Effects of different supports on catalytic performance of iron-based catalysts in CO₂ hydrogenation to olefins

The combustion of carbon-based fossil fuels (coal, oil and natural gas) for power generation is accompanied by large emissions of anthropogenic CO₂ into the atmosphere, leading to global climate change and ocean acidification. Direct conversion of CO₂ and H₂ via reverse water–gas shift (RWGS) and subsequent Fischer–Tropsch synthesis (FTS) is well-known as a sustainable alternative for the synthesis of olefins, replacing conventional petrochemical process of naphtha stream-cracking or separation. The structure and properties of the inert support have significant effects on the activity, selectivity and stability of the catalyst, but there are few reports about the effects of different supports on the active phase formation and olefins selectivity of Fe-based catalysts in CO₂ hydrogenation. Therefore, the study of the influence of different support on the CO₂ hydrogenation performance of Fe-based catalysts has important guiding significance for the design and synthesis of efficient catalysts. Herein, the Fe-based catalysts with different supports (Q-30, Q-10, Al₂O₃ and TiO₂) were prepared by impregnation method, and the effects of supports on the catalytic activity and olefins selectivity in CO₂ hydrogenation were studied. The catalyst samples were characterized by N₂ adsorption-desorption, XRD, HAADF-STEM, TEM, XPS, H₂-TPR, CO₂-TPD and NH₃-TPD. The N₂ adsorption-desorption isotherms of Q-10, Q-30, Al₂O₃ and TiO₂ support were all classified as type IV by IUPAC. At the same time, there are hysteresis loops in the absorption and desorption curves of these samples, indicating the existence of mesoporous structures. Combined with the reaction data, it can be seen that the specific surface area and pore structure of the support are not regularly correlated with the catalyst activity and olefin selectivity. No XRD diffraction peak attributed to Fe phase was observed in all fresh catalysts. Combined with the results of XPS and STEM-EDX of the fresh catalysts, it can be inferred that this is due to the low content of iron oxide and good dispersion. In the TEM images of the catalysts after the reaction, a large number of lattice fringes belonging to the (501) crystal face of Fe₅C₂ phase were observed in both FeNa/Al₂O₃ and FeNa/TiO₂ catalysts, while no Fe₅C₂ phase was found in FeNa/Q-30 and FeNa/Q-10 catalysts. XRD peaks of Fe₅C₂ species appeared on both FeNa/Al₂O₃ and FeNa/TiO₂ catalysts after reaction at 2θ=40°−50°, indicating that the Fe₂O₃ species was partially carbonized to Fe₅C₂ after reaction. However, FeNa/Q-30 and FeNa/Q-10 catalysts did not have significant iron carbide diffraction peaks. This may indicate that the iron oxide in FeNa/Al₂O₃ and FeNa/TiO₂ catalysts are more likely to be reduced and carbonized to produce Fe₅C₂.The catalytic results show that the Fe-based catalyst with Al₂O₃ as support has the highest activity, highest olefins selectivity, and good stability. The single-pass CO₂ conversion is 28.2%, the selectivity of all olefins reach 68.1%, and the selectivity of long chain olefins is 45.1%. The reduction and carburization of iron species were promoted in the high dispersed FeNa/Al₂O₃ catalyst, due to the moderate metal-support interaction. Meanwhile, it exhibits strongest CO₂ adsorption ability and more surface acid sites, which can enhance CO₂ activation and C−C coupling process. The chain growth factor (α) was as high as 0.74. The CO₂ conversion of FeNa/TiO₂ catalyst was 24.2%, but showed higher CO by-product selectivity and lower C₂₊ olefin selectivity than FeNa/Al₂O₃. In contrast, the activity of Fe-based catalysts with SiO₂ (Q-30, Q-10) support are very low (~10%), and the products are mainly CO and methane from RWGS and CO₂ methanation reaction, probably due to the low reduction and carburization ability, low CO₂ adsorption ability and less acid sites of catalysts.