Comparative Study of Combustion Methods, Opto-Magnetic and Catalytic Properties of Spinel CoAl₂O₄ Nano- and Microstructures

Spinel CoAl₂O₄ nano- and microstructures were successfully synthesized by a facile microwave combustion (MCM) and solution combustion (SCM) methods, respectively, using glycine as the fuel. Powder XRD, FT-IR, EDX spectra, SAED, HR-SEM and HR-TEM results showed that the as-prepared samples were pure phase spinel CoAl₂O₄ nano- and microstructures without any other secondary phase impurity. The optical band gap value of the samples CoAl₂O₄-MCM and CoAl₂O₄-SCM are 3.72 eV and 3.39 eV, respectively, which were measured from UV-Visible diffuse reflectance spectra (DRS) and photoluminescence (PL) spectroscopy. The magnetic properties of the samples were measured by VSM analysis. The relatively lower M _s (19.34 × 10^–3 emu/g) of CoAl₂O₄-SCM shows that it is weak ferromagnetic in nature and high M _s (43.26 × 10^–3 emu/g) of CoAl₂O₄-MCM confirms its ferromagnetic behavior. The sample CoAl₂O₄-MCM prepared by the MCM route was found to possess a higher surface area, and lower crystallite size than the CoAl₂O₄-SCM prepared by the SCM route, which in turn leads to the improved performance towards the selective oxidation of benzyl alcohol into benzaldehyde, with high activity, good reusability, remarkable stability and environmentally friendly and also promising materials for the industrial applications. It was found that the sample CoAl₂O₄-MCM show higher conversion (95.73%) of benzyl alcohol into benzaldehyde than the sample CoAl₂O₄-SCM (64.52%). Microwave irradiation has produced sufficient energy for the formation of spinel CoAl₂O₄ nanostructures than SCM route, because of its homogeneous distribution of microwaves within the raw materials.