Facile C-H Bond Cleavage and Deep Oxidation of Propane on a PdO(101) Thin Film

We investigated the adsorption and reactions of propane on a PdO(101) thin film under ultrahigh vacuum conditions and find that C-H bond cleavage of propane is highly facile on this NO surface. At 85 K, propane adsorption on the PdO film produces two distinct states that desorb at temperatures of 120 and 190 K during temperature programmed measurements. The 190 K desorption peak is consistent with propane molecules that interact strongly with coordinatively unsaturated surface atoms, possibly through a donor-acceptor interaction, whereas the 120 K peak is attributed to physically adsorbed propane. Upon further heating, reaction of propane with the PdO(101) surface generates an H2O desorption peak at 345 K and then simultaneously produces H2O and CO2 desorption peaks at 463 K. The relative yields of H2O at 345 and 463 K provide strong evidence that the dissociation of molecularly adsorbed propane occurs by cleavage of a single C-H bond. The resulting propyl fragments selectively undergo complete oxidation above 400 K. From measurements of the CO2 yields as a function of the surface temperature, we estimate that the initial dissociation probability of propane on PdO(101) decreases from 49% to 21% with increasing surface temperature from 250 to 300 K. We show that this temperature dependence is accurately described within the context of a precursor-mediated mechanism and estimate a negative, apparent activation energy of -16.2 kJ/mol for propane dissociation on PdO(101). Finally, we present evidence that the more strongly bound molecular state acts as the precursor to C-H bond cleavage of propane on the PdO(101) surface. Such behavior is analogous to C-H bond cleavage by mononuclear transition metal compounds for which alkane sigma complexes act as reaction intermediates.