Catalytic effect of CuO nanoplates, a graphene (G)/CuO nanocomposite and an Al/G/CuO composite on the thermal decomposition of ammonium perchlorate

Abstract

Tenorite CuO nanoplates, a graphene (G)/CuO nanocomposite and an Al/G/CuO composites were investigated in this paper as potential catalysts for the thermal decomposition of ammonium perchlorate (AP); the most common oxidant used in composite propellant formulations. Tenorite CuO nanoplates and the G/CuO nanocomposite were successfully obtained by a facile hydrothermal method; the Al/G/CuO composite was prepared using physical mixing of the G/CuO nanocomposite and aluminum powder with sonication and dispersion. The catalytic activity of these materials was investigated by thermogravimetric (TG) and differential thermal analysis (DTA). The results of DTA indicated that the three materials ameliorate the thermal decomposition of AP, especially the G/CuO nanocomposite and Al/G/CuO composite, where the high temperature decomposition (HTD) of AP decreases from 432 °C to 325 °C and 315 °C, respectively. Significant decrease in activation energy (E_a) (from 129 kJ mol⁻¹ to 71.47 kJ mol⁻¹ and 56.18 kJ mol⁻¹) was also achieved in the presence of these two materials, showing their strong catalytic activity on the thermal decomposition of ammonium perchlorate.

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1. Introduction

The performance of composite solid rocket propellants depends strongly on the characteristics of thermal decomposition of their oxidizers; the lower the decomposition temperature the shorter the delay time of propellant ignition and the higher the burning rate.¹ Ammonium perchlorate (AP) is the most common oxidant used in composite solid propellants; its thermal decomposition is strongly sensitive to additives, and especially nanoadditives due to their reduced dimensions and high surface area.^2,3

CuO nano-powder is widely used in heterogeneous catalysts because of its high activity and selectivity in oxidation/reduction reactions, as well as its availability and low production cost. This type of metal oxide exhibits superior properties like a direct band gap of 1.2 eV,⁴ non-toxicity, excellent thermal and chemical stability, high-temperature superconductivity and electrochemical activity.^5,6 Numerous methods have been developed to prepare CuO nanostructures; however, hydrothermal synthesis is the most used due to its easy manipulation, scalable production, low temperature required and low cost.⁷

Nanomaterials have many advantages; however, due to their small size they are likely to aggregate and expose fewer active sites, resulting in a decrease of their catalytic activity. To prevent the aggregation phenomenon and to improve catalytic activity of the CuO nanoplates, graphene is used here to ameliorate their distribution because of its high surface area.

To facilitate an improved distribution of CuO nanoplates in the graphene nanosheets the G/CuO nanocomposite was prepared using an in situ synthesis which is a simple method that allows one-step fabrication of nanocomposites with in situ generated nanomaterials from corresponding precursors.

Metal powders are widely used in composite propellant as fuel, comprising about 14% to 20% of the propellant composition to give more energy and to ameliorate the propellant ballistic properties. Aluminum is the most used metallic element because of its low cost and non-toxicity.⁸ For this reason a new energetic component (Al/G/CuO) was prepared by physical mixing of aluminum powder and G/CuO nanocomposite with sonication and dispersion.

The present work provides strong argument for the application of both G/CuO nanocomposite and Al/G/CuO composite in the AP-based propellant as catalyst to improve the thermal decomposition of ammonium perchlorate and therefore improve the ballistic properties of the composite propellant. As far as we know, these two composites have not been investigated as a catalyst in the thermal decomposition of ammonium perchlorate before. In this paper the catalytic effect of the three materials CuO, G/CuO and Al/G/CuO on the thermal decomposition of AP is investigated by TG/DTA analysis and compared with pure ammonium perchlorate.

2. Experimental section

2.1 The materials

The aluminum powder was provided from Tianjin Zhiyuan Chemical Reagent Co., Ltd. Graphite powder was obtained from Qingdao Furunda graphite CO., Ltd. H₂SO₄ was purchased from Luoyang Chemical Reagents Limited Company. KMnO₄ was obtained from Heilongjiang Acheng province Chemical Reagent factory. HCl was provided from Shanghai Institute of fine chemical technology. H₂O₂ was obtained from Tianli Chemical Reagent Co., Ltd. NaNO₃ was purchased from Tianjin Bodi Chemical Industry Co., Ltd. NaOH was provided from Tianjin Institute of Optics and fine chemicals. Cupric acetate Cu (CH₃COO)₂·H₂O was obtained from Tianjin chemical reagents Limited Company. Ammonium perchlorate was purchased from Sinopharm Chemical Reagent Co., Ltd. All the reagents are of analytically pure grade (AR) and they were used without further purification.

