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Texture and nanostructure of chromia aerogels preparedby urea-assisted homogeneous precipitation and
low-temperature supercritical drying
M. Abecassis-Wolfovich a, H. Rotter a, M.V. Landau a,*, E. Korin b,A.I. Erenburg c, D. Mogilyansky c, E. Gartstein c
a Chemical Engineering Department, The Blechner Center for Industrial Catalysis and Process Development,
Ben-Gurion University of the Negev, Beer-Sheva 84105, Israelb Chemical Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israelc The Institutes for Applied Research, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
Received 14 February 2002; received in revised form 24 July 2002
Abstract
Mesoporous chromia aerogels with a surface area of 484735 m2 g1, a pore volume of 0.40.9 cm3 g1 and a porediameter of 39 nm were prepared by urea-assisted homogeneous precipitation from an aqueous Cr(NO3)3 solution,
followed by continuous supercritical extraction with CO2 under dierent conditions (pressure and time) after re-
placement of the water with a hexane/2-butanol mixture. The texture and chemistry of the aerogels transformed by
heating in air or an inert atmosphere and the structure of the nanoparticles were characterized by means of N2-ad-
sorption isotherms, AA, HRTEM, FTIR, a variety of thermoanalytical methods (TPD, DSC, TGA, TPOTPK) and
X-ray diraction in combination with structure modeling. At the CO2 extraction stage, a pressure of about 400 bars was
critical for production of aerogels with surface areas >700 m2 g1. The fresh chromia aerogels consisted of closelypacked almost globular, 3- to 5-nm nanoparticles with a structure analogous to that of monoclinic a-CrOOH, in whichhalf of the O atoms and OH groups were replaced with coordinately bonded water molecules. After dehydration at 550
600 K, the materials retained their texture, being converted to faceted 3- to 5-nm nanoparticles, consisting of two-
dimensional fragments (clusters) of a-CrOOH crystals built on [Cr(OH)3O3] octahedra without bonding along theZ-axis. The texture of dehydrated chromia aerogels was stable at temperatures up to 650 K in air and up to 773 K in aninert atmosphere. At higher temperatures, the material underwent a glow transition, yielding microcrystalline 50-nm
particles with the well-dened structure of a-Cr2O3 and a surface area
variety of uses, particularly as green pigments,coating materials for thermal protection and wear
resistance, heterogeneous catalysts, and transpar-
ent colorants, inter alia. A great deal of eort has
thus been invested in developing dierent synthesis
strategies that facilitate reliable control of the
texture and structural parameters of these ultrane
oxides. The most popular synthetic route is pre-
cipitationgelation from an aqueous Cr(III) saltsolution (usually the nitrate); this route gives a
surface area in the range of tens to hundreds of
square meters per gram and a structure that varies
from orthorhombic Cr(OH)3 through amorphous
to hexagonal a-Cr2O3 [111]. The most ecientmethod for providing a large surface area is ho-
mogeneous precipitation, i.e., slow alkalinization
of an aqueous Cr(NO3)3 solution by hydrolysis ofdissolved urea [27,9,11]. Dry xerogels obtained by
this method are microporous materials with a
surface area of 250350 m2 g1 and a pore volumeof 0.100.12 cm3 g1, corresponding to a pore dia-meter of 0.62.0 nm. The gel structure may be
stabilized by formation of additional chemical
bonds by condensation during hydrothermal
treatment at 573 K before conventional drying toyield a xerogel with a surface area of 486 m2 g1
and a pore diameter of 1.6 nm [5]. Implementation
of methods that prevent the collapse (caused by
capillary stress) of the initial gel texture during the
drying step facilitated a further increase in the
surface area of such urea-assisted chromia gels and
shifted the pore size distribution (PSD) into the
mesoporous range. Replacement of water as thesolvent with pentane, which has a lower surface
tension, followed by supercritical release of the
pentane at >470 K produced an aerogel with asurface area of 447 m2 g1 [5]. If methanol wasused instead of pentane (with supercritical release
at 578 K (13.1 MPa)), mesoporous aerogels were
obtained with a surface area of 503785 m2 g1
and a pore volume of 2.43.7 cm3 g1, which cor-responds to a pore diameter of 20 nm [12,13].Freeze-drying of urea-assisted chromia gels did
not prevent their collapse and gave cryogels with a
surface area of 150160 m2 g1 and a pore volume0.5 cm3 g1 [14].
Until recently, no attempts had been made to
dry urea-assisted chromia gels by low-temperature
(316373 K) supercritical solvent extraction withCO2 [15], despite the fact that this method had
been used successfully for preventing thermal de-
gradation or degradation induced by capillary
forces of metal-oxide gels [16]. However, this ex-
traction method has now been used successfully
for the preparation of chromia aerogels with a
surface area of 420520 m2 g1 from materialsprepared by gelation of chromium salt precursorswith propylene oxide in ethanol [17].
