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Some Infrared Spectroscopic Studies of the Adsorption of Molecules on MqO Surfaces and MgO-Supported Palladium Catalysts
M. Che and G.C. Bond (Editors), Adsorption and Catalysis on Oxide Surfaces 0 1985 Elsevier Science Publishers B.V.. Amsterdam -Printed in The Netherlands
SOME IUFPAPED SPETTQOSCnPIC S T U n I F S OF THE ADSORPTInhl OF POLECULES
247
Oh'
WqO
SURFACFS AND PgO-SUPPnRTED PPLLADIUP CATALYSTS C.C.
CHU and N. SHFPPARD
School of Chemical Sciences, University of Fast Pnqlia, blorwich
hlR4,
7TJ, U K
ABSTRACT
Infrared sriectroscooic studies have been made of the adsorption of C02, C O a n d O2 on finely-divided magnesium oxide. The MgO samoles were prepared by decomnosing Mg(OH), a n d heatinq in vacuum a t 5Oo0C o r 8n0°C respectively. After preparation a t each temperature the adsorption of C02 and f n was measured a f t e r s t r a i g h t evacuation, o r a f t e r treatment of the MgO a t these temperatures in oxygen ( t o qive a n oxygen-rich surface) or in hydroaen ( t o qive an oxyaen-free s u r f a c e ) . The adsorntion of Ul2 occurred very s t r o n o l y , seemingly on faces as well as A t d i f f e r e n t staqes of adsorotion edqes o r corners of t h e WqO c r y s t a l 1 i t e s . bidentate and unidentate surface carbonate species aave senarately i d e n t i f i a b l e difference s n e c t r a . On surfaces w i t h OH aroutx ( a s nrepared a t 500°C o r a f t e r hydroqen treatment a t 80OoC) snectra from surface bicarbonate ions were c l e a r l y observed. C O adsorbed, as expected, t o a much smaller extent than C02 on the WgO samoles. Q u a n t i t a t i v e adsorption measurements suggested t h a t bidentate carbonate ions were formed from CO by i n t e r a c t i o n mainly with edoe o r corner s i t e s on t h e c r y s t a l l i t e s . I t i s sneculated t h a t these a r e t h e oxidising s i t e s described i n the 1 i t e r a t u r e as 0-. Adsorntion of n2 on FlgO gave weak h u t c l e a r absorptions a t 1130, 1085 and 1065 cm-', t h e f i r s t of these bands (ca. - 5 % absorotion) being t h e s t r o n g e s t . These bands can reasonably be a t t r i b u t e d t o t h e nresence of i o n i c 0; species on t h e surface. Palladium c a t a l y s t s were prepared sunported on Pg0 ( a ) because o f the w,ider infrared transparency of MgO compared w i t h Si02 o r A1203 normally used a s sunports and ( 6 ) because of t h e p o s s i b i l i t y of e p i t a x i a l qrowth o f Pd c r y s t a l l i t e s on t h e cube faces of MgO. Adsorntion o f CO on Pd gave p r i n c i a a l l y a much sharner-than-normal infrared band from the 2-fold bridged 1 s p e c i e s , between 2000 and 1990 cm- . T h i s observation sunported the idea of uniform surface s i t e s t h a t have resulted from e p i t a x i a l growth. However the r e l a t i v e weakness of t h e CO adsorntions implied t h e Dresence of r a t h e r l a r g e Pd c r y s t a l 1i t e s corresnondinq to a 1 ow-area metal c a t a l y s t .
248
DISCUSSION S t o n e ( U n i v e r s i t y of B a t h , U K )
P r . F.S.
The f o r m a t i o n o f c a r b o n a t e ions a s a r e s u l t o f CO i n t e r a c t i n g w i t h MgO d o e s n o t n e c e s s a r i l y i m p l y t h a t o x i d i z i n g s i t e s a r e p r e s e n t . One may h a v e a d i s p r o p o r t i o n a t i o n r e a c t i o n , w h e r e b y d i m e r i c o r p o l y m e r i c CO c a r b a n i o n s a r e formed simultaneously with carbonate ion formation ( 1 , Z )
by t h e o v e r a l l r e a c t i o n :
The c a r b a n i o n f o r m a t i o n i s l e s s e a s i l y d e t e c t e d b y I R t h a n t h e c a r b o n a t e , b u t
i s r e a d i l y o b s e r v e d b y UV r e f l e c t a n c e ( I ) .
