Chapter 1 Surface Spectroscopic Techniques

A1 Chapter 1

SUKFACE SPECTROSCOPIC TECHNIQUES

J.L. G. F i e r r o I n s t i t u t o de C a t a l i s i s y P e t r o l e o q u i m i c a , C . S . I . C . , (Spai n ) 1.1

Serrano 119, 28006 M a d r i d

INTKODUCTION

As t h e growth o f i n d u s t r i a l heterogeneous c a t a l y s t s c o n t i n u e s t o a c c e l e r a t e , t h e r o l e and c o n t r i b u t i o n s o f s u r f a c e s c i e n t i s t s and mechanism s p e c i a l i s t s become i n c r e a s i n g l y v i t a l . Researchers i n t h e s e f i e l d s a r e making m a j o r e f f o r t s i n d e v e l o p i n g t h e fundamental methods t o p r o v i d e t h e knowledge needed t o s u p p o r t r a p i d progress i n b a s i c and a p p l i e d c a t a l y s i s . Many r e c e n t developments t o c h a r a c t e r i z e c a t a l y t i c p r e p a r a t i o n s and r e a c t i o n mechanisms have r e s u l t e d i n i m p o r t a n t advances i n o u r u n d e r s t a n d i n g o f c a t a l y t i c phenomena. F o r a g i v e n c a t a l y z e d process a c t i v i t y and s e l e c t i v i t y a r e two i m p o r t a n t parameters which i n d i c a t e how good a c a t a l y s t i s . However, t h e s e parameters r e s u l t from measurements c a r r i e d o u t a t a macroscopic s c a l e , i . e . u s u a l l y by gas-chromatography, which i s c o n s i d e r a b l y g r e a t e r t h a n t h e a t o m i c s c a l e o f t h e m o l e c u l a r events c o n f i n e d t o t h e c a t a l y t i c s u r f a c e . From a s c i e n t i f i c p o i n t o f view, t h e i n v e s t i g a t i o n o f t h e s u r f a c e c o m p o s i t i o n and l o c a l s t r u c t u r e o f c a t a l y s t s a t t h e atomic l e v e l and t h e c o r r e l a t i o n o f t h e s e d a t a w i t h c a t a l y s t p e r formance r e s u l t e x t r e m e l y u s e f u l i n u n d e r s t a n d i n g t h e r o l e t h a t s u r f a c e atoms p l a y i n t h e c a t a l y t i c r e a c t i o n . T h i s b a s i c i n f o r m a t i o n on t h e s t r u c t u r e - p r o p e r t y r e l a t i o n s h i p f o r e x i s t i n g c a t a l y s t systems w i l l u l t i m a t e l y b e of v a l u e i n t h e d e s i g n o f new c a t a l y s t s , e s p e c i a l l y more e f f i c i e n t ones. On t h e same lines,much work i s undoubtedly

carried out i n the investigation o f catalyst f a i l u r e o r

gradual d e a c t i v a t i o n t o i d e n t i f y c a t a l y s t p o i s o n i n g mechanisms, e.g. by segreg a t i o n o f c a t a l y s t i m p u r i t i e s , d e p o s i t i o n f r o m i m p u r i t i e s i n t h e r e a c t a n t stream o r from side-reaction products. The above o b j e t i v e s o f t h e a n a l y s i s o f c a t a l y t i c s u r f a c e s have been a r t i c u l a t e d n e a r l y f i f t e e n y e a r s ago w i t h t h e advent o f many new techniques, developed through s u r f a c e s c i e n c e and r e i t e r a t e d many t i m e s (see (1-10)). Perhaps i t i s i m p o r t a n t h e r e t o keep i n mind t h a t t h e concept o f c a t a l y s t s u r faces i s n o t w e l l d e f i n e d , because t h e c o m p o s i t i o n o f t e n d e v i a t e s f r o m t h e i r b u l k values f o r depths o f m i c r o n s . As can be seen i n t h e n e x t s e c t i o n , d u e t o t h e fact t h a t each i n d i v i d u a l s u r f a c e a n a l y s i s t e c h n i q u e shows i t s own sampling depth, each p r o v i d e s i t s unique v i e w o f t h e inhomogeneous c a t a l y s t s u r f a c e r e g i o n . T h i s f e a t u r e i s a m a j o r b a r r i e r t o q u a n t i t a t i v e a n a l y s i s and as such has

A2

m o t i v a t e d e f f o r t s t o e s t a b l i s h c a l i b r a t i o n standards. Surface a n a l y s i s o f heterogeneous c a t a l y s t s i n v o l v e s t h e measurement o f t h r e e d i f f e r e n t q u a n t i t i e s . The most i m p o r t a n t o f these i s t h e qua1 i t a t i v e i d e n t i f i c a t i o n of surface species, i.e.,

what atom-type i s a t t h e surface. The

second concerns t h e chemical s t a t e o f these atoms, i.e.,

the oxidation state o f

surface species. F i n a l l y , i t i s necessary t o determine t h e s p a t i a l l o c a t i o n o f t h e s u r f a c e s t r u c t u r e s . For instance, i n a supported c a t a l y s t i s i t c r u c i a l t o know i f t h e a c t i v e i n g r e d i e n t i s on t o p o f t h e c a r r i e r s u r f a c e o r d i f f u s e s i n i t , o r perhaps i t forms l o c a l i z e d c r y s t a l l i t e s a t v a r i o u s p o s i t i o n s across t h e surface. T h i s t y p e o f surface a n a l y s i s i s by f a r t h e most e x t e n s i v e l y used form o f a n a l y s i s o f c a t a l y s t surfaces and i s t h e s u b j e c t o f most of t h e p u b l i s h e d papers. A complete review i n t h i s area covering t h e l a s t t h r e e y e a r s has been very r e c e n t l y pub1 i s h e d (11).

A d i s a p p o i n t i n g f e a t u r e , however, i s t h e non-planar n a t u r e o f t h e c a t a l y s t surfaces. Most o f t h e s u r f a c e spectroscopies r e q u i r e an ordered surface, i.e., a s i n g l e c r y s t a l . The e l e c t r o n spectroscopies f a l l i n t h i s category, although Auger spectroscopy i s n o t so r e c t r i c t e d , and has been s u c c e s s f u l l y a p p l i e d t o t h e study o f pronioter d i s t r i b u t i o n i n a commercial ammonia s y n t h e s i s K20-A1203Fe c a t a l y s t ( 1 2 ) . The small area samples, o f t e n s i n g l e c r y s t a l o r p o l y c r y s t a l l i n e f o i l s , b u t w e l l s u i t e d t o c h a r a c t e r i z e surfaces i n u l t r a h i g h vacuum by low energy e l e c t r o n d i f f r a c t i o n (LEED) , Auger spectroscopy (AES) o r o t h e r s u r f a c e s e n s i t i v e techniques, can be used as model c a t a l y s t s , s i n c e a s p e c i a l c o n f i g u r a t i o n provides p a r a l l e l k i n e t i c s t u d i e s under c o n d i t i o n s t h a t a r e v i r t u a l l y i d e n t i c a l t o those used i n t h e chemical technology. For d e t a i l s , t h e reader i s r e f e r r e d t o t h e works o f Goodman (13) and Somorjai ' s group ( 1 4 ) . When one t u r n s , however, $0 supported c a t a l y s t s c o n s i s t i n g o f small c r y s t a l s dispersed on h i g h s u r f a c e area c a r r i e r s , e.g.,

z e o l i t e s , A1203, Si02, e t c , m o s t l y w i t h i n t h e

pores, these spectroscopies become i m p r a c t i c a l . T h i s i s due t o t h e f a c t t h a t t h e a c t i v e i n g r e d i e n t i s n o t a c c e s i b l e by e l e c t r o n probes. S t u d i e s o f t h e atomic s t r u c t u r e and composition o f these l a t t e r c a t a l y s t s r e q u i r e o t h e r techniques. Among them, s o l i d s t a t e NMR (15-17), extended X-ray a b s o r p t i o n f i n e s t r u c t u r e (EXAFS) (5, 18. 19). X-ray p h o t o e l e c t r o n spectroscopy (XPS) (20-33), and Mossbauer spectroscopy (23, 24), o f l a t e have been found p a r t i c u l a r l y useful i n studies o f t h i s class o f catalysts.

1.2

SPECTROSCOPIES AND RELATED TECHNIQUES S c i e n t i s t working i n s u r f a c e science and c a t a l y s i s a r e now r a t h e r f a m i l i a r

w i t h many s u r f a c e spectroscopies f o r which t h e y have adopted conventional, b u t no l e s s confusing, acronyms (XPS, EAS. LEED, I S S , EELS, e t c . ) .

I n the past

f i f t e e n years t h e number o f these techniques has increased i n a such manner t h a t i t becomes d i f f i c u l t t o devise new acronyms f r e e o f ambiguity. Hence,many

A3

Fig. 1.1. P i c t o r i a l v i e w of surface analysis technlques. Various combinatSons o f probes i n and p a r t i c l e s out determine the various surface analysis techniques. attempts have been made t o overview i n a systematic way the plethora o f surface spectroscopies (25). Perhaps the m s t descriptlve and simplest method t o categorize them i s the Propst representation (Fig. 1.1).

The crossed c i r c l e represents the surface to be analyzed. In-going drrws represent the various probes t o e x c i t e the sample, while out-going arrows correspond t o t h e excitations which convey i n f o m a t i o n about the sample. In p r i n c f p l e , every spectroscopy can be represented by a combination o f dn

an ingoing

and

out arrow. On the basis of the 6 surface probes, i t r e s u l t s t h a t there are

6 2 combinations, about h a l f of them are I n c m n use. However, the number o f e x i s t i n g spectroscopies i s much larger. This i s due t o the f a c t that a s l n g l e

combination of an in-going arrow andaout-going arrow may lead t o several q u i t e d i s s i m i l a r spectroscopies, dependtng on what propertjes o f the probe and o f the emitted p a r t i c l e s a r e measured. For instance, when photons are used as the incident probe, t h e i r wavelength may widely vary frm radiofrequencies t o 'f-rarn. I n t h i s broad range o f energies the complete

diatinn, t y p i c a l l y I02 t o lo-''

electrwnagnetic spectrum comprises various narrower regions o f photon energy,

namely, radi ofrequencey , microwaves, in f rared , v l s 1 b l e-u 1. trav lo1 e t , X-rays, and ,,-radiation. Therefore, a single combination o f an i n and out photon (Fig, 1.1) may lead t o NMR, ESH, I R , UV-visible, XRD, XRF. EXAFS, and Mossbauer spectroscopies. I n order t o visualize t h i s f a c t i n a clearer way, the Propst diagram

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A6

has been more e x p l i c i t l y c o n s t r u c t e d i n F i g . 1.2. A l t h o u g h few o f t h e s e s p e c t r o s c o p i e s a r e e x t e n s i v e l y r e v i e w e d i n t h e f o l l o w i n g c h a p t e r s o f t h i s volume and a l s o i n p a r t B o f t h i s s e r i e s , a v e r y s h o r t d e s c r i p t i o n o f t h e s p e c t r o s c o p i e s summarized i n F i g . 1.2. which i n c l u d e s t h e i r p h y s i c a l bases and a p p l i c a t i o n s , i s o u t l i n e d below. F o r t h e t e c h n i q u e s t h a t a r e n o t d e a l t w i t h i n t h e s e c h a p t e r s , t h e l i t e r a t u r e mentioned has been r e s t r i c t e d t o o n l y few b a s i c r e f e r e n c e s . 1.2.1

Photons-Photons N u c l e a r Magnetic Resonance (NMR). T h i s t e c h n i q u e i s based on t h e i n t e r a c t i o n o f n u c l e a r s p i n s w i t h an e x t e r -

n a l magnetic f i e l d which s p l i t s t h e c o r r e s p o n d i n g energy l e v e l s . T r a n s i t i o n s between them e x a c t l y corresponds t o t h e energy o f t h e i n c i d e n t wave, t y p i c a l l y o f 100 MHz ( F i g . 1 . 3 ) . T h e s p l i t t i n g i s v e r y s e n t i s i v e t o t h e symmetry and conf i g u r a t i o n o f t h e nuclei, t h e r e f o r e t h i s technique allows t o study t h e environment o f t h e atoms and t h e i r m o t i o n . A e x t e n s i v e r e v i e w o f t h e a p p l i c a t i o n s o f IIMR i n c a t a l y s i s i s g i v e n i n p a r t 6, c h a p t e r 4.

E l e c t r o n S p i n Resonance (ESR)

.

I n t h i s case, t h e i n t e r a c t i o n o f t h e e l e c t r o n s p i n s o f t h e atoms w i t h a e x t e r n a l magnetic f i e l d causes a s p l i t t i n g o f t h e c o r r e s p o n d i n g energy l e v e l s . T r a n s i t i o n s between t h e s e l e v e l s o c c u r a t s l i g h t l y h i g h e r e n e r g i e s t h a n f o r NMR, t y p i c a l l y o f 10 GHz ( F i g . 1.3). As f o r NMR, t h e s p l i t t i n g o f t h e e l e c t r o n l e v e l s i s a f f e c t e d by t h e environment o f paramagnetic atoms, i o n s o r r a d i c a l s . A r e v i e w o f t h i s t e h c h n i q u e a p p l i e d t o c a t a l y s i s and r e l a t e d phenomena i s g i v e n i n p a r t B, c h a p t e r 3 . I n f r a r e d Spectroscopy ( I R ) . I n t h e a b s o r p t i o n mode, t h i s t e c h n i q u e i s based on t h e a b s o r p t i o n by a so-

l i d which e x c i t e s m o l e c u l a r v i b r a t i o n s . t h e energy o f t h e s e v i b r a t i o n s depends on t h e n a t u r e and b i n d i n g o f t h e groups (26-28).

I n t h e emission (29) o r r e f l e c -

t i o n (30, 31) modes, t h e s p e c t r a o f e m i t t e d r a d i a t i o n upon h e a t i n g o r t h e r e f l e c t e d i n f r a r e d r a d i a t i o n by t h e s o l i d can be a l s o r e c o r d e d . Ranian Spectroscopy. K a d i a t i o n f r o m t h e v i s i b l e wavelength window, t y p i c a l l y l a s e r beams o f 500-600 nm ( F i g . 1.3), i s i n e l a s t i c a l l y s c a t t e r e d by a s o l i d as a r e s u l t o f t h e

e x c i t a t i o n o f m o l e c u l a r v i b r a t i o n s . The i n f o r m a t i o n p r o v i d e d by t h i s t e c h n i q u e i s e s s e n t i a l l y s i m i l a r t o t h a t g i v e n by i n f r a r e d spectroscopy, and f r e q u e n t l y i s complemented by i t ( 3 2 ) . Atoniic A b s o r p t i o n Spectroscopy (AAS). The a b s o r p t i o n o f photons i n t h e v i s i b l e f r e q u e n c y window, i n t h e o r d e r 100-900 nm wavelength, induces e l e c t r o n i c t r a n s i t i o n s between t h e e l e c t r o n i c energy l e v e l s o f atoms. Since t h e energy o f t h e absorbed photon i s c h a r a c t e r i s -

A7

t i c o f each atom, t h i s p r o p e r t y i s e x p l o i t e d f o r q u a n t i t a t i v e a n a l y s i s (33, 3 4 ) . This technique r e s u l t s extremely useful i n c a t a l y s i s l a b o r a t o r i e s f o r determining t h e atomic composition o f the c a t a l y s t s . D i f f u s e R e f l e c t a n c e Spectroscopy (DRS). T h i s t e c h n i q u e i s based on t h e r e f l e c t i o n o f U V - v i s i b l e r a d i a t i o n by f i n e l y d i v i d e d m a t e r i a l s . The c o o r d i n a t i o n and 1 i g a n d charge t r a n s f e r phenomena i n t r a n s i t i o n metal i o n s o f heterogeneous c a t a l y s t s may be e a s i l y s t u d i e d f o r m t h e i n t e n s i t y and p o s i t i o n o f t h e U V - v i s i b l e a b s o r p t i o n bands ( 3 5 ) . P h o t o a c o u s t i c ( PAS) and P h o t o - D e f l e c t i o n Beam Spectroscopy (PDS). These techniques d i f f e r f r o m i n f r a r e d and U V - v i s i b l e s p e c t r o s c o p i e s o n l y i n t h e way t h e a b s o r p t i o n i s d e t e c t e d . I n t h e case o f PAS t h e t e m p e r a t u r e i n crease induced by a b s o r p t i o n o f an i n c i d e n t modulated r a d i a t i o n produces an a c o u s t i c wave t h a t may be p r o p e r l y r e c o r d e d by a microphone (36, 3 7 ) . The PDS c o n s i s t s o f p a s s i n g a l i g h t beam o v e r a heated s u r f a c e . The r e f r a c t i v e i n d e x g r a d i e n t i n t h e medium o v e r t h e s u r f a c e causes t h e l i g h t beam t o be d e f l e c t e d (38, 39).

X-Ray D i f f r a c t i o n (XRD) and X-ray Fluorescence (XRF). L i k e AAS, X-ray d i f f r a c t i o n i s an e x t r e m e l y u s e f u l t e c h n i q u e i n c a t a l y s i s l a b o r a t o r i e s , because i t a l l o w s t o c h a r a c t e r i z e t h e c r y s t a l s t r u c t u r e o f s o l i d m a t e r i a l s . From t h e broadening a n a l y s i s o f t h e most i n t e n s e d i f f r a c t i o n peaks o r f r o m t h e X-ray d i s t r i b u t i o n a t l o w d i f f r a c t i o n angles, t h e c r y s t a l s i z e o f a g i v e n c r y s t a l l i n e phase

can be determined (40, 4 1 ) . I n f l u o r e s c e n c e t h e i n c i -

d e n t X-ray photons can e j e c t e l e c t r o n s from t h e i n n e r l e v e l s o f t h e atoms. Furt h e r d e e x c i t a t i o n i n v o l v e s t r a n s i t i o n s o f e l e c t r o n s f r o m upper l e v e l s w i t h emission o f c h a r a c t e r i s t i c X-ray r a d i a t i o n . T h i s p r o p e r t y can be a1 so e x p l o i t e d f o r a n a l y t i c a l purposes ( 4 2 ) . Extended X-ray A b s o r p t i o n F i n e S t r u c t u r e (EXAFS). T h i s t e c h n i q u e i n v o l v e s t h e a b s o r p t i o n o f X-ray r a d i a t i o n by t h e atoms, which a l l o w s t h e excape o f t h e c o r e e l e c t r o n s f r o m t h e a t o m i c p o t e n t i a l w e l l . The i n t e r a c t i o n between t h e wave a s s o c i a t e d w i t h t h e e x c i t e d e l e c t r o n s and t h e n e i g h b o u r i n g atoms produce a f i n e s t r u c t u r e i n t h e X-ray spectrum i n an energy range o f 50-100 eV h i g h e r t h a n t h e energy o f t h e a b s o r p t i o n a t t h e edge. The o b s e r v a t i o n and f u r t h e r a n a l y s i s o f t h e f i n e s t r u c t u r e spectrum r e q u i r e s a h i g h l y powered X-ray source such as t h a t p r o v i d e d by s y n c h r o t r o n r a d i a t i o n . The d e t e r m i n a t i o n o f l o c a l s t r u c t u r e parameters around t h e e x c i t e d atom, e.g.,

in-

t e r a t o m i c d i s t a n c e s and c o o r d i n a t i o n numbers, can be d e r i v e d f r o m t h e a b s o p r t i o n spectrum. An e x t e n s i v e r e v i e w concerning t h i s t e c h n i q u e i s g i v e n i n Chapter 4. MSssbauer Spectroscopy. T h i s t e c h n i q u e i s based on t h e r e c o i l - f r e e e m i s s i o n and a b s o r p t i o n o f low energy gamma r a y s by t h e n u c l e i o f atoms i n s o l i d s . The photon e m i t t e d b y a r a d i a c t i v e nucleus may be absorbed by a n o t h e r nucleus, i f t h e c o r r e s p o n d i n g

A8 nuclear t r a n s i t i o n energies a r e p r o p e r l y matched. The f a c t t h a t t h e MGssbauer t r a n s i t i o n s have v e r y narrow l i n e w i d t h s p e r m i t s t h e use o f r e l a t i v e l y simple methods f o r modulating t h e gamma energy. By moving t h e r a d i a t i o n source o r t h e absorber r e l a t i v e t o t h e o t h e r , t h e energy o f t h e gamma r a y can be brought i n t o resonance by v i r t u e o f t h e Doppler e f f e c t . The energy o f t h i s resonance i s a f f e c t e d by t h e i n t e r a c t i o n o f t h e nucleus w i t h i t s environment. The study o f t h e e m i t t e r o r absorber environment i s o n l y f e a s i b l e f o r a few atomic p a i r s , e . g.,

57Fe-57Co, 119Sn-119Sb,

195Pt-195Au.

The a p p l i c a t i o n s i n c a t a l y s i s a r e

reviewed i n Chapter 5. Photon-Electrons

1.2.2

UV-Photoelectron (UPS) and X-ray P h o t o e l e c t r o n Spectroscopy (XPS)

.

The p r i n c i p l e o f p h o t o e l e c t r o n spectroscopy i s t h e r e l a t i o n hv = Eb

+

KE

(1.1)

where hu i s t h e energy o f t h e i n c i d e n t photon, E,

t h e b i n d i n g energy o f t h e

e l e c t r o n t o t h e atomic l e v e l s , and KE t h e k i n e t i c energy o f t h e e l e c t r o n s escaping from t h e atoms. Due t o t h e lower energy o f t h e i n c i d e n t photons ( F i g . 1.3), UPS a l l o w s t o study t h e valence and conduction bands, whereas XPS i s a powerful technique t o examine t h e chemical n a t u r e and b i n d i n g o f t h e atoms from t h e a n a l y s i s o f core e l e c t r o n photoemission peaks. Since t h e i n e l a s t i c mean f r e e path o f t h e photoelectrons i s w i t h i n t h e range 0.5-5

nm, b o t h techniques

prove t o be e s p e c i a l l y s e n s i t i v e f o r t h e f i r s t twenty upper-most s u r f a c e l a y e r s . 1.2.3

Photons-Molecules Photodesorption. Photon a b s o r p t i o n may be expected t o produce e l e c t r o n i c t r a n s i t i o n s i n ad-

sorbed molecules l e a d i n g t o desorption. This technique r e q u i r e s , i n p r i n c i p l e , photons i n t h e U V - v i s i b l e r e g i o n ( 4 3 ) , however o t h e r sources, such as synchrot r o n r a d i a t i o n ( 4 4 ) and gamma r a y s from 6oCo can be used. Mechanisms o f desorpt i o n and o f s u r f a c e r e a c t i o n s can be e l u c i d a t e d by t h i s technique. Laser Microprobe Mass Spectrometry (LMMS)

.

I n t h i s case, bombardment o f a s o l i d w i t h a l a s e r beam produces t h e emiss i o n o f i o n i z e d fragments which can be i d e n t i f i e d by mass ( a n a l y s i s ) spectrometry. The method a l l o w s t o deterniine t h e composition o f t h e sample w i t h a r e s o l u t i o n down t o t h e range o f 1-10 urn. By v a r y i n g t h e i n c i d e n t power d e n s i t y t h e in-depth r e s o l u t i o n may w i d e l y change from lo-’

to

m (46).

A9 1.2.4

Electrons-Photons Appearance P o t e n t i a l Spectroscopy (APS)

.

The t r e s h o l d p o t e n t i a l f o r i n e l a s t i c s c a t t e r i n g o f e l e c t r o n s f r o m atoms can be c o r r e l a t e d w i t h t h e appearance o f c h a r a c t e r i s t i c l i g h t emission. The r e s o l u t i o n i s much b e t t e r t h a n i n e l e c t r o n s p e c t r o s c o p i e s , s i n c e an e l e c t r o n spect r o m e t e r i s n o t r e q u i r e d . The t e c h n i q u e s which i n v o l v e appearance p o t e n t i a l s , e.g.,

s o f t X-ray- (SXAPS), Auger e l e c t r o n - (AEAPS), and disappearance p o t e n t i a l

spectrsocopy (DAPS), p r o v i d e i n f o r m a t i o n about t h e c o n d u c t i o n s t a t e s (47, 48). E l e c t r o n Probe M i c r o a n a l y s i s (EMPA). The atoms e x c i t e d upon bombardment o f a s o l i d w i t h h i g h energy (10-100 keV) e l e c t r o n s decay by emission o f c h a r a c t e r i s t i c X-ray photons w i t h e n e r g i e s equal t o t h e energy d i f f e r e n c e between t h e e x c i t e d and ground s t a t e s . The EPMA method a l l o w s q u a n t i t a t i v e compositon o f s o l i d s w i t h a s p a t i a l r e s o l u t i o n o f 1 pm ( 4 9 ) . 1.2.5

Electrons-Electrons Scanning (SEM) and Transmission E l e c t r o n Microscopy (TEM). I n t h e scanning mode, t h e t e c h n i q u e a l l o w s f o r t h e imaging of t h e t o p o g r a -

phy o f a s o l i d s u r f a c e by means o f b a c k s c a t t e r e d o r

secondary e l e c t r o n s . I t s

p r e s e n t r e s o l u t i o n i s b e t t e r t h a n 5 nm, TEM, which i n v o l v e s a v a r i e t y o f imaging techniques, e.g.

b r i g h t f i e l d , dark f i e l d , h i g h r e s o l u t i o n , a l l o w s f o r t h e e l u -

c i d a t i o n o f i n i c r o t e x t u r e and m i c r o s t r u c t u r e o f e l e c t r o n t r a n s p a r e n t samples w i t h an a c t u a l r e s o l u t i o n b e t t e r t h a n 0.5 nm ( 5 0 - 5 2 ) . E l e c t r o n Energy Loss Spectroscopy (EELS). T h i s t e c h n i q u e i s based on t h e i n e l a s t i c s c a t t e r i n g s u f f e r e d by e l e c t r o n s on s o l i d s u r f a c e s . The i n t e n s i t y o f t h e e l e c t r o n s i s r e c o r d e d as a f u n c t i o n o f t h e energy l o s s w i t h r e s p e c t t o t h e i n c i d e n t energy. T h i s i s a v i b r a t i o n a l t e c h n i q u e which a l l o w s t o s t u d y t h e s t a t e o f adsorbed molecules. The a p p l i c a t i o n s o f t h i s t e c h n i q u e a r e reviewed i n c h a p t e r 3 o f volume 6 . Auger E l e c t r o n Spectroscopy (AES). I n t h i s case, e n e r g e t i c (1-2.5 keV) e l e c t r o n s e j e c t c o r e e l e c t r o n s o f t h e atoms. I n t h e f o l l o w i n g d e e x c i t a t i o n process, a second e l e c t r o n (Auger) may be e j e c t e d , whose energy i s c h a r a c t e r i s t i c o f t h e energy d i f f e r e n c e between t h e l e v e l s , and t h e r e f o r e o f t h e t y p e o f atoms. Due t o t h e i n e l a s t i c mean f r e e p a t h o f t h e e l e c t r o n s , AES p e r m i t s t h e a t o m i c c o m p o s i t i o n o f t h e surface. The e j e c t e d Auger e l e c t r o n s may be r a s t e r e d on t h e s u r f a c e and imaged, t h u s p r o v i d i n g a mapping o f t h e

s u r f a c e composition. The t e c h n i q u e i s t h e n c a l l e d Scanning

Auger Spectroscopy (SAM). When u s i n g a h i g h d e n s i t y focussed e l e c t r o n beam, t h e s p a t i a l r e s o l u t i o n may be i n t h e o r d e r o f 0.1 pm. The t e c h n i q u e i s reviewed i n c h a p t e r 2 o f t h i s volume.

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Low Energy E l e c t r o n D i f f r a c t i o n (LEED). T h i s t e c h n i q u e i s based on t h e e l a s t i c s c a t t e r i n g o f e l e c t r o n s w i t h e n e r g i e s below 300 eV t h r o u g h t h e topmost l a y e r s o f s i n g l e c r y s t a l s u r f a c e s . From analysis o f the d i f f r a c t e d electron intensities, the structure o f the surface atoms and o f t h e cheinisorbed l a y e r can be r e v e a l e d ( 5 3 , 5 4 ) . R e f l e c t i o n High Energy E l e c t r o n D i f f r a c t i o n (RHEED). I n t h i s case, t h e s u r f a c e i s bombarded w i t h a monoenergetic e l e c t r o n beam,

t y p i c a l l y o f 10 keV. The a n a l y s i s o f t h e a n g u l a r d i s t r i b u t i o n o f e l a s t i c a l l y scattered electrons allows a l s o t h e study o f t h e surface s t r u c t u r e o f s i n g l e c r y s t a l s . One advantage o f RHEED w i t h r e s p e c t t o LEED i s t h a t t h e h i g h energy e l e c t r o n beam used i n t h e f o r m e r e x c i t a t e s X-rays c h a r a c t e r i s t i c of t h e elements, t h u s p e r m i t i n g simultaneous e x a m i n a t i o n o f t h e s t r u c t u r e a n d composit i o n (55). 1.2.6

Electron-Ions E l e c t r o n S t i m u l a t e d D e s o r p t i o n (ESD). If a s u r f a c e i s bombarded, w i t h low-energy ( < 500 eV) e l e c t r o n s , a t l o w

power d e n s i t i e s t o p r e v e n t thermal e f f e c t s , one can observe t h e d i s p e r s i o n o f n e u t r a l atoms and molecules, e x c i t e d n e u t r a l and p o s i t i v e i o n s , as w e l l as c o n v e r s i o n between b i n d i n g s t a t e s o f t h e adsorbates. The mass and k i n e t i c e n e r gy o f t h e desorbed s p e c i e s can t h u s r e v e a l t h e c r o s s s e c t i o n s o f d e s o r p t i o n . and t h e t h r e s h o l d e l e c t r o n energy f o r d e s o r p t i o n , t h e complex i n t e r a c t i o n s o f gases on w e l l c h a r a c t e r i z e d s u r f a c e s ( 5 6 - 5 8 ) . 1.2.7

Ions-Photons I o n Emission X-rays (IEXR). As occurs w i t h e l e c t r o n s , t h e bombardment o f s o l i d s u r f a c e s w i t h i o n s ( I -

5 MeV) produces t h e e m i s s i o n o f X-rays. T h i s p r o p e r t y i s a l s o e x p l o i t e d f o r q u a n t i t a t i v e a n a l y s i s . The advantage o f t h i s t e c h n i q u e o v e r EPMA i s t h a t i t does n o t r e q u i r e s a vacuum chamber. The s p a t i a l r e s o l u t i o n o f IEXP i s i n t h e o r d e r o f a few micrometers (59, 6 0 ) . 1.2.8

Ions-Electrons I o n N e u t r a l i z a t i o n Spectroscopy ( I N S ) . The approach o f a slow i o n t o a s u r f a c e p r o v i d e s a v a c a n t l o w - l y i n g l e v e l

which w i l l be f i l l e d by an e l e c t r o n t u n n e l i n g downward f r o m t h e v a l e n c e band o f t h e s u r f a c e . The energy r e l e a s e d i n t h i s t r a n s i t i o n can be t a k e n up by a second valence e l e c t r o n i n an Auger t r a n s i t i o n , The energy o f t h e e m i t t e d Auger e l e c t r o n depends on t h e l e v e l s w i t h i n t h e v a l e n c e band f r o m which t h e n e u t r a l i z i n g e l e c t r o n t u n n e l s . The energy d i s t r i b u t i o n t h u s r e f l e c t s t h e d e n s i t y o f s t a t e s

(DOS) o f t h e v a l e n c e band ( 6 1 ) .

