Unusual approaches to the preparation of heterogeneous catalysts and supports using water in subcritical and supercritical states

Article abstract of DOI:10.1023/A:1010496830238

The prospective ways of using water in sub- and supercritical states for the preparation of nanocrystal oxide catalysts Ce_0.5Zr_0.5O₂, Ce_0.1Y_xZr_0.9-xO₂, Zr_1-xY_xO₂, Zr_1-xIn_xO₂, La₂CuO₄, supported catalysts Pd/Rh/ZrO₂ and Pd/Rh/TiO₂, and supports CeO₂, ZrO₂, TiO₂ are discussed. The proposed method is characterized by high productivity, is ecologically friendly, and allows one to obtain multicomponent oxide catalysts with chemical and phase composition and properties that can be changed within large ranges. However, the content of carbon and nitrogen in the samples was much higher than expected. It could be caused by either the presence of unreacted starting materials in prepared oxides or the adsorption of nitric or acetic acids formed during the hydrolysis of metal salts. The La₂CuO₄ sample prepared in supercritical water showed unusual catalytic properties in the reaction of CO oxidation in a pulse microcatalytic system. In the case of the stoichiometric CO and oxygen mixture, the activity of the supercritical sample increased ～ 2.5 times in comparison with ceramic lanthanum cuprate due to its considerably larger surface area. If the pulses of the reagents were separated in time, the CO oxidation rate on the supercritical sample increased ～ 8 times due to the higher oxygen mobility.

Full text of DOI:10.1023/A:1010496830238

Kinetics and Catalysis, Vol. 42, No. 2, 2001, pp. 154–162. Translated from Kinetika i Kataliz, Vol. 42, No. 2, 2001, pp. 172–181.
Original Russian Text Copyright © 2001 by Galkin, Kostyuk, Kuznetsova, Turakulova, Lunin, Polyakov.
Unusual Approaches to the Preparation
of Heterogeneous Catalysts and Supports Using Water
in Subcritical and Supercritical States1
A. A. Galkin*, B. G. Kostyuk*, N. N. Kuznetsova*,
A. O. Turakulova*, V. V. Lunin*, and M. Polyakov**
*
Department of Chemistry, Moscow State University, Moscow, 119899 Russia
*
* Nottingham University, Nottingham, England, United Kingdom
Received August 25, 2000
Abstract—The prospective ways of using water in sub- and supercritical states for the preparation of nanoc-
rystal oxide catalysts Ce0.5Zr0.5O , Ce Y Zr O , Zr Y O , Zr In O , La CuO , supported catalysts
2
0.1
x
0.9 – x
2
1 – x
x
2
1 – x
x
2
2
4
Pd/Rh/ZrO and Pd/Rh/TiO , and supports CeO , ZrO , TiO are discussed. The proposed technique is charac-
2
2
2
2
2
terized by high productivity. It is also ecologically friendly and enables one to obtain multicomponent oxide
catalysts with chemical and phase composition and properties that can be changed within large ranges. The
physicochemical properties of sub- and supercritical water are discussed. A brief review of the present studies
on the use of critical media in various physicochemical processes is given.
1
In 1822, the French chemist baron Cagniard de erties. Let us trace the changes in the properties of a sol-
Latour discovered the critical point of a substance vent which take place when approaching the critical
while performing experiments with different solvents point, using water as an example. It should be noted
at high temperatures and pressures [1]. The supercriti- that because of the high values of the critical parame-
cal state of a substance was defined as a state when ters (see Table 1), water in the critical state is not widely
there is no difference between the liquid and gas used yet. But, at the same time, it is the cheapest, safest,
phases. After this discovery, the chemistry of supercrit- and most ecologically friendly solvent.
ical media has been developing rather slowly, and even
0 years later, in 1896, Willard wrote the first detailed
The properties of pure water and water solutions
depend strongly on the temperature and pressure.
Changes in the thermodynamic properties are reflected
in the phase diagrams of water and appropriate multi-
component systems. These changes are associated with
changes in the dielectric constant, electric conductivity,
ionic strength, and the structure of hydrogen bonds.
Finally, changes in the viscosity, density, heat capacity,
and diffusion coefficients result in changes of the trans-
port properties of aqueous solutions. It should be noted
that all the above-mentioned properties do not alter in a
stepwise manner, when approaching the critical point.
