State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

Keywords:Chemical-looping combustion Fe2O3 oxygen carrier H2 adsorption Density functional theory Reaction mechanism

ABSTRACT An atomic-level insight into the H2 adsorption and oxidation on the Fe2O3 surface during chemicallooping combustion was provided on the basis of density functional theory calculations in this study.The results indicated that H2 molecule most likely chemisorbs on the Fe2O3 surface in a dissociative mode.The decomposed H atoms then could adsorb on the Fe and O atoms or on the two neighboring O atoms of the surface.In particular, the H2 molecule adsorbed on an O top site could directly form H2O precursor on the O3-terminated surface.Further, the newly formed H-O bond was activated, and the H atom could migrate from one O site to another, consequently forming the H2O precursor.In the H2 oxidation process, the decomposition of H2 molecule was the rate-determining step for the O3-terminated surface with an activation energy of 1.53 eV.However, the formation of H2O was the ratedetermining step for the Fe-terminated surface with an activation energy of 1.64 eV.The Feterminated surface is less energetically favorable for H2 oxidation than that the O3-terminated surface owing to the steric hindrance of Fe atom.These results provide a fundamental understanding about the reaction mechanism of Fe2O3 with H2,which is helpful for the rational design of Fe-based oxygen carrier and the usage of green energy resource such as H2.

1.Introduction

Chemical-looping combustion (CLC) is a promising combustion technology paving the way for the inherent CO2capture [1-5].In this process, oxygen carriers undergo a repeated redox between air and fuel reactors to transfer lattice oxygen for fuel combustion avoiding the direct contact of fuel and air.Compared to the traditional combustion, CLC has higher energy efficiency because the two-step reaction can reduce exergy loss [6].Moreover, NOxformation in this combustion scheme also can be largely eliminated [7].

Oxygen carrier is regarded as one of the vital factors for the successful application of CLC [8].Fe2O3used as oxygen carrier is inexpensive, abundant and environmentally sound.In addition,Fe2O3owns other advantages like low tendency to carbon deposition [9]and high resistance to sulphation deactivation [10-12].Despite a relatively weak reactivity with coal and CH4,Fe2O3shows acceptable reactivity with H2and CO both at atmospheric [13,14]and pressurized conditions [15].On the other hand, the high-cost oxygen carriers with high reactivity are not necessary for the coal-fueled CLC because the life-time of oxygen carriers may be restricted by the presence of coal ash.Therefore, the low-cost Fe-based materials having enough reactivity towards H2and CO may prove more qualified[16].Leion et al.[17]further investigated a number of various solid fuels to react with two Fe-based oxygen carriers (e.g.Fe2O3/MgAl2O4and ilmenite).They found that the oxygen carriers could react fast with gasification products such as H2and CO.Song et al.[18]studied the reactivity performance of Fe2O3with coal in a 1 kWthCLC unit.The results indicated that no tendency of deactivation of Fe2O3was observed during 10 h operation and the carbon conversion efficiency was above 81.2%.In addition, Fe-based materials have been proven to be the attractive oxygen carriers for coal utilization without severe slagging and fouling problems [19].

The reactions occurring in coal-fueled CLC are very complicated.In the fuel reactor,coal first devolatilizes volatiles to char,which is then gasified by steam to produce syngas.Subsequently, the volatiles and syngas (mainly H2, CO and CH4) reacts with the oxygen carrier to complete the coal combustion.Therefore,oxygen carriers mainly react with the gas products during coal fueled CLC process.Many experimental and theoretical investigations of the reaction performance of Fe2O3in the CLC process have focused on CH4and CO fuels[20-28].However,H2used as fuel in CLC was paid less attention.Li et al.[29]studied the role of TiO2support in redox reactions of Fe2O3with H2as fuel gas.They found that the addition of TiO2support could enhance the O2-diffusivity in Fe2O3.Liu et al.[30]investigated the reduction process of iron titanium based oxygen carriers with H2,and the results indicated that Fe2TiO5showed the highest reactivity with H2.

