v ratio on the phase composition and catalytic activity of vanadium phosphate based catalysts

231

Applied Catalysts, 6 (1983) 231-244 Elsevier Science Publishers B.V., Amsterdam

INFLUENCE

- Printed in The Netherlands

OF P/V RATIO ON THE PHASE COMPOSITION

VANADIUM

PHOSPHATE

B.K. HODNETTa,

AND CATALYTIC

Ph. PERMANNE

Groupe de Physico-chimie

OF

and 8. OELMON

Min&ale

et de Catalyse,

Universite

Louvain,

Place Croix du Sud 1, B-1348 Louvain-la-Neuve,

aPresent

address:

Ontario,

ACTIVITY

BASED CATALYSTS

Oepartment

of Chemistry,

University

Catholique

de

Belgium. of Waterloo,

Waterloo,

Canada N2L 361.

(Received

10 November

1982, accepted

15 March

1983)

ABSTRACT The influence of the bulk P/V ratio of V-P-O catalysts on their phase composition, surface P/V ratio and catalytic activity for n-butane partial oxidation is reported. Catalysts were prepared by reduction of V2O5 in lactic acid followed by addition of o-H3P04. Excess lactic acid was evaporated off and the solid precursor calcined in air at 773 K. The bulk P/V ratio was varied in the range 0.94 to 1.07. This range was sufficient to bring about large variations in the nature of the final catalvst obtained. Catalvsts with low P/V ratios formed as BVPO5 with vanadium in the +5 oxidation siate. Those with high P/V ratios formed in a phase which we label B in which vanadium was in the +4 oxidation state. Scanning electron microscopy indicated that the catalysts increased in crystallinity as the P/V ratio decreased and EPMA indicated that the P/V ratio was homogeneous throughout each catalyst. Overall conversion of n-butane was at a maximum for P/V close to unity whereas selectivity was highest for the highest P/V ratio studied . The XPS study indicated that the surface P/V ratio did not change in the range of bulk compositions in which selectivity increased from ca. 10 to 50%. Arguments are presented which suggest that it is the solid state properties of these catalysts which largely determine selectivity and that the primary interaction of the hydrocarbon occurs with the aid of a V+5 related surface phase.

INTRODUCTION Whereas originated

in the past most of the world's from benzene,

costly and recent years describing

maleic

phosphate

patented

material

clearcut

understanding

many studies or phases,

and the growing

through

11, 21. industrial

of maleic

growth

has

in the number of patents

n-butane

However,

or butene oxidation

of selective to this problem

of this process,

oxidation has

has been placed on trying

no

on this catalyst

been attempted.

to identify which

are active or selective

0 1983 Elsevier Science Publishers B.V.

over

in spite of the bulk of

importance

of approaches

in these catalysts,

anhydride

have made this process more

of the mechanism

a strong emphasis

present

0166-9834/83/$03.00

synthesis

based catalysts

A variety

production

developments

have seen an enormous

anhydride

vanadium

has emerged.

economic

In phase

for this reaction

232 [ 3-71 .

Some of these phases,

such as aVPO5 18, 101 (with principal

d spacings

in

at 3.57, 3.07 and 3.01), BVP05 [9, 101 (d = 3.48, 3.40 and 3.07) and (VO)2P2O7 [lOI

(d = 3.87, 3.14 and 2.99) have already

activity

has sometimes

a'VPO5 [lo]

been assigned

been fully determined,

to less well determined

(d = 3.10, 3.00 and 1.96), a phase simply

3.07 and 3.00) which appears

identical

but catalytic

phases

labelled

such as

X [61

(d = 3.57,

to nVPO5 [8] or a phase labelled

6 [3,4]

(d = 3.90, 3.13 and 2.99) which Trifiro

et al.have

the R phase [ll] structure

to it.

(VO)2P207

[ 101.

seems to contain a mixture of V+4 and V+5. +4 the V containing part of this B phase [3, 41 as

labelled

(d = 3.87, 3.14 and 2.98) and have assigned On the other hand, Bordes

for catalysts

poorly crystalline

material

The oxidation

heating

in air.

