Oxygen vacancy enriched Cu-WO₃ hierarchical structures for the thermal decomposition of ammonium perchlorate

Abstract

The design and fabrication of efficient catalysts for ammonium perchlorate (AP) decomposition is crucial for the performance of composite solid propellants. Herein, a novel hierarchical structure material of Cu-WO₃ nanowire arrays on a nanoflower flake (Cu-WO₃ NANF) was synthesized by a facile hydrothermal reaction. The negatively charged (001) polar facets of hexagonal WO₃ and the action of Cu²⁺ induced growth were two important factors for the formation of hierarchical structures. The Cu-WO₃ NANF exhibited remarkable catalytic activity for the thermal decomposition of AP. The temperature and activation energy of high temperature AP decomposition were significantly decreased to 378.3 °C and 147.01 kJ mol⁻¹, respectively, which are attributed to the more oxygen vacancies and lower band gap energy of the Cu-WO₃ NANF. Thus, it was excited at a lower heat to produce activated species. Under the strong adsorption of Cu-WO₃ NANF surface Lewis acid, the activated species can react with NH₃ quickly and deeply to produce N₂, N₂O, NO₂ and NO gases, accompanied by a heat release of up to 1118 J g⁻¹. The proposed catalytic mechanism was further corroborated by the in situ TG-MS result. This facile strategy provided a new idea for the design of hierarchical catalysts, and our research opened up a new field for WO₃ applications.

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1. Introduction

Because of the high oxygen content, high enthalpy and ease of availability, ammonium perchlorate (AP) is often used as a significant oxidizer for composite solid propellants.¹ Its thermal decomposition mainly goes through two stages: low temperature decomposition (LTD) and high temperature decomposition (HTD). In the end, AP can be decomposed into gas mixtures of NO, N₂O, NH₃, O₂ and ClO₂ by releasing a lot of heat and thereby producing a powerful pushing force.² Aiming at more powerful solid fuel rockets, researchers have been working to acquire the decomposition of AP at a lower temperature with higher heat release.³ The most effective strategy is to add a thermal combustion catalyst to AP decomposition. Various transition metal oxides were applied to catalyze AP decomposition, such as CeO₂,⁴ Co₃O₄,^5,6 MnO₂,⁷ Fe₂O₃,⁸ CuO,⁹ and ZnO.¹⁰

Tungsten oxide (WO₃) is widely used in many fields due to its narrow band gap, including chemical catalytic oxidation,^11,12 photocatalysis,^13,14 and electrochemical supercapacitors.^15,16 But so far, AP decomposition catalyzed by WO₃ has not been reported. Generally, the catalytic performance of metal oxides is highly sensitive to oxygen vacancies and surface acidic sites.^4,11,17,18 The WO_x structure with a non-stoichiometric ratio often appears during the preparation of WO₃, accompanied by lots of oxygen vacancies.^19,20 It has been reported that the key factor limiting AP decomposition is the accumulation of NH₃ on the AP surface, resulting in a stagnating stage.⁶ The N atom of NH₃ gas has stronger ability to donate electrons, and Lewis acid sites are just good electron acceptors. From this point of view, increasing the amount of Lewis acid sites could facilitate the adsorption of NH₃, and then weaken the accumulation of NH₃, which is critical to the subsequent decomposition reaction. As it proceeds, WO₃ is a typical acidic metal oxide with abundant acid sites on its surface.

Moreover, the structural characteristics and morphologies also affect the catalytic performance of metal oxides.^6,16,21 Compared with other dimensions, one-dimensional WO₃ nanostructures have the advantages of smoother ions and charge transfer, and the hierarchical structures assembled from them have a larger specific surface area, ensuring the crystallinity. Therefore, a useful strategy is expected to achieve superior performance for the design of catalysts with hierarchical structures based on one-dimensional nanostructures.

Some achievements have been made towards the synthesis of hierarchical WO₃ consisting of one-dimensional nanostructures, but these reports have been based on the introduction of growth substrates. For example, Diao et al. prepared a WO₃ hierarchical material by virtue of nickel foam, which provided efficient pathways for electron and mass transfer, showing excellent catalytic activity for the oxygen evolution reaction.¹⁷ Zhang et al. synthesized a series of hierarchical hexagonal WO₃ films on fluorine-doped tin oxide glass, and they exhibited better photocatalytic O₂ generation activity.²² The above hierarchical structure material grown on the substrate can only be applied for a specific reaction. The preparation of substrate-free WO₃ hierarchical structures in a simple way remains a challenge.

According to the theoretical calculations, Chen et al. confirmed that one-dimensional WO₃ structures can be fabricated by the action of monovalent cations in the absence of substrates.²³ Jia et al. proved that WO₃ nanorod arrays could be prepared without substrates. In their work, the nucleation and growth of WO₃ proceeded continuously, leading to a flake structure in the initial growth stage. After that, WO₃ crystal nuclei grew along the [001] direction and eventually developed into a nanorod array.²⁴ On the basis of these studies, we tried to limit the growth of nanorods and make them into nanowires in the later stage, so as to get the three-dimensional hierarchical structure of nanowire arrays grown on nanoflakes. How to limit the growth of nanorods? Our group's previous work shows that high valence metal cations (Fe³⁺ and Cr³⁺) could not only increase the oxygen vacancy concentration, but also effectively prevent the crystallization and growth of WO₃, and finally, WO₃ with different morphologies and crystal structures were obtained.^11,13,25 Therefore, it is theoretically feasible and interesting to prepare a three-dimensional hierarchical WO₃ structure material by means of metal cation induction.

