Influence of cobalt on the textural, redox and catalytic properties of stoichiometric vanadium phosphate

245

Applied Catalysis, 6 (1983) 245-259 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

INFLUENCE OF COBALT ON THE TEXTURAL, STOICHIOMETRIC

B.K. HODNETTa

VANADIUM

REOOX AND CATALYTIC

Min6rale

et de Catalyse,

Universite

Louvain,

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

aPresent

address:

(Received

OF

and B. DELMON

Groupe de Physico-chimie

Ontario,

PROPERTIES

PHOSPHATE

Department

of Chemistry,

University

Catholique

de

Belgium. of Waterloo,

Waterloo,

Canada N2L 361.

10 November

1982, accepted

15 March

1983)

ABSTRACT The influence of coprecipitated cobalt on the catalytic, redox and textural properties of BVPO5 is reported. Four catalysts were prepared in which the Co/V ratio was varied between 0 and 0.05 and the P/V ratio was held at unity. Cobalt strongly reduced the surface area of BVP05 and diminished reactivity toward hydrogen was observed as a monotonous feature as the cobalt content increased. showed complex kinetic behaviour during By contrast this material reoxidation from reduced states. When reoxidation was confined to surface layers a maximum in rate was observed for Co/V ratios of ca. 0.02. Bulk reoxidation was strongly sensitive to small amounts of cobalt but it was independent of Results of catalytic testing for n-butane cobalt content above Co/V = 0.01. partial oxidation to maleic anhydride indicate that conversion was at a maximum for the undoped material but selectivity showed an optimum value for Co/V of ca. 0.02. Above this value a sharp drop in selectivity was observed. A strong increase in area1 reoxidisability was observed for Co/V above 0.02. X.P.S. data indicated that some surface segregation of cobalt had occured for Co/V = 0.05. Two domains of influence by cobalt on the catalytic activity of BVPO5 were identified; one in which it influenced the solid state properties and increased selectivity, the other in which a surface enrichment in cobalt provoked total oxidation of n-butane.

INTRODUCTION Vanadium

phosphate

based catalysts

butane and butene to maleic being carried studies

for the catalytic

On a more practical selectivity

of adding

0166-9834/83/$03.00

activity

line of approach V-P-O phases

attempts

to increase

is

the activity,

have been made. Many of these

of a third metal

0 1983 Elsevier Science Publishers B.V.

as modifier

[8-151.

of are

in these

11-71.

of these catalysts

small quantities

oxidation

number of investigations

One strong

which of the many reported

level, numerous

and lifetime

A growing

this system.

is aimed at identifying

responsible

consist

anhydride.

out concerning

are used for the selective

246 The role of some additives, sublimation work 116,

of phosphorus 171.

However

action of dopants

little explanation

such as Zn [8-111,

all of which are claimed We present

Our approach

selectivity

in patented

of a study of the influence

with the main elements,

to this problem

oxidation

processes

of the mode of

on the catalytic

catalysts.

of cobalt,

introduced

properties

is based on the reduction-oxidation

(Mars and Van Krevelen mechanism

still holds true in many cases.

can be explained reagent,

by a mechanism

with incorporation

reoxidation

in the field of selective

oxidation,

of

involved

properties

of undoped

determining

activity

isolate as clearly

followed

by

our study on the behaviour

of cobalt on the solid state

phosphate

and textural

on the textural

on the reducibility

are of crucial

importance

in

[3, 19, 201.

have been reported

this study should concentrate

and

These have shown

based catalysts.

properties

for industrial

vanadium

Many of these are as yet poorly defined.

as possible

and

This work

has also been studied.

in these laboratories

and selectivity

[ l-51.

oxidation

As with many investigations

Its influence

phosphate

vanadium

A large number of phases based catalysts

into the product,

on the influence

of vanadium

that solid state chemistry

selective

of

a concept

of the solid by the hydrocarbon

we have focussed

in these processes.

other investigations

oxidisability

speaking,

nature

[18]),

and reoxidation.

