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Sintering of Alumina Supported Platinum
B. Delmon and G.F. Froment (Editors), Catalyst Deactivation 0 1980 Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands
159
S I N T E R L N G OF A L U M I N A SUPPORTED PLATINUM
J.P.
BOURNONVILLE and G .
MARTINO
I n s t i t u t F r a n q a i s du P @ t r o l e , B . P .
311, 92506 Rueil-Malmaison,
France
ABSTRACT The i n f l u e n c e s of n a t u r e of t h e atmosphere and of t h e t r e a t m e n t t e m p e r a t u r e on t h e s i n t e r i n g of p l a t i n u m have been examined. I t a p p e a r e d t h a t , under o x i d i z i n g c o n d i t i o n s , no s i n t e r i n g o c c u r e d i f c h l o r i n e c o n t e n t on t h e higher than about 1 w t
%.
d
alumina remained
T h i s c h l o r i n e e f f e c t was n o t o b s e r v e d under hydrogen.
But
i t was shown t h a t , i n t h a t c a s e , t h e p r e s e n c e of c h l o r i n e dropped s i n t e r i n g and
t h a t a c t i v a t i o n energy w a s h i g h e r . The s t a b i l i t y of p l a t i n u m i n t h e p r e s e n c e of a i r i s due t o a v e r y h i g h r e d i s p e r s i o n r a t e . The s m a l l e r s i n t e r i n g r a t e under hydrogen i n t h e p r e s e n c e of c h l o r i n e i n d i c a t e s a s t r o n g e r i n t e r a c t i o n between t h e s m a l l c r i s t a l l i t e s and t h e c a r r i e r .
INTRODUCTION
The a g i n g of r e f o r m i n g c a t a l y s t s ( 1 ) i s a t t r i b u t e d t o t h e p o i s o n i n g of b o t h f u n c t i o n s of t h e c a t a l y s t by coke d e p o s i t and t o t h e r e d u c t i o n of t h e m e t a l l i c f u n c t i o n by s i n t e r i n g . S i n t e r i n g of m e t a l s ( 2 ) and e s p e c i a l l y of p l a t i n u m s i n t e r i n g
( 3 - 4 ) have been
w i d e l y i n v e s t i g a t e d . The i n f l u e n c e s of t h e t r e a t m e n t c o n d i t i o n s , atmosphere ( 5 t o 11) and t e m p e r a t u r e (12 t o 1 5 ) and of loading
(14-16-17)
t h e s p e c i f i c p r o p e r t i e s of t h e c a t a l y s t s , m e t a l
and n a t u r e of t h e c a r r i e r (7-9-12-18)
on t h e r a t e e q u a t i o n s and
t h e a c t i v a t i o n e n e r g i e s of t h e s i n t e r i n g p r o c e s s e s have been d e t e r m i n e d . Most r e s u l t s have been o b t a i n e d on b a d e l y d i s p e r s e d
(D
<.5) p l a t i n u m
;
t h e y do n o t l e a d t o a
g e n e r a l agreement a s f a r a s o r d e r s , a c t i v a t i o n e n e r g i e s or mechanisms
(19 t o 25) are
concerned. T h i s s i t u a t i o n l e d u s t o reexamine t o s i n t e r i n g of w e l l d i s p e r s e d p l a t i n u m (D
> .85) on
gamma alumina. The c a t a l y s t s used were v e r y c l o s e t o i n d u s t r i a l o n e s .
EXPERIMENTAL TECHNICS
S t a r t i n g m a t e r i a l u s e d w a s .6 w t % p l a t i n u m s u p p o r t e d on a from R . P .
.d
alumina p u r c h a s e d
P l a t i n u m w a s impregnated i n p r e s e n c e of c h l o r h y d r i c a c i d , d r i e d and
c a l c i n a t e d i n t h e p r e s e n c e of a i r a t 530 OC and t h e n r e d u c e d a t 500
OC.
