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Kinetics of Thermal Decomposition of Nano Magnesium Oxide Catalyzed Ammonium Perchlorate.

Byline: Zaheer-ud-din Babar and Abdul Qadeer Malik

Summary Arrhenius kinetic parameters of ammonium perchlorate (AP) catalyzed with nano-sized magnesium oxide (MgO) has been determined in this work. Nano particles of MgO with an average size of approximately 20 to 30 nm have been used to catalyze the AP. The particles were characterized using Scanning electron microscope (SEM) and X-ray diffraction (XRD) techniques before mixing with AP. Simultaneous Thermal Analysis (STA) shows that MgO nanoparticles have a strong catalytic effect on the thermal decomposition of ammonium perchlorate. The addition of MgO nano particles reduces the two stage decomposition of ammonium perchlorate to a single stage. Arrhenius kinetic parameters of pure and the catalyzed AP have been calculated using non isothermal kinetic approach based on Kissinger method. The comparison of the thermal behavior and kinetic parameters of pure and catalyzed AP has also been carried out to elucidate the reaction mechanism.

The results show that the activation energy of the catalyzed AP has increased from 138.1 kJ/mole to 159.1kJ/mole. The rate of reaction however has increased in the catalyzed AP showing that it has become more reactive by the addition of MgO nano particles. The enthalpy of activation has increased by 16 percent in the catalyzed AP.

Keywords: Kinetic parameters Ammonium perchlorate Magnesium oxide Nanoparticles. Introduction

Ammonium perchlorate finds extensive use as inorganic oxidizer for composite solid propellants. It is one of the most widely researched oxidizer due to its high utility in various propellant and pyrotechnic compositions [1 2]. A remarkably large quantity of this oxidizer is also used in space shuttles [3]. More than a million pounds of ammonium perchlorate is consumed in space shuttles in a single launch. The nature of the thermal decomposition of the ammonium perchlorate plays an important role in the combustion behavior of the propellants. Thermal decomposition of ammonium perchlorate is known to exhibit significantly high sensitivity to different kinds of additives. The additives are assumed to influence the rate of decomposition of AP based composition [4]. Effect of nano metals and nano sized transition metal oxides on the thermal decomposition of ammonium perchlorate has therefore been an area of interest for the researchers [5-15].

The effect of addition of MgO nano particles on the thermal as well as the kinetic behavior of ammonium perchlorate has been investigated. The nano metal oxides are considered to be more efficient in terms of their catalytic activity due to greater exposed area as compared to micro sized particles [16 17]. The MgO nanoparticles having an average size of 20 to 30 nm were used as a catalyst to increase the reactivity. The nano particles were characterized by using Scanning electron microscopy (SEM) and X-ray diffraction. Decomposition kinetics of ammonium perchlorate strongly depends upon the morphology and size of the nano additives. The catalytic effect of nano MgO on thermal decomposition of AP has been reported previously by Guorong Duan et al. [18].The earlier work reports the effect of different percentages of nano magnesium oxide on the thermal decomposition of AP and shows that two decomposition stages completely merge with each other by using 4 percent MgO as a catalyst.

At lower percentages of MgO the decomposition stages do not completely merge into each other. The present work however focuses on the effect of MgO nano particles on the kinetic parameters of the modified AP which have not been reported in the earlier work.

Thermal analysis is an effective tool for the study of energetic materials and their ingredients. It is widely used to obtain thermal and kinetic data to gain an insight into the decomposition reactions [19-21]. The kinetic data gives useful information regarding various mechanisms that take place during the course of decomposition. Simultaneous thermal analysis has been used to accomplish this experimental work. The kinetic parameters of pure and catalyzed ammonium perchlorate have been calculated using non isothermal approach based on Kissinger method.

Experiments show that nano MgO has made a strong catalytic effect on the decomposition of ammonium perchlorate and reduced the two distinct stages of decomposition of pure ammonium perchlorate to only one stage. The focus in this work has been to determine the kinetic parameters such as activation energy frequency factor reaction rate constant as well as the enthalpy of activation. Comparative analysis of the pure and the catalyzed ammonium perchlorate has been carried out to monitor the changes in thermal and kinetic parameters. This work attempts to elucidate the decomposition mechanism of the pure AP and the one catalyzed with a small amount of nano MgO on the basis of data obtained during the analytical experiments.

