Alkali- and Sulfur-Resistant Tungsten-Based Catalysts for NOx

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Alkali- and Sulfur-Resistant TungstenBased Catalysts for NOx Emissions Control Zhiwei Huang, Hao Li, Jiayi Gao, Xiao Gu, Li Zheng, Pingping Hu, Ying Xin, Junxiao Chen, Yaxin Chen, Zhaoliang Zhang, Jianmin Chen, and Xingfu Tang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03972 • Publication Date (Web): 20 Nov 2015 Downloaded from http://pubs.acs.org on November 24, 2015

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Alkali- and Sulfur-Resistant Tungsten-Based Catalysts for NOx Emissions Control Zhiwei Huang,† Hao Li,§ Jiayi Gao,† Xiao Gu,‡ Li Zheng,† Pingping Hu,† Ying Xin,§ Junxiao Chen,† Yaxin Chen,† Zhaoliang Zhang,§ Jianmin Chen,† and Xingfu Tang*† †

Shanghai Key Laboratory of Atmospheric Particle Pollution & Prevention (LAP3), Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China

§

School of Chemistry & Chemical Engineering, University of Jinan, Jinan 250022, China

‡

Department of Applied Physics, Chongqing University, Chongqing 401331, China

1

ABSTRACT

2

The development of catalysts with simultaneous resistance to alkalis and sulfur poisoning is of

3

great importance for efficiently controlling NOx emissions using the selective catalytic reduction

4

of NOx with NH3 (SCR), because the conventional V2O5/WO3-TiO2 catalysts often suffer severe

5

deactivation by alkalis. Here, we support V2O5 on a hexagonal WO3 (HWO) to develop a

6

V2O5/HWO catalyst, which has exceptional resistance to alkali and sulfur poisoning in the SCR

7

reactions. A 350 µmol g-1 K+ loading and the presence of 1,300 mg m-3 SO2 do not almost

8

influence the SCR activity of the V2O5/HWO catalyst, and under the same conditions, the

9

conventional V2O5/WO3-TiO2 catalysts completely lost the SCR activity within 4 hours. The

10

strong resistance to alkali and sulfur poisoning of the V2O5/HWO catalysts mainly originates


1

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11

from the hexagonal structure of the HWO. The HWO allows the V2O5 to be highly dispersed on

12

the external surfaces for catalyzing the SCR reactions and has the relatively smooth surfaces and

13

the size-suitable tunnels specifically for alkalis’ diffusion and trapping. This work provides a

14

useful strategy to develop SCR catalysts with exceptional resistance to alkali and sulfur

15

poisoning for controlling NOx emissions from the stationary source and the mobile source.

16

INTRODUCTION

17

Alkali metals and sulfur oxides are two kinds of the well-known catalyst poisons in the selective

18

catalytic reduction NOx with NH3 (SCR) for controlling NOx emissions from both stationary and

19

mobile sources.1-3 Numerous attempts have been made to solve these two problems because

20

catalyst deactivation is costly in that processes have to be shut down while regeneration or

21

replacement of deactivated catalysts is taken. Although catalysts simultaneously suffer severe

22

deactivation by the two poisons under practical conditions, efforts have typically focused on one

23

or the other individually, because it is extremely difficult to protect catalysts simultaneously

24

against poisoning from “basic” alkali metals and “acidic” SO2 molecules.4

25

Alkali poisoning during SCR process is often addressed by blocking catalytically active

26

sties with alkalis.5,6 The intrinsic nature of catalysts’ deactivation by alkalis is in detail described

27

by a newly-developed exchange-coordination mechanism, which, in turn, assists rational design

28

of improved alkali-resistant SCR catalysts.7 A catalyst with rich alkali-trapping sites independent

29

from catalytically active sites is often favorable for trapping alkalis,1 and strong Brønsted acidity

30

and suitable sizes are characteristics of the alkali-trapping sites.7 According to this mechanism,

