(189an) DFT Study on the Catalytic Activity of ALD-Grown Iron Oxide Nanoclusters for the Partial Oxidation of Methane to Methanol

The current route for converting natural gas into more useful chemicals is through complex, multi-step industrial processes that requires multiple reactors, separations, and high temperatures and pressures. Methane monooxygenase (MMO) is an enzyme found in biological systems that is capable of oxidizing methane to methanol using O₂ at mild conditions. Using density functional theory, we have studied the catalytic activity of ALD-grown iron oxide nanoclusters that mimic the structure of the active site in MMO for the oxidation of methane to methanol. Two mechanisms for the oxidation of methane are proposed using N₂O as the oxidant and a bridging oxygen (O_b) or a terminal oxygen (O_t) as the active site: (1) a concerted mechanism in which the C-H bond is cleaved heterolytically and (2) a rebound mechanism in which the oxygen atom abstracts a hydrogen atom from methane.

We have found that the radical rebound pathway is preferred over the concerted pathway by 40-50 kJ/mol. The desorption of methanol and the regeneration of the oxygen site exhibit high barriers for the direct conversion of methane to methanol. As demonstrated by a population analysis, the O_x (x= b or t) site behaves as an oxygen radical during the H-abstraction, and the [Fe⁺-O_x^-] site behaves as a Lewis-acid base pair during the concerted C-H cleavage. Molecular orbital decomposition analysis further demonstrates the electron transfer during the oxidation and reduction steps of the reaction. Understanding how these systems behave during the proposed reaction pathways provide new insights into how they can be tuned for alkane oxidation reactions.