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UV Diffuse Reflectance Spectra of Ammonia Adsorbed on Alkaline Earth Oxides And Analogy with Charge-Transfer-To-Solvent Spectra
M . Che and G.C. Bond (Editors), Adsorption and Catalysis on Oxide Surfaces 0 1985 Elsevier Sciencc Publishers B.V., Amsterdam -Printed in The Netherlands
97
UV DIFFUSE R EFLEC TANCE SPECTRA OF AMMONIA ADSORBED O N A L K A L I N E EARTH OXIDES A N D
A N A L O G Y WITH CHARGE-TRANSFER-TO-3OLVENT S P E C T R A
E . G A R R O N E ' and F. S. STONEL I
I n s t i t u t e of Physical Chemistry, U n i v e r s i t y of T u r i n , I t a l y
2School of Chemistry, U n i v e r s i t y of Bath, England
ABSTRACT Ammonia a d s o r p t i o n on w e l l - o u t g a s s e d MgO, CaO and SrO g e n e r a t e s U V bands which extend t h e a b s o r p t i o n edge due t o e x c i t a t i o n a t s u r f a c e i o n s from -40,000 cm-l(MgO), 35,000 cm-l(Ca0) and 32,000 c m l l ( S r 0 ) t o 25,000 cm-l. The e f f e c t s o f d i s s o c i a t i v e a d s o r p t i o n , producing M2+-NH2 (and O H - ) s p e c i e s a t l o w - c o o r d i n a t e M2+-02- s i t e s , a r e d i s t i n g u i s h a b l e from t h o s e of n o n - d i s s o c i a t i v e r e v e r s i b l e a d s o r p t i o n . The l a t t e r produces bands e x t e n d i n g t o 25,000 cm-1 and t h e s e a r e a s c r i b e d t o a s u r f a c e analogue of c h a r g e - t r a n s f e r - t o - s o l v e n t ( c t t s ) s p e c t r a , g e n e r a t e d a s NH3 molecules envelop NH; i o n s and t h e r e s i d u a l s u r f a c e 02- i o n s . INTRODUCTION S p e c t r o s c o p i c s t u d i e s of a d s o r p t i o n on a l k a l i n e e a r t h o x i d e s f e a t u r e d e x t e n s i v e l y i n t h e work o f Dr. Tench, and i n c l u d e d work p u b l i s h e d i n 1972 ( 1 , 2 ) on t h e a p p l i c a t i o n of i r s p e c t r o s c o p y t o N H 3 a d s o r p t i o n on MgO.
Coluccia e t a 1 . ( 3 )
took t h e s u b j e c t f u r t h e r some 10 y e a r s l a t e r , a g a i n using i r s p e c t r a , and we have now extended t h i s and r e l a t e d work ( 4 , 5 ) u s i n g UV r e f l e c t a n c e s p e c t r o s c o p y . Tench and G i l e s ( 1 ) showed t h a t the a d s o r p t i o n of N H 3 on o u t g a s s e d MgO a t room t e m p e r a t u r e was mainly r e v e r s i b l e , with a small f r a c t i o n ( < l o % ) irreversible.
T h e i r i r s t u d y concerned o n l y t h e r e v e r s i b l e a d s o r p t i o n , which
t h e y a s c r i b e d t o S p e c i e s ( i i ) below.
A weaker r e v e r s i b l e a d s o r p t i o n , found by
Tench ( 2 ) on h y d r o x y l a t e d MgO, was a t t r i b u t e d t o S p e c i e s ( i ) .
Coluccia e t a l .
( 3 ) were a b l e t o s t u d y a l s o t h e i r r e v e r s i b l e a d s o r p t i o n on o u t g a s s e d MgO, and t h e y i d e n t i f i e d two such forms ( S p e c i e s ( i i i ) and ( i v ) ) , the more s t r o n g l y held o f which was p o s t u l a t e d t o be d i s s o c i a t i v e l y chemisorbed ammonia i n v o l v i n g lowc o o r d i n a t e (3- o r 4- c o o r d i n a t e ) MgZtO2-
ion p a i r s .
T h u s t h e i r work has l e d
t o t h e view ( 3 ) t h a t ammonia a d s o r p t i o n on MgO l e a d s t o f o u r s p e c i e s , two r e v e r s i b l y and two i r r e v e r s i b l y h e l d a t room t e m p e r a t u r e , r e p r e s e n t a b l e i n o r d e r of i n c r e a s i n g a d s o r p t i o n s t r e n g t h a s f o l l o w s :
98
Coluccia e t a1.(3) a l s o investigated ammonia a d s o r p t i o n on CaO and S r O : t h e y
1
f o u n d t h e same s p e c i e s , b u t i n d i f f e r e n t proportions.
S p e c i e s ( i i i ) was d e s o r b -
a b l e a t room t e m p e r a t u r e i n t h e s e c a s e s , and S p e c i e s ( i v ) was more p r e v a l e n t on
R:,
CaO t h a n on MgO. I n p a r a l l e l w i t h work on a l k a l i n e e a r t h o x i d e s i n H a r w e l l and T u r i n ,
0
associated studies using UV-visible
I
d i f f u s e r e f l e c t a n c e s p e c t r o s c o p y were i n progress i n Bath (4-6).
UV r e f l e c t a n c e
shows t h e p r e s e n c e o f e x c i t o n i c a b s o r p t i o n i n well-outgassed m i c r o c r y s t a l l i n e I
1
I
60
40
-3-
10
MgO, CaO and S r O w h i c h i s a t t r i b u t a b l e
20
t o t h e surface, s p e c i f i c a l l y t o low-
I)lcm-'
F i g . 1 . R e f l e c t a n c e s p e c t r a o f MgO, CaO and S r O , showing e x c i t o n a b s o r p t i o n ( I , 11, 1 1 1 ) due t o s u r f a c e i o n s . The s o l i d s were o u t g a s s e d a t 800OC and s p e c t r a were measured u n d e r c o n d i t i o n s o f quenched f l u o r e s cence. The r e f l e c t a n c e s c a l e i s d i s placed v e r t i c a l l y i n order t o avoid o v e r l a p o f t h e s p e c t r a . The d o t t e d portions t o the l e f t o f the vertical dashed l i n e s a r e e x t r a p o l a t i o n s . ( A d a p t e d f r o m Garrone, Z e c c h i n a and Stone ( 5 ) )
c o o r d i n a t e (LC) s u r f a c e a n i o n s .
The
s p e c t r a ( F i g . 1 ) enable t h r e e absorpt i o n s ( s u r f a c e e x c i t o n s I, I 1 and 1 1 1 ) t o b e d i s c r i m i n a t e d , and t h e s e have been ascribed t o 0 ' -
i o n s i n 5-,
c o o r d i n a t i o n (0;;. ively (5).
0;;.
4- and 3-
0 23c), r e s p e c t -
Luminescence s t u d i e s b y
Tench and h i s c o l l e a g u e s ( 7 - 9 ) have a l s o p r o v i d e d v a l u a b l e i n f o r m a t i o n on t h e p r o p e r t i e s o f LC s u r f a c e i o n s .
A d s o r p t i o n of r e a c t i v e gases and v a p o u r s ' e r o d e s ' t h e s u r f a c e e x c i t o n s p e c t r a and may a l s o g e n e r a t e new bands.
The new bands may be due t o i n t e r n a l
t r a n s i t i o n s i n new a d s o r b e d s p e c i e s p e r se ( a s when c a r b a n i o n s a r e p r o d u c e d
( 4 ) ) o r t o t h e d e v e l o p m e n t o f c h a r g e t r a n s f e r between t h e s u r f a c e a n i o n s and t h e adsorbate.
The l a t t e r b e h a v i o u r was f i r s t i d e n t i f i e d i n T u r i n ( 1 0 ) i n
s t u d i e s o f t h e a d s o r p t i o n o f I 2 and CC14 on K I m i c r o c r y s t a l s . significantly,
More
however, t h e same work ( 1 0 ) h i g h l i g h t e d t h e f a c t t h a t i n t e r -
a c t i o n o f s u r f a c e I-i o n s w i t h a d s o r b a t e s c a n a l s o b e o f a y e t m i l d e r k i n d , where t h e e l e c t r o n t r a n s i t i o n s a r e t h o s e o f l o w - c o o r d i n a t e s u r f a c e I - i o n s whose e n v i r o n m e n t i s c o m p l e t e d b y ' s o l v a t i n g ' a d s o r b a t e m o l e c u l e s . surface analogue o f t h e 'charge-transfer-to-solvent'
This i s the
( c t t s ) s p e c t r a w h i c h have
been e x t e n s i v e l y s t u d i e d f o r a n i o n s ( e s p e c i a l l y t h e i o d i d e i o n ) i n s o l u t i o n chemistry (11).
I t i s i n t e r e s t i n g t o speculate whether t h e analogue o f c t t s
e f f e c t s m i g h t a l s o be observable f o r a d s o r p t i o n on a l k a l i n e e a r t h oxides. Ammonia i s t h e m o l e c u l e o f c h o i c e f o r such a t e s t .
I t g i v e s a marked s h i f t o f
99
the absorption edge of KI c r y s t a l s (10) a n d when used a solvent f o r I - ions i t produces a c t t s red s h i f t larger t h a n t h a t of any other molecule so f a r studied ( 1 1 , l Z ) . I t follows t h a t a U V reflectance study of ammonia adsorption on the a l k a l i n e earth oxides i s of i n t e r e s t f o r several reasons. There a r e b o t h the i r r e v e r s i b l e a n d r e v e r s i b l e e f f e c t s t o be s t u d i e d , and the a c t i v i t y of three oxides t o be compared (BaO has t o be excluded since i t cannot be prepared cleanly with s u f f i c i e n t l y high s p e c i f i c surface a r e a ) . The onward reaction of adsorbed ammonia with o t h e r gases i s a l s o of i n t e r e s t , a n d we s h a l l r e f e r b r i e f l y t o i n t e r a c t i o n with added oxygen a n d C O . EXPERIMENTAL
MgO, CaO and SrO powders were prepared as previously described (13,14). Typical s p e c i f i c surface areas a f t e r outgassing a t 8OO0C were 200 m2g-l (MgO), 100 rn’9-l (CaO) a n d 5 m2g-1(Sr0). The UV d i f f u s e r e f l e c t a n c e c e l l was of s i m i l a r design t o t h a t described by Zecchina e t a 1 . ( 1 3 ) , b u t modified with the addition of a side-tube ( 6 ) t o enable oxides t o be outgassed o u t of contact with the optical face. Diffuse reflectance spectra were measured from 20,000 t o 52,000 cm-’ using a Pye Unicam SP 700C spectrometer, with f i n e l y ground s i l i c a ( S u p r a s i l ) as the reference
material. Spectra determined in vacuo were subject t o an a r t e f a c t caused by fluorescence (luminescence), as discussed elsewhere ( 6 , 1 3 ) . The e f f e c t i s t o increase a r t i f i c i a l l y the reflectance value f o r the spectrum a t high values of v , b u t the positions of the absorption bands ( r e f l e c t a n c e minima) a r e n o t
s i g n i f i c a n t l y affected. efficiently.
