Impact of 1% Lanthanum Dopant onCarbonaceous Fuel Redox Reactions with anIron-Based Oxygen Carrier in ChemicalLooping ProcessesLang Qin,†,§ Zhuo Cheng,†,§ Mengqing Guo,†,§ Mingyuan Xu,† Jonathan A. Fan,‡ and Liang-Shih Fan*,†

†Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio 43210,United States‡Department of Electrical Engineering, Ginzton Laboratory, Spilker Engineering and Applied Sciences, Stanford University, 348 ViaPueblo Mall, Stanford, California 94305, United States

*S Supporting Information

ABSTRACT: The cyclic redox reactivity of metal oxides plays animportant role in many energy fields such as fuel cells, photocatalysis,and chemical looping. In chemical looping systems, oxygen carriers arerequired to have high reactivity, recyclability, and high oxygen carryingcapacity. We utilize catalytic lanthanum dopants to dramatically changethe reactivity with carbonaceous fuels while maintaining or evenimproving the recyclability of iron-based oxygen carriers. A lowconcentration of La dopant is applied to maintain the high oxygencarrying capacity. These results are substantiated by ab initio DFT+U andthermochemistry analysis and will have a significant impact on futurechemical looping oxygen carrier design.

Cyclic redox reactions of metal oxides are the key tomany energy conversion systems such as fuel cells,1

photocatalysis,2 and chemical looping.3 Consequently,improvements in the redox reactivity of metal oxides, madepossible by a deep understanding of the associated reactionmechanisms, are critical in energy-related fields. Chemicallooping is one of the most promising techniques in the cleanenergy industry. In chemical looping reforming (CLR) systems,carbonaceous fuels can be converted to syngas or collectableCO2 with heat through reactions with metal oxide oxygencarriers. The reduced form of the oxygen carriers can beregenerated by air or steam to complete a full cycle of the redoxreaction.The most critical factors in oxygen carrier design are the

reactivity and recyclability, along with the oxygen carryingcapacity.4 Hundreds of materials have been screened in the pastfew decades. Transition metal oxides such as Fe2O3 have beensingled out as the desired candidates for their high oxygencarrying capacity, good reactivity, and low cost amongtransition metal oxides.5−7 The recyclability of Fe2O3 can beincreased by the addition of inert supportive oxides to promoteoxygen ion conductivity and enhance mechanical strength.8

Thus, increasing the reactivity in Fe2O3-based oxygen carriersthrough material modification is highly promising due to its

direct effect on improving the molecular adsorption and ionicdiffusion process in the redox reactions.The modifications of both oxygen carriers and support

materials have been discussed by a number of research groupsfrom the viewpoints of increasing recyclability and reactivity.9

However, high concentrations of dopants were utilized in mostworks, which modify the crystal phases of the oxygen carriers,thus lowering the oxygen carrying capacity of the carriers whilechanging the reactivity of chemical looping reactions. Forexample, Ryden et al.10 doped CaMnO3 oxygen carriers with12.5% Ti for chemical looping oxygen uncoupling to increasethe oxygen carrier reactivity with improved redox recyclability.Li et al.11 modified CaMnO3 with 25% Sr to effectively stabilizethe perovskite structure. CuO-based oxygen carriers modifiedwith 40% CeO were demonstrated to have a more stableoxygen release rate compared to those without modifications.12

In addition, alkali earth metals, transition metals, and rare earthmetals have been screened as high-concentration modifications(10−50%) in supportive perovskite materials.13,14 However, asubstantial reactivity increase with high oxygen carrying

