Ultrasonics Sonochemistry 27 (2015) 474–479
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
MnO2/CeO2 for catalytic ultrasonic decolorization of methyl orange: Process parameters and mechanisms He Zhao, Guangming Zhang ⇑, Shan Chong, Nan Zhang, Yucai Liu School of Environment & Natural Resource, Renmin University of China, 59 Zhongguancun Street, Beijing 100872, China
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Article history: Received 26 December 2014 Received in revised form 14 June 2015 Accepted 14 June 2015 Available online 20 June 2015 Keywords: MnO2/CeO2 Catalytic ultrasonic process Methyl orange Manganese ion Hydroxyl free radical
a b s t r a c t MnO2/CeO2 catalyst was prepared and characterized by means of Brunauer–Emmet–Teller (BET) method, X-ray diffraction (XRD) and scanning electron microscope (SEM). The characterization showed that MnO2/CeO2 had big specific surface area and MnO2 was dispersed homogeneously on the surface of CeO2. Excellent degradation efficiency of methyl orange was achieved by MnO2/CeO2 catalytic ultrasonic process. Operating parameters were studied and optimized. The optimal conditions were 10 min of ultrasonic irradiation, 1.0 g/L of catalyst dose, 2.6 of pH value and 1.3 W/ml of ultrasonic density. Under the optimal conditions, nearly 90% of methyl orange was removed. The mechanism of methyl orange degradation was further studied. The decolorization mechanism in the ultrasound-MnO2/CeO2 system was quite different with that in the ultrasound-MnO2 system. Effects of manganese and cerium in catalytic ultrasonic process were clarified. Manganese ions in solution contributed to generating hydroxyl free radical. MnO2/CeO2 catalyst strengthened the oxidation ability of ultrasound and realized complete decolorization of methyl orange. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The discharge of dye wastewater has brought many challenges to water environment protection. Color of dye wastewater causes aesthetic problems and organic pollutants are harmful to aquatic organisms and humans. Azo dyes are the main organic pollutants in dye wastewater, and they have one or more azo groups ðR1 —N N—R2 Þ. These complex aromatic-conjugated structures give azo dyes intense color and resistance to decolorization in normal conditions [1,2]. Advanced oxidation processes (AOPs) have been found as promising technologies to remove persistent pollutants from contaminated water. AOPs are based on physico-chemical processes that enable to oxidize and mineralize pollutants into CO2, N2, water and mineral acids such as sulfuric and hydrochloric by means of highly oxidizing agents, mainly hydroxyl radicals [3]. Sonochemistry oxidation as one of AOPs has been used in treatment of organic pollutants, and this technique is also effective in decolorization of dyes. Ultrasonic irradiation results in the formation, growth and collapse of cavitation bubbles. Subsequently,
⇑ Corresponding author. E-mail address: [email protected] (G. Zhang). http://dx.doi.org/10.1016/j.ultsonch.2015.06.009 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.
millions of hot spots with extreme conditions (5000 K and 1000 atm) are generated in water, and hydroxyl free radicals and other oxidative species are created [4,5]. Dyes can be decolorized by hot spots effects and hydroxyl free radical oxidation. With addition of catalyst, higher decolorization efficiency could be achieved. It is reported that more than 90% of methyl orange and acid red B are removed by ultrasonic process in presence of TiO2 and MnO2, respectively [6,7]. In our previous work, a novelty catalyst MnO2/CeO2 has been prepared, which showed high catalytic activity in decolorization of methyl orange [13]. However, in the catalytic ultrasonic process, degradation efficiency of pollutants was greatly affected by different operating parameters, including pH value [8,9], catalyst dose [10,11] and ultrasonic density [12]. In addition, active sites, which were mainly composed of metal oxide, were closely related with the degradation of pollutants. Reactions on the catalyst surface might enhance the generation of hydroxyl free radical, leading to higher degradation efficiency. The manganese and cerium in MnO2/CeO2 and their behaviors needed to be investigated to reveal the degradation mechanism of pollutants. In this paper, detailed investigation on operating parameters was conducted. The role of manganese and cerium on catalyst surface was discussed in catalytic ultrasonic process. The aim was to achieve a promising way for the treatment of azo dyes.
