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Catalytic properties of metallic oxides in partial oxidation reactions
Materials
Chemistr!,
CATALYTIC
and Physics,
PROPERTIES
J. C. VEDRINE,
13 (1985)
OF -METALLIC OXIDES
G. COUDUPJER,
365
365-378
IN PARTIAL
M. FORISSIER
Institut de Recherches sur la Catalyse, 2 Av. A. Einstein, F 69626 Villeurbanne
OXIDATION
REACTIONS
and J.C. VOLTA
CNPS, aSsOCie (France)
a l'UCB, Lyon I
ABSTRACT
The purpose of this survey is to presentrecentexperimental results and main ideas from our Laboratory in the field of metallic oxides. Structure sensitivity of oxidation reactions on simple oxides as Moo3 and Sb2Oq is clearlydemonstrated and is shown to depend on the surface atomic arrangements related to bulk structure, on the nature of the transition metal cations and on the chemical properties and size of the reactant molecules. Catalytic properties of isomorphous oxides as Sb2Moo6 and Bi2MoO6 or orthorhombic bronzes A~+M~M$~O62 with A=Sb or Bi and M=Mo or W are presented and discussed in terms of electron lone pair location, lattice oxygen lability and nature of transition metal ions. Extension is presented to multicomponent catalysts and a special emphasis is placed upon structural fitting between active phases and a support, and upon the importance of catalyst preparation conditions.
INTRODUCTION The importance
of oxides
Indeed
it is a consequence
oxides
are directly
active phase.
cracking
and reforming of catalytic
classifed
as insulators
mina, silica-alumina, acidic or basic
reactions
molecules
processes.
This explains
oxidation
based on oxides
and as semi conductors.
magnesia,
which
so far.
chemistry.
for another
Many
catalytic
are the most economical
stemming
way
from cracking,steam-
the importance
of industrial
as catalysts.Oxides The insulators
clays, zeolites...
are principally
or total oxidation
0254-0584/85/$3.30
oxidation
based
may be
silica,
are principally
alu-
catalystsfor
type reactions.
Semi conductors in partial
partial
of petroleum
or as supports
the simple hydrocarbon
processes
has not to be demonstrated
of the success
used as catalysts
Moreover,
to functionalize
in catalysis
reactions.
transition We will
metal ion oxides
focus our interest
are involved in this
0 ElsevierSequoia/PrintedinThe Netherlands
366
survey on
transition metal oxide systems studied in our laboratory which
intervene as a basic component of most industrial mild oxidation processes. From the recent demonstration in our laboratory that oxidation reactions are sensitive to the structure of simple oxides, we will then discuss mixed oxides and multicomponentmixed oxide
catalysts and show how the catalytic properties
of these systems can be influenced by the superficial structure of the different crystallographicphases.
STRUCTURE
SENSITIVITY
0~ PAHIIAL OXIDATION
FEACTION~
0~ SIMPLE
OXIDES
Numerous parameters have been considered up to now, to explain the catalytic properties of the oxidation catalysts formulaelikeelectron mobility, metallic cation co-ordination,specific metal-oxygen bonds, lattice oxygen mobility, etc. The basic Mars and Van Krevelen mechanism [l] involved in most partial oxidation reactions of olefins has to be considered at this point : the first step is the formation of an ally1 intermediate on a cationic site on the surface by the abstraction of one hydrogen atom of the organic molecule followed by the insertion of a lattice oxygen atom into the molecule and the second hydrogen abstraction. These two steps are now well established. The transition metal cations can intervene in a dualistic mechanism either as adsorption sites for the II allylic intermediate or as sites responsible for the oxygen insertion. This has been discussed largely in the case of MoBi. systems for which some dispute remains on the respective role of the two cations [I?]. On account of the Mars and Van Krevelen mechanism, it appears that electron and lattice oxygen mobilities,-i.e.the redox properties of the catalysts,are important factors, able to influence the catalytic activity. Now, these redox properties depend on the arrangement of the cations and anions into the bulk structure and of course on the surface of the crystal. Consequently, one may reasonably expect structure sensitivity of oxides for catalytic oxidation reactions. But the questions which first arise are : what is the surface atomic arrangement with respect to the bulk ? What role does this atomic arrangement play in catalysis and how can they be determined by the bulk or modified by an additive or a support ? The
main
difficulty is to prepare oxide crystals Large enough
to
be cut off
as for metal crystals or to change the natural growth of the crystals in order to obtain crystals with various shape and size. One of us has developed an original method of crystal growth for MoO3 and SbZ04 oxides, by oxyhydrolysis of ~ooC15and SbC15intercalatedbetween the graphite sheets [3,4]. For example at temperatures higher than 4OO*C, MOO3 crystallitesemerge from the graphite sheet with their (010) planes parallel to the (001) planes of graphite as shown in Fig. 1.
367
graphite Moo3 crystal
(010) planes)--
&g@ --*OOl)
planes
-
Fig. 1. Crystal growth process of Moo3 during oxyhydrolysis of G -MoC15 intercalation compound.
A transmission electronmicroscopy analysisperformedon solids prepared between 400 and 500°C has allowed us to measure the length of the MOO3 crystals along the different crystallographicdirections and thus to estimate the area of the different faces exposed to the reactant [5]_ Let us consider in Fig. 2, the three planes
(b : basal, s : side and a : apical)
of Moo3 crystal, whose relative amounts vary with the preparation conditions of the solid.
(001,101
or 109
Fig. 2. Identification of the different crystal faces by electron diffraction and scanning electron microscopy.
368 The activity
for a given reaction
is given by
:
z
A=Ax+AY+A
x, y, z being three different
products
of the catalytic
reaction
on the Moo3
crystal. If CL, S and y designate the proportions of the three different faces b, s, a b and a, A,, AS and A (i = x, y, z) the intrinsic activities of the reaction i 1 i products on the three faces expressed per unit area, one has : Ai = uA; + SAT
with
+ yA;
and the selectivity
for product
The ratio of selectivities
sx
-=
@As x
+
yAa x
b CXA Y
BA;
+
YA;
is : S
x,
This formula shows that if a product one face (b, for example) for example),
s S
0.
Ab
B
A;
-_xx
-2-c
Y
one has
y=l
of products
CXAb + x +
a+S+
(x
and another
= Ax/A x x and y is then
:
for example) is
exclusively
one
(y, for example)
formed on
on another
face (s,
:
If one plots the variations
of Sy/Sv vs. a/B which is experimentally
obtained,
one then gets a straight a product
line with a slope Ah/As going through the origin. If x Y is formed on several faces, the above relationship is more complex.
In the case of propylene
oxidation,
Fig. 3, the linear correlation
observed
for S acro/SC02 shows that acrolein is exclusively formed on the "", B/ci s (100) face and ~02 on the b (010) face, while in the case of isobutene oxidation
(Fig. 4) methacrolein
One may calculate for propene,
is mainly
the contribution
isobutene
formed also on the
and 1-butene partial
oxidation
It is worth noting that the rates of product to another
are reported
formation
differ
Such results in Table 1.
from one face
one, the (100) face being the most active in all cases. CO2 is formed
exclusively onthe
(010) face for propene
face for i-butene
oxidation.
allylic
(100) s face.
of each face to a given product.
oxidation,
Concerning
Acrolein,
are principally
the oxidation
and 1-butene oxidation
methacrolein
formed on the
dehydrogenating case of methanol
and dehydrating. and ethanol
Similar
oxidation
products
of
in the case of isopro-
while the
(100) face is both
results werepreviouslyobtained
[171.
