Introduction

With the development of industry, air pollution becomes more and more serious. Nitrogen oxides (NOx) as the main pollutants have attracted intense attention. The selection catalytic reduction (SCR) of NOx by NH3 is a widely used technology in commercial SCR systems to control NOx emissions from stationary sources. V2O5-WO3/TiO2 is extensively used as the SCR denitrification catalyst in power plants (Topsoe 1994), and its optimum operating temperature range is 300–400 °C (Phil et al. 2008). To avoid the additional costs of reheating the flue gas, SCR unit is located upstream of the electrostatic precipitator and desulfurization tower (Liu et al. 2012). Flue gas contains a large amount of fly ash, K+, Na+, and SO2 because it is not treated by the electrostatic precipitator and desulfurization tower (Tang et al. 2007). These can cause catalyst poisoning and accelerate catalyst deactivation. Therefore, a low-temperature SCR catalyst placed downstream of the electrostatic precipitator and desulfurizer is extraordinarily necessary (Yang et al. 2016a, b).

Carbon-based materials such as multi-walled carbon nanotubes (Huang et al. 2007; Luo et al. 2012; Pan et al. 2013), single-walled carbon nanotubes (Gorji 2015; Zhang et al. 2018), activated carbon (Kim et al. 2015; Lázaro et al. 2006), activated coke (AC) (Wang et al. 2014a, b; Lin et al. 2017; Huang et al. 2014; Gao et al. 2018), activated semi-coke (Wang et al. 2014a, b), and carbon-coated materials (Ouzzine et al. 2008; Gao et al. 2011) as catalyst supporters attracted the attention of researchers owing to their large specific surface areas and developed pore structures. Especially, AC with high mechanical strength is more suitable to be used as the carrier of deNOx catalysts. Yang et al. used AC as a carrier and a pyrolusite as active component to prepare a series of low-temperature deNOx catalysts, which obtained 74.2% deNOx efficiency at 150 °C (Lin et al. 2017). Wang et al. (2017a, b) prepared a series of catalysts using activated semi-coke (ASC) as a carrier. The experimental results showed that the Ce/ASC catalyst had the highest deNOx efficiency at 300 °C, which the NOx conversion was 80% and N2 selectivity was 90%. Gao et al. (2018) found that AC-supported 6% Ce can effectively remove mercury from flue gas. Although there are some studies on AC as the carrier of deNOx catalysts, the deNOx efficiency will need to be improved, especially at low temperature.

The active component is another important factor that plays a crucial role in the SCR deNOx reaction. Transition metal oxides such as Fe (Klukowski et al. 2009; Skarlis et al. 2014), Cr (Chen et al. 2012), Ni (Gao et al. 2017), Mn (Yang et al. 2016a, b), and Cu (Gao et al. 2013) show excellent performance in the low-temperature SCR reaction of NO with NH3, especially for Mn and Fe. Mn-based catalysts have weak SO2 and H2O resistance (Yang et al. 2016a, b). However, Fe-based catalysts have good SO2 and H2O resistance (Wang et al. 2016; Xing et al. 2017), so some studies concentrated on the deNOx performance of AC-supported Fe-based catalysts (Du et al. 2018; Wang et al. 2017a, b). However, the NOx conversions of AC-supported Fe-based catalysts should be further improved at low temperature. Adding metallic oxides can enhance the SCR deNOx performance of catalysts at low temperature (Qiu et al. 2013). Therefore, it is necessary to study the modification of AC-supported Fe-based catalysts to improve their NOx conversions. Cr has various valences such as Cr2+, Cr3+, Cr5+, and Cr6+, which is prone to the redox reactions. Some researchers found that Cr has a promoting effect on the deNOx catalysts (Amin et al. 2003; Chen et al. 2012; Yang et al. 2015 ). Chen et al. (2012) found that Cr can effectively improve the deNOx performance of Mn-based catalysts at 80–220 °C. However, there are few studies on the effect of Cr on the deNOx performance of Fe/AC catalyst at low temperature. Therefore, in this paper, a series of Cr-modified AC-supported Fe-based catalysts were prepared by an impregnation method. The SCR deNOx activities were tested at 120–240 °C. In addition, the SO2 and H2O resistance of Cr-modified AC-supported Fe-based catalyst was also studied. Furthermore, the catalysts were characterized by BET, XRD, XPS, temperature-programmed reduction of H2 (H2-TPR), temperature-programmed desorption of NH3 (NH3-TPD), and FTIR, and the deNOx and SO2 poisoning mechanisms were discussed according to the experimental and characterization results.

