Hydrothermal Synthesis of Porous Al2O3/Al Metal Ceramics: III. Effect of the Thermal Dehydration of Bayerite on the Formation of a Porous Al2O3/Al Composite

Article abstract of DOI:10.1023/B:KICA.0000016116.77306.08

It was found that the diffusion permeability of a porous cover on aluminum particles increased as a result of the structural transformations of bayerite into active aluminum oxide on the thermal treatment of a porous Al(OH) ₃/Al composite. The cyclic treatment (each cycle included a treatment with water at 100°C and a thermal treatment at 550°C) eliminated diffusion limitations and resulted in the formation of porous Al ₂O₃/Al metal ceramics with a required set of adsorption-structure and mechanical properties. The following two competing processes were found to affect the formation of the mechanical properties of the synthesized material: the formation of new contacts in the course of the precipitation of aluminum hydroxide and the degradation of old contacts under the action of mechanical stresses that appear in the course of synthesis.

Full text of DOI:10.1023/B:KICA.0000016116.77306.08

Kinetics and Catalysis, Vol. 45, No.1, 2004, pp. 156–163. Translated from Kinetika i Kataliz, Vol. 45, No. 1, 2004, pp. 169–176.
Original Russian Text Copyright © 2004 by Rat’ko, Romanenkov, Bolotnikova, Krupen’kina.
Hydrothermal Synthesis of Porous Al O /Al Metal Ceramics:
2
3
III. Effect of the Thermal Dehydration of Bayerite
on the Formation of a Porous Al O /Al Composite
2
3
A. I. Rat’ko*, V. E. Romanenkov**, E. V. Bolotnikova*, and Zh. V. Krupen’kina*
*
Institute of General and Inorganic Chemistry, Belarussian Academy of Sciences, Minsk, 270072 Belarus
*
* Institute for the Advanced Training and Retraining of Personnel, Ministry of Education of the Republic of Belarus,
Minsk, Belarus
Received December 5, 2001; in final form, March 14, 2003
Abstract—It was found that the diffusion permeability of a porous cover on aluminum particles increased as a
result of the structural transformations of bayerite into active aluminum oxide on the thermal treatment of a
porous Al(OH) /Al composite. The cyclic treatment (each cycle included a treatment with water at 100°C and
3
a thermal treatment at 550°C) eliminated diffusion limitations and resulted in the formation of porous Al O /Al
2
3
metal ceramics with a required set of adsorption–structure and mechanical properties. The following two com-
peting processes were found to affect the formation of the mechanical properties of the synthesized material:
the formation of “new” contacts in the course of the precipitation of aluminum hydroxide and the degradation
of “old” contacts under the action of mechanical stresses that appear in the course of synthesis.
INTRODUCTION
EXPERIMENTAL
Thermograms, which illustrate thermal effects in
the course of structural transformations in the thermal
decomposition of bayerite, were obtained on a Q-500D
instrument (Paulik, Hungary) at a heating rate of
Previously [1, 2], we reported on the studies of the
kinetics of oxidation of aluminum powder with water
and the structure formation of a porous Al(OH) /Al
3
composite, proposed a mechanism of synthesis, and
calculated the diffusion coefficient of the hydroxo com-
5
K/min in air; the DTA, DTG, and TG curves were
recorded simultaneously. The electron-microscopic,
plexes of aluminum through a bayerite layer. We found adsorption, and texture characteristics and the
that the synthesis of the porous composite obeys the mechanical strength of synthesized samples; changes
in the conversion of aluminum with time; and the
phase composition of thermal degradation products
were determined with the use of procedures described
previously [1].
Wagner law, when the formation of a solid product in
the course of an oxidation reaction results in the inhibi-
tion of diffusion. The elimination of diffusion limita-
tions in the Al–Al(OH) –H O system is of considerable
interest because it provides an opportunity to control
the synthesis and to prepare porous metal ceramics with
required properties. It is well known [3–5] that the
dehydration of all modifications of aluminum hydrox-
ide occurs within the temperature range 300–550°C
3
2
The porousAl(OH) /Al composite samples as cylin-
3
ders (10 × 10 mm) were formed by oxidation with water
at 100°C for 2 h at atmospheric pressure. Next, they
were dried at 120°C for 30 min and calcined at 550°C
for 1 h until the complete conversion of bayerite into
active alumina. Thereafter, this cycle of sample treat-
with the formation of an active γ-Al O structure and a ment was repeated several times. Simultaneously, the
2
3
considerable change in the pore structure of the mate- evolution of the adsorption and texture characteristics,
the mechanical strength, and the microstructure of the
porous Al O /Al composite was studied after each
rial due primarily to an increase in the specific surface
area and the sorption pore volume. The bayerite layer
on aluminum particles would be expected to change its
adsorption–structure characteristics upon thermal
dehydration. Therefore, in this (third) paper, we report
the results of experimental studies on the effect of
bayerite structure transformations on the kinetics of
oxidation of aluminum powder and on the formation
2
3
cycle.