This paper is thus centred

catalytic

of reduction

by gas phase oxygen.

in reduction

chemistry

Generally

of lattice oxygen

of the lattice

of the catalyst

follows

has been presented

phosphate.

selective which

is to supress

at which these catalysts

Ti [9], U [12], Cu [8, 13, 141 or Si 1151,

to increase

here the results

by coprecipitation vanadium

such as the alkali earth metals,

at the high temperatures

the effects

of added cobalt,

on the influence

phosphate

order to

In

it was decided

that

of cobalt on a well defined

phase of vanadium phosphate. The P/V ratio is an important final catalyst maximum

is obtained [l-3,

rate of reduction

factor

the phase in which

Previous

indicated

and reoxidation

This composition

to unity 119, 201.

in determining

19, 201.

studies

were observed

corresponded

for a P/V ratio close

to the inflection

plot of selectivity

versus P/V ratio t3]

near the inflection

point of the curve giving variations

state of vanadium

versus P/V ratio 1191.

(i.e. maximum

is different oxidation

91 .

understood.

However

state of vanadium

reduced precursor

brought about by cobalt might be

catalysts,

and all feature

was carried

and was

in average oxidation

it must be pointed out that this composition

from those of industrial

The small P/V ratio adopted

sensitivity)

point of a

A P/V ratio of unity was thus chosen

in this work hoping that in so doing changes more easily

the

that the

which

seek

to favour the +4

P/V ratios greater

here insured

out in oxidizing

than 1.1

that when the calcination conditions,

[ 1, 2, of a

the +4 oxidation

247 state of vanadium was. not strongly stabilized t5 in a single phase was obtained. mostly V

131 but a catalyst

comprising

EXPERIMENTAL The method

of preparation

fully described dissolution

[ 3, 19, 201.

off and the solid obtained were prepared

and 0.05 and in which Reduction

was

using oxygen

to determined

The reduction

elsewhere

process was normally by pumping

pi.

were followed

on the

The extents

before reoxidations

of @PO5

in the tempera-

as reductant

extents

losses which occured,

are presented

gas was then removed

spiral spring balance

gas at 1 atmosphere

to a total reduction

Full details

In all,

ratios were 0, 0.01, 0.025

ratio was 1.

to proceed

from the weight

by addition

was then evaporated

at PO2 = 150 torr, as'oxidant.

were allowed

loss corresponds v4+.

prereduced

assuming

to (VO)2P2O7,

that a 5% weight

i.e. of all V5+ to

@a.

continued

for about 15 hours.

and the samples

cooled

The hydrogen

to close to room tempera-

from the balance

for X-ray analysis.

reduction

out in a SETARAM

electronic

experiments

the BET method

without

Full details surface

cross section

co

I co2p3/2

V

1 V2s

where

I represents

cross section Catalytic constructed

are given elsewhere

"'Zp3/2

JEk

t

[ 31.

"2p3/2

I CO2pl/2

Calculations

of

(1) 1231

SCOFI ELD

:

x-=oco2p1/2

I '2s

the area under the indicated

peaks; u the photoionization

energy of the photoelectron.

measurements

were made on a differential

of Pyrex glass. The feed gas was composed

flow reactor

of 1.3% n-butane

in air

This feed was passed over 2 grams of catalyst,

which was held at 673 K in the reactor. hand at 700 K for several

so by

and l/2 and the V2s peaks, for

factors l22l as in equation

xm

at a flow rate of 60 ml min-1.

microbalance

could be measured

bulk ratio of 0.05, were made using

and Ek the kinetic activity

samples

by the C02~3/2

oV2s

x-x

reduced

Some

to air.

as indicated

with Co/V nominal

photoionization

-=

exposure

of XPS measurements

Co/V ratios,

the catalyst

were carried

areas of some partially

to

were commenced,

ture in vacua before being removed

that the surface

been

of V2O5 and

followed

in air at 773 K for 16 hours.

out in a McBain

samples

reduction

lactic acid

the Co/V atomic

K using hydrogen

same apparatus

were calculated

Excess

was calcined in which

Reoxidationsof

reductions

solution.

