Dispersion of Metallic Derivates on Carbon Supports - American

21

Petroleum-Derived Carbons Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/18/19. For personal use only.

Dispersion of Metallic Derivates on Carbon Supports Pierre Ehrburger and Jacques Lahaye Centre de Recherches sur la Physico-Chimie des Surfaces Solides, C N R S , 24 avenue du Président Kennedy, 68200 Mulhouse, France

The dispersion of metallic derivates on a carbon surface is dependent upon the surface properties of the carbon support. Edge carbon atoms which constitute the majority of the active surface area of graphitized carbon blacks may interact with the catalyst precursor or the catalyst itself thus providing a higher state of dispersion. Moreover specific interactions of the metallic derivates with the edge carbon atoms tend to decrease the sinte ring rate of the supported solid. D u r i n g t h e l a s t two decades the use o f carbon as a c a t a l y s t support has g a i n e d growing i n t e r e s t . Depending on the p r e c u r s o r and on t h e p r o c e s s i n g , t h e carbonaceous m a t e r i a l s w i l l p r e s e n t d i f f e r e n t types of p o r o s i t y and o f c r y s t a l l i n e t e x t u r e thus e n a b l i n g wide p o s s i b i l i t i e s f o r t a i l o r i n g a c a t a l y s t c a r r i e r (J_) . F o r i n s t a n c e a c t i v a t e d c a r b o n s , d e v e l o p i n g h i g h p o r o s i t i e s and s u r f a c e areas a r e commonly used as s u p p o r t s f o r n o b l e m e t a l s i n heterogeneous c a t a l y s i s ( 2 - 4 ) . M e t a l l i c o x i d e s l i k e the w e l l known chromium and copper o x i d e s f o r the removal o f t o x i c s from t h e a i r a r e a l s o c u r r e n t l y d i s p e r s e d on carbons w i t h h i g h s u r f a c e a r e a s . More r e c e n t l y , supported molybdenum s u l f i d e appears t o be v e r y p r o m i s i n g f o r t h e h y d r o d e s u l f u r i z a t i o n o f p e t r o l e u m f e e d s t o c k s ( 5 ) . Other a p p l i c a t i o n s f o r carbon s u p p o r t s a r e i n t h e f i e l d o f e l e c t r o c h e m i s t r y . Carbon e l e c t r o d e s w i t h few amounts of n o b l e m e t a l s ( P t , Ag) o r m e t a l p h t h a l o c y a n i n e s a r e found i n m e t a l - a i r b a t t e r i e s o r a l k a l i n e f u e l c e l l s ( 6 - 8 ) . Supported m e t a l s o r o x i d e s a r e a l s o o f prime importance f o r changing t h e r e a c t i v i t y of t h e c a r r i e r i t s e l f l i k e f o r i n s t a n c e i n c a t a l y t i c g a s i f i c a t i o n o f carbonaceous m a t e r i a l s ( 9 - 1 0 ) . The most i m p o r t a n t a p p l i c a t i o n s o f carbon supported c a t a l y s t s a r e i n d i c a t e d i n T a b l e I . I t i s now w e l l r e c o g n i z e d t h a t t h e e f f i c i e n c y o f a m e t a l c a t a l y s t i s enhanced by i n c r e a s i n g i t s d i s p e r s i o n w h i c h may be d e f i n e d as t h e r a t i o o f t h e number o f s u r f a c e m e t a l atoms t o t h e t o t a l number o f m e t a l atoms, A s i m p l e way t o a c h i e v e h i g h degree o f d i s p e r s i o n c o n s i s t s i n d e p o s i t i n g t h e c a t a l y s t on h i g h s u r f a c e a r e a s u p p o r t s . T h e r e f o r e t h e p r i m a r y r o l e s o f t h e support a r e t o p r o v i d e a s t r u c t u r a l framework w h i c h enables 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 phase and t o i n c r e a s e i t s s t a b i l i t y when s u b m i t t e d t o s i n t e r i n g c o n d i t i o n s . 0097-6156/86/0303-0310$06.00/0 © 1986 American Chemical Society

21.

