Surface State and Active Surface Layer Formation of Fe2O3-Sb2O4 Catalyst in Propene Oxidation

1239

SURFACE STATE AND ACTIVE SURFACE LAYER FORMATION OF Fe, 0, Sb, O4 CATALYST IN PROPENE OXIDATION N . YAMAZOE,

I . ASO, T. AMAMOTO, and T. SEIYAMA

Department o f Materials S c i e n c e and Technology, G r a d u a t e School o f E n g i n e e r i n g S c i e n c e s , Kyushu U n i v e r s i t y , Hakozaki, Fukuoka, 812 Japan.

ABSTRACT: The s u r f a c e s t a t e and p r o p e r t i e s o f Fe203-Sb204 c a t a l y s t s were s t u d i e d t o e l u c i d a t e t h e i r c a t a l y t i c b e h a v i o r . TPD and relev a n t e x p e r i m e n t s showed t h a t s u r f a c e oxygen w a s v e r y i m p o r t a n t i n t h i s c a t a l y s t s y s t e m and t h a t , u n l i k e t h e c a t a l y s t w i t h S b / F e = l , t h e Sb r i c h c a t a l y s t s ( S b / F e >1) accommodated t h e t y p e o f s u r f a c e oxygen which c o n v e r t e d p r o p e n e t o a c r o l e i n s e l e c t i v e l y .

I n ac-

c o r d a n c e w i t h t h i s , s u r f a c e a n a l y s e s by XPS and SIMS r e v e a l e d t h a t ,

a t Sb r i c h c o m p o s i t i o n s , t h e s u r f a c e o f FeSb04 g r a i n s w a s e n r i c h e d i n Sb c o n t e n t t o a c o m p o s i t i o n Sb/Fe c l o s e t o 2 , f o r m i n g a s u r f a c e thick. The s u r f a c e oxygen is c o n s i d e r e d t o b e l a y e r less t h a n 6 bonded t o Sb i o n s i n p r e f e r e n c e t o Fe i o n s a s r e v e a l e d by SIMS measurements combined w i t h 1802 a d s o r p t i o n .

In conclusion, t h e

s e l e c t i v e a c r o l e i n f o r m a t i o n o v e r Sb r i c h c a t a l y s t s i s a s c r i b a b l e t o t h e Sb e n r i c h e d s u r f a c e l a y e r which accommodates t h e s e l e c t i v e t y p e o f s u r f a c e oxygen.

1. INTRODUCTION The mixed o x i d e c a t a l y s t Fe203-Sb204 o x i d a t i o n of o l e f i n s . s t u d i e s on t h i s

is i m p o r t a n t f o r t h e a l l y l i c

N e v e r t h e l e s s t h e r e have been few b a s i c A c h a r a c t e r i s t i c f e a t u r e is t h a t , while

o n l y FeSb04 is d e t e c t e d a s a complex metal o x i d e compound i n t h i s c a t a l y s t system, t h e s e l e c t i v i t y o f t h e a l l y l i c o x i d a t i o n can be much improved by a d d i n g t h e Sb204 component i n e x c e s s o f t h e FeSb04 composition.

Thus t h e r e must b e a c e r t a i n t y p e o f promoting e f f e c t

a r i s i n g between FeSb04 and Sb204.

Although some p r o p o s a l s have been

made c o n c e r n i n g t h e a b o v e phenomenon by Boreskov e t a1.'l2),

et a

~

~ and ) ,S a l a and T r i f i r o ' ) ,

established.

Fattore

none o f them a r e f a r from b e i n g

I n a n a t t e m p t t o e l u c i d a t e t h e phenomenon, w e have

s t u d i e d t h e s t a t e and p r o p e r t i e s o f c a t a l y s t s u r f a c e by u s i n g v a r i o u s t e c h n i q u e s i n c l u d i n g XPS and SIMS, and r e a c h e d t h e c o n c l u s i o n

1240

N. Yamazoc, I .

Aso, T. Amamoto,

T. Seiyama

that an Sb enriched layer which forms on the surface of FeSbOq grains is responsible for the selective allylic oxidation. This paper is concerned with the surface layer formation and its role in the selective oxidation of propene. 2. EXPERIMENTAL

The Fe203-Sb204 mixed oxide catalysts were prepared by mixing an aqueous solution of Fe(N03)3 with a water suspension of Sb203. The slurry was evaporated to dryness, and calcined at 900°C for 2 hr. Tho mixed oxides with the atomic ratio Sb/Fe=l (Catalyst-I) and Sb/Fe=2 (Catalyst-D)were mainly used, which consisted of FeSb04 and FeSb04 + S b 2 0 4 , respectively, according to X-ray diffraction analyses. FeyOy and Sb204 were obtained from Fe(N03)3 and Sba03 by usual procedure, and calcined at 900°C and GOO"C, respectively. The catalytic oxidation of propene as well as the reduction behavior of catalysts in propene atmosphere was studied by using a conventional fixed bed flow reactor. For the study of oxygen adsorption, temperature programmed desorption (TPD) chromatograms of oxygen were measured at a heating rate 10"C/min in a helium stream. Surface analyses of catalysts by XPS and secondary ion mass spectrometry (SIMS) were performed with a VG ESCA3 MkII spectrometer ~ ,MgKccl,2 ~ radiation was used equipped with a SIMS system. A ~ K L X or in XPS and binding energies were calibrated with the A ~ 4 f ~ level , ~ (83.6 eV) of evaporated gold. Negative secondary ions were measured in SIMS using primary Ar' beams. 3. RESULTS AND DISCUSSION

