Reduction and oxidation of simple oxocuprates

Article abstract of DOI:10.1023/A:1010121913036

Oxidation-reduction reactions were studied for the following oxocuprates: LiCuO, Li₂CuO₂, SrCu₂O₂, Sr₂CuO₃, SrCuO₂, BaCu₂O₂, BaCuO₂, LaCuO_2<

Full text of DOI:10.1023/A:1010121913036

Journal of Thermal Analysis and Calorimetry, Vol. 60 (2000) 219–227
REDUCTION AND OXIDATION OF SIMPLE
OXOCUPRATES
R. Kucharski and Z. Gontarz
Department of Inorganic Chemistry, Faculty of Chemistry, Warsaw University of Technology
Warsaw, Poland
Abstract
Oxidation-reduction reactions were studied for the following oxocuprates: LiCuO, Li2CuO2,
SrCu2O2, Sr2CuO3, SrCuO2, BaCu2O2, BaCuO2, LaCuO2, La2CuO4, Ca2CuO3 and Bi2CuO4.
Transformation schemes have been proposed for the anionic sub-lattices of these salts and the
effect of cations on the properties of the anionic sub-lattices in oxidation-reduction reactions
was determined.
Keywords: oxidation-reduction reactions, oxocuprates, TG-DTG-DTA
Introduction
The work has been devoted to investigation of fundamental chemical properties of
simple oxocuprates. Scientific centers throughout the world are working on possibili-
ties of increasing the critical temperature and on new methods of synthesis. Little at-
tention has been paid to simple oxocuprates, which have been the precursors of super-
conducting systems, thus the study of chemical properties of simple oxocuprates is
extremely important for the knowledge of chemistry of the superconducting systems.
Nowadays copper compounds are being studied very intensely because of their appli-
cation in high-temperature superconductors. Their structures are determined and con-
ditions of their syntheses are established. However, there are very few comprehen-
sive studies on their chemical properties, in particular, on their reactivity toward
oxidizing and reducing agents, such as O2 and H2, or on the manner in which cations
influence the reactivity of anionic sub-lattices, or finally – what is the effect of an-
ionic sublattice in systems with identical cations. Copper forms oxo-compounds at
three oxidation states. Simple species of copper are presented in the Górski’s classifi-
cation system in Table 1.
The aim of the work was to find relationships between the different kinds of
copper oxocompounds in their redox reactions. The studies were based on com-
pounds comprising the following kinds of oxocuprate species:
CuO33− , CuO43− , CuO22− , CuO, CuO23− , CuO– and Cu2O.
1418–2874/2000/ $ 5.00
Akadémiai Kiadó, Budapest
© 2000 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

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KUCHARSKI, GONTARZ: SIMPLE OXOCUPRATES
Table 1 Simple species of copper in the Górski’s classification system
ez
8
7
6
5
4
3
2
1
0
Copper and oxocuprates
CuO64−
CuO33−
CuO62−
CuO43−
Cu2O65−
CuO22−
Cu2O23−
CuO
CuO32−
Cu2O43−
CuO–
Cu2O
Cu+
Cu3+
8
Cu2–
9
Cu°
11
0
ev
10
Gox
+3
+2
+1
Experimental
The following compounds were synthesized: LiCuO [1], Li2CuO2 [2], SrCu2O2 [3],
Sr2CuO3 [4], SrCuO2 [5], BaCu2O2, BaCuO2 [6], LaCuO2, La2CuO4 [7], Ca2CuO3 [5]
and Bi2CuO4 [8]. The syntheses were carried out as solid state reactions in tubular
furnace in air or nitrogen atmospheres. Raw materials used for the syntheses were:
Li2O obtained by decomposition of commercial 90% Li2O2 (Aldrich), CaCO3 of pu-
rity grade 98% (BDH), BaO of purity 97% (Aldrich), SrO obtained by decomposition
of SrO2 (Aldrich), La2O3 purum (Fluka AG), and Bi2O3 from BiO(OH) decomposi-
tion (Chemapol Praga). Conditions of the syntheses are listed in Table 2.
Table 2 Conditions of the syntheses of oxocuprates
Raw material
Li2O+Cu2O
LiOHH2O+CuO
SrO+Cu2O
Molar ratio
1:1
Temp./°C
650
Reac. time/h Atmosphere Product
3
3
N2
air
N2
air
air
N2
air
N2
air
air
air
LiCuO
2:1
650
Li2CuO2
SrCu2O2
SrCuO2
1:1
750
12
5
SrO+CuO
1:1
900
SrO+CuO
2:1
900
5
Sr2CuO3
BaCu2O2
BaCuO2
LaCuO2
La2CuO4
Ca2CuO3
Cu(BiO2)2
BaO+Cu2O
BaO+CuO
1:1
800
5
1:1
950
3
La2O3+Cu2O
La2O3+CuO
CaCO3+CuO
Bi2O3+CuO
1:1
950
20
15
16
2
1:1
900
2:1
900
1:1
750
J. Therm. Anal. Cal., 60, 2000

