Construction of Covalent Triazine Framework-Supported MnCo2O4.5 Nanoneedles via Enhanced Dispersion Strategy to Promote Ammonium Perchlorate Thermal Decomposition

TL;DR

Researchers created a new composite material that improves the thermal decomposition of ammonium perchlorate in solid rocket propellants.

Contribution

A novel 2D CTF/MnCo2O4.5 composite was developed with enhanced dispersion for better catalytic performance.

Findings

CTF/MnCo2O4.5 composites reduced AP decomposition temperature by 81.3 °C at 2 wt% loading.

The composite decreased the required catalyst content to 30 wt% compared to pure MnCo2O4.5.

XRD and XPS confirmed successful synthesis and interaction between CTF and MnCo2O4.5.

Abstract

Enhanced catalytic activity for composite solid propellants (CSPs) can be achieved through high-efficiency dispersion of active sites on the surface of two-dimensional (2D) materials. In this study, we report the in situ formation of MnCo2O4.5 nanoneedles on the surface of covalent triazine frameworks (CTFs), resulting in 2D CTF/MnCo2O4.5 composites with outstanding catalytic properties for the thermal decomposition of ammonium perchlorate (AP). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses confirmed the successful preparation of the CTF/MnCo2O4.5 composites and revealed the interaction between CTFs and MnCo2O4.5. Scanning electron microscopy (SEM) and elemental mapping further demonstrated the uniform anchoring and dispersion of MnCo2O4.5 nanoneedles on the layered CTF surfaces. Additionally, the obtained CTF/MnCo2O4.5 composites exhibited promising catalytic capacity for AP decomposition. When added at a loading of 2 wt%, the CTF/MnCo2O4.5 composites significantly reduced the thermal decomposition temperature of AP by 81.3 °C, while simultaneously decreasing the content to 30 wt% compared to pure MnCo2O4.5 catalysts.

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1. Introduction

Composite solid propellant (CSP) serves as the core power material for missiles and carrier rockets [1]. As a primary constituent of CSP, ammonium perchlorate (AP) typically accounts for 60–80% of the total mass. The critical combustion properties of AP, including thermal decomposition rate, temperature and activation energy, dictate the overall performance of the propellant. The reduction in decomposition temperature stands as the most widely used indicator to evaluate the performance of AP decomposition, a value that varies widely depending on the experimental protocol and catalyst system [2,3,4]. To facilitate complete AP decomposition at minimal temperatures, two primary approaches have been explored: reducing the particle size of AP crystals or introducing combustion catalysts [5]. However, the preparation of ultrafine AP crystals is both challenging and hazardous, rendering the addition of combustion catalysts a more practical and applicable solution [3].

Transition metal oxides (TMOs) are widely employed as catalysts for AP–based propellants, owing to their stable catalytic activity under high-temperature and high-pressure conditions [6]. As a binary TMO, MnCo_2_O_4.5_ exhibits remarkable catalytic activity and adaptability. This outstanding performance originates from its considerable diversity in phase relations, chemical composition, and metal ion valences, coupled with variations in the distribution of ions and vacancy sites [7,8,9]. The catalytic activity of TMOs toward AP thermal decomposition is closely linked to the number of active catalytic sites they possess. Studies indicate that reducing the particle size of TMOs significantly enhances their specific surface area while simultaneously increasing the number of lattice defects. These structural changes further augment the active catalytic sites, thereby improving catalytic activity [10]. Nevertheless, the high surface energy of TMOs renders them susceptible to agglomeration during storage and practical applications, leading to insufficient exposure of active catalytic sites and ultimately diminishing their actual catalytic efficiency [11].

In the field of catalyst design, dispersing active sites on the surface of support materials has emerged as a mainstream strategy for enhancing catalytic efficiency [12]. A series of high-surface-area two-dimensional (2D) materials, including MXene, graphene, and g-C_3_N_4_, have been employed to promote the thermal decomposition of energetic oxidizers [13,14,15,16,17]. Covalent triazine frameworks (CTFs) represent an emerging class of 2D porous materials that combine high specific surface area and permanent microporosity with a nitrogen-rich covalent organic frameworks structure [18]. These materials feature aromatic triazine rings as basic linking units, endowing them with a strong conjugated system and excellent thermochemical stability. Additionally, the abundant nitrogen atoms on their surface can effectively bind metals or metal oxides through coordination anchoring [19,20]. Furthermore, through rational modification strategies such as defect engineering, the composite effect between CTFs and nanomaterials can be further enhanced [21]. Studies demonstrate that surface defects in CTFs provide abundant vacancies and unsaturated coordination sites, which act as “traps” to capture metal precursors and anchor metal atoms or clusters during subsequent treatment processes [22,23]. Meanwhile, these defects can enhance the stability of metal species through interaction with certain metal atoms or groups [24,25]. Consequently, utilizing vacancies in the defective structure of CTFs to immobilize metal precursors and adopting an in situ reduction strategy are promising approaches for achieving highly efficient dispersion of TMO catalysts.

