On the structure and dehydration of hydrous zirconia and hafnia xerogels

Article abstract of DOI:10.1134/S0036023610060070

Hydrous zirconia and hafnia xerogels of compositions ZrO₂ · 2.5H₂O and HfO₂ · 2.3H₂O are dehydrated in two steps upon heating. First, molecular water is mostly removed from the structures to form phases of compo

Full text of DOI:10.1134/S0036023610060070

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2010, Vol. 55, No. 6, pp. 866–875. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © A.V. Kostrikin, F.M. Spiridonov, L.N. Komissarova, I.V. Lin’ko, O.V. Kosenkova, B.E. Zaitsev, 2010, published in Zhurnal Neorganicheskoi Khimii, 2010,
Vol. 55, No. 6, pp. 929–939.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
On the Structure and Dehydration of Hydrous
Zirconia and Hafnia Xerogels
a
b
b
c
A. V. Kostrikin , F. M. Spiridonov , L. N. Komissarova , I. V. Lin’ko ,
a
c
O. V. Kosenkova , and B. E. Zaitsev
a
Michurinsk State Pedagogical University, Michurinsk, Russia
b
Moscow State University, Moscow, 119991 Russia
c
Russian University of Peoples’ Friendship, Moscow, Russia
eꢀmail: Radi1@rambler.ru
Received October 20, 2008
Abstract—Hydrous zirconia and hafnia xerogels of compositions ZrO · 2.5H O and HfO · 2.3H O are dehyꢀ
2
2
2
2
drated in two steps upon heating. First, molecular water is mostly removed from the structures to form phases
of compositions ZrO · H O and HfO · 0.5H O. Second, polycondensation of OH groups occurs. Both proꢀ
2
2
2
2
cesses are easier for ZrO · 2.5H O. Apart from these steps, the interaction of water molecules with zircoꢀ
2
2
nium–oxygen bridges was found to occur during dehydration of the zirconium compound. The composition
of HfO · 0.5H O should actually read as Hf O (H O)(OH)2.
2
2
4
7
2
DOI: 10.1134/S0036023610060070
Hydrous dioxides of Group IV elements are used as not widely used because of the high coast of the preꢀ
catalysts for a number of chemical processes [1], as cursor zirconium compound. Noteworthy is the synꢀ
adsorbents for the recovery of platinum metals, merꢀ thesis of hydrous zirconia ZrO2
⋅
n
H O (n = 0.3–0.1) by
2
cury, and copper from complexing media [2], as well as means of ZrO(NO3)2 H O decomposition [15].
⋅
n
2
fission and corrosion radionuclides from aqueous, and
as liquidꢀmetal cooling agents of reactors and in sewꢀ
age decontamination [3].
Hydrous hafnia with the degree of hydration
depending on the precipitation parameters is recovꢀ
ered from solutions by analogy with zirconium comꢀ
Here we are concerned with the properties of pounds [16].
hydrous zirconia and hafnia xerogels and their variaꢀ
tions in response to thermal dehydration; we also genꢀ
eralize our results and compare them with the related
literature.
Having regard to the strengths and weaknesses of
the prior art methods and the availability of precursors
for hydrous zirconia and hafnia, we used the following
procedure. A weighed portion of zirconium or
hafnium tetrachloride(chemically pure grade) was disꢀ
solved in aqueous hydrochloric acid, which was preꢀ
pared by diluting concentrated HCl (36 wt %) with
EXPERIMENTAL
Hydrous zirconia is precipitated from acid soluꢀ distilled water to reach the ratio 1 : 1. Hydrous dioxides
tions prepared by dissolving ZrCl4 Zr(SO4)2
were precipitated from the resulting hot solution by
ZrO(NO3)2 H O, zirconyl chloride, and zirconyl concentrated aqueous ammonia. Precipitates were
,
,
⋅
n
2
sulfate using aqueous alkalis and ammonia or, very selꢀ washed by decanting hot water until a negative test for
dom, gaseous ammonia as precipitators. Stateꢀofꢀart chloride ion was obtained. As the washes gave the negꢀ
processes for preparing hydrous zirconia are listed in ative silver nitrate test for chloride ion, the sample was
Table.
When sulfuric acid derivatives and sodium or dried at 50
washed with hot water three or four time and then
to obtain a xerogel.
