Sintering of a model nickel/alumina catalyst

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SINTERING OF A MODEL

NICKEL/ALUMINA

NICKEL/ALUMINA

CATALYST

PROEFSCHRIFT

TER VERKRUGING VAN DE GRAAD VAN DOCTOR

AAN DE TECHNISCHE UNIVERSITEIT DELFT, OP

GEZAG VAN DE RECTOR MAGNIFICUS, PROF.DRS.

PA. SCHENCK, IN HET OPENBAAR TE VERDEDIGEN

TEN OVERSTAAN VAN EEN COMMISSIE AANGEWEZEN

DOOR HET COLLEGE VAN DEKANEN, OP DINSDAG

23 MEI 1989 TE 16.00 UUR

DOOR

LUCRETIA AGNES CORREIA

SCHEIKUNDIG INGENIEUR

GEBOREN TE PARAMARIBO,

SURINAME

TR diss

1725

«fc-uk Wfftru(l".<.iirt.i!Mrikiihhit!i| iwUiKawJ

This investigation was supported by the Netherlands Foundation for

Chemical Research (SON) with the financial aid from the Netherlands

Organization for Advancement of Pure Research (Z.W.O.).

hebben geleverd aan de totstandkoming van d i t p r o e f s c h r i f t . Enkele van hen wil ik gaarne a p a r t vermelden.

- Mevr. W.H. Batenburg - Van der Vegte van de vakgroep B i o t e c h n o l o g i e , voor de a s s i s t e n t i e verleend b i j het Microscopisch onderzoek. - Dhr. T h . L . J . de Haan van de t u s s e n a f d e l i n g der Materiaalkunde, voor

de e l e c t r o n e n d i f f r a c t i e en EDX-analyse.

- Dhr. C.G. Borsboorn van de t u s s e n a f d e l i n g der Materiaalkunde, voor de verleende ondersteuning b i j het ESCA-onderzoek.

- Dhr. I r . J . L u i j e r i n k van de a f d e l i n g der C i v i e l e Techniek, voor de verleende f a c i l i t e i t e n op het gebied van b e e l d a n a l y s e .

- Dhr. J . H . F . Grondel voor z i j n technische adviezen en ondersteuning b i j het o p z e t t e n van de f a c i l i t e i t e n t . b . v . het dunnelaagonderzoek. - De medewerkers van de instrumentmakerij van de a f d e l i n g , voor, met

name, de opdampklok.

- Mijn werkgever, het ECN, ben ik zeer e r k e n t e l i j k voor de f a c i l i t e i t e n , mij geboden om d i t p r o e f s c h r i f t t e v o l t o o i e n .

In het bijzonder gaat mijn dank u i t naar de dames van de typekaner, o . a . mej. E. Warmerdam en mej. A. Bruin.

1. INTRODUCTION 1 1 . 1 . G e n e r a l s u r v e y 1 1 . 2 . Lay o u t of t h e s i s 6 1 . 3 . R e f e r e n c e s

2 . ELECTRON MICROSCOPIC STUDY OF THE SINTERINC OF THE METAL

PARTICLES IN A N i / A l203 MODEL CATALYST 9

2 . 1 . I n t r o d u c t i o n 9 2 . 2 . P r e p a r a t i o n of t h e model c a t a l y s t 9 2 . 2 . 1 . P r e p a r a t i o n o f t h i n a l u m i n a s u p p o r t s 9 2 . 2 . 2 . D e p o s i t i o n of m e t a l s 11 2 . 3 . S i n t e r i n g of t h e model c a t a l y s t i n h y d r o g e n 14 2 . 3 . 1 . E q u i p m e n t 14 2 . 3 . 2 . P r o c e d u r e 16 2 . 3 . 3 . A n a l y s i s by TED and EDX 18

2 . 4 . R e s u l t s of t h e a n a l y s i s by TED and EDX 19 2 . 4 . 1 . C o m p o s i t i o n of t h e m o d e l c a t a l y s t 19 2 . 4 . 2 . D i s c u s s i o n 21 2 . 5 . E l e c t r o n m i c r o s c o p e r e s u l t s a b o u t p a r t i c l e g r o w t h and p a r t i c l e s i z e d i s t r i b u t i o n 22 2 . 5 . 1 . S i n t e r i n g a t 873 K 22 2 . 5 . 2 . S i n t e r i n g a t 973 K 27 2 . 6 . D i s c u s s i o n and c o n c l u s i o n s 35 2 . 7 . R e f e r e n c e s 37

3 . ANALYSIS OF THE CHEMICAL COMPOSITION AND STRUCTURE AT

NICKEL-ALUMINA INTERFACES 39 3 . 1 . I n t r o d u c t i o n 39 3 . 2 . P r e p a r a t i o n of s p e c i m e n s 39 3 . 2 . 1 . Alumina s u b s t r a t e s 39 3 . 2 . 2 . M e t a l d e p o s i t s 41 3 . 2 . 3 . S t a n d a r d s a m p l e s 41 3 . 3 . The XPS-method 42 3 . 3 . 1 . P r i n c i p l e s 42 3 . 3 . 2 . E v a l u a t i o n of XPS d a t a 4 5 3 . 4 . R e s u l t s and d i s c u s s i o n 4 9

3 . 5 . Conclusions 61

3 . 6 . R e f e r e n c e s 62

KINETICS OF THE GROWTH OF INDIVIDUAL PARTICLES

4 . 1 . I n t r o d u c t i o n 63 4 . 2 . Approximations for the r a t e s and the e q u i l i b r i a of

elementary r e a c t i o n s t e p s 63 4 . 2 . 1 . T r a n s f e r between bulk n i c k e l ( f l a t s u r f a c e ) and

n i c k e l vapour 65 4 . 2 . 2 . Transfer between n i c k e l bulk and adsorbed n i c k e l 68

4 . 2 . 3 . Adsorption and d e s o r p t l o n of n i c k e l vapour on and

from a f l a t n i c k e l s u r f a c e 68 4 . 2 . 4 . Adsorption and d e s o r p t l o n of n i c k e l vapour on and

from a f l a t alumina s u r f a c e 69 4 . 2 . 5 . Diffusion to and from a small sphere (embedded in

v a p o u r ) , a small c i r c u l a r disk ( i n i n t e r a c t i o n with adsorbed l a y e r ) and a f l a t s u r f a c e (embedded In

vapour) 69 4 . 2 . 6 . Diffusion of n i c k e l atoms over the s u r f a c e towards

the edge of a half sphere of n i c k e l , supported by a f l a t alumina s u r f a c e , and the t r a n s i t i o n a t the edge from the n i c k e l p a r t i c l e to the alumina

s u r f a c e 70

4 . 2 . 7 . The d i f f u s i o n c o e f f i c i e n t s 71 4 . 3 . General formulae for growth r a t e s 72

4 . 3 . 1 . General remarks 72 4 . 3 . 2 . Formulae for maximum growth r a t e s 73

4 . 4 . Q u a n t i t a t i v e e v a l u a t i o n of the maximum r a t e s of the

i n d i v i d u a l growth s t e p s 78 4 . 4 . 1 . Heats of a d s o r p t i o n , a c t i v a t i o n e n e r g i e s for

d i f f u s i o n and s u r f a c e free e n e r g i e s 78

4 . 4 . 2 . F i n a l r e s u l t s 80 4 . 5 . The r o l e of s u p e r s a t u r a t l o n i n determining growth r a t e s 85

4 . 6 . Nucleation i n h i b i t e d growth as a p o s s i b l e r a t e determining 86 s t e p

1 . INTRODUCTION 1 . 1 . G e n e r a l s u r v e y I n many c a t a l y s t s f o r h y d r o g e n a t l o n o r c a r b o n monoxide c o n v e r s i o n p r o c e s s e s , t h e a c t i v e m a t e r i a l i s a t r a n s i t i o n m e t a l d i s p e r s e d on an o x i d i c s u p p o r t t o p r e v e n t c o n t a c t b e t w e e n t h e s m a l l p a r t i c l e s and t h u s t h e s i n t e r i n g of t h e m e t a l p h a s e . I t w i l l be c l e a r t h a t t h e s t a b i l i t y of t h e l a r g e a c t i v e s u r f a c e a r e a u n d e r o p e r a t i n g c o n d i t i o n s i s a r e q u i r e m e n t of e v e r y i n d u s t r i a l c a t a l y s t . Many c h e m i c a l c a u s e s may be r e s p o n s i b l e f o r c a t a l y s t d e t e r i o r a t i o n , s u c h a s p o i s o n i n g o r c a r b o n f o r m a t i o n . One p h y s i c a l e f f e c t , h o w e v e r , w i l l a l w a y s be p r e s e n t , namely s i n t e r i n g of t h e s m a l l m e t a l p a r t i c l e s a n d / o r t h e p o r o u s s u p p o r t due t o t r a n s p o r t of m a t e r i a l . S i n t e r i n g o r c o a r s e n i n g of s m a l l s u p p o r t e d m e t a l p a r t i c l e s I s d e f i n e d a s t h e p r o c e s s by which t h e mean p a r t i c l e s i z e of t h e d i s p e r s i o n i n c r e a s e s , a c c o m p a n i e d by a d e c r e a s e of t h e a c c e s s i b l e s u r f a c e a r e a . T h e r e a r e d i f f e r e n t p h y s i c a l p r i n c i p l e s I n v o l v e d d u r i n g t h i s p r o c e s s : a . When t h e p a r t i c l e s a r e s m a l l , t h e y c a n m i g r a t e on t h e s u b s t r a t e d u r i n g which t h e y c a n a p p r o a c h e a c h o t h e r and c o a l e s c e . b . When t h e s u p p o r t s i n t e r s , t h a t I s t h e s p e c i f i c s u r f a c e a r e a d e c r e a s e s , t h e d i s t a n c e b e t w e e n t h e p a r t i c l e s d e c r e a s e s and In t h e e x t r e m e c a s e p a r t i c l e s c o l l i d e and c o a l e s c e . c . I f t h e p a r t i c l e s a r e i m m o b i l e , c o a r s e n i n g c a n p r o c e e d by i n t e r p a r t i c l e t r a n s p o r t of a t o m i c ( m o l e c u l a r ) s p e c i e s e i t h e r by s u r f a c e d i f f u s i o n a l o n g t h e s u b s t r a t e o r by t r a n s p o r t t h r o u g h t h e v a p o u r p h a s e . I t I s t h e s i n t e r i n g of s m a l l m e t a l p a r t i c l e s t h a t w i l l be t h e s u b j e c t of t h i s t h e s i s .

