Hydrothermal Synthesis of H 3 PW 12 O 40 /TiO 2 Nanometer Photocatalyst and Its Catalytic Performance for Methyl Orange

Transcription

1 CHEM. RES. CHINESE UNIVERSITIES 2012, 28(3), Hydrothermal Synthesis of H 3 PW 12 O 40 /TiO 2 Nanometer Photocatalyst and Its Catalytic Performance for Methyl Orange FENG Chang-gen * and SHANG Hai-ru State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing , P. R. China Abstract H 3 PW 12 O 40 /TiO 2 nanometer photocatalyst was prepared by one step hydrothermal synthesis from H 3 PW 12 O 40 nh 2 O and Ti(OBu) 4, simultaneously realizing the load and modification of H 3 PW 12 O 40. The catalyst was characterized by Fourier transform infrared spectroscopy(ftir), powder X-ray diffraction(xrd), nitrogen adsorption-desorption analysis and scanning electron microscopy(sem). The results show that the catalyst is Keggin structure and crystallized in anatase structure, the diameter and specific area of the prepared catalyst are 3.8 nm and m 2 /g, respectively, and its dispersity is better. The photocatalytic properties were compared for TiO 2, H 3 PW 12 O 40 /TiO 2 prepared by impregnation and H 3 PW 12 O 40 /TiO 2 prepared by hydrothermal method with methyl orange as a probe. The effects of H 3 PW 12 O 40 loadings, crystallization method, initial ph and concentration of dye solution on the degradation of methyl orange were investigated. Keywords H 3 PW 12 O 40 /TiO 2 ; Hydrothermal synthesis; Photocatalytic performance; Degradation; Methyl orange Article ID (2012) Introduction A new area for ployacid chemistry and environmental chemistry has been developed via the researches on heteropolyacid(hpa) as a catalyst under the irradiation of light of a certain wavelength for organic wastewater treatment. Heteropolyacid is playing a major role in organic degradation and fine chemical synthesis as an excellent green catalyst [1]. Especially phosphotungstic acid(h 3 PW 12 O 40, HPW) with Keggin structure has been used as an important acid catalyst and has wide application in clean catalytic reactions because of its special structure, high catalysis activity, good stability and physicochemical properties [2 5]. However, the freely soluble property of HPA makes it difficult to recycle, and HPA with a low specific surface area of 1 10 m 2 /g [6] severely limits its catalytic activity, which makes it necessary to seek for the proper carrier and technics to overcome these drawbacks. As one of the most extensively investigated photocatalysts, TiO 2 shows relatively high efficiency, low cost, non-toxicity, and high stability [7 11], which could be used as an excellent carrier. To avoid catalytic active components losing during the reaction, some researchers [12 14] loaded H 3 PW 12 O 40 on the surface of TiO 2 or molecular sieve by impregnation method, improving the utility of catalyst. Nevertheless, the composite catalyst prepared by impregnation method is unstable, the load is unbalanced, solid acid can easily take apart, and the above mentioned modification is carried out in a separate step after the synthesis of TiO 2 or molecular sieve, which extends the preparation cycle. In this paper, a series of nanosize photocatalyst H 3 PW 12 O 40 /TiO 2 (H 3 PW 12 O 40 loadings of 10%, 20%, 30% and 40%, mass fraction) was synthesised in one step by hydrothermal method. H 3 PW 12 O 40 with Keggin structure acted as a template [15] where hydrogen bonds between water molecule and H 3 PW 12 O 40 molecule were formed, simultaneously realizing the load and modification of H 3 PW 12 O 40 as well. The removal ability for methyl orange(mo) was investigated in view of application to water treatment. Here MO was selected as a typical azo dye. This new technology could efficiently avoid the decomposition of HPA due to the high-fired(500 C) process to obtain the anatase TiO 2. Moreover, it overcomes the shortcomings of sol-gel technology which has a long preparation cycle. The catalyst with higher catalysis activity, better stability, milder reaction condition and simpler synthetic method has great prospect for industrial application. 2 Experimental 2.1 Reagents The reagents used for preparing H 3 PW 12 O 40 /TiO 2 included phosphotungstic acid hydrate(h 3 PW 12 O 40, Sinopharm Chemical Reagent Co., Ltd., China) and tetrabutyl titanate[ti(obu) 4, 98.0%, Tianjin Guangfu Fine Chemical Research Institute, China] as a titanium dioxide precursor with pure ethanol(etoh, 99.7%, Beijing Chemical Works, China) as solvent; hydrochloric acid(hcl, Beijing Chemical Works, China) was used to adjust ph and distilled water was used for hydrolysis. *Corresponding author. cgfeng@wuma.com.cn Received December 20, 2011; accepted February 20, Supported by the Fund of Institution of Chemical Materials, China Academy of Engineering Physics.

