Thermoanalytical Investigations of Hexaamminenickel(II) …shodhganga.inflibnet.ac.in/bitstream/10603/13162/12/12... · 2015-12-04 · Thermoanalytical Investigations 115of Hexaamminenickel(II)

Chapter 4

Thermoanalytical Investigations of Hexaamminenickel(II)

sulphate and Tris(ethylenediamine)nickel(II) sulphate

Complexes in the Solid State

4.1 Introduction

Chapter four discusses the thermoanalytical investigations of

hexaamminenickel(II) sulphate and tris(ethylenediamine)nickel(II)

sulphate. Thermal decomposition studies of transition metal amine

complexes especially those of nickel amine complexes have been well

studied.1-3

Amines like ammonia, ethylenediamine and propylenediamine

are well known for their complexing capacity and they form well defined

complexes with transition metals. Upon heating these complexes undergo

stepwise decomposition to form various intermediates.2-3

In order to have an unambiguous picture on the evolved gases and the

transient intermediates formed during heating, techniques like TG-MS

and TR-XRD can be of immense use. Conventional thermoanalytical

techniques like TG, DTA and DSC do not detect the nature of the gaseous

products. Evolved gas analysis (EGA) offers a useful tool to identify the

fragments which cannot be detected by other analytical techniques.4-5

Temperature resolved X-ray diffractometry (TR-XRD) is a powerful tool

to study the structural/phase changes occurring during a solid state

reaction as well as in identifying the reaction intermediates insitu during

pyrolysis.6 Present investigation makes use of these two insitu analytical

techniques exhaustively.

114 Chapter 4

This chapter also focuses on the thermal decomposition kinetics of the

complexes by model-free isoconversional methods, non-mechanistic and

mechanism based equations.

4.2 Experimental

4.2.1 Preparation of the complexes

The complex, [Ni(NH3)6]SO4 was synthesized as per the procedure in the

literature.7 Stoichiometric amount of ammonia was added to nickel

sulphate solution with stirring and cooled in ice bath. The complex was

precipitated out by adding ethanol. The isolated complex was washed with

ethanol and then with ether and dried over vacuum. The resulting complex

was characterized by various spectral and analytical methods. In order to

synthesize [Ni(en)3]SO4, stoichiometric amount of ethylenediamine was

added to nickel sulphate solution with stirring and cooled in ice bath.8 The

complex was precipitated out by adding ethanol. The isolated complex

was washed with ethanol and then with ether and dried over vacuum. The

resulting complex was characterized by spectral and chemical methods.

The nickel content in the complexes was determined by gravimetry.9

4.2.2 Instrumentations

TG/DTA-MS studies were carried out in a Rigaku TG-8120 instrument

combined with mass spectroscopy (Anelva, M-QA200TS) under high-

purity He (99.9999%) and at a flow rate of 200 ml min-1

. The heating rates

employed were 5, 10, 15 and 20ºC min-1

.

The elemental analyses were carried out using Vario El III instrument, the

details are given in Table 3.1. X-ray powder patterns were recorded on a

Bruker D8 Advance diffractometer attached with a programmable

temperature device (TTK 450) from Anton Paar, (using Cu Kα radiation,

Thermoanalytical Investigations of Hexaamminenickel(II) sulphate… 115

λ = 1.542 Å). The measurements were performed by placing the sample

on a flat sample holder, while the samples were heated by a

programmable temperature controller. Crystallite size was calculated

using Scherrer equation,

t = 0.9 λ / β cos θ,

where t is the thickness of the particle, λ is the wave length, β is the line

broadening (FWHM) and θ is the corresponding angle.

For the kinetic analysis of tris(ethylenediamine) complex,

thermogravimetric experiments were carried out using Shimadzu DTG-60

instrument connected to TA60 online analyser. The experiments were

conducted in flowing air at a rate of 50 ml min-1

. Samples were loaded in

a platinum crucible and the mass was kept constant around 10 0.1 mg.

The heating rates employed were 5, 10, 15, 20ºC min-1

. The morphology

of the complexes, intermediates and the residues were determined using

JEOL JSM-6390 Scanning Electron Microscope. The samples were

spread on a carbon tape and made uniform by blowing air.

4.3 Mathematical Treatment of Data

4.3.1 Isoconversional methods

Nowadays, model-free isoconversional methods have got wide spread

attention as a versatile method to investigate the kinetics of the solid state

reactions as these methods possess several advantages over the

conventional methods.10-13

Even though it does not permit direct

estimation of reaction model or pre-exponential factor, it shed light on the

complexities of solid state reaction. FWO, Friedman and KAS are

isoconversional methods and are frequently employed to study the

116 Chapter 4

kinetics. These methods yield effective activation energy (E) as a function

of extent of conversion ().

In order to study the kinetics using the model-free methods, several TG

measurements were carried out at different heating rates. Flynn-Wall-

Ozawa (FWO),14-15

Friedman16

and Kissinger-Akahira-Sunose (KAS)17-18

methods are based on the multiple heating rate experiments.

Flynn-Wall-Ozawa equation is

ln ln ln ( ) 5.3305 1.052AE E

gR RT

(1)

Where is the heating rate, is the degree of conversion, g() is the

mechanism function, E is the activation energy, A is the pre-exponential

factor and R is the gas constant.

Friedman equation is

ln ln[ ( )]d E

Afdt RT

(2)

Where d

dt

is the rate of conversion and ( )f is the mechanism function.

