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

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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.