ARTICLE pubs.acs.org/JPCC
Facile Microwave-Assisted Hydrothermal Synthesis of CuO Nanomaterials and Their Catalytic and Electrochemical Properties Guohong Qiu,†,‡ Saminda Dharmarathna,‡ Yashan Zhang,‡ Naftali Opembe,‡ Hui Huang,‡ and Steven L. Suib*,‡ †
Key Laboratory of Subtropical Agricultural Resources and Environment, Ministry of Agriculture, College of Resources and Environment, Huazhong Agricultural University, Wuhan, 430070, People's Republic of China ‡ Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, Connecticut, 06269-3060, United States of America
bS Supporting Information ABSTRACT: Copper oxides have been widely used as catalysts, gas sensors, adsorbents, and electrode materials. In this work, CuO nanomaterials were synthesized via a facile microwaveassisted hydrothermal process in Cu(CH3COO)2(0.1 M)/ urea(0.5 M) and Cu(NO3)2(0.1 M)/urea(0.5 M) aqueous systems at 150 °C for 30 min. The formation processes of copper oxides were investigated, and their catalytic activities were evaluated by the epoxidation of alkenes and the oxidation of CO to CO2. Their electrochemical properties were compared as supercapacitor electrodes using cyclic voltammetry. Experimental results indicated that copper acetate solution could be hydrolyzed to form urchin-like architectured CuO, and the addition of urea accelerated this transformation. CuO nanoparticles were formed and aggregated into spheroidal form (CuO-1) in Cu(CH3COO)2/urea aqueous solution. Cu2(OH)2CO3 was formed as an intermediate, and then thermally decomposed into CuO nanorods (CuO-2) in the Cu(NO3)2/urea aqueous system. The synthesized copper oxide nanomaterials exhibited excellent catalytic activities for the epoxidation of alkenes, the oxidation of CO, and pseudocapacitance behavior in potassium hydroxide solution. The increase of specific surface area promoted the catalytic activities and conversions for olefins and CO. CO was oxidized to CO2 when the applied temperature was higher than 115 °C, and conversion of 100% was obtained at 130 °C. CuO-1 showed higher catalytic activities and capacitance values than those of CuO-2 likely due to the former having a larger specific surface area. This work facilitates the preparation of nanosized CuO materials with excellent catalytic and electrochemical performance.
microspheres at 100 °C for 6 h.1 As for the above conventional methods, high temperature, high pressure, and relatively long times are required for the preparation of CuO nanomaterials. In contrast, unconventional methods based on chemical synthesis could provide an alternative and intriguing strategy for generating nanostructures in terms of material diversity, cost, throughput, and the potential for high-volume production.18 Microwave-assisted chemistry is based on dipolar and electrical conductor mechanisms.19,20 For the dipolar mechanism, the polar molecules follow the field in the same alignment when a very high frequency electrical field is applied, and then the molecules release enough heat to drive the reaction forward. In the electrical conductor mechanism, the irradiated sample is an electrical conductor and the charge carriers, ions, and electrons, move through the material under the influence of the electric field leading to polarization within the sample. These induced
1. INTRODUCTION Copper oxides function very efficiently in advanced materials for catalysts,13 gas sensors,46 adsorbents,7 superconductors,8 and electrodes of photocells,8 lithium-ion batteries,911 and supercapacitors.12,13 The structural morphology and particle size play significant roles in determining the physicochemical properties of CuO.1,8,9,13 Various synthesis methods have been used to control the morphologies and microstructures of copper oxides. Wirelike CuO particles with diameters in the range of 80200 nm were synthesized by heating copper foil at 500 °C for 5 h.6 Precipitation reactions of Cu(NO3)2 and NH4OH in aqueous solution were conducted to produce a precursor, and then calcined in air at 400 °C for 3 h to form flower-like CuO nanostructures.14 One dimensional CuO nanostructures were fabricated by means of single crystalline Cu2(OH)2CO3 nanoribbons as precursors for sacrificed templates via heat-treatment at nearly 300 °C for 1 h.15 Copper oxides with different morphologies have been widely prepared by hydrothermal reactions in organic and aqueous solutions at high temperatures and pressure.25,8,10,16,17 A facile reflux method was used to fabricate urchin-like CuO r 2011 American Chemical Society
Received: October 14, 2011 Revised: December 2, 2011 Published: December 03, 2011 468
