Fe基费托催化剂上低碳烯烃形成机理及动力学特征

Fe-catalyzed Fischer-Tropsch synthesis to lower olefins (FTO) is of prime scientific and industrial importance in enabling direct conversion of CO-rich syngas derived from coal and biomass for the sustainable production of lower olefins. Rational design of the Fe-based catalysts and fine-tuned control of product selectivity are, however, limited by the complex catalyst structural nature and reaction pathways...To this end, this proposal aims to unravel the underlying mechanism and kinetic behavior for the formation of lower olefins on the Fe-based Fischer-Tropsch catalysts by a combination of density functional theory (DFT) calculations with steady-state isotopic transient kinetic analysis (SSITKA). At first, systematic DFT calculations followed by microkinetic analysis will be carried out to understand effects of typical promoters on the mechanism of FTO reaction over different χ-Fe5C2 crystal facets. Subsequently, based on the supported iron-based FTO catalysts in the presence or absence of promoters, the methodology of multicomponent steady-state isotopic transient kinetic analysis and the coupling of DFT calculations with SSITKA studies will be developed to realize the kinetic isotopic effects and discriminate the FTO reaction mechanism. Finally, the promoter effects on the properties of the active sites and the kinetic behaviors will be elucidated. . .This research is valuable not only for rational design of Fe-based FTO catalysts, but also for in-depth understanding of the crucial role of coupling DFT calculations with SSITKA in the discrimination of the reaction mechanism and the properties of the active sites in complex heterogeneous catalytic reactions.

Fe基催化剂因能直接转化煤、生物质等来源的合成气经费托合成直接制低碳烯烃（FTO），引起了学术界与工业界的高度关注。然而，催化剂理性设计及产物选择性精细调控受限于复杂催化剂结构特性及反应路径。为此，本项目提出结合DFT计算和SSITKA阐明Fe基催化剂上低碳烯烃形成机理及动力学特征的新思路。采用DFT计算研究有无助剂存在条件下Fe基催化剂主要活性相χ-Fe5C2不同晶面上FTO反应路径，并对其进行微观动力学分析，进一步结合多组分SSITKA方法，以发展用于甄别Fe基催化剂上FTO反应机理的DFT计算与SSITKA耦合技术，从而解析动力学同位素效应及甄别FTO反应机理，最终阐明助剂对Fe基FTO催化剂活性中心结构及动力学特征的影响规律。本项目研究成果不仅可用于指导Fe基FTO催化剂的理性设计，还有助于深入理解DFT计算与SSITKA耦合技术在甄别复杂反应体系反应机理以及活性中心结构的重要性。

费托合成直接制低碳烯烃（FTO）是实现非石化基合成气（CO、H2）向各种烃类化合物和其他各种有机含氧化合物催化转化的重要路径。Fe基催化剂在FTO发展应用中扮演着重要的角色，而Fe基催化剂理性设计及产物选择性精细调控受限于复杂催化剂结构特性及反应路径。本项目基于DFT计算，阐明了Fe基费托合成活性相（χ-Fe5C2）催化剂上FTO的起始关键步骤—CO的活化主要以CO直接解离为主，揭示了不同晶面的表面配位环境对CO活化行为的调变机制，进一步提出了χ-Fe5C2催化CO活化的活性描述符为基于空间分布划分的表面参与反应的Fe原子的电荷；发展了SSITKA与DFT计算耦合的技术，揭示了χ-Fe5C2的典型表面上甲烷形成和C-C偶联反应路径，发现着χ-Fe5C2催化剂中的C原子更倾向于进一步C-C偶联形成C2+产物，而通过CO活化获得的解离C原子更容易甲烷化形成甲烷，建立了描述甲烷形成速率、C-C偶联反应速率以及甲烷选择性的动力学方程；此外，发现Mn、K等助剂的同时添加能有效减小低碳烯烃的脱附能而促进低碳烯烃的形成，而进一步提高K助剂含量可以有效抑制甲烷形成并提高C5+产物选择性，在此基础上最终制备出高催化活性与稳定性的纯相χ-Fe5C2催化剂，为高效Fe基FTO催化剂的可控制备与精细调控提供重要指导。