landauer en burg

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


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.


(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.


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.


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.


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.


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.


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).


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.


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.


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.


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.


step, resulted in a glow transition into large (50nm) crystals (Fig. 2(d)) of well-dened a-Cr2O3(Fig. 5) with a low surface area of

[29] M. Maciejewski, K. Koohler, H. Schneider, A. Baiker, J.Solid State Chem. 119 (1995) 13.

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1838.

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[32] A. Zecchina, S. Coluccia, E. Guglielminotti, G. Ghiotti,

J. Phys. Chem. 75 (1971) 2774.

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[34] A.N. Cristensen, P. Hansen, M.S. Lehmann, J. Solid.

State. Chem. 21 (1977) 325.

[35] R.A. Young, A.C. Larson, C.O. Paiva-Santos, DBWS-

9807: Rietveld analysis of X-ray and neutron powder

diraction patterns (1998).

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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

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