Chapter 3
Surface Oxide-Support Interactions
in the Molecular Design of Supported Metal
Oxide Selective Oxidation Catalysts
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Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch003
Goutam Deo and Israel E. Wachs
Department of Chemical Engineering, Zettlemoyer Center for Surface
Studies, Lehigh University, Bethlehem, PA 18015
A series of metal oxides were deposited on the
surface of different oxide supports to study the
surface
oxide
support
interactions.
The
dehydrated Raman spectra of the supported metal
oxide
catalysts
reveal
the
presence
and
structure of the supported metal oxide phases.
The same surface metal oxide species were found
on the different oxide supports for each of the
supported metal oxide systems. The reactivity of
the
surface
metal
oxide
species,
however,
depends
on
the
specific
oxide
support
(TiO ~ZrO >Nb O >Al O ~SiO ). For a given oxide
support, the reactivity depends on the specific
surface metal oxide species (e.g. VO > MoO ).
The redox activation energy for a l l the surface
metal oxide phases l i e in the range of 18-22
kcal/mole.
The similar
activation
energies
suggests that the number of active sites and/or
the a c t i v i t y per site is responsible for the
difference in reactivity. The redox TON for the
methanol oxidation reaction correlates with the
reduction temperature during TPR experiments,
which suggests that the bridging M-O-Support
bond
controls
the
activity
during
redox
reactions.
2
2
2
5
2
3
2
x
y
Supported metal oxide catalysts are formed when one
metal oxide component ( i . e . , Re O , CrO , MoO , WO , V O ,
Nb O ,
etc.),
the supported metal oxide phase,
is
deposited on a second metal oxide substrate ( i . e . , A l O ,
T i O , SiO , e t c . ) , the oxide support [1]. The supported
metal oxide phase is present on the oxide support as a
surface metal oxide species. The reactivity of these
2
2
7
3
3
3
2
5
2
3
5
2
2
0097-6156/93/0523-0031$06.00/0
© 1993 American Chemical Society
In Catalytic Selective Oxidation; Oyama, S., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
CATALYTIC SELECTIVE OXIDATION
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32
supported
metal
o x i d e c a t a l y s t s have been
intensivelyi n v e s t i g a t e d o v e r t h e p a s t d e c a d e i n numerous c a t a l y t i c
a p p l i c a t i o n s and t h e main e m p h a s i s h a s been t o r e l a t e
the r e a c t i v i t y with t h e s t r u c t u r e o f t h e s u r f a c e metal
oxide
species
[1,2] . In d e t e r m i n i n g t h e s t r u c t u r e o f
t h e s e s u r f a c e m e t a l o x i d e s p e c i e s Raman s p e c t r o s c o p y h a s
proven
t o be i n d i s p e n s a b l e b e c a u s e o f t h e a b i l i t y o f
Raman
spectroscopy
t o d i s c r i m i n a t e between
different
m e t a l o x i d e s p e c i e s t h a t may s i m u l t a n e o u s l y be p r e s e n t
in
the
catalyst.
The
reactivity
studies
have
demonstrated
t h a t these s u r f a c e metal oxide s p e c i e s a r e
t h e a c t i v e s i t e s f o r many c a t a l y t i c r e a c t i o n s [3] . The
combined s t r u c t u r a l and r e a c t i v i t y i n f o r m a t i o n c u r r e n t l y
a v a i l a b l e about
these oxide c a t a l y s t s
i s beginning t o
allow
us t o d e v e l o p
an u n d e r s t a n d i n g
of the surface
o x i d e s u p p o r t i n t e r a c t i o n s and t o a p p l y t h i s knowledge
for
the molecular
design
of supported
metal
oxide
catalysts.
The
molecular
design
of
supported
metal
oxide
c a t a l y s t s r e q u i r e s t h a t we s p e c i f y t h e s y n t h e s i s method,
oxide
support,
catalyst
composition,
calcination
temperature,
location
of
the
surface
metal
oxide
s p e c i e s , as w e l l as i t s r e a c t i v i t y . Consequently, t h e
influence
o f each
o f t h e above
parameters
upon t h e
s t r u c t u r e and c a t a l y t i c
p r o p e r t i e s of supported
metal
o x i d e c a t a l y s t s n e e d s t o be e x a m i n e d . The p r e s e n t s t u d y
p r i m a r i l y f o c u s e s on t h e m o l e c u l a r d e s i g n a s p e c t s o f
supported
vanadium
oxide
catalysts
because
these
catalysts
constitute
a
very
important
class
of
heterogeneous
o x i d e c a t a l y s t s . However, c o m p a r i s o n
with
other
supported
metal
oxide
systems
(Mo0 ,
Re 0 ,and
C r 0 ) w i l l a l s o be made.
