X-Ray Diffraction Study of Palladium Y Zeolite - Advances in

5

Downloaded by NORTH CAROLINA STATE UNIV on July 18, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch005

X-Ray Diffraction Study of Palladium Y Zeolite Location of Palladium Atoms before and after Hydrogen Reduction P. GALLEZOT and B. IMELIK Institut de Recherches sur la Catalyse, 69 Villeurbanne, France

The crystal structure of Pd Y zeolite was determined before and after hydrogen reduction at different temperatures. When the zeolite is evacuated at 600°C, Pd ions are mainly found to occupy SI' sites within the sodalite cages. Hydrogen adsorption at 25° C results in a complete withdrawal of Pd from SI' sites. This displacement out of cation sites is attributed to the reduction P d --> Pd(O) consistent with hydrogen volumetric measurements. Reduced palladium remains atomically dispersed inside the sodalite cages up to about 200° C. Between 200 and 300° C, Pd(O) atoms migrate toward the outer surface of the zeolite where they agglomerate into 20-A diameter crystallites. 12.5

2+

2+

2 +

Preparation and properties of highly dispersed metals on carrier continue to be important subjects of research in catalysis. In this respect, zeolites appear to be a unique material because reduction of initially dispersed cations may provide isolated metal atoms encaged in the porous framework or, at least, very finely divided metal catalysts. To achieve this, the reduction temperature of the cation and/or the metal vapor pressure must be as low as possible to prevent the migration of metal atoms to the outer surface of the zeolite where they agglomerate to form crystallites. The reduction of palladium in Y zeolite was undertaken because palladium best satisfies the requirements above. P d ions have a very high reduction potential (electromotive force +0.987 V) and Pd(II) compounds are known to be reduced to metal at room temperature by numerous reducing agents, for example, ethylene or carbon monoxide (i) while hydrogen produces an incandescent reduction of PdO. A

2 +

66 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

5.

GALLEZOT AND IMELIK

67

Palladium Y Zeolite

T h e c r y s t a l s t r u c t u r e a n a l y s i s of p a l l a d i u m - e x c h a n g e d zeolite allows t h e d e t e r m i n a t i o n of i n i t i a l c a t i o n positions i n t h e d e h y d r a t e d p o r o u s framework.

S i m i l a r studies after r e d u c t i o n b y h y d r o g e n at v a r i o u s t e m -

peratures s h o u l d p e r m i t t h e o b s e r v a t i o n of p a l l a d i u m r e m o v a l f r o m t h e c a t i o n sites a n d t h u s t h e e s t i m a t i o n of the r e d u c t i o n l e v e l .

Moreover, the

presence of m e t a l o n t h e e x t e r n a l surface is easily detected.

Hence, x-ray

d i f f r a c t i o n techniques s h o u l d give a g o o d p i c t u r e of h y d r o g e n r e d u c t i o n of


p a l l a d i u m i n Y zeolites. Experimental A L i n d e N a Y zeolite w i t h o u t b i n d e r was i o n exchanged i n a n a m m o n i a c a l s o l u t i o n of P d C l w h i c h p r o v i d e s exchangeable ( P d ( N H ) 4 ) cations. T h e s o l u t i o n was s t i r r e d at r o o m t e m p e r a t u r e for 24 h r a n d t h e n f i l t e r e d . T h e zeolite was w a s h e d w i t h a m m o n i a s o l u t i o n t o e l i m i n a t e C l ~ ions. The desired exchange l e v e l was r e a d i l y o b t a i n e d b y a l l o w i n g t h e zeolite t o e q u i l i b r a t e i n a s o l u t i o n where a s u i t a b l e a m o u n t of p a l l a d i u m has been i n t r o d u c e d . C h e m i c a l a n a l y s i s for p a l l a d i u m a n d s o d i u m s h o w e d t h e c o m p o s i t i o n of t h e c a l c i n e d s a m p l e t o be P d ^ . s N a i g . s H n . s A l s e S W ^ (10 w t % of P d ; p r o t o n c o n c e n t r a t i o n d e t e r m i n e d b y difference). 2

