Catalytic decomposition action of hollow CuFe₂O₄ nanospheres on RDX and FOX-7

Abstract

Two hollow CuFe₂O₄ nanospheres (CuFe₂O₄-p and CuFe₂O₄-d) were successfully prepared using a hydrothermal method. CuFe₂O₄-p has a smaller size and higher BET specific surface area compared to CuFe₂O₄-d. The catalytic activity of nano-CuO, nano-Fe₂O₃, physically mixed CuO + Fe₂O₃, CuFe₂O₄-p, and CuFe₂O₄-d on the thermal decomposition of RDX and FOX-7 were comparatively investigated by differential scanning calorimetry (DSC) method. The results showed that CuFe₂O₄-p and CuFe₂O₄-d exhibited higher catalytic activity than the others, and the apparent activation energies of decomposition were reduced by 37.7 and 28.4 kJ mol⁻¹ for RDX, 40.1 and 30.5 kJ mol⁻¹ for FOX-7, respectively. The catalytic performance of CuFe₂O₄-p for the decomposition of RDX and FOX-7 was superior to that of CuFe₂O₄-d.

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1. Introduction

Cyclotrimethylenetrinitramine (RDX) and 1,1-diamino-2,2-dinitroethylene (FOX-7), known as two common oxidizers, play key roles in composite solid propellants. Their thermal decomposition characteristics directly influence the combustion behavior of solid propellants: the lower thermal decomposition temperature and apparent activation energy, the shorter the ignition delay time and the higher burning rate of the propellant.^1,2 It is widely acknowledged that catalysts can accelerate the oxidizer decomposition and enhance the burning rate of the propellant.^3–6

Nano metal oxides exhibit high catalytic activity because of their large surface area, the high mobility of their surface atoms, and varied lattice defects.^7–11 In the past decades, the thermal decompositions of RDX and FOX-7 have been extensively investigated by taking advantage of the catalytic activities of a single metal oxide and two physically mixed components. The most commonly used catalysts are transition metal oxides, such as Fe₂O₃,^12,13 CuO,^14,15 MnO₂,¹⁶ and V₂O₃.¹⁷ As previously demonstrated, binary mixed metal oxides have better catalytic effect than a single one, because the performance of two kinds of metal oxides would be complementary and have a “synergistic effect”.^18–21

Nano metal composite oxides possess specific structure-properties and higher chemical stability (acid and alkali resistance), and two types of metals can generate a “synergistic effect”.^22,23 Therefore, they have been extensively studied in recent years and used in many high and new fields, such as photoelectric materials, magnetic materials and photocatalysis.^24–26 Among ferrites, CuFe₂O₄ has a cubic close-packed arrangement of oxygen ions with Cu²⁺ and Fe³⁺ ions at two different crystallographic sites.^27,28 Owing to the unique electronic configuration of its valence shell (3d10 4s1), low price, environmental benignity, and the large abundance of Cu, CuFe₂O₄ has attracted extensive attention and is widely applied in sensors,^29,30 batteries,^31,32 and catalysts.^33–35 In our work, we attempt to use CuFe₂O₄ as burning catalyst in the special field of solid propellant, instead of single metal oxides and their physical mixtures.

In this paper, we report the preparation of two hollow CuFe₂O₄ nanospheres and explore its catalytic decomposition action on RDX and FOX-7 in compared with other related metal oxides.

2. Experimental

2.1 Preparation

Hollow CuFe₂O₄ spheres were synthesized using a hydrothermal method. We dissolved 4 mmol CuCl₂·2H₂O, 8 mmol FeCl₃·6H₂O, and 12 mmol NaAc in 70 mL ethylene glycol, following which we added 1.0 g polyvinylpyrrolidone (PVP) (denoted as CuFe₂O₄-p) or dodecyl trimethyl ammonium bromide (DTAB) (denoted as CuFe₂O₄-d) as dispersants. The mixtures were stirred vigorously for 1 h, sealed in a 100 mL Teflon-lined stainless-steel autoclave, and heated at 180 °C in air for 12 h. The resulting black solid products were collected through centrifugation and washed with ethanol and deionized water several times. All the generated samples were dried at 60 °C. Finally, the CuFe₂O₄-p and CuFe₂O₄-d samples were prepared.

For comparison, nano-CuO, nano-Fe₂O₃ were prepared according to the methods in ref. 12 and 14, respectively. Briefly, CuCl₂·2H₂O (15 mmol) were dissolved in 100 mL NaOH solution (3.2 M), 100 mL H₂O₂ (50 wt%) was then added drop wise under constant stirring. The precipitate products were washed and dried at 110 °C. After calcined at 350 °C for 2 h, CuO sample was synthesized. For nano-Fe₂O₃, FeCl₃·6H₂O (4 mmol), urea (15 mmol), glycine (6.5 mmol), and 30 mL distilled water were added into a Teflon-lined stainless-steel autoclave (50 mL) with magnetic stirring, and then maintained at 160 °C for 10 h. Subsequently, the products were washed and dried at 60 °C. The resulting powder was used as the Fe₂O₃ sample. Besides, physically mixed CuO and Fe₂O₃ (1 : 1 mol mol⁻¹, denoted as CuO + Fe₂O₃) was obtained through mixing and grinding with CuO and Fe₂O₃ in the molar ratio of 1 : 1.

