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Highly Efficient Performance and Conversion Pathway of Photocatalytic NO Oxidation on SrO-Clusters@Amorphous Carbon Nitride Wen Cui, Jieyuan Li, Fan Dong, Yanjuan Sun, Guangming Jiang, Wanglai Cen, Shun Cheng Lee, and Zhongbiao Wu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00974 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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Environmental Science & Technology

1

Highly Efficient Performance and Conversion Pathway of

2

Photocatalytic NO Oxidation on SrO-Clusters@Amorphous

3

Carbon Nitride

4

Wen Cui a, Jieyuan Li b, Fan Dong a,*, Yanjuan Sun a, Guangming Jiang a,

5

Wanglai Cen b, S. C. Lee c, Zhongbiao Wu d

6

a

7

of Environment and Resources, Chongqing Technology and Business University,

8

Chongqing 400067, China.

9

b

Chongqing Key Laboratory of Catalysis and New Environmental Materials, College

College of Architecture and Environment, Institute of New Energy and Low Carbon

10

Technology, Sichuan University, Sichuan 610065, China.

11

c

12

University, Hong Kong, China.

13

d

14

China.

Department of Civil and Environmental Engineering, The Hong Kong Polytechnic

Department of Environmental Engineering, Zhejiang University, Hangzhou 310027,

15 16 17 18

* To whom correspondence should be addressed. E-mail: [email protected] (Fan Dong). Phone: +86 23 62769785 605. Fax:+86 23 62769785 605.

ACS Paragon Plus Environment


19 20

ABSTRACT: This work demonstrates the first molecular-level conversion pathway

21

of NO oxidation over a novel SrO-clusters@amorphous carbon nitride (SCO-ACN)

22

photocatalyst, which is synthesized via co-pyrolysis of urea and SrCO3. The inclusion

23

of SrCO3 is crucial in the formation of the amorphous carbon nitride (ACN) and SrO

24

clusters by attacking the intralayer hydrogen bonds at the edge sites of graphitic

25

carbon nitride (CN). The amorphous nature of ACN can promote the transportation,

26

migration, and transformation of charge carriers on SCO-ACN. And the SrO clusters

27

are identified as the newly formed active centers to facilitate the activation of NO via

28

the formation of Sr-NOδ(+), which essentially promotes the conversion of NO to the

29

final products. The combined effects of the amorphous structure and SrO clusters

30

impart outstanding photocatalytic NO removal efficiency to the SCO-ACN under

31

visible-light irradiation. To reveal the photocatalytic mechanism, the adsorption and

32

photocatalytic oxidation of NO over CN and SCO-ACN are analyzed by in situ

33

DRIFTS, and the intermediates and conversion pathways are elucidated and compared.

34

This work presents a novel in situ DRIFTS-based strategy to explore the

35

photocatalytic reaction pathway of NO oxidation, which is quite beneficial to

36

understand the mechanism underlying the photocatalytic reaction and advance the

37

development of photocatalytic technology for environmental remediation.

38


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1. INTRODUCTION

40

With the improvement in the quality of life and growing environmental awareness

41

among the public, strong emphasis has been placed on mitigating air pollution.1-3

42

Nitric oxide (NO), which is one of the major contributors to photochemical smog,

43

acid rain and ozone depletion and is primarily responsible for respiratory and

44

cardiopulmonary diseases, has triggered much social concern.4,

45

methods based on physical adsorption, biofiltration, and thermal catalysis were

46

employed to remove NO from industrial emissions. However, these methods are not

47

economically feasible for NO removal at parts per billion (ppb) levels.6, 7 As a green

48

technology, photocatalysis has gained considerable attention in view of its feasibility

49

and high efficiency for solar energy utilization and environmental remediation.8-10

5

Conventionally,

50

Graphitic carbon nitride (CN), a metal-free layered conjugated semiconductor, was

51

firstly reported as a visible-light photocatalyst by Wang et al.11 Owing to its appealing

52

electronic band structure, physicochemical stability, and earth-abundant nature, CN

53

has become a new research hotspot in the arena of environmental remediation and

54

solar energy conversion.12-14 However, to improve the photocatalytic efficiency of CN

55

and its adaptability in various application fields, further optimization on the CN

56

performance is desirable and then the developed strategy including inner architecture

57

modification and surface functionalization (elemental doping, copolymerization, and

58

formation of heterojunctions) was proposed.15-19 For inner architecture modification,

59

Kang et al. recently synthesized novel amorphous carbon nitride (ACN) by breaking

60

the in-plane hydrogen bonds between strands of polymeric melon units via post-heat

61

treatment of the partially crystalline CN at a high temperature of 620 °C for 2 h.20, 21

62

With the as-prepared ACN, the high localization of photogenerated charge carriers

63

within each melon strand was eliminated; the large potential barrier between the

64

layers and across the hydrogen bonds located regions was reduced; the transfer of

65

charge carriers was facilitated; and the light absorption range was broadened.