Despite the fact that investigation of the struc-
ture of chromia gels started as long ago as 1950
[18], structural characterization of urea-assisted
chromia xerogels and aerogels has never been
systematically undertaken. In contrast, the gela-
tion of chromia from aqueous solutions of
Cr(NO3)3 is well documented [19,20]. This gelationis based on the hydrolysis of the hexaaquacation
[Cr(OH2)6]3 upon alkalinization. Rapid alkalini-
zation to pH > 10 with NH4OH yields a crystal-line gel as a result of extensive condensation of
hydrolyzed aquaions by olation. The gel, having
the formula Cr(OH)3, exhibits the well-dened X-
ray diraction (XRD) patterns of an orthorhom-
bic system similar to that of alumina bayerite[8,10,21]. The gel is built up from polymeric la-
ments
held together in the b and c directions by hydrogenbonds. It was found that the crystalline structure,
whose thermal stability is very low, was trans-
formed into an amorphous material after partial
dehydration at 333 K [8]. After gradual dehydra-
tion by exposure to temperatures up to 523543K, the amorphous solid, which consisted of small
573 K the material exhibited well-denedXRD patterns [8,10,21].
The structure of urea-assisted chromia gels ob-
tained at lower pH values of 69.5 is substantially
96 M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111
dierent from the structure described above. Suchchromia xerogels are always amorphous [4
7,11,18,22]; they contain bonded water corre-
sponding to a H2O/Cr2O3 ratio of up to 5 [4,6,22],
and after endothermic dehydration at 423573 K,
they also crystallize exothermally at >673 K intohexagonal a-Cr2O3 with a well-dened XRD pat-tern and a surface area of 673 K.It has previously been reported that high-surface-
area chromia aerogels obtained after supercriticalrelease of methanol at 578 K are crystalline [12].
These aerogels exhibited XRD patterns that could
not be attributed to any known crystalline Cr oxide
or Cr hydroxide phase, but which could perhaps
be explained by large lattice distortion [12]. There
is no information in the literature on the texture
and structure of chromia aerogels further trans-
formed at temperatures higher than 578 K.In the present work, we focus on the two
problems that have so far been overlooked in
preparation of nanostructured chromium oxides:
(1) the eect of the conditions of low-tempera-
ture CO2 supercritical extraction on the texture
and structure of urea-assisted chromia aerogels,
and (2) texture/structure transformations during
their heating-dehydration. N2-adsorption (BET,PSD), HRTEM, DTADTG, AA analysis, FTIR
spectroscopy, thermoprogrammed desorption
(TPD)thermoprogrammed oxidation (TPO)
thermoprogrammed reduction (TPR) methods and
XRD in combination with structure modelingwere used for characterization and identication
of the texture and structure of low-temperature
chromia aerogels.
2. Experimental
2.1. Sol-gel aerogel synthesis
A wet gel of chromium(III) hydroxide was
prepared by mixing aqueous solutions of urea (0.1
M, Aldrich) and CrNO33 9H2O (0.038 M, Ri-edel de Haen) in ratio of 1.5:1 (v/v), agitating the
mixture at 368 K for 6 h, and nally allowing
the mixture to age at room temperature for 16 h.
The wet gel was separated by ltration, washed afew times with distilled water, and loaded into the
ask of an apparatus for solvent replacement by
distillation, as described in [23]. The ask was l-
led with a 1:1 (v/v) mixture of 2-butanol and cy-
clohexane. The distillate, composed of a mixture of
water and the organic phase, was collected in a
separating funnel; the organic material was recy-
cled to the ask; and the process was continueduntil all the water had been removed from the gel.
After solvent replacement, the wet gel was con-
verted to an aerogel by supercritical drying (ex-
traction) with CO2 in a supercritical extraction
system (Model SFX 220, ISCO, UK). The eects
of supercritical drying conditions at dierent
pressures (116456 bars) and drying times (0.252
h) were studied at a temperature of 313 K and aow rate of 1 mlmin1. The discharged aerogelwas further dehydrated under vacuum (85 mbar)
at 373593 K for 16 h to yield a high-surface area
nanostructured chromium oxide material. Cr-
xerogel was prepared by drying the wet chromia
gel at 373 K in air for 16 h and further evacuation
(85 mbar) for 16 h at 593 K. The Cr content in the
solid samples was determined by AA (k 357 nm,Varian Spectra 250 spectrophotometer) afterdissolution in aqueous HNO3H2SO4.
2.2. Characterization of chromia aerogels
X-ray diractograms were recorded on a Phil-
lips diractometer PW 1050/70 (CuKa radiation)
M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111 97
equipped with a graphite monochromator. Datawere obtained at a 0.02 step size with 2-s expo-sition. The peak positions and the instrument peak
broadening b were determined by tting eachdiraction peak by means of APD computer
software. The crystal domain size was determined
from the Sherrer equation: 1 Kk=B2 b20:5 cos2h=2, where K 1:000; k 0:154 nm; Bpeakbroadening at 2h 35:0 was 63.5 for nanostruc-tured chromia samples and at 2h 54:7, 65.2 forhighly crystalline Cr2O3. Representation of the
structures was performed with CarIne Crystallog-
raphy, version 3.1 crystallographic software.
Infrared spectra were recorded by Nicolet Im-
pact 460 FTIR spectrometer in KBr pellets (0.005
g sample and 0.095 g KBr), scan number 36, res-
olution 2 cm1 and analyzed by OMNIC software.Dierential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA) measurements
were obtained on a TA 8200 Mettler Toledo sys-
tem. The DSC instrument was calibrated with pure
indium and zinc. The analyses were performed in
an Al crucible (of mass 4752 mg) under both ni-
trogen (99.99% pure) and air atmospheres.
