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Studies of the Deactivation of Supported Palladium Catalysts by Halogenocarbons
B. Delmon and G.F.Froment (Editors),Catalyst Deactivation 1980 Elsevier Scientific Publishing Company,Amsterdam - Printed in The Netherlands
213
0
STUDIES OF THE DEACTIVATION OF SUPPORTED PALLADIUM CATALYSTS BY HALOGENOCARBONS
D. J. HUCKNALL, B. M. WILLATT and R. J. HOCKHAM J. and S. Sieger Ltd., Poole, Dorset, U.K.
ABSTRACT Halogenocarbons have been observed to inhibit the oxidation of methane over supported palladium catalysts and the nature of the interaction has been investigated by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and transmission electron microscopy.
With alumina-
supported catalysts, XPS and AES studies showed that poisoning can be accounted for in terms of the interaction of the inhibitor and catalytically-active palladium (11) oxide to give metallic palladium.
Adsorption of halogen on the
metal and the deposition of an organic residue on the catalyst surface also occur.
Tin (IV) oxide- and, possibly, titanium (IV) oxide-supported catalysts
behave differently to those based on alumina and a possible explanation is that these oxides act as oxygen reservoirs and, by maintaining palladium in an oxidised state, reduce the effects of the interaction. Although there is little difference between chloro- and bromocarbons in their inhibiting effects, differences are observed when attempts are made to recover the poisoned catalysts.
Finally, during poisoning, the size of the
palladium particles increases and this, under some circumstances, can adversely affect the performance of regenerated catalysts.
INTRODUCTION An important use of supported precious metal catalysts is in the detection and estimation of combustible gases in, for example, coal mines and oil- and gas-production platforms (refs. 1,2).
The most widely used flammable gas
detectors are those based on catalytic combustion of the gas and these have been described elsewhere (refs. 3 , 4 ) .
The sensing element consists
essentially of a coil of platinum wire (acting as both a heater and a platinum resistance thermometer) embedded in a bead of an oxide such as alumina on which the catalyst is deposited by the thermal decomposition of a solution of the appropriate metal salt.
Catalytic combustion of a flammable vapour causes a
rise in temperature of the detecting element.
214
Although catalytic gas detectors perform satisfactorily over lonij periods under laboratory conditions and in most environments, certain locations are particularly hostile to these devices and bring about a dramatic l o s s in sensitivity of the detector.
When such failure occurs, poisoning of the
catalyst appears to be the only reasonably explanation.
In view of the
considerable importance of this phenomenon to the design and manufacture of commercial gas detectors, studies have been made of the effects of certain substances on precious metal catalysts.
Preliminary work was particularly
concerned with the behaviour of gas sensors towards methane oxidation in the presence of various types of inhibitor, and the effects of some halogenated hydrocarbons, hydrogen sulphide and hexamethyldisiloxane on these devices have been described
(
ref. 1).
During these investigations, it was observed that
vapourising liquids containing halogenocarbons, which are used extensively in industry as fire-fighting materials, refrigerants etc., had a particularly marked inhibiting effect on the catalysts. The present studies have been designed to elucidate the nature of the fundamental changes involved in the interaction of supported precious metal catalysts, particularly palladium, with simple halogen-containing hydrocarbons (halogenomethanes and 1,2-dichloroethane).
The influence of the additives on
catalytic activity has been established using a microreactor technique whilst the nature of the interaction has been investigated by means of X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and transmission electron microscopy (TEM).
The significance of the findings is discussed.
EXPERIMENTAL Materials Supported Catalysts The preparation of palladium-containing catalysts supported on aluminium (111) oxide, thorium (IV) oxide, titanium (IV) oxide and tin (IV) oxide has been described elsewhere (ref. 5) . Gases Helium, nitrogen and oxygen (Air Products; ultra-high purity grade) and methane (Air Products; CP grade) were obtained from cylinders.
Helium was
further purified by passage through a Matheson 462 gas purifier containing
13 X molecular sieve, whilst nitrogen was passed through a liquid nitrogencooled trap.
Oxygen and methane were condensed at liquid nitrogen
temperatures and the middle fractions were used.
215 Inhibitors Chloroform (BDH A r i s t a r g r a d e ) , d i c h l o r o m e t h d n e (BDH AnalaR g r a d e ) and dibromornethane and I,2 - d i c h l o r o e t h d n e
( d l 1 BDH LR rqrade) were d e g a s s e d and used
without f u r t h e r p u r i f i c a t i o n .
A p p a r a t u s and P r o c e d u r e S t u d i e s o f t h e e f f e c t s of v a r i o u s h a l o g e n o c a r b o n s on t h e a c t i v i t y o f s u p p o r t e d p a l l a d i u m c a t a l y s t s were c a r r i e d o u t u s i n g a m i c r o r e a c t o r i n g a s - h a n d l i n g system.
conjunction with a U.H.V. c o n s t r u c t e d o f acid-washed
(2 mm i . d . 1
The m i c r o r e a c t o r was
s i l i c a t u b i n g and c o n s i s t e d of an i n n e r t u b e
s u r r o u n d e d by a n o t h e r t u b e ( 6 mm i . d . ) .
2 0 mm) w a s l o c a t e d a t t h e b a s e of t h e i n n e r t u b e .
The c a t a l y s t bed
(length
The r e a c t o r was mounted i n a
v e r t i c a l f u r n a c e , t h e t e m p e r a t u r e of which w a s c o n t r o l l e d a u t o m a t i c a l l y t o within 0.1K.
R e a c t a n t g a s m i x t u r e s , p r e p a r e d i n t h e U.H.V.
system were
i n j e c t e d i n t o a s t r e a m of c a r r i e r g a s ( h e l i u m ) v i a a P e r k i n Elmer 6-way g a s sample v a l v e f i t t e d w i t h a sample l o o p (1 o r 5 c m 3 a t s . t . p . ) pre-evacuated
to
Torr.
which c o u l d b e
G a s reached t h e c a t a l y s t through t h e annular
s p a c e between t h e r e a c t o r t u b e s and t h e c a r r i e r g a s t r a n s f e r r e d t h e r e a c t a n t p u l s e t h r o u g h t h e c a t a l y s t t o t h e a n a l y s i s system.
The m i c r o r e a c t o r c o u l d b e
c o n n e c t e d t o , o r i s o l a t e d from, t h e c a r r i e r g a s stream by means of a second 6-way v a l v e . A n a l y s i s was c a r r i e d o u t u s i n g e i t h e r a s i l i c a g e l column ( 3 m x 4 mm; 60-80mesh) m a i n t a i n e d a t 323K o r a Porapak Q column ( 2 . 1 m x 4 mm;
a t 373K w i t h h e l i u m ( 4 2 p s i g ) a s c a r r i e r g a s .
60-80 mesh)
A n a l y s i s w a s completed u s i n g a
Pye k a t h a r o m e t e r d e t e c t o r and a f l a m e i o n i s a t i o n d e t e c t o r h e a t e d t o 3 2 8 and 37 3K, r e s p e c t i v e l y . The b a k e a b l e , U.H.V.
gas-handling
system w a s c o n s t r u c t e d o f s t a i n l e s s s t e e l
and comprised a working chamber e v a c u a t e d by a n o i l d i f f u s i o n pump (Edwards E 0 4 ; 600 L s-l) backed by a r o t a r y pump (Edwards ED 2 0 0 ) . l i q u i d nitrogen-cooled
A 5L c a p a c i t y , b a f f l e d ,
t r a p (Vacuum G e n e r a t o r s VG CCT 100) e n s u r e d t h a t t h e
working chamber w a s f r e e from o i l . T o r r were r e a c h e d r o u t i n e l y .
With t h i s a p p a r a t u s p r e s s u r e s o f 2 x lo-’
R e a c t a n t p r e s s u r e s w i t h i n t h e r a n g e 1.0 t o
1000.0 T o r r w e r e r e a c h e d r o u t i n e l y MKS B a r a t r o n gauge (Model 221A;
a c c u r a c y , 1%of r e a d i n g ) and w i t h i n t h e r a n g e 0.001 t o 10 T o r r u s i n g a temperature-compensated
B a r a t r o n (MKS model 2 2 0 AHS-3AX-BlO;
a c c u r a c y , 0.15% of
reading). A f t e r a s e r i e s o f e x p e r i m e n t s , o r on changing t h e i n h i b i t o r , t h e g a s h a n d l i n g a p p a r a t u s was c l e a n e d and baked (523K) under vacuum f o r s e v e r a l hours.
216 Phvsicochemical P r o p e r t i e s E l e c t r o n Microscopv T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s were o b t a i n e d u s i n g a J E O L JAMP 100 CX microscope.
Samples were p r e p a r e d by two methods.
I n t h e f i r s t , d r o p s of an
u l t r a s o n i c d i s p e r s i o n of l i g h t l y - c r u s h e d c a t a l y t i c m a t e r i a l i n 2-methyl-propan2 - 0 1 were t r a n s f e r r e d t o a c l e a n , g l a s s microscope s l i d e and t h e d r y m a t e r i a l c o a t e d w i t h a t h i n ( c a . 50 nm) l a y e r of c a r b o n i n an e v a p o r a t i o n u n i t . T h e r e a f t e r , t h e c a r b o n ( c o n t a i n i n g t h e c a t a l y s t ) was f l o a t e d from t h e s l i d e and t r a n s f e r r e d t o a copper g r i d ( 3 mm d i a m ) .
According t o t h e second method, t h e
c a t a l y s t w a s mixed w i t h S p u r r l o w - v i s c o s i t y
r e s i n and t h e c u r e d p r o d u c t
s e c t i o n e d i n an u l t r a m i c r o t o m e ( R e i k e r OMU3) f i t t e d w i t h a diamond k n i f e b e f o r e being t r a n s f e r r e d t o a carbon-coated copper g r i d .
Auger e l e c t r o n - and X-Ray P h o t o e l e c t r o n S p e c t r o s c o p y XPS was performed u s i n g a VG S c i e n t i f i c ESCA 3 Mark I1 s p e c t r o m e t e r f i t t e d w i t h an a r g o n i o n gun f o r e t c h i n g t h e s u r f a c e .
Powdered c a t a l y s t s were p r e s s e d
i n t o indium m e t a l p r i o r t o mounting i n t h e i n s t r u m e n t and t h e s p e c t r a o b t a i n e d u s i n g aluminium Km r a d i a t i o n a t 1486.6 e V . spectrometer w i l l be given w i t h t h e R e s u l t s .
D e t a i l s of t h e c a l i b r a t i o n of t h e Auger s t u d i e s were performed
u s i n g a JEOL JW-10 Scanning Auger E l e c t r o n Microprobe ( r e f s . 5 , 6 ) , which a l l o w s a b s o r b e d e l e c t r o n c u r r e n t s of < ~ o - ~ O At o b e u s e d i n o r d e r t o r e d u c e damage t o t h e samples by t h e e l e c t r o n beam.
RESULTS C a t a l y t i c Oxidation S t u d i e s General From t h e p o i n t o f view o f g a s d e t e c t i o n , i t i s d e s i r a b l e t o e f f e c t r e a d i l y t h e c o m p l e t e combustion of flammable b u t u n r e a c t , i v e compounds s u c h a s methane. I n t h e p r e s e n t work, t h e a c t i v i t i e s w i t h r e s p e c t t o methane o x i d a t i o n ,
Of
a
number o f s u p p o r t e d p a l l a d i u m c a t a l y s t s , i n t h e p r e s e n c e a n d a b s e n c e o f t h e i n h i b i t o r s , were measured.
Decay i n t h e a c t i v i t y o f a c a t a l y s t may a r i s e
b e c a u s e of v a r i o u s p r o c e s s e s ( r e f . 7 ) , and i n o r d e r t o d i f f e r e n t i a t e between d e a c t i v a t i o n by t h e i n h i b i t o r s and t h a t d u e t o o t h e r f a c t o r s , s u c h a s f o u l i n g and a g e i n g , p r e l i m i n a r y e x p e r i m e n t s were performed t o e s t a b l i s h t h e i m p o r t a n c e of t h e l a t t e r d u r i n g methane o x i d a t i o n . Even a t t h e h i g h e s t r e a c t i o n t e m p e r a t u r e s s t u d i e d , t h e t e x t u r a l and m e c h a n i c a l p r o p e r t i e s o f t h e c a t a l y s t s u p p o r t s used i n t h i s work remained u n a f f e c t e d d u r i n g experiments.
I n v e s t i g a t i o n s w e r e a l s o made o f t h e
217 contribution made by the supports during the combustion of methane.
In the
absence of oxycjen and at temperatures up to 873K, the supports (with the exception of tin (IV) oxide) were inert towards methane.
