Study On The Structure-function Relationship Of Co Based Catalysts For Higher Alcohol Synthesis Via Fischer-tropsch Reaction By XAFS

Stable energy supply is one of the most crucial factors towards the sustainable development of human’s society.Fossil oil,which is one of the most important natural resources for the people’s livelihood and energy strategy,is relatively low in reserve storage in our country.However,the deposit amount of coal is much higher compared with that of fossil oil.Thus,the Chinese scientists and researchers have recently proposed a“Coal-converting-to-petroleum”strategy to transform the excess coal into fossil oil.Fischer-Tropsch synthesis is one of the most important technologies for “Coal-converting-to-petroleum”and has attracted much attention in energy field.Fischer-Tropsch synthesis is a typical reaction that converts the syngas of H₂ and CO derived from coal,natural gas or biomass into more valuable chemicals,such as C_5-11gasoline、C_8-16 jet fuel、C_10-200-20 diesel fuel、C_2-4 olefin、C₂₊higher alcohol and so on.Among which,C₂₊higher alcohol can not only be used as clean fuels or fuel additives,but also applied as feedstocks of surfactant,lubricant and other raw materials in the chemical industry.There are many kinds of catalysts for Fischer-Tropsch synthesis to higher alcohol and Co-Co₂C catalytic system is one of the most classic.According to the CO insertion mechanism,Co₂C plays a vital role for CO non-dissociative absorption to form the intermediate CO^*and Co is responsible for the dissociative absorption of H₂and CO to form the C_n H_m^*intermediate,at last CO^*is inserted into the metal catalyst surface and the C_nH_m^*intermediate to form the final C_n+1H_mO,higher alcohol.Many previous reports have proved that Co-Co₂C interfaces are efficient for higher alcohol production and fabrication of Co-Co₂C dual active sites is similar involving initial pure Co₂C synthesis and partial decomposition of Co₂C under high-pressure Fischer-Tropsch reaction,where the key points lie in the pure Co₂C preparation and tunable control of Co-Co₂C interfaces.Because Na can act as the electrical denote which can benefit the carbonization of Co and formation of Co₂C,the only obstacle for higher alcohol synthesis via Fischer-Tropsch reaction is the precious design of Co-Co₂C structures which is crucial for fabrication of the catalyst and further improvement of the catalytic performance.However,there are still some questions to be solved:1)effect of Na content towards the structural change of Co-Co₂C;2)effect of bulk and surface Na towards the structural change of Co-Co₂C;3)effect of Mn towards the structural change of Co-Co₂C,and so on.As the fast development of synchrotron radiation technology,especially the vast application of X-ray Absorption Fine Structure,we can deeply and accurately analyze the changes of valent states and variations of the atomic structure of the active sites during the Fischer-Tropsch synthesis,which enables us to investigate the Structure-Function-Relationship and provide theoretical guidance for more efficient catalysts’design.Particularly,the in situ XAFS experiment can take us to the insight of the changes of catalysts in realistic reaction condition.Based on these,the main work in this paper includes:1)To study the effect of Na towards the structural change of Co-Co₂C and the subsequent influence towards the higher alcohol production via Fischer-Tropsch synthesis,we prepared the Co₃O₄ without Na by co-precipitation method using cobalt nitrate and ammonium carbonate and then immersed 0.1wt%,0.2wt%,0.4wt,0.6wt and 1wt%of Na using sodium nitrate.XRD,ICP,TEM,XPS and XAFS were conducted to study the structure change.Although XRD and HRTEM revealed that there was no sign of metallic Co in the Na-promoted spent catalysts,XPS and XAFS confirmed the existence of Co⁰.The results of XANES,EXAFS,LCF and ICP showed that the selectivity of higher alcohol is in direct proportion to the ratios of Co to Co₂C when the Na contents are in the range of 0～1wt%.More Co is beneficial to the higher alcohol production.In addition,the ratios of the coordination number of Co-Co to Co-C was initially proposed to describe the change of Co-Co₂C interfaces.2)To study the effect of bulk and surface Na towards the structural change of Co-Co₂C and the subsequent influence towards the higher alcohol production via Fischer-Tropsch synthesis,bulk Na catalysts were prepared by co-precipitation of cobalt nitrate and sodium carbonate,and surface Na catalysts were prepared by impregnation of similar content of Na into Co₃O₄ without Na.XRD,ICP,TEM,XPS and XAFS were conducted.What’s more,high-pressure in situ XAFS experiment was conducted to reveal the effect of bulk and surface Na to the formation of Co₂C,which then enabled us to know the intrinsic of the structural change Co-Co₂C and its relation to the higher alcohol production.The result showed that the formation rate of Co₂C for surface Na is much slower than the bulk Na,which then caused the incomplete transformation of Co to Co₂C.This is the first time to reveal the effect of bulk and surface Na towards the structural change Co-Co₂C using high-pressure in situ XAFS experiment.3)Different Co/Mn ratios（1:1,2:1,4:1 and 6:1）of CoMn catalysts were prepared.Then similar contents of Na were impregnated using sodium nitrate.XRD,ICP,TEM,XPS and XAFS,especially in situ XAFS,were conducted to study the effect of Mn towards the the structural change of Co-Co₂C and the subsequent influence towards the higher alcohol production via Fischer-Tropsch synthesis.The results showed that CoMn21 catalyst under 240℃presented the highest CO conversion and higher alcohol selectivity.XAFS was initially used to study the relationship between the structure of Co-Co₂C and higher alcohol production.The Function-Performance-Relationship proposed here provided theoretical guidance to the fabrication of the catalysts and further improvement of the catalytic performance in Fischer-Tropsch synthesis.