Heterogeneous photocatalytic removal of toluene from air on building materials enriched with TiO2

https://doi.org/10.1016/j.buildenv.2007.01.016Get rights and content

Abstract

This paper reports the potential of heterogeneous photocatalysis as an advanced oxidation technology for removal of toluene from air using TiO2 as a photocatalyst in building materials. First, the photocatalytic activity of two types of TiO2 containing building materials, i.e. roofing tiles and corrugated sheets, has been investigated at ambient conditions (T=25.0 °C; relative humidity RH=47%; toluene inlet concentration [TOL]in=17–35 ppbv). Toluene removal efficiencies up to 63% were observed at a gas residence time (τ) of 17 s. Second, the effect of RH (1–77%), [TOL]in (23–465 ppmv) and τ (17–115 s) on toluene removal has been systematically investigated using TiO2 containing roofing tiles as photocatalytic building materials. Results revealed lower toluene removal efficiencies at higher RH and [TOL]in, whereas a positive effect was observed with increased τ. Under optimal conditions, toluene removal efficiencies up to 78±2% and elimination rates higher than 100 mg h−1 m−2 roofing tile were obtained. A decline in photocatalytic activity by a factor of 2 was observed after operation at gas residence times shorter than 69 s and [TOL]in higher than 76 ppmv. Washing the building materials with deionized water, simulating rainfall, could partially (i.e. by a factor 1.3) regenerate the catalyst activity.

Introduction

Titanium dioxide mediated heterogeneous photocatalysis has become an innovative technology with attractive application potentials such as self-cleaning and environmental pollution remediation, mainly due to the TiO2 hydrophilic properties and its ability to degrade a wide range of inorganic and organic compounds in both aqueous and gaseous phase [1], [2], [3], [4], [5]. The initial reaction step consists of electron–hole pairs production by irradiating the semiconductor with light having an energy content higher than the band gap [6]. For anatase and rutile TiO2, the band gap amounts 3.2 and 3.0 eV, respectively, corresponding with wavelengths of 388 and 410 nm [4]. Ultraviolet light is thus needed for TiO2 activation. Photogenerated electron–hole pairs can either recombine or participate in chemical reactions. After separation due to trapping by species adsorbed on the semiconductor, redox reactions occur between trapped electrons and holes and adsorbates. As a result, adsorbed pollutants undergo direct oxidation or reduction. Alternatively, highly reactive hydroxyl and superoxide radicals are formed through reaction of electron–hole pairs with adsorbed water or hydroxide ions and oxygen, respectively. These radicals induce mineralization of both inorganic and organic compounds [7].

With respect to air purification, research on TiO2 heterogeneous photocatalysis focuses on catalyst preparation and characterization, reactor design, reaction mechanisms and kinetics, so far mainly for treatment of industrial effluents loaded with relatively high concentrations (ppmv levels) of pollutants [2], [3], [5], [8], [9], [10], [11], [12], [13]. In recent review articles, however, it is concluded that heterogeneous photocatalysis may be of main use for treatment of relatively small pollutant loads, mainly because of the relatively moderate photocatalytic removal rates and the occurrence of catalyst deactivation after treatment of highly concentrated effluents [5], [13]. Therefore, photocatalytic treatment of ambient indoor or outdoor air, containing lower pollutant concentrations (ppbv levels), appears to be a promising field of application [13], [14], [15], particularly because most traditional air purification technologies such as incineration and biological processes are less efficient and economically non-competitive for treatment of diluted air streams.

This paper focuses on heterogeneous photocatalysis as an innovative technology for outdoor air remediation. In this context, TiO2 incorporation in building materials and activation by the near-UV fraction of incident solar irradiation (up to 5% [16]) offers promising potential. In Europe, white cementitious materials containing TiO2 in the top layers have been used for the construction of the ‘Dives in Misericordia’ church in Rome (Italy) and ‘l’Ecole de Musique’ in Chambéry (France). The primary driving force for TiO2 use in these projects was its self-cleaning properties maintaining the aesthetic characteristics of the white concrete structures by degrading accumulated coloured organic pollutants on their surfaces [17]. Less attention has been paid towards air purification effects. These can particularly be expected in confined spaces such as canyon streets in densely populated cities, where pollution originates from automobile exhaust and/or industrial processes. Indeed, since photocatalytic transformations are restricted to adsorbed pollutants, confined locations offer more favourable conditions than open spaces to obtain relatively high rates of clean air delivery from TiO2 compared to the total pollutant emission rates and influxes from outside [13].

Despite growing interest in the removal of micropollutants from ambient air by TiO2 photocatalysis on building materials, only a limited number of systematically acquired experimental data are available so far [13], [18], [19]. Lackhoff et al. [19] investigated photocatalytic atrazine (1 μg L−1) degradation on white Portland cement samples modified with semiconducting oxides such as TiO2 and ZnO (2–10% by weight) as a model reaction for pollutant decomposition on building surfaces. First order kinetics were observed with the highest rate constant (0.016 min−1) measured for Degussa P25 modified cement. Cassar reports NOx removal from air due to adsorption and photocatalysis on TiO2 containing (5% by weight) cement [17]. On lab-scale, NOx removal efficiencies up to 92% were obtained after a 7 h exposure to a 300 W lamp. Combined use of TiO2 and cement showed a synergetic effect towards NOx removal. A full-scale experiment was performed in Segrate (Italy) where photocatalytic mortar was applied to 6000 m2 of an urban road with a vehicle traffic of 1200 units per hour. On a sunny summer day (illuminance >90 000 lx, wind speed <0.7 m s−1), a 50% reduction in NOx was measured, with a stability for at least one year of application [17]. Very recently, photocatalytic removal of BTEX compounds from air using TiO2 Degussa P25 containing white Portland cement was investigated by Strini et al. [20]. At concentrations of 2.5–3.7 μmol m−3, BTEX removal efficiencies between 5% and 54% (i.e. oxidation rates of 0.2–1.3 μmol m−2 h−1) were noticed using 1% (by weight) TiO2 containing cement. The highest photocatalytic activity was observed towards o-xylene degradation, followed by ethylbenzene>toluene>benzene. Oxidation rates linearly increased with higher BTEX concentrations (0.2–5.8 μmol m−3) and UV-A light intensities (750–1350 μW cm−2). A levelling off effect was noticed in the effect of the TiO2 content (0.5–6% by weight), probably due to formation of catalyst clusters or catalyst segregation processes [20].

