HYDROGENATION OF ISOLATED ATOMS AND SMALL CLUSTERS OF CARBON ON PT(111) SURFACE - HREELS TDS STUDIES

The reaction of hydrogen with isolated atoms and small clusters of carbon adsorbed on Pt(111) surface was investigated by HREELS and TDS. The carbon adsorption layers were prepared by evaporation of carbon atoms onto the metal surface cooled down to 100 K. The surface carbon produced by this method reveals a high activity towards hydrogen: the reaction occurs at T greater-than-or-equal-to 170 K The initial carbon concentration is found to determine the chemical content of products in the adlayer. At n(C) < 2 X 10(14) at/cm2, when isolated atoms C(ads) prevail in the initial adlayer, methine CH(ads) formation is chiefly observed revealing two bands in HREELS: delta(CH) at 800 and nu(CH) at 2960 cm-1. The CH(ads) particles dissociate at 510 K leading to hydrogen evolution and formation of carbon islands with a graphite-like structure. At higher concentration, the carbon adlayer contains small clusters C(xads) in addition to the isolated atoms leading to more complex reaction products. An ethylidyne species, --> C-CH3, is detected among the products with characteristic bands delta(s)(CH3) at 1360 and nu(CC) at 1130 cm-1. It is assumed that ethylidyne molecules are produced in the course of a consecutive hydrogenation of the C2ads cluster. Dehydrogenation of hydrocarbon surface species causes hydrogen evolution at T almost-equal-to 350, 410, 450, 510 and 600-700 K. Ethylidyne dissociation is associated with the desorption peak at T almost-equal-to 450 K; an ethynyl CCH(ads) being the product. It is shown that the highest temperature stage of the dehydrogenation in the adlayer occurring at T > 600 K is accompanied by an increase of the carbon content in the C(x)CH(ads) molecules, finally resulting in the formation of islands with a graphite-like structure.