Promotion mechanism of CeO₂ addition on the low temperature SCR reaction over MnO_x/TiO₂: a new insight from the kinetic study

Abstract

CeO₂ addition showed a notable improvement on the low temperature selective catalytic reduction (SCR) performance of MnO_x/TiO₂. In this work, a new insight into the promotion mechanism was established from the steady state kinetic study. The kinetic rate constants of the SCR reaction through the Eley–Rideal mechanism, those of the SCR reaction through the Langmuir–Hinshelwood mechanism, and those of the non-selective catalytic reduction (NSCR) reaction over MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂ were obtained according to the steady state kinetic analysis. After comparing the reaction kinetic constants of NO reduction over MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂, the mechanism of the addition of CeO₂ for NO reduction over MnO_x/TiO₂ was discovered according to the relationship between the reaction rate constants and the catalyst properties. Because the oxidation ability of MnO_x/TiO₂ increased, the rate constant of the SCR reaction over MnO_x/TiO₂ increased remarkably after CeO₂ addition, resulting in a notable promotion of N₂ formation. The oxidation of NH₃ to NH over MnO_x/TiO₂ required two Mn⁴⁺ cations on the adjacent sites. However, the probability of two Mn⁴⁺ cations occurring on the adjacent sites on MnO_x/TiO₂ obviously decreased after CeO₂ addition, although the oxidation ability of MnO_x/TiO₂ increased. Therefore, the rate of N₂O formation during NO reduction over MnO_x/TiO₂ did not vary notably after CeO₂ addition. As a result, both the SCR activity and N₂ selectivity of NO reduction over MnO_x/TiO₂ improved after CeO₂ addition.

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1. Introduction

The removal of nitrogen oxides (NO and NO₂), which originate from automobile exhaust gas and industrial combustion of fossil fuels, has been a major challenge for air pollution control.^1–3 Selective catalytic reduction (SCR) using V₂O₅–WO₃(MoO₃)/TiO₂ as the catalyst and NH₃ as the reductant is the major technology used to control NO_x emission from coal-fired power plants.⁴ The temperature window of V₂O₅–WO₃(MoO₃)/TiO₂ is 300–400 °C;⁵ thus, it is located upstream of the electrostatic precipitator. However, it is very difficult to retrofit the SCR units into many existing power plants due to the limitation of space and access upstream of the electrostatic precipitator.⁶ Therefore, there has been a significant demand for a low temperature SCR catalyst, which is placed downstream of the electrostatic precipitator and desulfurizer.⁷ Recently, MnO_x/TiO₂ has been widely studied as a low temperature SCR catalyst.^8–11 However, a lot of N₂O is formed from the non-selective catalytic reduction (NSCR) during NO reduction over MnO_x/TiO₂. There is a general agreement that the low temperature SCR reaction over MnO_x/TiO₂ mainly follows two mechanisms. One mechanism is the Eley–Rideal mechanism (i.e. the reaction of gaseous NO with adsorbed NH₃ species), and the other mechanism is the Langmuir–Hinshelwood mechanism (i.e. the reaction of adsorbed NO_x with adsorbed NH₃ species on the adjacent sites).^12,13 Both the Eley–Rideal mechanism and the Langmuir–Hinshelwood mechanism contribute to N₂O and N₂ formation.¹³ Furthermore, many researches demonstrated that CeO₂ addition showed a remarkable improvement on the SCR activity, N₂ selectivity and SO₂ tolerance of MnO_x/TiO₂.^6,14–19 Wu et al.¹⁹ and Lee et al.¹⁷ both regarded that the improvement of the SCR activity of MnO_x/TiO₂ after CeO₂ addition was attributed to the increase in the redox property and acidity. However, the increase in N₂ selectivity, due to CeO₂ addition, cannot be well interpreted.

