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Sites for Methane Activation on Lithium‐Doped Magnesium Oxide Surfaces
Author(s) -
Kwapien Karolina,
Paier Joachim,
Sauer Joachim,
Geske Michael,
Zavyalova Ulyana,
Horn Raimund,
Schwach Pierre,
Trunschke Annette,
Schlögl Robert
Publication year - 2014
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201310632
Subject(s) - catalysis , magnesium , activation energy , chemistry , lithium (medication) , oxygen , radical , oxidative coupling of methane , methane , homolysis , inorganic chemistry , photochemistry , organic chemistry , medicine , endocrinology
Density functional calculations yield energy barriers for H abstraction by oxygen radical sites in Li‐doped MgO that are much smaller (12±6 kJ mol −1 ) than the barriers inferred from different experimental studies (80–160 kJ mol −1 ). This raises further doubts that the Li + O .− site is the active site as postulated by Lunsford. From temperature‐programmed oxidative coupling reactions of methane (OCM), we conclude that the same sites are responsible for the activation of CH 4 on both Li‐doped MgO and pure MgO catalysts. For a MgO catalyst prepared by sol–gel synthesis, the activity proved to be very different in the initial phase of the OCM reaction and in the steady state. This was accompanied by substantial morphological changes and restructuring of the terminations as transmission electron microscopy revealed. Further calculations on cluster models showed that CH 4 binds heterolytically on Mg 2+ O 2− sites at steps and corners, and that the homolytic release of methyl radicals into the gas phase will happen only in the presence of O 2 .