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Procedia Engineering 121 ( 2015 ) 521 – 527

9th International Symposium on Heating, Ventilation and Air Conditioning (ISHVAC) and the 3rd International Conference on Building Energy and Environment (COBEE) Photocatalytic Decomposition of by Combination of Ozone and AC Network with UV365nm, UV254nm and UV254+185nm M.Y. Wanga,Y.W.Lua*, F Wua, X.J Zhangb,C.X.Yangb

aKey Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education of China, college of environmental and energy engineering, University of Technology, Beijing 100124, China; bSchool of Aeronautic Science and Engineering, Beihang University, Beijing 100191, China.

Abstract

Formaldehyde (HCHO) is one of the most common indoor . Long time exposure to the environment with HCHO above 0.1mg/m3 can cause harm to human body. In order to remove HCHO efficiently, The TiO2/AC network film was prepared by loading nanometer particle TiO2 on the surface of activated . The photocatalytic degradation of gaseous HCHO was performed by TiO2/AC network film under the irradiation of UV365nm, UV254nm and UV254+185nm, respectively. The results show that the adsorption of AC network and byproduct ozone (O3) oxidation simultaneously can promote the degradation of the HCHO; under the illumination of UV254+185nm, HCHO can be degraded more effectively. The produced ozone can also be degraded below 0.214 mg/m3 that are required by Chinese standard. © 2015 2015 The The Authors. Authors. Published Publis byhed Elsevier by Elsevier Ltd. This Ltd. is an open access article under the CC BY-NC-ND license (Peer-reviewhttp://creativecommons.org/licenses/by-nc-nd/4.0/ under responsibility of the organizing). committee of ISHVACCOBEE 2015. Peer-review under responsibility of the organizing committee of ISHVAC-COBEE 2015

Keywords: HCHO; O3;TiO2; AC network; UV

1. Introduction

At present, people are paying more and more attention on the indoor air quality along with the improvement of living environment [1]. As one of volatile organic compounds (VOCs), HCHO is a typical indoor air .This

* Corresponding author. Tel.: 010-67396662-8043; fax: 010-67392774. E-mail address: [email protected]

1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of ISHVAC-COBEE 2015 doi: 10.1016/j.proeng.2015.08.1023 522 M.Y. Wang et al. / Procedia Engineering 121 ( 2015 ) 521 – 527

Nomenclature

ĭ removal efficiency, % 3 Cin inlet , mg/m 3 Cout outtlet concentrations, mg/m

pollutant can be found in many indoor products, such as pressed wood, coated products, paints, insulation, and combustible materials. Long time exposure to the environment at HCHO concentration of 0.1 ppm or higher can cause harm to human body health, such as nausea, chest tightness, skin rashes and allergic reaction [2-3]. Traditional methods for removing HCHO from indoor air include absorption, dilution with fresh air and . Adsorption filter has the inherent limitation of adsorption saturation and needs to be replaced periodically. Dilution with refresh air has the shortage of higher energy cost. Photocatalysis is an emerging and promising technology for indoor air purification by using TiO2 as a catalyst under the illumination of UV light, which can oxidize the VOCs into CO2 and H2O at and atmospheric pressure [4]. However, most of the studies on the degradation of contaminants show that UV/TiO2 alone was not enough in degradation of gaseous contaminant [5-8]. Researches show that when the HCHO below 1 ppm, the photocatalytic decomposition of HCHO decrease rapidly and then almost terminates on the way. Therefore, immobilizing TiO2 on an adsorbent material was used to improve the HCHO removal efficiency [9]. Studies show that the UV light wavelength can influence the photocatalytic removal of HCHO [10-11]. Because the byproducts O3 produced by UV light with primary wavelength of 185nm can not only react with HCHO directly [12-13], but also can form hydroxyl group in air stream when ozone is irradiated by UV light with primary wavelength lower than 254 nm [14-16]. The hydroxyl group in air stream can react with HCHO, which increase the HCHO removal. In order to confirm if the hydroxyl group in air stream can participate in HCHO removal, the photocatalytic decomposition of HCHO at different condition was performed by loading nanometer TiO2 on the surface of AC network. The effects of UV light wavelength and the O3 on the removal of HCHO was studied. The results can be a base for the application of photocatalytic air purification with UV254+185 nmDŽ

2. Reaction mechanism

HCHO can be photocatalytic degraded on the surface of nanometer TiO2 under the illumination of UV light, during which the photo-production holes and electronics can react with the that adsorb on the catalyst surface to generate hydroxyl. The produced hydroxyl plays an important role on the photocatalytic removal of HCHO [17].

