Strong Metal-Support Interactions - ACS Publications - American

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13 Metal-Support Interactions in Ni Catalysts A Comparative Study of Kinetic and Magnetic Behavior BetweenNb2O5and Phosphate Supports

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Ε. I. Ko1, J. E. Lester2, and G. Marcelin2,3 1Departmentof Chemical Engineering, Carnegie-Mellon University, Pittsburgh, PA 15213 2Gulf Research & Development Company, Pittsburgh, PA 15230 Different mechanisms of metal-support interactions were found from kinetic and magnetic studies over nickel catalysts on niobia- and phosphate-containing supports. All the catalysts showed a suppression in hydrogen chemisorption after they had been reduced at high temperatures. However, the niobia-containing samples as a group behaved very differently from the phosphate-containing samples in ethane hydrogenoly­ sis and carbon monoxide hydrogenation. When reduced at still higher temperatures, the phosphate-contain­ ing catalysts exhibited a change from the normal magnetization-temperature behavior, but the niobia­ -containing catalysts did not. Instead, the latter showed an apparent loss of nickel metal and a de­ crease in crystallite size. Furthermore, the extent of interaction in the niobia-containing catalysts could be moderated by using a niobia-silica surface phase oxide. The work of Tauster and coworkers ( l , 2 ) showed that hydrogen chemisorption i s suppressed on group VIII metals supported on a series of oxides after these samples have been reduced at high temperatures. The term strong metal-support interactions (SMSI) was introduced to describe this behavior. A similar suppression i n hydrogen chemisorption has since been reported for many other supported metal systems (3-5). However, the use of other chemical probes (4,5) demonstrated that different mechanisms of metalsupport interactions could exist for different types of oxides. Furthermore, even for a so-called SMSI oxide, the degree of interaction could be influenced by many parameters such as c r y s t a l ­ l i t e size and reduction temperature. It would thus be desirable to find an approach to systematically compare c a t a l y t i c behavior of d i f f e r e n t systems. 3Current address: Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA 15261 0097-6156/ 86/0298-0123$06.00/0 © 1986 American Chemical Society

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

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STRONG METAL-SUPPORT INTERACTIONS

AS AN ATTEMPT I N THIS D I R E C T I O N , A HIERARCHY WAS RECENTLY D E VELOPED FOR NICKEL CATALYSTS ( 6 ) . T H E B A S I C IDEA I S TO MONITOR THE CHEMICAL PROPERTIES OF A CATALYST AS PROBED BY HYDROGEN CHEMISORPTION, ETHANE HYDROGENOLYSIS, AND CARBON MONOXIDE HYDROGÉNATION. THE HIERARCHY, ORIGINALLY DEVELOPED FOR N I / N B 0 , C A T A L Y S T S , WAS LATER EXTENDED TO NICKEL SUPPORTED ON PHOSPHATE-CONTAINING M A T E RIALS AND A N I O B I A - S I L I C A SURFACE PHASE O X I D E . I N THIS PAPER THE USEFULNESS OF THE HIERARCHY WILL BE I L L U S T R A T E D BY I T S A B I L I T Y TO D I F F E R E N T I A T E BETWEEN SUPPORT E F F E C T S OF NIOBIA AND PHOSPHATE, AND TO E S T A B L I S H THE INTERMEDIATE DEGREE OF INTERACTION OF N I B O I A SILICA.

