DFT+U Study of the Electronic, Magnetic and Mechanical Properties of Co, CoO, and Co₃O₄

Abstract

Cobalt nanoparticles play an important role as a catalyst in the Fischer-Tropsch synthesis. During the reaction process, cobalt nanoparticles can become  oxidized leading to the formation of two phases: CoO rock-salt and Co₃O₄ cubic spinel. Experimentally, it is possible to evaluate the phase change and  follow the catalyst degradation by measuring the magnetic moment, as each material presents a different magnetic structure. It is therefore important to  develop a fundamental description, at the atomic scale, of cobalt and its oxide phases which we have done here using density functional theory with  the Dudarev approach to account for the on-site Coulomb interactions (DFT+U). We have explored different Ueff values, ranging from 0 to 5 eV, and found  that Ueff = 3.0 eV describes most appropriately the mechanical properties, as well as the electronic and magnetic structures of Co, CoO and  Co₃O₄. We have considered a ferromagnetic ordering for the metallic phase and the antiferromagnetic structure for the oxide phases. Our results  support the interpretation of the catalytic performance of metallic cobalt as it transforms into its oxidized phases under experimental conditions.