Hydrothermal Synthesis of Co3O4–Graphene for Heterogeneous

Article pubs.acs.org/IECR

Hydrothermal Synthesis of Co3O4−Graphene for Heterogeneous Activation of Peroxymonosulfate for Decomposition of Phenol Yunjin Yao,*,† Zeheng Yang,† Hongqi Sun,‡ and Shaobin Wang*,‡ †

School of Chemical Engineering, Hefei University of Technology, Hefei 230009, P.R. China Department of Chemical Engineering, Curtin University, G.P.O. Box U1987, Perth, WA 6845, Australia

‡

ABSTRACT: This paper reports the synthesis of Co3O4−reduced graphene oxide (rGO) hybrids and the catalytic performance in heterogeneous activation of peroxymonosulfate (PMS) for the decomposition of phenol. The surface morphologies and structures of the Co3O4−rGO hybrids were investigated by field emission scanning electron microscopy (SEM), energydispersive X-ray spectrometer (EDS), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). Through an in situ chemical deposition and reduction, Co3O4−rGO hybrids with Co3O4 nanoparticles at an average size of 33 nm were produced. Catalytic testing showed that 20 mg/L of phenol could be completely oxidized in 20 min at 25 °C on Co3O4−rGO hybrids, which is mostly attributed to the generation of sulfate radicals through Co3O4-mediated activation of PMS. Phenol oxidation was fitted by a pseudo-zero-order kinetic model. The rate constant was found to increase with increasing temperature and PMS dosage, but to decrease with increasing initial phenol concentration. The combination of Co3O4 nanoparticles with graphene sheets leads to much higher catalytic activity than pure Co3O4. rGO plays an important role in Co3O4 dispersion and decomposition of phenol. ZnSe,13 CeO2,14 MnO2,15 TiO2,16,17 and ZnO,18 show high activities in electrochemical catalysis, capacitors, photocatalytic degradation, etc. Since graphene hybrids can exhibit enhanced performance, it has become a priority for researchers to prepare these hybrids. We have reported the synthesis of a magnetic Fe3O4@graphene composite and utilization in dye removal from aqueous media.19 However, to our best knowledge, little work has been done on the preparation of Co3O4−graphene hybrids for the heterogeneous activation of PMS. The objective of the present study was to present a facile approach for preparing Co3O4−reduced graphene oxide (rGO) via hydrothermal synthesis under basic conditions. The catalyst performance of Co3O4−rGO hybrids in the heterogeneous activation of PMS was evaluated in the degradation of highly toxic and poorly biodegradable phenol, which is commonly used in different branches of industry.20,21 The effects of reaction time, initial concentration, oxone dosage, and reaction temperature have been investigated. It was found that the catalytic activities of the as-prepared hybrids could be remarkably enhanced. The materials have potential applications in wastewater treatment.

1. INTRODUCTION Presently, a great development has been achieved in decomposing persistent organic pollutants in wastewaters by advanced oxidation processes (AOPs), involving various chemical, photocatalytic, electrocatalytic, and Fenton oxidation methods.1 Among these AOPs, the Fenton reaction is known as an efficient way to degrade organic pollutants, where hydroxyl radicals (·OH) are usually the main reactive and oxidizing species generated to degrade organic contaminants. However, the production, transport, and storage of H2O2 is expensive, and it requires pH adjustments as well.2 Similar to H2O2, an alternative chemical, peroxomonosulfate ion (HSO−5 , PMS), is considered as an inexpensive and environmentally friendly oxidant in several different applications, especially in chemically mineralizing various organic contaminants.3 In addition to high reactivity, its main advantage over H2O2 is easy handling. It is available on a large scale in the form of a reasonably stable solid, which is commercially sold under the brand name of Oxone (2KHSO5·3KHSO4·3K2SO4).4 Cobaltous mediated PMS for degradation of contaminants has attracted much interest due to better efficiencies than the Fenton reaction in a wide pH range from 2 to 9.5,6 However, the homogeneous Co2+/PMS system has a major drawback in the toxicity of cobalt ions, because Co can induce several health problems. Several attempts have been made using Co3O4 as heterogeneous catalysts for activation of PMS.5,7 To enhance the catalytic performance, it was immobilized on various supports such as TiO2,8 SiO2,9 zeolite,8 Bi2O3,10 and coal fly ashes.11 Graphene, as a robust two-dimensional sheet of sp2-hybridized carbon, has been receiving significant attention as a support for catalysts because of its unique 2D structure with high surface area and special electronic transport properties.12 It has been recently reported that graphene incorporated with nanoparticles, such as © 2012 American Chemical Society

