Mechanism of acetylene oxidation on the Pt(111) surface using in situ fluorescence yield near-edge spectroscopy

In situ studies of acetylene oxidation on Pt(111) have been performed with both fluorescence yield near-edge spectroscopy (FYNES) and temperature-programmed FYNES (TP-FYNES) for temperatures up to 600 K and flowing oxygen pressures LIP to 0.009 Torr. Low-temperature spectroscopic (FYNES) results indicate that at 150 K acetylene adsorbs with the C-C backbone tilted slightly up from the Pt(111) Surface. consistent with the formation of an eta(2)-mu(3)-CCH2 surface intermediate. When acetylene is preadsorbed on the Pt(111) surface, forming the eta(2)-mu(3)-CCH2 intermediate, oxidation occurs in a single step over the 330-420 K temperature range. In the presence of excess oxygen, little effect on the rate of oxidation is seen when the oxygen pressure is increased. as observed previously for oxidation of adsorbed propyne on the Pt(111) surface. Comparison between the intensity of the C-H sigma* resonance and the intensity in the carbon continuum clearly shows that the adsorbed hydrocarbon intermediate maintains a 1:1 C-H stoichiometry throughout oxidation, suggesting that oxydehydrogenation and skeletal oxidation occur Simultaneously above 330 K. Identical reaction temperature profiles observed for (1) reaction of coadsorbed acetylene and atomic oxygen, and (2) reaction of acetylene and oxygen from the gas phase signify that in both cases a direct, bimolecular surface reaction mechanism occurs during oxidation. For the catalytic Studies. a large temperature hysteresis, which is associated with inhibition of oxygen adsorption by acetylene, is observed during thermal cycling. Detailed isothermal kinetic studies performed in flowing oxygen pressures indicate that the apparent activation energy for oxidation is 20.3 +/- 2.0 kcal/mol with a pre-exponential factor of 10(10.3 +/- 1.0) s(-1). Taken together, these results clearly indicate that surface properties and reactant properties both play major roles in controlling oxidation reactions over an extended ran-e of reaction conditions. (c) 2004 Elsevier Inc. All rights reserved.