*4.2. Photocatalytic Oxidation*

Photocatalytic reactions have drawn a lot of attention and have been well developed in recent years since Fujishima found the splitting water to H<sup>2</sup> and O<sup>2</sup> over TiO<sup>2</sup> [55,179,180]. Different kinds of photocatalysts have been developed to treat VOCs containing waste water, such as TiO2, WO3, ZnO2, CdS, g-C3N4, and BiOBr (Table 9) [56,180–185]. The mechanism of photocatalysis is that when the light with a suitable wavelength radio on catalysts (semiconductors), the electrons and holes were separated and generated on the surface of catalysts, then the radicals of •OH and O2<sup>−</sup> was formed on the surface of catalysts, the VOCs reacted with these radicals and decomposed to CO<sup>2</sup> and H2O at last [186,187]. In the water solution, water can react with the catalysts and form •OH, which is positive for the decomposition of VOCs in water. The photocatalytic elimination of VOCs in the gas phase follows a similar mechanism, but the radicals are main O2- due to the shortage of humidity. In this section, the progress in the elimination of VOCs in the gas phase by photocatalytic methods was mainly discussed.

The most studied photocatalyst for the elimination of VOCs is TiO<sup>2</sup> [44]. Wilson F. Jariam et al. degraded 17 kinds of VOCs with the concentration range of 400–600 ppmv over TiO<sup>2</sup> under the radiation of ultraviolet light [98]. The results showed that trichloroethylene (99.9%), isooctane (98.9%), acetone (98.5%), methanol (97.9%), methyl ethyl ketone (97.1%), t-butyl methyl ether (96.1%), dimethoxymethane (93.9%), methylene chloride (90.4%), methyl isopropyl ketone (88.5%), isopropanol (79.7%), chloroform (69.5%), and tetrachloroethylene (66.6%) were decomposed efficiently over TiO2. The photodegradation of isopropylbenlene (30.3%), methyl chloroform (20.5%), and pyridine (15.8%)

on TiO<sup>2</sup> was not as efficient as other VOCs. The catalytic lifetime was also tested by toluene. The conversion of toluene decreased to 20.9% after a 150 min test, but the deactivated catalysts can be easily regenerated by washing with H2O<sup>2</sup> and illumination. F. B. Li et al. prepared La ion doped TiO<sup>2</sup> by the sol-gel method for photodegradation of benzene, toluene, ethylbenzene, and o-xylene in the gas phase [188]. The results showed that the La ion doped TiO<sup>2</sup> performed much better than the pure TiO2. This was due to the improved adsorption ability and the enhanced electron–hole pairs separation by the presence of Ti3<sup>+</sup> and the electron transfer between the conduction band/defect level and lanthanide crystal field state. Tânia M. Fujimoto et al. supported the palladium on TiO<sup>2</sup> for the photocatalytic decomposition of octane, isooctane, n-hexane, and cyclohexane in a low concentration (100~120 ppmv) [99]. The modified catalysts showed excellent catalytic activity in the decomposition of VOCs rapidly. V. Héquet et al. used a closed-loop reactor to study the mixture effect over the P<sup>25</sup> TiO2/SiO<sup>2</sup> mixture (Figure 14) [189]. They have developed the accurate analytical methods to identify and quantify the majority of the potential formed intermediates, which provide an efficient way to study the reaction mechanism. Yajie Shu et al. used Mn doped TiO<sup>2</sup> to degrade benzene by O<sup>3</sup> under vacuum ultraviolet (VUV) irradiation [62]. The doped TiO<sup>2</sup> showed better performance than the undoped one and P<sup>25</sup> due to the formation of highly reactive oxidizing species. Jian Ji et al. showed that compared with the one without UV radiation, the UV radiation can improve the removal efficiency of benzene by about 10% (Figure 15) [63]. Marta Stucchi et al. developed a simultaneous photodegradation system for the VOC mixture elimination by TiO<sup>2</sup> powders, which showed a good efficiency [86]. Huiling Huang et al. developed Mn modified ZSM-5 as catalysts for VUV photolysis combined with ozone-assisted catalytic oxidation and studied the mechanism [102]. The catalysts showed good efficiency. Although most of the catalysts showed excellent activity in the elimination of VOCs under UV light, the light utilization efficiency was still low due to the low percent of the UV light in nature light. Xufang Qian et al. designed mesoporous TiO<sup>2</sup> films coated with carbon foam for photodegradation of acetone and toluene, which can converse more than 90% of VOCs to CO<sup>2</sup> under visible light due to the plausible carbon doping and the strong interaction between the TiO<sup>2</sup> precursor and the hydro-carbon foams [100]. The graphite-SiO2-TiO<sup>2</sup> composite and BiOBr@SiO<sup>2</sup> flower-like nanospheres were also used for photodegradation of VOCs under visible light and showed good catalytic activity [64]. The low solar utilization ratio was the main obstacle to improve photocatalytic efficiency. In order to improve the efficiency, materials with a good light adsorption ability was applied. Yun-En Lee et al. prepared black-TiO<sup>2</sup> and LFO/black-TiO<sup>2</sup> and they showed excellent photo catalytic activity for the removal of toluene and IPA due to their good light adsorption ability [77]. The modification of g-C3N<sup>4</sup> by hydroxyl groups can enhance visible light-driven photocatalytic properties of g-C3N<sup>4</sup> obviously which can improve the adsorption energy of g-C3N<sup>4</sup> for water and phenol [88]. The structure of catalysts was also carefully tuned to improve the catalytic activity [190]. Bettini S et al. insulated a layer of SiO<sup>2</sup> between zinc oxide and nano silver and controlled the thickness of the insulated layer, which enhanced the photocatalytic oxidation ability of the catalyst significantly [78]. The photocatalytic activity of ZnO can also be improved by doping CuO [191]. The photocatalytic elimination of VOCs is one of the most promising methods. However, the industrial application of this method is still a problem.

