| Fischer Tropsch is one of the most important routes for optimal utilization of non-petroleum carbon based resources.The core and key of F-T catalysis is the selection and regulation of the preferred reaction pathway.Fe-based catalyst is an ideal catalyst for F-T synthsis(FTS),either for high temperature(300-350℃)or low temperature(200-240℃),and can be used to produce oil products with high gasoline and diesel content.In addition,Fe-based catalysts have the advantages of high WGS activity,direct ues of coal-based syngas,low cost,high resistance to toxity,and low methane selectivity at high temperatures,and hence receive close attention from academia and industry.Under the condition of FTS,the great challenges in the area,which are the establishment of structure-activity relationship and the regulation of product selectivity,come from the complex phase composition and dynamically changing structure of Fe-based catalysts.In recent years,a lot of researchers focus on the active phase identification,methane generation pathway and chain growth mechanism of Fe-based catalysts.However,for research of the dominant active phase Fe2C catalysts in low-temperature FTS,is rarely noticed.Therefore,to current state it is still not well understood the cause of the structural sensitivity and the nature of active sites and the selectivity modification of Fe-based catalysts,meanwhile the structure-activity relationship including high-temperature FTS and low-temperature FTS is also not established,hence until now it is not realized for the rational design of Fe-based catalysts and the precise-control of structure.This dissertation focus on low-temperature FTS Fe2C catalyst,by means of spin-polarized density functional theory calculations,investigating the structural characteristics,CO adsorption and activation,methane formation process and C-C coupling mechanism.Together with DOS and Bader charge analysis,the evolution mechanism and internal relations of C species are elucidated and predicted on different crystal surfaces,from the aspect of difference among the surface electronic structure.The results could help for providing guidance for the rational design of-based catalysts and precise regulation of product selectivity.The main research contents are listed as follows:(1)The differences in the structural characteristics are illustrated between the two crystalline phases of Fe2C,from the point of view of electronic structure.The charge diagams and total densities of states show the different interactions between C atoms and Fe atoms,leading to the different charge values around Fe atoms in η-Fe2C and ε-Fe2C.Eight mostly exposed surfaces with unique features are predicted by Wulff construction.ε-Fe2C(221)surface is the only terrace-like surface compared with other step-like surfaces and ε-Fe2C(101)surface is the only one without surface C atoms.Due to the difference of electronic structure and surface morphology,η-Fe2C and s-Fe2C display different catalytic properties.(2)The influence of surface electronic structure of Fe2C catalysts on CO adsorption behavior is elucidated.On the perfect Fe2C surfaces,the CO adsorption is the most stable on the surface Fe atoms with low coordination numbers,while on the defective Fe2C surfaces,the most stable CO adsorption sites are transferred to 4F vacancy composed of surface Fe atoms with high coordination numbers,with higher binding enegies of CO and catalytic surfaces.the vibration frequency of C-O bond in the adsorbed CO provides an important means of evaluating CO adsorption sites and energies.The less number of bond between CO and surface Fe atom is,the higher vibration frequency of C-O bond is.And the vibrational frequency of C-O bond follows linear relations with CO adsorption energies at different adsorption sites on the perfect Fe2C surfaces,but on defective Fe2C surfaces,the vibration frequency of C-O bond has little change with CO adsorption energies.The Bader charge of Fe atoms on the perfect Fe2C surfaces is proportional to the vibration frequency of C-O bond.The lower the Bader charge around the Fe atoms,the stronger the electron donating ability of the Fe atoms,that is to say,the lower the vibration frequency of the C-O bond.The DOS and Bader charge analysis indicate that the electron donating ability of surface Fe atoms directly affects the CO adsorption behaviors,and provides the further evaluation of the CO activation behaviors.It should be noted that the Bader charge of Fe atoms is not sensitive to the vibration frequency of C-O bonds on the defective Fe2C surfaces.(3)Bader charge is the ideal descriptor to elucidate the CO activation on Fe2C catalysts,which is also reasonable to describe the CO activation behaviors on iron carbide catalysts combined with the data of χ-FesC2.H-assisted CO dissociation via the HCO*intermediate is suggested as the preferred CO activation pathway on the perfect Fe2C surfaces,while the direct CO dissociation is the optimal activation path on the corresponding defective Fe2C surfaces and the activation energy barriers are also greatly reduced.The formation of surface defective sites not only reduce the CO activation energy barriers,but also change the CO activation pathways,which is more beneficial to CO activation.This is because the generation of surface C vacancy enhances the electron-donating ability of surface Fe atoms related to the active sites.The comprehensive analysis of CO activation behaviors on Fe2C and χ-Fe5C2 catalysts indicates that the atomic charges follow nearly linear relations with the CO activation barrier on perfect surfaces.Apparently,in the region with lower Bader charge value,the CO direct dissociation is favorable,while in the region with higher Bader charge value,the H-assisted CO activation is favorable.Combined with the CO active sites analysis,it’s seen that the Bader charge values of different active sites are different.H-assisted CO activation prefers to occur at the active sites around Fe atoms with low coordination numbers which have high atomic charges,in contrast,direct CO activation occur easily at the active sites around Fe atoms with high coordination numbers which have low atomic charges.However,on defective surfaces the active sites of CO activation consistently locate at the C-vacant sites,suggesting the CO activation barrier to change weakly with the atomic charges when carbon vacancy formed.(4)The methanation and C-C coupling processes on Fe2C catalysts are highly sensitive to the surface structures.The effective barrier difference between the CH4 formation and C1-C1 coupling(ΔEeff)is used to compare the difference of the FTS selectivity among the iron carbide catalysts.The calculated results show that the selectivity of methane formation on the η-Fe2C(011)surface is the highest,and that on the η-Fe2C(211)surface is the lowest,next to 0-Fe3C(03 1)surface-which indicates that it is feasible to suppress the methanation by changing the crystal orientation of the catalysts.Meanwhile,the cycle evolution of C species on Fe2C catalysts is investigated.It’s found that the hydrogenation of surface C atoms is the initial step to trigger the reaction,and the surface C vacancy is the key to the cyclic evolution of C species.The CO activation hardly occurs on perfect Fe2C surfaces,only when the surface C atoms are hydrogenated to form CH4 or participate in C-C coupling,escaping from the surface to form a vacancy,then the CO activation can occur.(5)△Ec+4H and Bader charge values are discriminated as good descriptors for methanation and C-C coupling processes.△EC-4H can well describe the methane formation,C-C coupling and methane selectivity and follow good linear relations with the effective barriers of methane formation.The stronger △EC-4H is,the less likely it is to form methane.But △EC-4H has weak influence on C-C coupling process.On the contrary,Bader charge have little effect on methane formation,but follows good linear relations with the effective barriers of C-C coupling.And the lower Bader charge is,the stronger the electron feeding ability of surface Fe atom is,the easier the C-C coupling process is.By comparing the ratio of surface Fe and C atoms(NFe/Nc),it’s interesting to find that the surfaces with most surface Fe atoms(high NFe/Nc)are beneficial to the inhibition of methane formation while the surfaces with most surface C atoms(low NFe/NC)are favorable to the promotion of methane formation.This indicates that the surface geometry effect has stronger influence on methanation process of iron carbides than surface structure effect. |
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