Studies Of Mechanism Of Carbon Monoxide Oxidation On Ceria-based Catalysts

Carbon monoxide is a noxious gas in automotive exhaust, which is detrimental to human health. One of the effective methods to reduce the influence of CO is to convert CO to CO2 which has relatively lower negative influence than CO. Many efforts have been paid to study CO catalytic oxidation, an energy-efficient and environment protection technique, because this technique can decrease the temperature of CO oxidation. Ceria has been widely applied in CO oxidation catalyst because of its unique oxygen storage property. However, the catalytic activity and thermal stability of pure ceria is limited. Many researches indicate that doping other element into ceria can greatly improve the CO oxidation activity. Furthermore, ceria and ceria-based oxide not only show good activity for CO oxidation, but also as the support of noble metal catalyst, which significantly enhances the catalyst activity. The wide attention has been paid to study how to further improve the CO oxidation activity of ceria-based catalyst. Though many researchers have studied factors which influence the CO oxidation activity of the ceria-based catalysts, the key factor which primarily influences the catalyst activity is still ambiguous. The insufficient researches greatly limit the scientific and effective catalyst design.The understanding of the catalytic mechnism of ceria-based catalysts relies on the investigation of the reaction mechanism. The identification of the rate-determining step (RDS) in the overall reaction can promote effectively the catalyst design. However, the reaction mechanism of the CO oxidation on ceria-based catalyst is still in debating, which indicates that more works are required to propose potential way to optimize the catalyst. Hence, in this study, the systematical investigations were conducted to reveal the factors that primarily influence the CO oxidation activity of ceria-based catalyst, by coupling with the studies on the reaction mechanism and on the relationship between the catalyst structure and the catalytic activity. The potential way to enhance the catalytic activity of the catalyst was proposed.First, the reaction mechanism of CO oxidation on ceria was studied to understand the origin of the catalytic activity of ceria, and to provide a potential method to improve the catalyst activity. The key factors which influenced the catalyst activity were also proposed. Two kinds of ceria were prepared by hydrothermal method and citric acid sol-gel method, and the as-prepared ceria were named as HY and CA, respectively. The results of kinetics investigation indicated that the lattice oxygen extraction was the RDS in the CO oxidation on both catalysts, which was influenced by the amount of active lattice oxygen and the rate of oxygen migration. The amount of active lattice oxygen and the rate of oxygen migration were related to the surface area and oxygen vacancies, respectively. So, the larger surface area and more surface oxygen vacancy of HY enhanced its CO oxidation activity via advancing the reaction rate of RDS.Second, the catalyst deactivation induced by carbon deposition has been reported in the literature when employed ceria as the support. In order to inhibit the carbon deposit produced by CO disproportionation, the mechanism of the CO disproportionation on the reduced ceria was studied. The results of pulse and thermal analysis experiments indicated the occurrence and the reversibility of the CO disproportionation on the reduced ceria. In situ DRIFTS spectra indicated that the asymmetrical inorganic carboxylate species were the key reaction intermediates in both the forward and reverse reactions of the CO disproportionation. Besides, the results of in situ DRIFTS showed that the vibration frequency of the C-0 bonds on Ce3+ ions is lower than that on Ce4+ ions, which indicated that the reduced ceria weakened the adsorbed C-0 bonds. Results of EPR experiments indicated that the number of unpaired electrons increased with increasing the reduction extent. These electrons would back donated to the C-O bonds and resulted in the weakening of the C-O bonds. Along with the C-O bonds dissociation, the CO disproportionation took place on the reduced ceria.On the other hand, many researchers improved the CO oxidation activity of ceria via doping. Cu-doped ceria (CeCu) has been widely focused because of its excellent CO oxidation activity. However, the key factor which influenced the activity of the CeCu is still in debating. In this study, CeCu catalysts with different Ce/Cu mol ratio were prepared, and the CO oxidation activities of these catalysts were studied. The results indicated that the catalyst activity was primarily influenced by the amount of the active lattice oxygen. The kinetics results indicated that the CO oxidation on these catalysts followed Mars-van Krevelen mechanism and the lattice oxygen extraction was the RDS. Therefore, the amount of the active lattice oxygen influenced the catalyst activity via influencing the RDS. The analysis of the catalyst structure indicated that solid solution structure which was formed by Cu doping was responsible for the active lattice oxygen. The CeCu catalyst with more solid solution showed better CO oxidation activity.Additionally, to confirm the influence of the amount of the active lattice oxygen on the catalytic activity of CeM (M=Cu, Ti, Zr, and Tb), M was selected and doped into ceria. The results of isotopic oxygen tracer experiment and kinetics investigations indicated that the CO oxidation on these catalysts followed Mars-van Krevelen mechanism and the lattice oxygen extraction was the RDS. Furthermore, the CO oxidation activity of CeM linearly enhanced with the increase of the amount of active lattice oxygen. And the amount of active lattice oxygen linearly increased with the increasing of the Pauling electronegativity of the M. These results indicated that the Pauling electronegativity of the M can influence the CO oxidation activity of the CeM catalysts via adjusting the amount of the active lattice oxygen.Besides doping other elements into ceria, the loading of noble metal on ceria was another effective method to improve the CO oxidation activity. In this study, the key factor that influenced the catalytic activity of Pt/CeO2 catalyst was studied. Two kinds of ceria supports were prepared by hydrothermal method and precipitation method, and named as HY and PR, respectively. The experimental results indicated that the CO oxidation on fresh and pre-reduced Pt/HY and Pt/PR followed Mars-van Krevelen mechanism and CO activation was the RDS. Furthermore, the research results indicated that reduced Pt improved the ability of the catalyst to activate CO. The study on the support structure indicated that the higher concentration of surface oxygen vacancy on the HY facilitated loaded-Pt existing in reduced state, which further improves the CO oxidation activity of the catalyst.The above-results indicated that the amount of the active lattice oxygen and concentration of oxygen vacancy were the key factors which influenced the CO oxidation activity of ceria-based catalyst. Doping ceria with other elements greatly increased the amount of the active lattice oxygen of the ceria. The higher Pauling electronegativey of the doping element, the ceria-based oxide had higher amount of the active lattice oxygen, and showed better CO oxidation activity. On the other hand, the ceria which had more surface oxygen vacancy greatly improved the CO oxidation activity of the supported Pt catalyst.