Heywood H. Kan, Robert J. Colmyer, Aravind R. Asthagiri, and Jason F. Weaver. Chemical Engineering, University of Florida, Department of Chemical Engineering, Gainesville, FL 32611
Palladium oxide (PdO) is an excellent catalyst for the oxidation of CH4 and CO under oxygen-rich conditions. Unfortunately, however, many fundamental questions about the surface chemistry of PdO have remained unanswered since it has been challenging to prepare well-defined PdO surfaces for detailed experimental investigations. We investigated the adsorption of water on a PdO(101) thin film using temperature programmed desorption (TPD), isotope exchange 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 two peaks centered at 318 and 350 K that originate from different forms of chemisorbed water. Desorption orders determined from TPD spectra as well as H-D exchange experiments provide evidence that the peak at 350 K corresponds to dissociatively chemisorbed water, while the peak at 318 K arises from molecularly chemisorbed water. DFT calculations predict that dissociation is unfavorable for isolated H2O molecules, but is highly facile for water dimers and selectively produces HO-H2O dimers adsorbed along rows of coordinatively unsaturated (cus)-Pd atoms. Consistent with the TPD results, DFT also predicts that water begins to chemisorb only in molecular form when the total coverage exceeds 50% of the cus-Pd density. Finally, we find that uptake into the first-layer molecularly-chemisorbed state 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 the cus-Pd rows that hinder adsorption into the first layer when more than about 75% of the cus-Pd sites are occupied with adsorbed H2O and OH species. Overall, the results of this study may provide new insights for understanding how co-adsorbed water influences the activity of PdO surfaces in catalytic oxidation processes.