Reactions of cobalt oxide with chlorine

Article abstract of DOI:10.1134/S0036023607120066

We report the thermodynamic calculations and experimental studies of the kinetics of the reaction of Co₃O₄ with chlorine at 300-850°C. The show that cobalt chloride sublimation is controlled by the rate of chloride evolution from the

Full text of DOI:10.1134/S0036023607120066

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 12, pp. 1840–1843. © Pleiades Publishing, Inc., 2007.
Original Russian Text © T.A. Anufrieva, L.E. Derlyukova, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 12, pp. 1956–1959.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Reactions of Cobalt Oxide with Chlorine
T. A. Anufrieva and L. E. Derlyukova
Institute of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia
e-mail: alv@icp.ac.ru
Received December 16, 2004
Abstract—We report the thermodynamic calculations and experimental studies of the kinetics of the reaction
of Co3O4 with chlorine at 300–850°C. The show that cobalt chloride sublimation is controlled by the rate of
chloride evolution from the surface. The chlorination specifics of the oxides of iron-family metals are com-
pared.
DOI: 10.1134/S0036023607120066
Anhydrous chlorides of iron-family metals are calculations show that the chlorination of cobalt oxide
widely used in engineering and technology, in particu- can occur over a wide temperature range.
lar as reagents in inorganic and organic synthesis and as
catalysts. The reactions of iron and nickel oxides with
chlorine are well understood [1–4]. Cobalt oxide is
known to react with chlorine at relatively low tempera-
tures; however, data on low-temperature chlorination of
cobalt oxide are rather controversial [5, 6]. Chlorina-
tion is a way to prepare pure anhydrous CoCl2, the most
widely used cobalt salt.
Below 600°ë the cobalt dichloride partial pressure
is negligible; therefore, cobalt chloride sublimation
should not occur. CoCl2 accumulation in the condensed
phase during the reaction of cobalt oxide with chlorine
can create diffusion hindrances to chlorination. High
cobalt volatilization is possible at temperatures above
700°C.
Cobalt oxide chlorination kinetics were studied in
Here, we report the results of thermodynamic mod- an isothermal mode on a flow-through setup [8] and on
eling of the system containing cobalt, oxygen, and a gravimetric setup with the weight change recorded
chlorine and experimental kinetic studies of the reac- over time [9]. A high purity grade ëo3O4 sample
tion between cobalt oxide and chlorine at 300–850°ë;
we also compare the specifics of this reaction with the
chlorination reactions of nickel and iron oxides studied
previously [1–4].
(73.4 wt % Co) was used in these experiments. The
BET specific surface area of the cobalt oxide sample
measured by low-temperature krypton adsorption was
3.5 m2/g. X-ray powder diffraction and chemical anal-
ysis were used to identify the starting sample and reac-
tion products. To determine the product cobalt chloride
in the condensed phase, the sample after an experiment
was treated with water. Analysis showed that almost all
CoCl2 dissolved, and the solid phase was ëo3O4. Then,
cobalt was determined in the solid residue and solution.
The difference between the cobalt weight in the starting
sample and the overall cobalt weight in the solution and
solid residue after the experiment was the amount of
cobalt that entered the gas phase in the chloride form.
The equilibrium compositions of the gas and con-
densed phases of the Co–O–Cl system with various
component ratios were calculated through free-energy
minimization using Astra software [7].
Figures 1a and 1b show the equilibrium composi-
tion of the gas and condensed phases, respectively, for
the stoichiometric component ratio of the reaction
ëo3O4 + 3Cl2 = 3CoCl2 + 2O2
In the range 400–600°ë, oxygen and chlorine are the
main gas-phase components. The CoCl2 partial pres-
sure monotonically increases with temperature to reach
10–3 atm at 650°C and 0.3 atm at 1000°C. The con-
densed phase consists of ëo3O4 and CoCl2; the ëo3O4
proportion decreases with increasing temperature. At
900°ë cobalt(II) oxide appears in the condensed phase.
The cobalt proportion in the oxide decreases with tem-
perature from 42.5% at 400°C to 20.6% at 1000°C. An
increase in the chlorine proportion in the batch leads to
an increase in the cobalt proportion in the chloride over
Isotherms of the degree of cobalt conversion to chlo-
ride in the reaction of cobalt chloride with chlorine are
plotted in Fig. 2. At 300°ë, the conversion does not
exceed 5%. At 400–550°ë, the conversion increases
rapidly for 20–40 min, then the chlorination rate drops.
An increase in temperature from 400 to 600°ë slightly
increases the process rate. At 500–550°ë, the degree of
cobalt conversion to chloride exceeds 90%. The chlori-
nation rate is high even when considerable amounts of
cobalt chloride are formed.
Weight-change versus time isotherms are shown in
the whole temperature range. Thus, thermodynamic Figs. 3a and 3b. At 350–550°C the reaction of cobalt
1840

