Abstract
Wet impregnation is broadly applied for the synthesis of carbon-supported metal/metal oxide nanostructures because of its high flexibility, simplicity and low cost. By contrast, impregnated catalysts are typified by a usually undesired nanostructural and morphological heterogeneity of the supported phase resulting from a poor stabilization at the support surface. This study on graphite-supported Fe-based materials from an Fe nitrate precursor is concerned with the understanding of the chemistry that dictates during the multistep synthesis, which is key to designing structurally homogeneous catalysts. By means of core-level X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy and atomic resolution electron microscopy, we found not only a large variety of particles sizes and morphologies but also chemical phases. Herein, thermally stable single atoms and few atoms clusters are identified together with large agglomerates of an oxy-hydroxide ferrihydrite-like phase. Moreover, the thermally induced phase transformation of the initially poorly ordered oxy-hydroxide phase into several oxide phases is revealed, together with the existence of thermally stable N impurities retained in the structure as Fe–N–O bonds. The nature of the interactions with the support and the structural dynamics induced by the thermal treatment rationalize the high heterogeneity observed in these catalysts.
Highlights
Fe oxides and oxyhydroxides exist in nature in a large variety of structures and play a primary role in many natural geological and biological processes [1]
A common preparation route for carbon-supported Fe oxide and oxyhydroxide nanoparticles (NPs) is wet impregnation which consists of three steps: a) the wetting of the solid support with an aqueous solution containing a precursor of the active metal, e.g., ferric or ferrous nitrate, chloride and acetate; b) the drying at a given temperature and normally in air; and c) the thermal treatment in an inert or reactive atmosphere to decompose the precursor and produce the desired phase [13]
The ferric solution with a measured pH of approximately 2.3 was quantitatively loaded onto the carbon support with the targeted total loading of 1 percent in weight expressed as metallic Fe
Summary
Fe oxides and oxyhydroxides exist in nature in a large variety of structures and play a primary role in many natural geological and biological processes [1]. A common preparation route for carbon-supported Fe oxide and oxyhydroxide nanoparticles (NPs) is wet impregnation which consists of three steps: a) the wetting of the solid support with an aqueous solution containing a precursor of the active metal, e.g., ferric or ferrous nitrate, chloride and acetate; b) the drying at a given temperature and normally in air; and c) the thermal treatment in an inert or reactive atmosphere to decompose the precursor and produce the desired phase [13]. The popularity of this preparation route is attributable to its simple execution and the use of inexpensive
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