characteristics of iron-loaded tio 2 -supported montmorillonite...

Research ArticleReceived: 15 January 2014 Revised: 26 March 2014 Accepted article published: 2 April 2014 Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jctb.4393

Characteristics of iron-loaded TiO2-supportedmontmorillonite catalysts: �-Naphtholdegradation under UV-A irradiationA. Neren Ökte,* Duygu Tuncel, A.Hazal Pekcan and Taner Özden

Abstract

BACKGROUND: In this study, a ternary system was synthesized by Fe loading on TiO2-supported montmorillonite catalysts(Fe-TiO2-MMT) and characterized by various means. The catalytic activities of the supported catalysts were examined in thepresence of �-Naphthol molecule under UV-A irradiation.

RESULTS: The in situ growth of TiO2 nanoparticles and Fe species on the MMT surface varied the morphology of the rawclay. The supported catalysts exhibited higher surface areas, pore volumes and extended absorption profiles through longerwavelengths. Themesoporous structures of the supported catalysts improved their dark adsorption capacities anddegradationabilities. Themixedvalence existenceof Fe enhanced theelectron transfer reactions in the catalytic runs. Kineticswas controlledby following a Langmuir–Hinshelwoodmodel.

CONCLUSION: The repeatability of the photocatalytic reactions provides an important practical advantage in the use ofas-prepared supported catalysts.© 2014 Society of Chemical Industry

Keywords: supported TiO2; iron loading; montmorillonite; �-Naphthol; photocatalytic degradation

INTRODUCTIONTiO2 has proven to be the most suitable photocatalyst forwidespread environmental applications. However, fine TiO2

powders easily agglomerate into larger particles in a suspen-sion and thus, reduce the catalytic performance. Recent studieshave focused on supporting TiO2 nanoparticles on adsorbentmaterials. Clays are ideal candidates for such materials with alayered structure offering a variety of applications such as cata-lysts, stabilizing medium for nanoparticles and acting as a hostfor guest species.1–6 Higher surface areas and porous struc-ture are the features responsible for the extended applicationareas of the modified clay minerals. Among them, montmoril-lonite is frequently employed with TiO2 systems since it is anexpanding layer silicate mineral and it can accommodate varioustypes of compounds in its interlayer spaces. Kaneko et al. studiedTiO2-montmorillonite in the decomposition reaction of carboxylicacids.7 A similar system was also employed for the oxidation ofbenzene and cyclohexane.8 The enhanced activity obtained forthe photocatalytic degradation of endocrine disruptors was dueto the enrichment of the reactant on the TiO2-montmorillonitesurface and crystallinity of TiO2 pillars formed between the claylayers.9 Mogyorosi et al. investigated two different methods;one included the adsorption of titanium alkoxides onto/intomontmorillonite platelets followed by hydrolysis and the otherinvolved heterocoagulation of previously crystallized TiO2 andclay mineral.10 They made detailed characterization experimentsof the as-prepared composites to determine basal spacing of claylayers, diameters of TiO2 particles, images of the compositions

and surface areas. Zhu et al. found that the positively chargedTiO2 sol particles with 1–2 nm size can easily replace sodium ionswithin the interlayers of montmorillonite.11 Thus, the resultantcomposite separated and immobilized nano-TiO2 species andalso increased the active surface sites. TiO2/montmorillonite com-posites prepared in the study by Kameshia et al. indicated partialintercalation of smaller colloidal TiO2 particles at 50–80 ∘C.12 Arecent study used TiO2 pillared montmorillonite as an adsorbentfor the removal of As (III) and As (V) from aqueous solution withor without UV irradiation.13 The photocatalytic CO2 reduction wasinvestigated in the presence of a montmorillonite/TiO2 coatedmonolith photoreactor with higher hydrocarbon yield rates.14

Based on these studies, binary systems of TiO2-loaded montmo-rillonite structures work as efficient catalysts due to the largersurface areas and higher adsorption abilities induced by TiO2

nanoparticles located between the clay’s silicate layers.The number of ternary systems in combination with metal

ion-loaded TiO2-supported montmorillonite composites is ratherless in comparison with the binary ones. Alumina-, zirconia- andtitania-loaded montmorillonites additionally modified with silverwere tested as catalysts for NO reduction.15 Liu et al. exploredthe enhanced photocatalytic activity of silver and TiO2 nanoparti-cles/montmorillonite composites.16 They found that larger surface

∗ Correspondence to: A. Neren Ökte, Department of Chemistry, BogaziçiUniversity, Bebek 34342, Istanbul, Turkey. E-mail: [email protected]

Department of Chemistry, Bogaziçi University, Bebek, 34342, Istanbul, Turkey

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www.soci.org AN Ökte et al.

areas caused by pillaring and loading of silver led to the fastadsorption and accumulation of model compound (methyleneblue) toward catalytically active sites of TiO2. Synthesis, char-acterization and photoactivity of TiO2-montmorillonites dopedwith vanadium and/or carbon was investigated in the study byChen et al.17 The increased degradation rate obtained under UVillumination in the presence of V-TiO2-MMT and C-V-TiO2-MMTin comparison with V-TiO2 and C-V-TiO2 was due to reducedcrystalline size, required for the depression of electron–holepair recombination rate. The performance of vanadia supportedon TiO2-montmorillonite composites was also studied for theoxidation of H2S in the presence of water and ammonia.18 Pil-laring of clay layers by titania increased the surface area andenabled good dispersion of the vanadia species which thenresulted in the efficient conversion of H2S into elemental sul-fur. Also, photodegradation rate of sulforhodamine B moleculewas found to increase in the presence of vanadium-dopedTiO2-montmorillonite composites.19 Fatimah et al. studied methylorange degradation under a combined photocatalytic and Fentonbased oxidation in the presence of TiO2 and Fe3+ ion. The effectof Fe3+ immobilization and TiO2 photocatalysis resulted in fasterandmore efficient degradation of the contaminant in comparisonwith the results obtained in the presence of TiO2-pillared mont-morillonite. Moreover, the concentration of H2O2 and catalystdosage increased the efficiency of the degradation process.20

All these studies exhibited formation of crystalline TiO2 pillarswithin the interlayer of montmorillonite. However, Bovey et al.reported that the basal spacing of nano-TiO2 pillared montmoril-lonite was only 2–3 nm.21 This value is far from the TiO2 crystallinesizes (3–10 nm) that can be produced frompositively charged tita-nium hydrate. Very recently, a new immobilizationmethod of TiO2

adsorption on the surface of montmorillonite clay was reported.22

Nano-TiO2 adsorption on the external surface of the clay with theassistance of a surfactant was regulated rather than being interca-lated into the layers of the clay. As a consequence of this method,the adsorption and photocatalytic performance of the compositeswere found to be improved by more than 10%.In this present study, a ternary system was established by Fe

loading on TiO2-supported montmorillonite. To understand thesynergism induced after Fe and TiO2 loading, the photocatalystshave been examined by X-ray diffraction (XRD), nitrogen adsorp-tion desorption isotherms (BET), scanning electron microscopywith energy dispersive spot analysis (SEM-EDX), atomic forcemicroscopy (AFM), X-ray photoelectron spectroscopy (XPS) andUV–vis diffuse reflectance spectra (UV–vis DRS). And, to elucidatethe UV-A assisted degradation ability of the supported catalysts,�-Naphthol was selected as a probe molecule from naphthalenehomologues of phenol pollutant. Kinetics was investigated by fol-lowing a Langmuir–Hinshelwood model. Recycling studies wereperformed to test the repeatability of the photocatalysts.