In our previous paper, a novel steady-state kinetic study was developed to simultaneously determine the reaction rate constant of the SCR reaction through the Langmuir–Hinshelwood mechanism, that of the SCR reaction through the Langmuir–Hinshelwood mechanism, and that of the NSCR reaction.²⁰ Then, the mechanism of H₂O effect on the low temperature SCR reaction over Mn–Fe spinel and MnO_x–CeO₂ were discovered after comparing the reaction rate constants.²¹ In this work, a new insight into the effects of CeO₂ addition on the low temperature SCR reaction over MnO_x/TiO₂ was drawn after comparing the reaction kinetic constants of the SCR reaction and the NSCR reaction over MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂. Furthermore, the relationship between the reaction kinetic constants and the catalyst properties were built according to the kinetic study.

2. Experimental

2.1 Catalytic preparation

MnO_x/TiO₂ (Mn loading was 10 wt%), CeO₂/TiO₂ (Ce loading was 2 wt%) and MnO_x–CeO₂/TiO₂ (Mn loading and Ce loading were 10 wt% and 2 wt%, respectively) were prepared by the impregnation method using Degussa TiO₂ P25 as support, and manganese nitrate and/or cerium nitrate as precursors. The samples were dried at 110 °C for 12 h, and they were then calcined at 500 °C under air atmosphere for 3 h.

2.2 Characterization

BET surface area was determined using a nitrogen adsorption apparatus (Quantachrome, Autosorb-1). XRD patterns were recorded on an X-ray diffractometer (Bruker-AXS D8 Advance) between 20° and 80° at a step of 7° min⁻¹ operating at 30 kV and 30 mA using Cu Kα radiation. H₂-temperature programmed reduction (H₂-TPR) was recorded on a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx). Temperature programmed desorption of ammonia (NH₃-TPD) and temperature programmed desorption of NO (NO-TPD) were carried out on the packed-bed microreactor. Mn 2p, Ce 3d, Ti 2p and O 1s binding energies were recorded on an X-ray photoelectron spectrometer (Thermo, ESCALAB 250) with Al Kα (hv = 1486.6 eV) as the excitation source and C 1s line at 284.8 eV as the reference for the binding energy calibration.

2.3 Catalytic test

The catalytic reaction was performed on a fixed-bed quartz tube reactor with an internal diameter of 6 mm. The mass of the catalyst with a particle diameter of 40–60 mesh was 250 mg. The total flow rate was 200 mL min⁻¹ (room temperature), and the corresponding gas hourly space velocity (GHSV) was 4.8 × 10⁴ cm³ g⁻¹ h⁻¹ (i.e. 40 000 h⁻¹). The feed contained 500 ppm of NO (when used), 500 ppm of NH₃ (when used), 2% of O₂, 5% of H₂O (when used) with balanced N₂. The concentrations of NO, NO₂, NH₃ and N₂O in the outlet were continually monitored using a Fourier transform infrared spectrometer (FTIR, Thermo SCIENTIFIC, ANTARIS, IGS Analyzer).

2.4 In situ DRIFT study

In situ DRIFT spectra were performed on another FTIR spectrometer (Nicolet NEXUS 870) equipped with a liquid-nitrogen-cooled MCT detector, collecting 32 scans with a resolution of 4 cm⁻¹. The diffuse reflectance FTIR measurements were carried out in situ in a high-temperature cell, fitted with ZnSe windows. The catalyst was finely ground and placed in a ceramic crucible and manually pressed. Prior to each experiment, the catalyst was heated to 300 °C under an N₂ atmosphere with a flow rate of 100 mL min⁻¹ for 60 min.

2.5 Reaction kinetic study

To obtain the reaction kinetic constants of NO reduction over MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂, a steady state kinetic study was performed. Gaseous NH₃ concentration in the inlet was maintained at 500 ppm, while gaseous NO concentration varied from 200 to 500 ppm. To overcome the diffusion limitation (including inner diffusion and external diffusion), a very high GHSV of 1 200 000 cm³ g⁻¹ h⁻¹ (the catalyst mass was 20 mg, and the total flow rate was 400 mL min⁻¹) was adopted to obtain a less than 20% of the NO_x conversion.