Oxygen a t 1 Torr quenches the fluorescence very
The standard pretreatment before adsorption of ammonia was t o outgas the specimen a t 800°C f o r 16 h . All spectra were measured with the c e l l a t room temperature. Ammonia was purified by freeze-pump-thaw cycling. Oxygen and CO were high purity gases supplied by B.O.C. RESULTS
A. 1.
Adsorption and desorption of ammonia
Magnesium oxide The influence of ammonia on the reflectance spectrum i s shown in Fig. 2. The e f f e c t of very small doses of NH3 ( n o t shown) i s t o erode s l i g h t l y the shoulder
a t A A ’ (exciton I11 of Fig. 1 ) and t o extend the t a i l a t B B ’ . Further dosing begins t o quench the fluorescence and a l s o produces new absorption L (curve 2 ) ; a l l gas admitted so f a r i s t o t a l l y adsorbed. Beyond t h i s s t a g e , doses a r e
incompletely taken up, b u t absorption develops now in two regions ( M and N ) and
100
1
Rh 0.5
0 Fig. 2. Reflectance spectra of ammonia adsorbed on MgO. 1 - outgassed MgO, in vacuo. 1 ' - as 1 , b u t with fluorescence quenched by addition of 1 t o r r O2 (standard reference spectrum). 2 - with i r r e v e r s i b l y adsorbed ammonia. 3 - in 3 t o r r NH3. 4 - in 23 t o r r NH3.
Fig. 3. Reflectance spectra of ammonia adsorbed on CaO. 1 - outgassed CaO, with fluorescence q u e n c h e d (st a n d a r d r e f e r e n c e s p e c t r um). 2 - with i r r e v e r s i b l y adsorbed ammonia. 3 - in 2 t o r r NH3. 4 - in 45 t o r r NH3.
fluorescence becomes f u l l y quenched (curve 3 , 3 t o r r ) . Increasing P N H ~t o 23 t o r r (curve 4 ) has r e l a t i v e l y l e s s e f f e c t , b u t the M and N absorptions develop f u r t h e r (MI, N ' ) . Evacuation of residual ammonia a t room temperature f o r 30 min reproduces curve 2 , which therefore represents the spectrum of MgO carrying i r r e v e r s i b l y adsorbed ammo';lia. We may note t h a t absorption N ( a n d N ' ) i s t o be distinguished from absorption L: i t occurs a t higher V and i s more akin t o the exciton I1 absorption (Fig. 1 ) decreased i n i n t e n s i t y a n d s h i f t e d t o lower energy. 2.
Calcium oxide The e f f e c t of adsorbing ammonia i s t o quench fluorescence more readily t h a n
with MgO a n d t o develop very intense unstructured absorption (Fig. 3 ) on the low V s i d e of the original absorption edge. The spectral change approaches a
101
1
l i m i t ( t h e c u r v e f o r 45 t o r r ) as NH3
pressure i s increased.
Evacuation
l e a v e s a s p e c t r u i n due t o i r r e v e r s i b l y a d s o r b e d ammonia w h i c h i s c o r r e s p o n d i n g l y much more i n t e n s e
R:
t h a n w i t h MgO, and w h i c h i s s u f f i c i e n t i n e x t e n t t o quench f l u o r escence a l m o s t c o m p l e t e l y . 3.
Strontium oxide The r e s u l t s o f ammonia a d s o r p t i o n
a r e shown i n F i g . 4.
The s p e c i f i c
s u r f a c e a r e a i s a b o u t 40 t i m e s
0
50
30
40
-3
10 s/cm’
20
s m a l l e r t h a n t h a t o f t h e MgO sample, y e t s i m i l a r f e a t u r e s c a n be seen i n
V r e g i o n . There i s absorpt i o n o f t y p e L ( a l t h o u g h much
t h e low
F i g . 4. R e f l e c t a n c e s p e c t r a o f ammonia adsorbed on S r O . 1 - outgassed S r O , i n vacuo. 1 ’ - as 1, b u t w i t h f l u o r escence quenched. 2 - w i t h i r r e v e r s i b l y a d s o r b e d ammonia. 3 - i n 3 t o r r NH3. 4 - i n 40 t o r r NH3.
n a r r o w e r t h a n on MgO) and a l s o a b s o r p t i o n o f t y p e s M ( M ’ ) and N ( N ’ ) . The n o v e l f e a t u r e w i t h S r O i s t h a t the exciton I region (cf. Fig. 1)
c a n be seen, and t h e e f f e c t o f ammonia a d s o r p t i o n a t 40 t o r r i s t o p r o d u c e a b l u e s h i f t ( t h e band a t 38,000
c m - l i n t h e vacuum s p e c t r u m i s d e s t r o y e d and a d d i t i o n a l a b s o r p t i o n a p p e a r s a t 41,000-43,000 cm- 1 ) . The i r r e v e r s i b l y
a d s o r b e d ammonia i s i n s u f f i c i e n t i n amount t o quench t h e f l u o r e s c e n c e c o m p l e t e l y ( c f . MgO).
B.
Interaction o f
O2 and CO w i t h i r r e v e r s i b l y a d s o r b e d ammonia
Room t e m p e r a t u r e e v a c u a t i o n a f t e r ammonia a d s o r p t i o n l e a v e s s i g n i f i c a n t amounts o f i r r e v e r s i b l y a d s o r b e d ammonia i n t h e c a s e o f MgO and CaO, as j u d g e d b y t h e s p e c t r a i n F i g s . 2 and 3.
The e f f e c t s o f oxygen and o f CO on t h e
s p e c t r u m o f i r r e v e r s i b l y - h e l d ammonia was examined f o r t h e s e o x i d e s . With MgO t h e e f f e c t o f oxygen on t h e s p e c t r u m shown i n c u r v e 2 o f F i g . 2 was
t o c o m p l e t e t h e q u e n c h i n g o f t h e f l u o r e s c e n c e and t o g i v e a s m a l l e r o s i o n o f t h e edge o f a b s o r p t i o n
L a t 33,000-38,000
cm-’.
W i t h CaO t h e r e was a l a r g e r e f f e c t
o n t h e edge, t h e d i s p l a c e m e n t t o h i g h e r V b e i n g c o n s t a n t o v e r t h e r e g i o n 30,000-33,000
cm-’
(Fig. 5).
Below 30,000 cm-l t h e r e was a modest i n c r e a s e i n
a b s o r p t i o n commensurate w i t h t h e f o r m a t i o n o f a s m a l l amount o f 07 ( a s i s f o u n d w i t h o u t g a s s e d CaO exposed t o oxygen ( 1 3 ) . The e f f e c t o f CO o n i r r e v e r s i b l y a d s o r b e d ammonia o n MgO i s shown i n F i g . 6. The r e s i d u a l f l u o r e s c e n c e i s a g a i n quenched and t h e a b s o r p t i o n L ( F i g . 2 ) i s i n
102
t h i s case strongly eroded. Evacuation of the unreacted CO leads t o f u r t h e r erosion a n d t o a fluorescence g r e a t e r t h a n before C O contact. I t i s evident t h a t CO has reacted with i r r e v e r s i b l y adsorbed ammonia. I f CO i s admitted a t higher pressure and l e f t in contact f o r several hours, the (CO)l(.- carbanion spectrum c h a r a c t e r i s t i c of CO on clean MgO ( 4 ) i s weakly developed. With CaO, there i s a s i m i l a r erosion of t h e ammonia edge t o t h a t shown in Fig. 6 , b u t the change i s much more rapid t h a n with MgO and no (CO)l(.- carbanion spectrum develops on long-term contact i n t h i s case.
I
Rh
I
I
R:
_1/ c h 40 30
I
I
-3
1 O S/cm-’
Fig. 5. CaO reflectance s p e c t r a , showing t h e e f f e c t of oxygen on i r r e v e r s i b l y adsorbed ammonia. 1 - outgassed CaO, contacted with ammonia and evacuated f o r 20 h a t room temperature. 2 - a f t e r dosing oxygen a t 4 t o r r . C.
I
I
0
40
I
I
30
I
-3
10q/cm-’
2(
Fig. 6 . MgO reflectance s p e c t r a , showing the e f f e c t o f CO on i r r e v e r s i b l y adsorbed ammonia. 1 - with i r r e v e r s i b l y adsorbed ammonia. 2 - in 12 t o r r C O , immediately. 3 - in 20 torr C O , a f t e r 40 minutes. 4 ,- in 20 t o r r C O , a f t e r 5 hours.
Triethylamine adsorption on MgO
The e f f e c t o f (C2H5)3N adsorption on the spectrum o f outgassed MgO i s shownin Fig. 7 . The r e s u l t i s t o produce a blue s h i f t of both exciton I11 and exciton 11. The e f f e c t was completely r e v e r s i b l e . The s i g n i f i c a n c e of t h i s experiment i s t h a t i t discriminates uniquely the e f f e c t of nitrogen lone p a i r i n t e r a c t i o n with Mg2+ ions, i . e . Species ( i i i ) c i t e d in the Introduction.
103 DISCUSSION
A.
S p e c t r a l changes due t o i r r e v e r s i b l y adsorbed ammonia F i g s . 2 , 3 and 4 show t h a t w i t h a l l t h r e e o x i d e s ammonia adsorbed i r r e v e r s i b l y
a t room temperature generates d i s t i n c t i v e a b s o r p t i o n ( d e s i g n a t e d as L ) on t h e low-energy ( l o w
i)
s i d e o f t h e a b s o r p t i o n edge.
