Novel Effect of H2O on the Low Temperature Selective Catalytic

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Novel Effect of H2O on the Low Temperature Selective Catalytic Reduction of NO with NH3 over MnOx-CeO2: Mechanism and Kinetic Study Shangchao Xiong, Yong Liao, Xin Xiao, Hao Dang, and Shijian Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512407k • Publication Date (Web): 02 Feb 2015 Downloaded from http://pubs.acs.org on February 10, 2015

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The Journal of Physical Chemistry

1

Novel Effect of H2O on the Low Temperature Selective

2

Catalytic Reduction of NO with NH3 over MnOx-CeO2:

3

Mechanism and Kinetic Study

4

Shangchao Xiong, Yong Liao, Xin Xiao, Hao Dang, Shijian Yang ∗

5

Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of

6

Environmental and Biological Engineering, Nanjing University of Science and Technology,

7

Nanjing, 210094 P. R. China

8 9 10 11

∗

Corresponding author phone: 86-18-066068302; E-mail: [email protected] (S. J. Yang). 1

ACS Paragon Plus Environment


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Abstract:

13

H2O generally shows a strong inhibition on the low temperature selective catalytic reduction

14

(SCR) reaction. However, a novel phenomenon was discovered that the low temperature SCR

15

reaction over MnOx-CeO2 was promoted by H2O. The rate constants of N2 and N2O formation

16

over MnOx-CeO2 were calculated using the steady-state kinetic study. It showed that both the rate

17

constants of N2 and N2O formation decreased in the presence of H2O, and the promotion of the

18

SCR reaction over MnOx-CeO2 by H2O was mainly attributed to the inhibition of N2O formation.

19

Meanwhile, the influence of H2O on the elementary reaction of N2O formation over MnOx-CeO2

20

was studied using the transient reaction study. It indicated that the inhibition of N2O formation

21

over MnOx-CeO2 by H2O was not only attributed to the competition adsorption of H2O with NH3

22

but also related to the decrease of the oxidation ability of MnOx-CeO2 in the presence of H2O.

23

Keywords: H2O effect; SCR reaction; NSCR reaction; transient reaction study; steady-state

24

kinetic study.

25

2


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1. Introduction

27

As Nitrogen oxides (NO and NO2) contribute to haze, photochemical smog and acid rain,1 the

28

emission of NOx from coal fired power plants is a serious concern.2 So far, selective catalytic

29

reduction (SCR) of NO with NH3 (i.e. Reaction 1) is the major technology to control NOx

30

emission from stationary coal fired power plants.3 V2O5-WO3/TiO2 has been widely used as a SCR

31

catalyst for several decades.4 The temperature window of V2O5-WO3/TiO2 catalyst is 300-400 oC,

32

so it is located upstream of electrostatic precipitator. 5

33

4NH3 +4NO+O2 → 4N2 +6H2O

(1)

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4NH3 +4NO+3O2 → 4N 2O+6H 2O

(2)

35

4NH3 +5O2 → 4NO+6H2O

(3)

36

However, the space and access in many existing coal-fired power plants upstream of the

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electrostatic precipitator for the operation of V2O5-WO3/TiO2 are limited.6 Therefore, there is a

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strong demand for the low temperature SCR catalysts, 7 which can be placed downstream of the

39

desulfurizer.8 So far, MnOx-CeO2 has been regarded as the most promising low temperature SCR

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catalysts.7, 9, 10 However, the non-selective catalytic reduction (NSCR) reaction (i.e. Reaction 2)

41

simultaneously happens during the SCR reaction over MnOx-CeO2.11 Therefore, a lot of N2O,

42

which is now considered as a pollutant due to its contribution to global warming and stratospheric

43

ozone depletion,12-14 forms during the SCR reaction over MnOx-CeO2.7 Moreover, the catalytic

44

oxidation of NH3 to NO (i.e. Reaction 3) may happen during NO reduction over MnOx-CeO2 at

45

higher temperatures.15, 16

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H2O, which is one of the main components in the flue gas, generally has a strong inhibition

47

on the SCR reaction over Mn based catalysts.7, 14, 17 However, H2O showed a novel promotion

48

on the low temperature SCR reaction over MnOx-CeO2. As in situ DRIFTS study is difficult to

49

be performed in the presence of a high concentration of water vapor, the mechanism of H2O

50

effect on NO reduction over MnOx-CeO2 was studied using the steady-state kinetic study and

51

the transient reaction study. The results showed that the promotion of the SCR reaction over

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MnOx-CeO2 by H2O was mainly attributed to the inhibition of N2O formation, and the

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inhibition of N2O formation was not only attributed to the competition adsorption of H2O with 3



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NH3 but also related to the decrease of the oxidation ability in the presence of H2O.

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2. Experimental

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2.1 Catalyst preparation

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MnOx-CeO2 (Mn/Ce =3/7) was prepared using the citric acid method.7, 15 The foam-like solid

58

sample obtained from the citric acid method was first dried at 120 oC for 12 h, and then calcined at

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650 oC for 6 h under air atmosphere.

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2.2 Catalytic test

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The catalytic reaction was performed on a fixed-bed quartz tube reactor.15, 18, 19 The mass of

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MnOx-CeO2 with 40-60 mesh was 200 mg, the total flow rate was 100 mL min-1, and the

63

corresponding gas hourly space velocity (GHSV) was 30000 cm3 g-1 h-1 (i.e. 1.1×104 h-1). The

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typical reactant gas contained 500 ppm of NO, 500 ppm of NH3, 2% of O2, 5% of H2O (when used)

65

and balance of N2. The gas composition in the outlet was continually monitored by a Fourier

66

transform infrared spectrometer (FTIR, Thermo SCIENTIFIC, ANTARIS, IGS Analyzer).

