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Removal of Ozone by Carbon Nanotubes / Quartz Fiber Film Shen Yang, Jingqi Nie, Fei Wei, and Xudong Yang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02563 • Publication Date (Web): 08 Aug 2016 Downloaded from http://pubs.acs.org on August 14, 2016

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Environmental Science & Technology

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Manuscript for Environmental Science & Technology

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Removal of Ozone by Carbon Nanotubes / Quartz Fiber Film

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Shen Yang, † Jingqi Nie, ‡ Fei Wei, ‡ Xudong Yang *, †

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†

Department of Building Science, Tsinghua University, Beijing 100084, PR China

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‡

Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR

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China

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*

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Tsinghua University, Beijing 100084, PR China. Telephone: (86)10- 62788845. Fax:

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(86)10-62773461. E-mail: [email protected]

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Address correspondence to Dr. Xudong Yang, Department of Building Science,

Notes: The authors declare no competing financial interest.

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ACS Paragon Plus Environment


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ABSTRACT

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Ozone is recognized as a harmful gaseous pollutant, which can lead to severe human

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health problems. In this study, carbon nanotubes (CNTs) were tested as a new

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approach for ozone removal. The CNTs/quartz fiber film was fabricated through

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growth of CNTs upon pure quartz fiber using chemical vapor deposition method.

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Ozone conversion efficiency of the CNTs/quartz fiber film was tested for 10 h and

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compared with that of quartz film, activated carbon (AC), and a potassium iodide (KI)

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solution under the same conditions. The pressure resistance of these materials under

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different airflow rates was also measured. The results showed that the CNTs/quartz

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fiber film had better ozone conversion efficiency but also higher pressure resistance

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than AC and the KI solution of the same weight. The ozone removal performance of

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the CNTs/quartz fiber film was comparable with AC at 20 times more weight. The

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CNTs played a dominant role in ozone removal by the CNTs/quartz fiber film. Its high

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ozone conversion efficiency, light-weight and free-standing properties make the

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CNTs/quartz fiber film applicable to ozone removal. Further investigation should be

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focused on reducing pressure resistance and studying the CNT mechanism for

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removing ozone.

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Keywords: Ozone removal; Carbon nanotubes; Quartz fiber

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GRAPHIC ABSTRACT

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INTRODUCTION

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Ozone is a regulated gaseous pollutant in both atmospheric and indoor environments.

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It can have adverse effects on human health, for example, exposure to ozone is closely

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related to asthma and respiratory symptoms such as coughing and upper airway

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irritation.1-3 Significant associations have been reported between outdoor ozone

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concentrations and both mortality and morbidity.4, 5 Indoor ozone may originate from

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ozone emission sources indoors and ventilation or infiltration from outdoors.

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Generally, indoor ozone concentrations are lower than outdoors due to removal by

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indoor surfaces and indoor gas phase reactions. However, even low concentrations of

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ozone can still lead to human health problems.6 The standard value of

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one-hour-average ozone concentration indoor is around 80 ppb in China.7 The limiting

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level of ozone in aircraft cabins is higher, 250 ppb, since the outside ozone

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concentration of aircraft cabins is much higher than that of buildings.8 As a result of

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humans spending most of their time in indoor environments, it is estimated that 25-60%

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of total daily inhalation intake of ozone comes from indoors.9 Moreover, indoor

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ozone-initiated chemistry can generate secondary emissions of fine and ultrafine

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particles and volatile organic compounds (VOCs),10-12 which may be more harmful

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than ozone itself.13 Therefore, it is necessary to remove ozone from indoor

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environments.

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Several methods have been developed to remove ozone. Activated carbon (AC) is

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well known for its large adsorptive capacity due to its high surface area and porous

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structure. A number of studies have investigated interactions between ozone and AC,

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indicating that AC is effective at removing ozone.14-17 According to long-term

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performance tests, AC can provide substantial and long-lasting ozone control.18, 19 The 4


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capability of ozone removal by AC may be attributable to the ozone’s reaction with

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oxide groups on the surface of the AC20, 21 and the catalytic action of AC for ozone

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decomposition.22-24 In terms of catalytic decomposition of ozone, the catalytic

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materials are usually noble metals and metal oxide, which are often supported on

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materials such as TiO2, SiO2, and AC. 25-27 Potassium iodide (KI) solution and wet KI

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is also used to remove indoor ozone, especially in sampling procedures as passive or

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active ozone scrubbers.

