2.1.1. Basic Data on Methanol Photodegradation on TiO2

In what comes, we should list the key factors controlling the photomineralization of methanol on different engineered materials. The methanol degradation pathway is related to methanol surface coverage on photocatalysts as well as to the density of surface hydroxyl groups and adsorbed oxygen. For example, by exposing the adsorbed methanol on anatase (101) to UV irradiation, Setvin et al. [11] observed that the main reaction products are formaldehyde (CH2O) and methyl formate (CH3−O−CHO), formed by distinct reaction pathways. The first results from methanol interaction with the co-adsorbed oxygen or terminal OH− groups leading to methoxy anion formation:

$$\mathrm{CH\_3OH + OH^- \to CH\_3O^- + H\_2O} \tag{5}$$

By accepting a hole, methoxy radical is generated:

$$\mathrm{CH\_3O^- + h^+ \to CH\_3O\bullet}\tag{6}$$

This is further converted to formaldehyde. On the other hand, methyl formate (CH3−O−CHO) is obtained at high methanol surface coverages. Shen et al. [12] reported an increased photoactivation rate of methanol on annealed rutile (110) surface, facilitating both oxygen and methanol adsorption. The authors identified the thermally activated cleavage of the O−H bond as being the first step, followed by the photo-catalytically driven C−H bond cleavage. Also, the surface defects of TiO2 photocatalyst proved to increase the photo dissociation rate of methanol [13]. Shen et al. [8] showed that the methoxy group, formed on the catalyst surface, acts as a more efficient hole scavenger compared to the molecular methanol. On the methanol-saturated surface of rutile nanoparticles exposed to UV irradiation [14], the conversion of the methoxy group to formate, requiring two photoelectrons, is enhanced in the presence of oxygen. In this case, oxygen also acts as the main electron scavenger, competing with methanol oxidation.

A great number of semiconductor materials have been developed for the removal of organic pollutants from air and water by photomineralization. Noble (Au, Pt, Ag, Pd) and d-(Cu) metals deposition on TiO2 is typically used to extend the light absorption range of TiO2 and ZnO [15–17] but to bust the photocatalytic performances via better charge separation at Schottky domains created at metal/semiconductor interfaces [18–20]. The great majority of works concerning the photooxidation of alcohols (methanol) advance the following mechanistic pathways: (a) direct hole oxidation route and/or (b) oxidation intermediated by hydroxyl radicals (•OH) resulting from the following reactions:

h+ + -OH (surface hydroxyl) → •OH (7)

$$\text{(O}\_2 + \text{e}^- \rightarrow \text{(O}\_2\text{}^-)\_\text{ads} \text{ (on catalyst surface)}\tag{8}$$

$$\text{H} \text{ (O}\_2\text{ $^{-}$ )}\_{\text{ads}} + 2\text{H}^+ + \text{e}^- \rightarrow \text{H}\_2\text{O}\_2 \rightarrow 2\bullet\text{OH} \tag{9}$$

When O2 is not able to reach the surface because of high organic coverage, the direct hole route is favored [21].

2.1.2. Methanol and Ethanol Oxidation in Gaseous Phase on Bare and Modified Catalysts

Despite the great number of papers focusing on the photooxidation of methanol, only a few of them discriminate between degradation (oxidative degradation to intermediates) and mineralization (oxidation to CO2). El-Roz et al. [22] carried out a mechanistic study using an operando-FTIR system coupled with gas phase analysis techniques (gas-IR and MS) on TiO2 P25 photocatalyst. The authors found that methanol concentration is a key factor in tailoring oxidation selectivity. Under light irradiation of 365 nm, the maximum methanol conversion to CO2 and H2O was observed for 500 ppm CH3OH in the gas phase (20% O2/Ar), according to the reaction:

$$\text{CH}\_3\text{OH} + \text{3/2O}\_2 \rightarrow \text{CO}\_2 + \text{3H}\_2\text{O} \tag{10}$$

For 1200 ppm CH3OH in the gas phase, the favored product was methyl formate:

$$2\text{CH}\_3\text{OH} + \text{O}\_2 \rightarrow \text{CH}\_3\text{OCHO} + 2\text{H}\_2\text{O} \tag{11}$$

According to the authors, the oxidation sequence starts with the dissociative chemisorption of methanol, leading to the formation of surface methoxy groups which are oxidized to formate and finally to CO2, the last step being considered as rate limiting for the methanol photooxidation. Additionally, methyl formate secondary product can result from a reaction between adsorbed formate and methoxy group.