67

2.3 Reaction kinetic study

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To obtain the reaction rate constants of N2 and N2O formation over MnOx-CeO2 in the presence

69

and in the absence of H2O, the steady-state kinetic study was performed. Gaseous NH3

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concentration in the inlet was kept at 500 ppm, while gaseous NO concentration varied from 200

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to 500 ppm.

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diffusion), a very high GHSV of 240000-8000000 cm3 g-1 h-1 (the catalyst mass ranged from 3 to

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50 mg, and the total flow rate was 200 or 400 mL min-1) was adopted to obtain less than 15% of

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NOx conversion.

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2.4 Transient reaction study

7, 14, 15

To overcome the diffusion limitation (including inner diffusion and external

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To clarify the influence of H2O on the elemental reaction of N2O formation over MnOx-CeO2,

77

the transient reaction study was performed.14, 15, 20 The concentrations of NO, NO2 and N2O in the

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outlet were recorded during passing 500 ppm of NO and 2% of O2 over 500 ppm of NH3

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presorbed MnOx-CeO2, passing 500 ppm of NO, 2% of O2 and 5% of H2O over 500 ppm of NH3

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presorbed MnOx-CeO2, and passing 500 ppm of NO and 2% of O2 over 500 ppm of NH3 and 5%

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of H2O presorbed MnOx-CeO2.

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3. Results 4


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3.1 Effect of H2O on the SCR reaction over MnOx-CeO2

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Figure 1a shows that NOx conversion over MnOx-CeO2 was higher than 95% at 120-160 oC

85

with the GHSV of 30000 cm3 g-1 h-1 and it decreased to 85% with the further increase of reaction

86

temperature to 200 oC. Figure 1a also shows that NH3 conversion over MnOx-CeO2 was higher

87

than NOx conversion above 160 oC. It suggests that the catalytic oxidation of NH3 to NO happened

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over MnOx-CeO2 above 160 oC.5,

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MnOx-CeO2, and N2O selectivity increased from 18% at 120 oC to 50% at 200 oC. After the

90

introduction of 5% of H2O, NOx conversion over MnOx-CeO2 decreased at 120-160 oC, while it

91

increased at 180-200 oC (shown in Figure 1b). Meanwhile, N2O formation over MnOx-CeO2 was

92

suppressed in the presence of H2O, resulting in an obvious decrease of N2O selectivity. Moreover,

93

the amount of N2 formed over MnOx-CeO2 in the presence of H2O was much higher than that in

94

the absence of H2O above 120 oC (shown in Figure 1c). It suggests that H2O generally showed a

95

novel promotion on the SCR reaction over MnOx-CeO2.

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3.2 Effect of H2O on the adsorption of NH3 and NO on MnOx-CeO2

21

A lot of N2O formed during the SCR reaction over

97

There is generally agreement that the effect of H2O on the SCR reaction was related to the

98

competition absorption of H2O with NH3.22-24 Therefore, the effect of H2O on the adsorption of

99

NH3 and NO+O2 on MnOx-CeO2 was investigated. The amounts of NH3 and NO+O2 adsorbed on

100

MnOx-CeO2 can be approximately calculated from the NH3-TPD and NO-TPD profiles (shown in

101

Figures 2 and 3),25 which was shown in Table 1. Table 1 shows that the concentrations of NH3 and

102

NOx adsorbed on MnOx-CeO2 in the presence of H2O were much less than those in the absence of

103

H2O. It suggests that both the adsorption of NH3 and that of NO+O2 on MnOx-CeO2 were

104

restrained by H2O due to the competition adsorption.

105

3.3 Effect of H2O on NO and NH3 oxidation

106

Table 2 shows that MnOx-CeO2 had an excellent ability for NH3 oxidation (NH3 conversion was

107

close to 100% above 160 oC). However, the catalytic oxidation of NH3 was obviously restrained

108

after the introduction of 5% of H2O. Meanwhile, N2 selectivity of NH3 oxidation over MnOx-CeO2

109

increased remarkably. Furthermore, Figure 4 shows that the catalytic oxidation of NO over

110

MnOx-CeO2 was also restrained after the introduction of 5% of H2O.

5



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The catalytic oxidation of NH3/NO mainly depended on the amount of NH3/NOx adsorbed on

112

MnOx-CeO2 and the oxidation ability of MnOx-CeO2.16, 26, 27 Tables 1, 2 and Figure 4 show that the

113

inhibition of NH3 oxidation and NO oxidation over MnOx-CeO2 by H2O was much more

114

remarkable than the inhibition of NH3 adsorption and NO+O2 adsorption. It suggests that the

115

oxidation ability of MnOx-CeO2 decreased in the presence of H2O.

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4. Discussion

117

4.1 Reaction mechanism

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In situ DRIFT study and the transient reaction study in our previous study demonstrated that

119

N2O formation over MnOx-CeO2 mainly resulted from the Eley-Rideal mechanism (i.e. the

120

reaction between over-activated NH3 and gaseous NO), and the Langmuir-Hinshelwood

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mechanism (i.e. the reaction between adsorbed NH3 species and adsorbed NOx) did not contribute

122

to N2O formation.

