Effect of H2 Treatment on the Catalytic Activity of Pt-SiO2 Catalysts

15 Effect of H2 Treatment on the Catalytic Activity of Pt-SiO Catalysts 2

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 15, 2016 | http://pubs.acs.org Publication Date: February 10, 1986 | doi: 10.1021/bk-1986-0298.ch015

H. Zuegg and R. Kramer Institut für Physikalische Chemie, Universität Innsbruck, A-6020 Innsbruck, Austria The deactivation of platinum catalysts for hydrocarbon reactions caused by high temperature reduction (HTR) has been observed by many authors (for review, see 1). When using titania as support the extent of deactivation is especially strong, but also with "nonreducible" supports like silica or alumina similar effects have been reported (2, 3). The aim of this work was to investigate the changes in activity of Pt/SiO2 model catalysts caused by HTR. Special interest has been paid to the activity change of the Pt/SiO2 phase boundary in relation to the bulk platinum surface, as was for instance observed recently by Resasco and Haller (4) with Rh/TiO2. We have shown earlier (5, 6) that the hydrogenolysis of methylcyclopentane (MCP) on platinum proceeds via two different pathways, (i) occurring at the "bulk" platinum surface and yielding exclusively 2-methylpentane (2-MP) and 3-methylpentane (3-MP), and (ii) occurring at the phase boundary Pt-support, where nearly statistical ringopening leads to the formation of 2-MP, 3-MP and n-hexane (n-H). The selectivity for n-hexane formation in this reaction was therefore taken to indicate possible activity changes of the phase boundary in relation to the Pt-surface. Parallel to the MCP hydrogenolysis the hydrogenation of benzene was chosen for examining in addition the effect of HTR on a structure insensitive reaction. Experimental The model catalysts were prepared by HV deposition of an amorphous S i 0 f i l m (by evaporation of SiO i n 10~ Pa of oxygen), onto which Pt was deposited by high vacuum evaporation, as described e a r l i e r (7). In order to get catalysts of d i f f e r e n t dispersion, the mean thickness of deposited Pt was varied between 0.1 and 1 nm. By TEM inspection of c a t a l y s t specimens the p a r t i c l e density and the p a r t i c l e size d i s t r i b u t i o n were obtained, from which data the platinum surface area and the dispersion were calculated. Additionally a conventional 6,3 % Pt/Sio^ catalyst (EUROPT-1, d = 1.7 nm) was used i n the experiments. The c a t a l y t i c reactions were c a r r i e d out i n an a l l - g l a s s r e c i r c u l a t i o n apparatus providing long reaction times. For a l l reactions the conversion-time behavior was measured up t o a reaction 2

0097-6156/86/0298-0145$06.00/0 © 1986 American Chemical Society

Baker et al.; Strong Metal-Support Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


