Study of Hydrogen and Carbon Monoxide Interactions with Palladium

24

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Study of Hydrogen and Carbon Monoxide Interactions with Palladium-Y Zeolite by ESR and IR Spectroscopy C. NACCACHE, M. PRIMET, and M. V. MATHIEU Institut de Recherches sur la Catalyse, C.N.R.S., 39 Boulevard du 11 Novembre 1918, 69100, Villeurbanne, France

Oxidation states of palladium-loaded Y zeolites were measured by ESR and IR spectrometry. After treatment by oxygen at 500°C the Pd is almost in the Pd(II) form, and few Pd (1%) are found in the Pd(III) form. After reduction by hydrogen at room temperature the Pd at zero oxidation state is almost atomically dispersed. The electron density of the Pd(0) is low because of its strong interaction with Lewis acid sites of the zeolite network; it could even form Pd(I) (8%) (detected by ESR). This species is easily reoxidizable to Pd(II) by treatment in oxygen at 300°C. For reduction temperatures above 250°C, crystallites of metallic palladium are dispersed on the surface.

transition metal ion-exchanged zeolites possess interesting properties. T h e y have been used to obtain well dispersed metal catalysts. Early experiments dealt with platinum-loaded zeolites (1,2). Kubo et al. (3) and Boudart et al. (4) showed the effectiveness of the zeolites for preparing well dispersed platinum catalysts. Ni(I)-zeolites were also subjected to hydrogen reduction; the data gave strong evidence that nickel was not atomically dispersed and that metal atoms diffuse out of the pores to form crystals at the external surface of the zeolite (5,6). Another characteristic feature of the hydrogen-reduced transition metal zeolites is their acidic properties, as demonstrated by their catalytic behavior (7). Naccache and Ben Taarit (8) gave I R evidence of the subsequent formation of protons on hydrogen-reduced Cu(II)-Y zeolite. Furthermore, transition metal ions have various oxidation states. Owing to the shielding effect caused by the zeolite network and the electric fields, the transition metal ions may be stabilized in unusual oxidation states-—i.e., Ni(I) (9). 266 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

24.

Palladium-Y

NACCACHE ETA L .

267

Zeolite

W e were i n t e r e s t e d i n t h e change i n t h e o x i d a t i o n state of P d ( I I ) , i n c o r p o r a t e d i n t h e zeolite, d u r i n g heat t r e a t m e n t i n o x y g e n o r in vacuo. H y d r o g e n a n d c a r b o n m o n o x i d e i n t e r a c t i o n s were also s t u d i e d . ments i n v o l v e d two techniques:

T h e experi

E S R , w h i c h provides direct identification

of p a l l a d i u m i n a n i o n i c state, a n d I R spectroscopy, w h i c h gives i n f o r m a t i o n o n t h e s u p e r f i c i a l s t r u c t u r e of t h e exchanged zeolite a n d o n t h e a d sorbed species.


Experimental T h e s t a r t i n g m a t e r i a l was a n N a - Y f o r m zeolite w i t h S i 0 / A l 0 = 4.8 s u p p l i e d b y t h e L i n d e C o . S o d i u m ions i n t h e z e o l i t e were e x c h a n g e d b y s t i r r i n g a suspension of t h e zeolite i n a t e t r a m m i n e p a l l a d i u m i o n s o l u t i o n prepared b y dissolving P d C l i n a n a m m o n i a solution. T h e exchanged zeolite was t h e n w a s h e d u n t i l t h e w a s h w a t e r was free of c h l o r i d e ions. T h e zeolite was d r i e d a t 8 0 ° C i n a i r . T h e s a m p l e w a s a n a l y z e d b y flame p h o t o m e t r y for t h e s o d i u m c o n t e n t a n d b y a c o l o r i m e t r i c m e t h o d for p a l l a d i u m . S i n c e t h e exchanged zeolite c o n t a i n e d 12.5 P d ( I I ) ions a n d 19.5 N a ions, we c o n c l u d e d t h a t 11.5 N a + were exchanged b y N H . T h e samples were a c t i v a t e d a c c o r d i n g t o one of t h e f o l l o w i n g t r e a t ments: (a) : c a l c i n a t i o n i n a s t r e a m of o x y g e n , t h e n in vacuo a t 5 0 0 ° C (sample A ) , (b) : s a m p l e A c o n t a c t e d w i t h h y d r o g e n a t r o o m t e m p e r a t u r e (sample B ) , (c) : c a l c i n a t i o n u n d e r a pressure of 160 t o r r o x y g e n i n t h e presence of a l i q u i d n i t r o g e n t r a p , t h e n e v a c u a t i o n a t 5 0 0 ° C (sample C ) . Carefully purified oxygen, carbon monoxide, a n d hydrogen were allowed t o react w i t h t h e samples. E S R measurements were c a r r i e d o u t u s i n g a V a r i a n V - 4 5 0 2 X - b a n d spectrometer w i t h 100 k H z field m o d u l a t i o n . V a r i a n d u a l c a v i t y p e r m i t t e d g values a n d spin concentration measurements b y comparison w i t h a s t a n d a r d D P P H ( d i p h e n y l p i c r y l h y d r a z i n e ) sample. Measurements were c a r r i e d o u t a t r o o m t e m p e r a t u r e o r a t 7 7 ° K . F o r I R measurements t h e c a t a l y s t was compressed a t 4 Χ 10 P a s c a l . T h e disc (18 m m d i a m e t e r , 2 0 - 3 0 mg) was m o u n t e d i n a q u a r t z s a m p l e h o l d e r w h i c h was i n t r o d u c e d i n t h e a d s o r p t i o n / i n f r a r e d c e l l (10). To a v o i d t h e r e d u c t i o n of t h e P d ( I I ) ions b y h y d r o c a r b o n s , t h e cell was grease free. S a m p l e s were a c t i v a t e d a c c o r d i n g t o t r e a t m e n t c. Spectra were r e c o r d e d o n a P e r k i n E l m e r m o d e l 125 g r a t i n g spectrometer. The reference b e a m was a t t e n u a t e d , a n d t h e i n s t r u m e n t was f l u s h e d w i t h a i r freed of H 0 a n d C 0 . 2

2

3

2

+

4

+

8

2

2

Results and Interpretation ESR.

