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FORMATION OF Ru-SUBCARBONYLS BY REACTION OF CO WITH Ru FIELD EMITTER
SURFACES
N. Kruse, G. Abend, J. Block, E. Gillet, M. Gillet
To cite this version:
N. Kruse, G. Abend, J. Block, E. Gillet, M. Gillet. FORMATION OF Ru-SUBCARBONYLS BY
REACTION OF CO WITH Ru FIELD EMITTER SURFACES. Journal de Physique Colloques, 1986,
47 (C7), pp.C7-87-C7-92. �10.1051/jphyscol:1986716�. �jpa-00225906�
JOURNAL DE PHYSIQUE
Colloque C7, supplbment au n o 11, Tome 47, Novembre 1986
FORMATION OF Ru-SUBCARBONYLS BY REACTION OF CO WITH RU FIELD EMITTER SURFACES
N. KRUSE, G. ABEND, J . H . BLOCK, E . GILLET' and M. GILLET'
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 0-1000 Berlin 3 3 , F.R.G.
' ~ a b o r a t o i r e de Microscopie et Diffractions Electroniques, U A 797, Facult6 des Sciences et Techniques de Saint JBrdrne, F-13397 Marseille Cedex 1 3 , France
A b s t r a c t
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The i n t e r a c t i o n o f CO w i t h Ru f i e l d e m i t t e r s u r f a c e s i s s t u d i e d by p u l s e d f i e l d d e s o r p t i o n mass s p e c t r o m e t r y (PFDMS). RU(CO):+ s p e c i e s up t o R u ( C O ) ~ a r e d e t e c t e d . These i o n s a r e formed f r o m t h e i r c o r r e s p o n d i n g n e u t r a l s . 2+F i e l d s t r e n g t h v a r i a t i o n measurements show t h a t t h e R u ( C O ) ~ c a n n o t f o r m under t h e i n f l u e n c e o f s t e a d y e l e c t r i c a l f i e l d s . Thus, t h e R u ( C O ) ~ i s t h e p r o d u c t o f a s u r f a c e r e a c t i o n o c c u r r i n g f i e l d f r e e i n t h e t i m e between t h e p u l s e s . V a r i a - t i o n o f t h i s r e a c t i o n t i m e a t d i f f e r e n t temperatures proves t h e R U ( C O ) ~ forma- t i o n t o be a s s o c i a t e d w i t h an a c t i v a t e d process. A model i s p r e s e n t e d w h i c h des- c r i b e s t h i s process as t h e removal o f Ru l a t t i c e atoms f r o m k i n k s i t e p o s i t i o n s and t h e i r d i f f u s i o n i n t o t h e t e r r a c e r e g i o n s where f u r t h e r r e a c t i o n towards t h e h i g h i n d e x R U ( C O ) ~ s p e c i e s o c c u r s .
I
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INTRODUCTIONThe i n t e r a c t i o n of CO w i t h macroscopic l o w i n d e x s i n g l e c r y s t a l s u r f a c e s has been s t u d i e d i n t e n s e l y i n t h e p a s t and a r a t h e r d e t a i l e d p i c t u r e on t h e c h a r a c t e r i s t i c s o f t h e adsorbed l a y e r has emerged, e s p e c i a l l y f o r CO on Ru(001) /1-3/. R e c e n t l y , t h e s t e p s i t e s o f R u ( l , l , l O ) have been f o u n d a c t i v e i n C-0 bond b r e a k i n g /4/. The a c t i v i t y o f s t e p s towards t h e f o r m a t i o n o f subcarbonyl s p e c i e s has n o t been c l e a r l y p e r c e i v e d so f a r i n s i n g l e c r y s t a l work. On t h e o t h e r hand, t h e r e i s mounting e v i - dence f o r m u l t i p l e CO bonding on account o f IR r e s u l t s on s u p p o r t e d Ru c a t a l y s t p a r t i c l e s / 7 / which c o n t a i n c o n s i d e r a b l e amounts o f l o w c o o r d i n a t i o n l a t t i c e s i t e s o r d e f e c t s a t t h e i r s u r f a c e s .
