Enhanced visible light induced degradation of benzene on Mg ferrite hematite PANI nanospheres

Journal of Hazardous Materials 241–242 (2012) 472–477

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous

Materials

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j h a z m a

t

Enhanced visible-light induced degradation of benzene on

Mg-ferrite/hematite/PANI nanospheres:In situ FTIR investigation

Yu Shen a ,b ,Qidong Zhao a ,Xinyong Li a ,c ,?,Deling Yuan a ,Yang Hou a ,Shaomin Liu c ,??

a

Key Laboratory of Industrial Ecology and Environmental Engineering and State Key Laboratory of Fine Chemical,School of Environmental Science and Technology,Dalian University of Technology,Dalian 116024,China b

School of Environmental and Chemical Engineering,Dalian Jiaotong University,Dalian 116028,China c

Department of Chemical Engineering,Curtin University,Perth,WA 6845,Australia

h i g h l i g h t s

Mg-ferrite/hematite/PANI photo-catalysts showed enhanced photocatalytic activity.

Ethyl acetate,carboxylic acid and aldehyde were the intermediate products.

CO 2was produced as the ?nal prod-uct during the reaction process.

The high ef?ciency of charge sep-aration was mainly ascribed to the hybrid effect.

g r a p h i c a l

a b s t r a c t

The dramatic enhanced visible-light photocatalytic activity of Mg-ferrite/hematite nanospheres pho-tocatalyst on benzene were obtained after hybridized by polyaniline (PANI)using the chemisorption method.The enhancement of photocatalytic degradation of benzene under visible-light irradiation was mainly ascribed to the high ef?ciency of charge separation induced by the hybrid effect of PANI and Mg-ferrite/hematite.By using the in situ FTIR technique,ethyl acetate,carboxylic acid and aldehyde could be regarded as the intermediate products,and CO 2is produced as the ?nal product during the reaction

process.

a r t i c l e

i n f o

Article history:

Received 6June 2012Received in revised form 30September 2012

Accepted 3October 2012

Available online 11 October 2012

Keywords:

Mg-ferrite/hematite/PANI Benzene Visible-light

Photocatalytic degradation In situ FTIR

a b s t r a c t

The dramatic enhanced visible-light photocatalytic activity of Mg-ferrite/hematite nanospheres photo-catalysts on benzene were obtained after hybridized by polyaniline (PANI)using the chemisorption method.The samples were characterized by scanning electron microscope,transmission electron microscopy,X-ray diffraction,Fourier transform infrared spectra and UV–Vis diffuse re?ectance spec-troscopy.The enhancement of photocatalytic degradation of benzene under visible-light irradiation was mainly ascribed to the high ef?ciency of charge separation induced by the hybrid effect of PANI and Mg-ferrite/hematite.By using the in situ FTIR technique,ethyl acetate,carboxylic acid and aldehyde could be regarded as the intermediate products,and CO 2is determined as the ?nal product during the reaction process.

? 2012 Elsevier B.V. All rights reserved.

?Corresponding author at:Key Laboratory of Industrial Ecology and Environmen-tal Engineering and State Key Laboratory of Fine Chemical,School of Environmental Science and Technology,Dalian University of Technology,Linggong Road,No.2,Dalian,Liaoning 116024,China.Tel.:+8641184707733;fax:+8641184707733.??Corresponding author.Tel.:+610892669056;fax:+610892662681.

E-mail addresses:xyli@https://www.360docs.net/doc/016319995.html, (X.Li),shaomin.liu@https://www.360docs.net/doc/016319995.html,.au (S.Liu).

1.Introduction

The applications on solar energy conversion and environmen-tal puri?cation by semiconductor photocatalysts have attracted considerable attention in modern society [1–4].So far,TiO 2is still the most investigated photocatalyst due to its advantages of

0304-3894/$–see front matter ? 2012 Elsevier B.V. All rights reserved.https://www.360docs.net/doc/016319995.html,/10.1016/j.jhazmat.2012.10.003

Y.Shen et al./Journal of Hazardous Materials241–242 (2012) 472–477473

being inexpensive,nontoxic,and a good photostable semiconduc-tor[5,6].However,TiO2can only be activated by the ultraviolet light( <400nm),which only makes up about4%of the solar spec-trum.Hence,it is necessary to develop ef?cient visible-light-driven photocatalysts,and the research in this?eld has received particular interest in recent years.

