VINYL SUBSTITUENT EFFECTS ON THE REACTIONS OF HIGHER OXYGENATES ON PD(111)

The reactions of vinylic oxygenates (allyl alcohol, acrolein and acrylic acid) were compared with those of their aliphatic analogues using TPD and HREELS. The principal reaction channels of the vinylic and aliphatic oxygenates were the same: allyl alcohol, 1-propanol, acrolein and propanal all underwent decarbonylation, yielding CO, C2H4, H-2 and adsorbed carbonaceous species. Acrylic acid and propanoic acid both underwent decarboxylation. HREEL spectra of the respective adlayers demonstrated that all of these reagents interacted with the surface via their oxygen-containing functions: allyl alcohol gave rise to an allyl alkoxide, acrolein to a strongly rehybridized eta4(C,C,C,O)-bound species, and acrylic acid to an eta5-acrylate. However, the surface-adsorbate interaction was not limited to the oxygen functions alone. In addition to the HREELS evidence for multiple coordination of these intermediates to the surface, TPD experiments also demonstrated that each of the vinylic oxygenates exhibited an additional reaction channel not available to its aliphatic counterpart. Allyl alcohol dehydrated to produce propylene plus water, in parallel with the decarbonylation pathway noted above; some acrolein was hydrogenated to form propanal; and acrylic acid gave rise to surface ethylidynes, water, and atomic carbon and oxygen, not seen for propanoic acid. These observations suggest that vinyl-surface interactions are responsible for multiple coordination of unsaturated oxygenates and for opening new reaction channels; the chemistry of these molecules is dominated, however, by their oxygen-containing functions and the interactions of these with the Pd(111) surface.