With the advent of stricter vehicle emission standards, the improvement of three way catalyst performance and durability remains a pressing issue. A critical consideration in catalyst design is the potential for variations in fuel sulfur levels to have a significant impact on the ability to reach TLEV, LEV, and ULEV emission levels. As a result, a better understanding of the role of PGM composition in the interplay between thermal durability and sulfur tolerance is required.
Three way catalysts representative of standard Pd-only, Pd/Rh and Pt/Rh formulations were studied over a variety of aging and evaluation conditions. The parameters investigated included aging temperature, air fuel ratio and sulfur level. Evaluations were performed on a 1994 TLEV vehicle using different sulfur level fuels. The effect of PGM loading was also included within the study.
As expected, Pd-only catalysts generally had superior hydrocarbon performance under all conditions, however significant deterioration in carbon monoxide and nitric oxide performance was noted in the presence of high sulfur level fuels. In particular, low temperature aging coupled with high sulfur levels produced the poorest performance from Pd-only catalysts. Under these conditions, however, the Rh containing catalysts exhibited far superior nitric oxide and carbon monoxide performance.
The loading study revealed a significant effect on nitric oxide conversion, notably a different relationship was observed when comparing the Pd-only and Rh containing catalysts. The Pd-only catalyst exhibited a large conversion efficiency improvement from relatively low to medium loading.
Several sulfur recovery experiments were performed and conditions under which catalyst activity is regained were determined. For Pt/Rh, and Pd/Rh catalysts, the deactivation of the nitric oxide and carbon monoxide performance was partially reversible.