La-TiO2

JOURNAL OF RARE EARTHS, Vol. 31, No. 1, Jan. 2013, P. 44

Photocatalytic degradation of imidacloprid by composite catalysts H3PW12O40/La-TiO2 FENG Changgen (冯长根)1,*, XU Gang (徐 刚)1, LIU Xia (刘 霞)2 (1. State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China; 2. College of Science, China Agricultural University, Beijing 100193, China) Received 24 May 2012; revised 6 November 2012

Abstract: A series of La-doped TiO2 with different mass fractions were prepared by sol-gel method. Composite catalysts H3PW12O40/La-TiO2 with different loading levels were synthesized using impregnation method. The prepared samples were characterized by fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (DRS) and nitrogen adsorption-desorption analysis. The Keggin structure of H3PW12O40 (HPW) remained intact on the surface of the composites, they had relatively uniform spherical grains of diameter less than 20 nm. The visible light activity of prepared composites were improved by loading HPW and doping La. The prepared composites were used as photocatalysts in degradation of pesticide imidacloprid. Results revealed that 20%H3PW12O40/0.3%La-TiO2 possessed the best photocatalytic activity. Thus, the degradation conversion of imidacloprid reached 98.17% after 60 min irradiation when 20%H3PW12O40/0.3% La-TiO2 was used as catalysts. The degradation of imidacloprid corresponded with first-order kinetic reaction, and the half life of the degradation of imidacloprid was 9.35 min in the optimal conditions. Keywords: photocatalytic; composite catalysts; rare earths; imidacloprid

Imidacloprid was widely used to protect crops from damaging by pests. Massive imidacloprid was discharged into the environment as pollutant, which caused a serious threat to the environment and human health[1–4]. Therefore, such pollutant should be disposed before discharged into the environment. Many methods have been used to eliminate contaminants, such as physical adsorption, chemical oxidation, microbial degradation and photocatalytic degradation[5]. In the latest years, photocatalytic degradation has been studied widely because it is green, non-pollution and effective. TiO2 had been most widely used in the study of photocatalytic degradation because it is inexpensive, acid-proof alkaline, non-toxic, and photostable[6]. In recent years, many elements have been doped on TiO2 to improve the photochemical ability, which may reduce the bandgap of TiO2, extend absorption wavelength range to the visible region, or enhance the utilization of the sun light[7–10]. Polyoxometalates (POMs) such as H3PW12O40 (HPW) or H4SiW12O40 were proved to be another kind of efficient photocatalysts in degradation of dyes, pesticides and other organic pollutants due to the similar d0 electron structure to TiO2[11–13]. Furthermore, synergistic effects between TiO2 and POMs were observed which significantly improved the photocatalytic performances of the catalysts while polyoxometalates were loaded on TiO2[14–18]. In previous studies of our group, Feng et al.[19] synthe-

sized series of photocatalysts K11[Ln(PW11O39)2]/PVA (Ln=La, Ce, Pr, Nd, Sm), and used them to degrade methyl orange, Congo red, ponceau 2R, found the maximal degradation conversions of the three kinds of dyes were 99.58%, 47.61%, 72.42%, respectively. Then, composite materials HPA/MCM-41 with concentration of 10 mg/L, 20 mg of HPW/ MCM-41 with 50 wt.% loading level were added, and 58.0% imidacloprid was degraded after 5 h irradiation[20]. In this paper, HPW/La-TiO2 photocatalysts were prepared for decontaminating imidacloprid wastewater as well as to improve the photocatalytic ability availably. Finally, photocatalytic activities of the prepared composites were tested by degrading aqueous imidacloprid.

