Identification of surface radicals by vibration spectroscopy: Reactions of C2H2, C2H4, and H2 on Pt (111)

High-resolution electron energy loss spectroscopy has been applied to study the adsorption of acetylene and ethylene on Pt (111) and the surface reactions of these adsorbates as a function of temperature and in the presence of chemisorbed hydrogen. Making use of the surface selection rule and the observed frequencies a rather detailed picture is developed about the chemical nature of the adsorption states and reaction intermediates. Below room temperature, acetylene rehybridizes so as to form one strong di/σ-like bond to the surface and an additional bond to the surface with the remaining π orbital. Ethylene is essentially just di/σ bonded. Around room temperature, ethylene converts to a new chemical species which we identify to be ethylidene (CH3–CH=). Ethylidene is also formed from acetylene when the surface is first exposed to hydrogen, then to acetylene at low temperatures, and subsequently annealed to 350 K. The same result was obtained by exposing adsorbed acetylene to atomic hydrogen at 350 K. We further find that the hydrogenation of adsorbed acetylene to ethylidene proceeds via the intermediate –CH2–C≡ (2-ethyl–1-ylidine).