MATERIALS ANDMETHODSChemicals andmaterialsThe raw sodium montmorillonite (Na0.3(Al,Mg)2 Si4O10(OH)24H2O)(MMT) used as a support in this study was kindly donated bySüd Chemie (Moosburg, Germany). Titanium tetraisopropoxide(98%,Aldrich), acetonitrile (99.9%,Merck), acetic acid (96%,Merck),Fe(NO3)3.9H2O (Merck) and �-Naphthol (99%Aldrich)were used asprovided by the suppliers without further purification. Water waspurified with an Elga Ultra-Raw Water Purification (UHQ II) systemwith a resistivity of 18.2 MΩ cm−1 and a pH value of 5.9.

Synthesis of the supported catalystsTiO2 was prepared by means of an acid-catalyzed sol–gel methodfrom an alkoxide precursor.23,24 Briefly, 20mL of titanium tetraiso-propoxide was added gradually to 80wt% acetic acid solutionunder continuous stirring for 2 h at 50 ∘C to produce a transpar-ent sol. The requisite amount of titania-sol was then added tothe aqueous suspension of the MMT (initially 2 g in water for 2 h).After agitation and extensive washings, the catalysts were dried(at 100 ∘C for about 12 h), calcinated (at 500 ∘C with a heating rateof 10 ∘Cmin−1 for 5 h in a muffle furnace) and ground into finepowder and labeled 25% TiO2-MMT. In previous studies TiO2 load-ing percentages for different support materials were 10, 25 and50%.25–27 An average value was preferred in this study in order toend up with some free adsorption sites to host the Fe species onthe 25% TiO2-MMT.For the loading of Fe on the 25% TiO2-MMT surface, the con-

tents were regulated as 0.5, 1, 3 and 5 at% by solving the requi-site amount of Fe precursor. Each of these solutions was stirredovernight at room temperature. After centrifugation (at 4000 rpm)and washing steps, the catalysts were dried (at 100 ∘C), calcinated(at 500 ∘C) and ground into fine powders and labeled 0.5% Fe-25%TiO2-MMT, 1% Fe-25% TiO2-MMT, 3% Fe-25% TiO2-MMT and 5%Fe-25% TiO2-MMT. The Fe content of the supported catalysts waschecked by atomic absorption using a Varian Spectraa 250 Plusatomic absorption spectrophotometer, equipped with a Varian Fehollow cathode lamp and it was found that iron content was con-sistent with the nominal weight percentages for all synthesizedcatalysts.

Instruments and analytical methodsThe synthesized composites were characterized using differentanalytical techniques. The X–ray powder diffraction (XRD) pat-terns were recorded on a Rigaku-D/MAX-Ultima diffractometerusing Cu K� radiation (�= 1.54 Å) operating at 40 kV and 40mAand scanning rate 2∘ min−1. The nitrogen adsorption/desorptionisotherms were obtained at liquid nitrogen temperature 77 Kby using a Quantachrome Nova 2200e automated gas adsorp-tion system. The specific surface areas were determined using amulti-point BET analysis and the pores sizes weremeasured by theBJH method of adsorption. Surface morphologies were analyzedusing scanning electron microscopy (SEM) in combination withenergy dispersive X-ray analysis on an ESEM-FEG/EDAX PhilipsXL-30 instrument operating at 20 kV using catalyst powderssupported on carbon tape. Microscopic features of MMT beforeand after TiO2 and Fe loading were examined using atomic forcemicroscopy (AFM) at room temperature with a universal scanningprobe microscope (USPM) (Queosent-Ambios Technology, SantaCruz, CA). Contact mode imaging was performed using a siliconnitride cantilever probe with a nominal tip radius of 5–10 nm. Thethin films of the catalysts were deposited by dip-coating (KSV-LM)in an aqueous solution containing 0.1wt% of the catalysts. Glassmicroscope slides were used as substrates for reference, cleanedfor 2 h in an ultrasound bath with ethanol, then dipped in theprecursor solution bath, and finally pulled out at constant speedto obtain films of uniform thickness. The very thin film of catalystsformed on the substrate was dried in air at room temperature.In X-ray photoelectron spectroscopy (XPS) tests, a Thermo Sci-entific K-Alpha X-ray Photoelectron Spectrometer equippedwith a hemispherical electron analyzer and Al-K� micro-focusedmonochromator was used. The areas of peaks were estimated bycalculating the integral of each peak after subtracting a Shirley

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�-Naphthol degradation over Fe loaded supported catalysts www.soci.org

Figure 1. XRD patterns of MMT, 25% TiO2-MMT, 0.5-5% Fe-25% TiO2-MMT (A: anatase).

background and fitting the experimental curve to a combinationof Lorentzian/Gaussian lines. The UV–vis diffuse reflectance spec-tra (UV–vis DRS) of the MMT and of all supported catalysts wasobtained using aUV–vis spectrophotometer (UV-2450, Shimadzu)equipped with an integrating sphere reflectance accessory. Thebaseline correctionwas doneby BaSO4. The spectrawere recordedin the range 200–600 nm using BaSO4 as reference.

Photocatalytic degradation of �-NaphtholSolutions with different initial concentrations of �-Naphthol wereprepared by gradual heating. Cooling in air was followed beforemixing these solutions with the catalysts. Reaction systems wereset up by adding 0.2 g of supported catalysts into 200mL of�-Naphthol solution in a pyrex flask, with an inlet for air circulationand an outlet for collection of aliquots. The flask was located in an‘irradiation box’, equipped with eight black light lamps (Philips TL15W/ 5BLB) with an emission maximum at �= 365 nm. The lampswere positioned to surround the flask with an incident photonflux of 4.7× 1015 photons s−1. Prior to illumination, to ensureequilibrium of the adsorption process, suspensions were magnet-ically stirred in the dark for 30min. At given time intervals, smallaliquots of the stirred suspension were withdrawn and filteredusing Millipore filters (0.45 μm). The concentration of �-Naphtholremaining in solutionwas analyzed using high performance liquidchromatography (HPLC) (CECIL 1100 series) set with a CE 1100liquid chromatography pump, a CE 1220 variable wavelengthmonitor and an ultraviolet (UV) detector. The stationary phasewas a Hypersil ODS column (length 25 cm, internal diameter4.6mm, particle diameter 5 μm). Mobile phase was composed ofacetonitrile and water (70:30) delivered by a pump at a flow rateof 1mLmin−1. The wavelength of the UV absorbance detectorwas 265 nm. Calibration was made by the standard curve method.The calibration curve was constructed bymeasuring four differentconcentrations of �-Naphthol ranging from 3 to 25mg L−1 inthree replicates. All calibration curves were linear with correlationcoefficients (R2) higher than 0.98. Blank samples were used to