123

activated NH3 and gaseous NO) and the Langmuir-Hinshelwood mechanism (i.e. the reaction

124

between adsorbed NH3 species and adsorbed NO2-) could contribute to N2 formation over

125

MnOx-CeO2.

126

15

However, both the Eley-Rideal mechanism (i.e. the reaction between

NO reduction over MnOx-CeO2 through the Langmuir-Hinshelwood mechanism can be

127

approximately described as: 9, 10, 14, 15, 28

128

NH3(g) → NH3(ad)

(4)

129

NO (g) → NO (ad)

(5)

130

Mn 4+ =O + NO (ad) → Mn 3+ -O-NO

(6)

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NH 3(ad) +Mn 3+ -O-NO → Mn 3+ -O-NO-NH 3 → Mn 3+ -OH + N 2 +H 2O

(7)

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M n 3+ -OH + C e 4+ =O → M n 4+ =O + Ce 3+ -O H

(8)

133

Ce 3+ -OH +

134

1 1 O 2 → Ce 4+ =O + H 2 O 4 2

(9)

Meanwhile, NO reduction over MnOx-CeO2 through the Eley-Rideal mechanism can be

135

approximately described as: 11, 18, 20, 28

136

NH3(g) → NH3(ad)

(4)

6


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NH 3(ad) + Mn 4+ =O → NH 2 + Mn 3+ -OH

(10)

138

NH 2 +NO (g) → N 2 +H 2 O

(11)

139

NH 2 + Mn 4+ =O → NH + Mn 3+ -OH

(12)

140

NH+NO (g) + Mn 4+ =O → N 2O+Mn 3+ -OH

(13)

141

Furthermore, the catalytic oxidation of NH3 to NO (C-O) may simultaneously happen, and it

142

can be described as: 16, 21

143

NH 3(g) → NH 3(ad)

144

NH 3(ad) + Mn 4+ =O → NH 2 + Mn 3+ -OH

(10)

145

NH 2 + Mn 4+ =O → NH + Mn 3+ -OH

(12)

146

1 NH+ O2 + Mn 4+ =O → NO+Mn 3+ -OH 2

(14)

147

(4)

The SCR reaction, the NSCR reaction and the catalytic oxidation of NH3 to NO all contributed

148

to NH3 conversion. Therefore, NH3 conversion can be described as:21

149

NH3 conversion%=ηSCR +ηNSCR +ηCO

150 151

(15)

Where, ηSCR, ηNSCR and ηCO were the contributions of the SCR reaction, the NSCR reaction and the catalytic oxidation of NH3 to NO to NH3 conversion, respectively.

152

The SCR reaction and the NSCR reaction over MnOx-CeO2 contributed to NO reduction, while

153

the catalytic oxidation of NH3 to NO contributed to NO formation. Therefore, NO conversion over

154

MnOx-CeO2 can be described as:21

155

NO conversion%=ηSCR +ηNSCR -ηCO

156

157

158

(16)

Hence,

ηCO =

NH3 conversion - NO conversion % 2

(17)

Furthermore, the contribution of the NSCR reaction can be calculated according to the

159

formation of N2O, and it can be calculated as follows:21

160

η NSCR =

[N 2 O]out × 100% [NH 3 ]in

(18) 7



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161

Therefore, the contributions of the SCR reaction, the NSCR reaction and the catalytic oxidation

162

of NH3 to NO to NH3 conversion during NO reduction over MnOx-CeO2 in the presence and in the

163

absence of H2O (shown in Figure 1) can be calculated according to the concentrations of NO, NO2,

164

N2O and NH3 in the outlet, which was shown in Figure 5.

165

Figure 5a shows that the contribution of the SCR reaction to NH3 conversion over MnOx-CeO2

166

in absence of H2O gradually decreased with the increase of reaction temperature from 120 to 200

167

o

168

increased. After the introduction of 5% of H2O, the small contribution of the catalytic oxidation of

169

NH3 to NO disappeared, and the contribution of the NSCR reaction obviously decreased (shown in

170

Figure 5b). Meanwhile, the contribution of the SCR reaction to NH3 conversion in the presence of

171

5% of H2O was much higher than that in the absence of H2O above 120 oC. They suggest that the

172

SCR reaction over MnOx-CeO2 was generally promoted after the introduction of H2O, while the

173

NSCR reaction and the catalytic oxidation of NH3 to NO were both restrained.

174

4.2 Reaction kinetic study

175

C, while the contributions of the NSCR reaction and the catalytic oxidation of NH3 to NO

The kinetic equation of N2 formation through the Langmuir-Hinshelwood mechanism (i.e. the

176

decomposition of NH4NO2 adsorbed on MnOx-CeO2) can be described as:

177

d[N 2 ] dt

L-H

= k1[Mn 3+ -O-NO-NH 3 ]

(19)

178

Where, k1 and [Mn3+-O-NO-NH3] were the decomposition rate constant of NH4NO2 and the

179

concentration of NH4NO2 adsorbed on MnOx-CeO2, respectively. Our previous study

180

demonstrated that the concentration of NH4NO2 adsorbed on MnOx-CeO2 at the steady state can

181

be regarded as a constant, which was approximately independent of the concentrations of gaseous

182

NH3 and gaseous NO.

