Chemical Engineering and Processing 45 (2006) 959–964

Removal of VOCs by photocatalysis process using adsorption enhanced TiO2–SiO2 catalyst Linda Zou a,∗ , Yonggang Luo b , Martin Hooper c , Eric Hu b a

Institute of Sustainability and Innovation, Victoria University, P.O. Box 14428, Melbourne, Vic. 8001, Australia b School of Engineering and Technology, Deakin University, Geelong, Vic. 3217, Australia c Gippsland Centre for Environmental Science, Monash University, Churchill, Vic. 3842, Australia Received 28 February 2005; received in revised form 5 July 2005; accepted 19 January 2006 Available online 4 April 2006

Abstract Volatile organic compounds (VOCs) exist widely in both the indoor and outdoor environment. The main contributing sources of VOCs are motor vehicle exhaust and solvent utilization. Some VOCs are toxic and carcinogenic to human health, such as benzene. In this study, TiO2 –SiO2 based photocatalysts were synthesized using the sol–gel method, with high surface areas of 274.1–421.1 m2 /g obtained. Two types of pellets were used as catalysts in a fixed-bed reactor installed with a UV black light lamp. Experiments were conducted to compare their efficiencies in degrading the VOCs. Toluene was used as the VOC indicator. When the toluene laden gas stream passed through the photocatalytic reactor, the removal efficiencies were determined using a FTIR multi-gas analyser, which was connected to the outlet of the reactor to analyse the toluene concentrations. As the TiO2 –SiO2 pellets used have a high adsorption capacity, they had dual functions as a photocatalyst and adsorbent in the hybrid photocatalysis and adsorption system. The experiments demonstrated that the porous photocatalyst with very high adsorptive capacity enhanced the subsequent photocatalysis reactions and lead to a positive synergistic effect. The catalyst can be self-regenerated by photocatalytic oxidation of the adsorbed VOCs. When the UV irradiation and feeding gas is continuous, a destruction efficiency of about 25% was achieved over a period of 20 h. Once the system was designed and operated into adsorption/regeneration mode, a higher removal efficiency of about 55% was maintained. It was found that the catalyst pellets with a higher surface area (421 m2 /g) achieved higher conversion efficiency (100%) for a longer period than those with a lower surface area. A full spectrum scan was carried out using a Bio-rad Infrared spectrometer, finding that the main components of the treated gas stream leaving the reactor, along with untreated toluene, were CO2 and water. The suspected intermediates of aliphatic hydrocarbons and CO were found in minimal amounts or were non detectable. The kinetic rate constants were calculated from the experimental results, it appeared that the stronger adsorption capacity, i.e. larger specific surface area, the higher conversion efficiency would be achieved. © 2006 Elsevier B.V. All rights reserved. Keywords: Volatile organic compounds (VOCs); Photocatalysis; Synthesize; Titanium dioxide; Efficiency; Adsorption; Toluene; Chemical composition

1. Introduction Titanium dioxide has been used as a catalyst for the UVinduced photocatalysis of pollutants in recent years. Photocatalytic reactions are attractive because they do not require high temperature operational conditions and they can be very selective in radiation absorption. Research indicates that almost any organic pollutants and many of the inorganic pollutants produced by electrical, electronic, agricultural, textile, petrochemical, metallurgical and many other industries can be completely

∗

Corresponding author. Tel.: +61 3 9919 8266; fax: +61 3 9919 8284. E-mail address: [email protected] (L. Zou).

0255-2701/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2006.01.014

destroyed or separated [1]. Volatile organic compounds (VOCs) are one group of air pollutants that degrade the air of the indoor and outdoor environment. BTEX is an acronym for benzene, toluene, ethylbenzene, and xzylene. This group of VOCs is found in petroleum hydrocarbons, such as petrol, and other common environmental contaminants. Toluene is used as an indicator of VOCs in this research, this is due to that it belongs to the concerned BTEX group of VOCs and it has less toxicity than benzene and safer to be used in the laboratory environment. Using photocatalytic oxidation to remove toxic air pollutants is a very promising process. Over the past decade, the photocatalysis of organic compounds in water has received considerable attention, but there is a rapidly increasing interest in the oxidation of volatile organic compounds in