The e f f e c t i s g r e a t e r w i t h CaO
t h a n w i t h MgO s o t h a t , b e a r i n g i n mind t h e i r r e s u l t s ( 3 ) a l r e a d y mentioned, i t can be c o n f i d e n t l y a s c r i b e d t o d i s s o c i a t i v e a d s o r p t i o n and t h e p r o d u c t i o n o f Species ( i v ) i l l u s t r a t e d i n t h e I n t r o d u c t i o n . A b s o r p t i o n L c o u l d be due
a priori
t o OH-,
t o NH;,
o r t o an e f f e c t on t h e
e x c i t a t i o n o f c o n t i g u o u s 0;; s u r f a c e i o n s such t h a t e x c i t o n I 1 i s s h i f t e d t o l o w e r energy.
OH- as t h e e x p l a n a t i o n can be r e a d i l y r u l e d o u t , s i n c e i t s form-
a t i o n s h i f t s a b s o r p t i o n t o h i g h e r energy, as i s shown by experiments w i t h w a t e r vapour ( 1 3 ) .
D i s t i n c t i o n between t h e two o t h e r p o s s i b i l i t i e s i s more
d i f f i c u l t , b u t we d e c i d e i n f a v o u r o f a b s o r p t i o n L b e i n g due t o e x c i t a t i o n o f t h e e l e c t r o n l o c a l i z e d on NH;. shown i n F i g s . 5 and 6.
One reason i s t h e marked e f f e c t o f O2 and CO
Those changes a r e more r e a d i l y understood as e f f e c t s on e x c i t a t i o n a t NH- t h a n on e x c i t a t i o n 2 i n v o l v i n g 0;;. For instance, i n f r a r e d experiments ( 1 4 ) show t h a t carbon mono x i d e r e a c t s w i t h NH;
on MgO.
As f a r
as O2 i s concerned, 02; ( e x c i t o n 1 1 ) i s known t o be s t a b l e towards oxygen ( 1 3 ) . We r e f e r l a t e r t o a n o t h e r reason why we b e l i e v e a b s o r p t i o n L t o be due t o NH; p e r se. Absorption L i s a t approximately 39,000 cm-'
i n MgO, 35,000 cm-l i n CaO
and 30,000 cm-l i n S r O .
The decrease
a l o n g t h e s e r i e s occurs f o r e s s e n t i a l l y t h e same reason as the c o r r e s p o n d i n g decrease i n band gap e n e r g i e s and t h e respective energies o f excitons I , I 1 and I11 ( F i g . 1 ) .
It i s a manifesta-
t i o n o f the increasing radius o f the cation. F i g . 7. R e f l e c t a n c e s p e c t r a o f t r i e t h y l a m i n e (TEA) adsorbed on MgO. 1 - i n TEA a t 0.7 t o r r ( i d e n t i c a l w i t h r e f e r e n c e spectrum o f outgassed MgO ( F i g . 2, c u r v e 1 ' ) . 2 - i n TEA a t 1 t o r r . 3 - i n TEA a t 36 t o r r .
Since t h e e x c i t a t i o n i n v o l v e s
some charge t r a n s f e r between NH; and t h e c a t i o n on which i t i s adsorbed, i t
is i n f l u e n c e d by t h e size o f t h e cation.
104
B.
R e a c t i o n s o f O2 and C C w i t h i r r e v e r s i b l y - a d s-__ o r b e d ammonia I n f r a - r e d work ( 3 ) has shown t h a t d i s s o c i a t i v e a d s o r p t i o n o f ammonia o n MgO
i s r e s t r i c t e d t o a v e r y s m a l l f r a c t i o n o f t h e s u r f a c e and i s o r o b a b l y a s s o c i a t e d
2+ 2-
o n l y w i t h s i t e s o f l o w e s t c o o r d i n a t i o n , v i z . Mg3C03C.
The m a j o r i t y o f t h e
ammonia h e l d i r r e v e r s i b l y o n HgO i s t h e n o n - d i s s o c i a t e d S p e c i e s ( i i i ) .
WithCaO,
o n t h e o t h e r hand, d i s s o c i a t i v e a d s o r p t i o n ( S p e c i e s ( i v ) ) i s much more p r e v a l e n t , commensurate w i t h a l l 4C ( a s w e l l as 3C) s i t e s b e i n g i n v o l v e d .
There i s no
e v i d e n c e w i t h CaO t h a t i r r e v e r s i b l y - h e l d gas i n c l u d e s any S p e c i e s ( i i i ) . The r e s u l t s w i t h oxygen and CO a r e c o n s i s t e n t w i t h t h e s e d i s t i n c t i o n s . 0 2 d o e s n o t i n t e r a c t w i t h a d s o r b e d NH3, b u t o n l y w i t h NH;. o f t h e NH; of NH;
The e f f e c t i s t h u s e r o s i o n
edge ( a b s o r p t i o n L ) , g r e a t e r w i t h CaO ( F i g . 5 ) w i t h i t s h i g h c o v e r a g e
t h a n w i t h MgO.
ESR (02 + NH;
=
T h i s i s accompanied by 0; f o r m a t i o n , as c a n be shown b y
0; + N H 2 ’ ) ( 1 4 ) .
The onward r e a c t i o n s o f amino r a d i c a l s w i t h
t h e m s e l v e s o r w i t h O2 do n o t change t h e UV s p e c t r u m s i n c e t h e y y i e l d p r o d u c t s l i k e l y t o absorb o n l y i n t h e f a r UV. CO g i v e s a marked e f f e c t w i t h b o t h MgO a n d CaO.
r e a c t i o n w i t h NH;
We i n t e r p r e t t h i s as a
t o f o r m s p e c i e s such as (HCONH)-(14).
( F i g . 6 ) i s s l o w o n MgO and f a s t o n CaO.
As Mg:
The s p e c t r a l change
s i t e s become c l e a r e d o f NH;
on
MgO, NH3 f r o m t h e r e s e r v e o f S p e c i e s ( i i i ) s l o w l y d i s s o c i a t e s and r e a c t s . R e a c t i o n o f CO w i t h b a r e 0;;
ions t o form polymeric CO carbanions (15) i s
s u p p r e s s e d , e s p e c i a l l y o n CaO. C.
S p e c t r a a s s o c i a t e d w i t h r e v e r s i b l y - a d s o r b e d ammonia Pronounced e f f e c t s o n t h e r e f l e c t a n c e s p e c t r a w e r e o b s e r v e d when ammonia was
p r o g r e s s i v e l y dosed a t p r e s s u r e s where t h e o n l y changes i n a d s o r p t i o n were t h o s e o f i n c r e a s i n g t h e c o v e r a g e o f p h y s i c a l l y - a d s o r b e d ammonia ( F i g s . 2 - 4 ) .
Large
changes w e r e o b s e r v e d even i n t h e c a s e o f t h e l o w s u r f a c e a r e a S r O , w h i c h i n d i c a t e d t h a t a l l s u r f a c e i o n s (3C, 4C and 5C) were p a r t i c i p a t i n g . Two d i f f e r e n t k i n d s o f new a b s o r p t i o n c a n b e c l e a r l y d i a g n o s e d i n t h e c a s e o f MgO ( F i g . 2 ) , and t h e s e a r e d e s i g n a t e d M and N r e s p e c t i v e l y .
The M a b s o r p t i o n
( t o b e r e g a r d e d a l s o as i n c l u d i n g M ‘ as p r e s s u r e i s f u r t h e r i n c r e a s e d ) i s a band a t v e r y l o w e n e r g y g i v i n g a w e l l - d e v e l o p e d s h o u l d e r a t ca. 32000 cm-’.
The N
a b s o r p t i o n ( t o b e r e g a r d e d a l s o as e m b r a c i n g N ’ ) i s a band a t a n e n e r g y c l o s e t o t h a t o f t h e a b s o r p t i o n edge o f t h e c l e a n s u r f a c e .
T h i s band has t h e c h a r a c t e r
o f e x c i t o n I1 a b s o r p t i o n b u t s h i f t e d t o l o w e r e n e r g y and i n c r e a s i n g l y b r o a d e n e d
with i n c r e a s i n g pressure.
The same t w o f e a t u r e s c a n be d i s c r i m i n a t e d i n t h e S r O
s p e c t r u m f o r 3 t o r r ( F i g . 4) and t h e r e i s a weak b u t d i s c e r n a b l e r i p p l e i n t h e i n d i c a t i v e o f a n a d d i t i o n a l s h o u l d e r . An
40 t o r r M ’ s p e c t r u m a t 28,000 cm-’
important point i s t h a t the
M and N a b s o r p t i o n s a r e d i s t i n c t f r o m t h e L
absorption a l r e a d y discussed.
They a r e a t l o w e r and h i g h e r
V,
respectively,
105
M and N r e g i o n s a r e presumed t o e x i s t
and a r e a l s o a p p r e c i a b l y more i n t e n s e .
a l s o i n t h e CaO s p e c t r u m , b u t t h e a b s o r p t i o n i n t e n s i t y i s so g r e a t t h a t t h e y c a n n o t be d i s t i n g u i s h e d . The M and N a b s o r p t i o n s have t h e h a l l m a r k s o f charge-transfer-to-solvent
(G) spectra,
where p h y s i c a l l y a d s o r b e d NH3 m o l e c u l e s a r e s i m u l a t i n g t h e
effect o f a solvent.
J u s t as
I- i o n s d i s s o l v e d i n l i q u i d ammonia e x h i b i t an
i n t e n s e r e d - s h i f t e d s p e c t r u m i n t h e UV ( l l ) , so i n t h e p r e s e n t c a s e we p r o p o s e t h a t m o b i l e a d s o r b e d NH3 m o l e c u l e s c o m p l e t e t h e envelooment o f a n i o n s exposed a t t h e o x i d e c r y s t a l l i t e s u r f a c e s and p r o d u c e a r e d - s h i f t e d a b s o r p t i o n .
The
e f f e c t s shown i n F i g s . 2 - 4 a r e l a r g e , b o t h i n i n t e n s i t y and i n t h e m a g n i t u d e o f t h e r e d s h i f t (more t h a n 5000 cm-’
i n t h e c a s e o f MgO).
