# Preparation of Few-Layered WS2 and Its Thermal Catalysis for Dihydroxylammonium-5,5′-Bistetrazole-1,1′-Diolate

Authors: Yiping Shang; Wu Yang; Yabei Xu; Siru Pan; Huayu Wang; Xiong Cao
Journal: Journal of Nanomaterials
 (2019)

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## Abstract

In this study, few-layered tungsten disulfide (WS2) was prepared using a liquid phase exfoliation (LPE) method, and its thermal catalytic effects on an important kind of energetic salts, dihydroxylammonium-5,5′-bistetrazole-1,1′-diolate (TKX-50), were investigated. Few-layered WS2 nanosheets were obtained successfully from LPE process. And the effects of the catalytic activity of the bulk and few-layered WS2 on the thermal decomposition behavior of TKX-50 were studied by using synchronous thermal analysis (STA). Moreover, the thermal analysis data was analyzed furtherly by using the thermokinetic software AKTS. The results showed the WS2 materials had an intrinsic thermal catalysis performance for TKX-50 thermal decomposition. With the few-layered WS2 added, the initial decomposition temperature and activation energy (Ea) of TKX-50 had been decreased more efficiently. A possible thermal catalysis decomposition mechanism was proposed based on WS2. Two dimensional-layered semiconductor WS2 materials under thermal excitation can promote the primary decomposition of TKX-50 by enhancing the H-transfer progress.


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## Body

## 1. Introduction

Dihydroxylammonium-5,5′-bistetrazole-1,1′-diolate (TKX-50) [1, 2] is a synthetic high-energy-density material which has a high detonation velocity, high density, and low toxicity properties. It is considered as one of the substitutes to conventional energetic materials (EM) [3–7], such as RDX and HMX. It is interesting and valuable to study thermal decomposition behavior and thermodynamics for its potential applications.Transition metal dichalcogenide (TMD) materials [8, 9] have attracted much attention for diverse applications in various fields due to their unique optical and electrical properties [10, 11]. Bulk TMD materials can be exfoliated to form few-layered materials through micromechanical and electrochemical methods. During the exfoliating process, sandwiched tungsten and sulfur layers were peeled off as strong interlayer bonding and weak interlayer van der Waals interactions were broken by solvent action and ultrasound waves. As a novel few-layered material, exfoliated TMD materials show superior properties than raw TMD materials [12–15]. Its catalytic activity would significantly improve, since there would be more exposed edges, better conductivity, and more exposed area on the material’s surface [16, 17]. Additionally, samples after the exfoliation process provide more surface areas and produce an abundance of surface-active sites [18].As a typical TMD material, tungsten disulfide (WS2) is widely used as a catalyst in photochemistry and electrochemistry fields due to its unique physical, optical, electrical, and structural properties [19–22]. However, the applications of WS2 as a catalyzer in the energetic materials have not been reported. It is of great importance and significance to study thermal decomposition performance of TKX-50 by using WS2 as thermal catalyst [23].In this study, few-layered WS2 was prepared by bulk exfoliation via the liquid phase exfoliation (LPE) method. The catalytic properties of raw and as-obtained WS2 for TKX-50 thermal decomposition were studied through STA. Furthermore, activation energy (Ea) was calculated by using thermal kinetic software AKTS. A possible thermal catalysis mechanism of TKX-50 was provided with the WS2 added. This work offers a new way to design and fabricate TKX-50-based composite with high thermal decomposition performances.


## 2. Experiment

### 2.1. Materials

N-Methyl pyrrolidone (NMP, >99.5% (GC)) and bulk WS2 (average 2 μm, 99.9%) were purchased from Shanghai Aladdin Bio-Chem Technology Co. Ethanol was bought from Tianjin Beilian Chemical Co. Ltd. TKX-50 was provided from the Institute of Chemical Materials, CAEP. These four chemicals were used as received without further purification. The few-layered WS2 were prepared by the LPE method. Deionized water was homemade in the lab.


