The mechanism of nano-CuO and CuFe 2 O 4 catalyzed thermal decomposition of ammonium nitrate

Abstract

A new method was used to prepare nanocomposites (copper oxide, copper iron oxide). The copper oxide and copper iron oxide were characterized by powder X-ray diffraction, transmission electron microscope, and X-ray photoelectron spectra. Copper oxide/graphene oxide nanocatalyst was prepared during which copper oxide nanoparticle was simultaneously anchored on graphene oxide sheets. Copper iron oxide/graphene oxide was also prepared. In this work, the catalytic performance of the synthesized materials on the thermal decomposition of ammonium nitrate was investigated by thermogravimetric differential scanning calorimetry and thermogravimetric mass spectrometry. The results of thermogravimetric differential scanning calorimetry experiments indicated that nanocomposites catalyzed thermal decomposition of ammonium nitrate significantly, especially copper oxide/graphene oxide. The synergistic effect of copper iron oxide was not found. The activation energy of ammonium nitrate mixture was calculated respectively by Kissinger method. The initial temperature, peak temperature, and activation energy were significantly decreased. A new phenomenon, nitrous oxide formed at a very low temperature, was observed by mass spectra. Two stages of thermal decomposition phenomenon of ammonium nitrate catalyzed by copper oxide/graphene oxide were first observed according to thermogravimetric mass spectrometry results. The thermal decomposition mechanism of ammonium nitrate with copper oxide/graphene oxide was proposed according to thermogravimetric differential scanning calorimetry and thermogravimetric mass spectrometry results.

Introduction

Ammonium perchlorate (AP) as the main oxidizer is using in most of the solid rocket propellants currently, but its products of combustion are toxic and not environmentally friendly. To overcome these environmental problems, it’s necessary to investigate the use of the so-called clean burning propellants. Thus, many researchers are going to develop environmentally friendly oxidizers. Ammonium nitrate (AN) has attracted attention many years ago as a potential propellant oxidizer for no HCl presented in production of AN decomposition. However, poor ignitability, hygroscopicity, low energy, low burning rate, and polymorphic transitions around room temperature limited the application of AN in solid rocket propellants. ^{1 –4} Many researchers have attempted to overcome these problems to develop a wide range of practical applications for AN-based propellants. It’s known that transition metal oxides (TMO) were effective additives to increase the burning rate and ignitability of AN propellants. Many researchers have reported that TMO was successfully used in AN-based propellant thermal decomposition. ^{5 –13}

Graphene has attracted a great interest in both fundamental and applied areas. ^14,15 It has excellent properties, especially huge surface area. ^{16 –18} In recent years, many papers have reported graphene or graphene oxide (GO) to catalyze AP, ^{19 –24} especially GO, which can easily absorb polar molecules to form different nanocomposites on carbon nanosheets. ^25,26 However, a few literature has reported graphene- or GO-supported nanocomposites to catalyze AN. Therefore, it attracted great interests in developing GO-supported nanocomposites to catalyze AN thermal decomposition.

Ferric oxide (Fe₂O₃) and copper oxide (CuO) have been studied for excellent catalytic activity in solid propellants. ^{27 –31} Copper iron oxide (CuFe₂O₄) has also been reported in AP catalytic decomposition for its synergistic effect. ^{32 –34} It has been reported that nano-CuO and Fe₂O₃ can catalyze decomposition in AN-based propellants. ^{3,7,8,11–13} However, no literature reported CuFe₂O₄ composite oxides catalyzed AN thermal decomposition. For synergistic effect of composite oxides, thermal decomposition of AN can also be catalyzed by CuFe₂O₄ composite oxides.

Since pure nanoparticles were likely to aggregate, the catalytic activity would be decreased notably. Therefore, a facile and reliable method was to anchor nanocomposites onto a supporter to prevent aggregation. Graphene/GO may be an ideal catalyst supporter. Based on this viewpoint, we expected that graphene/GO can dramatically improve the catalytic activity of nanoparticles as a catalyst supporter. Herein, we report a facile synthesis of CuO and CuFe₂O₄ nanocomposites by an emulsion combustion method. Firstly, nanoparticles were characterized by powder X-ray diffraction (XRD), transmission electron microscope (TEM), and X-ray photoelectron spectra (XPS). The catalytic effect of CuO, CuO/GO, CuFe₂O₄, and CuFe₂O₄/GO nanocomposites on the thermal decomposition of AN was investigated by thermogravimetric differential scanning calorimetry (TG-DSC). Thermal decomposition kinetics of AN mixture were analyzed. The thermal decomposition mechanism was also analyzed by thermogravimetric mass spectrometry (TG-MS). The probable mechanism was also proposed.