All 1.2.9.

Ions-Ions I o n S c a t t e r i n g Spectroscopy ( I S S ) . D u r i n g t h e bombardment o f a s o l i d s u r f a c e w i t h l o w energy (0.5-3 keV) i o n s ,

some p a r t o f t h e i r k i n e t i c energy i s t r a n s f e r r e d t o t h e atoms o f t h e s u r f a c e . As t h e l o s t energy depends on t h e mass o f t h e t a r g e t atoms and o f t h e s c a t t e r i n g angle, a n a l y s i s o f t h e k i n e t i c energy o f t h e s c a t t e r e d i o n s p r o v i d e s t h e mass spectrum o f t h e atoms a t t h e s u r f a c e (62, 6 3 ) . Secondary I o n Mass Spectrometry (SIMS)

.

T h i s t e c h n i q u e i s based on t h e e r o s i o n o f a s o l i d s u r f a c e w i t h an i o n beam (1-10 keV). The i o n i z e d fragments a r e f u r t h e r analyzed by mass s p e c t r o m e t r y . The t e c h n i q u e p r o v i d e s t h e r e f o r e c o m p o s i t i o n as a f u n c t i o n o f depth. I t s sens i t i v i t y i s h i g h enough t o a l l o w f o r n o n - d e s t r u c t i v e a n a l y s i s o f t h e f i r s t t o t h i r d topmost l a y e r s by means o f a l o w d e n s i t y i o n beam (64, 6 5 ) . R u t h e r f o r d B a c k s c a t t e r i n g Spectroscopy (RBS)

.

The p r i n c i p l e i s t h e same as f o r I S S b u t RBS uses a h i g h energy ( 1 - 3 MeV) i o n beam. I t p r o v i d e s i n f o r m a t i o n concerning deeper l a y e r s . The d i s t r i b u t i o n o f heavy elements i n a l i g h t m a t r i x can be r e v e a l e d by t h i s t e c h n i q u e ( 6 6 ) . 1.2.10

Neutrals-Neutrals

Atom- and M o l e c u l a r Beam S c a t t e r i n g (AS and MBS). I n t h i s case, t h e bombardment o f s i n g l e c r y s t a l s u r f a c e s w i t h a mono-energ e t i c beam o f atoms o r molecules a l l o w s t o s t u d y t h e i r s t r u c t u r e t h r o u g h t h e a n a l y s i s o f t h e a n g u l a r d i s t r i b u t i o n and number o f s c a t t e r e d p a r t i c l e s (67, 6 8 ) . Neutron S c a t t e r i n g (NS). The s c a t t e r i n g o f neutrons i s used f o r s e v e r a l purposes. As t h e s c a t t e r i n g c r o s s - s e c t i o n f o r hydrogen i s c o n s i d e r a b l y g r e a t e r t h a n t h a t f o r any o t h e r atom,

NS i s e s p e c i a l l y s e n s i t i v e t o hydrogen. C r y s t a l s t r u c t u r e s , v i b r a t i o n a l t r a n s i t i o n s , atomic o r m o l e c u l a r d i f f u s i o n , and p o r e s i z e s can be s t u d i e d by NS (69, 70).

1.3 GENERAL FEATURES From t h e p o s s i b l e ways t o v i e w t h e s u r f a c e o f heterogeneous c a t a l y s t s , as a l r e a d y r e p r e s e n t e d by t h e P r o p s t diagram ( F i g . 1.1) o r more e x p l i c i t y b y F i g . 1.2, one would ask whether a r a n k i n g o r d e r can be e s t a b l i s h e d a c c o r d i n g t o t h e r e l e v a n c e o f i n f o r m a t i o n t h e y p r o v i d e . The answer i s n o t easy and would r e s u l t i n an e n t e r t a i n i n g e x e r c i s e , s i n c e each t e c h n i q u e analyzes t h e c a t a l y s t s u r f a c e frow a d i s t i n c t l y d i f f e r e n t perspective. I t i s not possible, therefore,

to

separate t h e r e l e v a n c e o f each spectroscopy f r o m t h e problems i t e x p e c t s t o solve. Several parameters, such as bandwidth, s e n s i t i v i t y , and d e p t h i n f o r m a t i o n a r e common t o a l l s p e c t r o s c o p i e s . However, t h e use o f t h e s e parameters must be t a k e n w i t h c a r e because few o f them a r e t o a l a r g e e x t e n t t r a n s m u t a b l e . F o r

A12

instance, s e n s i t i v i t y and r e s o l u t i o n a r e u s u a l l y balanced from bandwidth over a wide range. The optimum balance between these parameters depends on what i n f o r mation we a r e i n t e r e s t e d i n . Therefore, i t r e s u l t s impossible t o s p e c i f y t h e s e n s i t i v i t y o f a s u r f a c e technique w i t h o u t h a f u l l d e s c r i p t i o n o f i t s r e l a t i o n s h i p t o t h e o t h e r parameters. I n a d d i t i o n , i t must be d i f f i c u l t t o d i s t i n g u i s h t h e i n t r i n s i c l i m i t a t i o n s o f t h e technique i t s e l f f r o m t h e l i m i t a t i o n s o f an a p p r o p r i a t e model instrument. These parameters a r e analyzed below. 1.3.1

Ins trumental Sens it i v i Every s u r f a c e spectroscopy re1 i e s on some d i s t i n g u i s h i n g c h a r a c t e r i s t i c s t o

d i s c e r n t h e d e s i r e d s i g n a l from t h e background o f unwanted emissions t h a t t h e probe has a l s o s t i m u l a t e d . The s e n s i t i v i t y o f every a n a l y t i c a l spectroscopy i s u l t i m a t e l y l i m i t e d by a background residue, which i s i n some way c o r r e l a t e d w i t h t h e expected s i g n a l . By i n c r e a s i n g t h e primary c u r r e n t , t h e background w i l l increase p r o p o r t i o n a l l y t o t h e s i g n a l , It i s thought, however, t h a t reducing a r b i t r a r i l y t h e s t a t i s t i c a l n o i s e c o n t r i b u t i o n by i n c r e a s i n g i n d e f i n i t e l y t h e t i m e o f data a c q u i s i t i o n , any s i g n a l c o u l d be d e t e c t e d on t h e background. Unfort u n a t e l y , n e i t h e r an extremely l o n g measurement t i m e n o r p a t i e n t experimentat i o n s can p r o v i d e d i s t i n g u i s h a b l e s i g n a l s on t o p o f t h e background. T h i s phenomenon i s a p p r o p r i a t e l y named 'If1 i c k e r e f f e c t " . Much has been w r i t t e n on t h e sources o f f l i c k e r noise, and explanations o f i t s o r i g i n abound i n l i t e r a t u r e (83). The s i g n a l - t o - f l i c k e r n o i s e r a t i o can be regarded as a measure o f t h e s t a b i l i t y o f t h e e n t i r e measurement c i r c u i t . I n c o n t r a s t t o t h e white-noise spectrum r e s u l t i n g from thermal n o i s e i n t h e measurement system, f l i c k e r noise i s described by l / f spectrum. Consequently, i t increases p r o p o r t i o n a l l y t o t h e t i m e r e q u i r e d f o r r e c o r d i n g t h e spectrum. F l i c k e r n o i s e i s a m p l i f i e d by t h e background l e v e l . Small changes over t i m e i n any measurement parameter may completely overshadow a small s i g n a l i n t h e presence o f l a r g e background. Thus i n c r e a s i n g s e n s i t i v i t y i s e q u i v a l e n t t o r e moving t h e low frequency components o f t h e spectrum. I t i s , t h e r e f o r e , i n f e r r e d t h a t simple

s u b t r a c t i o n o f t h e background, however s o p h i s t i c a t e d i t be (84) may

simp1 i f y spectrum a n a l y s i s , s e n s i t i v i t y remaining unchanged. One i n t e r e s t i n g p o i n t t o analyze i s when t h e d e t e c t e d s i g n a l i n t h e spectrometer comes o n l y from t h e s u r f a c e region, o r i t comes from

both

surface

and b u l k regions, b u t i t i s p o s s i b l e t o d i s t i n g h i s h t h e s u r f a c e component from t h e b u l k . Most o f t h e XPS and AES s i g n a l s we a r e i n t e r e s t e d i n correspond t o t h e e l e c t r o n s which a r e detected w i t h o u t s u f f e r i n g i n e l a s t i c c o l l i s i o n s and which appear t h e r e f o r e a t t h e i r o r i g i n a l c h a r a c t e r i s t i c energies i n t h e e l e c t r o n spectrum. The IblFP i s s h o r t so these e l e c t r o n s can o n l y have o r i g i n a t e d from t h e inmiediate s u r f a c e environment, which proves t h e techniques t o be s u r f a c e s e n s i t i v e . The thickness from which t h e d e t e c t e d e l e c t r o n s can escape depends m o s t l y

A13

on k i n e t i c e n e r g y ( E q . 1 . 2 ) , b u t a l s o on t h e n a t u r e o f t h e compound ( s h o r t e s t i n m e t a l s , l a r g e s t i n metal o x i d e s ) and on t h e geometry o f t h e experiment. V a r y i n g t h e k i n e t i c energy o f t h e d e t e c t e d e l e c t r o n s b y v a r y i n g t h e energy o f t h e i n c i d e n t photon o r t h e a n g l e o f d e t e c t i o n o f t h e p h o t o e l e c t r o n s p r o v i d e s a means o f v a r y i n g t h e depth f r o m which t h e s i g n a l i s generated (71, 85). The sampled d e p t h can v a r y i n t h e range o f 0.5-10nm,

t a k i n g i n t o c o n s i d e r a t i o n a l l t h e above f a c -

tors. I o n s c a t t e r i n g (ISS) has t h e g r e a t e s t s u r f a c e s e n s i t i v i t y o n l y i n terms o f d e p t h sampled, b u t n o t i n terms o f l o w e s t l i m i t s o f d e t e c t i o n (secondary i o n s mass spectroscopy, SIMS, i s much more p o w e r f u l ) . Although t h e i m p i n g i n g He'

ions

can p e n e t r a t e t h e l a t t i c e , t h e single-He+-atom-coll i s i o n o c c u r s e n t i r e l y w i t h atoms f r o m t h e topmost l a y e r , so t h e ISS spectrum c o n t a i n s i n f o r m a t i o n a b o u t t h a t l a y e r . The d e t e c t i o n l i m i t i s i n t h e o r d e r o f 10-2-10- 3 monolayers. F o r SIMS i n t h e s t a t i c mode w i t h r e l a t i v e l y l o w energy i o n s (1-2 keV) t h e m a j o r i t y

o f e j e c t e d secondary i o n s i s b e l i e v e d t o o r i g i n a t e m a i n l y f r o m t h e f i r s t a t o m i c l a y e r , and o n l y a small f r a c t i o n f r o m t h e second l a y e r (86). I n t h e dynamic mode, t h e f a s t s p u t t e r i n g r a t e and t h e subsequent broadening e f f e c t s mask t h e d e f i n i t i o n o f s u r f a c e s e n s i t i v i t y , because t h e s u r f a c e i s 1.3.2

r a p i d l y changing.

1-nformation DepthAtoms exposed a t t h e s u r f a c e o f s o l i d s d i f f e r s u b s t a n t i a l l y i n symmetry and

a t o m i c p o t e n t i a l s f r o m atoms p o s i t i o n e d deeper w i t h i n t h e s o l i d . There a r e , however, o t h e r s u b s t r a t e atoms which sense t h e a1 t e r e d chemical environment imposed by those o f t h e s u r f a c e . For c l e a n metals, t h e s u r f a c e r e g i o n can i n c l u d e no more t h a n t h e topmost two a t o m i c l a y e r s . The sample d e p t h o f a givenmeasurement, which i s c o n f i n e d t o t h i s r e g i o n , i s f r e q u e n t l y c a l l e d s u r f a c e s e n s i t i v i t y . The s h a l l o w sampling d e p t h o f s u r f a c e s p e c t r o s c o p i e s d e r i v e s e i t h e r from t h e e x c i t i n g probe o r f r o m

t h e a t t e n u a t i o n o f t h e escaping p a r t i c l e s , which c a r r y i n -

f o r m a t i o n about t h e s u r f a c e . F o r most e l e c t r o n s p e c t r o s c o p i e s t h e c h a r a c t e r i s t i c

surface information i s

c o n t a i n e d i n t h e energy and momentum o f t h e e l e c t r o n s escaping from t h e s o l i d . Consequently, t h i s i n f o r m a t i o n i s l o s t a f t e r e l a s t i c o r i n e l a s t i c s c a t t e r i n g events t h a t t h e e l e c t r o n s u f f e r s on i t s way between t h e p o i n t where i t was gener a t e d and t h e s u r f a c e . F o r b u l k m a t e r i a l t h e i n e l a s t i c s c a t t e r i n g p r o b a b i l i t y i s p r o p o r t i o n a l t o t h e p a t h l e n g t h i n t h e s o l i d . The f l u x o f e l e c t r o n s decays expon e n t i a l l y w i t h distance, I = Io.e

-x/X

where X i s t h e i n e l a s t i c mean f r e e p a t h (IMFP) o f t h e e l e c t r o n . Representing t h e s o l i d by a f r e e e l e c t r o n gas o f p r o p e r d e n s i t y , a dependence o f X on t h e k i n e t i c

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energy o f t h e e l e c t r o n can be e a s i l y d e r i v e d (71). There have been d i f f e r e n t values f o r t h i s energy dependence, X a En, w i t h n

%

0.5-1.0

Szajman e t a l . ( 7 2 )

have d e r i v e d an a n a l y t i c a l expression t o compute IMFPs i n f r e e e l e c t r o n metals, and by e x t e n s i o n t o semiconductors, i n s u l a t o r s and non-free e l e c t r o n - l i k e met a l s . Good agreement w i t h experimental r e s u l t s was r e p o r t e d f o r a number o f mat e r i a l s a t energies g r e a t e r t h a n 200 eV. These authors then proposed t h a t t h e e l e c t r o n mean f r e e path has t h e form A.E0*75,

where A i s a c o n s t a n t dependent

upon t h e m a t e r i a l f o r energies above 200 eV ( 7 3 ) . Seah and Dench ( 7 4 ) examined IMFPs from a l a r g e number o f m a t e r i a l s . T h e i r a n a l y s i s i n d i c a t e d t h a t t h e r e were t h r e e separate classes o f m a t e r i a l s : metals, i n o r g a n i c , and o r g a n i c compounds. A l l t h e m a t e r i a l s f o l l o w e d a general e q u a t i o n o f t h e form, A = A . E - ~ + B.E+

where A and B a r e constants ( B i s p r o p o r t i o n a l t o atomic s i z e f o r t h e elements and i n o r g a n i c m a t e r i a l s ) . I t has been suggested, however, t h a t t h e energy dependence a t energies above 100 eV should more c l o s e l y f o l l o w a power dependency o f 0.65-0.75.

A graph o f t h e e x p e r i m e n t a l l y determined e l e c t r o n escape depth versus

e l e c t r o n energy i s g i v e n i n F i g . 1.4 f o r most o f t h e d a t a r e p o r t e d i n B r u n d l e ' s review (75). Note t h a t h i s v e r y small i n t h e energy range 20-50 eV. A t h i g h e r energies h increases almost 1 i n e a r l y and i s remarkably independent o f m a t e r i a l , however, below 30 eV t h e v a l u e o f X r i s e s f a s t w i t h decreasing e l e c t r o n energy and i s more dependent on m a t e r i a l (76). From F i g . 1.4,

i t i s i n f e r r e d t h a t elec-

t r o n spectroscopies would be more s e n s i t i v e t o t h e s u r f a c e o f d i f f e r e n t mater i a l s i n t h e e l e c t r o n energy range o f 20-50 eV. The mean f r e e p a t h o f an e l e c t r o n i s n o t always e q u i v a l e n t t o t h e sampling depth. F o r instance, i n disappearance p o t e n t i a l spectroscopy (DPS) and energy l o s s spectroscopy (ELS)

, the c h a r a c t e r i s t i c electron i s the primary electron

i t s e l f . I t penetrates t h e s u r f a c e l a y e r t w i c e and t h e r e f o r e t h e i n f o r m a t i o n depth i s X/2. For an e l e c t r o n l e a v i n g t h e s u r f a c e a t an angle

e from t h e normal,

t h e sampling depth i s p r o p o r t i o n a l t o A . Cos 8 . However, f o r s u r f a c e e x c i t a t i o n s i n ELS, sampling depth i s n o t r e l a t e d t o X a t a l l , b u t i s determined by t h e e l e c t r o n energy and r e f l e c t i o n angle. From these c o n s i d e r a t i o n s , i t r e s u l t s e v i dent t h a t sampling depth i s a v e r y u n c e r t a i n q u a n t i t y i n e l e c t r o n spectroscopy and one has t o be extremely c a u t i o u s and a t t e n t i v e , i f q u a n t i t a t i v e a n a l y s i s i s desired. For q u a n t i t a t i v e a n a l y s i s , h i g h energy e l e c t r o n s m i g t h t h e r e f o r e be used a t t h e expense o f a h i g h s u r f a c e s e n s i t i v i t y . S u b s t a n t i a l d i f f e r e n c e s e x i s t , when t h e s u r f a c e i s bombarded w i t h i o n s . A t energies o f a few k i l o v o l t s t h e displacement o f l a t t i c e atoms, i n c l u d i n g sputt e r i n g , accounts f o r most o f t h e energy loss. A t much h i g h e r (ca. 1 MeV) energ i e s , t h e p r i n c i p a l l o s s mechanism i s again by e l e c t r o n i c e x c i t a t i o n , and i o n s

A15

10"

10'

loL

ld Electron Energy (ev)

F i g . 1.4. Experimental e l e c t r o n escape depth as a f u n c t i o n o f t h e e l e c t r o n energy f o r s e v e r a l elements i n d i c a t e d . Each d a t a i n d i c a t e s t h e n a t u r e o f t h e e x p e r i mental source: 0 , XPS; 0 , UPS; 0 , AES. The f i g u r e has been r e p l o t t e d f r o m t h e Brundle's data (75). may p e n e t r a t e deeply. D e s p i t e t h e d e s t r u c t i o n o f t h e s u r f a c e , t e c h n i q u e s t h a t measure t h e r e c o i l momentum i n t h e presence o f an e x c i t i n g s l o w i o n , o r a n a l y z e t h e mass-to-charge r a t i o o f i o n s a r i s i n g f r o m t h e surface, a r e h i g h l y s p e c i f i c t o t h e topmost s u r f a c e l a y e r ( 7 7 ) . 1.3.3.

Statistical

Noise-

A common f e a t u r e o f t h e s u r f a c e s p e c t r o s c o p i e s i s t h a t t h e y r e q u i r e a vacuum, m o s t l y u l t r a - h i g h vacuum (UHV), environment. By UHV one u s u a l l y means p r e s s u r e s o f lo-'

T o r r o r b e t t e r . T h i s i s a necessary o p e r a t i o n a l c o n d i t i o n ,

s i n c e a s u r f a c e which i s h i g h l y r e a c t i v e , i . e . whose s t i c k i n g p r o b a b i l i t y app r o a c h e s l f o r r e s i d u a l gases, such as N i , Co, Rh, e t c . , w i l l t a k e f i f t e e n minut e s t o a few hours t o become contaminated t o monolayer q u a n t i t i e s , i . e . ,

long

enough t o make measurements w h i l e t h e s u r f a c e i s s t i l l c l e a n . If one i s i n t e r e s t e d i n t h e s t u d y o f t h e i n i t i a l a d s o r p t i o n r a t e s o f gases on c l e a n m e t a l s , one must work under UHV c o n d i t i o n s (78, 79). I n g e n e r a l , none o f t h e s p e c t r o s c o p i c t e c h n i q u e s has t o be performed a t UHV from a fundamental p h y s i c s o r i n s t r u m e n t a l p o i n t o f view. I t i s necessary t h a t i n t e r a c t i o n s o f t h e p r o b i n g s p e c i e s (photons, e l e c t r o n s , i o n s o r m o l e c u l e s ) w i t h t h e environmental gases do no i n t e r f e r e w i t h t h e s u r f a c e measurement and t h e r e

A16

may be some s u b s i d i a r y instrumental f a c t o r s t o be considered (extreme c a u t i o n w i t h t h e X-ray source, e l e c t r o n gun o r e l e c t r o n m u l t i p l i e r ) . I n c a t a l y s i s r e search one i s o f t e n i n t e r e s t e d i n r e a c t i o n s which occur, a t atmospheric pressure o r above, and then most o f t h e spectroscopic techniques become unusable. However, i n r e c e n t years work has been p u b l i s h e d which demostrates t h a t w i t h some instrumental refinement, techniques such R u t h e r f o r d b a c k s c a t t e r i n g (RBS) ( 8 0 ) and arrangements w i t h mass spectrometry (MS), Auger gun (AES) and low-energy e l e c t r o n d i f f r a c t i o n (LEED) o p t i c s (14, 81) may remain o p e r a t i v e a t much h i g h e r pressures (100 -10 4 T o r r ) . For t h e o p e r a t i o n under UHV c o n d i t i o n s , i t i s t h e contamination t h a t d e t e r mines t h e maximum t i m e t h a t can be t o l e r a t e d i n a s u r f a c e measurement

. The

above requirement o f a f i n i t e t i m e i n t e r v a l , w i t h i n which t h e measurement must be completed, bears s t a t i s t i c a l u n c e r t a i n l y . T h i s e f f e c t , which was named by Schottky (82) t h e " s h o t e f f e c t " ,

i s a consequence o f t h e f a c t t h a t t h e s i g n a l

c o n s i s t s o f d i s c r e t e quanta. According t o Poisson's law, t h i s e f f e c t can be expressed i n terms o f t h e s i g n a l - t o - n o i s e r a t i o S/Ns, S/Ns = (n.1.t)

as

f

where n i s t h e number o f s i g n a l events counted per each i n c i d e n t probe, and I i s t h e r a t e a t which t h e sample i s probed f o r t h e measurement t i m e t. The shot e f f e c t produces a w h i t e n o i s e spectrum whose c o n t r i b u t i o n depends e x c l u s i v e l y on bandwidth ( r e c i p r o c a l o f measurement t i m e t ) . According t o Eq. 1.4, any s i g n a l - t o - n o i s e r a t i o can be achieved by i n c r e a s i n g I o r t, however t h e r e a r e l i m i t s on how l a r g e I can be made, j u s t as t h e measurement w i l l be perturbed, i f t becomes a s i g n i f i c a n t p a r t o f t h e t i m e r e q u i r e d t o form a monolayer o f con-

tamination. 1.3.4.

R e s o l u t i o n o f t h e Instrument

Chemistry and physics f r e q u e n t l y g i v e us c l e a r l i m i t i n g g o a l s f o r t h e r e s o l u t i o n o f our spectroscopic techniques. I n t h e case o f t h e mass a n a l y s i s technique, f o r example, t h e r e i s n o t h i n g t o be gained by a r e s o l u t i o n b e t t e r than one atomic mass u n i t unless a t t e n t i o n i s focussed on t h e d e t e r m i n a t i o n of t h e mass d e f e c t o f a chemical bond. I n t h e case o f core l e v e l spectroscopies t h e r e i s l i t t l e i n t e r e s t i n r e s o l v i n g s p e c t r a l f e a t u r e s separated by l e s s t h a n t h e energy corresponding o f a l i f e t i m e o f a core h o l e . F o r many purposes, i t i s n o t important t o approach these l i m i t s , however, c o r r e c t i o n f o r t h e d i s t o r t i o n o f t h e spectrum by t h e instrument may be necessary i n o t h e r cases. The d i s t o r t i o n e f f e c t o f a spectrometer can be t r e a t e d a n a l y t i c a l l y through t h e use o f a broadening f u n c t i o n . For a recorded spectrum, S(E)exp, t h e r e i s t h e p o s s i b i l i t y o f r e - w r i t t i n g t h i s as t h e c o n v o l u t i o n p r o d u c t o f t h e ''clean" spec-

A17

trum, S(E), w i t h t h e spectrometer response,

where T(E) i s t h e f u n c t i o n t h a t t h e spectrometer would r e c o r d f o r a h y p o t h e t i c a l spectrum c o n s i s t i n g of a u n i t impulse. T h e r e f o r e , t h e knowledge o f T(E) g i v e s us t h e p o s s i b i l i t y t o compute t h e o u t p u t o f t h e spectrometer f o r an a r b i t r a r y i n p u t . There are, i n p r i n c i p l e , two approaches t o determine T(E) o f an i n s t r u m e n t : i ) by d e r i v i n g t h e response f r o m t h e known parameters o f t h e spectrometer, and i i ) by r e c o r d i n g t h e response o f a t e s t s i g n a l n e a r l y equal t o an impulse. The usual d i f f i c u l t y t o o b t a i n T(E) from t h e l a t t e r approach i s t o g e n e r a t e a s u f f i c i e n t l y narrow t e s t s i g n a l . The problem o f d e r i v i n g T(E) i s g r e a t l y s i m p l i f i e d by i s o l a t i n g i t s v a r i o u s c o n t r i b u t i o n s . A p h o t o e l e c t r o n spectrum i s broadened by b o t h t h e pass energy o f t h e e l e c t r o n a n a l y z e r and by t h e energy spread o f t h e e x c i t i n g photons. Assuming a monochromatic X-ray source, t h e response f u n c t i o n f o r t h e a n a l y z e r Ta(E) can be d e r i v e d , and assuming a p e r f e c t a n a l y z e r , t h e response i s j u s t t h e convolut i o n p r o d u c t o f t h e i n d i v i d u a l responses,

The w i d t h T(E) i s a measure o f t h e r e s o l u t i o n o f t h e s p e c t r o m e t e r . To e s t i mate t h e r e s o l u t i o n , i t i s c o n v e n i e n t t o assume t h a t each i n d i v i d u a l response f u n c t i o n i n Eq. 1.6 can be d e s c r i b e d as a Gaussian l i n e , t h e w i d t h o f T(E) measured a t h a l f maximum (FWHM), b e i n g g i v e n by t h e q u a d r a t i c sum of t h e i n d i vidual widths. 1.4. STRUCTURAL CHARACTERIZATION METHODS The u n d e r s t a n d i n g o f t h e c a t a l y t i c r e a c t i o n s r e q u i r e s t h e m i c r o s c o p i c desc r i p t i o n o f t h e environment and chemical s t a t e o f t h e i n d i v i d u a l s u r f a c e atoms. U n f o r t u n a t e l y , t h e r e i s no general method a l l o w i n g t h e c h a r a c t e r i z a t i o n o f t h e s t r u c t u r e o f t h e o u t e r e l e c t r o n i c l e v e l s , and t h e environment of t h e atoms. One has f r e q u e n t l y t o t r y a v a r i e t y o f techniques, each o f which i s s u i t a b l e f o r a p a r t i c u l a r aspect o r p r o v i d e s b u t p a r t i a l i n f o r m a t i o n . A s s t a t e d above, NMR, ESR, and M8ssbauer s p e c t r o s c o p i e s a r e h i g h l y s e n s i -

t i v e techniques f o r s t u d y i n g t h e chemical environment o f s p e c i f i c atoms l o c a t e d e i t h e r a t t h e s u r f a c e o r i n t h e b u l k o f s o l i d c a t a l y s t s . However, o n l y t h o s e atoms possessing a n u c l e a r magnetic moment, paramagnetic c h a r a c t e r , and a v a i l a b i l i t y o f an e m i t t e r - a b s o r b e r p a i r can be s t u d i e d by NMR, ESR, and MSssbauer techniques, r e s p e c t i v e l y . Consequently, t h e u s e f u l n e s s o f t h e s e s p e c t r o s c o p i e s depends on t h e chemical c o m p o s i t i o n o f t h e c a t a l y s t s . EXAFS i s d o u b t l e s s l y an

A18

exceptional technique, and can be, i n p r i n c i p l e , a p p l i e d t o any element. The most oustanding f e a t u r e o f EXAFS i s t h e p o s s i b i l i t y o f determining t h e l o c a l s t r u c t u r e o f each s e l e c t e d atomic species i n any phase. From an a n a l y s i s o f EXAFS data, one can o b t a i n i n f o r m a t i o n on t h e number and type o f neighbouring atoms around a s e l e c t e d absorber atom, on i n t e r a t o m i c d i s t a n c e s , and on t h e mean square r o o t d e v i a t i o n o f t h e i n t e r a t o m i c d i s t a n c e s from t h e e q u i l i b r i u m . D e s p i t e t h e relevance o f EXAFS i n c a t a l y s i s research, r e s t r i c t e d machine hours a v a i l a b l e a t synchrotron r a d i a t i o n s t a t i o n s have imposed a s e r i o u s 1 i m i t a t i o n on t h e popul a r i t y o f EXAFS. Other spectroscopic techniques e x t e n s i v e l y used i n t h e c h a r a c t e r i z a t i o n o f c a t a l y s t s t r u c t u r e a r e U V - v i s i b l e , I R and Raman spectroscopies, and t o a much l e s s e r e x t e n t , photoacoustic spectroscopy. U V - v i s i b l e i s m o s t l y employed i n t h e s t u d y o f t h e c o o r d i n a t i o n o f t r a n s i t i o n metal i o n s . U n f o r t u n a t e l y , t h e q u a n t i t a t i v e i n t e r p r e t a t i o n o f s p e c t r a i s complex, which means t h a t most s t u d i e s a r e c a r r i e d o u t from a q u a l i t a t i v e p o i n t o f view.

I R and Raman are, however, more

f r e q u e n t l y used f o r t h e study o f s u r f a c e s i t e s through a n a l y s i s o f t h e v i b r a t i o n a l modes and b i n d i n g energies o f s u i t a b l e probe-molecules (see p a r t B) s t u d i e s o f t h e v i b r a t i o n a l modes o f t h e c a t a l y s t s alone a r e r e s t r i c t e d t o a l i m i t e d number o f s y s t e m . Photoacoustic (PAS), and more s p e c i f i c a l l y photothermal beam d e f l e c t i o n (PDS) spectroscopies, undoubtedly o f f e r promising prospects f o r t h e study o f a wide range o f m a t e r i a l s , i n c l u d i n g carbons.

