Development of Beaded Three-Way Catalysts With Reduced H ₂ S Emissions

The introduction of more active and durable Three-Way Catalysts (TWCs) on vehicles with advanced fuel management systems has created a new emissions problem. Hydrogen sulfide emissions are responsible for the odor readily apparent during certain modes of vehicle operation such as sudden braking (deceleration) and quick starts (acceleration). Experimental work aimed at designing a laboratory test to characterize catalysts for their hydrogen sulfide emissions behavior has revealed the existence of a narrow oxidant/reductant window where H₂S emissions reach a maximum. Efforts to develop catalysts with low H₂S emissions when operating in that A/F window led to the conclusion that cerium oxide while an excellent choice for TWCs is a key contributor to the H₂S problem. Manufacturers of beaded catalysts have been restricted from using nickel oxide, an effective H₂S scavenger, over concern about its potential entry into the environment. This has prompted the search for novel preparative methods and new compositions that result in low H₂S emissions while maintaining good TWC performance. The performance of several experimental catalysts having that combination of properties is illustrated by means of comparison with conventional beaded TWCs.