## 3 Results and Discussions
### Catalytic Experiments  
A comparison between the RhB degradation over MnO\({}_{2}\) under solar irradiation and in dark conditions at pH = 3 is emphasized in Figure 10. After two hours of reaction, the degradation of RhB reached 74% under dark conditions. Enhanced results were obtained under solar irradiation so that only after 30 min did it reach the degradation of 74% and the maximum degradation of 97.2% was obtained after 90 min, with 50% higher than the degradation resulting under dark conditions. As the RhB aqueous solution was stable under solar irradiation, the reaction occurred because of the catalytic and respectively photocatalytic activities of MnO\({}_{2}\). In the time-dependent visible spectra obtained for the RhB aqueous solution during the catalytic and photocatalytic reactions over \(\alpha\)-MnO\({}_{2}\) (Figure 11), it can be noticed that major changes in shape have occurred for the absorbance peaks with the maximum absorption at 556 nm and respectively at 498 nm. As shown in the inset of Figure 11, the rhodamine B solution displayed a large band in the visible region with the maximum absorbance at 553 nm, which can be ascribed to \(\pi\)\(\rightarrow\)\(\pi^{\star}\) electronic transitions of the dye monomer and a shoulder at 515 nm corresponding to dimers [71; 72]. In the acidic medium, a redshift occurred, from \(\lambda_{\text{max}}\) = 553 nm to \(\lambda_{\text{max}}\) = 556 nm, that was due to the carboxylic group dissociation as a result of interaction between RhB and the acid [73; 74]. During the catalytic reaction, it can be observed that the characteristic peak of RhB shifted to lower wavelength values (\(\lambda_{\text{max}}\) around 539 nm) simultaneously with the appearance of a  
Figure 10: The degradation of RhB at pH = 3 is expressed by the A/A\({}_{0}\) ratio in the dark (marked with \(\bullet\)), under solar irradiation (marked with \(\blacktriangle\)) in the presence of a catalyst, and under solar irradiation without a catalyst (photolysis, marked with \(\blacksquare\)). peak at \(\lambda_{\rm max}\) = 498 nm. This suggests that the RhB catalytic degradation mechanism followed a deethylation pathway and the presence of the absorbance bands at \(\lambda_{\rm max}\) = 541 nm (t = 80 min) and \(\lambda_{\rm max}\) = 498 nm can be attributed, as reported in the literature, to the N-deethylated intermediates: N,N-diethyl-N'-ethyl-rhodamine 110 (DMRh) and rhodamine 110 (Rh110) [75, 76, 77]. The hyposochromic shift, the decrease in the intensity of the RhB characteristic absorbance band, and the formation of peaks at \(\lambda_{\rm max}\) around 513 nm and 494 nm (Figure 12), which are associated with the deethylation process, as in the case of the catalytic reaction process, occurred also for the photocatalytic reaction of RhB degradation over the \(\alpha\)-MnO\({}_{2}\). The multiple bands corresponding to the N-deethylation byproducts overlapped during the evolution of the photocatalytic reaction and the intermediates identified based on the visible absorbance spectrum analysis by scientific literature were N,N-diethyl-N'-ethyl-rhodamine 110 (\(\lambda_{\rm max}\) = 540 nm, t = 30 min), N-ethyl-N'-ethyl-rhodamine 110 (\(\lambda_{\rm max}\) = 529 nm, t = 90 min), and rhodamine 110 (\(\lambda_{\rm max}\) = 494 nm) [75,78]. As represented in Figure 13, the hyposochromic shift and the absorbance intensity decrease are higher in the case of the photocatalytic reaction considering that the maximum  
Figure 11: The peak shape evolution of RhB aqueous solution overtime during the reaction of (**a**) catalytic and (**b**) photocatalytic RhB degradation; inset—the redshift of the absorption spectrum for the initial RhB aqueous solution with a variation of the solution pH. Figure 12: The absorption spectrum evolution during photocatalytic RhB degradation with emphasis on the RhB degradation byproducts corresponding to absorbance peaks. hypochromic shift and the intensity decrease were 26 nm and 98% under light irradiation and, respectively, 20 nm and 57% under dark condition.