Size Adjustment of Iron Phosphate Nanoparticles by Using MixedAcidsTongbao Zhang, Yangcheng Lu,* and Guangsheng Luo

State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, 10084, Beijing

ABSTRACT: In this work, the size adjustment of FePO4 nanoparticles in the range of 9−50 nm was conducted on a generalplatform of coupling fast precipitation in a microreactor and thermal treatment process. Specifically, we used the mixture of nitricacid and phosphoric acid as the continuous fluid to change the free Fe3+ concentration and control the supersaturation of FePO4in fast precipitation. Inductively coupled plasma optical emission spectroscopy (ICP-OES) and X-ray diffraction (XRD) verifiedthat as-prepared nanoparticles were high-purity amorphous FePO4·2H2O. Transmission electron microscopy (TEM) verifiedtheir good dispersity and narrow size-distribution (standard deviation, ∼5 nm). Brunauer−Emmet−Teller (BET) and Cr (III)adsorption verified their property accordant as ion adsorbents. The effect of mixed acids on species with respect to iron was alsoinvestigated by UV−vis spectra. The technique would be easily scaled up for size controllable and property accordant FePO4nanoparticles preparation.

1. INTRODUCTIONThe preparation of nanoparticles with controllable size andgood dispersion has drawn consistent attention in thenanomaterial and nanotechnology area, which has significantimportance for fundamental researches1−3 as well as criticalrelevance for numerous size-dependent practical applicationslike energy conversion and storage,4 catalysis,5 adsorption,6

chemical and biological sensing,7,8 drug delivery,9 optical andspectroscopic response,10 biotagging and bioimaging,11 etc.For the past few years, iron phosphate nanoparticles have

gained considerable research interest in virtue of abundantsources, environmental compatibility, and remarkable perform-ance in the fields of lithium battery,12−14 catalysis,15−17

adsorption,18,19 ferroelectrics,20 and glass and steel industries.21

Iron phosphate nanoparticles have also been proposed as analternative for iron (Fe) nutritional supplementation and/orfood fortification recently.22 The application of iron phosphateis highly size dependent. For example, the small particle size ofiron phosphate is crucial for assuring high electrochemicalperformance of a lithium battery.23−25 When used as a catalystfor selective oxidative dehydrogenation of lactic acid to pyruvicacid, nanosized iron phosphate particles possess much bettercatalytic activity than bulk ones.26 Moreover, smaller ironphosphate particles are verified to achieve higher bioavailabilityand lower subchronic toxicity by experiments in rats.22

Many efforts have been devoted to the preparation of ironphosphate nanoparticles. By now, well established syntheticmethods include template-free precipitation in a micro-reactor,27 flame spray pyrolysis,22 microwave-assisted method,28

phase-transfer strategy,29 and other kinds of surfactant-assistedtechniques,30,31 etc. However, researches on the size adjustmentof iron phosphate nanoparticles are really limited. To our bestknowledge, Yi et al. reported the manipulation of ironphosphate nanoparticles size in the range of 50−150 nm,28

and other regions remain unclear.Synthesis of nanoparticles with controllable size requires a

detailed understanding of the process mechanism and control-lable reaction conditions. For iron phosphate preparation, the

reaction mechanism is dependent on the starting concentrationof iron and phosphate ions and pH.32 In our previous work,27 anew and facile method by coupling fast precipitation in amembrane microreactor and thermal treatment for ironphosphate nanoparticles preparation was successfully devel-oped, and the process mechanism has been carefully explored.The size of the primary particles generated in the membranemicroreactor was confirmed to determine the size of finalproduct mostly. Since the membrane microreactor had superiorand reproducible mixing performance, the size of the primaryparticle was almost dependent on the degree of supersaturationof iron phosphate and iron hydrogen phosphate directly, inwhich the concentration of Fe3+ is a critical factor. In ourprevious experiments, phosphoric acid was added to Fe(NO3)3solution as the continuous feed to suppress the hydrolysis ofFe3+. Consequently, the actual concentration of Fe3+ was lowerthan the theoretical value for the coordination effect of Fe3+

