Competition of selective catalytic reduction and non selective catalytic reduction over MnO_x/TiO₂ for NO removal: the relationship between gaseous NO concentration and N₂O selectivity

Abstract

In this work, a novel phenomenon was discovered that N₂O selectivity of NO reduction over MnO_x/TiO₂ was related to the concentration of gaseous NO and that lower concentration of gaseous NO would cause higher N₂O selectivity. In situ DRIFTS and transient reaction studies demonstrated that both the Eley–Rideal mechanism (the reaction of over-activated NH₃ with gaseous NO) and the Langmuir–Hinshelwood mechanism (the reaction of adsorbed NO₃⁻ with adsorbed NH₃ on the adjacent sites) could contribute to the formation of N₂O. Kinetic study demonstrated that N₂O selectivity would be independent of gaseous NO concentration if NO reduction over MnO_x/TiO₂ mainly followed the Langmuir–Hinshelwood mechanism. If NO reduction over MnO_x/TiO₂ mainly followed the Eley–Rideal mechanism, there was competition between the selective catalytic reduction (SCR) reaction and non selective catalytic reduction (NSCR) reaction. As gaseous NO concentration increased, more –NH₂ was used to reduce gaseous NO to form N₂ and the further oxidization of –NH₂ to –NH was restrained, resulting in an obvious decrease of N₂O selectivity. The Eley–Rideal mechanism played an important role in NO reduction over MnO_x/TiO₂, especially at higher temperatures. Therefore, N₂O selectivity of the low temperature SCR reaction over MnO_x/TiO₂ decreased especially at higher temperatures after the increase of gaseous NO concentration.

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1. Introduction

Nitrogen oxides (NO and NO₂), emitted from automobiles and stationary sources, greatly contribute to the formation of smog, acid rain and ozone.¹ Selective catalytic reduction (SCR) of NO with NH₃ has been an efficient technique for the control of NO_x emission from coal fired power plants and automobiles.² The standard SCR process is based on the following reaction between NH₃ and NO:³

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O (1)

V₂O₅–WO₃(MoO₃)/TiO₂ has been widely used as a SCR catalyst to control the emission of NO from stationary coal fired power plants for several decades.² The temperature window of V₂O₅–WO₃(MoO₃)/TiO₂ is about 300–400 °C, so the SCR unit is located upstream of the desulfurizer and electrostatic precipitator in order to avoid reheating of the flue gas.⁴ However, retrofitting the SCR devices into existing systems is difficult because the space and access in many power plants are extremely limited.⁵ Therefore, there has been strong demand in developing highly active SCR catalysts at low temperatures, which will be placed downstream of the electrostatic precipitator and desulfurizer.⁶ Mn based catalysts, for example MnO_x–CeO₂,^7,8 MnO₂/TiO₂,^9–13 MnO_x–CeO₂/TiO₂¹⁴ and Fe₂O₃–MnO₂/TiO_2,¹⁵ show excellent low temperature SCR activity among the first row transition metal based catalysts.^16–20 However, some N₂O would form during the low temperature SCR reaction over Mn based catalysts.^6,7,21 The non selective catalytic reduction (NSCR) reaction is based on the following reaction between NH₃ and NO:²²

4NH₃ + 4NO + 3O₂ → 4N₂O + 6H₂O (2)

N₂O is now considered as a pollutant due to its greenhouse effect and its depletion of the ozone layer.^22–25 However, only a little work focused on the mechanism of N₂O formation during the low temperature SCR over Mn based catalyst.²¹ Some groups have demonstrated that one of the two N atoms in N₂O originates from NH₃ and the other from NO under the SCR conditions.^3,22 However, there is no agreement on the mechanism of N₂O formation: by (1) reaction of gaseous NO with over-activated NH₃ (–NH) to N₂O (i.e. the Eley–Rideal mechanism), or (2) adsorption of NO₃⁻ on the adjacent sites of adsorbed NH₃, followed by reaction to an activated transition state (i.e. NH₄NO₃) and then decomposition to N₂O (i.e. the Langmuir–Hinshelwood mechanism).³ Recently, a novel phenomenon has been found that N₂O selectivity of the low temperature SCR reaction over MnO_x/TiO₂ was related to the concentration of gaseous NO. N₂O selectivity obviously decreased after the increase of gaseous NO concentration in the inlet. Herein, the mechanism of N₂O formation during the low temperature SCR reaction over MnO_x/TiO₂ was studied, and the effect of NO concentration on N₂O selectivity was investigated.

