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The Interaction of CO and NO at the Surface of MgO: AN IR and ESR Study
M. Che and G.C. Bond (Editors), Adsorption and Catalysis o n Oxide Surfaces 0 1985 Elsevier Science Publishers B.V.. Amsterdam -Printed in The Netherlands
225
THE I N T E R A C T I O N OF CO AND NO AT THE SURFACE OF MgO: AN I R A N D ESR STUDY
E . GARRONE
1
and E . GIAMELLO
2
I s t i t u t o d i Chimica F i s i c a , C.so Massimo d ' A z e g l i o 4 8 , 10125 Torino ( I t a l y ) 2
I s t i t u t o d i Chimica Generale ed I n o r g a n i c a , F a c o l t 5 d i Farmacia Via P . G i u r i a 9 , 10125 Torino ( I t a l y )
ABSTRACT CO and NO form on p a r t i c u l a r l y r e a c t i v e sites o f MgO a paramagnetic spec i e s , which h a s been r e c e n t l y c h a r a c t e r i z e d by i n f r a r e d s p e c t r o s c o p y and e l e c t r o n paramagnetic resonance s p e c t r o s c o p y ( E . Giarnello e t a l . , t o a p p e a r i n J . Chem. SOC. Faraday Trans. I ( 1 9 8 4 ) 1 . New d a t a a r e r e p o r t e d about t h e r e a c t i v i t y and s p e c t r a l f e a t u r e s . L a b e l l i n g o f t h e oxygen i n C O p r o v i d e s e v i dence a b o u t t h e s t r u c t u r e o f t h e s p e c i e s , a n d two p o s s i b l e models a r e d i s c u s sed.
INTRODUCTION
Because b o t h g a s e s
are present
i n t h e automobile e x h a u s t , t h e r e a c t i o n
between CO and NO h a s been e x t e n s i v e l y s t u d i e d on s u p p o r t e d metals, and t o a much lesser e x t e n t on p u r e o x i d e s . A s f a r a s MgO i s concerned,
t h e c o a d s o r p t i o n o f t h e two g a s e s h a s been
i n v e s t i g a t e d i n 1976 ( r e f . I) by UV-vis
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 . By -1 t h e p r e s e n c e o f a band a t about 30,000 c m , t h e i n f e r e n c e h a s been made t h a t
a CNO
2-
2 follows.
s p e c i e s i s formed a t t h e s u r f a c e . The b a s i c r e a s o n s f o r t h i s was as Both CO
portionation
(refs.
reaction.
2,
3 ) and NO
(ref.
4)
Carbonates and n i t r i t e s
undergo on MgO a d i s p r o -
are r e s p e c t i v e l y formed as
o x i d i z e d p a r t n e r s , w h i l s t t h e reduced ones a r e ,
i n t h e NO c a s e , h y p o n i t r i t e s 2N 0 i n c i s c o n f i g u r a t i o n and, i n t h e CO case, C 0 s p e c i e s of k e t e n i c 22 -r i 2 2 n a t u r e . A s d i m e r i c reduced s p e c i e s X 0 (X = C , N ) a r e formed i n t h e adsor2 2 2p t i o n o f s i n g l e g a s e s , t h e f o r m a t i o n o f a "mixed" d i m e r i c s p e c i e s CNO in 2-
2
t h e i r i n t e r a c t i o n appeared p l a u s i b l e .
W e have r e c e n t l y ( r e f . 5 ) i n v e s t i g a t e d i n some d e t a i l t h e CO/NO i n t e r a c t i o n at
the
MgO
surface,
and shown
that
a paramagnetic s p e c i e s i s formed i n
a p p r e c i a b l e amounts, s o t h a t i t s v i b r a t i o n a l s p e c t r a could be t a k e n . By t h e
226
use o f b i n a r y and t e r n a r y m i x t u r e s o f N and C i s o t o p e s , i t h a s been shown t h a t t h e paramagnetic s p e c i e s c o n t a i n s one carbon and one n i t r o g e n atom, and a t 2l e a s t two oxygen atoms, s o t h a t t h e proposed formula CNO was confirmed. 2 Both I R and ESR e v i d e n c e was i n agreement w i t h a s t r u c t u r e f o r t h e new s p e c i e s of a "mixed" dimer, i n t e r m e d i a t e between t h a t o f c i s h y p o n i t r i t e and ketenic species. In
p r e s e n t p a p e r , w e g i v e f u r t h e r evidence about t h e s t r u c t u r e and
the
mechanism o f f o r m a t i o n / d e p l e t i o n o f t h e new s p e c i e s , c o m i n g f r o m : i ) I R s p e c t r a , 12 18 14 16 of C 0 / N 0 m i x t u r e s ; i i ) computer s i m u l a t i o n o f ESR s p e c t r a ; i i i ) successive adsorption/desorption cycles.
