Synthesis of gallium-aluminum dawsonites and their crystal structures

Article abstract of DOI:10.1111/j.1551-2916.2010.03968.x

Solid solutions of dawsonites, ammonium aluminum carbonate hydroxide-ammonium gallium carbonate hydroxide, were obtained by the addition of aqueous solutions of mixtures of aluminum and gallium nitrates to an excess ammonium carbonate solution. The dawson

Full text of DOI:10.1111/j.1551-2916.2010.03968.x

J. Am. Ceram. Soc., 93 [11] 3908–3915 (2010)
DOI: 10.1111/j.1551-2916.2010.03968.x
r 2010 The American Ceramic Society
ournal
J
Synthesis of Gallium–Aluminum Dawsonites and
their Crystal Structures
z,y
z
z
z
z
,w,z
Tsunenori Watanabe, Takeo Masuda, Yoshihisa Miki, Yuya Miyahara, Hyung-Joon Jeon,
Saburo Hosokawa, Hiroyoshi Kanai, Hiroshi Deguchi, and Masashi Inoue*
z
z
y
z
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Kyoto 615-8510, Japan
yPower Engineering R&D Center, The Kansai Electric Power Company Inc., 3-11-20,
Nakoji, Amagasaki 661-0974, Japan
Solid solutions of dawsonites, ammonium aluminum carbonate
hydroxide–ammonium gallium carbonate hydroxide, were
obtained by the addition of aqueous solutions of mixtures of
aluminum and gallium nitrates to an excess ammonium carbon-
ate solution. The dawsonite solid solutions were calcined at
are thermodynamically stable. However, synthesis of diaspore
solid solutions is extremely difficult, especially for Al-rich com-
2
9
positions. Actually, Hill et al. reported that the samples of the
gel obtained by coprecipitation from Al and Ga nitrates could
not be converted to the diaspore structure when the Ga content
3
1
7
001C to give c-Ga O –Al O solid solutions. The structures of
was o25%. Escribano et al. examined coprecipitation of Al
and Ga nitrates with ammonia and reported that diaspore-type
compounds were formed for Ga-rich compositions, whereas
2
3
2
3
these two types of solid solution were discussed on the basis of
their X-ray diffraction patterns and X-ray absorption fine struc-
ture spectra.
31
Al-rich precipitates were amorphous. Incorporation of Al
ions in the GaOOH structure was suggested but 12% content
of Al ions in the GaOOH (diaspore) was estimated for the
31
31
precipitate obtained from 75% Ga to 25% Al composition.
The g-Ga –Al solid solutions are usually prepared by
impregnation of pseudoboehmite
lution of Ga nitrate and subsequent calcination. However, these
I. Introduction
2
O
3
2 3
O
5,6,8
4,32
ALLIUM OXIDE has unique catalytic functions for dehydro-
genation of paraffins, isomerization of hydrocarbons,
or g-alumina
with a so-
1
,2
3
G
4–7
nitrogen oxide reduction with hydrocarbons, decomposition
31
31
methods do not guarantee the homogeneity of Al 31and Ga
ions in Ga –Al because diffusion of Ga ions in
the alumina matrix is required. As far as the authors know, the
8
9
of CF
also has photocatalytic activity
port effects on the performance of noble metal
4
, and epoxidation of olefins with hydrogen peroxide. It
2
O
3
2 3
O
1
0,11
and exhibits interesting sup-
1
2–16
and Ni–Mo
Gallium-containing zeolites are known to have
most successful synthesis of the g-Ga O –Al O solid solutions by
2
3
2 3
1
7,18
33
catalysts.
high catalytic activities for aromatization of light paraffins,
NO reduction with methane, and aniline synthesis from
the precipitation method was reported by Otero Area
´
n et al.
