Kinetic and thermodynamics studies on the decompositions of Ni3C in different atmospheres

Article abstract of DOI:10.1016/j.tca.2008.04.003

The thermal decompositions (including TG and DSC) of nickel carbide were studied under different atmospheres of Ar, air and H₂. X-ray diffraction combined with element analysis indicated that nickel metal, together with solid amorphous carbon,

Full text of DOI:10.1016/j.tca.2008.04.003

Thermochimica Acta 473 (2008) 14–18
Contents lists available at ScienceDirect
Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Kinetic and thermodynamics studies on the decompositions
of Ni3C in different atmospheres
Yonghua Lenga, Lei Xiea, Fuhui Liaoa, Jie Zhenga, Xingguo Lia,b,∗
a Beijing National Laboratory for Molecular Sciences (BNLMS), The State Key Laboratory of Rare Earth Materials Chemistry and Applications,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
b College of Engineering, Peking University, Beijing 100871, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The thermal decompositions (including TG and DSC) of nickel carbide were studied under different atmo-
spheres of Ar, air and H2. X-ray diffraction combined with element analysis indicated that nickel metal,
together with solid amorphous carbon, was formed during Ni3C decomposition in Ar atmosphere, accom-
panying mass invariant in this process. While in H2 atmosphere nickel metal was the only residual from
reactions. The carbon component of nickel carbide reacted with H2 to form methane as the main volatile
gases. Both the nickel and carbon components of Ni3C reacted with O2 in the air to form their correspond-
ing oxides. Moreover, we calculated the activation energy for the decomposition process and the molar
enthalpy of formation of Ni3C based on the thermal analysis.
Received 7 December 2007
Received in revised form 2 April 2008
Accepted 7 April 2008
Available online 12 April 2008
Keywords:
Nickel carbide
Thermal decomposition
Kinetic
© 2008 Elsevier B.V. All rights reserved.
Metastable
1. Introduction
to distinguish these two materials. In this paper, we reported the
decomposition of Ni3C phase in different atmospheres, including
Ni3C has a metastable phase at room temperature and decom-
poses at the temperatures above 430 ◦C in inert atmosphere, which
make it hard to be synthesized [1]. Although many literatures have
reported the formation of thin Ni3C film [2,3], in the case of pure
Ni3C powder, only few groups succeeded to convert nickel metal
fully into Ni3C phase [4,5]. As a metastable phase resulted from the
low solubility of carbon into nickel at high temperature and lack of
ionic bonding [6], however, no detailed study has been reported on
the decomposition of Ni3C.
Synthesis of pure Ni3C powder becomes the key to study the
thermodynamics of Ni3C and calculate its kinetic parameters.
Recently, our group reported that pure Ni3C powder could be trans-
formed from Ni nanoparticles in organic solutions in the presence of
surfactants [7], which provide us the possibility to study the ther-
mal stability of Ni3C phase. In addition, it is hard to distinguish
hexagonal-close-packed Ni metal (JCPDS 45-1027) from Ni3C phase
(JCPDS 06-0697) according to their XRD patterns [8–10]. By ana-
lyzing the decomposition products, it may offer us an effective way
the inert argon, the oxidative air and the reductive hydrogen. The
activation energies and the molar enthalpy of formation of Ni3C
were also calculated based on the thermal analysis.
2. Experimental
Well-characterized Ni3C nanoparticles were prepared in 1-
octadecene by the decomposition of nickel formate, according
to a slightly modified procedure reported by our group [7]. The
nanoparticles were found to consist of pure Ni3C phase, which was
used for further decomposition in different atmospheres.