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currents and any electrical resistance will heat the sample.1921 The growing interest in novel microwave processing can be attributed to several attractive features. Significant reduction in reaction time and energy costs is attributed to shorter processing times, rapid heating, and the formation of homogeneous products.2224 Microwave-assisted hydrothermal reactions were carried out to synthesize manganese oxides and α-Fe2O3.19,2224 Copper(II) chloride reacted with ammonium hydroxide to form [Cu(NH3)4]2+ complex, which was then hydrolyzed to Cu(OH)2, and further transformed into CuO in a microwave-hydrothermal system.25 However, different morphologies of CuO might be obtained by using other copper(II) salts, since CuO micromorphologies were affected by the species of divalent copper salts for different formation mechanisms.1 Copper oxides with various micromorphologies possibly need to be directly synthesized with a simplified one-step procedure. Copper acetate and urea were used to synthesize hierarchical cocoon shaped CuO by microwave irradiation with refluxing, however, the exact reaction process and formation mechanism of CuO architectures need to be further investigated.26 The application and physio-chemical properties of the products resulting from microwave irradiation have not yet been reported. Investigation of the formation process, function, and properties of CuO synthesized by microwaveassisted reactions would further promote the development of this novel method in the fabrication of advanced materials, and also be favorably viewed for commercial applications. CuO nanostructures have been investigated to catalyze olefin epoxidation to substitute for noble-metal-containing catalysts, such as gold and silver oxides.1,2 When CuO nanoparticles participated in these catalytic reactions for 24 h, the conversion and selectivity were recorded,1 and catalytic performance was compared using different samples obtained by reflux methods, conventional methods, and purchasing. The key influential factors controlling differences in the catalytic performance of CuO samples need be further studied. CuO colloidal nanocrystal clusters coated with mesoporous SiO2 shells were synthesized and exhibited excellent catalytic activity and stability for olefin epoxidation reactions, and specific surface area played an important role in the catalytic process.2 However, the transformation process of the intermediate products, particularly for byproducts, needs to be further investigated to scale-up the process. Catalytic oxidation of CO is not only an important reaction for many applications, such as removal of exhaust gas and in fuel cells, but is also a fundamental process involving the participation of surface oxygen and oxygen vacancies.3,24,27 CuO nanomaterials showed excellent catalytic activity for the oxidation of CO to CO2, and the activity was affected by shapes and particle sizes.3,28,29 CO could be completely converted to CO2 at low temperatures of 150160 °C, likely due to the higher probability of some exposed crystal planes and larger specific surface area.3,28 However, the influence of temperature on the conversion of CO needs to be further studied, which would help determine the lowest temperature that CO could be transformed into CO2 by using CuO catalysts. Increasing the specific surface area of these catalysts might enhance their catalytic activity at lower temperatures and reduce energy consumption. Therefore, CuO catalysts should be synthesized with larger specific surface area, and all influencing factors need to be further investigated during the catalytic process. CuO nanostructures were used as pseudocapacitor electrode materials and exhibited superior performance in terms of specific capacitance, cyclability, energy density, and power density.12,13,30
Electrochemical performance and morphologies were compared for the synthesized and purchased CuO samples. Specific surface area might play an important role in improving the specific capacitance and influence of surface area on electrochemical properties needs to be investigated. In this work, copper oxide nanostructures were synthesized by facile microwave-assisted hydrothermal reactions using Cu(CH3COO)2/urea and Cu(NO3)2/urea aqueous solutions, respectively. The formation mechanism of CuO nanomaterials was studied in detail. Conversions and selectivity for the epoxidation of olefins, such as styrene, norbornene, and trans-β-methylstyrene, were obtained and compared when the synthesized CuO nanomaterials catalyzed the reactions. Gas phase catalytic oxidation of CO to CO2 was performed, and the influence of specific surface area, temperature on the conversions, and the stability of catalysts were investigated. The capacitance of these two samples fabricated in different solutions was compared by cyclic voltammetry.
