Applied Surface Science 271 (2013) 164–170 Contents lists available at SciVerse ScienceDirect Applied Surface Science jou rn al h om epa g e: www.elsevier.com/locate/apsusc Humidity effect on photocatalytic activity of TiO 2 and regeneration of deactivated photocatalysts Myung-Geun Jeong a , Eun Ji Park a , Hyun Ook Seo a , Kwang-Dae Kim a , Young Dok Kim a,∗ , Dong Chan Lim b a Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Republic of Korea b Materials Processing Division, Korea Institute of Materials Science, Changwon 641-010, Republic of Korea a r t i c l e i n f o Article history: Received 3 December 2012 Received in revised form 21 January 2013 Accepted 23 January 2013 Available online 29 January 2013 Keywords: Photocatalysis Humidity effects Volatile organic compounds Oxidation a b s t r a c t Photocatalytic decomposition of toluene was studied using commercially available TiO 2 nanocatalysts (Degussa P25) under different ranges of relative humidity (RH). The toluene removal efficiency (sum of toluene adsorption, and partial and total oxidation) decreased with increasing RH, whereas the selec- tivity to CO 2 in toluene oxidation enhanced. The enhancement in activity in terms of total oxidation at higher RH led to suppressing deactivation of TiO 2 with time. On the basis of the humidity effect on photocatalytic reaction, deactivated TiO 2 catalyst could recover their activity through the regeneration process consisting of exposure of the deactivated catalysts to the UV light under high RH conditions. The regenerated catalysts under high RH showed a higher total oxidation yield of toluene than that of the original catalysts due to the formation of more hydroxyl groups on TiO 2 surface during the regeneration process. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The volatile organic compounds (VOCs) are carbon compounds which are easily released into the atmosphere owing to low vapor pressure and typical examples of them are benzene, toluene and xylene (BTX), formaldehyde, acetaldehyde, etc. Among various VOCs, toluene is widely known as a substance that can cause illness in the human body and serious environmental contaminations. Accordingly, various methods for removing toluene have been of interest to many scientist and engineer in the past 10 years. Typical examples of chemical and physical removing methods of toluene include thermal oxidation of toluene using diverse catalysts [1–4], photocatalytic oxidation using semiconductive materials [5–18] and removal by porous materials as adsorbents [19–21]. TiO 2 has been attracting attention for its applications in various fields (e.g. UV protecting agent, vanishà, solar cell, etc.) [22–26]. Among them, in particular, abatement of VOCs using photocatalytic oxidation in TiO 2 nanoparticles is being spotlighted in environ- mental science due to its outstanding chemical and optical stability [5–18]. When TiO 2 nanoparticles are irradiated to UV light with a higher energy than the TiO 2 band gap (∼3.2 eV), electron–hole pairs can be generated, which can create strong oxidizing agents such as O 2 - and OH radicals by reacting with O 2 and water molecules ∗ Corresponding author. Tel.: +82 31 299 4564; fax: +82 31 290 7075. E-mail address: ydkim91@skku.edu (Y.D. Kim). [27–31]. These oxidizing agents can ultimately convert organic pol- lutants to CO 2 or H 2 O. There are, however, some hurdles to be overcome for real appli- cation of TiO 2 photocatalysts: TiO 2 catalyst can be deactivated as the photocatalytic reaction proceeds. Recently, much effort has been devoted to clarify the origin of the photocatalyst deactivation [6,17,18,32–34] and finding solutions for the deactivation problem [7,17,33,35–37]. Cao et al. have claimed that deactivation of TiO 2 is induced by accumulation of reaction intermediates during pho- tocatalytic oxidation and their catalytic activity can be completely recovered by thermal treatment at and above 420 ◦ C [6]. In the present work, the effect of water vapor on photocat- alytic oxidation using Degussa P25 nanopowder (average diameter: 25 nm), which has been widely utilized as photocatalysts, was investigated in three levels of RH, which are 1, 35 and 70%. Degussa P25 is TiO 2 nanoparticles composed of anatase and rutile phases with 7:3 ratio, which is known to be optimal for the formation of OH radical [30]. A plenty of studies have been devoted to find out the humidity effect on photocatalytic performance and mechanism [38–43]. The evaluation of photocatalytic efficiency was carried out in terms of both toluene removal efficiency, which is sum of adsorption, partial oxidation and total oxidation activities, and selectivity toward total oxidation (production of H 2 O and CO 2 out of organic pollutants). The toluene vapor was chosen as model pol- lutant in photocatalytic reaction of TiO 2 because toluene is well known for its harmfulness to human body and can be easily emitted in our daily life. Subsequently, photocatalytic regeneration test was 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.155