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C: Physical Processes in Nanomaterials and Nanostructures

Influence of Atmospheres on the Initial Thermal Decomposition of 1,3,5Trinitro-1,3,5-triazinane (RDX): Reactive Molecular Dynamics Simulations Kai Zhong, Jian Liu, Linyuan Wang, and Chaoyang Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10360 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on January 2, 2019

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The Journal of Physical Chemistry

Influence of Atmospheres on the Initial Thermal Decomposition of 1,3,5-Trinitro1,3,5-triazinane (RDX): Reactive Molecular Dynamics Simulations Kai Zhong, †,‡,§ Jian Liu, *,†,§ Linyuan Wang, ‡ and Chaoyang Zhang*,†,$ †

Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P.O. Box 919-311, Mianyang, Sichuan 621999, China ‡ School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, China $ Beijing Computational Science Research Center, Beijing 100048, China. Abstract: Energetic materials (EMs) are usually surrounded in atmospheres during manufacture, storage, transportation and application. Thus, an insight into the influence of atmospheres on the EM decay becomes of significance. In the present work, molecular dynamics simulations are separately performed on ten systems of one 1,3,5-trinitro-1,3,5-triazinane (RDX) nanoparticle in vacuum and in nine atmospheres including CO2, CO, H2O, H2, N2, NH3, O2, NO and NO2 to identify the influence. No evident reaction between RDX and any atmosphere is found at room temperature, suggesting a negligible such influence. Nevertheless, the influences of the nine atmospheres are variable at high temperatures, and can be classified into three cases. NH3, CO, NO and NO2, in particular NH3, promote the RDX decay by consuming RDX and the intermediates generated from it. Because NO2 that serves as a catalyzer to accelerate the RDX decay is considerably consumed by O2, the decay is significantly prohibited by O2. And a few prohibition effects of CO2, H2O, H2 and N2 on the decay are confirmed, due to their dilution effects, with a few or even without reaction involving them. Besides, the NO2 partition dominates the initial steps of the RDX decay, followed by the ring cleavage. In addition, the population of ONDNTA that is an intermediate by partitioning one O atom from RDX is found to be indicative of the decay degree of RDX in various atmospheres. This work presents a comprehensive insight into the influence of atmospheres on the thermal decay of RDX.

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1. INTRODUCTION Energetic materials (EMs) are a class of metastable substances that can transiently release a large quantity of gas and heat by decaying themselves. Due to a high work capacity, they have extensively been applied for civilian and military purposes.1,2 In general, safety and energy are the two most important properties concerning EMs;3-5 meanwhile, keeping EMs stable under various conditions of manufacture, stock, transportation and application is also of significance. So far, numerous tests of environmental adaptability, material compatibility and storability have been carried out for the viability assessment of EMs under different conditions.6-12 A large number of studies have focused upon the response of EMs to external environments or media. For example, in alkaline water, EMs undergo hydrolysis.6 In reed canary grass leaves, 1,3,5trinitro-1,3,5-triazinane (RDX) undergoes aqueous photolysis when exposed to sunlight.7 Under anoxic conditions, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is decayed through two different reaction pathways.8 In Fe-rich subsurface environments, green rusts may contribute to the abiotic natural attenuation of RDX.9 In addition, nano additives were found to enhance the decomposition rate and performance of EMs by changing the thermal conductivity, energy barrier of thermolysis, heat of reaction, and gas-phase reaction mechanisms. For example, due to a strong interaction between Al and O atoms, the thermal decomposition of RDX on the surface of Al nanoparticles can occur spontaneously without potential barrier.10-12 Besides, EMs usually exist in atmospheres with various components. Nevertheless, to our knowledge, the influence of atmospheres on the stability of EMs has seldom been reported. It is probably attributed to that only a few relevant experiments have been implemented with a few data, or a negligible such influence has been observed. In principle, atmospheres may react with EMs and/or the intermediates originated from their decomposition. Under ambient conditions, the reactions between EMs and atmospheres can be overlooked;13,14 while, they will become remarkable 2 / 29


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when heated to a much higher temperature, therein the influence of atmospheres may become remarkable too. However, hardly can this prediction be clarified experimentally, as it is difficult to maintain an invariable testing condition to thermally analyze an EM in vacuum and in various atmospheres for comparison. Fortunately, molecular dynamics (MD) simulations can remedy the shortage of experiments and may make such tests ready.15,16 To examine the reactivity variation of EMs in atmospheres, we take RDX, a most extensively applied EM, in combination with various common atmospheres including CO2, CO, H2O, H2, N2, NH3, O2, NO and NO2. These common atmospheres are representative of air and those of the manufacture, stock, transportation and application conditions, as they are all the intermediates or products of the RDX decay.13,17,18 As expected, different atmosphere can lead to different consequences on the thermal decay of EMs. In this work, our MD simulations show that NH3, CO, NO and NO2 promote the thermal decay of RDX. On the contrary, O2 prohibits it remarkably, and CO2, H2O, H2 and N2 inhibit it too to a smaller extent. By tracing the MD trajectories, the reaction mechanisms are further revealed. In a word, the present work presents a comprehensive insight into the influence of atmospheres in decaying EMs. It may put forward some innovations beyond only single EMs considered for MD simulations, which disagrees with the practical environment. 2. METHODOLOGIES This work aims at understanding the influence of various atmospheres on the thermal decomposition of RDX. To achieve this goal, a spherical RDX nanoparticle, with a large surface-tobulk ratio, is modelled in a close contact with the surrounding atmosphere composition molecules. In addition, the RDX nanoparticle lowers the decomposition temperature substantially relative to the bulk phase.19 While an unreasonable high temperature (>1500 K) is necessary for the previous MD 3 / 29



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simulation of bulk phase RDX,20 a reduced temperature is applied in this work. It not only narrows the gap between the experiment (500 K)14 and simulation, but also is helpful to distinguish the most kinetically favored pathways from numerous complex ones that can occur at high temperature.