P r e l i m i n a r y w o r k (11) s h o w e d t h a t s a m p l e C , because of t h e

influence of o x y g e n a n d a m m o n i a d u r i n g t h e a c t i v a t i o n of t h e s a m p l e , e x h i b i t e d t w o E S R signals s i m u l t a n e o u s l y . g

±

= 2.10 a n d ^|| = 2.33, s i g n a l 2 : g

iso

T h e g v a l u e s were, s i g n a l 1:

= 2.223.

T h e relative intensities

d e p e n d s t r o n g l y u p o n t h e t h i c k n e s s of t h e c a t a l y s t b e d a n d o n h e a t i n g c o n d i t i o n s (11).

I t w a s suggested t h a t t h e presence of these t w o species

was t h e r e s u l t of t h e influence of o x y g e n a n d a m m o n i a .

T o avoid the

s i m u l t a n e o u s presence of t h e t w o signals, heat t r e a t m e n t (a) w a s a d o p t e d .

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

268

MOLECULAR SIEVES

S a m p l e A gave o n l y a s y m m e t r i c a l single l i n e w i t h g = 2.223 ( F i g u r e 1). T h i s s i g n a l was observed a t 77° Κ b u t n o t at r o o m t e m p e r a t u r e . W h e n t h e P d - Y zeolite was h e a t e d at 5 0 0 ° C in vacuo w i t h o u t o x y g e n p r e t r e a t m e n t , o n l y s i g n a l 1 was observed. T h e l i n e w i d t h of the s i g n a l s h o w e d no t e m p e r a t u r e dependence. T h e s i g n a l was c h a r a c t e r i s t i c of powder p a r a m a g n e t i c species i n a n a x i a l c r y s t a l field s y m m e t r y . T h e p r i n c i p a l g values were g\\ = 2.33 a n d g = 2.10.


±

Figure 1.

ESR spectra of Pd-Y zeolites at 77°K heated at 600°C: (1) in vacuo, (2) in oxygen

W h e n t h e different samples were exposed t o o x y g e n , s i g n a l 1 d i s a p peared, b u t s i g n a l 2 r e m a i n e d u n c h a n g e d . I n c o n t r a s t , w h e n h y d r o g e n was a l l o w e d to react at r o o m t e m p e r a t u r e o n t h e different sample, s i g n a l 2 d i s a p p e a r e d w h i l e s i g n a l 1 increased i n i n t e n s i t y . T h e c o n d i t i o n s for f o r m a t i o n of species 1 (in r e d u c i n g atmosphere) a n d t h e i r s e n s i t i v i t y t o w a r d o x y g e n suggest t h a t t h e y m a y be P d ( I ) ions. For m a t i o n i n a n o x i d i z i n g a t m o s p h e r e a n d s e n s i t i v i t y t o w a r d h y d r o g e n for species 2 suggest t h a t t h e y m a y be P d ( I I I ) ions. T h i s assignment based u p o n t h e c h e m i c a l b e h a v i o r of species 1 a n d 2 m a y be also d e r i v e d f r o m t h e o r e t i c a l considerations. P a l l a d i u m ions were i n t r o d u c e d i n t h e zeolite as [ P d ( N H ) ] ions. P d ( I I ) forms a square p l a n a r complex w i t h a m m o n i a . T h e 4 d a n d 5d t r a n s i t i o n m e t a l ions experience a stronger c r y s t a l field t h a n t h e 3d ions. H e n c e t h e P d ( I I ) ions w i t h a 4 d electronic c o n f i g u r a t i o n are d i a m a g n e t i c a n d c a n n o t be i n v e s t i g a t e d b y E S R . O n t h e o t h e r h a n d , P d ( I I I ) a n d P d ( I ) are p a r a m a g n e t i c . P d ( I ) i o n has a 4 d electronic c o n f i g u r a t i o n . C o n s e q u e n t l y , t h i s i o n behaves i n a t e t r a g o n a l field i n t h e same m a n n e r as C u ( I I ) i o n . T h e g values c a l c u l a t e d f r o m t h e c r y s t a l field t h e o r y (12) a r e : 3

4

2 +

8

9


24.

269

Palladium-Y Zeolite

NACCACHE ET A L .

011 = 9e + 8λ/Δ#ι and g = g + 2\/AE L

e

4

where g , λ, AE a n d Δ2£ are, r e s p e c t i v e l y , t h e free e l e c t r o n g v a l u e , t h e s p i n - o r b i t c o n s t a n t , a n d t h e c r y s t a l field s p l i t t i n g of t h e d o r b i t a l s . Since λ is a l w a y s p o s i t i v e , one c a n expect g\\ > g± > g w h i c h is consistent w i t h t h e observed g values for species 1. I t is i n t e r e s t i n g t o compare t h e m a g n i t u d e of t h e g values for t h e P d ( I ) a n d C u ( I l ) ions i n d e h y d r a t e d Y zeolites. T h e g v a l u e s for C u ( I I ) i n zeolite are g\\ = 2.32 a n d g = 2.06 (8) ; as t h e v a l u e s for λ are 1412 c m " " for P d ( I ) a n d 828 c m " for C u ( I I ) (12), t h e c r y s t a l field s p l i t t i n g is larger for p a l l a d i u m t h a n for copper. H e n c e , t h e p a l l a d i u m ions m u s t be more c o v a l e n t l y b o n d e d t o t h e l a t t i c e o x y g e n ions t h a n are t h e copper ions. e

