Catalyst Characterization Science - American Chemical Society

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Spectroscopy of Metal-Titanium Dioxide Systems Y.-M. Sun, D. N. Belton, and J. M. White Department of Chemistry, The University of Texas—Austin, Austin, TX 78712

We have investigated thin film models of catalysts which are known to show strong metal-support inter actions. These models consist of vapor-deposited platinum or rhodium on titanium dioxide films prepared in ultrahigh vacuum by oxidation of Ti(0001). Thermal desorption spectroscopy, Auger electron spectroscopy and static secondary ion mass spectrometry have been used to characterize the surfaces of these films before and after thermal treatment in vacuum and with or without chemisorbed carbon monoxide. The results indicate that heating in vacuum to temperatures near 750 Κ leads to the migration of a reduced form of titania to the surface of the metal films. This migration is accompanied by significant suppression of carbon monoxide and hydrogen chemisorption just as is found in powdered oxide-supp orted transition metal catalysts. Sputtering removes the segregated titanium-oxygen species, and the chemi sorption of carbon monoxide i s nearly completely recov ered. In thin films of metal that are one to two mono layers thick with no surface titania species, there i s no significant suppression of hydrogen chemisorption, indicating that surface segregation i s , by itself, not fully responsible for the observed changes in the extent and energetics of hydrogen chemisorption on these sur faces. Clearly, both site-blocking and electronic (bonding) effects play a role in the observed strong metal-support interaction effects. R e c e n t l y t h e r e has been a g r e a t d e a l o f i n t e r e s t i n t h e systems w h i c h e x h i b i t s t r o n g m e t a l - s u p p o r t i n t e r a c t i o n s (SMSI) (1-12). T h i s a c t i v i t y was s t i m u l a t e d by t h e work o f T a u s t e r e t a l . (1,2) r e p o r t e d i n 1978 w h i c h showed t h a t Group V I I I t r a n s i t i o n m e t a l s s u p p o r t e d on r e d u c i b l e m e t a l o x i d e s were s u b j e c t t o l a r g e - s c a l e s u p p r e s s i o n o f c h e m i s o r p t i o n o f hydrogen when t h e o x i d e s were reduced a t h i g h t e m p e r a t u r e s . S i n c e then t h e r e have been a number o f papers and c o n f e r e n c e s on t h i s s u b j e c t . Proposed e x p l a n a t i o n s i n c l u d e t h e m i g r a t i o n o r s e g r e g a t i o n o f o x i d e s p e c i e s over t h e t r a n s i t i o n m e t a l 0097-6156/ 85/0288-0080506.00/ 0 © 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: October 16, 1985 | doi: 10.1021/bk-1985-0288.ch007

7.

SUN ET A L .