3
2
7
3
Experimental
The
Ti0
Si0
o x i d e s u p p o r t s employed i n t h e p r e s e n t s t u d y were:
(Degussa,
- 5 5 m^/g) , A 1 0
(Harshaw, - 1 8 0 m /g) ,
(Cabot,
-300 m /g) , Z r 0
(Degussa,
- 3 9 m /g) and
2^5
( N i o b i u m P r o d u c t s Co., — 5 0 m /g) . Many d i f f e r e n t
s y n t h e s i s methods have been u s e d t o p r e p a r e
supported
m e t a l o x i d e c a t a l y s t s . In t h e c a s e o f s u p p o r t e d v a n a d i u m
o x i d e c a t a l y s t s , t h e c a t a l y s t s were p r e p a r e d by v a p o r
phase
grafting
with
V0C1 ,
nonaqueous
impregnation
(vanadium
alkoxides),
aqueous
impregnation
(vanadium
oxalate),
as
well
as
spontaneous
dispersion
with
crystalline V 0
[ 4 ] . No d r a s t i c
reduction of surface
a r e a o f t h e c a t a l y s t s was o b s e r v e d .
Structural
characterization
of the surface
metal
o x i d e s p e c i e s was o b t a i n e d by l a s e r Raman s p e c t r o s c o p y
u n d e r a m b i e n t and d e h y d r a t e d c o n d i t i o n s . The l a s e r Raman
spectroscope
consists of a Spectra Physics A r
laser
p r o d u c i n g 1-100 mW o f power measured a t t h e s a m p l e . The
s c a t t e r e d r a d i a t i o n was f o c u s e d i n t o a Spex T r i p l e m a t e
s p e c t r o m e t e r c o u p l e d t o a P r i n c e t o n A p p l i e d R e s e a r c h 0ΜΑ
I I I o p t i c a l m u l t i c h a n n e l a n a l y z e r . About 100-200 mg o f
2
2
2
3
2
2
2
2
2
N b
3
2
5
+
In Catalytic Selective Oxidation; Oyama, S., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch003
3. DEOANDWACHS
Surface Oxide-Support Interactions
33
the
pure
catalysts
were
pelletized
and
used
for
o b t a i n i n g t h e Raman s p e c t r a i n t h e d e h y d r a t e d mode. F o r
a m b i e n t s p e c t r a 5-20 mg o f c a t a l y s t s was p l a c e d on a K B r
backing.
The s u p p o r t e d m e t a l o x i d e c a t a l y s t s were e x a m i n e d f o r
t h e i r r e a c t i v i t y i n t h e m e t h a n o l o x i d a t i o n r e a c t i o n . The
reactor
was
operated
using
milligram
amounts
of
c a t a l y s t s that provide d i f f e r e n t i a l reaction conditions
by
keeping
conversions
below
10%.
A
methano1/oxygen/he1iurn m i x t u r e o f ~6/13/81 (mole %) a t 1
atm p r e s s u r e was used a s t h e r e a c t a n t g a s f o r a l 1 t h e
data
presented.
The
analysis
was
performed
with
an
o n l i n e g a s c h r o m a t o g r a p h (GC) (HP 5840A) c o n t a i n i n g two
c o l u m n s ( P o r o p a k R and C a r b o s i e v e S I I ) and two d e t e c t o r s
(FID and TCD) . R e a c t i o n d a t a a t 230 °C a r e p r e s e n t e d i n
the
form
of turnover
number
(TON) - d e f i n e d a s t h e
number
o f moles
o f methanol
converted
p e r mole o f
vanadium
per
second.
The
reaction
data
f o r some
c a t a l y s t s were a l s o o b t a i n e d a t 200, 230, and 240 'C t o
calculate
the
activation
energy
and
check
for
d i f f u s i o n a l l i m i t a t i o n s i n t h e r e a c t o r . No mass and h e a t
t r a n s f e r l i m i t a t i o n s were o b s e r v e d .
R e s u l t s and D i s c u s s i o n
The v a n a d i u m o x i d e s p e c i e s i s f o r m e d on t h e s u r f a c e o f
the oxide support during the p r e p a r a t i o n of supported
vanadium
oxide
catalysts.
This
i s evident
by
the
consumption
of
surface
hydroxyIs
(OH)
[5] and t h e
s t r u c t u r a l t r a n s f o r m a t i o n o f t h e supported metal
oxide
phase
that
takes
place
during
hydration-dehydration
s t u d i e s and c h e m i s o r p t i o n o f r e a c t a n t g a s m o l e c u l e s [ 6 ] .
Recently,
a
number
o f s t u d i e s have
shown
that the
s t r u c t u r e o f t h e s u r f a c e vanadium o x i d e s p e c i e s depends
on t h e s p e c i f i c c o n d i t i o n s t h a t t h e y a r e o b s e r v e d u n d e r .
F o r example, u n d e r a m b i e n t c o n d i t i o n s t h e s u r f a c e o f t h e
oxide supports possesses a t h i n l a y e r o f moisture which
p r o v i d e s an aqueous e n v i r o n m e n t o f a c e r t a i n pH a t p o i n t
of z e r o charge
(pH a t p z c ) f o r t h e s u r f a c e v a n a d i u m
o x i d e s p e c i e s and c o n t r o l s t h e s t r u c t u r e o f t h e v a n a d i u m
o x i d e phase [7] . Under r e a c t i o n c o n d i t i o n s (300-500 °C) ,
m o i s t u r e desorbs from t h e s u r f a c e o f t h e o x i d e
support
and
t h e vanadium oxide s p e c i e s i s f o r c e d t o d i r e c t l y
interact
with
the oxide
support
which
results
in a
different
structure
[8] .
