X.P.S. Study of HY Zeolites: Characterization of Superficial

8 X.P.S.

S t u d y of

HY

Zeolites: C h a r a c t e r i z a t i o n of

Superficial C o m p o s i t i o n a n d

Acidity

Downloaded by CORNELL UNIV on August 27, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch008

C. DEFOSSE and B. DELMON Groupe de Physico-Chimie Minérale et de Catalyse, Université Catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium P. CANESSON Laboratoire de Chimie XI, U.E.R. Sciences Fondamentales et Appliquées, 40, Avenue du Recteur Pineau, F-86022 Poitiers, France ABSTRACT The superficial composition of germanium - and silicon-aluminum zeolites has been investigated by X.P.S. The aluminum concentration in the upper layers increases when the c r y s t a l l i nity decreases. Pyridine adsorption at 150°C on an HY zeolite induces a reduction of superficial s i l i c o n . Adsorption at room temperature allows a quantitative measurement of Brönsted and Lewis acid centers. Introduction The f i e l d of application of electron spectroscopy (called X.P.S. or E.S.C.A.) has broadened very quickly and i t s usefulness in catalysis has been conjectured very early (1J . Indeed, the high specificity for surface analysis achieved by this technique allows the qualitative and quantitative characterization of the outermost layers of solids as well as that of adsorbed species. Very recently, some studies have been published dealing with the application of X.P.S. to the study of zeolites (2-6). This communication concerns some applications of X.P.S. to the characterization of surface properties of zeolites. When catalytic phenomena are considered, diffusion effects may limit the actually active part of zeolites to the few f i r s t unit cells below the external surface of the grains, or, at least, give prominent importance to this external region. Since X.P.S. signals originate from the 1 5 - 3 0 outermost Angstroms, they may give information that is more narrowly related to the observed catalytic activity. Two topics w i l l be considered in the communication. F i r s t , the chemical composition of the superfic i a l layers for ordinary ( i . e . Si-Al) γ zeolites and for germa nium faujasite-type molecular sieves has been determined quan t i t a t i v e l y . Second, the superficial acidity of ordinary HY zeolites has been investigated by the adsorption of pyridine 86

Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

8.

DEFOSSE

ET

AL.

XPS

Study of ΗΎ

Zeolites

87

used as a p r o b e m o l e c u l e . The a d s o r p t i o n s t a t e o f t h e m o l e c u l e can be m o n i t o r e d by X.P.S. and a q u a l i t a t i v e d i s t i n c t i o n o f B r o n s t e d and L e w i s a c i d i t y can be e s t a b l i s h e d , q u i t e s i m i l a r t o what i s r o u t i n e l y r e a l i z e d u s i n g I.R. t e c h n i q u e s .


Experimental 1_. M a t e r i a l . The germanium f a u j a s i t e t y p e z e o l i t e has been s y n t h e t i z e d a c c o r d i n g to a procedure d e s c r i b e d elsewhere ( 7 ) . The b u l k A l / G e r a t i o i s e x a c t l y u n i t y . Sodium and ammonium ex changed (exchange l e v e l : 30%) s i e v e s have been s t u d i e d as w e l l as t h e ammonium f o r m a f t e r f r a m e w o r k c o l l a p s e s u b s e q u e n t t o h e a t i n g o v e r n i g h t i n a i r a t 180°C. Two s e r i e s o f s i l i c o n z e o l i t e s have been s t u d i e d . The f i r s t one ( U n i o n C a r b i d e l o t 3606-385) i s c h a r a c t e r i z e d by a b u l k S i / A l r a t i o o f 2.2. T h r e e d i f f e r e n t s a m p l e s have been i n v e s t i g a t e d : t h e o r i g i n a l Na Y , a 72% exchanged NH Υ , and t h e same NHLY. sample c a l c i n e d i n a i r a t 900°C. The p a r e n t NaY- z e o l i t e o f t h e s e c o n d s e r i e s has been s y n t h e t i z e d i n o u r l a b o r a t o r y ; i t s b u l k S i / A l r a t i o i s 2.4. 60% e x changed CaNaY and 90% exchanged NiNaY s a m p l e s have been p r e p a red from the o r i g i n a l N a Y The c r y s t a l l i n i t y o f b o t h s e r i e s has been m o n i t o r e d by X - r a y powder d i f f r a c t i o n . P y r i d i n e (Merck A.G.) i s d i s t i l l e d u n d e r vacuum. A f t e r t h o r o u g h o u t g a s s i n g by t h e f r e e z e - pump - thaw p r o c e d u r e , i t i s stored over a c t i v a t e d 5 A molecular sieve. 2

