Tuning The Formation Of Effective Active Phase Of Fe-based Catalyst For CO/CO₂ Catalytic Hydrogenation

At present,with the development of society,the renewable energy is recognized as an effective supplement to energy crisis has aroused concern.The coal-rich and oil-poor energy pattern of China makes the Fischer Tropsch synthesis from syngas(CO/H2)has been considered as a promising industrial technology route to produce the clean fuels and light olefins(C2=-C4=)from coal.In addition,although carbon dioxide as a greenhouse gas causes a series of environmental problems,the carbon dioxide can also be used as a carbon source for catalytic hydrogenation reactions.Carbon dioxide hydrogenation can not only alleviate environmental pressure,but also produce high value-added chemicals,which is consistent with the economic development needs.Herein,the purpose of this paper is tuning the formation of effective active phase of Fe-based catalyst for CO/CO2 catalytic hydrogenation with industrial application prospects.With this in mind,combined with a variety of bulk and surface characterization methods,as well as in-situ characterization methods;the structure and properties of catalysts,the synergistic effect of promoter and active metal,the different species of iron carbide,and the synergistic effect between iron carbide and iron oxide on the conversion of syngas(CO/H2)and the hydrogenation of carbon dioxide were deeply studied,and the reaction mechanism of CO2 hydrogenation was revealed.The main works are as follows:(1)In this chapter,three iron-based catalysts with different activity for Fischer-Tropsch synthesis were prepared by simple physical mixing method:?Fe2O3,SiO2/?-Fe2O3=1,Al2O3/?-Fe2O3=1.With the mind of avoiding diffusion limitation,metal-support interaction and other factors,the non porous structure?-Fe2O3 catalyst was selected as the Fischer-Tropsch synthesis catalyst.Under the same conditions of reduction and reaction,it is noted that the CO conversion of the ?-Fe2O3 catalyst,which was modified by Al2O3,can reach up to 61.6%,which is about 3.3 times and 1.4 times of that of ?-Fe2O3 catalyst and SiO2/?Fe2O3=1 catalyst,respectively.The XRD,N2-physical adsorption desorption,SEM,H2-TPR,In-situ XPS,HR-TEM,DRIFTS and Mossbauer spectroscopy(MES)were used to understand the effect of different iron carbide species on CO hydrogenation activity in Fischer-Tropsch synthesis.(2)In this chapter,iron-based catalysts for Fischer-Tropsch synthesis with highly selectivity of light olefins(C2=-C4=)were prepared by a simple mechanical mixing method.With the mind of avoiding diffusion limitation,metal-support interaction and other factors,the non porous structure ?-Fe2O3 catalyst was selected as the Fischer-Tropsch synthesis catalyst.It is noted that the electronic state of Fe atoms on the surface ?-Fe2O3 catalyst was changed due to the promoter effect of MnO2,and the HR-TEM results indicated that the interface of MnO-Fe3C was formed on the MnO2 modified ?-Fe2O3 catalyst.The characterization results of catalysts showed that MnO2 was beneficial to the formation of special structure of Iron carbide(?-Fe3C).In addition,in-situ XPS and Mossbauer spectroscopy(MES)were used to characterize and quantify the species of iron carbide.Combined with the catalystic performance,it is considered that the MnO2 plays an important role in enhancing the formation of effective active phase of ?-Fe3C for facilitating the formation of light olefins(C2=-C4=)in Fischer-Tropsch synthesis.(3)The promoter can effectively regulate the active phase of iron carbides.However,the promoter also plays a role of catalyzing and activating CO to a certain extent,which makes it difficult to identify the exact role of iron carbides in FTS reaction.In this chapter,in order to exclude the influence of promoters,diffusion limitation,metal-support interaction and other factors,the ?-Fe catalysts without promoters and supports were designed to understand the real role of iron carbides in Fischer Tropsch synthesis.By controlling the oxidation degree of ?-Fe precursor,the different components of iron carbide phases were obtained under the same carburizing conditions.Significantly,combined with the various characterization techniques and under the same reaction conditions,we have found that the selectivity of alkenes(especially C5+alkenes)were highly depends on the two components(?-Fe5C2,?-Fe3C),rather than pure phase ?-Fe5C2 or ?-Fe3C,and the optimal ratio determined by Mossbauer spectroscopy was ?-Fe3C/?-Fe5C2=1.5.(4)Owing to the chemical inertness and thermodynamic stability of CO2,efficiently regulate the product selectivity in CO2 hydrogenation is still a big challenge.In this chapter,the selective direct conversion of CO2 into alkenes and ethanol by Na-promoted Fe3O4 microsphere catalysts are investigated.Without impairing the CO2 reactivity and stability of the Na/Fe3O4 catalyst,the outstanding selectivity of 73.2%alkenes(the olefin to paraffin molar ratio at 12.9 in the C2-C4 range hydrocarbons)and 97.9%C2+oxygenates(ethanol selectivity at 87.4%in the total oxygenates)were achieved at as low as to 0.5MPa and 3MPa,respectively.Meanwhile,all the catalysts also exhibited as low as possible selectivity for by-product CO approximately 8%.Based on various characterization results,including XRD,N2-physical adsorption desorption,Mossbauer spectroscopy(MES),SEM,H2-TPR,In-situ XPS,HRTEM,NAP-XPS,CO2-DRIFTS and prepared catalysts were characterized,the effect of different iron carbide species and iron oxide on the selectivity of products during CO2 hydrogenation were investigated,and the reaction mechanism of CO2 hydrogenation was revealed by the CO2-DRIFTS.