The l a c k o f d e t a i l e d i n f o r m a t i o n a b o u t adsorbed Ru-subcarbonyls i s t h e m o t i v a t i o n f o r o u r s t u d y . We employed p u l s e d f i e l d d e s o r p t i o n mass s p e c t r o m e t r y t o s t u d y t h e i r f o r - m a t i o n k i n e t i c s on stepped Ru f i e l d e m i t t e r s u r f a c e s . PFDMS was a l r e a d y s u c c e s s f u l l y a p p l i e d i n s i m i l a r s t u d i e s on K i - s u b c a r b o n y l f o r m a t i o n f r o m N i and CO / 8 / .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986716
JOURNAL DE PHYSIQUE
I 1
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EXPERIMENTALThe e x p e r i m e n t a l s e t - u p o f PFOMS has been d e s c r i b e d i n d e t a i l elsewhere /9/. B r i e f l y , h i g h n e g a t i v e f i e l d p u l s e s (maximum a m p l i t u d e s 20 kV, r e p e t i t i o n r a t e s up t o 100 kHz, h a l f w i d t h s some 100 n s ) a r e a p p l i e d t o a c o u n t e r e l e c t r o d e w i t h a h o l e , a t a d i s - t a n c e o f - 0 . 1 mm i n f r o n t o f t h e Ru f i e l d e m i t t e r . Thereby t h e l a y e r , adsorbed a t t h e Ru s u r f a c e , i s desorbed. The r e s p e c t i v e i o n s a r e c h e m i c a l l y i d e n t i f i e d by t i m e - o f - f l i g h t mass s p e c t r o m e t r y . A channel p l a t e image i n t e n s i f i e r i s mounted a t t h e en- t r a n c e o f t h e f l i g h t t u b e and a l l o w s t o d e t e r m i n e t h e o r i e n t a t i o n o f t h e t i p .
T i p temperatures a r e measured by means o f a thermocouple spotwelded t o t h e t i p base.
The Ru e m i t t e r i s s p a r k e r o s i o n c u t f r o m a b o u l e s u p p l i e d by M a t e r i a l s Research C o r p o r a t i o n , and e l e c t r o l y t i c a l l y etched i n d i l u t e HC1.
PFOMS experiments a r e performed by c o n t i n u o u s l y d o s i n g t h e t i p w i t h gaseous CO. The k i n e t i c s o f s u r f a c e r e a c t i o n s a r e i n v e s t i g a t e d by s y s t e m a t i c a l l y v a r y i n g t h e r e a c t i o n t i m e , tR, between t h e f i e l d p u l s e s . U s u a l l y a range tR=lOO ps
...
1s i s scanned. De- t a i l s on g e n e r a l procedures f o r k i n e t i c s t u d i e s have been d i s c u s s e d elsewhere/ l o / .
An a r b i t r a r y f i e l d s t r e n g t h , FR, between t h e p u l s e s can be a p p l i e d i n o r d e r t o i n - v e s t i g a t e t h e i n f l u e n c e o f t h e e l e c t r i c a l f i e l d on t h e s u r f a c e processes.
I 1 1
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RESULTSThe r e s u l t s t o be p r e s e n t e d h e r e have been o b t a i n e d by p r o b i n g t h e stepped s u r f a c e r e g i o n i n t h e v i c i n i t y o f t h e (001) p o l e o f t h e Ru f i e l d e m i t t e r . The s i z e o f t h e m o n i t o r e d a r e a i s 8.8 nm 2
,
c o n t a i n i n g a b o u t 140 a t o m i c s i t e s o f t h e Ru(001) p l a n e .V a r i o u s i o n i c s p e c i e s a r e d e t e c t e d d u r i n g t h e i n t e r a c t i o n o f CO w i t h t h e Ru e m i t t e r s u r f a c e . Besides CO* and Ru*, Ru2+, s i n g l y and d o u b l y charged subcarbonyl s p e c i e s RU(CO):* ( x = l . . . 4 ) a r e observed. A l s o
c2*, c*,
0* as w e l l as v a r i o u s Ru-carbides and - o x i d e s a r e seen i n t h e mass s p e c t r a . T h i s i n d i c a t e s CO d i s s o c i a t i o n t o t a k e p l a c e .a ) S'I'EADY F I E L D INFLUENCE
