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Microporous and Mesoporous Materials 225 (2016) 303e311

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate/micromeso

Preparation and characterization of bimetallic FeeCu allophanenanoclays and their activity in the phenol oxidation by heterogeneouselectro-Fenton reaction

E.G. Garrido-Ramírez a, b, *, J.F. Marco c, N. Escalona d, e, M.S. Ureta-Za~nartu f

a Departamento de Ecología y Biodiversidad, Universidad Andres Bello, República 440, Santiago, Chileb Sustainability Research Centre, Universidad Andres Bello, República 440, Santiago, Chilec Instituto de Química Física “Rocasolano”, CSIC, C. Serrano 119, 28006 Madrid, Spaind Departamento de Ingeniería Química y Bioprocesos, Escuela de Ingeniería, Pontificia Universidad Cat�olica de Chile, Av. Vicu~na Mackenna 4860, Macul,Santiago, Chilee Facultad de Química, Pontificia Universidad Cat�olica de Chile, Chilef Departamento de Ciencias del Ambiente, Facultad de Química y Biología, Universidad de Santiago de Chile, Av. Libertador Bernardo O�Higgins 3363,Santiago, Chile

a r t i c l e i n f o

Article history:Received 20 October 2015Received in revised form3 January 2016Accepted 8 January 2016Available online 18 January 2016

Keywords:Heterogeneous electro-Fenton reactionBimetallic (FeeCu) allophane nanoclaysPhenol oxidation

* Corresponding author. Departamento de EcologíaAndres Bello, República 440, Santiago, Chile. Tel.: þ56

E-mail address: elizabeth.garrido@unab.cl (E.G. Ga

http://dx.doi.org/10.1016/j.micromeso.2016.01.0131387-1811/© 2016 Elsevier Inc. All rights reserved.

a b s t r a c t

Bimetallic (FeeCu) allophane nanoclays were synthesized using a two-step wet impregnation methodwith different Fe/Cu ratios. The catalytic activities of bimetallic (FeeCu) allophane were studied forphenol oxidation by heterogeneous electro-Fenton reaction (HEF) at different initial pHs (3.0 and 5.5),and were compared with Fe-allophane and Cu-allophane catalysts. A glassy carbon electrode modifiedwith the bimetallic allophane nanoclays was used as working electrode. FTIR, SEM, X-ray diffraction, XPS,M€ossbauer spectroscopy and N2 adsorptionedesorption were used to characterize the catalysts, andindicated the formation of small copper oxide particles stabilized by iron oxide species. Phenol con-version by HEF process at initial pH 3.0 was near 100% for all bimetallic (FeeCu) allophane nanoclays inless than 2 h of reaction, following an exponential decay. The chemical oxygen demand (COD) removalwas less than 47% for Cu-allophane and 65% for Fe-allophane, whereas for the bimetallic (FeeCu) allo-phane nanoclays the COD removal decreased with the amount of copper oxide in the catalyst, achievingan 80% COD removal with Fe5.4Cu0.6 catalyst. These results showed the synergetic effect between the Fe3þ

and Cu2þ ions present in the bimetallic (FeeCu) allophane nanoclays. Similarly, when the reactions wereperformed at initial pH 5.5 the phenol conversion was near 100% after 4 h for Fe-allophane and bimetallic(FeeCu) allophane with lower copper content. In the bimetallic (FeeCu) allophane clays the leaching ofiron and copper into the solution was less than 1.25 mg/L and 0.638 mg/L, respectively, indicative of thehigh stability of the bimetallic (FeeCu) allophane catalysts.

© 2016 Elsevier Inc. All rights reserved.

1. Introduction

Heterogeneous Fenton reaction and electro-Fenton process havebeen shown to be interesting alternatives for the degrading ofrecalcitrant organic pollutants from wastewater and industrial ef-fluents. Heterogeneous Fenton reaction (HFR) involves the use of asolid catalyst, in which, iron or copper species are “immobilized” in

y Biodiversidad, Universidad226615809.

rrido-Ramírez).

the structure of the catalyst. Thus, the reaction can be performed ina wide pH range, avoiding the iron hydroxide precipitation [1]. Onthe other side, the electro-Fenton process is based on the contin-uous electro-generation of hydrogen peroxide by oxygen reductionon the cathode whereas the regeneration rate of Fe2þ is increasedby a direct cathodic reaction [2].

By combining the efficiency of electro-Fenton with the advan-tages of heterogeneous catalysts, a new process has been devel-oped, called heterogeneous electro-Fenton (HEF). In this process, ahigher catalytic efficiency is expected, while the reaction can beperformed in a wide range of pH, with low concentration of solubleiron and therefore, with minimal oxyhydroxide precipitation [3,4].

E.G. Garrido-Ramírez et al. / Microporous and Mesoporous Materials 225 (2016) 303e311304

In a previous work [4] the atrazine oxidation by heterogeneousFenton and heterogeneous electro-Fenton processes using ironoxide-supported allophane clay (AlSi2Fe6) as catalyst was studied.The reactions were performed at different initial pHs. In the case ofHEF, AlSi2Fe6 was incorporated on glassy carbon electrodes. Theresults showed that both processes were efficient in oxidizingatrazine under acidic conditions (pH ~3). Besides, it was possible tocarry out the reaction at more neutral pH (~6), achieving 68%atrazine oxidation in HFR after 48 h and 76% in HEF after 8 h. In thiswork it was concluded that HEF is more efficient than HFR, mainlydue to the continuous electro-regeneration of iron species (Fe3þ/Fe2þ). However, incomplete atrazine mineralization was observedwith desethyl-atrazine and desethyl-deisopropyl-atrazine reportedas the main degradation products.

