Participation of Surface Oxygen in the Stabilization of the Rh/HOPG System with Respect to NO2

Abstract

In this work, we used X-ray photoelectron spectroscopy (XPS) to perform a comparative study of the interaction of NO₂ with two samples of highly oriented pyrolytic graphite (HOPG), on the surfaces of which rhodium was preliminarily deposited by evaporation in a vacuum, at room temperature and a pressure of 10^–5 mbar. Before metal deposition, one of the HOPG samples was annealed in a vacuum at 600°C, and the other was bombarded with argon ions followed by exposure to air at room temperature for 1 h in order to introduce strongly bound oxygen atoms into the surface composition. After the deposition of rhodium onto the two HOPG samples, two model catalysts designated as Rh/C and Rh/C(A)–O were prepared. It was found that the interaction of NO₂ with Rh/C led to the oxidation of graphite with the destruction of the surface layer. The Rh particles remained in a metallic state, but they were introduced into the near-surface layer of the carbon support. On the contrary, when the Rh/C(A)–O sample was treated with NO₂, the deposited rhodium was partially converted into Rh₂O₃, while the graphite was oxidized to an insignificant degree and retained its original structure. The role of surface oxygen in the stabilization of graphite with respect to oxidation in NO₂ was discussed.