高指數(shù)晶面結(jié)構(gòu)氧化鐵化學(xué)鏈燃燒反應(yīng)活性及深層還原反應(yīng)機(jī)理

覃 吳 林常楓 龍東騰 肖顯斌 董長(zhǎng)青(華北電力大學(xué)可再生能源學(xué)院,生物質(zhì)發(fā)電成套設(shè)備國(guó)家工程實(shí)驗(yàn)室,北京102206)

高指數(shù)晶面結(jié)構(gòu)氧化鐵化學(xué)鏈燃燒反應(yīng)活性及深層還原反應(yīng)機(jī)理

覃 吳*林常楓 龍東騰 肖顯斌 董長(zhǎng)青
(華北電力大學(xué)可再生能源學(xué)院,生物質(zhì)發(fā)電成套設(shè)備國(guó)家工程實(shí)驗(yàn)室,北京102206)

采用密度泛函理論計(jì)算和實(shí)驗(yàn)研究形貌可控制備氧化鐵作為高效載氧體用于化學(xué)鏈燃燒的可行性.首先從理論上對(duì)比分析Fe2O3高指數(shù)晶面[104]和低指數(shù)晶面[001]的反應(yīng)活性及深層還原反應(yīng)機(jī)理.表面反應(yīng)結(jié)果顯示,Fe2O3[104]氧化CO的反應(yīng)活性遠(yuǎn)高于Fe2O3[001],Fe2O3[104]被還原成為低價(jià)的鐵氧化物或單質(zhì),這些低價(jià)的鐵氧化物或單質(zhì)可被O2氧化再生.載氧體和CO深層反應(yīng)結(jié)果顯示Fe2O3[104]可被CO徹底還原成Fe單質(zhì),Fe2O3[104]釋放氧能力強(qiáng),反應(yīng)活性高;而Fe2O3[001]還原到一定程度后反應(yīng)能壘高,抑制表面進(jìn)一步還原,釋放氧能力有限.最后,實(shí)驗(yàn)結(jié)果進(jìn)一步證明了Fe2O3[104]作為載氧體用于化學(xué)鏈燃燒的高反應(yīng)活性及穩(wěn)定性.

燃燒;表面;吸附;Fe2O3;密度泛函理論

1 Introduction

Chemical looping combustion(CLC)has received great attraction for its efficient use of energy and inherent separation of CO2.1-3CLC is a promising technology that produces heat and energy using oxygen carrier(OC)to supply oxygen instead of air for combustion of fuel,which results in the generation of a sequestration-ready stream of CO2that is not diluted with N2or flue gas and also reduces NOxemissions.4-10Fig.1 shows the principleof CLC.

?Editorial office ofActa Physico-Chimica Sinica

Fig.1 Principle of chemical-looping combustion Me:metal

OC acts as one of the most important factors deciding the efficiency of CLC.Till now,many OCs mainly including the oxides of Fe,Ni,Co,Cu,Mn,and Cd,have been developed for CLC. Among these OCs,NiO exhibits high reactivity but thermodynamic limitation and potential carcinogenic tendency of NiO,11CuO shows exothermic characteristic but low melting point and thus is low resistance to sintering,12CoO shows satisfactory reactivity and oxygen transfer capacity but high cost,13Fe2O3becomes the candidate for CLC process recently due to its environmentally harmlessness,high oxygen capacity,good mechanical strength,and low price.14However,enhancing the oxygen transfer capacity and reactivity of Fe2O3becomes urgent problem.

