Stability of surface chromate ± A physicochemical investigationin relevance to environmental reservations about

calcined chromia catalysts

Mohamed I. Zakia,*, Muhammad A. Hasana, Nasr E. Fouadb

aChemistry Department, Faculty of Science, Kuwait University, PO Box 5969, Safat 13060, KuwaitbChemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt

Abstract

The present investigation presents and correlates observed and reported characterization results of calcined, supported and

unsupported chromias obtained via various precursor compounds. Studies performed employed a range of surface and bulk

analytical techniques, in hopes of a proper assessment of the stability of surface chromate (Cr(VI)±O) species thus generated

to thermal decomposition, hydrolysis and reduction. Impacts on the redox catalytic activity were probed towards the

decomposition of H2O2 solutions and the CO oxidation in the gas phase. The results have revealed that chromate species

established on bulk and dispersed a-Cr2O3 particles enjoy high stability against thermal and chemical treatments, and yet

contribute to the formation of surface sites that are catalytically active in redox reactions. This should help lessening

environmental reservations about the industrial application of calcined chromia catalysts. # 1998 Elsevier Science B.V. All

rights reserved.

Keywords: Chromia; Calcined chromia catalysts; Surface chromate; Environmental reservations

1. Introduction

The title term `̀ surface chromate'' refers to Cr(VI)±

O species generated on oxidized surfaces of chromia

(Cr2O3) based catalysts, prepared by heating in air

(calcination) of adequate parent materials [1]. The

catalysts thus obtained are potential in redox reactions

[2] and promising in deep oxidation (combustion)

processes [3]. The activity and selectivity of the

catalysts depend critically on whether the calcined

chromia is supported or unsupported, and promoted or

unpromoted [4].

Calcination of chromia based catalysts leads not

only to Cr(IV) surface species, but also to Cr(IV) and

Cr(V) species [1,2,5±7]. However, the hexavalent

(chromate) species dominate at �6008C [8,9], assum-

ing monomeric �CrO2ÿ4 � [10,11] and/or polymeric

�Cr2�xO7�3x�2ÿ [12,13] surface structures. Under cer-

tain circumstances, they may form interaction species

with nearby Cr(III) species (e.g., chromium chromate

like [14]) and metal oxide support surfaces (e.g.,

aluminum chromate-like on alumina [10]). Conse-

quently formed Cr(VI)±Cr(III) pair-sites have been

considered to promote the redox catalytic activity of

chromia in several reaction incidents [14±16]. Accord-

ingly, a catalytic mechanism has been preferred [14],

whereby localized adsorption occurs on coordina-

Applied Catalysis A: General 171 (1998) 315±324

*Corresponding author. Tel.: (965) 4811188/5606; fax: (965)

4816482; e-mail: zaki@kuc01.kuniv.edu.kw

0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.

P I I S 0 9 2 6 - 8 6 0 X ( 9 8 ) 0 0 0 8 8 - X

tively unsaturated Cr(III) sites [17], but electron avail-

ability occurs through interaction with nearby Cr(VI)

ions.

Despite numerous laboratory applications, the indus-

trial application of calcined chromia catalysts has been

hampered by the instability of surface chromate to

hydrolysis in aqueous media [15], reduction in reduc-

tive gas atmospheres [18], and high temperature treat-

ments in air [19].VolatileCr(VI)-compoundsare known

to be environmentally detrimental [19]. Some earlier

reports [5,20] claim, however, that the instability is the

concern of only a portion of surface chromate, and that

thermally and chemically stable chromate species

coexist on surfaces of calcined chromia. Based on

these reports, and without suf®cient experimental

control, irreducibility of calcined chromia in H2-TPR

experiments has been explained [21].

The present study explores observed and reported

characterization results of thermal and chemical sta-

bility of oxidized surfaces of supported and unsup-

ported chromias prepared by calcination in air.

According to previous studies [11,22,23], the chosen

parent materials yield calcination products exposing

variously structured surface chromate species estab-

lished on different substrates (a-Cr2O3, alumina and

silica particles). The thermal stability was probed by

thermogravimetry (TG), whereas the chemical stabi-

lity was tested versus hydrolysis and temperature-

programmed reduction in H2 atmosphere (H2-TPR).

The study correlates ®ndings with the catalytic per-

formance of test chromias in H2O2 decomposition

(liquid phase) and CO oxidation (gas phase) reactions.

