Catalytic Activity of Co 3 O 4 Nanomaterials with Different Morphologies for the Thermal Decomposition of Ammonium Perchlorate

Nano-Co 3 O 4 with different morphologies was successfully synthesized by annealing CoC 2 O 4 ⋅2H 2 O 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 Co 3 O 4 nanomaterials while their original morphologies were maintained. The catalytic effect was investigated for nano-Co 3 O 4 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 Co 3 O 4 nanorods with larger BET surface area and pore volume had the highest catalytic activity.


Introduction
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 [1], shape [2], and size [3].Therefore, many efforts have been made to develop costeffective synthesis methods of nanomaterials with different structures and morphologies for enabling novel intrinsic properties and applications of nanomaterials.
Co 3 O 4 , as one of the most intriguing magnetic p-type semiconductors, is of special interest due to its potential applications in heterogeneous catalyst [4], lithium-ion battery [5], supercapacitor [6], gas sensor [7], and many other aspects [8].Up to now, shape-controlled Co 3 O 4 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 Co 3 O 4 nanostructures [9][10][11][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 [9].Hu et al. synthesized -Co(OH) 2 nanosheet at 180 ∘ C for 12 h and Co(CO 3 ) 0.5 (OH) 0.11 H 2 O nanobelt at 140 ∘ C for 12 h via a solvothermal method [10].Wang et al. prepared onedimensional and layered parallel folding of cobalt oxalate nanostructures using N,N-dimethylacetamide (DMA) and dimethyl sulfoxide (DMSO) as solvents at ambient temperature [11].In our past work, we prepared shape-controlled synthesis of Co 3 O 4 nanostructures derived from coordination polymer precursors [12].However, for some shapecontrolled 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][14][15].Recently, Co 3 O 4 nanomaterials with various morphologies have been used as effective catalyst to accelerate thermal decomposition of AP [12,[16][17][18][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,Ndimethylformamide (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 Co 3 O 4 nanomaterials while their original morphologies had been well maintained.
To study their potential applications, the as-prepared nano-Co 3 O 4 with different morphologies had been applied in the thermal decomposition of AP, which exhibited good activity.

Experimental
All chemicals and solvents are of analytical grade and were used as received without further purification.In a typical experiment, 1 mmol Co(NO 3 ) 2 ⋅6H 2 O 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 H 2 C 2 O 4 ⋅2H 2 O 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 Co 3 O 4 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 K  radiation ( = 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 N 2 adsorption.
To test the catalytic effect of Co 3 O 4 nanostructures with different morphologies on the thermal decomposition of AP, the mixture of AP and Co 3 O 4 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 N 2 atmosphere over the temperature range of 30-500 ∘ C. The mass percentage of Co 3 O 4 nanostructures to AP in the mixture was 2%.

Results and Discussion
Figure 1 shows the XRD patterns of the precursors prepared under the different volume ratio of DMF and water.All of  Figure 2(d) illustrates nanorods in sample prepared by using 8 mL of H 2 O and 12 mL of DMF.In addition, when 4 mL of H 2 O and 16 mL of DMF were used or 20 mL of DMF was used without addition of H 2 O, no products could be obtained.The above facts showed that DMF/water volume ratio played an important role in the information of CoC 2 O 4 ⋅2H 2 O.According to the previously reported studies, when only water was used as solvent, CoC 2 O 4 ⋅2H 2 O 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 [11].
The thermal behavior of CoC 2 O 4 ⋅2H 2 O 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    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 Co Nitrogen adsorption-desorption isotherms of nano-Co 3 O 4 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 m 2 /g and 190.3 mm 3 /g, 61 m 2 /g and 226.3 mm 3 /g, 62 m 2 /g and 241.3 mm 3 /g, and 83 m 2 /g and 277.1 mm 3 /g for the Co 3 O 4 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-Co 3 O 4 for the thermal decomposition of AP. Figure 7 shows DSC curves for thermal decomposition of pure AP and its mixture with nano-Co 3 O 4 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)) [13].When nano-Co 3 O 4 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 Co 3 O 4 microrods, spindle-like architectures, nanorod bundles, and nanorods, respectively (Figure 7

Conclusions
In summary, we synthesized porous nano-Co
Figure 2(c)  shows the morphology of sample prepared in the presence of 12 mL of H 2 O and 8 mL of DMF.It was observed that the sample consisted of nanorod bundles.

3 O 4 (
JCPDS 43-1003), indicating that the pure phase of Co 3 O 4 was obtained by annealing CoC 2 O 4 ⋅2H 2 O precursor directly.Figure 5 shows TEM images of the Co 3 O 4 products, revealing that the Co 3 O 4 products were composed of numerous Co 3 O 4 nanoparticles with a size of several tens of nanometers and abundant pore structures were formed among the nanoparticles.
(a)-7(d)).The present catalytic activity of Co 3 O 4 nanorods was higher than Co 3 O 4 nanoparticles, nanosheets, and octahedral particles [12, 18, 19].The above results indicate that Co 3 O 4 particles have a significant effect on the decomposition temperature of AP and Co 3 O 4 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, Co 3 O 4 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.
3 O 4 with different morphologies via annealing CoC 2 O 4 ⋅2H 2 O precursors prepared under ambient condition without the assistance of template or surfactant.The as-prepared porous nano-Co 3 O 4 with different morphologies have good catalytic properties for the thermal decomposition of AP due to their large BET surface area and pore volume.Co 3 O 4 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.