Capture, conversion and utilization of CO2 for chemicals and fuels are important parts of integrated solutions to reaching carbon neutrality, or net zero carbon. Catalytic processing is the key to realize the effective utilization of CO2 as a carbon source with hydrogen produced using renewable energy for sustainable chemicals and fuels. Due to the complexity of the elementary reactions involved in CO2 hydrogenation and lack of fundamental understanding on the key factors controlling the selective reaction pathways on the catalysts, it remains challenging to achieve selective synthesis of desired chemicals and fuels from CO2 hydrogenation. Based on the recent progress reported from experimental and computational studies, this presentation will discuss the active phase, active sites and structural sensitivity of iron-based heterogeneous catalysts involving Fe carbides and Fe oxides for CO2 hydrogenation to hydrocarbons (CH4, C2-C4, C5+) and alcohols (methanol and ethanol). There are different forms of Fe carbides including Fe3C, Fe5C2, other FeCx and of Fe oxides including Fe2O3, Fe3O4, and FeO, which are all involved in various stages of catalyst activation and CO2 conversion with time on stream. There exists a dynamic competition between surface carburization and oxidation during CO2 hydrogenation, with the former contributes to creating more active sites while the latter causes oxidation of the active sites, where the deactivation is related to H2O produced for every CO2 molecule converted on Fe catalyst. The presence of excess surface O* on the catalysts make the CO2 hydrogenation different from CO hydrogenation. The conventional wisdom that CO2 hydrogenation goes through CO as an intermediate is also being challenged by recent experimental and computational studies. The recent studies point to a proposed elementary reaction pathway leading chemisorbed CO2 to HCOO* then HCO* then to HCOH followed by HC* whose coupling produce C2 species. Thus emerges a new path for CO2 hydrogenation that does not go through gas-phase CO, neither the chemisorbed CO, which has been shown for Fe-based bimetallic catalysts such as Fe-Cu-K catalyst. CO2 hydrogenation is structure sensitive, and the dependence of activity and selectivity on the particle size of Fe catalysts has been observed. From DFT computational study, certain crystal facets are more active for CO2 hydrogenation to form HCOO* intermediate.

Mechanistic insights into active phases and reaction pathways on Fe-based catalysts for CO2 hydrogenation to chemicals and fuels

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