高选择性合成气直接制低碳烯烃Co2C基纳米催化研究

The direct production of lower olefins from syngas via Fischer-Tropsch to olefins (FTO) reaction is one of the most challenging subjects in the field of C1 chemistry. It is considered as an attractive alternative non-oil-based production route due to its simplified process and low energy consumption. A major challenge for further development and application of FTO catalysts is to improve the product selectivity. The main objective is to maximize lower olefins selectivity and to reduce methane production by using new catalyst system which deviates from the classical ASF distribution. Although it is always considered as one of the main deactivation reasons for Co-based Fischer-Tropsch reaction, Co2C-based catalyst system will be chosen for FTO and the relative nanocatalysis study will be carried out in this project. The detailed surface structure of Co2C and its effect on the process during C-O activation and cleavage, carbon chain growth and termination will be investigated. Several Co2C-based model catalysts with different particle size and exposed facet will be controllable prepared and the effect of size and surface facet on catalytic performance will be studied. In addition, the structure evolution during Co2C formation and reaction process will be investigated in detail by various in-situ and ex-situ characterization methods. The change of catalytic performance for FTO and probe reactions will be traced with time-on-stream. The structure-performance relationship and deactivation mechanism will be discussed based on the correlation between structure evolution and the change of catalytic performance, and theoretical computation. The nature of the active site benefited for lower olefins formation will be clarified and a new mathematical model for product distribution will be established based on kinetics study. The implement of this project concerning Co2C-based FTO nanocatalyst will provide a new route and valuable information for the development of new FTO catalysts with promising catalytic performance.

合成气直接制备低碳烯烃(FTO)是C1领域中一个极具挑战性的研究课题，具有流程短、能耗低的优势，已成为非石油路径生产烯烃的新途径，但该课题尚存在选择性控制不佳的瓶颈问题。采用何种全新的催化剂体系，使产物分布打破ASF限制，同时实现较高的低碳烯烃选择性及较低的甲烷选择性是研究关键。在本项目中，我们将围绕Co2C开展FTO相关的纳米催化研究。深入研究Co2C表面层结构及所涉及的CO活化与碳链增长行为，可控设计制备Co2C基纳米催化剂，研究Co2C粒径及晶面对FTO性能的影响。同时，综合利用各种相关的原位及离线表征手段，研究Co2C形成过程及反应过程中的结构演变，关联同一时间过程中FTO及探针反应催化性能的变化趋势，结合理论计算，深化构效关系及失活机理的认识，澄清低碳烯烃生成的本质。本项目的执行可发展Co2C基全新的FTO催化系统，为高性能FTO催化剂的研发提供一条新的探索途径及理论指导。

合成气经费托路线直接制备低碳烯烃(FTO)是C1领域中一个极具挑战性的研究课题，具有流程短、能耗低的优势，已成为非石油路径生产烯烃的新途径，但该课题尚存在选择性控制不佳的瓶颈问题。采用何种全新的催化剂体系，使产物分布打破ASF限制，同时实现较高的低碳烯烃选择性及较低的甲烷选择性是研究关键。在本项目中，我们围绕常被视为Co基费托失活原因的Co2C基催化体系开展FTO相关的纳米催化研究。主要成果如下：.（1）发现暴露面为{101}和{020}的Co2C纳米棱柱结构对合成气转化具有异乎寻常的催化性能，在温和的反应条件下可实现高选择性合成气直接制备烯烃，总烯烃选择性高达80%以上，产物分布显著偏离经典的偏离经典的ASF模型。为了解释该独特的偏离现象，提出了基于三个链增长因子的修正ASF分布模型。.（2）通过构效关系研究，揭示了Co2C存在显著的晶面效应，相比于其它暴露面，{101}晶面非常有利于烯烃的生成，同时{101}和{020}晶面可有效抑制甲烷的形成。此外，Co2C纳米结构还存在独特的尺寸效应。当粒径小于7 nm时，催化剂的本征活性随着粒径减小逐渐降低，而当粒径大于7 nm时，本征活性不再随粒径的变化而改变。较小粒径的催化剂具有较高的甲烷选择性而C5+选择性较低，但粒径大于7 nm时产物选择性基本保持不变。.（3）针对Co2C纳米结构的可控制备，通过对助剂影响进行深入研究，发现Na离子能促进CO吸附，弱化C-O键，促进Co2C物相的形成；Mn助剂则通过与钴形成复合氧化物，影响碳化过程中的成核-长大行为，进而有利于棱柱状Co2C纳米结构的形成。同时，载体及还原过程也对Co2C纳米结构的形貌及最终催化性能起着显著作用，较弱的钴-载体相互作用及较为温和的还原过程有利于棱柱状Co2C纳米结构的形成，从而体现出优异的合成气直接制烯烃催化性能。