Characterization of Oxidized Surface Phases on VOx/ZrO2 Catalysts

Article abstract of DOI:10.1021/jp983156i

VOx/ZrO2 samples were prepared by three methods: (i) adsorption from aqueous solutions of ammonium metavanadate (AV) at pH 1 or 4; (ii) dry impregnation with AV aqueous solutions at pH 4; and (iii) adsorption from solutions of vanadyl acetylacetonate in toluene. The catalysts were characterized as prepared (a.p.), after evacuation at increasing temperature up to 773 K, after heating in dry oxygen at 773 K (standard oxidation, s.o.), and after reduction in H2 (or CO), by means of XPS, ESR, and FT-IR spectroscopies. The morphology of pure ZrO2 and VOx/ZrO2 particles was investigated by HRTEM and XRD analysis. Both ZrO2 and VOx/ZrO2 samples with V content <= 3.5 atoms nm^-2 showed only zirconia particles with a monoclinic structure. Samples with higher V content contained segregated vanadium phases (V2O5 and ZrV2O7). On a.p. and s.o. samples with surface V content <= 3.5 atoms nm^-2, XPS showed vanadium species, uniformly spread on the zirconia surface. Depending on the V content, on all reduced samples ESR detected mononuclear V(IV) in a square pyramidal configuration, and magnetically interacting V(IV). On a.p. samples, FT-IR spectra showed the presence of vanadate or metavanadate-type species for V content up to 1.5 atoms nm^-2 and of decavanadates for V content in the range 1.5-3.5 atoms nm^-2. Heating in oxygen at 773 K led to a variety of vanadium species anchored to the zirconia surface: isolated vanadates (prevalent in samples with V content <0.2 atoms nm^-2), low-nuclearity polyvanadates (prevalent in samples with V content up to 1.5 atoms nm^-2), and high-nuclearity polyvanadates (prevalent in more concentrated samples, up to 3.5 atoms nm^-2). The relative amount of surface species on s.o. samples mainly depended not on the preparation method, but on the vanadium content.

Full text of DOI:10.1021/jp983156i

10316
J. Phys. Chem. B 1998, 102, 10316-10325
Characterization of Oxidized Surface Phases on VOx/ZrO2 Catalysts
Federica Prinetto and Giovanna Ghiotti*
Dipartimento di Chimica IFM, UniVersit a` di Torino, Via P. Giuria 7, I-10125 Torino, Italy
Manlio Occhiuzzi and Valerio Indovina
Centro SACSO CNR c/o Dipartimento di Chimica, UniVersit a` “La Sapienza”,
Piazzale Aldo Moro 5, I-00185 Roma, Italy
ReceiVed: July 24, 1998; In Final Form: September 30, 1998
x 2
VO /ZrO samples were prepared by three methods: (i) adsorption from aqueous solutions of ammonium
metavanadate (AV) at pH 1 or 4; (ii) dry impregnation with AV aqueous solutions at pH 4; and (iii) adsorption
from solutions of vanadyl acetylacetonate in toluene. The catalysts were characterized as prepared (a.p.),
after evacuation at increasing temperature up to 773 K, after heating in dry oxygen at 773 K (standard oxidation,
s.o.), and after reduction in H
of pure ZrO and VO /ZrO particles was investigated by HRTEM and XRD analysis. Both ZrO
ZrO samples with V content e 3.5 atoms nm showed only zirconia particles with a monoclinic structure.
Samples with higher V content contained segregated vanadium phases (V and ZrV ). On a.p. and s.o.
2
(or CO), by means of XPS, ESR, and FT-IR spectroscopies. The morphology
2
x
2
2
and VO
x
/
-
2
2
2
O
5
2 7
O
-
2
samples with surface V content e 3.5 atoms nm , XPS showed vanadium species, uniformly spread on the
zirconia surface. Depending on the V content, on all reduced samples ESR detected mononuclear V in a
square pyramidal configuration, and magnetically interacting V . On a.p. samples, FT-IR spectra showed
the presence of vanadate or metavanadate-type species for V content up to 1.5 atoms nm and of decavanadates
IV
IV
-
2
-
2
for V content in the range 1.5-3.5 atoms nm . Heating in oxygen at 773 K led to a variety of vanadium
species anchored to the zirconia surface: isolated vanadates (prevalent in samples with V content < 0.2
-
2
-2
atoms nm ), low-nuclearity polyvanadates (prevalent in samples with V content up to 1.5 atoms nm ), and
-
2
high-nuclearity polyvanadates (prevalent in more concentrated samples, up to 3.5 atoms nm ). The relative
amount of surface species on s.o. samples mainly depended not on the preparation method, but on the vanadium
content.
Introduction
Vanadia-based catalysts, mainly supported on TiO2, Al2O3,
and V2O5(-WO3)/TiO2. The SCR process on vanadium-
containing catalysts has been extensively reviewed by Bosch
30
and Janssen. Within this framework, we have recently studied
and SiO2, have been extensively studied because of their
catalytic activity for the selective oxidation of hydrocarbons and
their derivatives. A recent issue of Applied Catalysis was entirely
the catalytic activity of VOx/ZrO2 for the SCR of NO with NH3.
Catalytic results have been reported in full, together with a short
summary of the characterization data relevant to the discussion
1
devoted to this topic. The preparation, characterization, and
31
of the catalytic activity. The SCR activity depended only on
activity of vanadium oxide monolayer catalysts have been
the V content, not on the method used for catalyst preparation.
The marked increase in SCR activity with the V content showed
that only specific vanadium configurations were active. This
aspect will not be further discussed here. In the present paper
we report the HRTEM, XPS, ESR, and FTIR characterization
of VOx/ZrO2 catalysts prepared by various methods (adsorption
from aqueous metavanadate solutions at different pH values,
dry impregnation, and adsorption from vanadyl acetylacetonate
in toluene). The characterization allowed us to specify the
structure of the vanadium polyoxoanions active in the SCR of
NO.
2
reviewed by Bond and Flamerz Tahir and by Wachs and
Weckhuysen.3
Previous research has addressed the dispersion, the nuclearity,
4
-13
and oxidation state of vanadium supported systems on TiO2,
Al2O3,8
,12-22
and SiO2.
13-15,18,23
Because the dispersion, nucle-
arity, and oxidation state of vanadium might depend on the
support, research also addressed other supports and particularly
ZrO2.3
,24-26
Zirconia is very stable to thermal treatments, has a
27
surface endowed with weak acid and basic sites, and is able
to maintain a high specific surface area up to about 1000 K if
28
transition metal ions are present. A recent issue of Catalysis
2
9
Today reviewed the use of ZrO2 as a support for transition
metal oxides.