2 >

2_. P r o c e d u r e . A l l t h e X.P.S. s p e c t r a have been r e c o r d e d on a Vacuum G e n e r a t o r s ESCA 2 s y s t e m e q u i p e d w i t h a s i g n a l a v e r a g e r ( T r a c o r N o r t h e r n NS 5 6 0 ) . The A l anode was powered a t 10 KV and 50 mA. The sample was s p r i n k l e a on a d o u b l e - s i d e d a d h e s i v e t a p e . I n t h e e x p e r i m e n t s d e a l i n g w i t h p y r i d i n e a d s o r p t i o n a t room t e m p e r a t u r e (R.T.) on NHLY^ z e o l i t e s , t h e t r e a t m e n t and s a m p l i n g sequence a r e d e s c r i b e d e l s e w h e r e (J3) . Samples o b t a i n e d t h i s way a r e l a b e l l e d Py 300, Py 400, Py 500 and Py 600, d e p e n d i n g upon t h e i r a c t i v a t i o n température, e x p r e s s e d i n °C. E x p e r i m e n t s have a l s o been c a r r i e d o u t on an HY z e o l i t e a c t i v a t e d a t 300°C. I n t h i s c a s e , p y r i d i n e was a d s o r b e d and o u t g a s s e d a t 150°C. The c e l l was k e p t i n l i q u i d n i t r o g e n o v e r n i g h t and a g o l d b a c k i n g was used i n s t e a d o f s c o t c h t a p e . The S i 2p- o r Ge 2 p g / " l i n e been used a s r e f e r e n c e f o r d e t e r m i n i n g b i n d i n g e n e r g i e s and i n t e n s i t i e s . I n t h e c a s e o f s i l i c o n - a l u m i n u m t y e z e o l i t e s , i t has been p r o v e d e l s e w h e r e t h a t t h e c a r b o n c o n t a m i n a t i o n o v e r l a y e r does n o t p e r t u r b e t h e i n t e n s i t y r a t i o s id) s i n c e t h e k i n e t i c e n e r g i e s o f e l e c t r o n s i s s u e d f r o m t h e r e f e r e n c e l i n e ( S i 2p) and f r o m t h e i n v e s t i g a t e d one ( A l 2p o r Ν I s ) have s i m i l a r v a l u e s . F o r germanium-aluminum t y p e z e o l i t e s , t h e Ge 2 p „ l e v e l has been chosen as r e f e r e n c e i n t h e a b s e n c e o f any o t h e r s u i t a b l e l i n e ; i n t h i s c a s e , i t n a s

2

3 /


88

MOLECULAR

SIEVES—II

s h o u l d be n o t e d t h a t t h e c a r b o n c o n t a m i n a t i o n o v e r l a y e r can a r t i f i c i a l l y i n c r e a s e t h e A l 2p/Ge 2^/2 i n t e n s i t y r a t i o because o f t h e l a r g e d i f f e r e n c e i n t h e mean f r e e p a t h o f e l e c t r o n s i s s u e d f r o m t h e Ge 2 ρ β / and A l 2p l e v e l s . The i n t e n s i t i e s a r e g i v e n a s i n t e n s i t y r a t i o s n o t e d R equal to R y Ix where I x i s t h e p l a n i m e t e r e d i n t e n s i t y o f t h e r e f e r e n c e peak and Iy i s the i n t e n s i t y f o r t h e l i n e o f i n t e r e s t . I n f r a r e d measurements i n c l u d e a s i m i l a r t r e a t m e n t t h a n f o r X.P.S. a n a l y s i s bu£, i n t h i s c a s e , t h e z e o l i t e i s compacted i n t o a w a f e r o f 3 mg.cm d e n s i t y and p y r i d i n e o u t g a s s i n g i s a c h i e v e d a t i n c r e a s i n g t e m p e r a t u r e s , namely a t R.T., 100°C, 200°C and 300°C. S p e c t r a i n t h e 1750-1400 cm" r e g i o n have been r e c o r d e d u s i n g a Beckman I.R. 12 s p e c t r o m e t e r o r a P e r k i n E l m e r 180 s p e c t r o m e t e r , w o r k i n g i n a b s o r b a n c e mode. I n t e n s i t i e s a r e t h e p l a n i metered s u r f a c e a r e a s , n o r m a l i z e d f o r t h e wafer t h i c k n e s s ; t h e y a r e e x p r e s s e d i n a b s o r b a n c e u n i t s (A-U.) χ wavenumber (W.N.; -1 ·» η -1 cm J x CITK χ g Assignments are taken from the l i t e r a t u r e (9,10). 2