### 2.2. Preparation of Few-Layered WS2

First, 20 mg of the bulk WS2 and 30 mL of NMP were mixed together into a glass vial. The solution was then sonicated in an ultrasonic machine at a frequency of 60 kHz, until the sample became a colloidal suspension. Next, large sediments and NMP were separated in a centrifuge washed by a mixture of deionized water and ethyl alcohol in different ratios. Finally, the few-layered WS2 was obtained in an oven after drying at 50°C. The exfoliating progress by the LPE method is shown in Figure 1(a). Bulk structure for raw WS2 in Figure 1(b) would be exfoliated into a few-layered structure for the processed WS2 in Figure 1(c).Figure 1
(a) Illustration of the exfoliate procedure of WS2. (b) Morphology of bulk WS2 in a scanning electron microscope (SEM) image. (c) Morphology of processed WS2 in a SEM image.
(a)
(b)
(c)


### 2.3. Characterization and Tests

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed to analyze the morphology of exfoliated WS2 in a Libra 200 microscope (ZEISS, German) operated at 200 kV and a field emission scanning electron microscope (Tescan Mira 3 LMH Czech Tescan Company), respectively. For crystal structure analysis, Raman spectra were collected on a Raman spectrophotometer (DXR-smart Raman) and X-ray powder diffraction (XRD) analysis was performed on a DX_2700 X-ray diffractometer (Dandong Haoyuan Co. Ltd.).Thermal gravity analysis (TGA) and differential scanning calorimetry (DSC) data were investigated by a simultaneous thermal analyzer (STA) 449F3 Jupiter (Germany, NETZSCH) at a temperature range of 40-400°C under condition of argon to analyze the WS2’s effects on the decomposition of TKX-50. Bulk and few-layered WS2 was mixed with TKX-50 severally for 1 wt% and 3 wt% at different heating rates (5, 10, 15, and 20 K·min-1) under 99.9% argon atmosphere. Samples were named B1, F1, B3, F3, and TKX-50 presented in Table 1 in order to represent the five samples conveniently. Kinetic analysis of thermal decomposition was carried out by a thermal analysis software AKTS (Setaram Co. Ltd. France).Table 1
Composition of samples (wt%).
Sample
B1
F1
B3
F3
Pure TKX-50
TKX-50
99
99
97
97
100
Bulk WS2
1
—
3
—
—
Few-layered WS2
—
1
—
3
—


## 2.1. Materials

N-Methyl pyrrolidone (NMP, >99.5% (GC)) and bulk WS2 (average 2 μm, 99.9%) were purchased from Shanghai Aladdin Bio-Chem Technology Co. Ethanol was bought from Tianjin Beilian Chemical Co. Ltd. TKX-50 was provided from the Institute of Chemical Materials, CAEP. These four chemicals were used as received without further purification. The few-layered WS2 were prepared by the LPE method. Deionized water was homemade in the lab.


## 2.2. Preparation of Few-Layered WS2

First, 20 mg of the bulk WS2 and 30 mL of NMP were mixed together into a glass vial. The solution was then sonicated in an ultrasonic machine at a frequency of 60 kHz, until the sample became a colloidal suspension. Next, large sediments and NMP were separated in a centrifuge washed by a mixture of deionized water and ethyl alcohol in different ratios. Finally, the few-layered WS2 was obtained in an oven after drying at 50°C. The exfoliating progress by the LPE method is shown in Figure 1(a). Bulk structure for raw WS2 in Figure 1(b) would be exfoliated into a few-layered structure for the processed WS2 in Figure 1(c).Figure 1
(a) Illustration of the exfoliate procedure of WS2. (b) Morphology of bulk WS2 in a scanning electron microscope (SEM) image. (c) Morphology of processed WS2 in a SEM image.
(a)
(b)
(c)