1
9–21
who synthesized g-Ga –Al solid solutions by calcination of
2
O
3
2 3
O
2
2
the amorphous gels obtained by coprecipitation from Al and Ga
nitrates. They used ethanolic solutions of Al and Ga nitrates
because the presence of excess water in the resulting gel leads to
crystallization of GaOOH (diaspore), which is not a precursor of
2
3
phenol. Because gallium oxide usually has a low surface
area, Ga supported on g-Al and g-Ga –Al solid
solutions are usually preferred for catalyst use.
In the alumina–water system, four polymorphs of Al(OH)
2
O
3
O
2 3
2
O
3
2 3
O
3
4
31
31
3
,
g-Ga O . However, homogeneity of Al and Ga ions in the
2
3
2
4,25
gibbsite, bayerite, nordstrondite, and doyleite
morphs of AlOOH, boehmite and diaspore, are known besides
; two poly-
precursor gel was not confirmed because of its amorphous nature.
Dawsonite is a mineralogical name for naturally occurring
26
3
5
tohdite (5Al O ꢀ H O). Gibbsite, bayerite, and nordstrondite
2
3
2
2 3
sodium aluminum carbonate hydroxide (NaAl(OH) CO ).
can be crystallized from aqueous solutions. Although boehmite is
also crystallized from aqueous media under ambient conditions,
crystallization of diaspore from aqueous media usually requires
high temperatures and pressures. In the gallia–water system,
Ammonium aluminum carbonate hydroxide (NH Al(OH) CO ,
4
2
3
AACH) is included in the group of dawsonite and is also called
ammonium dawsonite. The crystal structure of AACH was de-
3
6
termined by Iga and Kato, who reported that it belongs to the
space group of Cmcm. This structure is slightly different from
that of dawsonite, because the space group of dawsonite
27,28
on the other hand, crystalline Ga(OH)
GaOOH having the diaspore structure is easily crystallized.
Synthesis of Ga–Al solid solution systems has been examined
3
is not formed,
and
2
7,29
3
5
(
Imma) cannot accommodate the ammonium ions having a
symmetry. AACH was prepared by the addition of an alu-
minum nitrate solution into a solution of ammonium carbonate
2
9
by several researchers. Hill et al. reported that the boehmite
solid solution extended to an approximate composition of
T
d
3
0
3
7–39
Al0.7Ga0.3OOH, and Zhao et al. also reported the synthesis
of gallium-doped boehmite (Gao20%). However, synthesis of
the boehmite solid solutions in the entire range from Al end to
Ga end appears to be impossible because GaOOH having the
boehmite structure has never been reported. On the other hand,
Al Ga OOH solid solutions having the diaspore structure
or bicarbonate.
This compound has been used as a suitable
depending
As far as the authors know,
only two papers have reported the formation of ammonium
starting material for the synthesis of a- and g-Al
2
O
3
3
9–42
on the calcination temperature.
4
3,44
43
gallium carbonate hydroxide (AGCH).
reported that AGCH crystallized in the Pbcn space group, which
is different from that of AACH (Cmcm ).
We found that g-Ga –Al solid solutions prepared by
solvothermal methods have high catalytic activities for selective
Sidorenko et al.
x
1ꢁx
3
6
seem to exist because both end members, AlOOH and GaOOH,
O
2 3
2 3
O
E. Slamovich—contributing editor
catalytic reduction (SCR) of NO with methane as a reducing
The activity of the catalyst prepared solvothermally
7
,45,46
agent.
using various solvents depended on the crystallite size of the
catalyst. The catalyst with a large crystallite size had high cat-
alytic activity in spite of the fact that the catalyst had low surface
Manuscript No. 27683. Received March 19, 2010; approved June 1, 2010.
*Member, The American Ceramic Society.
Author to whom correspondence should be addressed. e-mail: inoue@scl.kyoto-u.ac.jp
w
3
908

November 2010
Synthesis of Gallium–Aluminum Dawsonites
3909
4
6
area. We recently found that calcination of the precursors
obtained by coprecipitation of gallium and aluminum nitrates
with ammonium carbonate yielded well-crystallized g-Ga O –
2
3
2 3
Al O solid solutions, which also showed high catalytic activities
4
7
for methane-SCR of NO. Other precipitants resulted in the
formation of catalysts with low activities. The X-ray diffraction
(
XRD) patterns of the precursors obtained by coprecipitation
with ammonium carbonate resemble that of AACH. Therefore,
solid solutions of AGCH–AACH are expected to be formed.
