Impact of sulfur on three-way automotive catalyst performance and catalyst diagnostics

In an effort to reduce emissions from mobile sources to help address acute ozone non-attainment problems, the State of California adopted a Low Emission Vehicle/Clean Fuel program in 1990. Low Emission Vehicles, or LEVs, are designed to meet significantly stricter HC, CO and NOx standards than vehicles sold in the remainder of the U.S., and are designed to operate on a strictly controlled reformulated fuel to achieve low emissions over the vehicle's lifetime. One of the characteristics of this fuel is a low sulfur content, which averages 30–40 ppm or less. Data provided by the Auto Oil Air Quality Research Program and other studies were instrumental in the development of the low emission properties of this fuel, and vehicle manufacturers now use this fuel in the development, calibration, and certification process for Low Emission Vehicles sold in California. Such vehicles may also be introduced federally through a national low emission vehicle program or by individual states adopting the California program, but there is concern that the low emission benefits obtained in California may not be achieved federally because of the variability in fuel properties, including sulfur content, which may be present in concentrations as high as 1000 ppm. This has caused increased interest on the impact of sulfur on the performance of automotive catalysts, particularly catalyst systems which will be used in Low Emission Vehicles. The impact of sulfur on catalyst oxygen storage is also of interest, due to the requirement of an on-board catalyst performance diagnostic which uses the loss of catalyst oxygen storage as an indicator of catalyst deterioration. This paper will review both laboratory and vehicle studies performed at GM and elsewhere which focus on the impact of sulfur on the performance and oxygen storage capacity of catalyst systems which are anticipated to be used in low emissions vehicles, and notably catalysts which incorporate palladium, and compare the magnitude of these impacts to those observed on earlier generation automotive catalyst systems. This paper will also review how these observations are consistent with conclusions obtained from fundamental studies of the interaction between sulfur-containing gas species in vehicle exhaust and the catalytic and oxygen storage sites in the catalyst.