CeO2 Hybrid Photocatalyst with Enhanced Photocatalytic Activity

Apr 29, 2014 - synthesis variables were optimized in the photocatalytic removal of phenazopyridine (PhP) as a model drug contaminant using response ...
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TiO2/CeO2 Hybrid Photocatalyst with Enhanced Photocatalytic Activity: Optimization of Synthesis Variables Hamed Eskandarloo,† Alireza Badiei,*,† and Mohammad A. Behnajady‡ †

School of Chemistry, College of Science, University of Tehran, Tehran 14176, Iran Department of Chemistry, College of Science, Tabriz Branch, Islamic Azad University, Tabriz 5157944533, Iran

‡

S Supporting Information *

ABSTRACT: In this study, TiO2/CeO2 hybrid photocatalysts were prepared from a powder mixture of the corresponding component solid oxides. X-ray diffraction (XRD), UV−vis diffuse reflectance spectroscopy (DRS), transmission electron microscopy (TEM), and N2 physisorption techniques were used for the characterization of prepared photocatalysts. The synthesis variables were optimized in the photocatalytic removal of phenazopyridine (PhP) as a model drug contaminant using response surface methodology (RSM). The results showed that the predicted data from RSM was found to be in good agreement with the experimental results with a correlation coefficient (R2) of 0.9518. Optimization results showed that maximum removal efficiency was achieved at the optimum synthesis conditions: Ti/Ce weight ratio of 0.84:0.16, calcination temperature of 502 °C, and calcination time of 62 min. Results also showed that coupling TiO2 with CeO2 could produce special electrons and holes transfer from TiO2 to CeO2 which is able to facilitate the separation of the electron−hole pairs and thus improve photocatalytic activity of the hybrid photocatalyst. Effect of synthesis variables on the removal efficiency of PhP was estimated by the response surface and contour plots. The obtained results clearly demonstrated that experimental design approach was one of the reliable methods for modeling and optimization of the synthesis variables.

1. INTRODUCTION Over the past years, a number of conventional methods has been employed for pharmaceutical waste treatment. Through conventional methods such as sewer or direct disposal, waste drugs are disposed into water; therefore, employment of these methods is not useful in removal of pharmaceutical wastes from waters.1,2 Hence, two strategies involve prevention of disposing waste drugs into the sewage system before appropriate treatment and purification of contaminated waters from contaminant drugs are needed for pharmaceutical waste treatment.1,2 Advanced oxidation processes (AOPs) are some of the most promising ways to deal with the destruction of various pollutants.3 Among the AOPs, heterogeneous photocatalysis involves combination of a catalyst (such as TiO2, ZnO, ZnS, Fe2O3, CdS, WO3, ZrO2, SrO2, CeO2, etc.) and UV or visible light irradiation.4 Titanium dioxide is extensively used as an active material for photocatalytic reactions, because of its high photocatalytic activity, availability, nontoxicity, high stability within a wide range of pH, low-cost, and a fast electron transfer to molecular oxygen.5 When TiO2 absorbs a photon with energy greater than or equal to the band gap energy, electrons are promoted from valence band (VB) to the conduction band (CB) to generate electron−hole pairs. The holes can oxidize water or hydroxide ions to produce hydroxyl radicals as a powerful oxidizing agent.6,7 Coupling TiO2 with electron-accepting materials can greatly enhance the photocatalytic activity of hybrid systems.8,9 Many studies related to TiO2 coupled with other metal oxides, such as ZnO−TiO2, ZrO2−TiO2, SnO2−TiO2, Cu2O−TiO2, WO3− TiO2, and CeO2−TiO2, have been reported. Among them, coupling TiO2 with CeO2 attracts more attention, due to the improved textural and structural properties of TiO2.10−12 Cerium dioxide is one of the most attractive materials and is used in © 2014 American Chemical Society