Figure 1. RDX nanoparticle in (a) vacuum and (b) CO2 atmosphere.

In our model, a RDX nanoparticle with 2.5 nm radius and 230 molecules was constructed within a 1000 nm3 cubic supercell, as shown in Figure 1. To mimic different atmospheres, 500 homogenous molecules of CO2, CO, H2O, H2, N2, NH3, O2, NO, or NO2 were filled into the cell for nine different RDX/atmosphere models, corresponding to a molar concentration of 0.89 mol/L or a pressure of 39 atm. For instance, Figures 1a and 1b shows the models of RDX/vacuum and RDX/CO2, respectively. The selection of the aforementioned gas molecules is based on their existence either as main compositions of air, or as intermediates and products of the thermal decay of RDX. Two kinds of isothermal–isochoric ensemble (NVT) MD simulations were carried out on these RDX/vacuum and RDX/atmosphere models to study the thermal decay process of RDX, namely, the programmed heating and the constant temperature heating. For the programmed heating simulation, an initial atomic relaxation was performed at 300 K for 10 ps, followed by a MD simulation at temperature increasing from 300 to 2800 K at a constant rate of 5 K/ps for another 500 ps with a temperature damping parameter of 100 fs. Therein, the RDX decay was observed to start at ~800 K. 4 / 29


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For the constant temperature heating simulations, an initial atomic relaxation was performed at 300 K for 3 ps, followed by a MD simulation at a constant temperature of 1000 K with a temperature damping parameter of 100 fs too. It should be noted that the heating rate (100 to 102 K/ps)21,22 usually adopted for decomposition simulations of EMs is considerably enhanced compared to the experiments conditions (10-3 to 101 K/s)14; nevertheless, the chemical mechanisms derived from the simulations with enhanced heating rates have been proven to be reliable.21,22 In this work, ReaxFFlg23 and LAMMPS package24 were used for MD simulations, where the MD timestep was set to 0.1 fs and the recording was set to every 100 fs. The OVITO25 and the VARxMD26 softwares were used to analyze the reaction details and to visualize the atomic structures, respectively. The principle of the ReaxFF method can be referred elsewhere in the literature.27 ReaxFF is a molecular force field based on the density functional theory (DFT), in which the formation and breakage of a chemical bond is determined by its bond order. ReaxFF has been successfully applied to study the reactions of shocked or heated EMs.28-30 ReaxFF-lg improves the description of dispersion force and usually provides more accurate predictions than ReaxFF.23 Because all nine gases as atmosphere components are the intermediates or products of common CHNO contained EMs, and most of them are only present in the training set as reaction products and reaction energies. Except from NO2 and NO, ReaxFF-lg was not trained for reaction barriers associated with the remaining gases. Accordingly, an additional validation was implemented by comparing energy barriers from the DFT and ReaxFF-lg calculations. Figure S1 of Supporting Information (SI) validates ReaxFF-lg, as the calculated results by the two methods are comparable to each other. 3. RESULTS AND DISCUSSION 3.1 Thermal decomposition of the RDX nanoparticle in vacuum. Nanocrystallization can usually make EMs more thermally sensitive. Comparisons in potential 5 / 29



energy (PE) and decay rate of the RDX nanoparticle and the bulk RDX in Figure S2 of SI confirm the case, as the nano-RDX possesses a higher PE and a higher decay rate. It also implied that nanogranulation of RDX makes it release more heat. To a certain extent, the RDX nanoparticle in Figure 1a can be employed to represent the RDX with a low filling degree of reaction volume. That is, a low filling degree results in more heat. This has experimentally been verified by Cosgrove and Owen,31,32 and Maksimov 33, but not in agreement with the data published by Hall 34.

800

100

Temperature, K 1200 1600 2000 2400 2800 200

RDX PE

80

150

60 100 40 50

20 I

III

II

0 0

100

PE, kcal/mol

400

RDX, %

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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200 300 Time, ps

400

0 500

Figure 2. Evolution of the percentage and potential energy (PE) of RDX as a function of time in the programmed heating simulation.

Figure 2 shows the evolution of the percentage and PE of RDX/vacuum in the programmed heating simulation. The RDX evolution can be divided into three stages: very slow reduction (I), rapid reduction (II) and slow reduction to complete decay (III), corresponding to the inducement, fast development and attenuation periods of the RDX decay, respectively. From Figure 2, we can find that the RDX molecules begin to analyze at ~800 K, reduce rapidly from ~1000 K and diminish at ~1800 K. This initial temperature (800 K) for the RDX nanoparticle decomposition is much lower than that (1100 K) for the bulk RDX,22 validating that the nano EMs are more thermally sensitive than the bulk ones. Moreover, due to the continuous temperature elevation, the PE successively increases till 2550 6 / 29


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K, despite a part counteracted by the decay induced reduction. Afterwards, the PE tends to reduce a little by more RDX molecules decayed to stable small product molecules with a more energy release.