Experimental sections

Materials

Copper nitrate (Cu(NO₃)₂·3H₂O; AR), ferric nitrate (Fe(NO₃)₃·9H₂O; AR), and ammonium nitrate (AN; AR) were used without further purification. Sorbitan monooleate (SMO) and diesel were industrial materials. GO was self-made in laboratory from purified natural graphite (230 mesh, Qingdao Zhongtian Company, China) according to the method reported by Hummers and Offeman. ³⁵

Synthesis and characterization of nanocatalyst

Synthesis of CuFe₂O₄ nanocatalyst by new method

Nano-CuO and CuFe₂O₄ were synthesized by a new method. This process of synthesis of CuO and CuFe₂O₄ nanoparticles was not reported in the previous study. It has two steps: (1) preparation of emulsion and (2) calcination of emulsion in furnace. The formation of emulsion was as follows:

Water (H₂O):AN:Cu(NO₃)₂·3H₂O:Fe(NO₃)₃·9H₂O:SMO:diesel = 16:38:9.19:30.81:1.5:4.5

H₂O:AN:Cu(NO₃)₂·3H₂O:SMO:diesel = 16:38:40:1.5:4.5

1. Preparation of emulsion: The procedure of emulsion prepared involved two steps. First, the oxidizer solution (H₂O, AN, Cu(NO₃)₂·3H₂O, and Fe(NO₃)₃·9H₂O) was prepared by dissolving the ingredients in a large stainless steel beaker heated to a temperature of approximately 80°C. The oil phase (SMO and diesel) was then poured into the mixer bowl, when it was heated to 60°C, and maintained. Then, it was mixed with a speed of approximately 200 r/min, and the hot oxidizer solution was slowly poured into the bowl. After pouring, the speed of mixing was accelerated (approximately to 1200 r/min) and continued for a few seconds to achieve the final refinement.

2. Calcination of emulsion in furnace: The emulsion sample was then calcined in furnace at a temperature above 600°C. After heating for 2 h, CuFe₂O₄ nanoparticles were collected. The solid was washed with ethanol and then used for further experiments. The nanoparticles were characterized by XRD and TEM. The preparation method of nano-CuO was similar to that of CuFe₂O₄.

Synthesis of CuFe₂O₄/GO nanocatalyst and sample preparation

The CuFe₂O₄/GO nanocomposite was synthesized in the following procedure. Hundred milligram of GO powder and 5 mL of n-butylamine were dispersed in 100 mL H₂O by sonication, forming stable GO colloid. After 1 h, a dispersion of 200 mg CuFe₂O₄ was sonicated in 100 mL H₂O, and 5 mL ethylenediamine was added to the GO colloid solution. Subsequently, the mixture was heated to 60°C with magnetic stirring for 6 h. The bulk samples can be obtained by centrifugation of the mixture, and then washed with deionized H₂O and ethanol for several times. The CuFe₂O₄/GO nanocomposite was obtained after the precipitate was dried at 60°C under vacuum for 12 h. The preparation method of nano-CuO/GO was similar to that of CuFe₂O₄/GO.

For the catalytic activity studies on CuO, CuFe₂O₄, CuO/GO, and CuFe₂O₄/GO, nanocatalyst was mixed with AN separately in a weight ratio 98:2 (wt%) by blending on a mechanical stirrer. The samples were named as follows:

1#: 2% CuO + 98% AN;

2#: 2% CuFe₂O₄ + 98% AN;

3#: 2% CuO/GO + 98% AN; and

4#: 2% CuFe₂O₄/GO + 98% AN.