Low-coordinate
s i t e s ( e . g k i n k and
c o r n e r s i t e s ) a r e c e r t a i n l y n e e d e d , b u t more b e c a u s e o f t h e d e m a n d i n g n a t u r e of t h e c a r b a n i o n f o r m a t i o n t h a n t h e accompanying c a r b o n a t e f o r m a t i o n . ( I ) A . Z e c c h i n a and F . S .
( 2 ) E. G u g l i e l m i n o t t i , J.C.S.
P r . N.
Stone, J.C.S.
Faraday I,
1978,
74,
2278.
S. C o l u c c i a , E . G a r r o n e . L. C e r r u t i a n d A. Z e c c h i n a ,
F a r a d a y I , 1979,
75,96.
S h e p p a r d ( U n i v e r s i t y o f E a s t A n g l i a , N o r w i c h , UK)
Your s u g g e s t i o n t h a t t h e o x i d a t i o n of C O t o c a r b o n a t e i o n s o n a n "oxygenf r e e " MgO s u r f a c e g o e s w i t h a p a r a l l e l r e d u c t i o n o f C O t o s u r f a c e - s p e c i e s t h e t y p e (CO);
of
i s a n i n t e r e s t i n g one. A l l I c a n s a y i s t h a t w e were, of c o u r s e ,
aware o f t h e e x i s t e n c e of s u c h n e g a t i v e l y c h a r g e d s p e c i e s and t h a t o u r e x p e r i m e n t s were u n d e r low CO p r e s s u r e s w h e r e t h e s e were n o t s p e c t r o s c o p i c a l l y i d e n t i f ied.
P r . J.H. L u n s f o r d ( T e x a s A & M U n i v e r s i t y , C o l l e g e S t a t i o n , USA) The c a t a l y t i c p r o p e r t i e s o f s u p p o r t e d p a l l a d i u m i n m e t h a n o l s y n t h e s i s a r e o f i n t e r e s t i n s e v e r a l l a b o r a t o r i e s . P r o f e s s o r P o n e c h a s s u g g e s t e d t h a t Pd
s+
.
is
a n e s s e n t i a l component o f t h i s c a t a l y s t . Do you b e l i e v e t h a t t h e I R b a n d a t ca.
2,000 cm-I
may r e s u l t f r o m CO a d s o r b e d o n a p a r t i a l l y o x i d i z e d f o r m o f Pd r a t h e r
t h a n on a h i g h - i n d e x
plane ?
P r . N. S h e p p a r d
I a g r e e t h a t a d e g r e e o f p o s i t i v e c h a r g e o n Pd a t o m s c o u l d b e a c a u s e o f a h i g h e r t h a n n o r m a l VCO f r e q u e n c y f o r t h e b r i d g e d s p e c i e s , a s a n a l t e r n a t i v e t o
my s u g g e s t i o n t h a t i t m i g h t b e c a u s e d b y a somewhat u n s y m m e t r i c a l l y b o n d e d b r i d g e d s p e c i e s on a s t e p p e d Pd s u r f a c e . Pr. G.C.
Bond ( B r u n e l U n i v e r s i t y ,
U x b r i d g e , UK)
What was t h e s i z e o f t h e p a l l a d i u m p a r t i c l e s i n y o u r c a t a l y s t ? I would g u e s s r a t h e r l a r g e , i f you c o u l d d e t e c t t h e i r s h a p e .
249 Pr. N.
Sheppard
I would e s t i m a t e 50-250 A a l t h o u g h t h e r e was a p r o p o r t i o n o f s m a l l o n e s J.D.F.
Dr.
Ramsay ( C h e m i s t r y D i v i s i o n , AERE, H a r w e l l , UK)
I t may b e of i n t e r e s t t o m e n t i o n some i n f r a r e d s p e c t r o s c o p i c m e a s u r e m e n t s o f CO a n d C O a d s o r b e d on o x i d e powders w h i c h w e r e c a r r i e d o u t b y D r . Gregg 2 a n d m y s e l f n e a r l y t w e n t y y e a r s a g o ( 1 , 2 , 3 ) and w h i c h showed s i m i l a r f e a t u r e s t o t h o s e you h a v e d e s c r i b e d . We f o u n d t h a t when C 0 2 was a d s o r b e d o n t o a dehyd r o x y l a t e d MgO, o u t g a s s e d a t h i g h t e m p e r a t u r e , t h e r e was a n i m m e d i a t e a p p e a r a n c e o f a band d u e t o p h y s i s o r b e d C02 which was f o l l o w e d b y t h e f o r m a t i o n o f a p a i r of bands, ascribed t o a b i d e n t a t e s p e c i e s ( F i g . ] ) .
