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Catalytic Activity of Co3O4 Nanomaterials with Different Morphologies for the Thermal Decomposition of Ammonium Perchlorate
Nano-Co3O4 with different morphologies was successfully synthesized by annealing CoC2O4·2H2O precursors. The as-obtained samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and low-temperature nitrogen adsorption-desorption. It was found that the volume ratio of N,N-dimethylformamide (DMF) and water played an important role in the formation of cobalt oxalate precursors with different morphologies. After calcination in air, cobalt oxalate precursors converted to Co3O4 nanomaterials while their original morphologies were maintained. The catalytic effect was investigated for nano-Co3O4 with different morphologies on the thermal decomposition of ammonium perchlorate (AP) by differential scanning calorimeter (DSC). The results indicated that all products showed excellent catalytic activity for thermal decomposition of AP and the Co3O4 nanorods with larger BET surface area and pore volume had the highest catalytic activity.
Morphology-controlled synthesis of inorganic nanomaterials is of extensive research interest in materials science because the electronic, optical, magnetic, and catalytic properties of nanocrystals are highly dependent on not only their composition, but also their structure , shape , and size . Therefore, many efforts have been made to develop cost-effective synthesis methods of nanomaterials with different structures and morphologies for enabling novel intrinsic properties and applications of nanomaterials.
Co3O4, as one of the most intriguing magnetic p-type semiconductors, is of special interest due to its potential applications in heterogeneous catalyst , lithium-ion battery , supercapacitor , gas sensor , and many other aspects . Up to now, shape-controlled Co3O4 nanostructures have been prepared by various approaches, in which morphology-conserved transformation of precursors has proved to be a promising approach for the synthesis of Co3O4 nanostructures [9–12]. For example, Zhu et al. reported the shape-controlled synthesis of cobalt carbonate/hydroxide intermediates via a solvothermal method at 220°C for 18 h . Hu et al. synthesized β-Co(OH)2 nanosheet at 180°C for 12 h and Co(CO3)0.5(OH)0.11H2O nanobelt at 140°C for 12 h via a solvothermal method . Wang et al. prepared one-dimensional and layered parallel folding of cobalt oxalate nanostructures using N,N-dimethylacetamide (DMA) and dimethyl sulfoxide (DMSO) as solvents at ambient temperature . In our past work, we prepared shape-controlled synthesis of Co3O4 nanostructures derived from coordination polymer precursors . However, for some shape-controlled synthesis methods, special instruments, complicated processes, long reaction times, and relatively high temperatures are required. Therefore, it is important to design a simple, rapid, low-temperature, and low-cost synthesis route to synthesize morphology-controlled cobalt precursors.
Over the past decades, ammonium perchlorate (AP) has received considerable attention because AP is an important oxidizer in solid composite propellants for solid fueled rockets and the combustion behavior of propellants is highly relevant to the thermal decomposition of AP. The lower the temperature at which AP begins to decompose, the higher the burning rate of propellants [13–15]. Recently, Co3O4 nanomaterials with various morphologies have been used as effective catalyst to accelerate thermal decomposition of AP [12, 16–19].
In the present work, we report morphology-controlled preparation of cobalt oxalate precursors from the reaction of cobalt(II) nitrate hexahydrate and oxalic acid under mild conditions. It was found that the volume ratio of N,N-dimethylformamide (DMF) and water played a crucial role in the formation of cobalt oxalate with different morphologies. After calcination in air, the as-prepared cobalt oxalate precursors subsequently converted to porous Co3O4 nanomaterials while their original morphologies had been well maintained. To study their potential applications, the as-prepared nano-Co3O4 with different morphologies had been applied in the thermal decomposition of AP, which exhibited good activity.
All chemicals and solvents are of analytical grade and were used as received without further purification. In a typical experiment, 1 mmol Co(NO3)26H2O was dissolved in a mixed solution of DMF and deionized water at room temperature (the total volume is 20 mL), followed by addition of 1 mmol H2C2O42H2O under vigorous stirring. After 5 min, the as-obtained precipitates were centrifuged, washed with distilled water and absolute ethanol several times, and dried in vacuum at 60°C for 5 h. In addition, a calcination process (350°C for 1 h in air with a heating rate of 2°C min−1) was performed to transform cobalt oxalate to black Co3O4 crystals. In the experiments, to obtain products with different morphologies, the volume ratio of DMF and water was adjusted while all other conditions were keeping unaltered.
