Thermal Behavior and Thermolysis Mechanisms of AP under the

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Thermal Behavior and Thermolysis Mechanisms of AP under the Effects of GO-Doped Complexes of Triaminoguanidine Ting An, Wei He, Shu-Wen Chen, Bei-Lin Zuo, Xiao-Fei Qi, Fengqi Zhao, Yunjun Luo, and Qi-Long Yan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09189 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 3, 2018

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The Journal of Physical Chemistry

Thermal Behavior and Thermolysis Mechanisms of AP under the Effects of GO-Doped Complexes of Triaminoguanidine Ting An1,3 , Wei He2 , Shu-Wen Chen2, Bei-Lin Zuo2, Xiao-Fei Qi2, Feng-Qi Zhao*3, Yunjun Luo1*, Qi-Long Yan2* 1. School of Material Science, Beijing Institute of Technology, Xi’an 710065, China. 2. Science and Technology on Combustion, Internal Flow and Thermo-structure Laboratory, Northwestern Polytechnical University, Xi'an 710072, China; 3. Science and Technology on Combustion and Explosion Laboratory, Xi’an Modern Chemistry Research Institute, Xi’an 710065, China.

Abstract: Several transition metal complexes of triaminoguanidine (TAG) nitrate, TAG-Ni, TAG-Co, GO/TAG-Ni (GT-Ni), GO/TAG-Co (GT-Ni), GO/TAG-Ni (GT-Ni)) were prepared using graphene oxide (GO) as a stabilizer. It has been shown that these complexes are promising energetic catalysts for decomposition of ammonium perchlorate (AP), which was uniformly coated with the mentioned catalysts. The decomposition kinetic parameters and mechanisms of the complexes have been studied by means of DSC/TG techniques. Results show that the presence of GO in such hybrid catalysts is able to stabilize AP before its decomposition, but the strong catalytic effects occur on the HClO4 as the intermediate of the initial decomposition of AP. In the cases of TAG-Co, GT-Co, and GT-Cu, they would make the two-step decomposition of AP into a single step. The heat releases have been greatly increased as well due to a more complete decomposition. More importantly, the thermolysis mechanism of AP could be changed by simply modifying the type of metal ions. Under the effect of these catalysts, the decomposition is more or less shifts to the two dimensional growth of nuclei model, which indicates the metal ions played the key roles as active centers. The decomposition usually starts from the surface of AP where the contact of the catalysts further enhanced surface reactions. The inclusion of GO on the surface of AP crystal may hinder the evaporation of NH3, and therefore the mechanism has been changed to the phase boundary controlled reaction.

1. Introduction It is well-known that ammonium perchlorate (AP) has been widely used in solid composite propellants as a powerful oxidizer. It is also believed that the burning behavior of propellants is highly dependent on the thermal behavior of AP.1,2 Therefore, in the past few decades, massive research works have focused on the thermal properties and decomposition mechanisms of AP.3,4 It has been found that a catalyst could play a significant role in the thermal properties as well as decomposition mechanisms of AP.5,6,7,8 For instance, nano-Ni powders could increase the decomposition rate of the low-temperature stage decomposition of AP, and decrease the peak temperature of the high-temperature decomposition stage.9 However, conventional catalysts including metal oxides, metallic salts, and organometallic compounds are not so energetic. The addition of such additives could have a negative effect on the energy performances of propellants. Besides, the storage performances could also be reduced due to lowered thermal stability caused by the poor compatibility between catalysts and the * Corresponding authors. Tel.: +86(029) 88492781; fax: +86(029) 88492781; E-mail: [email protected]; (Q.-L. Yan); [email protected] (F.-Q. Zhao); [email protected] (Y. Luo). These authors are equally contributed to this work.

