*2.4. Evaluation of Optimal Dosage of Nb2O5 to Ensure Maximum Performance*

Optimizing the dosage of catalysts is a critical engineering parameter when designing sustainable reactors for solar photocatalytic applications at large scale [16,17]. The impact of Nb2O5 dose on decolorization kinetics was evaluated within the range from 0.25 g L−<sup>1</sup> up to 2.00 g L−<sup>1</sup> by treating solutions of 5 mg L−<sup>1</sup> of MO azo dye at pH 5.0 in the presence of 0.20 M of H2O2. Figure 8 reports an enhancement in the decolorization rate with increasing dosage of Nb2O5. These improved performances can be explained due to the increasing number of active sites resulting from the higher total specific surface of Nb2O5 available, thereby accelerating the photocatalytic generation of oxidants in solution by Reactions (1) and (2). The effective reduction of photocatalytic efficiency at excessively high catalyst dosages is commonly reported in the literature, and is attributed to the detrimental effects on light transport in solution caused by: (i) the increase of the solution opacity, which diminishes the radiation penetration and consequently the photogeneration of vacancies [9,40]; (ii) the aggregation of suspended catalyst particles diminishing the specific area [27,36]; and (iii) light scattering effects that also reduce the UV light penetration [22,42]. In addition, the surface available for adsorption processes is also increased, favoring the mechanisms of organic degradation. However, in our experiments, no appreciable loss of efficiency was observed, although the decolorization rate increase became lower from1gL-1, resulting in a plateau. The kinetic analysis of MO color removal enabled the estimation of pseudo-first-order rate constants of decolorization (kdec). These analyses showed excellent fits for a pseudo-first-order reaction, assuming that the -OH radicals achieve a pseudo-constant concentration on the Nb2O5 surface. Increasing kdec of 3.18·10−<sup>4</sup> <sup>s</sup>−<sup>1</sup> (R<sup>2</sup> <sup>=</sup> 0.996) with 0.25 g L−<sup>1</sup> of Nb2O5, 6.12·10−<sup>4</sup> <sup>s</sup>−<sup>1</sup> (R<sup>2</sup> <sup>=</sup> 0.997) with 0.50 g L−<sup>1</sup> of Nb2O5, 1.00·10−<sup>3</sup> s−<sup>1</sup> (R<sup>2</sup> = 0.997) with 1.00 g L−<sup>1</sup> of Nb2O5 and 1.07·10−<sup>3</sup> s−<sup>1</sup> (R2 = 0.998) with 0.25 g L−<sup>1</sup> of Nb2O5 were observed. It is important to remark that the slight increase in decolorization rate from 1.0 to 2.0 g L−<sup>1</sup> is presumably related to the loss of photocatalytic efficiency at excessively high catalyst dosages. Thus, a catalyst dosage of 1.0 g L−<sup>1</sup> was identified as the optimal condition, because it is the lowest dosage at which a faster decolorization rate can be achieved.

**Figure 8.** Influence of Nb2O5 photocatalyst dose on the photocatalytic performance. Dosing: (-) 0.25 g L−1, (-) 0.5 g L<sup>−</sup>1, () 1.0 g L<sup>−</sup>1, and () 2.0 g L<sup>−</sup>1.
