Enhanced Synergistic Catalytic Effect of a CTF-Based Composite via Constructing of a Binary Oxide System for Thermal Decomposition of Ammonium Perchlorate

TL;DR

A new composite material using a binary oxide system on a 2D framework improves the thermal decomposition of ammonium perchlorate.

Contribution

A novel CTF/CuO–NiO composite with enhanced synergistic catalytic performance is developed using a hydroxylation strategy.

Findings

The CTF/CuO–NiO composite significantly lowers the decomposition temperature of ammonium perchlorate.

Hydroxylation of CTF surfaces enables uniform dispersion of CuO–NiO nanoparticles.

The composite achieves excellent catalytic performance with only 2 wt% loading.

Abstract

As a widely used catalyst class, transition metal oxides (TMOs) face the challenges of detrimental nanoparticle agglomeration. The newly developing two-dimensional (2D) covalent triazine frameworks (CTFs) offer a promising solution as catalyst supports, capable of yielding composites with excellent dispersibility and synergistic catalytic enhancement. Building on this, and employing a hydroxylation functional modification strategy, this article introduces a binary oxide system to construct a CTF/CuO–NiO composite that exhibits excellent catalytic performance for the thermal decomposition of ammonium perchlorate (AP). Specifically, polyvinyl alcohol (PVA) was first employed to introduce -OH anchoring sites onto the CTF surface. A subsequent co-precipitation yielded a uniform dispersion of CuO–NiO nanoparticles across the functionalized CTF support. DSC analysis revealed that incorporating merely 2 wt% of the CTF/CuO–NiO composite into AP significantly alters its high-temperature decomposition (HTD) peak temperature, shifting it from 404.6 °C to 332.1 °C. This work highlights the construction of a binary oxide system through an effective dispersion strategy to enhance the synergistic catalytic performance of CTF-based composites.

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1. Introduction

In recent years, ultrathin materials, particularly two-dimensional (2D) nanosheets, have garnered considerable research interest due to their unique electron transport properties, which are responsible for their exceptional performance in frontier areas such as heterogeneous catalysis and chemical sensing [1,2,3,4,5]. Among the extensive family of 2D materials, covalent triazine frameworks (CTFs) have emerged as a promising class, exhibiting distinct advantages for various applications owing to their remarkable thermochemical stability, ultrahigh framework nitrogen content, and precisely tunable pore structures [6,7,8]. Notably, the highly regular nitrogen atoms within the triazine rings serve as ideal chemical anchoring sites, capable of forming strong coordination bonds with metals or metal oxides via lone-pair electrons, thereby enabling the construction of high-performance composite materials [9,10]. For instance, Kang et al. fabricated 2D composite nanosheets by loading the bimetallic oxide CoFe_2_O_4_ [11] onto triazine-derived carbon materials, achieving electrocatalytic cyclic stability comparable to that of noble metals [12]. Similarly, Rajagopal et al. utilized an ortho-tolidine-based triazine covalent organic framework to support Cu and Ni nanoparticles, significantly optimizing the kinetic processes of both the oxygen evolution reaction and the hydrogen evolution reaction. These studies collectively underscore the profound research value of developing CTF-based nanocomposite systems in catalysis.

Among various catalytic active components, transition metal oxides (TMOs) are highly favored for their flexible redox properties and abundant surface-active sites [13,14]. TMOs can undergo rapid switching between multiple oxidation states during reactions, acting as “charge transfer bridges” to accelerate electron transfer throughout the catalytic cycle. Their rich surface-active sites effectively adsorb and activate reactant molecules [15]. By compositing with CTF, the thermodynamic agglomeration of TMOs during the loading process can be significantly suppressed, enabling high dispersion of the active components [16]. This structure not only maximizes the exposure of surface/interface active sites but also enhances the structural stability of the catalyst through the anchoring effect of the support framework [17]. Although recent studies on CTF-supported single-metal oxides have made considerable progress, single-component systems still face bottlenecks such as limited types of active sites and restricted electron transfer rates [18]. In contrast, bimetallic oxides can effectively regulate lattice defects and electronic band structures through strong interactions between different metal species, generating a pronounced synergistic effect that provides higher intrinsic catalytic activity than their single-component counterparts [19,20,21]. Therefore, developing CTF-supported bimetallic oxide composite catalytic systems holds great promise for achieving a leap enhancement in catalytic performance through inter-component synergy.

