Enhanced Performance and Conversion Pathway for Catalytic

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Remediation and Control Technologies

Enhanced Performance and Conversion Pathway for Catalytic Ozonation of Methyl Mercaptan on Single-Atom Ag Deposited Three-Dimensional Ordered Mesoporous MnO2 Dehua Xia, Wenjun Xu, Yunchen Wang, Jingling Yang, Yajing Huang, Lingling Hu, Chun He, Dong Shu, Dennis Y. C. Leung, and Zhihua Pang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03696 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018

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Environmental Science & Technology

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Enhanced Performance and Conversion Pathway for Catalytic

2

Ozonation of Methyl Mercaptan on Single-Atom Ag Deposited

3

Three-Dimensional Ordered Mesoporous MnO2

4 5

Dehua Xia a, b,, Wenjun Xu a, e,, Yuncheng Wang a, Jingling Yang a, Yajing Huang a,

6

Lingling Hu a, Chun He a, b, *, Dong Shu c, Dennis Y.C. Leung d,**, Zhihua Pang e

7 8

a School

of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China

9 b

10

Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou, 510275, China

11 12

c

13

Guangdong Universities, School of Chemistry and Environment, South China Normal

14

University, Guangzhou, 510006, China

Key Lab of Technology on Electrochemical Energy Storage and Power Generation in

d

15

Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong

16 e

17

South China Institute of Environmental Science, Ministry of Environmental Protection (MEP), Guangzhou 510655, PR China

18 19 20

Corresponding author: School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China. Email address: [email protected] (C. He); [email protected] (D.Y.C. Leung). *

1

ACS Paragon Plus Environment


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Abstract

22

In this study, Ag deposited three-dimensional (3D) MnO2 porous hollow

23

microspheres (Ag/MnO2 PHMSs) with high dispersion of the atom level Ag species are

24

firstly prepared by a novel method of redox precipitation. Due to the highly efficient

25

utilization of downsized Ag nanoparticles, the optimal 0.3% Ag/MnO2 PHMSs can

26

completely degrade 70 ppm of CH3SH within 600s, much higher than that of MnO2

27

PHMSs (79%). Additionally, the catalyst retains a long-term stability and can be

28

regenerated to its initial activity through regeneration with ethanol and HCl. The results

29

of characterization of Ag/MnO2 PHMSs and catalytic performance tests clearly

30

demonstrate that the proper amount of Ag incorporation not only facilitates the

31

chemi-adsorption but also induces more formation of vacancy oxygen (Ov) and lattice

32

oxygen (OL) in MnO2 as well as Ag species as activate sites to collectively favor to the

33

catalytic ozonation of CH3SH. Ag/MnO2 PHMSs can efficiently transform CH3SH into

34

CH3SAg/CH3S-SCH3 and then oxidize them into SO42- and CO2 as evidenced by in situ

35

diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs). Meanwhile,

36

electron paramagnetic resonance (EPR) and scavenger tests indicate that •OH and 1O2 are

37

the primary reactive species rather than surface atomic oxygen species contributed to

38

CH3SH removal over Ag/MnO2 PHMSs. This work presents an efficient catalyst of

39

single atom Ag incorporated MnO2 PHMSs to control air pollution.

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Keywords: Single atom Ag; Catalytic ozonation; Chemi-adsorption; MnO2 hollow

41

microspheres; Methyl mercaptan. 2


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1. Introduction

43

Methyl mercaptan (CH3SH), one of the typical sulfur-containing volatile organic

44

compounds (VOCs) with high toxicity and corrosive, is a representative odorous gas with

45

a very low odor detection threshold around 0.4 ppb/v [1]. CH3SH is widely produced in

46

urban waste, sewage treatment and industry wastes [2]. The presence of 5 ppm CH3SH in

47

the atmosphere will give us an unpleasant feeling, while the exposure of high

48

concentration of CH3SH can cause significant poisoning [3]. Till now, many

49

conventional technologies for eliminating odorous VOCs have been widely applied,

50

including adsorption, biological treatment, incineration, and plasma technology, but

51

which may not suitable for CH3SH from industrial activities with the concentrations

52

range from a few ppb to tens of thousands [4-8]. Currently, the catalytic conversion

53

process (including catalytic oxidation, catalytic decomposition) that does not need any

54

additional reagents (e.g. H2, O2) and produces low waste (e.g. H2S and inorganic

55

hydrocarbons), is considered as a suitable technique for the complete removal of CH3SH

56

[9, 10]. However, catalytic conversion process always operated under a higher

57

temperature (over 300 °C) and the catalyst can easily deactivate due to coke deposit [11,

58

12]. In comparison, catalytic ozonation is a more efficient advanced oxidation process for

59

non-selective mineralization of CH3SH with different concentrations at room temperature,

60

relying on the generated reactive oxygen species (ROS) as well as the ozone molecules

61

[13]. Due to the efficient decomposition of VOCs, catalytic ozonation cause no great

62

accumulation of intermediates on the catalysts and limited deactivation of catalysts [14]. 3



63

Moreover, ozone is an electrophilic molecule and it especially reacts with high electronic

64

density sites of molecules such as CH3SH, CH3S-SCH3 having carbon-carbon or

65

carbon-sulfur bonds [15].

66

During catalytic ozonation, the catalyst is important as it can accelerate

67

decomposition of ozone and enhance utilization of ozone simultaneously. Till now,

68

increasing interest has been paid in developing catalysts such as transition metal oxides

69

(MnO2, CeO2), noble metal (Ag/SBA-15, Pd-Mn/Al2O3) and carbon materials

70

(graphene), which exhibit promising performance for VOCs removal through catalytic

71

ozonation [16-19]. Among them, the widely studied MnO2 is more promising for

72

application, as it exhibits plentiful valence states, versatile structures (rod, wire, tubular,

73

and sphere), and high stability [20-23]. In recent, a three-dimensional (3D) ɑ-MnO2

74

porous hollow microsphere (PHMS) was reported as an efficient ozone catalyst to

75

remove bisphenol A. Compared with nonporous MnO2, the high surface area of 3D

76

ɑ-MnO2 can enhance its adsorption ability and prolong the retention of ozone, as well as

77

the rich lattice oxygen of ɑ-MnO2 can promote the ozone decomposition [24-26]. The

78

identified property of ɑ-MnO2 PHMSs infers its’ great potential for catalytic ozonation of

79

CH3SH.

80

To further enhance its catalytic ozonation property for CH3SH removal, the

81

cooperation of noble metals like Ag atom into ɑ-MnO2 PHMSs provides a feasible

82

strategy to modulate its structure. First, Ag can provide sufficient active sites and display

83

better catalytic ozonation than other metals (Co, Ni, Fe, Mn, Ce, Cu, etc.) [27, 28]. 4


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Second, Ag is benefit for the removal of sulfur-containing VOCs because it has high

85

affinity to S-H functional group [29, 30]. However, extensive application of the noble

86

metal catalysts is still limited by their high cost. To save the cost of noble metal, reducing

87

its deposited amount is a feasible method.

