First Principles Study of Molecular O₂ Adsorption on the PdO(101) Surface

Interactions of O₂ with the PdO(101) surface were studied using spin-dependent density-functional theory (DFT) with both the PBE and the non-local hybrid HSE exchange–correlation functional. The adsorption energies are strongly overestimated (by 40–60 kJ/mol) with PBE, whereas HSE predicts adsorption energies that are within ~5 kJ/mol of values derived from temperature programmed desorption (TPD) experiments. A detailed partial density of states analysis indicates that the band gap between the PdO d-band center and the LUMO of O₂ plays an important role in determining the adsorption strength. This gap is larger for the HSE functional and leads to a decrease in the back donation of the metal d-states to the O₂ LUMO orbital resulting in weaker adsorption. Based on the DFT–HSE calculations, three adsorption minima are found to be stable. The most favored configuration, with an adsorption energy of −67 kJ/mol, consists of an O₂ molecule lying flat and interacting with two coordinatively unsaturated Pd (Pd_cus) surface atoms. The other two configurations have weaker adsorption energies of about −25 kJ/mol and bind to a single Pd_cus atom with the O₂ molecule oriented away from the surface. The HSE results can be correlated with the observed TPD spectra, which shows only one type of O₂ configuration at low coverages with a subsequently lower temperature (more weakly bound) peak evolving at higher coverages associated with the singly coordinated O₂ adsorption configurations that start to populate when two adjacent Pd_cus sites start to become unavailable.