3.1. Behavior of the Fe203-Sb204 catalyst in propene oxidation Catalytic properties of the Fe2O3-Sb2Oq system vary markedly with the catalyst composition as shown in Fig. 1. Very notable is the fact that, although acrolein formation and deep oxidation proceed with comparable rates at Sb/Fe=l (FeSb04), a further increase in Sb content suppresses the deep oxidation drastically giving rise to the high acrolein selectivity in the Sb rich region. To reveal why the selectivity to acrolein is improved in the Sb rich region, the following studies were made mostly by using two catalysts of Sb/Fe=l (Catalyst-I) and Sb/Fe=2 (Catalyst-II). The kinetics of the acrolein formation was well described by assuming a redox mechanism over both Catalyst-I and Catalyst-I17). Reduction studies of the catalysts in a propene flow showed that, in both catalysts, the surface oxygen of less than a surface monolayer was much more reactive than the bulk oxygen. An interesting fact was that the surface oxygen of Catalyst-IIwas very selective for the acrolein formation while that of

Surface State of Fez03-SbzOb Catalyst in Propene Oxidation

Fe203

FeSb04

1241

Sb204

I

Sb Content/atomic % Fig.1. Propene oxidation over Fe203-Sb20q catalysts (4OOOC). =0.05 atm, Po =0.20 atm. 2

Catalyst-I was not, in agreement with the results of the catalytic These results indicate that the origin of the oxidation (Fig. 1). difference in selectivity between the two catalysts should be sought in the properties of the surface oxygen. 3.2. Oxygen adsorption and reactivity of adsorbed oxygen The TPD chromatograms of oxygen from Catalyst-I and Catalyst-II are shown in Fig. 2. When oxygen was preadsorbed during cooling from 800°C to room temperature (SOO"C+RT)(Fig. 2(a)), oxygen desorption began at about 500°C for Catalyst-I, while it shifted to a slightly higher temperature followed by a steep increase for Catalyst-II. The difference of the two catalysts was made clearer when oxygen was preadsorbed at a constant temperature 400°C as shown in Fig. 2(b), in which Catalyst-I exhibits a desorption peak (denoted a ) around 580°C, while Catalyst-IIshows a rather monotonous increase ( 6 ) with a small shoulder about 600°C which may be ascribable to a . Thus it is considered that a and 6 oxygen are typical of Catalyst-I and -It, respectively. These chromatograms were significantly different from those for the component oxides, Fe203 and Sb204. The reactivity of the surface oxygen a and B was studied by admitting pulses of propene at 300°C after the specified oxygen preadsorption 800°C+RT. The lower parts of Fig. 3 show the changes of conversion (X) to acrolein or C02 as well as the selectivity to

1242

:

(a)

$

1. Catalyst-1

2/

\

;0.4-

e; b

(Sb/Fe=l)

I

:

0.1

. *E

-

(b) 1. Catalyst-I

(Sb/Fe=l)

a, v)

,

0.2-

a,

400

600 Temperature/"C

800

600

400

800

Temperature/"C

Fig.2. TPD chromatograms o f oxygen from Fe203-Sb204 catalysts. Spectra were normalized to unit surface area of catalysts. Oxygen preadsorption, (a) 8 O O 0 C -+ RT; (b) 400°C.

TPD Spectra 1.Without propene

> E

;

s.

b

a,

pulse

0.5-

3 TemDe rat u rc / C

(a)

Number of P r o p e n e P u l s e Catalyst-II ( S b / F c = Z )

Temperatur e / C

Number of Propene Pulse ( b ) Catalyst-I (Sb/Fe=l)

F 1 g . 3 . Reaction of aclsorbed oxygen with propene pulse (SOOOC). Pr o p v n e p u l \ ( , sizc' , 2 . 2 3 pmol / p u l s e .

Surface State of Fe203-Sb204 Catalyst in Propene Oxidation

1243

acrolein ( S ) with the number of propene pulse, while the upper parts illustrate the TPD chromatograms of oxygen recorded with or without the preceeding admission of propene pulses. In the case of Catalyst-11, the pulsed propene was converted to acrolein selectively except for the first pulse in which a considerable amount of propene was trapped on catalysts in an irreversibly adsorbed form. The change of the TPD chromatograms with the propene pulses clearly shows that the preadsorbed oxygen (mainly $ ) is actually consumed. In the case of Catalyst-I, on the other hand, propene was mainly converted to C02 with preferential consumption of a oxygen. It is thus concluded that a and $ oxygen are endowed with very different selectivity for propene oxidation. 3.3. Surface layer formation of Fe203-Sb204 catalysts In order to know why the properties of surface oxygen change with the catalyst composition as mentioned above, Fe203-Sb204 catalysts were subjected to surface analyses by XPS and SIMS. In the XPS study, the signal intensity ratios of Sb3d312 to Fe2p312, ISb/IFe, were measured for a series of catalysts with various bulk compositions. As a reference, the same measurements were also performed for mechanical mixtures of Fe203 and Sb204. The results are shown in Fig. 4. For the mechanical mixtures, ISb/IFe was proportional to the bulk Sb/Fe ratio as expected. On the other hand, the change was very characteristic in the case of catalyst samples. In the lower Sb content region (Sb/Fe < l ) , the intensity ratio almost coincides

Bulk composition, Sb/Fe Fig.4. XPS analyses of the surface composition of Fe203-Sb204 catalysts.

