Determination Of Catalytic Activity In N₂O Decomposition And The Study On Structure-Performance Relationship

Metal modified zeolite material is one of the the most promising catalytic materials applied in N₂O decomposition. In this thesis, acid and alkaline pretreatment was applied as an effective way to modify the zeolite supports. The obtained catalysts were characterized by a variety of physicochemical techniques and the catalytic activities of N₂O decomposition were evaluated. The relationship between structure-performance was established and the key factors to decide the catalytic activities were discussed. Some understanding are summarized as followed:1. The Influence of Acid Pretreatment on Fe-ZSM-5 and Fe-beta Catalysts for N₂O DecompositionFe-zeolites with different acid-treated time were prepared and applied in the N₂O decomposition. From the characterization results, it is concluded that the acid corrosion on ZSM-5 and beta zeolite support is different. Only small part of Al in the ZSM-5 surface is etched under mild acid treatment. While for the beta support, amorphous structure is preferentially dissolved and part of micropore is widened. Acid pretreatments on Fe-ZSM-5 and Fe-beta show distinct positive effects on the activity. The catalytic activity curves of Fe-ZSM-5 and Fe-beta move to low temperature range by about 10-15 ℃ and 30 ℃ respectively. Based on the activity enhancement, we can conclude that only a fraction of the extra Fe becomes active, with the increasing degree of exchange.2. Comparison of Alkaline Pretreatment of MFI Zeolites for N₂O Decomposition: Different Zeolite SourcesMFI zeolites obtained from three sources are modified by alkaline treatment. Different structural transformation is observed over these three samples. For the Fe-Seed-S and Fe-Com-S samples, obvious micro-mesoporous structure is developed by the corrosion of alkaline solution. However, for the Fe-Solid-S sample, the amorphous substance is inclined to be dissolved and then the zeolite structure is etched. From the evaluation result, it is shown that the catalytic performance could be improved by lowering 50-100 ℃ after alkaline pretreatment. Especially, for the Fe-Solid-S sample, with the help of NO, the catalytic performance has the potential to meet the application in the real condition. It is believed that the hierarchical properties and the removal of amorphous substance is benefit for the ion exchange process. Therefore, the increase of active iron sites could explain the improvement of catalytic performance. Therefore, it is believed that the alkaline leaching is an effective method to improve the catalytic performance of Fe-MFI catalysts.3. The Influence of Si/Al on the Construction of Mesoporous ZSM-5 Zeolites and the Catalytic Performance for N₂O DecompositionA series of Fe-MFI zeolites with micro-mesoporous structure were constructed by alkaline treatment and the catalytic activity was evaluated in N₂O decomposition after iron exchange. The experiment results show that the performance of Fe-ZSM-5 catalyst could be greatly improved with the hierarchical zeolite structure. Amongst, for the Fe-Z5-250-S sample, the temperature for the N₂O decomposition could be lowered by >100 ℃. In addition, its hydrothermal stability could be well preserved, which was evaluated under the simulated real condition. The characterization results show that the increased active Fe loading and the well preservation of physicochemical environment around the active site lead to the great improvement of catalytic performance of Fe-Z5-250-S sample. It is believed the alkaline treatment is an effcive way to develop high-silica zeolite catalyst with excellent activity for N₂O decomposition.