y


X

Results 1_. S u p e r f i c i a l c o m p o s i t i o n . a_. G e r m a n i u m _ z e o l i t e s . F o r a l l t h e samples s t u d i e d the d i f f e r e n c e i n b i n d i n g e n e r g y between t h e Ge 2p3/2 and A l 2p l e v e l s i s c o n s t a n t , showing t h a t t h e Ge and A l c a t i o n s a r e i n a s i m i l a r surrounding. F o r t h e v a r i o u s s a m p l e s , namely NaGeX, NaNH^GeX, and t h e h y d r o x y s o d a l i t e s t r u c t u r e o b t a i n e d by h e a t i n g t h e NaNH^GeX a t 180°C ( 1 1 ) , t h e A l 2p/Ge 2 p i n t e n s i t y r a t i o s , noted R ^ , are g i v e n i n Table I . 3 / 2

2

Si b_. S i l i c o n _ z e o l i t e s . Concerning the R 2p r a t i o s , t h e i n f l u e n c e o f t h e c a l c i n a t i o n t e m p e r a t u r e a s w e l l as t h a t o f t h e i o n e x c h a n g e have been i n v e s t i g a t e d on Y z e o l i t e s . The r e s u l t s f o r t h e two s e r i e s a r e g i v e n i n T a b l e I . A 1

TABLE I ,x i n t e n s i t y r a t i o s f o r t h e v a r i o u s samples Germanium Silicon zeolites " A l 2p Second s e r i e s zeolites F i r s t s e r i e s Y^ Sample Na R

.D193

NH .0257

NH 180°C

Na

.0268

.21

NH * 4

.20^.01

NH 900°C

Na

Ca

,43

.24

.27

Κ U n a c t i v a t e d sample and s a m p l e s a c t i v a t e d a t r a n g i n g f r o m 300°C t o 600°C.

temperatures


Ni .58

Y

2

8.

DEFOSSE

ET

XPS

AL.

Study of HY

89

Zeolites


2. O r g a n i c m o l e c u l e s a d s o r p t i o n . The b e h a v i o r o f p y r i d i n e a d s o r b e d on NH^Y z e o l i t e s o u t g a s s e d a t v a r i o u s t e m p e r a t u r e s has been i n v e s t i g a t e d . A l l t h e e x p e r i m e n t s have been made w i t h t h e NH^Y^ z e o l i t e series. a_. A d s o r g t i o n _ a t _ 1 50°C. F o r t h e sake o f c o m p a r i s o n w i t h some i n f r a r e d s t u d i e s ( 1 2 , 1 3 ) , p y r i d i n e a d s o r p t i o n has been c a r r i e d o u t a t 150°C. In t h i s c o n d i t i o n , an u n e x p e c t e d r e s u l t has been o b s e r v e d , as shown i n f i g u r e 1. A s e c o n d S i 2p l i n e a p p e a r s a f t e r a d s o r p t i o n a t 150°C ( f i g u r e 1-C). T h i s second l i n e i s n o t o b s e r v e d i n any o f t h e f o l l o w i n g s a m p l e s : ( i ) s i m p l y o u t g a s s e d z e o l i t e ( I ) ( c u r v e a ) , ( i i ) sample e x p o s e d t o p y r i d i n e a t room t e m p e r a t u r e (R.T.) ( I I ) , ( i i i ) p r e v i o u s sample I I f u r t h e r o u t g a s s e d a t R.T. ( I l l ) ( c u r v e b ) , ( i v ) sample I I s l o w l y h e a t e d f o r 1 hour a t 150°C (IV) ( c u r v e n o t shown, s i m i l a r t o b ) . S i m i l a r r e s u l t s a r e o b t a i n e d i f e t h y l e n e i s used i n s t e a d o f p y r i d i n e ; namely a second S i 2p l i n e i s o b s e r v e d a f t e r a d s o r p t i o n a t 150°C ( f i g u r e 1, c u r v e d ) . b. A d s o r g t i o n _ a t _ r o o m _ t e m p e r a The most r e m a r k a b l e feature i t h e X.P.S. s p e c t r a a f t e r R.T. a d s o r p t i o n i s a s h i f t o f -2 eV o f t h e Ν I s l i n e f r o m a d s o r b e d p y r i d i n e when t h e a c t i v a t i o n t e m p e r a t u r e o f t h e z e o l i t e e x c e e d s 400°C. We have e v a l u a t e d t h e r a t i o o f t h e number o f a d s o r b e d p y r i d i n e m o l e c u l e s t o t h e t o t a l number o f s i l i c o n atoms i n t h e n e a r - s u r f a c s l a y e r s . I n t e n s i t y v a l u e s , noted R j ^ and N^1 (2)' P° r e s p e c t i v e l y t o t h e low b i n d i n g e n e r g y l i n e and l o trie h i g h one. These v a l u e s , a f t e r c o r r e c t i n g f o r t h e r e m a i n i n g Ν I s i n t e n s i t i e s f o u n d i n t h e c o r r e s p o n d i n g b l a n k sam p l e s Î Q ) a r e summarized i n t a b l e I I . n