2.2 Preparation of tenorite CuO nanoplates

The tenorite CuO nanoplates were obtained by a simple hydrothermal method using copper acetate as precursor. In a typical procedure 2.5 g of cupric acetate was added into 90 mL of deionized water and dissolved after constant stirring for 30 min. 1.6 g of NaOH was then added to the solution with stirring for another 30 min, then the mixture was transferred into a 100 mL stainless steel autoclave and heated in an oven at 180 °C for 3 h. After cooling to room temperature, the resulting products were separated via centrifugation and washed several times with deionized water and ethanol, then dried at 60 °C.

2.3 Preparation of graphene oxide

Graphene oxide (GO) was synthesized from natural graphite by modified Hummers method.⁹ Graphite powder (1 g) and sodium nitrate (0.5 g) were mixed with 23 mL of concentrated sulfuric acid and stirred for 30 min in presence of an ice bath, then 3 g of potassium permanganate was added gradually and stirred for 15 min. After that the mixture was transferred to a water bath and stirred for 30 min at 35 °C. Next, 46 mL of deionized water was slowly added, and the temperature quickly rises from 35 °C to 98 °C; the mixture was then maintained at this temperature for 30 min. 140 mL of deionized water and 5 mL of hydrogen peroxide were then added to the mixture and stirred for 2 h. In this stage the color of the mixture changed from dark brown to brilliant yellow. Finally, the mixture was separated by centrifugation, washed repeatedly with 5% HCl solution, followed by deionized water until the pH was neutral and then dried at 60 °C.

2.4 Preparation of G/CuO nanocomposite

After the preparation of GO, the G/CuO nanocomposite was prepared using an in situ synthesis in order to prevent the agglomeration of CuO nanoplates while maintaining an even distribution in the graphene nanosheets. The G/CuO nanocomposite was obtained as follows: 0.24 g of graphene oxide (GO) was dissolved in 100 mL of deionized water and stirred overnight. Then 2.5 g of cupric acetate and 1.6 g of NaOH were added to the GO solution and stirred for 3 h; the mixture was then transferred to a stainless steel autoclave for hydrothermal treatment at 120 °C for 24 h. The nanocomposite was obtained after centrifugation and washing with deionized water and ethanol several times, then dried at 60 °C.

The graphene is obtained using the same method but in the absence of cupric acetate (Cu (CH₃COO)₂·H₂O) and NaOH.

2.5 Preparation of Al/G/CuO

The Al/G/CuO composite was prepared by physical mixing of the G/CuO nanocomposite and the aluminum powder with sonication and dispersion in presence of ethanol, the samples were prepared with a stoichiometric ratio of Al : G/CuO nanocomposite, 82 : 18% wt, respectively.

2.6 Characterization of materials

Powder X-ray diffraction (XRD) analyses of the samples were carried out with a Rigaku TTR-III equipped with CuKα radiation (λ = 0.15406 nm). The morphologies of the samples were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), using Model (JEM-2100). The composition of the powders was characterized at room temperature by Fourier transform infrared spectroscopy (FT-IR spectra; Perkin Elmer Spectrum 100) in the range of 500 to 4000 cm⁻¹. The catalytic effect of the as-prepared additives on the thermal decomposition of AP was detected by TG/DTA at a heating rate of 10 °C min⁻¹ in a static N₂ atmosphere over the range of 50 to 500 °C.

3. Results and discussion

3.1 Structural and morphological characteristics

The XRD patterns of all the as-prepared materials are shown in Fig. 1a–c. Fig. 1a shows the XRD patterns of graphene oxide (GO) and graphene (G), the characteristic diffraction (002) peak of graphene oxide appears at about 11°. After thermal reduction of GO, the XRD pattern of graphene (G) shows a broad diffraction peak at about 25° and the disappearance of the diffraction peak at 11°, this reveals that the graphene oxide is successfully reduced to graphene.

Fig. 1 XRD patterns of (a) GO and G, (b) CuO and G/CuO, (c) Al and Al/G/CuO.

