pseudo photocatalysis

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Remediation and Control Technologies

Deep oxidation of NO by a hybrid system of plasma-N type semiconductor: High energy electron activated “pseudo photocatalysis” behavior Si Chen, Haiqiang Wang, Mengpa Shi, Haoling Ye, and Zhongbiao Wu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00655 • Publication Date (Web): 04 Jul 2018 Downloaded from http://pubs.acs.org on July 4, 2018

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Environmental Science & Technology

1

Deep oxidation of NO by a hybrid system of plasma-N type semiconductor: High

2

energy electron activated “pseudo photocatalysis” behavior

3 Si Chen1,2, Haiqiang Wang1,2*, Mengpa Shi1,2, Haoling Ye1,2, Zhongbiao Wu1,2

4 5 6

1. Key Laboratory of Environment Remediation and Ecological Health, Ministry of Education, College

7

of Environmental & Resources Science, Zhejiang University, Hangzhou 310058, P.R. China;

8

2. Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas

9

Pollution Control, Hangzhou 310027, P.R. China;

10 11 12 13

*

14

(H. Wang) E-mail: [email protected] ; Tel. / Fax: +86-571-87953088.

15

Full postal address: Key Laboratory of Environment Remediation and Ecological Health, Ministry of

16

Education, College of Environmental & Resources Science, Zhejiang University, Hangzhou 310058,

17

P.R. China.

Corresponding author:

1

ACS Paragon Plus Environment


18

ABSTRACT

19

A “pseudo photocatalysis” process, being initiated between plasma and N-type

20

semiconductors in the absence of light, was investigated for NO removal for the first time

21

via dynamic probing of reaction processes by FT-IR spectra. It was demonstrated that

22

N-type semiconductor catalysts could be activated to produce electron-hole (e--h+) pairs

23

by the collision of high-energy electrons (e*) from plasma. Due to the synergy of plasma

24

and N-type semiconductors, major changes were noted in the conversion pathways and

25

products. NO can be directly converted to NO2- and NO3- instead of toxic NO2, owing to

26

the formation of O2- and ·OH present in catalysts. New species like O3 or ·O may be

27

generated from the interaction between catalyst-induced species and radicals in plasma at

28

a higher SIE, leading to deep oxidation of existing NO2 to N2O5. Experiments with added

29

trapping agents confirmed the contribution of e- and h+ from catalysts. A series of

30

possible reactions were proposed to describe reaction pathways and the mechanism of

31

this synergistic effect. We established a novel system and realized an e*-activated

32

“pseudo photocatalysis” behavior, facilitating the deep degradation of NO. We expect that

33

this new strategy would provide a new idea for in-depth analysis of plasma activated

34

catalysis phenomenon.

35

KEYWORDS

36

Plasma; Semiconductor catalysts; Pseudo photocatalysis; Deep oxidation; FT-IR spectra 2


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37

TOC Art:

38 39

1. INTRODUCTION

40

NO, one of the primary air pollutants, is mainly removed via selective catalytic

41

reduction (SCR) using ammonia under high temperatures in power plants1. But, it isn’t

42

the best choice for the middle and small industrial coal-fired boilers2. Use of plasma

43

allows selective transfer of input electrical energy to electrons, without heating any other

44

species3, 4, and further contribute to the form of highly active species by collision

45

reactions. This method has been considered as a potential alternative for NO removal at

46

normal environment without using toxic ammonia5-8. However, high energy consumption

47

and formation of unwanted products like poisonous NO2 are some of the bottlenecks

48

faced during the industrial application of plasma instruments5. This led to the

49

development of “plasma-catalysis” technology, which integrates the use of plasma and

50

catalysts.