In recent years,density functional theory(DFT)methods play a more and more important role in clarifying oxygen carriers’ electronic and structural properties [31,32], and acquiring the unified understanding of atoms,molecules,solids and their interaction/reaction mechanism [33-39].These studies are very helpful for the selection and performance optimization of oxygen carriers employed in CLC process.Although some DFT studies have been carried out to better understand the reaction mechanism between Fe2O3oxygen carrier and fuel gas like CO and CH4, a detailed understanding of the reaction mechanism of Fe2O3with H2from theoretical study is still lacking.In CLC process, it is important for Fe2O3to convert the volatiles and gasification products completely to H2O and CO2.In this study, a detailed reaction mechanism between Fe2O3and H2was investigated at the atomistic level.The elementary steps of the reaction pathways involving both adsorption and oxidation were systematically examined.The results revealed that H2adsorption and oxidation on the O3-terminated surface is more favorable than that on the Feterminated surface.The O atoms on the O3-terminated surface are benefit for H2O formation, and Fe atoms on the Feterminated surface are conductive to H2decomposition.The objective of this investigation is to shed light on the chemical reaction in fuel reactor and help to design highly efficient Fe-based oxygen carriers for its application in CLC.

2.Models and Computational Details

2.1.Bulk and surface models

Fe2O3naturally crystallizes in the corundum structure in space group R-3c (Fig.1(a)).Along the(0 0 0 1) orientation,the layers of distorted hexagonally packed oxygen atoms were separated by the double Fe layer with Fe3+occupying two-thirds of the octahedral sites.The Fe2O3(0 0 0 1) surface is a predominant growth face and also a catalytically active surface [40].Hence, the interactions between this surface and small molecules have been the subject of extensively experimental and theoretical studies [22-24,41-43].Along the (0 0 0 1) direction [28], the layers on the surface have an alternated stacking sequence of Fe-Fe-O3-R, where R denotes the remaining layers in the bulk-phase structure and the subscript represents the number of atoms in a layer.

Two chemically stable terminations of Fe2O3(0 0 0 1) surface including single iron layer and three oxygens layer were considered, named as Fe-terminated surface (Fig.1(b)) and O3-terminated surface (Fig.1(c)).Nine-layer slabs for Fe2O3(0 0 0 1)surface were constructed based on the optimized Fe2O3bulk structure.In a wide range of oxygen partial pressures, Fe-terminated surface is the most stable one,and thus has been extensively used to investigate the interactions between small molecules and Fe2O3(0 0 0 1) surface [23,24,42].O3-terminated surface is stable in high oxygen pressures [40,44].A vacuum space of 12 ? was introduced to prevent interactions between the slabs.It has been proved that relaxing more than four outmost layers is enough to obtain adequately converged results on the Fe2O3(0 0 0 1) surface[45].Hence, the outmost five layers were fully relaxed, while the bottom four layers were fixed in all calculations.

2.2.Calculation methods

All the calculations performed in this work were based on DFT as implemented in the CASTEP codes [46].The exchange-correlation energy was treated by the generalized gradient approximation(GGA)[47]in the form of Perdew-Burke-Ernzerhof(PBE)functional[48].Previous DFT studies of Fe2O3system based on this functional have provided the satisfying calculation results [22,24,42,49].The electronic wave function was expanded in a plane wave basis set and the ionic cores were described by ultra-soft pseudopotentials[50].The energy cutoff was set to 340 eV to obtain accurate energies.Brillouin-zone integrations according to the Monkhorst-Pack scheme [51]were performed within 6 × 6 × 2 for the bulk optimization, and within 6 × 6 × 1 for surface relaxation as well as for H2adsorption.All self-consistent field (SCF) calculations were converged until the total energy difference between two electronic iterations smaller than 1.0 × 10-6eV per atom.The convergence criterions for the Broyden Fletcher Goldfarb Shanno (BFGS) algorithm were set to: (a) an energy tolerance of 1.0 × 10-5eV per atom,(b)a maximum force tolerance of 0.3 eV·nm-1and(c)a maximum displacement tolerance of 0.0001 nm

The adsorption energy is a key quantity describing the strength of the interaction of H2with the surface.In this work, the adsorption energy (Eads) of the H2is calculated using the expression:

where Eslabrepresents the energy of a clean slab,Emoleculerepresents the energy of H2in the gas phase, and E(slab+molecule)represents the total energy after H2adsorption.Based on this definition,a negative adsorption energy corresponds to an energetically favorable adsorption.