The two XRD patterns of the as-prepared tenorite CuO nanoplates and the G/CuO nanocomposite are shown in Fig. 1b, this figure shows several well-defined diffraction reflections at 2θ = 32.51°, 35.54°, 38.71°, 46.26°, 48.72°, 53.48°, 58.26°, 61.52°, 66.22°, 68.12°, 72.37°, and 75.24°, which correspond to the monoclinic form of tenorite CuO with lattice planes of (110), (11−1), (111), (11−2), (20−2), (020), (202), (11−3), (31−1), (220), (311) and (22−2) respectively, and with lattice constants of a = 4688 Å, b = 3423 Å, c = 5132 Å (JCPDS 48-1548), however the intensity of the peaks in the G/CuO nanocomposite are much lower than that of pure CuO nanoplates. The XRD pattern of the G/CuO nanocomposite shows also the apparition of the diffraction peak of graphene at about 25°.

The XRD patterns of Al powder and Al/G/CuO composite are shown in Fig. 1c. Similar peaks at 2θ = 38.47°, 44.71°, 65.09°, 78.22°, 82.43° are clearly observed, which correspond well with the crystal planes of (111), (200), (220), (311), (222), respectively for the Al powder (JCPDS File No. 65-2869). However, the XRD pattern of Al/G/CuO shows other peaks which correspond to the nanocomposite G/CuO, these peaks are much less pronounced compared with those of aluminum powder, because of the very small amounts they represent in the composite.

In addition, to confirm the identification and the composition of the as-prepared materials, Fourier transform infrared spectroscopy (FT-IR) investigations of graphene oxide, CuO nanoplates and G/CuO nanocomposite give corresponding spectra, as shown in Fig. 2. The three spectra show a strong and broad absorption band that appears at 3400 cm⁻¹ which is attributed to the O–H stretching vibration from adsorbed water molecules.¹⁰ For the FT-IR spectrum of GO, the C=O stretching of COOH groups is observed at 1722 cm⁻¹. The three peaks at 1356 cm⁻¹, 1217 cm⁻¹ and 1080 cm⁻¹ are attributed to the stretching vibrations of carboxyl C=O, epoxy C–O and alkoxy C–O, respectively.¹¹ The absorption peak at 2800 cm⁻¹ and 2900 cm⁻¹ represent the anti-symmetric stretching vibration of CH₂ and the stretching vibration of C–H respectively.¹² The FT-IR spectra of both CuO and G/CuO nanocomposite show two similar absorbance bands located at 514 cm⁻¹ and 587 cm⁻¹, which are attributed to the Cu(II)–O vibration.⁷ However, the FTIR spectrum of G/CuO nanocomposite shows additional peaks at 1550 cm⁻¹, which corresponds to aromatic C=C stretching,¹³ and at 1244 cm⁻¹, which is due to the C–O vibration and which shows the existence of residual oxygen groups.¹⁴

Fig. 2 FT-IR spectra of pure GO (black), CuO (red) and G/CuO (blue).

The morphology and size of the as-synthesized materials were observed by using SEM and TEM. As shown in Fig. 3a–d the CuO nanostructures are composed of irregular nanosheets with aggregate morphology. The width of the nanosheets is about 250 nm, the length about 700 nm and the thickness about 30 nm. The TEM analysis (Fig. 3d) also shows that the nanosheets are almost transparent, which indicates that the thicknesses of the obtained CuO nanosheets are very thin. The HRTEM image (Fig. 3e) shows that the distance between adjacent planes is measured to be about 0.23 nm, which correspond to the lattice spacing of (111) plane of the monoclinic phase. The SEM image of GO (Fig. 4a) shows smooth sheets with folded shapes; however, the SEM image of graphene (Fig. 4b) depicts well-exfoliated graphene sheets which have crumpled thin voile-like structures with folds.

Fig. 3 SEM (a, b, c), TEM (d) and HRTEM (e) images of CuO.

Fig. 4 SEM image of (a) graphene oxide (GO), (b) graphene (G), (c) G/CuO nanocomposite, (f) Al powder, (g) Al/G/CuO composite, TEM image (d) of G/CuO nanocomposite, and SAED pattern of (e) G/CuO nanocomposite, and (h) Al/G/CuO composite.

The SEM (Fig. 4c) and the TEM (Fig. 4d) images of G/CuO nanocomposite show the apparition of CuO nanoplates on the surface of graphene nanosheets, some of them appear on the surface of the graphene nanosheets, while others have been encapsulated within the graphene nanosheets.