with [HnPO4]−(3‑n) (n = 0, 1, 2). The strategy for adjusting the

size of iron phosphate nanoparticles naturally occurs to us byusing non/weak coordination acids as a substitute forphosphate acid.Therefore, we focused on the size adjustment of iron

phosphate nanoparticles in the range below 50 nm by usingnitric acid to substitute phosphoric acid partially or totally as aninhibitor for Fe3+ hydrolysis. The changing of the existing stateof iron in feedstock was checked by UV−vis measurements.Transmission electron microscopy (TEM), X-ray powderdiffraction (XRD), inductively coupled plasma−optical emis-sion spectroscopy (ICP-OES), and Brunauer−Emmet−Teller(BET) analyses were employed to characterize the purity,dispersity, and size distribution of iron phosphate products.Cr(III) adsorption experiments were carried out to investigate

Received: January 17, 2013Revised: April 25, 2013Accepted: May 9, 2013

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their accordant chemical property. And a simple and newavenue for synthesizing size controllable and propertyaccordant iron phosphate nanoparticles was established finally.

2. EXPERIMENTAL SECTION2.1. Materials. For the synthesis of FePO4 nanoparticles,

ferric nitrate nonahydrate (Fe(NO3)3·9H2O), ammoniumphosphate tribasic ((NH4)3PO4·3H2O), phosphoric acid(H3PO4), and nitric acid (HNO3) with analytical reagentgrade were used during the experiments. All of these chemicalswere purchased from Sinopharm Chemical Reagent Co., Ltd.and used directly without any further treatment.2.2. Preparation of FePO4 Nanoparticles. The prepara-

tion of FePO4 nanoparticles consists in creating a well-dispersed nanoparticle precursor in the microreactor andpurification of impurities (mainly Fe2(HPO4)3 here) by thermalaging treatment. Fe(NO3)3·9H2O and an acid mixture solutionserved as the continuous feed. (NH4)3PO4·3H2O solution wasused as the dispersed feed. Driven by the pressure differencebetween two sides of the dispersion medium, the dispersed feedcontaining (NH4)3PO4·3H2O was delivered to mix with thecontinuous feed. The well-dispersed nanoparticle precursor wasgenerated immediately due to the high mass transfer efficiencyand well-controlled reaction environment in the micromixingchamber. Then the precursor was aged in an oil bath at 110 °Cfor the convertion from impurities to FePO4 product. Theentire aging process continued under 1200 rpm stirring andatmospheric pressure. Soon afterward, the powders werefiltered from the slurry and washed with distilled water atroom temperature three times or more. Finally, FePO4nanoparticle product was obtained after the mixture wasdried at 105 °C in air overnight.2.3. Cr (III) Adsorption Experiments. Cr (III) adsorption

experiments were carried out according to Zhang’s work.18 In atypical batch adsorption experiment, a different amount ofFePO4 nanoaprticles with different size were added to 30 mLCrCl3·6H2O aqueous solution (Cr3+, 13.6 mg/L). The pH ofthe solution was adjusted to 5.9 by 0.1 mol/L sodiumhydroxide aqueous solution. The quantity of FePO4 nano-particles of sample 1, sample 2, sample 3, sample 4, and sample5 was 0.78, 1.05, 1.07, 1.04, and 1.13 g/L, respectively. Then,the mixture was shaken in a 30 °C, 150 rpm oscillating waterbath for 48 h. After that, the mixture was separated by 10 mincentrifugation at 5000 rpm. The supernatant was carefully takenout and analyzed by atomic absorption spectrometry (AAS) toquantify the residual chromium in the solution.2.4. Analysis and Characterization. The morphology of

as prepared FePO4 nanoparticles was observed by transmissionelectron microscopy (TEM, JEOL-2010, 120 kV). Thenanoparticles were dispersed in ethanol (analytical grade) anddried in air after dripping one/two drop(s) to the TEM grid.Particle size distribution was processed in Image-Pro Plus 6.0(Media Cybernetics, USA).Phase purity was characterized by XRD (D8-Advance, 40 kV,

