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This is due to thefact that the toluene concentrations in the reactor box were almostequal to those in the 1m3 room in the decomposition of tolueneusing only the photocatalytic reactor. Likewise, at the desorptiontemperature in the range of 130–160 ◦C, the toluene concentrationsin the reactor box are not remarkably different from those in the direct decomposition of toluene in 1m3 air using the photocatalyticreactor. If the toluene concentration in the reactor box was concen-trated by the continuous adsorption/desorption unit, toluene wouldhave been decomposed more efficiently. The cause of why toluenewas not successfully concentrated in the reactor box is based on thefact that toluene was strongly adsorbed onto zeolite particles.As evident from Figs. 5 and 6, the air purifier can drop the tolueneconcentration to theWHO guideline (0.26mgm−3 or 70 ppbv)within10min. Since the time required to purify the air using the air puri-fier is shorter than that using only the photocatalytic reactor withincreasing the amount of treated air (Shiraishi et al., 2003, 2005b,2007). Therefore, the use of the air purifier becomes more advanta-geous.3.5. Durability of the air purifierAll the decomposition experiments were performed using thesame zeolite rotor and photocatalytic reactor over six months, andrun more than 100 times. Nevertheless, no distinct decrease wasobserved in the decomposition activity of the photocatalyst and theability of the zeolite rotor to adsorb toluene, indicating that thesematerials can perform consistently over a long period of time.Several studies found that surface species formed during the pho-tocatalytic decomposition of toluene rapidly deactivate the catalyst(Blount and Falconer, 2001, 2002; Marci et al., 2003). In our study,however, no drop in the reactor performance was observed through-out the entire experimental period of time. Likewise, there was nodrop in the reactor performance in the photocatalytic decompositionof HCHO using the same reactor (Shiraishi et al., 2005b); the forma-tion of HCOOH, an intermediate in this reaction, was negligible. Thedifference between other researchers' findings and ours may be ex-plained by the following two reasons. One is due to the fact that otherresearchers mostly treated toluene at a high concentration such as100 ppmv, while we repeatedly treated toluene of 1–10 ppmv and,as a consequence, titanium dioxide in our reactor had a lower bur-den to the toluene decomposition. The other is due to the fact thatour photocatalytic reactor decomposed toluene into carbon oxidewithout significantly accumulating intermediates, since the photo-catalyst films in our reactor are erected under the irradiation withUV light of a large intensity per unit surface area. In fact,
it was noteasy to detect and measure the intermediate concentrations. For ex-ample, in the decomposition of HCHO, the intermediate HCOOH wasonly 9.86mgm−3 when HCHO was decomposed at an initial concen-tration of 1150mgm−3; the indoor HCHO concentration is normallyless than 0.369mgm−3 (Shiraishi et al., 2005a).3.6. Operational method of the air purifier in practical applicationIf the desorption temperature is increased further, a largeramount of toluene is desorbed and the rate of photocatalytic de-composition is increased, because the toluene concentration in thereactor box is increased. Clearly this operation is advantageous if onesimply wishes to decompose toluene quickly. However, an increasein the desorption temperature significantly increases the power re-quired to heat the rotor; the air purifier consumes about 250W ofenergy for heating purposes alone. Moreover, if the desorption tem-perature is increased up to around 200 ◦C, it is necessary to use anexpensive desorption fan that can endure high temperatures. Also,if the operation is conducted at such temperatures for long periods,the risk of fire in the reactor box is potentially increased. Thus, weconsider that the desorption temperature of 90–120 ◦C would bereasonable for practical application. Nevertheless, when such an airpurifier is repeatedly used over a long period, the rotor will sooneror later become saturated with toluene even when toluene is de-composed using the photocatalytic reactor. To solve this problem, we propose that the rotor be periodically regenerated by desorbingtoluene at a high temperature and then decomposing it using thephotocatalyst.4. ConclusionThe experimental results obtained in the present work clearlyshow that the miniaturized air purifier can reduce the tolueneconcentration in the 1m3 room to a value near zero in the first10–15min. This high performance is based on the adsorption oftoluene by the continuous adsorption/desorption unit. The tolueneconcentration in the reactor box increases for the first 10min, butdecreases to almost a value of zero after 40–150min owing to thephotocatalytic decomposition. The desorption process of toluenefrom the rotor into the reactor box is deduced to be rate-limitingstep. It is thus considered that the photocatalytic decomposition oftoluene in the reactor box is highly restricted. The decompositionactivity of the photocatalyst and the ability of the zeolite rotor toadsorb toluene are sufficiently stable for repeated use of the samematerials. However, the zeolite rotor should be periodically regen-erated since it would be sooner or later become saturated withtoluene as a result of long-term operation.ReferencesBarraud, E., Bosc, F., Edwards, D., Keller, N., Keller, V., 2005. Gas phase photocatalyticremoval of toluene effluents on sulfated titania. J. 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