extensive applications, such as pollution control applications and fuel cells, due to its distinctive characteristic properties, such as unique UV absorbing ability, high optical transparency in the visible region, stability at high temperature, and high hardness and reactivity.11,13 TiO2 and CeO2 coupling can produce special electrons and holes transfer from TiO2 to CeO2 which is able to facilitate the separation of the electron−hole pairs and thus improves photocatalytic activity of the hybrid photocatalyst.10,14 A literature review revealed that the preparation conditions of TiO2/CeO2 were not investigated in detail to achieve high activity photocatalyst, suitable for industrial applications. For this reason, we employed response surface methodology (RSM) for statistically optimizing the preparation conditions of TiO2/CeO2 by using a minimum number of experiments. So far, no report has been presented in the optimization of TiO2/CeO2 synthesis variables using RSM. The RSM is a mathematical and statistical technique that is widely employed in process optimizing and modeling. RSM technique is capable of analyzing the interactions of possible influencing factors and determining the optimum region of the factors level just by using a minimum number of designed experiments.15,16 Box−Behnken and central composite design (CCD) are the most commonly selected methods in the RSM technique. In the present study, TiO2/CeO2 hybrid photocatalyst was prepared from a powder mixture of the corresponding component solid oxides and the effect of different synthesis variables was optimized for the photocatalytic removal of phenazopyridine (PhP) as a model drug contaminant using Received: Revised: Accepted: Published: 7847

October 14, 2013 April 17, 2014 April 17, 2014 April 29, 2014 dx.doi.org/10.1021/ie403460d | Ind. Eng. Chem. Res. 2014, 53, 7847−7855

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Table 1. Experimental Ranges and Levels of the Synthesis Variables symbol

ranges and levels

synthesis variables

xi

−2

−1

0

+1

+2

weight ratio of Ti/Ce calcination temperature (°C) calcination time (min)

x1 x2 x3

0.84:0.16 416 26

0.7:0.3 450 40

0.5:0.5 500 60

0.3:0.7 550 80

0.16:0.84 584 94

and ν is the frequency of the radiation. The Eg values were calculated by plotting (αhν)2 versus hν, followed by extrapolation of the linear part of the spectra to the energy axis.20 Size of the samples was obtained by the TEM instrument (EM 208 Philips, 100 kV). Nitrogen adsorption−desorption was carried out using a Belsorp mini II instrument to measure the specific surface area and total pore volume of prepared samples using the Brunauer−Emmett−Teller (BET) and the Barret− Joyner−Halender (BJH) methods. 2.4. Photocatalysis Experiments. Photocatalytic removal processes were carried out at room temperature in a batch quartz reactor. Artificial irradiation was provided by a 15 W (UV−C) mercury lamp (Philips, Holland) emitting around 254 nm, positioned in top of the batch quartz reactor. In each run, 40 mg of catalyst was dispersed in 100 mL of water for 15 min using a probe sonicator; then, the desired concentration of PhP (12 mg L−1) and photocatalyst suspension were transferred into the batch quartz reactor and stirred for 30 min to reach the adsorption equilibration in the dark before irradiation. The photocatalytic reaction was initiated with turning on the light source. Distance between UV lamp and the solution was maintained at 6 cm, in all the measurements. At given irradiation time intervals, the samples (5 mL) were taken out and centrifuged (Sigma 2−16P), and then, PhP concentration was analyzed by a UV−vis spectrophotometer (Rayleigh UV−1600) at λmax = 430 nm. 2.5. Experimental Design. In this study, a central composite design (CCD) was used to propose and estimate a mathematical model of the photocatalytic process behavior. Computational analysis of the experimental data was supported by the Design−Expert (version 7) software. In order to evaluate the effect of independent synthesis variables, three key factors were chosen: Ti/Ce weight ratio, calcination temperature (°C), and calcination time (min). The photocatalytic removal efficiency of PhP was selected as the response. A total of 18 experimental runs were performed in this work with four replications at the center point. For statistical calculations, three chosen synthesis variables were converted to dimensionless ones (x1, x2, x3), with the coded values at levels: −2, −1, 0, +1, and +2. The experimental ranges and the levels of the synthesis variables are presented in Table 1. It should be noted that the preliminary experiments were performed to determine the extreme values of the synthesis variables.

the RSM technique. PhP is a widely used analgesic drug in relieving urinary tract pain, burning, irritation, and discomfort.17 The presence of the azo group in molecular structure of PhP makes it a resistant compound to biodegradation.18 The effect of synthesis variables on the removal efficiency of PhP was established by the response surface and contour plots. X-ray diffraction (XRD), UV−vis diffuse reflectance spectroscopy (DRS), transmission electron microscopy (TEM), and Brunauer−Emmett−Teller (BET) techniques were used for characterization of different photocatalysts.

