Adsorption and abstraction of oxygen atoms on Pd(1 1 1): Characterization of the precursor to PdO formation

We investigated the adsorption and abstraction of atomic oxygen on Pd(1 1 1) using oxygen atom beams in ultrahigh vacuum. Similar to results obtained with other oxidants, we find that oxygen atoms first populate a chemisorbed layer up to 0.25 ML (monolayer) at a surface temperature of 500 K, followed by formation of an ordered two-dimensional (2D) oxide. Decomposition of the 2D oxide produces a desorption peak at about 700 K in O2 TPD spectra. A distinct desorption feature at 620 K then grows with increasing coverage from 0.7 to 1.2 ML, but diminishes above 1.2 ML as a sharp desorption peak due to PdO decomposition intensifies at higher temperature. We attribute the TPD peak at 620 K to a precursor state to PdO formation corresponding to oxygen atoms adsorbed on top of the 2D oxide. Direct rate measurements of 18O abstraction from Pd(1 1 1) by an 16O atom beam show that the abstraction of 18O atoms chemisorbed on the metal surface and in the precursor state occurs with relative ease compared with oxygen abstraction from the 2D oxide and PdO. We find that the abstraction of 18O atoms chemisorbed on the metal surface is insensitive to the surface temperature, whereas the rate of 18O abstraction from the precursor state increases with increasing surface temperature from about 300-550 K. We show that this temperature dependence is well described by a kinetic model which assumes that thermally activated interchange of 18O atoms between the 2D oxide and the precursor state occurs during the abstraction experiments by a mechanism analogous to surface diffusion by concerted substitution. From analysis of the abstraction rate data, we predict an activation energy of 15.2 kJ/mol for atomic oxygen exchange between the precursor state and the 2D oxide.