31

hollandite manganese oxides1,7 and protonated titanate supports8,9 have successfully been

32

developed and showed excellent alkali resistance in the SCR reactions. Similarly, hexagonal


2


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33

WO3 (HWO) has attracted significant attention as an efficient catalyst due to its strong acidity10

34

and as a potential ion-sieve due to its size-suitable open-tunnel structure.11 The HWO is

35

constructed by the corner-sharing WO6 octahedra to form hexagonal tunnels oriented along the c

36

axis with the space group P6/mmm.12 The size of the HWO tunnels is ~5.4 Å, and thus alkalis are

37

readily inserted into the tunnels by ion-exchange reactions.11,13 Therefore, the HWO should show

38

a strong alkali resistance during the SCR reactions.

39

Sulfur-poisoning predominantly originates from SO2 adsorption on catalysts’ surfaces

40

and subsequent oxidation. Preferential adsorption of SO2 molecules due to stronger acidity than

41

NO on catalytic surfaces accounts for SO2 poisoning,9 especially in Langmuir-Hinshelwood SCR

42

reactions. More severely, thermally stable sulfate species such as (NH4)2SO4 and M2/nSO4 (M

43

represents active metal component of catalysts, and n does the oxidation state of M) are formed

44

via the SO2 oxidation to SO3 and subsequent reactions with NH3 and catalysts, respectively,

45

which are deposited on catalysts’ surface and blocking catalytic sites, leading to catalysts’

46

deactivation.14,15 The former is due to strong chemisorption of acidic SO2 at basic surface

47

hydroxyls and O2- sites, and the latter mainly results from the oxidation ability of catalysts.

48

However, a certain oxidation ability of catalysts is necessary to obtain high SCR efficiency,16

49

and thus one possible strategy is to develop an “acidic” catalyst with a weak adsorption ability to

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acidic SO2 molecules. Catalysts with acidic V2O5 as an active component are proven to have an

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excellent resistance against SO2 poisoning than other “basic” metal oxides such as MnOx,17

52

CuO,18 CeO2,14,15 and so on, although the vanadium-based catalysts are prone to poisoning by

53

alkalis due largely to their strong acidity.2,6,19,20

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In this work, we develop a V2O5/HWO catalyst with strong resistance simultaneously to

55

alkalis and SO2 poisoning. Two acidic oxides with strong resistance to SO2 poisoning, V2O5 and


3

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HWO, are designed as an active component and a support, respectively. The HWO have rich

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size-suitable alkali-trapping sites with highly specificity, and can professionally trap alkalis in

58

the presence of high-concentration SO2, even though alkalis are initially accumulated on the

59

V2O5 surfaces under normal SCR conditions. Some characterization techniques such as

60

synchrotron X-ray diffraction (SXRD), transmission electron microscopy (TEM), and scanning

61

TEM (STEM) and energy dispersive X-ray spectroscopy (EDX) mapping are used to correlate

62

the structure of the catalysts with the alkali- and sulfur-resistant performances.

63

EXPERIMENTAL SECTION

64

Materials Preparation. The HWO was prepared by a hydrothermal method. Ammonium

65

paratungstate [(NH4)10W12O41·xH2O, 0.7 mmol], ammonium sulfate [(NH4)2SO4, 63.0 mmol],

66

and oxalic acid (H2C2O4, 23.3 mmol) were dissolved in deionized H2O (80 mL). The resulting

67

solution was transferred to a 100 mL autoclave and kept at 180 oC in an oven for 12 h with

68

stirring. The final slurry was filtered, washed with deionized H2O, and dried at 105 oC. The

69

V2O5/HWO catalyst was prepared by wet impregnation of the HWO with an ammonium

70

metavanadate (NH4VO3) solution. NH4VO3 (2.26 mmol) was added to 8 mL deionized H2O at 80

71

o

72

added. The suspension was kept at 80 oC under vigorous stirring till all H2O was evaporated after

73

0.5 h. Finally, the obtained solid was calcined at 400 oC for 4 h in air to give a V2O5/HWO

