Purification of BTEX at Indoor Air Levels Using Carbon and Nitrogen Co-Doped Titania under Different Conditions

21(11); 1321~1331; November 2012 http://dx.doi.org/10.5322/JES.2012.21.11.1321 ORIGINAL ARTICLE

Purification of BTEX at Indoor Air Levels Using Carbon and Nitrogen Co-Doped Titania under Different Conditions

Wan-Kuen Jo

, Hyun-Jung Kang

Department of Environmental Engineering, Kyungpook National University, Daegu 702-701, Korea

Abstract

To date, carbon and nitrogen co-doped photocatalysts (CN-TiO

) for environmental application focused mainly on the aqueous phase to investigate the decomposition of water pollutants. Accordingly, the present study explored the photocatalytic performance of CN-TiO

photocatalysts for the purification of indoor-level gas-phase aromatic species under different operational conditions. The characteristics of prepared photocatalysts were investigated using X-ray diffraction, scanning emission microscope, diffuse reflectance UV-VIS-NIR analysis, and Fourier transform infrared (FTIR) analysis. In most cases, the decomposition efficiency for the target compounds exhibited a decreasing trend as input concentration (IC) increased.

Specifically, the average decomposition efficiencies for benzene, toluene, ethyl benzene, and xylene (BTEX) over a 3-h process decreased from 29% to close to zero, 80 to 5%, 95 to 19%, and 99 to 32%, respectively, as the IC increased from 0.1 to 2.0 ppm. The decomposition efficiencies obtained from the CN-TiO

photocatalytic system were higher than those of the TiO

system. As relative humidity (RH) increased from 20 to 95%, the decomposition efficiencies for BTEX decreased from 39 to 5%, 97 to 59%, 100 to 87%, and 100 to 92%, respectively. In addition, as the stream flow rates (SFRs) decreased from 3.0 to 1.0 L min

the average efficiencies for BTEX increased from 0 to 58%, 63 to 100%, 69 to 100%, and 68 to 100%, respectively. Taken together, these findings suggest that three (IC, RH, and SFR) should be considered for better BTEX decomposition efficiencies when applying CN-TiO

photocatalytic technology to purification of indoor air BTEX.

Key Words : Decomposition efficiency, Aromatic species, Input concentration, Relative humidity, Stream flow rate

1. Introduction

In recent, heterogeneous photocatalysis using titanium dioxide (TiO

) has been considered a promising technique in cleaning of environmental pollutants (Keshmiri et al., 2006; De Witte et al., 2008). This technique is known as an advanced oxidation process, which converts light energy into chemical reaction energy. However, although TiO

has outstanding photocatalytic activity and stability toward photocorrosion, its application requires light irradiation above the band gap energy of 3.2eV

(anatase) to produce electron-hole pairs that can generate superoxide anion radicals and hydroxyl radicals (Fujishima et al., 2008). This energy corresponds to UV light irradiation of wavelength

<387 nm; therefore, a variety of strategies for modifications to enhance its light absorption and photocatalytic activities under visible light irradiation have been investigated. These strategies have generally involved dye sensitization (Ding et al., 2005), transition metal doping (Zhu et al., 2006; Rauf et al., 2011), and nonmetallic element doping (Ohno

received 24 August, 2012; revised 24 September, 2012;

accepted 12 November, 2012

*

Corresponding author : Wan-Kuen Jo, Department of Environmental Engineering, Kyungpook National University, Daegu 702-701, Korea Phone: +82-53-950-6584

E-mail: [email protected]

ⓒ The Korean Environmental Sciences Society. All rights reserved.

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://

creativecommons.org/licenses/by-nc/3.0) which permits unrestricted

non-commercial use, distribution, and reproduction in any medium,

provided the original work is properly cited.

et al., 2004; Yin et al., 2005; Li et al., 2011).