Eventually, the study goals an objective assessment of

the environmental apprehensions of calcined chromia

catalysts.

2. Experimental

2.1. Calcined chromias

Supported and unsupported chromias examined in

the present study were obtained by calcination at 300±

6008C for 5 h (in a static atmosphere of air) of the

following parent materials:

� Unsupported chromias (denoted by Cr) were pro-

ducts of (I) chromia gel (Cr2O3�5H2O) prepared by

NH4OH alkalization of an aqueous solution of

AR-grade Merck Cr(NO3)3�9H2O [11], (II)

99.9% pure Riedel-de-Haen CrO3 [22], and (III)

AR-grade Merck (NH4)2Cr2O7 [23]. The resulting

chromias are discerned below by a designatory

combination of Cr, the indicative Roman numeral

(I)±(III) of the parent, and the calcination tem-

perature applied. Thus, CrI-600 means unsup-

ported chromia obtained by 6008C calcination

of the chromia gel.

� Alumina-supported chromias (CrAl) were calci-

nation products of Degussa aluminum oxide C

(100 m2 gÿ1) coated with chromia gel (I) [11]

and wet-impregnated with CrO3 solution (II)

[22] at 10 mol% Cr2O3. They are indicated below

adopting the designatory system of the corre-

sponding unsupported chromias. Thus, CrAlII-

300 indicates the 3008C calcination product of

alumina impregnated with the CrO3 solution.

� Silica-supported chromias (CrSi) were calcination

products of Degussa aerosil-200 (200 m2 gÿ1)

coated with chromia gel (I) [11] and wet-impreg-

nated with CrO3 solution (II) [22] at 10 mol%

Cr2O3. For identification, the above designatory

system has been applied. Thus, CrSiI-600 signifies

the 6008C calcination product of silica coated with

the chromia gel.

2.2. Characterization methods and techniques

� X-ray powder diffractometry (XRD) was carried

out to elucidate the crystalline bulk structure of

test materials [24], employing a model D5000

Siemens diffractometer (35 kV and 30 mA)

equipped with Ni-filtered CuK� radiation.

� UV±visdiffuse reflectancespectroscopy(DRS)was

applied to identify surface chromate structures and

electronic interactions [11], using a Cary-5 Varian

spectrophotometer equipped with MgO-coated

integrating sphere and a data acquisition system.

� Thermogravimetry (TG) was performed to moni-

tor the thermal stability of surface chromate in a

stream of air (50 cm3 minÿ1) [22], by means of a

TGA-50 Shimadzu analyzer.

� Temperature-programmed reduction (TPR) was

the technique used to probe the chemical stability

of surface chromate in a stream of 5% H2/He at

30 cm3 minÿ1 [21], implementing a common flow

system equipped with a quartz microreactor and a

316 M.I. Zaki et al. / Applied Catalysis A: General 171 (1998) 315±324

Pd diffusion cell connected to a model 100 Carle

thermal conductivity detector.

� Colorimetry was the analytical method adopted to

estimate water-leachable surface chromate [6],

employing a Spekol±Carl±Seiz colorimeter.

� Iodometry was the analytical method applied to

titrate total surface chromate [7], using KI/0.12 N

HCl as a reagent and a solution of thiosulfate as a

titrant.

2.3. Catalytic activity tests

� In solution the catalytic activity was tested towards

H2O2(!H2O�12O2) decomposition at 20±408C

and zero-order kinetics; the reaction rate was

measured volumetrically [6].

� In the gas phase, CO(�12O2!CO2) oxidation in

oxygen-rich stream of He was the test catalytic

reaction, and the rate was followed using a com-

mon flow system equipped with plug-flow micro-

reactor, and a model 3400 Varian chromatograph

equipped with a TC-detector, propack-N column,

and a model 4290 Varian electric integrator [25].

3. Results and discussion

3.1. Crystalline bulk structure

XRD-probed characteristics of crystalline bulk

structure for test chromias are compared in Table 1.

Accordingly, the sole detectable CrOx crystalline

phase is a-Cr2O3. It assumes relatively strongest

crystallinity and largest particle size in CrII-600.