183

mechanism was approximately independent of gaseous NO concentration.15

184

15

Therefore, the rate of N2 formation through the Langmuir-Hinshelwood

According to Reactions 11 and 13, the kinetic equations of N2 formation and N2O formation

185

over MnOx-CeO2 through the Eley-Rideal mechanism can be described as:15, 18

186

d[N 2 ] dt

E-R

=−

d[NO (g) ] d[NH 2 ] =− = k2 [NH 2 ][NO (g) ] dt dt

8


(2 0 )

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d[NO (g) ] d[N 2 O] d[NH] =− =− = k3 [NH][NO (g) ][Mn 4+ =O] dt dt dt

(21)

188

Where, k2, k3, [NH2], [NH], [Mn4+=O] and [NO(g)] were the kinetic constants of Reactions 11

189

and 13, the concentrations of NH2, NH and Mn4+ on MnOx-CeO2, and gaseous NO

190

concentration, respectively.

191 192

Meanwhile, the kinetic equations of NH and NH2 formation over MnOx-CeO2 (i.e. Reaction 10 and 12) can be described as: 5, 15, 20

193

d[NH 3(ad) ] d[NH 2 ] =− = k4 [NH 3(ad) ][Mn 4+ =O] dt dt

(22)

194

d[NH 2 ] d[NH] =− = k5 [NH 2 ][Mn 4+ =O] dt dt

(23)

195

Where, k4 and k5 were the kinetic constants of Reactions 10 and 12.

196

As the reaction reached the steady state, NH concentration on MnOx-CeO2 would not vary.15

197

Therefore,

198

d[NH] = k5 [NH2 ][Mn 4+ =O] − k3[NH][NO(g) ][Mn 4+ =O]=0 dt

199

Thus, NH concentration on MnOx-CeO2 at the steady state can be described as:

200

[NH]=

201

Hence, N2O formation over MnOx-CeO2 (i.e. Equation 21) can be transformed as:

202

k [NH2 ] d[N2O] = k3 5 [NO(g) ][Mn 4+ =O] = k5[NH2 ][Mn 4+ =O] dt k3[NO(g) ]

203

Our previous study demonstrated that NH2 concentration on MnOx-CeO2 at the steady state was

204

independent of gaseous NO and NH3 concentrations,15 which was mainly related to k4, and the

205

concentration of NH3 adsorbed and Mn4+ on MnOx-CeO2 (hinted by Equation 22). Therefore, N2O

206

formation over MnOx-CeO2 through the Eley-Rideal mechanism was independent with gaseous

207

NO concentration (shown in Equation 26), while the reaction order of N2 formation over

208

MnOx-CeO2 through the Eley-Rideal mechanism with respect to gaseous NO concentration was

209

nearly 1 (shown in Equation 20). 15

210

(24)

k5 [NH 2 ] k3[NO(g) ]

(25)

As a result, the rate of NO reduction over MnOx-CeO2 can be approximately described as: 9


(26)


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211

d[NO(g) ]

d[N 2 ] d[N 2 ] d[N 2O] E-R + L-H + dt dt dt dt 3+ = k2 [NH 2 ][NO(g) ] + k1[Mn -O-NO-NH3 ] + k5 [NH 2 ][Mn 4+ =O] −

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=

(27)

= kN2 (E-R) [NO(g) ] + kN2 (L-H) + kN2O(E-R) 212

Where, kN2(E-R), kN2O(E-R) and kN2(L-H) were the reaction rate constants of N2 formation and N2O

213

formation over MnOx-CeO2 through the Eley-Rideal mechanism, and that of N2 formation through

214

the Langmuir-Hinshelwood mechanism, respectively. Meanwhile, kN2(E-R), kN2O(E-R) and kN2(L-H) can

215

be described as:

216

k N2 (E-R) = k2 [NH 2 ][NO(g) ]

(28)

217

k N2 (L-H) = k1[Mn 3+ -O-NO-NH 3 ]

(29)

218

k N2O(E-R) = k5 [NH 2 ][Mn 4+ =O]

(30)

219

To determine these reaction rate constants, the steady-state kinetic study was conducted. Figures

220

6b and 6d both show that the rate of N2O formation over MnOx-CeO2 was nearly independent of

221

gaseous NO concentration, which was consistent with the application of Equation 26.14,

222

Therefore, kN2O(E-R) can be obtained directly from Figure 6b and Figure 6d. Meanwhile, Figure 6a

223

and 6c show that there was an excellent linear relationship between the rate of NO reduction and

224

gaseous NO concentration, which was consistent with the application of Equation 27. Therefore,

225

kN2(E-R) and kN2(L-H) can be calculated from Figures 6a and 6c after the linear regression (the slope

226

is kN2(E-R) and the intercept is the sum of kN2(L-H) and kN2O(E-R)).

15

227

Table 3 indicates that the reaction rate of NO reduction over MnOx-CeO2 through the

228

Eley-Rideal mechanism was much higher than that through the Langmuir-Hinshelwood

229

mechanism. It suggests that the Eley-Rideal mechanism predominated over NO reduction over

230

MnOx-CeO2 especially at higher temperatures. Table 3 shows that both kN2(E-R) and kN2O(E-R) of NO

231

reduction over MnOx-CeO2 obviously decreased after the introduction of 5% of H2O. It suggests

232

that both N2 and N2O formation over MnOx-CeO2 through the Eley-Rideal mechanism were

233

restrained by H2O. This result was not consistent with the suggestion of Figure 5 that the SCR

234

reaction over MnOx-CeO2 was promoted by H2O. However, Table 3 also shows that the decrease

235

of kN2O(E-R) due to the presence of H2O was 10 times than that of kN2(E-R). Therefore, the promotion 10


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236

of the low temperature SCR reaction over MnOx-CeO2 by H2O with a lower GHSV (shown in

237

Figure 1) was mainly related to the inhibition of N2O formation.

