Photocatalytic conversion of gaseous ethylbenzene on lanthanum-doped titanium dioxide nanotubes

doi:10.1016/j.jhazmat.2013.03.037

Journal of Hazardous Materials

Volumes 254–255, 15 June 2013, Pages 354-363

https://doi.org/10.1016/j.jhazmat.2013.03.037 Get rights and content

Highlights

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1.2%-La³⁺-TNTs was synthesized via sol–gel method combined with nanotube-formation.
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1.2%-La³⁺-TNTs had a higher photocatalytic activity for EB conversion than pure TiO₂.
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Rate controlling steps were different under various RHs of reaction media.
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Active radicals by photocatalysis played dominant roles during the conversion of EB.
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A detailed pathway was proposed for the photocatalytic conversion of EB.

Abstract

The photocatalytic properties of titanium dioxide (TiO₂) make it an attractive material for environmental remediation. In the present study, lanthanum (La³⁺)-doped TiO₂ nanotubes with excellent photocatalytic activity were fabricated by a combination of sol–gel method and hydrothermal technique. The optimal preparation parameters were determined by the structural characterization using a range of methods and the photocatalytic degradation of gaseous ethylbenzene (EB). Compared with pure TiO₂ nanoparticles, 1.2%-La³⁺-doped – titania nanotubes (1.2%-La³⁺-TNTs) exhibited higher activity under 254 nm UV for conversion of EB. The initial EB concentrations and relative humidity (RH) obviously influenced the photocatalytic activity of 1.2%-La³⁺-TNTs. Kinetic analysis showed that surface adsorption and surface reaction controlled the rate-determining step for RH of 40–50% and >80%, respectively. Gas chromatography and mass spectrometry were used to analyze the intermediates generated in the conversion of EB, allowing a tentative decomposition pathway to be proposed. The prepared photocatalyst exhibited enhanced EB conversion compared with undoped TiO₂, and showed a promise for the decomposition of recalcitrant compounds before subsequent biopurification.

Graphical abstract

Introduction

Volatile organic compounds (VOCs) are emitted from a wide range of industries, including chemical, petrochemical, pharmaceutical, food processing and paint [1], [2]. VOCs are regarded as priority hazardous substances by several governments because of their high toxicity, confirmed carcinogenicity and environmental persistence. Conventional treatments such as adsorption, absorption and reverse osmosis simply transfer these hazardous compounds from one phase to another, without actually removing them [3]. Methods such as the advanced oxidation process (AOP) and biotreatment have been developed to completely convert these pollutants to CO₂, H₂O and other inorganic compounds [4], [5], [6]. Because biotreatment is ineffective for treatment of recalcitrant or insoluble compounds, AOP is regarded as the most efficient technique to remove these VOCs. Recently, some researchers indicated that proper AOPs could directly generate readily biodegradable or water soluble intermediates, thus providing the possibility for their complete removal by subsequent biotreatment [7], [8]. Therefore, AOP, either alone or a combined technique, is widely used to reduce environmental pollution.

One type of AOPs is heterogeneous photocatalysis using semiconductor compounds, which is very effective for converting organic pollutants [9], [10], [11]. TiO₂ is a wide band-gap semiconductor that can readily induce oxidation (holes in the valence band) and reduction (electrons in the conduction band) by absorbing photons [12], [13]. Because it is non-toxic, inexpensive, and has a high oxidative capacity and good stability, TiO₂ is a promising candidate for photo-assisted pollutant conversion [14], [15], [16]. However, relatively low conversion efficiency is often obtained using commercial TiO₂ because of both a high recombination rate of electron–hole pairs and a small specific surface area to adsorb VOCs [17], [18]. As a result, development of improved TiO₂-based photocatalysts for environmental applications is still an attractive research topic.

