JOURNAL OF RARE EARTHS, Vol. 29, No. 9, Sep. 2011, P. 866

Foundation item: Project supported by Institution of Chemical Materials, China Academy of Engineering Physics

Corresponding author: WANG Yajun (E-mail: yajunwang@bit.edu.cn; Tel.: +86-10-68912941)

DOI: 10.1016/S1002-0721(10)60557-1

Photocatalytic degradation of methyl orange by polyoxometalates supported on yttrium-doped TiO2

WANG Yajun ( ), LU Kecheng ( ), FENG Changgen ( ) (State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China)

Received 9 March 2011; revised 7 April 2011

Abstract: A series of novel photocatalysts, H3PW12O40-Y-TiO2 nanocomposites with different H3PW12O40 loading levels (10% 40%) were prepared by impregnation method. And the Y-TiO2 support, doped with yttrium, was synthesized via sol-gel technique. The prepared catalysts were characterized by Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffraction (XRD), nitrogen adsorption-desorption analysis and scanning electron microscopy (SEM). The processes allowed obtaining Keggin structure and crystallized anatase with large BET surface area as well as uniform distribution. The effects of H3PW12O40 loadings, catalyst dose, initial pH and concentration of dye solution on the degradation kinetics of methyl orange under UV light ( 365 nm) were discussed. Kinetics studies showed that the photocatalytic degra-dation of methyl orange fitted the apparent first-order reaction. Methyl orange was totally degraded in 21 min under optimum conditions: 20% loading, 0.03 g dose and pH 1.0. The catalyst was stable and easily to be separated from reaction system for recovery.

Keywords: polyoxometalates; titanium dioxide; rare earths; photocatalytic degradation; kinetics; azo dyes

Azo dyes waste water, released into water bodies without degradation is toxic to the ecosystem and also has significant influence on human health[1,2]. Therefore, such pollutants have to be treated prior to discharging into the environment. They can be eliminated efficiently by photocatalytic degra-dation, an advanced oxidation process. As green and effec-tive photocatalysts, polyoxometalates (POMs) share many features with semiconductors and can be considered as the analogues of the latter. The photocatalysis of POMs, which originated from the photoexcitation of the oxygen-to-metal charge-transfer bands of POMs, is based on the electron-hole pair separation followed by reductive and oxidative reactions with surrounding molecules[3–5].

Supporting POMs on TiO2 has proved to be an efficient way to promote the photocatalytic performance of POMs, attributing to the synergistic effect between POMs and TiO2

[6–9]. As effective electron capture agents, POMs receive electrons generated by light irradiation of TiO2, and then ex-tend the recombination time of electrons and holes[10].

The photocatalytic activity of TiO2 can be enhanced by doping with rare earths[11,12]. The present research is to de-velop novel photocatalysts for treating the azo dye-contain-ing contamination. H3PW12O40 (HPW) was selected as pre-cursor and TiO2 doped with yttrium as support to prepare photocatalysts H3PW12O40-Y-TiO2 (HPW-Y-TiO2) with dif-ferent loadings, by using impregnation method and sol-gel technique. The obtained catalysts were characterized by various means. The photocatalytic degradation kinetics of azo dye, methyl orange (MO), under UV light irradiation

( 365 nm) was employed to investigate the photocatalytic activity of the composites.

1 Experimental

1.1 Materials

All chemicals were purchased from Sinopharm Chemical Reagent Co., China and employed without any further puri-fication. Phosphotungstic acid hydrate (H3PW12O40·nH2O), yttrium nitrate hexahydrate (Y(NO3)3·6H2O), absolute ethyl alcohol (EtOH), hydrochloric acid (HCl), perchloric acid (HClO4) and methyl orange (MO) are analytical grade re-agents. And tetrabutyl titanate (Ti(OBu)4) is chemical grade.

1.2 Catalysts preparation

To avoid decomposing of Keggin structure during calci-nation of the catalysts, HPW-Y-TiO2 composites were pre-pared by impregnation method while the supports Y-TiO2 were firstly synthesized via sol-gel technique.