51

Great effort has been made for developing the hybrid processes involving various 3



52

types of cooperation methods, catalyst selection9,

53

parameters11, 12, et al. Catalysts tip the scales in a plasma-catalysis system. Most reports

54

mainly focused on transition metal oxide catalysts like Ce13, Co14, Cu15, Mn16, and Ni17;

55

noble metal oxide catalysts like Ag18, Ru18, Pd18, and Pt19, or mixed oxide catalysts like

56

Cu-Ce20, Mn-Ce21, Mn-Co22, and Ag-Mn23. It has been proposed that the formation of

57

reactive oxygen species (ROS) on catalysts mainly contributed to efficiency enhancement

58

20, 21, 24

59

The production of O3 and its decomposition on catalysts surface played a key role in the

60

formation of active oxygen species28. However, the studies discussed so far aimed at

61

degrading VOCs, and are not applicable for NO, since O3 was not generated at all under

62

this condition because of the precedence of competitive reactions of NO2 and O2 with O

63

radicals reported in previous literatures29. Besides, catalysts can also be activated by the

64

increasing temperatures of DBD reactor during the long time discharge24, however, it was

65

not desired owing to NO generation in the background. Another method proposed for NO

66

removal was a two-stage combination of plasma and SCR processes30-32. On the contrary,

67

a new strategy which might be useful for NO removal was to improve the usage of O

68

radical and realize the deep oxidation of NO with the synergy contribution of catalysts

69

and plasma.

70

10

and impact of operational

and is also affected by the physical and chemical properties of the catalysts25-27.

Since high-energy electrons (e*) and active radicals are continuously generated in a 4


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71

plasma process, it is expected that catalysts can take advantage of such plasma generated

72

species for activation. In photocatalysis process, semiconductor materials—with an

73

ingredient-dependent band gap energy (Eg) of 0.1–5 eV33—are employed and electrons(e-)

74

in the valence band jump into the conduction band along with holes generation in the

75

valence band, further driving a series of reactions for pollutant degradation when the

76

energy of the absorbed photon is greater than Eg34. Such photocatalysis behavior might be

77

realized in plasma if efficient energy is provided to semiconductors (which are then

78

activated) through the collision of e* and semiconductor. The difference between

79

photocatalysis and e* activated “pseudo photocatalysis” behavior would only be in the

80

form of energy loading. Many e* of energy 1–25 eV3,

81

requirement of semiconductors, can just be generated in the plasma process. It was also

82

demonstrated that increasing the specific input energy (SIE) in plasma was expected to

83

effectively enhance the reduced electric field and consequently increase the mean

84

electron energy20. Literatures also reported that “dark photocatalysis” can be triggered if a

85

voltage applied to the material causes anodic schottky barrier breakdown36,

86

conjecture thus has feasibility. High efficiency but low energy consumption can be

87

realized if this assumption was verified, as the two effects can be obtained through the

88

consumption of one energy. Attempts have been made to enhance the efficiency via

89

cooperating with TiO2

38, 39

35

, which covers the energy

37

. This

, but mechanism involved has not been investigated 5



90

extensively. It has still not been clarified which kind of generated species in plasma can

91

be used by catalysts. The co-effect between the plasma and the catalyst also needs to be

92

interpreted clearly. Moreover, which kind of semiconductors can be activated is scarcely

93

discussed.

94

NO degradation was carried out for the first time under the in situ synergy of plasma

95

and semiconductor catalysts. It is expected that e* or active radicals can be best utilized

96

by semiconductors to enhance NO conversion and reduce secondary pollution. Both

97

P-type and N-type semiconductors were investigated and the reaction process was

98

dynamically probed by FT-IR spectra. A series of catalysts with different Eg and

99

conduction/valence band position were chosen for tests. The relevance of catalysts’

100

bandgap energy and SIE of plasma for NO removal is discussed in detail, to illuminate

101

the relationship between the mean electron energy of plasma and the prerequisite energy

102

for catalysts to be activated, SIE playing the role of a bridge, since bandgap energy was

103

the minimum energy required to be activated for semiconductors, and by increasing SIE

104

can enhance the mean electron energy in plasma. Synergistic effects and the changed

105

reaction pathways were studied by adding different trapping agents, with the

106

quantification of NO2- and NO3- by ion chromatograph (IC). A feasible promotion

107

mechanism and pathways are proposed.