Results and Discussion

Analysis of Nano particles

The SEM image of the nano sized MgO is shown in Fig. 1. The nano particles show good regularity in terms of their morphology and are found to be very well dispersed. The particles are elliptical in shape and not perfect spheres. The average diameter of the elongated side of particles measured from SEM images is between 20 to 30 nm. The measurements show that the average diameter of the shorter side of these particles is between 15 to 20 nm. The MgO powder therefore can be truly regarded as nano sized and considered to be very suitable for the catalytic activity.

The p-XRD pattern of nano sized MgO powder is shown in Fig. 2. Five main diffraction peaks can be seen in the spectra and these peaks are located at 37.92o 42.56 o 62.19 o 73.26 o 78.42 o. These peaks typically represent MgO and the peak data is in fair agreement with the reference pattern No 01-074-1225. Miller Indices have been assigned to all the five peaks. The peaks are in general broad showing that the particle size is fairly small. The crystallite size of the MgO particles has been calculated using Scherer formula for three different faces and the average crystallite of the sample was found close to 9nm.

Analysis of Pure AP

The heat flow curve of pure ammonium perchlorate (Fig. 3) shows three major peaks representing three distinct events. First endothermic peak of ammonium perchlorate appears near a temperature of 241C and corresponds to solid phase transformation during which the crystal structure of ammonium perchlorate changes from orthorhombic to cubic [22]. Next two peaks are exothermic in nature. The first exothermic peak appears at 310.7C and it marks the low temperature decomposition stage of ammonium perchlorate. The second exothermic peak appears at 384.1C and marks the high temperature decomposition stage of ammonium perchlorate. Fig. 3 also shows the weight loss curve of ammonium perchlorate. The weight loss curve shows two events as compared to three events exhibited in heat flow curve. The first endothermic peak corresponding to phase change is not associated with any weight loss.

It is confirmed from TG curve that decomposition of pure ammonium perchlorate is a two stage process. The first weight loss stage is between 280C and 320C resulting in more than 20 percent weight loss. The second weight loss stage starts at 340C and ends close to 400C causing complete decomposition of ammonium perchlorate Ammonium perchlorate decomposes to form various products. The decomposition equation of ammonium perchlorate has been reported as follows [2].Equation

The effect of heating rate on the heat flow curve has been shown in Fig. 4. The figure shows that the first endothermic peak does not significantly change with the heating rate; however there is a noticeable change in low temperature as well as high temperature decomposition peak temperature. These peaks shift to a higher temperature with increase in the heating rate. The onset and peak temperatures for low temperature and high temperature decomposition stages change significantly when the heating rate is increased from 2oC/min to 20oC/min (Table-1). The data presented in Table-1 has been used to calculate the kinetic parameters of pure AP.

Table-1: Effect of heating rate on decomposition peaks of pure AP.

###Low Temperature###High Temperature

Heating

###Stage###Stage

###Rate

###Onset###Peak###Onset###Peak

(oC/min)

###Temp (oC) Temp (oC)###Temp (oC) Temp (oC)

###2###258.7###276.3###326.4###357.9

###6###267.8###299.1###338.6###374.2

###10###283.9###310.7###342.2###384.1

###20###299.5###332.9###357.6###412.1

kinetic parameters of low stage decomposition of AP based on Kissinger method is shown in Fig 5(a) while the graph for calculation of the kinetic parameters of high temperature decomposition stage has been shown in Fig 5 (b). The data points of the plot of ln AY/Tp against the reciprocal peak temperature (1/Tp) fit into a straight line and the slope of this line gives the value of the activation energy. The activation energy of AP by Kissinger method has been found to be 103.7 kJ/mole and 138.1 kJ/mole for low temperature and high temperature decomposition stages respectively. The data points obtained from the plot of ln AY/T 2 against (1/T ) also fit into a straight line and the activation energy is determined from the slope of this line. The corresponding values of frequency factors are 1.17x 107 sec-1and6.13x 108 sec- 1 for low and high temperature decomposition stages respectively.

The rate of reaction constant for low temperature stage is 6.1 x 10-3 sec-1 and for the high temperature stage is 6.4 x 10-3 sec-1. The values of activation energy frequency factor rate of reaction constant and enthalpy of activation have been presented in Table-3 for both the stages separately. Analysis of Catalyzed AP

The heat flow curve of AP catalyzed with 4 percent MgO nano particles is shown in Fig. 6. This curve is quite different from the one for pure AP.