EXPERIMENTAL
The
procedure
for
the
preparation
of
high-area
2 -1
( a b o u t 200 m g
)
Mg0
samples h a s been d e s c r i b e d e l s e w h e r e ( r e f . 2 ) . T h e samples have been o u t g a s s e d a t a b o u t 1100 K t o e l i m i n a t e a l l s u r f a c e atmospheric i m p u r i t i e s . s p e c t r a have been
IR
t a k e n on a
Perkin
Elmer 580 B s p e c t r o p h o t o m e t e r ,
equipped w i t h a Data S t a t i o n . ESR s p e c t r a have been r e c o r d e d on Varian E 109 machine o p e r a t i n g i n t h e X band ( a b o u t 9.4 GHz). Varian p i t c h (g=2.0029) was used
as
a
r e f e r e n c e . High p u r i t y g a s e s were employed. 13 15 18 c o n t e n t was 92% for CO and 99% for NO, and C 0.
The heavy
isotope
The computer program a v a i l a b l e a t t h e " P i e r r e e t Marie C u r i e " U n i v e r s i t y of P a r i s h a s been used f o r t h e s i m u l a t i o n o f EPR s p e c t r a .
RESULTS Computer s i m u l a t i o n of ESR s p e c t r a 12 15 Upon c o n t a c t o f a 1:l CO/ NO m i x t u r e
( t o t a l p r e s s u r e 600 Nm
-2
),
an
i n t e n s e and s t r u c t u r e d spectrum ( F i g . l a ) was r e c o r d e d . Other i s o t o p i c mixt u r e s gave even more c o m p l i c a t e d s p e c t r a , because of t h e h i g h e r n u c l e a r s p i n s
of
13
C and I 4 N
( I = 1 i n b o t h c a s e s ) . The i n t e r p r e t a t i o n o f t h e s p e c t r a was
c a r r i e d o u t i n o u r p r e v i o u s p a p e r , or a s e m i q u a n t i t a t i v e b a s i s , b y c o n s i d e r i n g 14 t h e s h i f t s caused by i s o t o p i c s u b s t i t u t i o n s , e . g . , N f o r I 5 N . T h i s worked reasonably w e l l for t h e o u t e r p a r t s of t h e s p e c t r a : t h e i n t e r p r e t a t i o n of t h e central portion (related t o two
downward peaks
at
the g
high
2
component) was i n s t e a d o n l y t e n t a t i v e . The
fields
reveal
the
presence
of
two
slightly
227
different
v e r s i o n s of t h e paramagnetic s p e c i e s , having s l i g h t l y d i f f e r e n t g
values (henceforth called g
and g ' 1 . 3 3
a)
3
b)
F i g 1. ( a ) Ex r i m e n t a l ESR spectrum o f t h e MgO sample a f t e r c o n t a c t w i t h 600 -2 !?? CO/ 15NO m i x t u r e . ( b ) Computer s i m u l a t i o n . Nm o f a 1:l
I n F i g . lb, t h e computer s i m u l a t i o n o f t h e e x p e r i m e n t a l spectrum i s g i v e n . The
agreement i s ,
i n our o p i n i o n , s a t i s f a c t o r y : t h e small f e a t u r e s i n t h e
e x p e r i m e n t a l spectrum n o t accounted f o r by t h e computation, a r e probably due t o small amounts o f o t h e r s p e c i e s .
sities of
these
features
This i s confirmed by t h e e r r a t i c i n t e n -
i n d i f f e r e n t e x p e r i m e n t s . Those,
however,
do n o t
correspond t o any o f t h e paramagnetic s p e c i e s , which are known t o form upon a d s o r p t i o n o f CO or N O a l o n e ( r e f s . 6-8). The s i m u l a t i o n i n F i g . 1 allowed u s 15 t o determine t h e p r i n c i p a l components o f t h e g and A ( N ) t e n s o r s . An analogous s i m u l a t i o n ( n o t r e p o r t e d ) 13 A( C) tensor.
of t h e
13
CO/
15
NO
spectrum y i e l d e d
the
228
The ESR features are collected in Table I: to our satisfaction, these data are not too far away from those computed in ref. ( 5 ) .
TABLE I
ESR parameters of the new species.
g
3'
1
2
2.0135
2.0060
2.0014
2.0022
3
A( I 5 N ) /G
9.7
10.0
25.0
23.0
A( 13C)/G
7.5
7.0
0.0
0.0
Infrared spectra of the new species.
A set of spectra similar to that in Fig. 2 has been already reported and discussed in ref. (5). The presence of bidentate carbonates (intense bands at 1160, 1330, 1010 and 850 cm
-2
)
-1
and nitrites (weak bands at 1220 and 1100 cm
)
is readily recognized.The new species is manifested by the two distinct bands at 1624 and 1091 cm seen at
h)
-1
800 cm
(labelled A and C respectively): a further weak band is
-1
(band D not shown in the figure). The reason for
re-examining this kind of experiments is the peculiar behaviour of the band at 1330 cm
-1
, which
been overlooked earlier. We may note that, before
has
outgassing, it is definitely more intense than
what expected from the inten-
sities of the partner carbonate bands at 1660 and 1010 cm (ref. 5) the demolition the 1660 and 1010 cm
-1
-1
.Outgassing causes
of the new species to yield carbonates: accordingly, -1
bands grow. That at 1330 cm
remains constant in
intensity (the set of curves in Fig. 2 have actually been chosen on purpose to show this feature), and only shifts slightly to lower frequencies: its intensity is now close to what expected. -1
Our interpretation is that the absorption around 1330 cm
is actually due
to the superimposition of two close bands, one due to bidentate carbonates, the other (band B) ascribed to the new species, this latter band having a higher intrinsic intensity. Upon isotopic substitution (
13
C for
12
C,
15 14 N for N), bands A , C and D
undergo the shifts in frequency which are collected in Table 11. As for band
229 B,
no
precise
companion
statement
c a r b o n a t e band.
