Photocatalytic removal of toluene vapor using TiO2-based materials in a lab-scale reactor

Nghiên cứu khoa học công nghệ 
Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san NĐMT, 09 - 2017 53
PHOTOCATALYTIC REMOVAL OF TOLUENE VAPOR USING 
TIO2-BASED MATERIALS IN A LAB-SCALE REACTOR 
Nguyen Hoang My Linh1, Tran Thi Tuong Van2, 
Nguyen Chi Hieu2, Nguyen Nhat Huy1* 
Abstract: In this study, we applied photocatalytic process for degradation of 
toluene vapor using two types of catalysts: TiO2 nanoparticle (P25) and titania 
nanotubes (TNTs). TNTs nanomaterial was prepared using P25 as precursor via 
alkali hydrothermal method. The morphology, particle and tube sizes were 
determined by scanning electron microscopy (SEM) and transmission electron 
microscopy (TEM). The specific surface area, pore size and pore volume were 
analyzed by Brunauer-Emmett-Teller (BET) method. A lab-scale photocatalytic 
reactor was developed to investigate the removal of toluene vapor with different 
photocatalysts and operational conditions. Results showed that P25 and TNTs 
showed similar removal efficiency at low toluene concentration of ca. 80 ppm (e.g., 
97% for both TNTs and P25). However, TNTs had better photocatalytic activity 
than P25 at high toluene concentration of ca. 375 ppm (e.g., 91% and 84% for 
TNTs and P25, respectively). This study also investigated the effects inlet 
concentration and retention time on the toluene removal efficiency. 
Keywords: Toluene, Environmental photocatalysis, TiO2, TNTs. 
1. INTRODUCTION 
Toluene, having the formula C6H5CH3, is an aromatic hydrocarbon used as a 
solvent in the paint, resin, gum, zinc, and zinc industries. This is also the raw 
material for organic compound synthesis. Toluene adversely affects human health 
as both acute and chronic poisoning. Therefore, WHO (2000) defines Guideline 
Values for Average ambient air concentration of toluene 5 - 150 μg/m3. In 
Vietnam, the maximum allowable concentration of toluene in ambient air is 
regulated at 190 μg/m3 (annual average concentration, QCVN 06:2009/BTNMT). 
In fact, there are many gaseous VOCs (including toluene) control technologies, 
including adsorption by activated carbon, condensation, thermal oxidation, 
catalytic oxidation, and biological filtration. Currently, photocatalysis is a 
preferable choice due to several advantages such as capability of decomposing 
organic air pollutants under room temperature, even for the pollutants that cannot 
be removed by traditional methods [1-3]. 
Nowadays, there are many studies focused on using TiO2 materials due to its 
suitable physico-chemical characteristics, durable material, and environmentally 
friendly photocatalyst. Some popular applications of TiO2 nanoparticles are 
environmental photocatalysis, air cleaning and deodorizing, and bactericidal effect. 
There are some studies to remove toluene vapor using TiO2 P25 material. It was 
reported that the removal efficiency reached about 68% using P25 at toluene 
concentration of 26 ppm but declined to about 20% at toluene concentration of 170 
ppm [4]. In another study, result from the effect of input toluene concentration 
showed that removal efficiency decreased from 100% to 23% when toluene 
concentration increased from 5 to 37 ppm [5]. Since these results showed very low 
Hóa học & Kỹ thuật môi trường 
N. H. M. Linh, T. T. T. Van, , “Photocatalytic removal of  in a lab-scale reactor.” 54 
removal efficiency at high toluene concentration, a study on looking for new 
photocatalytic materials with high removal efficiency at high concentration of 
toluene is attractive. 
The objective of this study is to investigate the removal efficiency of toluene 
vapor using TiO2 and TNTs. The effects of UV light, inlet concentration, and 
retention time on the removal efficiency was investigated. 
