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A global kinetic study of NO reduction by NH3 over V2O5-WO3/ TiO2: Relationship between the SCR performance and the key factors Shangchao Xiong, Xin Xiao, Yong Liao, Hao Dang, Wenpo Shan, and Shijian Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03044 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015

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Industrial & Engineering Chemistry Research

A global kinetic study of NO reduction by NH3 over V2O5-WO3/TiO2: Relationship between the SCR performance and the key factors Shangchao Xiong, Xin Xiao, Yong Liao, Hao Dang, Wenpo Shan, Shijian Yang*

School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing, 210094 P. R. China ∗

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

1

ACS Paragon Plus Environment


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Abstract: The non selective catalytic reduction (NSCR) reaction and the catalytic oxidation of NH3 to NO (C-O reaction) simultaneously happened during the selective catalytic reduction (SCR) of NO with NH3 over V2O5-WO3/TiO2 especially at higher temperatures. There was an excellent linear relationship between the SCR reaction rate and gaseous NO concentration, and the intercept and slope can be used to describe the rate constant of NO reduction through the Langmuir-Hinshelwood mechanism and that through the Eley-Rideal mechanism respectively. However, the NSCR reaction rate was nearly independent of gaseous NO concentration, and the reaction order of the C-O reaction with respect to gaseous NO concentration was much less than zero. According to the kinetic study, the relationship of the SCR performance (i.e. SCR activity and N2 selectivity) with the key factors (for example V2O5 content, H2O effect and reactant concentration) was built, which can be used to predict the SCR performance. Keywords: the SCR reaction; the NSCR reaction; the C-O reaction; reaction order; kinetics.

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1. Introduction The emission of nitrogen oxides (NO and NO2) from coal fired power plants and automobiles is a serious concern due to their contribution to haze, photochemical smog and acid rain.

1-6

So far,

selective catalytic reduction (SCR) of NO by NH3 (i.e. Reaction 1) with V2O5-WO3/TiO2 as the catalyst is the major technology to control NOx emission from coal fired power plants. 7, 8

4NH3 +4NO+O2 → 4N2 +6H2O

(1)

The mechanism of the SCR reaction over V2O5-WO3/TiO2 was widely studied using in situ DRIFT spectra.8,

9

There is generally agreement that the SCR reaction over V2O5-WO3/TiO2

mainly follows the Eley-Rideal mechanism (i.e. the reaction of activated NH3 with gaseous NO), 8, 10

and the Langmuir-Hinshelwood mechanism (i.e. the reaction of adsorbed NH3 with adsorbed

NOx) hardly contributes to NO reduction over V2O5-WO3/TiO2 as the adsorption of NOx cannot be clearly observed. The kinetic data of NO reduction are often modeled using the following kinetic equation: 10-12

δ =k[NO(g) ]α [NH3(g) ]β [O2(g) ]γ

(2 )

Where δ, k, [NO(g)], [NH3(g)], [O2(g)], α, β and γ are the rate of NO reduction, the reaction rate constant, the concentrations of gaseous NO, NH3 and O2, and the reaction orders with respect to gaseous NO, NH3 and O2, respectively. The effect of gaseous O2 on NO reduction can be neglected due to its large excess in practical conditions, so NO reduction is generally independent of gaseous O2 concentration. Although the mechanism and kinetics of the SCR reaction over V2O5-WO3/TiO2 have been widely studied in the past thirty years,

13-19

there are still some issues

needed to be further clarified. Firstly, the reaction order of NO reduction with respect to gaseous NO concentration should be 1 in the absence of the Langmuir-Hinshelwood mechanism.20 However, some authors measured the parameter in the range of 0.5-0.8.

10

Furthermore, the

reaction order of NO reduction with respect to gaseous NH3 concentration should be nearly zero.20, 21

However, some authors found that the rate of NO reduction over V2O5-WO3/TiO2 often

increased with the increase of gaseous NH3 concentration.

14, 22, 23

Secondly, some side reactions

(i.e. Reactions 3-4) including the non selective catalytic reduction (NSCR) reaction and the 3



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catalytic oxidation of NH3 (C-O reaction) simultaneously happen during the SCR reaction over V2O5-WO3/TiO2 especially at higher temperatures, resulting in the formation of N2O and the decrease of NOx conversion. So far, the NSCR reaction and the C-O reaction are seldom taken into account in the kinetic study of NO reduction over V2O5-WO3/TiO2.

22-24

As a result, the

relationship of the SCR performance (i.e. SCR activity and N2 selectivity) with the key factors (for example V2O5 content, H2O effect and reactant concentration) is now highly uncertain. This lack of understanding presents a severe limitation in predicting the SCR performance.

4NH3 +4NO+3O2 → 4N2O+6H2O

(3)

4NH3 +5O2 → 4NO+6H2O

(4)

In this work, a global kinetics of NO reduction over V2O5-WO3/TiO2 (including the SCR reaction, the NSCR reaction and the C-O reaction) was studied according to the steady state kinetic experiment and the kinetic deduction based on the reaction mechanism. Then, the relationship of the SCR performance of V2O5-WO3/TiO2 with the key factors was built according to the kinetic study.

2. Experimental 2.1 Catalyst preparation A series of V2O5-WO3/TiO2 with 1, 3 and 5 wt % V2O5 and 10 wt % WO3 were prepared by the conventional impregnation method

7, 9

using NH4VO3, (NH4)10W12O41 and H2C2O4⋅2H2O as

precursors, and P25 as support. 2.2 Catalytic test The SCR reaction was performed on a fixed-bed quartz tube micro-reactor, whose diameter was 6 mm and length was 45 cm. The mass of V2O5-WO3/TiO2 with 40-60 mesh was 100 mg, and the total flow rate was 200 mL min-1 (room temperature and 1 atm), resulting in the gas hourly space velocity (GHSV) of 1.2×105 cm3 g-1 h-1. The typical reactant gas contained 500 ppm of NO, 500 ppm of NH3, 2% of O2 and balance of N2, which were prepared by 1% of NO/N2, 1% of NH3/N2, 100% of O2 and 100% of N2 using mass flow controllers. Furthermore, 5% of H2O was introduced when used by the bubble method. All the reactant gases were completely mixed in a 20 mL of 4


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mixing tank before they were introduced into the reactor. The reaction temperature ranged from 150 to 500 oC. An infrared spectrometer (Thermo SCIENTIFIC, ANTARIS, IGS Analyzer) was used to determine the concentrations of NO, NO2, NH3 and N2O in the inlet or outlet online. Meanwhile, a steady state kinetic study of NO reduction over V2O5-WO3/TiO2 was conducted. Gaseous NO concentration or NH3 concentration was kept at 500 ppm, while the other varied from 100-700 ppm.