h

4

e

L


1

1

P d ( I I I ) ions h a v e a 4 d electronic c o n f i g u r a t i o n . I n a s t r o n g c r y s t a l field, t h e electrons t e n d t o be p a i r e d , a n d t h e g r o u n d state corresponds t o a single u n p a i r e d electron w h i c h has a t w o f o l d o r b i t a l degeneracy, w h i c h is n o t r e m o v e d b y a t r i g o n a l or cubic c r y s t a l field. T h e o r b i t a l degeneracy m a y be r e m o v e d b y a t e t r a g o n a l d i s t o r t i o n . I n t h i s case, one c o u l d be expect t h e g tensor t o h a v e a n a x i a l s y m m e t r y . K r i g a s a n d R o g e r s (13) r e p o r t e d v a l u e s of g\\ = 2.012 a n d g = 2.149 for ( P d C l ) ~ i o n i n a t e t r a g o n a l c r y s t a l field. I t appears t h a t t h e i s o t r o p i c E S R s i g n a l observed m a y be i n t e r p r e t e d b y t h e effect of t h e d y n a m i c J a h n - T e l l e r d i s t o r t i o n . T h e J a h n - T e l l e r t h e o r e m s t a t e d t h a t a degenerated g r o u n d state s y s t e m is d i s t o r t e d t o r e m o v e t h e o r b i t a l degeneracy. Since there are three e q u i v a l e n t s t a t i c d i s t o r t i o n s , t h e J a h n - T e l l e r d i s t o r t i o n a x i s m a y resonate a m o n g t h e t h r e e e q u i v a l e n t directions t o produce t h e observed i s o t r o p i c s p e c t r u m . T h e d y n a m i c J a h n - T e l l e r effect has been suggested t o a c c o u n t for t h e isotropic E S R s i g n a l of P d ( I I I ) of a l u m i n a (14). 7

±

5

2

E S R q u a n t i t a t i v e measurements s h o w e d t h a t t h e c o n c e n t r a t i o n of P d ( I I I ) a n d P d ( I ) ions generated d u r i n g t h e c a l c i n a t i o n of t h e P d - Y zeolite i n o x y g e n o r in vacuo were s m a l l c o m p a r e d w i t h t h e t o t a l p a l l a d i u m c o n t e n t ; a b o u t 1 % of t h e p a l l a d i u m w a s f o u n d as either P d ( I I I ) or P d ( I ) forms. N e v e r t h e l e s s , t h e f o l l o w i n g reactions m i g h t describe t h e processes o c c u r r i n g d u r i n g t h e c a l c i n a t i o n i n o x y g e n or in vacuo (11): 2 Pd

2 +

+ H 0 + Vi 0 — 2 P d 2

2

3 +

+ 2OH"

in vacuo 2 N H + 3 O " + 6 Pd 3

2

2+

N + 3 H 0 + 6 Pd 2

2

+

H e n c e , t h e f o r m a t i o n of P d ( I ) d u r i n g t h e c a l c i n a t i o n in vacuo results f r o m the r e d u c i n g effect of a m m o n i a w h i c h desorbs d u r i n g t h e heat t r e a t m e n t . A t t a c k of l a t t i c e o x y g e n ions b y a m m o n i a w o u l d f o r m t r i g o n a l a l u m i n u m ions.


270

MOLECULAR SIEVES

EFFECTS

OF

HYDROGEN

ADSORPTION ON

P D - Y

ZEOLITE

SAMPLES.

T h e o x i d i z e d samples i n w h i c h P d ( I I I ) ions h a v e b e e n detected, w h e n ex posed t o h y d r o g e n a t 2 5 ° C , t u r n e d b l a c k i n s t a n t a n e o u s l y , a n d E S R m e a surements showed t h a t t h e P d ( I I I ) ions h a d d i s a p p e a r e d . S i m u l t a n e o u s l y , a strong E S R signal developed. T h e signals i n F i g u r e 2 are assigned t o P d ( I ) ions l o c a t e d a t t w o different sites, A a n d B . T h e i r respective g v a l u e s a r e : site A,g\\ = 2.41, g = 2.11 a n d site B , g = 2.28, g = 2.10. E S R q u a n t i t a t i v e measure m e n t s s h o w e d t h a t a b o u t 8 % of t h e l o a d e d p a l l a d i u m was i n t h e f o r m of P d ( I ) after h y d r o g e n a d s o r p t i o n . Subsequent t o t h e E S R e x p e r i m e n t s , t h e h y d r o g e n u p t a k e b y s a m p l e ( A ) was m e a s u r e d b y a v o l u m e t r i c m e t h o d .


±

u

β / /

±

:2.41

g„:2.28

J dpph Figure 2.

ESR spectrum at 77°K of Pd-Y zeolite heated at 500°C after hydrogen adsorption at 25° C

A t l o w h y d r o g e n pressures < 5 0 t o r r , h y d r o g e n a d s o r p t i o n was a s l o w process. H o w e v e r , a t pressures > 1 0 0 t o r r a n d after 48 hours, 7.6 X 1 0 ~ m o l e / g r a m of h y d r o g e n was c o n s u m e d . B a s e d o n the r e d u c t i o n scheme: 4

Pd

2 +

+ H -*Pd° + 2 H 2

+

t h e h y d r o g e n c h e m i s o r p t i o n d a t a suggest t h a t u n d e r these e x p e r i m e n t a l c o n d i t i o n s , a b o u t 8 0 % of t h e l o a d e d p a l l a d i u m was c o n v e r t e d i n t o a m e t a l form.

T h e s a m p l e r e d u c e d b y h y d r o g e n a t 25 ° C w as t h e n degassed a t r o o m r

temperature a n d allowed to react w i t h oxygen at r o o m temperature. o x y g e n u p t a k e was o b s e r v e d .

No

M o r e o v e r , if t h e s a m p l e was a g a i n degassed

a t r o o m t e m p e r a t u r e , n o h y d r o g e n u p t a k e was o b t a i n e d b y a d d i n g h y d r o g e n to the sample at 25°C. H e n c e , i t appears t h a t u p o n r e d u c t i o n a t r o o m t e m p e r a t u r e , the r e d u c e d p a l l a d i u m exists i n t h e zeolite c a v i t i e s , a n d perhaps t h e a t o m i c a l l y dispersed p a l l a d i u m i n t h e zeolite f r a m e w o r k loses i t s m e t a l l i c properties


24.

P(dladium-Y Zeolite

NACCACHE ET A L .

