Reaction kinetics of some actinide oxalates by differential thermal analysis

J. inorg,nucLChem.,1969.Vol.31, pp. 3789to 3795. PergamonPress. Printedin Great Britain

REACTION OXALATES

KINETICS OF SOME BY DIFFERENTIAL ANALYSIS

ACTINIDE THERMAL

M. S. S U B R A M A N I A N , R. N. SINGH* and H. D. SHARMA? Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay-74, India

(First received 24 December 1968; in revised form 15 April 1969) A b s t r a c t - Kinetic parameters such as the energy of activation and order of reaction for the stepwise

thermal decomposition in air of the hydrated oxalates of Th(IV), U(IV), Np(1V), Pu(1V), Pu(VI) and Pu(lll) have been evaluated by differential thermal analysis. The heats of reaction involved in the various stages of the decomposition have been determined from the peak areas of the DTA curves. The dehydration of the tetravalent hydrated oxalates excepting Np(IV) oxalate gives rise to a major endothermic peak followed by a smaller endotherm which could actually involve a partial decomposition also. The oxidation of the anhydrous oxalates gives a single exothermic peak. INTRODUCTION

THE METAL oxalates have been the subject of numerous thermal investigations from a theoretical as well as practical point of view[l]. Plutonium(IV) oxalate, in particular has widely been used as a suitable intermediate for the preparation of plutonium metal and hence, a knowledge of the temperatures at which various intermediates are formed during its thermal decomposition is of great interest. Recently, Jenkins and Waterman[2] have studied the decomposition of plutonium(IV) oxalates, the results of which were partly contradictory to those of Dawson and Elliott[3] and Kartushova, Rudenko and Fomin[4] but somewhat agreed with that of Rao e t al.[5]. The thermogravimetric and differential thermal analysis studies of hydrated thorium oxalate by Padmanabhan e t a/.[6] did not agree with those of Wendlandt e t al.[7] and Srivastava and Murthy [8]. However, it is a well known fact that DTA data is not very reproducible due to a variety of experimental conditions and materials used in obtaining such data. In the present study, DTA data on the thermal decomposition of the hydrated oxalates of plutonium(IV), neptunium(IV), uranium(IV), thorium(IV), plutonium(VI) and plutonium(III) along with those of calcium oxalate monohydrate have been *Present address: Molecular Biology Unit. Tata Institute of Fundamental Research, Bombay, India. ?Present address: Department of Chemistry, The University of Waterloo, Waterloo, Ontario, Canada. I. 2. 3. 4. 5. 6. 7. 8.

W. W. Wendlandt, Thermal Methods o f Analysis, p. 125. lnterscience, New York (1964). I. L. Jenkins and M. J. Waterman, J. inorg, nucl. Chem. 26, 131 (1964). J. K. Dawson and R. M. Elliott, U.K.A.E.A. Document, .4ERE C/R 1207 (1957). R.C. Kartushova, T. 1. Rudenko and V. V. Fomin, Atomn. Energ. 5, 24 (1958). G. S. Rao, M. S. Subramanian and G. A. Welch, J. inorg, nacl. Chem. 25, 1293 (1963). V. M. Padmanabhan, S. C. Sariya and A. K. Sundaram, J. inorg, nucl. Chem. 12, 356 (1960). W. W. Wendlandt, T. D. George and G. R. Horton, J. inorg, nucl. Chem. 17, 273 (1961). O. K. Srivastava and A. R. Vasudeva Murthy, J. scient ind. Res. 21B, 525 (1962). 3789

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M . S . S U B R A M A N I A N , R. N. S I N G H and H. D. S H A R M A

obtained under identical conditions which are likely to influence the data. The heat of reaction for the dehydration and decomposition in air of the various hydrated oxalates were determined by comparison of the thermograms with that of copper sulphate pentahydrate for which A H values are known. Kinetic para. meters, such as the energy of activation and order of reaction, have been obtained from the specific rates evaluated from the D T A traces using an approximation of the technique described by Borchardt and Daniels[9] and Freeman and Carroll [ 10]. EXPERIMENTAL