66

Consequently, much superior activity was observed for hydrogen generation in

67

comparison to that of pristine CN.20, 21

68

Besides, the conversion pathway for pollutant removal over a photocatalyst is key



69

to understanding the underlying reaction mechanism, estimating the possible

70

generation of toxic intermediates, and optimizing the photocatalyst performance.

71

Although numerous efficient photocatalysts have been developed for NO removal,

72

little attention has been paid to the conversion route of photocatalytic NO oxidation

73

process.5, 10, 19, 22, 23 In situ DRIFTS is an effective tool for gas-phase reaction analysis

74

because signals are observed even for slight changes at the molecular level; thus, it is

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well suited for investigating the related reaction pathway of photocatalytic NO

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oxidation.24-29

77

Here, a facile method involving the co-pyrolysis of urea and SrCO3 was developed

78

to prepare SrO clusters@amorphous carbon nitride (SCO-ACN), which exhibited

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substantially high visible-light photocatalytic NO removal efficiency. The high

80

efficiency is attributed to (1) the amorphous nature of ACN that can promote the

81

transportation, migration, and transformation of charge carriers and (2) the enhanced

82

activation of NO via the formation of Sr-NOδ(+). To reveal the photocatalytic

83

mechanism, the adsorption and photocatalytic oxidation of NO over CN and

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SCO-ACN were analyzed by in situ DRIFTS, and the intermediates and conversion

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pathways were elucidated and compared. Notably, SrO clusters were identified as the

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newly formed active centers to facilitate the activation of NO, which could effectively

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promote the conversion of NO to the final products. This work presents a novel in situ

88

DRIFTS-based strategy to explore the photocatalytic reaction pathway of NO

89

oxidation, which is quite beneficial to understand the mechanism of the photocatalytic

90

reaction and advance the development of photocatalytic technology for environmental

91

remediation.

92

2. EXPERIMENTAL AND THEORETICAL SECTION

93

2.1 Preparation of photocatalysts

94

All chemicals employed in this study were analytical grade and were used without

95

further treatment. In a typical synthesis procedure, 10 g of urea and a certain amount

96

of SrCO3 were added in an alumina crucible (50 mL) with 20 mL distilled water. The

97

obtained solution was transferred to an oven and dried at 60 °C. Then, the crucible

98

with a cover was calcined at 550 °C for 2 h at a heating rate of 15 °C/min in static air.


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To investigate the effect of the CN-SrCO3 ratio, the SrCO3 content was controlled at

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0.06, 0.1, and 0.18 g, respectively, and the prepared samples were labeled as

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SCO-ACN-X (X represents the amount of SrCO3). For comparison, an ex situ

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mechanical mixture of CN and SrCO3 was prepared and named as SCO-CN.

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2.2 Characterization methods

104

The crystal phase of the prepared samples was analyzed by X-ray diffraction (XRD)

105

with Cu Kα radiation (model D/max RA, Rigaku Co., Japan). X-ray photoelectron

106

spectroscopy (XPS) with Al Kα X-rays (hν = 1486.6 eV) radiation at 150 W (Thermo

107

ESCALAB 250, USA) was used to investigate the surface properties. Fourier

108

transform infrared (FT-IR) spectra were recorded on a Nicolet Nexus spectrometer

109

using samples embedded in KBr pellets. Scanning electron microscopy (SEM, model

110

JSM-6490, JEOL, Japan) and transmission electron microscopy (TEM, JEM-2010,

111

Japan)

112

adsorption-desorption isotherms were obtained on a N2 adsorption apparatus (ASAP

113

2020, Micromeritics, USA). UV-vis diffuse-reflectance spectrometry (UV-vis DRS)

114

measurements were performed on dry-pressed disk samples using a scanning UV-vis

115

spectrophotometer (TU-1901, China) equipped with an integrating sphere assembly,

116

with 100% BaSO4 as the reflectance sample. Photoluminescence (PL) studies (F-7000,

117

HITACHI, Japan) were conducted to investigate the optical properties of the samples.

