Received: 31 January 2023 Revised: 26 April 2023 Accepted: 27 April 2023 DOI: 10.1002/cjce.24978 R E V I EW AR T I C L E A review of prospects and challenges of photocatalytic decomposition of volatile organic compounds (VOCs) under humid environment Girma Masresha1 | S. Anuradha Jabasingh1 | Shimelis Kebede2 | David Doo-Arhin3 | Mekdim Assefa1 1Process Engineering, School of Chemical and Bio-Engineering, Addis Ababa Abstract Institute of Technology, Addis Ababa Volatile organic compounds (VOCs) are harmful for humans and the University, Addis Ababa, Ethiopia surrounding ecosystem. Emissions from these pollutants have caused a signifi- 2Environmental Engineering, School of cant reduction in air quality, which has an effect on people’s health. Alkanes, Chemical and Bio-Engineering, Addis Ababa Institute of Technology, Addis alkenes, alcohols, aromatics, and other VOC pollutants have all been broken Ababa University, Addis Ababa, Ethiopia down by TiO2 photocatalytic processes. Due to several operating inefficiencies 3Department of Materials Science and and deactivation issues in humid environments, the practical application of Engineering, School of Engineering Sciences, University of Ghana, photocatalysis has not been realized on a broader scale. The effectiveness of Legon-Accra, Ghana photo-oxidation of VOCs is impacted by a variety of environmental conditions. In the photocatalytic oxidation of the VOCs, relative humidity (RH) is critical. Correspondence Girma Masresha, Process Engineering, Therefore, it is important to review the recent findings on how humidity School of Chemical and Bio-Engineering, affects the photocatalytic breakdown of VOCs in air. To satisfy this need, this Addis Ababa Institute of Technology, work provides a critical review of the related literature with focus on the fun- Addis Ababa University, Addis Ababa, PO Box 385 Ethiopia. damentals of photocatalysis, photocatalytic degradation of air pollutants, and Email: girma.masresha@aait.edu.et the influence of humidity on the photocatalytic process degradation for selected air pollutants. It also highlights the kinetic models and typical photocatalytic reactor and supports for VOC removal. KEYWORD S air quality, photocatalysis, pollutants, titanium dioxide, volatile organic compounds 1 | INTRODUCTION porous material adsorption, membrane separation, ozonation, and photocatalytic oxidation (PCO).[2–10] These Volatile organic compounds (VOCs) have recently become treatment methods, however, involve more chemicals, a major public concern due to their pollution of both high energy requirements, operationally demanding situa- indoor and outdoor air. VOCs are primarily produced by tions, and low efficiencies. In this regard, heterogeneous the industrial emissions, urban public facilities, and agri- photocatalysis can be an appropriate choice when contam- cultural and animal husbandry emissions. These pollut- inants cannot be removed properly or efficiently using ants can come from a variety of sources, and long-term other treatment methods. Furthermore, because the cata- exposure to them causes sick building syndrome (SBS), a lyst is inexpensive and capable of mineralizing the major- health problem.[1] Currently, a number of methods exist ity of VOCs, photocatalytic treatment is less expensive for reducing the emission of gaseous pollutants. These than other most common methods, such as chemical include condensation, adsorption, solvent absorption, scrubbers or activated carbon adsorbers.[11] Can J Chem Eng. 2023;1–14. wileyonlinelibrary.com/journal/cjce © 2023 Canadian Society for Chemical Engineering. 1 2 MASRESHA ET AL. Photocatalysis is a type of chemical process in which factors such as light source, catalyst, temperature, light energy is used to drive two chemical reactions.[12] humidity, and residence time.[22,23] Among these factors, The pioneer work by Fujishima and Honda[13,14] has many authors have focused on the effect of relative motivated profound interests on the photocatalytic pro- humidity (RH) on the photocatalytic degradation of gas- cesses applied to environmental issues. Several studies eous VOCs.[24] The influence of water on the kinetics of have shown that TiO2 based photocatalysis with a UV photocatalytic degradation and the lifetime of the catalyst light source can be successfully applied to water and air is still up for debate. In real-world applications, it was purification processes in outdoor settings.[15,16] This is discovered that a high concentration of water vapour in due to TiO2’s unique properties, which include its inex- the feed stream had a negative impact on the removal pensiveness, chemical and biological stability, non-toxic- efficiency of VOCs from the catalysts. The effect of ity, and relatively high photoactivity (especially in its water’s inhibition has been discussed by a number of anatase or anatase-rutile combination forms).[17] authors. Re-engineering the catalyst’s natural structure UV photocatalytic treatment techniques are presently and enhancing VOCs’ ability to adsorb on the TiO2 sur- employed extensively for the degradation of VOCs. face are two potential solutions to this issue. The objec- Figure 1 depicts the photocatalytic mechanism for the tive of this work is to review and evaluate the existing VOC mineralization. The PCO processes of the catalyst literature related to the effect of water vapour on the on the degradation of VOCs have been frequently attrib- PCO process of VOCs and to propose strategies for photo- uted to two major photochemical oxidants: OH radical catalyst modification to improve the adsorption capacity. and reactive oxygen species (ROS). In general, removing VOCs in high concentrations has proven to be tedious. However, photocatalysis can be effective for some low 2 | PRINCIPLE OF concentrations of indoor VOCs such as trichloroethylene PHOTOCATALYTIC DEGRADATION (TCE), acetone, methanol, and toluene, as well as carbon monoxide (CO) and nitrogen oxides (NOx).