Journal of Hazardous Materials 283 (2015) 680–688
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Photocatalysis of sub-ppm limonene over multiwalled carbon nanotubes/titania composite nanofiber under visible-light irradiation Wan-Kuen Jo ∗ , Hyun-Jung Kang Department of Environmental Engineering, Kyungpook National University, Daegu 702-701, South Korea
h i g h l i g h t s • • • • •
A multiwalled carbon nanotube/titania composite nanofiber (MTCN) was synthesized. Photocatalytic function of visible-activated MTCN was examined using tubular reactor. MTCNs could be effectively used for the purification of sub-ppm gas-phase limonene. The experimental results agreed well with Langmuir–Hinshelwood model. Certain gas-phase intermediates were determined, but not for adsorbed intermediates.
a r t i c l e
i n f o
Article history: Received 7 June 2014 Received in revised form 22 September 2014 Accepted 29 September 2014 Available online 15 October 2014 Keywords: Intermediate Mineralization Gas-phase Continuous-flow tubular reactor
a b s t r a c t This study was conducted under visible-light exposure to investigate the photocatalytic characteristics of a multiwalled carbon nanotube/titania (TiO2 ) composite nanofiber (MTCN) using a continuous-flow tubular reactor. The MTCN was prepared by a sol–gel process, followed by an electrospinning technique. The photocatalytic decomposition efficiency for limonene on the MTCN was higher than those obtained from reference TiO2 nanofibers or P25 TiO2 , and the experimental results agreed well with the Langmuir–Hinshelwood model. The CO concentrations generated during the photocatalysis did not reach levels toxic to humans. The mineralization efficiency for limonene on the MTCN was also higher than that for P25 TiO2 . Moreover, the mineralization efficiency obtained using the MTCN increased steeply from 8.3 to 91.1% as the residence time increased from 7.8 to 78.0 s, compared to the increase in the decomposition efficiencies for limonene from 90.1 to 99.9%. Three gas-phase intermediates (methacrolein, acetic acid, and limonene oxide) were quantitatively determined for the photocatalysis for limonene over the MTCN, whereas only two intermediates (acetic acid and limonene oxide) were quantitatively determined over P25 TiO2 . Other provisional gas-phase intermediates included cyclopropyl methyl ketone and 2-ethylbutanal. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Limonene (1-methyl-4-(1-methylethenyl)-cyclohexene) is an abundant volatile organic compound (VOC) found in a variety of indoor residential and commercial environments because its “lemony” fragrance and cleaning properties lead its use in air fresheners and cleaning products [1,2]. However, limonene shows high reactivity for indoor oxidizing agents such as ozone and hydroxyl radicals and can produce secondary toxic indoor air pollutants [3]. Principal oxidation products include formaldehyde, acrolein, carbonyl compounds, and secondary organic aerosols [3]. Moreover,
∗ Corresponding author. Tel.: +82 53 950 6584; fax: +82 53 950 6579. E-mail address: [email protected] (W.-K. Jo). http://dx.doi.org/10.1016/j.jhazmat.2014.09.067 0304-3894/© 2014 Elsevier B.V. All rights reserved.
