Iron-based Catalyst Structure Regulation And Its Catalyitic Synthesis Of Carbon Dioxide To Olefins

Social development is greatly promoted by the use of fossil fuels,but it has also caused a rapid increase in the carbon dioxide content in the atmosphere at the same time.At present,all countries are facing tremendous pressure to reduce CO₂ emissions.Although carbon dioxide is the main cause of the greenhouse effect,it is also a potential carbon resource.In recent years,the attention of scholars has been attracted by the reuse of carbon dioxide.Among them,the hydrogenation of carbon dioxide to olefins is one of the most potential ways to realize the reuse of carbon dioxide.The reuse of carbon dioxide can realize the resource utilization of carbon dioxide as well as make up for China's dependence on oil imports.In the reaction of carbon dioxide hydrogenation to olefins,iron-based catalysts not only have good performances of reverse water gas shift reaction?RWGS?and Fischer-Tropschesis?FTS?,but also have the advantages of low price and good anti-toxicity,therefore,it has been widely concerned.At present,promoters are often added to modify the iron-based catalyst to improve its stability and activity.In this paper,the promoter effect of alkali metal Na and transition metal Mn on the control of iron-based catalyst and its consequent effect on the carbon dioxide hydrogenation to olefins were investigated.Firstly,the precipitation method was used to add ammonium carbonate solution to the iron ion solution to produce a precipitate.?-Fe₂O₃ was prepared after being washed,dried and calcined.Then Na was impregnated on the surface of the iron oxide catalyst by impregnation method.A series of iron-based catalysts with different Na contents were prepared and recorded as:x%Na/Fe₂O₃.By evaluation of the catalyst,it was found that the introduction of Na can significantly increase the selectivity of olefins and decrease the selectivity of methane and alkanes.When the content of the Na is increased from 0%to 5%,the C_2-7 olefin selectivity of the catalysts can be more than60%,and the selectivity of CH₄ can be about 26%.Through in situ characterization,it was found that the catalyst structure changes as?-Fe₂O₃?Fe₃O₄??-Fe₅C₂ during the catalyst activation process.With the increase of Na content,the reduction temperature of iron carbides increased,indicating that the Na promoter inhibites the reduction of the catalyst and improves the stability of Fe₅C₂.The introduction of Na can also reduce the particle size of Fe₅C₂ and adjust the surface carbon species distribution.The Mn and Na elements were impregnated on the surface of the iron oxide catalyst through excessive impregnation to prepare iron-based catalysts with different Mn contents,which were recorded as x%Mn-1%Na-Fe₂O₃.Through the evaluation of the prepared catalyst,it was found that the carbon dioxide conversion rate gradually decreased when the Mn content increased,while the methane selectivity continued to increase,and the olefin selectivity decreased.When the Mn content was increased from0.5%to 10%,the carbon dioxide conversion rate decreased from 31.76%to 22.83%,the methane selectivity increased from 39.08%to 50.86%,and the selectivity of C_2-4olefins decreased from 38.73%to 30.58%.Through characterization of XRD,TPR,TPD,it confirms that Mn is mainly distributed on the surface of the catalyst.The addition of Mn reduces the reduction temperature of the catalyst,promotes the reduction of the catalyst and generates different iron carbide after the reduction of iron oxide catalyst.At the same time,the addition of Mn improves the adsorption of iron carbide on carbon dioxide.