[11,15,18–21] The In PCO, a semiconductor catalyst (oftentimes TiO2) is efficiency of PCO is primarily influenced by several used in the presence of a light source to degrade pollut- ants into primarily oxidation products (usually CO2 and H O).[25]2 Photocatalytic reaction primarily depends on the wavelength or light (photon) energy and the catalyst. Under the influence of photons, the catalyst generates free radicals that obliterate the contaminants that have bonded to its surface.[26] Light absorption, the formation and separation of electron (e) and hole (h+) pairs, and oxidation–reduction surface reactions between electrons (e) and adsorbed oxygen molecules (O2), which can produce superoxide radicals (˙O 2 ), are all photocatalytic reaction mecha- nisms for air treatment. Subsequently, additional electron holes (h+) and water (H2O) molecules can generate hydroxyl radicals (OH˙). Thus, given that both O 2 and OH˙ radicals are powerful oxidants, they can mineralize harmful chemical species (i.e., VOCs) in air, converting them into CO2 and H2O in the case of complete mineralization.[27] As demonstrated in Figure 2, the basic mechanism of the electron–hole generation in TiO2 photocatalysis involves three major steps: (i) light absorp- tion and generation of electron–hole pairs; (ii) separation of charge carriers; (iii) oxidation and reduction reactions; and (iv) recombination of electron and electron–hole at the surface of semiconductor. TiO2 as a photocatalyst is used to produce electron  + FIGURE 1 Photocatalytic mechanism for volatile organic (e ) and holes (h ), subsequently facilitating the redox compounds (VOCs) mineralization. CB, conduction band; (reduction and oxidation) reactions. UV radiation can VB, valence band. stimulate electrons and holes, resulting in the generation MASRESHA ET AL. 3 O2 þH2O!OOHþOH ð5Þ 2 OOH!O2þH2O2 ð6Þ Electron removal from the CB is as follows: OOHþH2Oþ eCB !H2O2þOH ð7Þ H2O þ e2 CB !OHþOH ð8Þ Oxidation of organic pollutant molecules is as follows: OHþPollutantþO2 !Products ðCO2, H2O, etc:,Þ ð9Þ FIGURE 2 The electron–hole generation in a photocatalyst 3 | FACTORS AFFECTING THE particle. CB, conduction band; VB, valence band. PHOTOCATALYTIC ACTIVITY of radical OH and O2 ions, which transform the Several environmental factors have an impact on the desired pollutant into CO2 and H2O. [28] An electron (e) efficiency of PCO processes used to degrade VOCs. The may be promoted from the valence band (VB) to the con- incoming gas concentration, gas flow rates, gas flow type, duction band (CB), creating an electron vacancy-hole (h+), and RH are all factors that influence how effective the when a semiconductor absorbs a photon with energy process is.[31–35] Understanding the relationship between equal to or greater than the band gap energy (hν ≥ EG) photocatalytic degradation rate and substrate concentra- (Equation (1)). In redox processes (Equations (2)–(6)) tion is required for the PCO system to function involving various species adsorbed on the catalyst sur- properly.[36] How much of the reaction will degrade is face, the electron and the hole can migrate to the cata- determined by how many organic pollutants are adsorbed lyst surface. Surface-bond H2O or OH and holes can onto the photocatalyst’s surface. Adsorption on the react to form the hydroxyl radical OH (Equations (2) photocatalyst is also influenced by the initial concentra- and (3)), whereas electrons during reaction with oxygen tion of the pollutant, and this typically decreases as pol- can generate superoxide radical anion O 2 (Equation (4)). lutant concentration increases. Hydroxyl radicals can be also formed following the path The intensity of light radiation also has an impact on represented by reactions in Equations (5)–(9).[29] Investi- the rate of photodegradation reaction. The likelihood of gating the photocatalytic elimination of a specific pollutant electron excitation increases as the light intensity requires a thorough understanding of the reaction mecha- increases.[37] The pollutant gas flow rate has a significant nism and mineralization yield.[30] The electrons can also impact on the rate of PCO because it affects the mass react with organic compounds to provide reduction prod- transfer of pollutants from the gas phase to the catalyst ucts (Equations (1)–(9)). Since oxygen (Equation (4)) can surface, where oxidation occurs. The airflow rate, on the interact with the photogenerated electrons, its involvement other hand, is an important factor in the PCO of VOCs. is crucial. The airflow rate of a reactant can influence the rate of Electron–hole pair formation is as follows: the reaction. The PCO response is affected differently by low and high airflow speeds.[38] In terms of surface chem- TiO þh þ 2 v ! hVBþ eCB ð1Þ istry, the photocatalytic efficiency and lifetime of the photocatalyst depends on how the target gas molecules H O ðAdsÞ!OHþHþ ð2Þ interact with the photocatalyst, as well as how they2 adsorb and desorb intermediates and products.