Oxygen Vacancies Promoted Selective Photocatalytic Removal of NO

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Oxygen Vacancies Promoted Selective Photocatalytic Removal of NO with Blue TiO2 via Simultaneous Molecular Oxygen Activation and Photogenerated Holes Annihilation Huan Shang, Meiqi Li, Hao Li, Shun Huang, Chengliang Mao, Zhihui Ai, and Lizhi Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b07322 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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Environmental Science & Technology

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Oxygen Vacancies Promoted Selective Photocatalytic Removal of

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NO with Blue TiO2 via Simultaneous Molecular Oxygen

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Activation and Photogenerated Holes Annihilation

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Huan Shang, Meiqi Li, Hao Li, Shun Huang, Chengliang Mao, 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 & Applied Chemistry, College of Chemistry, Central China Normal University,

9

Wuhan 430079, People’s Republic of China

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

12

[email protected].

13

Phone/Fax: +86-27-6786 7535

14 15 16 17 18 19 20 21 22

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ABSTRACT Semiconductor photocatalytic technology has great potential for removal of dilute

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gaseous NO in indoor and outdoor atmosphere, but suffers from the unsatisfactory NO removal

25

selectivity due to the undesirable NO2 byproducts generation. In this study, we demonstrate that 99%

26

selectivity of photocatalytic NO oxidation towards nitrate can be achieved over blue TiO2 bearing

27

oxygen vacancies (OVs) under visible light irradiation. First-principles density functional theory

28

(DFT) calculation and experimental results suggested that OVs of blue TiO2 with localized electrons

29

could facilitate the molecular oxygen activation through single-electron pathway to generate •O2-,

30

and simultaneously promote the photogenerated holes annihilation. The generated •O2- directly

31

converted NO to nitrate, while the holes annihilation inhibited the side-reaction between holes and

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NO to avoid toxic NO2 by-product formation, resulting in highly selective removal of NO. This

33

study reveals the dual functions of OVs in defective photocatalyst, and also provides fundamental

34

guidance for selective purification of NO with photocatalytic technology.

35 36

Keywords: Photocatalytic NO removal; Oxygen vacancies; Molecular oxygen activation; Hole

37

annihilation; Selectivity

38 39

Introduction

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Nitric oxide (NO), derived from the fossil fuel combustion and automotive exhaust emission, is

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deemed as a major contributor to acid rain, haze and photochemical smog, harming human health

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with a risk for respiratory and cardiopulmonary diseases.1, 2 Conventionally, industrial techniques,

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such as adsorption, selective catalytic oxidation and thermal catalysis, have been applied to eliminate

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gaseous NO at high concentrations (parts per million levels, above 500 ppm) from industrial 2 ACS Paragon Plus Environment

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emission sources. However, these strategies, which were usually performed under the condition of

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high temperatures and/or pressures with using noble metals (Ru, Pd, Pt) and/or reducing reagents,3-5

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are unfeasible for the elimination of ppb-level NO in indoor and urban environments in the

48

economic and technological perspectives.

49

Photocatalysis technology using solar energy has exhibited great potential for efficient removal of

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dilute NO in an economically attractive and environmentally friendly manner.6-8 Unfortunately, in

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addition to oxidizing NO to nitrate, ordinary photocatalysts would trigger large amounts of

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undesirable NO2 during photocatalytic NO removal.9, 10 For instance, the selectivity of NO oxidation

53

towards nitrate was just 64% over g-C3N4-TiO2, implying that about 40% of NO was converted to

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more toxic NO2 byproduct.11 Therefore, it is still a challenge to completely transform NO to nitrate

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with photocatalyst and solar light.

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Generally, aerobic semiconductor photocatalytic NO removal is mediated by photogenerated

57

electron and hole.8, 12-14 The photogenerated electrons are captured by molecular oxygen to generate

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various reactive oxygen species (ROS) including hydrogen peroxide (H2O2), superoxide radical

59

(•O2- ) and hydroxyl radical (•OH), which react with NO via different pathways. Among them, •O2-

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is able to completely transform NO to the final nitrate (NO + •O2- → NO3-). For instance, our group

61

demonstrated that side-on •O2- of BiOCl with oxygen vacancy enabled to convert, selectively, NO

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into nitrate.13 Nevertheless, •OH would oxidize NO to the undesirable NO2 (NO + 2•OH → NO2 +

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H2O).14 Meanwhile, photogenerated holes, similar to electron-deficient molecular oxygen (O-),

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might also contribute to the NO2 byproduct generation (h+ + O2- → O-, NO + O- → NO2-).12-14

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Therefore, we are able to realize complete NO oxidation without NO2 emission by promoting the

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•O2- generation and inhibiting the reactions between photogenerated holes and NO as well as 3 ACS Paragon Plus Environment


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suppressing the •OH production during photocatalysis.