A total of two peaks can be seen from the figure as opposed to three peaks of pure AP. First endothermic peak which represents the phase change of AP remained unchanged and appeared almost at the same temperature showing that the catalyst did not interfere much with the phase transformation of AP. The second peak which is exothermic in nature represents the rapid decomposition of AP due to addition of MgO. This peak appears at a temperature of 364oC and shows that the two stages of AP decomposition have now reduced to one. The decomposition of the catalyzed AP takes place in a single stage at a temperature which is higher than the low temperature decomposition stage and lower than the high temperature decomposition stage. The TG curve of the catalyzed AP confirms this result and a sharp decline in the weight of the sample can be seen in a single step as opposed to two distinct weight loss steps for pure AP.

The effect of heating rate on the thermal decomposition of the MgO catalyzed AP is shown in Fig. 7. The curve shows that the first endothermic peak related to the phase transformation remains almost unaffected. On the other hand the decomposition peak shifts to higher temperatures with increasing heating rate. The decomposition peak temperature increases from 335oC to 381oC when the heating rate is increased from 2oC/min to 20oC/min. Peak temperature data at different heating rates has been used to determine the kinetic parameters of decomposition reaction of catalyzed AP. The kinetic parameters of AP catalyzed by MgO were estimated by using Kissinger method for comparison with the kinetic parameters of pure ammonium perchlorate. The peak temperature data obtained from multiple heating rate experiments is presented in Table-2. Representative graph of Kissinger method for AP+ 4% MgO is shown in Fig. 8.

The plot of ln AY/T 2 against the reciprocal peak temperature (1/Tp) gives a nearly straight line and slope of the line gives the activation energy value of 159.1 kJ /mole for the catalyzed AP. The values of rate of reaction constant frequency factor activation energy and enthalpy of activation for decomposition of the pure and catalyzed AP are presented in Table-3.

Comparison of thermal and Kinetic Parameters

There is a noticeable difference in the thermal behavior and kinetic parameters of pure and catalyzed ammonium perchlorate. The heat flow curve of pure AP shows one endothermic and two exothermic peaks. The addition of nano particles of MgO reduces two exothermic peaks of AP decomposition to one peak showing that both the stages have merged and the decomposition takes place in single stage. TG curve presented in Fig. 6 also confirms this result and shows that the weight loss takes place in a single step. The addition of MgO however has no effect at all on the endothermic peak related to the phase transformation. The decomposition peak of the catalyzed AP is quite sharp as compared to the DTA peak for pure ammonium perchlorate.

Data presented in Table-3 shows that the activation energy of catalyzed AP is higher than pure AP. The activation energy value increases from138.1 kJ/mole to 159.1kJ/mole for the high temperature decomposition stage of ammonium perchlorate as calculated by Kissinger method. Increase in activation energy apparently means that the AP after addition of catalyst has become difficult to decompose. It is generally observed that the addition of catalyst lowers the activation energy; however this effect may differ from case to case. Many researches have reported contrary to this observation based on the experimental results where the activation energy of ammonium perchlorate has increased due to the addition of the catalyst [17 23 24]. The rate of reaction however has increased despite increase in the activation energy in our work due to the addition of catalyst. This is considered to be due to the reason that the rate of reaction is directly proportional to the frequency factor (A).

In our case frequency factor has increased significantly and therefore the overall rate of reaction has also increased.

The value of frequency factor increases from 6.13x 108 s-1to 8.7x1010 s-1 due to the addition of MgO as a catalyst. The rate of reaction constant of catalyzed AP is 7.85 x 10-3 as compared to 6.41 x 10- 3 for Pure AP showing that addition of nano particles of MgO has increased the overall reactivity. Moreover the catalyzed AP gives higher value of the enthalpy of activation. Experimental

High purity defence grade ammonium perchlorate and commercial MgO nano powder have been used to carry out this work. The AP and MgO were mixed in the weight ratio of 96 percent and 4 percent respectively. Thorough mixing of the ingredients was carried out and the samples were pre heated for six hours before the conduct of experiments to make sure that there was no moisture in the sample. MgO nano particles were characterized using Scanning electron microscope (SEM) and X- ray diffraction. XRD instrument by STOE Germany is used for the analysis. The sample was scanned from 10o to 70o. JSM- 6490 scanning electron microscope was used to get the micrographs of nanoparticles. The methods of thermogravimetery (TG) and differential thermal analysis (DTA) have been used to study the decomposition process of pure and catalyzed ammonium perchlorate. Diamond TG/DTA instrument by Perkin Elmer has been used for simultaneous thermal analysis of the sample.