Photocatalytic oxidation of methanol in visible light over AuNPs modified WO3 was studied by DePuccio et al. [23] using a continuous flow gas-phase reactor. Distinct photocatalytic tests performed comparatively on AuNPs/SiO2 and bare WO3 indicated that the surface plasmon resonance (SPR) phenomenon (induced by AuNPs) and the band gap excitation of WO3 are responsible for two distinct mechanisms involved in methanol oxidation. One mechanism is responsible for methyl formate generation, whereas the other leads to the formation of formaldehyde. When AuNPs are deposited on WO3, the SPR triggered by light absorption enhances the separation of the photogenerated charges by WO3, increasing thus the photocatalytic activity. Over bare WO3, the formation of CO2 was not observed, contrasting with Au/WO3, where CO2 was the major reaction product (39%).

Ethanol photocatalytic oxidation is also highly important for depollution applications since it largely emerges from industrial activity. In addition, it is noteworthy to investigate the differences in photoreactivity brought by the carbon–carbon bond in comparison to methanol.

Muggli et al. [24] studied the photocatalytic oxidation of ethanol using transient reaction techniques and isotope labeling. The Carbon-13 labeled ethanol (CH3 13CH2OH) adsorbed on Degussa P-25 was exposed to light (maximum intensity ≈ 390 nm) and 0.2% O2 in He. The CO2, H2O, and acetaldehyde formation have been observed for ethanol saturation coverage, whereas for lower coverage, only CO2 and H2O were obtained. The α-carbon of ethanol proved to be preferentially oxidized. This finding is also certified by the resulting intermediates (acetaldehyde and acetic acid) [25]. The reactions below depict the whole process.

$$\text{Ethanol-derived Acetaldehyde}\_{\text{(ads)}} \rightarrow \text{(a)} \& \text{(b)}\tag{12}$$

	- (b) → formic acid + formaldehyde → formic acid → CO2 (slow) (14)

According to the authors, the presence of acetaldehyde on the catalyst surface, together with other intermediates, decreases the reactivity of (b). The enhancement of the acetaldehyde oxidation rate is required to increase the overall CO2 formation.

Yu et al. [26] studied the photocatalytic conversion of ethanol to CO2 in the presence of O2 by using in situ infrared (IR) spectroscopy. The surface coverage of adsorbed H2O and ethanol on P25 Degussa was identified as a key parameter in deciding the favored reaction pathway. The involved oxidizing species involved in the reaction are formed according to the following equations:

$$\text{TiO}\_2 + \text{hv} \rightarrow \text{h}^+ + \text{e}^- \tag{15}$$

$$\rm{H\_2O\_{ads}} \rightarrow \rm{OH^-} + \rm{H^+} \tag{16}$$

$$\bullet \text{OH}^- + \text{h}^+ \rightarrow \bullet \text{OH} \tag{17}$$

$$\text{O}\_{2\text{ads}} + \text{e}^- \rightarrow \text{O}\_2{}^- \tag{18}$$

$$\text{2O}\_2^{-} + 2\text{H}\_2\text{O}\_{\text{ads}} \rightarrow 2\bullet\text{OH} + 2\text{OH}^- + \text{O}\_2 \tag{19}$$

The authors summarized the ethanol photo-oxidation by using h<sup>+</sup> and •OH as it comes:

$$\text{(CH}\_3\text{CH}\_2\text{OH}\_{\text{ad}} / \text{CH}\_3\text{CH}\_2\text{O}\_{\text{ad}} \rightarrow \text{H}\_2\text{O} + \text{CO}\_2\text{)}\tag{20}$$

I. For low ethanol coverage and adsorbed H2O on the TiO2 surface, an •OH-initiating oxidation mechanism leading to HCOO−ads as a major intermediate was proposed by the authors. The envisaged parallel/series reactions were:

$$\text{CH}\_3\text{CH}\_2\text{OH}\_{\text{ads}}/\text{CH}\_3\text{CH}\_2\text{O}\_{\text{ads}} \to \text{.} \tag{21}$$

$$\text{(a)} \rightarrow \text{adsorbed}\,\text{C}\_1\text{-oxygen}\,\text{and}\,\text{species}\,\text{(HCHO}\_{\text{ads}}\,\text{HCOOH}\_{\text{ads}}\,\text{and}\,\text{HCOO}^-\,\text{ads})\tag{2}$$

$$\text{C}\_2\text{(b)} \rightarrow \text{adsorbed C}\_2\text{-oxygen} \\ \text{ded species (CH}\_3\text{COOH}\_{\text{ads}} \text{ and } \text{CH}\_3\text{COO}^-\text{)}\_{\text{ads}}) + \text{C}\_1\text{-oxygen} \\ \text{ated species} \tag{25}$$

