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Sequential Process Combination of Photocatalytic Oxidation and Dark Reduction for the Removal of Organic Pollutants and Cr(VI) using Ag/TiO

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Yeoseon Choi, Min Seok Koo, Alok D. Bokare, Dong-hyo Kim, Detlef W. Bahnemann, and Wonyong Choi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06303 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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Environmental Science & Technology

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Sequential Process Combination of Photocatalytic Oxidation and

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Dark Reduction for the Removal of Organic Pollutants and Cr(VI)

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using Ag/TiO2

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Yeoseon Choi,† Min Seok Koo,† Alok D. Bokare,† Dong-hyo Kim,† Detlef W. Bahnemann,‡

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and Wonyong Choi*,†

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†

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‡

Department of Chemical Engineering and Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Korea 37673 Laboratory “Photoactive Nanocomposite Materials” Saint-Petersburg State University, SaintPetersburg, Russia and “Photocatalysis and Nanotechnology”, Institut fuer Technische Chemie, Gottfried Wilhelm Leibniz Universitaet Hannover, Hannover, Germany

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Submitted to

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(Revised)

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*Corresponding author.

Phone: +82-54-279-2283

E-mail: [email protected]

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1

ACS Paragon Plus Environment


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ABSTRACT

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We investigated a sequential photocatalysis-dark reaction, wherein organic pollutants were

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degraded on Ag/TiO2 under UV irradiation and the dark reduction of hexavalent chromium

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(Cr(VI)) was subsequently followed. The photocatalytic oxidation of 4-chlorophenol (4-CP),

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a test organic substrate, induced the generation of degradation intermediates and the storage

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of electrons in Ag/TiO2 which were then utilized for reducing Cr(VI) in the post-irradiation

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period. The dark reduction efficiency of Cr(VI) was much higher with Ag/TiO2 (87%),

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compared with bare TiO2 (27%) and Pt/TiO2 (22%). The Cr(VI) removal by Ag/TiO2 (87%)

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was contributed by adsorption (31%), chemical reduction by intermediates of 4-CP

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degradation (26%), and reduction by electrons stored in Ag (30%). When formic acid, humic

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acid or ethanol was used as an alternative organic substrate, the electron storage effect was

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also observed. The post-irradiation removal of Cr(VI) on Ag/TiO2 continued for hours, which

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is consistent with the observation that a residual potential persisted on the Ag/TiO2 electrode

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in the dark whereas little residual potential was observed on bare TiO2 and Pt/TiO2 electrodes.

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The stored electrons in Ag/TiO2 and their transfer to Cr(VI) were also indicated by the UV-

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visible absorption spectral change. Moreover, the electrons stored in the pre-irradiated

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Ag/TiO2 reacted with O2 with showing a sign of low-level OH radical generation in the dark

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period.

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Keywords: post-irradiation effect, advanced oxidation processes, aquatic pollutant transformation, hexavalent chromium, electron storage in silver

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INTRODUCTION

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The deposition of noble metals on semiconductor surface is an efficient way to enhance the

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photocatalytic activity by creating a Schottky barrier at the semiconductor-metal interface,

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which helps separating the photo-excited electrons in the conduction band (CB) of

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semiconductor into the metal nanoparticles (NPs).1-3 Such separation of charge pairs through

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the Schottky barrier facilitates the subsequent charge transfer at the semiconductor interface

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with enhancing the overall photocatalysis and reactive oxygen species (ROS) generation.4-6

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Various noble metals such as Pt, Au, and Ag have been frequently immobilized on TiO2 and

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other semiconductors to increase the efficiency of photocatalytic oxidation (PCO) of organic

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pollutants.5-8

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The photocatalytic activities of these TiO2-metal composites show different behaviors

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depending on the kind of the loaded metal. Although the electron transfer from the

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semiconductor CB to noble metal NPs is very fast (~10 ps),5 the electron entrapment and

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subsequent interfacial transfer to the aqueous electrolytes critically depends on the type of

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noble metal. While the capacitive nature of Au and Ag impedes the charge transfer of

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entrapped electrons out of the metal surface, the existence of an ohmic contact in the case of

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Pt and Pd renders quick discharge of electrons at the solid/liquid interface.9-11 Wood et al.10

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demonstrated that the electron transfer from photocatalyst (ZnO) to aqueous electrolytes was

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delayed by the presence of Ag NPs, which serve as an electron storage that keeps electrons

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for a longer time. The transfer rate of accumulated electrons was much slower for Ag/ZnO

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than Au/ZnO, whereas Pt/ZnO did not exhibit any electron storage property at all. To

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compare the electron storage properties depending on the kind of noble metals, Takai and

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Kamat11 investigated the electron storage and discharge properties of M/TiO2 (M = Ag, Au

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and Pt) colloid in deaerated ethanol under UV irradiation and concluded that the electron 3



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storage is most efficient in Ag/TiO2 compared to Au/TiO2 and Pt/TiO2. To be specific, they

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estimated that Ag/TiO2 can store up to 51 electrons per an Ag atom under UV irradiation.