The support was synthesized as below. A mixture of Ti(OBu)4 (10 ml) and EtOH (30 ml) was stirred under room temperature. HCl was added into the mixture to obtain pH 1.5, and the solution was marked A. 0.08 g Y(NO3)3·6H2O, the rare earth precursor, was dissolved into the solution con-taining H2O (2 ml) and EtOH (4 ml), which was marked B. Then, B was added dropwise into A during 10 min approxi-mately. The resulting acidic mixture was stirred constantly for about 3 h until sol was obtained. The sol was maintained

WANG Yajun et al., Photocatalytic degradation of methyl orange by polyoxometalates supported on yttrium-doped TiO2 867

for 24 h till gel formation. The gel was dried at 373 K for 4 h and then calcinated at 823 K for 3 h with a heating rate of 5 K/min. The rare earth doping amount (calculated by Y2O3) in the support is 1.0%. Pure TiO2 powder was also prepared by the same process without doping.

The impregnation process is as following: 1 g Y-TiO2 sup-port was dispersed in water solution containing a given amount of HPW. The suspension was stirred continuously under room conditions. After 24 h, water was removed by evaporation at 373 K. The final product was obtained by drying at 373 K for 2 h. The loading levels of HPW in four prepared composites by mass are 10%, 20%, 30% and 40% (by theoretical calculation), respectively.

1.3 Characterization

The IR spectra of the composites were investigated in the wavenumber range of 4000 400 cm–1 using Thermo Nicolet 6700 Fourier transform infrared (FT-IR) spectrometer. Crystalline phases of the prepared catalysts were character-ized by powder X-ray diffraction (XRD) on a PANalytical X' Pert PRO MPD diffractometer using Cu K radiation which was operated at 40 kV and 40 mA. The textural prop-erties were determined from adsorption-desorption isotherms of nitrogen by a Beishide 3H-2000PS2 instrument using multipoint BET and BJH methods. Surface morphologies were observed by scanning electron microscopy (SEM) (Hi-tachi S-4800N) operating at 5.0 kV.

1.4 Photocatalytic procedures

The photocatalysis experiments were performed on an open photoreactor. The light source is provided by a PLS- SXE300UV Xe lamp (300 W) with emission of 365 nm, which is positioned above the photoreactor.

The initial concentration (C0) of MO was fixed at 10 mg/L, except for the tests investigating the effect of C0. A given amount of prepared catalyst was suspended into a fresh MO solution (50 ml). Thirty-minute adsorption-desorption time in dark condition was allowed prior to photoreaction. The beaker was transferred to the photoreactor after the intensity of UV light was stable. The suspension was vigorously stirred during the whole process. At given interval of illumi-nation, a sample of suspension (ca. 3 ml) was taken out and filtered with microporous membrane (0.45 m). The ab-sorbance of residual MO solution was analyzed by an APL 752 UV-vis spectrometer at 464 nm when pH 4.0 and 510 nm when pH 3.0. The decrease of absorbance was used to indicate the degradation of MO[13].

2 Results and discussion

2.1 Characterization of the catalysts

2.1.1 FT-IR analysis Fig. 1 shows the FT-IR spectra of Y-TiO2 and HPW-Y-TiO2 with different loadings. The characteristic absorption peaks of Keggin unit at 1080, 982, 888, 797 cm–1 attribute to as(P–Oa), as(W=Od), as(W–

Ob–W), as(W–Oc–W), respectively[10]. From Fig. 1(1) it can be seen that the spectroscope of Y-TiO2 shows an intense, broad, indistinct region between 1100 and 400 cm–1. As a result, some characteristic peaks of Keggin unit in HPW-Y- TiO2 are overlapped in this area (see Fig. 1(2–5)). However, some vibration peaks of Keggin unit still can be seen clearly, which indicates that the Keggin structure has not been de-stroyed. 2.1.2 XRD analysis Fig. 2 shows the XRD testing results. It can be observed that the XRD patterns of Y-doped TiO2 and pure TiO2 are similar, both crystallized in the anatase structure with characteristic diffraction peaks of 2 values located at 25.3°(101), 37.8°(004), 48.1°(200), 54.0°(105), 62.7º(211), respectively[14]. However, no characteristic peak of Y oxide is found, which implies that Y oxide content is very small and highly dispersed[15].

