Catalytic decomposition mechanism of aqueous ammonium dinitramide solution elucidated by thermal and spectroscopic methods

Journal Pre-proof Catalytic Decomposition Mechanism of Aqueous Ammonium Dinitramide Solution Elucidated by Thermal and Spectroscopic Methods Rajanna Gugulothu, Macharla Arun Kumar, Kranthi Chatragadda, Anuj A. Vargeese

PII:

S0040-6031(19)30912-8

DOI:

https://doi.org/10.1016/j.tca.2020.178544

Reference:

TCA 178544

To appear in:

Thermochimica Acta

Received Date:

15 October 2019

Revised Date:

28 January 2020

Accepted Date:

3 February 2020

Please cite this article as: Gugulothu R, Kumar MA, Chatragadda K, Vargeese AA, Catalytic Decomposition Mechanism of Aqueous Ammonium Dinitramide Solution Elucidated by Thermal and Spectroscopic Methods, Thermochimica Acta (2020), doi: https://doi.org/10.1016/j.tca.2020.178544

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Catalytic Decomposition Mechanism of Aqueous Ammonium Dinitramide Solution Elucidated by Thermal and Spectroscopic Methods

Rajanna Gugulothu, Macharla Arun Kumar, Kranthi Chatragadda, Anuj A Vargeese* [email protected]

Advanced Centre of Research in High Energy Materials (ACRHEM), University of

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Hyderabad, Hyderabad 500046, India

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*Ph: +91-40-23138708; Fax: +91-40-23012800;

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Ammonium dinitramide (ADN) is environmentally benign, energetic, oxygen rich molecule Condensed phase decomposition of ADN water solution studied using TG-FTIR technique Addition of CuO nanocatalyst changes thermal decomposition pathway of ADN solution Nanocatalyst presence during decomposition leads to formation of higher amount of NO2 Ratios of IR transmittance values of N2O to NO2 confirmed increased amount of NO2

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Graphical abstract

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Abstract Ammonium dinitramide (ADN) is a green, energetic and oxygen rich molecule, developed to replace the commonly used oxidizer ammonium perchlorate (AP). On combustion ADN produces relatively environmental-friendly species such as H2O, NO2, N2O, NO etc., compared to HCl, Cl2, CO2, CO, H2, H2O, N2 and highly reactive chlorine species produced by AP combustion. The ADN synthesized in our laboratory was dissolved in deionized water to prepare aqueous ADN solutions (aq. ADN). The thermal decomposition of solid ADN and aq. ADN were studied and the products of decomposition are identified using a

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thermogravimetric analyser hyphenated with a FTIR instrument. Nano CuO catalyst was synthesized and aq. ADN was decomposed over the nanocatalyst and the catalytic

decomposition mechanism is also elucidated. The study shows that, the condensed phase

decomposition of aq. ADN is influenced by the CuO nanocatalyst leading to the generation of

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higher concentrations of NO2, which autocatalyzes the remaining ADN decomposition. Keywords: Autocatalysis • Thermochemistry • IR spectroscopy • Nanotechnology •

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Hyphenated thermal analysis

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Introduction

Ammonium dinitramide (ADN) is a relatively new energetic oxidizer with excess of stoichiometric oxygen and known to produce environmentally benign combustion products

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[1,2] relative to ammonium perchlorate combustion. The dinitramide anion was first synthesized at the Zelinsky Institute in Russia in 1971 and is one of the most significant discoveries in the field of energetic materials[3,4]. ADN is highly hygroscopic, with critical

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relative humidity (CRH) of ~52% and hence, much efforts are still being devoted to overcome the hygroscopic nature of ADN. The physical properties of ADN is summarized in

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Table 1. Apart from the applications of ADN in solid propellants, there are reports which suggest the applications of ADN-water based liquid propellants for practical applications[5,6].