Therefore, some efforts can be made in order to improve thecatalytic efficiency of the heterogeneous electro-Fenton to obtain acomplete mineralization of recalcitrant organic pollutants. In thisway, it has been suggested that the performance of iron catalysts isoften improved by adding certain metals, such as Cu2þ species,which can contribute to the production of additional hydroxylradicals according to Fenton-like reactions [5e7]. Recently the useof ironecopper bimetallic catalyst has attracted great attention[6e12]. For instance, Guimaraez et al. [7] showed higher hydrogenperoxide decomposition rate over surface containing Cu2þ in thegoethite structure when compared to the surface containing onlyFe3þ species. Frank et al. [8] reported that ironecopper bimetalliccatalyst supported on MCM-41 had the highest catalytic activitycompared with the monometallic catalysts (FeM41 and CuM41). Inaddition, this catalyst was able tomaintain its high catalytic activityafter 10 consecutive runs and operate in a wide pH range. Han et al.[6] reported that CueFe-bimetal amidoximated-polyacrylonitrile(PAN) fiber complexes were better catalysts for aqueous dyedegradation in water than the respective Fe complexes. Similarresults were reported for CuFe-mesoporous carbon [9], CuFeZSM-5zeolite [10,11] and FeeCu bimetallic oxide-supported aluminium-containing MCM-41 [12].

The synergetic effects could also be a consequence of highlydispersed Fe and Cu oxides entrapped in the matrix of orderedmesoporous structure, increasing the numbers of active sitesleading to efficient catalytic activity for the activation of H2O2 [9].

Therefore, the aim of this work is to improve wastewatertreatment based on the heterogeneous electro-Fenton process, inorder to achieve an effective degradation of recalcitrant organicpollutants using bimetallic (FeeCu) allophane nanoclays asFenton reagent. Allophane is a nanoclay size mineral, which isabundant in volcanic ash soils and is the most importantconstituent of clays-fraction of Chilean southern soils (mostlyAndisols). Due to the allophane properties, such as, large surfacearea, unique structure and high porosity, they have been proposedas highly adsorbing compounds [13], enzyme support [14] andsolid catalysts [1]. Iron oxide-supported allophane clays with ahydrous feldspathoids structure were reported to be highly activeand stable in the phenol oxidation by catalytic wet peroxideoxidation [15].

For that purpose, we have synthesized for the first time bime-tallic (FeeCu) allophane clays with different Fe/Cu proportionsusing a two-step wet impregnation method. The solid catalystswere characterized by various techniques, such as nitrogenadsorptionedesorption isotherm, scanning electronic microscopy,FTIR, XRD, XPS and 57Fe M€ossbauer spectroscopy. The catalyticperformance of the heterogeneous electro-Fenton process wasstudied in the phenol oxidation. Phenol has been used as a model ofrecalcitrant chemical compound because it is one of the mostcommon organics in wastewater discharged from petrochemical,chemical and pharmaceutical industries [16].

2. Materials and methods

2.1. Chemicals

All chemicals were used as received. Phenol and Nafion® per-fluorinated ion-exchange resin were purchased from Sigma-eAldrich, whereas, Na2SO4, H2SO4 (analytical grade) and methanol(HPLC grade) were supplied by Merck. Graphite powder, Riedel-deHa€en, was washed in dilute nitric acid, filtered, washed thoroughlywith twice-distilled water until neutral pH was achieved, dried inan oven under argon atmosphere at 120 �C for 4 h, and cooled andstored under nitrogen.

2.2. Catalysis preparation

Allophane nanoclays with a theoretical Si/Al ratio of 3.0 andwith a similar structure as hydrous feldspathoids were synthesizedby co-precipitation method [17]. Part of the allophane nanoclayswere impregnated with 6 wt.% iron oxide (Fe-allophane) and theother with 6 wt.% copper oxide (Cu-allophane) using a wetimpregnation method, as previously reported [15]. The bimetallicFeeCu allophane nanoclays were prepared by a two-step wetimpregnation method. In the first step, different amounts of ironoxide were deposited into 60 mL of a dispersion containing 1 g ofallophane. The dispersionwas evaporated at 25 �C and at a pH near3.0, in a rotary evaporator, to obtain 1/3 of the initial volume. Thedispersion finally was washed three times with 1 M KCl; after-wards, it waswashedwith distilled water by centrifugation in orderto remove the chloride anions. In the second step, differentamounts of copper oxide were deposited into 60 mL of a dispersioncontaining 1 g Fe-allophane at 25 �C and at a pH near 5.0, followingthe same methodology described above.

