Changes in the surface structure of Pd/Ta₂O₅ by oxygen and CO studied using X-ray Photoelectron Spectroscopy (XPS)

Behaviors of Pd structures with different thicknesses supported by Ta ₂O₅/Ta in the reaction with oxygen and CO were studied by XPS and SEM. For the samples with a Pd thickness of 3 nm, a new low-binding-energy component appeared in the Pd 3d level upon O₂ exposure at ∼200 °C and was reduced in intensity after a subsequent CO exposure at 150 and 200 °C. The change in the Ta 4f state could also be found upon oxygen and CO exposure, indicating that both Pd and the Ta-oxide substrate participate in the chemical reactions. For the sample with a higher Pd thickness, a positive shift in the Pd 3d level due to the oxidation of Pd was observed after exposure to O₂ at a higher temperature (280 °C). A subsequent CO exposure at ∼150 °C could not reduce Pd-oxide layers, as confirmed by the unchanged Pd 3d spectra after CO treatment, i.e. Pd-oxide was not reactive for CO oxidation.