One-Step Synthesis AlCo₂O₄ and Derived “Al” to Double Optimise the Thermal Decomposition Kinetics and Enthalpy of Ammonium Perchlorate

Abstract

The solution combustion method is widely used because of its simple operation and ability to produce porous structures. The chemical composition and morphological structure of the material can be regulated by different oxidiser-to-fuel ratios (φ). In this work, AlCo₂O₄ derived “Al” catalytic materials were successfully synthesised by adjusting the fuel-to-oxidiser ratio using a one-step solution combustion method. On the one hand, the aluminium nanoparticles act as a part of the metal fuel in the composite solid propellant and, at the same time, serve as a catalytic material. In contrast, the thermal decomposition performance of AP was significantly improved by the synergistic catalysis of AlCo₂O₄. Among the samples prepared under different fuel ratios, considering all aspects (high-temperature decomposition temperature, activation energy, and decomposition heat) comprehensively, the AlCo₂O₄ prepared with φ = 0.5 had a more excellent catalytic effect on AP thermal decomposition, and the T_HTD of AP was reduced to 285.4 °C, which is 188.08 °C lower. The activation energy of the thermal decomposition of AP was also significantly reduced (from 296.14 kJ/mol to 211.67 kJ/mol). In addition, the ignition delay time of AlCo₂O₄-AP/HTPB was drastically shortened to 9 ms from 28 ms after the addition of 7% AlCo₂O₄ derived “Al” catalytic materials. Composite solid propellants have shown great potential for application.

1. Introduction

With the development of science, technology, national defence, and aerospace, research on solid propellants is becoming increasingly important [1,2]. Composite solid propellants (CSPs), which are used as the power source of carrier rockets and weapon missiles, have the advantages of high energy density, good safety performance, and excellent mechanical properties. [3,4] The most widely used of these is the terminal hydroxyl polybutadiene (HTPB)-based composite solid propellant, which is mainly composed of oxidisers, binders, and metals.

Metals and their compound powders are widely used in solid propellants due to their high combustion calorific value [5,6,7]. Currently, the metal fuels commonly used in solid propellants include Be, B, Mg, Al, and Zr [8]. Among them, aluminium (Al) has become a typical metal fuel by virtue of its large reserves, low cost, non-toxicity, high calorific value, and high density [9,10]. The Al content of the fuel generally accounts for 14~20% of the total mass of the propellant [11]. However, aluminium is easily oxidised to form a dense Al₂O₃ film on its surface, which is the main factor affecting combustion efficiency [12]. Problems such as agglomeration, insufficient combustion, and the formation of condensed phase product during combustion also affect the combustion performance of composite solid propellants [13,14]. Agglomeration can be reduced by adding other metal elements or by uniformly dispersing the Al powder. In order to further improve the effect of various components in composite solid propellants, it is crucial to increase the interfacial contact between the propellant components and reduce the coalescence of the metal particles. In existing research, it is common to form a core-shell structure through morphological modulation, which makes the metal fuel, oxidant, and catalyst form a whole and improves the catalytic effect. For example, Zhou et al. prepared Al@AP composites and achieved combustion rate control and reduction of aluminium agglomeration phenomenon under the catalytic effect of CuO [15]. Wang et al. formed Al/FeF₃/AP capsule microspheres, which ensured full contact between the components, and the micro-explosive combustion was intense, which significantly improved the combustion performance of the end-hydroxylated polybutadiene (HTPB)-based propellant, and the combustion rate reached 2.975 mm/s [9]. The micro-explosive effect produced by the ignition of the aluminium powder itself also shortens the ignition delay and improves the combustion efficiency to a certain extent. At the same time, the small size of fuel particles increases the interfacial area and effective mass diffusion rate.

The content of oxidising agent components can reach 60–90% of the solid propellants. Therefore, the combustion performance of the oxidant can be used to initially evaluate the combustion performance of the composite solid propellant [16]. Ammonium perchlorate (AP), the most commonly used oxidant, has the advantages of high enthalpy of combustion, low price, and good compatibility with other components [17,18,19,20]. Under the premise of experimental safety, scholars have generally improved the thermal decomposition of AP by adding catalysts [21,22,23].

Numerous studies have shown that binary transition metal oxides (TTMOs) exhibit better structural stability and catalytic properties due to their unique spinel structure and the synergistic effect of two metal elements [24,25]. Among them, cobaltate (MCo₂O₄) is easier to accept electrons and accelerate electron transfer through many incompletely filled d orbitals; on the other hand, the spinel structure makes more defects in the crystals, which generates more active sites [26,27,28]. Gao et al., through a facile free-template hydrothermal method, successfully prepared the MgCo₂O₄ nanosheets, and the decomposition temperature of AP was 155.9 °C from 431 to 275.1 °C with the addition of 5 wt% MgCo₂O₄ [29]. Wang et al. successfully prepared a NiCo₂O₄ catalytic material via a facile hydrothermal-annealing method and demonstrated its good catalytic performance for AP thermal decomposition [30]. However, the application of AlCo₂O₄ to AP thermal decomposition catalytic materials has rarely been reported.

