THE NUMBER OF DISTINGUISHABLE CATALYTIC SITES ON RH-SILICA, PT-SILICA AND PD-SILICA CATALYSTS IN SOME RING-OPENING AND EXCHANGE-REACTIONS

The number of different catalytic sites, phi(s), is the number which can be distinguished by catalytic data alone. This number may differ from the number of distinguishable sites identifiable by physical means, IR of adsorbed CO for example. This paper surveys the problem as exemplified by analysis of catalytic data from the hydrogenolysis of methylcyclopropane and the isotopic exchange between D2 and alkanes on three sets of catalysts, Ph/SiO2, Pd/SiO2 and Pt/SiO2, the members of which varied widely in percentage exposed. All measurements of turnover frequency were accompanied by one or more selectivity data. In the hydrogenolysis experiments the selectivity was the ratio of isobutane to butane in the product and, in alkane exchange, the concentrations of the various exchanged species, delta1, delta2, delta3 ... provide the selectivity data. Analysis of these data provide a powerful tool for diagnosing phi(s). Application of selectivity data is aided by accompanying turnover frequency data particularly when these derive from two or more different conditions of catalyst pretreatment, but some conclusions can often be drawn from turnover frequency data for two or more reactions or conditions of pretreatment.