Carbon interaction with the rhenium surface

Abstract

The conditions of formation and the properties of several chemical states of carbon on the rhenium surface, namely, the chemisorbed atoms and clusters, surface carbide and graphite, have been studied by the AES and TDS methods and by CsCl molecule dissociation. The carbon adatoms are found to dissolve actively in rhenium at 800 K with an activation energy of ≈2.0 eV. The carbon from the clusters dissolves at considerably higher temperatures, T1300 K, with an activation energy of 2.6E3.9 eV (the rate limiting stage being C−C bond rupture in the clusters). The surface carbide on rhenium has been revealed for the first time. It forms effectively at T1500 K. The carbon from carbide desorbs at T2600 K with an activation energy of ≈7.9 eV. Evidence has been found for the existence at T ~2200 K of β-carbide with a concentration Nβ≈1×1015 cm−2. The CsCl molecule dissociation technique has been used to study the effect of surface carbon on the catalytic activity of rhenium. It was shown that all states of surface carbon, namely, the surface carbide, carbon clusters, carbon film (≈2–3 atomic layers thick), except for graphite, are catalytically active, the degree of dissociation being γ=1. A monolayer of graphite reduces γ(T) down to 10−3–10−6. We attribute the unique catalytic passivity of the graphite layer to its valence bond saturation. The nature of the adsorption bonding between the graphite islands and the rhenium surface has been investigated. It is shown that the central, valence bond-saturated part of an island is raised above the surface (by 3–4 ) while being coupled to it by van der Waals forces. The valence-active edges of the island are lowered down onto the surface and chemically bound to it. High-resolution (ΔE/E≈0.1%) C KVV Auger spectra have been obtained for the various surface carbon states, and the peculiarities of their shape are discussed.