(100)
(100) face.
of alcohols , we have observed
panol that the (010) Moo3 face is dehydrogenating
and on the
and butadiene,
in the
369
S
S acre
4.0
i- SC02
1.:
O.! j0
/
2.0
1.0
P/a
P/a
/ I
I
m
L
0.1
0
3.0
0
i
/
%02
J
1.c) -
0.5
0
Fig. 3. Linear correlation observed for propylene oxidation on Moo3 crystals (MoO3/graphite catalysts)
Table
1. Relative
intrinsic
PHC/P02/PHe
0.1
0.2
Fig. 4. Correlation isobutene oxidation catalysts
activities
= 76/152/530
Products
Reactants
It
MA
of the different
torr from ref
observed for on MoOj/graphite
faces of aMoO
(101)
and
acrolein carbon oxides Total
0.06 1.00 1.06
2.26 0 2.26
0.73 0 0.73
i-Butene
methacrolein carbon oxides acetone Total
0 0 0.06 0.06
0.55 1.00 0 1.55
0.13 0 0.06 0.19
1-Butene
butadiene carbon oxides Total
2.9 1 3.9
9.3 0 9.3
1.9 0 1.9
is therefore
clear that the configuration
and the stereochemistry
parameters
which
The knowledge the atomic
orientate
of the bulk
arrangements
of the organic
the oxidation
of the surface planes reactants
reactions
crystalline
structure
at the surface
of the
with
(loi)
a
S
Propene
crystals
I
0.4
[6] at 360 to 375'C.
Faces (100)
(010) b
0.3
of Moo3
are two important
and thus the selectivity.
of &loo3 allows one to describe (OlO), (100)
and (101) or ClOi,
w
370
planes and to show that chemical and geometrical are more favorable to allylic-type two types of molybdenum
Mo'O band
: these sites correspond
with the breaking
ii) dehydrogenating
Mo2 centers
under
(100) plane
catalytic
conditions,
on this face (Fig.5) to unsaturated
of the long MO-O bonds
MO cations
(2.25 A).
: these sites are associated with the short
(1.73 A) with a covalent
the (010) face
features of the
Indeed,
centers have to be considered
i) acidic Lewis Mel centers associated
oxidation.
(the MO = 0 band
character equivalent
to that observed
in
length is 1.67 n in this face).
a
b
Fig. 5. Structure of the (100) Moo3 face. a) (100) plane of the bulk crystalline structure b) Scheme of the propene adsorption site Mel and H abstracting
oxygen
Allylic oxidation should first proceed by the chemisorptionof olefin (basic reagent)
on the acidic Mel center,
and the further
on the dehydrogenatingO-M02 center [6].
capture of an allylic H atom
371 PARTIAL
OXIDATION
Tin-antimony
OF PROPYLENE
mixed oxide
has been industrially The subject
mixture
for oxidation
reviewed
of oxides
catalytic
point of view, tin-antimony
strongly
oxide catalysts
at low Sb content
depends
We have prepared
have been shown
and an heterogeneous
at high Sb content.
on time and temperature
in our laboratory
solutionsof
Sn Sb 0 catalysts
Sn4+ and Sb3+ chlorides
The
of calcina-
by coprecipitation
for solidscalcined reached
at 500°C,
ratios higher
the solid solution Best catalytic
grains,
properties
the surface
solution, of the
The structure
XPS
by
with a superficial b4]
connected
and EDX-STEM
to the step
showed beyond
of the c1 and
5+
dissolution are not
of
Sb enrichment
of the
[4], the improvement nucleation
The detailed
of
of Sb204
examination
the rutile structure
of
of the
or B Sb204 phase with a strong
(004) a Sb204 or (400) 8 Sb204 lines for 20 and 40% Sb/(Sb+Sn)r present common features 5+ rows for Sb and Sb3+ cations 15
study, it was concluded during the crystal
Sn (Sb) O2 solid solution grainsfor development
Such an orientation (001) a Sb204/graphite (84%) at variance
of the Sb
[13]. In so far as high
of the Sn Sb 0 oxide solution.
a spectrum
of
at high temperature presence
of these two polymorphs
crystallographic
preferential
formation
was associated
on Fig. 6 with alternative
preferentially
calcined
to show that large Sb204 crystals
for acrolein
was
Sb enrichment
and a(or 8) Sb204. By a selective
it was possible
temperature
for solids
with the simultaneous
the X ray lines of the best catalysts
promotion
the solid
131.
associated
grains as evidenced
the catalytic grainsat
beside
(>,750°C) a superficial
p,
results were obtained
very active nor selective calcination
10% at., Sb204 is obtained
temperature
Sn Sb 0 solid solution
catalyst
Sb5+ as high as 40% may be
b2],
than
occurred
and for high Sb/Sn ratio,
of aSb204
allowed
of Sb5+ into SnO2,
iii) for high calcination
enriched
concentrationsof
in the solid solution
solution
analysis
:
us to show that
ii) for Sb/Sn
of
by NH40H at pHs1. The solids were
then dried and fired at 500, 750 and 95O'C. Physicochemical
solid
[9].
@l].
aqueous
i)
of olefins
[lo].
(Sn Sb 0 solid, Sb204 and/or Sb6013)
of these oxides
formula which
and ammoxidation
by F.S. Berry
in a Sn Sb 0 solid solution
proportion tion
systemsconstituteanother
developed
has been recently
From a structural to consist
ON Sn Sb 0 AND Sn Sb Fe0
of the
that a Sb204 surface
grnjrth process
these intermediary
catalyst presenting
with non oriented
Sb204
of the
Sb/Sn ratios with a (100) 8 Sb204
by the catalytic a high selectivity (42%).
. From the radiolayer was oriented
of Sb204 at the surface
(001) CY Sb204 and/or
effect was confirmed
as can be seen
faces.
study of an oriented for acrolein
372
Sb V
OS
Sb III
8
I (001)
plane ,I
I I
Sb V
Sb Ill
z
I
(100)
X
plani Y I--
I I
P Sb204 Fig. 6. Structure
We studied observed
of the two polymorphs
also the influence
[4] that this additive
6 antimony
oxide phases
significant
modification
principally
increases
conditions improving Finally importance
of Fe as additive
[15]
to the Sn Sb 0 system.
changes the relative
concentration
and the Sn Sb 0 solid solution particle of the catalytic
the stability
properties.
of the material
with a less easy elimination
It was
of the u or size without
It was concluded
that Fe
with time under working
of Sb from the solid solution,
thus
the life time of the catalysts. our results on Sn Sb 0 and Sn Sb Fe0 systems of activation
temperature
Sb oxide layers on the surface yield
of Sb204 from ref.
very efficient
catalysts
and duration
of solid solution with
structure
show clearly
in the formation
of surface
This may 2' features.
of Sb 5+ into SnO sensitivity
the great
a
373 BISMUTH
MOLYBDATES
Several
phases of bismuth
type structure se, Bi2MoO6 without
with ordered
koechlinite,and
vacancies,
ammoxidation
molybdates, cationic
the X-phase,
are knowntobe
ofolefins
such as the a-phase,
vacancifs,the
[16-181
Bi3M02Fe012
excellent
The main question
molecu1e.A gnized,
oxygen
redox mechanism
while cationic
a 71 ally1 complex
an excess
proposed
that in general
catalytic
an equimolar are better detected
CY + ycoprecipitate
than those of either
which
reaction,
supposed
all solids exhibiting
6 The difference
weaker
between
favors high catalytic we have observed one component
[18] that catalytic
mixture
with one
of both components.
namely Bi2Mo06
(y-phase) [20].
of the free
to Moo6 octahedra.
