The enhanced performance of a CeSiO_x support on a Mn/CeSiO_x catalyst for selective catalytic reduction of NO_x with NH₃

Abstract

A series of Mn/CeSiO_x catalysts were prepared by the wet impregnation method and used for selective catalytic reduction of NO with NH₃. As can be seen from the experimental results, the Mn/CeSiO_x catalyst with a Ce/Si molar ratio of 2/1 showed excellent low-temperature SCR activity, high N₂ selectivity and excellent SO₂ and H₂O tolerance. The relationship between the CeSiO_x support and the SCR performance of Mn/CeSiO_x (2 : 1) catalyst was investigated based on the characterization results of N₂ adsorption, XRD, XPS, H₂-TPR, NH₃-TPD and in situ DRIFT. The strong interaction between Ce and Si resulted in the good dispersion of Mn species on the support; correspondingly, the redox ability and NH₃ adsorption capacity were greatly enhanced. The results of in situ DRIFT study revealed that the NH₃-SCR reactions over Mn/CeO₂ and Mn/CeSiO_x (2 : 1) mainly obeyed both the E–R mechanism and the L–H mechanism. Furthermore, the formation of more Mn⁴⁺ and chemisorbed oxygen greatly facilitates the oxidation of NO to NO₂, as a result, promoting the low-temperature SCR performance of Mn/CeSiO_x (2 : 1).

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Introduction

Several environmental problems such as acid rain, photochemical smog and haze are closely related to the emission of NO_x from the combustion process of fossil fuels from stationary sources and mobile sources.^1–3 In the past several decades, several techniques have been developed and applied for the control of NO_x emission. Among them, the selective catalytic reduction of NO_x with NH₃ using vanadium-based catalyst has been put into industrial application for reducing NO_x from stationary sources such as coal-fired power plants for about 40 years.⁴ This catalyst exhibits high activity in the narrow temperature window of 300–400 °C.⁵ However, several inevitable drawbacks are still associated with it including the toxicity of vanadium species, high conversion of SO₂ to SO₃ and the deactivation by alkali metals in the fly ash.^6–10 Therefore, it is of great demand to develop vanadium-free catalysts with high activities at low temperatures.

In recent years, manganese-based catalyst has received much attention due to the presence of various types of labile oxygen in MnO_x, which could promote the catalytic cycle in SCR reactions.^11,12 In previous studies, MnO_x supported on different carriers such as TiO₂,^13,14 Al₂O₃,¹⁵ carbon nanotube^12,16 and palygorskite¹⁷ exhibited excellent low-temperature SCR performances. However, the deactivation of manganese-based catalyst by SO₂ is still a barrier to its industrial application.¹⁸ It is generally accepted that the support has a crucial impact on the performance of SCR catalyst. For the past few years, some carriers based on composite oxides such as TiO₂–Al₂O₃,¹⁹ CeO₂–ZrO₂,^20–22 TiO₂–SiO₂,²³ CeTiO_x (ref. 24) and TiWO_x (ref. 25) have been successfully applied as the support of SCR catalysts to enhance their SCR performances. Therefore, MnO_x catalyst supported on composite oxides may be a competitive candidate for future industrial application. In this study, Mn/CeSiO_x catalyst was synthetized and used in NH₃-SCR reaction. The experimental results revealed that the Mn/CeSiO_x catalyst with a Ce/Si molar ratio of 2/1 showed excellent low-temperature SCR activity, high N₂ selectivity and excellent SO₂ and H₂O tolerance. The promotion mechanism of CeSiO_x support on its SCR performance was also investigated based on the characterization results.

Experimental

All chemicals used in this study were of analytical grade and supplied by Aladdin Reagent Inc., China. Firstly, the CeSiO_x support was prepared by citric acid method. Cerium nitrate (CN), tetraethyl orthosilicate (TEOS) and citric acid (CA) were mixed in the desired molar proportions, and the mole ratio of CA/(Si + Ce) was set as 1.0 (Ce/Si molar ratio = 2 : 1; 1 : 1 and 1 : 2 respectively). After stirred at room temperature for 1 h, the mixture was dried at 100 °C for 12 h, a foam-like solid would come into being. Then the foam-like solid was calcined in air at 500 °C for 5 h to obtain the final CeSiO_x support. Similarly, the pure CeO₂ and SiO₂ supports were prepared by using CN and TEOS as the precursors of Ce and Si respectively.

