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SiO2 system
Applied C’atulysis, 31(1987) 73-85 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlends
CHARACTERI~TION
OF SUPPORTED
CHEMISORPTION
II.
TECHNIQUE:
VANADIU~~ OXIDE THE V205/SiO2
CATALYSTS
73
BY LOW TE~FERATURE
OXYGEN
SYSTEM
Nabin K. NAG*' Department
of Fuels
Engineering,
University
of Utah, Salt Lake City, Utah 84112,
and Komandur
V.R. CHARY,
Regional
Research
'Present address: Boulevard, (Received
B. Rama RAO and Varanashi
Laboratory,
Harshaw/Filtrol
Los Angeles, 21 July
S. SUBRAHMANYA~~
Hyderabad-500009, Partnership,
California
1986, accepted
India.
98023,
Filtrol
Division,
3250 E. Washington
U.S.A.
22 December
19861
ABSTRACT LOW TeMperatUre Oxygen Chemisorption (LTOC) has been applied to characte ize a series of silica-sup orted V-oxide catalysts containing up to 5 umol V m- 5 . LTOCdata, coupled wit R ancillary information obtained from ESR, IR, XRD, ESCA and surface area measurements, have thrown light on the nature of the dispersed vanadia which has been found to consist of two distinct highly dispersed V-oxide phases. Based on the experimental results, a model of the surface structure of the dispersed phases has been proposed. It has been envisioned that a "monolayer" phase containing, in small part, isolated (VO4)4- species and in great part, a dimeric V5+ oxide species, both in tetrahedral coordination, is formed at lower vanadia loading region, while at lo dings beyond the so-called "monolayer" coverage a second phase consisting of only Vf+ -oxide in square pyramida? or octahedral surroundings (i.e., a two dimensional structure akin to that of bulk V205) is formed preferentially on the top of the "monolayer" phase. The completion of the 'Imonolayer~ and the beginning of the formation of the "post mono~ayer~ phases have been clearly identified by a maximum in the oxygen uptake capacity of the samples.
INTRODUCTION Low temperature applied offers sulfide studied
oxygen
to characterize a means
of determining
phases.
Supported
uptake
zation
(HDS) reactions MO-containing
catalysts
(LTOC) at -78°C or below and sulfide
the "active"
can be directly
hydroprocessing
low t~perature
correlated
with
has recently
area of supported
oxide
and
have been extensively
have been made
studies
to correlate for hydrodesulfuri-
on a series
catalysts,
oxygen
been
[1,2]. This method
their activities
From the systematic
sulfided
conditions
with
catalysts
catalysts
and attempts
of the catalysts [2,3].
surface
and -sulfide
recently,
capacities
that under certain
oxide
MO-oxide
by this technique
oxygen
supported
chemisorption
supported
uptake
of A1203-
it has been demonstrate1 capacities
their HDS and hydrogenation
of these activities
c31. Similar
studies
0166~9834/87/$03.50
on supported
V-oxide
catalysts
are rare and only some fragmentar:
0 1987 Elsevier Science Publishers B.V.
information
relative
to this system
is found
oxide catalysts
act as redox systems
since partially
reduced
oxygen
chemisorption
candidates the results
Following
of a systematic
zed for measuring
active
the surface
the amount
surface
of chemisorbed
phase
of V/A1203
catalysts,
on different
supports
state of supported
metals
with
and this in turn may modify
of the supported
in order to verify gation
the applicability
of Si02-supported
applying
other
vanadia
techniques
order to better
phase
The present
of the LTOC technique Additional
the results
was standardi-
obtained
V/A1203
information
on the
on systems
containing because
to specific
the catalytic
activity
work was undertaken for structural
data were obtained
such as ESR, IR, XRD and surface
understand
of
[6].
oxides may be subject
[8-II].
catalysts.
and the merit
catalysts
are important
interaction
and selectivity
or metal
time,
of the supported
the surface
the supports,
are suitable
at -78°C on reduced
from LTOC data. The studies
but dispersed
are ideal for
a method
and dispersion
V-
and
for the first
[6] that specific
area, as well as the nature
the same active
metals
V-oxides
oxygen
in our work
phase could be obtained
conditions,
of the applicability
structure
[I,71 on Mo/A1203
It was demonstrated
[4,5]. Since
reaction
transition
supported
investigation
in studying
literature
In view of this, we reported,
the work of Weller
catalysts.
of polyvalent
at low temperatures,
for such studies.
this technique
vanadia
oxides
in.the
under oxidation
investiby
area measurements
in
by the LTOC technique.
EXPERIMENTAL The details acquisition a series
of the experimental
of oxygen
of catalysts
containing
procedures
chemisorption supported
on silica
up to ca. 18% by weight
nation method. of vanadia.
Ammonium
(Ketjen
of V205 were
metavanadate
The V contents
for catalyst
preparation,
data have been given elsewhere
(E. Merck,
of the samples
were
and
[6]. In short,
F-5, 672 m * g -I, 1.1 ml g-')
prepared Analar)
by standard
wet impreg-
was used as the source
determined
by atomic
absorption
spectroscopy. Oxygen
chemisorption
experiments
(up to 10-6 Torr)
system
in situ reduction
of the catalysts
ESR experiments.
were performed
connected
In a typical
to a specially prior to oxygen
experiment,
500°C for 6 h in a flow of hydrogen and 4A zeolite was performed was determined
molecular
by the double
sample
tribution
by following
The ESR spectra meter with
recorded
100 KHz modulation.
PW 1051 diffractometer IR spectrometer
using
and subsequent
about 0.5 g catalyst was reduced at -1 , and pretreated by Pd "Deoxo", at -196°C)
method
and oxygen
area,
chemisorption
of chemisorbed
standardized
the BET surface
the BJH method
were
chemisorption
at -78°C. The amount
isotherm
same system was used to determine
high vacuum
cell that permitted
(40 ml min
sieve maintained
on the reduced
with a static designed
by Weller
oxygen
[7]. The
and the pore size dis-
[12].
on a JEOL, X-band
(W = 9.278 GHz) spectro-
The XRD and IR spectra
using Ni-filtered
Cu K
the KBr pellet method
were
radiation,
rzspectively.
recorded
with a Philips
and a Perkin-Elmer
283B
75
5 300
I 2
0
4I
6
Vanadia Loading, p mol V mm2
FIGURE
1
BET surface
unreduced
a
area of V/SiO2
catalysts;
v
catalysts
reduced
as a function
of vanadia
loading;
catalysts.
RESULTS The BET surface as a function
from the measured surface
area of the reduced
of vanadia
loading
BET surface
area of the catalysts
Reduction
does not lead to any change
constant
of the filling
in the surface
ESR spectra
previously
reported
at a temperature but only slightly
in Figure
V/Al O3 system
with
increases
increasing
a much
at which well-resolved
loading
the axially distorted
in average
catalysts
vanadia
-164°C
elsewhere
symmetrical
tetrahedral
loading.
spectra, ligand
pore diameter
in the case of the
spectra
with hyperfine
samples
could be recorded
of the spectra
Reduction
however,
increases.
is quite
as depicted
In contrast with
of the
to these hfs only at
in the temperature
significant
in Figure
increases
of the catalysts
the intensity
[6]. This difference
are obtained
[13-151.
in average
essentially
with low and high
2. It is noted that unlike [6], well-resolved
later. The ESR spectra,
described
and remained
were found to show ESR spectra
namely
spectra
area of the catalysts.
ca. 10% decrease
thus leading to a -2 up to 3.0 umol V m .
and reduced
as the loading
catalysts
lower temperature,
dealt with method
This decrease
in the hfs pattern;
appreciably
V/A1203
that the specific
pores by vanadia,
as high as 25"C, and that the intensity
does not lead to any change
observations,
surface
showed
(hfs) due to 5f V (I = 7/Z) of the unreduced
splitting
spectra
of larger
area with vanadia
are given
here)
was loaded with vanadia,
of some of the unreduced
content
was calculated
It is noted
in the specific
(not given
at 2.5 nm for all the samples.
decrease
vanadia
data
when the support
is indicative
have been plotted
loading
decreases rather sharply as the vanadia loading -2 and then becomes almost independent of V loading.
up to ca. 3 urn01 V m
pore diameter
catalysts
1. Vanadia
area of each catalyst.
increases
The pore size distribution
and unreduced
in Figure
and will be
2, were
analyzed
by a
of the principal components of 4species in highly characteristic of (V04)
field,
The values
of the catalysts
are given
in Table
1.