146

S T R O N G METAL-SUPPORT INTERACTIONS

time of one hour. The p a r t i a l pressures of MCP and benzene were 1 KPa and 3 KPa respectively, hydrogen was admitted to a t o t a l pressure of 1 atmosphere. The reaction temperatures were 523 Κ f o r MCP hydrogenolysis and 323 Κ f o r benzene hydrogénation. Before each experiment the catalysts were pretreated by heating i n oxygen at 673 K. After cooling down to room temperature i n flowing helium the catalysts were reduced by heating up i n hydrogen (a) to 523 Κ f o r low temperature reduction (LTR) and (b) to 673 Κ f o r high temperature reduction (HTR). The LTR was continued f o r at least one hour at 523 K, i n the HTR the time of treatment was varied i n order to investigate the extent of deactivation as a function of HTR treatment time. Results A. Hydrogenolysis of Methylcyclopentane. After LTR of the catalysts the product d i s t r i b u t i o n of the MCP hydrogenolysis shows the expected dependence on p a r t i c l e size.(Table 1). The a c t i v i t i e s of a l l LTR-catalysts agree within experimental error, leading to an approximate turnover number of 40 h , independent of the metal dispersion. The conversion increases nearly l i n e a r l y with reaction time, indicating that no s i g n i f i c a n t deactivation occurs during reaction. The HTR causes generally both a decrease i n the s e l e c t i v i t y f o r n-hexane and a loss of c a t a l y t i c a c t i v i t y , but these changes exhibit a d i f f e r e n t time dependence. After HTR f o r only 15 minutes the f u l l s h i f t i n s e l e c t i v i t y i s reached, while the a c t i v i t y has changed only moderately. A f t e r HTR f o r several hours the remaining a c t i v i t y reaches a stationary l e v e l of about 10 to 40 % of the i n i t i a l value (Fig. 1). No general dependence of the deactivation on dispersion could be established. However, with the EUR0PT-1 c a t a l y s t only a change i n the reaction s e l e c t i v i t y but no s i g n i f i c a n t loss i n the a c t i v i t y was observed. The changes of the c a t a l y t i c behavior turned out to be reversible and i n i t i a l s e l e c t i v i t y could be reestablished by treatment i n oxygen, but again s e l e c t i v i t y was changed more e a s i l y than the a c t i v i t y . While treatment of the catalysts with oxygen even at room temperature gave the i n i t i a l s e l e c t i v i t y , the a c t i v i t y was only p a r t i a l l y restored a f t e r room temperature oxygen treatment. Further increase of a c t i v i t y was ob served by oxygen treatment at successively higher temperatures (Fig. 2). Only when treated at 673 Κ i n oxygen did the catalysts regain t h e i r f u l l i n i t i a l a c t i v i t y . B. Hydrogénation of benzene. After LTR the a c t i v i t i e s of the catalysts agree within experimental error r e s u l t i n g i n a turnover number of approximately 300 h . Again, a f t e r HTR the a c t i v i t y of the model catalysts was decreased, while the EUR0PT-1 catalyst retained the i n i t i a l a c t i v i t y . With the model catalysts the extent of deactivation by HTR i s smaller f o r benzene hydrogénation than f o r MCP hydrogenolysis. However, oxygen treatment at room temperature has a smaller e f f e c t on benzene hydrogénation a c t i v i t y than on the a c t i v i t y f o r MCP hydrogenolysis. For both reactions the same f r a c t i o n of i n i t i a l a c t i v i t y i s regained a f t e r t h i s treatment and the further recovery of a c t i v i t y due to oxygen treatment at successively higher temperatures proceeds i n a s i m i l a r way f o r both reactions.


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H Effect on Pt-Si0 Catalytic Activity 2

2

Table I. E f f e c t of HTR on c a t a l y t i c behavior. Catalyst

S e l e c t i v i t y change


Pt mean Disper% n-hexane i n thickness sion MCP hydrogenolysis (nm) % a f t e r LTR after HTR

A c t i v i t y change A c t i v i t y r a t i o HTR/LTR f o r MCP hydro- f o r benzene genolysis hydrogénation

