Face-Raised Octahedral Co3O4 Nanocrystals and Their Catalytic

Face-raised Co3O4 octahedral crystals were successfully constructed through a carbon-assisted method using cellulose as carbon resource and used for a...
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Face-Raised Octahedral Co3O4 Nanocrystals and Their Catalytic Activity in the Selective Oxidation of Alcohols Yue Teng,† Le Xin Song,*,†,‡ Liang Bing Wang,‡ and Juan Xia† †

CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China ‡ Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China S Supporting Information *

ABSTRACT: Face-raised Co3O4 octahedral crystals were successfully constructed through a carbon-assisted method using cellulose as carbon resource and used for a catalyst for selective oxidation of alcohols. The face-raised Co3 O 4 octahedra are about 200 nm in edge length and assembled to microtubes with a length of ca. 10 μm and a width of 2−3 μm. Our analysis showed that the thermal decomposition and carbonization of cellulose has contributed to the generation of the octahedral structure and the assembled process. Moreover, compared with normal Co3O4 octahedra, the face-raised Co3O4 octahedral crystals have higher remanence and saturation magnetization, which is attributed to their more uniform small grain sizes. In particular, the face-raised Co3O4 octahedra gave both higher activity and higher selectivity than the normal Co3O4 octahedra in the catalytic oxidation reactions of alcohols. More importantly, after eight cycles, the face-raised Co3O4 octahedral crystals still exhibited considerably high catalytic activity, suggesting promising applications in heterocatalysis.

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INTRODUCTION Simultaneous control of particle size and morphology on the nanoscale is fundamental for nanotechnology progress and has attracted much attention.1−12 In recent years, nanomaterials of iron series transition metal oxides such as Co3O4 have been found to have wide applications as supercapacitors,13,14 gas sensors,15,16 batteries,17−19 and catalysts.20−23 Although many novel methods have been developed for the synthesis of nanocrystals of Co3O4,24−28 a cheap, environmental friendly and convenient synthetic process is still desired, especially for what concerns the large-scale synthesis of Co3O4 nanoparticles with polyhedral configuration and excellent properties. On the other hand, much effort is dedicated to the fine modification of nanopolyhedral structures by crystal cut, etching, and decoration in order to improve the physical and functional properties of materials.29−33 For example, Yang’s group reported silver nanoparticles with intraparticle gaps formed by anisotropic etching,29 indicating that these etched particles can serve as excellent substrates for single-particle surface-enhanced Raman scattering compared with original polyhedra. Very recently, Huang and co-workers revealed that the catalytic property of face-raised Cu2O nanocrystals depended on the kind of facets that the crystals have.33 Therefore, it should be interesting to explore a simple, effective, green, and operable method for the fine structure modification of nanopolyhedra of iron series transition metal oxides. Starting from the two points, we hope to reach a twofold goal: to give an idea to proceed with the gram scale synthesis of nanopolyhedral Co3O4 crystals by one-step reaction and to give © 2014 American Chemical Society

an insight into the structural modification of the Co3O4 nanopolyhedra. In the study, we successfully accomplished the two goals. First, we constructed face-raised Co3O4 octahedra (Co3O4-1, eight raised triangle faces and 12 hollow edges) from the oxidation of CoCl2 through a very simple carbon-assisted synthesis. Carbon materials were produced in situ from the thermal decomposition and carbonization of cellulose and contributed to the generation of octahedral structure since without carbon source provided by cellulose, the sintering product of pure CoCl2 showed very poor morphology. Moreover, when the cellulose is replaced by active carbon, it could only lead to the normal octahedral Co3O4 structure. Second, and more importantly, this method (no organic solvent or surfactant is used) is suitable for large-scale synthesis. For example, 100 g of Co3O4-1 was synthesized through this method at a time. This work sheds light on the general mechanism of Co3O4 synthesis.

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EXPERIMENTAL SECTION Materials. CoCl2·6H2O was purchased from Guangdong Xilong Chemical Reagent Company. Cellulose, active carbon, alcohol, and di-tert-butyl peroxide were obtained from Shanghai Chemical Reagent Company. All other chemicals were of general-purpose reagent grade unless otherwise stated.

Received: December 12, 2013 Revised: February 16, 2014 Published: February 20, 2014 4767

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Figure 1. (a) SEM image, (b) HRSEM image, (c) XRD pattern, (d) TEM image, (e) high-resolution TEM image, and SAED pattern (inset) of Co3O4-1. (f) Photograph of a glass dish containing 100 g of Co3O4-1; the quarter dollar coin in the image is for comparison.