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Oxygen vacancy mediated photocatalysis has been proved to be capable of tuning active species

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in photocatalytic NO oxidation. For example, oxygen vacancy was reported to regulate the localized

70

electron behavior to favor the •O2- generation for the selective NO removal.13-16 However, the

71

interaction between oxygen vacancy and photogenerated holes still remains unexplored, although

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photogenerated holes may trigger the formation of NO2. Normally, photogenerated holes can be

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eliminated by the addition of sacrificial agents such as methanol during photocatalytic reaction,

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which is not feasible for the gas/solid interface reaction.17, 18 As photogenerated holes are essentially

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electron-deficient center,17, 18 while OVs are rich in localized electrons, photogenerated holes might

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be recombined by the localized electrons of OVs to suppress the reaction between NO and holes,

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thus realizing the selective transformation NO to the final nitrate without the NO2 production.

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Herein, we report that blue TiO2 possessing abundant OVs is capable of photocatalytic NO

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removal with high efficiency and selectivity towards nitrate under visible light irradiation. The roles

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of OVs during photocatalytic NO oxidation are first investigated with DFT calculations. Then the

81

structure of blue TiO2 is carefully characterized with XRD, TEM, Raman, XPS and EPR spectra.

82

Finally, a mechanism of OVs-mediated photocatalytic NO oxidation is proposed based on the

83

theoretical and experimental results.

84 85

Experimental Section

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Chemicals and Materials. The materials for blue TiO2 synthesis and other related chemicals were

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all analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd, China. And all

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chemicals employed in this work were not further purified before use. 4 ACS Paragon Plus Environment

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Preparation of Blue TiO2. Blue TiO2 was prepared by chemical reduction with NaBH4, a modified

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method as previous schemes reported.19 For the preparation of blue TiO2, 4 g TiO2 nanoparticle

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powder (Degussa P25) was completely mixed with 2 g NaBH4 by mortar. Then the white mixture

92

was transferred into a porcelain crucible with cap, annealed to 350 °C and maintained for 1h. After

93

naturally cooling down to room temperature in air, the obtained blue sample was washed with

94

deionized water and ethanol at least three times to remove unreacted NaBH4, then dried at 70 °C.

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The blue TiO2 was denoted as TiO2-OV. And the perfect TiO2 counterpart without OVs was the

96

widely used commercial P25-TiO2, denoted as TiO2. The blue TiO2 counterpart was prepared by

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Ultra-high vacuum (UHV) method without the introduction of hydrogen source.20 Typically, 0.2 g

98

TiO2 was homogeneously scattered in a porcelain crucible and then annealed in the vacuum

99

condition at 400 °C for 2h, the resultant TiO2 sample was denoted as TiO2-OV-Vacuum.

100

First-principles Density of Functional Theory (DFT) Calculation. All molecular simulation

101

calculations were carried out using the DFT + U with the GGA-PBE exchange-correlation energy

102

functional, as implemented in the Vienna Ab-initio Simulation Package (VASP) using the projector

103

augmented-wave (PAW) method.21-23 The valence electronic states were described by wave

104

functions which expanded in plane wave basis sets with an energy cutoff of 400 eV and the first

105

principles theoretical value (U) for Ti3d was 3.5 eV.24-26 In the geometrical optimization, the

106

tolerances of energy and force convergence were 10-5 eV/atom and 0.02 eV/Å, respectively. The

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corresponding k-points of the TiO2(101) was 3×3×1.27 The vacuum for all calculated models was 20

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Å. In the simulation for NO or O2 adsorption on the TiO2 surface, the adsorption energy was defined

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as ΔEad (ΔEad = E(TiO2-mol) - E(TiO2) - E(mol)), while the reaction energy was defined as ΔE (ΔE

110

= Efin - Eini). To trace the electron behavior, the corresponding charge density difference of the 5 ACS Paragon Plus Environment


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system was conducted as: ∆ρ = ρ(TiO2-mol) - ρ(TiO2) -ρ(mol), where ρ(TiO2-mol) is the density of

112

the adsorption structure, whereas ρ(TiO2) and ρ(mol) are the charge density of the interacting surface

113

and adsorbate respectively.

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Materials Characterization. The phases of the as-prepared TiO2 were obtained with the powder

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X-ray diffraction operating with monochromatized Cu Kα radiation (XRD, Rigaku D/MAX-RB

116

diffractometer, λ = 0.15418 nm). The surface morphology of as-prepared TiO2 was analyzed by the

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Hitachi H-7650 transmission electron microscopy (TEM) and JEOL-2010FEF high-resolution

118

transmission electron microscopy (HRTEM). The as-prepared TiO2 Raman spectra were analyzed by

119

Thermo DXR Microscope spectrometer with 532 nm laser irradiation. UV-visible diffused

120

reflectance spectra of as-prepared TiO2 were recorded on the UV-2550 spectrophotometer

121

(Shimadzu, Japan). X-ray photoelectron spectroscopy of as-prepared TiO2 (XPS, Thermo Scientific

122

Escalab 250 Xi) was applied to evaluate surface electronic states. Electron paramagnetic resonance

123

(EPR) spectra (Bruker E500, Billerica MA) were conducted to quantitatively determine OVs of

124

samples and the formation of •O2- adsorbed on solid samples at 120 K and quantitatively detect the

125

concentration

126

(5,5-dimethyl-1-pyrroline-N-oxide, DMPO). The settings for the EPR spectrum were: modulation

127

frequency 100 kHz, modulation amplitude 2 G, microwave frequency about 9.065 GHz, power 1

128

mW. The NO and O2 temperature-programmed desorption (TPD) using the TCD as detector were

129

performed in a quartz reactor to evaluate the gas molecules adsorption on as-prepared TiO2 surface.