76
Oxidized
Reduced
Field,
FIGURE
2
ESR spectra
(A) 2.2% V205/Si02;
(recorded
catalysts
Inserts
in the series stretching
further
for a sample
This composition
of the catalysts
of +OSi++O
(recorded
V/Si02
catalysts;
(D) 15.3% V,O,/SiO,
at -164°C)
corresponded
loading
on reduction,
the bands of the reduced
of 10%
any characteristic
a peak around
a mechanical
mixture
1015-
of 18%
to the highest vanadia loading -1 due to assymetric
The band at 960 cm showed
a slight
with low vanadia greater
than
samples
shift(from
960 to 970
content, and became brcader -2 . This band became
1.5 umol V m
and for the samples
and unreduced
showed
1015 cm -I, although
studied.
catalysts
or reduced,
containing
Cl63 bonds of silica
with vanadia
broadened
unreduced
IR band [16] around
cm-') for the u nreduced and broader
are the spectra
catalysts,
1020 cm-' was discernible V205 in silica.
and reduced
(C) 13.0% V205/Si02;
taken from ref. 6.
None of the supported V=O bond stretching
Oe
at 25°C) of unreduced
(6) 8.7% V,O,/SiO,;
and (E) 18.1% V,O,/SiO,. V205/A1203
id*x
containing
looked
2 umol V ms2 or more,
indistinguishable.
77 TABLE
1
ESR parameters
of V/SiO2
catalysts
determined
at 25°C.
Unreduced
Reduced
1.93
1.93
911
(f.Y22)a g1
2.00
2.00
(1.982)a A11
186.8
+ 1.8 G
184.4 f 1.5 G
(182.0)a AL
82.8 t 2.8 G
74.4 * 1.8 G
(72.01a
aThe numbers
within
the brackets
are taken ,from Van Reijn and Cossee's
4 Vanadia
FIGURE
3
Oxygen
loading;0,@,V/Si02
uptake
8
12
0,
n
[26].
16
)r mol V ti2
Loading,
and oxygen-titrated
catalysts;
work
surface
area as a function
, V/A1203 catalysts.
of vanadia
78 The unreduced at -78°C.
catalysts
Oxygen
is shown
uptake
in Figure
with the V/A1203 the specific of alumina,
surface
capacity
to a certain loading.
The loading supports,
catalysts,
of comparison,
was much
are reported
similar
results
linearly
at which maximum namely
oxygen
3. Since
than that
area for better
that for both series,
as a function
rapidly
loading
obtained
in Figure
(ca. 3.8 times)
It is observed
properties.
increases
larger
of oxygen
of vanadia
on the basis of unit surface
value and then decreases
different
chemisorption
as a function
(taken from ref. 6) are also shown
area of silica
of the surface
chemisorption
catalysts
3. For the purpose
catalysts
the results
comparison
did not show any detectable
by the reduced
of vanadia
with further
oxygen
loading
increase
up
in vanadia
uptake occurs is different for -2 for silica- and alumina-supported
3.5 and 8.5 umol V m
respectively.
DISCUSSION Surface
state of silica-supported
The nature
of the surface
has been investigated
vanadia
by various
the state of the supported
workers
vanadia
coordinated
dispersed
the support
as well as the extent
of supports
in dispersing
that when the loading as isolated
(V04)4-
V-oxide
distorted
square-pyramidal
of three surface
investigation
types of V-oxide
vanadia
surface
chemisorption
sites,
of vanadia
oxygen
chemisorbing
3.5 urn01 V mW2. (to be denoted reduced oxygen
that
loadings,
species effect
differing
it has been reported
of V on the support
species
increases sharply.
concentration
of vanadia
loading
on the nature
the catalyst
that the extent
of
of oxygen
of the samples,
and then declines
samples
under the same conditions in weight
on the structure
that the number
the surface
information
phase)
and the decrease It was found
uptake
a maximum region
Raman the presence
[18,22].
as a function
to get further
indicates
3, two types of V-oxide
It is observed
loading
increases
vanadia
(V04)4-
V205 crystallites
just mentioned.
of the supports
in Figure
reaches
V as V4+ in
three-dimensional species
by the oxygen
loading,
as the "monolayer"
of time.
of
the specificity
on the top of the isolated
has also been noted
[32] in a Cahn microbalance
a function
that
of variously
indicating
in the symmetry
of the nature
as reported
in the lower phase
bulky
to the other
as reflected
In order
chemisorption,
reveal
by the nature
(V05),, units with
are formed
are discernible.
as a function This indicates
Thus, for instance,
with alumina-supported
phase
From the LTOC results, on the silica
loading,
and alumina,
studies
and the distribution
two-dimensional
in addition
[21]. A specific
the dispersed
on silica
These
are very much dictated
of vanadia
symmetry
[14]. At still higher
spectroscopic
of the LTOC sites therein
low, vanadia on silica remains highly dispersed 4+ in tetrahedral symmetry, and with with V as V
polymeric
to form
[4,5,17-311.
phase,
species
vanadia.
species
loading,
species
supported
is very
increased
are reported
and the nature
state of vanadia,
were
up to ca.
of this phase isothermally
as used prior to
due to reduction of reduction,
of the
was noted as
following
the
79 V205 + V203, was between
stoichiometry:
ESCA investigation
[32] revealed
in conformity
60 and 70% for all the samples.
the presence
Detailed
of both V3+ and V5+ in the reduced
the reduction results. Additionally, although the 4+ reduced samples contained some V species, as indicated by strong ESR signals, 4+ remaining ESCA did not show any peak due to V4+. This means that the amount of V samples,
in the reduced reduction
with
samples
was very small.
the net conclusion is that upon 3+ to V , about 30% remained 4+ V5+ state and a very small fraction remained stabilized as V .
most of the supported
in the unreduced
Therefore,
vanadia
was converted
In the light of the above observations, the surface term
structure
"monolayer"
of the "monolayer"
has not been applied
surface;
it rather
discussed
further
indicates
two types of V-oxide
species
complete
of certain
It is envisaged
are formed
during
can now be made to elucidate
of silica-supported
to indicate
the covering
in the text.)
an attempt phase
coverage
patches
that within
V-oxides.
of the support
of the surface, the "monolayer"
the preparation
(The
as region
step. These
species
are:
0
Both the species 1 and 2 have tetrahedral the ESR signal
shown by the unreduced
and stabilization
of species -1 on silica It is observed
intensity
samples
of the unreduced This
indicates
an ESR signal, was converted
must
have coexisted
-2 is tentatively
species -2 satisfies
the valence
As mentioned
earlier,
whereas
the major
fraction
for oxygen
in the scheme
below:
samples.
in the unreduced
of about
samples,
is reduced
chemisorption
2 upon regives
rise to
and this precursor species
1. It is to be noted
(i.e., 5+) as well as symmetry
only a minor
observed
In view of these observations, of species
for
:'he formation
2 that the ESR signal
by a factor
of the species 1, which
as the precursor
ments.