2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Copper acetate monohydrate (>98%) and cupric nitrate 2.5-hydrate (>98.8%) were purchased from Alfa Aesar and J. T. Baker, respectively. Nickel foil (thickness: 0.125 mm, g 99.9%), urea (g99%), norbornene (99%), styrene (g99%), and trans-β-methylstyrene (99%) were supplied by Sigma-Aldrich. Acetonitrile and t-butylhydroperoxide (TBHP) were purchased from Honeywell Burdick & Jackson and Fluka, respectively. All reagents used were of analytical grade, unless otherwise noted. 2.2. Synthesis and Characterization of CuO Nanomaterials. In a typical experiment, a mixture of 10 mL of 0.1 mol L1 Cu(CH3COO)2 (or Cu(NO3)2) and 0.5 mol L1 urea aqueous solution was prepared in a 20-mL reactor vial and then sealed. The apparatus was equipped with a magnetic stirrer. The reactions were carried out in a Biotage Initiator microwave synthesizer, which was programmed to maintain at a fixed temperature with holding times from 5 to 30 min. All newly synthesized products were washed with distilled deionized water (DDW) several times to remove any possible impurities, such as the adsorbed Cu2+, NH4+, and anions, and then dried in an oven at 60 °C overnight. The as-obtained products were characterized by X-ray diffraction (XRD) at room temperature using a Scintag XDS 2000 instrument with Cu Kα radiation. The diffractometer was operated at a tube voltage of 45 kV and a tube current of 40 mA. Fourier transform infrared spectroscopy (FTIR, Nicolet 8700) was carried out with a DTGS detector by making pellets with KBr powder and CuO/KBr mass ratio about 1: 50, and the resolution was set at 4 cm1 with a scan number of 32. The BET surface area of CuO powder samples was tested with a Quantachrome Autosorb-1-C automated N2 gas adsorption system. About 0.25 g sample was weighed and degassed at 120 °C for 12 h before N2 physisorption measurements. Before and after catalytic reactions, the morphologies were characterized using a Zeiss DSM 982 Gemini field-emission scanning electron microscope (FESEM). High-resolution transmission electron microscopy (HR-TEM) experiments were performed using a JEOL 2010 instrument with an accelerating voltage of 200 kV. CuO powder was dispersed in ethanol, and then a drop of the homogeneous dispersion was loaded on a carbon coated copper grid and allowed to dry before analysis. 469
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2.3. Catalytic Activity for Olefin Epoxidation. The olefin epoxidation reactions were carried out in a 50-mL three-necked round-bottom flask connected with a reflux condenser. Reactants of 1 mmol of alkene, 10 mL of acetonitrile, and 10 mg of CuO catalyst were added into the 50 mL round bottomed flask, and then 5 mmol TBHP was added dropwise to the mixture. The reaction was maintained at 70 °C for different times under vigorous stirring, and the temperature was controlled using a paraffin oil bath. At the end of each reaction, about 0.2 mL solution was collected, and suspended catalysts were separated by filtration through a Millipore filter with pore size of 0.45 μm. Gas chromatographymass spectroscopy (GCMS) methods were used to identify and quantify the starting materials and products. GCMS analyses were conducted using an HP 5890 series II chromatograph with a thermal conductivity detector coupled with an HP 5970 mass selective detector. An HP-1 column (nonpolar cross-linked siloxane) with dimensions of 12.5 m 0.2 mm 0.33 μm was used in the gas chromatograph. 2.4. Catalytic Performance for the Oxidation of CO. The catalytic performance tests were carried out at atmospheric pressure in a quartz tubular fixed bed flow reactor. The temperature of the reactor was monitored using a K-type thermocouple in direct contact with the catalyst upstream of the bed and was regulated with a proportional-integral-derivative (PID) temperature controller. CuO samples (150 mg) were used in the experiment. The feed gas was composed of 1% CO, 20% O2, and 5% N2 by volume in helium, and the space velocity was controlled at 35 000 mL h1 gcat1 using mass flow controllers. Nitrogen was included as an internal standard for gas chromatography (GC). The reaction products were analyzed using an online gas chromatograph [SRI 8610C Gas Chromatograph (GC)] equipped with a 60 molecular sieve, a 60 silica gel column, and a thermal conductivity detector (TCD). Helium was used as the carrier gas for the GC. Before each experiment, the catalysts were pretreated by flowing He (50 mL min1) for 2 h at 110135 °C. GC samples were injected after 10 min stabilization at any given temperature. 2.5. Electrochemical Properties of CuO Nanomaterials. The electrochemical properties of the synthesized copper oxides were studied by cyclic voltammetry. The working electrode was prepared as follows: 10 mg copper oxide, 10 mg acetylene carbon black, and 5 drops of 60% polytetrafluoroethylene solution were ultrasonically dispersed in 5 mL of distilled deionized water, and 20 μL of the suspension was pipetted onto a glassy carbon substance. Nickel foil (1.0 cm 1.0 cm 0.125 mm) was used as the counter electrode, and a saturated calomel electrode (SCE) was used as a reference electrode with a salt bridge in the experiment. The working and the counter electrodes were vertically dipped into a 6.0 mol L1 KOH solution, and the capacitive characteristics of copper oxides were compared by cyclic voltammetry in a potential window of 00.4 V (vs SCE).
Figure 1. XRD patterns of the products synthesized at different temperatures for 30 min in different solutions: (a) 0.1 M Cu(CH3COO)2 + 0.5 M urea, (b) 0.1 M Cu(NO3)2 + 0.5 M urea.