The key to understand the fundamental processes of reaction is the transition state (TS).In this study, the complete LST/QST approach[52]was employed to locate transition state for H2oxidation.In the TS calculations, the tolerance for root-mean-square(RMS) forces on each atom was set at 2.5 eV·nm-1.The activation energy (Ea) is defined as following:

where ETSis the energy of TS structure and ERis the energy of reactant in each elementary reaction.

3.Results and Discussion

3.1.Bulk optimization and surface relaxation

The magnetic structure shows an important effect on the structural parameters of Fe2O3[44].To obtain the ground state of magnetic structure of Fe2O3, four Fe atoms along Z axis were aligned and four magnetic states were considered: ↑↑↑↑, ↑↑↓↓, ↑↓↑↓and↑↓↓↑,where ↑and ↓were assigned as up-spin and down-spin directions of the Fe atoms (see Fig.1(a)).In addition, the nonmagnetic state was also optimized in order to consider all possible situations.The calculated energies were listed in Table 1.

Table 1 Lattice parameters of bulk Fe2O3, energy and its changes in bulk optimization

From an energy viewpoint, Fe2O3with the antiferromagnetic magnetic state of ↑↓↓↑having the lowest energy is the most stable structure.In addition,the calculated lattice parameters of ↑↓↓↑are a = b = 0.5023 and c = 1.3927 nm, which is consistent with the experimental values of a = b = 0.5035 and c = 1.3747 nm [53].Accordingly,the magnetic state of ↑↓↓↑was selected for the following studies.H2molecule was optimized in a cubic cell with a constant of 1.0 nm.The obtained H-H bond length is 0.0752 nm,which is in good agreement with the experimental values of 0.0741 nm [54].

Fig.1.Bulk structure of α-Fe2O3 with magnetic configuration (a).The Fe-terminated surface (b) and O3-terminated surface (c).Probable adsorption sites on Fe-terminated surface (d) and O3-terminated surface (e).Sticks in the bottom of (b) and (c) represent the fixed atoms (1 ?=0.1 nm).

In general,all the surfaces will relax to some extent varied from the coordination environment[55].Surface relaxation was considered to provide further accuracy in the present study.Considering the accuracy and computation efficiency [45], two ideal surfaces with five upside layers relaxed were calculated to determine the surface relaxation of the Fe2O3(0 0 0 1) surface.The changes of the total surface energy and total atomic relaxation along z-axis were examined.The results are listed in Table 2.It can be noticed that the layers for relaxation move towards the center of bulk.Additionally, the relaxed surface has lower energy than corresponding unrelaxed one, and the relaxed Fe-terminated surface has the lowest energy.

Table 2 Surface energy and total displacement changes in surface relaxation

According to the above discussion,the surface relaxation causes slab models more compact and stable, making the surface models more close to the actual situation.Therefore,the surface relaxation cannot be neglected for the adsorption calculation.Therefore, all the following calculations are based on the two relaxed terminated surfaces to obtain higher accuracy.

3.2.Adsorption of H2 on Fe2O3 (0 0 0 1) surface

In the CLC process, H2first adsorbs on the Fe2O3surface, and then is oxidized by the surface lattice oxygen to produce H2O.In this section, we primarily focus on the interactions between H2and two important terminated Fe2O3(0 0 0 1) surfaces.To investigate the adsorption behavior between H2molecule and Fe2O3(0 0 0 1) surface, the structure is optimized as H2is placed at all possible sites on the surface.Six active sites and the vertical or parallel H2molecule to the surface were considered, as displayed in Fig.1d and e.The adsorption energies and structural features, as well as the electronic properties were analyzed.

3.2.1.H2adsorption on O3-terminated surface

O3-terminated surface is an oxygen-rich surface with three surface O atoms, and the Fe atoms in the sub-layer are saturated.Therefore, four original configurations of H2molecule reacting with the surface oxygen atom are built.