The r e s u l t s a r e com-
p l e m e n t a r y t o t h o s e r e p o r t e d b y C h i o r i n o e t a1 ( 1 0 ) f o r a d s o r p t i o n on K I .
In
t h a t s t u d y t h e same s o l i d ( K I ) was exposed t o v a r i o u s v a p o u r s and t h e m a g n i t u d e s ( a n d d i r e c t i o n s ) o f t h e s p e c t r a l s h i f t s were compared w i t h t h e e f f e c t s o f t h o s e same m o l e c u l e s as s o l v e n t s on I - i o n s i n s o l u t i o n .
Here we have t a k e n t h e
m o l e c u l e (NH3) known f r o m t h a t work ( 1 0 ) and s o l u t i o n s t u d i e s ( 1 1 ) t o show t h e g r e a t e s t c t t s r e d s h i f t a n d have s t u d i e d i t i n a d s o r p t i o n on t h r e e d i f f e r e n t solids. We must now d i s c u s s t h e a n i o n s i n v o l v e d . red-shifted i n unison w i t h absorption i s NH;
bound t o a c a t i o n .
The f a c t t h a t t h e M a b s o r p t i o n i s
L suggests t h a t t h e anion concerned t h e r e
The l a r g e m a g n i t u d e o f t h e s h i f t i s c o m p a t i b l e w i t h
t h e i o n b e i n g v e r y w e l l exposed, i . e . n o t o n l y p r o u d o f t h e s u r f a c e b u t a l s o a f f i x e d t o a l o w - c o o r d i n a t e 3C o r 4C i o n , and t h u s c a p a b l e o f b e i n g i n f l u e n c e d by s e v e r a l NH3 m o l e c u l e s s i m u l t a n e o u s l y .
The N a b s o r p t i o n , o n t h e o t h e r hand,
i s r e d - s h i f t e d f r o m e x c i t o n 11, and t h i s band i s t h e r e f o r e a s c r i b e d t o a c t t s s h i f t o f t h e ;0;
excitation.
These i o n s a r e r e g a r d e d as s u f f i c i e n t l y exposed t o
b e a f f e c t e d , and on MgO ( w h i c h shows a l a r g e e f f e c t ) t h e y a r e n o t s i g n i f i c a n t l y i n v o l v e d i n b i n d i n g NH;.
R e f e r r i n g back t o t h e f i r s t p a r t o f t h e D i s c u s s i o n ,
l e t us n o t e i n p a s s i n g t h a t t h e s i z e o f t h e c t t s s h i f t has h e l p e d u s t o d e c i d e i n favour o f absorption
L b e i n g a s c r i b e d t o NH; r a t h e r t h a n t o
.;0;
I t i s s i g n i f i c a n t t h a t t h e M a b s o r p t i o n i s n o t a t t h e same v a l u e o n a l l t h r e e
oxides.
The p r o p e r t i e s o f t h e u n d e r l y i n g o x i d e e x e r t a n i n f l u e n c e , t h e n a t u r e
o f s u r f a c e e x c i t o n i c and c t t s e f f e c t s b e i n g s u f f i c i e n t l y s i m i l a r t h a t a t r a n s i t i o n showing m i x e d c h a r a c t e r o c c u r s .
On S r O t h e
a p p r o a c h i n g t h e c t t s v a l u e o f 28,000 cm-l f o r f r e e NH; I t i s p o s s i b l e t h a t p a r t o f t h e l o w - 3 t a i l , e.g.
may b e t h e c t t s band o f ;0:
M
absorption
V
is
i o n s i n l i q u i d NH3 ( 1 1 ) .
t h e a r r o w e d s h o u l d e r i n F i g . 4,
( e x c i t o n 111).
The I - i o n i s c o n s i d e r e d ( 1 1 ) t o g i v e l a r g e s o l u t i o n c t t s e f f e c t s because e x c i t a t i o n of
I- l e a d s e a s i l y t o i o n i z a t i o n ( I -
=
I t e).
h e r e w i t h t h e s u r f a c e analogue o f c t t s on o u r oxides. r e a d i l y t r a n s f e r s an e l e c t r o n t o g i v e O;(ads)
There i s a p a r a l l e l
Thus NH;(ads)
on MgO
i n t h e p r e s e n c e o f oxygen, as
106
2shown by ESR ( 1 4 ) . Secondly, a s r e g a r d s 0 4 c , t h e p r o c e s s ; : 0 = 0- + e on a l k a l i n e e a r t h o x i d e s i s regarded a s i m p l i c a t e d i n (C0)'- carbanion f o r m a t i o n
2- n
ion i s more c o o r d i n a t e d with C O ( 1 5 ) . By t h e same t o k e n , when t h e s u r f a c e 0 i t i s l e s s e a s i l y i o n i z e d ; e q u a l l y i t no l o n g e r shows with N H 3 t h e analogue of c t t s . We f i n d no e v i d e n c e , f o r i n s t a n c e , t h a t e x c i t o n I (0;;) shows c t t s . Indeed, on SrO, where e x c i t o n I i s c l e a r l y o b s e r v a b l e , t h e e f f e c t of N H 3 i s t h e o p p o s i t e o f t h a t i n c t t s : N H 3 produces a b l u e s h i f t o f e x c i t o n I ( F i g . 4 ) . The b l u e s h i f t i s i n d i c a t i v e of S p e c i e s ( i i i ) . The key t o t h i s i s t h e e x p e r i ment w i t h (C2H5)3N, which g i v e s a b l u e s h i f t t o t h e e x c i t o n bands ( F i g . 7 ) . On o u t g a s s e d o x i d e t h i s molecule can o n l y a d s o r b a s S p e c i e s ( i i i ) . The i n f e r e n c e is t h a t ; : 0 i s t o o well s h i e l d e d t o engage s i g n i f i c a n t l y i n c t t s or t o a c t w i t h
2+ a s a s u f f i c i e n t l y b a s i c c e n t r e t o g i v e h e t e r o l y t i c d i s s o c i a t i o n of NH3. MSC 2t Adsorption o c c u r s i n s t e a d a s S p e c i e s ( i i i ) on MSC. The e f f e c t on the ion p a i r M$O$ i s l i k e t h a t of i n c r e a s i n g the Madelung p o t e n t i a l : hence t h e b l u e s h i f t .
To sum u p , t h e s u r f a c e analogue of t h e c h a r g e - t r a n s f e r - t o - s o l v e n t e f f e c t has now been shown f o r t h r e e a n i o n s , namely f o r ILc on KI ( 1 0 ) and f o r 0;; and NHi(ads) on MgO, CaO and SrO. I t s s t u d y i s an i n t e r e s t i n g new a p p l i c a t i o n o f
UV d i f f u s e r e f l e c t a n c e s p e c t r o s c o p y t o t h e s u r f a c e c h e m i s t r y of i o n i c c r y s t a l s . We acknowledge with much g r a t i t u d e t h e many s t i m u l a t i n g d i s c u s s i o n s we have had w i t h P r o f e s s o r Adriano Zecchina d u r i n g t h i s work. REFERENCE5 1 A. J . Tench and D. G i l e s , J . C . S. Faraday I , 68 (1972) 193. 2 A. J . Tench, J . C. S. Faraday I , 6 8 (1972) 197. 3 S. C o l u c c i a , E. Garrone and E. B o r e l l o , J . C. 5. Faraday 1 , 79 (1983) 607. 4 F. S. Stone and A. Zecchina, Proc. 6 t h I n t e r n . Congr. C a t a l y s i s (London 1 9 7 6 ) , p. 162, Chemical S o c i e t y , London, 1977. 5 E. Garrone, A. Zecchina and F. S. S t o n e , P h i l o s o p h . Mag., B , 42 (1980) 683. 6 F. S. S t o n e , i n ' S u r f a c e P r o p e r t i e s and C a t a l y s i s by Non-Metals' ( e d . 3. P. B o n e l l e , B . Delmon and E . Derouane), p. 237, R e i d e l , N e t h e r l a n d s , 1983. 7 5. C o l u c c i a , M. Deane and A. J . Tench, Proc. 6 t h I n t e r n . Congr. C a t a l y s i s (London 1 9 7 6 ) , Chemical S o c i e t y , London, p. 171. 8 S. C o l u c c i a , A. M. Deane and A. J . Tench, J . C . S . Faraday I , 74 (1978) 291 3. 9 W . W. Duley, Philosoph. Mag. B , 49 (1984) 159. 10 A. C h i o r i n o , E . Garrone, G. G h i o t t i and A. Zecchina, J . C . S . Faraday I , 78 ( 1 982) 857. 11 M. J . Blandamer and M. F. Fox, Chem. Rev. 70 (1970) 59. 12 I . Burak and A. Treinin, J . C. S. Faraday I , 59 (1963) 1490. 13 A. Zecchina, M. G. Lofthouse and F. S. S t o n e , J . C. S . Faraday I , 71 (1975) 1476. 14 E. Garrone and F. S . S t o n e , Proc. 8th I n t e r n . Congr. C a t a l y s i s ( B e r l i n 1984), Paper 111-441, Dechema, Frankfurt-am-Main, 1984. 15 A. Zecchina and F. S. S t o n e , J . C. S. Faraday I , 74 (1978) 2278.
107
0I SC U SS I OPI Professor K . Klier (Lehiqh University): ( a ) Do you have evidence f o r a n analogue of surface amide NH; t h a t w o u l d be formed by d i s s o c i a t i v e chemisorntion of mono- a n d dialkyl amines RNH2 and R2NH? (b)
Mill CO r e a c t with RNH- o r R2N- in a s i m i l a r fashion as you report f o r the CO + NH;
co +
reaction, i . e . NH;
+
HN=CH-O-,
over the Ma0 surface? Professor F.S. Stone (University of Bath):
( a ) \Ale do n o t have evidence f o r the formation of RNH- and R2N- from RNH2 and R2NH ( R = a l k y l ) because we have not studied them, b u t I would expect d i s s o c i a t i v e chemisorption t o occur in b o t h these cases;
the
a c i d i t i e s of NH3, W H p and R2NH a r e not g r e a t l y d i f f e r e n t . (b)
I would expect RNH- t o r e a c t s i m i l a r l y t o NH; already a t room temperature, b u t t h e reaction with R2N- i s l e s s easy t o p r e d i c t . alkyl migration t o produce R-N=CP-O- i s l i k e l y t o be a d i f f i c u l t reaction on the a l k a l i n e e a r t h oxides.
