Thermolysis of [Co(NH3)6][Cr(C2O 4)3]

Article abstract of DOI:10.1134/S0036023608080123

Thermolysis of the double-metal complex [Co(NH₃) ₆][Cr(C₂O₄)₃] was studied in air at 200, 350, and 500°C and in a hydrogen atmosphere at 200, 350, 500, 700, and 900°C, as well as the composition and properties of thermolysis products. Oxidative thermolysis produces mixed oxides CoCr₂O₄ and Co₂CrO₄; reductive thermolysis produces Co + Cr ₂O₃ mixture. Specific surface areas were measured for reductive thermolysis products; the maximal specific surface area and, therefore, maximal dispersion are reached at 500°C. The morphology of the reductive thermolysis products and the thermolysis chemism were studied in relation to the nature of the complex anion.

Full text of DOI:10.1134/S0036023608080123

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 8, pp. 1221–1226. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © S.I. Pechenyuk, D.P. Domonov, A.T. Belyaevskii, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 8, pp. 1313–1319.
COORDINATION
COMPOUNDS
Thermolysis of [Co(NH ) ][Cr(C O ) ]
3
6
2
4 3
S. I. Pechenyuk, D. P. Domonov, and A. T. Belyaevskii
Tananaev Institute of Rare Element and Mineral Chemistry and Technology, Kola Research Center, Russian Academy of
Sciences, ul. Fersmana 14, Apatity, Murmansk oblast, 184200 Russia
Received April 27, 2007
Abstract—Thermolysis of the double-metal complex [ëÓ(NH ) ][Cr(C é ) ] was studied in air at 200, 350,
3
6
2
4 3
and 500°ë and in a hydrogen atmosphere at 200, 350, 500, 700, and 900°C, as well as the composition and
properties of thermolysis products. Oxidative thermolysis produces mixed oxides CoCr O and ëo CrO ;
2
4
2
4
reductive thermolysis produces Co + Cr O mixture. Specific surface areas were measured for reductive ther-
2
3
molysis products; the maximal specific surface area and, therefore, maximal dispersion are reached at 500°ë.
The morphology of the reductive thermolysis products and the thermolysis chemism were studied in relation to
the nature of the complex anion.
DOI: 10.1134/S0036023608080123
Work [1] describes synthesis and characterizes new Quant-AFA atomic absorption spectrometer after a
double-metal complexes (DMCs) of transition metals sample was dissolved in hydrochloric + nitric acid mix-
3+
containing the cation [Co(NH ) ] and complex anions ture. As in [1], the test sample contained water and had
3
6
2
2
–
3–
4–
3–
[
Fe(CN) ] , [Fe(CN) ] , [ër(CNS) ] , [Cu(C O ) ] ,
6 6 6
2 4
2
3–
and [Cr(C é ) ] . We propose to use these complexes
2
4 3
Table 1. Analysis of the products of [Co(NH ) ][Cr(C O ) ]
as precursors in manufacturing powdered binary oxides
and metals with high degrees of blending homogeneity.
Their homogeneity is due to the regular crystal struc-
ture of DMCs. For this purpose, thermolysis in air and
3 6
2 4 3
thermolysis in air
Weight percentage
hydrogen
atmospheres
was
studied
for
[
Co(NH ) ][Fe(CN) ] and [Co(NH ) ] [Fe(CN) ] [2],
3
6
6
3 6 4
6 3
Co : Cr :
C
Tdec, °C
[
Co(NH ) ] C O [Cu(C O ) ]
and
[Co(NH ) ]Cl
3
6 2
2
4
2
4 2 2
3 6
[
Cu(C H O ) ] [4] (the last compound was described in
7
4
3 2
Co
Cr
C
[3]), and [Co(NH ) ][ër(CNS) ] [5]. Thermolysis of
3 6 6
these complexes in air at sufficiently high temperatures
produces mixtures of individual oxides or mixed oxides
of both metals; thermolysis in a hydrogen atmosphere
produces nanosized powders of the intermetallic com-
pound CoFe [2], cobalt + copper heterogeneous mixtures
Initial complex* 11.06
9.86
10.83
30.63
–
14.12 1 : 1 : 6.3
[4], and metallic cobalt + CoCr S mixtures [5]. There
2 4
were grounds to believe the that intermetallic compound
CoCr can be obtained [6], and we decided to use the
complex [ëÓ(NH ) ][Cr(C é ) ] for this purpose; this
2
00
12.42
36.06
–
15.20
0.07
0.05
1 : 1 : 6
3
6
2
4 3
complex is free of sulfur, which inhibits the appearance
of a metallic phase at sufficiently low temperatures [5].
350
–
–
EXPERIMENTAL
5
00**
The sample used in this work was synthesized
according to [1]; analyzed for cobalt, chromium, and
1
carbon (Table 1); and identified by X-ray powder dif-
fraction. Metals in the complex were determined on a
*
*
For the initial complex, anal. calcd. (wt %): Co, 11.07; Cr, 9.77;
3.53; ,C,5.79; Co + Cr, 20.84; CO , 49.62.
* Thermolysis products are insoluble in acids.
1
2
1
Analyses, including nitrogen determination, were performed at
the Analytical Department of the Tananaev Institute of Rare Ele-
ment and Mineral Chemistry and Technology.
1
221