The Mn/CeSiO_x catalyst was prepared by the impregnation method. Firstly, the CeSiO_x support (3 mL) was impregnated by incipient wetness with an aqueous solution of manganese nitrate (the mass ratio of Mn/CeSiO_x = 0.15). After that, the sample was dried at 100 °C for 12 h, followed by calcination in air at 500 °C for 5 h. Based on the same method, Mn/CeO₂ and Mn/SiO₂ catalyst samples with a Mn/support mass ratio of 0.15 were prepared and used as the counterparts. For convince, the Mn/CeSiO_x catalyst was denoted as Mn/CeSiO_x(y), where y represents the molar ratio between Ce and Si.

Nitrogen adsorption measurements were performed on a Quantachrome Autosorb-iQ-AG instrument based on nitrogen gas sorption at 77 K and analyzed by Quantachrome AS1Win software (Quantachrome Instruments Version 2.01). Prior to the experiments, the samples were heated to 200 °C under vacuum for 1 h. The specific surface area was evaluated by the Brunauer–Emmett–Teller (BET) method. And the pore volumes and average pore diameters were determined by the Barrett–Joyner–Halenda (BJH) method from the desorption branches of the isotherms.

To determine the crystal structures of the catalyst samples, X-ray powder diffraction patterns were collected by a Bruker D8 Advance diffractometer using a graphite monochrometer with Cu Kα radiation (λ = 0.15406 nm). The 2θ scans cover the range of 20–80°.

To evaluate the chemical states for all the elements on the surfaces of the catalyst samples, X-ray photoelectron spectroscopy was performed on a Thermo ESCALAB 250 spectrometer. An Al Kα monochromatized radiation (hν = 1486.6 eV) was employed as the X-ray source. Survey spectra were recorded with a pass energy of 100 eV and high resolution spectra of the XPS O 1s–V 2p, C 1s and valence band region were recorded with a pass energy of 20 eV. The shift of the binding energy was calibrated on the basis of C 1s level at 284.8 eV.

Raman spectra were recorded on a Renishaw inVia Raman Microscope (Renishaw, Britain) with Ar⁺ radiation (514 nm) and the laser power of 20 mW.

Temperature-programmed reduction of H₂ (H₂-TPR) and temperature-programmed desorption of NH₃ (NH₃-TPD) were carried out on an Autosorb-iQ-C TPR/TPD automatic chemisorption analyzer (Quantachrome Instruments, USA) using 50 mg catalyst samples. Prior to the TPR experiments, the catalyst sample was pretreated at 400 °C in He for 1 h. Then the TPR runs were carried out with linear heating rate (10 °C min⁻¹) up to 700 °C in pure Ar containing 5% H₂ at a flow rate of 40 mL min⁻¹. For NH₃-TPD experiments, the samples were also pretreated in He at 400 °C for 1 h. Then the samples were purged under 500 ppm NH₃ at a flow rate of 30 mL min⁻¹ for 30 min. Then the desorption was carried out by heating the catalyst sample in He (30 mL min⁻¹) form 100 to 750 °C. The signal of H₂ or NH₃ was recorded by a thermal conductivity detector (TCD).

The in situ DRIFT measurements were performed on a FTIR spectrometer (Nicolet iS50, Thermo, USA) equipped with a smart collector and an MCT/A detector cooled by liquid nitrogen. In the DRIFT cell (ZnSe window) connected with a gas flow system, the samples preheated at 500 °C for 2 h in N₂ flow and cooled down to 200 °C to get a background spectrum. Then the background spectra were automatically subtracted from the sample spectrum. During the in situ DRIFT measurements, the gas mixture contains 500 ppm NH₃, or/and 500 ppm NO + 5% O₂, balance N₂, with a flow rate of 300 mL min⁻¹. All the spectra were recorded by accumulating 100 scans with a resolution of 4 cm⁻¹. The infrared spectra in the region of 1100–2000 cm⁻¹ were recorded.