Even at 300°C and 220 atm, water is similar to acetone,
and in the critical state water is a nonpolar solvent that
can be mixed with oxygen and hydrocarbons without
any limits.
Let us consider the density of water as an example
to trace the changes in the properties when approaching
the critical state. In the subcritical region, water can
exist both in the liquid and gas states, which are sepa-
rated by a sharp border. At a constant temperature,
while changing the pressure, the density of water varies
continuously within the intervals of the existence of
these two phases. The discontinuity of density is
observed only at the gas–liquid interface. In the critical
state, this interface disappears, and the density of water
7
review devoted to the properties of a number of sub-
stances in a supercritical state [1]. In the 20th century,
the trend to enhance the efficiency and ecological
safety of various chemical processes stimulated intense
studies of the properties of diverse substances under
supercritical conditions. Nowadays supercritical liq-
uids are widely used in the processes of extraction and
chromatographic separation of chemicals, the produc-
tion of various materials, and the abatement of toxic
liquid wastes [1, 2]. Besides, new possibilities of using
supercritical liquids in organic and inorganic syntheses,
as well as in homogeneous and heterogeneous catalysis
are being developed [3–6]. The characteristics of the
critical state (temperature, T ; pressure, P ; and den-
cr
cr
sity, ρ ) for most frequently used solvents are given in
cr
Table 1 [3, 7]. As can be seen from the table, these char-
acteristics are considerably different: they range from
9
°C and 50 atm for ethylene up to 374°C and 220 atm
for water.
The permanently growing interest in supercritical
liquids arises from their unique physicochemical prop-
1
Proceedings of the Seminar in Commemoration of professor
Yu.I. Ermakov, Novosibirsk.
0
023-1584/01/4202-0154$25.00 © 2001 MAIK “Nauka /Interperiodica”

UNUSUAL APPROACHES TO THE PREPARATION
155
3
is approximately 0.3 g/cm [7]. Water possesses unlim- Table 1. Critical parameters of several solvents [3, 7]
ited compressibility near the critical point; that is why
Solvent
Tcr, °C
Pcr, atm
ρcr, g/cm3
even insignificant changes in temperature or pressure in
this region allows one to change considerably the den-
sity in a wide range. For instance, raising the tempera-
ture by 6°C at 220 atm halves the density of water [8].
Other characteristics, such as dielectric constant, ionic
strength, viscosity, and heat capacity change in parallel
with the changes in density. Therefore, we can operate
with the properties of water as a solvent and control the
physicochemical processes which take place in water
only by changing in a controllable way the water den-
sity in sub- and supercritical states.
C H
9.1
16.5
50.41
58.40
73.75
48.84
72.55
113.50
61.37
220.60
0.214
1.110
0.468
0.203
0.452
0.235
0.276
0.322
2
4
Xe
CO2
30.9
C H
32.2
2
6
N O
36.4
2
NH3
132.3
240.7
373.9
The ionic strength and the dielectric constant are the
most important characteristics for preparing individual
and composite oxide catalytic systems in sub- and
supercritical water. These properties determine the rates
of hydrolysis of inorganic salts in water with further for-
mation of the metal oxides and hydroxides. An increase
in the ionic strength of water in the subcritical state, with
a simultaneous decrease in its dielectric conductivity,
favors the hydrolysis of inorganic compounds [9].
The experiments in sub- and supercritical water are
usually performed both in batch reactors and flow-type
systems. The batch reactor represents an autoclave
fixed inside a powerful heater and equipped with a pres-
sure gauge. Since the discovery of the critical state of
substances in the 19th century, the majority of the
experiments in sub- and supercritical media have been
performed in batch reactors. However, this system is
not suitable for the preparation of oxide systems from
the corresponding inorganic salts for many reasons. For
example, the hydrolysis of acetates leads to the forma-
tion of acetic acid. Its partial decomposition is followed
by an incontrollable increase of the pressure in the sys-
tem. Many multicomponent oxide systems cannot be
prepared in batch reactors because of different hydrol-
ysis rates for the metal oxide components composing
inorganic salts. Therefore, in this case, gradual auto-
clave heating results in the separate formation of indi-
vidual oxides. Besides, the materials prepared in batch
systems are usually characterized by a nonuniform par-
ticle size distribution.
