Theoretical Study Of Surface Oxidation Over Fe₅C₂ Catalysts In Fischer-tropsch Synthesis

Understanding the structure of catalysts accurately under reaction conditions is crucial for building a bridge between structure and performance and for designing catalysts that meet expectations.Fischer-Tropsch synthesis（FTS）is one of the most intensively studied reactions in heterogeneous catalysis,converting the sygas transfered from coal to a series of chemical products.And as one of the route for utilizating coal clean and efficiently,this reaction provides an effective method for reducing environmental pollution and alleviating oil shortages.Iron-based catalysts have achieved their applications in industry due to their low cost and large operating space.Among the whole process,iron carbide is the main active phase,so understanding its structure under the reaction conditions is the top priority.Experimental studies have shown that the oxidizing atmosphere affects structure of iron carbide.However,there are many stumbling blocks on the road of structural understanding,for example,complex phases caused by the existence of oxygen,carbon,hydrogen and dynamic conditions,the inevitable introduction of supports and even impurities.With the premise,excluding these factors to study the structural changes,mechanisms,and the modification of promoters of pure iron carbide in the oxidation-related processes experimentally is challenging.The model of iron carbide Fe₅C₂ in this work was built aiming to understand its structure and related mechanism with pinpoint accuracy in the oxidation process.The surface structures,electronic properties and morphology evolution under the reaction conditions during the oxidation of iron carbide were revealed;and the potential energy surface for getting rid of adsorbed oxygen atoms on the surface through different paths was analyzed;and various promoters were further introduced to explore the effect on oxidation.This work enriches the comprehension about the structure and mechanism in the oxidation process,and provides a certain support for the development and design of efficient catalysts from a theoretical point of view.The contents are as follows:（1）Morphology evolution of Fe₅C₂ during oxidation processSeveral representative surfaces exposed under FTS conditions were selected to systematically explore the adsorption behavior of oxygen atoms from low to high coverage.It was known from the calculation that oxygen atoms are preferentially adsorbed on iron sites,and the binding strength of oxygen atoms with carbon-rich surface is weaker than that with iron-rich surface.The influence of temperature and pressure on the adsorption was studied,and the calculations demonstrated that reducing the temperature or increasing the partial pressure ratio of H₂O and H₂ would enhance the adsorption of oxygen atoms on the surface.The phase diagram indicates that there is a linear correlation of exposed carbon/iron ratio with the coverage,and the higher the carbon content,the lower the coverage.All these means the oxidation-resistant capacity of carbon-rich surface.The morphology changes under varied conditions were studied,and it was found that decreasing the oxygen chemical potential is beneficial due to an increasement in the proportion of the antioxidant surfaces.（2）Interaction between hydrogen and surface of Fe₅C₂ during the process of drcreasing oxidationHydrogen could be used as one of the effective ways to reducing adsorbed oxygen atoms,and its interaction with the surface and influence on the surface structure and morphology have been comprehensively studied.The dissociation of hydrogen molecule on the surface showed that the hydrogen molecule dissociates spontaneously on the iron-rich surface,and the energy barrier of dissociation on the carbon-rich surface is low（<0.5 eV）,indicating the existence of atomic hydrogen on the surface.The adsorption of hydrogen atoms on different surfaces showed that the hydrogen atom tends to locate on the Fe-top site on iron-rich surface,and C-top site on the carbon-rich surface,and meanwhile the adsorption energy is related to its summed bond valence（SBV_H）,higher SBV_H corresponds to more stable adsorption strength.And then,the morphology changes showed that exposed area of（510）is always maximum and carbon-rich surfaces increases with increasing temperature under selected hydrogen atmosphere from 400 to 700 K.In addition,the reduction of surface carbon to methane at 400–700 K is thermodynamically feasible at lower CH₄pressures.（3）Removal path of adsorbed oxygens on the surface of Fe₅C₂Adsorbed oxygens on Fe₅C₂ surface may occupy the active sites,and excessive coverage would lead to the oxidation of bulk.The reaction potential energy surface for removing adsorbed oxygen from each surface was analyzed.In the route for forming H₂O,the reaction energy of disproportionation is lower than that of hydrogenation after the generation of O-H bond on all surfaces,indicating that the former is superior from the thermodynamic point of view.And the energy barrier for disproportionation is also lower except for the（111）,suggesting that the formation of H₂O on most surfaces proceeds primarily through OH disproportionation.In the pathway for the CO₂ generation,the reaction of oxygen atoms with adsorbed CO is more favorable on（510）,（010）,（110）and（11-1）,while forming CO₂ via COOH intermediates are easier on（001）,（-411）and（111）.Comparing these two routes,it was concluded that the mechanism of eliminating adsorbed oxygen atoms is surface-dependent:adsorbed oxygen atoms on（010）could be removed through direct reaction in the form of CO₂;removing oxygen atoms in the H₂O route via OH disproportionation is favorable on（510）,（001）,（110）,（11-1）and（-411）;the elimination through both H₂O and CO₂ is feasible on（111）.（4）The modification of promoters on the oxidation of Fe₅C₂The effects of K₂O,Ce O₂,Cu,Pd,Pt and Mn on oxidation were explored.It was found that the effect of promoter is different on the adsorption energy of oxygen atoms on different surfaces at low coverage,for example,the adsorption energy of oxygen adsorbed on K₂O-promoted（001）and（111）is increased,while decreased on promoted（510）and（010）surfaces.As more oxygens binds to the surface,the situation becomes different,for instance,the adsorption energy of single oxygen atom on the K₂O-promoted（-411）increases,whereas decreases with the increase of coverage.Comparing the effect of promoters on the coverage of oxygen atoms under FTS-related conditions(600 K,p _H2O/p _H2=1)in thermodynamic phase diagram,it was concluded that the impact on the coverage over different surfaces is diverse:the coverage of oxygen atoms on（510）decreases after the pre-adsorption of Cu,while the coverage does not change after pre-adsorption on（11-1）.In terms of Wulff theory,the morphology of Fe₅C₂ due to the effect of promoters on the adsorption of oxygens under the conditions(600 K,p _H2O/p _H2=1)was constructed.The effect on the contribution of each surface is different:Ce O₂ reduces the exposure of（-411）,（110）,（11-1）and（111）,whereas opposite for（221）,（001）,（010）and（100）.Given that the effect of promoters is related to both structure of surface and coverage of adsorbed oxygen atoms,the average oxygen coverage in the oxygen chemical potential range of-8.02 to-7.29 eV after considering the proportion of each surface and the coverage was used to measure the effect on oxidation:K₂O has a prohibitive effect at the low oxygen chemical potential,but a promoting influence as the oxygen chemical potential increases;the average oxygen coverage increases with the existence of Mn,while opposite for Ce O₂,Cu,Pd and Pt.