This results
in
Bi and 0 from Moo4 layer than in the case of Sb. The oxygen expected
to be larger in Moo4 layers for Bi2MOO6
mild oxidation,Sb2MoO6
than Bi2MoO6
determine
It was observed
and Bi2Mo06
in accordance
Comparison
at low conversion products
than
for
the reoxidation lability
properties
on the relative
plateau
i-butene
for methacrolein is observed
catalysts
is difficult
that the rate of formation of olefin
at constant
formation
while
and oxygen
study
of
over the rate of
oxygen pressure.
a regular
for CO2 when increasing
since
in a kinetic
the plot of the formation
pressure
for
above.
role. For instance
pressure
by
are similar
rate is ten times weaker
as described
bl]
Fig. 8 represents
and CO2 versus
and that of reoxidation
of different
level it was observed
is obtained
an asymptotic
step by propene
(88%) for
allows one to
that the former rates of reduction
with oxygen
For instance
methacrolein
study of the reaction
may play a determining
is dependent
catalysts.
while
of catalytic
conditions
is less active but more selective
(52%).A kinetic
the rate of reduction
oxygen.
for Sb2MOO6
Amaximum
molybdates
This synergy effect morphology
is the orientation
In propene
reaction
properties
(Fig. 7).
acrolein
Sb2MOO6
When preparing
(a or y). The same effect has been
of the same phases.
two isomorphic
have shown that
properties.
these two structures
bonds between
gaseous
as
lone pair on Bi or Sb with respect
lability is therefore Sb2MoO4
reco-
ascheelite-typestruc-
, which exhibit M202 layers alternate with Moo6 octahedra
and Sb2Mo0
electron
[l] is widely
Moreover XFSmeasurements
laying on top of the other but from an intimate recently
oxygens having in the product
in part I.
p8] for a mechanicalmixture
We have studied
lattice
and
arises is
to adsorb the olefin
has also been shown by XPS to stem not from a geode-type phase
structure oxidation
immediately
by Mars and Van Krevelen
behaviour.
of MO at the surface
for partial
[19] to be incorporated
site as MO is usually
[2] as described
We have observed ture have better
experiments
Bi MO 0 scheelite2 312 Bi2Mo209, the y-pha-
scheelite-type
catalysts
what role do both cations play in the catalytic been shown by isotopic
B-phase,
increase
i-butene
up to
pressure.
374
B~aMo06 9
Sb&a&, l
5
R -
4 3 -
1. A.
O-
Fig. 7. Schematic
representation
Of Sb2 MO 06 and Bi2MoO6
melhacrolein
50 i - butene
pressure
structures.
h
75 l
(tow)
Fig. 8. Rate of methacrolein and CO2 formation at 360°C versus i-butene = 100 torg) for the X-phase pressure at constant oxygen pressure (P 02 (Bi3M02FeO12).
partial
375 It follows
that selectivity
to oxygen pressure.
is greatly
Moreover
the maximum
L21] _ For instance if oneextrapolates following
rrder in selectivity
at other i-autene literature
on olefin pressure
position
i-butene
is dependent
pressure
tozero
with respect
on the samples one may define
the
X > > u + y > c1 > y > B_ This order is different
pressures
which
can explain
discrepancies
observed
in
data.
Perovskites catalysts. active
relative
dependent
may also be used as catalysts
However
such materialsformed
and selective
in partial
particularly
with transition
oxidation
reactions
as total oxidation metal ions knownto
have been synthesized
be [22]
as A3+ M5+ M6+ 0 where A = Sb or Bi and M = MO or W. Their structure results 4 8 12 62 from an intergrowth between an hexagonal bronze and a perovskite and exhibits a direct
filiation
character. the surface
Table
with the y-phase
Their catalytic area being
2. Features
koechlinite(Bi2Mo06)
not measurable
of orthorhombic
properties are summarized 2 -1 (S
bronzes.
Catalytic
low conversion ( 1%) at 4OO'C with -1 -1 * in mol s g x 108
Compounds
MO 0
and a quasi-metallic
and electric
in Table
data were obtained
2, the
at
P = 100/100/560torr C3"6 'P02'Pii2
Selectivity for acrolein (8)
Rate of * acrolein formation
Electrical characteristics
67
41.3
Bi 4 Mo20 '62
79
3.1
semi conductor
p
Bi4 '20 '62
51
2.3
semi conductor
n
81
11.9
Mott conductor
53
1.5
semi conductor
0.2
semi metal
Bi
2
Bi4 '16
6
MO
4
0
62
Sb4 Mo20 '62 Sb4 '20 '62
Catalytic
activity
is rather
the nature
of cations,
industrial
catalysts.
Bi
4 These
W
low whereas
selectivity
being
in acrolein
depends
the best, but far inferior
on
to
16Mo 4 ' 62 data show that for the same atomic arrangements,
catalytic
properties
favorable
effect when both Wand
are related
to the nature Bi cations
of the cations with a peculiar
are present.
p
376
MULTICOMPONENT
CATALYSTS
This class of catalysts presents
very interesting
ammoxidation to complex
developed
in the seventies,
catalytic
properties
high activity
with chemical
formulae
and high selectivity
such as for instance
formula
:
catalyst
was patented
techniques
Bil Fe
following
andBi3Fe
the same procedure,
Mo03 for lo-15 % was much
less
acrolein ?r 80% and activity did not show a geode-type
role. For example,
by Rhdne Poulenc
attractive
23
with the following
and selectivity
(XRD and IR principally)
CoMo04 for 85%, Bi2 Mo3 012(c-phase) even prepared
and
Co7 Nil B2 Sboe2 12 3 that the synergy effect mentionned above for an
Bi Fe Cola Mo12 0 x with high activity
However physical
by Sohio,
olefin oxidation
(>>90%). It corresponds
MO
It is obvious Ko.07 Rbo.07 Ox. intimate mixture of a and y phases play an important a multicomponent
particularly
for partial
allowed
Mo2 O12(phase containing
up to 99%.
one to detect X): Co10Mo12
for catalytic
with
for instance
oxide,
(a+b)CoMoO 4 for 80% and use
(selectivity
30 times less) in the same conditions.
structure
(a+b)
in
XPS analysis
the c- or X-phase
at the
surface. However
a kinetic
study of these samples by varying
relative to oxygen pressure those of X- and a-phases quite
different
dispersed (sensitive
to high order)
an 'onion'
a core of CoMo04
components
of the catalyst
been proposed.
and the y-phase
For instance
by the presence
no direct way exists to unambiguously the dispersion.
that such
layer
in structure
sensitive
identify
at the
reactions
such
[26]
Co8 Fe3 Bi MoI3 0, with
It may also be possible of small amounts
of other
in finding the truth is that
suchbnion'top
It seems however
exists
at low concentration.