2.2 Equipments and conditions

X-ray diffraction (XRD) patterns were acquired using a Rigarku Mini Flex 600 X-ray diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer. Scanning electron microscopy (SEM) images were recorded using a Zeiss SIGMA scanning electron microscope. Transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images were obtained using an FEI Tecnai G2 microscope operated at 200 kV, and energy dispersive spectrum (EDS) mappings were acquired using an Oxford 51-XMX. N₂ adsorption–desorption isotherms at 77 K were measured using an Autosorb-iQ automated gas sorption system (Quantachrome Instrument). The specific surface area was calculated according to the Brunauer–Emmett–Teller (BET) method and the pore volume was calculated based on the Barrett–Joyner–Halenda (BJH) equation.

The as-prepared CuO, Fe₂O₃, CuO + Fe₂O₃, CuFe₂O₄-p, and CuFe₂O₄-d were fully mixed with RDX and FOX-7 in the molar ratio of 1 : 4, and the mixtures were used as samples for DSC tests, which were performed using a 200F3 differential scanning calorimeter (Netzsch, Germany) with the following conditions: nitrogen gas purity, 99.999%; N₂ flow rate, 40 mL min⁻¹; sample mass, approximately 0.5 mg; heating rate (β), 5.0, 10.0, 15.0, and 20.0 °C min⁻¹.

3. Results and discussion

3.1 Characterization

Fig. 1 shows the XRD diffraction patterns of the as-prepared samples. The patterns of Fig. 1a and b could be indexed to CuO (JCPDS file no. 45-0937) and Fe₂O₃ (JCPDS file no. 33-0664), respectively. It can be seen from Fig. 1c and d that almost all the diffraction peaks can be well assigned to CuFe₂O₄ with a cubic spinel structure (JCPDS file no. 77-0010, space group: Fd3̄m [227] with lattice parameters a = b = c = 8.37 Å). The diffraction peaks are at 2θ = 18.344°, 30.175°, 35.543°, 43.199°, 53.596°, 57.135° and 62.744°, which correspond to the crystal planes of (111), (220), (311), (400), (422), (511) and (440), respectively.

Fig. 1 XRD patterns of (a) CuO, (b) Fe₂O₃, (c) CuFe₂O₄-p and (d) CuFe₂O₄-d.

The surface composition and chemical state of CuFe₂O₄ nanoparticle were further investigated by XPS. In the survey spectrum (Fig. 2a), the elements Cu, Fe and O can be observed clearly. The appearance of C1s peaks was mainly due to the adsorbed or bonded organic species. The O1s spectrum (Fig. 2b) can be deconvoluted into two peaks at 529.4 and 531.5 eV, which correspond to the O²⁻ ions and –OH groups, respectively. From Fig. 2c, the main peaks at 710.9 eV and the shoulder peak at 724.6 eV can be assigned to Fe 2p_3/2 and Fe 2p_1/2, respectively. The accompanying satellite peaks at around 718.2 eV were indicative of the presence of Fe³⁺ cations. No Fe²⁺ peak was observed. Fig. 2d shows the binding energies of Cu 2p_3/2 and Cu 2p_1/2 at 933.5 and 954.2 eV respectively, with a typical satellite peak of Cu³⁺ at 943.3 eV. These results further confirm that CuFe₂O₄ has been synthesized successfully.

Fig. 2 XPS characterization of CuFe₂O₄, (a) survey spectra, (b) O1s spectra, (c) Fe 2p spectra, and (d) Cu 2p spectra.

The morphology and structure of the as-prepared samples were characterized by SEM. It can be observed that the CuO sample consists of irregular blocks of sizes in the range of 400–500 nm (Fig. 3a). As shown in Fig. 3b, a large amount of Fe₂O₃ nanoparticles aggregate and form a well-defined spherical structure, the diameter of which is estimated as approximately 500 nm. From Fig. 3c, we can obviously found that Fe₂O₃ particles were dispersed on the surface of CuO samples. Owing to the limitation of physical blending, Fe₂O₃ and CuO were not well-mixed. Fig. 3d and e clearly show that CuFe₂O₄-p and CuFe₂O₄-d are composed of quasi-spherical particles with an average size of approximately 200 nm. The surfaces of the two CuFe₂O₄ spheres are rough, and the diameters are uniform. Obviously, the particle size of CuFe₂O₄-p is smaller than that of CuFe₂O₄-d. Thus, PVP is better than DTAB as the dispersant for the preparation of CuFe₂O₄ nanospheres. Fig. 3f displays the hollow structure of the CuFe₂O₄ nanosphere. The outer diameter of the hollow is approximately 200 nm and the inner diameter is about 150 nm on average. Such a hollow structure is very favourable to increase the specific surface areas and diffusion rate of reaction molecules.