C H OH
2
5
H O
2
Fig. 1d), a mixture of metallic copper and cuprous
oxide (at the molar ratio salt : acid = 1 : 2, Fig. 1c), or
a mixture of copper(I) and copper(II) oxides (at the
molar ratio salt : acid = 3 : 1, Fig. 1b). The content of
each phase is determined by the amount of oxalic acid
added and, thus, can easily be controlled. Such a tech-
nique can be very efficient for the preparation of cata-
lysts and magnetic materials with a specified concen-
tration of paramagnetic centers. However, in any other
case of the preparation of oxide systems, it is better to
use flow-type reactors. Then, we can obtain nanocrystal
materials with a large surface area, which is hard to
achieve using conventional methods. Besides, flow
type systems exhibit other advantages in comparison
with autoclaves: the experimental conditions can easily
be controlled and the productivity of the process is
rather high.
The group of Professor Arai (Tohoku University,
Japan) pioneered the synthesis of nanocrystal metal
oxides in sub- and supercritical water. Presently, indi-
vidual oxides of Ce, Zr, Co, Ni, Fe, Al, and Ti are pre-
pared in this laboratory using a flow-type reactor.
Adshiri (a member of Arai’s team) synthesized several
composite oxides such as BaO · 6Fe O , Al (Y +
2
3
5
The main advantage of synthesis in a batch reactor
in comparison with flow-type systems is the possibility
to control the oxidation states of the elements and to
prepare systems with the desirable ratio of phases which
contain one element in different oxidation states. For
example, copper hydroxycarbonate decomposes into
copper(II) oxide (see the diffraction pattern in Fig. 1a):
Tb) O , and LiCoO in basic media [10]. Recently, the
3
12
2
group of Professor Polyakov (Nottingham University,
England) prepared a number of composite nanocrystal
oxides with the composition Ce1 – xZr O in a flow-type
x
2
reactor in supercritical water without any basic addi-
tives [11].
The aim of our work was to develop a technique that
allows the preparation of composite oxide nanocrystal
(
CuOH) CO
2CuO + CO + H O.
2 2
2
3
Oxalic acid under the same conditions decomposes catalytic systems using a flow-type reactor and that
with the formation of carbon monoxide:
makes it possible to control the phase composition and
physicochemical properties of the catalysts. The activ-
ity and selectivity of heterogeneous catalysts are known
H C O CO + CO + H O.
2
2
4
2
2
The combination of these two reactions in supercritical to be a function of their chemical composition and their
water (T = 385°C and P = 350 atm) allowed us to obtain micro- and macrostructure. The defectiveness of the
metallic copper (in the case of an excess of oxalic acid, crystal and ceramic structure, the surface structure as
KINETICS AND CATALYSIS Vol. 42
No. 2 2001

1
56
GALKIN et al.
(
(
(
(
‡)
b)
c)
d)
CuO
Cu O
2
Cu
3
0
35
40
45
50
55
60
65
70
θ, deg
2
Fig. 1. XRD patterns of copper-containing systems obtained in supercritical water in a batch reactor: (a) CuO, (b) CuO + Cu O,
2
(
c) Cu O + Cu, and (d) Cu. The experimental conditions are given in the text.
2
well as the nature of active sites, and the concentration
The conditions of all experiments are summarized
and distribution of admixed phases depend strongly on in Table 2. Zirconium oxide and composite materials
the synthesis conditions and further thermal treatment based on zirconium oxide were prepared in supercriti-
of the catalysts. Therefore, we believe that the use of cal water at 380°C and 250 atm. The residence time was
sub- and supercritical water will enable us both to pre- 4.5 s. Cerium and titanium oxides as well as Pd–Rh cat-
pare new catalysts and to vary the properties of tradi- alysts supported on titanium oxide were prepared in
tional catalysts and supports.
subcritical water. In this case, the residence time was
.5–2 times higher. The salt concentration 0.2 mol/l
was shown to be the optimal for the reaction mixture.
The feed rates were 10 and 5 ml/min for water and a
solution of metal salts, respectively.
