JKAU:Sci.. vol. 9, pp. 73-81 (1417A.H./1997A.D.)

The Kinetics of Thermal Decomposition of Nickel FormateDihydrate in Air

A.H.

QUSTI, A.A. SAMARKANDY, S. AL- THABAITI and EL-H. M. DIEFALLAH

Chemistry Department, Faculty of Science,King Abdulaziz University, leddah, Saudi Arabia

ABSTRACT. Differential thermal analysis-thermogravimetry (DTA-TG), andelectron scanning microscopy (ESM) were used to study the decomposition of

nickel formate dihydrate in air. The results showed that the decomposition pro-

ceeds in two successive TG steps, the first is due to dehydration and the secondis due to decomposition of the anhydrous salt and oxidation to form NiO. Ki-netic analysis of the isothermal TG data of the two steps in view of various het-erogeneous solid-state reaction models showed that the dehydration step is hest

described by phase boundary models whereas the second step reactions arebest described by the branching nucleation model. The changes in morphologythat accompany the thermal decomposition of samples as revealed by ESM mi-

crographs, were described and the kinetics and mechanism of the thermal de-

composition reaction were discussed.

Introduction

The chemical transformations of solids play an increasingly important role in moderntechnology, as sophisticated and costly solids can be produced by reaction of other, pre-cursing solids[I]. The kinetics and mechanism of thermal dehydration and de-composition reactions of simple salts have attra~ted the interest of several investigators[2].In calculating the kinetic parameters from the experimental data for a particular re-action, a comparison of the "fractional reaction (a) -time curves with theoretical expres-sions derived for appropriate nucleation and growth models, has been widely used asevidence for the identification of the geometry of interface advance. There are a varietyof factors which may control, determine, influence or modify the rate limiting pro-cesses[I.2]. In general. isothermal kinetic measurements are more reliable than non-isothermal data[3], since the latter are subject to possible inaccuracies due to the in-

fluence of sample size and shape on heat flow.

The decomposition of nickel formate has been the subject of numerous kinetic

7~

74 A.H. Qu.\"tietul.

studies[4]. In Nz-atmosphere, the decomposition is to the metal and this occurs by twoconcurrent routes[5]: '.

Ni(OOCH)z = Ni + COz + CO +HzO

and Ni(OOCH)2.= Ni + Hz +2 COz

The Ni metal is produced with a high surface area and is immediately oxidized in air withviolent exothermic reaction to form NiO. However, the decomposition to Ni powder inhardened oil flakes provides an unusual inert environment for the preparation of Ni cat-

alyst[5].

In the present study, the thermal dehydration and decomposition ofNi(OOCH)z' 2 HzOcrystals were investigated using DT A- TG and ESM techniques. The changes in mor-phology of samples and the kinetics of the isothermal decomposition reactions analyzedin view of various heterogeneous solid state reaction models, were discussed.

Experimental

Nickel formate dihydrate BDH grade, was used in this study without further pur-ification. DTA-TG curves were determined using a Shimadzu DT 30 thermal analyzer.Samples weighing about 9 mg were placed in aluminum crucibles 0.1 cm3 in volume,which was loosely covered. The samples were heated at a temperature rate of 20°Cmin-l .under a pressure of 1.0 atm in air at a flow rate of 3.0 L h- .

TO kinetic experiments were performed under isothermal conditions in air in an oventhermostated at different temperatures. Samples (weighing accurately about 100 mg) insmall test tubes, were placed in the oven, then removed from the oven at different timeintervals, cooled to room temperature in a dessicator, weighed and the weight loss wascalculated. From the weight loss, the fractional reaction (a) was calculated.

Scanning electron microscopy micrographs were obtained for samples formed duringthe thermal decomposition using a JEaL T -300 scanning electron microscope.

Results and DiscussionFig. I shows the DT A-TO curves for the thermal decomposition of nickel formate di-

hydrate in air. The DTA curve shows one endo form peak at about 180°C due to de-hydration and an exo peak at about 240°C due to decomposition and oxidation of the an-hydrous salt to form NiO. The TO curve shows two steps which correspond closely tothe endo and exo effects observed in the DT A curve. The first step shows a weight lossof about 20% (theoretical value = 19.2%) corresponding to the dehydration step. Thesecond step shows a weight loss of about 47% (theoretical value = 49.5%), cor-responding to the decomposition of the anhydrous salt to Ni powder and oxidation of Nito NiO.

Samples of the salt hydrate, the dehydrated salt apd the nickel oxide formed at theend of the thermal decomposition reactions were examined by scanning electron mi-croscopy. ESM micrographs of these samples are shown in Fig. 2. The original sampleconsists of small crystallites (Fig. 2a), most of them have particle size diameter less than

75The Kinetics of Thermal Deconlpo.,ition.

IOmm. Dehydration leads to rounding of crystal edges and faces due to release of waterof hydration (Fig. 2b). The SEM photograph of nickel oxide formed by heating Ni(OOCH)2 .2 H2O in air for lhr at about 2800C (Fig. 2c), shows aggregates of smallcubes of blackish NiO and the particle size is reduced to less than one-half in compari-son to the original crystals. The decrease in particle size increases significantly the sur-face area of the final product.

rI)rI)

0...J.c

b/)

~~

DT A- TG curves of nickel formate dihydrate in air.FIG.

A.H. Qu./; e/ al.76

(a)

(b)

(c)

FIG. 2. ESM microscope of (a) Ni(OOCH)2 .2 H2O); (b) Anhydrous nickel formate. Ni(OOCH)2 ; (c) Cal-

cined nickel formate. at 280°C for I hr in air x 1500.

77The Kinetics of Thermal Decomposition.