Cu2(OH)2CO3 was formed (JCPDS: 100399) at 120 °C (Figure 1b). As the reaction temperature increased from 120 to 150 °C, CuO was obtained (Figure 1b). The prepared copper oxides were designated as CuO-1 and CuO-2 when Cu(CH3COO)2 and Cu(NO3)2 were used as divalent copper sources in the above reaction systems at 150 °C for 30 min, respectively. Microwave-assisted synthesis technique can shorten the reaction time and reduce energy cost.2025 As shown in the Supporting Information, SI, Figure S1, the temperature, pressure, and power profiles were recorded when both reactions were carried out at 150 °C for 30 min. For systems of Cu(CH3COO)2/urea and Cu(NO3)2/urea, the reaction temperature in the reactor vial was rapidly increased and well maintained at 150 °C, and the pressure was kept at about 8.0 bar, and the power ranged from 26 to 36 W as the reaction proceeded, which indicated that the energy consumption was very low during this process. Further characterization of the synthesized samples, including CuO-1 and CuO-2, was conducted using FTIR spectroscopy as shown in the SI, Figure S2. Similar absorption spectra were obtained as reported in the literature.1,25,3133 The strong absorption bands at 539 and 594 cm1 were due to CuO stretching vibrational modes.31,32 The typical peaks at 1641 and 3441 cm1 corresponded to the stretching and bending vibrational modes of adsorbed water, respectively.3133 The morphologies of CuO-1 and CuO-2 were characterized by field emission scanning electron microscopy and high resolution transmission electron microscopy. Figure 2 shows the SEM images of the as-obtained CuO-1 and CuO-2. Spherical CuO-1 materials were formed with diameters of less than 1 μm and composed of nanosized particles (Figure 2a). Rod-like CuO-2 particles were formed and assembled into flower-like morphologies with a single nanorod having a width of less than 100 nm (Figure 2b). Morphologies and crystalline structures of CuO-1 and CuO-2 were further examined by HR-TEM (SI Figure S3). Nanosized CuO-1 particles were aggregated to form solid spheres, while rod-like CuO-2 was radially arranged to form flower-like structures. These morphologies were in good agreement with the observed results from FESEM. As insets of SI Figure S3, the width of 0.25 nm from neighboring fringes of single nanoparticles corresponded to (111) planes, which were consistent with the XRD results, and suggested that both the as-obtained products were copper oxides. Nitrogen gas adsorption measurements were used to determine the BrunauerEmmettTeller (BET) specific surface area of the synthesized copper oxides (SI, Figure S4). The specific surface areas of CuO-1 and CuO-2 were determined to be 45.7 and 25.6 m2 g1, respectively. These results were consistent with the morphologies and particle sizes of CuO-1 and CuO-2
3. RESULTS 3.1. Preparation and Characterization of CuO Nanomaterials. Microwave-assisted hydrothermal reactions were conducted in
different solution systems at 120 and 150 °C for 30 min, respectively. X-ray diffraction was used to study the phase purity of the obtained copper oxides. As shown in Figure 1, CuO (JCPDS: 050661) samples were obtained when 0.1 M Cu(CH3COO)2 and 0.5 M urea were used at both 120 and 150 °C (Figure 1a). However, when Cu(NO3)2 was used instead,
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Figure 2. Typical SEM images of the synthesized products: (a) CuO-1 and (b) CuO-2.
conducted for 8 h, the conversions were as high as 96% and 97% with selectivity of 100% when CuO-1 and CuO-2 participated in the reaction at 70 °C, respectively. 3.3. Catalytic Activity for the Oxidation of CO. For catalytic studies in the oxidation of CO, mixed gases (1 vol. % CO + 5 vol. % N2 + 20 vol. % O2 + 74 vol. % He) flowed through 0.15 g copper oxides (CuO-1, CuO-2) in the bed at different temperatures. As shown in Figure 4, when CuO-2 was used as catalyst, conversion to CO2 was zero when the temperature was below 115 °C, and increased to 46.0%, 65.9%, 80.8%, and 100.0% at 120, 125, 130, and 135 °C, respectively, at a flow rate of 10 mL min1 (Figure 4). When CuO-1 was used instead, the conversions remarkably increased to 53.0%, 75.7%, 100.0%, and 100.0% at 120, 125, 130, and 135 °C, respectively. 3.4. Capacitance Characteristics of Copper Oxides. Figure 5 shows the cyclic voltammograms (CVs) of CuO-1 and CuO-2 in 6.0 mol L1 KOH solution. These CVs exhibited pseudocapacitance properties of synthesized copper oxides, and rectangular and symmetric currentpotential curves for both oxides. There are two pairs of redox current peaks in the range of 0.20.3 and 0.30.4 V. The first pair of redox peaks in the range of 0.20.3 V is likely attributed to the oxidation of Cu2O (or CuOH) to CuO, and the second one might be due to the oxidation of Cu2O (or CuOH) to both CuO and Cu(OH)2.12 The peak potential depended on the hydroxyl concentration, and the specific capacitance of active materials is directly proportional to the voltammetric current for the same scanning rate.12,13 In this work, the intensity of the current using CuO-1 was remarkably higher than that of CuO-2 at the same scanning rate, suggesting that the specific capacitance of CuO-1 was superior.
Figure 3. Conversion of styrene oxidized by TBHP using 0.01 g CuO catalysts at 70 °C for different times.
