TiO 2_MgAl layered double hydroxides mechanical photocatalysts in phenol degradation

TiO 2/MgAl layered double hydroxides mechanical mixtures as ef?cient photocatalysts in phenol degradation

Silvia P.Paredes a ,Miguel A.Valenzuela a ,n ,Geolar Fetter b ,Sergio O.Flores a

a Lab.Cata

′lisis y Materiales,ESIQIE-Instituto Polite ′cnico Nacional,Zacatenco,07738,Mexico D.F.,Mexico b

Facultad de Ciencias Qu?

′micas,Universidad Auto ′noma de Puebla,72570Puebla,Pue.,Mexico a r t i c l e i n f o

Article history:

Received 3November 2010Received in revised form 7March 2011

Accepted 25March 2011

Available online 10May 2011Keywords:

A.Inorganic compounds

B.Chemical synthesis

C.X-ray diffraction C.Electron microscopy

D.Crystal structure

a b s t r a c t

MgAl layered double hydroxides (MgAl LDH)were synthesized by the sol–gel method using ultrasound irradiation in the crystallization step.The interlayer anions were nitrate and acetylacetonate-ethoxide.The solids were characterized by XRD,N 2-physisorption and TEM.TiO 2/MgAl LDH mixtures were prepared by mixing the MgAl LDH (as prepared)or the calcined sample with TiO 2(Aldrich,99.9%anatase)in different weight ratios.Photocatalytic activities of the TiO 2/MgAl LDH mixtures were evaluated through the degradation of phenol as model pollutant.TiO 2/MgAl LDH mixture (1:1)was more photocatalytically active for the degradation of phenol than pure TiO 2.The synergy effect was attributed to a higher production of OH radicals,which were formed from the structural hydroxides.Also,the hydrotalcite phase enhanced the phenol adsorption and transfer to the TiO 2sites where the phenol was photocatalytically degradated.

&2011Elsevier Ltd.All rights reserved.

1.Introduction

The hydrotalcite-like compounds or layered double hydroxides (LDH)or simply called the hydrotalcite are the anionic equiva-lents of cationic clays.LDH structure may be easily visualized from brucite as hexagonal close packing of hydroxyls ions and magnesium ions occupying all the octahedral positions.If Mg 2tions are substituted by Al 3tions during the synthesis process,the LDH in brucite-type structure is obtained.The brucite-type layer (HO–Mg–O–Al–OH)is,then,positively charged and one positive charge is generated for each aluminum substituted by magne-sium ion.To maintain the electrical neutrality,anions are required,and they are located in the interlamellar spaces [1–4].These anions are normally hydrated.Fig.1shows the LDH structure.The LDH can be represented by the general formula

?eM II T1àx eM III Tx eOH T2 x teA m à

x =m TU nH 2O ,where M II is a divalent metal,M III is a trivalent metal and A is a compensating anion

(CO 32à,SO 22à,NO 3à

,Cl à,y )with charge m and n is the number of water molecules.The most common LDH,known as hydrotalcite is constituted by magnesium and aluminum as M II and M III metals and the positive charges are compensated by carbonate anions.

The structure of these kinds of compounds,based on the stacking of positively charged layers with anions and water

confers relatively high mobility to the anions.It has been observed that the anions are easily exchanged in the following order [5]:

CH 3COO à4NO à34ClO à44Cl à4SO 2à44CO 2à

3

The LDH can be obtained by direct or indirect synthesis [6].Indirect synthesis can traditionally be conducted using three main techniques:(a)direct anion exchange,(b)anion exchange by elimination of the interlamellar precursor species susceptible to acid attack and (c)regeneration of the calcined LDH.The calcined LDH,which is composed by metal oxides,may be rehydrated again by contacting the solid with an anionic aqueous solution;the hydrotalcite structure is then recovered by incor-porating the solution anions.Direct synthesis can be carried out by the coprecipitation or sol–gel methods.In a previous work [7]we studied the effect of microwave irradiation on the synthesis of sol–gel hydrotalcites intercalated with ethoxide and acetylaceto-nate species compared with a nitrated sample.The crystallite sizes of the microwaved LDH’s were smaller than those obtained by the conventional method and the resulting speci?c surface areas were bigger [8].