1 Experimental 1.1 Materials H3PW12O40, Ti(OC4H9)4, La(NO3)3, C2H5OH, CH3COOH, HCl and CH3COCH2COCH3 are analytical regents. CH3OH is of chromatographic grade. All chemicals were purchased from Beijing Chemical Reagent Co., China. Imidacloprid (99.99%) was provided by Institute of Plant Protection, Chinese Academy of Agricultural Sciences. 1.2 Preparation of composite catalysts La-TiO2 was obtained by sol-gel method. 17.3 ml

* Corresponding author: FENG Changgen (E-mail: [email protected]; Tel.: +86-10-68912764) DOI: 10.1016/S1002-0721(12)60232-4

FENG Changgen et al., Photocatalytic degradation of imidacloprid by composite catalysts H3PW12O40/La-TiO2

EtOH was mixed with 15 ml Ti(OC4H9)4, the mixture solution was stirred vigorously. The pH of the liquor was adjusted to 2–3 by adding 2.5 ml CH3COOH and appropriate amount of HCl with stirring, the solution was marked A. A mixture of 2.4 ml deionized water, 8.7 ml EtOH and a certain amount of La(NO3)3 were stirred for 30 min and was marked as B. B was added dropwise into A, followed by adding 2 drops of acetyl acetone. After 6 h, a yellowish sol was obtained, and then it was gelatinized for 1 h. The gel was aged for 24 h at room temperature, and dried at 333 K, calcined at 823 k for 3 h. The doping levels of La in the support were 0.3%, 0.6%, 0.9% and 2.0%, respectively. Pure TiO2 was also synthesized using the same procedure except that no La(NO3)3 was added in solution B. HPW/La-TiO2 were prepared via impregnation process. HPW and La-TiO2 were mixed with different quality scores, the suspension was stirred for 24 h at room temperature and evaporated at 373 K by water-bath heating. Finally, the farinose product was dried at 373 K. In this study, composite catalysts with different loading levels of HPW were prepared, the mass fraction of HPW were 10%, 20%, 30% and 40% (by theoretical calculation) respectively. 1.3

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2 Results and discussion 2.1 XRD analysis X-ray diffraction patterns of the powdered photocatalysts are presented in Fig. 1. It is observed that TiO2 exhibits broad peaks located at 2θ of 25.28º, 37.79º, 48.03º, 53.90º, 62.70º and 75.10º, which are typical of anatase TiO2[8]. The results indicate that composites with different HPW loadings show pure anatase phase structure consistently. However, no characteristic peak of HPW is found, which suggests that HPW tended to be highly dispersed in the mesopores of TiO2. 2.2 FT-IR analysis So as to confirm the retention of the structural integrity of HPW in the composite catalysts, the structure of prepared materials were characterized by FT-IR spectra (Fig. 2). From Fig. 2, TiO2 shows an intense, broad, indistinct region extending from 1100 to 400 cm–1. The typical Keggin anion skeletal vibration bands are at 1080, 982, 889, 800, 596 and 525 cm–1. Some vibration bands corresponding to the HPW structures are overlapped in

Characterization of H3PW12O40/La-TiO2 catalysts

The crystalline phase of the prepared samples were analyzed by XRD on a PANalytical X' Pert PRO MPD diffractometer with Cu Kα radiation which was operated at 40 kV and 40 mA. To investigate the Keggion structure of HPW, FT-IR spectra were monitored by a Tensor 27 Fourier transform infrared spectrometer, USA. The specific surface properties of the powders were measured by dynamic Brunner-Emmet-Teller (BET) method using a Nova2200e high automatic specific surface and porosity analyzer, USA. Surface morphologies and microstructures were observed by scanning electron microscopy (SEM) (Hi-tachi S-4800N) operating at 10.0 kV. UV-vis DRS were recorded by a Shimadzu UV-3600 UV-VIS spectrophotometer.

Fig. 1 XRD patterns of prepared catalysts (1) Pure TiO2; (2) 20% HPW/TiO2; (3) 20% HPW/0.3% LaTiO2; (4) 40% HPW/0.3% La-TiO2

1.4 Photocatalytic degradation In order to test the performance of those composites, they were used to degrade imidacloprid. The light source of the photodegradation instrument is PLS-SXE300UV Xe lamp (300 W), the wavelength is ≥365 nm. A typical experiment of the photocatalysis degradation was as follows. A suspension of 50 ml imidacloprid (10 mg/L) with a certain amount of catalysts were stirred in dark for 30 min to reach the adsorption-desorption balance. Then, the degradation reaction started and the concentration of imidacloprid was monitored by high performance liquid chromatography.