3. Results

3.1 Catalytic performance

CeO₂/TiO₂ showed a poor SCR activity at low temperatures (shown in Fig. 1a). However, it showed an excellent N₂ selectivity and slight N₂O formation during the low temperature SCR reaction over CeO₂/TiO₂ (hinted by Fig. 1b). Both NO conversion and N₂O selectivity over MnO_x/TiO₂ obviously increased with the increase in reaction temperature (shown in Fig. 1). After CeO₂ addition, NO reduction over MnO_x/TiO₂ was obviously promoted, while N₂O selectivity remarkably decreased (shown in Fig. 1).

Fig. 1 Effect of Ce addition on the SCR reaction over MnO_x/TiO₂: (a), NO_x conversion; (b), N₂O selectivity. Reaction conditions: [NH₃] = [NO] = 500 ppm, [O₂] = 2%, catalyst mass = 250 mg, total flow rate = 200 mL min⁻¹ and GHSV = 4.8 × 10⁴ cm³ g⁻¹ h⁻¹.

H₂O is one of the main components in the flue gas, which strongly inhibits NO reduction and N₂O formation during the low temperature SCR reaction.⁶ The low temperature SCR activities of MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂ both obviously decreased after the introduction of H₂O (shown in Fig. 2a). Fig. 2a also shows that the SCR activity of MnO_x–CeO₂/TiO₂ was still considerably better than that of MnO_x/TiO₂ in the presence of H₂O. When 5% of H₂O was introduced, N₂O formation over MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂ were suppressed, and N₂O selectivity of NO reduction over MnO_x–CeO₂/TiO₂ was slightly less than that over MnO_x/TiO₂ (shown in Fig. 2b). This suggests that CeO₂ addition showed a notable promotion for the SCR reaction over MnO_x/TiO₂, which was consistent with the results of previous studies.^17,19

Fig. 2 Effect of Ce addition on the SCR reaction over MnO_x/TiO₂ in the presence of H₂O: (a), NO_x conversion; (b), N₂O selectivity. Reaction conditions: [NH₃] = [NO] = 500 ppm, [O₂] = 2%, [H₂O] = 5%, catalyst mass = 250 mg, total flow rate = 200 mL min⁻¹ and GHSV = 4.8 × 10⁴ cm³ g⁻¹ h⁻¹.

3.2 Characterization

3.2.1 XRD and BET. XRD patterns of TiO₂ (P25), CeO₂/TiO₂, MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂ are shown in Fig. 3. The reflections of both anatase (JCPDS, PDF 21-1272) and rutile (JCPDS, PDF 21-1276) appeared in the XRD pattern of TiO₂ (P25), and anatase predominated in the XRD pattern.²² After the loading of MnO_x and/or CeO₂, no additional characteristic peaks appeared (shown in Fig. 3). This suggests that MnO_x and/or CeO₂ were well dispersed on P25.

Fig. 3 XRD patterns of P25, CeO₂/TiO₂, MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂.

BET surface areas of P25, CeO₂/TiO₂, MnO_x/TiO₂, and MnO_x–CeO₂/TiO₂ were 33.6, 34.2, 37.3 and 34.5 m² g⁻¹, respectively.

3.2.2 H₂-TPR. Fig. 4 shows the TPR profiles of MnO_x–CeO₂/TiO₂, MnO_x/TiO₂ and CeO₂/TiO₂. CeO₂/TiO₂ shows no obvious reduction peak in the range 30–800 °C.²³ MnO_x/TiO₂ shows two obvious reduction peaks. The peak at 350 °C was related to the reduction of highly dispersed and easily reducible MnO₂ species. The peak at 426 °C was assigned to the reduction of Mn₃O₄.²⁴ After the addition of CeO₂, a strong displacement of the first reduction peak to lower temperatures occurred in the TPR profiles (shown in Fig. 4). This suggests that the strength of the Mn–O bond had been lowered due to the addition of CeO₂, resulting in an easier detachment of oxygen. Therefore, the oxidizing ability of MnO_x–CeO₂/TiO₂ was higher than that of MnO_x/TiO₂.

