Oxygen Vacancies Mediated Complete Visible Light NO Oxidation via

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Oxygen Vacancies Mediated Complete Visible Light NO Oxidation via Side-On Bridging Superoxide Radicals Hao Li, Huan Shang, Xuemei Cao, Zhiping Yang, Zhihui Ai, and Lizhi Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01849 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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Oxygen Vacancies Mediated Complete Visible Light NO

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Oxidation via Side-On Bridging Superoxide Radicals

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Hao Li,‡ Huan Shang,‡ Xuemei Cao, Zhiping Yang, Zhihui Ai* and Lizhi Zhang*

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

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Environmental Chemistry, College of Chemistry, Central China Normal University, Wuhan 430079,

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P. R. China

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* To whom correspondence should be addressed. E-mail: [email protected];

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[email protected].

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Phone/Fax: +86-27-6786 7535

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ABSTRACT It is of a great challenge to seek for semiconductor photocatalysts with prominent

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reactivity to remove kinetically-inert dilute NO without NO2 emission. In this study, complete

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visible light NO oxidation mediated by O2 is achieved over a defect-engineered BiOCl with

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selectivity exceeding 99%. Well-designed oxygen vacancies on the prototypical (001) surface of

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BiOCl favored the possible formation of geometric-favorable superoxide radicals (•O2-) in a side-on

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bridging mode under ambient condition, which thermodynamically suppressed the terminal end-on

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•O2- associated NO2 emission in case of higher temperatures, and thus selectively oxidized NO to

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nitrate. These findings can help us to understand the intriguing surface chemistry of photocatalytic

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NO oxidation and design highly efficient NOx removal systems.

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Keywords: NO oxidation; Photocatalysis; Oxygen vacancy; Superoxide radical; Selectivity

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Introduction

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Anthropogenically-derived nitric oxide (NO), even in an infinitesimal concentration (sub-ppm or

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ppb levels), is still regarded as the major contributor to acid rain, photochemical smog, ozone

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depletion, and bears a principal responsibility for respiratory and cardiopulmonary diseases in highly

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populated cities.1 Conventionally, dilute NO can be pre-concentrated by absorbents like active

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carbon and zeolites for subsequent selective catalytic reduction (SCR) removal, while the heavy use

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of noble metals (Pt, Ru, Pd) and reducing reagents under high operating temperatures (>500 K)

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largely restrict its practical application.2 Driven by the stringent environment regulations,

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semiconductors-based photocatalysis has received considerable attention in view of its potentiality

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for efficient dilute NO removal.3,4 Unfortunately, photocatalytic NO removal still suffers from the

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toxic intermediates generation and nitrification products handling, as NO2 with higher toxicity may

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be generated. Although selective NO reduction kinetics with ammonia (2NH3 + NO + NO2 → 2N2 + ACS Paragon Plus Environment

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3H2O) will be maximized to reduce the emission of NO2 in SCR system by unifying the ratio of

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NO2/NO in a relatively closed chamber,2,5 NO2 is an extremely undesirable intermediate or product

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in open photocatalytic system. Therefore, the complete NO oxidation to nitrate during photocatalytic

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NO removal remains a great challenge, which will avoid risky NO2 emission and also provide

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potential metabolic nitrogen for micro-organisms.

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Unavoidable NO2 generation in contemporary photocatalytic NO removal systems is normally

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attributed to the presence of one-oxygen reactive species such as holes (O-) and hydroxyl radicals

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(•OH), which detrimentally lead to the partial NO oxidation (NO(g) + O-/•OH → NO2(g)). In the light

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of this fact, tuning the formation of oxygen reactive species in the molecular form to dominantly

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govern complete NO oxidation, particularly superoxide radicals (O2 + e- → •O2-; NO(g) + •O2- →

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NO3-), has been proposed previously.3,6 Unfortunately, such a •O2--based solution still remains

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debatable because some scientists found that •O2- could also cause the partial NO oxidation (NO(g)

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→ NO2(g)/NO2(aq)-).7-9 The controversy over this issue intrinsically stems from the poor

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understanding of mechanistic function schemes of •O2- for NO oxidation on the surface molecular

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level, especially the influences of specific geometric structures of •O2- on the reaction selectivity

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have never been taken into consideration.