Methods have been conceived to increase electron–hole separation efficiency and reduce the recombination rate of electron–hole pairs in TiO₂, such as noble metal doping, coupling with transition metal or transition metal oxides, and dye sensitization [19], [20], [21]. Numerous studies have suggested that the deposition of metal nanoparticles on TiO₂ can significantly improve its photocatalytic efficiency through electron trapping in the Schottky barrier conduction band leading to a longer electron–hole pair lifetime. Therefore, metal doping is commonly regarded as more effective and simpler than other methods [22], [23]. To obtain TiO₂ with high specific surface area, TiO₂-based nanotubes, a type of one-dimensional nanostructure, have been proposed by several researchers [24]. This hollow, layered structure with a larger specific surface structure than bulk TiO₂ could be an excellent adsorbent for VOCs and thus enhance their photocatalytic degradation. Li et al. [5] synthesized Ag-doped TiO₂ nanotubes for photocatalysis of gaseous toluene. The composites exhibited a degradation efficiency of 98%, which was higher than those of pure P25 (a type of commercial TiO₂) and Ag-doped P25. The differences between these structures further suggested that nanotubes would be suitable for environmental remediation.

Despite numerous reports on the photocatalytic and self-cleaning activity of highly ordered titania nanotubes (TNTs) against liquid pollutants [25], [26], [27], they have seldom been used for air purification. The use of TNTs for photocatalysis of VOCs under industrial conditions requires systematic investigation.

In the present study, a facile method to prepare a highly catalytic active La³⁺-TiO₂ nanotubes (La³⁺-TNTs) by a sum of sol–gel and hydrothermal techniques is reported. The relationship between the photocatalytic activity of the La³⁺-TNTs for degradation of ethylbenzene (EB) and synthesis parameters is discussed based on conversion efficiency and structural characterization. The influence of initial concentration and relative humidity (RH) on the photocatalytic conversion of by La³⁺-TNTs is determined. The mechanism of gaseous EB conversion is also investigated, as well as the effect of RH on the kinetics of this reaction.

Section snippets

Catalyst preparation

All chemicals used in this work were of analytical grade and were used without further purification. La³⁺-TNTs were synthesized with the following procedure: a mixture of solution was first prepared with tetrabutyl titanium (17 ml Ti(O-Bu)₄), ethanol (40 ml), glacial acetic acid (3 ml) at a molar ratio of [Ti⁴⁺]:[ethanol]:[acetic acid] = 1:13.7:1.04. The obtained solution was added into another mixture composed of ethanol (18 mL) and La(NO₃)₃·6H₂O (4 mL) under stirring. After stirring for 1 h, the

Results and discussion

Before the photocatalysis of EB, a series of control experiments, including direct photolysis and adsorption, were carried out. Since the conversion of EB by photolysis or adsorption were much lower (<5% and <3% at the residence time of 20 s), they could be considered negligible in the photocatalytic system. This suggested that the conversion of EB was mainly caused by the photocatalysis, and this conclusion was similar to that reported by Hinojosa-Reyes et al. [28].

Conclusions

Photocatalysts can efficiently convert recalcitrant organic compounds. Here, a novel TiO₂-based catalyst driven by 254 nm-light was successfully synthesized via the sol–gel reaction combined with nanotube-preparation (hydrothermal techniques). Under optimized conditions with a RH of 45–50% and residence time of 20 s, the conversion efficiency for 50 mg m⁻³ EB achieved by 1.2%-La³⁺-TNTs exceeded 70%, nearly twice that achieved by unmodified TiO₂. Higher initial EB concentration and RH inhibited the

Acknowledgments

This study was sponsored by the National Natural Science Foundation of China (21207115), the International S&T Cooperation Program of China (2011DFA92660), and The 51th Postdoctoral Science Foundation of China (2012M510155). We also thank anonymous reviewers for helpful comments on the manuscript.

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Photocatalytic conversion of gaseous ethylbenzene on lanthanum-doped titanium dioxide nanotubes

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Catalyst preparation

Results and discussion

Conclusions

Acknowledgments

Environ. Res.

Energy Buildings

Chemosphere

Environ. Int.

Chemosphere

Biotechnol. Adv.

J. Hazard. Mater.

Catal. Today

Appl. Catal. B

Appl. Catal. A

Catal. Today

Chem. Eng. J.

Appl. Catal. B

Appl. Catal. B

J. Hazard. Mater.

Appl. Catal. B

Catal. Commun.

Appl. Catal. B

J. Hazard. Mater.

Appl. Surf. Sci.

J. Hazar. Mater.

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Appl. Catal. B

J. Mol. Catal. A

Appl. Catal. B

J. Solid State Chem.

J. Solid State Chem.