6


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108

2. EXPERIMENTAL

109

2.1 Experimental setup. Figure. S1 exhibits an overall principal diagram for the

110

experimental system. The inlet concentration of NO was 200 ppm, with 6% O2 fraction,

111

N2 as a carrier gas, the total gas flow was 3 L min-1. A packed-bed cylindrical DBD

112

reactor with a discharge gap of 3.5 mm and discharge length of 200mm was employed.

113

The detailed information about the DBD reactor design, the analysis system, the

114

assessment of energy consumption were discussed in Text S1 of Supporting Information.

115 116

To assess energy consumption, the specific input energy (SIE) and NO removal efficiency were defined and calculated as follows:

117

SIE (J/L) = P/Q = discharge power (W) × 60 (s/min) / total flow rate (L/min)

(1)

118

P (W) = E × f = input energy (J) × frequency (Hz)

(2)

119 120 121

E (J) = u × id !"

=

transient voltage V × transient current Ad

η=(C(NO)inlet - C(NO)outlet) / C(NO)inlet × 100%

(3) (4)

122

2.2 Catalyst preparation. Glass balls with diameter of 2.5 mm were chosen as catalyst

123

supports to avoid the strong adsorptions of conventional catalyst supports such as γ-Al2O3.

124

Prior to catalyst preparation, a binder based on pseudo-boehmite was prepared for

125

catalysts loading effectively. Pseudo-boehmite (10 g) was dissolved in 200 ml deionized

126

water, adding saltpeter solution (1M, 20 ml), stirring at 80–85°C for 0.5 h and natural 7



127

cooling. Reflux condensation was required during the experiment.

128

Catalysts used in this study consisted of two types of semiconductor materials listed

129

below: representative P-type materials like Co3O440, BiOI41, and Ag3PO442; and N-type

130

materials such as commercial P25, ZnO43, WO344, CdS45, In2O346, and Fe3O447, which

131

included variable-width Eg and ensured the generation of O2- and ·OH radicals alone or

132

concurrently, with their detailed information described in Text S2. All were prepared in

133

advance according to literatures.

134

To prepare P25/glass balls (taking P25 as example), a given amount of glass balls

135

was soaked in a solution of pseudo-boehmite and drained; P25 powder was whisked onto

136

the surface-wet glass balls until well blended, glued with the assistance of a spoon, and

137

dried at 60°C.

138

2.3 Products analysis. An ion chromatograph (IC, Shimadzu LC-20A, Japan) was

139

adopted for measurements of NO2- and NO3-. Prior to the measurements, the used

140

catalysts were washed with ultrasound in 20 mL deionized water for 30 min, and the

141

liquid supernatant obtained after centrifugal separation.

142

2.4 characterization. Related characterization methods were discussed in Text S3.

143

3. RESULTS AND DISCUSSION

144

3.1 Plasma-catalytic removal of NO. 3.1.1 Plasma cooperating with P-type

145

semiconductor materials. Initially, plasma degradation of NO with blank glass balls 8


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146

was carried out and its IR spectra with SIE is presented in Fig. 1. The peak at 1900 cm-1

147

was attributed to NO48 and its intensity showed a sharp decrease (and even disappeared)

148

with the increasing of SIE. Intensities of peaks at 1600 cm-1 and 2238 cm-1 —assigned to

149

NO249 and N2O50, 51, respectively—grew sustainably. NO experienced oxidation by O

150

radicals (mainly to NO2) with the input of energy and reduction via N radicals to N2O.

151

Some N2 may exist in products originating from N radicals, and bands at 2919 cm-1 and

152

1300 cm-1 were deemed the concomitant peaks of NO2 and N2O, respectively. In addition,

153

NO could be detected again under higher SIE, which was ascribed: (A) The degree of

154

ionization of N2 and O2 would be larger when input energy increased, which facilitated

155

the generation of NO from background (Table S2, R1-R5); (B) There existed a fast NOx

156

reaction (Table S2, R6) leading to the excessive consumption of O radical, whose

157

reaction rate (5.5×10-12 cm3/(mole·s)) balanced the reaction rate of NO reacting with O

158

radical (Table S2, R7)(2.99×10-11 cm3/(mole·s)). Thus excessive NO would obtain in

159

off-gas.

