Gas-phase aromatic compounds degradation by a partially TiO2 coated photoreactor assisted with ozone

Process Safety and Environmental Protection 135 (2020) 265–272

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Gas-phase aromatic compounds degradation by a partially TiO2 coated photoreactor assisted with ozone Bárbara Maria Borges Ribeiro ∗ , Tânia Miyoko Fujimoto, Bianca Gvozdenovic Medina Bricio, Ursula Luana Rochetto Doubek, Edson Tomaz ∗ Department of Process Engineering, School of Chemical Engineering, University of Campinas, Av. Albert Einstein, 500, CEP, 13083-852, Campinas, SP, Brazil

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Article history: Received 5 November 2019 Received in revised form 11 December 2019 Accepted 24 December 2019 Available online 18 January 2020 Keywords: Volatile organic compound Aromatic compound Heterogeneous p Hotocatalysis Ozone Titanium dioxide

a b s t r a c t Heterogeneous photocatalysis with TiO2 have been studied for VOCs degradation; however, aromatic VOCs degradation lead TiO2 deactivation. The objective of this study was to degrade aromatic VOCs by heterogeneous photocatalysis avoiding TiO2 deactivation. The experiments were conducted using a UV-C lamp (254 nm), relatively humidity (RH – 26–60 %), and ozone (O3 – 2.38 to 5.35 %). Due to O3 addition, it was necessary to have uncoated regions in the quartz tube around the lamp to allow UV photons to reach the gas bulk and generate radicals from O3 . Different quartz tube coating fractions were tested to analyze its influence on VOCs degradation. The highest toluene degradation was 99.2 % for the reactor with 90 % of coating, 3.4 % of O3 , space time of 123 s, and RH of 26 %. It was observed RH greater than 26 % affected the degradation achieved. Ozone addition allowed TiO2 use for 77 h avoiding early catalyst deactivation. Besides that, the requirement of uncoated regions indicated that VOC oxidative reactions occur both on solid and gas phases. This suggested that the addition of O3 combined with the reactor configuration is a promising alternative for aromatic compound degradation by heterogeneous photocatalysis. © 2019 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.

1. Introduction The presence of volatile organic compounds (VOCs) in the atmosphere intensify environmental problems such as the greenhouse effect and the tropospheric ozone formation (Yan et al., 2017) in the atmosphere in the presence of sunlight and nitrogen oxides (NOx ) (Mannucci et al., 2015; Kamal et al., 2016; Yang et al., 2018). Besides, they can directly affect the health of the population next to their emission sources (Song et al., 2019; Lamplugh et al., 2019). Due to this, The National Institute for Occupational Safety and Health (NIOSH) has value of recommended exposure limits (RELs) and The U.S. Environmental Protection Agency (U.S. EPA) has recommended concentrations (RfC) values for different VOCs. For example, REL is 100 ppm by volume (ppmv) for ethylbenzene, toluene and xylene (The National Institute for Occupational Safety and Health, 2020); and the RfCs are 0.23 ppmv, 18 ppmv, and 0.023 ppmv, respectively, above the mentioned concentrations these VOCs can affect nervous and developmental systems (United

∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (B.M. Borges Ribeiro), [email protected] (T.M. Fujimoto), [email protected] (B. Gvozdenovic Medina Bricio), [email protected] (U.L. Rochetto Doubek), [email protected] (E. Tomaz).