118

Photocurrent measurements were carried out using an electrochemical system

119

(CHI-660B, Chenhua, China), wherein the working electrode was irradiated by a 300

120

W Xe lamp with a 420 nm cut-off filter. Steady and time-resolved fluorescence

121

emission spectra were recorded at room temperature with a fluorescence

122

spectrophotometer (Edinburgh Instruments, FLSP-920). Electron spin resonance

123

(ESR) of radicals spin-trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was

124

recorded on a JES FA200 spectrometer. Samples for ESR measurements were

125

prepared by mixing them in a 50 mM DMPO solution tank (aqueous dispersion for

126

DMPO-•OH and methanol dispersion for DMPO-•O2−) and irradiated by visible light.

127

2.3 Evaluation of photocatalytic activity

128

were

used

to

characterize

the

morphology

and

structure.

N2

The photocatalytic activity was evaluated based on the removal efficiency of NO at



129

ppb levels in a continuous flow reactor with 0.2 g prepared samples. The

130

concentration of NO was continuously detected by a NOx analyzer (Thermo

131

Environmental Instruments Inc., model 42c-TL). A 150 W commercial tungsten

132

halogen lamp (the average light intensity was 0.16 W/cm2) that was vertically placed

133

above the reactor glowed when the adsorption-desorption equilibrium was achieved.

134

A detailed description of the photocatalytic apparatus is available in Supporting

135

Information.

136

2.4 In situ DRIFTS investigation

137

In situ DRIFTS measurements were conducted using a TENSOR II FT-IR

138

spectrometer (Bruker) equipped with an in situ diffuse-reflectance cell (Harrick) and a

139

high-temperature reaction chamber (HVC), as shown in Scheme 1. The reaction

140

chamber was equipped with three gas ports and two coolant ports. High-purity He,

141

high-purity O2, and 100 ppm of NO (in He) mixture could be fed into the reaction

142

system, and a three-way ball valve was used to switch between the target gas (NO)

143

and purge gas (He). The total gas flow rate was 100 mL/min, and the concentration of

144

NO was adjusted to 50 ppm by dilution with O2. The chamber was enclosed with a

145

dome having three windows, two for IR light entrance and detection, and one for

146

illuminating the photocatalyst. The observation window was made of UV quartz and

147

the other two windows were made of ZnSe. A Xe lamp (MVL-210, Optpe, Japan) was

148

used as the irradiation light source. Before measurements, the prepared samples were

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placed in a vacuum tube and pretreated 1h at 300 oC.

150 151 152

Scheme 1. The designed reaction system for the in situ DRIFTS signal recording.

2.5 DFT calculations


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All the spin-polarized DFT-D2 calculations were performed by applying the code

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VASP5.3.5,30,

31

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exchange and correlation functional.32 The projector-augmented wave (PAW) method

156

was employed, with a cut-off energy of 400 eV.33 The Brillouin zone was set using 5×

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5 × 1 K-points. All atoms were allowed to be relaxed and converged to 0.02 eV/Å.

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The nudged elastic band (NEB) method 34, 35 was used to search the reaction pathways

159

from the initial state (IS) to the respective final state (FS). The transition state (TS)

160

was determined using the climbing image method and verified with a single

161

imaginary frequency (f/i).

162

3. RESULTS AND DISCUSSION

163

3.1 Chemical composition and phase structure

utilizing the generalized gradient approximation with the PBE

164

The chemical structure and composition of CN and SCO-ACN-0.1 are examined by

165

XPS measurements. In XPS survey spectra (Figure S1a and S1b in Supporting

166

Information), C1s, N1s, Sr3d and O1s signals can be observed for the SCO-ACN-0.1

167

sample. As shown in Figure S1c, the corresponding binding energies of C1s at 284.6

168

and 288.1 eV are ascribed to the sp2 C−C bonds and sp2-bonded carbon in the

169

N-containing aromatic rings (N–C=N), respectively.36,

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deconvoluted into four peaks at 398.8 eV, 400.4 eV, 401.6 eV and 404.5 eV (Figure

171

1a). The main peak centered at 398.8 eV originates from the sp2-bonded N involved in

172

the triazine rings (C–N=C), and the weak peak at 400.4 eV is due to the tertiary

173

nitrogen N–(C)3 groups in CN.36, 37 The C–N=C, N–(C)3, and N–C=N groups make up

174

the basic substructure units of CN polymers and construct the heptazine heterocyclic

