MgAl₂O₄ nanoparticles: A new low-density additive for accelerating thermal decomposition of ammonium perchlorate

Abstract

Developing low-density additives with high activities is significantly important for thermal decomposition of ammonium perchlorate and the relative technical applications. In this paper, nanoparticles of magnesium aluminate (MgAl₂O₄) with a low density of 3.02–3.17 g cm⁻³ were synthesized by a self-generated template path, in which citric acid was introduced to ensure homogeneous distribution of metal cations at atomic level and serve as the carbon source of the self-generated carbon template. Carbon template was generated by pyrolysis and carbonization of citrate after calcinations at 800 °C in N₂, and then removed with annealing in air at high temperatures, resulting in porous MgAl₂O₄ with a specific surface area as high as 291 m² g⁻¹. Depending on the annealing temperatures, the primary sizes of MgAl₂O₄ nanoparticles that built up the porous structures were adjusted from 6.2 to 24.7 nm. The structural characteristics of MgAl₂O₄ nanoparticles were systematically studied by X-ray diffraction, transmission electron microscopy, Braun-Emmet-Teller analysis, and the catalytic role were evaluated by thermal decomposition of ammonium perchlorate. All samples were indicated to be X-ray-pure MgAl₂O₄, among which MgAl₂O₄ nanoparticles with a largest surface area of 291 m² g⁻¹ and pore volume of 0.24 cm³ g⁻¹ was proved to have an optimum catalytic activity: exothermic peaks of AP thermal decomposition shifted towards the lower temperatures by 78.3 °C for high-temperature decomposition process. The relative kinetic process was also investigated.

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1. Introduction

Catalyzed thermal decomposition of ammonium perchlorate (AP) plays a key role in the fields of aerospace, universal exploration, and satellite launching. This is because that AP is an important oxidizer in solid composite propellant for rockets, and its burning behavior greatly influences the ballistics of the rockets.^1–3 The kernel of catalytic technologies for AP thermal decomposition is the additives, which can realize the highly efficient decomposition of AP at relatively low temperatures. Up to date, numerous additives (e.g.TiO₂, CoO, Fe₂O₃, ZnO, CuO, and NiO)^4–12 for AP thermal decomposition have been studied and some additives with high catalytic properties towards thermal decomposition of AP have been obtained. Nevertheless, these additives suffer from relatively high densities, which may bring the rocket extra load. Therefore, hunting for low-density additives with high catalytic activities is one of the future directions for AP thermal decomposition and the relevant research fields, and possibly to be a hot research topic in both materials and chemistry fields.

By systematically analyzing the substantial literature data, we anticipate that MgAl₂O₄ has a high possibility to be a potential low-density additive with a high catalytic activity for AP thermal decomposition, which is based on the following considerations: (1) Metals Mg and Al are two typical elements with relatively low densities, the atomic numbers of which are 12 and 13, respectively. Additionally, the theoretical density of MgAl₂O₄ is as low as 3.58 g cm⁻³;¹³ (2) MgAl₂O₄ exhibits a very high thermal stability, and its melting point is as high as 2135 °C;¹⁴ (3) MgAl₂O₄ is a substance that has shown a catalytic role in many important reactions,^15,16 like improving the dimerization reaction of cyclohexanone,¹⁵ or as SO_x transfer catalyst for oxidation reaction of sulfur dioxide.¹⁶ However, up to date, there have not been enough evidences to prove that MgAl₂O₄ possesses a high catalytic role towards the thermal decomposition of AP.

Herein, we developed a novel self-generated template method to synthesize MgAl₂O₄ nanoparticles. The specific surface area, porous structure, and primary size of these MgAl₂O₄ nanoparticles were tailored by removing the self-generated carbon template and subsequent high-temperature annealing. All MgAl₂O₄ samples were demonstrated to have an apparent catalytic role towards the thermal decomposition of AP.

