苯的催化

https://www.360docs.net/doc/962271053.html,/Langmuir ?2009American Chemical

Society

Nanostructuring Cadmium Germanate Catalysts for Photocatalytic

Oxidation of Benzene at Ambient Conditions

Jianhui Huang,?,?Kaining Ding,?Xinchen Wang,*,?and Xianzhi Fu ?

?

Research Institute of Photocatalysis,State Key Laboratory Breeding Base of Photocatalysis,Fuzhou University,Fuzhou 350002,P.R.China,and ?Environmental and Life Science Department,Putian University,

Putian 351100,P.R.China

Received February 12,2009.Revised Manuscript Received April 16,2009

A nanostructured Cd 2Ge 2O 6photocatalyst was successfully prepared by a hydrothermal process.The photocatalyst was characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM),transmission electron microscopy (TEM),UV/vis,N 2adsorption -desorption,and Fourier transform infrared (FTIR)techniques.The photocatalytic property of the material was evaluated via the decomposition of benzene in the gas phase with light illumination and was compared with that of commercial titania (Degussa P25)and Pt/TiO 2.The electronic band structure of Cd 2Ge 2O 6was analyzed by density functional theory (DFT)calculation.Results reveal that the prepared Cd 2Ge 2O 6has unique geometric and electronic properties,which in combination with its superior textural properties makes it a new semiconductor photocatalyst for environmental purification of benzene in air with molecular oxygen under ambient conditions.It was also found that the Cd 2Ge 2O 6was more active and stable than TiO 2-based catalysts in the photocatalytic decomposition of other volatile aromatic pollutants including toluene and ethylbenzene.The enhanced photocatalytic performance of Cd 2Ge 2O 6can be explained by the special band structure,and geometric and electronic feature,in unison with the high surface area nanoporous framework.

1.Introduction

Benzene has been classified as a Class A carcinogenic by the Environmental Protection Agency (EPA)due to its ubiquity in the urban atmosphere and known toxic effects in humans,causing many physiological effects including drowsiness,dizziness,head-ache,lightheadedness,nausea,and even cancers such as leukemia.1,2The development of a green approach to remove benzene from the polluted environment is therefore of vital importance,but it remains a grand challenge due to the stable aromatic benzene structure,especially at ambient conditions.

Various approaches have been developed for the environmental purification of benzene and its derivatives.For example,physical adsorption using activated carbon,3zeolite,4,5and charcoal 6is one of the common methods to remove aromatic hydrocarbons from polluted air.This is however a phase-transfer process that requires further treatment of the adsorbents before final disposal,giving rise to processing problems during post-treatment.Biofilters and hybrid bioreactors have been investigated for the biodegradation of benzene,7-9with activity showing strong dependence on the types of microorganisms used,typically limited to “narrow”

operation conditions.10Chemical destruction of benzene has also been extensively investigated,including catalytic calcinations,11,12oxidation with ozone or supercritical water,13,14degradation using a plasma-based system,15and photocatalytic oxidation.16,17Among these approaches,photocatalytic oxidation could thermo-dynamically allow the destruction of benzene to CO 2and H 2O with clean molecular oxygen at ambient conditions.Intensive studies have been focused on light-harvesting semiconductor TiO 2material for a number of years.Despite the efforts spent on the modification of TiO 2and the optimization of reaction conditions for benzene photooxidation,18-25there remains a general problem of catalytic deactivation for TiO 2-based catalysts in benzene photocatalysis.This intrinsic limitation of TiO 2was reported to be associated

*To whom correspondence should be addressed.E-mail:xcwang@https://www.360docs.net/doc/962271053.html,.Telephone:86-591-83773729.