The i n c e n t i v e t o t h i s work came from p a r a l l e l work i n o u r l a b o r a t o r y , w h e r e t h e p r e p a r a t i o n was s t u d i e d of a n i c k e l a l u m i n a c a t a l y s t f o r t h e c o n v e r s i o n of s y n t h e s i s g a s (CO + H?) t o m e t h a n e u n d e r o p e r a t i o n c o n d i t i o n s r e q u i r e d f o r i t s p o t e n t i a l a p p l i c a t i o n In t h e e x o t h e r m a l s t e p of an I n t e r m e d i a t e d i s t a n c e e n e r g y t r a n s p o r t c y c l e ( N . F . E . p r o j e c t of K . F . A . J i f l i c h ) [ l ] > A s p e c i a l f e a t u r e of t h e d e v e l o p e d c a t a l y s t I s t h e h i g h m e t a l t o s u p p o r t volume r a t i o ( 1 : 1 ) combined w i t h a h i g h t h e r r a o s t a b i l i t y . A p p a r e n t l y a t a c e r t a i n p a r t i c l e s i z e ( ~ 2 0 n m

s t a b i l i z a t i o n of p a r t i c l e s i z e , which I s observed e x p e r i m e n t a l l y . But, combinations of normal growth p r o c e s s e s and n u c l e a t l o n I n h i b i t i o n a r e very f l e x i b l e and t h e o r e t i c a l p r e d i c t i o n s can be e a s i l y adjusted to experimental d a t a .

The present t h e s i s I s concerned e s s e n t i a l l y with the s i n t e r i n g of n i c k e l p a r t i c l e s on an alumina s u p p o r t .

Two main l i n e s of i n v e s t i g a t i o n have been followed:

- The s i n t e r i n g has been followed e x p e r i m e n t a l l y by s t u d y i n g a model c a t a l y s t system c o n s i s t i n g of a d i s p e r s i o n of small n i c k e l p a r t i c l e s on a very t h i n non-porous alumina f o i l . P a r t i c l e s i z e s and s i z e d i s t r i b u t i o n s have been determined by means of e l e c t r o n microscopy. S i n t e r i n g i s performed by h e a t i n g in a hydrogen atmosphere a t 873 K and 973 K.

- An attempt Is made to put the t h e o r e t i c a l c o n s i d e r a t i o n s of B.K-Chakraverty and o t h e r s on a more q u a n t i t a t i v e b a s i s by making e s t i m a t e s for the frequency f a c t o r s of the r a t e s of a l l r e a c t i o n s t e p s by using E y r l n g ' s theory for r e a c t i o n r a t e s and by r e l a t i n g a l l p o s s i b l e energy terms i n the Arrhenlus f a c t o r s to the e x p e r i m e n t a l l y known value of the heat of e v a p o r a t i o n of n i c k e l atoms from bulk n i c k e l . In t h i s way i t Is hoped to come to a b e t t e r d i s c r i m i n a t i o n between t h e v a r i o u s p o s s i b l e r a t e determining s t e p s in the growth process and to a more s i g n i f i c a n t comparison of t h e o r e t i c a l p r e d i c t i o n s and experimental o b s e r v a t i o n s .

As a s i d e l i n e a l s o i n the experimental work some a t t e n t i o n i s devoted to the p o s s i b l e e f f e c t s of dopes on the s i n t e r i n g of the n i c k e l p a r t i c l e s . Of course the e f f e c t of s o - c a l l e d promoters on a c t i v i t y and s t a b i l i t y of supported metal c a t a l y s t s Is a well known t o p i c in the f i e l d of c a t a l y s i s . I t has been mentioned a l r e a d y , t h a t promoters may s t a b i l i z e porous s u p p o r t s a t high temperatures and thus enhance

s t a b i l i t y a g a i n s t s i n t e r i n g . A much more d i f f i c u l t q u e s t i o n i s , whether promoters a l s o w i l l i n f l u e n c e the s i n t e r i n g p r o p e r t i e s of small metal p a r t i c l e s due to Ostwald r i p e n i n g p r o c e s s e s , In the way they have been d i s c u s s e d thus f a r .

A term o f t e n used In d e s c r i b i n g the favourable e f f e c t of a support in p r e v e n t i n g the s i n t e r i n g of metal p a r t i c l e s i s m e t a l - s u p p o r t

diameter) a q u i t e s a t i s f a c t o r y s t a b i l i t y i s achieved, with a c a t a l y t i c a c t i v i t y s t i l l p r o p o r t i o n a l to the metal s u r f a c e exposed [ 2 ] .

R e l a t i v e l y few s y s t e m a t i c i n v e s t i g a t i o n s of s i n t e r i n g have been r e p o r t e d for M/AI2O3 c a t a l y s t s . C.H. Bartholomew e t a l . [3] have reported on the s i n t e r i n g behaviour of impregnated c a t a l y s t s loaded with 15% wt of n i c k e l ; E.B.M. Doesburg e t a l . [2] on the s i n t e r i n g behaviour of c o p r e c l p l t a t e d c a t a l y s t s loaded with up t o 75% wt of n i c k e l . Despite the d i f f e r e n c e in p r e p a r a t i o n c o n d i t i o n s a l l c a t a l y s t s showed a f a s t i n c r e a s e of the mean p a r t i c l e s i z e during the f i r s t 60 hours of s i n t e r i n g in hydrogen, a t for I n s t a n c e 973 K, a f t e r which the p a r t i c l e s i z e s t a b i l i z e s . The s p e c i f i c s u r f a c e a r e a s of support and metal show a s i m i l a r behaviour, v i z . a f a s t i n i t i a l decrease followed by a s t a b i l i z a t i o n . Thus I t i s concluded, t h a t s i n t e r i n g of the metal i s Induced by s i n t e r i n g of the s u p p o r t . This Idea i s reinforced by r e s u l t s of H. Schaper [ 4 ] , where s t a b i l i z a t i o n of the c a r r i e r a g a i n s t s i n t e r i n g by using 1-5 mol % L a - 0 , as dopant led to a b e t t e r s t a b i l i t y of t h e a c t i v i t y of the c a t a l y s t .

Despite the information so g a t h e r e d , i t I s very d i f f i c u l t to come to a q u a n t i t a t i v e understanding of s i n t e r i n g , a s i t i s hard to d i s t i n g u i s h between e f f e c t s caused by the c a r r i e r and the metal phase. The only way to do so i s by using a thermostable s u p p o r t . Such a system was f i r s t reported by E. Ruckensteln [5] by using e l e c t r o n microscopy to study d i s p e r s i o n s of very small n i c k e l p a r t i c l e s on t r a n s p a r a n t films of c r y s t a l l i n e alumina. Recently v a r i o u s i n v e s t i g a t i o n s of the s i n t e r i n g of such model c a t a l y s t s , u s i n g e l e c t r o n microscopy, have been

p u b l i s h e d .

P. Wynblatt e t a l . [6] thoroughly s t u d i e d the behaviour of a Pt/y-Al-O system in an oxygen environment. Using model c a t a l y s t s with a mean p a r t i c l e diameter of 6 up t o 15 nra, growth of the p a r t i c l e s was mainly observed within the very f i r s t few h o u r s , a f t e r which, as discussed above, the mean p a r t i c l e s i z e s t a b i l i z e s .

J . J . Chen e t a l . [ 7 ] , u s i n g a P d / A 1 . 0 , system with a mean p a r t i c l e s i z e of 3.5 run, a l s o observed a f a s t i n c r e a s e of the mean p a r t i c l e s i z e within the f i r s t hours of s i n t e r i n g in hydrogen.

metal phase i s s u b s t a n t i a l and of the same order of magnitude as for metal p a r t i c l e s in supported c a t a l y s t s .

In the l a s t decade a l s o a number of fundamental s t u d i e s have appeared, emphasizing various t h e o r e t i c a l a s p e c t s of the s i n t e r i n g of ensembles of small supported metal p a r t i c l e s [6, 8 - 1 0 ] .