From Fig. 2(3–6) we can see that all the samples with dif-ferent loading levels have anatase phase. For 40% loading, it is easy to identify the main diffraction peaks of HPW at 2 =10.4° and 25.4°[16]. However, for 10% 30% loading, only anatase phase is present and no separate polyoxotung-state-related phase is observed. This may be the reason that HPW is either in the octahedral interstitial sites or the sub-stitutional positions of Y-TiO2

[7]. The particle diameters were calculated from Scherrer for

Fig. 1 FT-IR spectra of the prepared samples (1) Y-TiO2; (2 ) HPW-Y-TiO2 (10% 40%)

Fig. 2 XRD patterns of prepared samples

(1) TiO2; (2) Y-TiO2; (3 ) HPW-Y-TiO2 (10% 40%)

868 JOURNAL OF RARE EARTHS, Vol. 29, No. 9, Sep. 2011

mula and listed in Table 1. The data show that all prepared powders are nanoparticles. The crystallite size decreases from 25.0 to 11.5 nm, indicating that Y-doping could re-strain the increase in grain size during calcination. 2.1.3 Nitrogen adsorption-desorption analysis Table 2 gives the results of nitrogen adsorption-desorption analysis for the samples. The BET surface area of Y-TiO2 is 69.0 m2/g, which is 1.8 times as that of undoped TiO2. The in-crease of surface area could be explained by small crystallite size and high dispersion of rare earth. The data also illustrate that the BET surface area of POMs is markedly increased by

Table 1 Crystallite size of prepared materials

Materials Crystallite size/nm

TiO2 25.0

Y-TiO2 11.5

HPW-Y-TiO2 (10%) 9.9

HPW-Y-TiO2 (20%) 9.1

HPW-Y-TiO2 (30%) 9.0

HPW-Y-TiO2 (40%) 14.5

Table 2 Microstructure of the prepared catalysts

Materials SBET/

(m2 g)

Average pore

size/nm

Pore volume/

(cm3 g)

TiO2 37.6 10.1 0.128

Y-TiO2 69.0 10.0 0.221

HPW-Y-TiO2 (10%) 67.9 9.2 0.261

HPW-Y-TiO2 (20%) 66.0 8.7 0.155

HPW-Y-TiO2 (30%) 56.0 8.2 0.123

HPW-Y-TiO2 (40%) 47.3 8.0 0.120

immobilization, comparing with the initial POMs (<10 m2/g)[17]. As the area of the Keggin anion is ca. 1.13 nm2 [18] and BET surface area of the support is 69.0 m2/g, a loading of about 20% would theoretically saturate the Y-TiO2 sup-port surface. BET surface area and pore volume decrease at high loading, and this may be because some pores of the support are blocked up by excess precursor. The supported POMs obtained are typical mesoporous materials with average pore diameter of 8 10 nm. 2.1.4 SEM observation The SEM images of the final products (Fig. 3) reveal that the supported HPW forms rela-tively uniform nanometer particles of diameter less than 20 nm, which is in accordance with the XRD results. The particles obtained are regular spherical.

2.2 Photocatalytic testing

No detectable degradation occurred on MO solution under UV light irradiation without catalysts and only about 5% concentration decreased under the action of catalysts in dark, implying that the degradation of MO originates from the combination of the UV light and the photocatalyst. The photocatalytic tests show that the degradation of MO follows Langmuir-Hinshelwood (L–H) apparent first-order kinetics. 2.2.1 The effect of different loadings Catalysts used in the evaluation were 0.02 g. MO was used without pH ad-justment and the pH was 5.3. The effect of different loadings on the kinetics is shown in Fig. 4. The results indicate that the best loading level is 20%, which is also the saturation value of loading level. At loading levels higher than 20%, excess HPW is easy to drop from the composite during the

Fig. 3 SEM images of HPW-Y-TiO2 composites with different loadings

(a) 10%; (b) 20%; (c) 30%; (d) 40%

WANG Yajun et al., Photocatalytic degradation of methyl orange by polyoxometalates supported on yttrium-doped TiO2 869

Fig. 4 Effect of HPW loadings on kinetics

photocatalytic process, and less reactant is adsorbed, result-ing in the decline of apparent first-order rate constant (kobs). This is in agreement with the conclusions obtained from the nitrogen adsorption-desorption analysis results.