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Article Huang et al.

with its active surface sites that strongly bind to the reaction intermediates of benzene photooxidation process.17Fundamental research regarding the surface deactivation mechanism is helpful for the rational design of TiO2-based materials,but the development of new materials as photocatalysts would be one of the alternative approaches to realize efficient benzene photodegradation. Indeed,some miscellaneous oxides such as Zn2GeO4, ZnGa2O4,and Sr2Sb2O7have recently been demonstrated to be new semiconductor photocatalysts capable of catalyzing the oxidation of benzene with molecular oxygen under ambient conditions.26-28Although such miscellaneous oxides have been widely exploited as water-splitting photocatalysts,they are rarely used for photocatalytic purification of aromatic hydrocarbons in polluted air.One of the remaining drawbacks of these water-splitting catalysts is that they are generally synthesized via solid-state reaction methods at high temperatures.The materials prepared in this manner have a very low surface area (ca.<2m2/g),lacking accessible catalytic surface sites and hence are not suitable for gas-solid heterogeneous photocatalysis. Fabrication of catalysts in nanostructural forms can in principle increase the number of available surface sites,facilitating hetero-geneous catalysis to proceed,with the enlarged surface area enhancing the light-harvesting ability and improving mass trans-fer of the materials.Herein,we select Cd2Ge2O6as a new photo-catalyst material due to its unique geometric and electronic structure(see Theoretical Calculations section).Nanoarchitectural fabrication of Cd2Ge2O6was accomplished with a surfactant-assisted soft-chemical synthesis under hydrothermal conditions at <180°C.Materials were fully characterized by XRD,SEM, TEM,UV/vis,N2adsorption-desorption,and FTIR techniques. The photocatalytic oxidation of benzene and its derivatives on Cd2Ge2O6under mild conditions was investigated and analyzed to demonstrate the utility of the as-prepared material.The electronic band structure of nanocrystalline Cd2Ge2O6was also investigated to reveal its high photocatalytic activity and stability.

2.Experimental Section

Preparation of Catalysts.Nanocrystalline Cd2Ge2O6was synthesized by a hydrothermal process.In a typical synthesis,

0.30g of cetyltrimethylammonium bromide(CTAB,Sinopharm

Chemical Reagent,>99.0%),0.52g of GeO2(Sinopharm Chemical Reagent,99.999%),and1.33g of Cd(CH3COO)23 2H2O(Sinopharm Chemical Reagent,>99.0%)were added to

15.0mL of D.I.water.The resulting mixture was adjusted to pH8

by adding NaOH(30wt%).The mixture was stirred for1h and then transferred to a stainless Teflon-lined autoclave of20mL inner volume.The autoclave was heated to100-180°C and then tempered at these temperatures for24h,followed by cooling to room temperature.The product was centrifuged,filtered,and rinsed with alcohol and D.I.water several times.Finally,the product was dried overnight.A white Cd2Ge2O6powder was obtained with a yield of～95%.

Characterization.X-ray diffraction(XRD)patterns were collected inθ-θmode using a Bruker D8Advance X-ray diffractometor(Cu K R1irradiation,λ=1.5406A).The accel-erating voltage and the applied current were40kV and40mA, respectively.Data were recorded at a scan rate of0.02o2θs-1in the2θrange of15-70°.The morphology of the samples was investigated by a field emission scanning electron microscopy (SEM)(JSM-6700F).Transmission electron microscopy(TEM)and high-resolution transmission electron microscopy(HRTEM) images were obtained by using a JEOL model JEM2010EX instrument at the accelerating voltage of200kV.The powder particles were supported on a carbon film coated on a3mm diameter fine-mesh copper grid.A suspension in ethanol was prepared via sonication and allowed to evaporate to dryness on the support grid prior to analysis.The Brunauer-Emmett-Teller (BET)surface areas were determined by nitrogen adsorption-desorption isotherm measurements at77K by using Micro-meritics ASAP2020equipment.The samples were degassed under vacuum at120°C until a pressure lower than10-6Torr was reached and kept for5h before data acquisition.A Varian Cary 500Scan UV-visible system equipped with a Labsphere diffuse reflectance accessory was used to obtain the reflectance spectra of the catalysts,utilizing BaSO4as the background reference material,with spectra acquired in the200and800nm ranges. Fourier transform infrared(FTIR)spectra were recorded on a Nicolet Nexus670FT-IR spectrometer at a resolution of4cm-1. The catalysts were diluted with KBr powder by a factor of2.5% by weight.FTIR spectra were recorded using40mg of the catalyst/KBr mixture.The amount of OH3produced in the photocatalytic reaction is detected by means of the terephthalic acid(TA)fluorescence(FL)probe method as follows.An aqueous solution containing0.01M NaOH and5.0mM TA was prepared.

