Some aspects of thermal decomposition of NiC2O4·2H2O

Article abstract of DOI:10.1016/j.tca.2007.10.010

NiC₂O₄·2H₂O decomposition in air and in helium was studied using TG, DTA and MS and FTIR analyses. It was found that the dehydration phase of reaction is not complete. Remaining water occluded in the pores of nickel oxalate is not liberated until the decomposition of oxalate ions starts. The residual water takes part in reaction of the basic nickel carbonate formation. The basic nickel carbonate decomposition is a source of traces of nickel oxide stated in the final product of decomposition along with metallic nickel. It was found that the double peak endothermic DTA signal which accompanies decomposition of nickel oxalate results from changing of the microscopic mechanism of reaction.

Full text of DOI:10.1016/j.tca.2007.10.010

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Thermochimica Acta 466 (2007) 57–62
Some aspects of thermal decomposition of NiC2O4·2H2O
B. Małeckaa,∗, A. Małecki , E. Drozdz-Ciesla , L. Tortet ,
P. Llewellynb, F. Rouquerolb
a
a
b
˙ ˙
´
a AGH University of Science and Technology, Faculty of Materials Science and Ceramics,
Al. Mickiewicza 30, 30-059 Krako´w, Poland
b MADIREL, CNRS-Universite´ de Provence, UMR 6121, Centre de St Je´roˆme,
13397 Marseille Cedex 20, France
Received 6 September 2007; received in revised form 18 October 2007; accepted 21 October 2007
Abstract
NiC2O4·2H2O decomposition in air and in helium was studied using TG, DTA and MS and FTIR analyses. It was found that the dehydration
phase of reaction is not complete. Remaining water occluded in the pores of nickel oxalate is not liberated until the decomposition of oxalate ions
starts. The residual water takes part in reaction of the basic nickel carbonate formation. The basic nickel carbonate decomposition is a source of
traces of nickel oxide stated in the final product of decomposition along with metallic nickel. It was found that the double peak endothermic DTA
signal which accompanies decomposition of nickel oxalate results from changing of the microscopic mechanism of reaction.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Nickel oxalate; Thermal decomposition; TG; DTA; MS; FTIR
1. Introduction
of composition 99.2% NiC2O4 and 0.8% H2O [5]. The residual
water was considered as the occluded water [6,7]. All the results
quoted indicate that complete dehydration of NiC2O4·2H2O to
obtain anhydrous nickel oxalate cannot be achieved. However,
in other papers full dehydration of NiC2O4·2H2O was stated and
the kinetics of decomposition of anhydrous nickel oxalate was
determined [8].
The second unexplained effect related to nickel oxalate
decomposition in inert atmosphere is the complex DTA/DSC
peak corresponding to oxalate ions degradation. The complexity
of DTA or DSC curve, manifested by a shoulder on the peak, was
pointed out in many works [2,5–7,9–11]. The same shape shows
the DTG curve [9]. The different explanations of this effect were
given. It was supposed to be connected with the phase transi-
tion of nickel at Curie point [7,9] or with the formation of solid
intermediate [8]. The secondary reaction of nickel with resid-
ual water was indicated as the cause of complex DTA signal in
Ref. [5]. In Ref. [12] three stage mechanism of decomposition
of NiC2O4 was proposed and the differences in the particular
reactions rate were assumed to be responsible for the complex
DTA curve.
Thermal decomposition NiC2O4·2H2O has been studied for
many years. In the extensive reports on thermal decomposition
of oxalates [1,2] the number of references concerning this pro-
cess are cited. Despite the numerous works and publications on
thermal decomposition of nickel oxalate dihydrate this reaction
still remains the subject of investigations.
There are several effects that accompany this reaction which
still have no clear explanation and give rise to many contro-
versies. One of them is incomplete dehydration process and
an inability to obtain anhydrous compound. The possibility of
obtaining anhydrous nickel oxalate was discussed in several
works. In paper [3] Authors stated that dried nickel oxalate can
take up to 15% of original water on storing on air and that this
water can be completely removed about 250 ◦C. Authors of Ref.
[4] stored dehydrated nickel oxalate at 200 ◦C in the desiccator
over P2O5 for 2 years to remove residual water. After this time
nickel oxalate re-absorbed 15% of water. Dehydration of nickel
oxalate dihydrate in the dynamic vacuum led to the compound
And finally there are still controversies concerning the pri-
mary and secondary reactions occurring during NiC2O4·2H2O
decomposition. In most experiments performed in air NiO was
∗
Corresponding author.
E-mail address: bmalecka@agh.edu.pl (B. Małecka).
0040-6031/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2007.10.010