In the present work, therefore, we have examined optimum
2 3 2 3
conditions to synthesize the Ga O –Al O solid solutions and
attention has been paid for the synthesis and structure of the
AGCH–AACH solid solutions as the precursors of the oxide
solid solutions.
II. Experimental Procedure
(
1) Preparation of c-Ga O –Al O Solid Solutions
2 3 2 3
Appropriate amounts of gallium nitrate hydrate (Ga(NO
nH O; Mitsuwa Chemical, Osaka, Japan) and aluminum nitrate
nonahydrate (Al(NO O; Nacalai Tesque, Kyoto, Japan)
ꢀ 9H
were dissolved in 100 mL of deionized water. This solution was
added to a 200 mL solution of excess (NH CO at once at a
)
3 3
ꢀ
2
)
3 3
2
)
4 2
3
prescribed temperature (usually 251C) with vigorous stirring and
the mixture was stirred for 1 h. The precipitate was centrifuged
and washed with deionized water twice and ethanol twice fol-
lowed by drying at 801C. The precursor was calcined at 7001C
2 3 2 3
for 3 h in air to yield the g-Ga O –Al O solid solutions.
(2) Characterization
Powder XRD patterns were obtained (Model XD-D1
Shimadzu, Kyoto, Japan) using CuKa radiation and a carbon
monochromator. Crystallite size was calculated from the half-
2 3 2 3
height width of the (440) diffraction peak of g-Ga O –Al O
using the Scherrer equation. For Rietveld analysis, the XRD
pattern was measured on another diffractometer (Rigaku Rint
Fig. 1. TG and DTA profiles of the products obtained by coprecipita-
tion of gallium and aluminum nitrates at 251C with various amounts of
(
4 2 3
NH ) CO .
2
500, Rigaku, Tokyo, Japan) and analyzed by RIETAN-2000
48
program. The TCH pseudovoigt profile function was used.
BET surface area was calculated using the single-point method
on the basis of the nitrogen uptake measured at 77 K. The sam-
TG profile of the product prepared with 1.5-fold excess of
(NH CO exhibited a gradual weight decrease in a wide tem-
)
4 2
3
ples were pretreated in an N
measurement.
X-ray absorption fine structure (XAFS) measurements for
Ga K-edge were carried out on BL14B2 beam line at SPring-8
2
flow at 3001C for 30 min before the
perature range. This result accords with the XRD result, which
showed the amorphous nature of this product. On the other
hand, the TG profile of the products obtained with three to
4 2 3
fivefold excess of (NH ) CO showed a sharp weight decrease at
(
Harima, Japan). An Si(311) monochromator was used to mono-
about 2001C associated with an endothermic response in DTA.
Theoretical weight loss of AGCH–AACH solid solution (Ga/
(Ga1Al) 5 0.225) is calculated to be 59.2%. The weight losses
of the products prepared with four and fivefold excess
chromatize the X-ray. The absorption spectra in the Ga K-edge
region were measured by the transmission mode at room temper-
ature. The proportions of T and O gallium sites in the samples
d h
were estimated by the linear combination fitting method from their
(NH
but weight loss of the product obtained with threefold excess
(NH CO was smaller than the theoretical, indicating that this
product contains a small amount of an amorphous phase.
4 2 3
) CO were essentially identical with the theoretical value
XANES data. The g-Ga O –Al O solid solution with the Ga/
2
3
2 3
(
Ga1Al) ratio of 0.1 prepared solvothermally in 2-methylamino-
ethanol and ZnGa having a normal spinel structure were used
as the T and O standard samples, respectively.
