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 Oads-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 Oads atoms on Pd(poly) (eaa = 5–10 kJ/mol).