160

For further conversion of NO, a series of typical P-type semiconductor materials

161

(Co3O4, BiOI, and Ag3PO4) was placed in plasma zone and their co-effects on the

162

removal of NO with SIE recorded via IR spectra (shown in Fig. S4). IR spectra obtained

163

were almost completely identical to that of the blank sample, and the conversion

164

pathways of NO remained unchanged with SIE. The oxidation pathway for NO to NO2 9



165

occupied a leading position, along with low yields of N2O and N2 via reduction pathway,

166

with or without catalysts assistance. In addition, the intensity of the peak at 1900 cm-1 for

167

NO in Fig. S4 was much stronger than in the blank at SIE of 46 J/L, i.e., an inhibiting

168

effect on NO removal efficiency may be introduced by these materials. In brief, these

169

catalysts were not activated by plasma and did not work at all.

170 171

Figure 1. Evolution of NO removal with SIE by plasma filled with blank glass balls.

172

3.1.2 Plasma cooperating with N-type semiconductor materials. A different and

173

interesting phenomenon was noticed when a set of N-type semiconductor catalysts played

174

the role of fortifiers for the degradation of NO by plasma. The catalysts adopted in this

175

stage were TiO2 (commercial P25), ZnO, WO3, CdS, In2O3, and Fe3O4. As shown in Fig.

176

2, the intensity of band at 1900 cm-1 to NO dropped to zero with SIE, as before. The band

177

at 1600 cm-1 for NO2 and band at 2238 cm-1 for N2O appeared and got stronger. Unlike

178

the previous situation, the bands at 1720, 1246, and 743 cm-1 corresponding to N2O549

179

formed when SIE was greater than 130 J/L, along with the formation of HNO352, whose 10


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180

absorption peaks were located at 1308 and 876 cm-1. The addition of N-type catalysts in

181

the plasma zone thus provided a new approach for the deep oxidation of NO to N2O5 and

182

HNO3. Considering Fig. 2d, 2e, and 2f, the intensity of the peak at 1600 cm-1 attributed to

183

NO2 showed no marked increase with SIE, even under total removal of NO,

184

corresponding to a low yield of NO2 in the presence of CdS, In2O3, and Fe3O4.

185

Modification of NO conversion pathways may thus be brought about by the introduction

186

of some N-type materials. The details of the pathway will be discussed later. Another

187

difference brought forth by these catalysts was the generation of O3 at some SIE with

188

infrared characteristic peaks at 2125 and 1050 cm-1 53. All in all, N-type semiconductor

189

catalysts may possess some characteristics motivated by plasma and advanced oxidation

190

of NO to the highest valence would be accessible with the cooperation of plasma and

191

catalysts in situ.

11



192 193

Figure 2. IR spectra of NO removal with SIE by plasma cooperating with N-type materials (a) P25, (b)

194

ZnO, (c) WO3, (d) CdS, (e) In2O3, and (f) Fe3O4.

195

3.1.3 Comparison and analysis. After detailed discussion of the dynamic conversion

196

of NO by plasma assisted by P-type and N-type semiconductor catalysts, the N-type was 12


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197

demonstrated to implement a real synergistic effect with plasma, which caused the deep

198

oxidation of NO. Such a phenomenon could be attributed to the following reasons: (A)

199

photocatalysis is a process where the semiconductor is excited by absorbing a photon to

200

produce photo-induced electrons (e-) and holes (h+) to form a redox system, when the

201

intensity of incident light matches or exceeds the bandgap energy of the

202

semiconductor54-56. The band gap energy of semiconductor materials here ranged from

203

about 0.1 to 5 eV33. The mean electron energy of plasma was 1–25 eV3,

204

increasing ratio of e* with the increase of SIE20. It was thus possible that the energy-rich

205

electrons originating from the DBD plasma collided with the electrons in the valence

206

band of semiconductors to activate them. The activated electrons (e-) then jumped into

207

the conduction band, leaving behind a hole in the valence band and further realize a

208

“pseudo photocatalysis” behavior. (B) While focused on the living environment of

209

catalysts, an external electric field can be provided by DBD, which can provide the LSPR

