简介概要

First principles study of Fenton reaction catalyzed by FeOCl:reaction mechanism and location of active site

来源期刊：Rare Metals2019年第8期

论文作者：Xuan-Xuan Ji Hai-Feng Wang Pei-Jun Hu

文章页码：783 - 792

摘要：Fe-based solid catalysts in promoting Fenton reaction to generate ·OH radical has drawn much attention,and interestingly,FeOCl was reported to have superior activity compared with the traditional Fe₂ O₃ catalysts.However,the mechanism of Fenton reaction on FeOCl and the origin of high activity remain unclear.Herein,by virtue of DFT+ U calculations,the H₂ O₂ decomposition and conversion mechanism on FeOCl（100）surface were systematically investigated.It is found that on clean FeOCl（100）surface,the exposed[Fe³⁺-Fe³⁺]sites can hardly break O-O bond of H₂ O₂ into OH groups,but instead H₂ O₂ tends to dehydrogenate by the surface lattice O,resulting in a series of side reactions and final conversion into O₂,while the left H atoms gradually saturate the surface lattice O and reduce Fe³⁺ into Fe²⁺.On fully H-covered FeOCl（100）,H₂ O₂ can efficiently dissociate at[Fe²⁺-Fe²⁺]sites into two OH,but OH binds with Fe²⁺ so strongly that it cannot desorb as OH radical as easily as that on Fe³⁺.Interestingly,FeOCl（100）tends to be partially protonated in the real acid solution,which,along with H₂ O₂ dehydrogenation,results in the formation of active unit [Fe²⁺-Fe³⁺].On[Fe²⁺-Fe³⁺]unit,H₂ O₂ can easily break its O-O bond and OH at Fe3+ can desorb as OH radical,while the other OH at Fe²⁺ couples with the surface H into H20 and finish the catalytic cycle.By comparison,Fe₂ O₃（012）cannot provide enough [Fe²⁺-Fe³⁺] active units due to the relative difficulty in H₂ O₂ dehydrogenation,which accounts for its inferior catalytic efficiency for Fenton reaction.

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稀有金属(英文版) 2019,38(08),783-792

First principles study of Fenton reaction catalyzed by FeOCl:reaction mechanism and location of active site

Xuan-Xuan Ji Hai-Feng Wang Pei-Jun Hu

Key Laboratory for Advanced Materials,Research Institute of Industrial Catalysis and Centre for Computational Chemistry,School of Chemical and Molecular Engineering,East China University of Science and Technology

School of Chemistry and Chemical Engineering,The Queen's University of Belfast

作者简介：*Hai-Feng Wang e-mail: hfwang@ecust.edu.cn;

收稿日期：7 August 2017

基金：financially supported by the National Natural Science Foundation of China(Nos.21622305 and21333003);the Young Elite Scientist Sponsorship Program by China Association for Science and Technology(No.YESS20150131);"Shu Guang"Project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation(No.17SG30);the Fundamental Research Funds for the Central Universities(No.WJ1616007);

First principles study of Fenton reaction catalyzed by FeOCl:reaction mechanism and location of active site

Xuan-Xuan Ji Hai-Feng Wang Pei-Jun Hu

School of Chemistry and Chemical Engineering,The Queen's University of Belfast

Abstract：

Fe-based solid catalysts in promoting Fenton reaction to generate ·OH radical has drawn much attention,and interestingly,FeOCl was reported to have superior activity compared with the traditional Fe₂ O₃ catalysts.However,the mechanism of Fenton reaction on FeOCl and the origin of high activity remain unclear.Herein,by virtue of DFT+ U calculations,the H₂ O₂ decomposition and conversion mechanism on FeOCl(100)surface were systematically investigated.It is found that on clean FeOCl(100)surface,the exposed[Fe³⁺-Fe³⁺]sites can hardly break O-O bond of H₂ O₂ into OH groups,but instead H₂ O₂ tends to dehydrogenate by the surface lattice O,resulting in a series of side reactions and final conversion into O₂,while the left H atoms gradually saturate the surface lattice O and reduce Fe³⁺ into Fe²⁺.On fully H-covered FeOCl(100),H₂ O₂ can efficiently dissociate at[Fe²⁺-Fe²⁺]sites into two OH,but OH binds with Fe²⁺ so strongly that it cannot desorb as OH radical as easily as that on Fe³⁺.Interestingly,FeOCl(100)tends to be partially protonated in the real acid solution,which,along with H₂ O₂ dehydrogenation,results in the formation of active unit [Fe²⁺-Fe³⁺].On[Fe²⁺-Fe³⁺]unit,H₂ O₂ can easily break its O-O bond and OH at Fe3+ can desorb as OH radical,while the other OH at Fe²⁺ couples with the surface H into H20 and finish the catalytic cycle.By comparison,Fe₂ O₃(012)cannot provide enough [Fe²⁺-Fe³⁺] active units due to the relative difficulty in H₂ O₂ dehydrogenation,which accounts for its inferior catalytic efficiency for Fenton reaction.