2. MATERIALS AND METHODS 2.1. Materials. Potassium peroxymonosulfate, graphite powder (purity 99.9995%), sulphuric acid (95−97%), and sodium hydroxide (50%) were purchased from Sigma-Aldrich, Inc. Hydrogen peroxide (30%) was purchased from ChemSupply. Potassium permanganate and phenol were obtained from Ajax Finechem. Hydrochloric acid (32%, analytical grade) Received: Revised: Accepted: Published: 14958

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Figure 1. Preparation route of Co3O4−rGO.

of a catalyst (10 mg) was added to the solution. The suspension was thermostatted at a specific temperature under continuous stirring. Just prior to starting the reaction, a predetermined amount of oxone was added to the phenol solution. Aqueous phase samples (0.5 mL) were withdrawn at periodic intervals (up to 60 min) and immediately filtered (0.8 μm) to remove essentially the catalyst solids. Then, 0.5 mL of methanol was added to quench the reaction. The concentrations of remnant phenol were analyzed using a HPLC with a UV detector at 270 nm.27,28 The reaction was carried on at different temperatures (25, 35, and 45 °C). Phenol degradation was tested at four initial phenol concentrations ranging from 20 to 80 mg/L. Three different catalysts, rGO, Co3O4, and Co3O4−rGO, were tested to investigate the effect of the different types of catalyst on the transformation of phenol. Oxone was also tested at several doses from 0.05 to 0.5 g. Each kinetic test was carried out in duplicate to ensure the reproducibility of the experimental data. The correlation coefficient (R2) was used to represent the variation of the regression analysis for the determination of the reaction rate and order.

and methanol (analytical grade) were obtained from Biolab. Cobalt acetate tetrahydrate was purchased from BDH/Merck. All of the chemicals were used as received without further purification. All solutions were prepared in 18.2 MΩ·cm Milli-Qwater produced on a Milli-Q Biocel water system. 2.2. Preparation of Co3O4−rGO. Grapheneo oxide (GO) was synthesized by oxidation of graphite powder under acidic conditions according to the Hummers method using a mixture of H2SO4, NaNO3, and KMnO4.22−24 In a typical synthesis of the Co3O4−rGO hybrids, first, 0.93 g of cobalt acetate tetrahydrate (Co(C2H3O2)2·4H2O) was dispersed in 20 mL of distilled water, and 0.3 g of GO was dispersed in 250 mL water by sonication for 2 h to achieve uniform dispersion of GO. Then, Co(C2H3O2)2·4H2O solution was gradually added to the GO solution. Meanwhile, 10 mL of ammonia solution (28%) were added to the above solution, which will be used for cobalt ion precipitation and GO reduction. Finally, the mixture was transferred into an autoclave for hydrothermal treatment at 180 °C under static condition for 12 h. The solid product was separated by centrifugation and washed thoroughly with water and absolute ethanol to remove any impurities.25,26 The product was labeled as Co3O4−rGO. For a comparison, pure Co3O4 and bare rGO were also synthesized in the same way in the absence of graphene oxide or Co(C2H3O2)2·4H2O. The synthesis process of the Co3O4−rGO hybrids is schematically illustrated in Figure 1. 2.3. Catalyst Characterization. Powder X-ray diffraction (XRD) analyses were performed on a Bruker D8-Advance X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å), and the scanning angle ranged from 5° to 70° of 2θ. Raman scattering was performed on a Dilor Labram model 1B dispersive Raman spectrometer using the helium−neon laser at 632.8 nm. Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Perkin-Elmer Spectrum 100 with a resolution of 4 cm−1 in transmission mode at room temperature. The morphology of the samples were investigated by using field emission scanning electron microscopy (FESEM) (Zeiss Neon 40EsB FIBSEM), and the nanostructures of the samples were characterized by a TEM system (JEOL 2011 TEM) equipped with an energydispersive X-ray spectrometer (EDS). To investigate the samples via transmission electron microscopy (TEM), a suspension of the Co3O4−rGO hybrids in ethanol was drop-casted onto carbon-coated copper grids and dried under ambient conditions. Thermogravimetric analysis (TGA) was performed by heating the samples in airflow at a rate of 100 mL/min on a Perkin-Elmer Diamond TG/DTA thermal analyzer with a heating rate of 10 °C/min. 2.4. Catalyst Performance. To evaluate the activity of catalytic oxidation of phenol, batch experiments were carried out in a 300 mL batch reactor with a magnetic stirrer, heating jacket, and a condenser. In a typical run, water containing phenol (Co = 20 mg/L) was transferred into the reactor and a known amount