1.4.1. I n - g o i n g Photons 1.4.1.1. Out-going Photons 1.4.1.1.1. S o l i d S t a t e NMR When molecules move more o r l e s s f r e e l y , as occurs i n t h e l i q u i d o r i n t h e weekly adsorbed s t a t e , t h e r e s u l t i n g NMR 1 i n e s a r e sharp, because t h e broadening i n f l u e n c e s a r i s i n g from d i p o l a r i n t e r a c t i o n s and o t h e r s h o r t range i n t e r a c t i o n s a r e averaged by t h e motion o f molecules. T h i s i s n i c e l y i l l u s t r a t e d by s o l i d

1 c l a y c a t a l y s t s , f o r which r a t h e r sharp 13C- and H-NMR l i n e s can be o b t a i n e d from

i n t e r c a l a t e d o r g a n i c molecules, p r o v i d e d they execute a t r a n s l a t i o n a l and r o t a t i o n a l motion w i t h i n t h e i r i n t e r l a m e l l a r regions. For instance, t h e p r o t o n c a t a l y z e d a d d i t i o n o f water o r methanol t o isobutene i n these i n t e r l a m e l l a r regions, y i e l d i n g t - b u t a n o l and m e t h y l - t - b u t y l

e t h e r (87, 88), can be f o l l o w e d by NMR,

s i n c e t h e i n v o l v e d molecules possess adequate t r a n s l a t i o n a l and r o t a t i o n a l degrees o f freedom. F i g u r e 1.5 d i s p l a y s t h e 13C spectrum recorded a f t e r a small dose of isobutene was added t o a sample o f A13+-exchanged h e c t o r i t e . known t o be a good Bronsted c a t a l y s t . For n u c l e i t h a t have l o n g s p i n l a t t i c e r e l a x a t i o n times t h e r e a r e two main d i f f i c u l t i e s which l i m i t t h e o b s e r v a t i o n o f h i g h r e s o l u t i o n NMR s p e c t r a o f

A19

C

1 7

C- 'C

F i g . 1.5. 13C-NIVIR s p e c t r a o f a ) A13+-exchanged h e c t o r i t e , and b ) a f t e r adding a small dose of i s o b u t e n e l e s s t h a n t h a t r e q u i r e d t o consume a l l t h e i n t e r l a m e l l a r water.

s o l i d s . One o f t h e s e i s t h a t n o r m a l l y t h e resonance l i n e s a r e broadened by a n i s o t r o p i c d i p o l e - d i p o l e i n t e r a c t i o n s and quadrupole f i e l d g r a d i e n t i n t e r a c t i o n s , g i v i n g r i s e t o l i n e w i d t h s i n t h e kHz range. The second problem i s chemical s h i f t a n i s o t r o p y . These a n i s o t r o p i c i n t e r a c t i o n s a r e a1 so p r e s e n t i n 1 i q u i d s b u t a r e averaged t o z e r o b y r a p i d Brownian m o t i o n . F o r s o l i d s , a s i m i l a r a v e r a g i n g may be r e a l i z e d by magnetic a n g l e s p i n n i n g (MAS), which can e l i m i n a t e d i p o l a r and quadrupole f i e l d i n t e r a c t i o n as w e l l as chemical s h i f t i n t e r a c t i o n (89, 90). The s p i n n i n g frequency must be o f t h e same o r d e r as t h e f r e q u e n c y d i f f e r e n c e s which a r e p r e s e n t i n t h e s o l i d , e.g.

f o r a 27Al nucleus, s p i n n i n g f r e q u e n c i e s o f

a few kHz a r e r e q u i r e d a t a f i e l d o f 4.698 T (200 MHz f o r

'H).

MAS may b e a l s o

combined w i t h c r o s s p o l a r i z a t i o n ( C P ) t o i n c r e a s e s e n s i t i v i t y of r a r e s p i n s and long r e l a x a t i o n times. 1.4.1.1.1.1.

*'Si

and 27Al N u c l e i

Numerous r e p o r t s o f s o l i d s c h a r a c t e r i z a t i o n by s i l i c o n and aluminium NMR have appeared q u i t e r e c e n t l y . F o r an e x t e n s i v e

overview t h e r e are e x c e l l e n t

r e v i e w a r t i c l e s (15-17), c o v e r i n g more t h a n 500 r e f e r e n c e s on m i n e r a l s i l i c a t e s p i l l a r e d c l a y s , z e o l i t e s , and s i l i c a - a l u m i n a s . Some o f t h e key p o i n t s r e g a r d i n g

A20

TABLE 1.1. C a p a b i l i t i e s of 2 9 S i and 27Al S o l i d S t a t e NMR i n Studies o f A l u m i n o s i l i c a t e s

2 7 ~ ~

29~i 1.

Quant it a t i v e l y d i s t ingui s h i ng a1 1 f i v e p o s s i b l e Si(nA1) b u i l d i n g u n i t s , Si(nA1) represents SiO4 t e t r a h e d r o n l i n k e d t o n A104 t e t r a hedra and t o 4-n o t h e r SiO4 t e t r a hedra.

1.

Unambiguouslyand q u a n t i t a t i v e l y d i s t i n g u i s h i n g between t e t r a h e d r a l l y and o c t a h e d r a l l y coord i n a t e d A l , even i n n o n - c r y s t a l l i n e materials.

2.

Q u a n t i t a t i v e d e t e r m i n a t i o n o f Si/A1 framework r a t i o s according t o Eq. (7). P o s s i b i l i t y t o study n o n c r y s t a l l i n e materials.

2.

I n s i g h t s i n t o t h e n a t u r e o f act i v e s i t e s , when combined w i t h 1~-MAS-NMR,

3.

Combined w i t h t h e i n t e n s i t i e s o f 29Si peaks, c a l c u l a t i o n o f framework Si/A1 r a t i o s as l a r g e as ca. 10.000.

4.

I n f u l l y dealuminated z e o l i t e s t h e number and i n t e n s i t y o f d i s t i n c t peaks y i e l d s t h e p r o p o r t i o n o f nonequivalent tetrahedral ( S i ) s i t e s i n t o the u n i t c e l l .

4.

Q u a n t i t a t i v e d e t e r m i n a t i o n o f cat a l y t i c a l l y a c t i v e s i t e s (when synonymous o f A1 s i t e s i n framework).

5.

D i s t i n g u i s h e d d i f f e r e n t peaks f o r c r y s t a l l o g r a p h i c non-eqQivalent Si(OSi), groupings.

3.

t h e power and usefulness o f r e c o r d i n g 2 g S i and 27Al MAS-NMR spectra of aluminos i l i c a t e s a r e summarized i n Table 1.1. The a p p l i c a t i o n o f MAS-NMR t o t h e study o f c a t i o n d i s t r i b u t i o n i n alumin o s i l i c a t e s i s i l l u s t r a t e d f o r t h e case o f v e r m i c u l i t e ( 9 5 ) . I n F i g . 6a. t h e 27Al MAS-NMR spectrum o f a v e r m i c u l i t e shows two main components and a s e r i e s o f s i d e bands a s s o c i a t e d w i t h t h e s p i n n i n g o f t h e sample. The l i n e a t t5.0 ppm should be assigned t o octahedral A l . w h i l e t h e l i n e a t 62.5 ppm corresponds t o t e t r a h e d r a l A1

. The

r e l a t i v e i n t e n s i t i e s o f t h e s i g n a l s agree s a t i s f a c t o r i l y

w i t h t h e s t r u c t u r a l formula o f t h e sample. The 2 9 S i MAS-NMR spectrum e x h i b i t s , however, t h r e e w e l l - r e s o l v e d components a t -92.0,

-88.0,

and -83.5 ppm which a r e

associated w i t h t h r e e d i f f e r e n t S i environments r e s u l t i n g from t h e Si/A1 d i s t r i b u t i o n i n t h e t e t r a h e d r a l sheet. The a n a l y s i s o f t h e r e l a t i v e i n t e n s i t i e s o f t h e t h r e e components i n t h e 29Si MAS-NMR spectrum p e r m i t s c e r t a i n aspects o f t h e S i / A l d i s t r i b u t i o n t o be

e l u c i d a t e d and furthermore a l l o w s f o r a check on t h e v a l i d i t y o f Loewenstein's r u l e , which excludes t h e A1 atoms from occupying neighbouring t e t r a h e d r a . The c r i t e r i o n used t o prove compliance w i t h Loewenstein's r u l e i s t h e good agreement o f t h e S i / A l r a t i o s o b t a i n e d from t h e s t r u c t u r a l formula and from t h e NMR spect r a . I n t h e absence of A1-0-A1 l i n k a g e s , S i / A l r a t i o s may be c a l c u l a t e d accor-

A21

200

o

100

-100

ppm

-

-50

-70

-90

-110

BPI

F i g . 1.6. 27Al and "Si MAS- MR s p e c t r a o f v e r m i c u l i t e ( L l a n o , Texas). Chemical s h i f t s a r e t a k e n f r o m Al(H20)) and TMS, r e s p e c t i v e l y . Readapted f r o m r e f . ( 9 5 ) . d i n g t o t h e expression:

where ISi(nAl)

i s t h e i n t e g r a t e d i n t e n s i t y o f t h e n-component i n t h e MAS-NMR

spectrum, and n t h e number o f A1 i o n s around t h e S i . Table 1.2. compares t h e Si/A1 r a t i o s , c a l c u l a t e d a c c o r d i n g t o Eq. 1.7, s t r u c t u r a l froniula

and t h o s e o b t a i n e d f r o m t h e

f o r t h r e e d i f f e r e n t p h y l l o s i l i c a t e s . The r e a s o n a b l y good

agreement o f t h e Si/A1 r a t i o s o b t a i n e d f r o m t h e s t r u c t u r a l f o r m u l a

and t h o s e

d e r i v e d f r o m Eq. 1.7 demonstrate t h e v a l i d i t y o f L o e w e n s t e i n ' s r u l e i n t h e r e ported p h y l l o s i l icates. TABLE 1.2. S t r u c t u r a l and C a l c u l a t e d Si/A1 R a t i o s o f P h y l l o s i l i c a t e s Silicate Muscovite Vermicul it e Phlogopite

S t r u c t u r a l Formula 3.7 2.6 2.7

C a l c u l a t e d by Eq. 1.7 3.6 2.6 2.6

S t u d i e s of p i l l a r e d c l a y s u s i n g 2%1 ( 9 6 ) and "Si and 2 7 A l MAS-NMR (97, 98) have been conducted i n t h e v e r y r e c e n t p a s t . P i n n a v i a e t a l . ( 9 5 ) s t u d i e d

A22

p i l l a r e d s m e c t i t e c l a y s by 2 7 A l and concluded t h a t , i r r e s p e c t i v e o f t h e d i f f e rence i n t h e p i l l a r i n g reagents, t h e same t y p e o f oxocation, probalby Al13 Kegg i n ions, i s formed on t h e i n t r a c r y s t a l l i n e s u r f a c e o f t h e c l a y s . These authors proposed t h e f o r m a t i o n o f an u n i f o r m monolayer o f hydrated polyoxo c a t i o n s i n t h e i n t e r l a y e r s , w i t h t h e concomitant achievement o f e l e c t r i c a l n e u t r a l i t y v i a t h e h d y r o l y s i s o f t h e p i l l a r e d c a t i o n s . 27Al and "Si

MAS-NMR have been used by

Plee e t a l . (98) t o d e f i n e t h e s h o r t range o r d e r - s t r u c t u r e i n p i l l a r e d smectites. They showed t h a t c a l c i n e d p i l l a r e d smectites d i d n o t undergo r e a c t i o n between t h e p i l l a r s and t h e c l a y i n t h e absence o f t e t r a h e d r a l s u b s t i t u t i o n , as occurs i n l a p o n i t e . However, a deep s t r u c t u r a l t r a n s i t i o n was observed i n beidel l i t e upon c a l c i n a t i o n , which has such a t e t r a h e d r a l s u b s t i t u t i o n . T h i s change was a t t r i b u t e d t o t h e growth o f a three-dimensional network g r a f t e d on t h e twodimensional network o f t h e c l a y , t h e f i n a l p r o d u c t showing a c i d i c p r o p e r t i e s comparable t o z e o l i t e s and s i g n i f i c a n t l y s t r o n g e r t h a n those of c a l c i n e d p i l l a r e d smectites w i t h o u t s u b s t i t u t i o n i n t h e t e t r a h e d r a l l a y e r s . Z e o l i t e s , and e s p e c i a l l y dealuminated z e o l i t e s , have been r e c e n t l y invest i g a t e d by MAS-NMR techniques, because t h e i r s p e c t r a can y i e l d d i r e c t l y t h e number and d i s t r i b u t i o n o f non-equivalent t e t r a h e d r a l s i t e s p e r u n i t c e l l o f t h e framework. By means o f NMR and I R techniques, Anderson e t a l . ( 9 9 ) showed t h a t t r e a t -

ment o f a h i g h S i / A l r a t i o ZSM-5 z e o l i t e w i t h A1C13 vapour a t e l e v a t e d temperat u r e s produces t h e isomorphic s u b s t i t u t i o n o f A1 f o r S i i n t h e framework w i t h t h e simultaneous f o r m a t i o n o f o c t a h e d r a l l y c o o r d i n a t e d A1 i o n s a t t h e non-framework. Jacobs e t a l . (100) s t u d i e d t h e sol i d t r a n s f o r m a t i o n s o c c u r i n g i n t h e ZSM-5 z e o l i t e s upon thermal treatments. Based on 27Al and 2 9 S i spectra, they p o s t u l a t e d t h a t z e o l i t e dealumination r e s u l t s i n m i g r a t i o n o f A13+ i o n s from t h e framework t o i n t e r s t i t i a l p o s i t i o n s , thus r e l i e v i n g t h e s t r a i n from t h e f o u r membered r i n g s . As a r e s u l t , pore i n t e r a c t i o n s become more open. These authors a1 so s t u d i e d t h e realuminated z e o l i t e and concluded t h a t s i m i l a r phenomena occur i n the reverse direction. Thomas (101) has r e p o r t e d "Si MAS-NMR s p e c t r a o f f i v e z e o l i t i c s o l i d s upon dealumination. These a r e summarized i n F i g . 1.7. The most r e l e v a n t features a r e as f o l l o w s : i ) With f a u j a s i t e i n t h e form o f Y - z e o l i t e w i t h S i / A l = 2.61, o n l y one Si(OSi)q remains upon dealumination.

2 ) T h i s i s t o be expected as t h e r e i s

o n l y one c r y s t a l l o g r a p h y c a l l y d i s t i n c t t e t r a h e d r a l s i t e i n t h i s s t r u c t u r e . i i ) Z e o l i t e omega shows two sharp s i g n a l s , i n t h e i n t e n s i t y r a t i o 2:1,

i n l i n e with

expectations based on t h e known c r y s t a l l o g r a p h i c s t r u c t u r e o f m a z z i t e (space group P63/mmc), where t h e r e a r e two f a m i l i e s o f t e t r a h e d r a l l y c o o r d i n a t e d S i i n t h e r a t i o o f 24:12 (102). i i i ) With o f f r e t i t e , t h e two peaks e x h i b i t e d by t h e dealuminated form, w i t h t h e 2 : l i n t e n s i t y r a t i o

,is

what would be expected on

A23

I Zeolite Y

A Offretite

4

I

Mordenit e

+idink

zsM-5

1

)o

i

-110

I

-120

ppm from TMS

F i g . 1.7. *'Si MAS-NMR s p e c t r a a t 79.8 MHz o f s e v e r a l z e o l i t e s b e f o r e ( z e o l i t e Y and z e o l i t e omega) and a f t e r ( o f f r e t i t e , mordenite, and ZSM-5) d e a l u m i n a t i o n . The number and t h e i n t e n s i t y o f t h e peaks o f t h e dealuminated samples r e f l e c t s t h e number and d i s t r i b u t i o n o f n o n - e q u i v a l e n t t e t r a h e d r a i n t h e u n i t c e l l . Readapted from Thomas ( 1 0 1 ) . t h e b a s i s o f t h e z e o l i t e s t r u c t u r e (space group P6m2, w i t h two f a m i l i e s o f t e t r a h e d r a l s i t e s i n t h e r a t i o 12:6 ( 1 0 2 ) ) . i v ) With m o r d e n i t e (space group Cmcm) f o u r peaks would be expected w i t h i n t e n s i t y r a t i o s 2:1:1:2,

but only three

peaks a r e observed. I t i s p r o b a b l e t h a t t h e l a r g e s t peak, a t -115.0 ppm f r o m TMS, i s composite. The observed peak i n t e n s i t y r a t i o s a r e 2:1:3, however s t a c k i n g f a u l t s , which t e n d t o occur, c o u l d presumably be p r e s e n t e i n m o r d e n i t e . v ) The spectrum o f dealuminated ZSM-5 z e o l i t e i s r a t h e r c o m p l i c a t e d and t h e assignment o f t h e i n d i v i d u a l peaks t o s p e c i f i c s i t e s i s n o t y e t f e a s i b l e as s i g n a l s f r o m a number o f d i s t i n c t S i s i t e s a r e superimposed. A n o t h e r c o m p l i c a t i o n a r i s e s f r o m t h e m o n o c l i n i c d i s t o r t i o n s t o t h e o r t h o r h o m b i c s t r u c t u r e which i n creases t h e number o f d i s t i n c t s i t e s f r o m 12 t o 24 ( 1 0 3 ) . The s t u r c t u r a l a n a l y s i s o f Y z e o l i t e s and m o r d e n i t e s has been c a r r i e d o u t by Nakata e t a1

.

(104) u s i n g 2 7 A l and 2 9 S i MAS-NMR. They showed t h a t t h e Ht-ex-

change s i m u l t a n e o u s l y accompanies d e a l u m i n a t i o n o f Y z e o l i t e s , however t h i s i s n o t t h e case w i t h m o r d e n i t e s . Such a comparison can b e t a k e n w i t h c a u t i o n s i n c e Y z e o l i t e s a r e much more A l - r i c h t h a n m o r d e n i t e s and, hence, more s u s c e p t i b l e t o

A24

dealutiiination. F u r t h e r s t u d i e s by A u k e t t e t a l . (105) showed t h a t even i n a h i g h s i l i c a z e o l i t e such as a ZSM-5 w i t h Si/A1%15, dealumination can occur upon Htexchange.

'H Nucleus

1.4.1.1.1.2.

The measurement o f t h e

1H MAS-NMR s p e c t r a o f a c i d i c m a t e r i a l s , such as

z e o l i t e s and r e l a t e d c a t a l y s t s i s complicated by t h e f a c t t h a t these s o l i d s a r e m o s t l y hygroscopic so t h a t i t becomes necessary t o s p i n sealed powder w i t h a speed o f 2-3 kHz. F o l l o w i n g t h i s procedure, many 'H-MAS-NMR

samples

s t u d i e s have

been conducted i n r e c e n t years (106-110). Among them, amorphous s i l ica-alumina and z e o l i t e s were p r e f e r e n t l y i n v e s t i g a t e d . The 1H MAS-NMR spectra o f amorphous s i l i c a - a l u m i n a o f d i f f e r e n t composit i o n , obtained by Hunger e t a l . (106), a r e shown i n F i g . l.SA.Twolines,

which

a r e denoted by ( 1 ) and ( Z ) , can be discerned, t h e maximum s i l i c a - a l u m i n a contai n i n g 25.0 w t % A1203. For t h i s sample, t h e t o t a l c o n c e n t r a t i o n o f protons ( c ) , as w e l l as those g i v i n g r i s e t o l i n e s ( 1 ) and ( 2 ) , and values f o r t h e chemical s h i f t o f l i n e ( Z ) , t a k i n g l i n e ( 1 ) as an i n t e r n a l r e f e r e n c e

1.8 ppm),

were c a l c u l a t e d and a r e summarized i n Table 1.3. TABLE 1.3. I n f l u e n c e o f Pretreatments on Chemical S h i f t and P r o t o n Concentrations. Pretreatment o f s i l i c a aluiiiina (25.0 w t % A1203) 300 350 400 450

DB( DB DB DB

*

C

C

(1)

(2)

Chemical s h i f t P Pm

p) H

3.1 2.0 1.5 0.6

0.8 0.8 0.75 0.6

2.4 1.2 0.75

-

6.85 6.50 6.25

( " I DB = deep bed, estimated e r r o r i n p r o t o n c o n c e n t r a t i o n s and i n chemical s h i f t s a r e i 0.1 -0.2 x 1 0 2 l and 2 0.1, r e s p e c t i v e l y .

L i n e ( 1 ) i s a s c r i b e d t o n o n - a c i d i c HO groups, s i n c e i t can be observed f o r a l l samples i n c l u d i n g t h e pure Si02 and A1203 p a r t n e r s , which a r e known t o be non-acidic c a t a l y s t s . I n c o n t r a s t t o l i n e ( l ) , f o r l i n e (2) t h e c o n c e n t r a t i o n o f protons and t h e chemical s h i f t decrease w i t h i n c r e a s i n g temperature. T h i s l a t t e r l i n e i s associated t o a c i d i c protons, which belong t o

NH; i o n s a f t e r c o n t a c t i n g

t h e sample w i t h amnonia. As a t e s t r e a c t i o n t h e authors s t u d i e d t h e c r a c k i n g o f cumene on s i l i c a alumina c o n t a i n i n g 10-90 w t % A1203. The r a t e c o n s t a n t kl o f t h e cumene c r a c k i n g process, as w e l l as t h e r e l a t i v e a c i d i t y , as estimated b y t h e c(')/c

ratio (cf.

Table 1.3), a r e p l o t t e d as a f u n c t i o n o f t h e A1203 c o n t e n t o f t h e s i l i c a - a l u m i n a ( F i g . 1.88). The inaxiina o f b o t h curves appeared a t t h e same composition ( % 3 0 w t %

A25

I

20

I

10

1

0

6, ( P P ~ )

F i g . 1.8. (A) 1H MAS-NMR spectra o f amorphous s i l i c a - a l u m i n a s of d i f f e r e n t comp o s i t i o n : a ) Si02; b) 20 w t % A1203; c ) 25 w t % AlzO3; d ) 50 wt% Al.03; e ) ?-A1203 ( 6 ) Rate s t a n t ( k l ) o f t h e cumene c r a c k i n g ( 0 ) a t 35OOC and t i e r e l a t i v e i n t e n s i t y cffy/c o f t h e l i n e ( 2 ) i n t h e 1H MAS-NMR spectrum (0) o f t h e amorphous s i l i c a - a l u m i n a s as a f u n c t i o n o f t h e A1203 content. Readapted from r e f . (106). A1203). These r e s u l t s demonstrate t h e predominant r o l e played by t h e a c i d i c OH groups which g i v e r i s e t o l i n e ( 2 ) i n t h e 'H MAS-NMR spectra.

A26

Freude e t a1

. (107)

have o b t a i n e d h i g h l y r e s o l v e d 'H MAS-NMR s p e c t r a o f

v a r i o u s types o f hydroxyl groups i n z e o l i t e s . They u s u a l l y found t h r e e l i n e s . The resonance a t t h e h i g h e s t magnetic f i e l d (1.8 ppm from TMS) a r i s e s from nona c i d i c OH groups attached t o S i atoms i n t h e framework. These a r e s o - c a l l e d t e r minal hydroxyl groups, comparable t o those p r e s e n t i n s i l i c a g e l s (108). The l i n e s a t medium f i e l d (ca. 3.9-4.6

and 4.8-5.6

ppm) a r i s e from t h e a c i d i c o r so-

c a l l e d s t r u c t u r a l HO groups, and a r e o f t h e k i n d :

F i n a l l y , t h e s i g n a l a t t h e l o w e s t f i e l d (7.0-7.5

ppm) i s a t t r i b u t a b l e t o

t

r e s i d u a l NH4 i o n s .

An e s s e n t i a l f e a t u r e o f t h i s approach t o a c i d i t y measurements i s t h a t i t provides s t r i c t l y q u a n t i t a t i v e data, which c o n t r a s t w i t h I R measurements, where i n t e n s i t i e s do o f t e n vary, because o f t h e changes i n t h e environment. Moreover, p r o t o n s t u d i e s by NMR surpass t h e approach based on t h e use o f Hammett's i n d i c a t o r s , since, even s t e r e o c h e m i c a l l y

i n a c c e s s i b l e protons t o l a r g e o r g a n i c mo-

l e c u l e s a r e detected by t h e NMR technique. I n t e r e s t i n g works on t h e a c i d i t y o f z e o l i t e s have a l s o been r e p o r t e d by Dessau and K e r r (109) and by O c c e l l i e t a l . (110). Dessau and K e r r (109) s t u d i e d t h e s t r o n g shape-selective a c i d s i t e s generated i n ZSM-5 and ZSM-11 z e o l i t e s by treatment w i t h A1C13, and f u r t h e r h y d r o l y s i s and c a l c i n a t i o n . T h e i r I R and NMR data revealed t h a t A1 i s r e i n c o r p o r a t e d i n t o t h e framework. Both I R and 'H-NMR techniques were a l s o used by O c c e l l i e t a l . (110) t o m o n i t o r t h e s u r f a c e a c i d i t y o f ZSM-34 and s y n t h e t i c o f f r e t i t e z e o l i t e s . O f f r e t i t e was shown t o y i e l d more propylene and h e a v i e r o l e f i n e s i n t h e c a t a l y t i c conversion o f methanol w h i l e ZSM-34 z e o l i t e d i s p l a y s b e t t e r s e l e c t i v i t y t o conversion t o e t h y l e n e . The authors were a b l e t o d i s t i n g u i s h t h e d i s t i n c t d i f f e r e n c e s among t h e charge-compensating protons and c l a s s i f y them i n t o t h r e e c a t e g o r i e s : i s o l a t e d , l o c a l i z e d p a i r s , and c l u s t e r s f o r which t h e average i n t e r n u c l e a r d i s t a n c e s a r e 1.0, 0.7 and 0.3 nm, r e s p e c t i v e l y . 1.4.1.1.1.3.

Other N u c l e i

Boron-containing z e o l i t e s have been i n v e s t i g a t e d by

11i3 MAS-NMR techniques

(111, 112). T h i s technique was used as complementary t o 2 7 A l MAS-NMR t o c o n f i r m t h e s u b s t i t u t i o n a l i n s e r t i o n o f boron i n t o t h e z e o l i t e framework f o r z e o l i t e s t r e a t e d w i t h BC13. From these data and t h e a c t i v i t y o f t h e c r a c k i n g o f n-hexane, Derouane e t a l . (111) concluded t h a t t h e a c t i v i t y i s c o n t r o l l e d by t h e framework t e t r a h e d r a l aluminium content. They discuss t h e c r i t i c a l v a r i a b l e s i n t h e NMR

A27

experiments t o q u a n t i f y t e t r a h e d r a l aluminum and a l s o propose a mechanism t o account f o r t h e a c i d i t y o f t h e s u b s t i t u t e d z e o l i t e s . S i m i l a r s t u d i e s have been conducted by S c h o l l e and Veeman ( 1 1 2 ) who i n v e s t i g a t e d t h e e f f e c t o f h y d r a t i o n and d e h y d r a t i o n on t h e c o o r d i n a t i o n o f boron H - b o r a l i t e . On t h e b a s i s o f t h e quadrupole i n t e r a c t i o n i t i s i n f e r r e d t h a t d e h y d r a t i o n s e v e r e l y d i s t o r t s t h e c o o r d i n a t i o n sphere o f boron t o such an e x t e n t t h a t boron a l m o s t l i e s i n t h e p l a n e o f t h r e e oxygen atoms.

A new and s e n s i t i v e method t o probe a c i d s i t e s i n z e o l i t e s has r e c e n t l y been developed by L u n s f o r d e t a l . ( 1 1 3 ) . The a u t h o r s s t u d i e d t h e a d s o r p t i o n o f t r i m e t h y l phosphine(TMPj u s i n g 31P MAS NMR t e c h n i q u e and were a b l e t o d i s t i n g u i s h TMP bound t o BrSnsted o r Lewis s i t e s on HY z e o l i t e . The advantage t o use t h i s phosphorus-containing m o l e c u l e as an a c i d i t y probe l i e s i n t h e f a c t t h a t 31P i s an 100% abundant nucleus and r e l a t i v e l y easy t o d e t e c t . Such an approach

t o measure a c i d i t y can be used as a complementary t e c h n i q u e f o r t h e convent i o n a l d i s p e r s i v e i n f r a r e d spectroscopy. 31P magic a n g l e s p i n n i n g has a l s o been employed by Segawa e t a l . ( 1 1 4 ) t o

e l u c i d a t e t h e enhanced c a t a l y t i c a c t i v i t i e s f o r butene i s o m e r i z a t i o n o f s e v e r a l a f t e r o u t g a s s i n g a t d i f f e r e n t temperatures. Among t h e z i r c o n i u m phosphates amorphous ( g e l ) o r t h e c r y s t a l l i n e (a and E ) l a y e r e d forms o f z i r c o n i u m phosphates,

E-Zr(HP02)2 and s y n t h e t i c ZrP207 evacuated a t h i g h temperatures ( c a .

800 K) e x h i b i t e d t h e h i g h e s t c a t a l y t i c a c t i v i t y . F o r t h e

E

f o r m o n l y one 31P

resonance l i n e i s observed ( F i g . 1.9) a t -21.9 ppm. The o r d e r o f t h e chemical s h i f t o f t h e resonance corresponds t o d e c r e a s i n g amounts o f c r y s t a l 1 i z a t i o n water. A f t e r e v a c u a t i o n a t 773 K ( a f t e r condensation o f t h e phosphate g r o u p s ) t h e s p e c t r a show a peak w i t h b r o a d e r l i n e w i d t h a t -37.8 ppm, which i s essent i a l l y s i m i l a r t o t h a t o f s y n t h e t i c ZrP207, b o t h having a s m a l l number o f phosphate groups niay enhance t h e p r o t o n i c c h a r a c t e r i s t i c s , s i n c e t h e s e c a t a l y s t s show h i g h e r magnetic f i e l d s t h a n t h e o t h e r s . When a c c u m u l a t i o n Of e l e c t r o n s around P atoms occurs, t h e e l e c t r o n s move f r o m t h e r e s i d u a l s u r f a c e phosphate t o l a t t i c e P atoms, t h u s f a c i l i t a t i n g t h e a c i d i c p r o p e r t i e s o f t h e c a t a l y s t s . The s t r o n g q u a d r u p o l a r i n t e r a c t i o n s i n e t a1

. (115)

59C0

have been e x p l o i t e d by Ledoux

t o c h a r a c t e r i z e cobal t-promoted h y d r o d e s u l p h u r i z a t i o n C a t a l y s t s .

These a u t h o r s found f o u r d i f f e r e n t Co s i t e s , and i n t e r p r e t e d t h e r e s u l t s i n ternis o f a new quasi-amorphous c o b a l t s u l p h i d e phase c o e x i s t i n g w i t h t h e regul a r Cogs8 phase, w i t h Co i n s i d e v e r y i r r e g u l a r s u l p h u r tetrahedra.Such a c o n f i g u r a t i o n can be i n dynamic e q u i l i b r i u m w i t h an i n a c t i v e o c t a h e d r a l c o b a l t w i t h two vacancies i n i t s c o o r d i n a t i o n sphere. They a l s o concluded t h a t t h e synerget i c e f f e c t o f Co on Mo can be s i m p l y i n t e r p r e t e d i n terms o f a d d i t i v i t y o f MoS2 phase a c t i v i t y t o t h a t o f t h e h i g h l y d i s p e r s e d t e t r a h e d r a l c o b a l t s u l p h i d e phase.

A28

I

3'P

0 -20 -60 -60

Sp

(ppm)

F i g . 1.9. "P MAS-NMR spectra o f €-zirconium phosphate outgassed a t d i f f e r e n t temperatures: a ) ambient temperature; b ) 573 K; c ) 773 K; e ) 1473 K. * S p i n n i n g s i d e band. Readapted from r e f . 114. 1.4.1.1.2.

