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ADSORPTION OF C l6O AND Cl8O ON RHODIUM,
OBSERVED BY INELASTIC ELECTRON
TUNNELING SPECTROSCOPY
S. de Cheveigné, S. Klein, A. Léger
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-998
ADSORPTION OF C'&O AND C180 ON RHODIUM, OBSERVED BY INELASTIC ELECTRON
TUNNELING SPECTROSCOPY
S. de Cheveigne, J. Klein and A. Leger.
Groupe de Physique des Solides de I'Eoole Normale Superieure,
Tour 23, 2 PI. Jussieu, 75221 Paris Cedex OS, France
Résumé.- La Spectroscopie par Effet Tunnel Inélastique apporte des renseignements nouveaux sur le problème de l'adsorption du CO, essentiel à la compréhension de la catalyse. L'étude des déplacements isotopiques permet l'identification des modes basse fréquence que la spectroscopie infra-rouge ne peut pas atteindre.
Abstract.- Inelastic Electron Tunneling Spectroscopy brings new information on the problem of adsorption of CO on metals, essential to the understanding of catalytic mechanisms. Isotopic shifts allow identification of the low frequency modes that infra-red spectroscopy cannot observe.
Catalysis, in spite of its enormous prac-tical interest, is a remarkably poorly known phe-nomenon. The study of the adsorption of molecules onto the catalyst is an important step on the way to understanding the process, and forms a parti-cularly interesting application of Inelastic Electron Tunneling Spectroscopy (IETS).
The adsorption of CO is one of the funda-mental problems, partly because CO takes part in many catalytic .processes (for example in anti-pollution devices for automobiles, or in making coal gas), and partly because it is a simple mole-cule with a well-known electronic structure. The catalysts used are generally pure or oxide-suppor-ted transition metals, hence the choice of CO on Rh, a system first studied in IETS by P. Hansma
IETS, which provides us with the vibra-tional spectra of molecules, is a precious tool in adsorption studies. It produces the same type of information as iijfra-red spectroscopy but in a wider frequency range (from 30 to 500 eV or 250
to 4000 cm- 1) because infra-red spectroscopy is
limited by the absorption of the substrate. As we shall see, this is particularly useful in the case of» CO adsorbed on metals because IR spectroscopy can only give the C-0 stretch modes ( =2000 cm- 1) and not the metal - carbon (M-C) modes (<500 c m- 1) .
To study the adsorption of CO on Rh by IETS, we use Al-oxide-Pb junctions. Rh is evapo-red onto the aluminium oxide: the average thick-ness is from 3 to 20 A, but no doubt islands
are formed (this is to be checked by electron mi-croscopy) . The Rh is exposed to CO at a pressure of about 5xlO-5 torr during evaporation. The Pb
electrode is then deposited. Electrons tunneling through the metal-insulator- (Rh-CO) - metal junc-tion will excite the vibrajunc-tional modes of Rh-CO (see reference 111 for a general description of the method).
Infra-red spectroscopy gives two C-0 stretch frequencies on Rh / 4 / , attributed to a bridged species (C bonded to two Rh atoms) for the lower frequency and to a linear species (C bonded to only one Rh atom) for the higher. As well as these modes (noted (3) and (4) here) IETS gives three low frequency ones L ( I ) , (1*), (2)J that cannot be observed by IR spectroscopy. To try to identify these new modes we examine the isoto-pic shifts between C1 60 and C1 80 . We use a simple model /3/ : the Rh is supposed infinitely heavy,
the C atom is attached to it by a spring of force constant f, and the 0 atom is attached to the C by a spring of force constant g (Figure 2 ) . (In the case of a bridged species, the symmetric M-C vibration can be represented with an effective force constant).
Taking two frequencies that we attribute to the C-0 and M-C stretch modes (Vg_0 + V M_C) °f a
same species (for example, linear), we can deduce the force constants f and g by inverting the equa-tions of motion of the system :
]
v
c - o "
p (f
»
8
'
M
c> V
\
VM - C " Q <f • 8.
MC V
Then t h e e f f e c t of a change i n t h e oxygen mass (Mo) on t h e f r e q u e n c i e s can be deduced. Compa- r i s o n w i t h t h e experimental v a l u e s of t h e i s o t o p i c s h i f t s t e l l s u s i f t h e model i s c o n s i s t e n t .