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the gas phase. Some research has been conducted on the application of the photocatalytic process to degrade toxic organic compounds in air streams [1–3]. Conventional methods currently used to treat VOCs include incineration, condensation, adsorption and absorption. Incineration and condensation are cost-effective only for moderate to high VOC concentrations. Adsorption and absorption do not destroy VOCs but simply transfer them to another medium. The humid gas stream can plug the condenser, and fill up the adsorption site of the adsorbent. None of these methods is cost-effective for gas streams with low to moderate concentration and large numbers of compounds, as the recovery and reuse of the compounds is not economically feasible. Photocatalytic systems do not have the above mentioned problems and are cost effective for treating low concentration gas streams. The heterogeneous photocatalytic process used in pollutant degradation involves the adsorption of pollutants on the surface sites, and the chemical reaction to convert pollutant into carbon dioxide and water. Titanium dioxide (TiO2 ) is a semiconductor and is used as a photocatalyst in the photocatalysis process. Activation of TiO2 is achieved through the absorption of a photon (hν) with ultra-band energy from UV irradiation source. This results in the promotion of an electron (e− ) from the valence band to the conduction band, with the generation of highly reactive positive holes (h+ ) in the valence band. This causes aggressive oxidation of the surface adsorbed toxic organic pollutants and converts them into CO2 and water: TiO2 + hν → TiO2 (ecb − + holevb + ) H2 O → OH− + H+ oxidative reaction holevb + OHads − → • OH • OH+VOCs/Dye+O

2 → CO2 + H2 O The photocatalyst for the reaction process needs to be: (a) photo-active; (b) able to utilise near-UV light; (c) biologically and chemically inert, (d) photo-stable and (e) inexpensive. TiO2 meets these criteria and is one of the best semiconductors for photocatalytic reactions. Photocatalytic oxidation using UV and titanium dioxide thin film coated on a glass or metal surface has been used to eliminate hazardous indoor air pollutants, for example benzene, toluene and formaldehyde [4–6]. However, technical difficulties with coating affect the consistency and quality of the film, leading to variable removal efficiencies and loss of catalyst during the reaction process [7]. In this study, nano-structured TiO2 –SiO2 pellets were synthesized using the sol–gel method, a hybrid system of adsorption and photocatalysis that has been trialled in the laboratory. The preliminary experiments showed that the method was able to treat a pollutant laden air stream with higher removal efficiencies [8,9]. As UV/TiO2 photocatalysis can oxidize a wide range of organic mattes, the research results obtained using toluene as VOC indicator can be expended for the treatment of other VOCs, although some operational dynamics need to be adjusted for different VOCs.

2. Experimental 2.1. Synthesis of novel titania–silica pellets Titania–silica pellets were made from titanium alkoxide and tetraethylorthosilicates (TEOS) using the sol–gel method. There are two steps involved to prepare TiO2 /SiO2 . The first stage involved the hydrolysis of Ti(OC4 H9 )4 to form the uniform sol. Titanium butoxide was diluted by ethanol with small amount of HCl to form yellow transparent colloid. Then deioned water with small amount of hydrochloric acid dropped into above colloid solution. The molar ratio of Ti(OC4 H9 )4 :C2 H5 OH:H2 O:HCl is 1:15:10:0.89. It is then left to stand by at room temperature for gelation. The second stage is that TEOS were hydrolysed under the molar ratio of TEOS:C2 H5 OH:H2 O:HCl = 1:7.6:25:0.28 and added into the above TiO2 sol to form transparent TiO2 /SiO2 sol mixture. The transparent mixture gel can be obtained after drying at 60 ◦ C in an oven. Further drying at 105 ◦ C was performed to form xerogel. Finally, the xerogel was sintered at 540 ◦ C in a furnace to obtain titania–silica pellets. 2.2. Experimental system set-up A photocatalytic reaction system was designed and built. The annular reactor consists of a double-layered and sealed cylinder, where the pellets are placed in the gaps of the cylinder walls. A UV lamp (NEC FL8BL-B) with a wavelength peaking at 365 nm is installed in the open cental region. The set-up of the reaction system is shown in Fig. 1. The pressurised synthetic air from the gas cylinder flows through both a toluene vaporizer and water vaporizer. The toluene and moisture laden air stream mixes completely in a large chamber and enters the inlet of the annular reactor. The initial concentration of toluene can be kept constant when the gas stream passes through the reactor loaded with catalyst pellets, the UV irradiation starts the photocatalytic oxidation and converts the VOCs into carbon dioxides and water. The VOC concentrations were measured by FTIR infra-red multi-gas analyser before and after the treatment. The