2. Experimental

2.1 Catalyst preparation

MnO_x/TiO₂ (Mn loading was 5 wt%) was prepared by the impregnation method using Degussa TiO₂ P25 as support and manganese nitrate as precursor. The sample was dried at 110 °C for 12 h, and it was then calcined at 500 °C under air atmosphere for 3 h.

2.2 Catalytic test

The reduction of NO was performed on a fixed-bed quartz tube reactor (6 mm of internal diameter). 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 total flow rate was 200 mL min⁻¹ (room temperature), and the mass of catalyst was 200 mg. The corresponding gas hourly space velocity (GHSV) was 6 × 10⁴ cm³ g⁻¹ h⁻¹ (i.e. 75 000 h⁻¹). The feed contained 500 or 1000 ppm of NO, 500 or 1000 ppm of NH₃, 2% of O₂, and balance of N₂. The concentrations of NO, NO₂, NH₃ and N₂O in the outlet were continually monitored by an FTIR spectrometer (MKS Instruments). The ratios of NO_x and NH₃ conversion, the amount of N₂ formed and N₂O selectivity were calculated using the following equations:where, [NH₃]_in and [NO_x]_in were the concentrations of NH₃ and NO_x (including NO and NO₂) in the inlet, and [NH₃]_out, [NO_x]_out and [N₂O]_out were the concentrations of NH₃, NO_x (including NO and NO₂) and N₂O in the outlet.

2.3 In situ DRIFTS study

In situ DRIFT spectra were recorded on a Fourier transform infrared spectrometer (FTIR, Nicolet NEXUS 870) equipped with a smart collector and an MCT detector cooled by liquid N₂.²⁶ The catalyst was finely ground and placed in a ceramic crucible and manually pressed. The FTIR spectra were recorded by accumulating 100 scans with a resolution of 4 cm⁻¹.

3. Results

3.1 SCR performance of MnO_x/TiO₂ for the low temperature SCR reaction

The performance of MnO_x/TiO₂ for the low temperature SCR reaction at 100–250 °C is shown in Fig. 1. Both the ratio of NO conversion and N₂O selectivity increased with the increase of reaction temperature, which is consistent with previous research on the low temperature SCR reaction over MnO_x/TiO₂.^13,27,28 As the concentrations of NO and NH₃ in the inlet increased from 500 to 1000 ppm, the ratios of NO and NH₃ conversion over MnO_x/TiO₂ slightly decreased. A similar result once happened on Mn–Fe spinel.⁶ However, N₂O selectivity obviously decreased after the increase of the concentrations of gaseous NO and NH₃ especially at higher temperatures (shown in Fig. 1). It suggests that N₂O selectivity of the low temperature SCR reaction over MnO_x/TiO₂ was related to the concentrations of gaseous NO and NH₃ in the inlet. This phenomenon was seldom previously reported in the literature.

Fig. 1 SCR performance of MnO_x/TiO₂: (a) [NO] = [NH₃] = 500 ppm; (b) [NO] = [NH₃] = 1000 ppm. Reaction conditions: [O₂] = 2%, catalyst mass = 200 mg, total flow rate = 200 mL min⁻¹, GHSV = 60 000 cm³ g⁻¹ h⁻¹.

3.2 Effect of gaseous NO concentration on N₂O selectivity

Fig. 2 shows the effect of gaseous NO concentration on NH₃ conversion and N₂O formation. As shown in Fig. 2a, little NH₃ can be oxidized over MnO_x/TiO₂ below 175 °C in the absence of NO. With the increase of reaction temperature from 175 to 250 °C, NH₃ oxidation was promoted (shown in Fig. 2a). However, more than 50% of NH₃ was oxidized to N₂O at 200–250 °C (shown in Fig. 2b).

