Abstract:
Water is a limited resource and pollution has become an increasing problem due to industrialization. Aromatic organic pollutants are resistant to biodegradation, and thus chemical methods like the Fenton reaction is required for degradation. The Fenton reaction is catalyzed by cobalt oxide. This study aims to investigate the effect of size and shape of cobalt oxide nanoparticles on the catalytic activity. Methylene blue (MB) was used as a model pollutant.
The size and shape of nanoparticles are known to influence the activity of catalysts. The study used a precipitation method to prepare spherical and cubic-shaped cobalt oxide nanoparticles of different sizes using preparation parameters like cobalt precursor, amount and type of oxidant and time of reaction.
The XRD patterns of all the prepared cobalt oxide nanoparticles showed a pure cubic Co3O4 phase. The shape of the nanoparticles changed from spherical to cubic when the cobalt precursor was changed from cobalt nitrate to cobalt acetate. The size of the nanoparticles increased when lower amounts of hydrogen peroxide and longer reaction times were used. Nanoparticles between 4.6 to 19.5 nm for spherical particles and between 6.6 and 43.3 nm for cubic particles were prepared. FTIR spectra analysis showed the presence of both nitrate and acetate ions on the surface of cobalt oxide nanoparticles. The TGA results indicated that the adsorption of the acetate ions is stronger than the nitrate ions on the surface of the cobalt oxide nanoparticles.
The rate of degradation of methylene blue, the pseudo first order rate constant and the amount of leaching increased with a decrease in the nanoparticles size. The Turn Over Frequency (TOF), which is the moles of methylene blue converted per mole of surface cobalt atoms, decreased with a decrease in the size for both the spherical and cubic nanoparticles. The TOF for the spherical and cubic nanoparticles were similar indicating that the catalytic activity may not be dependent on the shape of the nanoparticles. FTIR analyses showed that degradation occurred, and that methylene blue was not just decolourised to leuco methylene blue.
