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Catalysis Communications 8 (2007) 2037–2042

Manganese ferrite nanoparticles synthesized through ananocasting route as a highly active Fenton catalyst

Teresa Valdes-Solıs *, Patricia Valle-Vigon, Sonia Alvarez, Gregorio Marban,Antonio B. Fuertes

Instituto Nacional del Carbon (CSIC), Francisco Pintado Fe, 26, 33011 Oviedo, Spain

Received 4 March 2007; received in revised form 27 March 2007; accepted 28 March 2007Available online 10 April 2007

Abstract

Spinel ferrite MnFe2O4 nanoparticles were synthesized by means of a nanocasting technique using a low-cost mesoporous silica gel asa hard template. The magnetic nanoparticles, of <10 nm diameter and with a surface area of around 100 m2/g, were tested as a heter-ogeneous Fenton catalyst for the decomposition of hydrogen peroxide under neutral and basic conditions. This catalyst shows a muchhigher activity than previous heterogeneous catalysts reported in the literature, which is mainly ascribed to its small particle size. Fur-thermore, the magnetic catalyst can be easily separated from the reaction medium by means of an external magnetic field. The effects ofresidual silica and the purity of the catalyst (hematite formation) on catalytic activity have been studied and correlated. The resultsobtained show this catalyst to be a suitable candidate for the removal of pollutants in wastewaters by means of the Fenton heterogeneousreaction.� 2007 Elsevier B.V. All rights reserved.

Keywords: Hydrogen peroxide decomposition; Template method; Manganese ferrite spinel; Nanoparticle; Heterogeneous Fenton reaction

1. Introduction

Heterogeneous catalysts made up of nanoparticles are,for many applications, an attractive alternative to classicalsupported catalysts [1,2]. They exhibit a high catalyticactivity derived from their large external surface area andtheir easy accessibility to active centres (low diffusionalresistance). A field of growing interest for the applicationof nanocatalysts is that of liquid phase catalysis.

Catalytic wet oxidation is a well known method forremoving organic pollutants from wastewaters [3,4]. Atambient temperature, ozone and hydrogen peroxide arepreferred as oxidants because they only produce innocuousO2 and H2O. H2O2 is preferred when treating solutionswith total carbon content in a medium range. MoreoverH2O2 is safer than ozone and more suitable for low biode-

1566-7367/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.catcom.2007.03.030

* Corresponding author. Tel.: +34 985 119 090; fax: +34 985 297 662.E-mail address: tvaldes@incar.csic.es (T. Valdes-Solıs).

gradability products because homolytic scission yields twohydroxyl radicals from H2O2 [5]. The catalytic generationof hydroxyl radicals by iron ions (Fenton mechanism) iswell known and can be basically described by the followingreactions [3]:

Fe2þ þH2O2 ! Fe3þ þOH� þHO� ð1ÞFe3þ þH2O2 ! Fe2þ þHþ þHOO� ð2Þ

However, the homogeneous Fenton process requires stoi-chiometric amounts of Fe2+ and large quantities of acid,usually H2SO4, to produce the optimum pH (pH 3). Afterthe process, the effluent must be neutralized with a base tobe safely discharged. This gives rise to significant amountsof sludge, which represents a serious drawback to the processdue to disposal problems. In order to overcome this obstacle,several heterogeneous Fenton catalysts have recently beendeveloped, including iron-exchanged Nafion membranes,iron modified clays and iron-exchanged zeolites, as recentlyreviewed by Pignatello et al. [3]. Additionally, a number of

