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for photocatalytic degradation of Orange G dye [18]. While Tzom-pantzi et al. synthesized lamellar double hydroxides of ZnAlLa andapplied them for removal of phenol [16]. These mixed oxides semi-conductors have a high surface area, basic properties in an aqueousmedium, thermal and structural stabilities, relatively low cost andare easily and reused [19], all desirable properties for photocata-lysts.

LDHs are interesting alternatives for dispersal and supportof TiO

2 nanoparticles since they do not harm semiconduc-

tor characteristics [12,19,20]. Furthermore, TiO2/LDH compositephotocatalysts have much higher sedimentation velocities thanTiO2nanoparticles, facilitating their removal by physical processes[21].

Combined ZnOTiO2photocatalysts have been described in theliterature [15,2225] but no mention has been made to date of photocatalysts synthesized with ZnO obtained by calcination ofdouble hydroxides layer by calcining the double ternary layer (Mg,Zn and Al) impregnated with TiO2 nanoparticles (TiO2/MgZnAl).ZnO formed by calcination of TiO2/MgZnAl composite may interactsynergistically with TiO2 leading to more efficient photodegra-dation of organic compounds. We therefore studied the effect ofvarying Zn2+/Mg2+ molarratiosinTiO2/MgZnAlcompositesonphe-nol photodegradation under UVvis irradiation (filter cut-off for>300nm). The novel TiO2/MgZnAl photocatalysts with poten-tial for phenol photodegradation prepared in this work at differentZn2+/Mg2+ molar ratios (1%, 5%, 10% and 15%) have not yet, to thebest of our knowledge, been reported in literature.

2. Experimental

2.1. Reagents and materials

TiO2 u sed was the P25 acquired from Degussa, and con-sists of 70% anatase and 30% rutile. The commercial reagentsMg(NO3)26H2O, Al(NO3)39H2O, Zn(NO3)26H2O, NaOH, Na2CO3and phenol were obtained from SigmaAldrich and used as

received. All solutions were prepared with analytical gradereagents and high purity deionized water produced by a Milli-Q

system (Millipore, Bedford, MA, USA).

2.2. Preparation of TiO2/MgZnAl

MgZnAl LDH with TiO2 nanoparticles was prepared at molarratio (Zn2+ + Mg2+):Al3+:Ti4+ equal to 2:1:1 in the presence of NaOH and Na2CO3. For this, 100 mL of solution containingMg(NO3)26H2O, Al(NO3)39H2O and Zn(NO3)26H2O were addeddropwise at a rate of 60mL/h, to 100mLof an alkaline solutioncontaining a fixed proportion of TiO2 nanoparticles (0.0255molTi), NaOH (2.45 mol/L) and the anion to be intercalated Na2CO3

(1.23 mol/L) under vigorous stirring at room temperature. Thegelatinous precipitate containing TiO2 were stirred at room tem-perature for a further 140min after which the pH was adjusted to1010.5 and the mixtures were transferred to a porcelain crucibleand placed in an oven at 60C for 18h. Subsequently, the suspen-sions were washed with deionized water until reaching pH7andthe precipitates obtained were dried at 80C for 16 h and thencalcined in a muffle furnace at 500C for 4 h.The calcinedphotocat-alyst composite weredesignatedusingthe formula TiO2/MgZnAl-R,where R represents the approximate percent molar ratio of Zn2+ toMg2+ (1%, 5%, 10% or 15%). According to this scheme the compos-ites were denominated TiO2/MgZnAl-1 (TiO2/Mg0.0505Zn0.0005Al),TiO2/MgZnAl-5 (TiO2/Mg0.0484Zn0.0025Al), TiO2/MgZnAl-10 (TiO2/Mg0.0459Zn0.0051Al)andTiO2/MgZnAl-15(TiO2/Mg0.0433Zn0.0076Al),

respectively. The chemical formulae in parentheses represent the

initial composition of preparation, where x and y stands for thecontent of magnesium and zinc, respectively.

The calcined composite without zinc was designated asTiO2/MgAl (TiO2/Mg0.0510Al) and the non-calcined compos-ites as TiO2/MgAl-CO3, TiO2/MgZnAl-CO3-1, TiO2/MgZnAl-CO3-5,TiO2/MgZnAl-CO3-10 and TiO2/MgZnAl-CO3-15.

The MgAl-CO3 (Mg0.0510Al-CO3) and MgZnAl-CO3-5(Mg0.0484Zn0.0025Al-CO3-5) LDHs were prepared by the proce-dure previously described but without incorporation of TiO

2.

The hydrotalcite calcined at 500 C was denominated MgAl(Mg0.0510Al).

2.3. Photocatalyst characterization

Determination of the final photocatalyst chemical compositionswas made by mixing 0.100g of each sample with 4 mLHNO3(65%),4 mLHCl(37%)and1mLHF (40%) followed by digestion in anindus-trial microwave oven (Milestone ETHOS) by heating to 230 C in15min and maintaining this temperature for 25min at 1.200 W.Thedigested samples were dilutedand metals concentrations weredetermined by ICP-MS, Perkin-Elmer model NexION 300D.

Textural properties of the samples were analyzed by their N2

adsorptiondesorption isotherms using a surface area and poresize analyzer (NOVA 2200e Quantachrome Instruments, Boyn-ton Beach, FL, USA). Prior to measurement, the samples weredegassed at 110 C for 4h. Surface areas were estimated by theBrunauerEmmettTeller (BET) method and pore volumes weredetermined by the HorvthKawasoe (HK) method applied to thedesorption branch.

