ADSORPTION AND DECOMPOSITION OF ETHYLENE AND ACETYLENE ON FE(100)

The adsorption of C2H4 and C2H2 on the clean, C- and O-covered Fe(100) surface was studied using HREELS, with additional information provided by TPD, AES and LEED. On the clean surface, C2H4 adsorbed molecularly on this surface in a di-sigma-bonded C2H4 desorbed at 240 K or decomposed to form the methylidyne (CH) and ethynyl (CCH) species below 260 K. Hydrogen is the only desorption product of C2H4 decomposition. Acetylene adsorbs on the clean Fe(100) surface in a structure with hybridization of similar to sp(3). At lower exposure (< 0.2 L), C2H2 decomposes to form CH and CCH at 253 K. At higher exposure (> 0.2 L), C2H2 partially dehydrogenates and hydrogenates to form CH, CCH and CHCH2 at the adsorption temperature of 100 K. When the surface is warmed to 393 K, a CCh(2) species is formed by the dehydrogenation of CHCH2. The final decomposition product of ethylene is a c(2 x 2)-C surface, formed by heating the adsorbate to 523 K. A ((11)(-12))-C surface with 0.81 ML coverage is obtained by heating the acetylene adsorbed layer to 553 K. Preadsorbed corbon and oxygen block the chemisorption of di-sigma-bonded C2H4 and induce physisorption of C2H4 at 100 K. Preadsorbed oxygen appears to inhibit the dehydrogenation of di-sigma-bonded C2H4 and induces the adsorption of pi-bonded C2H4. A carbon precovered surface shows a site-blocking effect for the adsorption of C2H2.