| The elimination of nitrogen oxides has received much attention all over the world due to the more and more serious pollution caused by the pollutant being contained in automobile exhaust with the number of cars increasing. Selective catalytic reduction of NOx by hydrocarbons (HC-SCR) has been believed to be one of the most effective ways to remove nitrogen oxides from the exhaust gas of lean-burn engines. For the HC-SCR investigation, ZSM-5 zeolite has exhibited preferable catalytic activity. However, it was also reported that the reaction of HC-SCR zeolite was controlled by diffusion of reactants and products in zeolite channels, due to the narrow pore with 0.5～0.6 nm of the zeolite. To solve this problem, the investigation being concerned in the thesis endeavored to create some mesopores on the zeolite crystals, since mesopores on the zeolite was expected to increase the diffusion of molecules being concerned in the reaction.To create mesopores on the zeolite, NaZSM-5 with Si/Al=12.5 was treated by NaOH solution at different conditions. It was found that with the concentration increase in the range of 0.2～3 mol/L, the particle size of the resulting material significantly decreased. However, for the resulting material obtained by the alkali-treatment, the diffraction intensity due to ZSM-5 zeolite in the X-ray diffraction (XRD) was decreased with the base concentration increasing in the range of 0.2～1mol/L, while increased in the range of 1～3mol/L. The morphology of alkali-treated zeolite were watched by scanning electron microscopy (SEM), which indicated that the resulting material obtained by treating the zeolite in 1 mol/L NaOH solution at 65℃for 90 min had obvious mesoporosity. The corresponding sample was also characterized by X-ray fluorescence and X-ray photoelectron spectroscopy, which indicated that selective desilication was accrued during the treatment of the ZSM-5 zeolite by 1 mol/L of NaOH solution. Based on these results, a model for the mesopores formation due to desilication of the zeolite was proposed. The porosity of ZSM-5 zeolite after alkali-treatment were further characterized by N2 adsorption-desorption. It indicated that a ZSM-5 zeolite with micro-mesoporosity was obtained. From FTIR of pyridine adsorption on the sample, it indicated that the sample after exchange in NH4NO3 and calcination had less Bronsted acid sites and more Lewis acid sites in population in comparison with the counterpart obtained by the parent zeolite.The activity of desilicated ZSM-5 zeolite for selective catalytic reduction of NOx by acetylene (C2H2-SCR) was evaluated. Comparing its counterpart obtained by the parent zeolite, the desilicated ZSM-5 zeolite displayed worse catalytic performance. Hence, the desilicated ZSM-5 zeolite was further hydrothermally treated at different conditions. It was found that the activity of the sample for C2H2-SCR could increased by hydrothermal treatment, and the sample after hydrothermal treatment at 170℃for 8 h exhibited slightly higher C2H2-SCR activity than the counterpart obtained by the parent zeolite.The C2H2-SCR activity and structure of ZSM-5 zeolite influenced by TiO2 coating and alkali-treatment was also investigated. It was found that the selectivity of C2H2-SCR was significantly increased by the zeolite doping with titanium. No promotional effect on the zeolite desilication could be observed for the zeolite coated by TiO2 submitting to alkali-treatment. |
PDF Full Text Request