238

3.4 Transient reaction study

239

To clarify the mechanism of the inhibition of N2O formation over MnOx-CeO2 by H2O, the

240

transient reaction study was performed. After NO+O2 passed over NH3 presorbed MnOx-CeO2 at

241

180 oC, 2.1 µmol of N2O formed (shown in Figure 7a). However, only 1.1 µmol of N2O formed

242

during passing NO+O2+H2O over NH3 presorbed MnOx-CeO2 (shown in Figure 7b). As NH3

243

adsorption and the activation of adsorbed NH3 (i.e. Reactions 4, 10 and 12) were not affected by

244

H2O, the decrease of N2O formation indicates that Reaction 13 was restrained by H2O. It suggests

245

that k3 decreased in the presence of H2O. Only 0.54 µmol of N2O formed during passing NO+O2

246

over NH3+H2O presorbed MnOx-CeO2 (shown in Figure 7c). As the interface reaction (i.e.

247

Reaction 13) was not affected, the decrease of N2O formation was mainly related to the decrease

248

of NH concentration. It suggests that NH concentration on MnOx-CeO2 in the presence of H2O

249

was only 26% of that in the absence of H2O. Meanwhile, NH3-TPD profiles show that the

250

concentration of NH3 adsorbed on MnOx-CeO2 (i.e. [NH3(ad)]) in the presence of H2O was 56% of

251

that in the absence of H2O. They suggest that k4 and k5 in the presence of H2O were much less than

252

those in the absence of H2O. The decrease of k4 and k5 was mainly related to the decrease of the

253

oxidation ability of MnOx-CeO2 in the presence of H2O, which was demonstrated by the study of

254

NH3 oxidation and NO oxidation.

255

Because k4 and the concentration of NH3 adsorbed on MnOx-CeO2 both decreased, NH2

256

concentration on MnOx-CeO2 would decrease in the presence of H2O (hinted by Equation 22).

257

Hence, kN2(E-R) would decrease in the presence of H2O (hinted by Equation 28), which was

258

demonstrated in Table 3. k5 and NH2 concentration on MnOx-CeO2 both decreased, so kN2O(E-R)

259

would decrease remarkably in the presence of H2O (hinted by Equation 30), which was

260

demonstrated in Table 3.

261 262

As the Eley-Rideal mechanism predominated over NO reduction over MnOx-CeO2, N2O selectivity of NO reduction over MnOx-CeO2 can be approximately described as:

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 28

263

d[N 2O] dt N 2 O selectivity = d[N 2 ] d[N 2 O] + dt dt 4+ k5 [NH 2 ][Mn =O] k5 [Mn 4+ =O] = = k2 [NH 2 ][NO(g) ]+k5 [NH 2 ][Mn 4+ =O] k2 [NO(g) ]+k5 [Mn 4+ =O]

264

k5 decreased remarkably due to the decrease of oxidation ability, so N2O selectivity of NO

265

reduction over MnOx-CeO2 would decrease notably in the presence of H2O (hinted by Equation

266

31), which was demonstrated in Figure 1.

267

5. Conclusion

(31)

268

H2O showed a marked inhibition of N2O formation during NO reduction over MnOx-CeO2,

269

resulting in a novel promotion on the SCR reaction. The adsorption of NH3 on MnOx-CeO2 was

270

restrained in the presence of H2O. Meanwhile, the oxidation ability of MnOx-CeO2 obviously

271

decreased in the presence H2O. Moreover, the interface reaction of NH with gaseous NO was

272

restrained by H2O. As a result, N2O formation during the low temperature SCR reaction over

273

MnOx-CeO2 was obviously inhibited by H2O.

12


Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


274

Acknowledgements:

275

This study was financially supported by the National Natural Science Fund of China (Grant No.

276

21207067 and 41372044), the Fundamental Research Funds for the central Universities (Grant No.

277

30920130111023), and the Zijin Intelligent Program, Nanjing University of Science and

278

Technology (Grant No. 2013-0106).

279

Supporting Information Available

280 281

XRD pattern and BET surface area of MnOx-CeO2. This information is available free of charge via the Internet at http://pubs.acs.org/.

282

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

283

References:

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285

Using Satellite Observations: Relative Roles of Fossil Fuel Combustion, Biomass Burning and

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287

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288

Study of Fe2(SO4)3/TiO2 Catalyst for Selective Catalytic Reduction of NOx by Ammonia. J. Phys.

289

Chem. C 2011, 115, 7603-7612.

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291

Vanadium Loadings for Selective Catalytic Reduction of NOx by NH3. J. Phys. Chem. C 2009, 113,

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N. Q. Fe-Ti Spinel for the Selective Catalytic Reduction of NO with NH3: Mechanism and

298

Structure-activity Relationship. Appl. Catal. B-environ 2012, 117, 73-80.

299

(6) Liu, Y.; Gu, T. T.; Weng, X. L.; Wang, Y.; Wu, Z. B.; Wang, H. Q. DRIFT Studies on the

300

Selectivity Promotion Mechanism of Ca-Modified Ce-Mn/TiO2 Catalysts for Low-Temperature

301

NO Reduction with NH3. J. Phys. Chem. C 2012, 116, 16582-16592.

302

(7) Qi, G. S.; Yang, R. T. Performance and Kinetics Study for Low-temperature SCR of NO with

303

NH3 over MnOx-CeO2 Catalyst. J. Catal. 2003, 217, 434-441.