-1 is responsible
has been repeatedly
from Figure
-1 upon reduction.
accepted
as the center
surface
increases
that a precursor
to species
Species
as well as the reduced
in the past [21,23,26,27].
duction.
symmetry.
that
(i.e., Td) require-
fraction
of -2 gets reduced to 1. 3+ to species _3 containing V that serves at -78°C.
The course
of reduction
is shown
80
I SCHEME
1
-
q
In this scheme LTOC occurs. tetrahedral
denotes
symmetry.
with
the silica
ments
(Figure
Species 2 maintains
surface
upon reduction.
I), ESR and ESCA also
surface
structure
absence
of any XRD pattern
the samples, dispersed
below,
important
samples,
indicating
on silica
surface
as well as its registry
obtained
phase occurs
splitting
states,
provide
(hfs) pattern
change
further
unlike
support
information
as V4+ (species _1) in both oxidized
in
the highly
sites.
From Figure
oxide
remains
and reduced
and reduced stabilized
states.
Second,
and the axially symmetrical ESR parameters, as given in Table 4+ are characteristic of V species in tetrahedral symmetry [26,27]. However, because
paramagnetic
ions (d' species
spaced
ground
of these
ions can be obtained
observed
here, unless
increased
energy
states
gap between
of some distorting
the ground ligand
1,
symmetry
[331, the ESR spectra
only at very low temperatures
the energy
by the influence
in this case) with tetrahedral
and excited
2
(I = 7/2) is
the hfs pattern
have very closely
above.
about the
in the case of the V/Al203
due to 51V4+
of vanadium
The
V205
phase as depicted
i.e., 25°C for both the unreduced
that a small fraction
of the
upon reduction.
strongly
chemisorption
First,
area measure-
of crystalline
of the "monolayer"
can be noted:
temperatures,
by surface
that no detectable
and unreduced
structure
phase and the oxygen
features
at higher
indicate
site (CUS) upon which
and the V ions in it have
its symmetry Results
"monolayer"
our ESR results
[6] the hyperfine
observable
unsaturated
and IR bands characteristic
both in the reduced
of the monolayer
several system
of the unreduced
and one dimensional
As elaborated nature
a coordinatively
Note that species _2 is one dimensional,
and not at 25"C, as
and excited
fields.
Since
states
is
in this study
81 the spectra distorted
are obtained
tetrahedral
interaction spectra,
symmetry.
of V with
earlier
stretching [6] showed
pyramidal
surroundings
as low as -164°C unreduced
trend
loading.
the exception
drastically,
and the signals
in the intensity
with
sites are generated
conclusions
reached
The question
in the reduced
earlier
now arises
about
increasing
As mentioned
uptake.
chemisorption
capacity
beyond
provides
two valuable
oxygen; oxygen
the
and that upon reduction,
observations
of the "monolayer"
support
the
phase.
both of them have CUS. It is not difficult
to
capable
of oxygen
constitutes
about
phase.
chemisorption
because
70% (as revealed
However,
by
it is not possible
or not species _1 also can chemisorb
the unreduced chemisorbing
catalysts,
centers,
which
oxygen
contained
species
did not show any measurable -1 chemisorbed
by the technique
question
concerns coverage
pieces
oxygen,
applied
phase
but
for oxygen
(V05)n species
3.5 umol V m-'). beyond
but also
uptake
of crystalline
it somehow
This behavior coverage,
the monolayer
does not show any change
phase
oxygen,
by XRD results.
that a highly
distorted
square region.
in the ESR spectra
does fail
the already
The "post monolayer"
V205 as revealed
beyond
prevents
phase from chemisorbing capacity.
phase containing
is formed
uptake
the monolayer
not only does this "post monolayer"
reduction,
V-oxide
in oxygen
and this phase upon reduction
et al. [18], it is envisioned
polymeric
upon reduction
First,
is formed,
in the oxygen
not to consist
decrease
(i.e., beyond
of information:
and second,
after
the sudden
species 1 and 3 in the "monolayer"
the line of Yoshida
phase
with
with
(species 2) that coexists
These
as well mean that species
vanadia
to a decrease
octahedral
as compared
of
whether
oxygen
the monolayer
surface
not chemisorb
dimensional
of
trend,
studies.
The next important
was found
of the
and only a slightly
show a similar
of the species
the amount was too small to be measured
leading
only at temperatures
1 and 3 is capable
earlier,
This might
samples.
of the "monolayer"
to ascertain
1 as the only potential
existing
in square
the ESR spectra
intensity
in the
alumina-supported of V centers
V loading,
the nature
this species
and ESCA studies)
to chemisorb
higher
the IR
some shift
is noted as a function
samples
from the precursor
as to which
because
catalysts
from our results
a different
showed
resolved,
of the spectra
that species -3 is definitely
at -78°C.
2). Third,
are well
in the unreduced
chemisorption,
reduction
section,
could be obtained
of the reduced
but moderately,
with the Vfdf centers
oxygen
catalysts
from the
by Si. Indeed
On the contrary,
in Figure
that they show a slightly
some new V;i
perceive
shared
with hfs characteristic
(see the inserts
The spectra
may have stemmed
atoms
are in
indicate that the number unreduced ones. The last two observations 4+ VTd sites (i.e., species _1) on the unreduced catalysts increases not
corresponding of isolated
oxygen
bonds.
ESR spectra
silica-supported
increasing vanadia
oxygen
in the "Results"
of O+SiOA
catalysts
that the V4+ centers
it is concluded
This distortion
the surface
as mentioned
assymetric
at 25"C,
thus
phase Following
dispersed
two
pyramidal
or
Obviously, (Figure
2)
this
82
because
most of it is reduced
nor does
it chemisorb
generated
oxygen
on this polymeric
Indeed there
is evidence
having
tetrahedral
symmetry,
VTd, chemisorbed
experiments
were conducted
ture which possible
energy
oxygen
without
symmetry
of chemisorption. at subambient
the CUS on VTd sites
of the
that V centers
activation
energy,
had to overcome
Since our oxygen
temperatures
at -78°C.
on V centers
on the symmetry
et al. [34] observed
pyramidal
oxygen
chemisorption
depending
studies,
that the cus
of chemisorbing
was even lower than that used by Khalif
that while
suggests
that oxygen
energy
Khalif
or square
of activatian
is not capable
to activation
the CUS. Thus,
those with octahedral amount
phase
All evidence
in the literature
may or may not be subject V centers
to V3+ state as found by ESCA and reduction at -78°C.
with
while
a considerable
chemisorption
(i.e., at -78"C),
a tempera-
et al. [34], it is quite
(species _1 and _3) could
chemisorb
those on the polymeric species containing VOh or VC4v centers, formed by 5+ species, failed to do so. If this is so, then the the reduction of polymeric V
oxygen,
sharp decline experimental
in oxygen
"post monolayer"
phase
top of the already the monolayer diffusion
chemisorption
observations
indicates
existing
phase
area
takes place thereafter.
This is perfectly
amount
surface,
for the formation
of vanadia
but rather studied
fore, we have been dealing phase which
contains
of polymeric
with
covers
crystalline
the that as
in the surface
area
hypothesis
level does not cover
on top of the monolayer loading
on
due to
indicates
with the proposed
the monolayer
gets deposited
did the vanadia
of three dimensional
decrease
in accord
beyond
1, also
on
if formed
by oxygen
phase which
in Figure
the
preferentially
be reached
no appreciable
is completed,
none of the samples
cannot
as shown
soon as the "monolayer"
the silica
is deposited
important
containing
and that the CUS, even
phase, probably
data,
is one of the most
of the catalysts
posed by the "post monolayer"
The surface
that an additional
(which
that this phase
monolayer
upon reduction,
restrictions
"monolayer".
capacity
of this investigation)
phase.
