Chen Wei, Kimpel Tobias F, Song Yuanjun, Chiang Fu-Kuo, Zijlstra Bart, Pestman Robert, Wang Peng, Hensen Emiel J M
Laboratory of Inorganic Materials Chemistry, Schuit Institute of Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, People's Republic of China.
ACS Catal. 2018 Feb 2;8(2):1580-1590. doi: 10.1021/acscatal.7b03639. Epub 2017 Dec 15.
One of the well-known observations in the Fischer-Tropsch (FT) reaction is that the CH selectivity for cobalt catalysts is always higher than the value expected on the basis of the Anderson-Schulz-Flory (ASF) distribution. Depositing graphitic carbon on a cobalt catalyst strongly suppresses this non-ASF CH, while the formation of higher hydrocarbons is much less affected. Carbon was laid down on the cobalt catalyst via the Boudouard reaction. We provide evidence that the amorphous carbon does not influence the FT reaction, as it can be easily hydrogenated under reaction conditions. Graphitic carbon is rapidly formed and cannot be removed. This unreactive form of carbon is located on terrace sites and mainly decreases the CO conversion by limiting CH formation. Despite nearly unchanged higher hydrocarbon yield, the presence of graphitic carbon enhances the chain-growth probability and strongly suppresses olefin hydrogenation. We demonstrate that graphitic carbon will slowly deposit on the cobalt catalysts during CO hydrogenation, thereby influencing CO conversion and the FT product distribution in a way similar to that for predeposited graphitic carbon. We also demonstrate that the buildup of graphitic carbon by CO increases the rate of C-C coupling during the CH hydrogenation reaction, whose products follow an ASF-type product distribution of the FT reaction. We explain these results by a two-site model on the basis of insights into structure sensitivity of the underlying reaction steps in the FT mechanism: carbon formed on step-edge sites is involved in chain growth or can migrate to terrace sites, where it is rapidly hydrogenated to CH. The primary olefinic FT products are predominantly hydrogenated on terrace sites. Covering the terraces by graphitic carbon increases the residence time of CH intermediates, in line with decreased CH selectivity and increased chain-growth rate.
费托（FT）反应中一个广为人知的现象是，钴催化剂的CH选择性总是高于基于安德森-舒尔茨-弗洛里（ASF）分布所预期的值。在钴催化剂上沉积石墨碳会强烈抑制这种非ASF CH，而对高级烃类的形成影响则小得多。碳是通过布多阿尔反应沉积在钴催化剂上的。我们提供的证据表明，无定形碳不会影响FT反应，因为它在反应条件下很容易被氢化。石墨碳会迅速形成且无法去除。这种无反应性的碳形式位于平台位点上，主要通过限制CH的形成来降低CO转化率。尽管高级烃产率几乎不变，但石墨碳的存在提高了链增长概率并强烈抑制了烯烃氢化。我们证明，在CO加氢过程中，石墨碳会缓慢沉积在钴催化剂上，从而以类似于预沉积石墨碳的方式影响CO转化率和FT产物分布。我们还证明，CO导致的石墨碳积累增加了CH加氢反应中C-C偶联的速率，其产物遵循FT反应的ASF型产物分布。我们基于对FT机理中基础反应步骤的结构敏感性的洞察，通过双位点模型来解释这些结果：在台阶边缘位点形成的碳参与链增长或可迁移到平台位点，在那里它会迅速氢化为CH。初级烯烃类FT产物主要在平台位点上被氢化。用石墨碳覆盖平台会增加CH中间体的停留时间，这与CH选择性降低和链增长速率增加一致。