Photocatalytic oxidation processes are highly effective advanced technologies for the degradation of a wide variety of pollutants in wastewater [9].The principle of photocatalysis is based on the generation of excited states of semiconducting materials by exposure to light.This allows exceptional redox reactions to take place.Nevertheless,rapid recombination of holes and electrons,large band gaps and mass transfer limitations

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Journal of Physics and Chemistry of Solids

0022-3697/$-see front matter &2011Elsevier Ltd.All rights reserved.doi:10.1016/j.jpcs.2011.03.017

n

Correspondance to:IPN-ESIQIE,Edi?cio 8,tercer piso,Zacatenco,07738

Mexico D.F.MEXICO Tel.:57296000x55293;fax:525555862728.

E-mail address:mavalenz@ipn.mx (M.A.Valenzuela).Journal of Physics and Chemistry of Solids 72(2011)914–919

are,among others,current issues to be improved for practical applications [10].

According to the extensive amount of publications related to titanium dioxide in several photocatalytic applications [9–11],it is clear that TiO 2will continue being used,due to the excellent photon absorption and excellent reactant adsorption compared with other semiconductors [12].However,TiO 2has a very low photonic ef?ciency (i.e.less than 10%for most degradation reactions)than several aspects concerning to the synthesis and design of photocatalytic systems covering;structure and mor-phology,doping,metal coating,surface sensitization and compo-site semiconductors are being extensively studied [13].

In particular,the layer structure may improve ef?ciency of photocatalytic reactions by inducing electron transfer,preventing recombination of electrons and holes and avoiding agglomeration of nanoparticles [14,15].LDH’s can be prepared with great ?exibility both for metal ions and incorporated anions,thus a large variety of photocatalysts can be synthesized [6].Works devoted to the use of LDH’s as photocatalysts have been focused to ZnAl-LDH,as-synthesized and calcined forms,obtaining homo-geneous mixtures of oxides with small crystal size and hence high surface area [16],as well as polyoxometalates intercalated in layered double hydroxides [17]or pillared p-tungstate ions [18]or intercalated p-aminobenzoic acid in MgAl-LDH and ZnAl-LDH as sun screen formulation [19,20].However,in our knowledge,there is no report about the synergist photocatalytic properties of mechanical mixtures of TiO 2with MgAl-LDH or with the calcined form.In this work,hydrotalcites were synthesized by the sol–gel method using ultrasound irradiation during the hydrothermal treatment step.These solids were mixed with titania (anatase)in different weight ratios and tested in the degradation of phenol in aqueous solution.

2.Experimental

2.1.Synthesis of MgAl-LDH with interlayered organic compounds (acetylacetonate-ethoxide)

A series of samples of hydrotalcite-like compounds were synthesized following the procedure described elsewhere [7].Magnesium ethoxide (Aldrich,99%)was mixed with aluminum acetilacetonate (Aldrich,99%),then,the mixture was added to ethanol and HCl aqueous solution as a catalyst.The obtained gel

was irradiated by ultrasound irradiation in an Elma D-78224cleaner container operating at 2.4kW and 25kHz,the tempera-ture was ?xed at 801C.The ?nal pH value of the mixture was adjusted to 11.5with ammonium hydroxide.The resulting mixture was again irradiated for 10,20or 30min under the same controlled pressure,temperature and stirring.The solid was separated by decantation and washed several times with ethanol.Ethanol was evaporated from the dispersion with a nitrogen ?ow.The solid was dried at 701C for 3h.Samples with 10,20and 30min of irradiation periods were referred to as UEA10,UEA20and UEA30,respectively.All the samples were prepared under N 2atmosphere and the metallic molar ratio (Al 3t/(Al 3ttMg 2t))was maintained constant at 0.25.2.2.Synthesis of MgAl-LDH nitrated

Nitrated hydrotalcite-like compound with an Al/(Mg tAl)molar ratio of 0.25was synthesized as reported in a previous work [7];a 1.86N NaOH aqueous solution was added to a 2.5M aqueous solution containing Mg(NO 3)2á6H 2O and Al(NO 3)3á9H 2O.The pH varied up to a ?nal value of 13.The obtained gel was treated by ultrasound irradiation for 10,20and 30min.The obtained samples were washed with deionized water and dried in an oven at 701C.These samples were designed as UNHT10,UNHT20and UNHT30,depending on the ultrasound irradiation time.Sample UNHT10was also calcined in air at 5001C for 18h (labeled as UNHT10C).2.3.Characterization

