Particle Size and Activity of Supported Plantinum Catalyst in Oxidation of Propane

Abstract

Particle size effect of supported metal catalysts on oxidation catalysis should be extensively investigated with reference to the optimization of catalyst preparations. This study deals with one of the particle size effects, i. e., "structure sensitivity", of the supported platinum catalysts in the oxidation of propane. While the fresh (untreated) catalysts prepared with FLPtCl6 showed fairly low activities, a remarkable increase of catalytic activity was observed after pretreatments of the catalysts at 750° C in O^2- or 112 stream and at 420° C in the reaction mixture (Table 2, Fig.2). This enhancement may be due to the removal of a small quantity of Cl ion, which remained on the surface of the catalysts during their preparations, from the catalysts. For the catalysts which attained a high level of catalytic activity after the pretreatments, a remarkable particle size effect on propane oxidation was observed in the wide range of platinum particle size from 20 to 2000A, that is, the specific activity (turnover frequency) increased almost proportionally with the increase of the mean particle size (Fig.3). This means that the catalytic activity (rate per unit catalyst weight) for propane oxidation is determined only by the supported amount of platinum regardless of the particle size. This effect was confirmed not to be due to some apparent factors, for example, the rate control by the boundary layer deffusion (Fig.4) or by the micropore diffusion (Table 2), and the increase of effective temperature of the platinum surface. This particle size effect was discussed on the basis of electron transfer from platinum to the supporting materials.