X-ray diffraction (XRD) was measured by 2 scans usinga Bruker model D8 Discover diffractometer using Cu K radi-ation (=0.1541nm) with an angular variation of 580 angle(2) and 0.05 s1 scan rate. The Powder Diffraction File (PDF)database (JCPDS, International Center for Diffraction Data) wasused to identify crystalline phases. Network parameters (dh k l) ofthe composites were calculated according to the Bragg equation:

= 2d sen where is the wavelength of X-rays (=0.1541nm),and is the diffraction angle [18].Scanning electron microscopy(SEM) images wereobtainedafter

sample mettalization with gold using a JEOL JSM-6010/LA micro-scope.The SEMequipment wasequipped withan energydispersivespectrometrysystem(EDS) foranalysis of samplechemical compo-sition.

Infrared spectroscopy(IR) were obtained using a VARIAN660-IRspectrophotometerequippedwith an attenuated reflectance acces-sory PIKE GladiATR in the region of 4004000cm1.

Diffuse reflectance spectra (RD) were acquired (referenceBaSO4) in a dual beam spectrophotometer model GBC 20 CINTRAat 0.5 nm intervals at a scanspeed of10 nm/minwith a 2.0 mm gapwidth.

2.4. Photocatalytic activity

Photocatalytic activity of the composites with different molarratios of Zn2+/Mg2+ was assessed by photodegradation of a phe-nol solution in an annular photoreactor shown schematically inFig. 1. The reactorconsisted of a Pyrex glass cylinder (40.0 cm long,4.0cm internal diameter, filter cut-off for>300nm) witha 125 Wmercury vapor lamp (HQL, Osram without the bulb protector) atits center within a concentric glass cylinder (7cm diameter, 60cmheight,1000 mLtotal capacity) surrounded by a recirculating waterbath maintained at 302 C. Reactions were run with 300 mLof a50mg/Lphenolsolutionand300mg of catalyst magnetically stirredin the dark for 60min to establish adsorptiondesorption equilib-

rium and then exposed to UVvis radiation for 360 min.

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M.F. de Almeidaet al. / Applied Surface Science 357 (2015) 17651775 1767

Fig. 1. Illustration of the annular photoreactor.

Three milliliters of the reaction solution were removed using asyringe every 60min for 360 min and filtered through a 0.45Mmembrane (Millipore) for phenol and total organic carbon (TOC)quantification.

The monitoring of the remaining phenol concentrations werecarried out by High Performance Liquid Chromatography using a

1260 Infinity system (Agilent Technologies) with a linear photo-diode array detector (HPLC-DAD) at 270nm, isocratic mode,reversed-phase Zorbax Eclipse Plus C18 (4.6mm150mm,5.0m) column, injected volume 10L, column temperature of30 C and a flow of mobile phase methanol:water (55:45, v/v) at1.0mL/min. Totalorganic carbonwas quantifiedusinga TOC-5000Aanalyzer from Shimadzu.

Photolysis was evaluated using 300mL of phenol solution(50 mg/L, initial pH 5.2) under UVvis radiation without catalyst.For adsorption tests, 300mg of sample were added to 300 mLofphenol solution (50mg/L, initial pH 5.2) without UVvis radiation.Photolysis and adsorption studies were also run for 360 min and3 mL were removed at 60min intervals for phenol quantificationby HPLC-DAD. All studies were performed in triplicate.

1.00.80.60.40.20.0

Volumeadsorbed(a.u

.)

Relative Pressure (P/P0)

A

B

C

D

E

F

G

H

Fig. 2. N2 adsorptiondesorption isoterms of photocatalyst samples: TiO2 (A),MgAl (B), TiO2/MgAl (C), MgZnAl-5 (D), TiO2/MgZnAl-1 (E), TiO2/MgZnAl-5 (F),TiO2/MgZnAl-10 (G)and TiO2/MgZnAl-15 (H).

2.5. Reuse assays

Photocatalysts were washed with deionized water and driedbefore reuse. The photocatalysts were characterized by X-raydiffraction techniques and infrared spectroscopy before and aftereach photodegradation cycle to verify its stability.

3. Results and discussions

3.1. Photocatalyst characterization

3.1.1. Chemical composition and textural analysesTable 1 presents the final metal composition of the synthe-

sized photocatalysts in which final (Zn2+ + Mg2+)/Al3+,Ti4+/Al3+ andZn2+/Mg2+ molar ratios found were lower than theoretically pre-dicted. These differences may have been caused by incompleteincorporation of cations inside the lamellar of LDH or preferen-tial precipitation of cations as hydroxide [12,18,26]. In either case,the molar ratios obtained for the photocatalysts proved that thecalcination process caused no significant loss of metal elements.

The N2 adsorptiondesorption isotherms are shown in Fig. 2.The prepared materials present type IV isotherms according to theIUPAC classification that correspond to mesoporous materials [17].

Table 1

Initial and final chemical composition, final molar ratio and textural analysis of synthesized photocatalysts.

Sample Chemical composition Molar ratio (final) SBET(m2/g) Pore v olume(cm3/g)

Initial Final aZn2+/Mg2+ b(Zn2+ + Mg2+)/Al3+ bTi4+/Al3+

TiO2 48.4 0.0178MgAl Mg0.0510Al0.0255 Mg0.0402Al0.0210 1.91 160.1 0.0428MgZnAl5 Mg0.0484Zn0.0025 Al0.0255 Mg0.0442Zn0.0021 Al0.0260 4.75 1.78 123.3 0.0495TiO2/MgAl TiO2(0.0255)/Mg0.0510Al0.0255 TiO2(0.0208)/Mg0.0504Al0.0281 1.80 0.74 100.5 0.0321TiO2/MgZnAl-1 TiO2(0.0255)/Mg0.0505 Zn0.0005Al0.0255 TiO2(0.0166)/Mg0.0418Zn0.0004 Al0.0221 0.92 1.91 0.75 99.4 0.0398TiO2/MgZnAl-5 TiO2(0.0255)/Mg0.0484Zn0.0025 Al0.0255 TiO2(0.0193)/Mg0.0470Zn0.0022 Al0.0270 4.68 1.82 0.71 89.7 0.0406TiO2/MgZnAl-10 TiO2(0.0255)/Mg0.0459Zn0.0051 Al0.0255 TiO2(0.0187)/Mg0.0436Zn0.0041 Al0.0269 9.41 1.77 0.69 85.0 0.0353TiO2/MgZnAl-15 TiO2(0.0255)/Mg0.0433Zn0.0076 Al0.0255 TiO2(0.0236)/Mg0.0496Zn0.0071 Al0.0324 14.30 1.75 0.73 79.9 0.0307

a Zn2+/Mg2+ molar ratiosin percent.b

The (Zn2+

+ Mg2+

)/Al3+

and Ti4+

/Al3+

molar ratios were kept constant.