(Figure 2). The size of spherical CuO-1 particles of about 500 nm in diameter was observed. However, the flower-like CuO-2 particles were greater than 1 μm in size. Single particles of CuO-2 were much bigger than those of CuO-1 (Figure 2, SI Figure S3), which might considerably affect their physicochemical properties. 3.2. Catalytic Performance for Olefin Epoxidation. Styrene was oxidized by TBHP using the synthesized CuO powder as catalyst at 70 °C for different time periods. The reaction temperature was controlled using the same paraffin oil bath to use the same experimental conditions. Figure 3 shows the time-dependent conversion of styrene when CuO-1 and CuO-2 were performed in these catalytic reactions. The oxidation rate of styrene followed a typical reaction kinetic curve. In the initial stages, the oxidation rate of styrene increased sharply then slowed down with time. After 24 h of reaction, the conversions of styrene approached 100% for both catalysts of CuO-1 and CuO-2 (Figure 3). However, the oxidation rate of styrene was higher when CuO-1 was used during the reaction process. For example, the conversions were about 70% and 54% when the reaction was carried out for 2 h using CuO-1 and CuO-2 as catalysts, respectively. As the reaction proceeded for 8 h, the conversions were increased to 92% and 86%, respectively (Figure 3). Table 1 shows the conversions and selectivities of styrene and norbornene when CuO-1 and CuO-2 were used in the catalytic reactions. Norbornene could be completely oxidized to norbornene epoxide with selectivity of 100%. Combined with the results of the epoxidation of styrene and norbornene, olefin epoxidation reaction might reach equilibrium at 8 h. When trans-βmethylstyrene was used instead and the catalytic reaction was
4. DISCUSSION 4.1. Formation Mechanism of Copper Oxide Nanomaterials. The formation mechanisms of copper oxides were compli-
cated in thermal reactions, and various reaction processes were proposed in different solution systems.1,13,16,26,34 The intermediate product of Cu2(OH)2CO3 was proposed when Cu(CH3COO)2 reacted with urea at 80 °C by microwave irradiation with refluxing for 30 min. However, Cu2(OH)2CO3 was not further produced during the reaction process.26 As reported in our previous work, OH produced from the decomposition of urea would first reacted with Cu2+ to form a precipitate of Cu2(OH)3NO3 when Cu(NO3)2 was used instead,1 suggesting that Cu2(OH)2CO3 was likely not formed in the process. In order to 471
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Table 1. Epoxidation of Alkenes with Synthesized Copper Oxidesa
a
1 mmol alkene, 5 mmol TBHP, and 10 mg catalyst, stirred in 10 mL of acetonitrile under reflux at 70 °C.
holder using ethanol and characterized by XRD. When the film was not thick enough, a wide diffraction peak at about 22° (2θ) was observed for the glassy substance of the sample holder (Figure 6a,b). This diffraction peak could not be observed when the product was dispersed as a thick film (Figure 6c,d). With the participation of urea, the crystallinity of the as-prepared CuO was enhanced for microwave-assisted hydrothermal reaction of 0.1 M Cu(CH3COO)2 at 120 for 5 min due to the increase of peak intensity in XRD patterns (Figure 6c,d). This trend was also observed as the reaction time was extended to 10 and 20 min. The amount of copper oxide precipitate increased substantially after urea was added to the Cu(CH3COO)2 aqueous solution at 120 and 150 °C, respectively. In this case, larger amounts of OH were produced, resulting in the formation of Cu(OH)2 with subsequent transformation into CuO. Copper hydroxide acetate [Cu2(OH)3OCOCH3 3 H2O] was formed when the hydrothermal reaction of Cu(CH3COO)2 3 H2O and NaOH aqueous solution with molar ratio of OH/Cu2+ about 1.0 was conducted at 140 °C for 1 h.8 In this work, copper hydroxide acetate was not formed because the concentration of OH, resulted from the decomposition of urea and hydrolysis of CH3COO, was not high enough. This synthesis of copper oxide is different from the proposed mechanism for producing CuO in the literature.1,8,16 When 0.1 M Cu(NO3)2 was used instead with 0.5 M urea at 120 °C for 30 min, Cu2(OH)2CO3 was synthesized as shown in Figure 1b, suggesting that Cu2(OH)2CO3 could be stable at 120 °C for 30 min. However, this intermediate product was not found in the reaction system of Cu(CH3COO)2 and urea. Therefore, Cu2(OH)2CO3 should not be produced during the reaction process of 0.1 M Cu(CH3COO)2 and 0.5 M urea when the temperature was below 120 °C since XRD experiments did not show the presence of this material. As the temperature was
Figure 4. Conversion of CO to CO2 oxidized by air using 0.15 g CuO catalysts at different temperatures.
