**3.2. Photocatalysis**

In advanced oxidation process, heterogeneous photocatalysis is a process with important potential for the degradation of recalcitrant organic contaminants in water. In this process, the use of a radiation source that generally is UV light and a semiconductor material as cata‐ lyst is necessary. The photon energy is converted into chemical energy which is capable of to degrade the organic pollutants. The photogenerated holes in the valence band diffuse to particle semiconductor surface and react with organic molecules present in aqueous solution forming hydroxyl radical (OH**·**). Meanwhile, electrons in the conduction band participate in reduction processes, reacting with molecular oxygen in the air to produce superoxide radical anions (O2 **·** ‐ ). Titanium dioxide (TiO2 ) is the most important semiconductor used in the het‐ erogeneous photocatalysis due to different properties such as superhydrophilicity, chemical stability, long durability, nontoxicity, low cost and transparency to visible light; however, other semiconductor materials have been used also in this process with good performance in the contaminants degradation from water. The size of the semiconductors particles is an important factor that has influence on the efficiency of the photocatalytic process. Others important factors in the photocatalysis are the specific surface area, pore volume, pore struc‐ ture, crystalline phase and the exposed surface. On the other hand, the charge separation is a problem that affects the efficiency of the photocatalytic process and whereby the improving of photocatalyts is a challenge. Thus, in this section, it is reviewed different works related to semiconductor particles and nanoparticles in the photocatalytic degradation of phenolic compounds. Also, we present some investigations associated with graphene nanomaterials focused to reduce the charges recombination in the photocatalytic degradation of phenolic compounds.

### *3.2.1. Semiconductor particles and nanoparticles*

Although TiO2 is the semiconductor more employed as catalyst in the heterogeneous photo‐ catalysis, at present, there exist others semiconductor materials that have been developed and employed in the degradation of pollutants from water obtaining good results. The reduction in the particle size is an important factor that can improve the photocatalytic performance due to an increase in the surface area. With the aim to improve the results on the degradation of contaminants from water, also, the combination of semiconductor particles has been devel‐ oped, obtaining composites and hybrids materials. A review of investigations about the deg‐ radation of phenolic compounds using semiconductor particles is presented.

Phenol is the phenolic compound more studied in the photocatalysis. Different semicon‐ ductor materials have been used in its degradation, but TiO2 is the semiconductor more used in the photocatalytics process. Ye and Lu [60] synthesized anatase TiO<sup>2</sup> nanocrystals with exposed {0 0 1} facets; these materials were obtained in the presence of fluoride ions. The photocatalytic results indicated that the oxidation of phenol increased with the rise in the percentage of {0 0 1} facets of TiO<sup>2</sup> . The main intermediates produced during the photocatalytic degradation of phenol were catechol and hydroquinone. The improving in the photocatalytic performance of TiO2 nanocrystales was attributed to the synergistic effects of the exposed {0 0 1} facets and surface fluorination. The TiO<sup>2</sup> doped have been used to improve the photocatalytic activity on the phenol degradation [61]. An important efficiency in the degradation of phenol was found. Best results of phenol degradation were found at high pH values. Composites of V<sup>2</sup> O5 /N,S–TiO<sup>2</sup> were used as photocatalyst for phenol degradation under direct solar light [62]. The sample of V<sup>2</sup> O5 /N,S–TiO<sup>2</sup> activated at 500°C showed the best photocatalytic performance, reaching a degradation of 88% of phenol solution (100 mg/L) in 4 h. The V<sup>2</sup> O5 component played a key role for the visible light activity of the composite system at longer wavelengths. The photocatalytic activity of the composite was mainly attributed to the acid sites present on the surface; however, other factors such as the surface area, anatase/rutile ratio and the absorption at longer wavelengths were important.

Definitely, the application of carbon materials on the removal of phenolic compounds by adsorption process is a good alternative for the remediation of the contamination problem of

In advanced oxidation process, heterogeneous photocatalysis is a process with important potential for the degradation of recalcitrant organic contaminants in water. In this process, the use of a radiation source that generally is UV light and a semiconductor material as cata‐ lyst is necessary. The photon energy is converted into chemical energy which is capable of to degrade the organic pollutants. The photogenerated holes in the valence band diffuse to particle semiconductor surface and react with organic molecules present in aqueous solution forming hydroxyl radical (OH**·**). Meanwhile, electrons in the conduction band participate in reduction processes, reacting with molecular oxygen in the air to produce superoxide radical

