Effect of Sulfation on the Selective Catalytic Reduction of NO with NH₃ Over γ-Fe₂O₃

Abstract

Because the site for NH₃ adsorption and the active site for NH₃ oxidization were separated after the sulfation, both the SCR reaction and the catalytic oxidization of NH₃ to NO over γ-Fe₂O₃ were restrained after the sulfation. As a result, the operation temperature window of γ-Fe₂O₃ for the SCR reaction shifted about 100 °C to higher temperature after the sulfation.

Graphical Abstract

1 Introduction

Selective catalytic reduction (SCR) with NH₃ is proven to be the most promising technology to control the emission of nitrogen oxides from automobile exhaust gas and industrial combustion of fossil fuels [1]. Although V₂O₅/WO₃–TiO₂ has been employed as a commercial SCR catalyst for several decades [2], it is still not satisfactory due to some drawbacks, such as the relatively narrow temperature window of 300–400 °C, the low N₂ selectivity at high temperatures, the toxicity of vanadium pentoxide to the environment [3], and the soaring price of W resource [4]. Therefore, a more cost-effective, better N₂ selectivity and more environmental-friendly SCR catalyst should be developed.

Recently, it is reported that Fe-based catalysts [5–7], for example Fe/ZSM-5 [8, 9], Fe³⁺ exchanged TiO₂-pillared clay [10], Fe–Ti spinel [11], Fe–Ti–V spinel [12], Fe₂(SO₄)₃/TiO₂ [13], and iron titanate [14], show excellent SCR activity and N₂ selectivity at 300–400 °C. γ-Fe₂O₃ is one of the simplest iron oxides, which adopts a cubic close packed cation deficient spinel structure [15]. γ-Fe₂O₃ shows the reduction–reoxidation properties, which is suitable for the use as an oxygen storage component in automobile exhaust catalysts [16]. Recently, it is reported that γ-Fe₂O₃ had excellent SCR activity and N₂ selectivity at 200–350 °C [17]. However, the operation temperature window of γ-Fe₂O₃ for the SCR reaction shifted 100 °C to higher temperature in the presence of SO₂ [17]. Generally, the deactivation of SO₂ on the SCR reaction was attributed to the deposition of NH₄HSO₄ and/or (NH₄)₂SO₄ at low temperatures [18]. The decomposition temperature of NH₄HSO₄ and/or (NH₄)₂SO₄ was less than 250 °C. Therefore, there could be another mechanism, which caused to the deactivation of SO₂ at 250–300 °C. Meanwhile, the SCR activity of γ-Fe₂O₃ was obviously promoted above 300 °C due to the presence of SO₂. Therefore, the mechanism of SO₂ effect on the SCR reaction over γ-Fe₂O₃ need to be further studied. The presence of SO₂ would cause to the sulfation of catalyst [19–21]. The mechanism of the SCR reaction over γ-Fe₂O₃ may differ from that over sulfated γ-Fe₂O₃. Therefore, the sulfation on the SCR reaction over γ-Fe₂O₃ was investigated in this work, which is helpful to understand the mechanism of SO₂ effect on the SCR reaction [22].

2 Experimental

2.1 Catalyst Preparation

Nanosized Fe₃O₄, the precursor of γ-Fe₂O₃, was prepared using a co-precipitation method at room temperature [23]. γ-Fe₂O₃ was obtained after the thermal treatment of Fe₃O₄ under air for 3 h at 400 °C [11]. Sulfated γ-Fe₂O₃ was obtained by pretreating γ-Fe₂O₃ (1.0 g) in a flow of 500 ppm SO₂ and 2 % O₂ (200 mL min⁻¹) at 300 °C for 8 h.

2.2 Catalytic Activity Measurement

The SCR reaction was performed on a fixed-bed quartz tube reactor. The mass of catalyst with 40–60 mesh was 100 mg. The total flow rate was 200 mL min⁻¹ (room temperature), and the corresponding gas hourly space velocity (GHSV) was 1.2 × 10⁵ cm³ g⁻¹ h⁻¹. The typical reactant gas composition was as follows: 500 ppm of NH₃, 500 ppm of NO, 2 % of O₂, 500 ppm of SO₂ (when used), 10 % of H₂O (when used), and balance of N₂. The concentrations of SO₂, H₂O, NO, NO₂, NH₃ and N₂O were continually monitored by an FTIR spectrometer (MKS Instruments).