Experimental Section
Vanadium catalysts are also used for environmental catalysis.
The commercial catalysts for the selective catalytic reduction
of NO with NH3 in the presence of O2 (SCR) are V2O5-TiO2
Sample Preparation. The zirconia support was prepared by
hydrolysis of zirconium oxychloride with ammonia as previously
3
2
described; the precipitate was then calcined in air at 823 K
for 5 h. After calcination, the BET surface area was SA ) 49
2
-1
m
g . The VOx/ZrO2 catalysts were prepared by three
*
Author to whom correspondence should be addressed. Phone: +39-
0
11-6707539. Fax: +39-011-6707855. E-mail: ghiotti@ch.unito.it.
methods: (i) adsorption from aqueous solutions of ammonium
1
0.1021/jp983156i CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/24/1998

Oxidized Surface Phases on VOx/ZrO2 Catalysts
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10317
metavanadate (AV) (7 × 10-2 to 32 mmol L ) at pH 1 or 4,
fixed by nitric acid; (ii) dry impregnation with AV aqueous
solutions at various concentrations and pH 4, fixed by nitric
acid; and (iii) adsorption from solutions of vanadyl acetyl-
acetonate, VO(acac)2, in toluene. The specimens were then dried
at 383 K for 24 h and ground into a fine powder. The VOx/
ZrO2 catalysts are designated as ZVx(a)pHy, ZVx(i), or ZVx-
-1
(
acac) where x specifies the analytical V content (weight %),
where (a), (i), or (acac) indicates the preparation method (i),
ii), and (iii), and y is the pH of the AV solution. After dissolving
(
a known amount of the solid in a concentrated (40%) HF
solution, the vanadium content was determined by atomic
absorption (Varian Spectra AA-30). The x range studied was
0
.02-4.65 wt %, corresponding to a surface density of 0.045
-
2
to 11.9 V atoms nm .
NH4VO3 was purchased from Carlo Erba (R. P.), VO(acac)2
from Merck-Schuchardt, and V2O5 from Aldrich. High-purity
gases from Matheson C. P. were used for catalyst treatments
without further purification.
Figure 1. Vanadium uptake on ZrO
solutions in toluene. Vanadium adsorbed (atoms nm ) as a function
of available vanadium in solution (atoms nm , referred to the ZrO
surface area).
2
from vanadyl acetylacetonate
-
2
The catalysts were studied as prepared (a.p.), after evacuation
at increasing temperature up to 773 K, after heating in dry
oxygen at 773 K (standard oxidation, s.o.), and after reduction
in H2 or CO. After s.o. treatment, all samples had SA values
-2
2
thickness, placed in an IR cell allowing thermal treatments in
vacuo or in controlled atmosphere. All thermal treatments were
performed in the presence of a trap at 77 K placed in the vicinity
of the activation chamber. After each treatment, FT-IR spectra
were run at RT on a Perkin-Elmer 1760-X spectrophotometer
equipped with an MCT cryodetector, working in the range of
2
-1
ranging from 45 to 47 m g , slightly smaller than that of pure
zirconia.
Characterization Techniques. XRD and HRTEM Measure-
ments. The X-ray diffraction analysis of a.p. and s.o. samples
was carried out by a Philips PW 1710 diffractometer at 45 kV
and 20 mA using the Cu KR radiation.
-
1
-1
wavenumbers 7200-580 cm at a resolution of 2 cm
(number of scans ∼100).
Transmission electron microscopies of a.p. and s.o. samples
were taken with a JEOL 2000 EX electron microscope equipped
with a top entry stage, working at 200 kV. The powders were
ultrasonically dispersed in hexane and the suspension was
deposited on a copper grid coated with a holey carbon film.
Data are reported as absorbance spectra or as absorbance
difference spectra, as appropriate. For band integration and curve
fitting we used the software program “Curvefit, in Spectra Calc”
(Galactic Industries Co).
4
5
The instrumental magnification was 2.5 × 10 to 6 × 10 .
XPS Measurements. The XPS spectra were obtained with a
Leybold-Hereaus LHS 10 spectrometer (Mg KR, 1253.6 eV,
Spectra of reference V O and VO(acac) compounds in KBr
2
5
2
were also recorded.
12 kV 20 mA) operating in FAT mode and interfaced to a 2113
HP computer. For the analysis, a.p. or s.o. samples were pressed
onto a gold-decorated tantalum plate attached to the sample
holder. The spectra were collected by computer in a sequential
manner: O1s, V2p 1/2, V2p 3/2, Zr3d 3/2, Zr3d 5/2. BE values were
referred to the O1s main peak, taken as 530.0 eV, and were
deduced by a best-fitting procedure of the composite peak. More
Results and Discussion
Vanadium Uptake. Because the vanadium uptake is a surface
phenomenon, we investigated the vanadium adsorbed per unit
-2
area of zirconia (atoms nm ) as a function of V atoms available
-2
in the solution used for preparation (atoms nm referred to
ZrO2 surface area). From vanadyl acetylacetonate in toluene
solution, zirconia adsorbed all vanadium, up to 3.8 atoms nm .
As the vanadium in solution increased further, uptake reached
an extended plateau, corresponding to about 4 V atoms nm
(Figure 1). Samples prepared by adsorption from AV solutions
at pH 2-4 showed an analogous trend, but the maximum uptake
was somewhat lower (about 3.5 V atoms nm ). The difference
in the V uptake between ZV(acac) and ZV(a) decreased
somewhat after the first evacuation or the first s.o. treatment;
the V content in ZV(acac) decreased by about 15%, whereas
that in ZV(a) remained unaffected.
26
details on spectrum analysis are given elsewhere.
-2
ESR Measurements. The ESR spectra were obtained at RT
or 77 K on a Varian E-9 (X band), equipped with an on-line
computer for data analysis. Absolute concentration of vanadium
-
2
-
2
species (spins nm ) were obtained by the ratio between
integrated areas of ZV samples and standards, recorded under
the same instrumental conditions. As standards, we used
vanadium compounds showing ESR spectra with similar hy-
perfine pattern, line-shape, and line-width: VO(acac)2 (Merck-
Schuchardt), VOSO5‚5H2O (Carlo Erba, RP), and vanadyl
tetraphenylporphyrin (Midcentury, IL), all in a polycrystalline
-2 26
2
6
state.
Based on the plateau value, ZV samples with V content <
-
2
Before ESR measurement, the specimen was placed in a silica
reactor equipped with an ESR silica tube and connected to a
circulation apparatus for the thermal treatment. The apparatus
was equipped with a magnetically driven pump, a pressure
transducer, and a trap placed downstream from the reactor.
During all thermal treatments, the trap was kept at 77 K.