1
The scheme of the flow-type system is shown in
Fig. 2. The high-pressure pump is used to supply water
(
10 ml/min) into the system of powerful heaters where
it is heated up to a desired temperature (Table 2). The sec-
ond pump is used to supply an aqueous solution of water
It was mentioned previously that the hydrolysis of a
salts (feed rate, 5 ml/min; concentration, 0.2 mol/l). Mix- metal salt in sub- and supercritical water is followed by
ing these two flows at the entrance to the reactor results the formation of a colloid solution of corresponding
in the rapid hydrolysis of the metal salts with further oxides. The microphotograph of zirconium oxide pre-
dehydration of hydroxides according to the following pared by the evaporation of such a colloid solution is
scheme:
presented in Fig. 3. The size of zirconium oxide parti-
cles is as low as 3–5 nm. All the prepared samples show
a nanocrystal structure with a narrow particle size dis-
tribution. The maximum size of the particles is 7–9 nm
in the case of titanium oxide systems.
ML + xH O
M(OH) + xHL,
x
2
x
M(OH)x
MOx/2 + 0.5xH O,
2
–
3
–
By varying the experimental conditions, we can
change the particle size of the synthesized materials.
For example, a temperature rise from 230 to 380°C dur-
where L = NO , ëç ëéé , M = Ce, Zr, Ti, Cu, Y, In,
Pd, and Rh.
3
After that the colloid solution of metal oxides is ing the synthesis of TiO leads to a simultaneous increase
2
cooled to room temperature, separated, and evaporated in the size of oxide particles from 7–9 to 16–18 nm. The
on the rotor. A constant pressure in the system is kept specific surface area, at the same time, decreases from
2
using a back-pressure valve.
200 to 50 m /g. The processes of titanium oxide recrys-
KINETICS AND CATALYSIS Vol. 42
No. 2 2001

UNUSUAL APPROACHES TO THE PREPARATION
157
Heater 2
Tmax = 500°C
R
e
a
c
t
o
r
Metal salt
solution pump
Heater 1
Tmax = 300°C
Water
pump
Pressure
controller
Heating
2
0 nm ZrO2
Colloid
solution of oxides
Cooling
Fig. 2. The scheme of the flow-type reaction system operat-
ing in sub- and supercritical conditions.
Fig. 3. Electronic micrographs of ZrO prepared in super-
2
critical water.
tallization most probably occur at elevated tempera-
tures, thus resulting in the growth of the particles. The
same phenomena are observed when the salt concentra-
tion increases.
tions up to 10%. The synthesis in supercritical condi-
tions is likely followed by the incorporation of only a
part of indium atoms into the lattice of zirconium
oxide, and individual indium oxide is formed in parallel
with the formation of the binary system Zr1 – xIn O .
Let us analyze the phase composition of oxides pre-
pared in sub- and supercritical water. The XRD patterns
of Ce, Ti, and Zr oxides are shown in Fig. 4. Zirconyl
nitrate, cerium ammonium nitrate, and titanium isopro-
poxyacetate were used as precursors. According to the
XRD analysis, the crystallization of titanium oxide in
subcritical water occurs with the formation of the tet-
x
2
The situation is much better in the case of the Zr–Y
system (Fig. 5). An yttrium concentration of 10% is
sufficient for the predominant formation of the tetrago-
nal modification of ZrO . The content of the mono-
2
ragonal modification, the lattice of CO has cubic sym- Table 2. Experimental conditions for the preparation of ox-
2
metry, and zirconium oxide forms a mixture of the tet- ide-type catalysts in sub- and supercritical water
ragonal and monoclinic modifications with the ratio
approximately 1 : 1.
P, atm ρ, g/cm3
Reaction
time, s
Sample
T, °C
The tetragonal and cubic modifications of ZrO are
2
known to be metastable at normal conditions [12]. At
the same time, the problem of the stabilization of such
structures at low temperatures is of a great importance,
especially in the case of oxidation catalysts, because
the higher symmetry of the crystal lattice of an oxide
favors the mobility of oxygen in the volume of the cat-
ZrO2
380
380
380
380
380
380
380
330
230
230
250
250
250
250
250
250
250
250
250
250
0.446
0.446
0.446
0.446
0.446
0.446
0.446
0.681
0.847
0.847
4.5
4.5
4.5
4.5
4.5
4.5
4.5
6.8
8.5
8.5
Zr1 – xIn O
x
2
Zr1 – xY O
x
2
alyst. A number of transition and alkali-earth metals are Ce0.5Zr0.5O2
used for the stabilization of the tetragonal and cubic
ZrO modifications. Several experiments on the prepa-
Ce0.1YxZr0.9 – xO2
2
ration of binary oxide systems, containing indium,
yttrium, and cerium in addition to zirconium were per-
formed in supercritical water with the purpose of
revealing the possibility of such stabilization.