88

Recently, single-atom catalysis has become a research hotspot, because the promoted

89

dispersion of noble metal via downsizing a nanoparticle to the atomic level can maximum

90

enhance its utilization [31]. Tang et al. used thermal treatment to make Ag nanoparticles

91

to migrate along the external surfaces and insert in the tunnels of Hollandite-type MnO2

92

nanorods, which can effectively catalyze oxidation of HCHO due to the high dispersion

93

of sub-nanosized Ag and even single atoms [32]. However, the decoration of catalysts

94

with highly dispersed noble-metal remains a tough task, because the Ag or Au

95

nanoparticles with high surface energy easily agglomerate into large particles via

96

Ostawald ripening [33]. Moreover, most noble-metal atoms are lost inside the bulk during

97

synthesis through conventional methods such as coprecipitation or photo-deposition,

98

where they are no longer effective as active sites [34, 35]. Furthermore, some studies also

99

indicate that more tight contact can be achieved between atomic metal and MnO2 than

100

that of Ag cluster and MnO2, thus can enhance the charge transfer of catalyst for catalytic

101

reaction [36, 37].

102

Inspired by the strategy for stabilizing single-atom Au on the surface of MnO2 rods

103

via trapping by Chen et al. [38], a modified redox precipitation was developed to

104

fabricate single-atom Ag-deposited 3D ɑ-MnO2 PHMSs for CH3SH removal in the 5



105

present work. In this strategy, H2O2 etches the surface of ɑ-MnO2 through consuming

106

released H+, thereby driving hydrolysis of AgNO3 into Ag(OH), which could be then

107

reduced into metallic Ag and subsequently trapped into surface defects of α-MnO2

108

PHMSs. The downsized single-atomic Ag NPs via this method was well-evidenced by

109

HAADF-STEM. The enhanced performance of Ag/ɑ-MnO2 PHMSs is attributed to the

110

highly dispersed Ag species, which not only facilitate chemi-adsorption of CH3SH, but

111

also induce more vacancy oxygen and lattice oxygen in MnO2 as well as atomic Ag itself

112

as activate site to collectively improve subsequent catalytic ozonation. Besides, the

113

proposed mechanism for CH3SH mineralization process was revealed by in situ diffuse

114

reflectance infrared Fourier transform spectroscopy (DRIFTS). The 3D structure catalyst

115

with atomic Ag thus may oﬀer great potential for improving the reaction activity to

116

eliminate odorous VOCs, holding promise for environmental remediation.

117 118

2. Experimental

119

2.1 Preparation of Ag/MnO2 PHMSs

120

MnO2 PHMSs were successfully synthesized by a self-template process at room

121

temperature [39]. Ag/MnO2 PHMSs were prepared by a modified redox precipitation

122

method (Text S1 and Figure S1). Different from Chen et al. work to use MnO2 nanorods

123

[38], 1.0 g MnO2 PHMSs was ultrasonically dispersed in deionized water, and then a

124

certain volume of AgNO3 solution (atomic Ag source) was added. The concentrated H2O2

125

(0.5 g, 30 wt.%) was dropwise added to the mixture of MnO2 PHMSs and AgNO3. 6


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During adding H2O2, O2 was emitted. MnO2 (s) + H2O2 (aq) + 2H+ (aq) = Mn2+ (aq) +

127

2H2O (l) + O2 (g), where the consumption of released H+ can trigger AgNO3 hydrolyze to

128

Ag(OH): AgNO3 (aq) + H2O (l) = Ag(OH) (s) + HNO3 (aq). Because these two reactions

129

occur simultaneously, the Ag species could be trapped by an in situ formed hole.

130

Moreover, Ag(OH) could be further reduced into Ag nanoparticles by H2O2: O2 (g) + 2H+

131

(aq) + 2e- ⇌ H2O2 (l) and Ag(OH) (s) + H+ (aq) + e- ⇌ Ag (s) + H2O (l). Later, the solid

132

was dried at 200 °C for 2 h. Ag/MnO2 PHMSs prepared with different silver contents

133

were labeled as x Ag/MnO2 (x represents the mass fraction of Ag, e.g., 0.3% Ag/MnO2).

134

Moreover, H2O2 treated MnO2 was also prepared and the photo-deposition method was

135

applied to synthesize PD-0.3% Ag/MnO2 as an aggregated-Ag deposited on MnO2 for

136

comparison. The characterization methods are described in the supporting information

137

(Text S2).

138 139

2.2 Catalytic ozonation of CH3SH with Ag/MnO2 PHMSs

140

The elimination of CH3SH with catalytic ozonation by Ag/MnO2 PHMSs was

141

carried out using a continuous flow reactor as shown in Figure S2. The fixed bed quartz

142

tube reactor (Φ1.5 cm) was padded with 0.1 g Ag/MnO2 PHMSs for each experiment.

143

The CH3SH and N2 from bottle gas were premixed in a container to adjust the initial

144

concentration of CH3SH maintained at 70 ppm. O3 was produced by an ozone generator

145

(YDG, YE-TG-02PH) and the initial concentration of O3 was controlled at 1.5 mg/L. The

146

concentrations of CH3SH and ozone in the inlet and outlet were continuously monitored 7



147

by a CH3SH sensor (Detcon, DM-400IS) and an ozone analyzer (IDEAL-2000),

148

respectively. The total gas stream velocity was kept at 0.1 L/min. A mixture of used

149

catalyst and activated carbon was used to catalyze and absorb the residual ozone and

150

exhaust gas, to ensure the emission of remaining gas meets the relevant standards of

151

environmental protection. 0.050 g Ag/MnO2 was respectively added into 20 mL of

152

trapping agents solution (20 mM of TBA, EDTA-Na, Ascorbic acid, Cr(VI), NaN3) and

153

ultrasonicated for 30 min. The suspension was then loaded on non-woven fabric,

154

followed by drying in air at 60 °C until water was completely removed. The next steps

155

were the same as catalytic ozonation of CH3SH.

156 157

3. Result and discussion

158

3.1 Structural analysis of the catalysts

159

In our strategy, the surface defects of the MnO2 PHMs can be created via H2O2

160

etching and are filled by downsizing and confining Ag NPs simultaneously, thus leading

161

to the well-dispersion of atomic Ag. The content of incorporated Ag in the MnO2 PHMSs

162

was determined by ICP-OES, which are listed in Table S1. The corresponding contents

163

are close to the nominal values. The crystalline structures were analyzed through XRD.

164

As shown in Figure 1a, the MnCO3 showed the sharp characteristic reflections

165

corresponding to the pure rhomb-centered hexagonal crystal phase of MnCO3 (JCPDS

166

No.44-1472) [39]. Meanwhile, the main peaks of MnO2 solid spheres (2θ =28.8°, 37.3°

167

and 56.7°) can be well indexed to the tetragonal α-MnO2 phase (JCPDS 44-0141) [40]. 8


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Similarly, the MnO2 PHMSs also match with the tetragonal α-MnO2 phase and no

169

observation of MnCO3 peaks, confirming that MnCO3 core is completely dissolved by

170

acid to form hollow structure. Moreover, after Ag deposition, the diffraction peaks of

171

MnO2 PHMSs become stronger and wider, indicating microscopic stress exists and leads

172

to the interaction of Ag with MnO2 support [41]. However, no diffraction peaks

173

corresponding to Ag can be observed, presumably owing to the ultrafine-sized and high

174

dispersion of Ag [42]. Moreover, the N2 adsorption and desorption isotherms for these

175

samples (Fig. S3a) are of type IV curves with obvious hysteresis loops at a relative

176

pressure P/P0 of 0.5-1.0, which is a character of capillary condensation occurring in

177

mesoporous solids [35]. The pore size distribution curves in Fig. S3b indicates the hollow

178

structure of both samples, while the average pore-size and specific surface area of 0.3%

179

Ag/MnO2 PHMs (100.85 nm, 104.11 m2 g−1) are smaller than MnO2 PHMs (138.13 nm,

180

124.13 m2 g-1), mainly due to the atomic Ag particles are highly dispersed and even inside

181

the porous hollow structure of MnO2 (Table S1).