1
222
PECHENYUK et al.
*
*
Table 2. Analysis of the products of [Co(NH ) ][Cr(C O ) ] reduction
3
6
2
4 3
In ignition residue, wt %
In gaseous products, wt % of initial weight
Weight loss on ignition,
wt %
Tdec, °C
Co
Cr
C
N*
CO2
2
3
5
7
9
00**
50
10.38
42.73
71.30
73.47
74.32
12.42
18.24
39.44
–
10.43
15.81
32.72
–
16.1
14.9
0.60
16.53
17.02
18.54
18.87
–
20.34
32.98
36.5
–
00
2.11
00
0.48
0.08
00
–
–
*
*
Highly overestimated relative to stoichiometry.
* For the anhydrous complex (wt %): Co, 12.35; Cr, 10.90; C, 15.09; N, 17.61.
the formula [ëÓ(NH ) ][Cr(C é ) ] · 3ç é. Thermal porcelain crucibles at 700°ë. The BaCO weight was
3
6
2
4 3
2
3
analysis of this compound in air was described in [1]; used to calculate the amount of ëé evolved. The
2
however, its thermolysis products in air have not been
studied.
results of chemical analysis of the reduction products
are compiled in Table 2. X-ray powder diffraction was
This work studies the composition of thermolysis measured on DRON-2 and DRF-2.2 diffractometers
products in air at 200, 350, and 500°ë and thermolysis (CuK radiation, graphite monochromator). The ASTM
products in a hydrogen atmosphere at 200, 350, 500, file [8] was used for phase identification. The X-ray dif-
00, and 900°ë. At 200°ë, the exposure duration was 2 h fraction patterns of solid oxidation and reduction prod-
α
7
in either case; in the other experiments, 1 h. This period ucts of the complexes are displayed in Figs. 1 and 2. For
does not include heating to the specified temperature. The reduction products, electron micrographs were
heating rate of two furnaces (see below) was 10 deg/min. obtained on an SEM LEO-420 scanning electron
Thermolysis in air was performed in porcelain cruci- microscope (Figs. 3–6). BET specific surface areas
bles in an SNOL 7.2/1100 muffle furnace. The results were measured on a FLOW-Sorb-II-2300 setup were
of chemical analysis of solid products are compiled in
measured for these products.2
Table 1. Thermolysis in a reductive atmosphere was
performed on a setup designed as described in [7]. A
sample of the complex in a corundum-coated boat was
reduced in a quartz tube inserted into an SNOL
RESULTS AND DISCUSSION
Work [1] demonstrated that thermolysis of
0
.2/1250 tubular furnace (a flow-through reactor). The
[
ëÓ(NH ) ][Cr(C é ) ] · 3ç é in air follows a far sim-
3
6
2
4 3
2
hydrogen flow rate was 10–15 L/h. The reactor was
purged with hydrogen for 5 min before heating and dur-
ing the entire heating and exposure period. Solid reduc-
tion products were also cooled in flowing hydrogen.
pler route than thermolysis of its analogue
[
ëÓ(NH ) ][Cr(NëS) ]. Thermolysis is accompanied
3
6
6
by an endotherm at 284°ë and two poorly resolved exo-
therms at 366 and 422°ë; weight loss stops at 400°ë. In
Two Drexel bottles, one filled with 1 M HCl and the thermolysis of the complex in air at 200°ë, the major
other with 1 M NaOH + 0.0225 M BaCl , were two peaks associated with the precursor are conserved
2
mounted in sequence at the outlet of the reactor in order in the X-ray diffraction pattern of the product (Fig. 1,
curve a) with a small shift toward smaller d (0.58
to trap gaseous reduction products. The first absorbing
solution was analyzed for nitrogen (ammonium ions).
2
S
measurements were performed by T.I. Lobacheva at the
Tananaev Institute of Rare Element and Mineral Chemistry and
Technology.
sp
From the second solution, BaCO precipitated; it was
3
filtered, washed with 700–800 mL water, and fired in
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 8 2008