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wheat board, and sunflower board, are applied as passive ozone removal

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mechanisms.30 Though the methods mentioned above have been proven to remove

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ozone in most indoor conditions, it is of interest to investigate new approaches for

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potential improved ozone removal.

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Carbon nanotubes (CNTs) have been applied in many fields since their discovery31

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due to their superior properties, such as high electrical conductivity,32 mechanical

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stability,33 and thermal conductivity.34 In recent years, this material has been

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introduced to the field of air filtration. CNTs have several advantages compared to

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traditional filters, including chemical stability and resistance of high temperatures.

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Previous studies have proven that CNTs and CNT-coated films can filter particles35

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and adsorb volatile organic vapors effectively.36, 37 As for interaction with ozone,

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several studies analyzed ozone adsorption on CNTs from micro-perspectives38-40 and

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focused on electrical properties of ozonized CNTs.41-43 However, one question yet to

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be addressed is how CNTs perform as filters for removing ozone? Furthermore, how

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do they compare with other ozone cleaners, such as AC and KI solutions?

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This study aims to investigate the active ozone removal performance of CNTs and

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conduct a comparative analysis with conventional ozone scrubbers. In this study, the

28, 29

Additionally, green building materials, e.g., bamboo,

5



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CNTs/quartz fiber film was fabricated through growth of CNTs upon pure quartz film

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using chemical vapor deposition method. Ozone conversion efficiency of the

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CNTs/quartz fiber film was tested for 10 h and compared with that of quartz fiber

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(QF), AC, and KI solutions under the same conditions. The pressure resistance of

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CNTs/QF film, pure QF, and AC under different airflow rates were measured and

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compared as well.

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MATERIALS AND METHODS

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Fabrication of Materials

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The CNTs/QF film was fabricated through in-situ growth of CNTs upon QF using

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floating catalyst chemical vapor deposition method. Figure 1 shows the manufacturing

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device. Ferrocene (purity > 99.99%, Tianjin Damao Chemical Reagent Factory,

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China), acting as the catalyst, was put in a quartz tube (38 mm inner diameter, 180

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mm length) at the entrance side of the heater (TF55030C-1, Thermal scientific, USA).

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After heating, the ferrocene slowly evaporated and the gaseous ferrocene was adhered

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onto the pure QF (50 mm in diameter, Membrane Solutions Inc.) and placed in the

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middle of the quartz tube. With the protection of 600 mL/min argon and 100 mL/min

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hydrogen, the temperature inside the quartz tube was increased to and maintained at

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760 °C. Next, ethylene flowed at 200 mL/min as the carbon source for the CNT

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growth on the catalyzed quartz film for 40 min. Afterward, the heater was turned off.

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The flow of ethylene was also switched off, while argon and hydrogen continued

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flowing until the temperature inside the quartz tube decreased to room temperature.

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The CNTs/QF film was finally obtained. The weight of the film was 0.30 ± 0.01g,

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measured by an electrical scale (HZT-A1000, Hz & Huazhi, USA.).

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Granular AC (Getu, Yunjia Company, China) was selected from 20 mesh to 40 mesh 6


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and divided into two parts. The first one (AC-1) was 0.30 ± 0.01 g, of the same weight

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as the CNTs/QF film. The second one (AC-2) was 6.00 ± 0.01 g, 20 times as heavy as

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AC-1. These two samples were packaged in circular, nonwoven fabric (40 mm in

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diameter) separately. Note that due to a small quantity of granular AC, the circular,

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nonwoven fabric of AC-1 was not fully filled in the cross-sectional area. Hence,

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airflow would bypass the granular AC in ozone removal by AC-1. In addition, 0.30 ±

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0.01g AC (AC-3) was prepared in free form to fully fill a smaller cross-sectional area

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as mentioned hereinafter. Furthermore, 0.30 ± 0.01 g KI was dissolved in 5.7 mL of

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deionized water to make a KI solution of 5% mass concentration.