KUCHARSKI, GONTARZ: SIMPLE OXOCUPRATES
221
For the sake of clarity the work has been divided into two parts. The first part has
been devoted to reduction, and the second to oxidation of the obtained oxocuprates.
For space-saving reasons only a limited number of the DTA curves and graphical re-
sults of the complex thermal analysis have been included.
The reactions of reduction were studied in a measuring setup that enabled to re-
cord thermal effects occurring as a functions of temperature. The reducing agent was
gaseous hydrogen diluted with nitrogen. The thermal effects were recorded by means
of a microprocessor device. All reductions were carried out at a constant rate of tem-
perature increase of 4.4°C min–1 under constant flow rate of hydrogen (2 dm3 min–1)
and nitrogen (0.8 dm3 min–1).
In the second part of the work the object of the studies were reactions of oxida-
tion of simple oxocuprates(I). The studies were based on complex thermal analysis
with the use of derivatograph Q 1500D (MOM, Budapest) and an X-ray phase analy-
sis with the use of HZG-3 Tur device (Germany) with CuKα radiation. The samples
were heated in air atmosphere.
Results and discussion
Reduction reactions
The following copper(II) compounds were subjected to reduction with hydrogen:
CuO, Li2CuO2, SrCuO2, BaCuO2, and (LaO)2CuO2.
The DTA curve obtained in reduction of CuO exhibits two effects with maxi-
mums at 196 and 312°C (Fig. 1a). The former is due to the reduction of CuO to Cu2O
in reaction:
2CuO + H2 → Cu2O + H2O
and the latter is connected with reaction:
Cu2O + H2 → 2Cu + H2O
Copper(II) oxide is reduced at the lowest temperature. Anionic sub-lattices con-
taining copper(II) are reduced at much higher temperatures.
The DTA curve of Li2CuO2 (Fig. 1c) is featured by two partly superposed ef-
fects with maximums at 409 and 440°C. At the first step the initial compound is re-
duced to LiCuO:
2Li2CuO2 + H2 → 2LiCuO + Li2O + H2O
At the second step, proceeding at a higher temperature, LiCuO is reduced to cop-
per metal:
2LiCuO + H2 → 2Cu + Li2O + H2O
A similar, two-step reduction is observed for BaCuO2 (Fig. 1d). At the first step,
with a maximum on DTA curve at 370°C, the compound is reduced to BaCu2O2:
2BaCuO2 + H2 → BaCu2O2 + H2O + BaO
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KUCHARSKI, GONTARZ: SIMPLE OXOCUPRATES
Fig. 1 DTA curves of reduction reactions; a – CuO; b – Cu2O; c – Li2CuO2;
d – BaCuO2; e – LiCuO; f – SrCu2O2
At the second step, with a maximum at 440°C, the cuprate(I) is reduced to cop-
per metal:
2BaCu2O2 + H2 → BaO + 2Cu + H2O
The reduction of SrCuO2 proceeds at temperature range 440–530°C, in a single
step reaction leading to formation of free copper.
SrCuO2 + H2 → SrO + Cu + H2O
A two-stage reaction is also observed in reduction of (LaO)2CuO2. In the first
stage, at 300°C, a Cu(I) compound is formed:
2La2CuO4 + H2 → 2LaCuO2 + La2O3 + H2O
The intermediate compound formed is further reduced at 520°C to give Cu metal:
2LaCuO2 + H2 → La2O3 + 2Cu + H2O
The reduction of two copper compounds containing the anionic sub-lattice of
CuO43− namely Ca2CuO3 and Sr2CuO3 proceeds in a single step reaction leading to
J. Therm. Anal. Cal., 60, 2000

