In-situ studies of acetylene oxidation on Pt(111) have been performed using both fluorescence yield near edge spectroscopy (FYNES) and temperature-programmed FYNES (TP-FYNES) to study surface oxidation for temperatures up to 600 K and oxygen pressures up to 1.2 Pa. Low temperature FYNES results indicate acetylene adsorbs with the C-C backbone tilted slightly up from the Pt(111) surface at 150 K. Oxidation of preadsorbed acetylene on the Pt(111) surface occurs in a single step that starts at 350 K and is complete at 420 K. In large excesses of oxygen, oxygen pressures have only a small effect on the rate of oxidation as observed previously during propyne oxidation on the Pt(111) surface. Comparisons between the intensity of the C-H s* resonance (magic angle) and the intensity in the carbon continuum provide an unambiguous indication of the C/H stoichiometry of surface species. These experiments clearly show that acetylene maintains a CH stoichiometry throughout oxidation. That is, oxydehydrogenation and skeletal oxidation occur concurrently above 350 K. Identical reaction temperature profiles observed for (1) reaction of preadsorbed acetylene and preadsorbed atomic oxygen and (2) reaction of acetylene and oxygen from the gas phase indicate that coadsorbed acetylene and atomic oxygen must be the dominant reacting species present during oxidation in both cases. Detailed isothermal kinetic studies performed with preadsorbed acetylene in oxygen atmospheres indicate that the activation energy for oxidation is 85.1 kJ/mol with a pre-exponential factor of 2.1 x 1010 s-1 for acetylene coverages ranging from 1.0 to 0.6 monolayers. In-situ oxidation studies in flowing oxygen and acetylene clearly show that the same one-step oxidation process is dominant. For these catalytic studies, a large temperature hysteresis is observed during thermal cycling associated with inhibition of oxygen adsorption by acetylene.
Citation: Journal of Catalysis
Pub Type: Journals
acetylene, intermediates, oxidation, platinum, soft x-ray, surface science