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 O 2 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. O 2 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 O 2 desorption for a chemisorbed-oxygen surface coverage of θ ≈ 0 is E des 0 = 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 ( E 2 and E 3 , respectively). As the oxide film is heated, O 2 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 O 2 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 des 0 = 230 kJ/mol, E 2 = 145 kJ/mol, and E 3 = 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 O 2 TPD spectra that are observed for Pd(poly) and are due to the decomposition of surface oxide films.