Ambient-Pressure XPS Studies of Reactions of Alcohols on SrTiO3(100)
2017
Zhang, Yafen | Savara, Aditya | Mullins, David R.
Ambient-pressure X-ray photoelectron spectroscopy (APXPS) and temperature-programmed desorption (TPD) have been employed to elucidate the adsorption and reaction of simple alcohols on SrTiO₃(100). TPD experiments indicate molecular desorption of alcohols with a small amount of aldehydes below 100 °C, whereas no gas-phase products are observed above this temperature. APXPS spectra at 0.1 Torr show that alcohols adsorb dissociatively onto SrTiO₃(100) to form alkoxies. Surface methoxides appear to react with each other to produce acetate as a surface intermediate. These surface species are eventually oxidized to gas-phase CO, CO₂, and H₂O. Ethoxide is readily oxidized to acetate species that undergo further reactions to form gas-phase products. CO₂ is the only C-containing product during ethanol oxidation, whereas methanol can also produce the partial oxidation product, CO. When no O₂ is present, alcohol oxidation yields gas-phase CO, CO₂, or H₂O and creates oxygen vacancies on the surface, resulting in the reduction of Ti⁴⁺. Without a source of oxygen replenishment, the availability of surface oxygen would be limited, and thus, the oxidation reactions could not progress indefinitely. At near-ambient pressures, the reactivity of the surface and the distribution of surface species and reaction products were changed by altering the alcohol/O₂ ratio, consistent with an interpretation that surface vacancies are being created and that their concentration is altered when an oxygen source is present. The conversion from acetate to CO₂ might be rate-limiting when sufficient O₂ gas is present.
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