To address the challenges of TMO agglomeration and further enhance catalytic performance for AP thermal decomposition, this work focuses on MnCo_2_O_4.5_ as the research object and defective CTFs as the matrix material. The aim is to achieve the uniform dispersion of MnCo_2_O_4.5_ on the CTF surface, forming CTF/MnCo_2_O_4.5_ composites. Structural analysis techniques are employed to confirm the dispersion effect and elucidate the mechanism of metal ions anchoring. Additionally, the catalytic capacity of the CTF/MnCo_2_O_4.5_ composites for AP thermal decomposition is investigated. In the final part, we analyze the accelerating process and catalyzing mechanism of the high-temperature thermal decomposition of AP.

2. Experimental Section

2.1. Materials

1,4-dycyanobenzene (DCB), Co(NO_3_)2·6H_2_O and MnCl_2_ were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). CH_2_Cl_2_ was obtained from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). CO(NH_2_)2, KCl, and CH_3_CH_2_OH were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). NaBH_4_ was provided by Xilong Scientific Co., Ltd. (Guangzhou, China). AP was provided by Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were used as received without further purification.

2.2. Preparation of CTF and CTF/MnCo2O4.5 Composites

The CTF and CTF/MnCo_2_O_4.5_ composites were synthesized via the route outlined in Figure 1. Firstly, 192 mg of DCB and 2 g of KCl were vacuum-sealed in a 10 mL ampoule and heated at 400 °C for 40 h in a tube furnace. After cooling, the sample was ground and sequentially washed with deionized water, ethanol, and dichloromethane. The washed product was freeze-dried under vacuum for 24 h to obtain CTF samples (the overall yield was 41%) [26]. Secondly, 0.2 g of CTF and 29.6 mg of NaBH_4_ were ground and dispersed in an agate mortar, and then the mixture was calcinated at 350 °C for 1.5 h in a tube furnace under N_2_ atmosphere [27]. After cooling to room temperature, the powder was washed repeatedly with ethanol and deionized water, then ultrasonically dispersed in 40 mL of deionized water. Thirdly, 30 mg of MnCl_2_, 134 mg of Co(NO_3_)2·6H_2_O, and 57 mg of CO(NH_2_)2 were added to the dispersion and stirred vigorously for 2 h. The homogeneous mixture was transferred into an 80 mL stainless steel autoclave lined with polytetrafluoroethylene (PTFE) and heated at 180 °C for 8 h in an oven. After natural cooling, the dark brown precipitate was thoroughly washed with deionized water and anhydrous ethanol alternately, then vacuum-dried at 60 °C for 8 h. Finally, the as-prepared precursor was calcined at 350 °C for 4 h to yield CTF/MnCo_2_O_4.5_ composites, with a calculated MnCo_2_O_4.5_ content of 30 wt%. For comparison, pure MnCo_2_O_4.5_ was synthesized following an identical procedure, except that no CTF dispersion was added.

2.3. Characterization

The phase composition and crystal structures of CTF, MnCo_2_O_4.5_, and CTF/MnCo_2_O_4.5_ composites were characterized by X-ray diffraction (XRD) (AXS D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany, Cu Kα radiation, λ = 1.54184 Å). Field-emission scanning electron microscopy (FESEM) (Merlin Compact, Jena, Germany, Secondary Electron Detector were used, and the acceleration voltage was 10 kV) was employed to examine the morphology and elemental distribution. X-ray photoelectron spectroscopy (XPS) analysis was performed using an ESCALAB250Xi instrument (Thermo Fisher Scientific Inc., Waltham, MA, USA) with Al Kα radiation as the excitation source to determine the surface elemental composition and chemical valence states. The obtained results were calibrated by referencing the C 1s peak to a binding energy of 284.8 eV.