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In this study, we demonstrate that complete NO oxidation to nitrate with selectivity exceeding

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99% can be achieved by the possible generation of side-on bridging •O2-, using a defect-engineered

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BiOCl as the model photocatalyst. BiOCl, as a characteristic UV-light-responsive oxide material,

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has received growing attention due to its inherent oxygen vacancies (OVs) associated

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photocatalysis.10,11 Well-designed catalytic OVs on its prototypical (001) surface can extend the

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photoresponse region of BiOCl to visible light, and also allow the selective and efficient activation

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of O2 to •O2- in different geometric structures possible for atomistic level investigation and practical

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application. ACS Paragon Plus Environment

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Experimental Section

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Chemicals and Materials. Synthetic materials for BiOCl and other related chemicals were bought

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from Sinopharm Chemical Reagent Co., Ltd.

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Preparation of BiOCl. Hierarchical BiOCl nanospheres were prepared according to the method

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reported by our group.12 First, we slowly added 3 mmol Bi(NO3)3·5H2O into 16 mL ethylene glycol

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solution, which contained 3 mmol KCl. Then, the mixture was kept stirring until Bi(NO3)3·5H2O was

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totally dissolved. Finally, we poured the above mixture into a Teflon-lined stainless autoclave (20

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mL). After 12 h of reaction at 160 °C, the precipitates were centrifuged and the surface residual ions

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were totally washed with deionized water and ethanol. The as-obtained powder, which was denoted

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as BiOCl-OV, was dried at 60 °C in air for further use. In order to prepare BiOCl-OV with lower

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concentration of OVs, we calcined BiOCl-OV in the air at 300 oC for different time, e. g. 30 min, 60

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min, 90 min, 120 min, and 180 min to gradually reoxidize the OVs. When the calcination time was

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lengthened to 3 h, we were able to obtain a nearly defect-free BiOCl counterpart sample. To prepare

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freestanding BiOCl nanosheets with (001) surface exposed, we added 1 mmol of Bi(NO3)3·5H2O

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into distilled water (16 mL), which contained 1 mmol KCl. pH value was then adjusted to 1 with

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using 1 M NaOH under continuous stirring. We then poured the above mixture into a Teflon-lined

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stainless autoclave (20 mL).13 After 24 h of reaction under 220 °C, resulting precipitates were

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centrifuged and the surface residual ions were totally washed with deionized water and ethanol.

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Final BiOCl powder was dried at 60 °C for further use. In order to introduce OVs on the

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freestanding BiOCl nanosheets, 0.1 g of BiOCl was uniformly dispersed in a crucible and annealed

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at 300 °C for 120 min under high vacuum with the heating rate being controlled at 5 oC/min.

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First-principles density of functional theory (DFT) calculation. Theoretical calculations were

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performed

via

DFT

+

U

calculations

(generalized

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approximation

with

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Perdew-Burke-Ernzerhof (PBE) exchange-correlation function).14 Thickness of BiOCl(001) surface

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was tested using a (1 × 1) surface, and was implemented by the CASTEP code. In this part of

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calculation, we used the ultrasoft pseudopotentials and plane-wave pseudopotential approach for all

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the atoms.15,16 Kinetic energy cutoff was set as 380 eV. During the optimization, the force and energy

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respectively converged to 0.03 eV/Å and 10-5 eV/atom.17 For subsequent NO or O2 adsorption

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calculation, a (2 × 2) supercell with a 20 Å vacuum was further used. This part of calculation was

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enabled by a VASP code with a kinetic energy cutoff of 520 eV.18,19 Location of the transition states

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(TS) was determined by nudged elastic band (NEB) method.20,21 We calculated the adsorption energy

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was calculated as via the following equation: ∆E = E(BiOCl + adsorbate) - E(BiOCl) - E(adsorbate).

