The adsorption geometry of the (2X 1) carbidic carbon on Ni(110), obtained by cracking ethylene at 520 K, has been studied by the electron-energy-loss fine-structure technique at the carbon K edge. We 0 found a carbon-nickel distance of 1.85+0.05 A, and a comparison of our experimental results with a model calculation within the extended x-ray-absorption fine-structure formalism strongly favors the threefold hollow chemisorption site inside the troughs in the [110]direction.Ethylene dehydrogenation on Ni(110) leads to the formation of carbonaceous overlayers whose character is strongly dependent on the reaction temperature. ' At temperatures in the range 520 -570 K, so-called carbidic carbon is formed, which is the active site in the Fisher-Tropsch synthesis.Heating the carbidic carbon-covered surface at -700 K, or directly exposing to ethylene at this temperature, results in the formation of the poisoning graphitic phase.Because of their relevance, both practical and theoretical, in the field of heterogeneous catalysis, some work has been devoted to the study of the electronic and structural properties of these carbonaceous phases.The structure of graphitic carbon on Ni(110) has been determined by the electron energy-loss fine-structure (EELFS) technique, and its electronic structure has been studied by photoemission with synchrotron radiation.We have also studied the carbidic carbon-covered Ni(110) by angle-resolved photoemission, while no investigations exist up to now to directly determine the geometric arrangement of carbon atoms in this phase. Some work has been done on the other two low-index Ni single-crystal surfaces." For carbidic carbon on Ni(100), the fourfold hollow site is generally favored, with a carbon-nickel distance of about 1.85 A. ' As far as the close-packed Ni (111) surface is concerned, the threefold hollow site suggested by Rosei et al. " is in agreement with the theoretical work performed by Feibelman for carbidic carbon on close-packed Ru and Rh surfaces. ' In this work we have determined the chemisorption geometry of carbon atoms in the carbidic phase on Ni(110), using the EELFS technique. Our findings give a C-Ni distance of 1.85+0.05 A, and favor the threefold hollow adsorption site over the other possible adsorption geometries. Experiments were performed at the Surface PhysicsLaboratory of the University of Calabria. EELFS measurements were made using a single-pass cylindrical mirror analyzer with a resolution that is 0.06% of the kinetic energy. The base pressure in the vacuum system was 5X10 "Torr. The Ni(110) single crystal was cleaned by repeated cycles of Ar+-ion sputtering and annealing (T=1000 K) until carbon contaminant coverage was less than 0.01 monolayers, with the low-energy electron diffraction (LEED) giving a sharp (1 X 1) pattern. Carbidic carbon was obtained by admitting ethylene in the vacuum system at pressures in the 10 -Torr range, with the sample kept at 520 K. Exposure to 150 L (1 langmuir = 10 Torr sec) saturated the surface: the LEED pattern was a (2 X 1) one...
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