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

Since the first industrial revolution,the massive extraction and use of fossil energy has not only led to global energy depletion,but also aggravated environmental pollution.At the75th United Nations General Assembly,China stated that it would strive to achieve carbon peaking by 2030 and carbon neutrality by 2060.In order to achieve this goal,China urgently needs to strengthen scientific and technological innovation in the development and utilization of energy.Fischer-Tropsch synthesis,which converts syngas（CO/H₂）into clean energy and chemicals,is considered as a promising way to alleviate the energy crisis.Compared with iron-based catalysts,cobalt-based Fischer-Tropsch catalysts process the following characteristics,such as low water gas shift reaction activity and long-chain hydrocarbon selectivity.In the cobalt-based Fischer-Tropsch synthesis,controlling the selectivity and catalytic activity of methane has always been the focus of research.However,the actual reaction conditions and processes are very complex,and the specific reasons for the results are unclear.Studying the relationship between the structural composition and performance of cobalt-based catalysts is of great importance to understand Fischer-Tropsch reaction mechanism,improve the activity,and regulate product distribution.In this paper,cobalt nanoparticles with controllable size and phase composition were prepared by hydrothermal method,which is then supported on CNTs to obtain supported cobalt-based catalysts.Combined with characterization methods including XRD,TEM,BET,Raman,H₂-TPR,H₂-TPD and CO-TPD,the influence of particle size and phase composition on Fischer-Tropsch activity,deactivation behavior and product distribution was systematically investigated.（1）The catalyst Co/CNTs supported cobalt nanoparticle with different size was synthesized and then used in the Fischer-Tropsch synthesis reaction.The results showed that:weaker H spillover results in much higher TOF value and lower/stable methane selectivity over large metallic Co particles.In contrast,smaller Co particles on CNTs lead to lower TOF value and continuously increased methane selectivity till late stage of the 20-h catalytic reaction.This is attributed to the fact that,the small Co particles are beneficial to expose defect sites on the surface of the carbon nanotubes,which exhibit strong hydrogen activation and spillover.Hydrogen spillover can lower atomic H*concentration on cobalt particle surface will reduce surface*CH_xconcentration owing to inefficient H-assisted CO dissociation and subsequent hydrogenation.As a result,the carbon-carbon coupling into longer carbon-chain hydrocarbons is substantially suppressed to lead to gradually increased selectivity toward methane.On the other aspect,the dissociated atomic H spillover will make much higher concentration of active H species near the interfacial sites of Co particle-CNTs.Thus,the relative lack of C*species at the interface also results in the product selectivity shifting to methane via successive hydrogenation.Since the hydrogen storage in CNTs surface is limited owing to the constant amount of surface defect,the spillover rate of hydrogen will become slower with TOS.Therefore,the selectivity to methane is getting stable owing to the relative stable surface H*and C*species distribution.（2）In this thesis,unsupported cobalt nanoparticle catalysts with the different ratios of Co（OH）₂and Co₃O₄crystal phases were prepared by a simple hydrothermal method for Fischer-Tropsch synthesis.The results showed that the catalysts with mixed phases presented higher catalytic activity than that of the single phase of Co（OH）₂and Co₃O₄.It was caused by the fact that Co（OH）₂and Co₃O₄exhibit different reduction properties.The close combination of the mixed phases favors the formation of higher surface area after the reduction of catalysts,which leads to the increase of exposed active sites and the enhanced catalytic activity.In addition,The CNTs-supported catalysts all exhibited higher activity than that of unsupported ones,which was attributed to the fact that CNTs can promote the reduction of cobalt species and inhibit the sintering and agglomeration behavior.Thus,more active sites can be exposed to adsorb and activate CO molecules.