Effect Of Reduction Time And Precipitation PH On Fischer-tropsch Iron Synthesis Catalyst

Using carbonaceous materials such as coal,biomass and natural gas as raw materials for synthesis gas,liquid fuel and high value-added chemical products can be obtained through a Fischer-Tropsch(FT)synthesis reaction.This FT process can solve the problems of insufficient oil supply and alternative energy resource to crude oil.Iron catalyst is widely used in coal-based commercial FT applications due to their abundant reserve,low price and high water-gas shift activity.Although iron catalysts have been well developed,there are still challenges in preparing improved iron catalyst with high activity,good stability and high selectivity for desired products.The performance of iron-based FT catalyst is closely related to the preparation parameters and the activation process as well.In this paper,iron-based FT catalyst(K-Cu-Fe/Si O₂)was prepared by using co-precipitation method with ferrous oxalate as the iron source.The effect of different CO reduction time on the catalyst activation process and catalytic performance in FT reaction was systematically studied.The influence of precipitation pH value on the structure and performance of precipitated iron catalyst was also investigated.The structural characteristics of iron catalysts were characterized by various characterization techniques such as SEM,XRD,H₂-TPR,N₂ adsorption and desorption,and Raman spectroscopy.In particular,in-situ XRD and in-situ Raman technology were used to study the change of iron phase during the reduction and activation process.It was found that two phase changes was observed in iron catalyst reduction process,i.e.,first from?-Fe₂O₃ to Fe₃O₄ and then from Fe₃O₄ to the active phase Fe₂C.As shown from the catalytic reaction evaluation data of iron catalysts,increase in catalyst reduction time can improve the stability,but excessive or insufficient reduction of the catalyst can yield negative impact on stability.The results of in-situ Raman data proved that the iron catalyst can form deposited carbon after excessive reduction.Carbon deposition is likely an important factor that leads to the deactivation of iron catalyst in an FT reaction.By studying the effect of different precipitation pH's on the properties of precipitated iron catalyst,it can be found that the particle size of prepared precipitated iron catalyst gradually increased with increasing pH,while specific surface area gradually decreased.However,precipitation pH generated little effect on the pore size distribution.The iron phase of the catalyst prepared in this study is mainly?-Fe₂O₃,plus a small fraction of Fe₃O₄.At precipitation pH of 9.0,iron was basically fully in the form of Fe₃O₄.Changing the precipitation pH generated certain effect on the reduction performance of the iron catalyst.High pH can result in increasing the reduction temperature,which is not favorable to the reduction of catalyst.The results of catalytic performance show that the change of pH yielded major influence on the selectivity of CO₂,but little effect on the selectivity of other products.