FT-IR Measurements. Powdered materials were pelleted in
3.5 atoms nm will be hereafter referred to as low-loading
-2
samples, those with V content > 3.5 atoms nm as high-loading
samples. We recall that a.p. ZV(a) samples prepared by
adsorption from AV solutions with a concentration higher than
-1
10 mmol L contain a segregated phase, which transforms into
2
6
V O after the s.o. treatment. Wachs et al., by using Raman
2
5
-
2
spectroscopy, have shown that V2O5 up to 6.8 V atoms nm
is molecularly dispersed on the zirconia surface.
-
2
22
self-supporting disks of 25-35 mg cm and 0.1-0.2 mm

10318 J. Phys. Chem. B, Vol. 102, No. 50, 1998
Prinetto et al.
5
Figure 2. HRTEM image of the as-prepared ZV1.05(i) sample. Instrumental magnification was 6 × 10 .
V
HRTEM and XRD Measurements. Irrespective of the
preparation method, pure ZrO2 and low-loading a.p. and s.o.
ZV samples consisted of aggregates of rounded particles with
average size 20-30 nm and a monoclinic structure, as revealed
by the diffraction-fringes distance, in agreement with the XRD
pattern (JCPDS file 36-420). The presence of vanadium left the
zirconia morphology unchanged (Figure 2). HRTEM of high-
loading a.p. ZV samples showed, in addition to a zirconia phase,
indicating the presence of V species only, as already observed
in ZV(a) catalysts.
2
6
In low-loading ZV(acac) and ZV(i) samples (a.p. and s.o.),
the intensity ratios, V2p/Zr3d, increased proportionally to the V
content and approached the corresponding values calculated by
34
the spherical model, as already found in the analysis of ZV(a)
31
samples. In high-loading ZV samples, the V2p/Zr3d values were
markedly larger than those calculated by the spherical model,
pointing to segregation of vanadium phases.
3
00-600 nm particles having a fibrous shape, possibly arising
from polyvanadates or segregated V2O5 (Figure 3a). Because
these particles were too thick to give electron transmission,
HRTEM provided no information on their crystalline structure.
In the high-loading s.o. samples, in addition to zirconia particles,
two segregated phases were present. One phase, consisting of
particles with sharp contours, rectangular shape, and size 300-
ESR Measurements. In a.p. ZV(acac) samples, a weak ESR
IV
signal of V species was detected. As in ZV(a) previously
2
6
studied, no signals were detected in a.p. ZV(i) samples. In all
s.o. samples, no signals were detected either after s.o. or after
s.o. followed by evacuation at 773 K.
Reduction with CO or H2 at 573 K of s.o. ZV(acac) and ZV(i)
caused the formation of ESR signals. Spectra consisted of a
signal showing a resolved hyperfine structure (Vh), overlapping
a broad and nearly isotropic band (Vb). In an earlier study we
detected the same signals in reduced ZV(a) and assigned them
1
000 nm, was the orthorombic V2O5 oxide (Figure 3b). For
some thinner particles, the distance between the diffraction
fringes (dhkl ) 4.28 and dhkl ) 3.40 Å) identified the (001) and
(110) planes. The assignment relies on the electron micrograph
of the reference compound and was confirmed by XRD analysis
JCPDS files 9.387, 41-1426). The second phase, consisting of
rare particles with rounded shape and average diameter 100-
00 nm, not showing diffraction fringes (Figure 3c), might be
attributed to ZrV2O7, in agreement with the XRD pattern
IV
(
to mononuclear V species in a square pyramidal configuration
(Vh) arising from the reduction of vanadates, and to magnetically
IV
2
interacting V (Vb), arising from the reduction of polyoxoanion
2
6,31
units.
The relative intensities of the two signals depended
(
JCPDS file 16-422). The ZrV2O7 compound formed from the
markedly on the V content (Figure 4, spectra 1-4). In particular,
the intensity ratio Vh/Vb (from integrated areas) strongly
decreased with increasing V content. We have previously
observed very similar dependence of the Vh/Vb ratio on the V
25,33
solid-state reaction between ZrO2 and V2O5.
XPS Measurements. In s.o. ZV(acac) and a.p and s.o. ZV(i)
samples, the binding-energy value of component V2p 3/2, deter-
mined by the curve-fitting of the region O1s-V2p, was 517.1 eV,
2
6,31
content for ZV(a) samples.

Oxidized Surface Phases on VOx/ZrO2 Catalysts
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10319
2
Figure 3. HRTEM images of the as-prepared ZV4.65(a)pH4 sample (a) and after heating in O at 773 K (b and c). Instrumental magnification was
5
4
4
1
× 10 (a), 2.5 × 10 (b), and 5 × 10 (c).
In dilute ZV(acac) and ZV(i) samples (up to 0.2 V atoms
ESR progressively decreased to 30-15%. In the high-loading
samples, ESR detected an even smaller percentage of total
vanadium.
-
2
IV
nm ), where isolated V (Vh) was the predominant species,
IV
the maximum V intensity corresponded to 90-60% of total
vanadium. In more concentrated samples (0.2 to 3.5 V atoms
Depending on the preparation method, the evacuation at 773
K of a.p. samples gave different results. The evacuation of
ZV(a)pH1 and ZV(i) at 773 K gave no ESR signals. Conversely,
-2
nm ), containing therefore increasing amounts of magnetically
IV
interacting V (Vb), the fraction of total vanadium detected by

10320 J. Phys. Chem. B, Vol. 102, No. 50, 1998
Prinetto et al.
Figure 5. FT-IR spectra of low-loading ZV as-prepared samples in
-
1
the 3800-1150 cm region. ZV0.83(a)pH4, (curve 1); ZV1.05(i),
(curve 2); and ZV0.30(acac), (curve 3).
Figure 4. ESR spectra at RT of ZV(acac) samples heated in O
73 K and reduced thereafter with CO at 573 K. ZV0.14(acac), (curve
a); ZV0.30(acac), (curve b); ZV0.58(acac), (curve c); ZV1.20(acac),
2
at
7
26
(
curve d). The asterisk indicates the marker at g ) 2.0008. The figures
on each spectrum give the relative gain.
the evacuation of ZV(a)pH4 at the same temperature reduced
V
IV
V to V , thus yielding ESR spectra similar to those obtained
for s.o. samples reduced with CO at 573 K. Irrespective of the
preparation method, the evacuation at 773 K of all s.o. samples
gave no ESR signal.
FT-IR of Low-Loading Samples. As-Prepared Samples. In
-
1
the 3800-1200 cm region (Figure 5), all samples showed a
-1
broad absorption at 3760-2600 cm and a sharp band at 1620
-
1
cm , due to surface hydroxyls and coordinated water.
