<figure><img src="image_24.png" /><figcaption>**Figure 20.** (A) Schematic VOC molecule absorption and degradation on the hydrophilic TiO₂-SiO₂ films[260]. (B) TEM image of the as-prepared TiO₂-SiO₂ photocatalyst[261]. Water contact angle of untreated TiO₂ (C) and plasma treated TiO₂[263]. (D) This figure is used with permission from the American Physical Society.</figcaption></figure>

In contrast, the plasma-treated TiO₂ shows a contact angle and is thus considered hydrophobic. The hydrophobic character of modified TiO₂ could enhance the adsorption of the xylene from the flowing air to the surface of the photocatalyst and thus favor the improved photocatalytic activity.

### Other metal oxides

In addition to TiO₂, other metal oxides such as WO₃ and Ga₂O₃ have also been explored for photocatalytic VOC elimination[264-267]. WO₃, as a typical semiconductor with a band gap value of about 2.5 eV, is widely used in photocatalytic oxidation reactions due to its positive VB potential (about 3.15 eV vs. NHE) and good chemical stability[264]. However, pure WO₃ photocatalyst usually displays insufficient photocatalysis oxidation ability because the lower CB edge (+0.3 - 0.5 eV vs. NHE) cannot afford enough reduction ability to reduce O₂ to produce strongly oxidizing radicals such as •O₂⁻ and •HO₂. The inability for O₂ reduction by photoinduced electrons on the CB of WO₃ may also cause the undesired recombination of electron-hole pairs and thus result in poor photocatalytic oxidation ability. It has been revealed that platinum nanoparticles loading can promote the multi-electron reduction of dioxygen to produce active oxygen related species on the CB of WO₃ at a relatively positive reduction potential and thus are favorable for high photocatalytic oxidation ability. Kim et al. reported that the Pt/WO₃ composite gives rise to higher activity for photocatalytic VOC [dichloroacetate (DCA), 4-chlorophenol (4-CP), tetramethylammonium (TMA), and arsenite] removal than that of pure WO₃[267]. Specifically, the loading of platinum nanoparticles led to concurrent oxidative reactions on the surface of WO₃; meanwhile, the OH radicals were also produced by the reductive decomposition of H₂O₂, which was formed via the in situ reduction of O₂. It is noteworthy that Pt/WO₃ presented a higher rate of H₂O₂ generation than that of WO₃, thus favoring the OH radical
formation and leading to the promoted photocatalytic oxidation performance [Figure 21A and B].

Hou et al. fabricated porous $Ga_2O_3$ for efficient photocatalytic benzene decomposition[265]. Significantly, the photocatalytic elimination rate of gaseous benzene over $Ga_2O_3$ was about two times greater than that of commercial P25. This enhancement can be attributed to that $Ga_2O_3$ possesses stronger oxidative capability due to its more positive CB level [Figure 21C] and higher specific surface area in comparison with P25 photocatalyst.

### Metal hydroxide

Apart from the above-mentioned metal oxides, metal hydroxide can also be used as a photocatalyst in adsorption-photocatalysis coupling for VOC removal[268-270]. For example, In (OH)₃ is a wide-gap semiconductor and can be used for UV light-driven photocatalytic reactions. Thus, using In (OH)₃ materials in adsorption-photocatalysis coupling has been conducted. Typically, In (OH)₃ nanocrystals with mesoporous structure have been fabricated by peptization of a colloidal precipitate with a post-heat treatment for acetone, benzene, and toluene degradation under UV light irradiation[268]. Specifically, the In (OH)₃ sample with a post-heat treatment temperature of 120 °C gives rise to the optimal photocatalytic performance for acetone degradation [Figure 21D], which can be ascribed to several major factors. Firstly, the In (OH)₃ sample post-treated at 120 °C exhibits the highest specific surface area. Therefore, more VOC molecules can be attracted on the surface of In (OH)₃, favoring enhanced photocatalytic ability. In addition to the surface area, the porous character of In (OH)₃ sample can also lead to its enhanced activity. Specifically, the existence of abundant mesopores in the In (OH)₃ nanoparticles brings out the fast diffusion and transfer of organic molecules during the photocatalytic process and increases the VOC removal rate. Finally, the crystal phase should be considered during the photocatalytic process. When the post-heat treatment temperature is over 120 °C, the new phase of $In_2O_3$ is formed due to the transformation from In (OH)₃. The reduced ratio of In (OH)₃ in the photocatalyst results in low photocatalytic performance. Thus, the post-heat treatment temperature of 120 °C is the optimal condition for obtaining the In (OH)₃ with the best photocatalytic activity.