74

catalyst. The conventional V2O5/WO3-TiO2 catalyst with 3.0 wt% V2O5 and 9.0 wt% WO3 was

75

prepared with continuous impregnations of a commercial anatase TiO2 powder (Aldrich) by

76

NH4VO3 (0.066 mol L-l) and (NH4)10W12O41·xH2O (0.0065 mol L-l) in an oxalic acid solution at

77

80 oC. After the evaporation of H2O, the resulting powder was dried at 105 oC for 12 h and

C under vigorous stirring to form a yellow solution, to which the HWO powder (2.000 g) was


4


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calcined at 400 oC for 4 h in air. The potassium loading was conducted by impregnating the

79

V2O5/HWO powder with a K2SO4 aqueous solution, and the calcium sulfate (CaSO4) loading

80

was prepared by successively impregnating the V2O5/HWO powder with a Ca(NO3)2 aqueous

81

solution and an equivalent (NH4)2SO4 solution. The amount of the potassium or calcium loading

82

was expressed in terms of molar concentration of K+ or Ca2+ with respect to the weight of

83

catalysts (µmol gcat-1).

84

Catalytic Evaluation. The SCR reactions were performed in a fixed–bed quartz reactor (i.d.

85

= 8 mm) under an atmospheric pressure. A certain amount of the catalyst (40–60 mesh) was

86

charged for each run. The feed gas was composed of 500 ppm NO, 500 ppm NH3, 3.0 vol% O2,

87

1,300 mg m-3 SO2 (when used), and balanced N2. The total flow rate was 1,000 mL min-1.

88

Different gas hourly space velocities (GHSVs) were obtained by changing the volumes of

89

catalysts. The concentrations of NO and NO2 in the inlet and outlet gas were measured by an on–

90

line chemiluminescence NO–NO2–NOx analyzer (42i–HL, Thermo Electron Corporation, USA).

91

The data were recorded after the reactions reached a steady state. The selectivity (SN2) to N2 in

92

the SCR reactions was measured by using a mass spectrometer (OmniStar GSD 301, Pfeiffer

93

Vacuum, Germany), and was calculated according to a following equation (1). The SO2

94

conversion (XSO2) to SO3 was studied with an on-line FTIR gas analyzer (MultiGas 2030, MKS

95

Instruments, USA), described as a following formula (2).

96

97

SN2 =

[CNOx ]in + [CNH3 ]in − [CNOx ]out − [CNH3 ]out − 2[CN2O ]

X SO2 =

[CNOx ]in + [CNH3 ]in − [CNOx ]out − [CNH3 ]out [CSO3 ]out [CSO2 ]in

(1)

× 100% (2)

98 Where [CNOx]in, [CNH3]in and [CSO2]in represent the concentration of NOx, NH3, SO2 in the inlet,


5

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99 and [CNOx]out, [CNH3]out, [CN2O] and [CSO3]out represent the concentration of NOx, NH3, SO3 in the 100 outlet. 101

Materials Characterization. X-ray diffraction (XRD) patterns of the catalysts were measured

102

by a D8 advance X-ray diffractometer (Bruker/AXS, Germany) at 40 kV and 40 mA using

103

monochromatized Cu Kα radiation (λ = 1.5405 Å). The SXRD patterns were performed at

104

BL14B of the Shanghai Synchrotron Radiation Facility (SSRF) at a wavelength of 1.2398 Å.

105

Rietveld refinement analyses of the diffraction profiles were conducted using Rietica v1.77

106

program. TEM and high-resolution TEM (HRTEM) images were obtained on a JEM 2100F

107

transmission electron microscope. STEM characterization with EDX mapping was performed

108

with an FEI Tecnai G2 F20 S-Twin transmission electron microscope. The Brunauer–Emmett–

109

Teller (BET) surface area of the sample was measured by N2 adsorption at liquid nitrogen

110

temperature using a Quadrasorb evo (USA, Quantachrome) autosorb gas sorption system. The

111

specific surface area of the V2O5/HWO is 11 m2 g-1 determined by using linear portion of the

112

BET model.

