Adsorption of ethylene on stoichiometric and reduced NiO(100)

The adsorption of ethylene has been studied on stoichiometric NiO(100) and on surfaces reduced to 40% of the stoichiometric oxygen content. The adsorption process was followed with XPS, Auger spectroscopy and LEED at substrate temperatures of 200 to 500 K and at ethylene pressure of 5 × 10^?7 Torr. At 200 K, two distinct ethylene species are observed on stoichiometric NiO(100). The first species saturates at 0.02 ML after 200 L and is adsorbed molecularly, interacting with both nickel and oxygen sites. A condensed species then forms which does not saturate for exposures up to 2100 L. Both adsorb reversibly with all traces of carbon absent by 270 K. At 200–300 K, reduced NiO(100) also adsorbs two molecular ethylene species, although with a preference for nickel sites. However, the uptake of ethylene increases only slightly with surface reduction. Adsorption is no longer reversible for the reduced surface and increasing the substrate temperature causes fragmentation of the adsorbed ethylene with a concomitant reduction in lattice oxygen content.