establish the limit of detection (LOD). LODs were calculated fromthe signal-to-noise (S/N) ratio 3.Periodic sampling and duplicate HPLC analysis was continued

during a 5 h illumination period. All experiments were performedat room temperature and at pH= 4.19 (pH value of �-Naphtholsolution in the presence of the supported catalyst). In order to con-trol the degradation at different pH values (in the range 3–11), pHof the solutionwas adjusted by adding dilute aqueous solutions ofHCl or NaOH before irradiation.�-Naphthol and degradation intermediates were characterized

by liquid chromatography–mass spectrometry (LC-MS) analysisusing a LCMS-2020-mass spectrometer system (Shimadzu, Japan)equipped with a C-18 5 μm 150× 4.6mm column. The mobilephase consisted of deionized water and HPLC grade acetoni-trile (ACN) with 1% formic acid using the following gradient;LC: 0–7min, 50–95% ACN; 7.01–11min, 95% ACN; 11.01–14min50% ACN.

RESULTS ANDDISCUSSIONXRD analysesX-ray diffractograms of the rawMMT and supported catalysts wereexamined in the range 2–70∘ (2� ) following MDI-JADE6 library(Fig. 1). The characteristic (001) reflection of MMT is observedat 7.04∘ (2�). In the patterns of the supported catalysts, thisreflection disappears, indicating that the layered structure of theclay is completely destroyed. Similar results were also found inthe studies by Tahir et al.,14 Chen et al.17 and Chen et al.19 Thepresence of a very small and broad crystalline peak in the lowangle range 2–10∘ (2� ) indicates the formation of some tactoids.Thus, partially exfoliated structure may enhance the adsorption ofTiO2 nanoparticles among themultilayer sheets andon the surfaceof the MMT. Meanwhile, the supported catalysts show broaderand lower intense diffractions. The existence of TiO2 nanoparticlesis confirmed in the high angle range (2� =10–70∘). In brief, thecatalysts are basically constituted of the anatase form of titania.

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Table 1. Crystalline sizes (danatase), surface areas (BET), total porevolumes (Vpore) andpore radius (rpore) ofMMTand supported catalysts

Catalysts danatase (nm)BET

(m2 g−1)aVpore

(cm3 g−1)brpore(Å)c

MMT - 28 0.027 14.9025% TiO2-MMT 9.75 94 0.115 16.350.5% Fe-25% TiO2-MMT 9.88 98 0.118 15.351% Fe-25% TiO2-MMT 12.8 117 0.158 14.843% Fe-25% TiO2-MMT 10.7 100 0.126 15.605% Fe-25% TiO2-MMT 9.46 106 0.139 16.58

a Determined from nitrogen adsorption–desorption isotherms usingBET equation.b Determined from cumulative adsorption pore volume using BJHmethod.c Determined from adsorption pore size using BJH method.

The characteristic (101) anatase peak is detected at 25.3∘ (2�).Other anatase diffractions of (112), (200), (105) and (211) appearat 38.2∘, 48∘, 54∘, and 55∘ (2�), respectively. Since there is noreflection at 27.4∘ (2� ), rutile phase does not form in the course ofthe synthesis. TheTiO2 phase remainsunchangeduponFe loading.Also, no crystalline phases of iron oxides and/or iron titanates arefound in the patterns of 0.5–5% Fe-25% TiO2-MMT catalysts. Somedetails about evidence of Fe in the supported catalysts can beachieved from the EDX, XPS and UV–vis analyses.The crystalline sizes of TiO2 nanoparticles, basedon thebroaden-

ingof themost intense (101) diffraction at 25.3∘ (2�), are calculatedby employing the Scherrer equation (Table 1). Fe loading increasesthe crystalline sizes of TiO2 nanoparticles. The highest crystallinesize (12.8 nm) is noticed in the presence of 1% Fe-25% TiO2-MMTcatalyst. However, 3%Fe-25%TiO2-MMTand5%Fe-25%TiO2-MMTcatalysts reveal smaller sizes of 10.7 nm and 9.46 nm, respectively.This may imply that the resultant matrix in the Fe species doesnot allow formation of TiO2 aggregates. Although TiO2 nanopar-ticles do not significantly differ in dimensions, the decrements inthe reflections of the raw support suggest their uniform distribu-tion throughout the surface and bulk. Moreover, such systems canbe readily separated from the suspension without filtration. Sinceit is very difficult to separate commercial TiO2 (Degussa P-25) fromthe aqueous solution due to the agglomeration of particles, it canbe suggested that the use of TiO2-supported catalysts provides animportant practical advantage.

Nitrogen adsorption–desorption isothermsA set of nitrogen adsorption–desorption tests were carried outon the raw MMT and supported catalysts (Fig. 2(a)). MMT exhibitsa Type II isotherm with a narrow hysteresis loop. This suggestsaggregates of plate-like particles forming slit shaped pores.28 Thesupported nanocatalysts reveal Type IV isotherms with hysteresisloops, characteristic of mesoporousmaterials.29 The initial parts ofthe isotherms (at lowP/P0) are attributed tomonolayer–multilayeradsorption on the surfaces of the supported catalysts. At higherP/P0 values, capillary condensation takes place within pores. For25% TiO2-MMT, capillary condensation starts at P/P0 = 0.44 whilethat of 1% Fe-25% TiO2-MMT proceeds at around P/P0 = 0.20.The pore size distribution curves show that the pores of thesupported catalysts are in the typical mesoporous region 20–40Å(Fig. 2(b)). An inevitable increase is noted in the pore volumesof the supported nanocatalysts, which results in higher surface

(a)

(b)

Figure 2. (a) Nitrogen adsorption desorption isotherms and (b) pore sizedistribution plots of MMT, 25% TiO2-MMT, 1% Fe-25% TiO2-MMT.

areas relative to those obtained in the presence of raw MMT(Table 1). The largest area is achieved in the presence of 1%Fe–25% TiO2-MMT. However, no noticeable difference is observedfor the higher percentages Fe contents, indicating the existenceof a saturation effect at 1% Fe loading. This is consistent with theXRDpatterns of the 0.5–5%Fe–25%TiO2-MMTcatalysts,wherenovariation in the XRD peak intensities is detected after Fe additionto the 25% TiO2-MMTmatrix.