Surface area, pore volume and PSD of thematerials were obtained from N2 adsorption
desorption isotherms (BET and BJH methods).
The isotherms were obtained at the temperature of
liquid nitrogen on a NOVA-1000 (Quantachrome,
version 5.01) instrument. Prior to measurements
the Cr-aerogels, directly after CO2-extraction and
after dierent thermal treatments, were degassed
at 373 K for 16 h. The equilibration time at eachsorption step was sucient to avoid the widening
of hysteresis loop [24,25]. It allowed the correct
estimation of the pore volume and PSD of chro-mia aero- and xerogels.
Samples for HRTEM were prepared by depos-
iting a drop of an ultrasonicated aqueous suspen-
sion on a carbon-coated Cu grid. The grid was
dried at 313 K under vacuum and mounted in the
specimen holder. Micrographs were recorded with
JEM 2010 microscope operated at 200 kV.
Water evolution (TPD), TPO and TPR spectrawere recorded with an AMI-100 Catalysts Char-
acterization System (Zeton-Altamira) equipped
with an Ametek 1000 mass-spectrometer. TPD
runs were performed in a He ow, TPO runs, in a
5 vol.% O2 in He, and TPR runs, in 10 vol.% H2 in
Ar; all were performed at a heating rate of 5 Cmin1.
3. Results and discussion
3.1. Texture of the aerogels and xerogel
The preliminary experiments showed that in-
creasing the evacuation temperature of Cr-aero-
gels from 313 K (CO2-extraction temperature) to373 K increases the surface area and pore volume
by a factor of 1.52. Further increase of the
evacuation temperature up to 593 K did not aect
the samples texture. Therefore all the N2-adsorp-
tion measurements were carried out after evacua-
tion at 373 K. The textural parameters of the
chromia aerogels prepared by CO2 extraction at
dierent pressures and times at a constant extrac-tion temperature are given in Table 1. Fig. 1 pre-
sents a nitrogen adsorptiondesorption isotherm,
Table 1
Textural properties of chromia aerogels
Sample # Extraction conditions qCO2(kgm3)
Surface area (m2 g1) Pore diameter (nm) Pore volume(cm3 g1)P (bar) Time (h) Total Micropore Average Mean
2M 116 0.25 0.643 484 334 3.1 3.6 0.37
4M 116 1.00 0.643 539 363 3.1 3.5 0.42
3M 116 2.00 0.643 540 364 3.5 3.6 0.45
1b 184 1.00 0.740 525 86 4.9 4.6 0.65
2b 252 1.00 0.840 651 108 5.5 4.7 0.89
3b 320 1.00 0.915 599 171 5.0 4.7 0.74
4b 387 1.00 0.958 735 247 4.8 4.7 0.87
5b 456 1.00 0.976 712 152 5.2 4.5 0.93
98 M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111
PSD, and the results of t-plot analysis for repre-sentative sample of Cr-aerogel designated 4b(pressure 387 bars, time 1 h; Table 1). The features
shown in Fig. 1 are typical for all the Cr-aerogels
obtained under dierent CO2 extraction conditions
when the further treatments under vacuum, ni-
trogen or air did not cause substantial sintering or
a decrease in the surface area below 200 m2 g1.The aerogel displayed a type IV isotherm, with
desorption hysteresis and the mesoporosity of arelatively narrow PSD with a mean pore size di-
ameter of 3.54.7 nm. After closure the hysteresis
loop as pressure approached saturation, the ad-
sorbed volume remained constant with further
increasing the pressure up to P=P0 1 (Fig. 1,
isotherm). According to [25] it is indicative thatchromia aerogels are sti enough to tolerate ni-
trogen condensation/desorption without signi-
cant deformationcontraction of the samples
texture during N2-adsorption measurements. It
yielded the correct information about the aerogels
pore volume and PSD. The HRTEM micrograph
(Fig. 2(a)) showed that the untreated chromia
aerogel consisted of disordered closely packed 3-to 5-nm, almost globular, primary particles.
Evacuation at 373 K resulted in formation of
faceted nanoparticles of the same size and shifting
the packing mode of primary nanoparticles from
dense to friable packing (Fig. 2(b)). It explains the
increase of the surface area and pore volume of the
Cr-aerogels by higher accessibility of the nano-
particles to the nitrogen adsorption.The xerogel obtained after drying the wet
chromia gel at 373 K in air for 16 h displayed the
total surface area of 349 m2 g1, micropore surfacearea of 340 m2 g1, pore volume of 0.17 cm3 g1
and average pore diameter of 1.9 nm. No signi-
cant changes of the xerogels textural parameterswere observed after evacuation at 593 K. The
substantially lower surface area and pore volumeof chromia xerogel relative to aerogels were caused
by the dense packing of 3- to 5-nm faceted primary
particles (HRTEM) as a result of degradation of
polymeric gel structure induced by capillary forces
during air drying. This produced a completely
microporous solid in agreement with the data re-
ported previously for chromia xerogels [9,11,22].