EHRBURGER

Table I .

Dispersion of Metallic Dérivâtes on Supports

A N D LAHAYE

311

D i s p e r s i o n of M e t a l l i c Dérivâtes on Carbons

-Electrocatalysis Metals ( P t , Ag, N i ) - Metal Phthalocyanines -Removal of t o x i c s Copper/Chromium o x i d e s -Change o f r e a c t i v i t y o f t h e carbon support Catalytic gasification I n h i b i t i o n of o x i d a t i o n S e v e r a l methods have been proposed f o r p r e p a r i n g h i g h l y d i s p e r s e d c a t a l y s t on c a r b o n . The t h r e e most common procedures a r e i m p r e g n a t i o n , i o n i c exchange and vapor phase c o n d e n s a t i o n . The former c o n s i s t s i n w e t t i n g t h e carbon by a s o l u t i o n of an a p p r o p r i a t e s a l t (11, 12) w h i l e t h e second i s based on t h e exchange o f carbon s u r f a c e groups w i t h c a t i o n i c complexes ( 1 3 , 1 4 ) . I n t h e vapor phase c o n d e n s a t i o n t h e support i s c o n t a c t e d w i t h t h e v o l a t i l e m e t a l o r m e t a l l i c d e r i v a t e ( 1 5 , 1 6 ) . When n e c e s s a r y , t h e supported p r e c u r s o r i s then reduced i n t o t h e m e t a l l i c s t a t e . I t has a l s o been e s t a b l i s h e d t h a t t h e support may i n t e r a c t w i t h the c a t a l y s t thus changing i t s c a t a l y t i c a c t i v i t y ( 1 0 , 1 7 ) . Systema t i c i n v e s t i g a t i o n s o f t h e i n f l u e n c e of t h e support on t h e c a t a l y s t properties (dispersion, s i n t e r i n g , catalyst-support interactions) have been c a r r i e d out w i t h i n o r g a n i c m a t e r i a l s l i k e s i l i c a , a l u m i n a and z e o l i t h e s (18) . I n c o n t r a s t , s i m i l a r s t u d i e s have n o t y e t been r e p o r t e d w i t h carbon s u p p o r t s . T h i s paper d e a l s w i t h t h e f a c t o r s a f f e c t i n g t h e d i s p e r s i o n of a c a t a l y s t on carbon b l a c k s and i t s s i n t e r i n g b e h a v i o r . The s e l e c t e d system i s i r o n - p h t h a l o c y a n i n e (PcFe) supported on g r a p h i t i z e d carbon b l a c k s which i s an e f f i c i e n t c a t a l y s t f o r t h e e l e c t r o - c h e m i c a l r e d u c t i o n of oxygen ( 7 ) . Carbon s u p p o r t . The s u r f a c e o f a g r a p h i t i z e d carbon c o n s i s t s o f two types of c r y s t a l l i n e p l a n e s , t h e b a s a l p l a n e s and t h e p r i s m a t i c o r edge p l a n e s . The r e a c t i v i t y o f t h e carbon atoms i s u s u a l l y h i g h e r i n t h e edge planes than i n the basal p l a n e s . I n p a r t i c u l a r , edge carbon atoms chemisorb oxygen and g i v e r i s e t o oxygenated f u n c t i o n a l g r o u p s . I t may be expected t h a t t h e d i s p e r s i o n of PcFe w i l l depend on t h e e x t e n t o f edge carbon atoms p l a n e s . I n t h e p r e s e n t study two types of carbon s u r f a c e s have been s e l e c t e d : - a homogeneous s u r f a c e w h i c h i s b u i l d - u p w i t h b a s a l p l a n e s ( g r a p h i t i z e d f u r n a c e b l a c k V u l c a n 3 , V3G) - a heterogeneous s u r f a c e c o n s i s t i n g i n b o t h types o f p l a n e s and o b t a i n e d by a c t i v a t i o n i n a i r o f t h e homogeneous s u b s t r a t e ( a c t i v a t e d V3G). The a c t i v a t i o n procedure has been d e s c r i b e d elsewhere ( 1 6 ) . D u r i n g the g a s i f i c a t i o n , the t o t a l s u r f a c e area (S^E-JO w e l l as the a c t i v e s u r f a c e a r e a ( S ) , i . e . t h e a r e a o f t h e edge carbon atoms p l a n e s i n c r e a s e . The s u r f a c e a r e a d a t a o f b o t h samples a r e shown i n Table I I . a

a

s

312

PETROLEUM-DERIVED CARBONS

Table I I .

