Photocatalytic oxidation of indoor toluene: Process risk analysis and influence of relative humidity, photocatalysts, and VUV irradiation

Abstract

Concentrations of 13 gaseous intermediates in photocatalytic oxidation (PCO) of toluene in indoor air were determined in real-time by proton transfer reaction mass spectrometry and desorption intensities of 7 adsorbed intermediates on the surface of photocatalysts were detected by temperature‐programmed desorption‐mass spectrometry. Effects of relative humidity (RH), photocatalysts, and vacuum ultraviolet (VUV) irradiation on the distribution and category of the intermediates and health risk influence index (η) were investigated. RH enhances the formation rate of hydroxide radicals, leading to more intermediates with higher oxidation states in gas phase. N doping promotes the separation of photo-generated electrons and holes and enhances PCO activity accordingly. VUV irradiation results in higher mineralization rate and more intermediates with higher oxidation states and lower toxicity e.g. carboxylic acids. Health risk analysis indicates that higher RH, N doping of TiO₂, and VUV lead to “greener” intermediates and smaller η. Finally, a conceptual diagram was proposed to exhibit the scenario of η varied with extent of mineralization for various toxicities of inlet pollutants.

Introduction

Volatile organic compounds (VOCs) such as toluene, benzene, and formaldehyde emitted from building materials, furnishings, and used electronic equipments have been widely detected in the indoor air of residential and commercial buildings, and have been associated with higher incidence of asthma, allergies, and building-related symptoms (Mendell, 2007, Takigawa et al., 2012). Photocatalytic oxidation (PCO) of VOCs with TiO₂ has been studied extensively (Li et al., 2011, Mo et al., 2009, Sleiman et al., 2009, Vildozo et al., 2011, Yu and Lee, 2007) owing to its mild reaction conditions and potential applications to purify indoor air.

Over the past decade or so, efforts have been made to modify TiO₂ by doping with metals (Aramendia et al., 2008, Liu et al., 2010, Xiang et al., 2010) and non-metals (Chen and Burda, 2008, Han et al., 2011, Khan et al., 2002). Among non-metal dopants, N doping of TiO₂ is considered as one of the most effective methods to extend the light absorption range, inhibit the combination of photo-generated electrons (e⁻) and holes (h⁺), and promote photo-generated e⁻ transfer to reducible adsorbates (D'Arienzo et al., 2009, Dong et al., 2009, Giamello et al., 2006, Giamello et al., 2009).

PCO process can be facilitated by photo-generated h⁺ from the valence band and by hydroxyl radical (OH) intrigued by water molecule (Maeda and Domen, 2007) in the presence of relative humidity (RH). The h⁺‐induced oxidation is strongly dependent on the structural and electronic properties of photocatalysts, whereas OH concentration is known to be affected by RH (Debono et al., 2011). Besides, water molecule can competitively occupy the active sites on the surface of photocatalysts to suppress the PCO process (Takeuchi et al., 2010).

Recently, Quici et al. (2010) revealed that irradiation of vacuum ultraviolet (VUV) light (emitting ca. 10% and 90% photons at 185 and 254 nm, respectively) can produce a number of strong oxidants such as O(¹D), O(³P), O₃, O₃⁻, O, and OH in PCO. These oxidants can minimize the formation of partially oxidized volatile species, eliminate non-volatile intermediates from the surface of photocatalysts, and then extend the lifetime of the photocatalysts.

It's worth mentioning that PCO is a process during which VOCs are decomposed stepwise and various intermediates can be generated before mineralization is achieved. Some volatile harmful intermediates such as formaldehyde, benzaldehyde, benzene, and cresol have been observed in PCO of toluene (d'Hennezel et al., 1998, Mo et al., 2009, Van Durme et al., 2007, Wang et al., 2007). This side effect may impede the application of PCO in the indoor air purification.

Proton transfer reaction mass spectrometry (PTR‐MS) has been proved to be a valid and powerful method to monitor volatile intermediates at pptv level in real-time (Mo et al., 2009, Zhao et al., 2011). While most of conventional off-line methods such as high performance liquid chromatography (HPLC‐UV), gas chromatograph–flame ionization detector (GC–FID), gas chromatography–mass spectrometry (GC–MS), and Fourier‐transform infrared spectroscopy (FTIR) (Augugliaro et al., 1999, d'Hennezel et al., 1998, Irokawa et al., 2006, Mendez-Roman and Cardona-Martinez, 1998, Patterson et al., 1999), can only detect limited intermediates because of their limitations in sampling and detection principles. Table S1 (see the Supplementary information) summarized the intermediates detected on PCO of toluene by many works with various detection methods mentioned above. Mo et al. (2009) and Zhao et al. (2011) identified the intermediates at ppbv level during PCO decomposition of toluene by PTR‐MS and GC–MS, and a health related index (HRI) was used to characterize the health risk associated with the process. However, the influence of experimental conditions on the intermediate formation and the relevant heath risk haven't been studied in detail.

In this work, a real-time PTR‐MS was used to monitor the concentrations of toluene and volatile intermediates, and a temperature programmed desorption mass spectrometer (TPD‐MS) was used to detect adsorbed intermediates on the surface of photocatalysts. The effects of RHs (0%, 45%, and 80%), photocatalysts (P25 and N‐doped TiO₂), and irradiation wavelengths (UV and VUV) on the distribution and categories of intermediates, and the health risk of the PCO process of toluene were investigated accordingly. Finally, a conceptual diagram was proposed to exhibit the scenario of health risk influence index (η) varied with extent of mineralization (ξ) for various toxicities of inlet pollutants.