Kissinger-Akahira-Sunose (KAS) equation is

2ln ln

( )

AR E

T Eg RT

(3)

From the above equations (1-3) the plots of ln versus 1/T, lnd

dt

versus

1/T, 2

lnT

versus 1/T give straight lines with slopes -1.052E/R, -E/R and -

E/R respectively for the FWO, Friedman and KAS equations. The slope of

the straight lines is directly proportional to the activation energy.


4.3.2 Non-mechanistic equations

The non-mechanistic equations are given as

The Coats–Redfern equation19

( ) 2ln ln (1 )

2

g AR RT E

E E RTT

(4)

The MKN equation20

1.9215

( ) 0.12039ln ln 3.7721 1.9215ln

g AE EE

T R T

(5)

The Horowitz-Metzger equation21

2

2ln ( ) ln

s

s s

ART E Eg

E RT RT

(6)

The MacCallum-Tanner equation22

10 10

30.435 (0.449 0.217 )10

log ( ) log 0.483AE E

g ETR

(7)

where, 1 (1 )

( )1

n

gn

where n 1 (8)

n is the order parameter and α is the extent of decomposition. Ts is the

DTG peak temperature and θ is (T-Ts).

The order parameter n was determined using the Coats-Redfern equation

by an iteration method. Linear plots of ln[g(α)/T2] against 1/T were

plotted by least square method, taking α and T values from the TG curve.

Linear curves were drawn for different values of n ranging from 0 to 2 in

increment of 0.01. The value of n is selected by the best fit having

maximum correlation coefficient.

118 Chapter 4

Plotting L. H. S of equations (4), (5) and (7) against 1/T and (6) against θ,

gives straight line. From the slope and the intercept E and A can be

calculated.

The entropy of activation was calculated using the following equation

#

expskT SA

h R

(9)

where k is the Boltzmann constant, h is the Planck’s constant and #S is

the entropy of activation.

Model fitting method

The mechanism of reactions from nonisothermal methods has been

discussed by Sestak and Berggren23

and by Satava.24

The procedure is

based on the assumption that the noniosthermal reaction may be

considered to proceed isothermally in an infinitesimal time interval, so

that the rate can be expressed by

( )exp ( )

d EA f

dt RT

(10)

For a linear heating rate eqn (10) can be written as

( )exp

( )

d A EdT

f RT

(11)

Where is the heating rate employed and dT

dt

Integrating of eqn (11) gives

0

( )exp

( )

T

o

d A Eg dT

f RT

(12)

where, g(α) is the integrated form of f(α). Mechanism based kinetic

studies based on the assumption that the form of f(α) or g(α) depends on

the reaction mechanism. The g(α) forms corresponding to nine probable


reactions have been proposed and the mechanism is obtained by fitting the

experimental data into these models and the equation which gives the best

fit i.e. with maximum correlation coefficient is selected. The forms of

g(α) corresponding to the different rate controlling processes are given in

Table 2.1. The Coats-Redfern method was used for solving the

exponential integral as it is one of the best approaches recommended by

several authors.25-27

Linear plots of given models of ln [g(α)/T2] versus

1/T were drawn for each model by the method of least squares (where

g(α) is the integral form of f(α), the reaction model) and the

corresponding correlation coefficients were also evaluated. E and A were

calculated in each case from the slope and the intercept respectively.

4.4 Results and Discussion

4.4.1 Thermal decomposition studies

4.4.1.1 Hexaamminenickel(II) sulphate

TG/DTA-MS plot of hexaamminenickel(II) sulphate at a heating rate of 10°C

min-1 is shown in Fig. 4.1. The complex has a four stage thermal

decomposition pattern in He atmosphere as follows.

Ni(NH3)6SO4→ Ni(NH3)2SO4 + 4 NH3 (Stage I)

Ni(NH3)2SO4→ Ni(NH3)SO4 + NH3 (Stage II)

Ni(NH3)SO4 → NiSO4 + NH3 (Stage III)

NiSO4 → NiO + SO3 (Stage IV)

The TG curve shows that the decomposition starts at 75°C and ends at 330°C.

The weight loss (26.5%) indicates that this decomposition is associated with

the liberation of four molecules of ammonia. The temperatures of inception

(Ti) for the second and third stages are 330°C and 390°C respectively. The

120 Chapter 4

final temperature (Tf) values of these two stages are 390°C and 440°C

respectively. For both these stages the weight loss observed (6.2 and 6.1%)

correspond to the loss of one molecule of ammonia each to give anhydrous

nickel sulphate. Nickel sulphate is stable up to 650°C and then starts to

decompose. This step has a weight loss of 31.2% corresponding to the

decomposition of nickel sulphate, eventually leading to the formation of nickel

oxide as the final residue. The thermal decomposition pattern of the

hexaamminenickel(II) complex is same in helium and air.

Fig. 4.1. TG/DTA-MS plot of [Ni(NH3) 6] SO4 in He at 10ºC min-1

The phenomenological details regarding the temperature of inception (Ti),

the temperature of completion (Tf) and the temperature of summit (Ts) for

the thermal decomposition of hexaamminenickel(II) sulphate at 10°C min-

1 is shown in Table 4.1.

200 400 600 800

-100

-80

-60

-40

-20

0

m/z 14

m/z 15

m/z 28

In

ten

sity

/a

. u.

m/z 32

ma

ss l

oss

/%

m/z 2 m/z 16

m/z 18

m/z 17

Temperature / ºC

e

nd

o

∆T

ex

o


Table 4.1

Phenomenological data for thermal decomposition of

hexaamminenickel(II) sulphate in He at = 10°C min-1

TG results Percentage weight loss

Stages Ti

(°C)

Tf

(°C)

Ts

(°C) Theoretical Observed Theoretical Observed Residue

Stepwise Cumulative

I 75 330 310 26.5 26.5 26.5 26.5 Ni(NH3)2SO4

II 330 390 260 6.6 6.2 33.1 32.7 Ni(NH3)SO4

III 390 440 425 6.6 6.1 39.7 38.8 NiSO4

IV 650 845 804 31.2 31.2 70.9 70 NiO

The recorded TG profile differs from those reported in the literature.1 As

per the published report the second step involves the loss of two

molecules of ammonia. Moreover, in that work studies were done only for

the deamination stages, till the formation of nickel sulphate.