### Factors affecting PCO performance

Generally, several factors such as humidity, airflow rate, light source, and the initial VOC concentration significantly influence adsorption-photocatalysis coupling for VOC removal. These factors are thoroughly outlined in this section.

#### Humidity

The relative humidity plays a great role during the photocatalytic oxidation process. In fact, the humidity can be both profitable and detrimental to VOC degradation[232,233]. Particularly, water molecules can take part in the photocatalytic removal process by serving as competitive adsorbents and resources of ·OH radicals. For the competitive adsorbents, water molecules absorbing on the surface active sites of photocatalysts will decrease the adsorption ability of VOC molecules and restrict the subsequent photocatalytic reactions[268]. As to the resources of ·OH radicals, the produced ·OH radicals from water molecules can accelerate the photocatalytic oxidation reaction, thus favoring photocatalytic VOC removal[271]. Hence, carefully modulating the relative humidity for efficient photocatalytic VOC removal is of great significance.

#### Airflow rate

Increasing the airflow rate can present two distinct effects for photocatalytic VOC removal[234,265,266,272]. On the one hand, a fast airflow rate will lead to the reduced residence time for photocatalytic VOC oxidation and thus decrease VOC degradation ability. On the other hand, the promoted mass migration between the VOC
<figure><img src="image_25.png" /><figcaption>**Figure 21.** (A) Formation of H₂O₂ over WO₃ and Pt/WO₃ under visible-light excitation. (B) ESR spectra of DMPO-OH• adducts in an aqueous dispersion of Pt/WO₃ and WO₃[267]. (C) Bandgap structure of WO₃ and Ga₂O₃[265]. (D) The photocatalytic acetone oxidation over In(OH)₂S treated at various temperatures: (a) 60 °C; (b) 120 °C; (c) 160 °C; (d) 200 °C; and (e) 250 °C. (f) P25 was adopted as a reference[268]. This figure is used with permission from the American Physical Society.</figcaption></figure>

molecules and photocatalysts under a high airflow rate can improve the pollutant removal efficiency[272,273]. Based on the above discussion, finding an optimum airflow rate to achieve maximum residence time of photocatalytic oxidation reaction and simultaneously optimal mass migration between the VOC molecules and photocatalysts is of significant importance to acquire the highest photocatalytic VOC molecule degradation efficiency.

Yang *et al.* studied the influence of various airflow rates on photocatalytic gaseous formaldehyde removal[273]. It was revealed that enhancing airflow rate brings out a fast mass transfer and a low concentration gradient between the bulk and the photocatalyst surface in laminar regimes. The influence of airflow rate on pollutant elimination falls under three distinct regimes. In low airflow rates, improving airflow rate favors VOC molecule elimination, which suggests that mass transfer on the surface of photocatalyst restricts VOC removal. In intermediate airflow rates, flow rate variation has no considerable effect on VOC removal. This means that surface reaction kinetics is the controlling stage. Finally, in high airflow rates, an increased airflow rate decreases the residence time for transferring VOCs contaminants from the gas phase to the catalyst surface, thereby decreasing the VOC removal rate.

### Light source

Photogenerated electron-hole pairs are the key factor during the photocatalytic VOC pollutant oxidation process. Frequently used catalysts such as TiO₂ need a UV light equivalent energy source for the
illumination to produce photoinduced charge carriers. Among a series of reports, high-pressure mercury lamps, xenon lamps, and UV lights are major light sources for photocatalytic VOC removal[271-273]. Generally, the VOC elimination rate is proportional to the power of the light source. The photocatalytic VOC removal rate can be modulated by the first order of depletion rate of photogenerated carriers and a half order of their recombination speed. Therefore, the power of the light source has a direct influence on the removal rate. As UV light is deleterious to human beings and possibly results in forming secondary pollutants such as strong oxidizing species in the atmosphere[273], visible-light-driven VOC removal should be further researched in detail.

