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We used density functional theory calculations to examine the initial stages of oxidation of the Pt͑111͒ surface. Consistent with prior studies, our calculations predict that oxygen atoms adsorb on fcc sites and form p͑2 ϫ 2͒ and p͑2 ϫ 1͒ structures at coverages of 0.25 and 0.50 ML, respectively. In addition to various surface configurations of oxygen on fcc sites, we examined subsurface oxygen and clustering of oxygen atoms on the surface. We find that subsurface oxygen is not the precursor to the oxidation of the Pt͑111͒ surface. Instead, we predict a strong preference for the formation and growth of one-dimensional Pt oxide chains within the p͑2 ϫ 1͒ structure. In particular, at coverages above 0.50 ML, additional oxygen atoms prefer to aggregate between the close-packed oxygen rows formed by the p͑2 ϫ 1͒ structure and induce large buckling ͑ϳ1.8 Å͒ and modification of the charge of the surface Pt atoms. The result is an oxide compound with threefold and fourfold Pt-O coordination that grows as a one-dimensional chain running parallel to the oxygen rows of the p͑2 ϫ 1͒ structure. Furthermore, half of the oxygen atoms in the Pt oxide chains reside near hcp sites, contrary to some reports that oxygen atoms reside only on the fcc sites on Pt͑111͒. Our results agree well with a recent scanning tunneling microscopy study and suggest a precursor mechanism to the oxidation of metal surfaces involving Pt oxide chain formation and growth on terraces at moderate oxygen coverages. Our results should have important implications to current models of NO and CO oxidation on Pt͑111͒ and potentially on studies of the initial oxidation of other transition-metal and bimetallic surfaces.

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.

We investigated the dissociative chemisorption and oxidation of H 2 and D 2 on a PdO(101) thin film using temperature-programmed desorption (TPD) experiments and density functional theory (DFT) calculations. We find that the dissociation of H 2 is highly facile on PdO(101), with more than 90% of a saturated H 2 layer dissociating below 100 K. Most of the dissociated hydrogen reacts with the surface to produce H 2 O that desorbs above 350 K during TPD. Our experimental data demonstrate that H 2 dissociation on PdO(101) occurs by a precursor-mediated mechanism in which a molecularly chemisorbed H 2 species acts as a necessary precursor to dissociation. The experimental data also reveal that a kinetic isotope effect strongly suppresses the dissociation of D 2 on PdO(101) terraces and causes the kinetic branching to shift toward desorption of the molecular D 2 precursor. DFT calculations predict that H 2 binds relatively strongly on PdO(101) by forming a σ complex on a coordinatively unsaturated (cus) Pd site. Using DFT, we identified only a single pathway for H 2 dissociation that generates stable products on PdO(101). In this pathway, the adsorbed H 2 σ complex dissociates by transferring an H atom to a neighboring cus-O site, thereby producing an OH species and an H atom bound to a cus-Pd site. Zero-point-corrected barriers determined for this pathway fail to explain our experimental observations of facile dissociation of H 2 on PdO(101) and a strong kinetic isotope effect that suppresses D 2 dissociation. We present evidence that quantum mechanical tunneling dominates the dissociation of H 2 on PdO(101) at low temperatures and that differences in tunneling rates are responsible for the large kinetic isotope effect that we observe experimentally.

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