210

effect. Its facilitation in the separation of catalysts’ electrons and holes has been

211

reported58, 59. (C) Electrons were the majority carriers in the N-type region, while holes

212

the carriers in the P-type area33. e* acted as the center of energy on account of the natural

213

form of loading energy in plasma3, and a large proportion of the collision reactions were

214

directed by the e* in the plasma. Hence, activation reactions may be easier to conduct on

215

N-type semiconductor catalysts due to the presence of excess electrons. This qualitatively 13


35, 57

, with


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216

agrees with our experimental results. (D) Ultraviolet light produced during the discharge

217

process was too weak to motivate the photocatalysis reaction as reported in earlier

218

studies60-62. The identical phenomenon of deep oxidation was also investigated when

219

Fe3O4 was adopted, Fe3O4 being acknowledged as an impracticable choice for normal

220

photocatalysis. Direct activation of N-type semiconductor materials by plasma can thus

221

be re-confirmed indirectly.

222

3.1.4 Connection between SIE and bandgaps of catalysts. Experiments were further

223

carried out to explore the influence of ingredient-dependent band gap energies of

224

catalysts. Fig. 3 displays the NO removal efficiency of the plasma-catalytic system as a

225

function of SIE (from 0 to 130 J/L) with different N-type catalysts. There existed a

226

positive correlation between the disposal efficiency of NO and the input energy, showing

227

that higher discharge power could induce more microdischarges and more active species

228

for chemical reactions15. It was noticed that the removal efficiency order was CdS > In2O3 > WO3 > Fe3O4 >

229 230

P25

>

ZnO,

which

negatively

correlated

with

231

composition-dependent band gap energy (Eg) of the semiconductor catalysts was

232

considered the minimum energy required to be excited by an absorbing photon36, when

233

referred to traditional photocatalysis system. Extremely analogously, with the collision of

234

e* from plasma of energy greater than Eg, the excitation and transition of e- and the 14


the

bandgap

energy.

The

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235

production of orbital holes could theoretically be achieved when these semiconductor

236

catalysts were exposed to an atmosphere of plasma full of e*. It has been demonstrated

237

that plasma possesses higher yield of e* and larger mean electron energy under greater

238

input energy20. At each SIE, there must be more energy-loaded electrons available for

239

catalysts with narrower bandgaps, further accelerating chemical reaction. In short,

240

narrower bandgap of catalysts improves the ease of activation, resulting in improved NO

241

removal efficiency. Fe3O4 may behave abnormally because of the recombination of e- and

242

h+ generated and weak oxidation capacity for its unusually narrow bandgap .

243 244

Figure 3. Effects of bandgap of N-type catalyst on the plasma-catalytic removal of NO with SIE

245

3.2 Exploration of reaction pathway and mechanism. To clarify the underlying

246

mechanism of the synergistic effect between the plasma and N-type semiconductor

247

catalysts, further study of the effect of e- and h+ produced by catalysts on NO removal

248

was carried out. CdS was as a typical case, and Fe3O4 and P25 were also taken here for 15



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249

supplementary discussion.

250

3.2.1 Deep oxidation to N2O5 and HNO3. Experiments with excessive addition of

251

electron-trapping agent (Na2C2O4)

252

developed as shown in Fig. 4. Initially, excessive amount of trapping agent with mass

253

ratio of 1:10 was employed to guarantee total consumption of h+ or e- during the process.

254

Peaks at 1720, 1246, and 743 cm-1 attributed to N2O5 and peaks at 1308 and 876 cm-1 for

255

HNO3 still arose at some SIE in both situations. A less pronounced but arguably more

256

crucial observation from the data that the intensity of peaks for N2O5 in Fig. 4a were

257

much weaker than those in Fig. 4b, corresponding to greater inhibition of deep oxidation

258

under the condition that holes produced by catalysts were captured and not used for

259

reaction. It revealed that both e- and h+ originating from catalysts contributed to the

260

further oxidation of NO to N2O5, and that the h+ played a vital role, assisted by e-. While

261

in traditional photocatalysis, the generation of e- and h+ from activated semiconductor

262

would generally induce the generation of O2- and ·OH with strong oxidability, which

263

plays dominant role in chemical reactions54, 65, 66, thus further investigation about O2-

264

and ·OH is necessary. In the case of CdS (ECB=-0.52ev, EVB=1.88ev)67, O2- (O2/O2- =

265

-0.33ev)68 can be generated but ·OH(OH-/·OH = 1.99ev)69 can’t, which indicated that

266

species like e-, h+ and O2- itself or their derivative can contribute to the N2O5 generation.