I t has been r e p o r t e d r e c e n t l y /11/ t h a t t h e i o n i c i n t e n s i t i e s o f t h e subcarbonyls, RU(CO):+, as w e l l as t h o s e o f t h e c a r b i d i c and o x i d i c s p e c i e s depend on t h e r e a c t i o n f i e l d , FR. T h i s o b s e r v a t i o n i s f u r t h e r s u b s t a n t i a t e d by f i g . 1. The d e s o r p t i o n f i e l d s t r e n g t h amounts t o FO=29 V/nm. T h i s v a l u e t u r n e d o u t t o be s u f f i c i e n t l y h i g h i n o r d e r t o desorb t h e adsorbed l a y e r q u a n t i t a t i v e l y by each f i e l d p u l s e /11/. The d a t a p r e s e n t e d i n f i g . 1 have been o b t a i n e d by v a r y i n g FR and a d j u s t i n g t h e p u l s e d f i e l d ,
F i g . 1 - Experimental i n t e n s i t i e s as f u n c t i o n o f t h e s t e a d y f i e l d s t r e n g t h , F ~ '
Parameters:
FD=29 V/nm, c o n s t a n t d u r i n g FR v a r i a - t i o n
,
t R = 1 0 ms, T.328 K, pCO=1.3x10-4~a0 1 2 3 L 5 6 7 8
field strength F, [ Vlnm I
Fp, such t h a t FD remained c o n s t a n t . Thus, complete d e s o r p t i o n o f t h e adsorbed s p e c i e s has been m a i n t a i n e d d u r i n g t h e s e measurements. We f i n d d i f f e r e n t t r e n d s f o r t h e i o n i n t e n s i t y o f t h e v a r i o u s s p e c i e s w i t h i n c r e a s i n g FR. F o r small FR v a l u e s c o n s i d e r - a b l e i o n i n t e n s i t i e s o f RU(CO):+ (x=2-4) a r e found, t h e Ruco2+, however, i s m i s s i n g i n t h e mars s p e c t r a . W i t h i n c r e a s i n g FR t h e RU(CO)? i n t e n s i t y decreases and f o r FR > 7 V/nm t h i s s p e c i e s c a n n o t be d e t e c t e d any l o n g e r . I n c o n t r a s t t o t h i s b e h a v i o u r we f i n d i n c r e a s i n g amounts o f R U C O ~ + , w i t h an o n s e t a t a f i e l d s t r e n g t h FR rr 3 V/nm.
The RU(CO);+ i o n s a l s o become s l i g h t l y more abundant w i t h i n t h e measured range o f FR values, whereas no change o f t h e amount o f RU(CO):+ i s observed. I t i s n o t e d t h a t t h e t o t a l CO c o n t e n t i n RU(CO):* (x>Z) remains n e a r l y c o n s t a n t d u r i n g FR v a r i a t i o n .
R U C O ~ ' a r e c o n s i d e r e d t o be formed by f i e l d d e s o r p t i o n o f COad w i t h simultaneous r e - moval o f t h e u n d e r l y i n g l a t t i c e atom. The occurrence o f t h e RU(CO):+ i o n s can be understood i n terms o f a c o n s e c u t i v e s u r f a c e r e a c t i o n i n v o l v i n g t h e i r n e u t r a l mole- c u l e s . These s p e c i e s must have d i f f e r e n t d i p o l e moments, p
,
i n o r d e r t o observe t h e s t e a d y e l e c t r i c a l f i e l d t o change t h e r e l a t i v e i n t e n s i t i e s . U n f o r t u n a t e l y , no u v a l u e s f o r adsorbed Ru-subcarbonyls a r e a v a i l a b l e . S i n c e t h e i n f l u e n c e o f FR i s t o i n c r e a s e t h e i n t e n s i t i e s o f t h e l o w i n d e x R U ( C O ) ~ sp e c i e s we expect, f r o m thermo- dynamic reasons, t h e s e s p e c i e s t o have s m a l l e r d i p o l e moments t h a n t h e h i g h i n d e x homologues.The i n t e n s i t i e s o f c a r b i d i c and o x i d i c s p e c i e s i s s m a l l a t l o w FR v a l u e s . However, a s h a r p o n s e t i s found a t FR= 7 V/nm as evidenced by t h e RUC" s p e c i e s . T h i s f i n d i n g
C7-90 J O U R N A L D E PHYSIQUE
suggests d i s s o c i a t i o n o f adsorbed CO t o take p l a c e and be promoted by the presence o f ( p o s i t i v e ) steady e l e c t r i c a l f i e l d s .