Figure 3. Snapshots showing the initial thermal decomposition of the RDX nanoparticle in vacuum: (a) a RDX molecule near the nanoparticle surface; (b) a RDX molecule transferred to the nanoparticle surface; (c) N-N bond fission of a RDX molecule; (d) a NO2 molecule partitioned from a RDX molecule.

We first observe a heat-induced volume expansion and a phase transition from orderliness to amorphousness of the RDX nanoparticle prior to its decomposition (Figures S3 and S4 of SI). Figure 3 illustrates the initial decomposition of the RDX nanoparticle. Due to the amorphousness of the RDX nanoparticle, a RDX molecule near the surface (Figure 3a) is gradually transferred to the surface (Figure 3b). Subsequently, a N-N bond of the RDX molecule is elongated and broken (Figure 3c), with a NO2 produced (Figure 3d). It shows that the initial decay of the RDX nanoparticle occurs from its surface, and the first step of RDX decomposition is the N-N bond cleavage. The results agree with previous experiments35,36 and calculations37. For example, Kuklja38 simulated the thermal decomposition of RDX through DFT calculations, showing a lowered energy barrier of the N-N bond fission in surface in contrast to bulk.

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400 2.5 2.0

800

Temperature, K 1200 1600 2000

RDX NO2

HCN NO

H2O

H2

N2

CO

2400

2800

1.5

Nm

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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1.0 0.5 0.0 0

100

200 300 Time, ps

400

500

Figure 4. RMN evolution of RDX and some main gaseous molecules. RMN of each species equal to its molecular numbers divided by the original molecular numbers of RDX. This is considered in the following figures.

Figure 4 shows the evolution of some gaseous molecules, which serve as main intermediates or products of the RDX decay. These molecules include HCN, NO2, H2O, NO, CO, N2O, N2, and H2, as reported by Litzinger et al.17 The relative molecular numbers (RMN) of NO2 molecules increases first to a maximum and reduces thereafter in the simulation time range of 500 ps. In this range, HCN, NO, H2O and H2 increase successively to higher populations; while N2 and CO increase successively too but with smaller populations. The small amounts of N2 and CO are attributed to the insufficient decay in the limited time of 500 ps. N2O is not found in this case, possibly due to the aforementioned incomplete decay of RDX.

Figure 5. Initial thermal decomposition pathways of RDX in vacuum with sequent numbers. The italic and underline blue number around the pathway number represents the frequencies of the relevant reaction with reverse reactions expunged. This is also considered in the following figures. 8 / 29


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Figure 5 lists six reactions in the highest frequencies in the total 500 ps of simulation, which represent

the

initial

thermal

decomposition

of

the

frame

of

RDX.

In

particular,

C3H6O6N6→C3H6O4N5+NO2 (Reaction 1) appears 216 times, suggesting that the decay of 94 % (216/230) RDX molecules starts by means of the NO2 partition. The subsequent NO2 partition in Reactions 2, 3 and 6 cause a large RMN in Figure 4. In addition, as shown in Figure 5, after the first partition of NO2 (Reaction 1), the RDX decay proceeds along two ways: one is the sequent NO2 fission till the formation of C3H6N3, through C3H6O4N5 (RDR, Reaction 2) and C3H6O2N4 (Reaction 3); and the other is along the ring breakage of RDR to form CH2O2N2 and C2H4O2N3 (Reaction 4), and subsequently, they are both decayed into CH2N (Reactions 5 and 6). The second pathway agrees with one proposed by Schroeder.39 We can find from Figure 5 that the RDX decay is along a way from C3 to C1, showing a way to smaller molecules.

400 0.5

800


2400

2800

C3H6O4N5 C3H6O2N4 C3H6N3 CH2O2N2 C2H4O2N3

0.4

RMN

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


0.3 0.2 0.1 0.0 0

100

200 300 Time, ps

400

500

Figure 6. Evolution of RDR and its subsequent intermediates.

As mentioned above, C3H6O6N6→C3H6O4N5 (RDR) +NO2 (Reaction 1) is the dominant step for the initial thermal decomposition. Thereby, more attention is paid to the evolution of RDR, as well as its subsequent intermediates. As shown in Figure 6, five intermediates appear in an order of RDR, C3H6O2N4, and C3H6N3/C2H4O2N3/CH2O2N2. It shows that the sequent NO2 partition path is followed 9 / 29



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by the ring breakage of RDR. As to the populations of these intermediates, they decrease in an order of CH2O2N2, RDR, C3H6O2N4, C3H6N3, and C2H4O2N3. In combination with this order and a higher frequency of Reaction 2 relative to Reaction 4, we can deduce that the N-N bond fission dominate the next decay of RDR. Schweigert studied the decomposition of RDR at 1750 K by ab initio MD, showing that RDR mainly conducts a further N-N bond fission, well in agreement with our result.40 Nevertheless, the comparison in frequency shows only an advantage of ~8 % of Reaction 2 over Reaction 4, implying that the ring fission is also the important step for decaying RDX. Both Reactions 4 and 5 produce CH2O2N2, leading to its maximum population among all five intermediates. As one of the most extensively applied EMs, numerous researches besides the aforementioned have been implemented to reveal its thermal behavior. For example, Rauch and Fanelli 41 found that the NO2 content has a positive relation to the increase in the filling degree of the reaction volume in RDX pyrolysis. Batten