SEM-EDX analysesSEMmicrographsofMMTand supported composites aredisplayedin Fig. 3. The raw MMT reveals randomly distributed stone-likeaggregates with no significant titania peak (Fig. 3(A) and inset). Inthe image of 25% TiO2-MMT, a distinct sharp edged and rathersmooth domain is noticed (Fig. 3(B)). An enhanced titania signal ofthis domainwith a decrease in the intensity of the Si peak suggestsadsorption of TiO2 nanoparticleswithin the silica framework of thesupport (Fig. 3(B) inset). Excluding these heterogeneously locatedtitania-enriched aggregates, the structure of MMT is retained fol-lowing TiO2 treatment. Similar aggregates are alsodistinguished inthe presence of 1%Fe-50%TiO2-MMT (Fig. 3(C)) with simultaneousdetectionofboth titania and iron signals (Fig. 3(C) inset ‘a’–‘b’). Thereduction in the silica peak intensity may correspond to the loca-tion of iron species in the silica matrix of the clay and the decreasein the intensity of the titania signal may imply adsorption of ironspecies on the surface of the TiO2 domains.

AFM analysesFigure 4 shows AFM images of MMT, 25% TiO2-MMT and 1%Fe-25% TiO2-MMT. For the supported composites, in addition



(a)

(b)

(c)

Figure 3. (A) SEM image of MMT. Inset: EDX spectrum of the whole image. (B) SEM image of 25% TiO2-MMT, Inset: EDX spectrum of an edged-aggregate.(C) SEM image of 1% Fe-25% TiO2-MMT, Inset (a) EDX spectrum of a bright edged-aggregate, and Inset (b) EDX spectrum of right-hand edged-aggregate.



Figure 4. Three-dimensional (3-D) AFM images of (a) MMT, (b) 25% TiO2-MMT, (d) 1% Fe-25% TiO2-MMT. Two dimensinonal (2-D) top views and particlesize distribution plots of (c) 25% TiO2-MMT and (e) 1% Fe-25% TiO2-MMT.



Figure 5. XPS survey analysis of 25% TiO2-MMT and 1% Fe-25% TiO2-MMT (a) core level spectra of Ti 2p (b) Fe 2p (c) and O 1 s (d) of 5% Fe-25% TiO2-MMT.

to the three-dimensional images, particle size distributionplots and two-dimensional images are presented. MMT alonereveals a rather smooth surface with a roughness value of0.3± 0.1 nm (Fig. 4(a)). TiO2 addition increases the roughnessvalue to 0.55± 0.1 nm. In addition, hemispherical aggregatesare detected in the three-dimensional image of 25% TiO2-MMT(Fig. 4(b)). One example is labeled with a size of 77.8 nm in the sizedistribution plot (Fig. 4(c)). In the image of 1% Fe-25% TiO2-MMT,in contrast to 25% TiO2-MMT, the number of dome-like aggregatesdecreases and also some are replaced by randomly distributedprotrusions (Fig. 4(d)). In the two-dimensional image, variationsin the sizes of particles are noticed. As an example, one particleis labeled with a size of 87.8 nm (Fig. 4(e)). Moreover, Fe loadingon the 25% TiO2-MMT substrate increases the roughness value(0.65± 0.1 nm) and creates a higher number of active adsorptionsites for the �-Naphthol molecule.

XPS analysesXPS analysis was performed on the 25% TiO2-MMT and 5% Fe-25%TiO2-MMT catalysts. A survey spectrum of the catalysts is illus-trated in Fig. 5(a) and the core level spectra of the characteristicelements of 5% Fe-25% TiO2-MMT are represented in Fig. 5(b)–(d).

Binding energy shifts are observed in the samples and hence, theinstrument is calibrated using the carbon peak (C-1 s) at 285 eVas in the other studies.30,31 The survey scan of 25% TiO2-MMTreveals Ti 2p peaks in addition to the Al (75.6 eV), Si (103 eV),Ca (347.07 eV), O (531.47 eV) and some Auger peaks of the clay(Fig. 5(a)). Fe loading does not alter the other peak positions andFe signals are clearly detected in the respective binding energies(Fig. 5(a)). Figure 5(b) displays a doublet corresponding to Ti 2p3/2and 2p1/2 core levels indicating the presence of a single oxida-tion state of Ti (Ti4+), typical of Ti in the TiO2 lattice (Fig. 5(b)).32,33

The Fe (2p) core level peaks are split into 2p3/2 (with total 68%spectral area) and2p1/2 (with total 32%spectral area) componentsdue to the spin–orbit coupling (Fig. 5(c)). Elemental Fe (706.3 eV)is not detected. Apart from the characteristic Fe 2p3/2 and Fe2p1/2 photoemission lines, the spectrum shows a shake-up satel-lite peak (713.55 eV) at c. 2.6 eV above the main Fe 2p3/2 peak(710.95 eV). This satellite structure is associated with the presenceof Fe2+.33,34 Moreover, the typical broadening of the lines in theFe 2p spectrum points to the presence of Fe3+. Therefore, 2p3/2and Fe 2p1/2 photoemission lines are deconvoluted by two sub-spectral components for Fe2+ (710.95 eV) and the Fe3+ (725.71)plus an additional contribution from the Fe2+ satellite peak at



Figure 6. Diffuse reflectance spectra (a) and Kubelka–Munk transformed reflectance spectra (b) of TiO2 (Degussa P-25), MMT and supported catalysts.

about 713.55 eV. Previous studies also reported that formation ofa satellite peak around 718.8 eV was a good indication of the exis-tence of Fe2O3.

35–38 However, such a peak is not clearly observ-able between 2p3/2 and 2p1/2 components. The peak noticedaround728.49 eVcannotbeassigned toFe3+ satellite peak, insteadit may be due to concomitant processes such as photoemissionand unresolved multiplet splitting.34 Consequently, an iron oxidewith mixed-valences, as in the form of Fe3O4, is expected to existin the supported catalyst system. This is confirmedwith the properfit of Fe2+ to Fe3+ ratio (0.32:0.68) to the stoichiometric constitu-tion of Fe3O4 (FeO.Fe2O3) (where Fe

2+ to Fe3+ is 1 to 2). Moreover,the mixed-valence existence of Fe is supposed to be importantfor the electron transfer reactions to enhance the photocatalyticactivities of the as-prepared catalysts. Meanwhile, the broad O 1 ssignal is deconvoluted by four subspectral components (Fig. 5(d)).MgO and TiO2 (529.71 eV, 11.5% spectral area), CaO and Al2O3

(531.61 eV, 42.8% spectral area), SiO2 and Fe3O4 (533.27 eV, 24.9%spectral area) and adsorbedwater (535 eV, 20.8% spectral area) areassigned as the constituents of the 5% Fe-25% TiO2-MMT.