Fig. 2 FT-IR spectra of prepared catalysts (1) Pure TiO2; (2) 20% HPW/TiO2; (3) 20% HPW/0.3% LaTiO2; (4) 40% HPW/0.3% La-TiO2

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composite materials, but some of them can be seen clearly, suggesting the remaining of the Keggin structure.

2.5 UV-vis DRS analysis

2.3 Nitrogen adsorption analysis Surface properties of the catalysts were calculated by micromeritics instrument through N2 adsorption using BET method. Table 1 shows the BET surface area, BJH pore volume and average pore diameter of these samples. There no significant difference of BET surface areas exists between pure TiO2 and 20% HPW/TiO2. However, after doping of La3+, the BET surface area of 20% HPW/0.3% La-TiO2 reach 62.12 m2/g, which is nearly 2 times as that of pure TiO2. It shows that the doping of La3+ may increase the catalytic activity of the catalyst effectively, thereby improve the photocatalytic degradation rate. 2.4 SEM analysis Fig. 3 shows the SEM morphologies and microstructures of pure TiO2 and 20% HPW/0.3% La-TiO2 samples. The SEM images of the catalysts reveal that they have relatively uniform spherical grains of diameter less than 20 nm, and good dispersion, which is in accordance with the XRD results. Table 1 Surface and porosity of catalysts SBET/

Pore volume/

Average pore

(m2/g)

(cm3/g)

diameter/nm

TiO2

37.71

0.111

9.672

20% HPW/TiO2

35.62

0.062

9.687

20% HPW/0.3% La-TiO2

62.12

0.133

9.720

Sample

Fig. 3 SEM micrographs of samples (a) TiO2; (b) 20% HPW/0.3% La-TiO2

UV-vis DRS was used to characterize the light absorption ability of the photocatalysts. The UV-vis diffuser reflectance spectra of pure TiO2, 20% HPW/TiO2 and 20% HPW/0.3% La-TiO2 are shown in Fig. 4. From Fig. 4, all samples have strong absorption in the range of 200–380 nm, corresponding to charge transfer from O2p to Ti3d. After formation of composites, the CT band shifted to a higher wavelength compared with pure TiO2, the absorption in the visible region was enhanced. This result suggests that both of Keggin unit and La3+ have influence on the electronic properties of TiO2 in the prepared composites. 2.6 Photocatalytic activity testing The photocatalytic activities of the prepared composite photocatalysts were tested by investigating the photodegradation effect of imidacloprid wastewater under UV light with the wavelength≥365 nm. Imidacloprid solution was prepared before degradation. Dark (adsorption) experiments were carried out for 30 min with stirring until adsorption-desorption balance. At the beginning of the experiments, 20 mg catalyst was added to the imidacloprid solution, and stirred 1 h in the dark, and no imidacloprid was degraded. The result showed that imidacloprid was not degraded without UV light. Then, imidacloprid solution of 50 ml was taken in an open glass reactor without catalysts under UV light and irradiated, after 1 h no degradation of imidacloprid was observed, too. Thus, we studied the influence factors of degradation. 2.6.1 The influence of different HPW loadings The different loadings of HPW over 0.3% La-TiO2 support affect the photocatalytic activity for degradation of imidacloprid. Photocatalytic degradation of imidacloprid was monitored with HPW 10 wt.%, 20 wt.%, 30 wt.% and 40 wt.% loadings over 0.3% La-TiO2. For solar experiments, imidacloprid solution of 50 ml was taken in an open glass reactor with 20 mg catalyst. Their degradation results are shown in Fig. 5. It indicates that 20% HPW/0.3% La-TiO2 is better than others, after 60 min 90.89% imidacloprid is degraded.

