The catalytic oxidation of benzene on the Pt(111) surface has been characterized, in pressures of flowing oxygen up to 1.33 Pa, using temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES). During temperature-programmed oxidation experiments in flowing oxygen a series of four adsorbed intermediates are formed. The dominant intermediates with increasing temperature are: n6-benzene, 1,4-di-sigma-2,5-cyclohexadiene, 1,1,4-tri-sigma-2,5-cyclohexadiene, and n5-cyclohexadienone. All of these intermediates are strongly adsorbed based on the molecular rehybridization indicated by spectroscopic (FYNES) measurements. Adsorbed benzene inhibits oxidation below 370 K by inhibiting oxygen adsorption. Over the temperature range 150 to 215 K the 1,4-di-sigma-2,5-cyclohexadiene surface intermediate with C6H6 stoichiometry is formed by rearrangement of the aromatic ring. Next the 1,1,4-tri-sigma-2,5-cyclohexadiene surface intermediate with C6H5 stoichiometry is formed by oxydehydrogenation over the temperature range 215 to 285 K. Above 350 K a fraction of this intermediate is oxidized to form carbon dioxide and water, while the remainder is oxygenated to form the n5-cyclohexadienone intermediate with C6H5O stoichiometry, which is dominant at 390 K. The reactivity of this intermediate is clearly demonstrated by rapid oxidation with increasing temperature. No change in the activation energy is observed with increasing oxygen pressures in the range 0.067 to 1.33 Pa.
Citation: Journal of Physical Chemistry B
Pub Type: Journals
NEXAFS, orientation, oxidation, soft x-ray, temperature-catalysis