XRD, TEM, and XPS analyses

The powder XRD analysis of the samples was carried out with a Bruker D8 (Bruker, Germany) SuperSpeed apparatus operating with a copper (Cu) K_α radiation. The morphologies of the samples were investigated by JEM-2100 TEM (JEOL, Japan). XPS recorded on a Thermo ESCALAB250 (Thermo Fisher Scientific, USA) X-ray photoelectron spectrometer, using aluminum K _α (hυ = 1486.6 eV). The binding energies were calibrated using C1 s peak of contaminant carbon (BE5284.6 eV) as an internal standard.

Thermal decomposition of AN samples

Thermal decomposition experiments of AN samples were carried out under a nitrogen atmosphere with a NETZSCH STA449C (NETZSCH, Germany) instrument. In all experiments, 10 mg sample was loaded in a closed ceramic crucible with a pinhole in the cap and heated up from room temperature to 400°C. Nitrogen was used as a carrier gas with a flow rate of 20 mL/min. The heating rate was 2.5 K/min, 5 K/min, 10 K/min, and 20 K/min.

TG-MS experiments

The TG-MS test was performed with NETZSCH STA449C system and MS (NETZSCHQMS403C, NETZSCH, Germany) system. NETZSCHQMS403C has the following conditions: ionizing electron energy of 70 eV, quartz capillary gas connector, and pressure injection 1000 mbar. Approximately 2 mg of sample was heated from 35°C to 400°C. The heating rate was 10 K/min, and the sample was carried in aluminum oxide crucibles. High-purity argon was used as purge gas with a gas flow rate of 20 mL/min.

Decomposition kinetic analysis

The kinetics of the AN thermal decomposition with and without additive can be determined using the Kissinger method, ³⁶ which uses the peak temperature (T _m) at which the reaction rate is at a maximum as obtained from the DSC curves of samples decomposition in experiments with different heating rates. The equation was as follows

$\text{ln}\left(\frac{\beta }{T_{\text{m}}^2}\right)=\ln \left(\frac{AR}{E_{\text{a}}}\right)-\frac{E_{\text{a}}}{R{T}_{\text{m}}}$

1

where β is the heating rate (K/min), T_m is the temperature, A is the pre-exponential factor, R is the gas constant, and E_a the is activation energy. The slope and intercept from the line plot of against 1/T _m were used to obtain the activation energy and pre-exponential factor, respectively.

Results and discussion

Characteristic of nanocatalysts

The nanocatalysts were characterized by XRD, TEM, and XPS. The XRD results were listed in Figure 1. The diffraction peaks located in the range from 20° to 80° were all ascribed to the characteristic Bragg reflections of CuO (JCPDS 34-0394), α-Fe₂O₃ (JCPDS 33-0664), and CuFe₂O₄ (JCPDS 30-1346), respectively. No obvious diffraction peaks of GO or graphene were observed. It indicated that CuO, α-Fe₂O₃, and CuFe₂O₄ particles were all attached to GO surface. Figure 2 showed the TEM images of as-synthesized CuO, CuFe₂O₄, CuO/GO, and CuFe₂O₄/GO. The results found a slight aggregation of metal oxide particles (CuO and CuFe₂O₄). The average sizes of CuO and CuFe₂O₄ were 83 and 13 nm, respectively. The GO prevented the aggregation of metal oxide particles , and most CuO and CuFe₂O₄ nanoparticles distributed randomly on GO sheets.

Figure 1.

XRD of as-synthesized (a) Fe₂O₃, (b) Fe₂O₃/GO, (c) CuO, (d) CuO/GO, (e) CuFe₂O₄, and (f) CuFe₂O₄/GO composites. XRD: X-ray diffraction; Fe₂O₃: ferric oxide; GO: graphene oxide; CuO: copper oxide; CuFe₂O₄: copper iron oxide.

Figure 2.

TEM images of (a) CuO, (b) CuO/GO, (c) CuFe₂O₄ and (d) CuFe₂O₄/GO composites. TEM: transmission electron microscope; CuO: copper oxide; GO: graphene oxide; CuFe₂O₄: copper iron oxide.