*Catalysts* **2020**, *10*, 668 36 of 49

*Catalysts* **2020**, *10*, 668 36 of 49

**Figure 14.** A 420‐L of continuous closed‐loop photocatalytic reactor: (**1**) Photocatalytic unit containing the TiO2 photocatalytic medium and the ultraviolet (UV) lamps, (**2**) fan, (**3**) air input and sampling points for analysis, (**4**) air tranquilization chamber, (**5**) flow rate measurement. Reprinted with permission from [176], 2018, Elsevier Ltd.. **Figure 14.** A 420-L of continuous closed-loop photocatalytic reactor: (**1**) Photocatalytic unit containing the TiO<sup>2</sup> photocatalytic medium and the ultraviolet (UV) lamps, (**2**) fan, (**3**) air input and sampling points for analysis, (**4**) air tranquilization chamber, (**5**) flow rate measurement. Reprinted with permission from [176], 2018, Elsevier Ltd.. **Figure 14.** A 420‐L of continuous closed‐loop photocatalytic reactor: (**1**) Photocatalytic unit containing the TiO2 photocatalytic medium and the ultraviolet (UV) lamps, (**2**) fan, (**3**) air input and sampling points for analysis, (**4**) air tranquilization chamber, (**5**) flow rate measurement. Reprinted with permission from [176], 2018, Elsevier Ltd..

**Figure 15.** The schematic diagram of the VUV‐PCO system. Reprinted with permission from [63], 2017, Elsevier Ltd. **Figure 15.** The schematic diagram of the VUV‐PCO system. Reprinted with permission from [63], 2017, Elsevier Ltd. **Figure 15.** The schematic diagram of the VUV-PCO system. Reprinted with permission from [63], 2017, Elsevier Ltd.

There is a large amount of reports on the photocatalytic elimination of VOCs. The advantages of this process are green, energy saving, and clean. However, the reported catalysts are not efficient enough for an industrial application at the present stage. Firstly, the reaction needs more time than other methods. Secondly, most of the reported catalysts could not use visible light as energy to start the reactions, which limited the improvement of quantum efficiency, so it still needs to develop efficient photocatalysts and design effective reaction systems. There is a large amount of reports on the photocatalytic elimination of VOCs. The advantages of this process are green, energy saving, and clean. However, the reported catalysts are not efficient enough for an industrial application at the present stage. Firstly, the reaction needs more time than other methods. Secondly, most of the reported catalysts could not use visible light as energy to start the reactions, which limited the improvement of quantum efficiency, so it still needs to develop efficient photocatalysts and design effective reaction systems. There is a large amount of reports on the photocatalytic elimination of VOCs. The advantages of this process are green, energy saving, and clean. However, the reported catalysts are not efficient enough for an industrial application at the present stage. Firstly, the reaction needs more time than other methods. Secondly, most of the reported catalysts could not use visible light as energy to start the reactions, which limited the improvement of quantum efficiency, so it still needs to develop efficient photocatalysts and design effective reaction systems.