2.4. Catalytic Decomposition of AP

The thermal decomposition properties of AP with and without CTF/MnCo_2_O_4.5_ composites were investigated using a simultaneous thermal analyzer (TG-DSC) (STA449F3, NETZSCH GmbH, Selb, Germany). The samples were heated at 10 °C·min^−1^ from 50 °C to 500 °C under N_2_ atmosphere. Sample preparation was conducted using the following procedure. First, 10 mg of CTF/MnCo_2_O_4.5_ composites and 10 mL of anhydrous ethanol were sonicated in an agate mortar for 10 min. Subsequently, 490 mg of AP was added to the mortar and subjected to ultrasonic grinding until a homogeneous paste was formed. Finally, the sample was dried in a 60 °C water bath oven for 1 h to obtain a mixture of AP and CTF/MnCo_2_O_4.5_ composites. For comparison analysis, the catalytic performances of both CTF and MnCo_2_O_4.5_ were evaluated using an addition amount of 2 wt%.

3. Result and Discussion

3.1. Characterization of Structure and Morphology

The crystal structures of CTF, MnCo_2_O_4.5_, and CTF/MnCo_2_O_4.5_ composites were initially characterized by XRD, as illustrated in Figure 2a. The pristine CTF exhibits three distinct diffraction peaks at 7.3°, 14.3°, and 26.2°, corresponding to the (100), (200), and (001) planes, respectively. This profile aligns well with the simulated AA-stacking model and previously reported literature, confirming that the CTF synthesized via the KCl template method possesses an ordered layered structure [26]. For pure MnCo_2_O_4.5_, diffraction peaks observed at 19.0°, 31.3°, 36.5°, 44.8°, 55.8°, 59.5°, and 64.9° are indexed to the (111), (220), (311), (400), (422), (511), and (440) planes of MnCo_2_O_4.5_ (JCPDS: 32-0297) [8]. This phase can be described as a cation-deficient spinel, where the excess oxygen (or metal deficiency) leads to a lattice similar to gamma Mn_2_O_3_ [10]. In this configuration, the charge balance is maintained by the coexistence of divalent and trivalent cations (Mn^2+^, Mn^3+^, Co^2+^, and Co^3+^) and the presence of cation vacancies within the octahedral sites [28,29]. In the CTF/MnCo_2_O_4.5_ composites, the characteristic peaks appear at all the aforementioned angles, verifying the successful preparation of MnCo_2_O_4.5_ and indicating that hydrothermal loading and sintering did not disrupt the original structure of the pristine CTF.

The morphology and elemental distribution of the samples were subsequently analyzed via FESEM, as shown in Figure 2b–j. CTF displays a nano-layered structure with a lateral size of approximately 100 μm (Figure 2b), which is attributed to the dual role of KCl: it acts not only as a template to promote the formation of nano-layer morphology but also as a solid catalyst that facilitates the trimerization of nitrile groups through chemical interactions between the salt lattice and nitrile monomers, promoting CTF growth on the KCl surface. Pure MnCo_2_O_4.5_ exhibits an acicular nanoneedle structure with a length of approximately 10 μm, accompanied by significant agglomeration (Figure 2c). In contrast, the CTF/MnCo_2_O_4.5_ composites (Figure 2d) show MnCo_2_O_4.5_ particles uniformly dispersed as nanoneedles (nanoscale diameter, micrometer-scale length) across the CTF surface. Elemental mapping of the composites (Figure 2e–j) further reveals a homogeneous distribution of Mn and Co on the CTF substrate. These FESEM observations provide empirical evidence for the successful and uniform integration of MnCo_2_O_4.5_ particles onto CTF, primarily due to strong coordination between Mn/Co atoms in MnCo_2_O_4.5_ and the N atoms in the CTF (forming Co–N/Mn–N bonds).