In particular, carbon (C)- or nitrogen (N) single doped photocatalysts (C-TiO

and N-TiO

, respectively) showed superior photocatalytic performance under visible-light irradiation because of their narrowing of the band gap energy of TiO

(Asahi et al, 2001;

Lindgren et al., 2003; Valentin et al., 2005; Chen and Burda, 2008). Moreover, co-doping techniques of TiO

with carbon and nitrogen (CN-TiO

) have been considered another promising method to extend the light absorbance into visible region with superior photocatalytic performance over single element-doped TiO

(Xie et al., 2007; Dolat et al., 2012). As regards the CN-TiO

, the doped C atoms could elevate adsorption of chemicals and the doped N atoms would increase the visible-light absorption. As a result, the superior performance of CN-TiO

could be attributed to a synergistic effect of co-doped elements (C and N). Several researchers (Chen et al., 2007; Yin et al., 2007; Zhang and Song, 2009; Wang and Lim, 2010; Dolat et al., 2012) have prepared CN-TiO

photocatalysts using a range of synthesis routes such as hydrothermal, sol-gel, hydrolysis-polymerization- calcination, and solvothermal processes and reported that CN-TiO

photocatalysts revealed high photocatalytic performance under visible-light irradiation. However, these studies focused mainly on the photocatalytic decomposition of the water pollutants, including as phenol and methylene blue. In fact, thephoton absorbance kinetics and reaction kinetics of environmental pollutants differ at the liquid-solid and gas-solid interfaces (Fujishima et al., 2008)

Nevertheless, studies on the photocatalytic performance of CN-TiO

photocatalysts for purification of air pollutants are hardly found in literature.

Accordingly, the present study explored the photocatalytic performance of CN-TiO

photocatalysts for the purification of indoor-level gaseous aromatic chemicals under different operational conditions. The target chemicals involved benzene, toluene, ethyl

benzene and xylene (BTEX), which are commonly detected at high concentrations in indoor and outdoor environments (Ohura et al., 2009). Moreover, these compounds are toxic or potentially toxic to humans (Chen et al., 2008). Major operational parameters involved input concentrations (ICs), relative humidity (RH), and stream flow rates (SFRs), which are associated with gas-solid photocatalytic reaction mechanisms (Demeestere et al., 2007; Fujishima et al., 2008). The experimental data obtained from this study can be used to the development of kinetic models for photocatalytic degradation of gas-phase pollutants via CN-TiO

photocatalysts.

2. Experimental

2.1. Preparation and characterization of photocatalysts The CN-TiO

photocatalysts were prepared using a solvothermal method. For this method, titanium isopropaoxide (4.8 mL) was added to 68 mL of isopropanol, which was then mixed with 4 mL of HNO

(0.5 M) and 0.78 g of hexamethylenetetramine and subsequently stirred for 1 h at room temperature to obtain a gel-type photocatalyst. The resulting gel-type photocatalyst was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated at 120

C for 4 h. After completing thermal treatment, the autoclave was cooled and then kept at room temperature for 20 h. Finally, the resulting CN-TiO

photocatalysts were dried at 90

C for 24 h and then calcined at 300

C for 4 h.

For the purpose of comparisons, single C-TiO

and N-TiO

photocatalysts were also prepared. C-TiO

photocatalysts were prepared by oxidizing titanium carbide powders (TiC, Taiji-Ring Co., China) at 350

°C. For this method, TiC powders were sprayed

uniformly onto a glass plate and then oxidized at the

specified temperature for 8 h to give grey powders. In

addition, N-TiO

photocatalysts were synthesized

using urea as an N source. Eight gram of commercially

available TiO

powder (Degussa P-25) was added to 20 mL of aqueous solution of the organic nitrogen compound (urea) and stirred at room temperature for 1 hr. The mixture was kept in the dark for 1 day and then dried under reduced pressure. N-doped TiO

powder was calcined at 350 °C for 1 h under aerated conditions to provide yellow powder. The calcined powders were clean edusing diluted sulfuric acid and then with deionized water, and vacuum-dried.

The prepared photocatalysts were characterized by X-ray diffraction (XRD), scanning emission microscope (SEM), diffuse reflectance UV-VIS-NIR analysis, and Fourier transform infrared (FTIR) analysis. XRD images were obtained from a Rigaku D/max-2500 diffractometer with Cu K

radiation operated at 40 kV and 100 mA in the range of 20-80

(2 θ). A scanning step size of 0.01

and a scintillation detector were used for this analysis. Curve fitting and integration were done using a proprietary software.