2 No.3 FENG Chang-gen et al Catalyst Preparation The hydrothermal method was introduced as a synthetic method in this study [16 19]. A mixture of Ti(OBu) 4 (15 ml) and EtOH(22.5 ml) was stirred at room temperature, then 1.8 ml of HCl was added to the mixture to achieve ph=1. A certain amount of H 3 PW 12 O 40 was dissolved in 0.75 ml of distilled water, which was then dropped into the mixed solution. The loading levels of H 3 PW 12 O 40 in four prepared composites by mass were 10%, 20%, 30% and 40%(theoretical calculation), respectively. After stirring for 2 h, another 22.5 ml of EtOH was added to the solution to reduce the acidity, which was favorable for the hydrolysis of organism. Finally, 2.25 ml of distilled water was added to the solution for hydrolysis. The molar ratio of Ti(OBu) 4 to distilled water was 1:4 to ensure the complete hydrolysis of Ti(OBu) 4. The resulting acidic mixture was stirred constantly for about 5 h until a gel was obtained. In the crystallization stage, the obtained gel of 40 ml in an hydrothermal reactor was heated to 120 C at a rate of 2 C/ min and kept for 2 h, then warmed up to 150 C at the same rate of 2 C/min and kept for 2 h. 2.3 Catalytic Characterization The infrared(ir) spectra of the H 3 PW 12 O 40 /TiO 2 were recorded in a wavenumber range of cm 1 on a Thermo Nicolet 6700 Fourier transform infrared spectrometer. Crystalline phase of the prepared catalyst was identified by powder X-ray diffraction analysis(xrd) with a Netherlands PANalytical X Pert Pro-MPD diffractometer. Nitrogen absorption-desorption isotherms were obtained on a autosorb IQ instrument via multipoint Brunauer-Emmett-Teller(BET) and Barrett-Joyner-Halenda(BJH) methods used for specific surface area, pore size and pore volume analysis. The morphology of the sample was observed by a scanning electron microscope (SEM, model Hitachi S-4800N) at an acceleration voltage of 5 kv. 2.4 Photocatalytic Degradation KBr pellets over a wavenumber range from 4000 cm 1 to 400 cm 1, at a resolution of 4 cm 1 with 32 scans. The characteristic absorption peaks of Keggin unit at 1080, 982, 888 and 797 cm 1 are attributed to v as (P O a ), v as (W=O d ), v as (W O b W) and v as (W O c W), respectively [21]. Fig.1 shows FTIR spectra of pure TiO 2 and H 3 PW 12 O 40 /TiO 2 samples with different loadings. It is obvious that compared with pure TiO 2, H 3 PW 12 O 40 /TiO 2 samples display four discernible peaks between 1100 and 800 cm 1, agreeing with Keggin unit well, which indicates that the Keggin structure of H 3 PW 12 O 40 has not been destroyed. Fig.1 FTIR spectra of TiO 2 (a) and H 3 PW 12 O 40 /TiO 2 (b e) Loading level(%, mass fraction): b. 10; c. 20; d. 30; e XRD Analysis The phase structure, crystallite size and crystallinity of H 3 PW 12 O 40 /TiO 2 are of great importance for its photocatalytic activity. XRD was used to investigate the changes of phase structure and crystallite size of the photocatalyst. In this paper, the prepared H 3 PW 12 O 40 /TiO 2 was identified by XRD with Cu Kα radiation(40 kv, 40 ma) at an angle of 2θ from 5 to 80. The scan speed was 10 /min and time constant was 1 s. A diffraction angle of 25.0 was selected to discuss the crystallinity of the sample. The XRD patterns of H 3 PW 12 O 40 /TiO 2 samples are shown in Fig.2. The photocatalysis