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2. Experimental 2.1. Materials Cerous nitrate, manganous nitrate, sodium hydroxide and methyl orange were of analytical grade and used without further purification. Solutions were prepared with water purified by a Millipore Milli Q UV plus system. CeO2 powder was obtained by a precipitation method which used NaOH and Ce(NO3)2 as starting reagents [14,15]. Subsequently, the obtained CeO2 was dipped in manganous nitrate solution for 2 h. Then the mixture was filtered and the solid was calcined for 2 h at 400 °C [13]. Finally, MnO2/CeO2 catalyst was obtained. 2.2. Catalyst characterization The characteristics of MnO2, CeO2 and MnO2/CeO2 were detected by Brunauer–Emmet–Teller (BET) method, X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and scanning electron microscope (SEM). The specific surface area, porosity of and average pore size of samples were measured by BET. The instrument used was 3H-2000PS2 (BeiShiDe Instrument-S&T Company). The structural features and mineralogy of samples were analyzed by XRD on the Bruker D8 Advance X-ray diffractometer at 2h from 10° to 90°. Surface morphology of samples was investigated using a Hitachi S 4700 SEM analyzer at different scales and magnifications. 2.3. Experimental runs Ultrasonic irradiation was performed with a JY92-II ultrasonic generator (Ningbo Xinzhi Technology Co., China) with an operating power of 650 W and frequency of 24 kHz. All experiments were conducted in 150 ml glass beakers. 100 ml of 20 mg/L methyl orange solution was tested each time. Methyl orange was detected at wavelength of 456 nm by using a Pu Xi TU-1900 UV–vis spectrophotometer. Concentration of manganese and cerium in solution at different reaction time was measured by a Hitachi Z-2000 atomic adsorption spectrometer.
Fig. 1. XRD pattern of MnO2, CeO2 and MnO2/CeO2 (a) MnO2; (b) CeO2; (c) MnO2/ CeO2.
energy dispersive spectroscopy (EDS) analysis. So peaks of MnO2 were not observed on the XRD pattern of MnO2/CeO2 catalyst. In addition, it might be difficult to detect diffraction peaks due to the homogeneous disperse of MnO2. As observed, the peaks intensity of MnO2/CeO2 was lower than that of CeO2, which was mainly due to the introduction of MnO2, and the formation of disordered structure and smaller particles [17]. The morphology of catalyst was analyzed by SEM and results are shown in Fig. 2(a). The surface of fresh MnO2/CeO2 was rough and irregular. Some attached particles were observed on the surface of MnO2/CeO2, which might be the implemented MnO2. The SEM image clearly showed that MnO2 was highly dispersed on the catalyst. After the catalyst used, small cracks could be observed on the catalyst surface in Fig. 2(b). Due to the micro-jets and shock waves generated by ultrasonic irradiation, the original morphology of fresh catalyst was destroyed in the catalytic ultrasonic process. In addition, the reaction occurred on the catalyst surface also affected the morphology of MnO2/CeO2 catalyst.
3. Results and discussion 3.1. Characterization of MnO2/CeO2 catalyst
3.2. Catalyst activity in ultrasonic decolorization of methyl orange
The BET specific surface area, average pore size and total pore volume of MnO2/CeO2, MnO2 and CeO2 are listed in Table 1. MnO2 had the largest specific surface area, followed by MnO2/CeO2, and then CeO2. Moreover, the pore size and total pore volume of MnO2/CeO2 were quite different with those of MnO2 or CeO2, illustrating the destruction of original channels. The XRD pattern of samples is shown in Fig. 1. Generally, the XRD pattern of MnO2/CeO2 catalyst was the same as that of CeO2 sample, while the diffraction peaks of MnO2 were not detected. It was reported that diffraction peaks of MnO2 were hardly detected when the content of Ce was more than 40% [16]. In this catalyst, the mass percent of Mn and Ce was 27% and 59% according to
CeO2, MnO2 and MnO2/CeO2 were used as catalyst in decolorization of methyl orange under ultrasonic irradiation. The experiment conditions were as following: 2.8 of initial pH value, 1.0 g/L of catalyst dose and 1.3 W/ml of ultrasonic density. Results are shown in Fig. 3. Without any catalyst, less than 14% of methyl orange could be removed with 60 min of ultrasonic irradiation. The addition of CeO2 catalyst slightly improved the methyl orange removal. The removal efficiency with MnO2 was much higher than with CeO2. So, for a catalyst composed by MnO2 and CeO2, MnO2 was chosen as the catalytically active component and CeO2 was chosen as the support. The composited MnO2/CeO2 catalyst showed excellent removal efficiency of methyl orange. Compared with decolorization efficiency of 24.5% and 87.2% with CeO2 and MnO2, complete decolorization of methyl orange could be achieved by MnO2/CeO2 catalytic ultrasonic process. Clearly, the catalytic ultrasonic mechanism with MnO2/CeO2 was different with that of MnO2 or CeO2. It was reported that cerium oxide could improve the catalyst redox properties of the composite oxide when associated with transition metal oxides [18]. So, the catalytic activity of MnO2/CeO2 was much higher, realizing complete decolorization of methyl orange.
Table 1 Specific surface area, average pore size and total pore volume of catalysts.
MnO2 CeO2 MnO2/CeO2
Specific surface area (m2/g)
Average pore size (nm)
Total pore volume (ml/g)
110 32 71
5.32 16.34 7.43
0.11 0.32 0.13
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As shown in Fig. 3, the decolorization efficiency of methyl orange increased rapidly in the early stage and reached 85% after 10 min, and then increased slower. Since the ultrasonic energy consumption was in proportional to the reaction time, the optimal ultrasonic irradiation time was selected as 10 min.