In
reach the level high enough V205 detectable
two types of vanadia
phases,
by XRD. There-
namely,
the monolayer
the species
_1 and 2, and the "post monolayer" phase comprised 5+ V-oxide with V in square pyramidal or octahedral
two dimensional
symmetry.
A surface
structural
model
In view of the above supported
vanadia
urn01 V me2 species
two species
in tetrahedral
coordination
oxygen
of the surface
Up to a certain on the silica
surface
surroundings.
at -78°C.
The maximum
coverage
partly species
surface
(3.5
as isolated the central
Upon reduction,
gives the ESR signal,
to what we call the "monolayer"
in this particular
state of silicasurface
species _2. In both these
to species _1 which
chemisorbs
corresponds
coverage
remains
as its precursor
converted
species -3 which
a model
as follows:
in this case) vanadia
V ion remains
layer"
is proposed
_1 and partly
-2 is partly
of V/SiO2
discussion,
species
and mostly coverage
to by these
coverage. Thus the "mono-2 . This phase covers
case is ca. 3.5 umol V m
83 only about one fifth of the total surface 0.103 nm2 as the diameter
rather
a "patchy monolayer", the "monolayer"
results
two dimensional
(V05)n
surroundings.
in the formation
while
monolayer"
phase fails
pyramidal
energy
of activation
further
as calculated
phase chemisorbs
phase
remains
Upon
the "post
of octahedral
requirements
[34]. A similar
type of picture
bound monolayer
phase,
been proposed
on the top
at low temperatures
and chemically
on the top of it has recently
beyond
polymeric
surface.
at -78"C,
consists
oxygen
of vanadia
containing
symmetry
oxygen
by taking
is actually
or octahedral
the silica
the latter
that do not chemisorb
this
addition
pyramidal
occupying
to do so, because
species
dispersed
of a second
phase with V in a different
the "monolayer"
square
Further
with V in square
phase without
reduction,
a highly
than a monolayer.
species
This second
of the "monolayer"
area of silica
of one VO2 5 unit [ZO]. Therefore,
comprised
and crystalline
for vanadia-titania
or due to of
V205
systems
by Wachs
et al. C311.
Vanadia-silica
versus
The effect capacity
of the nature
of the dispersed
to silica-
by LTOC)
layer coverage the maximum
accommodated This
from reduction is formed
Thus,
that the alumina
chemical
solution
[20],
it can be concluded
chemical is much
reaction higher
why vanadia certain
is higher
than that between
The implications
shared
of these
will be the subject
vanadia
be expected
by infrared
of future
however,
is different to be
coverage.
capacity
by Yoshida
than the
et al. [18]
of alumina
planes)
to form,
of the active of
and vanadate
Selective
ions
be the reason exposure
of
may also play a part in it.
structural
OH groups
on the
the extent
This may well
than on silica.
models
attached
of the surface
It would
Whether
species
to V and OH groups
and that these OH groups
spectroscopy.
investigation.
OH groups
phase"
in the impregnating
concentration
and silica.
(as surface
species
"monolayer"
In other words,
of OH groups
of the above discussed
to be detected
at full mono-
are found
the active
ions present
that the surface
on alumina
planes
by both V and Si would
be possible
(i.e., the area
coverage -2
dispersion
earlier
between
than in silica.
gets more dispersed
are that upon reduction
area
3
specific
also the fact that the "monolayer
vanadate
via the condensation
crystallographic
has higher
interaction
and the oligomeric
at full
was reached
Recalling
surface
in alumina
in Figure
area of both the systems
respectively
surface
support
OH groups
surface
phase at full monolayer
conclusion
and ESR studies.
by a strong
given
about four times more
ca. 8.5 and 3.5 umol V m
and silica
A similar
chemisorption pertaining
the same. This may be just a coincidence,
of the dispersed supports.
by alumina
surface.
possesses
to the total BET surface
amount
indicates
silica
silica
and oxygen
by the results
From the results
catalysts.
the ratio of the active
is practically
for the different
on the dispersion
is well demonstrated
that although
area than alumina,
titrated
of support
phase
and alumina-supported
it can be concluded surface
vanadia-alumina
should
or not this happens
also he interesting
to study
84 the effect
of dehydroxylation
of the resulting interaction
Oxygen
surface
i.e., the dispersion,
ment of the central
the vanadia monolayer"
loading
for recording
to be formed
in conjunction
tool for studying
chemisorption
formal
on the nature by the
and that of the V salt.
the surface
data are found
area of the supported
phases.
where
phase starts
The authors
they are envisaged
oxidation
V ions of the dispersed
as the "post monolayer"
(prior to V loading)
work shows that LTOC,
can be a useful
V-oxides.
on: (a) the "active"
because
of the supports
the present
techniques,
Si02-supported
nature
species
of the OH groups
In conclusion, standard
surface
of the supports
vanadia
with other structure
to provide
phase;
and (b) the
state and the coordination
species
The LTOC results
the "patchy monolayer"
in the "monolayer"
allow one to clearly becomes
complete
of
information
environas well
identify
and the "post
forming.
thank Dr. C.S. Sunandana
of the University
of Hyderabad
(India)
the ESR spectra.
REFERENCES
: 3 4 5 6
IO II I2 13 I4 I5 I6 17 18 I9 20 21 22 23 24 25 26 27
S.W. Weller, Act. Chem. Res., 16 (1983) 101. N.K. Nag, 3. Catal., 92 (1985) 432, and references therein. B.M.P. Reddy, K.V.R. Chary, V.S. Subrahmanyam and N.K. Nag, J. Chem. Sot., Faraday Trans. I, 81 (1985) 1655. J.L.G. Fierro, L.A. Gambaro, T.A. Cooper and G. Kremenic, Appl. Catal., 6 (1983) 363. J.L.G. Fierro, L.A. Gambaro, A.R. Gonzalez-Elipe and J. Soria, Colloids and Surfaces, II (1984) 31. N.K. Nag, K.V.R. Chary, B.M.P. Reddy, B.Rama Rao and V.S. Subrahmanyam, Appl. Catal., 9 (1984) 225. B.S. Parekh and S.W. Weller, J. Catal., 47 (1977) 100. M. Boudart, Adv. Catal. Relat. Subj., 20 (1969) 153. G.-M. Schwab, Trans. Far. Sot., 42 (1946) 689; Disc. Far. Sot., 8 (1950) 166; Naturwiss., 44 (1957) 32. F. Solymosi, Catal. Rev., 1 (1967) 233. G.-M. Schwab, Adv. Catal., 27 (1978) I. E.P. Barrett, L.G. Joyner and P.H. Halenda, J. Amer. Chem. Sot., 73 (1951) 373. H.G. Hecht and T.S. Johnstone, J. Chem. Phys., 46 (1967) 23. B. Bleany, Phil. Mag., 42 (1952) 441. K.V.R. Chary, B. Mahipal Reddy, N.K. Nag, V.S. Subrahmanyam and C.S. Sunandana, J. Phys. Chem., 88 (1984) 2622. A. Anderson, J. Catal., 76 (1982) 144. K. Tarama, S. Yoshida, S. Ishida and H. Kakioka, Bull. Chem. Sot. Japan, 41 (1968) 2840. Y. Yoshida, T. Iguchi, S. Ishida and K. Tarama, Bull. Chem. Sot. Japan, 45 (1972) 376. F. Roozeboom, P. Cordingley and P.J. Gellings, J. Catal., 68 (1981) 464. F. Roozeboom, T. Fransen, P. Mars and P.J. Gellings, 2. Anorg. Allg. Chem., 440 (1979) 25. F. Roozeboom, J. Medema and P.J. Gellings, J. Phys. Chem., III (1978) 215. M. Inomata, K. Mori, A. Miyamoto and Y. Murakami, J. Phys. Chem., 87 (1983) 761. A. Miyamoto, Y. Yamazaki, M. Inomata and Y. Murakami, J. Phys. Chem., 85 (1981) 2366. H. Ueda, Bull. Chem. Sot. Japan, 52 (1979) 1905. M. Akimoto, M. Ushami and E. Echigoya, Bull. Chem. Sot. Japan, 51 (1978) 2195. L.L. Van Reijen and P. Cossee, Disc. Far. Sot., 41 (1966) 277. V.B. Kazanskii, Kinet. Catal. (Eng.), 11 (1970) 378.