)
4 2
3
2 4
O
d
h
(2) Preparation of AGCH–AACH Solid Solutions with
Various Ga/Al Ratios
III. Results and Discussion
1) Effects of Amount of (NH ) CO
Figure 2 shows the XRD patterns of the products (Ga/
(
4
2
3
(
Ga1Al) 50.225) obtained at various temperatures with fivefold
excess of (NH CO as a precipitant. In a temperature range of
1–801C, the crystallinity of the dawsonite sample increased with
The effects of the amount of (NH
dawsonite at 251C were reported previously. The product
prepared with 1.5-fold excess of (NH CO in relation to the
stoichiometric amount was amorphous. However, large excess
of (NH CO yielded products that exhibited the XRD patterns
due to dawsonite.
equation was assumed for the preparation of dawsonite:
4 2 3
) CO on the preparation of
)
4 2
3
4
7
0
)
4 2
3
the increase in the precipitation temperature. However, further
increase in the crystallinity was not attained by prolonging the
aging period from 1 to 4 h at 801C (data not shown).
Figure 3 shows the XRD patterns of AGCH–AACH pre-
4 2 3
pared at 801C using fivefold excess of (NH ) CO from various
)
4 2
3
3
9,40,49
Note that the following stoichiometric
charged ratios of Ga/(Ga1Al). The peak patterns of the prod-
ucts are almost the same as that of AACH reported in the lit-
3
þ
ꢁ
Al þ ðNH4Þ CO3 þ 3OH
2
(
1)
3
9,40,49
!
NH4AlðOHÞ CO3 þ NH3 þ H2O
erature.
with (NH
it has the same structure as that of AACH. With the increase in
AGCH was similarly prepared from Ga(NO
)
3 3
2
4 2
)
CO , and its XRD pattern (Fig. 4) indicates that
3
Figure 1 shows TG–DTA profiles of AGCH–AACH
obtained with various amounts of (NH ) CO at 251C. The
the Ga/(Ga1Al) ratio, the diffraction peaks gradually shifted
4
2
3

3
910
Journal of the American Ceramic Society—Watanabe et al.
Vol. 93, No. 11
Fig. 4. X-ray diffraction patterns of AGCH–AACH samples prepared
from mixtures of gallium and aluminum nitrates at 801C with various ra-
4 2 3
tios of Ga/(Ga1Al) by coprecipitation with threefold excess (NH ) CO .
Fig. 2. X-ray diffraction patterns of the products obtained by copre-
cipitation of gallium and aluminum nitrates at various temperatures with
4 2 3
fivefold excess of (NH ) CO as a precipitant.
Cmcm and Pbcn are quite similar to each other and we could not
find any reason why AGCH has a lower symmetry than AACH.
Figure 6 shows the observed, calculated, and difference profiles
for the final refinement of XRD of AGCH. The empirical for-
mula of AGCH calculated based on the Cmcm space group was
close to the ideal one. The observed, calculated, and difference
profiles for the AACH refined using Cmcm and AGCH using
Pbcn are given in the supporting information (Figs. S1 and S2).
Table I shows the results of Rietveld analyses of AACH and
AGCH refined with the space group of Cmcm. Although AGCH
gave a slightly large Rwp, both refinements gave reasonable Rwp
values. Table II shows the results of Rietveld analyses of AGCH–
AACH with Ga/(Ga1Al) 5 0.50 on the basis of the space group
to the lower angle side, indicating that the unit cell parameters
are enlarged by incorporation of Ga ion because the ionic size
31
31
31
˚
˚
of Ga (0.76 A for CN 6) is larger than that of Al (0.675 A
for CN 6). These results indicate that the AGCH–AACH solid
solutions are formed in the whole range of Al–Ga composition.
The XRD analyses of the dawsonites indicated that the crys-
tallinity of the product increased with the increase in the Ga/
4
7
(
Ga1Al) ratio, just as the products obtained at 251C.