S t a r t i n g with B.K. Chakraverty [9 |, the notion a r o s e , t h a t the s i n t e r i n g of small metal p a r t i c l e s on an i n e r t support can be t r e a t e d along the same l i n e s as the growing of c o l l o i ' d a l p a r t i c l e s in a

s o l u t i o n . Here the s o - c a l l e d Ostwald r i p e n i n g process o c c u r s , where the d r i v i n g force c o n s i s t s of the d i f f e r e n c e In s u r f a c e energy of small and l a r g e p a r t i c l e s and where the mass t r a n s p o r t c o n s i s t s of the d i f f u s i o n of atoms and molecules, d i s s o l v i n g a t the surface of the small

p a r t i c l e s and condensing a t the s u r f a c e of the l a r g e p a r t i c l e s . The t h e o r e t i c a l treatment of these phenomena Is due to C. Wagner [10 ], with Improvements due to I.M. L i f s h l t z and V.V. Slyozov [ l l |. Altogether t h i s d e s c r i p t i o n is well known as the I,.S.W.-theory. B.K. Chakraverty has been the f i r s t author to apply the L.S.W.-theory to d e s c r i b e the s i n t e r i n g of ensembles of small metal p a r t i c l e s . Two r a t e determining s t e p s for the t r a n s p o r t of metal atoms from one p a r t i c l e to another were c o n s i d e r e d : atom d i f f u s i o n over the s u b s t r a t e and atom t r a n s f e r a t the i n t e r f a c e between p a r t i c l e and s u b s t r a t e . I t was shown, t h a t 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 , i f r e l a t e d to the average p a r t i c l e s i z e , tends to a unique asymptotic function for each of the p r o c e s s e s , while the r a t e of i n c r e a s e of the average p a r t i c l e s i z e Is p r o p o r t i o n a l to r~3 and r ~ ' , r e s p e c t i v e l y , for the two processes c o n s i d e r e d . T.M. Ahn and J . K . Tien [6] improved B.K. C h a k r a v e r t y ' s t r e a t m e n t , by a l s o c o n s i d e r i n g the r e t a r d a t i o n In the growth process t h a t occurs If the growing p a r t i c l e has welt developed c r y s t a l f a c e s . I n c o r p o r a t i o n of atoms in such a face r e q u i r e s the formation of two dimensional n u c l e i , which can be a very slow p r o c e s s . F i n a l l y H.H. Lee [12 [ has pointed out an u n j u s t i f i e d approximation in B.K. C h a k r a v e r t y ' s t r e a t m e n t ,

i n v a l i d a t i n g the simple power law growtli r a t e s obtained e a r l i e r .

The c o n f r o n t a t i o n of the p r e d i c t i o n s of t h e s e t h e o r i e s with the

experimental r e s u l t s for Pt and Pd d i s p e r s i o n s on alumina s u p p o r t s does not lead to c o n c l u s i v e r e s u l t s . There I s no s i n g l e mechanism t h a t e x p l a i n s the combination of fast I n i t i a l growth and u l t i m a t e

support new compounds a r e formed t h a t bind well to metal and to support leading to an Improved cohesion between metal and s u p p o r t . An a n c i e n t example Is the formation of h y d r o s l l i c a t e s In n l c k e l - s i l l c a c a t a l y s t s , discovered by J . J . de Lange and G.H. Visser f l 3 ] and e x t e n s i v e l y discussed by J.W.E. Coenen [14, 15, 16 |. During prolonged b o i l i n g of a mother suspension or a l s o during s l i g h t l y hydrothermal c o n d i t i o n s a t

the beginning of r e d u c t i o n , n i c k e l oxide or hydroxide on one hand and s i l i c a on the other hand can r e a c t to form h y d r o s l l l c a t e l a y e r s a t t h e I n t e r f a c e between n i c k e l oxide and s i l i c a , which p e r s i s t a f t e r

r e d u c t i o n , a c t i n g as a kind of glue between metal and s u p p o r t . When nickel-alumina c a t a l y s t s a r e made by c o p r e c l p l t a t l o n , a l r e a d y In the hydroxide s t a g e a c a t a l y s t p r e c u r s o r Is formed c o n s i s t i n g of a mutual s o l i d s o l u t i o n of n i c k e l - and aluminum-ions in one s i n g l e hydroxide l a t t i c e . Here the unknown f a c t o r I s to what e x t e n t such an I n t e r m e d i a t e compound p e r s i s t s during the phase s e p a r a t i o n between n i c k e l and

alumina which occurs in the f u r t h e r s t e p s of c a t a l y s t p r e p a r a t i o n . Here too i t i s p o s s i b l e , t h a t a s p i n e l type NlAl^O^ glue s t i l l s u b s i s t s a t the I n t e r f a c e between n i c k e l and alumina.

S t r i c t l y speaking such a glue would not I n t e r f e r e with s i n t e r i n g through an Ostwald r i p e n i n g p r o c e s s , a s not the metal p a r t i c l e s have t o move, but only the metal atoms e v a p o r a t i n g from small p a r t i c l e s and condensing on the l a r g e r ones. But I t Is obvious, t h a t t r a c e s of compounds, t h a t lead to Improved cohesion between metal p a r t i c l e s and s u p p o r t , a l s o may s t r o n g l y Influence the m o b i l i t y of i n d i v i d u a l atoms, d i f f u s i n g over the s u p p o r t .

Guided by the experimental technique used for the p r e p a r a t i o n of the model c a t a l y s t s for s i n t e r s t u d i e s and the a v a i l a b i l i t y of X-ray Photoelectron Spectroscopy (XPS), s e v e r a l experimental s t u d i e s have been made in t h i s c o n t e x t :

- The p o s s i b l e formation of i n t e r m e d i a t e compounds a t the i n t e r f a c e between n i c k e l and alumina has been studied by XPS. To t h i s end a 300 nm thick layer of non-porous alumina Is made by r e a c t i v e e v a p o r a t i o n of aluminum In an oxygen atmosphere onto a q u a r t z s u b s t r a t e . On top of t h i s l a y e r a 100 nm thick layer of n i c k e l Is deposited by

evaporation i n vacuum. Depth p r o f i l e s of composition and s t r u c t u r e a t the i n t e r f a c e have been obtained with the X P S - f a c i l i t y by

I n t e r m i t t e n t l y e t c h i n g by means of a c c e l e r a t e d Ar - i o n s and measuring XPS-spectra.

- In order to study p o s s i b l e e f f e c t s of promoters on the formation of compounds a t the metal-alumina i n t e r f a c e t h i n l a y e r s of chromium and manganese have been introduced between the alumina and the n i c k e l

layer by preceding the e v a p o r a t i o n of n i c k e l with the evaporation of chromium or manganese. Depth p r o f i l e s of composition and s t r u c t u r e by means of XPS have been made in the same way as described b e f o r e . - The e f f e c t of chromium and manganese on the s i n t e r i n g of d i s p e r s i o n s

of n i c k e l p a r t i c l e s i s s t u d i e d by i n t r o d u c i n g t h i n l a y e r s of chromium and manganese between the t r a n s p a r e n t alumina support of Che model c a t a l y s t s and the n i c k e l film. Again chromium and manganese have been

Introduced by e v a p o r a t i o n onto the support preceding e v a p o r a t i o n of n i c k e l .

1.2. Lay out of t h e s i s

The t h e s i s i s divided I n t o t h r e e p a r t s , v i z an experimental s e c t i o n c o n s i s t i n g of c h a p t e r 2 and 3 , a t h e o r e t i c a l s e c t i o n c o n s i s t i n g of c h a p t e r s A and 5 and an appendix, and g e n e r a l I n t r o d u c t i o n s and c o n c l u s i o n s In c h a p t e r 1 and 6 r e s p e c t i v e l y .

In c h a p t e r 2 the e l e c t r o n microscope measurements a r e presented of average p a r t i c l e s i z e s and s i z e d i s t r i b u t i o n s of n i c k e l model c a t a l y s t s , supported on t r a n s p a r e n t alumina f i l m s , in the v a r i o u s s t a g e s of a s i n t e r i n g t r e a t m e n t .

In c h a p t e r 3 a n a l y s e s by X-ray p h o t o e l e c t r o n spectroscopy are presented of the composition and s t r u c t u r e of the I n t e r f a c e s of t h i n film

composites of n i c k e l and alumina, In some cases separated by t h i n l a y e r s of chromium or manganese.

In c h a p t e r 4 r a t e e q u a t i o n s have been presented for the r e a c t i o n s t e p s , t h a t may lead to the growth of I n d i v i d u a l n i c k e l p a r t i c l e s . After making e s t i m a t e s for the k i n e t i c and e n e r g e t i c parameters involved, q u a n t i t a t i v e r a t e s have been estimated and d e c i s i o n s have been made as

to which s t e p s have to be considered as r a t e d e t e r m i n i n g .

Chapter 5 i s devoted to the f i n a l t h e o r e t i c a l a n a l y s i s of the

of ensembles of n i c k e l p a r t i c l e s according to t h r e e s e l e c t e d r a t e determining s t e p s . A d i s c u s s i o n i s given of the s e n s i t i v i t y of the c a l c u l a t e d growth curves to the choice of numerical values of the v a r i o u s parameters i n v o l v e d .

In c h a p t e r 6 the experimental r e s u l t s of c h a p t e r 2 are compared with the t h e o r e t i c a l p r e d i c t i o n s of chapter 4 and 5. Also, a comparison Is made with l i t e r a t u r e data about the P t / A l O , system. Conclusions a r e drawn as to the e f f e c t of a d d i t i v e s on the r a t e of s i n t e r i n g . Also, some g e n e r a l c o n c l u s i o n s a r e presented as to the a p p l i c a b i l i t y of t h e r e s u l t s of t h i s t h e s i s to r e a l c a t a l y s t s .

The appendix c o n t a i n s an a n a l y s i s and f u r t h e r j u s t i f i c a t i o n of a number of assumptions, made In the k i n e t i c and therraodynamic b a s i s of chapter 4 and 5.

1.3. References

[ l ] H.W. NUrnberg; G. Wolff: Naturwlss. 63_ (1976) 190.

[ 2] E.B.M. Doesburg; P.H.M, de Korte; H. Schaper; L.L. van Reyen: Appl. Cat. U_ (1984) 155.

[ 3] C.H. Bartholomew; W.L. Sorensen: J . Cat. 81^ (1983) 131. C.H. Bartholomew; R.B. Pannell; R.W. Fowler: J . C a t . 79^ (1983) 34.

D.G. Mustard; C.H. Bartholomew: J. Cat. 67_ (1981) 186.

C.H. Bartholomew; R.B. Pannell; J.L. Butler: J. Cat. 6_5_ (1980) 335.