85 28 29 30 31 32 33 34
V.M. Vorotyntsev, V.A. Shvets and V.B. Kazanskii, Kinet. Catal. (Eng.1, 12 (1971) 597. V.B. Kazanskii, V.A. Shvets, M. Ya Kon, V.V. Nikisha and B.N. Shelimov, Proc. 5th Intern. Cong. Catal., Miami Beach, 1972, p.1423. B. Horvath, J. Strutz, J. Geyer-Lippmann and E.G. Horvath, Z. Anorg. Allg. Chem., 483 (1981) 181-218. I.E. Wachs, R.Y. Saleh, S.S. Chau and C.C. Cherisch, Appl. Catal., 15 (1985) 339. N.K. Nag and F.E. Massoth, To be submitted for publication. B.R. McGarvey, in "Transition Metal Chemistry", Vo1.3, p.90, R.L. Carlin, ed., Marcel Dekker, New York, 1966. V.A. Khalif, E.L. Aptekar, O.V. Krylov and G. Ohlmann, Kinet. Catal. (Eng.), 18 (1977) 867.
CHARACTERI~TION
OF SUPPORTED
CHEMISORPTION
II.
TECHNIQUE:
VANADIU~~ OXIDE THE V205/SiO2
CATALYSTS
73
BY LOW TE~FERATURE
OXYGEN
SYSTEM
Nabin K. NAG*' Department
of Fuels
Engineering,
University
of Utah, Salt Lake City, Utah 84112,
and Komandur
V.R. CHARY,
Regional
Research
'Present address: Boulevard, (Received
B. Rama RAO and Varanashi
Laboratory,
Harshaw/Filtrol
Los Angeles, 21 July
S. SUBRAHMANYA~~
Hyderabad-500009, Partnership,
California
1986, accepted
India.
98023,
Filtrol
Division,
3250 E. Washington
U.S.A.
22 December
19861
ABSTRACT LOW TeMperatUre Oxygen Chemisorption (LTOC) has been applied to characte ize a series of silica-sup orted V-oxide catalysts containing up to 5 umol V m- 5 . LTOCdata, coupled wit R ancillary information obtained from ESR, IR, XRD, ESCA and surface area measurements, have thrown light on the nature of the dispersed vanadia which has been found to consist of two distinct highly dispersed V-oxide phases. Based on the experimental results, a model of the surface structure of the dispersed phases has been proposed. It has been envisioned that a "monolayer" phase containing, in small part, isolated (VO4)4- species and in great part, a dimeric V5+ oxide species, both in tetrahedral coordination, is formed at lower vanadia loading region, while at lo dings beyond the so-called "monolayer" coverage a second phase consisting of only Vf+ -oxide in square pyramida? or octahedral surroundings (i.e., a two dimensional structure akin to that of bulk V205) is formed preferentially on the top of the "monolayer" phase. The completion of the 'Imonolayer~ and the beginning of the formation of the "post mono~ayer~ phases have been clearly identified by a maximum in the oxygen uptake capacity of the samples.
INTRODUCTION Low temperature applied offers sulfide studied
oxygen
to characterize a means
of determining
phases.
Supported
uptake
zation
(HDS) reactions MO-containing
catalysts
(LTOC) at -78°C or below and sulfide
the "active"
can be directly
hydroprocessing
low t~perature
correlated
with
has recently
area of supported
oxide
and
have been extensively
have been made
studies
to correlate for hydrodesulfuri-
on a series
catalysts,
oxygen
been
[1,2]. This method
their activities
From the systematic
sulfided
conditions
with
catalysts
catalysts
and attempts
of the catalysts [2,3].
surface
and -sulfide
recently,
capacities
that under certain
oxide
MO-oxide
by this technique
oxygen
supported
chemisorption
supported
uptake
of A1203-
it has been demonstrate1 capacities
their HDS and hydrogenation
of these activities
c31. Similar
studies
0166~9834/87/$03.50
on supported
V-oxide
catalysts
are rare and only some fragmentar:
0 1987 Elsevier Science Publishers B.V.
information
relative
to this system
is found
oxide catalysts
act as redox systems
since partially
reduced
oxygen
chemisorption
candidates the results
Following
of a systematic
zed for measuring
active
the surface
the amount
surface
of chemisorbed
phase
of V/A1203
catalysts,
on different
supports
state of supported
metals
with
and this in turn may modify
of the supported
in order to verify gation
the applicability
of Si02-supported
applying
other
vanadia
techniques
order to better
phase
The present
of the LTOC technique Additional
the results
was standardi-
obtained
V/A1203
information
on the
on systems
containing because
to specific
the catalytic
activity
work was undertaken for structural
data were obtained
such as ESR, IR, XRD and surface
understand
of
[6].
oxides may be subject
[8-II].
catalysts.
and the merit
catalysts
are important
interaction
and selectivity
or metal
time,
of the supported
the surface
the supports,
are suitable
at -78°C on reduced
from LTOC data. The studies
but dispersed
are ideal for
a method
and dispersion
V-
and
for the first
[6] that specific
area, as well as the nature
the same active
metals
V-oxides
oxygen
in our work
phase could be obtained
conditions,
of the applicability
structure
[I,71 on Mo/A1203
It was demonstrated
[4,5]. Since
reaction
transition
supported
investigation
in studying
literature
In view of this, we reported,
the work of Weller
catalysts.
of polyvalent
at low temperatures,
for such studies.
this technique
vanadia
oxides
in.the
under oxidation
investiby
area measurements
in
by the LTOC technique.
EXPERIMENTAL The details acquisition a series
of the experimental
of oxygen
of catalysts
containing
procedures
chemisorption supported
on silica
up to ca. 18% by weight
nation method. of vanadia.