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energetic fillers. In order to overcome the abovementioned difficulties, newly developed catalysts are made based on the high energetic ligands with high thermal stability and compatibility. Graphene oxide (GO) modified energetic complexes are typical promising catalysts of this kind, since GO itself is energetic and could readily undergo fast exothermic decomposition.10,11 It has been reported that the intrinsic exothermicity of GO (∼1600 J g−1) is comparable to some hazardous chemicals and explosives.12 In addition, GO could stabilize RDX and some other energetic components, resulting in reduced sensitivity and improved catalytic activity.13,14 Therefore, GO could be considered as an energetic additive itself, but at the same time as a stabilizing agent of the contacted EMs. In our previous study, several GO-doped transition metals (Cu, Co and Ni) complexes of triaminoguanidine (TAG) have been prepared, and their effects on the thermal properties and decomposition mechanisms of RDX has been studied.15 It was found that these materials can not only catalyze the decomposition of RDX by decreasing its apparent activation energy, but also enhance the thermal stability of RDX due to increased thermal conductivity. In addition to decomposition mechanisms, the synergistic effect of GO could further change the decomposition chemical pathways of RDX, since GO also behaves as a catalyst. As for AP, such GO-based energetic coordination polymers have been shown even stronger catalytic effects on its decomposition. However, the catalytic effects on decomposition of AP have been found to be very different from the case of RDX due to the presence of perchlorate acid as condensed intermediate of AP thermolysis. Therefore, this paper is going to show such new findings as a further study. The decomposition of AP is rather complicated even though it is a simple salt, mainly because this molecule consists of four elements with large differences in property.16 It has been found that decomposition of AP generally includes two-step mass loss processes, which should correspond to the thermolysis of NH4ClO4 and the intermediate HClO4. Both two processes are dependent on the experimental conditions. In addition, an additive could also play a significant role in changing the decomposition mechanism of AP.17, 18, 19 Because of that, the inherent chemical mechanisms under the effects of catalysts are still not well understood.20 In the past few decades, various thermal analysis techniques have been widely used to evaluate the thermal properties of AP. It is accepted that those techniques could provide reliable information about the physical properties and decomposition mechanisms.21,22,23 However, previous studies mainly focused on the thermal decomposition temperatures and basic kinetic parameters including apparent activation energy, pre-exponential factors as well as gaseous products. Physical model of decomposition has not been widely studied although it should be determined in order for a complete kinetic description of the overall reaction. Therefore, the objective of this study is to study the catalytic effects of GO-based transition metal complexes on the thermal decomposition of AP. The decomposition kinetics is fundamentally characterized in terms of its thermal properties and kinetic models. 2 / 17


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2 Experiments 2.1 Samples The detailed information about the preparation of GT-M and TAG-M complexes could be found in our recently published paper [15]. Based on these GO-based catalysts, 160 mg AP was dissolved to20 ml acetone first. Then 40 mg TAG-M or GT-M was added to the mixture (M = Ni, Co and Cu), stirring the mixture for 3 h. The AP@GT-M and AP@TAG-M composites were prepared after freeze drying. Caution! It is strongly not recommended to prepare the TAG-Cu by any amateur without strong knowledge of explosives, because it is very unstable in the air under dry condition. It may cause unexpected explosion after drying.

2.2 Experimental techniques The decomposition kinetics of involved samples were calculated based on DSC/TGA data. DSC and TGA were performed on METTLER TG-DSC. The measurements were carried out with 40 ml min-1 N2 ambient purge in the temperature range of 45-500 °C with the sample mass of about 2.0 mg. The heating rates were 5, 10, 15, 20 °C min-1.

3 Results and Discussion 3.1 Thermal decomposition kinetics 3.1.1 Mass loss properties The TG/DTG curves for thermal decomposition of AP with or without catalysts are shown in Figure 1. It is clear that the decomposition of AP generally includes two-step mass loss processes, which should correspond to the thermolysis of NH4ClO4 and the intermediate HClO4 according to the literature.24,25,26 Under the effect of these GO-based catalysts, the initial decomposition temperature (Ti) of the main decomposition peak are all decreased.

80

3 7 8 .8 3 8 3 .3

60 40

-0.2 -0.3

3 9 4 .5

A P /T A G -N i -1 5,1 0,1 5,2 0  C m in

0

-0.4 1 .8 9 % 1 .5 9 %

50

100

150

200

250

300

Tem perature

350 400 / C

450

80

Mass loss/%

-0.1

3 5 6 .6

Deriv mass loss%/min

343.6

100

20

0.0

100

(a)

-0.5 500

-0.1

(b) -0.2

354.4 60

-0.3 368.2 372.7

40

-0.4 -0.5

20 A P /G T -N i -1 5,10,15,20  C m in

0

0.88% 0.78%

50

100

150

3 / 17


200

250

300

Tem perature

350 400 / C

450

-0.6

-0.7 500


0.0

120

Mass loss/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



-0.4

297.1 305.9 A P /T A G -C o

5,10,15,20  C m in

0 50

100

150

5.7%

-1

200

30 9.6 250

300

Tem perature /  C

-0.6

4.3 %

(e) 3 08 .4

60

-0.2 32 8 .5

-0.3

3 3 5.4 3 4 3 .8 A P /G T -C u -1 5,10,15,20  C m in

0

20

100

150

200

2 .6 6 %

250

300

-0.7

A P /G T -C o

5,10,15,20  C m in

0

100

150

-1

200

329.9

250

1.9 8% -0.8 1 .2%

300

350

-0.9 400

0.0

-0.4

(f)