The
97
UV DIFFUSE R EFLEC TANCE SPECTRA OF AMMONIA ADSORBED O N A L K A L I N E EARTH OXIDES A N D
A N A L O G Y WITH CHARGE-TRANSFER-TO-3OLVENT S P E C T R A
E . G A R R O N E ' and F. S. STONEL I
I n s t i t u t e of Physical Chemistry, U n i v e r s i t y of T u r i n , I t a l y
2School of Chemistry, U n i v e r s i t y of Bath, England
ABSTRACT Ammonia a d s o r p t i o n on w e l l - o u t g a s s e d MgO, CaO and SrO g e n e r a t e s U V bands which extend t h e a b s o r p t i o n edge due t o e x c i t a t i o n a t s u r f a c e i o n s from -40,000 cm-l(MgO), 35,000 cm-l(Ca0) and 32,000 c m l l ( S r 0 ) t o 25,000 cm-l. The e f f e c t s o f d i s s o c i a t i v e a d s o r p t i o n , producing M2+-NH2 (and O H - ) s p e c i e s a t l o w - c o o r d i n a t e M2+-02- s i t e s , a r e d i s t i n g u i s h a b l e from t h o s e of n o n - d i s s o c i a t i v e r e v e r s i b l e a d s o r p t i o n . The l a t t e r produces bands e x t e n d i n g t o 25,000 cm-1 and t h e s e a r e a s c r i b e d t o a s u r f a c e analogue of c h a r g e - t r a n s f e r - t o - s o l v e n t ( c t t s ) s p e c t r a , g e n e r a t e d a s NH3 molecules envelop NH; i o n s and t h e r e s i d u a l s u r f a c e 02- i o n s . INTRODUCTION S p e c t r o s c o p i c s t u d i e s of a d s o r p t i o n on a l k a l i n e e a r t h o x i d e s f e a t u r e d e x t e n s i v e l y i n t h e work o f Dr. Tench, and i n c l u d e d work p u b l i s h e d i n 1972 ( 1 , 2 ) on t h e a p p l i c a t i o n of i r s p e c t r o s c o p y t o N H 3 a d s o r p t i o n on MgO.
Coluccia e t a 1 . ( 3 )
took t h e s u b j e c t f u r t h e r some 10 y e a r s l a t e r , a g a i n using i r s p e c t r a , and we have now extended t h i s and r e l a t e d work ( 4 , 5 ) u s i n g UV r e f l e c t a n c e s p e c t r o s c o p y . Tench and G i l e s ( 1 ) showed t h a t the a d s o r p t i o n of N H 3 on o u t g a s s e d MgO a t room t e m p e r a t u r e was mainly r e v e r s i b l e , with a small f r a c t i o n ( < l o % ) irreversible.
T h e i r i r s t u d y concerned o n l y t h e r e v e r s i b l e a d s o r p t i o n , which
t h e y a s c r i b e d t o S p e c i e s ( i i ) below.
A weaker r e v e r s i b l e a d s o r p t i o n , found by
Tench ( 2 ) on h y d r o x y l a t e d MgO, was a t t r i b u t e d t o S p e c i e s ( i ) .
Coluccia e t a l .
( 3 ) were a b l e t o s t u d y a l s o t h e i r r e v e r s i b l e a d s o r p t i o n on o u t g a s s e d MgO, and t h e y i d e n t i f i e d two such forms ( S p e c i e s ( i i i ) and ( i v ) ) , the more s t r o n g l y held o f which was p o s t u l a t e d t o be d i s s o c i a t i v e l y chemisorbed ammonia i n v o l v i n g lowc o o r d i n a t e (3- o r 4- c o o r d i n a t e ) MgZtO2-
ion p a i r s .
T h u s t h e i r work has l e d
t o t h e view ( 3 ) t h a t ammonia a d s o r p t i o n on MgO l e a d s t o f o u r s p e c i e s , two r e v e r s i b l y and two i r r e v e r s i b l y h e l d a t room t e m p e r a t u r e , r e p r e s e n t a b l e i n o r d e r of i n c r e a s i n g a d s o r p t i o n s t r e n g t h a s f o l l o w s :
98
Coluccia e t a1.(3) a l s o investigated ammonia a d s o r p t i o n on CaO and S r O : t h e y
1
f o u n d t h e same s p e c i e s , b u t i n d i f f e r e n t proportions.
S p e c i e s ( i i i ) was d e s o r b -
a b l e a t room t e m p e r a t u r e i n t h e s e c a s e s , and S p e c i e s ( i v ) was more p r e v a l e n t on
R:,
CaO t h a n on MgO. I n p a r a l l e l w i t h work on a l k a l i n e e a r t h o x i d e s i n H a r w e l l and T u r i n ,
0
associated studies using UV-visible
I
d i f f u s e r e f l e c t a n c e s p e c t r o s c o p y were i n progress i n Bath (4-6).
UV r e f l e c t a n c e
shows t h e p r e s e n c e o f e x c i t o n i c a b s o r p t i o n i n well-outgassed m i c r o c r y s t a l l i n e I
1
I
60
40
-3-
10
MgO, CaO and S r O w h i c h i s a t t r i b u t a b l e
20
t o t h e surface, s p e c i f i c a l l y t o low-
I)lcm-'
F i g . 1 . R e f l e c t a n c e s p e c t r a o f MgO, CaO and S r O , showing e x c i t o n a b s o r p t i o n ( I , 11, 1 1 1 ) due t o s u r f a c e i o n s . The s o l i d s were o u t g a s s e d a t 800OC and s p e c t r a were measured u n d e r c o n d i t i o n s o f quenched f l u o r e s cence. The r e f l e c t a n c e s c a l e i s d i s placed v e r t i c a l l y i n order t o avoid o v e r l a p o f t h e s p e c t r a . The d o t t e d portions t o the l e f t o f the vertical dashed l i n e s a r e e x t r a p o l a t i o n s . ( A d a p t e d f r o m Garrone, Z e c c h i n a and Stone ( 5 ) )
c o o r d i n a t e (LC) s u r f a c e a n i o n s .
The
s p e c t r a ( F i g . 1 ) enable t h r e e absorpt i o n s ( s u r f a c e e x c i t o n s I, I 1 and 1 1 1 ) t o b e d i s c r i m i n a t e d , and t h e s e have been ascribed t o 0 ' -
i o n s i n 5-,
c o o r d i n a t i o n (0;;. ively (5).
0;;.
4- and 3-
0 23c), r e s p e c t -
Luminescence s t u d i e s b y
Tench and h i s c o l l e a g u e s ( 7 - 9 ) have a l s o p r o v i d e d v a l u a b l e i n f o r m a t i o n on t h e p r o p e r t i e s o f LC s u r f a c e i o n s .
A d s o r p t i o n of r e a c t i v e gases and v a p o u r s ' e r o d e s ' t h e s u r f a c e e x c i t o n s p e c t r a and may a l s o g e n e r a t e new bands.
The new bands may be due t o i n t e r n a l
t r a n s i t i o n s i n new a d s o r b e d s p e c i e s p e r se ( a s when c a r b a n i o n s a r e p r o d u c e d
( 4 ) ) o r t o t h e d e v e l o p m e n t o f c h a r g e t r a n s f e r between t h e s u r f a c e a n i o n s and t h e adsorbate.
The l a t t e r b e h a v i o u r was f i r s t i d e n t i f i e d i n T u r i n ( 1 0 ) i n
s t u d i e s o f t h e a d s o r p t i o n o f I 2 and CC14 on K I m i c r o c r y s t a l s . significantly,
More
however, t h e same work ( 1 0 ) h i g h l i g h t e d t h e f a c t t h a t i n t e r -
a c t i o n o f s u r f a c e I-i o n s w i t h a d s o r b a t e s c a n a l s o b e o f a y e t m i l d e r k i n d , where t h e e l e c t r o n t r a n s i t i o n s a r e t h o s e o f l o w - c o o r d i n a t e s u r f a c e I - i o n s whose e n v i r o n m e n t i s c o m p l e t e d b y ' s o l v a t i n g ' a d s o r b a t e m o l e c u l e s . surface analogue o f t h e 'charge-transfer-to-solvent'
This i s the
( c t t s ) s p e c t r a w h i c h have
been e x t e n s i v e l y s t u d i e d f o r a n i o n s ( e s p e c i a l l y t h e i o d i d e i o n ) i n s o l u t i o n chemistry (11).
I t i s i n t e r e s t i n g t o speculate whether t h e analogue o f c t t s
e f f e c t s m i g h t a l s o be observable f o r a d s o r p t i o n on a l k a l i n e e a r t h oxides. Ammonia i s t h e m o l e c u l e o f c h o i c e f o r such a t e s t .
I t g i v e s a marked s h i f t o f
99
the absorption edge of KI c r y s t a l s (10) a n d when used a solvent f o r I - ions i t produces a c t t s red s h i f t larger t h a n t h a t of any other molecule so f a r studied ( 1 1 , l Z ) . I t follows t h a t a U V reflectance study of ammonia adsorption on the a l k a l i n e earth oxides i s of i n t e r e s t f o r several reasons. There a r e b o t h the i r r e v e r s i b l e a n d r e v e r s i b l e e f f e c t s t o be s t u d i e d , and the a c t i v i t y of three oxides t o be compared (BaO has t o be excluded since i t cannot be prepared cleanly with s u f f i c i e n t l y high s p e c i f i c surface a r e a ) . The onward reaction of adsorbed ammonia with o t h e r gases i s a l s o of i n t e r e s t , a n d we s h a l l r e f e r b r i e f l y t o i n t e r a c t i o n with added oxygen a n d C O . EXPERIMENTAL
MgO, CaO and SrO powders were prepared as previously described (13,14). Typical s p e c i f i c surface areas a f t e r outgassing a t 8OO0C were 200 m2g-l (MgO), 100 rn’9-l (CaO) a n d 5 m2g-1(Sr0). The UV d i f f u s e r e f l e c t a n c e c e l l was of s i m i l a r design t o t h a t described by Zecchina e t a 1 . ( 1 3 ) , b u t modified with the addition of a side-tube ( 6 ) t o enable oxides t o be outgassed o u t of contact with the optical face. Diffuse reflectance spectra were measured from 20,000 t o 52,000 cm-’ using a Pye Unicam SP 700C spectrometer, with f i n e l y ground s i l i c a ( S u p r a s i l ) as the reference
material. Spectra determined in vacuo were subject t o an a r t e f a c t caused by fluorescence (luminescence), as discussed elsewhere ( 6 , 1 3 ) . The e f f e c t i s t o increase a r t i f i c i a l l y the reflectance value f o r the spectrum a t high values of v , b u t the positions of the absorption bands ( r e f l e c t a n c e minima) a r e n o t
s i g n i f i c a n t l y affected. efficiently.