1.0

7

3

3

0.3

0.7

0.52

17

6

8

0.3

0.8

0.3

20

12

11

0.1

0.8

0.23

30

28

23

0.4

1.0

0.1

40

37

32

0.15

0.7

60

40

33

1.0

1.0

EUROPT-1

Discussion The results obtained with the MCP hydrogenolysis indicate that HTR causes two d i f f e r e n t e f f e c t s on the c a t a l y t i c behavior of the Pt/SiO^ model catalysts: (i) a f a s t change of the s e l e c t i v i t y to less n-hexane formation followed by ( i i ) a slow decrease of a c t i v i t y . As was mentioned above the nonselective pathway of MCP hydrogenolysis (formation of n-hexane additional to 2-MP and 3-MP) takes place a t the phase boundary platinum-support (6). From the change of s e l e c t i v i t y towards less n-H formation i t i s concluded that HTR causes a p r e f e r e n t i a l i n h i b i t i o n of c a t a l y t i c ensembles composed by platinum and support s i t e s . This process occurs rather f a s t but requires reduction temperatures higher than 600 K. In e a r l i e r work (B) we were able to show that a t these temperatures atomic hydrogen can be adsorbed a t the support surface v i a hydrogen s p i l l o v e r . The support surface next to the platinum should quickly be saturated by t h i s spilled-over hydrogen r e s u l t i n g i n a p a r t i a l reduction of the s i l i c a surface. This reduction of the support surface i s assumed to be responsible f o r the change of s e l e c t i v i t y by i n h i b i t i o n of the c a t a l y t i c s i t e s situated a t the phase boundary p l a t i n u m - s i l i c a . The i n h i b i t i o n of these "adlineation ensembles" may occur either by poisoning of support s i t e s by atomic hydrogen or by formation of a bond between platinum and s i l i c a , as has been proposed recently by Frennet and Wells (9), r e s u l t i n g i n a changed electronic structure of the platinum atoms adjacent to the support. A l o c a l i z e d charge transfer has been proposed recently by Resasco and Haller (4) to account for the a c t i v i t y loss of the Rh/Ti0 interface. The f a s t attainment of the i n i t i a l s e l e c t i v i t y by an oxygen treatment at room temperature i s also understandable on the basis of t h i s mechanism, since atomic hydrogen adsorbed a t support s i t e s adjacent to the platinum should e a s i l y be oxidized even at room temperature. The o v e r a l l deactivation due to the HTR i s reversed by oxygen treatment at 673 K. Hence sintering of the platinum p a r t i c l e s i s not responsible f o r the deactivation since redispersion i s not l i k e l y to 2

American Chemical Society Library 1155 16th St., N.W. Baker etWashington, al.; Strong Metal-Support Interactions D.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


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STRONG METAL-SUPPORT

INTERACTIONS

occur i n oxygen at 673 K. Furthermore the deactivation i s only observed with the model c a t a l y s t s , while the a c t i v i t y of the EUROPT-1 catalyst i s hardly affected by HTR. The support of the model catal y s t s i s prepared d i f f e r e n t l y than that of the EUROPT-1 and we therefore conclude that the supporting material i s involved i n the deactivation process. This r e s u l t rules out the proposal during HTR i n h i b i t platinum s i t e s , since i n t h i s case the e f f e c t of HTR should not depend on the support. The HTR a f f e c t s more strongly the a c t i v i t y f o r MCP-hydrogenolysis than that for benzene hydrogénation. However, as i s seen i n Figure 2, even an oxygen treatment at room temperature results i n a recovery of a c t i v i t y for MCP hydrogenolysis to reach the same f r a c t i o n of i n i t i a l a c t i v i t y as f o r benzene hydrogénation. The stronger deactivation vs MCP hydrogenolysis due to HTR can be related to the change i n s e l e c t i v i t y , i . e . both are caused by HTR and are reversed by oxygen treatment at room temperature. Therefore we conclude that the p a r t i a l reduction of the support also influences the a c t i v i t y of the platinum, at least f o r hydrogenolysis of MCP. Oxygen treatment at room temperature most l i k e l y r e s u l t s i n the reoxidation of the support surface thereby reversing the special deactivation and the change of s e l e c t i v i t y However sthe f u l l a c t i v i t y ±s not regained by t h i s oxygen treatment at room temperature. As the course of reactivation by further oxygen treatments agrees for both reactions, we assume that part of the platinum surface might be i n h i b i t e d geometrically by support material, a mechanism well established f o r t i t a n i a supported catalysts exhibiting SMSI behavior (10). This i n h i b i t i o n of the platinum surface could occur either by surface migration of s i l i c a due to wetting conditions i n the reducing ambient causing the formation of a s i l i c a skin or by d i f f u s i o n of molecular SiO species onto the platinum surface. The formation of a s i l i c a skin has been already proposed by Schuit et a l . (jj.) for explaining deactivation e f f e c t s i n N i / S i 0 c a t a l y s t s . From electron microscopic observations the beginning of skin formation i n the Pt/SiO- system at high temperatures was deduced (12). On the other hand, the segregation of s i l i c a on platinum surfaces has been reported to s t a r t at about 600 Κ by bulk d i f f u s i o n through platinum (13). Van Langeveld et a l . (14) have shown that the structure of the substrate (quartz or Pyrex) determines, whether SiO species can migrate through evaporated films of platinum. Hence, the f a c t that EUROPT-1 does not suffer from t h i s deactivation could be due to the d i f f e r e n t preparation procedures of the s i l i c a . The support fojj the model c a t a l y s t s , which i s prepared by deposition of SiO i n 10~ Pa of oxygen, may have retained some structural "memory" on the formation from SiO and may therefore be reduced more e a s i l y than the conventional s i l i c a of the EUROPT-l c a t a l y s t . The islands formed by segregation of s i l i c o n from bulk platinum are reported to be e a s i l y oxidized to S i 0 at temperatures lower than 400 K, when they are exposed to atomic oxygen (^5). Thus assuming molecularly dispersed s i l i c o n species to be formed during HTR, these species should be readily oxidized to S i 0 even at room temperature, since atomic oxygen necessary f o r t h i s oxidation i s c e r t a i n l y provided by d i s s o c i a t i v e l y adsorbed oxygen on platinum. While oxygen treatment at room temperature i s assumed to be s u f f i c i e n t to cause reoxidation of either the s i l i c a skin or the 2