2.3. Catalyst characterization

Textural properties of the allophane, Fe-allophane, Cu-allo-phane and bimetallic (FeeCu) allophane were evaluated by N2adsorptionedesorption at 77 K on a Tristar II micromeritics in-strument. Prior to N2 adsorption, the samples were degassed undervacuum at 250 �C for at least 6 h. The specific surface area (SBET) wasdetermined from the linear part of the BET plot [18]. The microporevolume (V0) was calculated with DR (Dubidin Radushkevish)equation and the total pore volume (Vpore) from the isotherm atrelative pressure of 0.99 (P/Po). The mesopore volume (Vm) wascalculated by subtracting the total pore volume and the microporevolume. The BJH method [19] was used to determine the averagepore diameter. Fourier transform infrared spectra (FTIR) wererecorded on a Spectrum Two IR Spectrometer over a wavenumberrange of 400e4000 cm�1. Samples were pressed into KBr discs(0.5 mg in 100 mg KBr). Powder X-ray diffraction (XRD) patternswere obtained in the 2q values range 10�e70� using a BrukerDiffractometer D8 Advance equipped with a CuKa radiation at40 KV, 30 mA and. The surface and morphology of the allophanesamples were examined by transmission electron microscopy(TEM) using a Jeol-1200 EXII instrument operating at 120 kV, aswell as by scanning electron microscopy (SEM) using a Jeol JSM-S410 apparatus. For SEM analysis, the samples were suspended inanalytical grade methanol, sonicated for 5 min before being coatedwith gold-palladium. Images were obtained at 20 keV and amagnification of 5000�. Energy dispersive X-ray spectroscopy(EDX) was also used for Si, Al, Fe and Cu spot detection over thesample surface. The EDX analyses were performed at 20 keV, anangle of 28.8� and a magnification of 500�. XPS datawere recordedusing a Leybold LHS-10 spectrometer withMg Ka radiation, under avacuum lower than 5 � 10�9 Torr and constant pass energy of

Table 1Chemical composition of allophane, Cu-allophane, Fe-allophane and bimetallic(FeeCu) allophane nanoclays determined by energy dispersive X-ray spectroscopy(EDX).

Sample Si/Al ratio Chemical composition (wt.%)

Si Al Fe Cu

Allophane 2.2 69.0 31.0 e e

Cu-allophane 2.9 64.2 20.4 e 7.6Fe3Cu3 2.9 64.6 22.3 6.7 6.4Fe4.5Cu4.5 3.6 73.5 20.7 4.6 1.2Fe5.4Cu0.6 3.5 68.8 19.6 11.2 0.4Fe-allophane 3.2 64.2 20.4 15.4 e

E.G. Garrido-Ramírez et al. / Microporous and Mesoporous Materials 225 (2016) 303e311 305

50 eV. The energy scale was calibrated using the C 1s binding en-ergy of the adventitious carbon contamination layer, which was setat 284.6 eV. The background of the spectrawas subtracted using theShirley method [20] and the data were fitted using mixed Gaus-sianeLorentzian functions. 57Fe M€ossbauer data were recorded at16 K in the transmission mode using a constant accelerationspectrometer, a 57Co (Rh) source and a He closed-cycle cryo-refrigerator. The velocity scale was calibrated using a 6 mm thickiron foil and the isomer shifts are referred to the centroid of thespectrum of metallic iron at room temperature.

Allophane, Fe-allophane, Cu-allophane and bimetallic (FeeCu)allophanewere also characterized by cyclic voltammetry (CV) usingmodified glassy carbon disk electrodes (CHI-electrode). CVs wereobtained with Autolab (Eco Chemie, The Netherlands) electro-chemical workstation using platinum as counter electrode and Ag/AgCl/saturated KCl as reference electrode. The CVs were registeredat 0.1 V s�1 and under nitrogen-saturated solution at pH 5.5, using0.05 M Na2SO4 as support electrolyte.

2.4. Electrodes preparation

Glassy carbon disk (GC) electrodes (32.3 cm2 geometric area,Carbone Lorraine, France and CHI-electrode, 0.071 cm2 area) weremodified by painting the electrode surface with 1.2 mL and 0.05 mLof a dispersion prepared according to Ureta-Za~nartu et al. [21].Briefly, 50 mg of graphite, 0.5 mL of Nafion® perfluorinated ion-exchange resin and 4 mL of an aqueous dispersion containing30 mg of allophane, Cu-allophane, Fe-allophane or bimetallic(FeeCu) allophane were mixed and ultrasonically homogenized for2 h. The solvent was evaporated in air, in a stove at controlledtemperature (60 �C) for 2 h. As a blank electrode, the same GCelectrode was painted with drops of a dispersion prepared as theprecedent but without the addition of solid catalyst.

2.5. Phenol oxidation by heterogeneous electro-Fenton reaction

Phenol oxidation by HEF process was studied using themodifiedglassy carbon as working electrode, platinum wire as counterelectrode and Ag/AgCl/saturated KCl as reference electrode. A singlecompartment electrochemical glass cell was employed using atypical three-electrode arrangement in order to hold both anodeand cathode as close as possible. Oxygen was bubbled during30 min before starting the electrolysis and then it was keptbubbling for the entire duration of the reaction in order to keep thesolution saturated in this gas. The solutionwasmechanically stirredin order to improve mass transfer and decrease the thickness of theNernst diffusion layer. The initial phenol concentration was5 � 10�4 M (100 mL), prepared in 0.05 M Na2SO4. Electrolysis wascarried out at different initial pHs (3.0 and 5.5), adjusted by addingdrops of 0.1MH2SO4 or 0.1 MNaOH at room temperature. Every 1 h1 mL samples were withdrawn and immediately filtered (0.2 mm)and analysed by HPLC. The electrolysis experiments were per-formed at the oxygen reduction potential (�0.6 V), which waspreviously determined by CV.

2.6. Analytical methodology

Decay in phenol concentration was determined by HPLCequipped with an array diode detector L-2455 (Elite LaChromHitachi) at 213 and 271 nm, using a reverse phase column RP18e(5 mm). Phenol determination was performed using 50% methanoland 50% water as mobile phase at 1 mL min�1 and 30 �C. Thehydrogen peroxide concentration was determined by the potas-sium titanium(IV) oxalate method at 400 nm [22]. The chemicaloxygen demand (COD) was measured by COD analyser (HANNA

HI83099) instrument. Total iron and copper were determined byinductively plasma atomic emission spectrometry (ICP).

