Catalytic activity of rare-earth orthoferrites and orthochromites

Catalytic activity of rare-earth orthoferrites and orthochromites

! Mat. Res. Bull., Vol. 16, pp. 97-103, 1981. Printed in the USA. 0025-5408/81/010097-07502.00/0 CopyMght (e) 1981 Pergamon Press Ltd. CATALYTIC ACT...

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Mat. Res. Bull., Vol. 16, pp. 97-103, 1981. Printed in the USA. 0025-5408/81/010097-07502.00/0 CopyMght (e) 1981 Pergamon Press Ltd.

CATALYTIC ACTIVITY OF RARE-EARTH AND ORTHOCHROMITES

T. Arakawa,

ORTHOFERRITES

S. Tsuchi-ya and J. Shiokawa

Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadakami, Suita-Shi, Osaka-Fu, Japan

(Received November 17, 1980; Communicated by J. B. GoodenouKh)

ABSTRACT The catalytic activity of LnFeO 3 and LnCrO3(Ln=La-Gd) for methanol oxidation has been studied by the measurement of conductivity change on methanol adsorption in the mixture gas (02(0.5%)+ N2(99.5%)) ; T~e activity of these compounds appeared above the Neel points. The sequence of the activity for LnFeO 3 was Gd > E u > Sm > N d > Pr > La, which was correlated with the N~el temperature; GdFeO3, which has the lowest N~el temperature, showed the highest activity. However, a relationship between activity and N~el temperature was not clearly observed in LnCrO 3.

Introduction We have studied the catalytic properties of rare-earth transition-metal mixed oxides in an attempt to obtain a gas sensor (1,2, 3). An interesting aspect of rare-earth perovskites(LnMO3) is that one is able to vary the dimensions of the unit cell by varying the lanthanide ion. Changes in the crystal dimensions may be expected to produce variations in the Ln-O and M-O interactions. It would be interesting, therefore, to study the effect of the rare-earth ion on the catalytic activity of rare-earth transition-metal mixed oxides. According to Goodenough's model (4,5,6), the character of the d-electrons in transition-metal perovskites is dependent on the overlap of the cationic d-orbitals with neighboring cations through the adjacent anion. On the basis of this model, LnMnO3, LnFe03 and LnCrO 3 should show local97

98

T . ARAKAWA, et al.

Vol. 16, No. 1

ized-electron behavior. In a previous paper(3), we reported the catalytic activity of rare-earth manganites. It was found that the catalytic activity increased with the Weiss constant @p, and that On increases with the concentration of mobile holes in the Mn3+; ~4 band (via double-exchange component). The purpose of this paper is to present the relationship between the catalytic activity and the magnetic behavior for LnFeO 3 and LnCrO 3 . Experimental and Procedures Catalyst preparation. The catalyst used was prepared by the solid-state reaction of dried Ln203 and Fe203 or Cr203. The well ground mixtures of components were fired at 1300°C in air for 10 hr. These compounds consisted of a single orthorhombic phase, as determined by x-ray diffraction (Table i). TABLE 1

X-ray lattice constants for LnFeO 3 and LnCrO 3 O

O

O

Sample

Symmetry

a(A)

b(A)

c(A)

LaFeO 3 PrFeO 3 NdFeO 3 SmFeO 3 EuFeO 3 GdFeO 3 LaCrO 3 NdCrO 3 SmCrO 3 EuCrO 3 GdCrO 3

Orthorhombic " " " " " " " " " "

5.556 5.495 5.441 5.394 5.371 5.346 5.479 5.430 5.368 5.332 5.308

5.565 5.578 5.573 5.592 5.611 5.616 5.513 5.488 5.494 5.502 5.510

7.862 7.810 7.753 7.711 7.686 7.668 7.756 7.692 7.641 7.617 7.597

Procedures. The synthesized powder (surface area; <2m2/g) was pulverized to about 300-mesh size and mixed with a small amount of nbutyl acetate-cellulose solution to make a paste. Oxide/paste weight-ratio was unity. Subsequently the paste was printed on an alumina plate(5mm x 2mm x imm) and dried. The sample was then heated at 1000°C for 10 minutes and cooled in air. Electrical constants were made using platinum mixed paint at both ends of the painted oxide film. The thin film (thickness; ca. 40~m) was set in a pyrex glass tube, of 10mm i.d., and a mixture gas(N2( 99.5%) + 02(0.5%)) was passed through. The electric circuit for measuring conductivity changes in these oxides is shown in Fig. i.