The thusꢀprepared MO2 H O samples were studꢀ
°С
potassium are the starting reagents, the relevant catꢀ
ions are occluded by the precipitate and hardly removꢀ
able by washing and subsequent drying [2, 4–14]. Proꢀ
cesses employing nitric or perchloric acid derivatives
⋅
n
2
ied by thermogravimetric analysis in combination with
crystalꢀoptical analysis, powder Xꢀray diffraction, and
IR spectroscopy. Thermal curves were recorded on
Qꢀ1500D and ODꢀ102 instruments in a dynamic
mode (weight scale sensitivity: 100 mg; DTG sensitivꢀ
ity: 250 mV; DTA sensitivity: 250 mV; heating rate in
–
are free of such drawbacks. Cl ions are readily solubiꢀ
lized from the precipitate [7], and it is far easier to
wash the precipitate from these anions.
An interesting method is to prepare hydrous zircoꢀ air to 1000
°С: 10 K/min; sample holder: an uncovered
nia by hydrolysis of isopropoxide Zr(C H O) in aqueꢀ platinum crucible) and in a quasiꢀisothermal mode
3
7
4
ous nitric acid solution [8]. However, this method is (heating in air to 1000 ; weight scale sensitivity:
°С
866

ON THE STRUCTURE AND DEHYDRATION
867
1
00 mg; decomposition rate: 0.4 mg/min; sample
holder: a covered conicalꢀshaped platinum crucible).
Sample sizes were varied within 200–300 mg.
Powder Xꢀray diffraction analysis was performed
on DRONꢀ3M, (Co
K
radiation, Fe
β
filter, 0.1° 2θ
α
scan steps, 5ꢀs scan time per point).
Crystalꢀoptical analysis was performed on an
MINꢀ8 polarizing microscope using an IZhꢀ1 immerꢀ
sion liquids kit.
IR spectra were recorded on Specord M 82 in the
–1
frequency range 4000–300 cm as potassium bromide
disks and Nujol mulls between potassium bromide
platelets.
Specific surface areas of test samples were derived
from thermal nitrogen adsorption measured on gas
meter in a GKh setup. Reference specific surface areas
were determined by measuring lowꢀtemperature krypꢀ
ton adsorption on a vacuum setup. The references
used were silica gel KSSꢀ3 (specific surface area:
2
6
00 m /g) and Silochrom SChꢀ3 (specific surface
2
area: 110 m /g).
We find it necessary to mention that IR spectrosꢀ
copy was used as a tool for verifying test samples for the
absence of impurities in addition to powder Xꢀray difꢀ
fraction and chemical analysis. Suspected impurities
2
3
–
+
4
in the test samples were as follows: CO ,
N
H ,
–
3
–
2
N
O , and
N
O . The last two ions are formed as a
+
result of transformations
N
H4 ions under heating in
2
3
–
–
3
–
air. Groups of light atoms (CO ,
N
O , and
N
O2 ),
which weakly interact with other atomic groups, give
rise to strong (narrow and sharp) absorption bands in
2
–
–
IR spectra with peaks at: CO3 , ~879 cm–1
(ν
2); NO3
,
–
O2
~
831 cm–1
(ν2); and
N
,
~1244 cm–1
(ν3) [17]. Such
bands were not observed in the IR spectra of the comꢀ
pounds described, which unequivocally signifies the
absence of the relevant ions in the test samples. For
+
–1
N
H4 ion, the strongest band at 1400 cm in the IR
spectrum of NH Cl is recognized as the analytical
4
Fig. 1. Thermal curves for HfO · 2.3H O hydrous hafnia
samples recorded (a) in a dynamic heating mode and (b) in
a quasiꢀisothermal mode.
2
2
band. Looking ahead, we mention that bands at simiꢀ
lar frequencies appear in the IR spectra of the comꢀ
pounds recovered at 300, 700, 900, and 1000 С. Howꢀ
°
ever, it is highly problematic whether NH Cl is conꢀ
tained in our test samples at such temperatures. There
4
therm (Fig. 1a) having a peak at about 110
tion, a second weak endotherm appears at 295
°
С. In addiꢀ
°С,
are no reflections intrinsic to NH Cl in the Xꢀray difꢀ
4
being in part hidden by the next strong exotherm. A
similar DTA curve (Fig. 1a) was earlier observed by
Sakharov et al. [4], who found the DTA curve to
ascend before entering the exotherm and who related
this fact to “certain structural alterations” of amorꢀ
phous dioxide in the “precrystalloization period.” The
DTG weightꢀloss peak (Fig. 1a) is clearly skewed, and
fraction patterns of these samples. Thus, the hydrous
dioxide formula units suggested from gravimetric and
thermogravimetric data are supported by powder
Xꢀray diffraction and IR spectroscopy.