1
1
% Rh–Pd/ZrO2
0% Rh–Pd/ZrO2
An increase in the indium content in the case of the CeO2
Zr–In systems results in the growth of the ratio between
TiO2
the tetragonal and monoclinic modifications of ZrO2.
Nevertheless, the complete stabilization of the tetrago-
1
% Rh–Pd/TiO2
nal structure is not achieved even at indium concentra-
KINETICS AND CATALYSIS Vol. 42 No. 2 2001

1
58
GALKIN et al.
TiO2
CeO2
ZrO2
70
2
0
25
30
35
40
45
50
θ, deg
55
60
65
75
80
2
Fig. 4. XRD patterns of Ce, Ti, and Zr oxides obtained in supercritical water.
clinic phase is rather low and we hope that further opti- tendency to sintering. The XRD patterns of three ZrO2
mization of experimental conditions will enable us to samples prepared in (a) supercritical water and after
exclude the formation of the monoclinic modification.
thermal treatment at (b) 600 and (c) 900°ë for 1 h are
presented in Fig. 6. It can be seen that the calcination of
the sample at 600°ë does not change the phase compo-
sition and surface area. Even after thermal treatment at
It is quite possible that the formation of the cubic
ZrO phase instead of the tetragonal modification
2
explains the shape of the XRD patterns presented in
Fig. 5. Such significant line broadening makes difficult
the unequivocal identification of the XRD patterns.
Additional information can be obtained by Raman
spectroscopy. Using this technique, we demonstrated
the principal possibility of the stabilization of the cubic
phase in supercritical water for a number of multicom-
9
00°ë, the XRD lines are rather wide, which is typical
for small oxide particles. The specific surface area
2
diminishes two times and is only 90 m /g. Even such a
low value of the surface area is considerably higher
than the specific surface area of the zirconium oxide
systems prepared by precipitation with ammonia from
zirconium-containing solutions. The content of the tet-
ponent oxide catalysts based on ZrO . Cubic symmetry
2
ragonal modification of ZrO significantly increases
is typical for Ce–Zr solid solutions with a metal ratio of
2
after calcination at 900°ë. Therefore, the stabilization
of the metastable tetragonal modification is observed at
much lower temperatures in the case of the samples
prepared in supercritical conditions.
1
: 1, Zr–Rh systems containing 10% Rh, and ternary
Ce–Zr–Y systems containing 10% Ce and 10% Y. All
these composite oxides are single-phase nanocrystal
systems with a developed specific surface area.
Thus, the preparation of oxide catalysts in sub- and
supercritical water enables one to obtain nanocrystal
homogeneous materials with a high surface area. How-
ever, this technique has also one significant disadvan-
tage. The elemental analysis of all prepared samples is
shown in Table 3. As can be seen from Table 3, the con-
tent of carbon and nitrogen in the samples is much
higher than expected. It can be caused by either the
presence of unreacted starting materials in prepared
oxides or the adsorption of nitric or acetic acids formed
during the hydrolysis of metal salts.
The specific surface areas of all prepared oxide cat-
alysts are given in Table 3. Titanium oxide and Rh–Pd
catalysts supported on titanium oxide have the maxi-
2
mum surface area (approximately 200 m /g). The spe-
cific surface area of the systems based on zirconia is
2
varied in the range 140–170 m /g. For CÂO , the sur-
2
2
face area does not exceed 110 m /g. It should be
emphasized that all the samples under discussion are
crystalline systems, and it is almost impossible to
obtain catalysts with such a high surface area by using
traditional techniques such as coprecipitation, impreg-
nation, sol–gel synthesis, etc.