Spectroscopy of Metal-Titanium Dioxide Systems

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p a r t i c l e s and e l e c t r o n i c e f f e c t s i n c l u d i n g P t - t i t a n i a bonding and/or charge t r a n s f e r from the support t o the m e t a l . We have u n d e r t a k e n a s e r i e s of experiments i n v o l v i n g t h i n f i l m models o f such powdered t r a n s i t i o n m e t a l c a t a l y s t s (13,14). In t h i s paper we p r e s e n t a b r i e f r e v i e w of the r e s u l t s we have o b t a i n e d to d a t e i n v o l v i n g p l a t i n u m and rhodium d e p o s i t e d on t h i n f i l m s o f t i t a n i a , the l a t t e r prepared by o x i d a t i o n o f a t i t a n i u m s i n g l e c r y s t a l . These systems a r e prepared and c h a r a c t e r i z e d under w e l l c o n t r o l l e d c o n d i t i o n s . We have used t h e r m a l d e s o r p t i o n s p e c t r o scopy (TDS), Auger e l e c t r o n s p e c t r o s c o p y (AES) and s t a t i c secondary i o n mass s p e c t r o m e t r y (SSIMS). Our r e s u l t s i l l u s t r a t e the power o f SSIMS i n u n d e r s t a n d i n g the p r o c e s s e s t h a t t a k e p l a c e d u r i n g t h e r m a l treatment of t h e s e t h i n f i l m s . Thermal d e s o r p t i o n s p e c t r o s c o p y i s used t o c h a r a c t e r i z e the a d s o r p t i o n and d e s o r p t i o n o f s m a l l mole c u l e s , i n p a r t i c u l a r , c a r b o n monoxide. AES c o n f i r m s the SSIMS r e s u l t s and was used t o v e r i f y the s u r f a c e c l e a n l i n e s s o f t h e f i l m s as they were p r e p a r e d . Experimental The e x p e r i m e n t s were conducted i n an u l t r a h i g h vacuum chamber equipped w i t h a c y l i n d r i c a l m i r r o r e l e c t r o n energy a n a l y z e r , a quadrupole mass spectrometer w i t h a B e s s e l box energy f i l t e r f o r a n a l y s i s of secondary i o n s , and an A r gun. A Ti(0001) s i n g l e c r y s t a l was mounted on a l i q u i d n i t r o g e n c o o l e d and r e s i s t i v e l y heated m a n i p u l a t o r assembly. The s u r f a c e s were c l e a n e d by argon i o n bom bardment and a n n e a l i n g c y c l e s . The t r a n s i t i o n m e t a l s were d e p o s i t e d u s i n g t h e r m a l e v a p o r a t i o n s o u r c e s . D u r i n g the e x p e r i m e n t s , p r e s s u r e s were r o u t i n e l y 3 χ 1 0 " " t o r r . The t i t a n i a - b a s e d t h i n f i l m c a t a l y s t models were c o n s t r u c t e d by f i r s t o x i d i z i n g the t i t a n i u m s u r f a c e i n 5 χ 10"" t o r r o f O2 f o r a p p r o x i m a t e l y 30 minutes a t 775 K. T h i s produced an AES l i n e s h a p e consistent with f u l l y oxidized T i 0 2 . The m e t a l was t h e n v a p o r d e p o s i t e d onto the o x i d e s u p p o r t w i t h the l a t t e r h e l d a t 130 K. The t h i c k n e s s of the m e t a l o v e r l a y e r and i t s c l e a n l i n e s s were v e r i f i e d by AES. A f t e r v a r i o u s a n n e a l i n g and a d s o r p t i o n p r o c e d u r e s , these t h i n f i l m s were f u r t h e r c h a r a c t e r i z e d u s i n g SSIMS, AES and TDS. For comparison, some work was done w i t h P t on A I 2 O 3 . I n t h i s case a Mo f o i l covered w i t h AI2O3 r e p l a c e d the T i ( 0 0 0 1 ) s u b s t r a t e . +

1 0

7

Results F i g u r e 1 shows AES d a t a f o r the o x i d i z e d t i t a n i u m s u r f a c e b e f o r e and a f t e r d e p o s i t i o n o f 30 X o f p l a t i n u m w i t h the s u b s t r a t e h e l d a t 130 K. The p l a t i n u m t h i c k n e s s was c a l c u l a t e d from the a t t e n u a t i o n of the oxygen AES s i g n a l assuming l a y e r e d growth of the m e t a l . From the s p e c t r a i t i s c l e a r t h a t the p l a t i n u m was s u f f i c i e n t t o com p l e t e l y a t t e n u a t e the u n d e r l a y i n g f e a t u r e s o f the t i t a n i u m o x i d e . The s p e c t r a of the o x i d e s u r f a c e p r i o r t o m e t a l d e p o s i t i o n i s c h a r a c t e r i s t i c of f u l l y o x i d i z e d t i t a n i u m . I n the r e g i o n j u s t below 435 eV the l i n e s h a p e i s s i g n i f i c a n t l y d i f f e r e n t f o r d i f f e r e n t o x i d e s of t i t a n i u m (15). F i g u r e 2 shows the r e s u l t s of h e a t i n g a model system c o n s i s t i n g o f a 30 X p l a t i n u m f i l m on o x i d i z e d t i t a n i u m . A l i n e a r temperature ramp was a p p l i e d u n t i l the f o i l reached 760 K, a f t e r w h i c h the temp-

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


CATALYST C H A R A C T E R I Z A T I O N SCIENCE

dN(E)

135

535

235 335 Kinetic Energy (eV)

F i g u r e 1. AES s p e c t r a o f o x i d i z e d T i ( 0 0 0 1 ) ( t o p ) and o x i d i z e d T i w i t h 30 X P t (bottom).

200

400

600

800

1000

Time(sec) F i g u r e 2.