E l e c t r o n Spin Resonance (ESR)

I n t h e c o n t e x t of heterogeneous c a t a l y s i s , t h e ESR technique has been f r e q u e n t l y used t o i n v e s t i g a t e t h e n a t u r e o f t h e c a t a l y t i c s i t e and i t s c o o r d i n a t i o n number. The a p p l i c a t i o n o f ESR t o heterogeneous c a t a l y s t s , i n c l u d i n g det e r m i n a t i o n of o x i d a t i o n s t a t e s , f o r m a t i o n o f i o n p a i r s , and m o n i t o r i n g o f i o n m i g r a t i o n , have been r e c e n t l y reviewed by Che and Ben T a a r i t (136). An e x t e n s i v e r e v i s i o n o f t h e p o s s i b i l i t i e s o f t h e technique u s i n g adsorbed molecules t o i n v e s t i g a t e t h e n a t u r e of t h e c a t a l y t i c s i t e s i s g i v e n i n p a r t B, chapter 5. Only

A29

a few r e c e n t examples o f t h e a p p l i c a t i o n o f ESR t o s o l i d c a t a l y s t s , w i t h o u t adsorbed molecules, a r e examined i n t h i s s e c t i o n . 1.4.1.1.2.1.

Zeolites

Narayana and Kevan (117) used ESR and e l e c t r o n s p i n echo m o d u l a t i o n t e c h niques t o i n v e s t i g a t e t h e l o c a t i o n and environment o f CuZt i n CaX z e o l i t e s . They proposed a t r i g o n a l b i p y r a m i d a l complex t o account f o r t h e observed s p e c t r a l f e a t u r e s w i t h t h e most p r o b a b l e l o c a t i o n b e i n g s i x r i n g windows between t h e s o d a l i t e u n i t s and supercages o f t h e z e o l i t e . I n a p a r a l l e l s t u d y M i c h a l i k e t a l . (118) i n v e s t i g a t e d t h e f o r m a t i o n o f N i t i o n s o v e r Ni-CaX z e o l i t e s a p p l y i n g ESR. The f o r i n a t i o n o f two N i t complexes was p o s t u l a t e d , s t a b l e o n l y i n t h e p r e -

sence o f hydrogen i n t h e z e o l i t e . I n a t h i r d s t u d y , Narayana e t a l . (119) det e r m i n e d t h e chemical s t a t e o f p a l l a d i u m i n Pd-NaX z e o l i t e by ESR and XPS t e c h niques. The c a l c i n a t i o n i n a i r o f t h e z e o l i t e ion-exchanged w i t h [Pd(NH3i41ClZ i n d i c a t e d t h e f o r m a t i o n o f Pd3' i o n s which c o u l d be e a s i l y reduced t o Pd

at

moderate temperatures. The ESR and XPS d a t a i n d i c a t e d t h e f o r m a t i o n o f s m a l l charged Pd c l u s t e r s d i f f i c u l t t o i d e n t i f y . I n a r e c e n t ESR work, Ghosh and Kevan ( 1 2 0 ) s t u d i e d t h e n a t u r e o f t h e s i t e s on Pd-exchanged Na-X and Ca-X z e o l i t e s used f o r t h e d i m e r i z a t i o n o f e t h y l e n e . 2+ ) c o c a t i o n s i n X z e o l i t e s has been shown t o i n f l u e n -

The presence o f Nat ( o r Ca

ce t h e l o c a t i o n o f t h e a c t i v e p a l l a d i u m s p e c i e s (119) f o r t h e e t h y l e n e d i i n e r i z a t i o n . I n Pd-NaX z e o l i t e s , p a l l a d i u m c a t i o n s occupy s i t e s S I I ' which a r e r e l a t i v e l y a c c e s s i b l e t o e t h y l e n e . As a r e s u l t , t h e r e a c t i o n was f o u n d t o be s t r o n g l y i n h i b i t e d i n Pd-NaX z e o l i t e c a t a l y s t s , and o n l y o c c u r s a f t e r a l o n g i n d u c t i o n p e r i o d due t o m i g r a t i o n o f p a l l a d i u m s p e c i e s toward more a c c e s s i b l e l o c a t i o n s . I n t h i s s t u d y , t h e paramagnetic species g i v i n g ESR s i g n a l s a t gl1 = 2.53 and gll = 2.33-2.34,

t

both w i t h gL = 2.10, were assigned t o Pd c a t i o n s c o o r d i n a -

t e d t o e t h y l e n e . The f a c t t h a t t h e s e species were d e t e c t e d p r i o r t o butene f o r mation, seems t o i n d i c a t e t h a t Pdt c a t i o n s a r e c a t a l y t i c a l l y a c t i v e s i t e s f o r ethylene dimerization.

A c a r e f u l i n v e s t i g a t i o n o f reduced Pd-NaY z e o l i t e s was c a r r i e d o u t by Z i n a e t a l . (121) a l s o u s i n g ESR. Hydrogen o r e t h y l e n e r e d u c t i o n

Of

this zeolite a t

room temperature, a f t e r c a l c i n a t i o n i n oxygen a t 773 K, c o n v e r t e d Pd3' s p e c i e s i n t o Pdt i o n s , which a r e s t a b i l i z e d i n t h e s o d a l i t e cages o f t h e z e o l i t e . Above 423 K, t h e f r a c t i o n o f Pd2' d i f f e r e n t Pd'

l o c a t e d i n i n a c c e s s i b l e p o s i t i o n s were reduced t o

i o n s w i t h t h e p a r a l l e l f o r m a t i o n o f some Pd m e t a l .

The Claus r e a c t i o n has a l s o been s t u d i e d by ESR and FTIR s p e c t r o s c o p i c methods on both X and Y z e o l i t e s ( 1 2 2 ) . A c i d i c h y d r o x y l groups were formed on the X-zeolite,

b u t n o t on t h e Y - z e o l i t e d u r i n g t h e course o f s u l p h u r f o r m a t i o n .

It appears t h a t t h e SO; r a d i c a l s , observed by ESR, and t h e adsorbed SO2, obser-

ved by FTIR, a r e n o t c o r r e l a t a b l e .