Some workers neous catalysts must become

calcined

at 773 K.

with principal

state of vanadium

involved

state [9] and the other such as (V0)2P207,with studies

has suggested

anhydride

have emphasized

determining

selectivity

selectivity

by stabilizing

a large concentration

the importance

is in its +5 oxidation

vanadium

in the +4 oxidation

indicated

that there is a good

the +4 oxidation

of V+4 favours

selectivity oxidation

for butene partial

process

Our approach catalysts

involved

solid state chemistry vanadium addition, measured

has been to prepare

with

vanadium

that Naka-

that the

phosphate

in the range 0.94 to 1.07.

known crysthllographic

states of vanadium.

in n-butane

but

devoid of Vt5 showed lower

the P/V ratio to a small rBnge we hoped to prepare

terised oxidation

It appears

process

oxidation.from which they concluded +5_v+4 redox couple.

to this problem

could readily be compared

activity

totally

oxidation

ratio in to enhance

the V

with P/V ratios which varied

to restrict

state of vanadium.

the selective

mura et al. [13] have shown that catalysts

interconvension

of the P/V atomic

High P/V ratios are reported

[13, 141.

that

is the interface

fit at the interface between these two phases thus facilitating +5_v+4 between the V redox couple. Some studies

on heteroge-

states of vanadium

Thus Bordes

to maleic

one such as gVPO5,in which vanadium

crystallographic

in

into @VP05 upon

oxidation

least two oxidation

in the process [7, 10, 131.

Detailed

it forms

6' [12].

have taken the view that since selective is a redox process,at

has been observed

In general

d values at 4.67, 4.07, 3.14 and

This phase will thus be labelled

two phases;

state [lo].

B phase (111 with

is +4 and it transforms

the "active phase" for l-butene oxidation between

the

In addition, another phase of V-P-O catalysts

in this laboratory

2.52.

has equated

a tripolyphosphate

oxidation

and the results

properties,

and the homogeneity

These catalysts

notably

were tested

are compared

phese composition,

by XPS, were also studied.

principally

catalysts

which

phases with well charac-

of the P/V ratio throughout

some surface properties,

based

By choosing

for catalytic

with some of their oxidation

the samples.

the surface

state of In

P/V ratio, as

233 EXPERIMENTAL Preparation

of catalysts

The catalysts

used in this work were prepared

150 grams of lactic acid in water and stirring

by adding

9.1 grams of V205 to

the resulting

suspension

at

reflux for 16 hours, at which stage a blue solution, characteristic of the for+4 complex, had developed. Thereafter the appropriate amount of 85% mation of V H3P04 in water was added and this solution The solvent was then

4 hours.

solid material

which

remained

were prepared

and the

about by the finely

divided

and sieved, with those particles

the range 0.035 to 0.5 mm being retained catalysts

stirring

at 773 K for 16 hours in air. In

drops brought

it was pellitized,broken

at reflux for a further

off with constant

was calcinated

order to avoid very high pressure solid obtained,

was stirred

evaporated

for catalytic

in this way with P/V atomic ratios

in

In all,five

testing.

in the range 0.94

to

1.07.

Characterization X-ray diffractograms

of fresh and worked

Philips

X-ray diffractometer

filtered

through

Surface

by double

areas were determined

oxidation

titration

by the BET method

using nitrogen

as adsorbate

state of vanadium

against

KMn04.

in 100 ml of 2M H2SO4 at 350 K.

to Vt4

of FeS04(NH4)2S04

(equation

(a t b t c) Vt5 An excess

2).

to ferric [15]. 3) gave a measure (a t b t c) Vt4 The average

using a radiation

About

with the aid of a SETARAM

(AV) in the catalysts

+

was cooled +

Thereafter

was the quantitative

a second titration

oxidation

(a t b t c)e-

+

of all the Vt5

to close to 273K in ice water.

t4 (a t b t c) V

of the total vanadium -

(1) reduction

t Fe

t2

was then added to the cool solution.

which occured

was dissolved

was thus carried out +4 in the of all Vt3 and V

was then added to achieve

(a t b t c)e-

was determined

100 mg of each catalyst

The first titration

The solution

of (NH4)2S208

the only reaction

using Cub

at 77 K.

(volume 1) which effectively involved the oxidation +5 solution to V i.e. +3 +5 * (a t b t c) Vt5 + b Vt4 + cv aV An excess

were recorded

counter

nickel.

MLB 10-8 microbalance The average

catalysts

with Geiger-MBller

in excess

(2)

In these conditions

oxidation

of ferrous

ions

(volume 2) with KMn04 (equation

content

of the solid.

(a t b t c) Vt5

state of vanadium

(3)

(AV) was calculated

after equation

4

t131.

