Effect of confinement in carbon nanotubes on the activity of Fischer-Tropsch iron catalyst
Effect of confinement in carbon nanotubes on the activity of Fischer-Tropsch iron catalyst
复制标题
碳纳米管约束对费托铁催化剂活性的影响
DOI：
10.1021/ja8008192
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发表时间：
2008-07-23
影响因子：
15
通讯作者：
Bao, Xinhe
中科院分区：
文献类型：
--
作者：
Chen, Wei;Fan, Zhongli;Bao, Xinhe
继我们先前发现碳纳米管（CNTs）内的限域作用能够改变封装的氧化铁的氧化还原性质之后，我们在此展示了这如何影响费托合成（FTS）中铁催化剂的催化反应活性。在接近反应条件下利用原位X射线衍射（XRD）进行的研究表明，在碳纳米管限域体系中，碳化铁和氧化铁相的分布受到调节。封装在碳纳米管内的铁物种更倾向于以一种更还原的状态存在，在反应条件下更易于形成更多的碳化铁，而碳化铁已被认为对获得高的费托合成活性至关重要。在相同条件下，封装的铁催化剂的碳化铁（FexCy）与氧化物（FeO）的积分XRD峰的相对比值约为4.7，而分散在碳纳米管外壁上的铁催化剂的该比值为2.4。这导致了催化性能的显著改变。封装的铁催化剂上C5 +烃的产率是碳纳米管外铁催化剂的两倍，是活性炭负载的铁催化剂的6倍多。催化活性的提高归因于铁催化剂在碳纳米管通道内的限域作用。正如在氢气和一氧化碳气氛中的程序升温还原所表明的那样，当铁物种被限域时，其还原性显著提高。通过在碳纳米管内的限域作用来改变氧化还原性质的能力预计对许多催化反应具有重要意义，因为这些催化反应高度依赖于活性组分的氧化还原状态。此外，已经观察到铁物种在还原和反应过程中的扩散和聚集，但由于通道的空间限制，这些在碳纳米管内受到阻碍。
Following our previous findings that confinement within carbon nanotubes (CNTs) can modify the redox properties of encapsulated iron oxides, we demonstrate here how this can affect the catalytic reactivity of iron catalysts in Fischer-Tropsch synthesis (FTS). The investigation, using in situ XRD under conditions close to the reaction conditions, reveals that the distribution of iron carbide and oxide phases is modulated in the CNT-confined system. The iron species encapsulated inside CNTs prefer to exist in a more reduced state, tending to form more iron carbides under the reaction conditions, which have been recognized to be essential to obtain high FTS activity. The relative ratio of the integral XRD peaks of iron carbide (FexCy) to oxide (FeO) is about 4.7 for the encapsulated iron catalyst in comparison to 2.4 for the iron catalyst dispersed on the outer walls of CNTs under the same conditions. This causes a remarkable modification of the catalytic performance. The yield of C5+ hydrocarbons over the encapsulated iron catalyst is twice that over iron catalyst outside CNTs and more than 6 times that over activated-carbon-supported iron catalyst. The catalytic activity enhancement is attributed to the effect of confinement of the iron catalyst within the CNT channels. As demonstrated by temperature-programmed reduction in H-2 and in CO atmospheres, the reducibility of the iron species is significantly improved when they are confined. The ability to modify the redox properties via confinement in CNTs is expected to be of significance for many catalytic reactions, which are highly dependent on the redox state of the active components. Furthermore, diffusion and aggregation of the iron species through the reduction and reaction have been observed, but these are retarded inside CNTs due to the spatial restriction of the channels.