THERMOLYSIS OF [Co(NH ) ][Cr(C O ) ]
1223
3
6
2
4 3
CoCr O
2
4
Co CrO
2
4
‡
b
c
d
d, nm
Fig. 1. X-ray diffraction patterns for (a) the double salt [ëÓ(NH ) ][Cr(C é ) ] and (b–d) its thermolysis products in air at (b) 200,
3
6
2 4 3
(c) 350, and (d) 500°ë.
instead of 0.62 nm and 0.45 instead of 0.47 nm); the corresponds to an X-ray amorphous substance, and in
other characteristic peaks of the precursor disappear. the X-ray diffraction pattern for the reduction product
Elemental analysis (Table 1) shows that this product is at 500°ë, there are two insignificant peaks at 2.03 and
an almost intact complex. Thermolysis products at 350 1.915 Å; we failed to assign these peaks. Reduction at
and 500°ë in air have almost identical diffraction pat- 700 and 900°ë produces Co + Cr O mixtures (Fig. 2;
2
3
terns (Fig. 1; curves c, d), which correspond to isostruc- curves d, e). X-ray powder diffraction implies a very
tural mixed-metal oxides CoCr O and ëo CrO (pro- high dispersion of these products; therefore, we thought
2
4
2
4
ceeding from the formula unit, we are dealing with an it necessary to measure their specific surface areas.
equimolar mixture).
After preparation in argon at 250°ë, S was 6.37, 78.6,
sp
2
1
1.45, and 7.77 m /g for the reduction products
For the reduction product obtained at 200°ë, the
X-ray diffraction pattern is completely the same as for
the thermolysis product in air (Cf. Fig 1, curve a; Fig, 2,
curve ‡). Unexpectedly, the X-ray diffraction pattern of ysis was repeated: the structure of these samples (or the
the thermolysis product obtained in hydrogen at 350°ë nonexistence of structure) did not change during Ssp
obtained at 350, 500, 700, and 900°C, respectively.
After S measurements, X-ray powder diffraction anal-
sp
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 8 2008

1
224
PECHENYUK et al.
Co
Cr O
2
3
‡
b
c
d
e
d, nm
Fig. 2. X-ray diffraction patterns for the products of thermolysis of the double salt in hydrogen at (a) 200, (b) 350, (c) 500, (d) 700,
and (e) 900°C.
3
measurements. As expected, S decreases with rising nor it was observed in [5], where metallic chromium
sp
reduction temperature because of particle aggregation. was also produced at 900°ë. Therefore, we suggest that
this oxygen was bounded to chromium during reductive
thermolysis of the complex, and it originated from
partial thermolysis. At 200°ë, as during heating in air, oxalate ligands.
Elemental analysis of the reduction products of the
complex at 200, 350, and 500°ë also shows products of
this is the dehydrated but nondegraded complex; at
Reduction of DMCs is an unsteady-state process:
gaseous reduction products are continuously removed
from a flow-through reactor. During long exposure at
3
50°ë, a product of partial thermolysis (Co : Cr : C = 1
1 :4); at 500°ë, the product containing 2.11% carbon
:
(
Co : Cr : C = 1 : 1 : 0.26). Weight loss on reduction is
2
00 and 350°ë in our experiments, the complex likely
always smaller than expected for metal phase forma-
tion. Calculations show that the residue weights from
reduction at 700 and 900°ë correspond to Co + Cr O
has not enough time to thermolyze completely; at
higher temperatures, thermolysis goes to completion
2
3
mixtures. The question arises regarding the origin of (Table 2).
oxygen in these products. We did not observe oxidation
Proceeding from the results of elemental analysis
and assuming that oxygen in Cr O originated from
of the metal phase after the cooled residue was trans-
ferred to air in either of our previous studies [2, 4, 5];
2
3
oxalate groups, we can suggest the following reaction
equation:
3
Before Ssp measurements, samples were degassed at 350°C.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 8 2008