Apparatus The furnace was made of a silica tube of 2 in. dia. and 6 in. length suitably wound with nichrome wire and insulated to minimise radiation losses. The rate of heating was controlled by a manually operated variable output transformer, which was calibrated to give a linear rate of heating of 10°C/min. The temperature of the furnace at various time intervals was measured by a chromel-alumel thermocouple positioned between the sample and the reference materials and connected to an external pyrometer, permitting temperatures to be read with an accuracy of +--2°C. The differential temperature was monitored by a chromel-alumel differential thermocouple dipping into the sample and reference material. The differential e.m.f., after proper amplification by a chopper d.c. amplifier, was recorded using a Honeywell strip chart recorder. Platinum microcrucibles of 0.2 mi capacity were used as sample holders and they were placed side by side in tight-fining cavities drilled in a semi-cylindrical ceramic brick inserted in the furnace. Preparation of the oxalates Thorium(l V) and uranium(l V) oxalates were precipitated by the addition of a calculated amount of oxalic acid to a solution of the respective tetravalent cation in 2 M HCI[I 1, 12]. Neptunium was precipitated as Np(IV) hydroxide from a solution which was reduced with 0.1 M hydrazine hydrochloride and 0.18 M potassium iodide in 5 M HCI by heating to 80°C for 1 hrll 3]. The hydroxide was dissolved in 1 M HCI and hydrated Np(IV) oxalate was precipitated by adding 0.1 M oxalic acid to the solution. Plutonium(Ill) oxalate was precipiated by adding 0.6 M oxalic acid to a solution of plutonium in dilute hydrochloric acid, which was previously reduced to the trivalent state by hydroxylamine hydrochloride[5]. Plutonium(IV) and plutonium(Vl) oxalates were prepared by the addition of oxalic acid solution to the respective valancy state of the metal ion in 1 M HNOa [5, 14]. In the latter preparation, the plutonium was oxidized beforehand to the hexavalent state by passing ozonised oxygen at 60°C. The oxalates were washed free of excess reagents by distilled water and air dried at room temperature. The number of molecules of water in the oxalates was calculated from the weight of the respective oxides obtained by the ignition of a weighed amount of the hydrated oxalate. (Table 1) In the case of plutonium(Vl) oxalate hydrate, however, the determination was carried out by a counting of a weighed sample of the material dissolved in nitric acid to estimate plutonium. Determination of the heat transfer coe~cient, K A known amount of copper sulphate pentahydrate ( - 35 mg) mixed with an equivalent quantity of pre-ignited chromatographically pure alumina reference material was filled in one crucible to two9. H.J. Borchardt and F. Daniels, J. Am. chem. Soc. 79, 41 (1957). 10. E. S. Freeman and B. Carroll, J. phys. Chem. 62, 394 (1958). 11. L. I. Katzin, The ,4ctinide Elements (Edited by G. T. Seaborg and J. J. Katz) NNES, Div. IV, Vol. 14A, p. 89(1954). 12. H. R. Hoekstra and J. J. Katz, The Actinide Elements (Edited by G. T. Seaborg and J. J. Katz), NNES, Div. IV, Vol. 14A, p. 158 (1954). 13. H. Taube, J. inorg, nucl. Chem. 15, 172 (1960). 14. E. Staritzky and A. L. Truitt, The Actinide Elements (Edited by G. T. Seaborg and J. J. Katz), NNES, Div. IV, Vol. 14A, p. 836(1954).

