Effect Of Surface Modification Of Fe/g-C₃N₄ Catalyst On The Product Distribution In Fischer-Tropsch Synthesis

The production of high-value chemicals and liquid fuels from syngas through Fischer-Tropsch synthesis(FTS)is one of the important ways to utilize carbon based energy.Exploring the construction strategy and mechanism of new catalysts is of great significance for regulating the products distribution of FTS.The strategy of adjusting catalyst hydrophilicity provides a new approach for the design of FTS catalysts.During the surface wettability modification process,the surface groups of the catalyst have a significant impact on the product distribution,not only affecting the dispersion,reduction,and activation of active centers,but also having an important impact on product distribution in CO hydrogenation reactions.Fe/OHC3N4 prepared by H2O2 treatment and ultrasonic impregnation of Fe increases the surface OH content,promotes chain growth and olefin generation in CO hydrogenation,and reduces C1 by-product.However,due to strong metal-support interaction,CO hydrogenation activity is relatively low,and the C1 by-product selectivity needs to be further reduced.This study focuses on the preparation and surface modification of Fe/g-C3N4.Based on the systematic characterization of the catalyst structure and surface properties,the influence of surface modification of Fe/g-C3N4 on the distribution of FTS products and related mechanisms were explored.The main research content and results are as follows:(1)Carbon nitride(g-C3N4)was prepared using thermal condensation of urea.H2O2,NH3.H2O pretreatment,and impregnation methods was used to obtain Fe/g-C3N4.The catalytic performance of the catalysts before and after modification was studied in CO hydrogenation.The influence of surface pretreatment on the texture properties of Fe/g-C3N4 catalysts and the product distribution of CO hydrogenation was examined by combining detailed characterizations,including XRD,SEM,FT-IR,TG,CO2-TPD,CO-TPD,H2-TPR,contact angle measurement,and N2 physical adsorption and desorption.The results show that different pretreatment methods significantly affect the textural characteristics and catalytic performance of the catalysts.The g-C3N4 prepared by thermal condensation of urea has a typical honeycomb structure,with strong interaction between Fe and g-C3N4 and high dispersion.The samples are hydrophilic before and after modification,and the hydrophilicity is improved after treatment with H2O2 and NH3·H2O.H2O2 treatment improves surface hydroxyl groups.NH3·H2O treatment improves surface amino groups,promotes CO adsorption,and facilitates the formation of Fe(NCN)phase.The surface basicity of all pretreated catalysts is enhanced.The two-step modified catalyst Fe/AM-g-C3N4 had a weaker water gas shift(WGS)reaction activity and CO2 selectivity reduced to 11.61%in CO hydrogenation.Due to the enhanced basicity of Fe/AM-g-C3N4,the secondary hydrogenation ability of olefins was inhibited and exhibits higher olefin selectivity with C2=-C4=of 32.37%and an O/P value of 3.23.(2)Basis on the preparation of carbon nitride(g-C3N4),a series of Fe/CN-t were further prepared by hydrothermal method.The effect of hydrothermal treatment time on the structure and catalytic performance of the catalyst was investigated.Combined with a series of characterizations of the surface and phase structure of the catalysts,it is found that the hydrothermal process promotes the introduction of amino and hydroxyl groups on the surface of g-C3N4,increasing the surface alkaline sites and CO adsorption site,uniformly dispersing the Fe active components.In CO hydrogenation,the adsorption capacity of CO is strong,inhibiting the adsorption of H2 and weakening the secondary hydrogenation reaction.The selectivity of C2=-C4=is significantly improved,and the selectivity of C2=-C4=on Fe/CN-12 catalyst reaches 30.06%,while the selectivity of C5+ decreases to 10.73%.