2.3 Catalyst Characterization

BET surface area was determined using a nitrogen adsorption apparatus (Quantachrome, Autosorb-1). XRD patterns were recorded on an X-ray diffractionmeter (Rigaku, D/max-2200/PC) between 20° and 80° at a step of 7° min⁻¹ operating at 30 kV and 30 mA using Cu Kα radiation. H₂-TPR was recorded on a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx) under a 10 % hydrogen-90 % nitrogen gas flow (50 cm³ min⁻¹) at a rate of 10 °C min⁻¹. Temperature programmed desorption of ammonia (NH₃-TPD) and temperature programmed desorption of NO (NO-TPD) were carried out on the packed-bed microreactor at a rate of 10 °C min⁻¹ from 50 to 600 °C. The binding energies of Fe 2p, S 2p and O 1s were recorded on an X-ray photoelectron spectroscopy (Thermo, ESCALAB 250) with Al Kα (hv = 1486.6 eV) as the excitation source and C 1s line at 284.6 eV as the reference for the binding energy calibration. In situ DRIFT spectra were performed on a Fourier transform infrared spectrometer (FTIR, Nicolet NEXUS 870) equipped with a liquid-nitrogen-cooled MCT detector, collecting 100 scans with a resolution of 4 cm⁻¹.

3 Results and Discussion

3.1 SCR Activity

As shown in Fig. 1a, γ-Fe₂O₃ showed an excellent SCR activity at 200–350 °C (NO _x conversion was higher than 80 %). During the SCR reaction over γ-Fe₂O₃, only a small amount of N₂O formed above 300 °C. However, NO _x conversion obviously decreased with the increase of reaction temperature from 300 to 400 °C, and a small amount of NO₂ was observed at 350–400 °C. However, NH₃ conversion reached 100 % above 300 °C. It suggests that some NH₃ was oxidized to NO over γ-Fe₂O₃ at 300–400 °C. As 500 ppm of SO₂ was introduced, the SCR activity of γ-Fe₂O₃ obviously decreased at 150–300 °C. However, the SCR activity of γ-Fe₂O₃ was obviously promoted due to the presence of SO₂ at 350–400 °C (shown in Fig. 2). After the further introduction of H₂O, no obvious change happened (shown in Fig. 2). The similar result of γ-Fe₂O₃ was once reported by Mou et al. [17]. As shown in Fig. 2, the SCR activity of γ-Fe₂O₃ in the presence of SO₂ was close to that of sulfated γ-Fe₂O₃ at 300–400 °C. It suggests that the sulfation of γ-Fe₂O₃ could contribute to the effect of SO₂ on the SCR reaction over γ-Fe₂O₃. Therefore, the sulfation on the SCR reaction over γ-Fe₂O₃ was studied.

Fig. 1

SCR reaction over:a γ-Fe₂O₃, b sulfated γ-Fe₂O₃

Fig. 2

Effect of SO₂ and/or H₂O on the SCR reaction over γ-Fe₂O₃

With the increase of reaction temperature from 150 to 400 °C, NO _x conversion over sulfated γ-Fe₂O₃ gradually increased (shown in Fig. 1b). Figure 1b also shows that the ratio of NO _x conversion was close to that of NH₃ conversion. It suggests that most NH₃ was used to reduce NO over sulfated γ-Fe₂O₃. Meanwhile, little NO₂ can be observed during the SCR reaction over sulfated γ-Fe₂O₃. In comparison with γ-Fe₂O₃, NO _x conversion over sulfated γ-Fe₂O₃ was much less at 150–300 °C. However, sulfated γ-Fe₂O₃ showed excellent SCR activity (NO _x conversion was higher than 80 %) and N₂ selectivity (>95 %) above 300 °C, which was much better than γ-Fe₂O₃. Figure 1 shows that the operation temperature window of γ-Fe₂O₃ for the SCR reaction shifted about 100 °C to higher temperature after the sulfation.

3.2 Characterization

3.2.1 XRD and BET

As shown in Fig. 3, the characteristic peaks of γ-Fe₂O₃ and sulfated γ-Fe₂O₃ correspond very well to the standard card of maghemite (JCPDS: 39-1346) [15]. It indicates that the spinel structure of γ-Fe₂O₃ was not destroyed after the sulfation.

Fig. 3

XRD patterns of γ-Fe₂O₃ and sulfated γ-Fe₂O₃

BET surface areas of γ-Fe₂O₃ and sulfated γ-Fe₂O₃ were 74.7 and 64.8 m² g⁻¹, respectively. It indicates that the BET surface area of γ-Fe₂O₃ slightly decreased after the sulfation.

