simulated miles.

Automotive three-way catalysts (TWC) were characterized using temperature-programmed reduction (TPR), x-ray diffraction (XRD), Raman spectroscopy, chemisorption measurements and laboratory activity measurements. Capabilities and limitations of these standard analytical techniques for the characterization of production-type automotive catalysts are pointed out. With the exception of chemisorption techniques, all appear to have general utility for analyzing exhaust catalysts. The techniques were used to show that the noble metals and ceria in fresh Pt/Rh and Pd/Rh catalysts are initially highly dispersed and contain a mixture of interacting and non-interacting species. Thermal aging of these catalysts (in the reactor or vehicle) caused both precious metal and ceria particles to sinter, thereby decreasing the interaction between the two.

A novel non-phosphorus antiwear additive, NP-1, was evaluated as a partial substitute for zinc dialkyldithiophosphate (ZDTP). ZDTP, an antiwear/antioxidant engine oil additive may under certain conditions cause three way catalyst (TWC) deactivation due to formation of an amorphous zinc pyrophosphate, Zn2P2O7, glaze. Antiwear and antioxidant properties of NP-1 alone and in combination with ZDTP were compared with ZDTP only containing formulations. The effects of NP-1 on TWC activity during pulsator modulation and steady-state conditions showed that the TWC maintained good overall activity during 24,000 simulated miles.