182

As depicted in Figure 2a, the SEM images show that the 0.3% Ag/MnO2 PHMSs

183

have regular 3D spherical shape, and the cavity of the broken MnO2 PHMSs confirms its

184

hollow structure, indicating Ag incorporation does not break the 3D structure of MnO2.

185

In contrast, some rough deposits appeared on the surface of PD-0.3% Ag/MnO2 PHMs,

186

indicating the occurrence of agglomerated Ag nanoparticles (Figure 2b). To further

187

inspect the defined structure of catalyst, HRTEM, HAADF-STEM, and EDS-mapping

188

were utilized. The TEM in Figure 2c further confirms the 3D structure of MnO2 PHMSs. 9



189

The HRTEM analysis (Figure 2d) of the optimal 0.3 % Ag/MnO2 PHMs reveals that the

190

polycrystalline walls of MnO2 are grown along the [110] direction with lattice spacing of

191

0.35 nm, but it is hard to find Ag nanoparticles within catalyst. Interestingly, due to the

192

difference in element contrast between Ag and Mn, the Ag species looks brighter than the

193

Mn in the dark field of Cs corrected HAADF-STEM (Figure 2e). According to

194

Cs-corrected HAADF-STEM images (Fig. 2e), the bright dots (Ag) with particle size of

195

approximately in 1 Ångstrom scale were well-dispersed in α-MnO2 PHMSs, which

196

matches the van der Waals diameter of a single Ag atom [42, 43]. The HAADF-STEM

197

image of 0.3% Ag/MnO2 PHMs at larger scale in Figure 2e shows no aggregation of the

198

Ag species. The mapping images of Au, Mn, and O shown in Figure 2f-h and EDX data

199

in Figure S4 indicates that Ag nanoparticles are homogenously dispersed on the MnO2

200

PHMSs.

201 202

3.2 Surface composition and reducibility

203

XPS tests were conducted to further analyze the surface chemical compositions and

204

element oxidation states of catalysts. As shown in Figure 3a, the Mn 2p3/2 region was

205

resolved into sub-bands, three individual component bands at 640.6 eV, 641.6 eV and

206

642.8 eV represent Mn2+, Mn3+ and Mn4+ respectively [44]. Table S2 displays that the

207

surface molar ratio of Mn2+ + Mn3+ to Mn4+ in Ag/MnO2 PHMSs (1.06) is higher than

208

that of MnO2 PHMSs (0.96), indicating the incorporated Ag leads to an increase in the

209

content of Mn2+ and Mn3+. In general, the increase of surface Mn at low valence state can 10


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improve the amounts of crystalline defects and oxygen vacancies in MnO2, which play

211

important roles as active sites for the catalytic ozonation to generate activated oxygen

212

species [44]. Moreover, all the O-1s XPS spectra in Figure 3b have three peaks at 529.7

213

eV, 531.1 eV and 532.1 eV, which are assigned to lattice oxygen (OL), adsorbed oxygen

214

(Oads, such as O2 − , O − and OH group) and surface oxygen (H2O), respectively [45].

215

Table S2 indicates that molar ratio of OL:Oads in Ag/MnO2 PHMSs (2.12) is higher than

216

that of MnO2 PHMSs (1.78), suggesting the incorporated Ag can enhance the formation

217

of OL. Tang et al reported that the Ag atoms have migrated along the surface and are

218

highly dispersed on/in MnO2, and the stronger interaction between Ag and MnO2

219

increase the density of crystalline [34]. Generally, OL with high mobility is more

220

favorable than Oads for the formation of active oxygen species through the migration

221

between surface lattice oxygen and oxygen vacancy with molecular oxygen [46].

222

Additionally, there are two peaks belonging to Ag-3d5/2 (B.E. = 368 eV) and Ag-3d3/2

223

(B.E. = 374 eV) orbital, which can be further separated and fitted into Ag(I) and Ag(0)

224

species (Figure 3c). The Ag species is considered as an active site to activate substrates,

225

which is beneficial to the chemi-adsorption and oxidation of CH3SH [38].

226

Raman spectroscopy was used to further analyze the interaction between Ag and

227

MnO2 (Figure 1b). MnO2 PHMSs shows a sharp band at 629 cm-1 ascribed to the

228

symmetric v2 (Mn-O) stretching vibrations of the MnO6 groups, and a weak band at 573

229

cm-1 due to the stretching vibration v3(Mn-O) in the basal plane of [MnO6] sheets, as well

230

as a weak band at 389 cm−1 attributed to the skeletal vibrations [47]. Comparatively, the 11



231

peak intensity ratio between the v2(Mn-O) and v3(Mn-O) peaks decreased after Ag

232

involved, and both peak red shift to 625 cm −1 and 560 cm −1, indicating the existence of

233

disorder crystal defects, which are favorable to form oxygen vacancies[48]. Oxygen

234

vacancies in MnO2 play key role in ozone destruction through transform O3 into

235

intermediate O22- [49]. Moreover, the peak intensity and width of Ag/MnO2 PHMSs are

236

larger than that of MnO2 PHMSs. This is attributed to the stronger interaction of Ag and

237

MnO2 support, which roots in the effect of surface stress induced by the larger radius of

238

Ag and increasing defects of MnO2 [48]. The results also confirm the surface stress is the

239

reason that the diffraction peak in the XRD pattern of Ag/MnO2 widens (Figure 1a) after

240

Ag deposition. As identified above, the stronger interaction between Ag and MnO2

241

increase the density of crystalline defects and oxygen vacancies, which are beneficial for

242

the adsorption, activation and migration of oxygen in the catalytic ozonation [50].

243

H2-TPR measurements were carried out to explore the reducibility of Ag/MnO2

244

PHMSs, because high reducibility of catalyst could favor to the catalytic ozonation [51].

245

As shown in Figure 1c and Table S3, with the increase of reduction temperature, the

246

sample will undergo the successive reduction of surface adsorbed oxygen species, and the

247

process of MnO2 →

248

deposition, the temperatures of MnO2 peaks decrease from 202 °C, 311 °C and 491 °C to

249

114 °C, 155 °C, and 253 °C, respectively. Same phenomena are also observed in other

250

study, the three peaks with decreased temperature are assigned to the successive

251

reduction of surface adsorbed oxygen species of MnO2 rather than the deposed limited

Mn2O3 →

Mn3O4 →

MnO, respectively [51]. After Ag

12


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Ag [52]. This indicates the Ag/MnO2 PHMs has stronger low-temperature reducibility

253

than MnO2. Reference indicates that activated hydrogen on the Ag surface can easily

254

migrate to the surface of the MnO2 PHMSs and thus facilitate the reduction reaction at

255

low temperatures [27, 53]. Meanwhile, the incorporated Ag also can activate surface

256

lattice oxygen species of MnO2 PHMSs, which are easier to be desorbed and react with

257

hydrogen gas at low temperature [53]. Therefore, the Ag incorporation improves the

258

reducibility of surface adsorbed oxygen species and promotes the mobility of lattice

259

oxygen in MnO2. Accordingly, the better reducibility and higher oxygen mobility causes

260

more oxygen to be adsorbed and further excited to active oxygen, which would then be

261

involved in the reaction of Ag/MnO2 PHMSs.

