# The Effect of Particle Size of Copper Oxide (CuO) on the Heat-Induced Catalysis Decomposition of Molecular Perovskite-Based Energetic Material DAP-4

Authors: Yuping Fu; Peng Deng; Xiaoxia Li
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

In this study, the effect of copper oxide (CuO) with different sizes on the heat-induced catalysis decomposition of typical molecular perovskite-based energetic material DAP-4 was studied. Pure CuO with different sizes and DAP-4 are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Heat-induced catalysis decomposition performances of DAP-4 with adding CuO were investigated by differential scanning calorimeter (DSC). Results demonstrated that with a lower-addition CuO at the nanoscale added (1 wt%), DAP-4 had decreased at least by 36°C with the heating rate of 10°C/min, compared with pure DAP-4. With the CuO particle size of 50 nm, theEa of DAP-4 heat-induced decomposition in DAP-4/CuO-50 had reduced from 159.8 kJ/mol to 120.2 kJ/mol, which is lower than DAP-4/CuO-150. The heat-induced catalysis decomposition mechanism of DAP-4 by using CuO as a nanoadditive was proposed.


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## Body

## 1. Introduction

Nanoscale metal oxides, such as Fe2O3, NiO2, Co3O4, MnO2, FeCo2O4, and CuO, have received much attention due to their prominent catalysis properties in energy, biology, environment, catalysis, adsorption, and so on [1–5]. Metal oxides at the nanoscale showed unique light, electric, chemical, and physical properties as well as had many potential applications [6–9]. For instance, Wang et al. reported that ultrathin MnO2 nanosheets had been prepared by the simplistic hydrolysis processes and showed high efficient vacancy-induced photothermal therapy [10]. Santo et al. used a facile one-step pyrolysis method to synthesis nanocopper/cobalt oxide particles, which were grafted on the surfaces of graphitic carbon nitride (g-C3N4) nanosheets. They suggested a good electrochemical properties in super capacitors [11]. Liu et al. showed the nano-Fe2O3 particles were obtained via the hydrothermal method and had the good degradation effect of explosive wastewater under the condition of assisted UV light [12]. These studies revealed that nanoscale metal oxides have good potential applications.DAP-4, as an important and typical class of molecular perovskite-based energetic materials with high energy, strong thermal stability, has a great potential to replace the traditional energy materials [13–16]. At the molecular level, DAP-4 integrated with fuel dabco and oxidizer ammonium perchlorate to show high-energy performances. But huge heat-induced decomposition threshold of molecular perovskite-based energetic materials is always not beneficial for the ignition and combustion [17, 18]. But high heat-induced decomposition temperature of molecular perovskite-based energetic materials usually restricted their novel and possible applications. Ammonium perchlorate is the traditional oxidant in the solid propellant. Lower energy, however, limited possible application in the high-energy solid propellant. But DAP-4 had not only high-energy performances but also strong oxidizing capacity to show a great potential [19–23]. Therefore, to use the nanoscale metal oxide catalysts to reduce thermal decomposition temperature is still of great significance to extend DAP-4 applications.Considering utilization of nanoscale metal oxides in the propellant [9, 24–27], nanoscale metal oxides also can be considered important additives to enhance the reaction activity of composite system. In this work, the effect of CuO with different particle sizes (50 and 150 nm) on the heat-induced catalysis decomposition of molecular perovskite-based energetic materials was studied. The morphology and structure of nanoscale CuO with different sizes and heat-induced catalysis decomposition performance of DAP-4 by using CuO with different sizes as catalysts were studied. The heat-induced catalysis decomposition mechanism of DAP-4 with CuO at the nanoscale added was provided. This work offered a novel idea for further application of molecular perovskite-based energetic materials with nanometal-based additives in the high-energy solid propellant.