In the presence of
oxygen, a small amount of methane (less than 10%) was converted to total oxidation products at 873K.
Tin (IV) oxide was catalytically very active and
oxidised methane at temperatures as low as 650K. Since the presence of metal seems to be particularly effective for the formation of carbon from carbon-containing gases, pyrolysis of methane was also studied for a number of catalysts.
Typically, in the presence of a pre-
reduced 5% (w/w) Pd on Tho2 catalyst, some reaction occurred at temperatures in excess of 540K and, at 873K, almost 50% of the methane introduced was cracked on the catalyst surface.
Even so, at temperatures typical of the inhibition
experiments (<730K), less than 2% of the methane was converted.
With regard
to the products of the cracking reaction, neither ethane nor ethylene were ever detected and only hydrogen was observed consistently, even at temperatures as low as 6 0 0 ~ . Attempts were made to estimate the amount of pyrolytic carbon deposited on the catalyst surface but, at best, only half of the carbon was recoverable.
This observation has relevance in the area of general catalyst
deactivation and re-generation.
Certainly, under conditions studied in
inhibition experiments, it was difficult to envisage any mode of deactivation other than that involving the halogenocarbons.
Oxidation of Methane in the Presence of Inhibitors Results have been obtained of the influence of some chlorocarbons, particularly dichloro- and trichloromethane (chloroform), and dibromomethane
on the oxidation of methane (methane: oxygen ratio
=
1:2).
Unless otherwise
stated, carbon dioxide and water were the products of reaction of methane. Trace amounts of chlorocarbons are known to have an advantageous effect during catalytic oxidation of certain hydrocarbons, for example, ethylene (ref. 8 ) , since they facilitate the isolation of partial combustion products although decreasing overall catalytic activity.
In the present work, an
initial investigation of the effects of di- and trichloromethane on the combustion of methane at 650K, revealed that small amounts (ca.
moles
pulse) of these compounds also promoted oxidation although larger concentrations were found to poison the catalyst rapidly. for a 2 . 7 % (w/w) Pd on y-A1203 catalyst.
This effect is shown in Fig.1
A comparative survey of the
inhibiting effects of various halogenocarbons(dibromo- and dichloromethane and 1,2-dichloromethane) on a similar catalyst is shown in Fig. 2, whilst Fig. 3 shows the effects of dibromo- and dichloromethane and trichloromethane
on palladium-alumina catalysts of different loadings.
A striking feature of
218
INITIAL ACTIVITY
1 2 3
1 ,
PROMOTION AND/OR INHIBITION
1 1 2
-
3 1 5 6 7 8 9 10 11 12 13 11 15 16 1 7 18 19 2 0 21 Number af Pulses
F i g . 1. E f f e c t of c o n c e n t r a t i o n o f i n h i b i t o r on methane o x i d a t i o n o v e r 2 . 7 8 Pd (w/w) on y - A 1 0 a t 650K16 C a t a l y s t w e i g h t , 0 . 0 4 - 0 . 0 8 g ; i n i t i a l methane moles; c o n c e n t r a t i o n ! 31.8 x 10 molgs; i n i t i a l oxygen c o n c e n t r a t i o n , 3 . 6 x moles CH,Cl,; +,1.63 A , no i n h i b i t o r ; O , 4 . 0 8 x 10- m o l e s C H C 1 3 ; 0 , 2.72 x L L x 10-8 m o l e s C H 2 C 1 2 ; 0 , 1 . 3 6 x 10 7 moles CH2C12.
RECOVERY l h i n f l o w i n g >20h in flowing Heat 650K I Heal 6 5 0 K
01 ' 1 3
1
I
I
I
I I
I
I
I
'
1 3 5 7
9 11 13 15 17 19 21 23 25
Number of Pulses
1 3 5 7
9 11 13 15
-.c
F i g . 2. The i n h i b i t i o n of methane o x i d a t i o n by some h a l o g e n a t e d h y d r o c a r b o n s and t h e r e c o v e r y of 2 . 7 % Pd on y-A1203. T e m p e r a t u r e = 650K; c a t a l y s t w e i g h t = 0 . 0 7 g ; i n i t i a l methane c o n c e n t r a t i o n , 1.8 x m o l e s ; i n i t i a l oxygen c o n c e n t r a t i o n , 3.6 x 10-6 m o l e s ; a , C l C H CH c1; +, CH CI ; 0, C H 2 B r 2 2 2 2 2
219
INITIAL ACTIVITY
:?{ I
go
1
I N H I 8 ITION 1.36xlO”moles inhihilor ndded 10 reactnnls
RECOVERY lhln ZOhlnflowing He flowing I , He !
I ~
~
~
10 0 1
1
1 ,
, I ,
5 1
5
9
13
17
211
Number o f P u l s e s
,
I
5
,
,
1
5
,
,
9
13
Fig. 3. The inhibition of methane oxidation by various halogenated hydrocarbons and the recovery of palladium on alumina catalysts. Temperature = 650K; initial methane concentration, 1.8 x moles; initial oxygen concentration, 3.6 x 10-6 moles; A , C ~ ~ B r ~ , 2Pd5 %on y-A1 0 ; +,CHC13, 25% Pd on y-A1 0 2 3 0 ,CH2C12, 2.7% Pd on y-Al2O3;D,CH2Br2,2.7%2 3Pd on y-A1203
these curves is that the extent of inhibition of the catalysts is almost independent of the nature of the halogenocarbon. By passing helium through the poisoned catalysts at the same temperature as
that at which poisoning occurred, attempts were made in most cases to recover the catalysts.
Depending on the temperature, the activity of, for example, a
2.7% (w/w) Pd on y-A1203 catalyst was gradually regained by this process.
This
is shown in Fig. 4, for inhibition by trichloromethane, and it can be seen that, although the degree of deactivation of the catalyst was almost independent of temperature (55
5% of the initial activity lost after exposure to 13 gas
pulses containing inhibitor), recovery was most marked at higher temperatures. Similar results were observed for dibromomethane (Fig. 51, although it can be seen that, even at temperatures at which chlorine-containing catalysts showed marked recovery, bromine-containing catalysts exhibited a very small degree
of regeneration.
In both poisoning and recovery experiments, it was
interesting to observe that, whereas the shape of the deactivation curves suggested the preferential poisoning of the most active catalytic sites, the shape of the regeneration curves for chlorine-containing catalysts suggested a non-selective reactivation process (Figs. 2,4,6
and 7).
220
F i g . 4. E f f e c t of t e m p e r a t u r e on t h e i n h i b i t i o n by t r i c h l o r o m e t h a n e and r e c o v e r y o f 2.7% Pd on y-A1203. C a t a l y s t w e i g h t , 0.049; i n i t i a l methane c o n c e n t r a t i o n , 1.8 X moles; i n i t i a l oxygen c o n c e n t r a t i o n , 3 . 6 x moles;A,730K;0,700K, t, 650K; 0,625K.
E f f e c t o f t e m p e r a t u r e on t h e i n h i b i t i o n by dibromomethane and r e c o v e r y Fig.. 5. C a t a l y s t w e i g h t , 0.07 - 0.089; i n i t i a l methane o f 2.7% Pd on y-A1203. c o n c e n t r a t i o n , 1.80 x 10-6 moles; i n i t i a l oxygen c o n c e n t r a t i o n ; 3 . 6 x mo1es;A , 730K; 0, 700K; 0 , 650K. The e f f e c t o f t h e c a t a l y s t s u p p o r t on i n h i b i t i o n by t r i c h l o r o m e t h a n e and t h e r e c o v e r y of p a l l a d i u m c a t a l y s t s a t 650K was i n v e s t i g a t e d and t h e r e s u l t s a r e shown i n F i g s . 6 and 7 . on y-A1203 c a t a l y s t .
A l s o shown i n F i g . 7 a r e r e s u l t s for a 2 . 7 % (w/w)
Pt
I t w a s found t h a t t h e a b i l i t y o f t h e c a t a l y s t t o r e c o v e r
221 after the introduction of inhibitor depended on the nature of the support as well as the temperature of prolonged exposure to helium.
For example, at 650K
(at-which temperature y-alumina-supported palladium catalysts were irreversibly
INITIAL ACTIVITY INHIBITION
I
1.36 x 10-7moles C H C l 3 added to reactants
I
I
I I
I
RECOVERY
/lhinflowing !lShinIlowing He He
I
,I
+.+
I I
-*+*
I I I I
+-+<*
1 3 51 3 5 7
-
9 11 13 15 17 19 21 23 25 271 3 5 7 1 3 5 7 N u m b e r of Pulses
Fig. 6. Inhibition by trichloromethane and recovery at 650K of palladium catalysts on different supports. Catalyst weight 0.03 to 0.06 g; initial methane concentration, 1.8 x mols; initial oxygen concentration, 3.6 x mols; +, 2.7% Pd on y-Al2O3;0, 2 . 1 % Pd on Ti02; 0, 2.1% Pd on Tho2.
f!
I N I T I A L ACTIVITY
61: y:!; !
C1.3H C 1 3 added toles
-1
.
D
I*\
60 I
reactants
R E COVE RV
I I
j l h in iflowing 1He a t
s 2 0 h in f l o w i n g He at 650K
,
j650K
p-,
,+**
it
1 5 1 5
9 13 1 1 2 1 25 1 5 Number of Pulses
13
7 11 1 5 1 9 23 27
Fig. 7. Inhibition by trichloromethane and recovery of some palladium catalysts at 6 5 0 K ; catalyst volume, 0.05 cm3; initial methane concentration, 1.8 x 10-6 mols; initial oxygen concentration, 3.6 x mols; 0 , 2.7% Pd on Y-A1203' + t 2 0 % Pd on Sn02;A, 2.7% Pt on y- A1203
222 p o i s o n e d by t r i c h l o r o m e t h a n e
( F i g s . 3 and 4 3 , a 2 . 7 % ( w / w ) Pd on t i t a n i a
c a t a l y s t showed some r e c o v e r y i n a c t i v i t y when h e a t e d i n f l o w i n g helium ( F i g . 6 ) w h i l s t a 2 0 % ( w / w ) Pd on t i n ( I V ) o x i d e c a t a l y s t e x h i b i t e d a l m o s t complete recovery.
XPS STUDIES The c a t a l y s t s s t u d i e d i n t h i s work were e l e c t r i c a l l y i n s u l a t i n g and c a l i b r a t i o n of t h e e l e c t r o n spectrometer w a s necessary before t h e evaluation of t h e b i n d i n g e n e r g i e s of t h e e l e m e n t s of i n t e r e s t .
The u s e of g o l d w i t h non-
c o n d u c t i n g specimens i n o r d e r t o o b t a i n c h a r g i n g c o r r e c t i o n s , and t h u s a b s o l u t e (ref. 9).
b i n d i n g e n e r g i e s , h a s been recommended
I n t h e p r e s e n t work, t h e u s e
o f c e r t a i n i n t e r n a l s t a n d a r d s a s a method o f measuring b i n d i n g e n e r g i e s i n c a t a l y s t s h a s been examined and t h e f o l l o w i n g c o n c l u s i o n s were r e a c h e d : (1)
The C
1S L
literature
'
l i n e c a n n o t be u s e d a s a r e f e r e n c e , as recommended i n t h e ( r e f . lo), s i n c e n o t o n l y d o e s t h i s l i n e a r i s e from s u b s t a n c e s
of unknown c o m p o s i t i o n , b u t a l s o t h e d e p o s i t s which g i v e r i s e t o it a r e i n e l e c t r i c a l e q u i l i b r i u m w i t h t h e specimen.
F u r t h e r , i n t h e p r e s e n t work, complex
l i n e s were o b t a i n e d f o r t h i s e l e m e n t which, a f t e r d e c o n v o l u t i o n , a p p e a r e d t o b e c h a r a c t e r i s t i c ( i n some c a s e s ) of t h e added i n h i b i t o r . U n l e s s t h e c o v e r i n g w a s c o n t i n u o u s , t h e u s e o f s p u t t e r e d g o l d and
(2)
measurement o f t h e c h a r a c t e r i s t i c 4 f 7 and 4 f 5 l i n e s as i n t e r n a l s t a n d a r d s ( r e f . 9 )
-
was unsatisfactory, since
-
2 e l e c t r i c a l e q u i l i b r i u m was a g a i n
2
e s t a b l i s h e d w i t h t h e specimen.
C e r t a i n l y , c h a r g e s h i f t s i n t h e 4f b i n d i n g
e n e r g i e s o f up t o 3eV from t h o s e p u b l i s h e d i n t h e l i t e r a t u r e , were o b s e r v e d w i t h t h i s element. (3)
The OIs and t h e I n
measures
'
(from t h e sample mounting) l i n e s were t h e b e s t 3d5 of sample - c h a r g i n g . 2
I n t h i s work, a l l b i n d i n g e n e r g i e s were measured r e l a t i v e t o t h e 0
line
IS%
a t 532.0 eV.
