REACTIONS OF CARBOXYLIC-ACIDS ON THE PD(111)-(2 X 2)O SURFACE - MULTIPLE ROLES OF SURFACE OXYGEN-ATOMS
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The reactions of formic acid, acetic acid, and propanoic acid were examined on the oxygen-dosed Pd(111) surface. Adsorption of these carboxylic acids on Pd(111)-(2 2)O at 170 K produced adsorbed carboxylate species via direct transfer of the acid hydrogen to the oxygen adlayer. In addition, the presence of adsorbed oxygen suppressed the formation of the hydrogen-bonded catemers observed for carboxylic acids on the clean surface. The stability of carboxylate species on Pd(111) was found to depend upon the substituent on the carboxyl group and upon the presence of adsorbed oxygen. Formate species were the least stable on Pd(111) and decomposed primarily through a dehydrogenation pathway at 280 K, evolving CO2 and releasing hydrogen atoms to the surface. The temperature required for formate decomposition was 20 K higher on the oxygen-dosed surface than on the clean surface, suggesting that the presence of oxygen adatoms stabilized the formate adlayer. Acetate decarboxylation occurred at 305 and 420 K on the oxygen-dosed surface. The lower temperature acetate decomposition pathway was also observed for acetate species adsorbed on the clean Pd(111) surface, and resulted in the desorption of primarily CO2. In contrast, the decomposition channel at 420 K was associated with acetate species stabilized by the presence of adsorbed oxygen. Decomposition of oxygen-stabilized acetates released CO2, CH1COOH, and H2. Propanoate species produced from propanoic acid on the Pd(111)-(2 2)O surface decomposed at 380 K. 25 K higher than on the clean surface. The multiple roles of oxygen adatoms in carboxylate formation and stabilization on Pd(111) may be generalized both to the reactions of other oxygenates and to the reactivity of other group-VIII and -IB metals. 1991.