Satisfyingly, after optimizing the experimental conditions, we successfully prepared a Cu-WO₃ hierarchical structure by the Cu²⁺ cation induced method, which was composed of nanowire arrays on nanoflower flakes (NANFs). The morphology, structure, surface chemical state and formation mechanism were carefully studied. The obtained sample showed outstanding catalytic performance for the thermal decomposition of AP. The reaction kinetics and distribution of gaseous products of AP decomposition were also systematically investigated, and a possible decomposition mechanism of AP was finally proposed.

2. Experimental

2.1 Materials

Sodium tungstate (Na₂WO₄·2H₂O, 99.5%) and copper acetate (Cu(CH₃COO)₂·H₂O, 99.0%) were obtained from Tianjin Fuchen Chemical Reagent Factory. Ammonium perchlorate (AP, 99.0%), hydrochloric acid (HCl, 36.0–38.0%) and anhydrous ethanol (CH₃CH₂OH, 99.7%) were obtained from Sinopharm Chemical Reagent Co. Ltd. All the above chemical reagents were used without further purification. Distilled water was used throughout the experiment.

2.2 Synthesis

Typically, Na₂WO₄·2H₂O (3.0 mmol) and Cu(CH₃COO)₂·H₂O (0.3 mmol) were dissolved in 30 mL of distilled water and treated with ultrasound for 15 minutes. The precursor solution appeared clear light blue. Then, the pH of the solution was adjusted to 2.0 by dropping 3.0 M HCl aqueous solution under stirring. After that, the solution was transferred into a 150 mL Teflon-lined autoclave, and then sealed and maintained at 180 °C for an hour. After the reaction, the autoclave was naturally cooled to room temperature. The obtained precipitate was separated by centrifugation, and washed with distilled water and ethanol several times, respectively. The product was dried in a vacuum drying oven at 60 °C and then calcined in a muffle furnace at 300 °C for 2 h. The synthesized sample was denoted as Cu-WO₃ NANF. By comparison, the pure WO₃ nanorod array in a saucer-shape was prepared and denoted as WO₃ NASS, which had the same operational steps except for the addition of Cu(CH₃COO)₂·H₂O to the precursor solution.

2.3 Characterization

The crystal phase of the product was determined by X-ray powder diffraction (XRD, AXS D8 advance diffractometer) using Cu-Kα radiation in the 2θ range of 10–70°. The morphology, elemental distribution and fine crystal structure of the sample were determined by scanning electron microscopy (SEM, JSM-7001F), X-ray energy dispersive spectroscopy (EDS, X-Flash 5010) and transmission electron microscopy (TEM, JEM-2100F), respectively. X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD, Al-Kα radiation) was used to determine surface elemental distribution. The specific surface area was calculated using the Brunauer–Emmett–Teller method (BET, Micromeritics ASAP 2000). The diffuse reflectance UV-Vis absorption spectrum was recorded on a Shimadzu UV-3150 spectrophotometer. The surface acidic properties of the catalysts were characterized using a Fourier transform pyridine infrared adsorption spectrometer (Py-IR, Bruker Tensor 27). An electron paramagnetic resonance spectrometer (EPR, Bruker EMXPLUS 10/12) was used to monitor the existence of oxygen vacancies at room temperature.

2.4 Decomposition of AP

To evaluate the catalytic performance of the sample in the thermal decomposition of AP, the Cu-WO₃ NANF and WO₃ NASS were homogeneously mixed with AP in an agate mortar containing 5 mL of ethanol with a mass ratio of 1: 49, respectively. The mixture was separately ground and then dried at 80 °C for 6 h. Pure AP and the two mixtures were analyzed using a thermogravimetric and differential scanning calorimeter (TG-DSC, Setaram's SETSYS Evolution 16/18). The TG analysis combined with mass spectra (TG-MS) coupling technology was performed on a Perkin Pfeiffer OMNI star mass spectrometer. For each procedure, a sample mass of 10 mg in a platinum sample cup was heated from 25 °C to 600 °C at a heating rate of 10 °C min⁻¹ under an Ar atmosphere.

3. Results and discussion

3.1 Morphological and structural characterization

The morphology of the Cu-WO₃ NANF was first detected by SEM. The low magnification image (Fig. 1a) shows that the flake crosses each other to form a flower. A partial enlargement image is shown in Fig. 1b, and we observed that a nanowire array of about 20 nm diameter is vertically anchored on the two sides of the nanoflower flake. The thickness of the flake was 100 nm and the length of the nanowire array varied greatly. The closer to the edge of the flake, the shorter the length of the array becomes. The longest array was up to 800 nm. To get information about the content and distribution of the Cu element on the Cu-WO₃ NANF, selected area analysis and elemental mapping by SEM-EDS were performed, and the results are presented in Fig. S1.† O, W and Cu elements were observed in the EDS spectrum, and the mass percentage of the doped Cu element was 3.8%. Also, these three elements uniformly distribute on the surface of the Cu-WO₃ NANF without obvious agglomeration (Fig. S1c–e†). For comparison, the morphology of the sample obtained without the addition of Cu²⁺ cation is shown in Fig. S2.† The particle exhibits a saucer-shaped structure (Fig. S2a†), which is wide in the middle and narrow at both ends. It was seen from the side of the high magnification SEM image (Fig. S2b†) that the two sides are exactly symmetrical with the centerline as the axis, and the saucer is self-assembled from nanorod arrays of different lengths with a diameter of 50 nm. Therefore, the introduction of the Cu²⁺ cation does prevent the growth of nanorods in a diameter direction, thus realizing the synthesis of the three-dimensional Cu-WO₃ hierarchical structure of the nanowire array growing on the nanoflower flake.

Fig. 1 (a) Low magnification, (b) high magnification SEM images and (c) XRD pattern of the Cu-WO₃ NANF.