The particle morphology was determined using a Hitachi S-4300 and EDX-350 FE-SEM at an acceleration voltage of 15 kV. The samples for SEM analysis were prepared by dispersing the final powders in ethanol. The dispersion was then dropped on carbon-copper grids. Visible absorption spectra were determined using a Varian CARY 5G spectrophotometer equipped with an integrating sphere. Polytetrafluoro- ethylene was used as a reference. Both absorbance and diffuse reflectance spectra were recorded for all samples and the Kubelka-Munk function was applied to obtain a magnitude proportional to the extinction coefficient. FTIR analysis was conducted on a Perkin Elmer Spectrum GX spectrophotometer at a resolution of 4 cm-

in the spectral range of 400-4,000 cm

. Measurements were conducted in transmission mode using spectroscopic-grade KBR pellets for the powder samples.

2.2. Purification tests of target compounds

The purification of BTEX was performed using an

annular-type Pyrex reactor inner-coated with CN-TiO

film. Air leakage from the reactor was examined by observing the flow rate at the reactor outlet and comparing this with the supply airflow rate. Visible-light radiation was provided utilizing a conventional visible light lamp, and its intensity was measured using a digital Lux Meter (INS Model DX-100). The outside of reactor was covered with aluminum foil to eliminate the light loss from the reactor lamp through the Pyrex tube and the light transmission gain from laboratory light sources. This reactor allowed for a uniform light distribution onto the surfaces of the CN-TiO

photocatalysts.

Moreover, the reactor was designed to allow the flow of incoming air to direct toward the UV light to increase air turbulence inside the reactor, which enhances the distribution of the target compounds onto the photocatalyst surface.

Zero-grade air was supplied from a compressed cylinder and passed through a charcoal filter for re-purification. Subsequently, dry air flowed through a humidifying device that was immersed in a water bath to generate humidified air. The RH values were measured immediately adjacent to the photocatalytic reactor inlet using a humidity meter. SFR was adjusted using rotameters calibrated against a dry test meter. The humidified air was mixed with the standard gas, which was prepared by injecting standard gases into a mixing chamber via a syringe pump (model 210, KdScientific Inc.). Finally, the mixed gas was allowed to flow through the CN-TiO

-coated photocatalytic reactor.

The three parameters examined in this study for the destruction efficiency of BTEX are as follows:

ICs, RH, and SFRs. The concentrations surveyed ranged from 0.1 to 2.0 ppm. The RH range for these experiments was 20-95% (20, 45, 70, and 95%), and the range of SFRs investigated was 0.5-3.0 L min

. Other parameters were fixed to the following values:

RH, 45%; IC, 0.1 ppm; SFR, 0.5 L min

. The

hydraulic diameter (HD is defined as the inside diameter of the annular reactor tube minus the outside diameter of the lamp) of the photocatalytic reactor was 35 mm. Visible radiation was supplied by an 8-W fluorescent daylight lamp (F8T5DL, Youngwha Lamp Co.). The visible radiation intensity was measured as 2.1 mW cm

at a distance from the visible-light lamp equal to half the HD of the reactor.

The weight of the CN-impregnated TiO

, C-TiO

, and N-TiO

film coated inside the reactor were 3.6, 3.3, and 3.4 mg cm

, respectively, which were similar to each other.

Air samples were collected at both the inlet and outlet using an empty Tedlar bag at a constant flow rate for 10 min every hour. Gas from the sampling bag was then drawn through an adsorbent trap containing 0.2 grams of Tenax TA using a constant flow-sampling pump (Aircheck Sampler Model 224-PCXR8, SKC). All sampling was conducted at ambient temperature. The target species collected on the adsorbent trap were analyzed by coupling a thermal desorption system (SPIS TD, Donam Inc.) to a gas chromatograph (Agilent 7890A) equipped with a flame ionization detector. The peak areas with respect to other compounds on the GC chromatogram were not significantly high and they could not be easily assigned to theoretically expected intermediates;

thus, they were not quantified.

The quality assurance and quality control program for measurements of gaseous target compounds included laboratory blank traps and spiked samples.