271

regarding hydrogen a n d / o r oxygen adsorption. P a r t i a l electron transfer between P t clusters a n d t h e c a r r i e r m a y o c c u r (4, 15). H e n c e , as t h e p a l l a d i u m is dispersed a t o m i c a l l y , we suggest t h a t a Pd° a t o m transfers one 4d electron t o a n e a r b y s t r o n g l y e l e c t r o p h i l i c site, t h u s b e i n g c o n v e r t e d i n t o P d ( I ) i o n . T h i s c o u l d e x p l a i n t h e f o r m a t i o n of P d ( I ) ions, as o b s e r v e d b y E S R measurements, u p o n h y d r o g e n a b s o r p t i o n . CARBON MONOXIDE ADSORPTION. S i n c e t h e use of i s o t o p i c l a b e l i n g molecules s h o u l d h e l p t o i d e n t i f y t h e p a r a m a g n e t i c species, e x p e r i m e n t s were c a r r i e d o u t w i t h C O a n d w i t h c a r b o n m o n o x i d e e n r i c h e d t o 9 0 % i n C O ; C has a n u c l e a r s p i n / . U p o n exposure of s a m p l e A t o C O , a s m a l l t r i p l e t E S R s i g n a l a p p e a r e d . T h e s i g n a l grew i n i n t e n s i t y for s e v e r a l d a y s . S i m u l t a n e o u s l y , t h e P d ( I I I ) s i g n a l decreased, a n d t h e a s y m m e t r i c a l E S R s i g n a l a t t r i b u t e d t o P d ( I ) developed.


1 2

1 3

1

1 3

2

S a m p l e Β b e h a v e d i n t h e same m a n n e r . T h e t r i p l e t f r o m C O a d s o r p t i o n g r e w w i t h t h e t i m e . F u r t h e r m o r e , t h e E S R s i g n a l of P d ( I ) ions i n sites Β decreased. C O a d s o r p t i o n o n s a m p l e C r a p i d l y r e s u l t e d i n t h e f o r m a t i o n of t h e t r i p l e t s i g n a l ( F i g u r e 3 ) . T h e a d d i t i o n a l s t r o n g s i g n a l at a r o u n d g = 2 . 1 is the r e s u l t of P d + ions p r o b a b l y l o c a l i z e d i n sites I , w h i c h are h i d d e n f r o m the surface a n d , hence, inaccessible t o c a r b o n m o n o x i d e . T h e g v a l u e s are P i = 2 . 1 9 2 , g = 2 . 0 6 1 a n d g* = 2 . 0 3 8 . T h e s p e c t r u m of F i g u r e 3 w a s n o longer o b s e r v e d after t h e s a m p l e h a d been degassed for 1 h o u r a t r o o m temperature. 2

Figure 3.

I

ESR spectrum at 77°K of CO adsorbed on Pd-Y zeolite heated at 500°C in vacuo 12


272

MOLECULAR SIEVES

1 3

C O a d s o r b e d o n s a m p l e G gave t h e s p e c t r u m of F i g u r e 4.

This

s p e c t r u m c o u l d be r e s o l v e d i n t o t h r e e sets of t r i p l e t s , each t r i p l e t i s c e n t e r e d a r o u n d gi, g a n d g*. 2

T h e f o r m a t i o n of a C O r a d i c a l is r u l e d o u t for t h e f o l l o w i n g reasons: (1) W i t h t h e C O - C O m i x t u r e , C O r a d i c a l s w o u l d g i v e t w o t r i p lets centered a r o u n d t h e C O t r i p l e t , a n d t h e i n t e n s i t y r a t i o s h o u l d be e q u a l t o t h e C / C r a t i o ; t h i s was n o t o b s e r v e d ( F i g u r e 4 ) . 1 2

1 3

1 3

1 2


1 3

1 2

(2) T h e e x p e r i m e n t a l g v a l u e s o b s e r v e d for t h e species f o r m e d u p o n C O a d s o r p t i o n are larger t h a n those c a l c u l a t e d for t h e C O r a d i c a l a d s o r b e d o n M g O (16). T h u s , we believe t h a t t h e species f o r m e d i n C O a d s o r p t i o n m a y be CO

/ a t t r i b u t e d t o a c o m p l e x of t h e t y p e P d

, t w o molecules b e i n g a d -

+

\

sorbed on a P d ( I ) ion. molecules.

CO T h e free 4d e l e c t r o n is h i g h l y d e l o c a l i z e d o n t h e C O

H e n c e , t h e h y p e r f i n e s t r u c t u r e is t h e r e s u l t of t h e i n t e r a c t i o n of

the free e l e c t r o n w i t h t w o

1 3

C n u c l e i g i v i n g rise t o three sets of t r i p l e t s as

s h o w n i n F i g u r e 4, w i t h r e l a t i v e i n t e n s i t i e s of 1 : 2 : 1 .

D e t a i l e d a n a l y s i s of

t h e E S R s p e c t r a w i l l be g i v e n elsewhere. Infrared Spectroscopy.

T h e s p e c t r u m of t h e s o l i d C s h o w e d o n l y w e a k

a n d u n r e s o l v e d h y d r o x y l b a n d s ( F i g u r e 5).

T h e i n t r o d u c t i o n of C O u n d e r

a n e q u i l i b r i u m pressure of 50 t o r r d i d n o t m o d i f y t h e *>OH b a n d s .

After

e v a c u a t i o n of t h e c a r b o n m o n o x i d e a t r o o m t e m p e r a t u r e , t h e I R s p e c t r u m s h o w e d t w o b a n d s a t 2135 a n d 2110 c m

Figure 4.

- 1

caused b y s t r o n g l y c h e m i s o r b e d

ESR spectrum at 77°K of CO adsorbed on Pd-Y zeolite heated at 500° C in vacuo n


24.

7

a

f I Downloaded by UNIV OF CALIFORNIA SANTA CRUZ on October 21, 2014 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch024

273

PaUadium-Y Zeolite

NACCACHE E T AL.

