Peculiarities of polythermic decomposition of iron, cobalt and nickel oxalates within pores of photonic crystals based on SiO2 in atmosphere with oxygen lack

Article abstract of DOI:10.1007/s10973-008-9024-0

Iron(II), cobalt(II) and nickel(II) oxalates were synthesized as nanofractals inside the voids of the photonic crystals based on SiO₂. Guest substances undergone polythermic decomposition within the pores of the photonic crystals in helium atmo

Full text of DOI:10.1007/s10973-008-9024-0

Journal of Thermal Analysis and Calorimetry, Vol. 92 (2008) 3, 747–750
PECULIARITIES OF POLYTHERMIC DECOMPOSITION OF IRON,
COBALT AND NICKEL OXALATES WITHIN PORES OF PHOTONIC
CRYSTALS BASED ON SiO2 IN ATMOSPHERE WITH OXYGEN LACK
A. N. Zakharov*, A. F. Mayorova and N. S. Perov
Moscow State Lomonosov University, Moscow, Russia
Iron(II), cobalt(II) and nickel(II) oxalates were synthesized as nanofractals inside the voids of the photonic crystals based on SiO2.
Guest substances undergone polythermic decomposition within the pores of the photonic crystals in helium atmosphere containing
of oxygen traces (~1 Pa) under static conditions. Pyrolysis of Fe(COO)2·2H2O, Co(COO)2·2H2O and Ni(COO)2·2H2O studied by
TG and DSC techniques results in the formation of the metal oxides. The nanoparticles of Fe2O3, CoO (Co3O4) and NiO populated
the interspheric voids of the photonic crystals exhibited no ferromagnetic effects indicating that no metallic inclusions were formed
in helium in the presence of O2 traces. The exothermic effect was observed by the thermal decomposition of the cobalt(II) oxalate
only under oxygen lack.
Keywords: cobalt oxalate, decomposition, interspheric voids, iron oxalate, nickel oxalate, photonic crystals
Introduction
under a linearly-programmed temperature. The rates
of heating were 2, 5 and 10 K min–1. The mass loss
and thermal effects were monitored by TG and DSC
techniques (Netzsch thermal analyzer STA 409).
Magnetic moment measurements were carried
out at room temperature using a vibrating sample
magnetometer (accuracy of 8·10–11 A m2) in the range
from –800 to 800 kA m–1.
Topochemical reactions within pores of crystal materi-
als are used for modification of solids by inclusions of
metals or its oxides. Photonic crystals (PC) based on
SiO2, synthetic opals, populated with ferromagnetic
fractals were applied to a construction of week magnetic
field sensors, in systems for record and store of informa-
tion, and other nanotechnological branches.
Results and discussion
Experimental
Thermal transformations of iron(II), cobalt(II) and
nickel(II) oxalate dihydrates were studied elsewhere
under variable conditions [2–7]. The topochemical
M(COO)2·2H2O (M=Fe, Co, Ni) decomposition was
observed to undergo two-step reactions. On the first
step, the dehydration (1) takes place.
Materials
Iron(II), cobalt(II) and nickel(II) oxalates were pre-
pared in situ by reactions of (NH4)2Fe(SO4)2·6H2O
(Sigma-Aldrich Chemical Co.), Co(NO3)2·6H2O
(Aldrich), NiCl2·6H2O (Aldrich) with H2C2O4
(Aldrich) according to [1]. Acetone and ethanol have
been purified by fractional distillation under atmo-
spheric pressure.
M(COO)2·2H2O=M(COO)2·+2H2O
(1)
The decomposition of the anhydrous materials
was found to be dependent on many factors such as
the nature of the metal and gas phase, shape and di-
mensions of the sample particles, prehistory of the
sample preparation, the conditions of the decay and so
on [3]. The pyrolysis of the dehydrated metal(II) oxa-
lates in oxygen or nitrogen containing traces of oxy-
gen was considered to result in the formation of a
metal phase according to the reaction (2) or metal ox-
ide (3) depending on the nature of the metal [3].
Photonic crystals based on amorphous poly-
condensed SiO2 were synthesized by Guryanov. PC
were precalcined at 800 K for 5 h. Metal(II) oxalates
were synthesized within interspheric voids of dehy-
drated PC according to [1].
Methods
The supported samples were heated in helium con-
taining remaining oxygen (~1 Pa) in a static system
M(COO)2·=M+2CO2
(2)
*
Author for correspondence: alxnz@zakhar.msk.ru
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

