Reduction and Reoxidation of Silver-Mordenites - ACS Symposium

40 Reduction and Reoxidation of Silver-Mordenites HERMANN K. BEYER Central Research Institute for Chemistry, Hungarian Academy of Sciences, Budapest II, Pusztaszeri ut. 57/69, Hungary

Downloaded by UNIV OF ARIZONA on April 26, 2017 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0040.ch040

PETER A. JACOBS Centrum voor Oppervlaktescheikunde en Colloïdale Scheikunde, Katholieke Universiteit Leuven, De Croylaan 42, B-3030 Heverlee, Belgium

ABSTRACT The reduction and reoxidation of s i l v e r exchanged Na-zeolon has been followed v o l u m e t r i c a l l y . Isothermal and temperature programmed methods, together with i n s i t u i n f r a - r e d spectroscopy and X-ray spectrometry have been applied to i n v e s t i g a t e the stoichiometry and the k i n e t i c behaviour of the chemical reactions involved. The r e s u l t s have been compared with the AgY system.

Introduction S i l v e r m e t a l - c o n t a i n i n g z e o l i t e s p r e p a r e d by hydrogen r e d u c t i o n o f s i l v e r i o n s a r e found a c t i v e and s e l e c t i v e i n t h e gasphase o x i d a t i o n o f e t h y l e n e (1_) . S i l v e r i o n s i n Y t y p e z e o l i t e s upon r e d u c t i o n a l s o form b u l k y m e t a l c l u s t e r s o u t s i d e t h e z e o l i t e s t r u c t u r e (2). However, t h e s i l v e r m e t a l whether i t i s l o c a t e d i n s i d e o r o u t s i d e t h e Y z e o l i t e can be r e o x i d i z e d t o the o r i ginal s i l v e r cation containing zeolite (3,4). R e c e n t l y , we r e p o r t e d on t h e r e d u c i b i l i t y o f A g i o n s i n Y z e o l i t e (A). The l o c a t i o n o f t h e c a t i o n s a t d i f f e r e n t s i t e s m a i n l y determines t h e r e d u c i b i l i t y . The A g i o n s i n t h e supercages a r e r e d u c i b l e a t lower temperatures and as a r e s u l t t h e f o r m a t i o n o f a h i g h l y d i s p e r s e d m e t a l phase c o n t a i n i n g unreduced s i l v e r has been advanced. The A g i o n s l o c a t e d i n t h e hexagonal p r i s m s a r e o n l y reduced a t temperatures a t which c o n s i d e r a b l e s i n t e r i n g has o c c u r r e d a l r e a d y . The m i g r a t i o n o f t h e i o n s away from the h i d d e n s i t e s becomes r a t e l i m i t i n g (A). In t h i s work, we t r i e d t o g e n e r a l i z e t h e c o n c e p t s d e r i v e d f o r Y z e o l i t e s using a completely d i f f e r e n t +

+

+

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

MOLECULAR SIEVES—II

494

m o l e c u l a r s i e v e s t r u c t u r e . The s t o i c h i o m e t r y o f t h e r e d u c t i o n o f Ag i o n s i n m o r d e n i t e and the k i n e t i c mechanism o f t h e r e d u c t i o n has been d e r i v e d . U n f o r t u n a t e l y the e x a c t l o c a t i o n o f exchangeable c a t i o n s i n mordenite i s o n l y known f o r K (5) : 3.34 K ions are l o c a t e d i n the "side pockets" o f the s t r u c t u r e ( s i t e II) and are c o o r d i n a t e d t o oxygens b e l o n g i n g t o a 8-membered r i n g and t o an oxygen atom a c r o s s a s m a l l c h a n n e l . The o t h e r p o s s i b l e c a t i o n l o c a t i o n s ( s i t e s IV and VI) a r e i n the b i g p o r e s and c o n t a i n 3.04 and 0.91 c a t i o n s r e s p e c t i v e l y . The i o n s i n the former s i t e s are l o c a t e d i n a 8-membered r i n g o f oxygen i o n s , t h e l a t t e r i n a d i s t o r t e d 6-membered r i n g .