## 2.3. Characterization and Tests

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed to analyze the morphology of exfoliated WS2 in a Libra 200 microscope (ZEISS, German) operated at 200 kV and a field emission scanning electron microscope (Tescan Mira 3 LMH Czech Tescan Company), respectively. For crystal structure analysis, Raman spectra were collected on a Raman spectrophotometer (DXR-smart Raman) and X-ray powder diffraction (XRD) analysis was performed on a DX_2700 X-ray diffractometer (Dandong Haoyuan Co. Ltd.).Thermal gravity analysis (TGA) and differential scanning calorimetry (DSC) data were investigated by a simultaneous thermal analyzer (STA) 449F3 Jupiter (Germany, NETZSCH) at a temperature range of 40-400°C under condition of argon to analyze the WS2’s effects on the decomposition of TKX-50. Bulk and few-layered WS2 was mixed with TKX-50 severally for 1 wt% and 3 wt% at different heating rates (5, 10, 15, and 20 K·min-1) under 99.9% argon atmosphere. Samples were named B1, F1, B3, F3, and TKX-50 presented in Table 1 in order to represent the five samples conveniently. Kinetic analysis of thermal decomposition was carried out by a thermal analysis software AKTS (Setaram Co. Ltd. France).Table 1
Composition of samples (wt%).
Sample
B1
F1
B3
F3
Pure TKX-50
TKX-50
99
99
97
97
100
Bulk WS2
1
—
3
—
—
Few-layered WS2
—
1
—
3
—


## 3. Results and Discussions

### 3.1. Structural Analysis

As shown in Figure2(a), the diffraction pattern of the bulk WS2 has five peaks appearing at 14.35°, 39.45°, 43.95°, 49.65°, and 75.85°, corresponding to (0 0 2), (1 0 3), (1 0 4), (1 0 5), and (0 0 10) of 2H-phase WS2 (JCPDS 04-003-5636). The XRD pattern of the processed WS2 was consistent with the bulk one indicating that each of the two WS2 was in the 2H phase. Raman modes located at 350 and 420 cm-1 in Figure 2(b) further confirmed that the processed WS2 had all the characteristics of the 2H phase WS2, which was consistent with the XRD results.Figure 2
(a) XRD patterns of the processed and raw WS2. (b) Raman spectra of the processed WS2.
(a)
(b)


### 3.2. Morphological Analysis

Morphological results were analyzed from the TEM and SEM images. The TEM image of bulk and treated WS2 shown in Figures 3(a) and 3(b) indicated that the exfoliated WS2 had a lamellar structure with thin layers. Examples of few-layered sheets were observed for the processed WS2 by SEM images shown in Figures 1(c), 3(c), and 3(d), which were smaller and much thinner than the bulk WS2 presented in Figure 1(b). The lateral sizes of the processed sheets varied from 500 nm to 2 μm. The TEM and SEM images indicated that the WS2 was successfully exfoliated into a few-layered nanostructure. As the uniform regular lattice was observed in the HR-TEM image in Figure 4, single crystal for WS2 was shown clearly.Figure 3
(a) TEM image of bulk WS2. (b) TEM image of treated WS2. (c, d) SEM images of the processed sample.
(a)
(b)
(c)
(d)Figure 4
HR-TEM image of bulk WS2.