160
characteristics are g i v e r : in t a b l e 1
TABLE 1
Characteristics of the starting catalyst S
Carrier
area
200
(m*/s)
Loading ( % wt)
Pt c1
Metal 1ic dispersion
>0.85
w
0.6
#p1.2
The evolution of platinum area was followed by the classical hydrogen-oxygen titration ( 2 6 ) , by vapor phase benzene hydrogenation and, for some typical samples, by electron microscopy. A good agreement between the dispersion measurements made by titration or by
benzene hydrogenation was observed. Platinum repartition through the pellet was checked by
scanning microprobe
;
platinum and chlorine contents were determined by X-ray fluorescence. 3
3
All the experiments were performed with a constant flow rate of 2000 cm /cm /hr.
Gases were dried
;
residual moisture content was about 15 ppm volume.
RESULTS To get a whole set of information, we used the different atmospheres to which a reforming catalyst may be exposed during its life
:
air, nitrogen or an other inert
gas, hydrogen. Temperatures were changed in the range of 500
OC
to 700
OC ;
the
alumina used looses its surface area at higher temperature and it was impossible to separate the contribution of that phenomenon to the total platinum sintering process. Air treatment The fresh catalyst was submitted to air treatments at different temperatures. The variation of the platinum area was
followed versus time
;
the observed results
are plotted in figure 1. Sintering rate is rather low at 540
OC
but becomes very high at 750
pointed out before this is connected to the 30
%
OC ;
as
surface area loss of the carrier,
negligible till 700 'C. The importance of that parameter was checked with the study of platinum ondalumina (S = 9 mL/g). In that case, in a few hours, platinum looses more than 90
%
of its area.
In the litterature, rates are presented in the following form _ -dS =
dt
k Sn
:
161
Spt
1
TOC
(m?/gPt)
1
1
10 20
I
I
50
125
100
Time (hours)
Fig.
1. A i r t r e a t m e n t of .6 w t
%
p l a t i n u m on
d,
alumina c a t a l y s t .
The t r a n s f o r m a t i o n of o u r r e s u l t s l e d u s t o n = 1 4 , t h i s v a l u e a g r e e s w i t h some a u t h o r s ( 2 7 ) and d i s a g r e e s w i t h o t h e r s ( 2 8 ) . R e s u l t i n g a c t i v a t i o n e n e r g y of
1 4 2 k J / a t g P t i s t h e r a n g e of t h e p u b l i s h e d v a l u e s . F u r t h e r a n a l y s i s o f o u r r e s u l t s showed u s a loss of c h l o r i n e on t h e c a t a l y s t . Thir l o s s as w e l l as s i n t e r i n g was t i m e and t e m p e r a t u r e d e p e n d a n t . To s e e i n which way
162 t h e c i i 1 o r i c c
WE
tk:e follo.tiinq e x p e r i m e n t s . WE,
pc:rformi.ii
compared tlie e f f e c t o f c a l c i n a t i n g a t 3 d i f f e r e n t t e m p e r a t u r e s on two c a t a l y s t s ; t h e iir.ct ,,,as t l i e norirlal one c o n t a i n i n g 1 .2 wt
c h l o r i n e , t h e second was o b t a i n e d
:'
by t r e a t i r i y t h e reduced form w i t h ammonia, t h e n d r y i n g and t r e a t i n g w i t h hydrogen a t 500 O C t o e l i m i n a t e t h e w a t e r . Without t h e p r e s e n c e of c h l o r i n e , p l a t i n u m l o s s e s most of i t s a r e a i n a f e w h o u r s a t 700
OC.
T h i s showed c l e a r l y t h e c i i l o r i n e e f f e c t
and made u s a s k what would happen a t c o n s t a n t c h l o r i n e c o n t e n t . T r e a t m e n t of tlie c a t a l y s t i n a i r m a i n t a i n i n g t h e c h l o r i n e c o n c e n t r a t i o n w i t h an i n j e c t i o n of c a r b o n t e t r a c h l o r i d e o r g a s e o u s s i n t e r i n g a t 600 'C
c h l o r h y d r i c a c i d d i d n o t l e a d t o any
as indicated i n figure 2 .
and even a t 650 'C
T h i s phenomenon c o u l d be t h e r e s u l t of an i n h i b i t i o n of s i n t e r i n g o r of an
2 gives the obtained r e s u l t s
e q u i l i b r i u m between s i n t e r i n y and r e d i s p e r s i o n . T a b l e s t a r t i n g from a c a t a l y s t of a even a t 4 0 0 ' C ,
low d i s p e r s i o n
(D
=
.4).
