Preparation and microstructural and textural characterization of single-phase aluminum oxides

Article abstract of DOI:10.1023/B:KICA.0000044984.09163.80

Conditions for the preparation of single-phase η-, -, and χ-aluminas were determined. The structures of η- and χ-aluminas were characterized. With the use of high-resolution electron microscopy, it was found that η-Al₂O₃ particles exhibited the most developed {111} face and consisted of coherently joined domains with a pronounced platelet shape. Planar defects in the (111) plane occurred in the η-Al₂O₃ particles. Microstructural differences between single-phase η-Al₂O₃ and γ-Al₂O ₃ with a defect spinel structure were revealed. It was found that the η-Al₂O₃, χ-Al₂O₃, and -Al₂O₃ oxides are characterized by uniformly porous structures with average pore diameters of 47, 55, and 110 A, respectively.

Full text of DOI:10.1023/B:KICA.0000044984.09163.80

Kinetics and Catalysis, Vol. 45, No. 5, 2004, pp. 714–721. Translated from Kinetika i Kataliz, Vol. 45, No. 5, 2004, pp. 754–762.
Original Russian Text Copyright © 2004 by Kul’ko, Ivanova, Litvak, Kryukova, Tsybulya.
Preparation and Microstructural and Textural Characterization
of Single-Phase Aluminum Oxides
E. V. Kul’ko*, A. S. Ivanova*, G. S. Litvak*, G. N. Kryukova*, and S. V. Tsybulya**
* Boreskov Institute of Catalysis, Siberian Division, Russian Academy of Sciences, Novosibirsk, 630090 Russia
** Novosibirsk State University, Novosibirsk, 630090 Russia
Received August 7, 2003
Abstract—Conditions for the preparation of single-phase η-, θ-, and χ-aluminas were determined. The struc-
tures of η- and χ-aluminas were characterized. With the use of high-resolution electron microscopy, it was
found that η-Al2O3 particles exhibited the most developed {111} face and consisted of coherently joined
domains with a pronounced platelet shape. Planar defects in the (111) plane occurred in the η-Al2O3 particles.
Microstructural differences between single-phase η-Al2O3 and γ-Al2O3 with a defect spinel structure were
revealed. It was found that the η-Al2O3, χ-Al2O3, and θ-Al2O3 oxides are characterized by uniformly porous
structures with average pore diameters of 47, 55, and 110 Å, respectively.
INTRODUCTION
that has been prepared by various procedures. The thermal
genesis of hydroxides depends on a number of factors,
including preparation conditions, phase homogeneity,
chemical purity, and particle size. According to Sato [1],
coarse and fine trihydroxide particles (in particular, bayer-
Alumina occupies one of the leading places among the
most important industrial inorganic substances. It is
widely used as a catalyst and in complex catalyst compo-
nents, supports, and adsorbents. Alumina is primarily pre-
pared by the thermal treatment of aluminum hydroxide ite or gibbsite) undergo dehydration as follows:
Bayerite
coarse bayerite particles
boehmite
η-
γ-
η-
θ-
δ-
θ-
α-Al2O3
θ- + α-
α-Al2O3
α-Al2O3
small bayerite particles
α-Al2O3.
Gibbsite
coarse gibbsite particles
boehmite
χ-
γ-
χ-
κ-
δ-
κ-
α-Al2O3
θ- + α-
small gibbsite particles
α-Al2O3.
Sato [1] believed that the observed reactions are due
to the development of intraglobular hydrothermal con-
ditions, which are favorable for boehmite formation, in
coarse bayerite and gibbsite particles. Consequently,
the resulting oxide is a mixture of low-temperature
η-Al2O3 or χ-Al2O3 and γ-Al2O3 oxides for bayerite or
gibbsite, respectively.
EXPERIMENTAL
Bayerite was prepared by the precipitation of a solu-
tion of aluminum nitrate (high-purity grade) with a con-
centrated aqueous ammonia solution (high-purity
grade) at constant pH 10.0 0.2 and at 22 2°ë, fol-
lowed by keeping the suspension (aging) at room tem-
perature. After aging, the precipitate was filtered off
The aim of this work is to prepare various single- and washed with distilled water until nitrates were
phase alumina modifications by the thermal decompo- absent from the filtrate. Samples with different aging
sition of bayerite and gibbsite and to characterize their times were prepared. The samples aged for 10 and 24
microstructure and texture.
days are designated B-1 and B-2, respectively.
