# Decolorization of Orange-G Aqueous Solutions over C\({}_{60}\)/Mcm-41 Photocatalysts
## 3 Results and Discussion
### Interesting Characteristics of Decolorization Procedure  
In order to investigate some details of the decolorization procedure, we have monitored the change with time in the absorption bands at 330 and 250 nm assigned to the naphthalene and benzene (interfered by AA) rings, respectively (Figure 6). Inspection of Figures 3 and 6 shows that similar kinetics are followed concerning the decrease of the intensity of the absorption band at 475 nm with those observed for the absorption bands at 330 and 250 nm over the sample 3C\({}_{60}\)/MCM41 taken as an example. This is valid in the dark and under irradiation as well. Concentrating our attention to the experiments taking place in the absence of light, we observe that the normalized absorption is decreasing from 1 to about 0.43-0.45 at t=200 min for the three bands at 475, 330 and 250 nm assigned, respectively, to azo-bonds, naphthalene, and benzene (and ascorbic acid) rings. This suggests that the adsorption of Orange G on the solid surface is not accompanied by its decomposition and selective adsorption of some of the resulting molecules. The relatively faster initial decrease of the absorbance at 250 nm ascribed to benzene ring of the azo-dye and to AA could be attributed to the lower molecular weight of the latter (about 2.5 times lighter than OG) which facilitates its mass transfer from the solution to the catalyst surface. Concerning the experiments taken place under irradiation, similar kinetics is observed for the decolorization (absorbance decrease at 475 nm, Figure 3) and absorbance decrease at 330 and 250 nm attributed, respectively, to naphthalene rings and to the benzene ring plus AA (Figure 6). In both cases, indeed, the normalized absorption is decreasing to 0.2-0.25 after 140 minutes of irradiation. This presumably suggests full destruction of Orange G, not only the simple cleavage of azo-bonds, as well as of the ascorbic acid. According to our adsorption and photocatalytic kinetic results, it seems that adsorption is the rate determining step (k\({}_{\text{R}}\)>>k\({}_{\text{A2}}\)), see Table 1. This finding probably explains why complete decolorization has not been achieved in the reaction time for which photocatalytic degradation of OG has been studied (Figure 3). As a critical point in relevant studies, the reusability of the photocatalysts was developed, five subsequent cycles of OG photo-destruction were performed over the same sample of 3C\({}_{60}\)/MCM-41 catalyst, which showed the highest photocatalytic activity. The catalyst was thoroughly washed with triple-distilled water and dried at 110 \({}^{\circ}\)C overnight between the subsequent photocatalytic experiments. The results presented in Figure 7 reveal that this catalyst is quite stable losing less than 5% of its initial activity after five cycles. Figure 6: Decrease of absorbance decrease determined at 330 and 250 nm with time over the sample 3C\({}_{60}\)/MCM41. The absorbance values are normalized over the absorbance at zero time. Taking into account our experimental findings, as well as the relevant literature [20; 44; 45], we can propose the following tentative reaction network for the photocatalytic degradation of OG over the C\({}_{60}\)/MCM-41 catalysts. Irradiation causes the excitation of C\({}_{60}\) from the fundamental singlet state, \({}^{1}\)C\({}_{60}\), to the excited singlet state \({}^{1}\)C\({}_{60}\)*. It is very probable that this state is transformed to the excited triplet state, \({}^{3}\)C\({}_{60}\)*, by intersystem crossing [46]. Electron transfer occurs from the \({}^{3}\)C\({}_{60}\)* to OG, being in the hydrozone form, adsorbed on catalyst surface and from AA to the \({}^{3}\)C\({}_{60}\)*.This electron circulation activates the N-N bond and causes the decomposition of hydrozone structure towards 7-hydroxy-8-(hydroxymethyl)naphthalene-1,3-disulfonic species and aniline. These intermediate products are further mineralized producing SO\({}_{4}\)\({}^{2-}\), NO\({}_{3}\)-, NO\({}_{2}\)-, NH\({}_{4}\)\({}^{+}\), and CO\({}_{2}\)[10].