R

c

o

r

r

e

s

n

d

Table I I R^

,.. v a l u e s f o r t h e v a r i o u s Py

( Outgassing ( temperature

R

S i

S 1

Nls

( W i t h o u t any t r e a t m e n t ( ( ( (

300 400 500 600

samples.

.0 .02 .18 .11

(1)

R Nls K

(2)

) j

.33

)

.13 .13 .02 .0

) ) ) )


90

MOLECULAR

SIEVES—II

Discussion 1_. S u p e r f i c i a l c o m p o s i t i o n . The a t o m i c A l / x r a t i o s i n t h e s u p e r f i c i a l l a y e r s a n a l y z e d by X.P.S. c a n be c a l c u l a t e d f r o m t h e R r a t i o s g i v e n i n t a b l e I by t h e r e l a t i o n : —

= R


X

χ A l 2p

ix i A l 2p

where χ i s Ge o r S i and i x / i A l 2P i s t h e r e l a t i v e l i n e i n t e n s i t y r a t i o t a k e n f r o m t h e littérature (14,15) . T h i s above r e l a t i o n assumes t h a t mean f r e e p a t h o f e l e c t r o n s i n s i d e t h e s o l i d and t r a n s m i s s i o n f a c t o r o f t h e s p e c t r o m e t e r a r e t h e same f o r χ and A l 2p. These c o n d i t i o n s a r e v e r i f i e d f o r t h e S i '2ρ l i n e : t h e k i n e t i c e n e r g y o f e l e c t r o n s f o r t h i s l e v e l (about 1385 sV) i s v e r y c l o s e t o t h a t o f t h e A l 2p l i n e (about 1412 eV) . F o r t h e Ge 2 p ^ l e v e l , b e c a u s e o f t h e v e r y h i g h v a l u e o f t h e b i n d i n g e n e r g y , t h e r e l a t i o n w o u l d a r t i f i c i a l l y enhanced t h e A l / G e r a t i o . The r e s u l t s o b t a i n e d t h i s way a r e g i v e n i n t a b l e I I I . The s u r f a c e l a y e r s o f a l l w e l l c r y s t a l l i z e d z e o l i t e s , i n t h e i r i n i t i a l Na f o r m , a r e d e p l e t e d i n a l u m i n a . T h i s f a c t had a l r e a d y been o b s e r v e d by o t h e r a u t h o r s . (4) On t h e o t h e r hand, a d e s t r u c t i o n o f t h e s t r u c t u r e by c a l c i n a t i o n a t 900°C r e s u l t s i n a s u p e r f i c i a l s e g r e g a t i o n o f aluminum i n t h e c a s e o f Y s i l i c o n z e o l i t e s . T h i s i s d e m o n s t r a t e d by t h e f a c t t h a t t h e A l / S i r a t i o s f o r NH^ 900 a r e t w i c e t h o s e o b s e r v e d w i t h NaY. The b u l k A l / S i r a t i o i s s i t u a t e d between t h e s e two e x tremes. The s i t u a t i o n f o r germanium z e o l i t e s i s somewhat d i f f e r e n t . The c a l c i n a t i o n i n a i r a t 180°C o f t h e NH^ exchanged m a t e r i a l leads t o a s u r f a c e Al/Ge r a t i o s i m i l a r t o t h a t o f t h e b u l k . T h i s d i f f e r e n c e r e f l e c t s t h e f a c t t h a t , i n t h e case o f s i l i c o n z e o l i t e s , t h e c a l c i n a t i o n l e a d s t o an amorphous m a t e r i a l , w h e r e a s w i t h g e r manium o n e s , t h e t h e r m a l t r e a t m e n t a t 180°C b r i n g s a b o u t t h e transformation o f the f a u j a s i t e type s t r u c t u r e i n t o hydroxysodalite M l ) . As l o n g as t h e e x c h a n g e - d o e s n o t a f f e c t c r y s t a l l i n i t y , t h e s u p e r f i c i a l c o m p o s i t i o n r e m a i n s unchanged, a s o b s e r v e d f o r t h e NH^Y^ z e o l i t e . When i o n e x c h a n g e l e a d s t o a parti£l l o s s o f c r y s t a l l i n i t y , t h e s u p e r f i c i a l c o n c e n t r a t i o n i n aluminum i n c r e a s e s . T h i s i s i l l u s t r a t e d by t h e Ca e x c h a n g e d Y z e o l i t e , t h e X - r a y d i f f r a c t i o n p a t t e r n o f w h i c h i n d i c a t e s a l e s s c r y s t a l l i n e mate r i a l t h a n t h e s t a r t i n g N a Y z e o l i t e , and by t h e e x t e n s i v e N i exchanged o n e , where t h i s e f f e c t i s s t i l l more e n h a n c e d . I n b o t h c a s e s , one o b s e r v e s an i n c r e a s e i n t h e A l / S i r a t i o s i n t h e l a y e r s a n a l y z e d by X.P.S. The same t r e n d i s o b s e r v e d f o r germanium zeolites. 3

