The chemisorption and decomposition of ethylene and acetylene on Ni(110)

Abstract

High resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS) have been used to study the adsorption and decomposition of ethylene and acetylene on a Ni(110) surface. HREEL spectra are reported as a function of hydrocarbon exposure, temperature (80–500 K), and scattering angle for protonated and deuterated ethylene, and at 80 K for protonated and deuterated acetylene. Both ethylene and acetylene adsorb molecularly at 80 K, but both show rehybridization from the gas phase: ethylene to ~ sp 3 and acetylene to ~ sp 2.5. All of the observable vibrational modes of ethylene are excited to different degrees by the dipole scattering mechanism, and its site group symmetry at 80 K is lower than C 2v. Ordered LEED patterns are formed on adsorption at low temperatures; a complex pattern for ethylene and a c(2 × 2) for acetylene. Ethylene begins to decompose above 200 K to form CCH intermediates with evolution of hydrogen. On additional heating the CCH species decompose to CH species. Finally by 500 K atomic carbon remains on the surface and forms a (4 × 5) ordered overlayer. The thermal decomposition of acetylene is more complex than that of ethylene, as evidenced by the TDS results. Possible bonding models of acetylene and the CCH species on Ni(110) are proposed.