简介概要

Photocatalytic degradation of organic dyes with H₃PW₁₂O₄₀/TiO₂-SiO₂

来源期刊：Rare Metals2016年第10期

论文作者：Shui-Jin Yang Yu-Lin Xu Wen-Peng Gong Yong-Kui Huang Guo-Hong Wang Yun Yang Chuan-Qi Feng

文章页码：797 - 803

摘要：H₃PW₁₂O₄₀/TiO₂-Si_O₂ was synthesized by impregnation method which significantly improved the catalytic activity under simulated natural light.The properties of the samples were characterized by Fourier transform infrared spectra（FTIR）,X-ray powder diffraction pattern（XRD）,Scanning electron micrographs（SEM）,and Zeta potential.Degradation of methyl violet was used as a probe reaction to explore the influencing factors on the photodegradation reaction.The results show that the optimal conditions are as follows:initial concentration of methyl violet of 10 mg·L^-1,pH of 3.0,catalyst dosage of2.9 g·L^-1,and light irradiation time of 2.5 h.Under these conditions,the degradation rate of methyl violet is 95.4%.The reaction on photodegradation for methyl violet can be expressed as the first-order kinetic model,and the possible mechanism for the photocatalysis under simulated natural light is suggested.After used continuously for five times,the catalyst keeps the inherent photocatalytic activity for degradation of dyes.The photodegradation of methyl orange,methyl red,naphthol green B,and methylene blue was also tested,and the degradation rate of dyes can reach81%-100%.

稀有金属(英文版) 2016,35(10),797-803

Photocatalytic degradation of organic dyes with H₃PW₁₂O₄₀/TiO₂-SiO₂

Shui-Jin Yang Yu-Lin Xu Wen-Peng Gong Yong-Kui Huang Guo-Hong Wang Yun Yang Chuan-Qi Feng

Hubei Collaborative Innovation Center for Rare Metal Chemistry,College of Chemistry and Chemical Engineering,Hubei Normal University

College of Chemistry and Chemical Engineering,Hubei University

收稿日期：22 March 2014

基金：financially supported by the Hubei Key Laboratory of Pollutant Analysis & Reuse Technology (No.KL2013M08);

Photocatalytic degradation of organic dyes with H₃PW₁₂O₄₀/TiO₂-SiO₂

Shui-Jin Yang Yu-Lin Xu Wen-Peng Gong Yong-Kui Huang Guo-Hong Wang Yun Yang Chuan-Qi Feng

Hubei Collaborative Innovation Center for Rare Metal Chemistry,College of Chemistry and Chemical Engineering,Hubei Normal University

College of Chemistry and Chemical Engineering,Hubei University

Abstract：

H₃PW₁₂O₄₀/TiO₂-Si_O₂ was synthesized by impregnation method which significantly improved the catalytic activity under simulated natural light.The properties of the samples were characterized by Fourier transform infrared spectra(FTIR),X-ray powder diffraction pattern(XRD),Scanning electron micrographs(SEM),and Zeta potential.Degradation of methyl violet was used as a probe reaction to explore the influencing factors on the photodegradation reaction.The results show that the optimal conditions are as follows:initial concentration of methyl violet of 10 mg·L^-1,pH of 3.0,catalyst dosage of2.9 g·L^-1,and light irradiation time of 2.5 h.Under these conditions,the degradation rate of methyl violet is 95.4%.The reaction on photodegradation for methyl violet can be expressed as the first-order kinetic model,and the possible mechanism for the photocatalysis under simulated natural light is suggested.After used continuously for five times,the catalyst keeps the inherent photocatalytic activity for degradation of dyes.The photodegradation of methyl orange,methyl red,naphthol green B,and methylene blue was also tested,and the degradation rate of dyes can reach81%-100%.

Keyword：

Heteropoly acid; TiO2; SiO2; Photocatalytic degradation; Organic dye;

Author： Shui-Jin Yang,e-mail:yangshuijin@163.com;

Received： 22 March 2014

1 Introduction

Since the photocatalytic degradation of organic compounds was introduced,the preparation of photocatalyst with high catalytic performance has attracted much attention because of its relation with solar energy conversion and environmental cleaning [ 1, 2, 3] .In particular,advanced oxidation processes (AOPs),which have the potential to completely mineralize organic compounds to CO₂ and H₂O,show a great potential as a low-cost and high-efficiency water treatment technology [ 4, 5] .These heterogeneous photocatalytic technologies are based on the semiconductor materials.The holes (h⁺) in the valence band and electrons(e^-) in the conduction band are immediately generated by the photoexcited materials [ 6] .The positive holes can react with organic compounds to oxidize them.Electrons will be trapped by oxygen molecules in the water and then react with water to form hydroxyl radicals.