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Measurement of Physical Properties

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The Brunauer–Emmett–Teller (BET) specific surface areas (SBET) of QF, CNTs/QF

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film, and AC were determined using a Quadra Sorb Station (Quantachrome

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Instruments Corp., Florida, USA) through nitrogen adsorption-desorption method.

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Additionally, to determine the proportion of CNTs in the CNTs/QF film,

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thermogravimetric analysis (TGA) measurements were conducted for the QF and the

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CNTs/QF film. With a mixed flow of 50 mL/min oxygen and 20 mL/min nitrogen, the

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tests were carried out on a thermogravimetric analyzer instrument (TGA Q500) at a

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scan rate of 20 °C/min from 30 to 900 °C. Then the CNT content of the CNTs/QF film

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was calculated by Equation (1): CNTs % =

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LWCNTs /QF − LWQF 1 − LWQF

× 100% .

(1)

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Where, LWCNTs/QF is the weight loss percentage of the CNTs/QF film at 900 °C, and

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LWQF is the weight loss percentage of the QF at 900 °C.

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Experimental Setting

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Figure 2 shows the schematic of the experimental system for ozone conversion tests. 7



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A 53 L stainless environmental chamber, equipped with a water bath for controlling

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temperatures in the chamber at 25 ± 0.5 °C, was used to provide a controllable

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environment for conversion tests. The material was held in the reactor, which was put

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inside the chamber. A 500 mL glass bottle with a temperature and humidity monitor

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inside, called a pre-heater, was placed in the upstream of the reactor to ensure that the

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temperature of the air passing through the reactor was identical to that inside the

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chamber. On the night before each experiment, the system was supplied with clean air

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via a synthetic air cylinder (20.9% oxygen and rest of nitrogen, Beijing Zhaoge Gas

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Technology, qualified by Tsinghua University, China) at 2.60 L/min, controlled by a

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pressure-reducing valve and flow controller. Before entering into the reactor, the clean

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air went through a humidity controller, to control relative humidity (RH) at 10 ± 5%,

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and a thin glass tube under an ultra-violet (UV) lamp, to generate ozone. The

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consistency of ozone generation is shown in Supporting Information. The ozone

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monitor (Model 205, 2B Technology, USA, precision of 1 ppb or ± 1%, whichever is

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greater) was placed at the outlet of the reactor to measure exhaust ozone concentration

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every one minute until the end of the experiment. The ozone monitor was zeroed and

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calibrated after every set of experiments. Note that all the steps mentioned above were

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conducted while wearing disposable gloves to avoid any contamination of lipids from

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the experimenter’s hands. In the experiment, the exhaust ozone concentration of the

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empty reactor was controlled at around 300 ppb, which could be regarded as the net

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inlet ozone concentration during the experiment. The ozone conversion efficiency, η,

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was calculated according to equation (2): η = (1

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Cout ) × 100 % Cin .

(2)

Where, Cin is the net inlet ozone concentration, ppb, and Cout is the exhaust ozone 8


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concentration, ppb. By error analysis, a 2% error was estimated when obtaining the

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ozone conversion efficiency.

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There were three different reactors used in experiments, shown in Figure 2: a) a

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stainless fixture (38 mm inner diameter, Merk Millipore), b) a glass gas-washing

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bottle, and c) a thin, stainless, hollow tube (8mm inner diameter, 250mm length). The

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stainless fixture was for QF, CNTs/QF film, AC-1 (0.30 g), and AC-2 (6.00 g), while

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the stainless, hollow tube was for AC-3 so that the granules could fully fill the

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cross-sectional area. The difference of ozone conversion performance between QF

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and CNTs/QF film can demonstrate the effect of CNTs, while that between CNTs/QF

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film and AC-1, AC-2, and AC-3 can illustrate comparisons of the two materials. The

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glass gas-washing bottle was used for the 5% KI solution.