The a.p. samples were distinguished by the following
features: (i) in ZV(a)pH4 (curve 1), the presence of carbonates
in a very small amount (νOdCdO asym mode at 1600-1450,
-
1 35
νOdCdO sym at 1350-1200, νC-O at 1060-50 cm , ); (ii) in
ZV(a)pH1 (not shown) and ZV(i) (curve 2), the presence of
unidentate nitrates (νOdNdO asym mode broad band at 1550,
Figure 6. FT-IR spectra of low-loading ZV as-prepared samples in
the 1080-830 cm region. (a) ZrO , (curve 1); ZV0.17(a)pH4, (curve
2
2); ZV0.34(a)pH4, (curve 3); ZV0.58(a)pH4, (curve 4); ZV0.83(a)pH4,
(curve 5); and ZV1.21(a)pH4, (curve 6). (b) ZV1.05(i), (curve 1);
ZV1.21(a)pH4, (curve 2); ZV1.03(a)pH1, (curve 3); and ZV0.30(acac),
-1
-
1 36
νOdNdO sym at 1330-1280, νN-O at 1050-20 cm-
), and
1
ammonium species (δasym mode at 1460-30 cm , δsym at
-1
1
680-60 cm , and stretching modes not easily detectable being
(curve 4).
superposed on hydroxyl and water modes). Ammonium was
present in smaller amounts on ZV(a)pH(1) than on ZV(i). In
ZV(a)pH1, nitrates were adsorbed from the concentrated HNO3
solution used to fix the pH of the AV solution, and in ZV(i)
the high nitrate concentration was due to the impregnation
method that causes all ions to remain on the surface; and (iii)
in ZV(acac) (curve 3), the presence of carbonate and acetyl-
acetonate species (broad bands at 1750-1500 and 1450-1250
cm- ), CH3 and CH2 groups (bands in the CH stretching
1 37
-
1
region at 3000-2900 cm ).
-
1
In the 1100-830 cm region, ZV(a)pH4 samples showed
-
1
bands at 940-920, 990-960, and 880-850 cm (Figure 6a).
The intensity of these bands depended markedly on the V
-
1
content. Notably, the band at 940-920 cm was prevalent in
-
2
samples with V content <1.5 atoms nm (curves 2 and 3),

Oxidized Surface Phases on VOx/ZrO2 Catalysts
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10321
TABLE 1: Most Prominent Raman (R) V-O Bands of
Reference Compounds1
3,38,39
and IR V-O Bands of ZV
-1
As-Prepared Samples in the 1100-830 cm Range
ν
asym
a
ν(VdO)
(V-O-V)
-
1
-1
species
(cm
)
(cm )
ref. compd (R)
3
-
[
[
[
[
HV
(VO
2
O
3 n
) ]
7
]
(at pH 8.5-5.5)
(at pH 8.5-5.5)
(at pH 8.5-5.5)
915, 877
945
990, 960
1005, 980
940, 920
928, 890
n-
6
-
V
H
10
O
V
28
]
840
850
(6-n)-
n
10
O
28
]
(at pH 5.5-1.5)
]3 (at pH < 1.5)
-
cis-[VO
cis-[VO
2
F
4
Cl
4
]3 (at pH < 1.5)
-
2
ZV a.p. (IR)
ZV(a or i)pH4 (<1.5 V atoms nm-2)
940-920
ZV (a or i)pH4 (1.5-3.5 V atoms nm-2) 990-960
880-850
-
2
ZV(a)pH1 (1.5-3.5 V atoms nm
)
990-960,
005, 968
1030-980
1
ZV(acac) (0-3.5 V atoms nm-2)
a
Including νsym and νasym
.
whereas bands at 990-960 and 880-850 cm-1 were prevalent
-
2
in samples with V content 1.5 to 3.5 atoms nm (curves 4, 5,
and 6). We suggest that the species adsorbed on ZrO2 are similar
to those present in AV solutions with analogous concentrations
-
3
and pH. Specifically, at pH 4 in dilute AV solution (<10 M)
-
vanadates, [H2VO4] are the most abundant species, whereas
5
-
in concentrated solutions decavanadates, [HV10O28] and/or
4
-
13,38
[
H2V10O28] , are the most abundant species.
According to
13,38,39
the Raman spectra
of the relevant reference compounds
Table 1), we assign (i) the 940-920 cm-1 band to the ν(V d
V
(
O) mode of vanadate or metavanadate-type species, (ii) the
-
1
V
9
90-960 cm band to the ν(V dO) mode and the 880-850
cm band to νasym (V-O-V) mode of decavanadate-type
-
1
species.
Spectra of ZV(i) samples closely resembled those of ZV(a)-
pH4 with comparable V content (compare curves 1 and 2 in
Figure 6b). The main difference was the presence in ZV(i) of
-
1
a band at 1050-20 cm , due to surface nitrates. Spectra of
-2
ZV(a)pH1, with V content > 1.5 atoms nm , showed the same
-1
broad absorption at 990-960 cm of ZV(i) and ZV(a)pH4 with
similar V content and, in addition, two peaks at 968 and 1005
-
1
cm (Figure 6b, curve 3). Because at pH 1 in a wide
+
Figure 7. Evacuation at increasing temperature of as-prepared samples
concentration range of AV solutions, VO2 is the most abundant
4 3
prepared by adsorption from NH VO solutions at pH 4. FT-IR spectra
-
1
species, we assign the 968 cm peak to the νasym and the 1005
-1
of ZV0.83(a)pH4 sample in the 3800-1150 cm (a), and 1080-830
-
1
+
cm peak to the νsym mode of VO2 . At pH 1 the ZrO2 surface
is extensively protonated, and therefore anionic species are
expected to adsorb. Previous evidence that AV solutions,
-1
cm regions (b and c). Experiments in sequence: as-prepared sample,
(curve 1); evacuated at RT, (curve 2); at 423 K, (curve 3); at 523 K,
(curve 4); at 773 K, (curve 5); subsequently heated in O at 773 K,
2
3
-
(curve 6); and reduced with H at 773 K, (curve 7).
adjusted to pH 1 with HCl or HF, contain cis-[VO2Cl4] and
cis-[VO2F4] , suggests that AV solutions adjusted with HNO3
2
3- 38
K of ZV(a)pH1 and ZV(i) a.p. transformed unidentate nitrates
3-
contain the analogous cis-[VO2(NO3)4] species (Table 1). The
adsorption of cis-[VO2(NO3)4]3 -type species on ZrO2 accounts
for the simultaneous presence in ZV(a)pH1 of IR bands from
VO2 , nitrates, and NH4 , the latter species being present as a
counterion. Spectra of ZV(acac) showed a broad absorption at
030-980 cm (Figure 6b, curve 4), namely, in the same
region as the ν(VdÃ) bands of pure VO(acac)2 in KBr.
