Abstract:
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Molybdenum oxides are finding increasing applications that rely on their
redox character. For the most common polymorph, α-MoO3, oxygen vacancy formation
leaves two electrons on the surface that can be stored as small polarons. Detailed density
functional theory calculations that properly account for the self-interaction term, Ueff = 3.5
eV, show that the vacancy generates two different configurations: either two Mo5+ centers
(Mo5+□ and Mo5+O) or a single double-reduced Mo4+. These states are separated by
0.22 eV with a barrier for interconversion of 0.33 eV, and thus both are populated at
catalytic temperatures, as shown by first-principles molecular dynamics. At higher
reduction levels, vacancies can only be accumulated along a preferential direction and the
energy difference between the 2×Mo5+ and Mo4+ configurations is reduced. These results
point out the need for a revision of the experimental assignments based on our
characterization that includes charges, vibrational frequencies, and XPS signatures |