42

confirmed that NO2 inhibits the thermal decomposition of RDX because

they remove catalysts of its thermal decomposition, the catalysts being hydroxymethylformamide and formaldehyde. And Thompson et al

43

reported the concerted fission of the triazinane ring

(depolymerization to 1-nitro-1-azaethylene) occurs predominantly in the gas phase thermal decomposition of RDX at higher temperatures. These results are all not found in our simulations, largely due to the difference in boundary condition between the experiments and simulations. As a matter of fact, the decomposition mechanism of RDX is significantly influenced by the testing conditions. From above discussion, we can know from the simulation that the RDX decay starts from the surface of the nanoparticle, the NO2 partition dominates all the initial steps of the decay, and the ring fission of RDR serves as an important sequent step too. All these set a base for revealing the effect of atmospheres on the thermal decay of RDX. 10 / 29


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3.2 Thermal decomposition of the RDX nanoparticle in various atmospheres. 3.2.1 Overview. An overview of the effect of various atmospheres on the thermal decomposition of the RDX nanoparticle is focused upon, with that of the RDX nanoparticle in vacuum as reference. In atmosphere, the RDX nanoparticle will be thermally analyzed after heated, together with the reactions between the atmosphere molecules and RDX or its derivatives (Figure S5 of SI). By checking the evolutions of the PE and the RDX molecules, we can identify some differences in the effect.

250

RDX CO2 H2 O H2 N2 NH3 NO2

200

PE, kcal/mol

800

150 100


2400

Programmed heating

400

2800

250

(a)

50

800 RDX CO O2 NO

200

PE, kcal/mol

400


2400

Programmed heating

2800

(b)

150 100 50 0

0 0

100

80

200 300 Time, ps

400

0

500

80

(c)

Constant T heating

100

200 300 Time, ps

400

500

(d)

Constant T heating

60

60

40

RDX H2O N2 O2

20 0

200

PE, kcal/mol

PE, kcal/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


CO2 H2 NH3 NO2

400 600 Time, ps

40 20

RDX CO NO

0

800

1000

0

200

400 600 Time, ps

800

1000

Figure 7. PE evolution of the RDX nanoparticle with the various atmospheres under the conditions of programmed heating and constant temperature heating.

Overall, in most cases under programmed heating except from that of the atmosphere of NO, the PE undergoes a first increase to a maximum and a reduction thereafter, similar to above case of the RDX in vacuum. In the case of the NO atmosphere, the PE is evolved as a first reduction, instead of 11 / 29



an increase; subsequently, it increases to a maximum and then reduces as in the common thermal decomposition of EMs.21,43 Regarding the PE peaks of the RDX nanoparticle in vacuum and atmosphere gases, they are variable, as those in CO2, H2O and NH3(Figure 7a), in H2, N2 and NO2 (Figure 7a), and in NO, CO and O2 (Figure 7b) are above, equal to, and below that in vacuum, respectively, under the condition of the programmed heating. The case of constant temperature heating is different from that of the programmed heating, as the PE of most systems increases gradually (Figure 7c) in the timescale of 1000 ps, with two exceptions, i.e., that of RDX/CO increases firstly and reduces thereafter, and that of RDX/NO decreases firstly rapidly to a minimum, then increases gently to a maximum, and finally tends to reduce again (Figure 7d).

400

800


2400

100

2800 100

(a)

80

(b)

80

RDX, %

RDX, %

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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60 40

RDX CO H2 NH3 NO

20 0 0

100

CO2 H2O N2 O2 NO2

200 300 Time, ps

60 RDX1 CO 1 H2 NH3 NO

40 20 400

500

0

200

CO2 H2O N2 O2 NO2

400 600 Time, ps

800

1000

Figure 8. Evolution of the RDX molecules in the various atmospheres under the conditions of programmed heating (a) and constant temperature heating (b).

Next, we focus upon the evolution of the RDX molecules in the various atmospheres under the conditions of the two styles of heating. The trends of RDX decomposition in various atmospheres are wholly consistent with that in vacuum: the RDX molecules undergo three typical stages of inducement, fast reduction and attenuation (Figure 8a) when programmed heating; while, when they are constant temperature heated at 1000 K, they reduce gradually and successively to the end of the simulations (Figure 8b). Meanwhile, we can find that the RDX molecules programmed heated vanish 12 / 29


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in the time range of the simulation (Figure 8a), and those heated at 1000 K constantly do not (Figure 8b). This agrees with above PE analysis. In addition, it is worth noting that the RDX in the NO atmosphere starts to decompose at ~350 K when programmed heating, showing an earlier decay relative to other cases. 50

44.7

45

40.2

40

RDX, %

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


35

29.3

30

31.1

32.3

35.4 35.8 33.9 34.4

25.8 25 20

NH3 CO

NO NO2 Vacuum H2 H2O

N2

CO2

O2

Figure 9. Percentage of residual RDX molecules after heated at 1000 K for 1000 ps.

