Kinetic features of thermal decomposition of cobalt acetylacetonate and development of catalysts for the oxidation of carbon monoxide, based on the decomposition products

Article abstract of DOI:10.1007/s11176-005-0078-4

The kinetic features of thermal decomposition of cobalt acetylacetonate in a vacuum were elucidated, and the volatile thermolysis products were identified. The homogeneous-heterogeneous character of the process was shown. Catalysts for the oxidation of CO to CO₂ were developed on the basis of cobalt acetylacetonate decomposition products and synthetic cellular ceramics, and the kinetic and activation parameters of the oxidation were determined. 2004 MAIK Nauka/Interperiodica .

Full text of DOI:10.1007/s11176-005-0078-4

Russian Journal of General Chemistry, Vol. 74, No. 11, 2004, pp. 1646 1650. Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 11, 2004,
pp. 1770 1774.
Original Russian Text Copyright
2004 by Tsyganova, Didenkulova, Shekunova, Faerman, Aleksandrov.
Kinetic Features of Thermal Decomposition
of Cobalt Acetylacetonate and Development of Catalysts
for the Oxidation of Carbon Monoxide,
Based on the Decomposition Products
E. I. Tsyganova, I. I. Didenkulova, V. M. Shekunova,
V. I. Faerman, and Yu. A. Aleksandrov
Research Institute of Chemistry, Lobachevsky Nizhni Novgorod State University, Nizhni Novgorod, Russia
Received December 4, 2002
Abstract The kinetic features of thermal decomposition of cobalt acetylacetonate in a vacuum were
elucidated, and the volatile thermolysis products were identified. The homogeneous-heterogeneous character
of the process was shown. Catalysts for the oxidation of CO to CO were developed on the basis of cobalt
2
acetylacetonate decomposition products and synthetic cellular ceramics, and the kinetic and activation param-
eters of the oxidation were determined.
In [1] we considered in detail the reactivity of
diketonates of Group I VIII metals in thermolysis
and the kinetic features and mechanisms of these proc-
esses. The use of these compounds for obtaining
active phases of metals, oxides, oxycarbides, and
metal complexes on the surface of various supports
has been described in [2 4].
lysis of Co(acac) is a complex homogeneous-hetero-
geneous process. When the decomposition is carried
out in a reaction vessel with walls coated by solid
2
-
products of the Co(acac) thermolysis obtained in the
2
previous experiment, after removal of the volatile
thermolysis products by heating in vacuum at 593 K,
the rate constant of the Co(acac) decomposition at
2
5
13 K increases by a factor of 1.5. The 5.6-fold
Proceeeding with systematic studies of the reac-
tivity of metal -diketonates, we examined in this
work the thermolysis of cobalt acetylacetonate
increase in the heterogeneous factor S/V (S is the
surface area of the vessel and V is its volume) de-
creased the rate constant by a factor of 1.5. A similar
Co(acac) in a vacuum under static conditions in the
2
range 513 603 K at the initial pressure of 40
Table 1. Kinetic parameters of gas-phase thermolysis of
Co(acac)2 in a vacuum (V 60 ml, S/V 1.8 cm )
1
33 hPa, using a manometric method. The thermol-
ysis of Co(acac) under these conditions is satisfac-
1
2
torily described by a first-order kinetic equation with
respect to both current and initial concentrations of the
starting compound (Table 1).
T, K 1
k 10 , s 1
3
p0, hPa
pf /p0
5
13
2.6
1.1
6.1
3.6
5.5
7.7
8.1
9.6
14.3
14.5
114
110
40
125
110
40
40
137
30
1.8
1.9
2.6
2.3
2.4
2.6
2.7
2.7
2.8
2.8
a
The temperature dependence of the first-order rate
513
518
531
b
constant is described by the following equation (E,
1
J mol ):
c
5
31
3
log k = (2.93 0.34)
(54.3 3.6) 10 /(2.3RT).
557
d
5
57
As a result of the thermolysis at temperatures
above 593 K, the reaction vessel walls are coated with
a dark opaque film of pyrolytic cobalt. When the
thermolysis is carried out in the presence of air in the
ratio p[Co(acac) ]:p = 1:2, the rate constant of the
578
593
593
133
a
c
1 b
S/V = 10 cm
.
Thermolysis in the presence of air (70 hPa).
2
air
thermolysis increases by a factor of 2, whereas the
presence of hydrogen does not appreciably affect the
rate constant (Table 1). It was shown that the thermo-
Thermolysis in the presence of solid thermolysis products from
the previous run, heated in a vacuum at 593 K. Thermol-
ysis in the presence of hydrogen (24 hPa).
d
1
070-3632/04/7411-1646 2004 MAIK Nauka/Interperiodica