These
structural
transformations
taking
place
during
hydration
and
dehydration c o n d i t i o n s o f the oxide support suggest t h a t
the
correlation
of the s t r u c t u r e - r e a c t i v i t y
data
s h o u l d be p e r f o r m e d
with the s t r u c t u r a l data obtained
under
dehydration
conditions,
and
correlating
such
s t r u c t u r a l information with the r e a c t i v i t y data.
A s e r i e s o f ~ 1 % ^2^5 c a t a l y s t s was p r e p a r e d by non
aqueous i m p r e g n a t i o n o f v a n a d i u m t r i - i s o p r o p o x i d e
oxide
( f i n a l c a l c i n a t i o n i n oxygen a t 450/500 °C) i n o r d e r t o
investigate the influence of d i f f e r e n t
oxide
supports
In Catalytic Selective Oxidation; Oyama, S., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
34
CATALYTIC SELECTIVE OXIDATION
upon t h e d e h y d r a t e d m o l e c u l a r s t r u c t u r e a n d r e a c t i v i t y
o f t h e s u r f a c e vanadium o x i d e s p e c i e s . Under
ambient
conditions
Raman
bands
due
to
orthovanadate,
p y r o v a n a d a t e , m e t a v a n a d a t e , and d e c a v a n a d a t e s p e c i e s a r e
observed.
Assignments
of
t h e Raman
bands
t o the
different
s p e c i e s a r e made e l s e w h e r e
[7, 9] . A t t h e s e
surface
coverages
a
single
surface
vanadium
oxide
s p e c i e s i s p r e d o m i n a n t l y p r e s e n t on t h e d i f f e r e n t o x i d e
s u p p o r t s a s i s e v i d e n t by t h e p r e s e n c e
o f a dominant
1015-1039 cm" band i n t h e d e h y d r a t e d Raman s p e c t r a a n d
potential
complication
due
to
additional
surface
v a n a d i u m o x i d e s p e c i e s a r e e l i m i n a t e d . The d e h y d r a t e d
Raman band due t o t h e V=0 bond was f o u n d t o v a r y f r o m
1015-1039 cm"
as a f u n c t i o n
of the different
oxide
s u p p o r t s a s shown i n F i g u r e 1. The s l i g h t d i f f e r e n c e i n
band
position
i s due t o s l i g h t l y
different
V=0 bond
l e n g t h s o f t h e i s o l a t e d s u r f a c e vanadium o x i d e s p e c i e s
on t h e d i f f e r e n t o x i d e s u p p o r t s and c o r r e l a t e s w i t h t h e
d i f f e r e n t oxygen c o o r d i n a t i o n o f t h e o x i d e s u p p o r t s . The
Raman s p e c t r a r e v e a l t h a t e s s e n t i a l l y t h e same s u r f a c e
v a n a d i u m o x i d e s p e c i e s i s p r e s e n t on a l l t h e d i f f e r e n t
o x i d e s u p p o r t s . T h i s s u r f a c e vanadium o x i d e s p e c i e s i s
described
as
a
distorted
four
coordinated
species
p o s s e s s i n g a s i n g l e t e r m i n a l bond (V=0) and t h r e e bonds
t o t h e s u p p o r t (V-O-S) . The s°ame c o n c l u s i o n i s r e a c h e d
from s o l i d s t a t e
V NMR s t u d i e s o f t h e s e c a t a l y s t s [ 9 ] .
Low s u r f a c e c o v e r a g e s ,
— 1 % metal
oxide, of supported
molybdenum
oxide
(Mo0 )
[10] , r h e n i u m
oxide
(Re 0 )
[ 1 1 , 1 2 ] , and chromium o x i d e ( C r 0 ) [11,12] a l s o i n d i c a t e
the presence
o f a s i n g l e s u r f a c e metal o x i d e s p e c i e s .
The
similar
Raman
band
positions
of the
supported
vanadium-oxygen
(V=0)
stretching
frequency
during
d e h y d r a t e d c o n d i t i o n s a r e g i v e n i n T a b l e I . In a d d i t i o n ,
the
structural
transformation
taking
place
due t o
d e h y d r a t i o n i s o b s e r v e d by c o m p a r i n g columns 2 and 3 o f
T a b l e I . T h u s , a t low c o v e r a g e s t h e d e h y d r a t e d s u r f a c e
vanadium
oxide
and
related
metal
oxide
molecular
s t r u c t u r e s (Re 07, C r 0 , and Mo0 ) a r e i n d e p e n d e n t o f t h e
s p e c i f i c oxide support.
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1
1
5 1
3
2
7
3
2
3
3
T a b l e I . D e h y d r a t e d Raman band p o s i t i o n f o r t h e V=0
t e r m i n a l s t r e t c h i n g v i b r a t i o n s f o r 1% V 0 on d i f f e r e n t
o x i d e s u p p o r t s a l o n g w i t h t h e h i g h e s t a m b i e n t Raman band
position
2
Oxide
Support
Si0
Nb 0
Ti0
Zr0
Α1 0*
2
2
5
2
2
9
-1
V=0 band (cm )
dehydrated c o n d i t i o n s
1039
1031
1027
1024
1015
5
1
H i g h e s t band (cm" )
ambient c o n d i t i o n s
1000
970-980
940-950
960
920-930
In Catalytic Selective Oxidation; Oyama, S., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch003
3.