113

Theoretical Calculations. For density functional theory (DFT) calculations, we employed a

114

general gradient approximation for the description of exchange and correlation effects. The

115

pseudo potentials of projector augmented wave method were used to describe the interactions

116

between the core and valence electrons. The energy cutoff for the plane-waves was set to 450

117

eV. In the calculation of the HWO, the lattice constant for a conventional hexagonal cell is 7.319

118

Å × 7.319 Å × 3.881 Å (space group: P6/mmm). In the surface calculations, a slab model

119

containing 8 formula units of the HWO was employed, where a vacuum of 12 Å was used to

120

simulate the surface in periodic boundary condition; A 2 × 2 × 6 monkhorst-pack grid was used

121

in the k-point sampling for the surface calculation. A supercell of 4 formula units of the HWO


6


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122

was employed in the calculations inside the tunnels. All the features were implemented in the

123

Vienna ab-initio Simulation Package (VASP). The nudged elastic band method, implemented in

124

VASP, was employed to identify the minimum energy paths for the diffusions on the {100}

125

surfaces and inside the tunnels.

126

RESULTS AND DISCUSSION

127

To investigate the simultaneous resistance of the V2O5/HWO against alkali and sulfur poisoning,

128

the catalysts are firstly impregnated by K2SO4 at a high loading (350 µmol gcat-1), and after

129

drying at 100 oC, directly tested in the SCR reactions in the presence of high-concentration SO2

130

(1,300 mg m-3). The SCR reactions are initially conducted by a temperature-programmed

131

procedure from 25 to 350 oC, and then the reaction temperature is kept at 350 oC for stable tests.

132

Figure 1 shows the NO conversions (XNO) of the catalysts under isothermal reaction conditions.

133

Obviously, the V2O5/HWO catalyst shows excellent resistance simultaneously against alkali and

134

sulfur poisoning, and under a GHSV of 200,000 h-1, the catalytic activity of the K2SO4-

135

impregnated V2O5/HWO quickly increases and arrives at a steady state with a XNO of 92%.

136

Similarly, at a higher GHSV of 400,000 h-1, the V2O5/HWO catalyst shows such a strong

137

resistance against alkali and sulfur poisoning that alkali and sulfur have almost no influence on

138

its catalytic activity. As expected, the conventional V2O5/WO3-TiO2 catalyst even with a low K+

139

loading (115 µmol gcat-1) exhibits rather poor alkali resistance and undergoes severe and

140

continuous deactivation with time on stream until complete loss of activity is observed after 4 h.

141

Likewise, the V2O5/HWO catalyst shows the excellent resistance against alkaline earth metals’

142

poisoning such as Ca2+, as shown in Figure S1 in the Supporting Information.


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1.0

V2O5/HWO, GHSV = 200,000 h-1

XNO

0.8

0.6

V2O5/HWO, GHSV = 400,000 h-1 0.4

0.2

V2O5/WO3-TiO2, GHSV = 200,000 h-1

0.0 0.0

0.3

0.6

0.9

4

6

8

Time on stream (h)

143

Figure 1. The XNO over the V2O5/HWO and the V2O5/WO3-TiO2 catalysts impregnated with

144

K2SO4 at a 350 and 115 µmol gcat-1 loading, respectively, at the reaction temperature of 350 oC.

145

Reaction conditions: 500 ppm NO, 500 ppm NH3, 3.0 vol% O2, 1,300 mg m-3 SO2, and balanced

146

N2, gas flow rate = 1,000 mL min-1. For comparison, the XNO (solid lines) over the V2O5/HWO

147

catalysts without a K2SO4 loading are also given with two different GHSVs under the same SCR

148

reaction conditions.