Among the methods currently used to prepare catalysts, solution combustion synthesis is widely used because of its advantages of simple operation, energy saving, rapidity, and low cost, and the products prepared by the solution combustion method have more pore structures, which provide more active sites for the subsequent reaction [31,32,33,34]. By adjusting the oxidant-to-fuel ratio, the phase composition, crystallinity, morphology, specific surface area, and pore volume of the product can be controlled. H. Vahdat Vasei et al. obtained different morphologies of zinc oxide (ZnO) powders via the solution combustion route using polyvinyl-pyrrolidone as fuel at various fuel to oxidant ratios [35]. Xiao et al. adjusted the oxidant-fuel ratios to successfully synthesise pure-phase binary metal oxide MCo₂O₄ using the solution combustion method [36].

In this work, we propose the synthesis of AlCo₂O₄ nanomaterials using a one-step solution combustion method and the obtainment of AlCo₂O₄ and derived “Al” materials by adjusting the oxidant-to-fuel ratios (φ). This approach is designed to tackle both the AP decomposition property and the contribution of metallic fuel enthalpy. The bimetallic synergistic catalysis of AlCo₂O₄, multiple active sites, and other advantages were utilised to significantly catalyse the thermal decomposition of AP. At the same time, the in situ generated Al nanoparticles not only act as a part of the metal fuel in the composite solid propellant but also serve as a catalytic material to catalyse the thermal decomposition of AP. The effect of double optimisation of the thermal decomposition of AP was achieved through the preparation of AlCo₂O₄ and the derived Al catalytic materials. The AlCo₂O₄ and derived “Al” materials with multiple mesopores and a large specific surface area, synthesised using a one-step solution combustion method, significantly reduce the decomposition temperature of pure AP, meanwhile significantly increasing the decomposition heat of AP thermal decomposition, which creates conditions for continuous combustion of composite solid propellant.

2. Experimental Section

2.1. Materials

All chemicals were obtained from commercial sources and used without further purification. Citric acid (C₆H₈O₇, AR), cobalt nitrate (Co(NO₃)₂·6H₂O, AR), and aluminium nitrate (Al(NO₃)₃·9H₂O, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd., (Shanghai, China).

2.2. Preparation of AlCo₂O₄

The samples were prepared using Al(NO₃)₃·9H₂O and Co(NO₃)₂·6H₂O as oxidising agents and the organic fuel citric acid as a reductant by one-step solution combustion method, as shown (Figure 1a). The stoichiometric ratio of the fuel-to-oxidiser(φ) is calculated using propellant chemistry theory, and the corresponding reaction equation for solution combustion is derived (Equation (1)).

Al(NO₃)₃ + 2Co(NO₃)₂ + 2φC₆H₈O₇ + (18φ−17)O₂ = AlCo₂O₄ + 7/2N₂ + 12φCO₂ + 8φH₂O

(1)

As can be seen from the above equation, when φ = 17/18, the stoichiometry is achieved without additional oxygen. The oxidants needed for combustion are all from metal nitrates, which do not require the participation of external O₂, and the combustion reaction is the most thorough and intense. When φ < 17/18 and φ > 17/18, the combustion agent is poor and rich, respectively. φ = 17/18, φ = 17/18 × 1/2 and φ = 17/18 × 2 AlCo₂O₄-0.5 was prepared as follows (Figure 1a). The specific steps are as follows: Firstly, 1 mmol of Al(NO₃)₃·9H₂O, 2 mmol of Co(NO₃)₂·6H₂O, and 1.889 mmol of C₆H₈O₇ were homogeneously dispersed in 10–15 mL of DI water to form a mixture, which was transferred to a 200 mL crucible, and magnetically stirred for half an hour. The crucible containing the precursor liquid samples was placed into a muffle furnace with a good lid and heated at a rate of 5 °C/min with the furnace up to 400 °C for 1 h and cooled with the furnace to obtain the AlCo₂O₄-1 sample. AlCo₂O₄-0.5 and AlCo₂O₄-2 were prepared using the same preparation method with the addition of 0.945 and 3.778 mmol of C₆H₈O₇.

2.3. Materials Characterisation

The physical structure of the materials was analysed using X-ray powder diffraction (XRD) analysis (DX-2700BH purchased from Rigaku, Tokyo, Japan), the instrument was equipped with a radiation source of Cu K_α1 (λ = 0.15405 nm), and the information of the samples within the test angle 10–90° was tested at a scanning speed of 5 °/min. The morphologies of the samples were analysed using field-emission scanning electron microscopy (FESEM, Nova Nanosem 450 purchased from FEI, Hillsboro, OR, USA) and transmission electron microscopy (TEM, JEM-2100 purchased from Nippon Electric Company, Tokyo, Japan). The elemental compositions of the samples were analysed using X-ray energy spectrometry (EDS). The elemental valences of the samples were analysed using X-ray photoelectron spectroscopy (XPS, ESCALAB Xi⁺ purchased from ThermoFische, Waltham, MA, USA). The particle size distribution and specific surface area of the AlCo₂O₄-X samples were measured at 77 K using a NOVE2200E purchased from Kontes Instruments, US after vacuum degassing at 280 °C for 3 h.