Fig. 3 SEM images of (a) CuO, (b) Fe₂O₃, (c) CuO + Fe₂O₃, (d) CuFe₂O₄-p, and (e and f) CuFe₂O₄-d.

The TEM images of CuFe₂O₄-p and CuFe₂O₄-d samples (Fig. 4a and b) further demonstrates their hollow nanospherical microstructure, which is consistent with the SEM results. In order to verify the crystalline structure of the CuFe₂O₄, we acquired the HRTEM image shown in Fig. 4c, which indicates that the CuFe₂O₄ nanoparticles show a well-defined lattice plane with a high degree of crystallinity. The lattice spacing of 0.248 nm corresponds to the (400) lattice planes and matches well with the value observed from the JCPDF file (no. 77-0010) of CuFe₂O₄. As shown in Fig. 4d, the corresponding selected-area electron diffraction (SAED) pattern indicates the polycrystalline feature of CuFe₂O₄ samples. EDS results (Fig. 4e) confirm the presence of Cu, Fe and O elements in CuFe₂O₄ nanoparticles. Atomic contents of Cu and Fe are 8.19% and 16.18% respectively, and the atomic ratio of the two elements is 1 : 1.98, which is close to theoretical molar ratio of CuFe₂O₄.

Fig. 4 TEM images of (a) CuFe₂O₄-p, (b) CuFe₂O₄-d; HRTEM image of (c) CuFe₂O₄-p; SAD pattern of (d) CuFe₂O₄-p; EDS analysis results of (e)CuFe₂O₄.

To evaluate the specific surface areas and porosity of the as-prepared samples, N₂ adsorption–desorption isotherms were measured. The corresponding textural parameters are summarized in Table 1. The BET specific surface area of Fe₂O₃ is as large as that of CuFe₂O₄, whereas the surface areas of CuO and CuO + Fe₂O₃ are much lower than that of CuFe₂O₄. Note that the surface area of CuFe₂O₄-p (32.66 m² g⁻¹) is higher than that of CuFe₂O₄-d (28.97 m² g⁻¹). The larger specific surface area of the nanocomposite can offer more adsorption and reaction sites, leading to enhanced catalytic activity.

Table 1 Textural properties of as-prepared samples

Sample	SBET^a (m^2 g^−1)	Vtotal^b (cm^3 g^−1)	Daver (nm)
a Total surface area calculated using the BET method.b Total pore volume calculated at P/P0 = 0.95.
CuO	6.70	0.005	6.03
Fe2O3	32.07	0.094	11.77
CuO + Fe2O3	17.43	0.048	11.07
CuFe2O4-p	32.66	0.093	11.36
CuFe2O4-d	28.97	0.081	15.43

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3.2 Influence on thermal decomposition of RDX

DSC results (Fig. 5) indicate that the peak temperatures of exothermic decomposition for pure RDX, RDX + CuO, RDX + Fe₂O₃, RDX + CuO + Fe₂O₃, RDX + CuFe₂O₄-p and RDX + CuFe₂O₄-d are 241.3, 239.9, 236.9, 237.7, 220.0 and 231.4 °C, respectively. Here, the effects of CuO, Fe₂O₃, and CuO + Fe₂O₃ are unconspicuous, but CuFe₂O₄-p and CuFe₂O₄-d significantly influence the decomposition of RDX. The peak shapes of RDX + CuFe₂O₄-p and RDX + CuFe₂O₄-d were also significantly different from those of other four curves. These results should be ascribed two major reasons: (1) the positive synergistic effect between Cu and Fe in CuFe₂O₄ nanoparticles led to higher catalytic activity than CuO and Fe₂O₃; (2) the hollow structure of CuFe₂O₄ increased its specific surface areas and improved the diffusion rate of the decomposed molecules. Furthermore, the decomposition heats of pure RDX, RDX + CuFe₂O₄-p and RDX + CuFe₂O₄-d are calculated to be about 776, 970, and 872 J g⁻¹ respectively, indicating that CuFe₂O₄-p and CuFe₂O₄-d not only accelerated the thermal decomposition of RDX, but also increased the decomposition depth, which would be favourable to improve the burning rate of solid propellant. Obviously, CuFe₂O₄-p has superior catalytic performance compared to CuFe₂O₄-d because of its smaller particle size and larger specific surface area, which is consistent with the SEM and BET results.