DEOANDWACHS
Surface Oxide-Support Interactions
35
The r e a c t i v i t y o f t h e s u r f a c e v a n a d i u m o x i d e s p e c i e s
( — 17c V2O5) on "the d i f f e r e n t o x i d e s u p p o r t s was p r o b e d by
t h e m e t h a n o l o x i d a t i o n r e a c t i o n . The m e t h a n o l o x i d a t i o n
reaction
i s very
sensitive
t o the nature
of surface
s i t e s p r e s e n t . S u r f a c e redox s i t e s form
formaldehyde,
methyl formate,
and d i m e t h o x y methane a s t h e r e a c t i o n
products.
Formaldehyde
being
formed
as
the
first
oxidation
product
from
the
methoxy
intermediate.
Subsequent
reactions
of
the
methoxy
intermediate
produces methyl formate
and d i m e t h o x y methane. S u r f a c e
a c i d s i t e s , Lewis as w e l l as Bronsted,
result
i n the
formation of dimethyl ether. Surface basic s i t e s
yield
C0/C0
as t h e r e a c t i o n
products
[9] . F o r a l l t h e
s u p p o r t e d vanadium o x i d e c a t a l y s t s , w i t h t h e e x c e p t i o n
of
alumina,
the surface vanadia
redox s i t e s
produced
formaldehyde
almost
exclusively.
On
alumina,
only
a
trace
o f f o r m a l d e h y d e was f o r m e d b e c a u s e t h e s u r f a c e
acid
sites
produced
dimethyl
ether.
Thus,
f o r the
V2O5/AI2O3 s y s t e m t h e f o r m a l d e h y d e p r o d u c e d was t a k e n a s
representative of the r e a c t i v i t y of the surface vanadia
redox
sites.
The
reactivity
of the surface
vanadia
s p e c i e s on d i f f e r e n t o x i d e s u p p o r t s was f o u n d t o depend
d r a m a t i c a l l y on t h e s p e c i f i c o x i d e s u p p o r t a s shown i n
Table
I I . As t h e s u p p o r t was changed f r o m
silica to
zirconia
the turnover
number
(TON) f o r t h e s u r f a c e
v a n a d i u m o x i d e s p e c i e s was f o u n d t o i n c r e a s e by t h r e e
o r d e r s o f m a g n i t u d e . S i m i l a r t r e n d s were a l s o
observed
f o r s u p p o r t e d molybdenum o x i d e [ 1 0 ] , r h e n i u m o x i d e [ 1 2 ] ,
and chromium o x i d e [12] .
2
T a b l e I I . The TON and s e l e c t i v i t y t o f o r m a l d e h y d e f o r
t h e m e t h a n o l o x i d a t i o n r e a c t i o n on v a r i o u s 1% s u p p o r t e d
vanadium o x i d e c a t a l y s t s
- 1
Oxide
Support
TON
Si0
A1 0
Nb 0
Ti0
Zr0
2.0*10"
2.0*10~
7.0*10
1.8*10°
2.3*10°
3
2
2
2
2
2
(sec )
2
3
_1
5
S e l e c t i v i t y t o HCHO ( 7 o )
-80
<1
-94
-98
-96
The
reactivity
of
the
supported
vanadium
oxide
catalysts
f o r other
oxidation
reactions
also
show
similar
trends
as the oxide
support
i s varied
from
t i t a n i a t o s i l i c a [ 1 3 ] . The a c t i v i t y and s e l e c t i v i t y f o r
partial
o x i d a t i o n p r o d u c t s o f vanadium o x i d e
supported
on t i t a n i a b e i n g h i g h e r t h a n v a n a d i u m o x i d e s u p p o r t e d on
s i l i c a . The o x i d a t i o n a c t i v i t y o f t h e s u p p o r t e d v a n a d i u m
oxide
catalysts
i s related
to the a b i l i t y
t o donate
oxygen t o form
the required oxidation products.
The
In Catalytic Selective Oxidation; Oyama, S., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
CATALYTIC SELECTIVE OXIDATION
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36
origin
of
this
support
effect
is
either
due
to
d i f f e r e n c e s i n t h e t e r m i n a l V=0 bond o r t h e b r i d g i n g VO - S u p p o r t bond. Many p u b l i c a t i o n s have p r o p o s e d t h a t t h e
terminal
bond
is responsible f o r catalysis
and
its
activity
i s d i r e c t l y r e l a t e d t o t h e V=0
bond s t r e n g t h
[14] . However, c o m p a r i s o n
o f t h e Raman p o s i t i o n o f t h e
V=0
bond
(shorter
bond
corresponds
to
higher
Raman
p o s i t i o n ) f r o m T a b l e I and TON v a l u e f o r t h e d i f f e r e n t
s u p p o r t e d v a n a d i u m o x i d e c a t a l y s t s f r o m T a b l e I I shows
t h a t no r e l a t i o n s h i p e x i s t s between t h e bond s t r e n g t h
and
activity.
