Benzene intermediates and mechanisms during catalytic oxidation on the Pt(111) surface using in-situ soft X-ray methods

The catalytic oxidation of benzene on the Pt(111) surface has been characterized, in flowing oxygen pressures up to 0.01 Torr, using temperature-programmed fluorescence yield near-edge spectroscopy (TP-FYNES). During temperature-programmed oxidation experiments in flowing oxygen pressures, a series of four adsorbed intermediates are formed. The dominant intermediates with increasing temperature are eta(6)-benzene, 1,4-di-sigma-2,5-cyclohexadiene, 1,1,4-tri-sigma-2,5-cyclohexadiene, and eta(5)-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 150-215 K, a 1,4-di-sigma-2,5-cyclohexadiene surface intermediate with C6H6 stoichiometry is formed by rearrangement of the aromatic ring. Next, a 1,1,4-tri-sigma-2,5-cyclohexadiene surface intermediate with C6H5 stoichiometry is formed by oxydehydrogenation over the temperature range 215-285 K. Above 350 K, a fraction of this intermediate is oxidized to form carbon dioxide and water, while the remainder is oxygenated to form a eta(5)-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 onset temperature for rapid oxidation or the rate of carbon removal is observed with increasing oxygen pressures over the pressure range 0.0005-0.01 Torr. Temperature-programmed reaction spectroscopy (TPRS) has been used to identify the gas-phase products during deep oxidation between coadsorbed benzene and oxygen on Pt(111). Carbon dioxide and water are formed over the 300-500 K temperature range when benzene is in excess, and over the 380-620 K temperature range when oxygen is in excess. This combination of temperature-programmed UHV and in-situ soft X-ray methods has provided a detailed mechanistic description of catalytic benzene oxidation on the Pt(111) surface.