## 2. Experimental Sections

All the pure materials were purchased commercially. CuO particles with the sizes of 50 and 150 nm were purchased by Brofos Nanotechnology (Ningbo) Co. Ltd. The samples of CuO were correspondingly named by CuO-50 and CuO-150, respectively. Ammonium perchlorate (NH4ClO4) and perchloric acid (HClO4) were purchased by Shanxi Jiangyang Chem. Co. Ltd. Triethylenediamine (dabco) was purchased by Shanghai Aladdin Chem. Co. Ltd.The molecular perovskite energetic material DAP-4 was prepared by the molecular assembly strategy in our laboratory. In a typical experiment, molecular perovskite-based energetic material DAP-4 particles were prepared by a facile molecule assembly method in our laboratory [18]. The 1 mmol NH4ClO4, 2 mmol HClO4, and 1 mmol dabco were dissolved completely in 50 ml beaker with 20 ml pure water. The molecular perovskite-based energetic material DAP-4 samples were obtained by crystallization after 7 days.The composite DAP-4/CuO particles with different sizes (50 and 150 nm) at the nanoscale were fabricated by mechanical mixing. 0.1 g composite involved pure DAP-4, and CuO particles with different addition contents were mixed by mechanical stirring and prepared the composites. The composites were named by DAP-4/X wt%CuO-50 and DAP-4/X wt%CuO-150 (X=1,3,and5, respectively).X-ray diffraction (XRD) data of pure DAP-4 and CuO at the nanoscale were collected on a Bruker D8 advances X-ray diffractometer with the Cu Kα and λ=1.5406Å. Scanning electron microscopy (SEM) images of pure DAP-4 and CuO were recorded in a S4800 microscope (Hitachi, Japan) with the electric voltage of 15KV and electric current of 10 mA. The heat-induced decomposition performances of pure DAP-4 and the composite DAP-4/CuO were tested by differential scanning calorimeter (DSC131 EVO, Setaram, France). Pure DAP-4 and the composite DAP-4/CuO with the weight of 1.0±0.05mg were used. Five, 10, 15, and 20°C/min were selected as the different heating rates. The samples were tested in an Ar with a flow rate of 30 ml/min. The DSC data was recorded at the range from 50 to 500°C.