TPSSIMS f o r 30 X P t on o x i d i z e d T i ( 0 0 0 1 ) ,


7. SUN ET A L .

83


e r a t u r e was h e l d c o n s t a n t f o r a p p r o x i m a t e l y 5 minutes and then the sample was c o o l e d . D u r i n g t h e s e temperature changes the T i SSIMS s i g n a l was f o l l o w e d . Near 615 Κ the T i s i g n a l b e g i n s t o r i s e s h a r p l y . T h i s s i g n a l c o n t i n u e s t o r i s e d u r i n g the p e r i o d when t h e temperature i s h e l d c o n s t a n t a t 760 K. Upon c o o l i n g t h e r e was no tendency f o r t h e s i g n a l t o d e c r e a s e . A s i m i l a r sample was heated as above and then s u b j e c t e d t o argon i o n s p u t t e r i n g , as i n d i c a t e d i n F i g . 3. An a r g o n i o n c u r r e n t of 0.3 μΑ was u t i l i z e d and t h e T i s i g n a l was f o l l o w e d as a f u n c t i o n o f s p u t t e r i n g t i m e . T h i s f i g u r e i n d i c a t e s a sharp decay o f the T i s i g n a l followetf by a r e l a t i v e l y wide r e g i o n where v e r y l i t t l e s i g n a l i s noted. Deeper i n t o t h e sample t h e T i s i g n a l r i s e s s t e a d i l y , maximizes a t about 600 seconds and then decays. On a t h i r d sample, t h e r m a l d e s o r p t i o n o f c a r b o n monoxide was c a r r i e d out b e f o r e and a f t e r a n n e a l i n g t o 760 Κ and a f t e r s p u t t e r i n g i n t o the r e g i o n where the T i was a minimum ( i . e . , about 180 seconds i n F i g . 3 ) . The TDS r e s u l t s a r e summarized i n Table I . The l a s t column g i v e s the i n t e g r a t e d peak d e s o r p t i o n a r e a f o r carbon monoxide. +

+

+


+

+

+

Table I .

CO Thermal D e s o r p t i o n

Substrate

Ti0 Ti0 Ti0 (S) A1 0 2

2

2

2

3

AI2O3

(S)=sputtered

Spectra.

Anneal Temp (K)

Desorption Peak (T)

525 775 525 525 775

400 355 400 400 400

Total Peak A r e a 1.00 0.33 0.95 1.00 0.98

(see t e x t )

Two CO d e s o r p t i o n peaks (400 and 510 K) were observed on the a s d e p o s i t e d P t l a y e r . However, f o r a sample annealed t o 525 Κ t h e i n t e n s i t y of t h e h i g h temperature CO peak was =one t h i r d i t s o r i g i n a l v a l u e . Thus, a f t e r a n n e a l i n g t o 525 K, the d e s o r p t i o n p r o f i l e c o n s i s t e d of a l a r g e peak a t 400 Κ w i t h a s h o u l d e r a t 510 K. For t h i s r e a s o n we r e p o r t the peak temperature f o r the low temperature peak o n l y ; however, the 510 Κ s h o u l d e r i s i n c l u d e d i n t h e r e p o r t e d peak a r e a . Comparison of our d a t a t o r e s u l t s from o t h e r s u r f a c e s shows r e a s o n a b l e agreement. P o l y c r y s t a l l i n e P t ( 4 1 5 K, 507 K) ( 1 6 ) , P t ( 1 1 0 ) (430 K, 530 K) (17) and P t ( l l l ) (420 K, 530 K) (18) a l l have two d e s o r p t i o n peaks i n the 400-550 Κ range. For b o t h t i t a n i a - and a l u m i n a - s u p p o r t e d P t , the a r e a s a r e n o r m a l i z e d t o t h e d e s o r p t i o n a r e a a f t e r a n n e a l i n g the 30 X o v e r l a y e r t o 525 Κ and a d s o r b i n g CO t o s a t u r a t i o n a t 130 K. For t i t a n i a annealed t o 775 Κ the peak de s o r p t i o n temperature i s decreased by about 45 K, and the t o t a l a r e a i s decreased by a f a c t o r o f 3. A f t e r s p u t t e r i n g t o t h e minimum and r e a n n e a l i n g t o 525 K, the peak temperature r e t u r n s t o 400 Κ and the t o t a l peak a r e a i n c r e a s e s t o a p p r o x i m a t e l y i t s v a l u e b e f o r e the



