Catalytic Ozonation in Arrayed Zinc Oxide Nanotubes as Highly

Jul 20, 2018 - Reactor design is significant to catalytic ozonation for an efficient mass transfer and exposure of the powerful but short-lived hydrox...
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Catalytic Ozonation in Arrayed Zinc Oxide Nanotubes as Highly Efficient Mini-Column Catalyst Reactors (MCRs): Augmentation of Hydroxyl Radical Exposure Shuo Zhang, Xie Quan, and Dong Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02103 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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Catalytic Ozonation in Arrayed Zinc Oxide Nanotubes as Highly

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Efficient Mini-Column Catalyst Reactors (MCRs): Augmentation of

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Hydroxyl Radical Exposure

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Shuo Zhang,† Xie Quan,*,† and Dong Wang†

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† Key Laboratory of Industrial Ecology and Environmental Engineering (MOE),

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School of Environmental Science and Technology, Dalian University of Technology,

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Linggong Road 2, Dalian 116024, China.

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Abstract: Reactor design is significant to catalytic ozonation for an efficient mass

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transfer and exposure of the powerful but short-lived hydroxyl radicals (HO·). Herein,

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five groups of zinc oxide nanotube arrays with pore sizes from 168 to 10 nm were

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produced as mini-column catalyst reactors (MCRs) for internal catalytic ozonation,

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whose performance was comparatively studied on the kinetics of ozone transfer,

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consumption, and radical probe interaction. Using an RCT value describing HO·

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exposure, all the MCRs with sufficient ozone transfer featured an RCT level of at least

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3.2×10-6, which is substantially higher than most values in referenced works

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(10-9~10-6) and that for microscale reactors in our work (~10-8). Furthermore, the HO·

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exposure dramatically increased with diminishing pore size, causing an elevated RCT

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up to 8.0×10-5 for the smallest MCR with 10-nm pore. The interphase formed in this

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flow-through system might have enriched HO· radicals produced via surface, and for

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a smaller MCR, the effect would be greater with a more confined microfluidic region.

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Investigations

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ozone-recalcitrant organics corroborated the nanoscale effect of MCR on

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augmentation of HO· exposure. This study offers a new way to design nanotube

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reactors for internal HO·-based heterogeneous catalysis.

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on

electron

paramagnetic

resonance

Table of Contents

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INTRODUCTION

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Investigations of heterogeneous catalytic ozonation (HCO) have increased, and

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more applications for water and wastewater remediation have been discovered

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benefiting from the surface-mediated exposure of hydroxyl radicals (HO·) as powerful

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intermediate products (E0(HO·/H2O) = +2.80 V).1-3 Compared to homogeneous

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catalytic ozonation4, HCO allows sustained reactions via active surface that does not

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require a continuous supply of reagents, and the catalyst in solid phase is generally

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easy to separate or reuse after a treatment. While great effort is directed toward

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improving surface chemistry or developing novel catalysts to enhance O3-to-HO·

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conversion,5,6 the design of reactors or the establishment of reaction systems

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frequently challenges the expectation that HCO will achieve kinetically enhanced

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HO·-based advanced oxidation. These challenges are likely due to the structural

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limitations of the catalyst7 or a state of heterogeneity8 that may lead to inadequate

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bulk-to-surface ozone transfer and renewal, becoming a rate-limiting step for

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subsequent surface-mediated catalysis that produces HO· radicals. Furthermore, HO·

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populations have extremely short lifetimes (10-9 s) and quickly decay after being

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catalytically generated from a surface, which may cause extremely weak radical

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exposure in the aqueous bulk phase during catalytic ozonation9 or other catalytic

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processes10. Conducting HCO in micro or nanostructures appears to address these

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problems to optimize the availability of the active surface, as indicated in recent years

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through various membrane/ozone catalytic systems used for targeted water or

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wastewater treatments.11-13 However, the current reported materials with shapeless

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and/or randomly distributed pores involving complex matrices have not been

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characterized well enough to conclude a nanostructural effect on HCO, and the tested

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porous structures could not satisfy the increasing demand to for a more uniform and

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efficient HCO platform with flexible nanoscale control of the kinetics of HO·

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exposure.