[39] Hole trapping is as follows: Although photocatalysis technology is an effective treatment method for VOC degradation, it suffers from an OHþhþ ! OH ð3Þ unknown reaction mechanism and photocatalyst deactiva-VB tion, which severely limits its practical application.[40] The   effects of humidity on catalytic activity were thoroughlyO2þ eCB !O2 ð4Þ examined and addressed in the sections that follow. 4 MASRESHA ET AL. 4 | EFFECT OF HUMIDITY ON PCO levels of water vapour content: 0% RH and 50% RH. They OF VOCS observed that water vapour had no effect on the PCO of the decane conversion. Figure 3 illustrates the general Water’s role in VOC catalytic oxidation is quite complex PCO mechanism of decane proposed by Debono et al., and is determined by a number of variables such as the which indicates that decane is first converted to alde- type of VOC, the catalyst, and the reaction conditions. hydes and then to CO .[46]2 Zhang and Liu [47] investigated Humidity is frequently required in gas phase photocataly- how RH affected the PCO of hexane. The results showed sis to maintain photocatalytic activity. The presence of that hexane conversion increased dramatically with an water molecules on the photocatalyst surface can increase in RH up to 20%, then nearly stabilized until increase the activity of the catalyst in two ways.[41] First, 45%, but decreased significantly when RH exceeded 45%. it acts as a precursor for the formation of OH radicals (OH˙), and second, it improves oxygen adsorption. When exposed to light, surface oxygen reacts with photogener- 4.2 | Unsaturated alkenes and alkynes ated electrons to form oxygen radicals, which are impor- tant for oxidizing the contaminants. Researchers studied ethylene breakdown and discovered The impact of water vapour on gas phase PCO varies that as water vapour mole fractions increased, the greatly depending on the substance to be oxidized, the reaction rate decreased.[48] Jimenez-Relinque and shape of the photocatalyst, and the amount of water Castellote[49] investigated the PCO of isobutylene at vapour present.[42] As a result of excessive humidity, 50%, 75%, and 90% RH. They observed an increase in hydroxyl groups cover the titanium dioxide surface, and PCO up to 75% RH, followed by a minor decrease at 90% the material active sites are occupied, reducing catalytic RH, indicating that 75% RH is the optimal humidity activity.[43] A VOC molecule needs to get through this level for this device to degrade VOCs. Acetylene was layer of water molecules on the photocatalyst’s surface removed at a rate of 27 ppm/min in dry air, and 16, 10, and be able to diffuse to the photocatalytic surface for a and 8.5 ppm/min when the amount of water vapour was reaction to happen.[44] Depending on the type of pollut- increased to 4000, 8000, and 10,000 ppm, according to ant, water can have a positive or negative impact on a gas Thevenet et al.[42] phase photocatalytic degradation reaction. The best con- dition for humidity for each of the contaminants was suggested in the following sections, and details on the 4.3 | Aromatic hydrocarbons impact of humidity on photocatalytic degradation during the removal of this pollutant is presented as well. According to Zhang et al.’s[50] study of PCO of toluene, the amount of eliminated toluene decreased sharply as RH increased. This is consistent with the findings of 4.1 | Saturated alkanes Park et al.,[51] who discovered that when toluene is decomposed using TiO2 at higher humidity, the compet- Boulamanti and Philippopoulos[45] reported that the reac- ing adsorption of water and toluene reduces the effective- tion rate decreased by an increase in humidity in the sys- ness of toluene removal. Sun et al.[52] discovered that tem, leading to more adsorption of water vapour increasing the RH from 0% to 60% improves the removal molecules on the surface catalyst and a lowering of the rate of toluene, but only slightly when the RH is raised to reaction rate. Debono et al.[46] investigated the effect of 80%. Cui et al.[53] chose RH levels ranging from 10% to RH on decane PCO by conducting the experiment at two 60% in their study. They discovered that as RH increased FIGURE 3 Photocatalytic oxidation (PCO) mechanism of decane as proposed by Debono et al.[46] MASRESHA ET AL. 5 up to 35%, degradation efficiency increased significantly, 4.4 | Oxygen-containing VOCs before gradually declining under more humid conditions. Fang et al.[54] investigated the effect of water vapour Zhang et al.[50] investigated the nano-TiO2/diatomite on the photocatalytic decomposition of toluene in composite’s ability to degrade ketone and alcohol VOCs. another study. The results demonstrated that the addition They found that the optimal RH for isopropanol, isobu- of water vapour significantly reduced the catalysts’ ability tanol, and 1-heptanol degradation was 5%, 15%, and to catalyze reactions. The results showed that the addi- 50%, respectively. When the humidity increased from tion of water vapour significantly reduced the ability of 34% to 41%, Lin et al.[56] discovered that the concentra- the catalysts to catalyze reactions. Kuo et al.[55] investi- tion of formaldehyde HCHO decreased quickly gated the toluene removal efficiencies at RH = 15%, 30%, after 25 min; however, when humidity increased from 45%, and 65%. They discovered that for all tested inlet tol- 47% to 57%, the concentrations of HCHO decreased uene concentrations, the toluene removal efficiency at rather slowly. In trials on the elimination of HCHO, RH = 30% was the best in the tested RH conditions. Xu Liu et al.