At present,the properties of delocalized conjugated structures in electron-transfer processes have been extensively studied for their rapid photoinduced charge separation and a relatively slow charge recombination[7,8].It was well-known that polyaniline (PANI)is an ef?cient conductive polymer with unique electron and hole transporting properties[9].Since the photocatalytic activity of the photocatalyst can be promoted by increasing the separation ef?ciency of photoinduced electron–hole pairs,the introduction of PANI to photocatalysts may be an ideal way to achieve an enhanced charge separation by photoinduced carrier transfer.Up to now,var-ious PANI modi?ed catalytic materials,such as PANI/TiO2[8,10,11], PANI/ZnO[12],PANI/CdS[13]and PANI/BiVO4[14]hybrid photo-catalysts have been reported.Some of them showed dramatic photocatalytic activity for the degradation of pollutants under ultraviolet and visible-light irradiation.Herein,PANI doped Mg-ferrite/hematite photocatalyst with enhanced visible-light activity was fabricated for the?rst time by the chemisorption method.

In the present study,photocatalytic degradation of benzene was selected as a model reaction to evaluate the photocatalytic perfor-mance of the prepared photocatalyst since benzene is widely used as a solvent in industrial processes.Benzene is also regarded as the priority hazardous substance because of its high toxicity,con-?rmed carcinogenicity,and environmental persistence[15].The development of an ef?cient and mild technology for removing benzene from the ambient environment is necessary.Fu and co-workers prepared ZnGe2O4[16]and ZnGa2O4[17]photocatalysts for degradation of benzene under ultraviolet light illumination. Zhong et al.[18]studied photocatalytic degradation of gas ben-zene over TiO2/Sr2CeO4and speculated the benzene degradation mechanisms in details.Chen et al.[19]reported that phenol was the major intermediate and CO2was the?nal product detected by GC/MS during the photocatalytic degradation on benzene vapor with visible-light-driven silver vanadates.On the other hand,in situ Fourier-transform infrared spectroscopy could provide a real-time monitoring of transient events occurring on the catalyst during the reaction,and the surface adsorbed species detected by the in situ study will give an important insight for identifying the possible intermediates.By using in situ measurements of infrared spec-troscopy,some intermediate products such as phenolic species [20],carbonyl or carboxylate groups[21]were monitored in the course of benzene photocatalytic degradation.

Therefore,by using in situ FTIR,the visible-light-induced pho-tocatalytic degradation of benzene on Mg-ferrite/hematite/PANI nanospheres were systematically studied,and interpreted in terms of the synergetic effect between Mg-ferrite/hematite and PANI in this paper.

2.Experimental

2.1.Preparation of the samples

2.1.1.Mg-ferrite/hematite nanospheres

Mg(NO3)2·6H2O(1.28g)and Fe(NO3)3·9H2O(4.04g)were dis-solved in ethylene glycol(80mL)to form a clear solution,followed by the addition of NaAc(7.2g)and polyethylene glycol(PEG)20,000 (2.0g).The mixture was stirred vigorously for90min and then sealed in a te?on lined stainless-steel autoclave(100mL capacity). The autoclave was heated and maintained at200?C for22h.Then it was allowed to cool to room temperature.Subsequently,black precipitates were collected,washed with ethanol,and then dried at60?C for6h.Finally,the samples were annealed at650?C for2h with a heating rate of2?C min?1.

2.1.2.Polyaniline(PANI)

PANI was synthesized via an oxidative polymerization from aniline monomer4.66g,the molar ratio of1.1of an ammonium persulfate(APS)/aniline was employed,in which the APS(11.41g) was used as an oxidant.The components were both dissolved in1M hydrochloric acid and slowly added to each other by stirring for2h at0?C.The resulting polymer precipitates were collected by?ltra-tion and washed with deionized water thoroughly.The precipitates were then dried at80?C for24h.

2.1.

3.Mg-ferrite/hematite/PANI nanospheres

0.225g Mg-ferrite/hematite powder was added to50mL of 0.45g L?1PANI(tetrahydrofuran THF)solution,sonicated for 30min to ensure that the sample was totally dispersed,and then stirred for24h.The suspension was?ltered,and the precipitate was washed with water three times and transferred to oven to dry at80?C for24h.