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The electrons accumulated on Ag metal can be in equilibrium with TiO2 where the electrons

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can be trapped as Ti(III) or in the conduction band with an elevated Fermi level, which may

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explain how an Ag atom can hold such many electrons. Although the actual number of

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electrons that can be stored in M/TiO2 should depend on many parameters such as the size of

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metal particles, the light irradiation intensity, and the kind of electron donors and solvents,

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the large electron storage capacity in Ag may enable its application for post-irradiation

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reactions. On the other hand, Pt/TiO2 has an ohmic contact at the interface and the

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photogenerated electrons on Pt/TiO2 are rapidly transferred to the aqueous electrolyte with

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little chance of electron accumulation on Pt. The much smaller interfacial charge transfer

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resistance of Pt/TiO2 than Ag/TiO2 was also confirmed in a recent study.12

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Ag/TiO2 has been employed for the photocatalytic removal of water contaminants on

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the basis of various properties which include bactericidal properties of silver species,13-15

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electron accumulation in silver metal to improve the reactivity of the photo-generated holes16

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and the plasmonic effect of silver nanoparticles to utilize the visible light spectrum.17, 18

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Although the electron storage properties of Ag/TiO2 have also been the subject of many

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fundamental investigations,10, 19, 20 the possible utilization of these stored electrons for the

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transformation of water pollutants has been largely unexplored. Gunawan et al.21 used pre-

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irradiated Ag/TiO2 as antimicrobial agent, but they focused on the effect of Ag+ dissolution

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by pre-irradiation not on the stored electrons in Ag NPs. This study aims to utilize and

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combine the characteristics of TiO2 photocatalyst and the silver NPs loaded on TiO2 for the

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redox transformation of aquatic pollutants in a sequential light-dark process. Although the

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photocatalytic activity of TiO2 and the electron storage capacity of Ag NPs are well known, 4


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the serial combination of those properties has not been proposed for the removal of water

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contaminants. The electrons stored in Ag/TiO2 (capacitive charging) during UV illumination

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might be utilized to activate redox conversions in the subsequent dark cycle (thermal

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discharging). Considering great interests in making solar water treatment technologies more

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viable, the idea of utilizing not only the irradiation period (daytime) but also the post-

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irradiation period (night time) for remediation purpose can be a conceptually interesting

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strategy.

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Herein, we investigated a sequential treatment of oxidation (light) and reduction (dark)

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using Ag/TiO2. For this purpose, 4-chlorophenol (4-CP) and Cr(VI) (hexavalent chromium)

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were employed as model pollutants for PCO and dark thermal reduction, respectively. Since

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both 4-CP and Cr(VI) are ubiquitous contaminants and have been extensively investigated for

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their photocatalytic removal,22-28 they are good model substrates that can be employed for the

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demonstration of this sequential process. During the UV-irradiation period, photo-generated

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electrons are charged in Ag while 4-CP is degraded by hole-mediated oxidation with

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generating intermediates. In the following dark period, the stored electrons in Ag and the

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degradation intermediates are used to reduce Cr(VI). Sequential removal of 4-CP and Cr(VI)

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was monitored and the dual activities of Ag/TiO2 were compared with those of bare TiO2 and

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Pt/TiO2 to understand the mechanism of the sequential process.

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EXPERIMENTAL SECTION

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Materials. Chemicals and materials used in this work include: A commercial TiO2 (P25),

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which is a mixture of anatase and rutile (8:2) with primary particle sizes of 20-30 nm was

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used as the substrate TiO2 sample, Na2Cr2O7·2H2O (Cr(VI), Aldrich), 4-chlorophenol (4-CP,

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Sigma), ethanol (EtOH, J. T. Baker), formic acid (Aldrich), hydroxyhydroquinone (HHQ, 5



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Aldrich), 4-chlorocatechol (4-CC, Kasei), 1,4-benzoquinone (BQ, Aldrich), perchloric acid

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(HClO4, Sigma-Aldrich), acetone (Samchun), sulfuric acid (H2SO4, Sigma-Aldrich),

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diphenyl-carbazide (DPC, Aldrich). Humic acid (HA) was purchased from the International

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Humic Substances Society (http://www.ihss.gatech.edu). Deionized water (D. I. water) used

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was ultrapure (18 MΩ-cm) and prepared by a Barnstead purification system.

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Experimental procedure. Ag/TiO2 and Pt/TiO2 were prepared by using a typical photo-

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deposition method.12 TiO2 was suspended in the aqueous solution containing methanol (4%,

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v/v) and noble metal precursor (H2PtCl6 or AgNO3) and then stirred and sonicated for 30 min

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before irradiation. After UV irradiation (200-W mercury lamp) for 30 min, the metal-loaded

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TiO2 particles were filtered, sufficiently washed with D. I. water, and dried at 80 °C. The

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prepared photocatalyst was suspended in an aqueous solution (0.5 g/L) containing target

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organic species (300 µM) in a pyrex reactor. The suspension was dispersed well by ultra-

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sonication and the pH was adjusted to 3.0 with 1 N HClO4. The suspension was stirred for 30

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min in the dark to allow equilibrium adsorption of the model organic substrate. A 300-W Xe

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arc lamp (Oriel) was used as a solar simulating light source. The incident light was filtered

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through a 10-cm IR water filter and a UV cutoff filter (λ > 320 nm) that blocks the light of

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wavelengths shorter than 320 nm to avoid the direct photolysis of organic substrates and the

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incident light intensity was 1.7 W/cm2 (optical power meter, Newport 1918-R). After a

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certain time of UV irradiation, the reactor was placed in the dark and then Cr(VI) was

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subsequently spiked into the pre-irradiated suspension that was well stirred during the entire

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dark reaction period. The sample aliquots were intermittently withdrawn and filtered through

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a 0.45-µm PTFE filter (Millipore) for the analysis of Cr(VI). For experiments studying the

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effect of oxidation intermediates, the photocatalyst was first filtered, and Cr(VI) was then

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added in the filtrate solution. 6


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Analysis. The concentration of Cr(VI) was measured spectrophotometrically using the DPC

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(diphenylcarbazide) method.27 An aliquot of 0.5 mL sample solution was added to a vial

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containing 2.5 mL of prepared DPC reagent (100 mL of D.I. water and 4 mL of DPC reagent

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that contains 5 mL of acetone, 50 µL of H2SO4, and 0.01 g of DPC). The vial was mixed

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vigorously and kept standing for 30 min before the analysis. The absorbance measurements at

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540 nm (ε = 6850 M-1cm-1) were done using a UV/visible spectrophotometer (Libra S22,

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Biochrom). Quantitative analysis of organic substrates was done using a high performance

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liquid chromatograph (HPLC Agilent 1100) equipped with a C-18 column (Nova-Pak C18)

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and a photodiode-array detector (Waters 996). The eluent was composed of 0.1% phosphoric

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acid aqueous solution and acetonitrile (75:25 v/v). The residual organic carbon content in the

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irradiated organic substrate solution was monitored by a total organic carbon (TOC) analyzer

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(TOC-VCSH, Shimadzu).