3.1.1. Eect of CO2 extraction conditions
Increasing the duration of CO2 extraction from
0.25 to 1.0 h increased the surface area of the
aerogels from 484 to 540 m2 g1, but a further in-crease of the extraction time did not aect the
texture (Table 1). Therefore, in further experi-
ments the extraction was conducted for periods of
1 h.Increasing the pressure of CO2 extraction from
116 to 456 bars gradually increased the surface
area of the chromia aerogels from 530 to 730m2 g1 and more than doubled the pore volumefrom 0.42 to 0.93 cm3 g1, while the mean mes-opore diameter remained almost constant in the
CO2 extraction pressure range of 184456 bars
Fig. 1. Typical nitrogen physisorption patterns for the chromia
aerogel designated 4b evacuated at 373 K.
M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111 99
(Table 1). Increasing the pressure caused a three-to vefold decrease in the microporosity of the
chromia aerogels, as reected by the contribution
of micropores to the total surface area. The con-
tribution of microporosity to the total surface area
of the chromia aerogels varied from 12% to69%, depending on the CO2 extraction condi-tions. This is a result of the formation of a more
open structure built up of primary nanoparticles.
Such a structure is formed due to the higher sol-ubility of the solvent mixture inside the gel struc-
ture as a result of the increasing density of the
supercritical uid (CO2) with increasing pressure
at xed temperature. This yielded more rapid and
ecient solvent removal resulting in a more open
texture of the aerogel.
The CO2 extraction pressure also aected PSD
in the chromia aerogels (Fig. 3). The PSD was
Fig. 2. HRTEM micrographs of a typical chromia aerogel (sample 4b): (a) after CO2 extraction; (b) after CO2 extraction and
evacuation at 373 K; (c) after CO2 extraction and evacuation at 650 K; (d) after CO2 extraction and evacuation at 593 K followed by
calcination in air at 723 K.
100 M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111
shifted to higher pore diameters by increasing the
pressure from 116 to 184 bars, while a further
pressure increase caused narrowing of the meso-pore size distribution due to more uniform and
faster solvent removal at high pressures.
3.1.2. Eects of calcination
The stability of the texture of the chromia
aerogels under heating depends both on the at-
mosphere in which the samples were calcined and
on the temperature. Heating from 313 to 373 Kcaused increase of the surface area and pore vol-
ume by a factor of 1.6 (Fig. 4) due to shifting the
shape and packing mode of primary nanoparticles
from dense packing to a friable packing without
changing of their size (Fig. 2(a) and (b)). Heating
at temperatures range of 373650 K did not aect
the shape and packing mode of the nanoparticles
(Fig. 2(b) and (c)). At temperatures up to 650 K,the surface area, pore volume, PSD and the con-
tribution of microporosity to the total surface area
remained almost constant and were not dependent
on the calcination atmosphere (air, vacuum or N2).
This typical behavior is shown in Fig. 4 for sample
4b heated in air. Heating to temperatures beyond
650 K caused dramatic changes in the texture pa-
rameters of the chromia aerogels (Fig. 4). At these
elevated temperatures, the thermostability of the
texture was strongly inuenced by the atmosphere.In air, the surface area dropped to 4050 m2 g1 inparallel with a decrease in the pore volume of an
order of magnitude and an approximately twofold
increase in the average pore diameter (Fig. 4).
Heating in an inert atmosphere (Fig. 4) or under
vacuum (not shown) shifted this texture (glow)
transition to a higher temperature of 773 K. The
surface area of the material heated at 823 K ex-ceeded 100 m2 g1. TEM showed that beyond theglow transition the chromia aerogel structure was
made up of separate 50-nm microcrystals with-out any secondary structure (Fig. 2(d)).
The above-described transformations in the
texture of the chromia aerogels during thermal
treatment were caused by phase transitions, which
included the formation of oxide phases that havenot previously been identied by XRD (Fig. 5).
Below the texture (glow) transition temperature,
the peaks of these phases in the diractograms of
high-surface-area chromia aerogels were broad
due to the small size of the nanocrystals compris-
ing the aerogels (1.52.0 nm in diameter, as esti-
mated by the Sherrer equation). The XRD
Fig. 3. PSDs derived from the desorption branches of N2-physisorption at 77 K (at STP) on chromia aerogels extracted under dierent
conditions.
M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111 101
patterns of the chromia aerogel obtained immedi-
ately after CO2 extraction displayed maxima cor-
responded to d-spacings of d1 0:47 nm; d2 0:25nm; d3 0:20 nm and d4 0:15 nm. After evacu-ation at 593 K, the XRD pattern changed, givingthree broad peaks that reected d-spacings ofd1 0:246; d2 0:201 and d3 0:148 nm. SuchXRD patterns were observed rst by Armor et al.
[12] for chromia aerogels obtained by supercritical
methanol release at 578 K from a chromia gel
prepared by urea-assisted homogeneous precipi-
tation, followed by replacement of water with
methanol. These latter XRD patterns were as-cribed to a non-identied oxide material with large
lattice distortion [12]. The XRD pattern of our
chromia aerogels did not change at temperatures
up to 773 K under vacuum or an inert atmosphere,
but heating in air at 593 K immediately after CO2-
extraction or after evacuation at 593 K yielded
broadened XRD peaks, corresponding to an
amorphous material. Only after sintering at >650K in air or at >773 K under vacuum or in inertatmosphere were the XRD patterns of our chro-
mia aerogels in a good agreement with those for
well-dened a-Cr2O3 crystals [26]. The oxidestructure of chromia aerogel nanoparticles ob-
tained after exposure to dierent temperatures and
atmospheres was characterized in terms of their
chemical compositions and thermoanalytical be-
havior and by modeling the XRD patterns on the
basis of structure simulations.