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8070605040302010

H

(003)

(003)G

A(200) (113)

(110)(009)

R(110)

A(101)

(006)

(003)

F(003)

E

(116)

(113)(110)

(018)(015)

(009)(006)(003)

D

A(204)(110)A(200)

R(110)(009)

R(110)

A(101)

(006)

(003)

C

(116)

(113)

B

(110)

(018)(015)(009)

(006)(003)

A(216)

A(220)A(116)A(204)

A(211)

A(105)

A(200)

R(111)

A(004)

R(110)

R(110)

Intensity(a.u.)

2

A

A(101)

Fig. 3. X-ray diffraction spectra of the non-calcined samples: TiO2 (A), MgAl-CO3(B), TiO2/MgAl-CO3 (C), MgZnAl-CO3-5 (D), TiO2/MgZnAl-CO3-1 (E), TiO2/MgZnAl-CO3-5 (F) , TiO2/MgZnAl-CO3-10 (G) and TiO2/MgZnAl-CO3-15 (H). A, anatase; R,rutile.

Hysteresis loops were of type H3, indicating the presence of asym-metric pores (non-uniform size and shape) created by the collapseof LDH lamella upon calcination [19]. The textural properties of all

samples arelistedin Table 1. The highestsurface area wasfound forMgAl and the lowest for TiO2. The textural properties determinedfor MgAl and TiO2were in agreement with values reported in theliterature [19,27]. Combined TiO2/MgAl had a much larger specificsurface area than pure TiO2because of the presence of MgAl LDH.The difference between TiO2/MgAl and MgAl surface areas wascaused by TiO2 nanoparticle blockage. These nanoparticles werepreviously shown to partially block supporter pores, resulting in adecrease in BET surface area of hydrotalcite (MgAl) [28]. As shownin Table 1, TiO2/MgZnAl-R composite surface areas decreased grad-ually with the increase in zinc (Zn2+/Mg2+) content, in agreementwith other studies [8,17,29]. TiO2/MgZnAl-5 presented the highestsurface area and pore volume of the zinc-containing composites(Table 1).

3.1.2. X-ray diffractionThe XRD spectra obtained for the non-calcined samples are

shown in Fig. 3. Fig. 3A shows the main peaks for TiO2. The peaksare labeled A(hk l) or R(hk l) relating to the anatase (JCPDS #21-1272)) or rutile (JCPDS #21-1276) phases, respectively, and agreethe crystallographic standards found in the literature [3032].

MgAl-CO3(Fig. 3B) exhibited diffraction peaks with interplanardistances of 7.69A(003),3.79A (00 6), 2.59A (009),2.32A(015),1.90A (01 8), 1.52A (11 0), 1.49A (1 1 3 ) and 1.41A (11 6), alsoagreeing with the values cited in the literature [3335]. The peaks(003)and(006)arebasalpeaksthatconfirmthesolidobtainedhasa double layer structure, with a high degree of crystallinity, whichcorresponds to hydrotalcite-like material (JCPDS # 22-0700) [35].

The ternary LDH of the MgZnAl-CO3-5 (Fig. 3D) showedinterplanar

Fig. 4. X-ray diffraction spectra of the calcined samples: MgZnAl-5 (A), TiO2 (B),TiO2/MgAl (C), TiO2/MgZnAl-1 (D), TiO2/MgZnAl-5 (E), TiO2/MgZnAl-10 (F) andTiO2/MgZnAl-15 (G). A, anatase; R, rutile.

distances of 7.59A (00 3), 3.80A (006),2.59A(009),2.33A(015),1.90A (01 2), 1.52A (11 0) and 1.49A (11 3), similar to the peaksin the literature [29].

In the synthesized composite impregnated with TiO2nanopar-ticles in the LDH (Fig. 3C and Fig. 3EH) one can observe thediffraction peaks for LDH (2= 11.6, 23.4, 34.6, 60.7 and 62) and fortheTiO2 in itsanatase andrutile phase.However,thereis a decreasein the intensity of the reflection peaks, related to the LDH. This isprobably due to the disorder caused by the random incorporationof TiO2nanoparticles inside the LDH lamellar structure can lead tobroad peaks anda lowintensity [20,21,36,37]. One canalso observean increase in the (0 0 3) inter-layer distance from 7.69 (MgAl-CO3)to 7.72A (TiO2/MgAl-CO3) and from 7.59A in MgZnAl-CO3-5 to7.62A in TiO2/MgZnAl-CO3-5 caused by TiO2incorporation.

On the other hand, the substitution of Mg2+ by Zn2+, there is adecrease in the interlayer distance, from 7.72, for TiO2/MgAl-CO3photocatalyst to 7.66A for TiO2/MgZnAl-CO3-1. This decrease in

interlayerdistance continues with the increase in Zn+2/Mg+2 molarratio in the composites TiO2/MgZnAl-CO3-5 (7.62A),TiO2/MgZnAl-CO3-10 (7.60A) and TiO2/MgZnAl-CO3-15 (7.56A). This behaviorcan be explained by an increase in the charge density in the layerbecause of the higher electronegativity of zinc compared to themagnesium [29]. Incorporation of a cation with a greater elec-tronegativity increased the forces of attraction and decreased theinterlayer distance between the hydrotalcite layers [29].