2038 T. Valdes-Solıs et al. / Catalysis Communications 8 (2007) 2037–2042

iron oxides such as c-FeOOH [6], Fe2O3/Fe2Si4O10(OH)2 [7],or transition metal ferrites MxFe3�xO4 (M = Cu, Mn, Coand Ni) [8–11] have been investigated. However, the hetero-geneous Fenton reaction is frequently too slow to be used forthe treatment of wastewater. Therefore, most of the hetero-geneous Fenton catalysts developed until now have relied onUV irradiation to accelerate the reaction. This implies theneed for specific equipment at an additional cost andrequires that the whole catalyst be accessible to light. Asindicated by Hyeon et al. [12] the development of novel cat-alysts with a greater surface area could facilitate the Fentonreaction, thus avoiding the need for UV irradiation to accel-erate the process. The nanocasting technique has recentlybeen proved to be an attractive alternative for synthesizingnanosized inorganic materials [13,14]. This synthetic strat-egy applied to the preparation of nanoparticles involvesthe use of porous solids (mainly silica or carbon) as sacrificialtemplates [15–18]. The pores of the template act as nanoreac-tors where synthesis reactions take place in the desired prod-ucts. Under these conditions, the growth of the solids formedis restricted, resulting in nanosized materials with a high sur-face area once the template framework is removed. In partic-ular, the use of mesoporous silica gel as template for thesynthesis of metal oxide nanoparticles is of great interestfor its availability, low cost and inertness. Recently wereported the preparation of various nanosized ferrites witha size <10 nm using a commercial silica gel as template [19].

The main purpose of the present work is to analyse thecatalytic decomposition of H2O2 in liquid phase using man-ganese ferrite (MnFe2O4) nanoparticles prepared by thenanocasting technique.

2. Experimental

2.1. Preparation of materials

Nanosized MnFe2O4 was prepared following the proce-dure developed in our laboratory [14,16] and alreadyapplied in the synthesis of magnetic nanoferrites [19].Briefly, stoichiometric amounts of hydrated metal nitrates(Mn/Fe atomic ratio of 0.5) were dissolved in ethanol. Asilica gel purchased from Aldrich (Ref. 28,8500) wasemployed as hard template. This material was impregnatedwith the nitrate solution until incipient wetness. Theimpregnated sample was dried at 100 �C and the impregna-tion–drying cycle was repeated until a theoretical molar Si/(Mn + Fe) = 4 molar relationship was attained. Subse-quently, this material was treated under a N2 atmosphereat 800 �C (5 �C/min) for 4 h. Spinel ferrite nanoparticleswere obtained after dissolution of the silica framework ina 2 M NaOH solution (two steps of �20 h).

2.2. Characterization of materials

Nitrogen adsorption isotherms were obtained at�196 �C on a Micromeritics ASAP 2010 volumetricadsorption system. The BET surface area was evaluated

from the isotherm analysis in the relative pressure rangeof 0.04–0.20. The silica contents of the samples werestudied by means of a scanning electron microscope(DSM 942, Zeiss) and an attached energy dispersiveX-ray detector (EDX). Transmission electron microscopy(TEM) images were obtained in a JEOL-2000 FXII.X-ray diffraction (XRD) patterns were recorded over awide-angle range (2h = 20–70�) on a Siemens D5000instrument operating at 40 kV and 20 mA and usingCu Ka radiation (k = 0.15406 nm). Crystal size valueswere estimated from the XRD patterns by using Scher-rer’s equation.

2.3. Catalytic activity measurements

Hydrogen peroxide decomposition experiments wereperformed in a flask subjected to orbital stirring. The cata-lyst (20 mg) was added to 25 mL of 0.2 M H2O2. Thedecomposition of H2O2 was analysed at two different pHvalues: (a) at a pH �13 (KOH 2 M) to minimise the self-decomposition of the hydrogen peroxide [9] and (b) at apH �6. The decomposition reaction was followed by mea-suring the formation of gaseous oxygen in a volumetricsystem.

3. Results and discussion

3.1. Structural characteristics of the catalyst

The procedure employed to obtain the nanosizedMnFe2O4 catalyst involves three simple steps: (1) fillingthe porosity of a silica gel with the inorganic precursor,(2) conversion of the infiltrated compound to inorganicnanoparticles through thermal treatments (nanocomposite)and (3) dissolution of the silica framework with an etchingagent (NaOH).

Evidence of the formation of the manganese ferrite spi-nel is obtained by X-ray diffraction analysis. The XRD pat-tern shown in Fig. 1a reveals that the diffraction peaks arelocated in the position expected for MnFe2O4 (JCPDS 10-319), although several peaks corresponding to a-Fe2O3

were also detected (JCPDS 33-664). The size of theMnFe2O4 crystallites as deduced by applying the Scherrerequation to the main diffraction peaks are in the 7–9 nmrange. In Fig. 1a the XRD pattern for the MnFe2O4/SiO2 composite (prior to the dissolution of silica withNaOH) is also presented. The particle size calculated forthe MnFe2O4 nanoparticles embedded in the silica matrixis similar to that deduced for the non-confined nanoparti-cles (7–9 nm).