However, limited by their highly conjugated skeletal structures, pristine CTFs typically exhibit pronounced chemical inertness and hydrophobic characteristics [22,23]. This results in poor interfacial compatibility with inorganic metal precursors (e.g., aqueous metal salt solutions), making it difficult for metal ions to effectively infiltrate and achieve uniform loading. Research has indicated that introducing oxygen-containing functional groups onto the surfaces of 2D support materials is an effective strategy to enhance hydrophilicity and strengthen interfacial bonding with metal components [24,25,26]. For example, Liu et al. introduced carboxyl and hydroxyl functional groups onto graphene surfaces, which significantly increased the binding energy between the substrate and active substances. They successfully constructed a V_2_O_3_-TiO_2_/graphene composite that effectively improved the hydrogen storage performance of MgH_2_ [11]. This insight informs us that the functionalized modification of CTFs is a viable strategy to address issues such as poor dispersibility and weak binding of active substances.

Based on the above considerations, this study selects CuO–NiO as a representative bimetallic TMO and employs hydroxyl-functionalized CTF as a high-performance support. An efficient synthesis strategy is proposed to induce the highly uniform dispersion of CuO–NiO on the CTF surface through strong coordinative interactions. The microscopic morphology and phase structure of the prepared CTF/CuO–NiO composite were systematically characterized by X-Ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and X-Ray photoelectron spectroscopy (XPS). The catalytic activity of the composite toward the thermal decomposition of ammonium perchlorate (AP) was primarily evaluated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Moreover, the synergistic catalytic mechanism of the CTF/CuO–NiO composite system in modulating the thermal behavior of AP was thoroughly investigated. This work aims to establish a critical theoretical foundation and provide an experimental paradigm for the design and development of high-performance burning rate catalysts for composite solid propellants. Furthermore, owing to the high thermal stability and uniform dispersion of transition metal oxides on the CTF, this catalytic system could be extended to gas-phase oxidation [27] and catalytic combustion applications [28].

2. Experimental Section

2.1. Materials

1,4-Dicyanobenzene (DCB), CuSO_4_·5H_2_O, NiSO_4_·6H_2_O, NaOH, polyvinyl alcohol (PVA), and AP were provided by Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). CH_2_Cl_2_ was procured from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). KCl was sourced from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). CH_3_CH_2_OH was supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the chemicals involved in this project were used as received without any further purification.

2.2. Preparation of CTF, CTF-OH and CTF/CuO–NiO

A schematic illustration of the entire preparation process is presented in Figure 1. The CTF was first synthesized via a salt-templating method [29]. Briefly, 192 mg of DCB and 2 g of KCl were vacuum-sealed in a 10 mL ampoule and heated in a tube furnace at 400 °C for 40 h. After cooling to room temperature, the resulting product was ground, sequentially washed with deionized water, ethanol, and dichloromethane, and finally freeze-dried under vacuum for 24 h to obtain the CTF (the synthesis yield was approximately 55%). Subsequently, 100 mg of the as-synthesized CTF was added to a PVA solution (prepared by dissolving 20 mg of PVA in 100 mL of deionized water at 80 °C), followed by magnetic stirring for 30 min. The solid was then collected, washed thoroughly with ethanol and deionized water to remove any unbound PVA, and freeze-dried to yield the hydroxyl-modified CTF, denoted as CTF-OH.

The CTF/CuO–NiO composite catalyst was subsequently prepared via a co-precipitation method. Initially, 100 mg of CTF-OH was dispersed in 100 mL of deionized water under ultrasonication for 1 h. Then, 47 mg of NiSO_4_·6H_2_O and 53 mg of CuSO_4_·5H_2_O were dissolved into the resulting suspension. The pH of the mixture was adjusted to 10 by adding a 0.1 M NaOH solution. Then, the solution was stirred at 60 °C for 6 h to facilitate anchoring and co-precipitation. The resulting solid was collected, alternately washed with ethanol and deionized water, and vacuum-dried at 60 °C for 8 h to obtain the precursor. Finally, the precursor was calcined in a tube furnace at 550 °C for 2 h (heating rate: 10 °C/min) under a N_2_ atmosphere. After grinding, the final product was obtained as a black powder, designated as CTF/CuO–NiO (the overall yield was approximately 36%), with a nominal CuO:NiO molar ratio of 1:1 and a total metal oxide loading of 30 wt%.

