TiZrO4 as a visible light responsive photocatalyst

Ultrasonics Sonochemistry 18 (2011) 135–139

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Sonophotocatalytic degradation of 4-chlorophenol using Bi2O3/TiZrO4 as a visible light responsive photocatalyst Bernaurdshaw Neppolian a, Luca Ciceri a,b, Claudia L. Bianchi c, Franz Grieser a, Muthupandian Ashokkumar a,* a b c

Particulate Fluids Processing Centre, School of Chemistry, University of Melbourne, Parkville, Melbourne, Victoria 3010, Australia Dip. Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, 20131 Milano, Italy Dip. Chimica Fisica ed Elettrochimica, Università di Milano, 20133 Milano, Italy

a r t i c l e

i n f o

Article history: Received 10 December 2009 Received in revised form 3 March 2010 Accepted 7 April 2010 Available online 11 April 2010 Keywords: 4-Chlorophenol Sonochemical Photocatalysis Sonophotocatalysis Bi2O3/TiZrO4

a b s t r a c t The oxidative degradation of 4-chlorophenol (4-CP) by sonolytic, photocatalytic and sonophotocatalytic processes was studied in aqueous solutions using Bi2O3/TiZrO4 as a visible light driven photocatalyst and with 20 kHz ultrasound. The results reveal that Bi2O3/TiZrO4 is an efficient photocatalyst capable of degrading 4-CP by both photocatalytic and sonophotocatalytic processes. During the sonolysis of 4-CP solutions, HPLC results showed the formation of a number of intermediate products, whereas, no such intermediates were formed during the sonophotocatalytic degradation of 4-CP. TOC results showed rapid mineralization of 4-CP during the sonophotocatalytic degradation process, relative to that observed with sonolysis alone. The results reveal a clear advantage in using a coupled method for the oxidation of 4-CP and a cumulative effect was observed. Further, the solution pH had no specific influence on the sonophotocatalytic degradation of 4-CP, unlike the situation for sonolysis alone where the degradation rate decreased as the pH was raised from acidic to basic conditions. The combined sonophotocatalytic degradation process was found to be simple to apply and has the potential to be a powerful method for the remediation of organic contaminants present in water and wastewater. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Chlorophenols are widely used in herbicides, certain dyes, pesticides, and wood preservatives and appear in contaminate soil, water and wastewater as persistent pollutants. Chlorophenols are hazardous chemicals and generally classified as non-biodegradable pollutants. Continuous consumption of drinking water contaminated with chlorophenols can affect human health [1–5]. Among the various chlorophenols, 4-chlorophenol (4-CP) is primarily found in the wastewater from pulp and paper, dyestuff, pharmaceutical and oil industries [1,6] and listed as one of the priority pollutants by the US Environmental Protection Agency (EPA) and the European Union (EU) [6]. The permissible limit of 4-chlorophenol in drinking water supply is 0.5 mg/L [6]. Hence, the complete removal or oxidation of 4-chlorophenol in wastewater has been the focus of a number of recent studies [1,6]. Heterogeneous photocatalysis using TiO2 has been employed as an efficient process for the complete oxidation of organic pollutants [7–11]. However, TiO2 can only absorb 5% of the solar light

* Corresponding author. Tel.: +61 3 83447090; fax: +61 3 93475180. E-mail address: [email protected] (M. Ashokkumar). 1350-4177/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2010.04.002

that reaches the earth’s surface [10,11]. In order to utilize the maximum energy of the solar spectrum, mixed oxide catalysts containing TiO2, which are responsive to UV radiation and visible light (VLR) (narrow band gap semiconductors, such as CdS, CdSe and Bi2O3) have been developed [12–15]. Bi2O3 has a band gap of about 2.8 eV and hence absorbs visible light. Further, the energy level of the conduction band of TiO2 is well below the conduction band of Bi2O3, thus the photogenerated electrons from Bi2O3 move to TiO2 thereby increasing the performance of TiO2 under visible light irradiation. Recently, sonochemical oxidation has gained much attention as an advanced oxidation process for the degradation of organic contaminants present in wastewater [16–20]. Combining photocatalysis and sonochemistry offers a potentially useful way of overcoming the existing problems of the individual methods: (i) the aggregation of catalyst particles in aqueous solutions is prevented by the physical effects of acoustic cavitation, leading to an increase in the active surface area, (ii) the catalyst surface is continuously cleaned, which has the ability to further increase the catalytic performance of the photocatalyst, (iii) increase in the mass transport of the pollutants to the catalyst surface [21], and (iv) sonochemically formed H2O2 is stable in the presence of low levels of pollutants [22], and can be cleaved into OH radicals during photolysis.

