Continuous Hydrothermal Synthesis of Mg-Doped Co3O4 Nanocubes: Precursors to Co/MgO Ammonia Decomposition Catalysts

Abstract

This study reports the continuous hydrothermal synthesis of Mg-doped Co3O4 spinel nanomaterials in supercritical water. By optimizing the synthesis parameters, crystalline MgxCo3−xO4 nanospinels (22−35 nm) were obtained directly at 391 °C with τ ≥ 0.1 s and a Mg/Co ratio of 0.25, without thermal post-treatment. Under these conditions, the formation of secondary (Mg,Co)(OH)2 phases was suppressed. Structural refinement indicates a moderately inverse spinel lattice, with Mg2+ incorporation limited to x ≤ 0.24, consistent with the formation of a thermodynamically stable Mg0.24Co2.76O4 spinel phase, as confirmed by HAADF-STEM-EDX analysis. Furthermore, the mixed oxides were evaluated as catalyst precursors for ammonia decomposition after reduction in H2 at 600 °C to generate active metallic cobalt particles embedded in a MgO matrix. Catalytic studies revealed that the performance is determined by a balance between reducibility and cobalt dispersion. On the one hand, increasing Mg content decreases the extent of reduction of the precursor, leading to a lower fraction of metallic cobalt in the catalyst. On the other hand, higher Mg concentrations improve the dispersion of metallic Co0 and its interaction within the MgO matrix, thereby suppressing metal particle sintering. This study demonstrates that precise control of the synthesis enables a detailed understanding of the phase formation in Co−Mg mixed oxides and establishes a reliable strategy for the continuous synthesis of catalyst precursors that yield highly efficient, sintering-resistant catalysts.