Structural studies of WC(0001) and the adsorption of benzene

We report on studies dealing with the structure of WC(0001) and the adsorpt ion of benzene on this surface. An I(V)-low-energy electron diffraction str ucture analysis has been performed to elucidate the surface structure of WC (0001). These studies indicate that the surface consists of a tungsten laye r covered by carbon randomly distributed on the hcp sites with a coverage o f 30% that of a full carbon layer. The distance between this carbon layer a nd 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 fro m the bulk value was found for the distance between the first tungsten laye r 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 less than or equal t o 200 K. On stoichiometric WC(00001), molecular benzene of (sub)monolayer c overage is found up to temperatures of T approximate to 230 K. After desorp tion of this species, small signals of fragments are visible in the photoel ectron spectra up to T approximate to 1000 K. Above this temperature, a gra phite covered surface remains. On a surface covered by a thin closed oxide phase (WO) only multilayer adsorption is found; above T approximate to 200 K no adsorption takes place under UHV conditions. Weakly oxidized WC(0001) interacts more strongly with benzene in that strong photoemission signals o f a (sub)monolayer species are visible up to a temperature of T approximate to 340 K. (C) 1998 Elsevier Science BV. All rights reserved.