2.4. Catalytic Test

Thermogravimetric/Differential Scanning Calorimetry (TG/DSC, HCT-3 thermal analyser purchased from Hengjiu Scientific Instrument Factory, Beijing, China) was used to evaluate the catalytic performance of AP for thermal decomposition. The AP and AlCo₂O₄ catalytic materials with a mass ratio of 98:2 were added to an agate mortar and physically ground sufficiently in a certain amount of ethanol environment to mix homogeneously. The above mixture (~10 mg) was placed in an alumina crucible under an N₂ atmosphere (50 mL/min) from 25 °C to 500 °C at a specific heating rate for TG-DSC analysis. The kinetic parameters of AP thermal decomposition were evaluated by varying the heating rate (10, 20, 30, and 40 °C/min), and the effect of different contents of catalysts on the catalytic effect was investigated by varying the mass ratios of the AlCo₂O₄ catalytic materials (98:2, 95:5, 93:7, and 90:10).

2.5. Combustion Measurements of AP/HTPB-Based CSPs

The AP/HTPB-based composite solid propellant (AlCo₂O₄-AP/HTPB) was prepared by vacuum casting according to a certain raw material ratio (AP:HTPB:Al:AlCo₂O₄ = 67:11:15:7) and cut into 10 mm × 10 mm × 3 mm flakes as the combustion ignition specimens. A high-speed video camera (Photron FASTCAM purchased from Xi’an Modern Chemistry Research Institute, Xi’an, China) was used to monitor the combustion performance of AlCo₂O₄-AP/HTPB in real time at an indoor temperature with a sampling rate of 2000 fps. During the test, a CO₂ continuous laser (SLC110) was used to ignite AlCo₂O₄-AP/HTPB with a laser beam spot diameter of 5 mm and an ignition voltage of 34 V. The laser was maintained during the ignition process until an ignition reaction occurred. It is worth noting that the ignition delay time is the time gap between energisation and specimen combustion, and the combustion time is the time difference between the start and the end of specimen combustion.

3. Results and Discussion

3.1. Characterisation of AlCo₂O₄

The X-ray diffraction patterns (XRD) of the AlCo₂O₄ samples prepared using different raw material fuel ratios are shown in Figure 1b. The results were compared with the JCPDS card NO:09-0418, which shows that all the diffraction patterns of the samples were well matched with the Co₃O₄ structures. The XRD diffraction peaks were slightly shifted to the right because Al atoms with smaller atomic radii replaced some Co atoms with larger atomic radii [37]. The characteristic peaks are sharp, the half-peak width is small, and the appearance of the rest of the impurity peaks is not observed, which indicates that the AlCo₂O₄ nanomaterials were successfully prepared with good crystallinity [38]. The grain size of the samples can be calculated using the Scherrer Equation (1) [39,40,41].

$${\beta }_L={\displaystyle \frac{\mathrm{K}\lambda }{L \cos \theta }}$$

(2)

where ${\beta }_L$ is the full width at half maximum (FWHM) in radians, K = 0.94, $\lambda$ is the X-ray wavelength, L is the average size of the grain, and θ is the Bragg angle in degrees. The average grain size of the AlCo₂O₄-0.5 nanomaterial was calculated to be 7.345 nm. The average grain sizes of AlCo₂O₄-1 and AlCo₂O₄-2 samples are 6.596 and 6.800 nm, respectively, which verifies that the AlCo₂O₄-0.5 samples have sharp characteristic peaks and better crystallinity. Higher crystallinity leads to the rapid onset of selective catalytic reactions, providing catalytic materials with superior catalytic effects [42].

On this basis, this work further investigates the morphology and structure of AlCo₂O₄-0.5 material using SEM and TEM, as shown in Figure 2. The SEM characterisation of AlCo₂O₄-0.5 (Figure 2a) presents a multistage porous structure. A number of nanoparticles agglomerate to form the pore wall (Figure 2b). Through high-resolution SEM (Figure 2c), it can be observed that the pore walls formed by the accumulation of nanoparticles are different in thickness, thus showing a multistage pore structure with different pore sizes. It is conjectured that the multistage porous structure is formed due to the large amount of gas produced when the sample is prepared by the solution combustion method, and the gas rushes away from the newly formed material and escapes in a high-temperature environment. These properties provide a larger surface area for the reaction and expose more active sites.

The morphological structure and elemental composition of the samples were further observed using TEM, as shown in Figure 2d–f. The shapes presented in Figure 2d show pore structures of various sizes, which confirm the SEM results of the morphological analysis. The darkest area is concentrated at the edge region, which also confirms the occurrence of particle agglomeration on the surface. A small amount of derived “Al” nanoparticles was distributed on the thin nanosheets. Figure 2e shows the HRTEM pattern of the sample, where a crystal plane spacing of 0.4768 nm was measured and calculated in this region, corresponding to the (111) crystal plane of AlCo₂O₄-0.5. The region with a crystallographic spacing of 0.305 nm corresponds to the (311) crystal plane of Al, which is in agreement with the results of the XRD analysis. The inset of Figure 2e shows the selected area electron diffraction (SAED) pattern of the sample, and the diffraction rings correspond to the (220), (222), (400), (511), and (440) crystal planes, sequentially from inside to outside. The distribution of elements in AlCo₂O₄-0.5 was determined by energy spectroscopy (EDS) area scanning (Figure 2f). The figure shows that the sample contains three elements, Co, O, and Al, and the various elements are uniformly distributed in the sample, which is consistent with the XRD and XPS analyses. These results prove the successful preparation of the AlCo₂O₄-0.5 material.

