Environ. Sci. Technol. 2009, 43, 538–543
Highly Thermostable Anatase Titania-Pillared Clay for the Photocatalytic Degradation of Airborne Styrene MELVIN LIM,† YAN ZHOU,‡ B A R R Y W O O D , § L I A N Z H O U W A N G , §,| VICTOR RUDOLPH,| AND G A O Q I N G ( M A X ) L U * ,†,| Australian Research Council Centre of Excellence for Functional Nanomaterials, Advanced Water Management Centre, Brisbane Surface Analysis Facility, and Division of Chemical Engineering, University of Queensland, QLD 4072, Brisbane, Australia
Received September 2, 2008. Revised manuscript received November 4, 2008. Accepted November 12, 2008.
Airborne styrene is a suspected human carcinogen, and traditional ways of mitigation include the use of adsorption technologies (activated carbon or zeolites) or thermal destruction. These methods present their own shortcomings, i.e., adsorbents need to be regenerated or replaced regularly, and relatively large energy inputs are required in thermal treatment. Photocatalysis offers a potentially sustainable and clean means of controlling such fugitive emissions of styrene in air. The present study demonstrates a new type of wellcharacterized, highly thermostable titania-pillared clay photocatalysts for airborne styrene decomposition in a customdesigned fluidized-bed photoreactor. This photocatalytic system is found to be capable of destroying up to 87% of 300 ppmV airborne styrene in the presence of ultraviolet (UV) irradiation. The effects of relative humidity (RH: 0 or 20%) are also studied, together with the arising physical structures (in terms of porosity and surface characteristics) of the catalysts when subjected to relatively high calcination temperatures of 1000-1200 °C. Such a temperature range may be encountered, e.g., in flue gas emissions (1). It is found that relative humidity levels of 20% retard the degradation efficiencies of airborne styrene when using highly porous catalysts.
Introduction Styrene (C6H5CHdCH2) is slightly polar, volatile substance commonly used in industries where resins are employed. With a human threshold odor of approximately 0.32 ppmV (2), and vapor pressure of 44.6 mmHg (20 °C) (3), it is also classified as a potential human carcinogen (4, 5). The effects of inhaling styrene have been studied extensively, and toxicological studies reveal that laboratory mice, when exposed to relatively higher amounts of styrene continuously, develop a high risk of contracting tumors (6). Common * Corresponding author phone: +61 7 33463883, fax: +61 7 33656074, e-mail:
[email protected]. † Australian Research Council Centre of Excellence for Functional Nanomaterials. ‡ Advanced Water Management Centre. § Brisbane Surface Analysis Facility. | Division of Chemical Engineering. 538
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 2, 2009
methods of styrene abatement include the use of adsorbents (activated carbon, zeolites, etc), thermal destruction, and biological degradation. These methods temporarily contain the pollutant (absorption), require large energy inputs (thermal destruction), or require relatively large land areas for building biological filters. Photocatalytic degradation offers an alternative and potentially sustainable method, to the traditional methods of styrene abatement. The most commonly reported photocatalyst, titania (TiO2), causes the oxidation of undesired substrates, when excited by ultraviolet (UV) irradiation. The final products of photocatalysis are ideally carbon dioxide and water. Although commercial P25 (Degussa), a benchmark titania of primary particle size 30 nm (specific surface area of ca. 50 m2/g) (7) is generally known and regarded to be one of the best standards for titania-based photoreactive processes under UV irradiation, the nanoparticulate nature of this material also presents a disadvantage when it is required to employ photocatalysts of larger sizes in different flow regimes. Pillared-materials occur naturally in geochemical systems in the form of clays (smectites, hectorites, etc.), and synthetic clays (8, 9), have been developed commercially for many applications. The development of pillared materials through ion-exchange processes has received much attention because the formation of new layered materials (10, 11) present or promise novel and exciting properties or characteristics for the field of catalysis. We have recently demonstrated that titania-pillared clays can be used in a number of photocatalytic reactions (8, 12, 13).The current study focuses on the use of a new type of highly thermostable anatase titaniapillared clay material of varying surface areas (