3. Results and discussions

3.1. Catalyst characterization

Chemical composition and textural properties of allophane, Fe-allophane, Cu-allophane and bimetallic (FeeCu) allophane arepresented in Tables 1 and 2. The chemical and textural properties ofallophane clays were modified during impregnation with iron and/or copper oxide. An increase of Si/Al ratio was observed in all cat-alysts (Table 1), which was highest for the allophane clays with thehighest content of iron oxide. This could be a consequence ofaluminium dissolution occurring under pH near 3.0, used toimpregnate the allophane with iron oxide, or also due to isomor-phic substitution of aluminium by iron on the octahedral network[23]. The EDX analysis is consistent with the expected presence ofFe and/or Cu in the different catalysts (Table 1). FTIR spectrum ofallophane clays (Fig. 1) shows the typical bands of hydrous feld-spathoids allophane. The bands centred at 3444 cm�1 and1635 cm�1 are assigned to OH stretching of structural hydroxylsgroups and OH bending of adsorbed waters, respectively. The bandsat 1043 cm�1, 722 cm�1 and 452 cm�1 are associated to the tetra-hedral aluminosilicate framework. The band at 593 cm�1 is due tooctahedral aluminium and the shoulder at 905 cm�1 can be asso-ciated to SieOH groups [24,25]. Changes in the FTIR spectra werealso observed after impregnation with iron and/or copper oxide.The bands associate to tetrahedral aluminium were weakened orshifted to higher wavenumber in the presence of iron and/or cop-per oxide. These changes in the FTIR spectra may be associated tothe formation of new bonds as a consequence of the interactionbetween SieO groups and Fe(OH)þ, Fe(OH)2þ, Fe(OH)3þ, Cu2þ orCu(OH)þ species, all present in solution as it has been previouslyreported [15,17,23,26]. It is important to mention that theimpregnation method used does not affect the framework of theallophane as was observed by FTIR (Fig. 1). BET surface area ofparental allophane clays was increased when the allophane wasimpregnated with iron oxide (Fe-allophane). This increase could beassociated to iron oxyhydroxide formation [23], which wasconfirmed by M€ossbauer analysis (see discussion below). BET sur-face area of the parental allophane clays decreases after theimpregnation with copper oxide (Table 2), possibly due to the for-mation of copper oxide nanoparticles covering the external surfaceand clogging the microspore of the allophane support [27]. A uni-form pore size and volume pore is observed for all the catalysts(Table 2). Fe-allophane, Cu-allophane and bimetallic (FeeCu) allo-phane exhibit N2 adsorptionedesorption type IV isotherms (datanot shown) typical of mesoporousmaterials. This result is similar tothat of the parental allophane, in total accordance with the FTIRdata which showed that the impregnation method did not change

Table 2Textural properties of allophane clays and bimetallic (FeeCu) allophane nanoclays.

Sample SBET (m2 g�1) Vp (cm3 g�1) V0 (cm3 g�1) Vm (cm3 g�1) Pore diameter (nm)

Allophane 144 0.26 0.07 0.19 3.69Cu-allophane 118 0.20 0.06 0.14 3.85Fe3Cu3 152 0.26 0.07 0.19 3.85Fe4.5Cu1.5 166 0.27 0.08 0.19 3.84Fe5.4Cu0.6 161 0.24 0.08 0.16 3.89Fe-allophane 161 0.23 0.08 0.15 3.81

4000 3500 3000 2500 2000 1500 1000 500

460

452

905

1052

593

722

1047

1043

1635

3444

Fe-allophane

Fe5.4Cu0.6

Fe4.5Cu1.5

Fe3Cu3

Cu-allophane

Allophane

Tran

smis

son

(arb

itrar

y un

ity)

Wavenumber (cm-1)

Fig. 1. FTIR spectrum of allophane, Fe-allophane, Cu-allophane and bimetallic (FeeCu)allophane nanoclays.

E.G. Garrido-Ramírez et al. / Microporous and Mesoporous Materials 225 (2016) 303e311306

the structural properties of the allophane clays. Similarly, SEMmicrographs of the catalysts (Fig. 2) show highly porous aggregatedparticles, in agreement with the unitary structure of allophaneclays.

X-ray diffraction patterns of allophane (Fig. 3) shows broad linescentred at 3.3 and 2.2 Å d spacing, indicating the presence ofallophane [25,28,29]. No diffraction of iron or copper oxide is

Fig. 2. Scanning electron microscopy of the allophane nanoclays, Fe-all

presented in the patterns of Fe-allophane, Cu-allophane andbimetallic (FeeCu) allophane nanoclays, suggesting that thesespecies are finely dispersed on the allophane surface or that theiron and copper oxides particles are very small particles [27].