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A differential gage (P3000T, BESTACE, USA) was connected at the ends of the

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reactor to measure pressure resistance of QF, CNTs/QF film, and AC-2, under

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different airflow rates, varying from 0.5 L/min to 5.0 L/min linearly.

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RESULTS

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Characterization of Materials

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The appearance of manufactured CNTs/QF film is shown in Figure 3 a), compared

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with that of pure QF shown in b). The most obvious change of the QF film after

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loading CNTs was that it turned black in color due to being coated with black visible

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CNTs. Through the TGA measurement mentioned above and calculated by Equation

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(1), the CNT content of the CNTs/QF film was 13.4%.

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The physical properties of measured materials are listed in Table 1. After loading the

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CNTs, the SBET of CNTs/QF film increased more than 10 times, while the weight and 9



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the thickness only grew 0.05 g and 0.09 mm, respectively. The CNTs greatly

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improved the adsorption capacity of pure QF. The SBET of the AC was more than 20

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times larger than that of CNTs/QF film. Moreover, AC-2 was 10 times as thick as the

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CNTs/QF film. In comparing the shapes of CNTs/QF film with granular AC and KI

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solution, the free-standing properties (acting as a filter itself without any containers to

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hold) of CNTs/QF film exhibited an advantage in active ozone removal.

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Ozone Conversion Efficiency of Materials

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The ozone conversion efficiency of different materials is shown in Figure 4. The

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CNTs/QF film demonstrated very high ozone conversion efficiency: over 96% during

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the 10 h-long experiment, with a slight downtrend starting at the 6th hour. In

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comparison, the subtract material, which was the pure QF, performed the worst with

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efficiency perceptibly reduced from over 40% to less than 10%. This indicated that

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CNTs play a dominant role in ozone removal by the CNTs/QF film. The ozone

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removal effect of AC-1 was far inferior to that of the CNTs/QF film, decreasing from

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60% to approximately 40%. Since the granule could not fully fill the cross-sectional

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area of the 40 mm diameter nonwoven fabric of AC-1, some ozone bypassed the AC

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granules, resulting in low ozone conversion efficiency. Even though airflow with

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ozone passed the AC granules thoroughly for AC-3, the ozone conversion efficiency

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of AC was still inferior to that of the CNTs/QF film of the same weight, resulting in

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an approximate 10% shortage, shown in Figure 5. As for the 5% KI solution, it

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functioned well at the beginning, but the efficiency dropped sharply at the 6th hour

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once the reactive I- in the solution was depleted. When comparing these materials

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using the same weight, the CNTs/quartz film performed the best, both in ozone

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conversion efficiency and persistency. As for AC-2, increasing the weight of AC by

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nearly 20 times (0.6 g) and fully filling the package, a noticeable improvement in 10


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ozone removal was exhibited. Under these circumstances, AC-2 maintained ozone

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conversion efficiency at around 98% and showed no appreciable downtrend. The

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ozone removal performance of the CNTs/QF film and AC-2 was roughly the same in

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the first eight hours. However, since the downtrend of the CNTs/QF was witnessed, it

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is reasonable to presume that the disparity between the CNTs/QF and AC-2 may

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increase in continuing experiments.

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Pressure Resistance of Materials

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The pressure drops over different flow rates for the three materials are shown in

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Figure 6. Noticeably, pressure resistance increased with the increase of airflow rates

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and indicated an obvious linear relationship for all three materials. The pressure drop

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of the CNTs/QF film was the largest, followed by the pure QF. The substrate pure QF

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contributed about 1/3 of the total pressure drop. The pressure resistance of AC-2 was

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only 1/8 of that of the pure QF. From the perspective of pressure resistance during

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filtration, AC performed the best.