262

To investigate the interaction of Ag and MnO2 on the electron transfer progress and

263

the redox ability, the CV curves of Ag/MnO2 PHMSs are studied (Text S3) [54]. As

264

shown in Figure 1d, the current of Ag/MnO2 PHMSs are much higher than that of MnO2

265

PHMSs, verifying higher surface charge on Ag/MnO2 PHMSs. Moreover, with ozone

266

purging, the current intensity of Ag/MnO2 PHMSs was signiﬁcantly increased and the

267

peak potential shifted more positive than MnO2 PHMSs, suggesting a reduction process

268

occurred between ozone and catalyst, and Ag/MnO2 PHMSs exhibit a more favorable

269

performance for interfacial electron transfer than MnO2 PHMSs.

270 271

3.3 Catalytic removal of CH3SH with Ag/MnO2 PHMSs

272

3.3.1 Surface adsorption of CH3SH by Ag/MnO2 PHMSs 13



273

Because MnO2 PHMSs have high specific surface area and Ag has high affinity to

274

CH3SH, the as-prepared Ag/MnO2 PHMSs were firstly employed for adsorb CH3SH to

275

test their adsorption ability (Figure 4a). In the presence of MnO2 PHMSs alone (124.13

276

m2g-1), the reaction reached adsorption equilibrium after 240 seconds and finally about

277

40.34% CH3SH was removed in 600 seconds. In contrast, Ag/MnO2 PHMSs displayed a

278

specific adsorption curve. The 0.1% Ag/MnO2 PHMSs exhibits lower adsorption in the

279

initial 400s than MnO2 PHMSs but over 43.8% removal of CH3SH within the same 600

280

seconds. Similar adsorption curve and higher adsorption performance (45%) was also

281

obtained by 0.3% Ag/MnO2 PHMSs. The lower adsorption in the initial period is due to

282

the lower surface area (104.11 m2g-1) of Ag/MnO2 PHMSs, but higher adsorption in the

283

following stage is mainly because the S-H functional group in CH3SH strongly absorbs

284

on Ag particle surface. The S-H bond of alkanethiols can be dissociated on Ag

285

nanoparticles to form alkanethiolate species and strongly chemisorbed on Ag surface

286

through S atom as an anchor [55]. Three BE peaks of S 2p in Fig. 3d attributed to S2-, S22-

287

and S6+ can be detected over the spent Ag/MnO2 PHMSs catalysts, indicating that various

288

intermediate and products of S species are generated over the surface of Ag/MnO2

289

PHMSs [56]. Obviously, the XPS results further supported the existence of Ag-S species

290

at S2p peak after the surface adsorption (Figure 3d), well coincident with the decrease of

291

Ag0/Ag+ mole ratio from 0.88 to 0.63 in 0.3% Ag/MnO2 PHMSs (Table S2). Meanwhile,

292

dimethyl disulfide (DMDS, CH3S-SCH3) was also detected because Ag could cleave the

293

S-H bond of CH3SH and two CH3S- molecular can easily form into DMDS [55, 56]. 14


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294

Although CH3S-SCH3 is an odor gas, the formation of CH3S-SCH3 from CH3SH act as a

295

deodorization effect since the odor threshold of DMDS is 1/2 to 1/10 of that of CH3SH

296

[24]. This implies Ag/MnO2 PHMSs has favorable adsorption characteristic, which are

297

beneficial for the effective contact and reaction between gas molecules and active sites on

298

catalysts.

299 300

3.3.2 Catalytic ozonation of CH3SH by Ag/MnO2 PHMSs

301

After the adsorption equilibrium was attained, the as-prepared Ag/MnO2 PHMSs

302

were also employed for catalytic ozonation of CH3SH from different aspects including

303

the effect of Ag content, relative humidity, and stability. As depicted in Figure 4a, within

304

600 seconds reaction, a 0.3% Ag/MnO2 PHMSs with rare Ag can realize the 95%

305

removal of CH3SH than pure MnO2 PHMSs (79%) and ozonation alone (28%), indicative

306

of enhanced catalytic effect from Ag incorporation. Meanwhile, the calculated

307

degradation rate of CH3SH (pseudo first-order kinetics, Table S1) by 0.3% Ag/MnO2

308

PHMSs (4.24 × 10-8 mol g-1s-1) is 1.3 times than that of MnO2 PHMSs (3.17 × 10-8 mol

309

g-1s-1), and 10.1 times than that of ozonation only (0.42 × 10-8 mol g-1s-1). Moreover, the

310

calculated turnover frequency (TOFs) of CH3SH for 3D-MnO2 is 0.0000437 s−1, while

311

that of 0.3% Ag/MnO2 is rapidly increased to 0.00228 s−1 (Table S4). In contrast, the

312

catalytic performance of H2O2-MnO2 PHMSs is similar to that of MnO2 PHMSs,

313

indicative of no influence by H2O2 modification onto MnO2 PHMs. As identified above,

314

incorporated Ag plays an important role to enhance its performance for catalytic 15



315

ozonation. The further experiments of individual Ag catalysis were carried out in Ag/O3

316

system for CH3SH removal and the result was showed in Fig. S5a. Compared with

317

ozonation only (30%), the CH3SH removal efficiency was merely promoted by 36%

318

(0.36 mg Ag) and 47% (3.6 mg Ag) in Ag/O3 system in Ag/O3 system, while CH3SH

319

removal efficiency was promoted by 67% in Ag/MnO2 PHMSs/O3 system. Therefore, the

320

individual Ag also has a little contribution to the whole catalytic ozonation reaction.

321

The preparation method and Ag content on MnO2 PHMSs exhibit a great influence

322

on the catalytic activity of CH3SH removal. As depicted in Figure 4b, although

323

containing the same content of Ag, the optimum 0.3% Ag/MnO2 PHMSs (95%) prepared

324

by redox precipitation shows a better activity than PR-0.3% Ag/MnO2 PHMSs (76%)

325

prepared via photo-deposition. This confirms the feasibility of redox precipitation to

326

prepare efficient catalyst for catalytic ozonation. Moreover, the removal efficiency of

327

CH3SH was improved from 83% to 95% with Ag content increased from 0.1% to 0.3%.

328

This is because the increased Ag content meant higher CH3SH adsorption and higher

329

density of crystalline defect and oxygen vacancy to accelerate ozone decomposition [57].

330

However, CH3SH removal efficiency was reduced to 77.0% with Ag loading over 0.75%,

331

even worse than that of MnO2 PHMSs (79%). This is presumably due to the excess Ag

332

nanoparticles block the pore structures and cover the active sites of MnO2 PHMSs,

333

evidenced well by the greatly decreased specific surface area in Table S1 (75.29 m2g-1 for

334

0.75% Ag/MnO2 PHMSs).

