Removal of volatile organic compounds (VOCs) by photocatalytic treatment has drawn extensive interest as an environmentally-benign technique over the last few decades. 1-3 Low molecular weight aldehydes are one of the major VOCs contaminating indoor air. 4,5 They are usually generated from new architectural ornaments, furniture, merchandize, and plastics, as well as from paint, glue, or other chemicals, and known to be carcinogenic, mutagenic or teratogenic. 6,7 Thus far, photocatalytic

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... ition of aldehydes with TiO 2 has been reported by several researchers, 8-15 but the decomposition characteristics of each aldehyde has not been fully investigated. Previously, we reported that the monolayer coverage of WO 3 on the surface of TiO 2 particle enhances the photocatalytic activity by 3-4 times in decomposing gaseous 2propanol or benzene. 16, 17 It was proposed that the major advantage of the WO 3 -modified TiO 2 originates from much higher adsorption of organic compound on its surface because of the high Lewis surface acidity of WO 3 covering TiO 2 surface. In the present work, we studied the decomposition trends of the three volatile aldehydes, formaldehyde, acetaldehyde, and propionaldehyde, with the WO 3modified TiO 2 photocatalyst. Unexpectedly, increase of photocatalytic activity was not observed in decomposing formaldehyde, differently from other aldehydes. We analyzed the surface of formaldehyde-adsorbed photocatalysts to investigate the unusual decomposition behavior of formaldehyde. The obtained result will also provide a new insight to understand the photocatalytic enhancement in the WO 3modified TiO 2 . The commercial TiO 2 , Degussa P25, with a surface area of 50 m 2 /g, was chosen as a standard. The 3 mol% WO 3 /97 mol% TiO 2 (denoted to WO 3 /TiO 2 ; the TiO 2 -based composite whose surface is covered with monolayer of WO 3 ) was prepared by an incipient wetness method. That is, 1.00 g of P25 TiO 2 was suspended in 40 mL of 14.0 M aqueous ammonia solution containing 3.87 × 10 −4 mol of the tungstic acid (99%, Aldrich), and dried in a water bath at 70 o C while stirring. The dried sample was then heat treated at 200 o C for 2 hr in a flowing oxygen. 16, 17 The TEM images in Figure 1 show the pure TiO 2 (Degussa p25) and WO 3 /TiO 2 nanoparticles. The uniform lattice fringes observed over an entire particle with the size of about 25 nm indicate that the individual TiO 2 nanoparticle consists of a single grain. The WO 3 /TiO 2 nanoparticles also showed the lattice fringes similar to those observed in pure