3.3. Effects of catalyst dose With 10 min of ultrasonic time, 1.3 W/ml of ultrasonic density, 2.8 of pH value, the influence of MnO2/CeO2 catalyst dose on decolorization of methyl orange was investigated in the range of 0–2.0 g/L. Results are shown in Fig. 4. When the catalyst dose was lower than 1.0 g/L, decolorization efficiency of methyl orange was significantly improved with catalyst dose increase due to more active sites were available in the solution. Furthermore, it was reported that the presence of catalyst particles would increase the cavitation activity of ultrasonic irradiation, and then generate more hydroxyl free radical [19]. Hydroxyl free radical might turn into hydrogen peroxide, which has much lower redox potential (Reactions (1)–(4)) [20,21]. With higher catalyst dose, more active sites could convert hydrogen peroxide into hydroxyl free radical (Reactions (5)) [22]. Then the concentration of hydroxyl free radical could be maintained at a high level. As a result, excellent decolorization efficiency was realized with more catalysts in catalytic ultrasonic process.
H2 O ! OH þ H
ð1Þ
H þ O2 ! HO2
ð2Þ
2 OH ! H2 O2
ð3Þ
HO2 þ HO2 ! H2 O2 þ O2
ð4Þ
Mn3þ þ H2 O2 þ 2Hþ ! Mn4þ þ OH þ H2 O
ð5Þ
However, when catalyst dose increased from 1.0 to 2.0 g/L, the decolorization efficiency was not further improved. This might because that with too many catalyst particles, the energy provided by ultrasonic irradiation was scattered and could not reach the surface of catalyst, leading to weaker cavitation effects [23]. Therefore, less hydroxyl free radical was generated in catalytic ultrasonic process. In general, the optimal dose of MnO2/CeO2 was chosen as 1.0 g/L. 3.4. Effects of solution pH
Fig. 2. SEM image of MnO2/CeO2 catalyst, (a) fresh catalyst; (b) used catalyst.
Fig. 3. MnO2, CeO2 and MnO2/CeO2 activities in ultrasonic removing methyl orange.
The pH value of solution is a key parameter for ultrasonic decolorization process, since the surface charge of catalyst and the existence form of methyl orange were strongly influenced by pH values. Initial screening showed that the decolorization of methyl
Fig. 4. Effects of catalyst dose on degradation of methyl orange.
H. Zhao et al. / Ultrasonics Sonochemistry 27 (2015) 474–479
orange was effective only in acidic conditions. But too high acidity brought greater corrosion to equipment. Therefore, pH value ranged from 2.0 to 6.0 was tested. The experiment was conducted with 1.0 g/L of MnO2/CeO2 catalyst dose, 1.3 W/ml of ultrasonic density and 10 min of ultrasonic time. The results are shown in Fig. 5. Clearly, lower pH benefited methyl orange removal. As the pH value increased above 4, the methyl orange decolorization efficiency became much lower. This phenomenon could be explained by following two reasons. Firstly, pH value has strong effects on the existence forms of methyl orange; it was in quinoid structure in acidic condition and transformed gradually to azo structure as the pH value increased (Eq. (6)) [24]. The methyl orange in quinoid structure was easier to be decolorized compared to that in azo structure. So, the decolorization efficiency of methyl orange was higher in acidic condition.
S
H3C N
N N
H3C
N S
O-
H+ OH-
H3C
N+ H3C
Secondly, the surface charge of catalyst was also influenced by pH value. The point of zero charge (pZc) of MnO2 was about 3.0 [7]. When pH was near 3.0, methyl orange was in the form of neutrally charged (Eq. (6)) and had strong electrostatic attraction with MnO2 on the surface of catalyst. Therefore, methyl orange was easier to contact with active sites, and then was effectively decolorized. When the solution pH was up to 4.0, methyl orange and the catalyst were negatively charged. So they were mutually exclusive. Methyl orange could not contact with active sites, thus the decolorization efficiency decreased significantly. In consideration of these two reasons, the optimal pH value of MnO2/CeO2 catalytic ultrasonic decolorization of methyl orange was chosen as 2.6. 3.5. Effects of ultrasonic densities The effects of ultrasonic density on decolorization of methyl orange were investigated in the range from 0 to 2.6 W/ml at 2.6 of pH, 1.0 g/L of catalyst dose and 10 min of ultrasonic time. Results are shown in Fig. 6. As observed, with the ultrasonic density increasing, decolorization efficiency increased up to 87% and
Fig. 5. Effects of solution pH value on degradation of methyl orange.
