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    Observation of oxygen vacancy filling under water vapor in ceramic proton conductors in situ with ambient pressure XPS
    (2013) Chen Q.; El Gabaly F.; Aksoy Akgul F.; Liu Z.; Mun B.S.; Yamaguchi S.; Braun A.
    The interaction of metal oxides with their ambient environment at elevated temperatures is of significant relevance for the functionality and operation of ceramic fuel cells, electrolyzers, and gas sensors. Proton conductivity in metal oxides is a subtle transport process which is based on formation of oxygen vacancies by cation doping and substitution and oxygen vacancy filling upon hydration in water vapor atmosphere. We have investigated the conductivity and electronic structure of the BaCeY-oxide proton conductor under realistic operation conditions from 373 to 593 K and water vapor pressures up to 200 mTorr in situ by combining ambient pressure X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. We provide element specific spectroscopic evidence that oxygen vacancies are filled by oxygen upon water exposure and partly oxidize Ce3+ and Y2+ toward Ce 4+ and Y3+. Moreover, the resonant valence band spectra of dry and hydrated samples show that oxygen ligand holes in the proximity of the Y dopant are by around 0.5 eV closer to the Fermi level than the corresponding hole states from Ce. Both hole states become substantially depleted upon hydration, while the proton conductivity sets on and increases systematically. Charge redistribution between lattice oxygen, Ce, and Y when BCY is exposed to water vapor at ambient and high temperature provides insight in the complex mechanism for proton incorporation in BCY. © 2013 American Chemical Society.
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    "pop-On and Pop-Off" Surface Chemistry of Alanine on Ni{111} under Elevated Hydrogen Pressures
    (American Chemical Society, 2018) Nicklin R.E.J.; Shavorskiy A.; Aksoy Akgul F.; Liu Z.; Bennett R.A.; Sacchi M.; Held G.
    The coadsorption of hydrogen with a simple chiral modifier, alanine, on Ni{111} was studied using density functional theory in combination with ambient-pressure X-ray photoelectron spectroscopy and X-ray absorption spectroscopy at temperatures of 300 K and above, which are representative of chiral hydrogenation reactions. Depending on the hydrogen pressure, the surface enables protons to "pop-on and -off" the modifier molecules, thus significantly altering the adsorption geometry and chemical nature of alanine from anionic tridentate in ultrahigh vacuum to predominantly zwitterionic bidentate at hydrogen pressures above 0.1 Torr. This hydrogen-stabilized modifier geometry allows alternative mechanisms for proton transfer and the creation of enatioselective reaction environments. © 2018 American Chemical Society.