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The diffuse reflectance FTIR spectroscopy measurements (Nicolet iS50, Thermo) was applied to

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monitor the time-dependent generation of intermediates and products on sample surfaces within the

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in situ catalytic NO oxidation. Photoluminescence spectra and decay curves were measured using the

of

•O2-

and

•OH

with

using

a

radicals

spin-trapped

reagent


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Edinburgh FLSP 920 fluorescence spectrometer. Solid state 1H Nuclear Magnetic Resonance spectra

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(NMR, Bruker Avance Ⅲ 500WB spectrometer) were employed to record the surface H in samples

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using Magic Angle Spinning direct polarization. Thermo Scientific Dionex ionic chromatography

136

(ICS-900) was employed to measure nitrate products.

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Photocatalytic NO Removal. The photocatalytic NO removal performance of as-prepared TiO2 was

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measured in a continuous flow reactor (4.5 L) at ambient temperature (298 K). In an experimental

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system, 0.15 g sample was ultrasonically dispersed into 10 mL of H2O, and put the aqueous

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suspension on the glass dish (diameter = 10 cm). The dish with coated sample was heated at 80 °C

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for water evaporation to obtain a catalyst film. A Xenon lamp (PLS-SXE300, Perfect Light, China, λ

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> 400 nm) was served as a visible light source in photocatalytic NO removal. The experimental NO

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gas (600 ppb) was obtained by diluting the compressed NO gas cylinder (100 ppm, Ar balance) with

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air stream. When NO and O2 in reactor reached adsorption-desorption equilibrium, the lamp was

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turned on immediately. The real-time concentration of NOx (NO and NO2) was recorded online by

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the Teledyne NOX analyzer (Model T200), which controls gas flow rate of 1 L/min.

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The photocatalytic NO removal efficiency (η) and selectivity (S) were calculated as Eqs. 1-2:

148

(1)

149

(2)

150

Where [NO]fin and [NO2]fin were the final concentrations of NO and NO2 respectively when the

151

lamp was turned on. [NO]ini and [NO2]ini were the initial concentration of NO and NO2 respectively,

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while the adsorption-desorption equilibrium is reached without turning on the lamp.

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Photoelectrochemical Measurement. The photoelectrochemical (PEC) performance was performed

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by a CHI660B electrochemical work station with a typical three-electrode process in a 0.5 M 7 ACS Paragon Plus Environment


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Na2SO4 aqueous electrolyte. The TiO2 as the working anode was prepared by following the

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procedure: 10 mg catalyst was dispersing in 4% naphthol solution and the obtained slurry was

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deposited onto the FTO glass under the irradiation of infrared lamp. The counter electrode and

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reference electrode in this measurement are platinum foil and saturated calomel electrode (SCE)

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respectively. All PEC measurements use a Xenon lamp (PLS-SXE300, Perfect Light, China, λ > 400

160

nm) as a visible light source.

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Results and Discussion

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Theoretical Calculation Analysis. To elucidate the electronic interaction schemes of OVs for the

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aerobic NO removal, at first, we theoretically modelled the adsorption and reaction of NO and O2 on

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the TiO2 (101) because the common anatase TiO2 catalysts are typically exposed with the

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thermodynamically stable (101) facets. Judging from the adsorption energy and the charge transfer,

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OVs with localized electrons can act as adsorption and activation sites for NO and O2, however, for

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a perfect (101) surface, the molecular oxygen would be weakly adsorbed on Ti5c sites on the (101)

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surface. And corresponding Ti-O bond length of TiO2 is 2.65 Å, which is much longer than that of

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TiO2-OV (1.92 Å) (Figure S1, Figure S2, Table S1).28, 29 We noticed that NO could be absorbed on

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OVs by bonding with two nearest Ti atoms (-2.07 eV, with 0.8 e charge transfer), in the bond length

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of 2.15 Å, shorter than the corresponding bond length on the perfect (101) surface (2.78 Å).

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Compared with NO, O2 could attach to OVs more tightly with releasing 2.84 eV energy, performing

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a •O2- activation state with the charge transfer by 1.1 e. We therefore deduced that O2 was

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preferentially adsorbed on OVs and then obtained electrons from the OVs to generate the •O2-,

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which directly oxidized gaseous NO to nitrate (NO3-) with releasing around 1.85 eV energy

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simultaneously.30 The generated NO3- would get a proton from surface hydrogen (adjoining 8 ACS Paragon Plus Environment

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hydroxyl and/or adsorbed water) on the surface of TiO2 to form an adjoining HNO3 with releasing

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1.55 eV energy. This released energy can sustain the energy requirement of NO3-/HNO3

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desorption/migration from the reactive center (0.82 eV energy), the continuous desorption/migration

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implied the high stability of the TiO2(101) defective surface during the photocatalytic NO removal

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process.