Loaded Fe2O3and combined iron based OC have received great interest for their enhanced reactivity,avoidance of carbon deposition,and good repeatability.15-26But the application of these compounds in CLC systems may not be feasible due to expensive production cost.Recently,controlled synthesis of metal oxide with a large percentage of certain exposed facets causesresearcher's attention,owing to their scientific importance associated with highly reactive surfaces.27,28The surface energies of hematite calculated based on density functional theory(DFT)for[001], [101],and[104]facets are 1.146,1.308,1.453 J·m-2,respectively,29which shows that surface energy of the[104]facet is higher than those of[001]and[101]facets.Single crystals of hematite with exposed[104]facets have been prepared for potential application as carbon-alternative anode materials for lithium ion batteries, showing high capacity,improved cyclability,and good rate performance.30The unique α-Fe2O3crystals with[104]high-index facets are highly advantageous for gas sensors with high performance.31Single crystals of hematite with nearly 100%exposed [012]facets show improved photocatalytic properties over nanoplates with dominant[001]facets.32

Fe2O3with high-index facets shows a noticeable catalytic activity.However,differing from catalytic reaction course,Fe2O3is gradually reduced and structurally destroyed during CLC process. Whether such high index Fe2O3surface is active enough for fuel CLC and whether the reduced surface can be regenerated remain unknown.In addition,the reaction mechanism between fuel molecule and the high index Fe2O3surface,especially the deep reduction reaction mechanism of the high index Fe2O3surface,has not been investigated before.To address these problems,we compared the electronic properties,surface reaction activity, oxygen transfer,and deep reduction of the high index Fe2O3surface[104]with those of the low index Fe2O3surface[001],one of the dominant growth faces of the natural hematite.33Then we experimentally verified the enhanced performance and re-use property of the high index Fe2O3surface.

2 Experimental and computational details

2.1 Preparation and chemical looping combustion

According to the reported method,315.38 g FeCl3·6H2O(AR, Sinopharm Chemical Reagent Beijing Co.,Ltd.)and 5.82 g urea (AR,Sinopharm Chemical Reagent Beijing Co.,Ltd.)were dissolved in 200 mL of distilled water to form a clear yellow solution,and then appropriate amount of Al2O3(AR,Sinopharm Chemical Reagent Beijing Co.,Ltd.)and 7.25 g formamide(AR, Sinopharm Chemical Reagent Beijing Co.,Ltd.)were added.The reacting solution was transferred to 200 mLTeflon-lined stainless autoclave and heated to 120°C for 12 h.After cooled down to room temperature,the product was collected by centrifugation and washed with distilled water and absolute ethanol(AR,Sinopharm Chemical Reagent Beijing Co.,Ltd.)4 times,and then dried at 80°C for several hours.The obtained solid was sintered at 900°C for 2 h.Finally,the as-synthesized Fe2O3[104]/Al2O3(60%,mass fraction)samples were further ground and sieved to 63-106 μm for use.

The referenced Fe2O3/Al2O3was prepared following the traditional precipitation method.34The morphological characterizations of the prepared and the regenerated OCs were performed by LEO-1450 scanning electron microscope(SEM,LEO,USA),H-9000NAR high-resolution transmission electron microscopy (HRTEM,Hitachi,Japan),D/MAX-RB X-ray diffraction(XRD, Rigaku,Japan),and Autosorb-iQ-MP Brunauer-Emmett-Teller (BET,Quantachrome,USA).

The reaction between the synthesized OCs and CO was conducted in a simultaneous thermal analyzer(TGq500)under N2at 900°C.The flow rate of N2is 100 mL·min-1.As comparison, reaction between CO and the referenced Fe2O3/Al2O3was also performed under the same conditions.Reactivity data as a function of time were obtained from the mass variations during the reduction-oxidation cycles.The degree of solid conversion χ is defined as follow:

where m is the actual mass of sample,moxis the mass of the sample when it was fully oxidized;mredis the mass of the sample in the fully reduced form.