The products were characterized by powder X-ray diffraction (XRD) on a Rigaku D/max 2500PC diffractometer with graphite monochromator and Cu Kα radiation (λ = 0.15406 nm) at a step width of 0.02°. SEM images of the products were obtained on scanning electron micro analyzers (HITACHI S-3400N,). Nitrogen adsorption-desorption isotherms, pore size distributions, and surface areas of the samples were measured by the instrument of NOVA 2000e using N2 adsorption.
To test the catalytic effect of Co3O4 nanostructures with different morphologies on the thermal decomposition of AP, the mixture of AP and Co3O4 was ground carefully for 10 min and was detected by a differential scanning calorimeter (DSC) using STA 449C thermal analyzer with a heating rate of 10°C min−1 in N2 atmosphere over the temperature range of 30–500°C. The mass percentage of Co3O4 nanostructures to AP in the mixture was 2%.
3. Results and Discussion
Figure 1 shows the XRD patterns of the precursors prepared under the different volume ratio of DMF and water. All of the diffraction peaks in Figure 1(a), 1(b), 1(c), and 1(d) can be indexed as the orthorhombic phase of CoC2O42H2O by comparison with the data of JCPDS card files number 25-0250. No impurity peaks are detected in the XRD pattern.
Morphologies of all the precursors were characterized by SEM and the images of the samples are shown in Figure 2. When 20 mL of H2O was used without addition of DMF, the result shown in Figure 2(a) reveals that sample is composed of CoC2O42H2O microrods with diameter of about 2–5 μm. When 16 mL of H2O and 4 mL of DMF were used, spindle-like CoC2O4·2H2O nanostructures were obtained, shown in Figure 2(b). Figure 2(c) shows the morphology of sample prepared in the presence of 12 mL of H2O and 8 mL of DMF. It was observed that the sample consisted of nanorod bundles. Figure 2(d) illustrates nanorods in sample prepared by using 8 mL of H2O and 12 mL of DMF. In addition, when 4 mL of H2O and 16 mL of DMF were used or 20 mL of DMF was used without addition of H2O, no products could be obtained. The above facts showed that DMF/water volume ratio played an important role in the information of CoC2O42H2O. According to the previously reported studies, when only water was used as solvent, CoC2O42H2O microrods were obtained because the ion-exchange reaction between the cobalt ion and the oxalate ion was very rapid in aqueous solution. In organic solvent medium, oxalic acid is a weak electrolyte that cannot be electrolytically dissociated into ions, so the cobalt ion and the oxalic acid do not react immediately. Therefore, when DMF and water were used, the reaction rate slowed down leading to smaller products, including spindle-like architectures, nanorod bundles, and nanorods. Furthermore, when the amount of DMF was increased to 16 mL or 20 mL, no products were obtained because the ion-exchange reaction was restrained .
The thermal behavior of CoC2O42H2O microrods was investigated by thermogravimetric analysis (TGA) in static air atmosphere. From Figure 3, it can be seen that there are two distinct weight loss steps. The first weight loss occurs at 110–220°C, which corresponds to the evaporation of crystallized water. When the temperature is above 300°C, the second weight loss was observed, which is attributed to the decomposition of anhydrous oxalate into Co3O4. The weight loss of two steps is about 19.7% and 53.3%, which is close to the theoretical value. The decomposition of CoC2O42H2O can be expressed as the following reaction:
According to the results of TGA, the thermal decomposition of the corresponding CoC2O42H2O was performed at 350°C. After being annealed at 350°C for 1 h in air, the as-synthesized CoC2O42H2O with different morphologies were completely converted to phase-pure spinel Co3O4. The morphology of the Co3O4 products is shown in Figures 2(e)–2(h), from which it can be seen that the original shape has been maintained after calcination. The crystallographic phase of all the samples is again examined by XRD (Figure 4). All diffraction peaks can be well indexed to the pure cubic phase of Co3O4 (JCPDS 43-1003), indicating that the pure phase of Co3O4 was obtained by annealing CoC2O42H2O precursor directly. Figure 5 shows TEM images of the Co3O4 products, revealing that the Co3O4 products were composed of numerous Co3O4 nanoparticles with a size of several tens of nanometers and abundant pore structures were formed among the nanoparticles.