UV–vis DRS analysesFigure 6 presents UV–vis absorption spectroscopies and band gapadsorption edges of the raw MMT, TiO2 (Degussa P-25) and thesupported catalysts. The raw MMT shows an extended absorptiontail between 200 and 600 nm (Fig. 6(a)). The characteristic absorp-tion edge of the TiO2 is not pronounced in the supported cata-lysts. The support seems to possess a masking role, however, thisdoes not affect the performances of the supported catalysts underUV-A irradiation, i.e. efficient degradation rates are noticeabledue the higher surface areas and larger pore volumes. The pres-ence of Fe increases the absorptions below 400 nm. In addition,profiles extend to the longer wavelength regions above 400 nm,corroborating the variations in the colors of the catalysts; frombeige (for 0.5 % Fe-25% TiO2-MMT) to light brown (for 5% Fe-25%TiO2-MMT). This can be attributed to the excitation of 3d electronsof Fe3+ to the conduction band of TiO2 or the charge transfersbetween interacting Fe ions (Fe2+ and Fe3+).39 Diffuse reflectancespectra of the catalysts after Kubelka–Munk (KM) transformationare shown in Fig. 6(b). Band gap energies are evaluated by lin-ear extrapolation and taking the intercept on the x-axis as 3.2 eVfor TiO2 (Degussa P-25), 3.1 eV for 1% Fe-25% TiO2-MMT, 3 eV for3% Fe-25% TiO2-MMT and 2.9 eV for 5% Fe-25% TiO2-MMT. For25%TiO2-MMT and 0.5% Fe-25%TiO2-MMT, band gap energies are

found to be higher than with the other catalysts (not shown). Thereductions in the band gap energies are consistent with absorp-tion profiles of 1–5% Fe-25% TiO2-MMT catalysts. The activities ofthese catalysts will be further investigated under visible light irra-diation.

Dark adsorption experiments and photocatalyticdegradation of �-naphtholPreliminary experiments involve photolysis of �-Naphthol anddark experiments over the rawMMT, TiO2 (Degussa P-25) and sup-ported catalysts (Fig. 7(a)). Negligible degradation of �-Naphtholis observed under UV-A irradiation without any catalyst. The per-centages of �-Naphthol remaining in solution were 86% after 1 hand 85% after 5 h mixing in the dark in the presence of only MMT.The �-Naphthol percentage remaining in solutionwas 62.5% in thepresence of TiO2 (Degussa P-25)within 120min and thenno signif-icant changewas noticed. However, 25% TiO2-MMT demonstratedrelatively lower percentages, indicating better adsorption capac-ity in comparison with the raw support and the TiO2 (DegussaP-25). The surface area differences of the MMT (28m2 g−1), TiO2

(Degussa P-25) (50m2 g−1) and 25% TiO2-MMT (94m2 g−1) mayexplore the variations in the adsorption abilities of these cata-lysts and the clay.40 The reduced dark adsorption capacities of3% Fe-25% TiO2-MMT and 5% Fe-25% TiO2-MMT catalysts in com-parison with 0.5% Fe-25% TiO2-MMT and 1% Fe-25% TiO2-MMTpoint out a limiting effect in Fe loading. The lowest percentagesof �-Naphthol were achieved as 52.7% and 50.7% in the pres-ence of 0.5% Fe-25% TiO2-MMT and 1% Fe-25% TiO2-MMT, respec-tively. The relatively higher percentages 71.5% (for 5% Fe-25%TiO2-MMT) and 67.3% (for 3% Fe-25% TiO2-MMT) remaining insolution can be attributed to the accumulation of Fe species onthe 25% TiO2-MMT substrate and hence, fewer available sites foradsorption of the �-Naphthol molecule.The activities of the supported catalysts were investigated under

UV-A irradiation (Fig. 7(b)). The following sequence was obtainedfor the efficiencies of the catalysts:1% Fe-25% TiO2-MMT> 0.5% Fe-25% TiO2-MMT> 25% TiO2-

MMT> TiO2 (Degussa P-25)> 3% Fe-25% TiO2-MMT> 5% Fe-25%TiO2-MMT.For all catalysts, immediately after equilibrium adsorption,

depletion in the concentration of �-Naphthol was continuous.The highest performances were in the presence of 1% Fe-25%TiO2-MMT and 0.5% Fe-25% TiO2-MMT signifying the promoting



Figure 7. (a) Results of �-Naphthol photolysis and dark adsorption experiments over the rawMMT and supported catalysts. (b) Photocatalytic degradationof �-Naphthol in the presence of supported catalysts under UV irradiation.

role of Fe loading in the resultant matrix. 25% TiO2-MMT demon-strates better photocatalytic activity than TiO2 (Degussa P-25).The superiority of 25% TiO2-MMT lies in the synergy createdbetween the clay and TiO2 nanoparticles, which eventually leadsto improved adsorption capacity and catalytic efficiency. 3%Fe-25% TiO2-MMT and 5% Fe-25% TiO2-MMT display the lowestactivities, consistent with the limited dark adsorption capacitiesof these catalysts.The photocatalytic degradation mechanism starts with the illu-

mination of TiO2 nanoparticles and production of electron–holepairs:

TiO2 + h� (UV − A) → e− + h+ (1)

These charged pairs can then either recombine or react withwater or oxygen molecules to produce reactive species:

e− + O2 → O•−2 (2)

h+ + H2O → H+ + HO• (3)

Formationof hydroperoxyl radical is possible by electron transferfrom TiO2 particles tomolecular oxygen and consecutive protona-tion:

e− + O2 + H+ → HO•−2 (4)

The addition of Fe3+ ions induces further reactions. Accordingto Soria et al., Fe3+ ions were accepted as trapping sites for bothe− and h+.41 Only h+ trapping ability was proposed by Moseret al.42 Regardless of these postulates, it has been accepted thatexistence of Fe3+ ions increases the photocatalytic efficiency sincethey can serve as shallow trapping sites. The importance of beinga shallow trapping site lies in the separation of arrival times ofelectrons and holes at the surface. If one of the charge carriersis temporarily trapped while the other migrates to the surfaceand is transferred to the adsorbate, the required condition for theseparationof electrons andholes canbe satisfied. In themeantime,mobility of the trapped carrier is possible by thermal excitation

from the trapping site. If this mobility can be achieved before thegeneration of a new e−/h+ pair, the recombination possibility isreduced. But, in the caseof adeeply trapped carrier, recombinationbecomes easier with the subsequent photon absorption and leadsto a drastic decrease in photocatalytic ability. Thus, the critical roleof the shallow trapping site is to allow the mobility of the carrierprior to the creation of new pairs. Although Fe3+ ions introduceshallow trapping sites, various research groups observed thathigher contents of Fe3+ ions reduce the efficiencies.43,44 At a highconcentration loading where Fe3+ ions act as trapping sites forboth e− and h+, the possibility for both charge carriers to betrapped will be high and hence, recombination of these pairsthrough quantum tunneling becomes inevitable.44 Similarly, at ahigh concentration loading, if Fe3+ ions act only as trapping sitesfor h+, the mobility of these charge carriers will be reduced sincethey may be trapped several times before reaching the catalystsurface. This, consequently, induces rapid recombination withthe electrons generated under continuing photon absorption.Therefore, anoptimum loading is expected; Fe3+ ions canact eitheras trapping sites for both charge carriers, or they may serve astrapping sites only for h+. This also expresses our experimentalresults under irradiation. Beyond 1% loading of Fe3+ ions, therecombination risk of charge pairs increases and reduces thecatalytic performance of the supported matrix.In this study, a simultaneous e− and h+ trapping role of Fe3+

ions is proposed in order to explain the formation of very reac-tive hydroxyl radicals (•OH) responsible for destruction of the�-Naphthol molecule. Fe2+ and Fe4+ ions form with the electronand hole trapping duty of Fe3+ ions, respectively:

Fe3+ + e− → Fe2+ (5)

Fe3+ + h+ → Fe4+ (6)

However, based on the crystal field theory, Fe2+ and Fe4+ arerelatively unstable on going from d5 to d6 (in the case of Fe2+)or from d5 to d4 (in the case of Fe4+) configurations. Hence, they



Scheme 1. Postulated mechanism for the photocatalyticdegradation route of �-Naphthol.

tend to return to Fe3+ (d5- stable half-filled high spin electronicconfiguration). Then, the reactionsmay proceedwith the Fe2+ ionswhich either react with adsorbedO2 on the surface of TiO2 or reactwith a neighboring surface Ti4+ ion:

Fe2+ + O2 (ads) → Fe3+ + O2− (7)

Fe2+ + Ti4+ → Fe3+ + Ti3+ (8)

For charge compensation, O2− can trap photogenerated holesand produce •OH radicals:

O2− + h+ → O− (9)

O− + h+ → O• (10)

O− + H2O (ads) →• OH + OH− (11)

In the meantime, the reduction of Fe4+ ions into Fe3+ form ispossible with again generation of •OH radicals:

Fe4+ + OH− (ads) → Fe3+ +• OH (12)

For the degradation mechanism of Naphthalene moleculedemonstrated by Hykrdová et al., many reaction pathways werefollowed taking into account all these reactive radical species,and molecular oxygen existing in the reaction media.45 Basedon that study, a mechanism was suggested for the degra-dation of �-Naphthol.25 Briefly, attack of a hydroxyl radicaldisturbs the Π-conjugation in the Naphthalene ring system(Scheme 1). Addition of oxygen and hydroperoxyl radical formsan unstable tetraoxide. The absence of degradation products,1,2-naphthalene-diol and 1,2-naphthalene-dione, in the LC-MSanalysis indicates their transformation to decarboxylated formswithin 180min irradiation times. Further intramolecular rear-rangements and hydroxyl radical attacks result in the productionof a ring-open structure. Accordingly, 2-hyroxycinamic acid(m/z= 164) (II) and muconic acid (m/z= 142) (III) are identified bythe molecular ion and mass fragment peaks, this being consistentwith the suggested photocatalytic degradation mechanism of�-Naphthol (Scheme 1).

Effect of pHPhotocatalytic degradation of �-Naphtholwas studied over the pHrange 3–11 (Fig. 8). During these experiments, pH of the solutionwas adjusted before irradiation and it was not controlled duringthe course of the reaction. Before the irradiation experiments, itwas observed that iron and TiO2 loading decreases the alkalinenature of the MMT solution (pH= 9.5) to pH= 4.5. Moreover,



Figure 8. Effect of pH on the photocatalytic degradation of �-Naphthol inthe presence of 1% Fe-25% TiO2-MMT.

a slight decrement is noticed in the natural pH of �-Naphtholsolution (pH= 5) with the addition of the catalyst matrix(pH 1%Fe-25%TiO2-MMT+�-Naphthol solution = 4.19). Following the irradi-ation, the remaining �-Naphthol percentages were 60.7%, 12.4%,44.2% and 81.2% at pH values 3, 4.19, 7 and 11, respectively.For the pH values (3, 4.19 and 7) lower than the pKa (9.51) of�-Naphthol, the possible interaction is expected to occur viahydrogen bonding between the –OH group of �-Naphthol andthe catalyst O-containing groups.46 However, the strong acidicmedia at pH= 3 protonates the catalyst O-groups and decreasesthe possibility of hydrogen bonding. On the contrary, the absenceof additional hydrogen ions at the natural pH value (4.19) enablesthe formation of sufficient hydrogen bonding and hence, facil-itates the photocatalytic activity. At pH= 7, the adsorption of

Table 2. LC-MS retention times, mass fragments and molecularstructures of intermediate products and �-Naphthol

Retentiontime (min)

Molecularweight (m/z) Molecular structure

2.3 142

COOHCOOH

6.7 144

OH

12.3 164

COOHOH

OH− anions may be preferred by the catalyst surface due to thesmaller sizes of these anions in comparison with the �-Naphtholmolecule. This accelerates transportation of the holes to thecatalyst surface and their reactions with OH− anions and H2Omolecules to generate hydroxyl radicals. Hence, the photocat-alytic degradation of �-Naphthol increases by depleting theamount of �-Naphthol remaining in solution. When the pH (11)exceeds the pKa value (9.51), both the –OH group on �-Naphtholand the O-containing groups on 1% Fe-25% TiO2-MMT catalyst areionized and the hydrogen bonding effect is significantly impeded.The dominating electrostatic repulsion and the enhanced com-petition between the anionic form of �-Naphthol and OH− anionsreduce the photocatalytic performance of the catalyst. Basedon these results, the optimized pH value (4.19), with the lowest�-Naphthol percentage remaining in solution, is used throughoutthe experiments.

Kinetics and catalyst stabilityKinetic analysis was performed by varying the initial �-Naphtholconcentration from 25 to 3mg L−1 in the presence of 1% Fe-25%TiO2-MMT (Fig. 9(a)). The linearity obtained between ln(C0/C)

(a) (b) (c)

Figure 9. (a) Kinetic analysis, inset: pseudo-first-order fit; (b) Langmuir–Hinshelwood kinetic model application to 1% Fe-25% TiO2-MMT; (c) results ofrecycling studies for 1% Fe-25% TiO2-MMT.



Table 3. Kinetic parameters at different initial concentrations of�-Naphthol in the presence of 1% Fe-25% TiO2-MMT

[�-Naphthol] (mg L−1) k (min−1) R t1/2 (min)

25 0.002 0.986 24014.4 0.007 0.996 307.2 0.009 0.994 303 0.012 0.985 30

versus t plot indicates pseudo-first-order kinetics where C0 is theinitial concentration of �-Naphthol (mg L−1) and C is the concen-tration of �-Naphthol (mg L−1) at irradiation time t (min) (Fig. 9(a)inset). The kinetic parameters are shown in Table 2 where the rateconstants and half-lives are listed. The rate-constants, k (min−1)hence calculated (slopes of the lines) are found to decrease withincreasing concentration of �-Naphthol from 0.012min−1 (for3mg L−1) to 0.002min−1 (for 25mg L−1) (Table 3). This can be aresult of blocking of the photocatalytically active sites on the sup-ported catalyst and reducing the interaction of photonswith thesesites. Half lives are about 30min for 3–14.4mg L−1 �-Naphtholconcentrations, however 25mg L−1 �-Naphthol concentrationreveals 240min half life.A Langmuir–Hinshelwood kinetic model was tested for 1%