Fig. 2 Effect of NO concentration on: (a) NH₃ conversion; (b) N₂O selectivity. Reaction conditions: [O₂] = 2%, catalyst mass = 200 mg, total flow rate = 200 mL min⁻¹, GHSV = 60 000 cm³ g⁻¹ h⁻¹.

As 500 ppm of NO was introduced, NH₃ conversion over MnO_x/TiO₂ was promoted (shown in Fig. 2a). Meanwhile, N₂O selectivity of MnO_x/TiO₂ for the SCR reaction was much less than that for NH₃ oxidation at 200–250 °C. As the concentration of gaseous NO increased from 500 to 1000 ppm, NH₃ conversion was further promoted (shown in Fig. 2a). Meanwhile, the formation of N₂O over MnO_x/TiO₂ was further restrained (shown in Fig. 2b). It suggests that N₂O selectivity over MnO_x/TiO₂ was related to the concentration of gaseous NO.

3.3 In situ DRIFTS study

3.3.1 Transient reaction at 150 °C. MnO_x/TiO₂ was first treated with 500 ppm of NH₃ at 150 °C for 30 min followed by N₂ purged for 5 min. 500 ppm of NO and 2% of O₂ were then introduced into the IR cell (shown in Fig. 3a). After the adsorption of NH₃, five characteristic vibrations at 1680, 1600, 1437, 1206 and 1165 cm⁻¹ appeared on MnO_x/TiO₂. The bands at 1600 and 1206 cm⁻¹ were assigned to coordinated NH₃ bound to the Lewis acid sites, and the bands at 1680 and 1437 cm⁻¹ were attributed to ionic NH₄⁺ bound to the Brønsted acid sites.^8,10 The band at 1165 cm⁻¹ could be attributed to –NH₂, which resulted from the activation of coordinated NH₃ bound to the Lewis acid sites.²⁹ After NO + O₂ passed over NH₃ pretreated MnO_x/TiO₂, the bands at 1680, 1600, 1437, 1206 and 1165 cm⁻¹ corresponding to adsorbed ammonia species gradually diminished. Meanwhile, three characteristic vibrations at 1609, 1529 and 1280 cm⁻¹ appeared. The band at 1609 cm⁻¹ was assigned to monodentate nitrite, and the bands at 1529 and 1280 cm⁻¹ were attributed to bidentate nitrate.³⁰ Moreover, adsorbed H₂O, which is the product of the SCR reaction, appeared at 1630 cm⁻¹. These bands suggest that adsorbed NH₃ can react with gaseous NO (i.e. the Eley–Rideal mechanism). The concentrations of N₂O and NO in the outlet during the transient reaction were simultaneously recorded (shown in Fig. 4a). After NO + O₂ passed over NH₃ pretreated MnO_x/TiO₂, NO concentration gradually increased to about 480 ppm. Meanwhile, the concentration of N₂O in the outlet rapidly increased to about 30 ppm, and it then gradually decreased to the background of N₂O in NO (9 ppm) (shown in Fig. 4a). It suggests that the reaction between adsorbed NH₃ and gaseous NO (i.e. the Eley–Rideal mechanism) at 150 °C can produce N₂O.

Fig. 3 (a) DRIFT spectra taken at 150 °C upon passing NO + O₂ over NH₃ presorbed MnO₂/TiO₂; (b) DRIFT spectra taken at 150 °C upon passing NH₃ over NO + O₂ presorbed MnO₂/TiO₂; (c) DRIFT spectra taken at 150 °C upon passing NH₃ + NO + O₂ over MnO₂/TiO₂.

Fig. 4 (a) Transient reaction taken at 150 °C upon passing NO + O₂ over NH₃ presorbed MnO₂/TiO₂; (b) Transient reaction taken at 150 °C upon passing NH₃ over NO + O₂ presorbed MnO₂/TiO₂.