can
be
made:
it
seems,
however,
to
follow
the
A much c l e a r e r d i s t i n c t i o n between band B and i t s
c a r b o n a t e companion band i s o b t a i n e d when d o s i n g a CI80/NO m i x t u r e
(Fig. 3 ) .
Before e v a c u a t i o n , t h e c a r b o n a t e band a p p e a r s as a s h o u l d e r o f t h e B band, and upon e v a c u a t i o n , a s h i f t o f some 15 cm
-1
is observed.
A
F i g . 2. I n f r a r e d s p e c t r a o f t h e M 0 sample. Dot-dash c u r v e : b a c k g r o u n d . Broken 12 14 c u r v e : a f t e r c o n t a c t w i t h 600 Nm of a 1:l CO/ NO m i x t u r e . S o l i d c u r v e : after s h o r t evacuation.
4
There are two o t h e r f e a t u r e s of i n t e r e s t i n F i g . 3 . F i r s t , band A , which, 13 1 2 upon C/ C s u b s t i t u t i o n behaves l i k e a p u r e C-0 s t r e t c h i n g mode ( r e f . 5 ) , i s now
observed
to
undergo a s h i f t d e f i n i t e l y s m a l l e r t h a n what e x p e c t e d ,
as
though t h e mass o f t h e oxygen atom were 1 7 i n s t e a d of 18. S e c o n d l y , band C , s h a r p and i n t e n s e i n t h e e x p e r i m e n t i n F i g . 2 , i s now broadened, a s though two components a t l e a s t were p r e s e n t .
230
TABLE I1 12
13
12
13
N
1624
1583
1091
l5N
1619
-
1081
C
14
C
BAND
A
12
13
1087
800
776
-
800
-
C
C
BAND
C
C
C
BAND
D
F i g . 3 . I n f r a r e d s p e c t r a of a d s o r b e d s p e c i e s ( d i f f e r e n c e s p e c t r a ) . Broken 18 -2 c u r v e : a f t e r d o s i n g a 1:l C O/NO (1,3 kNm ) . S o l i d c u r v e : a f t e r a s h o r t evacuation.
231 Adsorption/desorption cycles A f t e r e v a c u a t i o n , which c a u s e s t h e d e p l e t i o n o f t h e new s p e c i e s , t o t h e i n c r e a s e o f c a r b o n a t e s ( s e e F i g s . 2 and 3 ) , a s u c c e s s i v e d o s i n g o f t h e CO/NO m i x t u r e s restores both t h e ESR and t h e I R s p e c t r a , a l t h o u g h a t weaker i n t e n s i t y . T h i s can be r e p e a t e d s e v e r a l times. Again a f t e r e v a c u a t i o n , we have cont a c t e d t h e s u r f a c e with CO and N O a l o n e , and with g a s e s l i k e N 0 and N
2
2
which
a r e a l s o p r o d u c t s o f t h e CO/NO r e a c t i o n . I n no c a s e s , t h e new s p e c i e s has been restored.
DISCUSSION Being c h a r a c t e r i z e d by a t l e a s t f o u r I R bands, t h e new s p e c i e s must be more than triatomic.
The smallest paramagnetic molecule w i t h one carbon and one
n i t r o g e n atom i s t h u s CNO
22
,
as a l r e a d y h y p o t e s i z e d . The s t r u c t u r e proposed
i n r e f . ( 5 ) was
0
,O'
\\
C--N 0 0
T h i s s t r u c t u r e a p p e a r s however n o t a b l e t o e x p l a i n t h e i s o t o p i c s h i f t s i n 18 F i g . 3 ( e . g . , band A ) , which i n d i c a t e t h a t two oxygen atoms ( o n e 0 and one 16 0 ) are l i n k e d t o t h e carbon.
W e c o n s i d e r t h u s a d i f f e r e n t arrangement o f t h e f o u r atoms
0
II C
/ \
OQ
hNQ
whereby a s t r u c t u r e c l o s e t o a c a r b o n a t e i o n i s o b t a i n e d . From a n a i v e p o i n t o f view, t h e I R s p e c t r a o f t h e new s p e c i e s are i n q u a l i t a t i v e agreement. A l l f o u r bands f a l l n o t t o o f a r away from c a r b o n a t e bands. Would t h e new s p e c i e s have s t r u c t u r e 11, i t s h o u l d be i s o s t r u c t u r a l
and i s o e l e c t r o n i c w i t h t h e
.
