Molecular adsorption of small alkanes on a PdO(101) thin film: Evidence of sigma-complex formation.

We investigated the molecular adsorption of methane, ethane, and propane on a PdO(101) thin film using temperature programmed desorption (TPD) and density functional theory (DFT) calculations. The TPD data reveal that each of the alkanes adsorbs into a low-coverage molecular state on PdO(101) in which the binding is stronger than that for alkanes physically adsorbed on Pd(111). Analysis of the TPD data using limiting values of the desorption prefactors predicts that the alkane binding energies on PdO(101) increase linearly with increasing chain length, but that the resulting line extrapolates to a nonzero value between about 22 and 26 kJ/mol at zero chain length. This constant offset implies that a roughly molecule-independent interaction contributes to the alkane binding energies for the molecules studied. DFT calculations predict that the small alkanes bind on PdO(101) by forming dative bonds with coordinatively unsaturated Pd atoms. The resulting adsorbed species are analogous to alkane sigma-complexes in that the bonding involves electron donation from C-H sigma bonds to the Pd center as well as backdonation from the metal, which weakens the C-H bonds. The binding energies predicted by DFT lie in a range from 16 to 24 kJ/mol, in good agreement with the constant offsets estimated from the TPD data. We conclude that both the dispersion interaction and the formation of sigma-complexes contribute to the binding of small alkanes on PdO(101), and estimate that sigma-complex formation accounts for between 30% and 50% of the total binding energy for the molecules studied. The predicted weakening of C-H bonds resulting from sigma-complex formation may help to explain the high activity of PdO surfaces toward alkane activation.