States Environmental Protection Agency, 2020). Despite this, these compounds are widely used by industries as solvents, formulation of cosmetics and medicines, fuels, formulation of paints and varnishes and others (Kamal et al., 2016). Due to their large use and the problems associated with their emission, the need for treatment of these compounds is justified. Among the techniques available to degrade VOCs, heterogeneous photocatalysis employing an UV-activated semiconductor catalyst has been extensively investigated in the last decades (Ibhadon and Fitzpatrick, 2013); and more recently the synergistic gains in pollutant degradation when combining it with other techniques such as non-thermal surface discharge plasma (TiO2 /UV/DBD-plasma) for instance, have drawn the attention of researchers (Zadi et al., 2018; Assadi et al., 2017; Abou Saoud et al., 2018). One of the main catalysts employed in this sort of processes is titanium dioxide (TiO2 ), a semiconductor that features high photoreactivity, chemical stability, low toxicity, low cost and ability to degrade different species of VOCs (Nakata and Fujishima, 2012; Mamaghani et al., 2017; Shayegan et al., 2018a). This compound is found in three crystal structures: anatase, rutile, and brookite (Mamaghani et al., 2017; Shayegan et al., 2018a; van der Meulen et al., 2007). The mixture of rutile and anatase phases is recommended to enhance the TiO2 photocatalytic activity (Wetchakun et al., 2019). Thus, TiO2 Evonik P25 (formerly Degussa P25), a

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commercial catalyst, is extensively used in heterogeneous photocatalysis due to its composition of 80 % anatase and 20 % rutile (Mamaghani et al., 2017); although the reason of its high photocatalytic activity is theme of discussion yet (Mamaghani et al., 2017). The wavelength must be less than or equal to 388 nm to activate this semiconductor (Shayegan et al., 2018a) in order to trigger the photocatalytic reactions. In this sense, the black light fluorescence lamp (300−370 nm) and the germicidal lamp (UV-C, 254 nm) are more frequently employed (Boyjoo et al., 2017). Heterogeneous photocatalysis with TiO2 is promising for degradation of VOCs under conditions of low operating temperature and low initial concentration of VOCs. However, the methods already studied are not efficient for the degradation of aromatic organic compounds. Some authors observed the catalyst deactivation and its color changed from white to yellow, when heterogeneous photocatalysis with TiO2 /UV was applied to treat benzene (Einaga et al., 2002; Boulamanti et al., 2008), toluene (Einaga et al., ´ 2002; Boulamanti et al., 2008; Moulis and Krysa, 2013), xylene (Boulamanti et al., 2008; Rochetto and Tomaz, 2015) and, ethylbenzene (Boulamanti et al., 2008). This deactivation can be explained by the adsorption of secondary products on the catalyst surface (Zhong and Haghighat, 2015). Some changes in the process have already been studied, such as the incorporation of metals into the catalyst (Stucchi et al., 2018a, 2016), which raises process costs and complexity of catalyst preparation. The catalysts Fe/TiO2 and Pt/TiO2 were not efficient for benzene, toluene and xylene degradation (Korologos et al., 2012). Benzene, toluene, ethylbenzene, and xylene degradation decreased with Fe/TiO2 /UV, this was explained by the excess of Fe3+ could produce Fe(OH)2+ , which could inhibit the absorption of photons over the wavelength range 290 − 400 nm, and as consequence decrease the photocatalytic activity (Yang et al., 2015). Therefore, according to research developed, TiO2 metal addition enhances VOCs degradation; however, it seems to not improve the degradation of aromatic compounds, which can lead the catalyst deactivation even in the metal presence. Another alternative is the adding of strong oxidants, such as ozone (O3 ) and hydrogen peroxide (H2 O2 ), to the photocatalytic process. Some authors have demonstrated that O3 addition at photocatalytic process with TiO2 improves toluene degradation and minimizes catalyst deactivation (Pengyi et al., 2003; Al Momani and Jarrah, 2009; Huang and Li, 2011; Marchiori et al., 2019). However, most studies were conducted for a single flowrate and VOC inlet concentration below 50 ppmv. This study analyzed the degradation of aromatic organic compounds (toluene, xylene and, ethylbenzene) in the gas phase at 100 ppmv inlet concentration by heterogeneous photocatalysis with TiO2 , UV-C, and ozone. Due to O3 presence, different percentages of TiO2 coating on the quartz tube outer wall were also evaluated. It sought to reach high degradation of aromatics VOCs under short space time avoiding the early catalyst deactivation. 2. Material and methods 2.1. Characterization of the catalyst In this study, the catalyst used was titanium dioxide (Aeroxide® TiO2 P25, Evonik Industries), well known in the literature for the properties described above. This catalyst is composed by 79 % anatase and 21 % rutile, determined by X-ray diffraction (da Silveira, 2017). Fresh catalyst was characterized by nitrogen physisorption (BET method) to determine the catalyst specific surface area and pore volume. For this, 1.00 g of TiO2 was dried at 300 ◦ C under vaccum and the physisorption analysis was conducted at −196 ◦ C in a Tristar Micromeritics ASAP 2010 (Austin, EUA) equipment. Besides that, some analyses were conducted to verify if the catalyst was modified after photocatalysis experiments.