Fig. 1. Flow diagram of photocatalytic reaction system set-up.

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conversion efficiencies of the treatment can be calculated based on the reduction of VOCs concentrations. Two types of pellets were used as catalysts in the separate experimental runs. The pellets had surface area of 421.1 and 50.4 m2 /g. As the pellets had high adsorption capacity, they had dual functions as photocatalysts and adsorbents in the hybrid photocatalysis and adsorption system. After adsorbing VOCs, the catalyst can be self-regenerated by photocatalytic oxidation of the adsorbed VOCs. A chemical composition analysis of the existing gas stream was conducted using a full scan Bio-Rad infrared spectrometer. This analysis aimed to determine the main components of the outlet gas from the reactor after the photocatalytic oxidation treatment and any intermediates formed from partial oxidation of toluene in the form of aliphatic hydrocarbons such as acetaldehyde, acetic acid and formaldehyde. The detection limits of this instrument are at approximate 1 ppm for above organic species. 3. Results and discussions 3.1. Surface area and pore size The synthesized TiO2 –SiO2 pellets were analysed for their surface areas, total pore volume and pore size distribution using gas adsorption principles. The primary objective was to generate the catalyst particles that have the highest surface area, and are the most porous. The synthesized samples had a surface area in the range of 274.1–421.1 m2 /g (Table 1), in contrast to the Degussa P25 pellets, with a surface area of 50 m2 /g. Fig. 2 shows Table 1 Surface area and pore information of synthesized TiO2 pellets Sample number

BET surface (area/m2 g−1 )

Pore volume (cm3 /g)

Pore size (nm)

1 2 3 4 5 P25 pellets

421.1 331.3 323.9 283.6 274.1 50.4

0.51 0.47 0.40 0.30 0.33 0.37

4.85 5.69 5.04 4.33 4.87 29.26

Fig. 2. Pore distribution for synthesised TiO2 –SiO2 catalyst.

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Fig. 3. TEM image of synthesized TiO2 –SiO2 catalyst.

the pore size distribution analysis of sample 1 (421.1 m2 /g). The analysed sample had a very narrow and uniform pore size distribution, which is supported by the very sharp and narrow peak obtained. The average pore diameter was 4.85 nm, as the dominant pore size. This is the direct result of the high surface area of the TiO2 –SiO2 pellets and the uniform nano-structure. 3.2. Morphology and composition analysis by TEM and XRD TEM was used to analyse the particle size and shape of the synthesized TiO2 –SiO2 samples. The TEM image of the synthesized sample with a surface area of 421.1 m2 /g showed that the sample consisted of nano-structured aggregates, which are made of very fine particles in few nanometres (Fig. 3). The sample had a very porous and rough surface, characteristic of a high surface area and high adsorptive capacity. The crystalline phases of the TiO2 –SiO2 catalyst was also analysed by electron microscope EDX and X-ray diffraction (XRD). It was found that catalyst material contained 72% TiO2 and 28% SiO2 , where the TiO2 was 100% anatase (Figs. 4 and 5).

Fig. 4. Electron microscope EDX spectrum of the composition of synthesized TiO2 –SiO2 catalyst.

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Fig. 5. XRD spectrum of anatase crystalline phase of TiO2 –SiO2 catalyst.

3.3. Destruction efficiencies with adsorption enhanced photocatalysis

Fig. 7. Effects of varying adsorption and desorption/photocatalysis periods (BET surface area 421.1 m2 /g).