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The generation of OH radicals in the irradiated or pre-irradiated suspension of bare TiO2,

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Ag/TiO2, and Pt/TiO2 was indirectly monitored by the fluorescence emission of 7-

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hydroxycoumarin (7-HC) that was produced from the reaction of OH radical and coumarin (a

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probe reagent for OH radical).29 The fluorescence emission intensity of 7-HC was monitored

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at 460 nm under the excitation at 332 nm using a spectrofluorometer (FluoroMax 4, HORIBA

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Jobin Yvon Inc.) The appearance of Ag plasmon band with stored electrons was observed by

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using a UV/visible spectrophotometer (Shimadzu UV-2401PC).

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Characterization of photocatalysts. The BET surface area of photocatalyst samples was

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measured by using nitrogen adsorption/desorption isotherms (Micromeritics Tristar 3000

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analyzer at 77K). JEOL JEM-2200FS with image Cs-corrector (200 KeV, 0.1nm) was used

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for scanning transmission electron microscopy (STEM) image of bright and dark field.

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Crystalline phases were determined by X-ray-powder diffraction (XRD) using Cu-Kα 7



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radiation (Mac Science Co. M18XHF). The chromium species deposited on the photocatalyst

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surface after reaction with Cr(VI) was analyzed by X-ray photoelectron spectroscopy (XPS)

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(Theta probe AR-XPS, Thermo Fisher Scientific) using Al-Kα (1486.6 eV) as the X-ray

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source.

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RESULTS AND DISCUSSION

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Three different catalysts (bare TiO2, Pt/TiO2 and Ag/TiO2) degraded 300 µM of 4-CP

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within 60 min (Figure 1a), but the efficiency of 4-CP removal was retarded by the

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coexistence of Cr(VI) (see Figure S1). A previous study also reported that the photocatalytic

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degradation of organic pollutants can be hindered by the co-existence of Cr(VI).30 We carried

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out a control experiment which employed bare TiO2 photocatalyst that was pre-adsorbed with

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Cr(VI) and observed that its photocatalytic activity for 4-CP degradation was markedly

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inhibited. This implies that the presence of Cr(VI) on the surface of TiO2 is an inhibitor of

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PCO activity although Cr(VI) is a good electron acceptor which may facilitate the hole-

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induced PCO. The most plausible explanation is that Cr(VI) serves as an external charge

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recombination center where Cr(VI) reacts serially with electrons and holes to make null

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photocatalytic cycles. Such role of Cr(VI) should block the pathways of ROS generation

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(e.g., O2−, H2O2, •OH) with retarding the PCO process. To test the inhibitory role of Cr(VI),

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the generation of OH radical was indirectly monitored by detecting 7-hydroxycoumarin (7-

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HC) which is produced from the PCO reaction between OH radical and coumarin (coumarin

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+ •OH → 7-HC). In the presence of Cr(VI), the generation of OH radical was indeed

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retarded for all three photocatalysts (see Figure S2). Since Cr(VI) is an intrinsic inhibitor of

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PCO, it is a better approach to remove 4-CP and Cr(VI) in a sequential process (as shown in

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Figure 1a) than in one batch reactor. 8


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Among the photocatalysts, the PCO degradation efficiency was in the order of Pt/TiO2

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> Ag/TiO2 > TiO2. Noble metal-loaded TiO2 was prepared by a typical photo-deposition

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method, which did not change the BET surface area and the crystal structure of the substrate

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TiO2 (P25) (see Figure S3). The average size of noble metal NPs of Pt/TiO2 and Ag/TiO2 was

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1.5 nm and 4 nm, respectively (see Figure S4). The positive effects of noble metals deposited

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on TiO2 on the PCO of organic compounds are well known and Pt is more efficient than Ag

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in most reported cases.8, 31, 32 The role of Pt is markedly prominent in the photocatalytic

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mineralization ability as shown in Fig. 1b. Ag/TiO2 was only slightly more efficient than bare

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TiO2 for 4-CP removal and no better than bare TiO2 for the mineralization. When the parent

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4-CP was completely removed (after 60 min of UV irradiation), the TOC removal was only

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partial with bare TiO2 and Ag/TiO2 but was almost complete with Pt/TiO2.

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On the other hand, the reactivity in the dark period that immediately followed the UV

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PCO process was significantly different among the three photocatalysts. After 60 min of UV

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irradiation (when all 4-CP was removed), the reduction of Cr(VI) under the dark condition

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(right after UV light turn-off) was investigated by spiking 100 µM Cr(VI) into the pre-

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irradiated catalyst suspension. The right panel of Figure 1a compares the time-dependent

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profiles of [Cr(VI)] which exhibit two distinct regions. An initial immediate decrease in

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[Cr(VI)] (regime I) was followed by a slow and gradual decrease (regime II). It is noted that

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Ag/TiO2 shows an outstanding activity for Cr(VI) removal compared with bare TiO2 and

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Pt/TiO2 systems. Although Pt/TiO2 was the most active for the PCO of 4-CP, it was the least

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active for the following dark thermal reduction of Cr(VI). The followings can be proposed as

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mechanisms that are responsible for the removal of Cr(VI) in the dark period: (1) adsorption

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on the catalyst surface, (2) chemical reduction by remaining intermediates of 4-CP

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degradation such as quinone derivatives,33-35 and (3) reduction by electrons in the noble metal 9



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phase (Pt or Ag), which were stored during the pre-irradiation period. Control experiments

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were carried out to estimate the contribution from each mechanism.