5 ~

op

i]

1

- D C 50URCE ~ 10 V

R=400 n op'

P.P':Junctions to recorder 5: Sensor FIG. 1 Schematic diagrams for conventional method.

Vol. 16, No. 1

R A R E - E A R T H ORTHOFERRITES

99

A fixed resistance Rs(400~) was connected in series with the thin film, and the electric current flowing through the film was measured with a electric recorder as the potential drop across Rs. As a source of direct current, a d.c. voltage generator (10V) was used. The catalytic reaction was carried out in a fixed bed flow reactor under atmospheric pressure. The gaseous mixture(60cm3/ min) of methanol(Tvol%) and O2(9voi%), diluted with N2(85vol%) , was passed through the catalyst bed at a fixed contact time, usually ig.sec/cm 3. The gas composition was analyzed before and after the reaction by gas chromatograph using the following column packing: Molecular sieve 13X for N 2 Jnd 02, Porapak Q for CO2, and Chromosorb 105 for CH3OH. The magnetic-susceptibility 6i EuSm Pr data for LnFeO 3 and LnCrO 3 were obtained with a Shimazu MB-II 5 tnFe03: Gd /Nd//La magnetic balance over the range ~ ~ - ~ . . / / 300-800K and 77-800K, respec~z tively. ~ / The temperature-programmed® desorption (TPD) experiments ~2i were performed as descrived in previous papers (1,2) =~ I 0 J

Results and Discussion The temperature dependence of relative sensitivity of LnFeO 3 and LnCrO 3 due to the injection of 1 pl methanol is shown in Fig. 2 and Fig. 3, respectively. Sensitivity of GdFeO 3 at 350°C,0.001mV/M~, and of SmCrO 3 at 250°C, 0.058 mV/K~, is taken as unity. A few isolated examples of the conductivity change are shown in the circules of Fig. 2 and Fig. 3. The increased conductivity for LnFeO 3 and the decreased conductivity for LnCrO 3 are restored to their initial values. Therefore, it would be thought that the state of oxidation of these samples did not change before and after the reaction. The Seebeck coefficient of LnFeO 3 and LnCrO 3 is shown in table 2. Thus it is understood that LnFeO3, except for PrFeO3, is an n-type semiconductor and LnCrO 3 is a p-type semiconductor. The property of semiconductor for PrFeO 3

i

250

~0

350

400

Temperature ('C)

450

560

FIG. 2 Temperature dependence of the sensitivity to a methanol pulse for LnFeO 3 thin-film detectors. 6 Nd//~m

thor03:

5 ~4 / ~ ~ =

~

y/

3 ~2

G

~I

/

~/ ~ "

6d~_

0 ~0

~0

~0 ~0 460 Temperature("C) FIG. 3

450

Temperature dependence of the sensitivity to a methanol pulse for LnCrO 3 thin-film detectors.

T. ARAKAWA, et al.

I00

TABLE

2 Seebeck

coefficient

of L n F e O 3 and L n C r O 3

Sample

Seebeck coefficient (mV/°C)

Sample

LaFeO 3

-0.30 (370oc - 500oc) +0.08 - -0.02 (530oc - 730oc) -0.22 (450oC - 500oC) -0.42 (440oC - 500oC) -0.40 (410oC - 500oC) -1.60 ~ -2.20 (420oC - 500oc)

LaCrO 3

PrFeO 3 NdFeO 3 SmFeO 3 EuFeO 3 GdFeO 3

Vol. 16, No. 1

NdCrO 3 SmCrO 3 EuCr03 GdCrO 3

Seebeck coefficient (mV/°C) +0.37 (230°C +0.40 (280°C +0.24 (290°C +0.69 (310°C +0.90 (270°C