REACTIONS OF COBALT OXIDE WITH CHLORINE
1841
p , atm
α, %
i
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
(‡)
100
90
80
70
60
50
40
30
20
10
1
5
3
2
4
1
3
2
0
20
40
60
80
100
120
τ, min
800
900
1000
1100
1200 T, K
weight fraction
(b)
Fig. 2. Co O chlorination isotherms at (1) 350, (2) 400,
(3) 450, (4) 500, and (5) 600°C.
3
4
0.6
0.5
0.4
0.3
0.2
2
δm/m , %
o
(‡)
1
5
60
3
40
0.1
2
3
4
6
1
20
0
800
900
1000
1100
1200 T, K
0
20
40
60
80
10
100
120
7
Fig. 1. Panel (a): the equilibrium composition of the gas
phase of the Co O + 3Cl system vs. temperature: (1) O ,
–20
–40
–60
3
4
2
2
9
(2) Cl , and (3) CoCl . Panel (b): the same for the con-
2
2
densed phase: (1) Co O , (2) CoCl , and (3) CoO.
3
4
2
8
oxide with chlorine is accompanied by weight gain
only. Cobalt chloride sublimation starts at 600°C,
where the degrees of conversion exceed 90%. The
weight of the sample decreases as a result of cobalt
chloride sublimation. Figure 3b illustrates more details
of the initial period of the reaction for 600–850°C. An
induction period, whose duration depends on tempera-
ture, is observed in all isotherms. At 600°ë, weight gain
starts after 2 min. At 700°ë, the duration of the induc-
tion period increases to 8 min, followed by weight gain.
After ∆m/m0 reaches a maximum (20%), which is lower
than achieved at 600°ë, the weight decreases. It is
known that the initial chlorination stage is chlorine
chemisorption on the oxide surface. When chlorine
chemisorption is the rate-controlling stage of the pro-
cess, an induction period can appear. No weight change
was detected during chemisorption because of the
insufficient sensitivity of the investigation method: the
weight change of the test sample (0.2 g) after mono-
layer surface coverage is 4 × 10–4 g, and the sensitivity
(b)
40
20
1
2
0
5
10
15
20
25
τ, min
–20
–40
–60
–80
3
4
Fig. 3. Panel (a): weight increment vs. run duration in a
flow-through setup at (1) 300, (2) 350, (3) 400, (4) 500,
(5) 600, (6) 650, (7) 700, (8) 750, and (9) 800°C. Penal (b):
the same in a gravimetric setup at (1) 600, (2) 700, (3) 800,
and (4) 850°C.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 12 2007

