Effect of reaction temperature on the chlorination of a Sm 2O3-CeO2-C mixture

Article abstract of DOI:10.1016/j.tca.2005.04.011

The chlorination of a Sm₂O₃-CeO₂-C mixture with gaseous chlorine was studied by thermogravimetry (TG). The effect of the reaction temperature between 400 and 950°C on the reaction rate was analyzed. The reaction products were studied by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Compared to the results previously found in the individual systems, it was concluded that each oxide evolves as being an individual in the mixture. Combined with TG results along with observations done using scanning electron microscopy (SEM), the stoichiometries of the reactions involved were obtained. From these results, a two-stage separation method for both oxides was proposed.

Full text of DOI:10.1016/j.tca.2005.04.011

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Thermochimica Acta 432 (2005) 47–55
Effect of reaction temperature on the chlorination of
a Sm2O3–CeO2–C mixture
M.R. Esquivela,∗, A.E. Bohe , D.M. Pasquevich
b
a,b
´
a
Comisio´n Nacional de Energ´ıa Ato´mica, Centro Ato´mico Bariloche, RG8402AGP S.C. de Bariloche, R´ıo Negro, Argentina
b
Consejo Nacional de Investigaciones Cient´ıficas y Te´cnicas, Centro Ato´mico Bariloche, RG8402AGP S.C. de Bariloche, R´ıo Negro, Argentina
Received 5 November 2004; received in revised form 17 March 2005; accepted 11 April 2005
Abstract
The chlorination of a Sm2O3–CeO2–C mixture with gaseous chlorine was studied by thermogravimetry (TG). The effect of the reaction
temperature between 400 and 950 ◦C on the reaction rate was analyzed. The reaction products were studied by X-ray diffraction (XRD) and
energy dispersive spectroscopy (EDS). Compared to the results previously found in the individual systems, it was concluded that each oxide
evolves as being an individual in the mixture. Combined with TG results along with observations done using scanning electron microscopy
(SEM), the stoichiometries of the reactions involved were obtained. From these results, a two-stage separation method for both oxides was
proposed.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Chlorination; Gas–solid; Samarium sesquioxide; Cerium dioxide
1. Introduction
400 ◦C [12] producing SmOCl (s) in one stage up to 650 ◦C
and SmOCl (s) and SmCl3 (s, l, g) in two successive stages
from 650 to 950 ◦C [12].
These differences in reactivity towards C (s) + Cl2 (g)
[12,13] suggest using this difference to separate these ox-
ides. The chlorides and oxychlorides formed occur in dif-
ferent agregation states and have differences in their vapor
pressures suggests separation by physical methods such as
volatilization or fractional distillation.
The importance of both cerium and samarium compounds
has increased markedly in the last decades. As pure oxides,
both lanthanides are used in fuel cells [1,2]. Cerium oxide
is used in catalysis [3,4] while samarium as either oxychlo-
ride, oxide or metal is used in permanent magnets and for
unique optical materials [5,6]. The marked chemical similar-
ities [7] between lanthanide oxides along with their high sta-
bility [8] make separation a difficult task. To overcome this
problem, different separation methods have been proposed
[9–11].
2. Experimental
The chlorination and carbochlorination of Sm2O3 and
CeO2 and their mixture were previously studied [12–16].
Reaction products were identified and by TG analysis, the
stoichiometry of these reactions was obtained [12–16].
CeO2 carbochlorination starts above 700 ◦C producing
CeCl3 (s, l, g) [13] and Sm2O3 carbochlorination starts over
2.1. Materials and procedure
Ar 99.999% purity (AGA, Argentina), Cl2 99.8% (Indupa,
Argentina) and raw Sm2O3, CeO2 and C were the reactants
used. These solids were previously characterized [12–16].
These reactants were weighed and mixed mechanically to
obtain a CeO2 (45 wt%)–Sm2O3 (45 wt%)–C (10 wt%) mix-
ture. The percentage of each oxide was determined us-
ing the Rietveld method [17] (see appendix for sample
∗
Corresponding author. Tel.: +54 2944 445293/5337;
fax: +54 2944 445299.
E-mail address: esquivel@cab.cnea.gov.ar (M.R. Esquivel).
0040-6031/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2005.04.011