Thermogravimetric and differential scanning calorimetric
(TG–DSC, NETZSCH STA 449 C) curves were recorded at 10 K/min
in Ar (99.99%), H2 (99.99%) and air atmospheres. Highly sintered
␣-Al2O3 was used as a reference material for the thermal measure-
ments. This thermal analyzer is equipped with a mass spectrometer
(MS) to detect the volatile components during the decomposition
process. During the measurements, the sample mass was ca. 10 mg
and all runs were conducted at a gas flow rate of 40 mL min−1. In
order to calculate the activation energy for the reaction of Ni3C
under different atmospheres, heating rates were varied 5, 10 and
20 K/min, respectively. X-ray diffraction (XRD, RIGAKU D/MAX-
2400) was conducted using Cu K␣ radiation to identify their
structure of the decomposition solid products. 2ꢀ scans were made
from 30◦ to 80◦ at a rate of 4◦/min with a step size of 0.02◦. The
∗
Corresponding author at: Beijing National Laboratory for Molecular Sciences
(BNLMS), The State Key Laboratory of Rare Earth Materials Chemistry and Applica-
tions, College of Chemistry and Molecular Engineering, Peking University, Beijing
100871, China. Tel.: +86 10 6276 5930; fax: +86 10 6276 5930.
E-mail address: xgli@pku.edu.cn (X. Li).
0040-6031/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2008.04.003

Y. Leng et al. / Thermochimica Acta 473 (2008) 14–18
15
measurements were conducted using a generator voltage of 40 kV
and a current of 100 mA. Microscale elements analysis instrument
(ELEMENTAR Vario EL) was adopted to detect the carbon content
in the decomposed products using combustion analysis.
3. Results and discussion
3.1. Ni3C decomposition in Ar atmosphere
Fig. 1 shows TG, DSC and MS curves of Ni3C decomposition in
Ar atmosphere at a heating rate of 10 K/min. The curves indicate
that Ni3C decomposes via a weight-loss step (process I) maxi-
mized at 540 K, ending up with solid products that produces an
exothermic peak centered around 688.2 K (process II). In process
I, the endothermic peak is attributed to the decomposition of the
adsorbed surfactants on the surface of the Ni3C particles. MS data
identified the formation of small fractions of C, CO, CO2, O, OH and
H2O during this process, corresponding to the mass loss by 2.9 wt%
from 500 to 570 K in the TG curve. There is also an exothermic peak
at 688.2 K in process II, which is associated to the decomposition of
Ni3C. The temperature for Ni3C to decompose is slightly lower than
that previously reported result, which is above 430 ◦C (703 K) [1].
Since no mass loss during process II is observed in the TG curve
and no volatile gases are detected by MS, this process could be
assigned to either a physical process, i.e. recrystallization of a solid
product, or a decomposition process, during which all the decom-
position products are solid matter. The decomposition products will
be validated below in detail.
The as-synthesized Ni3C nanoparticles was heated to 600 K at a
heating rate of 10 K/min and then cooled to room temperature, that
is, after the endothermic peak is completed (process I), but before
the emergence of the exothermic peak (process II), the decom-
posed product was measured with XRD. As shown in Fig. 2(b),
no structural change was observed compared with the XRD pat-
tern of the as-synthesized Ni3C (Fig. 2(a)), which confirms that the
endothermic peak belongs to the decomposition and desorption of
the adsorbed surfactants. This result agrees well with the MS data
by detecting the small molecules coming from the surfactants (pro-
cess I). According to the Scherrer equation, the average crystallite
size of Ni3C was estimated to be 40 nm, which was slight smaller
than that calculated from the TEM image (supporting information).
Microscale element analysis by combustion experiment confirms
Fig. 2. XRD patterns for: (a) the as-synthesized Ni3C nanoparticles, (b) the powders
obtained after heating to 600 K in Ar and (c) the powders obtained after heating to