40 mA; 3% phase detection limit) using Cu Kα radiation at a6°/min scanning rate. The powders were calcined from 30 to400, 500, 600, 800 °C at a heating rate of 10 K/min in airatmosphere.Elemental composition (referring to P and Fe) of the

product was determined by an inductively coupled plasma−optical emission spectrometer ICP-OES (IRIS Intrepid II XSP;10−6 g/g detection limit for P and Fe). For the characterization,the samples were first dissolved in 1:1 volume ratio

hydrochloric acid solution. The power of the plasma was1150 W. Peristaltic pump rotation speed was 100 r/min.The specific surface area, pore size distribution, and pore

volume of the product were measured at −196 °C (77 K) byBET (Quantachrome Autosorb-1-C). Prior to analysis, thesamples were outgassed under vacuum at 180 °C overnight.The pore size distribution was calculated from the desorptionbranches using the Barrett−Joyner−Halenda (BJH) method.Cr(III) concentration in aqueous solution was determined by

a Polarized Zeeman atomic absorption spectrophotometer(AAS, Hitachi Z −5000; 10−6 g/g detection limit for Cr(III))with the slit width of 1.3 nm, wavelength of 359.3 nm, lampcurrent of 9 mA, and air-C2H2 gas mixture flame type.The UV−vis experiments were performed on a UV−vis

spectrophotometer (UV-2450, Shimadzu Corporation) with 5nm slit width.

3. RESULTS AND DISCUSSION3.1. Characterization of FePO4 Nanoparticles. To

understand the size adjustment of FePO4 nanoparticles, thecomposition of the continuous feed was adjusted by changingthe ratio of nitric acid and phosphoric acid, as shown in Table1 . The concen t r a t i on o f Fe(NO3) 3 ·9H2O and

(NH4)3PO4·3H2O were both 0.1 mol/L. The pH value of thedispersed feed was 9.10. Two feeds were mixed in themembrane microreactor with equal volume flow rates of 50mL/min.

3.1.1. Physical Characterizations of FePO4 Nanoparticles.TEM Observation. The dispersity of nanoparticles is signifi-cantly important for their applications, which is also difficult tomaintain due to high surface energy when the characteristicdimension reduces to the nanoscale. FePO4 nanoparticlesproduced under different preparation conditions were observedby TEM. All the samples were redispersed by ethanolevaporated before determination. Corresponding differentmagnification TEM photographs are illustrated in Figure 1.The lower magnification TEM images, shown in the leftcolumn, demonstrate that all of the FePO4 nanoparticlesproduced under different conditions are well dispersed. Thehigher magnification TEM images, shown in the right column,reveal that the particles are irregular polyhedrons in appearance.

Particle Size Distribution. The particles were irregularpolyhedron, and we defined the characteristic size of eachparticle by the hydraulic diameter of its projection on the TEMimage. To represent the particle size distribution moreaccurately, two hundred or more particles with clear boundaryin different TEM photographs were counted. For each singleparticle, three replicate determinations were performed and theaverage was used as a valid data. The results were shown inFigure 2. As seen, all of the FePO4 particles produced underdifferent conditions have a narrow size distribution range.When H3PO4 concentration gradually increased, average size of

Table 1. Composition and pH Value of the Continuous Feed

samplecomposition of the continuous feed

Fe(NO3)3:HNO3:H3PO4 pH value

1 1:1:0 0.902 1:0.8:0.2 0.803 1:0.5:0.5 0.684 1:0.2:0.8 0.655 1:0:1 0.65

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the particles was 9.0, 16.9, 25.8, 33.0 and 50.7 nm, respectively.Corresponding standard deviation is 3.2, 3.1, 4.0, 4.7 and 3.4nm, respectively. The narrow size distribution was mainly abenefit from the good mixing performance of the membranemicroreactor. When two feeds contacted in the microchamber,the mass transfer between the reactants was enhanceddramatically because of the miniaturized mixing scale. As aresult, a uniform concentration field propitious to homoge-neous nucleation could be achieved instantaneously, whichdetermined the narrow size distribution of FePO4 nanoparticlesfinally.BET Characterization. The specific surface area, pore

volume, and pore size distribution were characterized by aBrunauer−Emmet−Teller analyzer. The adsorption isothermcurve with hysteresis loop at high relative pressure, shown inFigure 3a, was a typical IV type isotherm curve according toInternational Union of Pure and Applied Chemistry (IUPAC)classification. The process for N2 gas adsorption was a singlemolecular layer adsorption at first, and the multimolecular layer