Wolf andBatist
compound
as an active phase.
are modified
oxide
while it is
grain in a very well
allow one to detect such phases
as Bi203 or Fe 20 3 r271. The difficulty
a top
between
XPS nor IR (sensitive to local order) nor XP.D
then to measure
ties
at low olefin pressure
layer of a muticomponent
that CoMoO4 properties
pressure
Such a feature may be due to the presence
at the surface
layer since neither
Other explanationshave suggested
with a maximum
for other materials.
of the X- or a- phases
the propene
[21] gives curves which are intermediate
reasonable
surface
layer and to us to suggest
and modifies
as partial
catalytic
proper-
oxidation.
CONCLUSIONS The main conclusion reactions
on metallic
sions developed
which may be
by Boudart
of oxides are therefore their size relative
drawn from our work is that oxidation
oxides are structure
sensitive.
in the sixties for metals.
related to the chemical
to surface
This expands The catalytic
the concluproperties
nature of the reactants
atomic arrangements.
Such arrangements
and to
are
377 obviously
dependent
parameters
type mechanism. two important Comoarison
For instance
which in turn determine
involving
other
a redox Mars and Van Yrevelen
lattice oxygen mobility
and electron
mobility
are
of such materials.
of samoles
formed with the same transition metal ions, as 3+ 5+ E6' o with A= Sb or Bi bronzes A4 M8 12 62 shows clearly that for similar structures (isomorphism) the
MO 06 and orthorhombic
and M = MO or W catalytic
structure
in such reactions
characteristics
Sb2 MO 06/Bi2
pair
on the bulk oxide
important
properties
location
are dependent
on the nature of the cations
and on the lone
in space. MO 0 our results, particularly type catalysts as BiFeCo 10 12x study since XRD, IR and XPS techniques did not give striking and
For multicomponent of a kinetics
seem to indicate
clear cut informations, bismuth
molybdate,
on Co molybdate
presumably
that the catalyst
the X-phase
as a support.
or a mixture
is composed
of X- and a-phases
We arrive at an identical
conclusion
Sn Sb 0 and Sn Sb Fe 0 systems with an active phase of Sb oxide .. .)
on the Sb" in SnO
stucture
is not clearly
possibility
However
solid solutibn.
2
demonstrated
of modification
for
(Sb204, Sb6 013.
such an onion-like
and one cannot
of molybdate
of a
completely
by addition
rule out the
of small amounts
of Bi
and Fe. Data on single
crystal-type
fitting
led us to suggest
surface
atomic
arrangements
catalysts
arrangements
are different
favorable
therefore
be controlled rather obvious
we
factor in partial oxides
activation
the improvement
reactions.
and even multicomponent
may be developed
with
Such
as a support.
of such a support
catalysts.
on the catalyst
to
Such a conclusion in the
here may help in the
of catalysts
of industrial
atomic arrangements
interest.
are a determining
This may hold true for simple oxides, oxides
surface,
as well,since
depending
specific
on preparation
faces
and/or
conditions.
1
P.Marsand
2
R. Grasselli, J.D. Burrington 72 (19811 203.
D.W.VanL(revelen, Chem. Eng. SC., 3 (1954) and J.F. Brazdil,
It
and the
parameters
know that aspect very well, particularly
the idea that surface
oxidation
properties.
are two very important
think that the idea presented
and hopefully
oxide-oxide
one is dealing
can be considered
and structure
of the catalysts
since chemists
We want to emphasize
complex
reaction
in order to obtain highly performing
industry. HOweVer understanding
to catalytic
growth on what
turns out that the nature conditions
structural
catalysts
from those of the buik but depend on them and are
formed by a kind of epitaxial
preparation
and peculiar
that in multicomponent
41.
Discussion
Faraday
Sot.,
is
378
3
J.C. Volta, Catal. L&t.
Desquesnes, B. Moraweck and G. Coudurier, React. Kinet. 12 (1979) 241.
4
J.C. Volta, B. Benaichouba, I. S (1983) 215.
5
J.C. Volta, W. Desquesnes, B. Moraweck and J.M. Tatibouet, Proceedings 7th -_cIInternational Conqress on Catalysis, Tokyo 1980, p. 1398, Kodansha/Elsevier,Tokyo/Amsterdam, 1981.
6
J.C. Volta, J.M. Tatibouet, C. Phichitkul and J.E. Germain, Proceedings 8th International Congress on Catalysis, Berlin,1984, Vol IV, 431, _____.--Verlag C!hemie, Weinhein, 1984.
7
J.M. Tatibouet and J.E. Germain, J. Catal., 72 (1981) 365. J.M. Tatibouet, J.E. Germain and J.C. Volta, J. Catal. 82 (1983) 240.
8
J.C. Volta and J.M. Tatibouet, J. Catal. 93
9
Distillers Co Ltd, Brit. Pat. 1953, 876446 ; 1961, 864666 ; 1962, 920952 ; 1965, 997490 ; US Pat. 1963, 3094565, Belg. Pat., 1963, 630153 ; French _.__._ Pat. 1965, 1429477 ; 1966, 1471983 ; Ugine Kuhlmann Co Ltd., French Pat., 1962, 1293088.
W.
Mutinand J.C. Vldrine, Appl. Catal.
(1985) (in press).
10 F.J. Berry, Adv. Catal., 30 (1981) 97. 11 D.R. Pyke, R. Reid and R.J.D. Tilley, J. Chem. Sot., Faraday I, 76 (1980) 1174. 12 S.C. Volta, P. Bussiere, G. Coudurier, J.M. Hermann and J.C. Vedrine, Appl. Catal. 16 (1985) 315. 13 J.C. Volta, G. Coudurier, I. Mutin and J.C. Vedrine, J. Chem. Sot., Chem. a (1982) 1044. 14 Y. Boudeville, F. Figueras, M. Forissier, J.L. Portefaix and J.C. Vedrine, J. Catal., 58 (1979) 52. 15 D. Rogers and A.C. Skapski, Proc. Chem. Sot. (1964) 400. P.S. Gopalakrishnan and M. Manohar, Cryst. Struct. Comm., G. Thornton, Acta. Cryst. B33 (1977) 1271.
4
(1975)
203.
16 See for instance : R.K. Grasselli and J.D. Burrington, Adv. Catal. 30 (1981) 133 and referencestherein. 17 B. Gxzybowska,A. Mazurkiewicz and J. S&oczynski, ADwl. Catal. 13 (1985) 223. J.R. Burrington, C.T. Kartisek and R.K. Grasselli J. Catal. 87 (1984) 363, 18 D. Carson, G. Coudurier, M. Forissier, J.C. Vedrine, A. Laarif and F. Theobald, J. Chem. Sot. Faraday Trans.1, 79 (1983) 1921. 19 G.W. Keulks and L.D. Krenzke J. Catal. 61 (1980) 316. 20
F. Theobald, A. Laarif and M. Forissier, Submitted to J. Catal. 1985.
21
D. Carson, M. Forissier and J.C. Vedrine, J. Chem. Sot. Faraday Trans. I, 80, (1984) 1017.
22
M. Dion, Dr es Sciences thesis, Nantes (1984);
23
J.C.Daumas, J.Y. Derrien and F. Van Den Bussche, French Patent (1976) 2,364,061.
24
J.C. Volta and J.L. Portefaix, APP~. Catal. 1985, in press. 27 (1979) 141.
