Preparation of nanocrystalline TiO ₂-coated coal fly ash and effect of iron oxides in coal fly ash on photocatalytic activity

In order to more easily separate TiO ₂ photocatalyst from the treated wastewater, TiO ₂ photocatalyst is immobilized on coal fly ash by a precipitation method. The titanium hydroxide precipitated on coal fly ash by neutralization of titanium chloride is transformed into titanium dioxide by heat treatment in the temperature range of 300-700°C. The crystalline structure of the titanium dioxide shows anatase type in all range of heat treatment temperature. The crystallite size of anatase increases with increasing heat treatment temperature, with the draw back that the removal ability of NO gas will decrease. When the coal fly ash coated with 10 wt.% of TiO ₂ is calcined at the temperatures of 300 and 400°C for 2 h, the crystallite sizes of anatase appeared about 9 nm, and the removal rate of NO gas shows 63% and 67.5%, respectively. The major iron oxide, existing in coal fly ash as an impurity, is magnetite (Fe ₃O ₄). The phase transformation into hematite (Fe ₂O ₃) by heat treatment improves the removal rate of NO gas for TiO ₂-coated coal fly ash.