Fuel sulfur content has long been recognized to affect the performance of automotive emission control catalysts. Numerous studies have shown that three-way catalyst (TWC) performance is inhibited by sulfur and that the sulfur poisoning effect is at least partially reversible under normal TWC operating conditions. In this presentation, a literature review is first provided of fundamental laboratory studies aimed at understanding the interaction of sulfur with three-way automotive catalyst components, such as platinum group metals (PGMs) and base metal promoters (e.g., ceria). Although exposure of automotive catalysts to SO2 formed during engine combustion leads to catalytic activity reduction as a result of blockage of the reaction sites on all platinum group metal surfaces by strongly bound elemental sulfur, Pd is particularly sensitive to sulfur poisoning due to sulfur migration into bulk Pd. SO2 can also react readily with ceria to form a stable cerium sulfate, impairing the cerium’s redox (and thus oxygen storage/release) property. This, in turn, severely inhibits the reactions catalyzed by the PGM/ceria interfacial sites (e.g., water-gas shift reaction) and can potentially present challenges for the dual oxygen sensor-based catalyst monitoring systems when vehicles designed to operate on low-sulfur fuel are exposed to higher-sulfur fuel. In agreement with the laboratory results described above, vehicle fleet test results indicate that Pd-based formulations with large amonts of Ce additives used in advanced three-way catalyst systems for late-model low-emission vehicles are more susceptible to sulfur poisoning than Pt/Rh formulations used in early-generation TWCs. Although the sulfur poisoning effects can be largely reversed under high speed/load conditions (e.g., US06 cycle), in-use vehicle test results show evidence for the presence of significant amounts of sulfur retained in the catalysts, indicating partial irreversibility of sulfur poisoning under real-world driving conditions.

Review of fuel sulfur effects on vehicle emissions and catalyst monitoring systems

2019 Spring National Meeting

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