3

2 +

T h e r m a l t r e a t m e n t s were p e r f o r m e d i n q u a r t z vessels c o n n e c t e d t o a grease-free v a c u u m l i n e . S t a n d a r d t r e a t m e n t includes a n o v e r n i g h t c a l c i n a t i o n i n o x y g e n a t 6 0 0 ° C , f o l l o w e d b y a 6-hr e v a c u a t i o n (5 X 1 0 ~ t o r r ) at t h e same t e m p e r a t u r e , t h e s a m p l e b e i n g i n a s h a l l o w - b e d g e o m e t r y . T h i s last c o n d i t i o n a n d the p r e t r e a t m e n t i n o x y g e n atmosphere are essential t o a v o i d p a l l a d i u m i o n r e d u c t i o n b y a m m o n i a molecules d u r i n g t h e t h e r m a l r e m o v a l of w a t e r a n d a m m o n i a . T h e d e h y d r a t e d zeolite was r e d u c e d b y h y d r o g e n d r i e d o v e r a c t i v a t e d m o l e c u l a r sieves at 77 ° K . A 100-torr h y d r o g e n gas pressure was i n t r o d u c e d t h r o u g h a b r e a k s e a l i n t h e vessel w h i c h was t h e n h e a t e d at the r e q u i r e d t e m p e r a t u r e for 15 h r . T h e t r e a t e d s a m p l e was t r a n s f e r r e d i n t o a c o n n e c t e d L i n d e m a n n glass c a p i l l a r y w h i c h was t h e n sealed off for x - r a y i n v e s t i g a t i o n s . T h e s t a n d a r d t h e r m a l t r e a t m e n t was a p p l i e d t o a l l the i n v e s t i g a t e d samples. S a m p l e A was n o t r e d u c e d , w h i l e samples A H 25, A H 200, a n d A H 300 were h y d r o g e n r e d u c e d at 25, 200, a n d 3 0 0 ° C , r e s p e c t i v e l y . T h e c r y s t a l s t r u c t u r e s were determ i n e d f r o m p o w d e r d a t a a c c o r d i n g t o e x p e r i m e n t a l techniques a n d r e s o l u t i o n methods described p r e v i o u s l y (2,3). 6

T h e m e t h o d of S i m p s o n a n d S t e i n f i n k (4, 5) w h i c h uses l i q u i d s c a t t e r i n g f u n c t i o n s was e m p l o y e d t o t a k e i n t o a c c o u n t t h e u n l o c a t e d a t o m s , a s s u m i n g t h a t t h e y are u n i f o r m l y d i s t r i b u t e d t h r o u g h o u t a sphere. Atomic p a r a m e t e r s were refined w i t h 235 s t r u c t u r e factors c o r r e s p o n d i n g t o a l l r e flections w i t h h + k + I < 3 9 6 except the 111 line. A G u i n i e r - t y p e c a m e r a w i t h m o n o c h r o m a t i z e d C u K « i r a d i a t i o n was used because of i t s l o w b a c k g r o u n d diffusion, t o detect t h e b r o a d d i f f r a c t i o n lines of e x t e r n a l 2

2

2

In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

68

MOLECULAR SIEVES

p a l l a d i u m c r y s t a l l i t e s . T h e average size of t h e p a l l a d i u m c r y s t a l l i t e s was c a l c u l a t e d f r o m t h e i n t e g r a l w i d t h of the 200 l i n e u s i n g the Scherrer f o r m u l a . D e t e r m i n a t i o n of fixed or desorbed a m o u n t s of h y d r o g e n was c a r r i e d o u t b y v o l u m e t r i c measurements. Results F o r s a m p l e A , t h e electron d e n s i t y a p p e a r i n g o n S I ' sites (6) at x = y = z = 0.045 was a t t r i b u t e d w i t h o u t a n y a m b i g u i t y t o p a l l a d i u m ions because of t h e short S I ' - 0 ( 3 ) distance (2.0 A ) a n d the large a m o u n t of s c a t t e r i n g m a t t e r c o r r e s p o n d i n g t o a b o u t 10 P d . Sites S I a n d S H were assigned t o p a l l a d i u m a n d s o d i u m ions, r e s p e c t i v e l y , a l t h o u g h some N a m a y be m i x e d w i t h P d + i n the f o r m e r .