A
more e l a b o r a t e a n a l y s i s
of the
TON
v e r s u s bond s t r e n g t h f o r v a r i o u s s u p p o r t e d m e t a l
oxide
c a t a l y s t s are performed elsewhere
[15] . A more p l a u s i b l e
conclusion
i s that
the
reactivity
is related
to
the
b r i d g i n g V - O - S u p p o r t bond s i n c e t h e o x i d e s u p p o r t has a
v e r y s i g n i f i c a n t e f f e c t on t h e r e a c t i v i t y . The t r e n d i n
reactivity
with s p e c i f i c
oxide
support
appears
to
be
related
to
the
surface
reducibility
of
the
oxide
s u p p o r t s . The more r e d u c i b l e o x i d e s u p p o r t s ( T i 0 , Z r 0 ,
and
Nb 0 )
always
exhibit
very
high
TON
while
the
irreducible
oxide
supports
(A1 0
and
Si0 )
always
e x h i b i t v e r y low TON
[ 1 6 ] . In a d d i t i o n , t h e
importance
o f t h e M-O-Support bond d u r i n g t h e m e t h a n o l o x i d a t i o n
r e a c t i o n s u g g e s t s t h a t t h e r e a c t i v i t y s h o u l d a l s o be a
f u n c t i o n of the s p e c i f i c supported metal o x i d e s p e c i e s .
Indeed,
s u p p o r t e d molybdenum o x i d e c a t a l y s t s a r e
about
one
order
o f magnitude
less
reactive
than
supported
vanadia catalysts.
Additional
information
about
the
reactivity
was
obtained
by d e t e r m i n i n g t h e k i n e t i c
parameters
during
m e t h a n o l o x i d a t i o n f o r v a n a d i a , m o l y b d e n a , r h e n i a , and
chromia
on
different
oxide
supports.
For
a l l these
s y s t e m s t h e a c t i v a t i o n e n e r g y i s a p p r o x i m a t e l y t h e same,
18-22
k c a l / m o l . The
activation
energy
corresponds
to
t h a t expected
f o r t h e b r e a k i n g o f t h e C-H
bond o f a
s u r f a c e m e t h o x i d e i n t e r m e d i a t e , C H 0 j , and s h o u l d
be
independent
of the
specific
catalyst
[ 1 7 ] . The
preexponential
factors,
however,
vary
by
orders
of
m a g n i t u d e as t h e o x i d e s u p p o r t
i s v a r i e d . The
similar
structures
of
the
supported
metal
oxide
catalysts
suggests that the d i f f e r e n c e in pre-exponential f a c t o r s
i s r e l a t e d t o e i t h e r t h e number o f a c t i v e s i t e s o r t h e
a c t i v i t y per s i t e .
I r r e s p e c t i v e of the exact parameter
that
a f f e c t s the pre-exponential f a c t o r ,
i t i s shown
h e r e t h a t t h e o x i d e s u p p o r t has a l a r g e i n f l u e n c e on t h e
r e a c t i v i t y of the s u r f a c e metal o x i d e s p e c i e s .
To i n v e s t i g a t e t h e e f f e c t o f t h e s y n t h e s i s method on
the s t r u c t u r e - r e a c t i v i t y r e l a t i o n s h i p of the
supported
metal
oxide c a t a l y s t s , a s e r i e s of V 0 / T i 0
catalysts
were
s y n t h e s i z e d by
equilibrium
adsorption,
vanadium
oxalate,
vanadium
a l k o x i d e s and
vanadium
oxychloride
g r a f t i n g [ 1 4 ] . The d e h y d r a t e d Raman s p e c t r a o f a l l t h e s e
catalysts
exhibit
a
sharp
band
at
—1030
cm"
characteristic
of the
isolated
s u r f a c e vanadium
oxide
species described previously.
Reactivity
studies with
2
2
2
5
2
3
3
2
a c
s
2
5
2
1
In Catalytic Selective Oxidation; Oyama, S., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
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Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch003
3.
Surface Oxide-Support Interactions
DEOANDWACHS
37
m e t h a n o l o x i d a t i o n e x h i b i t e d s i m i l a r t u r n o v e r numbers.
T h u s , t h e s y n t h e s i s method d o e s n o t a f f e c t t h e f i n a l
s t r u c t u r e o r r e a c t i v i t y o f t h e s u r f a c e vanadium o x i d e
s p e c i e s on t i t a n i a . S i m i l a r c o n c l u s i o n s were a l s o f o u n d
for
molybdenum o x i d e s u p p o r t e d on t i t a n i a
[14], s i l i c a
[18] , and a l u m i n a
[18] . C o n s e q u e n t l y ,
the preparation
method i s n o t a c r i t i c a l
parameter t o c o n s i d e r f o r t h e
d e s i g n o f s u p p o r t e d m e t a l o x i d e c a t a l y s t s . However, c a r e
must be t a k e n t o f o r m t h e s u r f a c e m e t a l
oxide
phase
without
d e s t r o y i n g the oxide
support
or introducing
extraneous i m p u r i t i e s during p r e p a r a t i o n .