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MAGNETIC MEASUREMENTS, WHICH INCLUDED M A G N E T I Z A T I O N - T E M P E R A TURE BEHAVIOR AND P A R T I C L E S I Z E DETERMINATION, WERE ALSO MADE ON THIS S E R I E S OF CATALYSTS AS A FUNCTION OF REDUCTION TREATMENT. THESE RESULTS, I N CONJUNCTION WITH THOSE OBTAINED FROM K I N E T I C STUDIES, PROVIDED A PHYSICAL PICTURE OF THE D I F F E R E N T MECHANISMS FOR THE NIOBIA AND PHOSPHATE SUPPORTS. T H E SAME PICTURE I S C O N S I S TENT WITH THE LESS INTERACTING NATURE OF N I O B I A - S I L I C A , WHICH SHOULD PROVE USEFUL AS A MODEL SYSTEM FOR THE STUDY OF M E T A L SUPPORT INTERACTIONS I N G E N E R A L . EXPERIMENTAL SUPPORTS AND C A T A L Y S T S . T H E PREPARATION OF THE SUPPORTS USED I N THIS STUDY WAS DISCUSSED I N D E T A I L ELSEWHERE. T H E TWO PHOSPHATE SUPPORTS, A L 0 ·2Α1Ρ0^ AND 4MGO 13AL C> Ί Ο Α Ι Ρ Ο ^ WERE C O - P R E C I P I ­ TATED USING TNE NECESSARY NITRATE SALTS, PHOSPHORIC A C I D , AND AMMONIUM HYDROXIDE AT A F I X E D PH ( ! ) . NIOBIA WAS P R E C I P I T A T E D BY ADDING AMMONIUM HYDROXIDE TO A METHANOLIC SOLUTION OF NIOBIUM CHLORIDE ( 8 ) . THE N I O B I A - S I L I C A SUPPORT WAS PREPARED BY IMPREGNAT­ ING S I O ^ (DAVISON 952) TO I N C I P I E N T WETNESS WITH A HEXANE SOLUTION OF NIOBIUM E T H O X I D E . T H E SAMPLE WAS THEN DRIED AND CALCINED TO OBTAIN A HOMOGENEOUS SURFACE PHASE OXIDE (9). T A B L E I SUMMARIZES THE CHARACTERISTICS OF NICKEL CATALYSTS PREPARED ONTO THESE SUPPORTS. FOR BREVITY THESE CATALYSTS WILL BE REFERRED TO BY A NOTATION I N THE FORM Α Α - Β . FOR EXAMPLE, 7 A A P - 5 7 3 REPRESENTS A 7 WT % N I CATALYST SUPPORTED ON A ^ O ^ •2A1P0^ REDUCED AT 573 Κ FOR 1 H . I N C I D E N T A L L Y , THIS SAMPLE DID NOT REDUCE UNDER THESE CONDITIONS AND WAS EXCLUDED FROM FURTHER KINETIC STUDIES. NOTATIONS FOR THE OTHER CATALYSTS ARE SHOWN I N THE FIRST COLUMN OF TABLE I . A L L SAMPLES WERE REDUCED AT THE S P E C I F I E D TEMPERATURE FOR 1 H UNLESS NOTED OTHERWISE. T H E PERCENT REDUCTION WAS DETERMINED BY MEASURING OXYGEN UPTAKE AT 673 Κ I N A COMMERCIAL THERMOGRAVIMETRIC SYSTEM (CAHN 1 1 3 ) . T H E AVERAGE P A R T I ­ C L E S I Z E WAS DETERMINED BY EITHER X - R A Y D I F F R A C T I O N L I N E BROADEN­ ING OR MAGNETIC MEASUREMENTS ( S E E BELOW). 2

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CHEMICAL MEASUREMENTS. A L L THE CATALYSTS I N T A B L E I SHOWED A SUPPRESSION I N HYDROGEN CHEMISORPTION TO VARYING E X T E N T . SUCH A SUPPRESSION MANIFESTED I T S E L F EITHER AS AN OVERESTIMATION OF C R Y S ­ TALLITE SIZE FROM CHEMISORPTION DATA OR AS A LOWER ADSORPTION STOICHIOMETRY OF H / N I , ν THAN WHAT WOULD BE EXPECTED OF A COMPAR­ ABLE NI/SIO^ CATALYST. IN ADDITION, THE SUPPRESSION WAS MORE SEVERE WITH INCREASING REDUCTION TEMPERATURE ( 8 - 1 0 ) . AS MENTIONED

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

Baker et al.; Strong Metal-Support Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 100 100 100 100 100 30 100

773K, 1 h 573K, 1 h 773K, 1 h 573K, 1 h 773K, 1 h 573K, 1 h 773K, 1 h

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Loading (wt. %) % Reduction

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Preparation

Summary of Supported Nickel Catalysts

Notation

Table I.

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3 10"' FIGURE 5 . METHANATION A C T I V I T Y OF CARBON MONOXIDE HYDROGENA TION OVER NICKEL SUPPORTED ON ( A ) N B ^ O , AND ( B ) N B ^ O ^ - S I O ^ A A FUNCTION OF REDUCTION TREATMENT, INDUCTION TREATMENTS WERE (·) 7 7 3 K , L H ; AND ( A ) 8 7 3 K , 1 6 H .

