Comparison of the Reactivity of O₂ and NO₂ Relative to Palladium Supported on HOPG

A comparative study of the interaction of palladium supported on the surface of highly oriented pyrolytic graphite (Pd/HOPG) with oxygen and nitrogen dioxide was carried out using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The samples were prepared by vacuum deposition of metallic Pd onto HOPG; the size of the resulting particles was ≤5 nm. The samples were treated in oxygen at room temperature or 150°C and a pressure of 20 mbar. The interaction with NO₂ was carried out at room temperature and a pressure of 10^–6 mbar. For the two oxidants used, O₂ and NO₂, fundamental differences in their effects on Pd/HOPG were found. At room temperature, the treatment in O₂ was limited by the formation of some amount of oxygen-containing compounds C_xO_y on the surface of the carbon support. After increasing the temperature to 150°C, a portion of palladium was converted into a PdO oxide. When NO₂ was used as an oxidizing reagent, intense oxidative destruction of graphite occurred to a depth of 10–15 graphene layers. Palladium remained in a metallic state, but its particles penetrated deep into the carbon support, which led to significant screening of its lines in the X-ray photoelectron spectrum and the disappearance of particles in SEM images. A possible mechanism for the interaction of Pd/HOPG with O₂ and NO₂ has been proposed, which allowed us to explain the different behaviors of these oxidizing molecules.