To clarify the origin of this phenomenon, we per-
It is important that oxides prepared in supercritical formed a number of thermal gravimetric analyses for
water display high thermal stability and have almost no several samples. The results are presented in Fig. 7. The
KINETICS AND CATALYSIS Vol. 42
No. 2
2001

UNUSUAL APPROACHES TO THE PREPARATION
159
Zr0.9Y0.1O2
Zr0.95Y0.05O2
Zr0.97Y0.03O2
ZrO2
2
0
25
30
35
40
45
50
θ, deg
55
60
65
70
75
80
2
Fig. 5. Dependence of the phase composition of ZrO samples prepared in supercritical water on the yttrium content.
2
DTG curve of zirconium oxide (upper curve) exhibits tion of nitric acid. The lower curve in Fig. 7 corre-
two peaks in the temperature range up to 600°ë corre- sponds to the decomposition of zirconyl nitrate, which
sponding to weight loss at 140 and 420°ë. The first was used as a precursor. The maximum rate of its
peak is likely the result of water elimination. The sec- decomposition is observed at 225°ë. The absence of
ond weight loss can be explained by either the decom- this maximum in the DTG curve of ZrO indicates that
2
position of unreacted zirconium nitrate or the desorp- the increased nitrogen content can be accounted for by
Table 3. Specific surface areas and the content of carbon, hydrogen, and nitrogen in the catalysts prepared in sub- and su-
percritical conditions
Element content, %
2
Sample
S, m /g
C
H
N
ZrO2
163–172
135–154
147–161
153
Not found
0.22–0.32
0.14–0.44
Not found
0.25–0.44
0.12
0.09–0.89
0.37–0.55
0.73–0.84
0–0.14
0.85–1.67
1.45–1.84
1.54–2.09
1.06–1.13
1.49–1.51
0.98
Zr1 – xIn O
x
2
Zr1 – xY O
x
2
Ce0.5Zr0.5O2
Ce0.1Y Zr0.9 – xO2
115–164
158
0.31–0.61
0.24
x
1
% Rh–Pd/ZrO2
0% Rh–Pd/ZrO2
1
141–159
110
0.16–0.30
Not found
0.41–3.64
2.52–4.27
0.63–0.80
Not found
0–0.65
0.97–1.11
1.78–1.82
Not found
0–1.20
CeO2
TiO2
189–201
195–213
1
% Rh–Pd/TiO2
0.32–0.64
KINETICS AND CATALYSIS Vol. 42 No. 2 2001

1
60
GALKIN et al.
(c)
(b)
(a)
2
0
25
30
35
40
45
50
θ, deg
55
60
65
70
75
80
2
Fig. 6. XRD patterns of ZrO samples: (a) fresh sample prepared in supercritical water, (b) after thermal treatment at 600 and
2
(c) 900°C.
the adsorption of nitric acid rather than by the presence temperature-programmed reduction (TPR) curves of
of unreacted zirconyl nitrate. Thermal treatment of all three samples of zirconium oxide and rhodium and pal-
the obtained oxide samples at 600°ë results in the com- ladium zirconia-based systems containing 10% of each
plete elimination of all carbon and nitrogen species metal are shown in Fig. 8. It is known from the litera-
from the surface according to the data of elemental ture [13] that ZrO cannot be reduced at temperatures
2
analysis.
below 1000°ë, but in our case, three steps of reduction
are observed in this region, and the first reduction peak
is observed at 232°ë. After introducing rhodium in the
catalyst, the consumption of hydrogen increases con-
siderably, and this fact is in agreement with the litera-
ture data [14]. The most interesting results are found in
the case of palladium-containing samples, where a suf-
ficient hydrogen uptake begins at room temperature,
and at 70°ë, when the desorption of hydrogen begins.
