Interaction of Sulfur with Automotive Catalysts and the Impact on Vehicle Emissions-A Review 1999-01-1543

The requirement to meet more stringent emission standards has focused attention on the effects of gasoline sulfur on automotive emissions. Numerous studies have shown that three-way catalyst performance is severely inhibited by sulfur. A literature review of laboratory studies on the interaction of sulfur with automotive catalyst components provides the basis for understanding impacts on catalyst activity under the variety of conditions encountered in vehicle operation. Under stoichiometric and rich conditions, SO₂ formed during combustion is dissociatively adsorbed on platinum group metal surfaces to form strongly bound S_ad. Sulfur inhibition results from both physical blockage and electronic effects of S_ad, such that low coverage of S_ad results in disproportionately higher levels of reaction site blockage. This is responsible for the nonlinear effects observed with increasing fuel sulfur level. Migration of sulfur into bulk Pd accounts for its greater sensitivity to, and irreversibility of, sulfur inhibition. Weaker interaction with SO₂ and easier removal of S_ad result in better sulfur tolerance for Rh. Overall, the order of sulfur sensitivity is Pd>Pt>>Rh. Under stoichiometric and lean conditions, SO₂ reacts with ceria to form Ce₂(SO₄)₃, diminishing the oxygen storage and release capacity of the catalyst.

Vehicle results are generally in agreement with laboratory results and indicate Pd formulations are more sensitive to the effects of sulfur than Pt/Rh formulations. Even with this increased sensitivity, advanced Pd catalysts have performance and thermal durability advantages that result in their increased use in advanced emission control systems. Large variability of sulfur sensitivity observed in vehicle fleet studies indicate that system effects such as the interaction of vehicle calibration with catalyst formulation play a key role in sulfur inhibition. Studies with advanced catalyst technologies show substantial improvements in performance, but do not provide evidence for fundamental improvements in sulfur tolerance. Reversibility of sulfur inhibition is critical in assessing the impact of periodic operation with high sulfur gasoline on low emission vehicle technologies designed to operate in regions with low sulfur reformulated fuel.