Oxide surfaces are usually considered to be static, even when they are catalyzing chemical reactions. But researchers at Instituto de Química-Física “Rocasolano” and the Sandia National Labs showed that this view is incorrect for magnetite (Fe3O4), an important industrial catalyst. Real-time microscopy reveals that magnetite’s surface steps advance continuously during oxygen exposure. The iron needed for this growth of new magnetite comes from the material’s interior. The first step of oxidation, dissociative oxygen adsorption, occurs uniformly over magnetite’s terraces. The common assumption in heterogeneous catalysis, in contrast, is that redox reactions occur at surface steps. Furthermore, this research establishes that catalytic redox cycles on magnetite do not involve creating and destroying oxygen vacancies, as usually assumed. Instead, catalytic cycles grow and etch the crystal through a different defect, iron vacancies.

(a-d) Low-energy electron microscopy images from Fe3O4(100) exposed to O2. Surface steps are at the boundaries between the bright/dark bands. The red lines show one step advancing. Field of view = 20 m. (e) Spiral step topography. (f) Model of Fe3O4 growth at the surface. e un escalón. Campo de visión = 20 µm. (e) Topografía de escalón espiral. (f) Modelo de crecimiento de Fe3O4 (100) en la superficie.

 (e)

Shu Nie,1 Elena Starodub,1 Matteo Monti,2 David Siegel,1 Lucía Vergara,2 Farid El Gabaly,1 Norman Bartelt,1 Juan de la Figuera,2 and Kevin McCarty1, Insight into magnetite’s redox catalysis from observing surface morphology during oxidation, J. Am. Chem. Soc., in press (2013). 1 = SNL, 2 = IQFR