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

134

STRONG METAL-SUPPORT INTERACTIONS

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APPEARS GENERALLY THAT FOR THESE CATALYSTS THE ENHANCEMENT I N ACTIVITY FIRST DIMINISHES AND THEN DISAPPEARS WITH INCREASING SEVERITY OF REDUCTION. A COMPARISON BETWEEN CATALYSTS SHOULD BE DONE WITH THIS OBSERVATION I N MIND. AT PRESENT THE EXACT CAUSE FOR THE ENHANCED CO HYDROGÉNATION A C T I V I T Y REMAINS UNCLEAR. SEVERAL AUTHORS HAVE PROPOSED THAT S P E C I A L A C T I V E S I T E S E X I S T AT THE SUBOXIDE-METAL INTERFACE (17,23). RECENTLY, RAUPP AND DUMESIC (24) SUGGESTED THAT THE ENHANCED A C T I V I T Y OF N I / T I O ^ CAN BE UNDERSTOOD I N TERMS OF MORE COMPETITIVE HYDROGEN ADSORPTION AND THAT SPECIAL INTERFACIAL S I T E S NEED NOT BE INVOKED. WITHOUT D I F F E R E N T I A T I N G BETWEEN THESE MODELS, WE SIMPLY SUGGEST THAT THE ENHANCED A C T I V I T Y I S RELATED TO THE PRESENCE AND DISTRIBUTION OF THE SUBOXIDE S P E C I E S ON THE METAL S U R F A C E . T H E SUBOXIDE S P E C I E S SHOULD BE WELL DISTRIBUTED AT LOW CONCENTRATIONS, THUS AFFORDING A LARGE INTERFACIAL AREA AT I T S P E R I M E T E R . WITH INCREASING CONCENTRATIONS THESE S P E C I E S ARE L I K E L Y TO AGGREGATE, RESULTING I N A LOSS OF INTERFACIAL AREA, AND EVENTUALLY LEAD TO SOME COMPOUND FORMATION AT THE S U R F A C E . T H I S Q U A L I T A T I V E ARGUMENT IS CONSISTENT WITH THE OBSERVED ACTIVITY TREND. T H E USE OF NIOBIA-SILICA PRESUMABLY SLOWS DOWN THE D E C L I N E I N A C T I V I T Y BY L I M I T I N G THE CONCENTRATION OF MIGRATING OXIDE THROUGH A LOW A V A I L A B I L I T Y OF N I O B I A . CONSEQUENTLY THE N I O B I A - S I L I C A - S U P P O R T E D SAMPLE I S MORE A C T I V E AT A G I V E N REDUCTION TREATMENT, WHICH WOULD C O R R E SPOND TO A MORE FAVORABLE D I S T R I B U T I O N OF THE SUBOXIDE S P E C I E S . I T SHOULD BE NOTED THAT OUR ARGUMENT ONLY REQUIRES THAT THE A C T I V I T Y IS RELATED TO THE A V A I L A B L E INTERFACIAL AREA AROUND THE PERIMETER OF THE SUBOXIDE S P E C I E S . WITH CURRENTLY A V A I L A B L E DATA I T WOULD BE DIFFICULT TO A S C E R T A I N WHETHER SUCH A RELATIONSHIP ARISES FROM A CREATION OF SPECIAL S I T E S AT THE INTERFACE OR A DIFFERENCE I N THE C O M P E T I T I V E ADSORPTION OF AND CO ON THE METAL SURFACE NEAR THE INTERFACE. SUMMARY COMBINED K I N E T I C AND MAGNETIC STUDIES E S T A B L I S H DIFFERENT MECHANISMS OF METAL-SUPPORT INTERACTIONS FOR NIOBIA AND PHOSPHATE S U P P O R T S . T H E TERM S M S I , ORIGINALLY COINED FOR T I 0 - S U P P O R T E D M E T A L S , SHOULD PERHAPS BE RESERVED FOR SYSTEMS WHOSE MODE OF INTERACTION IS SIMILAR TO THAT OF T I 0 « I N ADDITION TO THE SUPPRESSION I N HYDROGEN CHEMISORPTION AND I T S R E V E R S I B I L I T Y WITH OXIDATION AS F I R S T REPORTED FOR SMSI BEHAVIOR, OTHER CHEMICAL PROBES CAN BE USED TO FURTHER D E F I N E THIS PARTICULAR MODE OF I N T E R A C T I O N . 2