T h i s l a t t e r i s known ( r e f . 9 ) t o have a C symmetry, 3 2v one bond h a s a more pronounced double bond c h a r a c t e r . If t h e same i s
r a d i c a l a n i o n CO
i.e.,
-
t r u e f o r s t r u c t u r e 11, one would e x p e c t t h a t band A ( r o u g h l y a C=O s t r e t c h ) 13 12 should respond t o a C/ C s u b s t i t u t i o n , j u s t as s h o u l d band B ( b a s i c a l l y a pseudoantisymmetric s t r e t c h o f t h e 0-C-N
metric
s t r e t c h of
0-C-N)
m o j e t y ) . Band C , i n s t e a d , (pseudosym-
s h o u l d respond
t o a change i n t h e mass of
the
232 n i t r o g e n and n o t t o a change i n t h e mass o f t h e carbon atom. Band D (C=O 13 12 bending mode) should a g a i n respond t o a C/ C s u b s t i t u t i o n . This i s roughly what observed ( T a b l e 11). Moreover, i f one admits t h a t w i t h i n s t r u c t u r e I1 t h e s c r a m b l i n g o f t h e oxygen atoms can o c c u r , t h e p r e s e n c e of two components i n 18 band C i n Fig. 3 i s j u s t i f i e d (pseudosymmetric s t r e t c h e s of 0-C-N and 16 0-C-N r e s p e c t i v e l y ) . The ESR s p e c t r a a r e a l s o i n agreement. From t h e d a t a i n Table I , we c a l c u l a t e through t h e u s u a l procedure
( r e f . 5 ) t h a t t h e s p i n d e n s i t y on t h e carbon
atom i s -0.075 and t h a t on t h e n i t r o g e n atom i s 0 . 2 2 8 . The CO
-
s p e c i e s has a s p i n d e n s i t y on t h e carbon o f -0.05 n o t t o o f a r from 3 what observed f o r our s p e c i e s . There a r e , however, two o b j e c t i o n s t o s t r u c t u r e 11. F i r s t , i n o r d e r t o have t h e u n p a i r e d e l e c t r o n on t h e oxygen atoms, one has t o l o c a l i z e b o t h c h a r g e s on the
nitrogen
observed
that
atom, the
activated, i . e . ,
which
appears
not
very
reasonable.
Secondly
we
have
f o r m a t i o n o f t h e new s p e c i e s i s l i t t l e (or n o t a t a l l )
i t does n o t seem t o come o u t o f t h e slow d i s p r o p o r t i o n a t i o n
reaction.
0
_cus
+ 2CO( g ) + N O ( g ) 4 C03
__ +
CNO
2-
2
Moreover, t h e decomposition o f t h e new s p e c i e s ( r e f . 5)
2CN02
2-
4
CO
22+ + C + % N + O 3 2 cus
s h o u l d l e a v e on t h e s u r f a c e v e r y r e a c t i v e atomic carbon, s o t h a t r e a d s o r p t i o n o f NO a l o n e s h o u l d r e s t o r e t h e new s p e c i e s
C
+ NO
(PI
+ 0
2+
4 CNO cus 2
2-
We c o n s i d e r t h u s a d i f f e r e n t s t r u c t u r e i n v o l v i n g one more oxygen atom
233
QN
/
/ O'
"
I
C
r
L
0 / \
0
0
0
The formation of such CN03
2-
OQ
'+
species does not involve any disproportiona-
tion and only requires a Mg-0 pair with high coordinative unsaturation:
CO
(g)
+ NO
2-
(g)
+ 0
cus
--3
CNO
2-
3
The decomposition of the new species would be in this case
so that no reactive species would be left on the surface.
Most probably the above reasoning about the isotopic effects should also hold for such species. These considerations however require a detailed normal mode analysis because of the complexity of structure 111. This, and the labelling of the oxygen in the nitric oxide, will be the subject of future work. REFERENCES 1 F.S. Stone and A. Zecchina, Proc. 6th Int. Congr. Catal., London 1976, The Chemical Society, 1977, p. 162. 2 E. Guglielminotti, S . Coluccia, E. Garrone, L. Cerruti and A . Zecchina, J. Chem. SOC. Faraday Trans. I , 75 (1975) 96. 3 A . Zecchina and F.S. Stone, J. Chem. SOC. Faraday Trans. I, 74 (1978) 2278. 4 L. Cerruti, A . Modone, E. Guglielminotti and E. Borello, J. Chem. SOC. Faraday Trans. I, 70 (1974) 729. 5 E. Giamello, E. Garrone, E. Guglielminotti and A . Zecchina, to appear in J. Chem. SOC. Faraday Trans. I ( 1 9 8 4 ) . 6 D. Cordischi, V. Indovina and M. Occhiuzzi, J. Chem. SOC. Faraday Trans. I, 76 (1980) 1147. 7 8
9
R.M. Morris, R.A. Kaba, T.C. Groshens, V.I. Klabunde, R.T. Baltisberger, N.F. Woolsey and V.I. Stenberg, J.A.C.S., 102 (1980) 3419. J. Lunsford, J. Chem. Phys., 46 (1967) 4347. G.W. Chautry, A . Horsfield, J.R. Morton and D.G. Whitten, Mol. Phys., 5 (1962) 506.