$$\text{(c)}\rightarrow\text{adsorbed CH}\_3\text{CHO}\_{\text{ads}}\tag{24}$$

II. For high ethanol coverage, decreased in the amount of adsorbed water on the TiO2 surface was registered due to the fact that the direct interaction between the photogenerated holes and adsorbed ethanol is favored. This leads to the hydrogen abstraction from αcarbon (CH3CH2OHads/CH3CH2Oads) and the formation of CH3COO<sup>−</sup>ads as primary, intermediate species, according to the reactions:

$$\text{CH}\_3\text{CH}\_2\text{O}\_{\text{ads}} + \text{h}^+ + \text{O}\_{\text{(lattice)}} \rightarrow \text{CH}\_3\text{COO}^- + 2\text{H}^+ \tag{25}$$

$$\text{CH}\_3\text{CH}\_2\text{OH}\_{\text{ads}} + 2\text{h}^+ + \text{O}\_{\text{(lattice)}} \rightarrow \text{CH}\_3\text{COOH} + 2\text{H}^+ \tag{26}$$

In this system, the oxidation of ethanol to CO2 is carried out preferentially by highly oxidizing •OH radicals to the detriment of holes.

Modifying P25 with Ag nanoparticles, Fukuhara et al. [27] obtained an active photocatalyst for the degradation of ethanol in the UV and visible range. They monitored the heat released by ethanol partial and total oxidation (Equations (27)–(29)):

$$\mathrm{C\_2H\_5OH} + 1/2\mathrm{O\_2} \rightarrow \mathrm{CH\_3CHO} + \mathrm{H\_2O}, \Delta\_rH^0 = -172.91 \text{ kJ mol}^{-1} \tag{27}$$

$$\text{C}\_2\text{H}\_5\text{OH} + 3\text{O}\_2 \to 2\text{CO}\_2 + 3\text{H}\_2\text{O}, \Delta\_\text{I}H^0 = -1277.38 \text{ kJ mol}^{-1} \tag{28}$$

$$\text{CH}\_3\text{CHO} + 5/2\text{O}\_2 \to 2\text{CO}\_2 + 2\text{H}\_2\text{O}, \Delta\_\text{r}\text{H}^0 = -1104.47\text{ kJ mol}^{-1} \tag{29}$$

The results revealed different light-induced acting mechanisms for Ag nanoparticles and TiO2. Under UV irradiation, TiO2 promotes the partial oxidation of ethanol to acetaldehyde, whereas O2 activation accounts for CO2 and H2O yield. Under visible light, the complete oxidation of ethanol to CO2 and generation of H2O occur over Ag0 nanoparticles.

The presence of oxygen was highlighted as a key factor for CO2 production on Ag-TiO2 catalyst under UV-Vis irradiation (Figure 2). Figure 2b shows that O2 is activated under UV light on TiO2, whereas the surface of the Ag nanoparticle promotes the partial oxidation of C2H5OH to CH3CHO and the cleavage of the C−C bond leading to CO2 and CH4 formation.

**Figure 2.** Photocatalytic reaction steps involving: (**A**) Ag-TiO2, ethanol, UV-Vis light irradiation and (**B**) Ag-TiO2, ethanol, O2, UV-Vis light irradiation. Reproduced with permission from ref. [25]. Copyright 2023 American Chemical Society.

2.1.3. Methanol, Ethanol, and Oxalic Acid Oxidation in Liquid Phase on Pristine and Modified Catalysts

Kawai et al. [28] proposed the following mechanism for methanol degradation on TiO2 suspended powder in deaerated aqueous media under 500 W Xe-lamp light irradiation:

$$\text{MeOH} \rightarrow \text{HCHO} + \text{H}\_{2\prime} \tag{30}$$

$$\text{HCHO} + \text{H}\_2\text{O} \rightarrow \text{HCO}\_2\text{H} + \text{H}\_2\text{O} \tag{31}$$

$$\text{HCO}\_2\text{H} \rightarrow \text{CO}\_2 + \text{H}\_2\text{O},\tag{32}$$

At the beginning of the process, H2 was the main gaseous product obtained along with small amounts of CO2. The photogenerated holes are used for the oxidation of MeOH, HCHO, and HCO2H intermediates, whereas the photogenerated electrons are responsible for H2 production via proton reduction.

Villareal et al. [29] explored the photoelectrochemical oxidation of methanol and formic acid dissolved in water on polycrystalline anatase electrodes. The authors found that methanol oxidation to CO2 is carried out by surface-bound hydroxyl radicals. This mechanism suggests weak interaction between methanol and oxide surface. In contrast, formic acid appears to be oxidized directly to CO2 by the valence band-free holes, indicating strong adsorption on the catalyst surface, despite the competitive adsorption of water.