175

ring (C6N7) units. Notably, the peak (at 401.6 eV) due to amino functions (C–N–H) is

176

clearly be observed in CN sample, but the intensity of this peak is low in

177

SCO-ACN-0.1, indicating that some of the hydrogen bonds in the intralayer

178

framework of CN have been eliminated. Furthermore, the atomic ratio of carbon to

179

nitrogen (C : N) gradually increases from 3:4.34 for the pristine CN to 3:3.60 for

180

SCO-ACN-0.1 (Table S1 in Supporting Information), demonstrating that some of the

181

amine groups (NH2/NH) from CN are lost along with the breaking of hydrogen

182

bonds.20 Hence the microstructure of CN would be changed when the hydrogen bonds

37


The N1s spectra can be


183

are broken via the reaction between CN and SrCO3 during co-pyrolysis process. The

184

related high-resolution spectra of Sr3d and O1s are shown in Figure 1b and S1d,

185

indicating that strontium oxide is formed on the CN surface.

186

Subsequently, X-ray diffraction is employed to elucidate the crystal structures of

187

the as-prepared samples. The formation of CN polymer is indicated by the two

188

characteristic diffraction peaks at 13.1° and 27.2°, which arise from the in-plane

189

structural repeating motifs of the aromatic systems and the interlayer reflection of a

190

graphite-like structure, respectively.38, 39 As shown in Figure 1c, the two peaks in the

191

case of the SCO-ACN-X samples gradually disappear or are diminished with the

192

addition of SrCO3. The disappearance of the peak at 13.1° intuitively reflects that the

193

in-plane periodicity of the aromatic systems has been destroyed. Correspondingly, the

194

formed irregular intralayer structure induce fluctuations in the interlayer structure and

195

disturb the periodic stacking of the layers. Hence, with the introduction of SrCO3, no

196

characteristic diffraction peaks of CN are observed for the SCO-ACN-X samples. And

197

characteristic diffraction peaks of SrCO3 also have not been detected in SCO-ACN-X

198

samples. However, the XRD pattern of SCO-CN developed by the ex situ method

199

displays both the characteristic diffraction peaks of SrCO3 and CN. Combining the

200

XPS results, we can conclude that the synergic interactions between CN and SrCO3

201

during the in situ thermal processes would break the intralayer hydrogen bonds,

202

resulting in the formation of ACN.

203

FT-IR spectra are measured to verify whether the basic atomic structures of CN

204

would be destroyed via the construction of amorphous structure by breaking intralayer

205

hydrogen bonds of CN (Figure 1d). A strong adsorption band of the heptazine

206

heterocyclic ring (C6N7) units at 1700-1200 cm-1 is detected.40 A sharp peak at 810

207

cm-1 corresponding to the breathing mode of the heptazine ring system can also be

208

observed, which indicates that the SCO-ACN-X samples maintain the basic CN

209

atomic structures.41 The broad peak located at 3500-3100 cm-1 can be attributed to the

210

residual N-H components and the O-H bands, associated with the uncondensed amino

211

groups and the absorbed H2O molecules, respectively. Notably, the absorption

212

intensity of the N-H components at 3500-3100 cm-1 decrease gradually with the


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addition of SrCO3. Also, the absorption band at 890 cm-1 assigned to the deformation

214

mode of N-H gradually diminishes. Furthermore, a newly generated absorption band

215

at 2166 cm-1, which is due to the stretching vibration of N=C=N, can be observed in

216

the spectrum of SCO-ACN-X. The change in the IR bands indicates that the

217

introduction of SrCO3 in the urea polymerization process only destroys the periodic

218

arrangement of the interlayers melon strands but maintains the basic atomic structures

219

of the strands to afford unique amorphous arrangements of short-range order and

220

long-range disorder. Therefore, a facile co-pyrolysis method can be employed to

221

synthesize ACN by breaking the hydrogen bonds to destroy the intralayer long-range

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atomic order arrangements.

223 224

Figure 1. The N1s XPS spectra of CN and SCO-ACN-0.1 (a), high-resolution Sr3d XPS spectra

225

of SCO-ACN-0.1 (b), XRD pattern (c) and FT-IR spectra (d) of CN and SCO-ACN-X.