Preparation of the Solid Materials. In a typical synthesis, 1 mmol of CoCl2·6H2O (238 mg) and cellulose (6 g) were added to a round-bottom flask containing deionized water (75 mL) and stirred for 1 min at 293 K (Figure S1 described the determination of the ratio of reactants). After water was removed by rotary evaporation below 323 K, a solid product (mixture of cellulose and CoCl2) was obtained and dried in vacuum at 333 K for 2 h to remove water molecules from the solid. The dried solid products were sintered at 773 K for 2, 4, and 6 h under air atmosphere in a muffle furnace, and the resultant powders were named as Co3O4-m, Co3O4-1, and Co3O4-2, respectively. Iron oxides and nickel oxides were prepared by similar procedures and named as Fe2O3-m, Fe2O31, and Fe2O3-2 and Ni2O3-m, Ni2O3-1, and Ni2O3-2. The mixture of CoCl2 with active carbon was obtained by replacing cellulose with active carbon (2 g). Catalytic Experiments. The alcohol (10 mmol) and resultant metal oxide (2 mg) were added to a glass reactor (50 mL). Di-tert-butyl peroxide (10.0 mmol) was continuously dropped into the solution in 12 h. The reaction mixture was vigorously stirred at 293 K for 12 h, and then methanol was added to completely dissolve the products (unreacted reactants and partially or fully reacted products) for GC-MS analysis immediately. Instruments and Methods. XRD experiments were performed on a Philips X’Pert Pro X-ray diffractometer with a monochromatized Cu Kα radiation source and a wavelength of 0.1542 nm. SEM images were obtained with a Supra 40 FESEM operated at 5 kV. TEM image, high resolution TEM image, and SAED pattern were collected on a JEOL-2010 fieldemission transmission electron microscope operating at 200 kV accelerating voltage. Magnetic measurements were carried out with a Quantum Design MPMS-XL5 SQUID magnetometer at 300 K. Raman spectra were collected at room temperature with

a LABRAM-HR confocal laser micro-Raman spectrometer in the range 500−2000 cm−1. TG analyses were performed on a Shimadzu TGA-50 thermogravimetric analyzer at a constant heating rate of 10 K·min−1 in air or in nitrogen with a gas flow of 25 mL·min−1.

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RESULTS AND DISCUSSION Formation and Characteristics of the Face-Raised Co3O4 Octahedra. Figure 1a and b shows the scanning electron microscopy (SEM) and high-resolution scanning electron microscopy (HRSEM) images of Co3O4-1, demonstrating that the octahedra could self-assemble to form rod-like microtubes with a length of ca. 10 μm and a width of 2−3 μm. Since the typical morphology of carbon from the sintering of cellulose at lower temperatures is microtubes,34−36 it is reasonable to believe that the cellulosic substrates act as templates for the assembly of the nanoscale particles. The octahedral structure of Co3O4-1 with eight raised faces could be easily identified in Figure 1b. The width of the gaps formed between upwardly protuberant faces was around 20−30 nm, while the average size of the octahedral particles is on the order of 200 ± 30 nm (defined as the distance between adjacent vertexes). Figure 1c shows the X-ray diffraction (XRD) pattern of the face-raised octahedra. All the peaks were indexed to the cubic Co3O4 (JCPDS 80-1542).37 Figure 1d,e illustrates the transmission electron microscopy (TEM), high-resolution TEM images, and selected area electron diffraction (SAED) pattern of the face-raised Co3O4 octahedra, indicating the octahedral structure is mainly enclosed by {111} facets, which possess the lowest surface energy.38,39 Raman spectrum of Co3O4-1 in Figure 2 exhibits a strong characteristic band of Co−O at 649 nm,40 and no signals of carbon materials appear. Also, no mass loss is observed in its thermogravimetric (TG) profile (inset of Figure 2). Both prove 4768

