Investigation on catalytic oxidation of CO over Mo-Sn catalysts

With the rapid development of industry and the gradual improvement of environmental awareness, air pollution has emerged as a prominent environmental issue. Carbon monoxide (CO), generated from the incomplete combustion of carbon-containing substances such as coal and oil, as well as produced by the exhaust emissions of vehicles, is one of the primary sources of atmospheric pollution. Herein, the elimination of low-concentration CO has been widely used in various applications, including industrial flue gas purification, automobile exhaust treatment, and hydrogen fuel cell purification, etc. Therefore, studying the elimination process of low-concentration CO has important practical significance. At present, low-temperature catalytic oxidation of CO is widely recognized as the most direct, simple, inexpensive, and effective method for CO elimination. Additionally, as a classic and simple reaction process, CO oxidation reaction can be often used as a probe reaction to investigate metal-support interactions, adsorption/dissociation behaviors, the structure of active centers, and the structure-activity relationship of catalysts. SnO₂ has been used as a catalyst for CO catalytic oxidation due to its low energy for oxygen vacancy formation and the proven reducibility and reactivity of lattice oxygen. However, due to less active sites on the surface of SnO₂ for CO catalytic oxidation, complete conversion of CO can only be achieved at higher reaction temperatures, thereby limiting its application. Specifically, molybdenum oxide is a typical non-precious metal catalytic material that is widely used in low-temperature selective oxidation reactions due to its diverse coordination structures and easily adjustable electronic states. Previous research has found that the directional oxidation of alcohol/ethers to target products can be achieved by regulating the active site structure of Mo-Sn catalysts, which exhibited unique and excellent low-temperature oxidation performance. Its adjustable structure, variable valence state, and highly active oxygen species provide new ideas for expanding the application of Mo-Sn catalysts in low-temperature oxidation of CO. In order to further investigate the mechanism and structure-activity relationship of Mo-Sn catalysts in CO catalytic oxidation reaction, in this work, a series of Mo-Sn catalysts with different Mo/Sn molar ratios were prepared by precipitation impregnation method. CO oxidation reaction was used as a probe reaction to investigate the effect of different Mo contents on the catalytic performance of CO oxidation, and further elucidate the active center structure and structure-activity relationships of the catalysts. The results showed that the Mo1Sn20 catalyst achieved complete conversion of CO at 300 ℃, which was 50 ℃ lower than that of pure SnO₂. XRD, Raman, XPS, H₂-TPR and in situ FT-IR were used to analyze the crystal structure, valence state of molybdenum species, and redox properties of the catalysts. Compared with pure SnO₂ catalyst, when a lower content of Mo species is introduced, the increase in specific surface area of Mo1Sn20 catalyst provides more active sites for the reaction. The enhanced interaction between Mo and Sn leads to the partial transformation of MoO₃ to MoO_x, producing more Mo⁵⁺ species. The presence of Mo⁵⁺ species promotes the adsorption activation of oxygen as well as the migration of oxygen atoms. The synergistic effect of lattice oxygen and Mo⁵⁺ species enhances the catalytic performance of the CO. This study not only provides a new strategy for efficient catalytic elimination of low concentration CO, but also deeply reveals the structure-activity relationship and active center structure of Mo-Sn catalysts in CO oxidation reactions. Also, it provides theoretical basis and experimental reference for the application of Mo-Sn catalysts in more important catalytic oxidation reactions.