304

(8) Chang, H. Z.; Chen, X. Y.; Li, J. H.; Ma, L.; Wang, C. Z.; Liu, C. X.; Schwank, J. W.; Hao, J.

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M. Improvement of Activity and SO2 Tolerance of Sn-Modified MnOx-CeO2 Catalysts for

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NH3-SCR at Low Temperatures. Environ. Sci. Technol. 2013, 47, 5294-5301.

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(9) Qi, G. S.; Yang, R. T. A Superior Catalyst for Low-temperature NO Reduction with NH3.

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Chem. Commun. 2003, 7, 848-849.

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Low-temperature Selective Catalytic Reduction of NO with NH3. J. Phys. Chem. B 2004, 108,

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over β-MnO2 and α-Mn2O3. Appl. Catal. B-environ 2010, 99, 156-162.

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(12) Zhang, X. Y.; Shen, Q.; He, C.; Ma, C. Y.; Cheng, J.; Li, L. D.; Hao, Z. P. Investigation of

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Selective Catalytic Reduction of N2O by NH3 over an Fe-Mordenite Catalyst: Reaction

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Mechanism and O2 Effect. ACS Catal. 2012, 2, 512-520.

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C.; Hori, K. Catalytic decomposition of N2O over Ni and Mg-magnetite catalysts. Catal. Sci.

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Technol. 2013, 3, 2288-2294.

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(14) Yang, S.; Xiong, S.; Liao, Y.; Xiao, X.; Qi, F.; Peng, Y.; Fu, Y.; Shan, W.; Li, J. Mechanism of

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N2O Formation during the Low Temperature Selective Catalytic Reduction of NO with NH3 over

322

Mn-Fe Spinel. Environ. Sci. Technol. 2014, 48, 21500-21508.

323

(15) Yang, S.; Liao, Y.; Xiong, S.; Qi, F.; Dang, H.; Xiao, X.; Li, J. N2 Selectivity of NO

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21500-21508.

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Improvement of the Activity of γ-Fe2O3 for the Selective Catalytic Reduction of NO with NH3 at

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High Temperatures: NO Reduction versus NH3 Oxidization. Ind. Eng. Chem. Res. 2013, 52,

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5601-5610.

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(17) Qi, G. S.; Yang, R. T. Low-temperature Selective Catalytic Reduction of NO with NH3 over

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Iron and Manganese Oxides Supported on Titania. Appl. Catal. B-environ 2003, 44, 217-225.

332

(18) Yang, S.; Wang, C.; Li, J.; Yan, N.; Ma, L.; Chang, H. Low Temperature Selective Catalytic

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Reduction of NO with NH3 over Mn-Fe Spinel: Performance, Mechanism and Kinetic Study. Appl.

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Catal. B-environ 2011, 110, 71-80.

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(19) Yang, S. J.; Guo, Y. F.; Chang, H. Z.; Ma, L.; Peng, Y.; Qu, Z.; Yan, N. Q.; Wang, C. Z.; Li, J.

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H. Novel Effect of SO2 on the SCR Reaction over CeO2: Mechanism and Significance. Appl.

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Catal. B-environ 2013, 136, 19-28.

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(20) Yang, S.; Fu, Y.; Liao, Y.; Xiong, S.; Qu, Z.; Yan, N.; Li, J. Competition of Selective Catalytic

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Reduction and Non Selective Catalytic Reduction over MnOx/TiO2 for NO Removal: The

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Relationship between Gaseous NO Concentration and N2O Selectivity. Catal. Sci. Technol. 2014, 15



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4, 224-232.

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on the Selective Catalytic Reduction of NO with NH3 over MnOx/TiO2: Key Factor of NH3

344

Distribution. Ind. Eng. Chem. Res. 2014, 53, 5810-5819.

345

(22) Hu, P. P.; Huang, Z. W.; Hua, W. M.; Gu, X.; Tang, X. F. Effect of H2O on Catalytic

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Performance of Manganese Oxides in NO Reduction by NH3. Appl. Catal. A-gen 2012, 437,

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139-148.

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(23) Lei, Z. G.; Han, B.; Yang, K.; Chen, B. H. Influence of H2O on the Low-temperature

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NH3-SCR of NO over V2O5/AC Catalyst: An Experimental and Modeling Study. Chem. Eng. J.

350

2013, 215, 651-657.

351

(24) Pan, S. W.; Luo, H. C.; Li, L.; Wei, Z. L.; Huang, B. C. H2O and SO2 Deactivation

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Mechanism of MnOx/MWCNTs for Low-temperature SCR of NOx with NH3. J. Mol. Catal.

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A-chem 2013, 377, 154-161.

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(25) Chen, L. A.; Li, J. H.; Ge, M. F.; Zhu, R. H. Enhanced Activity of Tungsten Modified

355

CeO2/TiO2 for Selective Catalytic Reduction of NOx with Ammonia. Catal. Today 2010, 153,

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77-83.

357

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Ammonia to Nitrogen on Transition Metal Containing Mixed Metal Oxides. Appl. Catal.

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B-environ 2005, 58, 235-244.

360

(27) Machida, M.; Uto, M.; Kurogi, D.; Kijima, T. Solid-gas Interaction of Nitrogen Oxide

361

Adsorbed on MnOx-CeO2: A DRIFTS Study. J. Mater. Chem. 2001, 11, 900-904.