267

For further exploration, Fe3O4 (ECB=1.23ev, EVB=1.33ev, can’t produce O2- and ·OH)

63

or hole-trapping agent (K2Cr2O7)64 in CdS was

16


70

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268

and P25 (ECB=-0.8ev, EVB=2.4ev, can produce both O2- and ·OH)

54

269

other two types of cases as displayed in Fig. S5, N2O5 and HNO3 were still obtained more

270

or less. Integrating the results, it can be proposed that e- and h+ itself or their derivative

271

such as O2- and ·OH may play a part in this deep oxidation course. In addition,

272

comparison experiments (Figure S6) were conducted to guarantee that the addition of

273

trapping agents almost do not affect the plasma process and the experimental results.

were taken as the

274 275

Figure 4. IR spectra of plasma cooperating with CdS with excess addition of (a) h+ trapping agent

276

Na2C2O4, and (b) e- trapping agent K2Cr2O7.

277

As has been noted, peaks at 1050 cm-1 attributed to O3 could be observed when

278

some N-type catalysts were introduced into the plasma zone, and its dynamic

279

concentration were in consistent with FT-IR spectra, as displayed in Fig. 5a. In2O3 owned

280

the largest yield of O3, followed by CdS and Fe3O4, whereas others did not. O3 generation

281

induced by catalysts may act as one of the sources for the deep oxidation of NO to N2O5.

282

A control test of charging the background gas (N2 + 6% O2) was carried out to study O3 17



283

generation, in order to avoid its consumption during normal experiments, and Fig. 5 (b-d)

284

exhibits the yield of O3 under different conditions. Highly enhanced production of O3

285

could be obtained under the co-effect of plasma and catalysts relative to blank glass balls,

286

and the suppression of e- or h+ may be beneficial for the generation of O3, and that greater

287

promotion was achieved when excessive K2Cr2O7 was appended to capture e-. Results

288

from CdS (Fig. 5b) can be obtained that species of h+ and e- or O2- would facilitate O3

289

generation. While for Fe3O4 (Fig. 5c), more yield of O3 can only be obtained under the

290

addition of excessive K2Cr2O7, comparing to blank glass balls, which further indicated

291

that e- itself may not contributed to O3 generation. In addition, yield of O3 can be

292

enhanced likewise under the co-effect of plasma and P25 (Fig. 5d), showing that ·OH

293

may also play a part. In sum, h+ and ·OH could generate O3 and play the dominant role,

294

while e- can get work only on condition that e- was transformed to O2-.

295

Based on the front discussion, except for O3 generation motivated by catalysts, other

296

possible reaction pathways may include the following three. (A) Holes produced by

297

catalysts during collision may directly oxidize NO2 to N2O5, when NO is oxidized to NO2

298

by the plasma in advance. (B) The e- formed can react quickly with O2 to produce O2-,

299

whose strong oxidizability may provide a pathway for NO2 to N2O5. (C) O- produced in

300

plasma could be converted to O radicals by h+, and further oxidation was achieved.

18


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301 302

Figure 5. (a) Yield of O3 for N-type catalysts during the course of NO degradation. And yield of

303

O3 when discharging in N2 + 6% O2 under different situations with SIE: (a) CdS, (b) Fe3O4 and (c)

304

P25.

