Alloy catalysis, ensemble effect

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

The behaviour of binary alloys with respect to catalysis is generally interpreted in terms of either the geometric ensemble effect , associated with the number of nearest neighbours of a given element for the catalytic reaction to occur [3-5] or to the electronic ligand effect , associated to the mutual electronic interaction between the associated elements. In addition, the influence of strains induced on surface atoms has also to be considered for binary alloys when the two partners have different atomic radii. The effect will strongly depend upon the surface composition, and upon the epitaxial relationsship between the outer atomic layer and the substrate layers. Therefore, one has to know precisely the local concentration and atomic arrangement of both components of the alloy at the outermost surface, and in the sublayer. 

More recently, the authors have further extended this work to include a comparison of the Au-Pt(lll) versus Au-Pt(lOO) alloy surfaces in n-hexane reactions (157). As shown in Fig. 12, the same general decreases in reaction rates and increases in isomerization selectivity with Au addition were seen for both Pt surface orientations, except for the fact that, in contrast to Pt(100), the rate of methylpentane production actually increases with Au addition. Other more subtle differences were also observed, demonstrating clearly the existence of structural sensitivity in alloy catalysis. Again, ensemble effects were used to explain the results. 

Ensemble Effect in Alloy Catalysis and the Creation of New Sites by Alloys... 

Chemical reconstruction of alloy and/or bimetallic surfaces will be more complex, and it is obvious that the catalysis of alloy and/or bimetallic surface can not be explained by the traditional idea of ligand effect and ensemble effect for the sites. That is, chemical reconstruction of alloy surface will occur by selective segregation or selective reaction of metal atoms. As a result, a new surface will be prepared sometimes, which is responsible to the prominent catalytic activity and/or selectivity of alloy and/or bimetallic surfaces. If this is the case, the catalysis of alloy surface is entirely different form the idiomatic idea of ligand effect" and " ensemble effect". 

The concept of site isolation is important in catalysis. On metal particles one usually assumes that ensembles of metal atoms are necessary to activate bonds and to accommodate the fragments of molecules that tend to dissociate or to recombine. We present here three examples of such effects the dehydrogenation of decane into 1-decene, the dehydrogenation of isobutane into isobutene and the hydrogenolysis of acids or esters into aldehydes and alcohols. In most cases the effect of tin, present as a surface alloy, wiU be to dilute the active sites, reducing thereby the yield of competitive reactions. 

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