Mechanism of the decomposition of the surface oxide film on polycrystalline palladium

The decomposition of thin surface oxide films on polycrystalline palladium Pd(poly) at 500–1300 K was investigated by mathematical modeling. This process was analyzed in terms of a model including O₂ desorption from the chemisorbed oxygen layer (O_ads) and the passage of oxygen inserted under the surface layer of the metal (O_abs) and oxygen dissolved in metal subsurface layers (O_dis) to the surface. O₂ desorption was modeled on a surface with a square lattice of adsorption sites, with account taken of the energy of the lateral repulsive interactions between adjacent O_ads atoms (ε_aa). At ε_aa = 10 kJ/mol and when the activation energy of O₂ desorption for a chemisorbed-oxygen surface coverage of θ ≈ 0 is E_des⁰ = 230 kJ/mol, the calculated spectra are in agreement with the oxygen temperature-programmed desorption (TPD) spectra obtained for Pd(poly) at θ ≤ 0.5. The passage of O_abs and O_dis atoms to the surface was calculated using a first-order equation, with account taken of the activation energy for these atoms coming out to the surface (E₂ and E₃, respectively). As the oxide film is heated, O₂ desorption is accompanied by the passage of O_abs and then O_dis to the surface, which leads to an increase in the O_ads surface coverage and, accordingly, to a buildup of lateral surroundings in the adsorbed layer. Owing to this fact and to the repulsive interactions between O_ads atoms, the bonds between O_ads and the surface weaken and E_des decreases. As a consequence, the O₂ desorption rate increases and a low-temperature peak with T_max ≈ 710 K, which is due to the passage of O_abs atoms to the surface, and then a high-temperature peak with T_max ≈ 770 K, which is due to the passage of O_dis atoms to the surface, appear in the TPD spectrum. At ε_aa = 10 kJ/mol, E_des⁰ = 230 kJ/mol, E₂ = 145 kJ/mol, and E₃ = 160 kJ/mol and when the number of inserted oxygen monolayers is θ_abs ≤ 0.3 and the number of oxygen monolayers dissolved in subsurface layers is θ_dis ≤ 10, the TPD spectra calculated for the given model are in agreement with the O₂ TPD spectra that are observed for Pd(poly) and are due to the decomposition of surface oxide films.