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THE CO OXIDATION REACTION ON Rh : A PULSED-LASER AND IMAGING ATOM-PROBE
STUDY
G. Kellogg
To cite this version:
G. Kellogg. THE CO OXIDATION REACTION ON Rh : A PULSED-LASER AND IMAG- ING ATOM-PROBE STUDY. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-365-C9-370.
�10.1051/jphyscol:1984961�. �jpa-00224448�
THE CO OXIDATION REACTION ON Rh :
A PULSED-LASER AND IMAG.ING ATOM-PROBE STUDY G.L. Kellogg
Sandia National Laboratories, Albuquerque, NM 87185, U.S.A.
Résume; On a étudié la réaction d'oxydation du CO sur des surfaces de Rh présentant une émission de champ dans des con- ditions de "haute pression" et de "basse pression"• Pour les expériences à haute pression, on a porté la surface de Rh a une température de 500 K dans des mélanges de O2/CO à des pressions de 1 à 2 torr, puis on l'a analysée au moyen d'un
spectroscope de masse à sonde de détection d'atomes et à formation d'images. Le fait que l'on a détecté la présence d'oxydes de surface seulement lorsque les rapports O2/CO étaient supérieurs à 30/1 a amené à identifier la formation d'oxydes comme étant le processus de surface responsable de la désactivation de la réaction. Pour les expériences "à basse pression, on a analysé la surface de Rh au moyen d'un spectroscope de masse à sonde de détection d'atomes et à laser puisé alors même que se produisait la réaction d'oxydation du CO. D'après l'influence exercée par le champ électrique et la température sur les intensités ioniques détectées, le champ électrique n' a pas de forte influence au niveau de la réaction d'oxydation du CO ni au niveau de la désorption thermique de CO. En ce qui concerne 1 ' adsorption d'oxygène ou de CO, on n'a observé aucune indication de spécificité cristallographique.
Abstract; The CO oxidation reaction on Rh field-emitter sur- faces has been investigated for both "high-pressure" and "low- pressure" conditions. In the high-pressure studies the Rh surface was heated to 500 K in 1-2 Torr of 0
2/CO mixtures and was subsequently analyzed by imaging atom-probe mass spectros- copy. The detection of surface oxides only at O2/CO ratios in excess of 30/1 identifies oxide formation as the surface pro- cess responsible for deactivation of the reaction. in the low-pressure experiments the Rh surface was analyzed by pulsed- laser atom-probe mass spectroscopy while the CO oxidation reaction was taking place. The electric field and temperature dependence of the detected ion intensities indicated that the electric field does not strongly influence the CO oxidation reaction or the thermal desorption of CO. No indication of crystallographic specificity in the adsorption of oxygen or CO was observed.
Introduction; There has been a considerable effort over the past two decades to characterize the atomic and molecular processes involved in surface catalyzed reactions[1]. The motivation for these studies is the expectation that an understanding of the reaction processes on a molecular level will lead to new and improved catalytic materials and a better understanding of technologically-important processes such as poisoning and promotion. Employing a wide variety of surface
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984961
C9-366 JOURNAL DE PHYSIQUE
a n a l y t i c a l t e c h n i q u e s , s u r f a c e s c i e n t i s t s have c h a r a c t e r i z e d t h e ad- s o r p t i o n and thermal d e s o r p t i o n of many c a t a l y t i c a l l y - i m p o r t a n t mole- c u l a r s p e c i e s on a v a r i e t y of t r a n s i t i o n metal surfacesCl]. More r e c e n t l y , t h e s e e f f o r t s have gone beyond c h a r a c t e r i z i n g simple ad- s o r p t i o n / d e s o r p t i o n p r o c e s s e s and have begun t o examine t h e d e t a i l s of simple r e a c t i o n p r o c e s s e s . In t h e s t u d y r e p o r t e d h e r e we continue t h i s t r e n d by examining t h e d e t a i l s of t h e CO o x i d a t i o n r e a c t i o n on Rh. Although it is one of t h e s i m p l e s t examples of a heterogeneous c a t a l y t i c r e a c t i o n , t h e o x i d a t i o n of CO on noble metals i s both com- m e r c i a l l y important and s c i e n t i f i c a l l y i n t e r e s t i n g [ 2 , 3 ] . The use of
f i e l d - i o n - r e l a t e d techniques f o r t h i s t y p e of s t u d y i s p a r t i c u l a r l y advantageous because t h e r e a c t i o n can be performed on a sample whose s i z e and s t e p d e n s i t y compares t o a commercial c a t a l y s t p a r t i c l e and whose s u r f a c e i s d e f i n e d on an atomic s c a l e .