477
then decreased slightly, which was similar to previous reports [25–27]. The highest decolorization efficiency was at 1.3 W/ml of ultrasonic density, which was chosen as the optimal condition. With higher ultrasonic density, more cavitation bubbles were generated, leading to stronger cavitation effects [28–30]. Then higher collapse temperature was achieved and more hydroxyl free radical was generated in catalytic ultrasonic process. In addition, more micro-jets generated by ultrasonic irradiation could improve the mass transfer of reactants and pollutants, and clean and sweep the active sites on the surface of MnO2/CeO2 catalyst [31]. However, when the ultrasonic density was above the optimal condition, cavitation bubbles grew to a bigger maximum radius during the rarefaction cycle and were unable to undergo complete collapse in the compression phase. These cavitation bubbles were not transient, and continued to either oscillate, or grow large enough to escape from the liquid [26]. So with the ultrasonic den-
S N NH
N O-
ð6Þ
S
sity above optimal condition, insufficient collapse of cavitation bubbles reduced the quantity of hydroxyl free radical, leading to lower decolorization efficiency. 3.6. Reusability of MnO2/CeO2 catalyst Reusability of MnO2/CeO2 catalyst was examined in five consecutive runs at optimal operating parameters: 10 min of ultrasonic irradiation time, 1.0 g/L of catalyst dose, 2.6 of pH value and 1.2 W/ml of ultrasonic density. After each run, the catalyst was filtered, washed thoroughly and dried at 100 °C. Then the catalyst was reused with a fresh methyl orange solution. The results are summarized in Fig. 7. Obviously, after five runs, the decolorization efficiency of methyl orange was still very high. Clearly, the catalyst, MnO2/CeO2, showed good reusability in catalytic ultrasonic process. Considering the dissolve of manganese ions, longer ultrasonic irradiation time was conducted. At the initial stage, the concentration of manganese ions in solution increased rapidly. With longer reaction time, the dissolve rate of manganese ions became slower. The amount of MnO2 on the catalyst will decreased slightly. The
Fig. 6. Effects of ultrasonic density on degradation of methyl orange.
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Fig. 9. Schematic diagram of reaction mechanism in catalytic ultrasonic process. Fig. 7. Reusability of MnO2/CeO2 catalyst.
dissolve rate of manganese ions was much slower with even longer time and finally became stable. 4. Potential mechanisms in catalytic ultrasonic process During catalytic ultrasonic process, manganese ions in the solution were detected. The results are shown in Fig. 8. In catalytic ultrasonic process, the concentration of manganese ions with methyl orange was three times higher than that of without methyl orange. Manganese ions concentration persistently increased and higher manganese ions concentration in treatment of methyl orange might be related with the dye decolorization. Dissolved cerium was not detected, indicating that reaction with cerium mainly occurred on the catalyst surface. In the MnO2/CeO2 catalytic ultrasonic process, the MnO2 on the catalyst could react with hydrogen peroxide generated by ultrasonic irradiation under acid condition (Reaction (7)), then dissolved in the solution. Subsequently, the generated Mn(II) might react with Mn(IV) to generate the Mn(III) (Reaction (8)) [32,33] which acted as an important role in the MnO2/CeO2 catalytic ultrasonic process. The generated Mn(III) could react with hydrogen peroxide to promote the formation of hydroxyl free radical (Reaction (5)). In addition, due to the lower standard oxidation
reduction potential of Mn(III) than that of Ce(IV), the Reaction (9) might be occurred in the MnO2/CeO2 catalytic ultrasonic process, leading to the formation of Ce(III). With the presence of Ce(III) on the catalyst surface, the Ce(III)/Ce(IV) redox couple could discompose the H2O2 into OH (Reaction (10)) [34]. Therefore, high quantity of hydroxyl free radical was generated, leading to excellent decolorization efficiency of methyl orange. A schematic diagram of the reaction mechanism in catalytic ultrasonic process was proposed and shown in Fig. 9. As observed, the catalytic ultrasonic reactions were mainly occurred on the catalyst surface. Some of manganese ions on the catalyst were dissolved into the solution and might react with hydrogen peroxide. After the catalyst used, the valence states of manganese and cerium were almost same with that of the fresh one, resulting in the good reusability of MnO2/CeO2 catalyst.
MnO2 þ H2 O2 þ 2Hþ ! Mn2þ þ 2H2 O þ O2
ð7Þ
Mn2þ þ Mn4þ $ 2Mn3þ
ð8Þ
Mn3þ þ Ce4þ ! Mn4þ þ Ce3þ
ð9Þ
Ce3þ þ H2 O2 þ Hþ ! Ce4þ þ OH þ H2 O
ð10Þ
5. Conclusions The catalyst of MnO2/CeO2 had a specific surface area of 71 m2/g, which was less than MnO2 and more than CeO2. MnO2 was well-distributed on the surface of CeO2. Compared to MnO2 and CeO2 as catalyst, the catalytic ultrasonic process with MnO2/CeO2 could realize complete decolorization of methyl orange. With the optimal parameters at 1.0 g/L of catalyst dose, 2.6 of pH value, and 1.3 W/ml of ultrasonic density, nearly 90% of methyl orange was decolorized after 10 min. Higher concentration of manganese ions was detected in the treatment of methyl orange, which contributed to the generation of hydroxyl free radical in solution. The catalytic ultrasonic process with MnO2/CeO2 as catalyst realized effective decolorization of methyl orange, and showed great potentials in treatment of azo dyes. Acknowledgments
Fig. 8. Mn leaching during catalytic ultrasonic process with MnO2/CeO2 catalyst.