After optimization, the adsorption energies and stable geometries are displayed in Fig.2.The results suggest that only H2molecule parallel to the surface is favorable for the stable adsorption.The adsorption energies of the two chemisorption configurations(see Fig.2A and B)are-2.34 and-2.08 eV,respectively.In configuration 2A,H2dissociatively adsorbs on two O sites of the surface.The bond lengths of the newly formed H-O bonds are about 0.983 and 0.987 ?(1? = 0.1 nm), respectively.As show in configuration(Fig.2B), H2molecule directly adsorbs on the O site to form H2O precursor.The H-O bond lengths are 0.0979 and 0.1003 nm,respectively.The breakage of a O-Fe bond and elongation of another O-Fe bond length imply that H2O desorption is energetically feasible.

Fig.2.The stable adsorption configurations of H2 on the O3-terminated and Fe-terminated surfaces.Bond lengths are in ? (1 ? =0.1 nm).

To further understand the electronic interactions between the H2and O3-terminated surface, we calculated the partial density of states (PDOS) of H atom and O atom in the two stable configurations, as shown in Fig.3.In Fig.3(a), two sharp resonance peaks appear at-20 and-7.5 eV,corresponding to the interaction of H-s state with O-s and O-p states,respectively.In Fig.3(b),the peaks of-22.2, -10 and -7.5 eV correspond to the resonance of H s state interaction with O s and p states.Comparing with Fig.3(a), the lower PDOS peaks of Fig.3(b)indicated that H atom in H2O precursor is more stable than H atom binding on the surface.Although the adsorption energies obtained for this surface are very high,the interaction among H-s state, O-s and O-p states in the newly formed H-O bond are relatively weak.This feature indicates that H atom diffusion on the surface is possible.

3.2.2.H2adsorption on Fe-terminated surface

Fe-terminated surface is a catalytic surface primarily due to the presence of Fe atom on surface layer.In the sub-layer, three O atoms coordinating with a Fe atom imply that H2can be oxidized by these lattice oxygens.Eight configurations for all possible interaction between H2and Fe, O atoms were taken into account.

Fig.3.PDOS of stable configurations for the interaction between H atoms and surface O atom on the O3-terminated α-Fe2O3 (0 0 0 1) surface: (a) H-s states and O atomic states in Fig.2A; (b) H-s states and O atomic states in Fig.2B.The vertical dash line indicates Fermi level at 0 eV.

Fig.4.PDOS of the interactions among H2,H,Fe and O atoms on the Fe-terminated surface:free H2 orbitals and H-s states(a);adsorbed H2 molecular orbitals and Fe atomic states in Fig.2E (b); H-s states and Fe atomic states (c) and H-s states and O atomic states (d) in Fig.2D; H-s states and O atomic states in Fig.2C (e).The vertical dash line indicates Fermi level at 0 eV.

After calculation, three relatively stable configurations with adsorption energies and structural parameters are displayed in Fig.2C,D and E.The adsorption energies of obtained configurations were-0.61,-0.29 and-0.15 eV,respectively.Two H2dissociative adsorption configurations were obtained.For configuration in Fig.2C, H2molecule decomposed into two H atoms binding with two O atoms.The bond distances of the newly formed H-O bond are 0.0982 and 0.0986 nm, respectively.Compared to the configuration 2A, the reduction of adsorption energy is about 1.73 eV,which may be due to the difference of the oxygen coordination environment.Another H2dissociative adsorption configuration is showed in Fig.2D, the decomposed H atom adsorbs on O and Fe atoms with the bond distances of 0.0987 and 0.1608 nm.Though the configuration in Fig.2E showed the lowest adsorption energy,the interaction between H2molecule and surface Fe atom makes H-H bond distance elongate from 0.0753 to 0.0771 nm.This implies the activation of H2molecule.On this surface, the surface Fe atom plays a key role in the activation of H2bond.

The electronic PDOS for the H2adsorption over the Feterminated surface was also examined, as illustrated in Fig.4.All the H2molecular orbital peaks shift downwards below Fermi level when H2molecule physically adsorbs on Fe site (see Fig.4(a) and(b)).However,H2molecular orbitals have weak hybridization with Fe s,p and d states at-8.5,-2.5 and 0.8 eV,respectively.The interaction of H atom and Fe atom mainly derives from the interaction of H s state and Fe d state at -4.0, -2.5 and -0.4 eV in configuration 2D.The orbital interaction of H and O atoms in newly formed H-O bond is at-20.4 and-7.8 eV.In configuration 2C,the s state of H atom primarily overlaps with the s and p states of O atom at about -20.9 and -10 eV.Since the adsorbed H s state has faintish hybridization with orbitals of surface atoms, indicating that the diffusion of H atom on the surface is energetically possible[24,56].

