# Photocatalytic Phenol Degradation by Silica-Modified Titanium Dioxide
## 3 Results
### Photocatalytic Activity Test of TiO\({}_{2}\)/SiO\({}_{2}\) Composites on Phenol
In photocatalysts exposed to UV light, photo-oxidation is commenced by absorbing photons of a higher energy than the photocatalyst's bandgap. This results in an electron jump from the valence (e\({}_{\text{cb}}\)) to the conduction band (h\({}_{\text{vb}}\)). Consequently, holes and electrons are formed in the h\({}_{\text{vb}}\) and e\({}_{\text{cb}}\), respectively. These react with oxygen (O\({}_{2}\)) and water (H\({}_{2}\)O) from the environment, provided by the photocatalyst, respectively, to produce \(\bullet\)OH radicals. The \(\bullet\)OH radicals formed reacted with the metal and the phenol degradation process occurred [46; 47; 48]. Meanwhile, for unexposed photocatalysts to UV light (dark state), there were no photons (UV light) to activate the photocatalyst's performance; therefore, there was no photo-oxidation reaction. The process of the decrease in the phenol concentration was the adsorption process from the catalyst that became larger in capacity due to the large  
Figure 7: The normalized concentration of the photocatalytic phenol removal using TiO\({}_{2}\) P25 and various TiO\({}_{2}\)/SiO\({}_{2}\) ratios. The experiment was conducted using 50 mL phenol solution 20 mg.L\({}^{-1}\) and 75 mg catalyst. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Time (min)**} & \multicolumn{4}{c}{**Phenol Concentration (mg.L\({}^{-1}\))**} & \multicolumn{4}{c}{**Percentage of Removal (\%)**} \\ \cline{2-9}  & **P25** & **TiO\({}_{2}\)/SiO\({}_{2}\)** & **TiO\({}_{2}\)/SiO\({}_{2}\)** & **TiO\({}_{2}\)/SiO\({}_{2}\)** & **TiO\({}_{2}\)/SiO\({}_{2}\)** & **TiO\({}_{2}\)/SiO\({}_{2}\)** \\  & **1:1** & **3:1** & **7:1** & **1:1** & **3:1** & **7:1** \\ \hline
0 & 20.00 & 20.00 & 20.00 & 20.00 & - & - & - & - \\
60 & 19.09 & 15.70 & 16.11 & 16.78 & 4.60 & 21.48 & 19.45 & 16.11 \\
90 & 12.97 & 10.97 & 10.34 & 9.81 & 35.13 & 45.14 & 48.28 & 50.95 \\
120 & 9.81 & 7.90 & 6.62 & 6.41 & 50.95 & 60.52 & 66.91 & 67.94 \\
150 silica content in the photocatalyst, considering that silica is a good adsorbent with a large adsorption capacity [15]. From Table 9, it was observed that the addition of SiO\({}_{2}\) increased the percentage of phenol degradation. SiO\({}_{2}\) substrate was an adsorbent that provided an adsorption side to support TiO\({}_{2}\) through the adsorption process; therefore, more pollutants were degraded [49]. The efficiency of the phenol concentration reduction is seen in Figure 7. After testing for 180 min, it was observed that the TiO\({}_{2}\) P25 Degussa photocatalyst had a phenol reduction percentage of 61.5%, while the percentage of phenol reduction for the TiO\({}_{2}\)/SiO\({}_{2}\) composites with the variations of 1:1, 3:1, and 7:1 were 75.65, 89.41, and 96.05%, respectively. Furthermore, the highest percentage of phenol reduction was shown by the photocatalyst of the TiO\({}_{2}\)/SiO\({}_{2}\) composites with the 7:1 variation. This occurred because this variation had the smallest and most homogeneous particle size among the samples; therefore, this photocatalyst provided a larger surface area, which improved its performance in decreasing the phenol concentration. The efficiency of reducing the phenol concentration is seen in Figure 8.  
The kinetics of the photocatalysis was observed using the Langmuir-Hinshelwood first-order and second-order kinetic model. The first-order kinetics was calculated using [50]:  
\[\text{-dC}/\text{dt}=k_{1}\text{C}, \tag{4}\]  
where C represents the concentration of phenol (mg.L\({}^{-1}\)) and \(k_{1}\) denotes the first-order rate constant (min\({}^{-1}\)). Meanwhile, at the start of the reaction, t = 0 and Ct = Ci. Therefore, the equation below is obtained after integration:  
\[\text{ln}(\text{Ct}/\text{Ci})=-k_{1}\text{t}+\text{b}, \tag{5}\]  
where, Ci signifies the initial concentration of phenol in mg.L\({}^{-1}\), Ct represents the concentration (mg.L\({}^{-1}\)) of phenol in the solution at t min, b denotes a constant, and \(k_{1}\) (min\({}^{-1}\)) is the first-order rate constant.