Table 1. Table Caption. Properties

Ammonium Dinitramide (ADN)

Molecular Formula

NH4N(NO2)2

Molecular Weight

124.07

92.9 °C

Decomposition Temperature (onset) [7]

135 °C

Density [7]

1.812 g/cc at 20 °C

Oxygen Balance

+25.80

Heat of Formation [7]

-148 kJ/mol

Heat of Explosion [7]

2668 J/g

Impact Sensitivity [7]

3.0-5.0 Nm (h50%)

Electrostatic Discharge [7]

0.45 J

Friction sensitivity [7]

72 N

Isp (monopropellant)[a]

202 s

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Melting Point [7]

calculated using EXPLO 5 ver. 6.03 (calculation details are provided in

supplementary material)

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Many reports are available on the analysis of decomposition mechanism of ADN[8, 9, 10]. The experimental investigations on the ADN near burning surface was investigated earlier

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and compared with that of ammonium nitrate (AN). The study [11] used T-Jump/FTIR technique and identified N2O, NO2, NO, HNO3, NH3 and small amount of ammonium nitrate

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as the decomposition products. Dissociation of ADN to form the corresponding acid, dinitramidic acid (HDA), and corresponding base NH3 and their subsequent decomposition to N2O, NO, H2O and O2 is also reported in literature [12, 13]Kinetics and mechanism of

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decompaction of ADN is discussed in another investigation [14]. The study [14] also reports the identification of NO, NO2 and N2O in the early stages of ADN decomposition and NH3 and H2O in the later stages.

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Another study with 15N and 2H istopomers of ADN suggests a similar decomposition mechanism [15]. The study concluded that, above 160 °C the dominant decomposition mode

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is free radical and ionic decomposition mode dominate below that temperature. The ADN aqueous thermolysis at 160 °C in PH buffer solutions identified the existence of NO3- [15] The theoretical investigation of ADN decomposition is also carried out by many investigators [16, 17, 18,19] and they report the possible formation of NO2, N2O, NO3, NO, H2O, NH3 and AN [16]. To understand the physical and chemical behavior of aqueous ADN solution (aq. ADN), samples are prepared and the decomposition behavior was studied. Based on the TG-FTIR

studies, catalyzed (by CuO nanocatalyst) and non-catalyzed decomposition mechanism of aq. ADN decomposition is elucidated. The application of micron sized CuO and impregnated alumina doped by copper oxide catalysts for ADN decomposition is reported in some of the earlier studies [6,20] however, we made a detailed investigation on the differences in mechanism of decomposition due to the presence of CuO nanocatalyst. Experimental Section Caution!!! ADN is a moderately sensitive energetic oxidizer. Although we have encountered no difficulties during handling, ADN should be handled with extreme care and appropriate standard safety precautions. Mechanical actions involving scratching or scraping of ADN

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must be avoided. Synthesis of Ammonium dinitramide (ADN)

The ADN required for the study was prepared by the nitration of nitramide [3][4]]. The

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formation and purity of synthesized ADN was confirmed by melting point (DSC &TG-DTA), FTIR, CHNS and HRMS. The as synthesized ADN with high purity (>99% by LC (Figure

Supplementary material.

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Preparation of aqueous ADN solution

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S1)) was used for further experiments. Details of analysis and characterization is provided as

ADN solution in water was prepared in three different concentrations, 0.3 g, 0.6 g and 1 g in 0.5 mL of deionized water referred as sample A, sample B and sample C, respectively. The

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solution was thoroughly mixed for 30 minutes and then used for further analysis or stored in sealed micro centrifuge tubes under refrigeration.

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Preparation of CuO nanocatalyst

CuO nanocatalyst was prepared through the aqueous thermolysis method using Copper

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acetate and glycine. The detailed procedure of synthesis is reported in our previous article [1]. The synthesized CuO nanocatalyst was characterized by High Resolution Transmission Electron Microscopy (HRTEM), selected area electron diffraction (SAED) and powder Xray diffraction (PXRD). Thermogravimetric analysis (TGA) Thermal or catalytic decomposition experiments of ADN liquid propellant was carried out under nitrogen atmosphere in a TA instruments SDT Q600 TG/DSC instrument. Nitrogen at a

flow rate of 100 mL min-1 was used as the purge gas in all the TG experiments. For the TG experiments, ~1 mg of CuO nanocatalyst was added to the 90 µL alumina pan and then 5 µL of aq. ADN, with varying concentrations, was added over the nanocatalyst. The nonisothermal TG runs were conducted at heating rate (β) 5 °C min-1 and the collected data was used for further analysis. Evolved gas analysis-simultaneous TG-FTIR TG-FTIR experiments were carried out using a TA instruments SDT Q600 TG/DSC coupled to a TG-FTIR cell placed in the Bruker Tensor II FTIR Spectrometer. The TGA experiments were conducted under the similar conditions as explained above. The transfer line

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temperature was maintained at 150°C and IR cell temperature was maintained at 180°C.

