2.2.3. Aromatic VOCs Abatement

Lyu et al. [43] tried to improve photocatalytic mineralization under UV irradiation (using four UV lamps at 8 W, 254 nm) of volatile organic compounds by developing a homojunction-adsorption layer on anatase TiO2. Injection and separation of photogenerated charge carriers can improve the mineralization efficiency of VOCs. Therefore, they used toluene as model VOC, and they grew microporous TiO2 onto the surface of anatase TiO2 to obtain a homojunction-adsorption layer, thus optimizing the adsorption ability and photoactivity of the catalyst for photocatalytic mineralization of the model VOC (toluene). Various techniques were used to analyze the physical properties and mineralization of toluene. Results showed that the growth of microporous TiO2 increased the surface area of the catalyst by 160% compared with anatase TiO2, as well as induced microporous structure. This enhanced the adsorption of toluene at a low concentration of 5.5 mg m−<sup>3</sup> and limited the release of the adsorbed organic compounds. The authors also reported that a homojunction occurred at the interface of microporous TiO2 and anatase TiO2, leading to improved separation of photogenerated charge carriers. They concluded that due to this enhanced adsorption ability and photoactivity, the mineralization efficiency of toluene over micropore-anatase TiO2 composite was 1.78 and 2.12 times higher than that over M-TiO2 and anatase TiO2, respectively.

Wang et al. [42] prepared the rGO-TiO2 composites through a modified refluxingsolvothermal method and used them as catalysts for photocatalytic degradation of single VOC (p-xylene and ethylene) and VOC mixtures (benzene, toluene, p-xylene) under simulated solar irradiation (using 250 W and 500 W xenon lamps). The authors reported that their developed catalyst had higher photocatalytic degradation activity for a single VOC and even 2.6 times higher activity for a VOCs mixture compared to commercial P25 TiO2. Also, the photodegradation efficiency of P25 decreased from 80% to 63.8%, while the synthesized catalyst kept its efficiency unchanged (around 93%). The improved performance of rGO-TiO2 was attributed to an enhanced separation efficiency of electron-hole, better light harvesting ability and increased VOC adsorption capacity.

Ji et al. [58] studied the photocatalytic oxidation of gaseous benzene using as photocatalyst mesoporous TiO2 prepared by one-step hydrolysis method and varying the calcination temperature. They observed that the calcination temperature interferes with the catalytic activity of synthesized titania, also affecting its structural properties. The authors also used commercial P25 titania for comparison reasons, but this had poor efficiency and deactivated quickly, while their obtained mesoporous titania had a much better stability and photocatalytic activity for benzene abatement. They reported that samples calcined at 400 ◦C had the best photocatalytic activity. Additionally, the authors used VUV (vacuum UV) irradiation and observed that it enhances benzene degradation, obtaining a removal efficiency of around 80%, while under UV irradiation (2 × 4 W, 254 nm, Cnlight) was just 10%. They concluded that this procedure, where VUV photolysis together with ozone generated from the VUV irradiation (2 × 4 W, Cnlight), is suitable for benzene degradation at room temperature if mesoporous titania is used.

Fiorenza et al. [59] developed ZnO-based photocatalysts for various VOCs mineralization. For the solar photocatalytic tests, they used an irradiation source, a solar lamp (Osram Ultra Vitalux 300 W, irradiance of 10.7 mW/cm2). They synthesized core-shell ZnO@Au NPs and used them for gas-phase oxidation of toluene (C7H8) (but also of formaldehyde (CH2O) and ethanol (C2H5OH)), obtaining 95% conversion for toluene and 85% conversion for formaldehyde resulting only in water and CO2 as by-products. The same catalyst used in the photooxidation of ethanol also performed well, leading to a conversion of almost 60% having as by-products acetaldehyde with its subsequent oxidation to CO2 (attaining up to 72% selectivity to CO2). When performing the stability of the catalysts, the authors reported very good stability even after five consecutive runs. In the end, they concluded that the performance of the developed catalysts was due to the interaction between the zinc shell and gold core that prevented Au agglomeration, thus improving the photo-stability of the material and total VOC oxidation.

Nanodiamond (ND)-decorated zinc oxide photocatalysts were developed by Liu et al. [60] and used for the photooxidation of toluene utilizing UV-365 irradiation. They prepared various types of ZnO with different exposed crystal faces in order to evaluate the performance of the catalysts and the differences in activity. They observed that bare nanodiamond decorated ZnO having a higher amount of active (0001) exposed crystal faces obtained a total toluene removal in 2 h, the active species being superoxide radicals and photogenerated holes. After performing DRIFT analysis, the authors also explained a decomposition pathway for toluene; namely, toluene reacts with •O2 − generating benzoic acid and benzaldehyde, and the benzoic acid is oxidized to oxalic acid and decomposed to carbon dioxide and water. In the end, the authors proposed a mechanism for toluene photodegradation by nanodiamond decorated zinc oxide catalyst (Figure 7). Thus, zinc oxide and nanodiamonds can generate e<sup>−</sup> and h+ if excited under UV light. After obtaining the ND-ZnO, the electrons can pass from the ND to ZnO and can form an electric field. Afterward, the h<sup>+</sup> transfers from ZnO to ND and reacts to toluene. At the same time, e<sup>−</sup> from ND transfers to zinc oxide and interacts with O2 forming •O2 −, thus mineralizing gaseous toluene and intermediates. As the valence band edge potential of the mentioned photocatalyst is more negative than the redox potential of •OH/H2O, there were no •OH radicals generated during the process. Therefore, the photo-generated h+ could have contributed directly to the reaction of toluene oxidation.

**Figure 7.** Proposed mechanism for photocatalytic degradation of toluene upon ND-decorated ZnO photocatalysts. Reproduced with permission from ref. [60]. Copyright 2019 Elsevier.

Zhang et al. [61] reported the preparation of Ag/ZnO/nBC photocatalyst consisting of zinc oxide, cellulose nanocrystal-derived nano biochar (nBC) and silver nanoparticles, which was used in the photodegradation of various volatile organic compounds using a 300 W xenon lamp. They observed that VOC molecules are adsorbed on the surface of the mentioned catalyst via oxygen-containing groups (–OH, –C=O, and –CO), thus obtaining high photocatalytic efficiencies for the degradation of methyl alcohol (92%), acetone (81%), formaldehyde (89%) and phenol (90%).
