费托合成催化剂中活性金属钴的晶相转变机理研究

Fischer-Tropsch synthesis technology has broad application prospects. The cobalt-based Fischer-Tropsch synthesis catalyst is a structurally sensitive system with obvious crystal phase sensitivity, and hcp-Co has better catalytic performance than fcc-Co. To this end, researchers in the field have carried out a large number of research work to convert fcc-Co into hcp-Co by carbonization and hydrogenolysis. However, due to the complex microstructure of the two cobalt crystalline phases and the unclear transitional relationship, the detailed mechanism involved in the crystal phase transformation process is difficult to research deeply. In this study, the intermediate CoO in the conversion process is taken as the research entry point, and several in-situ characterization techniques are combined to track the conversion of Co-Co bond and CO bond into Co-O bond, Co-C bond and Co-Co bond. The occurrence form of oxygen atoms in CO and their effects of inducing cobalt atom rearrangement in the conversion process were studied. Combined with quantum chemical calculation, the intrinsic relationship between the intermediate phase and the microstructure of fcc-Co/hcp-Co, and the evolution law of crystal phase structure are revealed, and the phase transition path is finally clarified. At the same time, the feasible path and mechanism of hcp-Co reverse conversion to fcc-Co were explored. This project will provide a useful reference for the study of metal catalysts with specific active crystalline and the transformation process of their different crystal phase.

费托合成技术具有广阔的应用前景。钴基费托合成催化剂属结构敏感性体系，晶相敏感性突出，hcp-Co较fcc-Co具有更优的催化性能。为此，领域内研究人员开展了大量将fcc-Co经碳化、氢解转化为hcp-Co的研究工作。但是，由于两种晶相钴的微观结构复杂、衔接过渡关系不明，导致晶相转变过程涉及的详细机理难以着手开展深入研究。本申请将以转化过程的中间物CoO为研究切入点，联合几种原位表征技术，追踪Co-Co键和C-O键逐步转变为Co-O键、Co-C键、Co-Co键的全过程，研究转化过程CO中氧原子赋存形态及其诱导钴原子重排的作用机制。结合量子化学计算，揭示中间物相与fcc-Co/hcp-Co微观结构的内在关联关系和晶相结构演变规律，最终阐明晶相转变路径。同时，探索研究hcp-Co向fcc-Co逆向转化的可行路径及作用机理。本项目将为研究具有特定活性晶相金属催化剂及其晶相转变过程提供有益参考。

在费-托合成反应中，hcp-Co比fcc-Co具有更优异的催化性能，因此，许多研究集中于通过“H2-Co-H2”工艺（还原-碳化-还原）从fcc-Co转化为hcp-Co。然而，碳化过程中CO的存在形式仍然不清楚，很难确定通过CO歧化生成的碳是否参与了碳化。在这项研究中，分别尝试构建了有CO无C和有C无CO的环境。通过原位拉曼光谱、XRD、TPH-MS和TEM研究了Co2C的形成和演化。结果表明：（1）fcc-Co向hcp-Co的转化可以在有C而无Co的环境中实现；（2） CO直接参与了“H2-CO-H2”过程中fcc CO向hcp CO的转化，CO浓度和碳化温度共同影响“H2-CO-H2”过程；（3） 在整个“H2-CO-H2”过程中，碳沉积是不可避免的；然而，在“H2-CO-H2”工艺的转变温度范围内，生成的碳不参与形成Co2C的反应。研究了碳化过程中CO的存在形式及其对CO原子重排和Co2C微观结构形成和演化的影响。碳化温度和CO浓度都显著影响碳化过程。在240℃下，50%及以上的CO是形成碳化钴晶体所必需的。通过RCR获得的hcp-Co比通过碳促进转化获得的hcp Co具有更好的催化性能。