Seminars Archive

HREELS and XPS investigation of atomic oxygen adsorbed on Ag(001)

Abstract
The adsorbed oxygen phase active in the partial oxidation reaction of ethylene catalyzed by Ag powders has escaped so far identification in spite of 20 years of research efforts. Two dissociated oxygen moieties are known to form on the (111) and (110) surfaces and are characterized by 1s binding energies of -528 eV and -530.5 eV, respectively, independently of crystal face. The former forms after gas phase deposition followed by thermal dissociation, while the latter forms by chemical deposition or when adsorption from gas phase O2 takes place in presence of other gases (as e.g. C2H4 on Ag(110)). No precise characterization of the two oxygen phases is present in literature. Oxygen adsorption on Ag(001) is even more intriguing as after dissociation, an abnormal mode at 130 meV is observed with HREELS in addition to the usual adatom surface vibration at 33.5 meV, while the work function decreases with coverage, contrary to expectation for an electronegative adsorbate. When heating the crystal slightly above room temperature the first mode disappears, while the latter shifts to 31 meV. The investigation of O/Ag(001) at the SuperESCA beamline at ELETTRA showed that indeed two different oxygen species are present below and above 350 K. The low T phase has a 1s binding energy of E(1s)=530.5 eV, while the high T phase is characterized by E(1s)=528 eV. Photoelectron diffraction shows that the latter corresponds to adsorption in the fourfold hollow. The site of the former species is more difficult to determine as surface reconstruction is present. Only the -528 eV specie reacts with C2H4, leading to total oxidation. We suggest therefore that the E(1s)=530 eV species occupies a subsurface site. When the high T phase is frozen by rapid cooling of the sample, conversion to the stable phase occurs over a time scale of minutes and matches therefore nicely the time resolution obtainable at SuperESCA.