N₂ adsorption/desorption isotherms were used to further analyse the specific surface area and pore size of the AlCo₂O₄-X material (Figure 3), and the inset shows the DFT pore size distribution curve of the material. All materials had slit mesoporous structured, according to the N₂ adsorption/desorption curves [43,44]. The specific surface areas of AlCo₂O₄-X (X = 0.5, 1, 2) were calculated using the BET (Brunauer−Emmett−Teller) method to be 178.825, 182.035, and 201.730 m²·g⁻¹, respectively. AlCo₂O₄-0.5, prepared using an oxidiser-to-fuel ratio (φ = 0.5), had a smaller specific surface area. The pore size distribution and pore volume of the AlCo₂O₄-X materials were calculated using the density flood theory (DFT) method. The calculations show that the pore size distributions of the AlCo₂O₄ materials are dominated by mesopores. The DFT pore sizes of AlCo₂O₄-0.5, AlCo₂O₄-1, and AlCo₂O₄-2 are 34.024, 33.158, and 32.152 nm, respectively. The DFT pore volumes were 2.64 cm³/g, 1.90 cm³/g, and 1.52 cm³/g, respectively (

Table 1. The structural parameters and BET specific surface areas of the samples.

Catalyst	BJH Pore Size (nm)	BJH Pore Volume (cm3/g)	BET (m2/g)
AlCo2O4-0.5	34.024	0.264	178.825
AlCo2O4-1	33.158	0.190	182.035
AlCo2O4-2	32.152	0.152	201.730

). The specific surface area of AlCo₂O₄-0.5 was the smallest, and the pore diameter was the largest compared with the other two samples; however, the difference between the three samples was negligible, indicating that the specific surface area of the AlCo₂O₄ and derived “Al” material was not the most important factor affecting its catalytic AP performance.

3.2. Catalytic Properties of AlCo₂O₄ Catalysts for AP Decomposition

The thermal decomposition of pure AP consists of three phases [45]. The first phase was the crystalline transformation process at 246.35 °C. The two exothermic peaks at 343.62 °C and 473.48 °C correspond to the low-temperature decomposition (LTD) and high-temperature decomposition (HTD) exothermic processes of AP, respectively. Compared with pure AP, when 2 wt% AlCo₂O₄-X materials were uniformly mixed with AP (Figure 4a), the crystal transition temperature did not change significantly, indicating that the addition of the catalytic material did not change the crystal transition process of AP. Both the HTD and LTD peaks decreased substantially, and even the phenomenon in the low-temperature decomposition peak and the high-temperature decomposition peak was combined into one peak (T_HTD). This indicates that the addition of AlCo₂O₄-X catalytic materials significantly improved the thermal decomposition of AP. The high-temperature decomposition temperature (T_HTD) of AP with the addition of 2 wt% AlCo₂O₄-0.5 was reduced to 285.4 °C, which was a greater decrease than that of AlCo₂O₄-1 and AlCo₂O₄-2 to 293.8 °C and 304.3 °C, respectively.

The heat of decomposition (ΔH), which is one of the main factors for evaluating the combustion performance of composite solid propellants [46], is shown in Figure 4b for AP after the addition of 2 wt% of AlCo₂O₄-X catalytic materials. The ΔH values were 1976.3 J/g, 2324.4 J/g and 2443.4 J/g after the addition of AlCo₂O₄-0.5, AlCo₂O₄-1 and AlCo₂O₄-2, respectively. Compared with the heat of decomposition of pure AP of 888.26 J/g, the heat of decomposition of AP was significantly increased by 1436.14 J/g, 1088.04 J/g, and 1555.14 J/g. The relatively small increase in AlCo₂O₄-0.5 may be caused by the fact that a small portion of the heat released during the decomposition of AP is used for the combustion of Al nanoparticles stacked on the catalytic material. These findings are beneficial for improving the combustion performance of composite solid propellants. As shown in

Table 2. Comparisons of catalytic activities for ammonium perchlorate of other catalysts.

Catalyst	Mass %	Reduction in HTD Temperature (°C)	Increase in Decomposition Heat (J⋅g−1)	Decrease of Ea (kJ⋅mol−1)	Refs.
MgO	2.0	114.9	511.6	—	[47]
LCOP-3	2.0	54.6	—	45.0	[48]
Al/FeF3	30.0	41.2	—	28.93	[9]
Al@AP	—	32.4	381.2	48.0	[12]
Al@AP/CuO	—	87.3	159.2	55.8
Zn-Co	10.0	135.4	—	82.28	[49]
Cu/C	5.0	60	—	82.12	[50]
AlCo2O4-0.5	2.0	188.08	1088.04	84.47	This work
AlCo2O4-1	2.0	179.68	1436.14	—	This work
AlCo2O4-2	2.0	169.18	1555.14	—	This work

, the AlCo₂O₄-0.5 catalytic material has a greater tendency to lower the high-temperature decomposition temperature (T_HTD) and increase heat of decomposition (ΔH), and is superior to those reported in the existing literature [9,12,47,48,49,50]. In order to better apply the catalytic material in composite solid propellants, the optimal amount of catalytic material was investigated in this work. The effect of the dosage of the catalytic material was investigated by varying the proportion of AlCo₂O₄-0.5 in the AP+AlCo₂O₄-0.5 system (2, 5, 7, and 10 wt%) using the high-temperature decomposition temperature as a reference. From the DSC test (Figure 4c), it can be seen that the T_HTD of AP decreases to 285.4 °C, 279.6 °C, 272.9 °C, and 286.3 °C for AlCo₂O₄--0.5 mass ratios of 2, 5, 7, and 10 wt%. With the increase in the amount of catalytic material, the high-temperature decomposition temperature decreases and then increases, and the inflection point was at 7 wt%, which indicates that the optimal addition of catalytic material was 7 wt%. The increase in the high-temperature decomposition temperature when the addition of catalytic material reaches 10 wt % is caused by the overloading of the catalytic material. Therefore, when applied to the composite solid propellant, 7 wt% of AlCo₂O₄-0.5 catalytic material should be added in order to achieve the best catalytic effect.