These properties are shown to have been suppressed

in low calcination temperature chromias, as well as

upon dispersion on support surfaces. Despite the

higher surface area of silica (200 m2 gÿ1), alumina

surfaces (100 m2 gÿ1) appear to be more capable of

dispersing a-Cr2O3 particles (Table 1). Consequently,

a-Cr2O3 is hardly detectable in CrAl-600 samples,

irrespective of the chromia precursor. Similar results,

obtained previously [22], have been interpreted on the

basis of stronger CrOx/support interactions in CrAl,

both during the precursor loading onto the support and

the subsequent calcination. It is to be noted that in all

supported chromias examined no CrOx/support inter-

action products were XRD-detectable. However, this

cannot exclude the likelihood of formation of surface

and/or noncrystalline bulk compounds, both being

XRD-undetectable. Compatibly, supported Cr(III)±O

species have been previously found [14,27] to exist in

various noncrystalline phases, namely d- (isolated

Cr(III) species), b- (Cr(III)-clusters), g- (Cr(III)

coupled with Cr(VI)) and a-phase (CrO1.5 species),

depending on the chromia content and calcination

temperature.

With respect to chromias derived from chromia gel

(NOÿ3 -contaminated), the a-Cr2O3 particle crystalli-

nity and size (Table 1) are signi®cantly higher than

those reported previously for corresponding chromias

obtained using a NOÿ3 -free gel [22]. This might be

Table 1

XRD-probed characteristics of crystalline bulk structure for unsupported and supported chromias

Chromia Chromia precursor Phase composition Cry.b (%) Dc (�2 nm)

CrI-400 Gela a-Cr2O3 55 28

CrI-600 a-Cr2O3 70 35

CrII-600 CrO3 a-Cr2O3 88 53

CrIII-400 (NH4)2Cr2O7 a-Cr2O3 63 34

CrIII-600 a-Cr2O3 75 42

CrAlI-600 Gela a-Cr2O3 12 10

g-Al2O3

CrAlII-600 CrO3 a-Cr2O3 12 15

g-Al2O3

CrSiI-600 Gela a-Cr2O3 58 30

CrSiII-600 CrO3 a-Cr2O3 73 44

aNH4NO3-contaminated.bCry.�crystallinity approximated relative to a well-crystalline Merck a-Cr2O3.cD�average particle size determined adopting XRD line broadening technique [26].

M.I. Zaki et al. / Applied Catalysis A: General 171 (1998) 315±324 317

attributed to NH4NO3-in¯uenced formation of

(NH4)2Cr2O7-like species on heating the parent gel

[22], which was shown to modify considerably the gel

decomposition course and products [28].

3.2. Surface chromate structure

UV±vis DRS spectra and band assignments exhib-

ited in Fig. 1 monitor the presence of chromate spe-

cies in the three gel-derived chromias examined, i.e.

CrI-600 (spectrum a), CrSiI-600 (spectrum c) and

CrAlI-600 (spectrum d). According to XRD results

(Table 1), these chromate species are not associated

with a crystalline bulk structure. Thus, they are either

involved in noncrystalline bulk phases, or established

as surface species on supported and unsupported

chromia particles, or on support particles. Direct

anchorage of chromates onto alumina surfaces in

CrAlI-600 (and CrAlII-600) is highly likely, in view

of the presence in these materials of hardly detectable

a-Cr2O3 particles (Table 1).

The surface structure of the chromate detected can

be drawn from the DRS spectra (Fig. 1) on the basis of

the following criteria. First, the band at 440±450 nm,

which is due to 1A1 !1 T1 charge transfer (CT)

transition is symmetry forbidden [29], and this

explains its presence and absence in the spectra

(Fig. 1) of K2Cr2O7 (spectrum e) and K2CrO4 (spec-

trum f), respectively. Therefore, it probes the chromate

structural symmetry and/or polymerization. Second,

the two bands at 270±280 nm (I1) and 370 nm (I2) are

due to different 1A1 !1 T2 CT-transitions [29±31];

they undergo an increase in the I1/I2 intensity ratio as

the polarization (covalency) of the structure increases.

Third, the band at 595 nm has been ascribed [14] to

d±d electron exchange interactions between inti-

mately coupled Cr(VI) and Cr(III) sites (denoted

g-phase [14]), or to d±d transitions of nonoctahedral

Cr(III) [31]. The former assignment is more likely

because it has been based on a magneto-chemical

evidence witnessed by Ellison et al. [14], and of the

fact that tetrahedral Cr(III) is immediately converted

into the octahedral symmetry in the presence of sur-

face water. Recently [32], an analogous band at 588±

590 nm has been ascribed to Cr(III) located in the

surface layer of alumina support. The absence of an

obvious absorption at 580±800 nm in the spectrum

exhibited by the present alumina-supported chromias

(see, e.g., spectrum d in Fig. 1) may justify to consider

the latter assignment as being less likely.