Figure 4 shows the XRD patterns of AGCH–AACH pre-
pared at 801C using threefold excess of (NH ) CO from various
4
2
3
charged ratios of Ga/(Ga1Al). The products with higher Ga
contents (Ga/(Ga1Al) ꢂ 0.5) were amorphous, while dawson-
ite crystals were obtained for the products with lower Ga con-
tents. Among the products obtained under this condition,
AACH had the highest crystallinity. In other words, the crys-
tallinity of dawsonite prepared at 801C using threefold excess of
31
31
of Cmcm. In the 4a sites, both Al and Ga ions are located,
indicating the formation of the AGCH–AACH solid solution.
Figure 7 shows the relationship between the starting compo-
sition for coprecipitation and unit cell parameters of the solid
solution calculated by Rietveld analysis. Although the data were
(
NH
ratio. These results indicate that the adequate amount
of (NH CO for the preparation of highly crystallized
4 2 3
) CO decreased with the increase in the Ga/(Ga1Al)
)
4 2
3
dawsonites depends on the Ga/(Ga1Al) ratio and precipitation
temperature.
Figure 5 shows the structures of AACH and AGCH refined
using the space group of Cmcm and AGCH refined using the
Pbcn space group. The crystal structures of AGCH refined using
Fig. 3. X-ray diffraction patterns of AGCH–AACH samples prepared
from mixtures of gallium and aluminum nitrates at 801C with various
ratios of Ga/(Ga1Al) by coprecipitation with fivefold excess (NH ) CO .
4 2 3
Fig. 5. Structures of (a) AACH and (b) AGCH; refined using the space
group of Cmcm and (c) AGCH refined using Pbcn space group.

November 2010
Synthesis of Gallium–Aluminum Dawsonites
3911
Table II. Results for Rietveld Analysis of AGCH–AACH
Obtained from Ga/(Ga1Al) 5 0.5
Element
Site
gw
x
y
z
Ga
Al
4a 0.394
0.502
0
0
0
C
4c 0.750
4c 0.811
8g 1.000 0.1943 (6) 0.0146 (6)
8f 0.750
4c 0.750
0
0
0.7810 (12) 0.2500
0.3473 (8)
NH4
OH
O1
O2
0.2500
0.2500
0
0
0.8433 (4)
0.6764 (8)
0.0626 (10)
0.2500
Space group: Cmcm (N0.63). Rwp: 9.98. wSite occupancy. AACH, ammonium
Fig. 6. Observed, calculated, and difference profiles obtained by Riet-
veld analysis of AGCH. The AGCH sample was prepared from a gal-
aluminum carbonate hydroxide; AGCH, ammonium gallium carbonate hydroxide.
lium nitrate solution by precipitation with fivefold excess (NH
4 2 3
) CO at
801C. The profile was refined using the space group of Cmcm.
were formed because the two-mode behavior was not observed
for other Raman bands.
slightly scattered presumably because of the presence of small
amounts of amorphous phase, the lattice parameters b and c
were expanded, while the lattice parameter a was slightly shrunk-
Figure 10 shows the Ga K-edge XAFS spectra of the daw-
sonites prepared with various Ga/(Ga1Al) charged ratios. The
peak positions for T -Ga and O -Ga (10 373 and 10376 eV,
31
31
d
h
31
4
en by the incorporation of Ga ions in the AACH structure.
