*3.2. Adsorption Kinetics and Results of Photodegradation Effect*

The methylene blue adsorption kinetics corresponding to mesoporous solids are shown in Figure 9. These curves show marked differences in dye adsorption rates obtained by the solids, depending on the methodology used in the incorporation of niobium, as well as on the percentage of this cation on the mesoporous silica MCM-41. The solids with the highest dye adsorption capacity were those containing 10% Nb, obtained through the sol-gel method, and 20% Nb obtained using the incipient impregnation method. This result indicates that greater uniformity of niobium distribution on the surface of the mesoporous solid guarantees higher adsorption efficiency, as indicated by the Raman spectra (Figure 2).

Further, solids with higher adsorption capacity did not show leaching or formation of Nb2O5 clusters. What is evident in the adsorption kinetics (Figure 9) is that the Nb percentage does not notably affect dye adsorption.

**Figure 9.** Adsorption kinetics of Nb-MCM-41 mesoporous solids obtained using the solgel (**a**) and impregnation method (**b**) (Factor *q*: mg methylene blue adsorbed/mg photocatalyst).

Although all the solids exhibit nearly the same total adsorption capacity, the behavior of the curves, as shown in Figure 9, is different, leading to the conclusion that the adsorption mechanisms were different. This could be due, in the main, to the differences in the microstructure of the samples, since as seen in the XRD at low angles, they clearly show a different mesophase, which could be closely related to the diffusion of the reagents. It is necessary, in future, to carry out a careful study to learn more about the mechanisms of adsorption as regards these samples and relate them to their characteristics (Table 1).

The process of photodegradation of methylene blue brought about by the Nb/MCM-41 mesoporous solids synthesized in this work is illustrated in Figures 10 and 11. The results show that for both methods of Nb incorporation, solids with 20% Nb showed a better photodegradation response. By comparing the curves, it is evident that the solid obtained using the sol-gel method showed a much higher degradation percentage than the solid obtained using incipient impregnation, with comparative figures of 60.6% and 16.7%, respectively (Figures 10 and 11).

Additionally, considering these results, and taking into account those of the Raman spectroscopy (Figure 2), it can be concluded that better niobium dispersion on the silica leads to a reduced photodegradation effect. In other words, although the uniform incorporation of niobium on mesoporous silica favored adsorption capacity, this condition notably diminished the photodegradation effect. This is not the case in the samples containing 10% Nb, where the reverse is true, *i.e.*, better niobium dispersion over the mesoporous silica indicates a greater photodegradation effect. This shows that it is necessary to conduct more systematic studies on both the niobium percentages in the sample and the uniformity of niobium dispersion, in the context of the effect of these on photodegradation capacity. Despite the fact that the 30% Nb-MCM-41 sample contains more niobium, as was determined in SEM-EDS, it may be that some of the Nb is not available to take part in photocatalysis, for example the Nb can be deposited in the zeolite porosities. Furthermore, as the amount of niobium in photocatalysts is increased, the surface area decreases, which indicates that there are fewer active sites available.

What is indeed evident in the results obtained is that the solids synthesized using the sol-gel method presented a greater photodegradation response than those obtained using the incipient impregnation method (Figures 10 and 11). This behavior could be justified considering that the pores observed in the TEM micrographs (Figure 8) of solids from the sol-gel method are more uniform than those observed in the incipient impregnation samples. This characteristic would provide a bigger surface area in the former samples, improving the photocatalytic process.

**Figure 10.** Degradation percentage of methylene blue, in function of irradiation time (sol-gel method).

**Figure 11.** Degradation percentage of methylene blue *vs.* irradiation time (incipient impregnation method).

The photocatalysts studied in this work can be considered, mainly those obtained by impregnation as a result of their behavior and the results obtained, as another type of photocatalyst, *i.e*., "single-site photocatalyst". In general, this single-site photocatalysis contains isolated polyhedral coordination transition metal oxides such as the oxides of Ti4+, V5+, Cr6+ and Mo6+ as active sites [30]; in this study, Nb5+ oxide. These species were designed on a mesoporous molecular MCM-41 surface and were considered to be highly dispersed at the atomic level on its surface (mainly in the samples obtained by impregnation) or within the frameworks and to be well-defined active sites. These local structures of our single-site Nb photocatalysts could explain the results obtained using Raman spectroscopy (Figure 2) and the behaviour of photocatalysts in the reduction of methylene blue (Figures 10 and 11). The photocatalytic properties of single-site photocatalysts Nb-MCM-41 could be attributed to the ligand to metal charge transfer (LMCT) process of the isolated niobium oxide with tetrahedral coordination. Under light irradiation, the charge transfer excited triplet state of niobium oxide was formed and electron-hole pairs were localized in close proximity to its photo-excited states [30]. Single-site photocatalysts exhibit unique and fascinating photocatalytic performances based on the reactions of specific photo-excited species. This process cannot be carried out on bulk photocatalysts [30]. Finally, the method of synthesis is clearly important in obtaining the single-site photocatalyst Nb-MCM-41 and for its functionality.