At the beginning of the day, a laboratory blank trap was analyzed to check for any trap contamination;

however, none was observed. An external standard was also analyzed daily to check the quantitative response. When the quantitative response differed by more than 10% from that predicted by the specified calibration equation, a new calibration equation was determined. The method detection limits ranged from 1.7 to 5.4 μ ^{g m-}

, depending on the compound.

3. Results and discussion

3.1. Characteristics of photocatalysts

The characteristics of CN-, C-, and N-TiO

powders were determined XRD, SEM, UV-visible spectroscopy, and FRIR analysis. Figure 1 exhibits the XRD patterns of CN

C, and N-TiO

photocatalyst. The XRD results of C- and N-TiO

presented both the anatase and rutile crystal phases, although the peak intensities differ from each other.

Specifically, both photocatalysts shoed an anatase phase of TiO

with a major peak at 25.2

2θ and a rutile phase with a distinct peak at 27.4

2θ. These findings are similar to those of previous studies (Jo and Kim, 2009; Nam et al., 2012). However, CN-TiO

did not reveal any distinct rutile crystal phase peak at 27.4 2θ, although it still showed a distinct anatase crystal phase peak at 25.2

2θ. These findings are consistent with those of previous studies (Chen et al., 2007; Zhang and Song, 2009; Wang and Lim, 2010; Dolat et al., 2012). The differences in XRD patterns between the C- and N-TiO

and CN-TiO

were ascribed to the calcination temperature of CN-TiO

and the role of CN elements for CN-TiO

. Certain studies (Chen et al., 2007; Zhang and Song,

Fig. 1. XRD patterns of CN-TiO

, C-TiO

, and N-TiO

powders.

Fig. 2. SEM images of CN-TiO

, C-TiO

, and N-TiO

powders.

2009; Wang and Lim, 2010) reported that crystallinity transformations from rutile TiO

to anatase TiO

occur during calcination processes and that these transformations increase as calcination temperature increases. The presence of CN elements for CN-TiO

could also influence the phase transformation (Zhang and Song, 2009).

Fig. 2 shows the SEM images of CN-, C-, and N-TiO

. Higher agglomeration was observed for the CN-TiO

than C- or N-TiO

samples. These findings are attributed to the sintering reaction among small particles for the CN-TiO

(Yu and Yu, 2009).

However, it should be noted that the particle size of CN-TiO

photocatalysts can vary with calcination temperature (Chen et al., 2007; Zhang and Song, 2009). Meanwhile, the size of agglomerated particles for CN-TiO

was larger than that of C- or N-TiO

. As demonstrated by the XRD results, CN-TiO

was

primarily consisted of rutile crystal phase, whereas C- and N-TiO

was consisted of both rutile and anatase crystal phases. Moreover, Wang and Lim (2010) reported that the crystallite size of rutile phase is larger than that of anatase phase. As a result, the larger particle size for the CN-TiO

is attributed to the rutile phase crystal phase with higher particle sizes.

The UV-visible absorbance spectra of three photocatalysts (CN, C- and N-TiO

) are presented in Fig. 3. The P25 TiO

revealed an absorption edge at λ

≈ 410 nm, which was consistent with that reported by other researchers (Jo and Kim, 2009; Horikawa et al., 2008; Znad and Kawase, 2009). However, unlike the unmodified TiO

, all three photocatalysts (CN-, C-, and N-TiO

) showed a shift of the absorbance spectrum towards the visible light region (400-800 nm). Similarly, previous studies (Dong et al., 2009;

Jo and Kim, 2009; Dolat et al., 2012) reported that

the CN-, C-, and N-TiO

photocatalysts prepared using other synthetic routes (sol-gel, magnetron sputtering, and hydrothermal methods) showed a light absorbance shift to the visible light region, although their absorbance intensities differed. Taken together, these findings indicate that the three photocatalysts prepared in the current study could be effectively activated by visible-light irradiation.

Fig. 3. UV-VIS spectra of CN-TiO

, C-TiO

, and N-TiO

powders.