Fig. 5 and Table 3 give the results of degradation kinetics in the presence of different catalysts. The kobs for Y-TiO2 is 0.006 min–1, which is 1.50 times as that of undoped TiO2, indicating that the doping of Y permits an improvement of photocatalytic activity of TiO2. This could be explained by the fact that Y-TiO2 has smaller crystallite size and good dispersion, which contributes to the quantum size effect[19]. In addition, Y-doping could extend the recombination time of electron-hole pairs, resulting in better photoactivity. It is obvious that the order of the photocatalytic performance is: HPW-Y-TiO2(20%)>Y-TiO2>TiO2. The kobs of HPW-Y- TiO2 (20%) is 3.50 times the value of Y-TiO2. 2.2.2 The effect of photocatalyst dose The pH of MO was not adjusted in the test (pH 5.3). Fig. 6 and Table 4 show the effects of HPW-Y-TiO2 (20%) dose on kinetics. The degradation rate is very slow at 0.01 g dose, implying

Fig. 5 Effect of different catalysts on kinetics

Table 3 Apparent first-order kinetics equations and relative parameters for photocatalytic degradation of methyl orange with different catalysts (pH 5.3)

Materials Kinetics equation kobs/min–1 t1/2/min R2

TiO2 ln(C0/C)=0.004t–0.037 0.004 173.3 0.992

Y-TiO2 ln(C0/C)=0.006t–0.012 0.006 115.5 0.999

HPW-Y-TiO2 (20%) ln(C0/C)=0.021t–0.204 0.021 33.0 0.982

Fig. 6 Effect of catalyst dose on kinetics

Table 4 Apparent first-order kinetics equations and relative parameters for photocatalytic degradation of methyl orange with different HPW-Y-TiO2 (20%) doses (pH 5.3)

Dose/g Kinetics equation kobs/min–1 t1/2/min R2

0.01 ln(C0/C)=0.001t+0.005 0.001 693.1 0.991

0.02 ln(C0/C)=0.021t–0.204 0.021 33.0 0.982

0.03 ln(C0/C)=0.027t–0.140 0.027 25.7 0.991

0.04 ln(C0/C)=0.021t–0.128 0.021 33.0 0.989

that activated species is not enough at this condition. It is clear that the optimum dose is 0.03 g. The degradation rate decreases when dose is up to 0.04 g. This is because the tur-bidity of the suspension increases with high catalyst dose, resulting in decrease in UV light penetration and photoacti-vated species. 2.2.3 The effect of initial pH In the test, the loading was 20% and the dose was 0.03 g. The effect of initial pH of the MO solution, which was adjusted with HClO4, was also in-vestigated. MO was not found degraded in the presence of HClO4 under UV light without catalyst, suggesting that HClO4 has no photocatalytic activity to MO. The depend-ence of degradation kinetics on pH is shown in Fig. 7 and Table 5. The results demonstrate that pH has significant im-pact on the photocatalytic degradation rate. The reaction rate increases with the decrease of pH. This could be explained by the fact that HPW is a super acidic catalyst which is sta-ble at ca. pH 1. So low pH is benefitial to the catalytic activ-ity. Another explanation is that MO is converted into quinoid

Fig. 7 Effect of initial pH on kinetics

870 JOURNAL OF RARE EARTHS, Vol. 29, No. 9, Sep. 2011

Table 5 Apparent first-order kinetics equations and relative parameters for photocatalytic degradation of methyl orange with different initial pH

pH Kinetics equation kobs/min–1 t1/2/min R2

1.0 ln(C0/C)=0.182t–0.122 0.182 3.8 0.981

2.0 ln(C0/C)=0.103t+0.004 0.103 6.7 0.999

3.0 ln(C0/C)=0.073t–0.111 0.073 9.5 0.988

4.0 ln(C0/C)=0.036t+0.020 0.036 19.3 0.997

structure at low pH, while quinoid structure is easy to be de-stroyed.