The recovered hydrous hafnia is a single phase as
identified by crystalꢀoptical analysis, and its composiꢀ
tion is HfO2 2.3H O. The water content of the sample
⋅
2
decreases under heating. The thermal decomposition its skewness may appear an argument in favor of the
of the compound in the dynamic mode starts at 50
°С
twoꢀstep thermolysis scheme for hydrous hafnia.
and is accompanied by a wellꢀdefined DTA endoꢀ Thus, the features of the differential curves unequivoꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 6 2010

868
KOSTRIKIN et al.
[
10]. In order to gain more data on the transformation
I/I0
of hydrous hafnia during dehydration, we prepared
samples containing less water by heating the intact
sample to constant weight at 120, 200, 300, 400, 700,
and 1000
for recovering the products of HfO2
ysis, we were guided by the results of our thermograviꢀ
metric investigations and the literature data on the
transformations of hydrous hafnia. The relevant Xꢀray
diffraction patterns are shown in Fig. 2, and specific
surface areas are in Table 2.
°С
. In choosing these heating temperatures
⋅
2.3H O thermolꢀ
2
d
c
The observed variations in specific surface area
(
Table 2) indicate that, most likely, the first thermolyꢀ
sis step involves the elimination of water mainly from
individual particles of the xerogel, which is responsible
for the increasing specific surface area of the sample.
At the second step, weight loss is due to active dehyꢀ
droxylation accompanied by particle coarsening and
the associated decrease in the specific surface area.
b
а
2
0
30
40
50
60
70
, deg
The samples prepared at 50, 120, and 200 С are
amorphous to Xꢀrays. Xꢀray diffraction patterns of the
samples prepared by heating hydrous hafnia above
°
2
θ
Fig. 2. Xꢀray diffraction patterns for HfO2
recovered at ( ) 50, ( ) 300, ( ) 400, and (
⋅
d
nH O samples
2
4
1
00
8, 19].
Thus, as hydrous hafnia is heated, water is elimiꢀ
°С contain lines from monoclinic HfO2 (Fig. 2) [4,
a
b
c
)
700
°
С
.
cally indicate the twoꢀstep dehydration of the sample. nated in two steps according to the scheme below.
As temperature increases further, the DTA curve feaꢀ
7
5–220
°
C
220–520°C
HfO2
–0.5H O
2
HfO ⋅ 2.3H O
HfO ⋅ 0.5H O
tures a strong narrow exotherm (with a peak at 540
°С)
2
2
–1.8H O
2
2
2
associated with the crystallization of monoclinic
hafnia [4, 18]. The positions of the first observed dehyꢀ
dration peak and the HfO2 crystallization peak agree
well with Kadoshnikova et al.’s data [19]; however,
Kadoshnikova et al. [19] came to the conclusion of
Weight loss calcd., %
12.9
12.9
3.6
3.8
Found, %
Hydrous zirconia xerogel has the following compoꢀ
HfO2
⋅
nH O dehydration having one step. Poorly
2
sition: ZrO · 2.5H O. Prozorovskaya et al. [10] preꢀ
2
2
defined signatures of a second decomposition step are
noticed only in comparatively large samples (weighing
more than 200 mg). Smaller sample sizes do not allow
these signatures to be detected for certain. Earlier [20],
pared this xerogel by drying the precipitate at 20
This is an Xꢀray amorphous compound. However, its
Xꢀray diffraction pattern (Fig. 3, curve ) contains
broad reflections from interplanar spacings of 4.6795
and 2.9208 Å. Probably, this fact served as the basis for
Nazarov [8] to infer that ZrO · 2.5H O is a partially
°С.
а
this also misled us to the conclusion of oneꢀstep HfO2
⋅
n
H O thermolysis.
2
2
2
During the quasiꢀisothermal heating of HfO2
⋅
crystallized cubic phase.