### 3.3. Thermal Analysis

The heat flow curves shown in Figure5 and the thermogravimetric curves in Figure 6 corresponded to each other. The differential scanning calorimetry (DSC) curves had two decomposition peaks related to the primary decomposition and the secondary decomposition process, respectively, which was supported by literature [24, 25]. When the 1 wt% few-layered WS2 was added, the initial decomposition temperature of the TKX-50 decreased by about 8.3°C, from 237.7 to 229.4°C. Upon adding the 1 wt% bulk WS2, the decomposition temperature of the TKX-50 decreased by 7°C, from 237.7°C to 230.7°C. When the concentration of the WS2 increased, its catalytic effect on the thermal decomposition of the TKX-50 increased. The initial decomposition temperatures of the TKX-50 were observed to be 13.1°C and 10.6°C depressed than the original ones, when the 3% few-layered and bulk samples were added. A summary of DSC results presented in Table 2 includes onset temperatures (T-onset) along with first (T-peak1) and second peak temperatures (T-peak2) for the TKX-50 containing different amounts of bulk and few-layered WS2. The catalytic effects of the WS2 were clearly observed as the T-onset, T-peak1, and T-peak2 decreased. Moreover, it was obvious that the WS2 catalyst content (either bulk or few-layered) in the samples slightly decreased the peak height, as is shown in Figure 5.Figure 5
DSC curves of TKX-50, B1, F1, B3, and F3.Figure 6
TG curves and DTG curves of TKX-50, B1, F1, B3, and F3 heating at 10 K·min-1.Table 2
The onset (T-onset) temperature and first (T-peak1) and second peak temperatures (T-peak2) of TKX-50, B1, F1, B3, and F3 in the DSC curves heating at 10 K·min-1.
Sample
T-onset
T-peak1
T-peak2
TKX-50
237.7
243.7
271.9
B1
230.7
237.4
269.2
F1
229.4
236.8
261.7
B3
227.1
235.7
270.0
F3
224.6
234.7
257.1TG curves and DTG curves presented in Figure6 showed the thermogravimetry of the five samples. It could be seen from the figure that the addition of WS2 led to the advance of the thermal weight loss of TKX-50 and the decrease of the weightlessness rate. The decrease of the weightlessness rate may be due to the fact that the added WS2 will not decompose before 500°C. The advance TG and DTG mass loss curves give more evidence of the catalytic of WS2 in the decomposition of TKX-50.A possible mechanism proposed in this work could be verified through the DTG curves in Figure6. The primary and secondary decompositions of TKX-50 were corresponding to two gas and energy release processes. The gas release could be reflected in the TG and DTG curves when the mass loss happened. The release of NH3 and H2O in the primary decomposition corresponded to the start temperatures and anterior peaks of DTG curves [26]. The release of CO2, N2, NH3, N2O, H2O, etc. in the secondary decomposition related to the secondary peaks of DTG curves. Evidently, the addition of WS2 led the initial temperature and primary maximum values of DTG curves ahead. A summary of the first (T-peak1) and second peak temperatures (T-peak2) of DTG curves is shown in Table 3. The T-peak1 of B1 and B3 at 236.2°C and 233.7°C was higher than F1 and F3 at 234.8°C and 231.0°C and lower than the TKX-50 at 240.5°C, indicating that both two WS2 could boost the primary decomposition of TKX-50 and few-layered WS2 had a better catalytic effect. However, the secondary peak temperatures of DTG curves had no obvious change with the addition of WS2, which could be concluded that the addition of WS2 could boost the primary decomposition of TKX-50, but not the secondary one, and the few-layered WS2 could be even more effective.Table 3
The first (T-peak1) and second peak temperatures (T-peak2) of TKX-50, B1, F1, B3, and F3 in the DTG curves heating at 10 K·min-1.
Sample
T-peak1
T-peak2
TKX-50
240.5
265.1
B1
236.2
265.2
F1
234.8
263.7
B3
233.7
266.1
F3
231.0
256.7