Toluene was used as an indicator of VOCs. Toluene laden gas streams were generated using the experimental set-up shown in Fig. 1. The initial concentrations of untreated gas were measured on line by an FTIR infra-red gas analyser (Innova 1301) as 1149 ± 38.3 mg/m3 (300 ± 10 ppmv). The TiO2 pellets with a surface area of 50 m2 /g were used. When the UV irradiation and feeding gas was continuous, a destruction efficiency of about 25% was achieved over the period of 20 h. Once the system was operated at adsorption/regeneration mode, a higher removal efficiency of about 55% was maintained. This showed that it is possible to use TiO2 as either adsorbent or photocatalyst. As the TiO2 regenerates immediately, it can be used for longer periods of operation without the need to be regenerated or changed (Fig. 6).

with higher surface area (421.1 m2 /g) achieved 100% conversion efficiency for a substantial period of about 4 h, whereas the catalyst with a low surface area only had a very brief period of 100% conversion efficiency. This can be explained by the higher surface area of the porous catalyst material, increasing the adsorption capacity. These experimental results demonstrated that it is possible to achieve desirable degradation efficiency using the innovative TiO2 –SiO2 photocatalyst. Within the regeneration limits, the adsorption enhanced the photocatalysis for better overall degradation.

3.4. Effects of different surface areas of pellets TiO2 –SiO2 pellets with different surface areas were used in the experiments, while the initial toluene concentration and other operational conditions were kept constant. The catalyst pellets

Fig. 6. Destruction efficiencies under different operational modes (BET surface area 50.4 m2 /g).

3.5. Dynamic equilibrium of adsorption and photocatalysis The adsorption and desorption/photocatalysis periods were varied, so that the dynamic kinetic relation could be observed experimentally. The tests were conducted using the same TiO2 –SiO2 catalyst with the surface area of 421.1 m2 /g, and under the same initial toluene concentration of 1149 ± 38.3 mg/m3 (300 ± 10 ppmv). Fig. 7 shows the results of these trials. One experiment was designed with the adsorption time equal to the desorption/photocatalysis time, and the second experiment was designed with longer desorption/photocatalysis time than the adsorption time to provide more opportunity for photocatalysis. It was observed that in experiment 2 an increased period with 100% conversion efficiency (about 6 h), and a higher degradation efficiency at about 80% at the end of 20 h operation were maintained. However, experiment 1 with equal adsorption and desorption/photocatalysis time showed shorter period of 100% degradation efficiency (about 4 h) and lower conversion efficiency (60%) at the end of the test. These results support that the adsorbed toluene can be sufficiently desorbed and followed by photocatalytic degradation so that the adsorptive capacity can be maintained to achieve a continuous operation for toluene removal the. As the experiments only chose two random parameters for adsorption and photocatalysis, it may not be optimum condition to maintain the dynamic kinetic equilibrium, instead, kinetic models can be used to compute the optimum parameters.

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3.6. Kinetics of photodegradation of toluene

Table 2 Kinetic parameters in different operation modes

The kinetics of photocatalytic oxidation has usually been discussed according to the Langumuir–Hinshelwood (L–H) kinetic model [10]. In the case of low initial concentration of toluene, the L–H kinetic equation could be reduced to be a pesuedo-firstoder rate equation [11], i.e.

Operation mode

Pesuedo-first-order rate constant (s−1 )

Conversion efficiency at equilibrium (%)

1. Continuous operation/illumination, BET 50.4 m2 /g 2. Desorption time equals to adsorption time, BET 50.4 m2 /g 3. Desorption time equals to adsorption time, BET421.2 m2 /g 4. Desorption time longer than adsorption time, BET421.2 m2 /g

23.935

24.0

19.267

51.5

5.890

64.1

7.523

83.4

dC = −kC. dt

(1)

If the conversion efficiency is defined as r=C/C0 , then Eq. (1) gives
 1 = kt + A, (2) ln r where C is the disappearance concentration of toluene from the reactor, i.e. C0 less the reactor exit concentration; C0 is the initial concentration of toluene, i.e. entering reactor concentration; k is the pesuedo-first-order rate constant (s−1 ), t is the time and A is a constant. Based on the experimental results with the catalyst of 50 m2 /g of surface area, which is shown in Fig. 6, the regression equations can be obtained as: for alternate adsorption/regeneration mode: r=

