Oxygen Vacancy Associated Surface Fenton Chemistry: Surface

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Oxygen Vacancy Associated Surface Fenton Chemistry: Surface Structure Dependent Hydroxyl Radicals Generation and Substrate Dependent Reactivity Hao Li, Jian Shang, Zhiping Yang, Wenjuan Shen, Zhihui Ai, and Lizhi Zhang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017

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Environmental Science & Technology

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Oxygen Vacancy Associated Surface Fenton Chemistry: Surface Structure Dependent Hydroxyl Radicals Generation and Substrate Dependent Reactivity

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Hao Li, Jian Shang, Zhiping Yang, Wenjuan Shen, Zhihui Ai, and Lizhi Zhang*

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Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of

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Environmental Chemistry, Central China Normal University, Wuhan 430079, P. R. China

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RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

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* To whom correspondence should be addressed. E-mail: [email protected]. Phone/Fax: +86-27-6786 7535 1

ACS Paragon Plus Environment


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ABSTRACT Understanding the chemistry of hydrogen peroxide (H2O2) decomposition and

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hydroxyl radicals (•OH) transformation on the surface molecular level is a great challenge for the

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application of heterogeneous Fenton system in the fields of chemistry, environmental and life

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science. We report in this study a conceptual oxygen vacancy associated surface Fenton system

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without any metal ions leaching, exhibiting unprecedented surface chemistry based on the oxygen

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vacancy of electron-donor nature for heterolytic H2O2 dissociation. By controlling the delicate

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surface structure of catalyst, this novel Fenton system allows the facile tuning of •OH existing form

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for targeted catalytic reactions with controlled reactivity and selectivity. On the model catalyst of

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BiOCl, the generated •OH tend to diffuse away from the (001) surface for the selective oxidation of

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dissolved pollutants in solution, but prefer to stay on the (010) surface, reacting with

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strongly-adsorbed pollutants with high priority. These findings will extend the scope of Fenton

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catalysts via surface engineering and consolidate the fundamental theories of Fenton reactions for

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wide environmental applications.

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Keywords: Fenton reaction; Oxygen vacancy; BiOCl; Hydroxyl radical; Selectivity; Surface

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exposed

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Introduction

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Generation of reactive oxygen species (ROSs) is a process of prime importance in nature, ranging

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from environmental chemistry, geochemistry to life sciences.1 Among various ROSs, hydroxyl

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radicals (•OH) with supreme oxidation potential are widely used for water treatment, soil

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remediation, and also participate many physiological processes.2-5 The most conventional way to

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produce •OH is via Fenton reaction (Fe2+/H2O2), which has been discovered for over 100 years. 2


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Unfortunately, traditional Fenton reaction suffers from poor recyclability and narrow working pH

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range. To avoid the shortcomings of traditional Fenton reaction, heterogeneous Fenton systems

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based on transitional metal or metal oxides of multiple oxidation states (e. g. Fe0, Cu0, Fe2O3,

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FeOOH, CeO2, MnO2, and TiO2) are developed.4,

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systems are not strictly surface confined, as free metal ions can leach from the catalyst surface.

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Although these free transitional metal ions (e.g. iron) may favor the H2O2 decomposition, they often

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result in rapid sludge formation without pH adjustment or the addition of organic chelators, thus

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deteriorating Fenton reaction.12-14 Meanwhile, undesirable transitional metal ions (e.g. cerium, cobalt

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and chromium) leaching may even cause the problem of biotoxicity, including direct acute

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cytotoxicity to aquatic life, plant species, and human beings, as well as indirectly oxidative cell

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damage by breaking intracellular normal ROS-antioxidant balance.4,

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intervention of free transitional metal ions strongly conceals basic surface chemistry of

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heterogeneous Fenton systems, especially the generation of •OH via heterolytic H2O2 dissociation

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and the •OH associated reactions on the catalyst surface.

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5, 6-11

However, many heterogeneous Fenton

5

Most importantly, the

It is known that the H2O2 decomposition on the catalyst surface can produce two kinds of •OH,

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namely free and surface-bound •OH.15,

16

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homogeneous Fenton system, possess superior reactivity with high diffusion capability in solution

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and very short lifetime, while more stable surface-bound •OH (•OHsurface), are remarkably restricted

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to the surface.4,15,17-21 Owing to the interference of abundant free •OH arisen from H2O2

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decomposition catalyzed by dissolved transitional metal ions, the molecular level conversion from

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H2O2 to two kinds of •OH on the surface and the intrinsic reactivity of •OHfree and •OHsurface for

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organic pollutants oxidation remain ambiguous. Therefore, developing novel heterogeneous Fenton

Free •OH (•OHfree), as often being discussed in

3



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systems without transitional metal ions leaching is crucial to deeply understand the fundamental

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molecular processes of surface Fenton reactions.

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To achieve this goal, a possible solution to utilize stable and low-toxic non-transitional metal

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oxides as heterogeneous Fenton reagents, which are intrinsically “Fenton-inert”, but offer a

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possibility to exclude the interference of homogeneous H2O2 decomposition by dissolved

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non-transitional metal ions. Obviously, the subsequent challenge is how to make non-transitional

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metal oxides “Fenton-active”, which could be realized by modulating their surface redox properties

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via the introduction of oxygen vacancies (OVs). It is well known that OVs possess abundant

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localized electrons that endow the oxygen-deficient surfaces an electron-rich character for the

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activation or dissociation of small molecules like O2, H2O, CO2, and N2.22-26 Given that the single

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O-O bond in H2O2 is much weaker than the O-O double bond in O2, back donation of localized

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electrons after the adsorption of H2O2 on OVs is highly possible to induce the heterolytic H2O2

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dissociation to generate •OH.

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To check this possibility, we select bismuth oxychloride (BiOCl), a typical V–VI–VII

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non-transitional metal oxide with high earth abundance and low toxicity, as the model catalyst.

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BiOCl has drawn remarkable attention in the environmental field because of its interesting layered

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structure and remarkable photoreactivity to degrade organic pollutants.27-31 Most importantly, similar

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with CeO2, the physical and chemical properties of BiOCl are strongly associated with OVs, which

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can be in situ generated and consumed during catalytic reactions.28, 32-34 In this study, we construct a

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conceptual surface Fenton system with BiOCl of OVs by using its non-transitional nature, not as the

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case of iron oxide with both Fe2+ and Fe3+ states. The OVs of electron donor nature are demonstrated

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to function as the “Fenton-catalytic” center to dissociate H2O2 for the •OH generation in a surface

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confinement pathway. Moreover, the tunable OVs of BiOCl allow us to clarify the influences of

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OVs structures on the H2O2 dissociation and •OH generation behaviors at the surface-atomic scale.

