Effect of molar ratio of citric acid to metal nitrate on the structure and catalytic activity of NiO nanoparticles

Abstract

NiO nanoparticles were prepared by means of sol-gel method via varying the ratio of citric acid to nickel nitrate. The samples were characterized by powder X-ray diffraction(XRD), Fourier transform infrared(FTIR) spectroscopy, transmission electron microscopy(TEM) and X-ray photoelectron spectroscopy(XPS). It was found that the molar ratio of citric acid to nickel nitrate has a great effect on the crystal structure and particle size of NiO. The increase of the molar ratio of citric acid to nickel nitrate is favorable to the formation of NiO smaller particles within the range tested. Compared to bulk NiO obtained by thermal decomposition, NiO nanoparticles possess more surface oxygen species O−. The activity test indicates that surface oxygen species O− plays a crucial role in the hydroxylation of benzene to phenol with hydrogen peroxide as oxidant. The active site may be originated from Ni2+ on the surface of the samples, while Ni0 does not contribute to the hydroxylation reaction.