226 227

3.2 Morphology and formation mechanism SEM images are presented to investigate the morphology differences between the



228

original CN and the ACN. As shown in Figure S2a-2d, the pristine CN is formed by

229

the stacking of silk-like nanosheets, and the SCO-ACN-X samples generally maintain

230

a similar morphology. After careful observation, a number of pores in the

231

SCO-ACN-X samples can be found, as opposed to the pristine CN. The formation of

232

this special porous structure should be associated with the interaction between CN and

233

SrCO3. Concretely, during co-pyrolysis, CO32- in SrCO3 will attack the hydrogen

234

bonds of CN to release H2 and CO2 gas, probably in the form of bubbles, thus

235

breaking the intralayer hydrogen bonds of CN and yielding ACN. Bursting of the

236

bubbles leads to the formation of a porous structure. Utilizing the DFT method, NEB

237

calculations are thus carried out to further confirm this deduction. As shown in Figure

238

S2f, a lower energy barrier and less energy adsorption are observed in the reaction at

239

the edge sites of CN. Specifically, HCO3− generation at edge site manifests lower

240

energy barrier ( Eb, 1.03 eV) to reach the transitional state (TS), compared with that of

241

bridge site (1.44 eV). Besides, less reaction energy (Er) is observed in the reaction at

242

edge site. This result indicates that CO32− in SrCO3 is preferable to attack hydrogen

243

bonds at the edge sites of CN, which is beneficial to the formation of ACN (Figure

244

S3). This result indicates that the special porous structure dominantly originates from

245

H2 and CO2 gas generation at the edge sites of CN. Therefore, the increased exposure

246

of the bare edge in SCO-ACN-X is certified, which contributes to the construction of

247

ACN. Besides, the EDX elemental mapping of SCO-ACN-0.1 (Figure S2e) suggests

248

that the C, N, Sr, and O elements are distributed uniformly. However, the actual form

249

of residual SrO remains debatable.

250

Next, TEM observations are carried out to further investigate the microstructure, as

251

shown in Figure 2. As opposed to the primary layered CN nanosheets, the

252

SCO-ACN-0.1 sample shows clear lattice fringes with a lattice spacing of 2.581 Å

253

(circled by the red dashed line), which match the spacing of the (200) crystal planes of

254

the SrO clusters (2-5 nm) formed by the thermal decomposition of SrCO3 during

255

co-pyrolysis. Hence a facile co-pyrolysis method has been developed to prepare

256

amorphous CN decorated with SrO clusters.


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257 258

Figure 2. TEM and HRTEM images of CN (a) and SCO-ACN-0.1 (b).

259

The N2 adsorption-desorption isotherms and Barrett-Joyner-Halenda (BJH)

260

pore-size distribution curves (Figure S4) also reflect the formation of mesopores. The

261

specific surface area and pore volume of SCO-ACN-X decrease, as shown in Table S1.

262

According to the comparison, the specific surface area and porous structures are not

263

the key factors responsible for the enhanced photocatalytic activity of SCO-ACN-X.

264

3.3 Optical properties, charge separation, and charge transfer

d200 = 2.581 Å

265

PL spectra and ns-level time-resolved fluorescence decay spectra are recorded

266

(Figure 3) to investigate the transfer of photogenerated carriers. In contrast to CN

267

which shows a strong band-to-band emission peak at 442 nm, the SCO-ACN-0.1

268

sample exhibits a much diminished PL peak (Figure 3a). The quenching of the PL

269

peaks can be ascribed to the inhibition of radiative recombination pathways, which

270

are associated with the unique short-range order but long-range disorder amorphous

271

arrangements of ACN that can eliminate the high localization of charge carriers within

272

each melon strand and decrease the large potential barrier in the regions between the

273

layers and across the hydrogen bonds to boost the separation of charge carriers.20, 21

274

The SrO has a band gap of 6.1 eV, with conduction band (CB) and valence band (VB)

275

positions at -3.08 and 3.02 eV, repectively.42, 43 Considering the band structure of CN,

276

the visible light-induced carriers from ACN could not be transferred to SrO. Thus, the

277

enhanced charge separation and transfer characteristics of SCO-ACN are irrelevant to



278

the SrO clusters. Correspondingly, the radiative lifetime of SCO-ACN-0.1 is longer

279

than that of CN (Figure 3b), further confirming the effective transfer of carriers to

280

inhibit the recombination of electron-hole pairs directly originating from the special

281

amorphous structure. Besides, the photocurrent density-time response plot via on-off

282

cycles of the samples under visible-light irradiation is employed to evaluate the

283

interfacial charge separation dynamics (Figure S5a). Owing to the enhanced

284

electron-hole separation and transfer, the photocurrent response intensity of

285

SCO-ACN-0.1 is higher than primary CN. As shown in Figure S5b, a typical

286

semiconductor absorption in the blue light range is observed for all samples. Owing to

287

the construction of unique amorphous arrangements of short-range order and

288

long-range disorder, a red-shift of the optical absorption band edge can be observed.

