contd..

The sintering of precious metal and wash coat are shown schematically on Fig. 5.26 ..

For the ceramic monolith catalytic converters, the temperature limit during 1990's was about 900° C. Improvements in the washcoat technology and use of metallic monoliths has raised this limit to around 1050°C . The correlation of different deactivation mechanisms of 3-way catalysts with operating temperature is summarized in Fig. 5.27 . The preferred operating range of the catalysts has gone up from 400 –700° C in 1980s to a maximum temperature of 1000° C in 1990s through development of better washcoat and catalyst formulations. The catalytic converters now are being mounted very close to the engine for cold engine emission control and hence higher catalyst temperatures Development of new substrate materials like SiC has contributed to catalyst operation at much higher temperatures than before.

	Temperature, ºC	Effect on Catalyst
	1700	High Temperature Ceramic melts
	1500	Standard Cordierite Monolith Melts
	1400
		Cordierite phase changes to Mullite
	1300	Washcoat deterioration ?– Alumina changes to a -            Alumina
	1100
	900	Pt-Pd alloy forms in oxidizing A/F
		Diffusion of Rh2O3 in Al2O3 , Alumina begins            sintering
	800
		Pt-Pd and Pt-Rh alloy forms in Reducing A/F
	700	Platinum sinters
		Reaction of Zn and P with washcoat
	500
	300	Catalyst Light-off
	100