Accordingly, the weak absorption near 450 nm and

the low I1/I2 intensity ratio displayed in the CT-

transition region for CrAlI-600 (spectrum d, Fig. 1)

indicate the presence of weakly covalent chromate

species of reasonably high structural symmetry. In

contrast, the relatively stronger absorptions at 440±

450 nm and the much higher I1/I2 intensity ratios

shown for the unsupported (spectrum a) and silica-

supported chromias (spectrum c, Fig. 1) reveal less

symmetric, strongly covalent chromate species.

According to Zaki et al. [11], the chromate detected

on CrAlI-600 is most probably monomeric species

anchored directly onto the support surface. The chro-

mate observed on CrI-600 and CrSiI-600 are estab-

lished essentially on a-Cr2O3 particles (i.e. a-

chromia-supported chromate) assuming a polymeric

structure. The latter chromate species is distinctively

Fig. 1. UV±vis DRS spectra and band assignments for CrI-600 (a),

CrSiI-600 (c), and CrAl1-600 (d). Results inset for K2CrO4 (f),

K2Cr2O7 (e) and a-Cr2O3 (b) are for comparison purposes.

318 M.I. Zaki et al. / Applied Catalysis A: General 171 (1998) 315±324

pertained by the emergence of the 595 nm band

(spectra a and c, Fig. 1) assignable to the g-(Cr(VI)±

Cr(III)) phase. The absence of a well-resolved band

near 595 nm in the spectrum (d, Fig. 1) of CrAlI-600

sustains that the material contains predominantly

chromate anchored directly onto the alumina support.

Nevertheless, the presence in CrAlI-600 of chromate

species established on noncrystalline chromia cannot

be excluded with certainty.

3.3. Stability of surface chromate

3.3.1. Thermal stability

Fig. 2 compares TG curves for CrO3 and two

chromia gel samples, a NOÿ3 -free and a NOÿ3 -con-

taminated sample. The nitrate inclusion is inherited

from the preparation source conceded by the NOÿ3 -

containing gel [11]. The trioxide, CrO3, was chosen to

facilitate probing the thermal stability of initial chro-

mate, whereas the gel samples are to allow for obser-

vation of the thermal stability of topochemically

generated chromate via oxidation of initial Cr(III)

[24]. It is obvious from Fig. 2, that CrO3 is stable

to heating to 2508C, where it commences decomposi-

tion into meta-stable mixed-valency bulk phases at

250±4508C [23]. These meta-stable intermediates

contain Cr(VI) species, and are XRD-detectable

[22,23]. They decompose into a-Cr2O3 (Cr(III)) near

5008C [23].

TG curve of the NOÿ3 -free gel sample (Fig. 2)

indicates, according to Zaki and Fouad [28], that

heating in air (calcination) leads to overlapping dehy-

dration processes ending up with a monohydrate

(Cr2O3�H2O�2CrOOH) near 2508C. The monohy-

drate is topochemically oxidized into the g-phase

(Cr2O3�CrO3) via a complex series of disproportiona-

tion reactions [28] at 250±3808C. The g-phase thus

produced decomposes into a-Cr2O3 near 4008C(Fig. 2). None of the intermediate products was

XRD-detectable [23].

The inclusion of NH4NO3 is shown (Fig. 2) to

modify considerably the decomposition course of

the gel. The initial decomposition curve is notably

steepened (kinetically enhanced) and completed near

2008C (instead of 3008C for the NOÿ3 -free gel). On the

other hand, the product is suggested to consist essen-

tially of a-Cr2O3 with minority g-phase (Cr2O3�x).

Thus formation of the a-phase is markedly enhanced

to occur near 2008C from the contaminated gel,

instead of 4008C from the pure gel, and 5008C from

CrO3 (Fig. 2). A previous study [28] disclosed the

formation of a (NH4)2Cr2O7-like phase in the NOÿ3 -

contaminated gel, which was considered responsible

for the new decomposition course and products. Park

[33] examining gas and solid phase decomposition

products of pure (NH4)2Cr2O7 has found NH3 being

released to catalyze a reductive conversion of Cr(VI)

species into a-Cr2O3 near 2108C.