2.1.4.N-TiO2

As a comparison photocatalyst,N-TiO2was prepared according to Ref.[22].The synthesis process are as follows:100mL ammo-nia aqueous solution was added dropwise into40mL tetrabutyl titanate(Ti(OBu)4)solution at room temperature.After continu-ously stirring for60min,the precursor was dried in an oven at60?C for24h.Finally,the TiO2precursor was calcined at500?C for2h to obtain N-TiO2photocatalyst.

2.2.Characterizations

The morphology of the Mg-ferrite/hematite nanospheres was characterized using an environmental scanning electron microscope(ESEM;Quanta200FEG)with an accelerating volt-age of30kV.Transmission electron microscopy(TEM)image was obtained with a Hitachi800system at200kV.The Brunauer–Emmett–Teller(BET)surface areas of the catalysts were measured by physical adsorption of N2at?196?C using NOVA1200 (Quanta Chrome).The sample was degassed at300?C for4h under vacuum prior to the BET determination.The crystallinity of the pre-pared samples was determined from X-ray diffraction(XRD,Rigaku D/max)with Cu K?radiation( =0.15418nm).Fourier transform infrared(FTIR)spectra were recorded on a VERTEX70-FTIR with the standard KBr pellet method.Light absorption properties were measured using UV–Vis diffuse re?ectance spectra(DRS)(JASCO, UV-550)with a wavelength range of200–800nm and compared to an MgO powder as a reference.

2.3.Photocatalytic activity evaluation

Photocatalytic activities of the catalysts were determined using the photocatalytic conversion of benzene under visible-light irra-diation in a self-made in situ quartz IR photoreaction cell.The cell(diameter,4cm;length,10cm)consisted of two NaCl win-dows and a sample holder(diameter,13mm)for the catalyst wafer (0.05g).After the catalyst was placed in the sample holder,a small amount of benzene was injected into the reactor with a microsy-ringe.The analysis of the benzene concentration in the reactor was conducted with a GC-FID(Agilent7890A).The benzene vapor was allowed to reach adsorption equilibrium in the reactor prior to irradiation.The initial concentration of benzene after adsorption equilibrium was controlled at280mg/m3.The Xenon lamp(XQ-500W)( >400nm)with the light intensity of about50mW cm?2 was turned on to allow the photocatalytic reaction to proceed under

474Y.Shen et al./Journal of Hazardous Materials 241–242 (2012) 472–

477

Scheme 1.Schematic illustration of the photocatalytic reactor system setup.

batch conditions.The IR spectra were continuously collected on the VERTEX 70-FTIR with a resolution of 1cm ?1and 20scans in the region of 4000–600cm ?1during the course of reaction.The same setup was both used for the wafers of Mg-ferrite/hematite and Mg-ferrite/hematite/PANI nanospheres.Scheme 1showed the schematic illustration of the photocatalytic reactor system setup.

3.Results and discussion

3.1.ESEM,TEM and BET analysis

Fig.1shows representative ESEM and TEM images of Mg-ferrite/hematite hollow nanospheres.The ESEM image shows that the synthesized samples are monodisperse spheres with diame-ter distributions about 200–400nm,mostly 380nm.Some small nanoparticles can be seen on the surface of one sphere,making it appear rough.From the TEM image,it can be further found that the spheres have the sharp contrast between the dark edges and the pale centers,unambiguously con?rming the well-de?ned hollow structures.The diameters of the Mg-ferrite/hematite nanospheres are about 400nm and the thickness of the wall of a hollow sphere is around 100nm.The BET speci?c surface areas of Mg-ferrite/hematite are 6.30m 2g ?1and 13.21m 2g ?1before and after PANI loading.

TEM and HRTEM images of the Mg-ferrite/hematite/PANI nanospheres are shown in Fig.2.It can be found from Fig.2a that a layer of PANI is covered on the surface of the nanospheres,and some dark spots can be observed within the hollow structures.Moreover the thickness of PANI is about 20nm obtained from the HRTEM image of the edge of the nanospheres (Fig.2b).Because of

the ?rst molecular is adsorbed chemically to the ferrite surface,the thickness of PANI adsorbed in chemisorption mode is less than one nanometer [8],while more thick outer layer is formed in physical adsorption.