dx.doi.org/10.1021/jp412175t | J. Phys. Chem. C 2014, 118, 4767−4773

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6CoO + O2 → 2Co3O4

The mixture of CoCl2 and cellulose was ultimately transformed into Co3O4 through sintering process under air (eq 1). This reaction can be divided into the following two stages. (1) A large quantity of carbon materials occurred (eq 2) and maintained for a long time (at least 2 h). Although the carbon materials were subsequently oxidized by oxygen and converted to carbon dioxide leaving this system (eq 3), it is likely that they indeed played an important role in the formation of the octahedral structures and self-assembled morphologies before they were oxidized. A controlled experiment supports the function of carbon materials. When no carbon source was added, the Co3O4 with an irregular geometric shape was obtained (Figures S1 and S9). Therefore, the carbon materials deriving from cellulose appear to offer a template for steering the formation of identical octahedral structures while thermally controlling their size. This template allows Co3O4 nanoparticles to self-assemble into structured microtubes. Further, we consider that the carbon materials may also contribute to modulate the reaction rates since the conversion of carbon to carbon dioxide is a relatively timeconsuming procedure. (2) A small proportion of CoO emerged at an early stage (eq 4), subsequently converting into Co3O4 (eq 5), more stable in air. Note that the fourth reaction could only happen in the presence of carbon materials because no CoO was observed in the sintering products of pure CoCl2· 6H2O at 773 K for 1 and 2 h (see XRD patterns in Figure S10). It indicates that the carbon materials derived from cellulose protected the CoO from oxidation. This could be one reason for the large difference in shape and size of the Co3O4 materials. Active carbon was applied as the carbon source to further demonstrate the function of cellulose, and no face-raised octahedra were obtained but normal ones (Figure S11). This may be due to the slow decomposition and carbonization process of cellulose, which exerts a long-term effect on the construction of Co3O4 particles. Figure 3 presents a continuous transformation from the initial mixture of CoCl2 with cellulose to three sintering

Figure 2. Raman spectrum of the face-raised Co3O4 octahedra. The inset is the TG curve of the Co3O4.

that a pure Co3O4 is obtained, which is in line with the result of XRD analysis. Furthermore, our facile synthesis of the face-raised Co3O4 octahedra can be easily scaled up for real applications. For example, by simply increasing the amount of the reactants and keeping the same molar ratio used in the standard synthesis, we have prepared as much as 100 g (Figure 1f) of the face-raised Co3O4 octahedral crystal (XRD, Figure S2). It confirms the validity, reliability, and repeatability of the synthesis method. This is of high importance in industrial applications. Construction Process of the Face-Raised Co 3 O 4 Octahedra. To clarify the construction mechanism of the face-raised Co3O4 octahedra, we have carried out parallel measurements through sintering the mixture of CoCl2 and cellulose at 2 and 6 h. At a relatively earlier stage (2 h), microtubes with smooth surfaces (Figure S3, similar size as those of Co3O4-1) emerged, which were found to be a mixture (Co3O4-m, Figure S4) of CoO (JCPDS 01-1227), Co3O4 (JCPDS 80-1542), and carbon from the decomposition and carbonization of cellulose (see TG curve of cellulose, Figure S5). Our result indicates that the complete decomposition temperature of cellulose is delayed by about 300 K after the introduction of CoCl2, giving enough time for the reaction of carbon and CoCl2. Interestingly, a few octahedra (with a mean edge length of 180 ± 20 nm, Figure S6) were embedded on the microtube surfaces. At a relatively later stage (6 h), the faceraised octahedra disappeared but normal octahedral particles (with a mean edge length of about 250−270 nm, Figure S7) appeared, while still fabricating microtubes by self-assembling. The sample (Co3O4-2) was determined to be a pure cubic phase of Co3O4 (Figure S8). These observations provide sufficient information on the time-dependent growth, remodeling, and coarsening of Co3O4, indicating that the initial carbonization of cellulose and subsequent oxidation/reduction reactions of carbon proceeded, accompanying the formation and transformation of cobalt oxides. Several equations are proposed to depict the reactions during the sintering process.

Figure 3. Schematic illustration describing the time-dependent growth of Co3O4.

products with increasing sintering time: small octahedra (2 h), faced-raised octahedra (4 h), and larger octahedra (6 h). This shows two important things. First, the as-obtained Co3O4 has a strong tendency to form octahedral structures through this synthetic method. Second, the heating-time-dependent structural transformation is a multistep process, including the oxidation and disappearance of CoO and the nucleation and growth process of Co3O4. Therefore, the construction of the face-raised octahedral structure of Co 3O 4 -1, in which continuous gaps appear at the edges of the octahedral crystals, may be explained by the difference in the growth speed of the

Δ

(C6H10O5)n + 6CoCl 2 + (6n + 1)O2 → 2Co3O4 + 6nCO2 + 12HCl + (5n − 6)H 2O

(1)

Δ

(C6H10O5)n → 6nC + 5nH 2O

(2)

Δ

C + O2 → CO2

(3)

Δ

CoCl 2 + H 2O → CoO + 2HCl

(5)