289

To be specific, the gradual destruction of the intralayer long-range atomic order by

290

breaking hydrogen bonds increases both the density and distribution of localized

291

states, which is responsible for the increased visible light absorption.20, 21

292 293 294

Figure 3. PL spectra (a), ns-level time-resolved fluorescence spectra (b) for as-prepared samples.

3.4 Photocatalytic activity and conversion pathway of NO oxidation

295

As shown in Figure 4a, all the SCO-ACN-X samples exhibit superior activity

296

compared to pristine CN. In particular, the SCO-ACN-0.1 sample reaches an

297

unprecedented high NO removal ratio of 50.0%. Thus, a facile co-pyrolysis method

298

has been developed to prepare highly efficient SCO-ACN and the optimized

299

preparation conditions are confirmed. To further demonstrate the interaction between

300

CN and SrCO3 during co-pyrolysis, the mechanically mixed SCO-CN sample is also


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301

tested, and a slight enhancement of photocatalytic activity is observed. Therefore, the

302

combined disruption of the intralayer long-rang atomic order structure of CN and

303

coupling with the SrO clusters are beneficial to enhance the photocatalytic activity.

304

And the reaction rate constant k of CN and SCO-ACN are determined to be 0.0915

305

and 0.1367 min−1, respectively (Fig. S6). Correspondingly, the apparent quantum

306

efficiency was estimated to be 19.12 and 28.57% for CN and SCO-ACN-0.1,

307

respectively (see the details about the apparent quantum efficiency calculations in the

308

Supporting Information), which indicates the SCO-ACN samples exhibit higher

309

apparent quantum efficiency than pristine CN. And the slight reduction in

310

photocatalytic activity after several minutes can be ascribed to the accumulation of

311

generated intermediates and final products occupying the active sites. The final

312

products (NO3−) can be easily removed by water washing.

313

The photocatalytic efficiency is strongly related to the number of the electron-hole

314

pairs generated under light irradiation, as well as their evolution route. The

315

photogenerated electrons and holes can migrate to the surface of the photocatalyst and

316

then be trapped, generally by the oxygen and surface hydroxyls, to ultimately form

317

superoxide radicals (•O2−) and hydroxyl radicals (•OH) that react with the adsorbed

318

pollutant. For further investigating the reactive species responsible for the

319

photocatalytic removal of NO, the ESR spin-trap with DMPO technique is employed

320

to detect the DMPO-•O2− and DMPO-•OH signals in the CN and SCO-ACN-0.1

321

suspension (Figure 4b). As expected, a much stronger DMPO-•O2− signal is observed

322

for SCO-ACN-0.1 than for CN. This improvement is associated with the improved

323

electron excitation properties and better charge transfer characteristics, which

324

facilitate the generation of photogenerated electrons to trap molecular oxygen and

325

produce more •O2−. Interestingly, a strong DMPO-•OH adduct signal generated by

326

SCO-ACN-0.1 is detected. The potential energy of the VB holes (1.40 eV) from CN is

327

more negative than the OH−/•OH and H2O/•OH potentials (1.99 and 2.37 eV) and

328

cannot directly oxidize OH−/H2O into •OH radicals. However, the observed •OH

329

radicals in Figure 4b can be formed through the reduction of •O2− via the following



330

route: •O2− → H2O2 → •OH (the detection of H2O2 has been demonstrated in

331

Figure S7).44 Therefore, we conclude that the SCO-ACN significantly promotes the

332

transportation, migration, and transformation of charge carriers and then facilitates the

333

generation of reactive radicals for NO oxidation.

334 335

Figure 4. Visible-light photocatalytic activities of as-prepared samples for NO removal (a), and

336

DMPO spin-trapping ESR spectra of samples (b).

