Synthesis and characterization of Fe-doped anatase/brookite TiO2 mixed phases and their behavior towards the degradation of the persistent pollutant simazine

This work of thesis analyzed the effect of iron doping on the photocatalytic properties of an anatase/brookite titanium dioxide (TiO2) mixed phase, tested for the degradation of simazine. The powders were obtained by a template-free sol-gel synthesis method based on pH control, followed by calcination at mild temperature, to favor the formation of anatase/brookite heterojunctions able to stabilize photogenerated electrons/holes. Titanium tetraisopropoxide (TTIP) and ferric chloride hexahydrate (FeCl3 x 6H2O) were used as Ti and Fe sources, respectively; to obtain the best results (in terms of brookite content) a pH = 2.00 and a temperature of calcination of 200°C were chosen. The studied nominal Fe contents were 0.00, 0.050, 1.0, and 2.5 wt.%. The powders have been characterized by XRD (X-Ray Diffraction), DR (Diffuse Reflectance) UV-Vis, XPS and IR spectroscopies, N2 adsorption/desorption isotherms at -196 °C, and z-potential measurement. Photocatalytic tests were carried out on a 1.73 10-5 M simazine solution both under simulated solar light (1 SUN) and UV light (55-60 mW/cm2). The XRD analysis showed that different relative percentages of anatase and brookite are obtained, as determined by Rietveld analysis. Fe-doping led to a decrease of the band gap from 3.1 to 2.85 eV. Some surface Fe-species were observed by XPS analysis only on the sample at 2.5 wt.% Fe. N2 adsorption at -196 °C showed that the powders are characterized by a type IV isotherm with H2 hysteresis loop, as typical of samples with disordered and interconnected mesoporosity. The surface area increased with the increase of dopant concentration from 236.5 for the undoped powder to 260.8 m2/g for the 2.5% wt.% Fe powder. Z-potential measurement showed that the pH at the isoelectric point is not markedly affected by Fe doping. The herbicide simazine showed to be stable to photolysis degradation after 5 hours of exposure to 1 SUN. In presence of the synthesized undoped Fe-doped powders, the photocatalytic tests under 1 SUN showed an increase in the degradation of simazine up to 19.6% observed for the non-doped sample. In presence of a stoichiometric amount of H2O2 in the solution, total degradation of simazine did not improve much, likely due to the low percentage of iron at the samples’ surface, too low to ensure photo-Fenton activity. The results of photolysis under UV exposure for totally 3 h, showed that simazine is degraded, with formation of various by-products. In the presence of the different powders, an increase in the degradation kinetics and the earlier formation of by-products was observed, compared to mere UV-photolysis. Different samples showed different degradation paths. The 0.050 wt.% Fe powder showed the highest activity. Under UV illumination, the addition of a stoichiometric amount of H2O2 brought an enhancement in the photocatalytic activity, underlined by the presence of more intense bands ascribable to some by-products of simazine degradation, earlier in time. The 0.050 wt.% powder showed an enhanced activity only after 60 min, while the other samples needed at least 120 min to reach a similar result. Interestingly, the undoped sample was also active, likely due to the occurrence of anatase/brookite heterojunctions, in agreement with the literature.