THE INFLUENCE OF OXYGEN ON THE SELECTIVITY OF ALCOHOL CONVERSION ON THE PD(111) SURFACE
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abstract
The reactions of methanol, ethanol, 1-propanol, and 2-propanol were examined on a Pd(111) surface containing one-quarter monolayer of adsorbed oxygen atoms. The presence of surface oxygen led to the oxidation of the primary alcohols to the corresponding aldehyde and carboxylate species, with carboxylate production the major reaction pathway. Methanol was oxidized on the oxygen-dosed Pd(111) surface to formaldehyde and surface formate species. Formaldehyde desorbed at 240 K and adsorbed formate species decomposed at 280 K to produce CO2, HCOOH, and surface hydrogen. The adsorption of ethanol on the oxygen-dosed Pd(111) surface resulted in the desorption of acetaldehyde at 220 K and the formation of surface acetate species. Adsorbed acetate species reacted via either decomposition at 410 K or hydrogenation to acetic acid at 280 K. The relative importance of the two acetate reaction channels was found to depend on the availability of surface hydrogen. Similarly, the adsorption of 1-propanol on O/Pd(111) produced both propanal and adsorbed propanoate species. The propanoate species were removed from the surface by either decomposition at 360 K or hydrogenation at 280 K. In contrast, oxidation of the secondary alcohol 2-propanol resulted mainly in the production of acetone, with acetate formation a relatively minor reaction pathway. The roles of oxygen adatoms in these reactions include(1) direct reaction as Brnsted bases with proton donors; (2) stabilization of surface alkoxide species; (3) alteration of the interaction of carbonyl compounds with the surface; (4) nucleophilic oxidation of adsorbed aldehydes to carboxylates; and (5) scavenging of surface hydrogen which would otherwise be available for hydrogenation reactions. 1988.