Then, the reactants were introduced to MnO_x/TiO₂ in the reverse order. MnO_x/TiO₂ was first treated with 500 ppm of NO and 2% of O₂ for 30 min at 150 °C followed by N₂ purged for 5 min. 500 ppm of NH₃ was then introduced into the IR cell (shown in Fig. 3b). After the adsorption of NO + O₂ at 150 °C, MnO_x/TiO₂ was mainly covered by monodentate nitrite (1609 cm⁻¹) and bidentate nitrate (1529 and 1280 cm⁻¹). After NH₃ was introduced into the cell, the bands corresponding to monodentate nitrite (at 1609 cm⁻¹) and bidentate nitrate (1529 cm⁻¹) firstly shifted to 1600 and 1506 cm⁻¹. Then, the intensities of the two bands gradually decreased (shown in Fig. 3b). They suggest that adsorbed NO_x can react with adsorbed NH₃ (i.e. the Langmuir–Hinshelwood mechanism). The concentrations of N₂O and NH₃ in the outlet during the transient reaction were simultaneously recorded (shown in Fig. 4b). After NH₃ was introduced to NO + O₂ pretreated MnO_x/TiO₂ for 5 min, little NH₃ was observed. Meanwhile, the concentration of N₂O in the outlet rapidly increased to about 13 ppm after the introduction of NH₃, and it then gradually decreased to about 5 ppm in 60 min. It suggests that the reaction between adsorbed NH₃ and adsorbed NO_x (i.e. the Langmuir–Hinshelwood mechanism) can also produce N₂O. Fig. 3b shows that the decrease of the band at 1600 cm⁻¹ corresponding to adsorbed NH₄NO₂ was much faster than that at 1506 cm⁻¹ corresponding to adsorbed NH₄NO₃. It suggests that the reaction through the nitrite route was faster than that through the nitrate route, which was consistent with the result of Mn–Fe spinel.⁶ Previous research demonstrated that the product of the nitrite route was N₂, while that of nitrate route was N₂O.²²

Finally, the IR spectra during the SCR reaction (i.e. 500 ppm of NH₃, 500 ppm of NO and 2% of O₂ were simultaneously introduced) at 150 °C were recorded. As shown in Fig. 3c, adsorbed H₂O (at 1630 cm⁻¹), coordinated NH₃ or adsorbed NH₄NO₂ (at 1600 cm⁻¹), adsorbed NH₄NO₃ (at 1566 and 1506 cm⁻¹) and ionic NH₄⁺ (at 1437 cm⁻¹) were all observed. It suggests that both the Langmuir–Hinshelwood mechanism and the Eley–Rideal mechanism could happen at 150 °C.

3.3.2 Transient reaction at 250 °C. MnO_x/TiO₂ was first treated with 500 ppm of NH₃ for 30 min at 250 °C, and 500 ppm of NO and 2% of O₂ were then introduced into the IR cell (shown in Fig. 5a). After the adsorption of NH₃, MnO_x/TiO₂ was mainly covered by coordinated NH₃ bound to the Lewis acid sites (at 1602 cm⁻¹). After NO + O₂ passed over NH₃ pretreated MnO_x/TiO₂, coordinated NH₃ rapidly diminished, and adsorbed H₂O (at 1620 cm⁻¹) appeared. Then, MnO_x/TiO₂ was mainly covered by monodentate nitrite (at 1607 cm⁻¹) and monodentate nitrate (1559 cm⁻¹).³⁰ The concentrations of NH₃, N₂O, NO and NO₂ during the transient reaction were simultaneously recorded (shown in Fig. 6a). As NH₃ was introduced to MnO_x/TiO₂, about 90 ppm of N₂O was observed, which resulted from the oxidation of NH₃ by the lattice oxygen of MnO_x/TiO₂. After NO + O₂ passed over NH₃ pretreated MnO_x/TiO₂ at 250 °C, N₂O concentration rapidly increased from 90 to 160 ppm, and it then decreased to the background of N₂O in NO (9 ppm). The concentration of N₂O from the transient reaction at 250 °C was much higher than that at 150 °C. It suggests that the formation of N₂O from the Eley–Rideal mechanism was obviously promoted with the increase of reaction temperature.