T o t a l S u r f a c e A r e a and A c t i v e S u r f a c e A r e a of the Carbon Supports S a (m /g)

S „ BET (m /g) 73 100 m

Carbon

2

2

V3G A c t i v a t e d V3G (50 % b u r n - o f f )

-

4.3

I t i s seen t h a t the s u r f a c e of V3G c o n s i s t s m a i n l y of b a s a l p l a n e s whereas about 4 % of the a r e a o f the a c t i v a t e d V3G sample i s b u i l d up w i t h edge carbon atoms. D e p o s i t i o n of the c a t a l y s t . The c h e m i c a l s t r u c t u r e of i r o n - p h t h a l o c y a n i n e i s shown on F i g u r e 1. PcFe i s a p l a n a r m o l e c u l e w i t h a c e n t r a l c a v i t y i n w h i c h the i r o n atom i s l o c a t e d . M e t a l p h t h a l o c y a n i n e s may be s u b l i m a t e d and the d e p o s i t i o n of PcFe on the carbon support has been done by vapor phase c o n d e n s a t i o n ( 1 6 ) . D u r i n g the c o n d e n s a t i o n , the carbon s u p p o r t s a r e k e p t a t 235°C. C o n d e n s a t i o n r a t e s were i n the range 2-4 mg PcFe p e r h o u r . Samples w i t h PcFe c o n t e n t r a n g i n g from 0.5 to 30 % i n w e i g h t have been p r e p a r e d w i t h b o t h c a r b o n s u b s t r a t e s . D i s p e r s i o n of P c F e . S u r f a c e a r e a o f supported P c F e . The s u r f a c e a r e a o f PcFe d e p o s i t s has been d e t e r m i n e d by measuring the amount of w a t e r m o l e c u l e s i r r e v e r s i b l y adsorbed on t h e i r s u r f a c e . We have shown t h a t each s u r f a c e m o l e c u l e of t h e PcFe c r y s t a l r e t a i n s on the average one m o l e c u l e of water a c c o r d i n g t o the f o l l o w i n g r e a c t i o n (16, 1 9 ) , PcFe

. + H 0 surface 2 o

+

PcFe

. - H 0 surface 2 o

The PcFe-H20 bond i s s t a b l e under vacuum up to 340 K . A t h i g h e r t e m p e r a t u r e s , the adsorbed w a t e r desorbs q u a n t i t a t i v e l y . The mean s u r f a c e a r e a of an a d s o r p t i o n s i t e f o r w a t e r has been e s t i m a t e d t o 0.83 nm from the l a t t i c e parameters of PcFe 3- T h i s v a l u e i s i n f a c t c l o s e to the geometric a r e a of a s i n g l e PcFe m o l e c u l e . The s u r f a c e a r e a developed by PcFe i s measured f o r d i f f e r e n t amounts of PcFe d e p o s i t e d on b o t h carbon s u p p o r t s , as shown on F i g u r e 2. As the two carbon s u b s t r a t e s have d i f f e r e n t t o t a l s u r f a c e a r e a s , the amount of supported PcFe i s e x p r e s s e d i n mg p e r m of carbon s u r f a c e i n o r d e r to f a c i l i t a t e the comparison between the two s u p p o r t s . On the homogeneous carbon s u p p o r t , the s u r f a c e a r e a o f PcFe i n c r e a s e s o n l y s l i g h t l y and reaches a maximum f o r a c o n t e n t of about 7 % by w e i g h t . F o r h i g h e r amounts o f P c F e , t h e r e i s a s m a l l decrease i n i t s s u r f a c e a r e a . The r e s u l t s f o r the heterogeneous carbon a r e s t r i k i n g l y d i f f e r e n t . The s u r f a c e areas of PcFe are h i g h e r on the a c t i v a t e d V3G and r e a c h a maximum v a l u e of 0,25 m / m f o r a l o a d of 20 % i n w e i g h t . There i s a more pronounced growth of the s u r f a c e o f the PcFe aggregates on the heterogeneous carbon s u b s t r a t e . C o n s i d e r i n g a s p h e r i c a l p a r t i c l e shape, the mean number of PcFe 2