337

To understand the mechanism of photocatalytic NO oxidation, in situ DRIFTS is

338

carried out to monitor time-dependent evolution of the reaction intermediates and

339

products over the photocatalyst surface, as shown in Figure 5. The background

340

spectrum is recorded before injecting NO into the reaction chamber. The NO

341

absorption bands appear once NO comes in contact with the photocatalyst at 25 oC

342

under dark conditions. Absorption bands of N2O at 2282 and 2244 cm−1 are detected,

343

indicating the adsorption of NO over CN.24, 25 In the OH stretching region, a negative

344

band at around 3550 cm−1 is observed, along with adsorption IR bands at 1193-1142

345

cm−1 due to NO−/NOH, and at 2087 and 934 cm−1 due to NO2.24-26 This result

346

indicates the disproportionation of NO on the surface of CN. The following reaction

347

can be proposed: 3NO + OH− = NO2 + NO− + NOH.24-27 The other absorption bands

348

developed progressively can be assigned to the stretching vibration of monodentate

349

(1060-1010 cm−1) and bidentate (at 1125 and 1109 cm−1) nitrites, or to bidentate

350

(1227 and 1060-1010 cm−1), bridging (1001cm−1), and chelating bidentate (986 cm−1)

351

nitrates.24-26,28,29,45 The formation of nitrites and nitrates over CN during NO

352

adsorption are mainly due to the active two-coordinated N atoms of CN that facilitate

353

the formation of the activated oxygen species and then enhance the oxidation capacity


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354

of the surface oxygen species for NOx oxidation.46, 47

355

In the case of SCO-ACN-0.1, the adsorption of nitro compounds can be observed

356

before the visible-light irradiation, as shown in Figure 5b, similar to NO adsorption

357

over the CN sample. However, significant differences between the two cases can be

358

identified. An outstanding band appears at 2126 cm−1 and is associated with the

359

nitrosyl (Sr-NOδ(+)) species, an intermediate formed during NO oxidation.48, 49 NO

360

molecules tend to interact with SrO to form the more stable nitrosyl intermediate.27, 49

361

Partial charge transfer from the 5s orbital of NO to Sr2+ results in the formation of

362

Sr-NOδ(+).25, 49 This is consistent with the observation that the vibration frequencies of

363

nitrosyls are higher than those of NO molecules (1876 cm−1) but lower than those of

364

NO+ free ions (around 2200 cm−1). The adsorbed nitrosyls, the main reaction

365

intermediates, would be preferentially oxidized to nitro compounds by reactive

366

oxygen species.49 Hence SrO clusters can be identified as the newly formed active

367

centers to facilitate the activation of NO via the formation of Sr-NOδ(+), which

368

effectively promotes the conversion of NO to the final products.

369

Once the adsorption equilibrium is achieved, a visible-light source is applied to

370

initiate the photocatalytic reaction. Figure 5b shows the IR spectra for CN under

371

visible-light irradiation in time sequence. The “baseline” spectrum is the same as that

372

of “NO + O2 20 min” in Figure 5a. In the range 2300-2050 cm−1, the absorption bands

373

at 2282 and 2244 cm−1 disappear, indicating the consumption of N2O/NO

374

accumulated during NO adsorption. Correspondingly, the peak intensities of some

375

intermediates (nitrito, NO−/NOH) and final products (nitrites, nitrates) significantly

376

increase. The ESR results demonstrate that the surface superoxide radicals are the

377

major radical species (Figure 4b). Thus, the superoxide radicals should be responsible

378

for the conversion of the intermediates into the final products under visible-light

379

irradiation. The adsorption-photocatalysis mechanism on the pristine CN is illustrated

380

in Scheme 2a.

381

The time-dependent IR spectra for the photocatalytic NO oxidation over

382

SCO-ACN-0.1 are recorded and shown in Figure 5d. In the range 1250-900cm−1, the

383

IR absorption bands show a similar pattern as that observed for CN. The increased



384

negative peak intensity of the OH groups at 3700-3350 cm−1 can attribute to the

385

consumption of OH groups for the generation of hydroxyl radicals, which is one of

386

the oxidation mediators for NO removal. A slightly intensified Sr-NOδ(+) band at 2126

387

cm−1 can be observed. Notably, a new absorption band at 2215 cm−1 is detected,

388

which can be ascribed to the fact that the electrons from the adsorbed NO are trapped

389

by the photogenerated holes and then converted into NO+ (free ions). Sr-NOδ(+) and

390

NO+ are the dominant products of NO activation on SCO-ACN.

391

Also, according to IR spectra in time sequence, the temporal evolution of

392

normalized absorbance of adsorbed Sr-NOδ(+) and NO3− species on the photocatalysts

393

surface during NO adsorption process and photocatalytic NO oxidation process can be

394

provided. For the concerned species, the normalized absorbance is calculated by

395

considering their individual maximum absorbance as 1. The normalized absorbance of

396

intermediates (Sr-NOδ(+)) and final product (NO3−) are illiustrated in Figure 5c and 5f.

