A NGR STUDY OF Pt-Sn/ Al2O3 CATALYSTS AND THEIR REACTIVITY

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A NGR STUDY OF Pt-Sn/ Al2O3 CATALYSTS AND THEIR REACTIVITY

R. Bacaud, P. Bussière, F. Figueras

To cite this version:

R. Bacaud, P. Bussière, F. Figueras. A NGR STUDY OF Pt-Sn/ Al2O3 CATALYSTS AND THEIR REACTIVITY. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-94-C2-95.

�10.1051/jphyscol:1979233�. �jpa-00218631�

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JOURNAL DE PHYSIQUE

Colloque C2, supplkment au no 3, Tome

40,

mars 1979, page C2-94

A N G R STUDY OF

P ~ - s ~ / A ~ ~ o ~

^CATALYSTS^ANDTHEIR REACTIVITY

R. Bacaud, P. B u s s i s r e and F. F i g u e r a s

I n s t i t u t de Recherches sur l a CataZyse du C. N. R.S., 79, boulevard du Onze-Novembre 1918, 69626 V i 2 Zeurbrmne cedex, France

RBsum6.- La s p e c t r o s c o p i e Miissbauer de 1 ' Q t a i n a permis d ' d t u d i e r l a s t r u c t u r e de c a t a l y s e u r s Pt-Sn/Al,O, B d i f f d r e n t e s t e n e u r s d e s deux mdtaux, a i n s i que l e u r comportement e n d o u b l e c a l c i - n a t i o n , e n d d s o r p t i o n , a d s o r p t i o n u l t g r i e u r e d'oxyggne e t d ' h y d r o g s n e , c h l o r a t i o n e t rdhydrogg- n a t i o n , c o n d u i s a n t B q u e l q u e s c o n c l u s i o n s s u r l e s p r o p r i d t C s s u p e r f i c i e l l e s e t c a t a l y t i q u e s de c e s s o l i d e s .

A b s t r a c t . - The s t r u c t u r e of Pt-Sn/Al,O, c a t a l y s t s o f d i f f e r e n t c o m p o s i t i o n s h a s b e e n s t u d i e d u s i n g t i n M6ssbauer s p e c t r o s c o p y and t h e i r b e h a v i o u r i n v e s t i g a t e d i n d o u b l e c a l c i n a t i o n , i n d e s o r p t i o n , s u b s e q u e n t a d s o r p t i o n of oxygen and hydrogen, c h l o r i n a t i o n and r e h y d r o g e n a t i o n . Some c o n c l u s i o n s a r e p r e s e n t e d on t h e s u r f a c e and c a t a l y t i c p r o p e r t i e s of t h e s e s o l i d s .

I . I n t r o d u c t i o n . - When a number o f p h y s i c a l and che- m i c a l methods f a i l i n g i v i n g a good p i c t u r e of t h e o x i d a t i o n s t a t e and s t r u c t u r e of t i n i n Pt-Sn/Al,O, c a t a l y s t s , Mzssbauer s p e c t r o s c o p y o f t h i s e l e m e n t a f f o r d s t o a c h i e v e t h i s c h a r a c t e r i z a t i o n . We u s e d i t i n a p r e l i m i n a r y s t u d y / I / , which r e s u l t e d i n t h e e v i d e n c e f o r d i f f e r e n t d e g r e e of r e d u c t i o n and a l l o y i n g a t d i f f e r e n t c o m p o s i t i o n s . These f e a t u r e s could s u p p o r t t h e h y p o t h e s i s o f i n t e r a c t i o n s of Sn m e t a l w i t h p l a t i n u m and of i o n i c t i n w i t h A1,0, a s f a c t o r s c o n t r o l l i n g t h e c a t a l y t i c p r o p e r t i e s . T h e r e f o r e t h i s r e s e a r c h was extended u s i n g improved tech- n i c a l c o n d i t i o n s and o t h e r p l a t i n u m and t i n c o n t e n t s , and s t u d y i n g t h e i r r e a c t i v i t y a s w e l l . Here we p r e - s e n t t h e r e s u l t s of such i n v e s t i g a t i o n s , a l s o paying a t t e n t i o n t o t h e work of B e r n d t e t a l . / 2 / on a ca- t a l y s t of t h i s k i n d .

