Sulfur Poisoning and Regeneration Behavior of Perovskite-Based NO Oxidation Catalysts

SO _x uptake and release properties of LaMnO₃, Pd/LaMnO₃, LaCoO₃ and Pd/LaCoO₃ perovskites were investigated via in situ Fourier transform infrared (FTIR) spectroscopy, temperature programmed desorption and X-ray photoelectron spectroscopy. Sulfation of the perovskite leads to the formation of surface sulfite/sulfate and bulk-like sulfate species. Pd addition to LaMnO₃ and LaCoO₃ significantly increases the sulfur adsorption capacity. Pd/LaMnO₃ sample accumulates significantly more sulfur than LaMnO₃; however it can also release a larger fraction of the accumulated SO _x species in a reversible fashion at elevated temperatures in vacuum. This is not the case for Co-based materials, where thermal regeneration of bulk sulfates on poisoned LaCoO₃ and Pd/LaCoO₃ is extremely ineffective under similar conditions. However, in the presence of an external reducing agent such as H₂ (g), Pd/LaMnO₃ requires much lower temperature (873 K) for complete sulfur regeneration as compared to that of Pd/LaCoO₃ (973 K). Sequential CO and SO _x adsorption experiments performed via in situ FTIR indicate that in the presence of carbonyls and/or carbonates, Pd adsorption sites may have a stronger affinity for SO _x as compared to that of the perovskite surface, particularly in the early stages of sulfur poisoning.