Fe-25%TiO2-MMTusing the rearranged formof the equation:47–49

1R= 1

kKC0

+ 1k

(13)

where R is the rate of degradation, K is the adsorption coeffi-cient of �-Naphthol onto the surface of the catalyst (Lmg−1), k isthe reaction rate constant (mg L−1 min−1). Validity of the modelis confirmed by the linearity of the plot of reciprocal of rates(1/R) against reciprocal of initial �-Naphthol concentrations (1/C0)(Fig. 9(b)). The values of K and k were found to be 0.37 Lmg−1 and0.0243mg L−1 min−1.The stability of 1% Fe-25% TiO2-MMT catalyst was examined by

recycling experiments (Fig. 9(c)). For each new cycle, the photo-catalyst was filtrated, washed and calcined at 500 ∘C for 2 h bykeeping other reaction conditions constant. After four cycles, per-centage degradation of �-Naphthol decreased by approximately6.5%, from 97% to 90.5%, indicating that the photocatalytic activ-ity of 1% Fe-25% TiO2-MMT has repeatability. The reduction inpercentage degradation over four cycles may be explained by theformation of by-products and their accumulation on the activesurface sites of the catalyst or by the loss of photocatalysts duringeach collection and rinsing step.

CONCLUSIONSIn this study, Fe-loaded TiO2-supported MMT catalysts were pre-pared and characterized by XRD, BET, SEM (EDX), AFM, XPS andUV–vis DRS techniques. Anatase reflections were observed in thehigh angle ranges of the XRD spectra. AFM and SEM imagesrevealed obvious variations in the surface morphology of the rawMMT after TiO2 addition and Fe loading. In the SEM images andEDX analysis, distinct-sharp edged aggregates were obtainedwithtitania-enriched signals. The evidence of Fe in the supported cat-alysts was achieved by EDX, XPS and UV–vis DRS analysis. Themixed-valence existence of Fe was found to be efficient in elec-tron transfer reactions improving the photocatalytic activities of

the supported catalysts in �-Naphthol degradation. The good per-formance was also attributed to the transformation of MMT to amesoporous structure. Based on these results, the ternary systemestablished in this study can be employed as a catalyst in photo-catalytic degradation reactions.

ACKNOWLEDGEMENTSThe author gratefully acknowledges the financial support ofBogaziçi University Research Foundation (Project No. 13B05P7).

REFERENCES1 Wicklein B, Darder M, Aranda P and Ruiz-Hitzky E, Bio-organoclays

based on phospholipids as immobilization hosts for biologicalspecies. Langmuir 26:5217–5225 (2010).

2 Papoulis D, Komarneni S, Nikolopoulou A, Tsolis-Katagas P, Pana-giotaras D, Kacandes P, Zhang HG, Yin S, Sato T and Katsuki H,Palygorskite- and Halloysite-TiO2 nanocomposites: synthesis andphotocatalytic activity. Appl Clay Sci 50:118–124 (2010).

3 Rossetto E, Petkowicz DI, dos Santos JHZ, Pergher SBC and PenhaFG, Bentonites impregnated with TiO2 for photodegradation ofmethylene blue. Appl Clay Sci 48:602–606 (2010).

4 Wang C, Shi H, Zhang P and Li Y, Synthesis and characterizationof kaolinite/TiO2 nano-photocatalysts. Appl Clay Sci 53:646–649(2011).

5 Mao H, Li B, Li X, Yue L, Xu J, Ding B, Gao X and Zhou Z, Facile syn-thesis and catalytic properties of titanium containing silica-pillaredclay derivatives with ordered mesoporous structure through anovel intra-gallery templating method. Micropor Mesopor Mater130:314–321 (2010).

6 Gao X, Mao H, LuM, Yang J and Li B, Facile synthesis route to NiO–SiO2intercalated clay with ordered porous structure: Intragallery interfa-cially controlled functionalization using nickel–ammonia complexfor deep desulfurization.MicroporMesoporMater 148:25–33 (2012).

7 Kaneko T, ShimotsumaHandKajikawaM, Synthesis andphotocatalyticactivity of titania pillared clays. J PorousMater 8:295–301 (2001).

8 Shimizu K, Kaneko T, Fujishima T, Kodama T, Yoshida H and KitayamaY, Selective oxidation of liquid hydrocarbons over photoirradiatedTiO2 pillared clays. Appl Catal A: Gen 225:185–191 (2002).

9 OokaC, YoshidaH,HorioM, Suzuki K andHattori T, Adsorptive andpho-tocatalytic performance of TiO2 pillared montmorillonite in degra-dationof endocrinedisruptors havingdifferenthydrophobicity.ApplCatal B: Environ 41:313–321 (2003).

10 Mogyorosi K, Dekany V and Fendler JH, Preparation and characteri-zation of clay mineral �ntercalated titanium dioxide nanoparticles.Langmuir 19:2938–2946 (2003).

11 Zhu HY, Li JY, Zhao JC and Churchman GJ, Photocatalysts preparedfrom layered clays and titanium hydrate for degradation of organicpollutants in water. Appl Clay Sci 28:79–88 (2005).

12 Kameshima Y, Tamura Y, Nakajima A and Okada K, Preparation andproperties of TiO2/montmorillonite composites. Appl Clay Sci45:20–23 (2009).

13 Li Y, Liu JR, Jia SY, Guo JW, Zhuo J and Na P, TiO2 pillared montmo-rillonite as a photoactive adsorbent of arsenic under UV irradiation.Chem Eng J 191:66–74 (2012).

14 Tahir M and Amin NS, Photocatalytic CO2 reduction with H2O vaporsusing montmorillonite/TiO2 supported microchannel monolithphotoreactor. Chem Eng J 230:314–327 (2013).

15 Chmielarz L, Zbroja M, Kustrowski P, Dudek B, Rafalska-Lasocha A andDaiembaj R, Pillared montmorillonites modified with silver. J ThermAnal Cal 77:115–123 (2004).

16 Liu J, Li X, Zuo S and Yu Y, Preparation and photocatalytic activityof silver and TiO2 nanoparticles/montmorillonite composites. ApplClay Sci 37:275–280 (2007).

17 Chen K, Li J, Li J, Zhang Y and Wang W, Synthesis and characteriza-tion of TiO2 –montmorillonites doped with vanadium and/or car-bon and their application for the photodegradation of sulphorho-damine B under UV–vis irradiation. Colloid Surf A: PysicochemEngAs360:47–56 (2010).

18 Bineesh KV, Kim DK, Kim MI and Park DW, Selective catalytic oxidationof H2S over V2O5 supported on TiO2-pillared clay catalysts in thepresence of water and ammonia. Appl Clay Sci 53:204–211 (2011).



19 Chen K, Li J, Wang W, Zhang Y, Wang X and Su H, The preparationof vanadium-doped TiO2-montmorillonite nanocomposites and thedegradation of sulforhodamine B under visible light irradiation.ApplSurf Sci 257:7276–7285 (2011).

20 Fatimah I, Shukla PR and Kooli F, Combined photocatalytic and fentonoxidation of methyl orange dye using iron exchanged titaniumpillared montmorillonite. J Appl Sci 9:3715–3722 (2009).