2.2 Computational model and methods

All density functional theory calculations were performed by the CASTEP35with the generalized gradient approximation(GGA)36for describing electron-electron interactions,since GGA can predict the ground state of bulk iron more accurately than local density approximation(LDA)or local spin density approximation (LSDA).37Hubbard parameter(Ueff)was set to be 4.5 eV in our calculation,which provides the best match to the experimentalband gaps of hematite.38The magnetic configuration(+--+)was set for Fe atoms in the rhombohedral unit cell of Fe2O3,which makes the optimized cell at the lowest total energy,39-42+anddesignated up-and down-spin directions with respect to the z-axis. The interaction between the valence electrons and the ionic core was described by the Perdew,Burke,and Ernzerhof(PBE)gradient-corrected functional and the effective core potential (ECP).43Valence electron wave functions were expanded by double numeric with polarization(DNP).The kinetic energy cutoff(Ecut)was 350 eV;the Monkhorst-Pack k-mesh was 4×4×1. Calculation used an energy convergence tolerance of 1×10-4eV and a gradient convergence of 5×10-1eV·nm-1.The formulation of the linear synchronous transit(LST)and quadratic synchronous transit(QST)methods were used to search the transition states for each reaction.44

Fig.2 3-layer model and site terminology of the stoichiometric(a)Fe2O3[001]2×2 super-cell surface and

Fe2O3[104]2×1 super-cell surface and Fe2O3[001]2×2 supercell surface slabs were cleaved from the optimized α-Fe2O3bulk structure,as shown in Fig.2.The stoichiometric[001]surface exhibits two nonequivalent Fe sites(Fe4-folded-1 and Fe4-folded-2)that bond to three O atoms on the top layer and one O atom on the second layer.The stoichiometric[104]surface exhibits four different Fe sites,including two nonequivalent Fe5-folded-1 and Fe5-folded-2 that bond to four O atoms on the top layer and one O atom on the second layer,respectively,Fe4-folded-1 that bonds to three O atoms on the top layer and one O atom on the second layer,and Fe4-folded-2 that bonds to four O atoms on the top layer.All the Fe atoms on[001]surface or[104]surface are unsaturated,since the coordination number of bulk Fe is six.Geometry optimization was performed using DFT calculations and the optimized structure was obtained after fully relaxation by minimizing total energy.Then the electronic properties,reaction behavior,and oxygen transfer were detected.

Fig.3 3dFe-DOS of(a)Fe2O3[104]and(b)Fe2O3[001]

3 Results and discussion

3.1 Electronic property

C atom of CO prefers to binding to the metal site of oxide mainly due to σ donation from 3pzof CO and 3dz2of metal oxide surface.45Therefore,we compared the partial density of states (PDOS)for these Fe atoms of Fe2O3[104]and Fe2O3[001],as shown in Fig.3.For Fe2O3[104],the DOS curves show that the 3d electrons of Fe atoms on the top layer delocalizes below and near the Fermi level(Ef=0.0 eV).The highest occupation molecular orbital(HOMO)of Fe5-folded-1 is at higher energy level than those of Fe5-folded-2,Fe4-folded-1,and Fe4-folded-2,while the lowest unoccupied molecular orbital(LUMO)of Fe5-folded-1 appears at lower energy level than those of Fe5-folded-2,Fe4-folded-1,and Fe4-folded-2 above theFermi level(0.0 eV).Therefore,the energy gap(Egap)between HOMO and LUMO for Fe5-folded-1 is the smallest(～1.0 eV),while Egapfor Fe5-folded-2 is～1.5 eV,and Egapfor Fe4-folded-1 and Fe4-folded-2 is about 2.0 eV.

The calculated Egapof Fe4-folded-1 and Fe4-folded-2 of Fe2O3[001]are about 2.0 eV,close to the results of 2.0-2.1 eV,46,47which is larger than that of Fe5-folded-1 on Fe2O3[104].The small Egapfor the Fe5-folded-1 atom of Fe2O3[104]will benefit the acceptance of electron.We adopt LUMO energies of Fe2O3[104]and Fe2O3[001]to characterize their Lewis acidity and the HOMO energy of CO to characterize their Lewis basicity,respectively.The energy gap between the two orbitals can be used to predict how CO interacts with Fe2O3,which can be calculated using the equation below:

where ELUMO(Fe2O3)and EHOMO(CO)are the LUMO energy of Fe2O3and the HOMO energy of CO,respectively.The ELUMO-HOMOof the CO-Fe2O3[104]system is-3.82 eV,while the ELUMO-HOMOof the COFe2O3[001]system is-4.91 eV.The ELUMO-HOMOimplies that Fe2O3[104]will be more active than Fe2O3[001]for initiating CLC reaction of CO.