Fig. 4 H₂-TPR profiles of CeO₂/TiO₂, MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂.

3.2.3 XPS. The ratios of Mn, Ce, Ti and O species on MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂, which were obtained from XPS spectra (shown in Fig. S1 in the ESI†), are listed in Table 1. The ratios of Mn to Ti on the surfaces of MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂ were both approximately 0.2, which were considerably higher than those added during the preparation (approximately 0.10). Moreover, the ratio of Ce to Ti on the surface of MnO_x–CeO₂/TiO₂ was 0.09, which was considerably higher than that added during the preparation (approximately 0.01). This suggests that MnO_x and/or CeO₂ were well dispersed on P25, which was consistent with the XRD analysis (shown in Fig. 3). Moreover, Table 1 shows that the Mn⁴⁺ concentration on MnO_x/TiO₂ decreased after the addition of CeO₂.

Table 1 Ratios of Mn, Ti, Ce and O species on MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂ (%)

		MnOx/TiO2	MnOx–CeO2/TiO2
Ti^4+		27.9	26.3
O^2−		65.9	65.8
Mn	Mn^2+	1.4	0.9
	Mn^3+	2.1	2.6
	Mn^4+	2.7	2.1
Ce	Ce^3+	—	0.9
	Ce^4+	—	1.4

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3.2.4 Adsorption of NH₃ and NO. The adsorption capacities of MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂ for the adsorption of NH₃ and NO can be calculated from NH₃-TPD and NO-TPD profiles (shown in Fig. S2 in the ESI†). As shown in Table 2, the adsorption capacities of MnO_x–CeO₂/TiO₂ for the adsorption of NH₃ and NO were both close to those of MnO_x/TiO₂. This suggests that the adsorption of NH₃ and NO on MnO_x/TiO₂ was not affected notably by the addition of CeO₂.

Table 2 Adsorption capacities of MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂ for NH₃ and NO adsorption at 50 °C (mmol g⁻¹)

	NH3	NO
MnOx/TiO2	1.62	0.70
MnOx–CeO2/TiO2	1.56	0.71

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3.3 Transient reaction

3.3.1 MnO_x/TiO₂. Fig. 5a shows in situ DRIFT spectra taken at 150 °C upon passing NO + O₂ over NH₃ pretreated MnO_x/TiO₂. After the adsorption of NH₃, three characteristic vibrations at 1597, 1221 and 1165 cm⁻¹ appeared on MnO_x/TiO₂ that were assigned to coordinated NH₃ bound to the Lewis acid sites.¹² After NO + O₂ was introduced, the bands corresponding to coordinated NH₃ rapidly diminished. This suggests that the reaction of adsorbed NH₃ species with gaseous NO (i.e. the Eley–Rideal mechanism) contributed to NO reduction over MnO_x/TiO₂. The N₂O concentration in the outlet rapidly increased to about 63 ppm (shown in Fig. 6a), and then it gradually decreased to about 6 ppm (the background of N₂O in gaseous NO) as NO + O₂ passed over NH₃ pretreated MnO_x/TiO₂. This suggests that the NO reduction over MnO_x/TiO₂ through the Eley–Rideal mechanism contributed to N₂O formation. Finally, seven characteristic vibrations at 1611, 1584, 1538, 1493, 1290, 1266 and 1244 cm⁻¹ appeared on MnO_x/TiO₂. The bands at 1611 and 1290 cm⁻¹ were assigned to monodentate nitrite,¹³ the band at 1493 cm⁻¹ was attributed to monodentate nitrate,⁶ and the bands at 1584, 1538, 1266 and 1244 cm⁻¹ were assigned to bidentate nitrate.¹⁸

Fig. 5 (a), DRIFT spectra taken at 150 °C upon passing NO + O₂ over NH₃ pretreated MnO_x/TiO₂; (b), DRIFT spectra taken at 150 °C upon passing NH₃ over MnO_x/TiO₂ pretreated by NO + O₂.