This structure effectively prevents the aggregation of CuO nanoparticles and makes their interaction with the graphene relatively strong. Fig. 4f shows that the morphology of Al powder contains micro-spherical grain not uniform in size and also in shape, the particle diameter ranged from 10 μm to 20 μm, however the SEM images of Al/G/CuO composite (Fig. 4g) shows that the G/CuO nanocomposite covers the whole surface of the micro aluminium powder. Fig. 4e and h show the selected area electron diffraction (SAED) pattern of G/CuO nanocomposite and Al/G/CuO composite respectively. Many concentric rings are observed in these two figures illustrating their polycrystalline structure.

Fig. 5 shows the EDS spectrums for the two composites and the percentages of their elements, for the G/CuO nanocomposite we can observe the presence of three elements which are copper (Cu), oxygen (O) and carbon (C), however; for the Al/G/CuO composite we can see the presence of a high percentage aluminum (Al) beside the previous three elements (Cu, O, C).

Fig. 5 EDS spectra of G/CuO nanocomposite and Al/G/CuO composite, inset tables show the percentages of their elements.

3.2 Effect of CuO, G/CuO and Al/G/CuO on the thermal decomposition of AP

The as-synthesized materials were investigated as additives to the thermal decomposition of AP. The catalytic effect of these additives was detected by TG/DTA analysis at a heating rate of 10 °C min⁻¹ in a static N₂ atmosphere over the range of 50–500 °C. The samples for TG/DTA analysis were prepared as follows: 0.03 g of each nanoadditive was ground with 1.6 g of AP in the presence of ethanol solution in an agate mortar and then dried at 60 °C. The SEM was also used to observe the morphology of the mixtures at room temperature.

The SEM image of pure ammonium perchlorate (Fig. 6a) shows that AP appears as connected clusters with a smooth surface. Fig. 6b shows the TG/DTA curves of pure AP. The DTA curve reveals that the thermal decomposition of AP takes place in three stages; same results are obtained in the literatures.^15–17 The first stage is the transition from orthorhombic to cubic form, this transition is characterized with an endothermic peak that appears at 250 °C and no weight loss is observed in this step. The second stage is the beginning of the thermal decomposition process it is characterized with an exothermic peak that appears at 320 °C, this step is also known as a low-temperature decomposition (LTD) step, which is solid–gas multiphase reaction, including decomposition and sublimation and it is characterized by the formation of intermediate products:

NH₄ClO₄ → NH₄⁺ + ClO₄⁻ → NH_3(g) + HClO_4(g) (ref. 18 and 19)

Fig. 6 (a) SEM image of pure ammonium perchlorate, (b) TG/DTA curves of pure ammonium perchlorate.

The third stage is characterized by another exothermic peak at 432 °C which indicates the complete decomposition of AP, it is also called high-temperature decomposition (HTD) step. And it is characterized by the interaction between the intermediate products and the formation of volatile products, such as; N₂O, O₂, Cl₂, H₂O and NO.^20–22

In the TG curve two weight loss steps are observed, the first weight loss is observed in the range from 320 °C to around 370 °C, and it is attributed to the LTD of AP. However, the second weight loss is observed in the range from 370 °C to 432 °C and it is attributed to the HTD of AP.

The SEM images of the mixtures of AP with additives (Fig. 7) show that all additives are uniformly distributed on the surface of ammonium perchlorate. The TG curves of these mixtures (Fig. 8) reveal that the mass loss begins at a temperature around the 320 °C for all the mixtures; however, it finishes at different temperatures, which are all lower than that of pure ammonium perchlorate (432 °C).

Fig. 7 SEM images of (a) AP + CuO, (b) AP + G, (c) AP + G/CuO, and (d) AP + Al/G/CuO.

Fig. 8 TG curves of AP, AP + CuO, AP + G, AP + G/CuO and AP + Al/G/CuO.

The DTA curves (Fig. 9) show that for all mixtures there is no change in the transition phase of ammonium perchlorate. The only change appears in the high-temperature decomposition step (HTD). In the presence of graphene, the HTD shifts from 432 °C to 400 °C; however, in the presence of CuO, G/CuO and Al/G/CuO. The LTD and HTD peaks of these mixtures have fused into one decomposition peak at 350 °C, 325 °C and 315 °C, respectively, that is the same as observed before in the TG curves (Fig. 8) where only one weight loss step is observed.

Fig. 9 DTA curves of AP, AP + CuO, AP + G, AP + G/CuO and AP + Al/G/CuO.

