Room Temperature Oxidation of Graphite by Nitrogen Dioxide with the Participation of Nanoparticles of Platinum Group Metals

This paper presents a review of the results obtained in studying the room temperature interaction of NO2 with model systems prepared by vacuum deposition of platinum group metals on the surface of highly oriented pyrolytic graphite (M/HOPG, M = Pt, Pd, and Rh) at a pressure of 10(-6)-10(-4) mbar. Particular attention was focused on establishing the chemical states of the supported metal particles and carbon support using X-ray photoelectron spectroscopy (XPS). Before treatment in NO2, the M/HOPG samples were characterized by scanning tunneling and/or scanning electron microscopy (STM and SEM, respectively). Upon interaction with NO2, supported palladium and rhodium remained in metallic states and, at the same time, exhibited catalytic activity in the oxidation of graphite. The process was accompanied by the destruction of >= 10-15 graphene layers with the penetration of metal particles deep into the carbon support. Rhodium was less active in the oxidation of graphite compared to palladium due to the filling of its surface with NO molecules arising from the dissociation of NO2. When the samples with deposited platinum were treated in NO2, the carbon support underwent minimal changes without disturbing its original structure. Platinum retained its metallic state when deposited on the surface of graphite annealed in a vacuum, and it was oxidized to PtO and PtO2 oxides on the surface activated by etching with argon ions. Based on the results obtained, a mechanism was proposed for the room temperature interaction of M/HOPG systems with NO2.