2. MATERIALS AND METHODS 
In the study (from Feb. to Jun. 2016), TiO2 P25 nanoparticles and TiO2 
nanotubes (TNTs) were applied for photocatalytic removal of toluene vapors. TiO2 
P25 is a high purity fine white powder made by Merck (Germany) with an average 
particle diameter of 21 nm and specific surface area of 50 m2/g. As seen in Figure 
1, TNTs was synthesized by hydrothermal method using nanoparticles TiO2 P25 
[6]. The SEM and TEM images of P25 and TNTs are displayed in Figure 2. Both 
the materials were then directly used without any pretreatment. 
Figure 1. Synthesis of TNTs by hydrothermal method. 
In order to study the photocatalytic removal of toluene vapors, a laboratory 
scale model was employed. Details of the research model are shown in Figure 3. 
The air flow from the compressed air tank was divided into two lines. The primary 
air went through the gas flow adjustment valve (3). The other line passed through 
Ultrasonic vibration 
for 20 minutes 
Hydrothermal at 
1350C, 24h 
Mixing for 20 
minutes 
Filtration with 
distilled water 
Calcination at 
5009C, 2h 
Ultrasonic vibration 
with HNO3 0,1N 
Drying at 1200C, 
12h 
Filtration with 
distilled water 
TNTs 
TiO2 P25 
NaOH 10N 
Nghiên cứu khoa học công nghệ 
Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san NĐMT, 09 - 2017 55
the toluene flow adjustment valve (5) to the impinger (7) containing the toluene 
solution (8). The gas flow containing evaporated toluene in the impinger was then 
mixed with the primary air and passed through the photocatalytic reactor. 
Figure 2. SEM images of (a) P25 and (b) TNTs 
and TEM images of (c) P25 and (d) TNTs. 
In the reactor, three glass supports coated with photocatalyst were placed 
under three UV-A lights (intensity of 1.25 mW/cm2, highest intensity at 
wavelength of 365 nm). The distance from the lamp to the surface of the catalyst 
material was 35 cm. Toluene concentrations in inlet and outlet were sampled and 
measured by using a KITAGAWA Gas Detector Tube System (Komyo Rikagaku 
Kogyo K.K., Japan) 
Figure 3. Diagram of experimental model. 
b a 
c d 
Hóa học & Kỹ thuật môi trường 
N. H. M. Linh, T. T. T. Van, , “Photocatalytic removal of  in a lab-scale reactor.” 56 
3. RESULTS AND DISCUSSION 
3.1. Effect of photocatalyst material 
Results from photocatalytic experiments using P25 and TNTs are shown in 
Figure 4. It can be observed that the removal efficiency of P25 material was 97.5% 
at a low inlet concentration of about 80 ppm. Removal efficiency by P25 and 
TNTs was comparable at about 97.5%. Hence, it was unable to compare the 
efficiency between the two materials. In addition, the results showed that the time 
for both materials to reach a steady-state condition was 60 min. Therefore, 
additional experiments were carried out under the same operating conditions but 
with higher toluene concentrations of about 375 ppm. 
Figure 4. Toluene removal efficiency over time using P25 and TNTs 
(Experimental condition: gas flow rate 0.7 l/min, retention time 20 s, toluene 
concentration 75 ppm). 
Figure 5. Toluene removal efficiency after 60 minutes using P25 and TNTs 
(Experimental condition: gas flow rate 0.7 l/min, retention time 20 s, toluene 
concentration 375 ppm). 
The removal efficiency after 60 min is presented in Figure 5, where declined to 
84 and 91% when using P25 and TNTs, respectively. As compared to literature, 
the removal efficiency of toluene at 26 ppm using P25 was about 68%, but that at 
concentrations higher than 170 ppm was only about 20% [4]. It can be concluded 
that the P25 material is only highly effective at low concentrations of toluene. On 
Nghiên cứu khoa học công nghệ 
Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san NĐMT, 09 - 2017 57
the other hand, TNTs showed lower efficiency decline when inlet concentration 
increased in this study (i.e. from 97 to 91%). This may be due to its more anatase 
crystalline structure, larger surface area, and higher density as compared to P25. 