4. Kinetics study

The activation energy of the thermal decomposition of ammonium perchlorate with and without the additives is determined using a thermogravimetric analysis. Our study is based on the method of Coats–Redfern.²³ Which uses the following equation:
where T is the temperature, α is the extent of conversion, β is the heating rate, A is the pre-exponential factor, E is the activation energy and R is the universal gas constant.

The value of the activation energy is obtained from the slope of the straight line, obtained by graphic representation of

as shown in Fig. 10.

Fig. 10 Coats–Redfern plot of the mixture of AP with the additives, (a) AP, (b) AP + G, (c) AP + CuO, (d) AP + G/CuO, and (e) AP + Al/G/CuO.

The values of activation energy of the thermal decomposition of pure ammonium perchlorate and ammonium perchlorate with the additives are represented in Table 1. From this table we can observe that for all the mixtures each diminution on the values of the HTD corresponds to a diminution in the activation energy too. The activation energy of pure AP is found to be 129 kJ mol⁻¹; however, in the presence of the as prepared additives, the value of activation energy decreases significantly, and the lowest one of 56.18 kJ mol⁻¹ is attributed to the mixture of AP + Al/G/CuO.

Table 1 Activation energy (E_a), high temperature decomposition (HTD) and correlation coefficient (r) for the decomposition of AP and the mixtures of AP with the additives

Compositions	HTD (°C)	Ea (kJ mol^−1)	r
AP	432	129	0.94
AP + G	400	123.41	0.98
AP + CuO	350	85.12	0.96
AP + G/CuO	325	71.47	0.94
AP + Al/G/CuO	315	56.18	0.91

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The above results show also that all the prepared additives; G, CuO, G/CuO and Al/G/CuO ameliorate the thermal decomposition of ammonium perchlorate by reducing its high decomposition temperature from 432 °C to 400 °C, 350 °C, 325 °C and 315 °C, respectively. The mechanism of thermal decomposition of AP in the presence of these additives is interpreted by an electron transfer process.²⁴ According to this mechanism the decomposition occurs due to electron transfer from anion to cation.

The improvement of the thermal decomposition of ammonium perchlorate is related to the nature of these additives. For the graphene nanosheets the improvement of thermal decomposition of AP is due to its high surface area and high electron mobility.²⁵ However, CuO is a transition metal oxide that contains metal cations with partially filled d-orbitals; when AP is mixed with these nanoplates the positive hole of each cation can accepts electrons from AP ions, thus enhancing the electron transfer process and, as a result, enhances the thermal decomposition of AP. The G/CuO gives better results than that of pure CuO, due to the good distribution of the CuO nanoplates and the presence of more active sites. After adding the aluminum powder to the G/CuO nanocomposite the new energetic material obtained (Al/G/CuO) is found to have the best catalytic activity for thermal decomposition of AP where the HTD process not only begins early but also completes early. We interpret these results by the presence of the aluminium powder that increase the heat transfer and therefore increase the chemical reaction during the thermal decomposition of AP and also to the presence of a large numbers of active sites in the Al/G/CuO composite that absorbs the gases obtained from the first decomposition step of AP, as a result the second decomposition step occurs quickly. A simplified mechanism of the thermal decomposition of AP in presence of these composites is given in Fig. 11.

Fig. 11 Mechanism scheme of the thermal decomposition of AP in presence of G/CuO and Al/G/CuO.

5. Conclusions

Tenorite CuO nanoplates, G/CuO nanocomposites and Al/G/CuO composites show excellent catalytic activity for the thermal decomposition of ammonium perchlorate. The results of DTA indicate that, compared to other additives, Al/G/CuO composite has the greatest catalytic activity on the thermal decomposition of AP, by reducing the high temperature decomposition (HTD) from 432 °C to 315 °C and the activation energy from 129 kJ mol⁻¹ to 56.18 kJ mol⁻¹.

Acknowledgements

This work was supported by Heilongjiang Province Natural Science Funds for Distinguished Young Scholar, Special Innovation Talents of Harbin Science and Technology for Distinguished Young Scholar (2014RFYXJ005), Fundamental Research Funds of the Central University (HEUCFZ), (HEUCFD1404), Natural Science Foundation of Heilongjiang Province (B201316), Program of International S&T Cooperation special project (2013DFR50060), Special Innovation Talents of Harbin Science and Technology (2014RFQXJ087), and the fund for Transformation of Scientific and Technological Achievements of Harbin (2013DB4BG011).

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