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A COMPARISON OF NO INTERACTION WITH STEPPED Pt AND Ru FIELD EMITTER SURFACES
N. Kruse, G. Abend, J. Block
To cite this version:
N. Kruse, G. Abend, J. Block. A COMPARISON OF NO INTERACTION WITH STEPPED Pt AND Ru FIELD EMITTER SURFACES. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-341-C2-346.
�10.1051/jphyscol:1986252�. �jpa-00225686�
A COMPARISON OF NO INTERACTION WITH STEPPED Pt AND Ru FIELD EMITTER SURFACES
N. KRUSE, G. ABEND* and J.H. BLOCK*
Institut fiir Technische Chemle der T.V. Berlin, Strasse des 17.
Jun±128, D-1000 Berlin 33, F.R.G.
*Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradaywegr 4-6, D-1000 Berlin 33, F.R.G.
Résumé - L'adsorption et la décomposition catalytique de NO à la surface de pointes émettrices de Pt et Ru, ont été étudiées par spectrométrie de masse de désorption de champ puisée. En l'absence de champ électrique, il y a ad- sorption moléculaire de NC sur les faces à marches (111) et (COI). La dépen- dance en température des durées de vie avant thermodêsorption, a permis d'évaluer 1'ênerqie d'activation (EJ = 153 KJ/mol) et le facteur préexponen-
"-15
tiel ( T = 5 x 10 sec). Ces valeurs sont représentatives d'une désorption à partir des marches.
La formation c'oxides de surface à une température de 535 à 6C2 K, a été observée pour le Pt(OOl) à marches, mais pas pour la face (111) à marches.
La dépendance en temps de l'intensité de PtO est due à un mêchanisme réactionnel postérieur.
Sur les faces (001) à marches du Ru, il y a adsorption moléculaire et disso- ciative de NO même à 300 K. A 5CC K, le Ru est plus fortement oxydé que le Pt; les ions RuO" jusqu'à RuC, sont observés. Cela implique une instabilité du Ru comme catalyseur de la décomposition de N0.
Abstract - The adsorption and the catalytic decomposition of NO on Pt and P.u field emitter surfaces has been studied by pulsed field desorption mass spec- trometry (PFDMS). A molecular adsorption of NO vas found on stepped Pt(lll) and (001) surfaces in the absence of electrostatic fields. From the tempera- ture dependence of the nean lifetimes before thermal desorption an activation energy Ed= 153 kJ/ral and a pre-exponential term T = 5 x 10" sec have been evaluated. These values are considered as representative for desorption from steps.
The formation of surface oxides at temperatures T=535...6C2 K was observed for stepped Pt(001), but not for stepped (111) surfaces. The time dependence of the PtO intensity is due to a consecutive reaction mechanism.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986252
JOURNAL DE PHYSIQUE
On stepped Ru(C01) surfaces a r:olecular as w e l l as a d i s s o c i a t i v e adsorption o f NO occurs even a t 300 K. A t 5QC K, Ru i s v o r e s t r o n ~ l y o x i d i z e d than Pt, PuCX ions up t o n+ RUC:' a r e observed. This i r r p l i e s an i n s t a b i l i t y o f Fu as a c a t a l y s t f o r NO d e c o ~ p o s i t i o n .
I
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INT'CDUCTIONThe p l a t i n u m ~ e t a l p l a y s an i m p o r t a n t r o l e as a component i n r u l t i f u n c t i o n a l cata- l y s t s f o r n i t r i c oxide removal from automobile exhaust gases. Ruthenium has a l s o pathered i n t e r e s t as a candidate f o r n i t r i c o x i d e decomnosition. However, i t forms v o l a t i l e oxides, Ru03 and Ru04, under t e c h n i c a l c o n d i t i o r s thus r e s t r i c t i n g i t s p r a c t i c a l appl i c a b i l i t y .
Various surface s e n s i t i v e techniques have been enployed t o e l u c i d a t e t h e f a c e selec- t i v i t y o f P t and Ru towards NO decomposition ( f o r a review see / l / ) .
