# Photocatalytic Phenol Degradation by Silica-Modified Titanium Dioxide
## 1 Introduction  
Phenolic compounds and their derivatives have been used as raw materials in various manufacturing and petrochemical industries [1]. Phenols contained in industrial waste products are considered to be the main source of pollution to the environment and are known to have low visibility and high stability. Like the majority of other organic substances used in industries, phenol is a carcinogenic compound that poses a high risk to human health and also damages aquatic ecosystems, even at low concentrations [2]. Furthermore, as a non-biodegradable pollutant, it tends to accumulate in an organism [3] and also forms different aromatic intermediates that are toxic, and thus makes it a serious threat [4]. Numerous techniques, including adsorption, precipitation, cross-flow microfiltration, electrodialysis, and reverse osmosis, have been used to actively investigate the removal of phenol from the environment. However, these methods are quite expensive and ofteninefficient at low concentrations. Therefore, a photocatalytic method was developed to serve as an alternative, using semiconductor materials with the potential for pollutant reduction. Photocatalysis is a process of accelerating reactions that is assisted by energy from light irradiation and a solid catalyst that is generally a semiconductor [5]. Semiconductor materials that have been used in photocatalysts for the degradation of organic pollutants in water include SnO\({}_{\text{x}}\)[6], ZnO [7], and TiO\({}_{2}\)[4]. Titanium dioxide (TiO\({}_{2}\)) is a semiconductor often used as a photocatalyst during wastewater purification. This compound is readily available and possesses various desirable properties, including high photocatalytic activity, oxidizing power, and chemical stability in acidic or alkaline conditions [8]. However, the single photoactivity of TiO\({}_{2}\) has the disadvantage of a low specific surface area and the occurrence of hole (h\({}^{+}\))/electron (e\({}^{-}\)) recombination, thus limiting the catalytic efficiency [9; 10]. Therefore, in previous studies, TiO\({}_{2}\) has been composited with various materials, such as carbon [11], WO\({}_{3}\)[12], Bi\({}_{2}\)O\({}_{3}\)[13], and SiO\({}_{2}\)[14]. SiO\({}_{2}\) was chosen as a composite for TiO\({}_{2}\) because it can increase the photocatalytic activity, thermal stability, mechanical strength, and active surface area [8]. In addition, SiO\({}_{2}\) can also stabilize the irreversible metastable progression from anatase to the rutile phase at 600-1100 \({}^{\circ}\)C and facilitate the transfer of TiO\({}_{2}\) to the adsorbed molecule by increasing the electron oxidation capability [15; 16]. In addition, it may be synthesized using a variety of methods, which includes solvothermal [17; 18], sol-gel [15; 17], impregnation [19; 20], and microwave radiation [21]. In our previous study [22], the synthesis of this composite photocatalysts was carried out using the solvothermal method, which was applied to reduce the concentration of inorganic waste, specifically a solution of Cr(VI) and Pb(II) metal ions. The results showed a fairly good percentage reduction of 93.77% and 93.55% for Cr(VI) and Pb(II), respectively. Furthermore, the silica successfully extracted from Bengkulu beach sand, Indonesia, using a potassium hydroxide (KOH) solvent, reached 90%, with a purity of 99.5% [23], while using a sodium hydroxide (NaOH) solvent, the purity was 97.3% [24]; the purity of silica obtained from Palangkaraya beach sand, Indonesia, was 91.19% [19]. In this study, we synthesized TiO\({}_{2}\) and composited it with natural silica. The crystal of TiO\({}_{2}\) (P25 Degusssa) is a standard material for photocatalytic reactions and was used as a precursor of TiO\({}_{2}\). In addition, the SiO\({}_{2}\) source used was natural silica extracted from the Bengkulu beach [24]. TiO\({}_{2}\) and SiO\({}_{2}\) were composited using the impregnation method, different from the previous literature [22], and which used the solvothermal method.