In order to determine the changes in the activation energy of AP thermal decomposition after the addition of catalytic materials, DSC-TG tests were conducted on AP with 2 wt% of AlCo₂O₄-0.5 added at different heating rates (10, 20, 30, and 40 °C/min) (Figure 5a,b). The DSC curves in Figure 5a show that the temperature of the pyrolysis peak (T_HTD) gradually shifted to a higher temperature, increasing the heating rate. The activation energy (E_a) of the thermal decomposition of AP can be calculated from the Kissinger equation [51], which explains the functional relationship between the heating rate and the high temperature of pyrolysis (Equation (2)).

$$\ln \left({\displaystyle \frac{\beta }{{T_P}^2}}\right)=-{\displaystyle \frac{E_a}{R{T}_P}}+\ln {\displaystyle \frac{AR}{{\mathrm{E}}_{\mathrm{a}}}}$$

(3)

where $\beta$is the heating rate (K/min), T_p is the T_HTD (K), R is the ideal gas constant (8.314 J/(min·K)), and A is the finger forward factor (1/s) [52]. From Figure 4d, it can be learnt that the thermal decomposition reactions of pure AP and 2 wt% AlCo₂O₄-0.5+AP show a good linear relationship. As shown in Equation (2), the activation energy (E_a) is determined based on the slope of the fitted curve. The E_a of pure AP was 296.14 kJ/mol, which decreased to 211.67 kJ/mol with the addition of 2 wt% AlCo₂O₄-0.5, which is 84.47 kJ/mol lower (Figure 4d). The decrease in the activation energy effectively confirmed the remarkable catalytic effect of the catalytic material.

Since the Kissinger method only considers the warming rate as a function of the AP thermal decomposition temperature, in order to comprehensively calculate the changes in the whole process of AP thermal decomposition and ensure the comprehensiveness and accuracy of the results, the present work took further action. The present work further explored the changes in the activation energy of the whole process using the Friedman kinetic model, which can be calculated by means of the amount of mass loss at different conversions using the TG curves and Equation (3).

$$\mathrm{l}\mathrm{n}{\displaystyle \frac{\mathrm{d}\alpha }{\mathrm{d}t}}=-{\displaystyle \frac{E_{aF}}{\mathrm{R}T}}+\mathrm{l}\mathrm{n}(\mathrm{A}\times f(\alpha \left)\right)$$

(4)

where α is the conversion rate, dα/dt is the mass loss at the α conversion rate, f(α) is the kinetic model function, and T is the temperature at which the specified conversion rate α is reached [53]. Using the TG tests at different heating rates in Figure 5b, the amount of mass loss at different conversion rates α (α = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9) can be obtained and fitted according to Equation (3). It can be seen that the AP thermal decomposition of 2 wt% AlCo₂O₄-0.5+AP also shows a good linear relationship (Figure 5c), consistent with the pattern obtained according to the Kissinger equation. Similar to the Kissinger method, the activation energy (E_aF) for the thermal decomposition of AP was calculated from the slope of the fitted curve. The specific theoretical calculations are presented in

Table 3. TG kinetic results calculated using the Friedman method.

Conversion (α)	Friedman Method (Ea, kJ·mol−1)
	Pure AP	2% AlCo2O4-0.5 + AP
0.1	199.54	102.01
0.2	221.38	191.91
0.3	228.81	202.67
0.4	236.42	216.64
0.5	249.76	221.41
0.6	272.13	226.89
0.7	284.64	211.06
0.8	295.87	194.71
0.9	300.28	160.51

. The E_aF varies in a certain region throughout the thermal decomposition of AP. The E_aF of the AP with 2 wt% AlCo₂O₄-0.5 floats in the range of 102.01~252.68 kJ/mol. The reaction mechanism of the thermal decomposition of AP varies with the temperature. Different conversion rate (α) stages may involve different control steps, and the differences in activation energy for different steps can lead to fluctuations in the calculated values [54]. From α = 0.4 to 0.8, the E_aF is maintained at a constant value, which is close to that calculated using the Kissinger method. At this time, the system enters the violent reaction stage of the pyrolysis process, which corresponds to the high-temperature decomposition stage in the DSC curve. The mass loss of the system reached 80% at lower temperatures, and AP tended to decompose completely, which effectively confirms the remarkable catalytic effect of AlCo₂O₄-0.5. Compared with pure AP (Figure 5d), the activation energies of the systems with catalytic materials were lower than those of pure AP at the same conversion, which further confirmed the significant catalytic effect of AlCo₂O₄-0.5 catalytic materials.

3.3. Catalytic Mechanism of AlCo₂O₄ Catalysts for AP Decomposition

The excellent catalytic performance of AlCo₂O₄-0.5 for AP was further studied using X-ray photoelectron spectroscopy (XPS) elemental analysis. The fine spectra of Al 2p, Co 2p, and O 1s are shown in Figure 6a–i.