The p h o t o e l e c t r o n s p e c t r a o f s o m e p u r e p a l l a d i u m compounds and b o t h p o i s o n e d and a c t i v e c a t a l y s t s were o b t a i n e d u s i n g aluminium K= r a d i a t i o n . e n e r g i e s measured f o r t h e C
and Pd3d
The b i n d i n g
l i n e s a r e shown i n T a b l e 1 and 2 . The
1S L 5 3 peak w a s p r e s e n t i n a l l samples -'- even t h o s e n o t d e l i b e r a t e l y exposed t o 1% 2 2 c a d o n - c o n t a i n i n g compounds, a n d p r o b a b l y r e p r e s e n t e d c o n t a m i n a t i o n a c q u i r e d i n
C
t h e a m b i e n t atmosphere i n p a l l a d i u m metal w a s v e r y c l o s e t o t h e
The b i n d i n g e n e r g y f o r t h e Pd l i t e r a t u r e v a l u e s (re-fs. l l a , b ) .
3d5
-
From T a b l e 1, it i s a p p a r e n t t h a t t h e
b i n d i n g e n e r g i e s i n p a l l a d i u m m e t a l were a p p r o x i m a t e l y 3eV pd3d5 and Pd3d 3 - lower t h a n t h o s e found f o r p u r e and s u p p o r t e d p a l l a d i u m ( I I j compounds. 2 2 I t a p p e a r e d , t h e r e f o r e , t h a t t h e a c t i v e form of t h e c a t a l y s t c o n s i s t e d , n o t o f
223 TABLE 1
E l e c t r o n Binding E n e r g i e s i n Some P a l l a d i u m C a t a l y s t s and Compounds
Binding Energy ( e V ) Catalyst/Compound
+
Pd metal'')
Pd3d5
Pd3d3
-
-
2
2
2 2
284.0
335.3
284.5
335.1
Pd (11) o x i d e ( 2 1
285.5
338.2
Pd (11) o x i d e (3)
285.2
337.2
342.6
287.0
338.7
344.2
Pd m e t a l
(2)
18.7% P d ( I 1 ) O
+
Ti02
0.1
340.7
0.1
0.05
340.5
0.05
343.6
2 . 7 % PdC12/Y-A1203
( 4 ) 286.0
337.3
342.5
2 . 7 % PdC12/Ti0
( 4 ) 286.8
338.5
343.8
6.0% Pd/Y-A1203
( 5 ) 285.4
337.9
2.7% Pd/Y-A1203
(6) 284.5
336.2
2.7% Pd/y-Al2O3
( 7 ) 283.8
)
342.9
2
0.5
341.3
335.4
340.7
335.4
341.4
337.4
342.5
337.2
342.4
2
0.5
285.3 2.7% Pd/Y-A1203
( 8 ) 283.7
2 . 7 % Pd/Y-A1203
(')
285.6 283.1 286.0 2.7% Pd/Y-A1203
(lo)283.8
(Sh)
99.99% Pd; 6 . 0 nm o f s u r f a c e removed See r e f . 1 A l f a P r o d u c t s Lot N o . 1116776 (4) Catalysts before conditioning i n H e (5) Conditioned c a t a l y s t ( 6 ) C a t a l y s t p o i s o n e d w i t h 3.0uL CH2C12 a t 675K. Recovery a t t e m p t e d by e x p o s u r e t o H (752K) and O2 (675 K) A c t i v i t y not recovered 2 C a t a l y s t poisoned w i t h 5 . 5 uL ( a t 67310 and 10 L ( a t 773K) CH2C12 C a t a l y s t p o i s o n e d w i t h 1 . 5 pL CH B r a t 675K 2 2 . -7 C a t a l y s t p o i s o n e d w i t h p u l s e s c o n t a l n l n g CH2BR (1.36 x 10 moles/ pulse). Recovery a t t e m p t e d by e x p o s u r e t o a few methane + O2 pulses; c a t a l y s t recovering. (10) C a t a l y s t a c t i v i t y a l m o s t r e c o v e r e d a f t e r p o i s o n i n g by CH C 1
.
2
2
p a l l a d i u m metal, b u t a p a l l a d i u m (11) compound which e a r l i e r work ( r e f . 5) had suggested w a s t h e oxide. A f t e r e x p o s u r e t o halogenocarbons, two marked changes were o b s e r v e d i n t h e s p e c t r a of p o i s o n e d alumina-supported c a t a l y s t s ( T a b l e 1 ) . F i r s t l y , t h e Pd3d b i n d i n g e n e r g i e s were s h i f t e d towards t h o s e t y p i c a l of m e t a l l i c p a l l a d i u m a n d , s e c o n d l y , t h e c a r b o n Is
JI
3
2 2 peak became b r o a d e r and more complex a n d , on de-
c o n v o l u t i o n , t w o p e a k s c o u l d b e r e s o l v e d which may p e r h a p s b e a s c r i b e d t o a
224 carbon-halogen residue in addition to the typical contamination.
In contrast
to these results, poisoned tin (IV) oxide-supported palladium catalysts showed no significant shifts in the positions of either the Pd 3d or C peaks (Table 2) Is% although the height of the CIS peak in contaminated samples appeared much larger ?r than that in catalysts not exposed to halogen-containing compounds. Finally, as can be seen from Table 1, recovery of palladium/alumina catalysts was marked both by a shift in the position of the Pd3d binding energies from -
those typical of Pd(2)towards those found in palladium (11) compounds and by a simplification of the C
IS
appeared that the first
’
signal.
During reactivation of these catalysts, it
step involved reoxidation of the palladium and
removal of the carbon-containing residue occurred somewhat later.
Thus, in
Table 1, a partially recovered catalyst (superscript (9); approx. 10% more activity than fully-inhibited material) had Pd3d binding energies typical of palladium (11) compounds whilst showing a complex CIS
signal.
A catalyst
51.
which had almost fully recovered its activity (superscript(l0)in Table 1) had almost identical Pd3d binding energies and, in addition, asimplerCIs
peak
in which the signal at ca. 286 eV had become attenuated to the extent’ in which it appeared as a shoulder on the peak at ca. 284
eV.
A E S Studies
In the Auger electron spectra of the catalysts, the carbon KLL peak usually occurs at about 270 eV which is very close to an Auger peak of palladium at 275 to 280 eV.
The presence of small amounts of carbon can, therefore, only
be inferred from the shape of the peak and any deviation from the expected ratios in the heights of the three palladium peaks at ca. 243, 279 and 326 eV. TABLE 2 Electron Binding Energies in Some Tin (IV) Oxide-Supported Palladium Catalysts
Binding Energy (eV) Catalyst
2.7 % Pd/Sn02 20.0% Pd/Sn02 (2) (3) 20.0% Pd/SnO‘ L
20.6% Pd/Sn02-A1203 (4)
286.3
337.3
342.6
286.4
337.9
342.9
286.4
338.2
343.4
284.8
338.0
342.7
(1) No inhibitor, heated to 87% (2) No inhibitor, heated to 720K -7 (3) Catalyst poisoned with pulses containing CHC13 1.36 x 10 moles/pulse at 650K; catalytic activity recovered (4) As (3) but catalytic activity not restored
225
During AES studies it was observed that bromine-containing residues were found consistently on the surface of catalysts exposed to that element whilst the occurrence of chlorine was much more localised in the corresponding chlorine contaminated material.
In an attempt to determine which were preferred areas
for chlorine adsorption, palladium-, oxygen- and chlorine-Auger images were obtained of the surface of a palladium/alumina catalyst exposed to trichloromethane, thus Plate 1 shows the secondary electron- and the absorbed electron images of this catalyst whilst the Auger images are shown in Plate 2. palladium-Auger image (Plate 2 (a)
),
From the
it can be seen that this element tends to
be concentrated in the vicinity of the spherical feature shown in the upper right hand corners of Plates 1 (a) and (b). oxygen (Plate 2 (b)
)
The corresponding distribution of
and chlorine (Plate 2 (c) ) indicated that in deactivated
catalysts, chlorine was more closely associated with palladium than was oxygen. Despite the uncertainties introduced by topographic effects in the examination of such specimens, this result supported the XPS findings that palladium oxide was no longer present on the surface of contaminated material.
Plate 1 (a) Secondary electron image (b) Absorbed electron image of a Pd/A1 0 2 3 (Pd > 2 0 % (w/w)) catalyst exposed to CHC13; magnification X200 Plate 2 (a) Pd-Auger image of area shown in Plate 1; (b) 0-Auger image of same area; (c) C1-Auger image of same area; magnification X200 Electron Microscopy Particle size histograms from transmission electronmicrographs of 2.7% (w/w) Pd on y-alumina catalysts after various treatments are shown in Fig. 8. In this diagram, histogram EN represents the particle size distribution of a typical catalyst after preliminary conditioning in flowing helium.
EO represents the
distribution of an equilibrium catalyst after oxidation of methane + oxygen
226 mixtures.
The two s y s t e m s were s i m i l a r , t.he most f r e q u e n t l y o b s e r v e d p a l l a d i u m
The remainder of t h e h i s t o g r a m s
p a r t i c l e d i a m e t e r s l a y i n g between 10 and 1 5 nm.
i n F i g . 8 ( E J , EK, EL, EP and EQ) r e p r e s e n t c a t a l y s t s which had been p o i s o n e d w i t h t r i c h l o r o m e t h a n e a t v a r i o u s t e m p e r a t u r e s a l t h o u g h , f o r c a t a l y s t s EK and E J , c a t a l y t i c a c t i v i t y had been f u l l y (EK) o r p a r t i a l l y (EJ) r e s t o r e d .
CAT € 0
l r
n
1
0 Diameter (nml
50 r
LO
CAT. E L
20
30 h0
50 60 70
Diameter I n m l +
50 I
CAT.
EP
LO
~
0
10
+
10
20 30
LO 50
Oiameler (nmi --c
0
10
20
30
LO
50
Oiameler l n m i
-
80
227 50
CAI EO
0
10
I
0
20
30 10 50 Oiameter i n m )
I
0
--
CAI E l
10
20 30 10
50
Oiomeler (nrn)
-
CAT. EN
10
20 30
LO
50
Oiomeler [ n m l
-
Fig. 8. Influence of exposure to trichloromethane on the particle size distribution of 2.7% Pd on -A1203 catalysts. Cat EN, conditioned at ??3K for 24h then at 723~ for 24h. Cat. EO, conditioned at ??3K for 24h then at 750K for 24h then 5 pulses of CH4 + O2 at 750K. Cat. EL, conditioned at ??3K for 16h; poisoned with CHC13 + CH4 + O2 at 580 - 730K. Cat. EP, conditioned at 773K for 16 h then pulses of CH4 + 0 at 750 and 625K; poisoned with CHC13 + O2 at 625K. Cat. EQ, conditioned at ??3K for 16h; poisoned with CHCl + CH4 + O2 at 650K. Cat EJ, conditioned at 750K for 60 h; poisoned with CHC?3 + CH4 + O2 at 700K. Cat EK poisoned with CHC13 + C H ~ + o2 at 730K. Particle counts 400 to 1000 over 5 to 14 (pm)*.
228
In all cases, exposure to the chlorocarbon had produced a gradual shift in the
distribution to particles of a larger diameter.
Particularly marked was the
growth of particles having diameters in the range 20 to 25 nm at the expense of those with diameters from 10 to 15 nm.
Eventually,after the introduction of very
large amounts of inhibitor, massive agglomeration of the catalyst occurred. This is shown in Plates 3 and 4 for a 2.7% (w/w) Pd on y-A1203 catalyst which had been exposed to approximately ten times the amount of dichloromethane normally used in poisoning experiments.
H
h
0.lV
0.1y
Plate 3 Transmission electron micrographs of 2.7% Pd on y - A 1 O3 after exposure to 5 U L dichloromethane at 673K, 10 pL dichloromethane at Plate 4 773K and 1 pL at 973K
DISCUSSION
Studies of the interaction of halogenocarbons with palladium catalysts have been reported previously (refs. 12, 13, 14) although the effects of these compounds is still not clear.
For example, Cullis et a1 (ref. 13) compared the
effect of various halogenocarbons on methane oxidation over palladium catalysts and found that, whilst the inhibiting efficiency of the four chloromethanes depended on both the number of chlorine atoms in the molecule and their reactivity, the effects of the different compounds were not very different at lower concentrations.
Comparison of the inhibiting effect of dichloromethane
with other halogenomethanes showed that this compound had approximately the same efficiency as difluorodichloromethane but was a much less effective poison than either dibromo- or diiodomethane (ref. 13).
