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J. of Supercritical Fluids 39 (2006) 135–141

Supercritical water synthesis and deposition of iron oxide (�-Fe2O3)nanoparticles in activated carbon

Chunbao Xu, Amyn S. Teja ∗School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, N.W., Atlanta, GA 30332-0100, United States

Received 29 November 2005; received in revised form 13 February 2006; accepted 20 February 2006

bstract

Iron oxide (�-Fe2O3) nanoparticles were deposited on the surface and in the pores of activated carbon pellets using supercritical water toynthesize the particles from a precursor solution of ferric nitrate. The dispersion of the particles in the activated carbon was found to dependainly on the immersion time in the precursor solution at room temperature. Two types of dispersions were obtained: egg-shell dispersions at low

mmersion times and uniform dispersions at high immersion times. The particle size and size distribution, on the other hand, were influenced mostly

y precursor concentration. Although the catalytic properties of the �-Fe2O3 in AC composites were not evaluated, the procedure of employingupercritical water to deposit metal oxide particles on hydrophobic surfaces inside support structures offers promise for carbon-supported catalystreparation without the use of toxic or noxious solvents.

2006 Elsevier B.V. All rights reserved.

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eywords: Activated carbon; Iron oxide; Catalyst support; Hydrothermal treatm

. Introduction

Supercritical water has received considerable attentionecently as a medium for synthesizing metal oxide particles viaydrolysis and dehydration of metal salts [1–3]. These studiesave shown that the high reaction rates and low metal oxide sol-bilities in supercritical water can lead to high supersaturationsnd the deposition of fine metal oxide particles from aqueousolutions of metal salts. More recently, Otsu and Oshima [4]ave exploited the reactive properties of supercritical water toeposit particles of several metal oxides in the pores of aluminaupports. The penetration of reactants in porous structures suchs alumina is facilitated by the gas-like transport properties andery low surface tension of supercritical water. In the presentork, we extend the work of Otsu and Kojima to the depositionf nanoparticles of iron oxide on hydrophobic surfaces such asarbon.

Carbon-supported metal or metal oxide catalysts are of inter-

st in a number of applications such as the destruction of VOCs5] because the porous carbon structure can be tailored to obtainigh surface area and pore-size distribution needed for specific

∗ Corresponding author. Tel.: +1 404 894 3098; fax: +1 404 894 2866.E-mail address: amyn.teja@chbe.gatech.edu (A.S. Teja).

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896-8446/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2006.02.004

Particle size

eactions. Moreover, the structure is resistant to temperature asell as acidic or basic conditions, and the metal or metal oxide

atalyst can easily be recovered by burning away the support6]. However, good dispersion of the catalyst in the support isequired in order to promote high conversions of the pollutants6].

Carbon-supported catalysts are usually prepared by imbibinghe support in a solution of the catalyst precursor, followed byrying, calcination, and reduction at high temperatures. The lat-er may result in undesirable changes such as sintering that leado changes in the active catalyst dispersion [7]. In addition, mul-iple impregnation steps may be required to obtain the desireduantity of catalyst on the support [8]. Since carbon is essen-ially hydrophobic in nature, it has low affinity for polar solventsuch as water. Therefore, when a catalyst precursor in water isontacted with carbon pellets, the metal precursor is found pri-arily on the outer surface of the pellets [9]. In contrast, theetal precursor penetrates into the pores of the pellets when

cetone is used as a solvent [9], eventually leading to a moreniform dispersion of the catalyst. However, as was pointed outy Alvim-Ferraz and Todo-Bom Gaspar [5], previous authors

id not recognize the effect of the nature of the carbon surfacen the distribution of the precursor in the pores.

In the present work, we use supercritical water to demonstratehat water can be used to synthesize and disperse nanoparticles

1 ritical Fluids 39 (2006) 135–141

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Table 1Summary of synthesis conditions

Sample Precursorconcentrationa

Th (◦C) Immersiontime τ1 (day)

Digestion timeτ3 (min)

c (M)

S1 0.05 394 5 90S2 0.15 394 0 90S3 0.15 394 5 90S4 0.15 394 15 90S5 0.15 394 30 90S6 0.15 394 30 30S7 0.5 383 5 90S8 0.5 394 30 90S9 0.15 394 45 minb 90

nSdmcaitawoppwmbpby TEM. Average particle sizes were estimated from long andshort axes measurements of at least 100 particles.

