Abstract
Li-doped MgO is a potential catalyst for the oxidative coupling
of methane, whereby surface Li+ O centers are suggested to
be the chemically active species. To elucidate the role of Li in
the MgO matrix, two model systems are prepared and their
morphological, optical and magnetic properties as a function
of Li doping are investigated. The first is an MgO film deposited
on Mo(001) and doped with various amounts of Li, whereas
the second is a powder sample fabricated by calcination of Li
and Mg precursors in an oxygen atmosphere. Scanning tunneling
and transmission electron microscopy are performed to
characterize the morphology of both samples. At temperatures
above 700 K, Li starts segregating towards the surface and
forms irregular Li-rich oxide patches. Above 1050 K, Li desorbs
from the MgO surface, leaving behind a characteristic defect
pattern. Traces of Li also dissolve into the MgO, as concluded
from a distinct optical signature that is absent in the pristine
oxide. No electron paramagnetic resonance signal that would
be compatible with Li+O centers is detected in the two Li/
MgO samples. Density-functional theory calculations are used
to determine the thermodynamic stability of various Li-induced
defects in the MgO. The calculations clarify the driving forces
for Li segregation towards the MgO surface, but also rationalize
the absence of Li+O centers. From the combination of experimental
and theoretical results, a detailed picture arises on
the role of Li for the MgO properties, which can be used as a
starting point to analyze the chemical behavior of the doped
oxide in future.