Representative TEM images obtained for the ferritenanoparticles and silica–ferrite nanocomposites are shownin Fig. 2a and b respectively. The MnFe2O4 sample consistsof nanoparticles of approximately 10 nm, which aggregateto form large clusters (see Fig. 2a). The size of these nano-particles as deduced by TEM inspection is in agreementwith that calculated by XRD analysis. The TEM image

10 30 50 70 90

u.a ,ytisnetnI.

(220) MnFe2O4

nanoparticles

MnFe2O4/SiO2

nanocomposite

JCPDS: 33-664 ( -Fe2O3)

JCPDS: 10-319 (MnFe 2O4)

(104)

(311)

(400)(511)

(440)

(440)

(511)

(220)

0

100

200

300

400

500

0 0.2 0.4 0.6 0.8 1

Relative pressure P/P0

mc ,emulov debrosd

A3

g/P

TS

MnFe2O4/SiO2

SiO2

MnFe2O4 nanoparticles

0.0

0.5

1.0

1.5

2.0

2.5

1 10 100

Pore size (D), nm

mc ,)D(gold/

Vd3

g/

12 nm

a b

Fig. 1. (a) XRD patterns of the synthesized materials. The spectra for MnFe2O4 and a-Fe2O3 from the database are included. (b) Nitrogen sorptionisotherms of silica template, silica–ferrite nanocomposite and MnFe2O4 nanoparticles.

Fig. 2. (a) TEM image of the MnFe2O4 nanoparticles. (b) TEM image of the MnFe2O4/SiO2 nanocomposite (the dark spots correspond to areascontaining MnFe2O4 nanoparticles).

T. Valdes-Solıs et al. / Catalysis Communications 8 (2007) 2037–2042 2039

obtained for the MnFe2O4/SiO2 composite (Fig. 2b) showsthat the MnFe2O4 (dark points in Fig. 2b) are randomlydispersed along the silica matrix, which exhibits a largeand open porosity.

Fig. 1b shows the nitrogen sorption isotherm and thepore size distribution (Fig. 1b, inset) obtained for theMnFe2O4 nanoparticles, MnFe2O4/SiO2 nanocompositesand silica template. The silica used as template has aBET surface area of 340 m2/g, a pore volume of0.89 cm3/g and a porosity resulting from mesopores ofapproximately 12 nm (Fig. 1b, inset). The compositeMnFe2O4/SiO2 still retains a large porosity with a BET sur-face area of 222 m2/g and a pore volume of 0.52 cm3/g,which consists of mesopores with a size similar to that ofthe template. This result demonstrates that the size of poresin the composite is unaffected by the deposited nanoparti-cles and, consequently, it has a porosity that is widely

accessible to reactants. This is coherent with the resultsdeduced from TEM inspection (Fig. 2b). The BET surfacearea and the pore volume calculated for the MnFe2O4

nanoparticles are 93 m2/g and 0.39 cm3/g, respectively.The nitrogen sorption isotherm of this material (Fig. 1b)does not exhibit a capillary condensation step and showsa large nitrogen uptake at high relative pressures(p/p0 > 0.8). These findings suggest that the material doesnot contain framework-confined pores but is made up ofindividual nanoparticles, which is in accordance with theresults deduced by TEM inspection (Fig. 2a). From thisresult we can estimate the effective size of the nanoparticleson the basis of the values of the BET surface area, whichcan be identified with the external surface area of the nano-particles. The value obtained in this way for the particlesize is �13 nm, which is slightly larger than that deducedfrom TEM inspection and XRD analysis.

2040 T. Valdes-Solıs et al. / Catalysis Communications 8 (2007) 2037–2042

3.2. Catalytic activity

The decomposition of H2O2 by the MnFe2O4 nanopar-ticles was examined at two pH values, 6 and 13. The resultsobtained under these conditions show that catalytic activityis similar at both pHs. Moreover, we also observed thatunder the reaction conditions used here, the leaching ofiron species into the solution does not take place, whichrules out the occurrence of homogeneous Fenton processes.