A total of80.0mg of Cd2Ge2O6powder was then suspended in

80.0mL of this solution in a Pyrex glass cell.The light source for the excitation of Cd2Ge2O6was three4W UV-lamps with a wavelength centered at254nm(Philips,TUV4W/G4T5).The OH3radicals formed in the system will react with(TA)and generate2-hydroxyterephthalic acid(HTA),the fluorescence of which is directly proportional to the generated OH3.The fluor-escence of HTA excited at321nm is426nm.The fluorescence spectra were measured using an Edinburgh Analytical Instru-ments FL/FSTCSPC920fluorescence spectrophotometer.29,30 Theoretical Calculations.The band structure and density of states(DOS)calculations were performed using CASTEP software utilizing a density functional theory(DFT)plane-wave pseudopotential method.The exchange and correlation terms were described with the local-density approximation(LDA)of Ceperly and Adler31parametrized by Perdew and Zunger.32The unit cell selected for calculation was Cd2Ge2O6.The kinetic energy cutoff was340eV.The fast Fourier transform(FFT)grid of the basis was40?30?30,and the self-consistent field(SCF) tolerance was1.0?10-6eV/atom.The ultrasoft pseudopotential and4?5?5k-point were adopted in this calculation. Photocatalytic Activity Measurements.Photocatalytic ex-periments were conducted with a fixed bed tubular quartz reactor operated in a single-pass mode.The catalyst(0.3g,50-70mesh) was loaded in the reactor surrounded by four4W UV-lamps with a wavelength centered at254nm(Philips,TUV4W/G4T5). Benzene diluted in a pure oxygen stream was used as the test reactant stream.The initial concentrations of benzene and carbon dioxide in the stream were determined as300and0ppm, respectively.The flow rate of the reactant stream was kept at 20mL/min.Simultaneous determination of benzene and CO2 concentrations was performed with an online gas chromatograph (HP6890)equipped with a flame ionization detector,a thermal conductivity detector,and a Porapak R column.The reaction temperature was controlled at29(1°C by an air-cooling system. In the case of photocatalytic degradation of toluene and ethyl-benzene,the experimental conditions were the same as those for the photooxidation of benzene reaction,except that the270ppm of the initial concentration of toluene and ethylbenzene was applied.Control experiments showed that,in the presence of

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Huang et al.Article light,but in the absence of catalysts,no degradations were

observed.For comparison,the photocatalytic activities of com-

mercial TiO2(Degussa P25)and Pt/TiO2were also tested under

the same reaction conditions as those employed for Cd2Ge2O6.

3.Results and Discussion

Hydrothermal synthesis is one of the common synthetic

methods for nanostructured materials,and it is generally accepted

that pH value plays an important role in controlling the crystal

structure,composition,and morphology of final products.

We have therefore investigated the effect of pH value on the

formation of Cd2Ge2O6under hydrothermal conditions.Figure1

shows the X-ray diffraction(XRD)patterns of the products

synthesized with different pH values in the presence of CTAB.

When the pH is in the range of4-6,the product is of a pure

tetragonal phase of Cd2Ge7O16(JCPDS:34-0287,a=b=

11.256,c=4.634),whereas pure monoclinic Cd2Ge2O6phase

(JCPDS:43-0468,a=10.184,b=9.652,c=5.377)can be

obtained at pH8and pH10.In addition,the intensity and

sharpness of the XRD peaks decreases with increasing pH value.

This result implies that product crystallinity is influenced

significantly by the pH value of the reaction medium.The sizes

of Cd2Ge2O6crystals calculated by the Scherrer equation and the

surface area values of Cd2Ge2O6prepared at different pH values

are listed in Table1.

