Mechanism of N2O Formation during the Low-Temperature Selective

Article pubs.acs.org/est

Mechanism of N2O Formation during the Low-Temperature Selective Catalytic Reduction of NO with NH3 over Mn−Fe Spinel Shijian Yang,*,† Shangchao Xiong,† Yong Liao,† Xin Xiao,† Feihong Qi,† Yue Peng,‡ Yuwu Fu,† Wenpo Shan,† and Junhua Li*,‡ †

School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China ‡ State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC), School of Environment, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: The mechanism of N2O formation during the low-temperature selective catalytic reduction reaction (SCR) over Mn−Fe spinel was studied. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and transient reaction studies demonstrated that the Eley−Rideal mechanism (i.e., the reaction of adsorbed NH3 species with gaseous NO) and the Langmuir−Hinshelwood mechanism (i.e., the reaction of adsorbed NH3 species with adsorbed NOx species) both contributed to N2O formation. However, N2O selectivity of NO reduction over Mn−Fe spinel through the Langmuir−Hinshelwood mechanism was much less than that through the Eley−Rideal mechanism. The ratio of NO reduction over Mn−Fe spinel through the Langmuir−Hinshelwood mechanism remarkably increased; therefore, N2O selectivity of NO reduction over Mn−Fe spinel decreased with the decrease of the gas hourly space velocity (GHSV). As the gaseous NH3 concentration increased, N2O selectivity of NO reduction over Mn−Fe spinel increased because of the promotion of NO reduction through the Eley−Rideal mechanism. Meanwhile, N2O selectivity of NO reduction over Mn−Fe spinel decreased with the increase of the gaseous NO concentration because the formation of NH on Mn−Fe spinel was restrained. Therefore, N2O selectivity of NO reduction over Mn−Fe spinel was related to the GHSV and concentrations of reactants. SO2 can be regenerated after water washing.14 N2O is now considered as a pollutant because of its contribution to global warming and stratospheric ozone depletion.16−18 Therefore, it is a major challenge for the application of Mn−Fe spinel to restrain N2O formation during the low-temperature SCR reaction. Generally, N2O formation during the low-temperature SCR reaction over a Mn-based catalyst results from the non-selective catalytic reduction (NSCR) reaction.19

1. INTRODUCTION Thus far, the most efficient technology for the control of nitrogen oxide (NO and NO2) emission from coal-fired power plants is selective catalytic reduction (SCR) of NOx with NH3.1 The standard SCR reaction is as follows:2 4NH3 + 4NO + O2 → 4N2 + 6H 2O

(1)

The commercial SCR catalyst is V2O5−WO3(MoO3)/TiO2, and its temperature window is about 300−400 °C.3 Hence, the SCR unit is located upstream of the desulfurizer and the electrostatic precipitator. However, the space and access between the air preheater and the electrostatic precipitator are limited in many existing coal-fired power plants;4 therefore, retrofitting the SCR unit in these coal-fired power plants is very difficult. Hence, there has been a strong demand in developing a low-temperature SCR catalyst, which is placed downstream of the electrostatic precipitator and desulfurizer.5 Mn-based catalysts, for example, MnOx−CeO2,6,7 MnO2/TiO2,8−12 MnOx−CeO2/TiO2,13 Fe2O3−MnO2/TiO2,5 and Mn−Fe spinel,14 show an excellent low-temperature SCR activity. However, Mn-based catalysts are currently extremely restricted in the application for at least two reasons: the formation of N2O and the deactivation of SO2.14,15 Our previous research has demonstrated that magnetic Mn−Fe spinel deactivated by © 2014 American Chemical Society

4NH3 + 4NO + 3O2 → 4N2O + 6H 2O

(2)

Thus far, only a few studies focused on the mechanism of N2O formation during the low-temperature SCR reaction over a Mnbased catalyst.19−22 An isotopic labeling experiment demonstrated that one of the two N atoms in N2O originated from NH3 and the other originated from NO under the SCR conditions.22 N2O formation during the low-temperature SCR reaction over Mn−Fe spinel mainly follows two mechanisms,14,21 which is similar to N2 formation. One is the Received: Revised: Accepted: Published: 10354

May 28, 2014 July 20, 2014 August 8, 2014 August 8, 2014 dx.doi.org/10.1021/es502585s | Environ. Sci. Technol. 2014, 48, 10354−10362

Environmental Science & Technology

Article

NO reduction over Mn−Fe spinel decreased (as shown in Figure 1). This phenomenon was seldom reported in previous studies.