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First, the removal of Cr(VI) by adsorption was estimated for bare TiO2, Ag/TiO2, and

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Pt/TiO2 (see Figure 2a). In the case of bare TiO2 and Pt/TiO2, the total amount of Cr(VI)

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removed by the adsorption is almost the same as observed in pre-irradiation-dark cycle

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experiments (Figure 1a). Second, the chemical reduction by 4-CP oxidation intermediates

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was negligible with bare TiO2 and Pt/TiO2 as shown in Figure 2b. This indicates the

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adsorption mechanism is mainly responsible for Cr(VI) removal for both bare TiO2 and

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Pt/TiO2 in the dark period. In contrast, both adsorption and chemical reduction mechanisms

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contribute to initial rapid Cr(VI) removal in the case of Ag/TiO2. The photocatalytic

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degradation of 4-CP is known to generate intermediates such as benzoquinone (BQ),

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hydroquinone

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hydroxybenzoquinone (HBQ), and other minor intermediates.36 We also confirmed the

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generation of those intermediates during PCO of 4-CP in the presence of bare TiO2, Ag/TiO2

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and Pt/TiO2 by HPLC analysis (Figure S5). To confirm that these intermediates of 4-CP

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degradation reduce Cr(VI), four different quinone species were tested. As shown in Figure 3,

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all 4 compounds (BQ, 4-CC, HQ and HHQ) exhibited Cr(VI) reduction capacity and HHQ

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showed the highest activity. Since 4-CP was almost completely mineralized after 1 h PCO in

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the case of Pt/TiO2, the Cr(VI) reduction by intermediates was negligible for Pt/TiO2 as

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shown in Figure 2b. On the other hand, it is interesting to note that the reduction by

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intermediates was significantly higher for Ag/TiO2 than bare TiO2 despite the fact that the

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residual TOC values after 1 h PCO were similar for both Ag/TiO2 and bare TiO2 (see Figure

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1b). This implies that intermediates generated from Ag/TiO2 PCO are more reactive towards

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Cr(VI) than those from bare TiO2 PCO.

(HQ),

4-chlorocatechol

(4-CC),

hydroxyhydroquinone

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(HHQ),

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However, they cannot be responsible for the gradual decrease in [Cr(VI)] observed at a

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longer time scale (Regime II in Figure 1a). These observations suggest that stored electrons

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in the noble metal are mainly responsible for dark Cr(VI) removal (in regime II) in the

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Ag/TiO2 system. It has been previously reported that the electrons trapped in bare TiO2 NPs

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during UV irradiation can be used for reducing metal ions such as Cu(II),37 Ag(I),38 and

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Cr(VI)39 immediately after turning off the light. However, the post-irradiation electron

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transfer rate on bare TiO2 was fast enough to be completed in a sub-second time scale, which

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could be monitored only by using the stopped flow technique. The presence of Ag NPs not

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only increases the electron storage capacity significantly but also enables the slow release of

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the stored electrons for a longer time. Based on the results of the aforementioned control

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experiments, Figure 4a quantifies each contribution of the adsorption, the chemical reduction

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by intermediates, and the reduction by stored electrons towards the total Cr(VI) removal. It

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can be clearly seen that the contribution of the reduction by the stored electrons is

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significantly higher in Ag/TiO2 (30%) compared to bare TiO2 (2.8%) and Pt/TiO2 (1.8%).

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The pH dependence of the photocatalytic removal of 4-CP and the subsequent removal of

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Cr(VI) in the post-irradiation (dark) period was compared (see Table S1). Although the PCO

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activity was little influenced by the pH change (pH 3-10), the dark removal efficiency of

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Cr(VI) was markedly influenced by pH, which dramatically decreased with increasing pH 3

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to 10. Being an oxyanion, chromate should be electrostatically attracted to the positively

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charged TiO2 surface (pHzpc ~ 6) at acidic condition and repelled from the negatively charged

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TiO2 surface at alkaline condition. As a result, the reductive conversion of chromate anions

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on Ag/TiO2 is favored at acidic condition and hindered at alkaline condition.

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On the other hand, the post-irradiation (dark) reduction of Cr(VI) was also carried out

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in the absence of dissolved O2 to investigate the effect of O2 on the Cr(VI) reduction 11



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pathways, which is shown in Figure 4b. The most outstanding feature of the anoxic system is

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the increased contribution of the reduction by stored electrons for all catalysts. In particular,

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the contribution of stored electrons was markedly enhanced for Pt/TiO2 from 2% (in the

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presence of O2) to 55% (in the absence of O2). This indicates that the storage of electrons and

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their subsequent transfer in the dark condition are enabled only in the absence of dissolved O2

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for Pt/TiO2 since platinum is a good catalyst for O2 reduction.40 Therefore, electrons in the Pt

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metal are immediately scavenged by dioxygen without leaving any electrons that would

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discharge slowly in the following dark period.5, 7, 10 On the other hand, for Ag/TiO2, the

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contribution by stored electrons was only moderately enhanced in the absence of O2 (Figure

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4a vs 4b). This supports that the capacitive electron storage and discharge from Ag is little

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affected by the presence of O2. That is, the storage of electrons in metals during the

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irradiation period and the subsequent discharge of them in the following dark period are

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enabled only with Ag/TiO2 regardless of the presence of O2.