FTIR data collection was manually triggered at the pre-determined temperature and the IR

data was collected at 40 seconds interval with a wavenumber resolution of 1 cm-1. For each

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spectrum 8 scans were conducted and each scan lasted for ~2.6 s. Results and Discussion

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ADN melts at 92.9 °C and the decomposition starts at 135 °C with peak temperature at 186 °C [7, 22] In the present study the synthesized ADN was confirmed by DSC, FTIR, CHNS

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and HRMS. The DSC (Figure S2) showed the melting peak at 91.48 °C and decomposition peak at 181 °C. The FTIR data (Figure S3) was compared with the results available in [22] and found to match without any deviation. The CHNS data also confirmed formation of

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ADN. (EA H4N4O4 (124.0233) calcd: H, 3.25; N, 45.16; found H, 2.99; N, 43.87%) During MS analysis, in the ESI- (Figure S4), base peak appeared at m/z 105.9885 (exact mas m/z= 105.9889), corresponding to the dinitramide [N(NO2)2]- ion. The TG-DTA data (Figure

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1) shows melting at 93.36 °C and decomposition at 185.49 °C. An endothermic peak at 210 °C was observed corresponding to ammonium nitrate (AN) decomposition, indicating the

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formation of AN as one of the recombination products [22]. Figure 2, shows the TEM images recorded for the CuO nanoparticles synthesised through aqueous thermolysis method. The TEM images showed CuO nanoparticle with 50 nm size and the particles are free standing and well separated. The PXRD reflections (Figure S5) confirmed the formation of monoclinic CuO as it matches well with JCPDS 05-0661 data.

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Figure 1. TG-DTA curves of ADN obtained at β=5 °C min-1.

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Figure 2. (a) TEM image of CuO nanoparticles with Electron Diffraction pattern as inset (b) HRTEM image of CuO nanoparticles.

Aq. ADN samples were prepared with varying concentration of ADN and designated as

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sample A (0.3 g/ 0.5 mL), Sample B (0.6 g/ 0.5 mL) and Sample C (1 g/ 0.5 mL). The TG & DTA data obtained for Sample A, Sample B and Sample C and the change in decomposition behaviour in presence of CuO nanocatalyst is shown in Figures 3. All the samples exhibited a

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broad endothermic peak below 100 °C indicating the evaporation of water. The solid ADN decomposition peak observed at 185 °C was shifted to slightly higher temperatures, ~189 °C

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for aq. ADN, irrespective of the concentration of ADN in the samples. The previous report on isotope labelled ADN decomposition [Error! Bookmark not defined.] revealed that, in solid ADN, the rate of loss of the ammonium cation was about half that of the dinitramide anion. In neutral solution, the rate of ammonium cation disappearance was less than one-tenth the rate of dinitramide anion loss. Possibly a similar behaviour has slowed down the decomposition of aq. ADN samples. However, similar to the case of solid ADN, the aq. ADN decomposition phenomenon was completed below 220 °C. Interestingly, the decomposition

peak corresponding to AN was completely disappeared, indicating a possible change in the decomposition behavior.

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(a)

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(b)

Figure 3. (a) TGA and (b) DTA curve of aq. ADN samples (β=5 °C min-1).

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When the catalytic decomposition process was analysed, the presence of CuO nanocatalyst induced remarkable changes in the aq. ADN decomposition behaviour. A substantial change

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was observed in the decomposition temperature as well as in the decomposition outline. Compared to the decomposition peak of aq. ADN samples, the peak temperature was lowered to 141 °C for CuO catalysed decomposition, indicating a staggering difference of ~50 °C. Also, the peak temperature was significantly lower than the solid ADN decomposition temperature. When the aq. ADN was decomposed over nanocatalyst, the TG data showed a sudden mass loss pointing towards a sharp increase in the decomposition rate. Analysis of the DTG curve (Figure S6), for all the samples including the solid ADN clearly indicates the

changes in decomposition temperature, decomposition behaviour and increase in the reaction rate. The TGA instrument was then coupled with FTIR instrument and the evolved gas analysis was performed. The thermal analysis parameters, such as sample quantity, heating rate and nitrogen flow rate were maintained as explained in the experimental section. The FTIR data obtained for evolved gases is shown in Figure 4 & Figure 5. The data for Sample A & Sample B is given as Supplementary material (Figure S7 & S8) The 3D plots show the variation in absorption with respect to wavelength and time/temperature. Also, The increase in the reaction rate was evident from the presence and absence of absorbing spices in the IR

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cell. Hence, for the catalysed decomposition, relatively a smaller number of spectra was obtained in the given time/temperature interval owing to the faster reaction rate. The TG-

FTIR data of solid ADN, is shown in figure 4. As it can be seen that, the data obtained for

varying concentration of aq. ADN samples were similar, the analysis details of only sample C

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0.90

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without (Figure 5 (a)) and with (Figure 5 (b)) nanocatalyst is discussed further.

