EXAFS STUDIES ON THE REDUCTION OF Pd(II) IN X ZEOLITES

HAL Id: jpa-00226165

https://hal.archives-ouvertes.fr/jpa-00226165

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

EXAFS STUDIES ON THE REDUCTION OF Pd(II) IN X ZEOLITES

K. Möller, T. Bein

To cite this version:

K. Möller, T. Bein. EXAFS STUDIES ON THE REDUCTION OF Pd(II) IN X ZEOLITES. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-231-C8-236. �10.1051/jphyscol:1986843�. �jpa-00226165�

EXAFS S T U D I E S ON THE REDUCTION OF P d ( 1 I ) I N X ZEOLITES

K. MOLLER( ) and T. BEIN* ( )

Center for Catalytic Science and rechnology, Department of Chemical Engineering, University of Delaware, Newark, DE 19716, U.S.A.

"E.I. Dupont d e Nemours and Company, Central Research and Development Department, Experimental Station, Wilmington, DE 19898, U.S.A.

ABSTRACT

P a l l a d i u m tetraammine i o n exchange i n t o z e o l i t e , t e m p e r a t u r e t r e a t m e n t s under d i f f e r e n t atmospheres, and hydrogen r e d u c t i o n have been s t u d i e d by EXAFS a t t h e Pd K edge. Complete a u t o r e d u c t i o n i s found t o t a k e p l a c e between 473 and 623 K under vacuum. A b i d i s p e r s e Pd m e t a l phase i s formed which agglomerates a t t h e expense of t h e s m a l l p a r t i c l e s upon p r o l o n g e d h e a t i n g a t 623 K. I n c o n t r a s t , a s i m i l a r t r e a t m e n t under oxygen suppresses a u t o r e d u c t i o n , and t h e Pd c a t i o n s remain i n z e o l i t e c o o r d i n a t i o n s i t e s . A d d i t i o n a l s m a l l amounts o f p a l l a d i u m o x i d e a r e observed under t h e s e c o n d i t i o n s . I f hydrogen i s a d m i t t e d t o t h e dehydrated, i o n i c system a t 295 K, t h e oxygen c o o r d i n a t i o n o f t h e Pd d i s a p p e a r s and evidence i s o b t a i n e d f o r t h e f o r m a t i o n o f Pd(0) dimers t h a t i n t e r a c t w i t h t h e framework v i a a l o n g Pd-0 bond.

INTRODUCTION

The use o f X-ray a b s o r p t i o n s p e c t r o s c o p y f o r t h e 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 became p o p u l a r i n r e c e n t y e a r s ( 1 ) . I t s s e n s i t i v i t y t o short-range o r d e r phenomena i s p a r t i c u l a r l y h e l p f u l i f t h e i n t e r a c t i o n o f m o l e c u l a r complexes o r s m a l l m e t a l c l u s t e r s w i t h t h e s u p p o r t i s examined.

The r e d u c t i o n process of Pd, i o n exchanged i n t o z e o l i t e s , has been s t u d i e d by u s i n g XRD (2.3). TPD/MS ( 4 ) . XPS and ESR t e c h n i q u e s (5,6). pdZf i o n s a r e found t o undergo a u t o r e d u c t i o n i f ammine l i g a n d s a r e p r e s e n t i n t h e z e o l i t e p o r e s ( 4 ) . P a r t i c l e s i z e s o f a few nm have been s t a b i l i z e d i n t h e z e o l i t e m a t r i x , and low t e m p e r a t u r e hydrogen r e d u c t i o n was r e p o r t e d t o r e n d e r

" a t o m i c a l \ y d i s p e r s e " Pd (2,3) and P d ( I ) s p e c i e s ( 6 ) .

The p r e s e n t s t u d y i s aimed a t a more d e t a i l e d u n d e r s t a n d i n g o f t h e Pd r e d u c t i o n process i n t h e z e o l i t e environment. EXAFS complements t h e use o f XRD f o r t h i s system s i n c e i t d e t e c t s m e t a s t a b l e s p e c i e s l a c k i n g l o n g range o r d e r .