25-28

Meanwhile, a very high GHSV (i.e. 4.8×106-1.2×107 cm3 g-1 h-1) was

adopted to keep NO conversion less than 15%, resulting in a minimum of the inner diffusion and external diffusion.

3. Results 3.1 SCR performance of V2O5-WO3/TiO2 Both NOx conversion and NH3 conversion obviously increased at 150-300 oC as V2O5 content in V2O5-WO3/TiO2 increased from 1% to 5% (shown in Figures 1a and 1b). However, NOx conversion decreased notably above 400 oC with the increase of V2O5 content (shown in Figure 1a) although NH3 conversions were all close to 100% (shown in Figure 1b). Meanwhile, the amount of N2O formed increased remarkably with the increase of V2O5 content, resulting in an increase of N2O selectivity (shown in Figure 1c). Both the SCR reaction and the NSCR reaction contributed to NO conversion and NH3 conversion.29, 30 Although the C-O reaction contributed to NH3 conversion, it contributed to NO formation.29, 30 Therefore, NH3 conversion (ηNH3) and NOx conversion (ηNOx) can be described as follows: 31

η NH =ηSCR +ηNSCR +ηC-O

(5)

η NO =ηSCR +η NSCR -ηCO

(6)

3

x

Where, ηSCR, ηNSCR and ηC-O were the contributions of the SCR reaction, the NSCR reaction and the C-O reaction, respectively. According to Reactions 5 and 6 , ηC-O can be calculated as:

ηC-O =

η NH − η NO 3

x

(7)

2 5



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Meanwhile, the contributions of the SCR reaction and the NSCR reaction can be calculated according to the formations of N2 and N2O respectively. The contribution of the SCR reaction to NH3 conversion below 300 oC obviously increased as the content of V2O5 in V2O5-WO3/TiO2 increased from 1% to 5%, while it above 400 oC obviously decreased (shown in Figure 2). Meanwhile, the contributions of both the NSCR reaction and the C-O reaction to NH3 conversion increased with the increase of V2O5 content. NOx conversion over 5% V2O5-WO3/TiO2 obviously decreased at 150-250 oC after the introduction of 5% of H2O (shown in Figure 1a). However, NOx conversion over 5% V2O5-WO3/TiO2 in the presence of 5% of H2O above 400 oC was much higher than that in the absence of H2O. Meanwhile, the formation of N2O over 5% V2O5-WO3/TiO2 was obviously restrained after the introduction of 5% of H2O, resulting in a notable decrease of N2O selectivity above 300 oC. The contribution of the SCR reaction to NH3 conversion over 5% V2O5-WO3/TiO2 below 250 oC obviously decreased after the introduction of 5% of H2O, while it above 350 oC obviously increased (shown in Figures 2c and 2d). Meanwhile, the contributions of the NSCR reaction and the C-O reaction to NH3 conversion over 5% V2O5-WO3/TiO2 both decreased after the introduction of 5% of H2O. The increase of gaseous NH3 and NO concentrations showed a dual effect on NO reduction over 5% V2O5-WO3/TiO2 that NOx conversion decreased remarkably at 150-200 oC, while it obviously increased above 300 oC (shown in Figure 3a). Meanwhile, NH3 conversion was obviously restrained with the increase of the concentrations of NH3 and NO from 250 to 1000 ppm (shown in Figure 3b). Furthermore, N2O selectivity decreased remarkably with the increase of gaseous NO and NH3 concentrations (shown in Figure 3c). As the concentrations of NH3 and NO increased from 250 to 1000 ppm, the contribution of the SCR reaction to NH3 conversion below 250 oC obviously decreased, while it obviously increased above 400 oC (shown in Figure 4). Meanwhile, the contribution of the C-O reaction obviously decreased with the increase of the concentrations of NH3 and NO. Figures 5a and 5b shows that both NOx conversion and NH3 conversion over 5% V2O5-WO3/TiO2 below 350 oC obviously decreased as the GHSV increased from 3.0×104 to 6


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4.8×105 cm3 g-1 h-1. However, both NOx conversion and NH3 conversion above 400 oC did not vary notably with the increase of GHSV. Meanwhile, N2O selectivity of NO reduction over 5% V2O5-WO3/TiO2 did not vary obviously. The contribution of the SCR reaction to NH3 conversion obviously decreased below 250 oC with the increase of the GHSV (shown in Figure 6). However, the contributions of the SCR reaction, the NSCR reaction and the C-O reaction to NH3 conversion over 5% V2O5-WO3/TiO2 above 400 oC all did not vary notably. The experiment results suggest that the SCR performance of V2O5-WO3/TiO2 was influenced by V2O5 content, the presence of H2O, reactant concentration and GHSV.

32

As most NH3 was

consumed above 300 oC, the SCR performance mainly depended on the competition among the SCR reaction, the NSCR reaction and the C-O reaction. To investigate the competition among the SCR reaction, the NSCR reaction and the C-O reaction, the steady-state reaction kinetic study was performed. 3.2 Steady state reaction kinetic study As gaseous NO concentration increased, the rates of NH3 conversion, NOx conversion and N2 formation all increased (shown in Figures 7a-7c). The curve fitting shows that the reaction orders of NH3 conversion, NOx conversion and the SCR reaction with respect to gaseous NO concentration were all in the range of 0 and 1, which were consistent with the result in previous study.

10

The NSCR reaction rate first increased notably with the increase of gaseous NO

concentration from 100-300 ppm, and it then slightly decreased with the further increase of gaseous NO concentration (shown in Figure 7d). This result was not completely consistent with the result in previous study that the reaction order of N2O formation with respect to gaseous NO concentration was nearly zero.

22, 25-28

Figure 7e shows that the C-O reaction rate of 5%

V2O5-WO3/TiO2 remarkably decreased with the increase of gaseous NO concentration. It suggests that reaction order of the C-O reaction with respect to gaseous NO concentration was less than zero. The reaction orders of NH3 conversion, NOx conversion, the SCR reaction, the NSCR reaction and the C-O reaction during NO reduction over 1% V2O5-WO3/TiO2 and those over 3% V2O5-WO3/TiO2 with respect to gaseous NO concentration (shown in Figures S1 and S2 in the 7



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Supporting Information) were similar to those over 5% V2O5-WO3/TiO2. It suggests that these reaction orders were generally independent of V2O5 content in V2O5-WO3/TiO2. The rates of NH3 conversion, NOx conversion, the SCR reaction, the NSCR reaction and the C-O reaction generally increased with the increase of gaseous NH3 concentration (shown in Figure 8). It suggests that the reaction orders of NH3 conversion, NOx conversion, the SCR reaction, the NSCR reaction and the C-O reaction with respect to gaseous NH3 concentration were all generally higher than zero. This result was consistent with the result in previous study. 22 Figures 8c-8e also show that the promotion of the SCR reaction due to the increase of gaseous NH3 concentration was much less than that of the NSCR reaction and the C-O reaction.