The observed thermal decomposition pattern is in agreement with the single

crystal structure of the complex28

(Fig. 4.2). The complex has a distorted

octahedral symmetry.28

The six ammonia ligands are crystallographically

different. The four ammonia molecules lying in the square planar plane

experience some strain due to the distortion which results from the

complicated hydrogen bonding between the SO42-

tetrahedra and the

different ammine ligands of the [Ni(NH3)6]2+

octahedra.28-29

This may leads

to the easier removal of four ammonia ligands during heating. Thereafter, the

complex adopts a linear structure with two ammonia molecules having Ni-N

bond distances 213.1 and 212.5 pm and N-Ni-N bond angle 179.6º. On

heating, the ammonia molecule which occupies at a distance 213.1 pm is

expected to get evolved first followed by the other ammonia molecule.

122 Chapter 4

Fig. 4.2. Crystal structure of [Ni(NH3) 6] SO4

The DTA curve shows four endothermic peaks corresponding to the four

stages of decomposition in TG. The results from the DTA curves viz., the

temperature of inception (Ti), final (Tf) and DTA peak temperature (Tp)

for the different decomposition stages are given in Table 4.2.

Table 4.2

DTA peak temperatures for the thermal decomposition of

hexaamminenickel(II) sulphate

Stages Ti

(°C)

Tf

(°C )

Tp

(°C )

I 121.6 176.6 140.8

II 276.2 341.8 308.3

III 400.9 444.9 420.2

IV 767.5 808.5 797.5

In the MS plot, ion peak with m/z value 18 correspond to the adsorbed

moisture in the complex. Peaks with m/z value 17 at 103.3, 389 and

433.8°C correspond to the evolved ammonia, which is in accordance with

the thermal decomposition of the complex (vide supra). Ion peaks with


m/z values 2 and 16 can be attributed to the presence of H2 and NH2

respectively which could be formed by the fragmentation of NH3. In the

mass spectra some other peaks with m/z values 15, 14 and 28 are also

seen. These peaks correspond to NH, N and N2 respectively and these

species can also be formed due to the fragmentation of evolved ammonia.

During the final stage of decomposition, SO3 (80) is evolved and is

fragmented into various species as shown below.

Scheme 1. Fragmentation of SO3

4.4.1.2 Tris(ethylenediamine)nickel(II) sulphate

Thermal decomposition in air

Tris(ethylenediamine)nickel(II) sulphate undergoes a four stage thermal

decomposition pattern in air. The various stages of decomposition are as

follows

[Ni(en)3]SO4 Ni(en)SO4 + 2 en (Stage I)

Ni(en)SO4 NiSO4 + en (Stage II)

NiSO4 mixture of NiO + NiSO4 (Stage III)

NiSO4 NiO + SO3 (Stage IV)

TG/DTA plot for the thermal decomposition at a heating rate of 10°C

min-1

in air and at a flow rate of 50 ml min-1

is shown in Fig. 4.3.

SO3

S (32) + O2 (32) or 2O (16)

SO2 (64) + O (16)

124 Chapter 4

Fig. 4.3. TG/DTA plot of [Ni(en)3]SO4 in air at 10°C min-1

From the TG curve at 10ºC min-1

, it is seen that the complex starts to lose

mass at 300ºC, indicating the thermal stability of the complex. This stability

is due to the bidentate nature of the ligand coordinating with the metal,

resulting in the formation of complex with octahedral geometry. The first

stage of the decomposition of tris(ethylenediamine)nickel(II) sulphate

corresponds to the loss of two molecules of ethylenediamine (36% loss) to

give mono ethylenediamine complex. The mono (ethylenediamine)nickel(II)

sulphate is very unstable and suddenly decomposes at 360ºC. The subsequent

stages are overlapping and hence not separable. The phenomenological data

for the thermal decomposition of the tris(ethylenediamine) complex in air at

10°C min-1

are shown in Table 4.3.

0 100 200 300 400 500 600 700 800 900

20

40

60

80

100m

ass

/ %

v

-100

0

100

200

300

400

500

600

Temperature / ºC

e

nd

o

∆T

ex

o


Table 4.3

Phenomenological data for the thermal decomposition of

tris(ethylenediamine)nickel(II) sulphate at = 10°C min-1

in air

TG results Percentage mass loss

Stages Ti

(ºC)

Tf

(ºC)

Ts

(ºC) Theoretical Observed Theoretical Observed Residue

Stepwise Cumulative

I 300 360 346 35.8 36 35.8 36 Ni(en)SO4

II 360 466 424 17.9 17.9 53.7 53.9 NiSO4

III 466 500 487

Mixture of

NiSO4 and

NiO

IV 696 786 760 23.9 23.9 77.6 77.8 NiO

Intermediate formed at 500ºC was separated and identified. This stage

corresponds to the partial dissociation of NiSO4 to a mixture of NiSO4

and NiO. This was further confirmed by IR and XRD analyses. Weight

loss of 77.8% at 786ºC corresponds to the formation of NiO. During the

temperature range (500-800ºC) an almost continuous process was

observed for which well defined DTA could not be obtained.

The DTA profile in Fig. 4.3 contains two endotherms and two exotherms.