Fig. 6 (a), Transient reaction taken at 150 °C upon passing NO + O₂ over NH₃ pretreated MnO_x/TiO₂; (b), transient reaction taken at 150 °C upon passing NH₃ over MnO_x/TiO₂ pretreated by NO + O₂.

After the adsorption of NO + O₂, MnO_x/TiO₂ was mainly covered by monodentate nitrite (at 1611 and 1290 cm⁻¹), bidentate nitrate (at 1584, 1538, 1266 and 1244 cm⁻¹) and monodentate nitrate (at 1493 cm⁻¹). As gaseous NH₃ was introduced, the bands at 1611 and 1290 cm⁻¹ corresponding to monodentate nitrite and the band at 1493 cm⁻¹ corresponding to monodentate nitrate gradually diminished (shown in Fig. 5b). This suggests that the reaction of adsorbed NH₃ species with adsorbed monodentate nitrite and monodentate nitrate (i.e. the Langmuir–Hinshelwood mechanism) contributed to NO reduction over MnO_x/TiO₂. However, the bands at 1584, 1538, 1266 and 1244 cm⁻¹ corresponding to bidentate nitrate shifted to 1576, 1501, 1280 and 1262 cm⁻¹ as they were bound with adsorbed ammonia.²⁵ However, these bands did not diminish with the further introduction of NH₃/N₂. This indicates that the bidentate nitrate route did not contribute to NO reduction over MnO_x/TiO₂. A similar phenomenon was once reported on MnO_x–CeO₂ by Qi and Yang.²⁶ At last, coordinated NH₃ (at 1201 and 1597 cm⁻¹) appeared on MnO_x/TiO₂. Fig. 7b shows that some N₂O formed during NH₃ passing over NO + O₂ pretreated MnO_x/TiO₂. There is a general agreement that the reaction product of the adsorbed NO₃⁻ with adsorbed NH₃ was N₂O.¹³ Therefore, the reaction of monodentate nitrate with adsorbed NH₃ (i.e. the Langmuir–Hinshelwood mechanism) contributed to N₂O formation over MnO_x/TiO₂. Fig. 6 also shows that the amount of N₂O formed during the induction of NO + O₂ to NH₃ pretreated MnO_x/TiO₂ was considerably higher than that formed during the induction of NH₃ to NO + O₂ pretreated MnO_x/TiO₂. This suggests that the N₂O formed during NO reduction over MnO_x/TiO₂ mainly resulted from the Eley–Rideal mechanism.

Fig. 7 (a), DRIFT spectra taken at 150 °C upon passing NO + O₂ over NH₃ pretreated MnO_x–CeO₂/TiO₂; (b), DRIFT spectra taken at 150 °C upon passing NH₃ over MnO_x–CeO₂/TiO₂ pretreated by NO + O₂.

3.3.2 MnO_x–CeO₂/TiO₂. In situ DRIFT spectra obtained from the introduction of NO + O₂ to MnO_x–CeO₂/TiO₂ pretreated by NH₃ and the introduction of NH₃ to MnO_x–CeO₂/TiO₂ pretreated by NO + O₂ (shown in Fig. 7) were similar to those for MnO_x/TiO₂ (shown in Fig. 5). This suggests that the reaction mechanism of NO reduction over MnO_x/TiO₂ did not vary after the addition of CeO₂.

4. Discussion

4.1 Reaction mechanism

In situ DRIFT spectra study demonstrated that the reaction mechanism of NO reduction over MnO_x/TiO₂ did not vary after CeO₂ addition, and both the Eley–Rideal mechanism and the Langmuir–Hinshelwood mechanism contributed to NO reduction (including N₂O and N₂ formation) over MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂.