KINETIC FEATURES OF THERMAL DECOMPOSITION
1647
decrease in the rate constant with increasing S/V was
observed in the thermolysis of Ca, Sr, Ba, and V
Table 2. Composition of gaseous products of cobalt
acetylacetonate pyrolysis (mol%) at various temperatures
(T) and degrees of decomposition ( )
-diketonates. This may be due to deceleration of the
limiting stage of the thermolysis of metal chelates,
namely, opening of the chelate ring followed by
elimination of the free ligand [5].
T,
K
,
H2
H O CO CO2 CH COCH
2 3 3
min
The products of the Co(acac) thermolysis in a vac-
2
5
5
5
6
6
33 60 1.0 10.5 1.8 12.0
33 10 0.3 10.0 2.9 17.9
8.6
1.8
67.1
uum were analyzed by mass spectrometry (Table 2).
The products of the thermolysis at low degrees of
decomposition at 533 K were acetone, carbon mon-
oxide, and hydrogen. As the temperature was in-
creased from 539 to 693 K, the content of hydrogen
and carbon monoxide in the products of complete
decomposition increased from 10.5 to 42.3% and from
67.4
65.1
9.8
73 10 0.5
93 30 1.0 42.3 0.7 40.05 7.0
93 10 0.8 46.0 0.7 23.0 7.8
8.1 2.5 13.8 10.5
22.4
Table 3. Composition of catalysts for the oxidation of CO
to CO , based on Co(acac)2 thermolysis products, and
conditions of their preparation and treatment
1
2.0 to 40.05%, respectively, and the content of
2
acetone decreased from 67.1 to 9.8%. This fact sug-
gests that acetone is formed in the initial stages of the
decomposition, and then it decomposes on the cobalt-
containing coating. Thus, the pathway of Co(acac)2
decomposition is similar to that characteristic of other
Application Application
technique
Catalyst
I
Support
conditions
Vacuum,
93 K, 2 h
The same
Annealing
compounds M(acac) (M = Y, Ti, V, n = 3; M = Ca,
n
SCCa
CVDb
Sr, Cu, Ba, n = 2) [5]. It is believed that the first stage
5
of the thermolysis of M(acac) is opening of the
n
c
CVDb
Impregnation
II
III
GAZ slime /SCC
chelate ring. Then the ligand undergoes a steric
rearrangement with migration of the -CH proton
from a nonactivated ligand to the activated ligand to
form free acetylacetone [5].
SCCa
with Co(acac)2 in air
solution in
acetone
GAZ slime /SCC The same
c
Comparison of the kinetic stability of Co(acac)2
IV
The same
and acetylacetonate complexes M(acac) of the metals
studied earlier (M = Cu, Ca, n = 2; M = V, Ti, n = 3),
decomposing in the gas phase [5], shows that
n
a
SCC is synthetic cellular ceramics with the composition
b
Fe O /P O /B O /KhIPEK [6].
CVD is chemical vapor
3
4
2
5
2 3
c
Co(acac) is the least stable with respect to the ther-
deposition. GAZ slime is the waste from the electroplating
2
molysis. Using the thermolysis rate constant as the
measure of reactivity, we can rank the acetylacetonate
complexes studied in the following order with respect
to their kinetic stability: V(acac) > Ca(acac) >
production of the GAZ Joint-Stock Company; composition, %:
3
+
2+
3+
2+
2+
3
Cr 0.017; Fe , Fe 4.5; Mn 0.01; Zn 4.5; PO 22.37;
4
2
+
2+
2
P O 16.72; Cu
0.01; Ni
0.87; SO 0.9; Cl 0.9.
2
5
4
3
2
Ti(acac) > Cu(acac) > Co(acac) .
under comparable conditions in a pulse microcatalytic
system [7] (Figs. 1, 2). All the catalysts were prelim-
inarily annealed for 3 h at 773 K in a helium flow to
achieve a steady catalytic activity.
3
2
2
We used Co(acac) to prepare catalysts I and II for
2
the oxidation of CO to CO by chemical vapor deposi-
2
tion (CVD) of a cobalt-containing coating in a
vacuum on synthetic cellular ceramics (SCC) of
various compositions. Catalysts III and IV were
prepared for comparison by the impregnation of a
Figures 1 and 2 show that the oxidation of CO with
oxygen in the presence of catalysts I and II obtained
by CVD proceeds in a complex oscillatory mode with
a hysteresis in an anti clockwise direction when the
cellular ceramics with a Co(acac) solution followed
2
by annealing in air at 873 K (Table 3). The coatings
conversion of CO into CO was different at increasing
2
obtained by the thermolysis of Co(acac) at 573
and decreasing temperatures. The occurrence of the
hysteresis is attributed to the critical phenomena and
the possibility of autooscillations [8], and also to local
overheating of active centers and low thermal con-
ductivity of the porous support [9].
2
1
073 K are composite carbon metal coatings of com-
plex composition, containing metals, their oxides,
carbides, and free carbon [1, 2]. The coatings obtained
by the impregnation of synthetic cellular ceramics
followed by annealing in air contain cobalt oxides and
free carbon.
An anticlockwise hysteresis was also observed
with catalysts III and IV prepared by impregnation
We have compared the activity of the catalysts
with a Co(acac) solution, but oscillatory phenomena
2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 11 2004

