Micro-morphology highly uniform mesoporous Co₃O₄ spheres: shape-controlled fabrication by a salt-assisted template-free method and enhanced catalytic performance of styrene epoxidation

In this work, the highly uniform microscale mesoporous cobalt oxide (Co₃O₄) spheres were successfully prepared via a facile salt-assisted template-free hydrothermal method. To the best of our knowledge, this is the first report on the synthesis of microscale mesoporous Co₃O₄ spheres. The regular Co₃O₄ microspheres with the average diameter of 8.5 μm are obtained. With the formation of mesoporous structure, these Co₃O₄ spheres have a high surface area (113 m²/g) and a narrowed pore size distribution (centered at 6.7 nm). Meanwhile, the formation process of these as-prepared Co₃O₄ spheres is in situ investigated via the time-dependent hydrothermal process, and the self-assembly and Ostwald ripening mechanisms are proposed to further understand the evolution of morphology structures. Due to the microscale sphere morphology and size-narrowed mesoporous structure, the contact and collision between reactant molecules and metal active sites are highly strengthened, and the residual time of the reactant molecules in the channels is relatively prolonged. Thus, compared with other Co₃O₄ in different morphologies, the as-prepared micro-morphology mesoporous Co₃O₄ spheres exhibit extremely high catalytic activity in styrene epoxidation reaction (92.3% conversion, 71.1% selectivity to styrene oxide). Moreover, these micro-morphology mesoporous Co₃O₄ spheres display excellent catalytic stability after five-time recycling tests. This strategy for construction of micro-morphology mesoporous Co₃O₄ spheres provides a new vision in the development of metal oxide materials with specific morphology for advanced applications. Graphical abstract: [Figure not available: see fulltext.]