The SCR activities of the catalyst samples were measured at atmospheric pressure in a fixed-bed continuous flow quartz reactor (i.d. = 8 mm). In each experimental run, about 0.55 mL catalyst sample was used. The reactant gas contains 600 ppm NH₃, 600 ppm NO, 5% O₂, 100 ppm SO₂ (when used), 5% H₂O (when used) and balance Ar, with a total flow rate of 1 L min⁻¹. So the gas hourly space velocity (GHSV) was about 108 000 h⁻¹. The concentrations of NO and NO₂ in the effluent gas stream were monitored by a Thermo NO-NO_x chemiluminescence analyzer (Model 42i-HL). The concentration of NH₃ in the outlet gas stream was measured by a NH₃ analyzer (Model 17i). And the concentration of N₂O was analyzed by a Nicolet iS50 spectrometer equipped with a gas cell with 0.2 L volume.

After the SCR reaction reached a steady state, the values of NO_x conversion and N₂ selectivity could be obtained accordingly:^26,27

Results and discussion

As show in Fig. 1(A), the catalyst samples supported on the composite oxides exhibit higher SCR activity than Mn/SiO₂ and Mn/CeO₂, especially in the low temperature range (<250 °C). It seems that the molar ratio of Ce/Si has a great impact on the SCR performance of Mn/CeSiO_x catalyst, and Mn/CeSiO_x (2 : 1) catalyst shows more than 90% NO_x conversion in the temperature range of 150–300 °C under a GHSV of 108 000 h⁻¹. Thus the modification of CeO₂ support by Si could greatly enhance the low-temperature SCR activity of Mn/CeSiO_x (2 : 1) catalyst. Combined with the results of N₂ selectivity (Fig. 1(B)), it can be observed that the composite oxides could also enhance the selectivity; moreover, Mn/CeSiO_x (2 : 1) catalyst is of the highest N₂ selectivity among the five catalyst samples. Therefore, we focus on Mn/CeSiO_x (2 : 1) catalyst in the following study.

Fig. 1 (A) NO_x conversions over the five catalyst samples as a function of reaction temperature. (B) N₂ selectivities over the five catalyst samples as a function of reaction temperature. (C) Normalized reaction rate of NO by BET surface area as a function of reaction temperature. Reaction conditions: [NO] = [NH₃] = 600 ppm, [O₂] = 5%, balance Ar, GHSV = 108 000 h⁻¹.

It is well known that the BET surface of SCR catalyst has a great impact on its catalytic performance. To eliminate the effect of specific surface area on the SCR performance, the reaction rate normalized by BET surface area was calculated by the following equation:^28,29

R_NOx = X_NOxQC_f/(WS_BET) (3)

where R_NOx is the normalized reaction rate of NO_x, mmol s⁻¹ m⁻²; X_NOx is the conversion of NO; Q is the volumetric flow rate of simulated flue gas, L s⁻¹; C_f is the feed concentration of NO, mmol L⁻¹; W is the weight of catalyst sample, g; S_BET is the specific surface area of catalyst sample, m² g⁻¹.

The results of NO reaction rate normalized by BET surface area are shown in Fig. 1(C). As can be seen from Fig. 1(C), the trend of normalized reaction rate is similar with that of NO_x conversions. The reaction rates of catalysts supported on the composite oxides are higher than that of Mn/CeO₂ and Mn/SiO₂, and Mn/CeSiO_x (2 : 1) exhibits the highest reaction rate among the five catalyst samples.

The deactivation effect of SO₂ and H₂O on SCR catalyst has been extensively reported in many studies.^30–34 As presented in Fig. 2, the NO_x conversion over Mn/CeSiO_x (2 : 1) at 250 °C only decreases a little after the introduction of 100 ppm SO₂ and 5% H₂O. In addition, the NO conversion over Mn/CeSiO_x (2 : 1) recovers to a great extent after the cut of SO₂ and H₂O. On the contrary, the NO_x conversion over Mn/CeO₂ decreases a lot in the presence of 100 ppm SO₂ and 5% H₂O, and the SCR activity is nearly unrecoverable after the cut of SO₂ and H₂O. Combined with its good SCR performance and high resistance to SO₂ and H₂O, it can be seen that Mn/CeSiO_x (2 : 1) is a promising deNO_x catalyst for future application.

Fig. 2 Effect of SO₂ and H₂O on the NO_x conversions over Mn/CeSiO_x (2 : 1) and Mn/CeO₂ catalysts at 250 °C. Reaction conditions: [NO] = [NH₃] = 600 ppm, [SO₂] = 100 ppm, [O₂] = [H₂O] = 5%, balance Ar, GHSV = 108 000 h⁻¹.

