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Materials Science and Engineering B 181 (2014) 64– 69

Contents lists available at ScienceDirect

Materials Science and Engineering B

jou rn al h om epa ge: www.elsev ier .com/ locate /mseb

mprovement of the photocatalytic activity of magnetite byn-incorporation

udson W.P. Carvalho1, Peter Hammer, Sandra H. Pulcinelli,elso V. Santilli, Eduardo F. Molina ∗

nstituto de Química, UNESP-Universidade Estadual Paulista, 14800-900 Araraquara, SP, Brazil

r t i c l e i n f o

rticle history:eceived 11 July 2013eceived in revised form 15 October 2013ccepted 13 November 2013vailable online 26 November 2013

a b s t r a c t

Mn-incorporated Fe3O4 photocatalysts were prepared by a simple co-precipitation method. Photocat-alytic discoloration of Methylene Blue (MB) was used to evaluate the performance of these catalysts.The DSC results have shown that the insertion of Mn into Fe3O4 lattice has increased converting Fe3O4

to �-Fe2O3. This is accompanied by a decrease of surface area and of crystallinity, as detected by XRD.The analysis of the chemical environment by XPS has shown that Mn2+ replaces Fe2+ preferentially inthe octahedral sites while Mn3+ replaces Fe3+ of inverse spinel sites. The Mn-incorporated samples were

eywords:agnetite catalystanganese doping

onding structurehotocatalytical activityolorant degradation

significantly more efficient in MB discoloration assisted by UVA irradiation and H2O2. It was also foundthat ascorbic acid prevents H2O2 decomposition, by scavenging preferentially •OOH radicals produced atMn sites. Finally, the results reported here can contribute for a better comprehension of the activity ofcomposite catalysts and the design of efficient systems for discoloration of organic pollutants.

© 2013 Elsevier B.V. All rights reserved.

PS

. Introduction

Industrial, agricultural, and domestic wastes increasingly con-ribute to the contamination of water sources with thousands ofrganic compounds, frequently toxic and non-biodegradable. Oneategory of substances with high environmental impact is organicolorants, produced at a high scale all over the world [1,2]. In gen-ral, dyes used by industries have high stability, most of which areecalcitrant to light, oxidation and aerobic digestion. Bioremedia-ion techniques have been developed to decolorize synthetic dyes,owever these treatments have often limited applicability due to

ow decomposition rates [3,4]. Among advanced oxidation pro-esses, the homogeneous photo-Fenton process is one of the mostfficient routes for the treatment of water polluted with recalci-rant chemicals [5–7]. In this degradation process, which combines

V light and Fenton reagents, H2O2 is converted to hydroxyl radical

n a catalytic cycle with Fe ions acting as catalyst [8,9]. In the lastecades, heterogeneous Fenton systems have been the subject of

∗ Corresponding author. Present address: Universidade de Franca, Av.r. Armando Salles Oliveira 201, 14404-600 Franca, SP, Brazil.

E-mail addresses: hudsonwpc@yahoo.com.br (H.W.P. Carvalho),eter@iq.unesp.br (P. Hammer), sandrap@iq.unesp.br (S.H. Pulcinelli),antilli@iq.unesp.br (C.V. Santilli), efmolina@iq.unesp.br (E.F. Molina).

1 Present address: Karlsruhe Institute of Technology (KIT), Engesserstr., 20, 76131arlsruhe, Germany. Tel.: +55 16 33019772.

921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.mseb.2013.11.008

interest for the treatment of toxic and refractory pollutants in soilsand wastewater due to their unique advantages, such as the longoperation life, simple recovery and easy separation of the catalystafter the reaction [10,11]. Several studies report on the removal ofcontaminants using UV-A radiation in iron/carboxylate oxidationsystems. In this case, oxalic, citric and tartaric acids have been usedin the UV-A/Fe(III) process to improve the removal of contaminantssuch as dyes, herbicides, benzene, etc., through the formation ofiron–carboxylate complexes that improve the generation of oxi-dizing free radicals [12,13].