The two endotherms correspond to the two deamination stages. The

exotherms at 449ºC and 492.5ºC represent the partial

decomposition/oxidation of NiSO4 to give a mixture of NiO and NiSO4.

The temperatures from DTA profile are given in Table 4.4.

126 Chapter 4

Table 4.4

DTA peak temperatures for the thermal decomposition of

tris(ethylenediamine)nickel(II) sulphate in air

Stages Ti

(0C)

Tf

(0C)

Tp

(0C)

I 354.3 358.8 347

II 395.5 415.8 405.9

III 444.5 457.1 449

IV 482.4 500 492.5

Fig. 4.4 shows the IR spectra of the complex, intermediate at 500ºC and

the final residue. IR spectrum of the intermediate isolated at 500ºC

contains the peaks corresponding to SO42-

groups. The peaks at 1110, 980

and 610 cm-1

are characteristics of SO42-

group.30

Fig. 4.4. IR spectra of (a) [Ni(en)3]SO4 (b) mixture of NiSO4 and NiO (c) NiO

Thermal decomposition studies of tris(ethylenediamine)nickel(II) sulphate

have been reported in the literature.1,31

The obtained decomposition profile

differs from those reported earlier. Wendlandt and George1 reported a


single stage decomposition pattern for this complex. The DTA profile

reported by them contained three endotherms. Mitra et.al31

reported two

stage decomposition pattern for tris(ethylenediamine) nickel(II) sulphate

and the DTA pattern contained two endotherms and two exotherms.

Thermal decomposition in helium

The plot of simultaneous TG/DTA coupled online with MS during the

decomposition of tris(ethylenediamine)nickel(II) sulphate in helium

atmosphere is shown in Fig. 4. 5. The phenomenological data for the

thermal decomposition of this complex in helium is given in Table 4.5.

The TG-MS analysis indicates the following two stage thermal

decomposition pattern.

Ni(en)3SO4 Ni(en)SO4 + 2 en (Stage I)

4 Ni(en)SO4 Ni3S2 + Ni + 4 en + 8 O2 + 2 S (Stage II)

Fig. 4.5. TG/DTA-MS plot of [Ni(en)3]SO4 in helium at 10ºC min-1

e

nd

o

∆T

ex

o

200 400 600 800

-100

-80

-60

-40

-20

0

m/z 28m/z 16

m/z 17

In

ten

sity

/arb

. un

its

m/z 2

mas

s lo

ss /%

Temperature /oC

128 Chapter 4

Table 4.5

Phenomenological data for the thermal decomposition of

tris(ethylenediamine)nickel(II) sulphate at = 10°C min-1

in helium

TG results Percentage mass loss

Stages Ti

(ºC)

Tf

(ºC)

Ts

(ºC) Theoretical Observed Theoretical Observed Residue

Stepwise Cumulative

I 270 347 330 35.8 35 35.8 35 Ni(en)SO4

II 330 495 420 - 44 - 79 Mixture of

Ni3S2 and Ni

The two stage thermal decomposition give a mixture of Ni and Ni3S2

(JCPDS no. 44-1418) phases as the decomposition products. The XRD

patterns of the residues are shown in Fig. 4.6.

Fig. 4.6. XRD pattern of mixture of Ni3S2 and Ni

The mass spectra of tris(ethylenediamine)nickel(II) sulphate show peaks

with m/z values 2, 15, 16, 17 and 28. These peaks are due to the presence

inte

nsi

ty/

a . u

Ni

Ni

Ni Ni

* *

* * *

10 20 30 40 50 60 70 80

two theta / degrees

* Ni3S2


of H2, NH, NH2, NH3 and N2 /CH2 = CH2. These species are formed due

to the fragmentation of the liberated en molecule as shown below.

NH2-CH2-CH2-NH2

Scheme 2. Fragmentation of ethylenediamine

4.4.2 Temperature resolved X-ray diffraction studies


The temperature resolved X-ray diffractogram of the complex is shown in

Fig. 4.7. At 40°C the pattern corresponds to the parent hexaammine complex.

On heating the peaks corresponding to planes of the hexaammine complex

disappears and new peaks start to appear. This is due to the formation of

diammine species by the release of four molecules of ammonia.

On further heating one ammonia molecule is evolved to get the

monoammine species. At 390°C, the pattern represents the monoammine

complex. The patten at 440°C represents the peaks corresponding to

nickel sulphate. Peaks corresponding to (110), (020), (021), (111), (112),

(130), (202), (132), (222), (042) and (311) planes represent nickel

sulphate (JCPDS card No: 13-0435). As the heating continues NiO is

formed at 780°C and the peaks corresponding to (111) and (100) planes

represent NiO (JCPDS card No: 47-1049). NiO has got well defined

peaks with cubic symmetry. The nickel oxide formed is in the nanometer

2 NH2 (16) + CH2 = CH2 (28)

NH3 (17) + NH (15) N2 (28) + 2 H2 (2)

130 Chapter 4

range. The crystallite size is calculated using the Scherrer equation and is

found to be 50 nm.

Fig. 4.7. TR-XRD patterns of [Ni(NH3)6]SO4


The XRD patterns of tris(ethylenediamine)nickel(II) sulphate, intermediate

and the residue are given in Fig. 4.8.

330ºC

390ºC

40ºC

440ºC

800ºC

o

o

#

# #

#

#

# #

o NiO

# NiSO4

*Ni(NH3)6SO4 In

ten

sity

/ a

.u.

