DFT Study on the Catalytic Activity of ALD-Grown Diiron Oxide Nanoclusters for Partial Oxidation of Methane to Methanol

Using density functional
theory (DFT), we studied the catalytic
activity of iron oxide nanoclusters that mimic the structure of the
active site in the soluble form of methane monooxygenase (sMMO) for
the partial oxidation of methane to methanol. Using N2O
as the oxidant, we consider a radical-rebound mechanism and a concerted
mechanism for the oxidation of methane on either a bridging oxygen
(Ob) or a terminal oxygen (Ot) active site.
We find that the radical-rebound pathway is preferred over the concerted
pathway by 40–50 kJ/mol, but the desorption of methanol and
the regeneration of the oxygen site are found to be the highest barriers
for the direct conversion of methane to methanol with these catalysts.
As demonstrated by a population analysis, the Ox (x = b or t) site behaves as an oxygen radical
during the H abstraction, and the [Fe+–Ox–] site behaves as a Lewis acid–base
pair during the concerted C–H cleavage. Molecular orbital decomposition
analysis further demonstrates electron transfer during the oxidation
and reduction steps of the reaction. High-level multireference calculations
were also carried out to further assess the DFT results. Understanding
how these systems behave during the proposed reaction pathways provides
new insights into how they can be tuned for methane partial oxidation.