25 B. Grzybowska and A. Mazurkiewicz, Bull. Acad. Pol. Sci. 26 N.W. Wolf and Ph. A. Batist, 3. Catal. 32 (1974) 25.
27 O.V. Isaev, L. Ya. Margolis and I. Ya. Kushnerev, Zh.Fiz.Khim, 47 (1973)2127.
Chemistr!,
CATALYTIC
and Physics,
PROPERTIES
J. C. VEDRINE,
13 (1985)
OF -METALLIC OXIDES
G. COUDUPJER,
365
365-378
IN PARTIAL
M. FORISSIER
Institut de Recherches sur la Catalyse, 2 Av. A. Einstein, F 69626 Villeurbanne
OXIDATION
REACTIONS
and J.C. VOLTA
CNPS, aSsOCie (France)
a l'UCB, Lyon I
ABSTRACT
The purpose of this survey is to presentrecentexperimental results and main ideas from our Laboratory in the field of metallic oxides. Structure sensitivity of oxidation reactions on simple oxides as Moo3 and Sb2Oq is clearlydemonstrated and is shown to depend on the surface atomic arrangements related to bulk structure, on the nature of the transition metal cations and on the chemical properties and size of the reactant molecules. Catalytic properties of isomorphous oxides as Sb2Moo6 and Bi2MoO6 or orthorhombic bronzes A~+M~M$~O62 with A=Sb or Bi and M=Mo or W are presented and discussed in terms of electron lone pair location, lattice oxygen lability and nature of transition metal ions. Extension is presented to multicomponent catalysts and a special emphasis is placed upon structural fitting between active phases and a support, and upon the importance of catalyst preparation conditions.
INTRODUCTION The importance
of oxides
Indeed
it is a consequence
oxides
are directly
active phase.
cracking
and reforming of catalytic
classifed
as insulators
mina, silica-alumina, acidic or basic
reactions
molecules
processes.
This explains
oxidation
based on oxides
and as semi conductors.
magnesia,
which
so far.
chemistry.
for another
Many
catalytic
are the most economical
stemming
way
from cracking,steam-
the importance
of industrial
as catalysts.Oxides The insulators
clays, zeolites...
are principally
or total oxidation
0254-0584/85/$3.30
oxidation
based
may be
silica,
are principally
alu-
catalystsfor
type reactions.
Semi conductors in partial
partial
of petroleum
or as supports
the simple hydrocarbon
processes
has not to be demonstrated
of the success
used as catalysts
Moreover,
to functionalize
in catalysis
reactions.
transition We will
metal ion oxides
focus our interest
are involved in this
0 ElsevierSequoia/PrintedinThe Netherlands
366
survey on
transition metal oxide systems studied in our laboratory which
intervene as a basic component of most industrial mild oxidation processes. From the recent demonstration in our laboratory that oxidation reactions are sensitive to the structure of simple oxides, we will then discuss mixed oxides and multicomponentmixed oxide
catalysts and show how the catalytic properties
of these systems can be influenced by the superficial structure of the different crystallographicphases.
STRUCTURE
SENSITIVITY
0~ PAHIIAL OXIDATION
FEACTION~
0~ SIMPLE
OXIDES
Numerous parameters have been considered up to now, to explain the catalytic properties of the oxidation catalysts formulaelikeelectron mobility, metallic cation co-ordination,specific metal-oxygen bonds, lattice oxygen mobility, etc. The basic Mars and Van Krevelen mechanism [l] involved in most partial oxidation reactions of olefins has to be considered at this point : the first step is the formation of an ally1 intermediate on a cationic site on the surface by the abstraction of one hydrogen atom of the organic molecule followed by the insertion of a lattice oxygen atom into the molecule and the second hydrogen abstraction. These two steps are now well established. The transition metal cations can intervene in a dualistic mechanism either as adsorption sites for the II allylic intermediate or as sites responsible for the oxygen insertion. This has been discussed largely in the case of MoBi. systems for which some dispute remains on the respective role of the two cations [I?]. On account of the Mars and Van Krevelen mechanism, it appears that electron and lattice oxygen mobilities,-i.e.the redox properties of the catalysts,are important factors, able to influence the catalytic activity. Now, these redox properties depend on the arrangement of the cations and anions into the bulk structure and of course on the surface of the crystal. Consequently, one may reasonably expect structure sensitivity of oxides for catalytic oxidation reactions. But the questions which first arise are : what is the surface atomic arrangement with respect to the bulk ? What role does this atomic arrangement play in catalysis and how can they be determined by the bulk or modified by an additive or a support ? The
main
difficulty is to prepare oxide crystals Large enough
to
be cut off
as for metal crystals or to change the natural growth of the crystals in order to obtain crystals with various shape and size. One of us has developed an original method of crystal growth for MoO3 and SbZ04 oxides, by oxyhydrolysis of ~ooC15and SbC15intercalatedbetween the graphite sheets [3,4]. For example at temperatures higher than 4OO*C, MOO3 crystallitesemerge from the graphite sheet with their (010) planes parallel to the (001) planes of graphite as shown in Fig. 1.
367
graphite Moo3 crystal
(010) planes)--
&g@ --*OOl)
planes
-
Fig. 1. Crystal growth process of Moo3 during oxyhydrolysis of G -MoC15 intercalation compound.
A transmission electronmicroscopy analysisperformedon solids prepared between 400 and 500°C has allowed us to measure the length of the MOO3 crystals along the different crystallographicdirections and thus to estimate the area of the different faces exposed to the reactant [5]_ Let us consider in Fig. 2, the three planes
(b : basal, s : side and a : apical)
of Moo3 crystal, whose relative amounts vary with the preparation conditions of the solid.
(001,101
or 109
Fig. 2. Identification of the different crystal faces by electron diffraction and scanning electron microscopy.
368 The activity
for a given reaction
is given by
:
z
A=Ax+AY+A
x, y, z being three different
products
of the catalytic
reaction
on the Moo3
crystal. If CL, S and y designate the proportions of the three different faces b, s, a b and a, A,, AS and A (i = x, y, z) the intrinsic activities of the reaction i 1 i products on the three faces expressed per unit area, one has : Ai = uA; + SAT
with
+ yA;
and the selectivity
for product
The ratio of selectivities
sx
-=
@As x
+
yAa x
b CXA Y
BA;
+
YA;
is : S
x,
This formula shows that if a product one face (b, for example) for example),
s S
0.
Ab
B
A;
-_xx
-2-c
Y
one has
y=l
of products
CXAb + x +
a+S+
(x
and another
= Ax/A x x and y is then
:
for example) is
exclusively
one
(y, for example)
formed on
on another
face (s,
:
If one plots the variations
of Sy/Sv vs. a/B which is experimentally
obtained,
one then gets a straight a product
line with a slope Ah/As going through the origin. If x Y is formed on several faces, the above relationship is more complex.
In the case of propylene
oxidation,
Fig. 3, the linear correlation
observed
for S acro/SC02 shows that acrolein is exclusively formed on the "", B/ci s (100) face and ~02 on the b (010) face, while in the case of isobutene oxidation
(Fig. 4) methacrolein
One may calculate for propene,
is mainly
the contribution
isobutene
formed also on the
and 1-butene partial
oxidation
It is worth noting that the rates of product to another
are reported
formation
differ
Such results in Table 1.
from one face
one, the (100) face being the most active in all cases. CO2 is formed
exclusively onthe
(010) face for propene
face for i-butene
oxidation.
allylic
(100) s face.
of each face to a given product.
oxidation,
Concerning
Acrolein,
are principally
the oxidation
and 1-butene oxidation
methacrolein
formed on the
dehydrogenating case of methanol
and dehydrating. and ethanol
Similar
oxidation
products
of
in the case of isopro-
while the
(100) face is both
results werepreviouslyobtained
[171.