0023-1584/04/4505-0714 © 2004 MAIK “Nauka /Interperiodica”

PREPARATION AND MICROSTRUCTURAL AND TEXTURAL CHARACTERIZATION
715
Table 1. Chemical composition and specific surface area of parent aluminum hydroxides
Chemical composition, wt %
Calcination loss (110–900°C)
Hydroxide
Ssp, m2/g
Fe
Si
Na
K
%
mol H2O/(mol Al2O3)
B-1*
B-2*
G-1*
G-2*
<0.001
<0.001
0.02
0.08
0.08
–
–
–
34.5
34.5
34.2
34.0
2.99
2.98
2.95
2.92
38
14
–
–
0.18
0.18
0.001
0.001
0.9
16
0.02
–
* The treatment temperature was 110°C.
Gibbsite was taken as two samples, G-1 and G-2, [3]; the error of the method was 10%; texture charac-
which differed in particle size. G-1, which contained teristics were calculated from low-temperature (−196°ë)
particles of size 20–45 µm, was prepared by screening nitrogen adsorption isotherms, which were obtained on
commercial alumina hydrate (gibbsite) using a Micromeritics ASAP-2400 system.
vibrosieves. G-2, which contained particles of size 1–5 µm,
was prepared by grinding G-1 particles on a planetary
mill for 280 min at a rate of 2.5 rps.
RESULTS AND DISCUSSION
Because the real structure of various aluminum
oxide species essentially depends on the chemical
purity of the parent hydroxides, we used high-purity
reagents with impurity contents of 0.005% or lower for
the synthesis of bayerite. The results of the chemical
analysis of the test samples demonstrated that the bay-
erite was of relatively high purity. It can be seen (Table 1)
that iron and silicon, whose concentrations were
<0.001 and 0.08 wt %, respectively, were the impurity
constituents of bayerite. The total concentration of
impurities in gibbsite (Table 1) was much higher than in
with bayerite. As would be expected [4], iron (0.02 wt %)
and sodium (0.18 wt %) were the gibbsite constituents;
the potassium content was low and equal to 0.001 wt %.
On average, the bayerite and gibbsite samples con-
tained 2.99 and 2.95 mol H2O/(mol Al2O3), respec-
tively (Table 1), which is close to the theoretical con-
centration of constituent water (3.0 mol H2O/(mol Al2O3))
in these hydroxides. The concentration of constituent
water in sample G-2 was somewhat lower.
All the hydroxide samples were dried at 110°C for a
day and calcined under isothermal conditions at differ-
ent temperatures: samples B-1 and B-2 were calcined at
600, 800, 900, and 1000°C for 4 h in an air atmosphere
(AA) (in this work, an air atmosphere refers to calcina-
tion with a fan turned on in a muffle furnace) and addi-
tionally at 900°C for 10 h in an air atmosphere or with-
out it (with a fan turned off in the muffle furnace); sam-
ples G-1 and G-2 were calcined at 500, 600, 800, 900,
and 1000°C for 4 h/AA. Sample G-1 was additionally
calcined at 500°C for 10 h/AA, and sample G-2 was
calcined at 880, 920, and 940°C for 4 h/AA and at
900°C for 12 h/AA.
The parent hydroxides were analyzed for impurities
using atomic absorption spectrometry [2]. The concen-
tration of constituent water was calculated from calci-
nation loss, which was determined as the ratio of the
weight difference between a sample dried at 110°C for
a day and this sample calcined at 900°C for 4 h to the
weight of the initial sample.
The specific surface areas of the hydroxides (Table 1)
decreased with the aging time of bayerite and upon
going from a relatively small fraction to a coarser frac-
tion in gibbsite. Electron-microscopic data support the
fact that more coarsely dispersed bayerite was formed
as the duration of aging was prolonged. According to
these results (Fig. 1a), in sample B-2, aged for 24 h,
platelike particles ~70 Å in thickness were formed,
which took the shape of a hourglass or truncated cones,
whose size was as large as 3500 Å; in sample B-1, aged
for 10 h, particles of size 1500 Å were mainly observed
(Fig. 1b). This allows us to assume that they will
undergo different thermal transformations.