To explore Cu and iron (Fe) elements on the surface of CuFe₂O₄ composites, XPS spectra were measured. Figure 3 depicted the Cu 2p and Fe 2p XPS spectra of different samples. The Cu 2p spectrum of CuO and CuFe₂O₄ samples showed the typical structure. The Cu 2p signal was composed of peak at 933 eV. The result was assigned to the presence of CuO species. The Cu 2p3/2 and Cu 2p1/2 peaks were accompanied by distinct shake-up at about binding energies of 944 eV and 963 eV, which were the characteristic of Cu^{2 +}. The results testified that a part of Cu species were incorporated into the framework in the form of CuFe₂O₄. Comparing CuO with CuFe₂O₄ XPS spectra, we can confirm the Cu species was on the external surface of CuFe₂O₄ catalysts. The XPS spectra of Fe 2p of CuFe₂O₄ catalysts were showed in Figure 3(b). For Fe 2p XPS spectra, each peak can be split. One of the Fe 2p3/2 doublet peaks was at 710.8 eV and the second one was at 713.8 eV. Hence, from XPS results, the peak of Cu2p and Fe2p almost has no change. From XPS results, we can also confirm the Fe species was on the external surface of CuFe₂O₄ catalysts. The species ratio of Cu:Fe = 16.62:33.1. The result was the same with mole ratio of CuFe₂O₄. The XPS results showed that ferric and Cu species were distributed well proportionly on the surface of CuFe₂O₄ sample.

Figure 3.

(a) Cu 2p XPS spectra of samples; (b) Fe 2p XPS spectra of samples. Cu: copper; XPS: X-ray photoelectron spectra; Fe: iron.

The influence on the thermal decomposition of AN with catalysts

The CuO, CuFe₂O₄, CuFe₂O₄/GO, or CuO/GO composite was explored as a catalyst to the thermal decomposition of AN, respectively. The reference has found that GO has little influence on thermal decomposition of AP even at high temperature (>350°C). ^20,21 Hence, GO was not investigated to catalyze thermal decomposition of AN again for AN decomposition temperature lower than 350°C. The performances of CuO, CuFe₂O₄, CuFe₂O₄/GO, and CuO/GO composite on the thermal decomposition of AN were investigated by TG-DSC. The results were showed in Figure 4. The DSC curve of Figure 4(a) revealed that prior to 170°C, appeared endothermic peak was phase transition of AN; after 170°C, thermal decomposition of AN was carried out. The other samples also have the same tendency. As can be seen from Figure 4, it’s shown that, compared with pure AN, decomposition temperature of AN mixture with 2% CuO and CuO/GO shifted to a low temperature notably. Aforementioned papers found that synergistic effect of CuFe₂O₄ presented in catalyzed AP. ^{32 –34} However, the present article found no synergistic effect of CuFe₂O₄. The reference found that Cu-based catalysts have been found to be highly active at relatively low temperatures (<350°C). But Fe-based catalysts were at high temperatures (>300°C). ³⁷ For thermal decomposition temperature of AN about 300°C at the present experimental condition, the Cu activity was stronger than Fe. The XPS results showed that Fe₂O₃ distributed on the surface of CuFe₂O₄ sample was well proportioned. Therefore, we can explain the synergistic effect of CuFe₂O₄ was not found in AN mixture thermal decomposition process.

Figure 4.

DSC curves for: (a) pure AN, (b) AN + CuFe₂O₄/GO, (c) AN + CuO/GO, (d) AN + CuFe₂O₄, and (e) AN + CuO at a heating rate of 10 K/min. DSC: differential scanning calorimetry; AN: ammonium nitrate; CuFe₂O₄: copper iron oxide; GO: graphene oxide; CuO: copper oxide.

When GO was added as a catalyst supporter, decomposition temperature of AN with CuO/GO shifted to a low temperature about 5°C. It means the catalytic activity of catalytic agent increased. However, to our surprise, the peak temperature of AN with CuFe₂O₄/GO shifted to a low temperature about 7°C. It must combine with activation energy to confirm catalysis results.