It can be seen from above analysis, H2prefers to adsorb on the surface with a dissociative mode.O3-terminated surface shows higher activity than Fe-terminated surface for H2adsorption, so both H atoms will adsorb onto surface O and not surface Fe [28].PDOS analysis of the stable adsorption configurations reveal that H s state has relatively weak hybridization with the states of surface O and Fe atoms.Therefore, the dissociative H atom can easily migrate on the Fe2O3surface to react with surface O completing the H2oxidation.

3.3.Oxidation of H2 by lattice oxygen to H2O

Based on the H2adsorption structures, the oxidation mechanism of H2over Fe2O3(0 0 0 1)surface was examined.Two possible H2oxidation schemes by lattice oxygen were proposed: (i) H2directly adsorbs on a surface O site and forms H2O, (ii) H2decomposes on Fe2O3surface, and then H atom migrates from an active site to another O site which already adsorbs a H atom.In the second scheme,H2adsorption behavior indicated that the dissociation of H2molecule could provide active sites and dissociative H atoms.In spite of the H2dissociative adsorption energies vary differently,H migration over the surface is a very important characteristic on oxygen-bearing transition metal surface [24,42,57]to form H2O.

3.3.1.H2O formation on O3-terminated surface

After H2adsorption,there was only one configuration with H2O precursor directly formed when H2approached to the O top site.However, the H2dissociative adsorption configuration was more stable.In this section, possible reaction pathways including the information of structural parameters for the reactants, transition states,intermediates and products are characterized and displayed in Fig.5.

Firstly,the transition state for H2decomposition(H2→H*+H*)was examined.It can be seen that H2decomposition via TS1 and TS3 has the energy barriers of 3.91 and 1.53 eV, respectively.The geometry of transition state of TS1 suggest the H2decomposition behavior: one H atom of tilted H2molecule binds to an O atom along with the elongation of H-H distance, and then another H atom moves to O atom to form another hydroxyl.Subsequently,H2O formation via H migration was also discussed in path1.The structure of transition state is demonstrated in TS2 of Fig.5.In the TS2, the dissociative H atom adsorbs on the bridge site of two adjacent O atoms with bond lengths of 0.1245 and 0.1267 nm.The bond length between the migrated H atom and hydroxyl O atom becomes shorter, which is convenient for the combination of H atom with hydroxyl to form H2O precursor.This process is found to be slightly endothermic by 0.26 eV and presents a lower activation barrier of 0.65 eV, showing that H diffusion on the surface is energetically favored.At last, H2O desorbs from the surface along with the breakage of Fe-O bond, and this process needs an endothermic energy of 0.64 eV.

Fig.5.Schematic potential energy profiles for H2 oxidation on O3-terminated surface, including the geometries of initial state,transition state,intermediate and final state.Bond lengths are in ? (1 ?=0.1 nm).

In summary,the entire process of H2oxidation by lattice oxygen on the O3-terminated surface is found to be exothermic with considerable energy of 1.44 eV.Comparing the two reaction pathways,H2adsorption via path1 has higher energy, but the decomposition process has an activation barrier as high as 3.91 eV.This indicates that path1 is difficult to occur,because the breakage of H-H bond on O3-terminated surface is a high energy consumption process.Hence, path2 with an activation barrier of 1.53 eV of the ratedetermining step is an easier oxidation pathway to produce H2O.

3.3.2.H2O formation on Fe-terminated surface

It can be clearly seen that the adsorption energy of H2molecule on the Fe-terminated surface is comparatively smaller than that on the O3-terminated surface.Three possible reaction pathways for H2oxidation on the Fe-terminated surface were proposed based on the three stable configurations in Fig.2C,2D and 2E.The structures in the three possible reaction pathways and the schematic potential energy profiles are shown in Fig.6.Since the direct H2O formation did not happen on this surface, we mainly focus on the H migration over the Fe-terminated surface.