We also investigated the impact of crystallization temperatures

on Cd2Ge2O6structure as prepared under hydrothermal condi-

tions.As shown in Figure2,the XRD patterns of material

prepared below100°C indicates that the product is amorphous.

At elevated temperatures of140and180°C,all of the diffraction

peaks for the samples can be assigned to the Cd2Ge2O6crystallites

having a monoclinic structure.Traces of the impurity phase were

not detected using the system resolution employed here for

analysis.The intensities of the diffraction peaks increase and

become sharper with the increase of hydrothermal temperature,

indicating the generation of enhanced crystallinity.The average

crystallite sizes are21and35nm for the Cd2Ge2O6prepared at

140and180°C,respectively.In addition,the intensity of the(131)

reflection of the Cd2Ge2O6of the sample prepared at140°C is

higher than expected,indicating a preferential crystal growth of

the materials under this condition.

Energy-dispersive X-ray(EDX)analysis of different sample

batches revealed that the chemical composition of the samples is

Cd2Ge2O6(Figure S1,Supporting Information).Even for the

material prepared at100°C,the EDX analysis of10dots at

different sample positions showed that the average ratio of Ge/Cd

is1:0.989,approaching the composition of Cd2Ge2O6.No other

element was found except for Pt left on the surface due to the

pretreatment of the sample with sputtering platinum.

Figure3a and b depicts the SEM and TEM images,respec-tively,of Cd2Ge2O6prepared at100°C for24h.The SEM image demonstrates that the as-prepared Cd2Ge2O6consists entirely of small particles with an average size of～10nm,whereas the TEM imaging shows that the material possesses a porous honeycomb structure with fine particulate morphology.However,at higher preparation temperatures,these small particles grow into rod-shaped Cd2Ge2O6with the aid of CTAB as a template.As shown in Figure3c,a large quantity of nanorods with20-80nm in width and150nm to1.5μm in length can be observed.The TEM image(Figure3d)also shows a nanorod-shaped Cd2Ge2O6 prepared at140°C.Figure3e highlights a typical shape of a Cd2Ge2O6nanorod.High resolution TEM(HRTEM)imaging the single crystal nature of the Cd2Ge2O6nanorod.The inter-planar distance of0.313nm observed in this image matches well with the(310)d spacing of the monoclinal Cd2Ge2O6structure. Figure3g corresponding to the fast Fourier transform of Figure3e further supports the single crystal nature of the Cd2Ge2O6nanorod.In combination with the results of XRD and HRTEM,it is clear that the Cd2Ge2O6nanorod grew along the[131]direction of the monoclinal Cd2Ge2O6crystal.

Figure4shows the nitrogen adsorption-desorption isotherms and Barrett-Joyner-Halenda(BJH)pore-size distribution plots of Cd2Ge2O6samples prepared at100and140°C,respectively. Both samples show similar type-IV isotherms,representative of the mesoporous nature of these solids.33The BJH

pore-size Figure1.XRD patterns of the Cd2Ge2O6samples prepared at 140°C for24h under different pH values.

Table1.Effects of Synthetic Conditions on the Compositions and

Crystal Sizes of Result Samples

synthetic conditions

catalyst T(°C)pH crystal size(nm)a S BET(m2/g)b Cd2Ge2O6100898

14082120

140102022

1808359

Cd2Ge7O16140642

140451

a The crystal sizes were calculated via the Scherrer equation:D=

0.89λ/βcosθ,where D is the average crystal size in nm,λis the Cu K R wavelength(0.15406nm),andβis the full-width at half-maximum.

b BET surface

area.

Figure2.XRD patterns of Cd2Ge2O6prepared at different temperatures.