2. E x p e r i m e n t a l . - The p r e p a r a t i o n by a d o u b l e impre- g n a t i o n a c c o r d i n g t o t h e p r o t o c o l p a t e n t e d by C.F.R / 3 / , t h e d e t e r m i n a t i o n of t h e d e g r e e of r e d u c t i o n , and t h e c a t a l y t i c a c t i v i t i e s and s e l e c t i v i t y f o r th h y d r o g e n a t i o n o f benzene and t h e c o n v e r s i o n o f n- h e p t a n e have been d e s c r i b e d e l s e w h e r e 141. Mgssbauer s p e c t r o s c o p y was performed a s u s u a l by t r a n s m i s s i o n through p e l l e t s , a b o u t 100 mg cm-2 t h i c k , p r e p a r e d u s i n g e n r i c h e d t i n , p u t i n t o a f u r n a c e a l l o w i n g t r e a t m e n t s i n vacuo o r i n d i f f e r e n t atmospheres and s p e c t r a w i t h o u t removing t h e sample. The temperature was lowered t o ambient f o r s p e c t r o s c o p y .

Because o f a n o f t e n a r i s i n g Karyagin-Goldanskii E f f e c t due t o a h i g h d i s p e r s i o n , e a c h s p e c t r u m was c o n s i d e r e d a s b u i l t from s i n g l e l i n e s . They were a t - t r i b u t e d t a k i n g i n t o a c c o u n t t h e known v a l u e s of NGR

p a r a m e t e r s of bulky t i n s p e c i e s / 2 , 5 , 6 / , s p e c i a l l y of Pt-Sn a l l o y s / 7 , 8 , 9 / , and w e l l e s t a b l i s h e d cherni- c a l and p h y s i c a l p r o p e r t i e s o f t h e system.

Three c a t a l y s t s were s t u d i e d ; No 1 h a s a p l a t i - num c o n t e n t 0,47% by w e i g h t and t i n 0 , 4 7 % , No 2 P t 0.95 and Sn 0.55, No 3 P t 0.5 and Sn 0.2.

3. R e s u l t s . - A s e t o f r e s u l t s , o b t a i n e d w i t h a sam- p l e o f t h e c a t a l y s t No 1 , i s shown i n f i g u r e 1 , S p e c t r u m s i s p r o v i d e d b e f o r e t h e r e d u c t i o n by hydrogen, i t i s t h a t of SnO,. Reduction r e s u l t s i n spectrum

b,

which was f i t t e d t o t h e l i n e s r e p o r t e d i n t a b l e I. The main f e a t u r e s o f t h e p o s s i b l e cha- r a c t e r i z a t i o n a r e t h e a t t r i b u t i o n of t h e l i n e a t 0 . 5 8 mm.s-' t o H2SnC16 o r s i m i l a r c h l o r i n a t e d Sn(1V) s p e c i e s , and t h a t of t h e l i n e a t 2.34 mm. s-' t o PtSn, r e s u l t i n g i n a r a t h e r l a r g e amount o f f r e e p l a t i n u m m e t a l . Small amounts of h i g h l y d i s p e r s e d PtSn and Sn (OH,C1)2 a r e p o s s i b l e a s w e l l .

T a b l e I F i t of s p e c t r u m

b

I S (mm s - l )

-

^0.16 ^0.58 ^2.34 ^4.06

i- (m s-l) 0.70 1.40 1 .80 0.75

exp

' r e l a t '%) 4 3 1 5 8 7

S t u d y i n g s p e c t r u m 2, o b t a i n e d a f t e r d e s o r p t i o n , l e a d s t o t h e s u r e c o n c l u s i o n of a n o x i d a t i o n of about one h a l f of t h e amount of PtSn, t o d i s p e r s e d Sn(OH,Cl),. Subsequent a d s o r p t i o n s a t f i r s t of oxygen t h e n of hydrogen a t room t e m p e r a t u r e do n o t g i - ve any a p p r e c i a b l e change i n t h e spectrum. A f t e r a c h l o r i n a t i o n by C C l , , a l m o s t a l l t i n i s c o n v e r t e d t o SnCl,, n H20 a c c o r d i n g t o spectrum

d.

Rehydroge- n a t i o n g i v e s apectrum 5, l i t t l e d i f f e r e n t from s p e c trum

A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979233

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A phase of f r e e p l a t i n u m m e t a l i s l i k e l y t o

Fig. 1 : S p e c t r a of c a t a l y s t No 1 a t room temperatu- r e a f t e r : a) c a l c i n a t i o n , b) r e d u c t i o n , c) desorp- t i o n and subsequent a d s o r p t i o n of oxygen and hydrogen, d) c h l o r i n a t i o n , e ) r e h y d r o g e n a t i o n .

The c a t a l y s t No 2 a p p e a r s a s more reduced, a- b o u t one h a l f of t h e spectrum a c c o u n t s f o r PtSn, t h e remaining p a r t i s a t t r i b u t e d t o o t h e r a l l o y s and SnO.