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By producing nanotube arrays and using the arrays in double-pass mode

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demonstrate advances in the development of state-of-art reaction platforms owing to

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the merits of well-defined open pore paths, relatively uniform architectures, and

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tunable pore sizes and arrangements. The pioneering works demonstrating the

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preparation of nanotube arrays with nanometer or sub-nanopores in the molecular size

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range were intended for size-selective exclusion applications, such as molecular 2

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sieving14,15 and desalination16,17. Larger nanotubes with mesoscopic pore sizes allow

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the delivery of the mixture solutions to the interior of the nanotubes and were used for

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the selective adsorption of solutes from liquid matrices via a functionalized inner

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surface.18 As an interesting extended application of nanotube arrays, the development

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of tubular catalysts and the performance of catalytic reactions, such as HCO, within

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the pseudo-one-dimensional hollow structures can thus employ each nanotube as a

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mini-column catalyst reactor (denoted MCR). Several attributes have motivated our

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studies on establishing MCRs. First, arrayed MCRs offer structurally regular

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nanovoids while preventing the aggregation of the encapsulated nanomaterials, thus

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preparing an extended available surface or interface for catalysis. Second, the mass

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delivery and transfer in a double-pass MCR are relatively uniform and sufficient

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compared to that in other porous structures that have blind holes or corners. Moreover,

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the pathway and internal catalytic kinetics can theoretically be engineered through the

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sophisticated control of the relevant physical and chemical properties of the nanotubes,

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which include the composition, dimension and arrangement.

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In this work, aligned zinc oxide nanotubes (ZnONTs) were template-synthesized to

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demonstrate MCRs and their performance for internal catalytic ozonation was studied.

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The preparation of ZnONTs as a target nanotube catalyst was based on our preliminary

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tests of HO·-assigned HCO using twelve kinds of metal oxide particles that were

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reported as ozone catalysts (Figure S1) and our consideration of the non-toxic and

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cost-effective advantages of zinc oxide. The as-synthesized ZnONT arrays following

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our developed solvothermal processes were used as MCRs characterized by structure,

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morphology and other relevant properties. To validate the MCR performance, we

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comparatively evaluated ozone transfer, conversion, and the final HO· exposure

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characteristics of five groups of MCRs (with average pores ranging from 168 to 10

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nm) and micrometer zinc oxide holes (with 180 and 850 µm pores) through the

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kinetics modeling, analysis of the RCT value as catalytic reactivity metrics, electron

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paramagnetic resonance (EPR), and the catalytic oxidation of recalcitrant model

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organics. The nanoscale effect of MCR on HO· exposure during HCO will mainly be

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discussed based on our results and the results from the referenced papers.

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EXPERIMENTAL SECTION

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Materials and Reagents. Double-pass anodized aluminum oxide (AAO) templates

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with 70 µm thickness were purchased from Hefei Pu-Yuan Nano Technology Co., Ltd. 3

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(China). Other materials and reagents are presented in the Supporting Information (SI),

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and all the chemicals were used as received without further purification.

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Synthesis of ZnONT Arrays. ZnONT arrays were synthesized within the pores of

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AAO templates via combined sol-gel and solvothermal processes. Five kinds of

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AAOs with different pore sizes (Figure S2) were used as templates for the growth of

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ZnONT arrays with different nanoscale sizes. In a typical procedure, 0.65 M zinc

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acetate and 0.016 M cetytrimethylammonium bromide were prepared in 20 mL of

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N-methyl-pyrrolidone as the solvent, which was treated by ultrasound at 65 °C for 25

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min until a clear and uniform sol was formed. The obtained solution was transferred

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into a Teflon-lined stainless-steel autoclave, and an AAO template was immersed into

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this sol at the middle of the vessel. The autoclave was sealed, maintained at 180 °C

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for 4 h, and naturally cooled down to room temperature. Afterwards, the obtained

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material was first removed from the vessel, washed by absolute ethanol several times

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under ultrasonic assistance, dried in an oven at 40 °C for 1 h, and heated at 400 °C for

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2 h to remove any organic/inorganic residues. Finally, we obtained the ZnONT-AAO

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composite products that were named “MCR-m”, for which “m” is the Sauter mean

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pore diameter (d32) determined statistically according to the recorded inner diameters

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of the nanotubes (Figure S3).

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Analytical Methods. The concentration of sodium benzenesulfonate (SBS) was

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detected by HPLC (Waters 2695 separations module). The stationary phase was a

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SunFire® C18 5 µm column (4.6 × 250 mm). The SBS was quantified at a wavelength

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of 212 nm with an eluent solution mixed by A (ammonium diphosphate solution) and

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B (methanol) under a flow rate of 1 mL min-1 (VA/VB = 30/70). A UV-Vis

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spectrophotometer (UV-2550, Shimadzu, Japan) was used to determine the

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concentration of dissolved ozone at 610 nm (the indigo method) following the

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Lambert-Beer

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5,5-dimethyl-pyrroline-oxide (DMPO) was detected

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spectrometer (Germany). The settings: center field, 3400 G; sweep width, 100 G;

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static field, 3350 G; microwave bridge frequency, 9.53 GHz; power, 16.45 mW;

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modulation frequency, 100 kHz; modulation amplitude, 2.00 G; conversion time,

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327.68 msec. Other analytical methods are presented in the SI.