Article Huang et al.

analysis (utilizing the isotherm desorption branch)demonstrates that the Cd 2Ge 2O 6sample prepared at 100°C has a wide pore-size distribution ranging from ～2to 120nm,peaking at 24nm with a pore volume of 0.41cm 3/g.For the Cd 2Ge 2O 6nanorod synthesized at 140°C,the pore volume decreases dramatically UV -vis diffuse reflectance spectra of Cd 2Ge 2O 6samples are presented in Figure 5.The wavelength at the absorption edge,λ,is determined as the intercept on the wavelength axis for a tangent line drawn on absorption spectra.It can be observed that the absorption edge of Cd 2Ge 2O 6prepared at 140and 180°C is 322and 325nm,respectively,corresponding to a band gap of ～3.9eV.The absorption edge of Cd 2Ge 2O 6samples prepared at 100°C equates to approximately 260nm,corresponding to a band gap of ～4.8eV.The blue shifts indicate the presence of a strong quantum confinement effect in the sample synthesized at low temperature.As a result of this confinement,the band gaps of Cd 2Ge 2O 6are increased,and accordingly its redox potentials changes.Thus,the photocatalytic performance of Cd 2Ge 2O 6would change significantly.

Photocatalytic activities of the samples prepared under differ-ent conditions were evaluated by the degradation of benzene in a signal-pass flow-type reactor at constant pressure.The results (see Figure S2in the Supporting Information)show the photo-catalytic performance of Cd 2Ge 2O 6prepared at different tem-peratures.All samples are capable of photocatalytically oxidizing benzene with molecular oxygen;however,the catalytic efficiency of Cd 2Ge 2O 6materials showed strong dependency on the synthetic temperature,with a reduction in efficiency observed with increasing hydrothermal preparation temperature.The concentration of produced CO 2over Cd 2Ge 2O 6samples follows a similar sequence,and all materials tested exceeded that produced over TiO 2.About 10%benzene was converted and 110ppm CO 2was produced for the sample prepared at 140°C.For the sample prepared at 180°C,the photocatalytic activity is slightly lower than that of 140°C.The Cd 2Ge 2O 6sample synthesized at 100°C shows the highest photocatalytic perfor-mance.Over 46%benzene in the stream was converted,and the concentration of produced was 700ppm.Note that GeO 2,CdO,and their mixture cannot convert benzene under the same experimental conditions.Considering the negligible conversion of excited-state benzene on dielectric oxides,34these results suggest that the reaction proceeded photocatalytically on the Cd 2Ge 2O 6samples.

The stability of the catalyst was also investigated.Figure 6shows results of a prolonged photocatalytic reaction over Cd 2Ge 2O 6against the reaction time,together with TiO 2data as a comparison.The initial benzene conversion rate on TiO 2is about 5%,but the activity is not sustainable.It declined with the reaction going on,and after 24h of reaction the conversion

rate

Figure 3.Morphology of Cd 2Ge 2O 6samples.(a)SEM and (b)TEM images of Cd 2Ge 2O 6synthesized at 100°C,(c)SEM and (d)TEM images of Cd 2Ge 2O 6nanorods,(e)typical TEM image of Cd 2Ge 2O 6nanorods,(f)HRTEM image of Cd 2Ge 2O 6nanorods,and (g)fast Fourier transformation pattern of the HRTEM

image.

Figure 4.N 2adsorption -desorption isotherms and pore size dis-

tribution plots for the Cd 2Ge 2O 6sample:(a)100°C and (b)140°

C.

Figure 5.Diffuse reflectance absorption spectra of Cd 2Ge 2O 6

synthesized at different temperature.

Huang et al.Article

decreased to ～1.3%with only ～12ppm CO 2concentration on the stream.After 40h of reaction,the white TiO 2turned black due to the deposition of reaction intermediates,leading to the deactivation of TiO 2.However,for the Cd 2Ge 2O 6sample,the catalyst did not exhibit any significant loss of activity;both the benzene conversion and CO 2concentration were kept steadily at about 46%and 700ppm,respectively,on the stream regardless of the reaction time,with a high mineralization ratio of ～85%.This fact confirms that the photocatalyst is essentially stable.The photocatalytic reaction in 48h produced 1.80mmol of CO 2,exceeding the amount of catalyst used (0.64mmol),further demonstrating that the reaction proceeded catalytically on the Cd 2Ge 2O 6sample.