Semiconductor materials,including Bi₂WO₆ [ 7] ,Ag₂WO₄ [ 8] ,TiO₂ [ 9] ,and ZnO [ 10] ,become a hot research topic.TiO₂ is one of the most important semiconductor materials due to its unique physicochemical properties.However,it has a broad band gap and absorbs only small fractions of sunlight energy.Owing to its poor thermal stability and low surface area,the catalyst still suffers from low catalytic efficiency.The modified TiO₂ [ 11, 12, 13, 14, 15, 16, 17] ,such as BiOI-sensitized TiO₂ [ 16] and Zr-doped TiO₂ [ 17] ,was investigated quite intensively in recent years to improve its visible light responsive.Anatase is a metastable titania polymorph,and the very fast recombination of electron-hole pairs on the TiO₂ surface will slow down its adsorption ability or catalytic activity.Thus,adding a second metal oxide (SiO₂,etc.) to titania was proven to be a potential strategy to overcome these drawbacks [ 18] .In particular,the composite of silica-supported TiO2 particles can not only take the advantage of both TiO2and SiO2,but also extend their applications through the generation of new catalytic activity sites due to a synergistic effect between the two components [ 19, 20, 21] .Therefore,the titania-silica composites become one of the most attractive photocatalyst for oxidation.

Recently,several researches [ 6, 22, 23, 24, 25] have demonstrated that the Keggin-type heteropolyacids (HPAs) are a good ground-state oxidant or electron acceptor resulting from their unique structure and can be utilized to facilitate the photocatalytic activity.They are constituted by d⁰transition metal and oxide ions and exhibit similar electronic characteristics with the semiconductor photocatalysts [ 26, 27] .However,several reports related to the photocatalysis of HPAs are concentrated on ultravioletvisible (UV) irradiation.Some attempts were made to increase the photocatalytic activity of these catalysts using a semiconductor as support.The improvements in their performance are ascribed to a synergistic effect between HPAs and support,which causes the efficient inhibition of the recombination of photogenerated electron-hole pairs [ 28, 29] .

In this paper,according to above consideration,TiO₂-SiO₂ was prepared by a sol-gel method and H₃PW₁₂O₄₀/TiO₂-SiO₂ was synthesized by an impregnation method.The photocatalytic degradation of some dyes with different chemical structures with the catalyst under simulated natural light irradiation was investigated.Those synthesized composite might have higher photocatalytic performances.

2 Experimental

2.1 Preparation of samples

TiO₂-SiO₂ was synthesized according to Ref. [ 22] by a solgel technique.Ti(OC₄H₉)4 and Si(OC₂H₅)₄ (TEOS) were used as precursors for TiO₂ and SiO₂,respectively.Ethanol and H₂O were used as mutual solvents,and hydrochloric acid was used as a catalyst for hydrolysis.n(TEOS):n(C₂H₅OH):n(H₂O):n(HCl)=1.00:5.00:2.00:0.03,and n[Ti(OC₄H₉)₄]:n(C₂H₅OH)=1.0:5.8.An acidic aqueous solution was prepared by the addition of HCl to H₂O.To the resultant solution,TEOS was added dropwise under vigorous stirring and was refluxed for 3.0 h at 80℃.Ti(OC₄H₉)₄ was mixed with C₂H₅OH,and the resultant was allowed to stir at room temperature for 2 h.The obtained Ti(OC₄H₉)₄ solution was added to the aqueous solution of TEOS under vigorous stirring,and51 ml hydrochloric acid was added dropwise to the aqueous solution.Then,a white precipitate was obtained.The precipitate was filtered,washed sequentially with ethanol and doubly distilled water,dried at 70℃,and calcined at 500℃for 4.0 h.

