A STUDY OF THE STRUCTURE SENSITIVITY OF THE PROPYLENE HYDROGENATION REACTION OVER SUPPORTED PLATINUM AND PALLADIUM CATALYSTS

The propylene hydrogenation reaction was investigated over a series of well characterized platinum on silica and palladium on silica catalysts. The purpose of this study was to investigate the effects of active surface morphology and pretreatment conditions on catalytic activity. The morphology of the active surface was characterized by measuring the percentage of metal exposed via hydrogen pulse chemisorption, hydrogen-oxygen titration, and x-ray line broadening.;The conversions to propane were measured in a differential flow reactor at one atmosphere and hydrogen : hydrocarbon feed ratio 17.;The turnover frequencies for the formation of propane, exhibited a mild structure sensitive behavior over both catalyst series. An increase of the turnover frequency with increasing metal percentage exposed was observed over the Pt/SiO(,2) catalysts. However, over the Pd/SiO(,2) catalysts, the turnover frequency passed through a maximum at an approximate percentage exposed of 65.;The activation energies did not vary much with percentage exposed.;The platinum catalysts were prepared either by impregnation with chloroplatinic acid or by ion exchange with platinum tetramine ions. The palladium catalysts were prepared by ion exchange with Pd(NH(,3))(,4)(NO(,3))(,2).;The temperature of catalyst treatment in hydrogen had a very significant effect on the catalytic activity. On the Pt/SiO(,2) catalysts, prepared by impregnation, the effect was a four-fold enhancement of the turnover frequency as the temperature of treatment in hydrogen decreased from 450(DEGREES)C to -50(DEGREES)C. The Pd/SiO(,2) catalysts, exhibited maxima in the turnover frequency at a hydrogen treatment temperature of about 100(DEGREES)C. These maxima were more profound for catalysts of high metal percentage exposed.;Some L(,III) x-ray absorption threshold resonance experiments suggested a correlation between catalytic activity and vacancies of the platinum d-band. It was argued that traces of oxygen left on platinum crystallites, after low temperatures of treatment in hydrogen, increased the platinum d-vacancies and the catalytic activity.;Formation of the (beta)-phase of palladium hydride, reduced the catalytic activity of the palladium catalysts. This fact was explained electronically.;The palladium catalysts were much more active than the platinum catalysts, in hydrogenating propylene.