1 1

4 » /

1141

1 /

Ή 3S4I

10V. 3800

cm"

f 3140 1700

3500

1

1500

Figure 5. Reduction by hydrogen. IR spectra of solid C: (a): initial sample, (b) 200 torr hydrogen for 6 hours at 25°C, (c) 200 torr hydrogen for 16 hours at 200°C, then evacuated at room temperature C O . T h e s e b a n d s decreased i n i n t e n s i t y after a subsequent d e s o r p t i o n a t 100°C a n d disappeared upon evacuation at 200°C. I f C O was again a d s o r b e d o n t h e s a m p l e , t h e p r e v i o u s b a n d s were o b s e r v e d w i t h w e a k e r i n t e n s i t y . W h e n t h e s o l i d t r e a t e d u n d e r o x y g e n a t 500° C was e v a c u a t e d a t 200° C , t h e C O b a n d s were w e a k e r t h a n f o r t h e s o l i d C . W h e n t h e d e a m m o n i a t i o n t e m p e r a t u r e was r e a c h e d a t a fast r a t e , C O a d s o r p t i o n also gave rise t o w e a k b a n d s a t 2100, 1935, a n d 1895 c m ; t h e responsible species were r e v e r s i b l y a d s o r b e d a t r o o m t e m p e r a t u r e . - 1

T h e i n t r o d u c t i o n of h y d r o g e n a t 100 t o r r o n s o l i d C p r o d u c e d a n increase of t h e J>OH b a n d s , w h i c h are n o w w e l l r e s o l v e d (3640-3540 c m ) ( F i g u r e 5 ) . T h e i n t e n s i t y of these b a n d s increased s l o w l y w i t h t h e t i m e ; t h e m a x i m u m v a l u e was reached after 6 h o u r s ; a t t h e same t i m e , t h e w a t e r f o r m a t i o n was detected b y i t s 6H O b a n d a t 1640 c m . A f t e r e v a c u a t i o n of h y d r o g e n a t r o o m t e m p e r a t u r e , t h e a d s o r p t i o n of c a r b o n m o n o x i d e g e n e r a t e d b a n d s a t 2135, 2110, 2100, 1935, a n d 1895 c m " . T h e l a s t three b a n d s were pressure dependent. E v a c u a t i o n a t 25 ° C p r o d u c e d a p a r t i a l r e m o v a l of t h e 2100 c m " b a n d , a n d t h e 1935-1895 c m bands d i s - 1

2

- 1

1

1

-

1


274

MOLECULAR SIEVES

a p p e a r e d a l m o s t c o m p l e t e l y a n d were s h i f t e d t o w a r d s lower frequencies ( F i g u r e 6). T h e 2135 a n d 2110 c m b a n d s were n o t r e m o v e d b y e v a c u a t i o n a t r o o m t e m p e r a t u r e , b u t t h e i r i n t e n s i t i e s were w e a k e r t h a n for t h e s o l i d C ( F i g u r e 6). - 1

R e d u c t i o n a t 2 0 0 ° C u n d e r h y d r o g e n p r o d u c e d a n increase of t h e von b a n d s ( F i g u r e 5). A d s o r p t i o n of C O gave b a n d s a t 2100, 1935, a n d 1895 c m " ( F i g u r e 7), r e m o v e d b y e v a c u a t i o n of t h e s a m p l e at 2 5 ° C . N o m o r e C O s t r o n g l y c h e m i s o r b e d was observed. I f t h e r e d u c t i o n at 200° C was f o l l o w e d b y a t r e a t m e n t u n d e r v a c u u m a t t h e same t e m p e r a t u r e , C O gave r e v e r s i b l e b a n d s a t 2105, 1935, a n d 1895 c m ( F i g u r e 7 ) ; w e a k b a n d s of


1

- 1

2000

1900

1 800

1700

1 BOB

Figure 6. CO adsorption. IR spectra of solid C: (a) treated with hydrogen at 200°C, evacuated at 25°C, (b) sample A + CO at room temperature (p = 50 torr), (c) sample Β evacuated at 25° C, id) sample C desorbed at 200°C, CO added at 25°C (p = 50 torr), (e) and evacuated at 25° C



24.

NACCACHE ET A L .

275

Palladium-Y Zeolite

2310

2100

Figure 7. CO adsorption. IR spectra of solid C: (a) contacted with CO at 25°C (p = 50 torr) and evacuated at room temperature, (b) treated by hydrogen (j) = 220 torr) for 3 hours at 25°C and evacuated at 25° C, CO added at room temperature (p = 50 torr), (c) sample (b) evacuated at room temperature i r r e v e r s i b l y a d s o r b e d C O were also d e t e c t e d a t 2135 a n d 2110 c m " "

(Figure

1

7). W a t e r seemed t o i n h i b i t t h e s t r o n g c h e m i s o r p t i o n of C O ; t o v e r i f y t h i s a s s u m p t i o n , w a t e r was c h e m i s o r b e d o n s o l i d C before t h e a d s o r p t i o n of C O . I n t h e first m i n u t e s , v e r y w e a k v o b a n d (2135, 2110 c m ) were o b s e r v e d , a n d a b a n d a t 2100 c m a p p e a r e d w h i c h increased s l o w l y i n C

- 1


- 1

276

MOLECULAR SIEVES

i n t e n s i t y . A f t e r one h o u r ' s c o n t a c t , t h e s o l i d seemed to be i n a c t i v e since n o m o r e c h e m i s o r p t i o n was d e t e c t e d ; t h i s p h e n o m e n o n w i l l be discussed elsewhere. R e d u c t i o n b y h y d r o g e n at 2 0 0 ° C is p a r t i a l l y reversible because t h e s a m p l e o b t a i n e d c o u l d be r e o x i d i z e d b y o x y g e n t r e a t m e n t between 200° a n d 400°C. T h e t w o b a n d s (2135, 2110 c m ) were present i n t h e s p e c t r u m of a d s o r b e d C O , b u t no b a n d c a n be o b s e r v e d b e l o w 2110 c m . - 1

- 1

W h e n treatment by on solid C treated b y H were e l i m i n a t e d a t r o o m of s t r o n g l y c h e m i s o r b e d under oxygen at 200°C.