Received: October 7, 2016Accepted: December 5, 2016Published: December 5, 2016

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Previous attempts have been made to increase the reactivityand recyclability of oxygen carriers using a low concentration ofdopants,11 but improvements to the reactivity of the oxygencarriers were not significant.16 Oxygen carrier doping levels ofless than or equal to 1%, which do not modify the crystalphases and structure of the carrier, are desirable because theydo not negatively impact the oxygen carrying capacity of thecarrier. It is highly challenging to largely increase metal oxidereactivity with such low concentrations of dopant.In this work, we propose and demonstrate a mechanism for

increased oxygen carrier reactivity using dopant concentrationsof only 1% with comparable or improved recyclability. We uselanthanum as an active dopant to provide extra reaction sites aswell as a lower reaction energy barrier, which will increase thereactivity to 200−400% in redox reactions. Lanthanum isselected as the dopant because of its high catalytic function inoxygen reaction with carbonaceous fuels. La2O3 is a provencatalyst for oxidative coupling of methane (OCM).17 Usingexperiments and DFT simulations, we examine how the dopantalters the redox reaction pathway in Fe2O3 crystals throughmodification of its surface structure. A higher dopingconcentration may result in the formation of other perovskitephases such as LaFeO3, which may decrease the oxygencarrying capacity of the metal oxide oxygen carriers. Dopantconcentrations less than 1% do not significantly change thereactivity of the oxygen carrier, based on our experiments andab initio calculations.The doped Fe2O3 has a rhombohedral structure, which is the

same as pure Fe2O3. SEM images in Figure 1a,b indicate that1% La-doped Fe2O3 has a smaller grain size and more grainboundaries than pure Fe2O3. La

3+ ions have a tendency tolocate at grain boundaries that would inhibit grain growth,resulting in more active sites for reaction with fuel gases.18

Meanwhile, the XRD spectrum of 1% La-doped Fe2O3 has ablue shift compared to the XRD spectrum of pure Fe2O3,

confirming that the dopant incurs lattice parameter variations.The lattice parameters a and c are 5.036 and 13.75 Å in pureFe2O3, respectively, and are 5.040 and 13.75 Å in 1% La-dopedFe2O3, respectively, as calculated from Figure 1c. This isconsistent with ab initio DFT+U calculations (DFT details aregiven in the Supporting Information), which yield a and c of5.04 and 13.83 Å in pure Fe2O3, respectively, and 5.05 and13.83 Å in 1% La-doped Fe2O3, respectively. No impurity phaseforms in the oxygen carriers.The reactivity of La-doped Fe2O3 oxygen carriers was studied

by cyclic redox reactions in TGA using two carbonaceous fuels:CO and CH4. (Details of TGA experiments setting are in theSupporting Information.) The oxygen carriers were regeneratedin air in each cycle. A comparison of the reactivity andrecyclability of both reduction and oxidation between undopedFe2O3 and La-doped Fe2O3 are shown in Figure 2 for fivecontinuous cycles. At a temperature of 700 °C, the La dopantdramatically increases the reactivity of oxygen carrier reductionin each cycle (Figure 2a). The conversion rate increases by125% in the first cycle and 245% in the fifth cycle. All of theother increasing conversion rates fall in between with asubstantially high average conversion rate increase of 170%.Similarly, the La dopant also promotes oxygen carrierregeneration. In Figure 2b shows the oxidation reactivity ofreduced oxygen carriers. The conversion rate increases by 200−300% in each cycle with an average value of 250%. It is worthnoting that the reactivity of La-doped Fe2O3 is equivalent tothat of the undoped Fe2O3 oxygen carriers at higher reactiontemperatures. In the redox reactions with CO, the La-dopediron oxide at 700 °C has a reactivity similar to that of iron oxideat 900 °C. For the same reaction, the doped iron oxide has areactivity at 1000 °C that is comparable to that of iron oxide at1150 °C.We also demonstrate that La-doped Fe2O3 exhibits enhanced

reactivity with CH4. Five typical continuous cycles are shown inFigure 2c,d. At a reaction temperature of 1000 °C, the reactivityof the La-doped oxygen carrier with methane increases between81 and 243% in each cycle, with an average increase of 178%.The reactivity of oxygen carrier regeneration in La-dopedFe2O3 increases by 111−199% in each cycle, with an averageincrease of 156%. To summarize the experimental analysis, theLa-doped Fe2O3 oxygen carriers strongly enhance the reactivityof both reduction and oxidization reactions. This suggests thatthe La dopants have high catalytic activity in both carbonaceouscombustion and oxidation of lower valence iron cations.To gain insight into the role of 1% La doping in the reactivity

of Fe2O3, we model the 1% La−Fe2O3 slab and examine COand CH4 activation in this slab system. We first determine thestable iron oxide surface termination state, assuming Fe2O3cleaving along the (001) surface, which was proven to be thedominant surface in the X-ray photoelectron diffraction study.19

