CO2 hydrogenation conducted on Fe-based catalysts consists of a wide range of reactions with CO2 and H2 reacting in the reverse water–gas shift (RWGS) to produce CO and CO and H2 reacting in the Fischer–Tropsch (FT) type reactions leading to hydrocarbons and oxygenates. Methanation and Boudouard side reactions are extremely detrimental to selectivity and stability of the Fe-based catalysts. The catalytic system is very complex, posing challenging issues that require fundamental understanding of the dynamics of changes in the catalytic phases, mechanism of key reactions, and effects of catalyst composition including key promoters. A comprehensive analysis of fundamental aspects of catalytic materials, phases, and promoters and the catalytic mechanisms are presented in this paper. It was established that the ratio of Fecarbide/Feoxide atoms at the surface of an activated catalyst responsible for its selectivity is determined by the environment of iron ions in oxide precursors changed by insertion of ions of other metals. Fungible liquid fuels were produced in bench scale reactors and demonstrated to be suitable as blending stock for transportation fuels. The techno-economic analysis of processes using CO2 and either water, biogas, or natural gas as feedstock was conducted. As expected, the production of eco-friendly, renewable fuels based on CO2 is not competitive with fuels based on crude oil because of the high cost of production of hydrogen.

Seven solid mono-, bi- and tri-metallic oxide matrices where Fe(2+,3+) ions are distributed in different chemical/spatial environments were synthesized and characterized by XRD, N2-adsorption and EDAX methods. After basification with potassium, all matrices were activated by carburization or reduction-carburization under conditions selected based on the TPC/TPR spectra, tailoring the carburization extent of iron. The performances of the activated Fe-based catalysts with respect to CO2 conversion and C5+ selectivity were measured in a fixed-bed reactor under standard conditions in transient and continuous operation modes in units containing one or three reactors in series with water separations between the reactors. The catalysts were characterized by XRD, N2-adsorption, HRTEM-EELS and XPS before and after steady-state operation in the reactors. It was found that the rate of CO2 conversion is not limited by thermodynamic equilibrium but is strongly restricted by water inhibition and it depends on the nature of the Fe-oxide precursor. The ratio between the FTS and RWGS rates, which determines the C5+ hydrocarbons productivity, is strongly affected by the nature of the Fe-oxide matrix. The catalysts derived from the Fe-Al-O spinel and Fe-Ba-hexaaluminate precursors displayed the best balance of the two functions RFTS/RRWGS = 0.77-0.78. They were followed by magnetite, CuFe-delafossite, K-ferrite, Fe-La-hexaaluminate and LaFe-perovskite with a gradual lowering of RFTS/RRWGS from 0.60 to 0.15 and a gradual decrease in the C5+ productivity. The active sites that enhance the RWGS reaction are located on the surface of the Fe-oxide phases, while the FTS and methanation reactions occur on the surface of the Fe-carbide phases.

The effects of chemical composition, texture, and promotion with potassium of coprecipitated and carbon‐templated BaFe‐hexaaluminate catalysts on their performance in reverse water gas shift (RWGS) reaction were studied in a fixed‐bed catalytic reactor at T=350 °C, p=20 bar, H2: CO2 of 3. The methane selectivity at these conditions did not exceed 0.1 %. It was established that the rate of RWGS is strongly affected by the catalysts iron content gradually increasing up to 45 % Fe. Decreasing the catalyst nanocrystals platelets size (diameter/thickness) from 500–800/50–80 nm to 150–250/30–50 nm (SEM) by implementing the CT increased the surface area and the rates of reaction. Modeling of the redox cycle of RWGS by CO2‐TPD, TP reaction and TP reduction indicated that H2 reduction is needed for regeneration of active sites Fe2+ ions associated with oxygen vacancies. Potassium displayed a strong promotion effect on the activity of Ba−Fe‐hexaaluminates. At optimal K content of 6 wt %, the RWGS rates increased by a factor of 12–15 without changing the TOF number. This was attributed to increasing concentration of active sites due to the reduction of Fe3+ ions (XPS).

The Fe-substituted Ba-hexaaluminates (BaFeHAl) are active catalysts for reverse water-gas shift (RWGS) reaction conducted in chemical looping mode. Increasing of the degree of substitution of Al3+ for Fe3+ ions in co-precipitated BaHAl from 60% (BaFeHAl) to 100% (BaFe-hexaferrite, BaFeHF), growing its surface area from 5 to 30 m2/g, and promotion with potassium increased the CO capacity in isothermal RWGS-CL runs at 350–450 °C, where the hexaaluminate/hexaferrite structure is stable. Increasing H2-reduction temperature converts BaFeHAl to a thermally stable BaFeHF modification that contains additional Ba-O-Fe bridges in its structure, reinforcing the connection between alternatively stacked spinel blocks. This material displayed the highest CO capacity of 400 µmol/g at isothermal RWGS-CL run conducted at 550 °C due to increased concentration of oxygen vacancies reflected by greater surface Fe2+/Fe3+ ratio detected by XPS. The results demonstrate direct connection between CO capacity measured in RWGS-CL experiments and calculated CO2 conversion.