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Experimental Section

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Chemicals and Materials. All chemicals used were purchased from Sinopharm Chemical Reagent

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Co., Ltd. (Shanghai, China), which were of analytical grade and used without purification.

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Well-defined BiOCl single-crystalline nanosheets were prepared by the method previously

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developed by our group.27 Typically, we first added Bi(NO3)3•5H2O into 18 mL distilled water

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containing stoichiometric amount of KCl. Then, pH of the solution was adjusted be either 1 or 6. At

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last, the solution was poured into an autoclave and then heated at 220 °C for 24 h. Resulting

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precipitates were collected and washed with deionized water and ethanol for several times and dried

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in air. The BiOCl nanosheets obtained under pH = 1 with (001) surface exposed was denoted as

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BOC-001 and the BiOCl nanosheets obtained under pH = 6 with (010) surface exposed was denoted

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as BOC-010. Ultra-high vacuum was adopted to create OVs on the surface of BiOCl according to

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the method reported by Xing et al.35 In a typical synthesis, 0.1 g of BOC-001 or BOC-010 was

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homogeneously spread in a crucible and then annealed at 400 K for 60 min with a heating rate of 5

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o

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or BOC-010-OV.

C/min under vacuum to obtain their oxygen-deficient counterparts, being denoted as BOC-001-OV

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Preparation of BiOCl Electrodes. To prepare BiOCl electrode, 10 mg of catalyst was dispersed

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in 1 mL of chitosan solution. The solution was then dip-coated onto the pretreated FTO surface,

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which was then allowed to dry under vacuum.

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Materials Characterization. The scanning electron microscope (SEM) images were recorded

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with a JEOL 6700-F field-emission scanning electron microscope. Electron paramagnetic resonance 5



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(EPR) spectra were conducted on a Bruker EMX EPR Spectrometer (Billerica, MA). The powder

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X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer with

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monochromatized Cu Kα radiation (λ = 0.15418 nm). X-ray photoelectron spectroscopy (XPS) was

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obtained with Perkin-Elmer PHI 5000C and all binding energies were calibrated by using the

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contaminant carbon (C1S = 284.6 eV) as a reference. Electrochemical measurements were

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conducted by an electrochemical analyzer (CHI660D Instruments) in a standard three-electrode

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system, with Ar-purged 100 mL 0.5 M Na2SO4 containing 0.01 mM H2O2 as the electrolyte. The

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dissolved Bi3+ was determined by ICP AES (Shimadzu, ICPS-8100). Raman spectra were obtained

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by a confocal laser micro-Raman spectrometer (Thermo DXR Microscope, USA) with a 532 nm

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laser. Total organic carbon (TOC) was determined by a Shimadzu TOC-V CPH analyzer.

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Analytical Method. •OH were first detected via EPR using 5,5-dimethyl-1-pyrroline-N-oxide as

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the radical spin-trapped reagent and 100 mM tert-butyl alcohol (TBA) or catalase was added during

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the test to scavenger •OH or H2O2, respectively. Free •OH were determined via photoluminescence

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according to the reaction of terephthalic acid with hydroxyl radicals that produces 2-OH-terephthalic

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acid with a fluorescence peak located at 426 nm after excitation at 312 nm.36 Measurement of

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fluorescence intensity was performed on a FluoroMax-P spectrophotometer.

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Pollutants Removal. For the removal of various pollutants, 0.05 g of catalyst was added into 50

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mL of 10 mg•L-1 aqueous solution in a container. The mixture was continuously stirred in the dark

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for one hour to ensure an adsorption-desorption equilibrium before adding 50 mM H2O2 aqueous

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solution into the reaction solution. Two milliliters of the solution was taken out each 2 h and after

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centrifuged to remove the catalyst, the concentration of pollutant or intermediates was monitored by

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colorimetry with the Shimadzu UV-2550 UV-vis spectrometer, high pressure liquid chromatography

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(HPLC, Ultimate 3000, Thermo) with an Agilent TC-C18 column (150 mm × 4.6 mm, 5 µm), and 6


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gas chromatograph-mass spectrometry (GC-MS, TRACE ISQ, Thermo) equipped with a TG-5

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column (30 m

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conducted by adding the corresponding scavenger (100 mM TBA for overall •OH and 10 mM KI for

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surface •OH) to the mixture before H2O2 was added. NaF (2 mM) was added to the reaction mixture

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to desorb •OH from the BiOCl surface before H2O2 was added.

0.25 mm ID, 0.25 Pm film thickness). Active species trapping experiments were

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H2O2 Decomposition by Fe2+ or Bi3+. As for H2O2 decomposition toward the generation of •OH,

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100 uL of 0.025 mol/L FeSO4 or Bi(NO3)3 was added into 100 mL of deionized water at about pH

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6.8. Then, the degradation reaction was initiated by adding 8 uL of 1.0 mol/L H2O2. Generated •OH

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were then determined via photoluminescence.

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Results and Discussion

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We first theoretically modelled the H2O2 adsorption and reaction on the prototypical BiOCl surfaces.

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The (001) surface of BiOCl (BOC-001) is of a close-packed structure terminated with a layer of

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hydroxyl groups, while the (010) surface (BOC-010) is of an open channel structure exposed with O,

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Bi and Cl atoms (Figure S1). On BOC-001, H2O2 was molecularly adsorbed via a hydrogen-bond

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(HB) network constructed by the interactions between hydroxyl groups of H2O2 and those on the

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BOC-001 (Figure 1a). According to the representative HB lengths, adsorption energy, and interfacial

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charge transfer, H2O2 was found to interact weakly with BOC-001 (Table S1). However, the

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introduction of an OV on BOC-001 caused spontaneous dissociation of OV-adsorbed H2O2

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(HaOh-OhHb), along with one–OhHb group being pinned at the OV and another –OhHa fragment

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diffusing away from the BOC-001 surface (Figure S1b and 1b). Remarkably, the dissociated –OhHa

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fragment accepted a long (2.27 Å) HB and donated a short (1.86 Å) HB to nearby hydroxyl groups,

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indicating its radical nature, which was consistent with the Bader charge calculation results (Table

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S1).37 Charge density difference revealed a distinct depletion of localized electrons on the Bi atoms 7



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around the OV and an accumulation of electrons on Oh-Oh (Figure 1b). Sufficient charge back

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donation from OV of BOC-001 to adsorbed H2O2 was responsible for the heterolytic dissociation of

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H2O2 toward the formation of a OH- bound on the OV and a free •OH ((001)-OV + H2O2

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(001)-OhHb- + •OhHa-free) as we expected.