2

2

2

21.

EHRBURGER AND LAHAYE

F i g u r e 1.


Chemical s t r u c t u r e of

iron-phthalocyanine.

F i g u r e 2. S u r f a c e a r e a of PcFe as a f u n c t i o n of l o a d i n g on V3G ( · ) and a c t i v a t e d V3G (*). "Reproduced w i t h p e r m i s s i o n from R e f . 16. C o p y r i g h t 1983, 'Academic P r e s s " . 1

313


314

p a r t i c l e s may be determined from the r e l a t i o n : x i - i - E 36ππι

(1)

2

S m η ρ

: : : :

s u r f a c e a r e a of PcFe per u n i t a r e a of carbon support weight of d e p o s i t e d PcFe per u n i t a r e a of carbon support mean p a r t i c l e number of PcFe per u n i t a r e a of carbon support d e n s i t y of PcFe (1.52) F i g u r e 3 shows the change of η as a f u n c t i o n of the amount of d e p o s i t e d P c F e . The number of PcFe p a r t i c l e s does not v a r y s i g n i f i c a n t l y on the a c t i v a t e d V3G support f o r l o a d i n g l e s s t h a n 20 % i n weight whereas i t d e c r e a s e s by a f a c t o r 10 on the g r a p h i t i z e d V3G c a r b o n . Moreover, by e x t r a p o l a t i n g b o t h c u r v e s a t low c o n t e n t of P c F e , i t appears t h a t the number of PcFe n u c l e i i s about the same on b o t h s u b s t r a t e s . Thus the s u r f a c e p r o p e r t i e s of the carbon a f f e c t more e x t e n s i v e l y the growth p r o c e s s of the p a r t i c l e s t h a n the i n i t i a l number of n u c l e i . C r y s t a l l i t e s i z e of PcFe p a r t i c l e s . The mean c r y s t a l l i t e diameters of the PcFe p a r t i c l e s c a l c u l a t e d from the b r o a d e n i n g of the (100) and (102) r e f l e x i o n s , are g i v e n i n T a b l e I I I . I t i s seen t h a t the c r y s t a l l i t e s i z e s i n b o t h d i r e c t i o n s are s i m i l a r w h i c h suggest t h a t t h e r e i s no s i g n i f i c a n t a n i s o t r o p y i n the shape of the PcFe c r y s t a l l i t e s . Table I I I .

Carbon

Mean C r y s t a l l i t e Diameter of P c F e , d X-Ray D i f f r a c t i o n support

V3G

A c t i v a t e d V3G

PcFe c o n t e n t (mg/m ) 2

0.71 0.77 1.36 2.03 2.14 0.80 1.17 1 .27 1 .80 2.50 2.80 4.30

determined by

fi

d

B

(nm) 102) 10 10 15 28 23 20 24 23 36 35 40 37

(100) 9 10 19 20 16 20 25 17 30 40 34 37

The d a t a of T a b l e I I a l s o i n d i c a t e t h a t f o r a g i v e n PcFe c o n t e n t the c r y s t a l l i t e s i z e i s g r e a t e r on the a c t i v a t e d c a r b o n . T h i s r e s u l t however does not mean t h a t the a c t u a l p a r t i c l e s i z e of PcFe i s g r e a t e r on the a c t i v a t e d c a r b o n . As w i l l be shown b e l o w , the p a r t i c l e s of PcFe may c o n t a i n s e v e r a l c r y s t a l l i t e s . The mean c r y s t a l l i t e diameter dg c a l c u l a t e d from the (100) r e f l e x i o n i s compared to the mean p a r t i c l e s i z e dg determined from the water uptake measurements i n F i g u r e 4 f o r b o t h carbon s u b s t r a t e s . I t appears t h a t 3 and 3$ i n f a i r l y good agreement i n the case of the heterogeneous carbon β