[ 4] H. Schaper: Thesis ( D e l f t ) 1984.

[ 5] E. Ruckenstein; M.L. Malhotra: J . Cat. 41^ (1976) 303.

[ 6] P. Wynblatt in Growth and P r o p e r t i e s of Metal C l u s t e r s , ed. J . Beurdon 1980, pg. 15.

T.M. Ahn; P. Wynblatt; J.K. Tien: Acta Metall. 29_ (1981) 921. T.M. Ahn; J.K. Tien: J . Cat. 66 (1980) 335.

T.M. Ahn; J . K . T I e n : J . P h y s . Chem. S o l i d s 3_7_ ( 1 9 7 6 ) 7 7 1 , 7 7 7 . P . W y n b l a t t ; N.A. G j o s t e l n : A c t a M e t a l l . 24_ ( 1 9 7 6 ) , 1 1 6 5 , 1 1 7 5 . [ 7 ] J . J . Chen; E. R u c k e n s t e l n : J . C a t . 69_ ( 1 9 8 1 ) 2 5 4 . [ 8 ] E. R u c k e n s t e l n ; D.B. D a d y b u r j o r : Rev. Chem. E n g . 1^ ( 1 9 8 3 ) 2 5 1 . E . R u c k e n s t e l n ; B. P u l v e n n a c h e r : J . C a t . 29_ ( 1 9 7 3 ) 2 2 4 , A. I . Ch. E. 19_ ( 1 9 7 3 ) 3 5 6 . [ 9 ] B.K. C h a k r a v e r t y : J . P h y s . Chem. S o l i d s 28_ ( 1 9 6 7 ) 2 4 0 1 . [ 1 0 ] C. Wagner: Z . E l e c t r o c h e r a l e 6_5 ( 1 9 6 1 ) 5 8 1 . [ 1 1 ] I . M . L i f s h l t z & V.V. S l y o s o v : J . P h y s . Chera. S o l i d s 19_ ( 1 9 6 1 ) 3 5 . [ 1 2 ] H.H. L e e : J . C a t . 63_ ( 1 9 8 0 ) 1 2 9 . [ 1 3 ] J . J . de Lange & C.H. V i s s e r : I n g e n i e u r 58_ ( 1 9 4 6 ) 2 4 . [ 1 4 ] J . W . E . C o e n e n : t h e s i s ( D e l f t ) 1 9 5 8 .

[ 1 5 ] G.C.A. S c h u i t ; L . L . van Reyen: Adv. C a t . 1£ ( 1 9 5 8 ) 2 4 2 .

[ 1 6 ] J . W . E . C o e n e n ; I n P r e p a r a t i o n o f C a t a l y s t s I I , e d s . B. Delmon, P G r a u g e , P . J a c o b s and G. P o n c e l e t , p g . 8 3 .

2 . ELECTRON MICROSCOPIC STUDY OF THE SINTERING OK THE METAL PARTICLES IN N i / A l203 MODEL CATALYSTS

2 . 1 . I n t r o d u c t i o n

The support of the model c a t a l y s t s c o n s i s t s of very t h i n non-porous alumina films (< 50 nm), t r a n s p a r a n t to the e l e c t r o n beam of the e l e c t r o n microscope, obtained by anodic o x i d a t i o n of an aluminum s h e e t , removal from the s u b s t r a t e , and subsequent thermal s t a b i l i z a t i o n . The d i s p e r s i o n of n i c k e l p a r t i c l e s i s obtained by f i r s t d e p o s i t i n g a very t h i n layer of n i c k e l (~ 1 nm) on the s u b s t r a t e by evaporation in vacuum from a h e a t i n g s o u r c e . During d e p o s i t i o n the t a r g e t i s a t room

t e m p e r a t u r e . The s u r f a c e t e n s i o n s a r e such, t h a t spreading of the n i c k e l In a t h i n l a y e r I s unfavourable. Thus, a f t e r h e a t i n g In

hydrogen, the n i c k e l film breaks a p a r t I n t o a fine d i s p e r s i o n of n i c k e l p a r t i c l e s . I n t e r l a y e r s of chromium or manganese, if r e q u i r e d , have been obtained by d e p o s i t i n g t h e s e metals p r i o r to n i c k e l and in the same way as n i c k e l .

In order to study the s i n t e r i n g of t h e n i c k e l p a r t i c l e d i s p e r s i o n of a p a r t i c u l a r sample, such a sample was heated for a number of s u c c e s s i v e periods in hydrogen a t 873 K or 973 K. Electron microscope photographs were made from these samples. As the n i c k e l p a r t i c l e s a r e so small t h a t they would o x i d i z e spontaneously In a i r a t room temperature and as they have to be t r a n s f e r r e d from the s i n t e r apparatus to the e l e c t r o n microscope and vice v e r s a , a c a r e f u l procedure had to be followed In order to avoid such o x i d a t i o n .

The s t r u c t u r e and chemical composition of the model c a t a l y s t s before and a f t e r s i n t e r i n g have been checked by e l e c t r o n d i f f r a c t i o n (T.E.D.) and energy d i s p e r s i v e a n a l y s i s of X-rays ( E . D . X . ) .

2 . 2 . P r e p a r a t i o n of the model c a t a l y s t

2 ^ 21li_ P r e 2 a r a t l o n _ o f _ t h l n _ a l u m l n a _ s u £ £ o r t s

Non-porous alumina films can be prepared by d i f f e r e n t techniques such as o x i d a t i o n or a n o d l z a t l o n of aluminum f o i l s [l ], r e a c t i v e evaporation d e p o s i t i o n of aluminum [ 2 ] , s p u t t e r i n g or chemical vapor d e p o s i t i o n

[ 3 ] . Because very t h i n films (<50nm) a r e needed for TEM s t u d y , a n o d l z a t i o n was the most proper way.

Alumina was formed using an aluminum f o i l , t h i c k n e s s 1.5 mm and p u r i t y 99,9995%. The f o i l was f i r s t polished e l e c t r o c h e m i c a l l y , using the r e c i p e of Schwartz [4] a s given in t a b l e 2 . 1 .

Table 2 . 1 . Electrochemical p o l i s h i n g of aluminum

P o l i s h i n g s o l u t i o n Condition 5% chromic acid 52% phosphoric acid 5% s u l f u r i c acid 10V Pb cathodes 65 C, 2 rain

After e t c h i n g , the f o i l was rinsed i n running water for one hour and washed with d e s t l l l e d w a t e r . The c l e a n aluminum was t h e r e a f t e r anodized a t 20V in a 3% wt t a r t a r l c acid s o l u t i o n , adjusted to a pH 5.5 [l ]. After 20 seconds a t room temperature the c u r r e n t drops and the process s t o p s . The thus formed amorphous non-porous l a y e r Is known to grow with 1.5 nm pro V o l t , hence the alumina layer was about 30nm t h i c k . The f o i l was washed in d e s t l l l e d water and cut to p i e c e s (3*3 mm) s u i t a b l e for use in the TEM.

The oxide film was detached from the f o i l by immersing t h e specimen i n a c o n c e n t r a t e d mercuric c h l o r i d e s o l u t i o n . When amalgamation s t a r t e d , the specimen was t r a n s f e r r e d i n t o d e s t l l l e d water where the

amalgamation could c o n t i n u e u n t i l the oxide film separated from the aluminum. The t h i n films were again t r a n s f e r e d i n t o fresh d e s t l l l e d water and picked up on e l e c t r o n microscope g r i d s , type o y s t e r , as shown

in

fig. 2 . 1 .

C r y s t a l l i z a t i o n and s t a b i l i z a t i o n of the amorphous alumina was achieved by h e a t i n g the specimen In a i r a t 925 or 1025K for 100 h o u r s . These thermally s t a b i l i z e d films were used as s u p p o r t .

2 . 2 . 2 . Deposition of metals 2 . 2 . 2 . 1 . Equipment

The equipment used for vapour d e p o s i t i o n , an a p p a r a t u s b u i l t under own s u p e r v i s i o n ( s e e f i g . 2.2a and b) c o n t a i n s :

- A vacuum b e l l g l a s with pumping system and vacuum gauges. - Measuring and c o n t r o l equipment.

- A r e s i s t a n c e heated s o u r c e , from which the metal i s e v a p o r a t e d . For d e p o s i t i o n of very t h i n l a y e r s a tungsten boat I s used, while for t h i c k e r l a y e r s c r u c i b l e s with a molybdenum w i r e - c o i l h e a t e r were used ( a ) . A l . 0 , and BN c r u c i b l e s were used to evaporate Ni and Al, r e s p e c t i v e l y .

- A specimen holder with a v a r i a b l e d i s t a n c e to the source of up to 20 cm ( b ) .

- A shutter (c). - A cold finger (d).

2.2.2.2. Procedure

During p h y s i c a l vapour d e p o s i t i o n ( p . v . d . ) of metals evaporation has to be c a r r i e d out i n high vacuum to ensure as l i t t l e as p o s s i b l e o x i d a t i o n to occur. The vacuum system, as shown b e f o r e , I s equlped with an o i l d i f f u s i o n pump a b l e to achieve a vacuum of 10~5 Pa In a c l e a n system. However, as the system c o n t a i n s a g l a s s c o n t a i n e r and rubber s e a l s I t I s Impossible to c l e a n the system by h e a t i n g . T h e r e f o r e , a cold finger was c o n s t r u c t e d on which r e s i d u a l gas can condense and a s h u t t e r was I n s t a l l e d between the h e a t i n g source and the s u b s t r a t e to prevent d e p o s i t i o n of the metal in the i n i t i a l s t a g e of evaporation during which the g e t t e r l n g power of the metal vapour Is used to clean up the system. Chemical c l e a n i n g up Is r e a l i z e d by the a b i l i t y of

e l e c t r o n e g a t i v e metals to bind r e s i d u a l g a s s e s such as water and oxygen, in t h i s way the adsorbed s p e c i e s are immobilized on the Inner s u r f a c e s of the system c e a s i n g out g a s s i n g .