+

+

Experimental Materials. A commercial sample o f N a - z e o l o n from Norton Company has been used. I t was p u r i f i e d by t r e a t i n g w i t h s e v e r a l NaCl s o l u t i o n s and was t h e n e x changed w i t h A g N 0 3 s o l u t i o n s . Exchange i n an e x c e s s o f a 0.05 mol dm~3 s o l u t i o n r e p e a t e d s e v e r a l times p r o duced the samples w i t h t h e f o l l o w i n g u n i t c e l l composition : NaAgZ- -75 NaAgZ- 84

l . 92 A g Na, Ag

6

NaAgZ- 94

Ag

?

N

a

20

N a

o . 15

5

.69

Al

7

.62

S i

4 0 . 4 °96

.42

Al

?

.62

S i

4 0 . 4 °96

.5

Al

?

.62

S i

40.4 °96

The o r i g i n a l N a - z e o l o n (Z) c o n t a i n e d 1.6 % by w e i g h t o f 2°3 A f t e r s e v e r a l e x t r a c t i o n s w i t h d i t h i o n i t e (6) a z e o l i t e Z c o n t a i n i n g o n l y 0.74 % by w e i g h t o f Fe2Û3 i s obtained. The samples are d e h y d r a t e d under vacuum a t 673 Κ f o r s e v e r a l h o u r s , then oxygen i s a d m i t t e d t o r e o x i d i z e Ag° p o s s i b l y formed by a p h o t o c h e m i c a l p r o c e s s (4) and the sample i s o u t g a s s e d a g a i n . F e

e

5C

Methods. The v o l u m e t r i c uptake o f hydrogen and oxygen i s measured e i t h e r i n the i s o t h e r m a l o r tempe r a t u r e programmed mode. The d e t a i l e d i n s t r u m e n t a t i o n has been d e s c r i b e d elsewhere (4). Also the i n f r a - r e d s p e c t r o s c o p i c measurements and the X-ray d i f f r a c t i o n t e c h n i q u e s have been d e s c r i b e d e a r l i e r (4).

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

40.

BEYER AND JACOBS

R e s u l t s and

Reduction

and

Reoxidation

495

Discussion

S t o i c h i o m e t r y o f hydrogen


of Silver-Mordenites

and oxygen uptake.

V o ^ u m e t r j L c ^ r e s u l t s ^ Gas uptake measurements o f dry NaAgZ mordenite a t d i f f e r e n t temperatures are shown i n t a b l e I . A l r e a d y a t 195 Κ hydrogen i s t a k e n up by the s o l i d , o n l y 41 % o f which i s h e l d i r r e v e r s i b l y a t t h a t temperature. This indicates that a considerable amount o f H 2 i s h e l d i n the z e o l i t e v i a a p h y s i s o r p t i o n p r o c e s s . A t ambient temperature the amount o f hydrogen consumed has d e c r e a s e d by 25 %, b u t more gas i s h e l d i r r e v e r s i b l y by the z e o l i t e . Below 423 K, t h e r e i s no f u r t h e r p r o g r e s s o f the degree o f r e d u c t i o n . However, a t t h i s temperature almost 96.0 % o f the hydrogen can be t i t r a t e d w i t h oxygen and i s removed from the system as water. A t h i g h e r r e d u c t i o n temperatures (643 Κ ) , the s i l v e r i o n s can be r e d u c e d c o m p l e t e l y , the r a t i o of hydrogen t o oxygen and o f water t o oxygen uptake i s c l o s e t o 2. R e o x i d a t i o n i s almost complete under t h e s e c o n d i t i o n s , w h i l e d u r i n g a second redox c y c l e the system can be brought c l o s e l y t o i t s o r i g i n a l s t a t e w i t h r e s p e c t t o the gas uptake v a l u e s . I n f r a - r e d e v i d e n c e ^ The appearance of d e u t e r o x y l groups i n the z e o l i t e c h a n n e l s upon r e d u c t i o n was p r o v e d by i n f r a - r e d s p e c t r o s c o p y ( F i g u r e 1.). Under m i l d con d i t i o n s ( F i g u r e l.b) a band appears a t 26 50 cm"" . T h i s band shows a c i d i c b e h a v i o r towards ammonia gas. The 2650 cm"" 0D band i n c r e a s e s upon r e d u c t i o n a t h i g h e r t e m p e r a t u r e s , w h i l e new bands around 2690 and 2750 cm" a l s o appear ( F i g u r e l . c , d ) . D e u t e r o x y l bands have been o b s e r v e d a t the same f r e q u e n c y i n e a r l i e r work (!). They c o r r e s p o n d t o OH bands w i t h the f o l l o w i n g f r e q u e n c i e s {]) : 3610, 3650 and 3735 cm" . The l a t t e r band has been a s s i g n e d t o amorphous S 1 O 2 i n c l u s i o n s (8) and the 3610 cm" band t o a c i d i c OH groups i n the main pores (7-9) ; the p r e s e n c e o f the 3650 cm"" band depends upon the p r e t r e a t m e n t o r the sample p r e p a r a t i o n (8). Upon oxygen t r e a t m e n t o f a reduced AgNaZ sample ( F i g u r e l . e , f ) , the 2690 cm" i s not completely e l i m i nated. The u l t i m a t e p r e s e n c e o f t h i s band t o g e t h e r w i t h the s l i g h t i r r e v e r s i b i l i t y as o b s e r v e d from the volumetric data, indicates that i r r e v e r s i b l e s t r u c t u r a l changes o c c u r t o a l i m i t e d e x t e n t , most p r o b a b l y o f the same n a t u r e (= "deep bed" e f f e c t s ) as d e s c r i b e d e a r l i e r {8) . The p r e s e n c e o f A g i o n s i n the z e o l i t e c h a n n e l s has been probed w i t h carbon monoxide. I t seems t h a t 1

