**4. Conclusions**

and 200 m<sup>2</sup>

298 Titanium Dioxide - Material for a Sustainable Environment

/g, the optimal concentration was found in the 0.5 to 3.0 g/L range depending on

**-LDH composites**

composites is their easy recovery and reuse over several degra-

,

High or low dosages of the photocatalyst may lead to a decrease in the reaction rate, so it is advisable to use the concentration of the photocatalyst near the point where its steady state is reached, i.e., the optimal concentration will correspond to the minimum quantity for which the maximum reaction is obtained, which is the highest proportion of material that remains exposed during radiation [19]. For all the composites, this state is observed at a concentration of 1 g/L where the highest performance is reached. In all the samples analyzed, it is observed that in quantities of 0.5 g/L a limiting effect occurs between the number of photocatalytic sites available for the reaction and the amount of phenol to degrade resulting in lower degradation rates [42]. When increasing the concentration of the photocatalyst, the radiation screening and dispersal phenomena—due to turbidity by the particles in suspension—gradually start to become significant, preventing the complete illumination of the solid due to the filtering effect of the excess particles, which mask part of the photosensitive surface. In addition, a bigger amount of the photocatalyst can lead to the deactivation of active molecules by particle colli-

the chemical characteristics and techniques of the irradiation system [19].

sion [19], as observed in the composites on increasing the concentration to 2 g/L.

dation cycles [43]. The results obtained on reusing a single solid from the synthesized composites over four rounds are shown in **Figure 10**, indicating the percentage of photodegraded phenol in each cycle. This behavior is favorable for the composites, since they can be reused, thereby demonstrating the synergy between mixed oxides derived from CLDH and TiO<sup>2</sup>

which, once they form the composite, cannot be separated and are therefore reusable.

**3.4. Phenol photodegradation in cycles with TiO2**

**Figure 10.** Phenol photodegradation using reutilized composites.

One advantage of using TiO<sup>2</sup>

TiO<sup>2</sup> -LDH composites were prepared from TiO<sup>2</sup> using three different methodologies, and LDHs were obtained by sol-gel synthesis, which are combined following different procedures. The different methodologies used to prepare the photocatalysts and the composites influence the photocatalytic activity of the materials, giving them different characteristics, being the most significant generation of mixed crystalline phases of TiO<sup>2</sup> with photocatalytic properties (anatase and rutile) in a ratio close to that reported as adequate (≈80:20%), the absence of TiO<sup>2</sup> phases without photocatalytic properties, and the particle size (between 15 and 50 nm), which allows an optimal balance between the production and recombination of photogenerated electron-hole pairs. The TiO<sup>2</sup> B precursor was the photocatalyst whose characteristics were closest to those described to promote greater photocatalytic activity. The composites originate distinct forms of interaction between the components affecting their photoactivity. Based on the characterization results in the TiO<sup>2</sup> T-LDH and TiO<sup>2</sup> I-LDH composites, a bigger chemical interaction and larger crystals are observed, which indicates the degree of diffusion of TiO<sup>2</sup> inside the composite. Meanwhile, in the TiO<sup>2</sup> B-LDH sample, the components remain segregated, with less chemical interaction, at the same time allowing minimal agglomeration and screening of the photocatalyst enhancing the photocatalytic mechanism. For the composites obtained, phenol elimination is attributed mainly to the degradation process through oxidation reactions produced by the formation of ●OH radicals, finding a minimal adsorptive capacity in the materials. In the analysis of the different concentrations of material, a dosage of 1 g/L was the most efficient, exposing the maximum amount of the composite to UV irradiation. In addition, the composites can be separated after use in the aqueous solution, allowing them to be reused with minimal loss of photocatalytic activity between each cycle.