Methanol photooxidation in aqueous media has also been studied by Haselman et al. [30] on platinum-modified TiO2. An ATR-FTIR setup was used for time-resolved investigations of both Pt particle growths during in situ photo deposition on TiO2 thin films and the photooxidation of methanol under UV irradiation in aqueous environments. The authors certified that methanol is oxidized in aqueous solutions to H2 and CO2 by the photogenerated holes. Also, for 2.7% Pt loading on TiO2, the methanol photooxidation to CO2 takes place via the formation of formaldehyde intermediate, while for lower Pt loading (1.4%), methyl formate is obtained. The reaction mechanism depends on the availability of specific active sites (on platinum and defects).

The solar light-driven generation of CO2 and H2 from a water/methanol mixture has been carried out on anatase and sodium titanates with tubular morphology [31]. The tubular morphology proved to have a beneficial effect on charge separation at the interface of semiconducting crystalline domains. The radical trapping experiments certified the formation of reactive oxygen species (•OH and O2 −) by reaction between photogenerated electrons and holes with adsorbed O2 and hydroxyl groups. The essential contribution ROS to the overall photocatalytic activity showed by tubular titania-based materials was clearly emphasized by experimental results. Papa et al. [32] synthesized PVP-protected bimetallic nanoparticles (Pt-Cu and Pt-Ag) by a modified protocol of the alkaline polyol method. Active photocatalysts for aqueous methanol mineralization have been obtained by supporting them on TiO2 (2.16 mmol CO2 h−<sup>1</sup> gcat−<sup>1</sup> for Pt-Ag/TiO2 and 1.68 mmol CO2 h−<sup>1</sup> gcat−<sup>1</sup> for Pt-Cu/TiO2) under 125 W medium pressure Hg lamp and 10% O2/Ar.

Photocatalytic experiments confirmed that not only semiconductor materials show photocatalytic activity but also large band-gap insulators such as tubular SiO2 may work as extremely active photocatalysts in the methanol/water mixture when exposed to solar light [33]. SiO2 nanotubes with a high surface density of light-absorbing defects were obtained by a modified sol-gel method, using DL tartaric acid as an organic template.

The formation of Si3+ defects by calcination enhanced light absorption characteristics, the importance of thermal treatment being illustrated in Figure 3a.

The CO2 generation rate in Ar flow was 2.4 μmol h−<sup>1</sup> and increased at 12 μmol h−<sup>1</sup> in the presence of O2. The evolvement of CO2 and H2 from aqueous methanol exposed to solar (AM 1.5) and visible light irradiation (λ > 420 nm) demonstrated that the light absorbing defects, having the energy levels located within the forbidden gap of SiO2, are able to work as photocatalytic sites (Figure 3b).

ROS photogeneration over the SiO2, SiO2-TiO2 nanotubes and P25, together with their impact on the aqueous methanol photodegradation, were also investigated [34]. The TiO2

proved to work as a photocatalyst by intermediation of •OH radicals, while the SiO2-TiO2 generated O2 −. In contrast, the organic substrate was activated and degraded on the surface of SiO2 by the intra-band gap, isolated surface quantum defects.

**Figure 3.** (**a**) Comparative UV-Vis spectra of silica nanotubes subjected to calcination in air for 3 h (SiO2-NT CALC) and 1 h (SiO2-NT CALC–BIS) (**b**) Representation of intraband gap defects in tubular SiO2 Reproduced with permission from ref. [33]. Copyright 2023 Elsevier.

Oxalic acid is found in biological systems but also as an emerging residue from industrial activities (textile industry, etc.). Oxalic acid is often chosen as a model pollutant for testing the photoreactivity of dicarboxylic acids because its photodegradation process conducts mainly to CO2. Other presumable intermediates, such as formic acid and carbonate, have not been clearly revealed [35].

The earlier mentioned SiO2 material with different morphologies and modified with platinum proved to be efficient for the photomineralization of oxalic acid in the 200–800 nm range [36]. For this system, the degradation mechanism proposed the following sequences:


$$\text{2h}^+ + \text{C}\_2\text{O}\_4^{2-} \rightarrow \text{2CO}\_2 \tag{33}$$


$$2\text{H}^+ + 0.5\text{O}\_2 + 2\text{e}^- \rightarrow 2\text{H}\_2\text{O} \tag{34}$$

The spherical SiO2 particles were inactive, whereas the rate of CO2 evolvement over tubular-shaped SiO2 particles was 45 μmol g−<sup>1</sup> cat h−1. Platinum deposition on tubular SiO2 further increased the CO2 formation rate to 428 μmol g−<sup>1</sup> cat h<sup>−</sup>1, the efficiency of the modified catalyst being comparable with that of semiconductor-based materials.