1244

N . Yamaaoe, I.

ASO,

T . Amamoto, T . Seiyama

w i t h Lhat o f t h e m e c h a n i c a l m i x t u r e s , i n d i c a t i n g t h a t t h e s a m p l e s

a r e s i m p l e m i x t u r e s o f t h e g r a i n s o f FeSbOq a n d F e 2 0 3 .

H o w ev er ,

I S b / I F e j u m p s s t e p w i s e by a c e r t a i n v a l u e a t S b / F e = l , f o l l o w e d by a s t e a d y i n c r e a s e a g a i n i n t h e r a n g e of S b / F e > 1 .

of I S b / I F e

The s u d d e n i n c r e a s e

s t r o n g l y s u g g e s t s t h a t t h e s u r f a c e o f FeSb04 g r a i n s is

On r e f e r r i n g t o

c o v e r e d by a l a y e r w h i c h is e n r i c h e d i n S b c o n t e n t .

t h e I S b / I F e l i n e o f t h e m e c h a n i c a l m i x t u r e s , t h e f o r m e d l a y e r is e s t i m a t e d from t h e s t e p h e i g h t t o have a composition Sb/Fe=1.8 as illustrated in the figure.

The s u c c e e d i n g s t e a d y i n c r e a s e a t

SI,/F'e > I i n d i c a t c s L h a t t h e c a t a l y s t s a r e m i x t u r e s o f t h e s u r f a c e -

c o v e r e d FeSbOq a n d S b 2 0 4 . I n accordance w i t h t h e s u r f a c e l a y e r f o r m a t i o n , t h e r e w a s a subt l e b u t s i g n i f i c a n t change i n t h e s h a p e o f Fe2p312

l u s t r a t e d i n F i g . 5.

s p e c t r a as il-

Whilst a s i n g l e peak w a s observed at b i n d i n g

e n e r g y ( B E ) = 7 1 1 . 3 i 0 . 3 e V f o r c a t a l y s t s w i t h S b / F e '1

(spectra (a)

a n d ( b ) ) , a s h o u l d e r d i d a p p e a r a t BE=709.2 t O . 3 e V f o r S b r i c h c a l a l y s t s a s t y p i c a l l y shown by s p e c t r u m ( c ) .

The p e a k a n d s h o u l d e r

can be a s s i g n e d t o Fe(III) and F e ( II ) , r e s p e c t i v e l y , b a s e d o n Lhe BE v a l u e s r e p o r t e d by

Brundle et a18).

The a p p e a r -

a n c e of F e ( II) i o n s s u p p o r t s t h e s u r f a c e m o d i f i c a t i o n i n t h e Sb r i c h r e g i o n as d i s c u s s e d later. A s f o r lhc s p e c t r a o f Sb 3d3,a,

t h e r e was n o d e t e c t a b l e c h a n g e over' a l l mixed o x i d e c a t a l y s t s e x a m i n e d , b u t t h e BE v a l u e ( 5 3 9 . 8 i O . l e V ) w a s f o u n d t o be 0.3 e V higher than t h a t f o r

S b ( II[ ) .

O r c h a r d a n d Thornton')

have r e p o r t e d a binding energy s h i f t of 0.6: e V b e t w e e n S b ( m ) and Sb(V).

The o b s e r v e d s h i f t

o f 0 . 3 e V i n o u r c a t a l y s t s may

J1

709.2 e V

s u g g e s t t h a t t h e s u r f a c e Sb i o n s e x i s t i n a mixed v a l e n c e

s t a t e of S b ( V ) a n d S b ( I I I ) . The d e p t h p r o f i l i n g , h y

u s i n g SIWS, p r o v i d e d i n f o r m a t i o n

on t h e t h i c k n e s s of t h e f o r m e d layer.

The m e a su r e m e n ts w e r e

made f o r t h e n e g a l i v e s e c o n d a r y

706

7 0 8 710 7 1 2 714 7 1 6 7 1 8

B i n d i n g Energy/eV

XPS s p e c t r a of Fe 2 p 3 / 2 Fig.5. l e v e l for c a t a l y s t s with ( a ) Sb/Fe=0.8, ( b ) 1 ( C a t a l y s t - I ) , and ( c ) 2 (Catalyst-II ) .

