Heterogeneous catalytic activity of NiO-silica composites designated with cubic Pm3n cage nanostructures

The fabrication of nanomaterials with the active network sites has led to efficient transport and easier diffusion of guest species in the catalytic application. In this regard, the NiO-supported cage monoliths show evidence to act as effective catalysts toward the oxidation of organic pollutants. Here, the cage-like NiO-silica catalyst with large particle size, cage-like pores, and ordered cubic Pm3n (HOM-9) structures could be fabricated by a simple, in short period (∼5 min), and direct strategy in which microemulsion liquid crystalline phase of Brij 56 (C₁₆EO₁₀) surfactant was used as a template. Our synthetic strategy revealed that the nickel oxide nanoparticles were wrapped onto the pore surface matrices of the cubic Pm3n monoliths, indicating the simplicity and flexibility to control the geometry, morphology and dispersion of particles. No significant change in the cubic Pm3n (NiO/HOM-9) phase structures was evident by using this synthetic manipulation; however, high nickel contents up to Si/Ni ratios ∼1 were added to the phase composition domains. Results from the analysis techniques including XRD, N₂ isotherms, TEM, XPS, and EDX revealed that the NiO nanoparticles with irregular sizes might be embedded, to some extent, into the pore cavity, particularly with low content of NiO. In turn, with low Si/Ni ratios, The NiO crystallite particles underwent the anisotropic growth to larger sizes (∼15 nm) resulted from the aggregation effect, leading to the difficulty to be wrapped into the pore cavity. Practically important results were that the NiO-supported cage monoliths were used as effective catalysts for the oxidation of aminophenols in aqueous solution. However, among all NiO-supported amorphous and ordered silica materials, the NiO/HOM-9 catalyst with open, uniform pore-cage architectures, high surface area and large pore volumes allowed efficient adsorption and diffusion of aminophenols to the active site of NiO clusters, leading to high degree of conversion and reaction rate. On such heterogeneous catalytic systems, the reaction affinity of aminophenols was substantially affected by the structural feature of the catalysts, amount and degree of dispersion NiO particles onto the cubic cage pore surfaces, and the controlled temperature conditions.