Eley−Rideal mechanism (i.e., the reaction of gaseous NO with overactivated NH3), and the other is the Langmuir−Hinshelwood mechanism (i.e., the reaction of adsorbed NO3− with adsorbed NH3 species on the adjacent sites).14,21 In this work, the mechanism of N2O formation during NO reduction over Mn−Fe spinel was studied using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra and the transient reaction, and the novel relationship of N2O selectivity with the concentrations of reactants and the gas hourly space velocity (GHSV) was discovered.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Mn−Fe spinel was prepared using a co-precipitation method.14,23,24 Ferrous sulfate, ferric chloride, and manganese sulfate were dissolved in distilled water with Fe2+/Fe3+/Mn2+ = 1:4:1. Then, the mixture was added to an ammonia solution with 800 rpm of stirring, leading to an instantaneous precipitation. The particles were then separated by centrifugation and washed with distilled water 3 times. After drying at 105 °C for 12 h, the particles were calcined under air at 400 °C for 3 h. 2.2. Catalytic Test. The reaction was performed on a fixedbed quartz tube reactor.14,25,26 The catalyst with 40−60 mesh was placed on the quartz wool held in the reactor, which was heated by a vertical electrical furnace. The catalyst mass ranged from 20 to 200 mg, and the total flow rate varied from 100 to 400 mL min−1. The corresponding GHSV ranged from 60 000 to 1 200 000 cm3 g−1 h−1. The typical reactant gas composition was as follows: 500 ppm NH3, 500 ppm NO, 2% O2, 5% H2O (when used), and balance of N2. The gas composition in the outlet (including NH3, NO, NO2, and N2O) was continually monitored by a Fourier transform infrared spectrometer (FTIR, Thermo Scientific Antaris IGS analyzer). In situ DRIFT spectra were recorded on another FTIR spectrometer (Nicolet 6700) equipped with a smart collector and a mercury cadmium telluride (MCT) detector by accumulating 32 scans with a resolution of 4 cm−1.27 N2O selectivity and the amount of N2 formed were calculated using the following equations:21

Figure 1. Effect of concentrations of reactants on N2O selectivity of NO reduction over Mn−Fe spinel (a) without H2O and (b) with 5% of H2O. Reaction conditions: [O2], 2%; catalyst mass, 100 mg; total flow rate, 200 mL min−1; GHSV, 120 000 cm3 g−1 h−1.

H2O, which is one of the main components in the flue gas, has a strong inhibition on NO reduction and N2O formation during the low-temperature SCR reaction.4 As 5% of H2O was added, NO reduction over Mn−Fe spinel was obviously restrained (as shown in Figure S2 of the Supporting Information). Meanwhile, N2O selectivity of NO reduction over Mn−Fe spinel remarkably decreased (as shown in Figure 1b). Furthermore, Figure 1b also shows that N2O selectivity of NO reduction over Mn−Fe spinel in the presence of H2O generally decreased as gaseous NH3 and NO concentrations increased from 500 to 1000 ppm. They demonstrate that N2O selectivity of NO reduction over Mn−Fe spinel was related to gaseous NH3 and NO concentrations. 3.2. Effect of Concentrations of Reactants. To study the effect of the gaseous NO concentration on N2O selectivity of NO reduction over Mn−Fe spinel, the gaseous NH 3 concentration in the inlet was kept at 500 ppm, while the gaseous NO concentration varied from 200 to 500 ppm (as shown in Figure 2).15 To study the effect of the gaseous NH3 concentration on N2O selectivity of NO reduction over Mn−Fe spinel, the gaseous NO concentration in the inlet was kept at 500 ppm, while the gaseous NH3 concentration varied from 200 to 500 ppm (as shown in Figure 3). To overcome the diffusion limitation, a very high GHSV of 1 200 000 cm3 g−1 h−1 was adopted to obtain the lower NOx conversion ( 0.990) between the rate of NO reduction and the gaseous NO concentration (as shown in

N2O selectivity = 2[N2O]out /([NH3]in + [NOx ]in − [NH3]out − [NOx ]out ) × 100%

(3)

N2 formation = ([NH3]in + [NOx ]in − [NH3]out − [NOx ]out − 2[N2O]out )/2

(4)

where [NH3]in, [NOx]in, [NH3]out, [NOx]out, and [N2O]out were the concentrations of NH3 and NOx (including NO and NO2) in the inlet and those in the outlet.