Presently we are studying the catalytic and physico-
chemical properties of oxide systems prepared in sub-
and supercritical water. The preliminary data testify
that such a method of catalyst preparation allows one to
vary the properties of the catalysts in a wide range. The
1
The contents of Zr3+ ions in the samples prepared
either by using supercritical water or by the traditional
coprecipitation technique are given in Table 4. The Zr
1
020°C 1180°C
3
+
1
40°C
cations are considered to be the active sites in the Fi-
scher–Tropsch process [15]. It is well known that the
4
20°C
2
3
+
content of Zr cations in zirconium oxide prepared by
traditional techniques is very low even after thermal
vacuum treatment and increases only after the introduc-
tion of modifying additives [16]. However, in the case
8
5°C
450°C
3
+
of the supercritical samples, the concentration of Zr
cations is rather high even in nonactivated samples, and
it is ten times higher than that in the Ce–Zr–Y systems
prepared by coprecipitation. The content of Zr ions in
the supercritical samples increases considerably after
the introduction of the modifying additives. The con-
centration of paramagnetic sites in the Ce–Zr system is
2
25°C
3+
T
Fig. 7. DTG curves of (1) ZrO sample prepared in super-
critical water and (2) zirconyl nitrate.
2
KINETICS AND CATALYSIS Vol. 42
No. 2
2001

UNUSUAL APPROACHES TO THE PREPARATION
294
511
826
161
1
13
7
0
(a)
1
70
4
70
(b)
1
129
1
238
2
96
632
2
32
(c)
0
300
600
900
1200 T, °C
Fig. 8. TPR curves for zirconium-based samples prepared in supercritical water: (a) 10% Pd/ZrO , (b) 10% Rh/ZrO , and (c) ZrO .
2
2
2
Temperatures (°C) of maximum hydrogen consumption (desorption) are shown above the curves.
two orders higher than in the Ce–Zr–Y samples pre-
pared by conventional methods.
TPR curves of two La CuO samples, which were
2 4
prepared either by (a) a ceramic technique or (b) by
using supercritical water are presented in Fig. 9. In both
cases, the first peak of hydrogen consumption is con-
nected with the elimination of weakly bonded oxygen
from the volume of the catalyst, and the second one
corresponds to the phase decomposition of the sample
followed by the formation of copper and lanthanum
oxides. The total hydrogen uptake is the same for both
catalysts, but in the case of the supercritical sample, the
elimination of weakly bonded oxygen is observed at a
temperature 100°ë lower and the process occurs faster
than in the case of ceramic lanthanum cuprate. This
opens up new possibilities for the preparation of effec-
tive catalysts in supercritical water, which can be used
as the sources of active oxygen.
The use of supercritical water is especially effec-
tive for the preparation of perovskite-type catalysts.
Perovskites are usually prepared by calcination at
1
000–1100°C [17]. The specific surface area of such
catalysts is usually rather low, and thus they cannot be
effectively used in real catalytic processes. Several
other techniques such as sol–gel synthesis, coprecipita-
tion, spray drying, and cryochemical techniques, which
allow one to diminish the temperatures characteristic
for precursor decomposition, are labor-consuming and
unprofitable on an industrial scale.
Using lanthanum cuprate as an example, we had
shown that the preparation of perovskite-type oxides in
supercritical water made it possible to increase the sur-
face area 50 times, as well as to optimize oxygen trans-
fer processes in the volume of the catalyst and to
improve its catalytic properties in the oxidation reac-
tions [17].
The increase of oxygen mobility in the supercritical
sample of La CuO is explained by the smaller particle
2
4
size in the catalyst due to the use of a more homoge-
neous precursor obtained in supercritical water and
lower calcination temperatures.
It was shown that the La CuO sample prepared in
2
4
In the case of lanthanum cuprate, the phase of the supercritical water reveals unusual catalytic properties
perovskite type is not formed directly in supercritical in the reaction of CO oxidation in a pulse microcata-
conditions. The hydrolysis of a stoichiometric solution lytic system. In the case of the stoichiometric CO and
of copper and lanthanum acetates yields a highly dis- oxygen mixture, the activity of the supercritical sample
persed precursor, which contains copper oxide and lan- increases ~2.5 times in comparison with ceramic lan-
thanum hydroxide. The calcination of this precursor at thanum cuprate due to its considerably larger surface
as low a temperature as 600°ë results in the formation area. If the pulses of the reagents are separated in time,
of homogeneous lanthanum cuprate.
the rate of CO oxidation on the supercritical sample
KINETICS AND CATALYSIS Vol. 42 No. 2 2001

1
62
GALKIN et al.
ical state mixes with oxygen and hydrocarbons without
(
‡)
any restrictions, therefore, homogeneous and heteroge-
neous catalytic reactions in sub- and supercritical water
could be of great interest. Moreover, catalysts prepared
in nonequilibrium sub- and supercritical conditions
would reveal proper stability in supercritical condi-
tions.