As mentioned above, the RDX molecules heated at 1000 K in any atmosphere do not vanish completely within 1000 ps. We list the percentage of the remaining RDX molecules of each case in Figure 9. It exhibits a decreasing order of the remaining molecules as O2, CO2, N2, H2O, H2, Vacuum, NO2, NO, CO, and NH3 (for simplicity, the composition molecule represents the system containing related atmosphere and the RDX nanoparticle). Comparing the remaining RDX molecular number in an atmosphere with that in vacuum, we can ascertain whether an atmosphere promotes or inhibits the RDX decay: the less remaining molecules suggests the more promotion, and vice versa. Thereby, we can tentatively conclude from Figure 9 that, NH3 and O2 significantly accelerates and restrains the RDX decay, respectively; following by NH3, CO, NO and NO2 can promote the RDX decay more or less; while, CO2, N2, H2O and H2 follows O2, and they may prevent from the decay. Besides, we checked the effect of atmospheres on the dominant steps. As shown in Table 1,

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Reactions 1 to 6 in atmospheres of NH3, CO, NO and NO2 take place less frequently than those of H2, H2O, N2, CO2 and O2. Interestingly, it seems that the atmospheres promoting the RDX decay prevent the six reactions, and vice versa. This implies some unknown mechanisms. Moreover, the frequency of Reaction 2 is greater than that of Reaction 4 in any atmosphere, indicating that N-N bond fission is still the dominant pathway of RDR decay, consistent with the above conclusion. Table 1. Frequencies of Reactions 1-6 of the RDX Decay in Various Atmospheres within Total 1000 ps at 1000 K. The Same Time Range is Adopted for Following Tables. Reaction No.

Vacuum

NH3

CO

NO

NO2

H2

H2O

N2

CO2

O2

1

109

95

92

67

97

117

102

101

108

99

2

50

37

46

34

50

53

58

50

58

39

3

2

7

4

2

1

4

0

2

6

4

4

25

6

15

12

10

29

17

19

23

21

5

21

5

12

11

9

27

15

18

23

20

6

6

2

4

4

1

8

2

7

3

10

600

Number of Molecules

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 14 of 29

500 400 CO2 H2O N2 O2 NO2

300 200 100

CO H2 NH3 NO

0 0

200

400 600 Time, ps

800

1000

Figure 10. Evolution of the atmosphere molecules in the thermal decomposition of RDX at 1000 K.

In addition, we are concerned ourselves about the evolution of atmosphere molecules at 1000 K. Figure 10 shows a rapid decrease of the NO molecules close to zero after 500 ps, implying their highest activity. NH3, CO, and NO2 molecules decrease about 100, 90 and 90, respectively, showing an evident reduction and a certain activity too. O2 decreases slightly. And H2, N2, CO2 and H2O molecules almost keep their numbers during the heating, with a slight increase of H2O, which should come from the RDX decay, as a main product. 14 / 29


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From above change tendencies of PEs, we can know that RDX is more completely decayed in the case of 500 ps of programmed heating than in that of 1000 ps of constant temperature heating at 1000 K. The evolution of PE, and numbers of earlier RDX molecules and atmosphere molecules of the RDX nanoparticle with the NO atmosphere are significantly different from those of other cases. Besides, NH3 and O2 exhibit remarkably promotion and prohibition effects on the RDX decay, respectively. All these are only of seeming results, and the relevant underlying mechanisms require further insights. 3.2.2 Case of the NH3 atmosphere.

Figure 11. Additional initial reaction pathways of the thermal decomposition of RDX in the NH3 atmosphere. Table 2. Main Reactions Involving NH3 and Their Frequencies in the Thermal Decomposition of RDX in the NH3 Atmosphere at 1000 K. Reaction