ZAKHAROV et al.
M(COO)2·=MO+CO+CO2
(3)
Iron(II) and nickel(II) oxalates were found to de-
compose by a formation of corresponding oxide,
Fe2O3 (FeO, Fe3O4) and NiO.
A concept that the decomposition of Co(COO)2·
causes to a formation of both CoO and Co in parallel
pathways according to the reactions (2) and (3) inde-
pendent of the nature of the gas phase supported by oth-
ers [4]. Also, it is unambiguously that the prior dehydra-
tion conditions effect upon the subsequent decomposi-
tion of the anhydrous material. It was for instance
shown [2] that the conditions of Co(COO)2·2H2O dehy-
dration resulted in variations of particle size and poros-
ity of the anhydrous oxalate and that this has a signifi-
cant effect on the kinetics of decomposition. The parti-
cles of the cobalt oxalate dehydrated in a vacuum at
200°C were of 1 m in diameter and less and broke up
into spherical ones [2]. The dehydration of
Co(COO)2·2H2O at lower temperature revealed more
porous and larger particles of about 5 m diameter.
The porosity of a material to be decomposed is evi-
dent to be of great importance for topochemical reac-
tions. Our work reports that the way of the synthesis of
Co(COO)2·2H2O effects the peculiarities of the kinetics
of the decomposition and mechanism of the decay.
The thermal behavior of metal oxalates within
pores of the photonic crystals based on amorphous
polycondensed SiO2 is unknown. On the one hand, it
should be no difference in thermal behavior of the
metal(II) oxalates supported on a solid. On the other
hand, it is of special importance for solid-phase trans-
formations the dimensions of species to decompose.
There are nanospecies of the metal oxalates supported
on PC that undergo a topochemical reaction. An aver-
age dimension of the metal oxalate particles prepared
inside the pores of fcc crystal structure of PC based on
SiO2 is limited by the effective diameter the pores
d£ D(2/ 3-1), D is effective diameter of the SiO2
nanospheres (~240 nm). We supposed that the par-
celed phases of the metal oxalates could not exceed
40 nm in diameter in accordance with a size of the
voids of the photonic crystallites.
Fig. 1 TG, DTG and DSC plots for the pyrolysis of
Co(COO)2·2H2O supported on the photonic crystals
based on SiO2 (in He); heating rate is 5 K min–1
Fig. 2 TG, DTG and DSC plots for the pyrolysis of unsup-
ported Co(COO)2·2H2O (in He); heating rate is
5 K min–1
Table 1 Characteristics of the samples of the photonic crys-
tals (PC) based on SiO2 containing M(COO)2·2H2O:
metal oxalate/photonic crystals mass ratio (r), theo-
retic (gt) and experimental (ge) ratio g=Dm3/Dm2 of
the mass losses of the 3rd (Dm3) to the 2nd (Dm2)
steps of the decomposition, by linear-programmed
heating in helium
b/
K min–1
ge (gt)
Sample
M2+
r/%
1
2
3
Fe
Co
Ni
0.39
6.88
1.07
10
10
2
2.58 (2.89)
2.41 (2.40)
2.38 (2.45)
photonic crystals undergone only endothermal reactions
in helium to form the corresponding metal oxides. The
decomposition of the supported cobalt(II) oxalate
dihydrate was found to reveal two endothermal and one
exothermal effects under the same conditions (Fig. 1).
The exothermal effect is evident to be due to the reac-
tion (4) which undergoes in helium containing remain-
ing O2.
Table 1 summarizes the characteristics of the sam-
ples obtained by a matrix synthesis of the iron(II), co-
balt(II) and nickel(II) oxalates. Figure 1 shows represen-
tative TG-, DTG- and DSC-traces for temperature-pro-
grammed decomposition of supported Co(COO)2·2H2O
up to 500°C in helium under static conditions. There are
some stages of a mass loss. The lowest-temperature loss
of the mass was due to a dehydration of the photonic
crystals. The samples released the main part of adsorbed
water in the range of 300–400 K. The second and third
mass losses were connected with the dehydration and
decomposition of the cobalt(II) oxalate, respectively.
The iron(II) and nickel(II) oxalates supported on the
2CO+O2=2CO2
(4)
No exothermal effects were found by decompo-
sition either of supported Ni(COO)2·2H2O or
Fe(COO)2·2H2O. It indicates that no reaction (4) oc-
curs in these cases although at least Fe(COO)2· de-
composes by CO evolution in accordance with the re-
748
J. Therm. Anal. Cal., 92, 2008