3. RESULTS AND DISCUSSION 3.1. Characterization of Samples. The phase structure of as-synthesized samples was first determined by XRD. The XRD patterns of rGO, Co3O4, and Co3O4−rGO hybrids are shown in Figure 2. The diffraction peaks in Figure 2b,c can be ascribed to

Figure 2. XRD patterns of (a) rGO, (b) Co3O4, and (c) Co3O4−rGO.

the well-crystallized Co3O4 with a face-centered cubic (fcc, Fd3m (227), a = 0.808 nm) structure (JCPDS No. 43-1003). No characteristic peaks of impurities were detected in the XRD patterns, implying the formation of single-phase spinel Co3O4.29,30 Compared to pure Co3O4 (Figure 2b), an additional small and weak broad diffraction peak (002) for Co3O4−rGO appears at 2θ of 24.5−27.5°, which can be indexed to the 14959

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disorderedly stacked graphene sheets (Figure 2c). Moreover, this broad peak is weaker than that of the as-prepared graphene (Figure 2a), suggestive of more disordered stacking and less agglomeration for graphene sheets in the hybrids. These results indicate that the hybrids consist of disorderedly stacked graphene sheets and well crystallized Co3O4. The average crystallite size of Co3O4 can be estimated according to the diffraction reflections by using the Debye−Scherrer formula.31 The average crystallite sizes of bare Co3O4, and Co3O4 in Co3O4−rGO were estimated to be 32.2, and 32.7 nm, respectively, which were consistent with the TEM observations (see Figure 5). Raman spectroscopy is a suitable technique to characterize carbonaceous materials, particularly for distinguishing ordered and disordered crystal structures of carbon. The typical Raman spectra of rGO and Co3O4-rGO hybrids are shown in Figure 3,

Figure 4. FTIR spectra of (a) GO, and (b) Co3O4−rGO.

