Catalytic and physico-chemical properties of La1−xSrxCoO3 perovskites

React.

Kinet.

Catal.

Lett.,

Vol.

53, No.

I, 223-229

(1994)

RKCL2506 CATALYTIC

AND P H Y S I C O - C H E M I C A L Lal_xSrxCoO3

L.A. S.V.

Isupova,

Tsybulya,

V.A.

M.P.

Boreskov

Institute

+Ural

Received Accepted

properties

num cobaltites

was

with the cobalt composition

cently.

Their

related

properties

and catalytic tention appear oxides,

differs

in oxidation

to correlate

(HCI, HF,

action of complex

cor-

on the surface;

to the den-

was noted.

of the catalytic

S02)

LaMnO3,

and simple oxides

and phys ~

have been published

(up to 1600 K)

[1,2] attract much at-

LaFeO3,

of ABO 3 per-

LaNiO3,

for transition

nature

LaCrO 3 metal

of the catalytic

[3]. Nevertheless,

turned out to depend on both the nature

re-

to that for Pt and Pdto heating

The order of activity

a similar

(3)

Under our experimental

with the same trend

which m a y evidence

com-

activity

from the bulk one;

(comparable

reactions

lanthaof CO oxiIt

is proportional

defects.

to studies

to these compounds.

ovskites

concentration

high resistance

poisoning

Russia

structure.

catalytic

of perovskites

high activity

containlng'catalysts),

Russia

the data about chemical

no role of point defects

Many papers ico-chemical

620083,

and bulk defect

activity

sity of bulk extended conditions

630090,

of strontium-substituted

(I) the initial

the steady-state

Petrov +

February 15, 1994 March 28, 1994

of the surface

(2) surface

Rar,

A.N.

Lal_xSrxCOO 3 in the reaction

found that

relates

A.A.

Novosibirsk

Ekaterinburg

dation were compared w i t h position

Ivanov,

Kolomilchuk,

Kononchuk +

of Catalysis,

State University,

Catalytic

V.P.

V.N.

and O.F.

OF

PEROVSKITES

Sadykov,

Andrianova,

PROPERTIES

of B cations Akad4miai

activity and the

Kiadd,

Budapest

ISUPOVA et al.:

presence

PEROVSKITES

of substituting

cations

in A sublattice.

data on the effect

of the degree

very c o n t r a d i c t i n g

[4,5], which may be explained

ence of the perovskite activity

of lanthanum

preparation

Literature

substitution

technique

are

by the influ-

on their catalytic

[1,2].

The aim of this work was to study the catalytic ico-chemical elucidate

properties

the reasons

of Lal_xSrxCOO3

perovskites

for the conflicting

and physand to

literature

data.

EXPERIMENTAL As starting pure),

La203

procedure perform

materials

Co304

(99.9% purity)

with repeated

the syntheses.

(pure),

were used.

intermediate

SrCO 3 (chemically

The solid-state

grindings

The final annealing

ceramic

was used to

was carried

out at

1373 K. The catalytic determined

activity

in a p u l s e / f l o w

with a v i b r o f l u i d i z e d position

was

in CO oxidation installation,

catalyst

bed.

1% CO + 1% 02 in He.

that,

the samples

The initial mixture

were

subsequent

first

termine

the initial

action mixture was analyzed

catalytic

after treatment

com-

activity.

was admitted

Steady-state

of the sample

for 1-2 h. Composition

cooling

at to

flushed with a helium

stream and then a pulse of CO + 02 mixture

were m e a s u r e d

K was

Samples were pretreated

673 K in the stream of dry oxygen with 413 K. After

at 373-500

using a mieroreactor

activities

in the flow of re-

of the reaction

by using gas c h r o m a t o g r a p h y

to de-

(thermal

mixture

conductivity

detector). The surface

chemical

composition

was analyzed

method using MS-7201 mass-spectrometer. Ar + ion beam was prevent were

supported

the dependences exposure ondary

the current during

onto high purity

density

indium

foil.

of Co + , La +, Sr + secondary

of the incident

beam were measured.

of corresponding

- 10-20 ~A/cm 2. To

ion bombardent,

ion yields of Co, La, Sr cations

the ratios 224

4 keV,

sample charging

by the SIMS

The energy of incident

ion currents

the samples In the experiments,

ion currents

on the

The relative

were determined

sec-

from

after attainment

of

ISUPOVA

the c o n s t a n t reference)

values

and were

and 3.6,

X-ray

patterns

using CuK

carried

of the r e l a t i v e

[6].

nickel

filter

and an a m p l i t u d e

specific

thermal

RESULTS

20 = of

In these

surface

desorption

experiments

was

of e x t e n d e d

X-Ray CuK

determined

Scattering

radiation

a n a l y z e r were

de-

with

a

used.

by the BET m e t h o d

of Ar.