exposure to the oxidation products has been linked to adverse health effects, including ocular irritation and bronchial damage [4]. This problem necessitates the development of methods for limiting the exposure of residential and commercial occupants to limonene. Photocatalytic techniques using titanium dioxide (TiO2 ) have been extensively applied to the removal of a range of VOCs from water and air. Specifically, researchers have reported that aqueous limonene in the wastewater of a citrus transformation plant could be photocatalytically degraded using suspended Degussa P25 TiO2 [5,6]. However, the application of stand-alone TiO2 has been limited by its low photocatalytic decomposition potential, which is a result of the recombination of photon-generated electron−hole pairs [7]. Carbon nanotubes (CNTs) have received attention as supporting substrates for TiO2 that reduce electron–hole pair recombinations. CNTs have a high electron-storage potential, which retards
W.-K. Jo, H.-J. Kang / Journal of Hazardous Materials 283 (2015) 680–688
the recombination rates of electron–hole pairs [8]. They can also improve the photocatalytic performance of TiO2 by functioning as a photosensitizer, thereby extending the photocatalytic response into the visible-light range [9]. In addition, CNTs in CNT/TiO2 composites, which have a high adsorption ability, may enhance the photocatalytic performance of the composites by concentrating the chemical species to be photocatalyzed at the TiO2 surface [10,11]. Moreover, researchers have applied multi-walled CNT/TiO2 composites prepared using a sol–gel or hydrothermal method for their environmental purification [12,13]. CNT/TiO2 composites can be further coupled to polymer conductors to form nanofibrous photocatalysts, which can be easily prepared using a simple and effective electrospinning process with the aid of a polymer base [14]. Previous studies have demonstrated the superior photocatalytic activity of multi-walled CNT/TiO2 composite nanofibers (MTCNs) consisting of multiple rolled graphene cylinders compared to stand-alone TiO2 for the decomposition of aqueous methyl orange or Rhodamine B dye under ultraviolet (UV) exposure [15–17]. However, the feasibility of a visible-light activated MTCN application for the photocatalytic decomposition of airborne VOCs is yet to be fully investigated. In addition to the photocatalytic decomposition, the mineralization of environmental pollutants on a developed photocatalyst is an important indicator for photocatalytic performance evaluation. This is because incomplete mineralization may result in gas-phase intermediates that are more toxic than the parent pollutant. In contrast to MTCNs, the mineralization efficiencies of P25 TiO2 and anatase phase TiO2 , as well as a range of resulting intermediates for the heterogeneous photocatalysis of VOCs, are relatively well documented [18–20]. Specifically, Sleiman et al. reported that the mineralization efficiency of TiO2 for the photocatalysis of toluene varied from 55 to 95%, while the decomposition efficiency was up to 90–100%, depending on the experimental conditions [19]. Additionally, these researchers observed several gas-phase intermediates, such as low-molecular-weight carbonyl and hydroxylated compounds, which are more harmful than the initial pollutant, and some intermediates adsorbed on the TiO2 surface, which were generated during the photocatalytic processing of the toluene. Consequently, our study was performed under visible light to examine the photocatalytic decomposition and mineralization efficiencies for limonene over an MTCN and to determine the potential intermediates using a continuous-flow tubular reactor. In addition, the photocatalytic decomposition kinetics for limonene over the MTCN was investigated. For comparison, the characteristics of two reference photocatalysts (P25 TiO2 and TiO2 nanofiber (TN)) for the conversion of limonene were also investigated. Airborne limonene at sub-ppm concentrations (
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Photocatalysis of sub-ppm limonene over multiwalled carbon nanotubes/titania composite nanofiber under visible-light irradiation Wan-Kuen Jo ∗ , Hyun-Jung Kang Department of Environmental Engineering, Kyungpook National University, Daegu 702-701, South Korea
h i g h l i g h t s • • • • •
A multiwalled carbon nanotube/titania composite nanofiber (MTCN) was synthesized. Photocatalytic function of visible-activated MTCN was examined using tubular reactor. MTCNs could be effectively used for the purification of sub-ppm gas-phase limonene. The experimental results agreed well with Langmuir–Hinshelwood model. Certain gas-phase intermediates were determined, but not for adsorbed intermediates.
a r t i c l e
i n f o
Article history: Received 7 June 2014 Received in revised form 22 September 2014 Accepted 29 September 2014 Available online 15 October 2014 Keywords: Intermediate Mineralization Gas-phase Continuous-flow tubular reactor
a b s t r a c t This study was conducted under visible-light exposure to investigate the photocatalytic characteristics of a multiwalled carbon nanotube/titania (TiO2 ) composite nanofiber (MTCN) using a continuous-flow tubular reactor. The MTCN was prepared by a sol–gel process, followed by an electrospinning technique. The photocatalytic decomposition efficiency for limonene on the MTCN was higher than those obtained from reference TiO2 nanofibers or P25 TiO2 , and the experimental results agreed well with the Langmuir–Hinshelwood model. The CO concentrations generated during the photocatalysis did not reach levels toxic to humans. The mineralization efficiency for limonene on the MTCN was also higher than that for P25 TiO2 . Moreover, the mineralization efficiency obtained using the MTCN increased steeply from 8.3 to 91.1% as the residence time increased from 7.8 to 78.0 s, compared to the increase in the decomposition efficiencies for limonene from 90.1 to 99.9%. Three gas-phase intermediates (methacrolein, acetic acid, and limonene oxide) were quantitatively determined for the photocatalysis for limonene over the MTCN, whereas only two intermediates (acetic acid and limonene oxide) were quantitatively determined over P25 TiO2 . Other provisional gas-phase intermediates included cyclopropyl methyl ketone and 2-ethylbutanal. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Limonene (1-methyl-4-(1-methylethenyl)-cyclohexene) is an abundant volatile organic compound (VOC) found in a variety of indoor residential and commercial environments because its “lemony” fragrance and cleaning properties lead its use in air fresheners and cleaning products [1,2]. However, limonene shows high reactivity for indoor oxidizing agents such as ozone and hydroxyl radicals and can produce secondary toxic indoor air pollutants [3]. Principal oxidation products include formaldehyde, acrolein, carbonyl compounds, and secondary organic aerosols [3]. Moreover,
∗ Corresponding author. Tel.: +82 53 950 6584; fax: +82 53 950 6579. E-mail address: [email protected] (W.-K. Jo). http://dx.doi.org/10.1016/j.jhazmat.2014.09.067 0304-3894/© 2014 Elsevier B.V. All rights reserved.