Kosani´c et al. [37] conducted the same photocatalytic process in oxygenated aqueous solution over TiO2 powder (Degussa P25) under UV irradiation (300–400 nm) and correlated the CO2 production with the illumination time. The authors advanced the following mechanism initiated by the •OH radicals:

$$\mathrm{HC\_2O\_4^- + \bullet OH \to OH^- + \bulletCOOH + CO\_2} \tag{35}$$

$$\bullet\bullet\text{COOH} + \text{O}\_2 \rightarrow \bullet\text{HO}\_2 + \text{CO}\_2\tag{36}$$

$$\bullet \text{HO}\_2 + \bullet \text{HO}\_2 \rightarrow \text{H}\_2\text{O}\_2 + \text{O}\_2 \tag{37}$$

Doping of TiO2 nanopowders with iron allowed the use of simulated solar light irradiation (AM 1.5) for the mineralization of oxalic acid in an aqueous solution. The redox sites created by introducing Fe3+ into the TiO2 lattice were able to absorb the visible component of solar light [38]. The photocatalytic activity was found to increase due to the enlargement of catalyst surface area and to the decrease in optical band-gap. The highest recorded CO2 formation rate was 245 μmol h<sup>−</sup>1. The proposed mechanism considered that the C2O4 <sup>2</sup><sup>−</sup> mineralization takes place either by •OH attack or by direct reaction with holes.

Kiatkittipong et al. [39] compared the photocatalytic generation of CO2 in an aqueous solution under UV irradiation (λmax = 254 nm) over various titanate nanoribbons. It was found the following sequence of the photocatalytic activity, in accordance with the increasing of the crystallinity (decreasing of bulk defects) due to the calcination step: TiO2 > Na2Ti6O13 > Na1.48 H0.52 Ti3O7~H2Ti3O7.

Cauxa et al. [40] used oxalic acid as an electron donor for water splitting performed on g-C3N4 and loaded with platinum (0.37 wt.%) under UV-Vis irradiation (using a cut-off filter for obtaining λ > 380 nm). Based on registered CO2: H2 ratios, the authors assumed that oxidation of water also occurs in addition to the photooxidation of oxalic acid since more hydrogen was produced. Also, the presence of hydrogen peroxide was identified in the oxalic solution after photocatalytic tests suggesting the free radical formation before the complete degradation of oxalic acid to CO2. This is in accordance with the mechanism proposed by Kosani´c et al. [37].

Karunakaran et al. [41] performed a systematic study on oxalic acid photomineralization under natural sunshine triggered by various particulate semiconductors (TiO2, CuO, ZnO, Pb2O3, PbO2, Bi2O3). The authors identified the operational parameters influencing CO2 production as being the following: oxygen presence, the concentration of the oxalic acid solution, and the area of the catalyst bed. The photocatalytic efficiency relative to the oxalic acid mineralization was the following: ZnO > CuO = TiO2 = Bi2O3 = Pb2O3 > PbO2 for the next reaction conditions: 50 mL of 0.25 M acid, [O2] dissolved = 24.7 mg L−1, 1.0 g—catalyst loading, 15.68 cm2—catalyst bed, 10 min sunshine. Additionally, for each catalyst and the same reaction conditions, the degradation of the formic, acetic and citric acid was also performed. Table 1 presents the registered reactivity sequence:


**Table 1.** The registered reactivity over the studied photocatalysts.

## *2.2. Volatile Organic Compounds (VOCs) from Air and Wastewater*

This section deals with the latest discoveries on VOC abatement from air and water using various photocatalysts and photocatalytic techniques.

### 2.2.1. Overview of Volatile Organic Compounds (VOCs)

Volatile organic compounds (VOCs), such as alcohols, aromatics (e.g., benzene, toluene, xylene), aldehydes, and halocarbons are a major group of air pollutants [42] produced mostly from outdoor (industrial processes and transportation) or indoor sources (domestic products, building materials, and food industry). They can cause major health issues like respiratory diseases, heart issues or even cancer [42–44]. VOCs also represent a precursor of PM 2.5 (particulate matter are fine inhalable particles with diameters of 2.5 μm or smaller), which determines important environmental challenges all over the world [43]. On the other hand, chlorinated VOCs cause severe contamination in aquatic environments, TCE (trichloroethylene) and PCE (perchloroethylene), used at a large scale as solvents or in dry cleaning, are the most encountered types of VOC contaminants [45,46]. In the specific case of chlorinated compounds, the monitoring of Cl− release in time in the water phase or TOC (total organic compound) evolution can be a measure of pollutant removal efficiency [47]. With the exception of a few studies [48–50], most investigations ignore this aspect focusing only on the quantification of the chlorinated compound by GC measurement.