The X-ray diffraction patterns were obtained using a Siemens D-500diffractometer coupled to a copper anode X-ray tube.A diffracted beam monochromator selected the K a https://www.360docs.net/doc/763845661.html,pounds were identi?ed by the conventional way using the JCPDS cards.The BET surface area,pore volume and mean pore diameter were determined by standard multipoint techniques using a Micromeritics ASAP 2010instrument.A Jeol 1020trans-mission electron microscope coupled to an EDX detector provided the sample micrographs and the corresponding elemental composition.2.4.Photocatalytic activity test

TiO 2(Aldrich,99%anatase)was mixed by kneading with the LDH samples (UEA10,UNHT10and UNHTC10)at weight ratios of:1:1,1:1.5and 2:1with each LDH.Photocatalytic phenol (Baker)degradation experiments were conducted using the following procedure.A solution of deionized water,phenol (50ppm)and the catalyst under test were dispersed in an ultrasound bath and then poured into the reactor.Air was dispersed in the aqueous solution through six 1/4inch rod spargers located in radial position to maintain a catalyst slurry.The solution was irradiated for 8h by one ?uorescent black light tube (UV Philips,8W,l max ?365nm)in a batch annular photoreactor thermostated at 251C.A sample of 4mL was extracted each hour from the photoreactor;the solids were separated by ?ltration of the sample (0.45m m,Millipore).Phenol analysis was done using a UV–vis CINTRA 20-GBC apparatus,with l max of phenol ?270nm.3.Results and discussion 3.1.Structure

Figs.2and 3correspond to the XRD patterns of UNHT and UEA series,respectively.These samples were synthesized by ultra-sound irradiation for 10,20and 30min.The X-ray

diffraction

Fig.1.Structure of a hydrotalcite-like compound.

S.P.Paredes et al./Journal of Physics and Chemistry of Solids 72(2011)914–919915

peaks in all the cases were identi?ed as LDH according to the JCPDS card 22-0700,then,pure hydrotalcite-like compounds were obtained.We observe in both series that by increasing the irradiation time,the crystallite size is also increased.Indeed,the smaller the particle size,a bigger surface area may result,which could favor the hydrotalcite adsorption capacity improving the photocatalytic process.Therefore,we selected the samples irra-diated at 10min (UEA10and UNHT10)in order to have suitable conditions in the evaluation of the photocatalytic properties of the mixtures TiO 2–MgAl LDH.

If we compare the samples UEA10and UNHT10,the inter-lamellar distances determined for UEA10sample shows a small displacement to the left in the 00l diffraction peaks corresponding to an increase in the interlamellar distance (d )based on the 003

diffraction peak.The d value changes from 7.8?A

for the card value to 7.88,7.81and 7.86?A

for the samples UNHT10and UEA10,respectively,which corresponds to an average of 3.05?A

of inter-lamellar space.This indicates that for the UNHT10sample nitrate is the compensating anion because the average size reported for

this anion was 3.99?A

[7].For the LDH with interlayered organic compounds (acetylacetonate-ethoxide),the d value is smaller

than any of the dimension of the acetylacetonate (4.81?3.64?A)

and ethoxide (3.73?2.20?A)

ions,this kind of samples may originate different species.

Fig.4shows the X-ray diffraction pattern of the UNHT10sample after calcination at 5001C for 18h.It can be observed that the characteristic peaks of magnesium oxide and a micro-crystalline phase corresponds to a mixture of Al and Mg oxides.

The transmission electron microscopy image of the sample UEA10as shown in Fig.5presents the local ordering of the brucite-type layers,in which the interlamellar distance (d )is clearly appreciated and represented by dark zones where the compensation anions (ethoxide and acetylacetonate)are located.The EDX measurements indicate that the corresponding metallic molar ratio (Al 3t/(Al 3ttMg 2t))was 0.25for UEA10and 0.25for UNHT10samples.

Table 1shows the textural properties of samples UEA10and UNHT10.As can be seen,the lowest speci?c surface area (SSA)was obtained with UNTH10,though it is higher than that reported by other authors using autoclave method [6].The calcined sample UNTH10C,showed a signi?cant decrease of its SSA from 97to 10m 2/g.It seems that the calcination temperature and time of treatment were not enough to complete the involved reactions that conventionally increase the surface area of LDH’s [21].Note that the SSA of the UEA10was 248m 2/g,this value is more than 100%higher than that of the UNHT10,and this result was attributed to the type of interlayered

anion.

I n t e n s i t y (a .u .)