Figure 8 shows IR spectra of the solid solutions with various
respectively ) are shown by broken lines. As shown in Fig. 10, all
the products exhibited essentially identical Ga K-edge XANES
spectra, which gave a peak at the midway between the peak
50–53
compositions. According to the literature,
the absorption
bands of AACH (Ga/(Ga1Al) 5 0.00) were assigned as follows:
3
1
31
positions of T
d
-Ga and O -Ga . Although the metal ions in
h
ꢁ
1
ꢁ1
36
nOH, at 3423 cm ; dOH, at 991 cm ; nNH, at 3151, 3007, and
the dawsonite structure have the octahedral coordination,
2
dawsonite has two types of M–O bonds: two M–O–CO bonds
ꢁ1
ꢁ1
2ꢁ
3
2
831 cm ; dNH, at 1839 and 1719 cm ; nC–O of the CO
ꢁ
1
groups, at 1543, 1447, 1380, 759, and 727 cm ; and nAl–O lat-
tice vibration, at 1103 cm . On the other hand, the absorption
bands of AGCH (Ga/(Ga1Al) 5 1.00) can be assigned as fol-
lows: nOH, at 3376 cm ; dOH, at 983 cm ; nNH, at 3155,
057, and 2851 cm ; dNH, at 1803 and 1720 cm ; nC–O, at
in the axial positions and four M–OH bonds in the equatorial
positions. This means that the structure is tetragonally distorted,
with metal ions having a D4h-like environment. Note that the
crystallographic symmetry of the metal sites (Wyckoff 4a sites)
of the Cmcm space group is C2h, whereas Wyckoff 4a sites of the
Pbcn space group (Ga sites if one assumes that AGCH belongs
ꢁ
1
ꢁ1
ꢁ1
ꢁ
1
ꢁ1
3
1
ꢁ1
522, 1446, 1362, 762, and 709 cm ; and nGa–O lattice vibra-
ꢁ1
tion, at 1089 cm . For the solid solutions, these bands appeared
at the frequencies between those observed for AACH and
AGCH and these two compounds had same numbers of
absorption bands. Therefore, Pbcn space group having a lower
symmetry can be ruled out for AGCH.
i
to this space group) have C symmetry. In AACH, the bond
˚
length of Al–O–CO calculated by Rietveld analysis is 1.88 A
2
˚
and that of Al–OH is 1.95 A. AGCH has a similar structure to
31
AACH. The distorted octahedral structure of Ga ions in
AACH–AGCH would give the absorption peak at an interme-
31
31
Figure 9 shows the Raman spectra of the solid solutions
with various compositions. All the bands were assigned to lattice
diate between those of T -Ga and O -Ga .
d h
5
2
vibrations. Because the same number of Raman band
was observed for AACH and AGCH, these compounds have
the same symmetry and the possibility that AGCH belongs to
the Pbcn space group can be ruled out. It is interesting to note
(
3) Preparation and Structure of c-Ga O –Al O Solid
2 3 2 3
Solutions with Various Ga/Al Ratios
Figure 11 shows TG–DTA profiles of the solid solutions. A
sharp weight loss associated with an endothermic response in
DTA was observed for all the products. This result accords with
the results obtained by FT-IR, Raman spectra, and XRD
patterns, which suggested that AGCH–AACH solid solutions
were obtained for the entire composition from Al to Ga end
members. The weight loss process is explained by the simulta-
neous desorption of water from hydroxide ions and of carbon
dioxide from carbonate ions. Although the observed weight
losses for the Ga-rich products were significantly smaller than
the theoretical values, those of the Al-rich products were essen-
tially identical with the theoretical values.
ꢁ
1
31
that the band at B550 cm split into two peaks when Al
and Ga ions are present in the structure. This is the so-called
31
5
4
‘two-mode behavior’’ and is found in the solid solution
‘
systems of metals, carbide, nitride, oxides, etc.
55–57
The origin
of the two-mode behavior is not yet fully understood, but it is
known that the intensity ratio of the two peaks approximately
corresponds to the composition ratio of the solid solution.