a r e



n. P c Fe p a r t i d e s / m

ι

ι

—J

1

2

3 loading,

PcFe

J ,

ο

mg/m*

4

F i g u r e 3 . Number o f PcFe p a r t i c l e s p e r u n i t a r e a o f carbon support as a f u n c t i o n o f PcFe l o a d i n g : V3G ( · ) , a c t i v a t e d V3G ( ) . "Reproduced w i t h p e r m i s s i o n from R e f . 16. C o p y r i g h t 1983, Academic P r e s s " . A

1

1

315


316

Pc Fe

l o a d i ng

,

mg / m

F i g u r e 4. Comparison between the mean p a r t i c l e s i z e dg ( · ) and the mean c r y s t a l l i t e s i z e 3 (•) of supported P c F e . "Reproduced w i t h p e r m i s s i o n from R e f . 16. C o p y r i g h t 1983, 'Academic P r e s s " β

1

21.



317

support f o r PcFe c o n t e n t l e s s than 20 % i n w e i g h t . I n c o n t r a s t , dfi i s always s m a l l e r than dg w i t h the homogeneous c a r b o n , the d i s c r e p a n cy i n c r e a s i n g w i t h the PcFe c o n t e n t . These r e s u l t s may be e x p l a i n e d by the f a c t t h a t the PcFe p a r t i c l e s are e s s e n t i a l l y formed by a s i n g l e c r y s t a l l i t e on the a c t i v a t e d carbon s u r f a c e whereas the PcFe aggregates are p o l y c r y s t a l l i n e on the g r a p h i t i z e d V3G s u r f a c e . Hence, the s u r f a c e p r o p e r t i e s of the carbon support a l s o a f f e c t the c r y s t a l l i n e t e x t u r e of the PcFe d e p o s i t s . Growth of PcFe P a r t i c l e s . D u r i n g the d e p o s i t i o n p r o c e s s , the PcFe p a r t i c l e s behave d i f f e r e n t l y on the two carbon s u p p o r t s . They a r e p o l y c r y s t a l l i n e on the homogeneous carbon s u r f a c e and t h e i r number d e c r e a s e s w i t h the PcFe l o a d i n g , i . e . w i t h the d e p o s i t i o n t i m e . These f a c t s p o i n t to a p a r t i a l c o a l e s c e n c e of the PcFe p a r t i c l e s on the g r a p h i t i z e d V3G s u r f a c e . T h e r e f o r e the s u r f a c e a r e a developed by PcFe w i l l depend on two mechanisms a c t i n g i n an o p p o s i t e way : growth of the n u c l e i by c a p t u r e of PcFe monomers and c o a l e s c e n c e of t h e p a r t i c l e s . C o a l e s c e n c e of supported c l u s t e r s may o c c u r i n two ways, s t a t i c a l l y due to the o v e r l a p p i n g of two growing p a r t i c l e s a n d / o r d y n a m i c a l l y as a r e s u l t of p a r t i c l e m i g r a t i o n . E x p e r i m e n t a l e v i d e n c e s of the motion of c l u s t e r s on a s o l i d s u r f a c e are now w e l l e s t a b l i s h e d ( 2 0 - 2 2 ) . Hence the s m a l l e r s u r f a c e a r e a developed by PcFe on the homogeneous carbon would r e s u l t from a h i g h e r m i g r a t i o n r a t e of the p a r t i c l e s as compared to t h e i r r a t e on a heterogeneous carbon. S i n t e r i n g of Supported PcFe S i n t e r i n g of supported s o l i d s can o c c u r by two d i s t i n c t mechanisms : p a r t i c l e m i g r a t i o n and c o a l e s c e n c e as a l r e a d y mentioned above and i n t e r p a r t i c l e t r a n s p o r t of atoms or m o l e c u l e s (Ostwald r i p e n i n g ) ( 2 3 ) . I n t e r p a r t i c l e t r a n s p o r t may be p o s s i b l e e i t h e r by s u r f a c e d i f f u s i o n a c r o s s the support or by vapor phase t r a n s p o r t . Depending on the supported systems and on the s i n t e r i n g c o n d i t i o n s the p a r t i c l e s may grow p r e d o m i n a n t l y v i a one of t h e s e p o s s i b l e r o u t e s . Supported PcFe d e p o s i t s have been s i n t e r e d i n e x p e r i m e n t a l c o n d i t i o n s c l o s e to t h a t of the c o n d e n s a t i o n p r o c e s s (T = 235°C, r e s i d u a l p r e s s u r e 10"^ t o r r ) . The s u r f a c e areas of PcFe a f t e r 2 and 3 hours of s i n t e r i n g a r e shown i n T a b l e I V . I t i s seen t h a t the s i n t e r i n g e f f e c t i s more pronounced w i t h a homogeneous carbon s u p p o r t . Table IV.