were dispersed uniformly on the basal planes of the graphene. From Figure 5b, the rGO sheets are distributed between the packed Co3O4 NP and the porous hybrids with a large amount of void space being formed. Moreover, the rGO sheets distributed between the Co3O4 NP can prevent the aggregation of Co3O4 NP to a certain extent, which can be of great benefit to reactions. From Figure 5c the average particle size of Co3O4 was determined to be as small as 33 nm, which is consistent with the average particle size from the XRD pattern (32.7 nm). Both the TEM and SEM images show the transparent properties of the rGO substrate, which directly demonstrated the formation of thin rGO sheets.35 Figure 5e shows the EDS spectra of Co3O4− rGO. These results further confirm the presence of Co, C, and O elements on the surfaces of the GO sheets (Cu peak from the Cu grid). The content of each component in Co3O4−rGO hybrids can be determined with the TGA technique via oxidative decomposition. Figure 6 shows the representative TGA and DSC curves of rGO and Co3O4−rGO. Pure rGO (Figure 6a) showed little residual weight due to the complete combustion of rGO. It was noted that the decomposition temperature of rGO ranged from 400 to 620 °C, and the obvious weight loss occurred at around 593 °C, which is assigned to the combustion and decomposition of the carbon skeleton.36 In contrast, TG/DSC curves of Co3O4−rGO present a characteristic step/peak in the range 250−450 °C with two strong exothermal peaks, which is lower than that of pure rGO. This phenomenon should be attributed to the catalytic effect of the metal centers, which is similar to a carbon nanotube−cobalt system.37,38 Moreover, Figure 6b exhibits a residual weight of about 58%, which suggests the weight percentages of Co3O4 and rGO in the Co3O4−rGO hybrids were 58 and 42 wt %, respectively. 3.2. Catalytic Evaluation. According to previous studies, the primary radical from Co3O4/PMS system is SO•− 4 which plays a key role in the degradation of phenol. PMS is activated by electron transfer mechanism from cobalt cations for the generation of mainly SO•− 4 (eqs 1 and 2). The biggest advantage of this system is that the reverse electron transfer from Co3+ to Co2+ is thermodynamically feasible.39 The regeneration of the catalyst makes the reaction proceed cyclically until PMS is completely consumed at enough reaction time. The reaction mechanism for the degradation of phenol is shown as follows:7,40

Figure 3. The Raman spectra of (a) rGO, (b) Co3O4−rGO.

the G band (∼1595 cm−1) corresponding to sp2-hybridized carbon and the D band (∼1352 cm−1) originating from disordered carbon are observed on both of the samples. The peaks of Raman shift at 191, 491, and 596 cm−1 can be attributed to the F2g mode of Co3O4, and the peaks at 465 and 669 cm−1 can be attributed to the Eg and A1g modes of Co3O4, respectively. These results demonstrate the existence of both graphene and Co3O4 in the as-prepared composites.32 FT-IR spectra of GO and Co3O4−rGO hybrids are shown in Figure 4. Several characteristic peaks of functional groups can be observed on GO, confirming the successful oxidation of graphite. The peaks at 1718 and 1612 cm−1 should be assigned to the antisymmetric and symmetric stretching vibration of COO groups, and the C−O vibration in the epoxy group was observed at 1035 cm−1.33 In contrast, most of the bands related with the oxygen-containing functional groups vanished in the FT-IR spectra (Figure 4b) of Co3O4−rGO. It is revealed that the bulk oxygen-containing functional groups were removed from GO in the process of reduction, and thus the GO was effectively transformed into rGO in the synthesis.33 The peaks at 1549 and 1347 cm−1 can be attributed to the stretching −COO vibrations of the free acetate ion.34 To investigate the morphology and structure of the products, FESEM and TEM images were taken for the obtained Co3O4− rGO. The morphology of the hybrids is almost consistent with the rGO sheets in the range of micrometers. The rGO sheets can be observed clearly in the Co3O4−rGO hybrids from the FESEM and TEM images (Figure 5a,c) and Co3O4 nanoparticles (NPs) 14960

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Figure 5. FESEM images: (a) low-magnification, (b) high-magnification). TEM images: (c) low-magnification, (d) high-magnification. (e) EDS of Co3O4−rGO. − Co2 + + HSO−5 → Co3 + + SO−• 4 + OH