225
THE I N T E R A C T I O N OF CO AND NO AT THE SURFACE OF MgO: AN I R A N D ESR STUDY
E . GARRONE
1
and E . GIAMELLO
2
I s t i t u t o d i Chimica F i s i c a , C.so Massimo d ' A z e g l i o 4 8 , 10125 Torino ( I t a l y ) 2
I s t i t u t o d i Chimica Generale ed I n o r g a n i c a , F a c o l t 5 d i Farmacia Via P . G i u r i a 9 , 10125 Torino ( I t a l y )
ABSTRACT CO and NO form on p a r t i c u l a r l y r e a c t i v e sites o f MgO a paramagnetic spec i e s , which h a s been r e c e n t l y c h a r a c t e r i z e d by i n f r a r e d s p e c t r o s c o p y and e l e c t r o n paramagnetic resonance s p e c t r o s c o p y ( E . Giarnello e t a l . , t o a p p e a r i n J . Chem. SOC. Faraday Trans. I ( 1 9 8 4 ) 1 . New d a t a a r e r e p o r t e d about t h e r e a c t i v i t y and s p e c t r a l f e a t u r e s . L a b e l l i n g o f t h e oxygen i n C O p r o v i d e s e v i dence a b o u t t h e s t r u c t u r e o f t h e s p e c i e s , a n d two p o s s i b l e models a r e d i s c u s sed.
INTRODUCTION
Because b o t h g a s e s
are present
i n t h e automobile e x h a u s t , t h e r e a c t i o n
between CO and NO h a s been e x t e n s i v e l y s t u d i e d on s u p p o r t e d metals, and t o a much lesser e x t e n t on p u r e o x i d e s . A s f a r a s MgO i s concerned,
t h e c o a d s o r p t i o n o f t h e two g a s e s h a s been
i n v e s t i g a t e d i n 1976 ( r e f . I) by UV-vis
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 . By -1 t h e p r e s e n c e o f a band a t about 30,000 c m , t h e i n f e r e n c e h a s been made t h a t
a CNO
2-
2 follows.
s p e c i e s i s formed a t t h e s u r f a c e . The b a s i c r e a s o n s f o r t h i s was as Both CO
portionation
(refs.
reaction.
2,
3 ) and NO
(ref.
4)
Carbonates and n i t r i t e s
undergo on MgO a d i s p r o -
are r e s p e c t i v e l y formed as
o x i d i z e d p a r t n e r s , w h i l s t t h e reduced ones a r e ,
i n t h e NO c a s e , h y p o n i t r i t e s 2N 0 i n c i s c o n f i g u r a t i o n and, i n t h e CO case, C 0 s p e c i e s of k e t e n i c 22 -r i 2 2 n a t u r e . A s d i m e r i c reduced s p e c i e s X 0 (X = C , N ) a r e formed i n t h e adsor2 2 2p t i o n o f s i n g l e g a s e s , t h e f o r m a t i o n o f a "mixed" d i m e r i c s p e c i e s CNO in 2-
2
t h e i r i n t e r a c t i o n appeared p l a u s i b l e .
W e have r e c e n t l y ( r e f . 5 ) i n v e s t i g a t e d i n some d e t a i l t h e CO/NO i n t e r a c t i o n at
the
MgO
surface,
and shown
that
a paramagnetic s p e c i e s i s formed i n
a p p r e c i a b l e amounts, s o t h a t i t s v i b r a t i o n a l s p e c t r a could be t a k e n . By t h e
226
use o f b i n a r y and t e r n a r y m i x t u r e s o f N and C i s o t o p e s , i t h a s been shown t h a t t h e paramagnetic s p e c i e s c o n t a i n s one carbon and one n i t r o g e n atom, and a t 2l e a s t two oxygen atoms, s o t h a t t h e proposed formula CNO was confirmed. 2 Both I R and ESR e v i d e n c e was i n agreement w i t h a s t r u c t u r e f o r t h e new s p e c i e s of a "mixed" dimer, i n t e r m e d i a t e between t h a t o f c i s h y p o n i t r i t e and ketenic species. In
p r e s e n t p a p e r , w e g i v e f u r t h e r evidence about t h e s t r u c t u r e and
the
mechanism o f f o r m a t i o n / d e p l e t i o n o f t h e new s p e c i e s , c o m i n g f r o m : i ) I R s p e c t r a , 12 18 14 16 of C 0 / N 0 m i x t u r e s ; i i ) computer s i m u l a t i o n o f ESR s p e c t r a ; i i i ) successive adsorption/desorption cycles.