To s t u d y t h e e f f e c t o f l o a d i n g , v a r i o u s amounts o f
v a n a d i a were d e p o s i t e d on t h e t i t a n i a s u p p o r t . The Raman
spectra
of t i t a n i a
supported
vanadia
catalysts
as a
f u n c t i o n o f vanadia loading reveal t h e presence o f three
different
vanadia
s p e c i e s on t h e T i 0
support
under
dehydration
c o n d i t i o n s a s shown
i n F i g u r e 2. A t low
l o a d i n g s ( — 1% V 0 ) , a s i n g l e s h a r p band i s p r e s e n t a t
— 1 0 2 7 cm" w h i c h i s a s s i g n e d t o an i s o l a t e d t e t r a h e d r a l
coordinated
s u r f a c e vanadium
oxide
species containing
one t e r m i n a l V=0 bond and t h r e e b r i d g i n g V - 0 - T i bonds
[19] . A t i n t e r m e d i a t e l o a d i n g s ( 2 - 5 % V 0 ) , a s e c o n d band
i s p r e s e n t a t 920-930 cm" w h i c h h a s been a s s i g n e d t o a
polymerized,
tetrahedral
coordinated
surface
vanadium
o x i d e s p e c i e s [19] i n a d d i t i o n t o t h e f i r s t
band a t
-1030
cm' . A t h i g h l o a d i n g s (>6% V 0 ) , a t h i r d
sharp
band i s p r e s e n t a t 994 cm" due t o c r y s t a l l i n e V 0
which
i n d i c a t e s t h a t t h e c l o s e - p a c k e d s u r f a c e vanadium o x i d e
m o n o l a y e r h a s been f o r m e d and a l l t h e r e a c t i v e s u r f a c e
h y d r o x y I s consumed. The f i r s t two bands a r e a l s o p r e s e n t
a t h i g h l o a d i n g s . S i m i l a r t r e n d s i n Raman b a n d s a r e a l s o
o b s e r v e d f o r vanadium o x i d e s u p p o r t e d z i r c o n i a , n i o b i a ,
alumina,
and s i l i c a
and t h e f o r m a t i o n o f m o l e c u l a r l y
dispersed
vanadium
oxide
s p e c i e s always preceeds t h e
formation
of
crystalline
^2^5' Thus,
the
catalyst
composition i s a c r i t i c a l
parameter s i n c e i t i n f l u e n c e s
t h e f o r m a t i o n o f d i f f e r e n t vanadium o x i d e s t r u c t u r e s .
The
reactivity
of the t i t a n i a
supported
vanadium
oxide
c a t a l y s t s was p r o b e d
by t h e m e t h a n o l o x i d a t i o n
reaction.
The o x i d a t i o n o f m e t h a n o l o v e r t h e t i t a n i a
supported
vanadia
catalysts
exclusively
yielded
f o r m a l d e h y d e , 95%+, a s t h e r e a c t i o n p r o d u c t . The t i t a n i a
support
i n t h e absence
of surface
vanadia
yielded
dimethyl
e t h e r and t r a c e
amounts o f C 0 . The a l m o s t
complete f o r m a t i o n o f formaldehyde demonstrates t h a t t h e
r e a c t i v i t y of the t i t a n i a supported vanadia c a t a l y s t s i s
due
t o t h e s u r f a c e v a n a d i a redox
s i t e s . The t u r n o v e r
numbers o f t h e v a r i o u s v a n a d i a t i t a n i a
catalysts are
presented
i n Table
I I I as a f u n c t i o n o f t h e v a n a d i a
l o a d i n g . The TON i n c r e a s e s somewhat w i t h i n i t i a l s u r f a c e
vanadium
oxide
coverage,
and d e c r e a s e s
slightly
at
surface
coverages
approaching
and e x c e e d i n g
monolayer
coverage.
Note t h a t t h e TON o f b u l k V 0
[16] i s two
orders
o f magnitude
less
than
the t i t a n i a
supported
vanadia
catalysts
indicating that
crystalline
V 0
is
2
2
5
1
2
5
1
1
2
5
1
2
5
2
2
5
2
In Catalytic Selective Oxidation; Oyama, S., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
5
CATALYTIC SELECTIVE OXIDATION
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38
In Catalytic Selective Oxidation; Oyama, S., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
3.
Surface Oxide-Support Interactions
DEO AND WACHS
39
s i g n i f i c a n t l y l e s s a c t i v e than surface vanadia s p e c i e s .
The
slight
increase
i n TON
with
increasing
surface
coverage
i s not
related
to
t h e presence
of the
p o l y m e r i z e d s u r f a c e v a n a d i a s p e c i e s (Raman band a t 920930 cm" ) a s t h e Raman a s s o c i a t e d w i t h t h e p o l y m e r i z e d
s p e c i e s i n c r e a s e s c o n t i n u o u s l y up t o m o n o l a y e r c o v e r a g e s
and
the turnover
numbers
are
comparable.
Similar
c o v e r a g e e f f e c t s were a l s o o b s e r v e d f o r v a n a d i u m o x i d e
s u p p o r t e d on z i r c o n i a , a l u m i n a , and n i o b i a c a t a l y s t s , a s
well
as f o r supported
rhenium o x i d e
[11] > molybdenum
o x i d e [10] and chromium o x i d e c a t a l y s t s [12] . T h u s , t h e
reactivity
of
the
surface
vanadium
oxide
species
e s s e n t i a l l y d o e s n o t depend on t h e s u r f a c e c o v e r a g e .
