EVALUATION OF THREE-WAY CATALYSTS. PART II

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.