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As shown in Figure 1a, theoretical DFT calculation confirmed the interaction of NO and OVs

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benefited the nitrate formation on TiO2(101) defective surface.31 Charge density difference

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calculation suggested the bonding O2 could accumulate electrons from the localized electrons of two

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unsaturated Ti atoms near the OVs of TiO2 surface in aerobic condition, simultaneously O2 was

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activated to •O2- via single-electron transfer pathway (Figure 1b). The generated •O2- would react

187

with NO accompanied with a remarkable charge depletion of N atom and a dramatic charge

188

accumulation of three O atoms, consistent with the previous Bader charge analysis (Figure 1b, Table

189

S1). This results suggested the OVs of TiO2(101) defective surface benefited molecular oxygen

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activation and the final nitrate production via efficient electron transfer in the photocatalytic NO

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removal. Unfortunately, the DFT calculation could not accurately predict the photogenerated holes

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behavior, which will be further discussed in the subsequent sections.

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Materials Characterization. As uncovered by XRD patterns and TEM images, the presence or

194

absence oxygen vacancy did not change the crystal structure and morphology of TiO2 catalyst

195

(Figure 2a and S3). The TiO2-OV and pristine TiO2 matched well to anatase TiO2 [JCPDS file no.

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01-562] with a weak peak for rutile TiO2 [JCPDS file no. 01-1292]. The clear lattice planes of both

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samples with a distance of 0.35 nm corresponded to the mainly exposed anatase TiO2(101) atomic

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planes, as revealed by HRTEM images (Figure 2b and 2c).19, 32-34 The Raman spectra indicated the 9 ACS Paragon Plus Environment


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peaks of pristine TiO2 were in good agreement with the previous report.34 Compared with TiO2, the

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peak intensity of TiO2-OV became weaker along with the peak at 149 cm-1 blue shifted slightly

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(Figure 2d). This phenomenon might result from the surface change by superimposed OVs in the

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blue TiO2.20, 34

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EPR spectra were used to verify the introduction of oxygen vacancy of blue TiO2. The

204

symmetrical signal at g = 1.998 was observed in TiO2-OV, while this signal was not found in the

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TiO2 counterpart (Figure 2e). It is generally assumed that signal at 1.94-1.99 was attributed to Ti3+

206

and 2.004 was attributed to oxygen vacancy.20, 35, 36 Since the signal for TiO2-OV is too close to the

207

two signals, it can be a superposition of two signals anisotropy, suggesting that both oxygen vacancy

208

and Ti3+ exist in the TiO2-OV catalyst. In the process of oxygen vacancy formation, part of the Ti4+

209

may be reduced by the electrons within the oxygen vacancy to produce Ti3+, resulting in the

210

existence of both signals.37, 38 Moreover, an extra XPS peak locating at 457.7 eV assigned to the

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peak of Ti3+ was observed in the TiO2-OV Ti 2p spectra (Figure S4), in good agreement with the

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EPR results.39 Additionally, no B and Na residue signal was observed in the full XPS survey of the

213

TiO2-OV, suggesting the reduction method only introduced oxygen vacancy in the sample (Figure

214

S4). As expected, TiO2-OV sample was blue, demonstrating that the introduction of OVs was

215

supposed to induce indirect excitation to modify the optical absorption of TiO2 (Figure 2f and S5).

216

Photocatalytic NO Removal. The NO removal over TiO2-OV significantly increased to 63% with

217

enhancement of 15% as compared with the pristine TiO2 (48%) under visible light irradiation. Apart

218

from the effect of BET specific surface area, the first-order rate constant was improved by 2 times

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with the introduction of OVs (Figure 3a and S6). Most importantly, the toxic NO2 byproduct was

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dramatically decreased to a negligible concentration (approximately 1 ppb), indicating that the 10 ACS Paragon Plus Environment

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selectivity of NO towards nitrate reached an unparalleled high level 99% over the blue TiO2 (Figure

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3b). In addition, TiO2-OV preserved excellent stability with negligible deactivation in photocatalytic

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performance (its activity maintains above 60% in cycles) and the corresponding selectivity of NO

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kept 98% within five consecutive cycles (Figure 3c, 3d and S7). This selectivity of photocatalytic

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NO oxidation towards nitrate generation was further confirmed by the nitrogen balance calculation

226

via ion chromatography (Figure S8 and S9). On the contrary, although the activity of the pristine

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TiO2 also was maintained, the production of NO2 boosted sharply during the cycles (the amount of

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NO2 reached 50 ppb in the fifth cycle). Except for the fact that pristine TiO2 itself can produce the

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NO2 byproduct, the enhancement of NO2 might also be attributed to the accumulations of nitrate,

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which was not beneficial to diffusion or desorption on the pristine TiO2 surface and then

231

decomposed to produce more NO2.10 Whereas, blue TiO2 with a negative electrical environment was

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conducive to the diffusion or desorption of nitrate on the surface, consequently preserving high

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stability and selectivity.