Experimental

Catalyst preparation

AC provided by Ningxia Zhongyou Activated Carbon Co., Ltd. was used as a supporter. Ferric nitrate (Fe(NO3)3·9H2O) and chromium nitrate (Cr(NO3)3·9H2O) were used as metal precursors of Fe and Cr, respectively. Firstly, the AC was crushed and sieved into granules with 40–60 mesh. Then, 2-g AC particles, ferric nitrate (Fe, AC = 0.04–0.1), and chromium nitrate (Cr, AC = 0.04, 0.06, 0.08) with a moderate amount of deionized water were mixed and stirred for 2 h on a magnetic stirrer. After the impregnation, the mixtures were dried at 105 °C for 6 h, followed by calcination in muffle furnace at 250 °C for 5 h, and cooled down to room temperature. The prepared catalysts are designated as xFeyCr/AC, where x is the Fe and AC mass percentage ratio and y is the Cr and AC mass percentage ratio.

Catalytic activity

The deNOx activities of xFeyCr/AC were investigated in a fixed-bed quartz reactor (inner diameter 3.5 mm) with continuously flowing gas at atmospheric pressure. The reactant gas was composed of 400 ppm NO, 400 ppm NH3, 5% O2, 5% H2O (when needed), and 100 ppm SO2 (when needed), and N2 was used as the balance. The total gas flow rate was 130 mL/min and the gas hourly space velocity (GHSV) was 10400 cm3 g−1 h−1. NOx concentrations in the inlet and outlet flue gas were analyzed by a multi-component online flue gas analyzer. The NOx (NO and NO2) conversion was calculated by the following equation:

$$ {NO}_X\ \mathrm{conversion}\ \left(\%\right)=\frac{{\left({NO}_X\right)}_{in}-{\left({NO}_X\right)}_{out}}{{\left({NO}_X\right)}_{in}}\times 100\% $$
(1)

where (NOx)in and (NOx)out are the concentrations of NOx in the inlet and outlet, respectively.

Catalyst characterization

The specific surface area, pore volume, and pore size were measured by a Micromeritics ASAP 2020 analyzer (Micromeritics Instrument Corp., USA). Prior to BET measurement, all catalysts were dried for 6 h at 105 °C and then degassed under vacuum at 220 °C for 6 h. The surface areas of xFeyCr/AC catalysts were measured by nitrogen adsorption at − 196 °C and calculated by the BET equation.

The XRD patterns were recorded on a Rigaku Ultima IV powder diffractometer (Rigaku Ultima, Japan) with Cu Kα radiation (λ = 0.154056 nm) to examine the crystallinity and dispersity of components on the supporter. The scanning range (2θ) was 10–80° and the scanning rate was 20° min-1. The operating voltage was 40 kV and the applied current was 40 mA.

The XPS spectra of Fe(2p) and Cr(2p) were determined by an Escalab 250Xi X-ray electron spectrometer (Thermo Scientific, USA) using Al Kα X-ray as the radiation source.

The H2-TPR and NH3-TPD were determined by the ChemBET Pulsar TPR/TPD (Quantachrome instruments of America) in the temperature range of 50–800 °C.

The Fourier-transform infrared spectra (FTIR) was obtained by a FT-IR Nicolet IS10 (Thermo Scientific, USA).

Results and discussion

SCR DeNOx performance

The NOx conversions of xFe/AC catalysts are shown in Fig. 1. The NOx conversion of AC catalyst is only 15 % at 120 °C and changes little as the temperature increases. However, it increases with increasing the loading amount of Fe on AC catalyst. When the Fe content is 8%, the NOx conversions of Fe/AC catalyst are the highest, which are 41% and 91% at 120 °C and 240 °C, respectively. This may be due to the addition of Fe which increases the specific surface area of the catalysts and the acid sites on the catalyst surface. However, the NOx conversions of 8Fe/AC catalyst are still relatively low, so they were modified by the addition of Cr.

