Effect of calcination temperature on the catalytic activity of nanosized TiO2 for ozonation of trace 4-chloronitrobenzene

Nanosized titanium dioxides were synthesized by hydrolysis of TiCl4 followed by calcination at different temperatures ranging from 300 to 1,000 degrees C. The as-prepared samples were characterized by X-ray diffraction, N-2 adsorption-desorption, and zeta potential analysis. The catalytic activities of the TiO2 nanoparticles were tested by catalytic ozonation of trace 4-chloronitrobenzene (4-CNB) in water. Moreover, the catalytic ozonation activity of a sample calcined at 400 degrees C (denoted as T400) was tested in aqueous solution using electron paramagnetic resonance (EPR) spin trapping technique with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap. It was found that with increasing calcination temperatures, the average crystallite size and average pore size increased, in contrast the BET surface areas decreased. However, the isoelectric point (IEP) first increased, and then decreased. The ozone adsorption on the catalyst surface played an important role in determining their catalytic activity. Sample T400 with the IEP of 7.0, farthest away from the 4-CNB solution pH value (pH = 5.3), showed the best catalytic activity. The EPR experiments further confirmed that the hydroxyl radicals TiO2-catalyzed ozonation followed a radical-type mechanism.