(4) 4769

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Co3O4-2. The enhanced Mr and Ms values may be associated with the more uniform small grain sizes of the face-raised octahedra. It may be related to the structural specificity of the face-raised octahedra. Catalytic Efficiency of the Face-Raised Co3O4 Octahedra in the Oxidation Reaction of Alcohol. Compared with other transition metal oxide catalysts, iron series transition metal oxide catalysts are cheap, nontoxic, and easier to recycle and reuse.46−48 In view of the facts that there are no capping agents on the octahedral Co3O4 substrates and that the faceraised octahedra seem to have a larger surface area (a smaller particle size and the presence of concaves on the surface), we investigated the possibility that they may be of importance for the catalysis of organic reactions. The oxidation of benzyl alcohol to benzaldehyde, an important reaction in industry, was employed to evaluate the catalytic activity and selectivity of the as-prepared materials. This reaction involves the use of di-tertbutyl peroxide (DTBP) as oxidant. Gas chromatograph−mass spectrometer (GC-MS) analysis was used to provide information that allowed us to identify the stable products. Table 1 shows the experimental results obtained by setting the initial weight of cobalt oxide catalysts to 2 mg. The catalytic

two kinds of Co3O4: one originating from the formation stage and the other from the further growth stage. When the mixture of CoCl2 and cellulose was replaced by those of FeCl3 and NiCl2 with cellulose, we got corresponding pure metal oxides (Fe2O3-1 and Ni2O3-1, Figures S12 and S13) after sintering for 4 h. However, no face-raised octahedral structure appears in these cases (only have a crude octahedral outline, Figure S14). Also, we found that Fe2O3-m and Ni2O3m, obtained by a similar method as described for Co3O4-m, consist of α-Fe2O3, γ-Fe2O3, and carbon (Figure S15) and NiO, Ni2O3, and carbon (Figure S16), respectively, while Fe2O3-2 (α-phase, Figure S17) and Ni2O3-2 (Ni2O3, Figure S18) are pure single crystals (octahedra). These results suggest that the template-sacrificed route could be used to create the octahedral constructs of iron series transition metal oxides, even though their morphologies are significantly different. Magnetic Properties of the Face-Raised Co 3 O 4 Octahedra. We also investigated the field dependence of magnetizations of the Co3O4-1 and Co3O4-2 using a superconducting quantum interference device (SQUID). Their hysteresis loops (Figure 4) indicate a weak ferromagnetic

Table 1. Selective Oxidation of Benzyl Alcohol to Benzaldehyde

Figure 4. Field dependencies of the magnetizations of Co3O4-1 and Co3O4-2 at 300 K.

state at 300 K, in agreement with the observation of Jiang and associates that Co3O4 exhibited a tiny hysteresis loop at room temperature.41 This result reveals that the Co3O4 crystals show a certain degree of ferromagnetism under the external magnetic field, even though they were regarded as typical antiferromagnetic materials. It may be because of the unpaired electronic spin orientation near the surface of nanocrystals and the interaction of atoms.42,43 Also, a tendency toward saturation is observed at 4 kOe for both the materials. However, there is a significant difference between them in coercivity (Hc), remanence (Mr), and saturation magnetization (Ms). The Co3O4-1 presents a lower Hc (111 Oe) and a higher Mr (0.45 emu·g−1) and Ms (1.76 emu·g−1) than the Co3O4-2 (Hc, 231 Oe; Mr, 0.07 emu·g−1; Ms, 0.38 emu·g−1). According to Brown’s equation,44 in which Hc inversely proportional to Ms, this is in accordance with our experimental results. The ratios of the Mr and Ms for the Co3O4-1 and Co3O4-2 are 0.26 and 0.18, respectively. The increase of 44% in the ratios is very significant because a higher ratio at 300 K is particularly encouraging for possible applications in recording media. Also, it should be noted that the Ms of the Co3O4-1 at 300 K is nearly 4 times that of the porous Co 3 O 4 microcubes reported by Cao and his colleagues.45 Their result is almost the same as that of the

entry

catalyst

yield (%)

TON

selectivity (%)