Fig. 1
figure 1

NOx conversions of the xFe/AC catalysts at 120–240 °C

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Figure 2 shows the NOx conversions of the 8FeyCr/AC catalysts at different temperatures. The NOx conversions first increase then decrease with increasing the addition amount of Cr. They reach the maximum values when the Cr content is 6%, which are 63% and 100% at 120 °C and 180 °C, respectively. Furthermore, the NOx conversion remains 100% at 240 °C after 4 h. Cr has different valence states, which are favorable to the catalytic redox reaction, especially in low-temperature section (Koehler et al. 2010), so the NOx conversions increase with increasing the addition amount Cr. While the Cr content reaches a certain amount and gathers on the surface of catalyst, the specific surface area and the number of acid sites can be reduced, so the activity of catalyst decreases.

Fig. 2
figure 2

NOx conversions of the 8FeyCr/AC catalysts at 120–240 °C

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To further understand the effects of Cr on the NOx conversions of the 8Fe/AC catalyst, the NOx conversions of the 8Fe/AC, 6Cr/AC, and 8Fe6Cr/AC catalysts were studied. The results are shown in Fig. 3. The NOx conversions of the 8Fe/AC catalyst are higher than those of the 6Cr/AC catalyst, while the NOx conversions of the 8Fe6Cr/AC catalyst have the highest, indicating that there is the interaction between Fe and Cr.

Fig. 3
figure 3

NOx conversions of the 8Fe/AC, 6Cr/AC, and 8Fe6Cr/AC catalysts at 120–240 °C

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Effects of H2O and SO2

The effects of H2O and SO2 on the NOx conversions of the 8Fe6Cr/AC catalyst were studied at 180 °C. The results are shown in Fig. 4. When H2O or SO2 were added to the reaction gas, the NOx conversions did not immediately decrease because it took a certain time for the reaction gas to arrive at the gas analyzer and catalyst poisoning caused by H2O or SO2 requires a process. As shown in Fig. 4a, the NOx conversion dropped from 100 to 73% after adding 5% H2O. Removing the H2O, the NOx conversion restored to 93%, indicating that the 8Fe6Cr/AC catalyst has a certain H2O resistance. The effect of SO2 on the NOx conversions of the 8Fe6Cr/AC catalyst is shown in Fig. 4b. When 100 ppm SO2 was introduced into the reaction gas, the NOx conversion dropped to 45%. After removing SO2, the NOx conversion did not restore, indicating that SO2 reacts with NH3 to form ammonium sulfate or ammonium hydrogen sulfate, which deposits on the surface of the catalyst and covers the active sites on the surface of the catalyst, so the NOx conversions of the 8Fe6Cr/AC catalyst decrease.

Fig. 4
figure 4

Effects of H2O (a) and SO2 (b) on the NOx conversions over the 8Fe6Cr/AC at 180 °C. Reaction conditions: [NH3] = [NO] = 400 ppm, [SO2] = 100 ppm, [H2O] = 5%, and GHSV = 10400 cm3 g−1 h−1

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Catalytic characterization

BET and XRD

The surface area, pore volume, and pore size of the xFeyCr/AC catalysts are summarized in Table 1. The surface area of AC is 163.2927 m2 g-1. While loading Fe on AC, its surface areas and pore volumes increase, so the activities of the catalysts increase. The specific surface area of the 8Fe/AC catalyst is similar to those of the 6Fe/AC and 10Fe/AC catalysts, but it is not the largest in these three catalysts, indicating that although the surface area of catalyst has effects on the NOx conversions, it is not a decisive factor. The specific surface area is further increased when Cr is added into the 8Fe/AC catalyst. With increasing the additional amount of Cr, the specific surface areas of the xFeyCr/AC catalysts first increase and then decrease owing to the excessive Cr gathering on the catalyst surface. The 8Fe6Cr/AC catalyst obtains the maximum specific surface area, so its deNOx efficiency is the highest.