Ammonium
(Ketjen
of V205 were
metavanadate
The V contents
for catalyst
preparation,
data have been given elsewhere
(E. Merck,
of the samples
were
and
[6]. In short,
F-5, 672 m * g -I, 1.1 ml g-')
prepared Analar)
by standard
wet impreg-
was used as the source
determined
by atomic
absorption
spectroscopy. Oxygen
chemisorption
experiments
(up to 10-6 Torr)
system
in situ reduction
of the catalysts
ESR experiments.
were performed
connected
In a typical
to a specially prior to oxygen
experiment,
500°C for 6 h in a flow of hydrogen and 4A zeolite was performed was determined
molecular
by the double
sample
tribution
by following
The ESR spectra meter with
recorded
100 KHz modulation.
PW 1051 diffractometer IR spectrometer
using
and subsequent
about 0.5 g catalyst was reduced at -1 , and pretreated by Pd "Deoxo", at -196°C)
method
and oxygen
area,
chemisorption
of chemisorbed
standardized
the BET surface
the BJH method
were
chemisorption
at -78°C. The amount
isotherm
same system was used to determine
high vacuum
cell that permitted
(40 ml min
sieve maintained
on the reduced
with a static designed
by Weller
oxygen
[7]. The
and the pore size dis-
[12].
on a JEOL, X-band
(W = 9.278 GHz) spectro-
The XRD and IR spectra
using Ni-filtered
Cu K
the KBr pellet method
were
radiation,
rzspectively.
recorded
with a Philips
and a Perkin-Elmer
283B
75
5 300
I 2
0
4I
6
Vanadia Loading, p mol V mm2
FIGURE
1
BET surface
unreduced
a
area of V/SiO2
catalysts;
v
catalysts
reduced
as a function
of vanadia
loading;
catalysts.
RESULTS The BET surface as a function
from the measured surface
area of the reduced
of vanadia
loading
BET surface
area of the catalysts
Reduction
does not lead to any change
constant
of the filling
in the surface
ESR spectra
previously
reported
at a temperature but only slightly
in Figure
V/Al O3 system
with
increases
increasing
a much
at which well-resolved
loading
the axially distorted
in average
catalysts
vanadia
-164°C
elsewhere
symmetrical
tetrahedral
loading.
spectra, ligand
pore diameter
in the case of the
spectra
with hyperfine
samples
could be recorded
of the spectra
Reduction
however,
increases.
is quite
as depicted
In contrast with
of the
to these hfs only at
in the temperature
significant
in Figure
increases
of the catalysts
the intensity
[6]. This difference
are obtained
[13-151.
in average
essentially
with low and high
2. It is noted that unlike [6], well-resolved
later. The ESR spectra,
described
and remained
were found to show ESR spectra
namely
spectra
area of the catalysts.
ca. 10% decrease
thus leading to a -2 up to 3.0 umol V m .
and reduced
as the loading
catalysts
lower temperature,
dealt with method
This decrease
in the hfs pattern;
appreciably
V/A1203
that the specific
pores by vanadia,
as high as 25"C, and that the intensity
does not lead to any change
observations,
surface
showed
(hfs) due to 5f V (I = 7/Z) of the unreduced
splitting
spectra
of larger
area with vanadia
are given
here)
was loaded with vanadia,
of some of the unreduced
content
was calculated
It is noted
in the specific
(not given
at 2.5 nm for all the samples.
decrease
vanadia
data
when the support
is indicative
have been plotted
loading
decreases rather sharply as the vanadia loading -2 and then becomes almost independent of V loading.
up to ca. 3 urn01 V m
pore diameter
catalysts
1. Vanadia
area of each catalyst.
increases
The pore size distribution
and unreduced
in Figure
and will be
2, were
analyzed
by a
of the principal components of 4species in highly characteristic of (V04)
field,
The values
of the catalysts
are given
in Table
1.
76
Oxidized
Reduced
Field,
FIGURE
2
ESR spectra
(A) 2.2% V205/Si02;
(recorded
catalysts
Inserts
in the series stretching
further
for a sample
This composition
of the catalysts
of +OSi++O
(recorded
V/Si02
catalysts;
(D) 15.3% V,O,/SiO,
at -164°C)
corresponded
loading
on reduction,
the bands of the reduced
of 10%
any characteristic
a peak around
a mechanical
mixture
1015-
of 18%
to the highest vanadia loading -1 due to assymetric
The band at 960 cm showed
a slight
with low vanadia greater
than
samples
shift(from
960 to 970
content, and became brcader -2 . This band became
1.5 umol V m
and for the samples
and unreduced
showed
1015 cm -I, although
studied.
catalysts
or reduced,
containing
Cl63 bonds of silica
with vanadia
broadened
unreduced
IR band [16] around
cm-') for the u nreduced and broader
are the spectra
catalysts,
1020 cm-' was discernible V205 in silica.
and reduced
(C) 13.0% V205/Si02;
taken from ref. 6.
None of the supported V=O bond stretching
Oe
at 25°C) of unreduced
(6) 8.7% V,O,/SiO,;
and (E) 18.1% V,O,/SiO,. V205/A1203
id*x
containing
looked
2 umol V ms2 or more,
indistinguishable.
77 TABLE
1
ESR parameters
of V/SiO2
catalysts
determined
at 25°C.
Unreduced
Reduced
1.93
1.93
911
(f.Y22)a g1
2.00
2.00
(1.982)a A11
186.8
+ 1.8 G
184.4 f 1.5 G
(182.0)a AL
82.8 t 2.8 G
74.4 * 1.8 G
(72.01a
aThe numbers
within
the brackets
are taken ,from Van Reijn and Cossee's
4 Vanadia
FIGURE
3
Oxygen
loading;0,@,V/Si02
uptake
8
12
0,
n
[26].
16
)r mol V ti2
Loading,
and oxygen-titrated
catalysts;
work
surface
area as a function
, V/A1203 catalysts.
of vanadia
78 The unreduced at -78°C.
catalysts
Oxygen
is shown
uptake
in Figure
with the V/A1203 the specific of alumina,
surface
capacity
to a certain loading.
The loading supports,
catalysts,
of comparison,
was much
are reported
similar
results
linearly
at which maximum namely
oxygen
3. Since
than that
area for better
that for both series,
as a function
rapidly
loading
obtained
in Figure
(ca. 3.8 times)
It is observed
properties.
increases
larger
of oxygen
of vanadia
on the basis of unit surface
value and then decreases
different
chemisorption
as a function
(taken from ref. 6) are also shown
area of silica
of the surface
chemisorption
catalysts
3. For the purpose
catalysts
the results
comparison
did not show any detectable
by the reduced
of vanadia
with further
oxygen
loading
increase
up
in vanadia
uptake occurs is different for -2 for silica- and alumina-supported
3.5 and 8.5 umol V m
respectively.
DISCUSSION Surface
state of silica-supported
The nature
of the surface
has been investigated
vanadia
by various
the state of the supported
workers
vanadia
coordinated
dispersed
the support
as well as the extent
of supports
in dispersing
that when the loading as isolated
(V04)4-
V-oxide
distorted
square-pyramidal
of three surface
investigation
types of V-oxide
vanadia
surface
chemisorption
sites,
of vanadia
oxygen
chemisorbing
3.5 urn01 V mW2. (to be denoted reduced oxygen
that
loadings,
species effect
differing
it has been reported
of V on the support
species
increases sharply.
concentration
of vanadia
loading
on the nature
the catalyst
that the extent
of
of oxygen
of the samples,
and then declines
samples
under the same conditions in weight
on the structure
that the number
the surface
information
phase)
and the decrease It was found
uptake
a maximum region
Raman the presence
[18,22].
as a function
to get further
indicates
3, two types of V-oxide
It is observed
loading
increases
vanadia
(V04)4-
V205 crystallites
just mentioned.
of the supports
in Figure
reaches
V as V4+ in
three-dimensional species
by the oxygen
loading,
as the "monolayer"
of time.
of
the specificity
on the top of the isolated
has also been noted
[32] in a Cahn microbalance
a function
that
of variously
indicating
in the symmetry
of the nature
as reported
in the lower phase
bulky
to the other
as reflected
In order
chemisorption,
reveal
by the nature
(V05),, units with
are formed
are discernible.