experiments were carried out on an open photoreactor. The light source was provided by a PLS-SXE 300 UV Xe lamp(300 W) with an emission of λ 365 nm. A given amount of prepared catalyst was suspended in a 50 ml of fresh MO solution with MO concentration augmented from 5 mg/l to 30 mg/l. After adsorption-desorption for 30 min under dark conditions, the suspension was put into the photoreactor and then was stirred during the whole process. The absorbance of the residual MO solution was analyzed on an APL 752 UV-Vis spectrometer at 510 nm when ph 3.0 and at 464 nm when ph 4.0 [20]. 3 Results and Discussion 3.1 Properties of Catalysts FTIR Analysis The FTIR spectra of the compounds were obtained with Fig.2 XRD patterns of H 3 PW 12 O 40 /TiO 2 at different H 3 PW 12 O 40 loading levels(mass fraction) a. 10%; b. 20%; c. 30%; d. 40%. Fig.2 curves a d shows the effects of different loading levels of H 3 PW 12 O 40. All the curves with characteristic diffraction peaks of 2θ values locate at 25.4, 30.7, 37.7, 47.9, 54.4, 62.3, 69.5 and The presence of a sharp peak at 25.4 which is the major peak for the anatase structure indicates the anatase structure of the H 3 PW 12 O 40 /TiO 2. Compared with pure TiO 2 with anatase structure, the H 3 PW 12 O 40 /TiO 2 shows a new peak at 2θ=30.7, indicating that the main diffraction peak of H 3 PW 12 O 40 is at 2θ=30.7. With

3 368 CHEM. RES. CHINESE UNIVERSITIES Vol.28 the enhance of H 3 PW 12 O 40 loading(10% 40%), the peak intensity increases Nitrogen Adsorption-desorption Analysis The specific surface area is an important representation of a catalyst. In this paper, the specific surface area, pore diameter and pore volume of the catalyst were characterized by BET and BJH methods. The degassing temperature was 150 C within 2 h retained. The results are shown in Table 1. Table 1 Pore structure of prepared catalyst Material S BET /(m 2 g 1 ) TiO 2 [12] Average pore diameter/nm Pore volume/ (cm 3 g 1 ) H 3 PW 12 O 40 /TiO According to the results, the specific surface area of H 3 PW 12 O 40 /TiO 2 prepared by hydrothermal method increased to m 2 /g, which is 5 times more than that of pure TiO 2 with anatase structure, and 20 times more than that of H 3 PW 12 O 40 (8.9 m 2 /g [1] ). Therefore, the hydrothermal method could greatly increase the specific surface area of catalyst. The H 3 PW 12 O 40 /TiO 2 used in this method is a typical mesoporous material based on the pore size SEM Observation The size, shape and distribution for the attained H 3 PW 12 O 40 /TiO 2 were observed by SEM at an acceleration voltage of 5 kv. The SEM images of H 3 PW 12 O 40 /TiO 2 samples are shown in Fig.3. Fig.3 SEM images of catalyst samples (A) TiO 2 obtained by sol-gel technology; (B) TiO 2 obtained by hydrothermal method. H 3 PW 12 O 40 /TiO 2 with different H 3 PW 2 O 40 loading(%, mass fraction): (C) 10; (D) 20; (E) 30; (F) 40. As shown in Fig.3, the diameter of H 3 PW 12 O 40 /TiO 2 is less than 20 nm, which is obviously smaller than that of TiO 2. Moreover, it could be identified that the particles obtained are regular spherical and their dispersity is better. 