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DISCUSSION

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Comparison between CNTs/QF Film and Conventional Ozone Removal Materials

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The experimental results above have demonstrated that the ozone removal

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performance of the CNTs/QF film was better than conventional materials, i.e., AC and

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KI solutions of the same weight, and even comparable with AC at 20 times the weight

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from the perspective of ozone conversion efficiency. Since the tested materials had

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different shapes and properties, it was difficult to guarantee all the test conditions for

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different materials were the same exactly. In this study, the mass of materials, the air

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flowrate and inlet ozone concentration were controlled at the same for different

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materials. The average contact time of ozone-AC-3 and ozone-CNTs/QF film was 11



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0.007s and 0.012s, respectively. The slightly longer contact time of ozone-CNTs/QF

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film may contribute to better ozone conversion of CNTs/QF film to some extent, but

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not the key point, seen from the supplementary experiment in Supporting Information.

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As shown in Table 1, the SBET of AC in this study was far larger than that of the

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CNTs/QF film. Why is it that the CNTs/QF performed better? Since the SBET was

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measured using the nitrogen adsorption-desorption method, there are two possible

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explanations for the phenomena according to our understanding: 1) chemisorption of

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ozone on CNTs was much stronger than that on AC; and 2) There were special

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connection bonds between the CNTs and ozone rather than the nitrogen. Besides, high

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pressure resistance of the CNTs/QF film comparing with AC and pure QF indicated

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that the flow paths inside the filter were more narrow and tortuous. It increased the

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possibility of ozone encountering inner surfaces of the filter, which in turn increased

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the ozone conversion efficiency. Though pressure resistance of the pure QF was much

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higher than the AC, the ozone removal performance of the pure QF was much worse.

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It indicated that the high ozone conversion efficiency of the CNTs/QF film may

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attribute to both CNTs and its microstructures. These hypotheses will be verified in

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our future studies.

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The experimental condition of 300 ppb ozone concentration and 10% RH set in this

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study was in accordance with the environmental condition in ozone-polluted aircraft

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cabins.44 For building environment, the ozone concentration to deal with is lower even

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for outdoor ozone

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ozone conversion efficiency of the CNTs/QF film was tested under lower inlet ozone

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concentration and showed good performance, as given in the Supporting Information.

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Due to competitive adsorption of water vapor on the CNTs/QF film, it is presumed

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that ozone conversion efficiency of the CNTs/QF film may decrease when RH

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and the RH is usually higher. In our preliminary experiment,

12


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increases, but still higher than that of the AC under same conditions. Since activated

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carbon usually contains oxygen functions while function-free CNTs do not, the

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activated carbon may be more hydrophilic than CNTs. Therefore, the adverse

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influence of water vapor on ozone adsorption may be larger to AC than the CNTs/QF

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film. However, hydrophilicity of the CNTs/QF film may increase with time, since the

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ozone and water vapor mixture can introduce some oxygen functions onto CNTs 46.

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High ozone conversion efficiency along with light-weight and free-standing properties

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enable the application of CNTs/QF film for ozone removal. However, the material’s

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high-pressure resistance could be a barrier since it leads to more power for driving air.

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The substrate, QF, did not remove ozone effectively itself, but contributed almost 1/3

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of the total pressure drop. Thus, more efforts could be made on the substrate to

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improve its performance. For example, carbon fiber is more incompact and could,

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adsorb ozone itself. Another possible approach is to reduce the CNTs’ growth time so

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that the relative content of CNTs can be smaller. Further investigation should be

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focused on reducing pressure resistance. Additionally, the downtrend of ozone

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conversion efficiency of the CNTs/QF film at the 6th hour may arouse concerns on

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sustainable usage and lifespan of the filter. It may owe to that more and more

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adsorption sites on the CNTs/QF film were occupied by ozone, the ability of

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removing ozone thus decreased. The lifespan and regeneration of the CNTs/QF film

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removing ozone will be discussed in our upcoming studies.