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On BOC-010, H2O2 was molecularly adsorbed in an end-on structure that its oxygen (Oh) was

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chemically bound to a surface 3-coordinated Bi atom (Bi3c) (Figure 1c). The appearance of charge

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localization in the region between –OhHa and a surface Cl suggested a HB-bond like interaction

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(Figure 1c). This structure was energetically more favorable than molecular H2O2 adsorption on

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BOC-001 via HBs according to the calculated adsorption energy (Table S1). The side-on adsorption

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structure of H2O2 on BOC-010 was excluded as Bi3c-Bi3c distance (3.89 Å) was too long for H2O2

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double bonding. As similiar with BOC-001, the generation of an OV on BOC-010 also led to

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spontaneous H2O2 dissociation. Differently, the dissociated •OhHa preferred to adsorb onto a surface

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Bi3c near the OV via a pathway of (010)-OV + H2O2 (010)-OHb- + •OhHa-surface, resulting in an

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in-between chemical bond (Figure S1d, 1d and Table S1).

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The above theoretical results suggested that localized electron donation from OVs to adsorbed

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H2O2 could directly facilitate the H2O2 dissociation in the manner of Fenton reaction, although

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surface Bi atoms (Bi3+) alone could not directly decompose H2O2 owing to the non-transition nature

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of BiOCl. More interestingly, chemical states (free or surface-bound) of •OH could be governed by

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the different structures of OVs confined on distinct BiOCl surfaces. For instance, surface oxygen

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atoms of high density impeded the binding of dissociated •OhHa to BOC-001 because of their steric

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hindrance. In contrast, the OV of BOC-010 exhibited an open structure, around which the

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neighboring Bi3c could provide a perfect Lewis-acid site for the well-stabilization of •OhHa fragment

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and the minimization of overall surface energy.34 Direct dissociation of H2O2 might be possible on 8


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defect-free BiOCl, but suffered from high thermodynamic energy barriers. On BOC-001, the

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transition state (TS1) over H2O2 dissociation involved a deprotonation process that -OhHb would

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approach the surface to abstract a proton, generating a HB-interacted H2O molecule and a free •OH

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via (001)-H + H2O2 (001)-H2O + •OhHa-free (Figure 2a). On BOC-010, the HB-like interaction

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between •OhHb and a surface Cl would stretch the Oh-Oh bond during the transiton state of H2O2

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dissociation (TS2), leading to the formation of a OH- and a surface-bound •OH via (010)-Cl + H2O2

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(010)-Cl-OHb- + •OhHa-surface (Figure 2b). Differently, H2O2 dissociations on the OVs of BOC-001

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and BOC-010 were both down-hill reactions induced by the sufficient charge donation from OVs to

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H2O2 and the consequent elongation of Oh-Oh of H2O2 toward heterolytic breaking (Figure 2c and

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2d). According to the corresponding energy expenditure, H2O2 dissociation on the OV of BOC-010

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was more thermodynamically favorable than that on the OV of BOC-001, highlighting the

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importance of surface •OhHa on minimizing the surface energy of BOC-010 (Figure 2e and 2f).34

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The interactions between OVs of BiOCl and H2O2 were then checked with electrochemical

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experiments. Grayish oxygen-deficient counterparts (BOC-001-OV or BOC-010-OV) of BiOCl

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single-crystalline nanosheets could be easily prepared by thermal treatment under high vacuum, as

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schematically illustrated in Figure 3a. We first adopted SEM and TEM to characterize the catalysts’

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morphology and estimated the relative proportions of exposed surface (Figure S2 and S3), and then

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employed low-temperature EPR, XPS, and Raman spectra to confirm the presence of OVs in the

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catalysts (Figure S4).38 Cyclic voltammetry (CV) was used to monitor the charge transfer at the

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BiOCl/H2O2 interfacial region in a designed reaction electrochemical reaction cell (Figure 3b).

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Continuous Ar purging was used to remove the dissolved O2. The CV curves of BOC-001-OV and

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BOC-001-OV showed distinct reduction peaks beginning at a low potential of -0.13 V, which were

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not observed for BOC-001, BOC-010 or bare FTO electrode (Figure 3c). These reduction peaks 9



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indicated the surface OVs served as the active sites for the H2O2 dissociation, which were

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diminished in the absence of H2O2 (Figure S5). The higher cathodic current density of BOC-010-OV,

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as compared with that of BOC-001-OV, indicated higher capability of OVs on BOC-010 for the

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H2O2 reduction (Figure 3c), which were further confirmed by electrochemical impedance

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spectroscopy (EIS) results, as BOC-010-OV showed a more significant decrease of Nyquist plot

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diameter than BOC-001-OV in the presence of H2O2 (Figure 3d).

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The H2O2 dissociation on OVs of BiOCl was then investigated with electron paramagnetic

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resonance (EPR) by detecting the spin reactive •OH adsorbed on BiOCl surfaces or dissolved in

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water with using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap. No EPR signal was

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observed over BOC-001 and BOC-010 in the presence of H2O2, confirming that defect-free BiOCl

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surfaces were not able to dissociate H2O2 for the •OH generation (Figure S6). As expected, strong

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four-line EPR spectra with the relative intensities of 1:2:2:1 corresponding to DMPO-•OH adduct

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were observed for both BOC-001-OV and BOC-010-OV reacting with H2O2 (Figure 4a). The EPR

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signals could be largely inhibited by adding the •OH scavenger tert-butyl alcohol (TBA) (Figure 4a).

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H2O2 decomposition induced by dissolved metal ions could be ruled out by non-leaching of Bi ions

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into the solution according to inductively coupled plasma-atomic emission spectroscopy

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measurement result. Moreover, unlike the case of iron with both Fe2+ and Fe3+ states, electron

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transfer between free Bi3+ and H2O2 did not happen (Figure S7). The H2O2 decomposition in this

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Fenton-like system was therefore considered as a strictly surface confined one. Plotting the intensity

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of the EPR signals against the reaction time produced two straight lines. The slopes gave the overall

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•OH radicals’ generation rates of 0.43 s-1 for BOC-001-OV and 0.85 s-1 for BOC-010-OV (Figure

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4b). Therefore, the rate of H2O2 dissociation on BOC-010-OV toward the generation of •OH was

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about 1.98 times that on BOC-001-OV, further confirming that the dissociation of H2O2 on OVs of 10


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BOC-010 was more thermodynamically favorable. Addition of catalase to quench H2O2 (via 2H2O2

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2H2O + O2) instantly decreased the concentration of •OH. Intriguingly, •OH generated by

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BOC-010-OV exhibited a remarkably slower decaying rate, whose lifetime was over 2 times longer

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than that in case of BOC-001-OV after normalizing the incident EPR signal intensities by the initial

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ones (Figure 4b and S8). This phenomenon suggested •OH of different chemical states might be

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generated by distinct BiOCl surfaces, as the theoretical calculations predicted.