-1
into bidentate bridged nitrates (νNdO at 1600-1500 cm ,
-
-1
νO)N-O asym at 1300-1200 cm , and νO)N-O sym at 1040-
-1 31,36
1
020 cm
). The evacuation completely removed am-
+
+
monium species at 523 K and nitrates at 773 K (Figure 8a, a′).
The evacuation of ZV(acac) at 773 K removed carbonates and
acetylacetonate species (spectra not reported).
-
1
1
-1
In the 1100-830 cm region, the evacuation of ZV(a)pH4
As-Prepared Samples EVacuated at Increasing Temperature.
The evacuation of all a.p. samples at increasing temperature
caused the complete removal of coordinated water at 423 K
and a progressive dehydroxylation. Evacuation up to 773 K
caused further dehydroxylation. After this treatment, the amount
of free hydroxyls decreased with the V content (Figures 7a and
and ZV(i) at increasing temperature up to 523 K progressively
shifted the bands of the different V species toward higher
frequency, resolved these bands into several components, and
enhanced their intensity (Figure 7b and Figure 8b, curves 1-4).
The changes caused by evacuation up to 773 K markedly
depended on the sample preparation. This treatment caused the
reduction of V species in ZV(a)pH4 but not in ZV(a)pH1 and
ZV(i) samples. In particular, spectra of ZV(a)pH1 and ZV(i)
evacuated at 773 K were nearly coincident with those of s.o.
8
a).
The evacuation of ZV(a)pH4 caused a progressive decreasing
of carbonates, up to 773 K (Figure 7a). The evacuation at 423
V

10322 J. Phys. Chem. B, Vol. 102, No. 50, 1998
Prinetto et al.
Figure 9. FT-IR spectra of low-loading ZV samples heated in O
7
2
at
73 K in the 3900-3000 cm (a) and 1080-830 cm regions (b).
ZrO , (curve 1); ZV0.17(a)pH4, (curve 2); ZV0.34(a)pH4, (curve 3);
ZV0.30(acac), (curve 3′); ZV0.21(a)pH1, (curve 3′′); ZV0.58(a)pH4,
-
1
-1
2
Figure 8. Evacuation at increasing temperature of as-prepared samples
prepared by dry impregnation with NH VO solutions. FT-IR spectra
of ZV1.05(i) sample in the 3800-1150 cm (a), 1720-1150 (a′), and
(
curve 4); ZV0.83(a)pH4, (curve 5); ZV1.21(a)pH4, (curve 6); ZV1.05-
4
3
(i), (curve 6′); ZV1.03(a)pH1, (curve 6′′).
-
1
-
1
1
080-830 cm region (b). Experiments in sequence: as-prepared
tentatively assigned to the ν(V-O-V) modes of polynuclear
species (Figure 9b, curves 2 to 6). The absorption edge of the
ZrO2 bulk vibrations shifted upward with increasing V content.
A similar effect has been previously observed on MoOx/ZrO2
and CrOx/ZrO2 systems,
Mo, Cr, V) stretching vibration modes localized at the surface
of zirconia.
sample, (curve 1); evacuated at RT, (curve 2); at 423 K, (curve 3); at
5
6
23 K, (curve 4); at 773 K, (curve 5); heated in O
).
2
at 773 K, (curve
4
2,43
and attributed to Zr-O-M (M )
samples (Figure 8b, curves 5 and 6); whereas spectra of a.p.
ZV(a)pH4 and ZV(acac) evacuated at 773 K were nearly
identical to those of the corresponding s.o. samples reduced in
H2 at 773 K (compare curves 5 and 7 in Figure 7c).
In the 1080-980 cm- region, seven components were
identified as peaks and shoulders and analyzed by a curve-fitting
procedure (Figure 10a,b). To this aim, the relative intensity of
the seven components were left free, the full width at half-
maximum (fwhm) were fixed, and a fully Gaussian shape was
assumed. The peak position (cm ) and fwhm (indicated in
parentheses, cm ) were as follows: peak 1, 1001 (5); peak 2,
1
peak 6, 1043 (10); and peak 7, 1053 (10). Irrespective of the
preparation method, the total integrated intensity of the seven
peaks increased proportionally to the V content (Figure 10c).
The relative intensity of the various components depended not
on the preparation method, but on the vanadium content. In
1
Samples Heated in O2 at 773 K. The s.o. treatment of (i) a.p.
samples and (ii) a.p. samples previously evacuated at 773 K
gave the same IR spectra. After the s.o. treatment, irrespective
of the preparation method, with the exception of ZV(a)pH1 with
-1
-
2
V content < 0.5 atoms nm , spectra of ZV samples with the
-1
same V content were nearly coincident (Figure 9a,b, curves 3,
009 (9); peak 3, 1019 (9); peak 4, 1026 (9); peak 5, 1037 (10);
3
′, 3′′ and 6, 6′, 6′′).
In the 3800-3000 cm region, the amount of free hydroxyls
main bands at 3770 and 3670 cm-1 40,41) decreased with the V
-1
(
-1
content (Figure 9a). In the region 3000-1100 cm , spectra
showed only a negligible amount of surface carbonates.
In the 1100-830 cm- region, spectra consisted of a
1
particular, in most dilute samples (up to 0.2 atoms nm )
-2
composite envelope of heavily overlapping bands assigned to
the vibrational stretching modes of terminal V dO groups and
to two weak bands at 894 and 874 cm . These two bands,
nearly absent in all samples with V content < 0.2 atoms nm
and in ZV(a)pH1 with V content < 0.5 atoms nm , are
component 3 predominated, the other components being absent
or present at very low intensity. An exception was ZV(a)pH1
samples, in which component 3 predominated up to 0.5 V atoms
nm . In samples with V loading up to 1.5 atoms nm ,
components 1, 2, and 4 increased together and became prevalent,
V
-
1
-
2
-2
-2
-
2

Oxidized Surface Phases on VOx/ZrO2 Catalysts
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10323
Figure 12. Reversibility of vanadium surface species in redox
treatments of low-loading ZV samples. FT-IR spectra of ZV0.83(a)-
pH4: sample heated in O
2
at 773 K, (curve 1); evacuated at 773 K,
at 773 K, (curve 3); heated in O at 773 K,
(curve 2); reduced with H
curve 4).
