Effect Of Glow Discharge Plasma Treating Parameters On The Structure And Catalytic Performance Of Cobalt-based Fischer-Tropsch Synthesis Catalysts

With the development of society and economy,people are faced with the problems of energy depletion and environmental degradation.Fischer-Tropsch synthesis reaction plays an important role in the production of sustainable and clean liquid fuels,alleviating the shortage of fossil fuels to a certain extent.The number of active sites of the cobalt-based Fischer-Tropsch synthesis catalyst is mainly determined by the degree of dispersion and reduction of cobalt species.The calcination step in the traditional catalyst preparation process usually causes the agglomerate of cobalt species due to high temperature,form strong interactions or even reacts to produce mixed oxides between cobalt and the supports.Glow discharge plasma has the characteristics of"high temperature and low energy",energy consumption in the treatment process is relatively low and the operation is rather simple,so it is quite practical to be used as a"new mode"for the treatment of catalysts.In the previous researches of our group,it was found that plasma treatment can remarkably improve the dispersion of cobalt-based catalysts and significantly improve their Fischer-Tropsch synthesis performance.However,the previous used catalyst systems are quite complex,the supports structure is disordered and promoters are employed.These factors would certainly interfere with the understanding of the influence of plasma treatment on the structure and performance of the catalyst.Therefore,in this thesis,we intend to use a structured carrier,without the addition of additive,and apply a single variable method to investigate a model catalyst,to systematically study the effect of glow discharge plasma parameters on the catalyst structure and corresponding catalytic performance and establish their structure-activity relationship.The specific research details are as follows:1.Using solid silica spheres with a smooth and non-porous surface as support,a series of Co/SP model catalysts precursors were prepared by impregnation method.The precursors were then undergone either traditional thermal calcination or plasma treatment under designed conditions(strength,time,vacuum).The results showed that the plasma treated catalysts have much higher cobalt dispersion as well as much stronger cobalt-support interaction and therefore,a much higher Fischer-Tropsch synthesis activity,in comparison with those of the calcined catalyst.Among the three investigated plasma treatment parameters,the treatment intensity had a greater impact on the catalyst structure and FTS performance of the catalyst,while the influence of the treatment time and the vacuum degree was not obvious.As the intensity of plasma treatment increased,the particle size of cobalt species gradually increased,and the interaction between Co and silica spheres also increased,which resulted in the increase of the formation of hardly reducible Co₂SiO₄.The particle size of cobalt species on all plasma-treated catalysts did not change much during the reduction process,however,severe agglomeration was likely to occur during the reaction process,which lead to an occurring of deactivation.2.Due to the poor stability of the Co/SP catalyst during the reaction process in the first part of the work,we modified the silica spheres support by coating a uniform layer of hydrothermal carbon on its outer surface,and the calcination as well as plasma treatment atmospheres were switched from air to nitrogen.The effect of plasma treating intensity on the structure and FTS performance of hydrothermal carbon coated silica spheres supported cobalt catalysts(Co/SP@C)were proceeded.Plasma treatment could also improve the cobalt dispersion on Co/SP@C catalysts.Compared with the Co/SP catalyst,the stability of all Co/SP@C catalysts was significantly enhanced.The cobalt particles on the Co/SP@C-PN catalysts underwent crystal grain growth during the high-temperature reduction process,and almost no particle size alternation was occurring during FTS reaction.Therefore,although the initial activities of the Co/SP@C-PN catalysts were lower than those of the Co/SP catalysts,during the entire reaction process,even if the CO conversion exceeds 80%,the stability of the plasma treated catalyst was still maintained.This was because the morphology and structure of the cobalt particles were relatively stable during the reaction process,and the catalyst maintained good reaction stability even under high CO conversion conditions.