3.3 XPS

As shown in Fig. 4a, the binding energies of Fe 2p 2/3 on γ-Fe₂O₃ mainly centered at about 710.2, 711.1 and 712.5 eV, which were assigned to Fe³⁺ in the spinel structure and Fe³⁺ bonded with hydroxyl groups respectively [15]. Furthermore, the satellite component appeared at about 719.0 eV, which is the fingerprint of Fe³⁺ species [23]. They both suggest that Fe species on γ-Fe₂O₃ were mainly Fe³⁺. The O 1s peaks on γ-Fe₂O₃ mainly centered at about 530.2 eV and 531.6 eV (shown in Fig. 4b), which were attributed to O²⁻ in transition metal oxides and that in –OH respectively [24]. As shown in Fig. 4c, no obvious S 2p band was observed on γ-Fe₂O₃.

Fig. 4

XPS spectra of γ-Fe₂O₃ and sulfated γ-Fe₂O₃ over the spectral regions of Fe 2p, O 1s and S 2p

As shown in Fig. 4d, a new peak at 713.8 eV appeared in the spectral region of Fe 2p after the sulfation of γ-Fe₂O₃, which could be attributed to Fe³⁺ in Fe₂(SO₄)₃ [23]. Meanwhile, XPS analysis shows that the percent of Fe³⁺ on γ-Fe₂O₃ decreased from 40 to 31.3 % after the sulfation. The presence of SO₄ ²⁻ on sulfated γ-Fe₂O₃ can also be supported by the XPS spectra over S 2p and O 1s regions. A new peak at 532.1 eV appeared in the spectral region of O 1s (shown in Fig. 4e), which could be assigned to O²⁻ in SO₄ ²⁻ [25]. The S 2p peaks mainly centered at 168.9 and 169.9 eV (shown in Fig. 4f), which could be assigned to SO₄ ²⁻ and HSO₄ ⁻ respectively [15]. Previous research on the heterogeneous uptake of SO₂ on γ-Fe₂O₃ also demonstrated that the formed S species on iron oxides were mainly HSO₄ ⁻ and SO₄ ²⁻ [26]. XPS analysis shows that the percent of S(SO₄ ²⁻) on γ-Fe₂O₃ increased to 5.4 % after the sulfation.

3.4 TPR

TPR profile recorded from γ-Fe₂O₃ showed two obvious reduction peaks. The peak centered at about 318 °C was assigned to the reduction of γ-Fe₂O₃ to Fe₃O₄ [27], and the slight fluctuation at about 350 °C may be related to the impurity on γ-Fe₂O₃. The broad peak at higher temperature was attributed to the reduction of Fe₃O₄ to Fe [27].

The active site on γ-Fe₂O₃ for H₂ oxidization was covered by SO₄ ²⁻ after the sulfation. Hence, a strong displacement of the first reduction peak to 423 °C happened in the TPR profiles of sulfated γ-Fe₂O₃ (shown in Fig. 5). It suggests that the oxidization ability of γ-Fe₂O₃ decreased after the sulfation. Meanwhile, no obvious changes happened on the broad peak at higher temperature. The reduction peak at 423 °C of sulfated γ-Fe₂O₃ could be attributed to the reduction of Fe³⁺–SO₄ ²⁻. TPR analysis shows that the area ratio of the first peak to the total TPR profile increased from 13 % for γ-Fe₂O₃ to 19 % for sulfated γ-Fe₂O₃. It also suggests that the reduction of γ-Fe₂O₃ to Fe₃O₄ and the reduction of SO₄ ²⁻ both contributed to the first reduction peak of sulfated γ-Fe₂O₃.

Fig. 5

H₂-TPR profiles of γ-Fe₂O₃ and sulfated γ-Fe₂O₃

3.5 NO-TPD and NH₃-TPD

The capacities of γ-Fe₂O₃ and sulfated γ-Fe₂O₃ for NH₃ and NO + O₂ adsorption were calculated from NH₃-TPD and NO-TPD (shown in Fig. S1). The capacity of sulfated γ-Fe₂O₃ for NH₃ adsorption was 5.1 mmol g⁻¹, which was about 4.3 times that of γ-Fe₂O₃ (1.2 mmol g⁻¹). However, the capacity of sulfated γ-Fe₂O₃ for NO + O₂ adsorption was 0.32 mmol g⁻¹, which was only about 1/5 that of γ-Fe₂O₃ (1.7 mmol g⁻¹). It suggests that the adsorption of NH₃ on γ-Fe₂O₃ was promoted and the adsorption of NO on γ-Fe₂O₃ was restrained after the sulfation.

