Effects of Synthesis Conditions on Rare Earth Doped Iron Oxide Catalyst for Selective Catalytic Reduction of NOx with NH3

Received: December 24, 2022
Revised: March 6, 2023
Accepted: March 15, 2023

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Cite this article:

Wei, Y., Wang, B., Ren, R., Wang, R. (2023). Effects of Synthesis Conditions on Rare Earth Doped Iron Oxide Catalyst for Selective Catalytic Reduction of NO_x with NH₃. Aerosol Air Qual. Res. 23, 220438. https://doi.org/10.4209/aaqr.220438

HIGHLIGHTS

• Different rare earth doped Fe based catalysts were prepared as SCR catalysts.
• Sm_0.075Fe_0.925 catalyst showed the optimal deNO_x performance.
• The catalytic activity at different calcination temperatures was investigated.
• The catalyst constructed with NH₃·H₂O has the best deNO_x performance.
• The Sm_0.075Fe_0.925 catalyst showed strong resistance for 0.05% SO₂.

Keywords: SCR, Rare earth, Fe oxide, Mesoporous structure, SO2 resistance

1 INTRODUCTION

Nitrogen oxides (NO_x) are recognized as one of the main atmospheric pollutants, which are not only the main component of acid rain, but also the culprit in the formation of photochemical smog, acid rain, greenhouse effect and ozone depletion (Eigenmann et al., 2006; Wei, 2014). In terms of economic and technological efficiency, SCR with ammonia is currently the most widely used method for eliminating NO_x from stationary sources (France et al., 2017; Chen et al., 2021a). The V₂O₅-WO₃/TiO₂ has been extensively used as a commercial SCR catalyst in the electric plant due to its excellent catalytic activity in the temperature range of 300–400°C (Kroecher and Elsener, 2008; Kompio et al., 2012). However, some shortcomings of this type of catalysts, such as narrow operation temperature window, biological toxicity of active component (V) and poor SO₂ resistance, constrain their application (Kobayashi and Miyoshi, 2007; Liu et al., 2009). Therefore, the development of eco-friendly SCR catalyst with high efficiency and high SO₂ tolerance is further imperative.

Iron is a low economic cost, and non-toxic transition metal and Fe₂O₃-based catalysts have attracted wide attention in NH₃-SCR process because of good medium-high SCR activity, satisfactory N₂ selectivity, excellent thermal stability and prominent SO₂-tolerant activity at high temperatures (> 300°C), but the operating temperature window of pure FeO_x is narrow (Chen et al., 2018; Han et al., 2019). In recent years, many efforts have been made to develop composite catalyst systems due to the synergetic effects. Therefore, a number of Fe-based catalysts doped with other metal components have been reported. For instance, Yang et al. (2011) prepared nanosized Fe-Mn oxide as a super catalyst for the low temperature SCR of NO_x with NH₃. Fan et al. (2017) rationally designed the porous nanoneedle-like MnO_x-FeO_x catalyst derived by heat-treating MOFs, which showed excellent NH₃-SCR activity in low temperatures. Fe-Ce mixed oxide catalyst was prepared and the SCR activity at low-temperature over iron oxide was improved significantly by adding a small amount of cerium (Xiong et al., 2013). Besides, the W-modified Fe₂O₃ catalyst can improve the activity and N₂ selectivity in a wider temperature window by tuning surface acidity and redox property (Xin et al., 2018; Wang et al., 2020a; Zhang et al., 2020). The modification of Fe₂O₃ catalyst with Ti species can inhibit the transformation of γ-Fe₂O₃ to α-Fe₂O₃ and weaken the oxidation ability of Fe₂O₃ for the oxidization of NH₃ (Liu et al., 2008; Yang et al., 2013). Besides, many researchers have paid great attention to Fe-exchanged zeolites, i.e., Fe/ZSM-5 (Brandenberger et al., 2011; Ruggeri et al., 2015), Fe/BEA (Shwan et al., 2015, 2012), Fe/SBA-15 (Li et al., 2018).