After calcination, there is a collapse of LDH structure formingmixed oxides of metals (Fig. 4) [29,38]. The calcined MgZnAl-5 (Fig. 4B) shows intense peaks related to ZnO and also MgO(periclase) similar to crystallographic descriptions in the litera-ture [7,8,30,36]. Peaks related to TiO2and magnesium oxide wereobserved in the TiO2/MgAl composite (Fig. 4C). In the composite

containing zinc (Fig. 4DG), a peak related to the ZnO (2=34.9

)

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Fig. 5. SEMimage of thecomposite TiO2/MgZnAl-CO3-5 (A) and TiO2/MgZnAl-5 (B).

Fig. 6. Energy dispersive spectra (EDS) for TiO2/MgZnAl-5.

was formed by zinc in the hydratalcite layers [39]. However, therewas a slight decrease in thepeak related to the MgO because of theincreased molar ratio of zinc in the MgAl LDH structure at 2equalto 43.2 and 62.8. A decrease in the MgO peak was also observedafter increasing the molar proportion of zinc in a calcined MgZnInLDH [8,40].

3.1.3. Scanning electron microscopy and energy dispersivespectroscopy

Fig. 5A shows the micrograph of the non-calcined photocata-lyst TiO2/MgZnAl-CO3-5 where there is a random coating of theLDH layers of the MgZnAl-5 by the nanoparticles of TiO2 formingagglomerates with a spongy appearance, which are responsible byincreasing the heterogeneity of the surface of the photocatalysts[37,41]. After thecalcination (Fig.5B), the lamellar structure collap-ses and a solid solution of mixed oxides with some large particlesareformed, further increasing theheterogeneityof thesurface.Thiscan probably be attributed to the collapse of the lamellar structure,which occurs during the calcination stage [26] as can be observedbytheabsence of the (00 3) and (00 6)peaks in the XRD data [37].

Energy dispersive spectroscopy (EDS) results (Fig. 6) confirmed

the expected metals composition in the TiO2/MgZnAl-5 compos-ite of15.2% Mg, 0.71% Zn, 7.52% Al and 7.86% Ti, corresponding to aZn:Mg:Al:Ti ratio of 2.01:1:1.04 anda Zn2+/Mg2+ ratio of 4.68%. Themolar ratio found by EDS agreed with the molar ratio used to syn-thesize the composite (2:1:1) and confirmed by ICP-MS (Table 1).

3.1.4. Infrared spectroscopy analysisFig. 7A shows the spectrum obtained for the TiO2nanoparticles,

where there is an absorption band in the low frequency region400800cm1 associated with TiOTi. The absorption band at1632cm1 due to the presence of thebending vibration of theOHdue to re-absorption of water from the atmosphere through theTiO2nanoparticles (Ti-OH) [42].

In Fig.7B, thewide absorption band at 3444cm1 isattributedto

vibratory stretching of the OH(OH) groups of inter-layer water

5001000150020002500300035004000

Wavenumber (cm-1

)

1632

3439

A

B

1639

3444Transmittance(%)

3430

1364

778825

C

16391371

D1632

3451

Fig. 7. Infrared spectra: TiO2 (A), MgZnAl-CO3-5 (B), TiO2/MgZnAl-CO3-5 (C) andTiO2/MgZnAl-5 (D).

molecules and also of the layers of metal hydroxides in the lay-ers MgZnAl-CO3-5 [18]. The weak band near 1632 cm1 is assignedfor to bending mode of water molecules. A band at 1364cm1 isattributed to the symmetric stretching of carbonate ions. In addi-tion, absorption bands below 1000cm1 are mainly correspondingto metaloxygen vibration and metaloxygenmetal layers of thebrucite type. According to the literature these bands centered at778cm1 and 825 cm1 are assigned to the LDH MgAl and ZnAl,respectively [18,43].

According to Fig. 7C, it is clear that the incorporation of TiO2tothe calcined MgZnAl-5 LDH did not impair the lamellar formation,because there is still the presence of bands at 825 and 778 cm1.After the heat treatment at 500 C (Fig. 7D), we can see a decreasein the bands related to the hydroxyls (3451 cm1) and carbonates(1371cm1) indicating the loss of water and CO2, as expected. Thepresence of TiO2 in the calcined composite TiO2/MgZnAl-5 canbe evidenced by the lead shoulder OTiO at 400800 cm1. Thechange of Zn2+/Mg2+ molar ratio has not introduced any significantdifferences in the infrared spectra.

3.1.5. Determination of band-gap valuesFig.8 shows the absorption spectraof photocatalyst obtained by

the UVvis diffuse reflectance. TiO2showed an intense absorptionband in the UV region below 385 nm that originated from chargetransfer that occurs between the 2p orbital of the oxygen atom tothe 3dof the Ti [44].

Maximum absorption shifted to longer wavelengths in the cal-

cined composites with higher Zn2+

/Mg2+

molar ratios indicating

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300 400 500 600 700

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Ab

sorbance

Wavelenght (nm)

TiO2/MgZnAl-15

TiO2/MgZnAl-10

TiO2/MgZnAl-5

TiO2/MgZnAl-1

TiO2/MgAl

TiO2

Fig. 8. UVvis absorption spectrum of the photocatalysts.