the P/V atomic

carried

ture range 573-723

it involved

in 85% lactic acid in water

to the resulting

4 catalysts

used in this study has already

Essentially,

of C0(N03)~.6H20

of o-H3P04

which

of catalysts

The catalysts

hours in a flow of air.

were outgassed

Full details,

before-

with the

248

definition

of conversion

formed to maleic

(percent

anhydride)

n-C4H10 transformed),

and selectivity

yield

(n-C4H10

(Y/C), have already

trans-

been presented

131. RESULTS X-ray diffraction principal

analysis

peaks characteristic

of the undoped

the 3' phase were also present] 3 1 is presented

in figure

catalyst

of the presence

.

after calcination

showed

of @/PO5 but small amounts

A typical

diffractogram

of

of this material

1, trace (a).

.,.,I (a.)

t

3

2.5

35

&

4.5 5 d/A'

Figure 1 6VPiI5,

X-ray diffractograms (L) after reduction

of cobalt free catalysts.

for 15 hours at 623 K.

(a) fresh undoped

(c) after reduction

for

15 hours at 723 K.

The X-ray diffractograms

of the cobalt doped catalysts

similar and did not revea! the Presence the formation

of any separate

were essentially

chases associated

with

of an oxide of cobalt.

On the other hand, analysis [22] of the XPS intensity

data for the sample

249

with a Co/V ratio of 0.05 indicated of 0.15 thereby

indicating

that the surface

that some segragation

Co/V ratio was of the order

of cobalt towards

the surface

had occured. The average significantly

oxidation

state of vanadium

throughout

the series did not vary

: it was between t4.85 and +4.9.

Figure 2(A) presents

the influence

area of the catalysts.

A nitrogen

of cobalt content

porosimetry

on the specific

study yielded

hysteresis

the type shown in figure 2(B) for each sample of the series, the catalysts plates

had morphologies

essentially

corresponding

Figure 2

A) Influence

B) Typical

hysteresis

of added cobalt on the surface

The horizontal

of the undoped dotted

loss.

A general

feature of these curves

Figure 4 shows the influence by hydrogen

different

temperatures

at'673

@PO5

(the data presented

corresponds

into (VO)2P207

are presented

in

to the level i.e. a 5%

is that no induction

of added cobalt on the reducibility

by presenting

of the cobalt

by hydrogen

period

process.

K and figure

and 723 K (i.e. the percentage as a function

material

was transformed

with the reduction

phosphate

study

line in this figure

at which all 8VPO5

was associated

porosimetry

area of

catalyst).

The curves of reduction

of reduction

PlpP

loop from nitrogen

to the undoped

figure 3.

weight

that parallel

[ 251.

CQN RATIO

relates

indicating

to non-porous

surface loops of

weight

content.

5 summarises

the initial rate of reduction loss registered

of vanadium

these and data from

during

Clearly,coprecipitated

at 573, 673

the first 0.5 hour) cobalt

strongly

250

c L

75* 10

15

Time I hrs

Figure 3

Influence

of temperature

on the reactivity

of undoped

@PO5

towards

hydrogen.

I

5

I

I

10

15

Timelhrs Figure 4 by hydrogen

Influence

of added cobalt on the initial rate of reduction

at 673 K (the Co/V ratio is indicated).

of @'PO5

251 inhibited

the reduction

of the reduced

catalysts

show that samples which peaks characteristic

of vanadium

phosphate.

are presented had experienced

of the formation

Typical

as traces weight

X-ray diffractograms

fb) and (c) of figure

losses

of a reduced

1, which

in excess of 5% showed

phase often

labelled

the

6 phase [4, 51. To check the

influence

of the extent of reduction

area, the catalyst

with Co/V = 0.01 was reduced

electronic

balance

to levels corresponding

6.6 weight

percent

losses.

catalyst

was measured

in surface

on the specific

in hydrogen

to 0, 0.7,

1.4,

2.5,

At each of these points the surface

without

its being exposed

area for the whole

to air.

range of reduction

surface

at 723 K in an 3.4,

5 and

area of the

No change was observed

levels checked.