The catalyst was characterized by scanning electron microscopy (SEM), thermogravimetric (TG/DTG), and Fourier-transform infrared spectroscopy (FTIR). The analyses were performed with powder samples of fresh TiO2 and used TiO2 (from the bottom and the top sections of the reactor) to evaluate the catalyst stability. Scanning electron microscopy (SEM) analysis (LEO 440i Leica) was realized to acquire catalyst images before and after its use to visualize its morphology. The samples were coated with 92 Å thickness of gold layer using a 3 mA current during 180 s, then analyzed on the LEO 440i Leica instrument. Thermogravimetric analysis (TG/DTG) was used to quantify the samples’ mass loss. This analysis was carried out on TGA/DSC1 METTLER TOLEDO (Schwerzenbach, Switzerland) equipment with a heating rate of 10 ◦ C/min and temperature varying from 25 ◦ C to 900 ◦ C, 50 mL/min of synthetic air was used. TG and DTG curves were obtained simultaneously. Fourier-transform infrared spectroscopy (FTIR) was realized to determine groups of chemical compounds present in the catalyst surface. The samples were prepared by grinding with KBr powder and then pressed to form pellets. FTIR spectra were obtained on the Nicolet 6700 Thermo Scientific (Madison, USA) instrument with 32 scans and wavenumber varying from 4000 cm−1 to 400 cm−1 . 2.2. Photocatalysis experiment Three compounds were selected for carrying out the experiments as representative of the class of aromatic compounds, which are more difficult to treat than other classes of organic compounds, as previously explained. These compounds were: toluene (purity > 99.5 %, Êxodo), ethylbenzene (purity > 99 %, Sigma Aldrich) and xylene (purity > 97.5 %, Merck), since they are very common in urban areas (Dehghani et al., 2018; de Castro et al., 2015) and frequently used in industrial processes (Upare et al., 2017). The selected compounds were transformed into gas phase by bubbling air in the liquid VOC, thus obtaining contaminated air. This flow was mixed with humid air, also obtained by bubbling air in distilled water. Ozone was generated in loco by an ozone generator (PANOZON P+25) and was also added to this airflow. The amount of ozone produced, in mol. min−1 was determined by the iodometric method, that consists in a titration analysis using 2 % potassium iodide solution, 1 mol.L−1 sulfuric acid, sodium thiosulfate (concentration of 0.0243 mol.L−1 ) and starch indicator (1 %) to easy recognition of the turning point (Marchiori et al., 2019). Therefore, the mixture of air, VOC, moisture, and ozone form the reactor feed, which flowed in the photocatalytic reactor from the bottom to the top. The input stream was introduced into the system at room temperature, however when passing through the reactor UV lamp heated this current which reached on average about 55 ◦ C. Temperature was only monitored since previous work with the system showed no significant differences in equipment performance with this parameter change (Rochetto and Tomaz, 2015). The experimental system is represented in Fig. 1. The photocatalytic reactor, already described (Rochetto and Tomaz, 2015; Marchiori et al., 2019; Fujimoto et al., 2017), is a tubular photoreactor composed by an external titanium cylinder (850 mm length and 69 mm intern diameter), in which a quartz tube with a closed bottom (850 mm length and 55 mm outer diameter) is the central position. A germicidal ultraviolet lamp (UV-MAX, Trojan Technologies) with typical UV C band emission (wavelength of 254 nm ranging about 5 %) and 100 W. The distance between the catalytic layer and the UV C lamp was 18 mm, this was considered as an appropriate intermediate distance because it is not too small which allows the presence of sufficient VOC molecules to be treated and at the same time it is not too large which prevents a possible deviation of the available photon beam. An accurate scheme figure of the reactor was already present in previous article (Rochetto and

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Fig. 1. Experimental system.