The results of BET analysis are summarized in Table 1. It is clear that the catalyst samples supported on composite oxides exhibit higher specific surface areas than Mn/CeO₂ and Mn/SiO₂, which should be originated from the strong interaction between Ce and Si in the support. The enhanced BET surface areas of the CeSiO_x-supported catalyst samples are favorable to the adsorption of reactants on them and facilitate the NH₃-SCR reaction. On the contrary, the pore volumes of Mn/CeO₂ and Mn/SiO₂ are larger than that of the CeSiO_x-supported catalyst samples, indicating that the effect of pore volume on BET surface area is very little.

Table 1 The BET analysis results of different catalyst samples

Samples	BET surface area (m^2 g^−1)	Average pore diameter (nm)	Pore volume (cm^3 g^−1)	Mean crystallite size of CeO2 (nm)
Mn/CeSiOx (2 : 1)	89.14	0.155	11.15	8.0
Mn/CeSiOx (1 : 1)	84.60	0.150	12.34	10.1
Mn/CeSiOx (1 : 2)	78.53	0.141	12.09	11.2
Mn/CeO2	58.09	0.121	16.05	16.8
Mn/SiO2	66.34	0.133	14.47	—

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The XRD patterns of the five catalyst samples are illustrated in Fig. 3. Noticeably, only the diffraction peaks of CeO₂ and SiO₂ could be detected, suggesting that Mn species in the catalyst samples are presented in amorphous structure. For Mn/SiO₂, only a very weak peak of SiO₂ could be observed, therefore, its crystallinity is very low. However, its SCR activity is not very high, which may be due to the participation of Ce species in the NH₃-SCR reaction for other samples. Mn/CeO₂ is of the highest crystallinity among the five catalyst samples, corresponding to the lowest SCR activity (Fig. 1(A)). For the catalyst samples with a composite oxides, their crystallinities are in the following order: Mn/CeSiO_x (2 : 1) < Mn/CeSiO_x (1 : 1) < Mn/CeSiO_x (1 : 2). Therefore, the strong interaction between Ce and Si could hinder the formation of CeO₂ crystal, leading to the decrease of its peak intensity. Furthermore, lower ceria crystallinity means the presence of more lattice defects, which is favorable to the formation of Ce³⁺.³⁵ The mean particle sizes of CeO₂ in different catalyst samples were also calculated by the Scherrer equation and the results are listed in Table 1. It can be seen that the crystallite size of CeO₂ in Mn/CeSiO_x (2 : 1) is much smaller than that in other catalyst samples, suggesting the presence of more lattice defects in Mn/CeSiO_x (2 : 1), agreeing well with the results mentioned above.

Fig. 3 XRD patterns of the five catalyst samples.

Compared with XRD analysis, Raman spectra are more surface sensitive for the characterization of the different catalyst samples. As shown in Fig. 4, the main band at about 463 cm⁻¹ could be assigned to the F_2g vibration mode of the cubic fluorite structure, which is due to the an octahedral local symmetric breathing mode of oxygen atoms around the ceria lattice.³⁶ As can be observed from Fig. 4, the characteristic F_2g vibration modes of the three Mn/CeSiO_x catalyst samples are weakened, broadened and shifted to lower frequency, indicating the presence of reduced state of cerium.^37,38

Fig. 4 Raman spectra of different catalyst samples.

The chemical states of Mn, Ce and O species on Mn/CeO₂ and Mn/CeSiO_x (2 : 1) are determined by XPS analysis, and the results are shown in Fig. 5.

Fig. 5 XPS spectra of the two catalyst samples: (A) Mn 2p_3/2; (B) Ce 3d; (C) Si 2p; (D) O 1s.

