3.2 Results of photocatalytic tests of diuron using TiO2-RE

The photocatalytic activity under sunlight in the photodegradation of diuron is described in Figure 7. All materials doped and calcined at 500°C obtained a higher yield than photolysis, pure TiO2 and P25-TiO2 (Figure 7a). This behavior is directly related to the high dispersion of dopants, which superficially modifies the titania, considerably increasing its specific area. The materials doped with La and Ce 0.1%, respectively, are the most active of the series when starting the dopant content, show an increase in the volume of pores, which indicates that the dispersion of these dopants do not obstruct the porous cavities, generating a porous material that has greater contact with the polluting solution, the opposite occurs with the rest of

#### Figure 7.

Photocatalytic degradation of diuron using solar light: (a) doped photocatalyst to 0.1% at 500°C. (b) Doped photocatalyst to 0.3% at 500°C. (c) Doped photocatalyst to 0.5% at 500°C and doped photocatalyst to 0.3–0.5% at 800°C. (d) Solar photodegradation of diuron with Ce 0.1% TiO2 500°C.

the doping ions, which upon insertion decrease the pore volume and its photoactivity decreases. For the sample with Eu, although it also increases its pore volume with respect to pure titania, its activity declines considerably and has the worst photocatalytic performance, when contrasting this phenomenon with the average particle sizes. It was found that this material maintains the same particle size; therefore, it can be attributed that photocatalytic activity in this material depends on the average particle size.

When increasing the content to 0.3% of the dopants in the titania, the materials doped with Eu and Gd do not increase the specific area with respect to the photocatalysts, in which the samples doped only with La, Ce, and Sm increase their photocatalytic performance, as seen in Figure 7b. The first two conserve the same pore diameter, showing a good dispersion by augmenting the dopant content that consequently raises the specific area, without blocking pores as reported in other investigations [19]. In the sample with Sm at this content, the dispersion improves remarkably with respect to 0.1%, which was observed in Figure 6b in the elemental mapping by EDS, which increases the pore diameter in the material. The tendency in the size of crystal when increasing concentration of dopant describes a reduction and conservation in values of this parameter, which already has been previously reported [54]; only the material doped with Ce increases this value, but not in a way significant and still showing a smaller size with respect to pure titania.

In Figure 7c, it is observed that the increase dopant content to 0.5% decrease photocatalytic efficiency; this gives us an idea of what is the content and the ideal ions of rare earth as dopants in the titania for solar photodegradation in aqueous medium of the diuron (Figure 7d), as in other investigations, at higher dopant concentrations, the pore diameter and average crystal size decrease [58]. Nevertheless, if the content of doping ions is excessively high, the recombination process of the photo-generated becomes easier, which led to the lower photocatalytic activity of titania. The photoactivity of some samples calcined at 800°C, presented a better efficiency than pure TiO2 and P25-TiO2; the trend of the specific area describes an increase with the presence of dopants, but with Eu, this parameter decreased considerably to a value below pure titania. The photocatalyst with Sm 0.3% was the most active, followed by Eu and Gd at this treatment temperature; this is attributed mainly to the presence of mixtures of crystalline phases (anatase-rutile), which incorporates Sm; the ratio of this mixture indicates a higher content of anatase (about 46%), as described above. This phase is more active than rutile and has a higher anatase ratio than that reported for P25-TiO2 (30% approx.); therefore, the performance is better. However, this same sample has a crystal size for the greater rutile phase compared to the rest of the materials, which indicates that the growth of the rutile crystals is directly proportional to the photocatalytic activity of the materials.

The materials doped and thermally stabilized at 500°C more active in the previous reaction, were selected to evaluate them photocatalytically under sunlight in the degradation of an organophosphorus insecticide (methyl parathion), to analyze the effect that photocatalysts have on different aqueous pollutants with different functional groups (phenylurea and thiophosphate). Although materials doped at 0.1 and 0.3% with La and Ce obtained similar yields, those with the least amount of dopant were chosen.

#### 3.3 Results of photocatalytic tests of methyl parathion using TiO2-RE

The photocatalytic behavior of the catalysts chosen for this test is described in Figure 8a; in Figure 8b, the photodegradation of methyl parathion with respect to time is shown in the most photoactive material of this series of materials doped and Photocatalytic Treatment of Pesticides Using TiO2 Doped with Rare Earth DOI: http://dx.doi.org/10.5772/intechopen.84677

Figure 8.

Photocatalytic degradation of methyl parathion using solar light: (a) La 0.1% TiO2, Ce 0.1% TiO2, Nd 0.3% TiO2, Sm 0.3% TiO2, Eu 0.3% TiO2, Gd 0.1% TiO2, pure TiO2, P25-TiO2 and photolysis. (b) Solar photodegradation of methyl parathion with Eu 0.3% TiO2 500°C.

calcined at 500°C. A different trend is observed when comparing the activity of the materials with the previous model molecule (diuron). The conversion by photolysis without catalyst was around 25%; however, conversions are reached above 95% with TiO2 doped with Eu at 0.3% and with the other catalysts, the conversions exceed 70%. Only samples doped with Eu 0.3%, Ce 0.1%, and Sm 0.3% were more active than P25-titania. The value of the average diameter of pores in these materials increased and was compared with the pure titania; this increment also happened with the doped catalyst with La, but as its specific area was the lowest, this reduced its photoactivity. This seems to indicate that the presence of pores and specific area large, allows a better diffusion between the polluting solution and the photocatalyst to increase the photoactivity of TiO2 doped with rare earth ions. With respect to the average crystal size, all materials decrease this value due to the presence of rare earth ions, which inhibit the growth of crystals in the process of synthesis and thermal stabilization. Finally, it can be stated that there is an affinity between the doped ions in the titania and the main functional groups of the molecules used for the photodegradation under sunlight. With phenylurea (diuron), solar photodegradation was more pronounced with the materials doped with La and Ce, meanwhile, for the thiophosphate (methyl parathion), the process of solar photooxidation had a greater affinity for the Eu.

### 4. Conclusions

The rare earth doping ions improve the textural, structural, electronic, and photocatalytic properties in TiO2. Due to the method of preparation and the treatment temperature, possibly the presence of N and the elimination of impurities produced a change in the absorption bands, which allows the titania to have a better photocatalytic behavior under sunlight. At 500°C, the materials present 100% of the anatase crystalline structure, the ideal amount of dopant was 0.1% and the most active rare earth ions were La and Ce in the diuron solar photodegradation. The increase in temperature of thermal treatment (800°C) showed the presence of mixtures of crystalline phases, which have a greater abundance of anatase, compared to P25-TiO2, where the catalyst doped with Sm 0.3% obtained the best performance. Photocatalysts treated at 500°C with greater activity in diuron

degradation were chosen to evaluate their solar photoactivity with a second pesticide (methyl parathion). Under these conditions, an affinity was found for the dopant ions in titania and the functional groups of the contaminating molecules (phenylurea and thiophosphate). Solar photodegradation of diuron was more effective with La and Ce, while for methyl parathion it was Eu at 0.3%.