EXPERIMENTAL

The Pd z e o l i t e s were o b t a i n e d by i o n exchange w i t h an 0.018 m aqueous s o l u t i o n o f Pd(NH3)4C12 H20 ( A l f a #88878) i n t o z e o l i t e X ( A l f a 13 X,

#020684; Si/A1=1.2) a t RT f o r 1 6 hours. e x c e s s i v e washing u n t i l c h l o r i n e f r e e and d r y i n g a t 300 K, y i e l d i n g a u n i t c e l l c o m p o s i t i o n o f

Na46Pdzo.5(Alo~)87(Si0~)105 235 H20.

(''present address : Department of Chemical Engineering, University of New Mexico. Albuquerque. NM 87131.

U.S.A.

"'present address : Department of Chemistry, University of New Mexico, Albuquerque. NM 87131, U.S.A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986843

C8-232 JOURNAL DE PHYSIQUE

Temperature programs were r u n a t a l i n e a r h e a t i n g r a t e o f 2 K/min. A l l t e m p e r a t u r e t r e a t e d samples were k e p t a t 363 K f o r a t l e a s t 1 0 hours.

Vacuum t r e a t m e n t was performed under dynamic c o n d i t i o n s under l e s s t h a n 10-4 t o r r . 600 t o r r o f hydrogen was a d m i t t e d a t RT f o r 1 hour, whereas r e d u c t i o n a t e l e v a t e d t e m p e r a t u r e s as w e l l as a r g o n o r oxygen t r e a t m e n t s were done under a gas s t r e a m i n t u b e r e a c t o r s . A l l samples were evacuated a f t e r h e a t t r e a t m e n t u n t i l c o o l e d down t o RT. Gases were p u r i f i e d by w a t e r o r oxygen a b s o r b i n g t r a p s .

Z e o l i t e samples were embedded i n a m i x t u r e o f octadecane, dodecane 1 : l a t 310'K under e x c l u s i o n o f oxygen and m o i s t u r e i n a glovebox t o g i v e a t o t a l X-ray a b s o r p t i o n o f l e s s t h a n 2 and edge s t e p s c l o s e t o 1. Wafers were k e p t u n d e r n i t r o g e n u n t i l measured a t t h e synchroton. A 1.8 m s o l u t i o n o f Pd(NH3)4C12, PdO powder and a 0.025 mm Pd f o i l were used as r e f e r e n c e s .

X-ray a b s o r p t i o n measurements were conducted a t t h e X-11A beamline a t NSLS i n December 1985 and May 1986 w i t h a n e l e c t r o n energy o f 2.5 GeV and r i n g c u r r e n t s between 40 and 120 mA. Data were c o l l e c t e d w i t h a S i ( 4 0 0 ) c r y s t a l p a i r a t t h e Pd K edge (24 keV) a t ca. 100 K.

Data a n a l y s i s was performed w i t h t h e U n i v e r s i t y o f Washington package.

The p h o t o e l e c t r o n energy o r i g i n Eo was chosen t o be a t t h e i n f l e c t i o n p o i n t o f t h e edges. N o r m a l i z a t i o n , background removal, F o u r i e r t r a n s f o r m a t i o n (FT) and F o u r i e r f i l t e r i n g (FF) o f unknown and r e f e r e n c e s were done o v e r s i m i l a r ranges t o a l l o w a d i r e c t comparison between s e v e r a l d a t a s e t s . FT o f f i l e s showing m a i n l y t h e ammine complex were done between 2.6 and 12.8 k and t h o s e o f t h e m e t a l c o n t a i n i n g samples between 2.6 and 17.5 k . The magnitudes o f t h e FT a r e compared w i t h o u t phase s h i f t c o r r e c t i o n .

E l e c t r o n m i c r o g r a p h s were t a k e n on a HITACHI H600 f r o m microtomed s e c t i o n s o f 80 t o 90 nm t h i c k n e s s . Samples were embedded i n EPON 812 under e x c l u s i o n o f a i r .

RESULTS AND DISCUSSION A) I o n exchanged PdX z e o l i t e s

The FT o f t h e Pdi+ tetraammine complex i n s o l u t i o n as w e l l as i n

c r y s t a l l i n e form show a m a j o r c o n t r i b u t i o n a t 1.61 A caused by t h e f o u r p l a n a r c o o r d i n a t e d ammine l i g a n d s and a d d i t i o n a l w a t e r molecules ( F i g 1 ) . F o l l o w i n g t h e l i t e r a t u r e ( l ) , t h e r e a l Pd-N d i s t a n c e i s s e t t o 2.06 A. The h i g h e r s h e l l s c a t t e r i n g i s proposed t o be due t o t h e c h l o r i d e c o u n t e r i o n as i s i n d i c a t e d by i t s dependence on d i f f e r e n t w e i g h i n g schemes.