5
4,55,57
Therefore, the peaks were deconvoluted and the results are given
in Table III. Apparently, the intensity ratio is approximately
identical with the starting composition ratio, indicating that
31
31
Al and Ga ions are well incorporated into the structure of
the solid solution at the coprecipitation stage. Although micro-
The XRD patterns of the samples obtained by calcination of
the AACH–AGCH solid solutions in air at 7001C were reported
previously. Aluminum oxide obtained by calcination of
5
8,59
segregation, which cannot be detected by XRD, cannot
be ruled out, we believe that homogeneous solid solutions
4
7
Table I. Results for Rietveld Analysis of Dawsonite Samples
Site occupancy
4
a
4c
4c
8g
8f
4c
Sample
Al
Ga
C
NH
4
OH
O1
O2
R
wp
AACH
AGCH
1.000
—
—
1.000
0.926
0.868
0.851(8)
0.737(9)
1.000
1.000
0.926
0.868
0.926
0.868
9.38
16.54
Space group: Cmcm (No. 63). AACH, ammonium aluminum carbonate hydroxide; AGCH, ammonium gallium carbonate hydroxide.

3
912
Journal of the American Ceramic Society—Watanabe et al.
Vol. 93, No. 11
Fig. 7. Lattice parameter (a, b, and c) of the AGCH–AACH solid solutions versus Ga content in AGCH–AACH.
Table III. Deconvolution of Raman Peaks
Ga/(Ga1Al)
Peak position
Relative
Peak position
ꢁ1
Relative
ꢁ
1
charged ratio
(cm
)
intensity (ꢁ)
(cm
)
intensity (ꢁ)
0
559.8
554.8
552.0
548.1
ꢁ
1
ꢁ
ꢁ
0
0
0
1
.25
.5
.75
0.71
0.47
0.33
ꢁ
534.8
534.5
532.7
532.2
0.29
0.53
0.67
1
Fig. 8. IR spectra of AGCH–AACH prepared at 251C with various
ratios of Ga/(Ga1Al).
Fig. 9. Raman spectra of AGCH–AACH prepared at 251C with var-
ious ratios of Ga/(Ga1Al).
Fig. 10. Ga K-edge X-ray absorption fine structure spectra of AGCH–
AACH prepared at 251C with various ratios of Ga/(Ga1Al) by copre-
cipitation with (NH ) CO .
4 2 3
AACH was amorphous. The samples obtained from AGCH–
AACH at Ga/(Ga1Al) 5 0.20–0.50 give the patterns similar to
2 3
that of g-Al O . With an increase in the Ga/(Ga1Al) ratio, the
diffraction peaks gradually shifted toward the lower angle side,
indicating Ga O –Al O solid solutions were obtained.
compared with pure b-Ga O , indicating that b-Ga O –y-Al O
2
3
2
3
2
3
2
3
2
3
The XRD pattern of the Ga O –Al O
2 3 2 3
mixed oxide obtained
solid solution was formed.
from Ga/(Ga1Al) 5 0.75 indicates that partial transformation
to b-Ga O -like phase occurred, and the proportion of this
phase increased with the increase of the Ga/(Ga1Al) charged
Figure 12 shows the correlation between the charged ratio of
Ga and lattice parameter calculated by the XRD patterns of the
Ga O –Al O mixed oxides, which is assumed to have a cubic
2
3
2
3
2
3
ratio. For the sample with Ga/(Ga1Al) 5 0.75, however, the
was observed at a slightly higher angle as
structure. Because the XRD pattern of the Ga mixed
oxide obtained from Ga/(Ga1Al) 5 0.75 showed b-Ga O -like
2 3
2
O
3
–Al
2
O
3
5
12 peak of b-Ga
O
2 3

November 2010
Synthesis of Gallium–Aluminum Dawsonites
3913
Fig. 13. Ga K-edge X-ray absorption fine structure spectra of Ga
2 3
O –
Al prepared by calcination at 7001C of AGCH–AACH solid solu-
2 3
O
tions with various ratios of Ga/(Ga1Al).
Fig. 11. TG–DTA profiles of AGCH–AACH prepared at 251C with
various ratios of Ga/(Ga1Al).