3.2 Catalytic Performance for Methyl Orange Decomposition The degradation of MO is fitted by ln(c 0 /c t )=kt. where c 0, c t, k and t represent the initial concentration, concentration at time t, rate constant and reaction time, respectively. The experimental data agree well with the fitting formula, showing that the reaction follows Langmuir-Hinshelwood apparent first-order kinetics Activities of H 3 PW 12 O 40 /TiO 2 with Different Loadings for MO Degradation In order to compare the effects of different H 3 PW 12 O 40 loadings, the loading levels of H 3 PW 12 O 40 in four prepared composites were of 10%, 20%, 30% and 40%(mass fractions), respectively. In Fig.4, the effects of different H 3 PW 12 O 40 loadings are exhibited. And the results of degradation kinetics parameters are given in Table 2. From Fig.4, it is obvious that the rate constant k for a H 3 PW 12 O 40 loading of 20% is 1.4 times that for a H 3 PW 12 O 40 Fig.4 MO decomposition by H 3 PW 12 O 40 with different loadings Reaction conditions: ph=2; catalyst content: 0.6 g/l; concentration of the wastewater: 10 mg/l. Loading level(%, mass fraction): a. 10 b. 20; c. 30; d. 40. Table 2 First-order kinetics parameters for MO degradation by H 3 PW 12 O 40 /TiO 2 with different loadings H 3 PW 12 O 40 loading(%) k t 1/2 /min R loading of 30%, nearly 3 times that for a H 3 PW 12 O 40 loading of 10% and a H 3 PW 12 O 40 loading of 40%. As H 3 PW 12 O 40 loading is higher than 20%, the value of k is declined. This could be

4 No.3 FENG Chang-gen et al. 369 explained by the fact that the optimum loading level is 20%, which is also the saturation value of loading level. As loading levels are higher than 20%, excess H 3 PW 12 O 40 lead to the specific surface of the catalyst is decreasing and less reactant is adsorbed, resulting in the decline of degradation activity. The results indicate that a H 3 PW 12 O 40 loading of 20% is the best quantity for this catalyst Effects of Different ph Values on MO Degradation The results of the experiment show that the ph has great influence on the MO degradation, so the ph values within a range of in 0.5 increments were investigated. The effects of different ph values on MO degradation are shown in Fig.5. HClO 4 was used in this experiment to obtain different ph values between 1 and 2.5. Fig.5 Effects of ph values on decomposition of MO a. ph=1; b. ph=1.5; c. ph=2; d. ph=2.5. Reaction conditions: H 3 PW 12 O 40 loading: 20%; catalyst content: 0.6 g/l; concentration of the wastewater: 10 mg/l. As shown in Fig.5 and Table 3, the rate constant k and half-life t 1/2 were investigated. MO decomposition is the highest at ph=2. From this result, it could be suggested that ph=2 is more beneficial to the MO converting into quinoid structure which is easily to destroy. It can also be confirmed that the MO decomposition may be sensitive to ph change, indicating the ph has significant impact on the photocatalytic degradation rate. Table 3 First-order kinetics parameters for MO degradation at different ph values ph k t 1/2 /min R Effects of Different Catalyst Amounts on MO Degradation To investigate the photocatalytic performance of the MO decomposition in the presence of different amounts of catalyst, g/l catalyst was added in this experiment. The results are shown in Fig.6 and Table 4. As shown in Fig.6, with the increase of the catalyst amount, the MO decomposition remarkably increases. In particular, the MO rapidly decomposes at a catalyst content of 0.6 g/l. The value