TNTs had specific surface area of 106.3 m2/g, which was two times higher than 
P25 (50 m2/g) and 100% anatase crystalline phase structure while P25 had a mixed 
anatase and rutile phase with ratio of 89/11. The crystal size of TNTs (12.4 nm) 
was smaller than that of P25 (anatase is 23 nm and rutile is 21.1 nm). Therefore, 
with the same amount of catalytic material, the density of TNTs is higher than that 
of P25. 
3.2. Effect of toluene concentration 
Inlet concentration is one of the important technical parameters affecting the 
removal efficiency of photocatalysis. Figure 6 demonstrates the effect of initial 
toluene concentration on the removal efficiency using TNTs. When inlet toluene 
concentration increased from 75 ppm to 375 ppm, the removal efficiency slightly 
decreased from 97% to 89%. Some other studies on photocatalysis of TiO2 
materials [4,5] also showed similar decline trends such as from 68% to 20% when 
concentration increased from 26 to 170 ppm [4] and 100% to 23% when 
concentration increased from 5 to 37 ppm [5]. These similar results may be 
explained by the fact that the intermediates generated during the reaction would 
increase as the inlet concentration increases. Consequently, adsorption competition 
occurs on the catalyst surface, which decreases the contact of toluene vapor and the 
surface of the photocatalytic material. On the other hand, the amount of products 
produced increases with the increase of inlet concentration. Therefore, the products 
cannot be completely removed from the catalyst surface, which then causes a 
decline in the reaction rate. In addition, the greater the amount of toluene vapor in 
the incoming air stream could also competitively absorb UV radiation with the 
photocatalytic material, which also causes a reduction in the removal efficiency. 
Figure 6. Effect of inlet concentration on toluene removal efficiency 
(Experimental condition: gas flow rate 0.7 l/min, retention time 20 s). 
3.3. Effect of retention time 
In addition to the inlet concentration, retention time (i.e. air velocity) is also an 
important technical parameter that determines the efficiency of the photocatalytic 
process. Figure 7 illustrates the effect of retention time on the removal efficiency. 
When retention time increased from 14 s to 20 s, the efficiency increased from 88 % 
Hóa học & Kỹ thuật môi trường 
N. H. M. Linh, T. T. T. Van, , “Photocatalytic removal of  in a lab-scale reactor.” 58 
to 93%. Theoretically, when the retention time increases, the time for the toluene to 
remain in the reactor and the contact between toluene and photocatalyst will 
increase, which is beneficial for the photocatalytic reaction. However, as the 
retention time increased from 20 s to 26 s, the efficiency began to decrease from 
93% to 85%. This may be because the increase of retention time lead to the 
accumulation of by-products on the surface of photocatalyst. This is also consistent 
with those reported by Liang et al. [7] where the removal efficiencies using TiO2 
were 20%, 60%, 50%, and 30% with flowrate of 1, 3, 5, and 7 liter per minute, 
respectively. In other work, Zhang and Liao [5] also found that the removal 
efficiency of toluene were 40%, 55%, 67%, 92%, 60%, and 52% at gas velocity of 1, 
2, 3, 4, 5, and 6 cm/s, respectively. These revealed that the retention time affects the 
removal efficiency in both intrinsic photocatalytic reaction rate and mass transfer of 
reactants, products, and by-products on the surface of the photocatalytic material. 
Figure 7. Effect of retention time on toluene removal efficiency 
(Experimental condition: TNTs, toluene concentration 225 ppm). 
4. CONCLUSIONS 
This study has successfully applied TNTs for toluene vapors removal by 
photocatalysis. The type of photocatalyst as well as the optimum conditions for the 
reaction including inlet concentration and retention time were assessed. Both P25 
nanoparticles and TNTs nanotubes were effectively applied for toluene removal at 
low concentrations, which could be applied in ambient condition. However, at high 
toluene concentration, TNTs showed superior activity due to its higher surface area 
and anatase crystallinity as compared to P25, which could be applied in industrial 
VOCs control. Future research should also focus on application and modification 
of some other TiO2 materials to obtain high removal efficiencies with low costs 
and investigation for other VOCs such as benzene, xylene, and ethylbenzene. 
Acknowledgement: This research is funded by Bach Khoa University - VNU-HCM, 
under grant number T-MTTN-2017-84. 