3.2.XRD and FTIR analysis

The XRD patterns of neat Mg-ferrite/hematite and PANI-modi?ed Mg-ferrite/hematite are compared in Fig.3a.In the patterns,the diffraction peaks are in agreement with the standard diffraction patterns of Mg-ferrite (JCPDS 36-0398)and hematite (JCPDS 33-0664).Moreover,PANI-modi?ed Mg-ferrite/hematite nanospheres did not cause any change in peak positions and shapes compared with neat Mg-ferrite/hematite.This shows that PANI-modi?ed Mg-ferrite/hematite prepared by this loading method did not change the crystalline structure of neat Mg-ferrite/hematite.The main peak of modi?ed catalyst,however,is slightly lower than that of neat Mg-ferrite/hematite.It also should be noted that no new diffraction peak appears in the pattern of PANI-modi?ed Mg-ferrite/hematite.This suggests that PANI is amorphous in the composite.

The average particle size of the prepared samples is deter-mined from the broadening of the peaks corresponding to the (311)diffraction of Mg-ferrite/hematite,using Scherrer’s formula:D =K /B cos ?,where D is the average crystallite size (nm), is the X-ray wavelength (15.405nm)and K is Scherrer’s constant (K =0.89).B is the full width at half maximum observed in radi-ans and ?is the angle of diffraction.The predicted crystallite sizes of Mg-ferrite/hematite and Mg-ferrite/hematite/PANI nanospheres are 32.99nm and 28.63nm,

respectively.

Fig.1.SEM image (a)and TEM image (b)of the Mg-ferrite/hematite nanospheres.

Y.Shen et al./Journal of Hazardous Materials 241–242 (2012) 472–477

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Fig.2.TEM image (a)and HRTEM image (b)of the Mg-ferrite/hematite/PANI

nanospheres.

Fig.3.XRD patterns (a)and FTIR spectra (b)of the Mg-ferrite/hematite nanospheres and the Mg-ferrite/hematite/PANI nanospheres.

The FTIR spectra of Mg-ferrite/hematite,PANI,and Mg-ferrite/hematite/PANI photocatalyst are shown in Fig.3b.The patterns show that the main characteristic peaks of Mg-ferrite/hematite and PANI all appear in the FTIR spectra of Mg-ferrite/hematite/PANI photocatalysts.Speci?cally,compared with the main bands of pure PANI at 1560cm ?1(C N and C C stretching mode),1294cm ?1(C N stretching mode)[12]and 1106cm ?1(in plane ring C H bending mode)[23],all bands appeared at the corresponding locations of the FTIR spectrum of Mg-ferrite/hematite/PANI.These results indicate that PANI and Mg-ferrite/hematite were successfully combined together.

3.3.UV–Vis diffuse re?ection spectrum

The optical property of the Mg-ferrite/hematite and the Mg-ferrite/hematite/PANI nanospheres are investigated by UV–Vis absorption spectroscopy.As shown in Fig.4,two catalysts both have strong photo absorption in the UV and visible spectral region,and the wavelength of the absorption edge is 688nm.The difference between the two spectra is not very obvious.Thus,PANI loading did not change the original band structure of the catalyst,which was also consistent with XRD results.

3.4.Photocatalytic conversion of benzene

When benzene reached adsorption equilibrium on the surface of the Mg-ferrite/hematite/PANI wafer,the reaction cell was irra-diated with visible-light.The characteristic peaks of benzene in infrared spectra are shown in Fig.5.The bands at 3047,3055,and 3089cm ?1are assigned to the C H stretching mode in the

benzene rings [20,24](Fig.5a).The 1485cm ?1band is related to C C stretching [21].The vibrational bands at 1802,1820cm ?1and 1948,1964cm ?1are assigned to C H out-of-plane bending mode [24](Fig.5c).The bands in the range of 1020–1050cm ?1could be ascribed to C H in-plane deformation vibrations [25](Fig.5d).In addition,the bands at 2360and 2340cm ?1are attributed to CO 2[21](Fig.5b).Increase of the irradiation time caused signi?cant increase in the intensity of characteristic peaks of CO 2(2360–2329cm ?1)and equally signi?cant decrease of the one of benzene (3089–3047,1964–1948,1820–1802,1485and 1020–1050cm ?1).

