Acetylene chemisorption and decomposition on the Co(1 1 2̄ 0) single crystal surface

Acetylene chemisorption and dissociation on the Co(1 1 2̄ 0) surface has been studied using high-resolution core level photoemission spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). The adsorbed acetylene molecules are found to dissociate at about 200 K, which is significantly lower than the dissociation onset reported for the system C2H2/Co(0 0 0 1). NEXAFS measurements show that acetylene hybridises strongly with the Co(1 1 2̄ 0) surface, forming antibonding states below the ionisation limit, which are not present in the gas-phase. In the temperature region from 200 to 300 K a dehydrogenated fragment, possibly of the form C2H or C2, is found to co-exist with molecular acetylene. Further heating to 450 K leads to decomposition of this fragment to graphitic carbon, while an ordered (5×2) carbon overlayer starts to form at the expensive of molecular acetylene. At 570 K this ordered overlayer is fully developed. By combining results from photoemission spectroscopy measurements, LEED and STM, a hard sphere model for the carbon overlayer relative to the Co substrate is proposed. Above ∼600 K, a substantial decrease in the amount of ordered carbon atoms is seen, leaving mainly graphitic carbon on the Co(1 1 2̄ 0) surface.