REFERENCES 
[1]. J. Mo, Y. Zhang, Q. Xu, J.J. Lamson, and R. Zhao, "Photocatalytic 
purification of volatile organic compounds in indoor air: A literature review," 
Atmos. Environ., Vol. 43 (14), pp. 2229-2246, (2009). 
Nghiên cứu khoa học công nghệ 
Tạp chí Nghiên cứu KH&CN quân sự, Số Đặc san NĐMT, 09 - 2017 59
[2]. P.V.L. Reddy, K.-H. Kim, and Y.-H. Kim, "A Review of Photocatalytic 
Treatment for Various Air Pollutants," Asian Journal of Atmospheric 
Environment, Vol. 5 (3), pp. 181-188, (2011). 
[3]. T.K. Tseng, Y.S. Lin, Y.J. Chen, and H. Chu, "A Review of Photocatalysts 
Prepared by Sol-Gel Method for VOCs Removal," International Journal of 
Molecular Sciences, Vol. 11 (6), pp. 2336-2361, (2010). 
[4]. J. Palau, J.M. PenyaRoja, C. Gabaldón, F. Javier ÁlvarezHornos, F. 
Sempere, and V. MartínezSoria, "UV photocatalytic oxidation of paint 
solvent compounds in air using an annular TiO2supported reactor," J. Chem. 
Technol. Biotechnol., Vol. 86 (2), pp. 273-281, (2011). 
[5]. X. Zhang and C. Liao, "Photocatalytic Degradation of Toluene by Nano-TIO2 
in a Fluidized Bed," Vol., pp. (2007). 
[6]. N.H. Nguyen and H. Bai, "Photocatalytic removal of NO and NO2 using titania 
nanotubes synthesized by hydrothermal method," J. Environ. Sci., Vol. 26 (5), 
pp. 1180-1187, (2014). 
[7]. W. Liang, J. Li, and H. He, in: (Ed.), D.M.A.F. (Ed.), "Advanced Aspects of 
Spectroscopy," InTech, (2012). 
TÓM TẮT 
NGHIÊN CỨU XỬ LÝ HƠI TOLUEN BẰNG CÔNG NGHỆ QUANG XÚC 
TÁC TRÊN VẬT LIỆU TIO2 TRÊN QUY MÔ PHÒNG THÍ NGHIỆM 
Bài nghiên cứu này cho thấy lần đầu tiên ứng dụng quá trình quang xúc 
tác để xử lý hơi toluen của hai vật liệu xúc tác là TiO2 dạng hạt (P25) và 
TiO2 dạng ống (TNTs). TNTs được điều chế từ P25 ban đầu bằng phương 
pháp thủy nhiệt ở nhiệt độ 5000C. Hình dạng, kích thước hạt và ống được 
xác định bằng kính hiển vi điện tử quét (SEM) và kính hiển vi điện tử truyền 
qua (TEM). Diện tích bề mặt riêng, kích thước lỗ và thể tích lỗ rỗng được 
phân tích bằng phương pháp Brunauer-Emmett-Teller (BET). Đề tài đã xây 
dựng một mô hình thử nghiệm để khảo sát hiệu quả xử lý hơi toluen của P25 
và TNTs ở các nồng độ khác nhau. Kết quả cho thấy P25 và TNTs cho hiệu 
quả xử lý tương đương nhau ở nồng độ thấp (>95%) và TNTs xử lý hơi 
toluen tốt hơn P25 ở nồng độ cao (>90%). Đồng thời, đề tài cũng khảo sát 
sự thay đổi của ba thông số vận hành mô hình: đèn UV; nồng độ đầu vào; 
thời gian lưu. 
Từ khóa: Toluene, TNTs, Xúc tác quang, TiO2. 
Received date, 07th Aug, 2017 
Revised manuscript, 12th Aug, 2017 
Published, 15th Sep, 2017 
Author affiliations: 1 Bach Khoa University, VNU-HCM; 
 2 Ho Chi Minh City University of Industry. 
 * Corresponding author: nnhuy@hcmut.edu.vn.