Tomaz, 2015). Besides that, the quartz tube outer wall was coated with TiO2 in different area percentages (Doubek and Tomaz, 2018). For this, a 2.5 % w/w TiO2 suspension was prepared with ethanol solution (70 %) and applied with a paint sprayer until a uniform coat was in view. VOC degradation efficiency was determined measuring VOC concentration in inlet and outlet flowrates by a total hydrocarbon with flame ionization detector continuous monitor (THC-FID - Thermo Scientific, 51iLT). This monitor provides VOC concentration in terms of total hydrocarbons, that is, the chemical speciation of possible by-products are not analyzed, thus complete VOC mineralization was considered. Photocatalytic experiments were conducted in order to analyze the influence of relative humidity (RH), inlet ozone concentration, TiO2 coating percentages, and space time. To study RH influence on toluene degradation a range from 26 % to 60 % (measured online by TSP01 sensor probe, ThorLabs) was established; space time was fixed at 123 s, and the chosen quartz tube area coating was 70 %. RH influence was then tested on different ozone concentrations. To verify the ozone concentration influence on toluene degradation, RH (26 %) and space time (123 s) were fixed and ozone inlet varied from 2.38 to 5.35 %. Quartz tube area coating variation from 0 to 100 %, was also investigated. For each coating area experiments were conducted with fixed RH (26 %) and space time (123 s) as ozone inlet varied from 2.38 to 5.35 %. Different TiO2 coating percentages at the quartz tube are object of study, because sections without the catalyst are necessary to allow the complete passage of UV radiation to generate oxygen radicals from O3 that will trigger oxidative reactions at gas bulk. Another studied parameter was space time and its influence on VOC degradation. After identifying the best configuration for toluene degradation, the experiments were performed for other VOCs (ethylbenzene and xylene) replicating the best obtained arrangement. In addition, for the optimal configuration, the stability of the catalyst for toluene degradation was tested through a time-on-stream (TOS). For this

test, space time was set at 70 s and ozone at 3.5 % and a run up to 77 h was conducted with this arrangement. For all experiments, the inlet VOC concentration was, on average, 100 ppm by volume (ppmv) and all data were collected in triplicate. 3. Results and discussion 3.1. Characterization of the catalyst The specific surface area of the catalyst determined by the BET method was 53.30 m2 g−1 , the specific pore volume (BJH method) was equal to 0.19 cm3 g−1 and the pore diameter was 13 nm. These results are closed to values found on the literature (Mamaghani et al., 2017; Kalan et al., 2016; Shayegan et al., 2018b). SEM analysis allowed to observe the morphology of the fresh and used (bottom and top sections) TiO2 (Fig. 2), which suggests the absence of significant difference among the samples. The TG curve of the fresh TiO2 (Fig. 3a) showed a total of 0.42 % mass loss for the range of temperature from 25 ◦ C to 900 ◦ C. For this sample, the DTG curve indicated that the most important event of mass loss occurred around 50 ◦ C, probably due to humidity. The mass loss of bottom and top samples were about 3.49 % and 2.01 % (Figs. 5b and c), respectively. It is noted that the DTG curve of the bottom sample is similar to the fresh sample. The bottom (inlet) sample had significant events of mass depletion around 50 ◦ C (likely due to humidity) and 180 ◦ C, which could indicate the desorption of byproducts. The top (outlet) sample had the most significant event of mass loss around 355 ◦ C, which could also indicate the desorption of by-products, formed during toluene photodegradation. As such, deactivated TiO2 is assumed to be regenerated by thermal desorption. The FTIR analysis (Fig. 4) revealed the presence of adsorbed by-products in both bottom (inlet) and top (outlet) samples after toluene photooxidation, which corroborates with the results obtained by TD/DTG analysis (Fig. 3). FTIR spectra showed a peak at

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Fig. 2. SEM images of TiO2 samples: (a) fresh; (b) bottom (inlet) after toluene photocatalysis reactions; (c) top (outlet) after toluene photocatalysis reactions.