EXPERIMENTAL
The
procedure
for
the
preparation
of
high-area
2 -1
( a b o u t 200 m g
)
Mg0
samples h a s been d e s c r i b e d e l s e w h e r e ( r e f . 2 ) . T h e samples have been o u t g a s s e d a t a b o u t 1100 K t o e l i m i n a t e a l l s u r f a c e atmospheric i m p u r i t i e s . s p e c t r a have been
IR
t a k e n on a
Perkin
Elmer 580 B s p e c t r o p h o t o m e t e r ,
equipped w i t h a Data S t a t i o n . ESR s p e c t r a have been r e c o r d e d on Varian E 109 machine o p e r a t i n g i n t h e X band ( a b o u t 9.4 GHz). Varian p i t c h (g=2.0029) was used
as
a
r e f e r e n c e . High p u r i t y g a s e s were employed. 13 15 18 c o n t e n t was 92% for CO and 99% for NO, and C 0.
The heavy
isotope
The computer program a v a i l a b l e a t t h e " P i e r r e e t Marie C u r i e " U n i v e r s i t y of P a r i s h a s been used f o r t h e s i m u l a t i o n o f EPR s p e c t r a .
RESULTS Computer s i m u l a t i o n of ESR s p e c t r a 12 15 Upon c o n t a c t o f a 1:l CO/ NO m i x t u r e
( t o t a l p r e s s u r e 600 Nm
-2
),
an
i n t e n s e and s t r u c t u r e d spectrum ( F i g . l a ) was r e c o r d e d . Other i s o t o p i c mixt u r e s gave even more c o m p l i c a t e d s p e c t r a , because of t h e h i g h e r n u c l e a r s p i n s
of
13
C and I 4 N
( I = 1 i n b o t h c a s e s ) . The i n t e r p r e t a t i o n o f t h e s p e c t r a was
c a r r i e d o u t i n o u r p r e v i o u s p a p e r , or a s e m i q u a n t i t a t i v e b a s i s , b y c o n s i d e r i n g 14 t h e s h i f t s caused by i s o t o p i c s u b s t i t u t i o n s , e . g . , N f o r I 5 N . T h i s worked reasonably w e l l for t h e o u t e r p a r t s of t h e s p e c t r a : t h e i n t e r p r e t a t i o n of t h e central portion (related t o two
downward peaks
at
the g
high
2
component) was i n s t e a d o n l y t e n t a t i v e . The
fields
reveal
the
presence
of
two
slightly
227
different
v e r s i o n s of t h e paramagnetic s p e c i e s , having s l i g h t l y d i f f e r e n t g
values (henceforth called g
and g ' 1 . 3 3
a)
3
b)
F i g 1. ( a ) Ex r i m e n t a l ESR spectrum o f t h e MgO sample a f t e r c o n t a c t w i t h 600 -2 !?? CO/ 15NO m i x t u r e . ( b ) Computer s i m u l a t i o n . Nm o f a 1:l
I n F i g . lb, t h e computer s i m u l a t i o n o f t h e e x p e r i m e n t a l spectrum i s g i v e n . The
agreement i s ,
i n our o p i n i o n , s a t i s f a c t o r y : t h e small f e a t u r e s i n t h e
e x p e r i m e n t a l spectrum n o t accounted f o r by t h e computation, a r e probably due t o small amounts o f o t h e r s p e c i e s .
sities of
these
features
This i s confirmed by t h e e r r a t i c i n t e n -
i n d i f f e r e n t e x p e r i m e n t s . Those,
however,
do n o t
correspond t o any o f t h e paramagnetic s p e c i e s , which are known t o form upon a d s o r p t i o n o f CO or N O a l o n e ( r e f s . 6-8). The s i m u l a t i o n i n F i g . 1 allowed u s 15 t o determine t h e p r i n c i p a l components o f t h e g and A ( N ) t e n s o r s . An analogous s i m u l a t i o n ( n o t r e p o r t e d ) 13 A( C) tensor.
of t h e
13
CO/
15
NO
spectrum y i e l d e d
the
228
The ESR features are collected in Table I: to our satisfaction, these data are not too far away from those computed in ref. ( 5 ) .
TABLE I
ESR parameters of the new species.
g
3'
1
2
2.0135
2.0060
2.0014
2.0022
3
A( I 5 N ) /G
9.7
10.0
25.0
23.0
A( 13C)/G
7.5
7.0
0.0
0.0
Infrared spectra of the new species.
A set of spectra similar to that in Fig. 2 has been already reported and discussed in ref. (5). The presence of bidentate carbonates (intense bands at 1160, 1330, 1010 and 850 cm
-2
)
-1
and nitrites (weak bands at 1220 and 1100 cm
)
is readily recognized.The new species is manifested by the two distinct bands at 1624 and 1091 cm seen at
h)
-1
800 cm
(labelled A and C respectively): a further weak band is
-1
(band D not shown in the figure). The reason for
re-examining this kind of experiments is the peculiar behaviour of the band at 1330 cm
-1
, which
been overlooked earlier. We may note that, before
has
outgassing, it is definitely more intense than
what expected from the inten-
sities of the partner carbonate bands at 1660 and 1010 cm (ref. 5) the demolition the 1660 and 1010 cm
-1
-1
.Outgassing causes
of the new species to yield carbonates: accordingly, -1
bands grow. That at 1330 cm
remains constant in
intensity (the set of curves in Fig. 2 have actually been chosen on purpose to show this feature), and only shifts slightly to lower frequencies: its intensity is now close to what expected. -1
Our interpretation is that the absorption around 1330 cm
is actually due
to the superimposition of two close bands, one due to bidentate carbonates, the other (band B) ascribed to the new species, this latter band having a higher intrinsic intensity. Upon isotopic substitution (
13
C for
12
C,
15 14 N for N), bands A , C and D
undergo the shifts in frequency which are collected in Table 11. As for band
229 B,
no
precise
companion
statement
c a r b o n a t e band.
can
be
made:
it
seems,
however,
to
follow
the
A much c l e a r e r d i s t i n c t i o n between band B and i t s
c a r b o n a t e companion band i s o b t a i n e d when d o s i n g a CI80/NO m i x t u r e
(Fig. 3 ) .