adsorption, capillary condensation, and external surfaceadsorption in following. The pore size distribution shown inFigure 3b indicated the existence of mesoporous structure. Theaverage pore size was 18.1, 28.9, 35.0, 44.7, and 31.4 nm whenH3PO4 concentration gradually increased. Referring to theTEM images, we considered that the pores were formed by theaggregation of FePO4 nanoparticles. The specific surface areawas 153.6 m2/g, 117.9 m2/g, 103.6 m2/g, 87.4 m2/g, and 55.1m2/g, respectively, with increasing H3PO4 addition, corre-sponding to the increase of particle size as well. Theinformation of the specific surface area, pore volume, andpore size distribution of FePO4 particles obtained underdifferent preparation conditions are summarized in Table 2.

XRD Analysis. The FePO4 nanoparticles require high purityfor successful applications since the existence of impurity maydeteriorate their performance.33 Thus, XRD analysis was usedto check the phase purity of obtained FePO4 nanoparticles. Theresults are illustrated in Figure 4. With 4 h of calcinations at 400°C, no diffraction peaks could be detected, suggesting as-prepared FePO4 nanoparticles were amorphous. Diffractionpeaks gradually emerged when the calcination temperatureincreases to 500 and 600 °C. After calcination at 800 °C for 4 h,sharp diffraction peaks appear and all of them are in accordancewith the standard (JCPDF, file No. 29-0715). No characteristicpeak in terms of impurity was observed, indicating the highpurity of all the products.

ICP-OES Characterization. P/Fe molar ratio is the mostimportant and quantitative index more preferred in industry forjudging the purity of FePO4 nanoparticles. Inductively coupledplasma−optical emission spectrometer was used to determinethe P/Fe molar ratio of FePO4 nanoparticles as prepared. Theresults for various samples are listed in Table 3. As seen, P/Femolar ratios for FePO4 nanoparticles with different sizes wereall very close to the stoichiometry value, confirming the nitricand phosphoric acids mixture method could guarantee thepurity of FePO4 nanoparticles. Herein, the determination errorsof Fe and P were 0.1% for duplicate samples, resulting in theerror of the Fe/P ratio of about 0.2%.

3.1.2. Chemical Characterization of FePO4 Nanoparticles.From the above-mentioned physical characterizations of FePO4nanoparticles as prepared, we may confirm that different sizesof FePO4 nanoparticles prepared with the method we suggestedappeared the accordance of some physical properties. However,considering on the size-dependent reactivity, the particles mayalso require the accordance of chemical properties. In thissection, Cr(III) adsorption was used as an indication toevaluate the chemical property of various FePO4 nanoparticles.The results are shown in Table 4, which clearly revealed thatCr(III) adsorption capacity was in proportion to the specificsurface area. However, they have nearly the same Cr(III)adsorption capacity (0.182 ± 0.005 mg/m2 adsorbent) whenexpressing the results as per m2 adsorbent. Cr(III) is specificallyadsorbed by FePO4. Thus, these results demonstrate thatFePO4 nanoparticles with different sizes have the accordance ofthe chemical property as well.

3.2. Process Mechanism. To explore the mechanism ofsize adjustment by mixed phosphoric acid and nitric acid, thecontinuous feed with specific formula was analyzed by a UV−vis spectrophotometer. The results are shown in Figure 5. Forsample 1 where HNO3 was being added to the continuous feed,two absorbance bands at 294.8 and 224.6 nm were observed.These two bands could be attributed to the charge transfertransitions of Fe−O (H2O). When adding H3PO4 to substitute

Figure 1. TEM images of FePO4 nanoparticles produced in thepreparation condition of sample 1 (a), sample 2 (b), sample 3 (c),sample 4 (d), sample 5 (e). The scale bar is 100 nm for the left columnand 20 nm for the right column.