Figure 1 k3 w e i g h e d FT o f (*)1.8 m Figure 2 k ¹ w e i g h e d FF ( - 1 s o l u t i o n o f Pd(NH 1 C 1 r e f e r e n c e 0.31 t o 2 . 6 A a n d f i t (..) o f a n d e x c h a n g e d i n t a i e o 3 i t e X Pd(NH3)* e x c h a n g e d z e o l i t e

o f NaX z e o l i t e ( F i g . 1 ) shows s t r o n g s i m i l a r i t i e s . B a c k s c a t t e r i n g of t h e c h l o r i d e i s vanished, and t h e absence o f o u t e r s h e l l s c a t t e r i n g i n d i c a t e s t h a t t h e [ P ~ ( N H ~ ) ~ ] ~ + i o n does n o t occupy p r e f e r r e d c o o r d i n a t i o n s i t e s a t t h e n e g a t i v e l y charged framework o f t h e z e o l i t e . A f i t o f t h e k l weighed CHI-function, backtransformed between 0.31 and 2.6 A, gave an e x c e l l e n t agreement w i t h t h e reference w i t h N=6.53, R=2.055 A, AEn=l eV and SIG2=0.0009 A Z(Fig.2).

These r e s u l t s suggest t h a t t h e i d e n t i t y o f t h e complex i s maintained upon i o n exchange and t h a t i t i s randomly o r i e n t e d i n t h e z e o l i t e supercages.

S i m i l a r observations were reported f o r Ni (8). Co o r Mn c a t i o n exchange i n t o Nay z e o l i t e (9.10).

b ) Vacuum treatment

Pd samples were t r e a t e d under vacuum a t 363 K f o r 10 hours, a t 473 K f o r 1 hour and a t 623 K f o r 1 and 10 hours. As shown i n a TPD/MS study o f t h e decomposition process ( 4 ) o f t h e z e o l i t e supported ammine complex, e v o l u t i o n o f NH3 occurs i n two steps a t 320-360 K and 420-470 K . The EXAFS data o f t h e samples taken a t 363 and 473 K are s t r i k i n g l y s i m i l a r t o those o f t h e s t a r t i n g m a t e r i a l . Even a t 473 K t h e r e i s o n l y a s l i g h t decrease i n magnitude o f t h e main peak. The magnitudes o f t h e FT i n F i g . 3 demonstrate t h a t a small new peak a r i s e s a t 473 K, supposedly from a d i f f e r e n t bonding t o t h e z e o l i t e framework .

This i s s t r o n g l y supported by t h e f i t t i n g r e s u l t s : o n l y t h e sample a t 363 K gave a reasonable r e s u l t w i t h N=6.00, R=2.069 A, AEo=O eV, and

~162=0.0008 ~ 2 . I n c o n t r a s t , an a d d i t i o n a l c o n t r i b u t i o n a t 473 K made i t impossible t o f i t t h i s sample w i t h t h e p r e c u r s o r alone.

R P D l l l L COORDIHRTE I ~ Y E S T P O W ) RUDIUL COORDIHllTE IRWGSTROIO

Figure 3 k3 weighed FT of 363 K and Figure 4 k3 weighed FT of 623 K 473 K ( * ) vacuum treated P ~ ( N H ~ ) ~ vacuum treated Pd(NH ) zeolite zeolite after 1 and 10 ( * ) h 8 u 4 ~

Since ammonia e v o l u t i o n must have taken p l a c e a t t h i s step, we e x p l a i n t h e s i x f o l d c o o r d i n a t i o n w i t h a replacement o f complex ammine l i g a n d s by oxygen o f t h e six-membered r i n g s o f t h e z e o l i t e . N i t r o g e n and oxygen can n o t e a s i l y be d i s t i n g u i s h e d w i t h EXAFS, because both b a c k s c a t t e r i n g amplitude and phase s h i f t o f these elements a r e n e a r l y i d e n t i c a l . We assume t h a t up t o 473 K, two o r t h r e e ammine l i g a n d s remain a t t h e Pd i o n i n a d d i t i o n t o t h e z e o l i t e oxygen c o o r d i n a t i o n .

