Powder Technology 210 (2011) 1–5

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Synergy effect on a suspended mixture of ceria and activated carbon for thephotocatalytic degradation of phenol

Jingjing Xu ⁎Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, College of Environmental Science and Engineering,Nanjing University of Information Science & Technology, Nanjing 210044, China

⁎ Tel./fax: +86 2558731090.E-mail address: xujj@seu.edu.cn.

0032-5910/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.powtec.2011.02.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 September 2010Received in revised form 2 January 2011Accepted 1 February 2011Available online 21 March 2011

Keywords:PhotocatalysisPhenolCeriaActivated carbonSynergy effect

Ceria, prepared by a precipitation method, was used as a new photocatalyst under UV light irradiation. Theas-prepared sample was characterized by XRD and SEM. Results showed that the ceria was cubic fluorite structureand sized in nanometer range. The photocatalytic degradation of phenol was investigated in the presence of asuspended mixture of ceria and activated carbon. A synergy effect was observed with an enhancement of theapparent rate constantbya factorof5.6 times. Theapparentquantumyieldof theceria–ACsystemwasalso increased2.9 times. The activated carbonwith strong adsorption activity provided sites for the adsorption of phenol. Then, theadsorbed phenolwouldmigrate continuously to the surface of ceria particles. Some phenol still remained adsorbingon the catalyst when no traces of phenol were detected in the solution. This adsorbed phenol could be degraded bymaintaining UV-irradiation.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In recent two decades, titanium dioxide has been extensivelystudied for environmental purification applications, due to its goodcharacteristics of powerful oxidation strength, chemical stability,nontoxicity and inexpensiveness [1–6]. However, it also shows somedisadvantages, one of the most important one is the deactivation ofthe photocatalyst, thus it cannot be long term utilized [7–9].Consequently, there are considerable attentions on the developmentof alternative photocatalyst with improved photocatalytic activity orlonger stability during prolonged application [10–12].

Ceria is an inexpensive material, and has some properties liketitania such as wide band gap, nontoxicity, and high stability.Therefore, ceria has been selected as an alternative photocatalyst forgeneration of hydrogen gas and photocatalytic degradation of organiccontaminants [13–16]. However, the photocatalytic efficiency stillcannot fulfill the practical application of ceria photocatalyst inwastewater treatment. Therefore, it is essential to find a way toimprove the photocatalytic activity of ceria. It is well known thatactivated carbon (AC) is one of the low-cost and widely availableporous materials with relatively large surface area. Commercial AChas been widely used as adsorbents and catalytic supporters in liquidmedia for removing of contaminants. It would be helpful for theachievement of enhanced photocatalytic activity if the ceria wascombined with AC in photocatalytic systems.

In the present work, the aim of us is to investigate the associativeor synergistic effect between ceria and AC in photocatalytic system oftheir suspended mixture.

2. Materials and methods

2.1. Sample preparation and characterization

All chemicals were of reagent grade or higher purity, and wereused without further purification. Activated carbon (AC) with specificsurface area around 1100 m2 g−1 (pore volume was 0.56 cm3/g,average pore size is 3.5 nm) was purchased from ShangHai activatedcarbon Ltd. The preparation process of ceria nanoparticles was asfollowing: first of all, 0.02 mol Ce(NO3)3·6H2O was dissolved into100 mL aqueous solution, then 5 mL NH3 (aq.) was added drop-wisedinto the solution. Afterwards, the solution was stirred for another12 h. The obtained products were centrifuged, washed, and redis-persed in water and ethanol for 3 cycles, respectively. The sample wasthen dried at 80 °C for 2 h under vacuum. Finally, the sample wascalcined at 400 °C for 6 h. The structure properties were determinedby X-ray diffractometer (XD-3A, Shimadazu Corporation, Japan) usinggraphite monochromatic copper radiation (Cu-Kα) at 40 kV, 30 mAover the 2θ range 20–80°. The morphologies were characterized witha scanning electron microscopy (SEM, Hitachi, S-4800).

2.2. Adsorption experiments

For the adsorption measurements, 0.05 g ceria, 0.025 g AC or 0.05 gceria+0.025 g AC were added into 100 mL of freshly prepared phenol

Fig. 1. (a) XRD pattern and (b) SEM image of the as-prepared ceria.

Table 1Langmuir isotherm parameters: total number of adsorption sites (nT) and adsorptionconstant (Kads).