Frequencies

NH3+C3H6O4N5→C3H9O4N6

31

NH3+OH→NH2+H2O

30

NH3+NO→H3N2O

14

NH3+NH2→H+N2H4

13

NH3+NO→H2N2O+H

11

NH3+C3H7O6N6→C3H6O5N6+H4NO

10

NH3+C3H6O4N5→C3H9O2N5+NO2

6

NH3+OH→H4NO

6

NH3+C3H6O4N5+NO2→C3H8O4N6+HNO2

4

NH3+NO2→HNO2+NH2

2 15 / 29



Page 16 of 29

As pointed out above, NH3 plays the most remarkable role in accelerating the RDX decay. Therefore, the reactions with NH3 as reactant were carefully checked. Table 2 lists 12 such reactions with frequencies no less than 2. Meanwhile, the initial decay pathways except from those in Figure 5 are described in Figure 11. In combination with data in Table 2 and Figure 11, we deduce that two reasons are responsible for the promotion of NH3. On the one hand, as shown in Figure 11, NH3 can directly consume RDR to form C3H9O4N6 by Reaction 7, which is able to consume RDX by Reaction 8. This reaction promotes RDX to trap one H atom to form C3H7O6N6, which consumes RDX molecules as aforementioned. Partitioning one OH from C3H7O6N6 by Reaction 9, C3H6O5N6 (ONDNTA) is obtained. By the consumption of NH3, the partitioned OH is converted into H2O and NH2 (Reaction 10). Thereby, beyond the routine in Figure 5, an additional one of sequent Reactions 1, and 7-10, can also consume RDX. These additional reactions are responsible for the promotion of NH3 despite the less frequencies of Reactions 1-6 in Table 2. On the other hand, the NH2 generated by Reaction 10 can be combined with a free H atom to form N2H5 through the reaction of H+NH2+NH2→N2H5 (with a frequency of 28); meanwhile, NH3 can be reproduced by N2H5→NH2+NH3 (with a frequency of 24), presenting a source of NH3 to continuously consume RDX. 3.2.2 Cases of the CO, NO and NO2 atmospheres. We classify CO, NO and NO2 atmospheres in a same group to discuss the relevant reaction mechanisms, due to their apparent roles in promoting the RDX decay too, following NH3. Table 3 lists the reactions with CO as reactant and frequencies no less than 2. The highest frequency of 28 appears in the reaction of CO+NO2→CO2+NO, in which CO is a reducer, and NO2 is an oxidizer and partitioned from RDX as an intermediate. The oxidation-reduction reaction between CO and NO2 leads to the consumption of NO2 and therefore that of RDX. Besides, CO can also capture one O atom 16 / 29


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from RDX to form CO2 and ONDNTA (Reaction 11), or can be directly combined with RDX. All these reduce RDX, as reasons for accelerating the RDX decay. Table 3. Main Reactions Involving CO and Their Frequencies in the Thermal Decomposition of RDX in the CO Atmosphere at 1000 K. Reactions

Frequencies

CO+NO2→CO2+NO

28

CO+C3H6O6N6→C3H6O5N6+CO2 (Reaction 11)

26

CO+HNO2→CHO2+NO

4

CO+NO3→CO2+NO2

3

CO+N2O→CO2+N2

3

CO+C3H6O5N6→C3H6O4N6+CO2

3

CO+C3H6O4N5→C4H6O5N5

2

CO+NO→CNO2

3

CO+CH2O2N2→CH2ON2+CO2

2

Section 3.2.1 shows several distinctive features of the RDX decay in the NO atmosphere, such as the PE evolution in Figures 7b and 7d, the rapid early reduction of RDX in Figure 8a, and the least frequency of Reaction 1 in Table 2. Therefore, many attempts have been made to understand these distinctive features. Table 4. Main Reactions Involving NO and Their Frequencies in the Thermal Decomposition of RDX in the NO Atmosphere at 1000 K. Reactions

Frequencies

NO+NO→N2O2

168

NO+C3H6O6N6→C3H6O5N6+NO2

52

NO+NO+NO→N3O3

23

NO+C3H6O4N5→C3H6O5N6

21

NO+C3H6O4N6→C3H6O3N5+N2O2

15


9

NO+C3H6O5N6→C3H6O4N5+N2O2

9


9

NO+C3H5O2N4→C3H5ON4+NO2

7


6

NO+C3H5O2N5→C3H5ON4+N2O2

5

NO+C3H6O6N6→C3H5O6N6+HNO

4

NO+C3H5ON4→C3H5O2N5

4

NO+C3H6O4N5→C3H6O3N5+NO2

3

NO+CH2ON2→CH2O2N3

3

NO+C3H6O3N5+NO2→C3H6O6N7

2

17 / 29



400

500


900 1000 200

NO N2O2

400

160

300

120

200

80

100

40

0 0

30

60 90 Time, ps

120

Number of N2O2

300 500

Number of NO

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 18 of 29

0 150

Figure 12. Evolution of NO and N2O2 during the first 150 ps of thermal decomposition of RDX in the NO atmosphere under programmed heating.

We first checked the relevant reactions with NO as reactant in the total 1000 ps of simulating the thermal decomposition of RDX in the NO atmosphere (Table 4). Distinctively, the highest frequency of 168 is of the dimerization of NO. Furthermore, the dimerization is identified by the evolutions of NO and N2O2 in Figure 12, as NO decreases with N2O2 increasing, and they both tend to equilibrium finally. This dimerization is different from other cases, as other atmosphere molecules can hardly be self-consumed. Presumably, it is also the reason for the distinctive features of the RDX decay in the NO atmosphere. For example, because of the exothermicity of the dimerization (see S6 of SI), that the PE of the RDX with the NO atmosphere decreases at the early stage (Figures 7b and 7d) is understandable.

Figure 13. MD snapshots showing the NO-induced RDX decay under programmed heating. 18 / 29


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Besides aforementioned the self-dimerization, it also found the reaction between RDX and NO, which is trapped by snapshots in Figure 13. As illustrated in the figure, the RDX nanoparticle is first immersed in the NO atmosphere (Figure 13a); as time proceeds to 10.7 ps and temperature reaches 350 K, one NO molecule appears above the surface of the particle (Figure 13b), and captures one O atom to form ONDNTA and NO2 (Figure 13c); and these two molecules remain till 30 ps (Figure 13d). That is to say, the reaction between RDX and NO takes place at the earlier stage and relatively low temperature, and leads to an earlier decrease of RDX (Figure 8a). As demonstrated in Figure 8a, from the 40th to 160th ps, RDX decreases slightly in the NO atmosphere, because most of NO are consumed by self-dimerization45 in the case of ~39 atm to reduce their number, for example, only ~60 % NO remained at the 40th ps (Figure 12). And the dimerization is almost finished at ~110th ps. In addition, the lowest frequency of Reaction 1 in the NO atmosphere (Table 1) is attributed to a large quantity of NO2 produced by the reaction between NO and RDX, which prevents Reaction 1 according to the chemical equilibrium theory.