Fig. 4 shows the FTIR spectra of the prepared CN-, C-, and N-TiO

photocatalysts. Major peaks were observed at 3411, 1630, and < 1000 cm-

. These results are similar to those of the matched photocatalysts prepared by other researchers (Dong et al., 2009; Jo and Kim, 2009; Dolat et al., 2012). The band at 3411 cm

was attributed to O-H stretching vibration, while the band at 163 cm

area was ascribed to O-H bending of water molecules absorbed onto the catalyst surface (Dolat et al., 2012; Nam et al., 2012). The bands at 460 and 694 nm were assigned to the stretching vibration of Ti-O (Nam et al., 2012). Regarding the peak at < 1000 cm

the frequency was shifted from 694 nm for TiO

to 460 nm for CN-TiO

. This frequency movement to a lower wavelength for the CN-, C-, and N-TiO

was attributable to the interaction between the impregnated C,N, and CN atoms.

Fig. 4. FTIR spectra of CN-TiO

, C-TiO

, and N-TiO

powders.

3.2. Purification efficiencies of BTEX determined via CN-TiO

under different conditions

The photocatalytic performance of a CN-TiO

composite

for purification of gas-phase BTEX was investigated

under a variety of operational conditions. The photo-

catalytic decomposition efficiencies of the four target

VOCs obtained from the CN-TiO

photocatalytic

system under visible-light irradiation, according to

IC, are presented in Fig. 5. In most cases, the

decomposition efficiency for the target compounds

exhibited a decreasing trend with increasing IC. The

average decomposition efficiencies for BTEX over a

3-h process decreased from 29% to close to zero, 80

to 5%, 95 to 19%, and 99 to 32%, respectively, as the

IC increased from 0.1 to 2.0 ppm. These findings are

consistent with those reported in other studies

(Pengyi et al., 2003; Jo et al., 2011) that used

unmodified TiO

under UV irradiation. The IC

dependence is ascribed to adsorptive competition

between BTEX molecules for the active adsorption

sites on the CN-TiO

surface. Regarding higher ICs,

the active adsorption sites on the photocatalyst

surface might be more limited for adsorption of

BTEX molecules. This decomposition-efficiency

decreasing pattern with IC is also related to the

Langmuir–Hinshelwood (LH) adsorption isotherm

Fig. 5. Decomposition efficiencies of BTEX determined via a photocatalytic system with CN‐TiO

, according to initial concentrations.

model that was most commonly used to link the photocatalytic reaction rate of VOCs to their ICs (Demeestere et al., 2007). Based on the LH model, the decreasing pattern in degradation efficiency indicates that the concentration range used in the current study corresponds to the intermediate regime, in which the photocatalytic reaction rate increased slowly but the degradation efficiency decreased. For the high regime, in which the photocatalytic oxidation is independent of ICs (zero-order reaction kinetics) the degradation efficiency would decrease.

The decomposition efficiencies obtained from the CN-TiO

photocatalytic system were higher than those of the TiO

system. The average photocatalytic degradation efficiencies for BTEX obtained from the CN-TiO

system were 27, 81, 95, and 99%, respectively.

Meanwhile, Jo and Kim (2009) reported much less photocatalytic performance of the reference Degussa P25 TiO

powders for decomposition of BTEX under

visible-light irradiation. This difference between the two studies indicated that the CN-TiO

photocatalyst was superior for photocatalytic decomposition of toxic gas-phase aromatic pollutants to the pure TiO

photocatalyst under visible-light irradiation. Moreover, these findings support the assertion that C and N elements embedded into TiO

networks could enhance the visible-light absorbance of TiO

and as a result, its photocatalytic performance. These findings are consistent with those of previous studies (Yin et al., 2007; Yang et al., 2008; Yu and Yu, 2009), in which CN-TiO

exhibited elevated photocatalytic performance for the decomposition of water-based or inorganic gas pollutants. As previously mentioned, theincreased photocatalytic performance of the CN-TiO

photocatalyst was attributed to the combined effect of C and N atoms in the TiO

networks. In addition, other studies (Yin et al., 2007;

Yang et al., 2008; Yu and Yu, 2009) indicated that C

Fig. 6. Decomposition efficiencies of BTEX determined via a photocatalytic system with CN‐TiO

, according to relative humidity.

atoms could act as a photosensitizer, which transfers excited electron to surface-adsorbed oxygen molecules to form reactive O

, thereby initiating the degradation of chemicals. Moreover, N atoms might produce low energy levels of N 2p and O 2p states or introduce a mid-gap (N 2p) energy level.