ZrO · 2.5H O under heating loses water rather
2
.3H O hydrous hafnia, the weight loss curve (Fig. 1b)
2
2
2
does not feature horizontal or vertical segments. At intensely. Its thermal decomposition in a dynamic
20°С, however, the curve changes its slope quite disꢀ mode (Fig. 4a) occurs within 55–900
and is accomꢀ
tinctly. This change is reproduced in TG curves panied by an endotherm (peaking at about 95
) and
recorded for various HfO2 2.3H O sample sizes and then an exotherm (peaking at 395
), the latter correꢀ
2
°С
°С
⋅
°С
2
corresponds to the elimination of 1.8 water molecules sponding to the crystallization of tetragonal ZrO2; its
within 75–220°С. The remaining 0.5 water mole is transition to the monoclinic phase occurs at a higher
removed within 220–520°С. The trend of the quasiꢀ temperature [14]. The clearꢀcut skewed weightꢀloss
isothermal curve implies that the thermolysis of DTG peak may signify the twoꢀstep thermolysis of
hydrous hafnia is a nonꢀzeroꢀorder reaction (the ZrO · 2.5H O. Two dehydration steps were noted in [6,
2
2
weight loss associated with the elimination of molecuꢀ 21, 22].
lar water is accompanied by OHꢀgroup polycondensaꢀ
tion). Presumably, the range 75–220°С is where water has a smooth run without horizontal arrests or vertical
molecules are eliminated and polycondensation has segments. At 145
, however, the curve changes its
not great contribution. Within 220–520°С, polyconꢀ slope. The break point corresponds to the elimination
densation dominates. A phase of composition HfO2
of 1.5 water moles. The remaining 1 water mole is
.5H O was earlier found to form during thermolysis eliminated within 145–900 ; the main loss is
The quasiꢀisothermal weightꢀloss curve (Fig. 4b)
°С
⋅
0
°С
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 6 2010

ON THE STRUCTURE AND DEHYDRATION
Table 1. Syntheses of hydrous zirconia
Initial species Precipitation process
ZrOCl2
869
Precipitation
temperature
Other parameters
Not indicated
Water washing and drying at various temp.
Source
Hydrolysis in 10% KOH or Not specified
aq. NH3
[4]
ZrO(SO4)
ZrOC12
ZrOC12
Precipit. by aq. ammonia
Precipit. by NaOH sln.
Not specified
Ambient
[10]
Water washing, pulp refluxing, drying at 80 [11]
and 140°C
ZrCl4
ZrOCl2
Precipit. by aq. NH3,
gas. NH3
Ambient
Ambient
pH control, water washing, drying at 120°C.
Gel freezing followed by defrosting and waꢀ
ter washing. Drying at 120°C
[7]
ZrO(SO4)
Zr(SO4)2
ZrC14
Treat. of solids by KOH (10% Ambient
Not indicated
[5]
and 20%) an NH aq. (1%)
3
ZrOCl2
ZrO(NO3)2
Hydrolysis in NaOH sln.
Ambient
pH control, water washing by decantation
with centrifuging
[2]
ZrO(NO3)2
ZrOCl2
Hydrolysis in aq. ammonia Ambient
pH 10; repeated water wash., hydrothermal [12]
heating at 423–523 K
ZrC14
Precipit. by NaOH sln.
Electrochemical precipit.
Precipit. by NaOH sln.
Not specified
pH 6; water wash., freez. at –12°C, again
water wash.
[13]
ZrOCl2
⋅
8H O
Ambient
Water washing and drying at room temperaꢀ [14]
ture to constant weight
2
ZrO(NO3)2
Not specified
pH 5.5, 7.0, 10.0; water washing by decantaꢀ [6]
tion, precipitate aging in NaOH and Na SO
2
4
sln. (1 molꢀequiv./L) at 80 and 96°C
ZrO(NO3)2
Zr(C H O)
4
Precipit. by aq. ammonia
Hydrolysis by aq. HNO3
Ambient
Ambient
pH 9; aging under mother sln., washing and
drying at room temperature and 100°C
[9]
[8]
Sol was investigated
3
7
observed below 485
°
С
. This run of the weightꢀloss heated from 50 to 120 С, however, its specific surface
°
2
curve indicates the nonꢀzero order of the thermolysis area decreases by ~112m /g; heating from 120 to 200
°С
reaction; that is, the weight loss associated with the brings about a 77 m /g decrease in specific surface
elimination of molecular water is accompanied by the area. Thus, we may infer the active dehydroxylation of
polycondensation of hydroxo groups. The pattern of the compound within 120–200
the quasiꢀisothermal curve suggests that within 55–
45 the compound loses molecular water and polyꢀ
condensation has not great contribution; within 145–
00 , polycondensation dominates.