## 3. Results and Discussion

The morphology of CuO particles with different sizes at the nanoscale was characterized firstly. CuO particles with different sizes are shown in Figure1 by SEM with different magnifications. CuO particles with different sizes showed an apparent agglomeration, in particular CuO-50 in Figures 1(a) and 1(b). The sample CuO-50 was composed of many nanoparticles, which had the average size of 50 nm in Figure 1(b). But CuO-150 has a cluster structure with nanoparticles. The particles have the size ranges from 60 nm to 150 nm, as shown in Figure 1(d). Irregular CuO-50 and CuO-150 clusters showed the agglomeration is easy to be occurred by CuO at the nanoscale, because of high surface energy of nanoparticles in its preparation progress from the nanosized effect.Figure 1
SEM images of CuO with different sizes: (a, b) CuO-50 and (c, d) CuO-150.
(a)(b)(c)(d)The structure of CuO particles at the nanoscale was confirmed by XRD tests in Figure2. The XRD results of the different-size samples CuO-50 and CuO-150 showed the same diffraction peak positions. The diffraction peaks of CuO are located at 32.5°, 35.6°, 38.6°, 48.7°, 53.4°, 58.3°, 61.6°, 66.3°, 68.2°, 75.2°, and 83.0°, respectively, which are reflected from crystal planes (110), (002), (200), (-202), (020), (202), (113), (-311), (220), (-222), and (222) of CuO nanoparticles. Compared to CuO-150, the weaker intensity of diffraction peaks of CuO indicated its lower crystallinity caused by the nanosize. Lower nanosize of Cu0-50 particles leads to the obvious widening of diffraction peaks, as shown in Figure 2. According to the experiential Debye-Scherrer formula depending on the two main strong peaks from crystal planes (110) and (002), the crystal size of CuO-50 was calculated as 30~50 nm, and the size of CuO-150 was calculated as 122-150 nm by half peak width of the main strong peaks. The calculated results confirmed the size change and its intrinsic relativity of CuO. This is consistent with SEM results.Figure 2
XRD patterns of CuO with different sizes.As a typical molecular perovskite-based energetic material, DAP-4 was prepared by a facile and efficient molecule assembly method. The appearance and structure of sample are shown in Figure3. The microscale morphology of pure DAP-4 samples is shown in Figure 3(a). DAP-4 micro particles have a cubic shape, which originated from the crystallization and growth process of the molecular cell with ABX3 perovskite structure. The SEM image of DAP-4 in Figure 3(a) shows the particle sizes ranged from ~5 μm to ~20 μm. And the XRD result of the molecular perovskite-based energetic material DAP-4 is shown in Figure 3(b). The main XRD peaks at 12.3°, 21.1°, 24.5°, 27.5°, 36.6°, and 27.1°, respectively, correspond to (200), (222), (400), (420), (531), and (600), of the crystal planes of DAP-4, respectively.Figure 3
(a) SEM images and (b) XRD patterns of DAP-4.
(a)(b)Compared to the simulated crystal structure data (CCDC:1528108) [13], XRD data of molecular perovskite-based energetic material DAP-4 is the same as the simulated data, revealing that molecular perovskite-based energetic material DAP-4 was successfully prepared by the facile and efficient molecule assembly method. In the molecular perovskite-based energetic material DAP-4 cell with ABX3 perovskite structure, H2dabco2+ cations are considered A sites, NH4+ cations are B sites, and ClO4- anions are X bridges. All the three components played the important roles in the cell and have been tightly stacked to form the cubic periodic structure.Furthermore, heat-induced decomposition performances of the molecular perovskite-based energetic material DAP-4 and the composite DAP-4/CuO (DAP-4/CuO-50 and DAP-4/CuO-150) with different CuO contents were studied. One, 3, and 5 wt% CuO were added into DAP-4 to obtain the composites DAP-4/CuO. The thermal catalysis decomposition properties with the 10°C/min are shown in Figure4. For pure DAP-4, two thermal decomposition peaks occurred at 275.4°C and 380.7°C. The weak endothermic peak at 264.3°C is caused by the heat-induced transformation. The heat-induced transformation of molecular perovskite-based energetic material DAP-4 occurred from the low-temperature crystalline phase to the high-temperature crystalline phase [28, 29]. And the violent exothermic peak was shown at 380.7°C, indicating huge and violent decomposition reaction between cations and anions.Figure 4
(a–c) DSC curves of pure DAP-4 and DAP-4/CuO particles with different sizes and (d) their decomposition heat release.
(a)(b)(c)(d)The strong heat-induced decomposition reaction occurred at the exothermic peak (380.7°C). Figure4(d) shows that the heat-induced decomposition heat release value of pure DAP-4 is 3900.4 J/g. With CuO at the nanoscale added, the heat-induced decomposition peak temperatures of molecular perovskite-based energetic material DAP-4 had obviously decreased, as shown in Figures 4(b) and 4(c). With the increasing contents of CuO at the nanoscale, a decreasing trend of thermal decomposition temperature was found. Different CuO-50 and CuO-150 contents were used, and thermal decomposition temperature had been reduced at least by 36°C. With 1 wt% CuO-150 used, the heat-induced decomposition temperatures of DAP-4 have reduced to 344.6°C in Figure 4(c), and the contents of CuO-150 were increased up to 5 wt%; the heat-induced decomposition temperatures of DAP-4 had reduced to 316.4°C. When the particle sizes had been reduced further, the CuO-50 was used to catalyze the heat-induced decomposition of DAP-4 in Figure 4(b). With 1 wt% CuO-50 used, the heat-induced decomposition temperatures of DAP-4 have reduced to 338.8°C and even had decreased to 325.6°C with 5 wt% CuO-50 added. The heat-induced decomposition heat release value of the composite DAP-4/1wt%CuO-50 is up to 4567.1 J/g that is more 666.7 J/g than pure DAP-4. And with 1 wt% CuO-150 added, the heat release value of DAP-4/1wt%CuO-150 has increased slightly. Generally speaking, the