## 3 Results and Discussion
### Photocatalytic Performance
These defects can act as recombination centers of electron-hole pairs [23], which decrease its photoactivity. The increase in \begin{table}
\begin{tabular}{l l l} \hline \hline
**Sample** & **k \(\times\) 10\({}^{-3}\) (min\({}^{-1}\))** & **k/S\({}_{BET}\)\(\times\) 10\({}^{-4}\) (min\({}^{-1}\) g m\({}^{-2}\))** \\ \hline Nb\({}_{2}\)O\({}_{5}\) & 1.68 \(\pm\) 0.071 & 14 \\ Nb\({}_{2}\)O\({}_{5}\):Mn 1.0 wt.\% & 0.57 \(\pm\) 0.032 & 3.1 \\ Nb\({}_{2}\)O\({}_{5}\):Mn 2.5 wt.\% & 0.92 \(\pm\) 0.045 & 2.9 \\ Nb\({}_{2}\)O\({}_{5}\):Mn 5.0 wt.\% & 1.38 \(\pm\) 0.136 & 2.0 \\ Nb\({}_{2}\)O\({}_{5}\):Mn 10.0 wt.\% & 0.78 \(\pm\) 0.035 & 0.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Exchange rate constant obtained from the Rhodamina B Photodegradation tests. Figure 10: Kinetic graphs of Rhodamine B photodegradation, for the pure sample and with the different percentages of the addition of Mn at 700\({}^{\circ}\) C.  
the specific surface area generates an increase in the number of active sites on the material surface, which increases the kinetic rate of the process [23]. Thus, the material can be standardized by surface area to reveal the photoactivity associated with the material properties. A suitable approach consists on dividing the k value of the process by \(S_{BET}\) of each sample (Table 4). The unit cell volume value obtained for 10.0% by weight is similar to the unit cell volume of Nb\({}_{2}\)O\({}_{5}\), which may indicate that a part of Mn was segregated in a secondary phase. The content of 10.0% by weight of Mn promotes the production of smaller nanoparticle agglomerates, this is in accordance with the pore size estimated by the BET method. The increase in Mn content established a defined morphology with a tendency to decrease in particle size and gap value, however, there is an increase in the specific surface area. We believe that the low photocatalytic performance is due to the loss of crystallinity with increasing Mn concentration. On the other hand, the IR results exhibited the presence of superficially adsorbed O-H molecules, which may have affected the photocatalytic response. According to other research in the literature, Activities of the Nitrogen-doped TiO\({}_{2}\) and TiO\({}_{2}\)-Nb\({}_{2}\)O\({}_{5}\) catalysts prepared by sol-gel technique have been evaluated on the photodegradation of benzene, toluene and xylenes solution under UV and visible light [25]. In that work, the \(N\)-doped TiO\({}_{2}\)-Nb\({}_{2}\)O\({}_{5}\) catalyst calcined at 500 \({}^{\circ}\)C showed promising catalytic activity in degrading the BTX in an aqueous solution under visible light. However, the N-TiO\({}_{2}\) catalysts showed the best result. Similar behavior was obtained in our work, where the Nb\({}_{2}\)O\({}_{5}\) had good photocatalytic performance. On the other hand, Zn-doped Nb\({}_{2}\)O\({}_{5}\) photocatalysts, obtained through the oxidant peroxide method, exhibited a diffraction pattern of Nb\({}_{2}\)O\({}_{5}\) orthorhombic-type phase [26]. These materials exhibited better crystallinity, high specific surface area and low band gap values, so zinc improved the photocatalytic activity of Nb\({}_{2}\)O\({}_{5}\) in Rhodamine B and caffe acid degradation. Hence, we attributed the low photocatalytic response of our materials at decreased crystallinity.