NO reduction over MnO_x/TiO₂ through the Langmuir–Hinshelwood mechanism can be approximately described as follows:^12,13,24

NH_3(g) → NH_3(ad) (1)

NO_(g) → NO_(ad) (2)

Mn⁴⁺=O + NO_(ad) → Mn³⁺–O–NO (3)

NH_3(ad) + Mn³⁺–O–NO → Mn³⁺–O–NO–NH₃ → Mn³⁺–OH + N₂ + H₂O (5)

Mn³⁺–O–NO₂ + NH_3(ad) → Mn³⁺–O–NO₂–NH₃ → Mn³⁺–OH + N₂O + H₂O (6)

Reaction 1 is the adsorption of gaseous NH₃ on the surface to form coordinated NH₃. There is a general agreement that the SCR reaction starts with the adsorption of NH₃.¹³ Reaction 2 is the physical adsorption of gaseous NO on the surface.^13,27 Physically adsorbed NO can be oxidized by Mn⁴⁺ on the surface to monodentate nitrite and monodentate nitrate (i.e. Reactions 3 and 4). Then, the adsorbed monodentate nitrite reacted with adsorbed NH₃ to form NH₄NO₂, which was then decomposed to N₂ (i.e. Reaction 5). Moreover, monodentate nitrate can react with adsorbed NH₃ to form NH₄NO₃, which was then decomposed to N₂O (i.e. Reaction 6).¹³

NO reduction over MnO_x/TiO₂ through the Eley–Rideal mechanism can be approximately described as follows:^12,13,25,28

NH_3(ad) + Mn⁴⁺=O → NH₂ + Mn³⁺–OH (7)

NH₂ + Mn⁴⁺=O → NH + Mn³⁺–OH (8)

NH₂ + NO_(g) → N₂ + H₂O (9)

NH + NO_(g) + Mn⁴⁺=O → N₂O + Mn³⁺–OH (10)

Adsorbed NH₃ can be activated to NH₂ (i.e. Reaction 7), which can be further oxidized to NH (i.e. Reaction 8). There is a general agreement that the reaction products of gaseous NO with NH₂ and NH (i.e. Reactions 9 and 10) are N₂ and N₂O, respectively.¹³

4.2 Reaction kinetic analysis

The kinetic equation of NO reduction through the Langmuir–Hinshelwood mechanism (i.e. Reactions 5 and 6) can be approximately described as follows:where, k₁, k₂, [Mn³⁺–O–NO–NH₃], and [Mn³⁺–O–NO₂–NH₃] are the decomposition rate constants of NH₄NO₂ and NH₄NO₃, and the concentrations of NH₄NO₂ and NH₄NO₃ on MnO_x/TiO₂, respectively. Our previous study demonstrated that the concentrations of NH₄NO₂ and NH₄NO₃ on MnO_x/TiO₂ at the steady state were independent of the concentrations of gaseous NH₃ and NO.^12,20,29 Therefore, the reaction orders of N₂ and N₂O formation through the Langmuir–Hinshelwood mechanism with respect to gaseous NO concentration were both nearly 0.

The kinetic equation of NO reduction through the Eley–Rideal mechanism (i.e. Reactions 9 and 10) can be described as follows:

where, k₃, k₄, [NH₂], [NH], [Mn⁴⁺=O] and [NO_(g)] are the reaction rate constants of reactions 9 and 10, the concentrations of NH₂, NH and Mn⁴⁺ on the surface and gaseous NO concentration, respectively.

The reaction kinetic equation of NH₂ and NH formation on the surface (i.e. Reactions 7 and 8) can be described as follows:

where, k₅ and k₆ were the reaction kinetic constants of reactions 7 and 8, respectively.

According to eqn (14) and (16), the variation of NH concentration can be described as follows:

As the reaction reached the steady state, NH concentration did not vary. Therefore,

Hence,

Then, eqn (14) can be transformed as follows:

Our previous study demonstrated that NH₂ on the surface was independent of the concentrations of gaseous NO and NH₃ at the steady state,^20,29 which was mainly related to k₅, the concentrations of NH₃ adsorbed and Mn⁴⁺ on the surface (hinted by eqn (15)). Therefore, the reaction orders of N₂ and N₂O formation through the Eley–Rideal mechanism with respect to gaseous NO concentration were nearly 1 and 0, respectively, (hinted by eqn (13) and (20)).

