Nanostructured iron oxides for heterogeneous catalysis

Abstract

Modulating the shape and crystal-phase of nano-sized iron-oxide particles play an essential role in the design of highly efficient heterogeneous catalysts. Iron oxides usually present as hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃), and magnetite (Fe₃O₄), where the coordination environments of Fe and O vary considerably. The diversity structures of iron oxides, in terms of chemical composition, particle size/shape, and crystal-phase, favor a flexible mediation on the geometric and electronic characters of surface Fe and O atoms that are intimately linked to the active sites for catalysis. Tuning the crystal-phase of size/shape-specified FeO_x particles alters the arrangements of Fe and O atoms both in the bulk and on the surface. While tailoring the particle shape, in a specific crystal-phase, enables to expose the more reactive facets featured by unique arrangements of Fe and O atoms. All these strategies could maximize the number of active sites for catalysis and regulate the adsorption and activation manner of reacting molecules. In addition, the shape and crystal-phase of FeO_x particles, when they are used to support the catalytically more active precious metals, affect the dispersion of the precious-metals via interfacial bonding and charge transfer. In this context, the precious-metals would show distinct electronic features via interaction with iron oxides, while their interfacial bonding is governed by the surface properties of iron oxides. Among them, precious-metal single-atoms, anchored on iron oxides, are characterized by the isolated sites, but a straightforward correlation between their electronic and geometric structures and the catalytic properties is controversial. Alternatively, inverse structures (iron-oxide layers on precious -metal particles) and core-shell geometries (a precious-metal core and an oxide shell) enable to construct active interfaces and describe the geometric and electronic characters. Moreover, the dynamic behavior of precious-metal-support interfaces, under reactive gases and at high temperatures, would provide accurate and realistic evidences for revealing the intrinsic structure-reactivity relationships.