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1
Table
I I I . The TON o f V 0 / T i 0
c a t a l y s t s as a f u n c t i o n
of loading
2
wt.7o V 0 / T i 0
2
5
0.5
1.0
2.0
3.0
4.0
5.0
6.0
7.0
bulk
V 0
2
5
2
T.O.N,
2
- 1
(sec )
1 .0
,
2 .0
2 .7
1 .6
,
1 .5
,
1 .3
,
1 ., 1
1 .2
,
0 .022
5
The n a t u r e o f t h e s u p p o r t e d v a n a d i u m o x i d e p h a s e i s
influenced
by t h e c a l c i n a t i o n
temperature.
Moderate
calcination
temperatures,
350-500 °C, a r e r e q u i r e d t o
decompose
the
metal
oxide
precursors
(oxalates,
alkoxides,
o x y c h l o r i d e s , e t c . ) t o form
the surface
vanadium o x i d e
species
[20] . I n s u f f i c i e n t
calcination
t e m p e r a t u r e s do n o t c o m p l e t e l y decompose t h e p r e c u r s o r s
and, c o n s e q u e n t l y , t h e p r e c u r s o r s do n o t r e a c t w i t h t h e
surface
hydroxyIs
t o form
the surface
metal
oxide
s p e c i e s . However, h i g h c a l c i n a t i o n t e m p e r a t u r e s , g r e a t e r
t h a n 600 C, c a n r e s u l t i n s h r i n k i n g o f t h e s u r f a c e a r e a
of
t h e oxide
support
and d e c r e a s i n g
the
available
surface
area
f o r the surface
metal
oxide
species.
Consequently, high c a l c i n a t i o n temperatures
increase the
surface
coverage
o f t h e metal
oxide
s p e c i e s and, i n
s e v e r e c a s e s , d e s t r o y t h e s u r f a c e m e t a l o x i d e p h a s e and
form c r y s t a l l i n e V 0
or solid state solutions
[7,20].
Thus, c a l c i n a t i o n temperature
i s an i m p o r t a n t p a r a m e t e r
that
controls
the
activation
and
deactivation
of
supported
metal
oxide
catalysts.
However,
supported
metal
oxide
catalysts
are
typically
prepared
by
c a l c i n i n g a t 400-500 °C w h i c h w o u l d e l i m i n a t e p r o b l e m s
w i t h c a t a l y s t a c t i v a t i o n and d e a c t i v a t i o n .
e
2
5
In Catalytic Selective Oxidation; Oyama, S., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
40
CATALYTIC SELECTIVE OXIDATION
T y p i c a l l y , t h e p r o p e r t i e s o f supported metal
oxide
c a t a l y s t s a r e m o d i f i e d by t h e a d d i t i o n o f p r o m o t e r s . T o
examine
the influence
o f promoters,
a
series
of
promoters
(tungsten
oxide,
niobium
oxide,
silica,
p o t a s s i u m o x i d e and p h o s p h o r o u s o x i d e ) were added t o a
17c V 0 / T i 0 2 c a t a l y s t . T h e i n f l u e n c e o f t h e d i f f e r e n t
promoters
upon t h e s t r u c t u r e
o f t h e s u r f a c e vanadium
o x i d e s p e c i e s was examined w i t h Raman s p e c t r o s c o p y a n d
t h e r e s u l t s a r e p r e s e n t e d i n T a b l e IV. T h e a d d i t i o n o f
m o n o l a y e r amounts o f t u n g s t e n o x i d e a n d n i o b i u m o x i d e t o
17c V 0 / T i 0
c a t a l y s t s show s i m i l a r e f f e c t s o b s e r v e d a s
t h e vanadium o x i d e
l o a d i n g i s i n c r e a s e d , namely, t h e
a p p e a r a n c e o f t h e Raman band a t 9 2 0 - 9 3 0 cm" . M o n o l a y e r
amounts o f s i l i c a
on p r e v i o u s l y p r e p a r e d
1% V 0 / T i 0
shows t h e p r e s e n c e o f a s i n g l e band a t 1024 cm" . I n a n y
c a s e t h e m a j o r Raman band i s a t 1024-1031 cm" i n d i c a t i n g
that
t h e vanadium
oxide
species
before
and
after
a d d i t i o n o f tungsten oxide, niobium
o x i d e , and s i l i c a
are
structurally
similar.
However,
the addition of
p o t a s s i u m o x i d e and p h o s p h o r o u s o x i d e had a s i g n i f i c a n t
e f f e c t on t h e s t r u c t u r e o f t h e s u r f a c e v a n a d i a s p e c i e s .