Fig. 5 (a) DRIFT spectra taken at 250 °C upon passing NO + O₂ over NH₃ presorbed MnO₂/TiO₂; (b) DRIFT spectra taken at 250 °C upon passing NH₃ over NO + O₂ presorbed MnO₂/TiO₂; (c) DRIFT spectra taken at 250 °C upon passing NH₃ + NO + O₂ over MnO₂/TiO₂.

Fig. 6 (a) Transient reaction taken at 250 °C upon passing NO + O₂ over NH₃ presorbed MnO₂/TiO₂; (b) Transient reaction taken at 250 °C upon passing NH₃ over NO + O₂ presorbed MnO₂/TiO₂.

Then, the reactants were introduced to MnO_x/TiO₂ in the reverse order. MnO_x/TiO₂ was first treated with 500 ppm of NO and 2% of O₂ for 30 min followed by N₂ purged for 5 min at 250 °C. 500 ppm of NH₃ was then introduced into the IR cell (shown in Fig. 5b). After the adsorption of NO + O₂ at 250 °C, MnO_x/TiO₂ was mainly covered by monodentate nitrite (1607 cm⁻¹) and monodentate nitrate (1559 cm⁻¹). After NH₃ was introduced into the cell, the band at 1607 cm⁻¹ corresponding to monodentate nitrite (at 1607 cm⁻¹) rapidly diminished. However, monodentate nitrate (1559 cm⁻¹) firstly shifted to 1534 cm⁻¹. Then, it gradually diminished. Finally, MnO_x/TiO₂ was mainly covered by coordinated NH₃ (at 1602 cm⁻¹). The concentrations of N₂O and NH₃ during the transient reaction were simultaneously recorded (shown in Fig. 6b). After NH₃ was introduced to NO + O₂ pretreated MnO_x/TiO₂ for 10 min, NH₃ in the outlet was observed. Meanwhile, the concentration of N₂O in the outlet rapidly increased to about 110 ppm after the introduction of NH₃, and it then gradually decreased to about 90 ppm in 50 min (shown in Fig. 6b). In this case, the origination of N₂O from NH₃ oxidation or the Langmuir–Hinshelwood mechanism was difficult to be differentiated. However, the concentration of N₂O in the first 5 min was slightly higher than the concentration of N₂O from the oxidation of 500 ppm of NH₃. It suggests that the Langmuir–Hinshelwood mechanism can also contribute to N₂O formation at 250 °C.

Finally, the IR spectra during the SCR reaction (i.e. 500 ppm of NH₃, 500 ppm of NO and 2% of O₂ were simultaneously introduced) at 250 °C were recorded. As shown in Fig. 5c, only adsorbed H₂O (at 1620 cm⁻¹) and coordinated NH₃ or adsorbed NH₄NO₂ (at 1602 cm⁻¹) can be clearly detected. However, the band at 1534 cm⁻¹ corresponding to adsorbed NH₄NO₃ can not be observed. Fig. 5a shows that the disappearance of NH₄NO₃ was much slower than that of NH₄NO₂. It suggests that NH₄NO₃ did not form during NO reduction over MnO_x/TiO₂ at 250 °C. Therefore, the contribution of NH₄NO₃ decomposition to N₂O formation can be neglected and N₂O formation at 250 °C mainly resulted from the Eley–Rideal mechanism.

4. Discussion

4.1 Mechanism of N₂O formation

4.1.1 N₂O formation from the Langmuir–Hinshelwood mechanism. The reduction of NO (including the SCR reaction and the NSCR reaction) through the Langmuir–Hinshelwood mechanism can be approximately described as:^3,6,22

NH_3(g) → NH_3(ad) (7)

NO_(g) → NO_(ad) (8)

Reaction 7 is the adsorption of gaseous ammonia on the acid sites (i.e. Brønsted acid sites and Lewis acid sites) to form adsorbed ammonia species including ionic NH₄⁺ and coordinated NH₃. Reaction 8 is the physical adsorption of gaseous NO on MnO_x/TiO₂. Then, adsorbed NO is oxidized by Mn⁴⁺ on MnO_x/TiO₂ to form adsorbed NO₂⁻ (i.e.reaction 9). Reaction 10 is the oxidation of adsorbed NO by Mn⁴⁺ on MnO_x/TiO₂ to NO₃⁻. Subsequently, adsorbed NO₂⁻ and NO₃⁻ react with adsorbed NH₃ species on the adjacent sites to form NH₄NO₂ and NH₄NO₃ (i.e.reactions 11 and 12), respectively. Finally, NH₄NO₂ and NH₄NO₃ are decomposed to N₂ and N₂O, respectively. Reaction 13 is the regeneration of Mn⁴⁺ on MnO_x/TiO₂.