C = 51.5 + 43.27 × e(−t/0.0519) , C0

3.7. Chemical composition analysis (3)

for continuous operation/illumination mode: r=

C = 24.01402 + 70.76526 × e(−t/0.04178) , C0

Eqs. (3) and (4) can be rearranged into the form: for alternate adsorption/regeneration mode: 
1 = 19.267t − 3.767, ln r − 51.5 for continuous operation/illumination mode 
1 = 23.935t − 4.259. ln r − 24.01402

(Figs. 6 and 7) is obtained from above regression equations and is listed in Table 2. It should be noted the operation modes 2 and 3 are the same, but catalysts used are with different surface areas. It appears that the stronger adsorptive capacity i.e. larger specific surface areas, the higher conversion efficiency.

(4)

(5)

(6)

The chemical composition analysis of gas stream was conducted to determine the main components of the treated gas from the reactor after the photocatalytic oxidation treatment. It aimed to provide information on any intermediates formed from partial oxidation of toluene in the form of aliphatic hydrocarbons. Fig. 8 shows that CO2 and H2 O appeared to be the main gas component in the treated gas stream with a small amount of untreated toluene. The suspected intermediates of straight chain hydrocarbons and CO were very minimal or undetectable in the treated gas stream. Some colour changes were observed on the surface of the TiO2 –SiO2 materials, indicating that oxidation intermediates may be formed at the surface of the catalyst. Further study of its speciation and how to minimise these reactions are recommended.

Similarly based on the experimental results (after the saturation of adsorption) with the catalyst of 421.1 m2 /g, which is shown in Fig. 7, the regression equations can be obtained: for desorption time longer than adsorption time:
 1 ln = 7.523t − 2.687, (7) r − 83.4256 for desorption time equals to adsorption time: 
1 = 5.89t − 3.481. ln r − 64.0512

(8)

The format of Eqs. (5)–(8) is similar to the general format of pesuedo-first-order reaction, i.e. Eq. (2), which indicates that the reactions of the photodegradation of toluene in this study are ruled by pesuedo-first-order rate equation. The pesuedo-firstorder rate constant (k) for each of the four operation modes

Fig. 8. Chemical composition of treated toluene gas stream.

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The above experiments support that using nano-structured TiO2 –SiO2 pellets can effectively treat the toluene as indicator of VOCs at a reasonable efficiency. The further increase of conversion efficiency can be achieved by two measures: (1) using the TiO2 catalyst with higher adsorption capacity; (2) increasing the regeneration time by photocatalytic oxidation, so that the TiO2 pellets will be sufficiently regenerated and ready to be used as an adsorbent. In comparison to using thin TiO2 film coated on supportive media, catalysts in pellet form have a number of advantages. Firstly, a larger quantity of pellets can be placed in the reactor, and the contoured shape of pellets provides a larger surface area to receive UV irradiation. When the air stream passes through the packed titania–silica pellets layer, better gas–solid contact is achieved through increased mass transfer and the increased illuminated surface area. Therefore, the number of TiO2 –SiO2 molecules available for the photocatalytic reaction is significantly increased resulting in higher removal efficiency. Secondly, the TiO2 –SiO2 pellets have a much higher adsorption capacity than thin film. Although some portions of the pellets do not participate in the photocatalytic reaction, it is active in adsorbing the pollutant molecules from the air stream on to the surface of the photocatalyst. The VOCs will be trapped in the pores of the TiO2 catalyst, and held for longer time and this adsorption step is essential for photocatalysis and can promote oxidation. 4. Conclusions Nano-structured TiO2 –SiO2 pellets were prepared using the sol–gel method in this study. The TEM image of the sample revealed that the sample consisted of agglomerates of nanostructured particles of about few nanometres in size. The sample has a very porous and non-smooth surface. Toluene was used as an indicator of VOCs in the experiments. The initial concentrations of untreated gas were measured on line as 1149 mg/m3 (300 ppm). The TiO2 pellets used had a surface area of 50 m2 /g. When the UV irradiation and feeding gas was continuous, the destruction efficiency of about 25% was achieved over the period of 20 h. Once the system was operated at adsorption/regeneration mode, a higher removal efficiency of about 55% was maintained. This supported that it is possible to use TiO2 –SiO2 as either adsorbent or photocatalyst. TiO2 –SiO2 pellets with different surface areas were used in the experiments, while the initial toluene concentration and other operational conditions were kept constant. It was found that the TiO2 –SiO2 pellets with higher surface area (421.1 m2 /g) achieved 100% conversion efficiency for a substantial period of time (4 h), whereas the catalyst with low surface area only had a very brief period of 100% degradation efficiency. The higher surface area indicates a more porous material, and higher adsorption capacity, and leads to higher conversion efficiency. This is confirmed by calculated kinetic rate constants. Within the regeneration limits, the adsorption enhanced catalyst can adsorb and then destruct more

