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Abstract
The partial oxidation of propene (CH$\sb2$ = CHCH$\sb3$) to acrolein (CH$\sb2$ = CHCHO) has been studied over Cu$\sb2$O(100) and (111) single crystal surfaces. Propene adsorption under ultra-high vacuum conditions yields no significant oxidation products, but the propene desorption temperatures are sensitive to the structural differences in the surfaces.
Propene adsorption at atmospheric pressure followed by thermal desorption in ultrahigh vacuum demonstrates that propene may be activated at higher pressures. Over the nonpolar Cu$\sb2$O(111) surface, lattice oxygen insertion occurs at 300 K and 1 atm. with the formation of the $\sigma$-bonded allyl intermediate. Once formed, this specie is stable in ultrahigh vacuum and produces acrolein during TDS via a reaction-limited process. A comparison of these data with studies of allyl alcohol decomposition over Cu$\sb2$O surfaces indicate that the $\sigma$-bonded intermediate is surface allyloxy (CH$\sb2$ = CH-CH$\sb2$O-) which dehydrogenates to acrolein via hydride elimination on the carbon $\alpha$ to the oxygen. Thus, oxygen insertion precedes the final hydrogen abstraction in the partial oxidation pathway. Propene is also observed during allyl alcohol decomposition indicating that the transformation of the $\pi$-allyl to the $\sigma$-allyl (allyloxy) during propene oxidation is reversible.
The structure sensitivity of the propene oxidation reaction is demonstrated by the lack of acrolein production from the Cu-terminated, Cu$\sb2$O(100) surface following 1 atm. propene exposures. The origin of the structure sensitivity is related to the absence of coordinately-unsaturated lattice oxygen anions on the (100) surface.