Fig. 3. TG/DTG curves of TiO2 : (a) fresh; (b) bottom (inlet) after toluene photocatalysis reactions; (c) top (outlet) after toluene photocatalysis reactions.

Fig. 5. Relative humidity influence on toluene degradation; space time = 123 s. Fig. 4. FTIR curves for the TiO2 : (a) fresh; (b) bottom (inlet) after toluene photocatalysis reactions; (c) top (outlet) after toluene photocatalysis reactions.

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Fig. 6. Coated area influence on toluene (100 ppmv) degradation, space time = 123 s.

1384.26 cm−1 presents at bottom and top samples, indicating that the formed and adsorbed compounds probably belong to the aldehyde, alkane, alcohol, and phenol functional groups. The spectrum of the top sample shows another peak at 1695.35 cm−1 , indicating that the compounds probably belong to the aldehyde, acid, and aromatic functional groups. This identification was based on FTIR reference data obtained from the Chemistry LibreTexts.1 These results are supported by the results already presented, which indicate that in the first step of toluene oxidation the benzyl radical is formed, which is oxidized and transformed into benzaldehyde, benzoic acid and benzyl alcohol (Binas et al., 2019). 3.2. Photocatalysis experiment Relatively humidity (RH) influence on toluene degradation was investigated for different ozone concentrations (Fig. 5). The results indicated that RH had more influence at 2.4 % ozone concentration; toluene degradation reached 74.8 % at low RH (26 %) whereas at high RH (60 %) the degradation obtained was 58.4 %. On the other hand, RH had no significant influence at high ozone concentration (5.4 %), toluene concentration reached 99.7 % for all RH selected. One possible explanation is that for low concentrations of ozone and high RH there is competition between radicals and they could recombine before oxidizing the VOC. Additionally, when there is a high concentration of ozone, the amount of radicals formed is too high, which would be enough to oxidize the VOC. Relative humidity, varying from 20 % to 60 %, lightly affected toluene (inlet concentration around 5.2 ppmv) degradation by O3 /TiO2 /UV (Pengyi et al., 2003). Toluene (inlet concentration of 900 ppmv) degradation by ZnFe2 O4 /UV was influenced by RH, ranged from 10 % to 75 %. In this case, higher toluene degradation occurred with RH of 75 % after 180 min of irradiation (Mehrizadeh et al., 2017). Therefore, it is reasonable to conclude that relative humidity influence depends on other operating conditions such as type of VOC, VOC inlet concentration, and the presence or absence of a different source of radicals. The influence of ozone concentration on toluene degradation was also analyzed. This study was conducted for different quartz tube coating area percentages (Fig. 6). The increase of ozone concentration allowed raising toluene degradation, which means that there were more radicals available to react with toluene. One exception was in the case in which the quartz tube area coated was 90 %. In this configuration, toluene degradation was 99.2 % for