305

3.2.2 “Lost” NO2. Some N-type catalysts like CdS, In2O3 and Fe3O4 exhibited nearly no

306

NO2 generation with the degradation of NO. Here, CdS was the typical case. Quantitative

307

study exhibited in Fig. 6a showed that high selectivity of NO2 was obtained in blank

308

sample, while NO2 could hardly be detected when CdS was used, with roughly constant

309

yield of N2O. Continuous discharge at 214 J/L for about 7 h was carried out and Fig. 6b

310

recorded its IR spectra with time. It can be found that the “lost” NO2 would gradually 19



311

reappear with time when focusing on the dynamic conversion of NO2. In addition, NO2

312

and NO3- were the only stable oxidation products of NO71, and the semiconductor

313

catalysts played dominant roles in photocatalysis oxidation; hence, we attributed the “lost”

314

NO2 to the formation of NO3-.

315 316

Figure 6. (a) Quantitative study with SIE for blank and CdS. (b) Dynamic IR spectra with time for

317

CdS under SIE of 214 J/L.

318

Further evidence for the formation of NO3- was provided by transmission infrared

319

and ion chromatography (IC) measurements, as displayed in Fig. 7. Detailed analysis for

320

the results of transmission infrared were discussed in Text S4 of Supporting Information.

321

From Fig. 7a, several IR bands attributed to surface absorbed NO3- (1395 and 1348 cm-1)72

322

and NO2- (1109 and 1071 cm-1)73 were detected evidently for the used CdS, indicating the

323

formation of NO2- and NO3-. Further, the role of catalysts for the formation of nitrates were

324

studied by IC tests (Fig. 7b), long-term experiments of about 7 h were conducted for

325

every case at fixed SIE of 214 J/L, Prior to tests. Significant quantitative difference in the 20


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326

cumulative yield of NO2- and NO3- was initially detected between the blank sample and

327

CdS (about 600 ppm NO2- and 6500 ppm NO3- production for CdS and only small

328

quantities for blank), which proves the occurrence of direct conversion from NO to NO3-

329

when catalysts existed. What contributed to this conversion was further explored by

330

comparing the yield of NO2- and NO3- when shielding e- or h+ by adding the relevant

331

trapping agent. Obviously, the output of NO2- and NO3- showed marked decreases when

332

only one active species was active, but these were still much greater than with the blank.

333

Hence, both kinds play parts in the formation of NO2- and NO3- as discussed before.

334

Moreover, more NO2- and NO3- were formed under the situation of CdS + Na2C2O4

335

(1:10), which indicated that the presence of e- may facilitate this course.

336 337 338

Figure 7. (a) Transmission infrared characterization of fresh and used CdS; (b) IC measurements of NO2- and NO3- after long-term experiments at 214 J/L for different samples.

339

Since there was no addition of water in the system, it revealed that the hydrogen and

340

OH came from the inevitable water vapor in cylinder gas74, and the surface adsorption of 21



Page 22 of 37

341

catalysts or the reactor. Initially, the bands for surface adsorption of water molecules

342

(3140 and 1630 cm-1)75 have been detected for fresh CdS on the transmission infrared

343

spectra in Fig. 7a, while its intensity weakened clearly for the used CdS, along with the

344

generation of nitrates. It revealed that the water molecules adsorbed on catalysts surface,

345

and could be activated and participate in reactions. Further verification experiment has

346

been done to testify the participation of water, which was operated under the same

347

condition as mentioned with addition of 100 ppm water vapor, and the results displayed

348

in Figure S7. The removal efficiency of NOx and the yield of N2O5 and HNO3 were highly

349

enhanced after the addition of water vapor. It indicated that the trace water vapor in the

350

system can be utilized by catalysts.

351

3.3 Proposed pathways and mechanism. On the basis of the observations and analysis,

352

possible reaction pathways and a synergistic mechanism are proposed as follows, when

353

plasma cooperated with N-type catalysts in situ:

354

First, O and N radicals were formed as well as the generation of an electron-hole

355

pair (e- and h+) by activation of catalysts (when the energy requirement was met) by the

356

collision of high-energy electrons (e*) originating from plasma discharging process, e*

357

itself transformed to normal electrons (e) without high energy and annihilated over time.