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Many researchers have explored photocatalysis in combination with ultrasound for the effective decomposition of pollutants using powerful UV irradiation [23–28]. However, very few studies have explored the VLR catalysts along with ultrasound for environmental remediation [21,29]. This study is mainly focused on the sonochemical and sonophotocatalytic degradation of 4-CP using Bi2O3/TiZrO4 under visible light irradiation. 2. Materials and methods

then with ethanol to remove all the PVP present in the solution and dried at 120 °C overnight. The dried mixed oxide catalysts was then calcined at 450 °C in an electric furnace (the temperature of the furnace was increased at 2 °C/min) and used for further studies. The particle size of Bi2O3/ZrTiO4, determined by TEM analysis was found to be 7 nm and its corresponding BET surface area was 49 m2/g.

2.3. Sonolytic and photocatalytic activity measurements

All the chemicals used in this study were purchased from Aldrich and were used without further purification. All solutions were prepared with 18 MX cm deionized water from a water purification system (Millipore, Synergy). For pH adjustment, 0.1 M HCl and 0.1 M NaOH solutions were used. A stock solution containing 1.0  103 M 4-CP was prepared and diluted to the required initial concentration. 2.2. Preparation and characterization of Bi2O3/TiZrO4 Bi2O3/TiZrO4 nanoparticles were synthesized by an ultrasonic assisted hydrothermal method. The required weight of bismuth(III) nitrate pentahydrate was dissolved in 30 mL of HNO3 acid (1 M) and 1.5 g of polyvinylpyrrolidone (PVP) was added to the above bismuth(III) nitrate pentahydrate solution [30]. TiO2/ZrO2 (2:1) was separately prepared and kept in 100 mL of HNO3 solution as reported earlier [31]. 0.2 M NaOH was added to TiO2/ZrO2 in a HNO3 solution and the solution pH was increased to 13. Bismuth(III) nitrate pentahydrate solution with PVP was added drop wise into the TiO2/ZrO2 in HNO3 solution under vigorous stirring and then the mixture was placed in a bath sonicator (45 kHz, Branson B8500R-DTH) for 1 h. Finally, the colloidal solution was placed in an autoclave (70% of its total volume) and sealed tightly. The autoclave was placed in a furnace and heated slowly until 170 °C (2 °C/min) and kept for another 15 h at the same temperature and then the product was cooled to room temperature. The prepared catalyst was washed repeatedly with distilled water and

The reactor system was made up of a borosilicate glass chamber with 85 mL capacity equipped with an ultrasonic horn-type

90 Sonophotocatalysis Sonolysis Photocatalysis

75

% 4-CP Degraded

2.1. Chemicals

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Sonication time (min) Fig. 2. 4-CP degradation (%) with respect to reaction time by using three different processes. Experimental conditions: [4-CP] = 1.25  104 M, pH = 5, 20 kHz horn diameter = 19 mm, power = 51 W.

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0.125 mM

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TOC removal (%)

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1.5 ln(C o /C)

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Fig. 1. Effect of different initial concentrations of 4-CP on the % degradation during sonophotocatalysis. Experimental conditions: pH = 5, 20 kHz horn diameter = 19 mm, power = 51 W.

Sonolysis Photocatalysis Sonophotocatalysis Different oxidation processes Fig. 3. TOC removal (%) of 4-CP using three different processes. Experimental conditions: [4-CP] = 1.25  104 M, time = 1 h, pH = 5, 20 kHz horn diameter = 19 mm, power = 51 W.

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transducer (Branson Digital Sonifier-450, 20 kHz, tip diameter was 19 mm, USA). The reactor vessel had a double wall with cooling water from a cooler unit circulating between the walls in order to maintain a constant temperature of 25 °C. The ultrasonic power was determined by calorimetry, as described by Koda et al. [32]. The photocatalyst (100 mg) was suspended in a quartz cell with an aqueous solution of 4-chlorophenol (2.5  104 M, 60 mL). Prior to light irradiation, the suspension was stirred for 20 min under an oxygen atmosphere in the dark. The sample was then irradiated at 25 °C using a 300 W Xe Arc lamp (Oriel, Model 66984; wavelength range 200–800 nm) with continuous stirring under air atmosphere. A 420 cut-off filter was used to eliminate the UV light from the lamp and allow only visible light to go through the reaction solution. During sonophotocatalysis, the 20 kHz horn was inserted from the top and light irradiation was sent through a flat window located on the side of the reactor. At regular intervals, up to 5 mL aliquots were taken from the system and filtered through a Millipore filter to remove the catalyst par-

a

ticles. The percentage of degradation was then analyzed by a HPLC (Shimadzu LC-10 AT VP system with a Shimadzu SPD-M10 A VP photodiode array detector with a Phenomenex reversed phase column) and the mineralization of 4-CP (TOC) was measured by a TOC analyzer (TOC–VCSH (Shimadzu) programmed by TOC-Control V Software).

