Scalable Synthesis of Three-Dimensional Meso/Macroporous NiO with Uniform Ultralarge Randomly Packed Mesopores and High Catalytic Activity for Soot Oxidation

Functional nanomaterials with uniform ultralarge mesoporosity possess great potential for advanced technological applications. However, their cost-effective synthesis at a large scale still remains a major challenge. In this work, a three-dimensional (3D) hierarchical meso/macroporous NiO catalyst with uniform ultralarge randomly packed 22 nm mesopores has been synthesized for the first time by a facile, economical, and highly scalable colloidal solution combustion synthesis (CSCS) and applied for catalytic soot oxidation. The structural parameters of the catalyst (specific surface area, pore volume, and pore size) can be readily tuned by the amount of colloids added, and NiO with a high surface area (221.6 m2/g), a large pore volume (0.39 mL/g), and a small crystal size (3–5 nm) has been produced. The addition of silica colloids confines the exothermic reaction in small nanovoids between closely arranged 22 nm hard-sphere colloids, leading to a combustion at the nanoscale level, which we denote as nanocombustion. Furthermore, the large surface area, uniform ultralarge mesoporosity, high crystallinity, and ultrasmall particle size of the material can be achieved at the same time, whereas post-synthesisprolonged high-temperature calcination for crystallization usually leads to the collapse of the mesostructure in other template-assisted methods. These can be achieved because the formation and crystallization of nuclei occur almost simultaneously through the heat generated from the exothermic reaction between the reactants in the CSCS. The CSCS-synthesized NiO shows excellent activity for soot oxidation resulting from the combination of interconnected accessible pores, which provide sufficient interfacial contact with the soot and a high concentration of oxygen vacancies. Maximum catalytic soot elimination is achieved at 394 °C, significantly better than that of commercial 20 nm NiO nanoparticles, which only oxidize 53% of the soot at 500 °C under tight-contact conditions. Considering the remarkable advantages offered, the CSCS method has the required features to make the commercial-scale synthesis of high-quality porous materials a reality