900 K in Ar.
that the carbon content is 6.42 wt% in the product after heating to
600 K, which accords well with the value of 6.38 wt% for pure Ni3C
phase. This result implies that after heating to 600 K, the product
changes to pure Ni3C with clean surfaces. After heating to 900 K,
Ni3C nanoparticles decompose to produce nickel metal (JCPDS 04-
0850), as shown in Fig. 2(c), indicating that the exothermic peak
was owing to the Ni3C decomposition. Because no mass loss was
observed in TG curves, no volatile carbon detected by MS and no
crystalline carbon observed from XRD pattern during process II,
microscale element analysis was performed to validate the pres-
ence of carbon in the decomposition products. It indicates that
6.39 wt% of carbon in the decomposed product after heating to
900 K, agreeing well with the carbon content in Ni3C. This result
confirms that the hcp phase should be Ni3C rather than hcp Ni
metal. Though such a large quantity of solid carbon was formed in
this process, no crystalline carbon was detected by XRD, suggesting
the carbon coexists with nickel metal in an amorphous state. The
decomposition equation for Ni3C in Ar atmosphere should be
Ni3C(s) = 3Ni(s) + C(s, amorphous)
(1)
The decomposition of Ni3C in Ar was identified to be a single step
reaction. In this case, the activation energy ꢁE could be calculated
for the decomposition process using Kissinger method based on the
following equation [11]:
ꢀ
ꢁ
ꢀ
ꢁ
C
Tp2
ꢁE
ln
= −
+ A
(2)
KBTp
where C is the heating rate, ꢁE the activation energy, Tp the peak
temperature, KB Boltzmann constant, and A is a constant. With
increasing heating rate, the peaks shift to higher temperatures.
From the peak shift, ꢁE could be calculated. As shown in Fig. 3,
when the heating rates increase from 5 to 10 K/min and 20 K/min in
Ar atmosphere, the peak temperature is 673.3, 688.2 and 698.0 K,
respectively. According to Eq. (2), the calculated ꢁE for Eq. (1) is
204 kJ mol−1, with a correlation coefficient r to be 0.991.
DSC measurements are also used to calculate some thermo-
dynamics parameters. The enthalpy (ꢁH, J g−1) of the thermal
events could be directly determined from the DSC data (recorded at
10 K/min) according to the peak area. The determined ꢁH is used to
calculate the specific heat capacity (Cp, J K−1 g−1) using the equation
of Cp = ꢁH/ꢁT, where ꢁT = T2 − T1, and T1 is the temperature from
Fig. 1. TG, DSC and MS curves recorded for Ni3C at 10 K/min in a dynamic atmo-
sphere of Ar (the dash line denotes the TG curve, the short-dash line denotes the
DSC curve and the solid lines denote the MS data).

16
Y. Leng et al. / Thermochimica Acta 473 (2008) 14–18
Fig. 5. XRD patterns for: (a) the as-synthesized Ni3C nanoparticles; (b) the powders
obtained after decomposing in H2 at 673 K.
Fig. 3. DSC curves recorded for Ni3C decomposing at various heating rates in Ar: (a)
5 K/min, (b) 10 K/min and (c) 20 K/min.
H2 via a weight-loss process. Meanwhile, volatile hydrocarbons are
formed, which are taken away by the flowing H2. The release of
carbon component from Ni3C powder results in the loss of weight.
The MS data indicates that the hydrocarbon gases comprise mainly
methane and small amount of ethane. Though the XRD pattern for
the decomposition product of Ni3C at 673 K in H2 (Fig. 5(b)) is sim-
ilar to that in Ar (Fig. 2(c)), only negligible amount of carbon is
detected in the decomposed product by microscale element anal-
ysis, indicating nickel metal the only residue after decomposition.
Distinct from the Ar gas just as a purge and protective gas, H2
participates in the reaction of Ni3C decomposition, which combines
the carbon component in Ni3C and forms volatile hydrocarbon. For
the multistep nature of solid state reaction, here isoconversional
method was used to calculate the activation energy [13,14], where
the activation energy showed dependence on experimental con-
ditions, such as the extent of conversion, sample mass and the
temperature and so on. In the case of nickel carbides in H2, the
activation energy was calculated from the TG curves at different
heating rates according to the Ozawa–Wall–Flynn equation [15]:
Table 1
Thermodynamic parameters for the decomposition of Ni3C in Ar atmosphere
ꢁH (J g−1
Cp (J K−1 g−1
ꢁS (J K−1 g−1
)
140.5
6.4
0.2
)
)
which the DSC curve begins to depart from the base line, while T2 is
the temperature at which the peak lands. Then using the equation
of ꢁS = 2.303Cp log(T2/T1), the entropy change (ꢁS, J K−1 g−1) could
be obtained [12]. From the DSC curves measured at the heating rate
of 10 K/min in Ar atmosphere, the thermodynamic parameters were
listed in Table 1. The molar enthalpy of formation (ꢁfH) for Ni3C
is calculated to be 26.4 kJ mol−1 (Mw × ꢁH, Mw is the molecular
weight of Ni3C).