2. MATERIALS AND METHODS 2.1. Materials. TiO2 dioxide (pure anatase phase, BET surface area 10 m2 g−1) and cerium dioxide powders were purchased from Merck Co. (Germany). PhP was donated by Tehran pharmaceutical company (Iran). Characteristics and chemical structure of PhP are given in Table S1, Supporting Information. 2.2. Preparation of Hybrid Photocatalysts. In this study, TiO2/CeO2 hybrid photocatalyst was prepared from a powder mixture of the corresponding component solid oxides, according to the following steps. First, 1 g of TiO2 and CeO2 powder mixtures with different weight ratios of Ti/Ce (0.84:0.16, 0.7:0.3, 0.5:0.5, 0.3:0.7, and 0.16:0.84) was ground thoroughly in an agate mortar. Then, the mixed oxides were dispersed in 100 mL of boiling deionized water and sonicated for 15 min using a probe sonicator (Bandelin HD 3200). The suspension solution was stirred for 24 h and then dried in an air oven at 80 °C for about 12 h. Then, the dried solids were calcined at different temperatures (416, 450, 500, 550, and 584 °C) for different times (26, 40, 60, 80, and 94 min). In addition, TiO2 and CeO2 powders were also treated by a similar procedure. 2.3. Characterization of Photocatalysts. The prepared samples were characterized by a Philips X’pert MPD diffractometer using Cu Kα radiation (λ = 0.15478 nm). The (101) reflection (2θ = 25.28°) of anatase TiO2 and the (111) reflection (2θ = 28.8°) of cubic CeO2 were used for analysis. The average crystallite size of the particles was calculated from the line broadening of corresponding reflections and according to the Scherrer’s equation19 D=

kλ β cos θ

(1)

where D is the average crystallite size (nm), λ is the wavelength of the X-ray radiation, k is a constant taken as 0.89, β is the full width at half-maximum intensity, and θ is the half diffraction angle. UV−vis DRS of samples was obtained using an AvaSpec− 2048 TEC spectrometer for determination of the optical band gap (E g) of pure TiO 2, CeO2, and coupled TiO2 /CeO 2 photocatalyst. For determination of the Eg, eq 2 was used α(hν) = B(hν − Eg )1/2

3. RESULTS AND DISCUSSION 3.1. Optimization of Synthesis Variables. To optimize the synthesis conditions in order to achieve TiO2/CeO2 hybrid photocatalyst with high photocatalytic activity, three synthesis variables, including Ti/Ce weight ratio, calcination temperature, and time were investigated. 3.1.1. Model Results. The mathematical relationship between the response and these variables can be approximated by a second-order polynomial equation as shown below:

(2)

where α is optical absorption coefficient, B is a constant dependent on the transition probability, h is the Plank’s constant, 7848

dx.doi.org/10.1021/ie403460d | Ind. Eng. Chem. Res. 2014, 53, 7847−7855

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Figure 1. The response surface and contour plots of the removal efficiency of PhP by TiO2/CeO2 hybrid photocatalyst as a function of (a) weight ratio of Ti/Ce and calcination temperature, (b) weight ratio of Ti/Ce and calcination time, and (c) calcination temperature and calcination time.

where Y is a predicted response of photocatalytic removal efficiency, b0 is a constant, b1, b2, and b3 are the regression coefficients for linear effects, b12, b13, and b23 are the regression coefficients for interaction effects, b12, b22, and b32 are the

Y = b0 + b1x1 + b2x 2 + b3x3 + b12x1x 2 + b13x1x3 + b23x 2x3 + b11x12 + b22x 2 2 + b33x32

(3)

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Figure 2. XRD patterns of prepared TiO2/CeO2 hybrid photocatalyst with different (a) weight ratios of Ti/Ce (calcination temperature (500 °C), calcination time (60 min)) (a1: 0.84:0.16, a2: 0.5:0.5, a3: 0.16:0.84), (b) calcination temperature (Ti/Ce weight ratio of 0.5:0.5; calcination time (60 min)) (b1: 416 °C, b2: 500 °C, b3: 584 °C), and (c) calcination time (Ti/Ce weight ratio of 0.5:0.5; calcination temperature (500 °C)) (c1: 26 min, c2: 60 min, c3: 94 min).