TABLE 1.4. Re1evant ESK Features o f Supported-Molybdena C a t a l y s t s

A Support Zr02

g1 g L 1.920 1.959 a t 295 K

Ti02

Si02

B ,g

1.963

1.958

1.980 1.959 removed a t T>473K

gII

91

9 11

91

1.889 1.953 1.861 outgassing a t 373 K

1.952 1.856 a t 773 K

1.945 1.886 a t 295 K ~

~~~~~~~

~

1.945 1.872 increases w i t h Tr

2'3 1.956 1.870 a t 773 K

1.944 1.870 a t 295 K

E or F 9 11

91

92

93

1.961 1.895 o n l y above 473 K

1.853 1.948 1.931 1.955 a t 373 K, i n t e n s i t y maxima a t 473 K

~

Si02 .A1 203

D

C

1.884

1.955 1.866 H2-reduction a t 773K

1.915 1.815 1.790 a t room temperature 1.944 1.938 1.899 o n l y a f t e r H2-reduction

A31

1.4.1.1.2.2. Supported C a t a l y s t s Supported vanadium oxides, comnonly used f o r t h e s e l e c t i v e o x i d a t i o n o f hydrocarbons, have been e x t e n s i v e l y s t u d i e d by ESR. An i n t e r e s t i n g example has r e c e n t l y been s u p p l i e d by Sharma e t a l . (123), who s t u d i e d monolayers and double l a y e r s of V205 supported on alumina, s i l i c a and magnesia. From t h e s p e c t r a , these a u t h o r s o b t a i n e d an e s t i m a t e o f t h e V=O bond s t r e n g t h and t h e d e l o c a l i z a t i o n o f t h e V4'

u n p a i r e d e l e c t r o n o n t o t h e c o o r d i n a t i v e l y bound oxygen l i g a n d s .

T h e i r a n a l y s i s o f t h e s p e c t r a was based on a t e t r a g o n a l l y (C4,,)

d i s t o r t e d aver-

age c o o r d i n a t i o n geometry; i n c r e a s i n g t e t r a g o n a l d i s t o r t i o n , which i s due e i t h e r t o s h o r t e n i n g of t h e V=O bond o r t o an i n c r e a s e d l i g a n d t o a h i g h e r bond strength. On alumina, an i n c r e a s e i n bond s t r e n g t h was observed upon second V 2 0 5 l a y e r i n c o r p o r a t i o n . H 2 - r e d u c t i o n weakened t h e V=O bond t o t h e same e x t e n t on b o t h monolayer and d o u b l e l a y e r s on alumina and i n c r e a s e d t h e e l e c t r o n d e l o c a l i z a t i o n o n t o t h e 1 i g a n d o r b i t a l s . I n c o n t r a s t , t h e s i l i c a - s u p p o r t e d monolayer V205 showed a s l i g h t l y weaker V=O bond t h a n t h e c o r r e s p o n d i n g double l a y e r c a t a -

l y s t . H 2 - r e d u c t i o n s t r e n g t h e n e d t h e bond c o n s i d e r a b l y i n b o t h c a t a l y s t s , b u t e l e c t r o n d e l o c a l i z a t i o n was e s s e n t i a l l y unchanged upon i n c r e a s i n g V205 l o a d i n g and upon r e d u c t i o n . Magnesia-supported vanadia e x h i b i t e d , however, t h e l a r g e s t e l e c t r o n d e l o c a l i z a t i o n and t h e s m a l l e s t t e t r a g o n a l d i s t o r t i o n , i n d i c a t i n g a weak V=O bond. H 2 - r e d u c t i o n o f t h e magnesia-supported monolayer c a t a l y s t d i d n o t i n f l u e n c e t h e V=O bond s t r e n g t h , b u t a weak c o n t r i b u t i o n (5-10%) of a spectrum corresponding t o V2'

i o n s was found superimposed on t h e V4'

o f o c t a h e d r a l l y c o o r d i n a t e d V2'

signals; the signals

i o n s being much more abundant i n t h e d o u b l e

1ayer p r e p a r a t i o n s . Molybdena-based c a t a l y s t s , commonly used f o r hydrodesul p h u r i z a t i o n and hyd r o d e n i t r o g e n a t i o n processes, c o n s t i t u t e a n o t h e r i m p o r t a n t c l a s s o f p r e p a r a t i o n s which have been e x t e n s i v e l y s t u d i e d by ESR. I n a v e r y r e c e n t work, Caceres e t a l . (124) s t u d i e d t h e presence o f Mo5' species on supported molybdena c a t a l y s t s w i t h subnionolayer l o a d i n g on T i 0 2 , Zr02, A1203, S i 0 2 and Si02.A1203 c a r r i e r s . Vacuum p r e t r e a t m e n t s o f t h e c a t a l y s t s gave r i s e t o v a r i o u s Mo5' ESR s i g n a l s whose i n t e n s i t y was s t r o n g l y dependent on b o t h temperature and s u p p o r t . The most r e l e v a n t f e a t u r e s o f t h e Mo5' s i g n a l s a r e summarized i n T a b l e 1.4. The s i n g a l A, narrow and o f l o w i n t e n s i t y , was observed when t h e o t h e r s i g n a l s B, C and D were v e r y weak o r absent. I t s g-values,

l o w e r t h a n ge, i n d i c a t e t h a t i t i s due t o

t r a p p e d e l e c t r o n s , p r o b a b l y i n d e f e c t s i n t h e Moo3. S i g n a l E has been a s s i g n e d t o iUlo5'

i n s u b s t i t u t i o n a l p o s i t i o n s i n T i 0 2 ( 1 2 5 ) . S i g n a l s 6, C and D showed g-

v a l u e s c l o s e t o t h o s e observed by L o u i s and Che (126) f o r Mo5' i n d i f f e r e n t co5+ 5+ o r d i n a t i o n environments, i . e . s i g n a l C t o M O ~ ~s i ,g n a l B t o M o and ~ ~s i g n a l D t o Moi:

i o n s . F i n a l l y , s i g n a l F, generated by H2-treatments, showed g-values v e r y

c l o s e t o those o f t h e hexacoordinated Mo5'05(OH)

species (127).

A32

F i g . 1.10. ESR spectra o f v a r i o u s supported molybdena c a t a l y s t samples subjected t o outgassing a t 473 K. The ESR spectra of t h e c a t a l y s t s outgassed a t 473 K a r e reproduced i n F i g . 1.10. I n general, t h e s i g n a l s a r e narrow and b e t t e r r e s o l v e d i n Mo03/M (M=Zr02,

Ti02) c a t a l y s t s than i n t h e o t h e r (M = Si02, A1203, Si02.A1203) p r e p a r a t i o n s which m o s t l y d i s p l a y a broad band. T h i s f a c t was i n t e r p r e t e d i n terms o f a good

Moo3 d i s p e r s i o n and a homogeneous oxygen environment around t h e Mo atom, when supported on L r 0 2 and Ti02. whereas t h e broadening o f t h e Mo5+ s i g n a l s i n Si02, A1203 and Si02.A1203 c a r r i e r s r e f l e c t e d s i g n i f i c a n t h e t e r o g e n e i t y i n oxygen coo r d i n a t i o n and probably d i p o l a r magnetic i n t e r a c t i o n s between c l o s e Mo5+ i o n s . The same c a t a l y s t s were H2-reduced i n t h e temperature range o f 295-773 K and s t u d i e d by ESR. The i n t e g r a t e d i n t e n s i t y o f t h e dominant ESR s i g n a l p e r gram Mo i s g i v e n i n F i g . 1.11, as a f u n c t i o n o f t h e r e d u c t i o n temperature ( T r ) . Sign i f i c a n t d i f f e r e n c e s i n s i g n a l i n t e n s i t y and Tr dependence were found among t h e c a t a l y s t s . For Mo03/Ti02, t h e curve showed a maximum a t ca. 473 K, i n d i c a t i n g t h a t t h e r e d u c t i o n o f Mo5+ species t o Mo4+ on t h e T i 0 2 c a r r i e r occurred a t v e r y low temperature. I n c o n t r a s t , f o r t h e Mo03/Zr02 c a t a l y s t , t h e Mo5+ concentra-

A33

O

400

600

C

T (K) *O0

F i g . 1.11. V a r i a t i o n o f t h e Mo5' ESR s i g n a l i n t e n s i t y (a.u.) o f H2-reduced c a t a l y s t s as a f u n c t i o n o f t h e r e d u c t i o n temperature. 0 , Mo03/Zr02; 0, Mo03/Ti02; A, Mo03/A1 203. t i o n i n c r e a s e d c o n t i n u o u s l y o v e r t h e e n t i r e t e m p e r a t u r e range. The Mo03/A1203 c a t a l y s t showed a r e l a t i v e l y much l o w e r Mo5'

i o n c o n c e n t r a t i o n , and t h e forma-

t i o n o f Mo5' s p e c i e s o c c u r r e d a t h i g h e r temperatures. I n t h i s case, t h e appearance o f a smooth and broad maximum a t 573-673 K suggested t h a t most o f t h e Mo"

species formed were r a p i d l y reduced t o a l o w e r v a l e n c e s t a t e and/or became

s p i n p a i r e d w i t h o t h e r Most i o n s i n t h e Moo3 c l u s t e r . F i n a l l y , t h e s i g n a l i n t e n s i t i e s f o r Mo03/Si02 and Mo03/Si02.A1203 c a t a l y s t were v e r y l o w and p r a c t i c a l l y constant. 1.4.1.1.3.

I n f r a r e d Spectroscopy

I R spectroscopy i s o n l y one o f many probes o f v i b r a t i o n a l and low energy

( < 10.000 cm- 1j e l e c t r o n i c e x c i t a t i o n s a t s u r f a c e s . Each o f t h e v i b r a t i o n a l

techniques possesses d e f i n i t e a t t r i b u t e s t h a t r e n d e r them s u i t a b l e t o p r o b e d i f f e r e n t aspects o f v i b r a t i o n s o f s u r f a c e s . The most r e l e v a n t v i b r a t i o n a l t e c h niques, t h e i r c h a r a c t e r i s t i c s , s t r e n g t h s and weakenesses a r e summarized i n Table 1.5 ( 1 2 8 ) . The a b s o r p t i o n o f I R r a d i a t i o n by molecules o r i g i n a t e s t r a n s i t i o n s between d i s c r e t e v i b r a t i o n a l (and r o t a t i o n a l ) energy l e v e l s . The r e s u l t i n g I R spectrum

TABLE 1.5. L i s t o f Surface V i b r a t i o n a l Spectroscopies EELS

I RS

SRS

ABS

INS

Probe

Low energy e l e c t r o n s ( - 5 eV)

Photons

Photons

Thermal atoms ( - 0.04 eV)

Neutrons

Mechanism

Inelastic

Resonant

Inelastic

Inelastic

I n e la s t i c

Analysis

Electrostatic analyzer

Grating o r i n t e r ferometer spect romet e r s

G r a t i n g spectrometer

Time o f f l i g h t

Grating o r time o f flight

Advantages

Sentitive (dipole High r e s o l u t i o n and non-dipol e ( < 1 cm-1) modes) Selection rules Broadband (10LOW and h i g h pres1000 meV) sures Momentum t r a n s f e r E l e c t r i c and mag( - 10 nm-1) netic fields Time r e s o l u t i o n < l s Time r e s o l u t i o n < l s

Good r e s o l u t i o n ( - 1 cm-1) Selection rules Low and h i g h pressures E l e c t r i c and magnetic fields

High r e s l u t i o n ( < 1 m-5 Low frequency (<400 cm-1) Large momentum t r a n s f e r (-10nm-I) Surface s e n s i t i v e

Broadband ( > 3 0 0 0 ~ m - ~ ) Sensitive t o H Q u a n t i t a t i v e measure o f a l l normal modes

Disadvantages

Low r e s o l u t i o n (-30 cm-1) Requires vacuum (10-4 T o r r ) R e f l e c t i v i t y strong l y depends on surface order

Current detectors Very i n s e n s i t i v e and r a d i a t i o n sour- except resonant ces l i m i t s e n s i t i - o r enhanced SRS v i t y a t low f r e quencies (<600cm-1)

R e s t r i c t e d t o low f a c e area high Requires frequencies (<400 cm-1) t o O n l y sensitive a v o i d mu1 tiphonon effects Requires long-range substrate order ( > 5 nm)

'"to

A35 may a l l o w f o r t h e stereochemical arranqements o f atoms i n

a molecule, s i n c e i t s

symmetry determines t h e a c t i v e v i b r a t i o n a l modes. The t y p e o f experiments t h a t can be t a c k l e d most s u c c e s s f u l l y by I R spectroscopy a r e t h o s e t h a t r e q u i r e h i g h r e s o l u t i o n , s t r i c t p o l a r i z a t i o n r u l e s , t i m e r e s o l u t i o n o r h i g h pressure. The s t u d y o f many c a t a l y t i c problems, which i n v o l v e a d s o r p t i o n and f u r t h e r r e a c t i o n o f molecules a t t h e s u r f a c e o f c a t a l y s t s , has t o r e l y on most o f t h e s e p r o p e r t i e s . The r e s o l u t i o n p o t e n t i a l makes i t p o s s i b l e t o r e s o l v e t h e c l o s e l y spaced normal modes o f t h e adsorbed molecules; s e n s i t i v i t y towards b o t h p a r a l l e l and p e r p e n d i c u l a r components of t h e modes i s c r u c i a l f o r s t r u c t u r e d e t e r m i n a t i o n ; and t i m e r e s o l u t i o n ( < 1s) i s c o m p a t i b l e w i t h t h e k i n e t i c s t a k i n g place, as t h e p r e s s u r e o f t h e adsorbate ( o r r e a c t a n t ) o r t h e temperature i s v a r i e d , even under c o n d i t i o n s where e l e c t r o n s p e c t r o s c o p i e s a r e no l o n g e r app i c a b l e.

1.4.1.1.3.1.

Transmission

I R spectroscopy i n i t s t r a n s m i s s i o n mode has been

SUC

e s s f u l l y used f o r

many y e a r s i n t h e s t u d y of adsorbed species on h i g h s u r f a c e area metal o x i d e s and supported m e t a l c a t a l y s t s . I n t h i s case, t h e I R beam passes t h r o u g h v e r y t h i n wafer-samples, and t h e f r a c t i o n o f t r a n s m i t t e d I R r a d i a t i o n i s r e c o r d e d as a f u n c t i o n o f t h e wavelength o f t h e I R source. The spectrum o f adsorbed s p e c i e s may b e o b t a i n e d by a p p r o p r i a t e

s u b t r a c t i o n f r o m t h e o v e r a l l spectrum and t h a t

of t h e gaseous phase. T h i s o p e r a t i o n i s now e a s i l y c a r r i e d o u t by computerized I R and by FTIR spectrophotometers. The a p p l i c a t i o n o f I R i s r e s t r i c t e d t o a some-

what

l i m i t e d s p e c t r a l r e g i o n because o f t h e s t r o n g a b s o r p t i o n o f I R r a d i a t i o n

by t h e s o l i d c a t a l y s t s t o which t h e adsorbates a r e bound. Consequently, n o t a l l t h e bands b e l o n g i n g t o t h e adsorbate can be observed, and t h e assignment o f s u r f a c e s t r u c t u r e s must be done by s i m i l a r i t y between t h e bands observed i n t h e spectrum and t h e corresponding bands o f t h e m o l e c u l e s i n t h e b u l k phase.

1.4.1.1.3.2.

R e f l e c t i o n - A b s o r p t i o n (RAIRS)

R e f l e c t i v i t y measurements o f a s u r f a c e c o n t a i n i n g an a d s o r b a t e can y i e l d t h e I R s p e c t r a o f t h e adsorbed s p e c i e s . There a r e two d i f f e r e n t r e f l e c t i o n - a b s o r p t i o n techniques: i n t e r n a l and e x t e r n a l

. These

t e c h n i q u e s a r e b r i e f l y exa-

mined t o g e t h e r , as t h e p h y s i c a l n a t u r e o f b o t h i s v e r y s i m i l a r .

I n i n t e r n a l r e f l e c t i o n an I R beam i s g u i d e d a l o n g t h e i n s i d e of an I R t r a n s p a r e n t s l a b , u s u a l l y s i l i c o n , s a p p h i r e , and germanium v i a i n t e r n a l r e f l e c t i o n , as shown i n F i g . 1.lla. The power l o s t a t each wavelength t h r o u g h i n t e r a c t i o n w i t h t h e adsorbed l a y e r i s measured. W i t h t h i s c o n f i g u r a t i o n a h i g h sens i t i v i t y can be achieved by u s i n g s u f f i c i e n t l y t h i n s l a b s t o g e n e r a t e m u l t i p l e r e f l e c t i o n s . T h i s t e c h n i q u e can be used q u a n t i t a t i v e l y w i t h c a l i b r a t i o n s , and i t s a p p l i c a t i o n i s r e s t r i c t e d t o s u r f a c e f i l m s on I R - t r a n s p a r e n t m a t e r i a l s which can be c u t and p o l i s h e d t o t h e r e q u i r e d s l a b dimensions. I t cannot be a p p l i e d ,

A36

1

I R beam

V

film

substrate

')a

prism

0

p r i s m g

substrate

F i g . 1.12. a ) T y p i c a l e x p e r i m e n t a l c o n f i g u r a t i o n o f t h e i n t e r n a l r e f l e c t i o n experiment f o r a f i l m d e p o s i t e d on a n I R t r a n s p a r e n t s l a b ; b ) d e s c r i p t i o n o f t h e external r e f l e c t i o n f o r a f i l m coated substrate; c ) experimental configur a t i o n o f surface e l e c t r o m a g n e t i c waves p r o p a g a t i o n .

A37

however, t o b u l k metals, u n l e s s v e r y t h i n ( < 10 nm) f i l m s o f t h e s e m a t e r i a l s can be p l a c e d on t h e s u r f a c e o f t h e r e f l e c t i o n s l a b so t h a t p a r t o f t h e evanesc e n t wave can p e n e t r a t e through t h i s l a y e r t o reach t h e adsorbate. F o l l o w i n g t h i s metodology, R i c e and H a l l e r (129) were a b l e t o r e c o r d RAIRS s p e c t r a o f CO adsorbed on a v e r y t h i n Pd f i l m d e p o s i t e d on a s a p p h i r e s l a b . I n e x t e r n a l r e f l e c t i o n t h e I R beam i s r e f l e c t e d o f f a metal s u r f a c e , as shown i n F i g . 1 . l l b . The power l o s t by i n t e r a c t i o n o f t h e s t a n d i n g wave near t h e metal s u r f a c e w i t h an adsorbed l a y e r g i v e s r i s e t o an a b s o r p t i o n spectrum. F o r i n s t a n c e , u s i n g a metal exposed t o ambient experiment, a f i l m o f t h e o x i d e w i l l cover t h e metal s u b s t r a t e , t h i s o x i d e f i l m b e i n g o f i n t e r e s t f o r e x t e r n a l r e f l e c t i o n . The e l e c t r i c f i e l d i n t e n s i t y o f I R r a d i a t i o n extends c o n t i n u o u s l y beyond t h e s u r f a c e , and t h e r e i s no l i m i t t o t h e t h i c k n e s s o f t h e sample t o be probed. R e l a t i v e l y i n e x p e n s i v e commercial attachments can be f i t t e d t o t h e spect r o p h o t o m e t e r s , and good s i g n a l t o n o i s e r a t i o s must be o b t a i n e d u s i n g e i t h e r FTIR (130) o r d i s p e r s i v e spectrophotometers w i t h wavelength (131) o r p o l a r i z a t i o n m o d u l a t i o n (132, 133), i n c o n j u n t i o n w i t h v e r y s e n s i t i v e d e t e c t o r s as i n t r i n s i c photoconductors, o f which MCT i s a t y p i c a l example. Conventional spect r o p h o t o m e t e r s can, however, be used f o r f i l m s t h i c k e r t h a n ca. 10 nm. I n t e r e s t i n g a p p l i c a t i o n s o f t h e e x t e r n a l r e f l e c t i o n t e c h n i q u e t o s u r f a c e a n a l y s i s have been reviewed (130, 134, 135).

1.4.1.1.3.3. Emission A l l samples which absorb I R r a d i a t i o n a l s o e m i t a t t h e same f r e q u e n c i e s , and i n p r i n c i p l e t h e I R spectrum can be o b t a i n e d by t h e d e t e c t i o n of t h i s emiss i o n . T h i s a1 t e r n a t i v e r e s u l t s e x t r e m e l y v a l u a b l e i n I R spectrum d e t e r m i n a t i o n , s i n c e i t does n o t r e q u i r e a f l a t r e f l e c t i n g s u r f a c e as i s t h e case w i t h t h e RAIRS technique, and t h u s t h e spectrum o f a rough s u r f a c e c o u l d be o b t a i n e d . There are, however, two s e r i o u s l i m i t a t i o n s . F i r s t , I R e m i s s i o n i s a f u n c t i o n o f temperature, and a t ambient temperature t h e l e v e l o f e m i s s i o n f r o m s u r f a c e f i l m s o r supported c a t a l y s t s near monolayer t h i c k n e s s i s v e r y weak compared t o t h e l e v e l s measured w i t h c o n v e n t i o n a l I R spectrophotometers. Second, s t i l l a t room temperature, t h e o p t i c s o f t h e i n s t r u m e n t e m i t s unwanted and i n t e r f e r i n g I R r a d i a t i o n , There a r e s e v e r a l ways t o overcome t h e s e d i f f i c u l t i e s : c o o l t h e opt i c s , h e a t t h e sample o r examine t h i c k f i l m s o r metal p a r t i c l e s w i t h h i g h emiss i o n l e v e l s . A l l t h r e e approaches have been experimented w i t h , b u t t h e few s t u d i e s t h a t have been p u b l i s h e d deal o n l y w i t h heated samples. A u s e f u l d i s c u s s i o n o f t h e a p p l i c a t i o n s o f e m i s s i o n I R has been p r o v i d e d by Chase (136).

1.4.1.1.3.4.

P h o t o a c o u s t i c (PAS) and Photothermal D e f l e c t i o n Beam Spectroscopy (PDBS)

PAS and PDBS a r e two r e l a t i v e l y new I R t e c h n i q u e s , which have been a p p l i e d t o study the surface properties o f m a t e r i a l s t h a t a r e d i f f i c u l t o r impossible t o

A38

examine by conventional means. The b a s i c p r i n c i p l e s o f b o t h techniques a r e v e r y simple. I t i s assumed t h a t a p o r t i o n o f t h e r a d i a t i o n impinging on a samp l e i s absorbed and degraded t o thermal energy. A p o r t i o n o f t h e thermal energy t r a v e l s t o t h e s u r f a c e and t h e r e heats t h e gas j u s t above t h e surface. The temp e r a t u r e d i s t u r b a n c e i n t h e gas i s q u i c k l y damped, so t h a t o n l y a v e r y t h i n gas l a y e r i s a f f e c t e d . I f t h e a b s o r p t i o n o f r a d i a t i o n and t h e h e a t i n g o f t h e g a s a r e p e r i o d i c , t h e gas l a y e r i n immediate c o n t a c t w i t h t h e s o l i d expands and cont r a c t s p e r i o d i c a l l y and thus can a c t as a p i s t o n on t h e r e s t o f t h e gas, producing an a c o u s t i c pressure s i g n a l t h a t t r a v e l s throughout t h e gas. The detect i o n o f t h a t s i g n a l i s p o s s i b l e w i t h PAS (36, 37). I n c o n t r a s t , w i t h PDBS, t h e r a p i d l y damped thermal e f f e c t s above t h e s u r f a c e a r e probed v i a t h e mirage e f f e c t (38, 39). The changes i n temperature i n t h e boundary l a y e r induce changes i n t h e r e f r a c t i v e index o f t h e gas. A beam o f l i g h t t r a v e r s i n g t h e l ' h o t zone'' i s thus d e f l e c t e d ; t h a t i s t h e mirage e f f e c t which Boccara e t a l . (38) have shown t o be t h e b a s i s o f a v e r s a t i l e and s e n s i t i v e spectroscopy. I t i s known t h a t t e x t u r e and p a r t i c l e s i z e e f f e c t s , as w e l l as l i g h t s c a t -

t e r i n g i n f l u e n c e t h e PAS s i g n a l s i g n i f i c a n t l y . S i m i l a r l y , p a r t i c l e s i z e and s c a t t e r i n g e f f e c t s a r e known t o have a bearing on PDBS measurements although a t h e o r e t i c a l treatment i n depth would be requ'red t o c l a r i f y t h e d i f f e r e n t i n t e r a c t i o n s . I n s h o r t , i t i s necessary t o c o n s i d e r parameters, such as p a r t i c l e s i z e and shape, surface t o volume r a t i o , s u r f a c e coverage by adsorbed molecules, pack i n g o f t h e s o l i d and e s p e c i a l l y t h e i n f l u e n c e o f such parameters on s c a t t e r i n g . Many examples o f t h e a p p l i c a t i o n o f PAS and PDBS e x i s t i n t h e l i t e r a t u r e , and t h e examination of a few r e l e v a n t systems o f i n t e r e s t f o r c a t a l y s i s a r e examined i n p a r t B, chapter 2. 1.4.1.1.3.5.

Surface Electromagnetic Waves (SEW)

External r e f l e c t i o n depends on t h e presence o f a standing wave generated by r e f l e c t i o n o f an I R beam from a surface. I t i s a l s o p o s s i b l e , by passing t h e i n - g o i n g I R beam a t a s p e c i f i c angle through a prism, t o launch a propagating wave along t h e s u r f a c e i n t e r f a c e . For metals, such as Cu, u s i n g mid-IR r a d i a t i o n , t h i s d i s t a n c e can be coupled o u t through another p r i s m and d i r e c t e d t o a d e t e c t o r . The SEW can i n t e r a c t w i t h t h e s u r f a c e and g i v e r i s e t o a b s o r p t i o n spectra. An experimental c o n f i g u r a t i o n o f t h e technique i s g i v e n i n F i g . 1.11~.

A general review o f t h e technique has been compiled by B e l l (1371, and i t s app l i c a t i o n t o t h e study o f hydrogen chemisorbed on W under UHV c o n d i t i o n s has been examined by Chabal and Sievers (138). The technique has n o t been g i v e n widespread use because o f t h e a d d i t i o n a l c o m p l e x i t y o f t h e o p t i c s , c o u p l i n g prisms and t h e narrow band c a p a b i l i t i e s o f l a s e r s (139).

A39

1.4.1.1.4.

UV-Visi b l e R e f l e c t i o n

L i g h t o f t h e U V - v i s i b l e r e g i o n o f t h e e l e c t r o m a g n e t i c spectrum can be used t o s t u d y t h e e l e c t r o n i c t r a n s i t i o n s o f s u b s t r a t e s . O p t i c a l s p e c t r a can b e obt a i n e d d i r e c t l y by e i t h e r i n t e r n a l o r e x t e r n a l r e f l e c t i o n - a b s o r p t i o n t e c h n i q u e s ( r e f l e c t a n c e s p e c t r o s c o p y ) . A m o d i f i c a t i o n o f r e f l e c t i o n spectroscopy, i .e. e l l i p s o m e t r y , i n which t h e p o l a r i z a t i o n p r o p e r t i e s o f r e f l e c t e d l i g h t a r e measured p r o v i d e s t h e above i n f o r m a t i o n , and, i n a d d i t i o n , can s u p p l y i n f o r m a t i o n on t h e t h i c k n e s s o f o v e r l a y e r s , even when t h e y do n o t absorb r a d i a t i o n . 1.4.1.1.4.1.

Ref1 ectance

Both i n t e r n a l and e x t e r n a l r e f l e c t a n c e spectroscopy a r e r e l a t i v e l y s i m p l e and w e l l developed ( c f . ( 1 4 0 ) ) . The attachments r e q u i r e d t o p e r f o r m t h e s e k i n d s o f measurement a r e commercially a v a i l a b l e and can be e a s i l y assembled t o s t a n d a r d U V - V i s i b l e spectrophotometers. S t i l l , even i n t h e 1 i g h t o f t h e s e advantages, t h e techniques have n o t been w i d e l y a p p l i e d t o s u r f a c e a n a l y s i s . The p r i n c i p a l reason appears t o be t h a t t h e U V - V i s i b l e s p e c t r a a r e v e r y b r o a d and a c c o r d i n g l y o f l i t t l e use i n a n a l y z i n g m o l e c u l a r s u r f a c e groups. 1.4.1.1.4.2.

Ellipsometry

I n t h i s t e c h n i q u e t h e changes i n t h e s t a t e o f l i g h t p o l a r i z a t i o n a r e measur e d upon r e f l e c t i o n r a t h e r t h a n t h e r e f l e c t e d power. The t h e o r y o f e l l i p s o m e t r i c measurements and d a t a a n a l y s i s i s w e l l known (141, 142) and t h e e x p e r i m e n t a t i o n t e s t procedures a r e r a t h e r s i m p l e . I t c o n s i s t s o f passing a w e l l c o l l i m a t e d monochromatic beam o f u n p o l a r i z e d l i g h t t h r o u g h a p o l a r i z e r t o o b t a i n a known p o l a r i z a t i o n s t a t e . T h i s l i g h t i s t h e n r e f l e c t e d o f f t h e sample, passed t h r o u g h a second p o l a r i z e r , which i s v a r i e d t o a n a l y z e t h e change i n t h e p o l a r i z a t i o n s t a t e , and f i n a l l y d i r e c t e d t o t h e d e t e c t o r . The o n l y r e q u i r e m e n t i s t h a t t h e sample r e f l e c t s l i g h t s p e c u l a r l y ; hence m e t a l s and non m e t a l s , i n p r i n c i p l e , can be analyzed, Consequently, when f l a t p a r a l l e l o v e r l a y e r s were developed on t h e s u b s t r a t e , i t became p o s s i b l e t o c a l c u l a t e b o t h t h e o p t i c a l p r o p e r t i e s and f i l m t h i c k n e s s f r o m e l 1i p s o m e t r i c s p e c t r a . 1.4.1.1.4.3.

Raman Spectroscopy (LRS)

Rainan spectroscopy i s based upon t h e Raman e f f e c t i n which l i g h t , u s u a l l y w i t h i n t h e v i s i b l e wavelength spectrum and d e l i b e r a t e l y monochromatic, can be adsorbed by a sample and be r e e m i t t e d as s c a t t e r e d l i g h t a t a d i f f e r e n t f r e q u e n cy. The frequency o f s c a t t e r e d l i g h t can be e i t h e r h i g h e r ( a n t i - S t o k e s ) o r l o w e r ( S t o k e s ) than t h a t o f t h e i n c i d e n t source, The d i f f e r e n c e i n f r e q u e n c i e s i s known as t h e Raman frequency. It corresponds t o a m o l e c u l a r v i b r a t i o n a l f r e q u e n cy, and t h u s t h e Raman spectrum g i v e s s i m i l a r i n f o r m a t i o n t o t h a t o b t a i n e d f r o m I R , except t h a t t h e v i b r a t i o n a l bands have d i f f e r e n t i n t e n s i t i e s and sometimes

A40

s l i g h t l y d i f f e r e n t frequencies. As Raman s c a t t e r i n g i s q u i t e i n e f f i c i e n t , o n l y w i t h t h e a p p l i c a t i o n o f l a s e r s w i t h a h i g h e r i n p u t power than t r a d i t i o n a l sources good q u a l i t y spectra a r e recorded. Lasers such as A r

+ , K r+

and He-Ne a r e

used as Raman e x c i t a t i o n sources. Raman s h i f t s a r e observed, whenever t h e v i b r a t i o n o f t h e molecule g i v e s r i s e t o a change i n p o l a r i z a b i l i b y , such v i b r a t i o n being known as Raman a c t i v e . Raman spectroscopy has been used s u c e s s f u l l y i n c a t a l y s i s (143-147)and s u r f a c e s t u d i e s (148-150).There are, however, experimental reasons, such as t h e weakness o f Raman l i n e s , t h e i r masking by f l u o r e s c e n c e e f f e c t s , and h e a t i n g on t h e coloured samples under t h e i n t e n s e t h e a p p l i c a t i o n o f LRS t o c a t a l y s t s t u d i e s

aser beam, which p u t s c o n s t r a i n t s on Considering t h a t a t y p i c a l monolayer

i s about 10l8 molecules rnm2, i t i s e v i d e n t t h a t t h e technique was though useless f o r s u r f a c e s t u d i e s u n t i l a few years ago. I n r e c e n t times, however, several Raman techniques have been developed capab e o f p r o v i d i n g Raman s p e c t r a o f monol a y e r s and t h i n f i l m s .

1.4.1.1.4.4. Surface Enhanced Raman Spectroscopy (SERS) The i m p o r t a n t d i s c o v e r y o f s u r f a c e enhanced Raman spectroscopy (SERS) has increased t h e e f f e c t i v e s e n s i t i v i t y o f t h e Raman e f f e c t , so t h a t i t i s now poss i b l e t o study adsorbates a t coverages as low as 1%o f a monolayer. The d i s c o v e r y o f SERS was made by Fleichrnan e t a1

. (164), who observed

i n t e n s e Raman

s i g n a l s from p y r i d i n e adsorbed on a roughened Ag e l e c t r o d e . I n t e n s e a c t i v i t y f o l l o w e d these i n i t i a l s t u d i e s and SERS was observed from a l a r g e number o f mol e c u l e s adsorbed on a r e l a t i v e l y small number o f surfaces. I n general, t h e s c a t -

t e r e d i n t e n s i t i e s can be 104-107 times l a r g e r than expected from gas-phase Raman s c a t t e r i n g cross-sections and t h e d e n s i t y o f adsorbed molecules. Some success i n enhanced s c a t t e r i n g has a1 so been achieved f o r c a t a l y t i c P t (152), N i (153) and Ag-Pd a1 1oy ( 154) surfaces. These developments have m o t i v a t e d a g r e a t number o f s t u d i e s over t h e l a s t years, so t h a t t h e general f e a t u r e s o f t h e mechanisms i n v o l v e d i n SERS a r e r a t h e r w e l l understood. The increased i n t e n s i t y o f SERS makes i t p o s s i b l e t o study s u r f a c e processes which occur a t v e r y l o w coverages, as w e l l as t i m e - r e solved measurements u s i n g conventional Raman i n s t r u m e n t a t i o n . Moreover, t h e surf a c e s e l e c t i v i t y o f SERS ensures t h a t t h e processes under o b s e r v a t i o n o r i g i n a t e a t o r v e r y c l o s e t o t h e surface. I n case one wishes t o study t h e s u r f a c e o f e s p e c i a l l y roughened o r prepared metals, such as Ag, Cu and Au, t h e method i s v e r y promising, and t h e v i b r a t i o n a l s p e c t r a can be o f b e t t e r q u a l i t y than t h e corresponding RAIRS spectra. The more general problem, however, i n v o l v e s t h e measurements o f t h e SERS spectrum o f a monolayer on o t h e r substrates, which would i n c l u d e o t h e r metals, metal oxides, e t c . U n f o r t u n a t e l y , SERS has n o t p r o ven t o be a general technique, because o f i t s s t r i n g e n t requirements r e g a r d i n g

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I 100

I

I

900

I

I

I

I

700

500 Wavenumber Icm-ll

F i g . 1.13. S u r f a c e enhanced Raman s p e c t r a (SERS) o f s i l v e r powder samples subj e c t e d t o v a r i o u s p r e t r e a t m e n t s : a ) background spectrum o f t h e sample exposed t o w a t e r vapour; b ) Raman spectrum o f t h e sample exposed f i r s t t o w a t e r vapour and t h e n t o NO; c ) Raman spectrum o f t h e sample exposed f i r s t t o w a t e r vapour and t h e n t o NO2; d ) Raman spectrum r e c o r d e d a f t e r i n t r o d u c t i o n o f a i r and s t a n d i n g f o r about 12 h. The l a s e r c o n d i t i o n s were 647.1 nm and 100 mW f o r a ) and b ) , and 488.0 nm and 50 mW f o r c ) and d ) . Readapted f r o m r e f . ( 1 5 5 ) . s u r f a c e morphology and d i e l e c t r i c c o n s t a n t . M a j o r improvements a r e expected t o o c c u r i n t h e near f u t u r e by combining u l t r a v i o l e t e x c i t a t i o n sources and s o p h i s t i c a t e d mu1 t i c h a n n e l d e t e c t o r s . To i l l u s t r a t e t h i s , t h e SERS s p e c t r a o f NO adsorbed on a s i l v e r powder s u r face a r e g i v e n i n F i g . 1.13 ( 1 5 5 ) . The Raman spectrum o f t h e sample exposed t o w a t e r vapour f o l l o w e d by evacuation, showed peaks a t 615 and 915 cm-',

due p r o -

b a b l y t o surface s p e c i e s , such as Ag20 ( 1 6 6 ) . When NO was c o n t a c t e d w i t h t h e Ag surface, a peak a t 815 cm-' was observed. However t h i s peak d i d n o t appear, i f t h e experiment was performed w i t h O2 i n s t e a d o f H20. A c c o r d i n g t o t h e s e r e s u l t s , Matsuta and Hirokawa (155) concluded t h a t H20 vapour i s necessary f o r t h e ads o r p t i o n of NO t o form NO;

s p e c i e s on t h e Ag s u r f a c e . The SERS spectrum o f t h e

A42

same sample exposed f i r s t t o water vapour and t h e n t o NO2 gave peaks a t 815 and 1040 cm-'

( F i g . 1.13, spectrum c ) , due t o NO;

and NO;

species, r e s p e c t i v e l y

(157). When a i r was introduced, t h e peaks due t o s u r f a c e o x i d e appeared again. These f a c t s show t h a t some NO2 was adsorbed on s u r f a c e Ag20 species t o form

NO; ions. However, i n t h e case o f NO, t h e i n t e n s i t y o f t h e SERS peaks due t o s u r f a c e species such as Ag20 d i d n o t change, as shown i n F i g . 1.13, spectrum b. T h i s i n d i c a t e s t h a t NO molecules were n o t adsorbed on Ag20. Since t h e decompos i t i o n o f water i n t o atomic oxygen on a s i l v e r s u r f a c e has been observed (158), i t so seems t h a t t h e atomic oxygen produced from adsorbed water molecules may

be accountable f o r t h e a d s o r p t i o n o f NO. 1.4.1.1.4.5.

Other Raman techniques

Other Raman techniques, i n c l u d i n g s t i m u l a t e d Raman g a i n (SRG) and i n t e r n a l R e f l e c t i o n Raman (IRR), have been r e c e n t l y used t o enhance t h e e l e c t r o m a g n e t i c f i e l d s t r e n g t h s o f a Raman e x c i t a t i o n beam. I n SRG, two synchronously pumped h i g h i n t e n s i t y , l a s e r beams o f s l i g h t l y d i f f e r e n t frequencies, cross t h e sample, When t h e d i f f e r e n c e i n frequencies o f t h e beams c o i n c i d e s w i t h t h a t o f a Raman a c t i v e v i b r a t i o n a l mode, one beam gains power a t t h e expense o f t h e o t h e r . The r e f l e c t e d beams a r e separated o p t i c a l l y and t h e Raman spectrum i s recorded by p l o t t i n g t h e power change as a f u n c t i o n o f t h e beam frequency d i f f e r e n c e . The I R R i s , however, o n l y a p p l i e d t o t h i c k e r and o p t i c a l l y t r a n s p a r e n t slabs. I n

t h i s case, t h e enhancement o f several o r d e r s o f magnitude i s due t o t h e w e l l understood p r o p e r t i e s o f t h e e l e c t r o m a g n e t i c f i e l d s a t smooth i n t e r f a c e s , thus r e s u l t i n g i n s u f f i c i e n t Raman s c a t t e r i n g s i g n a l s t o p r o v i d e u s e f u l spectra. 1.4.1.1.5. X-ray D i f f r a c t i o n (XRD) and Small Angle S c a t t e r i n g (SAS) X-ray d i f f r a c t i o n (XRD) f o r many y e a r s has been r o u t i n e l y used t o i d e n t i f y c r y s t a l l i n e phases i n heterogeneous c a t a l y s t s . The X-ray d i f f r a c t i o n l i n e s broaden when t h e c r y s t a l s i z e o f c a t a l y s t p a r t i c l e s f a l l s below about 100 nm. The simples approach t o analyze l i n e broadening i s t o assume t h a t p a r t i c l e s i z e c o n t r i b u t e s m a i n l y t o l i n e w i d t h i n excess o f i n s t r u m e n t w i d t h . The mean diameter, dp, o f c r y s t a l l i t e s i s d e l i n e d by t h e S c h e r r e r ' s e q u a t i o n (159).

where X i s t h e X-ray wavelength, K i s S c h e r r e r ' s constant, which takes values o f 0.84-0.89,