36 C. Xu, A.S. Teja / J. of Superc

f hematite (�-Fe2O3) in activated carbon. Hematite was useds a model oxide to demonstrate the supercritical synthesis andispersion method, since hematite is easily obtained via hydrol-sis and dehydration of ferric nitrate. We also explore the effectf several processing variables on the size and morphology ofhe particles deposited in the pores.

. Experimental

.1. Materials

Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, ACS reagent)f minimum purity 98 wt.% was purchased from Sigma–AldrichWI, USA). Norit RX 3-extra commercial activated carbon (inhe form of cylindrical pellets of surface area 1370 m2/g) wasupplied by NORIT Americas Inc. Deionized water was maden the lab and all materials were used without further treatment.

.2. Apparatus

The synthesis reaction and deposition were accomplished in250 ml stainless steel autoclave (Parr Instrument Co., IL, USA,odel 4576) equipped with a magnetic stirrer and a jacket for

irculating water for heating or cooling the contents. The reactoras rated for pressures of 33.5 MPa at 773 K.

.3. Procedure

The aqueous precursor solution used in the experiments wasrepared by dissolving known amounts of Fe(NO3)3·9H2O in50 ml of deionized water. Approximately the same amount ofctivated carbon (AC) pellets (∼3 mm diameter × 5 mm long)ere then added to each solution, and allowed to soak in therecursor solution at ambient temperature Ta and pressure Paor time τ1. After time τ1 had elapsed, each mixture was trans-erred to the autoclave reactor and heated at a rate of 5 K min−1

nder constant stirring until a final temperature Th was attainedthe time interval τ2 for this heating step was about the same in allxperiments). The mixture was then maintained at this temper-ture (Th) for time τ3 (usually 30 or 90 min). Finally, the reactoras cooled to ambient conditions by circulating water through

he reactor jacket at time τ = τ1 + τ2 + τ3. Reaction conditionsre summarized in Table 1. The AC pellets were separated fromhe mixture by filtration, washed several times with deionizedater, and dried overnight in an oven prior to analysis. The filter-

ng, washing, and drying procedure was also followed for ironxide nanoparticles in the bulk solution.

.4. Sample analysis

The impregnated AC pellets were characterized by powder X-ay diffraction using a PANalytical X-ray diffractometer (Model

W 1800, Phillips, MA, USA) with a Cu K� radiation source.he X-ray analyses were performed on the powder obtained bycraping off deposited particles from the surface of the pellets,nd grinding the scraped pellets to a powder.

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a Moles Fe(NO3)3 per liter H2O.b Ultrasonification time.

�-Fe2O3 particles obtained from the interior of the impreg-ated pellets and from the bulk solution were analyzed byEM (LEO 1530 FEG field-emission instrument), energy-ispersive X-ray spectroscopy (EDS), and transmission electronicroscopy (JEOL 100C TEM instrument). Images of lateral

ross-sections of several cylindrical pellets (see Fig. 1) werenalyzed by SEM at the center of each cross-section (position Cn Fig. 1 where r = 0), at approximately half the distance betweenhe center and the surface (position B in Fig. 1 where r ∼ 0.5R),nd near the edge of each cross-section (position A in Fig. 1here r ∼ R). Average particle sizes were estimated from imagesf at least 500 particles. TEM analyses of ground impregnatedellets were also performed after scraping off deposited oxidearticles from the surface of the pellets. The ground powderas dispersed in deionized water and a drop of the resultingixture was placed on a carbon-coated copper grid for analysis

y TEM. A drop of the bulk liquid in each experiment was alsolaced directly onto a carbon-coated copper grid and analyzed

ig. 1. Schematic of an AC pellet showing lateral cut and center (C), half wayB), and near-surface (A) regions.

C. Xu, A.S. Teja / J. of Supercritical Fluids 39 (2006) 135–141 137

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Particle sizes estimated from TEM and SEM images areshown in Table 2. Particles from the bulk solution were ana-lyzed by TEM, whereas particles deposited in the AC (in thesame experiment) were analyzed by SEM. Estimates of the size

Table 2Particle size determined from TEM and SEM data

Sample TEM SEM

Radius(nm)

Standarddeviation

Radius(nm)

Standarddeviation

S1 15.9 2.8 20.4 3.6S2 22.9 2.9 33.2 6.1S3 24.4 3.3 32.6 5.9S4 23.5 3.6 30.8 6.1S5 24.7 2.9 33.3 6.3

Fig. 2. Representative SEM micrographs of sample S2: (a) region A, ×1

. Results and discussion

Representative SEM micrographs for sample S2 are shown inig. 2, and confirm the formation of a large number of nanopar-

icles at 394 ◦C. An EDS spectrum of the sample exhibitedharacteristic peaks from iron, oxygen and carbon, indicatinghe existence of these elements in the sample. Subsequent XRDnalysis confirmed that these peaks were from �-Fe2O3.