Although first experiments are encouraging, the limited number of data available necessitates further systematic research in this field [13]. Therefore, this work focuses on toluene removal from air using TiO2 as photocatalyst in roofing tiles and corrugated sheets. First, the photocatalytic activity of different building materials is compared at ambient conditions. The second part deals with the effect of relative humidity (RH), gas residence time and toluene inlet concentration on photocatalytic toluene removal. Finally, attention is paid to the deactivation and regeneration of the TiO2 photocatalyst.

Section snippets

Materials

Toluene (TOL, 99.5+%) and methanol (99.5%) were purchased from Acros Organics (Geel, Belgium) and Fluka (Bornem, Belgium), respectively, and were used without further purification. Clean and dry air ([H2O]<3.0 ppmv; [CO2]<1.0 ppmv; CxHy<0.5 ppmv) was provided by Air Liquide (Luik, Belgium).

The TiO2 dispersion S5-300B (anatase; 17.3% by weight in water, pH=12) was obtained from Millennium Chemicals (Grimsby, UK). Stated impurities include P2O5 (⩽0.4%), Na2O (⩽0.01%), K2O (⩽0.01%), SO3 (⩽0.6%) and

Comparison of the photocatalytic activity of different TiO2 containing building materials for toluene removal from air at ambient conditions

The photocatalytic activity of four roofing tiles and one corrugated sheet was investigated for toluene removal from air at ambient conditions, i.e. at T=25.0±0.1 °C, RH=47±1%, and toluene inlet concentrations [TOL]in between 17 and 35 ppbv. The gas residence time (τ) was 17 and 21 s during experiments with the roofing tiles and the corrugated sheet, respectively. As an illustration, Fig. 1 shows TOL inlet and outlet concentrations measured as a function of time before and during near-UV

Conclusions

This paper demonstrates the potential of heterogeneous photocatalysis for outdoor air purification through TiO2 incorporation in building materials. Toluene removal from air was obtained by near-UV irradiation of TiO2 containing roofing tiles and corrugated sheets. At ambient conditions, i.e. at T=25 °C and RH=47%, and toluene concentrations [TOL]in between 17 and 35 ppbv, toluene removal efficiencies between 23% and 63% were achieved, with the TiO2 containing roofing tiles giving higher activity

Acknowledgement

The authors would like to acknowledge IWT-Flanders for their support. Koramic Building Products and Eternit Redco NV are acknowledged for supplying the building materials. Tinne Vangheel and Yves Vanhellemont from the Belgian Building Research Institute are thanked for providing useful information on the building materials.

References (42)

  • J. Dewulf et al.

    Solid-phase microextraction of volatile organic compounds. Estimation of the sorption equilibrium from the Kováts index, effect of salinity and humic acids and the study of the kinetics by the development of an “agitated/static layer” model

    Journal of Chromatography A

    (1997)
  • J. Dewulf et al.

    Measurement of Henry's law constant as function of temperature and salinity for the low temperature range

    Atmospheric Environment

    (1995)
  • R.L. Pozzo et al.

    Supported titanium oxide as photocatalyst in water decontamination: state of the art

    Catalysis Today

    (1997)
  • A. Rachel et al.

    Comparison of photocatalytic efficiencies of TiO2 in suspended and immobilised form for the photocatalytic degradation of nitrobenzenesulfonic acids

    Applied Catalysis B: Environmental

    (2002)
  • A. Fernández et al.

    Preparation and characterization of TiO2 photocatalysts supported on various rigid supports (glass, quartz, stainless steel). Comparative studies of photocatalytic activity in water purification

    Applied Catalysis B: Environmental

    (1995)
  • L. Cao et al.

    Photocatalytic oxidation of toluene on nanoscale TiO2 catalysts: studies of deactivation and regeneration

    Journal of Catalysis

    (2000)
  • C.H. Ao et al.

    Photodegradation of volatile organic compounds (VOCs) and NO for indoor air purification using TiO2: promotion versus inhibition effect of NO

    Applied Catalysis B: Environmental

    (2003)
  • W. Wang et al.

    Photocatalytic degradation of gaseous benzene in air streams by using an optical fiber photoreactor

    Journal of Photochemistry and Photobiology A: Chemistry

    (2003)
  • O. d’Hennezel et al.

    Benzene and toluene gas-phase photocatalytic degradation over H2O and HCl pretreated TiO2: by-products and mechanisms

    Journal of Photochemistry and Photobiology A: Chemistry

    (1998)
  • Z. Pengyi et al.

    A comparative study on decomposition of gaseous toluene by O3/UV, TiO2/UV and O3/TiO2/UV

    Journal of Photochemistry and Photobiology A: Chemistry

    (2003)
  • K.H. Wang et al.

    Heterogeneous photocatalytic degradation of trichloroethylene in vapor phase by titanium dioxide

    Environment International

    (1998)
  • Cited by (156)

    • Photocatalytic recycled aggregate concrete for air-purifying purpose

      2023, Multi-functional Concrete with Recycled Aggregates
    View all citing articles on Scopus
    View full text