The reaction of ethylene and acetylene with Pt(111) at room temperature: The formation of vinyl-like species

Abstract

Temperature programmed thermal desorption spectroscopy and UV photoemission spectroscopy have been used to investigate the chemisorption and reaction of acetylene and ethylene with Pt(111) at T ∼ 100K and T ∼ 300K, respectively. The combination of both techniques allows us to demonstrate that ethylene dehydrogenates on Pt(111) at room temperatures to form a C 2H 3 species: one hydrogen atom per ethylene molecule becomes chemisorbed to the surface. The ionization energy levels obtained from our filtered hν = 40.8eV studies of the room temperature phases of both acetylene and ethylene on Pt(111) are used to identify the chemical nature of these phases. We present and discuss a technique for correlating the observed low-lying levels to those calculated for various free radicals and closed shell lithium-hydrocarbon homologues. For the acetylene-derived phase we find that either a di-σ HC CH species or a H 2C C species describes these levels - the latter being most consistent with all experimental results. The C 2H 3 ethylene-derived phase is shown to have levels which are consistent with a vinyl-like species, H 2C CH. We also provide evidence that the expected vibrations for such species are consistent with those observed in recent high resolution electron energy-loss measurements. The relationship of these new results to other results as well as their possible implications to surface reactions on Pt are briefly discussed.