REACTIONS OF UNSATURATED OXYGENATES ON RHODIUM(111) AS PROBES OF MULTIPLE COORDINATION OF ADSORBATES

The interactions of acrolein, CH2 = CHCHO, and allyl alcohol, CH2 = CHCH20H, with the Rh(111) surface are dominated at low temperatures by the oxygen function of each molecule. Acrolein is bound initially in an eta-2(C,O) configuration; allyl alcohol dissociates to form an allyl alkoxide. For both species, vibrational spectra show significant rehybridization of the olefinic groups upon heating to 200 K and suggest that the surface intermediates are eta-4(C,C,C,O)-acrolein and eta-3(C,C,O)-allyl alkoxide. Both intermediates eliminate CO between 200 and 300 K. However, like that of their aliphatic analogues, decarbonylation of acrolein and allyl alcohol does not proceed via a common pathway. Acrolein decomposes to C2H6, C2H4, CO, H-2, and surface ethylidyne intermediates, but neither allyl alcohol nor any primary alcohol examined on Rh(111) to date produces volatile hydrocarbon products. Thus, although inclusion of the C = C group in these molecules alters the stability of the corresponding surface intermediates, the divergence of reaction pathways for alcohols and aldehydes is unaffected. These results suggest that surface reaction pathways for CO insertion and elimination are considerably more complex than previously recognized.