3. Results and discussion During the sonolysis of water, it is well known that acoustic cavitation generates highly reactive primary radicals such as OH and H due to the thermal decomposition of water as shown in reaction (1) [33,34]. A number of recombination and other reactions (reactions (2)–(5)) occur within the bubble following primary radical generation. Of these many radicals, the OH radical is a nonselective oxidant with a high redox potential (2.8 V), which is able to oxidize most organic pollutants.

4

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Intensity (a. u.)

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5 4 3 Retention time (min)

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Fig. 4. HPLC spectra of 4-CP degradation against reaction time. (a) sonolysis; (b) sonophotocatalysis. Experimental conditions: [4-CP] = 1.25  104 M, pH = 5, 20 kHz diameter = 19 mm, power = 51 W.

B. Neppolian et al. / Ultrasonics Sonochemistry 18 (2011) 135–139 ÞÞÞÞÞÞ

H2 O ! HO þ H HO þ H ! H2 O HO þ HO ! H2 O2 H þ H2 O ! HO þ H2 H þ O2 ! HO2

ð1Þ ð2Þ ð3Þ ð4Þ ð5Þ

where ‘‘))))))” refers to sonication. Similarly, during photocatalysis, it is well known that OH radicals are formed due to the oxidation of water by valence band holes, if light of sufficient energy is absorbed by the catalyst. The OH radical is a primary oxidizing radical in both methods but as solutes have different capacities to adsorb to a catalyst surface compared with the surface of bubbles other degradation processes can occur in these systems. Volatile solutes may enter the core of a collapsing bubble and then be thermally degraded, whereas direct oxidation by the hole on the surface of a photocatalyst is also possible. These additional processes may lead to synergistic effects in combined treatments particularly if intermediates in the degradation process have chemical characteristics different to their parent molecules. 3.1. Effect of initial concentration of 4-CP The effect of initial concentration of 4-CP on the percentage decomposition of the parent compound was studied with two different concentrations (0.125 and 0.25 mM) for the three different methods (sonolysis, photocatalysis and sonophotocatalysis) and the results are shown in Figs. 1–3. It is clearly seen that during sonophotocatalysis, the percentage of 4-CP degraded increases with an increase in the concentration of 4-CP (Fig. 1). For example, about 0.125 mM (50% of 0.25 mM (initial concentration)) 4-CP was decomposed in 75 min whereas only 0.09 mM (75% of 0.125 mM) was decomposed for the same duration when the initial concentration was 0.125 mM. A similar trend was observed with both sonolysis and photocatalysis. The sonochemical oxidation process usually follows first-order like kinetics [35]. A plot of ln (C0/C) against sonication time (min) is shown in Fig. 1, suggesting firstorder like kinetics. Similar kinetics was observed with other degradation data. Among the three methods, sonophotocatalysis showed the highest efficiency for 4-CP oxidation compared to the photocatalytic and sonochemical processes (Fig. 2). Furthermore, TOC results also showed a similar trend for the mineralization of 4-CP as depicted in Fig. 3. During the sonochemical oxidation of 4-CP, many intermediate products were formed (Fig. 4a), whereas no such intermediates were formed during sonophotocatalysis (Fig. 4b) and hence the sonophotocatalytic process enhanced the mineralization of 4-CP compared to the sonochemical oxidation alone. It is possible that the products generated were simultaneously decomposed during the sonophotocatalytic process. The intermediates formed during the degradation of 4-CP were benzoquinone, 4-chlorocatechol, catechol and hydroquinone, in addition to many other minor intermediate products as reported previously [1]. The photocatalytic activity of Bi2O3/TiZrO4 was compared to that of the standard Degussa TiO2 P-25; the decomposition rate of 4-CP was two times higher with the VLR catalyst under sonophotocatalysis compared to that obtained with Degussa P-25 (22% degradation of 4-CP with P-25 and 60% degradation with VLR catalyst). 3.2. Effect of catalyst loading