A,, =

3a + 2b + C = at

btc

5

(2a t b) atbtc

=

5 _ Volume

1

Volume 2

(4)

234 XPS measurements a signal averager sided adhesive

were carried

out using a V.G. ESCA 3 apparatus

(TRACOR NORTHERN

tape.

Excitation

energies were referenced

TN 1710).

Samples

equipped

were supported

was with the AlKo line (1486.6 eV).

to the CIs line (B.E. 285 eV).

with

on double Binding

The spectra

of CIs,

were recorded in this sequence for each sample studied. 02s' P2p' "2s and c1s Tbe relative intensities,i.e. the P/V ratio,were taken as the simple relation between the areas under the P For the electron

microscopy

2P

with a KEVEX E.D.S. was used. (STEM) facilities analysis

provided

of thin electron

and the V2s peaks. study, a JEOL 100 CX TEMSCAN The Scanning

Transmission

a highly focussed transparent

electron

microscope

Electron

equipped

Microscope

probe which made possible

samples with a resolution

of ca. 20 nm.

In general, the local P/V ratios of areas of the catalysts of diameter 0.1 to 1 um were checked

by Electron

Probe Micro Analysis

(EPMA) at ca. 50 different

spots for each sample.

Catalytic

activity

: apparatus and measurement

procedures

RF

G.C. -a R

Apparatus

Figure 1 R:

M

reactor;

The apparatus of the various n-butane

ment

outlinedhereabove

vanadium

phosphate

flow reactor

the catalyst

(R).

was packed

of the temperature

catalyst

BI_7

was outgassed

: valves;

: rotaflowmeter

to maleic anhydride.

differential which

RF

within

;

: 6-way injection valves;

: gas chromatograph.

was used to measure catalysts

, 03 ‘& . : 8s .

tests.

VI-2

G.C.

86

B7 . .

used to carry out catalytic

: gas mixer;

: flowmeters; Fl-2

,

the catalytic

for the partial

A charge of 3 grams of catalyst This reactor and featured the catalyst

consisted

a thermowell bed.

activities

oxidation

of

was placed

of a Pyrex U-tube which allowed

In normal operation,

in situ for 16 hours at 723 K, cooled to the

in the into

measurethe

reaction

235 temperature

(673 K) and then sealed off from the remainder

means of the 3 way valves BI and B2.

of the system by

Feed gases for the reaction,

comprising

1.3 vol % n-butane in air,were composed as follows : a large flow of air (1500 -1 ml min ) was established via valve B4 and,with B7 closed,this flow was vented to the atmosphere

via B6.

value, the required measured

When this gas flow had stabilized

flow of n-butane

at the desired

(19.5 ml min-') was set via B3 and

with the aid of the calibrated

rotaflowmeter

(RF).

Finally,

any

desired

part of this gas mixture could be flowed through the reactor via B7. -1 A flowrate of 90 ml min , corresponding to a gas hourly space velocity (GHSV) of 4680 hr-', was used throughout be checked suitable

this study.

The composition

by means of the G.C. system prior to its exposure

manipulation

Analysis

products

using an INTERSMAT

rometer

detector

reactor

with the chromatograph

was carried

one of two 6-way valves.

gas.

of reaction

Gases were injected

via

The first of these led to a 0.5 m x 2 nnn stainless

components

Oil 500 on chromosorb

was separated

the

steel and this was heated

products.

and flowed

on which

from all other

n-butane

was separated

into one cell of the detectors.

The second 6-way valve led to a 2 m x 4 mm stainless 20% Silicon

gas

fitted with a kata-

The piping which connected

was of 4 mm stainless

packed with PORAPAK Q, 80-100 mesh,

from all other gaseous

out by means of on-line

IGC 1214 gas chromatograph

using helium as carrier

at 100°C to avoid condensation

steel column

by

of BI and B2.

of the reaction

chromatography

of the gas could to the catalyst

steel column

packed with

WHP, 80-100 mesh, on which maleic

reaction

products.

These

separations

anhydride were carried

out at 368 K. In what follows, Y

=

yield % (Y) in maleic

No of moles of product No of moles of reactant

Conversion c

=

=

is defined

as

x 100

introduced

% (C) as

N" of moles of reactant

consumed

N" of moles of reactant

introduced

and selectivity s

anhydride

formed

(Y/C)

% (S) in maleic

anhydride

x 100

as

x 100

RESULTS X-ray diffractograms measured

directly

of the catalysts

after calcination

values of AV and specific presented

in table

1.

surface

with P/V ratios

are presented areas, measured

before

These data show the influence

P/V ratio on the composition

of the final catalyst

of 0.97 and 1.07

in figure

2.