0.93

0.96

0.99

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199.93

193.27

( C )

186.61

3500

3000

2500

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133.33

4000

2000

1500

1000

-1

Wavenumber (cm )

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Figure 4. TG-FTIR of solid ADN.

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m

Te

153.31

146.65

139.99

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ra t

173.29

166.63

159.97

e

179.95

0.918

(a)

(b)

0.912

0.935

0.931

0.952

0.950

0.969

0.969

0.986

0.988

163.27

226.57 219.91 213.25 206.59 199.93 193.27 186.61 179.95 173.29 166.63 159.97 153.31 146.65 139.99 133.33

156.61 149.95

3500

3000

0.918

2500

2000

Wavenumber (cm-1 ) 1500 A

(b)

ra

pe

m

109.99 103.33

4000

1000

Te

116.65

tu r

e

pe ra tu re

123.31

Te m

4000

o (C

)

(oC

)

143.29 136.63 129.97

3500

3000

2500

2000

Wavenumber (cm-1 ) A

1500

1000

0.912

0.935

0.931

0.952

0.969

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0.950

0.969

0.986

0.988

163.27

226.57 219.91 213.25 206.59 199.93 193.27 186.61 179.95 173.29 166.63 159.97 153.31 146.65 139.99 133.33

156.61 149.95

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m Te

Te m

ra

129.97 123.31

116.65

tu r

e

pe ra tu re

o (C

)

(oC

)

143.29 136.63

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109.99 103.33

4000

3500

3000

2500

2000

Wavenumber (cm-1 ) A

1500

1000

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Figure 5. TG-FTIR of (a) sample C (b) sample C + CuO.

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In all the samples the major species observed remains to be N2O, NO2, H2O and NH3. The species are confirmed by their characteristic absorption bands at 1275, 1302, 2217, 2457 (N2O), 1621,2910 (NO2), 3600-3900, 1400-1700 (H2O) and 970, 928 cm-1 (NH3). Apart from

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these species, very weak absorption by NO was observed. Along with these species, the possibility of evolution of IR inactive homonuclear diatomic species N2 and O2 is predictable. Figure 6 - Figure 10 (and Figure S9-S11) shows the specific absorbing region of a particular

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species highlighted for better understanding and confirmation of the species. The data is compared with NIST chemistry webbook [23] data obtained for that particular species. In all

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the spectra shown in Figure 6 - Figure 10, the curve overly was prepared in such a way that the y-axis. Ie., transmittance, scale interval remains exactly same, excluding that of NIST chemistry webbook data used for comparison. The plot stacking shown in Figure 6 is independent, however, for other figures the curves were stacked very close for incorporating more curves and for better comparison. It may be noted that a direct comparison of FTIR spectra was not possible owing to the difference in the peak decomposition temperature. To overcome this difficulty, we looked at the ratios of absorption values of N2O to NO2 and

given in Table 2. The data indicate that the NO2 concentration increases during aq. ADN decomposition and much more during the CuO catalysed aq. ADN decomposition. Table 2. Ratios of peak transmittance value observed in the FTIR data Sample

N2O:NO2[a] N2O:NO2[b]

N2O:NO2[c]

ADN (@166 °C)

1:9609

1:9614

1:9612

Sample C (@186 °C)

1:9786

1:9795

1:9818

Sample C + CuO (@136 °C)

1:9869

1:9812

1:9831

Highest; [b]Second Highest; [c]Third Highest N2O intensity peak

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[a]

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Figure 6. FTIR spectra of ADN, Sample C and sample C+CuO, for peak N2O transmittance.

Figure 7. H2O absorption compared with gases evolved during decomposition of different ADN samples.

Figure 8. N2O absorption compared with gases evolved during decomposition of different

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ADN samples.

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ADN samples.