Oxygen a t 1 Torr quenches the fluorescence very
The standard pretreatment before adsorption of ammonia was t o outgas the specimen a t 800°C f o r 16 h . All spectra were measured with the c e l l a t room temperature. Ammonia was purified by freeze-pump-thaw cycling. Oxygen and CO were high purity gases supplied by B.O.C. RESULTS
A. 1.
Adsorption and desorption of ammonia
Magnesium oxide The influence of ammonia on the reflectance spectrum i s shown in Fig. 2. The e f f e c t of very small doses of NH3 ( n o t shown) i s t o erode s l i g h t l y the shoulder
a t A A ’ (exciton I11 of Fig. 1 ) and t o extend the t a i l a t B B ’ . Further dosing begins t o quench the fluorescence and a l s o produces new absorption L (curve 2 ) ; a l l gas admitted so f a r i s t o t a l l y adsorbed. Beyond t h i s s t a g e , doses a r e
incompletely taken up, b u t absorption develops now in two regions ( M and N ) and
100
1
Rh 0.5
0 Fig. 2. Reflectance spectra of ammonia adsorbed on MgO. 1 - outgassed MgO, in vacuo. 1 ' - as 1 , b u t with fluorescence quenched by addition of 1 t o r r O2 (standard reference spectrum). 2 - with i r r e v e r s i b l y adsorbed ammonia. 3 - in 3 t o r r NH3. 4 - in 23 t o r r NH3.
Fig. 3. Reflectance spectra of ammonia adsorbed on CaO. 1 - outgassed CaO, with fluorescence q u e n c h e d (st a n d a r d r e f e r e n c e s p e c t r um). 2 - with i r r e v e r s i b l y adsorbed ammonia. 3 - in 2 t o r r NH3. 4 - in 45 t o r r NH3.
fluorescence becomes f u l l y quenched (curve 3 , 3 t o r r ) . Increasing P N H ~t o 23 t o r r (curve 4 ) has r e l a t i v e l y l e s s e f f e c t , b u t the M and N absorptions develop f u r t h e r (MI, N ' ) . Evacuation of residual ammonia a t room temperature f o r 30 min reproduces curve 2 , which therefore represents the spectrum of MgO carrying i r r e v e r s i b l y adsorbed ammo';lia. We may note t h a t absorption N ( a n d N ' ) i s t o be distinguished from absorption L: i t occurs a t higher V and i s more akin t o the exciton I1 absorption (Fig. 1 ) decreased i n i n t e n s i t y a n d s h i f t e d t o lower energy. 2.
Calcium oxide The e f f e c t of adsorbing ammonia i s t o quench fluorescence more readily t h a n
with MgO a n d t o develop very intense unstructured absorption (Fig. 3 ) on the low V s i d e of the original absorption edge. The spectral change approaches a
101
1
l i m i t ( t h e c u r v e f o r 45 t o r r ) as NH3
pressure i s increased.
Evacuation
l e a v e s a s p e c t r u i n due t o i r r e v e r s i b l y a d s o r b e d ammonia w h i c h i s c o r r e s p o n d i n g l y much more i n t e n s e
R:
t h a n w i t h MgO, and w h i c h i s s u f f i c i e n t i n e x t e n t t o quench f l u o r escence a l m o s t c o m p l e t e l y . 3.
Strontium oxide The r e s u l t s o f ammonia a d s o r p t i o n
a r e shown i n F i g . 4.
The s p e c i f i c
s u r f a c e a r e a i s a b o u t 40 t i m e s
0
50
30
40
-3
10 s/cm’
20
s m a l l e r t h a n t h a t o f t h e MgO sample, y e t s i m i l a r f e a t u r e s c a n be seen i n
V r e g i o n . There i s absorpt i o n o f t y p e L ( a l t h o u g h much
t h e low
F i g . 4. R e f l e c t a n c e s p e c t r a o f ammonia adsorbed on S r O . 1 - outgassed S r O , i n vacuo. 1 ’ - as 1, b u t w i t h f l u o r escence quenched. 2 - w i t h i r r e v e r s i b l y a d s o r b e d ammonia. 3 - i n 3 t o r r NH3. 4 - i n 40 t o r r NH3.
n a r r o w e r t h a n on MgO) and a l s o a b s o r p t i o n o f t y p e s M ( M ’ ) and N ( N ’ ) . The n o v e l f e a t u r e w i t h S r O i s t h a t the exciton I region (cf. Fig. 1)
c a n be seen, and t h e e f f e c t o f ammonia a d s o r p t i o n a t 40 t o r r i s t o p r o d u c e a b l u e s h i f t ( t h e band a t 38,000
c m - l i n t h e vacuum s p e c t r u m i s d e s t r o y e d and a d d i t i o n a l a b s o r p t i o n a p p e a r s a t 41,000-43,000 cm- 1 ) . The i r r e v e r s i b l y
a d s o r b e d ammonia i s i n s u f f i c i e n t i n amount t o quench t h e f l u o r e s c e n c e c o m p l e t e l y ( c f . MgO).
B.
Interaction o f
O2 and CO w i t h i r r e v e r s i b l y a d s o r b e d ammonia
Room t e m p e r a t u r e e v a c u a t i o n a f t e r ammonia a d s o r p t i o n l e a v e s s i g n i f i c a n t amounts o f i r r e v e r s i b l y a d s o r b e d ammonia i n t h e c a s e o f MgO and CaO, as j u d g e d b y t h e s p e c t r a i n F i g s . 2 and 3.
The e f f e c t s o f oxygen and o f CO on t h e
s p e c t r u m o f i r r e v e r s i b l y - h e l d ammonia was examined f o r t h e s e o x i d e s . With MgO t h e e f f e c t o f oxygen on t h e s p e c t r u m shown i n c u r v e 2 o f F i g . 2 was
t o c o m p l e t e t h e q u e n c h i n g o f t h e f l u o r e s c e n c e and t o g i v e a s m a l l e r o s i o n o f t h e edge o f a b s o r p t i o n
L a t 33,000-38,000
cm-’.
W i t h CaO t h e r e was a l a r g e r e f f e c t
o n t h e edge, t h e d i s p l a c e m e n t t o h i g h e r V b e i n g c o n s t a n t o v e r t h e r e g i o n 30,000-33,000
cm-’
(Fig. 5).
Below 30,000 cm-l t h e r e was a modest i n c r e a s e i n
a b s o r p t i o n commensurate w i t h t h e f o r m a t i o n o f a s m a l l amount o f 07 ( a s i s f o u n d w i t h o u t g a s s e d CaO exposed t o oxygen ( 1 3 ) . The e f f e c t o f CO o n i r r e v e r s i b l y a d s o r b e d ammonia o n MgO i s shown i n F i g . 6. The r e s i d u a l f l u o r e s c e n c e i s a g a i n quenched and t h e a b s o r p t i o n L ( F i g . 2 ) i s i n
102
t h i s case strongly eroded. Evacuation of the unreacted CO leads t o f u r t h e r erosion a n d t o a fluorescence g r e a t e r t h a n before C O contact. I t i s evident t h a t CO has reacted with i r r e v e r s i b l y adsorbed ammonia. I f CO i s admitted a t higher pressure and l e f t in contact f o r several hours, the (CO)l(.- carbanion spectrum c h a r a c t e r i s t i c of CO on clean MgO ( 4 ) i s weakly developed. With CaO, there i s a s i m i l a r erosion of t h e ammonia edge t o t h a t shown in Fig. 6 , b u t the change i s much more rapid t h a n with MgO and no (CO)l(.- carbanion spectrum develops on long-term contact i n t h i s case.
I
Rh
I
I
R:
_1/ c h 40 30
I
I
-3
1 O S/cm-’
Fig. 5. CaO reflectance s p e c t r a , showing t h e e f f e c t of oxygen on i r r e v e r s i b l y adsorbed ammonia. 1 - outgassed CaO, contacted with ammonia and evacuated f o r 20 h a t room temperature. 2 - a f t e r dosing oxygen a t 4 t o r r . C.
I
I
0
40
I
I
30
I
-3
10q/cm-’
2(
Fig. 6 . MgO reflectance s p e c t r a , showing the e f f e c t o f CO on i r r e v e r s i b l y adsorbed ammonia. 1 - with i r r e v e r s i b l y adsorbed ammonia. 2 - in 12 t o r r C O , immediately. 3 - in 20 torr C O , a f t e r 40 minutes. 4 ,- in 20 t o r r C O , a f t e r 5 hours.
Triethylamine adsorption on MgO
The e f f e c t o f (C2H5)3N adsorption on the spectrum o f outgassed MgO i s shownin Fig. 7 . The r e s u l t i s t o produce a blue s h i f t of both exciton I11 and exciton 11. The e f f e c t was completely r e v e r s i b l e . The s i g n i f i c a n c e of t h i s experiment i s t h a t i t discriminates uniquely the e f f e c t of nitrogen lone p a i r i n t e r a c t i o n with Mg2+ ions, i . e . Species ( i i i ) c i t e d in the Introduction.
103 DISCUSSION
A.
S p e c t r a l changes due t o i r r e v e r s i b l y adsorbed ammonia F i g s . 2 , 3 and 4 show t h a t w i t h a l l t h r e e o x i d e s ammonia adsorbed i r r e v e r s i b l y
a t room temperature generates d i s t i n c t i v e a b s o r p t i o n ( d e s i g n a t e d as L ) on t h e low-energy ( l o w
i)
s i d e o f t h e a b s o r p t i o n edge.