Table 1 Physical properties of different catalysts
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The crystalline phases of active components in these catalysts were examined by XRD and the results are shown in Fig. 5. A broad band in the range of 17–33° is found in the AC catalyst, corresponding to the amorphous carbon structure (Wang et al. 2017a, b). In addition, the characteristic peaks of the AC catalyst at 20.81°, 26.6°, 36.26°, 44.08°, 50.12°, 59.96°, and 68.04° correspond very well to the standard PDF card (ICDD, #46-1045), which belong to the SiO2. With the addition of Fe and Cr in the AC catalyst, the intensity of the diffraction peaks of SiO2 and carbon structure constantly weakens and no new diffraction peaks appear in the XRD spectrum, indicating that Fe and Cr are amorphous and have excellent dispersion on the catalyst surface. At the same time, the presence of Fe and Cr makes the original carbon and SiO2 disperse on the catalyst surface. This is the reason that the BET specific surface areas of xFeyCr/AC catalysts increase.

Fig. 5
figure 5

XRD patterns of the catalysts (a) AC; (b) 4Fe/AC; (c) 6Fe/AC; (d) 8Fe/AC; (e) 10Fe/AC; (f) 8Fe4Cr/AC; (g) 8Fe6Cr/AC; and (h) 8Fe8Cr/AC

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XPS

The surface chemical states of Fe, Cr, and O species for the xFeyCr/AC catalysts are shown in Fig. 6. Figure 6a shows the Fe 2p XPS spectra of the 8Fe/AC, 8Fe4Cr/AC, 8Fe6Cr/AC, and 8Fe8Cr/AC catalysts. The two characteristic peaks located at 709–716 eV and 723–729 eV correspond to the orbitals of Fe 2p3/2 and Fe 2p1/2, respectively. Fitting the Fe 2p3/2 spectra, the spectra of Fe2+ (peak at about 710.9 eV) and Fe3+ (peak at about712.8 eV) are obtained. Adding of Cr into the 8Fe/AC, the Fe 2p3/2characteristic peak shifts to the low binding energy, indicating that the interaction can happen between Fe and Cr. The binding energy is high, indicating that the element is more stable, while the binding energy is low, demonstrating that the element is more active (Shen et al. 2014). The contents of Fe2+, Fe3+, and the ratios of Fe3+/Fe2+ are shown in Table 2. Compared with the 8Fe/AC catalyst, the Fe2+contents of the 8Fe6Cr/AC catalyst decrease from 18.83 to 14.05% and the Fe3+contents increase from 28.13 to 30.53%, indicating that adding a certain amount (6%) of Cr to the 8Fe/AC catalyst can increase the content of Fe3+ and decrease the content of Fe2+. Increasing the amount of Cr from 6 to 8%, the contents of Fe2+ increase from 14.05 to 18.24% and the contents of Fe3+ decrease from 30.53 to 27.28%. This indicates that excessive Cr can increase the concentration of Fe2+ and reduce the concentration of Fe3+. High valence metallic ions are favorable for the adsorption of NO on the catalyst surface and the oxidation of NO (Wang et al. 2015). Low-temperature SCR reaction is a complex process, and one of the main reaction paths is NH3 adsorbing at the Brønsted acid site on the catalyst surface to form NH4+ ions, which then reacts with NO2 to form H2O and N2 (Lin et al. 2004). Therefore, to some extent, the oxidation rate of NO to NO2 can determine the NOx conversion in the low-temperature SCR reaction. For the 8Fe6Cr/AC catalyst, the high ratio of Fe3+/Fe2+ is one of the reasons for its high deNOx activity at low temperature.