Another interesting oxidation process is the combination of UV-A radiation and iron, in the form of metallic particles or oxides.Recent studies have shown that the efficiency of oxidizing systemscan be improved by the presence of cations in the iron lattice, suchas Cr, Co, Ni, Cu, Zn, and Ti, substituting Fe at tetrahedral (A) andoctahedral (B) sites in Fe3O4 and thus inducing changes in the cat-alytic properties of the material [14,15]. They have suggested amechanism for Cr-doping of magnetite, according to which •OHradical generation occurs at iron sites of pristine magnetite by theHaber Weiss mechanism. Furthermore, Cr-doping allows also thedirect degradation of methylene blue colorant, thus creating a sec-ond pathway of discoloration [16]. The dopants induce shallowdonor or acceptor levels for effective ionization under illumina-

tion, leading to prolonged carrier diffusion length, thus improvingthe photocatalytic activity [17]. Theoretical modeling has shownthat among the 3d metals, Mn presents the highest potential inpermitting significant optical absorption in the visible or even the

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nfrared spectral range, through the combined effects of narrowinghe optical band gap and introducing intermediate bands within theap [18,19]. Beside the high catalytic efficiency of iron oxides addi-ional aspects, which favor their use as photocatalysts are simplepplicability and large abundance.

Currently, extensive efforts have been devoted to understandhe mechanism over which the photocatalytic decontaminationroceeds, via photon-generated h+, e−, •OH, •O2− or H2O2, andhis controversy still exists. For a better comprehension of the rolef dopants in photocatalytic processes, this work focuses on thenvestigation of the influence of concentration and valence of man-anese in the magnetite lattice on the structural features and thusn the photocatalytic activity, investigated by methylene blue (MB)ecomposition in the presence of hydrogen peroxide and UV-A

rradiation.

. Experimental

.1. Synthesis of Fe3O4–Mn

Magnetite and Fe3O4–Mn powders were prepared by the chem-cal co-precipitation method. Typically, 30 mL of FeCl3•6H2O acidolution (HCl 2.0 M) were mixed with 20 mL of Na2SO3 (1 M), andhe resulting solution was added to aqueous solution of 50 mL ofH4OH (1 M). The precipitated specie was recovered with a mag-et and washed with distillate water. Then it was dried for 12 ht 60 ◦C in air. The manganese was incorporated by mixing MnCl2nd FeCl3•6H2O acidic solution. The 1% and 10% specimen, referredo as Mn-1% and Mn-10%, corresponds to molar Fe to Mn ratios,mployed during synthesis.

.2. Characterization

Nitrogen adsorption isotherms were recorded at liquid nitro-en temperature in a static volumetric apparatus supplied byicromeritics (ASAP 2010). Samples (0.2 g) were evacuated prior

o measurements at 130 ◦C for 24 h under vacuum of about 10 �Pa.he specific surface area was calculated following the BET method20].

The X-ray diffraction (XRD) measurements were performedt room temperature with a Siemens D5000 diffractometer,sing CuK-� radiation (� = 1.542 A) selected by a curved graphiteonochromator, over the 2� range between 25◦ and 65◦ at a reso-

ution of 0.01◦.Differential scanning calorimetry, DSC, measurements were car-

ied out using a TA Instrument model Q100. Powder samples ofpproximately 20 mg were placed into closed aluminum cruciblesnd heated from 25 ◦C to 400 ◦C at a rate of 10 ◦C min−1. The purgeas was high purity nitrogen supplied at a flow rate of 75 sccm.

The XPS analysis was carried out at a pressure of less than0−7 Pa using a commercial spectrometer (UNI-SPECS UHV). Theg K� line was used (h� = 1253.6 eV) and the analyzer pass energy

or the recording of high-resolution spectra was set to 10 eV. Thenelastic background of the Mn 2p, Fe 2p, O 1s and C 1s elec-ron core-level spectra was subtracted using Shirley’s method. Theomposition of the near surface region was determined with anccuracy of ±10% from the ratio of the relative peak areas correctedy Scofield’s sensitivity factors of the corresponding elements. Thepectra were fitted without placing constraints using multiple Voigtrofiles. The width at half maximum (FWHM) varied between 1.2nd 2.1 eV and the accuracy of the peak positions was ±0.1 eV.

