Genesis of supported carbon-coated Co nanoparticles with controlled magnetic properties, prepared by decomposition of chelate complexes

Abstract

Following procedures formerly developed for the preparation of supported heterogeneous catalysts, carbon-coated cobalt nanoparticles dispersed on porous alumina have been prepared by impregnation of γ-Al2O3 with (NH4)2[Co(EDTA)] and thermal decomposition in inert atmosphere. Below 350 °C, Co(II) ions are complexed in a hexa-coordinated way by the EDTA ligand. The thermal treatment at 400–900 °C leads to the EDTA ligand decomposition and recovering of the support porosity, initially clogged by the impregnated salt. According to X-ray absorption spectroscopy, and due to in situ redox reactions between the organic ligand and Co(II), both oxidic and metallic cobalt phases are formed. Characterisation by transmission electron microscopy, X-ray diffraction and magnetic measurements reveals that an increase in the treatment temperature leads to an increase of the degree of cobalt reduction as well as to a growth of the cobalt metal particles. As a consequence, the samples prepared at 400–700 °C exhibit superparamagnetism and a saturation magnetisation of 1.7–6.5 emu g−1 at room temperature, whilst the sample prepared at 900 °C has a weak coercivity (0.1 kOe) and a saturation magnetisation of 12 emu g−1. Metal particles are homogeneously dispersed on the support and appear to be protected by carbon; its elimination by a heating in H2 at 400 °C is demonstrated to cause sintering of the metal particles. The route investigated here can be of interest for obtaining porous magnetic adsorbents or carriers with high magnetic moments and low coercivities, in which the magnetic nanoparticles are protected from chemical aggression and sintering by their coating.