The spectrum of Al 2p of AlCo₂O₄-0.5 is shown in Figure 6a, and the peak with a binding energy of 74.00 eV indicates that most of the Al is present in AlCo₂O₄-0.5 as Al³⁺ [55]. The peak at 71.61 eV indicates that a small portion of Al existed in the sample in the form of metallic Al [56]. This indicates that AlCo₂O₄ and the derived “Al” can be obtained by adjusting the oxidiser-to-fuel ratio (φ) using a one-step solution combustion method. As shown in Figure 6a–c and

Table 4. Peak position values and relative proportions of different forms of Co, Al, and O elements present in AlCo₂O₄-X (x = 0.5, 1, 2).

Samples		AlCo2O4-0.5				AlCo2O4-1				AlCo2O4-2
Co 2p	Co Ion Valence State	+3		+2		+3		+2		+3		+2
	Co 2p3/2 peak position (eV)	780.74		781.23		780.27		781.79		780.13		781.38
	Co 2p1/2 peak position (eV)	795.78		797.78		795.24		796.71		795.18		796.55
	Relative atomic content (%)	62.11		37.89		50.77		49.23		48.72		51.28
	Co3+/Co2+	1.64				1.03				0.95
Al 2p	Al elements form	+3		metallic Al		+3		metallic Al		+3		metallic Al
	Al 2p Peak position (eV)	74		71.61		73.75		71.33		73.79		71.85
	Relative atomic content (%)	78.82		21.12		90.16		9.84		87.42		12.58
	Al/Al3+	0.27				0.11				0.14
O 1s	O element existence form	OLatt	OV		-OH	OLatt	OV		-OH	OLatt	OV		-OH
	O 1s Peak position (eV)	530.22	531.13		532.46	530.18	531.34		532.39	530.27	531.34		532.68
	Relative atomic content (%)	0.41	0.43		0.16	0.57	0.31		0.12	0.58	0.34		0.08
	O V/(OLatt+-OH)	0.76				0.45				0.52

, all the AlCo₂O₄-X samples prepared with different φ contained “Al”, The Al /Al³⁺ content ratio values of AlCo₂O₄-0.5, AlCo₂O₄-1, and AlCo₂O₄-2 were 0.27, 0.11, and 0.14, respectively. Sample AlCo₂O₄-0.5 has the largest percentage of derived “Al”. The spectrum of Co 2p is shown in Figure 6d–f, which belongs to two chemical states: Co 2p_3/2 (780 eV) and Co 2p_1/2 (795 eV). The peaks with binding energies of 780.74 eV and 795.78 eV belong to Co³⁺, and the peaks with binding energies of 781.23 eV and 797.78 eV belong to Co²⁺ in AlCo₂O₄-0.5 [57,58]. The Co³⁺/Co²⁺ values of AlCo₂O₄-0.5, AlCo₂O₄-1, and AlCo₂O₄-2 were 1.64, 1.03, and 0.95, respectively. The results showed that the Co³⁺/Co²⁺ ratio decreased with increasing φ, exposing more Co³⁺ active centres in AlCo₂O₄-X and improving its catalytic activity, due to the Co³⁺ on octahedral coordination sites is known as the catalytic active site for AP thermal decomposition [59]. The O 1s (Figure 6g) fine spectra of AlCo₂O₄-0.5 were deconvoluted into three major components, and the peaks with binding energies of 530.22 eV, 531.13 eV, and 532.46 eV correspond to the lattice oxygen (O_Latt), oxygen vacancies (O_V), and surface physically and chemically adsorbed oxygen (-OH), respectively [60]. Oxygen vacancies, a type of point defect, can optimise the adsorption energy of reactants on the surface of the oxidant, thus lowering the reaction energy barrier, providing more active sites, and improving the catalytic activity [60,61]. Therefore, AlCo₂O₄-0.5 has the highest O_V content (Table 4). Based on the above XPS analysis, it can be concluded that AlCo₂O₄-0.5 has the best AP thermal catalytic performance based on the fact that AlCo₂O₄-0.5 has the highest in situ derived “Al” content, Al and AlCo₂O₄-0.5 jointly catalysed AP thermal decomposition, and AlCo₂O₄-0.5 also has the highest proportion of Co³⁺ and O_V, so that it has more active centres and active sites.

The thermal decomposition of AP is a complex process involving solid, liquid, gas, and other phases, and there are many decomposition mechanism theories, among which the electron transfer theory is the most studied. In the LTD stage of AP based on electron transfer theory, the thermal decomposition of AP forms $\mathrm{C}\mathrm{l}{\mathrm{O}}_4^{-}$ and $\mathrm{N}{\mathrm{H}}_4^{+}$, and with the transfer of electrons (e⁻) from $\mathrm{C}\mathrm{l}{\mathrm{O}}_4^{-}$ to $\mathrm{N}{\mathrm{H}}_4^{+}$, and rapidly generate $\mathrm{Cl}{\mathrm{O}}_4^{\left[0\right]}$ and $\mathrm{N}{\mathrm{H}}_4^{\left[0\right]}$ [62]. The localised Shockley-type surface energy levels make electron transfer on the AP surface easier, and the $\mathrm{N}{\mathrm{H}}_4^{\left[0\right]}$ rapidly decomposes into NH₃ and H atoms. However, at the decomposition temperature, the HClO₄ products are extremely unstable and easily decompose into chlorine oxides such as H₂O, ClO₃, and ClO [63], which may rapidly leave the AP surface before oxidising the adsorbed NH₃. The final result is that the adsorbed state of NH₃ cannot be oxidised and remains on the surface of the AP, inhibiting the electron transfer process, thereby further inhibiting the thermal decomposition process of the AP. With the increase of temperature and entering the HTD stage, the decomposed O₂ in HClO₄ converts into superoxide ions (${\mathrm{O}}_2^{-}$), which accelerates the ammonia oxidation reaction, leading to the desorption of the adsorbed state of NH₃ from the AP surface, and the rapid completion of the AP thermal decomposition [17].