2.0 2.5 3.0 3.5 4.0

0

1

2

3

4

5

6

7

[F(R)h]1

/2

Eg (eV)

TiO2/MgZnAl-15

TiO2/MgZnAl-10

TiO2/MgZnAl-5TiO

2/MgZnAl-1

TiO2/MgAl

TiO2

Fig. 9. Determination of band-gap values (Eg) for photocatalysts.

that less energywouldbe necessary forphotocatalyticactivity.Thisshift was purportedly due to electronic interaction of molecularorbitals between Zn, Mg, Al, with TiO2 building a new molecu-lar orbit and thus, reducing the difference in the band of energy.Similar phenomenon was observed for TiO2 in associated withother compounds such as zinc oxide [45] graphene oxide [46],carbon graphene nanotubes [47] and lamellar double hydroxide-hydrophobic MgAl [37].

The band-gap energy (Eg) was determined by the Tauc relationusing the transformed KulbelkaMunk function according to thefollowing equations [48]:

h = A(h Eg)n (1)

[F(R)h]n

= A(h Eg) (2)

F(R) =(1 R)2

2R (3)

where h is Plancks constant, is the frequency of vibration, coef-ficient A is a constant, Eg is the band-gap value, R is the diffusereflectance, valuen is equal to 1/2 for direct transitions. The deter-mination of band-gap energy for photocatalysts were calculatedby extrapolating the linear region to thex-axis, plotting the trans-formed KulbelkaMunk equation [F(R)h]1/2 versus the energyband-gap (h) as shown in Fig. 9. It can be seen that the energyband-gap decreased from 3.2eV for TiO2to 3.12eV for the compos-ite TiO2/MgAl. However, after substitution of Mg by Zn cations inthe hydrotalcite structure, the band-gap energy shifted to the visi-

bleregion. This wasattributed to the formationof zincoxideduring

-50 0 50 100 150 200 250 300 350

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

C/C

0

Time (min)

TiO2

TiO2/MgAl

Photolysis

MgZnAl-5

TiO2/MgZnAl-1

TiO2/MgZnAl-5

TiO2/MgZnAl-10

TiO2/MgZnAl-15

Adsorption (TiO2/MgZnAl-5)

Fig. 10. Phenol concentration as a function of time in different photocatalytictreatments. Reaction conditions: initial phenol concentration of 50mg/L, 300mgphotocatalyst, reaction temperature 302 C and initial pH5.2.

calcination of the composites, since this oxide has a band-gap ofabout3.0eVandwhencombinedwiththeTiO 2 (3.2 eV)it decreasedband-gap [6,49]. Thus, it is observed that the increase in composite

Zn2+/Mg2+ molar ratio decreases the band-gap energy from 3.12 eVin TiO2/MgAl to 3.08eV in TiO2/MgZnAl-1, 3.06 eV in TiO2/MgZnAl-5,3.04eVinTiO2/MgZnAl-10and3.00eVinTiO2/MgZnAl-15.Otherstudies have also reported that an increase in the molar ratio of Znreduced band-gap values [17,29].

3.2. Photocatalytic activity

In the studies on the absence of UVvis the removal of thephenol was evaluated by means of adsorption. According toFig. 10, the TiO2/MgZnAl-5 photocatalyst adsorbed 3% of thephenol after 360 min in the absence of UVvis radiation. In all(photo)degradation assays the phenol solution pH increased from5.2 to 9.7 after 30 min and then gradually increased to 10.6

over 360min. The increase in the pH value during the experi-ments provides evidence for the lamellar reconstruction of calcinedhydrotalcite that also observed in other studies [29,34]. Accordingto Chen et al. [50], during the lamellar reconstruction of hydro-talcite the interleaving with the phenol molecule is reduced. Thebenzyl ring phenolat pH10.7 is hydrophobic and bulky and sodoesnot provide any electrostatic interaction with the layers of hydrox-ides present in the layers of the composite; and also has a weakaffinity for the phenolate anion. These authors suggest that theaffinity of phenolatefor LDHshould be much weakerthan hydroxylion(OH). Although more than 60% of phenolcan dissociate to phe-nolate anion at pH 1011 (pKa = 9.8), the degree of interleaving ofthephenolateislow,mostoftheaqueousphaseremains[50]. Alliedto this, there is a preference in the reconstruction of the LDH by the

CO32

, principally under basic conditions, thus reducing the pos-sibility of merging with the phenol molecule [50]. The presence ofcarbonate in the TiO2/MgZnAl-5 photocatalyst was confirmed byits adsorption band at 1380 cm1 in the IR spectrum of the reusedmaterial (Fig. 15C).

UVvis photolysis alone degraded 14% of the initial phenolwhile the addition of photocatalysts increased phenol degrada-tion. The TiO2/MgAl catalyst removed 60% of the phenol whereasthe photocatalysts containing zinc produced greater phenol pho-todegradation. As shown in Fig. 10, it can be seen that with theincrease in the Zn2+/Mg2+ molar ratio from 1% to 5% increased phe-nol photodegradation 88 to approximately 100%. The increase inmolar ratio of Zn to 10% and 15%provided a decrease in the amountof phenol removed of 93% and 82%, respectively. These results may

be due to the large surface area, pore volume (Table 1) and the

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addition of optimum amount of the zinc oxide in the hydrotalcitelayers in the composite TiO2/MgZnAl-5, which imply a larger num-berof sites availablefor interaction with theTiO2 and enhancementof photocatalytic properties. Large surface areas of the compositephotocatalysts produce more electronhole pairs and thus increasethe hydroxyl radical formation to photodegradation of phenol [51].Furthermore, in the composite prepared with a lower molar ratioof zinc (1% and 5% versus 10% and 15%) this element is proba-bly more uniformly distributed along the hydrotalcite layers [29].On the other hand, the incorporation of an excess of the Zn (10%and 15% zinc, observed in this study) to replace the Mg, indicatedan enhanced stress in the hydrotalcite crystal structure becauseof the differences between the ionic radii of Zn (0.074nm) andMg (0.072nm) [11,52]. Therefore, the incorporation of Zn into thestable layer structure is limited, otherwise the structure will bedamaged [8].

