A Study on Sulfur Resistance of Metal Oxide Catalysts for Carbon Monoxide Oxidation

Abstract

In this work, the modification of metal oxide catalysts was conducted to improve the sulfur resistance of catalysts and the prepared catalysts were applied to CO oxidation. First, two kinds of mesoporous cerium-zirconium-aluminum metal oxide were prepared by a single-step epoxide-driven sol-gel method and by a co-precipitation method to investigate the effect of preparation method for sulfur resistance. Palladium catalysts supported on these materials were then prepared by a wet impregnation method. The catalyst prepared by sol-gel was denoted as Pd/SGCZA and the catalyst prepared by precipitation was denoted as Pd/PCZA. The prepared catalysts were applied to the CO oxidation reaction before and after sulfur aging. XRD and N2 adsorption-desorption analyses revealed that these two catalysts retained different physicochemical properties. Pd/SGCZA had higher surface area and larger pore volume than Pd/PCZA before and after sulfur aging. H2¬-TPR (Temperature-programmed reduction), CO chemisorption, FT-IR, and XPS analyses showed that the catalysts were differently influenced by sulfur species. Pd/SGCZA formed less sulfate and retained higher palladium dispersion than Pd/PCZA after sulfur aging. In the CO oxidation, Pd/PCZA showed better activity than Pd/SGCZA before sulfur aging. However, Pd/SGCZA showed higher CO conversion than Pd/PCZA after sulfur aging. We concluded that Pd/SGCZA was less poisoned by sulfur species than Pd/PCZA.

In the second part, La1-xCaxCoO3 perovskite catalysts were prepared by using citrate sol-gel method with various calcium ratios (denoted as La1-xCaxCoO3 (x=0, 0.1, 0.2, 0.3, 0.4)). The prepared catalysts were poisoned with simulated gas including SO2 to investigate sulfur resistance of the catalysts. The catalysts were then applied to CO oxidation reaction. X-ray diffraction analysis revealed that the catalysts containing calcium showed perovskite structure. In the CO oxidation activity test, La0.8Ca0.2CoO3 showed the best activity. H2-temperature-programmed reduction showed that reduction temperature of Co3+ presented similar behavior with catalytic activity, indicating that proper addition of Ca to LaCoO3 promotes the facile reduction of Co3+ explaining the higher activity for CO oxidation. The catalytic activity of sulfur-aged catalysts were also investigated. Surprisingly, small amounts of SO2 poisoning were observed to be helpful for CO oxidation activity in Ca-substituted catalysts, although further exposure of SO¬2 gave rise to the decline of catalytic activity. Ca-substituted catalysts had better catalytic activity than LaCoO3 after sulfur aging. XPS analysis indicated that Ca-substituted catalysts have less residual sulfate on the surface than LaCoO3, confirming that the perovskite structure substituted with Ca is more resistant to sulfur poisoning, leading to the higher CO oxidation activity.