I t i s c l e a r l y shown t h a t ,
r e d i s p e r s i o n i s v e r y r a p i d , f i n a l c h l o r i n e c o n t e n t s of t h e r e d i s p e r s e t
c a t a l y s t s were i n t h e r a n g e 1 . 2 t o 1 . 3 w t %.
TABLE 2
I n f l u e n c e of t i m e , Pcc14,
400 450 500 450 4 50 450
t e m p e r a t u r e on r e d i s p e r s i o n
2 2
1
2 2
1 2 1 0.5
2 4
: V.V.H.
3
: 2000 cm /cm
.95 .97
- C1
S t a r t i n g c a t a l y s t : P t : .64 w t % Conditions
.98 .98 .97 .98
1
3
/hr
-
: .7
P
T
wt :
% - D
5
10
:
.4
Pa
The o b t a i n e d r e s u l t s c l e a r l y show t h a t l e a c h i n g o u t of t h e c h l o r i n e i s t h e most important,
i f n o t t h e o n l y f a c t o r r e s p o n s i b l e f o r t h e s i n t e r i n g i n an o x i d i z i n g
atmosphere of any i n d u s t r i a l r e f o r m i n g c a t a l y s t
(C1 2 1 . 0 w t
%).
163
(12 1
1,21
O/O
I
Cl
(04 1
20
10 0,O:
with injection of CCL4
0 : without
0
10
20
injection of CCL4
30
40 Time (hours)
F i g . 2 . A i r t r e a t m e n t a t c o n s t a n t c h l o r i n e c o n t e n t (T = 650 " C ) N e u t r a l atmosphere Argon was used i n t h a t c a s e . S i n t e r i n g was v e r y f a s t . But a s p o i n t e d o u t , even i n t h a t c a s e , l e a c h i n g o u t o f t h e c h l o r i n e w a s i m p o r t a n t . Two d i f f e r e n c e s were o b s e r v e d w i t h what was happening i n o x i d i z i n g a t m o s p h e r e was o b s e r v e d
;
:
t h e f i r s t w a s t h a t no r e d i s p e r s i o n
t h e second w a s t h a t d i f f e r e n t o r d e r s ( n ) c o i n c i d e d b e s t w i t h t h e
r e s u l t s . We do n o t have a n e x p l a n a t i o n for t h a t , b u t changing i n r e a c t i o n mechanism o r i n t h e n a t u r e of t h e p l a t i n u m p r e s e n t on t h e c a t a l y s t c a n b e i n v o l v e d .
164 Hydrogen Hydrogen and gaseous hydrocarbons are the most representative of the atmosphere present over a reforming catalyst in normal working conditions. Due to experimental problems, we only used hydrogen which may be considered as the major component present Well dispersed reduced catalysts were treated at different temperatures in a stream of hydrogen. These results best fit with an apparent order (n) of 8 and led to an activation energy of 138 kJ/atg Pt.
550
20
10( 0 with injection of HCL 0 without injection of HCL
I
(
10
20
30
I
1
40
50 Time(hours1
Fig. 3. Treatment in hydrogen with and without chlorine injection.
165 As in t ~ h eother cases, chlorii.ewas lost and work at constant chlorine content
was performed. Typical results are given in figure 3
;
sinterinq was slown down.
Plotting of these results led to an apparent order n equal to 8 but the activation energy was increased by about 30 kJ/atg Pt compared with the previous case. We could not find any redispersion when starting with a sintering material.
INTERPRETATION - CONCLUSION
The differences observed if the treatment atmosphere is changed can be explained by the fact that in each case, different platinum compounds are present at the surface of the carrier and the
possible migrating species are not the same.