2 +

+

2

T h e r e s o l u t i o n of the A H 25 c r y s t a l s t r u c t u r e shows t h a t q u i t e a d i f ferent c a t i o n d i s t r i b u t i o n occurs. T h e s c a t t e r i n g m a t t e r o n SI'(0.045) sites has a l m o s t c o m p l e t e l y disappeared. R e f i n e m e n t of S I , S I ' ( 0 . 0 4 5 ) , S I I , a n d f r a m e w o r k a t o m i c p a r a m e t e r s gives 0.145 for t h e R i n d e x (R = s | F — KF \/H\F \). A l s o , m a n y discrepancies were observed a m o n g t h e first 20 observed (F ) a n d c a l c u l a t e d (F ) s t r u c t u r e factors, a n d t h e difference F o u r i e r m a p s h o w e d electron d e n s i t y peaks t h r o u g h o u t t h e sodalite cage, p a r t i c u l a r l y a t x = y = z = 0.08. A second refinement w i t h a d d i t i o n a l p a l l a d i u m o n SI'(0.08) sites produces a n R d r o p t o 0.105 a n d i n d i c a t e s t h a t f o u r P d o c c u p y these sites. A t t h i s stage s c a t t e r i n g l i q u i d f u n c t i o n s were i n t r o d u c e d ; m u l t i p l e t r i a l s s h o w e d t h a t t h e best agreement b e t w e e n t h e first 20 s t r u c t u r e factors was o b t a i n e d w h e n a b o u t 10 p a l l a d i u m a t o m s were d i s t r i b u t e d i n a sphere of 2 . 5 - A r a d i u s centered o n x = y = z = 0.125 c o r r e s p o n d i n g t o the center of t h e sodalite cage. T h e last refinement i n v o l v i n g t h e p r e v i o u s p a r a m e t e r s p l u s t h e o c c u p a n c y f a c t o r of t h e r a n d o m p a l l a d i u m sphere gives a steep decrease of the R i n d e x t o 0.084. However, the p o p u l a t i o n of the dispersed p a l l a d i u m a n d i t s s t a n d a r d error g i v e n b y t h e final refinement are u n r e l i a b l e . T h e m a i n m e a n i n g of t h i s result is t h a t p a r t of t h e p a l l a d i u m atoms w h i c h h a v e been s h i f t e d o u t of i n i t i a l S I ' c a t i o n sites s t i l l d o o c c u p y t h e s o d a l i t e cages, b u t t h e y are i n r a n d o m p o s i t i o n s i n s i d e t h e 2 . 5 - A r a d i u s sphere as f a r as x - r a y s c a t t e r i n g is c o n cerned. 0

C

0

Q

c

2+

T h e difference F o u r i e r m a p shows a p e a k at x = y = z = 0.17 w h i c h m a y c o r r e s p o n d t o w a t e r molecules. I n c l u d i n g these species i n t h e refinem e n t s l i g h t l y decreased R, b u t t h e r e l e v a n t p o p u l a t i o n was n o t s i g n i f i c a n t . S t r u c t u r e refinements of t h e A H 200 a n d A H 300 samples were c o n d u c t e d i n t h e same w a y . U n i t c e l l constants, final a t o m i c p a r a m e t e r s , a n d R indexes are g i v e n i n T a b l e I . (Observed a n d c a l c u l a t e d s t r u c t u r e f a c t o r s are a v a i l a b l e f r o m the authors.) I n t e r a t o m i c distances a n d angles are g i v e n i n T a b l e I I . E s t i m a t e d errors o n the p o p u l a t i o n a n d p o s i t i o n of t h e cations m a y i n some cases be g r e a t l y u n d e r e s t i m a t e d especially for a t o m s w i t h l o w o c c u p a n c y factors.


GALLEZOT A N D IMELIK

5.

