JOURNAL OF CATALYSIS 127, 366--376 (1991) Structural Studies of Platinum/ZSM-5 Catalysts EFIM S. SHPIRO,* RICHARD W. JOYNER,t A KHABIB M. MINACHEV,* AND PAUL D. A. PUDNEY? *N. D. Zelinksy Institute of Organic Chemistry, USSR Academy of Sciences, Moscow, USSR; and tLeverhulme Centre for Innovative Catalysis and Surface Science Research Centre, University of Liverpool, PO Box 147, Grove Street, Liverpool L69 3BX, United Kingdom Received December 15, 1989; revised July 3, 1990 Pt/ZSM-5 catalysts have been prepared, characterised by extended X-ray absorption fine struc- ture (EXAFS), and electron microscopy, and their activity in ethane hydrogenolysis has been measured. Calcination temperature is important in determining the size of the platinum particles formed. For a 0.5% Pt/H-ZSM-5 catalyst calcined at 720 K and reduced at 620 K or 790 K, small particles within the zeolite framework are observed, with a nearest neighbour coordination number of ca. 6 and an average diameter of about 8 A. Calcination of a similar catalyst at 790 K resulted in larger metal particles, with the average diameter in the range 12-15 A, estimates from EXAFS and electron microscopy being in agreement within experimental error. Contractions of up to 4% in the average Pt-Pt interatomic distance were observed, but there is clear evidence that the larger particles at least retain the face-centred cubic structure. EXAFS indicates the presence of Pt-O bonding as a result of coordination to the zeolite framework, with bond lengths of 1.92-2.03 ,~. These catalysts have higher specific activity in ethane hydrogenolysis and alkane aromatization than materials where the platinum is in the form of large particles, (d > 100 ,~), at the external surface of the zeolite, and reasons for this are discussed. © 1991AcademicPress, Inc. INTRODUCTION There continues to be considerable inter- est in the properties of catalysts in which noble metals are combined with zeolites. The intrinsic catalytic activity of platinum or other metals may be enhanced and there is also the possibility of synergy between the metal and the zeolite. In this paper we discuss mainly structural aspects of small platinum clusters in H-ZSM-5 zeolite cata- lysts. These materials exhibit interesting catalytic properties in C2 and C3 alkane aromatization (1), and it has been sug- gested, based on XPS evidence, that the metal particles are electron deficient (2). The technique of choice is X-ray absorp- tion spectroscopy, and in particular ex- tended X-ray absorption fine structure (EXAFS). The structure of platinum parti- cles has been studied in faujasitic zeolites, 1 To whom correspondence should be addressed. 0021-9517/91 $3.00 Copyright© 1991 by AcademicPress, Inc. All rightsof reproduction in any form reserved. e.g., by Moraweck and Renouprez (3) us- ing EXAFS, and by Gallezot et al. (4), using EXAFS and radial distribution func- tion measurements. The only previous structural study of the zeolite ZSM-5 re- lates to the location of Ni 2+ ions exchanged into the framework (5). The EXAFS technique has been applied to the study of catalysts for about 10 years (6), but can still arouse scepticism about its quantitative accuracy (witness some of the discussion at the 1988 International Congress on Catalysis (7)). A number of approaches have been used to extract inter- atomic distances and coordination numbers from the spectra, and experimental error has been either ignored completely or esti- mated subjectively. Recently Joyner et al. (8) published a statistical test which allows a quantitative estimate of the error bar on any individual result and considers correla- tion between parameters; here this ap- proach is applied rigorously. 366