Kinetics and Thermal Decomposition of Tetrahydrazinelanthanum ( III ) 2-hydroxy-1-naphthoate

Tetrahydrazine lanthanum 2-hydroxy-1-naphthoate [La2(NA)3L4] (where NA and L indicate C10H6(2-O)(1-COO) and N2H4 respectively) was synthesised and characterised by micro elemental analysis, IR spectroscopy, simultaneous TG-DTA and X-ray diffraction (XRD) analysis. The results indicate that the compound loses hydrazine molecules in the temperature range 65 150 °C and decomposes to La2O3 at 675 °C through different intermediates, [La2O(CO3)2], [La2O2CO3] at 426 °C and 580 °C respectively. The intermediates of thermal decomposition and final product were confirmed by IR and X-ray diffraction. Coats and Redfern method has been implemented for the estimation of various kinetic parameters viz. energy of activation (E), pre-exponential factor (A),order of decomposition (n).The thermodynamic parameters like enthalpy of activation (H) entropy of activation (S) and free energy of activation (G) have been calculated by using thermogravimetric data.


INTRODUCTION
2][3][4][5] Complexes of aromatic carboxylates with hydrazine on thermal decomposition yield oxide in nanosize with high purity due to the fueling effects of hydrazine and high carbon content. 6Rare earth metal oxides are used in wide high-tech applications.The hexagonal structure phase of La 2 O 3 is stable up to 2050 °C.It is an important component of automobile exhaust gas conversion 7 and it is a catalyst support in the formation of gas conversion catalyst. 8La 2 O 3 is an ingredient for the manufacture of piezoelectric and thermoelectric materials 9 and as a catalyst of oxidative coupling of methane. 10It is also used as a refractory oxide for calcium light, optical glass 11 and in the creation of ceramics as core arc electrodes. 12oreover, the thermo-analytical study by Bakiz et  al reveal that all lanthanum complexes decompose through various intermediates, LaOHCO 3 , La 2 O 2 CO 3 forming lanthanum oxide as final product. 13& 15 Though many lanthanum aromatic carboxylates have been reported, complexes using 2-hydroxyl-1-naphthoic acid (NA) are little known.In this paper, preparation and characterisation of a new lanthanum complex using the above acid and hydrazine, a good bridging ligand.The thermal decomposition of the complexes has been thoroughly studied and kinetic study has also been performed using Coats Redfern equation.

Preparation of Tetrahydrazinelanthanum (III) 2hydroxy-1-naphthoate
The title compound was prepared by adding slowly lanthanum nitrate solution [prepared by dissolving lanthanum oxide (0.325 g, 1mmol) in minimum amount of conc.HNO 3 (99.9%pure)] into a solution of ligand (60 mL).It is formed by mixing the acid (0.188 g, 1 mmol) and hydrazine hydrate (0.2 g, 4 mmol) in 60 mL of water, and stirring the solution at pH 5 for about 1 hour.The solution was kept for evaporation for ½ hour over hot water bath at 90 °C.A cluster of micro crystalline solid developed on cooling.The compound separated was filtered, washed with water, alcohol and then with ether and dried in a desiccator.

Thermal analysis
The TG-DTA experiments were carried out using Perkin Elmer, Diamond TG/DTA analyzer, in air atmosphere at a heating rate of 10°C/min, from room temperature to 721 °C using 5 to 10 mg of the samples.Nitrogen was used as a burger gas.Platinum cups were employed as sample holders and alumina as reference.

XRD analysis
The X-ray powder diffraction (XRD) patterns of the complexes were recorded using Bruker X-ray diffractometer (model AXS D8 Advance) employing Cu-K ( = 1.5406) radiation with nickel filter.The operational settings for all the XRD scans are voltage: 40 kV, current: 30 mA, range: 3°-60° (2¸), scanning speed: 8°/min, slit width: 0.02°.For identification purpose, the relative intensities (I/ I°) and the d-spacing (A°) are compared to standard diffraction patterns were matched with JCPDS standards.

Electron Microscopy
Samples of the final decomposition products (675 °C) were examined in a JEOL model JSM-6390 LV scanning electron microscope to characterize in the texture.

IR spectrum
The IR spectrum of the complex is shown in (Figure .1. a).It shows a strong absorption band at 979 cm -1 due to N-N stretching vibration and a broad band stretching from 2800-3500 cm -1 most probably due to  N-H .These observations indicate that hydrazine is present as a bidentate bridging ligand in the complex.The former peak is not found in the case of pure acid.Moreover, a sharp peak  observed at 1680 cm -1 for carbonyl stretching is not observed in the case of complex but it is found to split into two peaks centred at 1572 and 1396 cm -1 owing to asymmetric and symmetric carbonyl stretching.Further, peaks observed at 591 and 500 cm -1 are attributed to stretching of La-O and La-N bonds respectively.These facts substantiate the complex formation of lanthanum with acid and hydrazine.

