Study On Structure Control And Fischer-tropsch Reaction Performance Over Supported Cobalt-based Catalysts

Fischer-Tropsch synthesis（FTS）is a green synthesis reaction that using catalysts directly convert syngas（H₂/CO）to high value-added clean energy.It is considered to be one of the most promising ways to solve energy problems.Cobalt-based Fischer-Tropsch catalysts have many advantages,such as high stability and Fischer-Tropsch activity,the pure hydrocarbon products and high selectivity of high-value long-chain hydrocarbons.They are widely used in the Fischer-Tropsch industry.Understanding the structure-performance relationship of cobalt-based Fischer-Tropsch synthesis is significant to the design and development of cobalt-based catalysts and the realization of high-efficiency regulation of target products.In this paper,a series of Co/SiO₂ and Co/ZrO₂ catalysts were prepared by different methods,and the related physical and chemical properties of the support and catalyst were characterized by some characterization methods,such as XRD,BET,TEM,H₂-TPR,CO-TPD.The process of structure evolution of the Co/SiO₂ catalyst during the FTS were detected and analyzed by In situ XRD method.Combing the results of characterization and quantum chemical calculations,we discuss the Fischer-Tropsch reaction performance of the cobalt-based catalyst in depth.In terms of structure control and structure-performance relationship of supported cobalt-based catalysts,the following conclusions are summarized:1)The structure control of the metal nanoparticles in the supported cobalt-based catalyst can be achieved through the selection of the carrier,preparation method,and promoter.The larger the specific surface of the carrier,the more beneficial it is for the dispersion of the metal,and the smaller the size of metal nanoparticles;the combustion method has more advantages for the formation of small-sized metal nanoparticles than the impregnation method,but this rule is only applicable to carriers with a larger specific surface;the addition of ZrO₂ promoter helps to increase the specific surface of the carriers thereby promoting the dispersion of metal particles over the catalysts.2)The size effect in the supported cobalt-based Fischer-Tropsch reaction:as the size of the metal particles in the Co/SiO₂ catalyst decreases,the number of active Co atoms with low coordination environment such as edge/corner sites will increase,which will help to improve the reaction activity of catalysts but easily oxidized to CoO by H₂O.Moreover,when the metal particle size is less than 6 nm,the catalyst still exists in the form of CoO phase during the reaction due to the lower reducibility.CoO easily reacts with Si-OH groups on the surface of the SiO₂ carrier to form Co₂SiO₄,which will not only reduce the active sites in the catalyst and reduce the catalytic activity,but also inhibit the progress of chain growth and change the trend of methane selectivity during the reaction.And,when the size of the metal nanoparticles in the catalyst is 9.2 nm,the structure transformation is readily to occur in the Fischer-Tropsch process,and the particle size changes during the reaction process.3)The promoter effect in the supported cobalt-based Fischer-Tropsch reaction:ZrO₂ as a structural promoter can inhibit the hydrolysis of the Si-O-Si bond on the surface of SiO₂ carrier and reduce the possibility of CoO reacting with Si-OH to form Co₂SiO₄;as an electronic promoter,ZrO₂ can form a Co-ZrO₂ interface with Co,and greatly improves the CO adsorption and dissociation ability with the charge transfer from metal Co to the Co-ZrO₂ interface.In addition,ZrO₂ itself has some acidic sites and basic sites,which have CO adsorption capacity,thus can provide more CO adsorption sites for the catalyst.4)The carrier effect in the supported cobalt-based Fischer-Tropsch reaction:the m-ZrO₂carrier with larger surface enable to prepare the Co/m-ZrO₂ catalyst with smaller particle size,which can expose more active sites and give a higher reaction activity,meanwhile the more basic sites is more conducive to promoting the CO adsorption capacity of the catalyst;t-ZrO₂carrier with smaller pore size is prone to produce liquid phase products to block the pores,and then affects the transfer of reactant molecules,promotes the hydrogenation reaction,and changes the CH₄ selectivity trend of the catalyst.