335

Moreover, humidity is beneficial for the catalytic ozonation [58, 59], evidenced by 16


Page 16 of 41

Page 17 of 41


336

the enhanced removal efficiency to ~100% of the 0.3% Ag/MnO2 PHMSs (Text S6 and

337

Figure S5b). Furthermore, the catalyst could also maintain a stable catalytic activity and

338

can be regenerated for continuous air purification (Text S7, Figures S7 and S8).

339 340

3.3.3 Role of reactive species

341

Generally, the catalytic ozonation is strongly related to the decomposed ozone

342

transform into atomic oxygen species (⁕O22 − ,⁕O-) or directly evolves into reactive

343

oxygen species (•O2 − , •OH, and 1O2) [60, 61]. Therefore, the ozone decomposition

344

efficiency by 0.3% Ag/MnO2 PHMSs was investigated to directly verify its catalytic

345

activity. As shown in Figure S9, 0.3% Ag/MnO2 PHMSs achieved higher ozone

346

decomposition efficiency (85.1%) than that of MnO2 PHMSs (67.3%), much faster than

347

the self-decomposition of ozone (negligible change). To identify the surface atomic

348

oxygen species caused ozone decomposition, Raman spectroscopy was conducted for

349

Ag/MnO2 PHMSs with O3 purging. As shown in Figure 1b, the catalyst did not exhibit

350

any peaks at approximately 828 and 938 cm − 1 after ozonation, which were usually

351

assigned to ⁕O22 − and surface ⁕O- respectively, indicating these species were not

352

involved in the catalytic ozonation [62]. However, the two diagnostic peaks of v2(Mn-O)

353

and v3(Mn-O) were blue-shifted after O3 purging, suggesting the energy of ozone could

354

efficiently activate Mn-O vibrations from ground state to the excited state [63]. Actually,

355

the activated Mn-O still can convert surface adsorbed O2/OH/H2O into reactive oxygen

356

species like •OH and •O2− [61]. Therefore, different chemical scavengers and a more 17



357

sensitive electron spin resonance (EPR) technique were collectively utilized to analyze

358

the radicals. As shown in Figure 4c, the removal efficiency of CH3SH dramatically

359

decreased in the present of 5 mM tert-butanol (TBA, kTBA×•OH = 6 × 108 M − 1s − 1),

360

indicating that •OH could dominate the catalytic ozonation process [54]. Coincidently,

361

the characteristic spectrum of the DMPO-•OH adducts (quartet lines with height ratio of

362

1:2:2:1) was observed to confirm the generation of •OH (Figure 4d). Similarly, when 5

363

mM AA (ascorbic acid) was added, the removal efficiency of CH3SH decreased a lot but

364

slightly better than TBA addition, indicating the moderate role of •O2 − for CH3SH

365

degradation [54]. The increasing characteristic spectrum of the DMPO-•O2− (1:1:1:1)

366

during catalytic ozonation was also observed (Figure 4e). Especially, the role of 1O2 was

367

always neglected in the previous study of catalytic ozonation. The generated •O2- is

368

proposed to recombine into 1O2, which prefer to oxidize electron-rich compounds like

369

phenols, thus it may be beneficial for CH3SH mineralization [64]. As seen in Figure 4c,

370

the addition of 5 mM FFA (furfuryl alcohol, a well-known scavenger for 1O2) greatly

371

inhibited the removal efficiency of CH3SH. The inhibition content by FFA (32.9%) is

372

similar with that of TBA addition (32.6%), which indicates the comparable role of 1O2

373

with that of •OH. The continuous generated 1O2 was evidenced well by its typical

374

three-line EPR spectrum (Figure 4f). In conclusion, radicals quenching experiments

375

identified that the •OH and 1O2 were the dominant ROSs for CH3SH decomposition.

376

Moreover, higher intensity of EPR peaks for Ag/MnO2 PHMSs were obtained than that

377

of MnO2 PHMSs, confirming that promoted ROS generation by incorporated Ag on 18


Page 18 of 41

Page 19 of 41

378


MnO2 PHMSs.

379 380

3.3.4 Conversion route for CH3SH removal

381

To understand the mechanism of adsorption and ozonation of CH3SH over

382

Ag/MnO2 PHMSs, in situ DRIFTS is carried out to monitor time-dependent evolution of

383

the reaction intermediates and products over the catalyst surface, as shown in Figure 5a.

384

The CH3SH absorption bands appear once CH3SH comes in contact with the catalyst. In

385

the OH stretching region, a negative band at around 3400 cm−1 is observed, along with

386

adsorption IR bands at 963 cm − 1 due to the vibration of S-H, 1446 cm − 1 due to

387

deformation mode of CH3, 1464 cm − 1 due to bending vibration of CH3 and 2840~3000

388

cm-1 due to stretch vibration of C-H, respectively, indicating the adsorption of CH3SH

389

over Ag/MnO2 PHMSs [65]. Simultaneously, two outstanding band appear at 1309 and

390

1428 cm-1 corresponding to δs(CH3) and δas(CH3) species were observed to increase over

391

time, indicating intermediates of CH3S- and dimethyl disulfide (DMDS, CH3S-SCH3)

392

were formed [31]. This result suggested that S-H bond of CH3SH was broken and

393

deprotonated methyl mercaptan (CH3S-) tended to form into CH3S-Ag complex and

394

CH3S-SCH3, well evidencing the occurrence of chemi-sorption. The following reaction

395

can be proposed: CH3SH + Ag → CH3SAg → CH3S-SCH3 [34]. Some studies reported

396

that the S-H functional group in mercaptans or other sulfur compounds could strongly

397

adsorb on metal M and M+ (Cu, Ag, or Au), in which the S-H bond will be cleaved due to

398

the activation by metals [31, 58]. The XPS results (Figure 3d) also supported the 19



399

existence of Ag-S species, as the mole ratio of Ag0/Ag+ decreased from 0.88 to 0.63 and

400

DMDS was greatly formed after surface adsorption (Table S5). Moreover, the other

401

absorption bands developed progressively can be assigned to the vibration of S=O and

402

S-O relevant to ν(S−O) of monodentate sulfonic acid (protonated acid CH3SO3H at 1158

403

cm−1 and deprotonated sulfonic acid CH3SO3− at 1215 cm−1) and SO42- at 1047 cm−1, as

404

well as stretch vibration of COO− at 1566 cm−1 [65, 66]. This suggests S-S and C-S bond

405

are also cleaved and thus induce the formation of CH3SO3-, SO42- and COO-. It is

406

proposed that Ag atom can activate the OL in MnO2 to generate active oxygen species at

407

ambient condition and these species could migrate to oxidize surface adsorbed CH3S- at

408

the perimeter of Ag nanoparticles [31, 32]. Consistently, XPS results also showed that

409

CH3SO3-, SO42- and COO- were greatly formed (Figure 3d), and the mole ratio of OL/Oads

410

decreased from 2.12 to 2.0 after adsorption due to the consumption of OL (Table S2).

411

During adsorption stage, it can be proposed that single atomic Ag provide active sites for

412

the chemisorption and partial oxidation of CH3SH, while MnO2 acts as a reservoir

413

providing OL species.