Fig. 6
figure 6

XPS spectra of Fe (a), Cr (b), and O (c) for the 8Fe/AC, 8Fe4Cr/AC, 8Fe6Cr/AC, and 8Fe8Cr/AC catalysts

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Table 2 Surface valence-state ratios of catalysts
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Figure 6b shows the Cr 2p XPS spectra of the 8Fe4Cr/AC, 8Fe6Cr/AC, and 8Fe8Cr/AC catalysts. Two characteristic peaks at 574–583 eV and 584–593 eV assigned to Cr 2p3/2 and Cr 2p1/2 are observed (Amin et al. 2003). Cr 2p3/2 spectrum is split into three peaks. They are Cr2+ (577.1 eV), Cr3+ (578.8 eV), and Cr6+ (582.4 eV). The contents of Cr2+, Cr3+, and Cr6+ are summarized in Table 2. The ratios of Cr3+/Cr2+ are 1.21, 1.37, and 0.97 for the 8Fe4Cr/AC, 8Fe6Cr/AC, and 8Fe8Cr/AC catalysts, respectively. The 8Fe6Cr/AC catalyst has the highest ratio of Cr3+/Cr2+, indicating that Cr3+ has obvious promotion effects on the deNOx performance. This result is consistent with the study of Wang et al. (2017a, b). The content of Cr6+ in the 8Fe6Cr/AC catalyst is slightly lower than that in the 8Fe4Cr/AC catalyst, but its deNOx efficiency is much higher than that in the 8Fe4Cr/AC catalyst, indicating that although the Cr6+ content has effects on the NOx conversions, it is not a decisive factor.

The O 1s XPS is shown in Fig. 6c. It is divided into two peaks at 531.12 and 532.28 eV, which are assigned to the binding energies of the chemisorbed oxygen (Oβ) (Wang et al. 2017a, b; Wu et al. 2011) and lattice oxygen (denoted as Oα) belonging to the Si–O bond in AC framework (Zhu et al. 2015), respectively. The surface contents of Oβ and Oα, and the ratio of Oβ/Oα are shown in Table 2. Compared with the 8Fe/AC catalyst, the ratios of Oβ/Oα in the 8Fe6Cr/AC and 8Fe8Cr/AC catalysts increase from 0.03 to 0.88 and 0.29, respectively, indicating that the Oβ content increases after the addition of Cr. Oβ has better reaction activity than Oα (Shen et al. 2011), so the 8Fe6Cr/AC catalyst has excellent NOx conversions.

H2-TPR

H2-TPR experiments were carried out to get insights into the redox properties of the different catalysts. Figure 7 shows the H2-TPR results of the catalysts. There are two peaks in the AC catalyst. The peak at 458.41 °C can be the reduction peak of metal oxides in AC itself. The peak at 661.19 °C is attributed to the reduction peak of oxygen-containing functional groups (Ding et al. 2017). However, after loading Fe, the reduction peak of metal oxides in AC is covered by the peak at 437.93 °C, which is assigned to the reduction of Fe3+ to Fe2+ (Yang et al. 2013, 2011) due to the high Fe content. It can be observed that the reduction peak temperature of Fe3+ to Fe2+ gradually shifts toward a low temperature when Cr is added, indicating that Cr can lower the reduction temperature of the xFeyCr/AC catalysts. In addition, a new reduction peak appears at 321.87, 305.96, and 308.04 °C for 8Fe4Cr/AC, 8Fe6Cr/AC, and 8Fe8Cr/AC, respectively, which assigns to the reduction of Cr3+ to Cr2+ (Yang et al. 2015). The peak temperature represents the catalyst reducibility. The lower the peak temperature is, the stronger is the catalyst redox ability (Liu et al. 2008). Comparing these catalysts, it can be found that the 8Fe8Cr/AC catalyst has the lowest reduction peak temperature, indicating that it has the best catalytic activity. However, we do not find the reduction peak of Cr6+ in the catalysts. Therefore, the effects of Cr6+ on the redox ability of the xFeyCr/AC catalysts are negligible. This result is also supported by the XPS analysis. It can be also found from Fig. 7 that the addition of Cr into the 8Fe/AC catalyst increases the reduction peak area of Fe3+ to Fe2+, indicating that the amount of reduced Fe3+ increases, so the NOx conversions of these catalysts increase significantly.