*

*

*

*

*

*

*

*

two theta / degrees

10 30 60


Fig. 4.8. TR-XRD pattern of (a) [Ni(en)3]SO4 (b) mixture of NiSO4

and NiO (c) NiO

The XRD pattern at 40ºC contains peaks corresponding to (100), (101),

(002), (110), (102), (200), (201), (112), (202), (211), (301) and (303)

planes of tris(ethylenediamine)nickel(II) sulphate (JCPDS no. 23-1796).

The XRD pattern of the intermediate at 500ºC has peaks of NiSO4 and

NiO. However, the majority peaks are those of NiO phase. The XRD

analysis revealed that the final residue at 800ºC is NiO and has well

defined peaks with cubic symmetry. From the peak broadening value the

crystallite size of NiO was calculated using Scherrer equation. Average

crystallite size was found to be 17.47 nm.

4.4.3 SEM analysis


The SEM images (Fig. 4.9) reveal that the morphology of the intermediates at

each stages (Fig. 4.9 b-d) and the residue differ from that of the parent

0 10 20 30 40 50 60

10 20 30 40 50 60 70 80 90 100

10 20 30 40 50 60 70 80 90 100

**

*****

***

*

*

###

o

o

o

o o

oo

o

o

o

#

o NiO

#NiSO4

* Ni(en)3SO

4

two theta / degrees

Inte

nsi

ty/

a.u

.

40oC

500oC

800oC

132 Chapter 4

hexaammine complex (Fig. 4.9 a). The intermediates appear as tiny rods. NiO

(Fig. 4.9 e) has an elongated morphology and are in the nanometer range.

Fig. 4.9. SEM images of (a) [Ni(NH3) 6] SO4 (b) Ni(NH3)2SO4

(c) Ni(NH3)SO4 (d) NiSO4 (e) NiO

(a)

(b)

(c) (d)

(e)



The SEM pictures of the complex tris(ethylenediamine)nickel (II) sulphate

and the residue in air are shown in Fig. 4.10 a-b. The complex has a plate like

appearance (Fig. 4.10 a) and NiO residue (Fig. 4. 10 b) is seen as a porous

structure due to the evolution of gaseous products during heating.

Fig. 4.10. SEM pictures of (a) [Ni(en)3]SO4 (b) NiO

4.4.4 Kinetics and mechanism

4.4.4.1 Kinetic analysis using isoconversional approach

Hexaamminenickel(II) sulphate

The kinetic parameters obtained using model-free methods for the three

deamination stages and decomposition stage are given in Fig. 4.11 a-d and Table

4.6. For the first deamination stage (Fig. 4.11 a) it was observed that the

activation energy values computed using FWO and KAS methods tends to

decrease with increase of conversion. The decreasing dependence of E with

indicates the reversibility of the deamination process. Moreover as the

decomposition progresses there may be chances of adsorption of gaseous

product on the solid surface and in this case the process is controlled by mass

transfer along the solid surface and desorption. These types of reactions are

characterized by lower activation energy.32-33

For the second and third

134 Chapter 4

deamination stages activation energy is found to be increasing with the extent of

conversion. This is an indication of the parallel or competing reactions occurring

in these solid state deamination reactions.

Fig. 4.11. Variation of activation energy with conversion for various

stages (a) first deamination (b) second deamination (c) third

deamination (d) decomposition (NiSO4 NiO).

Among the decomposition reactions, the third deamination stage (Ni(NH3)SO4

→ NiSO4 + NH3) show lower values of activation energy, probably because of

the unstable crystal structure of the monoammine complex eventually facilitates

sudden decomposition. The final decomposition reaction (NiSO4 NiO) has

higher activation energy values and can be attributed to the decomposition of

stable NiSO4 structure. From the activation energy vs. conversion plot for the

final decomposition stage (Fig. 4.11 d), the activation energy is found to be

varying with the extent of conversion and this variation is irregular. This non

0.0 0.2 0.4 0.6 0.8 1.0

70

80

90

100

110

alpha

E /

kJ

mo

l-1

FWO

KAS

Friedman

a

0.0 0.2 0.4 0.6 0.8 1.0

50

100

150

200

250

E /

kJ

mo

l-1

alpha

FWO

KAS

Friedman

b

0.0 0.2 0.4 0.6 0.8 1.0

15

20

25

30

35

40

45

E /

kJ

mo

l-1

alpha

FWO

KAS

Friedman

c

0.0 0.2 0.4 0.6 0.8 1.0

250

275

300

325

350

E /

kJ

mo

l-1

alpha

FWO

KAS

Friedman

d


uniform dependence of E with indicates the complex nature of the solid state

decomposition of NiSO4 to NiO.

Table 4.6

Activation energy and conversion () values for hexaamminenickel(II)

sulphate for various decomposition stages

Stage 1 Stage II

Stage 1II Stage IV

alpha

E (kJ mol-1

)

FWO KAS Friedman

0.05 97.6 98 110.2

0.1 97.8 98.9 100

0.2 98.9 99 98.9

0.3 99.3 99.2 99.3

0.4 92.3 92.3 95.2

0.5 85.9 84 90.1

0.6 76.2 74 86.7

0.7 76.1 72 80.1

0.8 71.5 70 75.7

0.9 65.3 66 70

alpha E (kJ mol-1

)

FWO KAS Friedman

0.05 59.3 59 60.2

0.1 51.7 51 58.3

0.2 52 52.5 62.1

0.3 48.4 48 68.5

0.4 51.7 52 67.7

0.5 71.7 72 75.2

0.6 73.2 73 83.6

0.7 99.3 99 88.9

0.8 95.8 96.3 99.3

0.9 95.8 96.1 105.5

alpha E (kJ mol-1

)