In order to analyze the catalysis of CuO, CuFe₂O₄, CuO/GO, or CuFe₂O₄/GO to AN thermal decomposition, the activation energy and initial decomposition temperature were listed in Table 1. From Table 1, the initial temperature and peak temperature significantly decreased when CuO/GO was added to AN, and also CuFe₂O₄/GO has excellent catalysis but catalytic activity was lower than CuO/GO. We also have found the activation energy required for the thermal decomposition of pure AN to be 162.7335 kJ/mol. Although the reported activation energy for pure AN decomposition varies from 86.2 kJ/mol to 206.9 kJ/mol, previous researchers in this area agreed that the overall decomposition reaction of AN was described by the first-order reaction kinetics. ³⁹ The activation energy of AN mixture decreased notably, especially in 3#. In other words, the reaction kinetics process of AN with additives perhaps was changed. The reaction kinetics process was not first-order reaction kinetics. Hence, according to initial temperature, peak temperature, and activation energy, we can obtain that additives catalyzed AN thermal decomposition significantly, especially CuO/GO. The difference of CuO/GO and CuFe₂O₄/GO was also observed. The activation energy of CuFe₂O₄ with and without GO almost has no discrepancy. But CuO was very notable. The reason perhaps was that Fe₂O₃ has little effect on AN thermal decomposition. ⁸ Hence, the effective content of catalytic activity materials of CuFe₂O₄/GO was fewer than CuO/GO. In other words, CuO/GO has more remarkable catalytic activity than CuFe₂O₄/GO to AN thermal decomposition.

Table 1.

Thermal properties of AN with additives.^a

Samples	T i (°C)	T p (°C)	E	lnA	R 2
AN + CuO	238.7220	305.0	115.0172	23.1159	0.98194
AN + CuFe2O4	244.9395	307.7	121.3096	24.3741	0.98052
AN + CuO/GO	235.5852	306.1	75.7915	14.4976	0.89875
AN + CuFe2O4/GO	240.9537	304.1	106.4146	21.2467	0.99285
Pure AN	247.3800	311.5	162.7335	58.4185	0.99859

AN: ammonium nitrate; CuO: copper oxide; CuFe₂O₄: copper iron oxide; GO: graphene oxide; T _i: initial decomposition temperature; T _p: peak temperature; E: activation energy; lnA: pre-exponential factor; R ²: correlation coefficient.

^a T _i, and T _p were valued at a heating rate of 10 K/min. T _i was temperature at the 5% mass loss according to Li et al. ³⁸

In the experiment process, we also found the sample mass notably effected the AN thermal decomposition. The results were listed in Online Supplementary Figure S1. Li et al. ³⁸ also found the same results. Koga found that the activation energy of AN decomposition decreased slightly with increasing sample mass in the range of 5–15 mg using TG. ^40,41 Hence, we selected less mass sample to analyze TG-MS in the next TG-MS experiments.

Evolved gas analysis

In order to analyze the decomposition mechanism of AN with additives, samples were chosen to perform the evolved gas analysis. Real-time mass spectral analysis of the evolved gases was also performed. The DSC curves of samples were listed in Online Supplementary Figure S2. The MS results of the evolved gases were listed in Online Supplementary Figures S3 to S6. It showed the ion current curves for mass-to-charge (m/z) ratios = 16, 17, 18, 30, 44, 46, 63. The peaks were assigned to ammonia (NH₃; m/z = 16, 17), H₂O (m/z = 17, 18), nitic oxide (NO; m/z = 30), nitrous oxide (N₂O; m/z = 44), nitric acid (HNO₃; m/z = 30, 46, 63), and nitrogen dioxide (NO₂; m/z = 46).

For dissociation reaction of AN presented even at low temperature, ⁴² the species (m/z = 17) at low temperature appeared, and it was ascribed to NH₃. From Online Supplementary Figures S3 to S6, the curve change showed the presence of NH₃. Then, NH₃ can further undergo pyrolysis but needs high temperature. The HNO₃ (g) (m/z = 63) was also detected, and it can demonstrate dissociation reaction of AN presented at very low temperature. The NH₃ was found at 130°C in isothermal experiments. ⁴³ Liu ⁴⁴ also detected species (m/z = 15, 16, ^·NH, ^·NH₂) at low temperature (100°C). However, Liu thought it presented at very high temperature. ^45,46 During the reaction process, NO₂ (m/z = 46) was also detected. The species (m/z = 44) was N₂O not CO₂ at present experimental condition.

Based on above analysis of the results from the present study, the thermal decomposition of AN with additives has been changed. And it needed to further analyze in the next paragraph.