H₃PW₁₂O₄₀/TiO_2-SiO₂ was synthesized by impregnation method:An amount of H₃PW₁₂O₄₀ was dissolved in30 ml anhydrous ethanol,and a stoichiometric amount of TiO₂-SiO₂ was mixed.The resultant was allowed to stir at room temperature for 12.0 h.The slurry mixture was filtrated and dried.The optimum conditions were found:m(H₃PW₁₂O₄₀):m(TiO₂-SiO₂) of 1:5,m(TiO₂):m(SiO₂) of20:1,calcining time of 1 h,and activated temperature of200℃.

2.2 Characterization

Fourier transform infrared spectra (FTIR) of the samples in KBr matrix were recorded on a Nicolet 5700 FTIR spectrometer in the range of 400-4000 cm^-1.X-ray powder diffraction (XRD) patterns of the samples were measured by a Bruker AXS D8-Advanced diffractometer (Bruker,Germany) employing Cu-Kαradiation.Scanning electron micrographs (SEM) were obtained on a Hitachi S-3400 N scanning electron microscope.The Zeta potential was supplied by Malvern Nano ZS 90.

2.3 Photocatalytic activity measurement

The photocatalytic activities of the samples were determined by measuring the degradation of dyes in an aqueous solution under simulated sunlight irradiation.Simulated sunlight irradiation was provided by a 500-W xenon lamp(Nanjing Xujiang Electromechanical Factory),and the intensity of the lamp was 1200μmol·m^-2·s^-1.Solution pH was adjusted with dilute aqueous HCl and NaOH solutions.The system was cooled by circulating water and maintained at room temperature.Before irradiation,the suspension was vigorously stirred in the dark for 30 min to reach the adsorption-desorption equilibrium of dyes on the catalyst surface.At given time intervals,about 3 ml suspension was continually taken out from the photoreactor and centrifuged.The change in methyl violet concentrations was analyzed by ultraviolet-visible (UV-Vis) spectrophotometer (Hitachi U-3010).

3 Results and discussion

3.1 Characterization of H3PW12O40/TiO2-SiO2catalyst

Figure 1 shows the FTIR spectra of H₃PW₁₂O₄₀,TiO₂-SiO₂ and H₃PW₁₂O₄₀/TiO₂-SiO₂.The absorption peaks of pure H₃PW₁₂O₄₀ at 1229.5,1091.9,972.5,757.6,and522.6 cm^-1 are due to the Keggin structure [ 30] .In addition,the band at 1615.7cm^-1,which is the bending mode of the water,indicates the presence of the water.When H₃PW₁₂O₄₀ is supported on TiO₂-SiO₂,these bands change somewhat.1209.4 cm^-1 is assigned to the vibration of Ti-O-Ti and Si-O-Si bonds.However,the characteristic bands for the Keggin anion could be observed for H₃PW₁₂O₄₀/TiO₂-SiO₂.The shifts indicate that a strong chemical interaction,not simple physical adsorption,exists between the H₃PW₁₂O₄₀ and the support TiO₂-SiO₂.

Fig.1 FTIR spectra of H₃PW₁₂O₄₀,SiO₂-TiO₂,and H₃PW₁₂O₄₀/SiO₂-TiO₂

Figure 2 shows the XRD patterns of H₃PW₁₂O₄₀,TiO₂-SiO₂,H₃PW₁₂O₄₀/TiO₂-SiO₂,and H₃PW₁₂O₄₀/TiO₂-SiO₂(recycled).The characteristic diffraction peaks of H₃PW₁₂O₄₀ at 8°-10°,17°-20°,26°-30°,and 32°-35°can be assigned to the diffraction characteristic peaks of crystalline H₃PW₁₂O₄₀ Keggin structure [ 30] .It can be seen that TiO₂-SiO₂,H₃PW₁₂O₄₀/TiO₂-SiO₂,and H₃PW₁₂O₄₀/TiO₂-SiO₂ (recycled) samples almost share the same peaks that can be indexed as characteristic (110) reflections of TiO₂.Standard anatase TiO₂ appears at 26=25.4°,and the signals of H₃PW₁₂O₄₀ disappear.So it is reasonable to consider that H₃PW₁₂O₄₀ highly disperses on the surface of TiO₂-SiO₂ support without any aggregation.