3.2. Ni3C decomposition in H2
To eliminate the influence of surfactants, Ni3C powder was pre-
heated to 600 K at a heating rate of 10 K/min in Ar and then cooled
to room temperature before decomposition under air and H2 atmo-
spheres. TG and MS curves for Ni3C decomposition in H2 at a heating
rate of 10 K/min are shown in Fig. 4. Ni3C powder decomposes in
ꢂ
ꢃꢂ ꢃ
ꢁE
R
1
T
log ˇ = 0.4567
+ A
where ˇ is the heating rate, ꢁE the activation energy, T the absolute
temperature and R is the gas constant. Fig. 6 shows the relationship
between the heating rate and reciprocal absolute temperature. It is
apparent that the gradient for several lines under different reaction
conversion is identical within error. The apparent activation energy
ꢁE as a function of conversion degree was shown in Table 2.
For the thermal decomposition of Ni3C in H2, the activation
energy increased a little when the extent of conversion not more
than 0.4.
The participation of H2 in the reaction accelerates the decom-
position process and decreases the decomposition temperature
Table 2
The apparent activation energy as a function of conversion degree (˛)
1 − ˛
E (kJ mol−1
)
0.9
0.8
0.7
0.6
0.5
71.3
75.7
76.9
78.2
77.4
Fig. 4. TG, DSC and MS curves recorded for Ni3C at 10 K/min in a dynamic atmo-
sphere of H2.

Y. Leng et al. / Thermochimica Acta 473 (2008) 14–18
17
Fig. 6. Isoconversional plots of Ni3C reacted in H2 with different conversion degrees.
Fig. 8. XRD patterns for the powder obtained after Ni3C was heated to 823 K in the
air at various heating rates (a) 5 K/min, (b) 10 K/min and (c) 20 K/min.
dramatically. The decomposition equation for Ni3C in H2 is
Ni3C(s) + 2H2(g) = 3Ni(s) + CxHy(g)
(3)
obtained due to the retention of carbon according to the following
equation
3.3. Ni3C decomposition in air
Ni3C powder was also pre-treated in Ar atmosphere to eliminate
the surfactants before decomposing in the air. Fig. 7 shows the TG,
DSC and MS curves of Ni3C decomposing in the air. Oxygen partici-
pates in the reaction, as indicated by the decrease of the O2 amount.
Thus to be more precise, in this case it is not a decomposition, but
oxidation of Ni3C instead. Though accompanied the release of car-
bon from Ni3C to form dioxide, the TG curve underwent a wide
weight-increased process from 550 to 740 K for the nickel compo-
nent reacted with oxygen to produce NiO (Fig. 8) and caused a net
mass increase in the solid product. The DSC curve which was com-
posed of three peaks seems a little complicated, which is somewhat
similar to the MS information of CO2. Shimada reported that oxi-
dation of the metal component in the carbides for MC (M = Zr, Ti
and Hf) prior to that of the carbon component though the proceed-
ing oxidation of M with subsequent oxidation of carbon was also
similarly observed [16]. As a result, exceeding 100% oxidation was
MC + O2 = MO2 + C
However, for our Ni3C sample, the case was quite different.