Heating t h e sample up t o 623 K causes dramatic changes i n t h e EXAFS.

F i g . 4 shows t h e f o r m a t i o n o f Pd metal a l r e a d y a f t e r 1 hour a t t h i s

temperature, w i t h b u l k - l i k e s h e l l s and t h e t o t a l disappearance o f t h e i o n i c

C8-234 JOURNAL DE PHYSIQUE

, '-'2 l$*:.. *; :- .:: ^" = ;

, .>+l . - , . ^{- Figure 5} electron micrographs of

94 f

. ^{, , Pd(NH )} zeolite after 1 and 10

. _.-+fa +;LA* :.. hour Jasuum, 10 hour argon and 10

: J "'.a"".-dq:'.,*- _* - * ,c. hour air treatment at a-

:n .-$:ir:.r..~r magnification of 3 0 0 000

Figure 6 k3 weighed FT of 523 K and 623 K ( * ) air treated Pd(NH3)*

Pd-OZ(zeo1ite) bond. The magnitude of all shells increases at prolonged heating. A fit of the first shell with Pd foil as reference gives N=7, R=2.73

A . AEo=O and 5162 = 0.0001 A?.

The average coordination number of N = 7 suggests an average particle size of about 1 nm, whereas the outershell scattering indicates significant contributions of larger particles. This requires the presence of a bidisperse system with one -undetected- fraction showing very low Pd-Pd coordination numbers or unreduced Pdn+ ions, and another fraction of bulk-like

particles. These larger particles (2-3 and 7-9 nm) are observed in the corresponding electron micrographs (Figure 5 top left). They agglomerate to a size of 7-9 nm upon prolonged heating (Figure 5 top right.) It is striking that all particles remain occluded in the zeolite matrix even when exceeding the supercage dimensions of 1.3 nm. Both findings are supported by XRD and XPS measurements (11).

Subsequent hydrogen treatment at 323 and 423 K shows that the samples are stable against further sintering. A similar procedure under argon atmosphere results in particles of 9-12 nm diameter (Fig. 5 bottom left).

These EXAFS results complete the earlier proposed picture of an autoreduction process of Pd(NH3)4 in zeolites: it is clear now that

ammonia liberat'lon and reduction are two distinct steps. The autoreduction is caused by the remaining 1 to 2 amrnine ligands.

C) Decomposition in air

I f Pd(NH3)4 zeolites are heated in air or oxygen up to 523 K , the corresponding EXAFS data resemble those obtained from the vacuum treatments.

In contrast, further heating in air to 623 K does not produce Pd metal as observed under vacuum (Fig. 6). The Pd cations still occupy lattice oxygen sites . showing that the autoreduction is completely suppressed in air. A minor fraction of larger PdO particles of ca. 15% is indicated in the

Sin' Onmb -5 TO o rtm m 4. 1 0. -n 0 034 0-7- 7 Yvl w 033 0 54.a OC n C m33-w lnwn lnwmw 7 Lo '" wm 33Cb rtm w w 2.1 0 *2 iTh nww 3-4 n w rD 7

OPPrt2. -5 2. a. S s -h-hA.rt rt-h-hw m om--'. 'I 7 0 rtu X -0 Cm7w Pu-om 0 -Y rt lrt H usm7 -0 2.nm . on GI w --Or 4.C 1 m1- now rt <!a-rtvl n sNm Sda- 70 wm w3-h 7 -r w-m (PCOV). n ID m 00 am -J.xa3 4.- '0 u n rt3 cnw w -r3 rt-5

cmmwow m7333cr ^rt

+ad.- m-5 nm a",E?.m m w-x m -5 -ID -hA.P7 . a-h*2.2. mmm3 rt7 -5.0 m omwm3 2 rt7 rt vrtcw m o"-02Xg* . P 7ort

wmmnS rtu

.A- 0 m w ow 2.2.$- -0 :zn57 A. 64.N d onm YX vrt5: nw3-L a. 2. ,+ w wm w m u 0 msITrw 'IP03Y w 7nw iD m hm rt 0.3 rtm mwm3 '0-5 SO CZlnOrtS -4-c w w om w ZP v w rtm CT -~3- m mrmwm wa. Y3 - n 5