Fig.2 XRD patterns of H₃PW₁₂O₄₀,TiO₂-SiO₂,H₃PW₁₂O₄₀/TiO₂-SiO₂,and H₃PW₁₂O₄₀/TiO₂-SiO₂ (recycled)

SEM measurements of as-prepared composites were taken to obtain the shape.Figure 3 shows the SEM images of H₃PW₁₂O₄₀,TiO₂-SiO₂,and H₃PW₁₂O₄₀/TiO₂-SiO₂ at different multiples.Surface morphology of TiO₂-SiO₂ is sphere,and it distributes evenly in the whole system.Surface morphology of H₃PW₁₂O₄₀ is petalage-like,and its specific surface area is relatively small.The results show that not only the catalyst uniformly disperses in the system,but also the specific surface area is bigger than before.Furthermore,Zeta potential measurements show that TiO₂and TiO₂-SiO₂ have a positive Zeta potential of 3.25 and2.91 mV,respectively,but H₃PW₁₂O₄₀/TiO₂-SiO₂ has a negative Zeta potential which is 10.8 mV.So H₃PW₁₂O₄₀/TiO₂-SiO₂ can generate more electron-hole pairs that benefit degradation methyl violet dye.

3.2 Investigation of photocatalytic activity of catalysts

3.2.1 Comparison of photo catalytic activity under light and catalyst or not

In order to detect the catalytic activity under light and catalyst or not,the degradation of methyl violet aqueous over the samples was carried out at the methyl violet concentration of 15 mg·L^-1,pH of 3,and 1.8 g·L^-1 catalyst,and the results are shown in Fig.4.

As shown in Fig.4,the degradation rate of methyl violet over H₃PW₁₂O₄₀/TiO₂-SiO₂ is only 10.2%and the degradation rate of methyl violet over light is only 30.2%,while after 2.5 h irradiation under the same conditions,the degradation rate over H₃PW₁₂O₄₀/TiO₂-SiO₂ is as high as75.1%.As shown in Fig.4,the comparison of photocatalytic activity under no light and without catalyst is inefficient.However,it is efficient with light and catalyst.

Fig.3 SEM images of H₃PW₁₂O₄₀ a,SiO₂-TiO₂ b,and H₃PW₁₂O₄₀/SiO₂-TiO₂ c

Fig.4 Comparison of photocatalytic activity under catalyst,light,and light and catalyst

Fig.5 Effects of initial dye concentration on photocatalytic activity of catalyst

3.2.2 Effect of initial concentration of dye

Figure 5 shows the effects of initial concentration on the photocatalytic degradation of methyl violet,indicating that the photocatalytic degradation rate of methyl violet decreases with an increase in the initial concentration of methyl violet.This phenomenon could be attributed to the excessive absorption of the methyl violet molecules on the surface of catalyst and a diminishing of light penetration through the solution at high initial concentration of methyl violet,which inhibits the process of activated catalyst.Then,the optimum of the initial concentration is10 mg·L^-1.

Fig.6 Effect of catalyst dosage on photocatalytic activity of catalyst

Fig.7 Effects of solution pH on photocatalytic activity of catalyst

3.2.3 Effect of catalyst dosage

Figure 6 shows the effects of catalyst dosage on the degradation rate of methyl violet.It is found that the degradation rate of methyl violet increases with an increase in the amount of catalyst from 0.6 to 4.1 g·L^-1 and then decreases when the catalyst dosage is higher than2.9 g·L^-1.This may be attributed to the scattering effect.The high turbidity of catalyst decreases the penetration depth of solar radiation.So,the optimum amount of catalyst is 2.9 g·L^-1.

3.2.4 Effect of pH

The pH value is one of the most important parameters in the photodegradation of organic compounds.Figure 7shows the effect of pH on the photodegradation rate of methyl violet,indicating that the degradation rate increases with the solution pH decreasing.This is mainly ascribed to the variations of surface charge properties of the photocatalyst.So,the optimum solution pH is 3.

Fig.8 Effect of H₃PW₁₂O₄₀,TiO₂-SiO₂,and H₃PW₁₂O₄₀/TiO₂-SiO₂ on photocatalytic activity

3.2.5 Comparison of photocatalytic activities ofH3PW12〇4〇, Ti02-Si〇2 and H3PW12O40/ Ti02-Si〇2

Figure 8 shows the effect of H₃PW₁₂O₄₀,TiO₂-SiO₂,and H₃PW₁₂O₄₀/TiO₂-SiO₂ on photocatalytic degradation of methyl violet.It can be found that the kind of catalyst plays an important role in the photocatalytic reaction.After 2.5 h irradiation under simulated natural light irradiation,the degradation rate of methyl violet over H₃PW₁₂O_4O,TiO₂-SiO₂,and H₃PW₁₂O₄₀/TiO₂-SiO₂ is 21.0%,63.1%,and95.4%,respectively.The activity of various components in the reaction follows the order of H₃PW₁₂O₄₀/TiO₂-SiO₂>TiO₂-SiO₂>H₃PW₁₂O₄₀.