+ Co3 + + HSO−5 → Co2 + + SO−• 5 + H

(1) 14961

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ments, and took around 20 min for complete phenol oxidation under the same conditions. Therefore, the combination of Co3O4 and graphene results in a synergistic effect in catalytic activity, which is similar to that of MnFe2O4−rGO41 and CoFe2O4− rGO28 catalysts. Mabayoje et al.42 synthesized cobalt(hydr)oxide/graphite-oxide composites for H2S adsorption, and they found a synergistic effect between cobalt hydroxide and GO for the reactive adsorption of H2S. Cobalt atoms were covalently attached to sheets of graphite oxide through the oxygen groups and the presence of graphite oxide produced a significant number of chemical defects for oxidation and reactive adsorption. In this investigation, characterizations from SEM (TEM), Raman, and FTIR show strong attachment of Co3O4 NPs on rGO and the presence of −COO and carbon defects. Although rGO has less catalytic activity, Co3O4−rGO hybrids exhibited better catalytic activity than pure Co3O4, indicating that the significant enhancement in catalytic activity can be attributed to the remarkable synergistic effect of the heterogeneous surface between Co3O4 and the graphene sheets. Such an enhancement in catalytic activity can be attributed to four factors: (i) rGO has peculiar electronic structure and possesses high migration efficiency of electrons, which plays an important role in electron transfer between surface CoO and Co2O3, enhancing the generation of sulfate radicals and catalytic activity for the degradation of phenol.43 (ii) As compared with bare Co3O4 NPs, rGO can also offer an environment to prevent aggregation of Co3O4 NPs and obstruct facile loss of activity. The improved activity of Co3O4−rGO hybrids for phenol degradation may be related to the strong interaction (Co−O−C) between Co3O4 and graphene in the hybrids and good dispersion of Co3O4 NPs.35 The Co−O−C sites will favor the formation of OH42 on Co3O4 and promote activation of HSO5− for sulfate radical production.40 (iii) Graphene is not only a support, but also a catalyst for activating oxone to produce sulfate radicals, the reaction of which is shown in eq 4.

Figure 6. TG and DSC curves of (a) rGO and (b) Co3O4−rGO in air atmosphere.

SO−• 4 + phenol → [...many steps...] → CO2 + H 2O

(3)

The catalytic activities of Co3O4−rGO hybrids and pure individual component (Co3O4 and rGO) were evaluated at 25 °C. As shown in Figure 7, for the pure Co3O4 sample, 100% of phenol was removed in 60 min. For pure rGO, the degradation of phenol reached 20% at 60 min, indicating that pure rGO is catalytically active. It should be noted that the degradation of phenol on Co3O4−rGO hybrids shows remarkable improve-

graphene + HSO−5 → graphene − H + SO−• 5

(4)

(iv) Graphene has extraordinary adsorption capacity and phenol has the ability to create π−π stacking interactions with the graphene aromatic domains.44−46 This adsorption process significantly increases the concentration of the organic molecules near the catalytic surface. The enriched environment of the substances closer to the catalytic surface is an important factor for achieving higher catalytic degradation rates.44 Therefore, we believe Co3O4−rGO as a catalyst will find many applications in various catalytic fields. The effect of initial phenol concentration was investigated by varying the phenol concentration in a range of 20−80 mg/L at 25 °C (Figure 8). An increase in the initial phenol concentration resulted in declined phenol oxidation efficiency. The phenol in the solution was completely removed within 30 min at the initial phenol concentration of less than 40 mg/L, while about 60% of phenol was removed within the same time at the concentration of 80 mg/L. It was found that phenol degradation in the Co3O4− rGO/PMS process follows a pseudo-zero-order reaction kinetics with respect to reduced phenol concentration (Ct/C0), which may be expressed as Ct/C0 = kobst, where t is the reaction time (min), kobs is the rate constant (min−1), and C0 and Ct are the phenol concentrations (mg/L) at times of t = 0 and t = t, respectively.47 At the same concentrations of Co3O4−rGO hybrids and PMS, a high amount of phenol in solution will require more time to achieve the same removal rate, thus lowering phenol degradation efficiency.

Figure 7. Phenol degradation using different catalysts (reaction conditions: [phenol] = 20 mg/L, [PMS] = 0.3 g/150 mL, [catalyst] = 10 mg/150 mL). 14962

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Figure 8. Phenol degradation using Co3O4−rGO/PMS: effect of phenol concentration. (Reaction conditions: [PMS] = 0.3 g/150 mL, [catalyst] = 10 mg/150 mL).