Figure 14 shows the Ga K-edge XAFS spectra of AGCH–
AACH prepared with Ga/(Ga1Al) 5 0.50 after heating at var-
ious temperatures. Significant change of XANES spectrum was
observed after calcination at 2001C. This result is consistent with
TG and DTA results, which showed that AGCH–AACH was
thermally decomposed at 2001C. After calcination at 2001C,
phase, its datum was not incorporated in Fig. 12. As is observed
in Fig. 12, the lattice parameter increased linearly with the
increase in the Ga/(Ga1Al) ratio, indicating that g-Ga
2 3
O –
Al solid solutions were obtained by the calcination of
2 3
O
the AGCH–AACH solid solutions. Because the precursors for
the g-Ga O –Al O solid solutions are the AGCH–AACH solid
2
3
2
3
3
1
31
solutions, Ga and Al ions must homogeneously distribute
d h
both T and O structures were observed and a further increase
in the calcination temperature did not alter the XANES spectra.
This result indicates that the product obtained by the calcination
in the oxide solid solutions. In other words, homogeneous dis-
tribution of Ga and Al ions in AGCH–AACH was main-
31
31
tained in the solid solutions of g-Ga O –Al O .
2
3
2
3
Figure 13 shows the Ga K-edge XAFS spectra of the mixed
oxides prepared with various Ga/(Ga1Al) charged ratios. With
31
the increase in the Ga/(Ga1Al) ratio, O -Ga peaks clearly
h
31
increased. However, Ga ions in the solid solutions prepared
structure predominantly.
with Ga/(Ga1Al)r0.50 have the T
d
31
This result suggests that Ga ions preferentially occupy the
tetrahedral sites over the octahedral sites in the defective spinel
2 3 2 3
structure of the Ga O –Al O mixed oxides.
Fig. 14. Ga K-edge X-ray absorption fine structure spectra of AGCH–
AACH solid solution (Ga/(Ga1Al) 50.5) by calcination at various
temperatures.
Fig. 12. Lattice parameter of the Ga
2 3 2 3
O –Al O mixed oxides versus Ga
O
2 3
2 3
content in the Ga –Al O solid solutions.

3
914
Journal of the American Ceramic Society—Watanabe et al.
Vol. 93, No. 11
9
P. P. Pescarmona, K. P. F. Janssen, and P. A. Jacobs, ‘‘Novel Transition-
Metal-Free Heterogeneous Epoxidation Catalysts Discovered by Means of High-
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of AGCH–AACH at 2001C has a microscopic structure similar
to that of the g-Ga O –Al O solid solution although it is X-ray
2 3 2 3
amorphous.
10
K. Ikarashi, J. Sato, H. Kobayashi, H. Saito, N. Nishiyama, and Y. Inoue,
1
0
‘
Configuration,’’ J. Phys. Chem. B, 106 [35] 9048–53 (2002).
‘Photocatalysis for Water Decomposition by RuO
2 2 4
-Dispersed ZnGa O with d
11
Y. Hou, L. Wu, X. Wang, Z. Ding, Z. Li, and X. Fu, ‘‘Photocatalytic
Performance of a-, b-, and g-Ga for the Destruction of Volatile Aromatic
Pollutants in Air,’’ J. Catal., 250, 12–8 (2007).
IV. Conclusions
2 3
O
AGCH is isomorphous with AACH and AGCH–AACH solid
solutions were prepared from aluminum and gallium nitrates
with ammonium carbonate by the use of fivefold excess of
ammonium carbonate. Calcination of the AACH–AGCH solid
solutions prepared with Ga/(Ga1Al)r0.50 at 7001C for 3 h
12
F. Domı
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14
AACH solid solutions were located in the distorted octahedral
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16
gave a structure similar to that of g-Ga
calcined product was X-ray amorphous.
2
3 2 3
O –Al O , although the
‘‘Development of an Active Ga O Supported Palladium Catalysts for the Syn-
2
3
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17
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3
and
The XAFS experiments were performed at BL14B2 in SPring-8 with the
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3
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Supporting Information
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20
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obtained by Rietveld analysis of AACH. The AACH sample
was prepared from a solution of aluminum nitrate by precipitation
with 5-fold excess (NH ) CO at 801C. The profile was refined
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