2. Experimental details

2.1. Sample preparation

We proposed a self-generated template method to prepare MgAl₂O₄ samples, which can be designed in term of three steps. As illustrated in Fig. 1, the first step involves the formation of a dried gel via a solution chemistry, in which citric acid plays dual roles: Ensuring the homogeneous distribution of metal cations at atomic level, and serving as the carbon source of the self-generated template. The second step is the in situ formation of carbon template, which was generated by pyrolysis and carbonization of citrate after calcinations at 800 °C in N₂. The third step involves the formation of MgAl₂O₄ nanoparticles with porous structure, which was achieved by removing the carbon template in air at high temperatures.

Fig. 1 Scheme that illustrates the self-generated template approach to porous MgAl₂O₄. Step 1: formation of dry gel that contains the homogenous mixed metal citrate precursor after solvent evaporation; step 2: formation of amorphous carbons template around the poorly crystallized MgAl₂O₄ by pyrolysis of citrate; and step 3: formation of well crystallized MgAl₂O₄ with porous structure by removing the template.

The typical synthesis procedure to the samples can be described as follows: Initially, analytical grade chemicals of Mg(NO₃)₂·6H₂O (≥ 99.0%), Al(NO₃)₃·9H₂O (≥ 99.0%), and citric acid (C₆H₈O₇·H₂O) (≥ 99.5%) were used as the starting materials without further purification. Secondly, 0.01 molar Mg(NO₃)₂·6H₂O and 0.02 molar Al(NO₃)₃·9H₂O were dissolved in 100 ml deionized water to form a transparent solution. Then, citric acid was added into the above solution with a molar ratio of citric acid to metal ions at 2 : 1. A lightly yellow transparent solution was formed after heating in 70 °C water-bath for several hours with an increased viscosity, resulting in the formation of yellow gels. The resultant gels were dried at 150 °C for 2 h to obtain a fluffy dry gel, the citrate precursor of MgAl₂O₄. After grinding, the precursor was heated in N₂ at 800 °C for 1 h, which gives rise to a black amorphous mixture. Ultimately, the resulting mixture was annealed in air at 600–900 °C for 1 h to obtain the final samples MgAl₂O₄. The sample before annealing was named as S-P, while that after annealing at 600, 700, 800, and 900 °C were named as S-600, S-700, S-800, and S-900, respectively.

It should be mentioned that the present self-generated template method is superior to the well-established Pechini methods in preparing porous structure, since when using the latter methods,^17–19 the precursor gel were always directly calcined in air, which does not facilitate the formation of carbon template necessary for porous structure.

2.2. Sample characterization

Phase structures of the samples were characterized by X-ray diffraction (XRD) on a Rigaku MiniFlex II X-ray diffractometer using a copper target. Crystallite sizes were calculated using the Scherrer formula as follows:

D = 0.9λ/(β cosθ) (1)

where λ(= 0.15418 nm) is the wavelength of X-ray employed, θ is the diffraction angle of the peak (311) for MgAl₂O₄, and β denotes the half-height width after subtracting the apparatus broadening effect. The chemical compositions of the samples were measured using energy-dispersive X-ray spectroscopy (EDS) on a field emission scanning electron microscopy (FE-SEM) on a JEOL JSM-6700 apparatus. Morphologies of the samples were determined using transmission electron microscopy (TEM) on a JEM-2010 apparatus with an acceleration voltage of 200 kV. Fourier transform infrared spectra (FTIR) of the samples were recorded on a Perkin-Elmer IR spectrophotometer using a KBr pellet technique.

Specific surface areas of the samples were measured by N₂ adsorption isotherms at 77 K on a Micromeritics ASAP 2000 surface area and porosity analyzer after degassing the samples in a flowing nitrogen atmosphere at 120 °C for 12 h in a separate degassing unit attached to the instrument. The actual densities of the samples were measured by a pycnometer method²⁰ using pure water as the immersion liquid.

Catalytic roles of the samples towards thermal decomposition of AP were studied by differential scanning calorimeter (DSC) using DTA404PC thermal analyzer at a heating rate of 15 °C min⁻¹ in N₂ over a temperature range of 30–600 °C. DSC experiments were performed using an open alumina crucible. AP and the samples were mixed at a mass ratio of 98 : 2 for thermal decomposition analyses. A mixture with a total mass of 5 mg was used in all runs.