XRD analysis (Fig. 1c) was carried out to identify the crystal structure of the Cu-WO₃ NANF. All peaks were indexed to hexagonal WO₃ (h-WO₃, JCPDS 33-1387), but only several (100) and (001) peaks and their double diffraction peaks are shown. Compared with the XRD pattern of the WO₃ NASS (Fig. S2c†), the crystallinity of the Cu-WO₃ NANF is significantly reduced. In addition, there is no peak corresponding to Cu or Cu compounds (Fig. S3†), which can be attributed to the high dispersion, low crystallinity or small particle size of the Cu species in the sample. It is noticeable that the (001) peak is stronger than the standard data. The intensity ratio of the (001) peak to the (200) peak was about 2.87, which should be 0.5 in the standard data. We presume the reason is that the material grows preferentially along the [001] direction, coming from the contribution of the growth characteristics of nanowires.

TEM and HRTEM analyses were carried out on the Cu-WO₃ NANF sample. Because the hierarchical structure was too compact, ultrasound was performed for 5 minutes before TEM characterization. A fraction of the nanoflower flake with the nanowire array was dropped from the sample. Fig. 2a shows the top view image from which we found that the density of the nanowire array is low at the edge of the flake. HRTEM analysis was used to analyze the marginal area (Fig. 2b), and the two interplanar distances were both 0.646 nm with an angle of 60°. This result is highly consistent with the crystal structure of the (001) plane (inset of Fig. 2b). Under the incident electron along the [001] direction, the selected area electron diffraction (SAED) pattern (Fig. 2c) shows (0–10), (1–10), and (100) planes and/or their equivalent planes, and the sharp diffraction spots reveal that the sample is well single crystalline. Fig. 2d shows the side view of a single flake in the Cu-WO₃ NANF. The nanowire array is almost uniformly distributed on either side of the flake, and the shortest array is located at the very edge. According to the HRTEM image in Fig. 2e, the fringe spacing was 0.328 nm, which matches the (200) plane. The SAED pattern (Fig. 2f) is attributed to the (001) and (200) planes and/or their equivalent planes under the incident electron along the [010] direction. On the basis of the above HRTEM and SAED analyses, we conclude that the growth direction of the array in the Cu-WO₃ NANF is [001]. This result is consistent with the XRD data, in which the (001) peak is very strong, demonstrating that the speculation about the preferential growth along [001] is right. Similar TEM, HRTEM and SAED data of the WO₃ NASS are shown in Fig. S4,† and the difference is that its array is made up of nanorods. The N₂ adsorption–desorption isotherm was used to demonstrate the advantage of the NANF hierarchical structure, as shown in Fig. S5.† The isotherms of both samples are similar to each other and exhibit the type III characteristics of an H3 hysteresis loop in the relative pressure range of 0.3–1.0. The calculated specific surface area of the Cu-WO₃ NANF reached up to 60.4 m² g⁻¹, which is much higher than that of the WO₃ NASS (12.7 m² g⁻¹).

Fig. 2 (a) Top view TEM image of a single flake in the Cu-WO₃ NANF, (b) HRTEM analysis of area b in 2a, (c) the corresponding SAED pattern taken from the red region in 2b, (d) side view of a single flake in the Cu-WO₃ NANF, (e) HRTEM analysis of area e in 2d, and (f) the corresponding SAED pattern taken from the red region in 2e. The inset in 2b shows the crystal structure of the (001) plane.

3.2 Growth mechanism study

We understood the detailed structure and morphology information of the Cu-WO₃ NANF material. But why do particles grow in this way? To gain more information about the growth process, time dependent experiments were carried out in both systems with and without the Cu²⁺ cation. Regardless of the presence or absence of the Cu²⁺ cation, there was no product when the reaction time is less than 25 min. When the reaction time is 30 min, the morphology of both samples was irregular flakes with different sizes. In the system without the Cu²⁺ cation, the size of the original flake was larger (Fig. 3a). With the reaction time increasing to 40 min, the flake crossed each other and became thicker. A lot of short nanorods were found on the surface of the flake (Fig. 3b). After growth for another 10 min, the nanorods became thicker and longer, but the flake became thinner (Fig. 3c). After the 60 min reaction, the product grew into an independent saucer composed of nanorod arrays, and the middle flake became faintly visible (Fig. S2b†). When the Cu²⁺ cation was added to the system, there were only a few flakes in the product after the reaction for 30 min (Fig. 3d). Increasing the time to 40 min and 50 min, the morphology of the product gradually changed from a smooth intersecting flake to an intersecting flake with many short nanowires on the surface (Fig. 3e and f). After another 10 min, the nanowires on the flake grew longer and thinner, and finally, the Cu-WO₃ NANF was formed (Fig. 1a). It can be seen that the Cu²⁺ cation not only restricts the lateral growth of the nanorod but also prevents the growth of the nanoflake at the initial stage, making it thinner and smaller in diameter.

Fig. 3 SEM images of the (a–c) WO₃ NASS and (d–f) Cu-WO₃ NANF at different reaction times, (g) illustration of the formation process of the WO₃ NASS and Cu-WO₃ NANF.

