Construction And Performances Of Iron-based Fischer-Trospch Catalysts Using Carbon-based Nanocages As Support

Fischer-Tropsch synthesis is a heterogeneous catalytic reaction that converts a mixture of CO and H2 into lower olefins,liquid hydrocarbons and other oxygenated organics.The key problem is to develop advanced catalysts with high selectivity,high stability,and low cost.Herein,.novel Fe-based Fischer-Tropsch catalysts were constructed with carbon based nanocages by loading the active phase on the cage surfaces or encapsulating into the cage-cavities.The catalytic performance was examined in a self-made fixed-bed reactor,and a corresponding evaluation method was established.The dependence of catalytic performance on the nanocage structure,nitrogen content,surface alkalinity,and confinement effect of nanocages has been extensively studied.The sintering mechanism of Fe nanoparticles,the sintering resistance of N-dopant and the confinement effect of nanocages were revealed for the first time.Finally,the Fe-based Fischer-Tropsch synthesis catalysts with high performance were optimized.(1)Study on the sintering mechanism of Fe-based Fischer-Tropsch catalyst:Through the investigation of the influence of reaction condition and N-doping in support on the sintering of catalysts,it is revealed for the first time that the sintering process of Fe based Fischer synthesis catalyst is an iron carbonyl-mediated Oswald ripening process.That is,during the FTO,the in situ-formed iron carbonyl species mediate the mass transfer from the small particles to the large ones,thereby the growth of large particle and shrink or even disappearance of small particles.And using the carbonyl iron-mediated sintering mechanism,the formation and evolution of the coreshell structures observed in the catalyst after reaction are explained.By in situ mass spectrometry,it was discovered that the formation of iron carbonyl can be effectively suppressed by using N-doped carbon supports.Our density functional theory(DFT)study on the formation of Fe(CO)5 on pristine or N-doped graphitic plane also supports this finding,in which the pyridinic-N possesses the best capability to suppress the iron carbonyl formation.(2)Construction of novel Fe-based Fischer-Tropsch catalysts for highly efficient production of lower olefins:Novel Fe-based Fischer-Tropsch catalysts for producing lower olefins were constructed by using the support of N-doped carbon nanocage(NCNC),featured with a super-large specific surface,hierarchical structure and high N content.The performances of the catalysts with pristine and nitrogen-doped carbon supports were studied in Fischer-Tropsch to lower olefins(FTO).The effects of nitrogen content on the dispersion of Fe nanoparticles,the selectivity to lower olefins,the ratio of olefin to paraffin,the resistance to sintering and carbon deposition were investigated.It was found that N-doping in the carbon supports was beneficial to improving the dispersion of Fe nanoparticles on the NCNC surface.Moreover,with increasing N content,the surface alkalinity of NCNC increases accrodingly,and the selectivity of low olefins also improves,while the formation of methane are suppressed.N-doping in the carbon supports can also effectively improve the carbon deposition resistance and the suppression of Fe nanoparticle growth in FTO.(3)The construction and performances of Fe catalysts encapsulated in carbon nanocages(CNC):According to the hollow nanocage structure and the rich microspores in cage-wall of CNC,a self-developed vacuum-filling method was used to fill Fe nanoparticles into the hollow cavities of nanocages.Novel Fe-based FischerTropsch catalysts(Fe@CNC)with yolk-shell structure were constructed.The confinement effect of nanocage on the catalytic performance in Fischer-Tropsch synthesis was revealed,which is that the product distribution of catalyst Fe@CNC deviates from the Anderson-Schulz-Flory distribution,resulting in higher selectivity for gasoline and diesel.Compared with CNC-supported iron catalysts,Fe@CNC also has a higher catalytic activity,lower methane selectivity,improved sintering resistance and suppressed of carbon deposition.