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for HNO3 gradually, these two bands shift blue. For the case ofHNO3 free, these two bands shift to 275.4 and 222.6 nm,respectively. The blue shift phenomenon could be ascribed toan increase of Fe3+ ions coordinating with [HnPO4]

−(3‑n) (n = 0,1, 2). Specifically, the Fe−O (H2O) charge transfer transitionswere progressively replaced by Fe−O ([HnPO4]

−(3‑n) (n = 0, 1,2)).

The change of the existing state of iron ions in thecontinuous feed has direct effect on the precipitation reactionfor generating iron phosphate precursors in the microreactor,which would further influence the original growth of theparticles, the particle purification, and their aging stage. Indetail, when HNO3 is added into the continuous feedcontaining Fe(NO3)3·9H2O, more free Fe3+ may be supplied.The fast precipitation, as illustrated in reaction 1, conducted

Figure 2. Size distribution of FePO4 nanoparticles. Images a−e correspond to samples from Figure 1. All the statistic results are plotted in image f,including average sizes and deviations as error bars.

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mostly in the microreactor, will generate primary ironphosphate particles with smaller size. The generation ofFe2(HPO4)3, shown as reaction 2, will be inhibited due tomore iron source being consumed by reaction 1. Furthermore,the increasing size of primary iron phosphate particles resultingfrom impurity transfer (FePO4 is more stable than Fe2(HPO4)3at high temperature), shown as reaction 3, will be also limited.Vice versa, with H3PO4 addition to the continuous feed, the

free Fe3+ concentration decreases due to coordination effect ofFe3+ with [HnPO4]

−(3‑n) (n = 0, 1, 2). The size of primary ironphosphate particles increases because of the gradual decrease ofthe supersaturation ratio of iron phosphate. Moreover, theamount of Fe2(HPO4)3 also may increase with the addition ofH3PO4. When the precursors are treated by thermal aging forproduct purification, the growth of primary particles originatingfrom the impurity conversion would make them even bigger.All in all, the fine size adjustment of FePO4 nanoparticles couldbe achieved in a comparatively wide range (9−50 nm in thiswork) by carefully adjusting the composition of mixed H3PO4and HNO3 in the continuous feed.

+ →+ −Fe PO FePO34

34 (1)

Figure 3. (a) A typical adsorption isotherm curve of as-prepared FePO4 nanoparticles and (b) pore size distribution of various samples.

Table 2. Summary of BET Results of Various Samples

samplespecific surface area

(m2/g)pore volume

(cc/g)average pore size

(nm)

1 153.6 1.39 18.12 117.9 0.77 28.93 103.6 0.91 35.04 87.4 0.98 44.75 55.1 0.99 31.4

Figure 4. XRD spectra of FePO4 products after calcinations. Frombottom to top corresponds to sample 1 at 400, 500, 600, 800 °C, andsample 2, sample 3, sample 4 and sample 5 at 800 °C, respectively.

Table 3. P/Fe Molar Ratio for Different Size FePO4Nanoparticles

sample P, wt % Fe, wt % P/Fe, molar ratio

1 17.44 31.17 1.0092 16.89 30.23 1.0083 16.94 30.56 1.0014 17.28 30.21 1.0335 16.16 28.58 1.021

Table 4. Cr (III) Adsorption Determination Using FePO4Nanoparticles with Various Sizes

Cr (III)concentration

(mg/L) adsorption capacity

sample initial residual (mg/g adsorbent) (mg/m2 adsorbent)

1 17.07 27.67 0.1802 16.94 20.63 0.1753 38.66 17.87 19.50 0.1884 22.27 15.81 0.1815 27.02 10.28 0.186

Figure 5. The UV−vis results for continuous feed with variouscompositions. The inset is the enlarged image for the left absorbancebands. The curves from the bottom to top corresponds to sample 1,sample 2, sample 3, sample 4, and sample 5, respectively.