1

1

1

1

1

1

+



2

2

2

643

423

2nd redox cycle

2

295.4

643

2

H

(K)

295.4

195

195

temp.

Adsorption

1 s t redox cycle

H

+

o u t g . 295.4K

H

o u t g . 195K + H

H

Treatment

0.750

0.830

0.500

0.010

0.463

0.371

0.633

(JJ j ^ r

0.00

0.00

0.00

0.00

9

0.00

o.oe-

0.00

z

(H 0) 9

0. 365

0. 386

0. 225

Δ

(0 ) Δ

9

0. 63

0. 71

0. 480

υ

1 )

(mmole

(H 0)^

gasr uptake ( 1 )

( 1 ) H 2 O formed a f t e r r e d u c t i o n (r) o r r e o x i d a t i o n ( 0 ) ; (2) a p p a r e n t degree o f r e d u c t i o n .

NaAgZ -75

:c

NaAgZ -75

5î

NaAgZ-84

Sample


97 .2

98 .2

14 .0

(13 .9)

(11 .3)

(2)

(2)

97 .3

93 .0

96 .0

reduction reoxida% tion %

1

g" )

40.

BEYER AND JACOBS

Reduction

and

Reoxidation


1

497

+


the 2175 cm" band, r e f l e c t i n g the i n t e r a c t i o n o f A g w i t h CO, d e c r e a s e s upon r e d u c t i o n ( F i g u r e l . a , b , c , d ) , and i s r e s t o r e d upon oxygen t r e a t m e n t ( F i g u r e l . e , f ) r e m a i n i n g s l i g h t l y below i t s o r i g i n a l i n t e n s i t y (Figure l . a , f ) . T h i s a l s o i s i n d i c a t i v e o f the s l i g h t i r r e v e r s i b i l i t y o f the system a f t e r a redox c y c l e . X-ray_ d i f f r a c t i o n data^_ X-ray d i f f r a c t i o n l i n e s o f the z e o l o n s t r u c t u r e and o f p o s s i b l e e x t e r n a l phases are shown i n F i g u r e 2. The r e l a t i v e i n t e n s i t y o f the d i f f r a c t i o n l i n e s changes t o a minor e x t e n t upon h y d r o gen r e d u c t i o n ( F i g u r e 2,a,b,c) b u t even a f t e r s e v e r a l redox c y c l e s the m a t e r i a l remains h i g h l y c r y s t a l l i n e ( F i g u r e 2.a,e). A t i n c r e a s i n g r e d u c t i o n t e m p e r a t u r e s , the i n t e n s i t y o f the (111) d i f f r a c t i o n o f c r y s t a l l i n e s i l v e r e x t e r n a l t o the z e o l i t e i n c r e a s e s , w h i l e X-ray l i n e b r o a d e n i n g t e c h n i q u e s show t h a t the average p a r t i c l e s i z e o f the Ag c r y s t a l l i t e s a l s o i n c r e a s e s . Upon o x i d a t i o n a t s u f f i c i e n t l y h i g h temperatures the e x t e r n a l s i l v e r phase d i