Many techniques have been used to remove those volatile organic compounds from the air or from water. These include adsorption, thermal catalysis, ozone oxidation or photocatalytic oxidation, the latter attracting more and more attention lately. The heterogeneous catalysis can offer various advantages if compared to other AOPs (advanced oxidation processes), such as moderate operation temperatures or pressure and low cost if sunlight is used as an irradiation source. On the other hand, even if photocatalysts offer good results, at least in lab-scale experiments, other AOPs proved to be as efficient, especially ozonation. For example, ozonation and activated carbon adsorption are accepted technologies for water remediation [51], but they do not meet the aim of our study.

Generally, three mechanisms regulate the photocatalytic mineralization of VOCs: light harvesting, photogenerated charge carrier (electrons and holes) separation, and charge injection into VOCs and their intermediates from the surface of the catalyst. In order to improve the effectiveness of charge carrier separation, various methods have been applied, such as metal doping, nonmetallic element doping, and the production of heterojunction from several materials with matching energy bands [43]. This can also be explained as follows: when a semiconductor photocatalyst is irradiated by light, an electron-hole pair is generated. Thus, an electron is excited from the valence band to the conduction band resulting in a hole (h+) remaining in the valence band. This oxidizing hole can interact directly with an adsorbed molecule or can oxidize water to produce hydroxyl radicals, which are themselves oxidizing agents [51].

Therefore, in this review we focused on chlorinated and aromatic VOCs removal from the environment (water and air) using various photocatalysts working under different irradiation sources.

### 2.2.2. Chlorinated VOCs Abatement from Air and Water

Monteiro et al. [52] studied the photocatalytic oxidation of perchloroethylene using a photoreactor having a compound parabolic collector. The TiO2-based paint was used as a coating for two configurations of monolithic structured cellulose acetate. The photocatalytic experiments were performed under solar irradiation (spectral range of 300 < λ < 800 nm). They obtained in the best experimental conditions (initial concentration = 1100 ppm, feed flow rate = 75 cm<sup>3</sup> min−1, relative humidity ≈ 40% and incident UV irradiance ≈ 38.4 W m−<sup>2</sup> in the presence of oxygen) a PCE conversion of around 60%. Also, they observed that if the relative humidity is low, Cl radical chain propagation reactions could be included in the photocatalytic oxidation mechanism of PCE and that the photoreaction can still occur in the absence of oxygen.

The authors proposed a reaction mechanism according to the schemes below (Figure 4). It relies on the supposition that the reaction of PCE degradation starts with the addition of •OH radicals, thus leading to a dechlorination reaction where •Cl radicals are formed. In detail, Figure 4a shows the attack of hydroxyl radical to PCE pursued by chlorine radical release yielding trichloroethenol. Figure 4b represents the addition of •Cl to PCE, thus forming chloroalkyl radical, which is then oxidized by superoxide radicals creating a peroxy radical. The peroxy radical can be transformed into chloroethoxy radical by reacting with a second peroxy radical which suffers a CC bond scission forming CCl2O and CCl3 radicals. CCl3 converts into chloroform or carbon tetrachloride by reacting with H+ or •Cl, while CCl2O produces phosgene that may be hydrolyzed into CO2 and HCl. Figure 4c shows the chlorination of PCE resulting in chloroalkanes. Here, PCE suffers a reaction with •Cl forming chloroalkyl radical that can be hydrogenated or chlorinated, resulting in pentachloroethane or perchloroethylene. Finally, Figure 4d represents the formation mechanism of trichloroethyl acetate and trichloromethyl acetate. If •OH is added to chloroalkyl radical and dichloroacetyl chloride, pentachloroethanol and dichloroacetic acid could be obtained. Dichloroacetic acid could be further chlorinated to obtain trichloroacetic acid. At the surface of the catalyst, dichloromethanol can be formed. In the end, ethyl,

trichloroacetate and methyl trichloroacetate may be produced after reactions between trichloroacetic acid and pentachloroethanol or dichloromethanol.

**Figure 4.** (**a**) •OH radical addition to PCE followed by •Cl radical generation; (**b**) •Cl addition to PCE followed by O2 − radical addition forming chloroalkanes and phosgene; (**c**) chlorination of PCE by addition of •Cl producing chloroalkanes; (**d**) esterification of ethyl, trichloroacetate and methyl, trichloroacetate. Reproduced with permission from ref. [52]. Copyright 2015 Elsevier.

