CATALYST DESIGN TO CHANGE SELECTIVITY OF PHOTOCATALYTIC OXIDATION

Different selectivities for photocatalytic oxidation (PCO) of ethanol were observed for two types of adsorption sites on TiO2, and this iden tification led to the design of a photocatalyst with enhanced selectiv ity to acetaldehyde, a partial oxidation product. Transient photocatal ytic oxidation (PCO), steady-state reaction, temperature-programmed de sorption (TPD), temperature-programmed oxidation, and isotope labeling were combined to determine the reactivity of the different adsorption sites. During PCO, weakly bound ethanol preferentially formed gas-pha se acetaldehyde, whereas strongly bound ethanol (which decomposes duri ng TPD) primarily produced CO2. Weakly bound ethanol appears to adsorb on sites that are not available for acetaldehyde adsorption. This inf ormation, combined with the fact that acetaldehyde decomposes to a str ongly bound intermediate during TPD, led to the design of a catalyst t hat was modified with acetaldehyde TPD products, which were stable dur ing PCO. The TPD products of acetaldehyde preferentially poison the si tes where ethanol is strongly bound so that selectivity to acetaldehyd e increased during ethanol PCO. Higher selectivity was seen during bot h transient and steady state experiments, and at 54-60% conversion the ratio of partial to complete oxidation on the poisoned catalyst was f ive times that on fresh TiO2. (C) 1998 Academic Press.