2.5

0

0

QO25

0.05 Co/V Ratio

Figure 5

Influence

by hydrogen

of added cobalt on the initial

at the temperatures

Figure 6 shows the influence of reoxidation hour exposure

(as indicated to oxygen)

been prereduced

of the level of prereduction

by the percentage

at 723 K such that it had experienced

level of prereduction prereduced reduction

increased.

for the enhanced

sively

samples.

This catalyst

weight

had

losses between

increased

loss could be fully reoxidized.

reoxidation

rate

in the first 0.5

0.5

as the

only those samples which

did not bring about an increase

cannot account

of 6VP05

on the initial

regained

rate of reoxidation

In addition,

to less than a 2% weight process

reduced

weight

for the sample with Co/V of 0.01.

It was found that the initial

and 5%.

rate of reduction

indicated.

had been

As the

in surface area, this factor

rates observed

for the more exten-

252

25 % Prereduction Figure 6

Influence

reoxidation

of the level of prereduction

on the initial

rate of

at 723 K of BVP05 with Co/V = 0.01.

10 Time/ Figure 7 phosphate

Influence

hrs

of added cobalt on the reoxidation

at 723 K.

(@) co/v = 0.0

(0) 0.01

(0) 0.025

(0) 0.05

of prereduced

vanadium

253 Figure

7 shows typical

reoxidation

curves for the series of catalysts

each had experienced

an initial weight

rates of reoxidation

were much less than the corresponding

by hydrogen

at 723K.

extent

to which

weight

losses,

ca. 0.02.

had proceeded

which

tended to inhibit

of the cobalt content

influenced

of the cobalt content

Figure 8

A. Influence vanadium

to which

an initial

prereduced

traces) weight

of‘these

but the extent

had proceeded

of cobalt but was nearly

level corresponding

Ratio

on the extent

had proceeded

after

to which

reoxidation

of

(s) 0.5 and (r) 5 hours for

to 5% (upper traces)

losses.

8. The data of part A normalised

feature

for a Co/V ratio of

reoxidation

by small amounts

on the

to 2 and 5%

above ca. 1 mole percent.

of added cobalt

phosphate

process

a minimum

(WV

prereduced

A general

the reoxidation

On the other hand, the extent

rates of reduction

had been reduced

after 0.5 and 5 hours.

after 0.5 hour passed through

after 5 hours was strongly independent

of samples,

after

It can be noted that the

Figure 8 shows the influence

reoxidation

data is that cobalt of reoxidation

loss of 5%.

in terms of surface

area.

and 2% (lower

254 The results

of the catalytic

activity

indicated

tion from n-butane

for a Co/V ratio between 0.01 and 0.02.

conversion

was observed

for the cobalt

per unit of surface

observed

as the cobalt content

specific

surface

in selectivity

are presented

These measurements

was expressed

a maximum

measurements

free catalyst.

area, a slight

of the catalysts

for maleic

in figure 9.

anhydride

A maximum

forma-

in

However when conversion

increase increased,

in conversion

was

i.e. as their

areas decreased.

Figure 9

Influence

conversion

per unit surface area (0).

of cobalt on conversion

(0); yield

(0); selectivity

(a);

DISCUSSION A summary added cobalt

of the most important influenced

findings

the textural

surface area and also its reactivity the rate of reduction

by hydrogen

of this work indicates

properties towards

that (i)

of DVPOS by diminishing

hydrogen;

its

(ii) for all samples

was much faster than the rate of reoxidation;

255

a monotonous minimum

decrease

was observed

(iii) catalytic

in reduction in initial

activity

per unit surface

sing Co/V ratio; maximum

In this discussion cobalt

selectivity

in the catalyst,

Cobalt

of cobalt

phosphate

in cobalt.

of this study supposing of part of the cobalt

reducibility

understood

additive.

in the surface

exactly

The fact that small amounts area suggests

The X-ray diffraction

information,

enrichment

of most physico-

we shall try to interpret

that the strong effects in solid solution

that, at least,

data did not

some surface

of data and the inability

observed

were due to

in 6VP05.