Authors are grateful for financial support from National Natural Science Foundation of China (51278489) and Fundamental
H. Zhao et al. / Ultrasonics Sonochemistry 27 (2015) 474–479
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
MnO2/CeO2 for catalytic ultrasonic decolorization of methyl orange: Process parameters and mechanisms He Zhao, Guangming Zhang ⇑, Shan Chong, Nan Zhang, Yucai Liu School of Environment & Natural Resource, Renmin University of China, 59 Zhongguancun Street, Beijing 100872, China
a r t i c l e
i n f o
Article history: Received 26 December 2014 Received in revised form 14 June 2015 Accepted 14 June 2015 Available online 20 June 2015 Keywords: MnO2/CeO2 Catalytic ultrasonic process Methyl orange Manganese ion Hydroxyl free radical
a b s t r a c t MnO2/CeO2 catalyst was prepared and characterized by means of Brunauer–Emmet–Teller (BET) method, X-ray diffraction (XRD) and scanning electron microscope (SEM). The characterization showed that MnO2/CeO2 had big specific surface area and MnO2 was dispersed homogeneously on the surface of CeO2. Excellent degradation efficiency of methyl orange was achieved by MnO2/CeO2 catalytic ultrasonic process. Operating parameters were studied and optimized. The optimal conditions were 10 min of ultrasonic irradiation, 1.0 g/L of catalyst dose, 2.6 of pH value and 1.3 W/ml of ultrasonic density. Under the optimal conditions, nearly 90% of methyl orange was removed. The mechanism of methyl orange degradation was further studied. The decolorization mechanism in the ultrasound-MnO2/CeO2 system was quite different with that in the ultrasound-MnO2 system. Effects of manganese and cerium in catalytic ultrasonic process were clarified. Manganese ions in solution contributed to generating hydroxyl free radical. MnO2/CeO2 catalyst strengthened the oxidation ability of ultrasound and realized complete decolorization of methyl orange. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction The discharge of dye wastewater has brought many challenges to water environment protection. Color of dye wastewater causes aesthetic problems and organic pollutants are harmful to aquatic organisms and humans. Azo dyes are the main organic pollutants in dye wastewater, and they have one or more azo groups ðR1 —N N—R2 Þ. These complex aromatic-conjugated structures give azo dyes intense color and resistance to decolorization in normal conditions [1,2]. Advanced oxidation processes (AOPs) have been found as promising technologies to remove persistent pollutants from contaminated water. AOPs are based on physico-chemical processes that enable to oxidize and mineralize pollutants into CO2, N2, water and mineral acids such as sulfuric and hydrochloric by means of highly oxidizing agents, mainly hydroxyl radicals [3]. Sonochemistry oxidation as one of AOPs has been used in treatment of organic pollutants, and this technique is also effective in decolorization of dyes. Ultrasonic irradiation results in the formation, growth and collapse of cavitation bubbles. Subsequently,
⇑ Corresponding author. E-mail address: [email protected] (G. Zhang). http://dx.doi.org/10.1016/j.ultsonch.2015.06.009 1350-4177/Ó 2015 Elsevier B.V. All rights reserved.
millions of hot spots with extreme conditions (5000 K and 1000 atm) are generated in water, and hydroxyl free radicals and other oxidative species are created [4,5]. Dyes can be decolorized by hot spots effects and hydroxyl free radical oxidation. With addition of catalyst, higher decolorization efficiency could be achieved. It is reported that more than 90% of methyl orange and acid red B are removed by ultrasonic process in presence of TiO2 and MnO2, respectively [6,7]. In our previous work, a novelty catalyst MnO2/CeO2 has been prepared, which showed high catalytic activity in decolorization of methyl orange [13]. However, in the catalytic ultrasonic process, degradation efficiency of pollutants was greatly affected by different operating parameters, including pH value [8,9], catalyst dose [10,11] and ultrasonic density [12]. In addition, active sites, which were mainly composed of metal oxide, were closely related with the degradation of pollutants. Reactions on the catalyst surface might enhance the generation of hydroxyl free radical, leading to higher degradation efficiency. The manganese and cerium in MnO2/CeO2 and their behaviors needed to be investigated to reveal the degradation mechanism of pollutants. In this paper, detailed investigation on operating parameters was conducted. The role of manganese and cerium on catalyst surface was discussed in catalytic ultrasonic process. The aim was to achieve a promising way for the treatment of azo dyes.