Initially, the transition states for the H2dissociation process(H2→H*+H*)on this surface were located.In path3,H2dissociative adsorption on two O atoms has the highest energy barrier of 1.83 eV,which means that this reaction channel is hard.Comparing with H2dissociative adsorption on the O3-terminated surface, the TS has similar structure and lower energy barrier.This may be associated with the Fe-induced change of the electronic structural character of Fe-terminated surface.The other two pathways(Path4 and Path5)via configuration in Fig.2D and configuration in Fig.2E show similar TS structures(see TS2 and TS3).However,the energy barrier of path4 is 0.73 eV, which is higher than that of path5(0.62 eV).Therefore, H2dissociative adsorption through TS3 in path5 is the energetically favorable pathway and will be discussed in detail subsequently.

Fig.6.Schematic potential energy profiles for H2 oxidation on Fe-terminated surface, including the geometries of initial state,transition state, intermediate and final state.Bond lengths are in ?.(1 ?=0.1 nm)

After H2dissociation, H atom of a hydroxyl migrates to the neighboring O site, forming a new H-O bond.In TS4, H atom adsorbs on the bridge of two O atoms with bond lengths of 0.1256 and 0.1228 nm,respectively.The H migration step is found to be an endothermic reaction with an activation barrier of 0.67 eV.Subsequently, the H atom of one hydroxyl reacts with another hydroxyl to generate a H2O precursor adsorbed at Fe site.In TS5,the distance of H atom and O atom in newly formed hydroxyl is shortened to 0.1427 nm, while the H-O distance of the original hydroxyl is elongated to 0.1180 nm.This H2O formation process is found to be endothermic and shows an energy barrier of 1.64 eV.Finally,the newly formed H2O molecule breaks away from the surface.

The overall reaction for H2oxidation on the Fe-terminated surface is endothermic by 0.95 eV,whereas on the O3-terminated surface is exothermic by 1.44 eV.This indicates that the energy of H2oxidation on O3-terminated surface of Fe2O3is energetically favored [28].The H2O formation is the rate-determining step of the minimum-energy path (Path5).Comparing H2oxidation paths on the two surfaces, it can be seen that O atom on the O3-terminated surface is conducive to the H2O formation step and Fe atom on the Fe-terminated surface prompts the H2decomposition process.However, the Fe atom on the Fe-terminated surface lowers the H2adsorption energy and generates steric hindrance,which is unfavorable for the dissociative H atom reaction with the surface O atom.This may be the reason that H2oxidation processes on the O3-terminated and Fe-terminated surfaces are exothermic and endothermic, respectively.

4.Conclusions

Theoretical investigations based on the density functional theory calculations were conducted to reveal the intrinsic reaction mechanism of H2with Fe2O3oxygen carrier during the chemical looping combustion.It is found that H2molecule mainly adsorbs on the Fe2O3(0 0 0 1)surface in a dissociative mode.The adsorption energies of H2dissociation on the O3-terminated surface are much higher than that on the Fe-terminated surface.Electronic analysis of the stable adsorption configurations reveal that H s state has relatively weak hybridization with the states of surface O and Fe atoms.This may be the reason that the dissociative H atom can easily migrate on Fe2O3surface to form H2O precursor during H2oxidation process.Two reaction pathways were found on the O3-terminated surface for the oxidation of H2.H2decomposition is the rate-determining step in both reaction pathways.The activation energy of H2decomposition step decreases obviously on the Fe-terminated surface due to the catalytic role of the Fe atom.However, the existence of Fe atom on the Fe-terminated surface generates steric hindrance that hinders H2O formation, which becomes the rate-determining step during H2oxidation process.Moreover, H2oxidation process on O3-terminated surface is exothermic,but H2oxidation process on the Fe-terminated surface becomes endothermic.The H2oxidation on O3-terminated surface is thermodynamically and kinetically favorable than that on Feterminated surface.This work shed light on the detailed reaction mechanism of Fe2O3oxygen carrier with fuel gas H2at an atomic level.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China (51976071) and Fundamental Research Funds for the Central Universities (2019kfyRCPY021).