3.6 Adsorption of NO and NH₃

The characteristic vibration corresponding to the adsorption of NO + O₂ on γ-Fe₂O₃ at 300 °C mainly appeared at about 1,600 and 1,578 cm⁻¹ (shown in Fig. 6a), which were assigned to monodentate nitrite and monodentate nitrate respectively [28]. However, the characteristic vibration corresponding to the adsorption of NO + O₂ on sulfated γ-Fe₂O₃ at 300 °C appeared at 1,385 cm⁻¹ (shown in Fig. 6a). Monodentate nitrite and monodentate nitrate adsorbed on γ-Fe₂O₃ were coordinated by oxygen atom. However, the oxygen atom on γ-Fe₂O₃, which can be used to bridge NO and Fe cation, was covered by SO₄ ²⁻ after the sulfation. Therefore, the characteristic vibration at 1,385 cm⁻¹ was assigned to nitro [28], which is coordinated via its N atom [19]. The intensity of the adsorption of NO + O₂ on sulfated γ-Fe₂O₃ was much less than that on γ-Fe₂O₃. It indicates that the adsorption of NO _x on γ-Fe₂O₃ was restrained after the sulfation.

Fig. 6

a DRIFT spectra of the adsorption of NO + O₂ on γ-Fe₂O₃ and sulfated γ-Fe₂O₃ at 300 °C, b DRIFT spectra of the adsorption of NH₃ on γ-Fe₂O₃ and sulfated γ-Fe₂O₃ at 300 °C

The characteristic vibration corresponding to NH₃ adsorption on γ-Fe₂O₃ at 300 °C mainly appeared at about 1,202 and 1,609 cm⁻¹ (shown in Fig. 6b), which were assigned to coordinated ammonia bound to the Lewis acid sites [14]. However, the characteristic vibration corresponding to NH₃ adsorption on sulfated γ-Fe₂O₃ appeared at 1,426 and 1,298 cm⁻¹ (shown in Fig. 6b), which were assigned to ammonium ions bound to the Brønsted acid sites [22]. The intensity of the adsorption of NH₃ on sulfated γ-Fe₂O₃ was much higher than that on γ-Fe₂O₃. It indicates that the adsorption of NH₃ on γ-Fe₂O₃ was promoted after the sulfation, which was consistent with the result of NH₃-TPD. Meanwhile, a negative peak at about 1,373 cm⁻¹ appeared on sulfated γ-Fe₂O₃. XPS analysis demonstrates that γ-Fe₂O₃ was covered by SO₄ ²⁻ after the sulfation. As is well known, SO₄ ²⁻ is a typical Brønsted acid [29], so NH₃ mainly adsorbed on SO₄ ²⁻ on sulfated γ-Fe₂O₃. Therefore, the negative peak at about 1,373 cm⁻¹ could be ascribed to SO₄ ²⁻, which was covered by NH₄ ⁺ after the adsorption of NH₃ [22].

3.7 Mechanism of the Sulfation on the SCR Reaction

Both the Eley–Rideal mechanism (the reaction of activated NH₃ with gaseous NO) and the Langmuir–Hinshelwood mechanism (the reaction between adsorbed NO _x and NH₃) could contribute to the SCR reaction over γ-Fe₂O₃ and sulfated γ-Fe₂O₃. Furthermore, the catalytic oxidization of NH₃ to NO [21] can happen at high temperatures (shown in Table S1).

The SCR reaction and the catalytic oxidization of NH₃ to NO both contributed to NH₃ conversion over γ-Fe₂O₃ and sulfated γ-Fe₂O₃. Therefore, the ratio of NH₃ conversion over γ-Fe₂O₃ and that over sulfated γ-Fe₂O₃ can be described as:

$$ {\text{NH}}_{ 3} {\text{ conversion \% = }}\delta_{\text{SCR}} + \delta_{{{\text{C}}{-}{\text{O}}}} , $$

(1)

where δ _SCR and δ _C–O were the contributions of the SCR reaction and the catalytic oxidization of NH₃ to NO to the ratio of NH₃ conversion, respectively.

The SCR reaction contributed to the reduction of gaseous NO, while the catalytic oxidization of NH₃ to NO contributed to the formation of gaseous NO. Therefore, the ratio of NO conversion over γ-Fe₂O₃ and that over sulfated γ-Fe₂O₃ can be described as:

$$ {\text{NO conversion \% = }}\delta_{\text{SCR}} \; - \;\delta_{{{\text{C}} - {\text{O}}}} $$

(2)

According to Eqs. 1 and 2, the amount of NH₃ conversion assigned to the SCR reaction and that assigned to the catalytic oxidization of NH₃ to NO can be calculated according to the difference between the ratio of NO conversion and that of NH₃ conversion.