In oxidizing atmosphere and in the presence of enough chlorine, mainly present species may be chloroplatinum aluminates ( 2 9 ) which are strongly bonded to the carrier and which sinter very slowly. Complexes like PtCl (A1C1 2
)
3 2
described (30)
or the same kind of oxychloroplatinum complexes may be the volatile species which are responsible for redispersion
:
equilibrium between sintering and redispersion
is related to experimental conditions (31-32) especially chlorine concentration, oxygen partial pressure and presence of OH groups on the carrier (33). If the chlorine concentration is too low, platinum oxide may exist and the equilibrium between sintering and redispersion may be obtained at much lower dispersion. In hydrogen, platinum is present as atoms, rafts clusters or small cristallites depending on dispersion. Surface atom (at least some of them) adsorbed hydrogen
;
are covered by
so that it is difficult to know the exact nature of the migrating
species. Our results show that chlorine changes the mobility of the migrating species. This result agrees with both mechanisms
;
one can say that interaction between the
platinum moiety and the support is enhanced by increasing the
acidity and so
reducing its mobility on the surface, or that the withdrawing of electrons decreases the volatility of the surface atoms and prevents their escaping from the cristallite. Further studies on the influence of the acidity will allow to confirm that. As a conclusion, we want to point out the very important role played by the presence of chlorine on the catalyst
:
it acts as redispersion agent in oxidizing
atmosphere and as stabilizing agent in reducing atmosphere. We may also emphasize that the big differences found in sintering rates in the litterature are largely connected to nature of supports used, chlorine contents, presence of impurities ( 3 4 ) , partial pressures of the different gases for instance.
ACKNOWLEDGMENT The authors thank Mrs Chenebaux and her coworkers for performing X-ray fluorescence and scanning microprobe determinations.
166 REFERENCE 5
1 J . F . L e Page and Al., C a t a l y s e d e C o n t a c t , Technip Ed., P a r i s 1 9 7 8 . 2 P . W y n b l a t t , R . A . D e l l a B e t t a and N.A. G ] o s t e i n , P h y s i c a l B a s i s f o r Iieteroqeneous C a t a l y s i s , Ed. Plenum ( N . Y . 1 9 7 5 ) a d r e f . t h e r e i n . 3 P . C . Flynn and S . E . Wanke, C a t a l . Rev. S c i . E n g . , 1 2 , 93 ( 1 9 7 5 ) . 4 G . A . Sornorjai, "X-ray and E l e c t r o n Methods of A n a l y s i s " ( H . Van Ophen and w. P o r r i c h E d s ) , chap. 6 , Plenum, New-York ( 1 9 6 8 ) . 5 M . B o u d a r t , A.W. Alday, L . D . P t a k and J . E . Benson, J . C a t a l . , 1 1 , 35 ( 1 9 6 8 ) . 6 H. S p i n d l e r , I n t . Chem. Eng., 1 4 , 7 2 1 ( 1 9 7 4 3 . 7 G . R . Wilson and W.K. H a l l , J . C a t a l . , 1 7 , 1 9 0 ( 1 9 7 0 ) . 8 G . A . M i l l s , S . Weller and E . B . C o r n e l i u s , A c t e s du 2eme c o n g r e s d e c a t a l y s e , P a r i s 1 9 6 0 , Ed. T e c h n i p , 2 , 2221 ( 1 9 6 1 ) . 9 J. F r e e l , J. c a t a l . , 2 5 , 1 4 9 ( 1 9 7 2 ) . 1 0 Y . F . Chu, E . R u c k e n s t e i n , J. C a t a l . , 5 5 , 3 4 8 ( 1 9 7 8 ) . 1 1 R.M. Fiedorow and S . E . Wanke, J . C a t a l . , 4 3 , 3 4 ( 1 9 7 6 ) . 