(100)
(100) face.
of alcohols , we have observed
panol that the (010) Moo3 face is dehydrogenating
and on the
and butadiene,
in the
369
S
S acre
4.0
i- SC02
1.:
O.! j0
/
2.0
1.0
P/a
P/a
/ I
I
m
L
0.1
0
3.0
0
i
/
%02
J
1.c) -
0.5
0
Fig. 3. Linear correlation observed for propylene oxidation on Moo3 crystals (MoO3/graphite catalysts)
Table
1. Relative
intrinsic
PHC/P02/PHe
0.1
0.2
Fig. 4. Correlation isobutene oxidation catalysts
activities
= 76/152/530
Products
Reactants
It
MA
of the different
torr from ref
observed for on MoOj/graphite
faces of aMoO
(101)
and
acrolein carbon oxides Total
0.06 1.00 1.06
2.26 0 2.26
0.73 0 0.73
i-Butene
methacrolein carbon oxides acetone Total
0 0 0.06 0.06
0.55 1.00 0 1.55
0.13 0 0.06 0.19
1-Butene
butadiene carbon oxides Total
2.9 1 3.9
9.3 0 9.3
1.9 0 1.9
is therefore
clear that the configuration
and the stereochemistry
parameters
which
The knowledge the atomic
orientate
of the bulk
arrangements
of the organic
the oxidation
of the surface planes reactants
reactions
crystalline
structure
at the surface
of the
with
(loi)
a
S
Propene
crystals
I
0.4
[6] at 360 to 375'C.
Faces (100)
(010) b
0.3
of Moo3
are two important
and thus the selectivity.
of &loo3 allows one to describe (OlO), (100)
and (101) or ClOi,
w
370
planes and to show that chemical and geometrical are more favorable to allylic-type two types of molybdenum
Mo'O band
: these sites correspond
with the breaking
ii) dehydrogenating
Mo2 centers
under
(100) plane
catalytic
conditions,
on this face (Fig.5) to unsaturated
of the long MO-O bonds
MO cations
(2.25 A).
: these sites are associated with the short
(1.73 A) with a covalent
the (010) face
features of the
Indeed,
centers have to be considered
i) acidic Lewis Mel centers associated
oxidation.
(the MO = 0 band
character equivalent
to that observed
in
length is 1.67 n in this face).
a
b
Fig. 5. Structure of the (100) Moo3 face. a) (100) plane of the bulk crystalline structure b) Scheme of the propene adsorption site Mel and H abstracting
oxygen
Allylic oxidation should first proceed by the chemisorptionof olefin (basic reagent)
on the acidic Mel center,
and the further
on the dehydrogenatingO-M02 center [6].
capture of an allylic H atom
371 PARTIAL
OXIDATION
Tin-antimony
OF PROPYLENE
mixed oxide
has been industrially The subject
mixture
for oxidation
reviewed
of oxides
catalytic
point of view, tin-antimony
strongly
oxide catalysts
at low Sb content
depends
We have prepared
have been shown
and an heterogeneous
at high Sb content.
on time and temperature
in our laboratory
solutionsof
Sn Sb 0 catalysts
Sn4+ and Sb3+ chlorides
The
of calcina-
by coprecipitation
for solidscalcined reached
at 500°C,
ratios higher
the solid solution Best catalytic
grains,
properties
the surface
solution, of the
The structure
XPS
by
with a superficial b4]
connected
and EDX-STEM
to the step
showed beyond
of the c1 and
5+
dissolution are not
of
Sb enrichment
of the
[4], the improvement nucleation
The detailed
of
of Sb204
examination
the rutile structure
of
of the
or B Sb204 phase with a strong
(004) a Sb204 or (400) 8 Sb204 lines for 20 and 40% Sb/(Sb+Sn)r present common features 5+ rows for Sb and Sb3+ cations 15
study, it was concluded during the crystal
Sn (Sb) O2 solid solution grainsfor development
Such an orientation (001) a Sb204/graphite (84%) at variance
of the Sb
[13]. In so far as high
of the Sn Sb 0 oxide solution.
a spectrum
of
at high temperature presence
of these two polymorphs
crystallographic
preferential
formation
was associated
on Fig. 6 with alternative
preferentially
calcined
to show that large Sb204 crystals
for acrolein
was
Sb enrichment
and a(or 8) Sb204. By a selective
it was possible
temperature
for solids
with the simultaneous
the X ray lines of the best catalysts
promotion
the solid
131.
associated
grains as evidenced
the catalytic grainsat
beside
(>,750°C) a superficial
p,
results were obtained
very active nor selective calcination
10% at., Sb204 is obtained
temperature
Sn Sb 0 solid solution
catalyst
Sb5+ as high as 40% may be
b2],
than
occurred
and for high Sb/Sn ratio,
of aSb204
allowed
of Sb5+ into SnO2,
iii) for high calcination
enriched
concentrationsof
in the solid solution
solution
analysis
:
us to show that
ii) for Sb/Sn
of
by NH40H at pHs1. The solids were
then dried and fired at 500, 750 and 95O'C. Physicochemical
solid
[9].
@l].
aqueous
i)
of olefins
[lo].
(Sn Sb 0 solid, Sb204 and/or Sb6013)
of these oxides
formula which
and ammoxidation
by F.S. Berry
in a Sn Sb 0 solid solution
proportion tion
systemsconstituteanother
developed
has been recently
From a structural to consist
ON Sn Sb 0 AND Sn Sb Fe0
of the
that a Sb204 surface
grnjrth process
these intermediary
catalyst presenting
with non oriented
Sb204
of the
Sb/Sn ratios with a (100) 8 Sb204
by the catalytic a high selectivity (42%).
. From the radiolayer was oriented
of Sb204 at the surface
(001) CY Sb204 and/or
effect was confirmed
as can be seen
faces.
study of an oriented for acrolein
372
Sb V
OS
Sb III
8
I (001)
plane ,I
I I
Sb V
Sb Ill
z
I
(100)
X
plani Y I--
I I
P Sb204 Fig. 6. Structure
We studied observed
of the two polymorphs
also the influence
[4] that this additive
6 antimony
oxide phases
significant
modification
principally
increases
conditions improving Finally importance
of Fe as additive
[15]
to the Sn Sb 0 system.
changes the relative
concentration
and the Sn Sb 0 solid solution particle of the catalytic
the stability
properties.
of the material
with a less easy elimination
It was
of the u or size without
It was concluded
that Fe
with time under working
of Sb from the solid solution,
thus
the life time of the catalysts. our results on Sn Sb 0 and Sn Sb Fe0 systems of activation
temperature
Sb oxide layers on the surface yield
of Sb204 from ref.
very efficient
catalysts
and duration
of solid solution with
structure
show clearly
in the formation
of surface
This may 2' features.
of Sb 5+ into SnO sensitivity
the great
a
373 BISMUTH
MOLYBDATES
Several
phases of bismuth
type structure se, Bi2MoO6 without
with ordered
koechlinite,and
vacancies,
ammoxidation
molybdates, cationic
the X-phase,
are knowntobe
ofolefins
such as the a-phase,
vacancifs,the
[16-181
Bi3M02Fe012
excellent
The main question
molecu1e.A gnized,
oxygen
redox mechanism
while cationic
a 71 ally1 complex
an excess
proposed
that in general
catalytic
an equimolar are better detected
CY + ycoprecipitate
than those of either
which
reaction,
supposed
all solids exhibiting
6 The difference
weaker
between
favors high catalytic we have observed one component
[18] that catalytic
mixture
with one
of both components.
namely Bi2Mo06
(y-phase) [20].
of the free
to Moo6 octahedra.