from 50 to 100 mg/60 mL, and then decreased (48%) at 150 mg catalyst loading over a 1 h reaction time ([4-CP] = 0.125 mM). The initial increase in 4-CP degradation can be ascribed to an increase in the availability of the active sites with an increase in the catalyst loading. However, at higher catalyst loading, the light penetration is reduced (due to light scattering from the suspension) and, hence, the photoactive volume of the suspension effectively shrinks [10]. Based on the results of different catalyst loadings, 100 mg/60 mL catalyst was found to be optimal for achieving the maximum % degradation of 4-CP. The remaining experiments were therefore performed with 100 mg/60 mL catalyst loading. 3.3. Effect of initial pH Generally, the solution pH plays an important role in the sonochemical as well as photocatalytic oxidation of organic compounds in an aqueous medium. Singla et al. [36] have reported that the sonochemical oxidation of benzoic acid is slow at high pH and faster at low pH. Ashokkumar et al. [37] have reported that alkyl carboxylic acids are more surface active at low pH than at high pH, i.e., sonochemical reactions are shown to be highly effective if the reactants are surface active. As expected, the sonolytic degradation of 4-CP was relatively faster at pH 3 than at higher pH values as shown in Fig. 5, similar to the previous findings with benzoic acid [36]. The pKa value of 4-CP is about 9.5. At pH >9.5, 4-CP is in its ionic form and present predominantly in the bulk of the solution. As 4-CP does not adsorb significantly to the surface of the cavitation bubbles under these conditions, it reacts with OH radicals only in the bulk solution. In contrast, during photocatalysis alone (data not shown), at pH 3 a lower efficiency for 4-CP degradation was obtained, which is similar to the 4-CP photocatalytic degradation as reported by Chen and Smirniotis [38]. Furthermore, it is interesting to note that the solution pH had no notable influence on the degradation of 4-CP under sonophotocatalytic oxidation in the pH range 3–9. This outcome indicates a clear advantage of this coupled method for the effective oxidation of 4-CP at any pH below 10. 3.4. Effect of ultrasonic power The ultrasonic power has a major influence on the rate of oxidation of pollutants [22]. The main sonicator used in this study [22] had a constant frequency output (20 kHz) with tunable amplitude.

50

40 % 4- CP Degraded

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0 3

The photocatalysis reaction was carried out with different amounts of the catalyst ranging from 50 to 150 mg/60 mL of the solution. It was observed that the % decomposition of 4-CP increased from 42% to 61% with increasing amounts of catalyst,

6 Initial pH values

9

Fig. 5. Effect of pH on the % degradation of 4-CP using sonolysis. Experimental conditions: [4-CP] = 2.5  104 M, 20 kHz horn diameter = 19 mm, reaction time = 2 h, power = 51 W.

B. Neppolian et al. / Ultrasonics Sonochemistry 18 (2011) 135–139

4. Conclusions

60 6W 15 W 29 W 44 W

% 4-CP Degraded

45

The synthesized Bi2O3/TiZrO4 photocatalyst showed a high efficiency for the sonophotocatalytic degradation of 4-CP in the presence of visible light. The solution pH did not influence the rate of the degradation of 4-CP under sonophotocatalysis, whereas both sonochemical and photocatalysis processes depended on the initial pH of the system. The efficiency of mineralization of 4-CP was higher during sonophotocatalysis than that observed under sonolysis alone. Enhanced degradation as well as enhanced mineralization of 4-CP were achieved using higher ultrasound power of the sonicator. The efficient degradation of 4-CP observed in this study suggests that the sonophotocatalytic process is a potentially useful method for the removal of organic pollutants in aqueous environment.

30

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Acknowledgments 0

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Fig. 6. Effect of ultrasonic power on the sonophotocatalytic degradation of 4-CP. Experimental conditions: [4-CP] = 1.25  104 M, pH = 5, 20 kHz horn diameter = 19 mm.

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30 TOC removal (%)

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Fig. 7. TOC removal (%) of 4-CP by using different ultrasonic powers on the sonophotocatalytic degradation of 4-CP. Experimental conditions: [4-CP] = 1.25  104 M, pH = 5, time = 1 h, 20 kHz horn diameter = 19 mm.

Generally, increasing the power (amplitude) delivered by the sonicator increases the OH radical concentration in the system as measured by the H2O2 yield [22]. Thus, the effect of power on the oxidation of 4-CP was performed in the range 6–44 W. The results are summarized in Fig. 6. It can be seen that the percentage of 4-CP degradation gradually increased with an increase in the acoustic power. The maximum power efficiency was reached at 29 W and further increase of power did not influence the degradation efficiency notably as shown in Fig 6. The % mineralization of 4-CP (TOC) using different powers is shown in Fig. 7. The % mineralization is also consistent with the % degradation of 4-CP, i.e., 29 W is the optimal power for the degradation of 4-CP. For the sonophotocatalytic degradation, % TOC removal was found to be 3 times higher than that observed when using the sonochemical process alone at 29 W (Fig. 7).

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