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2. Experimental 2.1. Materials Cerous nitrate, manganous nitrate, sodium hydroxide and methyl orange were of analytical grade and used without further purification. Solutions were prepared with water purified by a Millipore Milli Q UV plus system. CeO2 powder was obtained by a precipitation method which used NaOH and Ce(NO3)2 as starting reagents [14,15]. Subsequently, the obtained CeO2 was dipped in manganous nitrate solution for 2 h. Then the mixture was filtered and the solid was calcined for 2 h at 400 °C [13]. Finally, MnO2/CeO2 catalyst was obtained. 2.2. Catalyst characterization The characteristics of MnO2, CeO2 and MnO2/CeO2 were detected by Brunauer–Emmet–Teller (BET) method, X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and scanning electron microscope (SEM). The specific surface area, porosity of and average pore size of samples were measured by BET. The instrument used was 3H-2000PS2 (BeiShiDe Instrument-S&T Company). The structural features and mineralogy of samples were analyzed by XRD on the Bruker D8 Advance X-ray diffractometer at 2h from 10° to 90°. Surface morphology of samples was investigated using a Hitachi S 4700 SEM analyzer at different scales and magnifications. 2.3. Experimental runs Ultrasonic irradiation was performed with a JY92-II ultrasonic generator (Ningbo Xinzhi Technology Co., China) with an operating power of 650 W and frequency of 24 kHz. All experiments were conducted in 150 ml glass beakers. 100 ml of 20 mg/L methyl orange solution was tested each time. Methyl orange was detected at wavelength of 456 nm by using a Pu Xi TU-1900 UV–vis spectrophotometer. Concentration of manganese and cerium in solution at different reaction time was measured by a Hitachi Z-2000 atomic adsorption spectrometer.
Fig. 1. XRD pattern of MnO2, CeO2 and MnO2/CeO2 (a) MnO2; (b) CeO2; (c) MnO2/ CeO2.
energy dispersive spectroscopy (EDS) analysis. So peaks of MnO2 were not observed on the XRD pattern of MnO2/CeO2 catalyst. In addition, it might be difficult to detect diffraction peaks due to the homogeneous disperse of MnO2. As observed, the peaks intensity of MnO2/CeO2 was lower than that of CeO2, which was mainly due to the introduction of MnO2, and the formation of disordered structure and smaller particles [17]. The morphology of catalyst was analyzed by SEM and results are shown in Fig. 2(a). The surface of fresh MnO2/CeO2 was rough and irregular. Some attached particles were observed on the surface of MnO2/CeO2, which might be the implemented MnO2. The SEM image clearly showed that MnO2 was highly dispersed on the catalyst. After the catalyst used, small cracks could be observed on the catalyst surface in Fig. 2(b). Due to the micro-jets and shock waves generated by ultrasonic irradiation, the original morphology of fresh catalyst was destroyed in the catalytic ultrasonic process. In addition, the reaction occurred on the catalyst surface also affected the morphology of MnO2/CeO2 catalyst.
3. Results and discussion 3.1. Characterization of MnO2/CeO2 catalyst
3.2. Catalyst activity in ultrasonic decolorization of methyl orange
The BET specific surface area, average pore size and total pore volume of MnO2/CeO2, MnO2 and CeO2 are listed in Table 1. MnO2 had the largest specific surface area, followed by MnO2/CeO2, and then CeO2. Moreover, the pore size and total pore volume of MnO2/CeO2 were quite different with those of MnO2 or CeO2, illustrating the destruction of original channels. The XRD pattern of samples is shown in Fig. 1. Generally, the XRD pattern of MnO2/CeO2 catalyst was the same as that of CeO2 sample, while the diffraction peaks of MnO2 were not detected. It was reported that diffraction peaks of MnO2 were hardly detected when the content of Ce was more than 40% [16]. In this catalyst, the mass percent of Mn and Ce was 27% and 59% according to
CeO2, MnO2 and MnO2/CeO2 were used as catalyst in decolorization of methyl orange under ultrasonic irradiation. The experiment conditions were as following: 2.8 of initial pH value, 1.0 g/L of catalyst dose and 1.3 W/ml of ultrasonic density. Results are shown in Fig. 3. Without any catalyst, less than 14% of methyl orange could be removed with 60 min of ultrasonic irradiation. The addition of CeO2 catalyst slightly improved the methyl orange removal. The removal efficiency with MnO2 was much higher than with CeO2. So, for a catalyst composed by MnO2 and CeO2, MnO2 was chosen as the catalytically active component and CeO2 was chosen as the support. The composited MnO2/CeO2 catalyst showed excellent removal efficiency of methyl orange. Compared with decolorization efficiency of 24.5% and 87.2% with CeO2 and MnO2, complete decolorization of methyl orange could be achieved by MnO2/CeO2 catalytic ultrasonic process. Clearly, the catalytic ultrasonic mechanism with MnO2/CeO2 was different with that of MnO2 or CeO2. It was reported that cerium oxide could improve the catalyst redox properties of the composite oxide when associated with transition metal oxides [18]. So, the catalytic activity of MnO2/CeO2 was much higher, realizing complete decolorization of methyl orange.
Table 1 Specific surface area, average pore size and total pore volume of catalysts.
MnO2 CeO2 MnO2/CeO2
Specific surface area (m2/g)
Average pore size (nm)
Total pore volume (ml/g)
110 32 71
5.32 16.34 7.43
0.11 0.32 0.13
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As shown in Fig. 3, the decolorization efficiency of methyl orange increased rapidly in the early stage and reached 85% after 10 min, and then increased slower. Since the ultrasonic energy consumption was in proportional to the reaction time, the optimal ultrasonic irradiation time was selected as 10 min.