To determine the adsorption and texture characteris-
tics, the adsorption–desorption isotherms of benzene
vapor at 20°C were analyzed, which were obtained
gravimetrically in a vacuum system with the McBain–
Bakr quartz spring balance. The samples were pre-
heated at 140°C in a vacuum for 4 h. Experimental
of the structure and properties of a porous Al O /Al
2
3
points were measured to the relative pressure P/P =
s
composite.
0.9; next, extrapolation to P/P = 1.0 was performed.
s
0
023-1584/04/4501-0156 © 2004 MAIK “Nauka /Interperiodica”

HYDROTHERMAL SYNTHESIS OF POROUS Al O /Al METAL CERAMICS: III
157
2
3
3
V , cm /g
s
7
45
DTA
08 221
0
.25
5
30
1
0
0
0
0
.20
.15
.10
.05
3
10
0
200
400
600
800
1000
T, °C
Fig. 1. DTA curve of a bayerite precipitate.
The specific surface area, average pore radius, and
sorption pore volume were calculated from the adsorp-
tion isotherms with the use of the BET theory.
The total pore volume of the material synthesized
was determined by gravimetry; the samples were pre-
dried at 120°C for 1 h and then impregnated with water
for 0.5 h. The macropore surface area was determined
by mercury porosimetry using a PA-3M instrument.
RESULTS AND DISCUSSION
0
0.2
0.4
0.6
0.8
1.0
s
Structure Transformations of Bayerite
in the Course of Its Thermal Dehydration
P/P
The DTA curve of a flakelike precipitate of bayerite,
which was obtained by the oxidation of aluminum pow-
der with water with intense stirring, exhibits four endo
peaks within the temperature range 100–550°C
Fig. 2. Isotherm of benzene adsorption–desorption at 20°C
on active alumina obtained by the thermal treatment of a
bayerite precipitate at 550°C.
(
Fig. 1). These peaks due to the thermal decomposition
water. The second and third endo peaks (295 and
20°C) conceivably are the consequence of the decom-
of the precipitate were interpreted as described below.
At 108°C, the removal of physisorbed water occurred
4
position of bayerite and pseudoboehmite, respectively.
The fourth endo peak (660°C) is due to the melting of
aluminum. The conversions of bayerite into pseudo-
boehmite and of pseudoboehmite into γ-Al O are
(
a broad endo peak). Two endo peaks of different inten-
sities at 220 and 310°ë resulted from the thermal
decomposition of amorphous aluminum hydroxide and
bayerite, respectively. Finally, a small endo peak at
2
3
accompanied by an increase in the density of the porous
precipitate and, consequently, by a decrease in its vol-
ume. As a result, spots of the metal nucleus of a particle
become naked. Therefore, the first broad exo peak at
80°C and the second exo peak at 648°C are associated
with heat release in the oxidation of aluminum with
5
30°C resulted from the decomposition of pseudoboe-
hmite. As found by X-ray diffraction analysis, this
resulted in the formation of active alumina. Upon the
thermal treatment of the precipitate, the sorption vol-
ume and the specific surface area increased from 0.045
3
3
2
to 0.260 cm /g and from 53 to 368 m /g, respectively.
