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 hydrocarbons (cyclopentane, cyclohexane, cyclopentene, cyclohexene, and benzene) with Pt(l 11) 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 the 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.

No abstract

No abstract

A new method which should have relatively general applicability for the identification and quantitative analysis of reactive adsorbed molecular intermediates in surface reactions will be described, and the first examples of its application will be presented. When a reactive intermediate is generated on a surface, it often has a tendency to dissociate before desorbing. Since dissociation generally requires additional free sites on the surface, dissociation can be suppressed and desorption correspondingly enhanced if the free sites on the surface can be properly poisoned. We have found that bismuth adatoms are very good inert site blockers, which can be postdosed to the surface of a transition metal containing a reactive adsorbed hydrocarbon without destroying the hydrocarbon. Whereas in the absence of bismuth, the hydrocarbon would completely dehydrogenate during thermal desorption spectroscopy (TDS) and liberate only H2 into the gas phase, after bismuth postdosing the reactive hydrocarbon desorbs intact for mass spectral identification and quantitative analysis. This method has been used to prove that adsorbed benzene is the initial product of the dehydrogenation of cyclohexane on Pt(111) at ∼235 K. In the absence of bismuth, this benzene all dissociates during TDS to liberate only H2, leaving graphitic carbon residue on the surface. When one-third monolayer of Bi is postdosed at 110 K, the dehydrogenation pathway is sterically poisoned and the adsorbed benzene quantitatively desorbs during TDS, where it is unambiguously identified by mass spectroscopy. By briefly heating the reactive adsorbed intermediate to increasing temperatures prior to Bi deposition, the thermal stability limits of the intermediate and the kinetic parameters for its dissociation can be established. This is demonstrated for the dehydrogenation reaction of adsorbed cyclopentene on Pt(111). Bismuth postdosing in thermal desorption mass spectroscopy (BPTDS) should be a very useful but inexpensive addition to surface analytical capabilities.

No abstract