-

+0.56 500oc) +0.80 500oC) +0.42 500oc) +1.50 500oc) +1.70 500oc)

m o v e s from a p - t y p e to an n- type. These c o n d u c t i v i t y changes e x c e p t for P r F e O 3 c o r r e s p o n d e d to the S e e b e c k c o e f f i c i e n t . For PrFeO3, it is s u g g e s t e d that the surface c u r r e n t is d o m i n a t e d by bulk electrons. The s e q u e n c e of the a c t i v i t y was Gd > Eu > Sm > Nd Pr > La for L n F e O 3 and Sm > N d > La > Gd > E u for LnCr03, w h e r e the act i v i t y is given by the t e m p e r a t u r e at w h i c h the r e l a t i v e s e n s i t i v ity attains 2. Thus, it is found that the a c t i v i t y for L n F e O 3 i n c r e a s e s as the radius of the r a r e - e a r t h ion decreases. But the a c t i v i t y for L n C r O 3 is not c l e a r l y c o r r e l a t e d w i t h the radius of the r a r e - e a r t h ion. The m a g n e t i c s t r u c t u r e s of the o r t h o f e r r i t e s have b e e n s t u d i e d by n e u t r o n d i f f r a c t i o n ( 8 ) and d i s c u s s e d by s e v e r a l a u t h o r s ( 9 , 1 0 , 1 1 , 12). Plots of inverse m a g n e t i c s u s c e p t i b i l i t i e s , X~ l, a g a i n s t t e m p e r a t u r e for L n F e O 3 are shgwn in Fig. 4. These a n t i f e r r o m a g n e t i c c o m p o u n d s have a h i g h Neel t e m p e r a t u r e T N that i n c r e a s e s w i t h i n c r e a s i n g lattice p a r a m e t e r ( F i g . 6). T r e v e s (12) has p o i n t e d out that the s t r e n g t h of the m a g n e t i c i n t e r a c t i o n d e c r e a s e s w i t h d e c r e a s i n g lattice p a r a m e t e r b e c a u s e of the d e c r e a s i n g c a t i o n a n i o n - c a t i o n (Fe3+-O2--Fe 3+) angle. The X~ 1 - T curves for L n C r O 3 are shown i 9 Fig. 5. These are also a n t i f e r r o m a g n e t i c compounds, b u t the N e e l t e m p e r a t u r e s are lower than those for LnFeO~. A l ~ o n a r d et ai.(14,15) have obs e r v e d an i n c r e a s e in T N w l t h i n c r e a s i n g lattice p a r a m e t e r for the s y s t e m s LnCrO3, but we did not c o n f i r m this o b s e r v a t i o n . SmCrO 3 and E u C r O 3 have two m a g n e t i c t r a n s i t i o n s , and G d C r O 3 has no N ~ e l p o i n t in the m e a s u r e d t e m p e r a t u r e range. The v a r i a t i o n of the a c t i v i t y of L n F e O 3 for m e t h a n o l o x i d a t i o n w i t h the atomic n u m b e r of the r a r e - e a r t h e l e m e n t s is shown in Fig. 6, w h e r e it is also c o m p a r e d w i t h that of the N~el t e m p e r a t u r e . The a c t i v i t y i n c r e a s e s as the r a r e - e a r t h ion decreases, w h i l e the NSel t e m p e r a t u r e d e c r e a s e s as the radius of the r a r e - e a r t h ion decreases. Since this r e l a t i o n s h i p was not c l e a r l y o b t a i n e d for L n C r O 3, we do n o t discuss it in f u r t h e r detail.

Vo]. 16, No. 1

RARE-EARTH ORTHOFERRITES

101

400 ,....... "~300

~~ ....................;~.'; ......... . Nd /...... !f"'"~'~Pr

~200

..."""

TE

\ LnCr03 : \

E

?"''!!--E u

~: :: ................~.'...'."". :.....................~..". .....-~'

100

1000

La

LnFeO 3

G~

....... ~:: .................................... ........... :::... .......

800

\ ...........!i ....

;E 400

!

~

i...................................i.~..'.r......

&

I

I

I

I

300

400

500

600

700

0 800

0

I

i

i

i

i

I

100

200

300

400

500

600

700

Temperature ( K )

4

Inverse s u s c e p t i b i l i t y p e r a t u r e for LnFeO 3.

............................................. Gd

I

Temperature (K)

FIG.

Sm

: ........... :::::::::::::::::::::: t-~-:E~!.-.7~ .......... ~-~:~ ........................... i~Nd

200

,~;~:"~'d 0

La ............... ..................................

,~.

