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  Li/MgO Catalysts Doped with Alio-valent Ions. Part II: Local Topology Unraveled by EPR/NMR and DFT Modeling

Simon, U., Alarcón Villaseca, S., Shang, H., Levchenko, S. V., Arndt, S., Epping, J. D., et al. (2017). Li/MgO Catalysts Doped with Alio-valent Ions. Part II: Local Topology Unraveled by EPR/NMR and DFT Modeling. ChemCatChem, 9(18), 3597 -3610. doi:10.1002/cctc.201700610.

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 Creators:
Simon, Ulla1, Author
Alarcón Villaseca, Sebastián2, Author              
Shang, Honghui2, Author              
Levchenko, Sergey V.2, Author              
Arndt, Sebastian3, Author
Epping, Jan D.4, Author
Görke, Oliver1, Author
Scheffler, Matthias2, Author              
Schomäcker, Reinhard3, Author
Tol, Johan van5, Author
Ozarowski, Andrew5, Author
Dinse, Klaus-Peter6, Author
Affiliations:
1Technische Universität Berlin, Institut für Werkstoffwissenschaften und -technologien, Fachgebiet Keramische Werkstoffe , Hardenbergstraße 40, 10623 Berlin (Germany), ou_persistent22              
2Theory, Fritz Haber Institute, Max Planck Society, ou_634547              
3Technische Universität Berlin, Institut für Chemie, Straße des 17.Juni 124, 10623 Berlin (Germany), ou_persistent22              
4Technische Universität Berlin, Institut für Chemie , Straße des 17.Juni 135, 10623 Berlin (Germany), ou_persistent22              
5National High Magnetic Field Laboratory , 1800 E. Paul Dirac Dr. , Tallahassee, FL 32310 (USA), ou_persistent22              
6Freie Universität Berlin, Fachbereich Physik , Arnimallee 14, 14195 Berlin (Germany), ou_persistent22              

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 Abstract: The role of Li in Li/MgO as a catalyst for oxidative coupling of methane (OCM) is to promote MgO surface morphology change rather than serve as a constituent of catalytically active sites. While Li/MgO is unstable at realistic conditions with respect to loss of Li, the resulting samples show enhanced selectivity towards C2 hydrocarbons versus CO2, although activity is low and close to pristine MgO. The way (co-)doping with aliovalent metal ions affects the catalytic performance of Li/MgO has now been explored. To analyze the structure and the stability of the samples, catalysts with well-defined stoichiometry were prepared using a co-precipitation method with freezedrying and subsequent annealing. Gd and Fe were used as dopants. Apart from their potential direct role in catalysis, these dopants are anticipated to stabilize Li in the catalyst under the reaction conditions, allowing further clarification of the role of Li. In the case of Gd/Li co-doping, changes observed in EPR and 7Li-NMR spectra indicate the formation of correlated, next-neighbor Li-Mg···Gd+Mg pairs co-existing with “isolated” Gd3+ ions at octahedral Mg lattice sites. For Li/Fe codoping, no significant change in the EPR pattern is observed in the presence of Li+ ions, indicating a larger distance between the Li+ and Fe3+ cations in the MgO lattice. Hybrid DFT calculations explain the difference between Fe and Gd co-doping by a less efficient screening of the Coulomb repulsion between Gd3+ and neighboring cations in Gd doped samples, leading to the stabilization of LiMg near GdMg.

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Language(s): eng - English
 Dates: 2017-06-062017-04-072017-06-122017-09-22
 Publication Status: Published online
 Pages: 14
 Publishing info: -
 Table of Contents: -
 Rev. Type: Peer
 Identifiers: DOI: 10.1002/cctc.201700610
 Degree: -

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Title: ChemCatChem
  Other : ChemCatChem
Source Genre: Journal
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Publ. Info: Weinheim : Wiley-VCH
Pages: 14 Volume / Issue: 9 (18) Sequence Number: - Start / End Page: 3597 - 3610 Identifier: ISSN: 1867-3880
CoNE: https://pure.mpg.de/cone/journals/resource/1867-3880