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To identify the chemical states of •OH generated on the BiOCl surfaces, we quantitatively

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determined the •OH concentration. Although BOC-010-OV exhibited a higher overall H2O2

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dissociation rate, BOC-001-OV showed a stronger capability to produce free •OH (Figure 4c). This

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observation was consistent with the above theoretical calculation result. In order to quantify the ratio

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of free •OH, we added F- in the solution to desorb •OHsurface by forming strong •OHsurface…F-

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HB.39,40 It was found that the concentration of free •OH slightly increased over BOC-001-OV, but

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remarkably enhanced in case of BOC-010-OV after the F- addition. Assuming all surface-bound •OH

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could be removed from the BiOCl surfaces by adding F-, the relative ratio of free •OH to overall

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•OH generated by BOC-001-OV was about 71%, much higher than that (15%) in the case of

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BOC-010-OV (inset of Figure 4c).To further probe the different •OH chemical states on the distinct

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BiOCl surfaces, we monitored the change of surface hydroxyl groups before and after the H2O2

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dissociation. The O1s XPS spectra of BOC-001 and BOC-010 showed a shoulder at 531.6 eV with a

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chemical shift of 1.5 eV relative to the oxide O 1s peak, being attributed to surface hydroxyl groups

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(Figure 4d).34 Saturated adsorption of H2O2 on BiOCl surfaces allowed the formation of a broader

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shoulder at higher binding energy of 532.0 eV, which was attributed to the surface peroxide groups

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(–O22-) of adsorbed H2O2 according to the following two reasons (Figure 4d). First, similiar XPS

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peaks ascribed to surface peroxide groups were observed over TiO2, and the binding energies of 11



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surface peroxide groups were much larger than those of surface hydroxyl groups.41,42 Second, our

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recent work revealed that the O 1s XPS peak at 531.9 eV during photocatalytic benzyl alcohol

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oxidation over Au decorated BiOCl was arisen from a surface adsorbed peroxide species on BiOCl,43

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which was similar with the newly-formed O 1s XPS peak of BOC-001 (BOC-010) adsorbed with

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H2O2. These peroxide groups over BOC-001 and BOC-010 no longer stably existed after the

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introduction of OVs owing to the H2O2 dissociation. After the H2O2 dissociation, the peak intensity

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of surface hydroxyl groups over BOC-001-OV did not obviously change, but significantly improved

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in the case of BOC-010-OV (Figure 4d). This improvement was intrinsically attributed to the H2O2

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dissociation over BOC-010-OV, where surface-bound •OH was the selective product.

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As aforementioned, BOC-001-OV favored the formation of free •OH, while BOC-010-OV

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preferred to generate surface bound •OH when they reacted with H2O2. This controllable generation

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of free and surface •OH enabled us to probe their intrinsic reactivity for organic pollutants oxidation.

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Similar with •OH, organic pollutants can exist either in solution or on catalysts surface in

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heterogeneous Fenton systems. We then selected six typical organic pollutants (formic acid, benzoic

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acid, benzene, phenol, rhodamine B and methyl orange) widely distributed in the environment.

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Intriguingly, BOC-001-OV exhibited distinct higher reactivity than BOC-010-OV for the removal of

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benzoic acid, benzene, and phenol, which were classified as group I pollutants, although its overall

267

•OH generation rate was significantly lower than that of BOC-010-OV (Figure 5a and S9a-c).

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Intriguingly, the Fenton degradation efficiencies of these pollutants over BOC-001-OV were in the

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order of benzoic acid > benzene > phenol. Since the TOC removal efficiencies of group I pollutants

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were less than 15% and there were many intermediates produced via the hydroxylation of benzene

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ring by •OH, including monohydroxybenzoic acid, pyrocatechol and hydroquinone (Table S3), the

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degradation of these aromatic compounds with free •OH generated in BOC-001-OV/H2O2 system 12


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involved the hydroxylation of benzene ring as the initial step. A previous study demonstrated that

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strong activating groups (such as -OH) for electrophilic substitution had a negative effect on the

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hydroxylation of benzene ring by •OH, whereas deactivating groups showed a positive activation

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effect.44 Therefore, the removal of benzoic acid in BOC-001-OV heterogeneous Fenton system was

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much faster than that of phenol. As for the Fenton removal of formic acid, rhodamine B or methyl

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orange, which were classified as group II pollutants, BOC-010-OV exhibited remarkably higher

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reactivity than BOC-001-OV (Figure 5a and S9d-f). Moreover, BOC-010-OV heterogeneous Fenton

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system could more efficiently oxidize formic acid with 91.6% of TOC removal efficiency than

281

rhodamine B or methyl orange, because plenty of phenyl substituted intermediates or small acids

282

were produced during the degradation of rhodamine B or methyl orange (Table S3). It was

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reasonable to notice that the intrinsic catalytic efficiency of H2O2 decomposition over the OVs of

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BOC-001-OV or BOC-010-OV toward the generation of •OH was lower than that of traditional

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homogeneous Fenton system (Figure S10), which accounted for the long duration (8 h) of pollutants

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degradation.2,8 But the catalytic reactivity difference of BOC-001-OV and BOC-010-OV for distinct

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organic substrates removal suggested •OH in different chemical states undeniably possessed

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different organic pollutants oxidation abilities. To gain deeper insight, we carefully studied the

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adsorption behaviors of these pollutants on BiOCl surfaces, and found group I pollutants were

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poorly adsorbed on the BiOCl surfaces, regardless of which facet was exposed (Table S2). Group II

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pollutants, however, either being small (formic acid) or rich in functional groups (dyes), showed

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high affinity to BiOCl (Table S2). Regarding that adsorption capacity of a catalyst is highly related

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to surface area, charging states and active adsorption sites,45-47 we first measured the specific areas