2

o x y g e n was p e r f o r m e d after c h e m i s o r p t i o n of C O a t 2 5 ° C , t h e b a n d s a t 2100, 1935, a n d 1895 c m " t e m p e r a t u r e w i t h f o r m a t i o n of C 0 . T h e b a n d s C O are m o r e stable a n d d i s a p p e a r e d b y h e a t i n g 1

2

B y i n c r e a s i n g t h e h y d r o g e n r e d u c t i o n t e m p e r a t u r e , t h e i n t e n s i t y of t h e 2100 c m b a n d of a d s o r b e d C O decreased p r o g r e s s i v e l y , a n d a t t e m p e r a t u r e s > 2 5 0 ° C , a new w e a k b a n d at 2070 c m appeared. T h i s b a n d was p a r t i a l l y r e m o v e d b y e v a c u a t i o n a t r o o m t e m p e r a t u r e . - 1

- 1

I N T E R P R E T A T I O N . T h e s o l i d C has few O H groups as seen f r o m F i g u r e 1. I f t h e c a t a l y s t is o n l y desorbed at 200° C , t h e observed b a n d s at 3640 a n d 3540 c m are s i m i l a r t o those of H Y zeolites (17). I n con t r a s t t o t h e a l k a l i n e e a r t h exchanged Y zeolites (18, 19), t h e s p e c t r a of o u r solids present no b a n d i n the 3610-3570 c m range, a n d t h e existence of ( P d O H ) + groups seems u n l i k e l y . - 1

- 1

T h e r e a c t i o n of H o n s o l i d C produces, besides w a t e r molecules, t h e same O H groups as above. T h i s r e d u c t i o n , as discussed b e l o w , gives Pd°. T h i s p h e n o m e n o n m a y be i n t e r p r e t e d i n t w o w a y s : 2

/

Ο

\

(1) b y r e m o v a l of a n o x y g e n a t o m f r o m a S i A l bridge w i t h f o r m a t i o n of w a t e r , p a r t of w h i c h c a n dissociate a n d g i v e O H groups. T h e electrons p r o d u c e d b y t h i s r e a c t i o n reduce t h e P d ( I I ) ions t o P d a t o m s . A s a m a t t e r of fact, t h e m o b i l i t y of t h e o x y g e n of N a Y zeolites increases if t h e N a + ions are exchanged b y C u ( I I ) (20)) P d ( I I ) ions seem t o h a v e t h e same effect, (2) a n d b y t h e f o l l o w i n g r e a c t i o n : Pd

2 +

+ H — Pd° + 2 H , 2

+

w i t h s t a b i l i z a t i o n of t h e p r o t o n s f o r m e d i n t h i s w ay b y t h e zeolite n e t w o r k . T h i s scheme c a n e x p l a i n t h e f o r m a t i o n of t h e O H groups b u t fails t o a c c o u n t for t h e presence of w a t e r . r

T h e s t r e t c h i n g f r e q u e n c y of c h e m i s o r b e d C O is sensitive t o t h e elec t r o n i c d e n s i t y of t h e site (21, 22). T h e r e f o r e , t h e v a r i a t i o n s of t h e s p e c t r a of a d s o r b e d C O after v a r i o u s t r e a t m e n t s of t h e s o l i d allows t h e d e t e r m i n a t i o n of t h e state of p a l l a d i u m i n t h e zeolite. T h e s t r o n g l y c h e m i s o r b e d C O is c h a r a c t e r i z e d b y t w o s h a r p b a n d s a t 2135 a n d 2110 c m - . T h e y a p p e a r together w i t h a c o n s t a n t r a t i o of i n 1


24.

NACCACHE ET A L .

tensities ( H F / L F ~

277

Palladium-Y Zeolite 3).

T h e site of t h i s s t r o n g c h e m i s o r p t i o n m u s t be

o x i d i z e d p a l l a d i u m ions because these b a n d s decrease u p o n h y d r o g e n t r e a t ment.

The P d

3+

a n d P d + i o n s detected b y E S R are present i n a m o u n t s t o o

s m a l l t o be responsible for s u c h s t r o n g b a n d s ;

therefore, o n l y t h e

Pd

2 +

ions c o u l d be t h e c h e m i s o r p t i o n sites. I n c o n n e c t i o n w i t h t h e s t r u c t u r e of c a r b o n y l m e t a l complexes,

these

bands seem t o be t h e r e s u l t of t h e s y m m e t r i c a n d t h e a n t i s y m m e t r i c s t r e t c h i n g v i b r a t i o n s of t w o C O molecules b o n d e d l i n e a r l y w i t h t h e same P d ( I I )


ion.

I m e l i k et al. (23) h a v e s h o w n t h a t p a l l a d i u m ions are t r i g o n a l l y co

o r d i n a t e d i n S i , sites (d O m - P d = 2 A ) .

B e c a u s e of c h e m i s o r b e d C O , t h e

p a l l a d i u m ions a c q u i r e a t r i g o n a l b y p y r a m i d a l c o o r d i n a t i o n . T h r e e s t r u c t u r e s (24) are possible for t h e ( O i i i ) P d ( C O ) 3

2

complex:

(1) T r a n s s t r u c t u r e was n e v e r r e p o r t e d for a n y c o m p o u n d , a n d u n d e r t h e selection rules o n l y one b a n d is e x p e c t e d : t h e s y m m e t r i c v i b r a t i o n is forbidden. T h e l a t t e r c o u l d be a l l o w e d because t h e P d ( I I ) ions are n o t e x a c t l y i n t h e p l a n e of t h e three Ο a t o m s , a n d one C O c o u l d be l o c a t e d i n the h e x a g o n a l p r i s m . I f t h i s is t h e case, t h e s y m m e t r i c v i b r a t i o n (2135 c m ) w o u l d have a smaller intensity t h a n the always allowed a n t i s y m m e t r i c v i b r a t i o n (2110 c m ) . F u r t h e r , t h e presence of C O i n t h e h e x a g o n a l p r i s m is v e r y u n l i k e l y . π ι

- 1

- 1

(2) C i s s t r u c t u r e , w i t h t w o C O ' s i n e q u a t o r i a l positions, is possible. T h e selection rules p r o v i d e t w o b a n d s , b u t t h i s s t r u c t u r e is n o t i n agreement w i t h t h e i n i t i a l t r i g o n a l s y m m e t r y of t h e p a l l a d i u m . (3) C i s s t r u c t u r e , where C O groups o c c u p y one a x i a l a n d one e q u a t o r i a l p o s i t i o n , w o u l d present t w o b a n d s ; t h i s c o n f i g u r a t i o n is t h e o n l y one w h i c h agrees w i t h a l l e x p e r i m e n t a l observations. T h e t w o C O ' s are free t o v i b r a t e i n t h e sodalite cage a n d p o i n t t o w a r d s t h e h e x a g o n a l w i n d o w s of t h e supercages. T h e i n t e n s i t i e s of t h e b a n d s of i r r e v e r s i b l y c h e m i s o r b e d C O change a c c o r d i n g t o t h e p r e l i m i n a r y t r e a t m e n t of t h e s a m p l e .

