Adsorption of Water on a PdO(101) Thin Film: Evidence of an Adsorbed HO−H2O Complex

We investigated the adsorption of water on a PdO(101) thin film using temperature-programmed desorption (TPD) measurements and density functional theory (DFT) calculations. TPD spectra obtained from high water coverages exhibit sharp peaks at 149 and 197 K that arise from water desorbing from a multilayer and a second layer, where the second layer appears to be stabilized by direct interactions with the PdO(101) surface. The TPD spectra also exhibit features at 318 and 350 K that originate from different forms of chemisorbed water. The feature at 350 K grows as a sharp peak with increasing water coverage, whereas the feature at 318 K develops as a broad peak only after the water coverage exceeds ∼50% of the density of coordinatively unsaturated (cus) Pd atoms (0.50 MLcus). Consistent with the experimental observations, DFT predicts that formation of an HO−H2O complex is highly favored at low water coverages on PdO(101), and that water chemisorbs into less stable molecular states at coverages above 0.50 ML...

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