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The E(adsorbate) and E(BiOCl) are respectively the energies of the two isolated (noninteracting)

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subsystems, while E(BiOCl + adsorbate) is total energy of the interacting adsorbate-surface system.

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We depicted charge density difference via: ∆ρ = ρ(BiOCl + adsorbate) - ρ(adsorbate) - ρ(BiOCl).

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ρ(adsorbate) and ρ(BiOCl) are respectively the densities of the two noninteracting subsystems, while

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ρ(BiOCl + adsorbate) is the density of the interacting adsorbate-surface system.

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Materials characterization. Phase structure of the as-prepared BiOCl was determined by the

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powder X-ray diffraction (XRD) measurements (Rigaku D/MAX-RB diffractometer, Cu Ka

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radiation, Λ= 0.15418 nm). Surface morphology was recorded via a field-emission scanning electron

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microscope (SEM, JEOL 6700-F) and transmission electron microscopy (TEM, JEOL JSM-2010).

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Light absorption capacity of BiOCl was evaluated via a UV-visible spectrophotometer (UV-2550,

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Shimadzu, Japan). To observe in situ catalytic NO oxidation, surface functional group change was

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recorded with a diffuse reflectance FTIR spectra (Nicolet iS50FT-IR). Concentration of

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photoinduced •O2- and OVs were quantitatively determined via electron paramagnetic resonance

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(EPR, Billerica, MA) using Cu2+ as the calibrator. NO and O2 adsorption on BiOCl were evaluated

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via temperature-programmed desorption experiments (TPD) in a quartz reactor using a TCD as ACS Paragon Plus Environment

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detector. NO2-TPD was analyzed with a flow reactor connected to a mass spectrometer (UTI 100C).

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Nitrate was measured by Thermo Scientific Dionex ionic chromatography (ICS-900). Valence state

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of Bi was analyzed with X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C).

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Photocatalytic activity test. We prepared BiOCl or TiO2 films by carefully coating the aqueous

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suspension of the corresponding photocatalysts onto a glass dish (diameter = 12 cm). A continuous

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flow reactor was adopted to simulate practical NO removal at ppb levels under ambient conditions.

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Volume of the rectangular reactor (30 cm × 15 cm × 10 cm [L × W × H]) was around 4.5 L. For

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photocatalytic NO removal over BiOCl, a Xenon lamp (λ > 400 nm) or UV light (λ ~ 360 nm) was

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used. A 480 nm monochromatic light (20 mW/cm2) was used as the light source for photocatalytic

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NO removal over TiO2-P25. NO gas was provided from a compressed gas cylinder with a

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concentration around 50 ppm balanced by Ar, which was further diluted to 500 ppb by an air stream.

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A gas blender was used to mix the gas streams, simultaneously controlling the flow rate at 1 L/min.

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To ensure the sufficient absorption of light by the as-prepared films, the lamp was placed vertically

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above the films, which sat in the middle of the reactor. After adsorption-desorption equilibrium were

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reached, the light source was turned on to trigger the photocatalytic reactions. The concentration of

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NO and NO2 were monitored by a chemiluminescence NOx (the sum of NO and NO2) analyzer

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(Teledyne, NOx analyzer, model T200).

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Results and Discussion

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DFT calculation revealed fully oxidized BiOCl(001) surface was quite inert towards NO or O2

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adsorption, while the introduction of an OV with localized electrons remarkably enhanced its surface

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reactivity. According to the adsorption energies and activated bond lengths, O2 in either terminal

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end-on or side-on bridging mode interacted more strongly with the OV as compared with NO, which

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is reasonable as O2 of lower 2π* orbital energy is a better electron-acceptor than NO (Figure S1). So, ACS Paragon Plus Environment

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aerobic catalytic NO removal, especially in case of extremely low concentration of NO, is supposed

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to take place via an Eley-Rideal mechanism, through which NO will attack the •O2- preferentially

