SELECTIVE VAPOR-PHASE EPOXIDATION OF PROPYLENE OVER AU TIO2 CATALYSTSIN THE PRESENCE OF OXYGEN AND HYDROGEN/

Gold has long been thought to be chemically inert, however, it has rec ently been proven that its catalytic performance is dramatically tunab le by control of the particle size and by careful selection of the sup port metal oxide. A typical example is the selective oxidation of prop ylene in a gas containing oxygen and hydrogen. When gold is deposited on TiO2 by a deposition-precipitation technique as hemispherical parti cles with diameters smaller than 4.0 nm it produces propylene oxide wi th selectivities higher than 90% and conversions of 1-2% at temperatur es of 303-393 K, The oxidation of hydrogen to form water is depressed by propylene, whereas propylene oxidation is not only enhanced but als o restricted to partial oxidation by hydrogen. The depression of hydro gen combustion by the presence of propylene and a new peak due to gold deposition in TPD spectra have indicated that propylene is adsorbed o n the surfaces spectra have indicated that propylene is adsorbed on th e surfaces of both gold particles and the TiO2 support. The reaction r ate is almost independent on the concentration of propylene and increa ses linearly with increasing concentrations of O-2 and H-2. The above results suggest that propylene adsorbed on a gold surface may react wi th oxygen species formed at the perimeter interface between the gold p articles and the TiO2 support through the reaction of oxygen with hydr ogen. The effect of gold loading is surprising in that the reaction pr oduct switches from propylene oxide to propane when gold loading is de creased to 0.1 wt%. Careful TEM observation indicates that gold partic les larger than 2.0 nn in diameter produce propylene oxide, whereas sm aller gold particles produce propane. (C) 1998 Academic Press.