Keyword：

Fenton reaction; FeOCl; Density functional theory; H₂O₂; Active site;

Received： 7 August 2017

1 Introduction

To efficiently treat water pollution,unremitting effort has been made for decades by different ways such as physical,biological and chemical methods [ 1, 2, 3, 4, 5] .However,the component of current sewage has become more and more complicated,and conventional methods cannot meet the increasing requirement because of high cost,low efficiency and intricate pollutant composition.The advanced oxidative processes (AOPs) possess outstanding oxidative capacity,and they can conduct under ambient temperature and pressure,thus attracting extensive attention [ 6, 7, 8] .Usually,most AOPs break down contaminants by the means of producing numerous hydroxyl radicals (·OH)which has strong oxidative ability and can oxidize the majority of contaminants into CO₂,H₂O and inorganic ion [ 9, 10, 11] .Fenton reagent serves as one of the most famous AOPs and exhibits remarkable capacity for organic decomposition [ 12, 13, 14, 15] .Traditional Fenton reaction is composed of hydrogen peroxide (H₂O₂) and fcrric ion(Fe²⁺) and widely studied because it can remove various organic pollutants in the industrial production [ 16, 17, 18, 19, 20] .But several drawbacks limit its long-term development,such as the narrow reactive pH range (pH 2.5-3.5) and the accumulation of iron-containing sludge which is considered as a secondary pollution and a loss of catalyst [ 21, 22, 23] .

To overcome these disadvantages,a promising alternative is to use solid iron-based materials as Fenton catalyst,e.g.,Fe₂O₃,Fe₃O₄ and FeOOH [ 24, 25, 26, 27, 28] .Such heterogeneous Fenton systems have made evident improvement because of their stable catalytic properties,wider tolerance range of pH and much reduced iron sludge.As Yang et al. [ 29] reported,when using Fe_3-xTi_xO₄ as catalyst in heterogeneous Fenton reaction,the decolorization of MB(methylene blue) at neutral pH values was almost complete.Zhang et al. [ 30] prepared the heterogeneous Fe₂O_3-pillared rectorite clay catalyst for photo-Fenton degradation of organic contaminants,which showed great catalytic activity and good stability after five recycles.Besides,heterogeneous Fenton systems also have an outstanding advantage of easy separation [ 31] .

However,the formation rate of·OH in heterogeneous Fenton reaction is not high compared to traditional Fenton reaction.Recently,Han et al.has discovered a new Fentonlike catalyst,iron oxychloride (FeOCl),which exhibits supreme catalytic ability in producing OH by H₂O₂decomposition,being superior than other iron-based catalysts such as Fe₂O₃ and FeOOH [ 32] .Accordingly,FeOCl has drawn a wide attention in the field of Fenton reaction.However,the corresponding reaction mechanism at the atomic level remains unclear,which limits the understanding of the origin of high activity of FeOCl,despite much mechanism research on the traditional Fenton reaction.

In the conventional Fenton reaction,.OH is generally considered to be produced by redox reaction between Fe²⁺and H₂O₂ as follows [ 33, 34] :

The formed Fe³⁺can be reduced and recovered into Fe²⁺by the following reactions [ 7] :

Aleksic et al. [ 35] proposed that homogeneous Fenton processes could take place on solid surface.Costa et al. [ 36, 37] suggested that on the catalyst surface which contains Fe²⁺,·OH would be generated as follows:

which is actually corresponding to the homogeneous system.