2

EVEN FOR AN SMSI O X I D E , THE EXTENT OF INTERACTION I S D E P E N DENT ON MANY PARAMETERS AND A COMPARISON AMONG SAMPLES MUST BE DONE S Y S T E M A T I C A L L Y . UNDER HIGH REDUCTION TEMPERATURES E N C A P S U L A TION OF T H E METAL P A R T I C L E LEADS TO A D E C L I N E I N CO HYDROGÉNATION IN NIOBIA-CONTAINING SUPPORT. THE N I O B I A - S I L I C A SURFACE PHASE OXIDE, WHICH SHOWS A SIMILAR MECHANISM OF INTERACTION TO NIOBIA BUT I S LESS I N T E R A C T I N G , SHOULD PROVE USEFUL AS A MODEL SYSTEM I N FUTURE S T U D I E S . ACKNOWLEDGMENTS ONE OF US ( E I K ) WOULD L I K E TO THANK THE NATIONAL S C I E N C E TION ( C P E - 8 3 1 8 4 9 5 ) FOR PARTIAL SUPPORT OF THIS WORK.

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

FOUNDA-

ko et a l .

13.

Kinetics in Nb 0 and Phosphate Supports 2

5

Literature Cited 1. 2. 3. 4.

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5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

Tauster, S.J.; Fung, S.C.; Garten, R.L. J. Amer. Chem. Soc. 1978, 100, 170. Tauster, S.J.; Fung, S.C. J. Catal. 1978, 54, 29. Adamiec, J . ; Wanke, S.E.; Tesche, B.; Klengler, U. In "MetalSupport and Metal-Additive Effects in Catalysis"; Β. Imelik et a l . , Ed.; Elsevier: Amsterdam, 1982, p. 77. Meriaudeau, R.; Dutel, J.F.; Dufaux, M.; Naccache, C. In "Metal-Support and Metal-Additive Effects in Catalysis"; Β. Imelik et a l . , Ed.; Elsevier: Amsterdam, 1982, p. 95. Maubert, Α.; Martin, G.A.; Prailaud, H.; Turlier, P. React. Kinet. Catal. Lett. 1983, 22, 203. Ko, E.I.; Hupp, J.M.; Wagner, N.J. J. Catal. 1984, 86, 315. Marcelin, G.; Vogel, R.F. J. Catal. 1983, 82, 482. Ko, E.I.; Hupp, J.M.; Rogan, F.H.; Wagner, N.J. J. Catal. 1984, 84, 85. Ko, E.I.; Bafrali, R.; Nuhfer, N.T.; Wagner, N.J. J. Catal. accepted for publication. Marcelin, G.; Lester, J.E. J. Catal. 1985, 93, 270. Yates, D.J.C.; Taylor, W.F.; Sinfelt, J.H. J. Amer. Chem. Soc. 1964, 86, 2996. Vannice, M.A. J. Catal. 1975, 37, 449. Selwood, P.W. In "Chemisorption and Magnetization"; Academic: New York, 1975; Chapter IV. Marcelin, G.; Ko, E.I.; Lester, J.E. J. Catal. accepted for publication. Ko, E.I.; Marcelin, G. J. Catal. 1985, 93, 201. Taylor, W.F.; Sinfelt, J.H.; Yates, D.J.C. J. Phys. Chem. 1965, 69, 3857. Burch, R.; Flambard, A.R. J. Catal. 1982, 78, 389. Bartholomew, C.H.; Pannell, R.B.; Butler, J.L. J. Catal. 1980, 65, 335. Iida, K. J. Magn. Mater. 1983, 35, 226. Simoen, A.J.; Baker, R.T.K.; Dwyer, D.J.; Lund, C.R.F.; Madon, R.J. J. Catal. 1984, 86, 359. Chung, Y.-W.; Xiong, G.; Kao, C.C. J. Catal. 1984, 85, 237. Raupp, G.B.; Dumesic, J.A. J. Phys. Chem. 1984, 88, 660. Vannice, M.A.; Sudhakar, C. J. Phys. Chem. 1984, 88, 2429. Raupp, G.B.; Dumesic, J.A. Prepr. Div. Petrol. Chem. Amer. Chem. Soc. 1985, 30(1), 137.

RECEIVED September 12, 1985

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

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