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of the as-prepared BiOCl, which were 0.92 m2/g for BOC-001-OV and 2.06 m2/g for BOC-010-OV

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(Figure S11a), respectively. Subsequently, we normalized the organic substrates removal constants of 13



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different BiOCl with their corresponding surface areas, and found that the normalized constants of

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BOC-001-OV to remove group I organic substrates were about 4 times higher than those of

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BOC-010-OV (Table S2), even though BOC-001-OV was of a smaller surface area. However, the

299

normalized constants of BOC-010-OV to remove group II organic substrates were comparable with

300

those of BOC-001-OV, highlighting that the superior adsorption capacity of BOC-010-OV favored

301

the removal of group II organic substrates (Table S2). As the superior adsorption capacity of

302

BOC-010-OV might be related to either surface charging states or active adsorption sites, we then

303

monitored the surface charge of BiOCl via zeta potential measurement, and found that (001) surface

304

of BiOCl was negatively charged (-2.9 mV), while the (010) surface was positively charged (4.2

305

mV), which were consistent with the high density oxygen atoms exposure characteristic of BiOCl

306

(001) surface and the cationic Bi atoms exposure feature of (010) surface (Figure S11b and S11c).

307

However, surface charge might not remarkably influence the adsorption capacity of BiOCl, because

308

the adsorption of rhodamine B, a typical cationic species, was not favored on the negatively charged

309

BOC-001-OV surface, but on the positively charged BOC-010-OV surface (Figure S11d-f). As

310

aforementioned in the introduction part of manuscript, the (001) surface of BiOCl was of a

311

close-packed structure terminated with a layer of hydroxyl groups, while the (010) surface was of an

312

open channel structure exposed with coordinating unsaturated O, Bi and Cl atoms (Figure S1).

313

Regarding that surface Bi atoms on BiOCl (010) surface could serve as the Lewis acid sites, while O

314

and Cl atoms could act as the Lewis base sites, strong interaction between BOC-010-OV and organic

315

substrates of rich functional groups was also supposed to be related to the Lewis acid/base sites on

316

BiOCl (010) surface. Accordingly, for group I pollutants that poorly interacted with BiOCl surfaces,

317

they could be easily oxidized by free •OH, while surface-bound •OH preferred to react with the

318

group II pollutants of high affinity to BiOCl surfaces. To further demonstrate this 14


Page 15 of 29


319

substrate-dependent removal behavior induced by different types of •OH, we selected benzoic acid

320

and formic acid as the model pollutants for the subsequent investigation. Addition of TBA to

321

scavenge overall •OH remarkably decreased the reactivity of BOC-001-OV and BOC-010-OV for

322

either benzoic acid or formic acid removal (Figure 5b). However, the addition of KI to scavenge

323

surface-bound •OH only significantly suppressed the reactivity of BOC-010-OV to remove benzoic

324

acid and formic acid, but hardly influenced that of BOC-001-OV (Figure 5b).15,16 Moreover, we

325

found the reactivity of oxygen-deficient BiOCl were not remarkably influenced by the thicknesses of

326

the single-crystalline nanosheets (Figure S12), but highly dependent on the amount of OVs (Figure

327

S13). For instance, appropriately higher calcination temperature or longer calcination time under

328

high vacuum significantly enhanced the concentration of OVs on the BOC-001-OV or BOC-010-OV

329

surface and thus improved their Fenton formic acid degradation performances (Figure S13).

330

However, overheating or overlong calcination time would not contribute to the catalytic reactivity

331

increase because of the OVs formation in the subsurface and/or bulk or the saturation of OVs on the

332

surface (Figure S13f). Interestingly, BOC-010-OV was able to selectively remove formic acid with

333

100% efficiency from a mixture solution of formic acid and benzoic acid, without obviously

334

degrading benzoic acid (Figure S14). As this extremely high selectivity occurred when both •OH and

335

pollutant were preferentially co-adsorbed, we called it “site selectivity”, which can broaden the

336

applications of •OH bearing non-selective oxidation characteristic. Besides, different existing forms

337

of •OH were also supposed to affect the degradation pathways of organic pollutant. For example,

338

BOC-010-OV exhibited much higher reactivity than BOC-001-OV for the Fenton degradation of

339

pentachlorophenol (Figure S15a). According to the detected intermediates and the chloride ion

340

formation, surface •OH was found to be more beneficial for the dechlorination of pentachlorophenol

341

than free •OH (Figure S15b and S15c). More importantly, this OVs-based Fenton system could 15



Page 16 of 29

342

effectively degrade organic pollutants in the pH range of 3 to 10 (Figure 5c) and thus have much

343

wider pH working window than traditional Fenton systems. The adsorption of OH- on surface Bi

344

atoms of BOC-010-OV competed with the adsorption of formic acid (Figure S16), accounting for

345

the slight reactivity decrease of BOC-010-OV along with the pH increase from 4 to 10. Although the

346

quenching of OVs for multiple organic pollutants removal could not be avoided, a simple thermal

347

treatment ensured the reusability of catalyst for organic pollutants oxidation (Figure S17).

348

Environmental Implications. Although only a case study of interaction between H2O2 and OVs of

349

BiOCl is given, while the catalytic efficiency of H2O2 decomposition over oxygen-deficient BiOCl

350

toward •OH generation is much lower than that of traditional homogeneous Fenton reaction, the

351

findings in this study still have important implications for environmental processes and pollutant

352

controls related to H2O2-based chemical transformation. First, OVs on oxide surface extend the

353

scope of cheap and stable heterogeneous catalysts for Fenton reaction to the non-transitional oxide

354

materials. Moreover, manipulating the delicate structures of OVs allows us to control the existing

355

form of •OH for targeted catalytic selectivity beyond reactivity, because different organic pollutants

356

show different affinities to oxide surfaces, which will consolidate the fundamental theories of Fenton

357

reactions for wide environmental applications with designing surface Fenton systems without any

358

metal ions releasing (Figure 5d). Second, dissociation of H2O2 on OVs represents a new H2O2/oxide

359

surface interaction mode for the thermodynamically-enhanced formation of highly-reactive •OH. We

360

believe that OVs, the most common defects on the surfaces of non-transitional oxide minerals or

361

dust in the atmosphere, are highly possible to be associated with the transformation of organic

362

pollutants in both water and gaseous atmosphere. This finding is of significant importance in the

363

fields of catalysis, geochemistry, and environmental chemistry.49-51 Third, many metal oxide

364

semiconductors are able to response to solar light and in situ generate H2O2 either via the activation 16


Page 17 of 29


365

of O2 by electrons (O2 + 2H+ + 2e- H2O2) and the oxidation of H2O by holes (2H2O + 2h+ H2O2

366

+ 2H+) in water or atmoshpere.52-54 For the oxides with surface OVs, interaction mode between OVs

367

and H2O2 (OV + H2O2 •OH + OH-) is a new but neglected reaction path for the consumption of

368

H2O2 during photocatalysis.

