| Diesel engines have attracted much attention due to their high efficiency, economy and low CO2 emissions. Correspondingly, with the main emissions of NOx and soot, the related problems of environmental pollution are serious day by day. In cylinder, there exist trade-off effects for NOx and soot control. Therefore, soot and NOx cannot be reduced efficiently by improving combustion alone. Simultaneous removal of soot and NOx is the optimal diesel engine after-treatment technique, in which NOx is reduced into N2 by soot, in return, soot is oxidized into CO2 at the same time. To investigate the essence of the redox and find the crtical factors that promote and restrict the reaction, will be of great interesting in developing the technique for simultaneous removal of soot and NOx.In this study, the catalytic activity of BaAl2O4 in simultaneous removal of soot and NOx was evaluated by Temperature Programmed Reaction (TPR) technique under various reaction conditions. Then, the micro mechanism for the catalytic reaction of soot with NOx on the surface of BaAl2O4 was investigated by DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) and LRS (Laser Raman Spectroscopy). The detailed research is summarized as following:(1) La0.8K0.2MnO3 and BaAl2O4 were prepared and characterized by XRD, FT-IR and SEM. The catalytical activity of La0.8K0.2MnO3 and BaAl2O4 was evaluated systematically by TPR. These two catalysts all exhibit high catalytic activity in simultaneous removal of soot and NOx, for exampe, Tig and Tm of soot decrease by more than 175℃and 240℃respectively, and the conversion of NOx into N2 is improved markedly.(2) The reaction of soot with NOx was studied under various conditions. It is found that low GHSV (Gas Hourly Space Velocity) and high mass ratio of soot to catalyst are benefitial to the redox. Oxygen may promote the catalytic reaction of soot with NOx. Both Tig and Tm of soot is decreased by ca. 30℃by increasing O2 content from 2% to 5%. It is interesting that O2 ehances the maximum conversion efficiencies of NOx into N2 and N2O with the catalysis of BaAl2O4, however, the conversion efficiencies were depressed by changing BaAl2O4 to La0.8K0.2MnO3.(3) Adsorption of ammonia was studied in order to obtain informaion on the surface acidity (Lewis and Br?nsted) of BaAl2O4 by DRIFTS. It is found that Lewis and Br?nsted acid exist simultaneously on the the surface of BaAl2O4. The dynamic behavior of NO, NO2 and O2 on the surface of the sample as well as their interactions was analyzed under different reaction conditions. NO adsorption produces linear nitrites and bridged nitrites in the absence of O2. NO2 adsorption produces monodentate nitrates, bridged nitrates and ionic nitrates besides nitrites species. The produced nitrites will further react with O? surf, O2-lattice and O2 to form nitrates, of which O ? surf owns the highest oxidative activity. Besides oxidizing NO into NO2, O2 oxidizes nitrites into nitrates, while this role is neglected by previous study. High oxygen content in the NO-containing flow and high temperature contribute to the formation of nitrates species.(4) The reaction of soot with NO, NO2 and O2 was analyzed by LRS (Laser Raman Spectroscopy). It is found that amorphous material and SP2 structure of soot are destroyed after soot is pretreated with NOx or O2. High temperature favors the reaction of soot with NOx and O2, and the reaction activity is weakened in the order of NO+O2> O2 > NO at the same temperature.(5) The reaction of soot and NOx was analyzed in situ on the surface of BaAl2O4 by DRIFTS. The chemical configuration of NOx in the redox is revealed. Nitrates species on the sample will react with C(O) preferentially due to their high oxidative ability, accompanied by the formation of CO2, N2 and N2O. When these products desorb from the catalyst, the catalyst is regenerated. It is presumed that the reaction of nitrates with C(O) plays the main role in the redox and is regarded as the rate determing step. The presumption is consistent with the dynamic behavior of NOx on BaAl2O4 during the TPR proceedure. The produced N2O at high temperature will react with C(O) as an oxidant before it can be detected by DRIFTS easily. The conversion efficiency of NOx into N2 and N2O directly depends on the quantity of nitrates on BaAl2O4, which is related to the performance of the catalyst in forming of nitrates.(6) The molecule model of BaAl2O4, the intermediates of adsorbed NOx, carbon-oxygen complexes and the reaction gases were constructed and optimized with DFT (Density Functional Theory) theory. The molecular orbital and the molecular orbital energy of the constructed model were calculated by EHMO (Extended Hückel Molecular Orbital) method. It is also revealed with frontier molecular orbital theory that nitrites will be oxidized into nitrates by O-surf, O2-lattice or O2, of which O-surf presents the highest oxidative activity by comparing the energy of HOMO (High Occupied Molecular Orbital) and LUMO (Low Unoccupied Molecular Orbital). The results of simulation calculation are in accordance with those of TPR and DRIFTS experiments.
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