The a d d i t i o n o f p o t a s s i u m o x i d e d e c r e a s e d t h e p o s i t i o n
o f t h e Raman band ( c o r r e s p o n d i n g t o an i n c r e a s e i n V=0
bond
length),
and t h e a d d i t i o n
o f phosphorous
oxide
resulted
i n t h e f o r m a t i o n o f c r y s t a l l i n e V0P0
(major
Raman bands a t 1038 and 928 cm" i n t h e 7 0 0 - 1 2 0 0 cm'
r e g i o n ) . The n o n - i n t e r a c t i n g p r o m o t e r s
( o x i d e s o f W, Nb,
and S i ) d i d n o t a f f e c t t h e a c t i v i t y o r s e l e c t i v i t y o f
the
17c V 0 / T i 0
catalyst.
However,
the interacting
promoters
(potassium
oxide
and
phosphorous
oxide)
significantly
reduced
t h e TON. T h u s ,
promoters
that
p r e f e r e n t i a l l y coordinate t o the oxide support (tungsten
oxide,
niobium
oxide
and s i l i c a )
do n o t a f f e c t t h e
s t r u c t u r e o r r e a c t i v i t y o f t h e s u r f a c e vanadium o x i d e
species,
whereas, promoters
that
coordinate with t h e
surface
vanadia
site
(phosphorous
and
potassium)
i n f l u e n c e t h e s t r u c t u r e and r e a c t i v i t y o f t h e s u r f a c e
vanadia species.
2
5
2
5
2
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Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch003
1
2
5
2
1
1
4
1
2
Table
5
1
2
IV. Raman band p o s i t i o n f o r p r o m o t e r s on
17c V 0 / T i 0
9
5
9
P r o m o t e r s on
17c V 0 / T i 0
Raman band p o s i t i o n s
(700-1200 cm' ) r e g i o n
None
67c N b 0
37c S i 0
77c W0
0.37c K 0
0.77c K 0
57c P 0 s
1027
1031,
1024
1031,
1023,
1009,
1035,
2
5
2
2
5
2
3
2
2
9
1
928
925
997
980
925
In Catalytic Selective Oxidation; Oyama, S., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1993.
3. DEOANDWACHS
Surface Oxide-Support Interactions
41
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Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch003
Conclusions
The above d i s c u s s i o n d e m o n s t r a t e s t h a t i t i s p o s s i b l e t o
m o l e c u l a r l y d e s i g n supported metal oxide c a t a l y s t s w i t h
knowledge o f t h e s u r f a c e
oxide - support
interactions
made
possible
by t h e a s s i s t a n c e
of characterization
methods s u c h
a s Raman
spectroscopy
and t h e m e t h a n o l
oxidation
r e a c t i o n . The f o r m a t i o n and l o c a t i o n o f t h e
surface
metal
oxide
species
are controlled
by t h e
s u r f a c e h y d r o x y 1 c h e m i s t r y , and t h e s u r f a c e m e t a l o x i d e
species
are located
i n t h e outermost
layer
of the
c a t a l y s t s a s an o v e r l a y e r . The s p e c i f i c o x i d e s u p p o r t i s
a c r i t i c a l parameter s i n c e i t d r a m a t i c a l l y a f f e c t s t h e
reactivity
of the surface
metal
oxide
species,
even
though t h e s u r f a c e metal oxide s t r u c t u r e i s independent
o f t h e s p e c i f i c o x i d e s u p p o r t . The c a t a l y s t c o m p o s i t i o n
i s a c r i t i c a l parameter s i n c e i t a f f e c t s t h e presence o f
d i f f e r e n t metal oxide s p e c i e s ( i s o l a t e d s u r f a c e
species,
polymerized
surface
species,
and c r y s t a l l i n e
phases),
and t h e r e a c t i v i t y , TON, i s s i m i l a r f o r s u r f a c e
metal
o x i d e c o v e r a g e . The p r e p a r a t i o n method i s n o t a c r i t i c a l
parameter s i n c e i t does not i n f l u e n c e t h e s t r u c t u r e o r
reactivity
of
the
surface
metal
oxide
species.
C a l c i n a t i o n temperature
i s an i m p o r t a n t p a r a m e t e r t h a t
c o n t r o l s a c t i v a t i o n and d e a c t i v a t i o n o f s u p p o r t e d m e t a l
oxide
c a t a l y s t s , but c a l c i n a t i o n temperature
i s not
c r i t i c a l i f m o d e r a t e t e m p e r a t u r e s , 350-450 °C, a r e u s e d .
Additives that
i n t e r a c t w i t h t h e o x i d e s u p p o r t do n o t
i n f l u e n c e t h e s t r u c t u r e and r e a c t i v i t y o f t h e s u r f a c e
metal
oxide species.
However, a d d i t i v e s t h a t
interact
w i t h t h e s u r f a c e m e t a l o x i d e do i n f l u e n c e t h e p r o p e r t i e s
of the surface
metal
oxide
species.
In summary, t h e
c r i t i c a l parameters t h a t a f f e c t t h e c a t a l y t i c p r o p e r t i e s
are t h e s p e c i f i c
oxide
support,
catalyst
composition
( t y p e o f a d d i t i v e s ) and s u r f a c e m e t a l o x i d e c o v e r a g e .
Acknowledgments
We w o u l d l i k e t o t h a n k D r . D.S.Kim, H.
Vuurman f o r t h e i r
helpful discussions.
support
by
NSF
grant
#
CTS-9006258
acknowledged.
Hu, and M. A.
The
financial
is gratefully
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