234

Subsequently, capture experiments were performed to identify the active species in photocatalytic

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NO oxidation. It was observed that the NO removal was sharply inhibited in the presence of electron

236

scavenger (K2Cr2O7) and •O2- scavenger (PBQ) separately, both for the TiO2-OV and TiO2.

237

However, the NO removal performance of the TiO2-OV did not depress significantly with the

238

addition of holes (K2C2O4) or •OH (TBA) scavengers (Figure 3e, S10). These results suggested that

239

the photogenerated electrons and •O2- played the key role in NO oxidation removal over the

240

TiO2-OV, rather than holes or •OH. Moreover, we noticed that the addition of K2C2O4 could

241

completely inhibit the NO2 generation with the pristine TiO2, confirming that the photogenerated

242

holes contributed to NO2 generation (Figure 3f). Combined with the NO2 production in the five 11 ACS Paragon Plus Environment


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consecutive cycles, we concluded that holes would account for 10% to 15% proportion of NO

244

oxidation. However, the addition of hole scavenger did not impact the photocatalytic NO removal

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performance with the TiO2-OV, it is reasonable to deduce that the contribution of photogenerated

246

holes be inhibited by OVs over the blue TiO2.

247

Mechanism of the selective photocatalytic NO removal. To verify the role of OVs on highly

248

selective removal of NO over the blue TiO2-OV, a series of techniques were carried out to explore

249

its surface adsorption chemistry and the reaction intermediates in situ. The O2 and NO adsorption

250

performance on the blue TiO2-OV were evaluated with temperature-programmed desorption (TPD).

251

Three regions of lower temperature zone (100 °C-250 °C), medium temperature zone (250 °C-500

252

°C) and higher temperature zone (500 °C-800 °C) in O2-TPD curve were observed, which

253

correspond to the physical oxygen adsorption, chemical oxygen adsorption and the lattice oxygen

254

desorption respectively (Figure 4a). Oxygen desorption at medium-temperature zone is generally

255

ascribed to the adsorbed superoxide (•O2-) on OVs.40, 41 Notably, the area zone of •O2- desorption on

256

the TiO2-OV was remarkably larger especially in 400 °C, indicating a higher O2 adsorption capacity

257

on the blue TiO2-OV and the subsequent more efficient O2 activation with its OVs. Moreover, a

258

weaker desorption peak at 500 °C observed in TiO2-OV was attributed to the lattice oxygen release.

259

As shown in Figure 4b , two NO-TPD peaks assigning to the NO chemical adsorption on OV with

260

*N

261

supposed to facilitate the adsorption of NO. According to the desorption temperature, we believed

262

OVs preferentially favored the O2 adsorption and activation as compared with NO, consisted with

263

the DFT calculations. EPR spectra with the spin trap obtained under irradiation in O2 atmosphere

264

confirmed the generation of •O2- in situ (Figure 4c), obvious •O2- signals with isotropic hyperfine

or *O-end were observed in the TiO2-OV,41,

42

the larger desorption area of TiO2-OV was


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couplings to the 14N nucleus of about 14 Gauss and to the beta 1H of about 10 Gauss were detected

266

in the blue TiO2-OV in contrast to the faint signal of TiO2 counterpart, suggesting TiO2-OV was able

267

to activate O2 to •O2-, responsible for selectively photocatalytic NO removal. These results was

268

specifically demonstrated by the O2 isotope FTIR spectra on TiO2-OV with using 18O2 as O source

269

(Figure S11).

270

Furthermore, in situ DRIFTS was utilized to examine the intermediates and products under

271

reaction conditions on the TiO2-OV surface with a continuous real-time monitoring (Figure 4f).

272

After the pretreatment of samples under argon gas, NO was pumped into the reaction cell and

273

initiated the adsorption in dark conditions (Figure S12). The NO absorption bands at 1063 cm-1,

274

1210 cm-1, 1397 cm-1, 1540 cm-1, and 1630 cm-1 were observed in the TiO2-OV (Figure S12a). The

275

band at 1063-1540 cm-1 could be assigned to nitrates species, and the broad band at 1630 cm-1 may

276

originate from surface undissociated water.43 In the case of pristine TiO2, the adsorption of nitrate

277

compounds was also observed, similar to NO adsorption over the TiO2-OV (Figure S12b). Once

278

sufficient O2 filled up the cell and the gas adsorption equilibrium was achieved, the photocatalytic

279

NO reaction was initiated by the visible light illumination. Notably, the extremely extended bands at

280

3800-3300 cm-1 on the pristine TiO2 could be assigned to the consumption of H2O or OH groups for

281

the extra hydroxyl radicals (h+ + OH- → •OH

282

NO to produce NO2.44-46 In contrast, the peak of H2O or OH groups on the TiO2-OV did not change

283

under the irradiation, indicating that the reaction of H2O/OH- and photogenerated holes would not