So, a f t e r achieving high vacuum (1.7*10~3 Pa) with the o i l d i f f u s i o n pump, l i q u i d n i t r o g e n Is poured i n t o the cold f i n g e r during which vacuum immediately improved up to 5*10"'' Pa. Then, with the s h u t t e r c l o s e d , h e a t i n g up of the metal s t a r t s which i s accompanied by a f a s t decrease of the p r e s s u r e and subsequent Increase of the p r e s s u r e up to the e q u i l i b r i u m vapour p r e s s u r e of the metal a t t h a t p a r t i c u l a r t e m p e r a t u r e . After equilibrium Is reached the s h u t t e r i s opened and d e p o s i t i o n s t a r t s .

The t h i c k n e s s of the deposited t h i n films Is estimated by an i n d i r e c t t e c h n i q u e , namely measuring the l i g h t t r a n s m i s s i o n of a film

simultaneously deposited on a q u a r t z s l i d e using a spectrophotometer Zelss PMQ-2. This appears to be a s u i t a b l e technique for measuring the t h i c k n e s s of very t h i n f i l m s . However, because of the high transparency of these films the accuracy i s f e l t to be of the o r d e r of 0.5 nm.

For the p r e p a r a t i o n of extreme t h i n films a very low growth r a t e i s d e s i r e d ; hence d e p o s i t i o n of the metals was studied in a preliminary s t u d y . A d e p o s i t i o n r a t e of approximately 0.5 nm min" : could be achieved a t an steady s t a t e vapour p r e s s u r e of 2.6*10"'' Pa. The corresponding temperatures of the h e a t i n g source for the d i f f e r e n t metals used during t h i s i n v e s t i g a t i o n a r e given in t a b l e 2 . 2 .

Table 2 . 2 . Deposition c o n d i t i o n s and parameters for t h i c k n e s s measurement. Metal Ni Cr Mn T(source) K 1625 1825 1475 R nm/min 0.6 0.5 0.6 n 1.70 1.87 1.89

<

2.40 2.69 2.59

n = film r e f r a c t i v e Index and < the a b s o r p t i o n c o e f f i c i e s t , from r e f . [ 5 ] .

Model c a t a l y s t s were prepared under the before mentioned

c o n d i t i o n s , d e p o s i t i n g the metal onto the s t a b i l i z e d alumina s u p p o r t , captured in a grid with the specimen holder a t room t e m p e r a t u r e . However, as t h e r e was only one h e a t i n g source in the vacuum chamber, the composite films had to be prepared i n s e v e r a l s t a g e s . After a film was deposited the vacuum chamber was opened, the h e a t i n g source changed, and the next layer was subsequently d e p o s i t e d onto the film grown b e f o r e . The sequence of metal films deposited was f i r s t chromium followed by manganese and subsequently n i c k e l . Dependent on the d e s i r e d model c a t a l y s t one or more s t e p s were skipped. However, one has to bear in mind t h a t in the case of m u l t i l a y e r s each t h i n film i s exposed to a i r before the next film i s d e p o s i t e d .

All f i l m s , up t o 10 nm, were shown to be amorphous. The p a r t i c l e ensemble was obtained by h e a t i n g up the model c a t a l y s t , both the nickel-alumina as well as the multimetal layer-alumina systems, in hydrogen. During c o n t r a c t i o n of the film n i c k e l r e c r y s t a l l l z e s and small p a r t i c l e s a r e formed. Preliminary experiments have shown t h a t p a r t i c l e s of l e s s than 10 nm could be obtained from n i c k e l films of about 3 nm t h i c k n e s s .

2 . 3 . S i n t e r i n g of the model c a t a l y s t In hydrogen

2 . 3 . 1 . Equipment

2 . 3 . 1 . 1 . The gas p u r i f i c a t i o n s e c t i o n

S i n t e r i n g i s c a r r i e d out in hydrogen and because f i n e l y d i s p e r s e d n i c k e l metal o x i d i z e s r e a d i l y , the m o d e l - c a t a l y s t has to be p a s s l v a t e d in a wet n i t r o g e n flow before exposure to a i r . The gases used are p u r i f i e d using s e v e r a l a g e n t s as i s given in t a b l e 2 . 3 .

Table 2 . 3 . Adsorbentla used for p u r i f i c a t i o n of the g a s e s .

"2 gas

Pd/Al203 catalyst Molecular Sieves 4A

N2 gas

BTS catalyst (Cu/diatomaceous earth) Molecular Sieves 4A

2 . 3 . 1 . 2 . The s i n t e r i n g device

This d e v i c e c o n s i s t s of an oven and a q u a r t z tube with a diameter of 12 mm. The tube i s devided i n t h r e e d i f f e r e n t chambers connected by t a p s , as can be seen in f i g . 2 . 4 .

F i g . 2 . 4 . Scheraatical drawing of the s i n t e r i n g device

• The q u a r t z tube oven (A), diameter 12 mm, • The quenching and p a s s i v a t i o n chamber ( B ) .

Here the sample to be analyzed can be s e l e c t e d and quenched. With a hook ( i ) the boat c o n t a i n i n g the sample i s d i r e c t e d i n t o the s i d e t u b e , while t h e remaining boats are pushed back i n t o t h e oven using a q u a r t z bar ( i i ) and tap n r . I I s c l o s e d . After quenching, the sample i s p a s s i v a t e d by l e a d i n g a N,/3% HO gas mixture through the chamber. The sample i s then ready for exposure to a i r and a n a l y s i s by TEM. After removing the specimen, the chamber i s flushed with hydrogen and t a p n r . I i s opened.

• Sample I n l e t chamber ( C ) .

After a n a l y z i n g , the sample i s r e t u r n e d i n t o the oven through the i n l e t chamber. Tap n r . I I i s c l o s e d , d i s c o n n e c t i n g the chamber from the oven. After f l u s h i n g with hydrogen, the tap i s opened and the specimen can be moved i n t o the oven with the thermocouple ( i i i ) .

The p r e c u r s o r of the model c a t a l y s t , prepared as described b e f o r e , c o n s i s t s of a t h i n amorphous n i c k e l film with or without a d d i t i v e s and a s t a b i l i z e d Y~alumina l a y e r . Heating t h i s composite r e s u l t s i n

c r y s t a l l i z a t i o n and the formation of a n i c k e l c r y s t a l l i t e d i s p e r s i o n . Experimental r e s u l t s on the d e s i n t e g r a t i o n of the metal film showed t h a t t h i s i s a r a t h e r slow p r o c e s s . At 873K c o n t r a c t i o n of the p a r t i c l e s s t i l l goes on a f t e r one hour ageing. So for convenience the f i r s t o b s e r v a t i o n was c a r r i e d out a f t e r t h r e e hours h e a t i n g a t the s i n t e r t e m p e r a t u r e . S i n t e r i n g i s then followed e x - s i t u in the TEM ( P h i l i p s EM 2 0 1 , 4 0 1 ) . After each a n n e a l i n g period the sample i s

photographed and analyzed with a Quantimed 720, Imanco. Analysis of the p a r t i c l e d i s p e r s i o n with the Quantlmed I s based on Image a n a l y s i s . Therefore, the photograph Is p r o j e c t e d onto a screen connected to a computer and by means of a sensor the screen Is a u t o m a t i c a l l y scanned on d i f f e r e n c e s in c o n t r a s t . By applying a c e r t a i n t h r e s h o l d In darkness

for the background, the c o o r d i n a t e s of every point on the screen with a darkness higher than the t h r e s h o l d value are fed i n t o the computer. By

scanning the screen a two dimensional a r r a y of s p o t s i s c r e a t e d with every closed ensemble of p o i n t s being a p a r t i c l e . Then, using t h i s d i g i t a l p i c t u r e , a number of procedures can be executed such as c a l c u l a t i o n of the p a r t i c l e area and counting of the number of p a r t i c l e s , analyzed r e s u l t i n g In a p a r t i c l e s i z e d i s t r i b u t i o n .

Table 2 . 4 . The composition of the model c a t a l y s t s used for s i n t e r i n g In hydrogen, as determined by l i g h t t r a n s m i s s i o n measurement.

Sample C a t a l y s t A B C D E F G H Film t h i c k n e s s (rim) Ni 3 2 1 5 1 1 1 1 Cr 1 1 Mn <1 1 T ( s l n t e r ) (K) 873 973 Number of samples analyzed 1 3 5 2 4 1 1 1

As i t was i m p o s s i b l e to l a b e l a c e r t a i n area of the c a t a l y s t , which could serve as a standard p a r t i c l e ensemble to be followed during s i n t e r i n g , d i f f e r e n t s p o t s were photographed to ensure good o v e r a l l p a r t i c l e s i z e a n a l y s i s . For each a n a l y s i s a t l e a s t 400 p a r t i c l e s have been examined, using up t o t h r e e p i c t u r e s . This e n s u r e s a d e v i a t i o n in the mean p a r t i c l e s i z e of l e s s than 5%.

Table 2.4 g i v e s a survey of the model c a t a l y s t s a n a l y s e d .

2 ^ 3 . 3 . Analysis_by_T1E1D;_and E.D.X.

Because of the small dimensions of the model c a t a l y s t s , f o i l s of approximately 30 nm t h i c k n e s s and 3 mm d i a m e t e r , the usual techniques such as X.R.D. (X-ray D i f f r a c t i o n ) and X . P . S . (X-ray Photoelectron Spectroscopy) can not be used. An e x c e l l e n t t o o l was found In T.E.D. (Transmission E l e c t r o n D i f f r a c t i o n ) and E.D.X. (Energy D i s p e r s i v e a n a l y s i s of X - r a y s ) , both a v a i l a b l e In the e l e c t r o n microscope P h i l i p s EM 4 0 1 .