~ 600 5

FIG. vs tem-

F r o m the e x p e r i m e n t of thermal p r o g r a m d e s o r p t i o n of oxygen, we r e c o g n i z e d that the a m o u n t of oxygen d e s o r p t i o n from L n F e O 3 was very small c o m p a r e d to that from LnMnO 3 (2). While, the catalytic r e a c t i o n was c a r r i e d out in a fixed b e d flow reactor. The r e s u l t of GdFeO 3 is shown in Fig. 7, c o m p a r e d w i t h the s e n s i t i v i t y w h e n the o x y g e n c o n t a i n e d in carrier gas was varied. The reaction p r o d u c t s were only CO 2 and H20. The d e c r e a s e of the methanol c o n c e n t r a t i o n in the o u t l e t gas begins to take place w h e n the t e m p e r a t u r e is h i g h e r than 210°C. The s e n s i t i v i t y d e c r e a s e d as the p a r t i a l p r e s s u r e of 02 increased. The t e m p e r a t u r e - c o n v e r s i o n curve c o i n c i d e d w i t h the s e n s i t i v i t y t e m p e r a t u r e curve in the case of n i t r o g e n (contained 02<50ppm). T h e r e f o r e , w h e n m e t h a n o l is adsorbed on LnFe03, it w o u l d seem that a bulk oxygen of L n F e O 3 reacted w i t h m e t h a n o l and o x y g e n ion v a c a n c y of 2- charge was produced, and then a v a c a n c y was im-

5

Inverse s u s c e p t i b i l i t y p e r a t u r e for LnCrO 3.

vs tem-

750

~

~\~

~700 ~ ~ ~ E ~650

600

\ \~ \ \i \T \\ ~ . Z \ Y-~ ~\

t a Pr Nd SmEu Gd

FIG.

6

C o m p a r i s o n of the c a t a l y t i c activity(~) of LnFeO 3 for o x i d a t i o n of m e t h a n o l w i t h the N~el t e m p e r a t u r e ( ~ ) of the oxides.

T. A R A K A W A ,

102

m e d i a t e l y consumed by oxygen in the gas phase a c c o r d i n g to the following e q u a t i o n ;

et al.

A

Vol. 16, No. 1

02(%)

6

-b- 20

F e 3 + _ O 2 _ _ F e 3 + 1/3 MeOH >

~ O

-4- 5 ---.- 05I

;

.

+ 2/3 tt20

(1)

.~

Fe3+-V''-Fe 3+ 1/2 02

o c

Fe 3+ O 2- Fe 3+ -

(2)

0

The authors are happy to a c k n o w l e d g e the support of S c i e n t i f i c Research Grant from the M i n i s t r y of Education, Japan, for part of this work.

2;,o

360

460 Temperature ( ' C )

FIG. 7 S e n s i t i v i t y to a methanol pulse and catalytic activity for the m e t h a n o l o x i d a t i o n reaction on GdFeO 3 .

References i. T. Arakawa, S. Takeda, Bull., 14, 507(1979).

G. Adachi

and J. Shiokawa, Mat. Res.

2. T. Arakawa, A. Y o s h i d a and J. Shiokawa, Mat. Res. Bull., 269(1980).

15,

3. T. Arakawa, A. Y o s h i d a and J. Shiokawa, Mat. Res. Bull., 347(1980).

15,

4. J.B. ~oodenough, ence publishers,

" M a g n e t i s m and the Chemical Bond ", IntersciNew York(1963).

5. J.B. Goodenough,

Bull.

6. J.B. Goodenough,

J. Appl. Phys.,

7. J.B. Goodenough,

Phys. Rev.,

Soc. Chim. Fr., 4, 1200(1965). 37, 1415(1966).

16__~4, 785(1967).

8. W.C. K o e h l e r and E.O. Wollan,

J. Phys. Chem.

9. R.L. White,

40, 1061(1969).

J. Appl.

Physics,

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22, 707(1956).

ii. H. Watanabe,

Soc. Jap.,

J. Phys.

Solids, ~,

12, 515(1957).

100(1957).

Vol. 16, No. 1

RARE-EARTH ORTHOFERRITES

103

12. P.K. Gallagher, J.B. MacChensney, D.N.E. Buchanan, J. Chem, Phys., 41, 2429(1964). 13. D. Treves, J. Appl. Phys.,

36, 1033(1965).

14. R. Aleonard, R. Pauthenet, J.P. Rebouillat and C. Veyret, J. AppI. Phys., 39, 379(1968). 8

15. R. Aleonard, R. Pauthenet, J.P. Rebouillat and V. Zarubica, Compt. Rend., 262, 866(1966). 16. G.V. Subba Rao, J.R. Ferraro and C.N.R. Rao, J. Appl. Spec., 24, 436(1970).