However,most researches of heterogeneous Fenton reaction were experimental ones,and less attention was paid on the crucial issues at the atomic level.The following questions remain to be answered.What is the active center on solid catalyst that can make H₂O₂ effectively decompose into·OH radicals?How does the whole Fenton0reaction occur step by step?What roles do Fe²⁺and Fe³⁺play,respectively?Specifically,for FeOCl system,to the best of our knowledge,there are few quantitative theoretical reports.Herein,therefore,FeOCl was selected and all the possible Fenton reaction pathways on the FeOCl surface were investigated in order to figure out the above mechanism.

In this study,an extensive density functional theory(DFT) investigation of the Fenton reaction was performed on the dominant facet FeOCl(100) surface.Three different kinds of FeOCl(100) surfaces were explored on:clean surface,the surface fully covered by H atoms and the surface partially covered by H atoms.Finally,a mechanism of Fenton reaction on FeOCl surface was proposed,and the key active site,[Fe³⁺-Fe²⁺],which makes FeOCl become an outstanding Fenton catalyst,was uncovered.To further explore the activity origin of FeOCl,the Fe₂O₃ catalyst was also investigated and compared with FeOCl and its inferior catalytic activity was explained.

2 Modeling and calculation

All the spin-polarized calculations were carried out using density functional theory (DFT) with Perdew-BurkeErnzerhof (PBE) functional using Vienna Ab-initio simulation package (VASP) code [ 38, 39] .The project-augmented wave (PAW) method was used to represent the core-valence electron interaction [ 40] .The valence electronic states were expanded in plane wave basis sets with a cut-off energy of 450 eV.The transition states (TSs) were searched using a constrained optimization scheme,which has been widely used in previous studies [ 41, 42, 43, 44] .

As shown in Fig.1,FeOCl crystal has a layered structure along (010) direction and its primitive bulk unit contains 2Fe,2O and 2C1 atoms;the optimized lattice parameters are a=0.3860 nm,b=0.7924 nm and c=0.3275 nm,which agree with experimental ones(a=0.3780 nm,b=0.7917 nm and c=0.3302 nm).To describe the on-site Coulomb interaction,the DFT+U approach was used to calculate all the elementary reaction steps,and the U term of O-2p and Fe-3d is 6.3 and 3.0 eV,respectively,as discussed in Refs. [ 45, 46, 47, 48] .Noteworthily,the favored magnetic order of FeOCl bulk was tested at a ferromagnetic or antiferromagnetic state with high or low spin for Fe³⁺,showing that FeOCl with a ferromagnetic and high-spin state (μB=5 for each Fe) is energetically most stable.To well explore the catalytic activity of FeOCl,three common FeOCl surfaces,i.e..the (010),(100),(101)/(10-1) surfaces,were initially tested with the surface energy calculations,as shown in Fig.2.To well describe their surface energies,the DFT-D3 method with the van der Waals dispersion effect included was used [ 49] .

Fig.1 Structure of a primitive bulk unit and b supercell of FeOCl crystal,showing a layered configuration along (010) direction

For Fenton reaction,FeOCl(100) surface was modeled as a p(4×1) periodic slab with 4 atomic layers and the vacuum was set to be～1.5 nm.The bottom two layers were fixed,and all other atoms were fully relaxed.For these surface slabs,a 2×2×1 k-point mesh was used.The force threshold for the optimization and transition state search was 0.5 eV·mm^-1.

In addition,Fe₂O₃ was also studied as contrasted with FeOCl to better understand the internal relations between surface structure and catalytic activity.The common Fe₂O₃(012) surface was selected with similar surface configuration as FeOCl(100) [ 50] .The Fe₂O₃(012) surface was modeled as a p(3×1) periodic slab with 4 atomic layers,and the vacuum was set to be～1.5 nm.The bottom two layers were fixed,and all other atoms were fully relaxed.As for the magnetic order of Fe₂O₃,the antiferromagnetic state was adopted [ 51] .

3 Results and discussion

First of all,the surface energies of three common facets by FeOCl were calculated and compared,i.e.,the (010),(100),polarized (101)/(10-1),to determine the most possible active surface exposed in reality.The results show that they are 0.30,0.56 and 1.30 J·m^-2,respectively;the surface configuration of each one is illustrated in Fig.2.One can see that (010) surface has the lowest surface energy and thus is the most exposed thermodynamically.However,there are no Fe exposed on (010) surface except Cl(Fig.2a),on which Fenton reaction can hardly occur,considering that Fe species is the active site for Fenton reaction.As the second stable one,the (100) surface has lots of five-coordinated Fe³⁺and three-coordinated lattice O (Fig.2b) which can offer sufficient active sites.Therefore,(100) surface was taken as the model to explore the catalytic behavior of FeOCl for Fenton reaction.