3. RESULTS 3.1. SCR Performance. Mn−Fe spinel showed an excellent SCR performance at low temperatures (shown in Figure S1 of the Supporting Information). The pseudo-first-rate constant of NO reduction over Mn−Fe spinel was close to that over MnOx−CeO2,14,15 which was much higher than those of MnOx/Al2O3,28 MnOx−WO3/γ-Al2O3,29 Fe−Mn/TiO2,5 and Mn/TiO2.5,30 Furthermore, N2 selectivity of NO reduction over Mn−Fe spinel was much higher than that over MnOx−CeO2.15 As gaseous NO and NH3 concentrations increased from 500 to 1000 ppm, NOx conversion decreased (as shown in Figure S1 of the Supporting Information), which was consistent with the result of our previous study.14 Meanwhile, N2O selectivity of 10355

dx.doi.org/10.1021/es502585s | Environ. Sci. Technol. 2014, 48, 10354−10362


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Figure 2. Effect of the gaseous NO concentration on (a) NO reduction, (b) N2O formation, (c) N2 formation, and (d) N2O selectivity. Reaction conditions: [NH3], 500 ppm; [O2], 2%; catalyst mass, 20 mg; total flow rate, 400 mL min−1; GHSV, 1 200 000 cm3 g−1 h−1.

Figure 3. Effect of the gaseous NH3 concentration on (a) NO reduction, (b) N2O formation, (c) N2 formation, and (d) N2O selectivity. Reaction conditions: [NO], 500 ppm; [O2], 2%; catalyst mass, 20 mg; total flow rate, 400 mL min−1; GHSV, 1 200 000 cm3 g−1 h−1.

Figure 2a). This result was consistent with that of MnOx− CeO2. Qi and Yang reported that the reaction order of NO reduction over MnOx−CeO2 with respect to the gaseous NO concentration was nearly 1.15 However, the amount of N2O

formed did not vary notably with the increase of the gaseous NO concentration (as shown in Figure 2b). It suggests that the reaction order of N2O formation over Mn−Fe spinel with respect to the gaseous NO concentration was nearly zero and 10356