T.E.D. o f f e r s the o p p o r t u n i t y for s t r u c t u r a l a n a l y s i s of small samples because of the high s c a t t e r i n g power of a t o m s / i o n s for e l e c t r o n s and the short e l e c t r o n wavelength compared with X - r a y s . The technique i s based on the same phenomenon as X.R.D., namely i n t e r f e r e n c e of

s c a t t e r e d r a d i a t i o n according to the Bragg's c o n d i t i o n : nX ■ 2d s i n 9. For a t y p i c a l small angle r e f l e c t i o n with s i n 9/X " 0.2 5~ , numerical e v a l u a t i o n [6] shows t h a t the i n t e n s i t y of s c a t t e r e d e l e c t r o n s Is 1 08 times l a r g e r than the I n t e n s i t y of s c a t t e r e d X - r a y s . Hence samples of e . g . 5 nm t h i c k n e s s d i f f r a c t an e l e c t r o n beam s u f f i c i e n t l y s t r o n g to give a d i f f r a c t i o n p a t t e r n , whereas the same sample u s u a l l y does not c o n t a i n enough s c a t t e r i n g power for X - r a y s .

Moreover, the l i n e ( a n g u l a r ) broadening, 8(2 9 ) , of the d i f f r a c t i o n l i n e s of powder p a t t e r n s i s r e l a t e d to the c r y s t a l l i t e s i z e and the wavelength a s :

As the wavelength (X) of 100 keV e l e c t r o n s i s 0.037A while the wave­ l e n g t h s of monochromatic X-rays a r e of the order of 1A ( e . g . 1.54 A for CuK ) , the d e t e c t i o n l i m i t , as r e s t r i c t e d by p a r t i c l e s i z e i s about 40 times b e t t e r .

The e l e c t r o n beam can a l s o be used for elemental a n a l y s i s with E.D.X. During exposure of the sample to the beam, an inner s h e l l vacancy i s formed ( l o n l z a t i o n ) . Subsequently, t h i s vacancy is f i l l e d by an e l e c ­ t r o n from a higher energy l e v e l , thereby e m i t t i n g a photon (X-ray) with a c h a r a c t e r i s t i c energy for the elements which a r e analyzed. In con­ t r a s t with X . P . S . ( s e e c h a p t e r 3 ) , however, because of the weak e n v i ­ ronmental dependency of t h i s p r o c e s s , only elemental a n a l y s i s can be performed.

2 . 4 . R e s u l t s of the a n a l y s i s by T.E.D. and E.D.X.

2i4:.l1_Com20sitlon_of_the_raodel_catal^sts

For the c a t a l y s t s s i n t e r e d a t 873K, the alumina support used was thermally s t a b i l i z e d a t 923K in a i r . Even a f t e r prolonged h e a t i n g (200h) the support was s t i l l amorphous, c o n t r a r y to the r e s u l t s of E. Ruckenstein [ 7 ] ,

After s i n t e r i n g the model c a t a l y s t s (Ni/araorphous-Al 20 3) for 150 hours in hydrogen, some c r y s t a l l i z a t i o n of the support appeared to have occured. No change in the n i c k e l p a t t e r n was observed.

The supports for the c a t a l y s t s s i n t e r e d a t 973K were s t a b i l i z e d a t L025K in a i r before metal d e p o s i t i o n . At the f i r s t measurement, the high n i c k e l loaded c a t a l y s t D showed to have an amorphous s u p p o r t , while the s u p p o r t s of the low n i c k e l loaded c a t a l y s t s E-H were c r y s t a l l i n e .

Also In t h i s case f u r t h e r h e a t i n g in hydrogen of raodel c a t a l y s t D leads to some c r y s t a l l i z a t i o n of the amorphous s u p p o r t . The e l e c t r o n

d i f f r a c t i o n p a t t e r n shows a few a d d i t i o n a l s p o t s , f i g . 2 . 7 , and the TEM p i c t u r e s some c r y s t a l boundaries within the amorphous phase. This can be seen from f i g . 2 . 5 , where the dark a r e a s bordered with s t r a i g h t l i n e s a r e the Y-A1203 c r y s t a l l i t e s . Also In t h i s c a t a l y s t no change In t h e n i c k e l p a t t e r n was observed.

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F i g . 2 . 5 . E l e c t r o n D i f f r a c t i o n p a t t e r n and the e l e c t r o n micrograph of modelsystera D.

F i g . 2 . 6 . Electron D l f r a c t i o n p a t t e r n of model systems E, F and H

The s i n t e r e d c a t a l y s t s E-H, s t a r t i n g with a c r y s t a l l i z e d s u p p o r t , do show d i f f e r e n t f e a t u r e s . At the f i r s t measurement in a l l systems, except for c a t a l y s t F, n i c k e l aluminate i s d e t e c t e d . In the N i - C r - A l20 j system of c a t a l y s t F no n i c k e l aluminate is found, while n i c k e l

chromate cannot be d e t e c t e d as a s e p a r a t e compound because i t s e l e c t r o n d i f f r a c t i o n p a t t e r n c o i n c i d e s with those of n i c k e l and alumina.

Addition of manganese to the model c a t a l y s t r e s u l t s in the formation of both n i c k e l and manganese aluminate as can be seen in t a b l e 2 . 5 .

At the time s i n t e r experiments a t 973K s t a r t e d , i t became p o s s i b l e to perform q u a n t i t a t i v e E.D.X. a n a l y s e s . Therefore only c a t a l y s t s E-H have been be c h a r a c t e r i z e d in t h i s way. The r e s u l t of the s t r u c t u r a l (TED) and e l e m e n t a l (EDX) a n a l y s e s a r e gathered in t a b l e 2 . 5 .

Table 2 . 5 . Chemical and p h y s i c a l composition of t h e model c a t a l y s t s .

C a t a l y s t D E F G H EDX r e s u l t s a f t e r 3h. s i n t e r i n g

Mol.% with respect to A1203 Ni Cr Mn 25 4.6 0.2 0 . 1 3.6 3.6 4.9 0.2 2.9 4.2 5.1 3.4 TED r e s u l t s a f t e r 300h. s i n t e r i n g Ni A 12 ° J I n t e r m e d i a t e c a c y N1A1204 c Y c Y MnAl.,0,,, SiAX20M c Y MnAl 0 ^ , N i A l ^ c = c r y s t a l l i n e ; a = amorphous 2.*i2^_Discussion

The alumina supports prepared by c l e a n i n g the aluminum f o i l in a fresh p o l i s h i n g s o l u t i o n were shown to be amorphous a f t e r heat t r e a t m e n t , while those cleaned in a r e p e a t e d l y used bath could be c r y s t a l l i z e d . In c o n t r a s t with t h e r e c i p e of E. Ruckenstein t h e aluminum f o i l used for t h e p r e p a r a t i o n of t h e alumina s u p p o r t s was polished i n a chromic acid c o n t a i n i n g b a t h . H. Konno [ 8 ] showed t h a t chromate ions s t r o n g l y adsorb on alumina films and as i t i s expected t h a t the polished

aluminum w i l l be covered immediately with a t h i n oxide film, a d s o r p t i o n of chromate ions can be expected to occur. Subsequently during

a n o d i z a t l o n the ions probably p e n e t r a t e i n t o the oxide film a f t e r having been reduced to C r ( I I I ) . The so formed chromium oxide then s t a b i l i z e s the alumina l a y e r .

from red to g r e e n , I n d i c a t i n g the r e d u c t i o n of chroraate Ions, C r ( V i ) , to chromium c a t i o n s , C r ( I I I ) . Depletion of chromate Ions in the

s o l u t i o n must have lead to d e c r e a s i n g c r y s t a l l i z a t i o n i n h i b i t i o n of the s u p p o r t s of c a t a l y s t s E-H.

So, although only a t r a c e of chromiuraoxlde could be d e t e c t e d i n a l l t h e s u p p o r t s , we a s c r i b e a c e r t a i n c r y s t a l l i z a t i o n I n h i b i t i o n to the presence of chromium Ions In alumina.

2 . 5 . E l e c t r o n microscope r e s u l t s about p a r t i c l e growth and p a r t i c l e s i z e d i s t r i b u t i o n

At t h i s temperature samples with d i f f e r e n t n i c k e l metal loading were used ( s e e t a b l e 2 . 4 ) . The mean p a r t i c l e s i z e s of the n i c k e l d i s p e r s i o n s of c a t a l y s t s A, B and C a r e given In t a b l e 2 . 6 . For t h i s experiment one sample of c a t a l y s t A, t h r e e samples of c a t a l y s t B and five samples of c a t a l y s t C were s i n t e r e d with specimens of the same composition placed s e p a r a t e l y In d i f f e r e n t alumina boats In the oven.

Table 2 . 6 . The mean p a r t i c l e s i z e of the c a t a l y 8 t S before and a f t e r s i n t e r i n g , with r In nm. C a t a l y s t A B C r ( 0 ) 6 . 5 3 . 5 2 . 0 r(end) 8 . 4 5 . 4 3 . 3 number of samples 1 3 5

In f i g . 2 . 7 , the change of the mean p a r t i c l e s i z e , for a l l samples, d u r i n g s i n t e r i n g i s shown. I t can be seen t h a t , although t h e r e Is some s c a t t e r i n g in the measured p a r t i c l e s i z e s of d i f f e r e n t samples of the same c a t a l y s t , t h i s amounts to 8% o n l y , hence s i n t e r i n g of the metal d i s p e r s i o n i s r e p r o d u c i b l e .