Catalytic mechanism of the thermal decomposition of AN catalyzed by nanocomposites

Thermal decomposition of AN has been researched widely. ^{1 –4} Many researchers have been proposed thermal decomposition mechanism. Oxley proposed a widely accepted mechanism. Oxley ^47,48 showed that the decomposition mechanism of AN followed two pathways: an ionic reaction and a radical reaction. The ionic reaction occurred at low temperature and has relatively low speed. The radical reaction occurred at high temperature.

AN melted at 169°C and began to decompose as soon as it melted. Reaction (2) was generally accepted as the thermal decomposition initiating reaction, which was an endothermic proton transfer. However, Izato and Miyake found that the molten AN was first dissociated to ${\text{NH}}_4^{+}$ and ${\text{NO}}_3^{-}$ . The ${\text{NO}}_3^{-}$ in molten AN was more stable than the HNO₃. ⁴⁹ When the molten AN was heated from 470 K to 500 K, exothermic decomposition reaction (3) occurred. ⁵⁰ Reaction (3) was more common and was the basis of commercial production of N₂O.

${\text{NH}}_4{\text{NO}}_3\to {\text{NH}}_3+{\text{HNO}}_3$

2

${\text{NH}}_4{\text{NO}}_3\to {\text{NH}}_3+{\text{HNO}}_3\to {\text{NH}}_3+{\text{NO}}_2+\text{OH}$

3

$\text{HO}-{\text{NO}}_2\to ·\text{N}{\text{O}}_2+·\text{OH}$

4

${\text{NH}}_3+\text{OH}·\to {\text{ H}}_2\text{O}+{\text{NH}}_2^{·}$

5

${\text{NH}}_2^{·}+·\text{N}{\text{O}}_2\to {\text{ H}}_2{\text{NO}}^{·}+\text{NO}$

6

${\text{NH}}_2^{·}+·\text{N}{\text{O}}_2\to {\text{ HNO}}_2+{\text{NH}}^{·}$

7

${\text{NH}}^{·}+{\text{NO}}_2^{·}\to \text{HNO}+\text{NO}$

8

$\text{HNO}\to {\text{N}}_2\text{O}+{\text{H}}_2\text{O}$

9

${\text{HNO}}_2\to {\text{H}}_2\text{O}+\text{NO}+{\text{NO}}_2$

10

However, since the activation energy of the homolysis of HNO₃ was very high at about 190 kJ/mol, it was the rate-controlling step. Hence, at high temperature, the ionic mechanism of AN decomposition was overtaken by radical reactions, ⁵¹ with homolysis of HNO₃ being the rate-controlling step ³ as shown in reaction (4). The MS also detected NO₂. The reference also found the energy barrier was lowest in different reaction pathways. ⁵² Then amidogen radical ( ${\text{NH}}_2^{·}$ ) was reacted as showed in reactions (5) and (6). But Cagnina et al. ⁵³ thought NH^· was formed as showed in reaction (7). It was further reacted as showed in reactions (8)–(10). Hence, we thought NO₂^· was easy to be formed at high temperature. When heated above 500 K, reaction (3) was the dominant decomposition pathway. On the basis of reaction (11), the following mechanism has been reported ^{3,47,54 –56}

${\text{HNO}}_3+\text{HX}\to {\text{NO}}_2+{\text{H}}_2\text{O }+\text{X}$

$\left(\text{X}={\text{H}}_3{\text{O}}^{+},{\text{ HNO}}_3,{\text{ NH}}_4^{+}\right)$

${\text{NH}}_3+2{\text{NO}}_2\to {\text{HNO}}_3+{\text{NH}}_2\text{NO}\to {\text{N}}_2+{\text{H}}_2\text{O}$

11

Fe₂O₃ has showed excellent catalysis in NH₃-Selective Catalytic Reduction (SCR). It was widely accepted that it can interact with NH₃. ⁵⁷ The reaction was as follows (reaction (12))

${\text{Fe}}^{3+}+{\text{NH}}_3\to {\text{Fe}}^{2+}+{\text{NH}}_2+{\text{H}}^{+}$