12 A . Renouprez, C . Hoang Van and P . A . Compagnon, J . C a t a l . , 3 4 , 411 ( 1 9 7 4 ) . 1 3 R . A . Herrmann, S.F. A d l e r , M.S. G o l d s t e i n and R.M. De Baun, J . Phys. Chem., 6 5 , 2189 ( 1 9 6 1 ) . 1 4 T.P.. D o r l i n g , B . W . J . Lynch and R . L . Moss, J. C a t a l . 2 0 , 1 9 0 ( 1 9 7 1 ) . 1 5 R.T.K. B a y e r , C . Thomas and R . B . Thomas, J . C a t a l . , 3 8 , 5 1 0 ( 1 9 7 5 ) . 16 J . A . B e t t , K. K i n o s h i t a , P . S t o n e h a r t , J. C a t a l . , 3 5 , 307 ( 1 9 7 4 ) . 1 7 Z . A . Fuhrmann, G . P a r r a v a n o , Paper B 8 " 6 t h Congress of C a t a l y s i s " , London ( 1 9 7 6 ) . 1 8 H . A . B e n e s i , R.M. C u r t i s and H . P . S t u d e r , J . C a t a l . , 1 0 , 3 1 8 ( 1 9 6 8 ) . 1 9 E . R u c k e n s t e i n and B. Pulvermacher, Aiche J . , 1 9 , ( 2 ) 3 5 6 ( 1 9 7 3 ) . 2 0 E . R u c k e n s t e i n and B. P u l v e r m a c h e r , J. C a t a l . , 2 9 , 2 2 4 ( 1 9 7 3 ) . 21 E. R u c k e n s t e i n and B. P u l v e r m a c h e r , J . C a t a l . , 3 5 , 1 1 5 ( 1 9 7 4 ) . 22 E . R u c k e n s t e i n and B. P u l v e r m a c h e r , J. C a t a l . , 3 7 , 4 1 6 ( 1 9 7 5 ) . 2 3 E . R u c k e n s t e i n and D . B . D a b y b u r j o r , J. C a t a l . , 3 8 , 73 ( 1 9 7 7 ) . 2 4 P.C. Flynn and S . E . Wanke, J. C a t a l . , 3 4 , 3 9 0 ( 1 9 7 4 ) . 2 5 P . C . Flynn and S . E . Wanke, J. C a t a l . , 3 4 , 4 0 0 ( 1 9 7 4 ) . 2 6 J . E . Benson and M . B o u d a r t , J. C a t a l . , 4 , 7 0 4 ( 1 9 6 5 ) . 27 H.L. G r u b e r , J . Phys. Chem., 6 6 , 4 8 ( 1 9 6 2 ) . 2 8 T . R . Hughes, R . J . Houston and R.P. S i e g , I n d . Eng. Chem.,Process D e s . Develop., 1 , 96 ( 1 9 6 2 ) . 2 9 J. E s c a r d , B. P o n t v i a n n e , M.T. Chenebaux and J . Cosyns, Bull. SOC. Chim. 3 4 9 ( 1 9 7 8 . 3 0 H . S c h e i f e r and M . F r e n k e l , Z . Anorg. A l l g . Chem., 4 1 4 , 1 3 7 ( 1 9 7 5 ) . 31 G . M a r t i n o , t o be p u b l i s h e d . 32 I . F . P . , French P a t e n t 7 3 / 1 1 7 1 6 (march 1 9 7 3 ) . 3 3 E . R u c k e n s t e i n and Y.F. Chu, J. C a t a l . , 5 9 , 1 0 9 ( 1 9 7 9 ) . 34 J.P. Bournonville, T h e s i s , P a r i s 1979.
159
S I N T E R L N G OF A L U M I N A SUPPORTED PLATINUM
J.P.
BOURNONVILLE and G .
MARTINO
I n s t i t u t F r a n q a i s du P @ t r o l e , B . P .
311, 92506 Rueil-Malmaison,
France
ABSTRACT The i n f l u e n c e s of n a t u r e of t h e atmosphere and of t h e t r e a t m e n t t e m p e r a t u r e on t h e s i n t e r i n g of p l a t i n u m have been examined. I t a p p e a r e d t h a t , under o x i d i z i n g c o n d i t i o n s , no s i n t e r i n g o c c u r e d i f c h l o r i n e c o n t e n t on t h e higher than about 1 w t
%.
d
alumina remained
T h i s c h l o r i n e e f f e c t was n o t o b s e r v e d under hydrogen.
But
i t was shown t h a t , i n t h a t c a s e , t h e p r e s e n c e of c h l o r i n e dropped s i n t e r i n g and
t h a t a c t i v a t i o n energy w a s h i g h e r . The s t a b i l i t y of p l a t i n u m i n t h e p r e s e n c e of a i r i s due t o a v e r y h i g h r e d i s p e r s i o n r a t e . The s m a l l e r s i n t e r i n g r a t e under hydrogen i n t h e p r e s e n c e of c h l o r i n e i n d i c a t e s a s t r o n g e r i n t e r a c t i o n between t h e s m a l l c r i s t a l l i t e s and t h e c a r r i e r .