Such a pore-structure transformation is characteristic of atmospheric oxygen. Effects of this kind, which are a
aluminum hydroxides [5]; this transformation is due to consequence of structure transformations in the oxide
an increase in the true density of the material upon film and the oxidation of the naked spots of an alumi-
dehydration. The isotherm of benzene adsorption–des- num nucleus, were observed previously [1] in the oxi-
orption on the precipitate thermally treated at 550°C, dation of the parent ASD-1 aluminum powder in air.
which is shown in Fig. 2, can be assigned to type 2 Thus, structural transformations of aluminum hydrox-
according to the IUPAC Classification [6].
ide and the oxidation of aluminum metal simulta-
neously occur in the course of thermal treatment of the
Almost all of the thermal effects accompanying the
thermal treatment of the porous Al/(OH) /Al composite porous Al(OH) /Al composite in air. As mentioned
3
3
(
Fig. 3) are weakly pronounced, and they differ signifi- above, a lower intensity of peaks related to the thermal
cantly from those considered above. The first endo peak decomposition of bayerite and a shift of the tempera-
(113°C) is also due to the removal of physisorbed ture ranges of the conversion of various aluminum
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

1
58
RAT’KO et al.
decomposition of pseudoboehmite at 420–450°C.
9
50
Moreover, an endo peak due to the melting of residual
aluminum at 660°C was sometimes observed. The pre-
dominant phase formed in the course of the chemical
reaction of aluminum with water under hydrothermal
conditions was pseudoboehmite with an effective pore
DTA
3
80
648
4
20
radius of 3 nm and a specific surface area of up to
2
3
20 m /g. The above data suggest that the phase com-
2
95
position of aluminum hydroxides formed in the course
of hydrothermal synthesis primarily depends on the
reaction temperature. The concentration of bayerite in
the precipitated products increased as the synthesis
temperature was decreased, whereas the concentration
of boehmite increased with temperature. This corre-
sponds to the phase diagram of the Al O –H O system
6
60
1
13
200
0
400
600
800
1000
T, °C
2
3
2
Fig. 3. DTA curve of the porousAl(OH) /Al composite syn-
thesized at 100°C for 2.5 h.
[11].
3
In the thermal decomposition of gibbsite prepared
by colloid-chemical methods [3], a small endo effect at
2
00°C was due to the partial degradation of the gibbsite
structure. In the region of the second endo effect at
75°C, gibbsite was completely decomposed to form
boehmite. A small amount of crystalline γ-Al O
hydroxide species are observed because of the superpo-
sition of thermal effects due to the above parallel pro-
cesses and the low concentration of bayerite in the
3
2
3
porous Al(OH) /Al composite.
3
appeared simultaneously with the boehmite. In the
The phase transformations of aluminum hydroxides region of the third endo effect (500°C), the structure of
in the course of the thermal treatment of a porous com- the boehmite completely decomposed, and it was con-
posite synthesized under hydrothermal conditions were verted into an amorphous state. The thermal decompo-
studied [7–10]. Anan’in et al. [7] found that monophase sition of aluminum hydroxide can occur in several
boehmite (AlOOH) was formed at T ≥ 180°C, whereas steps; in this case, the removal of water molecules cor-
responds to each of the endothermic effects (two mole-
cules for gibbsite or one molecule for boehmite). Usu-
ally, the endothermic effects of the dehydration of gibb-
site, which is structurally similar to bayerite in many
respects [11], correspond to temperatures of 225–275,
the presence of aluminum hydroxide (Al(OH) ) (no
3
more than 20%) in reaction products was observed at a
lower temperature. Tikhov with coauthors [8, 9], who
used thermal analysis, X-ray diffraction, and IR spec-
troscopy, found that, in the course of hydrothermal syn-
thesis at 150–250°C for 0.5–6.5 h, an amorphous layer
of boehmite (AlOOH) was formed on the surface of
aluminum particles; this boehmite was converted into
crystalline γ-Al O upon the thermal treatment of the
2
00–300, and 510–550°C. Thus, an amorphous
hydroxide, bayerite, gibbsite, pseudoboehmite, or boe-
hmite can be formed depending on the temperature at
which the porous composite ceramics were synthe-
sized. At temperatures of ≤100°C, bayerite of gibbsite
is formed, the dehydration of which always initially
resulted in the formation of boehmite as a transient
phase in the formation of active γ-Al O .