3.2. Aerogel structure
3.2.1. Chemical composition and thermoanalytical
behavior
An analysis of the samples designated 2b and 4b
(extracted for 1 h at 252 and 387 bars, respectively)
showed their compositions to correspond to a for-mula of Cr2O3 5H2O or CrOOH 2H2O (Table 2),in agreement with the results of Burwell et al.
[22] for chromia xerogels. TGADSC showed
that heating of these aerogels resulted in three-
steps transformations (Fig. 6). The rst step was a
highly endothermic loss of 15 wt% of the sample
weight corresponding to elimination of one water
molecule from CrOOH 2H2O in the temperaturerange of 323443 K, irrespective of the atmosphere
(air or N2). The second step carried out in inert
atmosphere involved a further much less endo-
thermic loss of additional 15 wt% of the sample
weight corresponding to release of the second
Fig. 4. Eect of calcination temperature on the texture parameters of chromia aerogel sample 4b.
102 M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111
water molecule from CrOOH 2H2O (Fig. 6(a)).The same weight loss recorded in oxidative atmo-
sphere (air) was accompanied by an exothermic
eect centered at 550 K that could be caused by
elimination of a second water molecule accompa-
nied by CrOOH to CrO2 oxidation yielding the
same weight loss. This correlates with the datameasured by Carruthers et al. [27] for heating the
chromia xerogel in air. They attributed the rst
exothermic peak in the DTA spectra to
CrIII ! CrIV oxidation. The third transfor-mation step was a quick exothermic weight loss in
the narrow temperature range of 693713 K in air
or 793873 K in an inert atmosphere. These weight
losses together with the measured chromium
Fig. 5. X-ray diractograms of chromia aerogel sample 4b after
CO2 extraction at 313 K (1), followed by further evacuation at
593 K (2) and calcination in air at 593 K (3), followed by fur-
ther treatment at 723 K in an inert atmosphere (4) or in air (5).
Table 2
Chemical composition of typical chromia aerogels
Sample # Evacuation temperature (K) Chromium content (wt%) Corresponding formula of chromia aerogel
2b 44.2 Cr2O3 5H2O or CrOOH 2H2O593 63.1 Cr2O3 H2O, CrOOH or CrO2693 69.7 Cr2O3
4b 45.1 Cr2O3 5H2O or CrOOH 2H2O593 61 Cr2O3 H2O, CrOOH or CrO2693 68.2 Cr2O3
Fig. 6. TGA and DSC curves recorded in nitrogen (a) and air
(b) for chromia aerogel sample 4b directly after CO2-extraction.
M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111 103
contents correspond to the formation of productswith the formulas CrOOH or CrO2 and Cr2O3,
after the second and third transformation steps
respectively (Table 2). The high-temperature exo-
thermic eects could be attributed to the glow
transition caused by the transformation of nano-
structured CrOOH (inert atmosphere) or CrO2(air) to microcrystalline a-Cr2O3 (Figs. 2(d) and 5).The oxidation of chromia aerogel (4b, Table 2)lead to almost complete amorphisation of the
material (Fig. 5), and therefore less thermal energy
was required for the glow transition of nano-
structured CrO2 to microcrystalline a-Cr2O3. Theglow transitions accompanied by a sharp decrease
of the surface area were observed also for air-dried
Cr-xerogels by several groups [6,11,18,27] in sim-
ilar temperature ranges. The thermoanalyticalbehavior of the high-surface-area Cr-aerogels in-
vestigated in this study diered from that of cor-
responded Cr-xerogels [6,11,18,27] by shifting of
the temperature of the rst dehydration step peak
maxima from 470 to 370 K.In agreement with the TGADSC data, TPD
spectra of water recorded in an inert atmosphere
(He ow) of chromia aerogel after CO2 extractiondisplayed three peaks, corresponding to water
evolution (m=z 18) centered at 373, 593 and 890 K(Fig. 7(a)). The ratio of the integral intensities of
these three water evolution peaks was 0.95:1:0.5, a
nding that correlates well with the chemical
compositions of products determined by AA and
TGA analysis. Some oxygen evolution was ob-
served in this experiment at temperature range573673 K probably due to decomposition of trace
amounts of surface carbonates. When TPD spec-
tra of dried chromia aerogels were recorded in the
presence of oxygen (5% O2 in He, Fig. 7(b)), the
water was evolved in two stages in temperature
ranges of 320440 and 450620 K. The second step
of water evolution was accompanied with signi-
cant oxygen consumption at 500570 K followedby oxygen evolution centered at 715 K (Fig. 7(b)).
It means that the weight loss at the third exo-
thermic stage of Cr-aerogels transformation in air
(Fig. 6(b)) was caused by oxygen and not water
evolution. This clearly demonstrates the dierence
in the chemistry of Cr-aerogel transformations in
air and in inert atmosphere. After the rst dehy-
dration step: CrOOH 2H2O! CrOOH H2O at320440 K, the material undergoes two fur-ther dehydration steps: CrOOH H2O! CrOOH(>550 K) and CrOOH! Cr2O3 (>773 K) in inertatmosphere. Heating in air after the rst dehy-
dration stage, results in oxidative dehydration
CrOOH H2O! CrO2 followed by thermal de-composition of produced CrO2 into Cr2O3 and
oxygen.
