Iron-based catalysts for CO2 conversion into higher hydrocarbons under dynamic condi-tions

如果氢是由风能或太阳能等可再生能源生产的，那么二氧化碳加氢（CO2- ft）转化为高级碳氢化合物为大规模生产增值化学品提供了一条可持续的途径。从基本观点来看，这个反应也很有趣，因为铁基催化剂会发生反应诱导的重组，从而导致催化剂的动态操作并影响产物的选择性。在第一轮项目中，我们研究了FexOyCz催化剂在预处理和反应条件下的动态结构变化。由于不同的动力学和FeCx形成对反应产物CO和H2O浓度的强敏感性，它们的组成也沿着催化剂床长变化。催化剂的合成方法、处理条件和如钠的掺杂改变了催化剂的行为和碳化物的形成。本文首次合成了具有软模板介孔结构的纳米碳化铁薄膜。建立了形成碳化物的结构模型、形成碳化物的机理以及形成相、活性和选择性之间的关系。计划在第二个资助期进行的研究将集中在以下几个基本方面：(i)反应或外部刺激诱导的产物形成和催化剂重组的动力学，（ii）反应诱导的FeCx碳化物的形成和分解，（iii） CO2-FTS反应器动态操作对碳化物形成和稳定的影响，（iv）评估连续和瞬态操作中CH4选择性的下限。我们将结合控制材料合成与催化试验和全面的催化剂表征在操作条件下。计划中的实验将集中在促进体、支撑和模型型纳米结构的FeOx薄膜上。由于CO2-FT反应沿反应器床层产生相梯度，我们的目标是通过动态切换流动方向的逆流反应器操作来控制FeCx的浓度。适当的周期性操作可以提高产品的时间平均产量。由于形成FeCx需要CO，因此将使用双反应器系统定期向进料中添加CO，以确定提高产品选择性的动态协议。一种新型拉曼剖面反应器将用于分析FeOx和碳质物质的形成/转化依赖于催化剂床层的位置。因此，将推导出结构和催化性能之间的一般关系，并为定制催化剂设计和确定最佳反应器稳态和瞬态/周期性操作提供基础。我们还将扩展我们在SPP内的富有成效的合作，例如开发的operando FTIR或拉曼反应器，用于二氧化碳加氢测试的多通道设置，加入动力学建模以及统计数据分析方法。

CO2 hydrogenation (CO2-FT) into higher hydrocarbons offers a sustainable path to the large-scale production of value-added chemicals if hydrogen is produced from renewable energy sources such as wind or solar power. This reaction is also intriguing from a fundamental viewpoint because Fe-based catalysts undergo reaction-induced restructuring, which causes dynamic catalyst operation and impacts product selectivity. In the 1st-round project, we studied dynamic structural changes of FexOyCz catalysts depending on pretreatment and reaction conditions. Their composition also changes along the catalyst bed length due to different kinetics and a strong sensitivity of the FeCx formation towards the concentration of the reaction products CO and H2O. Catalyst synthesis method, treatment conditions and doping with e.g. sodium change the catalyst behavior and carbide formation. We report the first synthesis of nanostructured iron carbide films with soft-templated mesopore structure. A structural model for the formed carbides, a mechanistic picture of carbide formation and correlations between the formed phases, activity and selectivity were established. The studies planned for the 2nd funding period will focus on the following fundamental aspects: (i) dynamics of product formation and catalyst restructuring induced by reaction or external stimuli, (ii) reaction-induced formation and decomposition of FeCx carbides, (iii) influence of dynamic CO2-FTS reactor operation on carbide formation and stabilization, (iv) assessing the lower limit for CH4 selectivity in continuous and transient operation. We will combine controlled material synthesis with catalytic tests and thorough catalyst characterization under operando conditions. The planned experiments will focus on promoted bulk, supported and model-type nanostructured FeOx films. Since the CO2-FT reaction induces a phase gradient along the reactor bed, we aim to control the concentration of FeCx by reverse-flow reactor operation with dynamic switching of flow direction. Suitable periodic operation may enable higher time-averaged product yields. As CO is required for FeCx formation, CO will be added periodically to the feed using a dual-reactor system to identify dynamic protocols for improving product selectivity. A novel Raman profile reactor will be used to analyze the formation/conversion of FeOx and carbonaceous species in dependence on the position along the catalysts bed. Thus, general relationships between structural and catalytic properties will be derived and provide the basis for tailored catalyst design and for identifying optimal reactor steady-state and transient / periodic operation. We will also extend our fruitful collaborations within the SPP offering e.g. the developed operando FTIR or Raman reactors, a multi-channel setup for CO2 hydrogenation tests, joined kinetic modelling as well as statistical data analysis methods.