PARTIAL OXIDATION WITHOUT ALLYLIC C-H BOND ACTIVATION - THE CONVERSION OF PROPENE TO ACETONE ON RH(111)-P(2X1)-O

Propene is selectively oxidized to acetone by adsorbed atomic oxygen below 290 K on Rh(111)-p(2 x 1)-O (theta-0 = 0.5) under ultrahigh vacuum conditions. The selectivity for acetone production over combustion is high, although the absolute yield is low, approximately 0.02 acetone molecules per Rh atom, because a large fraction of the propene desorbs prior to oxygen addition. An oxametallacycle, formed by a direct addition of oxygen to the 2-carbon, is proposed to be the intermediate that results in acetone formation. Isotopic labeling experiments demonstrate that the C-H bond at the 2-position of the carbon chain is selectively broken and transferred to the 1-position during acetone formation. This is supportive evidence for oxygen addition prior to dehydrogenation, since oxygen addition to the 2-carbon would decrease the C-H strength at the 2-carbon. Propene oxidation is dramatically different from reactions on Ag. Significantly, the allylic hydrogens of the propene are not activated during the oxidation. Furthermore, the ketone, not the epoxide or aldehyde, is formed on Rh. An important role of the surface oxygen is the inhibition of C-H bond activation. On clean Rh(111), propylidyne is formed. Propylidyne is totally oxidized into CO, CO2, and H2O when coadsorbed with oxygen. The inhibition of dehydrogenation allows oxygen addition to compete favorably with dehydrogenation on surfaces with oxygen coverages greater than 0.45 monolayers. For lower oxygen coverages, only combustion products are formed.