Figure 14. Additional initial reaction pathways of the thermal decomposition of RDX in the NO atmosphere.

Also, ONDATA can be formed by the combination of NO with RDR, which is a product of Reaction 1 (Figure 14). This is supported by simultaneous thermogravimetry modulated beam mass spectrometry (STMBMS) measurements of Behrens18 and Maharrey46 et al, in which it showed that the reaction of RDX with NO could produce NO2 and ONDNTA when the temperature below the 19 / 29



Page 20 of 29

melting point of RDX, and afterwards ONDNTA could be broken into CH2O and N2O. CH2O and N2O are observed with low populations in our simulation, due to the insufficient simulation time. Patidar et al47 studied the reaction of RDX with NO using DFT too, showing that the combination of NO and RDX can generate ONDNTA in two ways shown in Figure 14, Reaction 12, and Reactions 1 and 13. Thus, NO can accelerate the RDX decay. Table 5. Main Reactions Involving NO2 and Their Frequencies in the Thermal Decomposition of RDX in the NO2 Atmosphere at 1000 K. Reactions

Frequencies

NO2+NO2→N2O4

94

NO2+C3H6O6N6→C3H6O5N6+NO3

29

NO2+C3H6O2N4→C3H5O2N4+HNO2

21

NO2+HNO2→HNO3+NO

13

NO2+NO2→NO3+NO

12

NO2+C3H6O3N5+NO→C3H6O6N7

4

NO2+C3H6O4N5→C3H5O4N5+HNO2

3

Figure 15. Additional initial reaction pathways of the thermal decomposition of RDX in the NO2 atmosphere.

We also examined the reactions with NO2 as reactant and listed them in Table 5. Similar to the case of NO, the self-dimerization of NO2 to N2O4 with a highest frequency of 94 consumes considerable NO2. Botcher et al48,49 studied the rapid thermal decomposition of RDX by pulse CO2 laser, and found that N2O4 was formed after breaking the N-N bonds of RDX, as observed in our simulation. In addition, another main reaction of NO2 is to reduce RDX to ONDNTA, and NO2 is converted into NO3, as shown in Figure 15. Irikura et al proved thermodynamically and kinetically that NO2 could be applied as a reducing agent to capture one O atom from RDX and to form NO3.50 20 / 29


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In our simulation, NO2 can also react with the large molecular fragments generated from RDX, such as C3H6O4N5 and C3H6O2N4, to form HONO. Liu et al studied the thermal decomposition of RDX by pyrolysis gas chromatography and mass spectrometry and showed that NO2 could catalyze the decomposition of RDX at high temperature.51 Wang et al’s calculation by DFT identified that NO2 can accelerate the thermal decomposition of a nitroamine compound of DMN, by promoting the dissociation of HONO.52 These results agree with Reactions 15 and 16. Previously, Batten42,53 confirmed that NO2 catalyzes the thermal decomposition of RDX negatively, as it removes catalysts of hydroxymethylformamide and formaldehyde. This negative effect of NO2 much different from aforementioned experiments and our simulation should be attributed to the difference in testing condition, or others. The decomposition mechanism of EMs is indeed complex and variable. For example, hydroxymethylformamide is not observed in our simulation, and formaldehyde is found will a small population. 3.2.3 Cases of the atmospheres of H2, N2, CO2 and H2O. N2, CO2 and H2O have not been found to participate the RDX decay as reactants, due to their high molecular stability at 1000 K. Behrens et al also considered that there is no autocatalysis effect of H2O on the thermal decomposition of RDX due to the fact that the thermal decomposition of RDX occur in the liquid phase and the cage effect is absent in decomposition process.54 In the H2 atmosphere,

only

three

reactions

take

place

with

H 2,

including

H2+NO2→H2NO2,

H2+C3H6O6N6→C3H8O6N6, and H2+OH→H3O, with low frequencies of 6, 3 and 2, respectively. Thereby, H2 almost doesn’t take part in the RDX decay. It shows a prohibition effect of the four atmospheres of H2, N2, CO2 and H2O on the RDX decay in Figure 9. This may be attributed to a dilution effect of these inertia molecules on the thermal decay. Interestingly, the prohibition effect of these four gases is just enhanced in an order of the increasing 21 / 29



of molecular sizes. That is, H2, N2, CO2 and H2O are inert to the RDX decay, and they can decrease the efficient intermolecular collision to chemical reactions to prohibit the RDX decay. 3.2.4 Case of the O2 atmosphere. Table 6. Main Reactions Involving O2 and Their Frequencies in the Thermal Decomposition of RDX in the O2 Atmosphere at 1000 K. Reactions

Frequencies

O2+NO2→NO4

28

O2+CH2N→HO2+CHN

3

O2+C3H6O2N4→C3H5O2N4+HO2

2

O2+C4H8O4N7→C4H7O4N7+HO2

2

Figure 16. Additional initial reaction pathways of the thermal decomposition of RDX in the O2 atmosphere.