Before e v a c u a t i o n , t h e c a r b o n a t e band a p p e a r s as a s h o u l d e r o f t h e B band, and upon e v a c u a t i o n , a s h i f t o f some 15 cm
-1
is observed.
A
F i g . 2. I n f r a r e d s p e c t r a o f t h e M 0 sample. Dot-dash c u r v e : b a c k g r o u n d . Broken 12 14 c u r v e : a f t e r c o n t a c t w i t h 600 Nm of a 1:l CO/ NO m i x t u r e . S o l i d c u r v e : after s h o r t evacuation.
4
There are two o t h e r f e a t u r e s of i n t e r e s t i n F i g . 3 . F i r s t , band A , which, 13 1 2 upon C/ C s u b s t i t u t i o n behaves l i k e a p u r e C-0 s t r e t c h i n g mode ( r e f . 5 ) , i s now
observed
to
undergo a s h i f t d e f i n i t e l y s m a l l e r t h a n what e x p e c t e d ,
as
though t h e mass o f t h e oxygen atom were 1 7 i n s t e a d of 18. S e c o n d l y , band C , s h a r p and i n t e n s e i n t h e e x p e r i m e n t i n F i g . 2 , i s now broadened, a s though two components a t l e a s t were p r e s e n t .
230
TABLE I1 12
13
12
13
N
1624
1583
1091
l5N
1619
-
1081
C
14
C
BAND
A
12
13
1087
800
776
-
800
-
C
C
BAND
C
C
C
BAND
D
F i g . 3 . I n f r a r e d s p e c t r a of a d s o r b e d s p e c i e s ( d i f f e r e n c e s p e c t r a ) . Broken 18 -2 c u r v e : a f t e r d o s i n g a 1:l C O/NO (1,3 kNm ) . S o l i d c u r v e : a f t e r a s h o r t evacuation.
231 Adsorption/desorption cycles A f t e r e v a c u a t i o n , which c a u s e s t h e d e p l e t i o n o f t h e new s p e c i e s , t o t h e i n c r e a s e o f c a r b o n a t e s ( s e e F i g s . 2 and 3 ) , a s u c c e s s i v e d o s i n g o f t h e CO/NO m i x t u r e s restores both t h e ESR and t h e I R s p e c t r a , a l t h o u g h a t weaker i n t e n s i t y . T h i s can be r e p e a t e d s e v e r a l times. Again a f t e r e v a c u a t i o n , we have cont a c t e d t h e s u r f a c e with CO and N O a l o n e , and with g a s e s l i k e N 0 and N
2
2
which
a r e a l s o p r o d u c t s o f t h e CO/NO r e a c t i o n . I n no c a s e s , t h e new s p e c i e s has been restored.
DISCUSSION Being c h a r a c t e r i z e d by a t l e a s t f o u r I R bands, t h e new s p e c i e s must be more than triatomic.
The smallest paramagnetic molecule w i t h one carbon and one
n i t r o g e n atom i s t h u s CNO
22
,
as a l r e a d y h y p o t e s i z e d . The s t r u c t u r e proposed
i n r e f . ( 5 ) was
0
,O'
\\
C--N 0 0
T h i s s t r u c t u r e a p p e a r s however n o t a b l e t o e x p l a i n t h e i s o t o p i c s h i f t s i n 18 F i g . 3 ( e . g . , band A ) , which i n d i c a t e t h a t two oxygen atoms ( o n e 0 and one 16 0 ) are l i n k e d t o t h e carbon.
W e c o n s i d e r t h u s a d i f f e r e n t arrangement o f t h e f o u r atoms
0
II C
/ \
OQ
hNQ
whereby a s t r u c t u r e c l o s e t o a c a r b o n a t e i o n i s o b t a i n e d . From a n a i v e p o i n t o f view, t h e I R s p e c t r a o f t h e new s p e c i e s are i n q u a l i t a t i v e agreement. A l l f o u r bands f a l l n o t t o o f a r away from c a r b o n a t e bands. Would t h e new s p e c i e s have s t r u c t u r e 11, i t s h o u l d be i s o s t r u c t u r a l
and i s o e l e c t r o n i c w i t h t h e
.