1 2 3 4 5 6 7 8 9 10

without Co3O4-m Co3O4-1 Co3O4-2 Fe2O3-m Fe2O3-1 Fe2O3-2 Ni2O3-m Ni2O3-1 Ni2O3-2

4 13 37 28 10 28 29 12 28 30

49 139 105 80 224 232 45 105 113

>99 96 90 82 22 60 74 90 69 75

properties of iron and nickel oxides with the same weight were also detected for comparison at the same conditions. The control experiment without the addition of any catalyst shows that only 4% of benzyl alcohol was oxidized into benzaldehyde but with high selectivity (>99%). Upon the addition of cobalt oxides, regardless of the composition and the microstructure, the conversion increased significantly, with a slight decrease on selectivity. Side products, such as benzoic acid, were observed by GC-MS analysis. With large amount of carbon materials, the Co3O4-m as well as the Fe2O3-m and Ni2O3-m displayed relatively low catalytic activity in comparison with pure oxide catalysts. In particular, the conversion reached 37% with an excellent benzaldehyde selectivity of 90% in the presence of the Co3O4-1 (Table 1, entry 3). This result (both yield and selectivity) is better than that of the Co3O4-2 (Table 1, entry 4) as well as those of the iron and nickel oxides. We believe that the remarkable enhancement of catalytic performance of Co3O4-1 is related to the larger surface area of the unique face-raised structure favorable to contacts with substrates.49 Our calculation result that surface areas decrease in the order Co3O4-1 > Co3O4-m > Co3O4-2 supports this point of view. The surface areas for the normal octahedra were 4770

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calculated based on the average edge lengths of triangle faces. The surface areas for the faced-raised octahedra were calculated based on the raised height of triangle faces and the average edge lengths of the raised triangle faces. All the lengths and heights are an average value of more than 20 individuals according to the SEM images. It should be noted that the face-raised structure may be able to contribute to the enhancement of the selectivity possibly due to multidentate interactions with substrates. Figure 5a shows the turnover numbers (TON, mol product/ mol catalyst) and the selectivity of the reaction (eq 6)

Table 2. Selective Oxidation of Alcohols to Aldehydes or Ketones Using Co3O4-1 as Catalyst

activity, and selectivity. What is more, a large-scale synthesis of the Co3O4 nanocrystals is available using our method. We believe that this work has the potential to have a large impact on the fields of heterogeneous catalysis, organic synthesis, and material science.

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ASSOCIATED CONTENT

S Supporting Information *

(1) SEM images of the sintering products derived from the processors of 238 mg of CoCl2 and different amounts of cellulose: 0 (a), 2 (b), 4 (c) and 8 g (d); (2) SEM image of Co3O4-m; (3) XRD pattern of 100 g of Co3O4-1; (4) XRD pattern of Co3O4-m; (5) TG curves of the composite of CoCl2 and cellulose (A) and pure cellulose (B) in air; (6) HRSEM image of Co3O4-m; (7) SEM and HRSEM images of Co3O4-2; (8) XRD pattern of Co3O4-2; (9) XRD pattern of the sintering product of CoCl2·6H2O at 773 K for 6 h; (10) XRD patterns of the sintering products of CoCl2·6H2O at 773 K for 1 and 2 h, respectively; (11) HRSEM image and XRD pattern of the sintering product from the mixture of CoCl2 and active carbon at 773 K for 4 h; (12) XRD pattern of Fe2O3-1; (13) XRD pattern of Ni2O3-1; (14) HRSEM images of Fe2O3-1 (a) and Ni2O3-1 (b); (15) HRSEM image and XRD pattern of Fe2O3m; (16) HRSEM image and XRD pattern of Ni2O3-m; (17) HRSEM image and XRD pattern of Fe2O3-2; (18) HRSEM image and XRD pattern of Ni2O3-2. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. (a) TON and selectivity of the benzyl alcohol oxidation reaction using Co3O4-1 as the catalyst (consecutively reused eight times). Typical error bars are shown in the figure. (b) HRSEM image of the Co3O4-1 after the eighth time in the reaction.

consecutively using the recycled face-raised Co3O4 octahedra as catalyst. Importantly, the catalyst only lost 4% of its initial activity and showed an increase of selectivity into benzaldehyde from 90 to 97% over the eight rounds of the experiment, demonstrating the stability, reusability, and durability of the catalyst. Additionally, the HRSEM observations confirmed that the particle sizes and shapes remained unchanged after catalysis, as shown in Figure 5b. To extend the scope of the use of the nanocrystal Co3O4-1 catalyst as an effective catalyst for oxidation reactions, in this work we investigated the oxidation of a series of alcohols to aldehydes or ketones using the catalyst (Table 2). This catalyst shows a consistently high selectivity (>99%) toward the conversion of various alcohol substrates after 12 h of reactions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.X.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to NSFC of China (No. 21071139) for financial support of this work.

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CONCLUSIONS In conclusion, the novel face-raised Co3O4 octahedra obtained through a green synthetic strategy have a tendency to enlarge the surface area of particles in relation to normal octahedra, which has contributed to their magnetic property, catalytic

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