The XPS and the Fe 2p XPS spectra recorded from the varioussamples (Fig. 4a and b) showed very broad lines and were fitted to acombination of spineorbit doublets, multiplet splitting peaks andshake-up satellite components. The different contributions andtheir respective assignments are listed in Table 3. The data indicatethe presence of both Fe2þ and Fe3þ in all the samples. Inspection ofFig. 4b clearly shows that the Fe2þ contributions increase withincreasing Cu content in the nanoclays. In contrast, the M€ossbauerdata at 16 K of the sample with the highest Fe content (Fe-allo-phane) do not show the presence of Fe2þ (Fig. 5a). This spectrum isdominated by an intense quadrupole doublet absorption, whichwas deconvoluted into two different quadrupole doublets. TheM€ossbauer parameters of these doublets (IS ¼ 0.44 mm�1;QS ¼ 1.10 mm�1, area 13%; IS ¼ 0.44 mm�1; QS ¼ 0.63 mm s�1, area44%) are characteristic of Fe3þ in octahedral oxygen coordinationand could correspond to Fe3þ substituting Al3þ ions in the allo-phane framework. The spectrum also shows broadened magneticcomponents, which were fitted using a discrete magnetic sextetand a broad single line. The parameters of the sextet(IS ¼ 0.44 mm�1; QS ¼ �0.14 mm s�1; H ¼ 44.6 T; area 9%) do notallow its assignment to any specific Fe3þ oxide or oxyhydroxide. Itsquadrupole shift is intermediate between that of goethite(�0.20 mm s�1) and that of ferrihydrite or lepidocrocite(�0.05 mm s�1) while its hyperfine magnetic field is more similarto that of lepidocrocite (ca. 46 T) than to those of goethite or fer-rihydrite (ca. 49e50 T). As pointed out byMurad and Schwertmann

ophane, Cu-allophane and bimetallic (FeeCu) allophane nanoclays.

Fig. 3. X-ray diffraction patterns of allophane, Fe-allophane, Cu-allophane and bime-tallic (FeeCu) allophane nanoclays.

E.G. Garrido-Ramírez et al. / Microporous and Mesoporous Materials 225 (2016) 303e311 307

[30], it is difficult to differentiate among the various Fe3þ oxy-hydroxides in the microcrystalline domain. This difficulty is morepronounced when the materials contain Al or Si substitutionswithin its structure, resulting in a dilution of the magnetic super-exchange interactions and, hence, in a reduction of the hyperfinemagnetic field. The broad singlet component (area ¼ 34%) tries tosimulate the fraction of the oxyhydroxide particles, which remainin superparamagnetic state at this low temperature. In any case, theresults indicate that for this particular sample a considerable pro-portion of the present Fe3þ pertains to an oxyhydroxide and doesnot substitute Al3þ in the allophane framework.

The 16 K spectrum recorded from the Fe3Cu3 allophane sampleshows, however, some subtle but significant differences (Fig. 5b).Firstly, no magnetic components are observed. Secondly, and moreimportantly, the spectrum is clearly asymmetrical in its central part(see absorption line at around 0.00 mm s�1). Therefore, we fitted

Fig. 4. XPS spectrum of allophane (a) and Fe 2p XPS data recorded from the Fe-allophane ared (For interpretation of the references to color in this figure legend, the reader is referre

the spectrum using two paramagnetic Fe3þ doublets having pa-rameters close to those obtained from the fit of the Fe-allophanesample plus a tiny Fe2þ doublet, in a similar fashion to what we didin a previous publication [4]. Given the low iron concentration inthe sample its signal-to-noise ratio is quite poor but, nevertheless,the quality of the fit increases significantly upon the introduction ofthe Fe2þ doublet (IS ¼ 1.11 mm s�1; QS ¼ 2.83 mm s�1, area 4%).Given the low relative area of the Fe2þ contribution it might havegone undetected in the spectrum of the previous sample due to thepresence of the broad magnetic components observed there. In anycase, the results endorse the XPS observations and, taken together,they strongly suggest that the Fe2þ component is mainly due to asurface species since its contribution to the XPS spectra is certainlyimportant.

A remarkable fact is that in the samples that contain both Fe andCu, Cu is not observed in the wide-scan XPS spectra, which in-dicates that, at least within the thickness which can be explored byXPS (ca. 3 nm), Fe is segregated preferentially to Cu in the allophanesurface. Note that, however, Cu is detected in these samples by EDXshowing that Cu is present in the bulk of the material.

The Cu 2p spectrum recorded from the Cu-allophane sampleshows a spineorbit doublet compatible with the presence of Cuþ

(Fig. 6). However, there is strong evidence that Cu is initially pre-sent as Cu2þ and that it is reduced to Cuþ upon irradiation underthe X-rays of the XPS spectrometer as previously reported byChusuei et al. [31]. Although with poor energy resolution, the widescan spectrum recorded from the Cu-allophane shows that the Cu2p region contains strong shake-up satellites close to the mainphotoemission lines (Fig. 7). This is indicative of the presence ofCu2þ. However, during the long periods of time needed to record aCu 2p spectrum with reasonable statistics and better energy reso-lution the shake-up satellites disappear indicating the reduction ofthe initial Cu2þ to Cuþ as a consequence of the irradiation of the XPSX-rays. This would indicate that Cu is present initially as very smallCuO nanoparticles in the allophane surface. As Chusuei et al. [31]reported, this effect is much more marked when the particle sizedecreases below 3 nm.

The presence of iron and copper oxide in the allophane was alsoconfirmed by cyclic voltammetry (CV). CV for Fe-allophane (Fig. 8a)presents a well-defined quasi-reversible couple (peaks 1C/1A)attributed to Fe3þ/Fe2þ process at 0.0 V and 0.33 V, for the cathodic

nd bimetallic (FeeCu) allophane (b). Fe2þ components in blue and Fe3þ components ind to the web version of this article.).

Table 3Fe 2p binding energies obtained from the fit of the Fe 2p spectra.