2 θ

Fig.2.X-ray patterns of ultrasound prepared LDH with acetylacetonate-ethoxide as interlayered organic compounds at different irradiation times

(UEA).

I n t e n s i t y (a .u .)

2θ

Fig.3.X-ray patterns of ultrasound prepared LDH with nitrate as interlayered inorganic compounds at different irradiation times

(UNHT).

I n t e n s i t y (a .u .)

2 θ

Fig.4.X-ray diffraction pattern of MgAl LDH calcined sample

(UNHT10).

Fig.5.Transmission electron micrograph of the MgAl LDH,UEA10sample.

S.P.Paredes et al./Journal of Physics and Chemistry of Solids 72(2011)914–919

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3.2.Photocatalytic degradation of phenol

Phenol degradation pro?les of UNHT10,UEA10LDHs as pre-pared samples,the photolytic reaction and the photocatalytic reaction with TiO2photocatalytic degradation are compared as shown in Fig.6.This?gure has the purpose of showing that only with the presence of TiO2a signi?cant degradation of phenol is observed(72%).The photolytic reaction and the UNHT10sample achieved15%of phenol degradation after8h of irradiation time although with a different degradation pro?le.This last sample de?nes the typical behavior of adsorption effect,whereas,the UEA10sample practically did not show adsorption.Note that the photolytic reaction and the photocatalytic conversion with TiO2 showed an induction period of about2h,in which the reaction started.This behavior has been attributed to a long period required for the formation of OH radicals to carry out the photo-oxidation[12].As shown in Table1,the speci?c surface areas(SSA)of samples UNHT and UEA10were97and248m2/g, respectively,in comparison with50m2/g belonging to TiO2,then, there is no correlation between the SSA with the adsorption effect on these solids.Even though,UEA10had a high SSA compared with UNHT,the higher adsorption capacity is attributed to the type of interlayer anion.This means that the presence of organic anions inside the interlayer spaces inhibits the phenol adsorption. Usually,phenol degradation has a low reaction rates and it depends on the light intensity mainly[22],in this work we use only one black light lamp of8W,which could explain the long period to start the degradation.

The phenol degradation pro?les obtained with LDHs/TiO2and mixed oxides/TiO2catalysts are shown in Fig.7.The weight ratio of TiO2and LDH or the corresponding mixed oxides was1/1.It is interesting to highlight that each mixture followed a different path. For instance,UNHT10-TiO2sample presented a linear increase in phenol degradation of all the irradiation time,reaching a phenol conversion of93%.If we compare the degradation path of the calcined sample UNHT10C/TiO2,it means that TiO2is mixed with MgO(main compound,shown in Fig.2),the reaction proceeds in the same way as that of the TiO2/LDH until2h of irradiation time, however,after this period the reaction is lowered signi?cantly,but the?nal conversion was almost the same.An explanation to this behavior should be done in terms of the structure of LDH and the mixed oxides,the former is highly hydroxylated,and then these species are easily converted to OH radicals.On the contrary,MgO does not have structural OH species and must be taken from water and converted to OH radicals.UEA10-TiO2mixture presented a lower conversion(80%)than UNHT10-TiO2;however,the degrada-tion pro?le was the same,which con?rms that the structural OH species surrounding TiO2after irradiation are easily converted to OH radicals.The lower conversion for this last catalyst was attributed to the interlayer organic anion,which inhibits the adsorption of phenol on its surface.It is clear from these results that the addition of the same amount of LDH to TiO2generates a kinetic synergy effect in phenol disappearance,with an increase of 9%and21%conversion units using UEA10-TiO2and UNTH10-TiO2, respectively,compared with pure TiO2anatase.A synergist effect has also been found by mixing titania with activated carbon(AC) and was explained in terms of extended adsorption of phenol on AC followed by a transfer to titania,where the photocatalytic degradation occurs[12].Nevertheless,samples UNTH10and UNTH10C with small surface areas(97and10m2/g)showed higher conversion when mixed with TiO2(Fig.7).

Table1

Textural properties of Mg/Al-hydrotalcites as-synthesized and calcined(C).

Catalyst Surface area

(m2/g)Pore volume

(cm3/g)

Mean pore

diameter(nm)

UEA102480.245 3.7 UNTH10970.082 3.3

UNTH10C100.036

2.6

Fig.6.Phenol degradation pro?les of MgAl LDH(UNHT10and UEA10samples, as-prepared),photolysis and photocatalytic degradation using TiO2

.

