Catalytic Ozonation in Arrayed Zinc Oxide Nanotubes as Highly

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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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Environmental Science & Technology

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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

4

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

9

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,

12

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

13

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.

25

on

electron

paramagnetic

resonance

Table of Contents

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1

ACS Paragon Plus Environment

and

the

treatment

of


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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

4


radicals

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.

167 168

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).

181 182

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))

196

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

197

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

198

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

200

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

201

, ,

∙ ,

(1)

202

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

203

respectively.

204

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

206

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

207

aqueous ozone causes an inhomogeneous distribution of downstream ozone alongside

208

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

209

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

210

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

211

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

212

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

213

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

214

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

215

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

216

increased background ozone concentration kinetically strengthened HCO for

217

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

218

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

219

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

220

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

223

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

224

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

225

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

229

(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

234

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

235

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

237

chemistry.

238

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

Catalytic Ozonation in Arrayed Zinc Oxide Nanotubes as Highly

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