4. Discussion 4.1 Reaction mechanism The SCR reaction through the Eley-Rideal mechanism, the NSCR reaction and the C-O reaction over V2O5-WO3/TiO2 can be approximately described as: 8, 10

NH3(g) → NH3(ad)

(8)

NH3 +V5+ =O → V 4+ -OH+NH 2

(9)

NH 2 + NO(g) → N 2 +H 2O

(10)

NH 2 + V5+ =O → NH+V 4+ -OH

(11)

NH + V5+ =O+NO(g) → V 4+ -OH+N 2O

(12)

1 NH+V5+ =O+ O2 → NO+V 4+ -OH 2

(13)

1 1 V 4+ -OH+ O2 → V5+ =O+ H 2 O 4 2

(14)

Firstly, gaseous NH3 adsorbs on the Brønsted acid sites on V2O5-WO3/TiO2 to NH4+ (i.e. Reaction 8), which is then activated by V5+ on V2O5-WO3/TiO2 to form NH2 (i.e. Reaction 9). Then, gaseous NO is reduced by NH2 on V2O5-WO3/TiO2 to N2 (i.e. Reaction 10). Meanwhile, NH2 on the surface can be further oxidized to NH (i.e. Reaction 11), which then reacts with 8


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gaseous NO to form N2O (i.e. Reaction 12). Reaction 13 is the deep oxidation of NH to NO, which is the key step of the C-O reaction. The formed NO from Reaction 13 can then react with NH2/NH on the surface to N2/N2O, which is the so-called selective/non-selective catalytic oxidation of NH3. 29, 30 There is generally agreement that NH3 cannot be directly oxidized to N2 or N2O and the formation of N2 and N2O from NH3 oxidation are mainly related to Reactions 10 and 12. 33-35 Reaction 14 is the regeneration of V5+ on V2O5-WO3/TiO2. Meanwhile, NO reduction over V2O5-WO3/TiO2 through the Langmuir-Hinshelwood mechanism can be approximately described as: 9, 34, 35

NO(g) → NO(ad)

(15)

NO(ad) + V5+ =O → V 4+ -O − NO

(16)

V 4+ -O-NO + NH3(ad) → V 4+ -O-NO-NH3 → V 4+ -OH+H 2O+N 2

(17)

Gaseous NO firstly adsorbs on V2O5-WO3/TiO2 (i.e. Reaction 15). Then, adsorbed NO is oxidized by V5+ on the surface to form NO2- (i.e. Reaction 16), which then reacts with adsorbed NH4+ to NH4NO2 (i.e. Reaction 17). At last, the formed NH4NO2 is decomposed to N2 and H2O. If the Langmuir-Hinshelwood mechanism did not contribute to NO reduction over V2O5-WO3/TiO2, its reaction rate would be close to zero. 4.2 Reaction kinetic study According to Reactions 10 and 12, the kinetic equations of N2 formation through the Eley-Rideal mechanism and N2O formation over V2O5-WO3/TiO2 can be described as: 21, 28

d[N 2 ] dt

E-R

=−

d[NO (g) ] d[NH 2 ] =− = k1[NH 2 ][NO(g) ] dt dt

d[NO(g) ] d[N 2 O] d[NH] =− =− = k2 [NH][NO (g) ][V 5+ =O] dt dt dt

(18)

(19)

Where, k1, k2, [NH2], [NH], [V5+=O] and [NO(g)] were the kinetic constants of Reactions 10 and 12, the concentrations of NH2, NH and V5+ on V2O5-WO3/TiO2, and gaseous NO concentration, respectively. Meanwhile, the kinetic equations of NH and NH2 formation over V2O5-WO3/TiO2 (i.e. 9



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Reactions 9 and 11) can be described as:

d[NH 3(ad) ] d[NH 2 ] =− = k3 [NH3(ad) ][V5+ =O] dt dt

(20)

d[NH 2 ] d[NH] =− = k 4 [NH 2 ][V 5+ =O] dt dt

(21)

Where, k3 and k4 were the kinetic constants of Reactions 9 and 11, respectively. Furthermore, the deep oxidation of NH on V2O5-WO3/TiO2 (i.e. Reaction 13) can be approximately described as:

d[NO] d[NH] =− = k5 [NH][V 5+ =O] dt dt

(22)

Where, k5 was the kinetic constant of Reactions 13. As the reaction reached the steady state, NH concentration on V2O5-WO3/TiO2 would not vary. Therefore,

d[NH] = k4 [NH 2 ][V5+ =O] − k2 [NH][NO(g) ][V5+ =O] − k5 [NH][V5+ =O]=0 dt

(23)

Thus, NH concentration on V2O5-WO3/TiO2 at the steady state can be described as:

[NH]=

k4 [NH 2 ] k2 [NO(g) ] + k5

(24)

Then, the NSCR reaction rate (i.e. δNSCR) of V2O5-WO3/TiO2 (i.e. Equation 19) can be transformed as:

δ NSCR =

d[N2O] k4 [NH2 ] k [NH2 ][V5+ =O] = k2 [NO(g) ][V5+ =O] = 4 k5 dt k2 [NO(g) ] + k5 1+ k2[NO(g) ]

(25)

Meanwhile, the rate of the C-O reaction (i.e. δC-O) over V2O5-WO3/TiO2 can be described as:

δ C-O = k5

k4 [NH 2 ] k [NH 2 ][V 5+ =O] [V 5+ =O] = 4 k2 [NO (g) ] k2 [NO (g) ] + k5 +1 k5

(26)

The kinetic equation of N2 formation through the Langmuir-Hinshelwood mechanism (i.e. the decomposition of NH4NO2 adsorbed on V2O5-WO3/TiO2) can be described as:

10


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d[N 2 ] dt

L-H

= k6 [V 4+ -O-NO-NH 3 ]

(27)

Where, k6 and [V4+-O-NO-NH3] were the decomposition rate constant of NH4NO2 and the concentration of NH4NO2 on V2O5-WO3/TiO2, respectively. Our previous study demonstrated that the concentration of NH4NO2 on the surface can be approximately regarded as a constant at the steady state (the deduction was shown in the Supporting Information), which was not related to gaseous NO and NH3 concentration.27,