3.2 Reaction activity of surface

CO was approached to the active site Fe5-folded-1 of Fe2O3[104] and Fe4-folded-1 of Fe2O3[001]to do optimization.Results show that CO binds to Fe5-folded-1 and Fe4-folded-1,respectively.For the stable configuration of CO-Fe2O3[104],the tilt angle is 171°414′,the bond length of the adsorbed CO is 0.115 nm,the distance between CO and Fe5-folded-1 is 0.180 nm.For the stable configuration of COFe2O3[001],the tilt angle is 171°414′,the bond length of the adsorbed CO is 0.116 nm,the distance between CO and Fe4-folded-1 is 0.186 nm.Hybrid occurred between C and Fe for both the COFe2O3[104]interaction system and the CO-Fe2O3[001]interaction system.The hybrid weakened the CO bond,resulting in a red shift of the C―O bond of the adsorbed CO to lower wavenumbers with respect to that of the gas-phase CO.

The binding energy for CO on Fe2O3[104]and Fe2O3[001], Ebind-CO,was calculated respectively using the following expression as mentioned in previous works:48,49

where E(CO-OC)and E(OC)are the total energies of the Fe2O3[104]or Fe2O3[001]with and without the presence of CO molecule,respectively,and E(CO)is the total energy of an isolated CO molecule.The interaction between CO and Fe2O3[104],in terms of binding energies,is-0.221 eV,while Ebind-COfor the interaction between CO and Fe2O3[001]is-0.786 eV.Both the absorption of CO on Fe2O3[104]and the absorption of CO on Fe2O3[001]are exothermic processes.The less heat release from the absorption process of CO on Fe2O3[104]will lead to better adsorption of CO on the surface at high temperature during CLC,while compared to the CO-Fe2O3[001]case.

Then we discussed the oxidation of CO into CO2by the surface lattice oxygen(①,②,③,and④)bonded to Fe5-folded-1 on Fe2O3[104],and by the surface lattice oxygen atom(①,②,and③) bonded to Fe4-folded-1 on Fe2O3[001],respectively.The reaction initiates from the stable adsorption configurations of CO on Fe5-folded-1 of Fe2O3[104]and CO on Fe4-folded-1 of Fe2O3[001].Fig.4 shows the calculated potential energy profiles for the most accessible channels of CO oxidation into CO2on Fe5-folded-1 of Fe2O3[104]and on Fe4-folded-1 of Fe2O3[001].Aqualitatively similar twostep reaction mechanism for the reaction between CO and Fe2O3[104]{and Fe2O3[001]}was obtained.The first step relates to the conversion from the initial state(IS)to a carbonate species at the intermediate state(MS).The second reaction step initiated from the MS and changed into the final state(FS)with the release of CO2from the surface.The total activation energy(Ea)and reaction enthalpy(Er)for CO+Fe2O3[104]→CO2+Fe2O3[104]red(reduced Fe2O3[104])are 0.681 and-0.776 eV,respectively,while Eaand Erfor CO+Fe2O3[001]→CO2+Fe2O3[001]red(reduced Fe2O3[001]) are 1.203 and-0.392 eV,respectively.The latter Eais almost twice as high as the former,which suggests that the oxidation of CO by Fe2O3[104]is more accessible than by Fe2O3[001].The reaction CO+Fe2O3[104]→CO2+Fe2O3[104]redreleases more energy than CO+Fe2O3[001]→CO2+Fe2O3[001]red.Results show that Fe2O3[104]surface is more chemically active than Fe2O3[001]surface for oxidizing CO during CLC.