2
3
material. The desorption of water and the thermal
decomposition of boehmite were accompanied by endo
effects at 120 and 520°C, respectively, whereas the
structural rearrangement of the oxy hydroxide was
accompanied by an exo effect at 320…330°C. The ther-
2
3
Table 1 summarizes the results of a study on the
mal-analysis curve of reaction products obtained by the effect of the thermal decomposition of bayerite on the
oxidation of aluminum powder in a free volume [10] adsorption and texture characteristics of the porous
exhibited three endothermic peaks of water release: the composite. Thermal treatment, which caused the trans-
removal of physisorbed (nonstructure) water at 120°C, formation ofAl(OH) /Al intoAl O /Al, almost doubled
3
2
3
the decomposition of boehmite at 230–290°C, and the the sorption pore volume and increased the specific sur-
Table 1. Effect of thermal treatment on the adsorption and texture characteristics of the porous composite
3
2
Sorption pore volume (V ), cm /g
Specific surface area (S ), m /g Average pore radius (r), nm
sp
s
Synthesis
duration, h
Al(OH) /Al
Al O /Al
Al(OH) /Al
Al O /Al
Al(OH) /Al
Al O /Al
2 3
3
2
3
3
2
3
3
2
0.035
0.035
0.04
0.05
0.058
0.08
0.08
26
28
34
36
28.3
45
2.0
2.3
2.4
1.9
3.0
2.8
2.8
3.05
3
5
7
.5
57
0.04
58
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROTHERMAL SYNTHESIS OF POROUS Al O /Al METAL CERAMICS: III
159
2
3
α
α
0
0
0
0
0
.28
0.6
0
0
0
.5
.4
.3
.24
.20
.16
.12
0.2
0
.1
0
1
00 200 300 400 500 600
1
2
3
4
n
T, °C
Fig. 4. Dependence of the degree of aluminum conversion
Fig. 5. Dependence of the degree of aluminum conversion
on the number of cycles including thermal treatment and
oxidation with water.
on the temperature of thermal pretreatment of the porous
Al(OH) /Al composite.
3
face area by 10 to 60%. The average pore radius, which
Figure 4 demonstrates the experimental dependence
was calculated using the equation r = 2V /S , was only of the conversion of aluminum on pretreatment temper-
s
sp
slightly affected by this treatment.
ature. The initial porous Al(OH) /Al composite with a
3
conversion of 0.14 was thermally treated in air under
stationary conditions for 1 h at 100–600°C. Thereafter,
it was treated with water at 100°C for 2 h at atmospheric
pressure. The conversion of the initial sample on the
The bayerite layer on aluminum particles is a porous
membrane semipermeable to diffusing species (water
molecules and the hydroxo complexes of aluminum). A
change in the structure of this membrane on thermal
decomposition is primarily related to an increase in the initial horizontal portion of the curve remained almost
porosity. At the little affected effective pore radius, this unchanged. This fact suggests that the thermal treat-
is equivalent to an increase in the number of pores per ment of the composite at 400°C cannot overcome diffu-
unit surface area of the membrane, that is, to an
increase in its permeability.
sion limitations and does not activate the chemical reac-
tion. An increase in the temperature of thermal treat-
ment to 500°C resulted in a stepwise increase in the
conversion α to ~0.23 and a twofold increase of α =
Effect of the Thermal Decomposition of Bayerite
on the Rate of Oxidation of the Porous
Composite with Water
0
.28 after thermal treatment at 600°C. A comparison of
the dependence obtained with the results of thermal
analysis demonstrates the coincidence between the
temperature range of pretreatment that results in the
activation of the chemical reaction and the temperature
The autoclave synthesis of a porous composite
exhibits undeniable advantages in terms of the control
of a chemical reaction and the obtaining of a required ranges of bayerite conversion into active alumina. This
aluminum conversion into hydroxides with different suggests the direct dependence of the chemical interac-
phase compositions because it provides an opportunity tion of aluminum powder with water on the pore struc-
to increase the temperature. In turn, the temperature is
responsible for dissolution, reprecipitation, and struc-
tural evolution processes in the precipitate, which pro-
vide a continuous access of water to the reaction sur-
face [7–9, 12, 13]. The capabilities of the synthesis at
ture of covers on aluminum particles. Consequently, the
pore structure of active alumina formed upon the ther-
mal decomposition of bayerite is no barrier for water
diffusion to the reaction surface.
1
00°C are considerably restricted by a rate-limiting
In a cyclic treatment, when each cycle included ther-
mal treatment at 550°C for 1 h and treatment with water
at 100°C for 2 h, the linear dependence of the conver-
sion on the number of cycles n was observed (Fig. 5).