### 3.4. Thermokinetic Analysis

In order to compare the changes ofEa intuitively, two thermokinetic analytic procedures, the Kissinger method [27] (Equation (2)) and the Ozawa method [28] (Equation (3)) based on the Arrhenius equation (Equation (1)), were applied by using the AKTS software. AKTS uses the TG or DSC testing parameters to derive and evaluate the Ea without explicitly guessing a particular form of the reaction model fα [29].
(1)lnβTp2=lnARE−ERTP,(2)lgβ=lnAEaRFα−2.315−0.4567EaRTp,(3)dαdt=Aexp−ERTtfα.In these equations,α is the reaction progress, fα is the model function, A is the preexponential factor, Ea is the activation energy, t is the time, R is the ideal gas constant (8.314 J·mol-1·K-1), and T is the absolute temperature.TheEa analyzed by the Kissinger method was presented in Table 4. Upon analyzing the DSC curves, the Ea of the TKX-50 decreased from 159.867 kJ·mol-1 to 155.617 and 150.316 kJ·mol-1 after adding 1 wt% of raw and few-layered WS2, respectively. Moreover, the Ea of the TKX-50 decreased more to 145.019 and 141.092 kJ·mol-1 after adding 3 wt% of bulk and few-layered WS2, respectively. The changing trend of Ea calculated by the TG curve analysis, as is shown in Table 4, was consistent with that by the DSC curves. However, the values were slightly different. After the addition of 1 wt% of raw and few-layered WS2, the Ea of the TKX-50 decreased from 169.66 kJ·mol-1 to 154.946 and 145.316 kJ·mol-1, respectively. Upon adding 3 wt% bulk and few-layered WS2, the Ea of the TKX-50 decreased to 141.271 kJ·mol-1 and 137.837 kJ·mol-1 which indicated the better catalysis of few-layered WS2.Table 4
The value of activation energy calculated by the Kissinger method.
Sample
TKX-50
B1
F1
B3
F3
Ea by DSC curves (kJ·mol-1)
159.867
155.617
150.316
145.019
141.092
Ea by TG curves (kJ·mol-1)
169.66
154.946
145.316
141.271
137.837TheEa calculated by the Kissinger method indicated that both raw and treated WS2 could reduce the Ea of TKX-50 thermal decomposition, and the few-layered WS2 could abate it more. Moreover, increasing the catalyst content in the material could enhance its catalytic effect.TheEa calculated by the Ozawa method were consistent with the Kissinger method. As shown in Figure 7(a), during the reaction progress from 0.1 to 0.9, the Ea decreased with the 1 wt% addition of the WS2, which demonstrated the catalysis of bulk and few-layered WS2. Besides, F1 and F3 had the lower Ea than B1 and F3 in Figures 7(a) and 7(b). The Ea curves in Figures 7(a) and 7(b) shared similar data trends, which indicated that these five samples likely followed the same reaction mechanism.Figure 7
(a)Ea of TKX-50, B1, and F1 calculated by DSC curves through the Ozawa method. (b) Ea of TKX-50, B3, and F3 calculated by DSC curves through the Ozawa method. (c) Ea of TKX-50, B1, and F1 calculated by TG curves through the Ozawa method. (d) Ea of TKX-50, B3, and F3 calculated by TG curves through the Ozawa method.
(a)
(b)
(c)
(d)TheEa calculated with the TG curves using the Ozawa method in Figures 7(c) and 7(d) showed similar trends with above results, though the relevancy of the Ea curve obtained by the analysis of the TG curve was not as good as that of the DSC curves.The result of thermal and thermokinetic analysis could be concluded as follows. Compared with the pure TKX-50, the addition of few-layered and bulk WS2 promoted its thermal decomposition. The sample in the absence of the WS2 catalyst had the highest initial decomposition temperature and Ea, while the sample containing 3 wt% of the few-layered catalyst had the lowest initial decomposition temperature and Ea. The increase in the concentration of the WS2 further promoted thermal decomposition, and the few-layered catalyst enhanced the decomposition of the TKX-50 more compared to the bulk one.As was seen in Figure8, a possible mechanism was proposed that WS2 materials were excited when the heating energy was greater than the band gap energy and then electron-hole pairs separated [30]. Separated conduction-band electrons and valence band holes would cause fast charge transfer and promote the proton transfer from H atom of NH3OH+ to O atom of bisteterazole in the TKX-50 [31]. NH3OH+ is not stable in high temperature and would decompose into NH3 and H2O with energy release. With more surface area, more active sites, more exposed edges, and better electrical conductivity, few-layered WS2 would increase the proton transfer activity than the raw material. Furthermore, supported by the DTG and DSC curves, the existence of WS2 promoted the decomposition and recombination of NH3OH+ with producing H2O and NH3 which caused gas and initial energy release to advance.Figure 8
Schematic diagram of the thermal decomposition with WS2.