DTA of some actinide oxalates

3791

Table 1. Determination of the number of water molecules in the oxalates

Compound Th(C204)2. xH20 U(C204)2. xH20 Np(C204)2 • xH20 Pu(C204)z • xH20 Pu~(C204)a. xH20 PUO2(C204) . xH20

Weight of the h} drated oxalate (rag)

Weight of dioxide (mg)

Number of moles of H20

52.168 34.171 4.013 12.364 13.610 14.550

26-891 18.409t 2.070 6.398 7.967 8.450*

5.81 5.96 6.02 6.05 10.20 2-92

-

Ignition temperature = 900°C. tWeight of UaO8. *Weight of plutonium by alpha assay. thirds its height. The other crucible was filled to the same height with the reference material. The crucibles were inserted in the two holes of the ceramic brick, which was then placed at the centre of the furnace longitudinally and heated at a rate of 10°C/min. From the DTA curve, the value of K was determined as 0.125 cal/min/°C following the method described by Borchardt [15].

DTA of the oxalate hydrates The DTA of the oxalates were performed with - 35 mg of the material. A minimum of three DTA curves was obtained for each compound. RESULTS T y p i c a l D T A c u r v e s are s h o w n in Fig. 1. T h e heat of r e a c t i o n , A H , is calculated f r o m the e q u a t i o n d e s c r i b e d b y Spiels et al.[ 16]. T h e f o l l o w i n g e x p r e s s i o n [ 1 0 ] w h i c h e l i m i n a t e s the u s u a l trial a n d e r r o r p r o c e d u r e has b e e n u s e d for the e v a l u a t i o n o f the o r d e r o f r e a c t i o n a n d a c t i v a t i o n e n e r g y for the d e h y d r a t i o n a n d d e c o m p o s i t i o n o f the v a r i o u s oxalates. T h e e x p r e s s i o n is:

--E*/RA (l/T) Aln[K(A--a)-CpAT]-

A In [ C p d A T / d t + K A T ] x-+ A l n [ K ( A - a ) - C p A T ]

w h e r e K = heat t r a n s f e r coefficient; A = area u n d e r the D T A c u r v e ; A T = differential t e m p e r a t u r e ; dA T / d t = rate o f c h a n g e o f differential t e m p e r a t u r e at the p o i n t w h e r e A T is m e a s u r e d ; Cp = total heat c a p a c i t y of the r e a c t a n t ; a = a r e a u n d e r the c u r v e up to t i m e T w h e r e AT a n d d A T / d t are m e a s u r e d ; E * = e n e r g y of a c t i v a t i o n , a n d x = o r d e r o f r e a c t i o n with r e s p e c t to o n e c o m p o n e n t . T h e v a l u e s of C p d A T/dt a n d CpA T c a n b e c o n s i d e r e d negligible c o m p a r e d to KA T a n d K (A -- a) r e s p e c t i v e l y [6, 17] a n d h e n c e a plot of A (I/T) A IogK(A--a)

A log KA T VS'A i o g K ( A - a )

15. H.J. Borchardt, J. chem. Edac. 33, 103 (1956). 16. S. Spiel, L. H. Berkeihamer, J. A. Pask and B. Davis, U.S. Bur. Mines, Tech. Paper No. 664 (1945). 17. R. P. Agarwala and M. C. Naik, Analytica chim. Aeta 24, 128 (1961).

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Time - rain Fig. 1. D T A traces of the oxlates. A, Th(C204)~. 6H20; B, U(C~O4)2 • 6H20; C, Np(C~O4)z • 6H~O; D, Pu(C204)2 • 6H~O; E, PuO2(C204). 3H~O; F, Pu~(C~O4)a • I O H 2 0 ; G, CaC204 • H~O.

. should yield a straight line with a slope of+__E*/2.3R and an intercept o f - x . The results are given in Table 2. Figure 2 is a typical activation energy plot for the dehydration and decomposition in air of hydrated plutonium(IV) oxalate. OBSERVATIONS

Dehydration of the tetravalent hydrated oxalates 1. The dehydration of thorium(IV), uranium(IV) and plutonium(IV) oxalate hydrates was found to give rise to a major endothermic peak followed by a smaller endotherm which could actually involve a partial decomposition also[7]. How-

23 22 28 13 43 18

141 137 136 142 183 227

U(C204) 2 • 6H20 Np(C204)2 • 6H20 Pu(C±O4)2 • 6H20 PuO 2 • C204 • 3H20

27 24 15 20 26 25 (22)[10] (27)[18]