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ozone concentration equal to or greater than 3.5 %. A non-thermal plasma (NTP) reactor associated with photocatalysis (O3 /TiO2 /UV) showed that increasing ozone concentration (formed on NTP reactor) improved toluene degradation, despite excessive ozone did not contribute to toluene oxidation due to ozone scavenging effect for hydroxyl radicals (Huang and Ye, 2009). The quartz tube 90 % area coated was also the configuration that presented better results for toluene degradation under different ozone concentrations, reaching 87.4 % for an ozone concentration equal to 2.4 %. The second best configuration was the quartz tube with 70 % of the area coated, however, for the ozone concentration of 2.4 % the degradation achieved was 74.5 %. On the other hand, as the concentration of ozone increased toluene degradation also rose and reached 97.4 % for an ozone concentration of 4.2 %. There was also no significant difference in toluene degradation obtained in the configurations in which the quartz tube was completely coated (100 %) and that the tube was not coated with TiO2 (0 %). These results indicate that when TiO2 is applied on the quartz tube outer wall, it is necessary to leave bands without the catalyst to allow the complete passage of UV C light to the gaseous flow to produce highly active oxygen (O•) from ozone. Water in gas flow reacts with active oxygen forming hydroxyl radicals (Huang and Ye, 2009). Therefore, the toluene decomposition mechanism by O3 /TiO2 /UV is complex. It includes toluene reactions with •OH and O•, in addition to the reactions occurring on solid phase (catalyst surface) and at gas bulk. The space time influence on toluene degradation was further studied for different TiO2 coated areas of the quartz tube. The experiments were conducted for 2.4 % and 3.5 % of ozone (Figs. 7a and b, respectively), selected based on Fig. 6 results. For all experiments, higher space time (123 s) increased toluene degradation; that happened because VOC had more contact time with both the catalyst and radicals. Besides, toluene degradation was close under space times of 70 s and 123 s for the experiments conducted at 90 % coated and 3.5 % of O3. Similar behavior was observed for 70 % and 100 % of coated design. N-octane degradation by TiO2 /UV reached 90 % of degradation in a space time of 20 s and the degradation was low for space time higher than 20 s (Rochetto and Tomaz, 2015). Photodegradation of cyclohexane by TiO2 /UV was 80.5 % for a space time of 48.8 s, and the degradation was only 26.5 % for 13.1 s (Marchiori et al., 2019). N-hexane degradation by TiO2-Pd/UV was about 95 % for space time of 43 s and 90 % for space time of 27 s (Fujimoto et al., 2017). Therefore, space time proved to be an important variable that directly affects the degradation of the studied VOCs, even for different photocatalysis processes and various VOCs. Furthermore, the experiment was conducted with a stainless steel reactor without TiO2 to analyze the influence of the reactor material on photocatalysis reactions once the main reactor was constructed with a titanium cylinder. The results showed that the titanium reactor improved toluene degradation compared to a stainless steel reactor. Some molecules that constitute the titanium cylinder are likely in their oxidized form (TiO2 ), so they can be activated by UV-C radiation and contribute to the photo-oxidation of toluene. The degradation of ethylbenzene and xylene was also verified (Fig. 8) under the best reactor configuration (90 % coating) as a function of space time and compared with toluene degradation. Of the three compounds analyzed, toluene is the one that reached greater degradation (93.2 %) with 3.5 % ozone concentration, followed by xylene, which presented a degradation of 87.2 % for the same condition. Considering a space time of 123 s, the worst scenario was the degradation of ethylbenzene (57.8 %) to 2.4 % ozone concentration. These results could indicate that the more complex the molecule, the more difficult to degrade the compound. Further studies should be developed to understand the molecular influence on its photodegradation and the influence of VOCs mix, since it is

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Fig. 7. Space time influence on toluene (100 ppmv) degradation for two different ozone concentration: a) 2.4 % of O3 ; b) 3.5 % of O3 .

