Ring-opening reactions of ethylene oxide on Pd(110) surfaces
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abstract
Ethylene oxide (EO) reactions on Pd(110) surfaces were investigated under ultra-high vacuum conditions using temperature programmed reaction/desorption. EO adsorbed on clean Pd(110) at 120-260 K underwent ring-opening, leading to the formation of methane at 325 K. The ratio of methane to CO produced exhibited a dependence on coverage. Furthermore, deuterium coadsorption experiments produced methane containing up to two deuterium atoms, suggestive of the presence of hydrocarbon species less hydrogen-rich than methyl groups on the surface. There was also evidence of C2 hydrocarbon dehydrogenation at ca. 450 K, reminiscent of ethylene dehydrogenation on the clean Pd(110) surface. These observations are consistent with the release of CH2 species in the course of EO decomposition on this surface. However, this is in contrast with acetaldehyde decarbonylation on the clean Pd(110) surface, where decarbonylation appears to lead to the release of methyl species on the surface. Studies of EO decomposition on oxygen-precovered Pd(110) surfaces indicated that the EO oxidation does not proceed via carboxylate intermediates; rather the epoxide decomposes to CO, which combines with oxygen on the surface to produce carbon dioxide. Again, in the light of the oxidation chemistry of acetaldehyde on oxygen-precovered metal surfaces, where acetate intermediates are formed, it is suggested that ring-opening of EO on Pd(110) does not proceed via isomerization to an acetaldehyde intermediate, but occurs by sequential C-O and C-C scission reactions.