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M.R. Esquivel et al. / Thermochimica Acta 432 (2005) 47–55
were treated and auxiliary measurements done in a globe box
[12–15].
Nomenclature
˚
cell parameter (A)
2.2. Expression of results
a
b
c
D
Eap
f
˚
cell parameter (A)
˚
For non-isothermal runs, TG data are expressed as ꢀw
versus T, where ꢀw and T are the mass change in milligrams
and the temperature in ◦C, respectively. For isothermal runs,
TG data are expressed as either the fractional carbon mass
loss given by αC = ꢀm(C)/m0(C), where ꢀm(C) and m0(C)
are the carbon mass change and carbon initial mass, respec-
tively, or the fractional samarium oxide mass loss given by
αSm O = ꢀm(Sm2O3)/m0(Sm2O3). Either Sm2O3 or car-
cell parameter (A)
diffusion coefficient (m2 s−1
)
apparent activation energy (kJ mol−1
)
stoichiometric correction coefficient (dimen-
sionless)
FW
g
formula weight (kg mol−1
)
coefficient that relates ꢀw to the mass change
of carbon, CeO2 and Sm2O3 (dimensionless)
characteristic dimension of the sample (m)
initial mass sample (mg)
2
3
bon consumption is related to the observed mass change by
ꢀm(C or Sm2O3) = g ꢀw. g is a coefficient which relates
the ꢀw to the mass change of carbon, CeO2 and Sm2O3,
and which depends on both the stoichiometry of the reaction
and the aggregation state of reactants and products. Then, the
reaction degree is transformed to:
L
m0
ꢀm
ꢀM
N
NTP
r
R
t
T
z
mass variation (mg)
balance mass variation (mg)
molar flow of chlorine (mol s−1
)
normal pressure and temperature conditions
reaction rate (s−1
)
g ꢀm(C, Sm2O3)
reaction rate (mol s−1
time (s)
)
αC,Sm O
=
(1)
2
3
m
0(C,Sm2O3)
temperature (◦C)
adjustable parameter
where the parenthesis indicates that the symbol con-
tained within the formula corresponds to either carbon or
Sm2O3.The reaction rate (s−1) is:
Greek symbols
cell angle (◦)
αC,Sm O carbon, Sm2O3 reaction degree (dimension-
dαC,Sm O
g
dm
2
3
r = −
= −
(2)
α
dt
m0(C,Sm O ) dt
2
3
2
3
less)
The reaction rate (mol Cl2 s−1) is:
β
γ
ν
cell angle (◦)
dnCl
dt
fm0(C,Sm O ) · r
2
2
3
cell angle (◦)
R =
=
FW(C, Sm2O3)
(3)
kinematic viscosity (m2 s−1
)
where n is the moles of chlorine, f a coefficient that relates
the stoichiometric consumption of moles of either carbon or
Sm2O3 to moles of chlorine and FW is the formula weight
of either carbon or Sm2O3.
parameters detail). Carbon percentage was determined by
weighing it before and after burning the mixture at 950 ◦C
[12,13,16].
A thermogravimetric analyzer based on a Cahn 2000 elec-
trobalance described elsewhere [18] was used to measure the
chlorination rate. The mass change resolution of the system
was 10 ␮g under the experimental conditions [12,13,15].
Samples of Sm2O3–CeO2–C of 10 mg were placed inside a
quartz crucible suspended by a quartz fiber inside a furnace.
Isothermal and non-isothermal measurements were made un-
der an overall pressure of 101.3 kPa and a total gas flow rate
of 7.9 l h−1 (NPT). In the isothermal experiments, samples
were first heated for 1 h in Ar at the desired operation tem-
perature, then Cl2 was injected into the system reaching a
pCl2 = 30.3 kPa.
In the temperature scanning experiments, samples were
heated from room temperature to 950 ◦C at 2.6 ◦C min−1 at
a pCl2 = 30.3 kPa in flowing Ar–Cl2. In both cases, mass
changes were measured and apparent mass changes were
corrected as explained elsewhere [18]. Since the reaction
products, SmCl3 and CeCl3, are hygroscopic [19], samples
3. Results and discussion
3.1. Reactivity of the CeO2–Sm2O3–C system with
chlorine and reaction products
Non-isothermal experiments were made to determine the
main products of reaction from room temperature to 950 ◦C.
The corrected TG curve is shown in curve (1) in Fig. 1. For
comparison, the corrected TG curves of the individual car-
bochlorination of both CeO2 (2) [13] and Sm2O3 (3) [12] are
also shown. In the CeO2–Sm2O3–Cl2–C system, the start-
ing temperature of reaction (190 ◦C) is indicated by “A” in
the curve. As temperature raises, the mass increases reach-
ing a maximum at 350 ◦C where a flat zone corresponding
to the formation of a condensed phase is indicated by “B”.
Slow formation of another condensed phase is observed up to
480 ◦C (“C”), followed by a flat zone. Above 650 ◦C, a fur-
ther slow mass gain extends up to 700 ◦C (“D”). Between 700