T h i s suggests t h a t

other adsorbates, e.g., w a t e r , c a n i n h i b i t t h e C O c h e m i s o r p t i o n o n t h e P d ( I I ) ions.

W e observe t h a t :

(1) D e s o r p t i o n at 2 0 0 ° C of t h e s o l i d after o x y g e n t r e a t m e n t gives C O bands w e a k e r t h a n after d e s o r p t i o n at 5 0 0 ° C . (2) P r e a d s o r p t i o n of w a t e r o n s o l i d (C) s t r o n g l y decreases t h e i n tensities of t h e p r e v i o u s bands, b u t t h e p h e n o m e n o n is c o m p l i c a t e d b y a s i m u l t a n e o u s r e d u c t i o n of P d ( I I ) ions i n presence of C O a n d w a t e r . I t has p r e v i o u s l y been s h o w n t h a t p a l l a d i u m (II) complexes i n aqueous solutions are r e d u c e d i n m e t a l b y c a r b o n m o n o x i d e (25). W e a k l y a d s o r b e d C O is c h a r a c t e r i z e d b y a b a n d a r o u n d 2100 a n d b y b a n d s b e l o w 2000 c m

- 1

(1935 a n d 1895 c m " ) . -

1

cm

- 1

A l l these b a n d s are

f o r m e d u p o n C O a d s o r p t i o n f o l l o w i n g h y d r o g e n r e d u c t i o n of t h e P d ( I I ) ions.

N o i n t e r m e d i a t e species c a n be detected d u r i n g t h i s r e d u c t i o n , a n d

the P d

+

ions observed b y E S R are present i n a m o u n t s too s m a l l t o g i v e

strong bands.

T h e reversible c h e m i s o r p t i o n m u s t t a k e place o n t h e Pd°.

C O adsorbed o n a m e t a l M , gives b a n d s a b o v e 2000 c m

- 1

w h i c h are


278

MOLECULAR SIEVES

assigned t o C O l i n e a r l y b o n d e d ( M - C O ) a n d b a n d s b e l o w 2000 c m " assigned t o C O b r i d g e d b e t w e e n t w o m e t a l a t o m s (26).

are

1

T h e f r e q u e n c y of t h e l i n e a r C O is h i g h e r t h a n t h a t f o u n d for P d films (vco = 2085 c m ) (27) or for s u p p o r t e d p a l l a d i u m ( P d / S i 0 ) , vco = 2060 c m ) (28). T h e increase i n f r e q u e n c y r e p o r t e d i n t h i s s t u d y is the r e s u l t of t h e decrease of t h e b a c k d o n a t i o n f r o m t h e d m e t a l o r b i t a l s t o t h e τ * o r b i t a l of C O . Y zeolites h a v e v e r y s t r o n g L e w i s a c i d sites; these sites s h o u l d be able t o decrease t h e electronic d e n s i t y of t h e p a l l a d i u m a t o m s b o n d e d t o C O . T h e decrease of t h e i n t e n s i t y of t h e b a n d a t 2100 c m " b y i n c r e a s i n g t h e h y d r o g e n r e d u c t i o n t e m p e r a t u r e c o u l d be e x p l a i n e d b y t h e f o r m a t i o n of agglomerates of p a l l a d i u m s t i l l i n i n t e r a c t i o n w i t h a L e w i s a c i d site. - 1

2

- 1


1

T h e i m p o r t a n t c a r r i e r effect is o n l y possible w i t h h i g h l y dispersed p a l l a d i u m — i . e . , easily o x i d i z e d p a l l a d i u m . I R results c o r r o b o r a t e t h i s a s s u m p t i o n . U p o n r e d u c t i o n b y h y d r o g e n a t 200° C , t r e a t m e n t w i t h o x y gen a t 3 0 0 ° C produces P d ( I I ) ions. R e v e r s i b i l i t y is o n l y p a r t i a l , a n d t h e disappearance of t h e m e t a l c a n be e x p l a i n e d b y t h e o x i d a t i o n of p a r t of t h e m e t a l l i c p a l l a d i u m i n t o b u l k p a l l a d i u m oxide. F o r r e d u c t i o n t e m p e r a t u r e s h i g h e r t h a n 2 5 0 ° C , t h e 2100 c m band is w e a k , a n d a n e w b a n d a t 2070 c m appears. T h e l a t t e r c a n be assigned t o C O c h e m i s o r b e d o n w e l l - o r g a n i z e d c r y s t a l l i t e s o n t h e outer surface of zeolites. T h e c a r r i e r effect is w e a k , a n d t h e o b s e r v e d f r e q u e n c y is v e r y close t o the r e p o r t e d v a l u e for a n o n - a c i d i c c a r r i e r (28). - 1

- 1

R e d u c t i o n b y h y d r o g e n produces w a t e r as r e p o r t e d p r e v i o u s l y . The increase of C O f r e q u e n c y f r o m 2100 t o 2105 c m w h e n w a t e r is desorbed a t 2 0 0 ° C u n d e r v a c u u m p r i o r t o t h e C O a d s o r p t i o n shows t h a t w a t e r is p r o b a b l y c o o r d i n a t e l y b o n d e d t o p a l l a d i u m a t o m s a n d s l i g h t l y increases t h e electronic d e n s i t y of t h e m e t a l , as expected for L e w i s bases. W e ob s e r v e d t h e same effect o n s u p p o r t e d p l a t i n u m (22). - 1

Conclusion L o a d e d p a l l a d i u m is u s u a l l y f o u n d as P d ( I I ) ions i n zeolites.