2

2



15.

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2

149

——τ

1 5

10 h Time of HTR treatment Figure 1. Loss of a c t i v i t y towards MCP hydrogenolysis as a func t i o n of HTR treatment time.

100 %

50 %

Ο 673 Η 523 (LTR)

Ο 673 Η 673 (HTR)

Ο Η

293 523

0 Η

373 523

Ο

MCP Hydrogenolysis

Λ

Benzene Hydrogénation

Ο Η

473 523

Ο Η

573 523

Ο 673 Η 523 (LTR)

Figure 2. Recovery of a c t i v i t y due to oxygen treatments at suc cessively higher temperatures.


S T R O N G METAL-SUPPORT

150

INTERACTIONS

s i l i c o n islands formed during HTR, higher temperatures are obviously necessary f o r removing the s i l i c a from the platinum surface thereby restoring the a c t i v i t y f o r MCP hydrogenolysis and benzene hydrogénation i n a similar way. The mobility i n the silica/platinum system also under oxidizing conditions has been demonstrated e a r l i e r (J5) by an experiment, using a P t / S i 0 model c a t a l y s t , exhibiting a platinum p a r t i c l e size of about 10 nm. This c a t a l y s t was covered by a 20 nm thick layer of S i 0 by HV deposition, r e s u l t i n g i n a t o t a l loss of c a t a l y t i c a c t i v i t y . However, a f t e r heating i n oxygen at 673 Κ followed by reduction at 523 Κ 26 % of the i n i t i a l a c t i v i t y was regained, indicating that the mobility i s s u f f i c i e n t to lead to the reexposure of a substantial part of the platinum surface. 2


2

Conclusion (i) High temperature reduction of platinum / s i l i c a model catalysts causes two d i f f e r e n t e f f e c t s (a) change i n product d i s t r i b u t i o n connected with a special deactivation of MCP hydrogenolysis and (b) slow o v e r a l l deactivation f o r both MCP hydrogenolysis and benzene hydrogénation. ( i i ) The special deactivation of MCP hydrogenolysis and the change i n product d i s t r i b u t i o n i s assumed to be due to a p a r t i a l reduction of the support next to the platinum v i a hydrogen s p i l l o v e r causing a deactivation of c a t a l y t i c ensembles at the phase boundary. The deactivation of the platinum support ensembles may be caused either by i n h i b i t i o n of support s i t e s by atomic hydrogen or by formation of a Pt-Si bond leading to a disturbed electronic structure of platinum adjacent to the support. ( i i i ) The o v e r a l l deactivation f o r MCP hydrogenolysis and benzene hydrogénation i s most l i k e l y due to coverage of platinum by s i l i c a . This coverage can occur either by s i l i c a skin formation due to wetting conditions i n a reducing atmosphere or by d i f f u s i o n of molecularly dispersed SiO species onto the platinum p a r t i c l e s . (iv) Recovery of i n i t i a l a c t i v i t y i s attained by oxygen treatment at 673 K. I t i s assumed that t h i s treatment causes the removal of the s i l i c a skin or the d i f f u s i o n of the S i 0 islands back to the support. 2