.3. Photocatalytic degradation

Methylene blue (MB, Aldrich) was employed as a model dye tovaluate the photocatalytic activity of the synthesized powders.

nd Engineering B 181 (2014) 64– 69 65

For each condition, 5 mg of the catalyst was dispersed in 50 mL ofthe 50 mg L−1 MB aqueous solution, containing 1 mL H2O2 (30%,v/v). The 100 mL beaker containing the catalyst and solution wasplaced on a magnetic stirrer plate to ensure a full dispersion ofthe particles during the experiment. A magnetic stirrer (200 RPM)was used for all photocatalytic essays. The photocatalytic reactionwas conducted at room temperature under UVA light from a 20 WPhilips lamp positioned horizontally above the liquid surface. Thedistance between the lamp and the base of the beaker was 15 cmwith 3 cm depth of the liquid. The chosen radiation (330–410 nm) iscompatible with the optical band gap of magnetite reported in therange of 2.9–5.0 eV for indirect and direct transitions, respectively[21]. Each experiment was conducted for 5 h with 2 mL samplealiquots. At given time intervals, to avoid the influence of parti-cles over the absorption measurements, samples were taken andplaced in a tube to centrifuge, ensuring only the solution of dyefor analysis. For all photocatalytic experiments, the concentrationdecay of MB in the solutions was determined via ultraviolet–visible(UV–vis) spectroscopy, using a Varian, Cary 5000 spectrometer, andthe absorption was determined at a wavelength of 665 nm, corre-sponding to the maximum absorbance of MB. De-ionized water wasused as reference in all experiments.

The H2O2 decomposition was monitored by O2 gas formation. Ina round-bottom flask were added 20 mL of deionized water, 5 mgof catalysts powders and 1 mL and H2O2 (30%, v/v). The quantity offormed O2 was determined by the shift of coupled water column.The essays were also carried out in presence of ascorbic acid (Asc). Inthis case, instead of water, 20 mL of 50 mg L−1 ascorbic acid aqueoussolution was used. All photocatalytic experiments were carried outin duplicate. The relative deviations are in the range of ±2.0%.

3. Results and discussion

3.1. Structural characteristics

The synthesis pathway led to fine black powders for both pris-tine Fe3O4 and Fe3O4–Mn samples. The surface area, determined byBET, decreased as function of Mn content. Pristine Fe3O4 presenteda surface area of 172 m2 g−1, while for the Mn-1% and Mn-10%incorporated samples 159 m2 g−1 and 143 m2 g−1 were detected,respectively. For comparison, the reported surface area for sim-ilar nanoparticles systems varied between 50 and 300 m2 g−1

[15,22,23].Fig. 1 displays the XRD patterns of pristine Fe3O4 and of

Fe3O4–Mn samples containing 1% and 10% of Mn. The presence ofthe NaCl peak is due to a small salt impurity in the samples. Allsamples patterns are characteristic of magnetite (JCPDS 1-1111)crystalline phase. It is interesting to note that with increasingdoping level the peak intensity decreases and the width increases.This effect is related to the isomorphic substitution of Fe ions by Mn,leading to a reduction of the crystallinity of these samples. Similarresults for the reduction of crystallinity with increasing Mn con-tent were reported by Varshey and Yogi [14]. These authors haveidentified only one crystalline phase, which they have assignedto successively isomorphic substitution of Fe2+ by Mn2+, for allFe3−xMnxO4 samples.

The substitutional incorporated effect and the associated phasetransition are supported by differential scanning calorimetry (DSC)results, presented in Fig. 2. The weak endothermic peak at about130 ◦C, observed for all the samples, is related to water loss. Theintense exothermic signals, which shift to higher temperatures

with Mn content, were attributed to the phase transition of Fe3O4 to�-Fe2O3. The heat release during this exothermic event was −4.8,−7.0 and −5.8 J g−1, for pure Fe3O4, Mn-1% and Mn-10% sample,respectively. The transition temperature of magnetite of 267 ◦C

66 H.W.P. Carvalho et al. / Materials Science and Engineering B 181 (2014) 64– 69

Fig. 1. X-ray power diffraction patterns of the Fe3O4 reference and of Mn-1% andMn-10% incorporated samples.