+ - TiO2+hv ՜ h +e (1)

+ + H2O+h ՜ gOH+H (2)

- - O2+ e ՜ gO2 (3)

HCHO+gOH ՜ gCHO+H2O (4)

gCHO+gOH ՜ HCOOH (5)

శ - - ାୌ ାୌେୌ୓ (gCHO+gO2 ՜ HCO3 ሱۛሮ HCOOOH ሱۛۛۛሮ CHOOH (6

శ శ ିୌ - ௛ + - (HCOOH ሱۛሮ HCOO ሱۛۛሮ H +gCO2 (7 M.Y. Wang et al. / Procedia Engineering 121 ( 2015 ) 521 – 527 523

[୓][·୓ୌ],௛శ (gCO - ሱۛۛۛۛۛۛሮ CO (8 2 2

Beyond to that, HCHO can also be degraded by O3 directly [12]:

HCHO+3O ՜ CO +H O+3O (9) 3 2 2 2 In order to improve photocatalytic HCHO removal, the UV light with primary wavelength of 185 nm can be used for the O3 can be inspired during illumination. The involved reactions are as follows [18]:

( ) ௛௩ ଵ଼ହ୬୫ 3 1 (O2ሱۛۛۛۛۛۛۛሮ O( P)+O( D) (10

3 O2+O( P) ՜O3 (11)

1 1 Also, researches show that ozone can be decomposed to O( D) and O2 , and O( D) can react with water molecules to generate hydroxyl. The hydroxyl in air stream can react with HCHO, which result in HCHO removal improved. The involved reactions are as follows [19].

1 O3+ hv(254nm)՜O( D)+O2 (12)

1 O( D)+H2O ՜2gOH (13)

O3+gOH՜gHO2+O2 (14)

gHO2+O3՜gOH+2O2 (15)

The above reactions show that photocatalytic degradation of HCHO can be enhanced when O3 participate it. In order to confirm it, photocatalytic decomposition of HCHO under illumination of UV light with different primary wavelength was performed.

3. Experiment

3.1 Experiment system

Figure 1 shows the experimental system for the photocatalytic removal of HCHO. Different reaction parameters, such as HCHO concentration, flow velocity, humidity, light intensity could be adjusted in this system. First, an air compressor drove a zero air generator to produce high pressure zero air. A dynamic gas calibrator was operated with the zero air generator to control the mass flow rate of the zero gas. The zero gas was split into three routes. The first route entered a saturator containing deionized water to humidify the air. A flow meter and valve were used to control the humidity. The second route gas entered infiltrative Teflon tube which was placed in a conical flask containing HCHO and the conical flask immerged in a constant temperature water bath, and then the concentration can be controlled by controlling the temperature of the constant temperature water bath. The third route was used to form the diluted gas. Then three way gases mixed in the mixed section, the tail gas emitted into the atmosphere. The reactor had a volume of 420 ml (3Hh28Lh5Wcm). The UV light with different primary wavelength was placed inside the reactor as the light source of photocatalysis and the catalyst TiO2/AC network film was placed on the top of the UV lamp; The HCHO concentration was measured by Interscan 4160 HCHO analyzer; The O3 concentration was measured by a Model 106-L O3 analyzer (America ECO sensor Inc. with 2% precision); The reaction temperature and humidity were measured by a temperature-humidity analyzer with a precision of ±1% RH. 524 M.Y. Wang et al. / Procedia Engineering 121 ( 2015 ) 521 – 527



1.Air compressor; 2.Zero air generator; 3.Gas calibrator; 4,7,10.Valve; 5.Flow meter; 6.Glass saturator; 8.HCHO solution; 9.Thermostatic water bath; 11.Mixer; 12.UV lamp; 13. TiO2/AC network film; 14.Reactor; 15. HCHO analyzer; 16. O3 analyzer; 17. Temperature-humidity analyzer; 18.Tail gas Fig.1. Experiment system

3.2 Photocatalyst TiO2 film

Nanometer TiO2 was used as the photocatalyst. An activated honeycomb carbon filter was coated with 5% weight by TiO2 in water suspension by dipping and then was calcined at 120qC for 1 h with a ramp of 5qC/min to form the TiO2-coated film. Films produced by this method are stable to repeated washing with water but are not stable to 2 mechanical abrasion. The TiO2/AC network film which had a coating area of 45 cm was show in Figure 2. The amount of TiO2 deposited was determined by the weight difference before and after the coating procedure. In all experiments, the TiO2 catalyst load was 0.2g r5%.