T h i s l a t t e r i s known ( r e f . 9 ) t o have a C symmetry, 3 2v one bond h a s a more pronounced double bond c h a r a c t e r . If t h e same i s
r a d i c a l a n i o n CO
i.e.,
-
t r u e f o r s t r u c t u r e 11, one would e x p e c t t h a t band A ( r o u g h l y a C=O s t r e t c h ) 13 12 should respond t o a C/ C s u b s t i t u t i o n , j u s t as s h o u l d band B ( b a s i c a l l y a pseudoantisymmetric s t r e t c h o f t h e 0-C-N
metric
s t r e t c h of
0-C-N)
m o j e t y ) . Band C , i n s t e a d , (pseudosym-
s h o u l d respond
t o a change i n t h e mass of
the
232 n i t r o g e n and n o t t o a change i n t h e mass o f t h e carbon atom. Band D (C=O 13 12 bending mode) should a g a i n respond t o a C/ C s u b s t i t u t i o n . This i s roughly what observed ( T a b l e 11). Moreover, i f one admits t h a t w i t h i n s t r u c t u r e I1 t h e s c r a m b l i n g o f t h e oxygen atoms can o c c u r , t h e p r e s e n c e of two components i n 18 band C i n Fig. 3 i s j u s t i f i e d (pseudosymmetric s t r e t c h e s of 0-C-N and 16 0-C-N r e s p e c t i v e l y ) . The ESR s p e c t r a a r e a l s o i n agreement. From t h e d a t a i n Table I , we c a l c u l a t e through t h e u s u a l procedure
( r e f . 5 ) t h a t t h e s p i n d e n s i t y on t h e carbon
atom i s -0.075 and t h a t on t h e n i t r o g e n atom i s 0 . 2 2 8 . The CO
-
s p e c i e s has a s p i n d e n s i t y on t h e carbon o f -0.05 n o t t o o f a r from 3 what observed f o r our s p e c i e s . There a r e , however, two o b j e c t i o n s t o s t r u c t u r e 11. F i r s t , i n o r d e r t o have t h e u n p a i r e d e l e c t r o n on t h e oxygen atoms, one has t o l o c a l i z e b o t h c h a r g e s on the
nitrogen
observed
that
atom, the
activated, i . e . ,
which
appears
not
very
reasonable.
Secondly
we
have
f o r m a t i o n o f t h e new s p e c i e s i s l i t t l e (or n o t a t a l l )
i t does n o t seem t o come o u t o f t h e slow d i s p r o p o r t i o n a t i o n
reaction.
0
_cus
+ 2CO( g ) + N O ( g ) 4 C03
__ +
CNO
2-
2
Moreover, t h e decomposition o f t h e new s p e c i e s ( r e f . 5)
2CN02
2-
4
CO
22+ + C + % N + O 3 2 cus
s h o u l d l e a v e on t h e s u r f a c e v e r y r e a c t i v e atomic carbon, s o t h a t r e a d s o r p t i o n o f NO a l o n e s h o u l d r e s t o r e t h e new s p e c i e s
C
+ NO
(PI
+ 0
2+
4 CNO cus 2
2-
We c o n s i d e r t h u s a d i f f e r e n t s t r u c t u r e i n v o l v i n g one more oxygen atom
233
QN
/
/ O'
"
I
C
r
L
0 / \
0
0
0
The formation of such CN03
2-
OQ
'+
species does not involve any disproportiona-
tion and only requires a Mg-0 pair with high coordinative unsaturation:
CO
(g)
+ NO
2-
(g)
+ 0
cus
--3
CNO
2-
3
The decomposition of the new species would be in this case
so that no reactive species would be left on the surface.
Most probably the above reasoning about the isotopic effects should also hold for such species. These considerations however require a detailed normal mode analysis because of the complexity of structure 111. This, and the labelling of the oxygen in the nitric oxide, will be the subject of future work. REFERENCES 1 F.S. Stone and A. Zecchina, Proc. 6th Int. Congr. Catal., London 1976, The Chemical Society, 1977, p. 162. 2 E. Guglielminotti, S . Coluccia, E. Garrone, L. Cerruti and A . Zecchina, J. Chem. SOC. Faraday Trans. I , 75 (1975) 96. 3 A . Zecchina and F.S. Stone, J. Chem. SOC. Faraday Trans. I, 74 (1978) 2278. 4 L. Cerruti, A . Modone, E. Guglielminotti and E. Borello, J. Chem. SOC. Faraday Trans. I, 70 (1974) 729. 5 E. Giamello, E. Garrone, E. Guglielminotti and A . Zecchina, to appear in J. Chem. SOC. Faraday Trans. I ( 1 9 8 4 ) . 6 D. Cordischi, V. Indovina and M. Occhiuzzi, J. Chem. SOC. Faraday Trans. I, 76 (1980) 1147. 7 8
9
R.M. Morris, R.A. Kaba, T.C. Groshens, V.I. Klabunde, R.T. Baltisberger, N.F. Woolsey and V.I. Stenberg, J.A.C.S., 102 (1980) 3419. J. Lunsford, J. Chem. Phys., 46 (1967) 4347. G.W. Chautry, A . Horsfield, J.R. Morton and D.G. Whitten, Mol. Phys., 5 (1962) 506.