Rare earth is an important mineral, especially for China. The 5d orbits of electronic structure for most rare earth elements under normal conditions are empty, which can be used as an electron transfer station for catalytic reaction (Da Silva et al., 2016; Xue et al., 2017). Thus, rare earth elements and their oxides have high catalytic activity. Rare earth catalytic materials have been crucial in a number of industries over the years, including petrochemical catalysis, vehicle exhaust catalysis, fuel cell catalysis, catalytic combustion, hydrogen production catalysis, and others (Guo and Lu, 2007; Zhan et al., 2014b). Ce is one of the most common rare earth metals and has been wildly reported as promoter in SCR reaction due to the high oxygen storage/release capacity and superior redox (Shan et al., 2012; Tan et al., 2021). As an important rare earth metal, Sm has also been explored in SCR reaction. Meng et al. (2015, 2016) reported that incorporating Sm into the MnO_x catalyst can significantly improve catalytic activity for NH₃-SCR while also improving sulfur and H₂O resistance. Sun et al. (2021) found that the incorporation of Sm enhanced adsorption and activation of NH₃ and facilitated the decomposition of NH₄HSO₄.

As well known, the morphology, crystal structure, and nanostructure of oxide catalysts are important to improve the SCR activity, which peculiarly depends on the regulation of their synthesis conditions. In addition, there are a few reports on Sm doped FeO_x catalyst. In this work, rare earth-doped Fe-based oxide catalysts were prepared by the coprecipitation method. The influence of catalyst preparation factors (various rare earth species, doping amount of Sm, calcination temperature and the kind of precipitant) on NH₃-SCR activity were evaluated. In addition, the ability of catalysts for sulfur resistance was also investigated. Various characterizations were used to investigate the effect of Sm addition on the structure and physicochemical properties of FeO_x. These results have important reference value for the optimization of catalyst preparation in further applications.

2 EXPERIMENTAL

 
2.1 Preparation of Catalysts

The rare earth-doped iron oxide catalysts were prepared by the co-precipitation method. Take SmFe oxide synthesis as an example. Typically, Fe(NO₃)₃·9H₂O and Sm(NO₃)₃·6H₂O with different stoichiometric ratios were dissolved into deionized water to obtain a uniform solution with a mixed metal cation concentration of 0.2 mole L^–1, and then the pH of the solution was adjusted to 8.5–9.5 using alkaline solution (2 mol L^–1) containing precipitant (NH₃·H₂O, (NH₄)₂CO₃ or NaOH) under vigorous stirring. To remove anion impurities from the formed slurry, it was filtered and washed several times. The crystalline product was dried at 105°C for 12 h and calcined at different temperatures (350, 400, 450 and 500°C) for 5 h in a muffle furnace. The obtained product was denoted as Sm_xFe_1-x (x = 0, 0.025, 0.05, 0.075, 0.1). Samples doped with different rare earths were denoted as Sm_0.05Fe_0.95, Nd_0.05Fe_0.95, Gd_0.05Fe_0.95 and Pr_0.05Fe_0.95. The pure FeO_x catalyst was prepared with the same method by comparison.

 
2.2 Catalyst Characterization

XRD patterns were acquired using a BRUKER-AXS D8 Adance X-ray Diffractometer operating at 40 kV and 60 mA with Cu-K radiation (λ = 0.15418 nm). The diffractograms were captured in the 10°–80° scanning angle (2) range with a 0.02° step size.

A Micromeritics Tristar 3020 surface area analyzer was used to measure the surface areas and pore characterization of the catalysts at liquid N₂ temperature (–196°C). The specific surface area, pore size and pore volume were calculated by the multi-point BET method and the BJH model, respectively.

SEM(JSM-6700F) analysis was done on the catalysts' morphological characteristics.

Using Al-Ka radiation, the catalysts were subjected to XPS examination on a scanning X-ray microprobe (PHI5300X). As a standard, the binding energies were calibrated using the C 1s peak (BE = 284.8 eV).