IRON, COBALT AND NICKEL OXALATES
action (3) [5]. So, the reaction (4) seems to have pre-
sumably a catalytic character. This conclusion is sug-
gested by well-known catalytic properties of cobalt
and cobalt oxides in respect with the above reaction.
An assignment of the reaction (4) to a catalytic
type is suggested unambiguously by the decomposition
of unsupported Co(COO)2. In helium, there are ob-
served exothermal and endothermal effects in the range
of thermal decomposition of Co(COO)2 (Fig. 2). The
temperature ranges of the exothermal and endothermal
effects of catalytic CO oxidation and decomposition of
unsupported Co(COO)2, respectively, are not coincided
in the low-oxygen atmosphere as it is shown in Fig. 2. In
the range of the decay of the Co(COO)2, a DTG-trace in
helium shows two peaks at 250–300 and 340–380°C in-
dicating changes in the rate of the mass loss by the de-
composition of Co(COO)2. The low-temperature pick
on the DTG-trace at 250–300°C is due to a change in the
rate of temperature enhancement owing heat release by
catalytic CO oxidation. This peak is attributed to the
exothermal effect on the DSC-trace. At higher tempera-
ture, there is no more oxygen to be consumed by the cat-
alytic oxidation of CO and the CO catalytic oxidation
gets to be controlled by the diffusive retardation in the
static system. According to the TG-trace, the mass loss
on this stage corresponds to the ‘normal’ tempera-
ture-programmed decomposition of the Co(COO)2 resi-
due. This peak on the DTG-trace is only due to the
endothermal effect of the Co(COO)2 decomposition.
In air, the exothermal effects are registered both
for supported and for unsupported Co(COO)2. A typi-
cal TG- and DSC-traces for the decomposition of sup-
ported Co(COO)2 are shown in Fig. 3. In air, the de-
crease in the temperature range of the decomposition
of Co(COO)2 supported on PC is probably connected
with hyper-programmed overheating of the sample by
the heat of the reaction (4) which provokes dramati-
cally increase in the rate of the Co(COO)2 decay.
Another feature of the supported Co(COO)2 de-
composition in low-oxygen atmosphere is the CoO for-
mation by pyrolysis instead of Co3O4 yielded by the
Co(COO)2 decomposition in air. Observed ratio of the
mass loss at the decomposition step to that of the dehy-
dration of Co(COO)2 is 0.509 which is in accordance
with a theoretic one (0.503). These data indicate that the
pyrolysis of supported Co(COO)2 in the presence of he-
lium containing traces of O2 follows the reaction (3).
Magnetic moment measurements suggest this
conclusion. The nanoparticles of Fe2O3, CoO (Co3O4)
and NiO populated the interspheric voids of the
photonic crystals exhibited no ferromagnetic effects
indicating that no metallic inclusions or other ferro-
magnetic phases were formed by the thermal decom-
position of metal(II) oxalates in helium in the pres-
ence of O2 traces. However, the reduction of the re-
Fig. 3 TG, DTG and DSC plots for the pyrolysis of
Co(COO)2·2H2O supported on the photonic crystals
based on SiO2 (in air); heating rate is 5 K min–1
Fig. 4 Plots of magnetization vs. magnetic field (H) for the
photonic crystals modified with the products of the
thermal decomposition of M(COO)2·2H2O [M=Fe, Co,
Ni] within the interspheric voids at room temperature
Fig. 5 DSC-curve for the decomposition of supported
Co(COO)2· in the temperature range 340–380°C in he-
lium atmosphere; heating rate is 5 K min–1
sulting samples with H2 at appropriate temperatures
revealed unambiguous evidence for the formation of
the ferromagnetic phases (Fig. 4).
It is noteworthy to analyze the kinetics of the sup-
ported Co(COO)2 decomposition in low-oxygen atmo-
sphere using the DSC-data in the range of temperatures
from 345 to 380°C (Fig. 5). In this interval there is no
influence of the exothermal effect of the catalytic oxida-
tion of CO by oxygen the endothermal effect of anhy-
drous cobalt(II) oxalate. The form of the kinetics is
J. Therm. Anal. Cal., 92, 2008
749