S u r f a c e S t a t e of Fe201-Sb204 C a t a l y s t i n Propene O x i d a t i o n

1245

ions containing Sb (SbOThe 2 and SbO; ) or Fe (FeO; and FeO;). result was expressed in terms of the intensity ratio of the Sb containing species to the Fe containing species, EISb/EIFe, in Fig. 6. While the ratio remains almost constant for Catalyst-I, that for Catalyst-11 decreases, starting from a high value, with increasing sputtering time (t,) until it reaches a steady value. Clearly, this indicates that Catalyst-11 contains particles whose surface is more enriched in Sb content than the bulk, supporting the results of the previous XPS study. From the sputtering rate (approximately 0.2 min) applied in this measurement, the thickness of the Sb enriched layer is estimated to be less than 6 1. A similar trend can alsobe seen for the impregnated catalyst which will be described later. On the basis of the XPS and SIMS study mentioned above, we conclude that, in the catalysts with Sb rich compositions (Sb/Fe >l), the surface of FeSb04 grains is enriched in Sb content. The estimated composition Sb/Fe=1.8 of the surface layer should be noted, since a compound FeSb206 (Sb/Fe=2), which has trirutile structure similar to the rutile structure of FeSb04, has been reported to form under particular conditionslO,ll). Formation of such a compound is consistent with the existence of Fe(II)in the Sb rich 1U,12) region, because FeSb206 can be described formally as FeESbgOg (an alternative is FemSbomf,Sby.506) while FeSb04 as FemSbV04. We

t/

consider therefore that the Sb enriched layer is composed of FeSb206 or something alike. The Sb enrichment is expected to occur also by modifying the surface of FeSbOq grains l5 with the Sb component. To con12 firm this, Catalyst-I (FeSb04) a, was impregnated with measured 2 9 w Sb-impregnated FeSb04 amounts of antimonic acid solu-

r------

--.

tion, and calcined at 800°C for 2 hr. The results of XPS measurements and catalytic propene oxidation on the impregnated samples are shown in Fig. 7. With an increase in impregnated Sb, the surface Sb/ Fe ratio determined from Isb/

IFe increased going through an inflection point at a surface coverage of Sb ions equivalent to 1.5 times the surface mono-

P

2

6

W

3 0

10 20 30 40 50 Sputtering Time, ts/min

Fig.6. Depth profiles of Catalyst-I, Catalyst-II, and the Sbim regnated FeSbOq. Primary Ar beams 4000 eV; 1.2 x 10-6 A/cm2.

P

1246

N. Yamazoe, I . Aso, T. Amamoto, T. Seiyama

Surface Coverage of Sb Ions

4

3

2

1

%+

0

0

10

20

30

40

Amount of Sb Impregnated/umol. I P - ~ Fig.7. Modification of catalytic properties of FeSbO4 with the impregnation of Sb oxide. Propene oxidation was carried out at 400°C. =0.05 atm, Po =0.20 atm. PC3H6 2

layer. Correspondingly, the acrolein formation went through a maxi'mum at the same surface coverage 1.5, while the C 0 2 formation decreased monotonously. It is noted that the surface Sb/Fe ratio at the inflection point is close to 2 in fair agreement with the case of Fig. 4. In accordance with the SIMS study on this catalyst (Fig. 6). the surface layer seems to be a few lattice layer thick, considering the surface coverage 1.5 of Sb ions needed to complete it. Further impregnation lowers the catalytic activity because the catalyst surface becomes partially covered by Sb204. These results show that the active surface layer can be prepared by the impregnation method. On the basis of such evidence, we conclude ,that the selective acrolein formation is ascribable to the Sb enriched surface layer. This is compatible with, among several proposals, the hypothesis of Sala and Trifiro') that the high selectivity be attributed to FeSb206 or Fe2Sb207 phase, not to FeSb04. 3.4. Adsorption sites of oxygen Why does the Sb enrichment bring about the modification of the properties of surface oxygen? Probably this question is deeply associated with which ions, Fe or Sb, accommodate the effective

Surface State of FenO,-SbxOb Catalyst in Propene Oxidation

1247

surface oxygen. To check the latter problem, we carried out the SIMS measurements combined with 1802 adsorption: the SIMS spectra were recorded for catalyst samples which, after evacuation at 700"C, had preadsorbed 1802 (99.9 atomic g) during cooling from 400°C to room temperature. In this case, negative secondary ions, SbO, and FeOi , contained l80 and/or l60. Figure 8 shows the l80 fraction in respective ionic species versus sputtering time (t,) for Catalyst-I and Catalyst-II. It is noted that in both catalysts the l80 fractions are far larger for SbOx than for FeO; at t,=O. Although the observed l80 fraction may in part be affected by the isotopic mixing of l80 with lattice l60, such large disparity in l80 fraction between Sb- and Fe- containing ions at ts=O would indicate that the adsorbed oxygen is bonded to surface Sb ions more preferentially than to Fe ions. A s the disparity is common for the two catalysts, it is considered that both a and 6 oxygen mentioned earlier are accommodated on the surface Sb ions. To account for their markedly different selectivity, however, there should be some difference in circumstances surrounding them. Sala and Trifiro6) have proposed the importance of the geminate Sb ions in the oxidative dehydrogenation of butenes. In view of the fact that a and B oxygen dominate respectively on FeSb04 and on the Sb enriched surface layer, it is inferred that the Sb ions surrounded by Fe ions accommodate ci oxygen while those adjacent to other Sb ions are responsible for 8 . The role of Fe ions seems to be the stabilization of the high valence state of Sb(V) by forming complex oxides such as FeSb04 and FeSb206.

