Adsorption and degradation of model volatile organic compounds by a combined titania–montmorillonite–silica photocatalyst
Highlights
► Adsorptive combined titania–montmorillonite–silica photocatalysts synthesized. ► All catalysts had relatively high adsorption capacities of multinary VOCs. ► All catalysts preferred to adsorb the VOCs with higher polarity. ► CTMS80 can effectively photocatalytically remove VOCs of various components.
Introduction
Volatile organic compounds (VOCs) mainly emitted from industrial processes and transport vehicles bring various environmental problems and have adverse effects on human beings. Kinds of physical, chemical and biological techniques have been utilized to remove them from air [1], [2], [3]. Among these different end-of-pipe abatement techniques, activated carbon adsorption is the most widely used. However, it just transfers organics from gas to solid phase. Comparatively, photocatalysis is an attractive technology for the removal of VOCs because it can completely mineralize these pollutants into CO2 and H2O without selection [4]. Hence, various photocatalysts such as metal oxides and chalcogenides have been developed [4], [5], [6], [7]. While TiO2 has been considered as the dominant photocatalyst due to its properties of superior photocatalytic oxidation ability, nonphotocorrosive, nontoxic, and inexpensive characteristics [8]. Unfortunately, the concentration of pollutants in ambient water and air is quite low and the adsorption of pollutants onto the TiO2 photocatalyst is often limited due to its small specific surface area, leading to very low photocatalytic degradation efficiency. A possible way to overcome this is to add a co-adsorbent to the TiO2 photocatalyst to combine high adsorptive ability of co-adsorbent with high photocatalytic activity of TiO2 [9], [10].
Clays have been investigated as a new class of composite materials during past few decades. Many organic [11], [12] or inorganic cations [13], [14] are intercalated into the silicate layers of clays to synthesize diversified pillared materials. Of them, extensive concern has been focused on the TiO2 pillared clay which can be applied to adsorb various organics from water and air [15], [16], [17], [18]. Nevertheless, photocatalytic activity of TiO2 pillared clay is still not high enough due to its low adsorption capacity [19], [20]. As known, photocatalytic oxidation occurs on the catalyst surface where the adsorption of the pollutants is essential process [21]. To increase the adsorption capacity of the TiO2 pillared clay, the immobilization of TiO2 pillared clay on porous materials may be a practicable method. Silica gel has attracted considerable interest as an excellent support since it is transparent to near-UV light, has large specific surface area and strong adsorbability [5]. In addition, the adsorption and condensation properties of the supports greatly depend on their surface hydrophilic–hydrophobic properties [22]. TiO2 pillared clay prefers to enrich and photocatalytically degrade organics with high hydrophobicity in water and air due to its hydrophobic interlayer surface [23], [24]. While the surface of SiO2 exhibits hydrophilic nature originated from the surface hydroxyl groups [25]. Therefore, by controlling the mass ratio of TiO2 pillared clay and silica gel, photocatalysts with different selective ability to abate different polarity VOCs in air can be prepared. However, to date, such photocatalysts have not been reported yet. Furthermore, VOCs always exist as a mixture of many species rather than single component in practical case. Unfortunately, very few publications to date have investigated the relationship between the competitive adsorption and the photocatalytic process in the presence of multiple VOCs.
In present work, by initially intercalated TiO2 into the montmorillonite and then immobilized onto the silica gel, a series of adsorptive photocatalysts were synthesized to remove low-level concentration VOCs from air. Toluene is a toxic compound listed in Title III of the 1990 Clean Air Act Amendment proposed by the Environmental Protection Agency of USA; ethyl acetate (EA) is a kind of irritative and explosive compound with fragrant odor [2], which is harmful to respiratory systems of mankind [26], while ethanethiol (EtSH) is high toxic pollutants with strong unpleasant odor and very low perceptibility threshold [27]. Three VOCs with different polarities as single and multinary VOCs were chosen as the model pollutants in a fluidized bed photocatalytic reactor to evaluate the adsorptive performance and photocatalytic activity of the prepared photocatalysts. Langmuir and Freundlich isothermal models were used to fit the isothermal adsorption data and the Langmuir–Hinshelwood (L–H) equation was applied to model the photocatalytic degradation reactions.
Section snippets
Materials and reagents
Silica gel (150–180 μm) was purchased from Qingdao Haiyang Chem. Co., Ltd., China. Toluene, EA and EtSH were all analytical grades from Tianjin Chemical Reagent Co., Inc., China. Tetra-n-butyl titanium (Ti (OC4H9)4) and glacial acetic acid were from Shanghai Chemical Reagent Co., Inc., China. Sodium montmorillonite was prepared in our laboratory method with montmorillonite from Lin’an, Zhejiang, China [28]. Certain silica gel was immersed into 0.35 mol/L nitric acid solution for 1 h and then
Characterization
The XRD patterns of the prepared photocatalysts and the pretreated silica gel are illustrated in Fig. 1a. Besides a broad characteristic peak of amorphous SiO2, the XRD patterns obtained from samples CTMS40, CTMS60, CTMS80 and CTS exhibit similar characteristics, where anatase peaks are clearly found at 2θ = 25.3°, 37.2°, 48.9°, 54.0°, 55.3° and 62.4°, suggesting that anatase is the only crystalline phase in samples [29]. In contrast, no characteristic peaks of the montmorillonite are discerned
Conclusions
In the present work, a series of adsorptive CTMS photocatalysts were prepared and applied in purification of single and multinary VOCs from air. Results showed that the prepared composite photocatalysts had excellent adsorption and photocatalytic degradation abilities to three model VOCs. The adsorption capacities of them on these photocatalysts increased in the following order: toluene < EA < EtSH, and the isothermal adsorption data were well described by Langmuir isotherm model compared to
Acknowledgements
This is contribution No. IS–1324 from GIGCAS. This work was financially supported by Science and Technology Project of Guangdong Province, China (2007A032301002, 2009B030400001, 2009A030902003 and 2006A36701002) and NSFC (40572173).
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