Less Mg in the hydrotalcite structure could lead to a smallersurface area, pore volume and an irregular distribution of the zincas observed in this study (Table 1) and evidenced by Valente et al.[29], Alanis et al. [17] and Snchez-Cant et al. [53]. Presence of Mgin the composite probably improved photocatalyst stability,similarto other ternary LDHs [8,54] and contributed to dispersion of ZnOon the surface of calcined samples [8].

The results obtained in this study are consistent with thoseof Valente et al. [29] who observed that increasing zinc contentabove 5% (m/m) in MgZnAl LDH photocatalysts decreased effi-ciency of photodegradation of 2,4-dichlorophenoxyacetic acid andphenol. According to the authors the superiority of hydrotalcitewith 5% Zn was due to the increase in catalytically active sitesand improved distribution of the ZnO along the layers of brookite,since the distribution of a second divalent cation is non-uniformafter the reconstruction the calcined LDH. Huang et al. studiedmethylene blue photodegradation using MgZnIn ternary LDHand found that adjusting the Mg2+/Zn2+ molar ratio to obtain amaximum surface area resulted in higher photocatalytic efficiency[8].

Tzompantzi et al. [49] found that a Zn/Al molar ratio of 1.47

resulted in higherphotocatalyticefficiencythan molarratios of 1.67and 3.81. This behavior was evident in the composites synthesizedin our work. Several studies have shown that the increase in theratio of Zn with respect to TiO2did not result in increased in photo-catalytic efficiency [6,7,55,56]. According to Zhao et al., the excessZn associated with TiO2 can lead to excessive formation of sur-face oxygen vacancy, which act as electron traps and thus slow theelectronic transfers, thereby reducing photocatalytic activity [57].Thus, an excessof zincpresentin thehydrotalcite structure impreg-nated with TiO2 (TiO2/MgZnAl) would reduce the photocatalyticefficiency of TiO2.

The MgZnAl-5 photocatalyst produced 73% phenol removalwhile its combination with TiO2 (TiO2/MgZnAl-5) increasedremoval by 27%, resulting in 100% photodegradation efficiency.

The increased efficiency was caused by increased hydroxyl radical(OH) generation because of the greater number of electronholepairs generated upon radiation of the TiO2-containing photocata-lyst. Therefore, it is necessary to have an appropriate molar ratioof Zn, Mg, Al and TiO2 in order to achieve efficient photocatalyticdegradation of phenol.

TOC removal wasalso quantified (Fig. 11) to measure the degreeof phenol mineralization. The TiO2/MgZnAl-5 catalyst that pro-duced the greatest phenol removal (Fig. 10) also resulted in thehighest TOC reduction, reaching about 80% after 360 min.

Theproposedschemefor phenoldegradationby TiO2/MgZnAl-5presented in Fig. 12 can be summarized as follows: when a photon(h) of energy equal to orgreater than the band-gapenergyhits theparticles of the photocatalyst TiO2/MgZnAl-5 an electron valence

band (VB) is promoted to the conduction band (CB), leading to the

0 50 100 150 200 250 300 3500.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

TOC/TOC

0

Time (min)

TiO2

TiO2/MgAl

Photolysis

MgZnAl-5

TiO2/MgZnAl-1

TiO2/MgZnAl-5

TiO2/MgZnAl-10

TiO2/MgZnAl-15

Fig. 11. TOC removal during photodegradation of a phenol solution.Reaction con-ditions: initial phenol concentration of 50mg/L, 300mg photocatalyst, reactiontemperature 302 C and initial pH5.2.

simultaneous generation of a hole in the valence band (h+

) andexcess electrons in the conduction band (e) (Eq. (4)) [58].

TiO2/MgZnAl-5 hTiO2/MgZnAl-5(h

+

VB)+ TiO2/MgZnAl-5(e

CB)

(4)

These photogenerated holes (h+) possess sufficiently positivepotential to generate OH radicals from water molecules adsorbedon its the photocatalyst surface (Eq. (5)) or from hydroxyl ions (Eq.(6)) that can subsequently oxidize the organic pollutant (Eqs. (7)and (8)) [16,58,59]

TiO2/MgZnAl-5(h+

VB)+H2Oads OH+H+ (5)

TiO2/MgZnAl-5(h+

VB)+OH OH (6)

TiO2/MgZnAl-5(h+VB)+ phenol oxidation of phenol (7)OH+ phenol degradation of phenol (8)

Allied to this, the hydroxyl groups present in the LDH struc-ture can react with the photogenerated hole in the VB of TiO2, andthereby promote the production ofOH [37,41,59].

The presence of oxygen in the CB can generate a super-peroxideanion (Eq. (9)), which can produce organic peroxide (Eq. (10)) andhydrogen peroxide (Eq. (11)) that also generate OH radicals (Eq.(12)) [16,5860].

TiO2/MgZnAl-5(e

CB)+O2 O2 (9)

O2 + Phen Phen-OO (10)

O

2+HO

2+H+ H2O2 +O2 (11)

TiO2/MgZnAl-5(e

CB)+H2O2 OH (12)

The presence of oxides formed during LDH calcination caninfluence generation of the hydroxyl radical (Eqs. (13) and (14)).Hydroxyl radical can be reduced by the electrons in the conduc-tion band and thereby promote the reconstruction of oxide withthe re-hydroxylation (Eq. (15)) [17,18].

Oxide Oxide(h+VB + e

CB) (13)

Oxide(h+VB) +H2Oads Oxide +OHads +H

+ (14)

Oxide(eCB)+OHads Oxide+

OHads (15)

Combination of the ZnO semiconductor obtained by the cal-

cining LDH with TiO2 made it possible to visible radiation for

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Fig. 12. Schematic diagram of thephotocatalytic processesoccurring on thesurface of theTiO2/MgZnAl-5.

