Investigations of Co(II) to Co(III) oxidation mechanisms at Fe 3-δ O 4 nanoparticle surface
Résumé
Understanding and predicting the interaction mechanisms between Co and spinel iron oxides (Fe 3-δ O 4 ; including magnetite, maghemite and their solid solutions) is of major interest in both the environmental and industrial contexts. While the surface complexation of Co 2+ , its polymerization and the surface precipitation of Fe 3-δ O 4 @CoO have been well been documented, the oxidation of Co 2+ to Co 3+ and interaction mechanisms between Co 3+ and Fe 3-δ O 4 nanoparticles in the presence of O 2 have attracted less attention. In this study, experimental and modeling results, combined with XAS and XMCD analyses at the L 2,3 -edges, as well as TEM and XRD analysis, allowed the differentiation of several Cospecies according to the Co concentration, at pH 8 under atmospheric O 2 . At the lowest Co concentrations investigated, Co 2+ prevailed as surface complexes, incorporated into Fe 3-δ O 4 nanoparticles, or as Co(OH) 2 -like surface precipitates. Increasing Co concentration lead to the formation of an additional Co 3 O 4 -like phase on the surface of Fe 3-δ O 4 nanoparticles. Quick-XAS measurements at the Co K-edge allowed to follow the kinetics of Co sorption and oxidation on Fe 3-δ O 4 nanoparticles, supporting the equilibrium observations. These results provide a more detailed understanding of Co sorption mechanisms onto Fe 3-δ O 4 nanoparticles under oxic conditions, which offers an environmentally friendly route for the synthesis alternative of Co-doped Fe 3-δ O 4 nanoparticles. In addition, this allows to understand the potential interaction between Co and Fe 3-δ O 4 nanoparticles in environmental systems.