The above results indicate that Cr(VI)-containing

bulk phases are produced during the decomposition

course of the chromia precursors examined (Fig. 2).

These bulk phases are destabilized thermally at

�4508C, giving rise to a-Cr2O3 as the eventual

decomposition product. A thermo-analytic study of

supported chromias [22] has brought about similar

results. Accordingly, DRS-detected chromate species

Fig. 2. TG curves obtained for CrO3, and NH4NO4-free and ±

contaminated chromia gel samples under the conditions indicated.

M.I. Zaki et al. / Applied Catalysis A: General 171 (1998) 315±324 319

in the 6008C calcined chromias (Fig. 1) are domi-

nantly surface species. Characterization studies

employing surface-sensitive techniques, e.g. XPS

[8,9], indicated persistence of the chromate surface

species on calcined chromia to heating to temperatures

well above 6008C.

3.3.2. Chemical stability

Chemical stability of chromate (bulk and surface

species) established in chromias obtained by calcina-

tion at 3008C and 6008C was tested versus H2-TPR

and hydrolysis in deaerated water.

H2-reduction. Observed H2-TPR pro®les are exhib-

ited in Fig. 3, and data therefrom derived are sum-

marized in Table 2. It is obvious that of the 6008Ccalcination products of unsupported and supported

chromias examined, CrAlII-600 is the sole product

that gave rise to a reduction behavior, as manifested in

the reduction peak displayed for the material at 3118C

(Fig. 3). These results indicate that a-Cr2O3 (the sole

detectable crystalline constituent of CrI- and CrII-

600) is irreducible under the experimental conditions

applied. Moreover, the DRS-detected a-Cr2O3-sup-

ported (or silica-supported, i.e. CrSiII-600) chromate

species (Fig. 1) also are irreducible. Hence, the spe-

cies responsible for the reduction peak of CrAlII-600

(Fig. 3) are most probably chromate species anchored

directly onto the support (or onto amorphous chromia)

(Fig. 1).

In view of the above results, 3008C calcination

products of the parent materials were TPR-examined.

Fig. 3 indicates that CrI-300 (derived from the NOÿ3 -

free gel) exhibits a low-temperature peak at 2858C and

a high-temperature one at 3508C, whereas the CrO3-

derived CrII-300 shows a broad reduction peak cen-

tered around 4208C. On the other hand, the 3008Ccalcination products of the CrO3-derived supported

chromias, whether on silica or alumina, give rise to a

much sharper peak at 330±3538C (Fig. 3).

The TPR results (Fig. 3) reveal that the stability of

chromate species to H2-reduction depends primarily

on whether they are contained in a bulk structure

(CrII-300) or exposed on the surface, and whether

the surface chromate species are anchored onto crys-

talline chromia particles (CrI-600 and CrII-600), non-

crystalline chromia (CrII-300), or onto the support

(CrAlII-300, CrAlII-600 and CrSiII-300). a-Cr2O3-

supported chromate species are shown to be the sole

stable chromate to H2-reduction amongst the various

chromates tested in the present investigation.

The fact that the CrOx stoichiometries derived from

the H2-consumption for the reducible phases differ in

magnitude, may be attributed to different chromate

Fig. 3. H2-TPR profiles for the materials indicated.

Table 2

Data derived from H2-TPR profiles of unsupported and supported

chromias

Chromia Tpa (8C) H2-consumption

(�10ÿ3 mol gÿ1)

CrOx

CrI-300b 285

350 1.64 CrO1.62

CrII-300 420 10.60 CrO2.48

CrSiII-300 353 2.82 CrO2.62

CrAlII-300 330 1.14 CrO2.20

CrAlII-600 311 0.44 CrO1.81

aTPR peak temperature.bDerived from a NH4NO3-free gel.

320 M.I. Zaki et al. / Applied Catalysis A: General 171 (1998) 315±324

compositions and structures. It is likely that O/Cr

ratios �2.2 (i.e., CrO2.2±CrO2.6) originate from sur-

face and bulk chromates, whereas O/Cr ratios �1.8

(i.e., CrO1.62±CrO1.8) are due mostly to surface chro-

mate species. This likelihood ®nds a strong support by

the fact that highly stable chromate species are exhib-

ited largely by CrO3-derived chromias, a precursor

that produces bulk chromates on heating at 250±4508C(Fig. 2).

