Non-porous stabilized ZrO2 particles as support for catalysts

305

Applied Catulysi~, 29 (1987) 305-310 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

NON-POROUS

STABILIZED

P. TURLIER*, *Institut

Claude

AS SUPPORT

G.A. MARTIN*

DALMON*,

J.A.

de Recherches

1'UniversitP Cedex,

ZrO2 PARTICLES

sur

Bernard,

and P. VERGNON** Laboratoire

la Catalyse,

I, 2, avenue

Lyon

FOR CATALYSTS

Albert

Propre

Einstein,

du CNRS, 69626

associ6

1

Villeurbanne

France.

**Laboratoire Bernard,

de Catalyse

Lyon

Villeurbanne (Received

I associP Cedex,

5 July

Appliquee

au

CNRS

et Cinetique

(L.A

de l'universitg

HPtgroggne

novembre

43 bd du 11

n”2311,

Claude

1918,

69622

France.

1986, accepted

5 November

1986)

ABSTRACT The textural stabilization of non-porous zirconia has been obtained by addition stability of the promoted zirconia of lanthanum or yttrium oxides. The improved has been traced to the structural stabilization of the tetragonal form of Zr02.

INTROOUCTION Zirconia promoter, In CO

is often

included

or more generally

hydrogenation

induces

for

instance,

a long term stability

selectivity

towards

used as a support Various

alcohol

13 I,

this

work

even

during

starting

the

they

of improved

area

oxides,

thermal La203

prevent

is expected

are

to non-porous

discrete

however,

Y2O3

of ZrO2

particles,

and

are

easily to

known

the

the sintering

sinter intented

aim

in the of this

of ZrD2,

141.

01987

catalysts

121 and

the

When zirconia

is

are needed. divided

zirconia

interesting

been

from

preferred

by conversion

:

results.

The condensation

has

151. This

remove

Elsevier Science Publishers B.V.

the

technology paper

leading

stability.

0166-9834/87/$03.50

porous.

as a

in

of volatile

particles.

pretreatments and

to yield

generally

Zr02 has been obtained

into oxide

stabilizers also

solution

however,

surface

divided

material.

structural extent

leads

high

highly

tolerance

to obtain

in a flame reactor

These

sulfur

supported

can be considered

which

and

the

to metal

both the activity

of preparation fran a liquid

ZrClq

addition

either

in about 25 patents.

stability

obtained,

phase,

and

systems,

and a good thermal

the precipitation

vapour

zirconia

111. It also enhances

The

thus

catalytic

Its use is suggested

a high dispersion

methods

solids

in heterogeneous

as a support.

sintering

occurs

chlorine

of the

of ceramics

is to examine

to La or Y-doped

as

to what zirconia

306 METHODS The flame reactor used is a multitubular

burner which has previously been

described 161. The zirconium chloride is vaporized and carried by a stream of oxygen or nitrogen into the central tube of the burner. Hydrogen, oxygen and nitrogen are conveniently fed into the concentric tubes in order to control the flame temperature, the residence time and the concentration of reactive species in the flame. In

the flame a reaction occurs between the volatile precursor

(anhydrous

chloride) with the steam (or the oxygen in excess) leading to an oxide vapour, the partial

pressure

of which

is not in equilibrium

theoretically stable at this temperature.

with

the condensed

This supersaturation

phase

of the vapour

results in a hcmogeneous nucleation process. The lower the temperature of the flame the higher the supersaturation

and then the higher the nucleation

rate. The

conditions for the formation of small solid particles are thus obtained. The residence time of reactive species, monitored by the flow rate of the carrier gas of the chloride, controls the growth of the primary particles and then the size of final particles which are collected in an electrostatic precipitator. When the flame temperature is higher than the melting point of the oxide formed, an intermediate liquid state is possible. On the contrary if the flame temperature is below the melting point, the solid phase is formed directly from the vapour phase. If the residence time and the concentration of reactive species are sufficient, spherical particles resulting frcm the coalescence of several nuclei are obtained. After their formation, the temperature of the oxide particles is lowered very rapidly and the structural form which comes up during the condensation step is generally preserved. As a consequence of this quenching, the crystalline form of oxide particles takes into account the structural arrangement of the previous gas or liquid phase (minimal consumption of energy). The crystalline phase is then in most cases a metastable phase. In the present case the tetragonal phase of zirconium dioxide will be maintained if the quenching is effective. When the quenching is less efficient (for the case, for instance, of large particles) the monoclinic form will appear. Another consequence of the mechanism of building up of particles is their lack of porosity.