Recently, a h i g h c a t a l y t i c a c t i v i t y has been discussed f o r c e r t a i n stepped surfaces l i k e P t (410), as expected from t h e o r e t i c a l c o n s i d e r a t i o n s /2/ and v e r i f i e d i n ex- perimental observations /3/.
I n t h i s study we cornpare adsorption and d e c o r p o s i t i o n o f NO on the stepped (001) surfaces o f P t and Ru f i e l d e m i t t e r s . Ve e r p l o y pulsed f i e l d d e s o r p t i o n mass spec- t r o r e t r y (PFDKS) i n o r d e r t o o b t a i n k i n e t i c i n f o r m a t i o n about these processes as r e c e n t l y r e p o r t e d f o r NO d e s o r p t i o n from stepped P t (111) surfaces /4/.
I 1
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EXPERIMENTALThe PFDMS technique has been described i n d e t a i l elsewhere / 5 / . Fast n e g a t i v e pulses o f up t o 20 kV amplitude are a p p l i e d t o an e l e c t r o d e i n f r o n t o f a P t o r a Ru f i e l d e o i t t e r t i p . F i e l d s t r e n g t h s of t h e order of some t e n V/nm cause t h e d e s o r p t i o n o f the adsorbed species. The r e s p e c t i v e i o n s a r e c h e ~ i c a l l y anal yzed by t i m e - o f - f l i a h t mass spectrometry.
K i n e t i c processes on t h e f i e l d e m i t t e r surfaces can be s t u d i e d by varyin! t h e repe- t i t i o n frequency o f t h e pulses, i . e . t h e t i n e tR between t h e pulses, u s u a l l y f r o r I C O usec up t o sone seconds. D e t a i l s - i n c l u d i n g e x p e r i r e n t a l procedures f o r r e a - surements o f t h e mean l i f e t i m e T b e f o r e t h e r ~ a l d e s o r p t i o n
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have been published e l sewhere /4/.NO (99.6 % p u r i t y ) was provided by Messer Griesheim. Both, P t (Goodfellow I.'etals, 99.99 % p u r i t y ) and Ru (spark e r o s i o n c u t from a boule s u p p l i e d by F a t e r i a l s Re- search Corporation) f i e l d e m i t t e r s were (001) o r i e n t e d . They vere cleaned i n s i t ~ by f i e l d evaporation.
F i r . 1 shows t h e r e s u l t s of t h e r e a c t i o n t i m e v a r i a t i o n reasurements. We s e l e c t e d t h e stepped 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 s o f (001) o r i e n t e d f i e l d e s i t - t e r t i p s o f P t and Ru. The ~ e a s u r e m e n t s have been p e r f o r r e d under c o n t i n u o u s s u p p l y o f NO f r o r t h e gas phase a t p = 1.3 x I O - ~ Pa. The f i e l d s t r e n n t h amounts t o F = 28 V/nm, s u f f i c i e n t t o desorb m o l e c u l a r l y adsorbed WC q u a n t i t a t i v e l y w i t h each p u l s e . Thus, t h e measured NO' i o n i n t e n s i t i e s a r e p r o o o r t i o n a l t o t h e NOad s u r f a c e c o n c e n t r a t i o n . S u b s t a n t i a l amounts o f Oad a r e rerroved by t h e f i e l d p u l s e s , so t h a t i t c a n n o t a c c u r u l a t e a t t h e s u r f a c e . Oad appears as P t o f o r R U O ~ ' i n t h e r a s s spec- t r a . D u r i n g t h e f i e l d pulses, NO molecules a r e a c c e l l e r a t e d by t h e f i e l d , and h i t t h e surface w i t h h i g h e r k i n e t i c energy, so t h a t t h e i r d i s s o c i a t i o n r a t e i s enhancea.
