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.