It is an inherited

feature of particles

formed by homogeneous

nucleation in a gaz phase. According to the adjusting conditions

of the flame

reactor two samples ZI and Z2 were prepared with specific surfaces areas

of 35 and

85 m2g-1, respectively. Both samples were cornparedwith a Zr02 powder DI (90 m2g-I) provided by Degussa and also prepared in a flame reactor. Addition of La203 or Y2O3 to Zr02 were performed using the incipient wetness method with solutions of La(N0313, 6H2O or Y(N0313, 5H20 frcm MERCK. The solids were then calcinated in air using a low heating rate (0.2 degree.mn-1) with a

307

15 hrs plateau at 673K. Solids with 4 and 1 mole % La203 and with 2 mole % Y2O3 were prepared from the Z2 zirconium oxide sample. The DI sample was prcmoted with 4 mole % La203. The sintering studies were performed by air heating during 15 hours at temperatures from 673 to 923K. RESULTS Figure 1 shows the effect of thermal treatment on the specific area of sample ZI. A small change is seen for this low area ZrO2. X-ray analysis shows that the initial monoclinic structure remains unchanged during the air heating.

S m? g-1 100,

50 -

I 673

I 773

I 873

temp

(K)

Fig.1 : Variatfons of the BET surface area of the ZI sample Figure 2 shows the change in specific area of the sample Z2 as a function of thermal treatment. Curve 1 is related to the unprolnotedsolid and curves 2,3 and 4 to Z2 promoted with 4 and 1 mole % La203 and 2 mole % Y2O3 respectively. The X-ray analysis shows that the starting solid Z2 is a mixture of monoclinic and tetragonal phases, the tetragonal one being the most abundant. Upon air heating the unprolnoted solid shows an increasing content of the monoclinic form (at 923K the monoclinic species

is clearly

prevailing).

In contrast

predaninant in the La or Y-promoted oxides.

the

tetragonal

phase

remains

308

S

2 -1 m.g 100 .initial S .

1 Q::;

50 .

IlJ

I

Fig.2

: Variations 0 zirconia

I

I

773

673 of the

temp (K)

873

BET surface

area of the Z2 sample

alone

* Zr02 promoted

with 4 moles % La203

0 2 moles % Y2O3 + 1 mole % La203 initial

S corresponds

to the unpromoted

solid after

standard

desorption

(10m5 torr, 1 hour at 67310

Figure is

3 shows

relative

textural

the behavior

to pure

effect

01 against

thermal

treatment.

curve 2 to the solid with 4 mole % La203.

zirconia,

stabilization

of the sample

is observed

for D1 when

adding

La203.

S m? g-l

initial

Fig. 3

100 S

: Variations

I

I

I

673

773

873

of the BET surface

0 zirconia

alone

* promoted

with 4 moles

temp

area of 01 sample

% La203

(K)

Curve

1

The same

DISCUSSION The

experimental

structural

and

poorly

divided,

flame

reactor

favoring

lead

low

a higher

have

a short

particle

growth.

from

metastable

is

(tetragonall

temperatures,

area,

time

limited

by

the

and

a structure

the stable monoclinic

affected

high

structure.

undergoes

thermal

the

stable

sample,

transformation

phase

phase

structure.

treatment

where

effective

This

and

the

the

products

when

reactor

leading

heated

corresponding

and in the final

the

zone or the flame

; the textural properties

treatment

of the

(Fig. 11. The Z2 sample,

in conditions

is

between

is rather

in the flame,

allows

(monoclinic)

affected

quenching Z2

which

conditions

the thermal

temperature

This

relation

sample,

quenching

throughout

is obtained

in the

ZI

time of the products

worse

is not drastically

surface

then

to

that a direct The

: the adjusting

resulting

maintained

surface

suggest exist.

long residence

metastable

residence

growth

towards

the

specific

work does

structure

The

is obviously

specific

with

this

to a rather

particle

structure

rather

of

properties

has the monoclinic

transformation stable

results

textural

at

to

;

the

increasing

to the transition

of the solid are also

state the Z2 sample

looks

like

the ZI solid. When

adding

textural the

structural

characteristics

thermal

specific

treatment

area

When effective

for

almost

the

view,

Y2O3

phenomenon

has

corresponding

been

structural

Y3+

in zirconia.

to 0.9

the

difference

the

for

ionic

point

Zr02.

radii

Y3+ and Zr4+,

of

This

of

the

respectively).

due to the substitution

for the case of yttria

is easier

is the most

(2 mole % Y2O3 yields

promoter

of

and the

solid.

