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In this work is presented a study of the shelf life of an iron-impregnated MgO (Fe/MgO) sample and an explanation of the effect of its aging on morphology and electroactivity of nitrogen-doped carbon nanostructures prepared by using this catalyst at different ages. Characterizations of Fe/MgO by thermogravimetric analysis (TGA), temperature-programmed desorption mass spectroscopy (TPD-MS), high-resolution X-ray powder diffraction (HR-XRPD), and X-ray photoelectron spectroscopy (XPS) have been performed at different aging times. The catalyst is formed by crystalline MgO, Mg­(OH)2, and FeO phases. During aging, CO2 adsorption induces the formation of amorphous carbonate, resulting in a decrease of specific surface area, a redistribution of porosity, a reduction of the iron percentage at the surface, and an increase of iron oxide particle size. Different surface and bulk properties are used to explain different morphologies and electroactivities of the nitrogen-doped carbon nanotubes synthesized at different catalyst aging times. The importance of a careful characterization of the catalyst as a function of shelf life, to optimize the synthesis and the morphology of the nitrogen-doped carbon nanotubes, is pointed out.