284

happen, in agreement with the trapping experimental results. The increased absorption bands in the

285

range of 1700-1100 cm-1 could be ascribed to the stretching vibration of monodentate (1326 cm-1)

286

and bidentate (1607 cm-1,1460 cm-1,1120 cm-1) nitrates over the TiO2-OV (•O2- ads + NO → NO3-ads)

ads,

h+ + H2O → •OH

ads

+ H+), which could oxidize



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(Figure 4d). As per pristine TiO2, besides the bands of nitrate species with monodentate (1282 cm-1)

288

and bidentate (1577 and 1469 cm-1) nitrates, new bands at 1690 cm-1 and 1142 cm-1 were identified

289

for NO+ and NO2- respectively (NO + •OH ads→ NO+ ads + OH-, NO+ ads + 2OH- → NO2- ads + H2O,

290

NO2- ads + •OH → NO2 gas+ OH-, NO2 gas + •OH → NO3- ads + H+), affirming that the photogenerated

291

holes of TiO2 are responsible for NO2 production in the absence of OVs (Figure 4e, S13).44-46

292

In order to further explore the relationship of OVs and photogenerated hole during photocatalytic

293

NO removal, we employed hydrogen peroxide (H2O2) to mildly fill the OVs on the surface of blue

294

TiO2.47 The NO removal performance over the H2O2 treated blue TiO2 significantly decreased

295

compared with untreated blue TiO2, following by 30 ppb NO2 accumulation. This result indicated

296

that OVs could inhibit the NO2 formation (Figure S14). Subsequently, we used EPR spectra to

297

measure the spectral response with light irradiation or not when exposing the samples to O2. Upon

298

visible light irradiation, the EPR spectrum of TiO2-OV changed drastically, the •O2- signal located at

299

g =2.002, 2.010 and 2.018 appeared (Figure 5a).48 The •O2- signal was overlapped by the oxygen

300

vacancy signal, which was attributed to that the oxygen vacancy signal located at 1.998 is too close

301

to the •O2- signal. Compared to the flat •O2- signal of TiO2, the •O2- signal was remarkable for

302

TiO2-OV, indicating that localized electrons within the OVs are excited by visible light irradiation

303

and then transferred to the adsorbed O2 to generate •O2-. When the light source was changed to UV

304

light, pristine TiO2 also generates the •O2- signal, due to the excitation of valence band electrons

305

under UV light irradiation (Figure S15). Notably, the •O2- signal with different g-values in TiO2 and

306

TiO2-OV indicated the different adsorption sites of molecular oxygen on the catalysts,49 in

307

agreement with theoretical calculations that the adsorption sites of molecular oxygen in the TiO2-OV

308

were oxygen vacancies and surface five-coordinate Ti atoms, while for TiO2, they were surface 14 ACS Paragon Plus Environment

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309

five-coordinate Ti atoms (Ti4+) (Figure S1, S2). And localized electrons activated molecular oxygen

310

was also demonstrated by the consumption of oxygen vacancies. This consumption of localized

311

electrons in photocatalytic NO oxidation was revealed by Ti XPS analysis, the OVs was slightly

312

quenched within blue TiO2 after repeated photocatalytic reactions, as shown by the 13.1% decrease

313

of the Ti3+ content (Figure S16). The EPR spectra, in combination with XPS spectra, demonstrated

314

that OVs as a defect states was able to capture conduction band (CB) electrons and directly transfer

315

electrons to activate oxygen along with its own localized electrons. On the other hand, as two

316

electrons are localized in the oxygen vacancy and the formation of a •O2- consumes only one

317

electron, the remaining one electron would recombine with the photogenerated hole left in the

318

valence band (VB), showing the formation of Ti4+ from Ti3+ oxidation (Figure S16). This result

319

implied that the localized electrons within OVs are capable of activating molecular oxygen and

320

recombining the photogenerated hole simultaneously. Based on the above results, the hypothesis

321

about the interfacial charge transfer path over the TiO2-OV is proposed, as shown in Eqs. 3-7 (Figure

322

5b).28, 50

323

TiO2-OV´´ + hv → h+ + e-

(3)

324

e- + O2 → •O2-

(4)

325

VO´´ + O2 → VO´ + •O2-

(5)

326

VO´ + h+ → VO + heat (light)

(6)

327

Net reaction: TiO2-OV´´ + hv + 2O2 → 2•O2- + TiO2-OV + heat (light)

(7)

328

To confirm this hypothesis, the interfacial charge transfer property over the TiO2-OV was

329

revealed by steady-state photoluminescence (PL) and time-resolved photoluminescence (tr-PL)

330

spectroscopy (Figure 6). The TiO2-OV exhibited an intense broad PL signal centered around 520 nm 15 ACS Paragon Plus Environment


Page 16 of 32

331

when excited by a 380 nm laser, which was ascribed to the indirect radiative recombination of

332

mobile electrons on the OVs-induced defect states with VB holes (Figure 6a). And the location of