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Figure 9. NO2 absorption compared with gases evolved during decomposition of different

Figure 10. NH3 absorption compared with gases evolved during decomposition of different ADN samples. Most of the energetic compounds are known to decomposes through different pathways depending on the temperature and pressure conditions and other analysis environment variables. Ammonium nitrate is a typical compound which is known to decompose through five different pathways depending on the temperature [24]. Similarly, ADN also proposed to have been decomposing in different pathways [11, 15]. In most of the onium salts (eg. NH4ClO4, NH4NO3), the first stage decomposition is dissociative sublimation through proton transfer and produces ammonia and the corresponding strong acid. The ammonia formed is

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further oxidized by the strong acid formed during the dissociation. According to the literature [14,15] the decomposition of ADN is proposed to have two different mechanisms as shown in (R1) & (R2). Reaction (R1) usually occurs below 130 °C and leads to the formation of AN

through the recombination of NH3 and HNO3. However, reaction (R2) proceeds through the

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formation of HDA and its subsequent decomposition to HNNO2 and NO2. Reaction (R3) is

the overall decomposition proposed [14] for the ADN decomposition. Further to add, earlier

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catalyses the ADN decomposition [15].

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studies have confirmed that N2O is exclusively formed from dinitramide and NO2 auto

Scheme 1. Degradation pathways of ADN.

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In the present study, the addition of CuO nanocatalyst has changed the absorbance values of N2O and NO2 indicating a concentration difference. This is accounted through the different

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possible mechanism of ADN decomposition. Solid ADN decomposition in the present study produced AN as the recombination product. The presence was indicated by the endothermic peak appeared at ~220 °C. The absence of such peak in aq. ADN samples indicate the possibility of following the path (R2). The AN formed in the case of solid ADN decomposition will further decompose to produce H2O and N2O as given in (R5). The aq. ADN decomposition releases slightly larger amount of NO2 at a given temperature as confirmed by the ratios between peak NO2 and N2O absorption values. This probably indicate

that during aq. ADN decomposition both pathways (R1) & (R2) is occurring. Though the major decomposition path remains to be (R1), small quantities of ADN is decomposing through (R2). When the aq. ADN was decomposed over CuO nanocatalyst, N2O presence was detected at very low temperatures as 100 °C with peak absorption at 136 °C, and the evolution was completed by 146 °C. This indicates the catalytic effect of CuO has on the aq. ADN. It may be noted that the N2O from aq. ADN (w/o catalyst) was observed above 133 °C. The CuO is known to interact with the basic and acidic species, NH3 and HN(NO2)2 in the present study, by hydrogen-bonding or coordination through Lewis acid sites and initially promotes the dissociation reaction [25]. This interaction and adsorption can lead to the

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formation of intermediates and this may promote the breakdown of ammonia [26] as well as the reactive HDA to smaller species.

To develop further insight, and explore the possibilities of formation of copper salts such as Cu(NO3)2 by the reaction between ADN and CuO and its subsequent decomposition, we

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analysed the thermal decomposition of aqueous Cu(NO3)2 solution by TG-FTIR. The TGDTA data is shown in Figure S12. The TG-FTIR data (Figure S13) shows that the water

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evaporation occurs till 100 °C and loses approximately 60% of mass. Above 100 °C the slow decomposition continuous till 210 °C and completely decomposes after that with a peak temperature at 238 °C. In the 100-210 °C region the IR analysis reveals H2O as the major

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species with small amount of N2O and NO2. Comparing this data with catalysed aq. ADN decomposition, and considering the high thermal stability of copper salts, the possibility of

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formation of any salt such as Cu(NO3)2 can be rules out. The reason for the lowering of decomposition temperature by nanocatalyst can be attributed to two reasons, the primary catalytic activity by CuO and secondary autocatalytic activity by

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the in-situ generated NO2. This may be concluded from the significant lowering of peak decomposition temperature of catalysed ADN decomposition.

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Conclusions

Free standing nano CuO has been proved to be a good decomposition catalyst for the aq. ADN. Decomposition of aq. ADN over nano CuO has brought down the thermal stability of the system to 100 °C, as confirmed by the evolution of N2O at this temperature. The interaction of ADN dissociation products NH3 and HDA with CuO has led to the catalytic activity. Also, the NO2 is known to auto-catalyse the decomposition reaction of ADN and which synergistically played a role in bringing down the decomposition temperature of ADN.

In conclusion, the aq. ADN decomposes through different pathways in presence and absence of nano CuO catalyst and exhibits a different thermal decomposition behaviour. Evidently, this is supported by the differences in the concentration of the species formed during the decomposition reactions.

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Acknowledgements

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The research is a part of ACRHEM Phase-III, which is fully funded by DRDO (India).

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