s a p p e a r s ( F i g u r e 2 . f ) . A f t e r a second redox c y c l e a t 643 K, a r e l a t i v e l y s m a l l amount o f s i l v e r remains l o c a t e d between the z e o l i t e c r y s t a l l i t e s ( F i g u r e 2.g). This again i s i n d i c a t i v e f o r the s l i g h t i r r e v e r s i b i l i t y o f the system. S t o i c h i o m e t r y o f the redox r e a c t i o n s . From the amount of gases t a k e n up o r e v o l v e d d u r i n g a redox c y c l e , from the appearance o f OH groups and disappear ance o f A g i o n s a f t e r r e d u c t i o n and from the o p p o s i t e b e h a v i o r upon oxygen t r e a t m e n t , the f o l l o w i n g o v e r a l l s t o i c h i o m e t r y appears s t r a i g h t f o r w a r d : +

2 Ag

+

2 Ag°

+ H

»

2

+ 2 H

+

+ i 0

2 Ag° >2

2

Ag

+ 2 H +

+

(1)

+ H0

(2)

2

The X-ray e v i d e n c e s u g g e s t s t h a t m e t a l a g g l o m e r a t i o n o c c u r s d u r i n g r e d u c t i o n , the system r e m a i n i n g r e v e r s i b l e t o a major e x t e n t : (Ag°) . η Ag°

x

>Ag° ^

1

> χ and the s u f f i x i and e stands f o r m e t a l agglomerates i n t e r n a l and m e t a l c r y s t a l l i t e s e x t e r n a l t o the z e o l i t e . Almost complete r e v e r s i b i l i t y o f the system appears d u r i n g o x i d a t i o n :


MOLECULAR SIEVES—Π

498

Figure 1. Infrared investigation of the reduction and reoxidation of NaAgZ-94 in the OD region (2800-2500 cm- ) and after CO physisorption (2300-2100 cm ), (a), After outgassing and oxygen treatment at 673 K; (b), after D treat ment for 1 hr at 313 K; (c), after reduc tion with Ό at 423 Κ for 1 hr; (d), after Ό treatment at 623 Κ for 1 hr; (e), after oxygen treatment at 623 Κ for 1 hr; (f), after D treatment at 573 Κ and oxygen treatment at 623 K.


1

1

2

2

2

2

I

ι

ι

2800

2600

'

ι

II

cm"

ι

2300 1

nm

Ag( 111)

II

I

I

9

I

ι

f

I

e

18-

I

8"

Figure 2. X-ray diffraction lines of NaAgZ*-75 upon various redox treatments, (a), Dehydrated zeo lite; (b), Η reduction at 432 K; (c), at 571 and (d), at 643 K; (f), 0 treatment of (d) at 571 K; (g), after one more redox cycle at 643 K.

d

I

I

1

1

. > I

II 1

I

îl

c

2

2

°'

5

a

Mi

d/


• I

2100

40.