3.3 Regeneration behavior

During CLC processes,after the CO was oxidized by Fe2O3[104]and Fe2O3[001],Fe2O3[104]and Fe2O3[001]were reduced; then the reduced Fe2O3[104]redand Fe2O3[001]redwould be transferred to the air reactor,where Fe2O3[104]redand Fe2O3[001]redwould be oxidized by O2into Fe2O3[104]and Fe2O3[001]. Therefore,the regeneration reaction was initiated from the adsorption of O2on Fe2O3[104]redand Fe2O3[001]red,which were then oxidized by the adsorbed O2.Actually,only one O atom is needed for oxidizing Fe2O3[104]redor Fe2O3[001]redinto Fe2O3[104]or Fe2O3[001].We pushed one O of O2to the defected site of Fe2O3[104]redor[Fe2O3[001]redto do geometrical optimization to define the final reaction state.At the stable final configuration of the O2-Fe2O3[104]redsystem,one O atom from O2fixes into the defected site of Fe2O3[104]redvery well in an one-step oxidation reaction mecha-nism,as shown in the upper reaction path in Fig.5a.However,the oxidation reaction of Fe2O3[001]redcrossed the barrier of 0.374 eV at the MS which subsequently overcome a barrier of 0.397 eV into the final state(FS),as shown in the lower reaction path in Fig.5a. At the final state of O2-Fe2O3[001]red,one O atom of O2embedded into the surface of Fe2O3[001]redand the other excess O atom is adsorbed on the top layer.Both the oxidation of Fe2O3[104]redand the oxidation of Fe2O3[001]redby O2are energetically accessible.

Fig.4 Calculated potential energy profiles for CO oxidation on Fe5-folded-1 of Fe2O3[104]and on Fe4-folded-1 of Fe2O3[001]

Fig.5 (a)Potential energy profiles for the oxidation of Fe2O3[104]redand Fe2O3[001]redby O2,and(b)comparison of DOS for the fresh and the regenerated OC

However,surface reconstructions help in understanding surface chemistry of oxygen carriers,especially in the case where the surfaces can be structurally regenerated.Table 1 lists the average relaxation of Fe and O atoms on the regenerated surfaces in the direction vertically to the slab models.The average relaxations of Fe and O atoms on Fe2O3[104]redare smaller than those on Fe2O3[001]red.But,in general,both Fe2O3[104]redand Fe2O3[001]redunderwent slight surface atom relaxation after regeneration.The change of electronic state will therefore occur with the atom relaxation,as the DOS shown in Fig.5b.Except for little change happened to the DOS before and after regeneration,the DOS of the regenerated surface,especially the regenerated Fe2O3[104], matches that of the fresh surface very well.Results of the oxidation reactions energetically and structurally verified that the possibility of regeneration of the reduced Fe2O3[104].

Table 1 Average relaxation of Fe and O atoms on the regenerated surfaces in the direction vertically to the slab models

3.4 Oxygen transfer

Oxygen carrying capacity and transfer in the OC system plays an important role in CLC.We examined the oxygen transfer on the surfaces of Fe2O3[104]and Fe2O3[001]by gradually removing the top layer O atoms to form non-stoichiometric surfaces during the further reductions of Fe2O3[104]and Fe2O3[001],results of which are listed in Fig.6.In Fig.6a,after the top layer O atoms of Fe22O33,Fe22O29,Fe22O22,and Fe22O15are removed,oxygen atoms of the inner layers transfer to the top layer.Then,Fe atom dominates the top layer after Fe22O15.In Fig.6b,the optimized Fe32O48, Fe32O44,and Fe32O36show relatively high O atom coverage;then the surface of the optimized Fe32O26is almost completely covered by Fe atoms except for only one O atom.Fe atoms hinder further oxidation of CO into CO2.Instead,hybrid between CO and Fe would promote Fe-catalyzed carbon deposit reaction(2CO?FeC+ CO2),48rather than CO oxidation reaction.In comparison with the low oxygen transfer on Fe2O3[001],the higher oxygen transfer capacity of Fe2O3[104]would result in more complete reduction of Fe2O3[104].