From the same f i g u r e I t can be seen t h a t , d e s p i t e the d i f f e r e n c e In the I n i t i a l mean p a r t i c l e s i z e s of the t h r e e c a t a l y s t s , they a l l show an I n i t i a l l y f a s t I n c r e a s e in the p a r t i c l e s i z e of l e s s than 75%. After 10 hours s i n t e r i n g the growth r a t e d e c r e a s e s a f t e r which a t approximately 50 h o u r s , the mean p a r t i c l e s i z e s tend to s t a b i l i z e a t a l e v e l of 8 . 4 , 5.4 and 3.3 nra as s p e c i f i e d in t a b l e 2 . 6 . However, some samples of model c a t a l y s t C, denoted as C[0, x, + ] in f i g . 2 . 7 , e x h i b i t an a 7 6 r (nm) t 5 < 3 2 I ■ 1 1 i 0 6 0 100 150 2 0 0 ► Khr)

F i g . 2 . 7 . The change of the mean p a r t i c l e s i z e s of t h r e e d i f f e r e n t model systems during heat treatment a t 873K i n hydrogen.

anomalous behaviour a f t e r 72 hours s i n t e r i n g . In c o n t r a s t with the o t h e r samples showing a s t a b i l i z a t i o n l e v e l a t 3.3 nm, the mean

p a r t i c l e s i z e of these samples drops a f t e r which growth proceeds a g a i n , s t e a d i l y but s l o w l y .

The development of 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 s for the c a t a l y s t s A, B and C a r e given i n f i g . 2 . 9 , 1 1 and 1 3 . F i g . 2.9 r e p r e s e n t s the P.S.D. f o r model c a t a l y s t C a f t e r heat treatment f o r 6 , 28 and 133 h o u r s , with the micrographs given in f i g . 2 . 8 . This system, having the s m a l l e s t mean p a r t i c l e diameter (4 nm), shows the most narrow

d i s t r i b u t i o n . After 133 hours the P.S.D. i s only s l i g h t l y broadend.

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Fig. 2 . 8 . E l e c t r o n micrographs of model c a t a l y s t C a f t e r 3h ( a ) , 28h (b) and 133h (c) s i n t e r i n g a t 873K i n hydrogen. Scale 1 cm 30 nra.

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■ H(~n) 3 8 1 11 18 F i g . 2 . 9 . P a r t i c l e s i z e d i s t r i b u t i o n evaluated for c a t a l y s t C a t d i f f e r e n t s t a g e s d u r i n g s i n t e r i n g a t 873K i n hydrogen.

For the somewhat c o a r s e r m o d e l c a t a l y s t B, t h e e l e c t r o n micrographs a r e given in f i g . 2.10 and the PSD's In f i g . 2 . 1 1 . The PSD of m o d e l c a t a l y s t B, f i g . 2 . 1 1 , for the ' f r e s h ' c a t a l y s t resembles t h a t of c a t a l y s t C. In t h i s case a s i g n i f i c a n t broadening of the d i s t r i b u t i o n i s observed.

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Fig. 2.10. Electron micrographs of model c a t a l y s t B a f t e r 5 h r ( a ) , 30 h r ( b ) and 153 h (c) s i n t e r i n g a t 873 K i n hydrogen. Scale 1 cm = 30 nm. i - I - 3 hr - 1 - 5 r» - 1 - 3 0 hr - 1 - 1 5 3 hr

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Fig. 2.11. Particle size distribution evaluated for catalyst B at different stages during sintering at 873K in hydrogen.

F i g . 2.12. E l e c t r o n micrographs of model c a t a l y s t A a f t e r 5 hr ( a ) , 30 hr (b) and 132 h (c) s i n t e r i n g a t 873 K i n hydrogen. Scale 1 cm ■ 30 nm. - 1 - 3 hr - l - a hr

XL

- 1 - 3 0 hr 1-132 hr 4 12 20 26 I ' l I I 1 4 12 20 29 F i g . 2 . 1 3 . P a r t i c l e s i z e d i s t r i b u t i o n evaluated for c a t a l y s t A a t d i f f e r e n t s t a g e s during s i n t e r i n g a t 873 K in hydrogen.

From f i g . 2.12 and 2.13 I t can be seen t h a t the PSD of t h e c a t a l y s t A i s extremely broad, while during s i n t e r i n g the f e a t u r e s of the PSD only change s l i g h t l y .

Now, as shown before in f i g . 2.7 samples C 0, x and + behave d i f f e r e n t l y . Therefore the development of the PSD of c a t a l y s t C 0 during s i n t e r i n g i s a l s o given in f i g . 2 . 1 4 . This n i c k e l p a r t i c l e ensemble c o n t a i n s a higher amount of p a r t i c l e s smaller than 5 nm than the one c h a r a c t e r i z e d by f i g . 2 . 9 .

- 1 - 3 ht -1-27 hf -1-73 h/ -1-158 hr

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d(nm) F i g . 2 . 1 4 . P a r t i e l e s i z e d i s t r i b u t i o n evaluated for c a t a l y s t C,0 a t d i f f e r e n t s t a g e s during heat treatment a t 873 K in hydrogen

Furthermore, t h i s sample shows a remarkable i n c r e a s e of the r e l a t i v e number of p a r t i c l e s of 3 nm, accompanied with a s i g n i f i c a n t broadening of the d i s t r i b u t i o n , and an Increased tendency for facet formation.

Four model c a t a l y s t s of d i f f e r e n t chemical composition, as given in t a b l e 2 . 7 , were used for the experiments a t t h i s t e m p e r a t u r e .

Table 2 . 7 . C h a r a c t e r i s t i c s of the model c a t a l y s t s used a t 973 K, witli r in nm. C a t a l y s t D E F G H Chemical composition Ni - A1203 Ni - A1203 Ni-Cr - A1203 Ni-Mn - A1203 Ni-Mn-Cr- A1203

r(0)

11.5 2 . 6 2 . 0 4 . 3 2 . 6 r(end) 12 4 . 1 3 . 1 6 . 4 5 . 0 number of samples 2 3 1 1 2

For a l l the samples the changes of the mean p a r t i c l e s i z e and the PSD during s i n t e r i n g have been p l o t t e d in f i g . 2.15 and 2.22. The e l e c t r o n micrographs e x h i b i t changing of the morphology of growing p a r t i c l e s while for t h e l a r g e p a r t i c l e s facet formation i s o b v i o u s .

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F i g . 2.15a. The change of the raean p a r t i c l e s i z e of the model system, Cat. E: N i / A l203, aged a t 973 K i n hydrogen.

(nm) Cal.G Cal.H 2 0 0 2 5 0 ► Khr) 3 0 0 3 5 0

F i g . 2.15b. The change of the raean p a r t i c l e s i z e s of the model s y s t e m s , Cat. F: Ni/Cr/Al203 > Cat. H: Nl/Mn/Cr/Al fl 3, and Cat. G:

2 . 5 . 2 . 1 . N i / A l203, c a t a l y s t E, 4.6 mol% NI

Three samples were s i n t e r e d In two s u c c e s s i v e c h a r g e s . The mean p a r t i c l e r a d i u s amounts to 2.6 nm and i n c r e a s e s very fast to 3.5 nm a f t e r 16 h o u r s , a f t e r which t h e r e i s only a very slow c o a r s e n i n g , as can be seen from the e l e c t r o n micrographs i n f i g . 2 . 1 6 . S t a r t i n g with narrow p a r t i c l e s i z e d i s t r i b u t i o n ( f i g . 2.17) resembling c a t a l y s t C, the d i s t r i b u t i o n g r a d u a l l y and s t e a d i l y broadens.

F i g . 2.16. Electron micrographs of model c a t a l y s t E a f t e r 3 hr ( a ) , 17 hr (b) and 350 hr (c) s i n t e r i n g a t 973 K i n hydrogen. Scale 1 cm = 30 nm. 4 0 -l 10 . 1 . 3 hi - 1 - 1 7 hr , 1*68 hr • 1 - 3 5 0 hr .r^P-r--»-7 I t 1S 19 23 d(nm)

Fig. 2.17. Particle size distribution determined for catalyst E (NI) at different stages of sintering at 973 K in hydrogen

2 . 5 . 2 . 2 . Ni/Cr/Y-Al203, c a t a l y s t F, 3.6 aolZ NI, 3.6 «olZ Cr

This c a t a l y s t , c o n t a i n i n g an e q u i v a l e n t amount of chromium and n i c k e l , e x h i b i t s a deviant behaviour during heat treatment ( f i g . 2 . 1 5 ) . With a mean p a r t i c l e r a d i u s of 2.0 run for the n i c k e l d i s p e r s i o n , a l i n e a i r growth r a t e i s observed in c o n t r a s t with growth so far observed. From 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 ( f i g . 2.19) and the e l e c t r o n micrographs ( f i g . 2 . 1 8 ) , i t can be seen t h a t t h i s p a r t i c u l a r n i c k e l

F i g . 2 . 1 8 . E l e c t r o n micrographs of model c a t a l y s t F a f t e r heat

treatment i n hydrogen a t 973 K for 6 hr (a) and 320 hr ( b ) . Scale 1 cm - 30 nm.

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- I . S hr - l - B B h»

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3 7 I I .1.178 f" -1.320 h' I-, ► d(nm) 3 7 I I 15 F i g . 2 . 1 9 . E v o l u t i o n of t h e 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 t h e c a t a l y s t F (Ni/Cr) during ageing in hydrogen a t 973 K

dispersion has an extremely narrow distribution. During ageing some broadening up to 15 nm occurs with hardly any change in the features of the distribution.