Egerton et al. [53] presented their work regarding the influence of platinum on the UV (irradiation by a Philips PL-L 36W 09 actinic lamp, UV peak intensity at ∼360 nm) photocatalytic degradation of dichloroacetate anion (DCA) by rutile and anatase forms of titania. They did the Pt deposition photochemically, with Pt(II) being more active than Pt(0). The activity of undoped rutile was lower than that of anatase, while after doping, the activity of rutile increased. In the end, both titania catalysts performed similarly. The authors observed that undoped Pt/anatase catalysts did not oxidize DCA when under visible light irradiation. On the other hand, Pt/rutile managed to oxidize DCA, being three times faster than experiments where Pt-free catalysts were used under UV irradiation.

Grzechulska-Damszel et al. [54] worked on a study regarding the photocatalytic decomposition of very low amounts (15 μg dm<sup>−</sup>3, these concentration levels corresponding to those available in groundwater) of trichloroethylene (TCE) and tetrachloroethylene (PCE) in water using TiO2. The authors performed various tests, including blank tests and tests

without light irradiation and observed that low (when catalysts were used in dark mode) or no degradation occurred. For the photocatalytic tests, the authors used commercial titania and a commercial reaction system (Trojan UVMax, Trojan Technologies, London, ON, Canada, UV/Vis radiation in the range of 250–800 nm, with high maximum at 254, 436 and 546 nm) commonly used for water disinfection. The photocatalyst was used either as suspended particles or as an immobilized refill (titania was coated onto a glass fabric). When subjected to light irradiation (UV-Vis lamp), either TCE or PCE alone was removed at about 95% in under 90 min, while when a PCE/TCE mixture was tested, around 95% degradation was still obtained after 150 min. Therefore, the authors concluded that low amounts of TCE and PCE can be removed from the water when using a titania-based photocatalytic process and recommend the use of the immobilized system, being more beneficial from an economic point of view.

Suarez et al. [55] studied the photocatalytic activity of TiO2/zeolite hybrids for VOC oxidation. Two UV-A lamps (8 W Philips and 6.5 mW cm−<sup>2</sup> irradiance) were used as the irradiation source. The authors synthesized titania with various morphologies (nanoparticles of almost 5 nm and decahedral anatase particles (DAP) having around 100 nm) and used commercial TiO2 for comparison reasons. They obtained the catalytic hybrid by incorporating TiO2 on ZSM5 using either freeze drying, incipient wetness impregnation or mechanical mixing. After characterization, they evaluated the photocatalytic performance of the studied catalysts on trichloroethylene photooxidation (and also formaldehyde) under UV-A irradiation and continuous airflow. The authors observed that the incorporation of titania, regardless of its type, into ZSM5 improved the photocatalytic performance up to 10 times compared to single titania particles. Even though DAPs had the highest VOCs reaction rate, it was concluded that TiO2 nanoparticles homogeneously distributed on the zeolite material showed the highest VOCs photooxidation and CO2 formation rates of the series (titania NP/ZSM5 > DAP/ZSM5 > commercial titania/ZSM5).

A study regarding the effect of photocatalysis on the degradation of trichloroethylene (TCE) in aqueous solutions using a photocatalyst-coated plastic optical fiber (POF) was done by Chen et al. [56]. They used TiO2 and ZnO as photocatalysts and two diodes (LEDs) with low light intensity as the light source (40 mW cm<sup>−</sup>2, 395 nm and 20 mW cm−2, 365 nm). Parachlorobenzoic acid (pCBA) was utilized as a hydroxyl radical for the calculation of hydroxyl radical conversion rate (ROH, UV). After performing experimental tests, the authors observed that titania-coated POF was more efficient in degrading TCE in basic solutions, while ZnO-coated POF had better results in acid solutions. This was expected due to the fact that the mechanism of TCE removal by titania is photocatalysis, while by zinc oxide is adsorption. Another noticeable fact was that if the coating time increases, thus increasing the coating thickness, the degradation efficiency decrease. On the other hand, the enhancement of light intensity improved the photocatalytic efficiency. pH played an important role in the photocatalytic tests and was observed by the authors that if it is increased from 4 up to 10, the (ROH, UV) increases from 2 × <sup>10</sup><sup>3</sup> to 8 × 103 <sup>M</sup>·s·cm2·mJ−<sup>1</sup> for titania and from 8 × 102 to 2 × <sup>10</sup><sup>3</sup> <sup>M</sup>·s·cm2·mJ−<sup>1</sup> when zinc oxide was used.

State et al. [45] investigated the photocatalytic removal of TCE (trichloroethylene) under simulated solar irradiation (AM 1.5) using Au/TiO2 and Pd-Au/TiO2. The authors obtained the mentioned photocatalyst using incipient wetness and deposition precipitation methods. After performing the photocatalytic tests, it was observed that TCE was converted to more than 80% of Cl− and CO2 no matter the catalyst used. Also, in order to obtain TCE conversion directly to Cl− and C2 (ethane, ethylene) using the H2 generated photo catalytically in situ, the authors performed the photocatalytic degradation of TCE over the same catalytic materials in the presence of methanol traces. Thus, it was evidenced that when Pd-Au/TiO2 was used, hydro dechlorination (HDC) and photomineralization reactions of TCE took place simultaneously (Figure 5), while Au/TiO2 was inactive. A reaction mechanism was proposed: the organic substrate was the source of protons (and thus of H2) and carbon of CO2. The •OH radicals supply the O2 for the development of oxidized organic intermediates and, finally, of CO2.

