The following content is copied from this reference (https://doi.org/10.1016/j.ccst.2022.100052) regarding the use of Fe as catalytic sites of DFMs for ICCU-RWGS.
“FeCrCu supported on hydrotalcite (PMG-20) with the modification of 10% K was further developed for the CO2 capture and conversion to CO from diluted CO2 stream (5.8%) containing both O2 (5%) and H2O (4%) (Bobadilla et al., 2016). Complete capture of CO2 was reached, with 45% being converted to CO (92% selectivity) and CH4 at 450°C. In the absence of H2O and O2, CO2 conversion efficiency could reach as high as 80%. With increasing the operation temperature, both the conversion efficiency and CO selectivity increased in the range of 450 to 550°C. Further, 750 cycles operation (45 h) was conducted to evaluate the stability of FeCrCu/K/PMG-20 at 550°C. The DFM activity was generally stable during the testing period under both model (CO2 in N2) or realistic (H2O and O2 contained) conditions, as shown in Fig. 18. The species that may cause the deactivation of DFM, such as coke, formate, and carbonates, were not observed, indicating the suitability of the developed DFMs for ICCC of CO production (Bobadilla et al., 2016).”
“Fig. 18. Cyclic ICCC performance of the FeCrCu/K/PMG-20 under ideal (5.8%CO2 /N2) and realistic conditions (5.8%CO2/5%O2/4%H2O/N2) over 750 cycles (Bobadilla et al., 2016).”
“FexCoyMg10CaO was developed with a one-pot sol-gel method and evaluated for its performance in CO2 capture and in-situ RWGS isothermally at 650°C (Shao et al., 2021a). The CO2 carbonation-decarbonation cyclic stability of CaO was significantly improved due to the prevention of CaO sintering by the melt-intercalation of Fe2O3 into CaO and the high-temperature refractory MgO. Fe5Co5Mg10CaO showed the best stability with a slight uptake capacity decrease from 9.58 to 9.20 mol/kg over ten cycles, much smaller than CaO (9.14 to 4.45 mol/kg), Fe10Mg10CaO (9.09 to 7.41 mol/kg), and Co10Mg10CaO (8.90 to 6.68 mol/kg). In the subsequent RWGS conversion stage, CO2 conversion efficiency of 90% was reached with a CO yield of 8.28 mol/kg for Fe5Co5Mg10CaO and remained stable for ten cycles. The varied valence states of Fe2+/Fe3+ and Co2+/Co3+ provided additional oxygen vacancies for CO2 carbonation enhancement. It was found that Fe2+/Fe3+ was the active catalytic site while Co acted as a promoter in the DFM. CO2 was first reduced to CO by Fe3O4, and then the formed Fe2O3 was further regenerated by H2 through the promoter of Co. The heterojunction-redox mechanism was proposed to understand the Fe-Co bimetallic catalytic RWGS reaction, as shown in Fig. 19. Due to the synergistic effect of Fe/Co, the newly formed Fermi level (0.74 V) in Fe5Co5Mg10CaO heterojunctions was much higher than the electric potential of CO2/CO (−0.52 V), and thus the absorbed CO2 around the bimetal catalytic sites could be easily reduced to CO (Shao et al., 2021a).”
“Fig. 19. Heterojunction redox mechanism of the Fe5Co5Mg10CaO for the ICCC (Shao et al., 2021a).”