28

Therefore, the rate of the SCR reaction over V2O5-WO3/TiO2 through the Langmuir-Hinshelwood mechanism was nearly independent of gaseous NO and NH3 concentrations. NH2 concentration on V2O5-WO3/TiO2 at the steady-state was approximately independent of gaseous NO and NH3 concentrations (the deduction was shown in the Supporting Information), 28 which was mainly related to k3, the concentration of NH3 adsorbed, and V5+ concentration on V2O5-WO3 /TiO2 (hinted by Equation 20). Therefore, the SCR reaction rate (i.e. δSCR) of V2O5-WO3/TiO2 can be described as:

d[N 2 ] d[N 2 ] L-H + dt dt = kSCR-LH + kSCR-ER [NO(g) ]

δ SCR =

E-R

= k6 [V 3+ -O-NO-NH 3 ]+k1[NH 2 ][NO(g) ]

(28)

kSCR-LH = k6 [V 3+ -O-NO-NH 3 ]

(29)

kSCR-ER = k1[NH 2 ]

(30)

Where, kSCR-LH and kSCR-ER were the rate constant of the SCR reaction through the Langmuir-Hinshelwood mechanism and that through the Eley-Rideal mechanism, respectively. If gaseous NO concentration was very high or k5 was very low (i.e. the C-O reaction did not happen), the value of k5/[NO(g)] was close to zero. Then, Equation 25 can be approximately transformed as:

k NSCR = δ NSCR ≈ k4 [NH 2 ][V5+ =O]

(31)

Where, kNSCR was the rate constant of the NSCR reaction over V2O5-WO3/TiO2. According to Equations 25 and 28, N2O selectivity can be described as:

11



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N 2 O selectivity =

δN O δN + δN O 2

2

= =

Page 12 of 37

2

k4 [NH 2 ][V5+ =O] k6 [V -O-NO-NH3 ]+k1[NH 2 ][NO(g) ] + k4 [NH 2 ][V5+ =O]

(32)

3+

k4 [V5+ =O] k6 [V -O-NO-NH3 ] +k1[NO(g) ] + k4 [V5+ =O] [NH 2 ] 3+

4.3 Model verification Figures 7c, S1c and S2c all indicate that there was an excellent linear relationship between the SCR reaction rate and gaseous NO concentration. This result was consistent with the hint of Equation 28. Therefore, kSCR-LH and kSCR-ER can be obtained after the linear regression of Figures 7c, S1c and S2c, and the intercept and slope can be used to describe kSCR-LH and kSCR-ER respectively.26, 36-38 If kSCR-LH was close to zero, the contribution of the Langmuir-Hinshelwood mechanism to NO reduction can be neglected. However, the intercepts in Figures 7c, S1c and S1c were generally much higher than zero (shown in Table 1). It suggests that the Langmuir-Hinshelwood mechanism possibly played an important role on NO reduction over V2O5-WO3/TiO2. Therefore, the adsorption of NOx on V2O5-WO3/TiO2 cannot be neglected although it was not clearly observed in the DRIFT spectra probably due to its lower concentration and unstability. Figures 7e, S1e and S2e show that the rate of the C-O reaction obviously decreased with the increase of gaseous NO concentration. This result was consistent with the hint of Equation 26. Hinted by Equation 25, the NSCR reaction rate of 5% V2O5-WO3/TiO2 would obviously increase with the increase of gaseous NO concentration as gaseous NO concentration was low. This deduction was demonstrated in Figures 7d, S1d and S2d. Equation 25 also hinted that the NSCR reaction rate would not vary notably with the increase of gaseous NO concentration if gaseous NO concentration was high. However, Figures 3d, S1d and S2d all show that the NSCR reaction rate slightly decreased with the further increase of gaseous NO concentration. Equations 25, 26 and 28 suggest that the reaction orders of the SCR reaction, the NSCR reaction and the C-O reaction with respect to gaseous NH3 concentration should all be zero. However, Figure 8 clearly shows that the rates of the SCR reaction, the NSCR reaction and the 12


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C-O reaction all increased with the increase of gaseous NH3 concentration. As both gaseous NH3 and NO can adsorb on V2O5-WO3/TiO2., there would be a competition between NH3 adsorption and NOx adsorption on V2O5-WO3/TiO2. Therefore, the concentration of NH3 adsorbed on V2O5-WO3/TiO2 increased with the increase of gaseous NH3 concentration, while the concentration of NO2- on V2O5-WO3/TiO2 decreased. Hinted by Equation 20, the concentration of NH2 on V2O5-WO3/TiO2 would increase with the increase of gaseous NH3 concentration. Equations 25 and 26 indicate that the rates of the NSCR reaction and the C-O reaction were both proportional to the concentration of NH2 on V2O5-WO3/TiO2. Therefore, both the NSCR reaction and the C-O reaction would be obviously promoted with the increase of gaseous NH3 concentration, which were demonstrated in Figures 8d and 8e. The SCR reaction over V2O5-WO3/TiO2 through the Eley-Rideal mechanism would be obviously promoted with the increase of gaseous NH3 concentration (hinted by Equation 18), while the SCR reaction over V2O5-WO3/TiO2 through the Langmuir-Hinshelwood mechanism would be restrained due to the decrease of adsorbed NO2- (hinted by Equation S13). As a result, the promotion of the SCR reaction over V2O5-WO3/TiO2 due to the increase of gaseous NH3 concentration was much less than those of the NSCR reaction and the C-O reaction (shown in Figure 8). The adsorption of NH3 and NO2- on V2O5-WO3/TiO2 were both related to the reaction temperature. The adsorption of NH3 on V2O5-WO3/TiO2 was obviously restrained with the increase of reaction temperature. Although the physical adsorption of NO was obviously restrained with the increase of reaction temperature, the oxidation of physically adsorbed NO to NO2- (i.e. Reaction 16) was promoted. Therefore, the adsorption of NO2- on V2O5-WO3/TiO2 was generally promoted with the increase of reaction temperature. The adsorption of NO2- at lower temperatures was weaker, while the adsorption of NH3 was stronger. Therefore, NH3 predominated over the adsorption at lower temperatures and the concentration of NH3 adsorbed on the surface did not vary notably after the increase of gaseous NH3 concentration. The adsorption of NH3 on V2O5-WO3/TiO2 was restrained with the increase of reaction temperature, while the adsorption of NO2- was enhanced. It suggests that the competition adsorption between NO2- and NH3 became more remarkable with the increase of reaction temperature. Therefore, the promotion 13



