Preoxidation of the surface of carbon support was found to increase the dispersity ... through KMnO 4 chemisorption from aqueous solutions (like in. Ref. [5]) with ...
React. Kinet. Catal. Lett., Vol. 41, No. I, 211-216 (1990) INFLUENCE OF CARBON SUPPORT P R E T R E A T M E N T
ON PROPERTIES
OF Pd/C CATALYSTS S.V. Gurevich,
P.A.
E.M. Moroz, Institute
Simonov,
A.L.
A.S.
Chuvilin
of Catalysis,
Lisitsyn,
and V.N.
V.A. Likholobov,
Kolomiichuk
Novosibirsk
630090,
USSR
Received June 28, ]989 Accepted August 7, ]989
Preoxidation
of the surface of carbon support was found
to increase the dispersity Pd/C
catalysts
of pal~dium and activity of
obtained by reduction o f supported Pd(OAc) 2,
in h y d r o g e n a t i o n
of olefins.
Probable
reasons
for this
effect are discussed.
YCTaHOB~eHO, yF~pO~HOFO
qTO npe~sapHTe~bHOe HOCHTe~fl H O B B O ~ e T
H aKTHBHOCTb
one~HHOB.
Pd(OAC),
06cy~eHN
B
BO3MOKHNe
The nature of carbon carriers and catalytic attributed
properties
peaN~HH
BOCCTaHOB-
FH~pHpOBaHHS
npHqlIHN 3TOFO 9~@eKTa.
is known to affect the state [I]. This has been
in the texture of carriers
of surface heteroatoms,
lysts has been studied due to the difficulty
on the properties insufficiently.
and an oxygen coverage
study of these effects
amounts of various admixtures
catathis is
the effects of carrier
of the surface.
is c o m p l i c a t e d
composi-
of supported
In particular,
in d i s t i n g u i s h i n g
and
in particular
But so far the effect of the chemical
tion of carbon surfaces
texture
no~yqeHH~X
of supported metals
to the difference
in the concentration oxygen [2,3].
HOBMCHTb ~ H C ~ e p c H O C T b
Pd/C KflTa~HgaTOpOB~
n e H H e M HaHeCeHHOFO
HoBepXHOCTH
OKHC~eHHe
Besides,
the
by the presence of large
in ordinary carbon carriers.
We
Akad~miai Kiad6, Budapest
GUREVICH et al.: Pd/C CATALYSTS have m a d e
an a t t e m p t
of the c a r b o n
to e x a m i n e
the role of a c h e m i c a l
s u r f a c e w i t h the use of a r e c e n t l y d e v e l o p e d
S i b u n i t c a r b o n as a s u p p o r t [4 ]. U n l i k e this m a t e r i a l admixtures
state
is c h a r a c t e r i z e d
and p r a c t i c a l l y
common
active
by low c o n c e n t r a t i o n
contains
carbons,
of m i n e r a l
no m i c r o p o r e s .
EXPERIMENTAL A s a m p l e of S i b u n i t w i t h BET content below
1% was u s e d as a c a r r i e r
Its m o d i f i c a t i o n i. The c a r b o n
surface
was c a r r i e d
Ref. [5]) w i t h s u b s e q u e n t
under mild
from aqueous
removal
10% HCI and w a s h i n g by w a t e r 2. The o x i d i z e d
(0.04-0.09
nm fraction).
out by two m e t h o d s .
s u r f a c e was o x i d i z e d
through KMnO 4 chemisorption
400 m 2 / g and ash
conditions
solutions
of m e t a l
ions by b o i l i n g
d o w n to a n e u t r a l
c a r r i e r was h e a t e d
(like in
at 500~
and then c o o l e d d o w n to r o o m t e m p e r a t u r e
in
reaction.
in v a c u u m
for 5 h
before contacting
w i t h air. Concentration
of d i f f e r e n t
of s u p p o r t s was d e t e r m i n e d with
solutions
of NaOH,
functional
N a 2 C O 3 and N a O E t
1% P d / C s a m p l e s w e r e p r e p a r e d I. B e f o r e treated
groups
by p o t e n t i o m e t r i c like
2. D e h y d r a t e d
titration,
in R e f . [ 6 ].
all c a r r i e r s w e r e
for 3 h to r e m o v e
c a r r i e r was
tion in a b s o l u t e b e n z e n e
on the s u r f a c e
as follows.
s u p p o r t i n g palladium a c e t a t e ,
in v a c u u m at 100~
back
a d s o r b e d water.
impregnated with
Pd(OAc) 2 solu-
and t h e n the s o l v e n t was r e m o v e d
in
v a c u u m at B0~ 3. The s a m p l e s at 250~
(if not
room temperature air.
obtained were
indicated
reduced
specially)
for 2 h, c o o l e d d o w n to
and p u r g e d by n i t r o g e n
Pd c o n t e n t was d e t e r m i n e d
using
in f l o w i n g h y d r o g e n
before
an a t o m i c
contacting with absorption
method. The solution
state of s u p p o r t e d m e t a l transmission
apparatus,
212
electron microscopy
small a n g l e X - r a y
c h a m b e r w i t h CuE
was e x a m i n e d
scattering
irradiation
using high-re-
(TEM), J E M - 1 0 0
(SAXS)
and a Ni f i l t e r
CX
in a C R M - I and w i d e
angle
GUREVICH et al.: Pd/C CATALYSTS X-ray scattering (with CuK
(WAXS)
on a HZG-4
irradiation
of catalysts
and a graphite monochromator).
in cyclohexene
a static reactor at 0~ absence of external
(GDR) X-ray diffractometer
hydrogenation
at
atmospheric
diffusion
in methanol was used.
control,
catalysts were p r e v i o u s l y
up to a mean
pressure
limitations.
c-hexene
Testing
was carried out in and in the
5% solution of
To minimize
internal diffusion
ground in an agate mortar
particle size of about 5 um.
RESULTS AND D I S C U S S I O N The results of studying carbon ous treatments
supports
and the Pd/C catalysts
in Table 1 and Fig.
subjected to vari-
obtained are represented
i.
As is seen from Table
I, the chemical
surfaces of three carriers
used differs
composition
of the
substantially.
Treat-
ment of the initial
Sibunit by a solution of potassium per-
manganate
considerably
increases
oxygen-containing contrary,
groups,
its thermal
is accompanied,
the concentration
particularly
treatment
indicates
as one would expect,
by surface deoxidation.
texture after treatments
TEM and SAXS data
dispersed
Pd particles,
BET surfaces of the samples (Table I). pre-
with a mean size of about 1 nm. At
upon the reduction pretreated
the initial carbon
This
in carrier
support leads to the formation of highly
ing in size from 1 to
of palladium acetate
Sibunit,
Pd crystallites
10 nm. Most Pd particles
support have small size
are also larger particles
in the sample.
with the WAXS data. Metallic lysts
as well
changes
(Fig. I) suggest that the oxidative
of carbon
ed on thermally
performed.
unchanged
treatment
the same time,
the spectra of small
scattering were undistinguishable.
the absence of any essential
remain p r a c t i c a l l y
On the
in vacuum at high temperature
It should be noted that for all carriers and wide angle X-ray
of surface
of carboxyls.
support-
are rang-
obtained on
(1-2 nm) but there
These results
agree
phase has been displaye d in cata-
1 and 3 (Table I), whereas
the catalyst prepared on
213
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al.:
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