## Deactivation and reactivation

The chemical and physical stability of photocatalysts is very important for long-term VOC removal. In fact, deactivation, regeneration, and reactivation often occur during the photocatalytic VOC treatment process[234]. For the adsorption-photocatalysis coupling system, the photocatalytic rate usually decreases with extending reaction time and the surface active sites simultaneously reduce due to the deactivation of the photocatalyst. The accumulation of reaction intermediates on the surface of photocatalyst leads to deactivation because these intermediates occupy active sites. For example, reaction intermediates such as formic acid are often adhered to the surface of TiO₂ photocatalyst due to its strong affinity during photocatalytic acetaldehyde elimination[235]. Although a certain amount of formic acid can be emitted into the atmosphere, the occupation of the surface active site results in the deactivation of TiO₂ photocatalyst. In practical application, the photocatalyst will be totally deactivated after 20 consecutive photocatalytic acetaldehyde degradation reactions because all the active sites capture reaction intermediates[274]. Therefore, the recovery of photocatalyst is put on the agenda. Typically, the absorbed reaction intermediates, including benzaldehyde and benzoic acid, could be totally removed with a heat treatment over 653 K. Hence, the reactivation of the photocatalysts by post calcination is often adopted.

## Pollutant concentration

The pollutant concentration has a non-negligible effect on photocatalytic VOC removal[275]. Many studies have suggested that the appropriately lower concentrations of VOC lead to the increased elimination ability of VOCs, which is ascribed to the active sites of photocatalysts possessing limited adsorption capacity[275-277]. Moreover, the produced reaction intermediates will prohibit the adsorption of VOC molecules, especially when the pollutant concentration is high in the reaction system. As a result, the adsorption-photocatalysis coupling reaction system is more likely to treat VOCs at a relatively low concentration. As to the high concentration VOC removal, the surface of the photocatalyst will become saturated because of the shielding effect of the pollutant molecules and is thus detrimental to the efficient VOC treatment[277]. Moreover, the saturation and competition of photocatalyst between adsorbed pollutants are higher when inlet VOCs are at a high concentration.

## CONCLUSION AND OUTLOOK

Human exposure to VOCs in the air is unavoidable in everyday life. Some VOCs are categorized as carcinogenic or reprotoxic by the European Union and the World Health Organization. Air-liquid interface exposure systems can be used to assess the toxicity of VOC emissions on respiratory symptoms. These techniques have the advantage of eliminating any direct interaction between the tested toxicants and the culture medium's components. Some experiments have used this technique to investigate the toxicity of single and mixed VOCs, as well as the compounds generated during the total catalytic oxidation of VOCs. By measuring the activation of XMEs, which are conventionally induced by polycyclic organic compounds, as well as the activation of AhR, in lung cells exposed to emissions from the catalytic degradation of toluene, the formation of high molecular weight compounds in total catalytic oxidation of VOCs can be deduced.
The toxicological study used to validate the system revealed the existence of by-products such as PAHs that were not discovered by the standard analytical method after the catalytic test. Therefore, adding an adsorption system seems to be a promising approach for thorough purification and validation of the catalytic processes. In this context, physico-chemical characterization of gaseous effluent is essential to implement an adapted and efficient VOC treatment technology. Data concerning the process, the chemicals used, and the effluent parameters (temperature, humidity, dust content, pressure, flow, etc.) must be collected first. Then, chemical analysis can be performed on a laboratory scale on the raw materials in order to prepare accurate on-site measurements. At last, continuous (e.g., THC analyzer) and semi-continuous (e.g., specific on-line analyzers, sampling on sorbent tubes, impingers, and containers) analytical techniques can be deployed on site for detailed evaluation. Continuous measurements give valuable information concerning the emission profile (cyclic or regular process) and the global concentration level, whereas semi-continuous techniques give detailed information about the specific VOCs involved. Depending on the industrial process studied, the VOC mixture to be treated can be quite complex and contain several chemical groups (e.g., aromatics, alcohol, esters, etc.), which may not react in the same way to the treatment technique.

