Interaction between oxygen and polycrystalline palladium at O2 pressures of 10−6–10 Pa

Abstract

The interaction between oxygen and polycrystalline palladium (Pd(poly)) at \( P_{O_2 } \) = 2.6 × 10−6–10 Pa and T = 300–1300 K was studied by the thermal desorption (TD) method. The interaction between O2 and Pd(poly) is governed by the O2 pressure and the sample temperature. At low pressures of \( P_{O_2 } \) (≤1.3 × 10−5 Pa), O2 is chemisorbed dissociatively on the Pd(poly) surface. During chemisorption, the Oads-surface bond energy and the O2 sticking coefficient gradually decrease as the surface coverage θ increases. At \( P_{O_2 } \) ≥ 10−2 Pa and T ≤ 500 K, after the saturation of the Oads layer (θ ∼ 0.5), Oads atoms penetrate under the surface layer of the metal to form surface palladium oxide. At \( P_{O_2 } \) ≥ 1 Pa and T > 500 K, after the saturation of the surface oxide film 2 ML in thickness (n ∼ 2), Oads atoms penetrate into the oxide film and then into the subsurface palladium layer and diffuse deep into the metal bulk. As a result, the oxygen uptake at 700 K is n ∼ 50. Upon heating, the surface oxides decompose, desorbing O2, which gives rise to a low-temperature TD peak with Tmax = 715 K. The release of oxygen inserted in the subsurface layers of palladium shows itself as a distinct high-temperature TD peak with Tmax ≥ 750 K.