[59] used an initial HCHO concentration of However, at high RH levels, water adsorption on the 5.5 0.2 ppm and a range of RH values between 40% and photocatalysts prevented toluene adsorption on the 80%. The outcomes showed that for the highest reaction catalyst. rate (3.47 ppmv/h), a RH of 55% was found to be ideal. Lin et al.[56] investigated the photocatalytic degrada- According to Hager and Bauer,[60] 2-propanol photooxi- tion of benzene under different humidity conditions. dation increased marginally with increasing water C6H6 degraded from 1.25 to 0.12 ppm in 45, 35, 45, 60, vapour concentrations and decreased with increasing and 65 min as humidity increased from 33% to 60%, water content. Boulamanti and Philippopoulos[61] inves- according to their findings. The highest degradation effi- tigated the effects of water vapour on methyl tert-butyl ciency of C6H6 was achieved at a humidity of 41%. When ether (MTBE) PCO at various RH values (0%–68% RH). the humidity increased from 33% to 41%, the degradation Higher concentrations of water vapour were discovered efficiency of C6H6 increased significantly, thus indicating to be inhibitory. the favourable effect of increasing the humidity on C6H6 Circumstantiae et al. [34] investigated the effects of degradation. However, as the humidity increased from humidity photocatalysis on the oxidation of NO2 from 41% to 60%, the degradation efficiency continued to vehicle exhaust by TiO2 immobilized on cement-based decline, indicating that high humidity has a negative road materials. According to the study’s findings, as the impact on the photo-catalytic degradation of C H .[56]6 6 humidity increased, the effectiveness of PCO of NO2 Wang et al.[57] examined how humidity concentration decreased significantly. Similarly, Nguyen et al.[62] on affected the UV/TiO2 technique used to break down gas- photocatalytic NO conversion found that increasing RH eous benzene. The air stream utilized in this experiment from 40% to 90% decreased the effectiveness of PCO. had a RH between 0.5% and 80%. With a RH of 0.1%–10%, From the RH range of 40% to 60%, there was no signifi- benzene removal and mineralization increased rapidly. cant change of the PCO efficiency. The maximum value According to the experiment results, a RH level of 10% of degradation of NO efficiency was obtained at about was required to achieve the highest reaction rate. 75% RH after 5 min of irradiation. However, when RH levels were greater than 10%, the effect of increasing RH shifted in the opposite direction. Ao et al.[58] reported that at 2100 ppm humidity, the 4.5 | Chlorinated VOCs conversions of benzene, toluene, ethylbenzene, and o-xylene (BTEX) were 24.9%, 35.6%, 55.2%, and 61.4%, Using pre-treatment TiO2 sol gel sheets, the degradation respectively. As the humidity increased to 2200 ppm, the process of TCE in gas phase was studied to determine the conversions of BTEX dropped to 2.2%, 3.8%, 7.9% and impact of humidity. No matter how the photocatalyst 10.5%, respectively. The experimental results clearly was treated, they discovered that when the RH was over showed that the increase in humidity levels reduced the 50%, the reaction rates dropped.[63] Similar result were conversion of BTEX. also reported by Kazemi et al.[64] A significant change in Jo and Kim[41] investigated the efficacy of target VOC degradation efficiency was not observed as the RH degradation at three RH levels: 10%–20%, 50%–60%, and increased from 10% to 15%, but when RH increased up to 80%–90%. The rate of decomposition slowed for the 30%, degradation efficiency increased to approximately majority of chemicals as RH increased. For two RH 99%. Further increasing of RH reduced the degradation ranges of 10%–20% and 50%–60%, four compounds efficiency due to competitive adsorption of perchloroeth- (ethyl benzene, o, m, and p-xylenes) had a degradation ylene (PCE) and water molecules on the photocatalytic efficiency of more than 90%. surface. 6 MASRESHA ET AL. 4.6 | Sulphur-containing VOCs photocatalysts prevented the adsorption of toluene on the catalyst, which decreases toluene removal efficiency. Ramakrishna et al.[65] reported that the PCO efficiency of diethyl sulphide (DES) decreases with the change of humidity from 87% (RH = 34%) to 78% (RH = 88%). Water 5 | KINETIC MODELS vapour has two opposing effects, competing with fimethyl sulphide (DMS) adsorption on the one hand and producing Equation (2) in a mathematical form shows that as hydroxyl radicals that speed up surface reaction kinetics humidity rises from zero, it always has a negative impact on the other, according to Demeestere et al.[66] The on the photocatalytic activity. In contrast, there are two former effect dominates at RH > 22%; the latter at lower additional ways to increase the humidity: (a) by continu- RH. Table 1 shows the literature reports mentioning ously promoting the reaction, or (b) by having no effect the optimal RH of humidity on photocatalysis for at all. It is therefore evident that Equations (9) and (11) various VOCs. cannot universally represent the influence of humidity as According to the above review, water vapour has both reported in literature. a negative and a positive influence on the activity of cata- lysts for the catalytic oxidation of VOCs. The effect will Kr¼ k BCBKWCW ð10Þ differ depending on the chemical and physical properties ð1þKBCBþKWC 2WÞ of the contaminants and catalyst materials, as well as the pollutant type, concentration, and possibly other experi- KBCB mental factors. For some pollutants, humidity had little r¼ k ð1þ ð11ÞKBCBþKWCW Þ effect on PCO, including for example benzene, ethyl ben- zene, o-, m-, and p-xylenes, TCE, and PCE. The absence where CB is the concentration of the organic reactant B of water for some chemicals hinders the reaction rates (ppm), CW is the concentration of water (humidity) common in toluene and formaldehyde PCO. The result of (ppm), k is reaction rate coefficient (ppm/min), and KB is the PCO of ethylene indicated an inverse correlation adsorption coefficient of the organic reactant B (ppm1). between water vapour concentration and reaction Zhang et al.