149

For V2O5-based catalysts, the presence of SO2 might have a negative influence on the

150

alkali resistance of catalysts.4,5 Thus, the effect of SO2 or alkali alone on the V2O5/HWO is also

151

tested in the SCR reactions under two different GHSVs (Figure S2 in the Supporting

152

Information), and still no deactivation is observed over the V2O5/HWO catalyst, regardless of the

153

presence of alkali or SO2 alone or both, indicating no interactions occurring between alkalis and

154

SO2 during the SCR process, unlike the reported results by Liu et al.4 It is well-known that the

155

V2O5-based catalysts have a strong SO2 tolerance and the weak alkali resistance.2,4,5 Therefore, it

156

is convincing that the strong alkali resistance of the V2O5/HWO originates from the HWO. The


8


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SCR reactions over the HWO with and without a loading of K2SO4 (115 µmol gcat-1 of K+) after

158

annealing at 400 oC are also tested and the results are shown in Figure S3 (Supporting

159

Information). The K2SO4 loading does not decrease the catalytic performance, but slightly

160

increases catalytic performance of the HWO. As a matter of fact, the reported WO3-

161

supported/promoted V2O5 catalysts do not have strong alkali resistance,2,21 and thus the strong

162

alkali-resistance of the current V2O5/HWO catalyst should be attributed to the structure of the

163

HWO.

164

A detailed study of the structure of the V2O5/HWO catalyst is crucial for thoroughly

165

understanding its strong alkali and sulfur resistance. We first investigate the morphology and the

166

microstructure of the HWO by using TEM, and then the structure of the V2O5/HWO by using the

167

XRD. The HWO has a one-dimensional rod-shaped morphology, as shown in the TEM image of

168

Figure 2a. A typical individual HWO rod with the side line parallel to the (100) plane is shown

169

in the HRTEM image of Figure 2b. As shown in Figure 2c, the fringe distances of 0.62 nm and

170

0.38 nm are in agreement with the lattice spacings of the (100) planes and the (001) planes

171

perpendicular to each other, respectively, indicating that the electron beam is parallel to the [210]

172

direction of the HWO nanorod. These results reflect that the HWO rod grows along the [001]

173

direction, like the reported HWO nanorods synthesized by the hydrothermal methods.22,23

174

According to the results above, we can reconstruct a structural model for the HWO nanorod

175

closed by six {100}/{1-10} side-facets and two {001} top-facets,24,25 as shown in Figure 2d. The

176

top-facets possess the openings of the one-dimensional tunnels, as shown in an inset model of

177

Figure 2d. The size of the HWO tunnels is ~5.4 Å, suitable for accommodating metal ions with

178

ionic radius less than 1.7 Å.11,13,25 The six side-facets are theoretically identical because of the

179

high hexagonal symmetry of the HWO crystal structure,25 which have a relatively smooth


9

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180

surface structure, shown as the atom arrangements in Figure S4 (Supporting Information).

181

Therefore, the relatively smooth surfaces and the size-suitable tunnels of the HWO are favorable

182

for diffusion and adsorption of alkali metal ions,26 respectively, like the structure and property of

183

our reported hollandite manganese oxides.1,7,26

184

Figure 2. TEM image (a), HRTEM images (b,c) and structural models (d) of the HWO. The

185

radius (R) of the HWO tunnel is ~2.7 Å.

186

After supporting V2O5, the crystal structure of the V2O5/HWO catalyst was determined

187

by the XRD, as shown in Figure 3. Apart from most of the strong reflections indexed to the

188

hexagonal WO3, three weak Bragg reflections are also observed, which should be assigned to the

189

V2O5 crystals, indicating the presence of the V2O5 nanoparticles on the HWO external surface

190

due to the size of the HWO tunnels being not enough to accommodate V2O5 nanoparticles.

191

Additionally, an atom rearrangement of the tunnel walls of the HWO is not suitable for the

192

distorted six-coordinated VO6 octahedral structure. This indicates that the vanadium species are

193

not inserted into the HWO tunnels, and should be dispersed on the external surface of the HWO.