Species Fe 2p3/2(BE, eV)

Multiplet splittingcomponent (BE, eV)

Shake-upsat. (BE, eV)

Fe 2p1/2

(BE, eV)Shake-upsat. (BE, eV)

Fe2þ 708.3 e 714.3 722.6 725.9Fe3þ 710.4 712.3 718.8 724.1 e

Fig. 5. M€ossbauer spectrum recorded at 16 K from the Fe-allophane (a) and Fe3Cu3 (b)allophane samples.

Fig. 6. Cu 2p recorded from the Cu-allophane clay after a long irradiation period underthe X-rays of the XPS spectrometer.

Fig. 7. Wide scan spectrum recorded from the Cu-allophane. The inset shows the Cu2p region in which very clear Cu2þ satellites are observed. Those satellites disappear asthe sample is irradiated for long times with the X-rays of the XPS spectrometer.

E.G. Garrido-Ramírez et al. / Microporous and Mesoporous Materials 225 (2016) 303e311308

and anodic process, respectively. In the case of Cu-allophane clays(Fig. 8b) a main couple with the reduction process at �0.205 V andthe associated oxidation process at 0.05 V are observed (peaks 1C/1A). Both main peaks shows a potential shoulder with higheroverpotential (2C/2A), which could be considered as an evidence ofcoexistence of two species with Cu atoms, two phases or well,species based on Cu with different states of oxidation (CuO andCu2O) [32e34]. The bimetallic Fe4.5Cu1.5 and Fe5.4Cu0.6 allophaneclays (line number 3 and 2, respectively, Fig. 8c) present a widthreduction peak (1C), which could have two contributions (bothpeaks well defined in the sample Fe3Cu3). The more negative peakis attributed at Cu2þ/Cuþ one-electron process, whereas the lessnegative is assigned to Fe3þ/Fe2þ, in accordance with the processesdescribed above. In the reverse scan to positive potentials, the twooxidation peaks at 0.024 V and 0.34 V are better defined. In the caseof Fe atoms, clearly the process in both sense of the potential scancan be associated with the Fe3þ/Fe2þ redox couple, in agreementwith Fig. 8a. However, for Cu atoms, it is possible to think that in theoxidation process the copper atoms pass for species with relativelystable Cuþ. From Fig. 8a and b, it is evident that the oxidationprocess for Fe2þ occurs in the same potential window as the copperoxidation and with markedly lower current. This, together with thefact that the charge associated to both anodic process in Fig. 8c arevery similar, and that the cathodic process charge is double of eachanodic peak, could be considered as evidence that copper passesfrom Cu to Cu2þ in two steps. The stability of glassy carbon elec-trodes modified with Fe-allophane, Cu-allophane and bimetallic(FeeCu) allophane clays was determined at pH 5.5 by repetitivecyclic voltammetry. The results (data not shown) indicate that allthe peaks remain invariant after 20 potential cycles at 0.1 V s�1.

3.2. Catalytic activity

3.2.1. Phenol oxidation over bimetallic FeeCu allophane nanoclaysCatalytic activities of Fe-allophane, Cu-allophane and bimetallic

(FeeCu) allophane nanoclays were determined in the phenol

-0.8 -0.4 0.0 0.4 0.8-0.6-0.4-0.20.00.20.40.60.8

-0.1

0.0

0.1

0.2

bCu-allophane

2C

2C

1C

1C

Potential (VAg/AgCl)

aFe-allophane

1C

1A

Cur

rent

(mA

)

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-0.4

-0.2

0.0

0.2

0.4

23

1

2C

2A1A

1C

C

Cur

rent

(mA

)

Potential (VAg/AgCl)

Fig. 8. Cyclic voltamograms of Fe-allophane (a), Cu-allophane (b) and bimetallic (FeeCu) allophane nanoclays (c). Blank in dash line, Fe3Cu3 (line number 1), Fe4.5Cu1.5 (line number3) and Fe5.4Cu0.6 (line number 2).

E.G. Garrido-Ramírez et al. / Microporous and Mesoporous Materials 225 (2016) 303e311 309

oxidation by heterogeneous electro-Fenton process. The results areshown in Figs. 9 and 10.

Phenol conversion of near 100% at initial pH 3.0 was observedfor all bimetallic FeeCu catalysts in less than 2 h of reaction,following an exponential decay (Fig. 9a and Table 4). In the case ofFe-allophane and Cu-allophane the phenol conversionwas 71% and62%, respectively, after 2 h, whereas in the absence of catalyst(blank) low phenol conversion was observed. The chemical oxygendemand (COD) removal after 4 hwas less than 47% for Cu-allophaneand 65% for Fe-allophane, whereas for the bimetallic FeeCu allo-phane the COD removal decreased with the amount of copper oxidein the catalyst (Table 5). Therefore, the best mineralization gradewas achieved for the Fe5.4Cu0.1 catalyst (80% COD decay). It isknown that the catalytic activity of bimetallic FeeCu catalyst isinfluenced by the availability of active sites (iron or copper oxide)on the surface of the catalyst, which react with the hydrogenperoxide in order to generate the hydroxyl radicals [9]. As waspointed by Frank et al. [8] when copper based compounds aredeposited after iron based compounds on the solid catalyst, it ispossible that at acid pH the deposited iron may be dominant forCOD degradationwhile a neutral pH the copper may be responsiblefor the reaction. Therefore, when the Fe/Cu ratio decreases it ispossible that the copper deposited on the allophane surface may

0 60 120 180 2400.0

0.2

0.4

0.6

0.8

1.0 a

[Phe

nol]/

[Phe

nol] 0

Time (min)

Fig. 9. Phenol conversion (a) and hydrogen peroxide generation during phenol degradation(FeeCu) allophane nanoclays at initial pH 3.0.

cover the catalyst surface limiting the activity of iron on the phenoldegradation, especially under acidic conditions.