To conrm this conclusion, we recorded TPRand TPO spectra for a chromia aerogel in which
the rst two dehydration steps were completed by
exposure to a temperature of 593 K in vacuo (Fig.
8(a)). The spectra showed that the chromia aerogel
did not consume hydrogen, in agreement with lit-
erature data that Cr(III) oxide species (Cr2O3 and
CrOOH) cannot be reduced to Cr(II) in the se-
Fig. 7. Water evolution spectra recorded in thermopro-
grammed desorption (TPD) runs in He (a) and 5%O2He ow
(b) for chromia aerogel sample 4b directly after CO2-extraction.
104 M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111
lected temperature range [28,29]. The TPO spec-
trum showed oxygen consumption by the sample,
with a mass spectrometric peak m=z 32 at 537 K(Fig. 8(b)). Mass spectrometry of the evolved gases
showed that the amount of consumed oxygen
corresponded to an O/Cr ratio of 0.5, whichcould be interpreted as the oxidative conversion of
CrOOH to CrO2, i.e., CrOOH 0:25O2 ! CrO20:5H2O. This type of oxidative conversion wasconrmed in a TPR experiment carried out at 560
K on the aerogel that had undergone TPO. The
sample consumed hydrogen (m=z 2, Fig. 8(c)) inamounts corresponding to the reduction of CrO2to CrOOH. These ndings are in a good agreementwith the results of Maciejewski et al. [29], who
showed reversible redox transformations of
CrOOH$ CrO2 phases with well-dened XRDpatterns formed by decomposition of Cr(III) ni-
trate.
Additional information on the chemistry of the
thermal transformations of the chromia aerogels
was obtained by FTIR spectroscopy (Fig. 9). The
following features were evident in the spectrum ofthe chromia aerogel after CO2 extraction (Fig.
9(a)): a broad band in the region of 450900 cm1,assigned to CrOCr vibrations [30]; a broad band
in the region 17002100 cm1, characteristic of OH stretching vibrations in OHO groups in crys-
talline CrOOH [31]; a band at 1627 cm1, assignedto the bending modes of non-dissociated water
molecules or OH stretching vibrations in OHOgroups [30]; and a broad band at 3405 cm1, as-signed by Zecchina et al. [32] to the OH stretching
vibrations of non-dissociated water molecules and
the stretching of surface hydroxyls in hydrated
Fig. 8. TPR and TPO spectra recorded for chromia aerogel
sample 4b after evacuation at 593 K: (a) TPR; (b) TPO,
m=z 32; (c) TPR after TPO, m=z 2.
Fig. 9. FTIR spectra of chromia aerogel (sample 4b, Table 1)
after CO2 extraction (a) and further evacuation at 593 K (b)
and calcination in air 693 K (c).
M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111 105
chromium oxides. These ndings are in agreementwith our previous conclusion that the untreated
chromia aerogel is hydrated CrOOH, containing
coordinately bonded water molecules, which could
be eliminated at temperatures up to 593 K. Evac-
uation of the chromia aerogel at 593 K (Fig. 9(b))
strongly reduced the intensities of the bands
characteristic of nondissociated water molecules
(3420 and 1628 cm1) but not those of the bandsassigned to OHO and CrOCr vibrations in
CrOOH. The spectrum of the chromia aerogel
heated further to the temperature of the exother-
mic glow transition of nanostructured CrOOH to
microcrystalline a-Cr2O3 (693 K) showed disap-pearance of the bands assigned to OHO and Cr
OCr vibrations in CrOOH and reduction in the
intensities of the hydroxyl bands, i.e., the bands at3430 and 1625 cm1 (Fig. 9(c)) assigned to theadsorbed water molecules. The bands present in
the 4001000 cm1 range were characteristic of IR-active fundamental and combination lattice modes
of crystalline a-Cr2O3 [32,33].From our studies on the chemical composition
and thermoanalytical behavior of chromia aerogels
obtained by urea-assisted homogeneous precipita-tion followed by low-temperature supercritical
drying, it became clear that such aerogels consist
of nanoparticles of hydrated CrOOH, which con-
vert to anhydrous CrOOH at 593 K in inert at-mosphere or vacuum or to CrO2 in air, and further
to a-Cr2O3 at a temperature dependent on thecalcination atmosphere (inert or oxidative). It
remained unclear the structure of the nanoparticlesin Cr-aerogels before the glow transition. From
the XRD data it follows that they display some
kind of order (Fig. 5). Hence further investigation
was aimed in more detailed analysis of their XRD
patterns using the structure modeling approach.
3.2.2. XRD analysis and structure models
X-ray diractograms of chromia aerogels ob-tained after CO2 extraction, either with or without
additional calcination at 593723 K in vacuum or
inert atmosphere (Fig. 5), provide evidence that
there is some order in the structure of the nano-
particles. Detailed analysis of the X-ray diracto-
grams in combination with structure modeling
provided further clarication of the ordering
modes. The d-spacings corresponding to the posi-tions of the maxima of the broadened XRD peaks
of the untreated chromia aerogels (d1 0:47 nm;d2 0:25 nm; d3 0:20 nm and d4 0:15 nm; Fig.5) were identical to those found by Douglass [30]
and Christensen et al. [34] in the crystal structure
of a-CrOOH. The latter is built up of polymeric[Cr3OOH]n layers packed such that the coor-dination number of the Cr ions is maintained at 6[30,34]. On the basis of these data, the structure of
CrOOH is presented in Fig. 10 as a three-layered
hexagonal close-packing of Cr ions located at the
centers of regular octahedra that are formed by
three O atoms (medium-sized black circles) and
three OH groups (large dark grey circles with H
atoms). In case of regular OOH distribution, the
Fig. 10. Three-layer hexagonal close-packing of Cr(III) octahedra in the crystal structure of a-CrOOH: (a) frontal view; (b) proleview.