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As for the chemical reduction by intermediates in the anoxic condition, its contribution

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was reduced with Ag/TiO2 but enhanced with Pt/TiO2 compared to that in the oxic condition.

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This implies that the concentrations of 4-CP degradation intermediates in the anoxic

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condition were reduced with Ag/TiO2 but enhanced with Pt/TiO2, compared with those in the

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oxic condition. To investigate this effect, TOC measurements were compared between the

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oxic and anoxic systems. After 1 h PCO of 4-CP, the residual TOC values were 36% and 0%

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in the oxic condition and 95% and 88% in the anoxic condition for Ag/TiO2 and Pt/TiO2,

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respectively. (see Figures 1b and S6) For Ag/TiO2, the PCO of 4-CP was significantly

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retarded in the anoxic condition and hence lower concentration of intermediates should be

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generated. As for Pt/TiO2, the PCO was also drastically retarded in the absence of O2, but

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about 50% of 4-CP was removed after 1 h irradiation. Considering that the residual TOC was 12


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88% after 1 h PCO of Pt/TiO2, a significant fraction of removed 4-CP should be converted

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into intermediates, which can reduce Cr(VI) in the dark phase (see Figure 4b). Since Pt/TiO2

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can completely mineralize 4-CP after 1 h PCO in the presence of O2 with leaving little TOC,

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the contribution of chemical reduction by intermediates was absent in the Pt/TiO2/oxic

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system.

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To further confirm the electron storage properties of Ag/TiO2, an additional control

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experiment was carried out to exclude the competitive effect of the PCO intermediates. The

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post-irradiation removal of Cr(VI) was carried out after 1 h UV irradiation of Ag/TiO2

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suspension containing no organic pollutants (D.I. water only, aerated). Since the production

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of organic intermediates should be absent in this case, the effect of stored electrons on Cr(VI)

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reduction can be easily demonstrated as shown in Figure 4c. Even in the absence of organic

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substrates, the intrinsic electron storage capacity of Ag/TiO2 is outstanding compared to

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Pt/TiO2. The effects of other organic substrates on the post-irradiation removal of Cr(VI)

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were also investigated with employing ethanol, formic acid, and humic acid. As seen in

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Figure 5, intermediates generated during the PCO of these alternative substrates showed very

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small or negligible Cr(VI) reduction capacity compared to the case of 4-CP. However, the

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amount of Cr(VI) reduced by stored electrons remained almost the same (20-30 µM)

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regardless of the kind of substrate organic compounds (even in their absence). The reduction

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mechanism by stored electrons in Ag seems to operate by itself, not depending on the PCO of

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organic substrates.

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The observation that the removal of Cr(VI) on Ag/TiO2 continued even after hours in

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the post-irradiation period implies that the stored electrons are very long-lived even in the

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aerated solution. To monitor this slow discharge of electrons directly, the decay of the

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photovoltage of the irradiated electrodes (bare TiO2, Pt/TiO2, and Ag/TiO2) upon turning off 13



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the light was recorded as a function of time as shown in Figure 6. The post-irradiation

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potential decay on the Ag/TiO2 electrode is significantly retarded compared with bare TiO2

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and Pt/TiO2 electrodes. Although the potential rapidly decayed within a minute, small

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residual potential on Ag/TiO2 persisted for a much longer time (far beyond 30 minutes after

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UV turn-off), which is consistent with the fact that the removal of Cr(VI) on Ag/TiO2

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continued for hours in the post-irradiation period (see Figure 1a). Another evidence for the

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stored electrons was obtained from UV/visible absorption spectra of Ag/TiO2 (see Figure 7).

314

Stored electrons in silver NPs induce the silver plasmon band.10, 11, 41 While Ag/TiO2 before

315

light irradiation exhibits no absorption peak in the range 300-900 nm, a peak at 450 nm was

316

observed only after UV illumination but not after visible light (λ > 420 nm) irradiation. Since

317

this peak cannot be ascribed to the plasmonic absorption peak of Ag NPs, it should be related

318

to the accumulation of electrons in the Ag metal (see Figure 7a). The UV-induced band at

319

450 nm did not change for 1 h even after the UV illumination was switched off, but decreased

320

in intensity upon spiking Cr(VI) (see Figure 7b). This confirms that the absorbance band at

321

450 nm is indicative of photo-induced electron accumulation, which disappears upon the

322

subsequent discharge on Ag/TiO2.

323

The post-irradiation catalytic effect of Ag/TiO2 might not be limited to Cr(VI)

324

reduction only. We tested several other chemical conversion reactions in the dark period

325

using the pre-irradiated Ag/TiO2. For example, the reductive conversion of nitrate (NO3-) by

326

electrons stored in Ag/TiO2 was negligible. The reductive discoloration of methylene blue in

327

the post-irradiation period, which had been demonstrated with the pre-irradiated Ag/TiO2 in

328

deaerated ethanol,11 was not observed in the present experimental condition (aerated aqueous

329

solution). However, we observed a sign of OH radical generation on Ag/TiO2 in the post-

330

irradiation period whereas bare TiO2 and Pt/TiO2 exhibited no such sign (see Figure 8). This 14


Page 14 of 33

Page 15 of 33


331

implies that electrons stored in Ag/TiO2 reacted with dioxygen sequentially in the dark to

332

generate OH radicals (i.e., O2 → HO2• → H2O2 → •OH). This observation is consistent with a

333

previous report that PdO/TiON photocatalyst exhibited a bactericidal activity up to 8 hours in

334

the post-irradiation period.42 Small but persisting generation of OH radicals (or other ROS)

335

through the reaction of stored electrons should be responsible for the dark bactericidal

336

activity. Although the electrons stored in Ag/TiO2 are not energetic enough to be utilized for

337

a wide variety of contaminants removal, they have long-lasting reactive effects for selected

338

substrates.