exposure to the oxidation products has been linked to adverse health effects, including ocular irritation and bronchial damage [4]. This problem necessitates the development of methods for limiting the exposure of residential and commercial occupants to limonene. Photocatalytic techniques using titanium dioxide (TiO2 ) have been extensively applied to the removal of a range of VOCs from water and air. Specifically, researchers have reported that aqueous limonene in the wastewater of a citrus transformation plant could be photocatalytically degraded using suspended Degussa P25 TiO2 [5,6]. However, the application of stand-alone TiO2 has been limited by its low photocatalytic decomposition potential, which is a result of the recombination of photon-generated electron−hole pairs [7]. Carbon nanotubes (CNTs) have received attention as supporting substrates for TiO2 that reduce electron–hole pair recombinations. CNTs have a high electron-storage potential, which retards
W.-K. Jo, H.-J. Kang / Journal of Hazardous Materials 283 (2015) 680–688
the recombination rates of electron–hole pairs [8]. They can also improve the photocatalytic performance of TiO2 by functioning as a photosensitizer, thereby extending the photocatalytic response into the visible-light range [9]. In addition, CNTs in CNT/TiO2 composites, which have a high adsorption ability, may enhance the photocatalytic performance of the composites by concentrating the chemical species to be photocatalyzed at the TiO2 surface [10,11]. Moreover, researchers have applied multi-walled CNT/TiO2 composites prepared using a sol–gel or hydrothermal method for their environmental purification [12,13]. CNT/TiO2 composites can be further coupled to polymer conductors to form nanofibrous photocatalysts, which can be easily prepared using a simple and effective electrospinning process with the aid of a polymer base [14]. Previous studies have demonstrated the superior photocatalytic activity of multi-walled CNT/TiO2 composite nanofibers (MTCNs) consisting of multiple rolled graphene cylinders compared to stand-alone TiO2 for the decomposition of aqueous methyl orange or Rhodamine B dye under ultraviolet (UV) exposure [15–17]. However, the feasibility of a visible-light activated MTCN application for the photocatalytic decomposition of airborne VOCs is yet to be fully investigated. In addition to the photocatalytic decomposition, the mineralization of environmental pollutants on a developed photocatalyst is an important indicator for photocatalytic performance evaluation. This is because incomplete mineralization may result in gas-phase intermediates that are more toxic than the parent pollutant. In contrast to MTCNs, the mineralization efficiencies of P25 TiO2 and anatase phase TiO2 , as well as a range of resulting intermediates for the heterogeneous photocatalysis of VOCs, are relatively well documented [18–20]. Specifically, Sleiman et al. reported that the mineralization efficiency of TiO2 for the photocatalysis of toluene varied from 55 to 95%, while the decomposition efficiency was up to 90–100%, depending on the experimental conditions [19]. Additionally, these researchers observed several gas-phase intermediates, such as low-molecular-weight carbonyl and hydroxylated compounds, which are more harmful than the initial pollutant, and some intermediates adsorbed on the TiO2 surface, which were generated during the photocatalytic processing of the toluene. Consequently, our study was performed under visible light to examine the photocatalytic decomposition and mineralization efficiencies for limonene over an MTCN and to determine the potential intermediates using a continuous-flow tubular reactor. In addition, the photocatalytic decomposition kinetics for limonene over the MTCN was investigated. For comparison, the characteristics of two reference photocatalysts (P25 TiO2 and TiO2 nanofiber (TN)) for the conversion of limonene were also investigated. Airborne limonene at sub-ppm concentrations (