350

-0.1

80 -0.2

378.4

60

-0.3

399.5 40

-0.4 405.6 429.2

20 AP

5,10,15,20  C min

0

1 .8 2 %

50

-0.6

317.1

100

-0.1

20

-0.5

40

50

0.0

40

-0.4

305.6


100 80

-0.3

60

-0.7 400

350

-0.2

297.8

-0.5 400


50

-1

-0.5

2.52% -0.6


20

-0.5

80

Mass loss/%

40

-0.3

-0.1

(d)

Mass loss/%

60


Mass loss/%

-0.2 282.1


(c)

80

0.0

100

-0.1


0.0

100

Mass loss/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.09%

-0.7 100 150 200 250 300 350 400 450 500 Temperature /  C

Figure 1. The TG/DTG curves of AP decomposition with/without catalysts at different heating rates.

It reveals that the catalytic activity of this kind of catalysts on decomposition of AP is significant. In particular, TAG-Ni and GT-Ni could catalyst AP partial decomposition at a much lower temperature, resulting more than two decomposition steps. It means a more complicated catalytic decomposition process occurred, since the thermal stability of TAG-Ni and GT-Ni catalysts are lower than AP.17 Unlike the TAG-Ni and GT-Ni, only a single mass loss peak presented in the cases of TAG-Co, GT-Co and GT-Cu catalyzed samples, which means the second decomposition step of AP was highly promoted and merged with the first one. This result indicates that those GO catalysts could greatly decrease the thermal stability of the intermediate HClO4, which decomposes consecutively with NH4ClO4. The presence of GO could slightly stabilize the TAG-metal complexes and postpone the catalytic effect. It can be observed that the Ti of AP/GT-Co is about 258.8 °C (10 °C min-1), while the Ti of AP/TAG-Co is 224.1 °C. Such stabilization effect is even more obvious for TAG-Cu, which undergoes fast decomposition/deflagration in ambient conditions, and it is extremely difficult to evaluate its catalytic properties. In presence of about 5 wt% of GO in GT-Cu, it becomes very stable in the air. It is important to note that, the condensed decomposition products also play a critical role in the catalytic combustion of propellants.27 As shown in Figure 1 and Table S1, the nickel or copper based catalysts showed higher amount of residues. It is believed that the increased condensed products come from the thermo-decomposition residues of catalysts, mainly metal oxides or chloride. In particular, the thermal decomposition of AP/GT-Co and AP/TAG-Co were more complete. The reason is that GT-Co and TAG-Co has higher catalytic efficiency than the other catalysts on AP, resulting in a more complete decomposition. Moreover, the peak mass loss rates of AP/TAG-Co and AP/GT-Co are higher than that of pristine AP, which further supports the fact that cobalt-containing complexes have better 4 / 17


Page 5 of 17

catalytic effects on thermolysis of AP.

3.1.2 Heat flow properties In order to verify the thermal behaviors of involved AP-based composites, the heat flow properties are also compared as a function of type of catalysts. Figure 2 shows the non-isothermal DSC curves of AP/TAG-Cu complex and pure AP.

344.2

AP/GT-Cu -1

Heat flow/w min-1C-1

246.0

243

-1 -1

433.3

AP 20 Cmin-1

20 C min

Heat flow/ W C min

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


337.4

-1

15 C min

242.5 331.4 -1

10 C min

426.1 15 C min

-1

244.8 409.3 10 C min-1

241.2

244.3

315.2 5 C min

-1

292.4

5 C min-1

241.3

243.1 100

200

300

400

100

Temperature / C

200

300

400

500

Temperature / C

Figure 2. The non-isothermal DSC curves of AP/TAG-Cu composite and pure AP at different heating rates..