Fig. 3g shows the illustration of the formation process. As seen from the time dependent experiments, the growth process of the WO₃ NASS and Cu-WO₃ NANF is quite similar. It mainly goes through three stages: the formation of the flake structure, the oriented growth of the nanorod (or the nanowire) array along the [001] direction, and an Ostwald ripening process between different facets. The first step is the formation of the flake structure exposed (001) facet. This is because both the two samples have hexagonal crystal structures (Fig. 1c and Fig. S2c†), which usually possess total oxide terminated (001) polar facets (Fig. S6†).^26–28 Because the isoelectric point (IEP) of WO₃ in water is smaller than 1.5, and the pH of the reaction system is 2.0, the (001) facet would concentrate much more negative charges in comparison with other facets.^29–32 Therefore, a lot of WO₃ nuclei with negatively charged (001) polar facets appear and get together quickly to reduce their high surface energy after the nucleation process. Under the electrostatic repulsion effect, these nuclei would mainly get together by the combination of other nonpolar facets, and the initial flake structure is formed. At the same time, the flake crosses each other, further reducing the energy of the whole system. According to the literature, polar facets diminish easily along the crystal growth direction as a result of surface energy minimization.^26,33,34 This means that polar facets possess a higher growth speed than non-polar facets, that is to say, the growth along the [001] direction would be faster. The exposed (001) facet on the flake is divided into many small areas by adsorbed cations (such as Na⁺ and H⁺), and thus the nanorod array is formed with the aggregation of WO₃ particles oriented in the [001] direction. Due to the steric hindrance of the intersecting flake, the edges of the flake have more chances of coming in contact with cations than the middle position. This restricts the growth of the nanorod at the edge and makes it shorter. Because of the limited space, the intersecting flake with the nanorod array falls off and grows into an independent saucer. The high valence Cu²⁺ cation existing in solution will also attach to the (001) polar facet, but it is more likely to adsorb on some equatorial oxygens of h-WO₃ and terminate the growth of the non-(001) crystal plane, according to our group's previous research.^11,13,25 Under the co-action of the three kinds of cation (Cu²⁺, H⁺ and Na⁺), the intersecting flake structure remains unchanged, but the nanorod array that grows on the flake becomes the nanowire array with a small diameter. As the nanowire array gets longer, the size of the underlying flake becomes smaller. This is because growth and decomposition is a dynamic equilibrium process. On (001) facets, growth obviously dominates the equilibrium and they would acquire more tungsten sources from the reaction solution. To maintain the balance of the concentration of the solution, decomposition would dominate the equilibrium on those slower growth facets. Actually, we can understand it as the Ostwald ripening process between different facets.

3.3 Thermal decomposition performance tests

To study the catalytic performance of Cu-WO₃ NANF and WO₃ NASS catalysts for AP decomposition, DSC and TG tests were conducted. As shown in Fig. 4a, pure AP decomposition can be divided into three stages: firstly, an endothermic peak at 250.2 °C is the phase transition of AP; then a weak exothermic peak at 317.3 °C is attributed to the partial decomposition of AP at the low temperature decomposition (LTD) stage; and finally, a main exothermic peak corresponding to the high temperature decomposition (HTD) stage appears at 449.3 °C. Seen from DSC curves, heterophase additives have little effect on phase transition and the LTD temperature. However, the HTD temperature obviously decreases. With the addition of the catalyst, the HTD peak decreases from 449.3 °C to 435.9 °C (WO₃ NASS) and 378.3 °C (Cu-WO₃ NANF), respectively. In addition, Fig. 4a shows that AP decomposition can release much more heat in the presence of a catalyst. The overall heat releases (ΔH) were determined to be 462, 808 and 1118 J g⁻¹ for pure AP, and AP with WO₃ NASS and Cu-WO₃ NANF samples, respectively. The TG curves (Fig. 4b) indicate that the addition of the Cu-WO₃ NANF decreases the ending mass loss temperature by more than 70 °C compared to that of pure AP (450.1 °C), which was 11.6 °C for the WO₃ NASS.

Fig. 4 (a) DSC and (b) TG curves for AP decomposition in the absence or presence of the Cu-WO₃ NANF and WO₃ NASS.

The catalytic decomposition reaction kinetics of pure AP and AP with the catalyst was performed by DSC tests at different heating rates (5, 10, 15 and 20 °C min⁻¹), as shown in Fig. S7.† It can be seen that there is almost no change in the endothermic peak (250.2 °C), but the positions of exothermic peaks change noticeably, especially for the HTD peak. With the decrease of the heating rate, the temperature of HTD drops more sharply, as shown in Table 1. The activation energy was figured out according to the Kissinger formula:

where β is the heating rate, T_p is the temperature at the HTD peak, R is the ideal gas constant, R′ is the linear correlation coefficient, E_a is the apparent activation energy, and A is the pre-exponential factor. The plot of ln(β/T_p²) against 1/T_p is shown in Fig. S8.† The calculated results of the apparent activation energy (E_a) values of pure AP, AP with WO₃ NASS or Cu-WO₃ NANF decomposition are also tabulated in Table 1. As for pure AP, the E_a value of 265.03 kJ mol⁻¹ is close to the value in the literature.¹⁸ A considerable decrease was observed in the catalytic systems, especially in the Cu-WO₃ NANF. Its E_a decreases to 147.01 kJ mol⁻¹, which is lower than that of 226.69 kJ mol⁻¹ for the WO₃ NASS.

Table 1 Summary of the activation energy calculated using the Kissinger formula

Samples	T p (°C)				E a (kJ mol^−1)
	5 °C min^−1	10 °C min^−1	15 °C min^−1	20 °C min^−1
Pure AP	437.67	449.29	451.86	454.56	265.03
WO3 NASS + AP	424.12	435.94	444.16	446.89	226.69
Cu-WO3 NANF + AP	369.48	378.31	392.28	396.72	147.01

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To find the reason for the low activation energy, diffuse reflectance UV-Vis spectra were recorded for the two samples, as shown in Fig. 5. Both the WO₃ NASS and Cu-WO₃ NANF have absorption in the wavelength range of ultraviolet and visible light. Compared with the WO₃ NASS, the light absorption of the Cu-WO₃ NANF shows a red shift, and an obvious absorption tail is present in the visible and near infrared regions. According to the literature,^20,35,36 this could be attributed to the existence of lots of oxygen vacancies. The energy band gap (E_g) is an important characteristic of semiconductor materials, and the Tauc plot (F(R_∞)hν)^mversus incident photon energy (hν) was applied to calculate E_g.¹³ The band gap E_g values were calculated to be 2.46 eV (Cu-WO₃ NANF) and 2.75 eV (WO₃ NASS), respectively. From this, we speculate that the Cu²⁺ cation may enter the WO₃ lattice during preparation to form effective doping, resulting in more oxygen vacancies, smaller band gaps and easier transition of charge carriers.