of k is about 3 times that of a catalyst content of 0.4 g/l and 6 times that of a catalyst content of 0.2 g/l. However, the degradation rate decreases when catalyst content increases from 0.6 g/l to 0.8 g/l. Fig.6 MO decomposition in the presence of different amounts of catalyst( g/l) a. 0.2 g/l; b. 0.4 g/l; c. 0.6 g/l; d. 0.8 g/l. Reaction conditions: H 3 PW 12 O 40 loading: 20%; ph=2; concentration of the wastewater: 10 mg/l. Table 4 First-order kinetics parameters for MO degradation by different amounts of catalyst Catalyst content/(g L 1 ) k t 1/2 /min R From the results of Fig.6, it is confirmed that with the increasing of the amount of catalyst, more effective photons could be produced, thus the degradation rate increases. However, excessive catalyst may scatter the light and decrease the penetration of the UV light. As a result, the activity for MO decomposition decreases Effects of Different Initial Concentrations on MO Degradation The degradation with different MO concentrations is shown in Fig.7 and Table 5. The initial MO concentration was augmented from 5 mg/l to 30 mg/l. Fig.7 shows the decomposition of MO at different initial Fig.7 MO decomposition at different initial concentrations of MO a. 5 mg/l; b. 10 mg/l; c. 20 mg/l; d. 30 mg/l. Reaction conditions: H 3 PW 12 O 40 loading: 20%; ph=2; catalyst content: 0.6 g/l. Table 5 First-order kinetics parameters for MO degradation at different initial concentrations of MO Concentration/(mg L 1 ) k t 1/2 /min R

5 370 CHEM. RES. CHINESE UNIVERSITIES Vol.28 concentrations. The degradation rates are , , and min 1, respectively when the initial MO concentrations are from 5 mg/l to 30 mg/l. This may be explained by the fact that the color of solution is more and more deeply with the increase of initial concentration, the spread of light is limited. Meanwhile, more MO would be absorbed on the catalyst surface at high initial concentrations. The production of effective photons is suppressed, and the degradation rate thus decreases with the increase of initial concentration. Owing to the degradation rate of 5 mg/l MO being too fast, we chose 10 mg/l as the initial concentration in the experiment Effects of Different Catalysts on MO Degradation The photocatalytic properties were compared for TiO 2, H 3 PW 12 O 40 /TiO 2 prepared by impregnation and H 3 PW 12 O 40 / TiO 2 prepared by hydrothermal method and the result is shown in Fig.8. Fig.8 Activities of different catalysts for MO decomposition a. Hydrothermal H 3 PW 12 O 40 /TiO 2 ; b. impregnation H 3 PW 12 O 40 /TiO 2 ; c. anatase TiO 2. Reaction conditions: H 3 PW 12 O 40 loading: 20%; ph=2; catalyst content: 0.6 g/l; concentration of the wastewater: 10 mg/l. As shown in Fig.8 and Table 6, it is obvious that the order of the photocatalytic performance is as follows: H 3 PW 12 O 40 / TiO 2 obtained from hydrothermal method>h 3 PW 12 O 40 /TiO 2 prepared by impregnation method>anatase structure TiO 2. The constant k of H 3 PW 12 O 40 /TiO 2 obtained from hydrothermal method is 1.5 times that of H 3 PW 12 O 40 /TiO 2 by impregnation method, and 1.9 times that of anatase structure TiO 2. The MO is almost degraded completely after 15 min by H 3 PW 12 O 40 /TiO 2 obtained from hydrothermal method. Table 6 First-order kinetics parameters for MO degradation by different