The reaction of acetylene with Ni(100) and Ni(110) surfaces at room temperature

Ultraviolet photoemission spectroscopy using hv = 21.2 eV and filtered 40.8 eV radiation as well as temperature programmed thermal desorption spectroscopy are used to investigate the chemical reaction of acetylene with Ni(100) and Ni(110) surfaces at room temperature. Striking crystallographic effects and several coexisting phases are observed and found to be coverage and temperature dependent. A methodology is described and used to predict the relative energy levels for a variety of adsorbed hydrocarbon fragments on Ni surfaces. Such levels together with the thermal desorption spectra are used to identify the observed species. In particular, CH and CCH species are isolated on Ni(100) and Ni(110) surfaces, respectively, via low temperature adsorption and subsequent pulsed sample warming experiments. The room temperature adsorption phases are deduced using these ionization levels together with those of chemisorbcd acetylene, atomic hydrogen and carbon. At room temperature on Ni(100), H, C, CH and C₂H₂ species form together below 2 L exposure while CH species form thereafter, up to a saturation exposure of ~10 L. On Ni(110), H and CCH species form below 1.5 L exposure followed by the formation of CH₂ and likely CH species. The relative stabilities of these species at elevated temperatures is: C₂H₂ < CCH ? CH < CH₂. A model for the bonding of acetylene and its reaction to form CCH species on Ni(110) is proposed.