AND D I S C U S S I O N

Initial Sr c o n t e n t tivities over,

a URD-63

on the d e g r e e

densities

out by the S m a l l A n g l e

(SAXS)

The

with

The scan r e g i o n was

of cell p a r a m e t e r s

Method

using

i (chosen as

(x) for La was also d e t e r m i n e d .

An e v a l u a t i o n fects was

0.ii,

were obtained

radiation.

4 ~ - 30 ~ . The d e p e n d e n c e Sr s u b s t i t u t i o n

to be

PEROVSKITES

respectively.

diffraction

diffractometer

found

et al.:

catalytic

increases

change

initial

tionary

activities (Fig.

values.

These

facts

centers

the d e g r e e

of d e a c t i v a t i o n

similar,

changes

composition

some

stability

found

imply while

to i n c r e a s e

i), w h i l e

manner

(Fig.

higher

that e i t h e r attaining

of t h e

2-5).

acMore-

than the sta-

the n a t u r e

of

the s t e a d y - s t a t e

of the same c e n t e r s

of the surface.

as the

steady-state

i, c u r v e s

are c o n s i d e r a b l y

the a c t i v e

the c h e m i c a l

i, curve

in a c o m p l e x

activities

are

depends

Since

steady-state

curves

centers

or

upon 2-5 are

up to 400 K

can be i n f e r r e d . The SIMS tration

s t u d y has

shown

does not c o r r e l a t e

that:

with

(i) s u r f a c e

the b u l k one;

cobalt

concen-

(2) l a n t h a n u m

and s t r o n t i u m

concentrations

change

(3) in c h a r g e

balance

at x > 0.I some a c c u m u l a t i o n

point O-)

defects

occurs

since

stant c o b a l t slow growth

in the

serious

analysis

of l a n t h a n u m

(Fig.

has r e v e a l e d

structural

changes.

structure

for x > 0.4 the s t r u c t u r e

with

directions;

(anion v a c a n c i e s ,

is c l e a r l y

of Sr c o n c e n t r a t i o n

ite has a h e x a g o n a l while

layer

a sharp decline

concentration

The X - r a y causes

terms, surface

in o p p o s i t e

of

Co 4+ or

content

not c o m p e n s a t e d

at conby the

2). that

Thus,

strontium pure

addition

lanthanum

cobalt-

a = 5.44 and c = 13.091,

is cubic.

The s a m p l e s

with 225

ISUPOVA

et al.:

PEROVSKITES

12

10

8 84

!

o x

_

_ i"

I

0

I

0l

~

0.4

gm~

I

06

0.8

X

Fig.

i.

Dependence

o f the

steady-state

X = 0.3

and

as c u b i c type of ~des

C O / m 2 s)

on the

strontium

0.4

of

expansion

226

The most

transition

of

strong

increase the

i) a n d

activities

(W.

1 - 413 K;

transient hexagonal of

the

and

c a n be

(increase the

For

content

each

pro-

o f c for h e x a g o n a l increase

o f the

de-

of the perovskite

structure

would

SAXS

demonstrate

data

defined

distortions.

strontium

in a c h a n g e

Indeed,

2 - 373 K;

5 - 500 K

Therefore,

disordered

region.

K;

lattice

results

2-5)

content:

4 - 458

structures).

substitution

structure. the

with the

(curve

for L a l _ x S r x C O O 3 p e r o v s k i t e s

structurally

structures

a n d a for c u b i c gree

are

structure,

some

(curves

molec.

3 - 413 K;

initial

be e x p e c t e d

in

a distinct

ISUPOVA et al.:

PEROVSKITES

0.6 ~

0.4 (J

02

I

I

J

02

04

0.6

r

X

Fig.

2.

Surface

concentration

thanum

[C3 of cobalt

(2) and strontium

(i), lan-

(3) cations

for

Lal_xSrxCOO 3 system

~

40

E 2o 0

I

'1

I

i

0.2

0.4

0.6

0.8

X

Fig.