3.3. Effects of catalyst dose With 10 min of ultrasonic time, 1.3 W/ml of ultrasonic density, 2.8 of pH value, the influence of MnO2/CeO2 catalyst dose on decolorization of methyl orange was investigated in the range of 0–2.0 g/L. Results are shown in Fig. 4. When the catalyst dose was lower than 1.0 g/L, decolorization efficiency of methyl orange was significantly improved with catalyst dose increase due to more active sites were available in the solution. Furthermore, it was reported that the presence of catalyst particles would increase the cavitation activity of ultrasonic irradiation, and then generate more hydroxyl free radical [19]. Hydroxyl free radical might turn into hydrogen peroxide, which has much lower redox potential (Reactions (1)–(4)) [20,21]. With higher catalyst dose, more active sites could convert hydrogen peroxide into hydroxyl free radical (Reactions (5)) [22]. Then the concentration of hydroxyl free radical could be maintained at a high level. As a result, excellent decolorization efficiency was realized with more catalysts in catalytic ultrasonic process.
H2 O ! OH þ H
ð1Þ
H þ O2 ! HO2
ð2Þ
2 OH ! H2 O2
ð3Þ
HO2 þ HO2 ! H2 O2 þ O2
ð4Þ
Mn3þ þ H2 O2 þ 2Hþ ! Mn4þ þ OH þ H2 O
ð5Þ
However, when catalyst dose increased from 1.0 to 2.0 g/L, the decolorization efficiency was not further improved. This might because that with too many catalyst particles, the energy provided by ultrasonic irradiation was scattered and could not reach the surface of catalyst, leading to weaker cavitation effects [23]. Therefore, less hydroxyl free radical was generated in catalytic ultrasonic process. In general, the optimal dose of MnO2/CeO2 was chosen as 1.0 g/L. 3.4. Effects of solution pH
Fig. 2. SEM image of MnO2/CeO2 catalyst, (a) fresh catalyst; (b) used catalyst.
Fig. 3. MnO2, CeO2 and MnO2/CeO2 activities in ultrasonic removing methyl orange.
The pH value of solution is a key parameter for ultrasonic decolorization process, since the surface charge of catalyst and the existence form of methyl orange were strongly influenced by pH values. Initial screening showed that the decolorization of methyl
Fig. 4. Effects of catalyst dose on degradation of methyl orange.
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orange was effective only in acidic conditions. But too high acidity brought greater corrosion to equipment. Therefore, pH value ranged from 2.0 to 6.0 was tested. The experiment was conducted with 1.0 g/L of MnO2/CeO2 catalyst dose, 1.3 W/ml of ultrasonic density and 10 min of ultrasonic time. The results are shown in Fig. 5. Clearly, lower pH benefited methyl orange removal. As the pH value increased above 4, the methyl orange decolorization efficiency became much lower. This phenomenon could be explained by following two reasons. Firstly, pH value has strong effects on the existence forms of methyl orange; it was in quinoid structure in acidic condition and transformed gradually to azo structure as the pH value increased (Eq. (6)) [24]. The methyl orange in quinoid structure was easier to be decolorized compared to that in azo structure. So, the decolorization efficiency of methyl orange was higher in acidic condition.
S
H3C N
N N
H3C
N S
O-
H+ OH-
H3C
N+ H3C
Secondly, the surface charge of catalyst was also influenced by pH value. The point of zero charge (pZc) of MnO2 was about 3.0 [7]. When pH was near 3.0, methyl orange was in the form of neutrally charged (Eq. (6)) and had strong electrostatic attraction with MnO2 on the surface of catalyst. Therefore, methyl orange was easier to contact with active sites, and then was effectively decolorized. When the solution pH was up to 4.0, methyl orange and the catalyst were negatively charged. So they were mutually exclusive. Methyl orange could not contact with active sites, thus the decolorization efficiency decreased significantly. In consideration of these two reasons, the optimal pH value of MnO2/CeO2 catalytic ultrasonic decolorization of methyl orange was chosen as 2.6. 3.5. Effects of ultrasonic densities The effects of ultrasonic density on decolorization of methyl orange were investigated in the range from 0 to 2.6 W/ml at 2.6 of pH, 1.0 g/L of catalyst dose and 10 min of ultrasonic time. Results are shown in Fig. 6. As observed, with the ultrasonic density increasing, decolorization efficiency increased up to 87% and
Fig. 5. Effects of solution pH value on degradation of methyl orange.