A very surprising result was
obtained by Otto and Montreuil (ref. 14) who found that the oxidation of hydrocarbons and carbon monoxide over palladium and palladium catalysts at 773K was unaffected by 1,2-dichloroethane whilst 1,2-dibromomethane acted as an inhibitor under the same conditions.
In the present investigation, it was
found, in contrast to the results of Cullis and co-workers (ref. 13) and
Otto and Montreuil (ref. 14), that there was little difference in the inhibiting efficiencies of a number of halogen-containing compounds, including dibromo- and dichloromethane, 1,2-dichloroethane and trichloromethane. Differences in the behaviour of chlorine- and bromine-poisoned catalysts were observed, however, when attempts were made to reactivate these systems. The following discussion will concentrate mainly on alumina-supported catalysts since material containing tin ( I V ) oxide and, possibly, titanium (IV) oxide, behaves differently
.
Catalysts containing these oxides will be briefly but
specifically mentioned at the end of the paper. Both the effect of halogen-containing compounds on supported palladium catalysts and the nature of the interaction appear to be complex. -8
amounts (ca. 10
Thus, small
moles) of dichloro- and trichloromethane enhanced the combustion
of methane on a 2.7% (w/w) Pd on y-Al2O3 catalyst at 650K,whilst larger quantities rapidly deactivated the catalyst.
Although experimental work to test the
promoting effect of other halogenocarbons was not performed, it seems likely that similar results would have been obtained.
It is well known that the selectivity
of a silver catalyst in the oxidation of ethylene to oxiran (ref. 9 ) is improved by the additions of small amounts of halogenated compounds to the gas-phase and. Rovida, Pratesi and Ferroni (ref. 15) examined the interaction of 1,2-dichloroethane and oxygen with a silver surface.
These authors concluded that the presence of
small amounts of chlorine on the metal surface affected significantly the strength of the oxygen-silver bond at the surface and lowered appreciably the temperature at which adsorbed oxygen became mobile (ref. 15).
Since the active state of
palladium oxidation catalystsinvolves oxygen adsorbed on the metal (probably palladium (11) oxide) (refs. 5 , 1 3 ) , which participates in hydrocarbon oxidation, the promoting effect of chloromethanes observed in this study was probably also due to the weakening of the palladium bond (ref. 15). It would appear that the influence of halogenocarbons on the interaction of palladium with oxygen was also responsible for catalyst poisoning by these compounds although evidence has also been obtained that carbon deposition played an important role.
For example, XPS measurements revealed that there was a change
in the oxidation state of palladium in active catalysts and those inhibited by halogenocarbons.
Thus the active phase of supported palladium catalysts appeared
to consist of palladium (11) oxide whilst palladium metal was present in the poisoned material (Table 1).
In the case of trichloromethane-poisoned catalysts,
these observations were confirmed by AES (Plates 1 and 2 ) which showed clearly that chlorine was more closely associated with palladium than was oxygen.
XPS studies
also showed that the interaction of halogenocarbons with the catalyst surface produced a carbon-containing residue in addition to adsorbed halogen.
For example,
the ubiquitous carbon peak detected on the catalysts became uncharacteristically
230
complex a f t e r e x p o s u r e t o i n h i b i t o r .
I n d e e d , i n t r o d u c t i o n of a l m o s t t w e n t y
t i m e s t h a t amount of d i c h l o r o m e t h a n e r e q u i r e d f o r p o i s o n i n q t o a 2 . 7 8 y-A1
0.
2 3
(w/w)
Pd on
c a t a l y s t , p r e v i o u s l y s a t u r a t e d w i t h oxyqt.n, a t 6 7 5 K showed t h a t , a l t h o u g h
o x i d e s o f c a r b o n w e r e p r o d u c e d , a s i g n i f i c a n t amount of c a r b o n was d e p o s i t e d on t h e catalyst.
Subsequent e x p o s u r e t o g a s e o u s oxygen a t 6 7 5 K produced c a r b o n d i o x i d e
from t h i s l a y e r .
Some e v i d e n c e was also o b t a i n e d which s u g g e s t e d t h a t t h e n a t u r e
of t h i s c a r b o n l a y e r was d i f f e r e n t depending on whether i t was d e p o s i t e d from a c h l o r o - o r a bromo-compound.
For example, a f t e r e x p o s u r e of c a t a l y s t s t o
dibromomethane, AES c o n s i s t e n t l y showed t h e p r e s e n c e of b o t h c a r b o n and bromine on t h e surface.
A f t e r e x p o s u r e t o chloro-compounds,
however, c a r b o n was a l w a y s d e t e c t e d
b u t a c a r e f u l s e a r c h had t o b e made f o r a r e a s o f t h e s u r f a c e c o n t a i n i n g b o t h c a r b o n and c h l o r i n e . The s h a p e of t h e o x i d a t i o n a c t i v i t y c u r v e s i n t h e p r e s e n c e o f i n h i b i t o r s s u g g e s t s t h a t t h e r e were two t y p e s of s i t e p r e s e n t on s u p p o r t e d p a l l a d i u m c a t a l y s t s , o f which t h e most a c t i v e was p r e f e r e n t i a l l y p o i s o n e d .
I n t h i s c o n n e c t i o n , C u l l i s e t a1
( r e f . 1 3 ) have s u g g e s t e d t h a t methane c a n b e a d s o r b e d on p a l l a d i u m i n two ways; t h e f i r s t i n v o l v e s "bridge-bonded'' linearly-bonded,
methylene r a d i c a l s , w h i l e t h e second i n v o l v e s
presumably m e t h y l , r a d i c a l s and t h a t o x i d a t i o n o c c u r r e d m a i n l y
d u e t o t h e r e a c t i o n of s u r f a c e l a t t i c e o x i d e w i t h t h e l e s s s t a b l e l i n e a r l y - b o n d e d radicals.
F u r t h e r , C u l l i s e t a1 ( r e f . 1 3 ) a l s o s u g g e s t e d t h a t d e p o s i t i o n o f
c a r b o n o n t h e s u r f a c e s e l e c t i v e l y b l o c k e d t h o s e s i t e s f a v o u r i n g bridge-bonding. The p r e s e n c e o f s i m i l a r s i t e s i n t h e p r e s e n t s y s t e m s would a c c o u n t f o r many o f o u r observations.
F o r example, i f t h e h a l o g e n i s a s s o c i a t e d w i t h o x y g e n - a d s o r p t i o n
s i t e s and t h e o r g a n i c r e s i d u e w i t h s i t e s which a d s o r b methane and t h e former a r e more a c t i v e t h a n t h e l a t t e r , t h e n t h e s h a p e of t h e d e a c t i v a t i o n c u r v e i s e x p l a i n e d . A l s o , t h e e x i s t e n c e of two t y p e s o f s i t e would e x p l a i n t h e two s t e p s which were
a p p a r e n t l y necessary f o r c a t a l y s t regeneration. The p r e s e n t work h a s shown t h a t , under some c i r c u m s t a n c e s , a c t i v i t y c a n b e r e s t o r e d t o p o i s o n e d c a t a l y s t s by e x p o s u r e t o f l o w i n g h e l i u m and p u l s e s o f methane
+
oxygen m i x t u r e s ( F i g s . 2 and 4 ) .
I t w a s i n t h e a b i l i t y of c a t a l y s t s t o r e c o v e r
from t h e i r e f f e c t s t h a t d i f f e r e n c e s i n t h e b e h a v i o u r o f v a r i o u s i n h i b i t o r s w a s observed.
Thus, a f t e r p o i s o n i n g w i t h dibromomethane, a c t i v i t y w a s n o t r e s t o r e d
t o a 2.7% (w/w) Pd on y-A1 0 c a t a l y s t by f l o w i n g helium and methane + oxygen 2 3 The same c a t a l y s t p o i s o n e d by t r i c h l o r o m e t h a n e began t o r e c o v e r
pulses a t 650K.
a c t i v i t y s l o w l y i n f l o w i n g h e l i u m a t 700K w h i l e , a t 7 3 0 K , t h e r a t e t o r e c o v e r y w a s much more r a p i d ( F i g . 4 ) .
The same c a t a l y s t , p o i s o n e d w i t h dibromomethane, showed
no r e c o v e r y a t 700K and v e r y l i t t l e r e c o v e r y a t 7 3 0 K ( F i g . 5 ) .
XPS s t u d i e s o f
r e c o v e r i n g and r e c o v e r e d c a t a l y s t s s u g g e s t e d t h a t removal o f c a r b o n a c e o u s d e p o s i t s
as w e l l a s r e o x i d a t i o n o f p a l l a d i u m , w a s n e c e s s a r y f o r r e a c t i v a t i o n .
It is
t e n t a t i v e l y s u g g e s t e d , t h e r e f o r e , t h a t t h e o v e r l a y e r formed by t h e i n t e r a c t i o n o f
231 dibromomethane with the catalysts, which app'eared to be different to that formed by chlorocarbons, was much more difficult to remove and that this accounted for the observed differences in catalyst recovery. required
111
Further work is
order to define the nature of such carbon deposits. One further questicr.
is raised by the results of catalyst recovery studies.
Bearing in mind the XPS
evidence, it is difficult to explain the apparently non-selective regeneration of chlorine-contaminated catalysts. Finally, the implications of the effects of halogenocarbons on the particle size distribution of palladium catalysts must be discussed. In an earlier paper (ref. i), it was suggested that the formation and subsequent reaction of palladium (11) oxide played an important role in the oxidation of methane in the presence of palladium. It had been shown (ref. 5) that the reversible decomposition of this oxide could explain many experimental observations.
Thus, with well-dispersed, supported,
catalysts, the amount of oxygen evolved during the high temperature decomposition of the oxide was identical to that adsorbed at lower temperatures during its formation from palladium salts.
With bulk palladium (11) oxide, however, the
amount of oxygen evolved during decomposition was greater than that readsorbed when the sample was cooled.
Although reoxidation is slow (ref. 5), with well-
dispersed catalysts, the palladium crystallites are small enough for a sufficient number of oxidised sites to be available to maintain efficient oxidation. With poorly-distributed catalysts of large metal particle size, the surface is only partially covered with oxygen, either because reoxidation is even more difficult or because surface reoxidation is competing with oxygen diffusion into the bulk of the particles.
Whatever the reason, oxidation activity is reduced unless a large excess
of oxygen is present in the ambient atmosphere.
It is likely, therefore that since
exposure to halogenocarbons causes an accelerated rate of growth of palladium particles in supported catalysts, either prolonged exposure to or exposure to large amounts of halogenocarbons will adversely affect the performance of these catalysts. Finally, as has been said, tin (IV) oxide - and, possibly titanium (IV) oxidesupported palladium catalysts appear to behave differently to those supported on alumina or thoria.
For example, not only was the palladium in Pd on Sn02
catalyst unaffected by exposure to halogenocarbons (Table 2) but this catalyst and Pd on TiOZ recovered their activity more quickly after exposure to poisons than other catalysts under comparable conditions (Figs. 6 and 7 ) .
A possible
explanation may be that these oxides act as oxygen reservoirs and maintain the palladium in an oxidised state and so reduce the effects of the interaction with inhibitor.
Certainly, this explanation was proposed to account for the beneficial
effects of tin additions to platinum/zinc aluminate dehydrogenation (ref. 16).