[44] investigated the influence of humidity rate. PCE degradation efficiency increased when RH on photocatalytic degradation of chlorobenzene. They increased. Further increasing of RH reduced the degrada- developed correlation based on the semi-empirical tion efficiency due to competitive adsorption of PCE and Langmuir–Hinshelwood bimolecular model. Use of the water molecules on the photocatalytic surface. For NO, model with experimental data indicates that the reaction increasing decreased the effectiveness of PCO. For tolu- rate coefficient of chlorobenzene is linearly related to the ene at high RH levels, the adsorption of water on the reciprocal of humidity. According to Lin et al.,[56] HCHO TABLE 1 Summary of literature reports mentioning the optimal relative humidity (RH) of humidity on photocatalysis for various volatile organic compounds (VOCs). Concentration Optimal Removal Pollutant (ppm) Catalyst used Light source RH (%) efficiency (%) References Toluene Carbon-TiO 15 W mercury 60 86.8 Guo et al.[67]2 NO [62]x 350 Iron chloride 150 W xenon 40 77 Nguyen et al. Dimethyl sulphide 275 TiO2 18 W UV 22 22.6 Demeestere et al. [66] Formaldehyde 5.5 TiOSO4 TiO2 8-W UV 55 - Xu Liu et al. [59] Diethyl sulphide 1200 Mn (NO ) 34 87 Ramakrishna et al.[65]3 2 Trichloroethylene 3000 TiO -C 4-W UV 30 99 Kazemi et al.[64]2 Benzene 20 TiO P-25 20-W UV 5 80 Wang and Ku[68]2 Hexane The TiO2 film UV lamp 45 Zhang and Liu [47] Toluene 100 Mo-TiO UVC 35 Jeong et al.[69]2 Isopropanol 10 TiO2/diatomite UVA lamps 5 - Zhang et al. [50] 1-Heptanol 10 TiO2/diatomite UVA lamps 50 - Zhang et al. [50] Isobutylene 4 TiO2 P-25 UV lamp 75 - Jimenez-Relinque and Castellote[49] MASRESHA ET AL. 7 degradation conformed to a pseudo-first-order kinetic (Pt/TiO2) (e.g., benzene, toluene, m-xylene, and styrene). reaction since the fitting plots of ln(C0/Ct) versus the deg- They discovered an increase in catalytic oxidation. radation time t were linear at any humidity and had Iwanaga et al.[74] investigated ethylene photocatalytic correlation coefficients (R2) above 0.98. decomposition. They discovered that humidity has a Toluene photocatalytic removal at the ppb level is likely significant effect on the elimination of ethylene. They dis- a pseudo-first-order process, according to Debono et al.[70] covered that co-depositing Pd and Pt on glass-immobilized However, it is difficult to precisely quantify the boost TiO2, as well as heating the immobilized photocatalysts, through toluene reaction rates. In contrast to the dispersion accelerated the breakdown of ethylene. Belver et al.[75] of the experimental data, the favourable effect of water investigated Pd/TiO2 photocatalysts for toluene vapour vapour on toluene disappearance is minimal. removal. They claimed that the Pd/TiO2 impact was reduced in dry conditions, but no discernible loss of photo- activity was observed during the study. Palladium is used to 6 | STRATEGIES FOR IMPROVING protect catalysts from deactivation caused by partially oxi- PHOTOCATALYTIC EFFICIENCY dized products of the reaction. AIMING AT REMOVING Tu et al.[76] developed aMe/TiO2-zeolite Y (Me = Au, Pd) POLLUTANTS FROM THE HUMID photocatalyst for the gas phase PCO of toluene under ENVIRONMENT highly humid conditions to overcome. Toluene photooxi- dation was enhanced as water vapour concentration The kind of photoreactor, catalyst modification, and sol- increased from 5.91 to 17.9 mg/L. Venezia et al.[77] investi- vent selection are only a few of the variables that gated a variety of Pd-based catalysts for the oxide. have been taken into account for the optimization of the photocatalytic process. The catalytic performance of the nano-catalyst is considerably improved by the surface 6.3 | Titania coated carbon and silica- modification, which is advantageous for the degradation titania composite of VOCs.[71] NiO nanoparticles were synthesized by Park et al.[51] and integrated into mesoporous silica for toluene oxidation at 6.1 | Novel cocatalytic LnOCl/BiOCl temperatures between 250 and 350C. They discovered composite photocatalysts that dampness had both favourable and unfavourable impacts on catalytic activity. They found both positive Okumura et al.[72] investigated nitric oxide (NO) photo and negative effects of humidity on catalytic activity. removal using the novel cocatalytic effects of mechanically Stokke and Mazyck[78] studied the effect of RH (12% and grafted new-type lanthanide oxychloride LnOCl (Ln = Sm, 95%). Due to competition with water vapour for adsorp- Nd) photocatalysis. They discovered that the maximum NO tion sites in a high humidity environment (RH I = 95%), photoremoval rate of the powder composite is approxi- the adsorption capacity of the silica titanium composite mately five times greater than that of pure BiOCl, or (STC) (1.2 mg/g) and titania coated activated carbon 20 times greater if BiOCl were the only active photocatalyst. (AC) (1.9 mg/g) was decreased. 