10


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194

Theoretically, it is easy for the exposed V2O5 to be attacked by alkalis due to the acid-base

195

reactions. After the K+ loading and annealing at 350 oC, the V2O5/HWO catalyst is denoted as

196

V2O5/Kin-HWO, and these reflections due to the V2O5 of the V2O5/Kin-HWO remain almost

197

unchanged. The intensity and position of these reflections are also not changed, as shown in the

198

inset XRD pattern of the V2O5(100) diffraction (Figure S5 in the Supporting Information), and

199

no reflections due to KVO3 are also observed, indicative of no strong interactions between the

200

V2O5 and the potassium ions.

6000 1000

Intensity (a.u.)

V2O5/HWO V2O5/Kin-HWO 4000

V2O5(001)

900 800 700 19.5

20.0

20.5

21.0

2 theta (°)

2000

10

20

30

40

2 theta (°) 201

Figure 3. XRD patterns of the V2O5/HWO and V2O5/Kin-HWO catalysts. Inset: the magnified

202

XRD patterns of the V2O5(001) diffraction showing the influence of the K+ on the V2O5 crystal

203

structure.

204 205

Still, no reflections of the XRD pattern of the V2O5/Kin-HWO catalyst in Figure 3 can be

206

attributed to K+ species such as K2SO4, K2O or K2CO3. Figure 4 shows the STEM image of the

207

V2O5/Kin-HWO and the corresponding EDX mapping, and EDX analysis, which evidence the

208

presence of highly dispersed potassium and vanadium. As discussed above, the vanadium species


11

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209

are dispersed on the HWO external surfaces. There are three possible locations for K+: i) on the

210

HWO surface not in conjunction with the V2O5, ii) on the HWO framework, and/or iii) in the

211

HWO tunnels. It is impossible for K+ to be located on the HWO surface because, even if WO3 is

212

used as a support for V2O5, K+ still readily adsorbs on the V2O5 surface,3 even partial diffusion

213

into the bulk to form the KVO3 crystal under normal SCR operating conditions2,3,6,28 in view of

214

the low Hüttig temperature (130 oC) of K2SO4.29 Additionally, if this were the case, the presence

215

of SO2 should have accelerated the deactivation of the V2O5/HWO by surface K+.4 However, the

216

SCR activity in Figure 1 is not influenced by the presence of both SO2 and K+, indicating that K+

217

is not on the catalyst’s surfaces. It is still little possible for K+ to incorporate into the HWO

218

framework by taking the ionic radium of K+ (1.33 Å)27 much larger than that (0.74 Å)27 of W6+

219

and the normal coordination configuration of K+ different from WO6 into account. Therefore, K+

220

should be located in the HWO tunnels at a stable state.

200 nm

(d)

(b)

(c)

K

V

K V W

2

(e)

W

W W

cps/ev

(a)

(f)

1 K

3

W

W

V

6

9

keV 12

221

Figure 4. STEM image (a), the corresponding EDX mappings (b-e) and EDX spectrum (f) of the

222

V2O5/Kin-HWO nanorod.

223 224

To determine the accurate location of the K+ in the HWO tunnel, the SXRD patterns of

225

the HWO after a 350 µmol K+ g-1 loading and annealing at 400 oC (labeled as Kin-HWO) and the

226

as-synthesized HWO are shown in Figure 5. To make a high-quality Rietveld refinement analysis


12


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227

of the SXRD patterns and to prevent V2O5 from possibly distorting the HWO structure,30,31 the

228

Kin-HWO without V2O5 is used to determine the K+ position. As shown in Figure 5, the (200)

229

and (001) Bragg reflections of the Kin-HWO shift to higher and lower Bragg angles, respectively,

230

in comparison with the HWO (Figure S6 in Supporting Information). Calculated lattice constants

231

show a contraction of a axis and an expansion of c axis after K+ insertion into the HWO tunnels

232

(Table S1 in the Supporting Information). The Rietveld refinement analyses show that K+

233

occupies exactly the (0,0,0) site or the Wyckoff 1a site (Figure S7 and Tables S1,S2 in the

234

Supporting Information) and thus K+ ions are coordinated to six oxygen atoms in the tunnels

235

with a mean K-O bond length of ~2.68 Å. A structural model of the local environment of K+ ion

236

is shown in the inset of Figure 5. Moreover, the highly dispersed K+ ions are also shown in the

237

EDX mapping of the Kin-HWO (Figure S8 in the Supporting Information).

