STRUCTURE AND ACTIVITY OF SUPPORTED METAL CATALYSTS .4. ETHYLENE HYDROGENATION ON PLATINUM/SILICA CATALYSTS

The specific activity for ethylene hydrogenation was measured over Pt/silica catalysts in which the mean crystallite size was varied from 13 to ~200 Å. Catalysts were prepared by adsorption of the ammine or by impregnation with chloroplatinic acid, dried or fired in air, and reduced in hydrogen under various conditions. A detailed description of the catalysts was built up from measurements of CO chemisorption, X-ray line-broadening, and electron micrographs of ultramicrotome sections. It was found that an apparent effect of crystallite size on specific activity arose in low Pt content catalysts or catalysts fired at high temperature because of the poisoning or self-poisoning of the small Pt areas. A less obvious effect occurred in impregnation catalysts when the reduction temperature was varied, due to changes in the level of residual chlorine, which was shown to promote the catalytic reaction. Eliminating these apparent crystallite-size effects, the specific activity was constant over a series of catalysts where variation in the surface topography of the Pt crystallites is believed to be responsible for differences in the ease of 14CO desorption. Comparison is made with the previously reported effect of crystal face on activity in ethylene hydrogenation. © 1969.