Probing Ensemble Effects in Surface Reactions. 1. Site-Size Requirements for the Dehydrogenation of Cyclic Hydrocarbons on Pt(111) Revealed by Bismuth Site Blocking

Article abstract of DOI:10.1021/j100339a056

Catalytic reactions on bimetallic surfaces are often thought to be controlled by ensemble effects, whereby a side reaction requiring a large ensemble of active sites can be selectively suppressed by diluting the active metal with a second, inert metal.Unfortunately, the lack of knowledge of surface structure and the complications due to coexisting electronic effects have, until now, precluded accurate determinations of ensemble requirements for surface reactions.We analyze here applications of a new method for determining ensemble sizes that partially overcomes these obstacles and allows for semiquantitative assessment of ensemble effects.The method involves the controlled blocking of sites on a well-defined transition-metal surface with a dispersed overlayer of inert bismuth adatoms.The interactions of five cyclic hydrocarbones (cyclopentane, cyclohexane, cyclopentene, cyclohexene, and benzene) with Pt(111) have been studied in this way in an accompanying series of papers.In particular, the influence of Bi upon the competing dehydrogenation and desorption kinetics of these adsorbed molecules has been qualitatively measured.This present paper correlates the results for those five molecules and fits them with a simple kinetic model to extract the absolute ensemble requirements for the surface dehydrogenation reactions.The method and model may have applicability to a broad range of surface reactions.In addition, an effective ensemble requirement is defined, whose value is useful in predicting ensemble effects in catalysis.Trends in the value of kinetic parameters and the ensemble requirements with hydrocarbon character are discussed. The absolute ensemble requirements for the dehydrogenation of these adsorbed hydrocarbons are surprisingly large and indicate in some cases that at least six additional free Pt atoms are necessary for dehydrogenation (beyond those required for adsorption).Mechanistic implications of these results are discussed.