238

Figure 5. SXRD patterns of the as-synthesized HWO (black) and the Kin-HWO (red). Inset:

239

structural models showing the K+ site in the HWO tunnel.

240 241

According to the above results and the ion-exchange properties of the HWO,11,13,32 the

242

immigration of the K+ ions into the HWO tunnels from the external surfaces should follow the


13

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243

exchange-coordination mechanism, like our recent report.7 The ion-exchange reaction of W-O-H

244

+ ½K2SO4 → W-O-K + ½H2SO4 is evidenced by detecting surface acidity on the catalyst, as

245

shown in Figure S9 (Supporting Information). Furthermore, the increase of the SCR activity of

246

the Kin-HWO compared with the HWO implies the presence of the surface SO42- (Figure S3 in

247

the Supporting Information), because the acidic surface can enhance absorption amount of NH3,

248

thus facilitating the SCR reactions.33 From the energetic point of view, it is not difficult for K+ to

249

diffuse on the surfaces or in the tunnels of the HWO. Owing to the smooth surfaces and the

250

tunnel structures of the HWO (Figure 2), the diffusing barriers (Ed) of K+ on the HWO(100)

251

planes and in the HWO tunnels are calculated to be only 0.47 and 0.27 eV (Figure S10 in the

252

Supporting Information), respectively. To evaluate the temperature required for the K+ diffusion,

253

a formula of Ed = ln(ν0)kBT is used,34 where kB is the Boltzmann constant and ν0 is the usual pre-

254

factor. By taking ν0 = 6 × 1012.34 When T = 400 K (~130 oC, the Hüttig temperature of K2SO4),

255

the Ed is ~1.0 eV, more than the Ed values (0.47 and 0.27 eV) when K+ diffuses on the HWO(100)

256

plane or in the HWO tunnel from the DFT calculations. This indicates that the K+ diffusion from

257

the HWO surface into the tunnels is relatively easy under the SCR reaction conditions even at

258

low temperature. Upon migrating into the HWO tunnels, the K+ ions are stably fixed in the HWO

259

tunnels with an energy savings of 2.31 eV with respect to that of the K+ ion on the HWO

260

surfaces, as predicted by the DFT calculations, indicating that the K+ ions in the tunnel are much

261

more stable than that on the surface.

262

Figure 6 shows the schematic models of the V2O5/HWO catalyst in a typical alkali-rich

263

environment in the presence of high-concentration SO2 during the SCR reactions at 350 oC.

264

When K2SO4 particles are accumulated on the external surface of the V2O5/HWO (see the TEM

265

image in Figure S11 of the Supporting Information), K+ ions will diffuse along the smooth


14


Page 16 of 23

266

surfaces of the HWO, and migrate into the HWO tunnels to form a stable coordination

267

configuration to oxygen atoms of the HWO tunnel walls. From the energetic point of view, K+

268

trapped in the tunnels arrives at a more stable state than on the catalyst’s surfaces, as evidenced

269

above. Thus, the HWO as a support can efficiently protects the active sites of V2O5 from alkali

270

poisoning, and also prevents the combined deactivation effects by both alkalis and SO2.4

271

Furthermore, the V2O5/HWO has the strong inherent resistance to SO2 poisoning during the SCR

272

conditions, as shown in Figure 1.

273

Figure 6. (a) A schematic model of the V2O5/HWO catalyst showing the alkali-resistant process.