A 15 Å thick vacuum layer was used to separate the surfacesand their images. The use of periodic models eliminates edgeeffects and allows for a more accurate description of surfacerelaxation. Depending on the cleavage position and the repeatof iron and oxygen atom ordering, three possible terminationscan be built: O−Fe(1)−Fe(2)− and Fe(2)−O−Fe(1)− andFe(1)−Fe(2)−O−. We calculate the surface free energy tocompare the stability of these three surface terminations at 700and 1000 °C by combined DFT calculations and the statisticalmechanics method. During the chemical looping process, theoxidation and reduction steps are alternated with anintermediate 5 min flushing step using N2 at 100 mL/min.

Figure 1. SEM image of (a) undoped Fe2O3 and (b) La-dopedFe2O3; (c) small-range XRD spectra of La-doped Fe2O3 andundoped Fe2O3.

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Therefore, it can be properly assumed that the surface is in athermodynamic equilibrium state. It was found that the freeenergy of the Fe(2)−O3−Fe(1)− terminated surface is 1.22 J/m2 at 700 °C and 1.24 J/m2 at 1000 °C, which is 1−3 J/m2

lower than the free energies of O3−Fe(1)−Fe(2)− and Fe(1)−Fe(2)−O3− terminated surface. Therefore, Fe(2)−O3−Fe(1)−is the dominant termination at both 700 and 1000 °C. Thisfinding is consistent with X-ray photoelectron diffractionstudies.19

Substitutions were made at the layers with Fe(2)−O3−Fe(1)− termination. Using O3 as the reference layer s, wereplaced Fe(2) at the s + 1 layer with La(1) to build the dopingsystem La−O3−Fe(1)−Fe(2)− as shown in Figure 3a.Similarly, we replaced Fe(1) at the s − 1 layer with La(2),

Fe(2) at the s − 2 layer with La(3), and Fe(1) at the s − 4 layerwith La(4) to build Fe(2)−O3−La−Fe(2)−, Fe(2)−O3−Fe(1)−La−, and Fe(2)···(s−2)−O3−La−, as shown in Figure3b−d, respectively. We found that the La−O3−Fe(1)−Fe(2)−terminated surface is the most stable, with a surface energy of1.39 J/m2. This analysis indicates that the 1% La dopant prefersto migrate to the outermost layer of the iron oxide surface. Onthe basis of the stable surface configurations, we can study theCO and CH4 adsorption and activation by DFT+U calculationsand thermochemistry analysis.We previously reported the adsorption of CH4 on the α-

Fe2O3(001) surface and found a weak interaction between CH4and the α-Fe2O3(001) surface.

20 To study the La dopant effect,various CH4 adsorption configurations on the α-Fe2O3(001)surface are modeled and optimized. In the best case, CH4adsorbs at the La dopant site with an adsorption energy of 0.12eV. The distance between the C atom and dopant site in thiscase is 2.66 Å. The O atop and La−O bridge adsorptions showlower stability with adsorption energies of 0.09 and 0.08 eV,respectively. Compared with the adsorption energy of CH4 tothe pure α-Fe2O3(001) surface, which is 0.06 eV, we found thatthe 1% La doping slightly facilitates CH4 adsorption. Similarly,with the increase in La distance to the outermost layer, the CH4binding energy to the 1% La-doped iron oxide decreases.To take into consideration realistic experimental conditions

