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 ML cus ). Consistent with the experimental observations, DFT predicts that formation of an HO−H 2 O 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 cus . The computed binding energy of the HO−H 2 O complex agrees very well with the desorption activation energy (∼99 kJ/mol) determined from analysis of the TPD peak at 350 K. DFT also predicts that water molecules chemisorbing above 0.50 ML cus can attach to the HO−H 2 O complexes to form different types of trimers. The broad TPD peak at 318 K is consistent with water desorption from a distribution of adsorbed trimers and other aggregates with varying binding energies. Finally, we find that uptake into the molecularly chemisorbed states effectively ceases prior to saturation and resumes only after the second-layer state nearly saturates. DFT suggests that strong orientation-dependent interactions between adsorbed species create unfavorable sites along rows of cus-Pd atoms that hinder adsorption into the first layer when more than ∼75% of the cus-Pd sites are occupied with adsorbed H 2 O and OH species.