The hydrogen peroxide concentration achieved a plateau after3 h reaction for all the catalysts (Fig. 9b), except for the Cu-allophane catalyst, implying different hydrogen peroxide de-compositions for the Fe and Cu based catalysts. The catalyticdecomposition of H2O2 by Cu2þ is slower at acidic conditions and isaccelerated with increase of pH since the deprotonated form ofH2O2 (HO�

2 , pKa of H2O2 is 11.6) is the major electro-donorresponsible for the reduction of Cu(II), whereas, acidic conditionsfavour the production of OH� in the Fe3þ/H2O2 system [35].

When the heterogeneous electro-Fenton reactions were per-formed at initial pH 5.5 (Fig. 10a), phenol conversion was nearlycomplete after 4 h for both Fe-allophane and the bimetallic (FeeCu)allophane nanoclays with lower copper oxide content (Fe5.4Cu1.5and Fe4.5Cu1.5). However, the COD removal was highest for thebimetallic catalysts, achieving a DOC removal of 69% and 68% forFe5.4Cu0.6 and Fe4.5Cu0.6, respectively, whereas for Fe-allophane a48% of COD removal was obtained. In the case of Cu-allophane andblank experiment (in the absence of catalyst) a COD removal of 21%and 6%, respectively, were observed (Table 5).

In order to correlate the decrease in phenol concentration withthe hydrogen peroxide availability during the electrolysis time at

0 60 120 180 2400

1

2

3

4b

[H2O

2] /m

M

Time (min)

blank Cu-allophane Fe-allophane Fe3Cu3 Fe4.5Cu1.5 Fe5.4Cu0.6

(b) by heterogeneous Fenton reaction over Fe-allophane, Cu-allophane and bimetallic

0 60 120 180 2400.0

0.2

0.4

0.6

0.8

1.0 a

[Phe

nol]/

[Phe

nol] 0

Time (min)0 60 120 180 240

0

1

2

3

4

5b

[H2O

2] /m

M

Time (min)

Blank Cu-allophane Fe-allophane Fe3Cu3 Fe4.5Cu1.5 Fe5.4Cu0.6

Fig. 10. Phenol conversion (a) and hydrogen peroxide generation during phenol degradation (b) by heterogeneous Fenton reaction over Fe-allophane, Cu-allophane and bimetallic(FeeCu) allophane nanoclays at initial pH 5.5.

Table 4Apparent exponential rate constant (kapp) and regression coefficient for phenoloxidation using different glassy-carbon electrodes modified with Fe-allophane, Cu-allophane and bimetallic (FeeCu) allophane by heterogeneous electro-Fenton pro-cess at initial pH 3.0 and 5.5

Catalyst kapp/min�1 R2 kapp/min�1 R2

pH 3.0 pH 3.0 pH 5.5 pH 5.5

Blank 0.0013 0.987 0.00126 0.980Cu-allophane 0.0052 0.988 0.00199 0.999Fe3Cu3 0.0105 0.987 0.00349 0.991Fe4.5Cu1.5 0.0143 0.988 0.00822 0.991Fe5.4Cu0.6 0.0157 0.962 0.00890 0.992Fe-allophane 0.008 0.952 0.00576 0.999

E.G. Garrido-Ramírez et al. / Microporous and Mesoporous Materials 225 (2016) 303e311310

initial pH 5.5, the hydrogen peroxide generation as a function ofelectrolysis time is plotted in Fig. 10b. It is observed that increaserate of hydrogen peroxide concentration is higher at initial pH 5.5for all the catalysts except for Fe-allophane, suggesting that an in-crease in the hydrogen peroxide generation implies a faster phenoldecomposition (curve for Fe5.4Cu0.6), whereas for all other samples,no correlation between both parameters is observed. It is generallyknown that phenol degradation begins with a reaction with hy-droxyl radical, and that the process at theworking electrode surfaceis precisely the formation of these radicals. Thus, as was pointedabove the presence of copper ions generates synergism in thehydroxyl generation from oxygen peroxide only if the copper is in asmall proportion. At higher content, copper oxide nanoparticlescover the external surface of the iron nanoclays, blocking the activesites and decreasing the catalytic activity.

Table 5Phenol conversion, chemical oxygen demand (DOC decay) and Fe leaching of Fe-allopha

Sample pH Phenol conversiona (%) DOC decayb

Blank 3.0 27 e

Cu-allophane 3.0 62 47Fe3Cu3 3.0 96 66Fe4.5Cu1.5 3.0 98 68Fe5.4Cu0.6 3.0 97 80Fe-allophane 3.0 71 65Blank 5.5 22 6Cu-allophane 5.5 45 21Fe3Cu3 5.5 49 48Fe4.5Cu1.5 5.5 80 68Fe5.4Cu0.6 5.5 76 69Fe-allophane 5.5 71 48

a After 2 h reactionb After 4 h reaction

As has been previously reported, the phenol degradation isoptimal at acidic pH for the iron catalytic systems, whereas theoptimal pH for Cu-based systems is about pH 5.5 [4,35]. In thiswork, we have been shown that simultaneous incorporation ofCu2þ and Fe3þ ions on the allophane surface significantly increasedthe catalytic activity for the phenol oxidation at both pHs (3.0 and5.5) due to synergetic effect in the bimetallic (FeeCu) allophanecatalyst.

