A Study On The Design Of Iron-based Nano-catalysts Confined By Graphite And Their Catalytic Performance For Fischer-Tropsch Synthesis

Shortage of crude oil and its associated environmental pollution have driven worldwide research on alternative energy technologies.Fischer-Tropsch Synthesis?FTS?still receives extensive attention nowadays because it can transform syngas derived from coal,natural gas or biomass to ultra-clean fuels and high-value chemicals via non-petroleum route.Iron-based catalysts are widely used in industry due to low price,low methane selectivity and wide operating range.Understanding the structure-performance relationship of iron-based catalyst can help design superb catalysts for industry.However,iron-phase changes intricately and deactivates fast in the process of FTS.Such complex transformations between different phases of Fe lead to great challenges for investigating the structure-performance relationship.Therefore,it is crucial to synthesize an iron-based catalyst with stable phase and define the performance of single Fe phase.In this thesis,Fe@G catalysts were synthesized by one step melting method,and the Fe/AC was synthesized by incipient wetness impregnation method.By changing the activation conditions,the?-Fe₅C₂,?-Fe₂C,?-Fe₂C@G and?-Fe₃C@G catalysts were synthesized.The as-synthesized catalysts with different iron carbides were characterized by Transmission Electron Microscopy?TEM?,Powder X-ray Diffraction?XRD?,⁵⁷Fe M?ssbauer Spectroscopy?MES?,Brunauer-Emmett-Teller?BET?,Thermogravimetry?TG?,Carbon Monoxide Temperature-Programmed Desorption?CO-TPD?,etc.The transformation of iron-phase in FTS and their FTS performances were compared and analyzed in fixed-bed reactor.The main contributions are summarized as following:?1?The?-Fe₂C phase is dominant at low temperature?200??while the?-Fe₅C₂ is dominant at high temperature?350??.The FTS activity follows the trend:?-Fe₂C@G>?-Fe₂C>?-Fe₃C@G>?-Fe₅C₂ at low temperature.The content of?-Fe₂C in 35Fe@G-200catalyst is much higher than that in 35Fe/AC-200 catalyst.It was found that the presence of graphite cladding layer can effectively inhibit the surface carbon segregation and is beneficial for the structural stability of the highly active?-Fe₂C phase.?2?Based on the abovementioned observations,we chose the relatively stable Fe@G catalyst as the research object.By changing calcination atmosphere,calcination temperature and Fe loading,the different pore structures of catalysts are prepared.After that,we selected35Fe@G-H₂/Ar-5 and 35Fe@G-N₂-H₂/Ar-5 to test the FTS performance.The results have indicated that with increasing calcination time,the pore volume,pore diameter and particle size of the catalyst increased gradually,and the specific surface area decreased gradually.The pore volume,pore volume and particle size of the catalyst decreased with the decreasing of the metal loading.The FTS results have demonstrated that the structure with abundant pores is in favor of the heat and mass transfer on the reaction,therefore,as a result,it improves the activity and stability of catalyst as well as the selectivity of C₅₊.?3?Based on the above study,we prepared the Fe@G catalyst with good stability and large pore volume,and then calcined part of the catalysts for 10 h under 450?in air,resulting the Fe₂O₃ catalyst.Afterwards,the FTS performance of both catalysts was tested on fixed-bed reactor.It turned out that the existence of graphite can increase the activity and stability of FTS,it is interested to noted that the Fe@G catalyst stay stable after 400 h.These results provide a new idea for improving the stability of industrial catalysts.