Shincho e t a l . , i n a r e c e n t paper /4/, r e p o r t on d i s s o c i a t i v e a d s o r p t i o n o f CO a t t h e step s i t e s o f a macroscopic Ru(l,l,lO) surface. Our measurements c o n f i r m t h i s r e - s u l t , however, t h e c o n c e n t r a t i o n o f c a r b i d i c and o x i d i c species i s small here because t h e h i g h frequency pulses (f=100 Hz, i .e. tR=10 ms) p r o v i d e f o r continuous f i e l d de- s o r p t i o n .
We mention t h a t t h e CO' i n t e n s i t y a l s o decreases s l o w l y d u r i n g FR v a r i a t i o n . The reason f o r t h i s observation i s s t i l l unknown. However, we s t a t e t h a t under pure pulsed f i e l d c o n d i t i o n s , i . e . FD=Fp=29 V/nm, t h e measured CO' i o n i c r a t e equals t h e a d s o r p t i o n r a t e o f CO from t h e gas phase. For d e t a i l s see r e f . /11/.
b ) KEACTIOPJ TIME VARIATIONS
The k i n e t i c s o f t h e R u ( C O ) ~ f o r m a t i o n can be s t u d i e d by v a r y i n g the r e a c t i o n time, tR. T h i s has been done by s w i t c h i n g o f f FR and a p p l y i n g pulsed f i e l d s o n l y . Under these c o n d i t i o n s t h e subcarbonyl f o r m a t i o n runs up t o t h e Ru(CO)~. The d e s o r p t i o n f i e l d s t r e n g t h i s t h e same as i n f i g . 1 and t h e adsorbed l a y e r i s q u a n t i t a t i v e l y removed by each p u l s e now as before. Thus, the i o n i n t e n s i t i e s o f the RU(CO)? spe- c i e s a r e a measure o f t h e surface concentrations o f t h e r e s p e c t i v e n e u t r a l molecules w i t h i n t h e monitored area.
F i g . 2
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Experimental RU(CO):+ i n t e n s i - t i e s as f u n c t i o n o f t h e r e a c t i o n time tR, monitored a t an area o f about 140 s u r f a c e s i t e s (impingement r a t e ~ 6 . 5 CO/s r e f e r r i n g t o a c a l i b r a t e d t r a n s - mission o f 18% f o r t h e mass spectro- meter).Parameters:
Fp=29 V/nm, FR=O, pC0=1.3x10 Pa, -4 a)T=328 K, b)T=458 K
ta-' C lo0
react~on time I sl
I n f i g . 2 we compare t h e t i m e dependence o f t h e Ru(CO)gt i n t e n s i t i e s f o r two d i f - f e r e n t temperatures, T1=328 K and T2=458 K, r e s p e c t i v e l y . High i n t e n s i t i e s f o r Ru(c0):' and RU(CO):' a r e o n l y found a t t h e lower temperature. The species f o l low a s i m i l a r t r e n d d u r i n g tR v a r i a t i o n as t h e Ru(cO)gt. A t t h e h i g h e r temperature these species do n o t form i n s i g n i f i c a n t c o n c e n t r a t i o n s i n c e adsorbed CO,,which i s a reac t i o n p a r t n e r f o r t h e i r formation, undergoes s u b s t a n t i a l thermal desorption. D e t a i l s
have been discussed elsewhere Ill/.
I n f i g . 2 we f i n d t h e RU(CO)~ concentrations t o increase f i r s t and t o l e v e l o f f l a - t e r on. Thus, a t s h o r t tR, t h e r a t e s o f t h e formation and t h e forward r e a c t i o n ( t o - wards t h e h i g h e r homologues) dominate over the r a t e s o f t h e back r e a c t i o n . A t l o n g tR, these r a t e s counterbalance and, consequently, steady concentrations e s t a b l i s h . The corresponding r e l a x a t i o n time, T
,
which i s t h e time necessary t o reach t h e ( 1 - l / e ) value o f the s a t u r a t i o n c o n c e n t r a t i o n i n a f i r s t order r e a c t i o n , i s found t o decrease w i t h i n c r e a s i n g temperature. We f i n d rl = 30 ms a t TI = 328 K and, r e s - p e c t i v e l y , r 2 = 3 ms a t T2 = 458 K. The steady Ru(CO)~ c o n c e n t r a t i o n f o l l o w s t h e same trend, i .e. i t drops by n e a r l y a f a c t o r o f ten. I n t e r e s t i n g l y , t h e RU(CO)~ f o r m a t i o n i s associated w i t h an i n d u c t i o n p e r i o d a t low T whereas t h e r e i s no delay t i m e a t h i g h T. This observation i n d i c a t e s an a c t i v a t e d process being i n v o l v e d i n the RU(CO)~formation.