333

the defect states was estimated at approximately 0.54 eV below the CB edge on the basis of the

334

visible emission band at 520 nm.51, 52 All the PL decay curves exhibited extremely fast relaxation,

335

fitted practically by a double-exponential model. The fast decay component (amplitudes A1 and

336

lifetimes τ1) was associated with the fluorescence emission via direct recombination of interband

337

exciton, and the relatively slower component (amplitudes A2 and lifetimes τ2) was associated with

338

OVs-mediated indirect recombination of electrons with the VB holes.52-54 The PL peak decay of

339

TiO2-OV (τ1 = 2.03 ns, τ2 = 27.17 ns) was much slower compared with pristine TiO2 (τ1 = 1.25 ns,

340

τ2 = 21.83 ns), suggesting OVs exceedingly enhanced the lifetime of photogenerated electrons with

341

promoting energetically the interfacial charge transfer within TiO2-OV (Figure 6b, Table S2). And

342

OVs-mediated indirect recombination of excitons would ultimately prevail in TiO2-OV, uncovered

343

by its higher proportion of long-lasting component (A2τ2, 73.82%). Therefore, OVs-states

344

remarkably increased the permanence of photogenerated carriers that can transfer easily to the

345

preactivated O2, the redundant electrons within OVs would further recombine holes of VB.

346

Usually the defect states of the blue TiO2-OV between the CB and VB could act as trapping sites

347

for photogenerated electrons which could activated molecule O2, moreover, OVs-induced defect

348

states can also work as electron donors further recombine with VB holes, significantly altering the

349

carrier recombination dynamics.53 It is commonly believed that, when holes were inhibited, the

350

oxidation capacity of the catalyst would be too weaker to completely oxidize NO. Therefore,

351

photoelectrochemical spectroscopy and electrochemical impedance spectroscopy were employed to

352

evaluate the oxidation capacity of VB holes.55, 56 A smaller electrochemical impedance semicircle 16 ACS Paragon Plus Environment

Page 17 of 32


353

found in the blue TiO2-OV sample indicated less resistance in the charge transfer on the surface,

354

while photocurrents of TiO2-OV was feeble compared to pristine TiO2 (Figure S17). These

355

observations revealed that carriers recombination of TiO2-OV sharply diminished the oxidation

356

capacity of VB holes, which lead to inefficient photoelectrochemical water oxidation reaction

357

induced by the photogenerated holes. The photogenerated •OH radical by holes oxidation detected

358

by EPR with DMPO spin-trapping also confirmed that •OH radical formation was suppressed on

359

TiO2-OV (Figure S18). As a result, efficient photocatalytic complete oxidation NO is explicated in

360

the TiO2-OV photocatalytic system. The photogenerated electrons migrated to CB and

361

photogenerated holes located at VB under light irradiation excitation. Then the photogenerated

362

electrons transferred from CB to OVs-induced defect states due to the localization effect of OVs,

363

where the preabsorbed O2 could be activated to •O2- with sufficient electrons (photogenerated

364

electrons captured from CB and localized electrons derived from unsaturated Ti3+) to directly

365

oxidize NO to nitrate. Meanwhile, surplus electrons are thermodynamically favorable to recombine

366

holes at VB, resulted in excellent selectivity by inhibiting the hole oxidation (Figure 6c).

367

Besides the selectivity towards nitrate in NO photocatalytic oxidation, the stability of the

368

photocatalyst is equally important for the engineering application, high stability needs the final

369

nitrate desorption or migration more likely to occur. Usually, apart from the contribution to NO2

370

generation, the •OH, OH- or H2O on the surface of TiO2 could also server as the proton source for

371

the nitrates desorption or migration during the photocatalytic NO removal process.10 However, OVs

372

inhibited the hole water oxidation and insufficient •OH or OH- was availably facilitate the nitrates

373

desorption or migration over TiO2-OV. Thus, the proton source for nitrates desorption or migration

374

on the TiO2-OV might be the surface H (interstitial protons), introduced by NaBH4 reduction 17 ACS Paragon Plus Environment


Page 18 of 32

375

procedure, which maintained the free migration of nitrate products and made the catalyst more stable

376

(Figure S19).57-59 To deeply understand the effects of H during NO oxidation, the TiO2-OV-vacuum

377

obtained by vacuum calcination without H was employed to remove NO at same conditions (Figure

378

S20). Stable and efficient activity of TiO2-OV-vacuum clearly showed that oxygen vacancy was the

379

main contribution to the NO oxidation rather than H proton. Therefore, surface H in the blue

380

TiO2-OV might merely serve as proton source to effectively facilitate nitrate migration or desorption,

381

thereby further inhibiting the generation of NO2 during prolonged cycle.

382

Environmental Implications.

383

Photocatalysis technology is an effective strategy to control dilute NO pollution in the atmosphere.