1648
TSYGANOVA et al.
1
00
1
00
2
8
0
8
0
6
0
6
0
1
1
3
2
4
2
0
0
4
0
0
4
3
2
4
4
00 500 600 700 800 900 1000 T, K
4
00 500 600 700 800 900 1000 T, K
Fig. 1. Comparison of the activities of catalysts I and III
Fig. 2. Comparison of the activities of catalysts II and IV
on a GAZ slime/SCC support (F 0.5 cm s , fraction
3
1
3
1
on an SCC support in a pulse system (F 0.5 cm s
,
fraction 0.3 0.5 mm). Sample: (1, 2) CO/O 1/8 and
0.3 0.5 mm). Sample: (1, 2) CO/O 1/8 and (3, 4) CO
2
2
(
3, 4) CO without O . Catalyst: (1, 4) I and (2, 3) III.
without O . Catalyst: (1, 3) II and (2, 4) IV.
2
2
were not observed. The catalysts catalysts are ranked
in the following order with respect to their catalytic
activity: II > I > III > IV (the comparison was carried
out using the ascending curve at 50% conversion).
Catalyst II had the highest activity; the conversion on
it started at 373 K, and 80% conversion was attained
at 623 K. Catalysts III and IV were less active.
To determine the mechanism of the reaction on
catalysts I IV, we carried out the oxidation of CO to
CO both in the presence and in the absence of
2
oxygen in the reaction mixture. The activation param-
eters of the oxidation decrease in the case of catalysts
I III and increase in the case of IV. Table 5 shows the
ratio of conversions ( / ) at various temperatures
2
1
( 1
is the conversion of CO to CO in the presence of
2
The catalyst Co O /SCC obtained by impregnation
3
4
O , when O and CO are adsorbed on active centers
2
2
of synthetic cellular ceramics with a solution of co-
balt(III) nitrate, followed by annealing [10], is much
inferior in the activity to catalysts I III. The conver-
of the catalyst and react with each other;
is the
2
conversion in the absence of O , when the oxidant is
2
the crystal lattice). The contribution of the second
pathway increases with temperature for catalysts I and
III and decreases for catalysts II and IV.
sion of CO to CO on it is 13% at 798 K.
2
The conversion on catalyst II at 373 573 K and on
catalyst IV at 423 723 K depends only slightly on
temperature (Fig. 1), apparently because of the occur-
rence of disproportionation [8].
EXPERIMENTAL
Crystalline Co(acac) (mp 482 K) was synthesized
2
according to [3]. Found, %: C 46.7; H 5.65; Co 23.0.
2CO
CO2 + C.
M 257. C H CoO . Calculated, %: C 46.69; H 5.45;
1
0
14
4
Co 22.96; O 24.9. M 257.
The experimental conditions and parameters of
heterogeneous catalytic oxidation of CO on the
catalysts under study are given in Table 4. The
activation energies show that the reaction is kinetical-
ly controlled.
The kinetics of the thermal decomposition of
Co(acac) was studied by the manometric method
2
[11]. The rate of the thermal decomposition was
determined from an increase in the total pressure
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 11 2004

