The Preparation Of Pd-Sn_1-xCe_xO₂ Catalysts And Their CO Catalytic Oxidation Performance

In the development of social economy and excessive consumption of fossil fuel,environmental pollution has developed to a very serious degree,which greatly affects economics and human beings.CO produced from the uncompleted combustion of fossil fuel is very harmful,and the most promising and effective method to remove it is the CO catalytic oxidation.In present work,a series of Pd-Sn_1-xCe_xO₂ catalysts with variable Ce/Sn ratios were synthesized by the co-precipitation method for CO catalytic oxidation.These catalysts were characterized by BET,XRD,TEM,H₂-TPR,CO-TPR,XPS and XAFS techniques.The SnO₂-CeO₂ binary catalysts exhibited higher activity than pure SnO₂ and CeO₂.The ratio of Sn/Ce greatly affected the catalytic performanc,and Sn_0.6Ce_0.4O₂ displayed the best CO oxidation performance with not only the lowest activation energy（Ea = 77 kJ mol-1）but also the lowest temperature for 50% conversion of CO（T50 = 245 °C）.Strong interaction existed between SnO₂ and CeO₂ due to the formation of SnO₂-CeO₂ solid solution.Compared to SnO₂ and CeO₂,the SnO₂-CeO₂ binary catalysts have smaller crystallite sizes and larger surface areas.As revealed by H₂-TPR and CO-TPR results,the improved oxidizability of the surface Sn⁴⁺ of Pd-Sn_1-xCe_xO₂ catalysts is due to the redox equilibrium of Sn⁴⁺+ 2Ce³⁺ Sn₂++ 2Ce⁴⁺,and the enhanced oxidizability of surface Sn⁴⁺ cations greatly promote the CO oxidation performance.The XPS and XAFS results investigated that SnO₂-CeO₂ binary catalysts with higher concentration of Ce³⁺ and oxygen mobility can contribute to the CO oxidation performance.A series of 1 wt.% Pd/Pd-Sn_1-xCe_xO₂ catalysts were also prepared through the incipient wetness impregnation method to evaluate the performance of CO catalytic oxidation.Many techniques such as BET,XRD,TEM,H₂-TPR,CO-TPR,CO-TPD,XPS and XAFS were engaged for catalyst characterization.Pd/Pd-Sn_1-xCe_xO₂ catalysts displayed higher CO oxidation activity as compared with Pd/SnO₂,and Pd/ Sn0.2Ce0.8O₂ exhibited the best catalytic performance than other Pd-doped catalysts.Pd/Sn0.2Ce0.8O₂ achieved the lowest temperature for 50% conversion of CO（T50 = 138 °C）and the lowest activation energy（Ea = 34 kJ mol-1）.The H₂-TPR and CO-TPR results indicated that the Pd/Pd-Sn_1-xCe_xO₂ catalysts have higher reducibility of supported PdO to metallic species,especially for Pd/Sn0.2Ce0.8O₂.The Pd/CeO₂ and Pd/ Ce-rich catalysts have more surface active oxygen species and improved oxygen storage and release property,which were favorable to the CO oxidation activity.The XPS and XAFS results showed that the Pd/Pd-Sn_1-xCe_xO₂ catalysts have higher concentration of surface Ce³⁺ cations combined with more oxygen vacancies.