Studies On TiN Supported Co-Fe Bimetallic Catalysts For Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis（FTS）is a gas-liquid conversion technology that can be used as a alternative to fossil fuel energy supply.The key to Fischer-Tropsch synthesis technology is the preparation of catalysts.Therefore,the development of catalysts with low cost,high activity,good stability,and reasonable product selectivity is of great significance to the industrialization of Fischer-Tropsch synthesis.Cobalt and iron are the two most studied active metals for Fischer-Tropsch synthesis.The price of iron is lower than that of cobalt.The synergitic catalytic performance of the cobalt-iron bimetallic catalyst has become a research hotspot.At present,there are still some contradictions in the research results of the cobalt-iron bimetallic catalyst.The final conclusion has not been drawn on the relationship between the catalyst structure and the catalytic performance.It is also not clear about the respective roles and active phases of cobalt and iron during the reaction.In this paper,TiN is used as a carrier to prepare a cobalt-iron bimetallic catalyst for Fischer-Tropsch synthesis.The relationship between cobalt and iron,and the active phase of iron were discussed.The main research contents are as follows:（1）The cobalt-iron bimetallic catalyst was prepared by co-impregnation method.The effect of the relationship between cobalt and iron on the reduction of the catalysts,Fischer-Tropsch synthesis activity and product selectivity was studied.The results show that the iron and cobalt species in the catalyst exist in the form of Fe₂O₃ and Co₃O₄,respectively.With increasing the content of iron,the ratio of Co²⁺/Co³⁺on the catalyst surface increases.indicating that a small part of Co³⁺on the catalyst surface becomes Co²⁺.The content of Fe on the catalyst surface is higher than that of the cobalt on the 10Co10Fe/TiN.It is implying that part of the cobalt is covered by iron,reduced the content of cobalt on the catalyst surface.H₂-TPR analysis shows that bimetallic catalysts have higher reducibility than single metal catalysts.The Catalyst with high iron content is easy to reduce.From the perspective of reaction performance evaluation,bimetallic catalysts have higher Fischer-Tropsch catalytic activity than single metal catalysts.A small amount of iron catalyst is beneficial for reducing methane selectivity and increasing C_5-12selectivity.The 10Co5Fe/TiN-C catalyst with a Co/Fe value of 2 has a CO conversion of 27.5%,a higher C_5-12 selectivity（55.4%）and a lower CH₄ selectivity（15.7%）.（2）Co₃O₄ particles was supported on the TiN carrier by ultrasonic impregnation method,and then iron was supported by means of incipient wetness impregnation method.The effect of Fe on the FTS performance of 10Co/TiN catalyst and the reactive phase of Fe were investigated.The results show that the cobalt and iron elements in the catalyst have high dispersion.The reduction temperature of cobalt-iron bimetallic catalyst is lower than that of single-metal catalyst.Compared with 5Fe10Co/TiN catalyst,10Co5Fe/TiN catalyst has higher CO conversion（28.7%）,lower CO₂ product selectivity（2.3%）,lower CH₄ product selectivity（13.3%）and higher C₅₊catalyst selectivity（75.9%）.The result of M（?）ssbauer spectrum analysis of the catalysts after reaction show that Fe²⁺and Fe³⁺exist in the catalyst,no iron carbide phase and matal iron.The order of the Fe²⁺content in the three catalysts is:10Co5Fe/TiN>5Fe10Co/TiN>15Fe/TiN,which is basically consistent with H₂-TPD results,CO-TPD results,and the order of the catalytic activity.We believe that iron adsorbed on the cobalt surface may participated in the reaction and increased the catalytic activity.Ionic Fe²⁺in the 10Co5Fe/TiN catalyst maybe comes from the oxidation of Fe⁰ after the reaction.Therefore,we believe that the active phase of the iron species on the cayalyst may be Fe⁰.（3）TiN carriers with different N content were prepared by oxidation TiN in air.Cobalt-iron bimetallic was loaded according to the method of iron first and then cobalt.The influence of nitrogen content on the performance was discussed.The results show that the interaction between the metal and the carrier in the catalysts is affected by the nitrogen content.The higher the N content,the greater the interaction between the metal and the carrier.Higher nitrogen content in the catalyst is beneficial for improving the stability of the reaction,but it is unfriendly for the Fischer-Tropsch synthesis activity.Appropriate nitrogen content content is usful for maintaining proper metal-support interaction,improving the stability and the activity.5Fe10Co/TiN-O-400-2 catalyst shows the best catalytic reaction activity.