T h i s p a r t of t h e i o n i n t e n s i t y i s c a l c u l a t e d and s u b t r a c t e d f r o m t h e t o t a l i o n c u r - r e n t s o f P ~ O ' and I?uO2'. \re f i r s t i n s p e c t a d s o r p t i o n o f NO on stepped P t (001),
f i c .
l a . Both, r o l e c u l a r and d i s s o c i a t i v e a d s o r p t i o n t a k e p l a c e as shown by t h e O C -curence o f NO' and P ~ O ' i o n s . R t s h o r t t i r e s , t h e PO t i o n i n t e n s i t y i n c r e a s e s l i n - e a r l y w i t h ta. A t l o n o t i r e s , t h e NO t i n t e n s i t y l e v e l s o f f . T h i s b e h a v i o u r i s c h a r - a c t e r i s t i c f o r t h e r ~ a l d e s o r p t i o n o f PIPad d u r i n g tR. I f we a s s u r e ( i ) a d s o r p t i o n t o be a s s o c i a t e d w i t h a c o n s t a n t s t i c k i n g p r o b a b i l i t y and ( i i ) t h e thermal d e s o r p t i o n t o obey f i r s t o r d e r k i n e t i c s , t h e s u r f a c e c o n c e n t r a t i o n o f NGad develops a c c o r d i n n t o c = c ( 1
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e-t17). S i n c e t h e measured NO' i o n i n t e n s i t i e s d i s p l a y t h e sace t i m e0
dependence, t h e mean l i f e t i m e r b e f o r e thermal d e s o r p t i o n o f NOad can be determined as t h a t v a l u e o f tR where t h e NO' i o n i n t e n s i t y reaches t h e ( 1
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l / e ) l e v e l o f t h e c o n s t a n t p a r t a t l o n g t i m e s . A t T = 543 K we f i n & T = 0.2 sec.Ey comparing t h e i n t e n s i t y i n c r e a s e o f NO' a t s h o r t times, i . e . t h e a d s o r p t i o n r a t e d c / d t , w i t h t h e impingement r a t e f r o n t h e fas phase i n t o t h e a o n i t o r e d a r e a
(checked by NC' dc f i e l d i o n i z a t i o n ) , v a l u e s o f 0.65.. .0.85 a r e d e r i v e d f o r t h e s t i c k i n g p r o b a b i l i t y . These v a l u e s a r e i n a c c o r d w i t h ~ e a s u r e ~ e n t s o f o t h e r a u t h o r s /6,7/.
Ide now i n s p e c t t h e p r o p e r t y o f t h e stepped ( O C 1 ) s u r f a c e t o o a r t i a l l y d i s s o c i a t e t h e adsorbed NO. I1e t a k e t h e P ~ O ' i o n i n t e n s i t i e s as a r-easure f o r t h e f o r m a t i c n o f s u r - f a c e o x i d e due t o d i s s o c i a t i o n o f VOad. N i t r o g e n c o n t a i n i n g i o n i c metal s p e c i e s a r e n o t d e t e c t e d . T h i s i s e x p l a i n e d by f a s t r e c o r b i n a t i o n and t h e r m a l d e s o r p t i o n ac- c o r d i n g t o 2Nad
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N 299'W i t h i n t h e measured t i m e range, t h e P ~ O ' i o n i n t e n s i t i e s a r e always c o n s i d e r a b l y s m a l l e r t h a n t h e NO' i n t e n s i t i e s . Thus, r o l e c u l a r a d s o r p t i o n p r e v a i l s o v e r d i s s o c i a - t i v e a d s o r p t i o n . The P ~ O ' i n t e n s i t y ( f r o m t h e f i e l d f r e e r e a c t i o n between t h e p u l s e s ) appears w i t h d d e l a y time, which i s o f a b o u t t h e same v a l u e as t h e t i m e r = 0.2 sec t o r e a c h t h e e q u i l i b r i u m c o n c e n t r a t i o n o f n o l e c u l a r l y adsorbed NO. T h i s i s i n a c c o r d w i t h a c o n s e c u t i v e r e a c t i o n nechanism, where a d s o r p t i o n o f NO f r o m t h e gas phase has t o t a k e p l a c e b e f o r e d e c o r r p o s i t i o n i n t o Oad can o c c u r . Values f o r t h e d i s s o c i a t i o n p r o b a b i l i t y , W, can be o b t a i n e d by c o r p a r i n y t h e r e a s u r e d PtO i n -
+
JOURNAL DE PHYSIQUE
Fig. l
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Dependence of ion i n t e n s i t i e s on r e a c t i o n time, tk. NO pressure: 1.3 XPa, impingement r a t e : 0.19 molecules/sec i n t o t h e monitored a r e a .