THERMOLYSIS OF [Co(NH ) ][Cr(C O ) ]
1225
3
6
2
4 3
1
0 µm
10 µm
(
‡)
(‡)
(b)
(c)
2
µm
10 µm
(
b)
3
00 nm
10 µm
(
c)
Fig. 3. Micrographs of the initial double salt
ëÓ(NH ) ][Cr(C é ) ]. Scale bar: (a) 10 µm, (b) 2 µm,
and (c) 300 nm.
Fig.
4.
Micrographs
of
the
products
of
[
[ëÓ(NH ) ][Cr(C é ) ] reduction at (a) 200, (b) 350, and
3
6
2
4 3
3 6 2 4 3
(c) 500°ë. Scale bar: 10 µm.
The micrographs in Figs. 3–5 show the crystal mor-
phology of the initial complex and the products of its
reduction at various temperatures. Figure 3 shows two
varieties of initial crystals. Most crystals are hexagonal
prisms with sizes up to 30 µm along axis L (Fig. 3a).
The crystals have sufficient mechanical strength. Under
2
[Co(NH ) ][Cr(C O ) ] + 9H
2Co
Cr O + 12NH + 9CO + 3CH + 3H O.
3
6
2
4
3
2
+
2
3
3
2
4
2
The results of elemental analysis (Table 2) conform
to this equation: the amount of carbon dioxide precipi-
tated as BaCO is 4.4 mol ëé /1 per mole of the com-
3
2
plex (4.5 in the equation). This ëé evolution agrees high magnifications, mosaic fracturing of faces and
2
with the suggestion that oxalate transfers its oxygen to dendritic layering on top of prisms are observed. Disin-
chromium.
tegration of prisms yields platy crystals (Figs. 3b, 3c)
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 8 2008

1
226
PECHENYUK et al.
initial crystal shape, but their sizes are less than one-
half the crystal sizes of the initial complex and progres-
sively decrease with rising reduction temperature (Figs.
4
a–4c). Moreover, enhanced aggregate fracturing and
an increased proportion of the progressively finer
aggregates of prismatic habit are observed under high
magnifications (Fig. 5). Although noncontact surface
defects exist on particles (they could hardly be called
crystals) and the evident nonexistence of internal com-
pactness, these particles without gold coating give rise
to steady gluing under an electron beam, signifying
hydrogen-induced surface metallization of their faces.
Due to the decreasing particle sizes and increasing
porosity, the sample reduced at 500°ë has the greatest
specific surface area. Figures 5a and 5b display micro-
graphs of the samples reduced at 700 and 900°ë,
respectively. Product aggregates still retain the shape of
the initial crystals, whose degradation progresses with
a rise in temperature. Aggregate sizes diminish, and the
proportion of macropores (with equivalent radii of 20–
3
00 nm
(
‡)
3
0 nm) in them increases; macropores likely substitute
for micropores and do not ensure considerable S val-
sp
ues. This effect is most prominent in samples reduced
at 900°ë. In micrographs we cannot distinguish phases
that would be clearly seen in other tested compounds
[4, 5].
To summarize, this work has revealed one more
thermolysis feature of bimetallic transition-metal com-
plexes: provided oxygen-containing ligands, the more
reactive metal of the pair in the complex can form oxide
even during reductive thermolysis.
3
00 nm
(
b)
Fig.
[
5.
3
Micrographs
of
the
products
of
ëÓ(NH ) ][Cr(C é ) ] reduction at (a) 700 and (b)
6
2 4 3
9
00°ë. Scale bar: 300 µm.
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1
2
. S. I. Pechenyuk,Yu. P. Semushina, G. I. Kadyrova, et al.,
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built by loose packing of hexagonal prisms, as revealed
by gluing of crystal faces under an electron beam with-
out surface metallization. The thickness of platy crys-
tals is less than 100 nm (Fig. 3b).
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5
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4
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(
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Figure 4 displays the products of reduction at 200,
50, and 500°ë. From the above data, we see that the
A. T. Belyaevskii, Zh. Neorg. Khim. 52 (7), 1104 (2007)
3
[Russ. J. Inorg. Chem. 52 (7), 1027 (2007)].
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plex. Most crystals are hexagonal prisms with sizes up
to 30 µm. The product of reduction at 350°ë retains the
ancient shape, but aggregates change their sizes and
acquire considerable fracturing (porosity) (Fig. 4b).
Inasmuch as the reduction product at 350°ë is an
incompletely destructed complex (the ratio Co : Cr : C
5
. D. P. Domonov, S. I. Pechenyuk, D. L. Rogachev, and
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6
7
=
1 : 1 : 4) and retains the initial crystal shape, its low
specific surface area and the absence of definite struc-
ture are natural (Fig. 2b). Most aggregates in the reduc-
tion product obtained at 500°ë (Fig. 4c) also retain the
8
. ASTM Diffraction Data Cards and Alphabetical and
Grouped Numerical Index of X- ray Diffraction Data
(ASTM, Philadelphia, 1946).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 8 2008