The thermal analysis of the samples was performed
on a Q 1500-D derivatograph over a temperature range
from 20 to 1300–1320°C at a heating rate of 7.5 K/min
in an air atmosphere; the sample weight was 0.5 g;
weight losses were determined with an accuracy of
0.5%. The X-ray diffraction analysis of samples was
performed on a Bruker D-8 diffractometer using mono-
chromatic CuKα radiation (λ = 1.5418 Å) over the angle
range 5°–80° (2θ). Electron-microscopic studies were
performed on a JEM-2010 transmission electron micro-
scope (the resolving power of the instrument was
14 nm, and the accelerating voltage was 200 kV) and a
JEM-100CX instrument (resolution, 30 nm; accelerat-
ing voltage, 100 kV).
The temperature-programmed heating of parent
The specific surface areas of the parent hydroxides hydroxides in an air atmosphere was accompanied by
were determined by the thermal desorption of argon the appearance of endo and exo effects in DTA curves
KINETICS AND CATALYSIS Vol. 45 No. 5 2004

716
KUL’KO et al.
DTG
(‡)
(a)
1315°C
DTA
1280°C
100°C
í, °ë
300°C
~
~
~
~
215
TG, mg
DTG
1280°C
(b)
65 nm
(b)
90°C
190°C
DTA
505°C
480°C
240°C
í, °ë
300°C
~ ~
~ ~
175
TG, mg
Fig. 2. TG, DTG, and DTA curves for bayerite: (a) B-1 and
(b) B-2.
to the crystallization of α-Al2O3; note that, at the spec-
ified temperature, the transition of metastable alumi-
num oxide species from the boehmite series into
α-Al2O3 was also detected [6]. Consequently, the ther-
mal genesis of B-2 includes the formation of boehmite
and η-Al2O3 followed by the formation of a mixture of
γ-Al2O3 and η-Al2O3.
65 nm
Fig. 1. Electron micrographs of samples (a) B-2 and (b) B-1.
The DTA curve of G-1 (Fig. 3a) exhibited endo
effects at 210–240–325°ë, due to hydroxide dehydra-
tion, with the formation of boehmite and a low-temper-
ature aluminum oxide. As in the case of bayerite, an
endo effect at 520°ë was due to boehmite dehydration
with the formation of γ-Al2O3. High-temperature exo
effects at 1240°ë were related to the crystallization of
α-Al2O3. In contrast to G-1, which was discussed
above, the thermal behavior of sample G-2 was as fol-
lows (Fig. 3b): the DTA curve exhibited endothermic
effects at 115, 310, and 500°ë, which were due to the
removal of physically adsorbed water, trihydroxide
dehydration, and the crystallization of a low-tempera-
ture aluminum oxide, respectively. This sample did not
exhibit endothermic effects at 210–240°ë, and an
endothermic effect at 500°ë was insignificant (weight
loss was 1.75%); therefore, it is believed that trihydrox-
(Fig. 2). Endo effects at 90–100°ë (B-1, B-2) were due
to the removal of physically adsorbed water. A compar-
ison between the thermal curves of B-1 and B-2
(Figs. 2a, 2b) demonstrated that they underwent differ-
ent transformations. B-1 dehydrated with an endother-
mic effect at 300°ë, which corresponds to the forma-
tion of a low-temperature aluminum oxide species. A
high-temperature exo effect at 1315°ë was due to the
crystallization of α-Al2O3. The dehydration of B-2 was
characterized by the occurrence of two endothermic
peaks at 240 and 300°ë; therefore, according to pub-
lished data [1], it is believed that the following two
phases were formed: boehmite and a low-temperature
aluminum oxide, η-Al2O3. This hypothesis was sup-
ported by the fact that a further increase in the temper-
ature was accompanied by the appearance of an endot-
hermic peak at 480–505°ë, which was observed in the
dehydration of boehmite with the formation of γ-Al2O3 ide dehydration in this sample occurred through a sin-
[5, 6].A high-temperature exo effect at 1280°ë was due gle-step reaction path without boehmite formation.