S i n t e r i n g of Supported PcFe P a r t i c l e s

Support V3G A c t i v a t e d V3G

S i n t e r i n g time (h) as prepared 2 3 as p r e p a r e d 2 3

S u r f a c e a r e a of (m /m ) 0.095 0.032 0.017 0.130 0.091 0.085 2

2

PcFe


318

The r a t e of s i n t e r i n g d S / d t i s g e n e r a l l y r e l a t e d to the time by a r a t e - p o w e r law §| = - KS

(2)

n

o r by i n t e g r a t i n g s

1-n_

s

ο

1-n

=

K

,

t

( 3 )

S and S are r e s p e c t i v e l y the s u r f a c e areas of the d i s p e r s e d s o l i d a t time t and t = o. K i s a c o n s t a n t and η i s an i n t e g r a t o r r a n g i n g g e n e r a l l y b e t ween 2 and 12 (18) . The v a l u e s of K and η have been determined f o r b o t h systems u s i n g a l e a s t square f i t t i n g method f o r e q u a t i o n 3 . The s i n t e r i n g r a t e s of PcFe on the homogeneous and heterogeneous carbon r e s p e c t i v e l y are : Q

1

1

(||)

horn.

(*§.) d t

= -15.3 S

. _

1 < 3

1 0

4

S in m /m ,

2

2

g

2

t

i n hour

6

het.

The i n i t i a l s i n t e r i n g r a t e of PcFe may be e s t i m a t e d by t a k i n g S = 0.1 m / m . Hence, the c o r r e s p o n d i n g r e l a t i v e s i n t e r i n g r a t e on the two s u p p o r t s i s : 2