6.2 | Noble metal supported 6.4 | Fabrication of hydrophobic TiO2 photocatalysts photocatalyst In the catalytic combustion of VOCs, precious metals and Surface modification has received a lot of attention base metal oxides are widely used. Due to their increased recently since TiO2 particle surfaces are hydrophilic, activity, the noble metals, particularly platinum and palla- while the majority of organic molecules are hydrophobic. dium, are recognized as the most common species of pre- Silanization, surfactant absorption, and hydrophobic sub- cious metal catalysts. Fu et al.[14] investigated the effect of strates might make it easier for organic molecules to Pt/TiO2 catalysts on the rate of ethylene oxidation over move from the solution to the surface of TiO2 particles. UV-illuminated TiO2. They discovered a significant Another approach for creating hydrophobic supports has increase in simultaneous heterogeneous catalytic oxidation been disclosed, and it comprises a number of silylation for the illuminated Pt/TiO2 catalyst at temperatures rang- techniques employing organosilanes. [79] Kuwahara et al.[80] ing from 40 to 110C. You et al.[73] investigated the removal proposed a method for creating hydrophobic surfaces uti- efficiencies of aromatic hydrocarbons by platinized TiO2 lizing triethoxyfluorosilane, an inorganic functional 8 MASRESHA ET AL. group containing silylation agent (triethoxyfluorosilane The application of different substrates used in gas [TEFS]). Lee et al.[81] created films with superhydropho- photocatalysis is depicted in Table 2. bic characteristics and photocatalytic activity by com- In order to immobilize photocatalytic powders on bining SiO2 and N-TiO2 in the right proportion, various substrates for photoreactor design, many strate- which is 5:5. Wang et al.[82] synthesized SiO2–TiO2– gies have been used. Titanium dioxide (TiO2) immobili- polydimethylsiloxane (PDMS) ternary compound, and zation on various support materials (e.g., silica gel, sol–gel composite SiO2–TiO2 particles were first modi- zeolites, glass beads, activated carbon, and alumina fied with PDMS. They discovered that the coating was quartz optical fibres, glass fibres, or wool) and coating practically superhydrophobic and that SiO2–TiO2– of TiO2 on walls of the reactor as a thin film are two PDMS had some photocatalytic activity. common forms of TiO2 handling in reactors. [98–100] It Likewise, Liu et al.[83] linked PDMS with the surface would be crucial to create a TiO2 photocatalyst sup- of metal-oxide photocatalysts. The photocatalytic activity ported on materials with a wide surface area to clean is enhanced for the superhydrophobic TiO2 with PDMS the environment of storage facilities where the ethylene layer of 2.2 nm compared with bare TiO2. The PDMS concentration is low under low-temperature, high-RH layer reduces the photocatalytic activity of the superhy- circumstances.[101] drophobic TiO2 when it is thicker than 5 nm. Shiraishi et al. [102] studied the effect of humid air on Widati et al.[84] synthesized from SiO2, TiO2, and the photocatalytic decomposition of ethylene. Under both methyltrimethoxysilane (MTMS) precursors. The addi- humid and dry conditions, the glass support, scarcely tion of TiO2 to SiO2–MTMS coated glass increased the adsorbs ethylene, while the glass-immobilized photocata- surface roughness and improved the hydrophobicity. lyst can continuously degrade ethylene quickly due to its Rocha Segundo et al.[85] used TiO2 and ZnO in combina- reduced sensitivity to humid air. TiO2 is advantageously tion to promote the ultra-hydrophobic photocatalytic deposited on glass fibres and glass frits because it reaction. Xing and colleagues[79] synthesized carbon-doped increases the surface area exposed to active surfaces and TiO2/methyltrimethoxysilane flourine doped (MCF-F) the interfacial charge carrier transfer rates, both of which hydrophobically modified photocatalysts by the hydrother- are essential to the effectiveness of photocatalytic mal method. Dioxin was degraded effectively using the systems.[74] synthesized catalyst.[86] Obuchi et al.[103] studied the use of porous silica as a support for TiO2. They reported that the use of Pt-TiO2/SiO2 catalyst enables us to construct a multifunc- 6.5 | Substrate/supported types tional reaction process for air purification, in which VOCs are photocatalytically decomposed. de Chiara et al.’s[104] Numerous studies have found that choosing the right investigation of silica support revealed that it had poor substrate is the best way to address these issues. ethylene adsorption performance and that the silica- TABLE 2 Summary of support and immobilized method reported in the literature. Model compound Support Immobilization method References Dichloromethane Activated carbon powder Torimoto et al.[87] Trichloroethylene Silica gel Sol–gel impregnation Dibble and Raupp[88] Toluene SiO2 support Sol–gel Jeong et al.[69] Propionaldehyde Silica/ferrite/zeolite Sol–gel Takeda et al.[89] Formaldehyde Activated carbon Sol–gel Huang and Saka[90] Toluene SiO2 Chemical vapour deposition Yao and Kuo [91] 2-Propanol X-Zeolite/Porous glass Hydrothermal dip coating Yasumori et al.[92] Acetylene Pyrex glass/organic fibres Dip coating Thevenet et al.[93] Ethylene Glass/silica/activated carbon Hydrothermal method Iwanaga et al.[74] Toluene Aluminium sheets Dip coating Tasbihi et al.