depending on t h e assumed p a r t i c l e s i z e , and 6 i s t h e angular l i n e -

width i n radians

due t o p a r t i c l e s i z e broadening and d e f i n e d by B2 = 8'

- b2

(6 standing f o r experimental w i d t h and b f o r i n s t r u m e n t a l width, which can be determined by c a l i b r a t i o n ) . T h i s technique i s a p p l i c a b l e t o p a r t i c l e s o f 3.5-6.0 nm; below 3.5 nm t h e

l i n e i s v e r y broad and d i f f u s e o r i s even absent, w h i l e

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F i g . 1.14. D i s t r i b u t i o n o f metal p a r t i c l e s i z e s i n a 0.6% Pt/A1 0 c a t a l y s t . The continuous l i n e i s from X-ray SAS, t h e dashed l i n e f r o m transmi$s?on e l e c t r o n microscopy ( f r o m r e f . ( 1 6 0 ) ) . above 60 nm t h e change i n l i n e s h a p e i s t o o s m a l l . F o r p a r t i c l e s w i t h s i z e s below 3.5 nm, t h e small a n g l e X-ray s c a t t e r i n g (SAS) t e c h n i q u e becomes v e r y v a l u a b l e . SAS i s based on t h e a n a l y s i s o f s c a t t e r e d r a d i a t i o n w i t h i n v e r y l o w angles

(<4")

o f t h e i n c i d e n t beam. The t h e o r y and p r i n c i p l e s o f t h e method have been w e l l e s t a b l i s h e d ( 4 0 ) , and a r e a p p l i c a b l e t o supported c a t a l y s t s . If t h e r a t i o between t o t a l s u p p o r t s u r f a c e and t h a t covered by s u p p o r t e d p a r t i c l e s i s favour a b l e w i t h respect t o the s c a t t e r i n g p r o p e r t i e s o f t h e support, analysis o f the Porod s l o p e s o f t h e SAS s u p p o r t curves, w i t h and w i t h o u t t h e c a t a l y t i c component, a l l o w s f o r r e l i a b l e measurement o f t h e t o t a l c a t a l y s t area. I t i s also p o s s i b l e t o determine c a t a l y s t p a r t i c l e s i z e d i s t r i b u t i o n f r o m SAS f o r known o r hypothet i c a l p a r t i c l e shapes, as Renouprez e t a l . (160) have p o i n t e d o u t f o r t h e case of P t p a r t i c l e s supported on alumina. The p a r t i c l e s i z e d i s t r i b u t i o n f u n c t i o n f ( d ) , as o b t a i n e d f r o m X-ray SAS V P and t r a n s m i s s i o n e l e c t r o n microscopy (TEM), i s shown i n F i g . 1.14. S p h e r i c a l p a r t i c l e s were assumed, and reasonable agreement was observed f o r t h e l o w e r peak o f t h e d i s t r i b u t i o n f u n c t i o n a t a d i a m e t e r o f 1.5-2.0 nm. The SAS d a t a do n o t reproduce t h e peak a t about 5 nm, p r i m a r i l y because o f a w i d e p o r e s i z e d i s t r i b u t i o n o f t h e c a r r e i r , which l e a d s t o a d d i t i o n a l s c a t t e r i n g . A l t h o u g h t h e d i s t r i b u t i o n f u n c t i o n s from SAS may n o t be a b s o l u t e l y c o r r e c t i n e v e r y d e t a i l , t h e y y i e l d v a l u a b l e i n f o r m a t i o n , e s p e c i a l l y f o r comparing d i f f e r e n t metal l o a d i n g s

A44

and a l s o f o r t h e d e t e r m i n a t i o n o f t h e changes induced by thermal treatments. For instance, Renouprez e t a l . (160) found t h a t i n P t / S i 0 2 c a t a l y s t s , i t i s econom i c a l l y u n p r o f i t a b l e t o e x c e e d a p t l o a d i n g o f 1 w t % because t h e excess o f P t i s p r i m a r i l y i n c o r p o r a t e d i n t o l a r g e r p a r t i c l e s w i t h a lower c a t a l y t i c e f f i c i e n c y . G a l l e z o t e t a l . (161) have s t u d i e d t h e l o c a t i o n and d i s p e r s i o n o f p l a t i n u m i n Pt-exchanged NaY z e o l i t e u s i n g X-ray SAS, a p a r t from o t h e r techniques. The r e s u l t s showed t h a t

P t O

c l u s t e r s form more e a s i l y upon r e d u c t i o n , i f t h e P t 2 +

i o n s a r e i n i t i a l l y i n supercages r a t h e r than i n s o d a l i t e cages. 1.4.1.1.6.Extended X-ray Absorption F i n e S t r u c t u r e (EXAFS) Extended X-ray a b s o r p t i o n f i n e s t r u c t u r e (EXAFS) has become an i m p o r t a n t t o o l i n t h e a n a l y s i s o f heterogeneous c a t a l y s t s . T h i s t r e n d i s p r o g r e s s i v e l y i n c r e a s i n g due t o t h e a v a i l a b i l i t y o f t h e new g e n e r a t i o n o f h i g h energy e l e c t r o n storage r i n g s p r o v i d i n g h i g h e r X-ray f l u x e s . The p h y s i c a l b a s i s o f EXAFS i s v e r y simple. When t h e photon energy o f t h e e x c i t a t i n g source becomes j u s t s u f f i c i e n t t o cause emission o f photoelectrons from any e l e c t r o n s h e l l , a sharp i n c r e a s e i n t h e c r o s s - s e c t i o n f o r photon abs o r p t i o n occurs. i.e.,

t h e a b s o r p t i o n edge. Above t h e a b s o r p t i o n edge an o s c i -

l l a t o r y v a r i a t i o n i n t h e a b s o r p t i o n cross s e c t i o n i s u s u a l l y observed. As Savers e t a l . (162) demonstrated, t h i s o s c i l l a t o r y s t r u c t u r e r e v e a l s t h e l o c a l s t r u c t u r e o f t h e atom under study. The g r e a t e r ease o f i n t e r p r e t a t i o n of d a t a i n t h e range 100 eV above t h e a b s o r p t i o n edge has g i v e n r i s e t o t h e t e r m Extended X-ray Absorption F i n e S t r u c t u r e . The study o f t h e o s c i l l a t o r y v a r i a t i o n s i n a b s o r p t i o n cross s e c t i o n s near t h e a b s o r p t i o n edge provides t h r e e types o f v e r y v a l u a b l e data: i ) i n t e r - a t o m i c distances; i i ) c o o r d i n a t i o n number; i i i ) Debye-Waller f a c t o r . These q u a n t i t a t i v e and s t r u c t u r a l data present many problems. F o r t u n a t e l y , t h e t h e o r y and i t s exper i m e n t a l v e r i f i c a t i o n has progressed v e r y much i n t h e l a s t decade (18, 163). On t h e b a s i s of EXAFS s p e c t r a o f m a t e r i a l s o f known s t r u c t u r e i n v o l v i n g t h e same elements as absorbers and s c a t t e r e r s , i t i s p o s s i b l e t o determine i n t e r a t o m i c distances w i t h an accuracy c u r r e n t l y estimated t o be

5 x 19-'

nm. The

d e t e r m i n a t i o n of t h e c o o r d i n a t i o n number i s more d i f f i c u l t . because t h e amplitude f u n c t i o n of t h e FT data i s more s t r o n g l y

a f f e c t e d than t h e phase by a low

s i g n a l - t o - n o i s e r a t i o , by f i n i t e l e n g t h i n t h e k space o f data, as w e l l as by d i f f e r e n c e s i n t h e sample and r e f e r e n c e compound. One must be concerned about t h e e l e c t r o n mean f r e e paths, t h e r e l a t i v e d i s o r d e r , and inhomogeneities between sample and r e f e r e n c e m a t e r i a l

. Very v a l u a b l e

i n f o r m a t i o n on supported metals

(and a l l o y s ) , and metal oxides (and s u l p h i d e s ) has been p r o v i d e d by EXAFS, which demonstrates t h a t these d i f f i c u l t i e s can be overcome (40, 164, 165). A review o f t h e most i n t e r e s t i n g a p p l i c a t i o n s o f EXAFS t o supported c a t a l y s t s i s g i v e n i n chapter 4, p a r t A.

A45

There a r e two r e l a t e d t e c h n i q u e s employing s y n c h r o t r o n r a d i a t i o n which p r o v i d e s t r u c t u r a l i n f o r m a t i o n , such as bond d i s t a n c e s w i t h i n absorbates and b i n d i n g s i t e s : S u r f a c e Extended X-ray A b s o r p t i o n F i n e S t r u c t u r e (SEXAFS) and Near Edge X-ray A b s o r p t i o n F i n e S t r u c t u r e (NEXAFS). SEXAFS i s an a d a p t a t i o n o f b u l k s t r u c t u r a l EXAFS, which r e v e a l s t h e bond l e n g t h and t h e c h e m i s o r p t i o n s i t e o f an adsorbate. NEXAFS determines, however, t h e o r i e n t a t i o n and bond l e n g t h s between l o w atomic number atoms w i t h i n an adsorbate. T h i s l a t t e r i n f o r m a t i o n i s o f p a r t i c u l a r i n t e r e s t , s i n c e most techniques a r e r a t h e r i n s e n s i t i v e a t o m i c number atoms, e.g.,

1.4.1.1.7.

t o low

hydrocarbons.

Mtissbauer Spectroscopy

MBssbauer spectroscopy i s based on t h e r e c o i l - f r e e e m i s s i o n o f g a m a r a d i a t i o n f r o m a nucleus i n an e x c i t e d s t a t e and i t s subsequent r e c o i l - f r e e r e s o n a n t a b s o r p t i o n and r e - e m i s s i o n by a nucleus o f t h e same i s o t o p e i n t h e ground s t a t e . I n a s o l i d t h e r e c o i l energy may be l e s s t h a n t h e l o w e s t q u a n t i s e d l a t t i c e v i b r a t i o n a l energy, so t h a t t h e gamma r a y may be e m i t t e d w i t h o u t l o s s o f energy due t o t h e r e c o i l o f t h e nucleus. Since t h e p r o b a b i l i t y o f t h e r e c o i l - f r e e p r o cess depends upon t h e energy o f t h e gamma photon, t h e Mossbauer e f f e c t i s r e s t r i c t e d t o c e r t a i n i s o t o p e s w i t h l o w - l y i n g e x c i t e d energy s t a t e s . Mossbauer s p e c t r a a r e c h a r a c t e r i z e d by t h e number p o s i t i o n , shape and r e l a t i v e i n t e n s i t y o f t h e v a r i o u s a b s o r p t i o n 1 i n e s . A l l t h e s e f e a t u r e s r e s u l t from t h e n a t u r e o f t h e v a r i o u s h y p e r f i n e i n t e r a c t i o n s and f r o m t h e p o s s i b l e m o t i o n o f t h e a b s o r b i n g n u c l e i . The t h r e e main h y p e r f i n e i n t e r a c t i o n s a r e p r i m a r i l y r e f l e c t e d i n t h e isomer s h i f t ( 6 ) , quadrupole s p l i t t i n g ( A ), and magnetic s p l i t t i n g . The isomer s h i f t r e p r e s e n t s a measure o f t h e d i f f e r e n c e i n t h e e l e c t r o n i c environments o f t h e source and a b s o r b e r n u c l e i . I t p r o v i d e s , t h e r e f o r e , a means o f m o n i t o r i n g s - e l e c t r o n d e n s i t y a t t h e nucleus, which depends o n e l e c t r o n d i s p o s i t i o n i n t h e o u t e r s h e l l s . Hence t h e isomer s h i f t g i v e s i n f o r m a t i o n on t h e o x i d a t i o n s t a t e and covalency. The second i m p o r t a n t i n t e r a c t i o n c a l l e d quadrupole s p l i t t i n g a r i s e s when t h e n u c l e a r quadrupole moment e x p e r i e n c e s an asymmetric e l e c t r i c f i e l d produced by an asymmetric e l e c t r o n i c charge d i s t r i b u t i o n o r l i g a n d arrangement. F o r instance, t h e 57Fe i n t h e e x c i t e d s t a t e w i t h

I = 3/2 and i n t h e presence o f a non-zero e l e c t r i c f i e l d g r a d i e n t s p l i t s i n t o a t w o - l i n e spectrum, w i t h t h e two l i n e s separated by A . The t h i r d i n t e r a c t i o n has i t s o r i g i n i n t h e i n t e r a c t i o n between any n u c l e a r magnetic moment and t h e magn e t i c f i e l d . Such i n t e r a c t i o n r i s e s t h e degeneracy o f a n u c l e a r s t a t e w i t h I>O and s p l i t s i t i n t o 21

+ 1 l i n e s . F o r 57Fe t h e ground s t a t e , w i t h I

= 3/2, s p l i t s

i n t o f o u r l i n e s , i n such a way t h a t t h e s i x t r a n s i t i o n s a p p r o p r i a t e t o t h e m = 0, 21 s e l e c t i o n r u l e g i v e a s e x t e t MBssbauer spectrum.

MBssbauer spectroscopy i s a v e r y s u i t a b l e t e c h n i q u e f o r t h e i n s i t u s t u d y o f c r y s t a l 1 i n e and n o n c r y s t a l l i n e s o l i d s i n c l u d i n g h i g h l y d i s p e r s e d p a r t i c l e s

A46

on a c a r r i e r . T h i s i s due t o t h e f a c t t h a t gamma r a y s w i t h energies t y p i c a l o f 1-2 keV can p e n e t r a t e a gaseous environment. D e s p i t e t h i s g r e a t advantage t h e r e e x i s t some n e g a t i v e features. The use o f t h e technique f o r c a t a l y s t c h a r a c t e r i z a t i o n a t temperatures above room temperature i s r e s t r i c t e d t o p r e p a r a t i o n s cont a i n i n g Fe, Sn, o r Eu. F u r t h e r r e s t r i c t i o n s a r e caused by t h e low n a t u r a l abundance o f some isotopes, which r e q u i r e s t h e i s o t o p i c enrichment of c a t a l y s t s cont a i n i n g these elements t o o b t a i n good q u a l i t y s p e c t r a .

A good account o f t h e

Mossbauer technique and a review o f t h e most r e l e v a n t a p p l i c a t i o n s t o c a t a l y t i c system i s g i v e n i n chapter 5, p a r t A. 1.4.1.2.

Out-going E l e c t r o n s

1.4.1.2.1.

U1 t r a v i o l e t P h o t o e l e c t r o n Spectroscopy (UPS)

U l t r a v i o l e t p h o t o e l e c t r o n Spectroscopy (UPS) i s based on t h e a n a l y s i s and i n t e r p r e t a t i o n o f t h e k i n e t i c energy d i s t r i b u t i o n o f t h e p h o t o e l e c t r o n s produced by u l t r a v i o l e t i r r a d i a t i o n . U s u a l l y two photon sources o f energy, e i t h e r o f 21.2 o r 40.8 eV, a r e used. These a r e produced by a discharge i n helium gas

according t o t h e f o l l o w i n g processes: t

He

+ e-

Hezt + e-

He

-+

-+

He'

(HeI, hv = 21.2 eV) ( H e I I , hv = 40.8 eV)

Due t o t h e low energy o f t h e e x c i t a t i o n source, UPS probes t h e valence e l e c t r o n l e v e l s . I t s p h y s i c a l b a s i s and experimental c o n f i g u r a t i o n i s t h e same as f o r XPS, b u t t h e r e s o l u t i o n o f gas phase s p e c t r a i s i n v a r i a b l y much b e t t e r i n UPS.

I n s o l i d s t h i s advantage i s f r e q u e n t l y l o s t , because t h e d i s c r e t e energy l e v e l s present i n i s o l a t e d atoms broaden i n t o energy bands. UPS i s u s u a l l y a p p l i e d i n t h e d i r e c t d e n s i t y d e t e r m i n a t i o n of s u r f a c e va-

lence s t a t e s o f s o l i d s , i n t h e assignment o f o r b i t a l s f o r adsorbed species, and i n t h e study o f t h e d i r e c t i o n a l dependence o f t h e photoemission process. The measured d e n s i t y of s t a t e s o f a s o l i d can o n l y be accepted as t h e t r u e d e n s i t y o f s t a t e s (DOS) , when t h e p h o t o i o n i z a t i o n process o f t h e valence e l e c t r o n s a r e e x c i t e d i n t o t h e f i n a l s t a t e which c o n s i s t s o f a f e a t u r e l e s s continuum. Theref o r e , i t i s i m p o r t a n t t o determine t h e DOS with m u l t i p h o t o n sources, and pref e r a b l y w i t h synchrotron photon sources. Since t h e geometry of t h e e l e c t r o n acceptance angle o f t h e a n a l y z e r can be a c c u r a t e l y defined, t h e angular d i s t r i b u t i o n o f t h e p h o t o e l e c t r o n i n t e n s i t y enables t h e d e t e r m i n a t i o n of t h e symmetry o f t h e s t a t e from which t h e e l e c t r o n was e m i t t e d . T h i s a n g u l a r dependence i s a l s o h e l p f u l i n t h e assignment o f o r b i t a l s f o r adsorbed molecules, a l t h o u g h t h e p r e c i s e c a l c u l a t i o n o f t h e energy s h i f t s undergone by t h e valence o r b i t a l s i n t h e gas phase molecule upon a d s o r p t i o n i s an unresolved problem.

-

@

A47

-@ .--

2s

II

...aAwere

2P

-

I

2~ or L2d

photoelectron

-

I

1s

U

-

IS

Or

II

F i g . 1.15. Diagram of t h e p h o t o e l e c t r o n process ( a ) and t h e Auger process ( b ) . 1.4.1.2.2.

X-ray P h o t o e l e c t r o n Spectroscopy (XPS)

X-ray p h o t o e l e c t r o n spectroscopy i s based on t h e a n a l y s i s o f t h e k i n e t i c energy d i s t r i b u t i o n of t h e p h o t o e l e c t r o n s e m i t t e d by a sample upon i r r a d i a t i o n w i t h a monoenergetic s o f t X-ray source. Usual p h o t o n sources employ e i t h e r MgK, r a d i a t i o n ( h v = 1253.6 eV) o r AlK, r a d i a t i o n ( h v = 1486.6 eV), a l t h o u g h t h e r e i s i n c r e a s i n g i n t e r e s t i n t h e use o f c o n t i n u o u s l y t u n a b l e h i g h f l u x photon synchrot r o n r a d i a t i o n . I n t h e absence o f monochromators, t y p i c a l l i n e w i d t h s a r e i n t h e o r d e r o f 0.8 eV b u t i n t h e presence o f c r y s t a l monochromators t h e s e w i d t h s a r e reduced t o l e v e l s as l o w as 0.3 eV. X-rays have a l i m i t e d p e n e t r a t i n g power i n a s o l i d , i n t h e o r d e r o f 1-10 pm, They i n t e r a c t w i t h t h e atoms i n t h i s s u r f a c e r e g i o n t h r o u g h t h e p h o t o e l e c -

t r i c e f f e c t causing e l e c t r o n s t o be e m i t t e d . A c c o r d i n g t o t h e p h o t o e l e c t r i c process, t h e k i n e t i c energy o f t h e e m i t t e d e l e c t r o n i s g i v e n by,

where hv i s t h e energy o f t h e i n c i d e n t photon, BE t h e b i n d i n g energy o f t h e e l e c t r o n , which r e p r e s e n t s t h e d i f f e r e n c e i n t h e energy o f t h e i n i t i a l ( n e u t r a l ) s t a t e and t h e f i n a l ( i o n i z e d ) s t a t e o f t h e system, and GS i s t h e s p e c t r o m e t e r work f u n c t i o n . I n a d d i t i o n t o t h e p h o t o e l e c t r o n s e m i t t e d i n t h e p h o t o e l e c t r i c process, Auger e l e c t r o n s a r e a l s o e m i t t e d due t o r e l a x a t i o n of t h e e n e r g e t i c i o n s l e f t a f t e r photoemission. T h i s Auger e l e c t r o n e m i s s i o n o c c u r s r o u g h l y 10 seconds a f t e r t h e p h o t o e l e c t r i c process. As i l l u s t r a t e d i n F i g . 1.15,

-14

i n the

Auger process an o u t e r e l e c t r o n f a l l s i n t o t h e i n n e r o r b i t a l vacancy, and a second e l e c t r o n i s e m i t t e d , c a r r y i n g o f f t h e excess energy. Thus, p h o t o i o n i z a -

A48

tion normally leads t o two emitted electrons, a photoelectron and an Auger electron; t h e i r energies being below that of the ionizing photon. The photoelectron spectrum consists of discrete peaks superimposed on a background due t o i n e l a s t i c a l l y scattered electrons, Frequently, each peak i s accompanied by other, l e s s intense peaks. Effects such as spin-orbit coupling and many electron processes r e s u l t in additional spectral features. Extra l i n e s , placed on the higher binding energy side of a principal peak, called s a t e l l i t e s or shake-up processes, are usually observed in the photoelectron spectrum. The binding energies ( B E ) are calibrated w i t h respect t o the Fermi level ( f o r solids) or vacuum level ( f o r gases) of the spectrometer, usually by reference t o a peak of accurately known BE. Binding energies f o r Cls a t 284.6 eV and f o r A ~ 4 f , , ~a t 83.8 eV are frequently used as calibrating standards. This l a t t e r procedure i s of particular use for poorly conducting samples where compensation of s h i f t s in the measured kinetic energy due to surface charging e f f e c t s i s necessary. The BE value f o r a particular photoelectron provides a q u a l i t a t i v e identification of the atom from which the electron was emitted, and i t i s particularly sensitive t o the chemical environment through a "chemical s h i f t " e f f e c t . Another very important characteristic of XPS i s t h a t i t provides a quantit a t i v e analysis of the surface. The intensity of the signal observed i s a function of the number of atoms present in the sample. In addition t o the concentration of the element producing the signal , XPS i n t e n s i t i e s depend upon the inel a s t i c mean f r e e p a t h (IMPF) of the electrons and the efficiency of absorption of the exciting X-rays by the solid. I f these factors are well understood or i f good standards are available quantitative analysis can be carried o u t by XPS. The relative concentration of the elements i s the quantity usually desired and i t can be obtained by the r e l a t i v e i n t e n s i t i e s i f the above factors are taken into account. Thus, theoretical models allow measured photoelectron i n t e n s i t i e s t o be related t o the concentration of the emitting atoms in the surface region (20-22). A large review of the XPS technique as well as examples of i t s application a r e given i n chapter 2. 1.4.2. In-going Electrons Electron probes have been used more extensively t h a n any other surface probes. This i s mainly due t o the f a c t that electron beams of controlled energy and density can be easily generated or are readily available a t low cost in the laboratory. When surfaces are bombarded by electrons the emission of photons, electrons, neutrals or ions can occur. Fig. 1.16 summarizes a sketch of the groups of general techniques involving in-going electrons, as well as the acronyms of the specific techniques. The general features of the most commonly used techniques are considered below, with special emphasis on those involving o u t going electrons because of the r e l a t i v e ease of electron detection.

+

A49

Electrons

Neutrals: € I D

Photons:

APS, CIS, EPHA

v/su ;a;

&'fl/

F i g . 1.16. Sketch summarizing t h e general group of t e c h n i q u e s i n v o l v i n g i n - g o i n g e l e c t r o n s . APS, appearance p o t e n t i a l spectroscopy; C I S , c h a r a c t e r i s t i c i s o c h r o mat spectroscopy; EMPA, e l e c t r o n microprobe analysis;LEED, l o w energy e l e c t r o n d i f f r a c t i o n ; HEED, h i g h energy e l e c t r o n d i f f r a c t i o n ; AES, Auger e l e c t r o n spect r o s c o p y ; HREELS, h i g h r e s o l u t i o n e l e c t r o n energy l o s s spectroscopy; RHEED, r e f l e c t i o n h i g h energy e l e c t r o n d i f f r a c t i o n ; EID, e l e c t r o n i n d u c e d n e u t r a l desorpt i o n ; E I I D , e l e c t r o n induced i o n d e s o r p t i o n .

1.4.2.1.

Out-going Photons

There a r e v a r i o u s t e c h n i q u e s a v a i l a b l e , such as appearance p o t e n t i a l spect r o s c o p y (APS) ( 1 6 6 ) , c h a r a c t e r i s t i c isochromat spectroscopy (CIS) (167), and e l e c t r o n probe m i c r o a n a l y s i s (EPMA) (168). A l l t h e s e t e c h n i q u e s p r o v i d e i n f o r m a t i o n from a r e g i o n c o n s i d e r a b l y t h i c k e r t h a n t h e s u r f a c e monolayer. A l t h o u g h t h e g e n e r a t i o n of t h e e l e c t r o n beam as t h e e x c i t a t i o n s o u r c e i s r e l a t i v e l y simp l e , photon d e t e c t i o n p r e s e n t s two m a j o r problems. The f i r s t i s r e l a t e d t o t h e f a c t t h a t t h e photons l e a v e t h e s u r f a c e i n a l l d i r e c t i o n s and o n l y a s m a l l f r a c t i o n o f them reaches t h e d e t e c t o r . The second problem i s t h e n o i s e l e v e l . I f one generates photons i n t h e UV o r v i s i b l e r e g i o n , s c a t t e r e d l i g h t from many sources , even w i t h we1 1-designed equipment , can produce a 1a r g e background n o i s e l e v e l . T h i s l a t t e r problem can be p r a c t i c a l l y e l i m i n a t e d by g e n e r a t i n g X-ray photons. T h i s approach r e q u i r e s o b v i o u s l y t h e use o f h i g h energy i n - g o i n g e l e c t r o n s as w i t h t h e EPMA technique, which t h e n p e n e t r a t e so d e e p l y , t h a t i t can no l o n g e r be termed a s u r f a c e - s e n s i t i v e t e c h n i q u e . I n a d d i t i o n , t h e bombardemenl o f t h e s u r f a c e by t h e e l e c t r o n beam g i v e s r i s e t o an i n c r e a s e of t h e background l e v e l o f photon n o i s e due t o Bremstrahlung. 1.4.2.2.

Out-going E l e c t r o n s

I n s u r f a c e s t u d i e s i n v o l v i n g i n - g o i n g and o u t - g o i n g e l e c t r o n s , t h r e e parameters, namely t h e i r energy, t h e i r s p a t i a l r e s o l u t i o n and t h e number o f e l e c -

A50

t r o n s l e a v i n g t h e surface, a r e o f prime importance. I f t h e r e f l e c t e d e l e c t r o n s a r e detected and t h e i r s p a t i a l r e s o l u t i o n i s considered, one i s , i n f a c t , d e a l i n g w i t h t h e well-known low energy e l e c t r o n d i f f r a c t i o n (LEED) technique (63, 169, 170). I n t h i s case, a monoenergetic c o l l i m a t e d e l e c t r o n beam i n t h e range o f 20-200 eV i s impinged on a surface, u s u a l l y d i r e c t e d p e r p e n d i c u l a r l y t o t h e surface, and t h e d i f f r a c t i o n o f e l a s t i c a l l y s c a t t e r e d e l e c t r o n s i s analyzed. Due t o t h e l a r g e s c a t t e r i n g cross s e c t i o n s o f atoms f o r l o w energy e l e c t r o n s , o f ca. nm2, as compared t o ca. 10-8 nm2 f o r X-ray photons, t h e s c a t t e r e d e l e c t r o n s come m a i n l y from s u r f a c e o r v e r y near t h e s u r f a c e atoms and hence c a r r y informat i o n concerning t h e s t r u c t u r e i n t h i s r e g i o n . LEED p a t t e r n s supply d a t a on t h e s p a t i a l p o s i t i o n i n g o f t h e atoms b u t do n o t p r o v i d e i d e n t i f i c a t i o n o f t h e t y p e o f atoms. Another p o i n t t o be considered i s t h e f a c t t h a t most o f t h e e l e c t r o n d i f f r a c t i o n s t u d i e s use l o w energy e l e c t r o n beams. However, h i g h energy e l e c t r o n s can a l s o be used t o produce d i f f r a c t i o n p a t t e r n s . The r e s u l t i n g technique i s c a l l e d h i g h energy e l e c t r o n d i f f r a c t i o n (HEED) (171). T h i s l a t t e r technique i s g e n e r a l l y a p p l i e d t o v e r y t h i n f i l m s , where t h e h i g h energy e l e c t r o n s p e n e t r a t e t h e f i l m , thus observing t h e t r a n s m i s s i o n e l e c t r o n d i f f r a c t i o n p a t t e r n . Theref o r e , w i t h t h i s technique, one g e t s i n f o r m a t i o n r e l a t e d t o t h e b u l k r a t h e r than t o t h e surface, as i t i s n o t s u r f a c e - s e n s i t i v e . HEED can, however, be made s u r f a c e - s e n s i t i v e by p r o b i n g t h e s u r f a c e a t a g l a n c i n g angle. Then, t h e s c a t t e r e d e l e c t r o n s a r e r e f l e c t e d from t h e s u r f a c e l a y e r s and appear on t h e same s i d e as t h e probe beam. The technique i s designed as r e f l e c t i o n h i g h energy e l e c t r o n d i f f r a c t i o n (RHEED) and can be used f o r s u r f a c e a n a l y s i s ( 1 7 2 ) . For smooth surfaces, LEED and RHEED techniques p r o v i d e s i m i l a r i n f o r m a t i o n , LEED being e a s i e r t o use. Yet f o r rough surfaces, RHEED can p r o v i d e s t r u c t u r a l i n f o r m a t i o n , f o r which LEED becomes i n e f f e c t i v e . The above techniques a r e concerned p r i m a r i l y with e l a s t i c a l l y s c a t t e r e d e l e c t r o n s , b u t many experiments have been conducted t o study t h e i n e l a s t i c a l l y s c a t t e r e d as w e l l as t h e secondary e l e c t r o n s . The energy a n a l y s i s o f t h e e m i t t e d e l e c t r o n s i s t h e main o b j e c t i v e i n t h i s l a t t e r category. Among these, h i g h resol u t i o n e l e c t r o n energy l o s s (HREELS) (173), and Auger e l e c t r o n (AES) spectroscopi c methods a r e predominant 1.4.2.2.1.

.

_High R e s o l u t i o n E l e c t r o n Energy Loss Spectroscopy (HREELS)

The HREELS technique i s used t o examine t h e v i b r a t i o n a l modes of adsorbed molecules on f l a t surfaces. I n t h i s case, a monoenergetic e l e c t r o n beam (usual Y 2-5 eV) i s impinged on t h e s u r f a c e and t h e s p e c u l a r l y r e f l e c t e d e l e c t r o n beam i s e n e r g e t i c a l l y analyzed. The e l e c t r o n s may i n t e r a c t w i t h t h e d i p o l e f i e l d o f t h e v i b r a t i n g d i p o l e on t h e adsorbed molecules, a c h i e v i n g v i b r a t i o n a l t r a n s i t i o n . The energy o f t h e t r a n s i t i o n reduces t h e k i n e t i c energy of t h e e l e c t r o n s

A5 1 t h e v i b r a t i o n a l spectrum r e s u l t s f r o m a p l o t o f t h e e l e c t r o n c u r r e n t a g a i n s t t h e energy l o s s . S e l e c t i o n r u l e s a r e t h o s e o f i n f r a r e d spectroscopy w i t h t h e a d d i t i o n t h a t v i b r a t i o n s p a r a l l e l t o the surface a r e i n a c t i v e . Resolutions f o r HREELS i n t h e o r d e r 3-10 meV a r e p o o r e r t h a n t h o s e o b t a i n e d by i n f r a r e d spect r o s c o p y , b u t t h e l a t t e r ' s s e n s i t i v i t y i s much h i g h e r , e.g.,

a b o u t one

thousandth o f a CO inonolayer can be d e t e c t e d on a s i n g l e c r y s t a l s u r f a c e . Ano t h e r i m p o r t a n t advantage o f t h i s t e c h n i q u e i s i t s c o m p a t i b i l i t y w i t h o t h e r u l t r a h i g h vacuum e l e c t r o n s p e c t r o s c o p i e s , v i z . , LEED, XPS and AES. A g e n e r a l o v e r v i e w o f t h e HREELS technique, as w e l l as i t s a p p l i c a t i o n t o model c a t a l y s t s i s g i v e n i n c h a p t e r 3, p a r t B. 1.4.2.2.2.

Auger E l e c t r o n Spectroscopy (AES)

The AES t e c h n i q u e i s concerned w i t h t h e s t u d y o f i n e l a s t i c a l l y s c a t t e r e d e l e c t r o n s . I t i s based on t h e a n a l y s i s o f k i n e t i c energy d i s t r i b u t i o n o f t h e e l e c t r o n s e j e c t e d f r o m s o l i d s by Auger t r a n s i t i o n s . The Auger t r a n s i t i o n f o r an i s o l a t e d i o n can be r e p r e s e n t e d by t h e e q u a t i o n : Ilt(i)-

M2'(j,k)

t e-

(1.10)

where Mt i s an i o n formed by t h e l o s s o f an e l e c t r o n f r o m t h e c o r e l e v e l i, as a r e s u l t o f e l e c t r o n bombardment w i t h e n e r g i e s i n t h e 1-2 keV energy range, o r from t h e X-ray r a d i a t i o n , as o c c u r s i n X-ray p h o t o e l e c t r o n spectroscopy (XPS). The Auger process occurs, when an e l e c t r o n f r o m a h i g h e r l e v e l j f a l l s i n t o t h e c o r e l e v e l i, and t h e energy r e l e a s e d s u f f i c e s t o e j e c t an e l e c t r o n f r o m a t h i r d l e v e l k, p l a c e d above i, b u t p o s s i b l y a t t h e same l e v e l as j. The l e v e l s i , j and k a r e such t h a t t h e o v e r a l l t r a n s i t i o n would r e l e a s e energy; t h i s appears an t h e k i n e t i c energy o f t h e e j e c t e d e l e c t r o n . The e l e c t r o n a c c o u n t a b l e f o r t h e Auger t r a n s i t i o n ( i j k ) , i s known as an Auger e l e c t r o n . The energy a n a l y s i s o f t h e s e e l e c t r o n s may r e s u l t i n t h e i r chemical i d e n t i f i c a t i o n , i n t h e same manner as a n a l y s i s o f e m i t t e d c h a r a c t e r i s t i c X-rays p r o v i d e s t h i s i n f o r m a t i o n . The usual n o t a t i o n designates t h e t r a n s i t i o n by t h e symbol o f t h e atom and t h e t r i p l e t s t a t e . The p r i n c i p a l quantum numbers 1,2,3

,...

a r e denoted b y K,L,M

,...,

and

t h e t o t a l a n g u l a r momentum of t h e e l e c t r o n j = 1 t s by t h e s u b s c r i p t s 1,2,3, ...

....

corresponding t o j = 1/2, 312, 5/2, Thus, a M M L ~ Lt ~r a n s i t i o n s t a r t s w i t h and 2p h o l e s i n M. an 1s h o l e i n t h e atom W and f i n i s h e s w i t h 2p 1/2 312 As f o r XPS, an irnportant f a c t o r t o be c o n s i d e r e d i s t h e escape d e p t h of t h e e l e c t r o n s f r o m t h e s o l i d . The i n c i d e n t e l e c t r o n beam p e n e t r a t e s t h e s o l i d t o a s i g n i f i c a n t depth. However, t h e Auger e l e c t r o n s produced i n subsurface l a y e r s

w i l l have a h i g h p r o b a b i l i t y o f s c a t t e r i n g i n e l a s t i c a l l y on t h e way o u t and t h e r e f o r e w i l l n o t be observed i n t h e Auger peak. The e l e c t r o n s r e s u l t i n g from t h e Auger process have escape depths v a r y i n g from 0.5 t o 1 nm

SO

t h a t AES i s

A52

surface-sensitive.

The c o n t r i b u t i o n o f t h e v a r i o u s subsurface l a y e r s decreases

e x p o n e n t i a l l y w i t h depth. So t h e s i g n a l i s s t r o n g l y surface-monolayer dependent. For q u a l i t a t i v e a n a l y s i s , AES i s q u i t e simple. However, i f one wishes q u a n t i t a t i v e a n a l y s i s , t h e r e l a t i o n s h i p o f t h e i n t e g r a t e d Auger peak t o a s p e c i f i c numb e r o f atoms o f t h e s o l i d r e q u i r e s s u i t a b l e c a l i b r a t i o n as w e l l as c a r e f u l cont r o l o f t h e e l e c t r o n energy analyzer. Moreover, w i t h these experimental r e q u i rements, i n f o r m a t i o n about t h e chemical s t r u c t u r e o f t h e atoms a t t h e s u r f a c e can be revealed. I n every case, t h e Auger e l e c t r o n s produce a v e r y weak impulse on a p l o t o f t h e i n t e n s i t y o f secondary e l e c t r o n s versus k i n e t i c energy. To d i s c r i m a t e among these components, t h e s i g n a l i s d i f f e r e n t i a t e d by r e l a t i v e l y simp l e e l e c t r o n i c procedures, and t h e a v a i l a b i l i t y o f l o c k - i n a m p l i f i e r s make t h i s a v i a b l e process. The sharpness o f t h e Auger peaks makes t h i s f e a s i b l e by measu2 2 rement o f d I/dK as a f u n c t i o n o f K, where I i s t h e c u r r e n t o f e l e c t r o n s and K i s t h e k i n e t i c energy. An Auger t r a n s i t i o n i n such a p l o t appears as a peak f o l l o w e d by a trough

trough; t h e v o l t a g e a t t h e i n f l e c t i o n p o i n t between peak and

i s t y p i c a l o f each element and i t s environment, and t h e peak-to-trough

energy i s taken as p r o p o r t i o n a l t o t h e s u r f a c e d e n s i t y o f t h a t element. The det e c t i o n o f Auger s i g n a l s as low as 0.1% o f t h e monolayer can be performed by t h a t device. A general overview o f t h e AES technique as w e l l as several i n t e r e s t i n g a p p l i c a t i o n s a r e g i v e n i n chapter 2.

1.4.2.3.

Out-going N e u t r a l s

The impact of e l e c t r o n s on adsorbed molecules can g i v e r i s e t o d e s o r p t i o n o f n e u t r a l fragments and i o n s . The desorbed n e u t r a l s can be i d e n t i f i e d , and q u a n t i f i e d t o p r o v i d e i n f o r m a t i o n about t h e adsorbed l a y e r . The method b a s i c a l l y i n v o l v e s two consecutive steps. F i r s t l y , t h e desorbed i o n s can be d i r e c t e d w i t h a p p r o p r i a t e e l e c t r i c f i e l d s t o t h e d e t e c t o r . Secondly, t h e desorbed n e u t r a l s from t h e s u r f a c e i n a l l d i r e c t i o n s can be i d e n t i f i e d by p r e v i o u s i o n i z a t i o n . The t y p i c a l r a t i o of c o l l e c t i o n e f f i c i e n c y f o r desorbed n e u t r a l s t o desorbed i o n s can be as low as

t o lom4. Since t h e s i g n a l o f desorbed i o n s i s small, t h e

s i g n a l o f desorbed n e u t r a l s i s t o o small t o be detected. I n a d d i t i o n t o t h i s , t h e secondary i o n i z i n g beam w i l l a l s o i o n i z e t h e r e s i d u a l gas components, which u s u a l l y a r e t h e same as t h e adsorbed species. I t i s , t h e r e f o r e , r e q u i r e d t o d e t e c t t h e small s i g n a l o f t h e desorbed component on t h e background component a t t h e same m/e v a l u e i n t h e mass spectrum. The r e s u l t i n g technique, known as e l e c t r o n induced n e u t r a l d e s o r p t i o n (EID), p r o v i d e s v e r y i m p o r t a n t i n f o r m a t i o n about t h e s u r f a c e monolayer. As occurs i n AES, t h e problem o f d e t e c t i n g these v e r y small s i g n a l s overimposed t o t h e h i g h background presents a f o r m i d a b l e t a s k . 1.4.2.4.

