Descriptors Affecting Methane Selectivity in CO₂ Hydrogenation over Unpromoted Bulk Iron(III)-Based Catalysts

Direct CO 2 hydrogenation to higher hydrocarbons, that is, CO 2 Fischer–Tropsch (CO 2 -FT) synthesis, is an attractive way to mitigate anthropogenic CO 2 emissions and to produce fuels alternatively to oil- or natural-gas-based processes. One of the major shortcomings of typically applied Fe-based catalysts in CO 2 -FT is high selectivity to methane. Herein, we provide descriptors relevant for purposeful preparation of unpromoted Fe 2 O 3 with the suppressed formation of this undesired product. This was possible owing to the combination of controlled material synthesis, sophisticated catalyst characterization (including along the catalyst bed) by state-of-the-art techniques with spatially resolved steady-state, and transient kinetic analyses. The defectiveness of in situ formed Fe 5 C 2, that reflects the deviation from the stoichiometric composition and expressed as the ratio of C/Fe, affects the ability to adsorb CO 2 and CO as well as to activate H 2 . CH 4 formation is hindered over less-defective iron carbides, which are characterized by strong adsorption of CO and CO 2 but low adsorption of H 2 . Thus, the catalyst efficiency in terms of C 2+ -hydrocarbon formation is improved. The imperfection of Fe 5 C 2 can be controlled through the morphology of iron(II) oxalate dihydrate used as a precursor for preparation of Fe 2 O 3 . In addition to the defectiveness, the size of crystallites of α-Fe 2 O 3 and the ratio of α-Fe 2 O 3 to γ-Fe 2 O 3 in fresh samples are two indirect descriptors affecting the ability of in situ formed Fe 5 C 2 to directly hydrogenate CO 2 to CH 4 . These structural characteristics depend on temperature, at which Fe 2 O 3 materials were prepared. The lowest achieved CH 4 selectivity is 0.05 at CO 2 conversion of about 0.2. The ratio of olefins to paraffins among C 2 –C 4 -hydrocarbons is about 6.