Frequency / cm-l 1500
1150 100
1250 I
1610
IT
$
1320
\
1 2 r
c
-0 + L a
z6C
D
4
I
Fig.
I.
On p r o l o n g e d e x p o s u r e t o C02 ( s e v e r a l d a y s ) t h e l a t t e r b a n d s merged t o g i v e a b r o a d b a n d a s c r i b e d t o CO C03'-
3
2-
.
i n a s y m m e t r i c e n v i r o n m e n t ; t h e f o r m a t i o n of t h i s
band w a s a c c e l e r a t e d by h e a t i n g i n C02 a t
100°C.
On e x p o s i n g MgO t o CO a t a m b i e n t t e m p e r a t u r e v e r y weak b a n d s a s c r i b e d t o t h e b i d e n t a t e s p e c i e s were e v e n t u a l l y a p p a r e n t a f t e r a d a y o r m o r e . T h e s e b a n d s i n t e n s i f i e d o n h e a t i n g i n CO (100°-2000C)( I ) .
The m e c h a n i s m o f CO c h e m i s o r p t i o n
was not c l e a r a t t h e t i m e of these experiments b u t your suggestion t h a t s i t e s a t e d g e s a n d c o r n e r s o f MgO c r y s t a l l i t e s a r e i n v o l v e d i s i n t e r e s t i n g . T h i s
250
mechanism could perhaps be investigated further by studying the effect of MgO crystallite size and the effect of compaction of the powder - which would possibly introduce more active sites due to fracture or deformation. (1)
J.D.F. Ramsay, Ph.D Thesis, Exeter University, 1965.
(2) S . J . Gregg and J.D.F. Ramsay, J. Phys. Chem., 73, 1243 (1969). ( 3 ) S . J . Gregg and J.D.F. Ramsay, J. Chem. Soc;(A), 2784 (1970).
Pr. N. Sheppard
All I can say is that our experiments reporting on the one hand the oxic'.atic;n of CO to carbonate o n the MgO surface, and on the other hand absorptions tentatively attributed to 02
, were made under the opposite experimental conditions.
The first phenomenon was observed on MgO which had been treated with H
2 to remove any surface oxygen species ; the second was observed after the MgO had
been heated in oxygen at high temperatures and cooled to room temperature in oxygen in order to maximise surface oxygen.
SOME IUFPAPED SPETTQOSCnPIC S T U n I F S OF THE ADSORPTInhl OF POLECULES
247
Oh'
WqO
SURFACFS AND PgO-SUPPnRTED PPLLADIUP CATALYSTS C.C.
CHU and N. SHFPPARD
School of Chemical Sciences, University of Fast Pnqlia, blorwich
hlR4,
7TJ, U K
ABSTRACT
Infrared sriectroscooic studies have been made of the adsorption of C02, C O a n d O2 on finely-divided magnesium oxide. The MgO samoles were prepared by decomnosing Mg(OH), a n d heatinq in vacuum a t 5Oo0C o r 8n0°C respectively. After preparation a t each temperature the adsorption of C02 and f n was measured a f t e r s t r a i g h t evacuation, o r a f t e r treatment of the MgO a t these temperatures in oxygen ( t o qive a n oxygen-rich surface) or in hydroaen ( t o qive an oxyaen-free s u r f a c e ) . The adsorntion of Ul2 occurred very s t r o n o l y , seemingly on faces as well as A t d i f f e r e n t staqes of adsorotion edqes o r corners of t h e WqO c r y s t a l 1 i t e s . bidentate and unidentate surface carbonate species aave senarately i d e n t i f i a b l e difference s n e c t r a . On surfaces w i t h OH aroutx ( a s nrepared a t 500°C o r a f t e r hydroqen treatment a t 80OoC) snectra from surface bicarbonate ions were c l e a r l y observed. C O adsorbed, as expected, t o a much smaller extent than C02 on the WgO samoles. Q u a n t i t a t i v e adsorption measurements suggested t h a t bidentate carbonate ions were formed from CO by i n t e r a c t i o n mainly with edoe o r corner s i t e s on t h e c r y s t a l l i t e s . I t i s sneculated t h a t these a r e t h e oxidising s i t e s described i n the 1 i t e r a t u r e as 0-. Adsorntion of n2 on FlgO gave weak h u t c l e a r absorptions a t 1130, 1085 and 1065 cm-', t h e f i r s t of these bands (ca. - 5 % absorotion) being t h e s t r o n g e s t . These bands can reasonably be a t t r i b u t e d t o t h e nresence of i o n i c 0; species on t h e surface. Palladium c a t a l y s t s were prepared sunported on Pg0 ( a ) because o f the w,ider infrared transparency of MgO compared w i t h Si02 o r A1203 normally used a s sunports and ( 6 ) because of t h e p o s s i b i l i t y of e p i t a x i a l qrowth o f Pd c r y s t a l l i t e s on t h e cube faces of MgO. Adsorntion o f CO on Pd gave p r i n c i a a l l y a much sharner-than-normal infrared band from the 2-fold bridged 1 s p e c i e s , between 2000 and 1990 cm- . T h i s observation sunported the idea of uniform surface s i t e s t h a t have resulted from e p i t a x i a l growth. However the r e l a t i v e weakness of t h e CO adsorntions implied t h e Dresence of r a t h e r l a r g e Pd c r y s t a l 1i t e s corresnondinq to a 1 ow-area metal c a t a l y s t .