Figure 3 and Fig. 4 show the isothennal fractional (a)-time data obtained for the de-hyration of the salt hydrate and for the decomposition and oxidation of the anhydroussalt, respectively. Kinetic analysis of the isothennal data were perfonned in view of var-ious solid state kinetic equation models[6-8] , using a computer program. Under iso-thennal conditions the a-t curves are expressed in the fonn (a) = kt, where k is the rateconstant and the function g(a) depends on the mechanism controlling the reaction andon the' size and shape of th~ reacting particles. Table I list the g(a) equation in thiswork. For the phase boundary controlled reactions, we have the contracting area or Rzmodel, and the contracting sphere or R3 model. For the diffusion-controlled reactions,analyses were perfonned with reference to a one-dimensional diffusion-controlled pro-cess in a cylinder, Dz function; lander's equation for a diffusion-controlled reaction in asphere, D3 function; the Ginstling-Brounshtein equation for a diffusion controlled re-action starting on the exterior of a spherical particle, D4 function; the Zhuravler-Losokhim- Tempelman diffusion equation, Ds function, and the Kroger-Ziegler diffu-sion equation, D6 function. If the solid-state reaction is controlled by nucleation fol-lowed by growth, then in this. case, we may have the Mampel unimolecular law, wherethe rate-detennining step is the nucleation process, described by the F 1 function the A v-rami equation for initial random nucleation followed by overlapping growth in two-dimensions.. Az function, the Erofeev equation for initial random nucleation followed byoverlapping growth in three dimensions, A3 function, and the Prout-Tompkins equation

for branching nuclei; Al function.

0 20 40 60 80 100 120

Timelt! minFIG. 3. Fractional reaction (IX) -time(tY curve for the isothermal dehydration of Nickel Formate dihydrate in

Air (a) 180; (b) 190; (c) 200; (d) 210; and (e) 2200C.

78 A.H. Qusl;l!lal.

1..0

--a- O.B'-'c:0

",0

~~~ 0.6c:0

',0u

J:0.4

0.2

00 10 20 30 40 SO 60

Timel~tY\)FIG. 4. Isothermill fractionill reaction (a) -time(t) curves for decomposition and oxidation of nickel formate to

NiO in air.

(a) 240"C;(b) 250"C; (c) 255°C; (d) 260"C; (e) 270"C; and (t) 280"C

TABLE I. Kinetic equations examined in this wodc.

79The Kinetics of Thernwl Decomposition..

TABLE 1. Contd.

The computer output of the kinetic data analysis showed that the best models whichdescribe the dehydration reaction are the phase boundary controlled reaction models.Calculation of the activation energy according to the Rz -and R3 -models gave a valueof 34.5kJ mol-I. This value is relatively small and possibly indicates the influence ofthe catalytic action of trace amounts of Ni and NiO powders formed at a slow rate dur-ing the dehydration reaction. Since dehydration occurs at a measurable rate at tem-peratures about 200°C, the possibility that the lost water molecules are lattice water,rather than coordinated water, must be eliminated[71. The regression analysis of kineticdata for the decomposition and oxidation of nickel formate in air to NiO showed thatthe reaction could be described by Mampel unimolecular equation where the rate-determining step is the nucleation process or by Prout-Tompkins equation where the re-action is controlled by the formation of branching nuclei. The average activation energyfor this step is about 70 kJ mol-I. For decomposition of the anhydrous salt in vacuum,Brown et aU3] found that the (X-t curves are sigmoid shaped, characteristics of solidphase nucleations and growth processes. The kinetic behaviour is significantly in-fluenced by the disposition of crystallites of the reactant and depends on the "rate of for-mation of Ni nuclei which is strQngly inhibited by traces of water of crystallization. Fordecomposition in air, it is possible that since one is dealing with a sequence of chem-ical reactions involving exo- and endothermic steps: the resultant activation energy wouldbe a composite value[9], is probably controlled .by a branching nucleation process sincethe experimental (X-t curves are sigmoid rather than deceleratory shaped.

Acknowledgement: Thanks are due to Mr. Eisa AI-Ansary and Mr. Ali AI-Malki fortheir help in doing the kinetic experiments and to Mr. M. Seif (production Eng. Dept.)for doing the SEM micrographs.

References[I] Boldyrev, V. V., Bulens, M. and Demon, B., The Control (if the Reactivity of Solids, Elsevier, Am-

sterdam, 1979.[2] Galwey, A,K., in: MTP International Review (Jj'Science: Inorganic Chemistry, Series Two, Solid State

Chemistry, Vol. 10, H,J. Emelius (Ed.), BullelWorths, London, 1975, p. 147.[3] Diefallah, El.H.M" Basahl, S.N., Obaid, A, Y., Abu-Eittah, R.H., Thermochim. Acta, 1978, 111,49.[4] Brown, M.E., Delmon, B., Galwey, A.K. and McGinn, M,J.,J. ChemiePhys., 1978,75,147.[5] Dollimore,D., Thennochim, Acta. 1991,177,59.[6] Dollimore, D., Thermochim. Acta. 1992, 203, 7.

80 A.H. Qusti et Ill.

[6] Dollimore, D., Thermochim. Acta. 1992, 203, 7.[7] DiefalIah, EI-H, M., Thermochim. Acta, 1991, 202, I.[8] Brown, M.E., Introduction to Thermal Analysis, Chapt. 13, Chapman and Hall. London. New York,

1988.[9] DiefalIah, EI-H.M., Obaid, A. Y., Samarkandy, A.A., Abdel Badei, M,M. and EI-BelIehi, A.A., J. Sol-

id State Chem., 1995,117, 122.

81The Kinetics iifThemuu Decomposition

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