I V
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DISCUSSIONThe i n t e r a c t i o n o f CO w i t h a stepped Ru f i e l d e m i t t e r s u r f a c e has been s t u d i e d and t h e i o n i c species RU(CO):+ up t o x=4 a r e found. A f i e l d induced f o r m a t i o n o f these species can be excluded on account o f t h e dependencies o f t h e i o n i n t e n s i t i e s on t h e f i e l d strength, FR, and on t h e r e a c t i o n time a t d i f f e r e n t temperatures. Since t h e RU(CO):' i s absent a t h i g h FR ( f i g . 1) the corresponding n e u t r a l species must be t h e product o f the f i e l d f r e e surface r e a c t i o n .
The k i n e t i c s o f t h e Ru(CO)~ formation have been studied. I t i s found t h a t t h e RU(CO)~
forms by an a c t i v a t e d process. This process i s l i k e l y t o be associated w i t h t h e r e - moval o f a Ru atom from a k i n k step s i t e p o s i t i o n
A f t e r d i f f u s i o n i n t o t h e (001) t e r r a c e regions, f o r m a t i o n o f h i g h index Ru(CO), spe- c i e s occurs by successive a d d i t i o n o f COad t o t h e r e s p e c t i v e precursor species.
A s i m i l a r mechanism was d e r i v e d f o r t h e f o r m a t i o n of Ni(C0)2 from N i and gaseous CO a f t e r PFDMS s t u d i e s 181. I n a d d i t i o n , f i e l d i o n microscopic s t u d i e s gave d i r e c t e v i -
C7-92 J O U R N A L DE PHYSIQUE
dence f o r t h e removal o f N i k i n k s i t e atoms f r o m t h e l a t t i c e /12/. S i m i l a r measure- ments a r e c u r r e n t l y b e i n g performed f o r CO/Ru.
The o u t l i n e d mechanism o f t h e R u ( C O ) ~ fo r m a t i o n c o n s i d e r s t h e c o n c e n t r a t i o n o f
[ I kink t o be s t a t i o n a r y . T h i s i s a good a p p r o x i m a t i o n f o r o u r experimental c o n d i - t i o n s s i n c e c o n t i n u o u s f i e l d e v a p o r a t i o n of t h e Ru s u b s t r a t e preserves t h e k i n k s i t e s . However, i n s t u d i e s on supported Ru c a t a l y s t p a r t i c l e s t h i s w i l l l e a d t o f a - c e t t i n g o f t h e s u r f a c e and t o m o r p h o l o g i c a l changes o f t h e m e t a l p a r t i c l e s as ob- served by Park e t a l . /13/.
The r e s u l t s o f t h e p r e s e n t s t u d y c o n f i r m t h e m u l t i p l e a d s o r p t i o n model as suggested f i r s t by Kobayashi and S h i r a s a k i f r o m I R r e s u l t s o f CO on supported Ru /5/. I t i s i n t e r e s t i n g t o n o t e t h a t we have a l s o evidence f o r subcarbonyl f o r m a t i o n on Pd and Rh f i e l d e m i t t e r s u r f a c e s . E a r l y I R d a t a i n d i c a t e Rh(C0)2 f o r m a t i o n on alumina sup- p o r t e d Rh c a t a l y s t s /14/. L a t e r r e s u l t s by t h e same method suggest t h i s species t o be formed i n an a c t i v a t e d process /15/. These f i n d i n g s c a l l f o r more d e t a i l e d PFDMS s t u d i e s i n t h e f u t u r e .
Acknowledgement
T h i s work was p a r t i a l l y supported by t h e Sonderforschungsbereich ( S f b 6 ) a t t h e F r e i e U n i v e r s i y a t B e r l i n .
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