Taking account of both the Langmuir–Hinshelwood mechanism and the Eley–Rideal mechanism, the rate of NO reduction and N₂O formation can be approximately described as follows:

where, k_NO, k_SCR-ER, k_SCR-LH and k_NSCR are the rates of NO reduction, the reaction rate constant of the SCR reaction (i.e. N₂ formation) through the Eley–Rideal mechanism, the reaction rate constant of the SCR reaction through the Langmuir–Hinshelwood mechanism, and the reaction rate constant of the NSCR reaction (i.e. N₂O formation), respectively.

To determine k_SCR-ER, k_SCR-LH and k_NSCR, a steady-state kinetic study was conducted (shown in Fig. 8). As shown in Fig. 8a and b, there is an excellent linear relationship between and the rate of NO reduction and gaseous NO concentration, which is consistent with eqn (21). Moreover, Fig. 8c and d show that the rate of formation of N₂O did not vary notably with the increase in gaseous NO concentration. This suggests that the reaction order of N₂O formation with respect to the gaseous NO concentration was nearly zero, which was consistent with eqn (22). Therefore, k_NSCR can be obtained directly from Fig. 8c and d, and k_SCR-ER and k_SCR-LH can be obtained after the linear regression of Fig. 8a and b (the slope is k_N2(E–R) and the intercept is the sum of k_SCR-LH and k_NSCR, which are listed in Table 3.

Fig. 8 Dependence of the rate of NO conversion on gaseous NO concentration during the SCR reaction over: (a), MnO_x/TiO₂; (b), MnO_x–CeO₂/TiO₂. Dependence of the rate of N₂O formation on gaseous NO concentration during the SCR reaction over: (c), MnO_x/TiO₂; (d), MnO_x–CeO₂/TiO₂. Reaction conditions: [NH₃] = 500 ppm, [NO] = 200–500 ppm, [O₂] = 2%, catalyst mass = 20 mg, total flow rate = 400 mL min⁻¹ and GHSV = 1 200 000 cm³ g⁻¹ h⁻¹.

Table 3 The reaction kinetic constants of NO reduction over MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂/μmol g⁻¹ min⁻¹

	Temperature/°C	kSCR(E-R)/10^6	kSCR(L–H)	kNSCR	R^2	kNO ([NH3]=[NO] = 500 ppm)
	120	0.016	1.9	1.4	0.999	11.3
	140	0.022	3.8	2.6	0.991	17.4
MnOx/TiO2	160	0.029	0.1	7.1	0.991	21.7
	180	0.030	3.1	14	0.995	32
	200	0.062	0	19	0.996	50
	120	0.024	5.0	2.5	0.979	19.5
	140	0.043	5.7	5.5	0.986	32.7
MnOx–CeO2/TiO2	160	0.068	3.0	8.6	0.993	45.6
	180	0.068	6.9	12	0.988	52.9
	200	0.088	2.8	18	0.990	64.8

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Table 3 shows that k_NO of MnO_x/TiO₂ obviously increased after CeO₂ addition, resulting in a remarkable promotion of NO reduction. The result was consistent with the result of Fig. 1a. Moreover, Table 3 indicates that the ratio of k_SCR-LH to k_NO is generally less than 20% over MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂. Furthermore, Fig. 7 shows that the amount of N₂O formed from the Eley–Rideal mechanism is considerably higher than that formed from the Langmuir–Hinshelwood mechanism (shown in Fig. 7). These findings suggest that NO reduction over both MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂ was mainly attributed to the Eley–Rideal mechanism, and only a small amount of the NO reduced was related to the Langmuir–Hinshelwood mechanism.

k_SCR-ER of MnO_x/TiO₂ increased remarkably after the addition of CeO₂ (shown in Table 3). This indicates that the SCR reaction over MnO_x/TiO₂ (i.e. N₂ formation) through the Eley–Rideal mechanism was obviously promoted after CeO₂ addition. However, k_NSCR of MnO_x/TiO₂ did not vary notably after CeO₂ addition. This suggests that the NSCR reaction (i.e. N₂O formation) over MnO_x/TiO₂ was not promoted after CeO₂ addition. Because N₂ formation over MnO_x/TiO₂ was promoted after CeO₂ addition and N₂O formation did not vary notably, N₂O selectivity of NO reduction over MnO_x/TiO₂ was considerably higher than that over MnO_x–CeO₂/TiO₂ (shown in Fig. 1b).