As known,hydroxyl radical (·OH) is the core species of Fenton system and its strong oxidation power makes Fenton reagent possess great ability to degrade complicated organic matter.In order to figure out how OH is generated from H₂O₂ conversion catalyzed by FeOCl,two mechanism aspects have to be included:(1) generation and desorption pathway of OH group into OH;(2) possible side reactions of surface OH group.Preliminarily,the adsorption energies of some crucial species on FeOCl(100) were calculated and listed in Table 1,which will be discussed later.

3.1 Fenton reaction on clean (100) surface

To clarify how H₂O₂ molecules decompose when they initially adsorb on FeOCl catalyst,the clean (100) surface was firstly studied.As shown in Fig.3a,c,on (100) surface there are unsaturated fivefold coordinated and threefold lattice O (O_latt) exposed,which provide basic adsorption sites.On site,H₂O₂ can adsorb with an adsorption energy of-0.54 eV,and then,it could dissociate in two ways:breaking O-O bond or breaking O-Hbond.The energy profiles of these dissociations are depicted in Fig.4a.At[_s]sites,the O-O cleavage into two adsorbed OH^*group[Reaction (5)]corresponds to a high barrier process of 0.74 eV (see the transition state structure in Fig.4b) and is endothermic with an energy cost of 0.23 eV.

Fig.2 Side views of common exposed surface structures of FeOCl crystal:a (010),b (100),c (101) and d

Table 1 Adsorption energies (E_ads) of key species on FeOCl(100)surface with and without surface H covered,as well as Fe₂O₃ (012)surface 下载原图

E_ads=E_X/surf-E_surf-E_x,where E_x/surf and E_surf being total energies of catalyst surface with and without adsorbate X on it,respectively,and E_x being energy of X in gas phase

Table 1 Adsorption energies (E_ads) of key species on FeOCl(100)surface with and without surface H covered,as well as Fe₂O₃ (012)surface

Assisted by the surface O_latt,H₂O₂ was found to prefer dehydrogenation,which will generate an OOH^*adsorbed on site and an H atom at O_latt site,as shown in Reaction (6).The corresponding barrier is as low as0.23 eV,and the reaction is endothermic by 0.03 eV.

Fig.3 Side (top) and top (bottom) views of optimized a,c FeOCl (100) surface and b,d Fe₂O₃(012) surface,in which surface-exposed active sites were marked

Fig.4 a Energy profiles of H₂O₂ decomposition process with Fenton reaction on clean FeOCl(100) surface,fully H-covered surface and partially H-covered surface;optimized TS of b O-O dissociation reaction and c dehydrogenation of H₂O₂ on clean FeOCl(100) surface;TS of O-O breaking reaction of H₂O₂on d fully or e partially H-covered surface;adsorption configuration of OH group sitting on f

With the formation of OOH*,it could further undergo a transformation by three kinds of pathways:(1) desorbing into·OOH radical,(2) breaking O-H bond,or (3) breaking O-O bond,and the calculation results are illustrated in Fig.4.It is found that it is hard for OOH^*to decompose into OH^*and O^*with a high barrier of 1.41 eV;moreover,this process is also endothermic.The direct desorption of*OOH needs to overcome a desorption energy of 0.75 eV.However,it is interesting that it would tend to continue dehydrogenation and release O₂ with a low barrier of only0.09 eV,leaving its H at the neighboring surface O_latt.

Overall,on the clean FeOCl(100) surface,H₂O₂ is inclined to carry on two successive dehydrogenations and finally leave two H atoms at nearby O_latt sites that eventually reduce into (see details in Sect.3.2);theoverall reaction is ,while there is no OH group effi-ciently generated during this process.This is mainly due to the fact that the surface[ ]sites cannot afford enough bonding ability to decompose H₂O₂ into OH groups,while the surface O_latt has strong dehydrogenation ability.Nevertheless,it is worth pointing out that the surface has relative weak binding ability toward OH^*group,which can guarantee OH^*(if present) to readily desorb as radicals with a desorption energy of 0.48 eV.