Fig. 5 Diffuse reflectance UV-Vis absorption spectra of the WO₃ NASS and Cu-WO₃ NANF, and the inset is their corresponding band gap data.

XPS analysis was performed to characterize the surface composition and chemical state. The XPS full spectrum of the Cu-WO₃ NANF is shown in Fig. 6a, showing the presence of W, O and Cu elements. The W 4f peaks of these two samples could be both fitted by using three doublets (Fig. 6b). In the WO₃ NASS sample, the first doublet centered at 35.84 eV and 37.98 eV is assigned to W atoms with an oxidation state of +6 (WO₃). The second doublet centered at 34.67 eV and 36.91 eV can be attributed to the W atoms near the oxygen vacancies (i.e. sub-stoichiometric WO_3−x). While the third doublet centered at 36.36 eV and 38.56 eV was reported to be associated with local variations in the vacuum-level energy caused by a surface oxygen vacancy (denoted as V_O).^37,38 In the Cu-WO₃ NANF sample, the positions of three doublets move to higher binding energies, which were 35.88, 38.04, 34.93, 37.13, 36.47 and 38.62 eV, respectively. This is due to the interaction between WO₃ and Cu, and the electrons migrate from WO₃ to Cu species, resulting in a decrease of the electron cloud density. Calculated from the peak area, the percentage of the oxygen vacancy was 16.8% on the Cu-WO₃ NANF, which is higher than that of the WO₃ NASS (6.7%). Fig. 6c shows the result of the Cu 2p dividing peak, in which 932.58 eV and 952.44 eV are attributed to Cu⁺, while 933.56 eV and 954.35 eV are allocated to Cu²⁺.³⁹ It can be seen from the peak area that Cu mainly exists as Cu⁺. Because there is no information about Cu in XRD and HRTEM, it is difficult to determine the existing form of the Cu species. Combined with the results of EDS and XPS, it may be speculated that Cu²⁺ is mainly adsorbed on the WO₃ surface and reduced to amorphous Cu⁺ species by sub-stoichiometric WO_3−x while restricting the growth of WO₃.²⁰ In addition, some Cu²⁺ or Cu⁺ might go into the WO₃ lattice in the manner of surface doping, realizing the substitution of the W atom lattice. It increases the oxygen vacancy on the surface, and the impurity level is observed in the WO₃ band gap. Thus the Cu-WO₃ NANF can be excited at a lower heat to produce reactive active species: holes (h⁺) and electrons (e⁻). In Fig. 6d, the O 1s of the WO₃ NASS can be deconvoluted into three component peaks: 530.71, 531.84 and 533.10 eV, corresponding to the lattice oxygen, oxygen vacancy and surface adsorbed hydroxyl species, respectively.^11,40 Also, all the peaks shift to higher binding energies in the Cu-WO₃ NANF (530.81, 531.93 and 533.45 eV). This is due to the fact that Cu²⁺ cations can attract electrons from oxygen, leading to a decrease in the electron cloud density and generating a high valence state of oxygen in the Cu-WO₃ NANF. According to the peak area estimation, we found that more oxygen vacancies and surface hydroxyl species are present on the surface of the Cu-WO₃ NANF.

Fig. 6 (a) Full XPS spectra of Cu-WO₃ NANF and WO₃ NASS, and XPS multi-peak separation patterns of (b) W 4f for the Cu-WO₃ NANF and WO₃ NASS, (c) Cu 2p for the Cu-WO₃ NANF, and (d) O1s for the Cu-WO₃ NANF and WO₃ NASS.

An EPR spectrometer was employed to further determine the defective structure, as shown in Fig. S9.† There was a faint EPR signal (g = 2.002) for the WO₃ NASS, while two remarkable symmetrical signals at a g values of 2.002 and 2.170 were detected for the Cu-WO₃ NANF. They correspond to free-electron-trapped oxygen vacancies and Cu²⁺ ions in the structure,^41,42 respectively. The result further proves that the oxygen vacancy concentration increases significantly in the Cu-WO₃ NANF due to the introduction of the Cu species.

3.4 Decomposition mechanism study

To understand the catalytic decomposition mechanism, the TG-MS coupling technology was adopted to analyze the decomposition products of AP. In the presence of the Cu-WO₃ NANF, a mixture of gaseous products was detected (Fig. 7), including N₂O, N₂, NO, O₂, NH₃, NO₂, ClO₂ and H₂O, which is consistent with the previous reports.^10,43 These products are released dramatically in the temperature range of 280–380 °C. For pure AP, the decomposition is mainly concentrated at 290–450 °C (Fig. S10†). In addition, while the release of NO decreases, an obvious peak of N₂ appears (Fig. S11†). The formation of N₂ is due to the deep dissociation of NH₃. The Lewis acid sites of the catalyst surface can act as acceptors to accept the isolated electrons from N atoms, promoting the adsorption and deep oxidation of NH₃.^6,44 The surface acidity of the catalyst was determined by Py-IR, as shown in Fig. 8. It can be seen that the introduction of Cu²⁺ has a great effect on the shape of the adsorption peak. There is a weak signal of pyridine adsorption on the WO₃ NASS, which may be due to the small specific surface area. The Cu-WO₃ NANF has several strong adsorption peaks. Generally, the adsorption peaks at 1540 cm⁻¹ and 1448 cm⁻¹ are considered as fingerprints, characterizing for Brønsted and Lewis acid sites, respectively.¹¹ Obviously, the Lewis acid sites occupy the main position on the surface of the Cu-WO₃ NANF, which is different from the acidic property of the WO₃ NASS based on Brønsted acid sites.⁴⁵ Combined with the structural features of the sample, we consider that some Cu⁺ sites and oxygen vacancies may be the main source of Lewis acid sites on the Cu-WO₃ NANF.

