Asian Journal of Chemistry; Vol. 25, No. 7 (2013), 3855-3859 http://dx.doi.org/10.14233/ajchem.2013.13820 Kinetic Studies of the Non-Isothermal Decomposition of Strontium Nitrate * SUNIL CULAS, ATHIRA SURENDRAN and JADU SAMUEL Department of Chemistry, Mar Ivanios College, Thiruvananthapuram-695 015, India *Corresponding author: Fax: +91 471 2530023; Tel: +91 471 2542124; E-mail: [email protected] (Received: 3 April 2012; Accepted: 14 January 2013) AJC-12710 The thermal decomposition kinetics of strontium nitrate, Sr(NO3)2 was studied by thermogravimetry using non-isothermal experiments. For the kinetic analysis, the TG/DTG data obtained in the temperature range 30-850 ºC at different heating rates (5, 10, 15 and 20 K/min) in the nitrogen atmosphere were processed by model fitting and model free methods. The thermal decomposition of Sr(NO3)2 occurred in a single stage without the formation of intermediate nitrite. The average apparent activation energies of thermal decomposition of Sr(NO3)2 as determined by Straink, Flynn-Wall-Ozawa, KAS and Friedman methods are 344.37, 341.39, 344.05 and 362.62 kJ/mol, respectively. The value of the invariant activation energy (344.83 kJ/mol) obtained by Invariant kinetic parameter method is in a good agreement with integral isoconversional methods. The appropriate conversion model of the process selected by means of the master plot method is "Diffusion model (D4)". Key Words: Kinetics, Non-isothermal decomposition, Strontium nitrate. INTRODUCTION Sr(NO3)2 under non-isothermal conditions with a single heating rate plot (4 K/min) and analyzed the data by model fitting methods. Strontium nitrate in the anhydrous form is a colourless They reported that the decomposition of Sr(NO3)2 sets in after crystalline powder. The principal uses of strontium nitrate are melting and the overall decomposition of the salt occurs as: in the manufacture of pyrotechnics, rescue signaling devices, red tracer bullets for the military etc., as it imparts a charac- Sr(NO3)2 → SrO + 2NO2 + 0.5O2 (3) teristic brilliant crimson colour to a flame. The oxidizing Historically, model-fitting methods were widely used properties of this salt are advantageous in such applications1. because of their ability to directly determine the kinetic triplet The thermal decomposition of common metal nitrates is involving activation energy (E) and frequency factor (A) and an important class of reaction in the chemical industry with conversion function, f(α). But it is well established that force- applications in the preparation of high surface area materials fitting non-isothermal data to different reaction models results for catalysts, molecular sieves and adsorbents as well as they in a widely varying Arrhenius parameters6. The only possible have interest for ecological and environmental reasons2. L'vov way to obtain trustworthy kinetic parameters is to evaluate and Novichikhin3 have suggested a gasification process them in a way that is independent of reaction model. Isocon- whereby the decomposition of metal nitrates proceeds through versional methods are known to allow for model-independent the gaseous metal oxide as an intermediate product: estimation of the activation energy, E, selected to different extents of conversion. So the main goal of this work is to study MNO3(g) → MO(g) + 2NO2 + 0.5O2 (1) the thermal decomposition kinetics of Sr(NO3)2 from non- followed by isothermal multi-heating TG data by a new approach that MO(g) → MO(s) (2) combines the power of isoconversional methods with model- 4 fitting methods. Duval reported that Sr(NO3)2 is stable upto 280 ºC and yields a perfectly horizontal level which starts at room tempe- EXPERIMENTAL rature. Decomposition then sets in and proceeds very slowly but becomes explosive above 600 ºC. This disintegration AR grade Sr(NO3)2 was used without further purification. appears to be completed over 820 ºC. The succeeding horizontal TG/DTG experiments were performed with a SDT Q 600 is due to the formation of strontium oxide. Nair et al.5 studied simultaneous DSC/TGA instrument, in the temperature range the effect of γ-irradiation on the thermal decomposition of of 30-950 ºC, under a dynamic atmosphere of nitrogen at a 3856 Culas et al. Asian J. Chem. flow rate of 50 mL/min. Samples with the mass in the range TABLE-1 of 9-12 mg were put into platinum crucibles, at a heating rate, PHENOMENOLOGICAL DATA FOR THE THERMAL DECOMPOSITION OF Sr(NO 3)2 β, of 5, 10, 15 and 20 K/min. The recorded total % mass-loss AT DIFFERENT HEATING RATES in all cases was 51.33 ± 0.05 confirming the complete conver- Temperature (ºC) sion of Sr(NO3)2 to SrO. The mass of material left behind after β (ºC/min) Ti Tf Tp thermal decomposition agreed with the stoichiometry of the 5 380 679 622 reaction (3) and also with the instrument reading. 