Finally, we focus on the case of the O2 atmosphere, in which the RDX decay is the most significantly inhibited as illustrated in Figure 9. Similarly, we checked the participation of O2 in the RDX decay, with results listed in Table 6. In comparison, the frequency sum of Table 6 is much less than that of any one of Tables 2-5, which belong to NH3, CO, NO and NO2, respectively. It shows a less activity of O2 on the RDX decay. The highest frequent reaction in Table 6 takes place between O2 and NO2; followed by NO4 (Reaction 17), HNO4 (Reaction 18) and NO3 (Reaction 19) are formed, without reaction between O2 and RDX, as described by Figure 16. 0.8

Decay in vacuum Decay in the O2 atmosphere

0.20

(a)

0.6

0.15

0.4

0.10

RMN

RMN

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 22 of 29

Decay in vacuum Decay in the O2 atmosphere

(b)

0.05

0.2

0.00

0.0 0

200

400 600 Time, ps

800

1000

0

200

400 600 Time, ps

800

Figure 17. Comparison in the NO2 (a) and NO3 (b) evolution in vacuum and the O2 atmosphere at 1000 K. 22 / 29


1000

Page 23 of 29

According to Reaction 17, NO3 increases at the cost of the consumption of NO2. This is identified by that the RDX decay in the O2 atmosphere produces less NO2 (Figure 17a) while more NO3 (Figure 17b) than that in vacuum. Experimental results shown that O2 strongly restrain the RDX decay for that O2 oxidize the solid residua, which could catalyze the RDX decay.53,55 In our simulations, the thermal decomposition of RDX in O2 is significantly slower than that in vacuum. Because NO2 that promotes the RDX decay is oxidized and consumed by O2, the RDX decay is inhibited by O2. Chemically, RDX is O-poor, and thus O2 may accelerate the RDX decay. However, at the early stage, the RDX decay is prevented by O2 due to this reason. 0.30

RDX N2

CO2 NH3

CO O2

H2O NO

H2 NO2

0.25 0.20

RMN

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


0.15 0.10 0.05 0.00 0

200

400 600 Time, ps

800

1000

Figure 18. Evolution of ONDNTA from the RDX decay in vacuum and various atmospheres at 1000 K.

From above discussion, we can know that NH3, CO, NO and NO2 can accelerate the thermal decay of RDX by considerable consumption of the intermediates from the RDX or of RDX itself. We find that among these intermediates, ONDNTA is an important indicator of the decay degree, i.e., a higher content of ONDNTA suggests a more complete decay; further, it can be indicative of the promotion or prohibition of the atmospheres on the RDX decay. As demonstrated in Figure 18, relative to RDX in vacuum, more ONDNTA are produced in the atmosphere of NH3, CO, NO and 23 / 29



Page 24 of 29

NO2, which can just accelerate the thermal decomposition of RDX; while O2 possess the strongest ability to inhibit the RDX decay with less ONDNTA formed. This finding agrees with an experimental observation that ONDNTA is able to generate the dynamic non-volatile residue (NVR), which can react with RDX to generate ONDNTA.46 That is to say, the thermal decomposition of RDX can be enhanced by the self-catalysis of ONDNTA.

4. CONCLUSIONS We have comprehensively studied the effect of atmosphere on the thermal decomposition of a common EM of RDX, by separately simulating the thermal decay of nanoparticles in vacuum and nine atmospheres of CO2, CO, H2O, H2, N2, NH3, O2, NO and NO2. By means of the analysis of apparent data relevant to the evolution of RDX and intermediates, and the insight into the mechanistic details, we identify that NH3, CO, NO and NO2 can promote the decay, O2 can prohibit it, and CO2, H2O, H2 and N2 don’t participate the decay reactions with a little prohibition due to their dilution effects. In particular, NH3 and O2 accelerates and prohibits the decay the most remarkably, respectively. The promotion of NH3, CO, NO and NO2 is resulted from their highly frequent reactions with the intermediates from RDX or RDX itself. The less influence of CO2, H2O, H2 and N2 is attributed to almost no reaction relevant to them. And the consumption of NO2 by O2 results in the prohibition, as NO2 can catalyze the RDX decay. Besides, the N-N bond fission is observed to initiate the thermal decay of RDX, followed by the ring breakage. Beyond these two pathways, some additional ones are found to deepen the decay. Among all the reactions, the evolution of ONDNTA can be indicative of the decay degree of RDX, i.e., a high content of ONDNTA suggests a fast RDX decay.

■ ASSOCIATED CONTENT Supporting Information 24 / 29


Page 25 of 29 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


Validation of ReaxFF-lg, comparison in PE and decay rate of the RDX nanoparticle and the bulk RDX, volume evolution of the RDX nanoparticle in the programmed heating simulation, radial distribution functions of the RDX nanoparticle and corresponding snapshots in different temperature, MD simulation snapshots of the thermal decomposition of RDX nanoparticle in the NO atmosphere, and thermodynamics for the dimerization reaction of NO. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J. Liu) and [email protected] (C.-Y. Zhang). Author Contributions §

K. Zhong and J. Liu contributed equally to this work.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS Authors are grateful for the financial support from the National Natural Science Foundation of China (11572296). Authors also thanks Dr. Weiyu Xie, ICM, CAEP, for his useful discussion.

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