Our s t u d i e s of t h e CO o x i d a t i o n r e a c t i o n on Rh involve two t y p e s of experiments: (1) "high-pressure" experiments i n which t h e t i p is heated i n t h e presence of r e a c t a n t g a s e s a t p r e s s u r e s up t o s e v e r a l Torr and subsequently t r a n s f e r r e d t o an imaging atom-probeC41 f o r s u r f a c e a n a l y s i s and ( 2 ) "low-pressure" experiments i n which t h e s u r f a c e i s analyzed by p u l s e d - l a s e r atom-probe mass spectroscopyC5,6]
while t h e r e a c t i o n i s t a k i n g p l a c e a t p r e s s u r e s - l o p 7 Torr. Our
c u r r e n t goal i n t h e high p r e s s u r e s t u d i e s i s t o i d e n t i f y t h e s u r f a c e p r o c e s s r e s p o n s i b l e f o r d e a c t i v a t i o n of t h e r e a c t i o n a t high O ~ / C O r a t i o s . Past studiesC7.81 have shown t h a t a t 500 K t h e CO o x i d a t i o n r e a c t i o n on rhodium changes from p o s i t i v e t o negative o r d e r i n 02 p a r t i a l p r e s s u r e when t h e 02/c0 r a t i o exceeds v a l u e s i n t h e range from 20/1 t o 40/1. To i d e n t i f y t h e s u r f a c e p r o c e s s r e s p o n s i b l e f o r t h i s d e a c t i v a t i o n , Rh t i p s were heated t o 500 K i n v a r i o u s mixtures of CO and 02 and t h e s u r f a c e s were subsequently examined f o r compo- s i t i o n a l changes. The primary goal i n t h e low-pressure s t u d i e s i s t o i d e n t i f y t h e temperature and c r y s t a l l o g r a p h i c dependence of de- s o r p t i o n and r e a c t i o n p r o c e s s e s while t h e r e a c t i o n proceeds. S i n c e a h i g h e l e c t r i c f i e l d i s p r e s e n t d u r i n g our a n a l y s i s , t h e e f f e c t of t h i s e l e c t r i c f i e l d on t h e CO o x i d a t i o n and thermal d e s o r p t i o n tem- p e r a t u r e was s t u d i e d . W e have a l s o begun p r e l i m i n a r y s t u d i e s t o determine i f CO o r oxygen adsorbs p r e f e r e n t i a l l y a t s p e c i f i c s u r f a c e l o c a t i o n s .
High-Pressure S t u d i e s : The high-pressure experiments were c a r r i e d o u t i n t h e a p p a r a t u s shown s c h e m a t i c a l l y i n Fig. 1. The system c o n s i s t s of a r e a c t i o n chamber i n which t h e CO o x i d a t i o n r e a c t i o n i s c a r r i e d out and a n a n a l y s i s chamber i n which t h e t i p s u r f p c e i s examined by f i e l d i o n microscopy[9] and imaging atom-probe mass spectroscopy[4].
I n our system t h e sample t i p i s spotwelded t o a platinum w i r e loop.
The loop ends a r e i n s e r t e d i n t o l e n g t h s of n i c k e l tubing which a r e
h e l d i n a c y l i n d r i c a l s a p p h i r e block. The s a p p h i r e block i s mounted
i n s i d e a copper cup and t h e e n t i r e assembly i s t r a n s f e r r e d between
t h e two chambers on a s t a i n l e s s - s t e e l rod m a g n e t i c a l l y coupled t o t h e
e x t e r i o r of t h e vacuum system. Four spring-loaded, gold-coated cop-
per p i n s make t h e e l e c t r i c a l c o n t a c t between t h e n i c k e l tubing and
high v o l t a g e l e a d s i n t h e a n a l y s i s chamber. During a n a l y s i s t h e
s t a i n l e s s - s t e e l rod i s detached from t h e sample assembly, and t h e
a n a l y s i s chamber i s i s o l a t e d from t h e r e a c t i o n chamber by an a l l - m e t a l
vacuum g a t e valve.
Elect.
Contacts Sapphire
L - - -
I - -
-
/
G a t e Valve
Tip Rod
1
LaserI L
T
DetectorImage
F i g . 1. A s c h e m a t i c d r a w i n g o f t h e a p p a r a t u s used i n t h i s s t u d y .
