One Oxygen Vacancy, Two Charge States: Characterization of Reduced α-MoO 3 (010) through Theoretical Methods

• Published in: The journal of physical chemistry letters 2018-05, Vol.9 (10), p.2568-2573
• Main Authors: Rellán-Piñeiro, Marcos, López, Núria
• Format: Article
• Language: English
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.8b00536

Molybdenum oxides are finding increasing applications that rely on their redox character. For the most common polymorph, α-MoO , 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, U = 3.5 eV, show that the vacancy generates two different configurations: either two Mo centers (Mo □ and Mo ═O) or a single double-reduced Mo . 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×Mo and Mo 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.