2
2
(
overlap, however, resembled strictly that of the corresponding
fundamental mode. We therefore suggest that all vanadyls are
mono-oxo species, because the spectral features of di-oxo
species would have differed, due to the combination of the asym
Figure 10. Curve fitting of IR spectra in the 1080-980 cm-1 region
2
for low-loading ZV samples heated in O at 773 K: ZV0.58(a)pH4 (a)
-
1
and ZV1.21(a)pH4 (b). Integrated intensity (cm ) of the overall
-
1
absorption in the 1080-980 cm region for ZV(a) (O), ZV(i) (9),
and ZV(acac) (4) samples as a function of the vanadium content (atoms
and sym ν(VdO) modes, as reported by Busca et al. for
-
2
-1
nm ) (c). Integrated intensity (cm ) of type I (4), type II (b) and
44-46
polycrystalline V2O5.
The three selected samples had lower
-2
type III (•) vanadyls as a function of the vanadium content (atoms nm
d).
)
measured overtone maxima values, 2ν(VdO)exp, than the
calculated values, 2ν(VdO)calc, this pointing to anharmonicity
of the VdO oscillators. The value, ∆ν ) [2ν(VdO)calc - 2ν-
(
(VdO)exp]/2, can be taken as a measure of VdO oscillator
anharmonicity. Type III vanadyls have higher anharmonicity,
-
1
∆
ν ) (2076 - 2050)/2 ) 13 cm , than both type I and type
II vanadyls, ∆ν ) (2044 - 2030)/2 ) (2054 - 2040)/2 ) 7
-
1
cm . If the frequency increase observed passing from type I
to type III vanadyls were due to an increase of VdO bond
strength, we would have found a parallel decrease in anharmo-
nicity, whereas we observed both the highest frequency and
anharmonicity for type III vanadyls. Therefore, we suggest that
the higher frequency of type III vanadyls results from a dynamic
coupling among a large-enough number of nearly parallel
vanadyls, belonging to polyvanadate species of high nuclearity
and showing a weaker VdO bond than that expected for an
isolated vanadyl in a monomeric species. Compared with
isolated vanadyls in a monomeric species, vanadyls of poly-
vanadates are expected to have a weaker double-bond character
and therefore a higher anharmonicity. Along the same lines,
we propose that type I and type II vanadyls belong to monomeric
species or to low-nuclearity polyvanadates.
Figure 11. FT-IR spectra of low-loading ZV samples heated in O
2
at
7
73 K in the ν(VdO) region (a) and in the 2ν(VdO) region (b).
ZV0.21(a)pH1, (curve 1); ZV0.34(a)pH4, (curve 2); and ZV1.21(a)-
pH4, (curve 3);
whereas component 3 increased little. In more concentrated
-
2
samples (V content of 1.5 to 3.5 atoms nm ) components 5,
, and 7 steeply increased, becoming predominant, whereas all
6
other components remained nearly constant (Figure 10d). On
the basis of these intensity changes, we ordered the seven
components into three groups: component 3 alone (hereafter
referred to as type I vanadyls); components 1, 2, and 4 together
S.O. Samples Heated at 773 K in Vacuo or in H . The
2
evacuation up to 773 K of all s.o. samples caused no changes
-
1
-1
in the 3800-1100 cm and 1100-830 cm spectral regions
(type II vanadyls); and components 5, 6, and 7 together (type
(Figure 12, curves 1 and 2).
III vanadyls).
Heating in H at 623 or 773 K of all s.o. samples markedly
2
-1
To obtain further information on the three vanadyl types, we
decreased the intensity of the 1100-830 cm bands and caused
-
1
compared the fundamental stretching modes, ν(VdO), with the
the formation of new components at 1070-1050 cm , due to
corresponding first overtones, 2ν(VdO),4
4-46
of ZV0.21(a)pH1
the reduction of V (curve 3). A subsequent heating in O2
V
(
mainly containing type I vanadyls), ZV0.34(a)pH4 (mainly
completely restored the spectrum of s.o. samples (curve 4). Full
reversibility was maintained when the redox cycles were
repeated several times.
containing type II vanadyls), and ZV1.05(a)pH4 (mainly
containing type III vanadyls) (Figure 11). Compared with the
fundamental modes, the overtones showed lower intensity (=
S.O. Samples Exposed to H2O. Exposure at RT to water vapor
for 30 min of s.o. ZV(a), ZV(i), and ZV(acac) caused the bands
1
/40) and resolution, so that the curve-fitting procedure failed
-
1
to yield a reliable analysis of these overtones. The 2ν(VdO)
in the 1100-830 cm region to broaden and shift to a lower

10324 J. Phys. Chem. B, Vol. 102, No. 50, 1998
Prinetto et al.
Figure 13. The effect of water vapor addition to low-loading ZV
samples previously heated in O at 773 K. FT-IR spectra of ZV1.05(i)
after various treatments. Experiments in sequence: sample heated in
at 773 K, (curve 1); contacted with H O vapor for 30 min at RT
2
O
2
2
(
4
p
eq ) 15 Torr), (curve 2); evacuated at RT, (curve 3); at 423 K, (curve
); at 773 K, (curve 5). The spectrum of the a.p. sample is reported for
comparison, (curve 6).
frequency (Figure 13, curves 1 and 2). After this treatment, the
spectra of ZV(a) and ZV(i) closely resembled those of the
corresponding a.p. sample (compare curves 2 and 6 in Figure
13), whereas spectra of ZV(acac) resembled those of a.p. ZV-
(a) and ZV(i) with similar V content. Evacuation up to 773 K
of all samples previously exposed to water restored progressively
the spectrum of the relevant s.o. sample (Figure 13, curves 3-5).
Exposure at RT to water vapor or atmosphere for several weeks
and evacuation at 773 K caused the reduction of vanadium,
leading to spectra similar to those detected on samples heated
in H2 at 773 K (spectra not reported).
Figure 14. FT-IR spectra of high-loading ZV samples in the 1080-
-
1
830 cm region. ZV4.65(a)pH4 after various treatments. (a) Experi-
ments in sequence: as-prepared sample, (curve 1); evacuated at 523
FT-IR of High-Loading Samples. The spectrum of high-
loading a.p. samples consisted of two broad and intense
K, (curve 2); at 773 K, (curve 3); heated in O
with H at 773 K, (curve 4). (b) Sample heated in O
); heated in O at 773 K after 3-4 redox cycles, (curve 2).
2
at 773 K and reduced
2
2
at 773 K, (curve
-
1
-1
absorptions at 1040-980 cm and at 950-830 cm . The
1
2
-
1
absorption at 1040-980 cm consisted of overlapping com-
-
1
-1
ponents at 1026, 1014, and 983 cm , that at 950-830 cm
overlapping of these components with strong bands from
segregated V2O5 did not allow reliable curve-fitting analysis.
