Study On The Design,Preparation And Catalytic Performance Of Iron Catalysts For CO₂ Hydrogenation

CO2 hydrogenation to light olefins via the reverse water-gas shift (RWGS) and Fisher-Tropsch (F-T) reactions is one route of CO2 cyclic utilization. Due to low catalyst activity and restriction by dynamics, there exist wide product spectrum and high C1 products. Besides, secondary reactions of the primary olefins, such as hydrogenation and polymerization, also affect the product distribution.The effect of pretreatment on the surface properties of Fe catalyst is firstly studied. Pretreatment conditions show significant effects on the phase structure and reaction behavior of the catalysts. At initial stage of CO2 hydrogenation, the O/P ratio on iron carbide is significantly higher than that on iron oxide. Iron oxide shows higher activity for secondary hydrogenation. Fe5C2 species results in the highest selectivity of olefin while Fe3O4 species in the highest conversion of CO2. CO2 hydrogenation follows the two-step reaction mechanism.Fe-Zr-K, Fe-Zr-Ce-K synthesized by microwave assisted-homogeneous precipitation shaped in small and uniform particles of 50-60 nm. Small and uniform particles favor in dispersion of iron species. Appropriate amount addition of Zr increases the catalyst surface basicity and CO2 adsorption. When the Fe/Zr ratio was 5, at conditions of H2/CO2=3,320° C,2MPa and 1000h-1, the CO2 conversion and the light olefins selectivity reached 54.36% and 53.63%, respectively. The selectivity of CO was as low as 3.00%. The excellent activity stability is possibly due to strong interaction between iron species and zirconia.CeO2 promoter restrains the growth of Fe2O3 crystallite with more defects in favor of the CO2 adsorptive activation resulting in significant improvement in the CO2 conversion and the product distribution. Fe-Zr-Ce-K catalysts showed high activity for hydrogenation of CO2 to light olefins via intensified bifunctional activity for both the RWGS reaction and F-T reaction. When the Fe/Ce ratio was 35, the CO2 conversion and the light olefins selectivity reached maxima,57.34% and 55.62%, respectively. The O/P ratio was 7.07.Iron templates prepared by solvent-thermal method, were transformed into α-Fe2O3, γ-Fe2O3 and Fe3O4 with nanostructure and high specific surface areas under heat treatment. Over K/a-Fe2O3 the light olefins selectivity was as high as 59.30%, O/P ratio reached 8.08 and the Cs+ selectivity reduced greatly. Over K/Fe3O4, the light olefins selectivity reached 61.03%. Larger specific surface area leads to stronger adsorption of CO2. Diffusion resistance is reduced through uniform nanolayers and chain growth on the surface suppressed, reducing the C5+ selectivity and secondary hydrogenation of olefins produced.