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Locating Catalytically Active Oxygen on Ag(1 1 1)—A Spectromicroscopy Study

Authors

  • Prof. Dr. Sebastian Günther,

    Corresponding author
    1. Chemie Department, Technische Universität München, Lichtenbergstr. 4, 85748 Garching (Germany), Fax: (+49) 89-289-13389
    • Chemie Department, Technische Universität München, Lichtenbergstr. 4, 85748 Garching (Germany), Fax: (+49) 89-289-13389

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  • Dr. Sebastian Böcklein,

    1. Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, 81377 Munich (Germany)
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  • Prof. Dr. Joost Wintterlin,

    1. Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, 81377 Munich (Germany)
    2. Center for NanoScience, Ludwig-Maximilians-Universität München, Schellingstr. 4, 80799 Munich (Germany)
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  • Dr. Miguel A. Niño,

    1. Elettra-Sincrotrone Trieste S.C.p.A. S.S. 14 - km 163.5 in AREA Science Park, 34149 Basozizza, Trieste (Italy)
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  • Dr. Tevfik O. Menteş,

    1. Elettra-Sincrotrone Trieste S.C.p.A. S.S. 14 - km 163.5 in AREA Science Park, 34149 Basozizza, Trieste (Italy)
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  • Dr. Andrea Locatelli

    1. Elettra-Sincrotrone Trieste S.C.p.A. S.S. 14 - km 163.5 in AREA Science Park, 34149 Basozizza, Trieste (Italy)
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Abstract

The loading of an Ag(1 1 1) sample with oxygen was monitored by in situ low-energy electron microscopy and X-ray photoemission electron microscopy during NO2 dosing at T≥480 K. At first, adsorbed oxygen populates the Ag(1 1 1) surface, which initiates the (4×4) reconstruction leading to the characteristic O 1s core level at 528.30 eV. The formation of this phase proceeds on a mesoscopic length scale by traveling fronts separating reconstructed from non-reconstructed surface areas. Continued NO2 dosing leads to the accumulation of a new oxygen species mainly at steps and step bunches. Characterized by an O 1s peak with two components at 530.20 eV and 530.75 eV, this species represents the active oxygen during the ethylene epoxidation reaction over Ag. The 530.20 eV component is attributed to surface oxygen, the 530.75 eV species to subsurface oxygen. This inhomogeneous accumulation of the active oxygen occurs at a very low rate. However, the preparation route can be changed, which strongly accelerates the population of the catalytically active oxygen species and leads to a homogeneous distribution of oxygen on the surface. This route involves the complete formation of the O(4×4) reconstruction by NO2 dosing, followed by a complete de-reconstruction of the surface by desorption of the oxygen adlayer. The faster population kinetics is related to the Ag adatom transport during such a reconstruction/de-reconstruction cycle.

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