.1. Effect of immersion time τ1

Immersion time (τ1) was varied from 0 to 30 days in order toetermine the effect of precursor solution penetration on theispersion of particles. Since activated carbon is essentiallyydrophobic [7] and was used without any surface modifica-ion, it was expected that the low degree of penetration of therecursor solution at short immersion times would result in �-e2O3 being deposited on or near the outer surface of the AChen the temperature was raised above the critical temperaturef water. At longer immersion times, a high(er) degree of pen-tration was expected to lead to better dispersion of �-Fe2O3articles throughout the AC structure.

Representative SEM micrographs of samples S2 and S5Figs. 2 and 3) confirm this hypothesis. Immersion time wasday in the case of sample S2 and 30 days in the case of S5,ith other variables (such as concentration of precursor, tem-erature, and τ3) the same in both cases. As expected, when1 = 0 day, most of the �-Fe2O3 nanoparticles were deposited

n the surface regions of S2, with few or no particles in thenterior. In contrast, the dispersion of nanoparticles in sample5 (τ1 = 30 days) was found to be uniform throughout the struc-

ure, as shown in Fig. 3. Since τ2 ∼ 75 min in both cases, this

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; (b) region A, ×60,000; (c) region B, ×10,000; (d) region C, ×15,000.

uggests that transport of precursor occurred mostly during themmersion phase and that diffusion of reactants into the centerf the AC was not significant during the heating (reaction) phasef the experiment.

XRD patterns for particles collected from the bulk solu-ion in each experiment exhibited intense iron oxide peaks,onfirming that crystalline �-Fe2O3 particles were obtainedfrom the solution and, therefore, also in the AC) at all immer-ion times (Fig. 4(a)). Fig. 4(b) shows XRD patterns of theround AC before and after impregnation. The intensity of the-Fe2O3 peaks can be seen to increase with immersion time

S5 > S4 > S2), confirming that more particles are deposited inhe AC as the immersion time increases.

6 24.4 3.4 31.0 6.67 35.7 4.8 43.3 9.28 36.1 5.1 42.6 9.49 23.6 3.6 32.2 5.9

138 C. Xu, A.S. Teja / J. of Supercritical Fluids 39 (2006) 135–141

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Fig. 3. Representative SEM micrographs of sample S5: (a) region A, ×

f the particles in the two cases were generally comparable andithin the errors (standard deviations) of the two methods. This

s validated in Fig. 5, which shows TEM images of particlesbtained from the AC and from the bulk solution. Note that allstimates of particle size were less than 100 nm, irrespective ofhe immersion time.

The morphology of the particles changed with immersionime. Most particles were circular and flat at low immersionimes (Fig. 2(b)) and rhombic or hexagonal and flat at highmmersion times (Fig. 3(b)). It is possible that the flat particlesre a result of confinement in the slit pores of the AC.

In summary, long immersion times resulted in uniform dis-ersion of �-Fe2O3 nanoparticles in the AC, whereas shortmmersion time resulted in egg-shell distributions. The num-er of particles deposited increased with increasing immersionimes, but particle size was not affected by immersion time.

.2. Effect of concentration

The precursor concentration was varied between 0.15 and.5 M at immersion times of 5 days (sample S1, S3 and S7)nd 30 days (samples S5 and S8). Fig. 6 and Table 2 show thathe particle size increased with precursor concentration. Repre-entative SEM micrographs of particles deposited in the AC inxperiment S3 are shown in Fig. 7(a)–(d), with precursor con-entration of 0.15 M and immersion time of 5 days. The averagearticle radius was found to be 32 ± 6 nm in these experiments.lso, the particles were well dispersed in the AC as shown in

ig. 7(a)–(c). Fig. 7(e) and (f) show that particles obtained inxperiment S7, in which the precursor concentration was 0.5 Mith other conditions the same, were significantly larger (aver-

ge radius 43 nm). Comparison of Fig. 7(c) and (e) shows that

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00; (b) region A, ×40,000; (c) region B, ×5000; (d) region C, ×5000.

he number of particles deposited increased with increasing pre-ursor concentration. Fig. 7(d) and (f) show that most particlesere circular and flat, although a few were rhombic, hexagonal,r irregular.

