From Understanding of Catalyst Functioning toward Controlling Selectivity in CO2 Hydrogenation to Higher Hydrocarbons over Fe-Based Catalysts

Abstract

The conversion of carbon dioxide (CO₂) with hydrogen (H₂), generated by renewable energy sources, into value-added products is a promising approach to meet future demands for sustainable development. In this context, the hydrogenation of CO₂ (CO₂-FTS) to higher hydrocarbons (C₂₊), lower olefins, and fuels should be mentioned in particular. These products are used in our daily lives but are currently produced by energy-intensive and CO₂-emitting oil-based cracking processes. The environmental compatibility and abundance of iron (Fe) used in CO₂-FTS catalysts are also relevant to sustainable development. The CO₂-FTS reaction was inspired by the experience accumulated in long-term research on Fischer–Tropsch synthesis with CO (CO-FTS). A simple grafting of catalyst formulations and reaction mechanisms from CO-FTS to CO₂-FTS has, however, been proven unsatisfactory, likely due to differences in surface adsorbates, chemical potentials of CO and CO₂, and H₂O partial pressure. These characteristics affect both the catalyst structure and the reaction pathways. Consequently, CO₂-FTS provides higher CH₄ selectivity but lower C₂₊-selectivity than does CO-FTS, which appeals to fundamental research to hinder CH₄ formation.

In this Account, our recent progress in identifying descriptors for purposeful catalyst design is highlighted. Different from the trial-and-error methods and chemist’s intuition strategies commonly used for catalyst design, our initial efforts were devoted to a meta-analysis of literature data to identify catalyst property–performance relationships in CO₂-FTS. The resulting hypotheses were experimentally validated and provided the basis for catalyst development. Our other distinguishing strategy is spatially resolved analyses of reaction-induced catalyst restructuring and reaction kinetics. As the catalyst composition changes downstream of the catalyst bed, it is critical to consider the respective profiles to establish proper correlations between the working catalyst phase and species and the kinetics of the formation of selective and unselective reaction products. The importance of in situ characterization studies for understanding reaction-induced catalyst restructuring is especially highlighted. We also demonstrate the power of transient kinetic methods, i.e., temporal analysis of products (TAP) and steady-state isotopic transient kinetic analysis (SSITKA), to identify the mechanism and microkinetics of the activation of CO₂, CO, and H₂ that characterize the efficiency of iron carbides for CO₂ hydrogenation. The SSITKA method is also instrumental in quantifying the abundance and lifetime of surface intermediates, leading to CO or CH₄. The global network of product formation is further established by analyzing selectivity–conversion relationships to identify primary and secondary products. Our spatially and time-resolved analyses of catalyst composition and product formation rates can be useful for various heterogeneous reactions studied in plug flow reactors because the partial pressures of feed components and reaction products change along the catalyst bed. Such changes can result in spatial profiles of active phases/species. Combining catalyst structural features with kinetic/mechanistic information allowed us to elucidate the fundamentals of controlling catalyst activity and product selectivity and the mechanism of catalyst deactivation. We also present how the derived knowledge aids in the design of robust Fe-based catalysts, paving the way for the current studies one step closer to the implementation of more sustainable CO₂ utilization.