°
С
, which leads to parꢀ
ticle coarsening.
1
°
С
The samples recovered at 50, 120, and 200 С
°
are
Xꢀray amorphous (Fig. 3, curves ). The absence of
crystalline phases was detected up to 400–450 [23],
in good agreement with our observed peak temperaꢀ
ture (395 ) of the exotherm associated with zirconia
а–c
9
°С
°С
To gain more dehydration data, ZrO · 2.5H O was
2
2
heated to 120, 200, 300, 700, and 900
°
С
. These temꢀ
°С
peratures for recovering ZrO · 2.5H O thermolysis
2
2
crystallization. The lengthening heat treatment leads
to the crystallization of the compound in the tetragoꢀ
nal phase and, then, a mixture of tetragonal and monꢀ
products were chosen proceeding from the results of
our thermogravimetric experiments and by analogy
with the hafnium compound. As the heating temperaꢀ
ture rises, the specific surface area of the sample
decreases monotonically (Table 2). After a sample is
oclinic ZrO2 phases [9, 14, 22] (Fig. 3, curve d).
Thus, the heatꢀinduced dehydration of the ZrO2 ·
2
.5H O xerogel has two steps according to the scheme
2
2
below.
Table 2. Specific surface areas (in m /g) of hydrous zirconia
and hafnia recovered at various temperatures
5
5–145
°
C
145–485°C
–H O
2
ZrO ⋅ 2.5H O
ZrO ⋅ H O
ZrO2
2
2
–1.5H O
2
2
2
Temperature 50°C
120°C
200°C
700°C
ZrO2
HfO2
⋅
n
H O 248.0
239.5
143.5
162.0
57.0
142.0
32.0
Weight loss calcd., %
Found, %
16.1
16.6
10.7
10.7
2
⋅
nH O 107.0
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 6 2010

870
KOSTRIKIN et al.
I
/I0
m
, %
100
(а)
90
80
70
d
c
TG
DTG
b
395°С
95
°
С
DTA
а
60
20
30
40
50
70
, deg
100
(
b)
2
θ
Fig. 3. Xꢀray diffraction patterns for ZrO2
recovered at ( ) 50, ( ) 120, ( ) 200, and (
⋅
d
nH O samples
2
90
a
b
c
)
700
°
С
.
In view of the detailed consideration of the properꢀ
ties of hafnium hydroxo salts by IvanovꢀEmin et al.
24], the IR spectroscopic data for HfO2 2.3H O are
80
[
⋅
2
interpreted with high consistency. This allowed us to
study the properties of hydrous hafnia in a detailed way
and, then, interpret experimental data on zirconium
compounds by analogy.
7
0
200
600
T, °C
Fig. 4. Thermal curves for hydrous zirconia ZrO2
samples recorded (a) in a dynamic heating mode and (b) in
a quasiꢀisothermal mode.
⋅
2.5H O
2
Figure 5 shows the IR spectra of an HfO2 2.3H O
⋅
2
sample and the products of its thermolysis recovered at
various temperatures. Assignment was performed on
the basis of related literature [24–27] and our experiꢀ
mental data (Table 3). Apart from this, we obtained IR
spectra from anhydrous hafnia, potassium hexahyꢀ
droxohafnate, and potassium oxohydroxohafnates (as
Nujol mulls in sodium chloride platelets). Comparaꢀ
tive analysis allowed the more consistent assignment.
is enhanced. This implies the elimination of relatively
weakly bound water molecules from HfO2
⋅
2.3H O
2
upon heating to 200 and the enhancement of interꢀ
°С
actions of the remaining water molecules with the
structural elements of the compound.