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of the SCR reaction, the NSCR reaction and the C-O reaction due to the increase of gaseous NH3 concentration at higher temperatures was much more remarkable than that at lower temperatures (shown in Figure 8). Furthermore, the concentration of NH3/NH2 on V2O5-WO3/TiO2 would decrease slightly with the increase of gaseous NO concentration due to the competition adsorption. Hinted by Equation 31, the NSCR reaction rate would slightly decrease with the increase of gaseous NO concentration. The deduction was consistent with the results in Figures 3d, S1d and S2d. As a result, the model verification demonstrates that the experiment results were nearly consistent with the kinetic analysis and the slight deviation was mainly related to the competition adsorption between NH3 and NO2- on V2O5-WO3/TiO2. 4.4 Relationship between the SCR performance and the key factors According to Equations 28 and 31, the reaction kinetic constants of the SCR reaction through the Langmuir-Hinshelwood mechanism and the Eley-Rideal mechanism (i.e. kSCR-LH and kSCR-ER), and the NSCR reaction (i.e. kNSCR) can be approximately calculated from Figures 7, S1 and S2, which were listed in Table 1. Figures 9a and 9b shows the dependences of NOx conversion rate and the NSCR reaction rate on gaseous NO concentration during NO reduction over 5% V2O5-WO3/TiO2 in the presence of 5% of H2O. However, gaseous NH3 concentration in the outlet during the steady state kinetic study in the presence of 5% of H2O cannot be accurately determined, so NH3 conversion rate in the presence of 5% of H2O cannot be obtained. Figure 2d shows that the contribution of the C-O reaction to NH3 conversion over 5% V2O5-WO3/TiO2 in the presence of 5% of H2O was nearly zero below 400 oC. Therefore, the C-O reaction rate over 5% V2O5-WO3/TiO2 was approximately neglected in the presence of 5% of H2O, and the SCR reaction rate was calculated according to the difference of NOx conversion rate and the NSCR reaction rate (shown in Figure 9c). Then, kSCR-LH, kSCR-ER and kNSCR were calculated from Figure 9 after the linear regression. Figure 4 shows that both the NSCR reaction and the C-O reaction hardly contributed to NH3 conversion over 5% V2O5-WO3/TiO2 below 300 oC and the SCR reaction predominated over NH3 conversion. The SCR reaction over V2O5-WO3/TiO2 can be achieved through two reaction routes: 14


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one is the Langmuir-Hinshelwood mechanism, and the other is the Eley-Rideal mechanism. Equation 28 suggests that the SCR reaction rate through the Langmuir-Hinshelwood mechanism was independent of gaseous NO concentration, while that through the Eley-Rideal mechanism was directly proportional to gaseous NO concentration. Therefore, the ratio of NO conversion would decrease with the increase of gaseous NH3 and NO concentrations if the Langmuir-Hinshelwood mechanism contributed to NO reduction. 20, 21 Table 1 shows that kSCR-LH was much higher than 0. It suggests that the Langmuir-Hinshelwood mechanism contributed to NO reduction over V2O5-WO3/TiO2. Therefore, the ratio of NO conversion would decrease below 300 oC with the increase of gaseous NO concentration, which was demonstrated in Figure 3a. The similar phenomena were also observed in the SCR reaction over Mn-Fe spinel and Fe-Ti spinel 20, 21. Most NH3 was consumed over 5% V2O5-WO3/TiO2 during NO reduction above 350 oC, so the SCR activity mainly depended on the contribution of the C-O reaction. Hinted by Equation 26, the C-O reaction was obviously restrained with the increase of gaseous NO concentration (shown in Figure 7e). Therefore, the contribution of the C-O reaction to NH3 conversion obviously decreased above 350 oC with the increase of gaseous NO and NH3 concentrations from 250 to 1000 ppm, resulting in an obvious increase of NOx conversion. Equation 32 suggests that N2O selectivity of NO reduction over V2O5-WO3/TiO2 would decrease with the increase of gaseous NO concentration. Figure 3c demonstrates that N2O selectivity of NO reduction over 5% V2O5-WO3/TiO2 obviously decreased with the increase of gaseous NO and NH3 concentrations from 250 to 1000 ppm. The occurrence of the SCR reaction through the Langmuir-Hinshelwood mechanism would cause to the decrease of the ratio of NOx conversion with the increase of gaseous NO concentration, while the occurrence of the C-O reaction would cause to the increase of the ratio of NOx conversion. As a result, the ratio of NOx conversion over 5% V2O5-WO3/TiO2 at 250 oC did not vary notably with the increase of gaseous NH3 and NO concentrations from 250 to 1000 ppm. Figure 2 suggests that the SCR reaction and the NSCR reaction predominated over NH3 conversion over V2O5-WO3/TiO2 below 350 oC and the C-O reaction hardly contributed to NH3 conversion. V5+ concentration on V2O5-WO3/TiO2 generally increased with the increase of the amount of V2O5 loaded. The capacities of V2O5-WO3/TiO2 for NH3 adsorption, which resulted 15



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from NH3-TPD analysis (shown in Figure S3 in the Supporting Information), were listed in Table 2. Table 2 suggests that the adsorption of NH3 on V2O5-WO3/TiO2 did not vary obviously with the increase of V2O5 content. As a result, NH2 concentration on V2O5-WO3/TiO2 would obviously increase with the increase of V2O5 content (hinted by Equation 20). According to Equations 18, S14 and 29, kSCR-ER, kSCR-LH and kNSCR would all increase with the increase of V2O5 content in V2O5-WO3/TiO2. Table 1 demonstrates that kSCR-ER, kSCR-LH and kNSCR all increased with the increase of V2O5 content. As a result, the SCR activity of V2O5-WO3/TiO2 below 350 oC increased with the increase of V2O5 content (shown in Figure 1a). Figure 1b also shows that most NH3 was consumed above 350 oC. Therefore, the SCR activity of V2O5-WO3/TiO2 above 350 oC was mainly related to the occurrence of the C-O reaction. Hinted by Equation 26, δC-O would obviously increase with the increase of V2O5 content. This deduction was demonstrated by Figures 7e, S1e and S2e. It suggests that the contribution of the C-O reaction to NH3 conversion would obviously increase with the increase of V2O5 content, which was demonstrated in Figure 2. Therefore, NOx conversion over V2O5-WO3/TiO2 above 350 oC obviously decreased with the increase of V2O5 content (shown in Figure 1a). Equation 32 hinted that N2O selectivity would increase with the increase of V5+ on V2O5-WO3/TiO2. This deduction was consistent with Figure 1d. Figures 2c and 2d show that the SCR reaction and the NSCR reaction predominated over NH3 conversion over 5% V2O5-WO3/TiO2 below 350 oC and the C-O reaction hardly contributed to NH3 conversion. Our previous studies demonstrated that the oxidation ability of the SCR catalyst would obviously decrease after the introduction of H2O.

