In Situ Active Site for CO Activation in Fe-Catalyzed Fischer–Tropsch Synthesis from Machine Learning

In situ-formed iron carbides (FeCx) are the key components responsible for Fischer–Tropsch synthesis (FTS, CO + H2 → long-chain hydrocarbons) on Fe-based catalysts in industry. The true active site is, however, highly controversial despite more than a century of study, which is largely due to the combined complexity in both FeCx structures and mechanism of CO hydrogenation. Herein powered by machine learning simulation, millions of structure candidates for FeCx bulk and surfaces are explored under FTS conditions, which leads to resolving the active site for CO activation. This is achieved without a priori input from experiment by first constructing the thermodynamics convex hull of bulk phases, followed by identifying the low surface energy surfaces and evaluating the adsorption ability of CO and H, and finally determining the lowest energy reaction pathway of CO activation. Rich information on FeCx structures and CO hydrogenation pathways is gleaned: (i) Fe5C2, Fe7C3, and Fe2C are the three stable bulk phases under FTS in producing olefins, where Fe7C3 and Fe2C have multiple energetically nearly degenerate bulk crystal phases; (ii) only three low surface energy surfaces of these bulk phases, namely, χ-Fe5C2(510), χ-Fe5C2(111), and η-Fe2C­(111), expose the Fe sites that can adsorb H atoms exothermically, where the surface Fe:C ratio is 2, 1.75, and 2, respectively; (iii) CO activation via direct dissociation can occur at the surface C vacancies (e.g., with a barrier of 1.1 eV) that are created dynamically via hydrogenation. These atomic-level understandings facilitate the building of the structure–activity correlation and designing better FT catalysts.原位生成的碳化铁（FeCx）是工业中铁基催化剂上费托合成（Fischer–Tropsch synthesis, 简称FTS，反应通式为CO + H₂ → 长链烃类）的核心活性组分。尽管历经一个多世纪的研究，其真实活性位点仍存在较大争议，这在很大程度上源于FeCx结构与CO加氢机理的双重复杂性。本研究依托机器学习模拟，在费托合成反应条件下对数百万个FeCx体相及表面结构候选体进行探索，最终解析出CO活化的活性位点。该研究无需依赖实验先验信息，具体步骤为：首先构建体相的热力学凸包，随后识别低表面能晶面并评估CO与H的吸附能力，最终确定CO活化的最低能反应路径。研究得到了关于FeCx结构与CO加氢路径的丰富信息：(i) 在制备烯烃的费托合成条件下，Fe₅C₂、Fe₇C₃与Fe₂C为三种稳定体相，其中Fe₇C₃与Fe₂C存在多种能量近乎简并的体相晶体结构；(ii) 上述体相仅存在三种低表面能晶面，即χ-Fe₅C₂(510)、χ-Fe₅C₂(111)与η-Fe₂C(111)，这些晶面暴露的Fe位点可放热吸附氢原子，对应的表面Fe:C比分别为2、1.75与2；(iii) CO活化可通过直接解离发生在表面碳空位处（例如能垒为1.1 eV），此类碳空位可通过加氢动态生成。这些原子级别的认知有助于建立结构-活性关联，并为设计更优异的费托催化剂提供指导。