The e f f e c t of hydrogen pretreatment temperature on the c a t a l y t i c behavior of Pt/SiO^ model catalysts has been studied f o r hydrogenol y s i s of methylcyclopentane (MCP) and hydrogénation of benzene. For benzene hydrogénation the catalysts exhibit only a s l i g h t l y lower a c t i v i t y a f t e r high temperature reduction compared to that a f t e r low temperature reduction. In the MCP hydrogenolysis both a loss of a c t i v i t y and a change of product d i s t r i b u t i o n i s observed after high temperature reduction. However, the i n i t i a l product d i s t r i b u t i o n of MCP hydrogenolysis i s reestablished a f t e r oxygen treatment at room temperature, and f u l l i n i t i a l a c t i v i t y f o r both reactions i s regained a f t e r oxygen treatment at 673 K. On the basis of these results the following mechanisms are deduced: (i) P a r t i a l reduction of the support adjacent to the p l a t i num v i a hydrogen s p i l l o v e r , r e s u l t i n g i n a special deactivation f o r MCP hydrogenolysis and i n a change of product d i s t r i b u t i o n . This


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151

reduction i s reversed by oxygen treatment at room temperature, ( i i ) The o v e r a l l deactivation f o r both reactions i s assumed to be due to platinum covered by supporting material. The s i l i c a cover layer i s assumed to be formed either by formation of a s i l i c a skin due to wetting conditions i n the reducing atmosphere, or by bulk or surface d i f f u s i o n of SiO species onto the platinum p a r t i c l e s . By oxygen treatment at 673 £ the s i l i c a cover layer i s removed and the f u l l platinum surface i s again exposed to the gas phase.


References 1. Bond, G.C. and Burch, R., in "Catalysis", ed. G.C. Bond and G. Webb, The Royal Society of Chemistry, London, 1983,Vol. 6,p. 27 2. Menon, P.G. and Froment, G.F., J. Catal. 59 , 138 (1979) 3. den Otter, G.J. and Dautzenberg, F.M., J. Catal. 53, 116 (1978) 4. Resasco, D.E. and Haller G.L., J. Catal. 82, 279 (1983) 5. Kramer, R. and Zuegg, Η., J. Catal. 80, 446 (1983) 6. Kramer, R. and Zuegg, Η., in Proceedings of the 8th Intern. Congress on Catalysis, Berlin 1984, Vol. 5, p. 275 7. Kramer, R. and Zuegg, Η., J. Catal. 85, 530 (1984) 8. Kramer, R. and Andre, Μ., J. Catal. 58, 287 (1979) 9. Frennet, A. and Wells, P.B., Applied Catal., in press 10. Anderson, J.B.F., Bracey, J.D., Burch, R. and Flambard, A.R., in Proceedings of the 8th Intern. Congress on Catalysis, Berlin 1984, Vol. 5, p. 111 11. Schuit, G.C., and van Reijen, L.L. in "Advances in Catalysis" Vol. 10, p. 242. Academic Press, New York/London 1958 12. Powell, B.R., and Whittington, S.E., J. Catal., 81, 382 (1983) 13. Niehus, H. and Comsa, G., Surf. Sci. 102 (1981) L 14 14. van Langeveld, A.D., Nieuwenhuys, B.E., and Ponec, V., Thin Solid Films, 105 (1983) 9 15. Bonzel, H.P., Franken, A.M. and Pirug, G., Surf Sci., 104 (1981) 625 RECEIVED September 17, 1985


Effect of H2 Treatment on the Catalytic Activity of Pt-SiO2 Catalysts

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