**Table 9.** *Cont*.

### *4.3. Non-Thermal Plasma Process 4.3. Non‐Thermal Plasma Process*

In the non-thermal plasma process, electrons and their surroundings are not in a thermal equilibrium, so the electrons are heated by electric discharges instead of the gas itself, and produce the electrons with high energy, active radicals and ions which can promote numerous chemical reactions in the ionized zones. It can be used to treat the high flow for both low (<100 ppmv) and high (>1000 ppmv) concentrations of VOCs, including toluene, benzene, acetone, trichloroethylene, etc. (Table 10) [47]. The main bottlenecks for the commercialization of the technology are the formation of poison byproducts and high energy consumption. The discharge methods are important for the VOCs removal efficiency, which includes corona discharge, surface discharge, microwave discharge, dielectric barrier discharge, and packed bed dielectric barrier discharge [190–194]. Among these discharge types, the packed bed dielectric barrier discharge shows the most potential in the industry application [195]. Savita K. P. Veerapandian et al. reviewed the packed bed DBD [47]. The influences of dielectric constant, packing materials size, shape, surface properties, and the byproducts formation were discussed. In the non‐thermal plasma process, electrons and their surroundings are not in a thermal equilibrium, so the electrons are heated by electric discharges instead of the gas itself, and produce the electrons with high energy, active radicals and ions which can promote numerous chemical reactions in the ionized zones. It can be used to treat the high flow for both low (<100 ppmv) and high (>1000 ppmv) concentrations of VOCs, including toluene, benzene, acetone, trichloroethylene, etc. (Table 10) [47]. The main bottlenecks for the commercialization of the technology are the formation of poison byproducts and high energy consumption. The discharge methods are important for the VOCs removal efficiency, which includes corona discharge, surface discharge, microwave discharge, dielectric barrier discharge, and packed bed dielectric barrier discharge [190–194]. Among these discharge types, the packed bed dielectric barrier discharge shows the most potential in the industry application [195]. Savita K. P. Veerapandian et al. reviewed the packed bed DBD [47]. The influences of dielectric constant, packing materials size, shape, surface properties, and the byproducts formation were discussed.

Using porous and catalytic materials as the packed bed can increase the resistant time of VOCs and decrease the unwanted byproducts, such as O3, NOx, and CO (Figure 16) [196]. The packed bed materials can be non-catalytic porous materials, such as activated carbon, porous Al2O3, glass, zeolite, graphene oxide, and the catalysts, such as metal oxides, noble metal loaded metal oxides, and catalytic porous materials [197]. For example, using Al2O<sup>3</sup> as the packed bed can concentrate VOCs molecules on its surface and weaken the bond energy of VOCs, which can enhance the dissociation when these adsorbed molecules encounter the active species in the plasma and increase the collision probability and deep oxidation of VOCs [198]. Lee et al. used porous γ-Al2O<sup>3</sup> as the packed bed to oxidize toluene to CO2. It showed a 100% conversion of toluene and high CO<sup>2</sup> selectivity [199]. Gandhi's research results showed that a large surface area and pore volume of alumina can not only increase the conversion of ethylene to CO<sup>2</sup> or CO, but also decrease the selectivity of unwanted byproducts, such as acetaldehyde, acetylene, methane, N2O, and O<sup>3</sup> [200]. Other researches also showed the same phenomenon in the abatement of benzene, acetone, formaldehyde, TCE, chlorobenzene in non-thermal plasma with the packed bed which has a large surface area and pore volume, such as porous alumina, TiO2, zeolite, and porous metal oxides [199,201–203]. Due to the large surface area, the retention time and concentration of VOCs molecules increased which led to the increase of collisions between VOCs and active species [200], the adsorption effect can weaken the chemical bond of VOCs [204], and form micro-discharges in the micro-pores in addition to the micro-discharges in the gas phase. Using porous and catalytic materials as the packed bed can increase the resistant time of VOCs and decrease the unwanted byproducts, such as O3, NOx, and CO (Figure 16) [196]. The packed bed materials can be non‐catalytic porous materials, such as activated carbon, porous Al2O3, glass, zeolite, graphene oxide, and the catalysts, such as metal oxides, noble metal loaded metal oxides, and catalytic porous materials [197]. For example, using Al2O3 as the packed bed can concentrate VOCs molecules on its surface and weaken the bond energy of VOCs, which can enhance the dissociation when these adsorbed molecules encounter the active species in the plasma and increase the collision probability and deep oxidation of VOCs [198]. Lee et al. used porous γ‐Al2O3 as the packed bed to oxidize toluene to CO2. It showed a 100% conversion of toluene and high CO2 selectivity [199]. Gandhi's research results showed that a large surface area and pore volume of alumina can not only increase the conversion of ethylene to CO2 or CO, but also decrease the selectivity of unwanted byproducts, such as acetaldehyde, acetylene, methane, N2O, and O3 [200]. Other researches also showed the same phenomenon in the abatement of benzene, acetone, formaldehyde, TCE, chlorobenzene in non‐thermal plasma with the packed bed which has a large surface area and pore volume, such as porous alumina, TiO2, zeolite, and porous metal oxides [199,201–203]. Due to the large surface area, the retention time and concentration of VOCs molecules increased which led to the increase of collisions between VOCs and active species [200], the adsorption effect can weaken the chemical bond of VOCs [204], and form micro‐discharges in the micro‐pores in addition to the micro‐discharges in the gas phase.