2.3. Characterization

The crystal phases of CTF and CTF/CuO–NiO composite were characterized by X-ray diffraction (XRD) (AXS D8 Advance, Bruker AXS GmbH, Karlsruhe, Germany; Cu Kα radiation, λ = 1.54184 Å). The operating voltage and current were set to 40 kV and 40 mA, respectively. The XRD patterns were recorded in the 2θ range of 5–80° with a scanning speed of 10°·min^−1^ and a step size of 0.02°). The morphological characterization was performed using field emission scanning electron microscopy (FESEM) (HitachiRegulus8100, Hitachi Scientific Instruments, Shanghai, China; acceleration voltage set at 10 kV). The elemental composition and distribution of the CTF/CuO–NiO composite were further investigated via energy-dispersive spectroscopy (EDS) attached to the same FESEM system. The surface elemental states and chemical composition of the CTF and CTF/CuO–NiO composite were studied by X-ray photoelectron spectroscopy (XPS) (K-Alpha, Thermo Fisher Scientific., RockFord, Tempe, AZ, USA; Al Kα radiation as the exciting source, C1s line 284.8 eV as a reference for calibration)

2.4. Catalytic Performance

The catalytic activity of the CTF/CuO–NiO composite toward the thermal decomposition of ammonium perchlorate (AP) was evaluated using a simultaneous thermal analyzer (TG-DSC) (STA449F3, NETZSCH GmbH, Selb, Germany). Measurements were conducted under a flowing N_2_ atmosphere at a heating rate of 10 °C·min^−1^ from 50 to 500 °C. The sample for thermal analysis was prepared as follows: First, 10 mg of the CTF/CuO–NiO composite was dispersed in 10 mL of anhydrous ethanol and sonicated in an agate mortar for 10 min. Subsequently, 490 mg of AP was added to the mortar, and the mixture was subjected to ultrasonic grinding until a homogeneous paste was formed. Finally, the paste was dried in a water bath oven at 60 °C for 1 h to obtain the well-mixed AP/CTF/CuO–NiO sample.

3. Results and Discussion

3.1. Characterization of Structure and Morphology

The crystal structures of CTF and CTF/CuO–NiO composite were analyzed using XRD, as shown in Figure 2a. The CTF samples exhibit characteristic diffraction peaks at 7.28°, 14.55°, and 26.24°, corresponding to the (100), (200), and (001) crystal planes of CTF, respectively. This diffraction pattern confirms that the CTF prepared by the KCl-template method exhibits a certain degree of crystallinity and a relatively regular structure, which is beneficial for generating a high specific surface area [30]. For the CTF/CuO–NiO composite, additional distinct peaks appear at 2θ values of 34.2°, 36.3°, 60.2°, 61.7°, 71.3°, 74.1°, and 75.2°. These peaks are indexed to the (110), (111), (202), (113), (220), (311), and (004) planes of monoclinic CuO (JCPDS No. 48-1548), respectively. Furthermore, peaks observed at 38.3°, 41.05°, and 61.5° are assigned to the (111), (200), and (102) planes of cubic NiO (JCPDS No. 07-5432). These results demonstrate the successful deposition of monoclinic CuO and cubic NiO phases onto the CTF surface, confirming the effective preparation of the CTF/CuO–NiO composite [31,32].

FESEM analysis reveals that the CTF possesses a nanosheet morphology with a lateral size of approximately 20 μm (Figure 2b). This morphology is attributed to the dual role of KCl during synthesis: it acts not only as a template guiding the formation of the nanosheet structure but also as a solid catalyst that promotes the trimerization reaction. In contrast, pure CuO–NiO exhibits an irregular nanoparticle structure with significant agglomeration (Figure 2c). As shown in Figure 2d, the CTF/CuO–NiO composite displays uniformly distributed oxide particles with an average size of about 400 nm on the lamellar CTF surface. To further elucidate the dispersion state of the CuO–NiO particles on CTF, elemental mapping was performed on the composite. Figure 2e shows the selected region and its corresponding full spectrum, while panels f–j present the distribution of C, N, O, Cu, and Ni elements, respectively. The images clearly reveal a homogeneous distribution of Cu, Ni, and O elements across the CTF surface. This uniform elemental distribution, combined with the XRD and morphological data, confirms the successful preparation of CTF/CuO–NiO composite with uniform dispersion.

The XPS survey spectrum of the CTF (Figure 3a) confirms the presence of carbon and nitrogen as the primary elements. High-resolution C 1s spectrum of CTF (Figure 3b) can be deconvoluted into three peaks at binding energies of 284.80, 286.86, and 288.82 eV, corresponding to C–C/C=C, –C≡N, and N–C=N species, respectively. The N 1s spectrum (Figure 3c) displays two peaks at 398.61 and 399.85 eV, assigned to C–N=C and C≡N groups within the CTF. These spectral features are in good agreement with previous reports, verifying the successful synthesis of CTF [33,34].