ZAKHAROV et al.
The fractional decomposition a¢ was calculated
by the endothermal effect of the DSC-trace observed
in the range 345–380°C (Fig. 5) as
Q
r
a¢=
Q
R
where Qr is the partial heat of the Co(COO)2 decom-
position registered by the temperature T and QR is the
total heat effect of the reaction.
The Arrhenius plot is presented in Fig. 7.
Equation (5) holds over the range 0.6<a<0.95.
The Arrhenius equation for the decomposition of the
Co(COO)2 is
Fig. 6 The fractional decomposition a¢ of anhydrous cobalt(II)
oxalate supported on PC based on SiO2 as a function of
temperature in He; heating rate is 5 K min–1
ln(k¢/min–1)=39.3–26800/RT
The activation energy over this period of the
Co(COO)2 decomposition is 222.8 kJ mol–1. This
value is in accordance with the data obtained for iso-
thermal decomposition of Co(COO)2 in a vacuum [2].
Conclusions
The thermal decomposition of Fe(COO)2·2H2O,
Co(COO)2·2H2O and Ni(COO)2·2H2O obtained by in
situ synthesis within the voids of the PC as nanoparticles
under static conditions in helium containing remaining
oxygen yielded oxides of the corresponding metal sup-
ported on the solid. The decay of Co(COO)2·2H2O only
was observed to show exothermal effect due to catalytic
oxidation of CO by the traces of O2 in helium. In the
presence of air the decomposition of Co(COO)2·2H2O
supported on PC occurred at lower temperature. There is
no endothermal effect which is masked on the
DSC-traces of the Co(COO)2·2H2O decomposition by
exothermal oxidation of CO.
Fig. 7 Arrhenius plots for the thermal decomposition of
Co(COO)2 supported on PC based on SiO2 in the tem-
perature range 340–380°C in helium atmosphere; heat-
ing rate is 5 K min–1
shown in Fig. 6 where the fractional decomposition a¢
of Co(COO)2·2H2O supported on the photonic crystals
is shown as a function of temperature. The initial decay
process commences as soon as the sample reaches the
temperature of the beginning of the reaction (3) and is
followed, after an induction period, by the acceleratory References
reaction connected with the catalytic CO oxidation. De-
1 Patent RF No 2 296 100.
noting the fractional decomposition at the end of the re-
action (4) by a0, then the fraction of Co(COO)2·decom-
posed in the succeeding ‘normally-programmed’ pro-
cess is a¢=a/a0.
2 D. Broadbent, D. Dollimore and J. Dollimore, J. Chem.
Soc. A, (1966) 1491.
3 V. A. Sharov, V. A. Zhilyaev, E. A. Nikonenko and
T. M. Zhdanovskikh, Koord. Khim., 6 (1980) 431.
4 B. MaÓecka, E. Drozdz-Cieíla and A. MaÓecka, J. Therm.
Anal. Cal., 68 (2002) 819.
The remainder of the reaction from a=0.6 to 0.95
is fitted by the Avrami–Erofeev equation with n=2, i.e.,
1–a¢=exp[-ka¢(t-t ¢ )2 ]
(5)
5 V. P. Kornienko, Ukr. J. Chem., 23 (1957) 159.
6 E. D. Macklen, J. Inorg. Nucl. Chem., 30 (1968) 2689.
7 D. Broadbent, D. Dollimore and J. Dollimore, J. Chem.
Soc. A, (1966) 278.
0
where t0¢ is the time of the decomposition of
Co(COO)2 accompanied by catalytic CO oxidation on
the product of the Co(COO)2 decay.
DOI: 10.1007/s10973-008-9024-0
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J. Therm. Anal. Cal., 92, 2008