106 M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111
structure belongs to R3m (N 160) space group.When O and OH groups are distributed randomly
as shown in Fig. 10 (i.e., each position in the oc-
tahedron may be lled by O or OH with equal
probability), the structure belongs to R3m (N 166)
space group. The layers are bonded by hydrogen
bonds (as shown by FTIR) between [O2(OH)]ion pairs located at the same axis along Z direc-tion.
On the basis of the similarity between the X-ray
diractogram of fresh chromia aerogels broadened
due to the presence of very small nanocrystals and
that of well-dened CrOOH crystals and existence
of structural water molecules proved by thermal
analysis and FTIR, it may be assumed that in fresh
aerogels water molecules replace O atoms and OH
groups in a certain order. In case of such substi-tution the lattice loses the third order axis and the
symmetry became reduced to the monoclinic
group m (space group P 1m1) if in the lattice (like
in space group R3m) is survived the ordering of
O2 and OH-ions. In case of random distributionof O2 and OH ions (as for space group R3m) thelattice symmetry became reduced to the group 2/m
(space group P 12/m1). The structure of a hydratedchromia aerogel corresponding to the formula
CrOOH 2H2O is presented in Fig. 11. This
structure is a monoclinic analogue of a-CrOOH(b ap3, the b axis is monoclinic), in whichhalf of the O ions medium-sized black circles and
OH groups large dark grey circles are replaced
with H2O molecules large light grey circles to form
two types of octahedron. In the rst light, H2O
molecules occupy four positions and O or OH, the
other two positions; in the second dark, H2O, O
and OH groups each occupy two positions. TheCr3 ions are located at the centers of octahedra,as in the a-CrOOH structure, but with the prob-ability that half of these positions are vacant.
These octahedra are partially bonded by [O2OH] ion pairs and partially by H2O pairs locatedalong the Z-axis (dipoledipole interaction or hy-drogen bonding).
Since the bonding between CrOH2 in theabove-described structure is substantially weaker
than CrO or CrOH bonding and since the latter
two bonds are weaker than those in the ideal a-CrOOH structure [30,34], the coordinates of the
oxygen atoms were specied by modeling the
XRD patterns of the material by means of a
Rietveld-based software program DBWS-9807
developed by Young et al. [35], as follows. Thematerial had high dispersion reected by broad-
ening of its XRD patterns (Fig. 5) and there was a
Fig. 11. Structure of an hydrated chromia aerogel with the formula CrOOH 2H2O.
M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111 107
possibility for existence of signicant amount ofthe amorphous phase. Therefore besides using in
the Rietveld program a simple background ap-
proximation with a fth power polynomial we
calculated also the contributions of the processes
of thermal and statistical disordering of O2, OH
and Cr3 ions and the H2O molecules as well as thenon-elastic Compton scattering of lattice electrons
[36,37]. X-ray diractograms of the untreated hy-drated chromia aerogel, after background sub-
traction, were used for renement of the structural
characteristics of CrOOH 2H2O. The unit cellparameters and the full width at half maximum of
the peaks were selected by the software to give the
best t to the experimental diractograms. The
coordinates of O and the OH and OH2 groups
were chosen on the basis of crystallochemicalconsiderations, keeping the CrO and OO dis-
tances within the permissible range.
The data in Table 3, which were obtained after
several simulation cycles, present the main char-
acteristics of the materials structure for the
monoclinic cell, space group P 1m1, that reect the
ideal structure of the hydrated CrOOH 2H2Omaterial. Fig. 12 shows the tting of the simulationresults to the experimental X-ray diractogram
after background subtraction. The relatively highRwp-factor of 0.14 reects the small deviationsfrom the ideal crystal structure in the structural
parameters of real nanocrystals of hydrated chro-
mia aerogel. More detailed analysis of the X-ray
diractograms by means of Fourier Transform
software is currently in progress for further veri-
cation of the real structure of the nanocrystals of
the hydrated chromia aerogel. As follows fromTable 3, the thermal (statistical) coecients for
H2O molecules are signicantly larger compared
with that for ions O2, OH and Cr3. This reectsthe weaker bonding of the structural water in the
crystal lattice compared with corresponding ions.
Nevertheless, in nanocrystals of 23 nm this
bonding strength is enough for the formation of a
3D lattice CrOOH 2H2O. The high degree of thewater molecules disorder makes visible contribu-
tion to the background scattering, caused by dis-
ordering of molecules and ions (Fig. 12).