**Figure 5.** Photomineralization and photomineralization/hydro dechlorination mechanisms. Reproduced with permission from ref. [45]. Copyright 2017 Elsevier.

Hsu et al. [46] evaluated the possibility of using LaFeO3 as a photocatalyst for trichloroethylene (TCE) degradation from the water via heterogeneous oxidation. The authors synthesized LaFeO3 using the sol-gel method. TCE was chosen as an organic pollutant to be removed because it is one of the most encountered water pollutants in Taiwan. The authors' results indicated that up to 95% removal efficiency from water could be obtained when using 2 g per liter photocatalyst (LaFeO3) and 1 h illumination from a Xe lamp up to 400 W. Other parameters were also studied, such as light intensity, catalyst loading, the influence of TCE concentration or pH contribution on TCE removal. They reported the highest energy efficiency to be 10.8 mg TCE/kWh. The TCE removal efficiency decreases if the initial TCE concentration increases, while the removal efficiency increases if the catalyst loading increases. Also, by increasing the pH value, the removal efficiency rises due to the addition of OH− groups. In the end, the authors revealed that the processes of adsorption, photodegradation and photocatalysis take place simultaneously and reach equilibrium after 1 h, and a removal mechanism had also been identified, suggesting that the overall removal efficiency reached 82% (20% accounting for adsorption and 39% for photolysis).

In the study of Raciulete et al. [48], the photocatalytic activity for TCE removal under simulated sunlight irradiation (using a 150 W short-arc Xe lamp (1000 W·m−2, Peccell-L01) over RbLaTa2O7 perovskites with mostly nanowire and platelet morphologies was investigated. Two RbLaTa-based layered perovskite samples were prepared via a solidstate synthesis route. The authors observed that the sample synthesized in mild conditions (e.g., 1200 ◦C for 18 h) favored the photo-mineralization of TCE to Cl− and CO2 due to the presence of a high density of hydroxyl groups (Figure 6). Contrariwise, the activity of the sample annealed in harsh conditions (e.g., 950 and 1200 ◦C, for 36 h) remained modest for TCE removal, whereas its surface carbonate was beneficial for the formation of intermediate products. With the purpose of enhancing the overall photocatalytic performances of RbLaTa-based layered perovskites, the samples were subjected to protonation [47]. The strategy involved the slow replacement of the interlayer Rb<sup>+</sup> of RbLaTa2O7 hosts by H<sup>+</sup> via the cation exchange route. The authors showed that the obtained HLaTa2O7 protonated perovskites were able to photomineralize TCE under simulated solar irradiation. The enhanced activity of protonate perovskites was ascribed to favorable roles played by their increased specific surface area and high density of hydroxyl groups.

**Figure 6.** Time-evolution of TCE (closed symbols) and Cl− (open symbols) concentrations over RbLaTa\_01(02) photocatalysts and precursors in the absence of methanol. Reaction conditions: initial concentration of TCE = 5 mg·L<sup>−</sup>1; simulated solar light, T = 18 ◦C, mass of catalyst = 0.05 g, volume of water = 110 mL. Reproduced with permission from ref. [48]. Copyright 2019 Elsevier.

Photocatalytic degradation of various chlorinated environmental pollutants (VOCs), such as various chlorinated ethene and methane derivatives, in real groundwater samples, was studied by Dutschke and coworkers [57]. They developed an appropriate experimental setup in order to apply advanced oxidation processes (AOP) to real groundwater samples. Their setup used an O3-bubble column reactor with a carrier-bound TiO2/UV system (365 nm LEDs). The authors did a comprehensive study and discussed the influence of flow rate, O3 concentration and radiation dose on the process performance. After parameter optimization (shown in Table 2) [57] using Box–Behnken experimental design, they obtained almost complete degradation rates for DCE: 99%, TCE: 99%, and PCE: 98%. A degradation rate of 85% was obtained for TCM (trichloromethane) without the formation of transformation products. The formation of tetrachloromethane (PCM) due to induced chlorination represented a problem during their photocatalytic experiments, but this was overcome by using suitable O3 doses and irradiation in order to produce enough hydroxyl radicals, thus PCM will not appear as a transformation product during the degradation of other chlorinated organic pollutants.