FWO KAS Friedman

0.05 14.9 15.1 14.5

0.1 15.1 15.5 14.3

0.2 15.3 15 15.2

0.3 16.9 17 16.2

0.4 18.3 18 17.1

0.5 19.2 19.5 18.9

0.6 20.4 21 21.2

0.7 20.9 20.9 21.5

0.8 21.9 22 21.7

0.9 20.8 21 22

alpha E (kJ mol-1

)

FWO KAS Friedman

0.05 304.4 304 240

0.1 255.6 256 256

0.2 284.1 285 270

0.3 282.6 283 275.2

0.4 280.1 280 280.1

0.5 296.1 296.5 292.2

0.6 296.9 297 300

0.7 295.9 296 305.2

0.8 296.8 294 316.5

0.9 285.4 283 320.2

136 Chapter 4

0.0 0.2 0.4 0.6 0.8 1.0

140

160

180

200

220

240

260

280

300

E /

kJ

mo

l-1

alpha

FWO

KAS

Friedman

Tris(ethylenediamine)nickel(II) sulphate

The kinetic analyses for the first deamination stage (i.e. [Ni(en)3]SO4

Ni(en)SO4 + 2 en) of tris(ethylenediamine)nickel(II) sulphate derived

using isoconversional methods and the corresponding kinetic plots are

shown in Fig. 4.12 and the corresponding values are given in Table 4.7.

Fig. 4.12. Variation of activation energy with conversion (α) for the

first deamination reaction of [Ni(en)3]SO4

The activation energy varies with the extent of conversion. Activation

energy was found to be decreasing with the extent of conversion in the

range 0.05 to 0.7 and then found to be increasing. The decreasing

dependence of activation energy on the conversion function can be

concluded as a kinetic scheme of endothermic reversible reaction

followed by an irreversible one. Increasing dependence of activation

energy with conversion indicates that reaction involves parallel reaction.34


Table 4.7

Activation energy and conversion () values for

tris(thylenediamine)nickel(II) sulphate for the first deamination stage

It is seen that the activation energy calculated by FWO and KAS methods

are comparable. However, the activation energies calculated by Friedman

method show higher values. This is because the Friedman method is very

sensitive to experimental noise and tends to be numerically unsound when

employing instantaneous rate values.35

4.4.4.1 Kinetic analysis using non-mechanistic approach


The kinetic parameters were evaluated from the TG curves using the four

non-mechanistic equations viz., Coats–Redfern, Madhusudanan-Krishnan-

Ninan, Horowitz-Metzger and MacCallum-Tanner and the values are

shown in Table 4.8.

alpha E (kJ mol-1

)

FWO KAS Friedman

0.05 159.7 188.7 232.9

0.1 147.2 160.8 216.3

0.2 143.4 146.2 217.7

0.3 146.9 146.2 229.8

0.4 141.6 146.2 228.3

0.5 139.8 146.2 207.6

0.6 135.2 141.5 265.7

0.7 149.2 160.8 282.8

0.8 153.5 160.8 300.6

138 Chapter 4

Table 4.8

Kinetic parameters for the thermal decomposition stages of

hexaamminenickel(II) sulphate using non-mechanistic equations

Stage1 Stage II Stage III Stage IV

n 1.25 0.6 1.48 0.47

E

(kJ mol-1

)

CR

MKN

HM

MT

84.6

84.7

90.7

84.5

42.3

42.7

60.7

42.4

43.1

43.4

61.2

43.3

311.8

312.2

320.9

315.1

A

(s-1

)

CR

MKN

HM

MT

3.8 × 108

4.3 × 108

2.2 × 109

3.9 × 10

8

2.5 × 101

3.0 × 101

1.8 × 103

2.7 × 10

1

4.1× 100

5.2 ×100

1.2 ×101

3.9 × 100

7.6 × 1012

8.4 × 1012

6.8 × 1014

8.6 × 10

13

∆S≠

(JK-1

mol-1

)

CR

MKN

HM

MT

-8.3 × 101

-8.2 × 101

-6.8 × 101

-8.1 × 101

-2.2 × 102

-2.2 × 102

-1.9 × 101

-2.2 × 10

1

-2.4 × 102

-2.4 × 102

-2.1 × 102

-2.3 × 101

-8.2 × 100

-8.9 × 100

-2.8 × 100

-2.2 × 100

r CR

MKN

HM

MT

0.9993

0.9993

0.9978

0.9994

0.9929

0.9929

0.9927

0.9953

0.9944

0.9945

0.9958

0.9946

0.9992

0.9992

0.9991

0.9993

CR, Coats–Redfern; MKN, Madhusudanan-Krishnan-Ninan; HM, Horowitz-Metzger;

MT, MacCallum-Tanner.

The correlation coefficients (r) are in the range 0.9929-0.9993, indicating

nearly perfect fits. The activation energy is highest for the IVth stage of

decomposition. Same trend is shown by the pre-exponential factor and entropy

of activation for stage IV. The first stage of deamination is having the next

highest value of activation energy. The values for kinetic parameters obtained

for IInd

and IIIrd

stages of deamination are lower. The kinetic parameters

obtained using the four non-mechanistic equations are almost same. However,

values obtained for Horowitz-Metzger equation are comparatively high and it

is due to the discrepancy involved in the approximation employed in the HM

equation. It can be seen that the order parameters are decimal value. It is

known from the literature36

that the order parameters can have decimal


number. The entropy of activation obtained are negative values. The negative

values indicate that the activated complex has an ordered structure than the

reactant and the reaction in this case is said to be slower than the normal.37


The kinetic parameters calculated by the four non-mechanistic equations

for tris(ethylenediamine)nickel(II) sulphate using the data obtained from

TG experiment are given in Table 4.9.

