*2.6. Evaluating Reutilization Capabilities of Nb2O5 Catalyst Aiming for Sustainable Implementation and Comparison with Other Catalysts' Performance*

Sustainable processes must consider the continuous reutilization of photocatalytic materials within catalytic converters [44]. Reutilization capabilities and stabilities of novel materials should be assessed in order to demonstrate material stability. Therefore, Nb2O5 was submitted to continuous treatment operation. Suspended Nb2O5 photocatalyst was recovered and tested over consecutive decolorization cycles of MO solutions. As depicted in Figure 10, the niobium metal oxide semiconductor presented excellent stability, and retained its catalytic properties over 10 cycles. Please note that consecutive cycles demonstrated reproducible MO removal performance. It is important to remark that previous reports in the literature reported higher stability of niobium oxides than for conventional TiO2 or ZnO catalysts [9,27,28]. In this context, the results reported here support these previous studies, indicating Nb2O5 as an emerging photocatalyst for environmental remediation. However, more studies related to catalyst aging and fouling during continuous operation are required to ensure the lifetime of these emerging catalysts.

**Figure 10.** Percentage of color removal achieved after 40 min of solar photoelectrocatalytic treatment of 5 mg L−<sup>1</sup> of MO with 1.0 g L−<sup>1</sup> of catalyst Nb2O5 in slurry at pH 5.0 with 0.20 M of H2O2 after consecutive reuse of Nb2O5.

A quick comparison with other photocatalytic materials reported in the literature demonstrates the promising performance of novel niobium oxide semiconductors for water treatment applications. As summarized in Table 2, the results reported in this work are highly competitive, since Nb2O5 makes it possible to reduce operational times by more than half when compared with doped TiO2 and other complex mixed oxides.

**Table 2.** Comparative performance of visible photocatalytic decolorization of Methyl Orange azo dye with different catalysts to attain over 95% color removal.


### **3. Materials and Methods**

#### *3.1. Chemicals*

MO azo dye of 85.0% purity and the H2O2 of 33% (*w*/*w*) were supplied by Sigma-Aldrich. The ammonium salt of the niobium oxalate complex (NH4[NbO(C2O4)2(H2O)]. 3H2O) of 99.0% purity used in the photocatalyst synthesis was purchased from CBMM. The pH was adjusted prior to experiments using H2SO4 or NaOH of analytical grade, supplied by Sigma-Aldrich. All solutions were prepared with high-purity water obtained from a Millipore Milli-Q system with resistivity >18 MΩ cm at 25 ◦C.
