Fs
+(H) color centers at the surface
of MgO have been studied using a combined EPR and quantum
chemical
approach. Fs
+(H) are paramagnetic
excess electrons centers where the unpaired electron is trapped in a
surface
anion vacancy. They are formed at the surface of thoroughly
dehydrated MgO (1073K) upon UV irradiation
under hydrogen in parallel with the formation of minor fractions of
different color centers. The whole EPR
spectrum resulting from irradiation has been analyzed by simulation of
the experimental profile.
Fs
+(H)
centers are characterized by an axial g tensor and display a
hyperfine interaction with a hydrogen nucleus
(belonging to an hydroxyl group stabilized nearby the vacancy) and two
distinct families of 25Mg nuclei
characterized by a large (10.5 G) and a small (0.7 G) hyperfine
coupling constant, respectively. Both EPR
and ab initio calculations on clusters of ions converge in indicating
that the features of the centers are due to
the polarization of the electron density by the positive charge of the
hydrogen in the OH group toward two
(or possibly three) equivalent magnesium ions of the vacancy close to
the OH group itself. The formation
mechanism of the centers is strictly analogous to that occurring upon
contact of low ionization energy metals
with the surface of MgO leading to the formation of another type of
color centers. Partially hydrated MgO
samples also give rise to another family of paramagnetic center based
on electrons trapped in anion vacancies.
This finding indicates that the structure of the partially
hydroxylated oxide and the mechanism of its dehydration
are still open questions.
Ein Eisen‐Kupfer‐Zeolith (Fe‐Cu‐ZSM‐5) katalysiert die selektive Oxidation von Methan zu Methanol mit wässrigem Wasserstoffperoxid. Das Eisen aktiviert die Kohlenstoff‐Wasserstoff‐Bindung, während das Kupfer dafür sorgt, dass Methanol als Hauptprodukt gebildet wird. Der Katalysator ist stabil und wiederverwendbar und aktiviert Methan mit >90 % Selektivität und 10 % Umsatz in einem geschlossenen Katalysezyklus (siehe Schema).
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