Cobalt-based Porous Composite Oxides For Catalysts CO Oxidation At Low-temperature

In recent years, low temperature CO oxidation technology has been paid more and more attention due to the improvement of people’s awareness of environmental protection and the strict requirements of the environmental laws and regulations. It has been widely used in automobile exhaust pollution control, safety of workers in mines, scuba diving, and indoor air purification. The noble metal catalysts have been widely applied in low temperature CO oxidation technology because of its high catalyst activity, good stability, long life, and other advantages. But it also was limited by its high cost and poor reserves. So in recent years, the study of non-noble metal catalyst is more and more attention, in which the cobalt-based catalysts showed a very good catalytic activity. In this thesis, a series of Co-based hydrotalcite-like compounds was prepared by coprecipitation method and their composite oxides were obtained after calcined under different temperature. The crystal structure, mirco morphology, pore properties, thermal stability of the catalysts was characterized by XRD, SEM, FTIR, TG-DTA, N₂ adsorption and desorption. At the same time, H₂-TPR, O2-TPO and in situ FTIR were used to test the redox performance, the catalytic performance of Co based catalysts, and the mechanism of CO oxidation reaction. The main conclusions are as follows:The cobalt chromium composite oxides with different ratio were prepared by co-precipitation method. The specific surface area of Co₃O₄ was 60 m²/g. The specific surface area of Co-Cr composite oxide was increased after adding Cr, which reached to 75⁸5 m²/g. Mixed of Cr did not improve the activity of Co based catalyst, which could be due to the poor redox ability of chromium species. Cr2O3 is not easy to be reduced. At about 280 ℃, there is a small reduction peak of Cr6+. The catalyst with cobalt chromium molar ratio of 3:1 and 2:1 showed relatively good catalytic activity compared to others and T50 < 100 ℃. According to the analysis of the precursors, the highly purity and crystallinity of hydrotalcite-like composites can be formed when the ratio of metal elements in the proper range. Because the structure of the hydrotalcite-like composites has contribute to the homogeneous distribution of the metal elements of the catalyst, which make the active site of the catalyst more exposure, and is more helpful to the catalytic oxidation reaction.After calcined at different temperatures, Co-A1 composite oxide catalyst obtained from Co-Al hydrotalcite precursor. In the process of increasing the calcination temperature, grain grows up gradually. The catalyst particles are agglomerated with the specific surface area is reduced from 171 m²/g（CA-400） to 50 m²/g（CA-800）. Due to the heat flow, the average pore diameter of catalyst increased from 11.2 nm to 20.6 nm as the temperature rose from 400 ℃ to 800 ℃. Co-Al catalyst showed good catalytic performance during CO oxidation. The best of the Co-Al catalytic is CA-400, in which CO half conversion temperature is 73 ℃. The experiment that the catalytic activity of CO is as follows: CA-400 > CA-500 > CA-600 > CA-700 > CA-800. In the process of increasing the calcination temperature, the catalyst particles are reunited, the specific surface area decreases, and the number of active sites on the catalyst surface is decreased. Therefore, the catalytic performance of the spinel oxide catalyst for CO was decreased.A series of Co Al Fe nonstoichiometric spinel-type oxides were synthesized from hydrotalcite precursors prepared through a co-precipitation method. Thermal decomposition hydrotalcites precursors at 500 ℃ leads to porous spinel-type oxides with specific surface area ranged from 90 to 117 m²/g. Co Al Fe nonstoichiometric spinel type mixed oxides derived from hydrotalcites were shown to be highly active catalysts for the CO oxidation. Compared to the binary oxides and Co₃O₄ catalyst, Co Al Fe ternary oxides displayed much larger catalytic activity for CO oxidation with half CO conversion（ T50） at about 50 ℃. The promotion effect of Fe incorporation into the Co-based nonstoichiometric spinels can be ascribed to the improved low temperature reducibility. After pretreatment with different atmosphere, the catalyst shows good activity. Even after the reduction atmosphere treatment, CO complete conversion at 140℃. In situ FTIR analysis showed that the Co Al Fe ternary oxides was exposed to stream of reactant gas. CO can be adsorbed on the catalysts surface and form cationic CO vibration peaks, accompanied by the formation and decomposition of surface carbonate species. At the same time, the small physisorbed CO2 peaks were observed which means the adsorbed CO reacted with surface lattice oxygen in oxide catalyst and formed CO2. Because of the large amount of CO adsorption and active oxygen sites on the catalyst surface, the Co based composite oxide catalysts showed good catalytic performance.Sn was introduced into Co-Al catalyst by co-precipitation method. The Co based composite oxide with spinel type non stoichiometric ratio was formed by calcination at 500℃. The specific surface area of CAS sample is 98.3 m²/g, and the pore volume is 0.48 cm3/g. The specific surface area and pore volume of the catalyst was significantly increased（the maximum of the samples is CAS1, SBET = 160.2 m²/g, Vp = 0.73 cm3/g）, the catalytic activity of the catalyst decreased with the addition of a small amount of Sn. This may be due to the reduced of the reducibility of the catalytic. In addition, we also have carried on the water resistance test. The study found that Sn doping can improve the water resistance of composite oxide catalyst.