The e f f e c t i s g r e a t e r w i t h CaO
t h a n w i t h MgO s o t h a t , b e a r i n g i n mind t h e i r r e s u l t s ( 3 ) a l r e a d y mentioned, i t can be c o n f i d e n t l y a s c r i b e d t o d i s s o c i a t i v e a d s o r p t i o n and t h e p r o d u c t i o n o f Species ( i v ) i l l u s t r a t e d i n t h e I n t r o d u c t i o n . A b s o r p t i o n L c o u l d be due
a priori
t o OH-,
t o NH;,
o r t o an e f f e c t on t h e
e x c i t a t i o n o f c o n t i g u o u s 0;; s u r f a c e i o n s such t h a t e x c i t o n I 1 i s s h i f t e d t o l o w e r energy.
OH- as t h e e x p l a n a t i o n can be r e a d i l y r u l e d o u t , s i n c e i t s form-
a t i o n s h i f t s a b s o r p t i o n t o h i g h e r energy, as i s shown by experiments w i t h w a t e r vapour ( 1 3 ) .
D i s t i n c t i o n between t h e two o t h e r p o s s i b i l i t i e s i s more
d i f f i c u l t , b u t we d e c i d e i n f a v o u r o f a b s o r p t i o n L b e i n g due t o e x c i t a t i o n o f t h e e l e c t r o n l o c a l i z e d on NH;. shown i n F i g s . 5 and 6.
One reason i s t h e marked e f f e c t o f O2 and CO
Those changes a r e more r e a d i l y understood as e f f e c t s on e x c i t a t i o n a t NH- t h a n on e x c i t a t i o n 2 i n v o l v i n g 0;;. For instance, i n f r a r e d experiments ( 1 4 ) show t h a t carbon mono x i d e r e a c t s w i t h NH;
on MgO.
As f a r
as O2 i s concerned, 02; ( e x c i t o n 1 1 ) i s known t o be s t a b l e towards oxygen ( 1 3 ) . We r e f e r l a t e r t o a n o t h e r reason why we b e l i e v e a b s o r p t i o n L t o be due t o NH; p e r se. Absorption L i s a t approximately 39,000 cm-'
i n MgO, 35,000 cm-l i n CaO
and 30,000 cm-l i n S r O .
The decrease
a l o n g t h e s e r i e s occurs f o r e s s e n t i a l l y t h e same reason as the c o r r e s p o n d i n g decrease i n band gap e n e r g i e s and t h e respective energies o f excitons I , I 1 and I11 ( F i g . 1 ) .
It i s a manifesta-
t i o n o f the increasing radius o f the cation. F i g . 7. R e f l e c t a n c e s p e c t r a o f t r i e t h y l a m i n e (TEA) adsorbed on MgO. 1 - i n TEA a t 0.7 t o r r ( i d e n t i c a l w i t h r e f e r e n c e spectrum o f outgassed MgO ( F i g . 2, c u r v e 1 ' ) . 2 - i n TEA a t 1 t o r r . 3 - i n TEA a t 36 t o r r .
Since t h e e x c i t a t i o n i n v o l v e s
some charge t r a n s f e r between NH; and t h e c a t i o n on which i t i s adsorbed, i t
is i n f l u e n c e d by t h e size o f t h e cation.
104
B.
R e a c t i o n s o f O2 and C C w i t h i r r e v e r s i b l y - a d s-__ o r b e d ammonia I n f r a - r e d work ( 3 ) has shown t h a t d i s s o c i a t i v e a d s o r p t i o n o f ammonia o n MgO
i s r e s t r i c t e d t o a v e r y s m a l l f r a c t i o n o f t h e s u r f a c e and i s o r o b a b l y a s s o c i a t e d
2+ 2-
o n l y w i t h s i t e s o f l o w e s t c o o r d i n a t i o n , v i z . Mg3C03C.
The m a j o r i t y o f t h e
ammonia h e l d i r r e v e r s i b l y o n HgO i s t h e n o n - d i s s o c i a t e d S p e c i e s ( i i i ) .
WithCaO,
o n t h e o t h e r hand, d i s s o c i a t i v e a d s o r p t i o n ( S p e c i e s ( i v ) ) i s much more p r e v a l e n t , commensurate w i t h a l l 4C ( a s w e l l as 3C) s i t e s b e i n g i n v o l v e d .
There i s no
e v i d e n c e w i t h CaO t h a t i r r e v e r s i b l y - h e l d gas i n c l u d e s any S p e c i e s ( i i i ) . The r e s u l t s w i t h oxygen and CO a r e c o n s i s t e n t w i t h t h e s e d i s t i n c t i o n s . 0 2 d o e s n o t i n t e r a c t w i t h a d s o r b e d NH3, b u t o n l y w i t h NH;. o f t h e NH; of NH;
The e f f e c t i s t h u s e r o s i o n
edge ( a b s o r p t i o n L ) , g r e a t e r w i t h CaO ( F i g . 5 ) w i t h i t s h i g h c o v e r a g e
t h a n w i t h MgO.
ESR (02 + NH;
=
T h i s i s accompanied by 0; f o r m a t i o n , as c a n be shown b y
0; + N H 2 ’ ) ( 1 4 ) .
The onward r e a c t i o n s o f amino r a d i c a l s w i t h
t h e m s e l v e s o r w i t h O2 do n o t change t h e UV s p e c t r u m s i n c e t h e y y i e l d p r o d u c t s l i k e l y t o absorb o n l y i n t h e f a r UV. CO g i v e s a marked e f f e c t w i t h b o t h MgO a n d CaO.
r e a c t i o n w i t h NH;
We i n t e r p r e t t h i s as a
t o f o r m s p e c i e s such as (HCONH)-(14).
( F i g . 6 ) i s s l o w o n MgO and f a s t o n CaO.
As Mg:
The s p e c t r a l change
s i t e s become c l e a r e d o f NH;
on
MgO, NH3 f r o m t h e r e s e r v e o f S p e c i e s ( i i i ) s l o w l y d i s s o c i a t e s and r e a c t s . R e a c t i o n o f CO w i t h b a r e 0;;
ions t o form polymeric CO carbanions (15) i s
s u p p r e s s e d , e s p e c i a l l y o n CaO. C.
S p e c t r a a s s o c i a t e d w i t h r e v e r s i b l y - a d s o r b e d ammonia Pronounced e f f e c t s o n t h e r e f l e c t a n c e s p e c t r a w e r e o b s e r v e d when ammonia was
p r o g r e s s i v e l y dosed a t p r e s s u r e s where t h e o n l y changes i n a d s o r p t i o n were t h o s e o f i n c r e a s i n g t h e c o v e r a g e o f p h y s i c a l l y - a d s o r b e d ammonia ( F i g s . 2 - 4 ) .
Large
changes w e r e o b s e r v e d even i n t h e c a s e o f t h e l o w s u r f a c e a r e a S r O , w h i c h i n d i c a t e d t h a t a l l s u r f a c e i o n s (3C, 4C and 5C) were p a r t i c i p a t i n g . Two d i f f e r e n t k i n d s o f new a b s o r p t i o n c a n b e c l e a r l y d i a g n o s e d i n t h e c a s e o f MgO ( F i g . 2 ) , and t h e s e a r e d e s i g n a t e d M and N r e s p e c t i v e l y .
The M a b s o r p t i o n
( t o b e r e g a r d e d a l s o as i n c l u d i n g M ‘ as p r e s s u r e i s f u r t h e r i n c r e a s e d ) i s a band a t v e r y l o w e n e r g y g i v i n g a w e l l - d e v e l o p e d s h o u l d e r a t ca. 32000 cm-’.
The N
a b s o r p t i o n ( t o b e r e g a r d e d a l s o as e m b r a c i n g N ’ ) i s a band a t a n e n e r g y c l o s e t o t h a t o f t h e a b s o r p t i o n edge o f t h e c l e a n s u r f a c e .
T h i s band has t h e c h a r a c t e r
o f e x c i t o n I1 a b s o r p t i o n b u t s h i f t e d t o l o w e r e n e r g y and i n c r e a s i n g l y b r o a d e n e d
with i n c r e a s i n g pressure.
The same t w o f e a t u r e s c a n be d i s c r i m i n a t e d i n t h e S r O
s p e c t r u m f o r 3 t o r r ( F i g . 4) and t h e r e i s a weak b u t d i s c e r n a b l e r i p p l e i n t h e i n d i c a t i v e o f a n a d d i t i o n a l s h o u l d e r . An
40 t o r r M ’ s p e c t r u m a t 28,000 cm-’
important point i s t h a t the
M and N a b s o r p t i o n s a r e d i s t i n c t f r o m t h e L
absorption a l r e a d y discussed.
They a r e a t l o w e r and h i g h e r
V,
respectively,
105
M and N r e g i o n s a r e presumed t o e x i s t
and a r e a l s o a p p r e c i a b l y more i n t e n s e .
a l s o i n t h e CaO s p e c t r u m , b u t t h e a b s o r p t i o n i n t e n s i t y i s so g r e a t t h a t t h e y c a n n o t be d i s t i n g u i s h e d . The M and N a b s o r p t i o n s have t h e h a l l m a r k s o f charge-transfer-to-solvent
(G) spectra,
where p h y s i c a l l y a d s o r b e d NH3 m o l e c u l e s a r e s i m u l a t i n g t h e
effect o f a solvent.
J u s t as
I- i o n s d i s s o l v e d i n l i q u i d ammonia e x h i b i t an
i n t e n s e r e d - s h i f t e d s p e c t r u m i n t h e UV ( l l ) , so i n t h e p r e s e n t c a s e we p r o p o s e t h a t m o b i l e a d s o r b e d NH3 m o l e c u l e s c o m p l e t e t h e envelooment o f a n i o n s exposed a t t h e o x i d e c r y s t a l l i t e s u r f a c e s and p r o d u c e a r e d - s h i f t e d a b s o r p t i o n .
The
e f f e c t s shown i n F i g s . 2 - 4 a r e l a r g e , b o t h i n i n t e n s i t y and i n t h e m a g n i t u d e o f t h e r e d s h i f t (more t h a n 5000 cm-’
i n t h e c a s e o f MgO).
The r e s u l t s a r e com-
p l e m e n t a r y t o t h o s e r e p o r t e d b y C h i o r i n o e t a1 ( 1 0 ) f o r a d s o r p t i o n on K I .