In situ mass spectrometry (TG-MS) was used to monitor and analyse the gas products during the thermal decomposition of AP in real time, in order to explore the mechanism of the role of AlCo₂O₄-0.5 catalytic materials in catalysing the AP thermal decomposition process (Figure 7). The present work is based on the electron transfer theory proposed by Bicromshaw and Newman et al. [64]. The TG-MS results (Figure 7a,c) show that the gaseous products produced during the thermal decomposition of pure AP were concentrated near 463.79 °C, corresponding to the high-temperature decomposition stage of AP. Among them, no obvious gaseous products were found in the low-temperature decomposition stage from 240 to 250 °C, which further confirmed that the NH₃ gaseous products were adsorbed on the surface of AP in the LTD stage. With the increase of temperature, the O₂ generated from decomposition was converted into superoxide ions (${\mathrm{O}}_2^{-}$), and NO₂, N₂O, and NO became the dominant products. At 464.81 °C, the content of NO reaches a maximum. HCl gas can be observed as a by-product of the slow decomposition of AP in the formation of NO₂. After the addition of AlCo₂O₄-0.5, the catalytic material mainly affects the HTD stage of AP (Figure 7b), accelerates the NH₃ desorption from the AP surface, followed by ammonia oxidation reaction with chlorine oxides obtained from the decomposition of HClO₄ to obtain the final gas products. Gas products were observed only at 287.5 °C. As shown in Figure 7d, the NOx increased significantly compared with that of pure AP, and there were products such as ClO and ClO₃. The increase in NO₂ also proved that the addition of the AlCo₂O₄-0.5 catalyst was conducive to the generation of reactive oxygen species, which promoted the thermal decomposition of AP. The N₂O peaks were more pronounced than those of pure AP, probably due to the abundant catalytic sites provided by the large specific surface area of the catalytic material.

Combined with the results of TG-MS, we propose the mechanism of the AlCo₂O₄-0.5 catalytic material on the thermal decomposition of AP, as shown in Figure 8. Due to the incompletely filled 3d orbitals, the AlCo₂O₄-0.5 catalytic material effectively accelerated the ammonia oxidation reaction by providing electrons for the step of conversion of O₂ to superoxide ions (${\mathrm{O}}_2^{-}$), which led to the merging of LTD and HTD peaks into one peak. Meanwhile, both Al³⁺ and Co³⁺ ions can accept electrons, and the synergistic effect of the bimetal effectively promotes the transfer of electrons (e⁻) from $\mathrm{C}\mathrm{l}{\mathrm{O}}_4^{-}$ to $\mathrm{N}{\mathrm{H}}_4^{+}$ and the ammonia oxidation process (Equation (4)).

$${\mathrm{C}\mathrm{l}\mathrm{O}}_4^{-}+{\mathrm{N}\mathrm{H}}_4^{+}\to {\mathrm{C}\mathrm{l}\mathrm{O}}_4^{\left[0\right]}+{\mathrm{N}\mathrm{H}}_4^{\left[0\right]}$$

(5)

$${{\mathrm{C}\mathrm{o}}^{3+}+\mathrm{C}\mathrm{l}\mathrm{O}}_4^{-}\to {{\mathrm{C}\mathrm{o}}^{2+}+\mathrm{C}\mathrm{l}\mathrm{O}}_4^{\left[0\right]}$$

(6)

$${\mathrm{C}\mathrm{o}}^{2+}+{\mathrm{N}\mathrm{H}}_4^{+}\to {{\mathrm{C}\mathrm{o}}^{3+}+\mathrm{N}\mathrm{H}}_4^{\left[0\right]}$$

(7)

$${{\mathrm{A}\mathrm{l}}^{3+}+\mathrm{C}\mathrm{l}\mathrm{O}}_4^{-}\to {{\mathrm{A}\mathrm{l}}^{2+}+\mathrm{C}\mathrm{l}\mathrm{O}}_4^{\left[0\right]}$$

(8)

$${\mathrm{A}\mathrm{l}}^{2+}+{\mathrm{N}\mathrm{H}}_4^{+}\to {{\mathrm{A}\mathrm{l}}^{3+}+\mathrm{N}\mathrm{H}}_4^{\left[0\right]}$$

(9)

As for the small amount of derived “Al” dispersed in the catalyst, it avoids the reaction of superoxide ions (${\mathrm{O}}_2^{-}$) generated with the increase of temperature with Al³⁺ ions to form [AlO₄] and [AlO₆], which inhibit the reaction of the main fuel Al [9]. Since the NH₃ and HClO₄ gas products generated in the LTD stage are believed to be generated several micrometres below the surface of the AP crystals in the cavity [17], the Al fuel particles located in the inner wall of the mesoporous pores effectively shorten the heat and mass transfer distances [9], while the addition of AlCo₂O₄-0.5 catalytic material to the system effectively reduces the activation energy of the thermal decomposition of AP. The microstructure of the good interfacial contact and the rapid rate of thermal decomposition of AP, these two aspects jointly lead to the fact that the Al fuel particles can be oxidised in a full oxygen atmosphere to gain a high combustion calorific value.

