Titania Catalysts

B. Imelik e t al. (Editors),Metal-Support and Metal-Additive Effects in Catalysis

193

0 1982 Elsevier ScientificPublishing Company,Amsterdam - Printed in The Netherlands

SOME CONSEQUENCES OF SMSI ON THE CATALYTIC ACTIVITY OF NI/TITANIA CATALYSTS

R. BURCH and A.R. FLAPIBARD Chemistry Department, The University, Whiteknights, Reading RG6 2AD, England

ABSTRACT Titania and silica-supported catalysts have been prepared by wet impregnation. The activities of the catalysts were determined, under continuous flow conditions, for the hydrogenation of benzene, the hydrogenolysis of n-hexane and ethane, and the hydrogenation of CO.

The results showed that the titania and

silica-supported catalysts behaved similarly, except in the case of the CO/H reaction.

2 It is concluded that the role of titania is to create at the interface

with the metal new active sites which are specific for the CO/H2 reaction. The importance of interfacial metal-support interactions is emwhasised.

RESUME Les catalyseurs supportes sur Ti02 et Si02 ont 6tB pr6par6s par voie humide (impregnation). Les activit6s catalytiques ont 6te determinees en systeme dynamique pour les reactions d'hydrogenation du benzene, d'hydrogenolyse du n-hexane et de 1'Qthane et l'hydrogenation de CO. Les r6sultats obtenus montrent que ces deux catalyseurs se comportent de fagon similaire, sauf dans le cas de la reaction CO/H2. On conclut que le rBle de TiOZ est de cr6er d l'interface avec le metal de nouveaux sites actifs specifiques de la reaction CO/H2. On insiste sur l'importance des interactions d l'interface metal-support.

INTRODUCTION The possible role of the support in modifying the catalytic prouerties of a metal particle has long been recognised (see, e.g., (1)).

Recent work has

uncovered many other examples of strong metal-support interactions (SMSI), especially for metals supported on transition metal oxides (see, e . g . ,

(2)).

Almost without exception, SMSI reduce the capacity of a metal to adsorb hydrogen or CO, and cause a marked decrease in catalytic activity ( 3 - 7 ) .

It is

surprising, therefore, that titania-supported catalysts exhibit a very high

194 activity for the CO/H2 reaction (8-10). This is particularly true for Ni catalysts,. for which there is evidence that the metal particles are fairly large (diameter about lOnm), since it is difficult to see how the support could materially affect the surface of such large particles.

Preliminary exDeriments

have indicated (11,12) that the high activity reported for titania-suuported Ni catalysts is restricted to the CO/H2 reaction.

The aim of the work presented

here is to investigate some of the consequences of SHSI on the catalytic properties of Ni/titania catalysts.

EXPERIMENTAL Preparation of catalysts Titania-supported catalysts (Degussa P25 titanium dioxide, 80% anatase, 20% 2 -1

rutile, surface area 50 m g

and a silica-supported reference catalyst 2 -1 (Davison grade 57 silica, surface area 300 m g ) were prepared by wetimprqnation ),

using nickel nitrate solutions of appropriate concentration. After the removal of residual water by rotary evaporation, the catalysts were dried overnight at 370 K, and stored in a vacuum desiccator.

The nickel content of the catalysts

(in the range 5-10%) was determined by wet analysis.

Activation of catalysts (a) Calcination and low temperature reduction Samples of the catalysts were dried overnight at 370 K, and then calcined in air for 2h

at 573 K.

The samples were transferred to the reactor (a Pyrex

glass U-tube reactor fitted with a sintered disc to support the sample), flushed -1 with hydrogen, (space velocity 0.3 x lo6 cm3 gNi h-'), and the temperature raised over a period of 10 minutes to 573 K for 2h, and then the sample was cooled to the appropriate reaction temperature. (b) Calcination and medium temperature reduction

Samples of the catalysts, calcined as above, were heated rapidly to 723 K in a flow of hydrogen (space velocity 1.8 x temperature for

lo6

cm3 9Ni-l h-l), held at this

lh, and then cooled.