This is likely due to increased access to the surface of
aluminum not only because of the formation of active
alumina upon the thermal dehydration of bayerite but
also as a consequence of oxide cover degradation under
the action of mechanical stresses, which appeared in
step, the diffusion supply of water to the surface of alu-
minum and the counterdiffusion of the hydroxo com-
plexes of aluminum through the growing porous layer
of bayerite to the deposition surface. However, the
above studies demonstrated that thermal dehydration is
responsible for a significant structural rearrangement of
the porous cover on aluminum particles. Consequently,
this process can be used not only as a final operation for
producing active alumina but also as a technique for
increasing the diffusion permeability of deposited the porous composite as the volume of the solid phase
products. increased.
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

1
60
RAT’KO et al.
Table 2. Dependence of the adsorption and texture characteristics of the porous Al O /Al composite and the oxide phase on
2
3
the degree of conversion
Degree P/P at the be-
3
2
Adsorption volume, cm /g
Specific surface area (S ), m /g
sp
s
Average pore
radius, nm
of conversion (α), ginning of
at the beginning
total
micro- and meso-
macropores
pores
%
hysteresis
of hysteresis
0
.01
.09
.14
.23
.30
.35
.42
.50
0.20
0.20
0.20
0.20
0.22
0.20
0.24
0.24
0.005
0.025
16.0
5.0
2.0
0
0
0
0
0
0
0
0.009 (0.1)
0.025 (0.167)
0.026 (0.113)
0.035 (0.117)
0.040 (0.114)
0.042 (0.1)
0.052 (0.104)
0.029 (0.32)
0.076 (0.507)
0.077 (0.335)
0.102 (0.34)
0.103 (0.294)
0.104 (0.248)
0.146 (0.292)
52.2
10.4
2.0
2.05
2.0
72.8 (485)
82.0 (356)
89.4 (298)
106.0 (303)
109.2 (260)
149.5 (299)
13.3 (89)
17.5 (76)
26.4 (88)
27.0 (77)
28.0 (67)
29.5 (59)
2.3
2.0
1.9
1.95
Note: Calculated values for the oxide phase are given in parentheses.
Evolution of the Texture Characteristics of the Porous
Composite Depending on the Degree of Conversion
ning of hysteresis was almost independent of the degree
of conversion. It is well known [3] that a decrease in the
coordination number and an increase in the diameter of
the primary particles that form the adsorbent structure
shift a hysteresis loop to the right (i.e., toward a higher
relative pressure of adsorbate vapor) and decrease its
width. The absence of a shift of the beginning of a hys-
Table 2 summarizes the results of a study on the
dependence of the adsorption and texture characteris-
tics of the porous Al O /Al composite, as well as the
2
3
calculated values for only the oxide phase (in parenthe-
ses), on the degree of conversion or the number of
cycles. Each cycle included treatment with water at
teresis loop toward higher values of P/P resulted from
s
the uniform packing (the same coordination number) of
the primary particles in all of the test samples regard-
less of the degree of conversion. The average pore
radius was also independent of conversion, and it was
approximately the same in all the samples; the set of
1
00°C for 2 h and thermal treatment at 550°C for 1 h.
As can be seen in Table 2, the adsorption volume of the
Al O /Al composite (both the total volume and the vol-
2
3
umes at the initial points of hysteresis loops) increased
with aluminum conversion, whereas the adsorption vol-
ume of the oxide phase, in both cases, initially these data suggests that the pore structure and the
increased, passed through a small maximum at α = mechanism of its formation remained unchanged with
0
.14, and then stabilized. The value of P/P at the begin- increasing conversion. The specific surface area and the
s
6
3
3
3
V × 10 , m /g
V × 10 , m /kg
s
0
0
0
0
0
0
0
.14
.12
.10
.08
.06
.04
.02
0
0
0
0
.15
.10
.05
0
1
2
1
2
0
.2
0.4
0.6
0.8
1.0
s
4
5
6
7
8
9
10
P/P
Pore diameter, nm
Fig. 6. Isotherms of benzene adsorption–desorption at 20°C
on the porous Al O /Al composites with the conversions
Fig. 7. Integrated mesopore-size distribution curves for the
2 3
(2) 0.14.
Al O /Al composites with the conversions α = (1) 0.5 and
2
3
α = (1) 0.50 and (2) 0.14.
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROTHERMAL SYNTHESIS OF POROUS Al O /Al METAL CERAMICS: III
161
2
3
×
1000
(
‡)
×
1000
(
b)
Fig. 8. Electron micrographs of the Al O /Al composites with the conversions α = (a) 0.14 and (b) 0.50.