As presented in Fig. 5(A), the XPS spectra of Mn 2p_3/2 contain three peaks after a peak-fitting deconvolution, which could be assigned to Mn²⁺, Mn³⁺ and Mn⁴⁺ respectively.^39–41 On the basis of XPS analysis results, the values of Mn⁴⁺ concentrations over the two catalyst samples could be calculated, as summarized in Table 2. It is noticeable that the Mn⁴⁺ concentration over Mn/CeSiO_x (2 : 1) is much higher than that over Mn/CeO₂. According to previous studies, species with higher oxidation state are more active for redox reactions over manganese-based catalysts.⁴² And the study of Liu et al.⁴³ also indicated that the enhanced NO oxidation to NO₂ by Mn⁴⁺ could greatly promote the NH₃-SCR reaction. Therefore, the relatively higher surface Mn⁴⁺ may partly contribute to the high low-temperature SCR activity of Mn/CeSiO_x (2 : 1).

Table 2 Surface atomic concentrations of Mn, Ce and O of the two catalyst samples (by XPS)

Samples	Mn (at%)	Ce (at%)	O (at%)	Mn^4+ (at%)	Ce^3+ (at%)	Oβ (at%)
Mn/CeO2	4.43	16.70	78.87	1.19	3.30	27.57
Mn/CeSiOx (2 : 1)	4.68	13.94	76.08	1.67	3.64	29.07

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The XPS spectra of Ce 3d for the two catalyst samples are illustrated in Fig. 5(B). It contains two multiplets corresponding to the spin orbit split 3d_5/2 and 3d_3/2 core holes respectively. According to previous studies,^44–46 the peaks denoted as u, u₂, u₃, v, v₂ and v₃ could be assigned to Ce⁴⁺, while the two peaks denoted as u₁ and v₁ could be ascribed to Ce³⁺. According to the XPS spectra of Ce 3d, Ce⁴⁺ and Ce³⁺ are concomitant on the surfaces of Mn/CeO₂ and Mn/CeSiO_x (2 : 1). Moreover, the Ce³⁺ concentration on Mn/CeSiO_x (2 : 1) is higher than that over Mn/CeO₂. It is well recognized that Ce³⁺ is originated from the defects of ceria defects, so the results of XPS analysis agree well with that of XRD analysis. Based on previous studies,^35,47 Ce³⁺ could form oxygen vacancies and unsaturated chemical bonds, facilitating the generation of chemisorbed oxygen on catalyst surface, which will be discussed in the following sections.

The XPS spectra of Si 2p of Mn/CeO₂ and Mn/CeSiO_x (2 : 1) are shown in Fig. 5(C). As can be seen from Fig. 5(C), the binding energy of Si 2p in the XPS spectra of Mn/SiO₂ is at 103.4 eV, which is in accordance with the results of Hu et al.⁴⁸ For Mn/CeSiO_x (2 : 1), the binding energy of Si 2p in the spectra of Mn/CeSiO_x (2 : 1) is lower than that in the spectra of Mn/CeO₂, indicating the formation of Ce–O–Si bond. And the charge disbalance formed in this process is favorable to the generation of acid sites,⁴⁹ as proven by the results of NH₃-TPD analysis.

Furthermore, the chemical states of oxygen species on the two catalyst samples were investigated by XPS analysis, as shown in Fig. 5(D). Obviously, the O 1s spectra could be fitted into two peaks: the lattice oxygen (529.0–530.0 eV, denoted as O_α) and the chemisorbed oxygen (531.3–531.9 eV, denoted as O_β).^50–52 From Table 2, it could be seen that the concentration of O_β over Mn/CeSiO_x (2 : 1) is higher than that over Mn/CeO₂, in good accordance with the different Ce³⁺ concentrations on them. As an active oxygen species in NH₃-SCR reaction, chemisorbed oxygen is of high mobility and could easily exchange with the oxygen in the gas or in the molecules adsorbed on catalyst, as a result, promoting the oxidation of NO to NO₂, accompanied by the enhanced “fast SCR” reaction.^12,53,54 According to the study of Chen et al.,⁵⁵ the oxidation of NO to NO₂ was of great importance to the SCR reaction below 250 °C, so the enrichment of Mn⁴⁺ and chemisorbed oxygen on the surface of Mn/CeSiO_x (2 : 1) by the introduction of Si is crucial to its excellent low-temperature SCR performance.