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PREPARATION AND MICROSTRUCTURAL AND TEXTURAL CHARACTERIZATION
717
DTG
440
400
311
í, °ë
1070°C
DTA
1240°C
520°C
220°C
222
220
111
422
511
1
2
325°C
173
~ ~
TG, mg
~ ~
(‡)
í, °ë
DTG
DTA
10
20
30
40
50
60
70
2θ, deg
500°C
115°C
Fig. 4. Diffraction patterns of bayerite calcined at 600°C for
4 h: (1) B-1 and (2) B-2.
310°C
~
~
71.5
TG, mg
~
~
(b)
Fig. 3. TG, DTG, and DTA curves for gibbsite: (a) G-1 and
(b) G-2.
440
1
400
2
3
311
222
220
111
4
511
1
2
5
6
3
10
20
30
40
50
60
70
2θ, deg
10
20
30
40
50
60
70
2θ, deg
Fig. 5. Diffraction patterns of various alumina modifica-
tions: (1) γ-Al O (600°C), (2) η-Al O (600°C), and
Fig. 6. Diffraction patterns of B-1 calcined at (1) 600,
(2) 800, (3) 900, and (6) 1000°C for 4 h and (4, 5) at 900°C
for 10 h in a flow of air (4) or with no flow of air (5).
2
3
2 3
(3) χ-Al O (600°C).
2
3
Thus, the above data indicate that the thermal genesis of was favorable for the formation of oxide structures, the
bayerite and gibbsite depends on particle size.
genesis of which is illustrated in Figs. 4–7 and Table 2.
The results of the thermal analysis of the test sam-
Samples B-1 and B-2 calcined at 600°ë were
ples were supported by X-ray diffraction (XRD) data.A η-Al2O3. Both of these samples exhibited similar dif-
comparison of XRD data showed that, based on the set fraction patterns (Fig. 4). Differences were observed in
of interplanar spacings, samples B-1 and B-2 consisted the shape and intensity of the (111) peak: it was less
of well-crystallized bayerite, the coherent-scattering symmetrical and less pronounced in B-2. This line
region (CSR) of which was >1000 Å. The isothermal shape (the superposition of two components, a rela-
treatment of these samples at different temperatures tively narrow diffraction maximum and a much-broad-
KINETICS AND CATALYSIS Vol. 45 No. 5 2004

718
KUL’KO et al.
α
α
sample B-2 (Table 2). Sample calcination at 900°ë
α
χ
facilitated further redistribution of the intensities of
corresponding lines (Fig. 6, curve 3) and the onset of θ-
Al2O3 crystallization in B-1; in this case, crystallization
in an air atmosphere occurred more slowly than in the
absence of air (Fig. 6, curves 4, 5). At 1000°ë, the sam-
ples were θ-Al2O3 (Fig. 6, curve 6; Table 2), and they
were very similar to the sample calcined at 900°ë for
10 h without air (Fig. 6, curve 5).
α
α
κ
α
κ
κ
α
κ
α
α
4
3
Thus, the formation of metastable aluminum oxide
species from bayerite depends on particle size. It can be
seen (Table 2) that the particle size affected not only the
phase composition of the resulting oxides (η-Al2O3 or
η-Al2O3 + γ-Al2O3) but also the rate of their transfor-
mations. In the case of B-2, the formation of θ-Al2O3
occurred at a lower temperature as compared with B-1.
In turn, it was found previously [6] that the higher the
degree of dehydroxylation in samples from the boeh-
mite series, the slower the processes of structural rear-
rangement. Consequently, in samples from the B-2
series, the degree of dehydroxylation was lower than
that in samples from the B-1 series; that is, dehydroxy-
lation occurred more effectively in small trihydroxide
particles. Moreover, we experimentally found that the
formation of aluminum oxide species from bayerite
depends not only on the temperature but also on the
time and atmosphere of calcination. The set of experi-
ments performed allowed us to determine the condi-
tions required for the preparation of single-phase η-
Al2O3 and θ-Al2O3.
χ
χ
χ
2
1
10
20
30
40
50
60
70
2θ, deg
Fig. 7. Diffraction patterns of G-2 calcined for 4 h at
(1) 600, (2) 800, and (3) 920°C or (4) for 12 h at 900°C.
ened base) suggests the platelike shape of the particles.
Other differences were not detected in the diffraction
patterns. With consideration for the dehydration of B-2
with the formation of boehmite and an oxide phase
(Fig. 2b), it is likely that an amount of γ-Al2O3 was
present in the sample calcined at 600°ë. However, it is
difficult to detect this by XRD analysis because of the
coincidence of the diffraction peaks of γ-Al2O3 and η-
Al2O3 (Fig. 5) due to their structural similarity. Accord-
ing to published data [7], γ-Al2O3 and η-Al2O3 exhibit
a defect spinel structure.