Table 2

Comparison of different photocatalysts used in phenol photodegradation.

Photocatalyst Initial concentration (mg/L) Experimental condition Dose (g/L) Rate (%) of degradation Ref.

Fe-TiO2 33.5 High-pressure lamp 125 W; UVvis 1.0 8 h of radiation: 66 [63]N-Si co-doping TiO2 10 500 W Xenon lamp; UVvis 1.0 8 h of radiation: 99 [64]ZnO/La2O3 (doped with K) 20 300 W halogen lamp; UVvis 1.0 2.5 h radiation: 98.5 [65]CeO2-ZnTi-LDHs 50 Mercury lamp; 300 W 1.0 After 7 h of radiation: 90 [66]ZnAlLa 40 Mercury lamp, 300 W; UVvis radiation 1.0 5 h radiation: 100 [16]CeO2-TiO2/SiO2 30 350 W; radiation 3002500 nm 2.0 3 h radiation: 96.2 [67]TiO2/MgZnAl-5 50 Hg vapor lamp 125 W; radiation >300 nm 1.0 After 6 h of radiation: 100 This study

photocatalysis (Fig. 8) [61]. Electron created in ZnO CB, after irradi-ation can migrate to the CB of TiO2, and promote catalytic process(Eq. (16)) [57,62].

ZnO(eCB) TiO2(e

CB) (16)

Table 2 presents a summary of photocatalytic phenol degrada-tion studies published in the literature. It can be seen that phenoldegradation by TiO2/MgZnAl-5 showed satisfactory performancecompared to other catalysts previously reported.

3.3. Kinetics of phenol photodegradation

The LangmuirHinshelwood is commonly used to describe the

kinetics of pseudo-first-order degradation of organic pollutants,which occur at the solidliquid interface [18]. This model relatesthe degradation rate r (mg/Lmin), reaction time t (min) and theconcentration of the organic compound C (mg/L), as expressed inEq. (17) [68]

r= dCdt =

krKadC

1+ KadC (17)

where kr is the intrinsic rate constant and Kad is the adsorptionequilibrium constant. At low initial organic compound concentra-tion KadC is negligible [29,69] and the model reduces to Eq. (18)that describes pseudo-first-order kinetics over the interval [C, C0].

lnC0

C

= Kappt (18)

where C0 is the initial phenol concentration atadsorptiondesorption equilibrium at t=0min and Kapp is theapparent rate constant (Kapp = krKad) [70]. In this model, the slopeof the ln(C0/C) versus time (t) plot is the apparent rate constant(Kapp).

Fig. 13 presents results of the phenol photodegradation kineticstudies that were used to determine apparent rate constants (Kapp)and phenol half-lives (t1/2). Half-life time (t1/2), the time at whichC=0.5C0, is one of the most useful values to compare pseudo-first-order reaction rates (Eq. (19)) [48],

t1/2 =ln(2)Kapp

(19)

Phenol (Fig. 13A) and TOC (Fig. 13B) photodegradation results

adjusted well to the pseudo-first-order kinetic model, withcoefficients of determination, R2 > 0.98 (Fig. 13). Apparent rateconstants (Kapp), half-lifes time (t1/2), and the coefficients of deter-mination (R2) are summarized in Table 3.

Half-lives of 248.4min (phenol by HPLC-DAD) and 545.8 min(TOC) were found for photodegradation with the TiO2/MgAl com-posite. Incorporation of Zn into the hydrotalcite layers increasedthe reaction rate, with TiO2/MgZnAl-5 reducing half-lives to60.8min (phenol by HPLC-DAD) and 248.4 min (TOC), correspond-ingtoKapp =0.0114min1 (phenol byHPLC-DAD)and 0.0040min1

(TOC). In comparison, Valente et al. [29] reported phenol degra-dation by a MgZnAl LDH of about 70% (initial concentration40mg/L) after 6h, with Kapp =0.0033min1 and t1/2 =208.2min.Prince et al. obtained nearly 80% phenol degradation by LDH-

3ZnAl-c after 6h for the same initial phenol concentration

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350300250200150100500

0

1

2

3

4

5

350300250200150100500

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6PhotolysisMgZnAl-5

TiO2

TiO2/MgAl

TiO2/MgZnAl-1

TiO2/MgZnAl-5

TiO2/MgZnAl-10

TiO2/MgZnAl-15

-ln(C/C

0)

Time (min)

A B

Photolysis

MgZnAl-5

TiO2

TiO2/MgAl

TiO2/MgZnAl-1

TiO2/MgZnAl-5

TiO2/MgZnAl-10

TiO2/MgZnAl-15

-ln(

TOC/TOC

0)

Time (min)

Fig. 13. Pseudo-first-order degradation kinetics for phenol with different photocatalysts, used to estimate LangmuirHinshelwood coefficients. Phenol measured by HPLC-DAD (A)and TOC(B) removals. Reaction conditions: initial phenol concentration of 50mg/L, 300mg photocatalyst, reaction temperature 302 C and initialpH 5.2.

Table 3

LangmuirHinshelwood apparent rate constants (Kapp), half-lifes (t1/2) and coefficients of determination (R2) for the photodegradation of phenol measured by HPLC-DADand TOC.