Table 4.9

Kinetic parameters for the first deamination reaction of

tris(ethylenediamine)nickel(II) sulphate using non-mechanistic equations

Equations E (kJ mol-1

) A (s-1

) ∆S≠

(JK-1

mol-1

) r

n 0.74

CR 254.4 2.2× 1019

1.2 × 102 0.9974

MKN 247.9 6.1 × 1018

1.1 × 102 0.9975

HM 258.9 5.0 × 1019

1.3 × 102

0.9963

MT 253.1 2.1× 1019

1 × 102 0.9977

CR, Coats–Redfern; MKN, Madhusudanan-Krishnan-Ninan; HM, Horowitz-Metzger;

MT, MacCallum-Tanner.

The values of kinetic parameters obtained from different equations are

comparable. The entropy is found to have positive value. The decomposition

stage with positive entropy indicates that the activated complex has a less

ordered structure and the reaction in this case is said to be faster than the

normal. It can be seen from the table that the order parameter for the

decomposition process is decimal numbers (Table 4.9).

140 Chapter 4

4.4.4.1 Deduction of reaction mechanism


The mechanism of the reaction is selected by choosing the equation which

gives the best linear fit with maximum correlation coefficient. The kinetic

parameters for mechanism based equations are given in Table 4.10.

In some cases more than one equation gives good linear curves, in such cases

it may become difficult to unambiguously assign the reaction mechanism

from the linearity of the kinetic curve alone. In such cases the mechanism is

selected by comparing the kinetic parameters with those obtained from non-

mechanistic equations.38

In the present case, a comparison with the values

obtained by the Coats–Redfern method is more appropriate, because the

same method was used here for solving the exponential integral. For first

deamination stage, the correlation coefficient is highest for Mampel equation

(eqn. no.5), and the rate controlling process for this stage is assumed to be

random nucleation with the formation of one nucleus on each particle. For

the second deamination stage one dimensional diffusion (eqn. no.1) shows

nearly perfect fits and the kinetic parameters obtained are fairly well

comparable with that of non-mechanistic equation (CR). The third

deamination stage adopts three dimensional diffusion Jander equation (eqn.

3) as the rate controlling process. The final decomposition stage adopts phase

boundary reaction with cylindrical symmetry (eqn. no.8) as the reaction

mechanism. In this case also the kinetic parameters are highly comparable

with that of non-mechanistic equation.


Table 4.10

Kinetic parameters for the thermal decomposition of

hexaamminenickel(II) sulphate from mechanistic equations

Mechanistic

equations no Stage1 Stage II Stage III Stage IV

1

E

A

r

100.3

7.8×109

0.9547

60.64

3.13×103

0.9964

131.8

3.6×106

0.9911

549.5

2.48×1024

0.9955

2

E

A

r

112.8

2.17×1011

0.9689

76.59

2.47×104

0.9954

117.27

8.31×105

0.9943

605.5

1.08×1027

0.9985

3

E

A

r

131.9

2.18×1013

0.9865

90.21

1.82×104

0.9908

105.4

1.43×105

0.9949

689.9

6.14×1030

0.9982

4

E

A

r

118.8

3.36×1011

0.9756

81.1

1.74×104

0.9942

122.1

4.93×105

0.9934

632.2

5.98×1027

0.9991

5

E

A

r

74

1.29×107

0.9965

48.07

1.22×102

0.9811

68.15

3.84×102

0.9842

387.1

7.12×1016

0.9905

6

E

A

r

33.6

5.59×101

0.9955

19.61

9.67×10-2

0.9727

28.59

1.69×10-1

0.9782

184.9

3.96×106

0.9897

7

E

A

r

201

7.09×10-1

0.9941

10.13

6.23×10-3

0.9568

15.41

9.28×10-3

0.9679

117.5

1.23×103

0.9888

8

E

A

r

57.8

3.2×104

0.9765

37.34

3.43×100

0.9918

56.84

6.74×101

0.9915

315.5

5.85×1012

0.9992

9

E

A

r

62.5

1.01×105

0.9848

48.7

5.66×100

0.9889

60.42

2.49×101

0.9895

336.3

5.02×1013

0.9982

E - kJ mol-1

, A - s-1


The kinetic parameters from the mechanism based equations for the

complex are given in Table 4.11. The mechanism of the reaction is found

142 Chapter 4

to be random nucleation (eqn.no.5) with the formation of one nucleus on

each particle.

Table 4.11

Kinetic parameters for the first deamination reaction of

tris(ethylenediamine)nickel(II) sulphate using mechanistic equations

4. 5 Conclusions

The thermal decomposition studies of hexaamminenickel(II) sulphate and

tris (ethylenediamine)nickel(II) sulphate complexes have been carried out

in helium and air. The hexaammine complex undergoes a four-stage

decomposition comprising three deamination stages and one

decomposition stage, which are in accordance with the crystal structure.

Simultaneous TG/DTA coupled online with mass spectroscopy (MS) in

helium atmosphere detected the presence of NH, N, S, O and N2

fragments during thermal decomposition. The intermediates as well as the

final residue were separated and identified. The thermolysis results in the

formation of NiO nano particles with particle size in the range of 50 nm.