In addition,

catalytic

of slight

testing,are

changes

and on its textural

in the proper-

236 ties.

Thus it can be clearly

phosphorus

seen from figure 2 that a slight deficiency

from the stoichiometric

crystallised phosphorus

1:l ratio promoted

phase with AV of ca. +5. strongly

stabilized

Figure 2

TABLE

state of vanadium

of a material

in

of a well

On the other hand, a slight excess

the +4 oxidation

nation at 773 K and led to the formation diffraction

the formation

during

of calci-

with very broad X-ray

lines.

X-ray diffractograms

of catalysts

with P/V of 0.97 and 1.07

1

Measured

values

and phases

of AV, before and after catalytic

P/V ratio AV before

testing,

specific

surface

present

testing

AV after

testing

S ecific m 5 g-1

surface

Phases present testing

area

0.94

0.97

1.0

1.03

1.07

4.97

4.90

4.86

4.39

4.04

4.98

4.92

4.90

4.59

4.24

2.7

5.6

9.6

4.9

4.9

t3VP05

;v,o,

Bn BVP05

iVPO5

w

before BVPO5

W

area

237 The results Measurements recorded

of the catalytic

at 673 K are presented

rature were

subject

across

measurements

However

the whole temperature in selectivity

for the catalyst

here since measurements

the general

range.

and yield

in figure 3.

Only those data

made above this tempe-

of maleic

trends observed

anhydride

were

on the

identical

The data recorded at 673 K show that a

for maleic

with the highest

the lowest P/V ratio

are presented

range 663 to 703 K.

to errors due to decomposition

walls of the reactor.

maximum

activity

were made in the temperature

anhydride

formation

was observed

P/V ratio tested and that the catalyst

(0.94) exhibited

with

zero selectivity.

P/V RATIO

Figure 3

Influence

(S) for maleic

By contrast, products

in overall

for intermediate

in surface areas

some oxidation

lines from catalysts

favour of SVPO5.

testing occured

catalysts

during

in the relative

recorded

tures as high as 703 K.

changes

to move towards

small changes were observed

of n-butane

P/V ratios.

By contrast,

P/V ratios less than unity. changes,tending

conversion

after catalytic

The data reveal that no significant

ting theoccurrenceof

(Y) and selectivity

to all reaction

This maximum

coincided

with

(table 1).

The values of AV measured

signficant

(C), yield

formation.

the maximum

was observed

the maximum

of P/V ratio on conversion

anhydride

after testing

are presented

in table 1.

in AV for catalysts

having

with P/V > 1 exhibited

higher values

for AV,thereby

the testing. intensities

In addition,

indicasome

of X-ray diffraction

for catalytic

activity

to tempera-

In general these showed a loss of the 6' phase in

238 The data presented tivity

in maleic

diffractograms

in figure 4 represent

anhydride

formation

before testing

used because

it was thought

tions during

testing

tions observed

an attempt

with the phases observed

for catalytic

activity.

that these were more

probably

occured

the selec-

in the X-ray

These diffractograms

representative

at 673 K, since the small changes

after testing

to correlate

were

of the composi-

in AV and phase composi-

at the highest temperatures

used.

WV RATIO

Influence

Figure 4 @PO6

of P/V ratio on the relative

(0) and (upper traces) AV before

curve marked

S replots

the selectivity

amounts

(a) and after

of the 13" phase

(0) catalytic

data of figure

(O),

testing.

3 for comparison

The

(same

scale).

However

the general

recorded after

trend was not affected

catalytic

testing.

in the X-ray diffractograms amount of each phase

selectivity

could

and a maximum Electron catalytic revealed

were taken directly For comparison

present.

and after testing,are

activity

these curves with data

of figure 4, the peak heights

as a relative measure

of the

the values of AV, measured

before

Thus the maximum in on the same diagrams. 4+ oxidation state present V with a maximum in

also presented

be correlated

in the phase labelled

microscopic

by plotting

For the purpose

examination

confirmed

6" in figure

2.