3. Results and discussion

3.1 Density comparison of the additives for AP thermal decomposition

Table 1 lists the density data of the additives reported in literature for AP thermal decomposition, along with other properties including melting point and catalytic roles. To emphasize the importance and possibility of MgAl₂O₄ as a new additive with low-density and high thermal stability, the relevant physical properties for MgAl₂O₄ spinel is also shown. From Table 1, it can be seen that the additives reported for AP thermal decomposition can be classified as three types: metals, metal oxides, and metal hydrides. For the metallic additives, both Ni and Co exhibit the relatively high thermal stability because of the high melting points (>1400 °C), while their densities are also relatively high, >8.9 g cm⁻³. Although the density of Al is lower than those of all metal oxides, the melting point of Al is only 660 °C. Metal hydride MgH₂ has the lowest density among all listed materials, while its application is limited by its poor thermal stability due to its low melting points of 327 °C. Comparatively, metal oxides can be the promising additives. The relevant additives reported to have a melting point higher than 2000 °C include CeO₂, NiO, and MgO, in which the densities of the former two are higher than 7 g cm⁻³, while that for the latter one is only 3.65 g cm⁻³. From Table 1, it is clear that among all metal oxides materials, MgAl₂O₄ possesses a very low density of 3.58 g cm⁻³ and a high melting point of 2135 °C. Therefore, MgAl₂O₄ is of the greatest possibility to be a new low-density additive for accelerating the thermal decomposition of AP.

Table 1 Comparison of the physical properties of various additives for thermal decomposition of AP

Materials	Density^a (g cm^−3)	Melting point^a (°C)	Catalytic role	Materials	Density (g cm^−3)	Melting point (°C)	Catalytic role
a The data of density and melting point without specific citation are referred to ref. 42.
Ni	8.91	1453	√ (ref. 21)	Cu2O	6.04	1235	√ (ref. 32)
Co	8.90	1494	√ (ref. 22)	ZnO	5.60	1975	√ (ref. 8,9)
CdO	8.15	1540	√ (ref. 23)	α-Fe2O3	5.25	1565	√ (ref. 6,7)
CeO2	7.65	2400	√ (ref. 24)	CoFe2O4	5.20 ^33	1567 ^34	√ (ref. 35)
NiO	7.45	2000	√ (ref. 25)	MnO2	5.08	530	√ (ref. 36)
LaCoO3	7.30 ^26	1740 ^27	√ (ref. 28)	MoO3	4.69	801	√ (ref. 37)
Nd2O3	7.28	1900	√ (ref. 29)	MgO	3.65	2800	√ (ref. 38)
CuO	6.32	1450	√ (ref. 30)	MgAl2O4	3.58 ^13	2135 ^14	?
CoO	6.44	1935	√ (ref. 5)	Al	2.70	660	√ (ref. 39)
Co3O4	6.07	900	√ (ref. 31)	MgH2	1.45	327 ^40	√ (ref. 41)

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In the following, we used a self-generated template strategy to prepare porous MgAl₂O₄ nanoparticles and further examined their catalytic role in thermal decomposition of AP.

3.2 Optimized conditions to the formation of MgAl₂O₄

The formation conditions to MgAl₂O₄ were optimized by tuning the annealing temperatures. Fig. 2 shows XRD patterns of the samples before and after annealing in air. As indicated in Fig. 2, the sample S-P before annealing in air showed several broad and dispersed diffraction peaks, which indicates a poor crystalline character. After annealing at 600 °C in air, the diffraction peaks for S-600 became stronger in intensity. The mean crystallite size of S-600 was calculated to be about 6.2 nm using Scherrer formula for the peak (311). When the annealing temperature increased to 700 °C, the diffraction intensity was further increased, and the peak widths became narrowed. The mean crystalline size of S-700 was calculated to be 9.4 nm. All diffraction peaks for S-700 matched well the standard data for MgAl₂O₄ (JCPDS, No. 73–1959), while no component phases like MgO and Al₂O₃ are detected, which indicates the formation of single-phase spinel MgAl₂O₄. These spinel MgAl₂O₄ nanoparticles were well crystallized, as supported by the following TEM observations. This spinel structure was retained with increasing the annealing temperatures to 900 °C, while the crystalline size increased to 11.5 nm for S-800 and 24.7 nm for S-900.