Fig. 5 DSC curves for pure RDX and RDX mixed with different samples.

3.3 Influence on thermal decomposition of FOX-7

DSC results (Fig. 6) suggests that the first peak temperatures of exothermic decomposition for FOX-7 + CuO, FOX-7 + Fe₂O₃, FOX-7+CuO + Fe₂O₃, FOX-7 + CuFe₂O₄-p and FOX-7 + CuFe₂O₄-d are 219.9, 221.0, 219.9, 219.9 and 218.5 °C respectively, which are all less than that of pure FOX-7 (234.6 °C) by approximately 15 °C. However, the apparent decomposition heats for the above mixtures are 620, 632, 528, 643 and 679 J g⁻¹ respectively, which are much larger than that of pure FOX-7 (489 J g⁻¹). FOX-7 + CuFe₂O₄-p and FOX-7 + CuFe₂O₄-d released more heat than others. Fig. 6 shows that the second peak temperatures for FOX-7 + Fe₂O₃, FOX-7 + CuO + Fe₂O₃, FOX-7 + CuFe₂O₄-p, and FOX-7 + CuFe₂O₄-d are less than that of pure FOX-7 (294.9 °C) by 6.6, 4.2, 12.2, and 9.1 °C, respectively. However, this temperature is even higher with the addition of CuO. Therefore, CuFe₂O₄-p and CuFe₂O₄-d play better catalytic role than others on the thermal decomposition of FOX-7. The catalytic activity of CuFe₂O₄-p is still stronger than CuFe₂O₄-d on FOX-7 thermal decomposition. Between the two energetic materials, CuFe₂O₄-p and CuFe₂O₄-d show higher catalytic decomposition activity on RDX than on FOX-7.

Fig. 6 DSC curves of pure FOX-7 and FOX-7 mixed with different samples.

3.4 Apparent activation energy

The apparent activation energy (E_a) of the decomposition process was evaluated by Kissinger method with DSC data from different heating rates (5.0, 10.0, 15.0, and 20.0 °C min⁻¹). The E_a values for four mixtures were obtained and listed in Table 2. We can see that the E_a values of the decomposition process for RDX + CuFe₂O₄-p and RDX + CuFe₂O₄-d are less than the literature value of pure RDX (172.6 kJ mol⁻¹)³⁶ by 37.7 and 28.4 kJ mol⁻¹, respectively. The E_a values of the decomposition process for FOX-7+CuFe₂O₄-p and FOX-7+CuFe₂O₄-d are less than the literature value of pure FOX-7 (249.9 kJ mol⁻¹)³⁷ by 40.1 and 30.5 kJ mol⁻¹, respectively. The reduction in the apparent activation energy is greater with CuFe₂O₄-p than with CuFe₂O₄-d, which is also ascribed to its smaller particle size and larger specific surface area. These results indicate that CuFe₂O₄-p and CuFe₂O₄-d can reduce the apparent activation energy of the decomposition process, increase the decomposition rate and show good catalytic decomposition activity.

Table 2 Apparent activation energies of the decomposition process for different samples

Sample	Ea (kJ mol^−1)	r
RDX	172.6	0.982
RDX + CuFe2O4-p	134.9	0.998
RDX + CuFe2O4-d	144.2	0.991
FOX-7	249.9	0.995
FOX-7 + CuFe2O4-p	209.8	0.984
FOX-7 + CuFe2O4-d	219.4	0.997

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4. Conclusion

In summary, hollow CuFe₂O₄-p and CuFe₂O₄-d nanospheres were successfully prepared and were characterized by XRD, XPS, SEM, TEM, HRTEM, EDS and BET methods. CuFe₂O₄-p has smaller size and higher BET specific surface area compared to CuFe₂O₄-d. DSC results indicate that CuFe₂O₄-p and CuFe₂O₄-d can greatly reduce the decomposition temperature, increase the decomposition heat, and show higher catalytic decomposition activity on RDX and FOX-7 compared to CuO, Fe₂O₃ and CuO + Fe₂O₃. The E_a values of the decomposition process with the catalytic action of CuFe₂O₄-p and CuFe₂O₄-d decrease by 37.7 and 28.4 kJ mol⁻¹ for RDX, 40.1 and 30.5 kJ mol⁻¹ for FOX-7, respectively. The reason for the good catalytic decomposition activity of CuFe₂O₄ should be the active chemical nature and hollow structure. The catalytic performance of CuFe₂O₄-p is superior to that of CuFe₂O₄-d because of its smaller particle size and larger specific surface area.

Acknowledgements

This investigation received financial assistance from the National Natural Science Foundation of China (21241003), Postdoctoral Science Foundation of China (2014M552480) and Postdoctoral Science Foundation of Shanxi Province.

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