Structure and Reactivity of Granular Noble Metal Catalysts
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STRUCTURE AND REACTIVITY OF GRANULAR NOBLE METAL CATALYSTS H. W. Deckman, S. C. Fung, M. G. Matturro and J. A. McHenry, Corporate Research Laboratory, Exxon Research and Engineering Co., Rt. 22 East Annandale, NJ 08801
ABSTRACT The deactivation / reactivation behavior of fresh granular Pt metal catalysts supported on alumina is used to obtain information about the nature of catalytically active sites. It is found that relatively fresh catalysts can in some cases be reactivated by evolving only -1/100 of a monolayer of molecules off the Pt metal. This implies that either less than -1/100 of the surface is catalytically active with a site turnover frequency of greater than -100 reactions/(site-sec) or that the reactivation process does not evolve small cracked molecules from the catalytic sites and involves some other physical or chemical change of the catalyst surface. Introduction Highly dispersed noble metal particles on high surface area metal oxides form an extremely important class of heterogeneous catalytic materials [1-3]. Often the noble metal coverage on the oxide support is -1/100 of a monolayer and the surface area in the microporous oxide is 10-1000 m 2 /gm [1-4]. In these systems, the noble metals are usually formed in 2-20 A sized granules with -100-50% of the metal atoms exposed at the granule surface. Although many aspects of the physical structure of these catalysts are known, very little is known about the nature of the actual catalytic sites. In several different systems information has been obtained about physical size [5-6], shape [6], migration [7] and agglomeration[8] of the metal species. Physical changes such as growth of carbonaceous overlayers which occur on catalysts in reaction environments have also been extensively studied. In this paper we study the nature of the actual catalytic sites by examining whether every exposed metal atom on the surface of a supported noble metal catalyst is catalytically active or whether only a small subset of the surface atoms are catalytically active. Overall reaction rates on supported metal catalysts appear to be low in comparison with homogeneous catalysts. Homogeneous catalysts have reported [9-10] turnover frequencies of 10 4- 10 6 (reactions/ [catalytic site-sec]} in a variety of reactions, some of which are listed in Table I. Reactions on supported metal catalysts [11] have turnover frequencies that are typically less than 100 (reactions/[exposed metal atom-sec]) and one of the more active is listed in Table I. Although precisely equivalent heterogeneously and homogeneously catalyzed reactions are not compared in Table I, there is a general trend towards higher potential reaction rates in reported homogeneously catalyzed reactions. The relatively low activity of heterogeneous supported metal catalysts may have a variety of explanations ranging from the presence of a large number of sites having an intrinsically low turnover frequency, to the possibility of the existence of only a few highly active sites. We have found that by studying the way a ca
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