Fig. 2. DSC thermogram of pristine Fe3O4 reference and of Mn-1% and Mn-10%incorporated samples.

Table 1Composition of the near surface region of the magnetite reference and of Mn-1% andMn-10% incorporated samples.

Sample Fe Mn O

Concentration (at.%)*

Fe3O4 35.5 – 64.5Fe3O4 1% Mn 35.0 0.4 64.7

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Fe3O4 10% Mn 33.6 2.0 64.4

* Experimental error: ±10%.

ncreased to 290 ◦C for Mn-1% and then further to 333 ◦C for then-10% sample. The conversion temperature of pristine magnetite

s in good agreement with the value of 266 ◦C, reported by Sandersnd Gallagher [24]. The retardation of the oxidation temperature ofe3O4–Mn samples is a complementary evidence of the isomorphicissolution of Mn atoms in the magnetite lattice.

Elemental composition of the near surface region of the cata-

ysts was obtained by XPS without sputter cleaning (Table 1). Theesults obtained for the Mn-10% sample showed slightly loweranganese content than expected. As the signal of Mn-1% was

Fig. 3. (a) XPS Fe 2p3/2 spectra of the pristine reference and (b) of the Mn-10%incorporated sample.

found close to the detection limit, a higher experimental error thanthe calculated value of ±10% should be considered in this case.With a estimated precision of about ±20% the concentration ofthe Mn-1% sample in the active surface region of the catalyst was0.3 ± 0.06 at.%. Magnetite has an inverse spinel structure in whichoxygen atoms form a face-centered-cubic lattice with one thirdof the Fe+2 cations occupying octahedral interlattice (B-sites), onethird (Fe+3) the tetrahedral interlattice (A-sites) and the last onethird (Fe+3) the octahedral interlattice (B-sites) [25,26]. Thus thedifferent coordination and valences of the Fe ions results in dif-ferent chemical shifts of the corresponding spectral components.Due to the equal abundance of each of these local environments,the Fe 2p3/2 high-resolution spectra of pristine can be fitted with-out placing constrains by three sub-peaks of equal intensity and ashake-up component, which accounts for the high energy tail, asshown in Fig. 3(a). The components were assigned to Fe2+ B-sites,Fe3+ A-sites and Fe3+ B-sites located at a binding energy of 709.8 eV,710.5 eV and 711.5 eV, respectively. Using the fitting parametersof pristine as input for the Mn-10% sample, the results indicate adecrease of the Fe2+ B-sites (∼3.1%), a slight reduction of Fe3+ A-sites (∼1.9%) and an increase of Fe3+ B-sites of about 4.4%, as canbe observed in Fig. 3(b). Considering the atomic concentrations ofincorporated Mn, these peak area changes indicate the substitu-tion of Fe2+ by Mn2+ ions at ocatahedral B sites and of Fe3+ by Mn3+

at ocatahedral B sites. This is supported by the deconvoluted Mn2p3/2 spectra, shown in Fig. 4, which indicates that Mn enters themagnetite lattice in two forms, as Mn2+ occupying octahedral sitesand as Mn3+ occupying tetrahedral sites. This is compatible withthe effective ionic radii of Mn and Fe reported by Shannon [26].Despite the noisy signal the fitted spectrum suggests a higher frac-

tion of Mn2+ sites in the Mn-1% sample than in that of Mn-10%. It isexpected that Mn at low content tends to form Mn2+ rather thanMn3+ sites because Mn2+ is the most favored oxidation state.

H.W.P. Carvalho et al. / Materials Science and Engineering B 181 (2014) 64– 69 67

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Fig. 5. Influence of Mn incorporation level of magnetite on the photocatalytic dis-

ig. 4. XPS Mn 2p3/2 spectra of (a) Mn-1% and (b) Mn-10% incorporated samples.

