Characterization of SO2-Poisoned Ceria-Zirconia Mixed Oxides

CeO2, ZrO2, and a series of CexZr1-xO2 catalysts with 1 wt% Pd were exposed to fixed exposures of SO2 under oxidizing environments and then characterized by FTIR, pulse-reactor studies with CO and O2, and temperature-programmed desorption (TPD). For exposures above 473 K, sulfates were formed on all of the materials; however, the results are consistent with the formation of bulk sulfates on CeO2 and only surface sulfates on ZrO2. For the mixed oxides, the quantity of sulfates formed at 673 K increased linearly with the Ce content. In TPD, the sulfates on ZrO2 were stable to higher temperatures than those formed on CeO2, which decomposed in a well-defined peak between 900 and 1050 K. The sulfates on both oxides were reduced by CO above 750 K. Even though XRD patterns for the mixed oxide were significantly different from that of the physical mixture, the TPD and pulse-reactor results were similar to what would be expected for physical mixtures of CeO2 and ZrO2, suggesting that sulfate species are associated with individual metal cations. Finally, pulse-reactor studies with CO and O2 at 873 K show that the sulfates can be reversibly reduced and oxidized on both CeO2 and ZrO2, so that sulfur poisoning gives rise to an apparent increase in oxygen storage, demonstrating that this method is not acceptable for measurement of this quantity.