Fig.2. The TiO2/AC network

3.3 The assessment index of HCHO

The HCHO removal (ĭ were used as evaluation indexes defined below:

100() C -C ĭ()%= in out (16) Cin

3 where Cin and Cout are the steady inlet and outlet concentrations (mg/m ) in reactor, respectively.

4. Results and Discussions

4.1 The effect of adsorption on the TiO2/AC network

As show in Figure 3, prior to the experiment, the HCHO concentration had reached equilibrium for more than 30min in the absence of TiO2/AC network. Only the stable data in 10 min was listed in the Figure 3. At time 10 min, the TiO2/AC network was put in the reactor, the concentration of HCHO dropped sharply owing to the adsorption of HCHO by the AC network. When the HCHO concentration reached point A the HCHO adsorption capacity of the TiO2/AC network decreased, causing the concentration of HCHO to augment gradually until the HCHO adsorption M.Y. Wang et al. / Procedia Engineering 121 ( 2015 ) 521 – 527 525

capacity of the TiO2/AC network reached saturation. From the above results, one can see that the AC network adsorbent existed adsorption saturation and needed to be regenerated after practical use for some time.

Fig.3. The adsorption capacity of AC network

4.2 The influence of different wavelength UV on degradation of HCHO

Figure 4 shows the concentration of HCHO change with time by TiO2/AC network under the illumination of UV light with different primary wavelength at humidity of 11.07g/kg and flow velocity of 3.08 cm/s. When the HCHO concentration reached point A, after which HCHO adsorption capacity of the TiO2/AC began to decrease, the UV light was turned on. Then the HCHO concentration would further decrease due to the photocatalytic degradation of HCHO under the illumination of UV light with different primary wavelength. It shows that the shorter the UV light primary length, the lower outlet HCHO concentration. The UV light with shorter primary length can excite TiO2 film produce more photon-production electrons and holes, so more hydroxyl group can be formed on the TiO2 surface. Therefore, more HCHO can be photocatalytic degraded with UV254nm than that with UV365nm. When the UV254+185nm light was turned on, ozone can be produced during illumination, as shown in reactions (10-11). Ozone can react with HCHO directly, as shown in reaction (9). Also, the produced hydroxyl groups in air stream can also react with HCHO, which result in HCHO removal improved, as shown in reactions (12-15). Therefore, HCHO can be easily degraded with UV254+185nm. Because ozone can also be photocatalytic degraded [4], so the outlet ozone concentration was as lower as 2.213 mg/m3 and near to 0.1 ppm that is required by Chinese “indoor air quality standards” (GB/T 18883-2002) [20]. Figure 5 shows HCHO removal by different UV light. Under the illumination of UV254+185nm, HCHO removal can reach 86%. Under this situation, if the reactor length can be enlarged, the HCHO outlet concentration can also reach 0.214 mg/m3 that is required by “indoor air quality standards” [20]. Therefore, it is feasible to photocatalytic degrade indoor HCHO by UV254+185nm.

Fig.4. HCHO and O3 concentration versus time under different UV lamp irradiation 526 M.Y. Wang et al. / Procedia Engineering 121 ( 2015 ) 521 – 527

Fig. 5. HCHO removal with different UV light

5. Conclusions

Ь Indoor HCHO can be photocatalytic degraded effectively by UV254+185nm and the outlet ozone concentration can reach below 0.1 ppm that is required by “Chinese indoor quality standards. Ь It is feasible to photocatalytic degrade indoor HCHO by UV254+185nm.

Acknowledgments

The research presented in this paper was financially supported by the National Key Basic Research and Development Program of China (the 973 Program) through grant No. 2012CB720100.

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