248
DISCUSSION S t o n e ( U n i v e r s i t y of B a t h , U K )
P r . F.S.
The f o r m a t i o n o f c a r b o n a t e ions a s a r e s u l t o f CO i n t e r a c t i n g w i t h MgO d o e s n o t n e c e s s a r i l y i m p l y t h a t o x i d i z i n g s i t e s a r e p r e s e n t . One may h a v e a d i s p r o p o r t i o n a t i o n r e a c t i o n , w h e r e b y d i m e r i c o r p o l y m e r i c CO c a r b a n i o n s a r e formed simultaneously with carbonate ion formation ( 1 , Z )
by t h e o v e r a l l r e a c t i o n :
The c a r b a n i o n f o r m a t i o n i s l e s s e a s i l y d e t e c t e d b y I R t h a n t h e c a r b o n a t e , b u t
i s r e a d i l y o b s e r v e d b y UV r e f l e c t a n c e ( I ) .
Low-coordinate
s i t e s ( e . g k i n k and
c o r n e r s i t e s ) a r e c e r t a i n l y n e e d e d , b u t more b e c a u s e o f t h e d e m a n d i n g n a t u r e of t h e c a r b a n i o n f o r m a t i o n t h a n t h e accompanying c a r b o n a t e f o r m a t i o n . ( I ) A . Z e c c h i n a and F . S .
( 2 ) E. G u g l i e l m i n o t t i , J.C.S.
P r . N.
Stone, J.C.S.
Faraday I,
1978,
74,
2278.
S. C o l u c c i a , E . G a r r o n e . L. C e r r u t i a n d A. Z e c c h i n a ,
F a r a d a y I , 1979,
75,96.
S h e p p a r d ( U n i v e r s i t y o f E a s t A n g l i a , N o r w i c h , UK)
Your s u g g e s t i o n t h a t t h e o x i d a t i o n of C O t o c a r b o n a t e i o n s o n a n "oxygenf r e e " MgO s u r f a c e g o e s w i t h a p a r a l l e l r e d u c t i o n o f C O t o s u r f a c e - s p e c i e s t h e t y p e (CO);
of
i s a n i n t e r e s t i n g one. A l l I c a n s a y i s t h a t w e were, of c o u r s e ,
aware o f t h e e x i s t e n c e of s u c h n e g a t i v e l y c h a r g e d s p e c i e s and t h a t o u r e x p e r i m e n t s were u n d e r low CO p r e s s u r e s w h e r e t h e s e were n o t s p e c t r o s c o p i c a l l y i d e n t i f ied.
P r . J.H. L u n s f o r d ( T e x a s A & M U n i v e r s i t y , C o l l e g e S t a t i o n , USA) The c a t a l y t i c p r o p e r t i e s o f s u p p o r t e d p a l l a d i u m i n m e t h a n o l s y n t h e s i s a r e o f i n t e r e s t i n s e v e r a l l a b o r a t o r i e s . P r o f e s s o r P o n e c h a s s u g g e s t e d t h a t Pd
s+
.
is
a n e s s e n t i a l component o f t h i s c a t a l y s t . Do you b e l i e v e t h a t t h e I R b a n d a t ca.
2,000 cm-I
may r e s u l t f r o m CO a d s o r b e d o n a p a r t i a l l y o x i d i z e d f o r m o f Pd r a t h e r
t h a n on a h i g h - i n d e x
plane ?