The spectrum of s.o. samples obtained after 3-4 redox cycles
markedly differed from that observed after the first s.o.
treatment. After several redox cycles, we observed a marked
was superposed on the zirconia bulk vibrations (Figure 14a,
curve 1). The spectrum was very similar to that of pure V2O5
in KBr, the band at about 1020 cm-1 being assigned to V dO
V
-
1
V
stretching modes and the band at about 820 cm to V dO
and V-O-V coupled vibrations.47
increase of a broad absorption at 1040-830 cm , possibly due
-1
The evacuation at increasing temperature up to 523 K
decreased the absorptions at 950-850 and 1014 cm and
shifted the 1026 cm component toward a higher frequency
Figure 14a, curve 2). Evacuation at 773 K caused a dramatic
to the formation of a new massive phase (Figure 14b, curve 2).
The Anchorage Process of Vanadium Species. Having
reported above the spectroscopic features and the assignment
of IR, ESR, and XPS spectra, in the following we will focus
on the surface chemistry processes taking place during the
heating of ZV samples under vacuum and in oxygen at 773 K.
In low-loading a.p. samples, in addition to the vanadium
species (hydrated vanadates, decavanadates, and vanadyls),
carbonates, nitrates, acetylacetonates, and ammonium are present,
depending on the preparation method.
-
1
-
1
(
loss of sample transparency in the whole IR region, a marked
intensity decrease of all bands, and the formation of new
-
1
components at 1069, 1052, 1042, 1033, and 1027 cm (curve
-
1
3
). In addition, new bands at 950, 920, 900, and 878 cm
became visible. These spectral features are possibly due to the
reduction of V2O5 to nonstoichiometric VOx oxides (x ) 0.8-
48
1
.3), because we observed similar spectra when the s.o. sample
In low-loading samples heated in O2 at 773 K, irrespective
of the preparation method and preceding treatments, carbonates,
nitrates, acetylacetonates, and ammonium are absent. In these
was subsequently reduced in H2 at 773 K (compare curves 3
and 4 in Figure 14a.
V
The spectrum of high-loading s.o. samples in the 1100-830
samples, XPS shows the presence of V species only, uniformly
-
1
cm region was similar to that of the corresponding a.p.
spread on the zirconia surface. Overall, ESR and IR results
identify three vanadium species: (i) isolated vanadates prevalent
-
1
samples, although the band at 1040-980 cm appeared more
-2
resolved, showing components at 1040, 1030, 1020, and 983
in samples with V content < 0.2 atoms nm ; (ii) low-nuclearity
-
1
cm (Figure 14b, curve 1). The higher-frequency components
most probably arose from supported V species similar to those
present in low-loading samples. For high-loading samples the
polyvanadates, prevalent in samples with V content up to 1.5
-
2
atoms nm ; and (iii) high-nuclearity polyvanadates, prevalent
-
2
in more concentrated samples, up to 3.5 atoms nm . The

Oxidized Surface Phases on VOx/ZrO2 Catalysts
J. Phys. Chem. B, Vol. 102, No. 50, 1998 10325
relative amount of vanadates and polyvanadates depends only
on the V content and not on the preparation method, with the
noticeable exception of ZV(a)pH1 samples, on which, in the
concentration range 0.2-0.5 V atoms nm , the amount of
monomers is much higher than that observed in the correspond-
ing ZV(a)pH4, ZV(i), and ZV(acac) samples, as discussed
above.
The vanadium anchorage to the zirconia surface takes place
through water elimination between adsorbed vanadium species
and the hydroxo groups of the zirconia surface, leading to
(5) Centi, G.; Giamello, E.; Pinelli, D.; Trifir o` , F. J. Catal. 1991, 130,
20.
2
(6) Soria, J.; Conesa, J. C.; Lopez Granados, M.; Mariscal, R.; Fierro,
J. L. G.; Garcia de la Banda, J. F.; Heinemann. H. J. Catal. 1989, 120,
457.
(7) Odriozola, J. A.; Soria, J.; Somorjai, G. A.; Heinemann, H.; Garcia
de la Banda, J. F.; Lopez Granados, M.; Conesa, J. C. J. Phys. Chem. 1991,
-
2
9
5, 240.
8) Gil-Llamb `ı as, F. J.; Escudey, A. M.; Fierro, J. L. G.; Lopez Agudo,
A. J. Catal. 1985, 95, 520.
9) Lars S.; Andersson, T. J. Chem. Soc., Faraday Trans. 1 1979, 6,
356.
10) Kantcheva, M. M.; Bushev, V.; Klissurski, D. J. Catal. 1994, 145,
(
(
1
9
(
(-OmVn-O-Zr-) species. In agreement, irrespective of the
6.
(11) Ciambelli, P.; Lisi, L.; Russo, G.; Volta, J. C. Appl. Catal., B
EnVironmental 1995, 7, 1.
12) Eckert, H.; Wachs, I. E. J. Phys. Chem. 1989, 93, 6796.
13) Deo, G.; Wachs, I. E. J. Phys. Chem. 1991, 95, 5889.
14) Van Rejen, L. L. Cossee, P. Discuss. Faraday Soc. 1966, 41, 277.
(15) Shvets, V. A.; Kazansky, V. B. J. Catal. 1972, 25, 123.
(16) Inomata, M.; Mori, K.; Miyamoto, A.; Murakami, Y. J. Phys. Chem.
983, 87, 761.
17) Jonson, B.; Rebenstorf, B.; Larsson R.; Lars, S.; Andersson, T.;
Lundin, S. T. J. Chem. Soc., Faraday Trans. 1 1986, 82, 767
(18) Sharma, V. K.; Wokaun, A.; Baiker, A. J. Phys. Chem. 1986, 90,
715.
preparation method, we detected a decreasing amount of
hydroxyl groups in s.o. ZV samples with increasing V content.
After evacuation at increasing temperature up to 773 K of
a.p. ZV(a)pH4, IR and ESR showed the reduction of vanadates
(
(
(
IV
and polyvanadates to species containing V . The evacuation
of ZV(i) and ZV(a)pH1 at increasing temperature did not reduce
vanadates and polyvanadates, most probably because NO and
NO2 evolving from the nitrate decomposition prevent reduction.
For all samples, once anchored to the zirconia surface,
vanadates and polyvanadates are neither reduced upon evacu-
ation at 773 K nor upon exposure for 30 min to H2O followed
by evacuation at 773 K. The heating with CO or H2 at 573-
1
(
2
(
19) Jhansi Lakshmi, L.; Narsimha, K.; Kanta Rao, P. Appl. Catal., A
993, 94, 61.
20) Eon, J. G.; Olier, R.; Volta, J. C. J. Catal. 1994, 145, 318.