10 392 693 635 15 401 702 644 RESULTS AND DISCUSSION 20 409 708 649 Non-isothermal decomposition: Fig. 1 shows the TG/ DTG curves at different heating rates of 5, 10, 15 and 20 ºC/ Evaluation of activation energy by using isoconver- sional methods: In order to apply different kinetic methods min. The decomposition of Sr(NO3)2 occurs totally in one- step process as can be concluded by the presence of only one on the thermal decomposition process of Sr(NO3)2, the peak in DTG. The main observations like temperature of dependence of α versus T at different heating rates for all the samples are plotted (Fig. 2). The sigmoid-shaped curves are inception (Ti), the temperature of completion (Tf) and peak shifted to higher temperatures with an increase of heating rates temperature of decomposition (Tp) obtained from thermal as reported earlier8,9. Initially, the activation energy for the curves at different heating rates are summarized in Table-1. In 10,11 decomposition of Sr(NO3)2 was estimated by using FWO , the present investigation Sr(NO3)2 melted at 570 ºC which is 12 13 14,15 in good agreement with the melting behaviour reported5,7. The KAS , FR and straink methods. The details of these methods have already been described16,17. general shift of Ti, Tf and Tp to higher temperature occurs when the heating rate is increased. This is typical for all non-iso- thermal experiments as described in the literature by different 1.0 researchers8,9. 5 0 C/min 10 0C/min 0 0.8 15 C/min a 0 20 C/min 100 0.6 0 α α 90 5 C/min α α 10 0C/min 15 0C/min 0.4 80 20 0C/min 70 0.2 % Mass loss Mass % 60 0.0 780 810 840 870 900 930 960 990 50 Temperature (K) 100 200 300 400 500 600 700 800 Fig. 2. α-T curves for the thermal decomposition of Sr(NO3)2 as a function of temperature at different heating rates Temperature ( 0C) FWO method is an integral method which is based on the b measurement of the adequate temperature to certain values of 0 the conversion α, for experiments effectuated to different rates of heating. The equation corresponding to this method is 1 1- 5 0C/min -5 0 2- 10 C/min 2 AE E 0 ln β = ln − .5 331 − .1 052 3- 15 C/min (4) 0 Rg (α) RT 4- 20 C/min -10 3 where g(α) is integral reaction model, α is extent of conversion, E is activation energy, A is pre-exponential factor, T is tempe- -15 4 rature and R the gas constant. For selected α values from 20-90 %, the plots of ln β versus 1/T give a group of straight Derivative mass (%/min) Derivative -20 lines (Fig. 3). The values of activation energy E were calcu- lated from the slopes of the regression lines and are given in 200 250 300 350 400 450 500 550 600 Table-2. Temperature ( 0C) KAS method is one of the best isoconversional methods Fig. 1. Experimentally obtained (a) thermogravimetric (TG) and (b) and is based on the equation differential thermogravimetric (DTG) curves at the different heating β AR E rates (β = 5, 10, 15 and 20 k/min), for the thermal decomposition ln = ln − 2 (5) of Sr(NO3)2 T Eg (α) RT Vol. 25, No. 7 (2013) Kinetic Studies of the Non-Isothermal Decomposition of Strontium Nitrate 3857 -9.6 425 FR -10.0 400 Straink KAS FWO -10.4 ααα = 0.2 375 0.3 1.92 0.4 /T -10.8 β 0.5 350 ln 0.6 E (kJ/mol) -11.2 0.7 0.8 325 0.9 -11.6 300 1.05 1.08 1.11 1.14 1.17 0.2 0.4 0.6 0.8 1.0 1000/T α Fig. 3. Straink plots for the determination of activation energy at different Fig. 4. Dependence of the activation energy (E) on the degree of conversion (a) determined using the FR, Straink, KAS and FWO methods for a for the thermal decomposition of Sr(NO3)2 the thermal decomposition of Sr(NO3)2 TABLE-2 ACTIVATION ENERGIES AT DIFFERENT EXTENT OF Determination of reaction model by using master plot CONVERSIONS ( α) OBTAINED FROM TG DATA BY method: Integral master plot method19 was used for the deter- DIFFERENT ISOCONVERSIONAL METHODS FOR THE mination of the reaction model for the decomposition of THERMAL DECOMPOSITION OF Sr(NO 3)2 Sr(NO3)2. Essentially the master plot method is based on the Conversion Activation energy, E (kJ/mol) comparison of theoretical master plot, which are obtained for (α) FR Straink FWO KAS a wide range of ideal kinetic models, with the experimental 0.2 359.15 338.47 335.12 338.18 master plot.