232 REFERENCES 1
2
3
4 5
6
7 8 9
lo
11
12 13 14 15 16
Hucknall, D.J.,Willatt, B.M. and Hockham, R.J., C.E.C. Symposium on New Technologies for Exploration and Exploitation of Oil and Gas Resources V01.1, Graham and Trotman Ltd., London 1979, pp.345-359. Firth J.G., Jones A. and Jones, T.A., Combust. ?lame,% (1973) 303-311 Denney, E.J., C.E.C. Symposium on New Technologies for Exploration and Exploitation of Oil and Gas Resources, Vol.1, Graham and Trotman Ltd., London 1979, pp.336-344. Firth, J.G., Trans. Faraday SOC., 62, (1966) 2566-2576 Hucknall, D.J., Willatt, B.M., and Benbow, A.W., Paper, Ref. 100, presented at the 6th Ibero-American Symposium on Catalysis, Rio de Janeiro 6th to 11th August 1978; Proceedings in the press. (a) Mogami, A,, Hotta, H., Hashimoto, H., Le Gressus, C. and Massignon, D. in Proc. 8th Internat. Congr. on Electron Spectroscopy, Canberra, Vol. 1 (1974), p.60; (b) Le Gressus, C., Massignon, D. and Sopizet, R., Surf. Sci., 68, (1977), 338-345. Butt, J.B. Adv. Chem. Series, 109 (Chem. React. Eng., 1st Symp 1st) (19721, 259-496. Hucknall, D.J., Selective Oxidation of Hydrocarbons, Academic Press, London 1974, p.8. Hnatowick, D.J. Hudis, J., Perlrnan, M.L. and Ragaini, R.C., J. Appl. Phys., (1971) 4883-4886. Siegbahn, K., Nordling, C., Fahlman, A., Nordberg, R., Hamrin, K., Hedman, L., Johansson, G., Bergmark, T., Karlsson, S.E., Lindgren, I. and Linbert, B., ESCA, Atomic, Molecular and Solid-state Structure by Means of Electron Spectroscopy, Almqvist and Wiksells, Uppsala 1967, pp. 174-175. (a) Johansson, G., Hedman, J., Berndtsson, A., Klasson, M. and Nilson, R.J, Electron Spectr., 2. (1973), 295 - 317 (b) Allen, G.C., Tucker, P.M. and Wild, R.K., JCS Faraday I1 (1978), 1226-1248. Anderson, J.R. and McConkey, B.H., J. Catal., 11 (1968) 54-70 Cullis, C.F., Keene, D.E. and T r i m D.L., J. Catal., 19, (1970) 378-385. Otto, K. and Montreuil, C.N., Environ. Sci. Technol., lo, (1976) 154-158. Rovida, G., Pratesi, F. and Ferroni, E., J. Catal., 41, (1976) 140-147 Gray, P.R. and Farha, F.E., Moessbauer Eff. Methodol., lo (1976) 47
213
0
STUDIES OF THE DEACTIVATION OF SUPPORTED PALLADIUM CATALYSTS BY HALOGENOCARBONS
D. J. HUCKNALL, B. M. WILLATT and R. J. HOCKHAM J. and S. Sieger Ltd., Poole, Dorset, U.K.
ABSTRACT Halogenocarbons have been observed to inhibit the oxidation of methane over supported palladium catalysts and the nature of the interaction has been investigated by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and transmission electron microscopy.
With alumina-
supported catalysts, XPS and AES studies showed that poisoning can be accounted for in terms of the interaction of the inhibitor and catalytically-active palladium (11) oxide to give metallic palladium.
Adsorption of halogen on the
metal and the deposition of an organic residue on the catalyst surface also occur.
Tin (IV) oxide- and, possibly, titanium (IV) oxide-supported catalysts
behave differently to those based on alumina and a possible explanation is that these oxides act as oxygen reservoirs and, by maintaining palladium in an oxidised state, reduce the effects of the interaction. Although there is little difference between chloro- and bromocarbons in their inhibiting effects, differences are observed when attempts are made to recover the poisoned catalysts.
Finally, during poisoning, the size of the
palladium particles increases and this, under some circumstances, can adversely affect the performance of regenerated catalysts.
INTRODUCTION An important use of supported precious metal catalysts is in the detection and estimation of combustible gases in, for example, coal mines and oil- and gas-production platforms (refs. 1,2).
The most widely used flammable gas
detectors are those based on catalytic combustion of the gas and these have been described elsewhere (refs. 3 , 4 ) .
The sensing element consists
essentially of a coil of platinum wire (acting as both a heater and a platinum resistance thermometer) embedded in a bead of an oxide such as alumina on which the catalyst is deposited by the thermal decomposition of a solution of the appropriate metal salt.
Catalytic combustion of a flammable vapour causes a
rise in temperature of the detecting element.
214
Although catalytic gas detectors perform satisfactorily over lonij periods under laboratory conditions and in most environments, certain locations are particularly hostile to these devices and bring about a dramatic l o s s in sensitivity of the detector.
When such failure occurs, poisoning of the
catalyst appears to be the only reasonably explanation.
In view of the
considerable importance of this phenomenon to the design and manufacture of commercial gas detectors, studies have been made of the effects of certain substances on precious metal catalysts.
Preliminary work was particularly
concerned with the behaviour of gas sensors towards methane oxidation in the presence of various types of inhibitor, and the effects of some halogenated hydrocarbons, hydrogen sulphide and hexamethyldisiloxane on these devices have been described
(
ref. 1).
During these investigations, it was observed that
vapourising liquids containing halogenocarbons, which are used extensively in industry as fire-fighting materials, refrigerants etc., had a particularly marked inhibiting effect on the catalysts. The present studies have been designed to elucidate the nature of the fundamental changes involved in the interaction of supported precious metal catalysts, particularly palladium, with simple halogen-containing hydrocarbons (halogenomethanes and 1,2-dichloroethane).
The influence of the additives on
catalytic activity has been established using a microreactor technique whilst the nature of the interaction has been investigated by means of X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and transmission electron microscopy (TEM).
The significance of the findings is discussed.
EXPERIMENTAL Materials Supported Catalysts The preparation of palladium-containing catalysts supported on aluminium (111) oxide, thorium (IV) oxide, titanium (IV) oxide and tin (IV) oxide has been described elsewhere (ref. 5) . Gases Helium, nitrogen and oxygen (Air Products; ultra-high purity grade) and methane (Air Products; CP grade) were obtained from cylinders.
Helium was
further purified by passage through a Matheson 462 gas purifier containing
13 X molecular sieve, whilst nitrogen was passed through a liquid nitrogencooled trap.
Oxygen and methane were condensed at liquid nitrogen
temperatures and the middle fractions were used.
215 Inhibitors Chloroform (BDH A r i s t a r g r a d e ) , d i c h l o r o m e t h d n e (BDH AnalaR g r a d e ) and dibromornethane and I,2 - d i c h l o r o e t h d n e
( d l 1 BDH LR rqrade) were d e g a s s e d and used
without f u r t h e r p u r i f i c a t i o n .
A p p a r a t u s and P r o c e d u r e S t u d i e s o f t h e e f f e c t s of v a r i o u s h a l o g e n o c a r b o n s on t h e a c t i v i t y o f s u p p o r t e d p a l l a d i u m c a t a l y s t s were c a r r i e d o u t u s i n g a m i c r o r e a c t o r i n g a s - h a n d l i n g system.
conjunction with a U.H.V. c o n s t r u c t e d o f acid-washed
(2 mm i . d . 1
The m i c r o r e a c t o r was
s i l i c a t u b i n g and c o n s i s t e d of an i n n e r t u b e
s u r r o u n d e d by a n o t h e r t u b e ( 6 mm i . d . ) .
2 0 mm) w a s l o c a t e d a t t h e b a s e of t h e i n n e r t u b e .
The c a t a l y s t bed
(length
The r e a c t o r was mounted i n a
v e r t i c a l f u r n a c e , t h e t e m p e r a t u r e of which w a s c o n t r o l l e d a u t o m a t i c a l l y t o within 0.1K.
R e a c t a n t g a s m i x t u r e s , p r e p a r e d i n t h e U.H.V.
system were
i n j e c t e d i n t o a s t r e a m of c a r r i e r g a s ( h e l i u m ) v i a a P e r k i n Elmer 6-way g a s sample v a l v e f i t t e d w i t h a sample l o o p (1 o r 5 c m 3 a t s . t . p . ) pre-evacuated
to
Torr.
which c o u l d b e
G a s reached t h e c a t a l y s t through t h e annular
s p a c e between t h e r e a c t o r t u b e s and t h e c a r r i e r g a s t r a n s f e r r e d t h e r e a c t a n t p u l s e t h r o u g h t h e c a t a l y s t t o t h e a n a l y s i s system.
The m i c r o r e a c t o r c o u l d b e
c o n n e c t e d t o , o r i s o l a t e d from, t h e c a r r i e r g a s stream by means of a second 6-way v a l v e . A n a l y s i s was c a r r i e d o u t u s i n g e i t h e r a s i l i c a g e l column ( 3 m x 4 mm; 60-80mesh) m a i n t a i n e d a t 323K o r a Porapak Q column ( 2 . 1 m x 4 mm;
a t 373K w i t h h e l i u m ( 4 2 p s i g ) a s c a r r i e r g a s .
60-80 mesh)
A n a l y s i s w a s completed u s i n g a
Pye k a t h a r o m e t e r d e t e c t o r and a f l a m e i o n i s a t i o n d e t e c t o r h e a t e d t o 3 2 8 and 37 3K, r e s p e c t i v e l y . The b a k e a b l e , U.H.V.
gas-handling
system w a s c o n s t r u c t e d o f s t a i n l e s s s t e e l
and comprised a working chamber e v a c u a t e d by a n o i l d i f f u s i o n pump (Edwards E 0 4 ; 600 L s-l) backed by a r o t a r y pump (Edwards ED 2 0 0 ) . l i q u i d nitrogen-cooled
A 5L c a p a c i t y , b a f f l e d ,
t r a p (Vacuum G e n e r a t o r s VG CCT 100) e n s u r e d t h a t t h e
working chamber w a s f r e e from o i l . T o r r were r e a c h e d r o u t i n e l y .
With t h i s a p p a r a t u s p r e s s u r e s o f 2 x lo-’
R e a c t a n t p r e s s u r e s w i t h i n t h e r a n g e 1.0 t o
1000.0 T o r r w e r e r e a c h e d r o u t i n e l y MKS B a r a t r o n gauge (Model 221A;
a c c u r a c y , 1%of r e a d i n g ) and w i t h i n t h e r a n g e 0.001 t o 10 T o r r u s i n g a temperature-compensated
B a r a t r o n (MKS model 2 2 0 AHS-3AX-BlO;
a c c u r a c y , 0.15% of
reading). A f t e r a s e r i e s o f e x p e r i m e n t s , o r on changing t h e i n h i b i t o r , t h e g a s h a n d l i n g a p p a r a t u s was c l e a n e d and baked (523K) under vacuum f o r s e v e r a l hours.
216 Phvsicochemical P r o p e r t i e s E l e c t r o n Microscopv T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s were o b t a i n e d u s i n g a J E O L JAMP 100 CX microscope.
Samples were p r e p a r e d by two methods.
I n t h e f i r s t , d r o p s of an
u l t r a s o n i c d i s p e r s i o n of l i g h t l y - c r u s h e d c a t a l y t i c m a t e r i a l i n 2-methyl-propan2 - 0 1 were t r a n s f e r r e d t o a c l e a n , g l a s s microscope s l i d e and t h e d r y m a t e r i a l c o a t e d w i t h a t h i n ( c a . 50 nm) l a y e r of c a r b o n i n an e v a p o r a t i o n u n i t . T h e r e a f t e r , t h e c a r b o n ( c o n t a i n i n g t h e c a t a l y s t ) was f l o a t e d from t h e s l i d e and t r a n s f e r r e d t o a copper g r i d ( 3 mm d i a m ) .
According t o t h e second method, t h e
c a t a l y s t w a s mixed w i t h S p u r r l o w - v i s c o s i t y
r e s i n and t h e c u r e d p r o d u c t
s e c t i o n e d i n an u l t r a m i c r o t o m e ( R e i k e r OMU3) f i t t e d w i t h a diamond k n i f e b e f o r e being t r a n s f e r r e d t o a carbon-coated copper g r i d .
Auger e l e c t r o n - and X-Ray P h o t o e l e c t r o n S p e c t r o s c o p y XPS was performed u s i n g a VG S c i e n t i f i c ESCA 3 Mark I1 s p e c t r o m e t e r f i t t e d w i t h an a r g o n i o n gun f o r e t c h i n g t h e s u r f a c e .
Powdered c a t a l y s t s were p r e s s e d
i n t o indium m e t a l p r i o r t o mounting i n t h e i n s t r u m e n t and t h e s p e c t r a o b t a i n e d u s i n g aluminium Km r a d i a t i o n a t 1486.6 e V . spectrometer w i l l be given w i t h t h e R e s u l t s .
D e t a i l s of t h e c a l i b r a t i o n of t h e Auger s t u d i e s were performed
u s i n g a JEOL JW-10 Scanning Auger E l e c t r o n Microprobe ( r e f s . 5 , 6 ) , which a l l o w s a b s o r b e d e l e c t r o n c u r r e n t s of < ~ o - ~ O At o b e u s e d i n o r d e r t o r e d u c e damage t o t h e samples by t h e e l e c t r o n beam.
RESULTS C a t a l y t i c Oxidation S t u d i e s General From t h e p o i n t o f view o f g a s d e t e c t i o n , i t i s d e s i r a b l e t o e f f e c t r e a d i l y t h e c o m p l e t e combustion of flammable b u t u n r e a c t , i v e compounds s u c h a s methane. I n t h e p r e s e n t work, t h e a c t i v i t i e s w i t h r e s p e c t t o methane o x i d a t i o n ,
Of
a
number o f s u p p o r t e d p a l l a d i u m c a t a l y s t s , i n t h e p r e s e n c e a n d a b s e n c e o f t h e i n h i b i t o r s , were measured.
