2.2 Synthesis of TiO2 doped with Ce3+, Pr3+, La3+, Nd3+ Sm3+, Eu3+, and Gd3+

Rare earth doping was made using nitrate salts of each element. Water solutions of these salts were prepared to calculate the stoichiometric amount in order to obtain 0.5, 0.3, and 0.1 wt% according to the desired composition of the sample. A mixture of ethanol, water, and salt solution of rare earth was stirred and maintained under reflux at 70°C. Enough NH4OH was added to the mixture to obtain pH 7. Titanium n-butoxide was added dropwise to this solution, stirring and refluxing was maintained for the period of 24 h until gel formation. The gels were dried in a rotating evaporator at 70°C under vacuum, subsequently, gels were placed into an oven at 120°C during 12 h. Samples were calcined at 500 and 800°C during 4 h with a heating ramp of 2°C/min. The same procedure was carried out but without adding the precursors of the rare earth ions to obtain the pure titania [51].

#### 2.3 Photocatalytic reactions

The photocatalytic activity was evaluated following the degradation with respect to the time of two aqueous solutions of diuron (249 nm) and another of methyl parathion (275 nm), respectively. The conversion was measured by UV-Vis spectroscopy of the peaks of maximum absorbance for each pollutant. For this process was used a borosilicate Ace photocatalytic reactor that was enabled with a quartz recirculation system to maintain a constant reaction temperature of 20°C. The reaction total volume was 300 mL, employing 0.5 g/L as a catalyst concentration, constant stir and flow of 60 ml/seg atmospheric O2. The total reaction time was 300 min. The reactions were carried out under sunlight, and the fully equipped reactor was placed on the roof of the laboratory building in full sunlight from 10:00 a.m. to 3:00 p.m.