(700 °C for CO oxidation and 1000 °C for CH4 oxidation), theeffect of the temperature T is included by explicitly taking intoaccount the surrounding gas phase in terms of ab initioatomistic thermodynamics. We previously developed amodified Brønsted−Evans−Polanyi relationship to calculatethe activation energy for the elementary steps of the metathesisreaction.21 Here, we extend this method to calculate theactivation energy for CO and CH4 activation on a 1% La-doped

Figure 2. Conversion of La-doped and undoped Fe2O3 for (a) 5 min reduction at 700 °C with CO and (b) 5 min regeneration with air at 700°C in five continuous cycles; (c) 5 min reduction at 1000 °C with CH4 and (d) 5 min regeneration with air at 1000 °C in five continuouscycles.

Figure 3. Optimized structure of α-Fe2O3(001) surfaces with 1% Ladopant in four different sites. Purple, red, and blue balls representFe, O, and La atoms, respectively.

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Fe2O3 surface at the chemical looping condition. The energyprofiles of CO activation are shown in Figure 4a. We see in theplot that 1% La reduces the energy barrier for CO activation oniron oxide to only 0.69 eV, compared to the 1.95 eV barrierfrom pristine Fe2O3. Similarly, we observed that the 1% Ladopant decreases the energy barrier of CH4 activation from1.72 to 0.84 eV on 1% La-doped Fe2O3 with La−O3−Fe(1)−termination (Figure 4b), compared to the correspondingundoped Fe2O3. Therefore, we conclude that the 1% Ladopant does significantly lower the energy barrier of CO andCH4 activation on Fe2O3, compared to the control case ofundoped iron oxide. Our results confirm that La dopants cansignificantly enhance the catalytic activity of iron oxide for COand CH4 oxidation. Actually, the activity is determined by aseries of oxidation states while being gradually reduced in thechemical looping combustion process. Our experiments werecarried out with a conversion of approximately 30% to avoidcarbon deposition, which means that the iron oxides in oursystem only have the Fe2O3, Fe3O4, and FeO phases with no Fephase. We further examined the 1% La dopant effect on Fe3O4

and FeO reactivity. The 1% La−Fe3O4 has a cubic inversespinel structure with a lattice constant of 8.402 Å (8.396 Å forFe3O4). The O anions form a close-packed face-centered cubic(fcc) structure with Fe2+ and Fe3+ ions located in the interstitialsites. On the basis of this model, we calculated the CO andCH4 activation energy and found that 1% La dopant alsosignificantly enhanced the activation by lowering the ∼1 eVbarrier. Similar results were obtained for 1% La−FeO.In summary, 1% of lanthanum dopants in Fe2O3 can

dramatically enhance redox reactivity while maintaining orimproving the recyclability of iron-based oxygen carriers. It isfound that the reactivity of La-doped oxygen carriers is higherthan that of undoped iron oxide oxygen carriers by 170% forCO combustion reactions and 250% for air regenerationreactions. Similarly, La-doped oxygen carriers support a 178%increase in reduction reactivity and a 156% increase inreoxidation reactivity for CH4 gasification reactions, comparedto updoped iron oxide. The mechanism for La dopant-basedreactivity enhancement stems from the ability of La dopants tolower the barriers of the C−O bond and C−H bond activationduring metal oxide redox reactions with carbonaceous fuels.Our findings are a pathway to dramatic metal oxide propertymodification using a relatively simple fabrication processes, andthey will impact future chemical looping particle design.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsenergy-lett.6b00511.

Description of the sample preparation, characterization,TGA experiments, DFT calculations, and a figureconcerning the conversion rate of oxygen carriers(PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: fan.1@osu.edu.ORCIDLiang-Shih Fan: 0000-0002-2075-6505Author Contributions§L.Q., Z.C., and M.G. contributed equally to this workNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors would like to acknowledge support from theCenter for Electron Microscopy and Analysis (CEMAS), theNanoSystems Laboratory (NSL) at The Ohio State University,and the Ohio Supercomputer Center.

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