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B. Małecka et al. / Thermochimica Acta 466 (2007) 57–62
claimed to be the primary product of NiC2O4·2H2O decomposi-
tion. However, Authors of Refs. [10,13] suggest that the primary
product of decomposition is Ni and NiO forms in result of sec-
ondary oxidation of nickel by oxygen. Almost everybody agrees
that decomposition of NiC2O4·2H2O in inert atmosphere leads
to the formation of Ni and CO2, however some Authors sug-
gest that Ni forms due to the reduction of the primary product
NiO by CO in secondary reaction [14]. During decomposition
in the atmosphere of CO2 metallic Ni was found as the product
of NiC2O4 decomposition by Authors of Ref. [10] whereas in
paper [15] the final product of decomposition was NiO.
The objective of this work is to present new results concern-
ing mentioned effects which shall throw some light on these
controversial points in thermal decomposition of nickel oxalate
dihydrate.
Fig. 1. XRD pattern of the initial sample of nickel oxalate dihydrate.
2. Experimental
X-ray diffraction analyses were carried out on Philips X-ray
diffractometer using Cu K␣ radiation.
2.1. Preparation of nickel oxalate
FTIR and HT-DRIFT (High Temperature Diffuse Reflexion
Infrared Fourier Transform) spectroscopy measurements were
performed on an Equinox 55.
Nickel oxalate dihydrate was prepared by the reaction of
nickel nitrate(V) with oxalic acid in aqueous solution at about
65 ◦C for 4 h (all chemicals were of analytical grade Pol-
skie Odczynniki Chemiczne S.A.). pH 4.0 of the mixture was
maintained with ammonia and hydrochloric acid to avoid pre-
cipitation of nickel hydroxide. Next, crystalline precipitate was
filtered, washed with dilute oxalic acid and finally with an abso-
lute alcohol. The product obtained was dried at 50 ◦C on air.
3. Results and discussion
The initial sample was analysed for the oxalate ions content
by wet analysis and for the nickel content by gravimetric analy-
sis with dimethylglyoxime. Analysis showed that the molar ratio
2−
Ni2+/C2O4 was 0.97 0.03. X-ray powder diffraction anal-
2.2. Experimental techniques
ysis confirmed that the only component of initial sample was
NiC2O4·2H2O (ICDD 14-0742) (Fig. 1).
Thermal decomposition measurements were performed with
an apparatus that enabled simultaneous recording of TG and
DTA signals (SDT 2960 TA INSTRUMENTS). The samples of
mass from the range 6 to 20 mg were heated in a standard plat-
inum sample pan. The experiments were carried out in dynamic
flow of helium (99.999%) or synthetic air (<15 ppm H2O). The
gas flow rate was 0.1 dm3 min−1 and the volume of thermoana-
lyzer was 0.06 dm3.
Figs. 2and3showTG/DTAandMS(MS, massspectrometry)
results of thermal decomposition of NiC2O4·2H2O obtained in
The analysis of gaseous products of decomposition was car-
ried out using a quadrupole mass spectrometer (QMD 300
THERMOSTAR BALZERS) connected on-line with SDT 2960
apparatus by the heated quartz capillary. The mass spectrom-
eter was operated with an electron impact ionizer with energy
0.112 aJ (70 eV). Mass spectra were recorded for m/z equal to
+
18, 28, 32 and 44 which correspond to ions: H2O+, CO+, O2
and CO2+, respectively.
CRTA (Constant Rate Thermal Analysis) measurements
[16] were performed on an apparatus designed and built in
MADIREL Laboratory. This apparatus controls the vacuum
pressure signal above the sample at a predetermined constant
value. The PID type regulation uses the pressure gauge input
to regulate the heating of sample such that the pre-set pressure
value, a resultant of vapour from the reaction, is obtained and
remains constant until the end of the experiment where there is
no more vapour produced by the thermolysis. In experiment the
pre-set pressure value was 2 Pa.
Fig. 2. Thermogravimetric and mass spectrometry results of decomposition of
NiC2O4·2H2O in air (heating rate β = 5 ◦C min−1).