4.3 Mechanism of CeO₂ addition

According to eqn (21) and (22), k_SCR-ER and k_NSCR can be described as follows:

k_SCR-ER = k₃[NH₂] (23)

k_NSCR = k₆[NH₂][Mn⁴⁺=O] + k₂[Mn³⁺–O–NO₂–NH₃] ≈ k₆[NH₂][Mn⁴⁺=O] (24)

After the addition of CeO₂, the oxidation ability of MnO_x/TiO₂ obviously increased (hinted by the TPR analysis). This suggests that k₅ of MnO_x–CeO₂/TiO₂ was considerably higher than that of MnO_x/TiO₂. Moreover, the concentration of NH₃ adsorbed on MnO_x–CeO₂/TiO₂ was close to that of MnO_x/TiO₂ (shown in Table 2). Although the concentration of Mn⁴⁺ on MnO_x–CeO₂/TiO₂ was less than that on MnO_x/TiO₂, the NH₂ concentration on MnO_x–CeO₂/TiO₂ would be considerably higher than that on MnO_x/TiO₂ (hinted by eqn (15)). As a result, k_SCR-ER of MnO_x/TiO₂ obviously increased after CeO₂ addition (hinted by eqn (23)), which is demonstrated in Table 3.

The oxidation ability of MnO_x/TiO₂ increased after CeO₂ addition, thus k₆ of MnO_x–CeO₂/TiO₂ was higher than that of MnO_x/TiO₂. Adsorbed NH₃ was first oxidized by Mn⁴⁺ on the sites adjacent to NH₂, and further oxidation of NH₂ to NH needed another Mn⁴⁺ on the adjacent sites. Therefore, the oxidation of adsorbed NH₃ to NH needed two Mn⁴⁺ cations on the adjacent sites. After CeO₂ addition, the concentration of Mn⁴⁺ and Mn species on MnO_x/TiO₂ both decreased (shown in Table 1). Moreover, 2.3% of the Ce species was well dispersed on P25. Therefore, the probability of two Mn⁴⁺ cations on the adjacent sites on MnO_x–CeO₂/TiO₂ was considerably lower than that on MnO_x/TiO₂. This suggests that the concentration of Mn⁴⁺ on MnO_x–CeO₂/TiO₂ for the oxidation of NH₂ to NH was considerably less than that on MnO_x/TiO₂. Although both k₆ and NH₂ of/on MnO_x–CeO₂/TiO₂ were higher than those of/on MnO_x/TiO₂, the amount of Mn⁴⁺ on MnO_x–CeO₂/TiO₂ for the activation of NH₂ to NH was considerably less than that on MnO_x/TiO₂. Therefore, k_NSCR of MnO_x/TiO₂ did not vary remarkably after CeO₂ addition (shown in Table 3).

5. Conclusion

The Eley–Rideal mechanism dominated the NO reduction (including the SCR reaction and the NSCR reaction) over MnO_x/TiO₂ and MnO_x–CeO₂/TiO₂. N₂ formation over MnO_x/TiO₂ through the Eley–Rideal mechanism was promoted after CeO₂ addition due to the improvement in the oxidation ability. However, N₂O formation over MnO_x/TiO₂ through the Eley–Rideal mechanism did not vary notably after CeO₂ addition. As a result, the SCR performance of NO reduction over MnO_x/TiO₂ (including the SCR activity and N₂ selectivity) improved after CeO₂ addition.

Acknowledgements

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

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Footnote

† Electronic supplementary information (ESI) available: XPS spectra, NH₃-TPD and NO-TPD profiles, and NH₃ oxidation and NO oxidation. See DOI: 10.1039/c5ra01767j

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