XRD analysis of samples G-1 and G-2 demon-
strated that they had the structure of gibbsite with very
strongly textured particles. Sample G-1 calcined at
500°ë for 4 h consisted of two phases: boehmite and
χ-Al2O3 (Table 2).A longer (10 h) treatment of the sam-
ples at the specified temperature did not result in boeh-
mite dehydration; only calcination at 600°ë facilitated
the formation of χ-Al2O3, whose structure was also
The thermal treatment of the initial samples at
800°ë was accompanied by changes in the intensity
ratio between 220, 311, and 222 lines; however, the
structure of η-Al2O3 was retained (Fig. 6, curve 2); the retained at 800°C. An increase in the treatment temper-
onset of a transition to θ-Al2O3 was observed only in ature to 900°C was favorable for the onset of κ-Al2O3
Table 2. Dependence of the phase composition of samples from the bayerite and gibbsite series on the conditions of thermal
treatment in air
Temperature, °C/calcination time, h
Sample
900/10
(away from air)
500/4
500/10
600/4
800/4
900/4
900/10
1000/4
B-1
B-2
G-1
G-2
–
–
–
–
η
η
η + OF θ
η + θ
η + θ
θ
θ
–
–
θ
η + γ
η + OF θ
χ + γ
χ
η + θ
θ
Boehmite + χ Boehmite + χ χ + γ
χ + OF κ
χ + α
–
–
κ + α
κ + α
χ
–
χ
Note: OF denotes the onset of formation of the corresponding oxide.
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PREPARATION AND MICROSTRUCTURAL AND TEXTURAL CHARACTERIZATION
719
crystallization, and α-Al2O3 appeared at 1000°C
(Table 2), although the fraction of κ-Al2O3 remained
significant.
(‡)
The isothermal treatment of G-2 at 600°ë resulted in
the formation of χ-Al2O3 (Fig. 7, curve 1). A compari-
son between the diffraction patterns of G-1 and G-2
demonstrated differences between the two, particularly
in the angle range 2θ = 30°–50°. The thermal treatment
of G-2 at 800°ë was not accompanied by a consider-
able change in the diffraction pattern (Fig. 7, curve 2),
and lines that indicate the onset of a structural rear-
rangement in the χ-phase appeared in the diffraction
pattern only above 900°C. The sample calcined at
1000°C was a mixture of κ-Al2O3 and α-Al2O3 phases
(Table 2).
To obtain individual κ-Al2O3, sample G-2 was cal-
cined at 880, 920, and 940°C for 4 h and at 900°C for
100 nm
12 h. Unfortunately, an analysis of the diffraction pat-
terns (Fig. 7) demonstrated that we failed to prepare
κ-Al2O3 free of other oxide impurity species under the
specified conditions.
(b)
Note that the thermal transformations of G-2 exhib-
ited special features. This appeared as a change in the
reaction sequence of formation of high-temperature
aluminum oxide species. Thus, after the thermal treat-
ment of sample G-2 at 900°C for 4 h, α-Al2O3 was
formed, but κ-Al2O3 was absent. An increase in the
treatment time to 12 h facilitated the appearance of
κ-Al2O3 in addition to α-Al2O3 (Fig. 7, curves 3, 4). A
similar phenomenon was noted by Andryushkova et al.
[8], who found that, after the mechanical treatment of
χ-Al2O3 followed by calcination, the oxide underwent
the following changes: χ-Al2O3
χ- + α-Al2O3
α-Al2O3; that is, κ-Al2O3 was not detected.
100 nm
According to Andryushkova et al. [8], accelerated
surface dehydroxylation at the stage of mechanical
treatment, accompanied by a concomitant redistribu-
tion of aluminum cations and a change in their coordi-
nation, may be a reason for the change in the sequence
of thermal reactions. It is likely that, in the preparation
of sample G-2 by milling G-1, local sites with
decreased water contents were produced (Table 1 indi-
cates that the concentration of constituent water in this
sample was reduced), which can subsequently serve as
crystallization centers for α-Al2O3 without the step of
κ-Al2O3 formation.