B. Małecka et al. / Thermochimica Acta 466 (2007) 57–62
59
Fig. 4. XRD pattern of the sample after dehydration.
Fig. 5 presents the FTIR spectra of the initial sample and
of the sample after dehydration at heating rate 2 ◦C min−1 until
300 ◦C. After this treatment, the bands at 617 and at 758 cm−1
,
present in the initial sample and corresponding to Ni–O–H and
water of crystallization vibration modes, respectively, have van-
ished. The band at 3400 cm−1, corresponding to OH stretching
vibration mode, is still present, but results from the presence
of the adsorbed H2O (not water of crystallization). This is the
consequence of the preparation of sample for the FTIR measure-
ment: during grinding the sample with KBr powder the water is
adsorbed very quickly.
Fig. 3. Thermogravimetric and mass spectrometry results of decomposition of
NiC2O4·2H2O in helium (heating rate β = 5 ◦C min−1).
helium and in air. The decomposition proceeds generally in two
steps, dehydration and decomposition of the anhydrous oxalate.
XRD analysis confirmed that the final product of decomposition
inairisNiOandinhelium–Ni. Intheresidueafterdecomposition
in helium, beside Ni, the traces of NiO have been stated on the
basis of measurements of reduction of the residue in hydrogen.
It was found that residue contains about 2 mol% of NiO.
A study of specific area of the sample after dehydration made
with the BET method with N2 revealed a value of 12.4 m2 g−1
.
The method t showed an external surface of 3.9 m2 g−1 and
a microporosity of 8.5 m2 g−1. This microporosity explains
adsorption of water in the product after crystallographic dehy-
dration (increase of weight and intense band in the FTIR spectra
relative to adsorbed water at 3400 and 1650 cm−1).
HT-DRIFT experiment has confirmed the total or quasi-total
departure of water at 250 ◦C at which the band at 3400 cm−1
vanished (Fig. 6).
3.1. Dehydration
NiC2O4·2H2O looses the main part of hydration water on
heating up to the temperature at which decomposition of the
oxalate ions starts. The remaining, small amount of water broke
free of the nickel oxalate structure together with decomposi-
tion of the compound. This is confirmed by the raise of line
m/z = 18 during decomposition above 300 ◦C (particularly evi-
dent in helium). To find out what is the character of the residual
water remaining after dehydration the series of XRD, FTIR
and HT-DRIFT analysis were performed. The samples of nickel
oxalate dihydrate were dried at 200 and 250 ◦C in static air and at
150 ◦Cindynamicvacuumforseveralhours. Afterthistreatment
the colour of the samples changed: from blue-green to yellow-
green, which means that the crystallization water was removed.
After taking samples out of the dryer their weight systematically
raised due to adsorption of water vapour. The final mass gain
was about 9.5 0.5%, which corresponds 8.3% of crystalliza-
tion water in nickel oxalate dihydrate. XRD diffraction patterns
of the sample dehydrated in air and for the sample dehydrated in
vacuum are similar to each other, but both are different from the
pattern in ASTM file. The X-ray diffraction patterns obtained are
close to the one published in Ref. [17] (Fig. 4). Stability of this
phase in the range of temperature 200–290 ◦C was confirmed by
high-temperature XRD.
Fig. 5. FTIR spectra of initial nickel oxalate dihydrate sample and sample after
dehydration at temperature raising with rate β = 2 ◦C min−1 up to 300 ◦C.