339

Finally, the stability and reusability of catalysts need to be mentioned since this is of

340

high practical importance. The catalyst performance of bare TiO2, Pt/TiO2 and Ag/TiO2 was

341

tested through multiple cycles for PCO removal of 4-CP, PCO removal of 4-CP in the

342

presence of Cr(VI), and PCO removal of 4-CP followed by dark reaction of Cr(VI) (see

343

Figure S7). Regardless of whether Cr(VI) was co-present with 4-CP during the irradiation

344

period or added in the post-irradiation period, the PCO activity of the tested catalysts (bare

345

TiO2, Pt/TiO2, and Ag/TiO2) was irreversibly deactivated. The dark activity of Ag/TiO2 for

346

Cr(VI) removal in the post-irradiation period was also markedly reduced with repeating the

347

irradiation-dark cycles (see Figure S8). This implies that the catalyst surface was deactivated

348

by the reductive deposition of Cr(III) species. The XPS analysis of the catalyst surface after

349

the post-irradiation reaction with Cr(VI) clearly showed that the Cr(III) species is present on

350

Ag/TiO2, but not on bare TiO2 and Pt/TiO2. (see Figure S9).

351 352

ENVIRONMENTAL IMPLICATIONS

353

Most photocatalytic water treatment applications have been focused on pollutants removal

354

under continuous illumination (UV or visible light) conditions. Excess charges can be 15



355

accumulated on the photocatalyst due to the difference between the interfacial charge transfer

356

rates of electron and hole to the aquatic redox species.43, 44 When electrons are accumulated

357

in the metal NPs during illumination, they can be subsequently discharged to suitable electron

358

acceptors in the absence of light. The utilization of such stored charges in the post-irradiation

359

process was applied to anticorrosion system.44 On the basis of this principle, this study

360

utilized the electron storage capacity of Ag NPs loaded on TiO2 surface to reduce Cr(VI) in

361

the following dark period. While the surface of Pt has ohmic contact for rapid discharge of

362

trapped electrons and exhibits a high electrochemical activity for O2 reduction,40 the

363

capacitive property of Ag and its lower catalytic activity for O2 reduction allows electron

364

accumulation and temporary storage even in the presence of dissolved O2. By coupling the

365

PCO of 4-CP with dark thermal reduction (Cr(VI)) reaction, the effect of metal loading on

366

TiO2 was examined in the presence of different organic substrates. The post-irradiation

367

removal of Cr(VI) was contributed by adsorption on catalyst, chemical reduction by organic

368

degradation intermediates, and reduction by electrons stored in noble metal NPs. In

369

particular, the reduction mechanism by stored electrons in Ag seems to operate for itself, not

370

depending on the PCO of organic substrates and the presence of dissolved O2.

371

The utilization of stored photo-electrons to initiate reductive conversions may

372

provide a useful concept to achieve continuous pollutant removal using both day

373

(illumination) and night (dark) cycles. This sequential combination of PCO and dark reaction

374

cycles can be employed in the design of photocatalytic water treatment reactors in which the

375

irradiated and non-irradiated parts can be integrated into a single treatment system. By

376

utilizing both UV PCO and dark reaction period, the overall process can be more energy

377

efficient for the removal of various pollutants. For example, the co-presence of 4-CP and

378

Cr(VI) in the photocatalyst suspension significantly retarded the PCO of 4-CP (see Figure S1) 16


Page 16 of 33

Page 17 of 33


379

but this present sequential combination of PCO-dark reactions successfully removed both

380

pollutants with consuming the same amount of photon energy. However, it should be

381

mentioned that the present study is at a proof-of-concept stage and the real application might

382

be hindered by many practical problems such as low reactivity of stored electrons and

383

catalyst deactivation. The present system of photocatalytic removal of 4-CP combined with

384

the dark reaction of Cr(VI) may not be of great environmental significance but the proposed

385

concept can be ideally suitable for target applications where the long-lasting reactivity of

386

stored electrons in Ag/TiO2 can be utilized for the removal of low-level contaminants.

387

Further investigations of the proposed system may find more suitable applications.

388

17



389

ASSOCIATED CONTENT

390

Supporting Information. PCO degradation of 4-CP with and without Cr(VI) (Figure S1);

391

Time profiles of 7-HC generation (Figure S2); XRD patterns and BET surface area (Figure

392

S3); STEM images (Figure S4); HPLC chromatograms of 4-CP degradation intermediates

393

(Figure S5); photocatalytic removal of 4-CP and TOC in the absence of O2 (Figure S6);

394

repetition tests of 4-CP degradation and Cr(VI) removal (Figure S7-S8); XPS analysis of

395

Cr(III) deposited on catalysts (Figure S9). This material is available free of charge via the

396

Internet at http://pubs.acs.org.

397

ACKNOWLEDGMENT

398

This work was supported by Global Research Laboratory (GRL) Program (NRF-

399

2014K1A1A2041044) and KCAP (Sogang Univ.) (2009-0093880), which were funded by the

400

Korea Government (MSIP) through the National Research Foundation (NRF).

18


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401


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of aqueous 4-chlorophenol by silica-immobilized polyoxometalates. Environ. Sci. Techno.

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2002, 36 (6), 1325-1329.

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chemical oxidation by TiO2 photocatalysis. Environ. Sci. Technol. 2005, 39 (16), 6251-6259.

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chromium(VI) over TiO2 particles in the presence of oxalate: involvement of Cr(V)

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(27) Kim, K.; Choi, W. Enhanced redox conversion of chromate and arsenite in ice. Environ. 21



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samples during simultaneous photoreduction of Cr (VI) and photooxidation of salicylic

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acid. J. Photoch. Photobio. A 2001, 138 (1), 79-85.