This results
in
Bi and 0 from Moo4 layer than in the case of Sb. The oxygen expected
to be larger in Moo4 layers for Bi2MOO6
mild oxidation,Sb2MoO6
than Bi2MoO6
determine
It was observed
and Bi2Mo06
in accordance
Comparison
at low conversion products
than
for
the reoxidation lability
properties
on the relative
plateau
i-butene
for methacrolein is observed
catalysts
is difficult
that the rate of formation of olefin
at constant
formation
while
and oxygen
study
of
over the rate of
oxygen pressure.
a regular
for CO2 when increasing
since
in a kinetic
the plot of the formation
pressure
for
above.
role. For instance
pressure
by
are similar
rate is ten times weaker
as described
bl]
Fig. 8 represents
and CO2 versus
and that of reoxidation
of different
level it was observed
is obtained
an asymptotic
step by propene
(88%) for
allows one to
that the former rates of reduction
with oxygen
For instance
methacrolein
study of the reaction
may play a determining
is dependent
catalysts.
while
of catalytic
conditions
is less active but more selective
(52%).A kinetic
the rate of reduction
oxygen.
for Sb2MOO6
Amaximum
molybdates
This synergy effect morphology
is the orientation
In propene
reaction
properties
(Fig. 7).
acrolein
Sb2MOO6
When preparing
(a or y). The same effect has been
of the same phases.
two isomorphic
have shown that
properties.
these two structures
bonds between
gaseous
as
lone pair on Bi or Sb with respect
lability is therefore Sb2MoO4
reco-
ascheelite-typestruc-
, which exhibit M202 layers alternate with Moo6 octahedra
and Sb2Mo0
electron
[l] is widely
Moreover XFSmeasurements
laying on top of the other but from an intimate recently
oxygens having in the product
in part I.
p8] for a mechanicalmixture
We have studied
lattice
and
arises is
to adsorb the olefin
has also been shown by XPS to stem not from a geode-type phase
structure oxidation
immediately
by Mars and Van Krevelen
behaviour.
of MO at the surface
for partial
[19] to be incorporated
site as MO is usually
[2] as described
We have observed ture have better
experiments
Bi MO 0 scheelite2 312 Bi2Mo209, the y-pha-
scheelite-type
catalysts
what role do both cations play in the catalytic been shown by isotopic
B-phase,
increase
i-butene
up to
pressure.
374
B~aMo06 9
Sb&a&, l
5
R -
4 3 -
1. A.
O-
Fig. 7. Schematic
representation
Of Sb2 MO 06 and Bi2MoO6
melhacrolein
50 i - butene
pressure
structures.
h
75 l
(tow)
Fig. 8. Rate of methacrolein and CO2 formation at 360°C versus i-butene = 100 torg) for the X-phase pressure at constant oxygen pressure (P 02 (Bi3M02FeO12).
partial
375 It follows
that selectivity
to oxygen pressure.
is greatly
Moreover
the maximum
L21] _ For instance if oneextrapolates following
rrder in selectivity
at other i-autene literature
on olefin pressure
position
i-butene
is dependent
pressure
tozero
with respect
on the samples one may define
the
X > > u + y > c1 > y > B_ This order is different
pressures
which
can explain
discrepancies
observed
in
data.
Perovskites catalysts. active
relative
dependent
may also be used as catalysts
However
such materialsformed
and selective
in partial
particularly
with transition
oxidation
reactions
as total oxidation metal ions knownto
have been synthesized
be [22]
as A3+ M5+ M6+ 0 where A = Sb or Bi and M = MO or W. Their structure results 4 8 12 62 from an intergrowth between an hexagonal bronze and a perovskite and exhibits a direct
filiation
character. the surface
Table
with the y-phase
Their catalytic area being
2. Features
koechlinite(Bi2Mo06)
not measurable
of orthorhombic
properties are summarized 2 -1 (S
bronzes.
Catalytic
low conversion ( 1%) at 4OO'C with -1 -1 * in mol s g x 108
Compounds
MO 0
and a quasi-metallic
and electric
in Table
data were obtained
2, the
at
P = 100/100/560torr C3"6 'P02'Pii2
Selectivity for acrolein (8)
Rate of * acrolein formation
Electrical characteristics
67
41.3
Bi 4 Mo20 '62
79
3.1
semi conductor
p
Bi4 '20 '62
51
2.3
semi conductor
n
81
11.9
Mott conductor
53
1.5
semi conductor
0.2
semi metal
Bi
2
Bi4 '16
6
MO
4
0
62
Sb4 Mo20 '62 Sb4 '20 '62
Catalytic
activity
is rather
the nature
of cations,
industrial
catalysts.
Bi
4 These
W
low whereas
selectivity
being
in acrolein
depends
the best, but far inferior
on
to
16Mo 4 ' 62 data show that for the same atomic arrangements,
catalytic
properties
favorable
effect when both Wand
are related
to the nature Bi cations
of the cations with a peculiar
are present.
p
376
MULTICOMPONENT
CATALYSTS
This class of catalysts presents
very interesting
ammoxidation to complex
developed
in the seventies,
catalytic
properties
high activity
with chemical
formulae
and high selectivity
such as for instance
formula
:
catalyst
was patented
techniques
Bil Fe
following
andBi3Fe
the same procedure,
Mo03 for lo-15 % was much
less
acrolein ?r 80% and activity did not show a geode-type
role. For example,
by Rhdne Poulenc
attractive
23
with the following
and selectivity
(XRD and IR principally)
CoMo04 for 85%, Bi2 Mo3 012(c-phase) even prepared
and
Co7 Nil B2 Sboe2 12 3 that the synergy effect mentionned above for an
Bi Fe Cola Mo12 0 x with high activity
However physical
by Sohio,
olefin oxidation
(>>90%). It corresponds
MO
It is obvious Ko.07 Rbo.07 Ox. intimate mixture of a and y phases play an important a multicomponent
particularly
for partial
allowed
Mo2 O12(phase containing
up to 99%.
one to detect X): Co10Mo12
for catalytic
with
for instance
oxide,
(a+b)CoMoO 4 for 80% and use
(selectivity
30 times less) in the same conditions.
structure
(a+b)
in
XPS analysis
the c- or X-phase
at the
surface. However
a kinetic
study of these samples by varying
relative to oxygen pressure those of X- and a-phases quite
different
dispersed (sensitive
to high order)
an 'onion'
a core of CoMo04
components
of the catalyst
been proposed.
and the y-phase
For instance
by the presence
no direct way exists to unambiguously the dispersion.
that such
layer
in structure
sensitive
identify
at the
reactions
such
[26]
Co8 Fe3 Bi MoI3 0, with
It may also be possible of small amounts
of other
in finding the truth is that
suchbnion'top
It seems however
exists
at low concentration.