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B. Małecka et al. / Thermochimica Acta 466 (2007) 57–62
Fig. 6. HT-DRIFT spectra of NiC2O4·2H2O samples dehydrated at temperature
raising with rate β = 2 ◦C min−1 up to different temperatures.
In conclusion the water present in the sample at the tem-
perature of decomposition in experiments performed at raising
temperature is the water occluded in highly porous product
obtained after dehydration process.
Fig. 7. Ion current of m/z = 28 measured and calculated from experimental signal
of m/z = 44 (helium, β = 5 ◦C min−1).
Let us consider if reaction (1) takes place. In that case the
amount of CO2 and CO has to be the same and the signals
from m/z = 28 and m/z = 44 should have the same shape, because
both gases evolve with equal amount and with identical rate.
The results obtained do not confirm the behaviour of MS sig-
nals described above which means that first reaction does not
happen. In conclusion evolution of CO at the beginning of
nickel oxalate decomposition can be the consequence of reac-
tion (2) with basic nickel carbonate and carbon monoxide as the
products.
3.2. Decomposition
The TG line between the end of first step (dehydration) and
the beginning of decomposition reveals the distinct slope con-
nected with about 3.5% loss of mass (in respect to the initial
mass). This change of mass is connected with evolution of CO2
and H2O. The value of ion current for m/z = 44 in this range is
raising and the value of ion current for m/z = 18 is continuously
approaching the baseline. This means that CO2 and H2O evolve
from the sample. For the measurement in helium it was pos-
sible to make comparison of ion current m/z = 28 to this being
the result of fragmentation of CO2 and calculated from ion cur-
rent m/z = 44 with fragmentation factor 0.0658 [18]. MS signal
for m/z = 28 can be a sum of CO+ ion current and ion current
resulting from fragmentation of CO2 (mass spectrum of CO2
consists of m/z: 44, 28, 22, 16, 12). If CO2 is present in a gas
mixture and CO is absent, signal m/z = 28 has to be the same as
ion current from fragmentation of CO2. If gas mixture contains
both CO2 and CO, m/z = 28 signal should have the value higher
than ion current from fragmentation of CO2. The comparison
showed (Fig. 7) that two lines do not overlap which means that
a small amount of CO evolves. Thus, 3.5% loss of mass results
from the evolution of H2O, CO2 and CO.
Very careful analysis of FTIR spectra of the decomposition
product of NiC2O4·2H2O obtained during isothermal heating
at 200, 260 and 300 ◦C for 10 h in air showed the formation
of nickel carbonate. In the spectrum of sample from 260 ◦C the
band at 1375 cm−1 appears which corresponds to the C–O vibra-
2−
tion in CO3 group [19]. It is partially masked by the band at
1360 cm−1 of the C–O stretching mode of the oxalate ion, but
in the sample from 300 ◦C the band at 1375 cm−1 is clearly seen
(Fig. 8). The same result was obtained for samples heated in air
with the rate 2 ◦C min−1 until 300 and 325 ◦C (Fig. 9). At 300 ◦C
we can see the band at 1360 cm−1 characteristic for oxalate
ion. At 325 ◦C this band has decreased that enabled to spot
the band at 1375 cm−1 of carbonate group. The vibration band
at 500 cm−1 originates from Ni–O bond. However, the XRD
analysis of product obtained at studied temperatures did not con-
firm the presence of nickel carbonate, which allows stating that
carbonate phase forms in small amount and/or in amorphous
form.
There are two ways of explaining the presence of carbon
monoxide in the gas evolved at the beginning of decomposition
by assuming the following reactions:
NiC2O4 → NiO + CO2 + CO
or
(1)
In conclusion, during decomposition of nickel oxalate dihy-
drate small amount of basic nickel carbonate forms. This
reaction is possible because reaction of dehydration of nickel
oxalate is not complete. NiCO3·H2O decomposes with forma-
tion of NiO. This explains the presence of traces of nickel oxide
in the final product of NiC2O4·2H2O decomposition in helium.
NiC2O4 + H2O → NiCO3·H2O + CO
Both proposed reactions have to occur in a very little scale.
(2)