INTRODUCTION
The a g i n g of r e f o r m i n g c a t a l y s t s ( 1 ) i s a t t r i b u t e d t o t h e p o i s o n i n g of b o t h f u n c t i o n s of t h e c a t a l y s t by coke d e p o s i t and t o t h e r e d u c t i o n of t h e m e t a l l i c f u n c t i o n by s i n t e r i n g . S i n t e r i n g of m e t a l s ( 2 ) and e s p e c i a l l y of p l a t i n u m s i n t e r i n g
( 3 - 4 ) have been
w i d e l y i n v e s t i g a t e d . The i n f l u e n c e s of t h e t r e a t m e n t c o n d i t i o n s , atmosphere ( 5 t o 11) and t e m p e r a t u r e (12 t o 1 5 ) and of loading
(14-16-17)
t h e s p e c i f i c p r o p e r t i e s of t h e c a t a l y s t s , m e t a l
and n a t u r e of t h e c a r r i e r (7-9-12-18)
on t h e r a t e e q u a t i o n s and
t h e a c t i v a t i o n e n e r g i e s of t h e s i n t e r i n g p r o c e s s e s have been d e t e r m i n e d . Most r e s u l t s have been o b t a i n e d on b a d e l y d i s p e r s e d
(D
<.5) p l a t i n u m
;
t h e y do n o t l e a d t o a
g e n e r a l agreement a s f a r a s o r d e r s , a c t i v a t i o n e n e r g i e s or mechanisms
(19 t o 25) are
concerned. T h i s s i t u a t i o n l e d u s t o reexamine t o s i n t e r i n g of w e l l d i s p e r s e d p l a t i n u m (D
> .85) on
gamma alumina. The c a t a l y s t s used were v e r y c l o s e t o i n d u s t r i a l o n e s .
EXPERIMENTAL TECHNICS
S t a r t i n g m a t e r i a l u s e d w a s .6 w t % p l a t i n u m s u p p o r t e d on a from R . P .
.d
alumina p u r c h a s e d
P l a t i n u m w a s impregnated i n p r e s e n c e of c h l o r h y d r i c a c i d , d r i e d and
c a l c i n a t e d i n t h e p r e s e n c e of a i r a t 530 OC and t h e n r e d u c e d a t 500
OC.
160
characteristics are g i v e r : in t a b l e 1
TABLE 1
Characteristics of the starting catalyst S
Carrier
area
200
(m*/s)
Loading ( % wt)
Pt c1
Metal 1ic dispersion
>0.85
w
0.6
#p1.2
The evolution of platinum area was followed by the classical hydrogen-oxygen titration ( 2 6 ) , by vapor phase benzene hydrogenation and, for some typical samples, by electron microscopy. A good agreement between the dispersion measurements made by titration or by
benzene hydrogenation was observed. Platinum repartition through the pellet was checked by
scanning microprobe
;
platinum and chlorine contents were determined by X-ray fluorescence. 3
3
All the experiments were performed with a constant flow rate of 2000 cm /cm /hr.
Gases were dried
;
residual moisture content was about 15 ppm volume.
RESULTS To get a whole set of information, we used the different atmospheres to which a reforming catalyst may be exposed during its life
:
air, nitrogen or an other inert
gas, hydrogen. Temperatures were changed in the range of 500
OC
to 700
OC ;
the
alumina used looses its surface area at higher temperature and it was impossible to separate the contribution of that phenomenon to the total platinum sintering process. Air treatment The fresh catalyst was submitted to air treatments at different temperatures. The variation of the platinum area was
followed versus time
;
the observed results
are plotted in figure 1. Sintering rate is rather low at 540
OC
but becomes very high at 750
pointed out before this is connected to the 30
%
OC ;
as
surface area loss of the carrier,
negligible till 700 'C. The importance of that parameter was checked with the study of platinum ondalumina (S = 9 mL/g). In that case, in a few hours, platinum looses more than 90
%
of its area.