369 370

Acknowledgements: This work was supported by National Natural Science Funds for Distinguished

371

Young Scholars (Grant 21425728), National Basic Research Program of China (973 Program) (Grant

372

2013CB632402), National Key Research and Development Program of China (Grant

373

2016YFA0203002), National Science Foundation of China (Grant 51472100), the 111 Project (Grant

374

B17019), Excellent Doctorial Dissertation Cultivation Grant from Central China Normal University

375

(Grant 2015YBZD018 and 2016YBZZ034), and the CAS Interdisciplinary Innovation Team of the

376

Chinese Academy of Sciences. We also thank the National Supercomputer Center in Jinan for

377

providing high performance computation.

378 379

Supporting Information: Details of theoretical calculation; synthesis and characterization of BiOCl

380

single-crystalline nanosheets; characterization of oxygen vacancies within BiOCl; organic pollutants

381

degradation profiles; catalytic performance of homogeneous Fenton reaction; surface area and

382

charge of the catalysts; effect of calcination temperature and time on catalytic performance of

383

oxygen-deficient BiOCl; intermediates detected during certain organic substrates removal; catalytic

384

removal of pentachlorophenol; reusability of the catalysts.

385 386

Literature Cited

387

(1) Herzberg, G. The spectra and structures of simple free radicals: an introduction to molecular

388

spectroscopy; Dover Publications, Inc.: Mineola, New York, 2003. 17



Page 18 of 29

389

(2) Yang, X.; Xu, X.; Xu, J.; Han, Y. Iron oxychloride (FeOCl): An efficient Fenton-like catalyst for

390

producing hydroxyl radicals in degradation of organic contaminants. J. Am. Chem. Soc. 2013,

391

135, 16058–16061.

392 393 394 395

(3) Gligorovski, S.; Strekowski, R.; Barbati, S.; Vione, D. Environmental implications of hydroxyl radicals (•OH). Chem. Rev. 2015, 115, 13051–13092. (4) Bokare, A. D.; Choi, W. Review of iron-free Fenton-like systems for activating H2O2 in advanced oxidation processes. J. Hazard. Mater. 2014, 275, 121–135.

396

(5) Voinov, M. A.; Pagán, J. O. S.; Morrison, E.; Smirnova, T. I.; Smirnov, A. I. Surface-mediated

397

production of hydroxyl radicals as a mechanism of iron oxide nanoparticle biotoxicity. J. Am.

398

Chem. Soc. 2011, 133, 35–41.

399 400

(6) Kwan, W. P.; Voelker, B. M. Rates of hydroxyl radical generation and organic compound oxidation in mineral-catalyzed Fenton-like systems. Environ. Sci. Technol. 2003, 37, 1150–1158.

401

(7) Navalon, S.; de Miguel, M.; Martin, R.; Alvaro, M.; Garcia, H. Enhancement of the catalytic

402

activity of supported gold nanoparticles for the Fenton reaction by light. J. Am. Chem. Soc. 2011,

403

133, 2218–2226.

404

(8) Zhang, A.-Y.; Lin, T.; He, Y.-Y.; Mou, Y.-X. Heterogeneous activation of H2O2 by

405

defect-engineered TiO2−x single crystals for refractory pollutants degradation: A Fenton-like

406

mechanism. J. Hazard. Mater. 2016, 311, 81–90.

407

(9) Bremner, D. H.; Burgess, A. E.; Houllemare, D.; Namkung, K. C. Phenol degradation using

408

hydroxyl radicals generated from zero-valent iron and hydrogen peroxide. Appl. Catal. B:

409

Environ. 2006, 63, 15-19.

410 411

(10) Leupin, O. X.; Hug, S. J. Oxidation and removal of arsenic (III) from aerated groundwater by filtration through sand and zero-valent iron. Water Res. 2005, 39, 1729-1740. 18


Page 19 of 29 412 413


(11) Joo, S. H.; Feitz, A. J.; Waite, T. D. Oxidative degradation of the carbothioate herbicide, molinate, using nanoscale zero-valent iron. Environ. Sci. Technol. 2004, 38, 2242-2247

414

(12) Sanchez, I.; Stüber, F.; Fabregat, A.; Font, J.; Fortuny, A.; Bengoa, C. Degradation of model

415

olive mill contaminants of omw catalysed by zero-valent iron enhanced with a chelant. J.

416

Hazard. Mater. 2012, 199–200, 328-335.

417 418

(13) Noradoun, C. E.; Cheng, I. F. EDTA degradation induced by oxygen activation in a zerovalent iron/air/water system. Environ. Sci. Technol. 2005, 39, 7158-7163.

419

(14) Chen, L.; Ma, J.; Li, X.; Zhang, J.; Fang, J.; Guan, Y.; Xie, P. Strong enhancement on Fenton

420

oxidation by addition of hydroxylamine toaccelerate the ferric and ferrous iron cycles. Environ.

421

Sci. Technol. 2011, 45, 3925–3930.

422

(15) Xu, L.; Wang, J. Magnetic Nanoscaled Fe3O4/CeO2 Composite as an Efficient Fenton-Like

423

Heterogeneous Catalyst for Degradation of 4-Chlorophenol. Environ. Sci. Technol. 2012, 46,

424

10145–10153.

425 426

(16) Liu, W.; Ai, Z.; Cao, M.; Zhang, L. Ferrous ions promoted aerobic simazine degradation with Fe@Fe2O3 core–shell nanowires. Appl. Catal. B Environ. 2014, 150–151, 1–11.

427

(17) Pignatello, J. J.; Oliveros, E.; MacKay, A. Advanced oxidation processes for organic

428

contaminant destruction based on the Fenton reaction andrelated chemistry. Crit. Rev. Environ.

429

Sci. Technol. 2006, 36, 1–84.

430

(18) El-Morsi, T. M.; Budakowski, W. R.; Abd-El-Aziz, A. S.; Friesen, K. J. Photocatalytic

431

degradation of 1,10-dichlorodecane in aqueous suspensions of TiO2: a reaction of adsorbed

432

chlorinated alkane with surface hydroxylradicals. Environ. Sci. Technol. 2000, 34, 1018–1022.