The kinetic equations of the formation of N₂ and N₂O over MnO_x/TiO₂ through the Langmuir–Hinshelwood mechanism can be approximately described as:

where, k₁, k₂, [NH₄NO₂] and [NH₄NO₃] are the decomposition rate constants of NH₄NO₂ and NH₄NO₃, and the concentrations of NH₄NO₂ and NH₄NO₃ on MnO_x/TiO₂, respectively.

[NH₄NO₂] and [NH₄NO₃] are mainly related to the concentrations of NO adsorbed ([NO_(ad)]) and Mn⁴⁺ on MnO_x/TiO₂ (the deduction is shown in the ESI†). The GHSV used in this work was quite high and there were generally large amounts of NO_x and NH₃ in the outlet (shown in Fig. 7a and b), so MnO_x/TiO₂ was almost saturated with the adsorption of NO and NH₃. Furthermore, there is generally agreement that the SCR reaction starts with the adsorption of NH₃, which is very strong compared to the adsorption of NO + O₂ and the products.³ Thus, the increase of gaseous NO concentration from 500 to 1000 ppm could not break the adsorption equilibrium of NH₃ and NO. It suggests that [NO_(ad)] and [NH_3(ad)] would not vary after increasing the concentrations of gaseous NO and NH₃. As a result, the concentrations of NH₄NO₂ and NH₄NO₃ on MnO_x/TiO₂ were independent of the concentrations of gaseous NO and NH₃.

Fig. 7 Concentrations of NO_x (a), NH₃ (b), N₂ formed (c) and N₂O (d) in the outlet of the reactor. Reaction conditions: [O₂] = 2%, catalyst mass = 200 mg, total flow rate = 200 mL min⁻¹, GHSV= 60 000 cm³ g⁻¹ h⁻¹.

4.1.2 N₂O formation from the Eley–Rideal mechanism. The reduction of NO (including the SCR reaction and the NSCR reaction) through the Eley–Rideal mechanism can be approximately described as:^3,6,22

NH_3(g) → NH_3(ad) (7)

NH_3(ad) + ≡Mn⁴⁺ → –NH₂ + ≡Mn³⁺ + H⁺ (16)

–NH₂ + ≡Mn⁴⁺ → –NH + ≡Mn³⁺ + H⁺ (17)

–NH₂ + NO_(g) → N₂ + H₂O (18)

–NH + NO_(g) → N₂O + H⁺ (19)

Reaction 16 is the activation of adsorbed ammonia species by Mn⁴⁺ on MnO_x/TiO₂ to form amide species (–NH₂). –NH₂ on MnO_x/TiO₂ can be further oxidized to –NH (i.e.reaction 17). Then, gaseous NO was reduced by –NH₂ and –NH on the surface to form N₂ and N₂O (i.e.reactions 18 and 19), respectively.

The kinetic equation of reaction 16 can be described as:

where k₃ and [–NH₂] are the kinetic constant of reaction 16 and the concentration of –NH₂ on MnO_x/TiO₂, respectively.

The kinetic equation of reaction 17 can be described as:

where k₄ and [–NH] are the kinetic constant of reaction 17 and the concentration of –NH on MnO_x/TiO₂.

The kinetic equations of reactions 18 and 19 can be described as:

where, k₅, k₆ and [NO_(g)] are the kinetic constants of reactions 18 and 19, and the concentration of gaseous NO, respectively.

