Selective Preparation Of ?-Fe₅C₂ Catalysts For Fischer-Tropsch Synthesis

Direct conversion syngas into clean liquid hydrocarbon fuels and light olefins via FischerTropsch synthesis(FTS)is a viable way that can optimize the energy supply network by using alternatively carbon source,i.e.coal.Iron-based catalysts are more competitive for FTS because of their low cost and potential high activity for reverse water-gas shift reaction.In most studies,?-Fe5C2 has been suggested as the dominant active phase.However,there is some unresolved issues of the preparation of ?-Fe5C2 catalyst such as complicated procedure,low ?-Fe5C2 content and the underlying mechanism of phase evolution.Herein,my research work focused on the selective preparation of ?-Fe5C2 catalyst.Firstly,an iron nanoparticle catalyst encapsulated by nitrogen-doped graphitic carbon was synthesized.The combined electronic and confinement effects of the nitrogen-doped graphitic carbon layers promote the formation of a high content of ?-Fe5C2 during FTS reaction conditions.Furthermore,by regulating the H2/CO ratio of atmosphere,the mechanism of phase evolution during the carburization process of iron oxide precursor was investigated.A facile and effective strategy was thus developed for the selective preparation of pure-phase ?-Fe5C2 catalyst.Finally,a functional configuration catalyst with a ?Fe5C2@Fe3O4 satellite-like structure was fabricated to address the potential high CO2 selectivity of the ?-Fe5C2 catalyst.The unique structure of catalyst can promote the formation of long-chain hydrocarbons on inner ?-Fe5C2 and further the depletion of by-product CO2 on outer Fe3O4 via reverse water-gas shift reaction.The detailed research contents are as following:(1)An iron nanoparticle catalyst encapsulated by nitrogen-doped graphitic carbon was prepared by a one-step high-temperature pyrolysis of a highly dispersed iron species complex,which was synthesized by coordination of ferric ions and glutamic acid.The formed nitrogendoped graphitic carbon layers can act as an electron donor to the iron nanoparticles to promote the CO activation,thus expediting the formation of ?-Fe5C2.The well-encapsulated structure can effectively inhibit the migration of iron nanoparticles during the reaction process,which is beneficial for good stability.The structure of carbon shell also influence the product distribution.A carbon shell with an appropriate thickness and a porous structure restricts the diffusion removal of reactive olefins,which can undergo further chain propagation reaction to longerchain hydrocarbons.Meanwhile,the carbon shell hinders the formation of C12+hydrocarbons,thus leading to a high selectivity toward Cs-C11 hydrocarbons.The catalyst pyrolyzed at 800?showed excellent catalytic activity(239.4 ?molco gFe-1 s-1)and high C5-C11 hydrocarbon selectivity(49%),outperformance most Fe-C catalysts.(2)The relationships of the H2/CO ratio of the pretreatment atmosphere with these competitive surface oxygen removal,carbon permeation,hydrogenation and carbon deposition reactions,and their effects on the structure evolution of iron phases have been established through the in-situ characterization.The atmosphere of 64%H2/32%CO/4%Ar yields a compromise among the competitive surface reactions,thus selectively forming pure-phase ?Fe5C2 catalyst.Such catalyst showed high CO conversion of 96%,selectivity to C2-C4 olefins of 30%and to C5+hydrocarbons of 40%(excluding CO2),and good stability.The simultaneously enhanced activity and selectivity are relate to the more terrace sites of ?-Fe5C2,and the pre-formed carbon coating during pretreatment process improves catalyst stability due to the inhibition effects on the nanoparticle aggregation and further carbon deposition during FTS reaction.(3)A ?-Fe5C2@Fe3O4 satellite-like structure catalyst was fabricated via an in-situ oxidation method during high pressure pretreatment process.The introduction of Fe3O4 around?-Fe5C2 promotes the depletion of primarily formed by-product CO2 via RWGS reaction to lower CO2 selectivity.The ?-Fe5C2@Fe3O4 satellite-like structure catalyst promotes the production of a high selectivity for C5+hydrocarbons of 45%(including CO2),while simultaneously suppresses the formation of CO2(only 7%)and CH4(10%).During the high pressure pretreatment process,carbon layers with a higher graphitization degree are formed on the surface of catalyst.The electronic promotion effects of formed carbon layers enhance surface C-C coupling ability and suppress hydrogenation ability,thus effectively improving the production of C5+hydrocarbons.