# Decolorization of Orange-G Aqueous Solutions over C\({}_{60}\)/Mcm-41 Photocatalysts
## 3 Results and Discussion
### Decolorization through Photocatalytic Splitting of Azo-Bonds  
Figure 4 shows the absorption spectra of an OG-AA aqueous solution in contact with the 3C60/MCM-41 sample recorded in the range 350-600 nm at various irradiation times after equilibration in the dark. One can observe additional decrease of the absorbance in the whole range of the spectra with the irradiation time which could easily be attributed to the photocatalytic degradation of the azo dye. Figure 3 (right part) illustrates the photocatalytic decolorization kinetics obtained over two samples with different C60 loadings (3 and 9 wt % C60) when the photoreactor lamp was switched on. One may observe that in both cases irradiation induces further reduction of the absorbance due to the photocatalytic decomposition of Orange G via the azo-bonds splitting. However, this is not the  
Figure 4: Absorbance spectra of OG-AA aqueous solution in contact with the 3C60/MCM-41 photocatalyst at various irradiation times after equilibration in the dark. case in the absence of ascorbic acid (Figure 3). This reveals the key role of the ascorbic acid in the photocatalytic degradation mechanism of Orange G.  
In order to further study decolorization through the photocatalytic splitting of azo-bonds, we determined the rate constants (\(k_{\text{R}}\)) of photocatalytic reaction for the samples studied. Assuming that the concentration of O\({}_{2}\) in the reaction mixture remains constant and equal to saturation due to the continuous feed with air, a pseudo-first-order photoreaction with respect to the Orange G (Equation 3) could be easily derived [10; 39; 40]. \[-ln\left(\frac{A}{A_{0}}\right)=k_{\text{R}}t, \tag{3}\]  
where A\({}_{0}\) and A stand, respectively, for the absorption at zero time and irradiation time t. The linear plots, -ln(A/A\({}_{0}\)) vs. t, obtained for all the samples studied corroborated the aforementioned assumption. Correlation coefficients (R\({}^{2}\)) greater than 0.93 have been obtained in all cases. An example, over the sample 9C\({}_{60}\)/MCM-41, is illustrated in Figure 5.  
The calculated photocatalytic reaction rate constant values are compiled in Table 1. These values are in good agreement with those reported in the recent literature for similar reaction conditions [39; 40; 41; 42; 43]. The \(k_{\text{R}}\) value was initially increased with the C\({}_{60}\) loading of the photocatalysts approaching its maximum over the 3C\({}_{60}\)/MCM41 sample. Then it was decreased considerably up to the sample 6C\({}_{60}\)/MCM41 and afterwards was slightly decreased with the fullerene content. The relatively high value for the photocatalytic reaction rate constant obtained over the sample 3C\({}_{60}\)/MCM41 can be effortlessly attributed to the very small C\({}_{60}\) clusters, not detectable by XRD, supported on MCM-41 [31]. These clusters, in effect, exhibited also very high intrinsic photocatalytic activity in other photocatalytic reactions as in the olefins photooxidation [31]. By increasing the C\({}_{60}\) content from 3 to 6 wt % the C\({}_{60}\) clusters size is increasing [32], bringing about a significant decrease in the intrinsic activity. The further decrease in the value of the rate constant determined over the photocatalysts with C\({}_{60}\) loading 9 and 12 wt % (Table 1) could be attributed to the additional size increase of C\({}_{60}\) clusters and therefore to the decrease of their intrinsic activity. However, the decrease of reaction constant value observed as the C\({}_{60}\) loading of the catalysts increases from 6 to 12 wt % is relatively low. This could be attributed to the partial compensation of the intrinsic activity decrease by the increase of photo sensitizer content per gram of catalyst. The high values obtained for the specific adsorption of Orange G and the specific rate of photocatalytic splitting of azo-bonds over the sample 3C\({}_{60}\)/MCM-41 justify the relatively high value obtained for the percentage decolorization of the Orange G aqueous solutions (Table 1).