384

However, this technology suffers from partial NO oxidation with undesirable NO2 emission. Seeking

385

for semiconductor photocatalysts with prominent selectivity for NO complete removal is a great

386

challenge. In this study, we demonstrated that the blue TiO2 photocatalyst abundant in OVs could

387

achieve highly selective photocatalytic NO oxidation to nitrate without NO2 generation under visible

388

light irradiation. It was believed that OVs could not only active molecule O2 to generate •O2- which

389

facilitate the selective NO transform towards nitrate under ambient conditions, but also could

390

annihilate the photogenerated holes to further inhibit the NO2 byproduct formation. This work could

391

deepen the mechanistic understanding of photocatalytic NO removal over the defective photocatalyst

392

surface, as well provide guidelines for practical photocatalytic NO removal application.

393 394

AUTHOR INFORMATION

395

Corresponding Author

396

*Phone/Fax: +86-27-6786 7535; e-mail: [email protected]; [email protected]. 18 ACS Paragon Plus Environment

Page 19 of 32


397

Notes

398

The authors declare no competing financial interest

399 400

ACKNOWLEDGEMENTS

401

This work was supported by the National Key Research and Development Program of China (grant

402

number 2016YFA0203000); Natural Science Funds for Distinguished Young Scholars (Grant

403

21425728); National Science Foundation of China (grant numbers 21876058, 21477044); and the

404

111 Project B17019.

405 406

ASSOCIATED CONTENT

407

Supporting Information

408

Surface coordination structure of the simulated TiO2(101) surface; NO and O2 adsorption on TiO2

409

and TiO2-OV surface; TEM images and XPS spectra of TiO2 and TiO2-OV; photographs of the

410

as-prepared TiO2-OV and TiO2; BET characterization and first-order kinetic constants calculation;

411

the selectivity of NO during five runs for photocatalytic NO oxidation; ionic chromatography of

412

nitrate and the calibration curve; product distributions on TiO2-OV and TiO2; influence of different

413

scavengers on photocatalytic NO oxidation; FTIR spectra for the NO oxidation with

414

spectra for the NO adsorption; the photocatalytic NO2 oxidation on TiO2-OV and TiO2; influence of

415

OVs on photocatalytic NO oxidation; the solid EPR measurement for •O2- radical; high-resolution Ti

416

2p XPS spectra of TiO2-OV and TiO2 after repeated use; electrochemical characterization; the EPR

417

measurement for •OH radical; the 1H NMR spectra; influence of surface hydrogen on photocatalytic

18O

2;

FTIR



Page 20 of 32

418

NO oxidation; calculated absorption energy and Bader charges; parameters of the time-resolved

419

photoluminescence decay curves.

420 421

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422

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Figure Captions



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576 577

Figure 1. (a) Free energy change against the reaction coordinate for the NO oxidation on TiO2(101)

578

defective surface. (b) Charge density difference of O2 and nitrate adsorbed on OV of TiO2(101)

579

surface. The yellow and blue isosurfaces with an isovalue of 0.005 a.u. represent charge

580

accumulation and depletion in the space. * was used to represent the absorption state.

581 28 ACS Paragon Plus Environment

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582

Figure 2. Characterizations of the TiO2-OV catalyst and its counterpart. (a) XRD patterns. HRTEM

583

images of (b) TiO2-OV and (c) TiO2. (d) Raman spectra, inset:the most intense Eg peak of TiO2 for

584

both, along with corresponding peak center positions and widths. (e) EPR spectra at 120 K. (f)

585

Diffuse reflectance spectra.

586 587

Figure 3. Photocatalytic performances. (a) Photocatalytic NO removal over the TiO2-OV and TiO2.

588

(b) The corresponding generation of NO2. (c) The stability of catalysts in five runs of NO removal.

589

(d) The generation of NO2 during five cycling runs. (e) Influence of different scavengers on the NO

590

removal. (f) The corresponding generation of NO2 with adding K2C2O4 for holes scavenging or not.



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591 592

Figure 4. Mechanism study of NO oxidation on TiO2-OV. (a) O2-TPD spectra and (b) NO-TPD

593

spectra of the TiO2-OV and TiO2 catalysts. (c) EPR spectra of the spin-reactive •O2- generated by

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TiO2-OV and TiO2 under visible light. In situ FTIR spectra of (d) TiO2-OV and (e) TiO2 during

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photocatalytic NO oxidation. (f) Schematic illustration of the reaction cell for the in situ FTIR study.


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Figure 5. (a) EPR spectra for TiO2-OV and TiO2 under visible (420 nm) light irradiation at 120 K.

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The corresponding simulated spectrum was indicated in the gray line. (b) Schematic illustration of

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the interfacial charge transfer path on the TiO2-OV. The red and green ellipses represent the electron

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clouds respectively. The oxygen vacancy with two localized electrons is essentially two unsaturated

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titanium (Ti3+).



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Figure 6. The charge carriers dynamics. (a) Room-temperature steady-state PL spectra of the

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TiO2-OV and (b) the corresponding decay curves. (c) Scheme: The illustration for the interfacial

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electron transfer processes during photocatalytic NO oxidation on TiO2 and TiO2-OV.

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TOC Art Figure

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Oxygen Vacancies Promoted Selective Photocatalytic Removal of NO

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