414

Once the adsorption equilibrium is achieved, ozone is purged to initiate the catalytic

415

ozonation. Figure 5b shows the IR spectra for Ag/MnO2 PHMSs with ozone purging in

416

time sequence. The decreased peak intensity of the OH groups at 3400 cm−1 can be

417

attributed to the consumption of OH groups for the generation of •OH, which is a critical

418

radical for CH3SH removal. Notably, the adsorption bands at 963, 1446, 1464 and

419

2840~3000 cm−1 corresponding to CH3S- species gradually decreased or shifted over 20


Page 20 of 41

Page 21 of 41


420

time, indicating the consumption of accumulated chemisorbed CH3S-. Correspondingly,

421

the peak intensities of some intermediates (protonated and deprotonated sulfonic acid,

422

CH3SO3H and CH3SO3− at 1158, 1215 cm−1) and final products (sulfate species, SO42- at

423

1047 cm−1) significantly increase, indicating S elements in adsorbed CH3SH are

424

efficiently oxidized into SO3− and SO42− over the catalyst. Meanwhile, XPS results of S

425

2p (Figure 3d and Table S5) also indicate that both peaks of Ag-S species and

426

CH3S-SCH3 greatly decreased, while that of CH3SO3- and SO42- increased, further

427

confirming the S-S scission and chemisorbed CH3S- was largely oxidized into CH3SO3-

428

and SO42-. In parallel, some new peaks were also greatly intensified, corresponding to the

429

ν(COO−) at 1630 cm−1 and ν(C=O) at 1762 cm−1 of carboxylic acids, as well as the

430

νs(COO−) at 1340 cm−1 and the ν(C-O) at 1412 cm−1 of monodentate carbonate,

431

evidencing the C elements in the adsorbed CH3S- are also oxidized into carboxylic acids

432

(R-COO-) and carbonate (CO32-) over the catalyst [65, 66]. Especially, carbonate species

433

should be main intermediate product because the intensity of carbonate peaks is much

434

higher than that of carboxylic acids. Additionally, two appearing peaks at 2334 and 2362

435

cm−1 were assigned to the Fermi doublet of CO2 [67], indicating intermediate product can

436

be finally mineralized into CO2. Therefore, the mechanism of catalytic ozonation

437

involves two reactions: (1) oxidation, in which the CH3S-/CH3S-SCH3 was oxidized to

438

CH3SO3- and further SO42-, (2) S-C/S-S bond scission, leading to an increase in the

439

amount of carboxylic acids and carbonate species as well as finally CO2. Also, according

440

to IR spectra in time sequence, the normalized absorbance of intermediates (CH3S-, 21



441

CH3S-SCH3) and final product (CH3SO3-, SO42-, R-COO-, CO32-) are illustrated in Figure

442

5c and 5d. The tendency of species evolution clearly indicated that the adsorption and

443

transformation of CH3S- are greatly boosted on Ag/MnO2 PHMSs.

444 445

3.3.5 Mechanism of adsorption and catalytic ozonation (electron transfer/redox couple)

446

XPS was also carried out to determine the chemical surface composition/state of

447

Ag/MnO2 PHMSs after adsorption and catalytic ozonation. After adsorption, XPS results

448

of Ag 3d (Figure 3c and Table S2) indicates the relative molar ratio of Ag0/Ag+ decreased

449

from 0.88 to 0.63, but limited change in the molar ratio of Mn2++Mn3+/Mn4+ from 1.06 to

450

1.02. This suggests the important chemisorption role of Ag rather than Mn during

451

adsorption stage. After catalytic ozonation, Figure 3c and Table S2 displays that the

452

molar ratio of Mn2++Mn3+/Mn4+ greatly decreased from 1.02 to 0.88, suggesting that

453

Mn2+/Mn3+ were the primary reactive sites and electrons were transferred from the

454

surface Mn2+/Mn3+ to ozone and result in the decomposition of ozone to ROSs. However,

455

XPS results of Ag 3d (Figure 3c, Table S2) display the molar ratio of Ag0/Ag+ slightly

456

increase from 0.63 to 0.69, but which is still lower than the initial 0.88, indicating that

457

chemisorbed CH3SAg was degraded but only partial Ag+ was reduced back to Ag0. This

458

may suggest that the Ag also work as active sites for the catalytic ozonation.

459

Generally, OL and OH groups play an important role in the oxidation reactions [25,

460

38]. After adsorption, the relative contents of OL negligibly reduced from 67.94 to

461

66.46% (Figure 3b, Table S2). The consumed OL is due to its activation into surface 22


Page 22 of 41

Page 23 of 41


462

oxygen species by Ag nanoparticles, which plays important role in the partial oxidation

463

of chemi-sorbed CH3SAg. After catalytic ozonation, the relative contents of OL reduced

464

significantly from 66.46 to 60.11%, while that of Oabs increased from 33.19 to 37.56%

465

and Osurf increased from 0.35% to 2.32% (Figure 3b and Table S2). The remarkable

466

decrease in the OL content indicated its utilization took place during catalytic ozonation,

467

which reduce the oxidized Mn4+ to Mn3+/Mn2+ with O2 releasing, thus to complete the

468

redox reaction and maintain the electrostatic balance [61]. Consequently, the great

469

consumption of OL would result in the oxygen vacancies on the catalyst surface. It is

470

proposed that the oxygen-deficient bulk on the Ag/MnO2 PHMSs surface can be

471

replenished by the development of surface OH groups (large OH consumption in Figure

472

5b), thereby causing the increase of Oabs and Osurf through the enhanced adsorption,

473

activation and migration of surface absorbed O3 [61].

474

To sum up the whole catalytic process, a cycling reaction in the sequence of

475

Mn2+/Mn3+ → Mn4+ → Mn2+/Mn3+ was accomplished. During the course of oxidation,

476

the transfer of electrons from the surface Mn2+/Mn3+ (partial Ag) induced ozone

477

decomposition to reactive oxygen species, while the OL converted/reduced Mn4+ to Mn3+

478

vice versa. Such an electrostatic balance/redox couple between Mn3+/Mn4+ and OL/O2

479

played a key role in the catalytic activity. Moreover, it was suggested that the higher

480

activity of Ag/MnO2 PHMSs was related to the electronic effect of highly dispersed Ag

481

on Mn by increasing Mn3+ electron occupancy and dissociate molecular oxygen to

482

supplement the consumed OL during the reaction [63, 68]. To have full understanding of 23



483

the above reactions, the conversion pathways are proposed in Text S7 and Figure 5f.

484 485

(1) Chemisorption:

486

CH3SH + Ag → CH3SAg or CH3S-SCH3 (Main)

487

CH3SAg + O2 → CH3SO3- or SO42- and R-COO- or CO32- (Partial)

488 489

(2) Catalytic ozonation:

490

≡Mn2+/3+(Ag)−OH + O3→≡Mn2+/3+(Ag)−OH−O3

491

≡Mn2+/3+(Ag)−OH−O3→≡Mn4+(Ag+) + OL + HO3•

492

≡Mn4+ + OL → ≡Mn3+ + OL → ≡Mn2+ + O2

493

HO3•+ HO3•→2•O2- + •OH

494

•O2- + H2O → 1O2

495

2•OH → •O2-

496 497

(3) Decomposition:

498

CH3SAg or CH3S-SCH3 + •OH or 1O2 or •O2-→ CH3SO3- → SO42- + HCOO- or CO32-

499

or CO2

500 501

Supporting Information

502

Additional experimental details. Materials and Synthesis of MnO2 PHMSs;

503

Characterization of Ag/MnO2 PHMSs; CV curves; Conclusion of characterization; 24


Page 24 of 41

Page 25 of 41


504

Conclusion of pathway; Specific surface area and degradation rate constant of catalyst;

505

XPS analysis of Mn, O, Ag, S in Ag/MnO2; Analysis of H2-TPR; TOFs, Schematic

506

illustration of Ag/MnO2 preparation, Setup scheme, EDS of Ag/MnO2; Influence of

507

humidity on activity of Ag/MnO2 PHMSs; Reusability and regeneration of Ag/MnO2; O3

508

decomposition; IC analysis and FT-IR spectra of Ag/MnO2.