433

(19) Sun, Y.; Pignatello, J. J. Evidence for a surface dual hole-radical mechanism in the titanium

434

dioxide photocatalytic oxidation of 2,4-D. Environ. Sci. Technol. 1995, 29, 2065–2072. 19



Page 20 of 29

435

(20) Mao, Y.; Schoeneich, C.; Asmus, K. D. Identification of organic acids and other intermediates in

436

oxidative degradation of chlorinated ethanes on titania surfaces en route to mineralization: a

437

combined photocatalytic and radiation chemical study. J. Phys. Chem. 1991, 95, 10080–10089.

438

(21) Kesselman, J. M.; Lewis, N. S.; Hoffmann, M. R. Photoelectrochemical degradation of

439

4-chlorocatechol at TiO2 electrodes: Comparison between sorption and photoreactivity. Environ.

440

Sci. Technol. 1997, 31, 2298–2302.

441

(22) Nowotny, J.; Alim, M. A.; Bak, T.; Idris, M. A.; Ionescu, M.; Prince, K.; Sahdan, M. Z.; Sopian,

442

K.; Mat Teridi, M. A.; Sigmund, W. Defect chemistry and defect engineering of TiO2-based

443

semiconductors for solar energy conversion. Chem. Soc. Rev. 2015, 44, 8424–8442.

444

(23) Bikondoa, O.; Pang, C. L.; Ithnin, R.; Muryn, C. A.; Onishi, H.; Thornton, G. Direct

445

visualization of defect-mediated dissociation of water on TiO2(110). Nat. Mater. 2006, 5,

446

189–192.

447

(24) Li, H.; Shang, J.; Ai, Z.; Zhang, L. Efficient visiblelight nitrogen fixation with BiOBr

448

nanosheets of oxygen vacancies on the exposed {001} facets. J. Am. Chem. Soc. 2015, 137,

449

6393–6399.

450 451 452 453 454 455

(25) Aschauer, U.; Chen, J.; Selloni, A. Peroxide and superoxide states of adsorbed O2 on anatase TiO2 (101) with subsurface defects. Phys. Chem. Chem. Phys. 2010, 12, 12956–12960. (26) Lee, J.; Sorescu, D. C.; Deng, X. Electron-induced dissociation of CO2 on TiO2 (110). J. Am. Chem. Soc. 2011, 133, 10066–10069. (27) Jiang, J.; Zhao, K.; Xiao, X. Y.; Zhang, L. Z. Synthesis and facet-dependent photoreactivity of BiOCl single-crystalline nanosheets. J. Am. Chem. Soc. 2012, 134, 4473.

456

(28) Zhao, K.; Zhang, L. Z.; Wang, J. J.; Li, Q. X.; He, W. W.; Yin, J. J. Surface structure-dependent

457

molecular oxygen activation of BiOCl single-crystalline nanosheets. J. Am. Chem. Soc. 2013, 20


Page 21 of 29 458 459 460 461 462 463 464


135, 15750. (29) Ye, L. Q.; Zan, L.; Tian, L. H.; Peng, T. Y.; Zhang, J. J. The {001} facets-dependent high photoactivity of BiOCl nanosheets. Chem. Commun. 2011, 47, 6951. (30) Cheng, H.; Huang, B.; Dai, Y. Engineering BiOX (X = Cl, Br, I) nanostructures for highly efficient photocatalytic applications. Nanoscale 2014, 6, 2009. (31) Li, J.; Yu, Y.; Zhang, L. Bismuth oxyhalide nanomaterials: layered structures meet photocatalysis. Nanoscale 2014, 6, 8473.

465

(32) Li, H.; Shi, J.; Zhao, K.; Zhang, L. Sustainable molecular oxygen activation with oxygen

466

vacancies on the {001} facets of BiOCl nanosheets under solar light. Nanoscale 2014, 6,

467

14168–14173.

468

(33) Li, H.; Shang, J.; Shi, J.; Zhao, K.; Zhang, L. Facet-dependent solar ammonia synthesis of

469

BiOCl nanosheets via a proton-assisted electron transfer pathway. Nanoscale 2016, 8,

470

1986–1993.

471 472

(34) Li, H.; Shang, J.; Zhu, H.; Yang, Z.; Ai, Z.; Zhang, L. Oxygen vacancy structure associated photocatalytic water oxidation of BiOCl. ACS Catal. 2016, 8276–8285.

473

(35) Xing, M.; Zhang, J.; Chen, F.; Tian, B. An economic method to prepare vacuum activated

474

photocatalysts with high photo-activities and photosensitivities. Chem. Commun. 2011, 47, 4947.

475

(36) Xiao, Q.; Si, Z.; Zhang, J.; Xiao, C.; Tan, X. Photoinduced hydroxyl radical and photocatalytic

476 477 478 479 480

activity of samarium-doped TiO2 nanocrystalline. J. Hazard. Mater. 2008, 150, 62–67. (37) Chen, J.; Li, Y.-F.; Sit, P.; Selloni, A. Chemical dynamics of the first proton-coupled electron transfer of water oxidation on TiO2 Anatase. J. Am. Chem. Soc. 2013, 135, 18774–18777. (38) Zhang, L.; Wang, S.; Lu, C. Detection of oxygen vacancies in oxides by defect-dependent cataluminescence. Anal. Chem. 2015, 87, 7313–7320. 21



Page 22 of 29

481

(39) Sheng, H.; Li, Q.; Ma, W.; Ji, H.; Chen, C.; Zhao, J. Photocatalytic degradation of organic

482

pollutants on surface anionized TiO2: Common effect of anions for high hole-availability by

483

water. Appl. Catal. B Environ. 2013, 138–139, 212–218.

484

(40) Xu, Y.; Lv, K.; Xiong, Z.; Leng, W.; Du, W.; Liu, D.; Xue, X. Rate enhancement and rate

485

inhibition of phenol degradation over irradiated Anatase and Rutile TiO2 on the addition of NaF:

486

New Insight into the Mechanism. J. Phys. Chem. C 2007, 111, 19024–19032.

487 488

(41) Xiang, G.; Wu, D.; He, J.; Wang, X. Acquired pH-responsive and reversible enrichment of organic dyes by peroxide modified ultrathin TiO2 nanosheets. Chem. Commun. 2011, 47, 11456.