Fig. 2 X-ray diffraction patterns of the samples: (a) S-P, (b) S-600, (c) S-700, (d) S-800, and (e) S-900.

Since Mg and Al may form spinel structure within a certain compositional range, we carried out a detailed study on the chemical compositions of the samples by elemental mapping and quantitative analysis using EDS. As indicated by the elemental mapping results in Fig. 3 and S1†, both elements Mg and Al are homogeneously distributed in all samples, which further confirms the single-phase character of the samples. The results of quantitative elemental analysis are summarized in Table 2. It can be seen that the atomic ratio of Mg to Al is approximately 1 : 2, closer to the stoichiometry of MgAl₂O₄. Therefore, using the present preparation method, MgAl₂O₄ spinel was formed after annealing at 600 °C, and higher temperature annealing did not significantly affect the compositions of MgAl₂O₄.

Fig. 3 SEM image (a) and X-ray mapping of Mg (b) and Al (c), and EDS spectrum (d) of S-700.

Table 2 Chemical compositions, specific surface area, pore volume, and actual density of the samples

Sample	Chemical compositions^a			Specific surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Actual density (g cm^−3)
	Mg	Al	Mg/Al ratio
a Chemical compositions were obtained by EDS measurements.
S-600	0.142	0.287	1 : 2	150	0.16	3.04
S-700	0.144	0.285	1 : 2	291	0.24	3.02
S-800	0.144	0.285	1 : 2	91	0.15	3.17
S-900	0.143	0.286	1 : 2	40	0.13	3.15

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3.3 Construction of porous MgAl₂O₄

Fig. 4a shows the adsorption–desorption isothermals of the samples. It can be seen that all single-phase MgAl₂O₄ exhibited a porous structure. For S-600, according to the IUPAC classification,⁴³ the isotherms are qualitatively of a type IV shape, which is characteristic of porous structures.^44,45 The hysteresis is of type H2, typical for capillary condensation inside the ink-bottle shaped mesopores. With increasing the annealing temperatures, the isotherms remain a type IV shape, while the hysteresis tends towards a H1 type, which indicates a greater regularity of the cross section along the longitudinal direction of the pores.

Fig. 4 (a) Adsorption–desorption isothermals and (b) BJH adsorption size distribution of the samples.

Generally, pore size distribution can be obtained from both adsorption and desorption branches of the isotherm data. It is well established⁴⁶ that in the case of the isotherm of type IV accompanied by a type H2 hysteresis loop, the pore size distribution derived from the desorption branch is often greatly affected by a tensile strength effect (TSE), which may lead to a narrow pore size distribution, not reflecting the real porous structures. To get insight into the impact of TSE on the pore size distribution, Barrett–Joyner–Halenda (BJH) adsorption and desorption size distribution of all samples were measured, as shown in Fig. 4b and S2†. BJH adsorption pore size distribution (Fig. 4b) indicates that all samples were mesoporous. The pore size distribution became broader as the annealing temperatures increased from 600 to 900 °C, while the maximum pore diameter shifted from 3.6 to 7.3 nm.

Such cases were apparently altered when the pore size distribution is derived from the desorption branch of the isotherm data. As shown in S2†, the pore size distribution for all samples became narrower and the maximum pore diameters were apparently changed: The maximum pore diameter for S-700 increased from 2.6 to 3.2 nm, while that for S-800 decreased from 6.7 to 4.7 nm, which are in accordance with the previous literature data where a narrow distribution of pores centered at 4 nm occurs due to TSE phenomenon.⁴⁶ Therefore, to avoid the impact of TSE, pore size distribution of the samples was derived from the adsorption branch of the isotherm. The total pore volume and specific surface areas for these porous samples are summarized in Table 2. It can be seen that S-700 has a largest total pore volume of 0.24 cm³ g⁻¹ and a highest specific surface areas of 291 m² g⁻¹. All these porous MgAl₂O₄ nanoparticles also showed low densities experimentally measured (Table 2).