 
2.3 Activity Measurements

An 8 mm internal diameter fixed-bed quartz tube reactor was used to test the evaluation of catalytic activity. The sample (0.5 g, 40–60 mesh) was fixed with quartz wool in a reactor before being placed in an electric tubular furnace that was temperature-controlled. A simulated reactant gas mixture containing 1000 ppm NO, 1100 ppm NH₃, 6% O₂, 500 ppm SO₂ (when used), and balance N₂ with a gas hourly space velocity (GHSV) of 15,000 h^–1. When the reaction reached a stable state at various temperatures, the associated activity statistics were recorded. The TH-990S NO and NO₂ analyzers were used to continuously monitor the amounts of NO and NO₂. The catalytic performance of NH₃-SCR reaction was presented in terms of NO_x conversion calculated as follows:

where the subscripts "in" and "out" represent the steady-state inlet and output concentrations, respectively.

 
3 RESULTS AND DISCUSSION

3.1 XRD Analysis

Fig. 1 shows the XRD patterns of Fe-based catalysts with different additions of samarium oxide. All the diffraction peaks observed on FeO_x can be assigned to α-Fe₂O₃ (Jayashree et al., 2019). After the doping of Sm on FeO_x, the intensity of the Fe₂O₃ diffraction peaks in the catalyst gradually decreased with the increase of Sm and the diffraction peak of α-Fe₂O₃ disappeared when the mole ratio of Sm was up to 0.075. It is difficult to observe the diffraction peaks of samarium oxide on all the mixed samples. This phenomenon suggests that samarium oxide and iron oxide interacted vigorously. The addition of Sm affected the nucleation process of iron oxide grains and inhibited the growth of iron oxide particles, reducing the crystallinity of α-Fe₂O₃. This means that the iron and samarium components form a solid solution with amorphous state.

Fig. 1. Powder XRD patterns of SmFe mixed oxide catalysts calcined at 400°C, (a) FeO_x, (b) Sm_0.025Fe_0.975, (c) Sm_0.05Fe_0.95, (d) Sm_0.075Fe_0.925 and (e) Sm_0.1Fe_0.9.

Fig. 2 shows the XRD patterns of Sm_0.075Fe_0.925 catalysts calcined at different temperatures. For the catalysts calcined at 350°C and 400°C, no diffraction peaks of iron oxide and samarium oxide were found. It can be seen that the diffraction peaks of Fe₂O₃ can be observed when the calcination temperature rises to 450°C. The diffraction peak of Fe₂O₃ at 500°C was slightly wider than it was at 450°C, indicating that the higher calcination temperature caused the Sm_0.075Fe_0.925 catalyst to generate more Fe₂O₃ grain crystals. Furthermore, the interaction between samarium oxide and iron oxide was weakened, affecting the formation of iron and samarium solid solution.

Fig. 2. Powder XRD patterns of Sm_0.075Fe_0.925 catalysts calcined at different temperatures. (a) FeO_x, (b) 350°C, (c) 400°C, (d) 450°C, (e) 500°C.

 
3.2 Porosity and Surface Area Analysis

Table 1 shows the results of the specific surface area and pore volume of iron-based catalysts with different Sm amounts calcined at 400°C. It can be seen that the doping of Sm significantly increased the specific surface area and pore volume of the iron-based catalyst. The specific surface area of pure FeO_x catalyst was only 39 m² g^–1 and with the increase of doping amount, the surface area of the catalyst Sm_0.1Fe_0.9 increased to 170 m² g^–1. In addition, the pore volume of the catalyst gradually increased as the increases of Sm amount. Simultaneously, more mesoporous pores with smaller pore diameter generated, which could be speculated that the substitution of smaller Fe by Sm with a larger cation radius reduced the pore diameters of the catalysts. To our best knowledge, high surface area and pore volume would greatly benefit the activity of catalysts, due to offering more active sites, efficient channels for mass transport and short diffusion length of reaction gases (Zhan et al., 2014a; Parlett et al., 2013).