The kinetic parameters were evaluated for the deamination and

Mechanistic eqns no. E (kJ mol-1

) A (s-1

) r

1 39 0.1

4.4 × 1030

0.9804

2 431.1

5.3 × 1033

0.9889

3 492.3

1.6 × 1038

0.9967

4 451.9

5 × 1034

0.9923

5 284.3

1.7 × 1021

0.9972

6 131.2

1.1 × 109

0.9969

7 851

7.6 × 104 0.9967

8 232.1

3.5 × 1016

0.9937

9 243.5

4.5 × 1017

0.9966


decomposition stages employing isoconversional methods and by using

both non-mechanistic and mechanism based equations. For

isoconversional methods a series of activation energy were obtained with

the extent of conversion. The activation energy varies with the extent of

conversion for all stages, indicating that the degradation process is multi-

step. The kinetic parameters calculated from the non-mechanistic

equations are comparable. The rate controlling processes for the

decomposition stages were determined by model fitting method

(mechanism based equations).

In air tris(ethylenediamine)nickel(II) sulphate decomposes in four steps and

in helium atmosphere this complex undergoes a two stage decomposition

resulting in a mixture of nickel and nickel sulphide as the final residues.

Simultaneous TG/DTA coupled online with mass spectra (MS) in helium

atmosphere revealed the presence of H2, O, NH, NH2, NH3 and N2 during

the decomposition. Kinetic analysis by isoconversional method for the first

deamination stage of tris(ethylenediamine)nickel(II) sulphate show

increasing as well as decreasing dependence of activation energy with

conversion, probably due to the simultaneous occurrence of various thermal

events in a single stage. The rate controlling process for the deamination

reaction was found to be random nucleation with the formation of one

nucleus on each particle.

144 Chapter 4

References

1. W. W. Wendlandt, J. P. Smith, Thermal Properties of Transition

Metal Amine Complexes, Elsevier, Amsterdam, 1967.

2. S. Mathew, C. G. R. Nair, K. N. Ninan, Thermochim. Acta, 144,

1989, 33.

3. G. Singh, C. P. Singh, S. M. Mannan, Thermochim. Acta, 437,

2005, 21.

4. F. Kooli, Thermochim. Acta, 486, 2009, 71.

5. J. P. Sanders, P. K. Gallagher, J. Therm. Anal. Calorim., 96, 2009, 805.

6. S. Mathew, N. Eisenreich, W. Engel, Thermochim. Acta , 269/270,

1995, 475.

7. T. D. George, W.W. Wendlandt, J. Inorg. Nucl. Chem., 25, 1963, 395.

8. E. G. Rochow (Ed.), Inorganic Synthesis, Vol. 6, McGraw-Hill,

New York, 1960.

9. A. I. Vogel, A Text Book of Quantitative Inorganic Analysis, 4th edn.,

Longmann, New York, 1978.

10. R. Ramani, J. Srivastava, S. Alam, Thermochim. Acta, 499, 2010, 34.

11. J. Liu, X. Jiang, L. Zhou, X. Han, Z. Cui, J. Hazard. Mater., 161,

2009, 1208.

12. A. A. Abu-Sehly, Thermochim. Acta, 485, 2009, 14.

13. A. S. Soltan, Physica B, 405, 2010, 965.

14. J. H. Flynn, L. A. Wall, J. Polym. Sci. Part B, 4, 1996, 323.


15. T. Ozawa, Bull. Chem. Soc. Jpn., 38, 1965, 1881.

16. H. L. Friedman, J. Polym. Sci. Part C, 6, 1963, 183.

17. H. E. Kissinger, Anal. Chem., 29, 1957, 1702.

18. T. Akahira, T. Sunose, Res. Rep. Chiba Inst. Technol., 16, 1971, 22.

19. A. W. Coats, J. P. Redfern, Nature, 201, 1964, 68.

20. P. M. Madhusudanan, K. Krishnan, K. N. Ninan, Thermochim.

Acta, 97, 1986, 189.

21. H. H. Horowitz, G. Metzger, Anal. Chem., 35, 1963, 1464.

22. J. R. MacCallum, J. Tanner, Eur. Polym. J., 6, 1970, 1033.

23. J. Sestak, G. Berggren, Thermochim. Acta, 3, 1971, 1.

24. V. Satava, Thermochim. Acta, 2, 1971, 423.

25. M. D. Judd, M. I. Pope, J. Therm. Anal., 4, 1972, 31.

26. J. Zsako, J. Therm. Anal., 5, 1973, 239.

27. V. M. Gorbachev, J. Therm. Anal., 8, 1975, 349.

28. C. Mockenhaupt, R. Eßmann, H. D. Lutz, Z. Naturforsch, 54b,

1999, 843.

29. F. A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, John

Wiley & Sons, 5th edn., 1988.

30. K. Nakamoto, Infrared and Raman spectra of inorganic and

coordination compounds; Theory and Applications in Inorganic

Chemistry, John Wiley & Sons, 6th edn., New York, 2009.

31. S. Mitra, G. De, N. R. Chaudhury, Thermochim. Acta, 71, 1983, 107.

146 Chapter 4

32. S. Vyazovkin, Int. Rev. Phys. Chem., 19, 2000, 45.

33. S. Vyazovkin, C. A. White, Chem. Mater., 11, 1999, 3386.

34. S. Vyazovkin, W. Linert, Int. J. Chem. Kinet., 27, 1995,73.

35. S. Vyazovkin, J. Comput. Chem., 22, 2001, 178.

36. A. C. Norris, M. I. Pope, M. Selwood, Thermochim. Acta, 41,

1980, 357.

37. A. A. Frost, R. G. Pearson, Kinetics and Mechanism, Wiley, New

York, 1961.

38. P. H. Fong, D. T. Chen, Thermochim. Acta, 18, 1977, 273.

Documents

Thermoanalytical Investigations of Hexaamminenickel(II) …shodhganga.inflibnet.ac.in/bitstream/10603/13162/12/12... · 2015-12-04 · Thermoanalytical Investigations 115of Hexaamminenickel(II)