(c) Reduction of uncalcined catalysts 3

Uncalcined samples were heated in hydrogen (space velocity 1.8 x lo6 cm 1 9Ni-l hat 7 K minute-' from room temperature to 723 K over a period of 1.5h, and then held at this temperature for a further Ih. To investigate the influence of reduction temperature on the catalytic properties, selected catalysts were reduced in hydrogen at temperatures up to 923 K.

Catalytic experiments The activity of the catalysts was determined for the hydrogenation of benzene, the hydrogenolysis of n-hexane, the hydrogenolysis of ethane, and the

195

hydrogenation of CO.

In these experiments,.ethane/hydrogen (10% ethane) and

CO/hydrogen (25% CO) were obtained as pure gas mixtures and used without further purification; hexane (Phillips Petroleum Co., 99.99% purity) and benzene (Fisons, purity 99.99%) were introduced into a hydrogen stream by motor-driven microsyringe. The reaction conditions for benzene hydrogenation were: H /benzene 2 3 -1 h-l, reaction temperature = 20: 1, benzene flow rate = 395 cm gNi

molar ratio 383-423 K.

For n-hexane the conditions were: H /hexane molar ratio = 5.58:1, 2 3 -1 -1 = 392 cm gNi h , reaction temperature 548 K. I n the hydro-

hexane flow rate

genolysis of ethane the flow of ethane was 6 x lo5 cm3 9Ni-l h-',

and the reaction

temperature was 478-673 K. The hydrogenation of CO was performed using a flow 5 of CO of 3 x 10 cm3 gNi-l h-' at a temperature in the range 518-553 K. Most catalytic experiments were performed using a continuous flow of reactants. However, in some cases, where it was desirable to prevent deactivation of the catalysts, a bracketing technique was used in which the activity was determined over a short period (10-20 minutes), after which the catalyst was cleaned at the reaction temperature in pure hydrogen. Chemisorption measurements The specific metal surface areas were determined by hydrogen chemisorption using a conventional static volumetric apparatus.

Adsorption isotherms were

determined at 273 K over the pressure range 0-50 mbar; and the quantity of hydrogen corresponding to monolayer coverage was calculated using the Langmuir equation for dissociative adsorption.

RESULTS AND DISCUSSION Chemisorption experiments The specific nickel surface areas determined by hydrogen chemisorption ( 1 H 2 assumed to cover 0.065 nm ) are given in Table 1. The average particle size varies from 7 to 18 nrn.

Uncalcined samples have higher surface areas than

calcined samples. The surface area for the titania-supported catalysts is lower by about a factor of 2. which is reasonable in view of the factor of 6 difference in surface areas of the supports, There is no evidence from these data for a suppression of hydrogen adsorption in titania-supported Ni catalysts. TABLE 1 Specific metal surface areas for catalysts activated by different procedures 2 -1 Surface area/m g Activation Catalyst procedure Ni9.8Si Ni9.8Ti (a) (b) (C)

54.6 45.6 106.0

38.4 22.3 52.5

Benzene hydrogenation The specific activity of the uncalcined catalysts for the hydrogenation of

196 benzene is given in Table 2, and the relative specific activities for the various catalysts is given in Table 3.

For calcined catalysts, the titania-supported

catalysts are more active by about a factor of 2; for the uncalcined catalysts the titania-supported catalysts are less active by a factor of 3.5.

In both

cases the activities are essentially independent of reaction temperature.

TABLE 2 Specific activity of nickel catalysts for the hydrogenation of benzene at various temperatures 3 Specific activity x 10 /molec s-l Ni-l Reaction temperature/K Catalvst

383

393

403

413

423

66 28

100 34

159 44

218 57

270 74

~

Ni9.8S1 (c) Ni9.8Ti (c)

TABLE 3 Relative specific activities of nickel catalysts activated by different procedures for the hydrogenation of benzene Catalyst Pretreatment (a) (b) (C)

Relative specific activitya Reaction temperature/K 383 393 403 413 423 0.80 0.73 0.48 0.45 2.36 2.94

0.76 0.47 3.61

0.69 0.44 3.82

0.58 0.43 3.65

adefined as activity of silica-supported catalyst divided by activity of titania-supported catalyst. A factor of 2-3 in specific activity can be considered close agreement.

