Ordering of ethylidyne on clean and adsorbate covered Pd(111): Influence of the coadsorption of oxygen

Abstract

The adsorption and ordering of acetylene on clean and on oxygen-covered Pd(111) at low temperature (150–170 K) and at room temperature were studied by low-energy electron diffraction and high-resolution electron energy-loss spectroscopy. At low temperature a poorly ordered overlayer of molecularly chemisorbed C2H2 was formed on both clean and O-covered Pd(111). At room temperature C2H2 converted to ethylidyne (C2H3), which weakly ordered in a ((3)1/2×(3)1/2) R30° structure. However, preadsorption of oxygen was found to induce better ordering of C2H3 on Pd(111) by (a) possibly removing the free hydrogen atoms on the surface and (b) interacting with the hydrocarbon species via the coadsorbate induced ordering mechanism. It is proposed that the Pd(111)-((3)1/2×(3)1/2) R30°–O+C2H3 ordered overlayer can arise from a combination of dipole–dipole interaction and charge transfer through the substrate.