Table I.

Atomic Coordinates, Temperature Factors (B, A ) , Occupancy Factors (O.F.), and Cation Site Population (P)° Sample Sample Sample Sample A AH 200 AH 300 AH 25 2

T_L X y


z B 01 x B 02

=

x z B 03

=

y

x z B 04

=

y

x = z B Pd(I) x = B O.F. P Pd(I') x = B O.F. P Pd(I') x = B O.F. P Na(II)

69

Palladium Y Zeolite

-y

y

z

y

=

y

=

z

y

=

z

0.1245(2) -0.0538 (2) 0.0344(2) 1.1(1)

0.1237 (2) -0.0520(2) 0.0358 (2) 1.2(1)

0.1236(2) -0.0525(2) 0.0358(2) 1-5(1)

0.1235 (2) -0.0529 (2) 0.0354(2) 1.1(1)

0.1070 (4) 1.7(4)

0.1063 (3) 1.4(3)

0.1058 (3) 1.4(3)

0.1063 (3) 2.5(3)

-0.0044 (5) 0.1366 (7) 4.9(5)

-0.0042 (4) 0.1433 (5) 2.9(3)

-0.0035 (3) 0.1424 (4) 3.1(3)

-0.0043 (4) 0.1416 (5) 3.5(3)

0.1819 (5) -0.0303 (7) 3.9(5)

0.1771 (4) -0.0317 (5) 2.5(4)

0.1782 (4) -0.0314 (5) 2.9(3)

0.1777 (3) -0.0322 (5) 2.1(4)

0.1748(5) 0.3164 (8) 5.4(5)

0.1750(4) 0.3181 (5) 2.8(3)

0.1746 (3) 0.3177 (5) 2.8(3)

0.1758 (3) 0.3191 (4) 0.7(3)

0.0 3.0 0.08 (1) 1.3(2)

0.0 3.0 0.11(1) 1.7(2)

0.0 3.0 0.11(1) 1-7(2)

0.0 3.0 0.12 (1) 1.9(2)

0.052(2) 3.0 0.047(6) 1.5(2)

0.056 (5) 3.0 0.025 (6) 0.8(2)

0.045 3.0 0.028 (6) 0.9(2)

0.078 (1) 3.0 0.125 (6) 4.0(2)

0.078 (1) 3.0 0.125 (6) 4.0(2)

0.078(2) 3.0 0.028 (6) 0.9(2)

0.0441 (3) 3.0 0.331 (6) 10.6(2)

0.2353 (7) 0.2352(6) 0.2373 (6) 0.2361 (6) x = y = z B 3.0 3.0 3.0 3.0 O.F. 0.59 (3) 0.56 (3) 0.53(3) 0.56 (3) P 19 (1) 18 (1) 18 (1) 17 (1) Pd(U) x = y = z 0.125 0.125 0.125 O.F. 0.03 (9) 0.32 (9) 0.29 (9) P 10 (3) 9(3) 1(3) 0.097 0.074 Final R 0.084 0.081 a, A 24.71 24.66 24.71 24.71 ° Estimated standard errors are given in parentheses. Pd(U) means palladium atoms dispersed within a 2.5-A radius sphere of which the center is at x = y = z = 0.125; the significance of the corresponding population is discussed in the text.


70

MOLECULAR SIEVES


Table II.

Interatomic Distances (A) and Bond Angles (degrees) Sample Sample Sample Sample A AH 25 AH 200 AH 800