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the N 2 O formation over Mn−Fe spinel was almost independent of the gaseous NO concentration. The rate of N2 formation was approximately equal to the difference between the rate of NO reduction and that of N2O formation. Therefore, N2 formation over Mn−Fe spinel was remarkably promoted with the increase of the gaseous NO concentration (as shown in Figure 2c). Because of the remarkable promotion of N2 formation and no effect on N2O formation, N2O selectivity of NO reduction over Mn−Fe spinel observably decreased with the increase of the gaseous NO concentration (as shown in Figure 2d). Figure 3a shows that NO reduction over Mn−Fe spinel was promoted with the increase of the gaseous NH3 concentration and the promotion gradually slowed. It suggests that the reaction order of NO reduction over Mn−Fe spinel with respect to the gaseous NH3 concentration was between 0 and 1. This result was not consistent with that of MnOx−CeO2. Qi and Yang reported that the reaction order of NO reduction over MnO x −CeO 2 with respect to the gaseous NH 3 concentration was nearly 0.15 Meanwhile, N2O formation was promoted remarkably with the increase of the gaseous NH3 concentration, especially at higher temperatures (as shown in Figure 3b). Therefore, the variation of N2 formation with respect to the gaseous NH3 concentration was generally close to a downward parabola (as shown in Figure 3c). As a result, N2O selectivity of NO reduction over Mn−Fe spinel remarkably increased with the increase of the gaseous NH3 concentration (as shown in Figure 3d). 3.3. Effect of the GHSV. As shown in Figure S3a of the Supporting Information, NOx conversion over Mn−Fe spinel obviously decreased with the gradual increase of the GHSV from 60 000 to 1 200 000 cm3 g−1 h−1. A similar result was widely reported in the study of the SCR reaction.31 However, N2O selectivity of NO reduction over Mn−Fe spinel surprisingly increased with the increase of GHSV from 60 000 to 1 200 000 cm3 g−1 h−1 (as shown in Figure 4a). Furthermore, N2O selectivity of NO reduction over Mn−Fe spinel in the presence of 5% H2O still remarkably increased with the increase of GHSV from 60 000 to 120 000 cm3 g−1 h−1 (as shown in Figure 4b). They suggest that N2O selectivity of NO reduction over Mn−Fe spinel was connected with the GHSV. 3.4. Transient Reaction. After the adsorption of 500 ppm NH3 at 120 °C for 30 min, Mn−Fe spinel was mainly covered by coordinated NH3 bound to the Lewis acid sites (at 1603 and 1190 cm−1) and ionic NH4+ bound to the Brønsted acid sites (1669 and 1440 cm−1) (as shown in Figure 5a).7,9,14 The negative peak at 1340 cm−1 was assigned to residual SO42− on Mn−Fe spinel after water washing, which was covered by NH4+.25,32,33 After the further introduction of 500 ppm NO and 2% O2, adsorbed ammonia species gradually diminished (as shown in Figure 5a). Meanwhile, adsorbed H2O (at 1620 cm−1) appeared, which is the typical product of NO reduction.7 Furthermore, adsorbed NOx cannot be observed during the remarkable decrease of adsorbed NH3 species in the first 1 min. They suggest that the reaction of adsorbed ammonia species with gaseous NO (i.e., the Eley−Rideal mechanism) contributed to NO reduction over Mn−Fe spinel. At last, Mn−Fe spinel was mainly covered by monodentate nitrite (at 1608 cm−1), monodentate nitrate (at 1548 cm−1), and bidentate nitrate (at 1584, 1531, 1277, and 1230 cm−1).14,34 Gaseous N2O and NOx concentrations in the outlet during the introduction of NO + O2/N2 to NH3 pretreated Mn−Fe

Figure 4. Effect of the GHSV on N2O selectivity of NO reduction over Mn−Fe spinel (a) without H2O and (b) with 5% of H2O. Reaction conditions: [NO] and [NH3], 500 ppm; [O2], 2%.

spinel were recorded (as shown in Figure 6a). After the introduction of NO + O2 to NH3 pretreated Mn−Fe spinel, the N2O concentration in the outlet rapidly increased to about 53 ppm and then gradually decreased. It suggests that NO reduction over Mn−Fe spinel through the Eley−Rideal mechanism contributed to N2O formation. According to the integration of NOx reduced and N2O formed, N2O selectivity can be approximately calculated. N2O selectivity of the transient reaction (i.e., NO reduction through the Eley−Rideal mechanism at 120 °C), which was calculated from Figure 6a, was approximately 10%. After the adsorption of 500 ppm NO and 2% O2 at 120 °C, monodentate nitrite (at 1608 cm−1), monodentate nitrate (at 1548 cm−1), and bidentate nitrate (at 1584, 1531, 1277, and 1230 cm−1) appeared on Mn−Fe spinel (as shown in Figure 5b). After NH3 was introduced, the band at 1608 cm−1 corresponding to monodentate nitrite and the band at 1548 cm −1 corresponding to monodentate nitrate gradually decreased (as shown in Figure 5b). It suggests that the reaction of adsorbed monodentate nitrite/monodentate nitrate with adsorbed NH3 on the adjacent sites (i.e., the Langmuir− Hinshelwood mechanism) contributed to NO reduction over Mn−Fe spinel. However, the bands at 1584, 1531, 1277, and 1230 cm−1 corresponding to bidentate nitrate shifted to 1556, 1507, 1286, and 1246 cm−1 because of the reaction with adsorbed ammonia species (as shown in Figure 5b).14 However, their intensities did not decrease after the further introduction of NH3/N2. It suggests that the reaction between ammonia and bidentate nitrate did not contribute to NO reduction over Mn−Fe spinel because of the stronger binding energy of bidentate nitrate with the interface.34 A similar result was reported on MnOx−CeO2 by Qi and Yang.15 Meanwhile, coordinated NH3 (at 1603 and 1190 cm−1) and ionic NH4+ 10357