B. Małecka et al. / Thermochimica Acta 466 (2007) 57–62
61
Fig. 8. FTIR spectra of NiC2O4·2H2O heated isothermally at different temper-
atures for 10 h in air.
Fig. 10. CRTA result of decomposition of NiC2O4·2H2O at pressure 2 Pa.
Themainstepofdecompositionofnickeloxalatecorresponds
to the reaction:
DTA line of NiC2O4 decomposition in helium shows double
peak. The shape of DTG and CO2 ion current signal m/z = 44
+
NiC2O4 → Ni + 2CO2
(3)
(MS analysis) copies DTA shape. This shape could be the evi-
dence that the process is complex. However, the double DTA and
DTG signal cannot be explained by occurring of parallel reac-
tions since this is the single step reaction with no intermediates
and Ni as a final product.
Reaction (3) starts just after dehydration step and develops
very slowly until the temperature reaches about 310 ◦C and
next accelerates. The final solid product of nickel oxalate
decomposition in helium and in vacuum is nickel. The way of
decomposition in air is the same but metallic nickel is oxidized
by oxygen from air giving NiO as a final product of reaction.
It is worth to notice that onset temperatures of DTA peaks of
decomposition are close to 310 ◦C, both in helium and in air.
The dihydrate of cobalt oxalate, CoC2O4·2H2O, metal very
similar to nickel, decomposes in a different way [20]. Decompo-
sitionofCoC2O4·2H2Oproceedsthroughtwoparallelreactions:
one leading to metallic cobalt, and second to cobalt oxide. The
amount of cobalt oxide in the residue was significant and depen-
dent on the heating rate.
One of the possible explanations of this unusual shape of
DTA line can be the change in the microscopic mechanism
of decomposition, for instance: in the beginning the rate of
decomposition is controlled by the process of decomposition
of oxalate ions with activation energy E1. This process is
very fast. Next rate of the whole reaction slows down and is
controlled by the nickel nuclei growth according to the dif-
fusion mechanism with activation energy E2. The results of
CRTA experiment seem in good agreement with this explana-
tion (Fig. 10). If we consider the part of temperature line in
Fig. 10 related to decomposition of nickel oxalate, i.e. above
250 ◦C, we can notice that in order to keep constant all at
once pressure and rate, it is necessary to increase then decrease
temperature. This is typical of a nucleation process. Then the
temperature of the reaction has to be continuously increased
to maintain the constant pressure which could correspond to
a diffusion mechanism of nickel nuclei growth. In paper [21],
where mechanism of decomposition of nickel oxalate at high
temperatures in Ar/5%H2 atmosphere was examined, Authors
stated that nanosized nickel grains form in result of homoge-
neous nucleation. Taking these considerations into account we
believe that the double DTA signal seems to be the consequence
of not the chemical changes but of changes of the kinetics of
reaction.
DTA signal obtained for decomposition of nickel oxalate
in air has the shape different from this characteristic for
decomposition in helium. The main reason of this is that
in air the process is exothermic as a whole. The growth of
nickel nuclei does not occur because nickel is at once oxi-
dized. Additionally relatively strong exothermic effect being
Fig. 9. FTIR spectra of NiC2O4·2H2O heated with rate 2 ◦C min−1 until 300
and 325 ◦C.

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B. Małecka et al. / Thermochimica Acta 466 (2007) 57–62
the result of metallic nickel oxidation increases the rate of
decomposition.
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3.3. Summary
Thermal decomposition of NiC2O4·2H2O proceeds in two
main stages: one is dehydration and second is decomposition.
Dehydration is never complete and water remains in the prod-
uct of dehydration as occlusion in highly porous structure. The
product after dehydration is very hygroscopic. At the begin-
ning of decomposition the presence of residual water enables
formation of small amount of basic nickel carbonate. The prod-
ucts of decomposition of NiC2O4 are nickel and carbon dioxide,
but in oxidizing atmosphere nickel is instantly oxidized giving
NiO. In the product of decomposition in helium, beside nickel,
a little amount of NiO is always present. NiO is a product of
nickel carbonate decomposition. The decomposition of NiC2O4
in inert atmosphere proceeds by two consecutive reactions: the
very fast reaction of homogeneous nucleation of metallic nickel
throughout the grain following by distinctly slower reaction
of nickel nuclei growth most probably with diffusion mecha-
nism.
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Acknowledgements
[18] NIST Standard Reference Database Number 69, June 2005 Release.
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Authors would like to thank Ministry of Science and Higher
Education of Poland for support of this work under project No.
11.11.160.110. Authors would also like to thank Dr. Marek
MaciejewskiforperformingreductionmeasurementofNiOwith
hydrogen.