The instrument performs simultaneous TG and DTA measurements on one and the same sample. The aluminum crucibles were used to hold the samples for the analysis. The nitrogen was used for producing inert atmosphere during the conduct of all the experiments. To investigate the effect of heating rate on decomposition peak temperature of ammonium perchlorate catalyzed with nano MgO the sample mass was kept close to 3 mg in all the experiments and the experiments were conducted at four different heating rates i.e. 2oC/min 6oC/min 10oC/min and 20oC/min. Ammonium perchlorate samples were heated up to 500oC till the completion of high temperature decomposition reaction using above mentioned heating rates and sample mass. Decomposition peak temperature data obtained from heat flow curve has been used to calculate the kinetic parameters of pure and catalyzed ammonium perchlorate using Kissinger method. The method is briefly described below.

Table-2: Effect of heating rate on decomposition peaks of catalyzed AP.

###Experiment No###Sample mass (mg)###Heating Rate (oC/min)###Onset Temp (oC)###Peak Temp (oC)

###1###3 mg###2###325.8###335.3

###2###3 mg###6###338.4###353.8

###3###3 mg###10###342.7###364.1

###4###3 mg###20###353.6###380.9

Table-3: Summary of Kinetic data of pure and catalyzed ammonium perchlorate.

###Ea (kJ/mole)###A (sec-1)###k (sec-1)###H#(kJ/mole)

###Composition

###Low###High###Low###High###Low###High###Low###High

###Pure AP###103.7###138.1###1.17x 107###6.13x 108###6.1 x 10-3###6.41 x 10-3###99.1###132.8

###Catalyzed AP###------###159.1###-------###8.7x1010###-------###7.85x10-3###------ 153.8

Kissinger method can be used for variety of applications because it is derived from basic kinetic equations pertaining to the heterogeneous materials. This method is based on the heat flow data obtained through Differential Thermal Analysis (DTA) or Differential Scanning Calorimetry (DSC). The Kissinger method assumes that the rate of reaction is maximum at the peak temperature (Tp) for a given heating rate AY. The sample is thus decomposed at different heating rates and the peak temperature data is used to plot a graph between ln AY/T 2 and reciprocal temperature (1/Tp). The slope of the resulting graph is then used to calculate the activation energy by using the following correlation as shown below [25].

Ea= R d ln[AY/T 2]/[d(1/T )] (2)

Where Ea is the activation energy in kJ/mole R is the universal gas constant AY is the heating rate at which the experiment was performed and Tp is the peak temperature corresponding to a specific heating rate. The value of activation energy obtained through this method can be further used to calculate the pre exponential factor or the frequency factor by the equation given below.

A = (AY E exp Ea/RTp) / (RT 2) (3)

Where A' is the frequency factor or pre exponential factor and rest of the parameters are same as defined above. When both activation energy and pre exponential factor are known Arrhenius relationship is used to determine the specific rate constant (k). k = A exp( -Ea/RTp) (4)

Conclusions

Catalytic effect of MgO nano particles on the thermal decomposition and kinetic parameters of ammonium perchlorate has been investigated using simultaneous thermal analysis including DTA and TG. MgO nano particles were characterized using SEM and XRD. The average size of the nano particles was found to be between 20-30 nm. MgO nano particles had a strong catalytic effect on the thermal decomposition of ammonium perchlorate. Two distinct stages of the decomposition of pure AP i.e. low temperature and high temperature decomposition were reduced to a single stage. High temperature decomposition of catalyzed AP took place at a temperature significantly lower than that of pure AP. Activation energy of the catalyzed AP increased by 21kJ/mole from 138.1kJ/mole for pure AP to 159.1kJ/mole for the catalyzed. The frequency factor A increased significantly and caused the overall increase in the rate of reaction despite increase in the value of activation energy.

The enthalpy of activation has also increased by approximately 16 percent.

Nomenclature

Ea = activation energy in( kJ/mole) R = R is the universal gas constant

AY = heating rate in (oC/min)

ln = natural logarithm

Tp = peak temperature in (Kelvin)

A = frequency factor or pre exponential factor in (sec-1)

k = rate constant H# =enthalpy of activation

Acknowledgement

The research work has been supported by National University of Sciences and Technology (NUST) Islamabad.

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Publication:Journal of the Chemical Society of Pakistan
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Geographic Code:9PAKI
Date:Dec 31, 2014
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