Phenol (HPLC-DAD) TOC

Kapp(min1) t1/2(min) R2 Kapp(min1) t1/2(min) R2

Photolysis 0.0005 1556.1 0.9865 0.0003 2539.0 0.9894MgZnAl-5 0.0036 193.8 0.9809 0.0019 362.9 0.9950TiO2 0.0016 433.2 0.9969 0.0009 718.3 0.9976TiO2/MgAl 0.0028 248.4 0.9880 0.0013 545.8 0.9958TiO2/MgZnAl-1 0.0062 111.3 0.9940 0.0025 275.1 0.9873TiO2/MgZnAl-5 0.0114 60.8 0.9956 0.0040 159.7 0.9910TiO2/MgZnAl-10 0.0079 88.2 0.9918 0.0032 217.3 0.9913TiO2/MgZnAl-15 0.0051 136.9 0.9930 0.0022 325.4 0.9903

with Kapp =0.0044min1 and t1/2 =156.0min [12]. These sameauthors used a calcined LDH of Zn(Ga)Al-5c and obtained 60%phenol removal (initial concentration of 80mg/L) in 6h, withKapp =0.0039min1 and t1/2 =174.0min [12]. In the study of Chwei-Huann et al. [71] reported phenol (initial concentration of50mg/L) was degraded in 180min with Kapp =0.014min1 andt1/2 =49.3min using1g/LofTiO2(Degussa,P25) butonly after addi-tion of H2O2 (0.05mg/L) and using a 400W UV lamp. Therefore,one can conclude that theTiO2/MgZnAl-5 composite photocatalystshowed a satisfactory performance kinetic with the potential toeliminate the phenolic compounds in an aqueous medium.

3.4. Reuse

Forthe purpose of practicalapplication, it is essentialto evaluate

the re-usability and stability of the photocatalyst. The experimentswere performed with the recovery of the material (300mg) andreuse, keeping all other parameters constant. The results revealedthat the composite TiO2/MgZnAl-5 showed good photocatalyticactivity after five successive cycles. The yield for phenol degrada-tion was 96%, 90%, 88%, 86% and 81% for the five tests, respectively.While with the TOC analysis the yield of the first to fifth cycle was77%, 74%, 70%, 68% and 65%, respectively.

X-ray diffraction and infrared spectroscopy analysis were usedto investigate the stability of TiO2/MgZnAl-5. According to the DRXanalysis shown in Fig. 14B, one can notice the reappearance ofpeaks related to LDH at 2equal to 11.53 (00 3) and 23.4 (006)relative to the calcined material (Fig. 14A). The appearance of thesepeaks shows the ability of the composite to partially or completely

regenerate when placed in contact with water or solution, a

Fig. 14. X-ray diffraction spectra of TiO2 /MgZnAl-5 photocatalyst before (A) andafter (B) five reuse cycles. Reaction conditions: initial phenol concentration of50 mg/L, 300mg photocatalyst, reaction temperature 302 C and initial pH5.2.

phenomenon known as the memory effect [19]. In addition,peaks related to zinc oxide and magnesium oxide remained afterreuse, although at lower intensity (Fig. 14B).

The IR spectrum of the reused photocatalyst (Fig. 15C), showsthe presence of a broad band around 3455cm1 attributed to OHstretching and at 1637cm1 of the bending mode of the watermoleculesadsorbedbycontactwiththeaqueoussolutionofphenol.Thebands1397,1380and1376cm1 were attributed to symmetric

stretching of carbonate ions intercalated in LDH.

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5001000150020002500300035004000

Wavenumber (cm-1)

A

1380

1397

1637

1632

3455

3447

C

3451

1376

1639

B

Transmittance(%)

Fig. 15. Infrared spectrum of the calcined TiO2/MgZnAl-5 (A) photocatalyst,hydrated TiO2/MgZnAl-5(B) andTiO2/MgZnAl-5 (C) afterfive reuse cycles. Reactionconditions: initial phenol concentration of 50mg/L, 300mg photocatalyst, reactiontemperature 302 C and initial pH5.2.

Analysis of the photocatalyst infrared spectrum in an aqueousmedium (hydrated) (Fig. 15B) and compared to the spectrum of

calcined material (Fig. 15C) shows no peaks corresponding to phe-nol or its possible intermediaries, been further evidence that thephenol molecule was mineralized by the TiO2/MgZnAl-5.

4. Conclusions

Novel composite photocatalysts containing zinc oxide obtainedby calcination of ternary (Mg, Zn andAl) layered double hydroxidesimpregnated with TiO2 nanoparticles were successfully synthe-sized for the first time. The TiO2/MgZnAl photocatalysts wereused in UV-vis (filter cut-off for >300nm) photodegradationof phenol in aqueous solution and presented high photocatalyticactivity. Optimum catalyst Zn2+/Mg2+ molar ratio wasabout 5% andthis TiO2/MgZnAl-5 composite photocatalyst resulted in approxi-

mately 100% phenol and 80% TOC removal from an aqueous phenolsolution of initial concentration equal 50mg/L after 360min. TheTiO2/MgZnAl-5showed good stabilityafter5 cycles, thus showing apromising potential for practical applications. Phenolremoval withthe novel catalyst was greater than that obtained with commercialTiO2(Degussa P25) because of its smaller band-gap energy(3.06 eVvs 3.20 eV) and lower electronhole pair recombination rate due tothe presence of zinc oxide. Since the TiO2/MgZnAl-5 photocatalystshifted thelight absorption to a higherwavelength, less energywasrequired for the photocatalytic activity and phenol removal effi-ciency increased. The new photocatalysts showed a synergy effectof the hydrotalcite support, the presence of zinc oxide associatedwith photoactivity of TiO2, resulting in formation of a compositewith high photocatalytic capacity.

Acknowledgements

The authors acknowledge the financial support of the Fundacode Amparo Pesquisa do Estado de Minas Gerais (FAPEMIG, Uni-versalDemand, process number: APQ-00416-11) and the ConselhoNacional de Desenvolvimento Cientfico e Tecnolgico (CNPq) andthe Departamento de Solos, Universidade Federal de Vicosa, Labo-ratrio de Geoqumica for N2adsorption isotherms (BET) analyses.

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