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+ = →+ − − nFe [H PO ] ( 0, 1, 2) Fe (HPO )nn3

4(3 )

2 4 3 (2)

→ ↓ +Fe (HPO ) 2FePO H PO2 4 3 4 3 4 (3)

3.3. Comparisons with Similar Techniques. In thissection, we give a detailed contrast of various iron phosphatepreparation techniques reported with aspects of the controll-ability of size, size distribution, and purity of the product. Table5 shows them briefly.Traditional precipitation34−36 is a fairly simple and widely

used way for nanosized iron phosphate preparation. However,big bulks of the product, wide size distribution, and unstablepurity are the cost of its simplicity. Though developments havebeen made by controlling the solution environment, like usingvarious templates/surfactants to hinder the aggregation,13,28,37

problems still remain including the difficulties in controlling themicroprecipitation environment in a macro-reactor, removal ofused organics, and increasing production efficiency.The oxidation method38,39 could easily fabricate iron

phosphate nanoparticles below 100 nm, but the relativelyhigh price of ferrite and strict requirement of an oxygen-freeenvironment have limited its popularization. Iron phosphatenanoparticles generated by this technique usually performpoorly in dispersity (mainly form a sponge-like structure) andpurity (impurities include Fe2O3, Fe4(P2O7)3, etc).The flame spray pyrolysis22 technique shows an advance in

fabricating small, dense, and spherical iron phosphate nano-particles. And the size distribution is also relatively narrow.However, the product is not pure with a P/Fe molar ratioaround 0.94, and large consumption of fuel gas and oxygen is acommon problem to such a technique.The electrochemical synthesis method40 brings a new way to

prepare crystal iron phosphate nanoparticles with high purity(P/Fe = 1.00), but it contains a high temperature calcinationstep (600 °C) for the conversion of Fe(OH)HPO4 precursornanoparticles to iron phosphate, which would lead to the fusionand heavy aggregation of the final product. Moreover, the sizeadjustment of iron phosphate nanoparticles is difficult andrarely reported for all of the above-mentioned techniques.Our technique takes advantage of the simplicity of a

traditional precipitation method and overcomes its drawbacksby using a microreactor to control the precipitation environ-ment at the microscale to produce a uniform and well-dispersedprecursor. The coupled thermal treatment at relatively lowtemperature could not only convert impurities, but also couldinherit the excellent property of the precursor. Additionally,through gradually changing the composition of continuousfluid, the size of iron phosphate could easily be regulated in therange of 9−50 nm. Therefore, we can expect more facile anddiversified industrial applications.

4. CONCLUSION

On the basis of the general platform of coupling fastprecipitation in a microreactor and thermal treatment process,we successfully developed a new and simple way for preparingFePO4 nanoparticles with controllable size in the range from 9to 50 nm. The goal was achieved by the strategy of adjustingthe degree of supersaturation of FePO4 during the fastprecipitation process in the microreactor. Specifically, thechange of actual concentration of free Fe3+ participating in thefast precipitation reaction was directed by adjusting the mixtureof nitric acid and phosphoric acid in the continuous fluid.Characterizations of TEM, XRD, BET, ICP-OES, and Cr(III)adsorption confirmed that these size controllable FePO4nanoparticles were pure, well dispersed, narrowly size-distributed, and property accordant. The effect of mixed acidson species with respect to iron was also checked by UV−visdetermination. The technique is general with respect to variousacids being able to coordinate with Fe3+ and can be potentiallyscaled-up for producing pure, property accordant FePO4nanoparticles with controllable size and narrow size distribu-tion.

■ AUTHOR INFORMATION

Corresponding Author* Tel.: +86 10 62773017. Fax: +86 10 62770304. E-mail: luyc@tsinghua.edu.cn.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors gratefully acknowledge the support of the NationalNatural Science Foundation of China (21036002, 21176136)and National Science and Technology Support Program ofChina (2011BAC06B01) on this work.

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Table 5. Contrast of Various Techniques for Nanosized Iron Phosphate Preparation

technique advantage disadvantage reference

precipitation simple, widely used wide size distribution,poor and unstable purity

13, 28, 34−37

oxidation easy for small particles preparation poor in dispersity and purity 38, 39

flame spray pyrolysis advance in small, dense and spherical particles preparation,relatively narrow size distribution

poor purity, large consumption of fuelgas and oxygen

22

electrochemical synthesis crystal nanoparticles, high purity heavy aggregation of products 40

our technique high purity, narrow size distribution,size adjustment in the range of 9−50 nm

relatively narrow range for size adjusting this work

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dx.doi.org/10.1021/ie400192y | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXXG