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Catalytic activity of copper (II) oxide prepared via ultrasound assisted Fenton-like reaction
Ultrasonics Sonochemistry 21 (2014) 854–859
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Catalytic activity of copper (II) oxide prepared via ultrasound assisted Fenton-like reaction Arzu Angı a,1, Deniz Sanlı b, Can Erkey b, Özgür Birer a,c,⇑ a
Department of Chemistry, Koç University, Rumeli Feneri Yolu, Sarıyer, 34450 Istanbul, Turkey Department of Chemical and Biological Engineering, Koç University, Rumeli Feneri Yolu, Sarıyer, 34450 Istanbul, Turkey c KUYTAM Surface Science and Technology Center, Koç University, Rumeli Feneri Yolu, Sarıyer, 34450 Istanbul, Turkey b
a r t i c l e
i n f o
Article history: Received 3 June 2013 Received in revised form 14 July 2013 Accepted 6 September 2013 Available online 15 September 2013 Keywords: Ultrasonic activation Fenton-like reactions Copper (II) oxide Catalysis
a b s t r a c t Copper (II) oxide nanoparticles were synthesized in an ultrasound assisted Fenton-like aqueous reaction between copper (II) cations and hydrogen peroxide. The reactions were initiated with the degradation of hydrogen peroxide by ultrasound induced cavitations at 0 °C or 5 °C and subsequent generation of the OH radical. The radical was converted into hydroxide anion in Fenton-like reactions and copper hydroxides were readily converted to oxides without the need of post annealing or aging of the samples. The products were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) surface area analysis. Catalytic activity of the nanoparticles for the hydrogen peroxide assisted degradation of polycyclic aromatic hydrocarbons in the dark was tested by UV–visible spectroscopy with methylene blue as the model compound. The rate of the reaction was first order, however the rate constants changed after the initial hour. Initial rate constants as high as 0.030 min1 were associated with the high values of surface area, i.e. 70 m2/g. Annealing of the products at 150 °C under vacuum resulted in the decrease of the catalytic activity, underlying the significance of the cavitation induced surface defects in the catalytic process. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Copper (II) oxide gained significant attention due to its several properties and ease of production. Several different morphologies of CuO such as nanowires [1–6], nanotubes [7], ribbons [8], flower shapes [9,10], nanorings [3], nanoplates [11], hollow microspheres [12–14] and disks [15] have already been synthesized. Copper (II) oxide particles, besides many other applications, are promising materials for heterogeneous catalysis [16–21]. CuO was utilized as an effective photo catalyst to increase the amount of OH radicals during hydrogen peroxide degradation [5]. Recent work demonstrated that the catalytic activity is also present in the dark [22]. Copper (II) oxide nanoparticles can be prepared with a variety of techniques, however chemical and hydrothermal methods with some form of surfactant are the most preferred methods. In these studies, copper ions are precipitated by a base as hydroxides and annealed to oxides. The rate of precipitation, hence the use of a ⇑ Corresponding author at: Department of Chemistry, Koç University, Rumeli Feneri Yolu, Sarıyer, 34450 Istanbul, Turkey. Tel.: +90 212 338 1357; fax: +90 212 338 1559. E-mail address: [email protected] (Ö. Birer). 1 Current address: Department of Chemistry, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching bei München, Germany. 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.09.006
strong or weak bases or slow alkali releasing sources, determine the morphology of the products. In recent years, ultrasound assisted synthesis of copper oxides were also reported following decomposition [23], hydrolysis [23], urea precipitation [13,24] and dispersed in polymer medium [25]. In this work, we demonstrated an alternate route to directly and quickly synthesize copper (II) oxide nanoparticles in a wet process. We have utilized ultrasound to decompose hydrogen peroxide and synthesize copper (II) oxide via a Fenton-like reaction. Fenton reactions are mostly studied for Fe2+/Fe3+ ion pairs [26–29], however in recent years copper ions were investigated as active ions in Fenton-like reactions [29–35]. Irradiation of liquids with high intensity ultrasound leads to cavitations, which is the formation, growth and sudden collapse of microbubbles. It only takes a few hundreds of microseconds (i.e. a couple cycles of the wave) from the formation to the collapse of these microbubbles. They collapse in less than a microsecond creating local extreme temperature (T 5000 K) and extreme pressure (P 200 bar) conditions [36,37]. We utilized these extreme conditions to break the O–O bond in hydrogen peroxide and obtain copper (II) oxide with high yield in a one-pot facile reaction. All the products obtained were catalytically active for degradation of a model polycyclic aromatic hydrocarbon, i.e. methylene blue, in the dark in the presence of hydrogen peroxide.
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2. Experimental methods Copper acetate monohydrate (Cu(CH3COO)2H2O), hydrogen peroxide (H2O2, CAS number: 7722-84-1, 30%) and methylene blue (C16H18N3SCl, CAS number: 61-73-4) were from Merck. All chemicals were used without further purification. In a typical synthesis, 5.5 103 mol Cu(CH3COO)2H2O was dissolved in 50 mL distilled water at room temperature until a clear blue solution was obtained. The pH of the solutions was slightly acidic, i.e. 5.50, due to the dissolved carbon dioxide. The solution was then transferred into a jacketed reaction flask and ultrasound was applied at 50 W/cm2 intensity with a Bandelin Sonopuls HD 3200 (20 kHz) horn immersed in the liquid for a total of 2 h. The solution was saturated with argon gas to facilitate higher cavitation temperatures before the reaction and the gas flow was maintained during the reaction. The temperature of the reaction medium was controlled with a circulating chiller. Hydrogen peroxide was diluted with water (1:2 ratio) and was delivered to sonicated reaction mixture with a syringe pump in approximately 40 min at 0.15 mL/min rate. At the end, a green solution of pH 3.7–4.2 was obtained. The black copper (II) oxide nanoparticles were collected by centrifuging and particles were washed with distilled water several times. The yield of the reaction was calculated by comparing the amount of the collected product (CuO) to the initial amount of copper in the solution since copper ions were the limiting reagent. After drying, the products were characterized by X-ray diffraction (Bruker D2 Phaser XRD) and field emission scanning electron microscope (Zeiss Ultra Plus FE-SEM) equipped with a Bruker XFlash 5010 energy dispersive X-ray spectrometer (EDS). The attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectra were recorded on a Thermo Scientific iS10 with Smart iTR. The Brunauer–Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) pore size distribution analysis were carried out with a Micromeritics ASAP 2020 system with nitrogen adsorption at liquid nitrogen temperature after annealing the samples to 150 °C. The catalytic activity of oxide samples towards peroxide degradation to treat polycyclic aromatic hydrocarbons was also tested. In a typical test, 1.0 ± 0.1 mg CuO sample and 1 mL of H2O2 (30%) were added to 10 mL of 14.4 lM methylene blue solution in a vial. The mixture was kept at room temperature in dark with continuous stirring. The visible region absorption spectra of the samples were recorded with an Ocean Optics HR4000 fiber spectrometer in 30 min intervals over several hours.
3. Results 3.1. Characterization of particles Irradiation of copper acetate solution with high intensity ultrasound resulted in the formation of black precipitate, which was thought to be copper (II) oxide. The yields of these reactions were so low that enough amounts of products could not be isolated for further analysis. Since the responsible mechanism was thought to be hydrogen peroxide formation due to sonolysis, the experiments were repeated with the addition of hydrogen peroxide. In this case, products could be isolated in high quantities when the reaction temperatures were low and copper ion to hydrogen peroxide mol ratio was 1:5. Copper (II) oxide was the only product isolated in these experiments. The yield of the reaction increased from 3% at 25 °C to 62% at 5 °C and to 78% at 0 °C. Here, the latter two products are discussed and they are labeled as ‘‘Sample A’’ and ‘‘Sample B’’ for the oxides prepared at 5 °C and 0 °C, respectively. The properties of these samples are summarized in Table 1.
Table 1 Summary of properties of Samples A and B. Property
Sample A
Sample B
Synthesis temperature (°C) Reaction yield (%)
5 62
0 78
Raw form Initial rate constant (min1) Final rate constant (min1)
0.030 0.013
0.018 0.011
Annealed form BET surface area (m2/g) Rate constant (min1)
68.5 0.0044
42.5 0.0031
Fig. 1. Powder XRD results of Samples A and B (raw and annealed versions).
The XRD patterns of the raw products are presented in Fig. 1 together with literature peak positions of Tenorite form of CuO (JCPDS 48-1548). The peaks at two theta positions of 35.6° and 38.7° are identified as the (0 0 2) and (1 1 1) lines of the monoclinic phase of copper (II) oxide. The Scherer equation puts the crystallite domain size in 9–10 nm range. Prior to the BET analysis, the samples had to be annealed in vacuum at elevated temperatures. It was determined that the annealing of CuO nanoparticles at 300 °C in vacuum for several hours transformed them mostly into Cuprite form of Cu2O (JCPDS 05-0667) as seen in the right panel of the figure. It should be noted that the transformation was not complete at this stage as evidenced by the remaining (0 0 2) and (1 1 1) peaks of CuO. Similar transitions were previously reported for thin copper (II) oxide films annealed at 200–300 °C [38]. It is also noteworthy that annealing also yielded much narrower peaks in the XRD indicating the crystallite domain size increased to approximately 30 nm. As a result of these transitions, the annealing operations before BET analysis had to be carried out at 150 °C, which was determined after several trials. FTIR spectra of the products, presented in Fig. 2, were used to determine the surface contamination of the samples after the XRD experiments identified CuO as the only crystalline product. The spectra of the Samples A and B, for both raw and annealed (at 150 °C) versions, are presented in the figure. In the raw form, the strong peaks between 1400 and 1560 cm1 were assigned to the shifted C@O stretching of the carboxylate ion bonded to the copper (II) oxide nanoparticles as bidentate ligand originating from the acetate residues on the surfaces [39–41]. In addition, surface adsorbed water was identified by its broad absorption around 3250 cm1. The spectra of the annealed samples were almost identical to the spectra of the raw samples, except for minute changes
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Fig. 2. FTIR spectra of Samples A and B (raw and annealed versions).
around 2000 cm1. Therefore, it can be inferred that annealing the samples at 150 °C in vacuum did not significantly clean the surfaces off contaminations. Nevertheless, the catalytic activity of the raw and annealed materials was tested for comparison. The SEM images of the copper (II) oxide particles are presented in Fig. 3. The particles were mainly spheroids with 100–200 nm size with clearly visible surface structuring. These structures were actually much smaller particles with sizes close to the crystallite domain size determined by the Scherer equation. While these particles protruded from the surface and increased the surface area, there were not any significant differences between Samples A and B in the SEM images. The EDS analysis of Sample A, presented in Fig. 4, showed 50–55% Cu and 50–45% O atomic composition supporting the CuO stoichiometry. It is important to mention that annealing apparently reduced the amount of carbon contamination off the surface, contrary to the FTIR analysis results. 3.2. BET analysis The BET analysis was run after the samples were annealed at 150 °C in vacuum for several hours. Annealing was attempted to
Fig. 4. EDS spectra of Sample A (raw and annealed versions).
get rid of the acetate and water residues on the surfaces. Since higher temperatures triggered tenorite to cuprite transition, the annealing temperature was set to 150 °C after several trials to eliminate unintentional phase transitions. Although annealing apparently cleaned some of the contamination, significant amount of chemisorbed contamination still remained on the surface as seen in Fig. 2 due to the low annealing temperature. Nevertheless, the BET analysis was performed on these samples. The nitrogen adsorption/desorption isotherms for the samples are presented in Fig. 5 together with the corresponding BJH pore size distribution semi-log curves. The isotherm is of type IV with very little hysteresis which is characteristic for mesoporous materials [42]. The pore size distribution curves demonstrate the presence of mesopores of 1 nm and a broad size distribution above 20 nm. The BET surface area values were measured as 68.5 and 42.5 m2/g for Samples A and B, respectively. Single point adsorption total pore volume was calculated as 0.13 and 0.09 cm3/g for the same samples, respectively. It should be pointed out that the surface areas of these samples were significantly larger than similar products reported in the literature despite the apparent surface contamination [13,22]. 3.3. Catalytic activity The catalytic activity of raw and annealed (at 150 °C) versions of Samples A and B nanoparticles for the degradation of methylene blue (MB) was studied using visible spectroscopy. The absorbance spectra of MB were recorded over several hours with 30 min
Fig. 3. SEM images of Samples A and B (raw version).
Fig. 5. BET isotherms and pore size distributions for Samples A and B (annealed form).
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intervals under different conditions. Initially, the spectra of MB were recorded with H2O2 in the absence of copper (II) oxide nanoparticles to establish that the degradation reaction had to be catalyzed. As presented in Fig. 6a, the absorption spectrum of MB did not change over 6 h under these conditions. Once we established the need for catalysis, the possibility of MB adsorption of the CuO particles had to be eliminated as a source for absorption decrease. The spectra of MB with 1.0 mg CuO nanoparticles in the absence of H2O2 are presented in Fig. 6b. The adsorption of MB on CuO surface was insignificant over 6 h and did not cause an important decrease in the absorbance. Finally, the catalytic activity of CuO nanoparticles were tested as described before. All versions of copper (II) oxide particles prepared in this study decreased the absorbance of MV in the presence of H2O2 albeit at different rates. The absorption spectra of MV in the presence of H2O2 and 1.0 mg of raw Sample A nanoparticles are presented in Fig. 6c. A significant decrease in the absorption was detected in the first half hour of the experiment. The absorption gradually decreased in time until the solution became colorless to the naked eye in about 5 h. The percentage degree of degradation was calculated by taking the difference of the initial and measured absorbance and dividing it by the initial absorbance value. The plot of percentage degree of degradation vs time is presented in Fig. 7a for Samples A and B, both for raw and annealed (at 150 °C) versions. The fastest degradation rate was observed for raw Sample A with about 96% degradation in 3 h. The degradation rate of raw Sample B was a little lower, reaching only 91% within the same amount of time. Annealing of samples at 150 °C decreased the degradation rate significantly. Only 55% and 45% of MB could be degraded in 3 h for the case of annealed Samples A and B, respectively. The reaction kinetics of degradation were modeled using the relative amounts, i.e. (A0/A) to remove the intercept of the plot. The plots of the logarithm of the ratio of the initial absorbance to the measured absorbance (ln(A0/A)) vs time are presented for the raw and annealed versions of Samples A and B in Fig. 7b and c, respectively. The plots yielded straight lines; therefore the kinetics of degradation was found to be first order in accord with the literature [22]. However, the plots of both of the raw CuO samples resulted in two straight lines with different slopes, i.e. rate constants. It was found that the rate constants changed after the initial hour, a cusp formed in the graphs isolating the first three points from the others. Therefore, first three points were fit together with the intercept forced to zero. Rest of the points was fit with linear regression without any restrictions. In all cases the chi square values were
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better than 0.995. The initial rate constants were determined to be 0.030 and 0.018 min1 for the raw versions of Samples A and B, respectively. After the initial hour, both rate constants decreased, i.e. to 0.013 and 0.011 min1, respectively. The annealed products were also tested under the same conditions for comparison. Only a single slope was detected in the plot of the annealed samples, however, the rate constants for these annealed products were much lower, i.e. 0.0044 and 0.0031 min1, for Samples A and B, respectively.
4. Discussion The reaction mechanism leading to copper (II) oxide deserves some consideration. It is evident that the reaction was driven by the free radicals originating from the peroxide degradation. Such reactions are well known for the Fe2+/Fe3+ redox system, i.e. the Fenton reactions. The involvement of copper ions in Fenton-like reactions has been debated for some time [29–34]. Recently, the participation of Cu (III) ions in such reactions as intermediate species were proposed [30]. The copper redox chemistry involving such transient higher oxidation states can be quite complex especially when assisted by high intensity ultrasound. Therefore, the proposed mechanism is merely a collection of possible reactions bearing in mind that their rate constants can be substantially different under current reaction conditions. We propose that the reactions were initiated with the degradation of peroxides by ultrasound induced cavitations. This hypothesis is based on the observation that we could not obtain any products when the temperature of the solution was higher than 50 °C indicating two competing degradation pathways, i.e. via cavitations and thermal reactions. The copper (II) oxide yield increased when the solution temperature was lowered, i.e. the possibility of thermal degradation was eliminated. Copper (II) oxide can only form when the hydroxide radical turns into the hydroxide anion via a Fenton-like reaction and the formed copper (II) hydroxide (Ksp = 2.2 1020) anneals in situ to the final product. The proposed mechanism starts with Eq. (1) i.e. the degradation of hydrogen peroxide within the cavitations induced by ultrasound. The copper (II) ions can be reduced to copper (I) ions by hydrogen peroxide as presented in Eq. (2). The copper (I) ion can participate in a Fenton-like reaction yielding the hydroxide anion as in Eq. (3) and recycle to the copper (II) ions. The hydroxide anion and copper (II) ions precipitate as copper (II) hydroxide and
Fig. 6. Absorption spectra of MB under various conditions.
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Fig. 7. Percentage degradation and degradation kinetics of MB.
anneal to copper (II) oxide in situ close to the collapsing cavitations as given in Eq. (4). The hydrogen peroxide can also react with hydroxide radicals forming the reactive oxygen radical anion, i.e. superoxide (Eq. (5)), which can reduce copper (II) ions as in Eq. (6). Under these reaction conditions, it is highly probable that copper (II) ions can also be oxidized in Fenton-like reactions yielding transient copper (III) ions as in Eq. (7). The copper (III) ions can be recycled back to copper (II) via reduction reactions with hydrogen peroxide or superoxide (Eqs. (8) and Eq. (9)). The decrease in the pH of the solution during the reaction supports reactions 2, 5 and 8 where superoxide and hydrogen ions form:
H2 O2 ! 2 OH
ð1Þ
þ Cu2þ þ H2 O2 $ Cuþ þ O 2 þ 2H
ð2Þ
Cuþ þ 2 OH ! Cu2þ þ OH þ OH
ð3Þ
Cu2þ þ 2OH ! CuðOHÞ2 ! CuO þ H2 O
ð4Þ
þ H2 O2 þ OH ! O 2 þ H þ H2 O
ð5Þ
þ Cu2þ þ O 2 !Cu þ O2
ð6Þ
Cu2þ þ 2 OH ! Cu3þ þ OH þ OH
ð7Þ
þ Cu3þ þ H2 O2 $ Cu2þ þ O 2 þ 2H
ð8Þ
2þ Cu3þ þ O þ O2 2 !Cu
ð9Þ
The reaction hypothesis was tested with organic peroxides, benzoyl peroxide ((C6H5CO)2O2, CAS number: 94-36-0) and dicumyl peroxide ((C6H5C(CH3)2)2O2, CAS number: 80-43-3) as well. These peroxides are water insoluble at low temperatures, and the experiments had to be carried out at higher temperatures. Although the reaction temperatures were 80 °C in both cases, the temperature was still below the required thermal degradation threshold [43]. Therefore, it is safe to assume that these symmetric organic peroxides were also degraded by ultrasonic cavitations into reactive radicals, which further generated the hydroxyl radicals in water. The hydroxyl radicals participated in the reaction mechanism as suggested above and copper (II) oxide particles were isolated in both cases. The yields of these reactions were rather low, i.e. 40–50%, and in addition the BET surface areas were much
lower, i.e. 5–10 m2/g, than their counterparts prepared with H2O2. These particles showed diminished catalytic activity, with initial rate constants as low as 0.008 min1, in accord with their smaller surface area. These reactions are currently under investigation. In the present work, the oxide yield increased with decreasing solution temperature. It is possible that lower temperatures not only diminished the thermal degradation contribution to the reaction, but also increased the cavitation efficiency by decreasing the vapor pressure of the solution, as well [44]. Cavitations must also play a crucial role in annealing the hydroxides to oxides in the solution. In the classical chemical route, when the copper ions are precipitated with a base, the temperatures must be higher than 80 °C for the oxide formation. In the current work, on the other hand, temperatures as low as 0 °C yielded crystalline oxide products. The annealing process must have taken place at or near the cavitations where local temperatures exceeded the ambient values. The initial rate constant of the raw version of Sample A particles for the catalytic degradation of MB was about 3.0 102 min1 which is 2.3-fold larger than the reported values in the literature [22], where the products were prepared by hydrothermal methods employing surfactants. The decrease in the rate constants of the raw versions of Samples A and B after the initial hour can be attributed to the irreversible adsorption to or poisoning of the most reactive defect sites. There is a correlation between the rate constants of both the raw and annealed samples and the BET areas, despite the fact the BET areas could only be measured for the annealed samples. Annealing the oxides did not alter the structure or the surface contamination significantly, however, resulted in much reduced rate constants. This observation indicates that the surface defects were responsible for the heightened activity of these products. 5. Conclusions CuO particles were successfully synthesized in an ultrasound assisted Fenton-like reaction starting from copper (II) ions and hydrogen peroxide. The oxides precipitated through the formation of the hydroxides via the peroxide radicals and in situ annealing of copper hydroxides to oxides. Higher yields were achieved at lower temperatures due to increased cavitation efficiency. CuO was found to be an effective catalyst for the degradation of methylene blue in the dark in the presence of H2O2. The initial rate constants were about 50% greater than previously reported values, whereas the rate constants reached the literature values in time. This
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decrease in the rate constant and the loss of catalytic activity after annealing of products both signify the importance of cavitation induced surface defects in the catalytic process.
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Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Catalytic activity of copper (II) oxide prepared via ultrasound assisted Fenton-like reaction Arzu Angı a,1, Deniz Sanlı b, Can Erkey b, Özgür Birer a,c,⇑ a
Department of Chemistry, Koç University, Rumeli Feneri Yolu, Sarıyer, 34450 Istanbul, Turkey Department of Chemical and Biological Engineering, Koç University, Rumeli Feneri Yolu, Sarıyer, 34450 Istanbul, Turkey c KUYTAM Surface Science and Technology Center, Koç University, Rumeli Feneri Yolu, Sarıyer, 34450 Istanbul, Turkey b
a r t i c l e
i n f o
Article history: Received 3 June 2013 Received in revised form 14 July 2013 Accepted 6 September 2013 Available online 15 September 2013 Keywords: Ultrasonic activation Fenton-like reactions Copper (II) oxide Catalysis
a b s t r a c t Copper (II) oxide nanoparticles were synthesized in an ultrasound assisted Fenton-like aqueous reaction between copper (II) cations and hydrogen peroxide. The reactions were initiated with the degradation of hydrogen peroxide by ultrasound induced cavitations at 0 °C or 5 °C and subsequent generation of the OH radical. The radical was converted into hydroxide anion in Fenton-like reactions and copper hydroxides were readily converted to oxides without the need of post annealing or aging of the samples. The products were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) surface area analysis. Catalytic activity of the nanoparticles for the hydrogen peroxide assisted degradation of polycyclic aromatic hydrocarbons in the dark was tested by UV–visible spectroscopy with methylene blue as the model compound. The rate of the reaction was first order, however the rate constants changed after the initial hour. Initial rate constants as high as 0.030 min1 were associated with the high values of surface area, i.e. 70 m2/g. Annealing of the products at 150 °C under vacuum resulted in the decrease of the catalytic activity, underlying the significance of the cavitation induced surface defects in the catalytic process. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Copper (II) oxide gained significant attention due to its several properties and ease of production. Several different morphologies of CuO such as nanowires [1–6], nanotubes [7], ribbons [8], flower shapes [9,10], nanorings [3], nanoplates [11], hollow microspheres [12–14] and disks [15] have already been synthesized. Copper (II) oxide particles, besides many other applications, are promising materials for heterogeneous catalysis [16–21]. CuO was utilized as an effective photo catalyst to increase the amount of OH radicals during hydrogen peroxide degradation [5]. Recent work demonstrated that the catalytic activity is also present in the dark [22]. Copper (II) oxide nanoparticles can be prepared with a variety of techniques, however chemical and hydrothermal methods with some form of surfactant are the most preferred methods. In these studies, copper ions are precipitated by a base as hydroxides and annealed to oxides. The rate of precipitation, hence the use of a ⇑ Corresponding author at: Department of Chemistry, Koç University, Rumeli Feneri Yolu, Sarıyer, 34450 Istanbul, Turkey. Tel.: +90 212 338 1357; fax: +90 212 338 1559. E-mail address: [email protected] (Ö. Birer). 1 Current address: Department of Chemistry, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching bei München, Germany. 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.09.006
strong or weak bases or slow alkali releasing sources, determine the morphology of the products. In recent years, ultrasound assisted synthesis of copper oxides were also reported following decomposition [23], hydrolysis [23], urea precipitation [13,24] and dispersed in polymer medium [25]. In this work, we demonstrated an alternate route to directly and quickly synthesize copper (II) oxide nanoparticles in a wet process. We have utilized ultrasound to decompose hydrogen peroxide and synthesize copper (II) oxide via a Fenton-like reaction. Fenton reactions are mostly studied for Fe2+/Fe3+ ion pairs [26–29], however in recent years copper ions were investigated as active ions in Fenton-like reactions [29–35]. Irradiation of liquids with high intensity ultrasound leads to cavitations, which is the formation, growth and sudden collapse of microbubbles. It only takes a few hundreds of microseconds (i.e. a couple cycles of the wave) from the formation to the collapse of these microbubbles. They collapse in less than a microsecond creating local extreme temperature (T 5000 K) and extreme pressure (P 200 bar) conditions [36,37]. We utilized these extreme conditions to break the O–O bond in hydrogen peroxide and obtain copper (II) oxide with high yield in a one-pot facile reaction. All the products obtained were catalytically active for degradation of a model polycyclic aromatic hydrocarbon, i.e. methylene blue, in the dark in the presence of hydrogen peroxide.
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2. Experimental methods Copper acetate monohydrate (Cu(CH3COO)2H2O), hydrogen peroxide (H2O2, CAS number: 7722-84-1, 30%) and methylene blue (C16H18N3SCl, CAS number: 61-73-4) were from Merck. All chemicals were used without further purification. In a typical synthesis, 5.5 103 mol Cu(CH3COO)2H2O was dissolved in 50 mL distilled water at room temperature until a clear blue solution was obtained. The pH of the solutions was slightly acidic, i.e. 5.50, due to the dissolved carbon dioxide. The solution was then transferred into a jacketed reaction flask and ultrasound was applied at 50 W/cm2 intensity with a Bandelin Sonopuls HD 3200 (20 kHz) horn immersed in the liquid for a total of 2 h. The solution was saturated with argon gas to facilitate higher cavitation temperatures before the reaction and the gas flow was maintained during the reaction. The temperature of the reaction medium was controlled with a circulating chiller. Hydrogen peroxide was diluted with water (1:2 ratio) and was delivered to sonicated reaction mixture with a syringe pump in approximately 40 min at 0.15 mL/min rate. At the end, a green solution of pH 3.7–4.2 was obtained. The black copper (II) oxide nanoparticles were collected by centrifuging and particles were washed with distilled water several times. The yield of the reaction was calculated by comparing the amount of the collected product (CuO) to the initial amount of copper in the solution since copper ions were the limiting reagent. After drying, the products were characterized by X-ray diffraction (Bruker D2 Phaser XRD) and field emission scanning electron microscope (Zeiss Ultra Plus FE-SEM) equipped with a Bruker XFlash 5010 energy dispersive X-ray spectrometer (EDS). The attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectra were recorded on a Thermo Scientific iS10 with Smart iTR. The Brunauer–Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) pore size distribution analysis were carried out with a Micromeritics ASAP 2020 system with nitrogen adsorption at liquid nitrogen temperature after annealing the samples to 150 °C. The catalytic activity of oxide samples towards peroxide degradation to treat polycyclic aromatic hydrocarbons was also tested. In a typical test, 1.0 ± 0.1 mg CuO sample and 1 mL of H2O2 (30%) were added to 10 mL of 14.4 lM methylene blue solution in a vial. The mixture was kept at room temperature in dark with continuous stirring. The visible region absorption spectra of the samples were recorded with an Ocean Optics HR4000 fiber spectrometer in 30 min intervals over several hours.
3. Results 3.1. Characterization of particles Irradiation of copper acetate solution with high intensity ultrasound resulted in the formation of black precipitate, which was thought to be copper (II) oxide. The yields of these reactions were so low that enough amounts of products could not be isolated for further analysis. Since the responsible mechanism was thought to be hydrogen peroxide formation due to sonolysis, the experiments were repeated with the addition of hydrogen peroxide. In this case, products could be isolated in high quantities when the reaction temperatures were low and copper ion to hydrogen peroxide mol ratio was 1:5. Copper (II) oxide was the only product isolated in these experiments. The yield of the reaction increased from 3% at 25 °C to 62% at 5 °C and to 78% at 0 °C. Here, the latter two products are discussed and they are labeled as ‘‘Sample A’’ and ‘‘Sample B’’ for the oxides prepared at 5 °C and 0 °C, respectively. The properties of these samples are summarized in Table 1.
Table 1 Summary of properties of Samples A and B. Property
Sample A
Sample B
Synthesis temperature (°C) Reaction yield (%)
5 62
0 78
Raw form Initial rate constant (min1) Final rate constant (min1)
0.030 0.013
0.018 0.011
Annealed form BET surface area (m2/g) Rate constant (min1)
68.5 0.0044
42.5 0.0031
Fig. 1. Powder XRD results of Samples A and B (raw and annealed versions).
The XRD patterns of the raw products are presented in Fig. 1 together with literature peak positions of Tenorite form of CuO (JCPDS 48-1548). The peaks at two theta positions of 35.6° and 38.7° are identified as the (0 0 2) and (1 1 1) lines of the monoclinic phase of copper (II) oxide. The Scherer equation puts the crystallite domain size in 9–10 nm range. Prior to the BET analysis, the samples had to be annealed in vacuum at elevated temperatures. It was determined that the annealing of CuO nanoparticles at 300 °C in vacuum for several hours transformed them mostly into Cuprite form of Cu2O (JCPDS 05-0667) as seen in the right panel of the figure. It should be noted that the transformation was not complete at this stage as evidenced by the remaining (0 0 2) and (1 1 1) peaks of CuO. Similar transitions were previously reported for thin copper (II) oxide films annealed at 200–300 °C [38]. It is also noteworthy that annealing also yielded much narrower peaks in the XRD indicating the crystallite domain size increased to approximately 30 nm. As a result of these transitions, the annealing operations before BET analysis had to be carried out at 150 °C, which was determined after several trials. FTIR spectra of the products, presented in Fig. 2, were used to determine the surface contamination of the samples after the XRD experiments identified CuO as the only crystalline product. The spectra of the Samples A and B, for both raw and annealed (at 150 °C) versions, are presented in the figure. In the raw form, the strong peaks between 1400 and 1560 cm1 were assigned to the shifted C@O stretching of the carboxylate ion bonded to the copper (II) oxide nanoparticles as bidentate ligand originating from the acetate residues on the surfaces [39–41]. In addition, surface adsorbed water was identified by its broad absorption around 3250 cm1. The spectra of the annealed samples were almost identical to the spectra of the raw samples, except for minute changes
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Fig. 2. FTIR spectra of Samples A and B (raw and annealed versions).
around 2000 cm1. Therefore, it can be inferred that annealing the samples at 150 °C in vacuum did not significantly clean the surfaces off contaminations. Nevertheless, the catalytic activity of the raw and annealed materials was tested for comparison. The SEM images of the copper (II) oxide particles are presented in Fig. 3. The particles were mainly spheroids with 100–200 nm size with clearly visible surface structuring. These structures were actually much smaller particles with sizes close to the crystallite domain size determined by the Scherer equation. While these particles protruded from the surface and increased the surface area, there were not any significant differences between Samples A and B in the SEM images. The EDS analysis of Sample A, presented in Fig. 4, showed 50–55% Cu and 50–45% O atomic composition supporting the CuO stoichiometry. It is important to mention that annealing apparently reduced the amount of carbon contamination off the surface, contrary to the FTIR analysis results. 3.2. BET analysis The BET analysis was run after the samples were annealed at 150 °C in vacuum for several hours. Annealing was attempted to
Fig. 4. EDS spectra of Sample A (raw and annealed versions).
get rid of the acetate and water residues on the surfaces. Since higher temperatures triggered tenorite to cuprite transition, the annealing temperature was set to 150 °C after several trials to eliminate unintentional phase transitions. Although annealing apparently cleaned some of the contamination, significant amount of chemisorbed contamination still remained on the surface as seen in Fig. 2 due to the low annealing temperature. Nevertheless, the BET analysis was performed on these samples. The nitrogen adsorption/desorption isotherms for the samples are presented in Fig. 5 together with the corresponding BJH pore size distribution semi-log curves. The isotherm is of type IV with very little hysteresis which is characteristic for mesoporous materials [42]. The pore size distribution curves demonstrate the presence of mesopores of 1 nm and a broad size distribution above 20 nm. The BET surface area values were measured as 68.5 and 42.5 m2/g for Samples A and B, respectively. Single point adsorption total pore volume was calculated as 0.13 and 0.09 cm3/g for the same samples, respectively. It should be pointed out that the surface areas of these samples were significantly larger than similar products reported in the literature despite the apparent surface contamination [13,22]. 3.3. Catalytic activity The catalytic activity of raw and annealed (at 150 °C) versions of Samples A and B nanoparticles for the degradation of methylene blue (MB) was studied using visible spectroscopy. The absorbance spectra of MB were recorded over several hours with 30 min
Fig. 3. SEM images of Samples A and B (raw version).
Fig. 5. BET isotherms and pore size distributions for Samples A and B (annealed form).
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intervals under different conditions. Initially, the spectra of MB were recorded with H2O2 in the absence of copper (II) oxide nanoparticles to establish that the degradation reaction had to be catalyzed. As presented in Fig. 6a, the absorption spectrum of MB did not change over 6 h under these conditions. Once we established the need for catalysis, the possibility of MB adsorption of the CuO particles had to be eliminated as a source for absorption decrease. The spectra of MB with 1.0 mg CuO nanoparticles in the absence of H2O2 are presented in Fig. 6b. The adsorption of MB on CuO surface was insignificant over 6 h and did not cause an important decrease in the absorbance. Finally, the catalytic activity of CuO nanoparticles were tested as described before. All versions of copper (II) oxide particles prepared in this study decreased the absorbance of MV in the presence of H2O2 albeit at different rates. The absorption spectra of MV in the presence of H2O2 and 1.0 mg of raw Sample A nanoparticles are presented in Fig. 6c. A significant decrease in the absorption was detected in the first half hour of the experiment. The absorption gradually decreased in time until the solution became colorless to the naked eye in about 5 h. The percentage degree of degradation was calculated by taking the difference of the initial and measured absorbance and dividing it by the initial absorbance value. The plot of percentage degree of degradation vs time is presented in Fig. 7a for Samples A and B, both for raw and annealed (at 150 °C) versions. The fastest degradation rate was observed for raw Sample A with about 96% degradation in 3 h. The degradation rate of raw Sample B was a little lower, reaching only 91% within the same amount of time. Annealing of samples at 150 °C decreased the degradation rate significantly. Only 55% and 45% of MB could be degraded in 3 h for the case of annealed Samples A and B, respectively. The reaction kinetics of degradation were modeled using the relative amounts, i.e. (A0/A) to remove the intercept of the plot. The plots of the logarithm of the ratio of the initial absorbance to the measured absorbance (ln(A0/A)) vs time are presented for the raw and annealed versions of Samples A and B in Fig. 7b and c, respectively. The plots yielded straight lines; therefore the kinetics of degradation was found to be first order in accord with the literature [22]. However, the plots of both of the raw CuO samples resulted in two straight lines with different slopes, i.e. rate constants. It was found that the rate constants changed after the initial hour, a cusp formed in the graphs isolating the first three points from the others. Therefore, first three points were fit together with the intercept forced to zero. Rest of the points was fit with linear regression without any restrictions. In all cases the chi square values were
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better than 0.995. The initial rate constants were determined to be 0.030 and 0.018 min1 for the raw versions of Samples A and B, respectively. After the initial hour, both rate constants decreased, i.e. to 0.013 and 0.011 min1, respectively. The annealed products were also tested under the same conditions for comparison. Only a single slope was detected in the plot of the annealed samples, however, the rate constants for these annealed products were much lower, i.e. 0.0044 and 0.0031 min1, for Samples A and B, respectively.
4. Discussion The reaction mechanism leading to copper (II) oxide deserves some consideration. It is evident that the reaction was driven by the free radicals originating from the peroxide degradation. Such reactions are well known for the Fe2+/Fe3+ redox system, i.e. the Fenton reactions. The involvement of copper ions in Fenton-like reactions has been debated for some time [29–34]. Recently, the participation of Cu (III) ions in such reactions as intermediate species were proposed [30]. The copper redox chemistry involving such transient higher oxidation states can be quite complex especially when assisted by high intensity ultrasound. Therefore, the proposed mechanism is merely a collection of possible reactions bearing in mind that their rate constants can be substantially different under current reaction conditions. We propose that the reactions were initiated with the degradation of peroxides by ultrasound induced cavitations. This hypothesis is based on the observation that we could not obtain any products when the temperature of the solution was higher than 50 °C indicating two competing degradation pathways, i.e. via cavitations and thermal reactions. The copper (II) oxide yield increased when the solution temperature was lowered, i.e. the possibility of thermal degradation was eliminated. Copper (II) oxide can only form when the hydroxide radical turns into the hydroxide anion via a Fenton-like reaction and the formed copper (II) hydroxide (Ksp = 2.2 1020) anneals in situ to the final product. The proposed mechanism starts with Eq. (1) i.e. the degradation of hydrogen peroxide within the cavitations induced by ultrasound. The copper (II) ions can be reduced to copper (I) ions by hydrogen peroxide as presented in Eq. (2). The copper (I) ion can participate in a Fenton-like reaction yielding the hydroxide anion as in Eq. (3) and recycle to the copper (II) ions. The hydroxide anion and copper (II) ions precipitate as copper (II) hydroxide and
Fig. 6. Absorption spectra of MB under various conditions.
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Fig. 7. Percentage degradation and degradation kinetics of MB.
anneal to copper (II) oxide in situ close to the collapsing cavitations as given in Eq. (4). The hydrogen peroxide can also react with hydroxide radicals forming the reactive oxygen radical anion, i.e. superoxide (Eq. (5)), which can reduce copper (II) ions as in Eq. (6). Under these reaction conditions, it is highly probable that copper (II) ions can also be oxidized in Fenton-like reactions yielding transient copper (III) ions as in Eq. (7). The copper (III) ions can be recycled back to copper (II) via reduction reactions with hydrogen peroxide or superoxide (Eqs. (8) and Eq. (9)). The decrease in the pH of the solution during the reaction supports reactions 2, 5 and 8 where superoxide and hydrogen ions form:
H2 O2 ! 2 OH
ð1Þ
þ Cu2þ þ H2 O2 $ Cuþ þ O 2 þ 2H
ð2Þ
Cuþ þ 2 OH ! Cu2þ þ OH þ OH
ð3Þ
Cu2þ þ 2OH ! CuðOHÞ2 ! CuO þ H2 O
ð4Þ
þ H2 O2 þ OH ! O 2 þ H þ H2 O
ð5Þ
þ Cu2þ þ O 2 !Cu þ O2
ð6Þ
Cu2þ þ 2 OH ! Cu3þ þ OH þ OH
ð7Þ
þ Cu3þ þ H2 O2 $ Cu2þ þ O 2 þ 2H
ð8Þ
2þ Cu3þ þ O þ O2 2 !Cu
ð9Þ
The reaction hypothesis was tested with organic peroxides, benzoyl peroxide ((C6H5CO)2O2, CAS number: 94-36-0) and dicumyl peroxide ((C6H5C(CH3)2)2O2, CAS number: 80-43-3) as well. These peroxides are water insoluble at low temperatures, and the experiments had to be carried out at higher temperatures. Although the reaction temperatures were 80 °C in both cases, the temperature was still below the required thermal degradation threshold [43]. Therefore, it is safe to assume that these symmetric organic peroxides were also degraded by ultrasonic cavitations into reactive radicals, which further generated the hydroxyl radicals in water. The hydroxyl radicals participated in the reaction mechanism as suggested above and copper (II) oxide particles were isolated in both cases. The yields of these reactions were rather low, i.e. 40–50%, and in addition the BET surface areas were much
lower, i.e. 5–10 m2/g, than their counterparts prepared with H2O2. These particles showed diminished catalytic activity, with initial rate constants as low as 0.008 min1, in accord with their smaller surface area. These reactions are currently under investigation. In the present work, the oxide yield increased with decreasing solution temperature. It is possible that lower temperatures not only diminished the thermal degradation contribution to the reaction, but also increased the cavitation efficiency by decreasing the vapor pressure of the solution, as well [44]. Cavitations must also play a crucial role in annealing the hydroxides to oxides in the solution. In the classical chemical route, when the copper ions are precipitated with a base, the temperatures must be higher than 80 °C for the oxide formation. In the current work, on the other hand, temperatures as low as 0 °C yielded crystalline oxide products. The annealing process must have taken place at or near the cavitations where local temperatures exceeded the ambient values. The initial rate constant of the raw version of Sample A particles for the catalytic degradation of MB was about 3.0 102 min1 which is 2.3-fold larger than the reported values in the literature [22], where the products were prepared by hydrothermal methods employing surfactants. The decrease in the rate constants of the raw versions of Samples A and B after the initial hour can be attributed to the irreversible adsorption to or poisoning of the most reactive defect sites. There is a correlation between the rate constants of both the raw and annealed samples and the BET areas, despite the fact the BET areas could only be measured for the annealed samples. Annealing the oxides did not alter the structure or the surface contamination significantly, however, resulted in much reduced rate constants. This observation indicates that the surface defects were responsible for the heightened activity of these products. 5. Conclusions CuO particles were successfully synthesized in an ultrasound assisted Fenton-like reaction starting from copper (II) ions and hydrogen peroxide. The oxides precipitated through the formation of the hydroxides via the peroxide radicals and in situ annealing of copper hydroxides to oxides. Higher yields were achieved at lower temperatures due to increased cavitation efficiency. CuO was found to be an effective catalyst for the degradation of methylene blue in the dark in the presence of H2O2. The initial rate constants were about 50% greater than previously reported values, whereas the rate constants reached the literature values in time. This
A. Angı et al. / Ultrasonics Sonochemistry 21 (2014) 854–859
decrease in the rate constant and the loss of catalytic activity after annealing of products both signify the importance of cavitation induced surface defects in the catalytic process.
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