Induced ordering of ethylidyne on the Pd(111) surface by the preadsorption of oxygen: a LEED and HREELS study

Abstract

Overlayers of oxygen coadsorbed with acetylene and ethylene on a Pd(111) surface were studied by low-energy electron diffraction (LEED) and high-resolution electron-energy-loss spectroscopy (HREELS) in the temperature range 150–320 K. Low temperature adsorption of both C 2H 2 and C 2H 4 resulted in non-dissociatively chemisorbed molecules, and the preadsorption of oxygen did not change the LEED or HREELS data. At room temperature both C 2H 2 and C 2H 4 formed ethylidyne (C 2H 3), which poorly ordered in a (√3 × √3)R30° structure. We found that preadsorption of oxygen induced better ordering for C 2H 3 formed from acetylene exposure, but not for C 2H 3 derived from ethylene adsorption. We propose that preadsorbed oxygen helped the ordering of the C 2H 3 overlayer by: (a) efficiently removing surface hydrogen and (b) attractively interacting with C 2H 3 to give coadsorbate induced ordering.