P r i o r t o t h e o x i d a t i o n e x p e r i m e n t s t h e t i p s u r f a c e s were c l e a n e d i n t h e a n a l y s i s chamber u s i n g a c o m b i n a t i o n o f neon c a t h o d e s p u t t e r i n g [ l O l and f i e l d e v a p o r a t i o n ( 9 1 . The c a t h o d e s p u t t e r i n g t e c h n i q u e was u s e d t o e l i m i n a t e t h e p o s s i b i l i t y o f o x i d e s m i g r a t i n g from t h e t i p s h a n k t o t h e a p e x d u r i n g t h e r e a c t i o n , and f i e l d e v a p o r a t i o n was u s e d t o remove t h e s p u t t e r damage and a n y r e m a i n i n g c o n t a m i n a t i o n from t h e a p e x . F o l l o w i n g t h i s c l e a n i n g p r o c e d u r e t h e t i p was t r a n s f e r r e d t o t h e r e a c t i o n chamber, and t h e sample was h e a t e d t o 500 K f o r 1 5 m i n u t e s i n m i x t u r e s o f 02 and CO. Sample h e a t i n g was a c c o m p l i s h e d b y p a s s i n g a d c c u r r e n t t h r o u g h t h e p l a t i n u m s u p p o r t l o o p , and resis- t a n c e measurements were used t o d e t e r m i n e t h e s a m p l e t e m p e r a t u r e .
T I M E
(ps)
F i g . 2. Imaging atom-probe mass s p e c t r a r e c o r d e d from a R h
t i p f o l l o w i n g h e a t i n g t o 500 K i n ( a ) a 30/1 02/C0
m i x t u r e and ( b ) a 4 0 / 1 02/C0 m i x t u r e .
C9-368 JOURNAL DE PHYSIQUE
To cover a wide range of o2/c0 r a t i o s , t h e oxygen p a r t i a l p r e s s u r e was h e l d f i x e d a t 1 Torr and t h e CO p a r t i a l p r e s s u r e was v a r r i e d from 0.01 t o 1 Torr. The r e a c t i o n was terminated by f i r s t d i s c o n t i n u i n g t h e h e a t i n g and t h e n pumping o u t t h e r e a c t a n t g a s s e s . When t h e p r e s s u r e i n t h e r e a c t i o n chamber f e l l t o t h e low Torr range, t h e t i p was t r a n s f e r r e d t o t h e a n a l y s i s chamber, and i t s s u r f a c e was examined by imaging atom-probe mass spectroscopy. Analog mass s p e c t r a were recorded using 2 kV v o l t a g e p u l s e s and i n c r e a s i n g l y h i g h e r holding v o l t a g e s . In a t y p i c a l run 20-30 mass s p e c t r a were recorded r e p r e s e n t i n g a t o t a l of t e n s of thousands of i o n s c o l l e c t e d . The mass s p e c t r a were t h e n analyzed f o r t h e presence of s u r f a c e oxides.
Imaging atom-probe mass s p e c t r a recorded from samples h e a t e d i n mix- t u r e s with an 02/c0 r a t i o of 30/1 o r l e s s showed no evidence f o r t h e formation of s u r f a c e oxides. The predominant s i g n a l s d e t e c t e d were CO and CO fragments. An example of an imaging atom-probe mass spec- trum t a k e n from a t i p heated t o 500 K i n a 30/1 mixture i s shown i n Fig. 2 ( a ) . When t h e 02/c0 r a t i o was i n c r e a s e d t o 40/1, however, s u r f a c e oxides were d e t e c t e d . Figure 2 ( b ) shows an imaging atom- probe mass spectrum from a t i p heated t o 500 K i n a 40/1 mixture.
The f o u r s i g n a l s d e t e c t e d were i d e n t i c a l t o t h e s i g n a l s d e t e c t e d when t h e sample was oxidized i n p u r e OZ[ 111. The q u a l i t a t i v e difference between mass s p e c t r a recorded a t 30/1 and 40/1 was r e p r o d u c i b l e over f i v e experiments on two s e p a r a t e t i p s . The 40/1 r a t i o where s u r f a c e oxides a r e d e t e c t e d c o r r e l a t e s well with t h e r a t i o where d e a c t i v a t i o n of t h e r e a c t i o n h a s been observed both on d i s p e r s e d p a r t i c l e c a t a l y s t s [ 7 ] and on t h e s i n g l e - c r y s t a l R h ( l l 1 ) planeL-81. This c o r r e l a t i o n l e a d s us t o conclude t h a t oxide formation i s t h e s u r f a c e p r o c e s s r e - s p o n s i b l e f o r d e a c t i v a t i o n of t h e r e a c t i o n . Q u a l i t a t i v e a n a l y s i s of t h e recorded mass s p e c t r a i n d i c a t e s t h a t t h e oxide formed under t h e s e c o n d i t i o n s i s s t o i c h i o m e t r i c Rh203[11].
