Adsorption of acetylene and hydrogen on Pd(111) : Formation of a well-ordered ethylidyne overlayer

• The C2H2(+H)/Pd(111) system has been studied using high resolution core level spectroscopy and low energy electron diffraction (LEED). Two ordered phases were found when adsorbing C2H2 at 125 K, giving rise to a (2 × 2) and a (√3 × √3)R30° LEED pattern, respectively. When preadsorbing H at 110-150 K, the sticking probability of the C2H2 molecules is strongly reduced and only the (√3 × √3)R30° overlayer is formed. In all three of the ordered structures the C2H2 molecules are suggested to occupy the hollow sites. Heating of the (√3 × √3)R30° C2H2 + H layer to 350 K yields a well ordered (√3 × √3)R30° overlayer consisting exclusively of ethylidyne (CCH3) positioned in the hollow sites. The formation of ethylidyne is furthermore found to be preceded by the formation of an intermediate species ("β") which probably is vinylidene.

Nyckelord

Alkynes

Chemisorption

Low index single crystal surfaces

Palladium

Soft x-ray photoelectron spectroscopy

Surface chemical reaction

Surface structure

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