of these materials

the findings

that as the P/V ratio increased,

before

testing

of the X-ray diffraction the crystallinity

for

study and

of the catalysts

239

Figure 5

Scanning

electron

micrograph

of the catalyst

with P/V = 0.94

Figure 6

Scanning

electron

micrograph

of the catalyst

with P/V = 0.97

240

Fiqure 7

Scanninq

electron

microqraph

of the catalyst

with P/V = 1.03

Fiqure 8

Scanning

electron

micrograph

of the catalyst

with P/V = 1.07

241 As shown in figure 5, the catalyst

decreased.

large particles

of a well crystallized

(figures 6 - 8), the amount extent

samples

material

were almost

Measurements

A summary presented

the different

of the ratio of intensities

in figure

these catalysts ;

below

9.

remained

relatively

presence

of any V2O5 in this catalyst.

layers,but

of the XPS study,the

at the extremes

It should

(greater than of

did not reveal the

be pointed

out here that for to cover the

the shape of the curve in figure 9. However i.e. those with P/V ratios of 0.9 and

activity.

a9

l.0

1

PNRATIO Influence

each catalyst

in the proportion

P/V ratio used was expanded

of the range,

1.1, were not tested for catalytic

increase

the X-ray diffractograms

range 0.9 to 1.1 in order to confirm

Figure 9

throughout

present.

of the PZp and the VZs XPS lines is

this value there was a drastic

06

that for all the

stable for high P/V ratios

in the surface

the samples

to such an

These data show that the P/V ratio at the surface of

vanadium

the purposes

increased

absent from the catalyst

by EPMA revealed

types of particles

of

but as the P/V ratio increased,

crystallized

particles

P/V ratio.

with P/V = 0.94 consisted

of the series the P/V ratio was homogeneous

and throughout

0.97)

of poorly

that well crystallized

with the highest

material

(BULK)

of the bulk P/V ratio on the ratio of the PZp to VZs XPS

peaks.

DISCUSSION Influence

of P/V ratio on phase composition

A striking

feature of the results

ratio was restricted catalysts

obtained

ratios favoured

presented

to a small range,its

and their catalytic

the formation

above is that,although

influence

activity

the P/V

on the nature of the final

was very pronounced.

of BVPO5 [9] with AV values

close to +5.

Low P/V On the

242 other hand, high P/V ratios favoured B"[ 12] and AV values close to +4. with P/V = 1.07 was composed Little

information

rial with a similar were not presented transforms strongly

XRD pattern

dependent

after calcination

influence

at 773 K[12j

The phase labelled

study confirmed

of calcination of reducing

mation or otherwise

agent,

B phase

13, 41&

In this

phase

The

of the SW phase as of this phase as

elsewhere

1121.

The

i.e. lactic acid, HCl or oxalic acid, on the for-

of the 0' phase is not yet fully understood.

The XPS data indicate

that some surface enrichment

samples with P/V ratios of less than about 0.97.

in vanadium

However

tion factors

( 17j , a surface P/V ratio of 2.4 was observed in the plateau Large errors

i.e. above 0.97.

vanadium

on the surface

ions on the surface,

in view of the uncertainties

are often associated

associated

between

Following

the arguments

outlined

position between

during

testing

selectivity

at 673 K.

for maleic

observed mostly

for catalysts

1181.

However

this aspect

activity presented

in figures

which did not change markedly

strong correlation

anhydride

phase and V+4 in these catalysts.

was

below.

above the results

A

this type

of individual

their selectivity

with these calculations,

P/V ratio and catalytic

and 4 can be taken to refer to catalysts

with

that phosphorus

to bring about isolation

thus increasing

of the results will not be pursued further

Relationship

cross section photoioniza-

but these data could be an indication

acting as a diluent

for

inten-

116 1 were calculated

of calculation

using the Scofield

occured

when absolute

sity values

region,

1111

work, only at 773 K.

of the evolution

will be presented

is

it is not observed

the poor crystallinity

conditions

and slowly

appearance

Its

after calcination

Full details

A mate-

but full details

precursor

.

in such circumstances.

by its XRD pattern.

a function

from an amorphous

of this phase were observed

microscopy

indicated

previously,

the P/V ratio, but in general,

above 873 K.

of the catalyst

of this material.

has been reported

calcination

[lo] is observed

trace amounts electron

upon

of a phase which we label

as to the nature of the 6' phase.

It originates

into 4VP05 during

or (VO)2P2O7

almost entirely

is available

[ 61 .

the formation

The X-ray diffractograms

formation

Conversely

was thus observed

and the presence

a minimum

3

in com-

of the BW

in selectivity

was

with P/V ratios less than 1, i.e. which were composed

of 6VPO6.