Decay i n t h e a c t i v i t y o f a c a t a l y s t may a r i s e
b e c a u s e of v a r i o u s p r o c e s s e s ( r e f . 7 ) , and i n o r d e r t o d i f f e r e n t i a t e between d e a c t i v a t i o n by t h e i n h i b i t o r s and t h a t d u e t o o t h e r f a c t o r s , s u c h a s f o u l i n g and a g e i n g , p r e l i m i n a r y e x p e r i m e n t s were performed t o e s t a b l i s h t h e i m p o r t a n c e of t h e l a t t e r d u r i n g methane o x i d a t i o n . Even a t t h e h i g h e s t r e a c t i o n t e m p e r a t u r e s s t u d i e d , t h e t e x t u r a l and m e c h a n i c a l p r o p e r t i e s o f t h e c a t a l y s t s u p p o r t s used i n t h i s work remained u n a f f e c t e d d u r i n g experiments.
I n v e s t i g a t i o n s w e r e a l s o made o f t h e
217 contribution made by the supports during the combustion of methane.
In the
absence of oxycjen and at temperatures up to 873K, the supports (with the exception of tin (IV) oxide) were inert towards methane.
In the presence of
oxygen, a small amount of methane (less than 10%) was converted to total oxidation products at 873K.
Tin (IV) oxide was catalytically very active and
oxidised methane at temperatures as low as 650K. Since the presence of metal seems to be particularly effective for the formation of carbon from carbon-containing gases, pyrolysis of methane was also studied for a number of catalysts.
Typically, in the presence of a pre-
reduced 5% (w/w) Pd on Tho2 catalyst, some reaction occurred at temperatures in excess of 540K and, at 873K, almost 50% of the methane introduced was cracked on the catalyst surface.
Even so, at temperatures typical of the inhibition
experiments (<730K), less than 2% of the methane was converted.
With regard
to the products of the cracking reaction, neither ethane nor ethylene were ever detected and only hydrogen was observed consistently, even at temperatures as low as 6 0 0 ~ . Attempts were made to estimate the amount of pyrolytic carbon deposited on the catalyst surface but, at best, only half of the carbon was recoverable.
This observation has relevance in the area of general catalyst
deactivation and re-generation.
Certainly, under conditions studied in
inhibition experiments, it was difficult to envisage any mode of deactivation other than that involving the halogenocarbons.
Oxidation of Methane in the Presence of Inhibitors Results have been obtained of the influence of some chlorocarbons, particularly dichloro- and trichloromethane (chloroform), and dibromomethane
on the oxidation of methane (methane: oxygen ratio
=
1:2).
Unless otherwise
stated, carbon dioxide and water were the products of reaction of methane. Trace amounts of chlorocarbons are known to have an advantageous effect during catalytic oxidation of certain hydrocarbons, for example, ethylene (ref. 8 ) , since they facilitate the isolation of partial combustion products although decreasing overall catalytic activity.
In the present work, an
initial investigation of the effects of di- and trichloromethane on the combustion of methane at 650K, revealed that small amounts (ca.
moles
pulse) of these compounds also promoted oxidation although larger concentrations were found to poison the catalyst rapidly. for a 2 . 7 % (w/w) Pd on y-A1203 catalyst.
This effect is shown in Fig.1
A comparative survey of the
inhibiting effects of various halogenocarbons(dibromo- and dichloromethane and 1,2-dichloromethane) on a similar catalyst is shown in Fig. 2, whilst Fig. 3 shows the effects of dibromo- and dichloromethane and trichloromethane
on palladium-alumina catalysts of different loadings.
A striking feature of
218
INITIAL ACTIVITY
1 2 3
1 ,
PROMOTION AND/OR INHIBITION
1 1 2
-
3 1 5 6 7 8 9 10 11 12 13 11 15 16 1 7 18 19 2 0 21 Number af Pulses
F i g . 1. E f f e c t of c o n c e n t r a t i o n o f i n h i b i t o r on methane o x i d a t i o n o v e r 2 . 7 8 Pd (w/w) on y - A 1 0 a t 650K16 C a t a l y s t w e i g h t , 0 . 0 4 - 0 . 0 8 g ; i n i t i a l methane moles; c o n c e n t r a t i o n ! 31.8 x 10 molgs; i n i t i a l oxygen c o n c e n t r a t i o n , 3 . 6 x moles CH,Cl,; +,1.63 A , no i n h i b i t o r ; O , 4 . 0 8 x 10- m o l e s C H C 1 3 ; 0 , 2.72 x L L x 10-8 m o l e s C H 2 C 1 2 ; 0 , 1 . 3 6 x 10 7 moles CH2C12.
RECOVERY l h i n f l o w i n g >20h in flowing Heat 650K I Heal 6 5 0 K
01 ' 1 3
1
I
I
I
I I
I
I
I
'
1 3 5 7
9 11 13 15 17 19 21 23 25
Number of Pulses
1 3 5 7
9 11 13 15
-.c
F i g . 2. The i n h i b i t i o n of methane o x i d a t i o n by some h a l o g e n a t e d h y d r o c a r b o n s and t h e r e c o v e r y of 2 . 7 % Pd on y-A1203. T e m p e r a t u r e = 650K; c a t a l y s t w e i g h t = 0 . 0 7 g ; i n i t i a l methane c o n c e n t r a t i o n , 1.8 x m o l e s ; i n i t i a l oxygen c o n c e n t r a t i o n , 3.6 x 10-6 m o l e s ; a , C l C H CH c1; +, CH CI ; 0, C H 2 B r 2 2 2 2 2
219
INITIAL ACTIVITY
:?{ I
go
1
I N H I 8 ITION 1.36xlO”moles inhihilor ndded 10 reactnnls
RECOVERY lhln ZOhlnflowing He flowing I , He !
I ~
~
~
10 0 1
1
1 ,
, I ,
5 1
5
9
13
17
211
Number o f P u l s e s
,
I
5
,
,
1
5
,
,
9
13
Fig. 3. The inhibition of methane oxidation by various halogenated hydrocarbons and the recovery of palladium on alumina catalysts. Temperature = 650K; initial methane concentration, 1.8 x moles; initial oxygen concentration, 3.6 x 10-6 moles; A , C ~ ~ B r ~ , 2Pd5 %on y-A1 0 ; +,CHC13, 25% Pd on y-A1 0 2 3 0 ,CH2C12, 2.7% Pd on y-Al2O3;D,CH2Br2,2.7%2 3Pd on y-A1203
these curves is that the extent of inhibition of the catalysts is almost independent of the nature of the halogenocarbon. By passing helium through the poisoned catalysts at the same temperature as
that at which poisoning occurred, attempts were made in most cases to recover the catalysts.
Depending on the temperature, the activity of, for example, a
2.7% (w/w) Pd on y-A1203 catalyst was gradually regained by this process.
This
is shown in Fig. 4, for inhibition by trichloromethane, and it can be seen that, although the degree of deactivation of the catalyst was almost independent of temperature (55
5% of the initial activity lost after exposure to 13 gas
pulses containing inhibitor), recovery was most marked at higher temperatures. Similar results were observed for dibromomethane (Fig. 51, although it can be seen that, even at temperatures at which chlorine-containing catalysts showed marked recovery, bromine-containing catalysts exhibited a very small degree
of regeneration.
In both poisoning and recovery experiments, it was
interesting to observe that, whereas the shape of the deactivation curves suggested the preferential poisoning of the most active catalytic sites, the shape of the regeneration curves for chlorine-containing catalysts suggested a non-selective reactivation process (Figs. 2,4,6
and 7).
220
F i g . 4. E f f e c t of t e m p e r a t u r e on t h e i n h i b i t i o n by t r i c h l o r o m e t h a n e and r e c o v e r y o f 2.7% Pd on y-A1203. C a t a l y s t w e i g h t , 0.049; i n i t i a l methane c o n c e n t r a t i o n , 1.8 X moles; i n i t i a l oxygen c o n c e n t r a t i o n , 3 . 6 x moles;A,730K;0,700K, t, 650K; 0,625K.
E f f e c t o f t e m p e r a t u r e on t h e i n h i b i t i o n by dibromomethane and r e c o v e r y Fig.. 5. C a t a l y s t w e i g h t , 0.07 - 0.089; i n i t i a l methane o f 2.7% Pd on y-A1203. c o n c e n t r a t i o n , 1.80 x 10-6 moles; i n i t i a l oxygen c o n c e n t r a t i o n ; 3 . 6 x mo1es;A , 730K; 0, 700K; 0 , 650K. The e f f e c t o f t h e c a t a l y s t s u p p o r t on i n h i b i t i o n by t r i c h l o r o m e t h a n e and t h e r e c o v e r y of p a l l a d i u m c a t a l y s t s a t 650K was i n v e s t i g a t e d and t h e r e s u l t s a r e shown i n F i g s . 6 and 7 . on y-A1203 c a t a l y s t .
A l s o shown i n F i g . 7 a r e r e s u l t s for a 2 . 7 % (w/w)
Pt
I t w a s found t h a t t h e a b i l i t y o f t h e c a t a l y s t t o r e c o v e r
221 after the introduction of inhibitor depended on the nature of the support as well as the temperature of prolonged exposure to helium.
For example, at 650K
(at-which temperature y-alumina-supported palladium catalysts were irreversibly
INITIAL ACTIVITY INHIBITION
I
1.36 x 10-7moles C H C l 3 added to reactants
I
I
I I
I
RECOVERY
/lhinflowing !lShinIlowing He He
I
,I
+.+
I I
-*+*
I I I I
+-+<*
1 3 51 3 5 7
-
9 11 13 15 17 19 21 23 25 271 3 5 7 1 3 5 7 N u m b e r of Pulses
Fig. 6. Inhibition by trichloromethane and recovery at 650K of palladium catalysts on different supports. Catalyst weight 0.03 to 0.06 g; initial methane concentration, 1.8 x mols; initial oxygen concentration, 3.6 x mols; +, 2.7% Pd on y-Al2O3;0, 2 . 1 % Pd on Ti02; 0, 2.1% Pd on Tho2.
f!
I N I T I A L ACTIVITY
61: y:!; !
C1.3H C 1 3 added toles
-1
.
D
I*\
60 I
reactants
R E COVE RV
I I
j l h in iflowing 1He a t
s 2 0 h in f l o w i n g He at 650K
,
j650K
p-,
,+**
it
1 5 1 5
9 13 1 1 2 1 25 1 5 Number of Pulses
13
7 11 1 5 1 9 23 27
Fig. 7. Inhibition by trichloromethane and recovery of some palladium catalysts at 6 5 0 K ; catalyst volume, 0.05 cm3; initial methane concentration, 1.8 x 10-6 mols; initial oxygen concentration, 3.6 x mols; 0 , 2.7% Pd on Y-A1203' + t 2 0 % Pd on Sn02;A, 2.7% Pt on y- A1203
222 p o i s o n e d by t r i c h l o r o m e t h a n e
( F i g s . 3 and 4 3 , a 2 . 7 % ( w / w ) Pd on t i t a n i a
c a t a l y s t showed some r e c o v e r y i n a c t i v i t y when h e a t e d i n f l o w i n g helium ( F i g . 6 ) w h i l s t a 2 0 % ( w / w ) Pd on t i n ( I V ) o x i d e c a t a l y s t e x h i b i t e d a l m o s t complete recovery.
XPS STUDIES The c a t a l y s t s s t u d i e d i n t h i s work were e l e c t r i c a l l y i n s u l a t i n g and c a l i b r a t i o n of t h e e l e c t r o n spectrometer w a s necessary before t h e evaluation of t h e b i n d i n g e n e r g i e s of t h e e l e m e n t s of i n t e r e s t .
The u s e of g o l d w i t h non-
c o n d u c t i n g specimens i n o r d e r t o o b t a i n c h a r g i n g c o r r e c t i o n s , and t h u s a b s o l u t e (ref. 9).
b i n d i n g e n e r g i e s , h a s been recommended
I n t h e p r e s e n t work, t h e u s e
o f c e r t a i n i n t e r n a l s t a n d a r d s a s a method o f measuring b i n d i n g e n e r g i e s i n c a t a l y s t s h a s been examined and t h e f o l l o w i n g c o n c l u s i o n s were r e a c h e d : (1)
The C
1S L
literature
'
l i n e c a n n o t be u s e d a s a r e f e r e n c e , as recommended i n t h e ( r e f . lo), s i n c e n o t o n l y d o e s t h i s l i n e a r i s e from s u b s t a n c e s
of unknown c o m p o s i t i o n , b u t a l s o t h e d e p o s i t s which g i v e r i s e t o it a r e i n e l e c t r i c a l e q u i l i b r i u m w i t h t h e specimen.
