Iron-Based Perovskites Catalyze CO₂ Hydrogenation To Light Olefins

Light olefins are an important basic raw material.The reaction of CO₂ with hydrogen to synthesize light olefins such as ethylene and propylene not only serves as an alternative to the traditional production of fuels and chemicals from fossil energy,but also converts CO₂ efficiently,mitigates the greenhouse effect,reduces people’s over-dependence on fossil energy.Due to the high structural stability of CO₂,a highly active and selective hydrogenation catalyst is the key for CO₂ hydrogenation to light olefins.Usually,CO₂ hydrogenation to light olefins is a two-step mechanism of Reverse Water-Gas Shift Reaction（RWGS）and Fischer-Tropsch synthesis（FTS）.The Fe-based catalysts have good catalytic performance in both reactions and are widely concerned,the current loaded catalysts have the problems of weak dispersion of active components and easy agglomeration.Based on the features of controlled structure,high catalytic activity and great elemental dispersion of perovskite-type oxides（ABO₃）,this paper uses Sr Fe O₃ as the catalyst precursor with K-doping at the A-site and Co-doping at the B-site,respectively,and their catalytic reaction activities were investigated in detail.The perovskite-type oxides Sr_1-xK_xFe O₃（x=0-0.6）and Sr_1-xK_xFe_0.5Co_0.5O₃（x=0-0.4）were prepared by sol-gel method,and the performance of CO₂ hydrogenation reaction was tested on a fixed-bed reactor.The effect of K on the catalyst structure and the reaction product distribution was investigated using relevant characterization.The phase transition process and rate control steps of the catalyst in this system were also investigated.The reaction performance of the perovskite-type oxides Sr_1-xK_xFe O₃（x=0-0.6）was first investigated,and the highest catalytic activity was observed when the catalyst was Sr_0.6K_0.4Fe O₃,the light olefins selectivity and CO₂conversion are 29.61%and30.82%,respectively,which are much higher than that of the undecomposed precursor catalyst Sr Fe O₃.With the increase of the hydrogen-carbon ratio in the reaction gas,the CO₂ conversion increases and the CO selectivity decreases,indicating that only RWGS reaction occurred at low hydrogen concentration and hydrocarbons were gradually produced only in the presence of sufficient amount of hydrogen,which is consistent with the literature.To improve the catalytic reaction performance,Sr_1-xK_xFe_0.5Co_0.5O₃（x=0-0.4）with Fe:Co=1:1 was prepared on the basis of Sr_1-xK_xFe O₃ with Co doping at the B-site,and the optimal amount of K doping at the A-site for Fe:Co=1:1 was investigated.The experimental results showed that the catalytic reaction performance was strongest when the catalyst was Sr_0.8K_0.2Fe_0.5Co_0.5O₃,and its CO₂ conversion and light olefins selectivity were 53.73%and 40.55%,respectively.According to the relevant experimental results,the FTS reaction is considered as a rate control step and the presence of Co facilitates the conversion of CO₂.The characterization results showed that the introduction of the alkali metal promoter K promoted the decomposition of the precursor catalysts Sr_1-xK_xFe O₃ and Sr_1-xK_xFe_0.5Co_0.5O₃ and improved the surface basicity and increased the adsorption of CO₂.No more obvious agglomerative sintering was found on the surface of the reacted catalyst,and the active substances such as Fe and Co were evenly dispersed on the catalyst surface.In addition the presence of Co and Fe Co alloy phases in the reacted Sr_1-xK_xFe_0.5Co_0.5O₃ further promoted the conversion of CO₂ and the generation of hydrocarbon substances.