Figure 10. Effect of reaction temperature on phenol degradation using Co3O4−rGO/PMS. Inset indicates Arrhenius curve. (Reaction conditions: [phenol] = 20 mg/L, [PMS] = 0.3 g/150 mL, [catalyst] = 10 mg/150 mL).

The degradation of phenol over Co3O4−rGO/PMS was further studied at varying amounts of Oxone (Figure 9). It is

Table 1. Kinetic Rate Constants and Activation Energy of Co3O4−rGO/PMS Oxidation of Phenol T, °C

kobs (min−1)

R2 of kobs

ΔE (kJ mol−1)

R2 of ΔE

25 35 45

0.1001 0.1231 0.1494

0.982 0.979 0.982

26.5

0.99

phenol removal rate. The activation energy (Ea) of the reaction was then evaluated by plotting ln(kobs) against 1/T (inset Figure 10), according to the Arrhenius equation of ln kobs = ln A − Ea/ (RT). The factor A represents the frequency of collisions between two molecules in the proper orientation for reaction to occur. R is the gas constant, 8.314 J/(mol K), T is the absolute temperature, and Ea is the activation energy. The Ea value was obtained as 26.5 kJ mol−1, which is much smaller than other Co materials reported in the references.48 Thus, in terms of high activity and low activation energy, Co3O4−rGO hybrids are promising catalytic materials for oxidation processes.

Figure 9. Phenol degradation using Co3O4−rGO/PMS: effect of Oxone dose. (Reaction conditions: [phenol] = 20 mg/L, [catalyst] = 10 mg/ 150 mL).

4. CONCLUSION Co3O4−rGO hybrids were prepared by hydrothermal synthesis under basic conditions, characterized by FESEM, TEM, EDS, XRD, FTIR, and TGA techniques and used for the degradation of phenol in aqueous solutions. TEM observations indicate that graphene sheets are fully exfoliated and decorated with Co3O4 NPs at an average size of 33 nm. The catalytic activity measurements show that coupling Co3O4 NPs with graphene sheets leads to higher catalytic activity for the degradation of phenol than pure Co3O4, attributed to the roles of rGO in nanoparticle dispersion and activation of PMS. Phenol degradation on Co3O4−rGO/PMS follows the pseudo-zeroorder kinetics model, and the activation energy is 26.5 kJ/mol. The rate constant of phenol degradation was found to increase with increasing temperature and Oxone dosage, but to decrease with increasing initial phenol concentration. Co3O4−rGO hybrids are thus believed to be effective catalytic materials for environmental applications.

observed that, with an increase of the Oxone dose (0.05, 0.1, and 0.3 g), the decomposition rate of phenol is increasing. It is shown that increased doses of Oxone led to the time of phenol decomposition reducing from 45 to 30 min. The increase in Oxone dose would provide more chance to the reaction with Co3O4−rGO, which enhances the rate of activation of PMS to generate active sulfate radicals, resulting in an increase in the rate of phenol removal. To further evaluate the catalytic performance of Co3O4−rGO/ PMS, the reaction was tested at different temperatures and the results are shown in Figure 10. It was found that phenol degradation in Co3O4−rGO/PMS process is well formulated by the pseudo-zero-order kinetics.47 The kobs values of phenol degradation were found to be 0.1001 min−1 (R2 = 0.982) at 25 °C, 0.1231 min−1 (R2 = 0.979) at 35 °C, and 0.1494 min−1 (R2 = 0.982) at 45 °C, respectively (Table 1), suggesting that increasing the temperature could significantly increase the 14963

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AUTHOR INFORMATION

Corresponding Author

*(Y.Y.) Tel.: +86 551 2901458. Fax: +86 551 2901450. E-mail: [email protected]. (S.W.) +61 8 9266 3776. Fax: +61 8 9266 2681. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the financial supports of the National Natural Science Foundation of China (Grants 20976033 and 21176054), the Fundamental Research Funds for the Central Universities (Grants 2012HGQC0010).

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