Sputtering Time, ts/min

Sputtering Time, ts/min

Yig.8 Change of 180 fraction in secondary ions with sputtering time. Primary Ar+ beams, 4000 eV; 6 x 10-6A/cm?

1248 4.

N. Yamazoe, I .

ASO,

T. Amamoto, T. Seiyama

CONCLUSION T h e a c t i v i t y a n d s e l e c t i v i t y o f Fe203-Sb204

c a t a l y s t s w i t h Sb

r i c h c o m p o s i t i o n s c a n b e a s c r i b e d t o t h e a c t i v e s u r f a c e l a y e r formed o n FeSb04 g r a i n s .

While t h e c a t a l y s t system forms a s i n g l e complex

oxide: p h a s e FeSb04, t h e s u r f a c e o f t h e FeSb04 g r a i n s is e n r i c h e d i n

Sb i n t h e p r e s e n c e of e x c e s s S b 2 0 4 . is a c t u a l l y v e r y t h i n ( l e s s than 6

!.

The s u r f a c e l a y e r t h u s formed thick) but has a rather w e l l

d e f i n e d c o m p o s i t i o n c l o s e t o S b / F e = 2 , s u g g e s t i n g a s u r f a c e compound l i k c FeSb206.

U n l i k e FeSb04, t h e Sb e n r i c h e d s u r f a c e l a y e r p r e f e -

r e n L i n l l y a c c o m m o d a t e s s u r f a c e o x y g e n of t h e t y p e t h a t o x i d i z e s o l e f i n s s e l e c t i v e l y , and t h u s e n a b l e s highly s e l e c t i v e a l l y l i c o x i dation.

I t is e s t i m a t e d t h a t t h e s u r f a c e S b i o n s p r o v i d e a d s o r p t i o n

s i t c s f o r oxygen w h i l e t h e Fe i o n s s t a b i l i z e t h e o x i d a t i o n s t a t e o f Sb(V). REFERENCES D.V. Tarasova, l . G . K . B o r e s k o v , S.A. V e n ' y a m i n o v , V . A : D z i s ' k o , V . N . D i n d o i n , N . N . S a n o b o b a , I . P . O l e n ' k o v a , a n d L.M. K e f e i l , K i n c t . K a t a l . , lo, 1530 ( 1 9 6 9 ) . 2.V.P. S h c h u k i n , G . K . B o r e s k o v , S.A. Ven'yaminov, and D . V . T a r a s o v a , K i n e t . K a t a l . , 11, 153 ( 1 9 7 0 ) . 3 . V . P . S h c h u k i n , S . A . V e n ' y a m i n o v , a n d G . K . B o r e s k o v , K i n e t . Katal., 1 1 , 1236 (1970). 4 . v - P . S h c h u k i n , S.A. V e n ' y a m i n o v , a n d G . K . U o r e s k o v , K i n e t . Katal., 1 2 , 621 ( 1 9 7 1 ) . 5 . r F a t t o r e , Z . A . F u h r m a n , G . l a n a r a , a n d B. N o t a r i , J . C a t a l . , 3 7 , 223 ( 1 9 7 5 ) . 13.S r a l a and F. T r i f i r o , J. C a t a l . , 1 (1976). 7 . I . A m , S. F u r u k a w a , N . Yamazoe, a n d T . S e i y a m a , * J . C a t a l . , submitted for publication. 8 . C . R . B r u n d l e , T . J . C h u a n g , a n d K . W a n d e l t , S u r f a c e S c i e n c e , E, 459 ( 1 Y 7 7 ) . 9 . A . F . O r c h a r d a n d G. T h o r n t o n , J. Chem. S o c . D a l t o n , 13, 1238 (1976). 1 O . A . 13ystr6m. B . H6k, a n d B . Mason, A r k i v K e m i . M i n e r a l o g . G e o l . , Udl5B, No. 4 ( 1 9 4 1 ) . N o . 15 ( 1 9 4 3 ) . l l . m a n d t , A r k i v K e m i . M i n e r a l o g . Geol., E, 1 2 . A . F. We1 I s , " S t r u c t u r a l I n o r g a n i c C h e m i s t r y " , p . 721, O x f o r d U n i v e r s i t y P r e s s , London, (1975).

41,

DISCUSSION R.K. Grasselli (Sohio Res., Cleveland) F i r s t I s h o u l d l i k e t o c o n g r a t u l a t e t h e a u t h o r s on t h e f i n e work t h e y p r e s e n t e d . N e x t I s h o u l d l i k e t o comment on some o f t h e similarities o f t h e p r e s e n t e d work t o t h a t which w e h a v e p u b l i s h e d some e i g h t y e a r s a g o o n t h e U-Sb-oxide

Grasselli and D.D.

Suresh, J. C a t a l . ,

25,

s y s t e m (R.K.