(c)
α-Al2O3
Thus, based on the experimental results, we can
conclude that the formation of oxide structures from
coarse and small gibbsite particles (as in the case of
bayerite) occurred in different ways. First, after the
thermal treatment of samples G-1 and G-2 at 600°ë,
single-phase χ-Al2O3 was formed in the latter sample,
whereas a mixture of γ-Al2O3 and χ-Al2O3 phases was
formed in the former sample. Second, it is likely that
the transition from χ-Al2O3 to κ-Al2O3 in the sample
with small particles occurred by a rapid structural rear-
rangement with simultaneous formation of α-Al2O3;
100 nm
Fig. 8. Electron micrographs and (insert) electron microdif-
fraction from particles: (a) η-Al O (600°C), (b) χ-Al O
2
3
2 3
(600°C), and (c) G-2 calcined at 940°ë.
KINETICS AND CATALYSIS Vol. 45 No. 5 2004

720
KUL’KO et al.
(‡)
{111}
2 nm
(b)
{111}
2 nm
Fig. 9. Micrographs of an η-Al O particle: (a) highly developed {111} plane and (b) planar defects in planes perpendicular to the
2
3
[111] direction.
because of this, pure κ-alumina cannot be obtained of α-Al2O3 as islands on the surface of plates was
under the chosen conditions.
observed on some plates (marked with an arrow in
Fig. 8c). Thus, the results are consistent with XRD data
and suggest that the formation of the α-Al2O3 structure
in the gibbsite series occurred locally within an isolated
particle.
Let us consider the structure and texture characteris-
tics of the synthesized single-phase (η-, χ-, and θ-) alu-
minum oxides.
According to electron-microscope data, η-Al2O3
The microstructure of η-alumina, which was pre-
particles (Fig. 8a) are thin (to 20 nm) aggregated plates. pared by bayerite dehydration at 600°ë, was studied
The microdiffraction pattern shown in the insert of using high-resolution electron microscopy. The parti-
Fig. 8a exhibits point reflections, which suggest that an cles of η-Al2O3 exhibited the most developed {111}
individual plate is a single-crystal plate with a devel- face and consisted of coherently joined domains with a
oped (110) plane; in this case, oxide particles with a pronounced platelet shape (Fig. 9a). A system of planar
developed (111) plane are also observed. The particles defects arranged in the (111) plane occurred in the par-
of χ-Al2O3 are similar to the above particles of η-Al2O3 ticles (Fig. 9b). In this case, the planar defects are
in morphology (Fig. 8b); however, an isolated χ-Al2O3 boundaries of a strictly defined orientation. The occur-
particle exhibited a ring microdiffraction pattern. This rence of planar defects of this type explains the shape
is indicative of a polycrystalline character in the struc- of the (111) diffraction peak (Fig. 4), which is the
ture of χ-Al2O3, which may be due to the fact that indi- superposition of two components, a relatively narrow
vidual plates consist of microblocks that are disoriented diffraction maximum and a much-broadened base. The
with respect to each other; analogously to the case of weakly broadened component corresponds to scatter-
polycrystalline aggregates, this results in the transfor- ing from one of the plane systems of the (111) type, that
mation of point reflections into ring reflections. After is, that in which planar defects lie. Evidently, the occur-
calcination at 940°ë, the agglomeration of microb- rence of such shear plane defects does not affect the
locks, which constitute a particle, occurred, and the coherent scattering of X-rays in a direction perpendic-
particle structure became single-crystalline (Fig. 8c). ular to the defect planes, and the half-width of a narrow
This manifested itself by the appearance of point reflec- component corresponds to the size of the entire particle
tions, which are characteristic of a single crystal, in the in this direction. At the same time, a considerable
electron diffraction pattern. In this case, the formation broadening of the second component is due to the fact
KINETICS AND CATALYSIS Vol. 45 No. 5 2004

PREPARATION AND MICROSTRUCTURAL AND TEXTURAL CHARACTERIZATION
Table 3. Texture characteristics of oxide phases found from the low-temperature adsorption of nitrogen
Pore size
721
Vmicropore
,
Aluminum
oxide species
Ssp, m2/g
V
pore, cm3/g
cm3/g
average pore
diameter, Å
pore diameter
pore diameter
(from adsorption), Å (from desorption), Å
η-Al2O3 (600°C)
θ-Al2O3 (900°C)
χ-Al2O3 (600°C)
280
110
250
0.33
0.30
0.34
0.044
0.003
0.037
47.2
110.6
54.5
55.8
100.9
64.5
52.4
88.4
55.9
that coherence is not observed in the other two direc- acteristics of the low-temperature aluminum oxides
tions of the closest packing of the 〈111〉 type; defect η-Al2O3 and χ-Al2O3 are similar.
boundaries are at a certain angle to these directions.
