Theoretical Study On Structure-performance Relationship Of Iron And Cobalt-based Catalysts In Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis?FTS?is one of the most attractive routes to convert syngas?CO+H₂?into liquid fuels and high value-added chemicals.Iron and cobalt-based catalysts are both widely used in Fischer-Tropsch synthesis.However,the products distribution of these two catalytic systems are different.The former one has high selectivity to light olefins and by-product CO₂,while the latter one leads to the formation of almost completely hydrocarbon products.In view of this significantly different catalytic performance,understanding the structure-performance relationship of iron and cobalt-based FTS catalysts can not only reveal the reaction mechanism on these catalysts,but also assist in the optimal design of more efficient catalysts to improve the target product selectivity.In this work,FTS reactions on iron-based and cobalt-based FTS catalysts were studied at the atomic scale using density functional theory?DFT?calculation.The formation mechanism of CO₂and light olefins on iron-based FTS catalysts was investigated.On the cobalt-based catalysts,the effect of cobalt phase structure of on CO activation,C₁ species hydrogenation,C-C coupling and CO insertion mechanism was studied,and the interfacial effect of Co-Co₃C on its catalytic performance was studied.The results are summarized as follows:?1?The x-Fe₅C₂?510??Fe₃O₄?111?and K₂O-promoted x-Fe₅C₂?510?surface models were constructed.The formation mechanism of CO₂ and light olefins on these crystal surfaces were studied in depth,and the structure-activity relationship on these surfaces were investigated.It was found that the Boudouard mechanism plays a predominant role in CO₂ formation on x-Fe₅C₂?510?phase comparing the energy barriers of the Boudouard reaction,WGS reaction and O*species hydrogenation reaction.The modification of potassium promoter did not alter the predominant reaction pathway for CO₂ formation over Fe-based FTS catalysts.The existence of Fe₃O₄ phase was favorable for the reverse water-gas shift?RWGS?reaction,leading to the decrease of CO₂selectivity and increase of generated H₂O amount.The experimental results also show that with the increase of the x-Fe₅C₂?510?phase content,the CO₂ selectivity increases,and the amount of produced H₂O decreases,which is consistent with the theoretical calculation results.In addition,CO activation and olefin formation mechanisms on x-Fe₅C₂?510?and?-202?surfaces were studied.It was found that x-Fe₅C₂?510?is the main active phase for CO activation and dissociation,and the high coverage of CH₂*species and the small energy barrier of CH₂*-CH₂*coupling leads to the high selectivity to light olefins on x-Fe₅C₂?510?.?2?The mechanisms of CO activation,methane formation,C-C coupling and CO insertion on three cobalt phases,namely Co,Co₂C and Co₃C,were investigated based on our previous experimental study.The structural and electronic properties were deeply analyzed to unveil the nature of active site.These three phases exhibit different catalytic performance.The surface H*-induced C-O dissociation via CH₂O intermediate and via CHOH intermediate are the predominant CO activation pathway for Co?111?and Co₂C?111?,respectively,while the C-O dissociation via COH intermediate is predominant for Co₃C?101?.Co?111?favors the production of alkanes due to its hydrogenation and CH₃-CH₂ coupling ability.Co₂C?111?has the highest selectivity toward CH₄,which is attributed to its valley-type surface structure and strong binding energy for CH₂*species.Co₃C?101?is exceedingly beneficial for the production of light olefins by controlling C-C coupling,suppressing deep hydrogenation,and preventing methane formation.The preferred production of light olefins on Co₃C?101?is originated from the synergistic effect between its ridge-type surface structure and the downward shift of the d-band center.?3?The effect of Co-Co₃C interface on CO activation and olefins formation was also studied.The calculated charge density difference results show that these occurs electron accumulation at the metal cobalt-cobalt carbide interface,which can significantly promote the adsorption and activation of CO.Meanwhile,the electron accumulation at the interface and the ridge-type structure of the Co₃C surface can promote the CH₂*-CH₂*coupling reaction and benefit the formation and desorption of light olefins.This study provides theoretical guidance for the rational design of effective Co-based FTS catalysts to tune the FTS selectivity toward the desired light olefins.