25, 26

k3, k4 and k5 were all related to the

oxidation ability of the SCR catalyst, so they would decrease notably after the introduction of H2O. Meanwhile, there is generally agreement that the adsorption of NH3 on the SCR catalyst is restrained in the presence of H2O.

25, 26, 39, 40

As the concentration of NH3 adsorbed and k3 both

decreased, NH2 concentration would obviously decrease after the introduction of H2O (hinted by Equation 20). Hinted by Equations 30 and 31, kSCR-ER and kNSCR would both decrease after the introduction of H2O, which were demonstrated in Table 1. As a result, the SCR activity of 5% V2O5-WO3/TiO2 below 350 oC obviously decreased after the introduction of H2O (shown in Figure 1a). Most NH3 over 5% V2O5-WO3/TiO2 was consumed above 350 oC, so the SCR activity 16


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depended on the contribution of the C-O reaction. As both k4 and [NH2] decreased, δC-O would obviously decrease after the introduction of H2O (hinted by Equation 26). It suggests that the contribution of the C-O reaction to NH3 conversion would decrease after the introduction of H2O, which was demonstrated in Figures 2b and 2c. As a result, the SCR activity of 5% V2O5-WO3/TiO2 above 350 oC obviously increased after the introduction of H2O (shown in Figure 1a). As both k5 and [NH2] decreased after the introduction of H2O, N2O selectivity of the SCR reaction over 5% V2O5-WO3/TiO2 would obviously decrease (Hinted by Equation 32). This deduction was demonstrated by Figure 1c. Figure 6 shows that the SCR reaction and the NSCR predominated over NH3 conversion over 5% V2O5-WO3/TiO2 with different GHSV below 350 oC and the C-O reaction hardly contributed to NH3 conversion. There was a positive relationship of the reaction rates of the SCR reaction (including through both the Langmuir-Hinshelwood mechanism and the Eley-Rideal mechanism) and the NSCR reaction with the catalyst mass. Therefore, they all increased with the increase of catalyst mass (i.e. the decrease of the GHSV). As a result, NOx conversion over 5% V2O5-WO3/TiO2 below 350 oC obviously increased with the decrease of GHSV (shown in Figure 5a). Figure 5b shows that most NH3 was converted over 5% V2O5-WO3/TiO2 above 350 oC even with the GHSV of 4.8×105 cm3 g-1 h-1. It suggests that the reaction was almost over above 350 oC with the GHSV of 4.8×105 cm3 g-1 h-1 and the catalyst added due to the decrease of the GHSV from 4.8×105 to 3.0×104 cm3 g-1 h-1 was superfluous. As a result, the SCR performance of 5% V2O5-WO3/TiO2 above 350 oC did not vary notably with the increase of the GHSV from 3.0×104 to 4.8×105 cm3 g-1 h-1 (shown in Figure 5).

5. Conclusions NO conversion over V2O5-WO3/TiO2 involved the SCR reaction, the NSCR reaction and the C-O reaction, so the kinetics cannot be accurately modeled by traditional kinetic equation. There was an excellent linear relationship between the SCR reaction rate and gaseous NO concentration, and the intercept and slope were related to the rate constant of the SCR reaction through the Langmuir-Hinshelwood mechanism and that through the Eley-Rideal mechanism respectively. However, the NSCR reaction rate was nearly independent of gaseous NO concentration, and the 17



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reaction order of the C-O reaction with respect to gaseous NO concentration was less than zero. Because of the competition adsorption between NH3 and NOx, the SCR reaction, the NSCR reaction and the C-O reaction all promoted with the increase of gaseous NH3 concentration. Furthermore, the relationship between the SCR performance and the key factors (i.e. the GHSV, reactant concentration, V2O5 content and H2O effect) can be well interpreted according to the kinetic analysis.

18


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Acknowledgements: This study was financially supported by the National Natural Science Fund of China (Grant No. 21207067 and 41372044), the Natural Science Fund of Jiangsu Province (Grant No. BK20150036), and the Zijin Intelligent Program, Nanjing University of Science and Technology (Grant No. 2013-0106).

Supporting Information The calculation method of the variables, the deduction of the kinetics, the steady state kinetic study of NO reduction over 1% V2O5-WO3/TiO2 and 3% V2O5-WO3/TiO2, and NH3-TPD profiles.

19



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Table 1 The rate constants of the SCR reaction through the Eley-Rideal mechanism (kSCR-ER) and /µmol g-1 min-1

the Langmuir-Hinshelwood mechanism (kSCR-LH), and the NSCR reaction (kNSCR)

δSCR = kSCR-ER [NO(g) ] + kSCR-LH

o

Temperature/ C

kNSCR kSCR-LH

k SCR-ER/106

R2

300

13.1

0.086

0.989

1.1

350

28.9

0.205

0.985

6.0

400

39.6

0.270

0.985

15.3

450

59.1

0.330

0.989

38.8

500

83.8

0.288

0.987

63.8

300

33.3

0.282

0.991

8.0

350

41.4

0.436

0.992

27.0

400

63.2

0.538

0.993

73.2

450

88.7

0.516

0.995

167

500

124

0.393

0.996

236

300

40.4

0.387

0.988

14.3

350

50.9

0.615

0.992

57.1

400

61.0

0.582

0.996

131

450

102

0.548

0.998

271

500

122

0.394

0.999

365

300

31.3

0.219

0.978

0.5

5% V2O5-WO3/TiO2 in

350

65.6

0.373

0.989

1.1

the presence of 5% H2O

400

92.0

0.479

0.980

2.1

450

89.3

0.499

0.982

7.9

1% V2O5-WO3/TiO2

3% V2O5-WO3/TiO2

5% V2O5-WO3/TiO2

24


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Table 2 The capacity of V2O5-WO3/TiO2 for NH3 adsorption Capacity for NH3 adsorption 1%V2O5-WO3/TiO2

