JOURNAL OF CATALYSIS 108, 304-322 (1987) An Integrated Study of Pt/W03/Si02 Catalysts for the NO-CO Reaction II. TEM Investigation of Overlayer Formation on Model Pt-WOdSiOn Catalysts J.R. REGALBUTO,' C. W. ALLEN,*,' AND E. E. WOLFP~ Departments of tChemica1 Engineering and *Metallurgical Engineering and Material Science, University of Notre Dame, Notre Dame, Indiana 46556 Received November 6, 1986; revised June 22, 1987 Model Pt/WOr/SiOr catalysts, prepared by “impregnation” of a silica-covered TEM grid using the same precursor salts and pretreatment steps as those used for supported catalysts, were characterized with TEM by analyzing the same catalyst regions through the series of pretreatment steps. Overlayers appeared after a second calcination step following Pt impregnation. By a series of control experiments the overlayers were shown not to arise from carbon contamination, from impurities in the silica or Pt precursor, or from aging under ambient conditions. On the other hand, the mobility of W03 was indicated. These observations qualitatively correlate to previous characterization of the real catalysts and suggest that the SMSI state in ptiWO,lSiOZ catalysts arises from mobile, monodisperse species of WOr formed during the second, low-temperature CakinatiOn Step. 0 1987 Academic Press, Inc. INTRODUCTION The morphology of the Pt/WOJSi02 cat- alysts is quite complex. In part I of this series of papers (I) on these catalysts, CO chemisorption, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the cata- lyst’s morphology. The results of the said studies indicated that chemisorption sup- pression occurred without the correspond- ing changes in crystallite size as indicated by XRD. Furthermore, associated with this, changes in the oxidation state of the catalysts were detected by XPS. These results were explained via a decoration model similar to that postulated for Pt/Ti02 catalysts, as presented recently in a review paper on the subject (2). While postulated 1 Present address: Department of Chemical Engi- neering, University of Illinois at Chicago, Box 4348, Chicago, IL 60680. ’ Present address: EM Center for Materials Re- search, Argonne National Laboratory, Argonne, IL 60439. 3 To whom correspondence should be addressed. for a new catalyst system (Pt/WOJSi02), the model is similar to others proposed for more common SMSI catalysts (2). For ex- ample, the SMSI state in Ti02 catalysts is produced by a high-temperature (500°C) reduction of the catalyst without an inter- mediate calcination step. Decoration is thought to occur as partially reduced moi- eties of the support, TiO, (x < 2), migrate onto the metal surface. While ample support for this type of morphological model has come from Auger electron spectroscopy, XPS, and infrared and chemisorption studies (2), direct detec- tion of decorating species by electron mi- croscopy has been difficult to obtain. No evidence for a decoration model has been obtained in a number of studies of both real (three-dimensional) and model (two-dimen- sional) Pt/TiO, (3-6) and Rh/TiOz (7) cata- lysts. In these, the characteristic features of the SMSI state were first associated with the formation of metal crystallites which assumed a hexagonal, flat “pillbox” mor- phology. It was also reported that Ti02 was reduced to T&O7 in the SMSI state (4, 5), 304 0021-9517/87 $3.00 Copyright 6 1987 by Academic Press, Inc. All rights of reproduction in any furm reserved.