.3. Effect of ultrasonic treatment

SEM micrographs of particles deposited from a 0.15 M pre-ursor solution (S9) subjected to ultrasonic treatment for 45 mint room temperature are shown in Fig. 8. Ultrasonic treatmentppears to improve nanoparticle dispersion in the AC, proba-ly by facilitating deeper penetration of precursor solution intohe pores. Ultrasonic treatment had no effect on particle size ashown in Table 2.

.4. Effect of τ3

The constant-temperature interval τ3 was varied from 30 to0 min with the temperature set at 394 ◦C in AC samples impreg-ated with a 0.15 M precursor solution for 30 days. Very littlehange could be observed in the dispersion, number, and size ofarticles as τ3 changed from 30 to 90 min. It is apparent that with1 = 30 days and τ2 ∼ 75 min, particle deposition and dispersionccurs mostly in the time interval τ1 + τ2. When τ > τ1 + τ2 (i.e.3 > 0), growth and secondary changes such as recrystallizationnd aggregation occur, but at an apparently negligible rate after3 = 30 min.

.5. Deposition mechanism

As discussed above, penetration of the carbon structure byn aqueous precursor solution at room temperature is gener-

C. Xu, A.S. Teja / J. of Supercritical Fluids 39 (2006) 135–141 139

Fig. 4. XRD patterns of iron oxide nanoparticles obtained from (a) the surfaceand (b) the interior of AC pellets immersed in precursor solution.

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Fig. 5. TEM images of �-Fe2O3 nanoparticles obtained from (a) the in

ig. 6. Effect of precursor concentration on the size of �-Fe2O3 nanoparticles.

lly very slow because of the high surface tension of water andhe hydrophobic character of the support surfaces. As a result,hen the immersion time is low, nanoparticles of �-Fe2O3 areeposited on the outer surface of the AC when the temperaturef the solution is raised to Th (above the critical temperature ofater). There is some diffusion of Fe3+ into the pores becausef the time taken to increase the temperature from Ta to Th.upercritical water aids this diffusion process. Nevertheless, angg-shell type of dispersion is obtained as shown in Fig. 3.

As large immersion times, penetration by the precursor solu-ion also increases until all the large pores are filled with pre-ursor solution. Increase in the temperature to Th now resultsn deposition of �-Fe2O3 nanoparticles throughout the carbontructure and a uniform dispersion of particles is obtained ashown in Fig. 5. Ultrasonic treatment accelerates this process byontinuous formation, growth and implosive collapse of bubblesn the liquid [10].

The precursor concentration has little effect on the disper-ion of particles, but strongly affects the number of particleseposited. As the concentration increases, more particles are

eposited and the particle density increases. Particle growthffects the particle density, and in turn is affected by reactionime.

terior of the AC and (b) from the bulk solution in experiment S3.

140 C. Xu, A.S. Teja / J. of Supercritical Fluids 39 (2006) 135–141

Fig. 7. Representative SEM micrographs of sample S3 from (a) region A, ×5000; (b) region B, ×5000; (c) region C, ×5000; (d) region C, ×40,000; and of sampleS7 from (e) region C, ×5000; (f) region C, ×40,000.

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. Conclusions

Supercritical water synthesis can be employed to obtain well-ispersed nanometer-sized �-Fe2O3 particles in activated car-on. The dispersion of particles in the AC depends mainly onhe time that the AC is immersed in the precursor solution at

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rom (a) region C, ×5000; (b) region C, ×40,000.

oom temperature and the concentration of the precursor. Twoypes of dispersions were obtained: egg-shell dispersions at low

mmersion times and uniform dispersions at high immersionimes. The particle size was mostly influenced by precursor con-entration. Although the catalytic properties of the �-Fe2O3 inC composites were not evaluated, the procedure of employing

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between the distribution of platinum, its dispersion, and activity of cat-alysts prepared by impregnation of activated carbon with chloropla-

C. Xu, A.S. Teja / J. of Superc

upercritical water to deposit metal oxide particles on hydropho-ic pore surfaces offers promise for catalyst preparation withouthe use of toxic or noxious solvents.

cknowledgements

The authors acknowledge NORIT Americas Inc. for provid-ng Norit RX 3-extra commercial activated carbon. They arerateful to Drs. Jaewon Lee and Tongfan Sun for help with somef the experiments, and to Dr. Sankar Nair and the Georgia Techlectron Microscopy Center for use of their equipment.

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