188

3%V2O5-WO3/TiO2

181

5% V2O5-WO3/TiO2

164

25


/µmol g-1


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Page 26 of 37

Figure captions Figure 1 SCR performance of V2O5-WO3/TiO2: (a), NOx conversion; (b), NH3 conversion; (c), N2O selectivity. Reaction conditions: [NH3]=[NO]=500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate=200 mL min-1 and GHSV=1.2×105 cm3 g-1 h-1. Figure 2 The contributions of the SCR reaction, the NSCR reaction and the C-O reaction to NH3 conversion during NO reduction over: (a), 1% V2O5-WO3/TiO2; (b), 3% V2O5-WO3/TiO2; (c), 5% V2O5-WO3/TiO2; (d), 5% V2O5-WO3/TiO2 in the presence of H2O. Figure 3 Relationship between the reactant concentration and the SCR performance of 5% V2O5-WO3/TiO2: (a), NOx conversion; (b), NH3 conversion; (c), N2O selectivity. Reaction conditions: [O2]=2%, catalyst mass=100 mg, total flow rate=200 mL min-1 and GHSV=1.2×105 cm3 g-1 h-1. Figure 4 The contributions of the SCR reaction, the NSCR reaction and the C-O reaction to NH3 conversion during NO reduction over 5% V2O5-WO3/TiO2 with: (a), [NH3]=[NO]=250 ppm; (b), [NH3]=[NO]=500 ppm; (c), [NH3]=[NO]=1000 ppm. Figure 5 Effect of the GHSV on the SCR performance of 5% V2O5-WO3/TiO2: (a), NOx conversion; (b), NH3 conversion; (c), N2O selectivity. Reaction conditions: [NH3]=[NO]=500 ppm, [O2]=2%, catalyst mass=50-200 mg, total flow rate=100-400 mL min-1. Figure 6 The contributions of the SCR reaction, the NSCR reaction and the C-O reaction to NH3 conversion during NO reduction over 5% V2O5-WO3/TiO2 with the GHSV of: (a), 3.0×104 cm3 g-1 h-1; (b), 6.0×104 cm3 g-1 h-1; (c), 1.2×105 cm3 g-1 h-1; (d), 4.8×105 cm3 g-1 h-1. Figure 7 Dependences of δNH3(a), δNOx (b), δSCR (c), δNSCR (d) δC-O (e) on gaseous NO concentration during NO reduction over 5% V2O5-WO3/TiO2. Reaction conditions: [NH3]=500 ppm, [NO]=100-700 ppm, [O2]=2%, catalyst mass=2-5 mg, total flow rate=400 mL min-1 and GHSV=4.8×106-1.2×107 cm3 g-1 h-1. Figure 8 Dependences of δNH3 (a), δNOx (b), δSCR (c), δNSCR (d) δC-O (e) on gaseous NH3 concentration

during

NO

reduction

over

5%

V2O5-WO3/TiO2.

Reaction

conditions:

[NH3]=300-700 ppm, [NO]=500 ppm, [O2]=2%, catalyst mass=2-5 mg, total flow rate=400 mL min-1 and GHSV=4.8×106-1.2×107 cm3 g-1 h-1. 26


Page 27 of 37

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


Figure 9 Dependences of δNOx (a), δNSCR (b) and δSCR (c) on gaseous NO concentration during NO reduction over 5% V2O5-WO3/TiO2 in the presence of 5% H2O. Reaction conditions: [NH3]=500 ppm, [NO]=300-700 ppm, [O2]=2%, catalyst mass=2-5 mg, total flow rate=400 mL min-1 and GHSV=4.8×106-1.2×107 cm3 g-1 h-1.

27



80

80

NH3 conversion/%

100

NOx conversion/%

100

60 1% V2O5-WO3/TiO2

40

3% V2O5-WO3/TiO2 5% V2O5-WO3/TiO2

20 0

5% V2O5-WO3/TiO2 with 5% of H2O

60 1% V2O5-WO3/TiO2

40

3% V2O5-WO3/TiO2 5% V2O5-WO3/TiO2

20

5% V2O5-WO3/TiO2 with 5% of H2O

0

150 200 250 300 350 400 450 500

150 200 250 300 350 400 450 500

o

o

Temperature/ C

Temperature/ C

a

b

50

N2O selecitvity/%

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 28 of 37

40 30 20

1% V2O5-WO3/TiO2 3% V2O5-WO3/TiO2 5% V2O5-WO3/TiO2 5% V2O5-WO3/TiO2 with 5% of H2O

10 0 150 200 250 300 350 400 450 500 o

Temperature/ C

c

Figure 1 SCR performance of V2O5-WO3/TiO2: (a), NOx conversion; (b), NH3 conversion; (c), N2O selectivity. Reaction conditions: [NH3]=[NO]=500 ppm, [O2]=2%, catalyst mass=100 mg, total flow rate=200 mL min-1 and GHSV=1.2×105 cm3 g-1 h-1.

28


Page 29 of 37

NSCR

C-O

100

SCR

C-O

80

80

60

60

40

40

20

20

0

0

150 200 250 300 350 o 400 450 500

150 200 250 300 350 400 450 500 o

temperature/ C

temperature/ C

b

a SCR

NSCR

C-O

100

SCR

NSCR

C-O

NH3 conversion/%

100

NSCR

NH3 conversion/%

SCR

NH3 conversion/%

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


80

80

60

60

40

40

20

20

0

0

150 200 250 300 350 400 450 500

150 200 250 300 350 400 450 500

o

temperature/ C

o

temperature/ C

d

c

Figure 2 The contributions of the SCR reaction, the NSCR reaction and the C-O reaction to NH3 conversion during NO reduction over: (a), 1% V2O5-WO3/TiO2; (b), 3% V2O5-WO3/TiO2; (c), 5% V2O5-WO3/TiO2; (d), 5% V2O5-WO3/TiO2 in the presence of H2O.

29



100

NH3 conversion/%

NOx conversion/%

100 80 60 40

[NH3]=[NO]=250 ppm [NH3]=[NO]=500 ppm

20

80 60 40

[NH3]=[NO]=250 ppm [NH3]=[NO]=500 ppm

20

[NH3]=[NO]=1000 ppm

[NH3]=[NO]=1000 ppm 0

0 150

200

250

300

350

400

450

150

500

200

250

300

350

400

450

500

o

o

Temperature/ C

Temperature/ C

b

a 50

N2O selectivity/%

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 30 of 37

40

[NH3]=[NO]=250 ppm

30

[NH3]=[NO]=1000 ppm

[NH3]=[NO]=500 ppm

20 10 0 150

200

250

300

350

400

450

500

o

Temperature/ C

c

Figure 3 Relationship between the reactant concentration and the SCR performance of 5%

V2O5-WO3/TiO2: (a), NOx conversion; (b), NH3 conversion; (c), N2O selectivity. Reaction conditions: [O2]=2%, catalyst mass=100 mg, total flow rate=200 mL min-1 and GHSV=1.2×105 cm3 g-1 h-1.

