LOW-PRESSURE OXIDATION MECHANISM AND REACTIVITY OF PROPYLENE ON AG(110) AND RELATION TO GAS-PHASE ACIDITY
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The reaction of propylene with adsorbed oxygen atoms was examined on the Ag(l 10) surface by means of thermal desorption and surface titration experiments. The presence of oxygen was observed to enhance the adsorption of propylene at 140 K. Propylene reacted facilely with adsorbed oxygen at 140 K to produce H2O and CO2 which desorbed from the surface upon heating. Adsorbed carbon atoms were also deposited on the surface following thermal desorption of the volatile products. At 300 K the probability for the reaction of propylene with adsorbed oxygen atoms was two to three orders of magnitude less than at 140 K, suggesting the involvement of a weakly chemisorbed precursor state in the reaction. Surface titration experiments with acetic acid demonstrate that the reaction of propylene with oxygen reduces the number of Bronsted base sites on the surface by a factor of 2; these data were interpreted in terms of a mechanism by which propylene reacts irreversibly to form hydroxyl groups plus adsorbed carbon on the surface. A portion of the carbon is subsequently oxidized by oxygen atoms liberated by the disproportionation of adsorbed hydroxyl species. These results indicate that propylene is more reactive than ethylene toward adsorbed oxygen atoms; this observation is interpreted on the basis of relative hydrocarbon acidities in the gas phase. 1983, American Chemical Society. All rights reserved.