509 510

Acknowledgments

511

The authors wish to thank the National Natural Science Foundation of China (Nos.

512

51578556, 21673086, 41603097 and 21876212), Natural Science Foundation of

513

Guangdong

514

S2011010003416), Science and Technology Research Programs of Guangdong Province

515

(No. 2014A020216009) for financially supporting this work. Dr. Xia was also supported

516

by the Start-up Funds for High-Level Talents of Sun Yat-sen University

517

(38000-18821111).

Province

(Nos.

2015A030308005,

S2013010012927,

and

518 519

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619

Immobilization of self-stabilized plasmonic Ag-AgI on mesoporous Al2O3 for efficient

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purification of industrial waste gas with indoor LED illumination. Appl. Catal. B:

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Environ. 2016, 185, 295-306.

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Catalytically active single-atom sites fabricated from silver particles. Angew. Chem., Int.

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629

low-temperature oxidatiion of formaldehyde. Environ. Sci. Technol. 2018, 52,

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4728−4737.

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632

Three-dimensional MnO2 porous hollow microspheres for enhanced activity as ozonation

to

synthesize

Au-doped

α-MnO2

with

30


high

dispersion

toward

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catalysts in degradation of bisphenol A. J. Hazard. Mater. 2017, 321,162-172.

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of MnO2 hollow nanospheres and their supercapacitive performance. New J. Chem. 2013,

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Zirconium-Porphyrin-based

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immobilization of noble-metal single atoms. Angew. Chem., Int. Ed. 2018, 57 (13):

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3493-3498.

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Confined-interface-directed

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graphene/amorphous carbon. Appl. Catal. B: Environ. 2018, 225: 291-297.

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wire-, tube-, and flower-like morphologies: highly effective catalysts for the removal of

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toluene. Environ. Sci. Technol. 2012, 46, 4034-4041.

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vacancy in birnessite-type MnO2 on room-temperature oxidation of formaldehyde in air.

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653

Three-dimensionally ordered macroporous La0.6Sr0.4MnO3 with high surface areas:

654

Active catalysts for the combustion of methane. J. Catal. 2013, 307, 327-339.

Metal-Organic

synthesis

of

Framework

Palladium

31


hollow

nanotubes

single-atom

catalysts

for

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solution combustion synthesis as catalysts for the total oxidation of VOCs. Appl. Catal.

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B: Environ. 2015, 163, 277-287.

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catalysts by Ag in catalytic oxidation of toluene. Appl. Catal. B: Environ. 2013, 132-133,

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353-362.

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[49] Yang, Y.; Zhang, S.; Wang, S.; Zhang, K.; Wang, H.; Huang, J.; Deng, S.; Wang,

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B.; Wang, Y.; Yu, G. Ball milling synthesized MnOx as highly active catalyst for gaseous

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POPs removal: significance of mechanochemically induced oxygen vacancies. Environ.

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Sci. Technol. 2015, 49, 4473-4480.

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666

Ultrasound-assisted nanocasting fabrication of ordered mesoporous MnO2 and Co3O4

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with high surface areas and polycrystalline walls. J. Phys. Chem. C 2010, 114,

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2694-2700.

669

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670

Li, J. Three-dimensionally ordered macroporous La0.6Sr0.4MnO3 supported Ag

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nanoparticles for the combustion of methane. J. Phys. Chem. C 2014, 118, 14913-14928.

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oxidation of toluene over Ag/MnO2 catalysts. Appl. Sur. Sci. 2016, 385, 234-240.

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675

MnO2/reduced graphene oxide as advanced electrode materials for supercapacitors. 32


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Chem. Eng. J. 2014, 252, 95-103.

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nanowires for highly efficient catalytic oxidation of carcinogenic airborne formaldehyde.

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ACS catalysis 2018, 8, 3435-3446.

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Luo, Y. Microwave-assisted rapid synthesis of CeO2 nanoparticles and its desulfurization

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processes for CH3SH catalytic decomposition. Chem. Eng. J. 2016, 289, 161-169.

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oxygen on the activity of manganese oxides towards the oxidation of volatile organic

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compounds. Appl. Catal. B: Environ. 2010, 99, 353-363.

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manganese dioxide nanosheets for formaldehyde removal and regeneration performance.

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Chem. Eng. J. 2016, 306, 1172-1179.

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mechanism of dimethyl disulfide in liquid hydrocarbon streams on modified Y zeolites.

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complete oxidation of formaldehyde in air with ozone over MnOx catalysts at room

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sulfur-containing compounds with Ferrate(VI). Environ. Sci. Technol. 2009, 43, 33



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Mn, Fe) perovskites for efficient catalytic ozonation and an insight into probable catalytic

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701

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Trifunctional C@MnO catalyst for enhanced stable simultaneously catalytic removal of

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formaldehyde and ozone. ACS Catal. 2018, 8, 3164-3180.

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unsaturated manganese monoxide-containing mesoporous carbon catalyst in wet peroxide

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oxidation. ACS Catal. 2012, 2, 2577-2586.

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710

P. K. Highly efficient performance and conversion pathway of photocatalytic CH3SH

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polyoxymethylenedimethyl ethers synthesis: The acid sites control and reaction

715

pathways. Appl. Catal. B: Environ. 2015, 165, 466-476.

716

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Appl. Catal. B: Environ. 2012, 117-118, 339-345.

719

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720

nanocrystal MnO2 and Ag/MnO2 catalytic oxidation of CO. J. Catal. 2006, 237, 426-430.

721

35



1.0% Ag/MnO2 PHMSs 310

211

600

301

Intenstiy(a.u.)

310

211

310 110 200

220

211

310 400

22000

002

211 301

MnO2 solid sphere

411

600 521 002 541



v2(Mn-O) (625)

v3(Mn-O) (560)

14000 12000

0.3% Ag/MnO2

10000

v3(Mn-O) (573)

8000

MnCO3

 













v2(Mn-O) (632) 0.3% Ag/MnO2 + O3

16000

MnO2 PHMSs 002

600

301

v3(Mn-O) (568)

18000

0.3% Ag/MnO2 PHMSs 600 002

301

(b)

20000

Intensity (a.u.)

(a)

Page 36 of 41

v2(Mn-O) (629)

6000

MnO2

4000



2000

10

20

30

40 50 2 Theta(degree)

60

70

80

300

400

500

600

700

-1

800

900

Wavenumber (cm ) 0.020

(c)

peak 3

peak 2 peak 1

MnO2+O3

0.015

0.3% Ag/MnO2

peak 3

MnO2

0.010

Current/A

TCD signal

peak 1

(d)

0.3% Ag/MnO2+O3

0.3% Ag/MnO2 MnO2

0.005

0.000

peak 2 -0.005

-0.010

40

140

240

340

440

Temperature (C)

540

-0.2

640

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Potenial / V

722

Figure 1. (a) XRD patterns of MnCO3, MnO2 solid spheres, MnO2 PHMSs, 0.3%

723

Ag/MnO2 PHMSs and 0.75% Ag/MnO2 PHMSs; (b) Raman spectra and (c) H2-TPR

724

profiles; (d) CV curves of 0.3% Ag/MnO2 PHMSs and MnO2 PHMSs.