Fig. 7 The in situ TG-MS curves during the thermal decomposition of AP with the Cu-WO₃ NANF.

Fig. 8 The Py-IR spectra of the Cu-WO₃ NANF and WO₃ NASS.

Fig. 9 shows the schematic diagram of the AP decomposition process in the presence of the Cu-WO₃ NANF catalyst. The Cu-WO₃ NANF with a large specific surface area and rich oxygen vacancies is excited to produce charge carriers (e⁻ and h⁺) at a lower heating temperature because of its narrow band gap.²¹ Generally, AP decomposes into NH₃ and HClO₄ at a low temperature. The resulting HClO₄ reacts with e⁻ to form ClO₂ and superoxide radicals (˙O₂⁻ and ˙O₂).^9,21 Under the action of the surface Lewis acid, the active species (h⁺, ˙O₂⁻ and ˙O₂) rapidly oxidized NH₃ to N₂O, NO₂, H₂O and N₂ with less NO release in a short time. The faster the AP decomposes to produce gases, the higher the combustion temperature, which is more favorable for the further oxidation of fuels in a propellant combustion system.

Fig. 9 The schematic diagram of the decomposition process of AP with the Cu-WO₃ NANF.

4. Conclusions

In this work, a Cu-WO₃ NANF hierarchical structure was successfully prepared by a rapid hydrothermal approach. We investigated the effect of Cu²⁺ cation introduction on the morphology, surface properties and growth process of WO₃. The growth mechanism was put forward based on time comparison experiments. Modified by the Cu species (Cu⁺ and Cu²⁺), the Cu-WO₃ NANF had a large specific surface area of 60.4 m² g⁻¹, a low band gap energy of 2.46 eV, rich oxygen vacancies and Lewis acid sites. In the synergy of multiple factors, the thermal decomposition of AP was promoted. The Cu-WO₃ NANF decreased the ending mass loss temperature by more than 70 °C and increased the heat release by 656 J g⁻¹. The excellent activity of the Cu-WO₃ NANF on the thermal decomposition of AP suggested its potential application as a high performing combustion catalyst.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21972158), the Fundamental Research Program of Shanxi Province (201901D211583 and 20210302124625) and the Doctoral Start-up Foundation of Shanxi Province (SQ2019006).