2

:

2

2



.88

: .66

4

( Surface ( Al/x ratio

NH

1

Na

1

:

C Bulk A l / x C ratio

[ Sample

.92

1

NH 180°C

Germanium z e o l i t e s

Calculated composition o f s u p e r f i c i a l

:

:

4

.33

.46

.33

.46

.67

.46

F i r s t s e r i e s Y. Na NH MH 900°C

Silicon

composition.

:

:

:

.37

.42

.42

.42

.90

.42

Second s e r i e s Y Na Ca Ni

zeolites

layers with respect to the bulk

TABLE I I I


)

)

) )

)

MOLECULAR

92

SIEVES—II


Thus, t h e p a r t i a l l o s s o f c r y s t a l l i n i t y by t h e e x c h a n g e p r o c e s s a f f e c t s t h e s u p e r f i c i a l c o m p o s i t i o n i n a way s i m i l a r t o a t h e r m a l t r e a t m e n t . The l o s s o f c r y s t a l l i n i t y r e s u l t s i n a p r e f e r e n t i a l l o s s o f aluminium t e t r a h e d r a i n t h e framework. These aluminum c a t i o n s a r e e x p e l l e d t o t h e e x t e r n a l s u r f a c e o f t h e crystallites. The above r e s u l t s s u g g e s t s t h a t t h e d e t e r m i n a t i o n o f t h e A l / S i r a t i o by X.P.S. c o u l d c o n s t i t u t e a v a l u a b l e method f o r evaluating c r y s t a l l i n i t y . 2. A d s o r p t i o n o f o r g a n i c m o l e c u l e s a_. Adsorgtion_at_150°C. The s e c o n d s i l i c o n l i n e s i t u a t e d a t a b i n d i n g e n e r g y l e v e l o f 98 eV c a n n o t be a t t r i b u t e d t o a s p e c i f i c c h e m i s o r p t i v e i n t e r a c t i o n between p y r i d i n e and s i l i c o n i n t h e f r a m e w o r k , s i n c e t h e same b e h a v i o r i s o b s e r v e d i n t h e c a s e o f a c o m p l e t e l y d i f f e r e n t a d s o r b e d m o l e c u l e , namely e t h y l e n e . The s e p a r a t i o n between t h e s i l i c o n l i n e s , a p p r o x i m a t e l y 4 eV ( f i g u r e 1, c and d ) , i s t o o l a r g e t o be a t t r i b u t e d t o any s p e c i f i c i n t e r a c t i o n between t h e a d s o r b e d m o l e c u l e s and s i l i c o n atoms. A s h i f t o f 4 eV i s c o m p a r a b l e n e a r t h a t o b s e r v e d between e l e m e n t a l s i l i c o n and S i 0 ( 1 6 ) . T h i s s h i f t i s n e i t h e r o b s e r v e d on t h e A l 2p l i n e n o r on t h e 01s o n e . We a r e t h e r e f o r e l e d t o c o n c l u d e t h a t , a s a c o n s e q u e n c e o f a d s o r p t i o n , some s i l i c o n i n t h e z e r o v a l e n c e s t a t e i s f o r m e d on t h e e x t e r n a l s u r f a c e o f t h e z e o l i t e . In s p i t e o f p o s s i b l e i n t e