2

Φa t

ιΦ

= 12

, dt , horn. het. The s i n t e r i n g r a t e i s more t h a n one o r d e r of magnitude h i g h e r on V3G t h a n on the a c t i v a t e d carbon V3G at the maximum coverage of V3G w i t h P c F e . T h i s r e s u l t c o n f i r m s q u a l i t a t i v e l y the h y p o t h e s i s of a more pronounced c o a l e s c e n c e r a t e of PcFe on the homogeneous support d u r i n g the d e p o s i t i o n p r o c e s s . S i n t e r i n g mechanism on g r a p h i t i z e d carbon b l a c k s have a l s o been i n v e s t i g a t e d w i t h o t h e r c a t a l y s t s l i k e p l a t i n u m . B e t t et a l . (24) showed t h a t the s i n t e r i n g of supported P t may be due to s u r f a c e d i f f u s i o n of m e t a l p a r t i c l e s . The Pt c r y s t a l l i t e s m i g r a t e from t r a p s i t e s on the support and c o a l e s c e on the carbon s u r f a c e . More r e c e n t l y , E h r b u r g e r and Walker (25) s t u d i e d the p a r t i c l e s i z e d i s t r i b u t i o n of P t p a r t i c l e s on a c t i v a t e d and n o n a c t i v a t e d V3G s u p p o r t s . On the homogeneous carbon s u b s t r a t e , t h e p a r t i c l e s i z e f o l l o w s a log-normal d i s t r i b u t i o n which i s c o n s i s t e n t w i t h a s i n t e r i n g mechanism based on p a r t i c l e m i g r a t i o n f o l l o w e d by c o a l e s c e n c e . On the a c t i v a t e d V3G s u p p o r t , t h e s i n t e r i n g b e h a v i o r of Pt i s d i f f e r e n t . P a r t i c l e growth i s d e t e c t a b l e a t much h i g h e r tempe r a t u r e t h a n on the homogeneous carbon and i t was c o n c l u d e d t h a t the m e t a l c l u s t e r s a r e t r a p p e d on s u r f a c e h e t e r o g e n e i t i e s . C o n s e q u e n t l y , s i n t e r i n g would o n l y s t a r t when the Pt p a r t i c l e s have enough energy to escape from the t r a p p i n g s i t e s . These few examples c l e a r l y show t h a t the s u r f a c e p r o p e r t i e s of the carbons i n f l u e n c e the d i s p e r s i o n and the s i n t e r i n g of supported c a t a l y s t s . fc

21.