heat-induced decomposition progresses of DAP-4 and DAP-4/CuO occur quickly in Figure 4. These above results indicated that CuO has inherent catalysis properties. CuO at the nanoscale catalyzed the heat-induced decomposition of DAP-4, resulting in reducing the decomposition temperature and increasing the heat release. With the contents of CuO increased, the heat-induced decomposition temperature of DAP-4 had reduced further. When the particles sizes of CuO at the nanoscale had been reduced, the heat release value had been increased.Compared with our previous literatures [19, 21, 30], two-dimensional nanomaterials, such as graphene and MoS2, also were considered catalysts. With the 10 wt% graphene, the heat-induced decomposition temperature of DAP-4/graphene composite was located at 372°C. And with 1 and 3 wt% MoS2, the heat-induced decomposition temperature of DAP-4 had reduced to 343.3 and 328.8°C, respectively. In comparison, CuO at the nanoscale has better catalysis properties for heat-induced decomposition of DAP-4.According to the corresponding relationships between the main heat-induced decomposition peak temperatures and the heating rates, main key thermodynamic data of the heat-induced decomposition progresses of pure DAP-4, DAP-4/1wt%CuO-50, and DAP-4/1wt%CuO-150 can be calculated. DSC curves of pure DAP-4, DAP-4/1wt%CuO-50, and DAP-4/1wt%CuO-150 at the different heating rates are shown in Figure5. By using the Kissinger model (Equation (1)), the value of apparent activation energy (Ea) was calculated as follows:
(1)InβTP2=InAREa−EaRTp,where β is the heating rate in degrees Celsius per minute, Tp is the exothermic peak temperature, A is the pre-exponential factor, R is the gas constant, and Ea is the apparent activation energy (J/mol).Figure 5
DSC curves of (a) pure DAP-4, (b, c) DAP-4/1wt%CuO-50 and DAP-4/1wt%CuO-150 at different heating rates, and (d) fitting lines displayed the linear fitting result of the tested data with the dependence of ln (β/Tp2) on 1/Tp for above samples.
(a)(b)(c)(d)As shown in Figure5, the value of Ea of pure DAP-4 was calculated to be 159.8 kJ/mol based on the heat-induced decomposition peak temperatures and the heating rates. The higher Ea value revealed pure DAP-4 showing the good thermal stability due to the strong perovskite structure. A lower deviation of DAP-4 resulted from only four heat-induced decomposition peak temperatures with four heating rates. With 1 wt% CuO-50 added, the Ea of heat-induced catalysis decomposition of DAP-4/1wt%CuO-50 was decreased to 120.2 kJ/mol by 39.6 kJ/mol, although the Ea of heat-induced catalysis decomposition of DAP-4/1wt%CuO-150 had been increased slightly. This revealed that CuO at the nanoscale has inherent catalysis properties and can be used as a potential catalyst for the heat-induced decomposition of DAP-4 with a lower addition of 1 wt%. CuO-50 showed a better heat-induced catalysis decomposition performances than CuO-150. As a result, CuO is significantly beneficial for reducing the heat-induced catalysis decomposition temperature, increasing the heat release value, and reducing the Ea of heat-induced decomposition process of DAP-4 in DAP-4/CuO.What is more, the heat-induced catalysis decomposition mechanism of typical molecular perovskite-based energetic material DAP-4 with CuO as a nanoadditive was proposed. As shown in Figure6, CuO at the nanoscale with a lower addition was introduced into the composites. CuO at the nanoscale still showed a good catalysis performance in the propellant due to the strong heat and mass transfer capabilities of nano-CuO nanoparticles or clusters. For pure DAP-4, the high thermal stability is from the cage-like framework of perovskite structure [17, 31–33]. Therefore, more energy needs to be provided to destroy the cage-like framework structure at a higher heat-induced decomposition temperature. Proton transfer of heat-induced decomposition of DAP-4 has occurred from protonated H2dabco2+ cations and NH4+ cations to ClO4- anions continually. The violent reaction between with anions and cations occurred at a shorter time. With CuO at the nanoscale added in Figure 6, nano-CuO in the composites contacted with the surfaces of DAP-4 particles can enhance proton transfer to DAP-4. Proton transfer from H atoms of H2dabco2+ and NH4+ to O atoms of ClO4− occurred fast during the heat-induced decomposition progress. The lower Ea also could promote the quick generation of HClO4. The HClO4 at the gaseous state can further form superoxide radical anion -O2· [34, 35]. CuO at the nanoscale with the inherent heat-induced catalysis properties is beneficial for enhancing the heat-induced decomposition of DAP-4. The composites DAP-4/CuO with a lower addition showed a lower decomposition temperature, lower apparent activation energy, and higher heat release value, indicating higher potential.Figure 6
The diagram scheme of heat-induced catalysis mechanism of DAP-4 with nano-CuO as a catalyst.


## 4. Conclusions

In summary, we studied the effect of CuO with different sizes on the heat-induced catalysis decomposition of DAP-4. We found that CuO particles at the nanoscale have the inherent catalysis characteristics on heat-induced catalysis performances of DAP-4. With a lower addition (1 wt%) of CuO added, the heat-induced decomposition temperature peaks of DAP-4 had reduced at least by 36°C at the heating rate of 10°C/min, compared from DAP-4. TheEa of thermal decomposition of DAP-4 had reduced from 159.8 kJ/mol to 120.2 kJ/mol, with the presence of lower-addition CuO nanoparticles with the size of 50 nm. The catalysis mechanism of nano-CuO for heat-induced decomposition of DAP-4 was studied. Nano-CuO is beneficial for accelerating the heat and mass transfer and enhanced the energy release of the system. This paper offered a novel reference for design and application of molecular perovskite-based energetic materials in high-energy solid propellant.



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