Out-going I o n s

E l e c t r o n bombardment o f atoms and molecules a t surfaces w i l l r a i s e them t o e x c i t e d and/or i o n i z e d s t a t e s . I n t h i s case, t h e mass r a t i o o f t h e e l e c t r o n s t o

A53

-I 5

9

W

e

+N

0

Energy (eV) F i g . 1.17. EIID i o n energy d i s t r i b u t i o n f o r CO; s p e c i e s f r o m a p o l y c r y s t a l l i n e rhodium r i b b o n exposed t o 1.6 L (po2/pCo = 2 ) a t 300 K. Readapted from ref.(174). atoms i s v e r y small and hence k i n e t i c energy exchanges a r e m i n i m a l . However, t h e e l e c t r o n i c t r a n s i t i o n s , i n v o l v i n g e n e r g i e s o f 0-50 eV, a r e expected t o o c c u r because of t h e i r h i g h

cross-sections. Therefore, electrons o f energies i n the

50-100 eV energy range w i l l cause v i r t u a l l y a l l p o s s i b l e e l e c t r o n i c t r a n s i t i o n s i n t h e adsorbed components. I f t h e adsorbed atoms o r m o l e c u l e s r i s e t o an e x c i t e d o r i o n i z e d s t a t e t h e y w i l l s t i l l remain bound t o t h e s u r f a c e , b u t i f t h e y r i s e t o a d i s s o c i a t i v e s t a t e , t h e n t h e fragments may l e a v e t h e s u r f a c e as i o n s . The r e s u l t i n g i o n s can be d i r e c t e d through a mass spectrometer f o r d e t e r m i n a t i o n o f t h e i r mass and number. Consequently, t h e t e c h n i q u e i n v o l v e s p r o b i n g t h e s u r f a c e w i t h an e l e c t r o n beam i n t h e energy range o f 100 eV and a n a l y z i n g t h e desorbed i o n s by mass spectrometry. The t e c h n i q u e i s known as e l e c t r o n - i n d u c e d i o n d e s o r p t i o n (EIID) o r e l e c t r o n - s t i m u l a t e d d e s o r p t i o n ( E S D ) . The i o n s i g n a l s d e t e c t e d by t h e mass spectrometer a r i s e o n l y from t h e outermost adsorbed l a y e r ; t h e t e c h n i q u e b e i n g s e n s i t i v e t o t h e adsorbed monol a y e r . A p a r t from t h i s advantage, t h e t e c h n i q u e has two m a j o r c o n s t r a i n t s . One i s t h a t no d i r e c t i n f o r m a t i o n i s o b t a i n e d about t h e s u b s t r a t e . The second i s t h a t n o t a l l adsorbed species g i v e r i s e t o E I I D s p e c t r a . There i s c o n s i d e r a b l e c o n t r a s t between t h e l a r g e s i g n a l s observed f o r some s t a t e s o f adsorbed CO and 02, and t h e absence o f e l e c t r o n - i n d u c e d i o n s i n adsorbed n i t r o g e n . However, t h e

t e c h n i q u e p r o v i d e s v a l u a b l e i n f o r m a t i o n about t h e a d s o r p t i o n and d e s o r p t i o n p r o cesses c o n f i n e d t o t h e s u r f a c e o f model c a t a l y s t s .

A54 The E I I D technique has been w i d e l y used t o study t h e n a t u r e o f adsorbed species on w e l l d e f i n e d surfaces, e.g.,

s i n g l e c r y s t a l s and f i l m s . I n a v e r y

n i c e p i e c e o f work Van Hieu and Craig (174) have shown t h a t CO and O2 adsorbates r e a c t a t ambient temperature on p o l y c r y s t a l l i n e Rh r i b b o n s t o y i e l d adsorbed C02. They observed electron-induced d e s o r p t i o n o f CO; t

i o n s . The i o n energy d i s -

t r i b u t i o n f o r C02 i o n s i s shown i n F i g . 1.17 f o r sample exposure of 1.6 L and p a r t i a l pressures Po2/Pco = 2. I t e x h i b i t s an unusual narrow i o n energy d i s t r i b u t i o n w i t h t h e FWHM v a l u e o f 1.6 eV and a peak energy around 2.5 eV. I t was p r e v i o u s l y observed t h a t t h e r e e x i s t s a c o r r e l a t i o n between t h e adsorbate b i n d i n g energy and t h e energy peak and FWHM a s s o c i a t e d w i t h t h e i o n energy d i s t r i b u t i o n o f e l e c t r o n i c a l l y desorbed species. For instance, f o r CO chemisorbed on N i ( 1 7 5 ) , W(176) and Pd(177), t h e t r o n impact. CO

t

Ot, 0- and Cot i o n s were found t o be r e l e a s e d by e l e c -

species was observed t o o r i g i n a t e from t h e most l o o s e l y bound

o f t h e a d s o r p t i o n s t a t e s . While being r e l e a s e d from such s t a t e s , t h e Cot i o n s showed a r a t h e r narrow energy d i s t r i b u t i o n . I n c o n t r a s t , 0' was observed t o be released from t h e s t a t e o f t h e l a r g e s t b i n d i n g energy, w i t h c o n s i d e r a b l y h i g h e r energy d i s t r i b u t i o n and a much h i g h e r peak v a l u e . Therefore, t h e narrow energy d i s t r i b u t i o n found by Van Hieu and C r a i g (174) f o r Cod i o n s i s due t o t h e v e r y weak b i n d i n g between t h e r e a c t i o n p r o d u c t C02(C0 t 02-,C02) I n a d d i t i o n , t h e i o n energy d i s t r i b u t i o n o f CO;

and Rh

atoms.

ions, measured a t i n c r e a s -

i n g sample temperature, e x h i b i t e d t h e same d i s t r i b u t i o n shape, b u t t h e CO;

ion

s i g n a l d r a s t i c a l l y increased. T h i s i s due t o t h e f a c t t h a t t h e h e a t i n g o f t h e sample p r i o r t o e l e c t r o n bombardment undoubtedly promoted t h e o x i d a t i o n r e a c t i o n o f CO by 02, thus i n c r e a s i n g t h e amount o f C02 formed, which then c o n t r i b u t e s t o t h e increase of t h e CO;

i o n s d e t e c t e d by t h e mass spectrometer.

A r e c e n t development o f t h e E I I D technique i n v o l v e s measurement o f angular v a r i a t i o n s i n t h e i o n d e s o r p t i o n (ESDIAD) , e l e c t r o n s t i m u l a t e d d e s o r p t i o n i o n angular d i s t r i b u t i o n . The angular d i s t r i b u t i o n o f e l e c t r o n induced i o n desorpt i o n may r e f l e c t t h e a n i s o t r o p y o f t h e f i n a l s t a t e o r t h e asymmetry o f t h e r e l a x a t i o n process. Arguments have a l s o been p u t f o r w a r d which r e l a t e t h e ESDIAD p a t t e r n t o t h e symmetry o f t h e a d s o r p t i o n s i t e i n t h e ground s t a t e , thus d i s playing directional properties. 1.4.3.

In-going Neutrals The techniques i n v o l v i n g n e u t r a l probes have a l i m i t e d use because o f t h e

r e l a t i v e l y g r e a t d i f f i c u l t y i n producing and c o n t r o l l i n g n e u t r a l p a r t i c l e beams. Among these, neutron i n e l a s t i c s c a t t e r i n g (NIS), atom beams (AB) and m o l e c u l a r beams (MB), have been a p p l i e d t o t h e s t u d y o f c a t a l y t i c surfaces.

A55

Fig. 1.18. Neutron i n e l a s t i c s c a t t e r i n g s p e c t r a obtained w i t h EUROCAT 6 w t % Pt/SiO2 c a t a l y s t a f t e r H2 a d s o r p t i o n a t 300 K and coverages o f 1.0 monolayer(a); and 0.3 monolayer ( b ) . Both s p e c t r a were recorded a t 100 K. Readapted from r e f . (185). 1.4.3.1.

Out-going Neutral s

1.4.3.1 .l. N e u t r o n - I n e l a s t i c S c a t t e r i n g (NIS) The N I S spectroscopy i s e s p e c i a l l y s u i t e d f o r t h e study o f adsorbed molec u l e s on surfaces (178-183). Due t o t h e l a r g e incoherent cross-section o f hydrogen atoms, t h e technique has been m o s t l y a p p l i e d t o t h e s t u d y o f hydrogen ads o r p t i o n on P t (178, 183), Pd(179, 183, 184) and Ni(181). An i n t e r e s t i n g example on t h e a p p l i c a t i o n o f N I S t o t h e a d s o r p t i o n o f H2 on t h e EUROCAT 6% Pt/Si02 c a t a l y s t has

r e c e n t l y been r e p o r t e d b y Renouprez and

J o b i c (185). The difference spectra a t coverages o f 0.3 and 1.0 monolayer ( F i g . 1.18) show peaks a t ca. 67 ( w i t h shoulders a t ca. 60 meV), 85, 125, 160 and 275 meV. I n agreement w i t h l i t e r a t u r e data f o r t h e a d s o r p t i o n o f H2 on Pd(100) (184) and t h e frequency v i b r a t i o n s c a l c u l a t e d by Nordlander e t a1

. (186)

f o r H-atoms

s i t t i n g i n t h e c e n t e r o f C4v s i t e s , t h e 67 and 85 meV peaks were assigned t o t h e p a r a l l e l and perpendicular v i b r a t i o n s a t t h e C4v o f Pt(100) faces. The shoulder a t 60 meV must be due t o v i b r a t i o n o f H-atoms with even h i g h e r c o o r d i n a t i o n than i n a C4v symmetry. To e x p l a i n t h i s , two i n t e r p r e t a t i o n s were advanced. The f i r s t assumes a t o t a l l y degenerate v i b r a t i o n l i k e t h a t o f H- atoms i n octahedral s i t e s (183), and t h e second considers an H-atom l o c a t e d i n a s i t e j u s t below t h e surface a t t h e c e n t e r o f a square-based pyramid a t t h e P t (100) f a c e (Scheme 1 ) . On t h e o t h e r hand, t h e assignment o f t h e 125 and 160 meV peaks t o t h e asymmetric s i t e i s commonly accepted (183, and symmetric s t r e t c h i n g o f H-atoms i n a C 3v 186, 187). The peak a t 85 meV has been e x p l a i n e d as due t o H-atoms i n a Cpv

A56 SCHEME 1. C o n f i g u r a t i o n o f P t S i t e s f o r Hydrogen Adsorption. Symmetry

C

cov

c2v

c3v

Subsurface

c4v

symmetry. The t h r e e expected v i b r a t i o n a l modes and t h e proposed frequencies agreed r a t h e r w e l l w i t h t h e N I S spectra: wagging mode a t 80 meV, symmetric s t r e t c h a t 120 meV and asymmetric s t r e t c h a t 160 meV. Since an equal N I S i n t e n s i t y should be expected f o r t h e t h r e e v i b r a t i o n s , t h e observed N I S i n t e n s i t y c o n t a i n s a l s o c o n t r i b u t i o n s from t h e C3v s i t e . F i n a l l y , t h e N I S peak a t ca. 275 meV was assigned t o a t e r m i n a l hydrogen (Cmv). From t h e above example i t i s c l e a r t h a t t h e c l a s s i c a l concept o f H-atoms s i t t i n g on t o p o f a P t atom t o form a monolayer is a rough approximation. The p i c t u r e revealed by NIS s p e c t r a i s more complex; t h e p r o p o r t i o n of t h e d i f f e r e n t s i t e s depends s t r o n g l y on c r y s t a l l o g r a p h y . Hence, t h e H / P t s t o i c h i o m e t r i e s above

1, commonly found when P t atoms a r e t i t r a t e d by H2 chemisorption, a r e p e r f e c t l y explained. The N I S technique has a1 so been used by W r i g h t e t a1 p r e f e r e n t i a l l o c a t i o n o f adsorbed Xe i n z e o l i t e rho. The

.

(182) t o reveal t h e

Dt -form o f t h e z e o l i t e

was exposed t o Xe gas, and approximately 6 Xe atoms were r e t a i n e d p e r u n i t c e l l . The neutron powder p r o f i l e s were recorded w i t h t h e sample a t 5K and 210K. A t 5K, a l l t h e Xe atoms were found t o be l o c a t e d a t t h e c e n t r e o f t h e octogonal prism. A t 210 K t h e degree o f occupancy o f t h i s s i t e decreased t o ca. 70% o f i t s former value, and a d d i t i o n a l s i t e s i n t h e supercage, j u s t o u t s i d e t h e octogonal prism, became p a r t i a l l y occupied. From these r e s u l t s , i t can be concluded t h a t , w i t h adequate neutron f l u x and v a r i a b l e temperature o f t h e sample, i n s i t u s t u d i e s o f t h e l o c a t i o n , movement and f r a g m e n t a t i o n o f molecules adsorbed i n z e o l i t e s might be possible, thereby g i v i n g g r e a t e r i n f o r m a t i o n on t h e dynamics o f c a t a l y t i c changes under r e a c t i o n c o n d i t i o n s . 1.4.3.1.2.

Molecular Beams (MB)

The experimental technique known as molecular beam (MB) spectrometry has become i n c r e a s i n g l y used i n t h e study o f t h e r a t e s and mechanisms o f s u r f a c e chemical r e a c t i o n s . Another a p p l i c a t i o n o f MB i s t h e n o n - r e a c t i v e i n e l a s t i c s c a t t e r i n g which r e v e a l s i m p o r t a n t i n f o r m a t i o n about t h e gas-sol i d energy exchange. The MB technique has been reviewed e x t e n s i v e l y (188-190).

It bassically

c o n s i s t s o f a w e l l - c o l l i m a t e d molecular beam i n t e r r u p t e d p e r i o d i c a l l y t o gene-

A57

L

1 Electrons: INS1 [Photons : IIR,PIX

1 \

I Neutrals: Sputtering 1

/

I Ions: ISS.RBS,SIMSI

F i g . 1.19. Sketch summarizing t h e general group o f t e c l i n i q u e s i n v o l v i n g i n - g o i n g i o n probes.IIR, i o n induced r a d i a t i o n ; P I X , p r o t o n induced X-rays; INS, i o n n e u t r a l i z a t i o n spectroscopy; ISS, i o n s c a t t e r i n g spectroscopy; RBS, R u t h e r f o r d b a c k s c a t t e r i n g spectroscopy; SIMS, secondary i o n mass spectroscopy. r a t e bem i n t e n s i t y modulation. T h i s process t i m e - t a g s t h e c r e a t i o n o f t h e i n p u t r e a c t a n t beam and p r o v i d e s a t i m e r e f e r e n c e f o r r e a c t i o n r a t e measurements. The modulated beam impinges upon t h e sample surface, which has been c l e a n e d and i s m a i n t a i n e d i n an u l t r a - h i g h vacuum environment, i n o r d e r t o ensure t h a t i t s c h a r a c t e r i s p r e s e r v e d t h r o u g h o u t t h e experiment. The modulated beam i n t e r a c t s w i t h t h e sample s u r f a c e and t h e s c a t t e r e d p r o d u c t s a r e d e t e c t e d by a mass spect r o m e t e r tuned t o t h e mass o f i n t e r e s t . A r e c e n t r e f i n e m e n t of t h i s t e c h n i q u e has been a p p l i e d by Sawin and M e r r i l l (191) i n a s t u d y o f t h e decomposition o f h y d r a z i n e on I r ( l l 1 ) . They demonstrated t h e power o f t h e F o u r i e r t r a n s f o r m t e c h n i q u e i n t h e a n a l y s i s o f MB data. F u r t h e r a n a l y s i s o f t h e F o u r i e r t r a n s f o r m method, as a p p l i e d t o modulated m o l e c u l a r beam s p e c t r o m e t r y ( 1 9 2 ) , l e d t o substant i a l improvements, which i n c r e a s e b o t h t h e accuracy and t h e g e n e r a l i t y o f t h e MB technique. 1.4.4.

In-going Ions I o n beams c o n s t i t u t e an i m p o r t a n t c l a s s o f s u r f a c e probes. As i n t h e case

o f photon o r e l e c t r o n probes, i o n beams can e m i t t h e f o u r t y p e s of p a r t i c l e s (photons, e l e c t r o n s , n e u t r a l s and i o n s ) ( F i g . 1.19). Among t h e s e , t h e most simp l e c o n f i g u r a t i o n i n v o l v e s t h e d e t e c t i o n o f e m i t t e d i o n s . When comparing i n going i o n t e c h n i q u e s w i t h those i n v o l v i n g i n - g o i n g photon o r e l e c t r o n t e c h n i ques, t h e m a j o r d i f f e r e n c e i s t h a t an i m p o r t a n t k i n e t i c energy exchange can o c c u r , because t h e masses o f t h e i o n p r o j e c t i l e s a r e comparable t o t h o s e o f t h e

A58 surface atoms. This energy exchange may cause i r r e v e r s i b l e s u r f a c e damage by s p u t t e r i n g . I t i s c r u c i a l , t h e r e f o r e , t o keep t h e k i n e t i c energy o f t h e i o n s a t a l e v e l a t which t h e s u r f a c e i s n o t destroyed b e f o r e t h e i n f o r m a t i o n i s obtained. 1.4.4.1. 1.4.4.1.1.

Out-going Photons Ion-Induced X-rays ( I I X R )

Ion-induced X-rays ( I I X R ) a n a l y s i s i s a r e l a t i v e l y simple technique t o apply i n an a c c e l e r a t o r l a b o r a t o r y , I t i s based on t h e e x c i t a t i o n of charact e r i t s t i c X-rays w i t h an i n c i d e n t i o n possessing g r e a t e r energy than t h e b i n d i n g energy o f core e l e c t r o n s i n t h e sample atoms. Protons (PIX) a r e used most f r e q u e n t l y (193), b u t h e a v i e r i o n s may have advantages i n some cases ( 5 9 ) . I n these cases, t h e r e l a t i v e l y weak I I X R s i g n a l s r u l e o u t t h e use o f d i s p e r s i v e a n a l y z e r s The i n s t r u m e n t a t i o n c o n s i s t s o f a cooled S i ( L i ) d e t e c t o r u s u a l l y i n i t s vacuum chamber and separated from t h e a c c e l e r a t o r by t h i n Be windows. Although t h e r e s o l u t i o n a t t a i n a b l e w i t h t h e S i ( L i ) d e t e c t o r i s c o n s i d e r a b l y h i g h e r than t h a t o f t h e e l e c t r o n spectrometers (more than two hudred), i t i s f r e q u e n t l y adequate f o r qua1 i t a t i v e a n a l y s i s . T h i s r e p r e s e n t s an i n t e r e s t i n g example o f t h e r e l a t i o n s h i p between s e n s i t i v i t y and r e s o l u t i o n . Ifone attempts t o use t h i s t y p e o f r e s o l u t i o n w i t h e l e c t r o n spectrometers, t h e i n c r e a s e i n background n o i s e would be dramatic. U n l i k e t h e b a c k s c a t t e r i n g , t h e energies o f t h e observed X-rays a r e charact e r i s t i c o n l y o f t h e elements p r e s e n t i n t h e samples and n o t o f t h e i r depth d i s t r i b u t i o n , T h i s f e a t u r e i s e x p l o i t e d t o p r o v i d e f o r elemental i d e n t i f i c a t i o n , e s p e c i a l l y i n cases where h i g h mass numbers cause d i f f i c u l t i e s u s i n g e l a s t i c i o n b a c k s c a t t e r i n g o r i n o t h e r cases t o i d e n t i f y l i g h t e r elements which may be masked by o t h e r masses i n o v e r l a p p i n g b a c k s c a t t e r i n g d i s t r i b u t i o n s . I t i s a l s o p o s s i b l e t o g e t q u a n t i t a t i v e i n f o r m a t i o n o f an element from

X-ray data. I n general, c o r r e c t i o n s a r e r e q u i r e d f o r decreasing c r o s s s e c t i o n and i n c r e a s i n g X-ray a b s o r p t i o n as a f u n c t i o n o f depth i n t h e s o l i d , except f o r cases o f elements deposited o n l y i n a t h i n l a y e r near t h e surface. 1.4.4.2.

Out-going E l e c t r o n s

It i s r a t h e r w e l l e s t a b l i s h e d t h a t i o n s impinging on a s u r f a c e cause e l e c -

t r o n emission. However, t h i s p a r t i c u l a r probe c o n f i g u r a t i o n has n o t been l a r g e l y e x p l o i t e d , m a i n l y because i t i s much e a s i e r t o produce secondary e l e c t r o n s by an i n c i d e n t e l e c t r o n beam. There e x i s t s another process, c a l l e d i o n n e u t r a l i z a t i o n (INS), c o n s i s t i n g b a s i c a l l y o f t h e n e u t r a l i z a t i o n o f l o w energy i o n s when approaching t h e s u r f a c e (194). The n e u t r a l i z a t i o n energy can be t r a n f e r r e d t o a s u r f a c e e l e c t r o n , and i f t h i s energy surpassesa t h r e s h o l d , t h e e l e c t r o n can be emitted. The a n a l y s i s o f t h e k i n e t i c energy o f t h e out-going e l e c t r o n s r e v e a l s

A59

i n f o r m a t i o n about t h e e l e c t r o n i c p r o p e r t i e s o f t h e s u r f a c e . The e x p e r i m e n t a l c o n f i g u r a t i o n o f I N S i s n o t simple, and t h e t e c h n i q u e has been a p p l i e d t o a l i m i t e d number of systems. F i n a l l y , i t i s i m p o r t a n t t o emphasize t h a t t h e p i o neer work of Hagstrum (194) on I N S was q u i t e s i g n i f i c a n t , b u t i t became o v e r shadowed by t h e advent, i n t h e e a r l y ~ O ' S , o f t h e newer s p e c t r o s c o p i c methods, r e g a r d i n g i n - g o i n g e l e c t r o n s ( s e e S e c t i o n 1 . 4 . 2 ) , as w e l l as t h e i n - g o i n g phot o n s (see S e c t i o n 1.4.1) capable o f s u p p l y i n g s i m i l a r i n f o r m a t i o n . 1.4.4.3.

Out-going N e u t r a l s

Most o f t h e d e s o r p t i o n p r o d u c t s f r o m ion-bombarded s u r f a c e s i n v o l v e neut r a l p a r t i c l e s . These p a r t i c l e s can be a n a l y z e d i n a s i m i l a r manner as i s done w i t h i o n s i n I S S , RBS and S I M S ( s e e S e c t i o n 1.4.5).

However, i n t h i s case ana-

l y s i s o f t h e desorbed n e u t r a l s i s much more d i f f i c u l t . I f one a t t e m p t s t o i o n i z e a small f r a c t i o n o f t h e n e u t r a l p a r t i c l e s l e a v i n g t h e s u r f a c e and t h e n analyzes t h e r e s u l t i n g i o n s , one w i l l encounter a s e r i o u s problem. T h e r e f o r e t h e s u r f a c e t e c h n i q u e s i n v o l v i n g i n - g o i n g i o n s and o u t - g o i n g n e u t r a l s a r e r e c e i v i n g scarce a t t e n t i o n . 1.4.5.

In-going Ions

1.4.5.1. 1.4.5.1.1.

Out-going I o n s I o n - S c a t t e r i n g Spectroscopy (1%)

T h i s t e c h n i q u e p r o v i d e s o n l y an atomic i d e n t i f i c a t i o n as t h e b i n a r y c o l l is i o n c a r r i e s no d i r e c t chemical t n f o r m a t i o n . I n p r i n c i p l e , a l l elements can be analysed from E1/Eo measurement, b u t i n p r a c t i c e elemental r e s o l u t i o n i s a lim i t i n g f a c t o r ( 1 9 5 ) . The a b s o l u t e and r e l a t i v e s e n s i t i v i t i e s of elements i n ISS depend on t h e s c a t t e r i n g c r o s s - s e c t i o n s f o r t h e i n c i d e n t i o n and on t h e i o n n e u t r a l i z a t i o n p r o b a b i l i t y . The s c a t t e r i n g c r o s s - s e c t i o n s i n c r e a s e w i t h i n c r e a s i n g p r o j e c t i l e mass and w i t h decreasing s c a t t e r i n g angle, b u t v a l u e s cannot be a c c u r a t e l y determined. The n e u t r a l ' i z a t i o n o f t h e i n c i d e n t i o n can a1 so o c c u r as i t approaches t h e s u r f a c e t o be analyzed, e.g.,

by Auger i o n n e u t r a l i z a t i o n ,

t h u s decreasing t h e d e t e c t e d s c a t t e r i n g i o n c u r r e n t . Using t h e ISS t e c h n i q u e one can g a t h e r i n f o r m a t i o n on t h e t y p e o f atoms exposed a t t h e o u t e r l a y e r o f c a t a l y s t s u r f a c e s and t h o s e which a r e p o s i t i o n e d i n t h e second o r deeper l a y e r s . S h e l e f e t a l . (196) were t h e f i r s t surface

tG

present

examination o f s e v e r a l s p i n e l s by ISS. The t e c h n i q u e has been promp-

t l y a p p l i e d by s e v e r a l r e s e a r c h e r s t o s t u d y t h e h i g h l y d i s p e r s e d components i n

inany supported c a t a l y s t s (197-203). Among these, molybdenum-based h y d r o t r e a t i n g c a t a l y s t s were e x t e n s i v e l y s t u d i e d . To i l l u s t r a t e t h i s , t h e I S S s p e c t r a of a commercial CoMo/A1203 c a t a l y s t reduced by hydrogen a t v a r i o u s temperatures were recorded

t

( 2 0 3 ) . I n t h i s case, t h e samples were bombarded w i t h He

ions, w i t h

an energy o f 1 keV and a t o t a l c u r r e n t o f 10 nA, a t normal i n c i d e n c e ; t h e i o n s

A60

0

.. I:

:.

..

a2

1

1

I

04

0.6

I

573

I

m

LC*a.

O8 E1/Eo14

I

I

I

973

Reduction Temperature (K)

F i g , 1.20. A ) I o n s c a t t e r i n g s p e c t r a o f CoMo/Al203 c a t a l y s t reduced by H a t 673 K ( a ) and 1073 K ( b ) . Dependence o f t h e r e d u c t i o n t e m p e r a t u r e o f c a t a f y s t on t h e (Mo/A1)ISS (B) and ( C O / M O ) ~ (~c ~) i n t e n s i t y r a t i o s . b a c k s c a t t e r e d i t a c o n i c a l s o l i d a n g l e o f 138" t o t h e p r i m a r y beam d i r e c t i o n b e i n g d e t e c t e d . F i g . 1.20A shows two t y p i c a l ISS s p e c t r a o f an i n d u s t r i a l CoMo/ A1203 c a t a l y s t reduced b y hydrogen a t 673

t

K. The d i s t r i b u t i o n o f He i o n s s c a t -

t e r e d b y t h e s u r f a c e atoms (0, A l , Co and Mo) i s g i v e n i n F i g . 1.20A. t i o n o f t h e energy r a t i o (E1/EO).

as a f u n c -

The ISS peak a r e a s have been e v a l u a t e d f o r t h e

d i f f e r e n t s u r f a c e atoms. From t h e s p e c t r a o f t h e sample s u b j e c t e d t o such ex-

A61

treme H2-petreatments i t r e s u l t s t h a t t h e Co i n t e n s i t y d i d n o t change essent i a l l y a f t e r H 2 - r e d u c t i o n , whereas a s t r o n g decrease o f Mo i n t e n s i t y was observed i n t h e c a t a l y s t reduced a t t h e h i g h e s t temperature. The i n t e n s i t i e s o f Mo r e l a t i v e t o A1 (Mo/A1)ISS,

and Co r e l a t i v e t o Mo ( C O / M O ) a~ r~e ~g i v e n i n F i g .

1.208 and C, r e s p e c t i v e l y , as a f u n c t i o n o f t h e r e d u c t i o n temperature.

I t should be n o t e d t h a t an i m p o r t a n t decrease o f t h e (Mo/A1)ISS r a t i o occurs a t temperatures above 673 K, whereas an o p p o s i t e t r e n d i s observed r e g a r d i n g t h e ( C O / M O ) r~a~t i~o . Such b e h a v i o u r i s n o t s u r p r i s i n g , because t h e (Co/ r a t i o s remained e s s e n t i a l l y unchanged. T h i s s u r p r i s i n g r e s u l t can be e x p l a i n e d by t h e s u p e r p o s i t i o n o f two a n t a g o n i s t i c phenomena. On t h e one hand, m e t a l l i c Co from t h e i n t e r a c t i n g Co-Mo phase p r o g r e s s i v e l y s i n t e r s . B u t , on t h e o t h e r hand, "new" m e t a l l i c Co i s formed by r e d u c t i o n o f t h e CoA1204 phase p r e s e n t a t t h e alumina i n t e r f a c e . T h i s l a t t e r phenomenon is much more marked a t h i g h e r r e d u c t i o n temperatures.

As r e v e a l e d b y chemical a n a l y s i s , t h e Mo c o n t e n t o f t h e c a t a l y s t s reduced a t t h e h i g h e s t temperatures d i d n o t change, i n d i c a t i n g t h a t no s u b l i m a t i o n o c c u r s f o r Mo upon h e a t i n g . T h i s f i n d i n g l e a d s t o t h e c o n c l u s i o n t h a t t h e l a r g e decrease o f t h e (Mo/A1)ISS

r a t i o observed a t r e d u c t i o n temperatures above 803 K

i s due t o t h e s t r o n g s i n t e r i n g o f t h e Moo2 s p e c i e s r e s u l t i n g from t h e r e d u c t i o n o f molybdena; t h e Moo2 b u l k - l i k e phase b e i n g t h e n much more d i f f i c u l t t o reduce. Many o t h e r examples o f r e l e v a n t a p p l i c a t i o n s t o t h e s t u d y o f s u r f a c e phenomena, e.g.,

a d s o r p t i o n . d e s o r p t i o n and phase s e g r e g a t i o n , as w e l l as c a t a l y s t s

c h a r a c t e r i z a t i o n have been r e c e n t l y reviewed b y H o r r e l l and Cocke ( 2 0 4 ) . I t i s i m p o r t a n t t o emphasize t h a t i n most cases t h e i n f o r m a t i o n p r o v i d e d by ISS cann o t be g a i n e d by o t h e r techniques, because o f i t s u n i q u e s u r f a c e s e n s i t i v i t y . However, t h e l a c k o f chemical i n f o r m a t i o n f r o m t h e ISS s p e c t r a and i t s semiquant i t a t i v e c h a r a c t e r i s an i m p o r t a n t shortcoming o f t h e t e c h n i q u e . TO overcome t h i s d i f f i c u l t y , I S S i s o f t e n used j o i n t l y w i t h XPS

and Raman spectroscopy. O f

p a r t i c u l a r i n t e r e s t i s t h e use of t h e XPS t e c h n i q u e a t a v e r y l o w t a k e - o f f a n g l e , which p r o v i d e s chemical i n f o r m a t i o n a b o u t t h e t o p 2-3 a t o m i c l a y e r s . 1.4.5.1.2.

R u t h e r f o r d B a c k s c a t t e r i n g (RBS)

The h i g h energy v e r s i o n o f i o n s c a t t e r i n g (RBS) uses a beam o f a c c e l e r a t e d low mass i o n s (H',

He+, He2+) w i t h an energy between 0.5 and 5 MeV (205-207).

Some o f t h e i o n s a r e b a c k s c a t t e r e d a f t e r c o l l i s i o n s w i t h t a r g e t atoms and a r e analyzed a t t h e d e t e c t o r . The s c a t t e r i n g process is a g a i n b i n a r y , however, a t t h i s energy t h e b a c k s c a t t e r i n g mechanism i s t r i g g e r e d f r o m t h e nucleus. T h i s means t h a t i o n s d i r e c t l y s t r i k i n g t h e nucleus a r e b a c k s c a t t e r e d , b u t most of t h e p r o j e c t i l e s s u f f e r small a n g l e s c a t t e r i n g , l o s e energy, t r a v e l on i n t o t h e l a t t i c e , and a f t e r a s e r i e s o f c o l l i s i o n s can be e v e n t u a l l y b a c k s c a t t e r e d . As i t occurs i n ISS, d i f f e r e n t masses of n u c l e i , i . e . d i f f e r e n t elements, can t h u s

A62

0

xl

1.5

A

Fe

24

&

Energy (Me'

F i g . 1.21. Rutherford backscattering spectrum f o r a t h i n MoS2 sputter-deposited f i l m . Conditions: He' ions normally i n c i d e n t a t 3.0 MeV and scattered ions detected a t 135" by a s u r f a c e - b a r r i e r diode detector. Note t h e scale f a c t o r f o r other than Mo and Si peaks. The sample l a y e r c o n f i g u r a t i o n i s i n d i c a t e d a t t h e upper l e f t . Readapted from r e f . (209). be i d e n t i f i e d by the energy o f the backscattered ion. I n a d d i t i o n , RBS, allows t o determine t h e composition o f the t a r g e t (208, 209). The number o f backscattered and detected p a r t i c l e s i s r e l a t e d t o t h e number o f i n c i d e n t p a r t i c l e s , t o the d i f f e r e n t i a l Rutherford s c a t t e r i n g cross section, t o the acceptance angle o f the detector and, of course, t o t h e concentration o f t a r g e t atoms o f the respective elements. As an i l l u s t r a t i o n o f t h i s technique, F i g . 1.21 shows t h e RBS spectrum f o r 2 a t h i n (401~9cm- ) MoS2 sputter-deposited f i l m (209). Since t h i s a n a l y s i s i s confined t o a r e l a t i v e l y t h i n l a y e r , i t provides a good example o f t h e e f f e c t s o f the e l a s t i c c o l l i s i o n process i n t h e i d e n t i f i c a t i o n o f t h e elements present w i t h i n the f i l m . As t h e He2+ p r o j e c t i l e s are s c a t t e r e d i n t o a narrow angular range, they w i l l have an energy dependent o n l y upon t h e mass of the nucleus from which they were scattered. This determines t h e energy spectrum o f t h e scattered ions, i n which each peak corresponds to a r e l e v a n t element. Another important f a c t o r t o be considered i s t h e r o l e o f energy loss, which defines t h e depth a t which t h e p a r t i c u l a r s c a t t e r i n g event took place. T h i s energy l o s s i s due t o i n e l a s t i c c o l l i s i o n s o f He2+ ions with the atoms present i n t h e f i l m .

I n this

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case, t h e r e l a t i v e l y t h i n MoS2 l a y e r d i s p l a y s RBS peaks w i t h an energy w i d t h n o t s i g n i f i c a n t l y g r e a t e r t h a n t h e energy r e s o l u t i o n o f t h e d e t e c t o r . However, f o r t h i c k e r l a y e r s , both t h e incoming and s c a t t e r e d He2+ i o n s l o s e energy a c r o s s t h e d i s t a n c e between t h e s u r f a c e and t h e s c a t t e r i n g atom, causing t o a broad d i s t r i b u t i o n of energy. T h i s d i s t r i b u t i o n c o n t a i n s a l a r g e amount o f i n f o r m a t i o n on t h e energy process. Thus, i t i s p o s s i b l e t o q u a n t i f y t h e d e p t h a t which t h e s c a t t e r i n g process t o o k p l a c e i n terms o f t h e energy loss i n t h e s c a t t e r e d He2+ i o n s as compared t o t h o s e s c a t t e r e d a t t h e s u r f a c e , f o r which no i n e l a s t i c l o s s e s occurred. F o r l o w e n e r g e t i c He2+ ions, i.e.,

below 3MeV. t h e e l a s t i c c o l l i s i o n p r o 2+

cess i s due e n t i r e l y t o t h e e l e c t r o s t a t i c r e p u l s i v e f o r c e s between t h e He

i o n s and t h e nucleus and does n o t i n v o l v e an a c t u a l c o n t a c t between t h e p r o j e c t i l e and t h e t a r g e t atoms. The advantage o f t h e p u r e l y e l e c t r o s t a t i c s c a t t e r i n g i s t h a t i t s p r o b a b i l i t y depends e x c l u s i v e l y on t h e atomic number of t h e n u c l e u s and n o t on s p e c i f i c n u c l e a r p r o p e r t i e s . T h e r e f o r e , f o r a g i v e n element t h e c r o s s s e c t i o n o f s c a t t e r i n g can be c a l c u l a t e d q u i t e s i m p l y . I t s h o u l d be emphasized t h a t a Z 2 dependence o f t h e c r o s s s e c t i o n s makes t h e process v e r y s e n s i t i v e f o r h e a v i e r elements , b u t p r o p o r t i o n a t e l y l e s s f o r l i g h t e r elements. T h i s e f f e c t i s observed i n F i g . 1.21 where t h e Au-peak i n d i c a t e s t h e presence of o n l y a b o u t 0.04% Au compared t o about 50% f o r t h e 0-peak. I n t h i s l a t t e r case, t h e s e n s i t i v i t y i s lowered b y t h e s u p e r p o s i t i o n o f t h e 0-peak f r o m t h e t h i n f i l m w i t h t h e broad d i s t r i b u t i o n f r o m t h e t h i c k u n d e r l y i n g s i l i c o n s u b s t r a t e . Note a l s o t h a t t h e s i l i c o n s u b s t r a t e was p r e v e n t e d f r o m i n t e r f e r i n g w i t h o t h e r elements by p l a c i n g a g r a p h i t e l a y e r on i t s s u r f a c e p r i o r t o d e p o s i t i n g t h e t h i n MoS2 film.

1.4.5.1.3. Secondary I o n Mass Spectroscopy ( S I M S ) I n t h e SIMS t e c h n i q u e a beam o f e n e r g e t i c i o n s , i . e . ,

Ar

+

t y p i c a l l y i n the

2-20 keV range, s t r i k e s t h e sample surface, p e n e t r a t e s t h e subsurface l a y e r s

and l o s e s energy by i n e l a s t i c c o l l i s i o n s w i t h t h e atoms. The energy t r a n s f e r r e d f r o m t h e l a t t i c e causes p a r t i c l e e j e c t i o n f r o m t h e s u r f a c e . Most o f t h e s e w i l l be n e u t r a l s , b u t a small f r a c t i o n a r e p o s i t i v e o r n e g a t i v e i o n s , which can be e a s i l y analyzed by a quadrupole mass spectrometer. The SIMS techniques, as a p p l i e d t o s u r f a c e a n a l y s i s , have been t h e s u b j e c t o f s e v e r a l r e v i e w s ( 6 4 , 210213). A comparison o f t h e advantages and disadvantages o f S I M S i n r e s p e c t t o t h e o t h e r i o n s c a t t e r i n g t e c h n i q u e s i s shown i n Table 1.6. SIMS analyses a r e performed i n e i t h e r o f two d i s t i n c t modes, i . e . s t a t i c o r dynamic. I n t h e s t a t i c mode, t h e p r i m a r y c u r r e n t d e n s i t y i s s u f f i c i e n t l y low t o ensure a small s p u t t e r i n g r a t e as compared t o t h e d a t a a c q u i s i t i o n r a t e (214). I n t h i s case, t h e r e l a t i v e i n t e n s i t i e s of a l l t h e monomer a n d c l u s t e r i o n s a r e recorded. From t h i s i n f o r m a t i o n , a p a r t from elemental c o m p o s i t i o n , chemical

TABLE 1.6.

Comparison of the Advantages and Disadvantages of the Ion Scattering Technique I ss

RBS

Advantages

-

Disadvantages Scattering cross sections not well understood; only semiquantitative - i n t r i n s i c a l l y destructive moderate l a t e r a l resolution ( 0 . 1 mm) - poor mass resolution f o r heavier el ements

-

-

-

Accurately known Rutherford crosssections - a t 100-300 keV quantitative with submonolayer s e n s i t i v i t y - beam damage very small ~~

SIMS

-

Single binary c o l l i s i o n interaction probing depth r e s t r i c t e d t o mono1ayer 0.01-0.001 monolayer s e n s i t i v i t y isotope separation detection of a l l elements except H and He

- Extreme surface s e n s i t i v i t y ( -

-

-

mono1 ayer) detection of a l l elements and isotopes good l a t e r a l resolution (1 urn) simultaneous depth profiling in the dynaini c mode limited chemical information; from peak i n t e n s i t i e s

An accelerator i s required

- no d i r e c t chemical information