In contrast, the XPS survey spectrum of the CTF/CuO–NiO composite (Figure 3a,d–h) displays characteristic peaks of five elements, with three additional elements O, Cu, and Ni compared to pure CTF, indicating the successful incorporation of metal oxides. The high-resolution C 1s spectrum (Figure 3d) exhibits four characteristic peaks at binding energies of 284.80 eV, 285.99 eV, 286.87 eV, and 288.63 eV, attributed to C–C=C, CO, –C≡N, and N–C=N in the structure, respectively. The distinct C–O peak confirms the successful hydroxylated modification of the CTF support [35,36]. The N 1s spectrum (Figure 3e) shows peaks at 399.77 eV and 401.15 eV, assigned to C–N=C and –C≡N, respectively. The O 1s spectrum (Figure 3f) is fitted with three peaks at 529.91, 531.71, and 532.84 eV, which are ascribed to metal-oxygen bonds (Cu–O and Ni–O) [37] and the C–O [38] bond from the functionalized CTF, respectively. This, combined with the XRD results, further corroborates the formation of the CTF/CuO–NiO composite.

In the Cu 2p region (Figure 3g), the characteristic doublet with peaks at 934.73 eV (Cu 2p_3/2_) and 954.33 eV (Cu 2p_1/2_) exhibits a spin–orbit splitting of 19.8 eV. The presence of pronounced shake-up satellite peaks at 941.8 and 961.8 eV is a clear signature of the Cu^2+^ oxidation state [37,39]. Together with the Cu–O component identified in the O 1s spectrum, these features confirm that copper exists predominantly as CuO in the composite. Similarly, the Ni 2p spectrum (Figure 3h) shows main peaks at 856.72 eV (Ni 2p_3/2_) and 874.73 eV (Ni 2p_1/2_), accompanied by satellite peaks at 862.17 and 879.84 eV. This spectral profile, along with the corresponding Ni–O bond signal in the O 1s spectrum, verifies that nickel is present in the form of NiO [40,41]. The collective XPS analysis across all elemental regions consistently demonstrates the successful preparation of the CTF/CuO–NiO composite, where CuO and NiO nanoparticles are anchored on the hydroxyl-functionalized CTF support.

3.2. Catalytic Behavior of CTF/CuO–NiO Composite

The catalytic activity of the samples for ammonium perchlorate (AP) decomposition was evaluated by differential scanning calorimetry and thermogravimetric analysis (DSC/TG). Measurements were performed with a 2 wt% loading of CTF, CuO–NiO, or CTF/CuO–NiO composite mixed with AP. The resulting DSC, TG, and derivative thermogravimetry (DTG) curves are presented in Figure 4. As shown in Figure 4a, the DSC curves of pure AP and AP mixed with the catalysts all display one endothermic peak and two distinct exothermic peaks. These correspond to the crystallographic phase transition, low-temperature decomposition (LTD), and high-temperature decomposition (HTD) stages of AP, consistent with its reported thermal behavior [42,43]. While the phase transition temperature remains nearly constant at approximately 246.8 °C regardless of the additive, the HTD peak temperature is significantly reduced upon the introduction of catalysts. Specifically, the HTD peak for pure AP occurs at 404.6 °C. With the addition of 2 wt% CTF/CuO–NiO composite, this peak shifts markedly to 332.1 °C, representing a substantial reduction of 72.5 °C (Figure 4b). This indicates that the catalyst does not interfere with the phase transition but exerts a pronounced catalytic effect specifically on the HTD process of AP.

The TG and derivative thermogravimetry (DTG) profiles provide further insight into the decomposition kinetics. Figure 4c shows that the thermal decomposition of both pure and catalyzed AP proceeds in two major mass loss stages, aligning with the LTD and HTD events observed in the DSC curves. The presence of two corresponding peaks in each DTG trace (Figure 4d) confirms this two-stage mechanism. Further analysis of the TG data reveals that the final decomposition temperature (taken as the point where mass loss ceases) is progressively lowered by the catalysts. For pure AP, this temperature is 405.3 °C. With the addition of 2 wt% CTF, CuO–NiO, and CTF/CuO–NiO composite, the final decomposition temperatures are advanced to 384.6 °C, 354.7 °C, and 344.7 °C, respectively (Table 1). This trend clearly demonstrates the superior catalytic activity of the CTF/CuO–NiO composite, which promotes the most complete and rapid decomposition of AP at the lowest temperature. In summary, comprehensive thermal analysis confirms that the CTF/CuO–NiO composite exhibits optimal catalytic performance for AP thermal decomposition, as evidenced by the most significant reduction in both the HTD peak temperature and the final decomposition temperature among all tested samples.