Furthermore,some new surface species also formed during the reaction,as evidenced by the FTIR spectra (Fig.5d).New bands

at

Fig.4.UV–Vis absorption spectra of the Mg-ferrite/hematite nanospheres and the Mg-ferrite/hematite/PANI nanospheres.

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Fig.5.FTIR spectra recorded as a function of irradiation time following the photocatalytic degradation of benzene on the Mg-ferrite/hematite/PANI nanospheres:(a)infrared spectra in the region 3300–2800cm ?1;(b)infrared spectra in the region 2500–2200cm ?1,(c)infrared spectra in the region 2200–1300cm ?1;(d)infrared spectra in the region 1200–750cm ?1.

1166and 1100cm ?1are attributed to C O C stretching vibration which existed in ester [26,27].The 950cm ?1band is assigned to O H out-of-plane deformation vibration of carboxylic acid [28],and 792cm ?1is assigned to C H deformation vibration of aldehyde [29].On the basis of the previous reports [18]and our results,ethyl acetate,carboxylic acid and aldehyde are the intermediate products during the photocatalytic degradation of benzene,CO 2is produced as the ?nal product.

Fig.6presents the photocatalytic conversion of benzene over different samples under visible light irradiation.Without the pres-ence of a catalyst,a small amount of benzene was transformed under visible light irradiation.The conversion of benzene was about 15.1%for N-TiO 2after 4h reaction,and Mg-ferrite/hematite and the Mg-ferrite/hematite/PANI nanospheres presented much higher photocatalytic conversion (52.0%and 62.5%,respectively)for benzene under the same conditions.N-TiO 2is a semiconduc-tor with wide band gap and its visible light induced photocatalytic activity is weaker than the ferrites.The photocatalytic activity of Mg-ferrite/hematite/PANI is higher than Mg-ferrite/hematite due to the doping of PANI.

3.5.Mechanism of the charge separation and visible light induced photocatalysis process

On the basis of the characterization results and photocatalytic tests,the enhancement of visible photocatalytic activity was mainly

due to the high ef?ciency of charge separation induced by the hybrid effect of PANI and Mg-ferrite/hematite.A schematic mech-anism of the charge separation and photocatalytic process over the Mg-ferrite/hematite/PANI photocatalyst is shown in Fig.7.Being hybridized by PANI,the conduction band (CB)position of Mg-ferrite [30]was lower than the lowest unoccupied molecu-lar orbital (LUMO)of PANI [31],so the former could act as a sink for the photogenerated electrons in the hybrid photocatalysts;

the

Fig.6.Photocatalytic conversion of benzene over different samples under visible light irradiation.

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Fig.7.Mechanism scheme of the charge separation and photocatalysis process over MgFe2O4/?-Fe2O3/PANI nanospheres photocatalysts under visible light irradiation.

valence band(VB)position of Mg-ferrite was lower than the high-est occupied molecular orbital(HOMO)of PANI,so the later could act as an acceptor for the photogenerated holes in the hybrid photocatalysts.Under visible-light irradiation,the excited elec-trons moved to the CB of Mg-ferrite,and then passed to the CB of hematite[32].Simultaneously,the photogenerated holes in the VB of hematite and Mg-ferrite moved to the composites surface through HOMO of PANI.Therefore,a rapid photogenerated charge separation and relatively slow charge recombination are achieved, which lead to an enhanced visible photocatalytic activity of the Mg-ferrite/hematite/PANI nanospheres.

4.Conclusions

In summary,PANI modi?ed Mg-ferrite/hematite nanospheres have been fabricated by the chemisorption method.The mod-i?ed nanospheres demonstrated some advancement over Mg-ferrite/hematite nanospheres on the photocatalytic degradation of benzene under visible-light irradiation.By using the in situ FTIR technique,phenol,carboxylic acid and aldehyde could be regarded as the intermediate products,and CO2is determined as the?nal product during the reaction process.The photocatalysts prepared in this work are expected to play an important role in environmental puri?cation by utilizing solar energy.

Acknowledgments

This work was?nancially supported by the National Nature Sci-ence Foundation of China(No.20837001,20877013and NSFC-RGC 21061160495),the National High Technology Research and Devel-opment Program of China(863Program)(No.2010AA064902),the Major State Basic Research Development Program of China(973 Program)(No.2007CB613306)and the Excellent Talents Program of Liaoning Provincial University(LR2010090).

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