[94] Isopropanol Mesoporous silica powders Sol–gel impregnation Tasbihi et al.[95] Toluene Activated carbon Chemical vapour deposition Kuo et al.[55] NOx Silica gel Sol–gel Matsuda and Hatano[96] Ethylene Glass plate Sol–gel Lin et al.[97] MASRESHA ET AL. 9 immobilized photocatalyst decomposed ethylene slowly. reactors have been developed, each with its own advan- However, the dried silica-immobilized photocatalyst tages and disadvantages, such as flat plate, multi-plate, significantly improved ethylene adsorption performance paper-based immobilized TiO2, annular, corrugated plate, and increased the photocatalytic activity for ethylene multi-annular, monolith, packed bed, foam packed bed, decomposition. Thevenet et al.[93] investigated the photo- and fluidized bed, as per Raza et al.[18] degradation of acetylene (C2H2) over powder, Pyrex Using computational fluid dynamics, Wang et al. [110] glass-supported, and non-woven fibre-supported tita- modelled the photocatalytic breakdown of formaldehyde nium dioxide in air into static conditions. The maximum in a honeycomb monolith reactor. It is frequently used in activity was achieved using fibre-deposited photocata- the reduction of NOx in power plant flue gases by catalytic lysts, and the chemical composition of the fibre had no reduction and the control of vehicle exhaust emission. For effect on the photodegradation. the acceleration of gas-phase photocatalytic oxidative dehydrogenation of cyclohexane, Palma et al.[111] used a fluidized bed reactor. 6.6 | Support immobilization techniques Amama et al.[112] used a cylindrical batch reactor for photocatalytic degradation of TCE. The batch reactor is Aqueous or gaseous methods can be used to deposit the simple type of photo reactor used for VOC degrada- TiO2-based catalysts on structured substrates. Sol–gel and tion. The batch reactor typically consists of a Pyrex glass electrophoretic deposition (EPD) are some examples of chamber. aqueous methods given in Table 2, while spray pyrolysis A novel packed bed reactor was proposed by deposition, chemical vapour deposition (CVD), physical Arabatzis et al.[113] for the photocatalytic destruction of vapour deposition, and other processes are examples of VOCs. The packed bed reactors are straightforward, sim- gas phase methods.[105] EPD is a coating method using ple to build, and effective. A cylindrical tube constructed the principle of generating movement of electrical of Pyrex glass, metal, or another material makes up this charges to anode or cathode by applying an electric field kind of reactor. The photocatalyst is located in the central to particles having each positive charges and negative part of the reactor. Shiraishi et al.[102] investigated the charges in a colloidal phase.[106] CVD is a technique effect of photocatalytic decomposition of ethylene con- where a solid material is deposited from a vapour by centration in a spiral-type reactor. They showed a spiral- some chemical reaction occurring on or in the vicin- type photocatalytic reactor that can decompose ethylene ity of a normally heated substrate surface.[107] The at low concentrations and used it to preserve the quality hydrothermal process is used in the autoclave under of agricultural products. A plug-flow, fixed-bed annular regulated temperature and/or pressure, with the reac- reactor that was operated continuously was used to study tions taking place in aqueous solution to create the decomposition of gaseous acetone by the UV/TiO2 nanoparticles.[108] reaction at room temperature (between 20 and 25C).[102] Dip coating is a process in which the substrate is Using a lab-scale continuous-flow annular photoreac- gently and steadily removed from a precursor solution of tor that was filled with glass beads, Kazemi et al.[64] TiO2. Dip coating is a procedure in which a precursor examined the PCO of PCE in the gas phase. In this study, solution of TiO2 is slowly and carefully withdrawn from a lab-scale continuous-flow tubular UV-photoreactor the substrate. The hydrothermal method of nanoparticle filled with coated glass beads was used to analyze photo- synthesis is carried out in the autoclave. The reaction catalytic degradation of PCE in air utilizing titanium requires an aqueous medium, and controlled temperature oxide base catalysts. Table 3 below shows common types and pressure are achieved. This technique is utilized for of photocatalytic reactors usually used for the air synthesizing nanoparticles of TiO2. [109] Among various treatment. common photocatalysts immobilization method, the sol– gel method is common. This method has the characteris- tics of a controllable process, simple operation, high 6.8 | Catalyst regeneration purity, and normal temperature.[83–86] Any use of photo-oxidation requires catalyst regeneration.[138] Recent research has examined the 6.7 | Reactor configuration electrochemical regeneration of saturated activated car- bon by Ti/SnO anode.[139]2 The PCO process, which The effectiveness of the catalytic process for the degrada- promotes the breakdown of organic substances tion of VOCs is significantly influenced by reactor design. when exposed to UV radiation, is another means of Numerous types of laboratory scale photocatalytic regeneration.[140] Satter et al.[141] used heat treatment of 10 MASRESHA ET AL. TABLE 3 Summary of the catalyst used and reactor type in heterogeneous photocatalysis reported in the literature. Pollutant Type of catalyst Reactor References Toluene TiO2 on the activated carbons Fluidized bed Kuo et al. [55] Propionaldehyde Silica, alumina, activated carbon, Pyrex reaction cell Takeda et al.