We

conclude, therefore, that these Ni/Ti02 catalysts exhibit normal hydrogenation characteristics. This is important because benzene hydrogenation is a structure insensitive reaction, so data for this reaction can be used as an independent check on the amount of nickel surface available for catalysis. Hexane reaction The hydrogenolysis reaction is structure sensitive so it is an excellent probe reaction with which to investigate small changes in the surface chemistry of a catalyst.

Both the activity and the selectivity can give information on

the nature of the active site. .The activities and selectivities of our catalysts for the hydrogenolysis of hexane are given in Table 4.

The specific

activities show that the titania-supported catalysts are slightly more active (by a factor of 2). The selectivities, which show typical cracking patterns for nickel catalysts, are similar for all the catalysts; the major product is methane which reflects the tendency of nickel to break the terminal C-C bond.

197 TABLE 4

A c t i v i t y and s e l e c t i v i t y of n i c k e l c a t a l y s t s f o r t h e h y d r o g e n o l y s i s o f n-hexane a t 548 K Conversiona Catalyst Ni9.8Si Ni9.8Ti Ni9.8Si Ni9.8Ti Ni9.8Si Ni9.8Ti

(a) (a)

(b) (b) (c) (c)

/%

Specific activity/ molec s-l N i - ' 0.019 0.042 0.027 0.054 0.086 0.133

3.20 4.95 3.72 3.74 28.2 21.5

ci

b P r o d u c t s e l e c t i v i t y /% c2 c3 c4 C5 c6

54.9 50.1 55.2 43.8 40.6 49.2

5.0 5.6 5.7 4.9 4.6 5.8

7.9 7.8 8.1 8.0 8.7 8.7

11.0

11.5 11.0 13.3 15.3 13.1

19.1 23.0 18.0 27.2 29.0 22.2

2.1 2.0 2.0 2.8 1.8 1.0

a d e f i n e d a s ( i n i t i a l conc. hexane - f i n a l c o n c . hexane) x 1 0 0 / i n i t i a l conc. hexane. b d e f i n e d a s S ( x ) / % = Cn x 100/n Cn, where Cn i s t h e c o n c e n t r a t i o n of hydrocarbon (x) and n i s t h e number of c a r b o n atoms i n Cn.

The f a c t t h a t t h e t i t a n i a - s u p p o r t e d c a t a l y s t s have h i g h e r s p e c i f i c a c t i v i t i e s c o u l d be i n t e r p r e t e d as e v i d e n c e o f a s u p p r e s s i o n o f hydrogen c h e m i s o r p t i o n . W e t h i n k t h i s i s u n l i k e l y s i n c e t h e o p p o s i t e t r e n d w a s observed f o r t h e s t r u c t u r e

i n s e n s i t i v e benzene h y d r o g e n a t i o n r e a c t i o n , and a l s o b e c a u s e t h e c r a c k i n g p a t t e r n s f o r t h e hexane r e a c t i o n i n d i c a t e t h a t t h e n i c k e l i s i n t h e same form i n a l l the catalysts.

W e c o n c l u d e t h a t t h e s e c a t a l y s t s e x h i b i t normal hexane

hydrogenolysis c h a r a c t e r i s t i c s . Ethane h y d r o g e n o l y s i s Only a few d a t a on t h e h y d r o g e n o l y s i s o f e t h a n e have been d e t e r m i n e d , s i n c e t h e y m e r e l y c o n f i r m t h e p a t t e r n o b s e r v e d f o r t h e h y d r o g e n o l y s i s o f hexane.