Moreover, in the presence of GO, the endothermic peak temperature increased slightly (shown in Figure S1). For pure AP, it is clear that no thermal event was observed before the phase transition near 244.3 °C. Subsequently, AP decomposes with the peak temperature 319.3 and 426.1 °C (10 °C min-1), which is in consistence with the TGA results above. Figure 3 presents typical DSC scans of AP/TAG-Ni and AP/GT-Ni complexes. In both samples, more than two overlapped peaks appeared between 250 and 400 °C, as well as an endothermic with a peak temperature around 240 °C (10 °C min-1). It is well known that this endothermic peak of AP is related to orthorhombic to cubic form transition. In comparison to pure AP, the endothermic peak temperature of AP/TAG-M was slightly decreased due to reducing effects of the catalysts on electrostatic interaction between NH4- and ClO4-. It means that, at a certain extent, the crystallographic transition temperature of AP could be changed by the additives. Also, one may notice that, before the endothermic peaks (Figure 3), there are small exothermic peaks due to decomposition of TAG-Ni and GT-Ni, which further supports the corresponding findings from TGA analyses (Figures 1a and b).

5 / 17



360.6

AP/TAG-Ni

min-1 243.3 344.7

10

min-1

min -1）

349.5

20

Heat flow/（

243.9

15

374.1

AP/GT-Ni

min-1

min -1 ）

20

Heat flow/（

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 17

15

min-1 245.4 370.4 min-1 246.3 355.0 -1

10

min

243.3

244.3

336.2 5

min-1

5

242.9 100

200

344.3

min-1 243.1

300

Temperature/

400

500

100

200

300

Temperature/

400

500

Figure 3. The non-isothermal DSC curves of AP/TAG-Ni and AP/GT-Ni composites at different heating rates.

It is believed that increased endothermic peak temperature is caused by the increased structure integrity of AP crystals by layered GO doping. It is believed that improved structure-stability to the heat is caused by the high heat conductivity and dispersion by GO nanosheets. In presence of TAG-M complexes as catalysts, the decomposition peak temperature (Tp) of AP decreased dramatically, which are in consistence with TGA results. It is also obvious that the use of the involved catalysts would result in the sharper exothermic peaks. Under the effects of the TAG-Ni and GT-Ni catalysts, the decomposition mechanism of AP has been largely changed, since they decomposes before the crystal transition of AP, where the condensed decomposition products would interact with the ions of AP.28 In comparison to TAG-Ni, the presence of GO would significant postpone the initial decomposition of AP, resulting in faster final reaction steps (see sharper peaks for AP/GT-Ni in Figure 3). It could be seen that, under the effects of TAG-Co and GT-Co, the second exothermic peak of pure AP at 409.3 °C (10 °C min-1) was merged with the first exothermic peak. It is well known that the second exothermic decomposition process corresponds to the heterogeneous decomposition of deprotonated HClO4 vapor on the solid surface of AP crystals

[28]

. The single exothermic peak means that cobalt based materials

are promising catalysts for the decomposition of HClO4.

6 / 17


Page 7 of 17

312.8

min-1

-1 min ）

20

244.3

327.5

AP/GT-Co

min-1）

AP/TAG-Co

309.1

20

min-1 243.3 319.8

min

-1

Heat flow/（

15

Heat flow/（

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


242.9 299.0 10

min-1

min-1

15

242.9 307.6 10

min

-1

242.4

242.1

298.9

284.0 5

min

-1

5

min

-1

241.1

240.6 100

200

300

Temperature/

400

100

200

300

Temperature/

400

Figure 4. The non-isothermal DSC curves of AP/TAG-Co and AP/GT-Co composite at different heating rates.

The GO-based energetic catalysts are a special group of materials that have stabilization effect on the RDX before its decomposition, whereas the catalytic effect occurs during thermolysis of RDX molecules. It is also the case for AP according to the literature as well as this research. As shown in Table 1, the carbon nanomaterials including carbon nanotubes (CNTs) and graphene oxide have been widely used as the catalysts carriers. The resulted composite materials mostly have stabilization effect on decomposition of AP. In comparison to the pure AP, the nano-catalysts carried/doped by CNTs, could increase the decomposition peak temperature, whereas the conventional nano-metals such as n-Cu would make AP decompose at a lower temperature. The first decomposition peak temperature of AP was improved as high as 52.5 °C by using pure CNTs, while it is in the range of 1.3-23.1 °C in the cases of GO-based complexes. More importantly, the heat release was sometime greatly increased as a result of a more complete catalytic reaction. For instance, the AP/NiO-CNTs composite is featured with a higher heat release (1226 J g-1) and higher initial decomposition temperature (342.2 °C). However, as a 3-D carbon nanomaterial, the nano-diamond has no such stabilization effect on the energetic molecules. Regarding to the heat releases from catalytic thermolysis of AP, GO-based materials are superior than those of the CNTs-based ones. Moreover, a more rapid decomposition rate of AP is observed by using GO-based catalysts in comparison to CNT ones. The typical evidence is that the decomposition duration of AP/GT-Ni is shorter than that of AP/CNTs-Ni. Table 1. DSC Parameters of catalytic decomposition of AP using carbon nanomaterials based additives Samples

To / °C

Tp / °C

∆H/J g-1

∆T/°C

Tp-To/° C

7 / 17


P-M

Refs.


