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

Abstract

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.