3.3. Catalytic Mechanism of CTF/CuO–NiO Composite

DSC analysis demonstrated that incorporating 2 wt% of the CTF/CuO–NiO composite into AP lowered its HTD peak temperature by 72.5 °C, underscoring the composite’s superior catalytic efficacy. To elucidate the origin of this enhanced activity, the thermal decomposition mechanism of AP and the role of the catalyst are discussed in Figure 5.

The thermal decomposition of AP typically proceeds via LTD and HTD stages. During LTD, simultaneous gas-phase and solid-phase reactions occur, involving proton (H^+^) transfer from NH_4_^+^ to ClO_4_^−^ to produce NH_3_ and HClO_4_. At these relatively low temperatures, the NH_3_ adsorbed on the AP crystal surface cannot be rapidly oxidized by the decomposition products of HClO_4_. This leads to the accumulation of an NH_3_-rich layer that hinders further electron transfer, causing the LTD reaction to decelerate and eventually cease [44]. In the subsequent HTD stage, HClO_4_ decomposes to release O_2_, which is then transformed into reactive superoxide ions (O_2_^−^) [28,45]. These superoxide ions, along with other gaseous products, facilitate the further oxidation of NH_3_ into final products such as N_2_O, NO_2_, and NO.

The catalytic function of the CTF/CuO–NiO composite can be understood within this framework. As typical semiconductor oxides, CuO and NiO can generate electron-hole pairs (e^−^/h^+^) upon thermal excitation. The thermally excited electrons can promote the reduction of surface-adsorbed oxygen (derived from HClO_4_ decomposition) to form superoxide radicals (O_2_^−^), thereby accelerating the rate-limiting HTD process of AP [46]. Furthermore, the CTF support plays a crucial multifunctional role. Firstly, it acts as a dispersion matrix that effectively inhibits the agglomeration of CuO–NiO nanoparticles, thereby exposing a greater number of accessible catalytic active sites. Secondly, the inherently high specific surface area of CTF aids in the adsorption of gaseous intermediates (e.g., NH_3_ and HClO_4_) released during AP decomposition. This adsorption promotes the separation of these reactive species from the AP crystal surface, alleviating the inhibitory coverage effect observed in LTD and enhancing the overall thermal decomposition kinetics [47].

In summary, the significant catalytic effect of the CTF/CuO–NiO composite on AP thermal decomposition arises from the synergistic combination of the intrinsic activity of well-dispersed CuO–NiO nanoparticles in generating reactive oxygen species, and the favorable structural properties of the CTF support that maximize active site availability and facilitate reactant/intermediate adsorption and transport.

4. Conclusions

To address the catalytic deactivation caused by severe agglomeration of TMOs during AP decomposition, this study proposes an efficient dispersion strategy. By leveraging the high specific surface area and unique covalent framework of CTF, a nitrogen-rich 2D material, a representative bimetallic oxide composite (CuO–NiO) was uniformly dispersed onto the CTF surface through optimized coordination interactions. This approach yielded a novel CTF/CuO–NiO composite with a controlled metal oxide loading of 30 wt%. Comprehensive characterizations via XRD, FESEM, and XPS confirmed the homogeneous anchoring of CuO–NiO nanoparticles throughout the CTF matrix. Catalytic performance evaluation demonstrated that incorporating only 2 wt% of the CTF/CuO–NiO composite into AP (forming a 98:2 AP/composite mixture) induced a significant reduction in the HTD peak temperature of AP, from 404.6 °C to 332.1 °C. Furthermore, the final decomposition temperature was advanced from 405.3 °C to 344.7 °C. This performance substantially surpasses that of pristine CuO–NiO, underscoring a profound synergistic effect imparted by the CTF support. Mechanistic insights indicate that the accelerated decomposition kinetics are primarily attributable to the highly accessible active sites of the dispersed CuO–NiO nanoparticles and the strong adsorption capacity of the CTF for gaseous decomposition intermediates. This work presents a promising design paradigm for advanced AP decomposition catalysts, effectively harmonizing high catalytic efficiency with minimized consumption of transition metals. This work not only provides an efficient strategy for enhancing AP decomposition but also suggests that CTF-supported bimetallic oxides could serve as versatile catalysts for other energetic systems and high-temperature catalytic processes, offering a broader paradigm for the design of robust heterogeneous catalysts.