The FTIR spectra of fresh and used Cd 2Ge 2O 6with a spectrum of TiO 2sample as comparison is presented in Figure S3in the Supporting Information.In the case of Cd 2Ge 2O 6,there is no obvious difference between the spectra of fresh and used samples.This suggests that no detectable reaction intermediates are deposited on the surface of Cd 2Ge 2O 6during the photocatalytic degradation of benzene.The strong redox power of Cd 2Ge 2O 6could allow for the conversion of most reaction intermediates to CO 2,H 2O,or other small molecular substances during the photocatalytic reaction,maintaining a clean active surface for photocatalysis.The clean surface of the used Cd 2Ge 2O 6is also confirmed by a solution NMR analysis (Figure S4,Supporting Information).However,for the TiO 2sample,several IR absorp-tion peaks were found for the used sample but were absent in the fresh sample.The small band at 1483cm -1is the characteristic of aromatic ring stretching vibrations,which demonstrates the deposition of some aromatic compounds on the used TiO 2.35Two bands at 1686and 1711cm -1are the characteristic frequen-cies of C d O stretching,suggesting that attack by the oxygen species upon the carbon deposits also occurred.25As the photo-catalytic reactions take place on the catalyst surface,a clean photocatalytic surface is important for sustaining photocatalysis by maintaining active surface sites for reactants.

In a further set of experiments,we extended this new photo-catalyst to the treatment of other toxic volatile aromatic organic compounds in air.Table 2displays the specific photocatalytic activity of Cd 2Ge 2O 6catalyst toward the degradation of benzene,toluene,and ethylbenzene in the gas phase.The data for com-plementary experiments performed with TiO 2(P25)and Pt/TiO 2are also shown for comparison.Results show that Cd 2Ge 2O 6exhibits high photocatalytic performance for degrading benzene

and its derivatives,whereas TiO 2is almost inactive at the steady state of the reactions.The modification of Pt improved the activity of TiO 2,but the activity was still much lower than that of pure Cd 2Ge 2O 6.In addition,we also tested the activity of samples under humidified conditions (2.5%water vapor)by passing reaction gas through a humidifier.It was found that the photocatalytic activity of Pt/TiO 2and TiO 2can be significantly enhanced,which is accord with the reference,18,36while it has not much influence for Cd 2Ge 2O 6.The degradation rate of benzene over TiO 2and Pt/TiO 2under the humidified condition was enhanced to 11and 12%,respectively.However,the photo-catalytic activity of TiO 2and Pt/TiO 2in the presence of water vapor is still much lower than that of Cd 2Ge 2O 6in dry conditions.An additional experiment was also performed to examine the capability of light-excited Cd 2Ge 2O 6to induce the formation of hydroxyl radicals (OH 3),which is the key reactive species in the degradation of organic compounds.It is well-known that tereph-thalic acid can capture OH 3and form 2-hydroxyterephthalic acid which can be easily detected by its fluorescence signal.29,30In the present work,we applied this fluorescence method for the direct measurement of the amounts of OH 3production.Figure 7a shows fluorescence spectra observed for the Cd 2Ge 2O 6suspen-sion containing 5mM terephthalic acid irradiated for various exposure times.As OH 3could also be produced by irradiation of terephthalic acid with 254nm UV,the fluorescence for the system without photocatalyst of Cd 2Ge 2O 6was also tested.Figure 7b shows the fluorescence intensity as a function of the duration of irradiation.The fluorescence intensity increases linearly with irradiation time for the systems with and without Cd 2Ge 2O 6.However,the slope of the system containing Cd 2Ge 2O 6is much higher than that of the system without the catalyst.This result is a strong indication that the photogenerated charge carriers in Cd 2Ge 2O 6possess strong redox ability,and are long-lived enough to react with H 2O and hydroxyl groups adsorbed on the catalyst surface to form the desirable OH 3radical species.