In this review, three coupling techniques are presented, outlining their advantages and characteristics for effective removal of hazardous VOCs:

*   **Adsorption-catalytic oxidation:**

Adsorption and catalytic oxidation are both efficient ways of environmental remediation of VOCs, whereas these techniques are sometimes not sufficient for some industrial applications, which produce low concentrated VOC emissions. Coupled, these systems become more adapted to treat diluted gaseous streams of VOCs (> 1%) since the adsorbent helps ensure a pre-concentration of VOCs before their elimination by catalytic oxidation. Adsorbents can act as solid catalysts, can be used as a support for a catalytic phase ensuring a high dispersion of the active species, and can be used prior to the catalytic set to eliminate organic or inorganic compounds (such as H₂S). The most used adsorbents cited in the literature are activated carbon (AC), biochar, zeolites, etc. Experimental conditions are important to surpass some of the drawbacks these materials can present. Hybrid adsorption-catalytic technology exhibiting high adsorption capacity, catalytic activity, and selectivity is provided as a new practical, efficient, and economical alternative to the conventional VOC removal treatments for controlling small concentrations of VOCs in industrial emissions.

*   **Adsorption-plasma catalysis:**

Although NTP has been proposed as an end-of-pipe technology for the removal of VOCs, the formation of toxic by-products is still the major hindrance to the commercialization of this technique. The main strength of plasma catalysis lies in the synergetic effect of the two individual technologies, which enhances the mineralization efficiency and reduces the formation of unwanted by-products by catalysis. For in-plasma catalysis (IPC), the catalytic material is placed either within the plasma discharge or in the downstream of the plasma discharge (post-plasma catalysis). Several points are important for a catalyst suitable for NTP environment: its stability in plasma and in situ regeneration by NTP exposure. Although plasma catalysis has been proven more efficient than the individual techniques for VOC abatement, the treatment of a large volume of exhaust gas containing a very low concentration of VOCs needs an alternative approach: adsorption-plasma catalysis (APC).
* Adsorption-photocatalysis:

The adsorption-photocatalysis coupling technology is also one of the effective and promising methods for VOC removal. Compared with the high temperature and pressure adopted during adsorption-catalysis and plasma catalysis processes, the adsorption-photocatalysis process gives rise to obvious advantages in VOC removal such as excellent elimination efficiency, low energy depletion, and being environmentally friendly. Metal oxides are major candidates for VOC elimination via adsorption-photocatalysis coupling. Generally, several factors such as humidity, airflow rate, light source, and the initial VOC concentration significantly influence adsorption-photocatalysis coupling for VOC removal. As a result, the adsorption-photocatalysis coupling reaction system is more likely to treat VOCs at a relatively low concentration.

Therefore, for any industrial emission, one of these coupling techniques should provide complete, low-energy VOC removal. On-site characterization is essential to making the choice, and a toxicological validation is recommended to determine if an additional coupling adsorbent is needed for the complete removal of VOCs.

# DECLARATIONS

## Acknowledgement

The authors want to acknowledge Karen Leus (Univ. Ghent, Research Unit Plasma Technology) for her contribution in obtaining the copyrights permissions of the figures and tables of the adsorption/plasma catalysis part in this review.

## Authors' contributions

This review is the result of a collaborative work between different teams and laboratories in the Interreg project:

* Toxicity of VOCs:

-Substantial contributions to conception, data curation and design of the study: Billet S, Landkocz Y, Méausoone C, Jaber N, Courcot D

-Writing-original draft preparation and review editing: Billet S

* Measurements of VOCs:

-Substantial contributions to conception, data curation and design of the study, writing-original draft preparation and review editing: Cazier F, Dewaele D, Genevray P

* Adsorption/catalysis of VOCs:

-Substantial contributions to conception, data curation and design of the study: El Khawaja R, Poupin C, Cousin R, Heymans N, De Weireld G, Siffert S

-Writing-original draft preparation and review editing: El Khawaja R, Siffert S

* Adsorption/plasma catalysis of VOCs:

-Substantial contributions to conception, data curation and design of the study: Veerapandian SKP, Bitar R, Abdallah G, Sonar S, Morent R, De Geyter N, Löfberg A, Lamonier JF, Giraudon JM

-Writing-original draft preparation: Veerapandian SKP, Bitar R

* Adsorption/photocatalysis of VOCs:

-Substantial contributions to conception, data curation and design of the study, writing-original draft preparation and review: Barakat T, Ding Y, Su BL

All authors contributed to the discussion and the revision of the paper. They have all read and agreed to the published version of the manuscript.
**Availability of data and materials**

Not applicable.