Samples Kads (mol−1) nT (mol)

Ceria 3.12×103 7.35×10−5

AC 2.64×104 4.62×10−4

Ceria–AC 2.98×104 3.37×10−4

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solutions of known concentration in the range 10 to 100 mg L−1 atnatural pH, respectively. The suspensions were sonicated for 5 min andthen stirred for 1 h at room temperature in the dark to reachadsorption–desorption equilibrium. After filtration through a Milliporefilter membrane (0.22 μm), the sample was taken to determine theconcentration of phenol. It the separation process, we abandoned theformer 1 mL solution to avoid the influence of the filter membrane onthe concentration of the phenol solution.

2.3. Photocatalytic experiment

In anordinaryphotocatalytic experiment, 0.2 g ceria and0.1 gACwereadded under stirring into 400 mL phenol aqueous solution whoseconcentration was 50 mg L−1. The suspension was maintained in thedark for 1 h to reacha complete adsorption–desorptionequilibrium. Then,thephotocatalytic reactionwas irradiated by an18WUV lamp(themajorwavelength is 365 nm). Samples of the suspension (5 mL)were removedat regular intervals for analysis. Theconcentrationsofphenol and themainintermediate productswere characterized byHPLC. TheHPLC systemwasAgilent 1100with tunable absorbance detector adjusted at 270 nm for thedetection of phenol. A reverse-phase column (length, 250 mm; internaldiameter, 4.6 mm) Aglient Eclipse XDB-C18 was used. The mobile phasewas composed of acetonitrile and deionized doubly distilled water. Thev/v ratio CH3CN/H2O was 10/90 and the flow rate was 1 mLmin−1.

3. Results and discussion

3.1. Characterization of sample

The crystal phase and texture of the synthesized ceria werecharacterized by XRD and SEM. Fig. 1 (a) shows the obtained XRD patternfor the ceria. Diffraction peaks, corresponding to cubic fluorite structure(JCPDS card No.: 43-1002), are clearly observed [13]. No peaks of anyother phase were detected, indicating the high purity of as-preparedsamples. The crystal size estimated by Scherrer equation: D=(Kλ)/(βcosθ),whereλ is thewavelengthof theX-ray radiation (λ=0.15418),Kthe Scherrer constant (K=0.9), θ the characteristic X-ray radiation and βis the full-width-at-half-maximum [17]. The obtained average crystal sizeis 9.5 nm. The SEM imageof the ceria is shown in Fig. 1 (b), fromwhichwecan see that the ceria nano-particles are all about 30 nm. The value islarger than the crystal size calculated by Scherrer equation above. Thedifference may be resulted from the reason that nano sized particlestended to aggregate into bigger particles. That is to say, the XRDestimatedvalue was the size of single crystallite while measured value of SEM wasthe size of agglomerates.

3.2. Adsorption activity

The investigation of phenol adsorption has been performed atroom temperature on pure ceria, on AC and on a suspendedmixture ofthem. The adsorptions in the dark were conducted during 1 h understirring for different initial concentrations of phenol between 10 and100 mg L−1. In the section, we choose 1 h for the adsorption since thatall samples reach adsorption–desorption equilibrium after about15 min. Therefore, 1 h is enough for the adsorption of phenol. Theconventional Langmuir isotherm model with a surface coverage θvarying as: θ=nads/nT=KadsC/(1+KadsC) was used to determine thetotal number of adsorption sites nT and the constant Kads from thelinear transform (1/nads)=ƒ(1/C) obtained with correlation coeffi-cients close to 0.99. The corresponding values are given in Table 1. Itcan be seen from Table 1 that the total number of adsorption sites ofceria–activated carbon system is slightly smaller than that of activatedcarbon. Similarly, the amounts of phenol adsorbed on the solids followthe same trend (Fig. 2). There is no enhancement of the adsorptioncapacities of ceria–activated carbon system compared to the activatedcarbon. This could be ascribed to the ceria close to the surface and

pores of the activated carbon, taking up some adsorption sites of theactivated carbon.

3.3. Kinetics of the photocatalytic degradation of phenol

The photocatalytic experiments were all performed at roomtemperature, and the results are shown in Fig. 2. The system wasmaintained in the dark for 1 h to reach a complete adsorption–desorption equilibrium. It can be seen that most of adsorption occurredwithin 15 min for all samples. The blank experiment without anycatalysis (direct photolysis) can be neglected with less than 2% ofdegradation of phenol within 4 h of UV irradiation. Similarly, thedegradation of phenol in the presence of AC under UV irradiation canalso be neglected with less than 2% of decomposition after 4 h. Thisillustrates that theAChas nophotocatalytic activity. After 4 h irradiation,the degradation percent of phenol is 36.8% in the presence of pure ceria.While for the CeO2–AC (0.2+0.1 g) system, the degradation percent is96.0%. The results illustrate that the addition of AC promoted thedegradation of phenol. For the determination of the mineralization rateof the phenol, TOC was also investigated. The TOC degradation percent

Fig. 2. Kinetics of phenol degradation in the presence of different photocatalyst. Thevertical line at time t=o separates the adsorption period from the UV irradiation period.