of SVP05 as the cobalt content

in terms of the changes

Indeed we have demonstrated

reducibility

and surface

The results process.

presented

the rate of reoxidation

influenced

initially

one possible

for catalysts

initially identical.

reduced

approach

reduction

vanadium

content

proceeded, to which

this comparison,

is to compare

reoxidation

rate was proportional

the extents

to which

To calculate

reduction

1 and 10 hours were taken from figures

factors was

Figure 8

reoxidation

state of vanadium.

rates for catalysts reduced

to surface

reoxidation

had proceeded

3-S.

two other

the catalyst

layers was

area [21] the

layer should be the same across the whole

time was held constant.

phosphate,

even though

by comparing

to the same oxidation

to measure

is rather arbi-

(fig 7 and fig 8).

in such a way that the depth of their

Since reduction

depth of the reduced

conditions

way of making

which were prereduced

An alternative

i.e. the extent

(fig 6) and the cobalt

of the reoxidation

experiments

study because,

in surface area as the reduction

the rate of reoxidation,

picture

should commence

true in the present

between

[21].

such as prereduced

the reoxidation

can be

by this

a proportionality

6-8 that, in designing

of a material,

This is particularly

there was no change

previously

here give a more complicated

the choice of point from which

increased

in surface area provoked

area for these catalysts

It is clear from figures

presents

to

and reoxidation

The diminished

reduced

data, attempt

there is little hope of determining

to give the necessary

the presence

trary.

Co/V ratios.

try to offer an explanation

but the XPS data did indicate

the results

readily

and finally

the lattice.

In view of the scarcity

techniques

Reduction

with increa-

the data we have on the state of

in these catalysts.

bring about a large change

any segregation

chemical

slightly

catalysts

and state of cobalt

part of this dopant entered indicate

a

Co/V ratios;

for intermediate

the reduction-reoxidation

activity

of its low concentration,

the position

area increased

was observed

then discuss

content was observed;

effects.

in vanadium

Because

cobalt

rate for intermediate

we shall first discuss

relate these data to catalytic for the observed

rate versus

reoxidation

series if the

rates for these

for each catalyst

after

The data of figure 8werereplotted

256 as reoxidation

rates versus extent

the type shown in figure 6. samples

reduced

dation

(2 or 5%) to give plots of

these plots, reoxidation

for 1 or 10 hours could be estimated.

has advanced

1OB presents

of prereduction

By combining

Extents

after 0 5 and 5 hours, are presented

the same data normalised

0.05

0

,

0

to which

in figure

in terms of surface

rates for reoxi-

10A.

Figure

area.

0.05 Co/V Ratio

Figure

A. Influence

10

vanadium traces)

phosphate,

of added cobalt on the extent

prereduced

hours had proceeded

after

B. The data of part A normalised

The curves pective

in figures

of whether

fixed depth of reduced In what follows,

(0) 0.5 and (0) 5 hours.

prereduced

essentially

layer or a fixed average

it will be assumed

between

catalytic

The lower conversion can be readily fact,

explained

conversion,

oxidation

that valid comparison

on the catalysts

in terms of the changes

in terms of a

state of'vanadium. in reoxidation

beha-

8 or 10.

and reduction-reoxidation

of n-butane

expressed

the same! shape irrds-

state was defined

viour can be made on the basis of either figures Relationship

of

and 10 (lower

in terms of surface area.

8 and 10 display

the startinq

to which reoxidation

at 723 K for 1 (upper traces)

properties

with higher cobalt

in surface

contents

area observed.

per unit surface area, far from decreasing,

In

actually

257 increased

when the cobalt

tivity observed be ascribed

content was increased

for the catalyst

to a consequence

sion was the highest

of its low overall

.

Within

mediate

that selectivity

can be correlated

bulk; the most selective 201

activity

the low selec-

i.e. 0.05, cannot

since its area1 conver-

of undoped vanadium

with the difficulty

catalyst

this perspective,

Co/V ratios

However

Co/V ratio,

of the series.