investigate the role of urea during the formation of CuO, the microwave-assisted hydrothermal reaction was conducted using 0.1 M Cu(CH3COO)2 without urea at 120 and 150 °C for 30 min, respectively. Single-phase copper oxides were formed (Figure 6a,b), implying that urea did not function as a crucial factor in the formation of CuO. In order to further investigate the formation process, the reaction was performed at 120 °C for 5 min, and only copper oxide was observed as shown in Figure 6c. Copper oleate could be hydrolyzed to form Cu(OH)2, and then CuO was obtained by further adding NaOH in the aqueous solution.35 In the present work, CuO was formed likely due to the decomposition of Cu(OH)2, in which OH was produced from the hydrolysis reaction of CH3COO. Cu(OH)2 was not observed as an intermediate product since that Cu(OH)2 could be decomposed into CuO in aqueous solution at less than 120 °C. As reported, Cu(OH)2 could be thermally decomposed into CuO in aqueous solution at low temperatures, such as 60 and 80 °C.30,36 Due to the low yield, the products had to be dispersed on a glass 472
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Figure 5. Cyclic voltammograms of the prepared copper oxides in 6.0 mol L1 KOH solution with different scanning rates, (a) CuO-1 and (b) CuO-2.
Figure 6. XRD patterns of the products synthesized under different conditions: (a) 0.1 M Cu(CH3COO)2, 120 °C, 30 min; (b) 0.1 M Cu(CH3COO)2, 150 °C, 30 min; (c) 0.1 M Cu(CH3COO)2, 120 °C, 5 min; and (d) 0.1 M Cu(CH3COO)2 + 0.5 M urea, 120 °C, 5 min.
increased to 150 °C, CuO was formed due to the thermal decomposition of Cu2(OH)2CO3 in Cu(NO3)2/urea reaction system. Solid Cu2(OH)2CO3 could be decomposed at 250 °C in air,15 however, the decomposition temperature decreased in aqueous solution likely due to the hydrolysis reaction, and a similar trend was also observed for Cu(OH)2 and Cu2(OH)3Br.16,36 These transformation processes were recorded and studied by SEM images. Urchin-like CuO was formed when hydrothermal reaction of 0.1 M Cu(CH3COO)2 was carried out under microwave radiation for 30 min (Figure 7a). Similar morphologies were also obtained by the hydrolysis of copper lactate at high temperature and high pressure.34 When the reaction temperature increased to 150 °C, the uniformity of the nanorods that assembled into spherical structures decreased (Figure 7b) likely due to the rapid dehydration of Cu(OH)2 at a higher temperature. When 0.5 M urea was added to the reaction system, spherical CuO particles assembled by nanorods were formed as shown in Figure 7c. The spherical diameter remarkably increased (Figure 7a,c) due to an increase in the concentration of OH produced from the decomposition of urea that facilitated precipitation of Cu(OH)2, which was subsequently transformed into CuO. As the reaction temperature was further increased to 150 °C, the size of the nanoparticles assembling into spheres decreased, although no obvious changes were observed (Figure 7d), possibly due to the rapid nucleation of Cu(OH)2 at a higher temperature, and then CuO particles were formed with smaller sizes. Compared with the morphologies of CuO products formed by using Cu(CH3COO)2 as shown in Figure 7, there was no significant difference, suggesting a similar formation mechanism. Only when 0.5 M urea was added to the reaction system, could a precipitate be formed in 0.1 M Cu(NO3)2 solution by
microwave-assisted hydrothermal reaction at both 120 and 150 °C. Nanosized Cu2(OH)2CO3 flakes were overlapped and assembled together at 120 °C as shown in the selected area in Figure 8a. When the reaction temperature increased to 150 °C, uniform CuO nanorods were generated to form flower-like structures (Figure 8b).37 Comparing the selected area in Figure 8a with the inset in Figure 8b, similar structures were observed. Therefore, CuO nanorods were possibly transformed from the decomposition of Cu2(OH)2CO3 flakes in aqueous solution. A small quantity of CuO was also observed as shown in Figure 8a, suggesting that Cu2(OH)2CO3 was not very stable at 120 °C for a long time. To further study the formation process of CuO, the above reaction system was performed at 150 °C for 5 and 10 min, respectively, and products were collected and characterized by XRD (SI, Figure S5). However, Cu2(OH)2CO3 was not obtained and CuO was formed likely due to the rapid thermal decomposition of the newly formed Cu2(OH)2CO3 at this high temperature. 4.2. Catalytic Activity for Olefin Epoxidation. Copper oxides have been used as an effective catalyst for olefin epoxidation with peroxide, and can substitute for noble-metal-containing catalysts, such as gold and silver oxides, due to their high catalytic activity, selectivity, environmentally friendly characteristics, and abundant resources.1,2,38 In this work, product distribution was determined during the reaction process. Figure 9 shows the timedependent selectivities of products, such as benzaldehyde, styrene oxide, and benzoic acid. A similar trend in selectivity was found when both CuO-1 and CuO-2 were used. Benzaldehyde was formed with the highest selectivity in the initial reaction stage, and then decreased remarkably. The selectivity of the target product styrene oxide increased as the reaction proceeded. 473
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Figure 7. SEM images of the products synthesized for 30 min under different conditions: (a) 0.1 M Cu(CH3COO)2, 120 °C; (b) 0.1 M Cu(CH3COO)2, 150 °C; (c) 0.1 M Cu(CH3COO)2 + 0.5 M urea, 120 °C; (d) 0.1 M Cu(CH3COO)2 + 0.5 M urea, 150 °C.