Fig. 3. Variations of ln(C0/C) as irradiation time in the presence of ceria and ceria–AC.

Fig. 4. (a) Influence of the mass of ceria on the photocatalytic degradation rate ofphenol, (b) Variations of ln(C0/C) as irradiation time from the data of Fig. 4 (a).

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was about 84.2% after 4 h UV irradiation for the CeO2–AC (0.2+0.1 g)system. This result illustrated that most of phenol are decomposed intoCO2 andH2O,while other phenol are transformed to intermediates (suchas hydroquinone etc.).

Furthermore, we investigated the kinetics of phenol degradationby different samples through Langmuir–Hinshelwood model:

−dC = dt = krKaC=1 + KaC ð1Þ

where (−dC/dt) is the degradation rate of phenol, C is the phenolconcentration in the solution, t is reaction time, kr is a reaction rateconstant, and Ka is the adsorption coefficient of the reactant. KaC isnegligible when value of C is very small. As a result, Eq. (1) can bedescribes a first-order kinetics. Setting Eq. (1) at the initial conditionsof the photocatalytic procedure, when t=0, C=C0, it can be describedas follows:

ln C0 = Cð Þ = kapp × t ð2Þ

where kapp is apparent rate constant. kapp was chosen as the basickinetic parameter for the different photocatalysts, since it enables oneto determine a photocatalytic activity independent of the previousadsorption period in the dark and the concentration of phenolremaining in the solution [18]. The variations in ln(C0/C) as a functionof irradiation time are given in Fig. 3. The obtained apparent rateconstants kapp are 0.098 and 0.55 h−1 for pure ceria and CeO2–AC,respectively. The results show that the photocatalytic activity of theCeO2–AC system is much higher than pure ceria (5.6 times). Theaddition of 0.1 g AC to 0.2 g ceria obviously creates a kinetic synergyeffect in phenol degradation.

The influence of the mass of ceria added on the rate of phenoldegradation has also been investigated, and the results are shown inFig. 4. Fig. 4 (a) shows the plots of variation of phenol concentrationagainst irradiation time. While Fig. 4 (b) shows the variations in ln(C0/C) as a function of irradiation time in the presence of differentamounts CeO2–AC system. The obtained apparent rate constants kapp are0.50, 0.55, 0.47and0.36 h−1 for (CeO2–ACsystem)0.1+0.1 g, 0.2+0.1 g,0.3+0.1 g and 0.4+0.1 g, respectively. The photocatalytic activity ofCeO2–AC system increases firstly and then decreases when the mass ofCeO2was increased. The decrease in activity beyond 0.2 g ceria is thoughtto be related to the increased scattering of photons (i.e. the utilization rateof light decreased).

3.4. Calculation of quantum yield

In heterogeneous photocatalysis, quantum yield has come to definethe number of molecules converted relative to the total number ofphotons incident on the photocatalyst. The overall quantum yieldΦoverall

Fig. 5. Kinetics of formation and disappearance of the intermediate specie hydroquinone.

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expresses thenumberofmoleculesNmol undergoing anevent (conversionof reactants or formation of products) relative to the number of photonsNph absorbed by the photocatalyst [19] (Eq. (3)).

Φoverall = Nmol mol·s−1� �

=Nph einstein·s−1� �

= rate of reaction = rate of absorption of photons:

ð3Þ

In the practical reactor, it is difficult to exactly determine the amountsof absorbed photons by the photocatalyst because of the reflection anddispersion of the photons by the photocatalyst and supporter or thetransmitting medium. So, the efficiency of heterogeneous photocatalysisoften depicted by apparent quantum yield [20]:

Φapp = amountof thedegradedmolecules=quantumnumberof incidentphotons:

ð4Þ

In thepresentwork,wedefinedΦapp as the ratio of initialmolars of thedegraded phenol in unit time and the incident photons:

Φapp = R⋅V= Iint⋅A=Uλ=365nmð Þ ð5Þ

whereR is the initial rate of phenol degraded (mol/L·S), V is the volumeofphenol aqueous solution(L), Iint is the intensityof incident light (μW/cm2),and U is the photonic energy of one einstein of the used UV light. Thevalues calculated from Eq. (3) are given in Table 2. It can be seen from thetable that the apparent quantumyield of CeO2–AC systems enhanced a lotcompared to that of pure ceria. It proves that the addition of activatedcarbon can enhance thephotocatalytic activity of thephotocatalyst,whichis in agreement with other investigators who studied the titania-coatedactivated carbon [21–24]. The enhanced photocatalytic activity can beascribed to the enhanced adsorbent activity of the composite system. Itcan be seen from the data of adsorption of phenol on different systems;the adsorption constant of the composite photocatalyst is almost 10 timesas that of single phase ceria. So, the phenol can be adsorbed onto thephotocatalyst more quickly and degraded there. We deduced that theapparent rate constant andquantumyield of the composite systemwouldbe 10 times as that of pure ceria. However, it is just 2.9 times. Thismay beascribed to that activated carbon, which has no photo-activity, absorbedsome incident photons.