The IR spectrum of the thermolysis product recovꢀ
HfO2 2.3H O is a compound built of the following
⋅
2
ered at 1000 С and exposed in air for several hours feaꢀ
°
atomic groups (Table 3): OH groups (either unbound
tures absorption bands associated with hydroxo groups
and water molecules. In all probability, the occurrence
of water vapor in the laboratory atmosphere leads to
rehydration upon cooling the instrument after experiꢀ
or bound by strong hydrogen bonds), H O molecules,
2
Hf–O–H terminal groups, and Hf–O(H)–Hf and
Hf–O–Hf bridging groups, these groups appearing in
the IR spectrum as individual or composite absorption
bands.
ments. The comparison of the intensity of the δ(H O)
2
absorption band with the
(HfOH) bands in the IR spectrum of the thermolysis
product and a similar intercomparison with the IR
spectrum of HfO2 2.3H O, implies the low rehydraꢀ
δ
(Hf–O(H)–Hf) and
As temperature rises, the absorption bands associꢀ
ated with the vibrations of hydroxo groups and water
molecules dramatically weaken until complete disapꢀ
δ
⋅
2
pearance (Table 3). At 120
to highꢀfrequency and lowꢀfrequency components. At
00 , the lowꢀfrequency component disappears and
°
С, the δ(H O), band is split
2
tion rates of Hf–O–Hf olic bonds.
2
°С
The assignment of ZrO · 2.5H O absorption bands
2
2
the highꢀfrequency shift of the other component peak in the IR spectrum (Table 4) was based on (a) the
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 6 2010

ON THE STRUCTURE AND DEHYDRATION
871
d
c
b
а
32
24
16
8
–1
–
2
ν × 10 , cm
Fig. 5. IR spectra of HfO2
⋅
n
H O samples recovered at (
a) 50, (
b
) 120, ( ) 200, and (
c
d)
700°C.
2
results of thermogravimetric experiments, (b) related to shift to high frequencies. For example, in the specꢀ
literature [24, 26, 28–32], and (c) by analogy with the trum of the SnO2 1.75H O analogue, the absorption
IR spectra of HfO2 2.3H O and its thermolysis prodꢀ band responsible for the
ucts. The ZrO · 2.5H O spectrum features several appears at 1406 cm , and the
⋅
2
⋅
δ
(Sn–O(H)–Sn) vibrations
(Sn–OH) mode
absorption bands signifying the presence of OH groups appears at 1270–1070 cm [35]. Therefore, the
unbound or bound in strong hydrogen bonds), H O absorption bands in the IR spectra of ZrO · 2.5H O
2
–1
δ
2
2
–1
(
2
2
2
molecules, Zr–O–H terminal groups and Zr–O(H)– and its thermolysis products with peaks in the region of
–
1
Zr and Zr–O–Zr bridging groups.
1530–1330 cm , which change their intensity to disꢀ
appear upon heating, can fairly arise from the vibraꢀ
–1
Notably, the absorption peak at 1575 cm is due to
tions of bridging bonds (δ(Zr–O(H)–Zr) mode).
the bending vibrations δ(H O) of water encapsulated
2
in mesopores in the form of vapor (Table 4). Trens [21]
detected this type of water in a ZrO2
Once the heating temperature is raised to 200
new absorption bands appear in a ZrO · 2.5H O samꢀ
⋅
nH O sample.
2
RESULTS AND DISCUSSION
°С,
Hydrous zirconia is prepared by hydrolyzing
oxochloride or another zirconium compound, but zirꢀ
2
2
ple (Table 4, Fig. 6). The intensities of these newly
–1
conium oxochloride ZrOCl is always involved in the
2
resolved bands (peaks at 1529 and 1490 cm ) decrease
dramatically in response to rising temperature. Thus,
these bands are associated with the vibrations of
conversion chain.
There is abundant literature on the hydrolysis of
hydroxylꢀcontaining groups, which are most likely chloride compounds of zirconium, and diversified
Zr–O(H)–Zr. The assignment of these absorption investigation tools were used, namely, diffusion [36–
bands to the vibrations of bridging bonds may be justified 39], dialysis [40], potentiometry [39, 41], cryoscopy
by the following reasons. Compounds with a high E–OH [39, 41], spectrophotometry [42], and partition [43].
bond covalence feature the bands of (
EОН) vibrations As to the structure of hydrous zirconia, a reasonable
δ
–1
in a highꢀfrequency region (1300–1400 cm [29, 33]); assumption is that the structure of the initial comꢀ
in the IR spectrum of gaseous methanol, for example, pound (in the case, zirconium oxochloride) dictates
–1
the
tion enhancement as a result of, for example, the crystallography [44] shows that hydrous zirconium
bidentate binding of an OH group will cause this band oxochloride ZrOCl 8H is built of tetrameric cyclic
δ
(СОН) band appears at 1340 cm [34]. Interacꢀ the structure of the precipitated product [4]. Xꢀray
⋅
O
2
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 6 2010

872
KOSTRIKIN et al.