3.

Integral

intensity

tended defects

of the scattering

(I, relative

units)

on ex-

versus

x

for Lal_xSrxCoO 3 maximum

of the integral

defects

at x = 0.3-0.4

intensity (Fig.

of scattering

on extended

3).

227

ISUPOVA et al.: PEROVSKITES

Hence,

Lal_xSrxCOO 3 samples

(oxygen vacancies, and extended

defects

Figures

catalytic

activity

activity.

So, one can expect

decline

perovskites.

to the steady-state

seems to be p r i m a r i l y in the number

also b y b i o c k i n g

that the ini-

surface)

varies

isolated

active

If this

value

determined

cobalt

of

the cations

sites on the clear

is so, the activity

caused by reaction by surface

of Co3+ cations),

of the surface

di-

Any variations

at x > 0.i do not influence

as c a t a l y t i c a l l y

of oxidized

crease

reconstruction.

cobalt concentration.

surface

incorporation

I) suggest

(clean oxidized

concentrations

(Co 3+) to function

by strontium

i) and 2 (curve

as the surface

point defects

contain both point defects

generated

caused by structural

1 (curve

tial catalytic rectly

etc.)

media

reduction

possibly

by strongly

(de-

accompanied

adsorbed

car-

bonates. On the other hand, ty correlates curves

2-5 and Fig.

concentration importance

in steady

cations defects

state clusters upon

function

surface

reduction) sites.

centers

reactive

forms of CO and oxygen,

bonates

due to a smaller

cobalt

thus making type

resistent number

ions,

at outlets

The reason

[7,83.

By that

localized

of extended

for high activity of weakly

possible These

to blocking

of basic

the

catalysis.

one can expect

to be the coordination

nism of the L a n g m u i r - H i n s h e l w o o d to be more

[7,8],

i,

i) suggests

for steady-state

oxides

activi-

(Fig.

w i t h the surface

2, curve

of low-charged

as active appears

(Fig.

of these

also expected

and SAXS data

3) and does not correlate

the pure cobalt

(or appearing defects

fact that the steady-state

structural

of cobalt

of extended

analogy with

the

well with

bound

a mecha-

clusters

by surface

Sr cations

are car-

as nearest

neighbors. The results

obtained

allow to explain

data on the effect of substitution for lanthanum. uncertainty

First, state).

caused by preparation

228

of alkaline

the discrepancies

in the conditions

or non-steady

Second,

the contradicting

of activity

measurements

an essential

procedure

earth cations

can be ascribed

determining

influence

to the (steady

may be

the bulk defect

ISUPOVA et al.: PEROVSKITES

structure and, hence,

the density of surface active centers

in

steady-state conditions.

REFERENCES i.

N. Yamazoe,

2.

M. Zwinkels, Rev.-Sci.

3.

Y. Teraoka:

Catalysis Today, 8,

S. Yaras, P.G. Menon,

Eng.,

G.K. Boreskov:

35, 319

175

T. Griffin:

(1990). Catal.

(1993).

Heterogeneous

Catalysis.

Nauka, Moscow

1986. 4.

K. Tabata,

5.

L. Wachowski:

I. Matsumoto:

6.

V.A. Sadykov~ ~ S.F. Tikhov, V.V. Popovskii, 7_44, 200

7.

Sci.,

125, 745

G.N. Kryukova,

V.N. Kolomiichuk:

22, 1882

(1987).

(1988). N.N. Bulgakov,

J. Solid State Chem.,

(1988).

V.A. Sadykov,

Yu.A. Lokhov,

M.N. Bredikhin, barov,

J. Mater.

Acta Chim. Hung.,

S.F. Tikhov,

V.V. Popovskii,

L.P. Soloveva,

VI Int. Symp. Heterog.

G.N. Kryukova,

N.N. Bulgakov,

I.P. Olenkova, Catalysis,

V.A. Razdo-

A.V. Golovin:

Proc.

Sofia, Bulgaria.

Part.

i,

1987. p. 359. 8.

V.A. Razdobarov, gakov,

G.N. Kryukova, 3_/7, 109

V.A. Sadykov,

O.N. Kovalenko,

S.A. Veniaminov,

Yu.D. Pankratiev,

S.F. Tikhov:

N.N. Bul-

V.V. Popovskii,

React. Kinet. Catal. Lett.,

(1988).

229