According to eqn (20)–(22), the variation of –NH₂ concentration on MnO_x/TiO₂ can be described as:

As the reaction reached the steady state, –NH₂ concentration on MnO_x/TiO₂ would not vary. Therefore,

Thus,

Then, the formation of N₂ (eqn (22)) can be transformed as:

According to eqn (21) and (23), the variation of –NH concentration on MnO_x/TiO₂ can be described as:

As the reaction reached the steady state, –NH concentration on MnO_x/TiO₂ would not vary. Therefore,

Thus,

Then, the formation of N₂O (eqn (23)) can be transformed as:

Taking account of the contributions of both the Langmuir–Hinshelwood mechanism and the Eley–Rideal mechanism, the formation of N₂ and N₂O can be approximately described as follows:

The concentrations of NH₄NO₂ and NH₄NO₃ on MnO_x/TiO₂ were independent of gaseous NO and NH₃, so the contribution of the Langmuir–Hinshelwood mechanism to the formation of N₂ and N₂O would not vary after the increase of gaseous NO concentration. However, the contribution of the Eley–Rideal mechanism to N₂ formation would increase after the increase of gaseous NO concentration (hinted by eqn (32)). Meanwhile, the contribution of the Eley–Rideal mechanism to N₂O formation would decrease after the increase of gaseous NO concentration (hinted by eqn (33)).

If the reduction of NO mainly followed the Langmuir–Hinshelwood mechanism, N₂O selectivity can be described as:

The concentrations of NH₄NO₂ and NH₄NO₃ did not change after the increase of gaseous NO and NH₃, so N₂O selectivity was independent of gaseous NO concentration.

If the reduction of NO mainly followed the Eley–Rideal mechanism, N₂O selectivity can be described as:

Eqn (35) suggests that N₂O selectivity would decrease after the increase of gaseous NO concentration.

There is generally agreement that the Langmuir–Hinshelwood mechanism plays an important role on the SCR reaction at lower temperatures.^3,6 Meanwhile, the Eley–Rideal mechanism can also contribute to NO reduction at lower temperatures. The Eley–Rideal mechanism was obviously promoted with the increase of reaction temperature and it predominated over the SCR reaction at higher temperatures.^3,6

4.2 Effect of gaseous NO concentration in the inlet on N₂O selectivity

If the concentrations of gaseous NO and NH₃ were sufficiently high, the whole catalyst bed was saturated with the adsorption of gaseous NO and NH₃. Thus, [NH_3(ad)], [NH₄NO₂] and [NH₄NO₂] can be regarded as constants on the whole catalyst bed. However, the concentration of gaseous NO at the bottom of the catalyst bed was much less than that at the top of the catalyst bed due to the reduction of NO. Therefore, the amounts of N₂ and N₂O formed over the whole catalyst bed should be described as follows:where t is the time how long gaseous NO passed through the catalyst column, which is inversely proportional to the GHSV.

Fig. 7a shows that the concentrations of gaseous NO in the outlet of the reaction with 1000 ppm of NO and 500 ppm of NH₃ were much higher than that with 500 ppm of NO and 500 ppm of NH₃. Meanwhile, the concentration of gaseous NO in the inlet of the reaction with 1000 ppm of NO and 500 ppm of NH₃ were twice that with 500 ppm of NO and 500 ppm of NH₃. They suggest that the concentrations of gaseous NO at each section of the catalyst bed during the reaction with 1000 ppm of NO and 500 ppm of NH₃ were all higher than those with 500 ppm of NO and 500 ppm of NH₃. Hinted by eqn (36), the amount of N₂ formed during the reaction with 1000 ppm of NO and 500 ppm of NH₃ was higher than that with 500 ppm of NO and 500 ppm of NH₃, which was demonstrated in Fig. 7c. Hinted by eqn (37), the amount of N₂O formed during the reaction with 1000 ppm of NO and 500 ppm of NH₃ was less than that with 500 ppm of NO and 500 ppm of NH₃, which was demonstrated in Fig. 7d. As a result, N₂O selectivity of the reaction with 1000 ppm of NO and 500 ppm of NH₃ was less than that with 500 ppm of NO and 500 ppm of NH₃ (shown in Fig. 2b).