In the litterature, rates are presented in the following form _ -dS =
dt
k Sn
:
161
Spt
1
TOC
(m?/gPt)
1
1
10 20
I
I
50
125
100
Time (hours)
Fig.
1. A i r t r e a t m e n t of .6 w t
%
p l a t i n u m on
d,
alumina c a t a l y s t .
The t r a n s f o r m a t i o n of o u r r e s u l t s l e d u s t o n = 1 4 , t h i s v a l u e a g r e e s w i t h some a u t h o r s ( 2 7 ) and d i s a g r e e s w i t h o t h e r s ( 2 8 ) . R e s u l t i n g a c t i v a t i o n e n e r g y of
1 4 2 k J / a t g P t i s t h e r a n g e of t h e p u b l i s h e d v a l u e s . F u r t h e r a n a l y s i s o f o u r r e s u l t s showed u s a loss of c h l o r i n e on t h e c a t a l y s t . Thir l o s s as w e l l as s i n t e r i n g was t i m e and t e m p e r a t u r e d e p e n d a n t . To s e e i n which way
162 t h e c i i 1 o r i c c
WE
tk:e follo.tiinq e x p e r i m e n t s . WE,
pc:rformi.ii
compared tlie e f f e c t o f c a l c i n a t i n g a t 3 d i f f e r e n t t e m p e r a t u r e s on two c a t a l y s t s ; t h e iir.ct ,,,as t l i e norirlal one c o n t a i n i n g 1 .2 wt
c h l o r i n e , t h e second was o b t a i n e d
:'
by t r e a t i r i y t h e reduced form w i t h ammonia, t h e n d r y i n g and t r e a t i n g w i t h hydrogen a t 500 O C t o e l i m i n a t e t h e w a t e r . Without t h e p r e s e n c e of c h l o r i n e , p l a t i n u m l o s s e s most of i t s a r e a i n a f e w h o u r s a t 700
OC.
T h i s showed c l e a r l y t h e c i i l o r i n e e f f e c t
and made u s a s k what would happen a t c o n s t a n t c h l o r i n e c o n t e n t . T r e a t m e n t of tlie c a t a l y s t i n a i r m a i n t a i n i n g t h e c h l o r i n e c o n c e n t r a t i o n w i t h an i n j e c t i o n of c a r b o n t e t r a c h l o r i d e o r g a s e o u s s i n t e r i n g a t 600 'C
c h l o r h y d r i c a c i d d i d n o t l e a d t o any
as indicated i n figure 2 .
and even a t 650 'C
T h i s phenomenon c o u l d be t h e r e s u l t of an i n h i b i t i o n of s i n t e r i n g o r of an
2 gives the obtained r e s u l t s
e q u i l i b r i u m between s i n t e r i n y and r e d i s p e r s i o n . T a b l e s t a r t i n g from a c a t a l y s t of a even a t 4 0 0 ' C ,
low d i s p e r s i o n
(D
=
.4).
I t i s c l e a r l y shown t h a t ,
r e d i s p e r s i o n i s v e r y r a p i d , f i n a l c h l o r i n e c o n t e n t s of t h e r e d i s p e r s e t
c a t a l y s t s were i n t h e r a n g e 1 . 2 t o 1 . 3 w t %.
TABLE 2
I n f l u e n c e of t i m e , Pcc14,
400 450 500 450 4 50 450
t e m p e r a t u r e on r e d i s p e r s i o n
2 2
1
2 2
1 2 1 0.5
2 4
: V.V.H.
3
: 2000 cm /cm
.95 .97
- C1
S t a r t i n g c a t a l y s t : P t : .64 w t % Conditions
.98 .98 .97 .98
1
3
/hr
-
: .7
P
T
wt :
% - D
5
10
:
.4
Pa
The o b t a i n e d r e s u l t s c l e a r l y show t h a t l e a c h i n g o u t of t h e c h l o r i n e i s t h e most important,
i f n o t t h e o n l y f a c t o r r e s p o n s i b l e f o r t h e s i n t e r i n g i n an o x i d i z i n g
atmosphere of any i n d u s t r i a l r e f o r m i n g c a t a l y s t
(C1 2 1 . 0 w t
%).