Nitrogen adsorption-desorption isotherms of nano-Co3O4 are shown in Figure 6, and the insets illustrate the corresponding Barrett-Joyner-Halenda (BJH) pore size distribution plots. The isotherms can be categorized as type IV with an H3 hysteresis loop, which is characteristic of mesoporous materials. The BJH pore size distribution indicates that all of the samples contain mesoscale pores. The Brunauer-Emmett-Teller (BET) surface areas and pore volumes of the samples are 42 m2/g and 190.3 mm3/g, 61 m2/g and 226.3 mm3/g, 62 m2/g and 241.3 mm3/g, and 83 m2/g and 277.1 mm3/g for the Co3O4 with microrods, spindle-like architectures, nanorod bundles, and nanorods, respectively.
Considering the porous structures and high BET surface area, we investigated the application of the synthesized nano-Co3O4 for the thermal decomposition of AP. Figure 7 shows DSC curves for thermal decomposition of pure AP and its mixture with nano-Co3O4 with different morphologies. For pure AP, an endothermic peak was observed at about 250°C, which is due to the crystal transformation of AP from orthorhombic to cubic phase (Figure 7(e)) . When nano-Co3O4 with different morphologies as catalyst was added to AP, all samples have similar endothermic peaks at about 250°C, indicating that the catalysts have little effect on the crystallographic transition temperature of AP. However, in the relatively high temperature region, the samples containing catalysts have dramatic changes in the exothermic peaks of AP decomposition. When 2 wt% catalysts were added to AP, the original exothermic peak of pure AP at 445.0°C disappeared and only one exothermic peak was observed. The exothermic peak temperature was 305.1, 299.7, 297.4, and 296.2°C for Co3O4 microrods, spindle-like architectures, nanorod bundles, and nanorods, respectively (Figure 7(a)–7(d)). The present catalytic activity of Co3O4 nanorods was higher than Co3O4 nanoparticles, nanosheets, and octahedral particles [12, 18, 19]. The above results indicate that Co3O4 particles have a significant effect on the decomposition temperature of AP and Co3O4 nanomaterials with different morphologies for decreasing the decomposition of AP are proportional to their BET surface area and pore volume. It is known that specific surface area and pore volume can be the primary reasons for the catalytic role, since more reactive sites can be generated [12, 20, 21]. Thus, Co3O4 nanorods with larger BET surface area and pore volume have the most effective catalytic activity and the thermal decomposition temperature of AP shifted downward about 148.8°C.
In summary, we synthesized porous nano-Co3O4 with different morphologies via annealing CoC2O42H2O precursors prepared under ambient condition without the assistance of template or surfactant. The as-prepared porous nano-Co3O4 with different morphologies have good catalytic properties for the thermal decomposition of AP due to their large BET surface area and pore volume. Co3O4 nanorods with larger BET surface area and pore volume show better catalytic activity than others and shifted the AP thermal decomposition temperature downwardly to about 148.8°C.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors acknowledge the financial support by National Natural Science Foundation of China (no. 21401081), China Postdoctoral Science Foundation (no. 2014M560397), Jiangsu Natural Science Funds for Distinguished Young Scholars (no. BK20140013), Jiangsu Postdoctoral Science Foundation (no. 1401051C), and the Senior Intellectuals Fund of Jiangsu University (nos. 14JDG058 and 11JDG098).
Nano-Co3O4 with different morphologies was successfully synthesized by annealing CoC2O4·2H2O precursors, and showed good catalytic property for the decomposition of ammonium perchlorate.
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