In-Situ TEM Investigation on Redox Mechanisms of Transition Metal Oxides

The physicochemical properties of transition metal oxides are determined by their phase structures, surface exposed atoms, morphology, and defects. Metal oxide catalysts usually undergo a series of activation and reaction processes in practical applications, which modulate their morphology and change their valence states and crystal phases. These structural variations alternate their properties significantly and have been believed to be a critical problem, e.g., in their catalytic applications. During reactions, metal oxides undergo either reduction or oxidation reactions depending on the ratio of reducing or oxidizing gases in the atmosphere. Although it is believed that <i>in situ</i> investigation of the reaction pathways is of importance to understand their intrinsic redox nature, as far as we know, the microstructural evolution during the redox of metal oxides is still not well understood. We have studied the dynamic reaction behaviors of iron oxides via <i>in situ</i> transmission electron microscopy (TEM) with a Climate system from DENSsolutions. We have achieved atomic-scale resolution imaging to visualize the evolution of phase boundary and the competitive transitions between Fe₃O₄, FeO, and Fe in the nanoreactor. Thus we found the inhomogeneous reaction pathways which are strongly affected by the trace amount of oxygen or water vapor in catalytic reactions. Our results could provide new insights into the redox reaction mechanism of transition metal oxides, and help to understand the role of these oxides in catalytic reactions.