No Sn(IV) i s d e t e c t a b l e . D e s o r p t i o n o x i d i z e s about one h a l f of t h e a l l o y s t o SnO. A d s o r p t i o n s have no a p p r e c i a b l e e f f e c t . C h l o r i n a t i o n l e a d s t o a m i x t u r e of SnO, and c h l o r i n a t e d Sn(IV), and r e h y d r o g e n a t i o n r e s u l t s i n a spectrum s i m i l a r t o t h a t of t h e s t a r t i n g reduced c a t a l y s t b u t l e s s d i s p e r s e d .

The c a s e of t h e c a t a l y s t No 3 i s somewhat t r o w blesome. It could c o n t a i n e i t h e r a l a r g e amount of c h a o r i n a t e d Sn(1V) i n s p i t e of a measured h i g h degree of r e d u c t i o n , o r a l a r g e amount of Sn i n s p i t e of a t r e a t m e n t a t .a t e m p e r a t u r e h i g h e r t h a n i t s m e l t i n g p o i n t . C h l o r i n a t i o n g i v e s o n l y SnCl,, n H,O and re@- d r o g e n a t i o n r e s t o r e s t h e i n i t i a l components.

C a l c i n a t i o n a f t e r t h e a d d i t i o n of H,PtCl,befo- r e t h e r e d u c t i o n d i d n o t change t h e composition of t h e reduced c a t a l y s t s .

4. D i s c u s s i o n and c o n c l u s i o n s . - The l a s t r e s u l t sug- g e s t s t h a t t h e improvements i n t h e c a t a l y t i c a c t i v i - t y i n t r o d u c e d b u t t h e c a l c i n a t i o n a r e c h i e f l y r e l e - v a n t t o t h e behaviour of p l a t i n u m a l o n e .

o c c u r i n t h e c a t a l y s t No 1 , i n a d d i t i o n t o a l l o y i n g . I n t h e c a t a l y s t No 2 a l l t h e p l a t i n u m i s a l l o y e d t o t i n . These f e a t u r e s s a t i s f a c t o r i l y account f o r t h e c a t a l y t i c a c t i v i t y i n benzene h y d r o g e n a t i o n , t h e d i - m i n u t i o n of which i n comparison t o p l a t i n u m a l o n e i s due t o a c o n t r i b u t i o n of t h e e l e c t r o n s of t i n t o t h e d-band of p l a t i n u m . I n t h e c a s e of t h e c o n v e r s i o n of n-heptane, where c y c l i z a t i o n proceeds a c c o r d i n g t o a b i f u n c t i o n n a l mechanism u s i n g s t r o n g a c i d i c s i t e s of t h e s u p p o r t , a n i m p o r t a n t r o l e i s p l a y e d by i o n i c s p e c i e s of t i n . Not o n l y t h e c o n t e n t i n o x i d i z e d t i n i s d i f f e r e n t f o r o u r c a t a l y s t s b u t even i t s o x i d a t i o n s t a t e , s i n c e Sn(1V) i s p r e s e n t i n t h e c a t a l y s t No 1 b u t n o t i n t h e c a t a l y s t No 2.

The p a r t i a l o x y d a t i o n of a l l o y s d u r i n g a de- s o r p t i o n t r e a t m e n t could be due t o t h e OH groups of the s u p p o r t , such a s i t appeared i n t h e c a s e of Fe/

XgO c a t a l y s t s / l o / and i n o t h e r systems. The e x t e n t of t h i s p r o c e s s l i k e l y depends on d i s p e r s i o n .

T h e r e f o r e i t i s n o t s u r p r i s i n g t h a t a subsequent a d s o r p t i o n of oxygen does n o t g i v e any d e t e c - t a b l e change i n t h e s t a t e of t i n , s i n c e i t could oc- c u r o n l y on a p l a t i n u m phase. Hydrogen a t room t e m p e r n t u r e c l e a r l y h a s l i t t l e e f f e c t i f any on t h e oxi- d i z e d t i n and i t i s adsorbed o n l y on platinum.

The c a t a l y s t s No 2 and No 3 do n o t s u f f e r any l a r g e d e p a r t u r e from t h e i r f i r s t composition and s t r u c t u r e a f t e r c h l o r i n a t i o n and r e h y d r o g e n a t i o n , o n l y a s m a l l d i m i n u t i o n of d i s p e r s i o n a p p e a r s . For the c a t a l y s t No I , more s i m i l a r t o t h e i n d u s t r i a l o n e s , t h i s t r e a t m e n t seems t o i n t r o d u c e d i f f e r e n t small changes i n t h e m e t a l l i c and i o n i c p h a s e s , w h i c h could account f o r t h e v a r i a t i o n i n t h e c a t a l y t i c se- l e c t i v i t y i n reforming.

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