Fig. 6 shows the photocatalytic decomposition efficiencies of the four target VOCs obtained from the CN-TiO

photocatalytic system under visible- light irradiation, according to RH. For the target compounds, a descending trend in decomposition efficiencies with increasing RH was observed.

Specifically, as RH increased from 20 to 95%, the decomposition efficiencies for BTEX decreased from 39 to 5%, 97 to 59%, 100 to 87%, and 100 to 92%, respectively. With respect to high RH conditions, excessive water molecules could preferably occupy active sites on the photocatalyst surface, causing a

decrease in the reaction rate. The changes in the toluene decomposition efficiencies with RH for the CN-TiO

photocatalytic system were similar to those of an unmodified TiO

photocatalytic system reported by Sleiman et al. (2009). Pengyi et al. (2003) reported that, using TiO

as a photocatalyst under similar experimental conditions (0.02-0.4 ppm), the toluene decomposition efficiency decreased with increasing RH within the RH range of 55-95%. Therefore, similar to the unmodified TiO

photocatalytic system, RH would also be an important operational parameter for the CN-TiO

photocatalytic system.

Fig. 7 represents the photocatalytic decomposition efficiencies for BTEX obtained from the CN-TiO

photocatalytic system for four different SFRs (0.5, 1.0, 2.0, and 3.0 L min

) under visible-light irradiation.

The decomposition efficiencies of all target

compounds increased with decreasing SFRs. As the

Fig. 7. Decomposition efficiencies of BTEX determined via a photocatalytic system with CN‐TiO

, according to stream flow rates.

SFRs decreased from 3.0 to 1.0 L min

, the average efficiencies for BTEX increased from 0 to 58%, 63 to 100%, 69 to 100%, and 68 to 100%, respectively.

Similarly, Kontos et al. (2012) investigated an unmodified TiO

photocatalytic unit and found that the decomposition of nitrogen oxide increased as SFR decreased. Taken together, these findings suggest that SFR is still an important parameter for the photocatalytic BTEX decomposition mechanism of the CN-TiO

unit. Decreased SFRs would result in a decrease in the bulk mass transport of target compounds from the gas-phase to the surface of the catalyst particle due to convection and diffusion, which is an important heterogeneous catalytic reaction process (Kontos et al. 2012). In this case, the removal rate would decrease with decreasing SFR, indicating that BTEX removal is limited to the catalyst surface. However, this assertion is inconsistent

with that obtained from the current study. Specifically, at higher SFRs, the gas residence time in the continuous-flow photocatalytic reactor might be too short to allow sufficient BTEX transfer from the gas phase to the catalyst surface (Kontos et al. 2012). The residence times in the present study, which were calculated by dividing the reactor volume by SFR, were 15.2, 7.6, 3.8, and 2.5 s for SFRs of 0.5, 1.0, 2.0, and 3.0 L min

, respectively. Accordingly, the lower efficiencies at higher SFRs suggest that an insufficient reactor residence time effect outweighs the bulk mass transport effect of BTEX on their decomposition efficiency.

4. Conclusions

This work examined the photocatalytic performance

of CN-TiO

photocatalysts for the purification of

indoor-level gaseous BTEX under different conditions.

The surface morphology of the CN-TiO

photocatalysts were successfully characterized using XRD, SEM, diffuse reflectance UV-VIS-NIR analysis, and FTIR analysis. Specifically, it was found that the characterstics of this photocatalyst differed from those of the C- or N-TiO

photocatalyst. The CN-TiO

powders showed a definite shift in the absorbance spectrum towards the visible region, suggesting that the CN-TiO

photocatalyst could be activated effectively by visible-light irradiation. The CN-TiO

photocatalytic system exhibited higher BTEX decomposition efficiencies to the reference TiO

under visible-light irradiations. In addition, the effects of the photocatalytic performance of CN-TiO

on operational conditions, including treatment concentration, indoor humidity, and air flow entering into air cleaning devices, were quantified. In most cases, the decomposition efficiency for the target compounds exhibited a decreasing trend with increasing IC. For the target compounds, a descending trend in decomposition efficiencies with increasing RH was observed. The decomposition efficiencies of all target compounds increased with decreasing SFRs.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2011-0027916).

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