After dehydration at 593 K in vacuum or at 723
K in nitrogen, the material with a composition
corresponding to the formula CrOOH displayed
similar XRD patterns and retained its nanostruc-
tured nature, i.e., the domain diameter was 1.52.0nm. Dehydration of the parent chromia aerogel
Table 3
Unit cell parameters and atomic positions in the hydrated chromia aerogel CrOOH 2H2O structureAtomic coordinates
x=a y=b z=c Occupation B (A2)
1. Cr3 0 0 0 0.5 0.4 (1)2. Cr3 1/2 1/2 0 0.5 0.4 (1)3. Cr3 1/3 0 2/3 0.5 0.4 (1)4. Cr3 1/6 1/2 1/3 0.5 0.4 (1)5. Cr3 2/3 0 1/3 0.5 0.4 (1)6. Cr3 5/6 1/2 2/3 0.5 0.4 (1)7. O (OH) 1/3 0 0.078 (7) 1.0 1.0 (2)
8. O (OH) 0 0 0.411 (7) 1.0 1.0 (2)
9. O (OH) 2/3 0 0.744 (7) 1.0 1.0 (2)
10. O (O2) 2/3 0 0.922 (7) 1.0 1.0 (2)11. O (O2) 1/3 0 0.256 (7) 1.0 1.0 (2)12. O (O2) 0 0 0.589 (7) 1.0 1.0 (2)13. O (OH2) 5/6 1/2 0.082 (8) 1.0 5 (1)
14. O (OH2) 1/2 1/2 0.415 (8) 1.0 5 (1)
15. O (OH2) 1/6 1/2 0.748 (8) 1.0 5 (1)
16. O (OH2) 1/6 1/2 0.918 (8) 1.0 5 (1)
17. O (OH2) 5/6 1/2 0.252 (8) 1.0 5 (1)
18. O (OH2) 1/2 1/2 0.585 (8) 1.0 5 (1)
a 0:488 0:01 nm, b 0:5316 0:002 nm, c 1:39 0:01 nm, b 94:2 0:1.
108 M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111
was reected in the disappearance of the X-ray
peak corresponding to the d-spacing of 0.47 nm(plane 0 0 3) that reects three-dimensional order-
ing in the lattice of the a-CrOOH material. Taking
into account that this sample does not containstructural water molecules (as follows from DTG,
TPD and FTIR), we may assume that the chromia
aerogel dehydrated at 593 K consists of two-
dimensional fragments (clusters) of octhaedra of a
a-CrOOH lattice, as presented in Fig. 13, withoutany bonding along the Z-axis. As may be clearlyseen in Fig. 13, the dimensions of the octahedra in
the a-CrOOH lattice correspond to the d-spacingscalculated from the X-ray diractograms of the
dehydrated material. These two-dimensional clus-
ters could not be fragments of the a-Cr2O3 lattice,because the octahedra of this latterlattice do not
have a spacing of d 0:20 nm [26]. The structureof the a-Cr2O3 lattice diers substantially fromthat of the a-CrOOH lattice: the Cr3 ions in thelatter lattice are located within of the two types ofoctahedron turned slightly relative to one another.
The Cr3 ions are located symmetrically relative tothe three upper and three lower oxygen atoms.
The XRD data taken together with the results
of the AA, TPD, TGA, DSC and FTIR studies
provide evidence for the crystallohydrate nature of
the parent chromia aerogel extracted with super-
critical CO2. Dehydration at 593 K in vacuum orat 723 K in nitrogen removed the structural water
in an endothermic process that yielded a meso-
porous nanocrystalline material made up of two-
dimensional nanoclusters with an a-CrOOHstructure and a high surface area (>500 m2 g1). Atthe higher temperatures depending on the treat-
ment atmosphere, exothermic dehydrationre-
crystallisation of nanostructured a-CrOOH, withor without oxidation to CrO2 as an intermediate
Fig. 12. Comparison of experimental XRD patterns of un-
treated chromia aerogel sample (4b, open circles) with the su-
perimposed rened Rietveld plot (line 1) for CrOOH 2H2Ostructure. The residuals between the experiment and the cal-
culation are shown by solid circles. The background as tted
with the fth-order polynomial (line 2); the same plus the
contribution due to the disorder (line 3); the same plus the
contribution due to the incoherent Compton scattering (line 4).
Fig. 13. Two-dimensional clusters fragments of an a-CrOOH lattice constituting the building blocks of the CrOOH nanoparticlesin chromia aerogels evacuated at 593 K.
M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111 109
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[33] R. Marshall, S.S. Mitra, P.J. Gielisee, J.N. Plende, L.C.
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[34] A.N. Cristensen, P. Hansen, M.S. Lehmann, J. Solid.
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[35] R.A. Young, A.C. Larson, C.O. Paiva-Santos, DBWS-
9807: Rietveld analysis of X-ray and neutron powder
diraction patterns (1998).
[36] P. Riello, G. Fagherazzi, D. Clemente, P. Canton, J. Appl.
Crystallogr. 28 (1995) 115.
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M. Abecassis-Wolfovich et al. / Journal of Non-Crystalline Solids 318 (2003) 95111 111
Texture and nanostructure of chromia aerogels prepared by urea-assisted homogeneous precipitation and low-temperature supercritical dryingIntroductionExperimentalSol-gel aerogel synthesisCharacterization of chromia aerogels
Results and discussionTexture of the aerogels and xerogelEffect of CO2 extraction conditionsEffects of calcination
Aerogel structureChemical composition and thermoanalytical behaviorXRD analysis and structure models
ConclusionsAcknowledgementsReferences