From above discussion, the optimum conditions for photo-catalytic degradation of MO solution (50 ml, C0=10 mg/L) are: 20% loading, 0.03 g dose and pH 1.0. Fig. 8 shows the photocatalytic behaviors of supported and starting HPW, Y-TiO2 under optimum conditions. It can be seen that MO is almost degraded in 21 min while just about 70% disappeared in 120 min in the presence of HPW-Y-TiO2 and HPW, re-spectively. As listed in Table 6, kobs for the supported HPW is 0.182 min–1, corresponding to more than 15 times the value of primary HPW, and 2.04 times that of Y-TiO2 under the same conditions. The photocatalytic performance of POMs is greatly improved by immobilization, which is at-tributed to the synergistic effect between Keggin-type and anatase support as well as the increased surface area after immobilization. 2.2.4 The effect of initial concentration Considering the effect of initial concentration, the test conditions were: 20% loading, 0.03 g dose and pH 1.0. When C0 is changed in the range of 5 30 mg/L, the degradation is apparent first-order reaction. It can be seen from Fig. 9 and Table 7 that the higher the C0 is, the lower the degradation rate is. This

Fig. 8 Photocatalytic activity of supported HPW, starting HPW, and

Y-TiO2 under optimum conditions

Table 6 Apparent first-order kinetics equations and relative parameters for photocatalytic degradation of methyl orange with HPW, supported HPW and Y-TiO2 un-der optimum conditions

Materials Kinetics equation kobs/min–1 t1/2/min R2

HPW ln(C0/C)=0.012t–0.174 0.012 57.8 0.989

HPW-Y-TiO2 (20%) ln(C0/C)=0.182t–0.122 0.182 3.8 0.981

Y-TiO2 ln(C0/C)=0.089t–0.190 0.089 7.8 0.989

Fig. 9 Effect of initial concentration on kinetics

Table 7 Apparent first-order kinetics equations and relative parameters for photocatalytic degradation of methyl orange with different initial concentration

C0/(mg/L) Kinetics equation kobs/min–1 t1/2/min R2

5 ln(C0/C)=0.241t–0.101 0.241 2.9 0.990

10 ln(C0/C)=0.182t–0.122 0.182 3.8 0.981

20 ln(C0/C)=0.093t–0.133 0.093 7.5 0.990

30 ln(C0/C)=0.036t–0.232 0.036 19.3 0.989

may be because more MO are absorbed on the catalyst sur-face at high C0, resulting in decline of the free radicals pho-toexcited by catalyst. In addition, UV light is hard to spread and less photons involve in the photocatalytic process.

2.3 Stability of the catalyst

The stability of HPW-Y-TiO2 (20%) catalyst was evalu-ated under optimum conditions. The degradation conversion of MO keeps at ca. 96% after the catalyst was reused eight times. The composite is easy to be separated from solution after reaction by natural sedimentation, which is of great importance for potential practical applications.

3 Conclusions

Novel solid Keggin-type POMs, HPW-Y-TiO2 with dif-ferent HPW loading levels were prepared by impregnation method. Both Keggin structure and anatase existed in the prepared materials. The optimum conditions for photocata-lytic degradation of MO (50 ml, 10 mg/L) were: 20% loading, 0.03 g dose and pH 1.0. The disappearance of MO followed Langmuir-Hinshelwood apparent first-order kinetics. The photocatalytic activity of POMs was markedly enhanced, owing to the synergistic effect between POMs and anatase Y-TiO2 as well as the increased BET surface area. The photocatalyst kept high activity after eight times’ recycle.

References:

[1] Feng C G, Zhuo X X, Liu X. Study on photodegradation of azo dye by polyoxometalates/polyvinyl alcohol. Journal of Rare Earths, 2009, 27(5): 717.