~~~~

poor l a t e r a l resolution (-1 m) ~

- I n t r i n s i c a l l y destructive - SIMS intensity strongly dependent on the environment, making quantitative analysis d i f f i c u l t

A65

s t a t e and s h o r t range s u r f a c e o r d e r d a t a can u s u a l l y be o b t a i n e d . F o r i n s t a n c e , f o r CO a d s o r p t i o n on N i , Hopster and B r u n d l e (215) found CO c o n t a i n i n g secondary i o n s i n SIMS s p e c t r a , i f CO was m o l e c u l a r l y adsorbed, whereas no such s p e c i e s were observed i n t h e case o f d i s s o c i a t i o n o f CO m o l e c u l e s . I n dynamic S I M S t h e s p u t t e r i n g r a t e i s k e p t h i g h , t h u s t h e i n f o r m a t i o n corresponds t o a p r o f i l e t h r o u g h o u t t h e s p u t t e r e d depth. Thus, t h e o b j e t i v e o f t h i s t y p e o f experiment

is t o o b t a i n an a t o m i c c o m p o s i t i o n depth p r o f i l e . An i m p o r t a n t p o i n t t o be cons i d e r e d h e r e is t h a t t h e secondary i o n y i e l d s a r e s t r o n g l y dependent on t h e chem i c a l s t a t e o f t h e elements a t t h e s u r f a c e . T h e r e f o r e , i f t h e chemical s t a t e o f a g i v e n element changes w i t h depth, t h e n i t s a t o m i c c o n c e n t r a t i o n p r o f i l e w i l l be d i s t o r t e d by changes o f t h e secondary i o n y i e l d s . S t a t i c SIMS nieasurements were used r e c e n t l y t o c h a r a c t e r i z e s u p p o r t e d metal o x i d e c a t a l y s t s (216, 217). The h i g h s u r f a c e s e n s i t i v i t y o f t h e s t a t i c SIMS t e c h n i q u e has been e x p l o i t e d by Rodrigo e t a l . (216) t o r e v e a l t h e changes i n t h e d i s p e r s i o n o f molybdena i n s i l i c a and alumina-supported molybdena c a t a l y s t s s u b j e c t e d t o c a l c i n a t i o n and w a t e r t r e a t m e n t s . They found a r e v e r s i b l e v a r i a t i o n o f t h e MOO'/MO+

r a t i o s on Mo/Si02 c a t a l y s t s as a f u n c t i o n o f t h e c a l c i n a t i o n -

h y d r a t i o n t r e a t m e n t s , w h i l e t h e s e r a t i o s remained e s s e n t i a l l y unchanged i n Ho/Al 203 p r e p a r a t i o n s . These r e s u l t s i n d i c a t e i m p o r t a n t d i f f e r e n c e s i n t h e nat u r e o f supported nlolybdena s p e c i e s s t a b i l i z e d by t h e s e s u p p o r t s . On Mo/A1203 c a t a l y s t s t h e s t r u c t u r e s o f supported molybdena s p e c i e s a r e s t a b l e , w h i l e t h e y undergo d r a s t i c m o d i f i c a t i o n s as a f u n c t i o n o f c a l c i n a t i o n - h y d r a t i o n t r e a t m e n t s on Mo/Si02 c a t a l y s t s . S t a t i c SIMS was a l s o used by Takahashi e t a l . (217) t o probe t h e l o c a l a t o m i c environment o f vanadium oxide-promoted r u t h e n i u m c a t a l y s t s used i n t h e Fischer-Tropsch r e a c t i o n , S I M S s p e c t r a f o r an

unpromoted and vanadium o x i d e -

promoted 10% Ru/A1203 c a t a l y s t a r e shown i n F i g . 1.22. The A l t and Rut s i g n a l s appear f o r b o t h c a t a l y s t s , w h i l e t h e V-promoted c a t a l y s t g i v e s t h e a d d i t i o n a l s i g n a l s o f Vt and VO

+

i o n s a r i s i n g from t h e promoter. I n a d d i t i o n , t h e RuO' s i g -

n a l is a l s o observed i n t h e S I M S spectrum o f V-promoted p r e p a r a t i o n . The absence o f t h e RuO' s i g n a l i n e i t h e r t h e unpromoted c a t a l y s t o r t h e mechanical m i x t u r e (Ru/A1203 t V205j was taken by Takahashi e t a l . (217) as c o n c l u s i v e t h a t t h e source o f t h e oxygen atom i n t h e RuO'

species must be t h e vanadium o x i d e , which

c o n t a c t s i n t i m a t e l y w i t h Ru metal p a r t i c l e s . T h i s model has been f u r t h e r supp o r t e d by t h e s t r o n g i n h i b i t i o n o f t h e a d s o r p t i o n o f CO on promoted vanadium o x i d e ruthenium c a t a l y s t s . Since t h e metal d i s p e r s i o n was e s s e n t i a l l y s i m i l a r f o r b o t h unpromoted and vanadium oxide-promoted Ru c a t a l y s t s , t h e marked decrease o f t h e a d s o r p t i o n s i t e s f o r CO must be a s c r i b e d t o blockage o f Ru s i t e s by p a r t i a l coverage of Ru p a r t i c l e s by VOx e n t i t i e s .

A66

1

1

40

1

1

80

1

1

120

1

mle

1

160

F i g . 1.22. SIMS spectra o f a prereduced unpromoted ( a ) and vanadium oxide-promoted ( b ) Ru/A1203 c a t a l y s t . Readapted from r e f . (217). 1.4.6.

In-going E l e c t r i c F i e l d s We t u r n now t o another cathegory o f s u r f a c e techniques based on t h e a p p l i -

c a t i o n o f e l e c t r i c f i e l d s , e s s e n t i a l l y d.c.

f i e l d s , as opposed t o electromagne-

t i c r a d i a t i o n . The techniques i n v o l v e d here a r e F i e l d Emission Microscopy (FEM), F i e l d I o n i z a t i o n Microscopy (FIM), and I n e l a s t i c E l e c t r o n Tunnel 1 i n g Spectroscopy (IETS). These techniques a r e b r i e f l y examined below. 1.4.6.1. 1.4.6.1.1.

Out-going E l e c t r o n s F i e l d Emission and F i e l d I o n i z a t i o n Microscopy

Two o f t h e most p r e c i s e s u r f a c e a n a l y s i s approaches a r e r e l a t e d t o t h e well-known FEM (218-221) and FIM (222-225) techniques. They b a s s i c a l l y i n v o l v e t h e a p p p l i c a t i o n o f h i g h e l e c t r i c f i e l d s t o a surface, so t h a t e l e c t r o n s can tunnel through t h e s u r f a c e work f u n c t i o n b a r r i e r . Because t h e tunnel1 i n g probab i l i t y depends on t h e e x a c t s t a t e o f t h e atom where t h e e l e c t r o n passes, t h e number o f these e l e c t r o n s w i l l vary from p o i n t t o p o i n t , as a f u n c t i o n of s u r f a c e topography v a r i a t i o n . Therefore, i n t e r e s t i n g i n f o r m a t i o n about s p a t i a l v a r i a t i o n s , which produce p i c t u r e s of t h e surface, can be o b t a i n e d w i t h r e s o l ut i o n s approaching t h e atomic scale. D e t a i l s on t h e p o t e n t i a l c o n f i g u r a t i o n of

A67

t h e atoms a t t h e s u r f a c e can a l s o b e r e v e a l e d b y measuring t h e number o f t u n n e l 1 ing e l e c t r o n s . The FEM t e c h n i q u e c o n s i s t s e s p e c i f i c a l l y o f t h e removal o f e l e c t r o n s t h r o u g h surface t u n n e l l i n g as a r e s u l t o f t h e a p p l i c a t i o n o f e x t e r n a l e l e c t r i c f i e l d s . The number of e l e c t r o n s t u n n e l l e d o u t a r e measured o r a c c e l e r a t e d t o a f l u o r e s c e n t screen t o p r o v i d e a p i c t u r e o f t h e s u r f a c e v a r i a t i o n s i n t h e work f u n c t i o n . The FIM t e c h n i q u e i s s l i g h l t y d i f f e r e n t . I n t h i s l a t t e r case, t h e f i e l d i s r e v e r s e d and t h e e l e c t r o n s a r e t u n n e l l e d o u t from gas atoms on t h e s u r face, t h e r e b y p r o d u c i n g p o s i t i v e i o n s . These i o n s a r e t h e n a c c e l e r a t e d t o a f l u o r e s c e n t screen t o p r o v i d e t h e same p i c t u r e s as i n

FEM. The most i m p o r t a n t d i s a d -

vantage o f b o t h t e c h n i q u e s i s t h a t v e r y h i g h e l e c t r i c f i e l d s ( > 10' V cm-')

are

r e q u i r e d t o produce measurable e l e c t r o n o r i o n c u r r e n t s , t h u s b e i n g u n p r a c t i c a l w i t h f l a t s u r f a c e s . T h i s problem has been p a r t i a l l y overcome by t h e use o f ext r e m e l y narrow needle-shaped e m i t t e r s , w i t h p o i n t r a d i i i n t h e neighborhood o f a few t e n t h s nm. The techniques a r e , t h e r e f o r e , r e s t r i c t e d t o t h o s e m a t e r i a l s which can be prepared i n t h i s f o r m and dimensions. The p r e p a r a t i o n o f f i e l d e m i t t e r s i s u s u a l l y c a r r i e d o u t by e l e c t r o l y t i c e t c h i n g . Many d i f f e r e n t mater i a l s have been t e s t e d t o d a t e i n FIM. and t h e p r e p a r a t i o n t e c h n i q u e s f o r t h e s e have been p u b l i s h e d

(222, 223, 226).

One f u r t h e r s t e p was t a k e n w i t h t h e a d v e n t o f t h e atom probe f i e l d i o n m i croscope, which i s capable o f d e t e c t i n g s i n g l e atoms (227, 228).

T h i s develop-

ment t o o k advantage o f t h e unique c a p a b i l i t y o f FEM t o examine t h e specimen atom by atom u s i n g t h e process o f f i e l d e v a p o r a t i o n . T i m e - o f - f l i g h t (TOF) of t h e e f f l u e n t i o n s t h e n p r o v i d e s an m/e a n a l y s i s w i t h s i n g l e i o n s e n s i t i v i t y . There a r e , however, s e r i o u s l i m i t a t i o n s i n a p p l y i n g t h i s t e c h n i q u e t o t h e s t u d y o f g a s - s o l i d i n t e r a c t i o n s . For i n s t a n c e , atoms and molecules o f t e n cannot b e f i e l d desorbed w i t h o u t f i e l d e v a p o r a t i o n o f t h e s u b s t r a t e atoms o c c u r r i n g a t t h e same t i m e . T h i s l a t t e r problem has been l a r g e l y overcome by t h e i n c o r p o r a t i o n o f t h e p u l s e d - l a s e r t e c h n i q u e i n t o t h e TOF atom probe (228, 229). I n p u l s e d - l a s e r f i e l d d e s o r p t i o n , a t o m and molecules a r e e i t h e r t h e r m a l l y desorbed f i r s t and subseq u e n t l y f i e l d i o n i z e d , o r t h e y a r e s i m p l y i o n i z e d by t h e r m a l l y enhanced f i e l d d e s o r p t i o n . The t r u e desorbed s p e c i e s and t h o s e species r e s u l t i n g from f i e l d d i s s o c i a t i o n can be d i s t i n g u i s h e d by t h e i o n energy d i s t r i b u t i o n . I n general, n e i t h e r FEM n o r F I M found w i d e a p p l i c a t i o n . T h i s i s due t o t h e f o l l o w i n g reasons. F i r s t l y , t h e t y p e s o f s u b s t r a t e q u a l i f y i n g f o r c o n s i d e r a t i o n a r e l i m i t e d . Secondly, one must e x t r a p o l a t e i n f o r m a t i o n f r o m v e r y f i n e needleshaped e m i t t e r s t o t h e p o s s i b l e b e h a v i o u r o f l a r g e and r e l a t i v e l y f l a t s u r f a c e s . F i n a l l y , t h e t i p s used a r e d e l i c a t e , and experiments must b e conducted i n a c a r e f u l l y c o n t r o l l e d UHV environment. T h i s means t h a t , a l t h o u g h t h e systems a r e r e l a t i v e l y s i m p l e , t h e y r e q u i r e optimum UHV c o n d i t i o n s . The above f a c t o r s are, t h e r e f o r e , r e s p o n s i b l e f o r t h e 1 i m i t e d use o f t h e s e t e c h n i q u e s .

A68

u

0

1

2

3

4

v (Volts)

5

F i g . 1.23. I n f u s i o n i n e l a s t i c e l e c t r o n t u n n e l i n g s p e c t r a f o r d e u t e r a t e d f o r m i c a c i d . Spectrum a was o b t a i n e d f r o m an A1-Pt j u n c t i o n doped i n s i d e t h e vacuum system, and s p e c t r a b and c were o b t a i n e d by vapour i n f u s i o n o v e r s o l u t i o n s o f 500 and 10 ppm of DCOOD i n D20. Readapted f r o m r e f . ( 2 3 4 ) . 1.4.6.1.2.

I n e l a s t i c E l e c t r o n Tunnel 1 i n g Spectroscopy (IETS)

The IETS t e c h n i q u e i s a s o l i d s t a t e method f o r o b t a i n i n g v i b r a t i o n s p e c t r a o f adsorbed molecules (230-232). About one monolayer i s adsorbed on t h e o x i d e t u n n e l l i n g l a y e r on t h e i n s i d e o f a t u n n e l j u n c t i o n i n t h e t y p i c a l f o r m o f A1-A1 oxide-mol e c u l a r 1ayer-Pb. The c u r r e n t - v o l t a g e c h a r a c t e r i s t i c s o f t h i s device, measured a t l i q u i d h e l i u m temperature, y i e l d s an IET spectrum d i r e c t l y r e l a t e d t o t h e v i b r a t i o n a l e n e r g i e s o f t h e molecule. I n c o n v e n t i o n a l IETS i t i s necess a r y t o d e p o s i t t h e m o l e c u l a r l a y e r as an i n t e r m e d i a t e s t e p i n t h e j u n c t i o n f a b r i c a t i o n sequence ( 2 3 3 ) . However, s m a l l metal p a r t i c l e s may be evaporated on t h e o x i d e l a y e r t o s i m u l a t e a s u p p o r t e d c a t a l y s t and t h e s e can be t h e a d s o r p t i o n s i t e s ( 2 3 2 ) . C h e m i s o r p t i o n u s u a l l y t a k e s p l a c e b e f o r e t h e second m e t a l ( e l e c t r o de) i s i n c o r p o r a t e d , a l t h o u g h t h e a d s o r b a t e can be i n t r o d u c e d i n t o t h e complete t u n n e l j u n c t i o n by an i n f u s i o n procedure ( 2 3 4 ) . To i l l u s t r a t e t h i s l a t t e r method, F i g . 1.23 shows t h e s p e c t r a o f d e u t e r a t e d f o r m i c a c i d (DCOOD) i n f u s e d i n t o t h e t u n n e l j u n c t i o n s ( 2 3 4 ) . Spectrum ( a ) was o b t a i n e d f r o m c o n v e n t i o n a l doping a d s o r p t i o n i n s i d e t h e vacuum system and i s compared w i t h spectrum ( b ) , o b t a i n e d when t h e j u n c t i o n was exposed t o t h e vapour o f a 5uO ppm s o l u t i o n o f DCOOD i n D20, and w i t h spectrum ( c ) , o b t a i n e d a f t e r

A69

exposure t o 10 ppm. The c h a r a c t e r i s t i c peaks a t ca. 269 and 114 mV a r e t h e CD s t r e t c h i n g and d e f o r m a t i o n modes, r e s p e c t i v e l y , w h i l e t h e b r o a d e r peaks a t 167 and 198 mV a r e CO v i b r a t i o n s , s i n c e t h e y a r e n o t s h i f t e d b y d e u t e r a t i o n . I n spectrum ( c ) , even though o n l y a 10 ppm s o l u t i o n was used, t h e CD peak i s s t i l l observed, i n d i c a t i n g a h i g h d e t e c t a b i l i t y f o r t h i s m o l e c u l e . A CH s t r e t c h i n g a t ca. 360 mV i s a l s o p r e s e n t due t o i m p u r i t i e s . 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 t h e OH peaks a r e v e r y weak f o r t h e s e s p e c t r a i n d i c a t i n g t h a t t h e r e s p e c t i v e s i t e s a r e p r o b a b l y taken up by t h e a d s o r p t i o n o f f o r m a t e groups. 1.4.7.

I n - g o i n g Heat

1.4.7.1.

Out-going N e u t r a l s

The thermal d e s o r p t i o n t e c h n i q u e (Temperature-programmed D e s o r p t i o n (TPD) and F l a s h F i l a m e n t ( F F ) ) i s w i d e l y used t o s t u d y s u r f a c e r e a c t i o n s , t h e energet i c o f s u r f a c e species and k i n e t i c s o f d e s o r p t i o n . The e a r l i e r a p p l i c a t i o n s o f TPD t o metal c a t a l y s t s were reviewed by Cvetanovic and Amenomiya (235), and a

more r e c e n t overview o f t h e work done s i n c e 1972 on TPD can be f o u n d i n t h e r e v i e w o f F a l c o n e r and Schwarz ( 2 3 6 ) . B r i e f l y , a gas i s adsorbed o n t o a c l e a n e d surface a t a g i v e n temperature and c o n t r o l l e d pressure, and t h e n a t e m p e r a t u r e programme, e.g.,

l i n e a r w i t h t i m e , i s a p p l i e d t o remove t h e adsorbate c o n t i -

nuously by pumping from t h e r e a c t i o n chamber. The r e s u l t i n g d e s o r p t i o n spectrum c o n s i s t s o f m o n i t o r i n g t h e gas p r e s s u r e o r a d e r i v e d p r o p e r t y , of a g i v e n spec i e s as a f u n c t i o n o f t i m e o r sample temperature. I n g e n e r a l , i t i s assumed t h a t t h e d e s o r p t i o n r a t e o f a s u r f a c e s p e c i e s f r o m a s i n g l e b i n d i n g s t a t e f o l l o w s an A r r h e n i u s - t y p e e q u a t i o n s : (1.11) where e i i s t h e coverage i n t h e adsorbed s t a t e i, A i s t h e p r e e x p o n e n t i a l f a c t o r , ni t h e

r e a c t i o n o r d e r f o r d e s o r p t i o n ( u s u a l l y 1 o r 2). and Ei r e p r e s e n t s

t h e a c t i v a t i o n energy f o r d e s o r p t i o n f r o m t h e s t a t e i. The values o f t h e a c t i v a t i o n energy f o r t h e d e s o r p t i o n (Ei)

and t h o s e o f

t h e r e a c t i o n o r d e r ni can be e s t i m a t e d f r o m t h e d e s o r p t i o n s p e c t r a . A good est i m a t e o f Ei v a l u e s can be made f r o m t h e peak temperature. Some i n f o r m a t i o n about t h e n a t u r e o f t h e a d s o r p t i o n o f d i a t o m i c (or p o l y a t o m i c ) molecules can be i n f e r r e d from t h e ni v a l u e . However, i f Ei and ni a r e coverage-dependent, t h e s e parameters cannot be used t o d e f i n e t h e adsorbed s t a t e . The d e s o r p t i o n s p e c t r a a r e u s u a l l y complex, o v e r l a p p i n g and broad peaks a r e a t t r i b u t e d t o more t h a n one b i n d i n g s t a t e , i n d i c a t i n g d i s t i n c t s u r f a c e adsorpt i o n s i t e s . I n t h i s case, t h e o v e r a l l d e s o r p t i o n process i s t h e summatory o f t h e i n d i v i d u a l processes. However, t h e appearance o f mu1 t i p l e thermal d e s o r p t i o n peaks, and/or i m p o r t a n t broadening can have a q u i t e d i f f e r e n t o r i g i n . O t h e r

A70

superimposed phenomena, may account f o r t h e complex d e s o r p t i o n s p e c t r a namely, domain growth, induced h e t e r o g e n e i t y by i n c r e a s i n g t h e number of adsorbed molecules, and t r a n s i t i o n s i n t h e s t r u c t u r e i n t h e adsorbed l a y e r as induced by l a t e r a l i n t e r a c t i o n s between adsorbed species. S t i l l considering these spureous e f f e c t s , owing t o t h e i r r e l a t i v e simp1 i -

c i t y and g r e a t s e n s i t i v i t y , thermal d e s o r p t i o n methods a r e commonly used t o i n v e s t i g a t e t h e g a s - s o l i d i n t e r f a c e . Other processes, such as i s o t o p i c exchange and s u r f a c e r e a c t i o n s may be monitored by thermal desorption

methods. The j o i n t

use o f thermal d e s o r p t i o n and o t h e r p h y s i c a l methods f o r fundamental s t u d i e s o f chemisorption and c a t a l y s i s on w e l l d e f i n e d surfaces, e.g.,

s i n g l e c r y s t a l s and

t h i n metal f i l m s , i s a powerful t o o l f o r t h e understanding o f surface processes a t an almost atomic scale. A p a r t o f these fundamental s t u d i e s , thermal desorpt i o n i s i n c r e a s i n g l y used t o c h a r a c t e r i z e l e s s d e f i n e d surfaces than adsorbents and porous c a t a l y s t s . An exhaustive t r e a t m e n t o f t h e TPD technique and i t s app l i c a t i o n s t o c h a r a c t e r i z e many c a t a l y t i c systems i s g i v e n i n chapter 6, p a r t B . 1.5. CONCLUSION I n t h i s i n t r o d u c t o r y chapter t o s u r f a c e spectrsocopic methods 1 i t t l e more can be done than p r o v i d e a c a t a l o g o f t h e r e l e v a n t techniques and s t r e s s those which i l l u s t r a t e t h e i r u n d e r l y i n g p h y s i c a l b a s i s . The author has chosen a c l a s i f i c a t i o n o f these methods based on t h e Propst diagram ( F i g . 1.1). T h i s approach may seem very simple and i n t u i t i v e , y e t i t accounts

f o r a l a r g e number o f

e x i s t i n g spectroscopic techniques, as t h e s o l e combination of an i n g o i n g arrow and an outgoing arrow may l e a d t o several

, quite

d i s s i m i l a r methods, depending

on what f e a t u r e o f t h e probe is made prominent t o measure what p r o p e r t y o f t h e emitted p a r t i c l e . Changes i n t h e chemical s t a t e o f t h e samples a r e expected t o take p l a c e when subjected t o a n a l y s i s . I n some cases, t h e use o f h i g h l y e n e r g e t i c probes may i n v o l v e i r r e v e r s i b l e sample damage, which can i n c l u d e minimal changes, such as t h e a l t e r a t i o n i n t h e o x i d a t i o n s t a t e o f t h e atoms. Nowadays, t h e improvement i n d e t e c t i o n and data a c q u i s i t i o n methods g i v e r i s e t o t h e hope t h a t t h e damage can be minimized by u s i n g lower dosage l e v e l s . While i t was deemed

i n a p p r o p r i a t e t o attempt a c r i t i c a l r e v i e w o f a l l t h e

techniques, i t i s , however, p e r t i n e n t t o e n q u i r e about t h e i r impact, p a s t and present i n t h e development o f c a t a l y t i c processes. From t h e p l e t h o r a o f methods t h e reader i s bound t o d e r i v e t h e common message: t h e r e does n o t e x i s t an i d e a l spectroscopy, t h e u n i v e r s a l s o l u t i o n t o a1 1 t h e problems. Several spectroscopic methods w i l l have t o be adequately combined, one complementary o f t h e other, t o remove a m b i g u i t i e s and r e f i n e i n t e r p r e t a t i o n s . As many of t h e methods presented i n t h i s chapter r e q u i r e v e r y w e l l - d e f i n e d surfaces, e.g.,

s i n g l e c r y s t a l s , one m i g h t q u e s t i o n t h e e x t e n t t o which such

A71 probes have i n c r e a s e d o u r u n d e r s t a n d i n g o f t h e c a t a l y t i c phenomena i n r e a l cat a l y s t s . It i s c l e a r t h a t c a t a l y s i s

i s c o n f i n e d t o t h e topmost l a y e r o f t h e

c a t a l y s t , so t h a t d e t a i l e d c h a r a c t e r i z a t i o n o f t h e s u r f a c e r e g i o n i n c l u d i n g s t r u c t u r e , s t o i c h i o m e t r y and s t a b i l i t y , as we1 1 as r e a c t i o n i n t e r m e d i a t e s ,become a m a t t e r o f prime importance. I n t h e l i g h t o f what has been s a i d i n t h i s chapter, i t i s l e g i t i m a t e t o conclude t h a t , even i f i n - d e p t h s t u d i e s o f t h e s e i d e a l i z e d systems do n o t i n m e d i a t e l y m a t e r i a l i z e i n t h e development o f more e f f i c i e n t conc e p t u a l models and guidance f o r f u t u r e a c t i v e research. LIST OF ACRONYMS AND DEFINITIONS

AAS

= Atomic A b s o r p t i o n Spectroscopy

ABS

= Atom Beam Spectroscopy

AEAPS

= Auger

AES

= Auger E l e c t r o n Spectroscopy

APS

= Appearance P o t e n t i a l Spectroscopy

BE

= B i n d i n g Energy

CIS

= C h a r a c t e r i s t i c Isochromat Spectroscopy

E l e c t r o n Appearance P o t e n t i a l Spectroscopy

DAPS

= Disappearance P o t e n t i a l Spectroscopy

DRS

= D i f f u s e R e f l e c t a n c e Spectroscopy

EELS

= E l e c t r o n Energy Loss Spectroscopy

EID

= E l e c t r o n Induced N e u t r a l D e s o r p t i o n

EIID

= E l e c t r o n Induced I o n D e s o r p t i o n

EPMA

= E l e c t r o n Probe M i c r o a n a l y s i s

ESD

= Electron Stimulated Desorption

ESDIAD = E l e c t r o n S t i m u l a t e d D e s o r p t i o n I o n Angular D i s t r i b u t i o n ESR

= E l e c t r o n S p i n Resonance

EXAFS

= Entended X-ray A b s o r p t i o n F i n e S t r u c t u r e

FEM

= F i e l d Emission Microscopy

FF

= Flash Filament

FIM

= F i e l d I o n i z a t i o n Microscopy

FTIR

= F o u r i e r Transform I n f r a r e d Spectroscopy

HEED

= High Energy E l e c t r o n D i f f r a c t i o n

HREELS = High R e s o l u t i o n E l e c t r o n Energy Loss SpeCtrOSCOpY IETS

= I n e l a s t i c E l e c t r o n Tunnel 1 i n g Spectroscopy

IEXR

= I o n Emission X-rays

IMFP

= I n e l a s t i c Mean Free Path

INS

= I o n N e u t r a l i z a t i o n Spectroscopy

IR

= I n f r a r e d Spectroscopy

IRR

= I n t e r n a l R e f l e c t i o n Raman

ISS

= I o n S c a t t e r i n g Spectroscopy

A72 LEED

= Low Energy E l e c t r o n D i f f r a c t i o n

LMMS

=

Laser Microprobe Mass Spectrometry Laser Raman Spectroscopy

LRS MAS

= Magnetic Angle Spinning

MBS

=

Molecular Beam Spectroscopy

MS

=

Mijssbauer Spectroscopy

NEXAFS

= Near Edge X-ray A b s o r p t i o n F i n e S t r u c t u r e

NMR

=

NS

= Neutron S c a t t e r i n g

PAS

= Photoacoustic Spectroscopy

PDBS

= Photothermal D e f l e c t i o n Beam Spectroscopy

PIXE

= Photon Induced X-ray Emission

RAIRS

= R e f l e c t i o n - A b s o r p t i o n I n f r a r e d Spectroscopy

RBS

= Rutherford B a c k s c a t t e r i n g Spectroscopy

RHEED

= R e f l e c t i o n High Energy E l e c t r o n D i f f r a c t i o n

SAM

= Scanning Auger Microscopy

SAS

= Small Angle S c a t t e r i n g

SEM

=

Scanning E l e c t r o n Microscopy

SERS

=

Surface Enhanced Raman Spectroscopy

SEW

= Surface Electromagnetic Waves

SEXAFS

= Surface Extended X-ray A b s o r p t i o n F i n e S t r u c t u r e

SIMS

= Secondary I o n s Mass Spectrometry

SRG

=

SXAPS

= Surface X-ray Apperance P o t e n t i a l Spectroscopy

Nuclear Magnetic Resonance

S t i m u l a t e d Raman Gain

TEM

= Transmission E l e c t r o n Microscopy

TPD

= Temperature-Programmed Desorption

UHV

=

U l t r a h i g h Vacuum U1 t r a v i o l e t P h o t o e l e c t r o n Spectroscopy

UPS

UV-Visible = U1 t r a v i o l e t - V i s i b l e Spectroscopy X-ray D i f f r a c t i o n

X RD

=

XRF

= X-ray Fluorescence

XPS

= X-ray Photoelectron Spectroscopy

REFERENCES G.A. Somorjai, i n "Proceedings 8 t h I n t e r n a t i o n a l Congress on C a t a l y s i s " , 1. B e r l i n , 1984, Verlag-Chemie, Weinheim 1984, Vol. I p. 113. I n F. Delannay, Ed., " C h a r a c t e r i z a t i o n of Heterogeneous C a t a l y s t s " , Marcel 2. Dekker, New York, 1984. M.W. Roberts, Pure U Appl. Chem., 53 (1981) 2269. 3. C.B. Duke, i n L.A. Casper and C.J. Powell, Eds. " I n d u s t r i a l A p p l i c a t i o n s of 4. Surface A n a l y s i s " , ACS Symposium S e r i e s 199, Am. Chem. SOC. Washington, 1982 p. 1. G.A. H a l l e r , Appl. Surf. S c i . , 20 (1985) 351. 5. 6. C.J. Powell, Appl. S u r f . Sci., 1 (1978) 143.

A 73

7. 8. 9. 13. 11.

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