H₂-TPR analysis was performed to investigate the redox behavior of the catalyst samples. As can be seen from Fig. 6, each TPR profile contains two reduction peaks. Based on the results of XRD and TPR, the first peak in the profile of Mn/CeSiO_x (2 : 1) at about 345 °C could be attributed to the reduction of Mn⁴⁺ to Mn³⁺, and the second one at about 461 °C may be ascribed to Mn³⁺ to Mn²⁺ and the reduction of amorphous CeO₂ on catalyst surface.^35,56–58 So the results of H₂-TPR analysis are in good accordance with the results of XPS analysis. The reduction of bulk CeO₂ is neglected because it happens above 750 °C.⁵⁹ For Mn/CeO₂ and Mn/SiO₂, the reduction peaks in their TPR profiles could also be assigned to Mn⁴⁺ → Mn³⁺ and Mn³⁺ → Mn²⁺ respectively. Compared with Mn/CeSiO_x (2 : 1), the reduction temperatures of Mn/CeO₂ and Mn/SiO₂ move to higher values, suggesting that the redox abilities of Mn/CeO₂ and Mn/SiO₂ are lower than that of Mn/CeSiO_x (2 : 1). Furthermore, larger reduction peaks of Mn/CeSiO_x (2 : 1) also indicates the presence of more active oxygen than the other two catalyst samples. It is generally accepted that the reducing ability of Mn-based catalyst plays an important role to complete the catalytic cycle in SCR reaction.⁶⁰ So the high redox ability of Mn/SiO_x (2 : 1) should also be responsible for its excellent SCR performance.

Fig. 6 H₂-TPR profiles of the three catalyst samples.

It is well recognized that the adsorption and activation of NH₃ species is a key step for NH₃-SCR reaction.⁶¹ Therefore, NH₃-TPD technique was used to study the surface acid properties of the catalyst samples. As shown in Fig. 7, there is a broad desorption peak could be observed in the profiles of Mn/CeO₂ and Mn/SiO₂ from 100–500 °C, suggesting the presence of acid sites with different thermal stabilities on them. For Mn/CeSiO_x (2 : 1), there are three desorption peaks located at 137, 202 and 364 °C respectively, which could be related to the desorption of physisorbed NH₃ and some NH₄⁺ bounded to the weak Brønsted acid sites, NH₄⁺ from strong Brønsted acid sites and the coordinated NH₃ bound to the Lewis acid sites, respectively.^62,63 Moreover, the total desorption peak area of Mn/CeSiO_x (2 : 1) is much larger than that of Mn/CeO₂ and Mn/SiO₂, indicating the presence of more acid sites on its surface. Therefore, the composite oxide support could provide more acid sites for NH₃ adsorption.

Fig. 7 NH₃-TPD profiles of the three catalyst samples.

To investigate the NH₃-SCR reaction mechanism over the catalyst samples, in situ DRIFT study was performed to identify the active sites, adsorbed species and intermediates under different conditions.

To determine the surface acid properties of the catalyst samples, the in situ DRIFT spectra of NH₃ adsorption over Mn/CeO₂ and Mn/CeSiO_x (2 : 1) at 200 °C were recorded, and the results are shown in Fig. 8. As can be seen from Fig. 8, several bands could be observed. The band at 1710 and 1433 cm⁻¹ could be assigned to NH₄⁺ species on Brønsted acid sites, and the bands at 1617, 1258 and 1156 cm⁻¹ could be attributed to NH₃ species adsorbed on Lewis acid sites.^64–67 The band at 1533 cm⁻¹ belongs to NH₂ species, as an important immediate in NH₃-SCR reaction, which could react with NO to form N₂ and H₂O.⁶⁸ While this band is not found in the spectra of Mn/CeO₂. It is noticeable that the intensities of the bands in the spectrum of Mn/CeSiO_x (2 : 1) are higher than that in the spectrum of Mn/CeO₂, suggesting more NH₃ species are adsorbed on the surface of Mn/CeSiO_x (2 : 1), as also reflected by the results of NH₃-TPD analysis.

Fig. 8 In situ DRIFT spectra of NH₃ adsorption over the two catalyst samples at 200 °C.

The in situ DRIFT spectra of NO + O₂ co-adsorption over the two catalyst samples are shown in Fig. 9. The bands at 1644 cm⁻¹ could be assigned to bridged nitrate; the band at 1575, 1563 and 1239 cm⁻¹ could be corresponded to bidentate nitrate; and the band at 1392 and 1284 cm⁻¹ should be originated from monodentate nitrate species.^69–71 Due to the presence of more acid sites on Mn/CeSiO_x (2 : 1), it shows less NO_x species adsorption amount than Mn/CeO₂.⁷¹

Fig. 9 In situ DRIFT spectra of NO + O₂ co-adsorption over the two catalyst samples at 200 °C.