84

a n n e a l i n g experiment was c a r r i e d o u t . S i m i l a r experiments were done w i t h P t on a l u m i n a . As i n d i c a t e d i n T a b l e I , a n n e a l i n g t o 760 Κ does n o t change e i t h e r t h e d e s o r p t i o n temperature o r t h e a r e a under the c a r b o n monoxide d e s o r p t i o n peak. S i m i l a r SSIMS and TDS r e s u l t s were o b t a i n e d f o r rhodium on t i t a n i a and fclr hydrogen c h e m i s o r p t i o n on b o t h s u b s t r a t e s . I n a b l a n k experiment i n v o l v i n g no m e t a l o v e r l a y e r , temperature program ming w h i l e f o l l o w i n g the T i and T i 0 SIMS s i g n a l s ( F i g . 4) shows t h a t the t i t a n i a t h i n f i l m does n o t b e g i n t o change u n t i l t h e temperature r e a c h e s about 760 K, w e l l beyond the 615 Κ where T i was f i r s t n o t e d t o i n c r e a s e on t h e systems w i t h t h i n m e t a l o v e r layers . To e x p l o r e the p o s s i b i l i t y o f e l e c t r o n i c i n t e r a c t i o n between t h e reduced t i t a n i u m o x i d e and P t , 1 ML o f P t was d e p o s i t e d on b o t h f u l l y o x i d i z e d and p a r t i a l l y reduced T 1 0 2 . The reduced sample was p r e p a r e d by A r s p u t t e r i n g of t h e T i 0 2 s u b s t r a t e p r i o r t o m e t a l d e p o s i t i o n , i n t r o d u c i n g reduced T i c e n t e r s a t t h e P t - T i i n t e r f a c e . H TDS was used as t h e h i g h e s t H d e s o r p t i o n temperature (370 K) o c c u r s below t h e temperature regime o f e n c a p s u l a t i o n . For t h e reduced sample t h e r e was a 70% d e c r e a s e i n H2 c h e m i s o r p t i o n and a 33 Κ s h i f t t o l o w e r t e m p e r a t u r e s when t h e unannealed sample ( f i r s t H TDS) was compared t o t h e sample annealed a t 370 Κ (second H2 TDS). No change i n t h e AES was observed a f t e r e i t h e r the f i r s t o r second TDS, showing t h a t t h e P t o v e r l a y e r does n o t i s l a n d o r encap s u l a t e . We t a k e t h e s e low P t coverage experiments t o i n d i c a t e an e l e c t r o n i c i n t e r a c t i o n ( p r e f e r a b l y bond f o r m a t i o n , w h i c h does n o t r e q u i r e s i g n i f i c a n t charge t r a n s f e r ) between P t and reduced T i s p e c i e s t h a t i s a c t i v a t e d a t about 370 K. F o r t h e f u l l y o x i d i z e d sample t h e r e s u l t s were somewhat d i f f e r e n t . A f t e r a n n e a l i n g a t 370 Κ t h e r e was 25% l e s s H2 a d s o r p t i o n , and t h e peak s p l i t i n t o two peaks, one s h i f t e d h i g h e r and one l o w e r , each by about 30 K. There were a l s o s m a l l changes i n t h e AES P t / T i r a t i o a f t e r t h e f i r s t TDS. S i n c e T i 0 2 m i g r a t e s a t l o w e r tempera t u r e s f o r reduced t i t a n i a , as compared t o f u l l y o x i d i z e d , we do n o t f a v o r T i 0 m i g r a t i o n as t h e e x p l a n a t i o n . R a t h e r , we suggest t h a t t h e changes observed a f t e r t h e 370 Κ a n n e a l of t h e o x i d i z e d sample a r e due t o s m a l l changes i n t h e morphology o f t h e P t o v e r l a y e r . These a l t e r the number and k i n d o f exposed P t s i t e s . +