KUCHARSKI, GONTARZ: SIMPLE OXOCUPRATES
223
formation of copper metal. The calcium salt is reduced within 400–500°C with a
maximum at 430°C.
Ca2CuO3 + H2 → 2CaO + Cu + H2O
and the strontium salt is reduced at 440–495°C:
Sr2CuO3 + H2 → 2SrO + Cu + H2O
Cu(BiO2)2, which comprises copper in the form of Cu2+ cations, is reduced by
hydrogen in a process with a maximum on DTA curve at 514°C to give free metals:
Cu(BiO2)2 + 4H2 → 2Bi + Cu + 4H2O
Reduction of copper(I) oxo-compounds by means of hydrogen proceeds at
higher temperatures. Copper(II) oxide is reduced by hydrogen with a maximum ef-
fect at 312°C. An oxide obtained from Cu(I) was reduced in a single step reaction
with a maximum at 267°C (Fig. 1b), and the one obtained by thermal decomposition
of CuO at 990°C was reduced at 340°C. These examples show that the reactivity of
Cu2O depends on the conditions of its synthesis.
The reduction of LiCuO proceeds according to the following reaction, with a
maximum at 430°C (Fig. 1e). The temperature, at which this reaction is effected, is
close to the temperature of reduction of this compound when obtained as an interme-
diate product in the reduction of Li2CuO2.
SrCu2O2 (Fig. 1f) is reduced to copper metal in the following reaction:
SrCu2O2 + H2 → SrO + 2Cu + H2O
The maximum of DTA effect due to this reaction is observed at 460°C, which is
close to a maximum temperature of SrCuO2 reduction. This is the reason why SrCu2O2 is
not formed in the reduction of SrCuO2.
BaCu2O2 is reduced by hydrogen in the following reaction having a maximum
on DTA curve at 420°C:
BaCu2O2 + H2 → BaO + 2Cu + H2O
The intermediate compound formed in the reduction of BaCuO2 is reduced by
hydrogen with a maximum on the DTA curve at 440°C.
In reduction of LaCuO2 the effect on DTA curve with a maximum at 485°C cor-
responds to the reaction:
2LaCuO2 + H2 → La2O2 + 2Cu + H2O
The effect on the DTA curve occurs at a lower temperature than that observed in
reduction of LaCuO2 obtained as an intermediate product in reduction of (LaO)2CuO2.
Oxidation reactions
All copper(I) compounds are oxidized by O2.
Thermogravimetric curves of LiCuO oxidation in the air are shown in Fig. 2.
The compound is oxidized in the temperature range of 180–440°C with a maximum
J. Therm. Anal. Cal., 60, 2000

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KUCHARSKI, GONTARZ: SIMPLE OXOCUPRATES
Fig. 2 TG, DTG, DTA curves of oxidation reaction of LiCuO
Fig. 3 TG, DTG, DTA curves of oxidation reaction of LiCuO (air and N2)
rate at 390°C and strong endothermic effect. The products contained Li2CuO2 and
CuO. The observed increase in mass of 9.2% correspomds to the following stoichio-
metry:
2LiCuO + 1/2O2 → Li2CuO2 + CuO
The oxidation products are stable up to 820°C. At higher temperatures two endo-
thermic effects are observed, with maximums at 840 and 880°C. In either case the
loss in mass was 1.8%. Analysis of the products obtained >880°C revealed the pres-
ence of Li2CuO2 and LiCu3O3, and the absence of CuO. Evidently it is due to the fol-
lowing reaction:
J. Therm. Anal. Cal., 60, 2000