A t

478 K t h e s p e c i f i c a c t i v i t y of an u n c a l c i n e d c a t a l y s t ( N i 8 . 5 T i ( c ) ) w a s 7.3 x molec s-l N i - * . ( 1 3 , 1 4 ) 6.7-7.6

x

A t y p i c a l s p e c i f i c a c t i v i t y f o r a normal N i c a t a l y s t i s

molec s

-1

Ni-l-

W e conclude, t h e r e f o r e , t h a t our N i /

t i t a n i a c a t a l y s t has t h e a c t i v i t y o f a t y p i c a l pure N i c a t a l y s t . The o b j e c t i v e i n p e r f o r m i n g t h e s e e x p e r i m e n t s w i t h benzene, hexane, and e t h a n e h a s been t o e s t a b l i s h t h e f a c t t h a t f o r a l l t h e s e r e a c t i o n s , whether s t r u c t u r e i n s e n s i t i v e or s t r u c t u r e s e n s i t i v e , o u r N i / t i t a n i a c a t a l y s t s e x h i b i t p e r f e c t l y normal b e h a v i o u r .

W e emphasise t h i s . p o i n t t o u n d e r l i n e t h e f a c t t h a t

when t h e s e same c a t a l y s t s a r e u s e d i n t h e CO/hydrogen r e a c t i o n q u i t e d i f f e r e n t p r o p e r t i e s are observed.

CO/H2

reaction

T a b l e 5 summarises t h e a c t i v i t y and s e l e c t i v i t y d a t a f o r t h e CO/H2 r e a c t i o n o v e r o u r c a t a l y s t s a t 553 K.

I n i t i a l l y , t h e t i t a n i a supported c a t a l y s t i s about

75 t i m e s more a c t i v e than a c o r r e s p o n d i n g s i l i c a - s u p p o r t e d c a t a l y s t .

This

h i g h e r a c t i v i t y i s o f t h e same o r d e r o f magnitude as r e p o r t e d by o t h e r s f o r Ni/titania catalysts.

(8-10)

However, e a r l i e r w o r k e r s have i n t e r p r e t e d t h i s

198 enhanced activity in terms of a strong metal-support interaction.

It is clear

from the data presented earlier that there is no SMSI in our catalysts, (it is difficult to see how a SMSI could be present without some effect being observed in hydrogenation or hydrogenolysis reactions).

Nevertheless, the activity of

the Ni/titania catalyst in the CO/H2 reaction is very high.

We consider that

this occurs because new active sites are created at the interface between the metal particles and the titania support.

Of course, it is important to

differentiate between an interfacial phenomenon and a bulk phenomenon.

A

modification of the properties of the nickel atoms at the interface, without any modification of the remainder of the surface Ni atoms, could account for the reaction specificity of the titania-supported catalysts.

All that is required

is that the active sites for the CO/H reaction should be unique. 2 consider a tentative model of the active site presently.

We shall

TABLE 5 Activity and selectivity of nickel catalysts for the CO/H2 reaction at 553 K b Conversiona Specific activity/ Product selectivity / % /% molec s-l Ni-l C1 C2 c3 c4 c5+ Ni9.8Si (c) Ni9.8Si (c) Ni9.8Ti(c) Ni9.8Ti (c)

1.64' 1. 46d 59.2c 9.5d

0.022 0.020 1,633 0.262

88.0 90.7 95.3 72.3

5.0 2.8 3.0 11.7

5.4 4.4 1.1 10.0

1.6 1.9 0.3

4.1

0.0 1.2 0.3 1.9

adefined as fraction of CO converted to hydrocarbon products. bdefined as weight of product Cn/total weight of hydrocarbon products. C.

initial activity

dactivity after 1.5h on stream. A further feature of titania-supported catalysts which has been reported, and

again taken as evidence of an SMSI, is their greater stability.

In fact, earlier

data were obtained using a bracketing technique which is designed to prevent deactivation.

Figure 1 shows how the activity of our catalysts declines with

time under continuous flow conditions. The unique nature of the CO/H2 reaction over these catalysts is illustrated in Figure 2 which compares the activity of the titania-supported catalyst (A ) Ti with that of the silica-supported catalyst (A ) for all the reactions we have Si

investigated. Since in none of our catalysts have we observed evidence of SMSI as defined in the literature, some further experiments were performed in which higher reduction temperatures were used in an attempt to induce SMSI. Figure 3 compares the activities of titania-supported catalysts reduced at 623 K with the same catalysts reduced at 923 K.

There is a decrease in activity by a

factor of 15-20 after high temperature reduction.