KINETIC FEATURES OF THERMAL DECOMPOSITION
1649
Table 4. Conditions of kinetic experiments and activation
The first order of the thermolysis was proved by
the fact that the rate constant is independent of the
parameters of catalytic oxidation of CO to CO on cobalt-
2
containing catalysts (F 0.5 cm s , CO/O ratio 1/8)a
3
1
Co(acac) initial concentration. The rate constant was
2
2
calculated at low conversions (up to 25 30%), which
allowed us to neglect the influence of decomposition
products on the kinetics of the main process. The
activation energy and the preexponential factor were
calculated by the Arrhenius equation k = k0e
using the least-squares method.
Tempera-
Cata-
lyst
T, K
( 50%)
ture
log k0
E , kJ mol 1
a
range,
K
E/RT
I
528 681
673
603
973
593
588
9.47 1.20 120.96 3.67b
7.10 0.30
3.45 0.52
6
6
53 550
81 946
87.69 2.93c
The mass-spectrometric analysis of the thermolysis
products was carried out on an MI-1305 mass spec-
trometer at the energy of ionizing electrons of 70 eV,
emission current of 1.5 mA, and room temperature of
the ion source and inlet system.
d
70.35 2.11
b
II
548 613
11.96 1.08 137.94 2.8
6
6
01 538
27 943
12.20 0.80 139.19 2.6c
0.85 0.13
5.03 0.30
7.12 0.12
3.80 0.20
1.83 0.10
2.50 0.16
4.46 0.16
27.21 1.92d
b
III
IV
668 811
768
698
898
838
783
76.45 4.02
7
6
58 628
78 906
97.89 2.30c
67.34 2.93d
The catalyst supports were prepared on the basis
of KhIPEK ceramics according to TU (Technical
Specifications) 5759-010-10657190 97 [6] by intro-
ducing various additives into the system.
b
548 883
32.69 3.76
9
7
03 573
59 943
37.87 2.09c
88.24 2.51d
To prepare catalysts I and II, we used samples of
synthetic cellular ceramics (SCC) annealed for 1 h at
a
b
F is the flow rate of the carrier gas. Ascending branch of the
773 K (fraction 0.3 0.5 mm). For this purpose, 0.26 g
c
d
hysteresis.
Descending branch of the hysteresis.
CO
of Co(acac) on 2 g of the support was placed in a
2
sample without oxygen.
60-ml glass ampule, which was evacuated and heated
for 2 h at 573 K. Catalysts III and IV were prepared
by impregnating samples of cellular ceramics with a
solution of Co(acac)2 in acetone [0.206 g of
Table 5. Temperature dependence of CO conversion to
CO2 in the presence ( 1) and in the absence ( 2) of
oxygen
Co(acac) in 2 5 ml of acetone on 2 g of the support].
2
The solvent was evaporated at room temperature, and
then the catalysts were kept for 1 h at 773 K in a
muffle.
Catalyst T, K
1, %
2, %
( 1/ 2) 100%
I
673
51
15
90
20
82
12
60
98
20
80
1
4
10
2
5
4
17
50
2
2
27
11
10
6.1
33.3
28.3
51
10
5
7
8
73
73
The kinetics of heterogeneous catalytic oxidation
of CO to CO was studied using a pulse version of
2
II
573
73
673
the gas-chromatographic method [7]. The conversion
6
was calculated from the CO peaks using calibration.
2
III
The technique of the kinetic measurements is
described in [12].
7
8
73
73
IV
773
73
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8
4
1
2
3
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