Fig.7.Phenol degradation pro?les of TiO2/Mg–Al LDH(UNHT10,UNHT10C

calcined and UEA10samples)using a weight ratio MgAl LDH/TiO2,

1:1.

Fig.8.Phenol degradation pro?les of Mg–Al LDH(UNHT10sample)/TiO2using

different weight ratios.

S.P.Paredes et al./Journal of Physics and Chemistry of Solids72(2011)914–919917

In order to understand the effect of the weight ratio of LDH-TiO 2,three experiments by varying the amount of UNHT10sample were carried out.As shown in Fig.8,higher phenol degradation is obtained with the 1:1weight ratio (93%)and by increasing the weight ratios from 1.5:1or 2:1a detrimental behavior is observed and the phenol conversion was 51%and 45%,respectively.The same experiments were performed with the UEA10-TiO 2and are shown in Fig.9.A similar tendency can be observed than that observed with the UNTH10-TiO 2sample.Note that in all cases the reaction started after 2or 3h of irradiation time,which explains that the adsorption of phenol is not enhanced with the addition of high surface area UEA10(220m 2/g)sample,see Table 1.These results revealed a linear photodegradation pro?le reaching a 81%conversion with the 1:1weight ratio,and 51%and 32%using 1.5:1and 2:1weight ratios,respectively.

The mineralization of phenol was checked in a qualitative way at the end of the reaction (8h)by the formation of barium

carbonate in a reservoir containing barium hydroxide.Although several reaction mechanisms have been reported for the photo-catalytic degradation of phenol,it is accepted that the highly oxidative holes (h t)with E 1?t2.8V,may directly react with the surface-sorbed phenol or indirectly be oxidized via formation of

OH radicals according to the following reactions [23–25]:TiO 2th u -e àth t(1)h ttH 2O - OH tH t(2)h ttOH à- OH

(3)

In the presence of molecular oxygen,the negatively charged electrons are preferentially trapped by oxygen as

O 2te à-O à

2

(4)O 2t2H tt2e à-H 2O 2(5)H 2O 2te à-OH àt OH

(6)

Therefore,materials rich in OH groups,as LDH,may favor the titania activity.Indeed,as shown in Figs.6–8,for mixed 1:1samples,the activities were higher than that of pure titania.Furthermore,a conversion of 99%is obtained for the sample UNHT10C-TiO2(1:1),which is constituted by calcined hydrotal-cite and titania.In this case,the surface OH groups should be more accessible than those of the hydrotalcite lamellar.On the other hand,the samples with organic interlayers (UEA10),may block more structural OH groups than the nitrated hydrotalcite (UNHT),which results in decrease of the photocatalytic activity,as shown in Fig.9.

When the amount of hydrotalcite:titania was increased to 1.5:1and 2:1the phenol conversion decreased to values lower than those for pure TiO 2,Fig.9.The OH radicals formation by electronic transfer mechanism could be affected by covering the TiO 2nuclei with hydrotalcite in excess.Thus,the distances from OH groups to Ti atoms could become bigger enough to block the electronic transfer.Also,a screen effect of the hydrotalcite,blocking the UV-light to TiO 2could not be discarded.A complete overview of the process was proposed and depicted in Fig.10

.

Fig.9.Phenol degradation pro?les with Mg–Al LDH (UEA10sample)/TiO 2using different weight

ratios.

Fig.10.A proposed scheme of the photocatalytic degradation of phenol in presence of a mixture of TiO 2/Mg–Al layered double hydroxides.

S.P.Paredes et al./Journal of Physics and Chemistry of Solids 72(2011)914–919

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4.Conclusion

MgAl-hydrotalcites prepared with ethoxide-acetylacetonate as interlayer anions presented higher surface area(220–248m2/g) than that prepared with nitrate as interlayer anion(97m2/g). Calcination at5001C of nitrate sample resulted in a signi?cant decrease of its surface area to10m2/g,by appearance of the MgO phase.Nevertheless,it was these catalyst components,with low surface areas,which shows higher photocatalytic activities in the phenol degradation reaction.Even if the hydrotalcite promotes the phenol sorption capacity,a synergist effect leading to a higher production of radical species could explain the TiO2/LDH’s super-ior performance in comparison with each component. Acknowledgments

The authors gratefully acknowledge the‘‘CONACYT’’for ?nancial support(Projects:44118-Q and79132)and Dr.Paz del Angel Vicente for the X-ray analysis.

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