Figure 8. SEM images of the products synthesized in 0.1 M Cu(NO3)2 and 0.5 M urea solutions at different temperatures for 30 min: (a) 120 °C and (b) 150 °C.
The selectivity for styrene oxide was stable after 8 h of reaction as shown in Figure 9. When CuO-1 was used as catalyst, the selectivity to styrene oxide was about 28% for 1 h. However, the selectivity was almost zero as CuO-2 was used when the reaction proceeded for 1 h, suggesting that the catalytic activity of CuO-1 was higher than that of CuO-2 for the transformation of styrene into styrene oxide in the initial stage because the former possessed larger surface area. Benzoic acid was not formed at the beginning of the reaction, and the proportion increased with a decrease in the content of benzaldehyde, indicating that benzaldehyde was further oxidized into benzoic acid in the catalytic process.
At the end of the catalytic reaction, the selectivities for benzaldehyde, styrene oxide, and benzoic acid were almost the same when CuO-1 and CuO-2 were used (Figure 9), further proving that catalysts only enhance the reaction rate, but cannot change the chemical equilibrium. After 24 h of reaction, styrene was completely oxidized as shown in Figure 3 and Table 1 using both CuO-1 and CuO-2 catalysts. Instead, when norbornene was used, conversions and selectivities of 100% were obtained for both CuO-1 and CuO-2 catalysts after refluxing for 24 h (Table 1). As reported in our previous work, the conversion was about 89.5% under the same conditions.1 The specific surface area of 474
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Figure 9. Selectivity of the oxidation products of styrene by TBHP using different catalysts: (a) CuO-1 and (b) CuO-2.
urchin-like CuO obtained by reflux at 80 °C for 6 h was about 15 m2 g1.1 Norbornene could be completely oxidized to norbornene epoxide with selectivity of 100% likely due to the larger surface areas of copper oxides obtained by microwave-assisted hydrothermal reactions in this work. As shown in Figures 3 and 9, the oxidation reaction of styrene could reach equilibrium after 8 h for the high catalytic activity of copper oxides obtained in this work. The catalytic oxidation of trans-β-methylstyrene was then conducted for 8 h, and conversions approached 96% and 97% with selectivity of 100% when CuO-1 and CuO-2 were used (Table 1), respectively, which further suggested that both these synthesized copper oxides were excellent catalysts. 4.3. Catalytic Activity for CO Oxidation and Capacitance Property. The catalytic performance for the oxidation of CO was studied and compared in the literature, and the exposed crystal planes and specific surface areas possibly affected their catalytic activity.3,28 The complete conversion of CO to CO2 could be obtained at temperatures of 150160 °C when the specific surface area of CuO catalysts was about 30 m2 g1.3,28 Specific surface areas of catalysts might play an important role in the oxidation of CO. In this work, CuO-1 exhibited better catalytic activity than CuO-2 (Figure 4) likely due to the larger specific surface area of the former. When the reactant gases were conducted at a flow rate of 10 mL min1, CuO-1 and CuO-2 exhibited catalytic activity above 115 °C, and CO could be completely converted to CO2 at 135 °C as shown in Figure 4, suggesting that these synthesized copper oxides had a higher catalytic activity. The superior catalytic activity is likely due to (i) the larger specific surface area of the as-obtained copper oxides and (ii) the slow flow rate of feed gas. The specific surface area of the synthesized copper oxides in this work were larger than these reported copper oxides from commercial samples (1.2 m2 g1),2 hydrothermal synthesis (30.04 m2 g1),3 hydrothermal synthesis with subsequent heat-treatment at 400 °C (5.5 m2 g1),8 and reflux reaction (15 m2 g1).1 The catalytic activity of CuO-1 was higher than that of CuO-2 since the former had a larger specific surface area. We have studied the influence of flow rate on the conversion of CO to CO2 using α-Fe2O3 as catalyst, and found that the conversion decreased with an increase in flow rate of CO.28 As reported, the reaction gases including 1.0% CO passed through the reactor containing copper oxide at a flow rate of 100 mL min1.3,28 The lowest temperature for copper oxides having catalytic activity was evaluated at a flow rate of 10 mL min1 in this work. The synthesized CuO-1 and CuO-2 exhibited better catalytic activity than α-Fe2O3 at the same flow rate of feed gas
Figure 10. XRD patterns of the synthesized catalysts after a series of catalytic oxidation reactions of CO to CO2: (a) CuO-1 and (b) CuO-2.