T-O(l) T-0(2) T-0(3) T-0(4) M

1.620 (7) 1.579 (6) 1.720 (9) 1.626 (6) 1.636

1.664 (5) 1.614 (5) 1.634 (6) 1.644(5) 1.638

1.645 (5) 1.621 (5) 1.643(6) 1.640 (5) 1.637

1.638 (5) 1.615 (5) 1.640 (6) 1.658 (5) 1.638

0(l)-0(2) 0(l)-0(3) 0(l)-0(4) 0(2)-0(3) 0(2)-O(4) 0(3)-0(4) M

2.635 (15) 2.705 (16) 2.689 (8) 2.541 (13) 2.659 (13) 2.781 (20) 2.668

2.684 (11) 2.706(11) 2.685 (5) 2.668(9) 2.633 (9) 2.681 (13) 2.676

2.686 (10) 2.689 (11) 2.692(5) 2.642(9) 2.635 (9) 2.697 (12) 2.673

2.669 (11) 2.691 (11) 2.676 (4) 2.643 (9) 2.653 (8) 2.714 (12) 2.674

Pd(I)-0(3) Pd(I')-0(3) Pd(I')*-0(3) Na(II)-0(2) 0(3)-Pd(I)-0(3) 0(3)-Pd(I')-0(3) 0(3)-Pd(I')*-0(3)« 0(2)-Na(II)-0(2)

2.49 (2) 2.01 (2) 2.52(2) 87.1 (6) 114.8(3) 108.3(6)

2.66 (1) 2.19 (3) 2.72 (3) 2.40(1) 86.6(4) 112 (2) 84 (1) 107.4(6)

2.62 (1) 2.23 (7) 2.72 (2) 2.39 (1) 86.7(4) 107 (6) 83 (1) 107.7(6)

2.65 (1) 2.13 (2) 2.75 (7) 2.41 (1) 87.1 (3) 115 (2) 83 (3) 108.9(5)

° (I')* means the Si' site at x = y = z = 0.078 Discussion Dehydrated Zeolite (Sample A ) . T h e conspicuous feature w h i c h comes o u t of t h e r e s u l t s i s t h e great preference of p a l l a d i u m ions f o r SI'(0.045) sites. T h e s e ions are t i g h t l y b o n d e d t o three 0(3) o x y g e n atoms of t h e h e x a g o n a l p r i s m s i x - m e m b e r e d r i n g . T h e short P d - 0 d i s t a n c e (2.0 A ) suggests a p a r t i a l c o v a l e n c y of these bonds, a n d t h i s is c o r r o b o r a t e d b y a n i m p o r t a n t l e n g t h e n i n g of the T - 0 ( 3 ) b o n d (1.72 A ) w h i c h d i s t o r t s t h e t e t r a h e d r o n . I t m u s t be n o t i c e d t h a t a b r i d g i n g o x y g e n b e tween two P d w o u l d be b e y o n d t h e scope of t h e m e t h o d ; h o w e v e r , t h e 10.6 P d t e n d t o be t h e m o s t dispersed a m o n g t h e eight sodalite cages, a n d b r i d g e d p a l l a d i u m ions are u n l i k e l y t o occur. I n conclusion, p a l l a d i u m ions e x h i b i t b e h a v i o r s i m i l a r t o t h a t of C u (3) whereas N i + ions m a i n l y o c c u p y S I sites i n Y zeolites (7-8). 2 +

2 +

2 +

H y d r o g e n R e d u c t i o n at 25°C (Sample A H 25).

2

Adsorption at room

t e m p e r a t u r e of c a r e f u l l y d r i e d h y d r o g e n o n s a m p l e A produces considerable change of l i n e i n t e n s i t i e s , a n d s t r u c t u r e r e s u l t s show t h a t cations u n d e r g o a complete r e d i s t r i b u t i o n w i t h i n t h e s o d a l i t e cage. H e n c e , 1.5 P d + o u t of 10.6 are left o n SI'(0.045) sites, 4 P d o c c u p y SI'(0.078) sites, a n d t h e others n o longer o c c u p y a definite site i n the sodalite cage. T h e SI'(0.078) cations are b o n d e d t o 3 0(3) a t a b o u t a 2 . 6 8 - A distance, a n d p r o b a b l y t o 2

2 +


5.

71

Palladium Y Zeolite

GALLEZOT AND IMELIK

S H ' w a t e r molecules detected o n the difference F o u r i e r m a p .

Therefore,

t h e i r c o o r d i n a t i o n w o u l d be s i m i l a r to t h a t of the p a r t i a l l y h y d r a t e d N i ions observed o n S I ' ( 0 . 0 8 ) sites i n N i faujasite (9) a n d N i Y zeolite

2 +

(8).