KUCHARSKI, GONTARZ: SIMPLE OXOCUPRATES
225
4Li2CuO2 + 4CuO → 2Li2CuO2 + 2LiCu3O3 + Li2O + 1/2O2
At temperatures above 980°C the liberation of oxygen continues and the prod-
ucts react with each other to give LiCuO. The process is confirmed by the thermo-
gravimetric curve shown in Fig. 3. In this process a LiCuO sample was oxidized by
heating in the air up to 500°C and heating was then continued in nitrogen atmosphere.
Two effects of mass loss observed on heating in the air join together and the effects
are shifted toward lower temperatures. Maximums on the DTA curve appear at 780
and 820°C. In nitrogen atmosphere the reaction of LiCuO formation begins as early
as at 900°C and the sample mass comes back to its initial value.
Fig. 4 DTA curves of oxidation reaction of LiCuO (O2)
LiCuO was also oxidized in a stream of gaseous oxygen, with recording of the
DTA curve (Fig. 4). The oxidation started at 200°C and attained a maximum at
300°C. The product mixture contained, besides CuO and Li2CuO2, also a copper(III)
compound, namely Li3Cu2O4. The oxidation in pure oxygen proceeded according to
the following stoichiometry:
10LiCuO + 3O2 → 2Li2CuO2 + 2Li3Cu2O4 + 4CuO
Thermogravimetric analysis of BaCu2O2 (Fig. 5) has shown that the oxidation
proceeds in the temperature range 340–500°C, and the maximum of the exothermic
effect is attained at 460°C. The products obtained in the first stage of the reaction
contained copper(II) compounds: Ba2Cu3O6 and CuO. The reaction proceeded at
620°C according to the following equation:
4BaCu2O2 + 3O2 → 2Ba2Cu3O6 + 2CuO
The resulting system was thermally stable up to 860°C, at which a 2.3% loss in
mass began and had its end at 880°C. The reaction had an endothermic effect, with a
maximum at 820°C, due to the reduction of Cu(III).
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KUCHARSKI, GONTARZ: SIMPLE OXOCUPRATES
2Ba2Cu3O6 → 4BaCuO2 + 2CuO + O2
A following mass loss begins at 910°C and ends at 1000°C.
2CuO → Cu2O + 1/2O2.
Investigation of the thermal effects occurring during oxidation of BaCu2O2 has
shown that the reaction has the highest rate at 350°C, and the products have the com-
position BaCuO2+x
>
BaCu2O2 + xO2 → BaCuO2+x/2 + CuO
Fig. 5 TG, DTG, DTA curves of oxidation reaction of BaCu2O2
Fig. 6 TG, DTG, DTA curves of oxidation reaction of SrCu2O2
J. Therm. Anal. Cal., 60, 2000

KUCHARSKI, GONTARZ: SIMPLE OXOCUPRATES
227
The thermogravimetric analysis of LaCuO2 has shown that the oxidation pro-
ceeds within 340–580°C and the product obtained is stable up to 1000°C. The results
obtained, along with those of X-ray pwoder diffraction analysis have shown that the
reaction proceeds with the following stoichiometry:
2LaCuO2 + 1/2O2 → La2CuO4 + CuO
The oxidation of SrCu2O2 (Fig. 6) has shown that the reaction proceeds in a sin-
gle step within 340–500°C. The results of the X-ray phase analysis have proved that
the oxidation products are limited to SrCuO2 and CuO. Above 980°C a loss in mass is
observed. The X-ray phase analysis of a sample heated in the air up to that tempera-
ture and then rapidly cooled in nitrogen has revealed the presence of SrCuO2 as the
main phase, and certain amounts of Sr2CuO3 and SrCu2O2.
The oxidation of SrCu2O2 by pure oxygen is effected in a temperature range
300–600°C. The maximum of the thermal effect is observed at 320°C. The oxidation
products of the reaction are: Sr1.75Cu3O5.13, SrCu2O3 and SrO.
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