SO2 Poisoning of Ceria-Supported, Metal Catalysts

RAYMOND J. GORTE and TIAN LUO 311 Towne Building, 220 S. 3 3 Street, University of Pennsylvania, Philadelphia, PA 19104 USA e-mail gorte seas.upenn.edu 

The current generation of catalysts would be sufficient to meet some of the most stringent requirements if sulfur were not present in essentially all hydrocarbon fuels. The combustion of sulfur-containing fuels produces SO2 in the engine exhaust. In high concentrations and for reducing environments, SO2 poisons sites on the precious-metal catalysts by forming adsorbed sulfur atoms, which in turn block a portion of the active metal sites and reduce the activity of adjacent metal sites [5]. 

However, for the relatively low sulfur concentrations to which three-way catalyst are typically exposed, the precious metals are usually unaffected. Poisoning of ceria is much more serious than poisoning of the precious metal at the levels of 5 to 20 ppm SO2 currently present in the typical automotive exhaust. At these concentrations, SO2 interacts primarily with the ceria-containing component of in the catalytic converter and it is this poisoning of ceria that appears to be the primary problem associated with sulfur poisoning [6-11]. The evidence for this is strong. For 

Clearly, understanding the interaction between SOj and ceria is extremely important in three-way, emissions-control catalysis. This chapter will review what is known about the nature of sulfur poisoning of ceria and how this affects the catalytic properties. 

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In the absence of SO2, Pt supported on silica and alumina show about the same activity for CO oxidation (blue squares) Upon addition of 20 ppm of SOj in the gas feed, the ignition temperature increases only 5°C during the first run in the alumina-supported sample, compared to 3UC in the case of the silica-supported catalyst. Because both samples have similar metal dispersion, the differences in LOT can only be attributed to the nature of the supports. In the literature, it has been proposed that the presence of SOj leads to the formation of sulfates on the alumina surface, which thus becomes a sulfur trap. This would allow the Pt surface to appear not to be affected as strongly as in the case of the silica support in this short time on stream (T-O-S) run. The increase in LOT is relatively small for the ceria-promoted alumina catalyst, probably for the same reason as for unpromoted alumina. Addition of cerium oxide to Pt/silica significantly decreases the LOT (by 26°C compared to Pt(0.63)/SiO2). In the presence of S, however, the activity is the same as that of the Pt without Ce, indicating poisoning of the Ce promotional effect. 

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