M o r e o v e r , the sodalite cages c o n t a i n a b o u t five p a l l a d i u m s w h i c h do n o t o c c u p y c a t i o n sites, i.e., sites where cations are b o n d e d t o a l a r g e r n u m b e r of f r a m e w o r k oxygens t o b a l a n c e the n e t n e g a t i v e charges. Such p e c u l i a r positions of highest charge (sites S I , S I ' , S H ' , a n d S I I ) are a l w a y s f i l l e d w i t h cations u n t i l some r e s t r i c t i v e rules s u c h as t h a t s t a t e d b y M o r t i e r et al. (10) a p p l y . H o w e v e r , sodalite cage sites i n the A H 25 s a m p l e are f a r f r o m c a t i o n o v e r c r o w d i n g ; therefore, the present results are w e l l i n t e r p r e t e d if the u n l o c a t e d P d are assumed t o be r e d u c e d to P d ( 0 ) . Indeed, s u c h a t o m i c p a l l a d i u m atoms h a v e no electrostatic r e q u i r e m e n t s t o o c c u p y a c a t i o n site, a n d t h e y w i l l be r a n d o m l y d i s t r i b u t e d a l o n g the cage w a l l s as suggested b y t h e results o b t a i n e d w i t h l i q u i d s c a t t e r i n g f u n c t i o n s . T h e r e d u c t i o n scheme of P d t o P d ( 0 ) is s u p p o r t e d b y t h e f o l l o w i n g e x p e r i m e n t a l evidence, (a) H y d r o g e n a d s o r p t i o n o n s a m p l e A a t r o o m t e m p e r a t u r e i n a few m i n u t e s causes t h e P d r e d - b r o w n color t o disappear w h i l e a grey t i n t appears, (b) V o l u m e t r i c measurements show t h a t sample A consumes h y d r o g e n . A t e q u i l i b r i u m the h y d r o g e n u p t a k e at 25 a n d 3 0 0 ° C was 5.6 a n d 8.9 H per u n i t cell. T h e s e figures c o r r e s p o n d i n g t o 4 5 % a n d 7 1 % r e d u c t i o n r a t i o are i n excellent agreement w i t h t h e x - r a y results. F o r samples A H 25 a n d A H 300 i t a p p e a r e d t h a t 5 a n d 8.8 P d + left S I ' ( 0 . 0 4 5 ) sites, a n d therefore 5 a n d 8.8 P d o u t of a t o t a l c o n t e n t of 12.5 are supposed t o be r e d u c e d . T h u s , the r e d u c t i o n r a t i o s of these t w o samples d e d u c e d f r o m t h e x - r a y i n v e s t i g a t i o n are 4 0 % a n d 7 0 % , r e s p e c t i v e l y , (c) I n f r a r e d experiments p e r f o r m e d i n t h i s l a b o r a t o r y (11) w i t h t h e same s a m p l e show t h a t h y d r o g e n a d s o r p t i o n gives rise t o h y d r o x y l s t r e t c h i n g b a n d s at 3550 a n d 3650 c m . T h i s result proves t h a t p a l l a d i u m ions are i n v o l v e d i n a redox r e a c t i o n f o l l o w e d b y a p r o t o n c a p t u r e b y 0 ( 1 ) a n d 0 ( 3 ) f r a m e w o r k oxygens. M o r e o v e r , c a r b o n m o n o x i d e a d s o r p t i o n o n t h e s a m p l e gives rise t o t y p i c a l b a n d s of C O a d s o r b e d o n m e t a l . 2 +