273 ( 1 9 7 2 ) ) .

( a ) Correlation of s e l e c t i v e a c t i v i t y t o a c r y l o n i t r i l e w i t h s t r u c t u r e a n d c o m p o s i t i o n a s a f u n c t i o n o f U/Sb r a t i o . ( F i g . 9

Surface State of FenOs-SbZO. Catalyst in Propene Oxidation

1249

in J. Catal., 14,93 (1969)). (b) USb3OI0 vs. USb05 atom positions and isolation of sites in USb3010 - i.e. the selective phase. (see below)

(c) Doping of USb05 (i.e. unselective phase) with very small amounts of Sb-oxide improves selectivity from ~ 1 4 %to ~ 7 5 % acrylonitrile. (d) Formation of USb3010, the selective phase on the surface of USb05 which is the unselective phase by Sb-oxide surface doping, was inequivocally identified and confirmed by ATR and XPS (e) Finally - isolation of sites in U-Sb-oxide catalysts, Fe-Sb-oxide catalysts, or Bi-molybdate catalysts is one of the necessary requirement for selective oxidation or arnmoxidation. The other two are proper M-0 bonding and lattice oxygen availability (J.L. Callahan and R.K. Grasselli, AIChE J., 2, 7-55 (1963)).

.

N. Yamazoe

I would like to express my thanks for Dr. Grasselli's com-

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N. Y a m a z o e , I . A s o , T . A m a m o t o , T . Seiyama

ments.

Your s t u d i e s on t h e U-Sb-0

g e s t i v e f o r oiir p r e s e n t work.

s y s t e m had i n f a c t been sug-

As you m e n t i o n e d , t h e r e is a

close s i m i l a r i t y on t h e e f f e c t o f e n r i c h i n g Sb c o n t e n t between Fe-Sb-0

s y s t e m and U-Sb-0

s y s t e m ; t h e S b - e n r i c h e d l a y e r and

USb3OI0 are f a r more s e l e c t i v e t h a n t h e i r r e l a t i v e p h a s e s c o n t a i n i n g less Sb, i . e . , FeSb04 and USb05, r e s p e c t i v e l y . On t h e s e l e c t i v i t y f a c t o r s , Grasselli.

w e g e n e r a l l y a g r e e w i t h Dr.

I n t h e mixed o x i d e s y s t e m s c o n t a i n i n g Sb5+, w e w i s h

t o suggest e s p e c i a l l y t h a t t h e s e l e c t i v e oxidation r e q u i r e s not o n l y t h e Sb5+-oxygen bond p r o p e r l y t i g h t e n e d by t h e f o r m a t i o n of compounds ( l i k e Fe-Sb-0

and U-Sb-0)

or s o l i d s o l u t i o n ( l i k e

b u t a l s o some c i r c u m s t a n c e s i n which t h e Sb5+ i o n is

Sn-Sb-0)

a d j a c e n t t o o t h e r Sb i o n s . Y . Moro-oka (Tokyo

Inst. Tech,)

You have s u g g e s t e d t h a t FeSb04 combined w i t h e x c e s s Sb204, p o s s i b l y FeSb20s p h a s e , is a c t i v e t o form a c r o l e i n .

However,

c o n s i d e r i n g F i g . 1, t h e r a t e o f f o r m a t i o n o f a c r o l e i n c h a n g e s only a l i t t l e w i t h c a t a l y s t composition.

Improved s e l e c t i v i t y

m a i n l y comes from s u p p r e s s i o n o f c o n v e r s i o n t o c a r b o n d i o x i d e . So I t h i n k a n o t h e r i n t e r p r e t a t i o n , t h a t e x c e s s Sb204 p r e v e n t s

t h e formation of a c t i v e sites (possibly f r e e i r o n oxide) t o produce c a r b o n o x i d e s , may a l s o b e p o s s i b l e .

Did you c o n s i d e r

this possibility? N. Yarnazoe

You are p e r f e c t l y correct i n s a y i n g t h a t t h e improved select i v i t y t o a c r o l e i n of t h e S b - r i c h c a t a l y s t s i n F i g . 1 r e s u l t e d

m a i n l y from t h e s u p p r e s s i o n of C 0 2 f o r m a t i o n :

The e x c e s s

Sb204 s u p p r e s s e d t h e C 0 2 f o r m a t i o n d r a s t i c a l l y w h i l e a f f e c t i n g t h e a c r o l e i n formation only s l i g h t l y . have been p r o p o s e d f o r t h i s phenomenon.

Several explanations The i n t e r p r e t a t i o n

s u g g e s t e d by you t h a t t h e e x c e s s SbZOq k i l l s t h e a c t i v e s i t e s f o r t h e C 0 2 f o r m a t i o n on f r e e i r o n o x i d e is e s s e n t i a l l y t h e same a s h a s a l r e a d y been p r o p o s e d by R u s s i a n r e s e a r c h e r s ( r e f 1, 2 ) .