Considering the formation of CO2 throughout the whole process
of weight increase caused by the formation of NiO, the oxidative
reactions for the carbon and nickel component with O2 take place
at the same time. We could not observe a clear gap between the
oxidation of nickel and the oxidation of carbon component. This
chemical phenomenon indicates the overlapping reaction of both
nickel and carbon components in Ni3C with O2. The complexity
of the DSC peaks and detectable nickel metal in the decomposed
product (Fig. 8) hinder us to calculate the activated energy. The
difference in the oxidation of Ni3C and ZrC may result from the
metastable properties of Ni3C. Also, the nano-scale particles may
be responsible for the high activity of carbon in Ni3C, leading to
the oxidation reaction for carbon occur together with the metal
oxidation. At least three exothermic peaks were observed, which
may attribute to the release of CO2 based on the almost identical
shapes of DSC curve and the MS for CO2. This phenomenon could
be explained by the high surface area of our sample. The small
particle size (Figure S1) favored adsorption of the gaseous decom-
position products. As a result, the exothermic decomposition was
controlled by mass-transfer and desorption along the solid surface,
which are usually characterized by relatively low activation ener-
gies. Although we could not estimate the activation energy of Ni3C
decomposition in the air, the low activation energy for Ni3C decom-
posing in Ar and H2 could testify that the high surface area played
a crucial role in the reaction.
4. Conclusions
In summary, the decomposition of Ni3C in Ar, H2 and air were
studied. Decomposition in Ar is a single-step reaction with acti-
vation energy of 204 kJ mol−1, yielding Ni metal and amorphous
carbon. Decomposition in H2 to give Ni and hydrocarbon has much
lower activation energy depending on the conversion degree. Reac-
tion with oxygen in the air shows a complicated exothermal peak,
without any clear indication of the sequence oxidation of the two
components.
Fig. 7. TG, DSC and MS curves recorded for Ni3C decomposing at 10 K/min in an
oxidative atmosphere of air (the dash line denotes the TG curve, the short-dash line
denotes the DSC curve and the solid lines denote the MS data).

18
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[3] S. Sinharoy, M.A. Smith, L.L. Levenson, Surf. Sci. 72 (1978) 710–718.
Acknowledgements
[4] K. Tokumitsu, Mater. Sci. Forum 235 (1997) 127–132.
[5] P. Hooker, J.T. Beng, K.J. Klabunde, S. Suib, Chem. Mater. 3 (1991) 947–952.
[6] L.P. Yue, R. Sabiryanov, E.M. Kirkpatrick, D.L. Leslie-Pelecky, Phys. Rev. B 62
(2000) 8969–8975.
[7] Y.G. Leng, H.Y. Shao, Y.T. Wang, M. Suzuki, X.G. Li, J. Nanosci. Nanotechnol. 6
(2006) 221–226.
The authors acknowledge NSFC (Nos. 20221101 and 20671004),
MOST of China (Nos. 2006AA05Z130, 2007AA05Z118) and MOE of
China (No. 707002).
[8] V. Tzitzios, G. Basina, M. Gjoka, V. Alexandrakis, V. Georgakilas, D. Niarchos, N.
Boukos, D. Petridis, Nanotechnology 17 (2006) 3750–3755.
[9] Y.T. Jeon, J.Y. Moon, G.H. Lee, J. Park, Y. Chang, J. Phys. Chem. B 110 (2006)
1187–1191.
[10] M. Richard-Plouet, M. Guillot, S. Vilminot, C. Leuvrey, C. Estournes, M. Kurmoo,
Chem. Mater. 19 (2007) 865–871.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.tca.2008.04.003.
[11] H.E. Kissinger, Anal. Chem. 29 (1957) 1702–1706.
[12] M.A. Mohamed, S.A.A. Mansour, M.I. Zaki, Thermochim. Acta 138 (1989)
309–317.
References
[13] S. Vyazovkin, Int. Rev. Phys. Chem. 19 (2000) 45–60.
[14] S. Vyazovkin, New J. Chem. 24 (2000) 913–917.
[15] T. Ozawa, Bull. Chem. Soc. Jpn. 38 (1965) 1881–1886.
[16] S. Shimada, Solid State Ionics 141–142 (2001) 99–104.
[1] D.L. Leslie-Pelecky, X.Q. Zhang, S.H. Kim, M. Bonder, R.D. Rieke, Chem. Mater.
10 (1998) 164–171.
[2] S. Sinharoy, L.L. Levenson, Thin Solid Films 53 (1978) 31–36.