AP

268.2

327.0/453.4

309

0

58.8

AP/CNTs

278.2

370.5/426.8

685

+52.5

92.3

AP/CNTs-Ni

289.6

351.9/none

1066 a

+24.9

62.3

AP

324.3

332.0/436

590

0

7.7

AP/n-Cu

278.5

297/332

1420

-35.0

18.5

AP/n-Cu-UDD

281.3

287/317

1400

-45.0

5.7

AP

312.5

327.0/453.4

309

0

14.5

AP/CNTs (1.3%)

362.3

370.5/426.8

709

+33.5

8.2

341.3

367.8/none

1199 a

+40.8

AP/NiO-CNTs (1.3%)

(1:2)

AP/NiO-CNTs (1.3%)

(1:6)

342.2

365.6/none

1226 a

Page 8 of 17

Chemical liquid deposition

Y. Liu, et al., 200828

Liquid reductive deposition

X.Q. Shi, et al., 200629

Co-precipitati on, the products NiO, 32 nm;

26.5

+38.6 23.4

AP

282.5

308.9/409.3

1088a

0

26.4

AP/TAG-Ni (3.2%)

281.5

308.3/344.7 /377.3

1398a

-0.6

27.8

+5.6

12.0

AP/GT-Ni (3.2%)

301.9

313.9/355.0

1367a

AP/TAG-Co (3.2%)

276.6

299.0/none

1337

-9.9

22.4

AP/GT-Co (3.2%)

282.1

307.6/none

1492

+1.3

25.5

AP/GT-Cu (3.2%)

293.4

331.4/none

1431

+23.1

38.0

NiO/CNTs (1:2), 15 nm; NiO/CNTs (1:6), 10 nm

Hydrothermal reaction and mixed by sonication

J.X. Liu, et al., 200830

This paper (10 °C min-1)

Notes: To ,the onset temperature; Tp , the peak temperature; ∆H, heat releases; ∆T, temperature differences between the first exothermic peaks with and without additives; P-M, preparation method of the catalysts; UDD, nano-diamonds; Superscript: a, the heat is for two peaks after exclusion of the heat release from the catalysts, whereas the others correspond to the first exothermic peaks.

3.2 Thermal decomposition kinetics 3.2.1 Kinetic parameters obtained by Kissinger method The kinetic parameters of the involved materials calculated by Kissinger method are listed in Table 2. As mentioned before, there are two exothermic peaks during the decomposition of pristine AP. As for the first exothermic peak, the activation energy (Ea) of AP determined by TGA data is about 102.6 kJ mol-1 with log A of 3.38 s-1, while the Ea for the second exothermic peak is 101.2 kJ mol-1 with log A of 1.90 s-1. In presence of involved GO-based catalysts, the values of Ea have been increased except AP/GT-Cu. In particular, the Ea of the first exothermic peak for AP under the effects of TAG-Ni, TAG-Co, GT-Ni, and GT-Co are 119.9, 146.1, 114.9 and 107.7 kJ mol-1, which were increased by 17.4, 43.5, 42.3, and 5.2 kJ mol-1 in comparison to that of the pristine AP, respectively. Only GT-Cu reduced the energy barrier for decomposition of AP, whereas the other catalysts herein would increase the energy barrier for thermolysis reaction, which makes AP more stable without 8 / 17


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mass loss if these values were validated. However, for AP/GT-Cu, the Ea determined by DSC shows a higher value. It reveals that the phase transition of AP was induced by GT-Cu, which improved the energy barrier, which would not be detected by TG experiments. In comparison to TAG-Ni complex, the Ea of AP thermolysis could be decreased when it is combined with GO. Table 2. The decomposition kinetic parameters of AP obtained by Kissinger method TGA Samples