BEYER AND JACOBS


Ag° + η H

Reduction

+

+

I0

2

and

Reoxidation

> η Ag

+


+ η H0

499

(4)

2

E s s e n t i a l l y the same b e h a v i o r has been o b s e r v e d f o r the AgY system ( 3 , 4 ) . However, upon r e p e a t e d redox c y c l e s , a s l i g h t degree o f i r r e v e r s i b i l i t y i s o b s e r v e d most p r o b a b l y due t o d e a l u m i n a t i o n o r "deep bed" e f f e c t s . I n c o n t r a s t t o o t h e r t r a n s i t i o n i o n z e o l i t e s such as copper (10) , i n no case has an e x t e r n a l o x i d e phase been d e t e c t e d . The p r e s e n c e o f the l a t t e r phase would e x c l u d e the r e v e r s i b i l i t y o f the system ( J O ) . The hydrogen uptake a t low and ambient tempera t u r e s , a l r e a d y suggests t h a t r e d u c t i o n has t a k e n p l a c e . However, a t 195 Κ no r e d u c t i o n o f A g i o n s c o u l d be d e r i v e d from i n f r a - r e d methods. A t ambient tempera t u r e s , a f a s t c h e m i s o r p t i o n o f hydrogen o c c u r s , w h i l e Ag° metal i s formed a t a much s l o w e r r a t e . This i s i l l u s t r a t e d i n F i g u r e 3, where the degree o f r e d u c t i o n deduced from the hydrogen uptake, the amount of OD groups formed and the d e c r e a s e o f the c o n c e n t r a t i o n o f the Ag ...C0 complex has been compared. The chemisorp t i o n o f hydrogen i s r e l a t e d t o the p r e s e n c e o f A g ions s i n c e no uptake o c c u r s o v e r NaZ, and t o the mordenite s t r u c t u r e s i n c e a l s o no c h e m i s o r p t i o n has been o b s e r v e d on AgY, A g - s t i l b i t e o r A g - c h a b a s i t e . The p r e s e n t d a t a however do not a l l o w t o advance a h y p o t h e s i s c o n c e r n i n g the c h e m i c a l n a t u r e o f the c h e m i s o r p t i o n s i t e . +

+

+

K i n e t i c study o f the redox b e h a v i o r . The k i n e t i c curves o f the hydrogen uptake o f NaAgZ-84 mordenite a t d i f f e r e n t temperatures are shown i n F i g u r e 4. At h i g h temperatures (> 473 K) the r a t e of uptake does not de pend on the hydrogen p r e s s u r e i n the p r e s s u r e r e g i o n from 3.99 t o 53.2 kNm" . The r e a c t i o n i s f i r s t o r d e r w i t h r e s p e c t t o the amount o f unreduced s i l v e r . The a c t i v a t i o n energy i s the same f o r the t h r e e samples, d i f f e r i n g by t h e i r degree o f i o n exchange and by t h e i r i r o n c o n t e n t and i s e q u a l t o 103.0 ± 0.50 k J m o l " . T h i s i s c l o s e t o the v a l u e r e q u i r e d f o r the r e d u c t i o n o f s i l v e r i o n s l o c a t e d i n the hexagonal prisms o f the Y f a u j a s i t e ( £ ) . The amount o f Ag* i n v o l v e d i n the h i g h temperature r e d u c t i o n p r o c e s s i s e q u a l t o 2 A g ions p e r u n i t c e l l f o r sample NaAgZ-84. I f i t i s assumed t h a t the i o n exchange o f A g f o r Na occurs p r e f e r e n t i a l l y i n the b i g pores and t h a t o n l y when the l a t t e r s i t e s are f i l l e d up exchange i n the s i d e p o c k e t s o c c u r s , t h i s amount c o r r e s p o n d s v e r y c l o s e l y t o the r e d u c t i o n of the A g i o n s l o c a t e d i n the s i d e p o c k e t s (3.3-1.2 ions per u n i t c e l l ) . A l l these arguments are i n f a v o r o f the h y p o t h e s i s t h a t the r e d u c t i o n a t h i g h tempera2

1

+

+

+

+


500

MOLECULAR SIEVES—Π

Q


1r

18000

1

s

Η

1

36000

1

Figure 3. Degree of reduction (a) with time of NaAgZ*-75 at 318 K. (%)> from hydrogen uptake; (O), from the intensity of the OD stretching band at 2650 cm ; (Φ), from the intensity of the CO stretching vibration (2175 cm' ) of the complex 1

1

Ag* . . . CO.

rooi 630 ' 5Θ2 .

~

' ' *

573 553Κ

,75 .

