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-σ-bonded configuration at 100 K. The di-σ-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 ∼ sp³. 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 CCH₂ species is formed by the dehydrogenation of CHCH₂. The final decomposition product of ethylene is a c(2 × 2)-C surface, formed by heating the adsorbate to 523 K. A ( 1 1 -1 2)-C surface with 0.81 ML coverage is obtained by heating the acetylene adsorbed layer to 553 K. Preadsorbed carbon and oxygen block the chemisorption of di-σ-bonded C₂H₄ and induce physisorption of C₂H₄ at 100 K. Preadsorbed oxygen appears to inhibit the dehydrogenation of di-σ-bonded C₂H₄ and induces the adsorption of π-bonded C₂H₄. A carbon precovered surface shows a site-blocking effect for the adsorption of C₂H₂.