DSC

Stages Ea / kJ mol-1

log A / s-1

r

Ea / kJ mol-1

log A / s-1

r

1st

102.6

3.38

0.9999

100.4

3.16

0.9999

2nd

101.2

1.90

0.9714

83.1

0.44

0.9380

1st

119.9

4.95

0.9730

110.2

4.00

0.9801

2nd

180.9

9.37

0.8725

168.0

8.25

0.9420

3rd

141.5

5.47

0.9711

144.6

5.69

0.9808

AP/TAG-Co

-

146.1

7.56

0.9891

132.2

6.24

0.9908

AP/GT-Co

-

144.9

7.16

0.9854

134.2

6.15

0.9885

1st

107.7

3.77

0.9953

109.4

2.98

0.9956

2nd

167.1

7.83

0.9564

135.4

5.22

0.9606

-

100.6

3.11

0.9944

121.1

4.63

0.9797

AP

AP/TAG-Ni

AP/GT-Ni

AP/GT-Cu

In most cases, the Ea determined by DSC is slightly lower than those determined from TGA data, since the TGA method does not consider the heat effect of condensed phase reactions which have little mass change signals. One also has to note that the kinetic parameters determined by Kissinger method would be very inaccurate if the reaction process does not follow the nth order model. Therefore, more precise calculations have been implemented in the following sections.

3.2.2 Dependence of activation energy on conversion rate To describe the thermolysis process more precisely, the decomposition Ea of the involved AP-based composites as a function of conversion rate using both DTG and DSC as the data sources for calculations are obtained and compared in Figures 5 and 6 (detail information is shown in Table S2). It shows that the dependence of their Ea on extent of conversion from both DTG and DSC data have the same tendency, which almost remained unchanged during the first exothermic process. However, it increases first and then slightly decreases after α = 0.55 in their second exothermic processes.

9 / 17



400

350

350

300

300

Ea (kJ.mol )

400

-1

100

-1

Ea (kJ.mol )

-1

Ea (kJ.mol )

150

250

50

150 100

100

AP/TAG-Ni1 (DSC) AP/TAG-Ni1 (DTG)

50

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

AP/TAG-Ni2(DSC) AP/TAG-Ni2(DTG)

AP/TAG-Ni3(DSC) AP/TAG-Ni3(DTG)

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0





300

200 175

250 -1

125

Ea (kJ.mol )

150 -1

50

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0



Ea (kJ.mol )

250 200

200 150

100 75

200 150

50 25

AP1 (DSC) AP1 (DTG) AP/GT-Ni1 (DSC) AP/GT-Ni1 (DTG)

0 -25 -50

100

AP2 (DSC) AP2 (DTG) AP/GT-Ni2 (DSC) AP/GT-Ni2 (DTG)

50 0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0





Figure 5. The dependences of Ea on conversion rate for two-step or three-step decomposition of AP under the effects of catalysts.

Under the effect Ni-based catalysts, the initial Ea of the first exothermic process was greatly decreased by using DSC data, which is quite different from the result from simple Kissinger method, which did not consider the physical model. One could also notice that under the effect of TAG-Ni, the differences between the DSC and TGA data resulted Ea is not significant for the first and the third decomposition reaction processes, which means that the mass changes largely correspond to the heat changes. There should be some condensed reactions involved with heat change but no mass changes for the second step of decomposition, resulting in large discrepancy in Ea from DSC and TGA data. It is difficult to simply compare the dependence of Ea for AP under the effects of TAG-Ni and GT-Ni, they have different number of reaction steps. It is well-known that the first exothermic process of AP is caused by the decomposition of NH4ClO4 forming NH3 and HClO4, whereas the second exothermic process is due to decomposition of HClO4 [24, 25]. It is also should be noted that the third decomposition process may be caused by the decomposition of the reduced GO stabilized nickel oxides with a Ea of 180 kJ mol-1. 200

200

150

150

300

-1

-1

-1

Ea (kJ.mol )

Ea (kJ.mol )

250

Ea (kJ.mol )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 17

100

100

50

150 100

50

AP/TAG-Co(DSC) AP/TAG-Co (DTG) 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0



200

AP/GT-Co(DSC) AP/GT-Co (DTG) 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0



50

AP/GT-Cu(DSC) AP/GT-Cu(DTG) 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0



Figure 6. The dependences of Ea on conversion rate of one-step catalytic decomposition of AP with TAG-Co, GT-Co, and GT-Cu as the catalysts.