even though the nanosized α-Fe2O3 possessed much larger specific surface area.24 After a series of catalytic oxidation reactions of CO, CuO catalysts were then characterized by XRD and SEM. The crystal structure of CuO-1 and CuO-2 did not noticeably change after reaction as shown in Figure 10, suggesting that the synthesized copper oxides are promising catalysts with excellent stability. The morphologies of the spent CuO powder were further characterized by SEM (SI, Figure S6). There was no obvious change in particle sizes and morphologies after the catalytic reaction of CO to CO2 at 110140 °C for more than 8 h. The CuO particles were not aggregated after catalytic reaction, indicating that the catalyst retained the high specific surface area during reaction. CuO-1 and CuO-2 exhibited catalytic oxidation activity when the applied temperature was over 115 °C, which may lead to further applications of CuO nanocrystals, such as in gas purification and carbon monoxide gas sensors, under mild conditions. The super capacitance behavior of CuO has attracted research interest in recent years, and cyclic voltammetric measurements are important methods to compare the capacitance properties of active materials.12,13,30,39 In this work, the CV current of CuO-1 was remarkably higher than that of CuO-2 at the same scanning rates in KOH solution, as shown in Figure 5. The specific surface areas of CuO-1 and CuO-2 were 45.7 and 25.6 m2 g1, respectively, suggesting that the capacitance of copper oxides could be enhanced by increasing specific surface area. Specific surface area and dopant affected the capacitance properties of active materials, such as CuO and MnO2.39,40 In order to improve the electrochemical properties of nanomaterials, chemical modification techniques, such as coating2,30 and doping,39 should be further conducted in the microwave-assisted synthesis of copper 475
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oxides. The chargedischarge cycling for CuO-1 and CuO-2 needs to be carried out at a constant current in future studies.
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5. CONCLUSIONS Nanosized copper oxides have been successfully prepared by microwave-assisted hydrothermal reactions in Cu(CH3COO)2/ urea and Cu(NO3)2/urea aqueous solutions at 150 °C for 30 min. In the Cu(CH3COO)2/urea reaction system, CuO-1 nanoparticles were formed and assembled into spheroidal form by hydrolysis of CH3COO, and Cu(OH)2 possibly acted as an intermediate product. In Cu(NO3)2/urea aqueous solution, CuO-2 nanorods were formed and assembled into flower-like structures by the decomposition of Cu2(OH)2CO3 intermediate product. Both the synthesized copper oxides (CuO-1 and CuO-2) exhibited high catalytic activity for olefin epoxidation. After 24 h of catalytic reaction, the conversions of styrene and norbornene reached 100% with selectivity of about 45% and 100%, respectively. The conversions of trans-β-methylstyrene approached 97% with selectivity of 100% when the reaction proceeded for 8 h. The prepared copper oxides showed excellent catalytic activity for the oxidation of CO to CO2 when the temperature was higher than 115 °C. CuO-1 exhibited better catalytic activity and capacitance properties than CuO-2 due to the larger specific surface area of the former. This work facilitates the preparation and application of nanosized copper oxides with different microstructures and understanding of the key factors influencing their catalytic activity and electrochemical performance. ’ ASSOCIATED CONTENT
bS
Supporting Information. This includes the representative profiles of temperature, pressure, and power when CuO-1 and CuO-2 were synthesized, FTIR spectra, nitrogen sorption isotherms, typical TEM and HRTEM images of CuO-1 and CuO-2, XRD patterns of the products synthesized in the solution of 0.1 M Cu(NO3)2 and 0.5 M urea at different times, and SEM images of CuO-1 and CuO-2 after a series of catalytic oxidation reactions of CO to CO2. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: 1 860 486 2797; fax: 1 860 486 2981; e-mail: Steven.
[email protected].
’ ACKNOWLEDGMENT The authors thank the National Natural Science Foundation of China (Grant Nos.: 41171375, 40830527, and 20807019) and the Fundamental Research Funds for the Central Universities (Program No.: 2011PY015) for financial support. The authors acknowledge the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical, Geochemical and Biological Sciences for support of this work. ’ REFERENCES (1) Xu, L.; Sithambaram, S.; Zhang, Y.; Chen, C. H.; Jin, L.; Joesten, R.; Suib, S. L. Chem. Mater. 2009, 21, 1253–1259. (2) Chen, C.; Qu, J.; Cao, C.; Niu, F.; Song, W. J. Mater. Chem. 2011, 21, 5774–5779. 476
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