Y2O3

from the structural

stabilizing

is probably

that

properties

and 0.8 A for La3+,

effect

The substitution

to the unpromoted

and

during

is observed

(Fig.21

Similarly,

better

structural

of La3+ or

since the ionic

of Y3+ and Zr4+ are nearer.

It appears

that for all the cases

changes

companion

phenomenon

mobility This

have

the phase Works

suggesting of phase

and a subsequent observation

of alumina

these

a

(1.1,

stabilization

structural

which

be

both

well maintained

transformation

of the textural

traced

cations

The

radii

to

zirconia,

are rather

it appears

as 4 mole % La2031.

reported

Z2

when compared

Y .promotors

stabilization

same effect is

phase

less affected

La and

the

to the material

: no important

is clearly

comparing

stabilizers

of the starting

shown

is that

that

studied, the

the textural

textural

transitions,

which

properties

evolution induce

of

parallel

zirconia

an increase

the is

a

of the ion

sintering. in good agreement

addition

and simultaneously

of La203

with

the recent

toy-Al203

the sintering

which

data of two groups

retards is believed

they-a to be

17-81

transformation associated

to

transformation. are

now

stabilized

in progress zirconia.

in our laboratory

to develop

catalysts

supported

on

310 CONCLUSION It can be therefore concluded that La203 and Y2O3 can be used as textural promoters of ZrO2, leading to an improved resistance to thermal sintering. This r study has also confirmed that the textural stability could be directly connected with the structural stability of solids. AKNOWLEDGEMENTS The authors are indebted to Rhone Poulenc Recherches and Gaz de France for fruitful1 discussions and financial support.

They also

gratefully

acknowledge

M.C. Durupty and F. Leconte for preparing pyrogenic zirconias, and DEGUSSA for providing a commercial sample. REFERENCES 1. K.J. Andersen, R. Candia and J. Rostrup-Nielsen, Ger. Offen. 760 (1974) 122. K.J. Andersen, R. Candid and J. Rostrup-Nielsen, U.S. Patent 3, 988, 262 (1976). 2. A.S. Lisitsyn, V.L. Kuznetsov and Yu.1. Yermakov, React. Kinet. Catal. Lett. 14 (1980) 445. L. Bruce and J.F. Matthews, Applied Catalysis 4 (1982) 353. L.A. Bruce, G.J. Hope and J.F. Matthews, ibid. 8 (1983) 349. T. Izuka, Y. Tanaka, and K. Tanabe, J. Mol. Catal., 17 (19821 381. 3. C.D. Chang and P.D. Perkins, U.S. Patent 4, 440, 668 (19841. M. Ichikawa, Bull. Chem. Sot. Jap. 51 (1978) 2268. 4. R.A. Dalla Betta, A.G. Piken and M. Shelef, Ger. Offen. 761, 111 (1975). R.A. Dalla Betta, A.G. Piken and M. Shelef, J. Catal. 35 (1974) 55. R.A. Dalla Betta, A.G. Piken and M. Shelef, J. Catal. 40 (19751 173. 5. R. Szymansky Thesis Lyon (19851. 6. M. Formenti, F. Juillet, P. Meriaudeau, S.J. Tefchner and P. Vergnon, J. Colloid. Interface Sci., 39 (1972) 79. 7. H. Schaper, E.B.M. Doesburg and L.L. Van Reijen, Applied Catalysis, 7 (1983) 211. 8. S. Matsuda, A. Kato, M. Mfzumoto and H. Yamashita, Proc. 8th Int. Cong. Catalysis, Verlag Berlin (19841, IV-879. 9. R. Ruh, K.S. Mazdiyasni, P.G. Valentine and H.O. Bielstein, J. Amer. Ceram. sot., 67 (19841 C190. 10. E. Tani, M. Yoshimura and S. Somiya, J. Amer. Ceram. Sot., 66 (1983) 11. 11. T. Sato and M. Shidama, J. Amer. Ceram. Sot., 10 (1984) C 212.