30


Page 31 of 37

SCR

NSCR

C-O

SCR

100

NSCR

C-O

NH3 conversion/%

100

NH3 conversion/%

80

80

60

60

40

40

20

20

0

0 150 200 250 300 350 400 450 500

150 200 250 300 350 400 450 500

temperature/ C

temperature/ C

o

o

a

b 100

SCR

NSCR

C=O

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


80 60 40 20 0 150 200 250 300 350 400 450 500 o

temperature/ C c Figure 4 The contributions of the SCR reaction, the NSCR reaction and the C-O reaction to NH3

conversion during NO reduction over 5% V2O5-WO3/TiO2 with: (a), [NH3]=[NO]=250 ppm; (b), [NH3]=[NO]=500 ppm; (c), [NH3]=[NO]=1000 ppm.

31



100

NH3 conversion/%

NOx conversion/%

100 80 60 4

3

-1

-1

3.0×10 cm g h 4 3 -1 -1 6.0×10 cm g h 5 3 -1 -1 1.2×10 cm g h 5 3 -1 -1 4.8×10 cm g h

40 20 0

80 60 4

3

-1

-1


40 20 0

150

200

250

300

350

400

450

500

150

o

200

250

4

3

350

400

450

500

Temperature/ C

a 80

300

o

Temperature/ C

b -1

-1


N2O selectivity/%

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 32 of 37

60 40 20 0 150

200

250

300

350

400

450

500

o

Temperature/ C c

Figure 5 Effect of the GHSV on the SCR performance of 5% V2O5-WO3/TiO2: (a), NOx conversion; (b), NH3 conversion; (c), N2O selectivity. Reaction conditions: [NH3]=[NO]=500 ppm, [O2]=2%, catalyst mass=50-200 mg, total flow rate=100-400 mL min-1.

32


Page 33 of 37

SCR

NSCR

C-O

100

SCR

80

C-O

80

60

60

40

40

20

20

0

0

150 200 250 300 350 400 450 500

150 200 250 300 350 400 450 500

o

temperature/ C

o

temperature/ C

a SCR

b

NSCR

C-O

100

SCR

NSCR

C-O

NH3 conversion/%

100

NSCR

NH3 conversion/%

NH3 conversion/%

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


80

80

60

60

40

40

20

20

0

0

150 200 250 300 350 400 450 500

150 200 250 300 350 400 450 500

o

o

temperature/ C

temperature/ C

d

c

Figure 6 The contributions of the SCR reaction, the NSCR reaction and the C-O reaction to NH3

conversion during NO reduction over 5% V2O5-WO3/TiO2 with the GHSV of: (a), 3.0×104 cm3 g-1 h-1; (b), 6.0×104 cm3 g-1 h-1; (c), 1.2×105 cm3 g-1 h-1; (d), 4.8×105 cm3 g-1 h-1.

33



o

o

800

o

350 C o 500 C

400 C

o

300 C o 450 C

-1

300 C o 450 C

800 600 400 200

100

200

300

400

500

600

500

200

100

700

200

300

o

400

o

350 C o 500 C

400 C

o

300 C o 450 C

o

400 C

400

500

350 C o 500 C

o

-1

400

700

b o

300 C o 450 C

600

NO concentration/ppm

δNSCR/µmol g min

300

300

-1

-1

500

-1

400

400

a

δSCR/µmol g min

o

600


200 100 0

200 100 0

100

200

300

400

500

600

700

100


200

500

o

600

700

d o

300 C o 450 C

300


c

-1

400 C

0

0

-1

o

350 C o 500 C

-1

δNOx/µmol g min

-1

δNH3/µmol g min

-1

1000

δC-O/µmol g min

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 34 of 37

350 C o 500 C

o

400 C

400 300 200 100 0 100

200

300

400

500

600

700


e Figure 7 Dependences of δNH3(a), δNOx (b), δSCR (c), δNSCR (d) δC-O (e) on gaseous NO

concentration during NO reduction over 5% V2O5-WO3/TiO2. Reaction conditions: [NH3]=500 ppm, [NO]=100-700 ppm, [O2]=2%, catalyst mass=2-5 mg, total flow rate=400 mL min-1 and GHSV=4.8×106-1.2×107 cm3 g-1 h-1.

34


Page 35 of 37

1500

o

o

800

o

-1

δNOx/µmol g min

δNH3/µmol g min

900

-1

400 C

-1

350 C o 500 C

-1

300 C o 450 C

1200

600 300

o

o

300 C o 450 C

350 C o 500 C

300

600 400 200

400

500

600

700

300

NH3 concentration/ppm

400

500

o

o

400 C

δNSCR/µmol g min

500

600

o

350 C o 500 C

-1

300 C o 450 C

700

b o

300 C o 450 C

500

o

350 C o 500 C

o

400 C

400

-1

400

-1

δSCR/µmol g min

-1

600

600


a

300 200 100 0

300 200 100 0

300

400

500

600

700

300


400

400

o

500

600

700


d

c

-1

o

400 C

0

0

o

o

300 C o 450 C

350 C o 500 C

400 C

400

500

600

300

-1

δC-O/µmol g min

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


200 100 0 300

700


e Figure 8 Dependences of δNH3 (a), δNOx (b), δSCR (c), δNSCR (d) δC-O (e) on gaseous NH3

concentration

during

NO

reduction

over

5%

V2O5-WO3/TiO2.

Reaction

conditions:

[NH3]=300-700 ppm, [NO]=500 ppm, [O2]=2%, catalyst mass=2-5 mg, total flow rate=400 mL min-1 and GHSV=4.8×106-1.2×107 cm3 g-1 h-1.

35



400

o

350 C o 500 C

80

o

o

400 C -1

o

300 C o 450 C

δNSCR/µmol g min

300

300 C o 450 C

200 100

300

400

500

600

700

60 40 20

300


400

-1 -1

500

o

300 C o 450 C

500

600

700


a

300

o

400 C

0

0

400

o

350 C o 500 C

-1

-1

δNOx/µmol g min

-1

500

δSCR/µmol g min

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 36 of 37

b o

350 C o 500 C

o

400 C

200 100 0 300

400

500

600

700


c Figure 9 Dependences of δNOx (a), δNSCR (b) and δSCR (c) on gaseous NO concentration during NO

reduction over 5% V2O5-WO3/TiO2 in the presence of 5% H2O. Reaction conditions: [NH3]=500 ppm, [NO]=300-700 ppm, [O2]=2%, catalyst mass=2-5 mg, total flow rate=400 mL min-1 and GHSV=4.8×106-1.2×107 cm3 g-1 h-1. .

36


Page 37 of 37

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For Table of Contents Only

37


Global Kinetic Study of NO Reduction by NH - ACS Publications

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