Unlike the Ni-based catalysts, TAG-Co and GT-Co could catalyze and accelerate the 10 / 17


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decomposition reaction of HClO4, resulting in a single complex reaction process. It has been widely accepted that there are two possible decomposition mechanisms for AP: 1) the electron transfer mechanism31 and 2) the proton transfer mechanism32 depending on the temperature. For the first mechanism, the decomposition of AP occurs due to electron transfer from anion to cation via ClO4- + NH4+ → ClO40 + NH40. The version of proton mechanism is as follows: NH4+ + ClO4- = NH3 (g) + HClO4 (g). In the adsorbed layer of AP crystals, where HClO4 was assumed to be desorbed more rapidly than NH3, which causes incomplete oxidation of NH3, creating a saturated atmosphere of NH3. Therefore, the second decomposition process is postponed for pristine AP due to the hindrance effect of NH3 by covering the active center. It has been shown that the second exothermic decomposition of AP produces NO, O2, Cl2, and H2O in the gas phase reactions. The presence of TAG-Co and GT-Co would decrease the inhibition of the second state decomposition. As shown in the Figure 6, the Ea values for AP/TAG-Co calculated both from DSC and DTG are dramatically decreased at the induction periods, and then remained constant after α = 0.25. This result reveals that the autocatalysis process of AP/TAG-Co occurs at the beginning and it was weakened later on. Due to the stabilization effect of GO, the first step of AP decomposition was postponed in presence of GT-Co.

3.2.3 Physical meaningful models for the catalytic decomposition processes Table 3 summarizes the kinetic parameters obtained by the Kissinger, Friedman, and Combined kinetic methods. It shows that Ea obtained by the Kissinger method differs a lot from those obtained by the other two methods, which simply implies that the inherent physical models that governing the decomposition processes of AP is far different from the nth order model. Also, the Kissinger method only takes into account of the peak values instead of the whole process, resulting in large discrepancies with the mean values obtained by the other two isoconversional methods. It is clear that all TAG-M catalysts could activate the first decomposition process of AP resulting in comparable lowered apparent activation energies, but they did play a different role in the second decomposition process. One could also notice that, in fact, the presence of all of the catalysts, the mean Ea would be decreased for the first steps. The GT-Co actually could increase apparent energy barrier for AP decomposition due to stabilization effect of GO surpasses the catalytic effect of TAG-Co complex. As shown in Table 3, the TAG-Ni has a high hindrance effect on the second decomposition of AP, whereas the TAG-Co has an excellent catalytic activity. In general, after doping of GO, the Ea of TAG-M all greatly increased. As an example, the Ea of the second decomposition process of AP/GT-Ni is as high as 300 kJ mol-1, while it is only 135 kJ mol-1 for AP/TAG-Ni, which indicates enhanced effect of GO-Ni on blocking of NH3 evolvement, which may hinder the decomposition of HClO4. In contrast, the Ea of AP/GT-Co is much lower than that of AP/TAG-Co, indicating the enhanced catalytic activity by combination of GO. It also reported that the morphology of GT-Co is mesoporous clusters templated by GO, which is much better than the irregular layered structure of GT-Ni in providing active sites and mass transfer.15 Table 3. Parameters for decomposition reaction models of AP under the effects of GO-based catalysts using non-isothermal TG/DTG data 11 / 17



Combined kinetic method Sample

Friedman method

Kissinger method

Steps m

n

Ea(1)

cA/min-1

Ea(2)

r

Ea(3)

Log A

1st

0.448

0.799

135.6±0.5

1.8±0.2E9

138.5±4.5

0.9989

102.6

3.38

2nd

0.211

0.379

151.0±1.1

3.2±0.4E9

162.6±10.2

0.9959

101.2

1.90

1st

0.446

0.808

113.2±1.8

1.7±0.6E7

112.9±23.8

0.9582

119.9

4.95

2nd

0.358

0.691

307.8±3.4

6.6±4.3E22

291.5±28.7

0.9875

180.9

9.37

3rd

-0.049

0.458

185.4±2.1

2.0±0.8E12

178.9±9.9

0.9961

141.5

5.47

AP/TAG-Co

-

0.501

0.730

104.7±0.6

3.2±0.4E6

102.3±0.5

0.9886

96.1

7.56

AP/GT-Co

-

0.406

0.685

158.7±1.6

1.5±0.5E12

153.7±16.6

0.9997

144.9

7.16

1st

0.371

1.033

117.1±1.1

3.1±0.7E7

123.8±7.4

0.9957

107.7

3.77

2nd

0.480

1.072

135.0±0.9

7.9±1.3E8

140.4±3.2

0.9992

167.1

7.83

-

0.292

0.857

122.8±1.4

2.1±0.6E8

122.8±1.9

0.9856

100.6

3.11

AP

AP/TAG-Ni

AP/GT-Ni

AP/GT-Cu

In order to check the inherent mechanisms relevant to the effects of these TAG-M and GT-M composites on AP decomposition, the physical models of involved composites are evaluated. Figure 7 showed a comparison of the normalized function curves of the obtained kinetic models for AP combined with and without catalysts. It is clear that the first decomposition process of AP is almost following the contracting area and contracting volume models. Under the effect of catalysts, the decomposition is more or less shifts to the two dimensional growth of nuclei model (A2). It indicates the metal ions play roles of active points and the decomposition begins at the surface of catalysts. After doping of GO, phase boundary becomes more critical. The inclusion of GO on the surface of AP crystal may hinder the evaporation of NH3, and therefore the mechanism has been changed to the phase boundary controlled reaction.