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adsorbed on OVs.22 A broader DFT study was then performed to explore the possible NO oxidation

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pathways and intermediate species initiated by surface •O2- in different geometries. When NO

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approached the terminal end-on •O2-, it was oxidized to peroxynitrite (OONO-), simultaneously

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releasing 1.32 eV energy (pathway I). Peroxynitrite is known to be a powerful and toxic oxidant that

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causes the direct biotoxicity of NO when NO acts as an intercellular messenger in vivo, and also a

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transient precursor towards the NO2 formation on metal or metal oxide surfaces.23,24 As expected,

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breaking of the O-O bond in peroxynitrite led to the release of gaseous NO2 with a barrier of 0.49 eV

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(Figure 1a).

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Figure 1. (a) Free energy change against the reaction coordinate for the oxidation of NO by •O2- on

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BiOCl(001) surface in different geometries. (b) Geometric transition from peroxynitrite to nitrate.

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TS represents transition state. Charge density difference and O2 partial DOS of the BiOCl(001) ACS Paragon Plus Environment

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surface adsorbed with (c) O2 and (d) nitrate. The yellow and blue isosurfaces with an isovalue of

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0.005 a.u. represent charge accumulation and depletion in the space. The vertical dashed line in the

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DOS shows the VBM.

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Differently, side-on bridging •O2- could directly oxidize NO into monodentate nitrate without

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any barrier (pathway II, ∆E = -3.10 eV), which was far more thermodynamically accessible than

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pathway I. Abstracting a proton from a neighboring hydroxyl or adsorbed water by nitrate towards

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the formation of HNO3* was also favorable with an energy release around 1.64 eV. Moreover, energy

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expenditure towards the diffusion of nitrate or HNO3 on the surface was only 0.20 eV, much smaller

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than that (0.49 eV) of NO2 desorption (Figure 1a). Since geometric transformation from

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peroxynitrite to its structural isomer of nitrate was up against a high barrier of 1.03 eV, NO oxidation

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via pathway I or II was rather independent (Figure 1b). Such a theoretical scenario can explain the

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controversy over the mechanistic roles of •O2- on the NO oxidation. Obviously, the side-on bridging

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•O2- (•O2-B) mediated complete NO oxidation (pathway II: •O2-B + NO(g) → NO3-) would be prior to

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the terminal end-on one (•O2-T) mediated partial NO oxidation (pathway I: •O2-T + NO(g) → OONO-

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→ NO2) according to the reaction thermodynamics.

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To uncover the remarkably promoted thermodynamics of NO oxidation by side-on bridging •O2-,

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charge density difference calculation was adsopted to trace the interfacial charge transfer. After O2

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was adsorbed on the OV of BiOCl(001) surface, instant charge back donation from the OV to O2

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occurred, which was depicted by the localized electrons depletion on two Bi atoms around the OV

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and electrons accumulation on the coordinating O2 (Figure 1c). Partial density of states (DOS) of O2

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contained two occupied spin-up majority states and one occupied spin-down minority state below

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the Fermi energy, as well as an empty spin-down minority states near the conduction band, revealing

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the filling of one O2 2π* orbital toward the •O2- formation (Figure 1c).25 Along with the nitrate

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formation, •O2- was then found to interact rigorously with NO according to a significant charge ACS Paragon Plus Environment

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depletion of N atom and an extraordinary electron gaining by three O atoms, manifesting an

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outstanding oxidative ability (Figure 1d). Meanwhile, 2π orbitals of •O2- underwent a considerable

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broadening and were shifted below the valence band maximum (VBM) of BiOCl, indicating the

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complete NO oxidation by side-on bridging •O2- (Figure 1d).22,26,27

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Figure 2. (a) Backscattered scanning electron microscopy image of BiOCl nanospheres and (b) their

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Barrett-Joyner-Halenda pore-size distribution plot. (c) NO-TPD and (d) O2-TPD profiles of the

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BiOCl photocatalysts. (e) EPR spectra of BiOCl-OV under different conditions.