In turn, it is well known [6, 9] that the particles of γ-
ACKNOWLEDGMENTS
Al2O3 prepared from boehmite exhibit the shape of
The structural studies of the samples were supported
by the program “Universities of Russia” of the Ministry
of Education of the Russian Federation (grant
no. UR.05.01.042).
extended (~100 nm) thin single-crystal plates with the
most developed (110) face. The main microstructure
peculiarity of these particles is the occurrence of spe-
cific closed defects that are formed by extended dislo-
cation walls. The latter result from the coalescence of
lattice vacancies in the course of structure formation.
Thus, the electron-microscopic study of the microstruc-
ture of η-Al2O3 and a comparison of this microstructure
with that obtained previously [6, 9] for γ-Al2O3 demon-
strate that, at the same defect spinel structure, the
microstructures of these oxides are different.
According to data on the low-temperature adsorp-
tion of nitrogen, η-Al2O3 was characterized by a spe-
cific surface area equal to 280 m2/g; the pore volume
was equal to 0.33 cm3/g; in this case, the fraction of
micropores was 13% of the total pore volume; the aver-
age pore diameter was 47 Å (Table 3). Upon going from
η-Al2O3 to θ-Al2O3, the specific surface area decreased
by 62%, and the total pore volume decreased insignifi-
cantly (10%); at the same time, the micropore volume
decreased by 94%. Consequently, microporosity
almost completely disappeared upon an oxide structure
transformation from η-Al2O3 to θ-Al2O3, and the sys-
tem became uniformly porous, with an average pore
diameter of 110 Å.
REFERENCES
1. Sato, T., Thermochim. Acta, 1985, vol. 88, no. 1, p. 69.
2. Price, W., Analytical Atomic Absorption Spectroscopy,
New York: Wiley, 1972.
3. Buyanova, N.E., Karnaukhov, A.P., and Alabuzhev, Yu.A.,
Opredelenie poverkhnosti dispersnykh i poristykh mate-
rialov (Surface Area Measurements of Dispersed and
Porous Materials), Novosibirsk: Institute of Catalysis,
1978.
4. Dzis’ko, V.A. and Ivanova, A.S., Izv. Sib. Otd. Akad.
Nauk SSSR, Ser. Khim. Nauk., 1985, no. 5, p. 110.
5. Ivanova, A.S., Skripchenko, E.V., Moroz, E.M., Lit-
vak, G.S., Kustova, G.N., and Krivoruchko, O.P., Izv.
Akad. Nauk SSSR, Sib. Otd., Ser. Khim. Nauk, 1989,
no. 6, p. 116.
6. Ivanova,A.S., Litvak, G.S., Kryukova, G.N., Tsybulya, S.V.,
and Paukshtis, E.A., Kinet. Katal., 2000, vol. 41, no. 1,
p. 137.
7. Stroenie i svoistva adsorbentov i katalizatorov (Structure
and properties of Adsorbents and Catalysts), Linsen, B.G.,
Ed., Moscow: Mir, 1973, p. 203.
The specific surface area of χ-Al2O3 prepared at
600°ë was 250 m2/g, which is somewhat lower than
that of η-Al2O3, whereas the pore volume and the
micropore volume were practically equal (Table 3).
χ-Alumina exhibited a uniformly porous structure; the
average pore diameter was 55 Å. Thus, the texture char-
8. Andryushkova, O.V., Kirichenko, O.A., and Ushakov, V.A.,
Stud. Surf. Sci. Catal., 1998, vol. 120, p. 587.
9. Kryukova, G.N., Klenov, D.O., Ivanova, A.S., and
Tsybulya, S.V., J. Eur. Ceram. Soc., 2000, no. 20,
p. 1187.
KINETICS AND CATALYSIS Vol. 45 No. 5 2004