3.5 Deep reduction

To further investigate the deep reduction of Fe2O3[104]and Fe2O3[001],we initiated the reactions from the stable physisorption of CO on the top layers of Fe22O33,Fe22O29,Fe22O20, Fe22O15,Fe22O10,Fe32O48,Fe32O44,Fe32O36,and Fe32O26,respectively. The energy profiles for the most possible redox reaction(with the lowest Ea)on these surfaces are illustrated in Fig.7,respectively, where y-ordinate is the energy in eV and the x-ordinate is the gradual reduction reaction coordinate for Fe2O3[104]and Fe2O3[001]by CO.Reduction of Fe22O33,Fe22O29,Fe22O20,Fe22O15,and Fe22O10by CO followed a two-step reaction mechanism,where the first step led to the generation of carbonate species on the surfaces while the second step was for the desorption of the carbonate species that changed into a free CO2,which is similar to our previous discovery on Fe2O3[112]49and Fe2O3cluster34.Eafor every reaction step on Fe22O33,Fe22O29,Fe22O20,Fe22O15,and Fe22O10is below 2 eV.Results show that Eaincreases slightly with the decrease of the valence states(Qv)of Fe atoms of Fe22O33,Fe22O29, Fe22O20,Fe22O15,and Fe22O10.

The reduction of Fe32O48,Fe32O44,and Fe32O36by CO followed a two-step reaction mechanism,while the reduction of Fe32O26followed a one-step reaction mechanism.Eafor the reduction of Fe32O48and Fe32O40is below 2 eV,respectively,while Eafor the reduction of Fe32O44and Fe32O36is a bit higher than 2 eV.However, the reaction between CO and Fe32O26crossed rather high barrier (Ea＞14 eV).The high Eatogether with the limited oxygen carrying capacity decides the lower reaction activity and reduction degree of Fe2O3[001],in comparison with Fe2O3[104].If completelycovered by the[001]facet,the reduction of the single crystal ironbased oxygen carrier should be very limited.However,it is known that different facets(including low and high index facets)expose on the iron-based oxygen carrier used for the chemical looping combustion in previous research works,14,34where the high index facet could benefit the deep reduction of Fe2O3into Fe.Obviously, surface morphology greatly decides the reaction performance of iron-based OC during CLC.

Fig.6 Oxygen transfer on(a)Fe2O3[104]and(b)Fe2O3[001]

Fig.7 Energy profile for oxidation of CO using(a)Fe2O3[104]and(b)Fe2O3[001]

3.6 Experimental control

Fig.8a shows the CLC reactions between CO and Fe2O3[104]/Al2O3and the referenced Fe2O3/Al2O3,which were conducted in a simultaneous thermal analyzer at 900°C.It could be observed that Fe2O3[104]/Al2O3reached a conversion(χ)of 100%without carbon deposit after 4 min,implying that Fe2O3[104]could be completely reduced by CO into metallic iron(Fe2O3+3CO→2Fe+ 3CO2).Compared to Fe2O3[104]/Al2O3,a two-stage reaction process was identified for the referenced Fe2O3/Al2O3.The first stage shows a rapid rise of reaction rates at the first minute, reaching the χ of 33.3%.The second reduction stage is at a relatively consistent reaction rate,showing an indicator of its poor reactivity,where the maximum χ is 72.0%and the formation of carbon should be in the ascendancy.50Results correspond to the theoretical analysis of oxygen transfer and deep reduction above. Five reduction-oxidation cycles were further performed to detect the effect of re-use of Fe2O3[104]/Al2O3on CLC,results of which are depicted in Fig.8b.The weight loss of 2%was found after its first use,whereas,the activity differences were inconspicuous among the second to the fifth re-use of Fe2O3[104]/Al2O3,which implies the stability of Fe2O3[104]/Al2O3for further cycles.