l a )
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NO' and P ~ O ' ion i n t e n s i t i e s , P t t i p temperature: 543 K , f i e l d s t r e n g t h : 20 V/nm. T = mean l i f e t i m e of molecularly adsorbed NO.10)
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~u0'' ion i n t e n s i t y , Ru t i p temperature: 503 K, f i e l d s t r e n g t h : 30 V/nm. For comparison with P ~ O ' of f i g . l a , t h e ion i n t e n s i t y has been r e l a t e d t o t h e same s i z e of t h e r o n i t o r e d a r e a .t e n s i t i e s with t h e impingement r a t e . Within t h e reasured time range, W i s small. A t a time t R = 1 s e c , w i s of t h e o r d e r of a few percent only.
In f i g . l b we compare t h e development of t h e s u r f a c e oxide on stepped Pt(OC1) with t h a t on stepped Ru(OC1). In c o n t r a s t t o t h e Pt s u r f a c e , t h e oxide build-up on Pu follows normal k i n e t i c s a s evidenced by a proportional i n c r e a s e of t h e R U C ~ + in- t e n s i t y with t i r e . Furthermore, t h e d i s s o c i a t i o n p r o b a b i l i t y of t h e MC i s consider- ably higher on t h e Ru s u r f a c e . By comparison w i t h t h e impingement r a t e from t h e gas phase we f i n d values of more than 5 %. Itithin t h e treasured time range t h e s u r f a c e oxiae coverage i s f a r below a monolayer. Further oxidation of P,u i s achieved a t higher NO gas p r e s s u r e s .
In f i g . 2 , d a t a of a pulse f i e l d s t r e n c t h v a r i a t i o n measurement a t p = 5 . 5 X 10 Pa -4 and t R = 0.25 sec a r e shown. A t low f i e l d s t r e n g t h values, 140' ion i n t e n s i t i e s
( i o n s / s e c ) doninate. They i n c r e a s e with t h e r i s i n g f i e l d s t r e n g t h . This i s due t o
h i g h d e s o r p t i o n r a t e s o f Ruo2'. L'e c o n c l u d e t h a t a t h i g h f i e l d s t r e n g t h s t h e s u r - face o x i d e l e v e l i s c o n s i d e r a b l y lowered by f i e l d d e s o r p t i o n . Thus oxygen vacancy s i t e s a r e c r e a t e d a l l o w i n g f o r NOad d i s s o c i a t i o n .
A t l o w f i e l d s t r e n g t h s , i . e . w i t h o u t d e s t r o y i n g t h e o x i d e l a y e r by t h e f i e l d p u l s e s , we observe RUO;' and RUO:' i o n s . T h e i r decrease a t h i g h e r f i e l d s t r e n g t h s i s more pronounced f o r RUO:' t h a n f o r RUO;'.