**Figure 16.** Schematic diagram of the plasma‐driven catalyst (PDC) reactor. Reprinted with permission from [196], 2007, Elsevier Ltd. **Figure 16.** Schematic diagram of the plasma-driven catalyst (PDC) reactor. Reprinted with permission from [196], 2007, Elsevier Ltd.

The addition of catalysts in the non‐thermal plasma process as the packed bed can enhance the VOCs removal efficiency significantly due to the plasma assisted reactions on the surface of the The addition of catalysts in the non-thermal plasma process as the packed bed can enhance the VOCs removal efficiency significantly due to the plasma assisted reactions on the surface of the catalyst.

catalyst. These plasma‐assisted reactions can generate some active radical species, such as

These plasma-assisted reactions can generate some active radical species, such as intermediates with electrons, O·, OH·, N2·, NO·, N2O· in the gas phase and O·, OH· on the surface of catalysts, which can improve the VOCs removal efficiency and increase the CO<sup>x</sup> selectivity [80]. The catalysts include metal oxides, such as CuO, MnO2, CeO2, NiO, CoO2, and Fe2O3, and noble metals, such as Pt, Au, and Ag [65,196,203,205–209]. The different kinds of oxygen species in catalysts, namely lattice oxygen, surface oxygen, and absorbed oxygen, are very helpful for the oxidation of VOCs, which can activate the oxygen molecules in the gas phase. P.J. Asilevi et al. established a laboratory scale DBD reactor for the removal of toluene by ·O and ·OH which were generated from the reaction between O<sup>2</sup> and H2O and the removal efficiency of VOCs can be improved by increasing the oxygen concentration and relative humidity [210]. Lu et al. used FeOx/SBA-15 as the packed bed to eliminate toluene in the non-thermal plasma process. The results showed that the presence of Fe2<sup>+</sup> can increase the toluene removal efficiency and CO<sup>x</sup> selectivity obviously and reduce the formation of unwanted and toxic byproducts [80]. Zhu et al. prepared 5 wt% CuO/-Al2O<sup>3</sup> as the packed bed to the abatement acetone. It showed better performance than the one without CuO due to the better reducibility and abundant active oxygen species [87]. Li et al. prepared Pt/Al2O<sup>3</sup> and it showed higher decomposition efficiency than the unloaded Al2O<sup>3</sup> due to the presence of Pt, which increases the number of active sites and reduces the activation energy of the decomposition reaction and suppresses harmful NO<sup>x</sup> formation [65]. The research by Hua Song et al. showed that CoMnOx/TiO<sup>2</sup> can obviously improve the plasma removal of VOCs [103].

There are other factors that influence the VOCs removal efficiency, such as humidity and the plasma structure. In industrial conditions, the humidity in the VOCs stream is the factor which must be considered. The humidity has both a positive and negative effect on the removal of VOCs in the non-thermal plasma process. The water molecules can modify the surface of the packed bed, especially the catalytic packed bed, quench the free electrons and active species, and produce OH radicals during the discharge process [103,202,207,211,212]. A lot of studies showed that the presence of humidity can reduce the VOCs removal efficiency by quenching the high energy electrons and reactive species, and modify the catalysts surface (BaTiO3) in the decomposition of toluene, benzene, xylene, and C2F<sup>2</sup> [103,207,212]. On the other hand, the humidity can suppress the formation of toxic byproducts, such as CO and NOx by deactivation of some oxygen species in the catalysts [213,214]. The optimization of the plasma structure also can enhance the VOCs decomposition efficiency. Muhammad Farooq Mustafa et al. designed a double dielectric barrier discharge reactor, in which the conversion of tetrachloroethylene, toluene, trichloroethylene, benzene, ethyl acetate, and carbon disulfide can be 100% with BaTiO<sup>3</sup> and HZSM-5 as catalysts, respectively [215].

There are also some disadvantages in the non-thermal plasma process, such as high energy consumption, low VOCs conversion (mostly about 30% to 70%), low CO<sup>x</sup> selectivity, and produced unwanted byproducts. The other barrier for the application of non-thermal plasma in the industry is the high cost and the high energy consumption. The way to solve these problems is the development of a highly efficient catalytic packed bed and the optimization of an electrode structure.


**Table 10.** Catalysts for VOCs elimination through the non-thermal plasma process.