362

(28) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Chemical and Mechanistic Aspects of the Selective

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Catalytic Reduction of NOx by Ammonia over Oxide Catalysts: A review. Appl. Catal. B-environ

364

1998, 18, 1-36.

365 366 367

16


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Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


368

/µmol g-1

Table 1 The amounts of NH3 and NOx adsorbed on MnOx-CeO2 NOx

NH3 120 oC

180 oC

120 oC

180 oC

in the absence of H2O

65

54

98

88

in the presence of 5% of H2O

35

30

33

33

369 370 371

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

372

Page 18 of 28

Table 2 Effect of 5% of H2O on NH3 oxidation over MnOx-CeO2.



Temperature/oC

NH3 conversion/%

N2 selectivity/%

N2O selectivity/%

120

45

42

58

140

72

43

57

160

97

45

55

180

99

42

58

200

99

40

56

120

5

-

-

140

6

-

-

160

10

74

26

180

22

59

41

200

46

51

49

373

Reaction conditions: [O2]=2%, [NH3]=500 ppm, catalyst mass=200 mg, total flow rate=100 mL

374

min-1 and GHSV=30000 cm3 g-1 h-1.

375

18


Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


376

Table 3 The reaction rate constants of N2 formation over MnOx-CeO2 through the Eley-Rideal

377

mechanism (kN2(E-R)) and the Langmuir-Hinshelwood mechanism (kN2(L-H)), and the reaction rate

378

constant of N2O formation over MnOx-CeO2 through the Eley-Rideal mechanism (kN2O(E-R))

− o

Temperature/ C

d[NO(g) ] dt

= kN2 (E-R) [NO(g) ] + kN2 (L-H) + k N2O(E-R) R2

kN2(L-H)

k N2(E-R)

kN2O(E-R)

/µmol g-1 min-1

/mol g-1 min-1

/µmol g-1 min-1

120

10.5

0.088

17.8

0.999

140

24.8

0.126

49.0

0.999

160

0

0.228

71.2

0.999

180

0

0.332

145

0.998

200

0

0.411

236

0.993

120

1.8

0.023

0.4

0.994

140

3.8

0.051

0.9

0.999

160

0.1

0.112

1.9

0.985

180

0

0.197

3.3

0.999

200

1.2

0.209

6.4

0.998



379

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

380

Figure captions

381

Figure 1 (a), SCR performance of MnOx-CeO2 in the absence of H2O; (b), SCR performance of

382

MnOx-CeO2 in the presence of 5% of H2O; (c), Effect of 5% of H2O on N2 formation over

383

MnOx-CeO2. Reaction conditions: [O2]=2%, [NH3]=[NO]=500 ppm, catalyst mass=200 mg, total

384

flow rate=100 mL min-1 and GHSV=30000 cm3 g-1 h-1.

385

Figure 2 NO-TPD profiles of MnOx-CeO2: (a), saturated with the adsorption of NO+O2 in the

386

absence of H2O at 120 oC; (b), saturated with the adsorption of NO+O2 in the presence of H2O at

387

120 oC.

388

Figure 3 NH3-TPD profiles of MnOx-CeO2: (a), saturated with the adsorption of NH3 in the

389

absence of H2O at 120 oC; (b), saturated with the adsorption of NH3 in the presence of H2O at 120

390

o

391

Figure 4 Effect of 5% of H2O on NO oxidation over MnOx-CeO2. Reaction conditions: [O2]=2%,

392

[NO]=500 ppm, catalyst mass=200 mg, total flow rate=100 mL min-1 and GHSV=30000 cm3 g-1

393

h-1.

394

Figure 5 Contributions of the catalytic oxidation of NH3 to NO (C-O), the SCR reaction, and the

395

NSCR reaction to NH3 conversion over MnOx-CeO2: (a), in the absence of H2O; (b), in the

396

presence of 5% of H2O.

397

Figure 6 Dependences of (a) NOx conversion rate and (b) N2O formation rate on gaseous NO

398

concentration during the SCR reaction over MnOx-CeO2 in the absence of H2O. Reaction

399

conditions: [NH3]=500 ppm, [NO]=200-500 ppm, [O2]=2%, catalyst mass=3-20 mg, total flow

400

rate=400 mL min-1 and GHSV=1200000-8000000 cm3 g-1 h-1.

401

Dependences of (c) NOx conversion rate and (d) N2O formation rate on gaseous NO concentration

402

during the SCR reaction over MnOx-CeO2 in the presence of 5% of H2O. Reaction conditions:

403

[NH3]=500 ppm, [NO]=200-500 ppm, [O2]=2%, catalyst mass=5-50 mg, total flow rate=200 mL

404

min-1 and GHSV=240000-2400000 cm3 g-1 h-1.

405

Figure 7 Transient reaction taken at 180 oC upon: (a), passing NO+O2 over NH3 presorbed

406

MnOx-CeO2; (b), passing NO+O2+H2O over NH3 presorbed MnOx-CeO2; (c), passing NO+O2

407

over NH3+H2O presorbed MnOx-CeO2.

C.