According to above discussion,one may see that more and more H atoms will gradually accumulate on FeOCl(100) as H₂O₂ dehydrogenation proceeds.Accordingly,two vital issues arise.What impact will H atoms make for Fenton reaction?How does H₂O₂ decompose on the (100) surface with a large number of H atoms adsorbed on O_latt sites?

3.2 Fenton reaction on fully H-covered surface

To clarify these questions,the (100) surface with H atoms fully covered was constructed,as shown in Fig.5a,and the related reactions on this surface were further examined.Firstly,Bader charge analysis was performed for the surface Fe [ 52] .The calculation results show that all the surface Fe have a valence state of+1.4,lower than that of on the clean surface (+1.8 from Bader charge).It indicates that H atoms donate their electrons to nearby surface and reduce them into Fe²⁺.This reduction process is illustrated in Fig.5a.As a result,H₂O₂ decomposition will be catalyzed by the surface Fe²⁺species on FeOCl(100) covered by H fully.

Interestingly,the key reaction of Fenton process,i.e.,H₂O₂ directly breaking O-O bond,has become much easier to occur when they adsorb on the surface [Reaction (7)],corresponding to a barrier of 0.18 eV,which is much lower compared to that of (0.74 eV) on the surface .The optimized transition state structure is shown in Fig.4.In other words,it illustrates that surface has greater ability than surface Fe³⁺to catalyze H₂O₂ decomposition into two OH groups and the surface H atoms play an important role for FeOCl to make it form surface Fe²⁺.

Fig.5 a Structure of FeOCl(100) surface fully covered by H atoms and schematic illustration of H atom reduction effect for surface

sites;b structure of FeOCl(100) surface with H atom and proton (H+)co-covered,illustrating formation of active[

]unit 下载原图

As mentioned in Sect.3.1,OH^*can weakly adsorb at the surface ,corresponding to an adsorption energy of-0.48 eV,and the O-Fe bond distance is long at0.216 nm (Fig.4g).However,when OH group adsorb at surface ,corresponding O-Fe bond is reduced to0.184 nm (Fig.4f) and the adsorption energy becomes as high as-1.32 eV (Table 1).Bader charge analyses show that it carries a charge of-0.38|e|,indicating the formation of OH^-.These results imply that the adsorbed OH can hardly desorb into OH radicals from the surface s on account of excessive adsorption capacity,although OH group can be easily produced from H₂O₂decomposition catalyzed by .

3.3 Fenton reaction on partially H-covered surface

By combining Sects.3.1 and 3.2,it can be suggested that on FeOCl(100) surface,neither[ ]surface (corresponding to the surface fully covered by H atoms) nor[ ]surface (corresponding to clean surface) can serve as the efficient active center for the overall Fenton reaction.One may ask how does Fenton reaction occur if it is catalyzed by Fe²⁺and Fe³⁺simultaneously.

In fact,Fenton reactlon usually (occurs under an acidic environment,which could possibly result in the protons adsorbing on the surfaice lattice O (Lewis base).In order to verify this,proton adsorption was calculated under four different coverages (25%,50%,75%and 100%) with the p(4×1) slab model of FeOCl(100),in which five H₂O molecules were put above FeOCl(100) surface and the implicit periodic continuum solvation model was employed to mimic the water solution environment [ 53] .To ensure the electro-neutrality of the periodic system in the presence of proton (H⁺),OH groups or F atoms were introduced as electron acceptor on the bottom surface of the slab,instead of removing electrons from the periodic system.Similar approaches were used in our previous work and others [ 44, 45] .Results show that proton adsorption energies on the coverage of 25%,50%,75%and 100%are-0.14,-0.08,+0.03 and+0.17 eV,respectively.These adsorption energies show a trend that the binding strength decreases progressively with the increase in proton coverage,ascribing to the proton-proton electrostatic repulsion;more importantly,below the proton coverage of 50%,the adsorption is an exothermic process and expected to occur spontaneously.In other words,the protons would occupy the surface O_latt site of the FcOCl(100) at about half a monolayer.

