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Abstract:
We report on studies dealing with the structure of WC(0001) and the adsorption of benzene on this surface. An I(V)-low-energy electron diffraction structure analysis has been performed to elucidate the surface structure of WC(0001). These studies indicate that the surface consists of a tungsten layer covered by carbon randomly distributed on the hcp sites with a coverage of 30% that of a full carbon layer. The distance between this carbon layer and the tungsten layer beneath is enlarged by 5% with respect to the spacing between carbon and tungsten layers in the bulk. Only a small deviation from the bulk value was found for the distance between the first tungsten layer and the carbon layer below. No indications of surface reconstruction have been observed. Benzene adsorption was studied on clear oxygen covered and oxidized WC(0001). The benzene multilayer desorbs at T≤200 K. On stoichiometric WC(00001), molecular benzene of (sub)monolayer coverage is found up to temperatures of T≈230 K. After desorption of this species, small signals of fragments are visible in the photoelectron spectra up to T≈1000 K. Above this temperature, a graphite covered surface remains. On a surface covered by a thin closed oxide phase (WO) only multilayer adsorption is found; above T≈200 K no adsorption takes place under UHV conditions. Weakly oxidized WC(0001) interacts more strongly with benzene in that strong photoemission signals of a (sub)monolayer species are visible up to a temperature of T≈340 K.
