Rational Construction Of Cobalt-based Catalysts For Fischer-tropsch Synthesis

With the rapid development of Chinese social economy,the contradiction between supply and demand of oil becomes intensified.In China,the dependence on foreign countries has been continuously increasing,which will seriously restrict the further development of the social economy.The fact that coal is the main resource is in line with the natural endowment of our country and can not be changed.The development of Fischer-Tropsch synthesis(FTS)technology,which is the core of indirect coal liquefaction technology,accords the energy strategy of clean and efficient utilization of coal resources.Recently,the rational construction of cobalt-based FTS catalysts with high performance is the focus and difficulty of the research areas.Herein,the candidate constructed a model catalyst of“watermelon seeds”(Co₃O₄)embedded in“watermelon pulps”(SiO₂)to study the size effect in FTS.The results show that the model catalyst has the advantages of a uniform cobalt particle size distribution and effectively inhibits the aggregation of cobalt species.The product selectivity can be successfully adjusted from diesel-range hydrocarbons(66.2%)to gasoline-range hydrocarbons(62.4%)by controlling the crystallite sizes of confined cobalt from 7.2 to 11.4 nm and modify the ASF law.A phenomenon which is different with traditional recognition is observed:the uniform small-sized cobalt crystallites can strongly adsorb active C*species,and the confined structure will inhibit the escape of reaction intermediates,inducing the higher selectivity towards heavier hydrocarbons.In oxides(SiO₂,etc.)supported cobalt catalyst,inevitably the cobalt species will strongly interact with the support,which is inactive in FTS.The carbon supports with a relatively inert surface can interact with cobalt to form a weaker interaction,and the cobalt species can be fully activated.However,the interaction tends to result in sintering of catalysts.Therefore,the contradictory problems can be solved by improving the interaction by chemical modification(N doping and carbon defect design).The N effect in FTS was studied by constructing controlled N-doped carbon nanospheres supported Co catalyst.The N dopants are precisely tailored by adjusting carbonization temperatures,and the interaction between cobalt and supports is significantly strengthened via donation electron function of N atoms,especially for pyrrolic N.It improves the dispersion of cobalt species to generate more electron-enriched cobalt sites,which can strongly adsorb CO and accelerate CO dissociation to C*,resulting in the enhanced catalytic activity.In addition,these in turn increase C*concentration on the surface of active cobalt sites,which is favorable to form a high concentration of chain initiators CH₂ to stimulate carbon-chain growth,inducing the high selectivity towards the C₅₊products.In order to solve the problems of poor mechanical strength and difficult formation over traditional carbon-based catalysts,the defective 3D high-density porous graphene-based macroassembly supported Co catalyst was constructed,and the carbon defect effect on FTS performance was studied.The radial crush strength of the catalyst reaches 449 N cm^-1.A size-uniformed cobalt catalyst was successfully deposited on the defective three-dimensional high-density porous graphene-based macroassembly.The selectivity for diesel fuel(C₁₀-C₂₀)as high as 61.3%at carbon monoxide conversion of 50.7%.No obvious deactivation is observed within 150hours.By the in-depth characterization and analyses,the interfacial electron interaction makes the catalytic surface favor the dissociation of CO and insertion of chain initiator CH₂ to stimulate a narrow distribution of the mediate products.The results of this paper provide new design ideas and insights for the development of catalytic systems with high performance for related industrial processes.