–
1
Table 3. Absorption peaks (in cm ), their relative intensities, and assignment in the IR spectra of oxygenꢀandꢀhydroxylꢀconꢀ
taining hafnium compounds recovered at various temperatures
HfO2
comp.
HfO2
⋅
2.3H O and its thermolysis products
K Hf(OH)
K Hf O (OH)
2 2 3 5
2
2
6
Assignment
5
0
°
C
120
°
C
200
°
C
700
°
C
1000
°
C
25
°
C
25
°
C
220
°
C
250°C
3
748
w
3748
3363
w
3721 m 3734 m 3729 w
3
526 sh
3661 3593 sh
w
3
484 sh
3403
3366
280
159
m
m
m
m
m
3470
3411
w
3404
s
s
w
w
w
3434 sh
3305 s
3154 s
ν
(OH)
3
3203
2916
w
w
3210 s
Not
3300 w
3200 s
3
2979
s
s
2924 sh 2920
1603
1616 sh 1629
m
1610
1365
Not
Not
Not
1576
s
1583
1401
w
w
appear. appear. appear. appear.
δ
(H O)
2
1
391
358
112
m
s
1460
1364
m
1480
w
Nujol
band
Nujol
band
Nujol
bands
Nujol bands
s
s
δ
(Hf–O(H)–Hf)
1
s
1392 m 1364
1146 sh
w
1
w
δ
(HfOH)
(HfO)
1
063
m
1056 sh 1088
002
m
m
1055 sh
995 s
885 w
844 sh
788 s
1
2
ν
967 sh
959 sh
963 sh
967 sh 967 w 979 sh
9
8
25 s
85 sh
819 sh
79 s
950 s
897 m
850 m
δ
(HfOH)
842
s
s
846 sh
785 sh
856
m
7
767
778 sh
663 sh
766
s
769 m 776 m
33 m
ν
(HfOHf)
7
652 sh
664 sh
547 sh
661 sh 661 sh Intens.
6
13
550 sh
480
s
640 sh
absorpꢀ
tion
617
560
w
w
625
w
5
97 sh
524
s
s
517 s
438 s
ν
(HfO)
464
s
468
s
s
473 s
433 sh
483 s
460 s
421
fragments [Zr (OH) ]8+. These fragments also contain of polymerization, which always exceeds four [40, 41]
4
8
and depends on the solution acidity.
1
6 water molecules (four molecules per zirconium
atom). When alkali is added, some water molecules,
Probably, hydrous hafnia HfO2 2.3H O is also built
⋅
2
clearly,
Zr4
enter
OH
groups
to
form
of tetrameric hafnium–oxygen–hydroxide fragments,
like those of which hydrous zirconia is built [11]. It
contains Hf–O(H)–Hf and Hf–OH atomic groups
4
+
b
t
⎡
⎣
OH)8 (OH
⋅12H O⎤ , fragments, where OHb
(
)
2
⎦
4
are bridging hydroxo groups and OHt are terminal and water molecules. These hydroxylꢀcontaining
hydroxo groups [39, 45]. The process ends with the groups and water molecules take part in hydrogen
0
bonding between structural elements. A particular
indication of this hydrous hafnia structure is the forꢀ
b
t
formation of ⎡Zr
OH)8 (OH
⋅8H O⎤ uncharged
(
)
4
2
⎣
8
⎦
fragments. These fragments are linked by hydrogen mation of a compound of the Na Hf O
⋅
5H O tetꢀ
2
4
9
2
and Zr–O(H)–Zr bridging bonds to form grains with rameric anion in the lowꢀalkali region (less than
sizes from 3 to 64 nm [8, 46]. Sometimes, they contain 3.18 mol/L) in the Na O–HfO aq–H O system [47].
2
2
2
up to 70 tetramers per grain [8]. This structure of the
polymer in zirconium oxochloride solutions is responꢀ
sible for its increased viscosity [39]. Bridging bonds
between tetramers are less strong (here, steric factors
are important) and are more easily destroyed by acids
8+
The existence of Hf (OH)8 tetrameric cation was
4
noted in perchloric acid solutions when hafnium conꢀ
–3
centrations were less than 10 mol/L [48].