725 726 727

36


Page 37 of 41


(b)

(a)

(h)

(c)

(g)

(f)

(d)

(e)

728 729

Figure 2. SEM images of 0.3% Ag/MnO2 PHMSs (a) and PD-0.3% Ag/MnO2 PHMSs

730

(b), TEM (c) and HRTEM (d) images of 0.3% Ag/MnO2 PHMSs, (e) Cs-corrected

731

HAADF-STEM images of 0.3% Ag/MnO2 PHMSs at the same district with partially

732

magnified pictures, (f − h) HAADF-STEM of 0.3% Ag/MnO2 PHMSs with element

733

distribution images corresponding to Mn, O and Ag.

734 735 736 737

37



(a) Mn 2p

Page 38 of 41

(b) O 1s 4+

3+

MnO 2

Olat

MnO2

Mn

Mn

Oads

2+

Mn

3+

Mn

0.3% Ag/MnO2

4+

Mn

4+

Mn

Oads

Intensity (a.u.)

Intensity (a.u.)

3+

0.3% Ag/MnO2 (After adsorption)

2+

Mn

3+

2

adsorption) Oads (After Osurf

0.3% Ag/MnO

2

Olat

4+

Mn

2+

0.3% Ag/MnO

Olat

0.3% Ag/MnO2 (After catalytic ozonation)

Mn

0.3% Ag/MnO 2

Olat

2+

Mn

Mn

(After catalytic ozonation)

Oads O surf

Mn

638

640

642

Osurf

644

646

648

528

Binding Energy (eV)

530

532

534

Binding Energy (eV) Ag 3d5/2

(c) Ag 3d +

Ag

0.3% Ag/MnO2

Ag 3d3/2

0

Ag

0.3% Ag/MnO2

(d) S 2p

(After adsorption)

0

Ag

+

Ag

2-

S2 (Dimethyl disulfide)

6+

S (Sulfonic acid)

2-

Ag 3d3/2

+

Ag

0

Ag

+

+

366

0

Ag

0.3% Ag/MnO2

0

Ag

368

370

+

Ag

372

Binding Energy (eV)

6+

S (Sulfonic acid)

0.3% Ag/MnO2

(After catalytic ozonation) 6+

S (Sulfate)

2-

2S (Ag-S species) S2 (Dimethyl disulfide)

Ag 3d3/2 (After catalytic ozonation)

Ag

6+

S (Sulfate)

(C)

(After adsorption)

Ag

Ag 3d5/2

364

S (Ag-S species)

0.3% Ag/MnO2

Intensity (a.u.)

Intensity (a.u.)

Ag 3d5/2

(D)

0

Ag

374

376

162

378

164

166

168

170

Binding Energy (eV)

738

Figure 3. XPS spectra of Mn 2p (a), O 1s (b), Ag 3d (c), and S 2p (d) for fresh MnO2

739

catalyst, fresh 0.3% Ag/MnO2 PHMSs catalyst, 0.3% Ag/MnO2 PHMSs catalyst after 38


172

Page 39 of 41


740

deep adsorption of CH3SH, 0.3% Ag/MnO2 PHMSs catalyst after deep ozonation of

741

CH3SH. (b)

Catalytic ozonation

0.1% Ag/MnO2

40

Adsorption MnO2

0.3% Ag/MnO2 (N2)

H2O2-MnO2 0.3% Ag/MnO2

20

0.1% Ag/MnO2

60

1.0% Ag/MnO2

40

0.75% Ag/MnO2 0.5% Ag/MnO2 0.3% Ag/MnO2 0.2% Ag/MnO2

20

0.1% Ag/MnO2

Ozonation O3

0

100

0.3% Ag/MnO2 + O3

MnO2 + O3

Time (s)

*

*

3420

3440

300

400

*

3460

600

0

*

*

3480

100

200

-

O2

(d)

*

*

500

*

*

O3

3400

200

*

OH

3380

MnO2

0

-100

90 80

Catalytic Ozonation 0.3% Ag/MnO2 (No scavenger)

70

0.3% Ag/MnO2+AA

60

0.3% Ag/MnO2+FFA

50

0.3% Ag/MnO2+TBA

40

O3

30 20 10 0 0

100

200

300

Time (s)

400

500

600

40

20

0.3% PD-Ag/MnO2

*

MnO2 + O3

Time (s) 



400



500

600

0 No Scavenger

(e)

3500

3520

3540

3460



3480

TBA

AA



1

O2

FFA

















O3

(f)

0.3% Ag/MnO2 + O3



O3

*

Magnetic field (G)



300

0.3% Ag/MnO2 + O3

Intensity (a.u.)

0 -200

60









3500

Intensity (a.u.)

MnO2

80

CH3SH removal efficiency (%)


0.3% Ag/MnO2 60

(c)

80

H2O2-MnO2

Intensity (a.u.)


80

100

100

(a)

CH3SH removal efficiency(%)

100

100



MnO2 + O3

O3



3520

3540

Magnetic Field (G)

3400

3420

3440

3460

3480

3500

3520

Magnetic field (G)

742

Figure 4. (a) Performances in adsorption and catalytic ozonation of CH3SH in different

743

system; (b) Influence of different catalysts on CH3SH removal; (c) Catalytic ozonation of

744

CH3SH by 0.3% Ag/MnO2 PHMSs with/without addition of chemical scavengers (TBA:

745

Tert-butanol, AA: Ascorbic acid, FFC: Feri-furoic acid); Electron paramagnetic

746

resonance (EPR) signal of hydroxyl radical (d), superoxide radical (e) and singlet oxygen

747

(f).

748 749 750 751

39


3540

3560

3580

3600


Page 40 of 41

752 0.036

Adsorp. Equil.

(c)

Normalized absorbance (a.u.)

0.032 0.028

S-H CH3S

0.024

CH3SO3

CH3S-SCH3 -

2-

SO4

0.020

R-COOH

0.016 0.012 0.008 0.004 0.000 1

0.55

Normalized absorbance (a.u.)

0.50

2

3

Time (min)

4

5

(d)

0.45

S-H CH3S

0.40

CH3SO3

-

2-

SO4

0.35

2-

CO3

R-COOH

0.30 0.25 0.20 0.15 0.10 0.05

Adsorp. Equil.

0.00 0

(f)

40


2

4

6

Time (min)

8

10

Page 41 of 41


753

Figure 5. In situ DRIFTS spectra of (a) adsorption of CH3SH, (b) catalytic ozonation of

754

CH3SH with 0.3% Ag/MnO2 PHMSs; Species evolution of (c) adsorption of CH3SH, (d)

755

catalytic ozonation of CH3SH with 0.3% Ag/MnO2 PHMSs, (f) Adsorption and catalytic

756

ozonation pathway of CH3SH by Ag/MnO2 PHMSs.

757

Graphical Abstract

758 759 760

41


Enhanced Performance and Conversion Pathway for Catalytic

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