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M.R. Esquivel et al. / Thermochimica Acta 432 (2005) 47–55
49
Fig. 1. (1) Non-isothermal curve of the chlorination of Sm2O3–CeO2–C.
XRD, EDS and SEM observations are indicated by full circles, hollow circles
and full squares, respectively. (2) Non-isothermal curve of the chlorination
of CeO2–C [13]. (3) Non-isothermal curve of the chlorination of Sm2O3–C
[12]. Reaction products [12,13] are labeled in the figure.
Fig. 2. Diffraction patterns of the reaction products at 550 ◦C.
only Sm2O3 reacts with the stoichiometry,
2Sm2O3 (s) + 2Cl2 (g) + C (s)
= 4SmOCl (s) + CO2 (g)
and 800 ◦C (“E”), a marked mass gain occurs. Above 800 ◦C,
an increasing rate of mass loss is observed up to 950 ◦C.
This mass loss is attributed to vaporization of the reaction
products. This curve resembles the curves for the individual
carbochlorination of Sm2O3 [12] and CeO2 [13] suggesting
formation of similar products. Differences in mass gain be-
tween the mixture and the individual oxide in each tempera-
ture range are attributed to the differences in the C content in
the initial mixtures [12,13]. The identity of phases formed in
each of the individual systems [12,13] are labeled in Fig. 1.
To identify the reaction products, isothermal experiments
were done at selected temperatures and the reaction products
were characterized by XRD, EDS and SEM at the points on
the TG curve of Fig. 1 indicated by full circles, hollow circles
and full squares, respectively. With the aid of mass balances,
the stoichiometries of the reactions involved were assessed.
(4)
XRD and SEM analysis performed at temperatures lower
than 400 ◦C indicated that only direct chlorination of Sm2O3
occurs [12,14]. Like the individual CeO2–Cl2 system [15],
no chlorination of the CeO2 present in the Sm2O3–CeO2–C
mixture is observed.
3.3. Stoichiometries and reaction products between 650
and 700 ◦C
Six hundred and fifty and 700 ◦C were selected for analy-
sis of the reaction products. The diffraction patterns obtained
at various times at both temperatures are shown in Fig. 3. At
3.2. Stoichiometries and reaction products between 400
and 625 ◦C
Four hundred and 550 ◦C were selected as representative
temperatures in this range. The diffraction patterns of the
reaction products at 3 min (a), 20 min (b) and 40 min (c) at
550 ◦C along with the reference patterns of CeO2 [20] (d)
Sm2O3 [21] (e) and SmOCl [22] (f) are displayed in Fig. 2.
SmOCl diffraction lines increase with time as the ones of
Sm2O3 decreases. No Ce compound other than CeO2 is ob-
served. No product of interaction between Sm and Ce is ob-
served in the diffraction patterns. SEM images of reaction
products at 400 ◦C show chemical attack in the shape of pits
on the carbon indicating it is involved in the reaction [12,13].
These results along with mass balances at various temper-
atures lead to the conclusion that in this temperature range
Fig. 3. Diffraction patterns of the reaction products at 650 and 700 ◦C.

