氮掺杂石墨化炭包覆的铁基催化剂的制备及其费托合成反应性能

doi:10.1016/S1872-2067(18)63158-4

摘要/Abstract

摘要：

由合成气经费托合成（FTS）直接制取液态燃油如汽油（C₅-C₁₁）或柴油（C₁₀-C₂₀），对缓解全球能源危机具有重要意义.但是，费托合成产物大多服从Anderson-Schulz-Flory（ASF）分布，C₅-C₁₁烃类选择性最大为45%.因此，高选择性地合成C₅-C₁₁烃仍具有挑战性.铁基催化剂价格低廉且能够在较宽温度区间内保持高活性，其中χ-Fe₅C₂纳米粒子催化剂表现出高活性及高C₅-C₁₁选择性.理论计算表明，Fe₅C₂中高米勒指数晶面如（510）晶面更易暴露，且C-C偶联反应更易发生在该晶面上.但纯相Fe₅C₂的制备流程复杂，操作条件苛刻，成本较高.此外，在反应过程中，因高温、高压及氧化性产物（如H₂O或CO₂等）的影响，Fe₅C₂易发生相转变，导致多物相共存.因此，制备在反应过程中能够保持高Fe₅C₂含量的催化剂意义重大.石墨化炭材料如石墨烯、碳纳米管等，因其具有大π电子结构和高电子密度，作为载体能够促进铁粒子的还原；氮掺杂石墨化炭能够进一步改善电子结构，增强载体与铁物种间的电子传导，进而促进氧化铁粒子的还原及后续碳化形成Fe₅C₂.大量研究表明，包覆结构具有独特的限域效应，能够促进碳化铁物相的生成和稳定存在.结合氮掺杂石墨化炭的电子效应和包覆结构的限域效应，有望得到高含量Fe₅C₂催化剂，实现高C₅-C₁₁选择性.因此，本文通过谷氨酸与Fe物种的配位作用，合成Fe高度分散的配合物，并热解得到氮掺杂石墨化炭包覆铁基催化剂（FeC-x，x为热解温度（℃）），通过改变热解温度调变炭层结构，并考察了其对催化剂费托性能的影响.
在不同热解温度下制备的催化剂的费托合成反应结果表明，FeC-800催化活性高达239.4μmol_CO g_Fe^-1 s^-1，分别是FeC-700的2倍和FeC-900的20倍.而且，FeC-800的C₅-C₁₁烃类选择性为49%，高于大多数报道的Fe/C催化剂.FeC-900则表现出较低的C₅-C₁₁烃类选择性.TG表征发现，热解温度升高，炭层石墨化过程中有损失，导致实际铁负载量增高.XRD和Raman结果表明，炭层石墨化程度随热解温度升高而增加.N₂吸附-脱附等温线表明催化剂存在介孔，有利于反应物及产物的扩散.TEM观察到铁纳米粒子被包覆在石墨化炭结构中.XPS测试结果显示，催化剂表面可探测到的元素为C，O，N和Fe.其中表面Fe的含量远低于实际负载量，说明铁纳米粒子大多存在于包覆炭层之内.通过对比反应60 h前后样品的TEM结果发现，催化剂铁纳米粒子尺寸无明显增加，说明炭层对铁纳米粒子具有限域作用.炭层的包覆可能对产物选择性造成影响：一方面，炭层能够抑制烯烃的扩散，促进二次反应，从而促进长链烃的生成；另一方面，炭层的空间效应也会抑制更长链烃（如C₁₂₊）的生成.因此，FeC-800表现出高C₅-C₁₁选择性.通过N 1s谱图可以发现，石墨化氮、吡啶氮及吡咯氮是主要的表面氮物种，说明N被成功掺杂进石墨化炭结构中.且随热解温度增加，石墨化氮含量增加.通过H₂-TPR及还原后XRD结果发现，FeC-700与FeC-800具有较低的还原温度，易被H₂还原为单质Fe，这有利于在反应过程中转变为活性相Fe₅C₂.CO-TPD结果显示，CO吸附强度随样品热解温度升高而显著增加.热解温度的提高促进了炭层的石墨化度，强化了炭层与Fe之间的电子转移，进而增强了Fe与CO间的相互作用，促进了H₂还原后生成的单质Fe碳化为Fe₅C₂，并且Fe₅C₂在反应过程中不易被氧化.高含量的Fe₅C₂和适宜的CO吸附强度使FeC-800催化剂表现出高催化活性及高C₅-C₁₁选择性.

关键词: 氮掺杂, 石墨化炭, 包裹结构, 铁基催化剂, 费托合成

Abstract:

Fischer-Tropsch synthesis (FTS) has the potential to be a powerful strategy for producing liquid fuels from syngas if highly selective catalysts can be developed. Herein, a series of iron nanoparticle catalysts encapsulated by nitrogen-doped graphitic carbon were prepared by a one-step pyrolysis of a ferric L-glutamic acid complex. The FeC-800 catalyst pyrolyzed at 800℃ showed excellent catalytic activity (239.4 μmol_CO g_Fe^-1 s^-1), high C₅-C₁₁ selectivity (49%), and good stability in FTS. The high dispersion of ferric species combined with a well-encapsulated structure can effectively inhibit the migration of iron nanoparticles during the reaction process, which is beneficial for high activity and good stability. The nitrogen-doped graphitic carbon shell can act as an electron donor to the iron particles, thus promoting CO activation and expediting the formation of Fe₅C₂, which is the key factor for obtaining high C₅-C₁₁ selectivity.

Key words: Nitrogen doping, Graphitic carbon, Encapsulation, Iron-based catalyst, Fischer-Tropsch synthesis

唐磊, 董晓玲, 徐薇, 贺雷, 陆安慧. 氮掺杂石墨化炭包覆的铁基催化剂的制备及其费托合成反应性能[J]. 催化学报, 2018, 39(12): 1971-1979.

Lei Tang, Xiao-Ling Dong, Wei Xu, Lei He, An-Hui Lu. Iron-based catalysts encapsulated by nitrogen-doped graphitic carbon for selective synthesis of liquid fuels through the Fischer-Tropsch process[J]. Chinese Journal of Catalysis, 2018, 39(12): 1971-1979.

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