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Figure 6. (a) Transient reaction taken at 120 °C upon passing NO + O2 over NH3 presorbed Mn−Fe spinel, (b) transient reaction taken at 120 °C upon passing NH3 over NO + O2 presorbed Mn−Fe spinel, and (c) transient reaction taken at 120 °C upon passing NH3 over fresh Mn−Fe spinel.

the introduction of 500 ppm NH3 to fresh Mn−Fe spinel at 120 °C (as shown in Figure 6c). It suggests that NO reduction over Mn−Fe spinel through the Langmuir−Hinshelwood mechanism contributed to N2O formation. Previous research demonstrated that the product of nitrate route was N2O, while that of the nitrite route was N2.35 Therefore, N2O formation over Mn−Fe spinel during the introduction of NH3/N2 to NO + O2 pretreated Mn−Fe spinel mainly resulted from the reaction of adsorbed monodentate nitrate with adsorbed NH3. N2O selectivity of the transient reaction (i.e., NO reduction through the Langmuir−Hinshelwood mechanism at 120 °C), which was calculated according to the integration of NH3 reduced and N2O formed in Figure 6b, was approximately 3%. It suggests that N2O selectivity of NO reduction through the Langmuir−Hinshelwood mechanism was much less than that through the Eley−Rideal mechanism. At last, the IR spectra during the SCR reaction (i.e., 500 ppm NH3, 500 ppm NO, and 2% O2 were simultaneously

Figure 5. (a) DRIFT spectra taken at 120 °C upon passing NO + O2 over NH3 presorbed Mn−Fe spinel, (b) DRIFT spectra taken at 120 °C upon passing NH3 over NO + O2 presorbed Mn−Fe spinel, and (c) DRIFT spectra taken at 120 °C upon passing NH3 + NO + O2 over Mn−Fe spinel.

(1669 and 1440 cm−1) were observed on Mn−Fe spinel. N2O and NH3 concentrations in the outlet during the introduction of NH3/N2 to NO + O2 pretreated Mn−Fe spinel were simultaneously recorded (as shown in Figure 6b). The N2O concentration in the outlet rapidly increased to about 15 ppm after the introduction of 500 ppm NH3 to NO + O2 pretreated Mn−Fe spinel, which was much higher than that resulting from 10358



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monodentate NO3− reacted with adsorbed NH3 species on the adjacent sites to form NH4NO2 and NH4NO3 (i.e., reactions 10 and 11), which were then decomposed to N2 and N2O, respectively.19 However, NH4NO3 on Mn−Fe spinel, which resulted from the reaction between bidentate NO3− and adsorbed NH3 (i.e., reaction 12), cannot be decomposed because of the stronger binding energy of bidentate NO3− with the interface.34 Therefore, the formation of bidentate NO3− on Mn−Fe spinel did not contribute to NO reduction at the steady state. Furthermore, the formation of bidentate NO3− on Mn−Fe spinel may cause the deactivation of Mn−Fe spinel because of the loss of active sites for NO2− adsorption. Reaction 13 was the regeneration of Mn4+ on Mn−Fe spinel by gaseous O2. NO reduction over Mn−Fe spinel through the Eley−Rideal mechanism can be approximately described as14,19,21

introduced) at 120 °C were recorded. As shown in Figure 5c, adsorbed H2O (at 1620 cm−1), coordinated NH3 (at 1603 and 1190 cm−1), ionic NH4+ (at 1669 and 1440 cm−1) adsorbed bidentate NO3− (at 1584 cm−1), and adsorbed NH4NO3 (at 1507 and 1246 cm−1) were all observed. A previous study has demonstrated that bidentate nitrate on the surface mainly resulted from the further oxidation of adsorbed NO2−.36 Therefore, the presence of bidentate nitrate on the surface indicates the formation of NO2− on Mn−Fe spinel, which was rapidly reacted with NH3. Therefore, both the Langmuir− Hinshelwood mechanism and the Eley−Rideal mechanism contributed to NO reduction over Mn−Fe spinel.