Therefore,evenly proton-covered FcOCl(100) at 50%was constructed to examine the Fenton reaction.In this case,H₂O₂ preferentially undergoes dehydrogenation assisted by the unoccupied surface O_latt[Reaction (6)],leading to two kinds of H adsorbed on FeOCl(100) surface:H atoms and H⁺(proton),which have different effects on the surface ;H atom can reduce the neighboring into ,but proton cannot.Thus,the adsorption of H⁺keeps neighboring unchanged.Bader charge analysis shows that Fe sites closest to H atom present a chemical valence of+1.45,while the one closest to proton (H⁺)gives+1.8.As a result,[ ]unit can be formed,as illustrated in Fig.5b.

To find out whether Fenton reaction can efficiently take place on the surface[ ]unit,H₂O₂ decomposition was explored.Interestingly,our calcul ations show that H₂O₂ can be easily cracked into two OH groups with a low barrier of 0.22 eV (versus 0.18 and 0.74 eV for[ ,respectively),which adsorb on site,respectively.The OH at can readily desorb into OH radical,and the other one binds strongly with and transforms into OH ion (OH^-),which can couple xwith the surface H into H₂O or be neutralized by protons in the acid solution to form H₂O,leaving the surface Fe site free.

A schematic illustration is shown in Fig.6,displaying the complete process of Fenton reaction that occurs on the surface[Fe²⁺-Fe³⁺]unit.Specifically,the mechanism of Fenton reaction can be elucidated as follows.(1) The first is the generation of surface[ ]units.As discussed above,the dehydrogenation reaction of H₂O₂ is a dominant process before the surface O_latt is saturated.On the proton pre-covered FeOCl(100),it would eventually result in the formation of[Fe²⁺-Fe³⁺],and H₂O₂ itself would be converted into gaseous O₂ or OOH^*intermediate adsorbed on .Notably,the adsorbed OOH^*can easily desorb from site with a desorption energy of 0.53 eV(Table 1),implying that the vitally important surface Fe²⁺can be readily released and exposed to the second H₂O₂molecule for the O-O cleavage.(2) The second is the generation of OH radicals.On the“in situ”-formed[ ]H₂O₂ can readily decompose and produce OH species co-catalyzed by both .Fe²⁺and Fe³⁺play different roles in the Fenton reaction:Fe²⁺is responsible tomake H₂O₂ break O-O bond to yield two OH groups;Fe³⁺ensures that the adsorbed OH group on it can readily desorb into OH radical.As for the other OH group,it will obtain electrons from Fe²⁺and become OH^-,finally neutralized by protons.In other words,Fe²⁺and Fe³⁺complement each other's advantages and act synergistically during Fenton process.

3.4 Side reactions of adsorbed OH*on

Considering that OH^*groups on may react with other intermediate species and compete xwith its desorption process as·OH radical,it is necessary to discuss the possible side reactions and their impact on the final formation of·OH radical.On the FcOCI catalyst,besides OH^*,there could be high-coverage surface species such as OOH^*groups,H atoms and H₂O₂.The corresponding side reactions involving these species on the clean (100) face were examined and illustrated in Fig.7a.

It is found that OH self-reaction can be triggered easily if the adsorbed OH encounters another one with a barrier of0.07 eV (TS in Fig.7b),as shown in Reaction (8).Fortunately,the enthalpy change is only 0.20 eV,which means that the reverse reaction is also easy,implying that the whole process could achieve a dynamic equilibrium on the surface.Thus,adsorbed OH will not be consumed severely.

Fig.6 Schematic illustration of Fenton reaction mechanism on proton pre-covered FeOCl(100) surface:a formation of[[

]unit as a result of H₂O₂ dehydrogenation and OOH^*desorption process;b H₂O₂ adsorption and decomposition co-catalyzed by[

]unit.as well as OH desorption into radicals on

site and OH^-neutralization reaction on site by coupling with proton or surface-adsorbed H

Fig.7 a Energy profiles of OH groups involved side reactions on FeOCl(100) surface including the coupling reaction between two adsorbed OH groups,adsorbed OH reacting with H adsorbed at surface lattice O,adsorbed OH reacting with H₂O₂ and adsorbed OH reacting with OOH group;b-e corresponding to optimized TS of above four reactions,respectively

Similarly,H atom can also easily react with adsorbed OH[Reaction (9)],but its reverse reaction can still occur with a very small barrier of 0.12 eV (TS in Fig.7c).Thus,this side reaction will not have a big influence on OH radicals.

