FORMATION OF ACTIVE-CARBON LAYERS ON PT(111) BY ELECTRON-BOMBARDMENT OF CO

The nature of carbon layers produced by electron bombardment of adsorb ed CO on Pt(lll) has been investigated by several surface-sensitive te chniques, namely high-resolution electron energy-loss spectroscopy (HR EELS), AES, XPS, temperature-programmed desorption (TPD) and low-energ y electron diffraction (LEED). The experiments were performed with CO coverages lower than 0.3 ML at 120 and 300 K using electron energies o f 1500-2000 eV, The formation of C- and C-2-containing species was eas ily evidenced after exposing the layer to H-2 because it resulted in t he formation of adsorbed methine (CH) and ethylidyne (C-CH3) radicals characterized by strong vibrational modes in HREELS spectra, The prese nce of these low carbon content species was also confirmed by XPS. It was found that the concentration of the isolated C atoms and C-2 clust ers depended on the temperature at which the electron bombardment was performed and on the carbon coverage: the relative amount of C-2 speci es, generated by coupling of the isolated C atoms increased with incre asing temperature and initial CO coverage. The maximum amount of carbo n species produced by electron bombardment of CO was determined by AES to be similar to 0.22 ML (1 ML equals the number of surface Pt atoms: 1.52 x 10(15) atoms cm(-2)). Both TPD and XPS data showed that at tem peratures above 400 K the CH and C-CH3 species dehydrogenate and agglo merate in larger clusters, resulting in an inactive graphite-like laye r after annealing above 800 K