The ability of MnFe2O4 nanoparticles to decomposehydrogen peroxide is exemplified in Fig. 3 which depictsthe temporal conversion of H2O2. Total decomposition isachieved in around 5 min, regardless of the initial H2O2

concentration (0.2 M or 3 M). For purposes of comparisonthe data reported by other authors, who used ferrites madeup of large particles (SBET = 14–16 m2/g [11], �28 lm[8,9]) are plotted in Fig. 3a. As illustrated, the nanosizedferrites prepared in our study exhibit a much higher cata-lytic activity than the literature catalysts, which clearlyrequire longer times for the decomposition of H2O2 to becompleted.

The reaction rate of H2O2 decomposition was analysedby assuming first order kinetics and defining tX=0.5 as thetime needed to achieve a conversion of 50%. The kineticconstant k was estimated by performing the integrationin the X = 0–0.5 range (Eq. (3)).

dXdt¼ kW catð1� X Þ !

Z 0:5

0

dXð1� X Þ

¼Z tX¼0:5

0

kW catdt! k ¼ 0:693

W cat � tX¼0:5

ð3Þ

We examined the H2O2 decomposition by using nanosizedMnFe2O4 catalysts of slightly different compositions (inresidual silica content and hematite content) and particlesizes as a result of the slight variations in drying and silicaetching times. The values obtained for the rate constant k

for these samples are in the range of 130–1600 g�1 s�1 range.These variations in the rate constant are associated with

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25

Time (min)

)X( noisrevnoc lanoitcar

F

MnFe2O4 (0.2 M H2O2)

Mn0.53Fe2.47O4 (Costa et al., 2006) [11]

MnFe2O4 (3M H2O2)

CuxFe3-xO4 (Onuchukwu, 1990) [8]

a b

Fig. 3. (a) Conversion of H2O2 with time in the presence of nanosized Mnconditions used by Onuchukwu [8] ([H2O2] = 0.2 M, 20 mg, 25 mL, [KOH] = 2conditions reported by Costa et al. [11] ([H2O2] = 3 M, 30 mg and 7 mL). (b)25 mL, [KOH] = 2 M).

three factors: (i) the differences in particle size of the nano-particles (7–9 nm), (ii) the negative effect of the remainingsilica, not completely dissolved by NaOH, and (iii) the pres-ence of small amounts of hematite (a-Fe2O3) in the nanom-aterials [11]. The effect of the residual silica content on thecatalytic activity of MnFe2O4 nanoparticles is illustratedin Fig. 3b. In this figure, a catalyst containing 5.4 wt% of sil-ica was subjected to successive NaOH treatments in order toreduce the silica content of the material to 1.6 wt%, whilekeeping the particle size and the hematite content constant.The reduction in silica content gives rise to a significant de-crease in tX=0.5 from 3.58 min to 1.59 min and consequentlythe rate constant (k) increases from 807 to 1291 g�1 s�1. Theproportion of a-Fe2O3 in the nanocatalyst was related toXRD peak area ratios for a-Fe2O3(104) andMnFe2O4(220). By correlating the experimental data of k

obtained for different catalysts with these parameters we de-duced the following expression for the rate constant:

kcalc ¼ k0ð1� a � rFe2O3� b � dp � c � SiO2Þ ð4Þ

where rFe2O3¼ Areað104Þ=Areað220Þ, dp the particle size

(nm) and SiO2 the silica content (wt%). In Fig. 4 the valuesof kcalc calculated from Eq. (4) are compared with theexperimental values of k. An excellent agreement betweenthe two values is observed. The influence of each parameter(subtracting terms in Eq. (4)) on the calculated kinetic con-stant differs. Thus, dp is the factor that has the strongestinfluence on the value of kcalc (66% of the terms in bracketsin Eq. (4)), while the average effect of the presence of hema-tite is 22% and that of silica content is 12%.