Fig. 7
figure 7

H2-TPR profiles of the catalysts

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NH3-TPD

NH3-TPD experiments were performed to determine the acid site distributions on the surfaces of the AC, 8Fe/AC, 8Fe4Cr/AC, 8Fe6Cr/AC, and 8Fe8Cr/AC catalysts. The results are shown in Fig. 8. The AC catalyst has only a broad desorption peak at 150.7 °C, corresponding to the NH3 desorption from weak acid sites (Lewis acid sites) and physisorbed NH3 (Yang et al. 2016a, b; Ding et al. 2017). Adding Fe into the AC catalyst, a new peak at 250–500 °C appears, which attributes to the NH4+ desorption from medium and strong acid sites (Brønsted acid sites) (Vishwanathan et al. 2004). Compared with the AC catalyst, the intensity of weak acid sites in the 8Fe/AC catalyst increases obviously, suggesting that the addition of Fe enhances the concentration of weak acid sites and adds a new medium and strong acid site to the catalysts. Adding Cr into the 8Fe/AC catalysts, the species of acid sites in xFeyCr/AC catalysts are not changed.

Fig. 8
figure 8

NH3-TPD profiles of the catalysts

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The desorption peak area of NH3 represents the number of acid sites in the catalyst (Zhu et al. 2017). To determine the concentration change of acid sites, the number of Lewis acid sites and Brønsted acid sites of the catalysts was calculated and the results are shown in Fig. 9. Adding Fe into the AC catalyst, the number of both Lewis acid sites and Brønsted acid sites increases. When Cr modified the 8Fe/AC catalyst, the number of Lewis acid sites and Brønsted acid sites firstly increases then decreases. The 8Fe6Cr/AC catalyst has the largest amount of the acid sites, indicating that its catalytic activity is the highest. In addition, the number of Brønsted acid sites in 8Fe6Cr/AC catalyst is significantly higher than that of the Lewis acid sites, which shows that the Brønsted acid sites play the main role in the SCR deNOx reaction. There are two main mechanisms such as E-R mechanism and L-H mechanism (Yang et al. 2016a, b), in which E-R mechanism mainly occurs at Lewis acid sites and L-H mechanism mainly occurs at Brønsted acid sites (Gao et al. 2013). For the 8Fe6Cr/AC catalyst, Lewis acid sites and Brønsted acid sites exist simultaneously, which indicates that the E-R mechanism and L-H mechanism co-exists in the NH3-SCR reaction at low temperature.

Fig. 9
figure 9

Lewis acid site and Brønsted acid site comparison for different catalysts

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FTIR

To clarify the reason for SO2 poisoning of the 8Fe6Cr/AC catalyst, the fresh catalyst and the catalyst poisoned by SO2 were studied by FTIR, and the results are shown in Fig. 10. Compared with the fresh 8Fe6Cr/AC catalyst, the 8Fe6Cr/AC catalyst poisoned by SO2 shows new peaks at 595, 894, 1049, 1030, and 1080 cm-1, which correspond to the moderate absorption peak of hydrogen sulfate, the weak absorption peak of sulfate, the strong absorption peak of hydrogen sulfate, the strong absorption peak of sulfate, and the weak absorption peak of sulfate, respectively. This indicates that when SO2 is introduced into the reaction gas, sulfates and hydrogen sulfates are formed on the catalyst surface, and these substances cover the active sites on the catalyst surface, thus reducing the deNOx efficiency (Gao et al. 2013).

Fig. 10
figure 10

FTIR spectra of the 8Fe6Cr/AC and 8Fe6Cr/AC poisoned by SO2

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Conclusions

The deNOx performance of Cr-modified activated coke–supported Fe-based catalyst was investigated at 120–240 °C. With increasing Cr content, the deNOx efficiency of the 8Fe/AC catalysts is greatly increased. When the Cr addition amount reaches 6%, the catalyst has the highest SCR activity, which the NOx conversions are more than 90% at 160–240 °C. A bigger specific surface area; higher amounts of Fe3+, Cr3+, and Oβ; stronger reduction ability; and more acid sites are responsible for the high deNOx activity of the 8Fe6Cr/AC catalyst. Both Brønsted acid sites and Lewis acid sites are present in the 8Fe6Cr/AC catalyst, and the L-H and E-R mechanism may contribute to the NOx reduction for 8Fe6Cr/AC catalyst. In addition, the 8Fe6Cr/AC catalyst has high stability in the presence of 5% H2O, but its ability of resistance to SO2 poisoning is poor due to formed sulfates and hydrogen sulfates on the catalyst surface covering the active sites.