Notes and references

1. B. C. Terry, T. R. Sippel, M. A. Pfeil, I. E. Gunduz and S. F. Son, Removing hydrochloric acid exhaust products from high performance solid rocket propellant using aluminum-lithium alloy, J. Hazard. Mater., 2016, 317, 259–266.
2. S. G. Hosseini, S. Gholami and M. Mahyari, Highly dispersed Ni-Mn bimetallic nanoparticles embedded in 3D nitrogen-doped graphene as an efficient catalyst for the thermal decomposition of ammonium perchlorate, New J. Chem., 2018, 42, 5889–5899.
3. W. D. Zhou, L. Wang, H. J. Yu and X. Xia, Synthesis of ferrocene-modified poly(glycidyl methacrylate) and its burning rate catalytic property, Appl. Organomet. Chem., 2018, 32, e3932.
4. J. Shi, H. X. Wang, Y. Q. Liu, X. B. Ren, H. Z. Sun and B. L. Lv, Rapid microwave-assisted hydrothermal synthesis of CeO₂ octahedra with mixed valence states and their catalytic activity for thermal decomposition of ammonium perchlorate, Inorg. Chem. Front., 2019, 6, 1735–1743.
5. H. Zhou, B. L. Lv, D. Wu and Y. Xu, Synthesis and properties of octahedral Co₃O₄ single-crystalline nanoparticles enclosed by (111) facets, CrystEngComm, 2013, 15, 8337–8344.
6. L. Y. Zhou, S. B. Cao, L. L. Zhang, G. L. Xiang, J. X. Wang, X. F. Zeng and J. F. Chen, Facet effect of Co₃O₄ nanocatalysts on the catalytic decomposition of ammonium perchlorate, J. Hazard. Mater., 2020, 392, 122358.
7. T. Zhang, Y. Guo, C. C. Li, Y. Y. Li, J. C. Li, F. Q. Zhao and H. X. Ma, The effect of LaFeO₃@MnO₂ on the thermal behavior of energetic compounds: An efficient catalyst with core-shell structure, Adv. Powder Technol., 2020, 31, 4510–4516.
8. J. X. Wang, W. C. Zhang, Z. L. Zheng, Y. Gao, K. F. Ma, J. H. Ye and Y. Yang, Enhanced thermal decomposition properties of ammonium perchlorate through addition of 3DOM core-shell Fe₂O₃/Co₃O₄ composite, J. Alloys Compd., 2017, 724, 720–727.
9. Y. H. Hu, Y. L. Yang, R. Q. Fan, K. F. Lin, D. Y. Hao, D. B. Xia and P. Wang, Enhanced thermal decomposition properties and catalytic mechanism of ammonium perchlorate over CuO/MoS₂ composite, Appl. Organomet. Chem., 2019, 33, e5060.
10. G. Tang, Y. W. Wen, A. M. Pang, D. W. Zeng, Y. G. Zhang, S. Q. Tian, B. Shan and C. S. Xie, The atomic origin of high catalytic activity of ZnO nanotetrapods for decomposition of ammonium perchlorate, CrystEngComm, 2014, 16, 570–574.
11. H. X. Wang, M. X. Tang, F. L. Shi, R. M. Ding, L. C. Wang, J. B. Wu, X. K. Li, Z. Liu and B. L. Lv, Amorphous Cr₂WO₆-modified WO₃ nanowires with a large specific surface area and rich Lewis acid sites: A highly efficient catalyst for oxidative desulfurization, ACS Appl. Mater. Interfaces, 2020, 12, 38140–38152.
12. H. X. Wang, X. B. Ren, Z. Liu, D. Jiang and B. L. Lv, Bubble-template synthesis of WO₃·0.5H₂O hollow spheres as a high-activity catalyst for catalytic oxidation of benzyl alcohol to benzaldehyde, CrystEngComm, 2019, 21, 1026–1033.
13. H. X. Wang, C. H. Wang, X. M. Cui, L. Qin, R. M. Ding, L. C. Wang, Z. Liu, Z. F. Zheng and B. L. Lv, Design and facile one-step synthesis of FeWO₄/Fe₂O₃ di-modified WO₃ with super high photocatalytic activity toward degradation of quasi-phenothiazine dyes, Appl. Catal., B, 2018, 221, 169–178.
14. Y. L. Ling and Y. Z. Dai, Direct Z-scheme hierarchical WO₃/BiOBr with enhanced photocatalytic degradation performance under visible light, Appl. Surf. Sci., 2020, 509, 145201.
15. F. Zheng, J. Wang, W. B. Liu, J. M. Zhou, H. Li, Y. Yu, P. F. Hu, W. Yan, Y. Liu and R. Li, Novel diverse-structured h-WO₃ nanoflake arrays as electrode materials for high performance supercapacitors, Electrochim. Acta, 2020, 334, 135641.
16. A. V. Salkar, A. P. Naik, S. V. Bhosale and P. P. Morajkar, Designing a rare DNA-like double helical microfiber superstructure via self-assembly of in situ carbon fiber-encapsulated WO_3−x nanorods as an advanced supercapacitor material, ACS Appl. Mater. Interfaces, 2021, 13, 1288–1300.
17. J. X. Diao, W. Y. Yuan, Y. Qiu, L. F. Cheng and X. H. Guo, A hierarchical oxygen vacancy-rich WO₃ with “nanowire-array-on-nanosheet-array” structure for highly efficient oxygen evolution reaction, J. Mater. Chem. A, 2019, 7, 6730–6739.
18. Y. L. Wang, T. An, N. Yan, Z. F. Yan, B. D. Zhao and F. Q. Zhao, Nanochromates MCr₂O₄ (M = Co, Ni, Cu, Zn): Preparation, characterization and catalytic activity on the thermal decomposition of fine AP and CL-20, ACS Omega, 2020, 5, 327–333.
19. J. F. Al-Sharab, R. K. Sadangi, V. Shukla, S. D. Tse and B. H. Kear, Synthesis of nanostructured tungsten oxide (WO_2.9) fibers and discs, Cryst. Growth Des., 2009, 9, 4680–4684.
20. G. C. Xi, J. H. Ye, Q. Ma, N. Su, H. Bai and C. Wang, In situ growth of metal particles on 3D urchin-like WO₃ nanostructures, J. Am. Chem. Soc., 2012, 134, 6508–6511.
21. J. H. Xu, S. Y. Li, L. H. Tan and B. Kou, Enhanced catalytic activity of mesoporous graphitic carbon nitride on thermal decomposition of ammonium perchlorate via copper oxide modification, Mater. Res. Bull., 2017, 93, 352–360.
22. S. Zhang, S. F. Wu, J. M. Wang, J. P. Jin and T. Y. Peng, Controllable syntheses of hierarchical WO₃ films consisting of orientation-ordered nanorod bundles and their photocatalytic properties, Cryst. Growth Des., 2018, 18, 794–801.