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different TiO2 photocatalysts and the effects of dissolved organic species. J. Hazard. Mater.

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photocatalytic performance of Ag/TiO2 and Pt/TiO2 photocatalysts. Separ. Purif. Technol.

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2007, 58 (1), 148-151.

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(33) Elovitz, M. S.; Fish, W. Redox interactions of Cr(VI) and substituted phenols: Products

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and mechanism. Environ. Sci. Technol. 1995, 29 (8), 1933-1943.

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immobilization by acinetobacter sp HK-1 with the assistance of a novel quinone/graphene

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oxide composite. Environ. Sci. Technol. 2014, 48 (21), 12876-12885.

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(36) Theurich, J.; Lindner, M.; Bahnemann, D. W. Photocatalytic degradation of 4-

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chlorophenol in aerated aqueous titanium dioxide suspensions: A kinetic and mechanistic 22


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study. Langmuir 1996, 12 (26), 6368-6376.

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(37) Mohamed, H. H.; Mendive, C. B.; Dillert, R.; Bahnemann, D. W. Kinetic and

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mechanistic investigations of multielectron transfer reactions induced by stored electrons in

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TiO2 nanoparticles: a stopped flow study. J. Phys. Chem. A 2011, 115 (11), 2139-2147.

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(38) Mohamed, H. H.; Dillert, R.; Bahnemann, D. W. Growth and reactivity of silver

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nanoparticles on the surface of TiO2: a stopped flow study. J. Phys. Chem. A 2011, 115 (11),

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12163-12172.

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(39) Meichtry, J. M.; Dillert, R.; Bahnemann, D. W.; Litter, M. I. Application of the Stopped

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Flow Technique to the TiO2-Heterogeneous Photocatalysis of Hexavalent Chromium in

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Aqueous Suspensions: Comparison with O2 and H2O2 as Electron Acceptors. Langmuir

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2015, 31 (22), 6229-6236.

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(40) Yu, T. H.; Hofmann, T.; Sha, Y.; Merinov, B. V.; Myers, D. J.; Heske, C.; Goddard III,

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W. A. Finding correlations of the oxygen reduction reaction activity of transition metal

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catalysts with parameters obtained from quantum mechanics. J. Phys. Chem. C 2013, 117

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512

(41) Hirakawa, T.; Kamat, P. V. Charge separation and catalytic activity of Ag@TiO2 core-

513

shell composite clusters under UV-irradiation. J. Am. Chem. Soc. 2005, 127 (11), 3928-3934.

514

(42) Li, Q.; Li, Y. W.; Wu, P.; Xie, R.; Shang, J. K. Palladium oxide nanoparticles on

515

nitrogen-doped titanium oxide: Accelerated photocatalytic disinfection and post-illumination

516

catalytic “Memory”. Adv. Mater. 2008, 20 (19), 3717-3723.

517

(43) Kim, S.; Park, H. Sunlight-harnessing and storing heterojunction TiO2/Al2O3/WO3

518

electrodes for night-time applications. RSC Adv. 2013, 3 (38), 17551-17558.

519

(44) Tatsuma, T.; Saitoh, S.; Ohko, Y.; Fujishima, A. TiO2-WO3 photoelectrochemical

520

anticorrosion system with an energy storage ability. Chem. Mater. 2001, 13 (9), 2838-2842. 23



521 522

Scheme 1. Conceptual illustration of photocatalytic degradation of 4-CP and the following

523

reductive removal of Cr(VI) in the dark. After the photocatalytic degradation reaction, the

524

reactor was placed in the dark and Cr(VI) was spiked. Cr(VI) can be removed by three

525

different mechanisms that include adsorption, reduction by 4-CP degradation intermediates,

526

and reduction by stored electrons.

527

24


Page 24 of 33

Page 25 of 33


300

in the Dark

(a) UV irradiation

80

200 TiO2

150

Ag/TiO2

Pt/TiO2

regime I

TiO2

Pt/TiO2

100

Ag/TiO2

60 40 20

50 regime II

0 0

10

20

30

40

50

irradiation time (min)

0

60

120

[Cr(VI)] (µM)

[4-CP] (µM)

250

100

0 180

240

300

Dark reaction time (min)

100

[TOC]/[TOC0] x 100%

(b)

After 60 min UV irradiation

80 60 40 20 0

TiO2

Ag/TiO2

Pt/TiO2

528 529

Figure 1. (a) Time profiles of the photocatalytic degradation of 4-CP under UV irradiation

530

and the subsequent Cr(VI) removal in the dark (after turning off the light) and (b) the

531

resulting removal of total organic carbon (TOC) after 60 min UV irradiation. The

532

experimental conditions were [Cat] = 0.5 g/L, [noble metal (Pt or Ag) loading on TiO2] = 3.0

533

wt%, [4-CP]0 = 300 µM, [Cr(VI)]0 = 100 µM, pH0 = 3.0, air-equilibrated, and λ > 320 nm

534

irradiation.

535

25



Page 26 of 33

100

[Cr(VI)] (µ M)

80 60 Pt/TiO2 TiO2

40

Ag/TiO2

(a) dark adsorption

20 0 0

30

60

90

120

150

180

time (min) 100

[Cr(VI)] (µM)

80 60

Pt/TiO2 TiO2

40

Ag/TiO2

(b) dark reduction by 4-CP oxidation intermediates

20 0 0

536

30

60

90

120

150

180

time (min)

537 538

Figure 2. Time profiles of Cr(VI) removal by (a) physical adsorption on catalyst in the dark,

539

and (b) dark thermal reduction induced by the remaining intermediates of 4-CP oxidation that

540

were generated in the pre-PCO period for 60 min. After the pre-PCO period, the catalyst was

541

filtered out and Cr(VI) was then added into the filtrate solution. The experimental conditions

542

were [Cat] = 0.5 g/L, [noble metal (Pt or Ag) loading on TiO2] = 3.0 wt%, [4-CP]0 = 300 µM,

543

[Cr(VI)]0 = 100 µM, pH0 = 3.0, air-equilibrated.