Although it has been stated that the total removal ofSiO2 has a positive effect on the kinetics of the decomposi-tion process, the use of MnFe2O4/SiO2 composites remainsa subject of interest. The use of nanocomposites instead ofnanoparticles affords two advantages: (i) the syntheticmethod is simpler because the silica removal step is avoidedand (ii) the use of an MnFe2O4/SiO2 composite couldreduce the aggregation between nanoparticles that usuallytakes place in magnetic nanomaterials. Recently, Hyeon

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25

Time (min)

)X( noisrevnoc lanoitc ar

F

SiO2 content: 5.4 wt%

SiO2 content: 1.6 wt%

Fe2O4. Conditions reported as 0.2 M corresponds with the experimentalM), experiments reported as 3 M were performed under the experimentalEffect of SiO2 content on H2O2 decomposition ([H2O2] = 0.2 M, 20 mg,

0

500

1000

1500

2000

0 500 1000 1500 2000

kcalc (g-1 s-1)

k (g

-1s

-1)

Fig. 4. Comparison between the experimental (k) and calculated (kcalc) kinetic constants for the decomposition of H2O2 ([H2O2] = 0.2 M, 20 mg, 25 mL,[KOH] = 2 M). The values of kcalc were obtained by means of Eq. (4) (k0 = 5340 g�1 s�1, a = 0.243 g�1 s�1, b = 0.062 g�1 s�1 nm�1, c = 0.021 g�1 s�1).

0.0

0.2

0.4

0.6

0.8

1.0

0 500 1000 1500 2000 2500 3000

Time (min)

)X( noisrevnoc lanoitcar

F

B: MnFe2O4/SiO2 (this work)C: FeAlSi (Hyeon, 2006) [12]

A: MnFe2O4 (this work)

Catalyst Conversion time (min) A 0.98 33 B 0.99 360 C 0.88 2858

Magnet

Fig. 5. Variation of the H2O2 decomposition with time for the MnFe2O4

nanoparticles, MnFe2O4/SiO2 and FeAlSi composite reported by Hyeonet al. [12] (active phase 0.2 g l�1, [H2O2] = 5 mM). Time values to achievethe highest conversion degrees are included in the table. Inset: illustrationof the magnetic separation of nanoparticles from the liquid media.

T. Valdes-Solıs et al. / Catalysis Communications 8 (2007) 2037–2042 2041

et al. [12] reported the preparation of two heterogeneousFenton catalysts consisting of iron oxide–silica (FeSi) andiron oxide–alumina–silica (FeAlSi) nanocomposites andanalysed their activity for H2O2 decomposition (0.005 M).They observed a notable increase in the reaction rate wheniron oxide nanoparticles were synthesized over the aluminacovered silica (FeAlSi) and assumed that this result wasrelated to the small particle size of the catalyst and a highdispersion of hematite in FeAlSi. The catalytic activity ofthe MnFe2O4 nanoparticles and MnFe2O4/SiO2 nanocom-posites prepared in this work was compared with the datareported by Hyeon [12] for the FeAlSi catalyst (Fig. 5). Itcan be seen that, even if the activity of the MnFe2O4/SiO2 nanocomposite (�72 SiO2 wt%) is considerably lowerthan the activity of MnFe2O4 nanoparticles, the MnFe2O4/SiO2 sample exhibits a very high activity compared to thatof the FeAlSi composite prepared by Hyeon et al. [12].

MnFe2O4 nanoparticles and MnFe2O4/SiO2 can easily berecovered from the reaction media by applying an externalmagnetic field. It was experimentally observed that the mate-rials synthesized are rapidly attracted (<1 min) by a conven-tional magnet placed close to the reaction vessel (inset ofFig. 5), demonstrating the efficacy of magnetic separation.

4. Conclusions

In summary, we have demonstrated that the MnFe2O4

nanoparticles obtained through a nanocasting route, usinga silica gel as a sacrificial hard template, serve as a veryactive heterogeneous Fenton catalyst for the decomposi-tion of H2O2. This catalyst exhibits a high performancefor a wide range of pH values (6 and 13) and H2O2 concen-trations (0.005–3 M). The activity measured for thesenanoparticles is clearly superior to that observed for thenanoparticles embedded in a porous silica matrix. Theresults obtained show that the catalytic activity ofMnFe2O4 nanoparticles is reduced by residual silica andalso by the presence of hematite impurities. A comparisonof the catalytic activity of the materials reported in thepresent work (MnFe2O4 nanoparticles and MnFe2O4/SiO2 nanocomposite) with that of other heterogeneousFenton catalysts reported in the literature shows that ourcatalysts exhibit a superior performance.

Acknowledgements

T.V.-S. thanks the CSIC-ESF for the I3P postdoctoralcontract. The financial support provided by MCyT(MAT2005-00262) and FICYT (IB05-001) is gratefullyacknowledged.

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