2 +

2 +

2

2

2 +

- 1

X - r a y results a n d h y d r o g e n v o l u m e t r y show t h a t t h e r e d u c t i o n is i n complete a t r o o m t e m p e r a t u r e . T h i s l i m i t of r e d u c t i o n m a y be caused b y t h e presence of w a t e r molecules f o r m e d d u r i n g t h e r e d u c t i o n a n d b o n d e d t o the S I ' ( 0 . 0 7 8 ) cations. F i n a l l y t h e r e d u c t i o n process m a y be a t least p a r t i a l l y reversible u n d e r t h e f o l l o w i n g c o n d i t i o n s . T h e A H 25 s a m p l e was e v a c u a t e d to r e m o v e h y d r o g e n . A 2 0 0 - t o r r pressure of o x y g e n was t h e n established, a n d t h e s a m p l e was s l o w l y h e a t e d t o 500° C . T h e c r y s t a l s t r u c t u r e of t h e s a m p l e t r e a t e d i n t h i s w a y was n o t d e t e r m i n e d , b u t t h e set of l i n e intensities is s i m i l a r t o t h a t of t h e A s a m p l e . I t seems t h e n t h a t m o s t of t h e p a l l a d i u m a t o m s h a v e p r o b a b l y been o x i d i z e d a n d h a v e m o v e d t o w a r d the i n i t i a l S I ' ( 0 . 0 4 5 ) sites.


72

MOLECULAR SIEVES


H y d r o g e n R e d u c t i o n at 200 a n d 300°C ( S a m p l e A H 200 a n d A H 300). T h e d i s t r i b u t i o n of cations i n A H 200 is v e r y s i m i l a r to t h a t f o u n d w i t h A H 25, a n d t h u s t h e a m o u n t of r e d u c e d p a l l a d i u m r e m a i n s u n c h a n g e d . T h i s result is n o t s u r p r i s i n g because w a t e r molecules subsequent t o t h e r e d u c t i o n of t h e first five a t o m s are s t i l l b o n d e d t o the c a t i o n a t t h i s t e m p e r a t u r e , a n d p r e v e n t subsequent r e d u c t i o n as suggested above. Besides, v e r y w e a k a n d b r o a d d i f f r a c t i o n lines o b s e r v e d o n G u i n i e r p h o t o g r a p h s i n d i c a t e a b e g i n n i n g of P d ( 0 ) m i g r a t i o n t o w a r d the zeolite e x t e r n a l surface. T h e s i t u a t i o n is e n t i r e l y different for t h e A H 300 s a m p l e . Palladium ions a n d a t o m s are a l m o s t c o m p l e t e l y r e m o v e d f r o m t h e sodalite cage. O n l y 0.9 P d c a n be detected b o t h o n SI'(0.045) a n d SI'(0.078) sites t o gether w i t h 1.9 P d + inside h e x a g o n a l p r i s m s . S t r o n g d i f f r a c t i o n lines of p a l l a d i u m m e t a l are n o w observed o n t h e x - r a y p a t t e r n . T h e average c r y s t a l l i t e size c a l c u l a t e d f r o m t h e l i n e - b r o a d e n i n g a n a l y s i s is a b o u t 18 A . T h i s v a l u e is also f o u n d w i t h a n o t h e r s a m p l e r e d u c e d at 2 5 0 ° C a l t h o u g h i n t h i s case the a m o u n t of m e t a l o n t h e surface is o n e - t h i r d as large. 2 +

2

T h e results o b t a i n e d w i t h samples r e d u c e d at 200, 250, a n d 300° C t e n d t o p r o v e t h a t P d ( 0 ) a t o m s m i g r a t e d i r e c t l y f r o m t h e sodalite cages t o t h e e x t e r n a l surface w i t h o u t a g g l o m e r a t i o n i n t h e supercages. This behavior m a y be u n d e r s t o o d b y c o n s i d e r i n g t h a t the P d ( 0 ) a t o m of 1 . 3 7 - A r a d i u s needs a large a c t i v a t i o n energy t o pass t h r o u g h t h e 2 . 3 - A sodalite a p e r t u r e ; consequently, t h e a c t i v a t e d a t o m s h o u l d r a p i d l y cross t h e supercages w h i c h are w i d e l y open t o w a r d t h e e x t e r n a l surface. N a t u r e of P a l l a d i u m i n H y d r o g e n - R e d u c e d S a m p l e s . I n t h e a b o v e discussion, p a l l a d i u m ions were a s s u m e d t o be r e d u c e d f r o m P d to Pd(0). T h i s a s s u m p t i o n needs f u r t h e r discussion. F i r s t , t h e p a t t e r n of o u r results is w e l l e x p l a i n e d b y a s s u m i n g t h a t i s o l a t e d p a l l a d i u m a t o m s m a y exist i n t h e sodalite cages after r e d u c t i o n b y H u p t o a b o u t 2 0 0 ° C u n d e r o u r exp e r i m e n t a l c o n d i t i o n s . M o r e o v e r , t h i s a s s u m p t i o n is i n good agreement w i t h i n f r a r e d results o b t a i n e d for s i m i l a r samples. S o m e P d + ions m a y be also f o r m e d d u r i n g t h e r e d u c t i o n , b u t q u a n t i t a t i v e measurements show t h a t t h e a m o u n t of P d does n o t exceed 1 0 % of t h e r e d u c e d p a l l a d i u m (11). 2 +