544

26


Page 27 of 33


HHQ HQ 4CC BQ 4CP

Cr(VI) removal (%)

100 80 60 40 20 0 0

50

100

150

200

250

300

intermediates (µΜ) 545 546

Figure 3. Cr(VI) removal by various intermediates of 4-CP degradation in the dark.

547

Hydroxyhydroquinone

548

benzoquinone (BQ) were used as model intermediates. The Cr(VI) removal efficiency was

549

tested as a function of the intermediate concentration. The effect of 4-CP itself was also tested

550

as a control. The experimental conditions were [Cr(VI)]0 = 100 µM, pH0 = 3.0, air-

551

equilibrated. The reaction was finished within 10 min.

(HHQ),

hydroquinone

(HQ),

4-chlorocatechol

552

27


(4-CC),

and


100

-

Cr(VI) removal (%)

(a) Oxic

stored e interm. adsorption

80 60 40 20 0

TiO2

Ag/TiO2

Pt/TiO2

Ag/TiO2

Pt/TiO2

100

(b) Anoxic Cr(VI) removal (%)

-

80

Stored e interm. adsorption

60 40 20 0

TiO2 100

-

Cr(VI) removal (% )

(c) DW

stored e adsorption

80 60 40 20 0

TiO2

Ag/TiO2

Pt/TiO2

553 554 555 556 557 558 559

Figure 4. Removal of Cr(VI) in pre-irradiated aqueous solution of bare TiO2, Ag/TiO2, and Pt/TiO2 (a) in air-equilibration and (b) in anoxic condition. (c) Removal of Cr(VI) in preirradiated deionized water suspension (without 4-CP, aerated) of bare TiO2, Ag/TiO2, and Pt/TiO2 (uncertainty of ±2%). The experimental conditions were [Cat] = 0.5 g/L, [4-CP]0 = 300 µM, [Cr(VI)]0 = 100 µM, pH0 = 3.0, air-equilibrated or Ar-saturated, and λ > 320 nm irradiation.

28


Page 28 of 33

Page 29 of 33


Cr(VI) removal (%)

100

-

stored e interm. adsorption

80 60 40 20 0

4-CP 560

EtOH Formic acid

HA

DW

561

Figure 5. Removal of Cr(VI) in the pre-irradiated aqueous suspension of Ag/TiO2 which

562

contained various organic substrates (4-CP, EtOH, Formic acid, or HA) or deionized water

563

only (DW, as a control) (uncertainty of ± 2%). The experimental conditions were [Ag(3.0

564

wt%)/TiO2] = 0.5 g/L, [4-CP, EtOH, or formic acid]0 = 300 µM, [HA]0 = 1 ppm, [Cr(VI)]0 =

565

100 µM, pH0 = 3.0, air-equilibrated, and λ > 320 nm irradiation.

566

29



1.4 Light OFF

Light ON

1.2

(E-E0)/(Eph-E0)

1.0 0.8 0.6

TiO2

0.4

Pt/TiO2 Ag/TiO2

0.2 0.0 0

10

20

30

40

50

60

70

time (min) 567 568

Figure 6. Time profiles of the normalized open circuit potential (OCP) of the bare TiO2,

569

Pt/TiO2, and Ag/TiO2 electrode before, during, and after UV irradiation. E0 is the OCP

570

stationary value in the dark, and Eph is the stationary potential under UV irradiation.

571

Experimental conditions were [4-CP]0 = 300 µM, [NaClO4]0 = 0.1 M, pH0 = 3.0, air

572

equilibrated, and λ > 320 nm irradiation.

573

30


Page 30 of 33

Page 31 of 33


1.0

(a)

Absorbance

0.8

0.6 Ag/TiO2 0.4

0.2

VL (10 min) UV (1 min) UV (10 min) UV (60 min)

1.0

(b)

Absorbance

0.8

0.6

0.4

0.2 350

574

Ag/TiO2 UV (60 min) UV (60 min) + Cr(VI) spiking in the dark (10 min) 400

450

500

550

600

650

Wavelength (nm)

575

Figure 7. UV/visible absorption spectral changes of Ag/TiO2 under UV-irradiation. (a) a

576

broad band around 450 nm appeared under UV irradiation, but not under visible light. (b) The

577

UV-induced broad band disappeared after spiking Cr(VI) in the dark. Experimental

578

conditions were [Ag/TiO2] = 0.5 g/L (in D.I. water), pH0 = 3.0, [Cr(VI)]add = 100 µM, air-

579

equilibrated, and λ > 320 nm irradiation.

580

31



Emission intensity (a.u.)

350 Ag/TiO2

300

TiO2 Pt/TiO2

250

Ag/TiO2 (Dark)

200 150 100 50 0

20

40

60

80

100

120

time (min)

581 582

Figure 8. Time profiles of 7-hydroxycoumarin (7-HC) production (monitored by its

583

fluorescence emission intensity) in the dark in the pre-irradiated suspensions of bare TiO2,

584

Pt/TiO2, and Ag/TiO2. Experimental conditions were [catalyst] = 0.5 g/L, [noble metal (Pt or

585

Ag) loading on TiO2] = 3.0 wt%, [coumarin]0 = 1 mM, pH0 =3.0, air-equilibrated, and pre-

586

irradiation (λ > 320 nm) for 1 hour.

587 588

32


Page 32 of 33

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589


TOC graphic

590

33


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