**Financial support and sponsorship**

The “DepollutAir” project (grant number 1.1.18) of the European Program INTERREG V France-Wallonie-Vlaanderen (FEDER), Région Wallone and Région Hauts-de-France are acknowledged for the funding and their support for this work.

**Conflicts of interest**

All authors declared that there are no conflicts of interest.

**Ethical approval and consent to participate**

Not applicable.

**Consent for publication**

Not applicable.

**Copyright**

© The Author (s) 2022.

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<figure><img src="image_26.png" /><figcaption>Rebecca El Khawaja</figcaption></figure>

Rebecca El Khawaja obtained her M.Sc. in physical chemistry of materials from the Lebanese University in 2017. She worked with the Rennes School of Chemistry on her final project to study the hydrogenation of aromatic compounds in the presence of ordered mesoporous catalysts using various alumina precursors and organic surfactants. During her PhD at the University of the Littoral Opal Coast, Rebecca worked on the development of hybrid systems based on adsorption/catalysis coupling for the catalytic oxidation of oxygenated and cyclic VOCs from industrial sources. Her interests include hybrid material, catalysis, depollution and recently waste valorization as she joined EPIC research team at Polytechnique Montreal as a postdoctoral fellow.

<figure><img src="image_27.png" /><figcaption>Savita Kaliya Perumal
Veerapandian</figcaption></figure>

Savita Kaliya Perumal Veerapandian received her PhD degree in Engineering Physics from Ghent University, Belgium in 2021. She was working on the Sequential adsorption plasma catalysis for the abatement of toluene under the supervision of Prof. Rino Morent. Her research work was carried out within the framework of INTERREG project DepollutAir. The main aspects of her research include reducing process cost by sequential treatment, reducing material cost by plasma assisted regeneration of the catalysts and improving the product selectivity and yield. During her research career, she published 14 articles in peer-reviewed journals and she actively participated in 12 International conferences and workshops.

<figure><img src="image_28.png" /><figcaption>Rino Morent</figcaption></figure>

Rino Morent obtained in 2004 his PhD in Engineering: Applied Physics at Ghent University. In 2012 he was appointed as professor in plasma technology at Ghent University, thanks to obtaining the ERC Starting Grant PLASMAPOR. At the moment he is Full Professor and leading the experimental Research Unit Plasma Technology (RUPT). RUPT focuses on the development of different atmospheric pressure plasma sources (including pulsed discharges, plasma jets & dielectric barrier discharges) and their applications in gas conversion, materials science, air cleaning, energy solutions, biomedical applications and plasma medicine.
<figure><img src="image_29.png" /><figcaption>Guy De Weireld</figcaption></figure>

Guy De Weireld is full professor in chemical thermodynamics, environment, industrial processes, and sustainable development at the Faculty of Engineering, University of Mons (Belgium). His main research field is adsorption in porous materials and catalysis. He has experience in CO2 capture, gas separation, gas purification (removing of acid compounds from natural gas, VOC from exhaust air) and CO2 conversion to fuel as well as techno-economic and environmental assessments in carbon capture, utilization, and storage. Since 2019, he is the coordinator of the H2020-MOF4AIR project: Metal Organic Frameworks for carbon dioxide Adsorption processes in power production and energy Intensive industries.

<figure><img src="image_30.png" /><figcaption>Yang Ding</figcaption></figure>

Yang Ding received his M.Sc. degree from College of Chemistry and Materials Science at Shanghai Normal University in 2018. He is currently a PhD student in the Laboratory of Inorganic Materials Chemistry (CMI) of University of Namur under the supervision of Prof. Bao-Lian Su. His research interests include porous materials synthesis and characterization, photocatalytic pollutants elimination and sustainable energy production.