Porous structures of all MgAl₂O₄ samples were confirmed by TEM observations. As shown in S3†, all MgAl₂O₄ samples were composed of many small nanoparticles, which are cross-linked with each other to form a porous structure. For S-600, the pores having a diameter of about 4 nm were randomly arranged but distributed homogeneously, as indicated by the light regions in Fig. 5a. The pore walls were constructed by many crystallized MgAl₂O₄ nanoparticles that are randomly oriented and overlapped from each other. These nanoparticles had an average size around 6 nm in good agreement with the mean crystal size of 6.2 nm calculated from XRD peak broadening. It is also noted that there are some amorphous portions without long-range ordering for S-600, which became less with the increase of the annealing temperature.

Fig. 5 HRTEM images of the samples: (a) S-600, (b) S-700, (c) S-800, (d) S-900. Typical pores are sketched in red.

With the annealing temperature increased to 700 °C, as shown in Fig. 5b, the pore structure of S-700 is similar to that of S-600 except that the pore size decreased to about 3 nm and the average size of the nanoparticles on the pore walls increased to about 9 nm. Such a pore structure retained up to an annealing temperature of 900 °C (Fig. 5c, d), while the pore size and crystalline size of nanoparticles were increased. It is consistent with the trend obtained from BJH adsorption size distribution and XRD measurements.

Surface chemistry of all samples was studied by FTIR. As shown in S4†, all MgAl₂O₄ samples showed similar infrared spectra, in which OH⁻ bonding characteristic of surface hydration layer was apparent. Therefore, all MgAl₂O₄ samples were terminated by surface hydration layers, closely related to the porous nature.

3.4 Thermal decomposition of AP in the presence of porous MgAl₂O₄

The heat released during AP decomposition was kinetically measured by DSC in a flowing nitrogen gas. As indicated in Fig. 6, all porous MgAl₂O₄ nanoparticles exhibited an apparent catalytic role towards the thermal decomposition of AP. With no addition of the porous MgAl₂O₄ nanoparticles, two endothermic peaks and one exothermic peak were observed: The first endothermic peak maximized at about 247.7 °C corresponds to a phase transition of AP from the orthorhombic to cubic,⁴⁷ in which no mass changes appeared in the corresponding temperature range. The second exothermic peak located at about 331.9 °C (T_a as marked in Fig. 6), corresponds to the low-temperature decomposition (LTD) process. The third endothermic peak maximized at 437.8 °C (T_b as shown in Fig. 6) is referred to the high-temperature decomposition (HTD) process.⁴⁸ Such cases were apparently altered when the porous MgAl₂O₄ nanoparticles were added to AP. As shown in Fig. 6, with the addition of MgAl₂O₄ nanoparticles, the decomposition temperatures of HTD process were all shifted: For S-600, the peak of HTD stage was exothermic, and the peak temperature was at 387.1 °C. For S-700, the peak temperature of HTD stage moved downwards to 359.5 °C, while the peak became stronger in intensity and narrower in width. For S-800, the peak temperature of HTD stage moved to 401.6 °C and became weaker in intensity. Further increasing the annealing temperature up to 900 °C, the peak temperature of HTD stage of AP was at 431.2 °C, while the HTD peak was changed to an endothermic process. Therefore, S-700 is proved to possess an optimum catalytic role in thermal decomposition of AP.

Fig. 6 DSC curves for AP (a) and that mixed with additives of S-600 (b), S-700 (c), S-800 (d), and S-900 (e).

Having these experimental results in mind, one question appeared: What is the primary reason for the optimum catalytic role of S-700? To answer this question, one has to take into account several factors that may contribute to the thermal decomposition of AP. For instance, particle size effect is popularly considered to be crucial for the catalytic decomposition of AP. For the present work, particle size effect can be first dismissed, since it cannot rationalize why S-700 with a particle size of 9.4 nm offers a higher performance than S-600 (with a smaller particle size of 6.2 nm) or S-800 and S-900 (with larger particle sizes of 11.5 nm and 24.7 nm, respectively). Alternatively, specific surface area and pore volume can be the primary reasons for the catalytic role, since more reactive sites can be generated in porous MgAl₂O₄ nanoparticles necessary for accelerating AP decomposition. Although the interactions between the porous MgAl₂O₄ nanoparticles and AP are still not fully understood due to the complexity of AP decomposition process, we believe that porous MgAl₂O₄ nanoparticles may accelerate ammonia oxidation and ClO₄⁻ species dissociation, the intermediates during the AP decomposition. Among all porous MgAl₂O₄ nanoparticles, S-700 showed a highest surface area and largest pore volume (Table 2), which explains its best performance in accelerating AP decomposition.