3.5. Analysis of intermediates of phenol

The intermediates in photocatalytic degradation process of phenolby titania had been studied by other investigators [25]. They reportedthat the main intermediates were experimentally identified ashydroquinone, catechol, benzoquinone and resorcinol. In the presentwork, the intermediates of phenol were detected by HPLC study. Theresults show that the major intermediates in the photocatalyticdegradation process by the as-prepared samples were detected to behydroquinone in our case. The variations plots of hydroquinoneconcentration in the degradation process were shown in Fig. 5.Catechol was also detected but only within pure ceria and in muchsmaller quantities. From the figure, the amount of hydroquinone islower for CeO2–AC system than pure CeO2 in the overall process. Theresults indicate that the CeO2–AC system shows a higher rate of

Table 2Apparent rate constant and quantum yield of different samples.

Samples Kapp (h−1) Apparent quantumyield

Correlationcoefficients

Ceria 0.098 0.063 0.99057Ceria–AC (0.2+0.1 g) 0.55 0.18 0.98831Ceria–AC (0.1+0.1 g) 0.50 – 0.99162Ceria–AC (0.3+0.1 g) 0.47 – 0.9928Ceria–AC (0.4+0.1 g) 0.36 – 0.98147

appearance and disappearance of hydroquinone, which is in goodagreement with the results observed for phenol degradation. All in all,addition of AC creates a kinetic synergy effect in phenol degradation.

3.6. Desorption of phenol and intermediates which adsorbed on thephotocatalyst

It was attempted to determine the quantity of phenol remainingpresent on the photocatalyst. Blank preliminary tests performed onAC alone have shown that phenol could be extracted from AC withefficiency higher than 95% by acetonitrile solution. The photocatalyst(CeO2–AC (0.2+0.1 g) system)was therefore filtered and phenol wasextracted in 20% acetonitrile solution under sonication for 30 min. Theresults are shown in Fig. 6. It can be seen from the figure that thequantity of adsorbed phenol on the catalyst decreased as theirradiation time passed. A quantity of 0.28 mg of phenol could beextracted from the photocatalyst after 4 h of UV-irradiation when allthe phenol had nearly disappeared from the solution. We can calculatethat the total degraded phenol is 18.92 mg, which is 94.6% of the initialamount of phenol. Furthermore, we investigated the intermediatesadsorbed on the photocatalyst and we find only hydroquinone existedon the photocatalyst, and the obtained amount is 0.08 mg. From the value

Fig. 6. Mass of phenol extracted from the catalyst (ceria–AC: 0.2+0.1 g) after beingexposed to UV light for different times.

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we can know that the phenol and intermediates were mostly photo-catalytic degraded but not adsorbed by activated carbon.

The quantity of phenol remaining adsorbed on the photocatalystdetermined after extraction in acetonitrile reduced to nearly zero whenthe irradiation time increased to 6 h. Because theAChas nophotocatalyticactivity, this figure clearly illustrates that molecules of phenol that havebeen adsorbed and accumulated on the AC during the 1 h adsorption andprocess of photocatalytic degradation are able to be transferred to titaniawhere they are decomposed under irradiation.

4. Conclusion

In summary,we investigated the photocatalytic degradation of phenolby ceria–activated carbon in a suspended system under UV lightirradiation. The ceria nanoparticles, which prepared by a precipitationmethod,were used as a newphotocatalyst. A synergy effectwas observedin ceria–AC system (0.2+0.1 g) by a factor of 5.6 times for the phenoldegradation. Furthermore, we calculated the apparent quantum yield.Results showed that the value enhanced 2.9 times after the addition of 0.1AC.

The desorption experiment of phenol which adsorbed on thephotocatalyst was also investigated. Results showed that the phenoladsorbed on the surface of activated carbon can migrate continuouslyto the surface of ceria particles. Continuous migration and subsequentphotocatalytic oxidation on the surface of ceria accelerated phenolremoval efficiency greatly.

Acknowledgments

We are grateful for grants from Open Foundation of State KeyLaboratory of Hydrology-Water Resources and Hydraulic Engineering(No. 2010490511) and the project supported by Nanjing University ofInformation Science & Technology (20100348).

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