(21) Roozeboom, F.; Fransen, T.; Mars, P.; Gellings, P. J. Z. Anorg.
Allg. Chem. 1979, 449, 25.
1
(
V
IV
7
7
73 K reduced V to V . The subsequent heating with O2 at
73 K reversibly restored surface vanadates and polyvanadates.
The exposure of all s.o. samples to the atmosphere or water
(
(
22) Wachs, I. E. Catal. Today 1996, 27, 437.
23) Che, M.; Canosa, B.; Gonzalez-Elipe, A. R. J. Phys. Chem. 1986,
vapor for several weeks, dis-anchored vanadates and polyvana-
dates from the surface. For all ZV samples, after dis-anchorage,
evacuation at 773 K reduced vanadates and polyvanadates.
The XPS of a.p. and s.o. high-loading ZV samples showed
poor spreading of vanadium species on ZrO2. The XRD,
HRTEM, and FT-IR of the s.o. high-loading samples showed
segregated V2O5 and ZrV2O7, the latter irreversibly formed
through the solid-state reaction between ZrO2 and V2O5.
9
0, 618.
(24) Szakacs, S.; Altena, G. J.; Fransen, T.; Van Ommen, J. G.; Ross,
J. R. H. Catal. Today 1993, 16, 237.
25) Sohn, J. R.; Cho, S. G.; Pae, Y. I.; Hayashi, S. J. Catal. 1996, 159,
70.
26) Occhiuzzi, M.; Tuti, S.; Cordischi, D.; Dragone, R.; Indovina, V.
J. Chem. Soc., Faraday Trans. 1996, 92, 4337.
(
1
(
(27) Tanabe, K. Mater. Chem. Phys. 1985, 13, 347.
(28) Valigi, M.; Cimino, A.; Cordischi, D.; De Rossi, S.; Ferrari, C.;
Ferraris, G.; Gazzoli, D.; Indovina, V.; Occhiuzzi, M. Solid State Ionics
1
993, 63-65, 136.
Conclusions
(29) Zirconium in catalysis. Moles, P. J., Ed.; Catal. Today 1994, 20.
(30) Bosch, H.; Janssen, F. Catal. Today 1988, 2, 369.
The heating in O2 at 773 K of VOx/ZrO2 with V content up
to 3.5 atoms nm- stabilizes surface vanadates of various
nuclearities, uniformly spread on the ZrO2 support. For sample
with a V content up to 1.5 atoms nm , monomeric and low-
nuclearity V species prevail, whereas in the concentration range
(31) Indovina, V.; Occhiuzzi, M.; Ciambelli, P.; Sannino, D.; Ghiotti,
2
G.; Prinetto, F. Studies in Surface Science and Catalysis; Hightower, J.
W., Delgass, N. W., Iglesia, E., Bell, A. T., Eds.; Elsevier Science Publ.,
Amsterdam, 1996; Vol. 101, p 691.
-
2
(32) Cimino, A.; Cordischi, D.; De Rossi, S.; Ferraris, G.; Gazzoli, D.;
Indovina, V.; Minelli, G.; Occhiuzzi, M.; Valigi, M. J. Catal. 1991, 127,
744-760, 761-776, 777-787.
-
2
1
.5-3.5 atoms nm higher-nuclearity V species, possibly
(33) Sanati, M.; Andersson, A.; Wallenberg, L. R.; Rebenstorf, B. Appl.
decavanadates, prevail. Samples with V content > 3.5 atoms
Catal., A 1993, 106, 51.
34) Cimino, A.; Gazzoli, D.; Valigi, M J. Electron Spectrosc. Relat.
Phenom. 1994, 67, 429.
-
2
nm contain V2O5 and ZrV2O7. The relative abundance of the
various V species mainly depends on the V content, not on the
method used for catalyst preparation. This finding strengthens
(
(35) Busca, G.; Lorenzelli, V. Mater. Chem. Phys. 1982, 7, 89.
(36) Laane, J.; Ohlsen, J. R. Prog. Inorg. Chem. 1980, 27, 465.
that the nuclearity of the adsorbed species is mostly controlled
by the acid-base properties of the support.42,49
(37) Pope, M. T. In Heteropoly and isopoly oxometalates; Springer-
Verlag: Berlin, 1983; Chapter 3.
The marked increase in the SCR activity of VOx/ZrO2 with
the vanadium content31 is due to the increased concentration of
polynuclear vanadium species.
(
38) Griffith, W. P.; Lesniak, P. J. B. J. Chem. Soc. A 1969, 1066.
(39) Went, G. T.; Oyama, S. T.; Bell, A. T. J. Phys. Chem. 1990, 94,
240.
40) Tsyganenko, A. A.; Filimonov, V. N. J. Mol. Struct. 1973, 19, 579.
41) Bensitel, M.; Saur, O.; Lavalley, J. C. Mater. Chem. Phys. 1987,
4
(
(
Acknowledgment. Financial support was provided by the
Ministero dell’Universit a` e della Ricerca Scientifica e Tecno-
logica (Progetti di rilevante interesse nazionale).
1
7, 249.
(42) Prinetto, F.; Cerrato, G.; Ghiotti, G.; Chiorino, A.; Campa, M. C.;
Gazzoli, D.; Indovina, V. J. Phys. Chem. 1995, 99, 5556.
(43) Ghiotti, G.; Chiorino, A. Spectrochim. Acta 1993, 49 A, N. 9, 1345.
(44) Busca, G.; Lavalley, J. C. Spectrochim Acta 1986, 42 A, N. 4, 443.
(45) Busca, G. Langmuir 1986, 2, 577.
References and Notes
(
1) Vanadia catalysts for selective oxidation of hydrocarbons and their
(46) Busca, G. Mater. Chem. Phys. 1988, 19, 157.
(47) Mori, K.; Miyamoto, A.; Murakami, Y. J. Chem. Soc., Faraday
Trans. 1987, 83, 3303.
(48) Greenwood, N. N.; Earnshaw, A. Chemistry of Elements; Pergamon
Press: U.K., 1993; p 279.
derivative. Grybowska-Swierkosz, B., Trifir o` , F., Vedrine, J. C., Eds.; Appl.
Catal., A 1997, 157.
(
(
(
2) Bond, G. C.; Flamerz Tahir, S. Appl. Catal. 1991, 71, 1.
3) Wachs, I. E.; Weckhuysen, B. M. Appl. Catal., A 1997, 157, 67.
4) Rusiecka, M.; Grzybowska, B., Gasior, M. Appl. Catal. 1984, 10,
(49) Kim, D. S.; Segawa, K.; Soeya, T.; Wachs, I. E. J. Catal. 1991,
136, 744.
1
01.