In addition to these factors, amorphous portion of the samples may contribute to the catalytic role, since in most cases, amorphous materials can be reactive.^49–51 For the present work, XRD and TEM measurements have demonstrated that the crystallinity of porous nanoparticles increased with the annealing temperature. Namely, the amorphous portion decreased with increasing the annealing temperature from 600 to 900 °C, which does not follow the sequence of the catalytic roles of the corresponding samples. For example, S-700 offers a highest catalytic activity among all samples, in which amorphous material is apparently less than that for S-600. Therefore, it is reasonable that crystalline wall of the porous MgAl₂O₄ nanoparticles is likely the primary part of the catalytic role.

S-700 was chosen as the additive to investigate the kinetic process of the thermal decomposition of AP, focusing on the activation energy of HTD stage. It is well established⁵² that the activation energy for the thermal decomposition of AP can be obtained by the following relationship between decomposition temperature and heating rate:

where β is the heating rate in degrees Celsius per minute, T_p is the peak temperature, R is the ideal gas constant, E_a is the activation energy, and A is the pre-exponential factor. Thus, the activation energy can be calculated from the slope of the line resulting from the plot of ln(β/T_p²) as a function of 1/T_p.

Fig. 7a shows the DSC curves of the mixtures of AP with additives of S-700 at given heating rates. When the heating rate is 5 K min⁻¹, there is a narrow exothermic peak in HTD stage. With an increase in heating rate, the HTD peak was widened and the peak temperature shifted from 354.0 °C at 5 K min⁻¹ to 365.1 °C at 30 K min⁻¹. Fig. 7b shows the experimentally measured ln(β/T_p²) versus 1/T_p with and without additives of S-700. As can be seen, for pure AP, the activation energy of HTD was calculated to be 108 kJ mol⁻¹, which is close to the value previously reported in literature.⁵³ In the presence of S-700, the activation energy of AP decomposition became as large as 483 kJ mol⁻¹. It appears that adding S-700 increases, rather than decreases the activation energy (energy barrier) of the AP decomposition reaction, we may conclude that S-700 is an effective additive, though it may also catalyze the high-temperature decomposition of ammonium perchlorate as anatase does.⁴

Fig. 7 (a) DSC curves of AP mixed with S-700 at given heating rates, and (b) dependence of ln(β/T_p²) on 1/T_p for AP decomposition with and without additive S-700. Scattered points are corresponding to the experimental data, while lines denote the data fit results.

4. Conclusions

This paper reports our findings on a new low-density additive, porous MgAl₂O₄ nanoparticles, towards thermal decomposition of ammonium perchlorate. The main results can be summarized as follows:

(1) MgAl₂O₄ nanoparticles were synthesized by developing a self-generated template method. Using this method, MgAl₂O₄ nanoparticles were achieved to be porous with pore walls being constructed by randomly oriented spinel nanoparticles.

(2) The pore volume of the porous MgAl₂O₄ nanoparticles was tailored from 0.13 to 0.24 cm³ g⁻¹ by removing the self-generated carbon template after high-temperature annealing in air.

(3) When acting as the additives, all porous MgAl₂O₄ nanoparticles catalyzed the high-temperature decomposition of ammonium perchlorate, a key component of solid propellant. An optimum catalyzed AP decomposition was obtained when porous MgAl₂O₄ nanoparticles were controlled to have a largest surface area of 291 m² g⁻¹ and pore volume of 0.24 cm³ g⁻¹.

The findings reported in this work are significantly important, which may help to explore more new additives towards thermal decomposition of ammonium perchlorate.

Acknowledgements

This work was financially supported by NSFC (No. 21025104, 91022018, 50972143), National Basic Research Program of China (2011CBA00501), Directional program CAS (KJCXZ-YW-M10), and FJIRSM fund (No. 2010KL002).

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c1ra00489a/

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