Fig.8 (a)Conversion(χ)as a function of reaction time for the reaction between CO and Fe2O3[104]/Al2O3,and the referenced reaction between CO and Fe2O3/Al2O3,at 900°C;(b)re-use of Fe2O3[104]/Al2O3at 900°C

Fig.9 (a)SEM image(with the inset HRTEM image)and(b)XRD pattern of Fe2O3[104]/Al2O3,(c)SEM image(with the inset HRTEM image)and(d)XRD pattern of the referenced Fe2O3/Al2O3

Fig.9 depicts the XRD patterns,SEM images,and HRTEM images of the fresh Fe2O3[104]/Al2O3and the regenerated Fe2O3[104]/Al2O3after five reduction-oxidation cycles,in comparison with those of the referenced Fe2O3/Al2O3.The SEM image shows that porosity of this rough structure mainly oriented along[104] on the surface of the prepared Fe2O3[104]/Al2O3according to the inset HRTEM images in Fig.9a.Fig.9b shows that all the iden-tified diffraction peaks of Fe2O3can be assigned to α-Fe2O3(JCPDS card No.33-0664),no other phase is observed,indicating a high phase purity and the well dispersion of Fe2O3on the support Al2O3.The sharp diffraction peaks suggest good crystallinity of the prepared Fe2O3[104]/Al2O3and confirm the regenerated renewable of the used Fe2O3[104]/Al2O3.Fig.9c shows that the surface of the referenced Fe2O3/Al2O3is also porous but smoother than Fe2O3[104]/Al2O3,and the surface of Fe2O3/Al2O3is oriented along different crystallographic directions.XRD pattern in Fig.9d verifies the high crystallization and regeneration of the referenced Fe2O3/Al2O3.Further,the regenerated Fe2O3[104]/Al2O3and the referenced Fe2O3/Al2O3keep a specific surface area of around 40 m2·g-1as those for the fresh Fe2O3[104]/Al2O3and the fresh referenced Fe2O3/Al2O3.

4 Conclusions

We have theoretically and experimentally demonstrated that morphological control of Fe2O3is very rewarding:Fe2O3[104] exhibits surprisingly high activity for CO CLC and can be regenerated for further reuse.The fundamental understanding shows that morphological control of metal oxides allows preferential exposure of active sites,improves reaction activity and oxygen transfer,decreases carbon deposit during chemical looping combustion.

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Reaction Activity and Deep Reduction Reaction Mechanism of a High Index Iron Oxide Surface in Chemical Looping Combustion

QIN Wu*LIN Chang-Feng LONG Dong-Teng XIAO Xian-Bin DONG Chang-Qing
(National Engineering Laboratory for Biomass Power Generation Equipment,School of Renewable Energy Engineering, North China Electric Power University,Beijing 102206,P.R.China)

The possibility of morphological control of iron oxide as an oxygen carrier for chemical looping combustion was investigated using density functional theory and experiment.First,we calculated the reactivity of Fe2O3with high-index facets[104]and low-index facets[001],as well as the deep reduction reaction mechanism of these two facets.Surface reaction results show that the activity of Fe2O3[104]for oxidizing CO is greater than that of Fe2O3[001].Fe2O3[104]was reduced into iron oxide at lower oxidation state or into iron, which could then be regenerated after being oxidized by O2.The deep reduction reaction mechanism between oxygen carrier and CO shows that Fe2O3[104]can be completely reduced into Fe,and Fe2O3[104]exhibits high oxygen transfer ability.However,Fe2O3[001]can only be reduced to a limited extent,with a high energy barrier preventing further reduction,while it also exhibits limited oxygen transfer capacity.Results of experiments further verify the high reactivity and stability of Fe2O3[104].

Combustion;Surface;Adsorption;Fe2O3;Density functional theory

O641

10.3866/PKU.WHXB201502061www.whxb.pku.edu.cn

Received:October 13,2014;Revised:February 6,2015;Published on Web:February 6,2015.

?Corresponding author.Email:qinwugx@126.com;Tel:+86-10-61772457.

The project was supported by the National Natural Science Foundation of China(51106051),111 Project,China(B12034),and Fundamental Research Funds for the Central Universities,China(2014MS36,2014ZD14).

國(guó)家自然科學(xué)基金(51106051),111計(jì)劃(B12034)和中央高校基金(2014MS36,2014ZD14)資助項(xiàng)目