Degradation of the herbicide 2,4-dichlorophenoxyacetic acid over TiO2–CeO2 sol–gel photocatalysts: Effect of the annealing temperature on the photoactivity

TiO2–CeO2 mixed oxides were synthesized by the sol–gel method and calcined at 473, 673 and 873 K. The preparation method enabled us to obtain TiO2–CeO2 mixed oxides with specific surface areas of 250, 174 and 99 m2/g. An important effect of the thermal treatment on the band gap of the TiO2–CeO2 mixed oxides was observed. The Eg band gap shifts from 3.1 to 3.2 eV, for the TiO2 bare catalysts, to 2.3–2.5 eV on the annealed mixed oxides. The X-ray Rietveld refinement shows that the calculated titanium deficiency diminishes as the annealing temperature of the mixed oxides is increased. A relationship between the titanium deficiency per unit cell (from 44.8 to 20.0%) the Eg band gap (2.3–2.5 eV) and the capacity for the herbicide photodegradation (from 85.6 to 94.5%) was observed.