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Motivated by the theoretical calculation results, we then synthesized specific hierarchical BiOCl

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nanospheres assembled by (001) surface exposed nanosheets containing OVs (BiOCl-OV) (Figure

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2a and S2). The presence and the relative concentration of OVs were estimated by XPS (Figure S3).

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The hierarchical nanostructured morphology with small mesopores (2~10 nm), which are beneficial ACS Paragon Plus Environment

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for adsorbates/products diffusion and transportation in many energy and environmental applications,

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offers the possibility of nitrate diffusion and storage for long term NO removal (Figure 2b and

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S4).28,29 To experimentally verify whether BiOCl-OV could achieve complete NO oxidation

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mediated by side-on bridging •O2- on (001) surface, a series of techniques were performed to in situ

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characterize the surface chemistry under reaction conditions. Temperature-programmed desorption

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(TPD) was first used to detect NO and O2 adsorption on BiOCl-OV. NO-TPD spectra of BiOCl-OV

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was dominated by two desorption at 455 K and 502 K (Figure 2c). Since hydroxyl groups and lattice

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O atoms on clean BiOCl(001) surface were inert for direct oxidation of NO to nitrite or nitrate, these

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two peaks were therefore ascribed to the chemical adsorption of NO on OV with *O or *N-end. As

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for O2-TPD, we observed a wide molecular O2 desorption peak from 450 K to 700 K over

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BiOCl-OV that surface side-on bridging •O2- were supposed to fall into this region (Figure 2d).

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Judging from the temperature and area of the desorption peaks, O2 was supposed to be adsorbed

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preferentially on the OVs as compared with NO, consistent with our DFT calculations. Spin-reactive

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•O2- were then monitored by room-temperature (298 K) electron paramagnetic resonance (EPR). As

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compared with BiOCl, BiOCl-OV possessed a typical signal at g = 2.001, corresponding to the

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inherent OVs (Figure 2e). Irradiation of BiOCl-OV with visible light in O2 atmosphere led to the

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formation of another anisotropic EPR peak with a g-tensor of 2.005, being ascribed to the

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photoinduced •O2- (Figure 2e).7 Such an •O2- EPR peak was not observed for BiOCl (Figure 2e).

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Interestingly, on exposure to NO gas, intensity of the EPR signal for •O2- completely disappeared,

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suggesting the intimate interaction between •O2- and NO (Figure 2e).

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Figure 3. (a) Schematic illustration of the reaction cell for the in situ FTIR study. (b) FTIR spectra

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of BiOCl-OV during photocatalytic NO oxidation. (c) Dynamic change of the •O2-/NO3- absorbance

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increase along with NO absorbance peak decrease. (d) NO2-TPD profiles of BiOCl-OV under

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different reaction conditions

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Diffuse reflectance FTIR spectroscopy was further employed to check the possible reaction

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intermediates and/or products on the BiOCl-OV in a special reaction cell (Figure 3a and Figure S5).

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In the absence of O2, NO at trace concentration in the vacuum cell gave two distinct bands at 1091

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cm-1 and 1630 cm-1 on the BiOCl-OV at 298 K in the dark (Figure 3b). The former band was

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possibly arisen from the negatively-charged OV-NO species, and the latter broad band was due to

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surface undissociated water, as specifically demonstrated by time-dependent in situ NO adsorption

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on BiOCl in the dark (Figure S6). After sufficient O2 was pumped in, visible light illumination was ACS Paragon Plus Environment

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then introduced to trigger photocatalytic reactions. Significant changes of FTIR spectra were vividly

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observed. First, the band at 1091 cm-1 gradually decreased and became negative, accompanied by the

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progressive increase of the band at 1003 cm-1 (Figure 3b). This band at 1003 cm-1 could be attributed

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to the O-O stretching mode of side-on bridging O2- species, suggesting the rapid replacement of NO

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by O2 on the OVs. Second, replaced NO was immediately converted to other nitrogen species

