Amama Placidus B, Lim Sangyun, Ciuparu Dragos, Yang Yanhui, Pfefferle Lisa, Haller Gary L
Department of Chemical Engineering, Yale University, 9 Hillhouse Ave., New Haven, Connecticut 06520, USA.
J Phys Chem B. 2005 Feb 24;109(7):2645-56. doi: 10.1021/jp047158g.
Fe-substituted MCM-41 molecular sieves with ca. 1, 2, and 3 wt % Fe were synthesized hydrothermally using different sources of colloidal silica (HiSil and Cab-O-Sil) and characterized by ICP, XRD, N2 physisorption, UV-vis, EPR, TPR, and X-ray absorption. Catalysts synthesized from Cab-O-Sil showed higher structural order and stability than those from HiSil. The local environment of Fe in the mesoporous material as studied by UV-vis reveals the dominance of framework Fe in all the as-synthesized Fe-MCM-41 samples. Dislodgement of some Fe species to extraframework location occurs upon calcination, and this effect is more severe for Fe-MCM-41 (2 wt %) and Fe-MCM-41 (3 wt %), as confirmed by EPR and X-ray absorption. These materials have been used as catalytic templates for the production of carbon nanotubes (CNTs) by acetylene pyrolysis at atmospheric pressure. A relationship between the Fe loading in MCM-41 and the carbon species produced during this reaction has been established. Using our optimized conditions for this system, Fe-MCM-41 with ca. 2 wt % Fe showed the best results with particularly high selectivity for single-wall carbon nanotube (SWNT) production. This catalyst was selective for carbon nanotubes with a low amount of amorphous carbon for a narrow range of temperatures from 1073 to 1123 K. To account for the different selectivity of these catalysts for CNTs production, the local environment and chemical state of Fe in the used catalyst was further probed by X-band EPR.
采用不同来源的胶体二氧化硅（HiSil和Cab - O - Sil）水热合成了铁含量约为1 wt%、2 wt%和3 wt%的铁取代MCM - 41分子筛，并通过电感耦合等离子体质谱（ICP）、X射线衍射（XRD）、N₂物理吸附、紫外可见光谱（UV - vis）、电子顺磁共振（EPR）、程序升温还原（TPR）和X射线吸收进行了表征。由Cab - O - Sil合成的催化剂比由HiSil合成的催化剂表现出更高的结构有序性和稳定性。通过紫外可见光谱研究的介孔材料中铁的局部环境表明，在所有合成态的铁 - MCM - 41样品中骨架铁占主导地位。经煅烧后，一些铁物种会迁移到骨架外位置，电子顺磁共振和X射线吸收证实，这种效应在铁 - MCM - 41（2 wt%）和铁 - MCM - 41（3 wt%）中更为严重。这些材料已被用作常压下乙炔热解制备碳纳米管（CNT）的催化模板。已经建立了MCM - 41中铁负载量与该反应过程中产生的碳物种之间的关系。在我们针对该体系优化的条件下，铁含量约为2 wt%的铁 - MCM - 41表现出最佳结果，对单壁碳纳米管（SWNT）的生产具有特别高的选择性。该催化剂在1073至1123 K的窄温度范围内对碳纳米管具有选择性，且无定形碳含量低。为了解释这些催化剂对碳纳米管生产的不同选择性，通过X波段电子顺磁共振进一步探究了使用过的催化剂中铁的局部环境和化学状态。