r f e r e n c e o f o t h e r c o n t a m i n a t i n g gases, c o n t r o l e x p e r i m e n t s i n d i c a t e t h a t t h e l i n e a t t r i b u t e d t o Si° d i s a p p e a r i n t h e p r e s e n c e o f t r a c e s o f 0^· I n a d d i t i o n , t h i s l i n e t e n d s t o d i s a p p e a r o v e r l o n g p e r i o d s o f x - r a y e x p o s u r e a t R.T., p r o b a b l y by a d i f f u s i o n i n t o t h e b u l k . N e v e r t h e l e s s , a p o s s i b l e r e o x i d a t i o n b e c a u s e o f t h e p o o r q u a l i t y o f t h e vacuum c a n n o t be e x c l u d e d . The c r i t i c a l f a c t o r f o r t h i s r e d u c t i o n o f S i t o Si° i s t h e a d s o r p t i o n t e m p e r a t u r e . The s p l i t t i n g i n t o two l i n e s o f t h e S i 2p s p e c t r u m i s n o t o b s e r v e d when t h e a d s o r p t i o n t a k e s p l a c e a t R.T. even a f t e r a s u b s e q u e n t s l o w o u t g a s s i n g a t 150°C. The r e d u c t i o n t o Si° a p p a r e n t l y c a n o n l y o c c u r a t t h e o u t e r m o s t s u r f a c e o f t h e c r y s t a l l i t e s s i n c e i t has n e v e r been o b s e r v e d by bulk techniques. This s u r p r i s i n g r e s u l t r a i s e s t h e question as to whether such a s u p e r f i c i a l r e d u c t i o n c o u l d modify s t r o n g l y c a t a l y t i c p r o p e r t i e s i n c e r t a i n working c o n d i t i o n s . 2

4 +

JD. A d s o r p t i o n _ a t _ r o o m _ t e m g e r a t u r . Since p y r i d i n e adsorption a t R.T. does n o t i n d u c e any r e d u c t i o n , X.P.S. c a n be used f o r s u p e r f i c i a l a c i d i t y d e t e r m i n a t i o n s , by m o n i t o r i n g t h e Ν I s l e v e l of t h e probe m o l e c u l e . T h e r e i s a s h i f t o f 2 eV o f t h e N1s l e v e l t o w a r d s l o w e r b i n d i n g e n e r g i e s between Py 400 and Py 500 s a m p l e s ( t a b l e I I ) . T h i s s h i f t must be a t t r i b u t e d t o d i f f e r e n c e s i n i n t e r a c t i o n s between t h e n i t r o g e n atom o f t h e p r o b e m o l e c u l e a n d , r e s p e c t i v e l y , a

I Katzer; Molecular Sieves—II ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


8.

DEFOSSE

XPS Study of HY Zeolites

E T AL.

(^Binding energy (eV)

la 154 ·> c m " „ ( a.u. w.n. c m m g "

Figure 1. Si 2p profiles for NH^ zeolite activated at 300°C. (a), with no pyridine adsorption; (b), upon pyridine adsorption at R.T.; (c), upon pyridine adsorption at 150°C; (d), upon ethylene adsorption at 150°C.