2
3
sorption volume of the porous ceramics increased with
conversion; this was due to an increase in the weight of adsorption on the porous Al
Figure 6 demonstrates the isotherms of benzene
O
/Al composite with con-
versions of 0.5 and 0.14. A portion of isotherms at
P/P = 0.8–1.0 is almost parallel to the axis of P/P ; this
2
3
the resulting active alumina. The dependence of the
specific surface area and sorption volume on the degree
of conversion was practically linear; this fact provides
support for the hypothesis of the removal of diffusion
limitations because of the conversion of aluminum
hydroxide into active alumina. The adsorption and tex-
ture properties of the porous composite depend on the
s
s
fact is indicative of almost complete adsorption at this
value of the relative pressure of the adsorbate. A hyster-
esis loop is characteristic of the adsorption–desorption
process over the entire rage of ascending isotherms. At
P/P ≈ 0.2, polymolecular adsorption changed to capil-
s
lary condensation. At the initial stage of desorption, an
oxide phase formed because of the thermal decomposi- isotherm declined smoothly; next, at P/P ≈ 0.5, it dra-
tion of the precipitated bayerite.
s
matically declined to the connection with an adsorption
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

1
62
RAT’KO et al.
σ, MPa
in the initial aluminum powder. As the degree of con-
4
3
2
1
0
0
0
0
version increased, the volume of the resulting oxide
phase increased so that the consolidation of the porous
material under the action of mechanical stresses took
place; this consolidation was accompanied by the deg-
radation of porous covers around aluminum particles.
In this case, the initially formed contacts were
destroyed and new contacts were formed. As a result,
an agglomerate of particles was formed; the size and
shape of these particles were essentially different from
the initial ones (Fig. 8b). The fraction of aluminum
metal considerably decreased, and the volume of ultra-
macropores decreased; the volume of micropores and
mesopores increased. The structure of the porous mate-
rial approached the structure formed by the methods of
colloid chemistry in its characteristics. The total pore
volume of the samples decreased somewhat from 0.45
0
0.1
0.2
0.3
0.4
0.5 α
Fig. 9. Dependence of the mechanical strength of the porous
Al O /Al composite on the degree of conversion.
2
3
3
to 0.34–0.38 cm /g as the degree of conversion
isotherm. This loop is typical of adsorbents with open
capillaries having approximately the same effective
radius (for example, aluminosilicates). The shape of the
isotherms of benzene vapor adsorption on the test mate-
rial (the slopes of adsorption–desorption branches, the
increased from 0.14 to 0.5.
Figure 9 illustrates the dependence of the compres-
sion strength of the porous Al O /Al composite on the
2
3
degree of conversion of aluminum. The strength
increased from 10 to 40 MPa with conversion and then
remained practically unchanged. Evidently, the
strength of composite metal ceramics depends on the
quantity and quality of contacts between aluminum
particles, as well as on the structure and properties of
these contacts, which are formed in the course of pre-
cipitation from solution followed by thermal decompo-
sition. The occurrence of two competing processes (the
formation of “new” contacts in the course of hydroxide
precipitation from solution and the mechanical degra-
dation of “old” contacts) resulted in the fact that an
increase in the mechanical strength of composite
ceramics was completed at the instant of the onset of
the intense consolidation of a porous solid.
width of a hysteresis loop, and the value of P/P before
s
hysteresis) remained unchanged with changes in the
concentration of active alumina. The only difference
consisted in the sorption pore volumes. The isotherms
can be assigned to type 2 according to the IUPAC Clas-
sification [6].
Based on the isotherms of adsorption, the pore-size
distribution was calculated with the use of the Thomp-
son equation (Fig. 7). As can be seen in Fig. 7, the plots
are uniform for porous Al O /Al composites with dif-
2
3
ferent conversions; they exhibit no maximum charac-
teristic of commercial active alumina. The pore size lies
within a very narrow range of 4.5–7.5 nm. This is due
to special features of the formation, diffusion, and pre-
cipitation of hydroxo complexes. The hydroxo com-
plexes of aluminum or clusters are formed in a
restricted volume: the pore volume of bayerite. They
are deposited as nanoparticles onto a solid support: the
growing layer of bayerite. This support is a stabilizer,
which almost completely restricts the mobility of the
nanoparticles and prevents agglomeration processes
Thus, we studied in detail the structure formation of
the porous Al(OH) /Al composite using a cyclic treat-
3
ment method. We found that this treatment provides an
opportunity to control the conversion of aluminum
because intermediate thermal treatment increases the
diffusion permeability of a porous cover on aluminum
particles. We found that the limiting sorption volume
and the specific surface area of the composites
increased in the subsequent cycles of treatment because
of the growth of an oxide cover without significant
structural changes. We also noted that the following
two competing processes affected the formation of the
strength properties of the material synthesized: the for-
[
14].