358

e* + N2 → ·N + ·N + e

(5)

359

e* + O2 → ·O + ·O + e

(6) 22


Page 23 of 37


360

e* + catalysts → e- + h+ + e

361

Electrons and holes from catalysts could react with surface-adsorbed oxygen and

362

hydroxyl groups, resulting in the generation of ·O2- and ·OH when EVB and ECB of

363

catalysts were greater than the OH-/·OH redox potentials (1.99 eV) and O2/·O2- (-0.33

364

eV).

(7)

365

e- + O2 → ·O2-

(8)

366

h+ + OH- → ·OH

(9)

367

O radical from plasma could oxidize NO to NO2 with high selectivity and a small

368

fraction could be reduced to N2O and N2 by N radicals.

369

NO + ·O → NO2

(10)

370

NO + ·N → N2 + O·

(11)

371

NO2 + ·N → N2O + O·

(12)

372

NO + ·N3 → N2O + N2

(13)

373

Meanwhile, pathways for NO removal may change markedly when catalysts were

374

present; NO2 may vanish and be replaced by NO2- and NO3-, based on the direct

375

conversion of NO by ·O2- or ·OH and the reaction of NO2 with ·OH, if the catalyst had

376

been activated. Narrower bandgap of catalysts led to faster reaction and higher removal

377

efficiency:

378

·O2- + NO → NO3-

(14) 23



Page 24 of 37

379

NO + ·OH → HNO2

(15)

380

HNO2 + ·OH → NO2 + H2O

(16)

381

NO2 + ·OH → HNO3

(17)

382

Further increasing SIE increased the mean electron energy of the plasma, exciting

383

the catalysts thoroughly. Hence, interactions between species from plasma and the

384

catalysts may take place to produce new species like O3 and O radicals, which would

385

contribute to deep oxidation from NO2 to N2O5 in the presence of spare NO2, N2O5 being

386

further converted to HNO3.

387

h+ + O- + O2 → O3

(18)

388

·O2- + O+ → O3

(19)

389

h+ + O- → ·O

(20)

390

O3 + NO2 → NO3 + O2

(21)

391

·O + NO2 → NO3

(22)

392

NO3 + NO2 → N2O5

(23)

393

H2O + N2O5 → HNO3 + HNO3

(24)

394

Meanwhile, it is possible that strong-oxidizing holes and ·O2- from catalysts may

395

oxidize spare NO2 to N2O5 when the energy requirement was satisfied.

396

h+ + NO2 + O2 → NO3 + O+

(25)

397

·O2- + NO2 → NO3 + O-

(26) 24


Page 25 of 37

398 399


NO3 + NO2 →N2O5

(27)

ACKNOWLEDGMENT

400

This work was financially supported by National Key Research and Development

401

Program of China (2016YFC0204100), Zhejiang Provincial “151” Talents Program, Key

402

Project of Zhejiang Provincial Science & Technology Program, the Program for Zhejiang

403

Leading Team of S&T Innovation (Grant No. 2013TD07) and Changjiang Scholar

404

Incentive Program (Ministry of Education, China, 2009).

405

ASSOCIATED CONTENT

406

Supporting Information. The overall principal diagram for the experimental system

407

(Fig. S1), the variation temperatures of DBD reactor (Fig. S2), the discharge

408

characteristic of DBD (Fig. S3), the IR spectra of NO removal cooperating with P-type

409

materials (Fig. S4), the IR spectra of NO removal under Fe3O4 and P25 with trapping

410

agents (Fig. S5), the comparison of discharging in P25/glass ball with the addition of 100

411

ppm water vapor or not (Fig. S7), the correlation between applied voltage and SIE (Table

412

S1) and reactions related to the process (Table S2) are present in supplemental section.

413

The IR spectra of NO removal with sole trapping agents (Fig. S6), the influence of

414

NO absorption capability of catalysts (Fig. S8), the stability of NO2- and NO3- during the

415

plasma process (Fig. S9), the stability of photocatalysts in the plasma process (Fig. S10)

416

and the influence of surface areas of catalysts (Table S3) were discussed in supplemental 25



417

section.

418

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37


pseudo photocatalysis

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