This synergetic effect could be a consequence of an increaseof the numbers of active sites (iron and copper oxide) on the surfaceof allophane, leading to efficient catalytic activity for the activationof H2O2. The plausible mechanism could be explained because theelectro-generated H2O2 molecules adsorbed on the allophanebimetallic surface (FeeCu) can reduce Fe3þ to Fe2þ and Cu2þ to Cuþ

ions through a Fenton-like process (Eqs. (1) and (3)), and thegenerated Fe2þ and Cuþ ions on the bimetallic allophane catalystare then oxidized immediately by H2O2 to complete Fe3þ/Fe2þ andCu2þ/Cu redox processes and produce OH* radicals (Eqs. (2) and(4)). Meanwhile, the formed Fe3þ ions can also react with the Cuþ

ions to form Fe2þ and Cu2þ to accelerate the transformation of Fe3þ/Fe2þ and Cu2þ/Cuþ, giving rise to additional OH* radical during thedegradation (Eq. (5)) [6,9].

Fe3þ þH2O2/Fe2þ þ �O2Hþ Hþ (1)

Fe2þ þH2O2/Fe3þ þ �OHþ OH� (2)

Cu2þ þ H2O2/Cuþ þ �O2Hþ OH� (3)

ne, Cu-allophane and bimetallic FeeCu allophane nanoclays.

Dissolved iron in solution (mg L�1) Dissolved copper in solution (mg L�1)

e e

e 0.940.9 0.091.12 0.140.95 0.070.81 e

e e

e 0.6540.118 0.3461.250 0.0750.432 0.6380.28 e

E.G. Garrido-Ramírez et al. / Microporous and Mesoporous Materials 225 (2016) 303e311 311

Cuþ þ H2O2/Cu2þ þ �OHþ OH� (4)

Fe3þ þ Cuþ/Fe2þ þ Cu2þ (5)

The stability of iron- or copper-rich minerals (as heterogeneouscatalysts) is strongly dependent on pH of the solution. It is knownthat that the leaching of iron increases at low pH, whereas in thecase of copper oxide a significant leaching of copper could occur atpH 5e5.5. In a previous work [15] the catalytic wet peroxideoxidation of phenol was studied using iron oxide-supported allo-phane and copper oxide-supported allophane at pH 3.7 and 5.0,respectively. It was noticed that copper oxide-supported allophanecatalysts are far less stable in the CWPO of phenol than their iron-based counterparts. The presence of soluble copper species in thereaction medium could account for the significant homogeneouscatalytic contribution. This behaviour could stem from an insuffi-cient stabilization of the impregnated copper oxide on the surfaceof allophane support or could be caused by the presence of inter-mediate reaction products leading to the complexation of Cu2þ

ions, thus forming soluble moieties [15]. In this sense, we havestudied the stability of glassy carbon electrodes modified with Fe-allophane, Cu-allophane and bimetallic FeeCu allophane at pH5.5 by repetitive cyclic voltammetry. The results (data not shown)indicate that bimetallic (FeeCu) allophane nanoclays were morestable than Cu-allophane after 20 potential cycles at 0.1 V s�1.However, when the heterogeneous electro-Fenton for phenoloxidationwas performed the leaching of iron and copper species atpH 3.0 and 5.5 was highest for the bimetallic FeeCu allophane(Fe4.5Cu1.5). In the case of initial pH 3.0 the leaching of iron andcopper oxide could be a consequence of minerals leaching intosolution due to acidic conditions. Whereas, at pH 5.5 we hypoth-esized that the iron and copper solubilisations are due to thecomplexation of Cu2þ and Fe3þ species by the presence of inter-mediate reaction forming soluble moieties. However, in all bime-tallic catalysts the iron concentration was less than 1.25 mg/L and0.638 mg/L after 4 h reaction, much lower than 5 mg/L and 1 mg/L,which is the legal limit imposed by the Chilean Normative DS90/2000 [36] for effluent disposition in superficial watercourse for ironand copper, respectively. Therefore, the bimetallic (FeeCu) allo-phane with lowest copper contents could be used as highly activeand stable heterogeneous catalysts for the heterogeneous electro-Fenton process in a wide pH range without a significant drop intheir catalytic efficiency and with minimal iron and coppersolubilisation.

4. Conclusions

In this work we have prepared, for the first time, bimetallic(FeeCu) allophane nanoclays and characterized them by diversemethods. The results showed that the bimetallic allophane nano-clays are composed of small copper oxide particles stabilized byiron oxide. The simultaneous incorporation of iron and copper ionson the allophane nanoclays improves the catalytic efficiency of thephenol oxidation by heterogeneous electro-Fenton reactioncompared with Fe-allophane and Cu-allophane nanoclays due tosynergetic effect. This synergetic effect could be explained bycooperation between iron and copper metals in the bimetallic(FeeCu)/H2O2 systems by Fenton-like reaction mechanism. Thecatalytic activity of bimetallic (FeeCu) allophane nanoclaysdecreased with the copper content, suggesting that a higher

amount of copper in the catalyst could cover the active sitesdecreasing the catalytic efficiency. In the bimetallic (FeeCu) allo-phane clays, the leaching of iron and copper into the solution waslow which is indicative of the high stability of bimetallic (FeeCu)allophane catalysts. Therefore we proposed that bimetallic (FeeCu)with low iron content allophane could be used as highly active andstable heterogeneous catalyst in HEF process for recalcitrantorganic pollutant under environmental friendly conditions.

Acknowledgements

This work was supported by FONDECYT Postdoctoral Project Nº3130725.

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