F u r t h e r , i n t h e p r e s e n t work, complex
l i n e s were o b t a i n e d f o r t h i s e l e m e n t which, a f t e r d e c o n v o l u t i o n , a p p e a r e d t o b e c h a r a c t e r i s t i c ( i n some c a s e s ) of t h e added i n h i b i t o r . U n l e s s t h e c o v e r i n g w a s c o n t i n u o u s , t h e u s e o f s p u t t e r e d g o l d and
(2)
measurement o f t h e c h a r a c t e r i s t i c 4 f 7 and 4 f 5 l i n e s as i n t e r n a l s t a n d a r d s ( r e f . 9 )
-
was unsatisfactory, since
-
2 e l e c t r i c a l e q u i l i b r i u m was a g a i n
2
e s t a b l i s h e d w i t h t h e specimen.
C e r t a i n l y , c h a r g e s h i f t s i n t h e 4f b i n d i n g
e n e r g i e s o f up t o 3eV from t h o s e p u b l i s h e d i n t h e l i t e r a t u r e , were o b s e r v e d w i t h t h i s element. (3)
The OIs and t h e I n
measures
'
(from t h e sample mounting) l i n e s were t h e b e s t 3d5 of sample - c h a r g i n g . 2
I n t h i s work, a l l b i n d i n g e n e r g i e s were measured r e l a t i v e t o t h e 0
line
IS%
a t 532.0 eV.
The p h o t o e l e c t r o n s p e c t r a o f s o m e p u r e p a l l a d i u m compounds and b o t h p o i s o n e d and a c t i v e c a t a l y s t s were o b t a i n e d u s i n g aluminium K= r a d i a t i o n . e n e r g i e s measured f o r t h e C
and Pd3d
The b i n d i n g
l i n e s a r e shown i n T a b l e 1 and 2 . The
1S L 5 3 peak w a s p r e s e n t i n a l l samples -'- even t h o s e n o t d e l i b e r a t e l y exposed t o 1% 2 2 c a d o n - c o n t a i n i n g compounds, a n d p r o b a b l y r e p r e s e n t e d c o n t a m i n a t i o n a c q u i r e d i n
C
t h e a m b i e n t atmosphere i n p a l l a d i u m metal w a s v e r y c l o s e t o t h e
The b i n d i n g e n e r g y f o r t h e Pd l i t e r a t u r e v a l u e s (re-fs. l l a , b ) .
3d5
-
From T a b l e 1, it i s a p p a r e n t t h a t t h e
b i n d i n g e n e r g i e s i n p a l l a d i u m m e t a l were a p p r o x i m a t e l y 3eV pd3d5 and Pd3d 3 - lower t h a n t h o s e found f o r p u r e and s u p p o r t e d p a l l a d i u m ( I I j compounds. 2 2 I t a p p e a r e d , t h e r e f o r e , t h a t t h e a c t i v e form of t h e c a t a l y s t c o n s i s t e d , n o t o f
223 TABLE 1
E l e c t r o n Binding E n e r g i e s i n Some P a l l a d i u m C a t a l y s t s and Compounds
Binding Energy ( e V ) Catalyst/Compound
+
Pd metal'')
Pd3d5
Pd3d3
-
-
2
2
2 2
284.0
335.3
284.5
335.1
Pd (11) o x i d e ( 2 1
285.5
338.2
Pd (11) o x i d e (3)
285.2
337.2
342.6
287.0
338.7
344.2
Pd m e t a l
(2)
18.7% P d ( I 1 ) O
+
Ti02
0.1
340.7
0.1
0.05
340.5
0.05
343.6
2 . 7 % PdC12/Y-A1203
( 4 ) 286.0
337.3
342.5
2 . 7 % PdC12/Ti0
( 4 ) 286.8
338.5
343.8
6.0% Pd/Y-A1203
( 5 ) 285.4
337.9
2.7% Pd/Y-A1203
(6) 284.5
336.2
2.7% Pd/y-Al2O3
( 7 ) 283.8
)
342.9
2
0.5
341.3
335.4
340.7
335.4
341.4
337.4
342.5
337.2
342.4
2
0.5
285.3 2.7% Pd/Y-A1203
( 8 ) 283.7
2 . 7 % Pd/Y-A1203
(')
285.6 283.1 286.0 2.7% Pd/Y-A1203
(lo)283.8
(Sh)
99.99% Pd; 6 . 0 nm o f s u r f a c e removed See r e f . 1 A l f a P r o d u c t s Lot N o . 1116776 (4) Catalysts before conditioning i n H e (5) Conditioned c a t a l y s t ( 6 ) C a t a l y s t p o i s o n e d w i t h 3.0uL CH2C12 a t 675K. Recovery a t t e m p t e d by e x p o s u r e t o H (752K) and O2 (675 K) A c t i v i t y not recovered 2 C a t a l y s t poisoned w i t h 5 . 5 uL ( a t 67310 and 10 L ( a t 773K) CH2C12 C a t a l y s t p o i s o n e d w i t h 1 . 5 pL CH B r a t 675K 2 2 . -7 C a t a l y s t p o i s o n e d w i t h p u l s e s c o n t a l n l n g CH2BR (1.36 x 10 moles/ pulse). Recovery a t t e m p t e d by e x p o s u r e t o a few methane + O2 pulses; c a t a l y s t recovering. (10) C a t a l y s t a c t i v i t y a l m o s t r e c o v e r e d a f t e r p o i s o n i n g by CH C 1
.
2
2
p a l l a d i u m metal, b u t a p a l l a d i u m (11) compound which e a r l i e r work ( r e f . 5) had suggested w a s t h e oxide. A f t e r e x p o s u r e t o halogenocarbons, two marked changes were o b s e r v e d i n t h e s p e c t r a of p o i s o n e d alumina-supported c a t a l y s t s ( T a b l e 1 ) . F i r s t l y , t h e Pd3d b i n d i n g e n e r g i e s were s h i f t e d towards t h o s e t y p i c a l of m e t a l l i c p a l l a d i u m a n d , s e c o n d l y , t h e c a r b o n Is
JI
3
2 2 peak became b r o a d e r and more complex a n d , on de-
c o n v o l u t i o n , t w o p e a k s c o u l d b e r e s o l v e d which may p e r h a p s b e a s c r i b e d t o a
224 carbon-halogen residue in addition to the typical contamination.
In contrast
to these results, poisoned tin (IV) oxide-supported palladium catalysts showed no significant shifts in the positions of either the Pd 3d or C peaks (Table 2) Is% although the height of the CIS peak in contaminated samples appeared much larger ?r than that in catalysts not exposed to halogen-containing compounds. Finally, as can be seen from Table 1, recovery of palladium/alumina catalysts was marked both by a shift in the position of the Pd3d binding energies from -
those typical of Pd(2)towards those found in palladium (11) compounds and by a simplification of the C
IS
appeared that the first
’
signal.
During reactivation of these catalysts, it
step involved reoxidation of the palladium and
removal of the carbon-containing residue occurred somewhat later.
Thus, in
Table 1, a partially recovered catalyst (superscript (9); approx. 10% more activity than fully-inhibited material) had Pd3d binding energies typical of palladium (11) compounds whilst showing a complex CIS
signal.
A catalyst
51.
which had almost fully recovered its activity (superscript(l0)in Table 1) had almost identical Pd3d binding energies and, in addition, asimplerCIs
peak
in which the signal at ca. 286 eV had become attenuated to the extent’ in which it appeared as a shoulder on the peak at ca. 284
eV.
A E S Studies
In the Auger electron spectra of the catalysts, the carbon KLL peak usually occurs at about 270 eV which is very close to an Auger peak of palladium at 275 to 280 eV.
The presence of small amounts of carbon can, therefore, only
be inferred from the shape of the peak and any deviation from the expected ratios in the heights of the three palladium peaks at ca. 243, 279 and 326 eV. TABLE 2 Electron Binding Energies in Some Tin (IV) Oxide-Supported Palladium Catalysts
Binding Energy (eV) Catalyst
2.7 % Pd/Sn02 20.0% Pd/Sn02 (2) (3) 20.0% Pd/SnO‘ L
20.6% Pd/Sn02-A1203 (4)
286.3
337.3
342.6
286.4
337.9
342.9
286.4
338.2
343.4
284.8
338.0
342.7
(1) No inhibitor, heated to 87% (2) No inhibitor, heated to 720K -7 (3) Catalyst poisoned with pulses containing CHC13 1.36 x 10 moles/pulse at 650K; catalytic activity recovered (4) As (3) but catalytic activity not restored
225
During AES studies it was observed that bromine-containing residues were found consistently on the surface of catalysts exposed to that element whilst the occurrence of chlorine was much more localised in the corresponding chlorine contaminated material.
In an attempt to determine which were preferred areas
for chlorine adsorption, palladium-, oxygen- and chlorine-Auger images were obtained of the surface of a palladium/alumina catalyst exposed to trichloromethane, thus Plate 1 shows the secondary electron- and the absorbed electron images of this catalyst whilst the Auger images are shown in Plate 2. palladium-Auger image (Plate 2 (a)
),
From the
it can be seen that this element tends to
be concentrated in the vicinity of the spherical feature shown in the upper right hand corners of Plates 1 (a) and (b). oxygen (Plate 2 (b)
)
The corresponding distribution of
and chlorine (Plate 2 (c) ) indicated that in deactivated
catalysts, chlorine was more closely associated with palladium than was oxygen. Despite the uncertainties introduced by topographic effects in the examination of such specimens, this result supported the XPS findings that palladium oxide was no longer present on the surface of contaminated material.
Plate 1 (a) Secondary electron image (b) Absorbed electron image of a Pd/A1 0 2 3 (Pd > 2 0 % (w/w)) catalyst exposed to CHC13; magnification X200 Plate 2 (a) Pd-Auger image of area shown in Plate 1; (b) 0-Auger image of same area; (c) C1-Auger image of same area; magnification X200 Electron Microscopy Particle size histograms from transmission electronmicrographs of 2.7% (w/w) Pd on y-alumina catalysts after various treatments are shown in Fig. 8. In this diagram, histogram EN represents the particle size distribution of a typical catalyst after preliminary conditioning in flowing helium.
EO represents the
distribution of an equilibrium catalyst after oxidation of methane + oxygen
226 mixtures.
The two s y s t e m s were s i m i l a r , t.he most f r e q u e n t l y o b s e r v e d p a l l a d i u m
The remainder of t h e h i s t o g r a m s
p a r t i c l e d i a m e t e r s l a y i n g between 10 and 1 5 nm.
i n F i g . 8 ( E J , EK, EL, EP and EQ) r e p r e s e n t c a t a l y s t s which had been p o i s o n e d w i t h t r i c h l o r o m e t h a n e a t v a r i o u s t e m p e r a t u r e s a l t h o u g h , f o r c a t a l y s t s EK and E J , c a t a l y t i c a c t i v i t y had been f u l l y (EK) o r p a r t i a l l y (EJ) r e s t o r e d .
CAT € 0
l r
n
1
0 Diameter (nml
50 r
LO
CAT. E L
20
30 h0
50 60 70
Diameter I n m l +
50 I
CAT.
EP
LO
~
0
10
+
10
20 30
LO 50
Oiameler (nmi --c
0
10
20
30
LO
50
Oiameler l n m i
-
80
227 50
CAI EO
0
10
I
0
20
30 10 50 Oiameter i n m )
I
0
--
CAI E l
10
20 30 10
50
Oiomeler (nrn)
-
CAT. EN
10
20 30
LO
50
Oiomeler [ n m l
-
Fig. 8. Influence of exposure to trichloromethane on the particle size distribution of 2.7% Pd on -A1203 catalysts. Cat EN, conditioned at ??3K for 24h then at 723~ for 24h. Cat. EO, conditioned at ??3K for 24h then at 750K for 24h then 5 pulses of CH4 + O2 at 750K. Cat. EL, conditioned at ??3K for 16h; poisoned with CHC13 + CH4 + O2 at 580 - 730K. Cat. EP, conditioned at 773K for 16 h then pulses of CH4 + 0 at 750 and 625K; poisoned with CHC13 + O2 at 625K. Cat. EQ, conditioned at ??3K for 16h; poisoned with CHCl + CH4 + O2 at 650K. Cat EJ, conditioned at 750K for 60 h; poisoned with CHC?3 + CH4 + O2 at 700K. Cat EK poisoned with CHC13 + C H ~ + o2 at 730K. Particle counts 400 to 1000 over 5 to 14 (pm)*.
228
In all cases, exposure to the chlorocarbon had produced a gradual shift in the
distribution to particles of a larger diameter.