274

(b) Illustration of the resistance of the V2O5/HWO catalyst against alkali and sulfur poisoning

275

during the SCR reactions. Operation temperature 350 oC, 500 ppm NO, 500 ppm NH3, 1,300 mg

276

m-3 SO2, under alkali poisoning. NPs represents nanoparticles.

277

All characterization data above provide the evidence that the V2O5/HWO catalyst has the

278

specific alkali-trapping sites in the HWO tunnels, which can trap the alkali-metal ions and stably

279

fix them in the HWO tunnels under the normal SCR reaction conditions, even if the alkalis are

280

initially accumulated on the catalyst’s surfaces such as on the surface of the V2O5. The trapped

281

alkali-metal ions are spatially separated from the catalytically active sites of the V2O5, so the

282

combined deactivation effects of alkali and SO2 are not observed in the SCR reactions. Although


15

Page 17 of 23


283

conventional V2O5/WO3-TiO2 catalysts show the strong SO2 tolerance, they suffer severe alkali

284

poisoning. Moreover, the V2O5/HWO catalyst gives a high selectivity to N2 and a low oxidation

285

rate of SO2 in the SCR reactions, as shown in Figures S12,S13 (Supporting Information),

286

respectively. Therefore, the V2O5/HWO catalysts with the exceptional resistance to alkali and

287

SO2 poisoning should have widespread applications in the SCR reactions for controlling NO

288

emissions from both the stationary source and the mobile source.

289

In conclusion, the V2O5/HWO catalysts with the exceptional resistance to alkali and

290

sulfur poisoning were successfully developed to control NOx emissions by the SCR reactions.

291

The V2O5/HWO catalysts are thoroughly characterized by HTREM, EDX mapping, XRD and

292

SXRD together with the Rietveld refinement analysis. The catalytic tests revealed that the

293

V2O5/HWO catalyst showed the strong resistance to alkali poisoning, and the catalysts with the

294

high K+ loading of 350 µmol g-1 did not decrease the high SCR activity even in the presence of

295

the high-concentration SO2, whereas the conventional V2O5/WO3-TiO2 catalysts almost

296

completely lost SCR activity under the same conditions. The experimental results coupled with

297

theoretical calculations demonstrated that the strong resistance of the V2O5/HWO catalysts to

298

alkali and sulfur poisoning mainly originated from the specific alkali-trapping sites of the HWO.

299

Alkalis accumulated on the catalytically active surface sites of the V2O5/HWO catalysts could

300

spontaneously migrate into the HWO tunnels during the SCR reactions, and arrived at a separate

301

state from the catalytically active sites, thus leading to simultaneous resistance to alkalis and SO2

302

poisoning. Therefore, the V2O5/HWO catalysts with a hexagonal structure of WO3 are promising

303

candidates for controlling NOx emissions from the stationary source and the mobile source

304

against alkali and sulfur poisoning.

305

ASSOCIATED CONTENT


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Page 18 of 23

306

Supporting Information. Some related tables and figures. This material is available free of

307

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

308

AUTHOR INFORMATION

309

Corresponding Author

310

*Tel: +86-21-65642997; fax: +86-21-65643597; e-mail: [email protected] (X.T.).

311

Notes

312

The authors declare no competing financial interest.

313

ACKNOWLEDGMENT

314

This work was financially supported by the NSFC (21277032, 21477023 and 21277060), the

315

STCSM (14JC1400400) and the NCET (12-0131). The SXRD measurements were conducted at

316

the SSRF.

317

REFERENCES

318

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Table of Contents 1.0

NOx conversion

408


V2O5/HWO

0.8

NO NH3 SO2 K2SO4 particles

0.6

0.4

N2

V2O5

H2O SO2

350 oC

0.2

V2O5/WO3-TiO2 0.0 0.0

0.3

0.6

0.9

4

6

8

Time on stream (h)


22

Alkali- and Sulfur-Resistant Tungsten-Based Catalysts for NOx

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