## 3.1. Structural Analysis

As shown in Figure2(a), the diffraction pattern of the bulk WS2 has five peaks appearing at 14.35°, 39.45°, 43.95°, 49.65°, and 75.85°, corresponding to (0 0 2), (1 0 3), (1 0 4), (1 0 5), and (0 0 10) of 2H-phase WS2 (JCPDS 04-003-5636). The XRD pattern of the processed WS2 was consistent with the bulk one indicating that each of the two WS2 was in the 2H phase. Raman modes located at 350 and 420 cm-1 in Figure 2(b) further confirmed that the processed WS2 had all the characteristics of the 2H phase WS2, which was consistent with the XRD results.Figure 2
(a) XRD patterns of the processed and raw WS2. (b) Raman spectra of the processed WS2.
(a)
(b)


## 3.2. Morphological Analysis

Morphological results were analyzed from the TEM and SEM images. The TEM image of bulk and treated WS2 shown in Figures 3(a) and 3(b) indicated that the exfoliated WS2 had a lamellar structure with thin layers. Examples of few-layered sheets were observed for the processed WS2 by SEM images shown in Figures 1(c), 3(c), and 3(d), which were smaller and much thinner than the bulk WS2 presented in Figure 1(b). The lateral sizes of the processed sheets varied from 500 nm to 2 μm. The TEM and SEM images indicated that the WS2 was successfully exfoliated into a few-layered nanostructure. As the uniform regular lattice was observed in the HR-TEM image in Figure 4, single crystal for WS2 was shown clearly.Figure 3
(a) TEM image of bulk WS2. (b) TEM image of treated WS2. (c, d) SEM images of the processed sample.
(a)
(b)
(c)
(d)Figure 4
HR-TEM image of bulk WS2.


## 3.3. Thermal Analysis

The heat flow curves shown in Figure5 and the thermogravimetric curves in Figure 6 corresponded to each other. The differential scanning calorimetry (DSC) curves had two decomposition peaks related to the primary decomposition and the secondary decomposition process, respectively, which was supported by literature [24, 25]. When the 1 wt% few-layered WS2 was added, the initial decomposition temperature of the TKX-50 decreased by about 8.3°C, from 237.7 to 229.4°C. Upon adding the 1 wt% bulk WS2, the decomposition temperature of the TKX-50 decreased by 7°C, from 237.7°C to 230.7°C. When the concentration of the WS2 increased, its catalytic effect on the thermal decomposition of the TKX-50 increased. The initial decomposition temperatures of the TKX-50 were observed to be 13.1°C and 10.6°C depressed than the original ones, when the 3% few-layered and bulk samples were added. A summary of DSC results presented in Table 2 includes onset temperatures (T-onset) along with first (T-peak1) and second peak temperatures (T-peak2) for the TKX-50 containing different amounts of bulk and few-layered WS2. The catalytic effects of the WS2 were clearly observed as the T-onset, T-peak1, and T-peak2 decreased. Moreover, it was obvious that the WS2 catalyst content (either bulk or few-layered) in the samples slightly decreased the peak height, as is shown in Figure 5.Figure 5
DSC curves of TKX-50, B1, F1, B3, and F3.Figure 6
TG curves and DTG curves of TKX-50, B1, F1, B3, and F3 heating at 10 K·min-1.Table 2
The onset (T-onset) temperature and first (T-peak1) and second peak temperatures (T-peak2) of TKX-50, B1, F1, B3, and F3 in the DSC curves heating at 10 K·min-1.
Sample
T-onset
T-peak1
T-peak2
TKX-50
237.7
243.7
271.9
B1
230.7
237.4
269.2
F1
229.4
236.8
261.7
B3
227.1
235.7
270.0
F3
224.6
234.7
257.1TG curves and DTG curves presented in Figure6 showed the thermogravimetry of the five samples. It could be seen from the figure that the addition of WS2 led to the advance of the thermal weight loss of TKX-50 and the decrease of the weightlessness rate. The decrease of the weightlessness rate may be due to the fact that the added WS2 will not decompose before 500°C. The advance TG and DTG mass loss curves give more evidence of the catalytic of WS2 in the decomposition of TKX-50.A possible mechanism proposed in this work could be verified through the DTG curves in Figure6. The primary and secondary decompositions of TKX-50 were corresponding to two gas and energy release processes. The gas release could be reflected in the TG and DTG curves when the mass loss happened. The release of NH3 and H2O in the primary decomposition corresponded to the start temperatures and anterior peaks of DTG curves [26]. The release of CO2, N2, NH3, N2O, H2O, etc. in the secondary decomposition related to the secondary peaks of DTG curves. Evidently, the addition of WS2 led the initial temperature and primary maximum values of DTG curves ahead. A summary of the first (T-peak1) and second peak temperatures (T-peak2) of DTG curves is shown in Table 3. The T-peak1 of B1 and B3 at 236.2°C and 233.7°C was higher than F1 and F3 at 234.8°C and 231.0°C and lower than the TKX-50 at 240.5°C, indicating that both two WS2 could boost the primary decomposition of TKX-50 and few-layered WS2 had a better catalytic effect. However, the secondary peak temperatures of DTG curves had no obvious change with the addition of WS2, which could be concluded that the addition of WS2 could boost the primary decomposition of TKX-50, but not the secondary one, and the few-layered WS2 could be even more effective.Table 3
The first (T-peak1) and second peak temperatures (T-peak2) of TKX-50, B1, F1, B3, and F3 in the DTG curves heating at 10 K·min-1.
Sample
T-peak1
T-peak2
TKX-50
240.5
265.1
B1
236.2
265.2
F1
234.8
263.7
B3
233.7
266.1
F3
231.0
256.7


