Catalytic Performance Of Carbon Supported Atomically Dispersed Iron Based Catalysts For Fischer-Tropsch Synthesis

China is rich in coal resources,but lacking in oil storage.It has been relying on imports for a long time.Effective utilization of coal resources is crucial to China’s economic development.Fischer-Tropsch synthesis as a key process in coal-to-oil technology can catalyze the conversion of syngas from coal resources into clean fuel oil and high value-added chemicals,which can effectively improve energy efficiency and promote the transformation of energy structure.Iron-based catalysts are widely used in Fischer-Tropsch synthesis due to their low cost,high operability and wide product distribution.Atomically dispersed active phases exhibit excellent properties in many fields due to their special coordination environment and electronic structure.Carbon material has gradually become an excellent support for supported catalysts because of its chemical inertia,large specific surface area and suitable interaction between metal and support.In this paper,a series of atomically dispersed iron-based catalysts were prepared by using carbon materials as support and their Fischer-Tropsch synthesis reaction performance was evaluated.Investigates the types of the support,the preparation process of calcination temperature and the load of the metal on the properties of catalyst structure and Fischer-Tropsch synthesis reaction performance.The study found that the atomic dispersion iron-based catalyst can be good for Fischer-Tropsch synthesis reaction,and showed a good catalytic performance,the high CO conversion at the same time also has the high C₅₊selectivity and low CO₂selectivity.The main research contents and conclusions are as follows:（1）Firstly,atomically dispersed Fe-based catalysts supported by carbon were successfully prepared by impregnation method.Graphene,carbon nanotubes and activated carbon were selected as the supports respectively.By comparing the reaction performance,it was found that the catalyst with carbon nanotubes and activated carbon as the support had higher CO conversion.This may be due to the introduction of N element in the process of nitric acid treatment of the support,which promotes the catalytic activity.Graphene-supported catalysts have higher C₅₊selectivity and only 18%CO₂selectivity at 300℃.Combined with the synchrotron radiation and XPS analysis results,the special structure formed by the graphene-supported iron catalyst is Fe-C species,while the other two carbon supported iron catalysts are N-Fe-C species.This special structure difference may be one of the reasons for the difference in the water-gas conversion reaction activity.At the same time,graphene has stronger hydrophobicity compared with the other two supports,which can weaken the interaction between catalyst and water.Therefore,graphene-supported catalyst has lower water-gas conversion activity and CO₂selectivity.（2）Choosing graphene-supported catalyst and changing the calcination temperature during the preparation process.Through a series of characterization,it was found that the calcination temperature affected the graphitization degree of the catalyst,and the graphitization degree increased gradually with the increasing of the calcination temperature,but had no obvious effect on the surface properties of the catalyst.900℃calcination on the catalyst for the anchoring effect of the metal is not strong,easy to spread unevenly,1300℃calcination catalyst for its calcination temperature is too high,easy to damage the thermal stability of catalyst,lead to generate a small amount of small clusters,however,1100℃calcined catalyst has the most suitable temperature and is dispersed in the support with atomic iron,which selectivity of CH₄,C₅₊and CO₂was 4.9%,81.2%and 10%respectively at 300℃,which were better than those of traditional iron nanoparticles catalysts under the same conditions.（3）On the basis of the previous chapter,determine the calcination temperature as1100℃to control the metal load.The results show that Fe species exist on the surface of graphene as a single atom only at low loading amount（0.1%）.When the loading amount is increased to 0.5%or above,the main iron phases are Fe₃O₄and metal iron.With the increase of the loading amount,the particle size gradually increases,mainly in the form of a cube.After calcination at high temperature,the catalyst has a self-reduction phenomenon,and part of iron oxide is reduced to metallic iron.Comparing atomic dispersion of iron-based catalyst and traditional differences in iron-based catalyst reaction properties of nanoparticles,found that When the loading amount increased,the CO conversion rate increased gradually,and the CO conversion of catalyst with 20%loading amount was 45.2%.However,the selectivity of the product was less affected by the loading amount,and theαvalues of the series catalysts were all above 0.8,showing good selectivity of long-chain hydrocarbons.Comparing the catalyst phases before and after the reaction,it was found that Fe₃O₄as the active phase of the water-gas conversion reaction in Fischer-Tropsch synthesis always existed in the reaction process,which promoted the water-gas conversion reaction and led to the increase of the selectivity of CO₂in the reaction.