[89] mordenite, femerite, X-type zeolite Toluene (TiO2/c-MDF) carbonized Batch reactor Lee et al. [114] medium density fibreboard Dichloromethane TiO2-1oaded activated carbon Pyrex reaction cell Torimoto et al. [87] Toluene TiO Batch reactor Jeong et al.[69]2 NOx TiO2/silica gel Fluidized bed system Matsuda and Hatano [96] Formaldehyde TiO2 crystallite-activated carbon Batch reactor Huang and Saka [90] 2-Propanol TiO2-zeolite-porous-glass composite Batch reactor Yasumori et al. [92] Toluene TiO2 Degussa P25 Fluidized bed photoreactor Prieto et al. [115] Trichloroethylene Fluidized bed Dibble and Raupp[88] Dimethyl Sulphide Titanium dioxide P25 Flat-plate reactor Demeestere et al.[66] Diethyl Sulphide MnO/Zeolite-13X Batch reactor Ramakrishna et al.[65] Acetone/acetaldehyde/toluene Commercial TiO2 Pyrex glass cylindrical reactor Bianchi et al. [116] Ethanol Nano-MnO2 decoration of TiO2 Batch reactor Stucchi et al. [117] microparticles Benzene BiPO4 Fixed bed tubular reactor Long et al. [118] Benzene Pt/TiO2 catalyst Fixed bed annular Fu et al. [119] Benzene Ga O Fixed bed tubular Hou et al.[120]2 3 Acetone, benzene, toluene Indium hydroxide nanocrystals In2O3 Batch reactor Yan et al. [121] Benzene Pt/TiO2 Batch reactor Li et al. [122] Benzene TiO  P25 Batch reactor Einaga et al.[123]2 2-Butanone Gallium oxide/reduced graphene oxide Batch reactor Ibrahim and Sreekantan[124] No Degussa P-25 titanium dioxide Annular Lim et al.[125] Trichloroethylene TiO2/SiO2 Annulus fluidized bed reactor Lim and Kim [126] Acetaldehyde Ternary g-C N /Ag-TiO composites A continuous gas flow reactor Wang et al.[127]3 4 2 N-Butanol Degussa P25 Cylindrical reactor Batch Kirchnerova et al.[128] 1-Propanol TiO2 P25 Degussa Annular reactor Vincent et al. [129] Propene TiO Batch reactor Bouazza et al.[130]2 Toluene TiO2 Fixed-bed cylindrical Augugliaro et al. [131] Benzene TiO2/Sr2CeO4 Closed stainless steel Zhong et al. [132] Ethylene Zirconia-titania catalyst Batch reactor Tibbitts et al.[133] Trichloroethylene, Degussa P-25 TiO2 powder Plug flow reactor Alberici and Jardim [134] isooctane, acetone Ethylene TiO2 obtained by plasma modification Batch reactor Kajita et al. [135] of Ti foil and oxidation Toluene Commercial TiO Annular reactor Tomašic et al.[136]2 Cyclohexane Pt doped-TiO2 Fluidized bed photoreactor Murcia et al. [137] the spent catalyst in a He flow and regenerated the 6.9 | Photodegradation by vacuum original activity for subsequent reaction. Another group ultraviolet (VUV) light of investigators demonstrated the use continuous adsorption and electrochemical regeneration using an Using VUV degradation as an alternate method to break air-lift reactor.[142] down gaseous organic molecules shows promise.[69] The MASRESHA ET AL. 11 key processes used in the technique are direct photolysis, enhance photocatalytic activity. New techniques for photocatalysis, and ozone oxidation, which is formed by creating catalysts and enhancing their overall perfor- the byproduct ozone and may effectively destroy organic mance should therefore be managed and given more molecules.[143] focus in the future research. The photodegradation of ethylene was studied using UV (254 + 185 nm) irradiation and a TiO2 photocatalyst. AUTHOR CONTRIBUTIONS It was observed that using UV irradiation considerably Girma Masresha: Conceptualization; writing – original improved the photodegradation of ethylene by Chang draft; writing – review and editing. S. Anuradha et al.[144] Jeong et al.[145] studied the degradation of NOx Jabasingh:Conceptualization; supervision; writing – review and toluene by means of UV-C254 + 185 nm/TiO2 irradi- and editing. Shimelis Kebede: Conceptualization; supervi- ation and reported that NOx and toluene were effectively sion; writing – review and editing. David Doo-Arhin: degraded to HNO3 and CO2, respectively. Conceptualization; supervision. Mekdim Assefa: Concep- The effectiveness of palladium-modified TiO2 coatings tualization; writing – original draft; writing – review and in the simultaneous breakdown of formaldehyde under editing. UV254 + 185 nm illumination was studied by Fu et al.[146] According to the findings, UV 254 + 185 nm photocatalysis FUNDING INFORMATION has a greater rate of HCHO breakdown and a longer life- No financial support was received. time of photocatalysts than conventional photocatalysis. CONFLICT OF INTEREST STATEMENT The authors declare that there are no conflicts of interest 6.10 | Conclusions and future to disclose. opportunities PEER REVIEW Since photocatalysis acts at room temperature, creates no The peer review history for this article is available at secondary pollutants, and has a high removal activity, it https://www.webofscience.com/api/gateway/wos/peer- is becoming acknowledged as a reliable and healthy review/10.1002/cjce.24978. treatment approach for VOC elimination. This article summarizes the PCO of VOCs focusing on the differing DATA AVAILABILITY STATEMENT pollutant types. Significant progress has been made in Data sharing is not applicable to this article as no new the development of photocatalysts for air purification. data were created or analyzed in this study. From what was discussed, the following conclusions can be drawn: Humidity will have a significant impact on REFERENCES VOC adsorption, because water molecules and reactants [1] S. Wang, H. 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