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Characterization. Scanning electron microscopy (SEM) (S4800, Hitachi, Japan) and

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transmission electron microscopy (TEM) (JEM-2000EX, Japan) were used to

law.

The

EPR

signal

of

free

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trapped

by

using a Bruker A200

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examine the structure and morphology of the as-synthesized ZnONT arrays. X-ray

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diffraction (XRD) patterns were acquired using a DX-3000 X-ray diffractometer

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(Shimadzu, Japan) with Cu Kα monochromatic radiation operated at 40 kV and 100

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mA. The nitrogen adsorption-desorption isotherms at -196.15 °C were measured by an

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Autosorb-iQ-C (Quantachrome, USA), and the specific surface area (SSA) was

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determined following the multipoint Brunauer-Emmett-Teller (BET) model. Atomic

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force microscopy (AFM) (Pico Scan 2500, USA) measurements were conducted in

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tapping mode.

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Catalytic Ozonation in MCR. A homemade two-pass reaction module was designed,

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as illustrated in Figure S4. The prepared ZnONT-AAO composite was fixed in the

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middle of the cell on top of a perforated plastic support and was then sealed by a

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concentrically shaped rubber pad to prevent liquid leakage. An ozone stock solution

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was prepared in ultrapure water by continuously bubbling gaseous ozone for 10 min,

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and then, peristaltic pumps were turned on to mix the ozone stock solution

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proportionally with a bypass solution (containing a radical probe or other target model

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organics) before entering the ZnONT. The influent ozone concentration was controlled

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at 225 µM for all the experiments. Both the ozone and organic stock solutions were

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controlled at 22 °C by thermostats, and catalytic reactions in the module were

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performed at room temperature within 20-22 °C. The permeating liquid after the

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reaction was collected and was used for further analysis.

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RESULTS AND DISCUSSION

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Structure and Microstructure Characterizations. The ZnONT arrays were designed

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to grow within the pores of the AAO templates, as illustrated in Figure 1a. Due to

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parallel channels inside the AAO template, the ZnONT-AAO composite has

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well-aligned, inserted two-pass nanotubes and, thus, appears to be semitransparent

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(Figure 1b). Although non-uniform zinc oxide nanostructure shapes were reportedly

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formed in the AAO templates (e.g., particles,19 rods,20 wires21), our developed method

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allows technically stable and reproducible ZnONT array, and MCR-168 as an example

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was further characterized (Figure 1c-1i). The ZnONT array consistently formed

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alongside the AAO channels according to the SEM images (Figure 1c and 1d), and the

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TEM results indicated a well-defined hollow structure (Figure 1e). Though the inner

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surface of the ZnONTs do not appear to be completely smooth (Figure 1f), the

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nanotube structure exhibited a relative roughness of no more than 1/16, as shown in 5

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the AFM report (Figure 1g). The XRD pattern illustrates the crystallographic phase of

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ZnONT belonging to hexagonal wurtzite (Figure 1h), and the selected area electron

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diffraction (SAED) result of an inner surface (inset figure) confirmed the

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multi-faceted orientation of crystallite units for the growth of ZnONT. The

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as-synthesized ZnONTs exhibit a larger specific surface area (approximately 428.5 m2

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g-1) than does the AAO template (7.9 m2 g-1) according to the nitrogen

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adsorption/desorption isotherms (Figure 1i). More details describing the formation of

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ZnONT array within the pores of AAO template are shown in Figure S5-S7.

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Figure 1. Characterization of MCR-168 as an example ZnONT array grown in an AAO template.

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(a) Schematic diagram of the fabrication of MCRs. (b) A photograph of the ZnONT-AAO

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composite. (c) Top-down view and (d) cross-sectional view of the SEM images of ZnONT-AAO.

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(e) TEM image of ZnONT-AAO (inset: magnified view of a template-tube interface region). (f)

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SEM image of the inner surface of a ZnONT in an AAO template. (g) AFM report for ZnONT-AAO

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and the internal nanotube roughness profiles. (h) XRD patterns of the ZnONT-AAO composite and

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the AAO template (inset: the corresponding SAED pattern). (i) Nitrogen adsorption/desorption

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isotherms of the ZnONT-AAO composite and the AAO template.

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Figure 2 presents SEM images of the other four MCR arrays used in this study, i.e.,

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MCR-96, MCR-61, MCR-26 and MCR-10. Our method can successfully produce

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10-nm scale nanotubes (detailed characterizations are in Figure S8) even as the AAO

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template pore sizes decrease to tens of nanometers (approximately 40-70 nm, Figure 6

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S2).

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Figure 2. SEM images for the top-down and cross-sectional view of the as-synthesized ZnONT

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arrays within different AAO templates. (a) MCR-96. (b) MCR-61. (c) MCR-26. (d) MCR-10.