The very poor selectivity understood

of the catalyst

in terms of the XPS data.

point where a marked

drop in surface

much more pronounced

than the corresponding

surface enrichment selectivity

in vanadium

with P/V = 0.94 can be further

This composition

P/V ratio was observed. diminution

would be expected

since it is phosphorus

coincides

which

This drop was

in bulk P/V ratio. Any

to strongly

is believed

with the

decrease

the

to render these catalysts

243 selective 1191,

an opinion

which

is fully confirmed

by our data for catalysts

with P/V > 1. A feature most,

of these results

i.e. between

variant,

within

P/V = 0.97 to 1.07,the

experimental

ratio is a necessary phosphate

are apparent

that the oxidation

influenced

by excess

the V+4 [17,

condition

surface

contention

phosphorus

in-

plateau

for a selective

P/V

vanadium

i.e. since excess

ratio in the P/V range where

phosphorus

inhibits

inhibits

reoxidation

lysts and strongly Reports

in the patent is the B phase

that selective

maleic

literature

[ 3, 41.

anhydride

phosphorus

strongly.

energies

changed

formation

of the catalysts of excess

It has been shown else-

of the bulk of these cata-

of the bulk of reduced

catalysts [ 20] .

that the active phase in V-P-O

The results

presented

here would

from n-butane

to this phase since the 6' phase was also selective.

amorphous

phases

have been reported

indicate

is not a property

exclusive

of V-P-O catalysts

catalysts.

selectivity

of the influence

the reduction

This

for the V2s and V2p peaks calcined

of the system.

indicate

stabilizes

selectivity

with bigh P/V ratios must have been a consequence

that excess

but

it can be

an invariant

that the increased

on the solid state chemistry

at the surface Secondly,

P/V ratio indicated

in this P/V range for freshly indicated

composition.

at the surface was not strongly

surface

is borne out by the XPS binding

phosphorus

surface

become concentrated

state of vanadium

bulk phosphorus,

These consideraions

catalysts

changed

(XPS) remained

into the bulk of the catalyst.

191 state the invariant

which were unchanged

where

P/V ratio

as a result of the invariant

little of the excess

vi5/C4

surface

Thus it seems that a certain

but not sufficient

must have been incorporated argued

error.

selectivity

based catalyst.

Two factors Firstly,

is that in the range where

Indeed reduced

in the literature

[ 111 .

Thus what would or otherwise the surface indicating

seem important is not the exact phase involved but the presence +4 . However the arguments presented above would indicate that of V

layers of the catalysts that the direct

of the catalyst excess

are in a fairly oxidized condition thus +5 phase. interaction occured with a V The AV value

could be seen as a consequence

phosphorus

which controls

the mobility

of the stabilizing of oxygen

anions

effect

through

of the

lattice [ 20] . An interesting version

feature

of n-butane

correlated

was observed

with a maximum

and their reducibility determines

overall

lattice oxygen

of these results

in

for intermediate

specific

by hydrogen

activity

P/V ratios.

in overall

This finding

[20].

indicates

of the catalyst

con-

This maximum

surface area for these samples

is the ability

in a non-selective

is that the maximum

(table 1)

that what

to give up its bulk

way such as it does in reacting

with hydrogen.

In summary, the results of this study indicate that it is not only the B phase which can be selective

for maleic

anhydride

formation.

The Bw phase is

244 also selective. catalytic

However

conditions,

direct interaction with the hydrocarbon may occur, in +5 with a V phase on the surface of these catalysts. Thus

the AV values shown by the catalysts

are the result of the influence

phosphorus surface

on the solid state processes, i.e. reduction-oxidation. +5 +4 ratios were effected by changes P/V ratio nor the V /V

for the bulk of these catalysts Further

details

selectivity

in the range where selectivity

of the mode of action of excess

will be presented

phosphorus

of excess Neither

the

in the former

was changed

in determining

most. the

soon.

ACKNOWLEDGEMENTS One of us (B.K. HODNETT) Politique

Scientifique"

for running

the scanning

wishes

(Belgium) electron

to thank the "Service de Programmation for a fellowship. micrographs

We also thank Michel

de la Genet

and for help with the XPS measure-

ments.

REFERENCES 1 2 3 4 5 6 7 8 9 10 lla b 12 13 14 15 16 17 18 19 20

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