erogeneous photocatalysis due to different properties such as superhydrophilicity, chemical stability, long durability, nontoxicity, low cost and transparency to visible light; however, other semiconductor materials have been used also in this process with good performance in the contaminants degradation from water. The size of the semiconductors particles is an important factor that has influence on the efficiency of the photocatalytic process. Others important factors in the photocatalysis are the specific surface area, pore volume, pore struc‐ ture, crystalline phase and the exposed surface. On the other hand, the charge separation is a problem that affects the efficiency of the photocatalytic process and whereby the improving of photocatalyts is a challenge. Thus, in this section, it is reviewed different works related to semiconductor particles and nanoparticles in the photocatalytic degradation of phenolic compounds. Also, we present some investigations associated with graphene nanomaterials focused to reduce the charges recombination in the photocatalytic degradation of phenolic

is the semiconductor more employed as catalyst in the heterogeneous photo‐

catalysis, at present, there exist others semiconductor materials that have been developed and employed in the degradation of pollutants from water obtaining good results. The reduction in the particle size is an important factor that can improve the photocatalytic performance due to an increase in the surface area. With the aim to improve the results on the degradation of contaminants from water, also, the combination of semiconductor particles has been devel‐ oped, obtaining composites and hybrids materials. A review of investigations about the deg‐

Phenol is the phenolic compound more studied in the photocatalysis. Different semicon‐

with exposed {0 0 1} facets; these materials were obtained in the presence of fluoride ions. The photocatalytic results indicated that the oxidation of phenol increased with the rise

radation of phenolic compounds using semiconductor particles is presented.

used in the photocatalytics process. Ye and Lu [60] synthesized anatase TiO<sup>2</sup>

ductor materials have been used in its degradation, but TiO2

) is the most important semiconductor used in the het‐

is the semiconductor more

nanocrystals

water by this kind of compounds.

**3.2. Photocatalysis**

anions (O2

compounds.

Although TiO2

**·** ‐

). Titanium dioxide (TiO2

358 Phenolic Compounds - Natural Sources, Importance and Applications

*3.2.1. Semiconductor particles and nanoparticles*

The phenol degradation also was investigated by Liu et al. [63]. In this investigation, BiPO4 synthesized through hydrothermal process was used as photocatalyst and the process was assisted with H2 O2 . The initial concentration of phenol solution was 50 mg/L and the cata‐ lyst concentration was 0.5 g/L. The results indicated that the phenol could be mineralized after of 4 h with BiPO4 but no by H2 O2 . The efficiency of BiPO4 was attributed to the high potential photogenerated holes in the valence band and the high separation efficiency of electron hole pairs.

ZnO, other important semiconductor also have been used in the phenol degradation. Europium‐doped flower like ZnO hierarchical [64], Ni‐loaded ZnO nanorods [65], cerium‐ doped ZnO hierarchical micro/nanospheres [66] and ZnO nanosheets immobilized on mont‐ morillonite [67] are some of the ZnO‐based catalysts that have been employed successfully on the phenol degradation.

Although Al2 O3 is known as insulator material, Tzompantzi et al. [68] synthesized an Al<sup>2</sup> O3 by the sol–gel method. The Al2 O3 was dried and annealed at 400, 500, 600 and 700°C. The samples were tested in the degradation of phenol from water. The best results of degradation were obtained with the sample calcinated at 400°C. The photocatalytic activity of the Al<sup>2</sup> O3 can be due to the modification in the Al‐O bonds distances and hydroxyl groups present in the Al2 O3 structure, delaying the recombination process. The sample annealed to 400°C also was tested in the p‐cresol and 4‐chlorophenol degradation. The major degradation was obtained on 4‐clorophenol, followed by phenol and p‐cresol, in this order.

The degradation of 4‐chlorophenol also have been studied by different investigation groups. A composite of anatase/titanate nanosheet was employed by Liu et al. [69] for phenol degra‐ dation from water. Titanate acted as the main adsorption site. The phenol degradation was carried out in a binary system. About of 99% of phenol was degraded within 120 min. The important photocatalytic efficiency was attributed to the synergetic effect on the photo‐oxida‐ tion of 4‐clorophenol and photoreduction in Cr (VI) due to the efficient separation of electron‐ hole pairs. In other investigation of phenol degradation, Naeem and Ouyang [70] investigated the degradation of 4‐chlorophenol on TiO<sup>2</sup> supported on materials as activated carbon, silica


**Table 1.** Photocatalytic materials used on the removal of phenolic compounds.

(SiO2 ) and zeolite (ZSM‐5). All materials TiO<sup>2</sup> ‐supported reached a better photocatalytic per‐ formance on the 4‐chlorophenol degradation than TiO<sup>2</sup> alone. AC was found to be the best support followed by ZSM‐5 and SiO<sup>2</sup> . The maximum degradation of 4‐chlorophenol using TiO2 ‐AC as photocatalyst was 89.7%. Others photocatalytic materials that have been studied on phenols degradation in water are shown in **Table 1**.