397

According to the tendency of species evolution, it can be clearly observed that the

398

adsorption and transformation of Sr-NOδ(+) and NO3− both in NO adsorption process

399

and photocatalytic NO oxidation process are greatly boosted on SCO-ACN, indicating

400

the construction of SrO clusters@amorphous carbon nitride could promote the ability

401

for NO activation. Consequently, the enhanced NO activation accelerates the

402

formation of intermediates and then facilitates the conversion from original pollutant

403

or intermediates to final products.

404 405

The conversion pathways for the NO adsorption and photocatalytic NO oxidation processes on SCO-ACN are proposed for the first time, as depicted in Scheme 2b.


Page 16 of 24

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406 407

Figure 5. In situ IR spectra of NO adsorption (a, b) and visible-light reaction processes (c, d) over

408

CN and SCO-ACN-0.1, temporal evolution of normalized absorbance of adsorbed Sr-NO

409

NO3 species on photocatalysts surface during NO adsorption process (c) and photocatalytic NO

410

oxidation process (f).

δ(+)

and

−

411

And there are some differences exist between the adsorption and photocatalysis

412

processes for CN and SCO-ACN-0.1. Firstly, owing to the introduction of SrCO3, the

413

NO molecules tend to be adsorbed on the SrO clusters to form Sr-NOδ(+), and the

414

reaction intermediates would be preferentially oxidized by reactive oxygen species.

415

Secondly, the increased consumption of OH groups in SCO-U-0.1 during the reaction

416

is not only beneficial for the conversion of intermediates but also contributes to the

417

generation of hydroxyl radicals, in accordance with the ESR results. Last, although

418

the adsorption and photocatalytic NO oxidation with SCO-ACN are slightly different

419

from those with the pristine CN, the SrO clusters do not change the overall conversion

420

pathway of photocatalytic NO oxidation. Significantly, the SrO clusters as newly

421

formed active centers facilitate the activation of NO via the formation of

422

Sr-NOδ(+)/NO+ and promote the conversion of NO to the final products (Scheme 2c).

423

Therefore, both the conversion pathway of photocatalytic NO oxidation and the

424

reasons for the enhanced photocatalytic activity are directly reflected in the in situ

425

DRIFTS spectra.



426 427

Scheme 2. The conversion pathways of adsorption and photocatalytic oxidation of NO over CN (a)

428

and SCO-ACN-0.1 (b), the illustration of the catalyst structure and key role of SrO clusters (c).

429 430

ASSOCIATED CONTENT


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431

Supporting Information.

432

Evaluation of photocatalytic activity. The XPS survey spectra of CN (a) and

433

SCO-ACN-0.1 (b), high-resolution C1s (c) and O1s (b) XPS spectra of SCO-U-0.1

434

sample. SEM images of as-prepared samples (a-d), EDX elemental mapping of N, C,

435

Sr and O in image for SCO-ACN-0.1 sample (e) and relative energy comparison for

436

CO32− → HCO3− reaction at bridge and edge sites of carbon nitride (f). Optimized

437

structures of pristine g-C3N4, CO32− and HCO3− ions (a). Reaction pathways of HCO3−

438

generation at edge (b) and bridge (c) sites in CN. N2 adsorption-desorption isotherms

439

curves (a) and pore-size distribution (b) of as-prepared samples. Table showing SBET,

440

pore volume, formula, and NO removal ratio of the samples. Transient photocurrent

441

densities (a) and UV-vis DRS spectra (b) for as-prepared samples. Reaction rate

442

constants k of the as-prepared samples. Visible-light-driven H2O2 formation over CN

443

in 60 minutes. Table listing assignments of the FT-IR bands observed during NO

444

adsorption over the photocatalysts. Table assignments of the FT-IR bands observed

445

during photocatalytic NO oxidation over photocatalysts. This material is available

446

free of charge via the Internet at http://pubs.acs.org.

447 448

ACKNOWLEDGMENTS

449

This research was financially supported by National Natural Science Foundation of

450

China (51478070, 21501016 and 51108487), the National Key R&D project

451

(2016YFC0204702),

452

(CXTDG201602014) and the Key Natural Science Foundation of Chongqing

453

(cstc2017jcyjBX0052).

the

Innovative

Research

Team

of

Chongqing

454 455

Author Contributions

456

The manuscript was written through contributions of all authors. All authors have

457

given approval to the final version of the manuscript.

458 459

REFERENCES

460

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Highly Efficient Performance and Conversion Pathway of

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