| With the rapid increase of motor vehicles in China, air pollution caused by automotive exhaust has been one of the most important pollution resources. To develop the catalysts with high performance is the key to solve this pollution. At present, there are two urgent difficulties for the automotive exhaust catalyst development:1. light-off temperature is high;2. Active components of precious metals are expensive. Therefore, to develop new catalysts or modify the present ones with low light-off temperature and low price is very important for improving the quality of our environment.In this thesis, a series of cerium-based solid solution and perovskite composite oxide were prepared using sol-gel method. Their catalytic performances were valued by CH4combustion reaction. The physical and chemical properties of catalysts were characterized by X-ray diffraction spectrometer (XRD), BET surface area measurements, scanning electron microscopy (SEM) images, and temperature-programmed reduction (TPR), etc. Our results suggested that the addition of Ce-based perovskite-type oxide greatly improves the activity of CH4combustion. The possible effects of perovskite were explored by the correlation between the performance and structures of catalysts.1. Modification at A site of La1-xCexFeO3and their CH4catalytic combustion performanceUsing nitrate as precursor, citric acid as sol agent and ammonia as precipitant, a series of La1-xCexFeO3(x=0-0.5) perovskite oxides were prepared by a sol-gel method. XRD, TEM, BET and H2-TPR were used to characterize their physical structures and redox properties. Catalytic methane combustion tests for La1-xCexFeO3(x=0-0.5) perovskite oxides show that the activity of LaFeO3was highly improved due to the introduction of Ce in the A-site of the perovskite catalysts. Among all the catalysts, La0.7Ce0.3FeO3has the maximum oxidative performance with the corresponding T90as low as510℃. Combining with density functional theory calculation, it was suggested that the electrons of Fe ions increase in La0.875Ce0.125FeO3due to the introduction of Ce4+ion, which leads to stronger interactions with adsorbed O2. Correspondingly, the adsorption energy of O2on La0.875Ce0.125FeO3increases and the O-O bond is activated. Thus, the Ce doped perovskite has higher oxidative activity than pure LaFeO3.2. Modification at B site of La0.9Ce0.1Fe1-xCoxO3and their CH4catalytic combustion performanceIt was reported that metal doping at B site of perovskite can also improve its catalytic oxidative performance. A series of Co-doped Lao.9Ceo iFe1-xCoxO3catalysts wre prepared in this work. On the basis of the performance evaluations of CH4combustion, it is indicated that Co ions doping at B site of La0.9Ce0.1FeO3improve the oxidative activity of perovskite. When the Fe/Co ratio is up to7/3, the activity of CH4combustion of La0.9Ce0.1Fe1-xCoxO3is optimum. Therefore, introducing proper metal ions at A and B sites can keep the original structure of perovskite and greatly improve its catalytic performance.3. Ce-Zr composite and perovskite catalysts and their CH4catalytic combustion performanceA series of Ce-Zr composite oxides with different Ce/Zr ratios were prepared and evaluated using the CO oxidation as probe reaction. On the basis of optimum Ce/Zr ratio (0.7), an industrial catalyst for automobile exhaust purification was prepared by mixture and ball-milling of γ-Al2O3, Ce-Zr composite powder, pseudo boehmite, distilled water and Pd solution. Our results suggest that the amount of Pd has a great effects on the oxidative performance of catalysts; the addition of rare earth based pervoskite can highly improve the activity of CH4combustion at low temperature and lower the concentration of Pd in catalysts with a higher performance. This can greatly reduce the cost of catalysts for automobile exhaust purifaction. |
PDF Full Text Request