JOURNAL OF CATALYSIS 98, 502-512 (1986) Characterization of Palladium Surfaces with (+)-Apopinene: Correlation of Reaction Paths with Surface Features G. V. SMITH,**?+' A.MoLNAR,*-$ M.M. KHAN,* D. OSTGARD,* ANDN. YOSHIDA*+~ *Department of Chemistry and ‘IMolecular Science Program, Southern Illinois University at Carbondale, Carbondale, Illinois, 62901; $Department of Organic Chemistry, Jozsef Attila University, Szeged Hungary; and SDepartment of Chemistry, Kobe University, Kobe, Japan Received March 18, 1985; revised October 18, 1985 The molecular probe (+)-apopinene is chosen for the characterization of metal surface sites because of its unique structural features. These features allow clear distinctions of adsorption side and double bond position. By measuring the relative rates of double bond migration and addition, kjk,, it is shown that (+)-apopinene is sensitive to 17 different Pd catalyst preparations. Among these, a set of alumina-supported Pd catalysts ranging in dispersions from 0.055 to I.00 (13 1 to 8.4 A by EM) yielded results consistent with the notions that jM-type sites (highly coordinatively unsaturated) catalyze principally addition and ‘M-type sites catalyze principally isomerization. (+)- Apopinene distinguishes between reduced PdOr samples which have undergone different extents of surface I’eCOnStrLICtiOn. 0 1986 Academic Press. Inc. INTRODUCTION Correlations of metal surface structure with catalytic reactivity have made great progress in recent years. This progress has occurred for two principal reasons. First, the development of sophisticated instru- mentation has made possible the examina- tion of crystalline surfaces in unprece- dented detail (I), and second, the development of an understanding of or- ganometallic complexes has furnished spe- cific systems after which surface complexes can be modeled (2). Growing out of this understanding is a picture of metal surfaces which includes a variety of sites on which specific chemical reactions are thought to occur. It is the pos- sibility of correlating these well-known sur- face sites with a well-understood family of interrelated reactions that has stimulated this study. Critical to such a study is the develop- ment of a variety of sophisticated molecular probes which will undergo unambiguous ’ To whom correspondence should be addressed transformations on specific surface sites. To start with we have chosen the transfor- mations associated with the hydrogenation and exchange of alkenes because of our ex- perience in that field and because of the rel- atively high level of mechanistic under- standing which now exists in it. These transformations are addition, exchange, c&tram isomerization, double bond mi- gration, and hydrogen shifts. These and other reactions have been identified and correlated with possible surface sites by Ledoux (3). Similarly, but approaching from the organometallic model side, some of these reactions have been correlated with possible surface sites by Siegel et al. (2) and elaborated on by Augustine (4). Together they present the following pic- ture. Metal surfaces contain three principal kinds of sites, planes (faces or terraces), edges (or steps), and corners (vertices or kinks or isolated atoms) (3). These sites ap- pear to be related to three kinds of organo- metallic coordination sites (‘M, 2M, and 3M, respectively) (2,4). Coordination sites sim- ilar to those on faces possess one coordina- tive unsaturation (one empty site), those 502 0021-9517/86 $3.00 Copyright 0 1986 by Academic Press. Inc. All rights of reproduction in any form reserved.