It has been shown elsewhere based catalysts

(fig 9).

with the highest

of reoxidation

being the most difficult

the higher

selectivity

is in line with this previous

thus seems to hold true for samples

phosphate of the

to reoxidize [3,

of the samples

observation.

with inter-

Our prediction

doped with cobalt up to a Co/V ratio of ca.

0.015. Two factors

suggest

that something

ble for the loss in selectivity is that the increase

in reoxidation Secondly,

loss in selectivity.

5 hours for highly prereduced observed measure

of the reoxidation

effect

reasonable

when extents

catalysts,

of the increased

surface

The known high activity [26]

the high area1 conversion oxidation

domains;

of cobalt

that its presence of n-butane

one in which reoxidation increased

high Co/V ratios surface

after 5 hours is a

may lie in the

of cobalt as indicated oxides

for catalysing

on the surface

observed,

indicates

by offering

highly active total

the occurrence

of the latter to activate

of the catalyst

ability

to reoxidise,

thus increasing

optimum

value cobalt

had little effect

alter their behaviour

oxygen [26].

Tentative

of the role of cobalt

however,

the solid state chemistry to make allowance suggest

remains

for surface

sites for non-selective

areas

its

above an but provoked

oxidations.

in the fNP05 lattice

The reoxidation (figures

that cobalt was only homogeneously

low concentrations

However,

as to how coprecipitated

of 6VPO5.

On the one hand

on the solid state chemistry

by presenting

for

For samples with

so as to diminish

its selectivity.

a loss in selectivity explanation

extents,

of two distinct

was observed.

in cobalt would deeply

the solid state chemistry

The question,

total

could have provoked

In summary, the influence of cobalt may be seen as twofold. it modified

by the XPS

was slow for Co/V < 0.025 and the other,

area1 reoxidation

enrichment

of the strong ability

for

was

for the bulk of the catalyst.

in terms of surface area,

Co/V = 0.05, where

because

were measured

In this regard the shape of the curves of reoxidation

sites.

normalised

The first than the

in this quantity

of reoxidation

concentration

analysis.

suggests

of reoxidation

for the loss in selectivity

oxidations

was responsi-

Co/V ratio.

less proportional

no increase

Extent

process

explanation

ease of reoxidation

for the highest

rate was much

above Co/V equal to 0.01.

A quite

besides

observed

cobalt

experiments,

influences recalculated

88 and 108) and the XPS data

dispersed

(Co/V less than ca. 0.015).

in the bulk of BVPO5 at

Consequently

this section

of

the discussion

will focus on that region of composition.

It has been suggested catalysts

proceeds

ding shear defects

by other workers

via a shear plane type mechanism should be considered

on the redox and textural preting

that the reduction

properties

[6].

in explaining

close to an oxygen anion, as is presented

If

so,

the correspon-

the influence

of the catalyst.

these results would be to envisage

of pure BVPO5

A possible

of cobalt

way of inter-

Co++ ions in interstitial formally

in scheme

positions

1.

Scheme 1

o=

o=

o=

o=

q

+

co++ This type of structure reduction

process

q

o=c&

could act in two ways to diminish

stabilize

for this reaction.

it, thus diminishing

as acting as a termination

thereby

the reduction

An alternative the structure Within occured

viewpoint

would

Vacancies

thus restricing

and the presence

would be expected

anion

in a purely formal manner,

a POi sublattice

in the VOttt sublattice

points for shear structures environment

be to visualise,

and a VOttt sublattice.

cobalt would act to fill in any stacking

in the VOttt sublattice

sublattice.

oxygen

point for a shear structure

process.

of BVPO5 as comprising

this context,

the number of nuclea-

2) This type of highly stabilized

could be envisaged inhibiting

the rate of the

: 1) The extra positive charge of the cobalt ion, localised

near an oxygen anion,would tion centres

q

fault

ease of movement could be regarded

which

through

this

as nucleation

of Co++ or Co'++ ions in this

to remove this source.

ACKNOWLEDGEMENTS One of us (B.K. HODNETT)

wishes

to thank the Belgian

S.P.P.S.

for a research

fellowship.

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