1

2

1

)

If

BRONSTED AC IDiTY 1

.1

^ 2 . Uïa 1619 c m " (a.u- W J I . c m

«Si R

NU2)

1

2

mg" ) 1

ι. k

Figure 2. Quantitative corrélation between X.P.S. R i and infrared absorption intensities for Bronsted and Lewis acid sites. Nls data collected at —90°C. I.R. wafer exposed to pyridine and outgassed at 20°C ( Β j , 200°C (Φ )and 300°C ( 0 λ N

R

Nls(0

8iZp

8


94

molecular sieves—π

Bronsted or a Lewis acid site since i t is well knoun that dehydroxylation of HY zeolites occurs between 400°C and 500°C [17,18) • Infrared measurements further support this conclusion. Figure 2 indicates a good correlation between the R|\| 1s (i) values characteristic respectively of Bronsted (i = 2) and Lewis (i = 1) sites and the intensity of the infrared bands at 1545 cm" (Bronsted sites) and 1619 cm (Lewis sites). The comparison cof N l s (2) i n i t i a l N H ^ zeolite and the Py samples snows that roughly one third of the near surface sites accessible to NH can be reached by pyridine molecules. From the stoichiometry, i t is possible to calculate the theoretical intensity ratios of the Ν Is and Si 2p lines for the NH^Y^ i n i t i a l zeolite (14). The theoretical intensity ratio is 0.68, and the experimental observed one is 0.33. This indicates that the surface NH^ content is about 1/2 that of the bulk. This suggests that the surface depletion in aluminum tetrahedra cor responds to a similar depletion in NH . The superficial layers of the crystallites are poorer in acidic centers (by a factor of about 1/2) than the bulk. This relation between Al and acidic center contents confirms that acidic centers are associated with the incomplete charge compensation of Al tetrahedra in the zeo l i t e or alumino-silicate frameworks. _>l

R

r

a

t

i

o

s

f

o

r

t

n

e


3

Acknowledgements. One of us (CD.) thanks the "Fonds National de la Recherche Scientifique" (Belgium) for an "Aspirant" fellowship. Niss M.L. Dubru and Mr. G. Decamp are gratefully acknow ledged for kindly supplying germanium zeolites and some silicon ones. Litterature Cited 1. Delgass W.N., Hughes T.R. and Fadley C . S . , Catalysis Rev., (1970), 4, 179. 2. Minachev Kh. M., Antoshin G . V . , Shpiro E.S. and Navruzov T . A . , Izv. Akad. Nauk. S.S.S.R., Ser. Khim., (1973), p. 2131. 3. Minachev Kh.M., Antoshin G . V . , Shpiro E.S. and Navruzov T . A . , Izv. Akad. Nauk. S.S.S.R., Ser. Kim., (1973), p. 2134. 4. Tempere J.F., DELAFOSSE D. and Contour J.P., Chem Phys. Letters, (1975), 33, 95. 5. Defossé C . , Friedman R.M. and Fripiat J.J., B u l l . Soc. Chim. France, (1975), p. 1513. 6. Defossé C. and Canesson P . , React. Kinet. Catal. Letters, (1975), 3, 161. 7. Lerot L., Poncelet G. and Fripiat J.J., Mat. Res. B u l l . , (1974), 9, 979.



defosse et al.

XPS Study of HY Zeolites

8. Defossé C. and Canesson P . , J. Chem. Soc., Faraday Trans. I, accepted for publication, in the press. 9. Ward J.W., J. Colloid Interf. Sci., (1968), 28, 269. 10. Pichat P . , Mathieu M.V. and Imelik B . , J. Chim. Phys., (1969), 66, 845. 11. Poncelet G. Personnal communication. 12. Jacobs P . Α . , Theng B.K.G. and Uytterhoeven J.B., J. Catalysis, (1972), 26, 191. 13. Bielanski A. and Datka J., Bull. Acad. Pol. Sci., Ser. Chim., (1974), XXII, 341. 14. Nefedov V.I., Sergushin N.P., Band I.M. and Trzhaskovskya N.B., J . Electron Spectrosc., (1973), 2, 383. 15. Nefedov V.I., Sergushin N . P . , Salyn Y . V . , Band I.M. and Trzhaskovskaya, J. Electron Spectrosc., (1975), 7, 175. 16. Hollinger G . , Jugnet Y . , Pertosa P. and Tran Minh Duc, Chem. Phys. Letters, (1975), 36, 441. 17. Uytterhoeven J.B., Christner L.G. and Hall W.K., J. Phys, Chem., (1967), 69, 2117. 18. Ward J.W., J. Catalysis, (1967), 9, 225.


X.P.S. Study of HY Zeolites: Characterization of Superficial

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