While being dried, the hydrolysis products experiꢀ
or alkalis or as the solution temperature increases. This ence the following transformations: sol
gel
explains the existence of species with strongly diversiꢀ xerogel. These transformations are accompanied by a
fied molecular weights (from 800 to 2600 [36] to decrease in the number of water molecules in polyꢀ
5
400–7000 a.m.u. [46]) in solutions of various comꢀ meric fragments. ZrO
positions. The same is responsible for a varied degree formed in this way.
· 2.5H O
2
and HfO 2.5H O are
⋅
2
2 2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 6 2010

ON THE STRUCTURE AND DEHYDRATION
873
–1
Table 4. Absorption peaks (in cm ), their relative intensities, and assignment in the IR spectra of ZrO2
⋅
n
H O samples recovꢀ
2
ered at various temperatures
ZrO · 2.5H O and its thermolysis products
2
2
Assignment
50
°
C
120
°
C
200
°
C
300
°
C
900
°
C
3
542 s
475 s
3545 m
3592 w
3588 w
3534 w
3
3492 s
3463 m
3417 m
3381 m
3344 m
3292 m
3237 m
3188 m
3146 m
3099 m
3040 m
3014 m
3
428 s
411 s
3448 w
3341 w
3245 w
3417 sh
3367 w
3277 w
3
3
337 s
261 s
3348 s
307 s
3240 s
3
3
ν
(OH)
3
203 sh
153 s
3
3155 w
3076 w
3110 s
3017 s
2863 s
3100 s
054 sh
3
3004 sh
2945 m
2849 s
2832 m
2720 m
2681 s
1
622 s
1648 w
1623 w
1631 w
δ
δ
(H O)
2
1
1
1
599 s
575 s
524 s
1598 s
1596 s
1571 s
(H O) in pores
2
1506 s
1354 s
1529 m
1532 w
1455 sh
1490 m
1406 sh
1350 sh
δ
(Zr–O(H)–Zr)
1357 s
1362 s
1398 sh
1326 sh
1333 sh
1269 w
1
169 sh
059 sh
1188 w
1057 sh
970 s
1197 w
1061 sh
976 s
1105 w
1034 w
1
δ
(ZrOH)
983 s
938 s
847 s
995 w
942 s
946 s
938 w
853 s
848 s
7
74 sh
775 s
721 s
692 s
785 sh
737 sh
665 s
7
03 s
61 s
704 sh
650 sh
743 s
6
681 sh
634 sh
592 s
562 s
515 s
473 s
439 s
404 s
372 s
606 s
583 s
509 s
424 s
ν
(ZrO)
563 s
525 s
478 s
515 s
482 s
489 s
424 s
404 s
372 s
346 s
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 6 2010

874
KOSTRIKIN et al.
d
c
b
а
34
28
22
16
10
ν × 10–2 cm–1
Fig. 6. IR spectra of ZrO2
⋅
n
H O samples recovered at (
a
) 50, (
b
) 120, (
c
) 200, and ( 900°C
d
)
.
2
Xerogel dehydration occurs in two steps. From gen bridges are enhanced in the same order, namely, in
ZrO · 2.5H O, unlike from hydrous hafnia, water is the Hf Zr Sn series of compounds. Rehydraꢀ
2
2
eliminated even at the first thermolysis step as a result tion is responsible for variations in zeroꢀcharge point
of the polycondensation of hydroxo groups being pHs (pHzcp.) of hydrous dioxide surfaces found from
rather intense. The second thermolysis step is almost the results of potentiometric titration [50]. As the dryꢀ
completely due to the polycondensation of hydroxo ing temperature of hydrous hafnia increases, the pHzcp.
groups. HfO2 2.3H O has the following noteworthy
⋅
2
increases monotonically, too; for ZrO2 ·
nH O and
2
dehydration feature: water molecules almost do not
interact with hafnium–oxygen bridges under heating.
SnO2 · H O, in contrast, these values shift down. The
n
2
interactions of water molecules with oxygen bridges in
the structure of the last two hydrous oxides increase
the amount of hydroxo groups on particle surfaces,
and this in turn decreases pHzcp.
1
Here, we will mention the results of the H NMR
investigation of HfO2 2.3H O and ZrO · 2.5H O perꢀ
⋅
2 2 2
formed by Prozorovskaya et al. [10]. They discovered
that the number of hydroxo groups per mole of
hafnium atoms in the structure of hydrous hafnia is
constant within 20–125°С.
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 6 2010