However, t h i s i n t e r p r e t a t i o n is n o t a t a l l c o n s i s t e n t

w i t h t h e p r e s e n t r e s u l t s , i . e . , t h e f o r m a t i o n of t h e s e l e c t i v e s u r l a c e which h a s a w e l l d e f i n e d c o m p o s i t i o n o f S b / F e 2 2 ( F i g . 4 ) and c o n t a i n s F e ( I 1 ) ( F i g . 5 ) , and t h e s m a l l b u t def i n i t e i n c r e a s e s of t h e r a t e of a c r o l e i n f o r m a t i o n i n t h e Sb r i c h r e g i o n ( F i g s . 1 and 7 ) .

The most r e a s o n a b l e c o n c l u s i o n

drawn from t h e s e r e s u l t s w i l l b e t h a t t h e improved s e l e c t i v i t y o f t h e Sb r i c h c a t a l y s t s i s a s c r i b e d t o t h e S b - e n r i c h e d s u r f a c e

Surface State of Fe203-Sb20YCatalyst in Propene Oxidation

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l a y e r ( p r o b a b l y FeSb206) which i s f a r more s e l e c t i v e t h a n t h e u n d e r l y i n g p h a s e FeSb04. J.C. Vedrine (Catalysis k s . I n s t . , Villeurbanne) I n a s i m i l a r work on Sb-Sn-0

s y s t e m s * ) , w e have shown t h a t

h e a t t r e a t m e n t of a s o l i d a t 900°C l e a d s t o a s u r f a c e e n r i c h W e then a r r i v e at t h e conclusion t h a t a c t i v e sites are p r o b a b l y composed of antimony o x i d e on a s u r f a c e of a s o l i d s o l u t i o n o f Sb i n Sn02. Our f e e l i n g s a r e t h a t t h e s u r f a c e i s n o t as homogeneous as you have d e s c r i b e d , b u t p r o b a b l y v e r y h e t e r o g e n e o u s . You have g i v e n u s c h a n g e s i n XPS l i n e i n t e n s i t y r a t i o s S b / F e and as you know b i n d i n g e n e r g y v a l u e s may depend on t h e o x i d a t i o n s t a t e s of Sb. You a l s o know t h a t Sb6013 h a s o f t e n been p o s t u l a t e d a s t h e a c t i v e s i t e s i n m i l d o x i d a t i o n o f propene t o a c r o l e i n . My q u e s t i o n i s : d i d you see any c h a n g e i n b i n d i n g e n e r g y v a l u e s or w i d t h o f Sb3d l i n e s as a f u n c t i o n of Sb c o n t e n t and ment i n Sb.

selectivity differences? *) J . C .

V e d r i n e e t a l , J. C a t a l . , ( 1 9 7 9 )

N. Yamazoe T h e r e w a s no s i g n i f i c a n t c h a n g e i n b i n d i n g e n e r g y (BE) of Sb3d312 p e a k s (BE = 539.8 f 0 . 1 eV) t h r o u g h a l l Fe203-Sb204 mixed o x i d e c a t a l y s t s ( S b / F e = 0 . 5

%

4.0),

w h i l e p u r e Sb204

0.1 eV. I t is s u g g e s t e d t h a t t h e s u r gave a lower BE, 5 3 9 . 5 f a c e Sb i o n s on FeSb04 g r a i n s e x i s t i n a mixed v a l e n c e s t a t e

o f Sb(V) and S b ( I I I ) , w h i l e t h o s e on Sb204 g r a i n s e x i s t i n S b ( I I 1 ) . A s f o r t h e l i n e w i d t h of 3d312 p e a k s , i t w a s d i f f i c u l t

t o d i s c e r n i t s c h a n g e w i t h Sb c o n t e n t due t o t h e sample c h a r g i n g ( 0 . 5 % 1 . 0 eV). C o n c e r n i n g t h e homogeneity o f t h e S b - e n r i c h e d s u r f a c e l a y e r ,

w e c a n n o t e x c l u d e a p o s s i b i l i t y t h a t a v e r y t h i n antimony o x i d e l a y e r i s d e p o s i t e d o n FeSb04 i n s u c h a v e r y p a r t i c u l a r manner t h a t l e a d s t o b o t h a w e l l d e f i n e d Sb/Fe r a t i o ( 2 2 ) and t h e f o r m a t i o n of F e ( I 1 ) . However, w i t h many t r i r u t i l e compounds of t h e t y p e MSba06 w e l l known, w e c o n s i d e r i t f a r more l i k e l y t h a t t h e s u r f a c e is composed of a compound l i k e FeSb206, n o t an a n t i mony o x i d e .

J. Haber (Inst. o f Catalysis, Krakow) Many y e a r s a g o , w e s u g g e s t e d ( 1 ) t h a t i n c a t a l y s t s of t h e oxysalt type such as molybdates, antimonates, t u n g s t a t e s etc. t h e c a t i o n i c s u b l a t t i c e is r e s p o n s i b l e f o r t h e a c t i v a t i o n o f

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N . Yamazoe, I . Aso, T . Amamoto, T . Seiyama

h y d r o c a r b o n and t h e a n i o n i c s u b l a t t i c e f o r oxygen i n s e r t i o n . W e c a r r i e d o u t e x p e r i m e n t s i n which a l l y l i o d i d e w a s p a s s e d

o v e r antimony o x i d e and o b s e r v e d h i g h c o n v e r s i o n t o a c r o l e i n , which shows t h a t it is i n f a c t antimony o x i d e which h a s t h e a b i l i t y t o i n s e r t oxygen.