Finally, the surface elemental compositions and chemical states were characterized by XPS, as displayed in Figure 3. The survey spectrum (Figure 3a) identifies C and N as the primary elements in pristine CTF, while C, N, O, Co, Mn, and B are detected in the CTF/MnCo_2_O_4.5_ composites, confirming successful integration of both components. As shown in Figure 3b,c, the C 1s spectrum of pure CTF exhibits three peaks at 284.80 eV, 286.68 eV, and 288.38 eV, corresponding to C–C=C, –C≡N, and N–C–N bonds, respectively [27]. The N 1s spectrum reveals two characteristic peaks at 398.78 eV and 399.88 eV, assigned to C–N=C and C=N within the CTF, consistent with literature [26]. For CTF/MnCo_2_O_4.5_ composites (Figure 3d,e), C 1s peaks appear at 284.80 eV, 286.88 eV, and 288.78 eV, while N 1s peaks are observed at 398.58 eV and 399.98 eV. Specifically, the C–N=C binding energy decreases by 0.20 eV relative to CTF, indicating a shift toward lower energy due to the formation of Co–N and Mn–N coordination bonds between CTF nitrogen atoms and Mn/Co sites [30,31,32]. Furthermore, the Mn 2p spectrum (Figure 3f) shows peaks at 653.1 eV and 642.2 eV for Mn^2+^ 2p_1/2_ and 2p_3/2_, while those at 654.5 eV and 643.4 eV for Mn^3+^. Similarly, the Co 2p spectrum (Figure 3g) exhibits four peaks at 780.5 eV (Co^3+^ 2p_3/2_), 781.9 eV (Co^2+^ 2p_3/2_), 795.6 eV (Co^3+^ 2p_1/2_), and 797.2 eV (Co^2+^ 2p_1/2_) [6]. Collectively, these XPS results substantiate the successful synthesis of CTF/MnCo_2_O_4.5_ composites and highlight strong interfacial coordination that facilitate uniform nanoparticle dispersion.

3.2. Catalytic Performance of CTF/MnCo2O4.5 Composites

The catalytic effect of CTF/MnCo_2_O_4.5_ composites on AP decomposition was evaluated via differential scanning calorimetry (DSC) and thermogravimetry (TG) techniques, with the results presented in Figure 4. As shown in the DSC curve of pure AP (Figure 4a), an endothermic peak appears at approximately 240.9 °C, corresponding to the phase transition from an orthorhombic to a cubic structure, which is consistent with previous reports [33,34]. Subsequently, two exothermic peaks are observed at 284.7 °C and 400.6 °C, assigned to the low-temperature decomposition (LTD) and high-temperature decomposition (HTD) stages, respectively [35,36]. The LTD peak is attributed to initial decomposition at subsurface sites with lattice dislocations, while the HTD peak represents complete dissociation. Since the overall decomposition process is governed by the ignition stage, catalytic intervention is crucial to initiate rapid heat release. The high thermal stability of CTF, which remains intact up to 400 °C, ensures that the CTF/MnCo_2_O_4.5_ catalyst maintains structural integrity and activity throughout these reactions [26,37,38,39].

Significant changes in the decomposition behavior were observed upon incorporating 2 wt% of CTF, MnCo_2_O_4.5_ or CTF/MnCo_2_O_4.5_ into the AP system. Specifically, the HTD temperature of AP was markedly reduced from 400.6 °C to 383.6 °C, 315.3 °C, and 319.3 °C, respectively. The introduction of CTF alone advanced the HTD by 17.0 °C, whereas the addition of MnCo_2_O_4.5_ and CTF/MnCo_2_O_4.5_ composites resulted in a more substantial reduction exceeding 81.3 °C and induced the merger of the LTD and HTD peaks into a single exothermic event. This pronounced decrease indicates that both MnCo_2_O_4.5_ and the CTF/MnCo_2_O_4.5_ composites effectively accelerate AP thermal decomposition even at low loadings.

These findings are further corroborated by the TG and DTG curves shown in Figure 4c,d. The TG curve of pure AP displays two distinct weight-loss stages between 250 °C and 400 °C. The presence of 2 wt% CTF lowered the final decomposition temperature by 11.6 °C. In contrast, the samples containing MnCo_2_O_4.5_ and the CTF/MnCo_2_O_4.5_ composites completed decomposition at significantly lower temperatures of 320.4 °C and 326.4 °C, respectively. The corresponding DTG results (Figure 4d) reveal a distinct negative shift in the two characteristic peak temperatures corresponding to the maximum weight-loss rates of AP. It is noteworthy that the actual TMO catalyst content in the CTF/MnCo_2_O_4.5_ composites is only 30% of that in the pure MnCo_2_O_4.5_ sample, based on the calculated composition.