Figure 7a shows that the SCR reaction over γ-Fe₂O₃ was obviously promoted with the increase of reaction temperature. However, the catalytic oxidization of NH₃ to NO happened over γ-Fe₂O₃ above 250 °C, resulting in a drop of NO _x conversion (shown Fig. 1a). After the sulfation, the SCR reaction over γ-Fe₂O₃ was obviously restrained, resulting in a remarkable decrease of NO conversion at 150–300 °C. Meanwhile, the catalytic oxidization of NH₃ to NO over γ-Fe₂O₃ above 250 °C was suppressed after the sulfation. As a result, the operation temperature window of γ-Fe₂O₃ for the SCR reaction shifted about 100 °C to higher temperature after the sulfation.

Fig. 7

Contributions of the SCR reaction and the catalytic oxidization of NH₃ to NO (C–O) to NH₃ conversion during the SCR reaction over: a γ-Fe₂O_3, b sulfated γ-Fe₂O₃

The acid sites on γ-Fe₂O₃ (Lewis acid) mainly resulted from the unsaturated coordination between Fe³⁺ and O²⁻. The unsaturated coordination was destroyed after the sulfation, and the acid sites on sulfated γ-Fe₂O₃ mainly resulted from SO₄ ²⁻ on the surface. Therefore, NH₃ mainly adsorbed on Fe³⁺ (or Fe–O band) on γ-Fe₂O₃, while it mainly adsorbed on SO₄ ²⁻ on sulfated γ-Fe₂O₃. It suggests that the site for NH₃ adsorption and the active site for NH₃ activation on γ-Fe₂O₃ were separated after the sulfation (illustrated in Fig. 8). Meanwhile, the oxidization ability of Fe³⁺ on sulfated γ-Fe₂O₃ was less than that on γ-Fe₂O₃, which was hinted by the TPR analysis (shown in Fig. 5). Moreover, the concentration of Fe³⁺ on sulfated γ-Fe₂O₃ was slightly less than that on γ-Fe₂O₃. As a result, the activation of adsorbed NH₃ over γ-Fe₂O₃ was restrained after the sulfation. It suggests that the SCR reaction over γ-Fe₂O₃ through the Eley–Rideal mechanism would be obviously restrained after the sulfation. Furthermore, NO-TPD analysis and in situ DRIFTS study show that the adsorption of NO + O₂ on γ-Fe₂O₃ was restrained after the sulfation. It suggests that the SCR reaction through the Langmuir–Hinshelwood mechanism would be restrained after the sulfation. As a result, the SCR reaction over γ-Fe₂O₃ was restrained after the sulfation (shown in Fig. 7).

Fig. 8

Illustration of sulfation on the SCR reaction over γ-Fe₂O₃

Figure 1b shows that most of adsorbed NH₃ on sulfated γ-Fe₂O₃ can be transformed above 300 °C, so the negative effect of sulfation on the SCR reaction over γ-Fe₂O₃ above 300 °C can be neglected. Therefore, the key factor of NO reduction over γ-Fe₂O₃ at high temperatures was the negative effect of the catalytic oxidization of activated NH₃ species (–NH₂) to NO.

–NH₂ mainly adsorbed on Fe³⁺ (or Fe–O band) on γ-Fe₂O₃, while it mainly adsorbed on SO₄ ²⁻ on sulfated γ-Fe₂O₃. It suggests that the site for –NH₂ adsorption and the active site for the further oxidization of –NH₂ were separated after the sulfation, which was similar to that for NH₃ activation. Meanwhile, the concentration of Fe³⁺ on sulfated γ-Fe₂O₃ was less than that on γ-Fe₂O₃. Moreover, the concentration of –NH₂ on sulfated γ-Fe₂O₃ was much less than that on γ-Fe₂O₃ due to the depression of NH₃ activation. As a result, the catalytic oxidization of NH₃ to NO over γ-Fe₂O₃ was obviously restrained after the sulfation (shown in Fig. 7).

4 Conclusions

Because the site for NH₃ adsorption and the active site for NH₃ activation were separated, the SCR reaction over γ-Fe₂O₃ was restrained at 150–300 °C after the sulfation. Meanwhile, the catalytic oxidization of NH₃ to NO over γ-Fe₂O₃ was restrained after the sulfation, so the drop of NO _x conversion at high temperatures postponed. As a result, the operation temperature window of γ-Fe₂O₃ for the SCR reaction shifted about 100 °C to higher temperature after the sulfation.