2

+

H o w e v e r , t h e p a l l a d i u m r e d u c e d a t 2 5 ° C has u n u s u a l p r o p e r t i e s : n e i t h e r h y d r o g e n n o r o x y g e n is c h e m i s o r b e d b y t h i s species. T h e h y d r o gen u p t a k e seems t o be l i m i t e d to t h e a m o u n t needed for r e d u c t i o n . I f w e assume t h a t some species l i k e P d - H is f o r m e d , one c a n e x p l a i n t h e a b sence of h y d r o g e n c h e m i s o r p t i o n . H o w e v e r , t h i s does n o t agree w i t h v o l u m e t r i c measurements of h y d r o g e n u p t a k e , a n d m o r e o v e r , if s u c h species exist, o x y g e n c h e m i s o r p t i o n s h o u l d be possible. I t seems, therefore, as s u p p o r t e d b y the results o b t a i n e d for C O c h e m i s o r p t i o n (11), t h a t i s o l a t e d p a l l a d i u m a t o m s do n o t h a v e properties u s u a l l y f o u n d for metals, l i k e c h e m i s o r p t i o n of H a n d 0 . A s a m a t t e r of fact, t h e c h e m i s o r p t i o n m a y d e p e n d u p o n collective properties of b u l k metals. Therefore, since the m e t a l o r b i t a l s do n o t exist for i s o l a t e d atoms, h y d r o g e n c h e m i s o r p t i o n a n d 2

2


5.

GALLEZOT AND IMELIK

Palladium Y Zeolite

73

other properties relevant to metal no longer hold. Further investigations are necessary for better understanding of the nature and properties of reduced palladium encaged in the zeolite. Acknowledgments


The authors gratefully acknowledge A. Theolier for helpful technical assistance and A. Dalmon for performing hydrogen volumetric measurements. Literature Cited 1. 2. 3. 4. 5. 6. 7.

Phillips, F. C., Amer. Chem. J. (1894) 16, 255. Gallezot, P., Imelik, B., J. Chim. Phys. (1971) 68, 34. Gallezot, P., Ben Taarit, Y., Imelik, B., J. Catal. (1972) 26, 295. Simpson, H. D., Steinfinck, H., J. Amer. Chem. Soc. (1969) 91, 6225. Simpson, H. D., Steinfinck, H., Acta Crystallogr., Sect A (1970) 26, Smith, J. V., ADVAN. CHEM. SER., (1971) 101, 171. Gallezot, P., Ben Taarit, Y., Imelik, B., J. Catal. (1972)

26,

158.

481.

8. Gallezot, P., Imelik, B., J. Phys. Chem., in press. 9. Olson, D. H., J. Phys. Chem. (1968) 72, 4366.

Mortier, W. J., Bosmans, H. J., Uytterhoeven, J. B., J. Phys. Chem. (1972) 76, 650. 11. Naccache, C., Primet, M., Mathieu, M. V., ADVAN. CHEM. SER. (1973) 121, 266.

10.

RECEIVED

November 28, 1972.


X-Ray Diffraction Study of Palladium Y Zeolite - Advances in

Recommend Documents