# Catalytic Degradation of Anionic Organic Dye on Greenly Synthesized CuO/ZnO Nanocomposites
## 3 Results and Discussion
From the figure, it is apparent that the pH\({}_{\text{ZPC}}\) of Z, 1C3Z, 1C1Z, 3C1Z, and 1C are 7.0, 7.3, 7.5, 7.2, and 6.8, respectively. These values imply that, for a solution with a pH of higher than 7.2, all the samples are expected to be deprotonated and their surfaces will become negatively charged. On the contrary, if these samples are placed in a solution with a pH of less than 6.8, they will be protonated with a positively charged surface. This concept is very important for any surface phenomena such as adsorption or heterogeneous catalytic reactions, since the first step is expected to be the diffusion of the target molecule, which is MO, to reach the active sites on the catalyst surface; since MO dye is known to be an anionic dye that forms anions in aqueous solution, it is expected that decreasing the pH of the solution to a lower value than the pH\({}_{\text{ZPC}}\) would enhance the degradation process by increasing the rate of reaching MO to the surface under the effect of electrical attraction. Accordingly, all the degradation tests were performed at a pH of 4.5, as will be discussed next. It is expected that under this pH value (4.5), the most positively charged surface will be 1C1Z, and this is maybe one reason for the best degradation efficiency obtained by this nanocomposite, compared to the other samples. Degradation results shown in Figures 8 and 9 suggest that the main two parameters affecting the degradation efficiency of the prepared samples are pH\({}_{\text{ZPC}}\) and the content of CuO. For samples Z, 1C3Z, and 1C1Z, there was a gradual increase in the degradation efficiency as a result of increasing both the CuO% and pH\({}_{\text{ZPC}}\). For samples 3C1Z and C, the degradation efficiency started to decline despite the increase in the CuO%. This may be related to the decrease in the pH\({}_{0}\) for these two samples. Figure 10 shows that for the five nanocomposites, gas bubbles appear after adding the nanocomposite to the MO solution, which indicates the formation of CO\({}_{2}\) gas as a result of MO degradation, as reported by other research groups [22; 23]. As the time passed, the number of gas bubbles decreased, as shown in the third row of Figure 10, indicating that the rate of reaction slowed over time. This is mainly because the concentration of MO declines over time as a result of degradation; accordingly, the driving force for the diffusion of MO from the bulk of the solution to the surface of the catalyst becomes smaller, especially as the tests were performed stagnantly without any stirring, leading to a smaller amount of MO  
Figure 7: pH of zero point charge of Z, 1C3Z, 1C1Z, 3C1Z, and C greenly prepared via hydrothermal route (pH = 10, T = 250 \({}^{\circ}\)C, t = 2.5 h) and calcination for 2 h at 300 \({}^{\circ}\)C.  
adsorbed on the surface of the catalyst and, thus, a slower rate of degradation. Figure 10 also shows the change in solution color as a result of degradation. To confirm the rule of the nanocomposite in the degradation process, a blank test (B) was performed by mixing the MO solution with H\({}_{2}\)O\({}_{2}\) and adjusting the pH at 4.5. As shown in the first column of Figure 10, there was no change in the solution color after 3 h or 24 h, indicating that no degradation took place. Another test was performed to confirm that catalytic degradation took place, rather than adsorption. This test was performed by mixing the MO solution with the nanocomposites without adding H\({}_{2}\)O\({}_{2}\), and the pH was adjusted at 4.5. The obtained results showed no decrease in the concentration of MO. Figure 8 confirms the same thing through the absorbance of UV at different intervals (the highest spectra of each figure represents the absorbance at time zero, before adding the catalyst, and each lower one represents the absorbance after 10 min, until the end of the experiment). There is no change in the absorbance even after 24 h. Another experiment was performed using 1C1Z nanocomposite without controlling the pH at 4.5, as shown in Figure 10, column 2.