desorption field strength [V/nml
F i g . 2
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Dependence o f i o n i n t e n s i t i e s on d e s o r p t i o n f i e l d s t r e n g t h . NO p r e s s u r e : 6.7 x 1 0 - ~ Pa, Ru t i p temperature: 552 K, r e a c t i o n t i m e : 0.25 sec, i . e . p u l s e f r e - quency: 4 Hz.IV
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DISCUSSIONA d s o r p t i o n o f NO on stepped (001) f i e l d e m i t t e r s u r f a c e o f P t and Ru o c c u r s mole- c u l a r l y as w e l l as d i s s o c i a t i v e l y . On t h e P t s u r f a c e m o l e c u l a r a d s o r p t i o n o f NO p r e - v a i l s o v e r d i s s o c i a t i v e a d s o r p t i o n i n t h e measured t i m e range a t T = 543 K . The mean l i f e t i m e T o f NOad b e f o r e thermal d e s o r p t i o n has been measured. We f i n d T = 0.2 sec a t T = 543 K. The t e m p e r a t u r e dependence o f T has been measured and w i l l be p u b l i s h e d elsewhere. We found, t h a t t h e r a t e parameters o f thermal d e s o r p t i o n (Ed=
153 kJ/mol, r0 = 5 X 10-''sec) a r e governed by t h e presence o f s t e p s . T h i s i s i n accord w i t h o b s e r v a t i o n s o f o t h e r a u t h o r s /6,7/. S i n c e s t e p s may be a c t i v e i n N-0 bond b r e a k i n g i t has t o be concluded t h a t m o l e c u l a r a d s o r p t i o n and d i s s o c i a t i o n a r e
c2-346 JOURNAL
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PHYSIQUEc o m p e t i t i v e processes a t t h e s e s i t e s . W i t h i n t h e measured t i m e range, NO' and P ~ O ' i n t e n s i t i e s a r e n o t i n f l u e n c e d by each o t h e r . Thus, Oad does n o t c o m p l e t e l y b l o c k t h e s t e p s i t e s f o r m o l e c u l a r a d s o r p t i o n .
D i s s o c i a t i v e a d s o r p t i o n was f o u n d a t t h e s t e p s o f a (001) P t s u r f a c e , b u t n o t a t t h e s t e p s o f ( 1 1 1 ) P t . There, NO adsorbed o n l y m o l e c u l a r l y / 4 / . Banholzer e t a l . /2/ de- veloped a symmetry ~ o d e l f o r e x p l a i n i n g t h i s s t r o n g i n f l u e n c e o f t h e c r y s t a l l o g r a p h - i c o r i e n t a t i o n .
It has been f o u n d t h a t t h e o x i d e b u i l d - u p a t 543 K e s s e n t i a l l y b e g i n s a f t e r t h e mo- l e c u l a r l y adsorbed NOad has reached i t s e q u i l i b r i u m c o n c e n t r a t i o n a t T = 0.2 sec.
T h i s b e h a v i o u r i s e x p l a i n e d by a c o n s e c u t i v e r e a c t i o n mechanism, i n v o l v i n g adsorp- t i o n f i r s t and decomposition s u c c e s s i v e l y . The s t e e p i n c r e a s e o f t h e P ~ O ' i n t e n s i t y a t l o n g t i m e s tR p o s s i b l y i n d i c a t e s d i s s o c i a t i o n k i n e t i c s o f h i g h e r t h a n f i r s t o r d e r
The d i s s o c i a t i o n p r o b a b i l i t y (compared w i t h t h e impingement r a t e f r o m t h e pas phase) i s c o n s i d e r a b l y h i g h e r f o r c l e a n Ru t h a n f o r c l e a n P t . A t l o w p u l s e d f i e l d s t r e n g t h s t h e s u r f a c e l a y e r i s n o t c o m p l e t e l y removed t h u s a l l o w i n g Oad t o accumulate a t t h e s u r f a c e and t o form an o x i d e l a y e r where RUO? and RUO;' i o n s can be desorbed from.
I t i s remarkable t h a t such an o x i d e l a y e r i s b u i l t u p n o t o n l y f r o m O2 /8/ b u t a l s o from NO. T h i s i n t e n s e o x i d a t i o n o f t h e r u t h e n i u m r e s t r i c t s i t s a p p l i c a b i l i t y as a c a t a l y s t f o r NO decomposition, i n c o n t r a s t t o p l a t i n u m which i s n o t so s t r o n g l y o x i d i z e d .
ACKNOWLEDGEMENT
T h i s work was p a r t i a l l y supported by Sonderforschungsbereich 6 a t t h e F r e i e U n i - v e r s i t a t B e r l i n
REFERENCES
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