Site density mechanism validity

Also, one might expect that many met catalysts would have a very high site density, perhaps near 10 cm In 1966 Horiuti and coworkers ( ), using a method we showed ( ) is almost equivalent to the TST method, examined the reported data for 66 metal-catalyzed reactions. They found their assumptions to be valid. (Miyahara, one of Horiuti s coworkers, and Kazusaka later (50) modified the method to take variable orders into account.) Again, a very high site density is required for the postulated mechanism of isopropanol-acetic acid esterification over silica gel, as reported by Fricke and Altpeter ( M). We showed (52), that such a high value for the site density is obtain usjng TST. 

Because there are physical limitations on allowable values of the site density, different mechanisms for a given reaction may be postulated and the site density calculated for a mechanism can be used to decide if that mechanism is a feasible one. In this way, many conceivable mechanisms have been eliminated. For example, in 1970 Cha and Parravano ( ) used such calculations to exclude certain mechanisms in the gold-catalyzed transfer of oxygen between CO2 and CO. We have reviewed the question of using the calculated site density as a criterion for the validity of a mechanism (25). 

The combustion of methane on platinum was modeled using the detailed heterogeneous scheme of Deutschmann et al. [7] (24 reactions, 11 surface and 9 gaseous species), coupled to the Cl/H/O elementary gas-phase mechanism of Warnatz et al. [8] (26 species, 108 reactions). In the catalytic mechanism, a surface site density F = 2.7 x 10 mol/cm was used. The catalytic mechanism has been validated against spatially-resolved measurements of major species concentrations across the boundary layer formed in a Pt-coated channel at pressures of 1-16 bar [9]. 

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