ο Ε

Figure

4.

Hydrogen uptake with time of NaAgZ-84

-

.251

ι 1000

ι

--528 "

460

-

-430 '298

I 3000


I


40.

BEYER AND JACOBS

Reduction

and

Reoxidation


501

t u r e s i n v o l v e s the m i g r a t i o n o f s i l v e r i o n s from the " s i d e p o c k e t s " o f the s t r u c t u r e ( S i t e I I ) . The r e d u c t i o n i s a c a t a l y z e d p r o c e s s : hydrogen most p r o b a b l y i s a c t i v a t e d on i r o n i m p u r i t y c e n t e r s p r e s e n t i n the z e o l i t e . Indeed, i n F i g u r e 5 i t i s c l e a r l y shown t h a t the r a t e o f r e d u c t i o n s t r o n g l y de pends on the i r o n c o n t e n t o f the s t r u c t u r e . Indeed, on the d e i r o n i z e d m a t e r i a l , the r a t e o f r e d u c t i o n of the s i l v e r i o n s i n the " s i d e p o c k e t s " o f the s t r u c t u r e shows a 5 - f o l d d e c r e a s e , w h i l e the r e d u c t i o n mechanism remains unchanged as deduced from the s l o p e o f the Arrhenius p l o t s . A k i n e t i c study o f the r e d u c t i o n a t lower tempe r a t u r e s (< 473 K) cannot be c a r r i e d out s i n c e o v e r l a p p i n g e x i s t s w i t h the c h e m i s o r p t i o n p r o c e s s o b s e r v e d at low t e m p e r a t u r e s . However, the uptake c u r v e s of F i g u r e 4 a t temperatures below 510 Κ show i n i t i a l l y v e r y f a s t r a t e s o f hydrogen u p t a k e , w h i l e t h i s r a t e g r a d u a l l y decreases at longer r e d u c t i o n times. From i d e n t i c a l o b s e r v a t i o n s on AgNaY (6), the c o n c l u s i o n was d e r i v e d t h a t some o f the s i l v e r i o n s a s s o c i a t e w i t h r e a c t i o n p r o d u c t s , a g g l o m e r a t i n g p r e f e r e n t i a l l y t o Ag"^ s p e c i e s which are more d i f f i c u l t t o r e d u c e . In the p r e s e n t c a s e , the r e d u c t i o n r a t e o n l y s t a r t s t o de c r e a s e when 80 % o f the A g i o n s a v a i l a b l e i n the b i g p o r e s ( s i t e s IV and VI) are r e d u c e d . T h i s means t h a t Age; s p e c i e s are formed p r e f e r e n t i a l l y upon hydrogen t r e a t m e n t under m i l d c o n d i t i o n s . +

D i s p e r s i o n o f Ag° i n the mordenite p o r e s . Reoxi d a t i o n o f Ag°Z z e o l i t e s a t 523 Κ i s o n l y complete when r e d u c t i o n i s c a r r i e d out under m i l d c o n d i t i o n s (below 523 K ) . In a temperature programmed o x i d a t i o n (TPO) experiment the r a t e o f oxygen uptake i s maximum around 423 Κ ( F i g u r e 6.a). A f t e r r e d u c t i o n a t h i g h e r tempe r a t u r e s and l o n g e r c o n t a c t t i m e s , a second maximum i n the r e o x i d a t i o n r a t e i s o b s e r v e d s l i g h t l y below 573 Κ ( F i g u r e 6.b,c). Comparison w i t h X-ray d i f f r a c t i o n r e s u l t s , shows always the c o n c o m i t a n t p r e s e n c e o f the second maximum and b i g Ag° c r y s t a l l i t e s . This i s a general observation f o r other metal loaded z e o l i t e s (11). The f i r s t maximum i s t h e r e f o r e a s s i g n e d t o the o x i d a t i o n o f metal agglomerates i n the z e o l i t e p o r e s , w h i l e the second maximum c o r r e s p o n d s t o the o x i d a t i o n of b u l k y m e t a l c r y s t a l l i t e s a t the o u t s i d e o f the zeo l i t e s t r u c t u r e . Under t h e s e assumptions 100, 75 and 40 % o f the reduced s i l v e r remains h i g h l y d i s p e r s e d i n the z e o l i t e s under the r e d u c t i o n c o n d i t i o n d e s c r i b e d sub a, b and c r e s p e c t i v e l y ( F i g u r e 6 ) .


