Effect of Na promoter and reducing atmosphere on phase evolution of Fe-based catalyst and its CO₂ hydrogenation performance

Although alkali metal promoters have a considerable effect on the product distribution of CO₂ hydrogenation over Fe-based catalysts, the mechanism of the active phase transformation is uncertain. The effects of Na modification and pretreatment atmosphere on phase evolution of Fe₂O₃ sample and the synergistic effect between iron oxides and carbides were investigated using in-situ X-ray diffraction ( in-situ XRD). The physiochemical properties of catalyst were characterized by H₂-TPR and CO+H₂-TPSR-MS. When the reducing environment is H₂, Na promoter inhibited the reduction of Fe₂O₃. However, when the reducing atmosphere is syngas (CO/H₂ = 1:2), a suitable amount of Na promoter decreased the reduction and activation temperatures, and increased the iron carbide concentration. The selectivity of light olefins increased from 0.3% to 20.2%, and CO₂ conversion increased from 7.3% to 25.8% when syngas was used as the reducing gas compared to using H₂ as the reducing gas. Compared with pure Fe₂O₃, the CH₄ selectivity of Na modified Fe₂O₃ decreased from 43.2% to 14.9%, while the selectivity of C₅₊ increased from 7.8% to 37.0%, owing to the fact that the Fe₅C₂ content increased from 8.5% to 38.4%. The ratio of Fe₃O₄ to Fe₅C₂ in the catalyst could be effectively controlled by changing the reducing atmosphere and the amount of Na promoter, thereby improving the CO₂ hydrogenation activity and target product selectivity.