regression coefficients for squared effects, and xi is the coded experimental level of the synthesis variables. The details of the designed experiments along with experimental results and predicted values for photocatalytic removal efficiencies of PhP drug as a function of synthesis variables are shown in Table S2, Supporting Information. Following the experimental design presented in Table S2, Supporting Information, an empirical relationship between the

response (Y) and independent synthesis variables (x1, x2, x3; see Table 1) was attained as shown below: Y = 63.6 + 2.47x1 + 0.29x 2 + 1.41x3 − 0.073x1x 2 − 0.78x1x3 + 3.67x 2x3 − 0.58x12 − 5.57x 2 2 − 2.49x32 (4)

Equation 4 is used to predict the photocatalytic removal efficiencies of drug contaminant by the prepared TiO2/CeO2 7850

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structure in samples were calculated from eq 1 using reflections of anatase at 25.3° and cubic structure at 28.8°, respectively. The results of XRD analysis are summarized in Table 2.

hybrid photocatalyst with varied synthesis variables within the selected experimental ranges. By using the resulted second-order polynomial equation (eq 4), the predicted values of photocatalytic removal of PhP are plotted versus corresponding experimental results in Figure S1, Supporting Information. Results confirm that the predicted photocatalytic removal efficiencies for PhP as a function of synthesis variables from the model are in good agreement with the experimental results. Analysis of variance (ANOVA) of the quadratic response surface model is a statistical procedure to test the significance and adequacy of the model.21 Table S3, Supporting Information, shows the ANOVA results for quadratic response surface model. According to the ANOVA results, the regression model presents high correlation coefficients (R2 = 0.9518) for the photocatalytic removal of PhP. The correlation coefficient is often used to characterize the goodness of fit between model and experimental data. For the model to fit better to the experimental data, the correlation coefficient value should be close to 1.22 The value of R2 implies a satisfactory representation of the photocatalytic removal process by the model. Adjusted R2 is also used to measure the goodness of fit between model and experimental data, but it is more suitable for comparing models with different numbers of independent variables. It corrects the R2 value for the data counts and the number of terms in the model by using the degrees of freedom on its computations, so if there are many terms in a model and not a very large sample size, adjusted R2 may be visibly smaller than R2.23,24 The effect of independent synthesis variables, adjusted R2 value of 0.8978, was very close to the corresponding R2 value. The F value is the ratio between the mean square of the model and the residual error and indicates the significance of each controlled factor on the tested model.25 The F value for the model is 17.59, and the corresponding p-value is TiO2 > CeO2. As observed from results of XRD patterns (Figure 4), 7852

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Figure 6. Plot of (αhν)2 versus hν for TiO2, CeO2, and optimized TiO2/ CeO2 hybrid photocatalyst. Figure 4. XRD patterns of TiO2, CeO2, and optimized TiO2/CeO2 hybrid photocatalyst.

the anatase TiO2 (25.3°) and cubic CeO2 (28.8°) reflections width became broader and weaker than the corresponding solid oxides, which means that optimized hybrid photocatalyst has lower crystallite and smaller particle size. The average crystalline sizes of both TiO2 and CeO2 are smaller in the optimized hybrid photocatalyst than in the corresponding solid oxides. Finally, photocatalytic efficiency of optimized TiO2/CeO2 hybrid photocatalyst was compared with commercial TiO2−P25 in the removal of PhP. It was found that TiO2/CeO2 hybrid photocatalyst has slightly higher photocatalytic activity than TiO2−P25. TEM images allow direct deduction of the particles mean size. The TEM images of the optimized TiO2/CeO2 hybrid photocatalyst have been shown in Figure 5. The mean crystallite

Figure 5. TEM image of the optimized TiO2/CeO2 hybrid photocatalyst.

size of TiO2/CeO2 nanoparticles is estimated to be about 30−40 nm, which is in agreement with the crystallite size calculated from the XRD pattern (Figure 4). To investigate the effect of CeO2 coupling on the optical absorption properties of TiO2 nanoparticles, DRS analysis has been carried out. The values of band gap energy (Eg) are calculated from Figure 6 by extrapolation of the linear part of the spectra to the energy axis. The Eg values for pure CeO2, TiO2, and optimized TiO2/CeO2 hybrid photocatalysts are 2.01, 3.26, and 3.28 eV. Results indicate that CeO 2 coupling to TiO 2 nanoparticles increased optical band gap energy; therefore, the electron−hole separation occurs relatively better in the coupled nanoparticles. Nitrogen adsorption and desorption isotherms of the hybrid photocatalyst prepared under optimized conditions (Figure 7a)

