摘要
Two series of ZSM-5 and beta zeolites were pretreated in 1.0 mol/L HNO3 solution at room temper-ature for various time periods. The catalytic performances of their Fe-exchanged products in N2O decomposition were evaluated. The Fe-zeolite catalysts were characterized using N2 adsorp-tion-desorption, inductively coupled plasma optical emission spectroscopy, X-ray diffraction, ultra-violet-visible spectroscopy, temperature-programmed desorption of NH3, and scanning and trans-mission electron microscopies. For the ZSM-5 zeolite, acid leaching primarily takes place on the crystal surface and the particle size is reduced, therefore the pore channels are shortened. However, because of the good stability of MFI zeolites, the acid does not greatly penetrate the pore channels and new mesopores are not created. For the beta zeolite, because the amorphous material is in-clined to dissolve（deagglomerate）, some of the micropores are slightly dilated. The improved cata-lytic activities can be explained by the increased active Fe loading as a result of structural changes.
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.
基金
supported by the National Natural Science Foundation of China (21307144,21307007)
Science of Technology Development Plan of Jilin Province of China (20140520150JH)~~