2.5

2.5 F1

R3

1.5

R2

1.0

L2 A2

0.5 0.0 0.0

0.2

0.3

0.4

0.5



0.6

0.7

0.8

0.9

1.0

AP (P1,DSC) AP (P1,DTG) AP/GT-Ni (P1,DSC) AP/GT-Ni (P1,DTG)

F1

R3

1.5

R2

1.0

L2

0.5

A3

0.1

D2

(b)

2.0

f()/f(0.5)

2.0

AP/TAG-Co (DSC) AP/TAG-Co (DTG) AP/GT-Co (DSC) AP/GT-Co (DTG) AP/GT-Cu (DSC) AP/GT-Cu (DTG)

D2

(a)

f()/f(0.5)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 17

0.0 0.0

A2 A3

0.1

0.2

12 / 17


0.3

0.4

0.5



0.6

0.7

0.8

0.9

1.0

Page 13 of 17

2.5

2.5 D2

(c)

(d)

AP (P2,DSC) AP (P2,DTG) AP/GT-Ni (P2,DSC) AP/GT-Ni (P2,DTG)

F1

2.0

R3

1.5

f()/f(0.5)

2.0

f()/f(0.5)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


R2

1.0

L2

R3

1.5 R2

1.0

L2 A2

A2

0.5 0.0 0.0

AP/TAG-Ni (P1,DSC) AP/TAG-Ni (P1,DTG) AP/TAG-Ni (P2,DSC) AP/TAG-Ni (P2,DTG) AP/TAG-Ni (P3,DSC) AP/TAG-Ni (P3,DTG)

D2

F1

0.5

A3

0.1

0.2

0.3

0.4

0.5



0.6

0.7

0.8

0.9

1.0

0.0 0.0

A3

0.1

0.2

0.3

0.4

0.5



0.6

0.7

0.8

0.9

1.0

Figure 7. A comparison of normalized curves of obtained kinetic models for AP with and without catalysts with the ideal models by combined kinetic analysis method. The physical models for all processes cannot compare by m and n values, and therefore they have to be plotted and normalized together with the ideal ones. It is advantageous to compare the physical models (SB function) obtained above with some of the most usual ideal models in the literature. The functions are normalized at α= 0.5. Notes: “D2: Two-dimensional diffusion; R2 and R3: Phase boundary controlled reaction (contracting area, and contracting volume); F1, First order reaction; A2 and A3: Random nucleation and two and three dimensional growth of nuclei”; L2: the random scission model.33,34

4. Conclusions The effects of several GO-doped transition metal complexes of TAG on the decomposition mechanisms of AP were studied. It has been found that those catalysts have different effects on the decomposition of AP. The decomposition mechanisms of AP could be changed largely by using different transition metals. The following conclusions could be made: 1) All TAG-M catalysts could catalyze the first decomposition process of AP, resulting in lowered apparent activation energies. The TAG-Co, GT-Co, and TAG-Cu have stronger catalytic effects than the others involved, as they could make the two-step decomposition of AP into a single step. 2) The presence of GT-M additives plays different roles in decomposition process of AP. Metal elements act as the major active sites and the decomposition usually starts from outside molecules of AP crystals, where the contact of the catalysts would enhance the surface reactions. More importantly, the coating of GO-based materials on the surface of AP crystals may hinder the evaporation of NH3, and hence the decomposition mechanism has been changed to the phase boundary controlled reactions.

Supporting Information This document includes the non-isothermal TG/DTG data of AP containing different catalysts, and the non-isothermal DSC curves of AP and AP/GO at a heating rate of 20 ℃ min-1.

ACKNOWLEDGMENT This work has been supported by the National Natural Science Foundation of China (grant number: 51776176), the Fundamental Research Funds for the Central Universities and Thousand Youth Talents Plan Program (W099107). 13 / 17



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TOC Graphic

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Thermal Behavior and Thermolysis Mechanisms of AP under the

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