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(Figure 3b). Sharp band at 1274 cm-1 was assigned to the ν3´´(split) mode of nitrate in a particular

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monodentate state and the broad band at 1477 cm-1 was ascribed to the almost symmetrical surface

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nitrate.30 The wide band from 1600 to 1800 cm-1 was typical of combination band associated with

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NO and free nitrate.31 Noticeably, IR absorbance intensity of •O2- and NO3- species linearly

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increased along with that of NO decrease, indicating that the oxidation of NO to nitrate was directly

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mediated with photoinduced •O2- (Figure 3c). TPD was then employed to more precisely

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differentiate the surface nitrogen species. 18O-labelled O2 was used to avoid the possible interference

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of lattice O or pre-adsorbed O2 on NO oxidation. For photocatalytic NO oxidation under 298 K,

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BiOCl-OV surface exhibited a recognizable NO2 desorption peak at 548 K with m/z of 48,

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suggesting one the O atoms in NO2 was 18O-labelled (Figure 3d). Since the chemical adsorption of

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NO2 on BiOCl-OV corresponded to a desorption peak at remarkably lower temperature of 438 K, the

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NO2 desorption peak at 545 K was therefore arisen from 18O-labelled nitrate thermal decomposition

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on BiOCl-OV surface (16O=N18O18O- → N16O18O) (Figure 3d).32 This observation was in consistent

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with the FTIR result that the selective product of photocatalytic NO oxidation under ambient

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condition was nitrate, not gaseous NO2 or surface NO2-, directly evidencing that complete NO

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oxidation toward the formation of nitrate was the major pathway on BiOCl-OV via

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geometric-favorable side-on bridging •O2-. It was interesting to note that NO2 desorption peak

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around 438 K appeared during the photocatalytic NO oxidation at 335 K and 355 K, suggesting the

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formation of chemically-adsorbed NO2 at higher reaction temperatures (Figure 3d). Apparently, the ACS Paragon Plus Environment

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appearance of chemically-adsorbed NO2 was associated with a coexisted NO oxidation pathway I,

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governed by terminal end-on •O2- according to the DFT calculations. This is reasonable because the

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barrier of the O-O bond breaking in peroxynitrite for gaseous NO2 desorption via pathway I is only

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0.49 eV, which could be overcame via thermal heating (Figure 1a). However, this terminal end-on

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•O2- associated partial NO oxidation could be largely inhibited by the side-on bridging •O2- mediated

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complete NO oxidation by performing the reaction at room temperature. We believe that the

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proposed NO removal pathway, mainly supported by theoretical calculation and in situ TPD

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measurements, will still require further experimental validation with sophisticated in situ

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characterization techniques, including XPS and scanning tunneling microscope.

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Figure 4. (a) Photocatalytic NO removal over the as-prepared BiOCl under visible light. (b)

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Influence of generated •O2- on the NO oxidation kinetic constants and the NO2 concentration. (c)

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Schematic illustration of photocatalytic NO removal on BiOCl-OV. (d) Transient photocatalytic NO ACS Paragon Plus Environment

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removal on BiOCl-OV.

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To confirm the above results and discussions, we thus employed the as-prepared BiOCl-OV film

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to remove dilute NO (500 ppb) in a continuous flow reactor at 298 K (Figure S7). Defect-free BiOCl

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could not remove NO efficiently under visible light irradiation (λ > 400 nm). As for BiOCl-OV, we

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observed a fast drop of NO concentration in 5 min, followed by a slower decrease with up to 70%

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NO removal efficiency being achieved within 15 min (Figure 4a). As expected, only 4 ppb NO2 was

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on-line detected, much lower than the regulation standard (0.24 ppm, GB/T18883-2002) of indoor

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NO2 and also suggesting most of NO was directly oxidized to nitrate with selectivity exceeding 99%.