w nD*Pwm3 CO 5C31C

v3nmmo<3 m

r+mu o w rtnrt5-m

17m-lOm0-5D r+w7

-5TJ 3 --.I 4. 7, w3-w -m2,w E(D1--I--UOO rt m wlna~-h-rt -5-lrt.C I . m m m -.I ONrtUl3 mom 3- u 7s uw-.1rt m 4onmm n rto-1 w 2.e mw-h PrtU~O 3 w rtm3 rt~rt3~mwcr-h La.JQA.7 a. urt-. rtm svl d.3-'I- o w - nwrt w--rv N d.r+a- rto nm mu0 0 2- ws I a.07. 3m 2.0 oln dm w 1 34-ww w m ww-r.,-+Cvwu noo-hm3 3w 312. wwnrt ananow- m ow wx-mw3 -5 Q3YYO wa.rtln rzz wm w -m 0 no 4. 3rtrtY3Xm3rt E w - Y rtPT -m1- an, -rmnoLmo-v N3m

33-srtmn - 02..ms 3 -'.an 0-h w m m -5 7-1- rto 1 rb 33 3A- nwrtrto'.

-

(D 5N Oln 4. P C -0~mxt n PQOY-3 rt -5 40m a. 0 0 ,lnrt3-$3 omm 33 vl. 0 . -hn n rt 3- 7w0n art3 r 2-43 rt' 7 m rm mY m A.E-5 9. n m 0 2.0 0 113w- 0 0 h0m 2.2nrw;;f a. 0 A. 2. w 300r+JQ w 37 3-5Y +-a 6 ' oo 3 w nw 0 P Nrt 5 J. +f

Lnwo wn w7m omma rtm3 rtC 0 -Jm 3gn 3.w I;;B LQ3 3 nu rt ww a-mw 0 £0- X (UY ln 3 m 2. s 0 7 vl wcm 3 w omo can 2.

C8-236 JOURNAL DE PHYSIQUE

J. H. S i n f e l t , G. H. V i a , F. W. L y t l e , C a t a l . Rev.-Sci. Eng., 26, 81, ( 1 984)

G. B e r g e r e t , P. G a l l e z o t , B. I m e l i k , J.Phys.Chem., 85, 411, (1981) P. G a l l e z o t , 8. I m e l i k , Adv. Chem. Ser., 121, 66, (1973)

D. Exner, N. I. Jaeger, K. M o l l e r , 6. S c h u l z - E k l o f f , J.Chem.Soc., Faraday Trans. 1, 78, 3537, (1978)

J. C. V e d r i n e , M. Dufaux, C. Naccache, B. I m e l i k , J.Chern.Soc.. Faraday Trans. 1. 74, 440, (1978)

M. Narayana, J. M i c h a l i k , S. C o n t a r i n i , L. Kevan, J.Phys.Chem., 89, 3895, (1985)

J. D. B e l l , J. C. Bowles, H. J . Curnming, D. H a l l , R . V . H o l l a n d , A c t a C r y s t . , 832, 634, (1976)

A. C h e s t e r , G. Kuehl. G. Woolery. T. B e i n , G. S t u c k y , t h i s volume, see a l s o f o r c a t i o n s i t e s i n z e o l i t e

T. M o r r i s o n , G. K. Shenoy. L. I t o n , G. D. Stucky, S. L. Suib, J.Chem.Phys., 76, 5665, (1982)

T. M o r r i s o n , L. I t o n , G. K. Shenoy, G. D. S t u c k y , S. L. S u i b , J.Chem.Phys., 75, 4086. (1981)

K. M o l l e r , T. B e i n . L. F i r m e n t , t o be p u b l i s h e d

J. 8. A . D. van Zon, D. C. K o n i n g s b e r g e r , H. F. J . v a n ' t B l i k , D. E.

Sayers, J.Chem.Phys., 82, 5742, ( 1 9 8 5 ) l 13) F. B. M. Duivenvoorden, D.

C. K o n i n g s b e r g e r , Y. S. Uh, 8. C. Gates, JACS, i n p r e s s