P r . N. S h e p p a r d
I a g r e e t h a t a d e g r e e o f p o s i t i v e c h a r g e o n Pd a t o m s c o u l d b e a c a u s e o f a h i g h e r t h a n n o r m a l VCO f r e q u e n c y f o r t h e b r i d g e d s p e c i e s , a s a n a l t e r n a t i v e t o
my s u g g e s t i o n t h a t i t m i g h t b e c a u s e d b y a somewhat u n s y m m e t r i c a l l y b o n d e d b r i d g e d s p e c i e s on a s t e p p e d Pd s u r f a c e . Pr. G.C.
Bond ( B r u n e l U n i v e r s i t y ,
U x b r i d g e , UK)
What was t h e s i z e o f t h e p a l l a d i u m p a r t i c l e s i n y o u r c a t a l y s t ? I would g u e s s r a t h e r l a r g e , i f you c o u l d d e t e c t t h e i r s h a p e .
249 Pr. N.
Sheppard
I would e s t i m a t e 50-250 A a l t h o u g h t h e r e was a p r o p o r t i o n o f s m a l l o n e s J.D.F.
Dr.
Ramsay ( C h e m i s t r y D i v i s i o n , AERE, H a r w e l l , UK)
I t may b e of i n t e r e s t t o m e n t i o n some i n f r a r e d s p e c t r o s c o p i c m e a s u r e m e n t s o f CO a n d C O a d s o r b e d on o x i d e powders w h i c h w e r e c a r r i e d o u t b y D r . Gregg 2 a n d m y s e l f n e a r l y t w e n t y y e a r s a g o ( 1 , 2 , 3 ) and w h i c h showed s i m i l a r f e a t u r e s t o t h o s e you h a v e d e s c r i b e d . We f o u n d t h a t when C 0 2 was a d s o r b e d o n t o a dehyd r o x y l a t e d MgO, o u t g a s s e d a t h i g h t e m p e r a t u r e , t h e r e was a n i m m e d i a t e a p p e a r a n c e o f a band d u e t o p h y s i s o r b e d C02 which was f o l l o w e d b y t h e f o r m a t i o n o f a p a i r of bands, ascribed t o a b i d e n t a t e s p e c i e s ( F i g . ] ) .
Frequency / cm-l 1500
1150 100
1250 I
1610
IT
$
1320
\
1 2 r
c
-0 + L a
z6C
D
4
I
Fig.
I.
On p r o l o n g e d e x p o s u r e t o C02 ( s e v e r a l d a y s ) t h e l a t t e r b a n d s merged t o g i v e a b r o a d b a n d a s c r i b e d t o CO C03'-
3
2-
.
i n a s y m m e t r i c e n v i r o n m e n t ; t h e f o r m a t i o n of t h i s
band w a s a c c e l e r a t e d by h e a t i n g i n C02 a t
100°C.
On e x p o s i n g MgO t o CO a t a m b i e n t t e m p e r a t u r e v e r y weak b a n d s a s c r i b e d t o t h e b i d e n t a t e s p e c i e s were e v e n t u a l l y a p p a r e n t a f t e r a d a y o r m o r e . T h e s e b a n d s i n t e n s i f i e d o n h e a t i n g i n CO (100°-2000C)( I ) .
The m e c h a n i s m o f CO c h e m i s o r p t i o n
was not c l e a r a t t h e t i m e of these experiments b u t your suggestion t h a t s i t e s a t e d g e s a n d c o r n e r s o f MgO c r y s t a l l i t e s a r e i n v o l v e d i s i n t e r e s t i n g . T h i s
250
mechanism could perhaps be investigated further by studying the effect of MgO crystallite size and the effect of compaction of the powder - which would possibly introduce more active sites due to fracture or deformation. (1)
J.D.F. Ramsay, Ph.D Thesis, Exeter University, 1965.
(2) S . J . Gregg and J.D.F. Ramsay, J. Phys. Chem., 73, 1243 (1969). ( 3 ) S . J . Gregg and J.D.F. Ramsay, J. Chem. Soc;(A), 2784 (1970).
Pr. N. Sheppard
All I can say is that our experiments reporting on the one hand the oxic'.atic;n of CO to carbonate o n the MgO surface, and on the other hand absorptions tentatively attributed to 02
, were made under the opposite experimental conditions.
The first phenomenon was observed on MgO which had been treated with H
2 to remove any surface oxygen species ; the second was observed after the MgO had
been heated in oxygen at high temperatures and cooled to room temperature in oxygen in order to maximise surface oxygen.