An evaluation of the single turnover (STO) procedure as a method for the determination of the active site densities on dispersed metal catalysts

Abstract

Results have been obtained which show that the standard single turnover (STO) reaction procedure can be used to determine the surface densities of the direct and two-step alkene saturation sites on various oxide supported Pt, Pd, and Rh catalysts if the carrier gas flow rate and reactant pulse size are such that diffusion control of the reaction is avoided. Rh catalysts are more susceptible to diffusion control than are Pt and Pd species. The extent of 2-butene formation during the STO reactions over Pd and Rh indicates that the alkene isomerization is nonstoichiometric. With Pt the extent of isomerization is also influenced by flow rate and reactant pulse size but to a lesser extent. Thus, the STO reaction procedure cannot be used for the determination of isomerization site densities on any of these catalysts. This procedure has been used to determine the saturation site densities on the EuroPt-1 Pt/SiO 2 and some northwestern Pt/SiO 2 catalysts. The STO reaction sequence has also been run over Pt, Pd, and Rh catalysts using each of the isomeric butenes as the reactant alkene. Over all three catalysts the same amounts of direct and two-step saturation were observed regardless of the starting alkene showing that these saturation sites are not sensitive to the geometry of the reactant olefin. With Rh a near equilibrium mixture of all three double bond isomers is formed from each of the three starting alkenes. With Pt the extent of isomerization is characteristically low regardless of the starting olefin, so the low isomerization observed during the STO reaction of 1-butene on Pt is not the result of the formation of a primary metalalkyl on a large number of isomerization sites. The STO determined reactive site densities for series of catalysts have been correlated with transition electron microscopy (TEM) measured metal particle sizes and turnover frequency (TOF) data for a number of reactions. From these results the sites on which specific reactions take place have been determined as has the site TOF for each of the active sites involved.