How

ever, o x y g e n t r e a t m e n t leaves few p a l l a d i u m ions i n t h e u n u s u a l P d ( I I I ) f o r m w h i l e in vacuo c a l c i n a t i o n forms P d ( I ) ions. O n a t h e o r e t i c a l basis, P d ( I I I ) a n d P d ( I ) ions are p r o b a b l y l o c a t e d i n a d i s t o r t e d o c t a h e d r a l or t r i g o n a l c r y s t a l field (Si or Si/, Su/ sites) r a t h e r t h a n i n a square p l a n a r e n v i r o n m e n t .

arise f r o m t h e P d ( I )

/

\

T h e ESR s p e c t r a of a d s o r b e d C O

CO complex.

T h e f o r m a t i o n of a τ b o n d

between

CO P d ( I I ) a n d C O w o u l d r e s u l t f r o m the o v e r l a p of t h e filled 4 d o r b i t a l s of the



24.

NACCACHE ET AL.

Palladium-Y Zeolite

279

metal ion with the τ* vacant antibonding orbitals of the CO molecule; this is consistent with our IR results which showed that vco of the ad sorbed molecule shifts toward a lower wave number compared with vco of the gas. Egerton and Stone (29), taking into account that synthetic sodalite zeolites did not adsorb CO molecules, concluded that CO does not enter the sodalite cages of the Y zeolites. However, the strong electric fields present in zeolites could also produce changes in the adsorptive properties of the solids; thus the energies associated with the cationic sites in crystal line zeolites must be considered. From our IR results, we concluded that CO molecules were located in the volume of the sodalite cages. Thus, the steric effect alone cannot explain the different adsorptive properties ex hibited by sodalite and faujasite. Palladium ions were reduced by hydrogen at room temperature. The zeolite thus formed has hydroxyl groups identical to those found in decationated Y zeolites and probably has a Bronsted acid character. Fur thermore, hydrogen reduction produces metallic palladium almost atomically, dispersed within the zeolite framework as demonstrated by our IR, volumetric, and x-ray (23) results. Palladium atoms are located near Lewis acid sites which have a strong electron affinity. Electron transfer between palladium atoms and Lewis acid sites occurs, leaving some palla dium atoms as Pd(I). Reduction by hydrogen at higher temperatures leads to a solid in which metal palladium particles are present. The be havior of these particles for CO adsorption seems to be identical to that of palladium on other supports. Acknowledgment The authors thank B. Imelik for helpful discussions, and P. Gallezot and B. Imelik who permitted us to use their x-ray data. Literature Cited 1. Rabo, J. Α., Pickert, P. E., Stamires, D. N., Boyle, J. E., Actes Cong. Int. Catal., 2nd, Ed. Technip, Paris, (1961) 2055. 2. Pickert, P. E., Rabo, J. Α., Dempsey, E., Schomaker, V., Proc. Int. Congr. Catal., 3rd, Amsterdam, Wiley, New York, (1965) 1, 714. 3. Kubo, T., Arai, H., Tominaga, H., Kunugi, T., Bull. Chem. Soc. Japan (1972) 45, 607. 4. Dalla Betta, R. Α., Boudart, M., Int. Congr. Catalysis, Vth, Palm Beach, (1972) paper 100. 5. Brooks, C. S., Christopher, G. L. M., J. Catal. (1968) 10, 211. 6. Reman, W. G., Ali, A. H., Schuit, G. C. Α., J. Catal. (1971) 20, 374. 7. Tsutsumi, K., Fuji, S., Takahashi, H., J. Catal. (1972) 24, 8. 8. Naccache, C., Ben Taarit, Y., J. Catal. (1971) 22, 171. 9. Rabo, J. Α., Angell, C. L., Kasai, P. H., Schomeker, V., Discuss. Faraday Soc. (1966) 41, 328.



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10. Mathieu, M. V., Pichat, P., La Catalyse au Laboratoire et dans l'Industrie, Masson, Paris, (1967) 319. 11. Che, M., Dutel, J. F., Naccache, C., J. Catal. to be published. 12. Griffiths, J. S., "Theory of Transition Metal Ions," Cambridge, England, (1961). 13. Krigas, T., Rogers, M. T., J. Chem. Phys. (1971) 54, 769. 14. Lacroix, R., Höchli, U., Muler, Κ. Α., Helv. Phys. Acta (1964) 37, 627. 15. Figueras, F., Mencier, B., de Mourgues, L., Naccache, C., Trambouze, Y., J. Catal. (1970) 19, 315. 16. Lunsford, J. H., Jayne, J. P., J. Chem. Phys. (1966) 44, 492. 17. Ward, J. W., Int. Conf. Molecular Sieve Zeolites, 2nd, Worcester, 1970, paper 64 18. Ward, J. W., J. Phys. Chem. (1968) 72, 211. 19. Uytterhoeven, J. B., Schoonheydt, R., Liengme, Β. V., Hall, W. K.,J.Catal. (1969) 13, 425. 20. Antoshin, G. V., Minachev, K. H. M., Sevastjanov, E. N., Kondratjev, D. Α., Chan-Zui-Newy, Int. Conf. Molecular Sieve Zeolites, 2nd, Worcester, 1970, pape 73. 21. Gardner, R. Α., J. Catal. (1964) 3, 22. 22. Primet, M., Basset, J. M., Mathieu, M. V., Prettre, M., J. Catal., in press. 23. Gallezot, P., Imelik, B., ADVAN. CHEM. SER. (1973) 121, 66. 24. Adams, D. M., "Metal-Ligand and Related Vibrations," p. 132, Arnold, London, 1967. 25. Stern, E. W., Catalysis Reviews (1967) 1, 73. 26. Little, L. H., "Infrared Spectra of Adsorbed Species," p. 54, Academic Press, London, 1966. 27. Garland, C. W., Lord, R. C., Troiano, P. F., J. Phys. Chem. (1965) 69, 188. 28. Eischens, R. P., Pliskin, W. Α., Adv. Catal. (1958) 10, 1. 29. Egerton, Τ. Α., Stone, F. S., Trans. Faraday Soc. (1970) 66, 364. RECEIVED November 30, 1972.


Study of Hydrogen and Carbon Monoxide Interactions with Palladium

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