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作为六大温室气体之一,N_2O的增温潜能是CO_2的310倍,甲烷的21倍,目前仍然以0.80 ppb/年的速度增长,但是减排成本很低,因此对N_2O的消除具有重要意义.在工业中金属修饰的微孔分子筛因其优良的催化活性,高水热稳定性,低成本等优点而成为研究重点.但是微孔分子筛狭窄的微孔孔道限制了金属前驱体的进入,导致活性金属含量低,进而限制了活性的提高.因此采用一定的改性手段减小分子筛颗粒尺寸,缩短微孔孔道长度或者扩大微孔孔道来增加活性铁物种的含量进而提高分子筛催化活性.本文选用商用的ZSM-5和beta分子筛作为母板分子筛,按照3 g:50 mL比例将分子筛母板与1.0 mol/L的HNO_3在室温下混合,分别搅拌0,2和24 h,然后采用液相离子交换法负载金属铁制备得到Fe-ZSM-5和Fe-beta.通过X射线衍射、N_2物理吸吸脱附、电感耦合等离子原子发射光谱仪、扫描电镜、透射电镜、NH_3程序升温脱附及紫外漫反射(UV-vis)等手段对不同时间处理的分子筛的形貌、酸性和铁物种等物理化学性质进行表征.对两种催化剂催化消除N_2O的反应性能进行了测试.结果显示,温和的酸处理下分子筛脱除了部分Al,其中,ZSM-5分子筛的表现为由外向内逐层刻蚀,颗粒尺寸减小,孔道长度缩短,但是由于MFI型分子筛较高的稳定性,酸处理对分子筛孔道大小的改变并不明显,而对于beta分子筛,首先是其中大量无定形物种的去除,然后对孔道进行修饰,使之略微扩大,但是对颗粒尺寸的影响不大.ICP结果显示,商用ZSM-5和beta分子筛经过温和的酸处理改性后,Si/Al比增大,负载Fe的含量明显增加,各催化剂催化消除N_2O的活性也出现了不同程度的提高.Fe-ZSM-5和Fe-beta分子筛上N_2O完全转化温度分别向低温段移动了10–15和30°C.UV-vis谱图显示,分子筛中存在着不同种类的铁物种,通过分峰计算发现,孤立的Fe~(3+)铁离子和低聚态的Fe_x~(3+)Oy均是催化活性铁物种,其含量的增加部分也解释了活性提高的原因.
As one of the six greenhouse gases, the warming potential of N 2 O is 310 times that of CO 2 and 21 times that of methane, which is still increasing at the rate of 0.80 ppb / year, but the emission reduction cost is very low, so it is of great significance for the elimination of N 2 O In the industry, metal-modified microporous molecular sieves have been the focus of research due to their excellent catalytic activity, high hydrothermal stability, low cost, etc. However, the narrow microporous pores of microporous zeolites restrict the entry of metal precursors, leading to The content of active metal is low, and then the activity is limited.Therefore, some modification methods are used to reduce the size of molecular sieve, shorten the length of micro-pore or enlarge the micro-pore to increase the content of active iron species and improve the catalytic activity of the molecular sieve. Commercial ZSM-5 and beta molecular sieves were used as master molecular sieves, and the molecular sieve master was mixed with 1.0 mol / L HNO3 at a ratio of 3 g: 50 mL, stirred at room temperature for 0, 2 and 24 h, respectively, Fe-ZSM-5 and Fe-β-Fe-ZSM-5 and Fe-β were prepared by exchange method. The structure and properties of Fe-ZSM-5 and Fe-β were investigated by X-ray diffraction, N 2 physical adsorption and desorption, inductively coupled plasma atomic emission spectroscopy, SEM, TEM, Temperature-programmed desorption and ultraviolet-visible diffuse reflectance spectroscopy were used to characterize the morphology, acidity and the physical and chemical properties of the zeolites treated with different time.The catalytic performance of the two catalysts for the catalytic elimination of N 2 O was tested. The results showed that part of Al was removed by mild acid treatment. Among them, the ZSM-5 zeolite was characterized by layer-by-layer etching and particle size reduction, and the pore length was shortened. However, due to the higher stability of MFI molecular sieve , Acid treatment on zeolite pore size change is not obvious, and for the beta zeolite, the first of which is a large number of amorphous species removal, and then modified the pore to make it slightly expanded, but little effect on particle size.ICP results The results showed that the commercial ZSM-5 and beta zeolites modified with mild acid treatment showed that the Si / Al ratio increased and the content of Fe increased obviously, and the activity of each catalyst for the catalytic elimination of N 2 increased to some extent.Fe-ZSM -5 and Fe-beta zeolites shifted to lower temperature by 10-15 and 30 ° C. The UV-vis spectra showed that different kinds of iron species exist in the molecular sieve, Now, isolated Fe 3+ iron ions and oligomeric Fex 3+ Oy are all catalytically active iron species, and their increased content partly explains the reason for their increased activity.