Figure 7. Nitrogen adsorption−desorption isotherms (a) and BJH pore size distribution curve (b) of optimized TiO2/CeO2 hybrid photocatalyst.

displayed type-III isotherm of the IUPAC classifications, which indicates a mesoporous structure.36,37 The BET surface area for optimized TiO2/CeO2 photocatalyst as determined by N2 7853

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physisorption experiments was 11.9 m2 g−1. The value of BET surface area for TiO2/CeO2 photocatalyst is not so remarkable and close to pure TiO2, which reveals that the CeO2 coupling to TiO2 does not have a considerable effect on the surface area of photocatalyst. Pore size distribution of the hybrid photocatalyst prepared under optimized conditions was obtained from the adsorption branch, using the BJH method (Figure 7b). It can be seen that the diameter range of pores was located from 1.5 to 10 nm. 3.1.4. Possible Photocatalytic Process Mechanism for TiO2/ CeO2. A simple representation of the coupled CeO2/TiO2 mechanism responsible for UV light activity is shown in Figure 8. The energy band gap of TiO2 is bigger than that of CeO2. From

photocatalysts for photocatalytic removal of the solution containing PhP drug has been performed. It can be observed that the highest removal efficiency (66.64%) was obtained using optimized TiO2/CeO2 hybrid photocatalyst, whereas at the same time, using TiO2 and CeO2 samples lead to 55.63% and 19.1% removal efficiency, respectively. Results showed that coupling TiO2 with CeO2 could produce special electrons and holes transfer from TiO2 to CeO2 which is able to facilitate the separation of the electron−hole pairs and thus improve photocatalytic activity of the hybrid photocatalyst. The results showed that the predicted values of removal efficiency were found to be in good agreement with the experimental results with a correlation coefficient (R2) of 0.9518. Optimization results showed that maximum removal efficiency (66.64%) was achieved at the optimum synthesis conditions: Ti/Ce weight ratio of 0.84:0.16, calcination temperature of 502 °C, calcination time of 62 min. The obtained results clearly demonstrated that response surface methodology (RSM) with a central composite design was one of the reliable methods for modeling and optimization of the synthesis variables.

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ASSOCIATED CONTENT

S Supporting Information *

Comparison between ANN predicted and experimental removal efficiencies of PhP by TiO2/CeO2 hybrid photocatalyst, Figure S1. Characteristics and chemical structure of PhP, Table S1. The 3-factor central composite design matrix for synthesis variables with the experimental and predicted responses, Table S2. ANOVA results of the response surface quadratic model for the photocatalytic removal of PhP, Table S3. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Possible photocatalytic mechanism of CeO2/TiO2 under UV light irradiation.

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DRS results, the Eg values for pure CeO2 and TiO2 nanoparticles were about 2.01 and 3.26 eV. In other words, the energy level of the conduction band of TiO2 is higher than the corresponding ones of CeO2, which can cause the electron transfer from TiO2 to CeO2. On the other hand, the energy level of the valence band of CeO2 is higher than the corresponding one of TiO2, which can cause the transfer of the holes from TiO2 to CeO2.38,39 Finally, the complete separation of electron−hole pairs in TiO2 occurs, resulting in the improvement of the photocatalytic activity of the TiO2 photocatalysts with an increase in O2•− and hydroxyl radical generation through the following equations:10 TiO2 + hν → eCB− + hVB+

(5)

Ce 4 + + eCB− → Ce3 +

(6)

Ce3 + + O2 → Ce 4 + + O2•−

(7)

O2•− + 4H+ → 2•OH

(8)

PhP +• OH → products

(9)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +98−2161112614. Fax: +98−2161113301. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank University of Tehran for financial support of this work. REFERENCES

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This is the reason why the CeO2/TiO2 hybrid phtocatalyst displays a higher photocatalytic activity than TiO2 and CeO2 photocatalysts.

4. CONCLUSIONS TiO2/CeO2 hybrid photocatalyst was prepared and used in the photocatalytic removal of PhP drug under a UV light irradiation. Response surface methodology was successfully employed in this study to optimize the individual and interactional effects of the synthesis parameters. A comparison of single and hybrid 7854

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dx.doi.org/10.1021/ie403460d | Ind. Eng. Chem. Res. 2014, 53, 7847−7855