.2. Photocatalytic activity

Fig. 5(a) shows the photocatalytic discoloration essays in pres-nce of hydrogen peroxide and UV-A irradiation. It shows thenfluence of structural changes induced by Mn doping of magnetiten the discoloration of MB dye. In the absence of the catalyst only

low MB photolysis was detected, leading to less than 10% dis-oloration after 5 h of reaction. A much higher efficiency of MBecomposition was observed after magnetite addition and evenigher for the Mn-incorporated catalysts. After 5 h of essays aiscoloration of 85%, 96% and 87% (relative deviations of ±2.0%)as detected for the Fe3O4, Mn-1% and Mn-10% containing solu-

ions, respectively. The highest activity has been observed for anntermediate Mn content, revealing the active role of Mn in thehotocatalytic process. On the other hand, at higher Mn doping

evels, the observed structural changes might cause a reductionf the catalytic efficiency of the material. In fact, Deng et al. [17]ave proposed for their Mn modified TiO2 material that a largeumber of defects could be induced at high doping levels, whichay serve as recombination centers reducing the photoactivity.nother interesting finding is that although the Mn-10% sampleresents the lowest surface area, it was more efficient in the dis-oloration process than pristine. This finding confirms that decisiveor the photocatalytic activity is the number of active centers andot necessarily the surface area.

The photocatalytic activity of the materials in the dark was alsovaluated from the MB discoloration curves. All essays performedn the dark, were less efficient compared to those in presence

f UV irradiation (Fig. 5(b)). However, also in this condition Mn-ncorporated samples showed the highest efficiency compared toristine Fe3O4, revealing the catalytic effect of Mn sites. After

h the discoloration levels were 19%, 34% and 31% for Fe3O4,

coloration of methylene blue in presence of H2O2 (a) irradiated by UV-A and (b) inthe dark.

Mn-1% and Mn-10%, respectively, values significantly lower thatthose obtained under UVA irradiation. This confirmed the impor-tant role of irradiation for the decomposition of organic pollutantsin the Fe3O4/Mn catalyst system. In this respect, Rodrigues et al.[27] reported for the Fe3O4/H2O2 system the formation of hydroxylradicals after UV-A irradiation. The authors suggest that upon irra-diation of magnetite the reduction process in the conduction bandof Fe(III) to Fe(II) leads to hydroxyl radical formation by reactionwith hydrogen peroxide through the heterogeneous Fenton pro-cess. The obtained results confirmed that the activity improvementby incorporation of Mn ions in the magnetite lattice is more efficientfor the photocatalytic process than the increase of the surface area.Furthermore, as shown in the XPS Mn 2p spectra of the Mn-1% sam-ple (Fig. 4), the higher Mn2+ fraction on the catalyst surface mighthave contributed to the improved photocatalytic performance ofthe material.

The improvement of Fe3O4 photocatalytic activity in presenceof H2O2 by Mn doping was also reported by Nguen et al. [28]. Theseauthors prepared Fe3O4 and Fe2MnO4 supported on activated car-bon and observed with higher pH an increasing photoactivity. Theyfound that the Mn-incorporated sample was more efficient thanpristine one, however poor information was provided about thecatalytic mechanism and structural features of Fe2MnO4.

In the presented system (catalyst + UVA + H2O2) there are sev-

eral ways to create radical species, which are able to decomposeMB. The H2O2 can either form OH radicals through the photolysisprocess (1), or can also react with Fe3O4 by Fenton reaction (2)–(4),

68 H.W.P. Carvalho et al. / Materials Science a

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ig. 6. Influence of Mn incorporation level of magnetite on the H2O2 degradationate in presence and absence of ascorbic acid.

r radical species can be generated by the Photo-Fenton reactionEq. (5)), as shown below [29].

� + H2O2 � •OH + •OH (1)

e2+ + H2O2 → •OH + OH− + Fe3+ (2)

e3+ + H2O2 � FeOOH2+ + H+ (3)

eOOH2+ → Fe2+ + •OOH (4)

e(OH)2+ + h� → Fe2+ + •OH (5)

oreover, it is expected that adsorption, photolysis and photo-atalytic processes occur simultaneously during the discolorationeaction. For a better understanding of the role of Mn ions in thehotocatalytic reaction, we have investigated the H2O2 degrada-ion process. Fig. 6 shows the H2O2 decomposition in two systems:i) catalyst + H2O2 and (ii) catalyst + H2O2 + ascorbic acid. In bothystems, the H2O2 decomposition rate is directly proportional tohe Mn concentration in the sample. Although the mechanisms of2O2 decomposition by oxides is not very well understood, Lin andurol [30] have proposed a mechanism for H2O2 degradation on thenO surface. In the first step H2O2 should interact with the Mn–O