+

+

2

2

2

x

Discussion From t h e SIMS, AES and TDS d a t a t h e f o l l o w i n g p i c t u r e emerges. O x i d a t i o n of t i t a n i u m i n s i t u leads to the formation of a f i l m of f u l l y o x i d i z e d t i t a n i a t h a t i s t h i c k enough t o c o m p l e t e l y a t t e n u a t e m e t a l and s u b o x i d e c o n t r i b u t i o n t o t h e AES s p e c t r a . H e a t i n g t h e s e o v e r l a y e r s , w h i c h a r e judged t o be more t h a n 60 A t h i c k , r e s u l t s i n no d e t e c t a b l e changes by SIMS o r by AES (not shown) u n t i l the tempera t u r e exceeds 760 K. S i n c e t h e r m a l e f f e c t s a r e observed a t s i g n i f i c a n t l y l o w e r temperature when m e t a l o v e r l a y e r s a r e p r e s e n t , we con c l u d e t h a t t h o s e o b s e r v a t i o n s a r e n o t due t o d e g r a d a t i o n o f t h e o x i d e l a y e r v i a d i f f u s i o n o f oxygen i n t o the b u l k m e t a l . The AES s i g n a l s observed a f t e r d e p o s i t i o n o f p l a t i n u m ( F i g . 1) i n d i c a t e t h a t p l a t i n u m goes down r e a s o n a b l y u n i f o r m l y under our c o n d i t i o n s . T h i s r e s u l t i s c o n f i r m e d by a more d e t a i l e d a n a l y s i s of t h e a t t e n u a t i o n o f the oxygen and t i t a n i u m s i g n a l s as a f u n c t i o n


7. S U N ET A L .



ι

0

200

ι

400

ι

600

85

r

800

1000

Time(sec) F i g u r e 3. T i SIMS depth p r o f i l e o f 30 X P t on o x i d i z e d T i ( 0 0 0 1 ) annealed t o 760 K. +

F i g u r e 4. TPSSIMS o f t h e o x i d i z e d T i ( 0 0 0 1 ) s u b s t r a t e w i t h o u t a metal overlayer.



86

o f p l a t i n u m d o s i n g time. From t h e s e r e s u l t s (not shown) we c o n c l u d e that platinum d e p o s i t i o n occurs i n a very n e a r l y l a y e r - b y - l a y e r f a s h i o n a t 130 K. A n n e a l i n g t h e s e f i l m s t o 525 Κ does n o t h i n g t o the T i SIMS s i g n a l , but a t around 600 Κ T i s i g n a l s b e g i n t o appear i n t h e SIMS s p e c t r a . These r i s e s h a r p l y t o a f a i r l y steady v a l u e which increases s l o w l y w i t h time. This i s a thermally i r r e v e r s i b l e change s i n c e , upon c o o l i n g , ( F i g . 2) the T i s i g n a l does not r e t u r n t o a low v a l u e . The s p e c i e s formed i s l a r g e l y s e g r e g a t e d t o the s u r f a c e as i n d i c a t e d by t h i s s p u t t e r i n g p r o f i l e o f F i g . 3. Sputter i n g removes t h i s o v e r l a y e r and l e a d s t o m e t a l l i c p l a t i n u m . A f t e r s p u t t e r i n g through the p l a t i n u m the o x i d e r e g i o n i s a g a i n reached. A f t e r 600 seconds the m e t a l l a y e r i s removed, and t h e T i s i g n a l b e g i n s t o drop. T h i s f i n a l drop i s the r e s u l t of a d e c r e a s i n g c r o s s - s e c t i o n f o r i o n d e s o r p t i o n w h i c h accompanies the p r e f e r e n t i a l removal o f oxygen. The o b s e r v e d t h e r m a l d e s o r p t i o n r e s u l t s a r e e n t i r e l y c o n s i s t e n t w i t h t h e p i c t u r e t h a t emerges from F i g s . 2 and 3. A n n e a l i n g t o 525 Κ ( T a b l e I ) g i v e s a t h e r m a l d e s o r p t i o n peak f o r c a r b o n monoxide l i k e t h a t observed from b u l k p l a t i n u m f i l m s . A n n e a l i n g t o 775 Κ l o w e r s t h e d e s o r p t i o n peak t e m p e r a t u r e and, more i m p o r t a n t l y , a t t e n u a t e s s h a r p l y the amount o f carbon monoxide t h a t w i l l a d s o r b . S p u t t e r i n g t o t h e minimum o f t h e T i s i g n a l f o l l o w e d by an a n n e a l t o 525 Κ and a d s o r p t i o n g i v e s a r e s u l t t h a t i s v e r y much l i k e t h a t observed p r i o r t o the h i g h - t e m p e r a t u r e a n n e a l . From these and o t h e r more d e t a i l e d r e s u l t s i n v o l v i n g Auger l i n e s h a p e s t u d i e s , we c o n c l u d e t h a t the s p e c i e s t h a t m i g r a t e s t o the s u r f a c e o f the p l a t i n u m i s p r o b a b l y TiO. For s i m i l a r samples on a l u m i n a , these e f f e c t s a r e not o b s e r v e d , as i n d i c a t e d i n T a b l e I . No m i g r a t i o n o f aluminum o r oxygen s p e c i e s i s observed i n AES, and t h e c a p a c i t y of the f i l m t o adsorb c a r b o n monoxide i s not a l t e r e d by c h a n g i n g the a n n e a l i n g temperature from 525 t o 760 K. A l l of these r e s u l t s a r e c o n s i s t e n t w i t h the n o t i o n t h a t s u r f a c e m i g r a t i o n o f t i t a n i u m o x i d e s p e c i e s i s an i m p o r t a n t f a c t o r t h a t c o n t r i b u t e s t o t h e s u p p r e s s i o n o f c a r b o n monoxide c h e m i s o r p t i o n . The H2 c h e m i s o r p t i o n e x p e r i m e n t s on 1-2 ML of P t , where no m i g r a t i o n i s o b s e r v e d , s t r o n g l y i n d i c a t e t h a t e l e c t r o n i c (bonding) i n t e r a c t i o n s a r e a l s o o c c u r r i n g . Thus, f o r the t i t a n i a system, b o t h e l e c t r o n i c i n t e r a c t i o n s and s u r f a c e s i t e b l o c k i n g due t o t i t a n i u m o x i d e s p e c i e s must be c o n s i d e r e d i n i n t e r p r e t i n g SMSI effects. +