 
3.3 SEM

In order to investigate the effect of different precipitating agents on the morphology and particle size of the SmFe composite oxide, SEM was carried out on the Sm_0.075Fe_0.925 catalyst prepared with NH₃·H₂O and (NH₄)₂CO₃ as the precipitating agent. It can be seen from the comparison of the low-magnification images in Figs. 3(A) and 3(B) that the catalyst presented large agglomeration, and the surface of the block was relatively smooth. As shown in the high-magnification images of Figs. 3(C) and 3(D) that the catalyst particles prepared with different precipitating agents were all composed of nanoparticles, and the catalyst particles prepared with NH₃·H₂O were smaller. In general, the dispersion of the catalyst prepared by the two precipitating agents was not uniform, the catalyst prepared with NH₃·H₂O had smaller particle size and lighter agglomeration, which may be related to the nucleation rate of the precipitate is faster compared to that prepared with (NH₄)₂CO₃.

Fig. 3. SEM images of Sm_0.075Fe_0.925 catalyst calcined at 400°C (A) and (C) with NH₃·H₂O, (B) and (D) with (NH₄)₂CO₃.

 
3.4 XPS

Considering the influence of valence state of catalyst surface element on the deNO_x activity, XPS of FeO_x and Sm_0.075Fe_0.925 was performed as shown in Fig. 4. Fig. 4(A) shows the Fe 2p spectra. Three prominent peaks over FeO_x catalyst were observed in the binding energy range of 705–735 eV. The peak at 709.0 eV could be attributed to the binding energy of Fe²⁺ two and the peaks at 711.6 and 724.0 eV are the binding energy of Fe³⁺ (France et al., 2017; Yang et al., 2011), indicating that Fe²⁺ and Fe³⁺ coexisted on the surface of FeO_x catalyst. While the less content of Fe²⁺ resulted in the failure to observe divalent iron oxides in the XRD pattern (Fig. 1). After the addition of Sm, the Fe 2p XPS Gaussian fitting curve of Sm_0.075Fe_0.925 was similar to that of FeO_x, indicating that there were Fe²⁺ and Fe³⁺ on the surface. The binding energy for Sm_0.075Fe_0.925 shifted to lower values of about 0.4 eV, indicating that the addition of Sm increased the density of the electron cloud around Fe atoms, and there is a strong interaction between Sm and Fe. The O1s spectra of the catalyst are shown in Fig. 4(B). Three Gaussian simulation peaks were observed at 527.4, 530.2 and 532.2 eV on FeO_x. The peak at 527.4 and 530.2 eV were attributed to the lattice oxygen and the peak at 532.2 eV was assigned to surface adsorbed oxygen (Kang et al., 2007; Parlett et al., 2013). Compared with FeO_x, the chemically adsorbed oxygen at of 532.2 eV disappeared over Sm_0.075Fe_0.92, indicating that the addition of Sm reduced the content of chemically adsorbed oxygen on the surface. Fig. 4(C) shows the Sm3d XPS curve of Sm_0.075Fe_0.925. The peak at 1081.2 eV corresponds to the 3d5/2 structure of Sm²⁺, and the peak at about 1110.0 eV is the 3d3/2 peak of Sm³⁺ (Sone et al., 2015; Liu et al., 2019). The weaker accompanying peak at 1097.4 eV further proved the existence of two valence states of Sm. However, the Sm³⁺ at 1084.8 eV moved about 1.6 eV to higher binding energy than the standard spectrum Sm³⁺ (1083.2 eV), indicating that the electron density around Sm atoms was decreased. This phenomenon was mutually corroborated by the increase in electron density around Fe atoms in the catalyst. This finding was related to the increase of the surrounding electron density around Fe atoms in the catalyst and further confirmed the existing strong interaction between samarium and iron.

Fig. 4. XPS of (A) Fe 2p, (B) O 1s, (C) Sm 3d of FeO_x and Sm_0.075Fe_0.925 calcined at 400°C.