Hydrolysis. Hydrolysis of surface chromate was

tested against 3 h shaking of known amounts of var-

ious chromias in distilled water. Leached chromate

species were determined in solution by colorimetry

[6]. To check on the presence of unleachable chromate

species, surface chromates remaining on the solid

residue were determined by shaking with KI/0.12 N

HCl solution, and a subsequent titration of liberated

iodine by a standard thiosulfate solution [7]. Quanti-

tative estimates based on the results are summarized in

Table 3. It is worth noting that of the 6008C calcined

supported and unsupported chromias, CrAlII-600 is

the sole product that showed positive test results. Thus

chromate species established on the other 6008Ccalcined chromias were stable to hydrolysis. Accord-

ingly, one may conclude that chromate species stabi-

lized against H2-reduction (Fig. 3 and Table 2), are

equally stable to hydrolysis and KI-reduction.

The hydrolysis results (Table 3) are in line with the

TPR results, and, therefore, support their conclusions.

Of the 3008C calcined chromias, the supported chro-

mias assume higher total contents of surface chromate

than the unsupported chromias. The highest content of

hydrolyzable chromate is shown for CrSiII-300,

whereas the lowest for CrII-300 (Table 3). The pro-

portion of hydrolyzable to total surface chromate

contents (Table 3) implies that alumina surfaces are

more capable of stabilizing surface chromate than

silica surfaces, and the stability is improved at the

higher calcination temperature of 6008C. Similar

results have very recently been reported by JoÂzÂwrak

and Dalla Lana [34].

3.4. Redox catalytic activity

3.4.1. H2O2 decomposition activity

Chromias tested as H2O2 decomposition catalysts in

the present study were chosen from the unsupported

set of chromias to avoid likely complications due to

hydration and amphoteric interactions of support

materials. The elected materials, which include stable

and unstable chromate species, were catalytically

tested before and after leaching by water. The leached

chromate species also were tested as a homogeneous

catalyst for H2O2 decomposition. Plots of volume of

oxygen liberated (VO2) versus time (t) determined at

408C for solid catalysts only, are displayed in Fig. 4.

The data in Fig. 4 help to arrange the water-

unleached catalysts in the following descending order

of the H2O2 decomposition activity:

CrI-500 > CrI-400 > CrIII-400 > CrIII-500:

These catalysts can be arranged in the following

descending order of total content of surface chromate

[7]:

CrI-400 > CrI-500 > CrIII-400 > CrIII-500:

Except for CrI-400, the two orders meet, thus

indicating that the higher the surface total content

of chromate species the higher the catalyst activity

towards H2O2 decomposition.

Water leaching of surface chromate species from

CrI-400 and CrI-500 resulted in a marked improve-

ment of the activity of the former catalyst and in a

slight suppression of the activity of the latter catalyst.

This implies that the relationship between the catalytic

activity and the surface content of chromate species

passes through a maximum, which seems to be located

at CrI-500. Thus, CrI-500 represents a catalyst case for

which the loss on surface chromate content is com-

pensated for by improved energetics of active sites

(Cr(VI)±Cr(III)) [35].

A similar upward swing to that occurring from the

initial linearity of the VO2ÿt plot of CrI-400 (Fig. 4)

Table 3

Quantitative estimates for surface chromate generated on unsup-

ported and supported chromias

Chromia CrO4(w)a

(�10ÿ4 g gÿ1)

CrO4(t)b

(�10ÿ4 g gÿ1)

CrO4(w)/CrO4(t)

(%)

CrI-300 27 84 32

CrII-300 17 70 24

CrSiII-300 147 190 77

CrAlII-300 65 149 44

CrAlII-600 23 119 19

aWater-leached surface chromate detected colorimetrically [6].bTotal surface chromate detected iodometrically [7].

M.I. Zaki et al. / Applied Catalysis A: General 171 (1998) 315±324 321

has been considered [15] to imply an in situ generation

of a homogeneous catalyst (soluble chromate) for

H2O2 decomposition. Indeed, chromate species lea-

ched out of CrI-400 and CrI-500 were used in a

separate, but similar, set of experiments to catalyze

H2O2 decomposition. They were found to catalyze the

decomposition, following an induction period of time

(a few minutes) in which a wine red colored inter-

mediate was developed [36]. The VO2ÿt plots (not

shown) were linear, and the slope (rate constant) was

directly proportional to the amount of chromate in

solution.