To identify the role of adsorbed NH₃ species in the NH₃-SCR reactions over the catalyst samples, the in situ DRIFT spectra of the reaction between the preadsorbed NH₃ species and NO + O₂ at 200 °C as a function of time were recorded, as shown in Fig. 10. From Fig. 10(A), two bands of adsorbed NH₃ species (1700 and 1533 cm⁻¹) in the spectra of Mn/CeO₂ could be observed after the exposure of NH₃. And these bands disappeared quickly after the introduction of NO + O₂, indicating that all the adsorbed NH₃ species are active in the NH₃-SCR reaction over Mn/CeO₂ catalyst. Meanwhile, two bands (1566 and 1256 cm⁻¹) of adsorbed NO_x species appeared and increased with time. As for Mn/CeSiO_x (2 : 1), a similar trend could also be observed in its DRIFT spectra of the reaction between NO + O₂ and preadsorbed NH₃ species. Compared with Fig. 10(A) and (B), it seems that the adsorbed NH₃ species on Mn/CeSiO_x (2 : 1) is more active than that absorbed on Mn/CeO₂. For example, the band at 1533 cm⁻¹ in Fig. 10(A) disappeared in about 5 minutes, while the band at 1433 cm⁻¹ vanished in about 2 minutes. Thus the composite oxides support may have an activation effect on adsorbed NH₃ species. The quick disappearance of adsorbed NH₃ species after the introduction of NO + O₂ indicates the existence of E–R mechanism.⁷²

Fig. 10 In situ DRIFT spectra of the reaction between NO + O₂ and preadsorbed NH₃ species over: (A) Mn/CeO₂; (B) Mn/CeSiO_x (2 : 1) at 200 °C.

On the other hand, the in situ DRIFT spectra of the reaction between NH₃ and preadsorbed NO_x species are shown in Fig. 11. As presented in Fig. 11(A), several bands of adsorbed NO_x species (1569, 1398 and 1240 cm⁻¹) were detected in the spectra of Mn/CeO₂ after the exposure of NO + O₂. After NH₃ was passed over the catalyst, the two bands at 1569 and 1240 cm⁻¹ vanished in about 2 minutes, while the band at 1398 cm⁻¹ kept rarely changed in the whole process. Thus only the bidentate nitrate is active in the NH₃-SCR reaction over Mn/CeO₂ catalyst, while the monodentate nitrate is a spectator in this process. Furthermore, several bands of adsorbed NH₃ species formed with the introduction of NH₃. For Mn/CeSiO_x (2 : 1), a similar phenomenon could also be observed (Fig. 11(B)), only the bidentate nitrate could participate in the NH₃-SCR reaction over it. Therefore, the adsorbed NO_x species are of different activities in the NH₃-SCR reaction, which is also reported by Liu et al.⁶⁵ Combined the results of in situ DRIFT study, we may conclude that the adsorbed NH₃ species and the bidentate nitrate species are all active in the NH₃-SCR reactions over the two catalyst samples, thus the L–H mechanism is also applicable for them.

Fig. 11 In situ DRIFT spectra of the reaction between NH₃ and preadsorbed NO_x species over: (A) Mn/CeO₂; (B) Mn/CeSiO_x (2 : 1) at 200 °C.

Conclusions

Mn/CeSiO_x (2 : 1) catalyst shows higher low-temperature SCR activity and better SO₂ and H₂O resistance than Mn/CeO₂ catalyst. From the characterization results, it was found that the modification of CeO₂ support of Mn/CeO₂ catalyst by Si leads to lower crystallinity, more reducible species and acid sites, which are all crucial to the NH₃-SCR reaction over it. The results of in situ DRIFT study reveal the coexistence of E–R mechanism and L–H mechanism. And the presence of more Mn⁴⁺ and chemisorbed oxygen should play a primary role for the greatly enhanced low-temperature SCR performance of Mn/CeSiO_x (2 : 1) catalyst.

Acknowledgements

This work was supported by the Natural Science Foundation of China (21546014) and the Natural Science Foundation of Shanghai, China (14ZR1417800).

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