4. DISCUSSION In situ DRIFT spectra and the transient reaction demonstrate that both the Langmuir−Hinshelwood mechanism (i.e., the reaction between adsorbed monodentate NO3− and adsorbed NH3) and the Eley−Rideal mechanism (i.e., the reaction between overactivated NH3 and gaseous NO) contributed to N2O formation over Mn−Fe spinel. However, N2O selectivity of NO reduction over Mn−Fe spinel through the Langmuir− Hinshelwood mechanism was much less than that through the Eley−Rideal mechanism. NO reduction over Mn−Fe spinel through the Langmuir− Hinshelwood mechanism can be approximately described as14,21 NH3(g) → NH3(ad) (5) NO(g) → NO(ad)

(6)

≡Mn 4 +O + NO(ad) → ≡Mn 3 +−O−NO

(7)

≡Mn 3 +−O−NO +

1 O2 → ≡Mn 3 +−O−NO2 2

(8)

≡Mn 3 +−O−NO +

1 O2 → ≡Mn 3 +(O)2 NO 2

(9)

NH3(g) → NH3(ad)

≡Mn −OH + N2 + H 2O

NH3(ad) + ≡Mn 4 +O → NH 2 + ≡Mn 3 +−OH

(14)

NH 2 + NO(g) → N2 + H 2O

(15)

NH 2 + ≡Mn 4 +O → NH + ≡Mn 3 +−OH

(16)

NH + NO(g) + Mn 4 +O → N2O + Mn 3 +−OH

(17)

≡Mn 3 +−OH +

1 1 O2 → ≡Mn 4 +O + H 2O 4 2

(13)

NO reduction through the Eley−Rideal mechanism starts with the activation of adsorbed ammonia species by Mn4+ on Mn− Fe spinel to form NH2 (i.e., reaction 14).19 Then, NH2 on Mn−Fe spinel reacted with gaseous NO to form N2 (i.e., reaction 15). Meanwhile, NH2 on Mn−Fe spinel can be further oxidized to NH (i.e., reaction 16), which can react with gaseous NO to form N2O (i.e., reaction 17). N2O formation through the Langmuir−Hinshelwood mechanism (i.e., reaction 11) can be described as

≡Mn 3 +−O−NO + NH3(ad) → ≡Mn 3 +−O−NO−NH3 → 3+

(5)

d[N2O] dt

(10)

= k1[Mn 3 +−O−NO2 −NH3] L−H

(18)

where k1 and [Mn −O−NO2−NH3] were the rate constant of Mn3+−O−NO2−NH3 decomposition and the concentration of Mn3+−O−NO2−NH3 on Mn−Fe spinel, respectively. N2 formation through the Langmuir−Hinshelwood mechanism (i.e., reaction 10) can be described as 3+

≡Mn 3 +−O−NO2 + NH3(ad) → ≡Mn 3 +−O−NO2 −NH3 → ≡Mn 3 +−OH + N2O + H 2O

(11)

≡Mn 3 +(O)2 NO + NH3(ad) → ≡Mn 3 +(O)2 NO−NH3

1 1 ≡Mn −OH + O2 → ≡Mn 4 +O + H 2O 4 2

d[N2] dt

(12)

3+

= k 2[Mn 3 +−O−NO−NH3] L−H

(19)

where k2 and [Mn3+−O−NO−NH3] were the rate constant of Mn3+−O−NO−NH3 decomposition and the concentration of Mn3+−O−NO−NH3 on Mn−Fe spinel, respectively. [Mn3+−O−NO−NH3] and [Mn3+−O−NO2−NH3] were both mainly related to the concentrations of NO adsorbed, NH3 adsorbed, and Mn4+ on Mn−Fe spinel (the deduction is shown in ref 14).14 Mn4+ on Mn−Fe spinel can be rapidly recovered through reaction 13; therefore, the Mn4+ concentration on Mn−Fe spinel was mainly related to the reaction temperature.14 Therefore, the Mn4+ concentration on Mn−Fe spinel can be approximately regarded as a constant at a specific temperature at the steady state. The GHSV used in this study (Figures 2 and 3) was quite high, and there were large amounts