ite by hydrogen bonding (1), then it decomposes forming theydroperoxide radical (2), which in the last step (3) evolves to O2(g).he reaction rate of H2O2 decomposition over goethite (FeO(OH)),ound by Wang et al. [31], was 0.0016 s−1 M−1; over hematite (�-e2O3) this constant was determined to be 0.0030 s−1 M−1, while

value of 0.0036 s−1 M−1 was reported for MnO by Do et al. [32].espite of the small difference between these rate constants, thebserved increase of the H2O2 decomposition rate with increasingn doping level indicates that Mn ion sites have higher affinity to2O2, thus being more efficient for its decomposition.

2O2(aq) + [Mn–O]OH(s) → [Mn–O]OH–H2O2(s) (1)

Mn–O]OH–H2O2(s) � [Mn–O](s) + •OOH(aq) + H2O(s) (2)

Mn–O](s) + •OOH(aq) → [Mn–O]H(aq) + O2(g)↑ (3)

omparing the different catalysts both in the presence and absence

f ascorbic acid, Fig. 6 shows that ascorbic acid reduces the H2O2ecomposition rate, an effect most clearly observed for the sam-le containing the highest Mn content. It is evident that at leastwo kinds of radicals, •OH and •OOH, are present in the reaction

[

nd Engineering B 181 (2014) 64– 69

medium. According to the discussed mechanisms of H2O2 decom-position, Fe2+ sites form preferentially •OH, while Fe3+ and Mn sitescontribute to •OOH formation. The ascorbic acid is able to scavengepreferentially the •OOH radical, and thus hinders the O2 forma-tion. Consequently, as the amount of •OOH radical increases withMn content, a stronger reduction of O2 formation in the presenceof ascorbic acid is observed. These results reinforce the proposedreaction pathway in which H2O2 is preferentially decomposed overMn sites. Thus the lower catalytic efficiency of the Mo-10% catalystmight be related to a larger extend to the lower surface area of thissample than the structural changes induced by excessive doping.From these results, it can be concluded that the main radical respon-sible for the discoloration of MB should be •OOH, a hypothesis thatwill be investigated in future experiments by mass spectroscopy.For a complete comprehension of the active role of the dopant inthe host matrix and its effect in the photocatalytic process a futurestudy will provide additional data for a number of different Mnconcentrations and photocatalytic process parameters.

4. Conclusions

Manganese incorporated Fe3O4 catalysts were synthesized bysimple co-precipitation method. Structural investigations haveshown that Mn ions were substitutionally incorporated in the mag-netite lattice. It was found that the degree of Fe substitution affectstextural properties of the catalyst, as reveled by the decrease of thesurface area and crystallinity. The effect of the isomorphic substi-tution was also evidenced by the shift in oxidation temperature,verified by DSC. XPS results have confirmed the incorporation ofexpected quantities of Mn, which are compatible with the degreeof isomorphic substitution, observed in the Fe 2p3/2 spectra. Theresults suggest two kinds of chemical environment of manganese,Mn2+ replacing Fe2+ in the octahedral geometry and Mn3+ replacingFe3+ in the tetrahedral sites. Photocatalytic experiments, performedby discoloration of MB under UV-A irradiation, have shown a supe-rior performance of Mn-incorporated Fe3O4 compared to pureFe3O4. Although to a less extent, this is also valid in the absenceof light. The enhanced photocatalytic activity of the Mn-1% sam-ple can be attributed mainly to the higher reaction rate principallyof Mn2+ sites but also to its higher surface area. In addition, H2O2degradation experiments have shown that ascorbic acid scavengespreferentially •OOH radicals formed at Mn sites rather than •OHformed at Fe sites. This hints on •OOH as the principal radicalresponsible for the MB degradation.

Acknowledgements

This work has been supported by CAPES, CNPq and FAPESP.

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