Fig. 9. Time-on-stream of toluene degradation, 90 % quartz tube coated with TiO2 , 2.4 % O3 concentration; space time = 70 s. Fig. 8. VOC degradation, 90 % quartz tube coated with TiO2 .

more complex and could be affected by adsorption kinetics (Stucchi et al., 2018b). The deactivation of the catalyst during the experiments was not noticed since the data reproduced well under all operating conditions. However, characterization analyses demonstrated the beginning of catalyst deactivation, since FTIR and GC/MS analyses indicated the presence of organic compounds in the bottom (inlet) and top (outlet) reactor samples. As such, it is important to know the stability of the catalyst, especially if it is of interest to scale up the process for possible applications on industrial effluent treatment or indoor air purification. For this purpose, a time-on-stream (TOS) experiment (Fig. 9) was performed using the best reactor configuration and operational condition: 90 % coating, space time of 70 s, 3.5 % ozone concentration and 100 ppmv initial toluene concentration. Toluene degradation showed some variations during the 77 h of the experiment. The highest degradation achieved was 86 % at 10 h

and the lowest was 76.3 % at 47 h. These results indicated that the ozone presence for aromatic compounds degradation avoids the early deactivation of TiO2 and makes the photocatalysis process more stable. However, it does not totally avoid the deactivation of the catalyst, since after the experiments it was noted that the catalyst changed its color from white to light yellow at the top region of the photoreactor; the TiO2 present at the bottom region of the reactor did not change its color. This is explained by the fact that the gaseous flowed upwards into the photoreactor and probably most of the ozone was consumed in the bottom region. Thus, there was not enough ozone at the top of the reactor to react with the VOCs analyzed and the catalyst deactivation started there. TiO2 deactivation was already observed by other authors (Einaga et al., ´ 2013; Rochetto 2002; Boulamanti et al., 2008; Moulis and Krysa, and Tomaz, 2015) and it is explained by the by-products adsorption on the surface of TiO2 (Zhong and Haghighat, 2015). Despite this study identified the beginning of by-products adsorption, it was not sufficient to cause the catalyst deactivation completely

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since after 77 h of continuous operation toluene degradation was still higher than 85 %. 4. Conclusion Aromatic VOCs (toluene, ethylbenzene, and xylene) degradation higher than 80 % was achieved by photocatalysis using TiO2 , UVC lamp (254 nm), humidity and O3 . Different operating conditions were studied, with varying relative humidity, initial O3 concentration, space time, and percentage of reactor area coated with the catalyst. The RH was shown to influence toluene degradation. The highest VOC degradation was obtained for RH by 26 %, demonstrating that the higher the RH, the lower the degradation of VOC. An O3 concentration equal to 3.4 % is enough to achieve 99.2 % toluene degradation under a space time of 123 s when RH was 26 % and the reactor area coated with TiO2 was 90 %. Besides, it was observed that the shorter the space time, the lower the degradation achieved for the three compounds studied. The coating area with TiO2 proved to be an important variable in the process, and better results were found for 90 % of coating. For smaller or larger percentages, the toluene degradation achieved was lower. This means that it is necessary to have an uncoated fraction of the quartz tube for the passage of UV-C light to generate the radicals that trigger the photocatalysis reaction since it occurs on the TiO2 surface and at gas bulk. The experiments for ethylbenzene and xylene were conducted for the best operating conditions obtained for toluene. Degradation of ethylbenzene reached a low value compared with toluene and xylene. This is likely due to the ethylbenzene molecule being more complex than other molecules. It is suggested further studies to understand and to improve the photocatalytic degradation of these VOCs. Besides that, it is suggested the study of the degradation of VOCs in mixture since it could approach to a real atmosphere. Lastly, catalyst life was prolonged due to the presence of O3 in the photocatalytic process. Thus, the use of this oxidant is an attractive solution for the photodegradation of aromatic VOCs, which could be scaled up to purify indoor ambient and to treat gaseous effluents containing VOC. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This study was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) – Finance Code 141387-2018-1. This was also partially financed by Coordenac¸ão de Aperfeic¸oamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. The authors are thankful to Espac¸o da Escrita – Pró-Reitoria de Pesquisa – UNICAMP – for the language services provided. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.psep.2019.12. 037. References Abou Saoud, W., Assadi, A.A., Guiza, M., Bouzaza, A., Aboussaoud, W., Soutrel, I., Ouederni, A., Wolbert, D., Rtimi, S., 2018. Abatement of ammonia and butyralde-

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