Cluster Chemistry on Surfaces: Characterization and Catalytic Studies of Phosphine-Stabilized Platinum-Gold Clusters on Silica and Alumina Supports Irina V. G. Graf, † Jeffrey W. Bacon, † Mark B. Consugar, † Michael E. Curley, † Larry N. Ito, ‡ and Louis H. Pignolet* ,† Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, and Central Research-Catalysis Laboratory, Dow Chemical Company, Midland, Michigan 48674 ReceiVed April 6, 1995 X Cationic, phosphine-ligated Pt-Au cluster compounds, [Pt(AuPPh 3 ) 8 ](NO 3 ) 2 (1) and [(PPh 3 )Pt(AuPPh 3 ) 6 ](NO 3 ) 2 (2), have been immobilized on silica and alumina supports. Characterization of the supported clusters by 31 P MAS NMR, IR, and UV-visible spectroscopy and study by chemisorption and temperature-programmed desorption of CO show that the clusters are immobilized intact without measurable fragmentation or irreversible transformation. This is confirmed by quantitative desorption of the clusters. The reactivity of the supported clusters with CO and H 2 was found to be similar to the known reactivities in solution and molecular solid phases. The cluster [(PPh 3 )- Pt(H)(AuPPh 3 ) 7 ](NO 3 ) 2 (3) was also immobilized on silica and found to be intact by MAS 31 P NMR and desorption experiments. The turnover rate (TOR) for H 2 -D 2 equilibration was determined for these supported clusters at room temperature. The clusters 1 and 2 were good catalysts on silica and alumina supports after treatment at 135 °C under vacuum (TOR ) 10-20 s -1 ), but showed significant activation by treatment at 110 °C under H 2 (TOR ) 85-220 s -1 ). Cluster 3/SiO 2 gave a similar rate (TOR ) 170 s -1 ) with no thermal activation. All samples subjected to the thermolysis conditions given above were found to be intact by 31 P MAS NMR and could be quantitatively desorbed. The high activities of the supported Pt-Au clusters are believed to result from support- promoted, partial PPh 3 dissociation. Desorption experiments show that this dissociation is reversible. Introduction Supported molecular metal complexes and organometallic compounds have been extensively studied as novel catalysts and metal particle catalyst precursors. 1 For such systems it is of interest to determine the effect of the support on structure and reactivity of the metal complex. The field of surface organo- metallic chemistry deals with this topic and has recently received significant attention. 1-4 Studies in this field have primarily used transition metal carbonyl cluster compounds deposited on a variety of metal oxide supports. 1 This area of research is important because oxide-supported metals and some metallic complexes (especially of the bimetallic variety) catalyze many significant reactions. 1-8 Although supported molecular com- pounds are not likely to be practical catalysts themselves due to their instability, the study of their surface chemistry can provide useful insight into cluster reactivity and the mechanism of thermal activation. A long-range goal of our research in this area is to evaluate the use of cationic, phosphine-ligated Pt- Au cluster compounds as precursors to uniform, highly dis- persed, supported Pt-Au particle catalysts. There has been significant interest in Pt-Au catalysts but in no case has a preformed bimetallic cluster been used in catalyst preparation. 