However, the most revealing

199 observation is that the activity for the CO/H reaction decreases exactly in 2 parallel with the decrease in activity for the other reactions. We must conclude that if SMSI is responsible for the loss of activity for the hydrogenation of benzene, or for hydrogenolysis, then SMSI results in a l o s s of activity for the CO/H2 reaction a l s o .

25

50 75 Ti me /mi nutes

100

Fig. 1. Change in activity with time of Ni/titania catalysts for the CO/H 2 reaction.O,NiS.OTi(c) tested at 518 K; 0 ,Ni9.8Ti(c) tested at 548 K.

COIH, I ethane I hexane benzene I I

I

I

I

200

COIH, ethane hexane

20

10

F i g . 3 R e l a t i v e a c t i v i t i e s of Ni/Ti02 c a t a l y s t s a f t e r r e d u c t i o n a t 623 K (A 623) and a t 923 K ( A g z 3 )

INTERFACIAL METAL-SUPPORT INTERACTIONS W e have i n d i c a t e d above t h a t o u r r e s u l t s s u g g e s t a unique s i t e f o r t h e CO/H

2

r e a c t i o n , a n d w e propose t h a t t h i s s i t e i s s i t u a t e d a t t h e i n t e r f a c e between t h e metal p a r t i c l e and t h e t i t a n i a s u r f a c e .

I t i s w e l l e s t a b l i s h e d , and h a s been

confirmed f o r o u r c a t a l y s t s , t h a t t h e r e i s some r e d u c t i o n of t h e s u r f a c e of t h e titania.

F i g u r e 4 d e s c r i b e s our model of t h e a c t i v e s i t e , i n which w e emphasise

t h e importance of anion v a c a n c i e s a t t h e i n t e r f a c e .

I t i s not possible t o

d i s t i n g u i s h a t t h i s s t a g e between a l t e r n a t i v e models of t h e i n t e r f a c i a l a c t i v e site.

W e merely wish t o propose

t h a t t h e most r e a s o n a b l e way i n which t h e

t i t a n i a could s e l e c t i v e l y promote t h e CO/H2

r e a c t i o n i s by modifying t h e

p r o p e r t i e s of t h e a c t i v e s i t e a t t h e i n t e r f a c e .

Whether t h i s m o d i f i c a t i o n

'involves t h e t i t a n i a d i r e c t l y , o r i s a r e s u l t of e l e c t r o n t r a n s f e r between t h e metal and t h e reduced s u p p o r t , i s n o t known.

Model of t h e a c t i v e s i t e i n t h e CO/H2 r e a c t i o n a t t h e i n t e r f a c e between Fig. 4. a n i c k e l p a r t i c l e and t h e t i t a n i a s u r f a c e : bulk N i ; & , N i atoms a t t h e i n t e r f a c e ; D , anion vacancy.

m,

201 CONCLUSIONS r e a c t i o n c a n be 2 F u r t h e r m o r e , i t h a s been shown

Our r e s u l t s have shown t h a t v e r y h i g h a c t i v i t y i n t h e CO/H o b s e r v e d under c o n d i t i o n s where SMSI a r e a b s e n t .

t h a t i f h i g h e r t e m p e r a t u r e s are u s e d t o t r y t o c r e a t e SMSI t h e e f f e c t i s t o d e s t r o y t h e a c t i v i t y o f t h e c a t a l y s t s f o r a l l t h e r e a c t i o n s which w e have investigated.

A model i s p r o p o s e d which e m p h a s i s e s t h e i m p o r t a n c e o f i n t e r f a c i a l

phenomena, and i n which it i s s u g g e s t e d t h a t t h e a c t i v e s i t e i n t h e CO/H2 r e a c t i o n i s unique t o t h i s r e a c t i o n .

ACKNOWLEDGEMENTS R.B.

t h a n k s Amax I n c . ,

f o r f i n a n c i a l support.

a n d A.R.F.

t h a n k s t h e S t a t e s o f t h e I s l a n d of J e r s e y

W e a r e g r a t e f u l t o Degussa and W.R.Grace

f o r supplying

samples o f t h e c a t a l y s t s u p p o r t s .

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