319

C a t a l y s t - C a r b o n Support I n t e r a c t i o n s . I n t e r a c t i o n s between the c a t a l y s t and the carbon s u p p o r t s o b v i o u s l y depend on the type of carbonaceous m a t e r i a l u s e d . I n the case of a c t i v a t e d c a r b o n , t h e * h i g h l y porous t e x t u r e a c t s as a framework f o r d i s p e r s i o n of the c a t a l y s t and the pores may be c o n s i d e r e d as t r a p p i n g s i t e s w h i c h h i n d e r s i n t e r i n g . F o r l e s s porous carbon l i k e amorphous carbon f i l m o r carbon b l a c k s the concept of t r a p p i n g s i t e s i s s t i l l a p p l i c a b l e . P h i l l i p s , e t . a l . (26) developed t h i s concept f o r e x p l a i n i n g the s i n t e r i n g mechanism of g o l d c l u s t e r s on c a r b o n . L a t e r on, B e t t , e t . a l . (24) suggested t h a t the b o u n d a r i e s of b a s a l p l a n e s may be the t r a p p i n g s i t e s r e q u i r e d f o r the i m m o b i l i z a t i o n of p l a t i num p a r t i c l e s . E h r b u r g e r , e t . a l . (27) a l s o found t h a t the degree of d i s p e r s i o n of p l a t i n u m i n c r e a s e s w i t h i n c r e a s i n g s u r f a c e h e t e r o g e n e i t y of g r a p h i t i z e d s u p p o r t s . C o n s i d e r i n g P t p a r t i c l e s s i z e d i s t r i b u t i o n and s i n t e r i n g b e h a v i o u r , they a t t r i b u t e d a h i g h e r degree of d i s p e r s i o n of the m e t a l to a s t r o n g e r i n t e r a c t i o n of the c h l o r o p l a t i n i c p r e c u r s o r w i t h the s u r f a c e h e t e r o g e n e i t i e s (28) . More r e c e n t l y B e n e d e t t i , e t . a l . (29) a l s o a t t r i b u t e d changes i n the d i s p e r s i o n of p a l l a d i u m on c h a r c o a l s t o s u r f a c e h e t e r o g e n e i t y e f f e c t s . I n the case of i r o n - p h t h a l o c y a n i n e , i t was p o s s i b l e t o i d e n t i f y the n a t u r e of i t s i n t e r a c t i o n w i t h the s u p p o r t . PcFe exchanges c o o r d i n a t i n g bonds w i t h a v a r i e t y of m o l e c u l e s l i k e p y r i d i n e and q u i n o l i n e (30) . A complex P c F e - ^ O w i t h a bond between the m e t a l atom and the water m o l e c u l e has a l s o been e v i d e n c e d i n CCI4 ( 3 1 ) . S i n c e the a c t i v e s u r f a c e of carbon i s u s u a l l y covered w i t h oxygen complexes ( m a i n l y c a r b o n y l groups) a h i g h e r s t a b i l i t y o f the PcFe p a r t i c l e s on a c t i v a t e d V3G may o r i g i n a t e from i n t e r f a c i a l bonds w i t h oxygen g r o u p s . A p o s s i b l e i n t e r a c t i o n between oxygen s u r f a c e groups and PcFe m o l e c u l e s i s drawn s c h e m a t i c a l l y on F i g u r e 5. I n o r d e r to v e r i f y t h i s h y p o t h e s i s , the oxygen groups of the carbon have been removed by H2 t r e a t m e n t a t 950°C. F o l l o w i n g t h i s t r e a t m e n t , the carbon was c o o l e d i n H2 and under these c o n d i t i o n s , the a c t i v e s u r f a c e a r e a w i l l be covered w i t h chemisorbed h y d r o g e n . Two samples h a v i n g r e s p e c t i v e l y 14.5 and 26 % PcFe i n weight have been p r e p a r e d w i t h the hydrogen t r e a t e d heterogeneous c a r b o n . The c o r r e s p o n d i n g s u r f a c e areas of PcFe are compared t o the p r e v i o u s one on F i g u r e 6. I t i s seen t h a t a f t e r removal of the oxygen complexes the d i s p e r s i o n of PcFe decreases s i g n i f i c a n t l y and t h a t the carbon behaves l i k e the i n i t i a l V3G s u p p o r t . T h e r e f o r e , the edge carbon atoms v i a the c h e m i sorbed oxygen p l a y an i m p o r t a n t r o l e i n the d i s p e r s i o n and s i n t e r i n g of P c F e . Melendres (32) i n v e s t i g a t e d c a r b o n - s u p p o r t e d PcFe by Mossbauer and Raman s p e c t r o s c o p y and found changes i n m o l e c u l a r e l e c t r o n i c s t r u c t u r e i n g o i n g from pure PcFe t o c a r b o n - s u p p o r t e d P c F e . These o b s e r v a t i o n s p r o v i d e f u r t h e r p h y s i c a l evidence of c a t a l y s t - s u p p o r t i n t e r a c t i o n and c o n f i r m our own r e s u l t s . Conclusion. The s u r f a c e p r o p e r t i e s o f carbon p l a y an i m p o r t a n t r o l e i n the d i s p e r s i o n and the s i n t e r i n g b e h a v i o r o f supported c a t a l y s t . The study of i r o n - p h t h a l o c y a n i n e i s of p a r t i c u l a r l y i n t e r e s t f o r the i n v e s t i g a t i o n of c a t a l y s t - c a r b o n support i n t e r a c t i o n s . The c o a l e s c e n c e of PcFe p a r t i c l e s on g r a p h i t i z e d carbon i s c o n s i d e r a b l y lowered by the

320


F i g u r e 5. I n t e r a c t i o n between an oxygen complex and a PcFe m o l e c u l e . "Reproduced w i t h p e r m i s s i o n from R e f . 16. C o p y r i g h t 1983, 'Academic P r e s s " . 1

F i g u r e 6. S u r f a c e a r e a of PcFe supported on the a c t i v a t e d and hydrogen t r e a t e d V3G (•) compared t o the c o r r e s p o n d i n g d a t a on V3G ( · ) and a c t i v a t e d V3G ( a ) . "Reproduced w i t h p e r m i s s i o n from R e f . 16. C o p y r i g h t 1983, 'Academic P r e s s ' " .

21.


Dispersion of Metallic Derivates on Supports

321

presence of oxygenated groups l o c a t e d on edge carbon atoms which a c t as anchors o f t h e supported p a r t i c l e s . I t i s r e l e v a n t t o note t h a t o n l y a few p e r c e n t of a c t i v e s u r f a c e a r e a ensures a h i g h e r d i s p e r s i o n o f PcFe on carbonaceous s u p p o r t s .

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Dispersion of Metallic Derivates on Carbon Supports - American

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