Low-Pressure S t u d i e s : The low p r e s s u r e s t u d i e s were conducted e n t i r e l y w i t h i n t h e a n a l y s i s chamber shown s c h e m a t i c a l l y i n Fig. I . The ex- perimental procedure c o n s i s t e d of c l e ning t h e t i p s u r f a c e by vacuum f i e l d evaporation, admitting 1 x lo-' Torr of a 3/1 C0/02 mixture t o t h e chamber, and recording pulsed l a s e r atom-probe mass s p e c t r a . The p u l s e d - l a s e r technique was employed i n t h i s s t u d y i n o r d e r t o be a b l e t o vary t h e e l e c t r i c f i e l d a t t h e time of desorption[6]. The amplitudes of t h e observed i o n i c s i g n a l s , which a r e p r o p o r t i o n a l t o t h e f i e l d d e s o r p t i o n r a t e , were measured a s a f u n c t i o n of f i e l d s t r e n g t h and t i p temperature.
The s p e c i e s d e t e c t e d i n t h e mass s p e c t r a depended on t h e f i e l d desorp- t i o n c o n d i t i o n s . A t f i e l d s t r e n g t h s and l a s e r powers below t h e o n s e t of Rh f i e l d evaporation, t h e mass s p e c t r a contained s i g n a l s a t m/n=18 and m/n=28 corresponding t o ~ ~ and 0 ' CO+. The temperature dependence of t h e H20+ s i g n a l (below) s u g g e s t s t h a t t h e H20+ s i g n a l i s produced by adsorbed oxygen r e a c t i n g with f i e l d adsorbed hydrogen. The ampli- t u d e of t h i s s i g n a l i s t h e r e f o r e r e l a t e d t o t h e c o n c e n t r a t i o n of s u r - f a c e oxygen and can be used t o monitor t h e s u r f a c e oxygen d u r i n g t h e r e a c t i o n . This assumption i s a l s o supported by t h e f a c t t h a t H 2 0 does not a d s o r b on R h a t room temperature o r aboveC121. Upon i n c r e a s i n g t h e l a s e r power t o t h e p o i n t where e v a p o r a t i o n of t h e s u b s t r a t e occurred, a d d i t i o n a l s i g n a l s corresponding t o Rh++, R ~ c o + + , Rh+, and R ~ C O + were d e t e c t e d . For t h e p r e s e n t study, measurements of t h e f i e l d and tem- p e r a t u r e dependences of t h e i o n s i g n a l s were r e s t r i c t e d t o c o n d i t i o n s where t h e s u b s t r a t e was n o t f i e l d evaporating.
The v a r i a t i o n of t h e H20+ and CO+ s i g n a l with a p p l i e d t i p v o l t a g e i s
shown i n Fig. 3 . The a b s c i s s a can be converted t o approximate f i e l d
5
E1
1 x Tor,- 300 K
TIP
VOLTAGE(kV)
F i g . 3 . The v a r i a t i o n i n t h e ~ ~ and 0 ' CO+ s i g n a l s from a Rh sample a s a f u n c t i o n of a p p l i e d t i p v o l t a g e .
s t r e n g t h s i n V/ A by m u l t i p l y i n g t h e v o l t a g e v a l u e s by 3/8. The on- s e t o f t h e H ~ O + s i g n a l o c c u r s a t lower f i e l d s t r e n g t h s t h a n t h e CO+
s i g n a l due t o i t s weaker b i n d i n g and lower i o n i z a t i o n p o t e n t i a l . Both s i g n a l s i n c r e a s e w i t h e l e c t r i c f i e l d and s a t u r a t e a t v a l u e s con- s i s t e n t w i t h t h e 3/1 r a t i o o f t h e background g a s e s .
More i n t e r e s t i n g a r e p l o t s o f t h e two s i g n a l s a s a f u n c t i o n o f b a s e t i p t e m p e r a t u r e shown i n Fig. 4. A s t h e s u b s t r a t e i s h e a t e d a t con- s t a n t f i e l d , t h e H20+ s i g n a l d i s a p p e a r s a t a t e m p e r a t u r e c o n s i s t e n t w i t h t h e CO o x i d a t i o n r e a c t i o n ( i . e . , t h e r e a c t i o n removes s u r f a c e oxygen) and t h e CO+ s i g n a l d i s a p p e a r s a t a t e m p e r a t u r e c o n s i s t e n t w i t h t h e t h e r m a l d e s o r p t i o n o f CO from t h e s t e p p e d s u r f a c e s o f ~ h C 1 3 1 . Thus, it d o e s n o t a p p e a r t h a t t h e a p p l i e d f i e l d i s s t r o n g l y i n f l u e n c - i n g e i t h e r t h e CO o x i d a t i o n r e a c t i o n o r t h e t h e r m a l d e s o r p t i o n of
TEMPERATURE (K)
- >
E 3 0 -
-
X
2E
2 2 0 - a Z
d
a z
(310-
rn
0
F i g . 4. The v a r i a t i o n i n t h e H ~ O + and CO+ s i g n a l s from a Rh sample a s a f u n c t i o n o f b a s e t i p t e m p e r a t u r e .
Rh 4.0
kV
1 x Torr