21

E* (kcal/mole)

1.1 1.0 1.0 1.0 0-8 0"9 (1.0([10]

0-4

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482"1"

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AH (kcal/mole)

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1.0

331 321 298 219 331 817

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-- 57 --47 --59 --25 --70 20

-- 18

60 (52)[6]* (55)[6] 77 61 67 78 35 39 (39)[10]

1.0 0.8 1-8 1.9 0.5 0"4 (0-4)[10]

0-9

E* Orderof (kcal/mole) reaction

Decomposition

E* O r d e r o f Tmax. AH (kcal/mole) reaction (°C) (kcal/mole)

Second dehydration

187

250

O r d e r o f Tma~. reaction (°C)

*Figures in parantheses indicate literature values. t Refers to the reaction CAC204 (s) + ½02(air) = CaCO3 (s) + CO~ (g). 18. W. W. Wendlandt, J. chem. Educ. 38, 571 (1961).

CAC204 • H20

Pu2(C204)3 • 1 0 H 2 0

20

152

Tm~. AH (°C) (kcal/mole)

Th(C204) 2 • 6H20

Substance

First dehydration

Table 2. Kinetic parameters for the decomposition of the oxalates

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Pu(C204)2 • 6H~O;• decompositionof Pu(C204)2, ever, with neptunium(IV) oxalate hydrate, only one such endotherm was observed. 2. The Tmax. (temperature at peak maximum) for the first dehydration of the tetravalent oxalates is in the range of 136°-152°C, the order being Th > U > Np > Pu, the values for U, Np and Pu lying close together. 3. Heat of reaction for the first dehydration increases with the atomic number. The AH value for thorium is 20 kcal/mole and the value progressively increases to 28 kcal/mole for plutonium. 4. The activation energy for the dehydration reaction does not show any particular pattern of variation amongst these actinide oxalates. 5. The order of reaction for the first dehydration step is 1 with the exception of Th(C204)2.6H20 for which the value is 0.4.

Decomposition of the anhydrous tetravalent oxalates 1. The decomposition of the anhydrous oxalates in air gives a single exothermic peak. 2. The Tmax.for the decomposition of the tetravalent oxalates also follows the same pattern as in the case of dehydration, viz. Th > U > Np > Pu. 3. AH value for decomposition appears to decrease with the atomic number from -18 kcal/mole for thorium(IV) oxalate to --59 kcal/mole for plutonium(IV) oxalate. However, AH value for uranium(IV)oxalate is lower as compared to neptunium(IV) oxlate.

DTA of some actinide oxalates

3795

4. Activation energy for the decomposition reaction does not show any particular pattern of variation. 5. The order of reaction for the decomposition in air is approximately one for thorium, uranium and neptunium oxalates whereas for plutonium(IV) oxalate, the value is nearly two.

Effect of valency A H values for the dehydration reaction are roughly proportional to the hydration number of the compounds and their values decrease with increase in valency of the metal ion. A trend in the reverse direction is observed for the AH values of decomposition reaction. For plutonium oxalates in the tri, tetra and hexavalent states, AH value for the first dehydration and decomposition reactions are 43, 28, 13 and - 7 0 , - 5 9 , - 2 5 kcal/mole respectively. The activation energy for the oxidation reaction is highest for the hexavalent oxalate and least for the trivalent oxalate. There is a broad exotherm v]sible in the D T A traces between 500°-550°C which is too high for any decomposition involving the breaking of the organic fragment. It is possible that this could be due to a phase change or a change in the stoichiometry of the oxide. The exact interpretation of this peak is rather difficult. The value of the kinetic parameters involved in the D T A study of calcium oxalate monohydrate (Table 2) determined in the same apparatus under identical conditions are found to be in good agreement with those reported by Freeman and Carroll using thermogravimetric techniques [10]. This agreement is remarkably good considering the limitations of the technique itself. Acknowledgements - T h e authors wish to thank Dr. M. V. Ramaniah, Head, Radiochemistry Division for his comments.