The calculated band structure supports the high catalytic performance of Cd 2Ge 2O 6.As shown in Figure 8,the top of the valence band of Cd 2Ge 2O 6is made up of O2p levels,and the bottom of the conduction band (CB)is the hybridized orbital of Ge4s4p and Cd5s5p states.37Large overlap among these states enlarges the dispersion of the CB,facilitating the diffusion of free charge carries.This unique electronic structure is a common feature for post-transition metal oxides which are believed to show great promise in photochemical applications.

The edge energy of the conduction band (E CB )of Cd 2Ge 2O 6estimated from the equation of X (Cd 2Ge 2O 6)-0.5E g 38is

Table 2.Photocatalytic Activity of Different Catalysts for Benzene,

Toluene,and Ethylbenzene Photooxidation Reactions

CR b

catalyst S BET (m 2/g)a

benzene toluene ethylbenzene

TiO 2c 510.7 1.70.9Pt/TiO 2c

51 3.1 4.4 1.3Cd 2Ge 2O 6-100d 9824.629.629.9Cd 2Ge 2O 6-140e

20

3.5

5.4

6.1

a

BET surface area.b Conversion rate:n (aromatic compound)per gram catalyst per hour (unit:μmol g -1h -1).c Data from ref 26.d

Cd 2Ge 2O 6prepared at 100°C.e Cd 2Ge 2O 6prepared at 140°

C.

Figure 6.Prolonged photocatalytic experiment of benzene decomposition over Cd 2Ge 2O 6with TiO 2(P25)as comparison.

(35)Hou,Y.D.;Wang,X.C.;Wu,L.;Ding,Z.X.;Fu,X.Z.Environ.Sci.Technol.2006,40,5799–5803.

(36)Einaga,H.;Futamura,S.;Ibusuki,T.Phys.Chem.Chem.Phys.1999,1,4903–4908.

(37)Sato,J.;Kobayashi,H.;Ikarashi,K.;Saito,N.;Nishiyama,H.;Inoue,Y.J.Phys.Chem.B 2004,108,4369–4375.

(38)Butler,M.A.;Ginley,D.S.J.Electrochem.Soc.1978,125,228–232.

Article Huang et al.

-2.86eV AVS (AVS =absolute vacuum scale)and is higher than that of TiO 2(-4.21eV AVS ).39The edge energy of the valence band (E VB )is estimated to be -7.66eV AVS ,lower than that of TiO 2(-7.41eV AVS ).This issue is illustrated in Figure 9,which shows the redox potentials of TiO 2and Cd 2Ge 2O 6.The high energy levels of the conduction and valence band edges allow the photogenerated holes to be kinetically more favorable to react with adsorbed H 2O,producing active OH 3radicals.The wide gap of Cd 2Ge 2O 6might be one drawback;however,it is not a serious problem for treatment devices using bactericidal lamps (emission wavelength 254nm).In addition,our initial experiments have shown that Cd 2Ge 2O 6can be easily modified by doping nitrogen

into the crystal lattice using a NH 3gas flow at high temperature,making the materials responsive to the visible spectrum (see Figure S5in the Supporting Information).Investigations into the utility of these materials in the visible-light induced photo-catalytic degradation of aromatic pollutants are currently under-way.

Figure 10shows the proposed three-dimensional crystal struc-ture of Cd 2Ge 2O 6.The Cd 2Ge 2O 6crystal cell is composed of GeO 4tetrahedrons and CdO 4tetrahedrons.These tetrahedrons are combined with edge oxygen.We have calculated the center of gravity of oxygen ions surrounding a Ge 4+ion by using the crystallographic data regarding the atom positions.There are two kinds of dipole moments of GeO 4tetrahedrons in the Cd 2Ge 2O 6crystal cell:one dipole moment of GeO 4is 0.2D,and the other one inside the tetrahedron is 0.4D.The dipole moment is known to induce the formation of local fields in the interior of the distorted polyhedra,which are considered to promote the separa-tion of electron -hole pairs in semiconductor materials.A corre-lation between the photocatalytic activity and the dipole moment (local fields)has been demonstrated by Inoue et al.40Their study indicated that CaIn 2O 4and SrIn 2O 4showed high photocatalytic activity as a result of the distorted InO 6octahedra,whereas the photocatalytic activity of LiIn 2O 4and NaIn 2O 4was negligible due to their distortion-free structure.