2.5.2.3. Hi/Mn/Y - AljOj, catalyst G, 4.9 molZ Ni, 2.9 molZ Hn

Sintering of this model catalyst results in a gradual Increase of the mean particle radius of 4.3 nm up to 6.4 nm, showing a stabilization level at 6.4 nm after 170 hours.

Fig. 2.20. Electron micrographs of model catalyst G after heat

treatment in hydrogen at 973 K for 3 hr (a), 80 hr (b) and 259 hr (c). Scale 1 cm = 100 nm. 40 I | 2 0 . 1-3 hr 1-17 hr I I T 10 18 - t - 8 0 hr 1-25» hr n i i i 2 10 18 28 3 4 4 0 d(nm)

F i g . 2 . 2 1 . P a r t i c l e s i z e d i s t r i b u t i o n determined for c a t a l y s t G (Ni/Mn) d u r i n g s i n t e r i n g a t 973 K i n hydrogen

Compared with the n i c k e l model c a t a l y s t E, 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 ( f i g . 2.21) i s somewhat b r o a d e r . During s i n t e r i n g extreme l a r g e c r y s t a l l i t e s grow with a p a r t i c l e diameter up to 42 nm.

Remarkable i s the high f r a c t i o n of p a r t i c l e s with diameter of 6 nm, during the whole period of heat t r e a t m e n t .

2 . 5 . 2 . 4 . Ni/Cr/Mn/Y-Al203, c a t a l y s t H, 4 . 2 «olZ Hi, 5.1 »olZ Cr, 3.4

molZ Mn

The growth curve for t h i s n i c k e l d i s p e r s i o n resembles t h a t of model c a t a l y s t E, e x h i b i t i n g a f a s t s i n t e r i n g up t o 4 . 4 nm a f t e r 40 hours then s t a b i l i z i n g a t a l e v e l of 5.0 nm. The development of 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 t h i s n i c k e l d i s p e r s i o n i s the same as for c a t a l y s t E. 2 . 5 . 2 . 5 . H i / A l203, 25 «olZ Ni r ( n m )

i,

1 4 1 0 6 o . a

-a L

•

o i

•

o 1 1

•

1 C a l D , C a l D2 i SO 1 0 0 1 6 0 2 0 0 2 5 0 I t h r ) 3 0 0 F i g . 2 . 2 2 . E v o l u t i o n of t h e mean p a r t i c l e s i z e i n c a t a l y s t D d u r i n g s i n t e r i n g a t 973 K i n hydrogen

The behaviour of the high n i c k e l loaded model c a t a l y s t D i s i l l u s t r a t e d i n f i g . 2 . 2 2 . Also In t h i s case the p e c u l i a r behaviour as observed for some samples of c a t a l y s t C ( f i g . 2.14) i s found. For c a t a l y s t D, no change in the mean p a r t i c l e s i z e Is observed, while 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 changes only s l i g h t l y , as can be seen In f i g . 2 . 2 4 .

F i g . 2 . 2 3 . E l e c t r o n micrographs of c a t a l y s t D a f t e r s i n t e r i n g for 3 hr ( a ) and 350 hr (b) a t 973 K i n hydrogen. Scale 1 cm - 100 nm.

30 20 I

Pn

L , 1-44 hr 1-3 hr —1-89 hr - 1-350 hr «101418 28 34 10 18 28 34 42 • cKnm)

F i g . 2 . 2 4 . P a r t i c l e s i z e d i s t r i b u t i o n determined for c a t a l y s t D, during s i n t e r i n g a t 973 K i n hydrogen

For c a t a l y s t D a decrease in the mean p a r t i c l e s i z e i s measured, while 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 ( f i g . 2.25) changes d r a m a t i c a l l y . During heat treatment the f r a c t i o n of p a r t i c l e s with a diameter of 10 nm, i n c r e a s e s from 10% up to 81%. This was accompanied with a change in the transparency of the support observed i n the microscope. The e f f e c t of the s t r u c t u r a l change of the support i s demonstrated in f i g . 2.26 and 2 . 2 7 , with the PSD's evaluated for a c r y s t a l l i n e and an amorphous p a r t of the s u p p o r t .

30-__E

1 . 3 h. «O 1 - 0 I»

n

-l.lt h. 1 . 3 5 0 I»

zfaffeb,

« 1 0 18 2 « 3 4 10 18 26 34 42 ■ d(nm)

F i g . 2 . 2 5 . P a r t i e l e s i z e d i s t r i b u t i o n evaluated for c a t a l y s t D during s i n t e r i n g a t 973 K in hydrogen. i Ca: Ü , Support ' a m o r p h o u s crystalllna r»1

TH.,v.-,-1a 2 8 34 42 ► d(nm)

F i g . 2 . 2 6 . 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 determined for the n i c k e l d i s p e r i o n of c a t a l y s t D2 on the amorphous and the c r y s t a l l i n e p a r t of the s u p p o r t .

F i g . 2 . 2 7 . E l e c t r o n micrographs of model c a t a l y s t D2 a f t e r c r y s t a l l i z a t i o n of the s u p p o r t , a: amorphous and b: c r y s t a l l i n e p a r t of the s u p p o r t . Scale 1 cm ■ 100 nra.

Although the boundary used was r a t h e r a r b i t r a r y , the d i f f e r e n c e I s remarkable. 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 on the c r y s t a l l i n e p a r t Is asymmetric with a high f r a c t i o n of small p a r t i c l e s (d ■ 10 nm). On the amorphous p a r t of the support a broad d i s t r i b u t i o n i s evaluated s i m i l a r to the d i s t r i b u t i o n of the s t a b l e c a t a l y s t Dt ( f i g . 2 . 2 4 ) .

2 . 6 . Discussion and c o n c l u s i o n s

P a r t i c l e s i z e d i s t r i b u t i o n s as presented in s e c t i o n 2 . 5 . 2 . show t h a t d u r i n g s i n t e r i n g the f r a c t i o n of p a r t i c l e s l a r g e r than a c e r t a i n s i z e

I n c r e a s e s a t the expense of the smaller o n e s . Such a phenomenon corresponds to the e x p e c t a t i o n for an Ostwald r i p e n i n g p r o c e s s . Except

for the chromium c o n t a i n i n g c a t a l y s t , growth curves of a l l model systems aged a t 873 K a s well as a t 973 K show an i n i t i a l s t a g e of fast s i n t e r i n g , followed by a s t a g e of very slow s i n t e r i n g . This I s i n agreement with t h e o b s e r v a t i o n s on Pt/alumina [9 ] and Pd/alumlna flO | model systems I n v e s t i g a t e d by o t h e r a u t h o r s , but not with the power law growth c u r v e s , p r e d i c t e d from t h e o r e t i c a l c o n s i d e r a t i o n s for Ostwald r i p e n i n g in l i t e r a t u r e .

At 873 K, i n c r e a s i n g i n i t i a l mean p a r t i c l e s i z e s lead to growth curves l e v e l l i n g off a t i n c r e a s i n g p a r t i c l e s s i z e s . In o t h e r words: the growth r a t e a p p a r e n t l y does not only depend on average p a r t i c l e s i z e , as i s to be expected for Ostwald r i p e n i n g , but a l s o on o t h e r f a c t o r s .

At 973, no obvious I n d i c a t i o n s for such an e f f e c t have been found, and for i n s t a n c e a sample with an i n i t i a l mean p a r t i c l e r a d i u s of 11 nm does not show any growth a t a l l . A p o s s i b l e explanation for t h e d i s c r e p a n c i e s a t 873 K must be sought in the alumina c a r r i e r . After high temperature s t a b i l i z a t i o n of t h e c a r r i e r and before d e p o s i t i o n of n i c k e l a l l c a r r i e r s s t u d i e d a t 873 K and some s t u d i e d a t 973 K appeared to be amorphous, In c o n t r a s t for I n s t a n c e to the c a r r i e r used by E. Ruckensteln [7 | . After d e p o s i t i o n of n i c k e l and the subsequent decomposition of t h e film I n t o s e p a r a t e p a r t i c l e s , In s e v e r a l of t h e c a t a l y s t s r e c r y s t a l l l z a t l o n of the c a r r i e r has been observed.

Apparently, n o t w i t h s t a n d i n g the high temperature p r e t r e a t m e n t , the c a r r i e r s have not been a s s t a b l e as e x p e c t e d . Nickel p a r t i c l e growth connected to r e c r y s t a l l l z a t l o n of the c a r r i e r must be held r e s p o n s i b l e for the anomalous growth a t 873 K of n i c k e l d i s p e r s i o n s with g r e a t i n i t i a l mean p a r t i c l e dimensions.

In the s i n t e r study a t 973 K, using chromium and manganese as

a d d i t i v e s , chromium d e c r e a s e s the growth r a t e during the i n i t i a l s t a g e of s i n t e r i n g while manganese I n c r e a s e s growth. Combination of the two metals c a n c e l s out both e f f e c t s .

Comparing 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 s of model c a t a l y s t s E, F and H ( f i g . 2 . 1 7 , 19, 2 1 ) , a d d i t i o n of chromium (F) shows t o i n c r e a s e t h e d i s p e r s i o n of the n i c k e l phase while on a d d i t i o n of manganese the o p p o s i t e i s a c h i e v e d . Obviously d e s i n t e g r a t l o n of the amorphous n i c k e l f i l m , governed by the n l c k e l - a l u m l n a i n t e r f a c e , i s s t r o n g l y Influenced by the presence of the a d d i t i v e s . But a l s o the subsequent s i n t e r i n g of the n i c k e l d i s p e r s i o n i s s t r o n g l y Influenced by the a d d i t i v e s .

A f u r t h e r d i s c u s s i o n of growth k i n e t i c s , in p a r t i c u l a r a t 973 K, w i l l be postponed to c h a p t e r 6, where experimental r e s u l t s can be compared with t h e o r e t i c a l p r e d i c t i o n s .