Paper SS1-TuM11
Facile C-H Bond Cleavage of Propane on a PdO(101) Thin Film

Tuesday, November 10, 2009, 11:20 am, Room M

Palladium oxide (PdO) is an excellent catalyst for the oxidation of alkanes under oxygen-rich conditions, but difficulties in preparing well-defined surfaces have hindered fundamental studies of PdO surface chemistry. We have recently found that a high-quality PdO(101) thin film can be grown on Pd(111) in ultrahigh vacuum by oxidizing the metal at moderate temperature using an oxygen atom beam. In this talk, I will discuss recent results showing that propane adsorbs into a strongly-bound molecular state on PdO(101) and undergoes facile C-H bond cleavage below 200 K. The adsorbed hydrocarbon fragments are completely oxidized by the PdO surface above about 400 K, yielding gaseous CO₂ and H₂O during temperature-programmed experiments. From measurements of product yields as a function of the surface temperature, we show that the initial C-H bond cleavage of propane on PdO(101) occurs by a precursor-mediated mechanism with a negative, apparent activation energy, and present evidence that the strongly bound molecular state serves as the precursor to the initial dissociation. We suggest that a donor-acceptor interaction between a C-H bond(s) of the propane molecule and a coordinatively unsaturated Pd atom enhances the binding of propane on PdO(101), beyond that expected for physisorbed molecules, and facilitates C-H bond activation. Finally, I will summarize recent experimental and computational results which show that CH₄ and H₂ molecules also interact strongly with PdO(101) through dative bonding. The activation of propane on PdO(101) is analogous to alkane reactions with mononuclear transition metal compounds for which alkane sigma complexes act as key intermediates.