MOLECULAR SIEVES—II


502

Figure 5. Rate of reduction of Ag ions in NaAgZ*-75 (O) and NaAgZ-85 (Φ). Arrhenius plot of the rate constants. +

Γ

1

/ 10 K"

1

3

•

10"* Τ

• ·'\ u :;:

f

ΙΛ

: · ' ;

Figure 6. Temperature-pro grammed oxidation of Ag° mor denite: rate of oxygen uptake (r ) at different temperature, (a), After reduction at 473 Κ for 1 hr; (b), at 523 Κ for 1 hr (c), at 673 Κ for 1 hr.

ί: ! ;

0

5

ίο"

hi

373

' 'X

b

\

I

473

573

;


40.

BEYER AND JACOBS

Reduction

and Reoxidation


503

Conclusion The r e d u c t i o n and subsequent r e o x i d a t i o n o f s i l v e r i o n s i n mordenite a r e r e v e r s i b l e p r o c e s s e s . During r e p e a t e d redox c y c l e s o n l y a s l i g h t i r r e v e r s i b i l i t y i s o b s e r v e d due t o deep bed e f f e c t s . The k i n e t i c laws o f the r e d u c t i o n a r e e s s e n t i a l l y t h e same as f o r t h e AgY system. The r e d u c t i o n o f A g i o n s i n t h e i n n e r s i t e s i s most d i f f i c u l t , w h i l e t h e r e i s k i n e t i c e v i d e n c e t h a t upon m i l d r e d u c t i o n t h e i o n s i n t h e main pores agglome r a t e as Ag^. The a c t i v a t i o n o f hydrogen o c c u r s by a c t i v a t i o n on the i r o n i m p u r i t i e s i n t h e z e o l i t e . Since s i l v e r par t i c l e s i n s i d e and o u t s i d e t h e z e o l i t e a r e o x i d i z e d a t a d i s t i n c t r a t e , t h e i r r e p a r t i t i o n may be determined easily.


+

Acknowledgments P.A.J, acknowledges a r e s e a r c h p o s i t i o n as "Aanges t e l d N a v o r s e r " from N.F.W.O. (Belgium).

L i t e r a t u r e Cited 1. Giordano, Ν., M o n t e l a t i c i , S . , and Zen, C., "Mole cular Sieves", p. 449, J.B. Uytterhoeven, e d . , Leuven University Press, 1973. 2. Tsutsumi, Κ . , and Takahashi, Η . , Bull. Chem. Soc. Japan, (1972), 45, 2332. 3. Riekert, L., Ber. Bunsenges. Phys. Chem., (1973), 73, 331. 4. Beyer, H., Jacobs, P.Α., and Uytterhoeven, J.B., J.C.S. Faraday I , (1976), 72, 674. 5. M o r t i e r , W . J . , P l u t h , J.J., and Smith, J.V., Proc. Conf. Natural Z e o l i t e s , Tucson, Pergamon Press, to be published, 1976. 6. Derouane, E., Mestdagh, Μ . , and Vielvoye, L., J. C a t a l y s i s , (1974), 33, 169. 7. Karge, H., Z e i t s c h r . Phys. Chem. N.F., (1971), 76, 133. 8. Karge, H., Z e i t s c h r . Phys. Chem. N.F., (1975), 95, 241. 9. Karge, H., and Klose, H., Z e i t s c h r . Phys. Chem. N.F., (1973), 83, 100. 10. Herman, R . G . , Lunsford, J.H., Beyer, Η . , Jacobs, P.Α., and Uytterhoeven, J.B., J. Phys. Chem., (1975), 79, 2388. 11. Jacobs, P . Α . , and Beyer, Η . , J.C.S. Faraday I , to be published.


Reduction and Reoxidation of Silver-Mordenites - ACS Symposium

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