[2] Bubacz K, Choina J, Dolat D, Morawski A W. Methylene blue

WANG Yajun et al., Photocatalytic degradation of methyl orange by polyoxometalates supported on yttrium-doped TiO2 871

and phenol photocatalytic degradation on nanoparticles of anatase TiO2. Polish Journal of Environmental Studies, 2010, 19(4): 685.

[3] Yamase T. Photo- and electrochromism of polyoxometalates and related materials. Chemical Reviews (Washington, D C), 1998, 98(1): 307.

[4] Yamase T. Photoredox chemistry of polyoxometalates as a photocatalyst. Catalysis Surveys from Asia, 2003, 7(4): 203.

[5] Guo Y H, Wang Y H, Hu C W, Wang Y H, Wang E B, Zhou Y C, Feng S H. Microporous polyoxometalates POMs/SiO2: synthesis and photocatalytic degradation of aqueous organo-cholorine pesticides. Chemistry of Materials, 2000, 12(11): 3501.

[6] Yang Y, Guo Y H, Hu C W, Jiang C J, Wang E B. Synergistic effect of Keggin-type [Xn+W11O39](12–n)– and TiO2 in macro-porous hybrid materials [Xn+W11O39](12–n) –-TiO2 for the photo-catalytic degradation of textile dyes. Journal of Materials Chemistry, 2003, 13(7): 1686.

[7] Yang Y, Wu Q Y, Guo Y H, Hu C W, Wang E B. Efficient degradation of dye pollutants on nanoporous polyoxotung-state-anatase composite under visible-light irradiation. Journal of Molecular Catalysis A: Chemical, 2005, 225(2): 203.

[8] Jin H X, Wu Q Y, Pang W Q. Photocatalytic degradation of textile dye X-3B using polyoxometalate-TiO2 hybrid materials. Journal of Hazardous Materials, 2007, 141(1): 123.

[9] Li K X, Guo Y N, Ma F Y, Li H C, Chen L, Guo Y H. Design of ordered mesoporous H3PW12O40-titania materials and their photocatalytic activity to dye methyl orange degradation. Ca-talysis Communications, 2010, 11(9): 839.

[10] Lee J, Dong X L, Dong X W. Ultrasonic synthesis and photo-

catalytic characterization of H3PW12O40/TiO2 (anatase). Ul-trasonics Sonochemistry, 2010, 17(4): 649.

[11] Ou X Q, Meng J P, Wang Q M, Yu J M. Enhanced photoactiv-ity of layered nanocomposite materials containing rare earths, titanium dioxide and clay. Journal of Rare Earths, 2006, 24(1): 251.

[12] Fan C M, Xue P, Sun Y P. Preparation of nano-TiO2 doped with cerium and its photocatalytic activity. Journal of Rare Earths, 2006, 24(3): 309.

[13] Li F B, Gu G B, Huang G F, Gu Y L, Wan H F. TiO2-assisted photo-catalysis degradation process of dye chemicals. Journal of Environmental Sciences, 2001, 13(1): 64.

[14] Zhu H Y, Orthman J A, Li J Y, Zhao J C, Churchman G J, Vansant E F. Novel composites of TiO2 (anatase) and silicate nanoparticles. Chemistry of Materials, 2002, 14(12): 5037.

[15] Shi Z L, Liu F M, Yao S H. Preparation and photocatalytic ac-tivity of B, Y co-doped nanosized TiO2 catalyst. Journal of Rare Earths, 2010, 28(5): 737.

[16] Dias J A, Osegovic J P, Drago R S. The solid acidity of 12- tungstophosphoric acid. Journal of Catalysis, 1999, 183(1): 83.

[17] Mizuno N, Misono M. Heterogeneous catalysis. Chemical Re-views, 1998, 98(1): 199.

[18] Caliman E, Dias J A, Dias S C L, Garcia F A C, de Macedo J L, Almeida L S. Preparation and characterization of H3PW12O40

supported on niobia. Microporous and Mesoporous Materials, 2010, 132(1-2): 103.

[19] Cai H S, Liu G G, Lv W Y, Li X X, Yu L, Li D G. Effect of Ho-doping on photocatalytic activity of nanosized TiO2 cata-lyst. Journal of Rare Earths, 2008, 26(1): 71.