Microwave Enhanced Catalytic Degradation of Methyl Orange in

Article pubs.acs.org/IECR

Microwave Enhanced Catalytic Degradation of Methyl Orange in Aqueous Solution over CuO/CeO2 Catalyst in the Absence and Presence of H2O2 Dongyan Xu,*,† Fei Cheng,† Qizeng Lu,† and Ping Dai‡ †

College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China College of Electromechanical Engineering, Qingdao University of Science and Technology, Qingdao 266061, People’s Republic of China

‡

ABSTRACT: This work focuses on the microwave enhanced catalytic degradation of methyl orange (MO) in aqueous solution over CuO/CeO2 catalyst in the absence and presence of H2O2. The prepared CuO/CeO2 catalysts were characterized with X-ray diffraction, Brunnauer-Emmett-Teller analysis, temperature-programmed reduction, and temperature-programmed desorption techniques to elucidate the effect of calcination temperature on its properties and catalytic performance. The results show that calcination temperature exerts remarkable influence on the catalytic performance of CuO/CeO2, with that calcined at 300 °C displaying the highest MO degradation ability. On the basis of Fourier transform infrared spectroscopy, ultraviolet−visible spectroscopy, and X-ray photoelectron spectroscopy measurement results, the mechanism of MO degradation under microwave irradiation in the presence of both CuO/CeO2 and H2O2 was suggested. A synergistic rather than additive effect of catalyst, microwave irradiation, and H2O2 contributes to the high degradation activity toward MO.

1. INTRODUCTION Azo dyes are the largest class of dyes commercially used in textile industries and have become one of the most serious pollutants in the effluents. To treat these organic contaminants in an effective and economic way, a large number of physical, chemical, and biological methods have been proposed and investigated during the past few decades. Among the possible wastewater treatment alternatives, the advanced oxidation processes (AOPs), which mainly involve the generation of strongly oxidative species like free hydroxyl radicals (•OH) and the subsequent attack of these radicals on the pollutants, are highly efficient methods for rapid oxidation/degradation of azo dyes. The most widely adopted AOPs include photodegradation,1 Fenton,2 photo-Fenton,3 ultrasonication,4 ozonation,5 catalytic H2O2 oxidation,6 and so forth. Although the AOPs can offer good performance for oxidation of refractory organic pollutants, the complexity, high chemical consumption, and relatively high operation costs constitute major barriers for their large-scale applications. Recently, microwave (MW) irradiation technique has received increasing attention as a means of removing the refractory chemicals present in the waste streams, owing to its advantages such as reduced reaction time, increased selectivity of reaction, lower activation energy, improved speed of reaction, reduced equipment size and waste, and ease of control.7 Because the energy of MW is insufficient to disrupt the chemical bonds of organic compounds, MW has to be incorporated with some MW-absorbing materials, i.e., aluminasupported copper oxide,8,9 activated carbon (AC) or ACsupported base metal,10,11 ZrOx/ZnO12 and MnO2 nanoplates,13 and/or AOPs14−16 to improve the degradation efficiency. Until now, several mechanisms were proposed to explain the synergistic effect between MW and MW-absorbing materials or catalysts. Generally, it was accepted that the © 2014 American Chemical Society

combination of MW with MW-absorbing materials can produce great amounts of “hot spots” on the surface of these materials, on which a more rapid oxidation and combustion of organic pollutant molecules could occur, thus leading to high degradation efficiency.7,12 Lai et al.17 suggested that the oxygen ions (O2−, O−, and O2−) are easily donated to participate in the degradation of 4-chlorophenol via the coupling effect between MW and active oxygen species on the surface of high-valence nickel oxide. CuO/CeO2 has been exhaustively studied as an active heterogeneous catalyst for many important reactions, including water-gas shift (WGS) reaction,18 selective CO oxidation,19 and reforming of hydrocarbons.20 The high activity of CuO/CeO2 is believed to be closely related to the redox behavior of CeO2 and its strong interaction between CuO and CeO2. To the best of our knowledge, the catalytic performance of CuO/CeO2 in the MW assisted catalytic degradation of organic pollutants in aqueous solution in the absence or presence of H2O2 has been rarely investigated. In the present work, CuO/CeO2 catalysts were prepared using a coprecipitation method and used to catalyze the degradation of MO in aqueous solution by combining with MW irradiation in the absence and presence of H2O2. The effects of calcination temperature on the properties and catalytic performance of CuO/CeO2 catalysts were mainly investigated. The roles of MW irradiation, CuO/CeO2, and H2O2 in the combined degradation process were also discussed in detail. Received: Revised: Accepted: Published: 2625

October 3, 2013 January 18, 2014 January 22, 2014 January 22, 2014 dx.doi.org/10.1021/ie4033022 | Ind. Eng. Chem. Res. 2014, 53, 2625−2632

Industrial & Engineering Chemistry Research

Article

Figure 1. N2 adsorption/desorption isotherms (A) and BJH pore diameter distributions (B) of CuO/CeO2 catalysts calcined at different temperatures.

2. MATERIALS AND METHODS 2.1. Chemicals. Copper nitrite (99.5 wt %) and methyl orange (99.0 wt %) were purchased from Tianjin Rgent Chemical Company. Cerium nitrite (99.0 wt %) was purchased from Shanghai Shanpu Chemical Company. Aqueous ammonia (25−28%) was purchased from Yantai Sanhe Chemical Company. Hydrogen peroxide (30.0 wt %) was purchased from Tianjin Bodi Chemical Company. All reagents were used without further purification. 2.2. Preparation of CuO/CeO2. A series of CuO/CeO2 catalysts with nominal Cu/Ce atomic ratio of 1 were prepared with conventional coprecipitation method. A mixture of Ce(NO3)3·6H2O and Cu(NO3)2·3H2O with a Cu/Ce molar ratio of 1 was dissolved in 100 mL distilled water to form a homogeneous solution. Ammonia solution was added into the solution containing Cu2+ and Ce2+ under continuous stirring until the pH value of the solution became 10. The formed precipitate was dried overnight at 110 °C. The obtained catalyst precursors were subjected to calcination in a muffle furnace at 200, 300, 400, and 500 °C for 2 h, respectively. The as-prepared catalysts are denoted as CuO/CeO2(x), where x represents the calcination temperature. 2.3. Catalyst Characterization. XRD patterns of the catalyst samples were recorded in a D/max-2500/PC X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation, operated at 40 KV and 30 mA. The surface area, total pore volume, and average pore size of samples were analyzed using a Micromeritics ASAP-2010 automated system. Prior to the measurements, all of the catalyst samples were outgassed at 350 °C for 2 h. TPR experiments were carried out in a quartz microreactor. The calcined catalyst sample (40 mg) was first purged with high purity Ar flow at 350 °C for 0.5 h to remove traces of water, followed by cooling to room temperature. TPR was performed by heating the samples from room temperature to 700 °C at a rate of 10 °C/min, in a 5% H2/Ar mixture (30 mL/min). The water produced during the reduction was removed by passing the outlet gas through a trap cooled by dry ice in ethanol. A thermal conductivity detector (TCD) was used to quantitatively measure H2 consumption. Prior to O2-TPD, 100 mg sample was heated from room temperature to 300 °C at a rate of 10 °C/min, and then purged in He for 30 min. The sample was cooled down to 100 °C and subjected to a pulse of 2% O2/ He mixture (30 mL/min) until the surface was saturated. O2TPD was performed by heating the samples from room temperature to 800 °C at a rate of 10 °C/min in the He flow

(30 mL/min). XPS experiments were carried out with an ESCALAB 250 spectrometer. The spectra were recorded using monochromatic Al Kα radiation (hv = 1486.6 eV) as the excitation source. Photoelectrons were selected in energy with a hemispheric electron analyzer. All of the binding energy (BE) values were referenced to the C 1s peak of carbon at 284.6 eV. FTIR was collected from 4000 to 400 cm−1 on a Bruker Tensor 27 IR spectrometer. Two milligrams of each solid sample were mixed with 200 mg of vacuum-dried IR-grade KBr. 2.4. Batch Microwave-Enhanced Degradation Process. The MW assisted degradation experiments were carried out in a modified microwave apparatus (Meide Household Electrical Appliances Group Co., Ltd., China) with a frequency of 2450 MHz and maximum output power of 700 W. All of the experiments were conducted in batch mode. In a typical reaction, MO solution (25 mL, 50 mg/L), CuO/CeO2 catalyst (0.2 g, 8 g/L), and 33% aqueous H2O2 (0.6 mL) were placed in a quartz beaker, which was then put into the microwave oven and subjected to MW irradiation at varied power levels for different times. For comparison, the experiments in the absence of catalyst or H2O2 were also conducted. However, the catalyst dose of 2 g/L was used in these contrast experiments. The initial pH of MO solution is in the range of 6−7. 2.5. Reaction Solution Analysis. After degradation experiment, the treated MO solution was centrifuged for 5 min at 2000 rpm and then filtered to remove the catalyst powders. The MO concentration (C) in solution can be determined by measuring the absorbance (A) at 462 nm using a TU 1810 spectrophotometer. A calibration curve was obtained by using the standard MO solutions with known concentrations (A = 0.0024 + 0.074 58C, R = 0.999 91). The calibration curve of standard MO solutions was used to estimate the percentage of degradation efficiency = [(C0 − C)/C0] × 100%, where C0 and C are the initial and instant (at reaction time t) concentrations of MO, respectively. UV−vis spectra of MO solutions treated with different methods were also recorded using a Varian Cary 500 UV−vis spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. Effects of Calcination Temperature on Textual, Structural and Redox Properties of CuO/CeO2. Figure 1 shows the nitrogen adsorption/desorption profiles (A) and the pore size distributions (B) of the CuO/CeO2 samples calcined at different temperatures. As shown in Figure 1, the catalysts exhibit typical isotherms of mesoporous structure, with uniform 2626

dx.doi.org/10.1021/ie4033022 | Ind. Eng. Chem. Res. 2014, 53, 2625−2632


Article

about 430 and 570 °C, belonging to surface and bulk oxygen species, respectively.21 Whereas the reduction profile of pure CuO is characterized by a single peak at 293 °C.19 As can be seen in Figure 3, two overlapping peaks, denoted as peak I

mesopores ranging from 3.7 to 4.3 nm for these samples. The textural data of all the catalysts are listed in Table 1. It can be Table 1. Physico-Chemical Properties of Different CuO/ CeO2 Catalysts crystal size (nm) sample

CeO2

CuO

specific surface area (m2/g)

CuO/ CeO2(200) CuO/ CeO2(300) CuO/ CeO2(400) CuO/ CeO2(500)

11.5

24.9

179

0.18

4.0

10.5

22.8

205

0.21

4.1

11.1

25.3

168

0.17

4.9

13.1

26.5

142

0.17

4.9

pore volume (cm3/g)

average pore size (nm)

found that the specific surface area increases first with increasing calcination temperature from 200 to 300 °C and then decreases with further increase of calcination temperatures up to 500 °C. Figure 2 shows the XRD patterns of CuO/CeO2 catalysts calcined at different temperatures. As can be noted, all of the

Figure 3. H2-TPR profiles of CuO/CeO2 catalysts calcined at different temperatures.

(low-temperature) and peak II (high-temperature), respectively, are present for all of the CuO/CeO2 samples. As the calcination temperature increases from 200 to 400 °C, these two peaks markedly shift to lower temperature, with peak I and peak II located at 188 and 220 °C, respectively, for CuO/ CeO2(400). Many studies on CuO/CeO2 have confirmed that at least two peaks are present in the TPR profiles although the peak shapes and positions may be different due to the discrepancy in both experimental conditions and preparation methods of the samples. It is generally recognized that the lowtemperature peak in the range of 120−170 °C is ascribed to the highly dispersed CuO strongly interacting with ceria, while the high-temperature peak above 200 °C is attributed to fairly large clusters of CuO.22−24 As pointed out by Martinez-Arias et al., CeO2 can enhance the reducibility of finely dispersed CuO clusters, leading to a much lower reduction temperature.25 Therefore, the increase of calcination temperature enhances the interaction between CuO and CeO2, which promotes the reduction of CuO and reduces the reduction temperature. For CuO/CeO 2 (500), the temperature of peak I remains unchanged, while that of peak II changes to higher value possibly because of a further growth of large CuO clusters under high calcination temperature. As to the reduction of CeO2, it is still under debate. Some authors suggested that the high-temperature peak is attributed to simultaneous reduction of CeO2 and bulk CuO.26,27 However, Djinović et al.18 proposed that the peaks in the low-temperature region (around 100 °C) belong to the reduction of finely dispersed CuO species strongly interacting with the CeO2 support and partial CeO2 at the Cu−CeO2 interface. Recently, Ciston et al.28 took an in situ XRD measurement of 5% CuO/CeO2 during reduction and confirmed a partial reduction of ceria (Ce4+ → Ce3+) that produces an abrupt expansion of the lattice at the same time as the reduction of CuO. The states and quantities of oxygen species in CuO/CeO2 catalyst could be detected by the O2-TPD measurement (see Figure 4). All of the profiles clearly display a desorption peak below 500 °C, with another one commencing above 750 °C. In general, the low-temperature peak is related to surface chemisorbed oxygen, while the high-temperature peak is ascribed to lattice oxygen.29 It is obvious that the increase of

Figure 2. XRD patters of CuO/CeO2 catalysts calcined at different temperatures.

examples show diffraction peaks at 28.7, 32.9, 47.5, and 56.0° that could be ascribed to (111), (200), (220), and (311) planes of CeO2 in a face-centered cubic fluorite structure (JCPDS, No. 43-1002), respectively. These peaks exhibit broad and diffuse characteristics, indicating a small crystallite and/or a very disordered structure. In addition to CeO2 phase, other peaks at 2θ values of 35.5, 38.8, 48.7, 58.4, 61.6, 66.0, and 68.1° are well associated with (1̅11), (111), (2̅02), (020), (1̅13), (3̅11), and (113) planes of CuO in a monoclinic crystal structure (JCPDS, No. 48-1548), respectively. For CuO/CeO2(200), the presence of two additional peaks ascribed to copper(II) hydroxynitrate (Cu2(OH)3NO3) suggests incomplete decomposition of copper precursor. By applying the Scherrer equation to the principal characteristic CeO2 (111) and CuO (11̅ 1) peaks, the average crystallite sizes of CeO2 and CuO were calculated and listed in Table 1. It can be found that the increase of calcination temperature from 300 to 500 °C caused a slight increase in the particle sizes of CuO and CeO2. Temperature-programmed reduction has been used extensively to characterize the reducibility of CuO/CeO2 catalyst. It was reported that pure CeO2 exhibits two reduction peaks at 2627



Article

Figure 6. The UV−vis absorption spectrogram of MO solution treated with different methods. MO = 50 mg/L, catalyst = 2 g/L, H2O2 = 0.02 mol/L, MW power = 380 W, and time = 7 min.

Figure 4. O2-TPD profiles of CuO/CeO2 catalysts calcined at different temperatures.

calcination temperature can induce the reduction in the amount of surface chemisorbed oxygen of CuO/CeO2 catalysts. 3.2. Effect of Calcination Temperature on the Performance of CuO/CeO2 Catalyst. MW enhanced catalytic degradation of MO in the absence of H2O2 was performed to investigate the correlation between calcination temperature and catalytic activity of the catalysts. The results demonstrate that the calcination temperature exerts remarkable influence on the catalytic performance of CuO/CeO2, with the CuO/CeO2(300) catalyst displaying the highest degradation efficiency of MO (Figure 5). For this MW/catalyst system, it is

The original MO aqueous solution gives two absorption peaks at 478 and 276 nm, corresponding to the conjugated structure linked by azo band and aromatic rings in MO molecule, respectively. It can be found that almost no MO degradation occurred when only MW irradiation (microwave power of 380 W, irradiation time of 7 min) was applied because the energy of the MW at frequency 2.45 GHz is not enough to destroy any bonds of MO (not shown in Figure 6). The absorption peak declines a little bit when MW irradiation coupled with H2O2 (0.02 mol/L) was employed. This indicates that the generation of hydroxyl radicals cannot be effectively induced by MW irradiation alone, therefore displaying slow degradation kinetics. Compared with H2O2, the combination of MW irradiation with CuO/CeO2 (2 g/L) can provide a better degradation of MO under the same conditions. The degradation efficiency can be drastically improved when the MO solution was heated by water bath in the presence of both H2O2 and CuO/CeO2. For comparison, the reaction system was maintained at 80 °C by water bath, which is close to the temperature of the MO solution treated with MW irradiation. This finding demonstrates that hydroxyl radicals can efficiently generate in the presence of heterogeneous CuO/CeO2 catalyst, therefore improving the catalytic degradation of MO. Furthermore, these two characteristic peaks of MO almost totally disappeared after treatment by combining MW irradiation with CuO/CeO2 and H2O2, implying a nearly complete degradation of MO. Therefore, the superior performance of MO degradation is ascribed to a synergistic rather than an additive effect of MW irradiation, catalyst, and H2O2. 3.4. Effect of Hydrogen Peroxide Concentration on MW Assisted Degradation of MO. Figure 7 displays the effect of H2O2 concentration on MW-assisted degradation of MO over CuO/CeO2 catalyst. As expected, increasing hydrogen peroxide concentration will speed the MO oxidation as more hydroxyl radicals are produced. The MO degradation efficiency increases from 57.7 to 85.2% with increasing H2O2 concentration from 0 to 16 mmol/L. However, some of the hydroxyl radicals could also be scavenged by excess H2O2 to form hydroperoxyl radicals, which reduced the probability of the attacking hydroxyl radical on the organic molecule.9 As a consequence, the MO degradation efficiency increases little with further increase in H2O2 concentration. 3.5. FT-IR Spectra of CuO/CeO2 before and after Reaction. To further elucidate the degradation behaviors of MO, the FT-IR spectra of CuO/CeO2 were also recorded

Figure 5. Effect of calcination temperature on the catalytic performance of CuO/CeO2 catalysts. MO = 50 mg/L, catalyst = 8 g/L, MW power = 700 W, and time = 7 min.

expected that the MO degradation efficiency is closely related to the amount of active oxygen species of the CuO/CeO2 catalyst. H2-TPR and O2-TPD measurement results have clearly indicated that the exposure to high calcination temperature causes the reduction of both the amount of surface chemisorbed oxygen and the specific surface area of CuO/ CeO2. Consequently, it is reasonable to conclude that the high activity of CuO/CeO2(300) can be ascribed to its optimal amount of active oxygen species and specific surface area. 3.3. UV−vis Spectra of MO Solutions before and after Various Treatment Processes. In order to determine the roles of CuO/CeO2, MW irradiation, and H2O2 in the degradation of MO in aqueous solution, a series of contrasting experiments were carried out. Figure 6 displays the UV−vis absorption spectra of the original and treated MO solutions. 2628



Article

Figure 7. Effect of H2O2 concentration on the degradation efficiency of MO. MO = 50 mg/L, catalyst = 8 g/L, MW power = 380 W, and time = 7 min.

(Figure 8). For fresh CuO/CeO2 samples (Figure 8a), the peaks centered at about 3392 and 1630 cm−1 are assigned to

Figure 8. The IR spectra of (a) CuO/CeO2; (b) CuO/CeO2 after MO adsorption; and (c) CuO/CeO2 after degradation reaction.

the hydroxyl groups of the superficial adsorbed or crystallized water. The peak centered at around 513 cm−1 can be assigned to vibration of Cu−O band,30 while the bands centered at 838, 1046, 1377, and 1475 cm−1 are related to CeO2 structure.31,32 Liu et al. reported that the IR spectrum of MO exhibits two aromatic ring stretching vibration peaks centered at 1607 and 1520 cm−1.33 However, no characteristic MO peaks were found in the IR spectrum of CuO/CeO2 after adsorption in MO solution (Figure 8b), indicating that the contribution of physical adsorption of MO to the removal efficiency can be ruled out in our study. None of the peaks corresponding to intermediate degradation species were detected in the spectrum of CuO/CeO2 after reaction (Figure 8c), meaning a negligible adsorption of these species on the surface of the catalyst and/or complete degradation of MO. To detect the intermediate degradation species, more detailed study is needed. 3.6. XPS of Spectra of CuO/CeO2 before and after Reaction. Figure 9 depicts the characteristic spin−orbit split of Cu 2p, Ce 3d, and O 1s signals for both fresh and used CuO/ CeO2 catalysts. As shown, all of the Cu 2p profiles present two sets of peaks, corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. A broad Cu 2p3/2 signal composed of two contributions centered at 932.7 and 934.9 eV, accompanied by a shakeup satellite in the 939−946 eV region, was observed

Figure 9. XPS profiles of the fresh and used CuO/CeO2 catalysts.

for the fresh CuO/CeO2 catalyst (CuO/CeO2-f). According to previous results in the literature,19,34 these two peaks centered at 934.9 and 932.7 eV could be ascribed to the presence of Cu2+ and reduced copper species (Cu+ or Cu0), respectively. The chemical states of Cu+ and Cu0 are not distinguishable on the basis of their Cu 2p3/2 binding energies, which appear at 932.4 and 932.6 eV, respectively.35 However, it is reasonable to speculate that the presence of reduced states (Cu+ or Cu0) strongly depends on the preparation method of the sample. Therefore, the existence of Cu+ rather than Cu0 is most possible in CuO/CeO2-f prepared by calcination in air. Hocevar et al. proposed that the formation of Cu+ might be induced by substitution at the interface of the two oxide phases because of 2629



Article

the analogous Ce4+ and Cu+ ionic radii.36 The main binding energies of Cu 2p are slightly higher than the data of pure CuO (933.6 eV), which is possibly caused by the interaction between CeO2 and CuO.37,38 In terms of the intensities of these two peaks, it can be inferred that the predominant Cu species is CuO in the fresh sample. Generally, the presence of reduced Cu species can also be estimated by the ratio between the intensity of the shakeup satellite peak and the principal Cu 2p3/2 peak (Isat/Ip), which is 0.55 for pure Cu2+ and zero for pure Cu+.19 From Figure 9, the Isat/Ip value for CuO/CeO2-f was calculated to be 0.46, which is lower than that observed for CuO, indicating the presence of reduced Cu species. In comparison with the XPS profile of CuO/CeO2-f, the used sample (CuO/CeO2-u) displays two obvious changes. First, the binding energies of Cu 2p shift to lower values. Second, a marked reduction in Isat/Ip from 0.46 of CuO/CeO2-f to 0.39 of CuO/CeO2-u was observed. This clearly suggests that the transformation of Cu2+ into Cu+ occurred to some degree during the microwave assisted degradation process, resulting in more Cu+ species in the used sample. As can be noted from Figure 9, the Ce 3d profiles are more complicated because of the mixing of Ce 4f levels with O 2p states. The Ce 3d XPS peaks are labeled for identification, where the peaks labeled v and u are from the spin−orbit coupling 3d5/2 and 3d3/2, respectively. The six peaks (u, u″, u‴, v, v″, and v‴) and the four peaks (u0, v0, u′, and v′) are associated with the Ce4+ initial state and the Ce3+ initial state of CeO2, respectively. The u‴/v‴ doublet is due to the primary photoemission from Ce4+−O2. The u/v and u″/v″ doublets are shakedown features resulting from the transfer of one or two electrons from a filled O 2p orbital to an empty Ce 4f orbital.34,39 For CuO/CeO2-f, the u0, v0, and u′ peaks are not present due to the low intensity. However, the v′ peak corresponding to Ce3+3d94f1O2p6 final state at 884.6 eV was observed. Thereafter, cerium is present at the surface in both 4+ and 3+ oxidation states. As to CuO/CeO2-u, a u′ peak with low intensity was found, indicating more Ce3+ species at the surface. It is believed that the reducibility of Ce4+ to Ce3+ forces the copper ions to adapt to a different oxidation state, maintaining the charge balance of the lattice.36 This is very consistent with the variation of Cu species in CuO/CeO2 before and after reaction. Therefore, it is reasonable to suggest that the redox cycles between Ce4+/Ce3+ and Cu2+/Cu+ are more facile to occur during the microwave irradiation process and plays an important role in the degradation reaction. As shown in Figure 9, the O1s photoelectron spectra show two contributions at 529.6 and 531.8 eV after peak modeling for CuO/CeO2-f. The peak centered at ca. 529.6 eV can be ascribed to lattice oxygen in CeO2 and CuO phases, while the signal centered at ca. 531.8 eV can be associated to the presence of hydroxyl groups or adsorbed water molecules on the surface.39 The relative intensity of the peak corresponding to the chemisorbed oxygen in CuO/CeO2-u decreases obviously compared with that in CuO/CeO2-f, indicating that the chemisorbed oxygen did participate in the degradation reaction. 3.7. Degradation Mechanism of MO. The above results clearly indicate that the CuO/CeO2 catalyst plays a significant role in the MW assisted MO degradation process. From all experimental results discussed above, we propose a possible reaction mechanism for MO degradation by microwave enhanced catalytic oxidation over CuO/CeO2 in the presence of H2O2. The schematic mechanism illustration is summarized in Figure 10. Various characterization results have proved that

Figure 10. Schematic illustration of MO degradation mechanism during MW/catalyst/H2O2 combined process.

the CuO/CeO2 catalyst possesses excellent redox ability, to which a high catalytic performance was attributed. Under MW irradiation, the oxygen ions derived from surface chemisorbed oxygen and lattice oxygen of CuO/CeO2 can effectively react with the adsorbed MO molecules on the catalyst surface, which is primarily responsible for the oxidative degradation of MO. The lost lattice oxygen can be supplemented by capturing oxygen dissolved in water. It was proposed that cerium is capable of redox cycling in the presence of H2O2 and produces •OHads, behaving similarly to iron in a Fenton-like reaction.40 Therefore, the added H2O2 can partly transform into •OH on the surface of CuO/CeO2 catalyst with the assistance of MW energy, further improving the degradation efficiency. In addition, some H2O2 may also decompose into nontoxic substances of H2O and O2 by MW irradiation, the produced oxygen can help to replenish the consumed lattice oxygen and facilitate the redox cycles of CuO/CeO2. In addition, it is generally accepted that metal oxides can produce a great amount of “hot spots” under MW irradiation, on which more rapid oxidation of organic pollutants can occur. Particularly, CuO is one of the metal oxides that best couples to microwaves and its temperature rapidly rises under MW irradiation.41 Although the exact temperature on the surface of CuO/CeO2 catalyst during reaction cannot be measured, the contribution of the “hot spots” to the degradation of MO should not be excluded.

4. CONCLUSIONS A microwave-assisted catalytic oxidation method over CuO/ CeO2 in the presence of H2O2 has been utilized for the degradation of MO in aqueous solution. This MW/catalyst/ H2O2 combined method has been proven to be more effective toward MO degradation than using MW alone, MW/H2O2, and MW/catalyst methods. The superior MO degradation performance in this process is ascribed to a synergistic rather than an additive effect of microwave irradiation, catalyst, and H2O2. Under MW irradiation, the oxygen ions derived from surface chemisorbed oxygen and lattice oxygen of CuO/CeO2 can effectively react with the adsorbed MO molecules on the catalyst surface, which is primarily responsible for the oxidative degradation of MO. In the presence of CuO/CeO2, the added H2O2 can effectively generate hydroxyl radicals by MW irradiation, thereby enhancing the degradation of MO. 2630



■

Article

of TiO2 Nanoparticles in Degrading Organic Dyes Using a Continuous-Flow Reactor. J. Phys. Chem., C 2012, 116, 14040−14051. (17) Lai, T. L.; Lee, C. C.; Huang, G. L.; Shu, Y. Y.; Wang, C. B. Microwave-Enhanced Catalytic Degradation of 4-Chlorophenol over Nickel Oxides. Appl. Catal., B 2008, 78, 151−157. (18) Djinović, P.; Batista, J.; Pintar, A. Calcination Temperature and CuO loading dependence on CuO−CeO2 Catalyst Activity for WaterGas Shift Reaction. Appl. Catal., A 2008, 347, 23−33. (19) Avgouropoulos, G.; Ioannides, T. Selective CO Oxidation over CuO−CeO2 Catalysts Prepared via the Urea−Nitrate Combustion Method. Appl. Catal., A 2003, 244, 155−167. (20) Djinović, P.; Batista, J.; Ć ehić, B.; Pintar, A. Utilization of High Specific Surface Area CuO−CeO2Catalysts for High Temperature Processes of Hydrogen Production: Steam Reforming of Ethanol and Methane Dry Reforming. J. Phys. Chem., A 2010, 114, 3939−3949. (21) Jiang, X. Y.; Lou, L. P.; Chen, Y. X.; Zheng, X. M. Effects of CuO/CeO2 and CuO/γ-Al2O3 Catalysts on NO + CO Reaction. J. Mol. Catal., A 2003, 197, 193−205. (22) Zheng, X. C.; Zhang, X. L.; Wang, X. Y.; Wang, S. R.; Wu, S. H. Preparation and Characterization of CuO/CeO2 Catalysts and Their Applications in Low-Temperature CO Oxidation. Appl. Catal., A 2005, 295, 142−149. (23) Luo, J. Y.; Meng, M.; Zha, Y. Q.; Guo, L. H. Identification of the Active Sites for CO and C3H8 Total Oxidation over Nanostructured CuO−CeO2 and Co3O4−CeO2 Catalysts. J. Phys. Chem., C 2008, 112, 8694−8701. (24) Pérez-Hernández, R.; Gutiérrez-Martínez, A.; Gutiérrez-Wing, C. E. Effect of Cu Loading on CeO2 for Hydrogen Production by Oxidative Steam Reforming of Methanol. Int. J. Hydrogen Energy 2007, 32, 2888−2894. (25) Martínez-Arias, A.; Fernández-Gercia, M.; Gálvez, O.; Coronado, J. M.; Anderson, J. A.; Conesa, J. C.; Soria, J.; Munuera, G. Comparative Study on Redox Properties and Catalytic Behavior for CO Oxidation of CuO/CeO2 and CuO/ZrCeO4 Catalysts. J. Catal. 2000, 195, 207−216. (26) Ayastuy, J. L.; Gurbani, A.; González-Marcos, M. P.; GutiérrezOrtiz, M. A. Kinetics of Carbon Monoxide Oxidation over CuO Supported on Nanosized CeO2. Ind. Eng. Chem. Res. 2009, 48, 5633− 5641. (27) Gurbani, A.; Ayastuy, J. L.; González-Marcos, M. P.; Herrero, J. E.; Guil, J. M.; Gutiérrez-Ortiz, M. A. Comparative Study of CuO− CeO2 Catalysts Prepared by Wet Impregnation and Deposition− Precipitation. Int. J. Hydrogen Energy 2009, 34, 547−553. (28) Ciston, J.; Si, R.; Rodriguez, J. A.; Hanson, J. C.; Martínez-Arias, A.; Fernandez-García, M.; Zhu, Y. M. Morphological and Structural Changes during the Reduction and Reoxidation of CuO/CeO2 and Ce1−xCuxO2 Nanocatalysts: In Situ Studies with Environmental TEM, XRD, and XAS. J. Phys. Chem., C 2011, 115, 13851−13859. (29) Luo, J. J.; Chu, W.; Xu, H. Y.; Jiang, C. F.; Zhang, T. LowTemperature CO Oxidation over CuO−CeO2/SiO2 Catalysts: Effect of CeO2 Content and Carrier Porosity. J. Nat. Gas. Chem. 2010, 19, 355−361. (30) Rahnama, A.; Gharagozlou, M. Preparation and Properties of Semiconductor CuO Nanoparticles via a Simple Precipitation Method at Different Reaction Temperatures. Opt. Quant. Electron. 2012, 44, 313−322. (31) Deraz, N. M. Characterization and Catalytic Performance of Pure and Li2O-Doped CuO/CeO2 Catalysts. Appl. Surf. Sci. 2009, 255, 3884−3890. (32) Zhang, D. S.; Fu, H. X.; Shi, L. Y.; Pan, C. S.; Li, Q.; Chu, Y. L.; Yu, W. J. Synthesis of CeO2 Nanorods via Ultrasonication Assisted by Polyethylene Glycol. Inorg. Chem. 2007, 46, 2446−2451. (33) Liu, Y.; Sun, D. Z. Development of Fe2O3−CeO2−TiO2/γAl2O3 as Catalyst for Catalytic Wet Air Oxidation of Methyl Orange Azo Dye under Room Condition. Appl. Catal., B 2007, 72, 205−211. (34) Rao, K. N.; Venkataswamy, P.; Reddy, B. M. Structural Characterization and Catalytic Evaluation of Supported Copper-Ceria Catalysts for Soot Oxidation. Ind. Eng. Chem. Res. 2011, 50, 11960− 11969.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work was supported by Shandong Provincial Natural Science Foundation, China (No. ZR2012EEM029).

■

REFERENCES

(1) Zhao, D. L.; Sheng, G. D.; Chen, C. L.; Wang, X. K. Enhanced Photocatalytic Degradation of Methylene Blue under Visible Irradiation on Graphene@TiO2 Dyade Structure. Appl. Catal., B 2012, 111−112, 303−308. (2) Rodríguez, A.; Ovejero, G.; Sotelo, J. L.; Mestanza, M.; García, J. Heterogeneous Fenton Catalyst Supports Screening for Mono Azo Dye Degradation in Contaminated Wastewaters. Ind. Eng. Chem. Res. 2010, 49, 498−505. (3) Macías-Sánchez, J.; Hinojosa-Reyes, L.; Guzmán-Mar, J. L.; Peralta-Hernández, J. M.; Hernández-Ramírez, A. Performance of the Photo-Fenton Process in the Degradation of a Model Azo Dye Mixture. Photochem. Photobiol. Sci. 2011, 10, 332−337. (4) Ince, N. H.; Tezcanli-Güyer, G. Impacts of pH and Molecular Structure on Ultrasonic Degradation of Azo Dyes. Ultrasonics 2004, 42, 591−596. (5) Koch, M.; Yediler, A.; Lienert, D.; Insel, G.; Kettrup, A. Ozonation of Hydrolyzed Azo Dye Reactive Yellow 84(CI). Chemosphere 2002, 46, 109−113. (6) Tizaoui, C.; Karodia, N.; Aburowais, M. Kinetic Study of the Manganese-Based Catalytic Hydrogen Peroxide Oxidation of a Persistent Azo-Dye. J. Chem. Technol. Biotechnol. 2010, 85, 234−242. (7) Remya, N.; Lin, J. G. Current Status of Microwave Application in Wastewater Treatment−A Review. Chem. Eng. J. 2011, 166, 797−813. (8) Bi, X. Y.; Wang, P.; Jiang, H.; Xu, H. Y.; Shi, S. J.; Huang, J. L. Treatment of Phenol Wastewater by Microwave-Induced ClO2− CuOx/Al2O3 Catalytic Oxidation Process. J. Environ. Sci. 2007, 19, 1510−1515. (9) Bi, X. Y.; Wang, P.; Jiang, H. Catalytic Activity of CuOn−La2O3/ γ-Al2O3 for Microwave Assisted ClO2 Catalytic Oxidation of Phenol Wastewater. J. Hazard. Mater. 2008, 154, 543−549. (10) Zhang, Y. B.; Quan, X.; Chen, S.; Zhao, Y. Z.; Yang, F. L. Microwave Assisted Catalytic Wet Air Oxidation of H-Acid in Aqueous Solution under the Atmospheric Pressure Using Activated Carbon As Catalyst. J. Hazard. Mater. 2006, 137, 534−540. (11) Bo, L. L.; Zhang, Y. B.; Quan, X.; Zhao, B. Microwave Assisted Catalytic Oxidation of p-Nitrophenol in Aqueous Solution Using Carbon-Supported Copper Catalyst. J. Hazard. Mater. 2008, 153, 1201−1206. (12) Liao, W. C.; Zheng, T.; Wang, P.; Tu, S. S.; Pan, W. Q. Efficient Microwave-Assisted Photocatalytic Degradation of Endocrine Disruptor Dimethyl Phthalate over Composite Catalyst ZrOx/ZnO. J. Environ. Sci. 2010, 22, 1800−1806. (13) Ai, Z. H.; Zhang, L. Z.; Kong, F. H.; Liu, H.; Xing, W. T.; Qiu, J. R. Microwave-Assisted Green Synthesis of MnO2 Nanoplates with Environmental Catalytic Activity. Mater. Chem. Phys. 2008, 111, 162− 167. (14) Prasannakumar, B. R.; Regupathib, I.; Murugesan, T. An Optimization Study on Microwave Irradiated Decomposition of Phenol in the Presence of H2O2. J. Chem. Technol. Biotechnol. 2009, 84, 83−91. (15) Yang, Y.; Wang, P.; Shi, S. J.; Liu, Y. Microwave Enhanced Fenton-Like Process for the Treatment of High Concentration Pharmaceutical Wastewater. J. Hazard. Mater. 2009, 168, 238−245. (16) Genuino, H. C.; Hamal, D. B.; Fu, Y. J.; Suib, S. L. Synergetic Effects of Ultraviolet and Microwave Radiation for Enhanced Activity 2631



Article

(35) Bera, P.; Priolkar, K. R.; Sarode, P. R.; Hegde, M. S.; Emura, S.; Kumashiro, R.; Lalla, N. P. Structural Investigation of Combustion Synthesized Cu/CeO2 Catalysts by EXAFS and Other Physical Techniques: Formation of a Ce1‑xCuxO2‑δ Solid Solution. Chem. Mater. 2002, 14, 3591−3601. (36) Hocevar, S.; Krasovec, U. O.; Orel, B.; Aricó, A. S.; Kim, H. CWO of Phenol on Two Differently Prepared CuO−CeO2 Catalysts. Appl. Catal., B 2000, 28, 113−125. (37) Huang, J.; Wang, S. R.; Zhao, Y. Q.; Wang, X. Y.; Wang, S. P.; Wu, S. H.; Zhang, S. M.; Huang, W. P. Synthesis and Characterization of CuO/TiO2 Catalysts for Low-Temperature CO Oxidation. Catal. Commun. 2006, 7, 1029−1034. (38) Chen, Y. Z.; Liaw, B. J.; Chang, W. C.; Huang, C. T. Selective Oxidation of CO in Excess Hydrogen over CuO/CexZr1‑xO2 Catalysts. Int. J. Hydrogen Energy 2006, 31, 427−435. (39) Li, G. S.; Zhang, D. Q.; Yu, J. C. Thermally Stable Ordered Mesoporous CeO2/TiO2 Visible-Light Photocatalysts. Phys. Chem. Chem. Phys. 2009, 11, 3775−3782. (40) Xu, L. J.; Wang, J. L. Magnetic Nanoscaled Fe3O4/CeO2 Composite As an Efficient Fenton-Like Heterogeneous Catalyst for Degradation of 4-Chlorophenol. Environ. Sci. Technol. 2012, 46, 10145−10153. (41) Reguera, E.; Díaz-Aguila, C.; Yee-Madeira, H. On the Changes and Reactions in Metal Oxides under Microwave Irradiation. J. Mater. Sci. 2005, 40, 5331−5334.

2632


Microwave Enhanced Catalytic Degradation of Methyl Orange in

Recommend Documents