Wolf andBatist
compound
as an active phase.
are modified
oxide
while it is
grain in a very well
allow one to detect such phases
as Bi203 or Fe 20 3 r271. The difficulty
a top
between
XPS nor IR (sensitive to local order) nor XP.D
then to measure
ties
at low olefin pressure
layer of a muticomponent
that CoMoO4 properties
pressure
Such a feature may be due to the presence
at the surface
layer since neither
Other explanationshave suggested
with a maximum
for other materials.
of the X- or a- phases
the propene
[21] gives curves which are intermediate
reasonable
surface
layer and to us to suggest
and modifies
as partial
catalytic
proper-
oxidation.
CONCLUSIONS The main conclusion reactions
on metallic
sions developed
which may be
by Boudart
of oxides are therefore their size relative
drawn from our work is that oxidation
oxides are structure
sensitive.
in the sixties for metals.
related to the chemical
to surface
This expands The catalytic
the concluproperties
nature of the reactants
atomic arrangements.
Such arrangements
and to
are
377 obviously
dependent
parameters
type mechanism. two important Comoarison
For instance
which in turn determine
involving
other
a redox Mars and Van Yrevelen
lattice oxygen mobility
and electron
mobility
are
of such materials.
of samoles
formed with the same transition metal ions, as 3+ 5+ E6' o with A= Sb or Bi bronzes A4 M8 12 62 shows clearly that for similar structures (isomorphism) the
MO 06 and orthorhombic
and M = MO or W catalytic
structure
in such reactions
characteristics
Sb2 MO 06/Bi2
pair
on the bulk oxide
important
properties
location
are dependent
on the nature of the cations
and on the lone
in space. MO 0 our results, particularly type catalysts as BiFeCo 10 12x study since XRD, IR and XPS techniques did not give striking and
For multicomponent of a kinetics
seem to indicate
clear cut informations, bismuth
molybdate,
on Co molybdate
presumably
that the catalyst
the X-phase
as a support.
or a mixture
is composed
of X- and a-phases
We arrive at an identical
conclusion
Sn Sb 0 and Sn Sb Fe 0 systems with an active phase of Sb oxide .. .)
on the Sb" in SnO
stucture
is not clearly
possibility
However
solid solutibn.
2
demonstrated
of modification
for
(Sb204, Sb6 013.
such an onion-like
and one cannot
of molybdate
of a
completely
by addition
rule out the
of small amounts
of Bi
and Fe. Data on single
crystal-type
fitting
led us to suggest
surface
atomic
arrangements
catalysts
arrangements
are different
favorable
therefore
be controlled rather obvious
we
factor in partial oxides
activation
the improvement
reactions.
and even multicomponent
may be developed
with
Such
as a support.
of such a support
catalysts.
on the catalyst
to
Such a conclusion in the
here may help in the
of catalysts
of industrial
atomic arrangements
interest.
are a determining
This may hold true for simple oxides, oxides
surface,
as well,since
depending
specific
on preparation
faces
and/or
conditions.
1
P.Marsand
2
R. Grasselli, J.D. Burrington 72 (19811 203.
D.W.VanL(revelen, Chem. Eng. SC., 3 (1954) and J.F. Brazdil,
It
and the
parameters
know that aspect very well, particularly
the idea that surface
oxidation
properties.
are two very important
think that the idea presented
and hopefully
oxide-oxide
one is dealing
can be considered
and structure
of the catalysts
since chemists
We want to emphasize
complex
reaction
in order to obtain highly performing
industry. HOweVer understanding
to catalytic
growth on what
turns out that the nature conditions
structural
catalysts
from those of the buik but depend on them and are
formed by a kind of epitaxial
preparation
and peculiar
that in multicomponent
41.
Discussion
Faraday
Sot.,
is
378
3
J.C. Volta, Catal. L&t.
Desquesnes, B. Moraweck and G. Coudurier, React. Kinet. 12 (1979) 241.
4
J.C. Volta, B. Benaichouba, I. S (1983) 215.
5
J.C. Volta, W. Desquesnes, B. Moraweck and J.M. Tatibouet, Proceedings 7th -_cIInternational Conqress on Catalysis, Tokyo 1980, p. 1398, Kodansha/Elsevier,Tokyo/Amsterdam, 1981.
6
J.C. Volta, J.M. Tatibouet, C. Phichitkul and J.E. Germain, Proceedings 8th International Congress on Catalysis, Berlin,1984, Vol IV, 431, _____.--Verlag C!hemie, Weinhein, 1984.
7
J.M. Tatibouet and J.E. Germain, J. Catal., 72 (1981) 365. J.M. Tatibouet, J.E. Germain and J.C. Volta, J. Catal. 82 (1983) 240.
8
J.C. Volta and J.M. Tatibouet, J. Catal. 93
9
Distillers Co Ltd, Brit. Pat. 1953, 876446 ; 1961, 864666 ; 1962, 920952 ; 1965, 997490 ; US Pat. 1963, 3094565, Belg. Pat., 1963, 630153 ; French _.__._ Pat. 1965, 1429477 ; 1966, 1471983 ; Ugine Kuhlmann Co Ltd., French Pat., 1962, 1293088.
W.
Mutinand J.C. Vldrine, Appl. Catal.
(1985) (in press).
10 F.J. Berry, Adv. Catal., 30 (1981) 97. 11 D.R. Pyke, R. Reid and R.J.D. Tilley, J. Chem. Sot., Faraday I, 76 (1980) 1174. 12 S.C. Volta, P. Bussiere, G. Coudurier, J.M. Hermann and J.C. Vedrine, Appl. Catal. 16 (1985) 315. 13 J.C. Volta, G. Coudurier, I. Mutin and J.C. Vedrine, J. Chem. Sot., Chem. a (1982) 1044. 14 Y. Boudeville, F. Figueras, M. Forissier, J.L. Portefaix and J.C. Vedrine, J. Catal., 58 (1979) 52. 15 D. Rogers and A.C. Skapski, Proc. Chem. Sot. (1964) 400. P.S. Gopalakrishnan and M. Manohar, Cryst. Struct. Comm., G. Thornton, Acta. Cryst. B33 (1977) 1271.
4
(1975)
203.
16 See for instance : R.K. Grasselli and J.D. Burrington, Adv. Catal. 30 (1981) 133 and referencestherein. 17 B. Gxzybowska,A. Mazurkiewicz and J. S&oczynski, ADwl. Catal. 13 (1985) 223. J.R. Burrington, C.T. Kartisek and R.K. Grasselli J. Catal. 87 (1984) 363, 18 D. Carson, G. Coudurier, M. Forissier, J.C. Vedrine, A. Laarif and F. Theobald, J. Chem. Sot. Faraday Trans.1, 79 (1983) 1921. 19 G.W. Keulks and L.D. Krenzke J. Catal. 61 (1980) 316. 20
F. Theobald, A. Laarif and M. Forissier, Submitted to J. Catal. 1985.
21
D. Carson, M. Forissier and J.C. Vedrine, J. Chem. Sot. Faraday Trans. I, 80, (1984) 1017.
22
M. Dion, Dr es Sciences thesis, Nantes (1984);
23
J.C.Daumas, J.Y. Derrien and F. Van Den Bussche, French Patent (1976) 2,364,061.
24
J.C. Volta and J.L. Portefaix, APP~. Catal. 1985, in press. 27 (1979) 141.
25 B. Grzybowska and A. Mazurkiewicz, Bull. Acad. Pol. Sci. 26 N.W. Wolf and Ph. A. Batist, 3. Catal. 32 (1974) 25.
27 O.V. Isaev, L. Ya. Margolis and I. Ya. Kushnerev, Zh.Fiz.Khim, 47 (1973)2127.