catalysts Catalyst k t 1/2 /min R 2 Hydrothermal H 3 PW 12 O 40 /TiO Impregnation H 3 PW 12 O 40 /TiO Anatase structure TiO These results indicate that the proper loading amount of H 3 PW 12 O 40 could significantly increase the photocatalytic capability. Furthermore, it reconfirms that the hydrothermal method is a very reliable method for the loading of H 3 PW 12 O Conclusions H 3 PW 12 O 40 /TiO 2 was prepared by a modified hydrothermal method. To investigate the photocatalytic performance, H 3 PW 12 O 40 /TiO 2 was utilized to sterilize the MO in an aqueous solution. The results show that the hydrothermal preparation of H 3 PW 12 O 40 /TiO 2 is simple and rapid, which can be performed within 24 h and overcomes the weakness of sol-gel technology with a long preparation cycle. Besides, this method has mild reaction conditions, and the crystallization temperature is between 120 and 150 C, which avoids the decay of H 3 PW 12 O 40 effectively. The optimum conditions for photocatalytic degradation of MO are: 20% loading of catalyst, ph=2, initial concentration of MO is 10 mg/l and catalyst amount is 0.6 g/l. The MO is sterilized completely within only 18 min under the above conditions and the degradation of MO fits in the first-order kinetics. In summary, the hydrothermal method mentioned above is very suitable to the preparation of H 3 PW 12 O 40 /TiO 2 with high photocatalytic activity. References [1] Wang E. B., Hu C. W., Xu L., Introduction to Polyacid Chemistry, Chem. Ind. Press, Beijing, 1998, 170, 201 [2] Zhao X. Q., Han Y. T., Sun X. L., Wang Y. J., Chin. J. Catal., 2007, 28(1), 91 [3] Mishra B. G., Kumar D., Rao V. S., Catal. Commun., 2006, 7(7), 457 [4] Jermy B. R., Pandurangan A., Catal. Commun., 2006, 7(12), 921 [5] Li L., Guo Y. H., Zhou P., Yu X. D., Kang W. L., Chin. J. Catal., 2005, 26(3), 209 [6] Guo Y. H., Wang Y. H., Hu C. W., Wang Y. H., Wang E. B., Zhou Y. C., Feng S. H., Chem. Mater., 2000, 12(11), 3501 [7] Cao J., Xu B. Y., Luo B. D., Lin H. L., Chen S. F., Applied Surface Science, 2011, 257, 7083 [8] Zang Y., Farnood R., Appl. Catal. B, 2004, 57, 273 [9] Yu J. C., Ho W., Yu J., Yip H., Wong P. K., Zhao J., Technol., 2005, 39, 1175 [10] Wolfrum E. J., Huang J., Blake D. M., Maness P. C., Huang Z., Fiest J., Jacoby W. A., Environ. Sci. Technol., 2002, 36, 3412 [11] Zhang L., Huang T., Guo X. J., Liu X. Y., Chem. Res. Chinese Universities, 2010, 26(6), 1020 [12] Wang Y. J., Lu K. C., Feng C. G., Journal of Rare Earths, 2011, 29(4), 866 [13] Feng C. G., Li Y. Z., Liu X., Chin. J. Chem., 2011, 29, 2421 [14] Liu X., Li Y. Z., Feng C. G., Spectroscopy and Spectral Analysis, 2012, 32(4), 1020 [15] Luo G. S., Huang D., Wang Y. J., A Method Synthesis TiO 2 Used H 3 PW 12 O 40 as Template, CN A, 2007 [16] Kang M., Appl. Catal. B, 2002, 37, 187 [17] Lü K. L., Yu J. G., Cui L. Z., Chen S. L., Li M., Journal of Alloys and Compounds, 2011, 509, 4557 [18] Lee J. H., Kang M., Choung S. J., Ogino K. J., Miyata S., Kim M. S., Park J. Y., Kim J. B., Water Research, 2004, 38, 713 [19] Xu Y. T., Li L., Yu X. D., Guo Y. H., Chem. J. Chinese Universities, 2004, 25(8), 1549 [20] Li F. B., Gu G. B., Huang G. F., Gu Y. L., Wan H. F., Journal of Environmental Sciences, 2001, 13(1), 64 [21] Lee J., Dong X. L., Dong X. W., Ultrasonics Sonochemistry, 2010, 17(4), 649