## 3.4. Thermokinetic Analysis

In order to compare the changes ofEa intuitively, two thermokinetic analytic procedures, the Kissinger method [27] (Equation (2)) and the Ozawa method [28] (Equation (3)) based on the Arrhenius equation (Equation (1)), were applied by using the AKTS software. AKTS uses the TG or DSC testing parameters to derive and evaluate the Ea without explicitly guessing a particular form of the reaction model fα [29].
(1)lnβTp2=lnARE−ERTP,(2)lgβ=lnAEaRFα−2.315−0.4567EaRTp,(3)dαdt=Aexp−ERTtfα.In these equations,α is the reaction progress, fα is the model function, A is the preexponential factor, Ea is the activation energy, t is the time, R is the ideal gas constant (8.314 J·mol-1·K-1), and T is the absolute temperature.TheEa analyzed by the Kissinger method was presented in Table 4. Upon analyzing the DSC curves, the Ea of the TKX-50 decreased from 159.867 kJ·mol-1 to 155.617 and 150.316 kJ·mol-1 after adding 1 wt% of raw and few-layered WS2, respectively. Moreover, the Ea of the TKX-50 decreased more to 145.019 and 141.092 kJ·mol-1 after adding 3 wt% of bulk and few-layered WS2, respectively. The changing trend of Ea calculated by the TG curve analysis, as is shown in Table 4, was consistent with that by the DSC curves. However, the values were slightly different. After the addition of 1 wt% of raw and few-layered WS2, the Ea of the TKX-50 decreased from 169.66 kJ·mol-1 to 154.946 and 145.316 kJ·mol-1, respectively. Upon adding 3 wt% bulk and few-layered WS2, the Ea of the TKX-50 decreased to 141.271 kJ·mol-1 and 137.837 kJ·mol-1 which indicated the better catalysis of few-layered WS2.Table 4
The value of activation energy calculated by the Kissinger method.
Sample
TKX-50
B1
F1
B3
F3
Ea by DSC curves (kJ·mol-1)
159.867
155.617
150.316
145.019
141.092
Ea by TG curves (kJ·mol-1)
169.66
154.946
145.316
141.271
137.837TheEa calculated by the Kissinger method indicated that both raw and treated WS2 could reduce the Ea of TKX-50 thermal decomposition, and the few-layered WS2 could abate it more. Moreover, increasing the catalyst content in the material could enhance its catalytic effect.TheEa calculated by the Ozawa method were consistent with the Kissinger method. As shown in Figure 7(a), during the reaction progress from 0.1 to 0.9, the Ea decreased with the 1 wt% addition of the WS2, which demonstrated the catalysis of bulk and few-layered WS2. Besides, F1 and F3 had the lower Ea than B1 and F3 in Figures 7(a) and 7(b). The Ea curves in Figures 7(a) and 7(b) shared similar data trends, which indicated that these five samples likely followed the same reaction mechanism.Figure 7
(a)Ea of TKX-50, B1, and F1 calculated by DSC curves through the Ozawa method. (b) Ea of TKX-50, B3, and F3 calculated by DSC curves through the Ozawa method. (c) Ea of TKX-50, B1, and F1 calculated by TG curves through the Ozawa method. (d) Ea of TKX-50, B3, and F3 calculated by TG curves through the Ozawa method.
(a)
(b)
(c)