Effect of the Cyclic Treatment on the Ultramicropore
Structure and Mechanical Strength
of the Porous Composite
Electron-microscopic studies of the samples synthe-
sized allowed us to distinguish different steps in the for-
mation of the composite depending on the degree of
conversion. Initially, aggregation occurred without a
detectable change in the surface morphology of parti-
mation of new contacts in the course ofAl(OH) precip-
3
itation and the degradation of previously formed con-
tacts under the action of mechanical stresses. Thus, the
cles with the formation of weakly pronounced contacts cyclic treatment allowed us to control the degree of
Fig. 8a). Individual particles retained their structural conversion and to form a porous material with the
identity; their size and shape were almost the same as required properties.
(
KINETICS AND CATALYSIS Vol. 45 No. 1 2004

HYDROTHERMAL SYNTHESIS OF POROUS Al O /Al METAL CERAMICS: III
163
2
3
REFERENCES
7. Anan’in, V.N., Belyaev, V.V., Romanenkov, V.E., et al.,
Vestsi Akad. Navuk BSSR, Ser. Khim. Navuk, 1987, no. 5,
p. 17.
1
2
3
. Rat’ko, A.I., Romanenkov, V.E., Bolotnikova, E.V., and
Krupen’kina, Zh.V., Kinet. Katal., 2004, vol. 45, no. 1,
p. 154.
8
. Tikhov, S.F., Sadykov, V.A., Potapova,Yu.V., et al., Stud.
Surf. Sci. Catal., 1998, vol. 118, p. 797.
9
. Tikhov, S.F., Salanov, A.N., Palesskaya, Yu.V., et al.,
React. Kinet. Catal. Lett., 1998, vol. 64, no. 2, p. 301.
. Rat’ko, A.I., Romanenkov, V.E., Bolotnikova, E.V., and
Krupen’kina, Zh.V., Kinet. Katal., 2004, vol. 45, no. 1,
p. 162.
10. Yakerson, V.I., Dykh, Zh.L., Subbotin, A.N., et al.,
Kinet. Katal., 1995, vol. 36, no. 6, 918.
. Ermolenko, N.F. and Efros, M.D., Regulirovanie poris-
toi struktury okisnykh adsorbentov i katalizatorov (Con-
trol of the Porous Structure of Oxide Adsorbents and
Catalysts), Minsk: Nauka i Tekhnika, 1971.
1
1. Physical and Chemical Aspects of Adsorbents and Cata-
lysts, de Boer J.H. and Linsen, B.G., Eds., London: Aca-
demic, 1970.
1
2. Tikhov, S.F., Fenelonov, V.B., Sadykov, V.A., Pota-
pov, Yu.V., and Salanov, A.N., Kinet. Katal., 2000,
vol. 41, no. 6, p. 907 [Kinet. Catal. (Engl. Transl.),
vol. 41, no. 6, p. 826].
4
. Anderson, J., Structure of Metallic Catalysts, London:
Academic, 1975.
5
. Dzis’ko, V.A., Karnaukhov, A.P., and Tarasova, D.V.,
Fiziko-khimicheskie osnovy sinteza okisnykh kataliza-
torov (Physicochemical Fundamentals for the Synthesis
of Oxide Catalysts), Novosibirsk: Nauka, 1978.
1
3. Tikhov, S.F., Zaikovskii, V., Fenelonov, V.B., Pota-
pova, Yu.V., Kolomiichuk, V.N., and Sadykov, V.A.,
Kinet. Katal., 2000, vol. 41, no. 6, p. 916 [Kinet. Catal.
(Engl. Transl.), vol. 41, no. 6, p. 835].
6
. Gregg, S.J. and Sing, K.S.W., Adsorption, Surface Area, 14. Suzdalev, T.P. and Suzdalev, P.I., Usp. Khim., 2001,
and Porosity, London: Academic, 1967.
vol. 70, no. 3, p. 203.
KINETICS AND CATALYSIS Vol. 45 No. 1 2004