(13)

There is general agreement that the SCR reaction starts with the adsorption of NH3, which is very strong compared to the adsorption of NO + O2 and the reaction products.37 Reaction 5 was the adsorption of gaseous ammonia on the Lewis acid sites and the Brønsted acid sites on Mn−Fe spinel to form coordinated NH3 and ionic NH4+. Reaction 6 was the physical adsorption of gaseous NO on Mn−Fe spinel,19 which was then oxidized by Mn4+ on Mn−Fe spinel to form adsorbed monodentate NO2− (i.e., reaction 7). Adsorbed NO2− can be further oxidized to monodentate NO3− and bidentate NO3− (i.e., reactions 8 and 9).36 Adsorbed monodentate NO2− and 10359



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Meanwhile, N2O formation through the Langmuir−Hinshelwood mechanism was independent of the gaseous NO concentration. Taking into account both the contribution of the Eley−Rideal mechanism and that of the Langmuir− Hinshelwood mechanism, N2O formation was not promoted as the gaseous NO concentration increased (as shown in Figure 2b). As a result, N2O selectivity of NO reduction over Mn−Fe spinel decreased remarkably with the increase of the gaseous NO concentration. The formation of N2O and N2 over Mn−Fe spinel through both the Eley−Rideal mechanism and the Langmuir−Hinshelwood mechanism was related to the concentration of NH3 adsorbed (i.e., [NH3(ad)]). Because Mn−Fe spinel was saturated with gaseous NH3, [NH3(ad)] can be approximately regarded as a constant. However, some of the active sites on Mn−Fe spinel for NH3 adsorption may also be the active sites for NO + O2 adsorption. Therefore, there was a competitive adsorption between gaseous NH3 and NO for the active sites. As the gaseous NH3 concentration increased, NH3 would prevail over NO + O2 in the competitive adsorption, resulting in an increase of [NH3(ad)]. Figure S4 of the Supporting Information demonstrates that the intensities of the bands at 1669, 1603, 1440, and 1190 cm−1 corresponding to adsorbed NH3 species increased with the increase of the gaseous NH3 concentration from 200 to 500 ppm. As [NH3(ad)] increased, NO reduction over Mn−Fe spinel through the Eley−Rideal mechanism would be promoted (hinted by eqs 20−23). However, the adsorption of NOx on Mn−Fe spinel was restrained because of the competitive adsorption as the gaseous NH3 concentration increased. Figure S4 of the Supporting Information demonstrates that the intensities of the bands at 1584, 1507, and 1286 cm−1 corresponding to adsorbed NOx species decreased with the increase of the gaseous NH3 concentration from 200 to 500 ppm. It suggests that NO reduction over Mn−Fe spinel through the Langmuir−Hinshelwood mechanism would be restrained as the gaseous NH3 concentration increased. The transient reaction demonstrates that N2O selectivity of NO reduction through the Langmuir−Hinshelwood mechanism was much less than that through the Eley−Rideal mechanism. Therefore, N2O selectivity of NO reduction over Mn−Fe spinel increased because of the promotion of NO reduction through the Eley−Rideal mechanism as the gaseous NH3 concentration increased (as shown in Figure 3d). If the concentrations of gaseous NO and NH3 were sufficiently high to keep Mn−Fe spinel saturated with the adsorption of NO and NH3, NO reduction through the Langmuir−Hinshelwood mechanism increased proportional to every increase of catalyst mass (i.e., the decrease of GHSV).14 The rate of NO reduction through the Eley−Rideal mechanism depended upon the gaseous NO concentration (hinted by eqs 20 and 21). However, the gaseous NO concentration at the added catalyst was much less than that at the former catalyst because of NO reduction. Therefore, the promotion of NO reduction through the Eley−Rideal mechanism because of the decrease of GHSV lagged behind the increase of the catalyst mass. They suggest that the ratio of NO reduction through the Langmuir−Hinshelwood mechanism generally increased with the decrease of GHSV. The transient reaction study demonstrates that N2O selectivity of NO reduction through the Langmuir−Hinshelwood mechanism was much less than that though the Eley−Rideal mechanism. Therefore, N2O selectivity of NO reduction over Mn−Fe spinel decreased with the decrease of GHSV (as shown in Figure 4).