477
then decreased slightly, which was similar to previous reports [25–27]. The highest decolorization efficiency was at 1.3 W/ml of ultrasonic density, which was chosen as the optimal condition. With higher ultrasonic density, more cavitation bubbles were generated, leading to stronger cavitation effects [28–30]. Then higher collapse temperature was achieved and more hydroxyl free radical was generated in catalytic ultrasonic process. In addition, more micro-jets generated by ultrasonic irradiation could improve the mass transfer of reactants and pollutants, and clean and sweep the active sites on the surface of MnO2/CeO2 catalyst [31]. However, when the ultrasonic density was above the optimal condition, cavitation bubbles grew to a bigger maximum radius during the rarefaction cycle and were unable to undergo complete collapse in the compression phase. These cavitation bubbles were not transient, and continued to either oscillate, or grow large enough to escape from the liquid [26]. So with the ultrasonic den-
S N NH
N O-
ð6Þ
S
sity above optimal condition, insufficient collapse of cavitation bubbles reduced the quantity of hydroxyl free radical, leading to lower decolorization efficiency. 3.6. Reusability of MnO2/CeO2 catalyst Reusability of MnO2/CeO2 catalyst was examined in five consecutive runs at optimal operating parameters: 10 min of ultrasonic irradiation time, 1.0 g/L of catalyst dose, 2.6 of pH value and 1.2 W/ml of ultrasonic density. After each run, the catalyst was filtered, washed thoroughly and dried at 100 °C. Then the catalyst was reused with a fresh methyl orange solution. The results are summarized in Fig. 7. Obviously, after five runs, the decolorization efficiency of methyl orange was still very high. Clearly, the catalyst, MnO2/CeO2, showed good reusability in catalytic ultrasonic process. Considering the dissolve of manganese ions, longer ultrasonic irradiation time was conducted. At the initial stage, the concentration of manganese ions in solution increased rapidly. With longer reaction time, the dissolve rate of manganese ions became slower. The amount of MnO2 on the catalyst will decreased slightly. The
Fig. 6. Effects of ultrasonic density on degradation of methyl orange.
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Fig. 9. Schematic diagram of reaction mechanism in catalytic ultrasonic process. Fig. 7. Reusability of MnO2/CeO2 catalyst.
dissolve rate of manganese ions was much slower with even longer time and finally became stable. 4. Potential mechanisms in catalytic ultrasonic process During catalytic ultrasonic process, manganese ions in the solution were detected. The results are shown in Fig. 8. In catalytic ultrasonic process, the concentration of manganese ions with methyl orange was three times higher than that of without methyl orange. Manganese ions concentration persistently increased and higher manganese ions concentration in treatment of methyl orange might be related with the dye decolorization. Dissolved cerium was not detected, indicating that reaction with cerium mainly occurred on the catalyst surface. In the MnO2/CeO2 catalytic ultrasonic process, the MnO2 on the catalyst could react with hydrogen peroxide generated by ultrasonic irradiation under acid condition (Reaction (7)), then dissolved in the solution. Subsequently, the generated Mn(II) might react with Mn(IV) to generate the Mn(III) (Reaction (8)) [32,33] which acted as an important role in the MnO2/CeO2 catalytic ultrasonic process. The generated Mn(III) could react with hydrogen peroxide to promote the formation of hydroxyl free radical (Reaction (5)). In addition, due to the lower standard oxidation
reduction potential of Mn(III) than that of Ce(IV), the Reaction (9) might be occurred in the MnO2/CeO2 catalytic ultrasonic process, leading to the formation of Ce(III). With the presence of Ce(III) on the catalyst surface, the Ce(III)/Ce(IV) redox couple could discompose the H2O2 into OH (Reaction (10)) [34]. Therefore, high quantity of hydroxyl free radical was generated, leading to excellent decolorization efficiency of methyl orange. A schematic diagram of the reaction mechanism in catalytic ultrasonic process was proposed and shown in Fig. 9. As observed, the catalytic ultrasonic reactions were mainly occurred on the catalyst surface. Some of manganese ions on the catalyst were dissolved into the solution and might react with hydrogen peroxide. After the catalyst used, the valence states of manganese and cerium were almost same with that of the fresh one, resulting in the good reusability of MnO2/CeO2 catalyst.
MnO2 þ H2 O2 þ 2Hþ ! Mn2þ þ 2H2 O þ O2
ð7Þ
Mn2þ þ Mn4þ $ 2Mn3þ
ð8Þ
Mn3þ þ Ce4þ ! Mn4þ þ Ce3þ
ð9Þ
Ce3þ þ H2 O2 þ Hþ ! Ce4þ þ OH þ H2 O
ð10Þ
5. Conclusions The catalyst of MnO2/CeO2 had a specific surface area of 71 m2/g, which was less than MnO2 and more than CeO2. MnO2 was well-distributed on the surface of CeO2. Compared to MnO2 and CeO2 as catalyst, the catalytic ultrasonic process with MnO2/CeO2 could realize complete decolorization of methyl orange. With the optimal parameters at 1.0 g/L of catalyst dose, 2.6 of pH value, and 1.3 W/ml of ultrasonic density, nearly 90% of methyl orange was decolorized after 10 min. Higher concentration of manganese ions was detected in the treatment of methyl orange, which contributed to the generation of hydroxyl free radical in solution. The catalytic ultrasonic process with MnO2/CeO2 as catalyst realized effective decolorization of methyl orange, and showed great potentials in treatment of azo dyes. Acknowledgments
Fig. 8. Mn leaching during catalytic ultrasonic process with MnO2/CeO2 catalyst.
Authors are grateful for financial support from National Natural Science Foundation of China (51278489) and Fundamental
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