Metal site isolation effect

Taming Carbon-Carbon Bond Cleavage on Metallic Surfaces, the "Site Isolation" Effect... 

Site isolation effects catalytically active centres such as metal complexes can be dispersed over the zeolite matrix. In this way, reaction between adjacent metal centres can be impeded for steric reasons (7). In other well-documented cases, intramolecular reactions are preferred over intermolecular reactions due to the spatial isolation of molecules adsorbed in low concentrations (8)... 

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. 

Heterogeneous catalysts for hydrocarbon conversion may require metal sites for hydrogenation-dehydrogenation and acidic sites for isomerisation-cyclisation and these reactions may be more or less susceptible to the effect of carbonaceous overlayers depending on the size of ensembles of surface atoms necessary for the reaction. In reality we must expect species to be transferred and spilled-over between the various types of sites and if this transfer is sufficiently fast then it may affect the overall rate and selectivity observed. If there is spillover of a carbonaceous species [4] then there may be a common coke precursor for the carbonaceous overlayer on the two types of site. Nevertheless, the rate of deactivation of a metal site or an acidic site in isolation may be very different from the situation in which both types of site are present at a microscopic level on the same catalyst surface. The rate at which metal and acid sites deactivate with carbonaceous material may of course not be identical. Indeed metal sites may promote the re-oxidation of a carbonaceous species in TFO at a lower temperature than the acid sites would allow on their own and this may allow differentiation of the carbonaceous species held on the two types of site. 

Each of the intermediate steps in the mechanistic sequence involved in spillover can differ. As an example, surface transport may occur as a two-dimensional gas, as species associated with an activated site, or as two-dimensional exchange (via hydroxyls). The literature has suggested a variety of mechanisms for spillover and mechanisms induced by spillover. The studies have not assumed that all of the mechanisms are possible and have not focused on discriminating between the alternative possibilities. This more open (albeit more disconcerting) approach is suggested to understand the phenomena of spillover. Further, novel experimental techniques are needed to isolate the catalytic effects occurring on the source of spillover (usually the metal) from the effects induced by spillover on the acceptor surface, and to access the relative contribution of each. Both the numbers of species involved and the relative rates need to be quantified. 

---