1977-02-01

EVALUATION OF THREE-WAY CATALYSTS. PART II 770196

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. The effects of deliberate misfire on the selectivity and activity characteristics are discussed. Further, the role of the water-gas shift and steam-reforming components in extending the “window” was studied. The effect of temperature and of inlet CO (H2) and hydrocarbon concentration on these reactions was investigated separately on single component samples and fully formulated catalysts. The extent of sulfate formation over typical fresh and dynamometer-aged 3-way catalysts is shown. The relatively small effect of Rh levels in 3-way catalysts on the extent of the sulfate formation is attributed to the possible enrichment of the catalyst surface by Rh, under oxidizing conditions, thereby resulting in a lower Pt/Rh ratio on the surface of the catalyst than in the bulk.

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