Synthesis and characterization of cobalt–based catalysts for selective oxidation of styrene and photodegradation of methylene blue

Abstract

Cobalt is one of the transition metals that has been widely used in various fields of modern research, especially as a catalyst and photocatalyst for various types of reactions. In this work, two types of cobalt-based catalyst, i.e. cobalt oxide and cobalt Schiff base complex, each supported on silica and silica-coated magnetite, have been investigated. Silica-coated magnetite (SiO2@Fe3O4) has been widely used as a catalyst support because it can be easily separated from the reaction mixture by applying only an external magnetic field. Herein, magnetite was synthesized by using the co-precipitation method before being coated with the silica via the sol-gel method in order to stabilize and reduce toxicity. Cobalt complex/SiO2@Fe3O4 was synthesized by the condensation between cobalt(II) salicylaldehyde and 3-aminopropyltrimethoxysilane (APTMS) on the surface of SiO2@Fe3O4. Cobalt oxide (Co3O4/SiO2@Fe3O4) was later produced after calcination of its complex at the temperature 550 ºC. The same method was used to synthesize cobalt complex/SiO2 and cobalt oxide/SiO2. These cobalt-based catalysts were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), adsorption-desorption nitrogen analysis, photoluminescence (PL) and diffuse reflectance ultraviolet visible (DRUV-Vis) spectroscopy. The performance of the cobalt-based catalysts was tested in the oxidation of styrene at room temperature (27 ºC) and 50 ºC for 24 h. The results revealed that the cobalt complex catalyst has the highest turnover number (TON) compared to the cobalt oxide catalyst. This is because the amount of Co2+ ions in the complex system which act as the active sites in the reaction is higher than in the oxides. Moreover, all of the active sites in the complexes are more accessible by the substrate, unlike those of the the oxide system, in which the active sites are confined inside the particles. The photocatalytic activity of the synthesized Co3O4/SiO2@Fe3O4 and Co3O4/SiO2 were also tested and showed positive results. The Co3O4/SiO2@Fe3O4 catalyst showed the highest percentage of methylene blue (MB) photodegradation under UV and visible light irradiations. This can be explained by the relationship between the PL spectra and photocatalytic activity. Hence, the photocatalytic activity is expected to be higher when PL with stronger intensity is used. This difference is due to the larger amount of oxygen vacancies and defects in the sample. It can be concluded that the amount of Co2+ ion plays an important role in the oxidation of styrene, while the oxygen vacancies and defects are crucial for the photodegradation of MB.