Catalytic properties of metallic oxides in partial oxidation reactions

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...

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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)--

[email protected] --*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.