of NOx and NH3 in the outlet; therefore, Mn−Fe spinel was almost saturated with the adsorption of gaseous NO and NH3. Therefore, the concentrations of NO adsorbed and NH3 adsorbed (i.e., [NO(ad)] and [NH3(ad)]) on Mn−Fe spinel were approximately independent of the concentrations of gaseous NO and NH314,26 if the competitive adsorption between gaseous NO and NH3 was neglected. Therefore, [Mn3+−O−NO−NH3] and [Mn3+−O−NO2−NH3] were approximately not related to gaseous NO and NH 3 concentrations. Hinted by eqs 18 and 19, the formation of N2O and N2 through the Langmuir−Hinshelwood mechanism was nearly independent of gaseous NO and NH3 concentrations. N2O formation through the Eley−Rideal mechanism (i.e., reaction 17) can be described as d[N2O] dt

= k 3[NH][NO(g)][Mn 4 +O] E−R

(20)

where k3, [NO(g)], [NH], and [Mn O] were the rate constant of reaction 17, gaseous NO concentration, and concentrations of NH and Mn 4+ on Mn−Fe spinel, respectively. N2 formation through the Eley−Rideal mechanism (i.e., reaction 15) can be described as 4+

d[N2] dt

= k4[NH 2][NO(g)] E−R

(21)

where k4 and [NH2] were the rate constant of reaction 15 and the concentration of NH2 on Mn−Fe spinel, respectively. The formation of NH2 on Mn−Fe spinel can be described as d[NH 2] = k5[NH3(ad)][Mn 4 +O] dt

(22)

where k5 was the reaction rate constant of reaction 14. Equation 22 shows that the formation of NH2 on Mn−Fe spinel was related to the concentrations of NH3 adsorbed and Mn4+ on Mn−Fe spinel. The formation of NH on Mn−Fe spinel can be described as d[NH] = k6[NH 2][Mn 4 +O] dt

(23)

where k6 was the reaction rate constant of reaction 16. There was a strong competition between reactions 16 and 15 for the consumption of NH2. Hinted by eq 21, reaction 15 was remarkably promoted as the gaseous NO concentration increased, resulting in a notable promotion of N2 formation. Although N2 formation through the Langmuir−Hinshelwood mechanism was independent of the gaseous NO concentration, that through the Eley−Rideal mechanism was promoted as the gaseous NO concentration increased. Taking into account both the contribution of the Eley−Rideal mechanism and that of the Langmuir−Hinshelwood mechanism, N2 formation over Mn− Fe spinel was notably promoted as the gaseous NO concentration increased (as shown in Figure 2c). However, the increase of NH2 on Mn−Fe spinel for gaseous NO reduction indicates the decrease of NH2 to be oxidized to NH, resulting in a notable decrease of NH on Mn−Fe spinel.21 Although the gaseous NO concentration increased, the NH concentration on Mn−Fe spinel decreased. Hinted by eq 20, N2O formation through the Eley−Rideal mechanism was not promoted as the gaseous NO concentration increased. 10360



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In summary, N2O selectivity of NO reduction over Mn−Fe spinel was related to the concentrations of gaseous NO and NH3 and the GHSV. The higher concentration of gaseous NO in the flue gas will cause a lower N2O selectivity; therefore, a low-temperature SCR reaction with Mn−Fe spinel as the catalyst is suitable for the flue gas with a higher concentration of gaseous NO. Furthermore, the decrease of the GHSV to increase the removal efficiency of NO can also decrease the N2O selectivity.

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ASSOCIATED CONTENT

S Supporting Information *

Simple rate expressions of NO reduction, N2O formation, and N2 formation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: 86-18-066068302. E-mail: yangshijiangsq@163. com. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Fund of China (Grants 21207067, 41372044, and 21325731), the Fundamental Research Funds for the Central Universities (Grant 30920130111023), the Zijin Intelligent Program, Nanjing University of Science and Technology (Grant 2013-0106), and the Environmental Scientific Research of Jiangsu Province (2012026).

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Mechanism of N2O Formation during the Low-Temperature Selective

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