21 Bovey J, Kooli LF and Jones W, Preparation and characterization ofTi-pillared acid activated clay catalysts. Clay Minerals 31:501–506(1996).

22 Yuan L, Huang D, Guo W, Yang Q and Yu J, TiO2/montmorillonitenanocomposite for removal of organic pollutant. Appl Clay Sci53:272–278 (2011).

23 KitayamaY, KodamaT, AbeM, ShimotsumaHandMatsudaY, Synthesisof titaniapillared saponite in aqueous solutionof acetic acid. JPorousMater 5:121–126 (1998).

24 KanekoT, FujiM, KodamaTandKitayamaY, Synthesis of Titania pillaredMica in aqueous solution of acetic acid. J Porous Mater 8:99–109(2001).

25 Ökte AN and Say�nsöz E, Characterization and photocatalytic activityof TiO2 supported sepiolite catalysts. Sep Purif Technol 62:535–543(2008).

26 Ökte AN and Yilmaz Ö, Photodecolorization of methyl orange byyttrium incorporated TiO2 supported ZSM-5. Appl Catal B: Environ85:92–102 (2008).

27 Ökte AN and Yilmaz Ö, La and Ce loaded TiO2-ZSM-5 catalysts: com-parative characterization and photocatalytic activity investigations.Micropor Mesopor Mater 126:245–252 (2009).

28 Cecilia JA, Garcia-Sancho C and Franco F, Montmorillonite basedporous clay heterostructures: influence of Zr in the structure andacidic properties.Micropor Mesopor Mater 176:95–102 (2013).

29 Ismail AA, Bahnemann DW and Al-Sayari SA, Synthesis and photocat-alytic properties of nanocrystalline Au, Pd and Pt photodepositedonto mesoporous RuO2 –TiO2 nanocomposites. Appl Catal A: Gen431:62–68 (2012).

30 Lupan O, Chow L, Chai G, Roldan Cuenya B, Naitabdi A, Schulte A andHeinrich H, Nanofabrication and characterization of ZnO nanorodarrays and branchedmicrorods by aqueous solution route and rapidthermal processing.Mater Sci Eng B 145:57–66 (2007).

31 Lupan O, Emelchenko GA, Ursaki VV, Chai G, Redkin AN, GruzintsevAN, Tiginyanu IM, Chow L, Ono LK, Roldan Cuenya B, Heinrich H andYakimov EE, Synthesis and characterization of ZnO nanowires fornanosensor applications.Mater Res Bull 45:1026–1032 (2010).

32 Biesinger MC, Lau LWM, Gerson AR and Smart RStC, Resolving surfacechemical states in XPS analysis of first row transition metals, oxidesand hydroxides: Sc, Ti, V, Cu and Zn. Appl Surf Sci 257:887–898(2010).

33 Yu X and Shen Z, Photocatalytic TiO2 films deposited on ceno-sphere particles by pulse magnetron sputtering method. Vacuum85:1026–1031 (2011).

34 Gracia M, Marco JF, Gancedo JR, Exel W and Meisel M, A surface spec-troscopic study of the corrosion of ultrathin 57Fe evaporated andLangmuir–Blodgett films inhumid SO2 environments. Surf InterfaceAnal 29:82–91 (2000).

35 Garrido-Ramírez EG, Mora ML, Marco JF and Ureta-Zañartuc MS, Char-acterization of nanostructured allophane clays and their use as sup-port of iron species in a heterogeneous electro-Fenton system. ApplClay Sci 86:153–161 (2013).

36 Yamashita T and Hayes P, Analysis of XPS spectra of Fe2+ and Fe3+ ionsin oxide materials. Appl Surf Sci 254:2441–2449 (2008).

37 Grosvenor AP, Kobe BA, Biesinger MC and McIntyre NS, Investigationof multiplet splitting of Fe 2p XPS spectra and bonding in ironcompounds. Surf Interface Anal 36:1564–1574 (2004).

38 Wang H, Fei X, Wang L, Li Y, Xu S, Sun M, Sun L, Zhang C, Li Y, YangQ andWei Y, Magnetically separable iron oxide nanostructures-TiO2nanofibers hierarchical heterostructures: controlled fabrication andphotocatalytic activity. New J Chem 35:1795–1802 (2011).

39 Palanisamy B, Babu CM, Sundaravel B, Anandan S and MurugesanV, Sol–gel synthesis of mesoporous mixed Fe2O3/TiO2 photocata-lyst: application for degradation of 4-chlorophenol. J Hazard Mater252–253:233–242 (2013).

40 Adan C, Bahamonde A, Oller I, Malato S andMartinez-Arias A, Influenceof iron leaching andoxidizing agent employedon solar photodegra-dation of phenol over nanostructured iron-doped titania catalysts.Appl Catal B Environ 144:269–276 (2014).

41 Qourzal S, Barka N, Tamimi M, Assabbane A, Nounah A, Ihlal Aand Ait-Ichou Y, Sol–gel synthesis of TiO2 –SiO2 photocatalystfor �-naphthol photodegradation. Mater Sci Eng C 29:1616–1620(2009).

42 Soria J, Conesa JC, Augugliaro V, Palmisano L, SchiavelloM and SclafaniA, Dinitrogen photoreduction to ammonia over titanium dioxidepowders doped with ferric ions. J Phys Chem 95:274–282 (1991).

43 Moser J, GrätzelMandGallay R, Inhibitionof electron–hole recombina-tion in substitutionally doped colloidal semiconductor crystallites.Helv Chim Acta 70:1596–1604 (1987).

44 Wang C, Bahnemann DW and Dohrman JK, A novel preparation ofiron-doped TiO2 nanoparticles with enhanced photocatalytic activ-ity. Chem Commun 16:1539–1540 (2000).

45 Litter M and Navia JA, Photocatalytic properties of iron-doped tita-nia semiconductors. J Photochem Photobiol A: Chem 98:171–181(1996).

46 Hykrdová L, Jirkovsky J, Mailhot G and Bolte M, Fe(III) photoinducedand Q-TiO2 photocatalysed degradation of naphthalene: compari-son of kinetics and proposal of mechanism. J Photochem PhotobiolA: Chem 151:181–193 (2002).

47 ChenW, Duan L, Wang L and Zhu D, Adsorption of hydroxyl and aminosubstituted aromatics to carbon nanotubes. Environ Sci Technol42:6862–6868 (2008).

48 Suchithra PS, Shadiya CP, Peer Mohamed A, Velusamy P and Anan-thakumar, One-pot microwave mediated growth of heterostruc-tured ZnO@AlSi as a potential dual-function eco-catalyst for treat-ing hazardous pollutants in water resources. Appl Catal B: Environ130–131:44–53 (2012).

49 Kim SH, Ngo HH, Shon HK and Vigneswaran S, Adsorption and pho-tocatalysis kinetics of herbicide onto titanium oxide and powderedactivated carbon. Sep Purif Technol 58:335–342 (2008).


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