163
(12 1
1,21
O/O
I
Cl
(04 1
20
10 0,O:
with injection of CCL4
0 : without
0
10
20
injection of CCL4
30
40 Time (hours)
F i g . 2 . A i r t r e a t m e n t a t c o n s t a n t c h l o r i n e c o n t e n t (T = 650 " C ) N e u t r a l atmosphere Argon was used i n t h a t c a s e . S i n t e r i n g was v e r y f a s t . But a s p o i n t e d o u t , even i n t h a t c a s e , l e a c h i n g o u t o f t h e c h l o r i n e w a s i m p o r t a n t . Two d i f f e r e n c e s were o b s e r v e d w i t h what was happening i n o x i d i z i n g a t m o s p h e r e was o b s e r v e d
;
:
t h e f i r s t w a s t h a t no r e d i s p e r s i o n
t h e second w a s t h a t d i f f e r e n t o r d e r s ( n ) c o i n c i d e d b e s t w i t h t h e
r e s u l t s . We do n o t have a n e x p l a n a t i o n for t h a t , b u t changing i n r e a c t i o n mechanism o r i n t h e n a t u r e of t h e p l a t i n u m p r e s e n t on t h e c a t a l y s t c a n b e i n v o l v e d .
164 Hydrogen Hydrogen and gaseous hydrocarbons are the most representative of the atmosphere present over a reforming catalyst in normal working conditions. Due to experimental problems, we only used hydrogen which may be considered as the major component present Well dispersed reduced catalysts were treated at different temperatures in a stream of hydrogen. These results best fit with an apparent order (n) of 8 and led to an activation energy of 138 kJ/atg Pt.
550
20
10( 0 with injection of HCL 0 without injection of HCL
I
(
10
20
30
I
1
40
50 Time(hours1
Fig. 3. Treatment in hydrogen with and without chlorine injection.
165 As in t ~ h eother cases, chlorii.ewas lost and work at constant chlorine content
was performed. Typical results are given in figure 3
;
sinterinq was slown down.
Plotting of these results led to an apparent order n equal to 8 but the activation energy was increased by about 30 kJ/atg Pt compared with the previous case. We could not find any redispersion when starting with a sintering material.
INTERPRETATION - CONCLUSION
The differences observed if the treatment atmosphere is changed can be explained by the fact that in each case, different platinum compounds are present at the surface of the carrier and the
possible migrating species are not the same.
In oxidizing atmosphere and in the presence of enough chlorine, mainly present species may be chloroplatinum aluminates ( 2 9 ) which are strongly bonded to the carrier and which sinter very slowly. Complexes like PtCl (A1C1 2
)
3 2
described (30)
or the same kind of oxychloroplatinum complexes may be the volatile species which are responsible for redispersion
:
equilibrium between sintering and redispersion
is related to experimental conditions (31-32) especially chlorine concentration, oxygen partial pressure and presence of OH groups on the carrier (33). If the chlorine concentration is too low, platinum oxide may exist and the equilibrium between sintering and redispersion may be obtained at much lower dispersion. In hydrogen, platinum is present as atoms, rafts clusters or small cristallites depending on dispersion. Surface atom (at least some of them) adsorbed hydrogen
;
are covered by
so that it is difficult to know the exact nature of the migrating
species. Our results show that chlorine changes the mobility of the migrating species. This result agrees with both mechanisms
;
one can say that interaction between the
platinum moiety and the support is enhanced by increasing the
acidity and so
reducing its mobility on the surface, or that the withdrawing of electrons decreases the volatility of the surface atoms and prevents their escaping from the cristallite. Further studies on the influence of the acidity will allow to confirm that. As a conclusion, we want to point out the very important role played by the presence of chlorine on the catalyst
:
it acts as redispersion agent in oxidizing
atmosphere and as stabilizing agent in reducing atmosphere. We may also emphasize that the big differences found in sintering rates in the litterature are largely connected to nature of supports used, chlorine contents, presence of impurities ( 3 4 ) , partial pressures of the different gases for instance.
ACKNOWLEDGMENT The authors thank Mrs Chenebaux and her coworkers for performing X-ray fluorescence and scanning microprobe determinations.
166 REFERENCE 5
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