Mechanisms of oxygen adsorption and desorption on polycrystalline palladium

Abstract

The adsorption and desorption of oxygen on a polycrystalline palladium (Pd(poly)) surface (10-to 100-μm crystallites; ∼32% (100), ∼18% (111), ∼34% (311), and ∼15% (331)) at P _O2 ≤ 1.3 × 10⁻⁵ Pa and T = 500–1300 K have been studied by TPD and mathematical modeling. The kinetics of O₂ adsorption and desorption on Pd(poly) are primarily governed by the formation and decomposition of oxygen adsorption structures on the (100) and (111) crystallite faces. The O₂ adsorption rate is constant at ϑ ≤ 0.15–0.25 owing to the formation of the p(2 × 2) structure with an O_ads-surface bonding energy of D(Pd-O) = 364 kJ/mol on the (100) and (111) faces. The adsorption rate decreases with increasing coverage at ϑ ≥ 0.15–0.25 because of the growth, on the (100) face, of the c(2 × 2) structure, in which D(Pd-O) is reduced to 324 kJ/mol by lateral interactions in the adsorption layer. A high-temperature (∼800 K) O₂ desorption peak is observed for ϑ ≤ 0.25, which is due to O₂ desorption from a disordered adsorption layer according to a second-order rate law with an activation energy of E _des = 230 kJ/mol. A lower temperature (∼700 K) O₂ desorption peak is observed for ϑ ≥ 0.25, which is due to O₂ released by the c(2 × 2) structure according to a first-order rate law with E _des = 150 kJ/mol. At ϑ ≥ 0.25, there are repulsive interactions between O_ads atoms on Pd(poly) (ε_aa = 5–10 kJ/mol).