as a function This indicates
Thus, for instance,
with alumina-supported
phase
From the LTOC results, on the silica
loading,
and alumina,
studies
and the distribution
two-dimensional
in addition
[21]. A specific
the dispersed
on silica
These
are very much dictated
of vanadia
symmetry
[14]. At still higher
spectroscopic
of the LTOC sites therein
low, vanadia on silica remains highly dispersed 4+ in tetrahedral symmetry, and with with V as V
polymeric
to form
[4,5,17-311.
phase,
species
vanadia.
species
loading,
species
supported
is very
increased
are reported
and the nature
state of vanadia,
were
up to ca.
of this phase isothermally
as used prior to
due to reduction of reduction,
of the
was noted as
following
the
79 V205 + V203, was between
stoichiometry:
ESCA investigation
[32] revealed
in conformity
60 and 70% for all the samples.
the presence
Detailed
of both V3+ and V5+ in the reduced
the reduction results. Additionally, although the 4+ reduced samples contained some V species, as indicated by strong ESR signals, 4+ remaining ESCA did not show any peak due to V4+. This means that the amount of V samples,
in the reduced reduction
with
samples
was very small.
the net conclusion is that upon 3+ to V , about 30% remained 4+ V5+ state and a very small fraction remained stabilized as V .
most of the supported
in the unreduced
Therefore,
vanadia
was converted
In the light of the above observations, the surface term
structure
"monolayer"
of the "monolayer"
has not been applied
surface;
it rather
discussed
further
indicates
two types of V-oxide
species
complete
of certain
It is envisaged
are formed
during
can now be made to elucidate
of silica-supported
to indicate
the covering
in the text.)
an attempt phase
coverage
patches
that within
V-oxides.
of the support
of the surface, the "monolayer"
the preparation
(The
as region
step. These
species
are:
0
Both the species 1 and 2 have tetrahedral the ESR signal
shown by the unreduced
and stabilization
of species -1 on silica It is observed
intensity
samples
of the unreduced This
indicates
an ESR signal, was converted
must
have coexisted
-2 is tentatively
species -2 satisfies
the valence
As mentioned
earlier,
whereas
the major
fraction
for oxygen
in the scheme
below:
samples.
in the unreduced
of about
samples,
is reduced
chemisorption
2 upon regives
rise to
and this precursor species
1. It is to be noted
(i.e., 5+) as well as symmetry
only a minor
observed
In view of these observations, of species
for
:'he formation
2 that the ESR signal
by a factor
of the species 1, which
as the precursor
ments.
-1 is responsible
has been repeatedly
from Figure
-1 upon reduction.
accepted
as the center
surface
increases
that a precursor
to species
Species
as well as the reduced
in the past [21,23,26,27].
duction.
symmetry.
that
(i.e., Td) require-
fraction
of -2 gets reduced to 1. 3+ to species _3 containing V that serves at -78°C.
The course
of reduction
is shown
80
I SCHEME
1
-
q
In this scheme LTOC occurs. tetrahedral
denotes
symmetry.
with
the silica
ments
(Figure
Species 2 maintains
surface
upon reduction.
I), ESR and ESCA also
surface
structure
absence
of any XRD pattern
the samples, dispersed
below,
important
samples,
indicating
on silica
surface
as well as its registry
obtained
phase occurs
splitting
states,
provide
(hfs) pattern
change
further
unlike
support
information
as V4+ (species _1) in both oxidized
in
the highly
sites.
From Figure
oxide
remains
and reduced
and reduced stabilized
states.
Second,
and the axially symmetrical ESR parameters, as given in Table 4+ are characteristic of V species in tetrahedral symmetry [26,27]. However, because
paramagnetic
ions (d' species
spaced
ground
of these
ions can be obtained
observed
here, unless
increased
energy
states
gap between
of some distorting
the ground ligand
1,
symmetry
[331, the ESR spectra
only at very low temperatures
the energy
by the influence
in this case) with tetrahedral
and excited
2
(I = 7/2) is
the hfs pattern
have very closely
above.
about the
in the case of the V/Al203
due to 51V4+
of vanadium
The
V205
phase as depicted
i.e., 25°C for both the unreduced
that a small fraction
of the
upon reduction.
strongly
chemisorption
First,
area measure-
of crystalline
of the "monolayer"
can be noted:
temperatures,
by surface
that no detectable
and unreduced
structure
phase and the oxygen
features
at higher
indicate
site (CUS) upon which
and the V ions in it have
its symmetry Results
"monolayer"
our ESR results
[6] the hyperfine
observable
unsaturated
and IR bands characteristic
both in the reduced
of the monolayer
several system
of the unreduced
and one dimensional
As elaborated nature
a coordinatively
Note that species _2 is one dimensional,
and not at 25"C, as
and excited
fields.
Since
states
is
in this study
81 the spectra distorted
are obtained
tetrahedral
interaction spectra,
symmetry.
of V with
earlier
stretching [6] showed
pyramidal
surroundings
as low as -164°C unreduced
trend
loading.
the exception
drastically,
and the signals
in the intensity
with
sites are generated
conclusions
reached
The question
in the reduced
earlier
now arises
about
increasing
As mentioned
uptake.
chemisorption
capacity
beyond
provides
two valuable
oxygen; oxygen
the
and that upon reduction,
observations
of the "monolayer"
support
the
phase.
both of them have CUS. It is not difficult
to
capable
of oxygen
constitutes
about
phase.
chemisorption
because
70% (as revealed
However,
by
it is not possible
or not species _1 also can chemisorb
the unreduced chemisorbing
catalysts,
centers,
which
oxygen
contained
species
did not show any measurable -1 chemisorbed
by the technique
question
concerns coverage
pieces
oxygen,
applied
phase
but
for oxygen
(V05)n species
3.5 umol V m-'). beyond
but also
uptake
of crystalline
it somehow
This behavior coverage,
the monolayer
does not show any change
phase
oxygen,
by XRD results.
that a highly
distorted
square region.
in the ESR spectra
does fail
the already
The "post monolayer"
V205 as revealed
beyond
prevents
phase from chemisorbing capacity.
phase containing
is formed
uptake
the monolayer
not only does this "post monolayer"
reduction,
V-oxide
in oxygen
and this phase upon reduction
et al. [18], it is envisioned
polymeric
upon reduction
First,
is formed,
in the oxygen
not to consist
decrease
(i.e., beyond
of information:
and second,
after
the sudden
species 1 and 3 in the "monolayer"
the line of Yoshida
phase
with
with
(species 2) that coexists
These
as well mean that species
vanadia
to a decrease
octahedral
as compared
of
whether
oxygen
the monolayer
surface
not chemisorb
dimensional
of
trend,
studies.
The next important
was found
of the
and only a slightly
show a similar
of the species
the amount was too small to be measured
leading
only at temperatures
1 and 3 is capable
earlier,
This might
samples.
of the "monolayer"
to ascertain
1 as the only potential
existing
in square
the ESR spectra
intensity
in the
alumina-supported of V centers
V loading,
the nature
this species
and ESCA studies)
to chemisorb
higher
the IR
some shift
is noted as a function
samples
from the precursor
as to which
because
catalysts
from our results
a different
showed
resolved,
of the spectra
that species -3 is definitely
at -78°C.
2). Third,
are well
in the unreduced
chemisorption,
reduction
section,
could be obtained
of the reduced
but moderately,
with the Vfdf centers
oxygen
catalysts
from the
by Si. Indeed
On the contrary,
in Figure
that they show a slightly
some new V;i
perceive
shared
with hfs characteristic
(see the inserts
The spectra
may have stemmed
atoms
are in
indicate that the number unreduced ones. The last two observations 4+ VTd sites (i.e., species _1) on the unreduced catalysts increases not
corresponding of isolated
oxygen
bonds.