7 In this paper we describe the preparation, characterization, and reactivity of the Pt-Au cluster compounds, [Pt(AuPPh 3 ) 8 ](NO 3 ) 2 (1), [(PPh 3 )Pt(AuPPh 3 ) 6 ](NO 3 ) 2 (2), and [(PPh 3 )Pt(H)(AuPPh 3 ) 7 ]- (NO 3 ) 2 (3), immobilized on silica and alumina supports. There have been very few studies of this type with cationic, phosphine- † University of Minnesota. ‡ Dow Chemical Co. X Abstract published in AdVance ACS Abstracts, January 1, 1996. (1) For an overview, see: (a) Gates, B. C. Chem. ReV. 1995, 95, 511. (b) Ichikawa, M. AdV. Catal. 1994, 38, 283. (c) Scott, S. L.; Basset, J.- M. J. Mol. Catal. 1994, 86, 5. (d) Yermakov, Y. I.; Kuznetsov, B. N.; Zakharov, V. A. Catalysis by Supported Complexes; Elsevier: Am- sterdam, 1981. (e) Marks, T. J. Acc. Chem. Res. 1992, 25, 57. (f) Bond, G. C. Chem. Soc. ReV. 1991, 20, 441. (g) Tailored Metal Catalysts; Iwasawa, Y. I., Ed.; D. Reidel Publishing Co.: Boston, 1986. (h) Braunstein, P.; Rose, J. In Heterometallic Clusters in Catalysis: Stereochemistry of Organometallic and Inorganic Compounds; Bernal, I., Ed.; Elsevier: Amsterdam, 1988; Vol. 3, Chapter 1, p 3. (i) Suss- fink, G.; Meister, G. AdV. Organomet. Chem. 1993, 35, 41. (j) Studies in Surface Science and Catalysis; Gates, B. C., Guczi, L., Knozinger, H., Eds.; Elsevier: Amsterdam, 1986; Vol. 29 and references cited therein. (k) Basset, J.-M.; Gates, B. C.; Candy, J. P.; Choplin, A.; Leconte, M.; Quignard, F.; Santini, C. C. Surface Organometallic Chemistry: Molecular Approaches to Surface Catalysis; Kluwer: Dordrecht, The Netherlands, 1988. (l) Hartley, F. R. Supported Metal Complexes; Reidel: Dordrecht, The Netherlands, 1985. (m) Zecchina, A.; Area ´n, C. O. Catal. ReV. Sci. Eng. 1993, 35, 261. (n) Hsu, L.-Y.; Shore, S. G.; D’Ornelas, L.; Choplin, A.; Basset, J.-M. J. Mol. Catal. 1994, 149, 159. (2) Scott, S. L.; Dufour, P.; Santini, C. C.; Basset, J.-M. J. Chem. Soc., Chem. Commun. 1994, 2011. (3) Soucek, M. D.; Chiou, H.-S.; Kyba, E. P. J. Organomet. Chem. 1993, 456, 255. (4) (a) Dufour, P.; Scott, S. L.; Santini, C. C.; Lefebvre, F.; Basset, J-M. Inorg. Chem. 1994, 33, 2509. (b) Nedez, C.; Choplin, A.; Basset, J.- M. Inorg. Chem. 1994, 33, 1094. (c) Eisen, M. S.; Marks, T. J. J. Mol. Catal. 1994, 86, 23. (d) Schwartz, J. Acc. Chem. Res. 1985, 18, 302. (5) Meyer, T. Y.; Woerpel, K. A.; Novak, B. M.; Bergman, R. G. J. Am. Chem. Soc. 1994, 116, 10290. (6) Clark, J. H. Catalysis of Organic Reactions by Supported Inorganic Reagents; VCH Publishers, Inc.: New York, 1994. (7) Some Pt-Au particle catalysts: (a) Schwank, J.; Balakrishnan, K.; Sachdev, A. In New Frontiers in Catalysis; Guczi, L., Solymosi, F., Tetenyi, P., Eds.; Elsevier: Amsterdam, 1993; p 905. (b) Rouabah, D.; Fraissard, J. J. Catal. 1993, 144, 30. (c) Balakrishnan, K.; Schwank, J. J. Catal. 1991, 132, 451. (d) Sachdev, A.; Schwank, J. J. Catal. 1989, 120, 353. (e) Sermon, P. A.; Thomas, J. M.; Keryou, K.; Millward, G. R. Angew. Chem., Int. Ed. Engl. 1987, 26, 918. (f) Balakrishnan, K.; Sachdev, A.; Schwank, J. J. Catal. 1990, 121, 441. (g) Yates, R. C.; Somorjai, G. A.; J. Catal. 1987, 103, 208. (h) Sinfelt, J. H. Bimetallic Catalysts; Wiley: New York, 1985. (i) Clarke, J. K. A.; Creaner, A. C. M.; Baird, T. Appl. Catal. 1984, 9, 85. (j) Clarke, J. K. A.; Manninger, L.; Baird, T. J. Catal. 1978, 54, 230. (k) van Schaik, J. R. H.; Dessing, R. P.; Ponec, V. J. Catal. 1975, 38, 273. (8) Some supported Au particle catalysts: (a) Sakurai, H.; Tsubota, S.; Haruta, M. Appl. Catal. A 1993, 102, 125. (b) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (c) Tanielyan, S. K.; Augustine, R. L. Appl. Catal. A 1992, 85, 73. 689 Inorg. Chem. 1996, 35, 689-694 0020-1669/96/1335-0689$12.00/0 © 1996 American Chemical Society