1
00°C
ACKNOWLEDGMENTS
We would like to thank L.N. Ikryannikova and
G.L. Markaryan for their assistance in the investigation
of the samples by ESR spectroscopy. This work was
supported by Russian Foundation for Basic Research
and INTAS, joint project no. IR-97-0402.
(
b)
REFERENCES
1
. McHugh, M.A. and Krukonis, V.J., Supercritical Fluid
Extraction: Principles and Practice, Boston: Butter-
worth–Heinemann, 1994.
2
. Pisharody, S.A., Fisher, J.W., and Abraham, M.A., Ind.
Eng. Chem. Res., 1996, vol. 35, no. 12, p. 4471.
3
4
5
. Baiker, A., Chem. Rev., 1999, vol. 99, p. 453.
. Savage, P.E., Chem. Rev., 1999, vol. 99, p. 603.
. Jessop, P.G., Ikariya, T., and Noyori, R., Chem. Rev.,
5
00
600
700
800
T, °C
1
999, vol. 99, p. 475.
Fig. 9. TPR curves of the La CuO sample prepared by
6. Adschiri, T., Kanazawa, K., and Arai, K., J. Am. Ceram.
Soc., 1992, vol. 75, no. 4, p. 1019.
2
4
(
a) ceramic technique and (b) using supercritical water.
7
. Van Eldik, R. and Hubbard, C.D., Chemistry under
Extreme or Non-Classical Conditions, NewYork: Wiley-
Spectrum, 1997.
increases ~8 times because of the higher oxygen mobil-
ity [17].
8
. Schmidt, E., Properties of Water and Steam in SI-Units,
Therefore, the use of supercritical water creates pro-
spective ways for the preparation of a variety of new
catalysts and supports. Note that water in the supercrit-
Berlin: Springer, 1969.
9. Kritzer, P., Boukis, N., and Dinjus, E., J. Supercrit. Flu-
ids, 1999, vol. 15, p. 205.
1
0. Adschiri, T., Proc. 5th Int. Symp. on Supercritical Flu-
ids, Atlanta, 2000.
Table 4. The results of the investigation of Zr-containing
samples by ESR spectroscopy
1
1. Cabanas, A., Darr, J.A., Lester, E., and Piliakoff, M.,
3
+
17
[
Zr ] × 10 , spin/g
Chem. Commun., 2000, no. 7, p. 901.
I. Oxides obtained in supercritical water
12. Murav’eva, G.P., Fionov, A.V., Lunina, E.V., et al., Dokl.
Akad. Nauk, 2000, vol. 371, no. 1, p. 52.
nonactivated
activated*
1
3. Hoang, D.L., Berndt, H., and Lieske, H., Catal. Lett.,
ZrO2
2.5
1.1
2.6
3.8
3.6
2.0
1995, vol. 31, nos. 2–3, p. 165.
Ce0.1Y0.06Zr0.84O2
14. Guglielminotti, E., Giamello, E., Pinna, F., et al.,
J. Catal., 1994, vol. 146, no. 2, p. 422.
Ce0.1Y0.1Zr0.8O2
2.9
Ce0.5Zr0.5O2
11.0
15. Wen, L., Yuanqi, Y., Liangbo, F., and Peiju, Z., Chin.
Chem. Lett., 1996, vol. 7, no. 3, p. 269.
II. Oxides prepared by coprecipitation [16]
1
6. Markaryan, G.L., Ikryannikova, L.N., Muravieva, G.P.,
ZrO2
Not found
Not found
Not found
0.1
et al., Colloids Surf. A, 1999, vol. 151, p. 435.
Ce0.1Y0.06Zr0.84O2
1
7. Galkin, A.A., Kostyuk, B.G., Lunin, V.V., and Poliakoff, M.,
Angew. Chem. Int. Ed. Engl., 2000, vol. 39, no. 15,
p. 2738.
*
Standard thermal vacuum treatment: first stage, T = 550°C, air,
–
5
2
h; second stage, T = 550°C, P = 10 torr, 2 h.
KINETICS AND CATALYSIS Vol. 42
No. 2
2001