.
Angewandte
Communications
DOI: 10.1002/anie.201310632
ꢀ
C H Activation on MgO
Sites for Methane Activation on Lithium-Doped Magnesium Oxide
Surfaces**
Karolina Kwapien, Joachim Paier, Joachim Sauer,* Michael Geske, Ulyana Zavyalova,
Raimund Horn,* Pierre Schwach, Annette Trunschke,* and Robert Schlçgl
Dedicated to the MPI fꢀr Kohlenforschung on the occasion of its centenary
Abstract: Density functional calculations yield energy barriers
for H abstraction by oxygen radical sites in Li-doped MgO that
are much smaller (12 ꢁ 6 kJmolꢀ1) than the barriers inferred
from different experimental studies (80–160 kJmolꢀ1). This
raises further doubts that the Li+OCꢀ site is the active site as
postulated by Lunsford. From temperature-programmed oxi-
dative coupling reactions of methane (OCM), we conclude that
the same sites are responsible for the activation of CH4 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 morpho-
logical changes and restructuring of the terminations as
transmission electron microscopy revealed. Further calcula-
tions on cluster models showed that CH4 binds heterolytically
on Mg2+O2ꢀ sites at steps and corners, and that the homolytic
release of methyl radicals into the gas phase will happen only in
the presence of O2.
in chemical industry to natural gas, there is renewed interest
in the formation of higher hydrocarbons, for example by
oxidative coupling of methane (OCM):[6]
2 CH4 þ O2 ! C2H4 þ 2 H2O
ð1Þ
The simplest catalysts for this reaction, among a large
number of complex solid oxides, is Li-doped MgO.[7] Early,
Lunsford proposed that the active sites are OCꢀ radicals
neighbored to Li+, with Li+OCꢀ formally replacing Mg2+O2ꢀ,[8]
ꢀ
and that the C H bond is activated by homolytic splitting
involving hydrogen atom transfer to the OCꢀ sites:[9]
ꢀ
þ
Cꢀ
C
H3CꢀH þ ½O LiþꢂMgO ! H3C þ ½HO Li ꢂMgO
ð2Þ
However, there is also evidence that the Li+OCꢀ site may
ꢀ
not be the active site and that the C H bond may be
heterolytically split,[6] as Lunsford already mentioned in his
1995 review.[7] Recently, crucial ENDOR experiments showed
that in none of the powder catalysts that were run under an
OCM atmosphere Li+Oꢀ centers could be found,[10,11]
although they were detectable in Li-doped MgO single
crystals prepared by arc fusion of MgO/Li2CO3.[10] Instead,
by careful multi-method characterization,[11,12] Li addition was
found to lead to restructuring of the MgO surface exposing
steps and corner sites and high-index crystallographic surfaces
alien to pristine MgO. Studies of thin MgO films by surface
science techniques reached the same conclusion.[10]
T
aylorꢀs active site concept[1] has stimulated catalysis
research over almost a century, but it took many decades
until surface science identified low-coordinated atoms at step
edges as active sites of metal catalysts.[2] Subsequently, the
complex nature of active sites at supported metal[3] or metal
oxide catalysts[4,5] has been revealed by combined experi-
mental and computational studies. With the raw material shift
[*] Dr. K. Kwapien, Dr. J. Paier, Prof. Dr. J. Sauer
Institut fꢀr Chemie, Humboldt-Universitꢁt
Unter den Linden 6, 10099 Berlin (Germany)
E-mail: sek.qc@chemie.hu-berlin.de
Herein, we provide theoretical and further experimental
evidence that the Li+OCꢀ site is not the active site and
conclude that the activity of OCM catalysts is connected with
morphological features of the crystallites that form under
reaction conditions and depend on the synthesis process.
Lunsford already points to a discrepancy[7] between the
measured apparent activation energy for the formation of
methyl radicals (96 ꢁ 8 kJmolꢀ1)[13] and quantum chemical
calculations. In 2005 Catlow et al. used density functional
theory and periodic models to calculate the energy barrier of
H abstraction by an OCꢀ site at the (001) surface of Li-doped
MgO.[14] The barrier obtained, 74 kJmolꢀ1, is more or less in
apparent agreement with the above value and reported
barriers of 85 kJmolꢀ1 (CH4/CD4 isotope exchange)[15] and
90 kJmolꢀ1 (C2 hydrocarbon formation).[16] Microkinetic
simulations yielded significantly higher values, namely
147 kJmolꢀ1.[17] More recently, Li-doped MgO catalysts have
been found unstable under reaction conditions, and after 24 h
Dr. M. Geske, Dr. U. Zavyalova, Prof. Dr. R. Horn, P. Schwach,
Dr. A. Trunschke, Prof. Dr. R. Schlçgl
Fritz-Haber-Institut der Max-Planck-Gesellschaft
Faradayweg 4-6, 14195 Berlin (Germany)
Prof. Dr. R. Horn
Current address: Institut fꢀr Chemische Reaktionstechnik
Technische Universitꢁt Hamburg-Harburg
Eißendorfer Strasse 38, 21073 Hamburg (Germany)
[**] This work has been supported by Deutsche Forschungsgemein-
schaft within the Cluster of Excellence “Unifying Concepts in
Catalysis”. We thank Wiebke Frandsen for recording of MgO images
by transmission electron microscopy. K.K. thanks the International
Max Planck Research School “Complex Surfaces in Materials
Science” for a fellowship. J.S. is grateful for a Miller Visiting
Professorship at Berkeley, during which parts of this paper have
been written.
Supporting information for this article is available on the WWW
8774
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 8774 –8778