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This high selectivity towards the nitrate formation was further verified by calculating the nitrogen

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balance via ion chromatography (Figure S8). Photocatalytic NO removal rates were linearly related

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with •O2- in different concentrations without generating detectable NO2 (Figure 4b). Together with

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reactive species trapping experiment, we further proposed that photocatalytic NO oxidation to nitrate

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was directly related to •O2- generated via BiOCl intraband excitation, through which both holes (O-)

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and •OH could be avoided (Figure 4c, S9 and S10). Interestingly, for commercial TiO2-P25,

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photocatalytic NO2 emission could also be inhibited after the introduction of OVs (Figure S11 and

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S12). We believe the dominant reactive surfaces, including the rutile(110) surface and anatase(101)

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surface, which allow the formation of side-on bridging reactive oxygen species, are responsible for

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promoted complete NO oxidation (Figure S13).27,28,33,34 Meanwhile, along with reaction temperature

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increase from 298 K to 375 K, the selectivity for photocatalytic NO2 generation over BiOCl-OV was

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gradually increased from 0.8% to 22.6%, agreeing well with the TPD results (Figure S14). In

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comparison with the poor long-term stability of free-standing BiOCl-OV single crystalline

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nanosheets with limited surface area and hierarchicality (Figure 4d and S15), BiOCl-OV

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nanospheres were stable for continuous NO removal, highlighting the beneficial effects of ACS Paragon Plus Environment

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mesopores for nitrate diffusion and storage (Figure S3). XPS analysis revealed Bi also maintained

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most of its valence state after repeated use. Although the area percentage of Bi(3−x)+ XPS peaks at

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163.8 and 158.4 eV of BiOCl-OV was slightly decreased from 23.5% to 20.3% (Figure S16a and

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S16b), this slight decrease of OVs could be easily recovered via thermal annealing under high

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vacuum at 250 oC for 10 min (Figure S16c).35 The stored nitrate after hours of use could be easily

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washed down, exhibiting a nitrate storage capacity of 7.3 × 10-3 mol g-1 h-1, much larger than that of

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active carbon (0.10 × 10-3 mol g-1 h-1) under the same condition.

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Environmental Implications.

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Complete removal of dilute NO without NO2 emission is a great challenge. Aiming to avoid partial

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NO oxidation and gain mechanistic insight into the intrinsic catalytic roles of •O2-, we have utilized a

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defect-engineered BiOCl to realize the complete oxidation of NO under visible light. It was

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demonstrated that well-designed catalytic OVs on the prototypical (001) surface of BiOCl favored

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the possible formation of geometric-favorable •O2- in a side-on bridging mode under ambient

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conditions, which thermodynamically suppressed the terminal end-on •O2- associated NO2 emission

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in case of higher temperatures, and thus selectively oxidized NO to nitrate. These findings will

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provide the instructive information on exploring the intriguing surface chemistry of photocatalytic

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NO oxidation and developing highly efficient NOx removal systems.

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AUTHOR INFORMATION

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Corresponding Author

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*Phone/Fax: +86-27-6786 7535; [email protected]; [email protected]

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Author Contributions

312



H.L. and H. S. contributed equally to this work. ACS Paragon Plus Environment

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Notes

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The authors declare no competing financial interest

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Acknowledgements: This work was supported by The National Key Research and Development

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Program of China (2016YFA0203000), National Natural Science Funds for Distinguished Young

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Scholars (21425728), National Science Foundation of China (21477044 and 51472100), 111 Project

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(B17019), Self-Determined Research Funds of CCNU from the Colleges’ Basic Research and

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Operation of MOE (CCNU16A02029). We also thank the National Supercomputer Center in Jinan

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for providing high performance computation.

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ASSOCIATED CONTENT

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Supporting Information: NO and O2 adsorption model on prototypical BiOCl(001) surface;

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reactive species trapping experiment; organic pollutants degradation profiles; photocatalytic NO

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removal over TiO2 with OVs; surface area and charge of the catalysts; influence of temperatures on

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photocatalytic NO removal; characterization of free-standing BiOCl single-crystalline nanosheets

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(PDF)

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References

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