toluene in the air stream. The chemical composition analysis by full scan Infrared spectrometer found that the suspected intermediates of straight chain hydrocarbons and CO were very minimal or undetectable. The experiments supported that the combined adsorption/photocatalysis systems can effectively removal and destruct VOCs from the pollutant laden air stream in either indoor or industrial environments. The findings will be useful when transferring the photocatalysis technology into industrial application, as the developed catalyst is more robust when is used to degrade the VOCs from air streams, compared to conventional adsorption only or photocatalysis only processes. The adsorption mode followed by photocatalytic regeneration makes it possible to achieve a continuous treatment operation for long period. Acknowledgments The authors wish to thank Mr. Steve Atkinson on providing technical assistance and contributing to the experimental work, and thank Dr. Larry Jordan from Nanotechnology Victoria for the microscopic analysis. References [1] T. Ibusuki, K. Takeuchi, Toluene oxidation on UV-irradiated titanium dioxide with and without O2 , NO2 or H2 O at ambient temperature, Atmos. Environ. 20 (1986) 1711–1715. [2] J. Lin, J. Yu, An investigation on photocatalytic activities of mixed TiO2 -rare earth oxides for the oxidation of acetone in air, J. Photochem. Photobiol., A 116 (1998) 63–67. [3] S. Yamazaki, S. Tanaka, H. Tsukamoto, Kinetics of oxidation of ethylene over a TiO2 photocatalyst, J. Photochem. Photobiol., A 121 (1999) 55–61. [4] S. Hager, R. Bauer, G. Kudielka, Photocatalytic oxidation of gaseous chlorinated organics over titanium dioxide, Chemosphere 41 (2000) 1219–1225. [5] K. Wang, H. Tsai, Y. Hsieh, The kinetics of photocatalytic degradation of trichloroethylene in gas phase over TiO2 supported on glass bead, Appl. Catal., B: Environ. 17 (1998) 313–320. [6] H. Yoneyama, T. Torimot, Titanium dioxide/adsorbent hybrid photocatalysts for photodestruction of organic substances of dilute concentrations, Catal. Today 58 (2000) 133–140. [7] L.M. Hitchman, F. Tian, Studies of TiO2 thin films prepared by chemical vapour deposition for photocatalytic and photoelectrocatalytic degradation of 4-chlorophenol, J. Electroanal. Chem. 538-539 (2002) 165–172. [8] R.R. Bansode, J.N. Losso, W.E. Marshall, R.M. Rao, R.J. Portier, Adsorption of volatile organic compounds by pecan shell- and almond shell-based granular activated carbons, Bioresour. Technol. 90 (2003) 175–184. [9] J. Pires, A. Carvalho, M.B. Carvalho, Adsorption of volatile organic compounds in Y zeolites and pillared clays, Microporous Mesoporous Mater. 43 (2001) 277–287. [10] Jose. Peral, Xavier Domenech, David F. Ollis, Heterogeneous photocatalysis for purification, decontamination and deodorization of air, J. Chem. Technol. Biotechnol. 70 (1997) 117–140. [11] Young Ku, Chi-Ming Ma, Yung-Shuen Shen, Decomposition of gaseous trichloroethylene in a photoreactor with TiO2 -coated nonwoven fiber textile, Appl. Catal., B: Environ. 34 (2001) 181–190.