12

CuO has also been found to have same reaction. When NH₃ was absorbed on the nanoparticle surface, the reaction carried out by the formation of CuO and NH₃ and amine (NH₂). ⁵⁸ However, Vargeese and Muralidharan proposed new viewpoint. ⁵⁹ There was no NH₂ formed during the reaction process. If NH₂ formed, the curve of ion current would change. The results of ion current curve of NH₂ were listed in Figure 5. The discrepancy of different samples was little except CuO/GO. At about 190°C, the ion current notably changed, which means NH₂ formed. It can be oxidized easily by oxidizing gas. To our surprise, ion current curve of N₂O was very strange. From Figure 6, we can find that N₂O formed at very low temperature of 3# at about 130°C. Chen et al. ⁶⁰ thought N₂O was formed when NH₃ reacted with NO_x at less than 200°C. According to Andersen et al.’s ⁵⁰ results, the NO₂ must be presented to form N₂O. However, at present condition, NO₂ was very difficult to form below 200°C or little amount. Oxidizing gas was just only HNO₃, it can also be found at very low temperature. ⁴² Another oxidizing species was ${\text{NO}}_3^{-}$ according to Izato and Miyake. ⁴⁹ That is to say, it presented a new thermal decomposition pathway. The possible mechanism of thermal decomposition was ${\text{HNO}}_3/{\text{NO}}_3^{-}$ -oxidized NH₂ directly. The reactivity of HNO₃ is stronger than ${\text{NO}}_3^{-}$ . In this condition, HNO₃ perhaps played an important role. Wang et al. ⁴³ and Xu et al. ⁶¹ also found HNO₃ can oxidize low-molecular-weight materials. According to these results, the reaction to form N₂O was proposed as follows:

${\text{HNO}}_3+{\text{NH}}_2\to {\text{N}}_2\text{O}+{\text{H}}_2\text{O}$

13

Figure 5.

Ion current curve of NH₂ (m/z = 16). NH₂: amine; m/z: mass-to-charge ratio.

Figure 6.

Ion current curve of N₂O (m/z = 44). N₂O: nitrous oxide; m/z: mass-to-charge ratio.

However, from Figure 6, we also found that N₂O was formed in two stages. When temperature was heated to more than 200°C, the NO_x was detected. It means NH₂ can be oxidized by NO_x. Hence, the two stages of N₂O mean the mechanism of AN thermal decomposition was changed. The mechanism of AN thermal decomposition was proposed. At low temperature (<200°C), most likely mechanism was that NH₂ was oxidized by HNO₃ or ${\text{NO}}_3^{-}$ to form N₂O, especially HNO₃. In the process, CuO played an important role. CuO absorbed NH₃ to form NH₂. This process promoted AN disassociation reaction to form large amount of HNO₃. As we all know, acid catalyzed AN thermal decomposition. ⁵⁴ Hence, CuO can catalyze AN thermal decomposition notably, especially CuO/GO. The fundamental reason was that HNO₃ catalyzed AN thermal decomposition at low temperature.

Zhang et al. thought NO played an important role in the NO_x and NH₃ catalytic reaction by CuO. ³⁷ However, Figure 7 showed no change in the ion current of NO. Hence, in the present research, the mechanism of AN thermal decomposition might be different from the results of Zhang et al. ³⁷ and Janssens et al. ⁵⁹

Figure 7.

Ion current curve of NO (m/z = 30). NO: nitric oxide; m/z: mass-to-charge ratio.

Conclusion

In summary, CuO/GO and CuFe₂O₄/GO nanocomposites were successfully obtained by a new emulsion combustion method and they showed an intensive catalytic effect on the thermal decomposition of AN. The catalytic agent was characterized by XRD, TEM, and XPS. The initial temperature, peak temperature, and activation energy were notably decreased when CuO/GO was added to AN. Fe₂O₃ distributed on the surface of CuFe₂O₄, and Fe₂O₃ has little effect on AN thermal decomposition. Therefore, the synergetic effect was not found when CuFe₂O₄/GO was added to AN. An important phenomenon was that N₂O was formed at very low temperature around 140°C during the AN thermal decomposition process. We considered that it was formed by HNO₃ and NH₂ reaction at low temperature. The mechanism of thermal decomposition of AN with CuO/GO was proposed. The formed intermediate of NH₂ was found to be a very important factor to progress the catalytic reaction of AN decomposition.