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50
M.R. Esquivel et al. / Thermochimica Acta 432 (2005) 47–55
range indicate that CeO2 is carbochlorinated according to
2CeO2 (s) + 3Cl2 (g) + 4C (s)
= 2CeCl3 (l, s) + 4CO (g)
(6)
which is the same stoichiometry as that of the individual car-
bochlorination [13]. Like the Sm2O3–Cl2–C system [12], the
mass balances indicated that Sm2O3 is carbochlorinated in
two steps. Between 700 and 850 ◦C, SmOCl is produced ac-
cording to Eq. (4). At higher temperatures, the formation of
SmOCl as a first step is observed but no stoichiometry can be
assigned. At 725 ◦C, the second step leads to the carbochlo-
rination of SmOCl to produce SmCl3 according to
2SmOCl (s) + 2Cl2 (g) + C (s)
= 2SmCl3 (l, s) + CO2 (g)
(7)
(8)
(9)
And between 750 and 950 ◦C:
Fig. 4. Diffraction patterns of the reaction products at 750, 800 and 900 ◦C
[20–24].
SmOCl (s) + Cl2 (g) + C (s) = SmCl3 (l, s) + CO (g)
The global reaction at 725 ◦C is described by:
3Sm2O3 (s) + 6Cl2 (g) + 3C (s)
= 4SmCl3 (l, s) + 3CO2 (g)
shorter times at both temperatures (Fig. 3a and d), the for-
mation of SmOCl is detected. At longer times, the further
carbochlorination of this oxychloride leads to the formation
of SmCl3. This product is incompletely obtained at 650 ◦C
(Fig. 3c). At 700 ◦C, the successive formation of both com-
pounds is observed but no SmOCl is detected at longer times
(Fig. 3e). No products of the carbochlorination of CeO2 are
observed in this temperature range (Fig. 3a–e). These results
are in agreement with those obtained from the individual car-
bochlorination of each oxide [12,13]. A SEM image of the
reaction products at 650 ◦C analyzed by EDS shows SmCl3
particles that were vaporized during the preparation of the
sample. These results along with mass balances led to the
conclusion that only Sm2O3 reacts in two steps, the first one
defined by Eq. (4) and the second one,
While the global reaction between 750 and 950 ◦C is de-
scribed by:
3Sm2O3 (s) + 6Cl2 (g) + 6C (s)
= 4SmCl3 (l, s) + 6CO (g)
Traces of both CeCl3 and SmCl3 were observed on SEM
images which is in agreement with their respective melting
points of 816 and 681 ◦C [25].
(10)
3.5. Effect of the temperature on the reaction rate
Isothermal TG curves are shown in Fig. 5. At T ≤ 675 ◦C,
the carbochlorination is achieved in one step corresponding
to the formation of SmOCl according to Eq. (4). The rate
increases as the temperature is raised, i.e., time needed to
achieve αSm O = 0.9 is 607, 90 and 20 s at 400, 500 and
2Sm2O3 (s) + 3C (s) + 6Cl2 (g) = 4SmCl3 + 3CO2 (g)
(5)
which is the same as the individual carbochlorination of
Sm2O3 [12].
2
3
600 ◦C, respectively. Table 1 displays reaction rates calcu-
3.4. Stoichiometries and reaction products between 700
and 950 ◦C
lated at various αSm O by Eq. (3) along with the theoretical
2
3
values obtained from the equation of Ranz-Marshall [12–15]
corrected as suggested [26]. Since both sets of values are
similar, it is concluded that carbochlorination of the mixture
is achieved under external mass transfer control which in-
creases as temperature increases [12,15]. The values are also
compared to those for the individual carbochlorination sys-
tems [12,13]. The values in the mixture are of the same order
of magnitude as those for carbochlorination of Sm2O3 [12]
indicating that no changes are produced due to the presence
of CeO2 in the mixture. CeO2 presents no reaction in this tem-
perature range which is also in agreement with the individual
system [13].
Seven hundred and fifty, 800 and 900 ◦C were selected
as the representative temperatures in this range. The reaction
productsobtainedatthesetemperaturesatvarioustimesalong
with the reference patterns are shown in Fig. 4. At shorter
times at 750 and 800 ◦C, formation of SmOCl is observed.
At longer times, both SmCl3 and CeCl3 are detected. These
results are the same as the individual carbochlorination of
each oxide [12,13]. As observed at the lower temperatures
(Figs. 2 and 3), no interaction between cerium and samarium
compounds is observed. Mass balances in this temperature