Thermal analysis
Simultaneous TG and DTA curves of the complex are shown in (Figure .2) and the results are shown in (Table .1).The thermo analytical curves indicate the degradation of the complex in four steps.
The decomposition begins with the elimination of hydrazine molecules in the temperature range 65-150 °C showing endotherm at 85 °C.All the four molecules of hydrazine are eliminated taking temperature from the system.Hence, the decomposition is endothermic.The elimination is confirmed by taking the IR of the residual at 150 °C (Figure .1.b).The spectrum shows a complete vanishing of peak at 970 cm -1 which appear in the IR of the complex (Figure .1.a).Further the stretching due to N-H is also lost in the IR spectrum of residual.
Step-2 indicates the decomposition of lanthanum carboxylate in to [La 2 O(CO 3 ) 2 ] which shows similar range of decomposition point, 426 °C and similarity in IR spectrum (Figure .1.b.) conforming to the data reported in the literature. 8his decomposition is accompanied by a sharp exothermic peak at 426 °C implying a sudden explosion reaction.This may be due to the association of fuelling effect of the hydrazine liberated and the carboxlyate moiety present in that complex.
Step-3 involves further decomposition of the compound to [La 2 O 2 CO 3 ] which is confirmed by its IR spectrum shown in (Figure .1.c).This is exhibited by a small exothermic hump centered at 580 °C.
Step-4 indicates the final decomposition of the compound to La 2 O 3 as evidenced from the similarity of IR spectrum of the residue with that reported.
The observed mass losses in TG are in good agreement with the composition of intermediate and final product.The thermal decomposition steps are shown in the equation. (2)

XRD analysis
The XRD patterns of the complex, intermediate and final product are shown in (Figure .3).The final product shows similarity with the JCPDS data of lanthanum oxide.All other intermediate show different diffraction patterns corresponding to [La 2 O(CO 3 ) 2 ] and [La 2 O 2 CO 3 ] 8 in (Figure .3.b-d).The diffraction pattern of the complex could not index, since it is a new compound.

SEM analysis
The particle size of lanthanum oxide (Figure .4)produced by the decomposition of the complex in revealed from sem image.The size of the particles varies from 29-50 nm.This is in good agreement with the values calculated from Scherrer's formula D = K/ cos¸, where  is the Xray wavelength,  is the full width of height maximum (FWHM) of a diffraction peak, ¸ is the diffraction angle and K is the Scherrer's constant of the order of 0.89. 18

Thermal decomposition kinetics
The thermogravimetric analysis is a useful analytical technique for evaluating kinetic parameters of solid state reactions.It provides valuable quantitative information regarding the stability of the material.The following Coats and Redfern 14,16 equation was used to evaluate kinetic data from thermogravimetric curves.
Vs. 1/T ... (4.3)where  is the fraction of sample decomposed at time t, n is order of decomposition reaction,  is heating rate in °C degree/min, T is temperature (K), A is frequency factor (s -1 ), R is gas constant (8.314J.k -1 mol -1 ) and E is the activation energy (kJ.mol -1 ).The order of decomposition was obtained by substituting different values of 'n' in to equation (4.2) to obtain the plots of best fit.The plots obtained are shown in (Figure . 5. a -d).Among all the plots for decomposition, the best fit curve gives the order of decomposition with slope of (-E/ 2.303R) and intercept of log AR/E[1-(2RT/E)].
The value of activation energy (E) and preexponential factor (A) are calculated from the slope and intercept of the best fit curve respectively.The entropy of activation (S), enthalpy of activation (H) and free energy of activation (G) are also calculated from the following equation.where h and k are Planck's constant (6.626x10 -34 J.s) & Boltzmann constant (1.38x10 -23 J.k -1 ) respectively.The data obtained from the best fit curve are given in (Table .2).
The kinetic data indicate that the activation energy for step-II and step-III is lower when compared to step -I and IV revealing their exothermocity.The higher value for the initial step indicates that the complexes more stable.

CONCLUSION
The results of the present studies led to the following conclusion: 1) The reaction of 2-hydroxy-1-naphthoic acid and hydrazine react with lanthanum nitrate at pH 5 yielded tetrahydrazinelanthanum 2-hydroxy-1-naphthoate. 2) The thermal decomposition course of tetrahydrazinelanthanum 2-hydroxy-1naphthoate May include the following path ways: ( 3) The tetrahydrazinelanthanum 2-hydroxy-1naphthoate is thermally stable but the Anhydrous naphthoate is unstable and the dehydrazination of naphthoate is responsible of the crystal coherency of The activation energy values calculated by Coats-Redfern method show that the prepared compound is more stable.