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Advantages of a Two-stage Structure of the CNTs/QF Film

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Now that it is known that the subtract, QF, of the CNTs/QF film did not contribute to

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effective ozone removal but considerable pressure resistance, the idea of cutting QF

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off the CNTs/QF film is reasonable. To realize this idea, an additional experiment was 13



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conducted. A free-standing pure CNTs film weighing 0.02 g was fabricated according

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to the literature.47 Ozone conversion efficiency and pressure resistance of the CNTs

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film was measured and compared with that of the CNTs/QF film, shown in Figure 7.

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To test the ozone removal of the CNTs/QF film under extreme high ozone level

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conditions and to tell the performance difference between the two materials better, the

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net inlet ozone concentration was elevated to 2.33 ppm, and other settings were

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implemented consistent with the method mentioned above.

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As seen in Figure 7, ozone conversion efficiency of the CNTs/QF film became lower

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when the net inlet concentration increased, which can be owing to more adsorption

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sites were occupied by ozone and the occupation was faster. Ozone conversion

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efficiency of the CNTs film dropped sharply and displayed an obvious gap with that

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of the CNTs/QF film at the beginning. This indicates that the ozone performance of

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the pure CNTs film was inferior to that of the CNTs/QF film. A possible reason is that

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the thickness of the CNTs film (scale of 1 µm) was much smaller than the CNTs/QF

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film. Furthermore, the pressure resistance of the CNTs film was about 25% higher,

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which may be attributed to its compact structure due to CNTs gathering with liquid

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bridge force. In addition, the mechanical strength of the pure CNTs film was poorer

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than that of the CNTs/QF film. The pure CNTs film can be easily broken under

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conditions of high airflow passing velocity. Hence, in the comparison between pure

291

CNTs film and the two-stage structure of the CNTs/QF film, the latter provided better

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ozone removal, lower pressure resistance, and stronger free-standing ability.

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ACKNOWLEDGEMENTS

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This study was supported by the National Basic Research Program of China (The 973

295

Program) through Grant No. 2012CB720100, and the Innovative Research Groups of 14


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the National Natural Science Foundation of China grant No. 51521005.

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Supporting Information Available

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Details of ozone generation (Section S1), the CNTs/QF film removing ozone under

299

lower ozone level condition (Section S2) and the supplementary experiment for AC-3

300

(Section S3) are provided, including three figures to show results. This information is

301

available free of charge via the Internet at http://pubs.acs.org.

302

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Page 21 of 25


Tables Table 1. Physical characterization of measured materials BET specific Thickness Materials

Weight (g)

Shape

surface area

(mm) (m2/g) QF

0.25

0.37

Free-standing film

3.37

CNTs/QF film

0.30

0.46

Free-standing film

42.84

AC-1

0.30

--*

Granule packaged in 913.20 nonwoven fabric Granule packaged in AC-2

4.60**

6.00

913.20 nonwoven fabric Granule piled up

AC-3

5.70***

0.30

913.20 inside the reactor

5% KI

6.00 (with --

solution *

Liquid

--

0.30g KI)

The AC granule could not fully-fill the cross-sectional area of 40 mm diameter

nonwoven fabric. Hence, the thickness of AC-1 was not measured. **

Average thickness when placed horizontally.

***

Average thickness after full-filling cross area of the thin, stainless, hollow tube.

21



Figures

Figure 1. Schematic of manufacturing device for CNTs/QF film.

Figure 2. Experimental system for ozone conversion tests. The reactors used in the experiments included, a) a stainless fixture, b) a glass gas-washing bottle, and c) a thin, stainless, hollow tube.

22


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a)

b)

Figure 3. Appearance of a) the CNTs/QF film and b) the pure QF.

Figure 4. Ozone conversion efficiency of different materials. Note that the graduations of Y-axis are not consistent for efficiency greater and less than 90%.

23



Figure 5. Comparison of ozone conversion efficiency between AC-3 and the CNTs/QF film.

Figure 6. Pressure drops over different flow rates for three materials.

24


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Figure 7. Ozone conversion efficiency and pressure resistance of the CNTs/QF film and CNTs film. Lines with dots represent pressure drops, while smooth curves represent ozone conversion efficiency.

25


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