This was done f o r t h e exchange 60/180. Figure 1 shows t h e s p e c t r a of c160 and c180 adsor- bed on Rh. As we s a i d , peaks (3) and (4) a r e a t t r i b u e d t o t h e CO s t r e t c h i n g mode of bridged and l i n e a r s p e c i e s r e s p e c t i v e l y . To f i n d t h e c o r r e s - ponding M-C s t r e t c h modes we t r i e d a s s o c i a t i n g f i r s t (3) then (4) each of t h e low frequency modes
(11,
( 1 ' ) and (2).I
(I\+(I') structure forI
I
F i g . a l : S p e c t r a of c160 and c180 adsorbed on Rh (12 A average t h i c k n e s s ) . The i n s g t g i v e s d e t a i l s of peaks ( I )
+
( 1 ' ) obtained on 4 A of Rh. The peak a t 115 meV i s due t o a mode of t h e Al203. F i g u r e s 2a and 2b show t h e p r e d i c t e d i s o t o p i c s h i f t s , a s compared t o t h e experimental r e s u l t s (experimental r e p r o d u c t i b i l i t y i s i n d i c a t e d through t h e mean v a r i a n c e ) .Peak (1) can be a s s o c i a t e d n e i t h e r with(3) nor w i t h ( 4 ) . Whats more i t s i s o t o p i c s h i f t i s v e r y v e r y small; i t could be due t o a s p e c i e s n o t con-
t a i n i n g 0 , such a s a c a r b i d e formed by decomposi- t i o n of CO. (This needs t o be confirmed by s i m i l a r experiments on 1 4 ~ / 1 3 ~ ) .
Peak ( 1 ' ) can f i t e i t h e r w i t h (3) o r w i t h ( 4 ) . It could correspond t o t h e M-C s'tretch of a
peak (2) probably does n o t f i t (3) nor ( 4 ) , b u t i t s s t r o n g i s o t o p i c s h i f t excludes an oxygen-less s p e c i e s l i k e ( I ) . A l e s s s i m p l i f i e d a n a l y s i s taking i n t o account t h e asymmetric M-C s t r e t c h mode and bending modes ( t h e l a t t e r a r e a t much lower f r e - quencies) may a l l o w us t o i d e n t i f y t h i s peak.
F i g . 2 : a ) I s o t o p i c frequency r a t i o s p r e d i c t e d i f we a t t r i b u t e mode (3) t o t h e C-0 s t r e t c h i n g mode of a p a r t i c u l a r s p e c i e s and s u c c e s s i v e l y mode ( I ) ,
( 1 ' ) and (2) t o t h e M-C s t r e t c h i n g mode of t h e same s p e c i e s . For a given a s s o c i a t i o n b3)
-
(ifl two i s o t o p i c r a t i o s ( p 3 and p . ) a r e deduced and p l o t t e d v e r s u s t h e energy hvi:~he v e r t i c a l b a r s( o r band f o r mode (3) ) r e p r e s e n t t h e average ex-
perimental r a t i o s f t h e mean v a r i a n c e . For t h e c o r r e c t a s s o c i a t i o n D 3 )
-
( j u
,
both p g and p jshould be p r e d i c t e d . Only [ ( 3 )
-
( 1 ' n f i t s w e l l .b) I s o t o p i c r a t i o s i f (4) i s a s s o c i a t e d t o ( I ) , ( I F ) o r (2).
I n conclusion, f u r t h e r experiments with 14) and 13C i s o t o p e s t o improve t h e experimental p r e c i s i o n a s w e l l a s a more a r e needed r e f i n e d a n a l y s i s t o confirm t h e assignments g i v e n t o t h e peaks observed, b u t i t i s c l e a r t h a t i s o t o p i c s h i f t s i n IETS b r i n g new i n s i g h t i n t o t h e problem of t h e a d s o r p t i o n of CO on m e t a l s .
References
/ I / Hansma, P.K., Kaska, W.C., Laine,R.d., J.Am.
Chem.Soc.98 (1976) 6064.
/2/ Klein, J., LSger,A., B e l i n , M., Defourneau,D., Sangster,R., Phys.Rev. (1973) 2336
.
Hatlsma, P.K., Phys. Rep.= (1977) 147. / 3 / Wilson, E.B., Decius, J . C . , Cross,P.C., Mole-c u l a r V i b r a t i o n s (Mc Graw-Hill, New York) 1951 141 Wells,M.G., Cant, N.W., Greenler, R.G., S u r f .
S c i .
67
(1977) 541. l i n e a r s p e c i e s and t o t h e synrmetric M-C s t r e t c h ofa bridged s p e c i e s , unresolved.