The p r e s e n c e of i r o n i o n s i n i r o n

o x i d e a c t i v a t e s oxygen t o a n e l e c t r o p h i l i c form, which is responsible f o r total oxidation.

Your e x c e l l e n t r e s u l t s a g r e e

v e r y w e l l w i t h t h i s mechanism.

(1) J . Haber, I n t e r n . Chem. Eng.,

15, 2 1

(1975)

N. Yamazoe W e t h i n k t h a t your m u l t i c e n t e r mechanism is u s e f u l f o r unders t a n d i n g t h e c a t a l y t i c b e h a v i o r o f s e v e r a l mixed o x i d e s . The p r e s e n t s t u d y c l e a r l y shows t h a t t h e a c t i v e s u r f a c e oxygen a t o m s are bonded t o Sb ( p r o b a b l y S b ( V ) ) . This result agrees w i t h y o u r mechanism as f a r a s t h e r o l e o f Sb i o n s f o r oxygen i n s e r t i o n is concerned. As f o r t h e f o r m a t i o n o f a l l y l i n t e r m e d i a t e , however, w e s u s p e c t t h a t Sb i o n s are a l s o r e s p o n s i b l e f o r i t i n t h i s c a t a l y s t s s y s t e m a s G r a s s e l l i and S u r e s h proposed i n t h e U-Sb-0 s y s t e m ( J . C a t a l y s i s , %, 273 ( 1 9 7 2 ) ) , w h i l e Fe i o n s a r e e f f e c t i v e f o r s t a b i l i z i n g t h e o x i d a t i o n s t a t e of Sb(V). Our f e e l i n g i s t h a t t h e a c t i v e sites c o n s i s t o f a s e t of s u r f a c e Sb(V) i o n s ; a c o o r d i n a t i v e l y u n s a t u r a t e d Sb(V) provides an adsorption site f o r t h e a l l y l intermediate, while t h e o t h e r s p r o v i d e a c t i v e s u r f a c e oxygen. S. Ogasawara (Yokohama National U n i v . ) 1) What d o you t h i n k a b o u t t h e s t a b i l i t y o f t h e c a t a l y s t s ?

I t seems t h a t s u c h a t h i n l a y e r o f Sb2O4 is n o t s o s t a b l e i n a

working c o n d i t i o n .

P l e a s e l e t m e know t h e r e p r o d u c i b i l i t y o f

the catalytic activity. 2 ) I f t h e a c t i v i t y o f Fe203-Sb204 c a t a l y s t w i t h e x c e s s Sb2O4 c a n b e a s c r i b e d t o t h e t h i n l a y e r o f Sb2O4, a n o t h e r Sb2O4 s y s t e m other than iron oxide could be obtained.

Have you s t u d i e d u s i n g

any o t h e r s y s t e m s c o n t a i n i n g SbaOq? 3 ) In t h i s connection,

I a m i n t e r e s t e d i n t h e a c t i v i t y of

t h e Sb204 c a t a l y s t i t s e l f , which w a s n o t shown i n F i g . 1. What v a l u e d i d you o b t a i n ?

N . Yamazoe 1) F i r s t I s h o u l d l i k e t o s a y t h a t t h e s u r f a c e a c t i v e l a y e r is n o t a s i m p l e Sb204 l a y e r b u t is e s t i m a t e d t o b e a s u r f a c e compound c o n t a i n i n g b o t h Sb and F e . The c a t a l y s t s were q u i t e s t a b l e f o r u s e u n d e r u s u a l working c o n d i t i o n s below 5OO0C

Surface State of Fe20r-Sb20v Catalyst in Propene Oxidation

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e x h i b i t i n g r e p r o d u c i b l e a c t i v i t y . However, t h e c a t a l y s t s , e s p e c i a l l y t h o s e w i t h t h e Sb-enriched s u r f a c e l a y e r , w e r e not s t a b l e t o e v a c u a t i o n a t h i g h t e m p e r a t u r e above 8OO0C. 2 ) We have t e s t e d a s e r i e s of compounds of MeSb206 t y p e where

M e s t a n d s for Cu, Co, Zn, Mg e t c .

These c a t a l y s t s w e r e e i t h e r

f a r less a c t i v e (Zn, Mg) o r f a r less s e l e c t i v e (Cu, Co) f o r propene o x i d a t i o n t h a n t h e Fe203-Sb204 system, s u g g e s t i n g t h a t t h e role of Fe i n t h e l a s t system is a l s o very important f o r catalysis. 3) The c a t a l y t i c a c t i v i t y of p u r e Sb2O4 under t h e c o n d i t i o n s of F i g . 1 w a s 0.02 wnol-m-2-min-1, which is s m a l l e r by almost two o r d e r s of magnitude t h a n t h o s e of c a t a l y s t s w i t h SbfFe = 1 and

2 , though t h e s e l e c t i v i t y t o a c r o l e i n ( -80% ) was r a t h e r high.