Particularly marked was the
growth of particles having diameters in the range 20 to 25 nm at the expense of those with diameters from 10 to 15 nm.
Eventually,after the introduction of very
large amounts of inhibitor, massive agglomeration of the catalyst occurred. This is shown in Plates 3 and 4 for a 2.7% (w/w) Pd on y-A1203 catalyst which had been exposed to approximately ten times the amount of dichloromethane normally used in poisoning experiments.
H
h
0.lV
0.1y
Plate 3 Transmission electron micrographs of 2.7% Pd on y - A 1 O3 after exposure to 5 U L dichloromethane at 673K, 10 pL dichloromethane at Plate 4 773K and 1 pL at 973K
DISCUSSION
Studies of the interaction of halogenocarbons with palladium catalysts have been reported previously (refs. 12, 13, 14) although the effects of these compounds is still not clear.
For example, Cullis et a1 (ref. 13) compared the
effect of various halogenocarbons on methane oxidation over palladium catalysts and found that, whilst the inhibiting efficiency of the four chloromethanes depended on both the number of chlorine atoms in the molecule and their reactivity, the effects of the different compounds were not very different at lower concentrations.
Comparison of the inhibiting effect of dichloromethane
with other halogenomethanes showed that this compound had approximately the same efficiency as difluorodichloromethane but was a much less effective poison than either dibromo- or diiodomethane (ref. 13).
A very surprising result was
obtained by Otto and Montreuil (ref. 14) who found that the oxidation of hydrocarbons and carbon monoxide over palladium and palladium catalysts at 773K was unaffected by 1,2-dichloroethane whilst 1,2-dibromomethane acted as an inhibitor under the same conditions.
In the present investigation, it was
found, in contrast to the results of Cullis and co-workers (ref. 13) and
Otto and Montreuil (ref. 14), that there was little difference in the inhibiting efficiencies of a number of halogen-containing compounds, including dibromo- and dichloromethane, 1,2-dichloroethane and trichloromethane. Differences in the behaviour of chlorine- and bromine-poisoned catalysts were observed, however, when attempts were made to reactivate these systems. The following discussion will concentrate mainly on alumina-supported catalysts since material containing tin ( I V ) oxide and, possibly, titanium (IV) oxide, behaves differently
.
Catalysts containing these oxides will be briefly but
specifically mentioned at the end of the paper. Both the effect of halogen-containing compounds on supported palladium catalysts and the nature of the interaction appear to be complex. -8
amounts (ca. 10
Thus, small
moles) of dichloro- and trichloromethane enhanced the combustion
of methane on a 2.7% (w/w) Pd on y-Al2O3 catalyst at 650K,whilst larger quantities rapidly deactivated the catalyst.
Although experimental work to test the
promoting effect of other halogenocarbons was not performed, it seems likely that similar results would have been obtained.
It is well known that the selectivity
of a silver catalyst in the oxidation of ethylene to oxiran (ref. 9 ) is improved by the additions of small amounts of halogenated compounds to the gas-phase and. Rovida, Pratesi and Ferroni (ref. 15) examined the interaction of 1,2-dichloroethane and oxygen with a silver surface.
These authors concluded that the presence of
small amounts of chlorine on the metal surface affected significantly the strength of the oxygen-silver bond at the surface and lowered appreciably the temperature at which adsorbed oxygen became mobile (ref. 15).
Since the active state of
palladium oxidation catalystsinvolves oxygen adsorbed on the metal (probably palladium (11) oxide) (refs. 5 , 1 3 ) , which participates in hydrocarbon oxidation, the promoting effect of chloromethanes observed in this study was probably also due to the weakening of the palladium bond (ref. 15). It would appear that the influence of halogenocarbons on the interaction of palladium with oxygen was also responsible for catalyst poisoning by these compounds although evidence has also been obtained that carbon deposition played an important role.
For example, XPS measurements revealed that there was a change
in the oxidation state of palladium in active catalysts and those inhibited by halogenocarbons.
Thus the active phase of supported palladium catalysts appeared
to consist of palladium (11) oxide whilst palladium metal was present in the poisoned material (Table 1).
In the case of trichloromethane-poisoned catalysts,
these observations were confirmed by AES (Plates 1 and 2 ) which showed clearly that chlorine was more closely associated with palladium than was oxygen.
XPS studies
also showed that the interaction of halogenocarbons with the catalyst surface produced a carbon-containing residue in addition to adsorbed halogen.
For example,
the ubiquitous carbon peak detected on the catalysts became uncharacteristically
230
complex a f t e r e x p o s u r e t o i n h i b i t o r .
I n d e e d , i n t r o d u c t i o n of a l m o s t t w e n t y
t i m e s t h a t amount of d i c h l o r o m e t h a n e r e q u i r e d f o r p o i s o n i n q t o a 2 . 7 8 y-A1
0.
2 3
(w/w)
Pd on
c a t a l y s t , p r e v i o u s l y s a t u r a t e d w i t h oxyqt.n, a t 6 7 5 K showed t h a t , a l t h o u g h
o x i d e s o f c a r b o n w e r e p r o d u c e d , a s i g n i f i c a n t amount of c a r b o n was d e p o s i t e d on t h e catalyst.
Subsequent e x p o s u r e t o g a s e o u s oxygen a t 6 7 5 K produced c a r b o n d i o x i d e
from t h i s l a y e r .
Some e v i d e n c e was also o b t a i n e d which s u g g e s t e d t h a t t h e n a t u r e
of t h i s c a r b o n l a y e r was d i f f e r e n t depending on whether i t was d e p o s i t e d from a c h l o r o - o r a bromo-compound.
For example, a f t e r e x p o s u r e of c a t a l y s t s t o
dibromomethane, AES c o n s i s t e n t l y showed t h e p r e s e n c e of b o t h c a r b o n and bromine on t h e surface.
A f t e r e x p o s u r e t o chloro-compounds,
however, c a r b o n was a l w a y s d e t e c t e d
b u t a c a r e f u l s e a r c h had t o b e made f o r a r e a s o f t h e s u r f a c e c o n t a i n i n g b o t h c a r b o n and c h l o r i n e . The s h a p e of t h e o x i d a t i o n a c t i v i t y c u r v e s i n t h e p r e s e n c e o f i n h i b i t o r s s u g g e s t s t h a t t h e r e were two t y p e s of s i t e p r e s e n t on s u p p o r t e d p a l l a d i u m c a t a l y s t s , o f which t h e most a c t i v e was p r e f e r e n t i a l l y p o i s o n e d .
I n t h i s c o n n e c t i o n , C u l l i s e t a1
( r e f . 1 3 ) have s u g g e s t e d t h a t methane c a n b e a d s o r b e d on p a l l a d i u m i n two ways; t h e f i r s t i n v o l v e s "bridge-bonded'' linearly-bonded,
methylene r a d i c a l s , w h i l e t h e second i n v o l v e s
presumably m e t h y l , r a d i c a l s and t h a t o x i d a t i o n o c c u r r e d m a i n l y
d u e t o t h e r e a c t i o n of s u r f a c e l a t t i c e o x i d e w i t h t h e l e s s s t a b l e l i n e a r l y - b o n d e d radicals.
F u r t h e r , C u l l i s e t a1 ( r e f . 1 3 ) a l s o s u g g e s t e d t h a t d e p o s i t i o n o f
c a r b o n o n t h e s u r f a c e s e l e c t i v e l y b l o c k e d t h o s e s i t e s f a v o u r i n g bridge-bonding. The p r e s e n c e o f s i m i l a r s i t e s i n t h e p r e s e n t s y s t e m s would a c c o u n t f o r many o f o u r observations.
F o r example, i f t h e h a l o g e n i s a s s o c i a t e d w i t h o x y g e n - a d s o r p t i o n
s i t e s and t h e o r g a n i c r e s i d u e w i t h s i t e s which a d s o r b methane and t h e former a r e more a c t i v e t h a n t h e l a t t e r , t h e n t h e s h a p e of t h e d e a c t i v a t i o n c u r v e i s e x p l a i n e d . A l s o , t h e e x i s t e n c e of two t y p e s o f s i t e would e x p l a i n t h e two s t e p s which were
a p p a r e n t l y necessary f o r c a t a l y s t regeneration. The p r e s e n t work h a s shown t h a t , under some c i r c u m s t a n c e s , a c t i v i t y c a n b e r e s t o r e d t o p o i s o n e d c a t a l y s t s by e x p o s u r e t o f l o w i n g h e l i u m and p u l s e s o f methane
+
oxygen m i x t u r e s ( F i g s . 2 and 4 ) .
I t w a s i n t h e a b i l i t y of c a t a l y s t s t o r e c o v e r
from t h e i r e f f e c t s t h a t d i f f e r e n c e s i n t h e b e h a v i o u r o f v a r i o u s i n h i b i t o r s w a s observed.
Thus, a f t e r p o i s o n i n g w i t h dibromomethane, a c t i v i t y w a s n o t r e s t o r e d
t o a 2.7% (w/w) Pd on y-A1 0 c a t a l y s t by f l o w i n g helium and methane + oxygen 2 3 The same c a t a l y s t p o i s o n e d by t r i c h l o r o m e t h a n e began t o r e c o v e r
pulses a t 650K.
a c t i v i t y s l o w l y i n f l o w i n g h e l i u m a t 700K w h i l e , a t 7 3 0 K , t h e r a t e t o r e c o v e r y w a s much more r a p i d ( F i g . 4 ) .
The same c a t a l y s t , p o i s o n e d w i t h dibromomethane, showed
no r e c o v e r y a t 700K and v e r y l i t t l e r e c o v e r y a t 7 3 0 K ( F i g . 5 ) .
XPS s t u d i e s o f
r e c o v e r i n g and r e c o v e r e d c a t a l y s t s s u g g e s t e d t h a t removal o f c a r b o n a c e o u s d e p o s i t s
as w e l l a s r e o x i d a t i o n o f p a l l a d i u m , w a s n e c e s s a r y f o r r e a c t i v a t i o n .
It is
t e n t a t i v e l y s u g g e s t e d , t h e r e f o r e , t h a t t h e o v e r l a y e r formed by t h e i n t e r a c t i o n o f
231 dibromomethane with the catalysts, which app'eared to be different to that formed by chlorocarbons, was much more difficult to remove and that this accounted for the observed differences in catalyst recovery. required
111
Further work is
order to define the nature of such carbon deposits. One further questicr.
is raised by the results of catalyst recovery studies.
Bearing in mind the XPS
evidence, it is difficult to explain the apparently non-selective regeneration of chlorine-contaminated catalysts. Finally, the implications of the effects of halogenocarbons on the particle size distribution of palladium catalysts must be discussed. In an earlier paper (ref. i), it was suggested that the formation and subsequent reaction of palladium (11) oxide played an important role in the oxidation of methane in the presence of palladium. It had been shown (ref. 5) that the reversible decomposition of this oxide could explain many experimental observations.
Thus, with well-dispersed, supported,
catalysts, the amount of oxygen evolved during the high temperature decomposition of the oxide was identical to that adsorbed at lower temperatures during its formation from palladium salts.
With bulk palladium (11) oxide, however, the
amount of oxygen evolved during decomposition was greater than that readsorbed when the sample was cooled.
Although reoxidation is slow (ref. 5), with well-
dispersed catalysts, the palladium crystallites are small enough for a sufficient number of oxidised sites to be available to maintain efficient oxidation. With poorly-distributed catalysts of large metal particle size, the surface is only partially covered with oxygen, either because reoxidation is even more difficult or because surface reoxidation is competing with oxygen diffusion into the bulk of the particles.
Whatever the reason, oxidation activity is reduced unless a large excess
of oxygen is present in the ambient atmosphere.
It is likely, therefore that since
exposure to halogenocarbons causes an accelerated rate of growth of palladium particles in supported catalysts, either prolonged exposure to or exposure to large amounts of halogenocarbons will adversely affect the performance of these catalysts. Finally, as has been said, tin (IV) oxide - and, possibly titanium (IV) oxidesupported palladium catalysts appear to behave differently to those supported on alumina or thoria.
For example, not only was the palladium in Pd on Sn02
catalyst unaffected by exposure to halogenocarbons (Table 2) but this catalyst and Pd on TiOZ recovered their activity more quickly after exposure to poisons than other catalysts under comparable conditions (Figs. 6 and 7 ) .
A possible
explanation may be that these oxides act as oxygen reservoirs and maintain the palladium in an oxidised state and so reduce the effects of the interaction with inhibitor.
Certainly, this explanation was proposed to account for the beneficial
effects of tin additions to platinum/zinc aluminate dehydrogenation (ref. 16).
232 REFERENCES 1
2
3
4 5
6
7 8 9
lo
11
12 13 14 15 16
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