3.4. Application in AP/HTPB-Based CSPs

The AP/HTPB-based composite solid propellant (AlCo₂O₄-0.5-AP/HTPB) was prepared by the vacuum casting method according to a certain raw material ratio (AP:HTPB:Al:AlCo₂O₄-0.5 = 67:11:15:7) (

Table 5. Base formulations of AP/HTPB-based composite solid propellants.

Samples	Ingredient	Content Ratio (wt)	Functionality
AP/HTPB	AP	74	Oxidising agent
	HTPB	11	Binding agent
	Al	15	Metallic fuel agent
AlCo2O4-0.5 -AP/HTPB	AP	67	Oxidising agent
	HTPB	11	Binding agent
	Al	15	Metallic fuel agent
	AlCo2O4-0.5	7	Catalyst

), evaluating the combustion performance of composite solid propellants.

The combustion of both AP/HTPB and AlCo₂O₄-0.5-AP/HTPB consists of a combustion ignition stage, an early combustion stage, a steady-state combustion stage, and an end of combustion stage [65] (Figure 9). Since the ignition stage of AP/HTPB and AlCo₂O₄-0.5-AP/HTPB combustion lasts for a very short period of time and emits a discontinuous flame, the time is recorded as the ignition delay time. This was followed by a short initial stage of combustion. As time passes, the flame spreads outwards from the centre and becomes progressively larger when the sample combustion enters the dominant steady-state combustion phase. The sample burns vigorously, and its flame brightness reaches its maximum after a certain time delay. At the same time, the decomposing gases drive the tiny solid particles upwards, and the large amount of energy released by combustion allows the solid particles to continue to react with the surrounding gaseous environment until the end of combustion. Subsequently, the centre flame gradually weakened until the end of the combustion. The time between the ignition of the combustion and the end of the combustion is defined as the duration of the combustion. The combustion results are shown in Figure 9, where the AP/HTPB was ignited after a longer delay of 28 ms at a laser ignition voltage of 34 V. It then enters the early stage of combustion at 28–139.5 ms and burns steadily after 139.5 ms until the end of combustion. With the addition of 7% AlCo₂O₄-0.5, the ignition delay of AlCo₂O₄-0.5-AP/HTPB was drastically reduced to 9 ms, and more intense and brighter flames were observed in the early combustion, steady-state combustion, and end of combustion stages. The steady-state combustion time was extended from 629.5 to 757.5 ms. Meanwhile, comparing the tiny solid particles ejected with the gas at the early combustion stage and steady-state combustion stage of AP/HTPB and AlCo₂O₄-0.5-AP/HTPB, the latter was obviously smaller in size and more uniformly dispersed, which indicated that the derived “Al” monomers stacked up in the catalytic material were more uniformly dispersed than those of the added metal fuels Al, and the agglomeration phenomenon was suppressed to some extent. The phenomenon of agglomeration is suppressed so that the Al nanoparticles can be burned and ejected with the generated gas in a short time, thus achieving sufficient combustion and reducing agglomeration.

4. Conclusions

In this paper, AlCo₂O₄ and derived “Al” catalytic materials were synthesised in one step via a φ-controlled solution combustion method, designed to tackle both AP decomposition property and contribute metallic fuel enthalpy in AP/HTPB composite solid propellant. The relevant characterisation tests showed that the AlCo₂O₄ catalytic material prepared with an oxidiser-to-fuel ratio (φ = 0.5) had a spongy porous structure, and a small amount of derived “Al” particles was generated synchronously, presenting a better catalytic effect. The XPS patterns confirmed the presence of the largest amount of metallic Al, as well as the exposure of more Co³⁺ active sites and O_V. The addition of 2 wt% AlCo₂O₄-0.5 significantly reduced the T_HTD of AP by 188.08 °C. In the kinetic study, the addition of catalytic materials led to a decrease in the activation energy (Ea) by 84.47 kJ/mol, from 296.14 to 211.67 kJ/mol. Under a 34 V laser ignition voltage, the ignition delay of the composite solid propellant was reduced to 9 ms after the addition of 7% AlCo₂O₄-0.5. Subsequently, we also explored the feasible mechanism of AlCo₂O₄-0.5 for AP thermal decomposition, and these excellent catalytic effects were mainly a result of AlCo₂O₄-0.5 has the highest in situ derived “Al” content, Al and AlCo₂O₄-0.5 jointly catalysed AP thermal decomposition, which provides a good opportunity for the composite solid propellant to it provides a feasible idea to reduce the addition of Al fuel particles and inhibit the agglomeration of Al nanoparticles in composite solid propellants. In a subsequent study, we will adopt more advanced characterisation methods to further explore the effects of active site density and aluminium content on the catalytic effect, to further confirm the significant catalytic effect of AlCo₂O₄-0.5 on the thermal decomposition of ammonium perchlorate, and to provide new ideas for the reduction of metal fuels in composite solid propellants.