Origin of vinylic CH bond activation in the combustion of alkenes on palladium: an HREELS study of propene on Pd(100)-p(2 × 2)-O

While the mechanism of many reactions with oxygen on Ag(110) can be successfully predicted according to the gas-phase acidity of the reacting molecule, recent temperature-programmed reaction spectroscopic study of propene reactions with oxygen on Pd(100) reveals that the most reactive CH bond is the vinylic rather than the most acidic, allylic, CH bond. Using vibrational spectroscopy it is found that two forms of propene species exist on the surface at 130 K: a π-bonded state and a di-σ-bonded state. The former desorbs molecularly upon heating, while the latter undergoes combustion. In the di-σ-bonded state, the sp3 rehybridization strongly reduces the acidity from its gas-phase value, rendering the methyl hydrogens unreactive. The mechanism for the combustion of propene is proposed.