Effect Of Na And Zn On Structure Regulation And Hydrogenation Of CO₂ To Olefin Over Iron-based Catalysts

Formation of olefins directly from CO₂ hydrogenation is one of the effective and valuable routs for utilization.Formation of high value-added products directly from CO₂-H₂ mixtures may occur via a combination of reverse water-gas-shift（RWGS）reaction and Fisher-Tropsch（FT）synthesis over Fe based catalysts.Recently,Fe based catalysts were extensively used for the synthesis of olefins from CO₂.However,the reverse water-gas-shift reaction,methanation,and Fischer–Tropsch synthesis occur concomitantly over Fe based catalysts with the CO₂ hydrogenation,which produce a series of carbon containing by-products such as CO,CH₄,and paraffins,and thus resulting unsatisfactory selectivities and yields for the formation of olefins.The Fe based was modified to obtain the desired reaction production.In this work,a series of Fe based catalysts with Na or Zn addition were synthesized to get a deep insight into the effects of Na and Zn decoration on the catalytic hydrogenation of CO₂ and the dynamic structures of Fe Zn catalyst at working state,by steady-state rate measurements,temperature programmed CO reduction（CO-TPR）and desorption（CO-TPD）,in-situ X-ray diffraction（in-situ XRD）,in-situ X-ray photoelectron spectroscopy（in-situ XPS）,in-situ diffuse reflectance infrared Fourier transform spectroscopy（in-situ DRIFTS）and ex-situ characterization.The reverse water-gas-shift reaction was chemically equilibrated during CO₂ hydrogenation over the series of Fe Znx Na catalysts under the reaction condition(593 K,1.5 MPa total pressure,H₂/CO₂=3,balanced by Ar,GHSV=10000 cm³ g^-1_cat h^-1),thus conversion of CO₂ and concentrations of CO,H₂,and H₂O were determined solely by the thermodynamic constraint of RWGS reaction.Addition of Na did not alter the structures of the fresh and CO reduced Fe based catalysts,but improved the stability of iron carbides during CO₂ hydrogenation by suppressing the phase oxidation from iron carbides to iron oxides.Decorating with Na facilitated both the surface reaction of CO*and hydrogen and the C-C bond formation by coupling surface CH*,thus lead to more effective olefins formation.Adding Zn O into Fe catalysts enhanced the rates of CO₂hydrogenation to all carbon containing products because Zn O facilitated the generation of iron carbide,as an active phase in Fe based catalysts,however,the relative rates among primary RWGS reactions,secondary CO hydrogenation to olefins and to paraffins were insensitive to the addition of Zn.