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Kinetics Study on Internal HCO. Catalytic ozonation was initiated if the ozone/SBS

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mixture solution entered the inner void of ZnONT, together with the resultant radical

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probe reaction between SBS and the generated HO·. SBS was employed as a radical

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probe compound since it reacts with HO· (kHO·/SBS ≈ 4×109 M-1 s-1 )22 in a much faster

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second-order rate constant than with molecular ozone (kO3 /SBS ≈ 0.23 M-1 s-1 )23. As

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shown in Figure 3, the passage of aqueous SBS through the ultrathin MCR groups

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caused considerable reduction of SBS within a short retention time (tR) of no more

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than 0.61 s (estimation of the tR is addressed in the SI). The retention time result

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demonstrated severe and sustained HCO occurring in the nanotube reactors, because

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either plain ozonation or sole MCR filtration did not cause much removal of SBS

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(Figure S9). The kinetics of the probe reaction, which are influenced by the pore size

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of the nanotubes, presented a pseudo-first-order rate constant ( , in Equation (1))

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for the smallest MCR (i.e., MCR-10) as high as 8.6 s-1, which was much higher than

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that of MCR-168 with 0.85 s-1 at a flowrate of 1.0×10-4 L min-1. We also designed

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microscale zinc oxide holes (MH, specified in Figure S10) for comparison, and the

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relevant probe reaction rate greatly decreased from 0.047 s-1 for MH-180 to 0.01 s-1

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for MH-850, as shown in the inset of Figure 3.

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, , 

  ∙ ,

(1)

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where , and , represent the inlet and outlet concentration of SBS in M,

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respectively.

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Aqueous ozone was continually consumed in MCR due to the sustained internal 7

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HCO, and as a result, the outlet ozone concentration differed much in different MCR

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systems with different retention time (Figure S11). The time-dependent decay of

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aqueous ozone causes an inhomogeneous distribution of downstream ozone alongside

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the inner wall as the mixture solution traverses the MCR, thus gradually attenuating

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the ozone-to-HO· conversion. Although the collection of internal data is difficult, for

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a given MCR, the disappearance of SBS is not linearly time-dependent since a longer

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tR results in lower probe reaction kinetics (Figure 3) due to the lower availability of

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HO· resulting from the gradual depletion of ozone inside the nanotubes. Take the

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MCR-168 system for an example. The enhanced flowrate from 1.0×10-4 to 1.5×10-3 L

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min-1 shortened the retention time from 0.61 to 0.04 s, which led to an elevated

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internal ozone concentration from 105 to 206 µM (Figure S12). Meanwhile, the

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increased background ozone concentration kinetically strengthened HCO for

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production of HO·, and in response, the radical probe reaction was largely accelerated

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with  from 0.85 to 2.66 s-1. Therefore, the eventual oxidative removal of a target

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organic would be dictated by achieving balance between a prolonged internal reaction

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and the expected ozone feeding efficiency. 0.27

MCR-26 MCR-168

MCR-10 MCR-96

0.24

MCR-61

0.21

△CSBS (mM)

0.18 0.15 0.12 0.10

△CSBS (mM)

0.09 0.06

MH-180 MH-850

0.08 0.06 0.04 0.02

0.03

0.00

0

1

2

0.00 0.0

0.1

0.2

0.3

4 tR (s)

5

0.5

6

7

8

0.6

tR (s)

221 222

0.4

3

Figure 3. Reduction of SBS concentration (△CSBS) versus retention time (tR) because of catalytic

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ozonation in MCR groups (inset: the reduction of △CSBS versus tR in MH systems). Conditions:

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injection concentration of SBS, 0.28 mM; initial pH, 7.5; flow rates were respectively 1.0×10-4,

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1.5×10-4, 3.0×10-4, 5.0×10-4, and 1.5×10-3 L min-1.

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To gain insight into the kinetics of HCO in nanotubes, we subdivided the whole 8

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catalytic process into the successive steps as follows: (i) bulk-to-surface ozone

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transfer, (ii) surface-mediated catalytic conversion of ozone into intermediate radicals

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(including HO·), and (iii) the HO·-enabled instantaneous advanced oxidation of

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surrounding substances. These steps delineate the occurrence of HCO after the

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injection of ozone into the MCR, for which the depletion of HO· (i.e., step (iii)) is

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extremely fast via non-selective reactions, including the radical probe interaction,

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which acts as a branching process. Prior to the generation of HO·, the decay of ozone

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in tiny channels is subject to a coupling effect of ozone transfer toward the surface,

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which closely correlates to the influx space and local hydrodynamics, and

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surface-mediated ozone conversion, which is mainly a function of the inner surface

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chemistry.

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Given the low Reynolds number (, dimensionless) for MCR (