ESR spectra
silica-supported
increasing vanadia
oxygen
in the "Results"
of O+SiOA
catalysts
that the V4+ centers
it is concluded
This distortion
the surface
as mentioned
assymetric
at 25"C,
thus
phase Following
dispersed
two
pyramidal
or
Obviously, (Figure
2)
this
82
because
most of it is reduced
nor does
it chemisorb
generated
oxygen
on this polymeric
Indeed there
is evidence
having
tetrahedral
symmetry,
VTd, chemisorbed
experiments
were conducted
ture which possible
energy
oxygen
without
symmetry
of chemisorption. at subambient
the CUS on VTd sites
of the
that V centers
activation
energy,
had to overcome
Since our oxygen
temperatures
at -78°C.
on V centers
on the symmetry
et al. [34] observed
pyramidal
oxygen
chemisorption
depending
studies,
that the cus
of chemisorbing
was even lower than that used by Khalif
that while
suggests
that oxygen
energy
Khalif
or square
of activatian
is not capable
to activation
the CUS. Thus,
those with octahedral amount
phase
All evidence
in the literature
may or may not be subject V centers
to V3+ state as found by ESCA and reduction at -78°C.
with
while
a considerable
chemisorption
(i.e., at -78"C),
a tempera-
et al. [34], it is quite
(species _1 and _3) could
chemisorb
those on the polymeric species containing VOh or VC4v centers, formed by 5+ species, failed to do so. If this is so, then the the reduction of polymeric V
oxygen,
sharp decline experimental
in oxygen
"post monolayer"
phase
top of the already the monolayer diffusion
chemisorption
observations
indicates
existing
phase
area
takes place thereafter.
This is perfectly
amount
surface,
for the formation
of vanadia
but rather studied
fore, we have been dealing phase which
contains
of polymeric
with
covers
crystalline
the that as
in the surface
area
hypothesis
level does not cover
on top of the monolayer loading
on
due to
indicates
with the proposed
the monolayer
gets deposited
did the vanadia
of three dimensional
decrease
in accord
beyond
1, also
on
if formed
by oxygen
phase which
in Figure
the
preferentially
be reached
no appreciable
is completed,
none of the samples
cannot
as shown
soon as the "monolayer"
the silica
is deposited
important
containing
and that the CUS, even
phase, probably
data,
is one of the most
of the catalysts
posed by the "post monolayer"
The surface
that an additional
(which
that this phase
monolayer
upon reduction,
restrictions
"monolayer".
capacity
of this investigation)
phase.
In
reach the level high enough V205 detectable
two types of vanadia
phases,
by XRD. There-
namely,
the monolayer
the species
_1 and 2, and the "post monolayer" phase comprised 5+ V-oxide with V in square pyramidal or octahedral
two dimensional
symmetry.
A surface
structural
model
In view of the above supported
vanadia
urn01 V me2 species
two species
in tetrahedral
coordination
oxygen
of the surface
Up to a certain on the silica
surface
surroundings.
at -78°C.
The maximum
coverage
partly species
surface
(3.5
as isolated the central
Upon reduction,
gives the ESR signal,
to what we call the "monolayer"
in this particular
state of silicasurface
species _2. In both these
to species _1 which
chemisorbs
corresponds
coverage
remains
as its precursor
converted
species -3 which
a model
as follows:
in this case) vanadia
V ion remains
layer"
is proposed
_1 and partly
-2 is partly
of V/SiO2
discussion,
species
and mostly coverage
to by these
coverage. Thus the "mono-2 . This phase covers
case is ca. 3.5 umol V m
83 only about one fifth of the total surface 0.103 nm2 as the diameter
rather
a "patchy monolayer", the "monolayer"
results
two dimensional
(V05)n
surroundings.
in the formation
while
monolayer"
phase fails
pyramidal
energy
of activation
further
as calculated
phase chemisorbs
phase
remains
Upon
the "post
of octahedral
requirements
[34]. A similar
type of picture
bound monolayer
phase,
been proposed
on the top
at low temperatures
and chemically
on the top of it has recently
beyond
polymeric
surface.
at -78"C,
consists
oxygen
of vanadia
containing
symmetry
oxygen
by taking
is actually
or octahedral
the silica
the latter
that do not chemisorb
this
addition
pyramidal
occupying
to do so, because
species
dispersed
of a second
phase with V in a different
the "monolayer"
square
Further
with V in square
phase without
reduction,
a highly
than a monolayer.
species
This second
of the "monolayer"
area of silica
of one VO2 5 unit [ZO]. Therefore,
comprised
and crystalline
for vanadia-titania
or due to of
V205
systems
by Wachs
et al. C311.
Vanadia-silica
versus
The effect capacity
of the nature
of the dispersed
to silica-
by LTOC)
layer coverage the maximum
accommodated This
from reduction is formed
Thus,
that the alumina
chemical
solution
[20],
it can be concluded
chemical is much
reaction higher
why vanadia certain
is higher
than that between
The implications
shared
of these
will be the subject
vanadia
be expected
by infrared
of future
however,
is different to be
coverage.
capacity
by Yoshida
than the
et al. [18]
of alumina
planes)
to form,
of the active of
and vanadate
Selective
ions
be the reason exposure
of
may also play a part in it.
structural
OH groups
on the
the extent
This may well
than on silica.
models
attached
of the surface
It would
Whether
species
to V and OH groups
and that these OH groups
spectroscopy.
investigation.
OH groups
phase"
in the impregnating
concentration
and silica.
(as surface
species
"monolayer"
In other words,
of OH groups
of the above discussed
to be detected
at full mono-
are found
the active
ions present
that the surface
on alumina
planes
by both V and Si would
be possible
(i.e., the area
coverage -2
dispersion
earlier
between
than in silica.
gets more dispersed
are that upon reduction
area
3
specific
also the fact that the "monolayer
vanadate
via the condensation
crystallographic
has higher
interaction
and the oligomeric
at full
was reached
Recalling
surface
in alumina
in Figure
area of both the systems
respectively
surface
support
OH groups
surface
phase at full monolayer
conclusion
and ESR studies.
by a strong
given
about four times more
ca. 8.5 and 3.5 umol V m
and silica
A similar
chemisorption pertaining
the same. This may be just a coincidence,
of the dispersed supports.
by alumina
surface.
possesses
to the total BET surface
amount
indicates
silica
silica
and oxygen
by the results
From the results
catalysts.
the ratio of the active
is practically
for the different
on the dispersion
is well demonstrated
that although
area than alumina,
titrated
of support
phase
and alumina-supported
it can be concluded surface
vanadia-alumina
should
or not this happens
also he interesting
to study
84 the effect
of dehydroxylation
of the resulting interaction
Oxygen
surface
i.e., the dispersion,
ment of the central
the vanadia monolayer"
loading
for recording
to be formed
in conjunction
tool for studying
chemisorption
formal
on the nature by the
and that of the V salt.
the surface
data are found
area of the supported
phases.
where
phase starts
The authors
they are envisaged
oxidation
V ions of the dispersed
as the "post monolayer"
(prior to V loading)
work shows that LTOC,
can be a useful
V-oxides.
on: (a) the "active"
because
of the supports
the present
techniques,
Si02-supported
nature
species
of the OH groups
In conclusion, standard
surface
of the supports
vanadia
with other structure
to provide
phase;
and (b) the
state and the coordination
species
The LTOC results
the "patchy monolayer"
in the "monolayer"
allow one to clearly becomes
complete
of
information
environas well
identify
and the "post
forming.
thank Dr. C.S. Sunandana
of the University
of Hyderabad
(India)
the ESR spectra.
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