23. L. Chen, S. W. Lam, Q. H. Zeng, R. Amal and A. B. Yu, Effect of cation intercalation on the growth of hexagonal WO₃ nanorods, J. Phys. Chem. C, 2012, 116, 11722–11727.
24. J. Z. Jia, X. C. Liu, R. Mi, N. Liu, Z. W. Xiong, L. Yuan, C. Y. Wang, G. M. Sheng, L. H. Cao, X. W. Zhou and X. D. Liu, Self-assembled pancake-like hexagonal tungsten oxide with ordered mesopores for supercapacitors, J. Mater. Chem. A, 2018, 6, 15330–15339.
25. H. X. Wang, R. M. Ding, C. H. Wang, X. B. Ren, L. C. Wang and B. L. Lv, Iron cation-induced biphase symbiosis of h-WO₃/o-WO₃·0.33H₂O and their crystal phase transition, CrystEngComm, 2017, 19, 3979–3985.
26. X. J. Zhang, A. X. Gu, G. F. Wang, B. Fang, Q. Y. Yan, J. X. Zhu, T. Sun, J. Ma and H. H. Hng, Enhanced electrochemical catalytic activity of new nickel hydroxide nanostructures with (100) facet, CrystEngComm, 2011, 13, 188–192.
27. B. L. Lv, Z. Liu, R. M. Ding, D. Wu and Y. Xu, Fast production of β-Ni(OH)₂ nanostructures with (001) and (100) plane exposure and their electrochemical properties, J. Mater. Chem. A, 2013, 1, 5695–5699.
28. M. L. Huang, Y. Yan, W. H. Feng, S. X. Weng, Z. Y. Zheng, X. Z. Fu and P. Liu, Controllable tuning various ratios of ZnO polar facets by crystal seed-assisted growth and their photocatalytic activity, Cryst. Growth Des., 2014, 14, 2179–2186.
29. Y. Y. Petrov, S. Y. Avvakumova, M. P. Sidorova, L. E. Ermakova and O. M. Merkushev, Electrosurface properties of tungsten(VI) oxide in electrolyte solutions, Colloid J., 2010, 72, 663–668.
30. A. Koliadima, L. A. Pérez-Maqueda and E. Matijević, Monodispersed colloidal salts of tungstosilicic acid, Langmuir, 1997, 13, 3733–3736.
31. M. Anik and T. Cansizoglu, Dissolution kinetics of WO₃ in acidic solutions, J. Appl. Electrochem., 2006, 36, 603–608.
32. G. M. Veith, A. R. Lupini, S. J. Pennycook, A. Villa, L. Prati and N. J. Dudney, Magnetron sputtering of gold nanoparticles onto WO₃ and activated carbon, Catal. Today, 2007, 122, 248–253.
33. H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng and G. Q. Lu, Anatase TiO₂ single crystals with a large percentage of reactive facets, Nature, 2008, 453, 638–641.
34. X. W. Xie, Y. Li, Z. Q. Liu, M. Haruta and W. J. Shen, Low-temperature oxidation of CO catalysed by Co₃O₄ nanorods, Nature, 2009, 458, 746–749.
35. D. B. Migas, V. L. Shaposhnikov, V. N. Rodin and V. E. Borisenko, Tungsten oxides. I. Effects of oxygen vacancies and doping on electronic and optical properties of different phases of WO₃, J. Appl. Phys., 2010, 108, 093713.
36. J. P. Wang, Z. Y. Wang, B. B. Huang, Y. D. Ma, Y. Y. Liu, X. Y. Qin, X. Y. Zhang and Y. Dai, Oxygen vacancy induced band-gap narrowing and enhanced visible light photocatalytic activity of ZnO, ACS Appl. Mater. Interfaces, 2012, 4, 4024–4030.
37. A. P. Shpak, A. M. Korduban, M. M. Medvedskij and V. O. Kandyba, XPS studies of active elements surface of gas sensors based on WO_3−x nanoparticles, J. Electron Spectrosc. Relat. Phenom., 2007, 156, 172–175.
38. C. Navío, S. Vallejos, T. Stoycheva, E. Llobet, X. Correig, R. Snyders, C. Blackman, P. Umek, X. X. Ke, G. V. Tendeloo and C. Bittencourt, Gold clusters on WO₃ nanoneedles grown via AACVD: XPS and TEM studies, Mater. Chem. Phys., 2012, 134, 809–813.
39. P. Mao, L. Y. Qi, X. D. Liu, Y. Liu, Y. Jiao, S. W. Chen and Y. Yang, Synthesis of Cu/Cu₂O hydrides for enhanced removal of iodide from water, J. Hazard. Mater., 2017, 328, 21–28.
40. B. B. Zhang, L. Wang, Y. J. Zhang, Y. Ding and Y. P. Bi, Ultrathin FeOOH nanolayers with abundant oxygen vacancies on BiVO₄ Photoanodes for efficient water oxidation, Angew. Chem., Int. Ed., 2018, 57, 2248–2252.
41. M. Q. Shen, C. X. Li, J. Q. Wang, L. L. Xu, W. L. Wang and J. Wang, New insight into the promotion effect of Cu doped V₂O₅/WO₃-TiO₂ for low temperature NH₃-SCR performance, RSC Adv., 2015, 5, 35155–35165.
42. Y. T. Wang, J. M. Cai, M. Q. Wu, J. H. Chen, W. Y. Zhao, Y. Tian, T. Ding, J. Zhang, Z. Jiang and X. G. Li, Rational construction of oxygen vacancies onto tungsten trioxide to improve visible light photocatalytic water oxidation reaction, Appl. Catal., B, 2018, 239, 398–407.
43. Y. H. Hu, Y. L. Yang, K. F. Lin, D. Y. Hao, L. L. Qiu, D. K. Wang, R. Q. Fan and D. B. Xia, Ammonium perchlorate encapsulating nanothermites as high energetic composites: Preparation, thermal decomposition and combustion properties, Chem. Eng. Sci., 2019, 207, 334–343.
44. K. Shojaee, B. S. Haynes and A. Montoya, The catalytic oxidation of NH₃ on Co₃O₄(110): A theoretical study, Proc. Combust. Inst., 2017, 36, 4365–4373.
45. G. Rodriguez-Gattorno, A. Galano and E. Torres-García, Surface acid-basic properties of WO_x-ZrO₂ and catalytic efficiency in oxidative desulfurization, Appl. Catal., B, 2009, 92, 1–8.

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Footnote

† Electronic supplementary information (ESI) available: The EDS spectrum and mapping of the Cu-WO₃ NANF; the XRD pattern, and SEM, TEM and HRTEM images of the WO₃ NASS; the atomic structure of the WO₃ (001) crystal plane; N₂ adsorption–desorption isotherms and EPR spectra of the Cu-WO₃ NANF and WO₃ NASS; DSC curves at different heating rates, kinetic equation lines, TG-MS curves and the comparison of N₂ and NO release for different reaction systems. See DOI: 10.1039/d1qi01027a

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