1842
ANUFRIEVA, DERLYUKOVA
Table 1. Cobalt distribution between gas and solid phases
Cobalt distribution between reaction products,
Cobalt conversion
to chloride, %
% of the batch concentration
T, °C
Time, min
Co3O4
CoCl2 condensed
CoCl2 sublimed
650
40
60
35.5
28.7
19.5
27.8
14.3
10.8
24.2
11.9
2.7
39.4
42.0
46.0
42.6
47.3
30.4
40.3
46.6
23.7
39.5
–
25.1
29.3
34.5
29.6
38.4
58.8
35.5
41.5
73.6
36.8
100.0
52.5
100.0
64.5
71.3
80.5
72.2
85.7
89.2
75.8
88.1
97.3
76.3
100.0
76.7
100.0
100
40
700
750
60
100
40
60
100
20
800
850
23.7
–
40
20
23.3
–
24.2
–
40
of the gravimetric setup is 1 × 10–3 g/mm scale. The
chemisorption stage is followed by the formation of a
reaction surface layer, in which CoCl2 nucleation
occurs. These processes end by the appearance of a new
cobalt chloride phase. The chemisorption rate increases
with rising temperature. At 700°ë, however, the induc-
tion period (8 min) is longer than at 600°C (2.5 min).
The extent of CoCl2 sublimation increases considerably
with temperature (Table 1). From Table 1, which illus-
trates the cobalt distribution between the reaction prod-
ucts, one can see that the ëo3O4 concentration in the
solid residue at 700–750°ë monotonically decreases,
the CoCl2 proportion in the sublimate monotonically
increases, while the CoCl2 proportion in the condensed
phase passes through a peak. This tendency is charac-
teristic of consecutive reactions with an intermediate, in
which the formation rate of the intermediate is higher
than the formation rate of the final product. In the case
at hand, condensed CoCl2 can be considered as an inter-
mediate and sublimed CoCl2 as a final product. The
increase in the induction period at 700°ë can be due to
the competition between the formation of condensed
cobalt chloride and its volatilization. Because the CoCl2
formation rate exceeds the sublimation rate, the
weight starts to increase after the induction period is
over due to accumulation of cobalt chloride in the
solid phase. The weight decreases when the extents of
cobalt oxide conversion to chloride are higher than
50%. The maximum weight gain and the period of
reaching this maximum decrease with temperature ele-
vation.At 800−850°ë, weight gain in the initial period of
time was not detected. The weight started to decrease
after the induction period (2 and 1 min, respectively), and
after 20 min cobalt sublimed completely (Figs. 2, 3).
The chloride sublimation rate at 800–850°ë was
high, but at short reaction times at these temperatures,
the solid phase contained noticeable cobalt dichloride
amounts, as indicated by the different extents of reac-
tion and CoCl2 sublimation (Table 1). This confirms
that cobalt dichloride formation in the condensed phase
until 850°ë is an intermediate stage of cobalt chloride
sublimation.
Table 2. Ratio of O2 and Cl2 equilibrium partial pressures
for the M3O4 + 3Cl2 (M = Fe, Co, or Ni) systems
p /p
O2 Cl2
T, °C
Fe
Co
Ni
Thus, cobalt oxide chlorination at 300−850°ë has
high rates. At 300–350°C, cobalt chloride pileup on the
surface dramatically slows down the process. Above
400°C, virtually all cobalt oxide converts to chloride.
The apparent activation energy at 350–550°ë is
3.5 kcal/mol. Cobalt chloride volatilization occurs
above 600°C. The apparent activation energy at
600−800°ë is 20.5 kcal/mol, which is close to the value
reported in [5]. CoCl2 sublimation at these temperatures
controls the chloride sublimation.
300
400
500
600
700
800
900
1.1 × 10–6
0.2 × 10–5
0.4 × 10–4
4.0 × 10–4
0.004
0.76
0.78
0.84
0.93
1.03
1.65
2.45
7745.2
878.9
178.9
52.7
18.1
0.025
20.6
0.084
61.0
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 12 2007

REACTIONS OF COBALT OXIDE WITH CHLORINE
1843
The high cobalt dichloride formation rate in a con- and not by the electrophysical properties of iron-family
densed phase can be due to the ëÓCl2-mediated race oxides. According to the literature [4], iron oxides (FeO
and Fe3O4) under a chlorine atmosphere transform to
mechanism of chlorine transport to the reaction inter-
face [10]. A similar phenomenon was observed in the Fe2O3; the chlorination product is FeCl3. Despite the
reaction between nickel oxide and chlorine [9]. How-
ever, unlike ëo3O4 (an n-type semiconductor), nickel
oxide is a -type semiconductor, chlorine chemisorp-
tion on which enriches the surface layer with charge
carriers [11]. It was pertinent to compare the chlorina-
tion laws discovered for these oxides with those for a
third metal of the same family, namely Fe2O3, which
has a high electronic conductivity, as ëo3O4.
fact that cobalt in ëo3O4 has two valences, this oxide
does not convert to ëo2O3 under a chlorine atmosphere,
and ëÓël2 is the only stable cobalt chloride.
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 12 2007