It is known that gas -solid heterogeneous photocatalysis is a surface-based process and therefore a large material surface will benefit such processes.41,42In this study,the samples have a specific surface area as high as 98m 2/g.This relatively large surface area provides more surface sites for the adsorption

of

Figure 7.(a)Fluorescence spectra obtained for the supernatant

liquid of the irradiated Cd 2Ge 2O 6sample suspension containing terephthalic acid (5.0mmol/L)at various irradiation periods.(b)Plots of the induced fluorescence intensity (426nm)against light irradiation

time.

Figure 8.(a)Total and atomic density of states and (b)band

structure for Cd 2Ge 2O 6.In each panel,the top of the valence band was set to

zero.

Figure 9.Position of the energy levels of the conduction band edge (E CB )and the valence band edge (E VB )of TiO 2and Cd 2Ge 2O 6with respect to the

AVS.

Figure 10.Three-dimensional crystal structure of Cd 2Ge 2O 6with

a unit cell.

(39)Xu,Y.;Schoonen,M.A.A.Am.Mineral.2000,85,543–556.

(40)Sato,J.;Kobayashi,H.;Inoue,Y.J.Phys.Chem.B 2003,107,7970–7975.(41)Hoffmann,M.R.;Martin,S.T.;Choi,W.Y.;Bahnemann,D.W.Chem.Rev.1995,95,69.

(42)Huang,J.H.;Wang,X.C.;Hou,Y.D.;Chen,X.F.;Wu,L.;Wang,X.X.;Fu,X.Z.Microporous Mesoporous Mater.2008,110,543–552.

Huang et al.Article

reactant molecules and also for light harvesting,and ultimately makes the photocatalytic processes more efficient.Furthermore, our group has reported a series of catalytic materials including Zn2GeO4and SrSb2O7suitable for the photocatalytic degrada-tion of benzene(see Table S1in the Supporting Information).26,28 These photocatalysts also show higher activity toward benzene degradation than TiO2-based catalysts under ambient conditions. However,none of these catalysts demonstrate as such a high photocatalytic activity under the same conditions as Cd2Ge2O6materials presented here,which together with the ease of modification of these porous materials presents possible future opportunities to further improve Cd2Ge2O6-based catalyst performance.

4.Conclusions

In summary,Cd2Ge2O6was prepared by a surfactant-assisted hydrothermal method and used as a new photocatalytic material. The catalysts can efficiently photocatalyze the oxidation of benzene and its derivatives without the problem of catalytic deactivation.The explanation for the high photocatalytic perfor-mance of Cd2Ge2O6is provided,arising from unique geometric and electronic structure and large surface area,which favors the formation of the reactive hydroxyl radical species.This study demonstrates the promise of complex metal oxide materials with favorable electronic and textural structures for environ-mental photocatalysis especially for the elimination of aromatic pollutants in air.

Acknowledgment.This work is financially supported by the NSFC(Grant Nos.20603007and20777011),the973Program (Grant Nos.2007CB613306and2007CB616907),the New Century Excellent Talents in University of China(Grant No. NCET-07-0192),and the NSF of Fujian Province(Grant Nos. 2006J0160and2008H0089).We are indebted to Prof.J.Q.Li and Dr.R.White for their help and discussion.We also thank the reviewers for their helpful suggestions and comments on the manuscript.

Supporting Information Available:EDX analysis result of Cd2Ge2O6,photocatalytic activity of Cd2Ge2O6prepared at different temperature,FTIR spectra of the fresh and used Cd2Ge2O6and TiO2,and light absorption property of nitrogen doped Cd2Ge2O6.This material is available free of charge via the Internet at https://www.360docs.net/doc/962271053.html,.