Kinetics of the catalytic oxidation of morin on γ-Al₂O₃ supported gold nanoparticles and determination of gold nanoparticles surface area and sizes by quantitative ligand adsorption

We report on the kinetic analysis of catalytic activity of γ-Al₂O₃ supported gold nanoparticles (AuNPs) on the rate of oxidation of morin by hydrogen peroxide as model reaction. Gold-based catalysts were revealed to be very active in the oxidation of morin under mild conditions, especially when Li₂O is used as additive, and cationic imidazolium ionic liquids are used as AuNPs stabilizer. All kinetic data could be modeled in terms of the Langmuir–Hinshelwood model; that is both reactants are assumed to be adsorbed on the surface of the nanoparticles. The apparent reaction rate could therefore be related to the surface of the nanoparticles, to the kinetics constant, related to the rate determining steps, and to the adsorption constant of the reactants. The organothiol adsorption-based technique for the determination of specific surface area and particle sizes of AuNPs on γ-Al₂O₃ supports is also reported. A fair agreement was found between particle sizes obtained from ligand adsorption and TEM methods. Atomic absorption spectrometry (AAS), X-ray diffraction spectrometry (XRD), N₂-physisorption (BET), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) are used for the characterization of the catalysts, while the oxidation reaction and the ligand adsorption was followed by ultraviolet-visible spectrometry (UV–vis).