<figure><img src="image_31.png" /><figcaption>Renaud Cousin</figcaption></figure>

Renaud Cousin received his PhD degree in Spectroscopy and Chemistry from Littoral Côte d'Opale University in Dunkirk, France, in 2000 on the topic "Soot Oxidation". After a postdoctoral position at the University of Strasbourg sponsored by Daimler, he worked as Assistant Professor at the Littoral Côte d'Opale University, France, from 2003 to 2016. In 2014 he obtained the accreditation to Supervise Research (Habilitation Thesis). Since 2016 he was promoted to Full Professor. Currently his research focuses on the development and characterization of heterogeneous catalysts for application to the elimination of environmental pollutants (Soot, CO, VOCs, ...). He has co-authored over 80 peer reviewed scientific papers and 1 patent.
<figure><img src="image_32.png" /><figcaption>Fabrice Cazier</figcaption></figure>

Fabrice Cazier obtained his PhD in Chemical Sciences at Lille University (France) in 1990 with works dealing with "Development of analytical methods for organic and nitrated pollutants in the atmosphere". He worked then as a researcher for the Air Quality monitoring network on VOC and nitrogen oxides monitoring. In 1993, he managed the creation of an analytical laboratory dedicated to risk evaluation and environmental analysis. He became the Director of this unit in 1995. Since then, as Director of the Common Centre of Measurements (CCM) of the Université du Littoral Côte d'Opale (ULCO), he has developed a large expertise in chemical analysis applied to the environment and manages a highly equipped laboratory available as well for research teams as for industrial partners. He is notably specialized in gaseous compounds and particulate matter measurements.

<figure><img src="image_33.png" /><figcaption>Sylvain Billet</figcaption></figure>

Sylvain Billet has been an associate professor in industrial and environmental toxicology at the Environmental Chemistry and Interactions with the Living Organisms at the University of the Opal Coast (UCEIV, ULCO), France since 2009. He is general secretary of the French Cellular and Molecular Toxicology Society (STCM). His PhD in toxicology focused on the biotransformation and genotoxicity of organic pollutants adsorbed on the surface of fine atmospheric particles. He has since set up a method of exposure of complex cultures of human respiratory cells in order to characterize the toxicity of gaseous air pollutants alone or in mixtures.

<figure><img src="image_34.png" /><figcaption>Jean-François Lamonier</figcaption></figure>

Jean-François Lamonier is a full Professor in Department of Chemistry and Deputy Director of the Chevreul Institute, Lille University (France). Jean-François Lamonier leads the team "Catalytic Remediation" of the « Unité de Catalyse et Chimie du Solide » laboratory. His research addresses the catalytic oxidation technologies for Volatile Organic Compounds emissions removal and the catalytic pyrolysis of plastic waste. His research comprises (i) the development of transition metal oxide nanomaterials with emphasis in the elucidation of the structure-chemical properties and catalytic activity relationship and (ii) the coupling of abatement technologies such as non-thermal plasma and heterogeneous catalysis. For this last-mentioned topic Jean-François Lamonier heads the International Associated Laboratory "Plasma & Catalysis" between Lille University and Ghent University (Belgium).
<figure><img src="image_35.png" /><figcaption>Bao-Lian Su</figcaption></figure>

Bao-Lian Su created the Laboratory of Inorganic Materials Chemistry (CMI) at the University of Namur, Belgium in 1995. He is Full Professor, Member of the European Academy of Sciences, Member of the Royal Academy of Belgium, Fellow of the Royal Society of Chemistry, UK and Life Member of Clare Hall College, University of Cambridge. He is also a strategy scientist at Wuhan University of Technology, China. His research fields include the synthesis, the property study and the molecular engineering of organized, hierarchically porous and bio-organisms for artificial photosynthesis, (photo) Catalysis, Energy Conversion and Storage, Biotechnology, Cell therapy and Biomedical applications.

<figure><img src="image_36.png" /><figcaption>Stéphane Siffert</figcaption></figure>

Stéphane Siffert is a full Professor in Department of Chemistry and Director of Unit of Environmental Chemistry and Interactions with leaving organisms (UCEIV, UR 4492, FR CNRS 3417), Dunkerque (France). He was coordinator of three European Interreg projects till 2022 on Volatile Organic Compounds (VOC) removal. His mean research is on catalytic treatment of pollutants, especially the oxidation of VOC, and also purification and valorisation of CO2. He works on oxides synthesized by "classic" and "hydrotalcite" way, zeolites and highly structured meso and macroporous compounds and noble metals catalysts.