3.3 Photocatalytic degradation of various dyes

In order to determine the feasibility of the catalyst in the treatment of some dyes with different chemical structures,it was attempted to choose other dyes such as methyl orange (MO),methyl red (MR),naphthol green B (NG),and methylene blue (MB) as reactants.The results are shown in Fig.9.The degradation rate of dyes can reach 81%-100%at the optimized conditions.Therefore,the catalysts have good prospects of application in treatment of organic pollutants.

3.4 Reuse of catalyst

Reusability of the catalyst was also studied under the optimized conditions.After the reaction finished,the catalyst was recovered and washed with distilled water and ethyl alcohol.The recovered catalyst was treated for the next degradation run.

The results are shown in Fig.10.It is observed that the catalytic activity is decreased after subsequent reuse.However,the degradation rate of methyl violet can reach94%in the fifth run.So,it can be concluded that the high catalytic activity is retained in a next run.These results demonstrate a strong interaction between the support and the HPA,not simple physical adsorption.This interaction is supposed to be strong enough to avoid significant leaching of the HPA [ 6, 28] .

Fig.9 Photocatalytic degradation of various dyes using H₃PW₁₂O₄₀/TiO₂-SiO₂ as catalyst

Fig.10 Recycling of catalyst

3.5 Kinetic analysis and mechanism of photodegradation

The kinetics of photocatalytic degradation of methyl violet by H₃PW₁₂O₄₀/TiO₂-SiO₂ was studied under optimized conditions.The results are shown in Fig.11.The results show that a plot of In(C₀/C_t) versus time exhibits a nearly straight line,and the linear correlation coefficient (R²) is0.993.It can be concluded that the photodegradation reactions follow the first-order kinetics,which follow Langmuir-Hinshelwood kinetics.The rate constant is calculated to be 1.2 h^-1.

Fig.11 Relationship between 1n(C₀/C_t) and time (t).C₀ and C_t being concentrations of dye in initial and over time

As we all know,photocatalytic activity is determined mainly by band gap energy and recombination rate of photogenerated electron-hole pairs.A possible mechanism of methyl violet photodegradation on H₃PW₁₂O₄₀/TiO₂-SiO₂ could be described as self-photosensitization.The electrons and holes may immediately generate when H₃PW₁₂O₄₀ and TiO₂ are irradiated under visible light.The adsorbed oxidants can capture the electrons to produce reactive oxygen radical,and the holes can react with water to form hydroxyl radicals.Reactive oxygen species and hydroxyl radicals are highly oxidizability,so methyl violet can be completely mineralized to CO₂ and H₂O.The photocatalytic activity of TiO₂ for the degradation of methyl violet may be enhanced by loading H₃PW₁₂O₄₀ and SiO₂ on TiO₂.The large surface area of TiO₂-SiO₂ enhances the absorption of methyl violet,which leads more electrons transition to the conduction band of TiO₂.Furthermore,H₃PW₁₂O₄₀ is a good electron acceptor,which will enhance the electron-hole separation when it contacts with TiO₂.

4 Conclusion

TiO₂-SiO₂ was prepared by a sol-gel method,and H₃PW₁₂O₄₀/TiO₂-SiO₂ was synthesized by an impregnation mefthod.Degradation of methyl violet was used as a probe reaction to explore the influencing factors on the photodegradation reaction.The optimal conditions are as follows:initial concentration of methyl violet of10 mg·L-¹,pH of 3.0,catalyst dosage of 2.9 g·L^-1,and light irradiation time of 2.5 h.Under these conditions,the degradation rate of methyl violet is 95.4%.The photodegradation of methyl orange,methyl red,naphthol green B,and methylene blue was also tested,and the degradation rate of dyes can reach 81%-100%.The high activity and stability of the catalyst are well retained after five runs.The reaction of photocatalysis for methyl violet can be expressed as the first-order kinetic model.The results show that H₃PW₁₂O₄₀/TiO₂-SiO₂ is an excellent catalyst which has a synergistic effect between HPAs and support.