In
t h a t s t u d y t h e same s o l i d ( K I ) was exposed t o v a r i o u s v a p o u r s and t h e m a g n i t u d e s ( a n d d i r e c t i o n s ) o f t h e s p e c t r a l s h i f t s were compared w i t h t h e e f f e c t s o f t h o s e same m o l e c u l e s as s o l v e n t s on I - i o n s i n s o l u t i o n .
Here we have t a k e n t h e
m o l e c u l e (NH3) known f r o m t h a t work ( 1 0 ) and s o l u t i o n s t u d i e s ( 1 1 ) t o show t h e g r e a t e s t c t t s r e d s h i f t a n d have s t u d i e d i t i n a d s o r p t i o n on t h r e e d i f f e r e n t solids. We must now d i s c u s s t h e a n i o n s i n v o l v e d . red-shifted i n unison w i t h absorption i s NH;
bound t o a c a t i o n .
The f a c t t h a t t h e M a b s o r p t i o n i s
L suggests t h a t t h e anion concerned t h e r e
The l a r g e m a g n i t u d e o f t h e s h i f t i s c o m p a t i b l e w i t h
t h e i o n b e i n g v e r y w e l l exposed, i . e . n o t o n l y p r o u d o f t h e s u r f a c e b u t a l s o a f f i x e d t o a l o w - c o o r d i n a t e 3C o r 4C i o n , and t h u s c a p a b l e o f b e i n g i n f l u e n c e d by s e v e r a l NH3 m o l e c u l e s s i m u l t a n e o u s l y .
The N a b s o r p t i o n , o n t h e o t h e r hand,
i s r e d - s h i f t e d f r o m e x c i t o n 11, and t h i s band i s t h e r e f o r e a s c r i b e d t o a c t t s s h i f t o f t h e ;0;
excitation.
These i o n s a r e r e g a r d e d as s u f f i c i e n t l y exposed t o
b e a f f e c t e d , and on MgO ( w h i c h shows a l a r g e e f f e c t ) t h e y a r e n o t s i g n i f i c a n t l y i n v o l v e d i n b i n d i n g NH;.
R e f e r r i n g back t o t h e f i r s t p a r t o f t h e D i s c u s s i o n ,
l e t us n o t e i n p a s s i n g t h a t t h e s i z e o f t h e c t t s s h i f t has h e l p e d u s t o d e c i d e i n favour o f absorption
L b e i n g a s c r i b e d t o NH; r a t h e r t h a n t o
.;0;
I t i s s i g n i f i c a n t t h a t t h e M a b s o r p t i o n i s n o t a t t h e same v a l u e o n a l l t h r e e
oxides.
The p r o p e r t i e s o f t h e u n d e r l y i n g o x i d e e x e r t a n i n f l u e n c e , t h e n a t u r e
o f s u r f a c e e x c i t o n i c and c t t s e f f e c t s b e i n g s u f f i c i e n t l y s i m i l a r t h a t a t r a n s i t i o n showing m i x e d c h a r a c t e r o c c u r s .
On S r O t h e
a p p r o a c h i n g t h e c t t s v a l u e o f 28,000 cm-l f o r f r e e NH; I t i s p o s s i b l e t h a t p a r t o f t h e l o w - 3 t a i l , e.g.
may b e t h e c t t s band o f ;0:
M
absorption
V
is
i o n s i n l i q u i d NH3 ( 1 1 ) .
t h e a r r o w e d s h o u l d e r i n F i g . 4,
( e x c i t o n 111).
The I - i o n i s c o n s i d e r e d ( 1 1 ) t o g i v e l a r g e s o l u t i o n c t t s e f f e c t s because e x c i t a t i o n of
I- l e a d s e a s i l y t o i o n i z a t i o n ( I -
=
I t e).
h e r e w i t h t h e s u r f a c e analogue o f c t t s on o u r oxides. r e a d i l y t r a n s f e r s an e l e c t r o n t o g i v e O;(ads)
There i s a p a r a l l e l
Thus NH;(ads)
on MgO
i n t h e p r e s e n c e o f oxygen, as
106
2shown by ESR ( 1 4 ) . Secondly, a s r e g a r d s 0 4 c , t h e p r o c e s s ; : 0 = 0- + e on a l k a l i n e e a r t h o x i d e s i s regarded a s i m p l i c a t e d i n (C0)'- carbanion f o r m a t i o n
2- n
ion i s more c o o r d i n a t e d with C O ( 1 5 ) . By t h e same t o k e n , when t h e s u r f a c e 0 i t i s l e s s e a s i l y i o n i z e d ; e q u a l l y i t no l o n g e r shows with N H 3 t h e analogue of c t t s . We f i n d no e v i d e n c e , f o r i n s t a n c e , t h a t e x c i t o n I (0;;) shows c t t s . Indeed, on SrO, where e x c i t o n I i s c l e a r l y o b s e r v a b l e , t h e e f f e c t of N H 3 i s t h e o p p o s i t e o f t h a t i n c t t s : N H 3 produces a b l u e s h i f t o f e x c i t o n I ( F i g . 4 ) . The b l u e s h i f t i s i n d i c a t i v e of S p e c i e s ( i i i ) . The key t o t h i s i s t h e e x p e r i ment w i t h (C2H5)3N, which g i v e s a b l u e s h i f t t o t h e e x c i t o n bands ( F i g . 7 ) . On o u t g a s s e d o x i d e t h i s molecule can o n l y a d s o r b a s S p e c i e s ( i i i ) . The i n f e r e n c e is t h a t ; : 0 i s t o o well s h i e l d e d t o engage s i g n i f i c a n t l y i n c t t s or t o a c t w i t h
2+ a s a s u f f i c i e n t l y b a s i c c e n t r e t o g i v e h e t e r o l y t i c d i s s o c i a t i o n of NH3. MSC 2t Adsorption o c c u r s i n s t e a d a s S p e c i e s ( i i i ) on MSC. The e f f e c t on the ion p a i r M$O$ i s l i k e t h a t of i n c r e a s i n g the Madelung p o t e n t i a l : hence t h e b l u e s h i f t .
To sum u p , t h e s u r f a c e analogue of t h e c h a r g e - t r a n s f e r - t o - s o l v e n t e f f e c t has now been shown f o r t h r e e a n i o n s , namely f o r ILc on KI ( 1 0 ) and f o r 0;; and NHi(ads) on MgO, CaO and SrO. I t s s t u d y i s an i n t e r e s t i n g new a p p l i c a t i o n o f
UV d i f f u s e r e f l e c t a n c e s p e c t r o s c o p y t o t h e s u r f a c e c h e m i s t r y of i o n i c c r y s t a l s . We acknowledge with much g r a t i t u d e t h e many s t i m u l a t i n g d i s c u s s i o n s we have had w i t h P r o f e s s o r Adriano Zecchina d u r i n g t h i s work. REFERENCE5 1 A. J . Tench and D. G i l e s , J . C . S. Faraday I , 68 (1972) 193. 2 A. J . Tench, J . C. S. Faraday I , 6 8 (1972) 197. 3 S. C o l u c c i a , E. Garrone and E. B o r e l l o , J . C. 5. Faraday 1 , 79 (1983) 607. 4 F. S. Stone and A. Zecchina, Proc. 6 t h I n t e r n . Congr. C a t a l y s i s (London 1 9 7 6 ) , p. 162, Chemical S o c i e t y , London, 1977. 5 E. Garrone, A. Zecchina and F. S. S t o n e , P h i l o s o p h . Mag., B , 42 (1980) 683. 6 F. S. S t o n e , i n ' S u r f a c e P r o p e r t i e s and C a t a l y s i s by Non-Metals' ( e d . 3. P. B o n e l l e , B . Delmon and E . Derouane), p. 237, R e i d e l , N e t h e r l a n d s , 1983. 7 5. C o l u c c i a , M. Deane and A. J . Tench, Proc. 6 t h I n t e r n . Congr. C a t a l y s i s (London 1 9 7 6 ) , Chemical S o c i e t y , London, p. 171. 8 S. C o l u c c i a , A. M. Deane and A. J . Tench, J . C . S . Faraday I , 74 (1978) 291 3. 9 W . W. Duley, Philosoph. Mag. B , 49 (1984) 159. 10 A. C h i o r i n o , E . Garrone, G. G h i o t t i and A. Zecchina, J . C . S . Faraday I , 78 ( 1 982) 857. 11 M. J . Blandamer and M. F. Fox, Chem. Rev. 70 (1970) 59. 12 I . Burak and A. Treinin, J . C. S. Faraday I , 59 (1963) 1490. 13 A. Zecchina, M. G. Lofthouse and F. S. S t o n e , J . C. S . Faraday I , 71 (1975) 1476. 14 E. Garrone and F. S . S t o n e , Proc. 8th I n t e r n . Congr. C a t a l y s i s ( B e r l i n 1984), Paper 111-441, Dechema, Frankfurt-am-Main, 1984. 15 A. Zecchina and F. S. S t o n e , J . C. S. Faraday I , 74 (1978) 2278.
107
0I SC U SS I OPI Professor K . Klier (Lehiqh University): ( a ) Do you have evidence f o r a n analogue of surface amide NH; t h a t w o u l d be formed by d i s s o c i a t i v e chemisorntion of mono- a n d dialkyl amines RNH2 and R2NH? (b)
Mill CO r e a c t with RNH- o r R2N- in a s i m i l a r fashion as you report f o r the CO + NH;
co +
reaction, i . e . NH;
+
HN=CH-O-,
over the Ma0 surface? Professor F.S. Stone (University of Bath):
( a ) \Ale do n o t have evidence f o r the formation of RNH- and R2N- from RNH2 and R2NH ( R = a l k y l ) because we have not studied them, b u t I would expect d i s s o c i a t i v e chemisorption t o occur in b o t h these cases;
the
a c i d i t i e s of NH3, W H p and R2NH a r e not g r e a t l y d i f f e r e n t . (b)
I would expect RNH- t o r e a c t s i m i l a r l y t o NH; already a t room temperature, b u t t h e reaction with R2N- i s l e s s easy t o p r e d i c t . alkyl migration t o produce R-N=CP-O- i s l i k e l y t o be a d i f f i c u l t reaction on the a l k a l i n e e a r t h oxides.
The