+


+

+

+

Acknowledgments T h i s work was

supported

i n p a r t by the O f f i c e of N a v a l R e s e a r c h .

Literature Cited 1. 2. 3.

Tauster, S. J . ; Fung, S. C.; Garten, R. L. J. Am. Chem. Soc. 1978, 100, 170. Tauster, S. J.; Fung, S. C. J . Catal. 1978, 55, 29. Imelik, B. et al., eds. "Metal-Support and Metal Additive E f f e c t s i n Catalysis"; E l s e v i e r : Amsterdam, 1982, and references cited therein.



7. S U N ET A L .


4. Huizinga, T., "Metal Support Interactions i n Pt and Rh A l 2 O 3 and TiO2 Catalysts," Ph.D. Thesis, Eindhoven University of Tech nology, 1983. 5. Jiang, X.-J.; Hayden, T. F.; Dumesic, J . A. J . Catal. 1983, 83, 68. 6. Resasco, D. E.; Haller, G. L. J . Catal. 1983, 82, 279. 7. Vannice, Μ. Α.; Twu, C. C. J . Catal. 198, 82, 213. 8. Short, D. R.; Mansour, A. N.; Cook, J . W. J r . ; Sayers, D. E.; Katzen, J . R. J . Catal. 1982, 82, 299. 9. Fang, S.-M.; White, J . M. J . Catal. 1983, 83, 1. 10. Tanaka, K.; White, J . M. J . Catal.1983, 79, 81. 11. Baker, R. T. K. J . Catal. 1980, 63, 523. 12. Fung, S. C. J . Catal. 1982, 76, 225. 13. Belton, D. N.; Sun, Y.-M.; White, J . M. J . Am. Chem. Soc. ( i n press). 14. Belton, D. N.; Sun, Y.-M.; White, J . M. J . Phys. Chem. ( i n press). 15. Davis, G. D.; Natan, M.; Anderson, K. A. Applic. Surface S c i . 1983, 15, 321. 16. Thrush, Κ. Α.; White, J . Μ., unpublished r e s u l t s . 17. McCabe, R. W.; Schmidt, L. D. Surface S c i . 1976, 60, 85. 18. C o l l i n s , D. M.; Spicer, W. E. Surface S c i . 1977, 69, 55. RECEIVED January 8, 1985


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