489

(42) Zhou, C.; Luo, J.; Chen, Q.; Jiang, Y.; Dong, X.; Cui, F. Titanate nanosheets as highly efficient

490

non-light-driven catalysts for degradation of organic dyes. Chem. Commun. 2015, 51,

491

10847–10849.

492

(43) Li, H.; Qin, F.; Yang, Z. P.; Cui, X. M.; Wang, J. F.; Zhang, L. Z. New reaction pathway induced

493

by plasmon for selective benzyl alcohol oxidation on BiOCl possessing oxygen vacancies. J. Am.

494

Chem. Soc. 2017, 139, 3513–3521.

495

(44) Augusti, R.; Dias, A. O.; Rocha, L. L.; Lago, R. M. Kinetics and Mechanism of Benzene

496

Derivative Degradation with Fenton’s Reagent in Aqueous Medium Studied by MIMS. J. Phys.

497

Chem. A 1998, 102, 10723–10727.

498

(45) Wu, N.; Wei, H. H.; Zhang, L. Z. Efficient removal of heavy metal ions with biopolymer

499

template synthesized mesoporous titania beads of hundreds of mcrometers size. Environ. Sci.

500

Technol. 2012, 46, 419–425.

501

(46) Zhang, M.; Yao, Q.; Lu, C.; Li, Z.; Wang, W. Layered double hydroxide–carbon dot composite:

502

high-performance adsorbent for removal of anionic organic dye. ACS Appl. Mater. Interfaces

503

2014, 6, 20225–20233. 22


Page 23 of 29


504

(47) Yao, Q.; Wang, S.; Shi, W.; Lu, C.; Liu, G. Graphene quantum dots in two-dimensional confined

505

and hydrophobic space for enhanced adsorption of nonionic organic adsorbates. Ind. Eng. Chem.

506

Res. 2017, 56, 583–590.

507

(48) Weng, S.; Pei, Z.; Zheng, Z.; Hu, J.; Liu, P. Exciton-free, nonsensitized degradation of

508

2-naphthol by facet-dependent BiOCl under visible light: Novel evidence of surface-state

509

photocatalysis. ACS Appl. Mater. Interfaces 2013, 5, 12380–12386.

510

(49) Pradhan, M.; Kyriakou, G.; Archibald, A. T.; Papageorgiou, A. C.; Kalberer, M.; Lambert, R. M.

511

Heterogeneous uptake of gaseous hydrogen peroxide by Gobi and Saharan dust aerosols: a

512

potential missing sink for H2O2 in the troposphere. Atmos. Chem. Phys. 2010, 10, 7127–7136.

513

(50) Möller, D. Atmospheric hydrogen peroxide: Evidence for aqueous-phase formation from a

514

historic perspective and a one-year measurement campaign. Atmos. Environ. 2009, 43,

515

5923–5936.

516

(51) Pradhan, M.; Kalberer, M.; Griffiths, P. T.; Braban, C. F.; Pope, F. D.; Cox, R. A.; Lambert, R.

517

M. Uptake of gaseous hydrogen peroxide by submicrometer titanium dioxide aerosol as a

518

function of relative humidity. Environ. Sci. Technol. 2010, 44, 1360–1365.

519 520 521 522

(52) Harbour, J. R.; Tromp, J.; Hair, M. L. Photogeneration of hydrogen peroxide in aqueous TiO2 dispersions. Can. J. Chem. 1985, 63, 204–208. (53) Chen, H.; Nanayakkara, C. E.; Grassian, V. H. Titanium dioxide photocatalysis in atmospheric chemistry. Chem. Rev. 2012, 112, 5919–5948.

523

(54) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D.

524

W. Understanding TiO2 photocatalysis: mechanisms and materials. Chem. Rev. 2014, 114,

525

9919–9986.

526

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Page 24 of 29

Figure Captions

528 529

Figure 1. Adsorption of H2O2 on (a) BOC-001, (b) BOC-001-OV, (c) BOC-010, and (d)

530

BOC-010-OV. The yellow and blue isosurfaces represent charge accumulation and depletion in the

531

space, respectively. For clarity, symmetric parts of the optimized slabs at the bottom are not shown.

532

The isovalue is 0.005 au. Dashed grey lines show the ordinary HBs.

24


Page 25 of 29


533 534

Figure 2. Schematic illustration of H2O2 dissociation processes on (a) BOC-001, (b) BOC-010, (c)

535

BOC-001-OV, and (d) BOC-010-OV. Calculated potential energy profiles for H2O2 dissociation

536

over (e) BOC-001 and (f) BOC-010 surfaces with and without the OV. Intermediate energy for H2O2

537

dissociation on the OV of BOC-001 or BOC-010 was calculated at the Oh-Oh lengthening bond

538

length around 1.70 Å.

25



Page 26 of 29

539 540

Figure 3. (a) Schematic formation of surface OVs on BiOCl and the corresponding color change

541

after thermal treatment. (b) Designed electrochemical reaction cell to record transient photocurrent

542

responses. The working electrode (WE) was prepared by dip coating BiOCl nanosheets onto the

543

FTO conductive glass. Pt foil and saturated calomel were used as the counter electrode (CE) and

544

reference electrode (RE), respectively. Ar-saturated H2O2 (0.01 mM)-Na2SO4 (0.5 M) electrolyte was

545

adjusted to pH = 6.8. (c) CV curves of the as-prepared BiOCl and bare FTO electrode. (d) EIS of

546

BOC-001-OV and BOC-010-OV with and without H2O2.

26


Page 27 of 29


547 548

Figure 4. (a) EPR spectra of spin-reactive •OH radicals produced by BOC-001-OV and

549

BOC-010-OV in the presence of H2O2. (b) Qualitative determination of •OH’s generation and

550

decaying. (c) Quantitative determination of free •OH radicals generated by BiOCl in the presence of

551

H2O2. (d) High-resolution O1s XPS spectra of BiOCl before and after H2O2 adsorption.

27



Page 28 of 29

552 553

Figure 5. (a) Kinetic constants of BOC-001-OV and BOC-010-OV for the removal of group I and

554

group II pollutants. (b) Influence of TBA and KI on the removal of benzoic acid and formic acid

555

over BOC-001-OV and BOC-010-OV. (c) Influence of pH on the formic acid removal over

556

BOC-001-OV and BOC-010-OV in the presence of H2O2. (d) Schematic illustration of different

557

H2O2 dissociation behaviors on BiOCl of OVs on the removal of benzoic acid and formic acid.

558

28


Page 29 of 29 559


TOC Art Figure

560

29


Oxygen Vacancy Associated Surface Fenton Chemistry: Surface

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