(d)TheEa calculated with the TG curves using the Ozawa method in Figures 7(c) and 7(d) showed similar trends with above results, though the relevancy of the Ea curve obtained by the analysis of the TG curve was not as good as that of the DSC curves.The result of thermal and thermokinetic analysis could be concluded as follows. Compared with the pure TKX-50, the addition of few-layered and bulk WS2 promoted its thermal decomposition. The sample in the absence of the WS2 catalyst had the highest initial decomposition temperature and Ea, while the sample containing 3 wt% of the few-layered catalyst had the lowest initial decomposition temperature and Ea. The increase in the concentration of the WS2 further promoted thermal decomposition, and the few-layered catalyst enhanced the decomposition of the TKX-50 more compared to the bulk one.As was seen in Figure8, a possible mechanism was proposed that WS2 materials were excited when the heating energy was greater than the band gap energy and then electron-hole pairs separated [30]. Separated conduction-band electrons and valence band holes would cause fast charge transfer and promote the proton transfer from H atom of NH3OH+ to O atom of bisteterazole in the TKX-50 [31]. NH3OH+ is not stable in high temperature and would decompose into NH3 and H2O with energy release. With more surface area, more active sites, more exposed edges, and better electrical conductivity, few-layered WS2 would increase the proton transfer activity than the raw material. Furthermore, supported by the DTG and DSC curves, the existence of WS2 promoted the decomposition and recombination of NH3OH+ with producing H2O and NH3 which caused gas and initial energy release to advance.Figure 8
Schematic diagram of the thermal decomposition with WS2.


## 4. Conclusion

In summary, a novel and effective few-layered catalyst for the energetic salt TKX-50 was prepared. A thermal analysis of the TKX-50 mixed with different ratios of bulk and few-layered WS2 was carried out to study the effects of bulk and few-layered WS2 on the thermal decomposition of TKX-50. The results are concluded as follows:
(i)
Raw WS2 tablets were peeled off using the LPE method, and few-layered sheets were found in the exfoliated samples.
(ii)
Both bulk WS2 and processed few-layered WS2 could promote the decomposition of TKX-50 and reduced its activation energy; at the same time, the few-layered WS2 had a better catalytic effect because it exposed more active sites.
(iii)
A possible mechanism was proposed: the WS2 catalyzed thermal decomposition of the TKX-50 because the WS2 improved the ability of proton transfer and promoted the initiation of the TKX-50 decomposition. The exposed active surface of the WS2 few-layered sheets can be more effective in facilitating the decomposition of the TKX-50 for its stronger ability to promote proton transfer, indicating that the LPE method is a promising approach to improve the catalytic performance of WS2



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*Source: 7458645-2019-12-06.xml*