408 20


Page 20 of 28

Page 21 of 28

409

80 NOx conversion

60

NH3 conversion

60

N2O selectivity

40

40 20

20 0

NOx/NH3 conversion/%

80

0 120

140

160

180

40

100 80

30

N2O selectivity/%

NOx/NH3 conversion/%

100

N2O selectivity/%

NOx conversion

60

NH3 conversion

20

N2O selectivity

40

10

20 0

200

0

120

o

140

Temperature/ C

160

o

180

200

Temperature/ C

a

b 500

N2 formation/ppm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


without H2O

with 5% of H2O

400 300 200 100 0 120

140

160

180

200

o

Temperature/ C

c 410 411

Figure 1 (a), SCR performance of MnOx-CeO2 in the absence of H2O; (b), SCR performance of

412

MnOx-CeO2 in the presence of 5% of H2O; (c), Effect of 5% of H2O on N2 formation over

413

MnOx-CeO2. Reaction conditions: [O2]=2%, [NH3]=[NO]=500 ppm, catalyst mass=200 mg, total

414

flow rate=100 mL min-1 and GHSV=30000 cm3 g-1 h-1.

415

21



NOx concentration/ppm

60 NO

NO2

50 40 30 20 10 0 100

200

300

400

500

600

o

Temperature/ C

a 60


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NO

NO2

50 40 30 20 10 0 100

200

300

400

500

600

o

Temperature/ C

b 416 417

Figure 2 NO-TPD profiles of MnOx-CeO2: (a), saturated with the adsorption of NO+O2 in the

418

absence of H2O at 120 oC; (b), saturated with the adsorption of NO+O2 in the presence of H2O at

419

120 oC.

420 421

22


Page 22 of 28

Page 23 of 28

NH3/NOx concentration/ppm

422

40 NO

NH3

30

N2O

20 10 0 100

200

300

400

500

600

o

Temperature/ C

a

NH3/NOx concentration/ppm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40 NO

NH3

30

N2O

20 10 0 100

200

300

400

500

600

o

Temperature/ C

b 423 424

Figure 3 NH3-TPD profiles of MnOx-CeO2: (a), saturated with the adsorption of NH3 in the

425

absence of H2O at 120 oC; (b), saturated with the adsorption of NH3 in the presence of H2O at 120

426

o

C.

427 428

23



429 60 without H2O

NO conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with H2O 40

20

0 120

140

160

o

180

200

Temperature/ C

430 431

Figure 4 Effect of 5% of H2O on NO oxidation over MnOx-CeO2. Reaction conditions: [O2]=2%,

432

[NO]=500 ppm, catalyst mass=200 mg, total flow rate=100 mL min-1 and GHSV=30000 cm3 g-1

433

h-1.

434 435

24


Page 24 of 28

Page 25 of 28

436

C-O

NSCR

SCR

NH3 conversion/%

100 80 60 40 20 0 120

140

160

180

200

o

Temperature/ C

a

100

NH3 conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C-O

NSCR

SCR

80 60 40 20 0 120

140

160

180

200

o

Temperature/ C

b 437 438

Figure 5 Contributions of the catalytic oxidation of NH3 to NO (C-O), the SCR reaction, and the

439

NSCR reaction to NH3 conversion over MnOx-CeO2: (a), in the absence of H2O; (b), in the

440

presence of 5% of H2O.

441 442

25


o

o

160 C

-1

140 C o 200 C

-1 -1

N2O formation/µmol g min

o

120 C o 180 C

300

/µmol g min

Rate of NOx conversion

400

200 100 0 200

300

400

500

500

o

120 C o 180 C

400

o

160 C

100 0 200

300

400

500

NO concentration/ppm

b 20

120 o

160 C

o

60 30 0 200

300

400

120 C o 180 C

o

o

140 C o 200 C

160 C

-1

15

-1

-1

90

140 C o 200 C

/µmol g min

o

120 C o 180 C

Rate of N2O formation

o

/µmol g min

o

140 C o 200 C

200

a

-1

Page 26 of 28

300


Rate of NOx conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1


500


10 5 0 200

300

400

500


c

d

443 444

Figure 6 Dependences of (a) NOx conversion rate and (b) N2O formation rate on gaseous NO

445

concentration during the SCR reaction over MnOx-CeO2 in the absence of H2O. Reaction

446

conditions: [NH3]=500 ppm, [NO]=200-500 ppm, [O2]=2%, catalyst mass=3-20 mg, total flow

447

rate=400 mL min-1 and GHSV=1200000-8000000 cm3 g-1 h-1.

448

Dependences of (c) NOx conversion rate and (d) N2O formation rate on gaseous NO concentration

449

during the SCR reaction over MnOx-CeO2 in the presence of 5% of H2O. Reaction conditions:

450

[NH3]=500 ppm, [NO]=200-500 ppm, [O2]=2%, catalyst mass=5-50 mg, total flow rate=200 mL

451

min-1 and GHSV=240000-2400000 cm3 g-1 h-1.

452 453

26


Page 27 of 28

454

500

80 60

NO NO2

300

40

N2O

200


400

20

100

0

0 -5

0

5

10

15

20

25

80

400

40

N2O

200

20

100

30

0

0 -5

0

5

10

t/min

t/min

a

b 500

15

20

25

30

80

400

60

NO NO2

300

40

N2O

200

20

100

0

0 -5

0

5

10

15

20

25

N2O concentration/ppm


60

NO NO2

300



500


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

t/min

c 455

Figure 7 Transient reaction taken at 180 oC upon: (a), passing NO+O2 over NH3 presorbed

456

MnOx-CeO2; (b), passing NO+O2+H2O over NH3 presorbed MnOx-CeO2; (c), passing NO+O2

457

over NH3+H2O presorbed MnOx-CeO2.

458

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

459 460

TOC

461 462 463

28


Page 28 of 28

Novel Effect of H2O on the Low Temperature Selective Catalytic

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