The concentrations of gaseous NO in the outlet and in the inlet during the reaction with 1000 ppm of NO and 1000 ppm of NH₃ were both close to those with 1000 ppm of NO and 500 ppm of NH₃ below 200 °C (shown in Fig. 7a). It suggests that the concentrations of gaseous NO at each section of the catalyst bed during the reaction with 1000 ppm of NO and 1000 ppm of NH₃ were all close to those with 1000 ppm of NO and 500 ppm of NH₃ below 200 °C. Therefore, the amounts of N₂ and N₂O formed during the reaction with 1000 ppm of NO and 1000 ppm of NH₃ were close to those with 1000 ppm of NO and 500 ppm of NH₃ below 200 °C, which is demonstrated in Fig. 7c and d. As a result, N₂O selectivity of the reaction with 1000 ppm of NO and 1000 ppm of NH₃ was close to that with 1000 ppm of NO and 500 ppm of NH₃ below 200 °C (shown in Fig. 2b). However, little NH₃ can be observed in the outlet of the reaction with 1000 ppm of NO and 500 ppm of NH₃ above 200 °C (shown in Fig. 7b). It suggests that some catalyst in the bottom of the catalyst bed did not take part in the reaction because gaseous NH₃ had been completely consumed. As the concentration of gaseous NH₃ increased from 500 to 1000 ppm, a large amount of NH₃ can be observed in the outlet above 200 °C (shown in Fig. 7b). Therefore, most of the catalyst took part in the reaction with 1000 ppm of NO and 1000 ppm of NH₃ above 200 °C. They suggest that t of the reaction with 1000 ppm of NO and 1000 ppm of NH₃ was higher than that with 1000 ppm of NO and 500 ppm of NH₃ above 200 °C. Hinted by eqn (36) and (37), the amounts of N₂ and N₂O formed during the reaction with 1000 ppm of NO and 1000 ppm of NH₃ above 200 °C were both much higher than that with 1000 ppm of NO and 500 ppm of NH₃ (shown in Fig. 7c and d). Gaseous NO concentration at the top of the catalyst bed was much higher than that at the bottom of the catalyst bed due to NO reduction. It suggests that N₂O selectivity at the top of the catalyst bed was much less than that at the bottom of the catalyst bed. As a result, N₂O selectivity of the reaction with 1000 ppm of NO and 1000 ppm of NH₃ above 200 °C were higher than that with 1000 ppm of NO and 500 ppm of NH₃, which was demonstrated in Fig. 2b.

Fig. 7a shows that the concentrations of gaseous NO at each section of the catalyst column during the reaction with 1000 ppm of NO and 1000 ppm of NH₃ were all higher than those with 500 ppm of NO and 500 ppm of NH₃. Hinted by eqn (35), N₂O selectivity of the reaction with 1000 ppm of NO and 1000 ppm of NH₃ was less than that with 500 ppm of NO and 500 ppm of NH₃ (shown in Fig. 2b).

5. Conclusion

N₂O selectivity of the low temperature SCR reaction over MnO_x/TiO₂ was related to gaseous NO concentration in the flue gas. The lower concentration of gaseous NO in the flue gas would cause the higher N₂O selectivity. If the concentration of gaseous NO in the flue gas is very low, low temperature SCR of NO with MnO_x/TiO₂ as the catalyst could not be the right choice for the control of NO emission due to the lower N₂ selectivity. Furthermore, N₂O selectivity at the bottom of the catalyst bed was much higher than that at the top of the catalyst column due to the lower gaseous NO concentration. Therefore, the decrease of GHSV to excessively pursue the removal efficiency of NO will cause lower N₂ selectivity.

Acknowledgements

This study was financially supported by the National Natural Science Fund of China (Grant No. 21207067 and 41372044), Environmental scientific research of Jiangsu Province (2012026), the Fundamental Research Funds for the Central Universities (grant no. 30920130111023), the Zijin Intelligent Program, Nanjing University of Science and Technology (grant no. 2013–0106), and special fund of State Key Joint Laboratory of Environment Simulation and Pollution Control.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cy00648d

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