钴基催化剂的水氧化机理及其改性的理论研究

Fischer-Tropsch synthesis (FTS) is a clean technology with good development prospects and application potentials, since it can convert natural syngas (CO and H2) into high quality clean motor fuels with low aromatic and sulphur, nitrogen free. Water, the most abundant byproduct of FTS, is considered as the main cause of oxidation deactivation of cobalt-based FTS catalysts, and it has received considerable attention. However presently, the mechanism of water oxidation on cobalt-based catalysts is still not fully understood, and theoretical work on it is rather lack. In the present project, we will use first-principles method to study the behaviors of water and its impact on monometallic and bimetallic Co-based catalysts.The stepped Co(0001) surface is modeled to simulate the steps and step-corner site on the monometallic cobalt terrace, and the bimetallic surface alloys on flat Co(0001) is used to examine the effect of transition metal promoters on cobalt-based catalyst by partially substituting cobalt with either Pd, Pt or Re. In this study,we will first simulate the properties of water adsorption and dissociation. Then, the reaction of water oxidation on O-preadsorbed Co(0001) surface is examined with related minimum energy pathways and activation barrier evaluation. Finally, we will discuss the effects of surface hydroxyl group (product of water oxidation) on the dissociative chemisorption of CO and C2H4 molecules in FTS reactions. Based on the atomic and electronic structural calculations, we hope to understand the oxidation mechanism of water and hence to obtain the deactivation mechanism of cobalt-based catalysts, and secondly, to shed light on the intrinsic synergetic effect between cobalt and different transition metal promoters. So, this study will be a helpful verification for the catalytic "structure-activity" of cobalt-based catalysts, and also provide a theoretical guide for the rational design and optimization of industrial cobalt-based catalysts by means of surface alloying.

费-托合成技术是以一氧化碳选择加氢来获得无硫、无氮和低芳烃优质清洁马达燃料的重要途径，具有良好发展前景和应用价值。费-托反应的副产物水被认为是导致金属钴基催化剂氧化而失活的主要原因，因而倍受关注。但其内在机制尚不明确且相关理论研究匮乏。 本项目拟采用第一性原理计算方法，构建台阶状钴表面和钴表面合金模型，分别针对台阶附近和不同过渡金属助剂钴表面合金上水分子的吸附、解离和氧化过程，以及水氧化产物对费-托反应中CO和C2H4分子催化性能的影响进行对比研究。通过研究以期①从原子层次探寻钴基催化剂表面水解离和氧化过程的最小能量路径和能量势垒，阐述水分子氧化的动态过程并结合电子结构阐明钴基催化剂因水氧化而失活的微观机理；②从原子层次对比不同助剂金属对钴表面催化性能影响的异同，并从电子结构来阐明不同助剂金属与钴之间协同作用的微观机理，探讨钴基催化剂的"构-效"关系并为其合金化改性设计和研究提供理论依据。

以金属钴为催化剂的费-托合成反应是以一氧化碳选择加氢来获得无硫、无氮和低芳烃优质清洁马达燃料的重要途径。水和氨分子会导致催化剂中毒而失活，然而相关的理论研究匮乏。. 本项目采用第一性原理计算并结合原子热力学方法，主要研究：金属钴表面含平坦的和台阶状表面上水分子和氨分子的吸附和解离过程及其相互作用微观机理，重要结果如下：（1）水分子弱吸附在清洁钴表面Top顶位，易脱吸附。当氧原子吸附在钴表面时，水分子解离脱氢势垒明显降低，在Co（110）表面，其脱氢势垒几乎为零，即氧原子促进水分子解离为羟基OH， OH在金属钴表面吸附较强，它对水分子的解离稍有抑制作用。由此可见，水分子与金属钴表面的相互作用具有结构敏感性，钴表面含氧物（O原子和OH分子）对水分子的吸附和解离有重要的影响。.（2）由于氢键作用，水分子簇H2O—H2O在金属Co（0001）表面的吸附较稳定，且解离脱氢势垒降低。理论预测：氧原子存在时，H2O—H2O解离脱氢为产物3OH+H的决速步的势垒约为0.96 eV，即：该脱氢反应2H2O+O→H2O+OH+H→3OH+H在一定温度下生成产物OH分子和H原子，与实验结果吻合较好。因此，水分子解离产物OH分子可能是导致金属钴表面氧化而失活的主要原因。.（3）在Fcc-Co表面，含氮分子在钴表面的吸附强度随NHx（x=0-3）逐步脱氢而增大。吸附在钴表面的氧原子对NHx的吸附和脱氢反应势垒有不同的影响。在Co(110)表面，氧原子促进氨分子的逐步脱氢，最终为N原子强吸附在表面上；而在Co(100)和Co(111)表面，产物NH分子的解离脱氢是限速步骤。因此，氨分子在有氧覆盖的金属钴表面上脱氢反应形成的产物NH和N很可能占据表面的活性位置从而使金属钴基催化剂失活，与实验符合较好。. 这些理论研究不仅从原子层次探寻钴基催化剂表面水分子和氨分子解离和氧化过程的最小能量路径和能量势垒，而且还阐述水分子和氨分子氧化的动态过程并理论上预测了钴基催化剂因水氧化和氨氧化而失活的微观机理，为钴基催化剂的“构-效”关系方面的探讨和研究提供理论上依据。