As for reaction with H₂O₂-like Reaction (10),the barrier is 0.58 eV (TS in Fig.7d),which is higher than OH adsorption energy,suggesting that the adsorbed OH at would prefer to desorb rather than react with H₂O₂.Besides,OOH group as one of products have a certain possibility to decompose into OH radicals,or they can desorb into radicals,with the fact that OOH radicals still have oxidation capacity to oxidize organic compound.

However,as shown in Fig.7,the reaction of OH^*group adsorbed at with the adsorbed OOH^*corresponds to a low barrier and strongly exothermic process,which would form undesired H₂O and O₂ (Reaction (11) and TS in Fig.7e).To obtain a high net generation rate of OH^*(prerequisite for the following OH radical),this side reaction should thus be inhibited.Nevertheless,based on the discussion in Sect.3.3,OH^*and OOH^*would not easily meet with each other during reaction process because their existence is at different reaction stages(Fig.6).Specifically,the dehydrogenation of the first H₂O₂molecule leads to the formation of OOH^*,and until OOH^*desorbs,the simultaneously formed reactive site can be exposed to the second H₂O₂ molecule's decomposition for OH^*generation.

It is clear that even though there are several interference reactions that may consume adsorbed OH^*on ,they will not seriously affect the main process.That is one of the important reasons why Fenton reaction can sustain high efficiency in radical's generation on FeOCl(100) surface.

3.5 Fenton reaction on Fe2O3(012) surface

So far,various kinds of solid iron-based catalysts for Fenton reaction have been studied and Fe₂O₃ is one of popular iron-based catalysts.Fe₂O₃ has the same valence state of Fe as FeOCl.Thus,Fe₂O₃ was also investigated and compared with FeOCl.The commonly exposed (012)was examined for Fenton reaction mechanism.Similar to FeOCl (100),all the surface Fe³⁺on Fe₂O₃(012) are fivecoordinated,and they can firmly adsorb H₂O₂ with an adsorption energy of-1.24 eV (Table 1).Besides,there is unsaturated threefold coordinated lattice O exposed on this surface.According to above discussion,surface lattice O has a crucial function during Fenton reaction,which is to adsorb H atoms and then provide a chance for surface Fe³⁺reduction into Fe²⁺.

The calculation results for the explored H₂O₂ decomposition reaction on the clean (012) surface are shown in Fig.8.Firstly,it is found to be difficult for H₂O₂ to break O-O bond with a high barrier of 0.91 eV because of the weak catalytic ability of Fe³⁺.Secondly,the barrier of H₂O₂ dehydrogenation reaction[Reaction (6)]is also a little bit high (0.76 eV) compared with that (0.23 eV) on FeOCl(100) surface,which largely hinders the formation of reactive Fe²⁺.Thus,one can speculate that[Fe²⁺-Fe³⁺]units cannot form as easily as that on the FeOCl(100)surface,which could account for inferior catalytic ability of Fe₂O₃ to FeOCl [ 32] .

4 Conclusion

By analyzing the three kinds of FeOCl(100) surfaces,it is found that the active center which makes FeOCl exhibit excellent catalytic ability is[ ]unit.The surface has great ability to catalyze H2O2 to break O-O bond and decompose into two OH groups,but the adsorbed OH cannot easily desorb;instead,it is converted into OH^-ion and neutralized by protons.Although does not have catalytic ability as good as ,it can make adsorbed OH readily desorb into OH radicals. possess a synergistic effect during Fenton reaction.Moreover,it is revealed that[ s]unit cannot form easily on the clean Fe₂O₃(012) surface on which H₂O₂ dehydrogenation is uneasy to occur,which provides an explanation of the relatively weaker catalytic ability of Fe₂O₃ for Fenton reaction.In general,this study gives a prerequisite to judge an excellent iron-based Fenton catalyst,and could enlighten further catalyst exploration.

Fig.8 a Energy profiles of H₂O₂ dehydrogenation and direct O-O dissociation reaction on Fe₂O₃ (012) surface;transition states of b H₂O₂ dehydrogenation reaction and c direct O-O breaking reactionhas great ability to catalyze H₂O₂ to break O-O bond