Collectively, these results demonstrate the superior catalytic performance of the CTF/MnCo_2_O_4.5_ composites in promoting AP decomposition. This enhanced activity is facilitated by the uniform dispersion of MnCo_2_O_4.5_ nanoneedles on the 2D CTF support, achieved through strong interfacial coordination, as evidenced by XPS analysis (Figure 3, Section 3.1).

3.3. Catalytic Mechanism of CTF/MnCo2O4.5 Composites

DSC analysis demonstrated that incorporating 2 wt% CTF/MnCo_2_O_4.5_ composites significantly reduced the decomposition temperature of AP, highlighting their superior catalytic efficacy. Elucidating the underlying mechanism is essential for understanding this behavior. As established, AP decomposition involves two distinct stages [40,41]. In the LTD stage, proton (H^+^) transfer from NH_4_^+^ to ClO_4_^−^ generates NH_3_ and HClO_4_, constituting the rate-determining step. During the HTD stage, liberated HClO_4_ decomposes rapidly, reacting with NH_3_ in the gas phase to yield N_2_O, NO_2_, H_2_O, and other products [4,42,43]. On the other side, TMOs typically possess partially filled d-orbitals, and surface oxygen vacancy defects can adsorb O_2_ molecules, promoting electron capture to form reactive O_2_^−^ species [44]. Compared to monometallic oxides, bimetallic oxides like MnCo_2_O_4.5_ offer enhanced redox sites and synergistic effect [45].

Based on the established framework, we propose the catalytic mechanism illustrated in Figure 5. Under thermal excitation, MnCo_2_O_4.5_ generates conduction band electrons (e_cb_^−^) and holes (h^+^) on its surface [46,47]. In the LTD stage, MnCo_2_O_4.5_, as a semiconductor, participates in the charge transfer between ClO_4_^−^ and NH_4_^+^, thereby promoting efficient separation and diffusion of NH_3_ and HClO_4_. During the HTD stage, the resulting HClO_4_ decomposes to produce O_2_, which reacts with e_cb_^−^ to form highly reactive superoxide ions (O_2_^−^). These potent O_2_^−^ anions, along with H^+^ species, accelerate the oxidation of NH_3_ into smaller molecules such as N_2_O, NO_2_, and NO [36,38,48]. The rapid consumption of gaseous NH_3_ shifts the dissociation equilibrium, consequently accelerating the overall decomposition kinetics of AP [49]. Notably, while electron–hole recombination typically limits the catalytic performance of pure MnCo_2_O_4.5_, the anchoring effect of the CTF support improves nanoparticle dispersion and exposes additional active sites [49]. Owing to its structural stability, MnCo_2_O_4.5_ can sustain synergistic interactions between superoxide ions and gaseous intermediates without structural degradation. Furthermore, the high specific surface area and boron-doping of CTF facilitate effective adsorption of reaction intermediates, driving the thermal decomposition forward [27,50]. Collectively, the CTF/MnCo_2_O_4.5_ composites synergistically accelerate the entire AP decomposition process, ensuring a more rapid and complete reaction.

4. Conclusions

To address the challenges of detrimental nanoparticle agglomeration and high catalyst loading requirements in AP thermal decomposition, this work developed a high-efficiency dispersion strategy to achieve the uniform distribution of MnCo_2_O_4.5_ on the surface of nitrogen-rich, 2D CTF. This approach successfully yielded novel CTF/MnCo_2_O_4.5_ composites with a reduced active phase content of 30 wt%. Comprehensive structural and morphological characterization via XRD, FESEM, and XPS confirmed the homogeneous dispersion of the MnCo_2_O_4.5_ phase on the CTF surface. Remarkably, incorporating only 2 wt% of this composite into the AP system (98 wt% AP, 2 wt% CTF/MnCo_2_O_4.5_) resulted in a substantial reduction in the HTD temperature peak by 81.3 °C. These results demonstrate the superior catalytic performance of the CTF/MnCo_2_O_4.5_ architecture and validate the effectiveness of the strategic design. Mechanistic discussion suggests that the accelerated decomposition stems from the highly anchored dispersion of MnCo_2_O_4.5_ nanoneedles and the large specific surface area of the CTF support. Overall, this study presents a promising paradigm for developing advanced AP decomposition catalysts that combine enhanced efficacy with minimized TMO consumption.