Fig. 4 UV-vis DRS spectra of catalysts

FENG Changgen et al., Photocatalytic degradation of imidacloprid by composite catalysts H3PW12O40/La-TiO2

Fig. 5 Photocatalytic degradation of different H3PW12O40 loadings (1) 10% HPW/0.3% La-TiO2; (2) 20% HPW/0.3% La-TiO2; (3) 30% HPW/0.3% La-TiO2; (4) 40% HPW/0.3% La-TiO2

2.6.2 The influence of different catalyst doses Different catalyst doses affect the photocatalytic activity to degrade imidacloprid. Photocatalytic degradation of imidacloprid was monitored with 10, 20, 30 and 40 mg of 20% HPW/0.3% La-TiO2 in 50 ml of imidacloprid containing solution respectively. As seen in Fig. 6, the photocatalytic activity increases with catalyst increasing doses from 10 to 40 mg. When the amount of catalyst is 30 and 40 mg, after 60 min, degradation rate are 98.17% and 98.78%, respectively, no significant difference is observed between the two amounts, so 30 mg is chosen as catalyst dosage. 2.6.3 The influence of doping amount of La3+ La-TiO2 with different La contents were prepared. 20% HPW was loaded on La-TiO2. Composite 20% HPW/B%La-TiO2 were synthesized. Then, they were used to degrade imidacloprid. Catalysts used in the evaluation were 30 mg. From the results, we can see that 20% HPW/0.3% La-TiO2 has the best degradation effect. After 60 min, imidacloprid was degraded 98.17%. Specific results are shown in Fig. 7. 2.6.4 The influence of initial pH In this step, HClO4 were used to adjust imidacloprid wastewater’s initial pH, HClO4 was also investigated. No degradation of imidacloprid was found in the presence of

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Fig. 7 Photocatalytic degradation of different La-doped. (1) 20% HPW/0.3% La-TiO2; (2) 20% HPW/0.6% La-TiO2; (3) 20% HPW/0.9% La-TiO2; (4) 20% HPW/2.0% La-TiO2

HClO4 under UV light without catalyst, suggesting that HClO4 has no photocatalytic activity to imidacloprid. The results are shown in Fig. 8. As is shown in the figure the initial pH of solution has no effect on the decomposition rate. 2.7 Dynamic fitting In order to do dynamic simulation to the process of degrading imidacloprid, lnC0/Ct–T linear fitting was carried out. The results show that the linear correlations are very anastomotic (specific results as shown in Fig. 9). All reactions follow the first-order kinetics, and the data analysis is listed in Table 2. From the different rate constants, we can see that TiO2 and HPW together have synergistic effect, La3+ doped on samples can increase the catalytic activity of the catalyst, as well as improve the photocatalytic degradation rate effectively.

Fig. 8 Photocatalytic degradation of different initial pH Table 2 Kinetic parameters for the process of degradation imidacloprid Catalysts

Fig. 6 Photocatalytic degradation of different catalyst doses

Equation

k/min–1 t1/2/min

R2

TiO2

lnC0/Ct=0.01628t–0.01718 0.01628 42.58 0.9989

20% HPW/TiO2

lnC0/Ct=0.03977t–0.38816 0.03977 17.43 0.9585

20% HPW/0.3% La-TiO2 lnC0/Ct=0.07418t–0.64929 0.07418

9.35

0.9821

48

Fig. 9 Photocatalytic decomposition first order kinetics profiles of imidacloprid on different catalysts (1) Pure TiO2; (2) 20% HPW/TiO2; (3) 20% HPW/0.3% La-TiO2

3 Conclusions In this study, we demonstrated the possibility of photodecomposition of imidacloprid using photocatalysts HPW/La-TiO2. The optimum conditions for degradation of 10 mg/L imidacloprid was 0.6 g/L TiO2 composites containing 20% HPW loading and 0.3% La-doped under initial pH 5.88 unadjusted. In this condition, imidacloprid was almost completely degraded after 60 min. The degradation of imidacloprid corresponded with first-order kinetic reaction, and the half life of the degradation of imidacloprid was 9.35 min in the optimal conditions. Experimental results showed that the La doped on samples could enhance their specific surface area compared with pure TiO2 and promote degradation rate effectively. From different rate constants between TiO2 and 20%H3PW12O40/TiO2, conclusion could be drawn that HPW and TiO2 had synergistic effect. Acknowledgments: This work was supported by Institution of Chemical Materials, China Academy of Engineering Physics. We are grateful to them for providing research funding. Also, we are grateful to Institute of Plant Protection Chinese Academy of Agricultural Sciences for the supply of the pesticides.

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