An Examination of Sulfur Poisoning on Pd/Ceria Catalysts

The species formed by exposure of a Pd/ceria catalyst to SO2 under various conditions have been studied using temperature-programmed desorption (TPD) and FTIR. For adsorption of SO2 between 298 and 473 K, a molecular SO2 species adsorbs on the surface, possibly as a surface sulfite; and this species converts to a sulfate above 473 K. Exposure of Pd/ceria to SO2 at temperatures above 473 K in the presence of O2 results in the formation of bulk sulfates. These sulfates decompose to form SO2 and O2 upon TPD in He, with O2 and SO2 peaks at 1023 K assigned to Ce+4 sulfates and peaks at 1123 K assigned to Ce+3 sulfates. When H2 is present in the TPD carrier gas, the sulfates are reduced and a significant fraction of the sulfur is removed as H2S, with the rest remaining as Ce2O2S. When CO is present in the TPD carrier gas, all of the sulfates are reduced to Ce2O2S, with the simultaneous formation of CO2. The formation of CO2 from the reduction of the sulfate occurs in the same temperature range as CO2 production from reduction of Pd/ceria, except that more CO2 is formed from the sulfur-poisoned catalyst. The implications of these results for understanding oxygen storage capacity (OSC) in automotive, three-way catalysts is discussed.