The above results underline the importance of sur-

face chromate to the H2O2 decomposition activity of

calcined chromia. They also indicate that unstable

surface chromate species are not important to the

formation of the catalyst active sites (cf. CrI-400),

i.e. electron-mobile Cr(VI)±Cr(III) ion-pairs moni-

tored by the DRS-band at 595 nm (Fig. 1).

3.4.2. CO oxidation activity

In the present (gas phase) experiments focus was

made on supported chromias, which were ignored in

the preceding H2O2 experiments. In order to mimic

conditions of water leaching of surface chromate, the

present test catalysts were pre-reduced by CO at

3508C and, then, examined as CO oxidation catalysts

at 25±3508C. The test reaction was run in oxygen-rich

environment furnished by 5% O2/He as a function of

reaction temperature at one-and-the-same space velo-

city (WHSV�15 000 hÿ1). The results are graphically

represented in Fig. 5.

Fig. 5 shows that the CO oxidation commences on

CrII-600 at a temperature as low as 1208C, and 100%

conversion is accomplished near 2208C. The reaction

commencement is delayed to 2008C on CrAlII-600

and CrSiII-600, and a conversion as low as 20% is not

achieved unless the temperature exceeds 2808C.

Hence, the unsupported chromia is much more active

Fig. 4. Plots of volume of oxygen liberated versus time for H2O2

decomposition on the catalysts indicated.Fig. 5. Plots of CO-conversion versus reaction temperature for CO

oxidation on the catalysts indicated.

322 M.I. Zaki et al. / Applied Catalysis A: General 171 (1998) 315±324

as a CO-oxidation catalyst than the supported chro-

mias. When the results shown in Fig. 5 are compared

at a given temperature (2808C), the activity of the

supported chromias can be ranked in the following

descending order:

CrAlII-600�r� � CrAlII-600�c�> CrSiII-600�c� > CrSiII-600�r�;

where r�reduced and c�calcined.

These results indicate that the CrII-600 catalyst,

which contains chromate species established on large

a-Cr2O3 particles (Tables 1 and 3) and involved in

electronic interactions with nearby Cr(III) sites

(Fig. 1), exhibits higher CO oxidation activity than

the catalysts containing similar sites however exposed

on noncrystalline chromias (CrAlII-600) or on small

a-Cr2O3 particles (CrSiII-600). A pre-reduction of the

catalysts with CO alters, but slightly, their CO oxida-

tion activity. Since the CO-treatment of calcined

chromia catalysts under similar conditions to those

applied here has been found [13,37] to eliminate

surface chromate species, particularly those estab-

lished on surfaces other than of crystalline chromia,

the catalysis results may attribute the CO oxidation

activity largely to thermally and chemically stable

chromates. Thus the unstable chromate species seem

not to contribute importantly to the generation of

redox catalytic sites (presumably Cr(VI)±Cr(III)).

4. Conclusion

Calcination of unsupported or supported chromias

(on silica and alumina surfaces) at 6008C generates

high valency surface CrOx species mostly of Cr(VI).

These species are anchored onto surfaces of the

chromia phase (crystalline or noncrystalline) and/or

surfaces of the support material. Chromates produced

on surfaces of noncrystalline chromia and silica sur-

faces are the least stable to thermal decomposition,

hydrolysis and chemical reduction. In contrast, chro-

mates established on crystalline a-Cr2O3 particles

interact electronically with nearby Cr(III)±O species

to form thermally and chemically persistive interac-

tion species (Cr(VI)±Cr(III)) thus facilitating the elec-

tron-mobile environment demanded by surface redox

reactions. Elimination of unstable chromates by a

brief CO-reduction at 3508C, or by hydrolysis at room

temperature, does not signi®cantly alter the surface

redox activity. Thus, chromia catalysts synthesized by

calcination and a subsequent elimination of unstable

chromates would still possess suf®cient catalytic

potential in redox processes, and pose no acute threat

to the environment.

Acknowledgements

MIZ and MAH thank Kuwait University for a grant

(SC. 076) and the SAF (SLC 063) of the Faculty of

Science for technical assistance. NEF thanks Prof. H.

KnoÈzinger of Munich University for the permission

granted to carry out the CO-oxidation and H2-TPR

experiments.

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