The Effects Of Pretreatment On The Structure And Property For Cobalt-Based Fischer-Tropsch Catalysts Supported On Carbon Nanotubes

Fischer-Tropsch synthesis（FTS）is a significant approach for the utilization of non-petroleum carbon resources such as natural gas,coal,and biomass into liquid fuels and high value-added chemicals.Cobalt-based catalysts is one of the most promising catalysts in industrial FTS reactions.In the case of cobalt-based FTS reactions,sintering and oxidation of metallic Co nanoparticles are the major reasons for catalyst deactivation.It was reported that the re-reduction treatment of the deactivating catalysts could lead to the re-dispersion of active phase and prolong catalyst lifetime.Reduction-oxidation-reduction（ROR）pretreatment is a process by successively reducing,oxidizing and re-reducing the FTS catalyst.It is applied as a key method of catalyst activation and also an important approach to understanding the structural evolution and structure-activity relationship of cobalt catalysts FTS reaction.Previous study focused on the effects of particle size,oxide support,and pre-treatment atmosphere on the structural evolution and catalyst performance during ROR process.However,the effects of morphology of cobalt nanoparticles and carbon support still lack insightful understanding.In this paper,spherical（Co-S）and cubic Co₃O₄（Co-C）were controllably prepared by hydrothermal method,which were deposited onto functionalized CNTs by ultrasonic assisted deposition method to obtain Co-S/CNTs and Co-C/CNTs model catalysts.Additionally,Co/CNTs and Co/Si O₂ model catalysts were also prepared through traditional impregnation methods.After pretreated by reduction-oxidation-reduction（ROR）processes,the four model catalysts were evaluated under FTS reaction conditions.Combined with catalytic test results,various characterizations over the fresh and ROR-pretreated catalysts and quantum chemical calculations were performed to elucidate the effects of morphologies and supports on cobalt catalyst evolution and corresponding catalytic performance under ROR pretreatment process.The main conclusions were summarized as follows:1)Similar structural evolution was observed over Co-S/CNTs and Co-C/CNTs during ROR pretreatment.In detail,the reduction and oxidation would increase the particle size of Co nanoparticles,while re-reduction would lead to re-dispersion.Correspondingly,both reduction and oxidation（RO）pretreatment would decrease catalyst activation,while enhanced activity was found after re-reduction process.However,these two catalysts exhibited different activity after reduction-oxidation pretreatment.Co-S/CNTs-RO showed only 1.8%CO conversion,while Co-C/CNTs-RO remained 8.8%CO conversion.Combined XRD,TEM and XPS characterizations reveled that Co-S/CNTs was hard to be reduced but easy to be oxidized,which exhibited major inactive phases of Co O and Co₃O₄after RO pretreatment.Oppositely,Co-S/CNTs was more facile to be reduced but harder to be oxidized with more active metallic Co phase after RO pretreatment.These results indicate that the initial morphology of Co nanoparticles is a key factor on phase evolution and FTS performance during ROR pretreatment.2)To further examine the effect of CNTs support on ROR pretreatment,the structural evolution and catalyst performance between Co/CNTs and Co/Si O₂ model catalysts were checked during ROR process.The two catalysts exhibited different structural evolution.As for Co/CNTs,Co nanoparticles gathered during RO pretreatment,and then re-dispersed during the re-reduction process.However,in the case of Co/Si O₂,the sintering of Co nanoparticles was found during reduction process,following by re-dispersion during oxidation process and re-gathering during re-reduction process.The former phenomena was related to the Kirkendall effect to give smaller nanoparticle from the splintering of larger-sized Co particles during reduction process.The later was owing to the water-induced formation of cobalt oxide species strongly interacting with Si O₂ support during oxidation process.Although the two catalysts exhibited similar changes of FTS activity after ROR process with decreasing activity after oxidation and increasing activity after re-reduction,RO-pretreated Co/Si O₂showed evidently lower deactivation from 15.8%to 3.5%in contrast to that for Co/CNTs from 15.4%to 8.5%.It suggests that the kind of support has effect on the particle migration behavior and catalytic performance after ROR pretreatment.Differently from oxide supports,carbon supported cobalt based Fischer-Tropsch catalysts exhibit more excellent oxidation resistance.3)Although the four model catalysts exhibited similar activity changing trend after ROR pretreatment,the product selectivity was different.In detail,the alkane selectivity of Co/CNTs was 2-3 times to that of Co/Si O₂.It was related to the hydrogen spillover phenomenon caused by the presence of many defect sites and surface oxygen-containing functional groups on the surface of functionalized CNTs.DFT calculation showed that defect sites were the prerequisite for hydrogen spillover.And the type of defect sites also affected hydrogen spillover performance.Among them,five-membered carbon defective sites and O sites in the oxygen-containing functional groups were favorable for both forward and reverse hydrogen spillover.These sites were key to change the H*concentration on the active sites,and then resulted in the high selectivity to alkanes.