3.5 Catalytic Performance

3.5.1 Effect of different rare earth doping on SCR activity

Fig. 5 shows the effect of different rare earth-doped iron-based oxides on the NO_x conversion rate. As shown in Fig. 5, for pure FeO, the NO_x removal rate was only 27.2% at 100°C, reached a maximum NO_x conversion rate of 87.4% at 250°C, and then rapidly decreased. After adding four different rare earth oxides, the similar conversion curves can be seen. The NO_x conversion rate was about 50.0% at 100°C and the maximum removal rate was nearly 100% between 200–250°C. The doping of rare earth oxides significantly increased NO_x conversion rate over iron-based oxide and broadened the whole temperature window, which could be due to the existing synergistic interaction between doping and Fe species for the bi-metal oxides. In detail, the addition of rare earth oxides increased the specific surface area of the catalyst, which is conducive to the exposure of more active sites and increases the oxygen storage capacity (Qi and Yang, 2003; Tang et al., 2006). Thus, more reactive gases can be quickly adsorbed on the catalyst surface and fully reacted with more active sites exposed, thereby improving the deNO_x activity of the catalysts.

Fig. 5. Effect of different rare earth-doping (Sm, Nd, Gd and Pr) on the NH₃-SCR activity of iron oxide catalysts.
 

 
3.5.2 Effect of Sm amount on SCR activity

Fig. 6 shows the effect of the amount of Sm addition on the NO_x conversion rate of the iron-based catalyst. The doping in rare earth Sm oxide had a significant effect of improving the activity of the catalyst. Compare to the catalyst FeO_x, the NO_x conversion rate of the catalyst added a small amount of samarium oxide was significantly improved. The deNO_x performance of the catalyst first increased and subsequently dropped as the ratio of doped Sm grew steadily from 0 to 0.1; the ideal doping ratio was 0.075. As the increase of Sm amount, the specific surface area and pore volume of the catalyst gradually increased (Table 1) and the optimum molar ratio of Sm doping with the best activity is 0.075. At the optimal doping rate, the deNO_x rate is close to 100% between 200 and 250°C. Compared with the NO conversion (90% and 92%) of the same type of catalyst (Mn@ZIF-8 and MnO_x) in recent years, the conversion is higher and the reaction temperature is lower (Chen et al., 2021b; Zhu et al., 2021). This means structural properties are not single decisive factors affecting catalytic activity, and the composition of the catalyst would also affect the catalytic activity due to the interaction of Sm and Fe species.

Fig. 6. Effect of the amount of Sm on the NH₃-SCR activity of iron oxide catalyst.

 
3.5.3 Effect of different precipitants on SCR activity

The effect of different precipitants (NH₃·H₂O, (NH₄)₂CO₃ and NaOH) on SCR activity over Sm_0.075Fe_0.925 was investigated. As shown in Fig. 7, in the temperature range of 150–250°C, the catalyst produced with NH₃·H₂O as precipitant had the highest activity with a maximum NO_x conversion exceeding 93%. There was no significant difference in the activity below 250°C for the catalyst prepared with the (NH₄)₂CO₃ and NaOH as the precipitating agent. In addition, at 300°C the catalyst prepared with (NH₄)₂CO₃ exhibited a slightly better NO_x conversion (66.7%) than the catalyst prepared with the other two precipitants, which may be due to the difference in the particle size and dispersity of catalyst. From the SEM analysis in Fig. 3, the catalyst prepared with NH₃·H₂O as the precipitant had a small particle size, loose structure and good particle dispersion, meaning that particle size and dispersity could affect the SCR activity.

Fig. 7. Effect of precipitations on the NH₃-SCR activity of SmFe mixed oxide catalysts.

3.5.4 Effect of calcination temperature on SCR activity

Fig. 8 shows the effect of calcination temperature on the catalytic activity of Sm_0.075Fe_0.925 catalyst. The order of catalytic activity at different calcination temperatures is 350°C ≈ 400°C > 450°C > 500°C, the Sm_0.075Fe_0.925 achieved 100% the de-NOx efficiencies at calcination temperatures of 350–400°C. The catalyst's catalytic activity rapidly reduced as the calcination temperature rose. This might be because differing calcination temperatures resulted in the presence of active substances on the catalyst surface in various forms. According to the XRD pattern of the Sm_0.075Fe_0.925 catalyst that was calcined at various temperatures (Fig. 2), the diffraction peaks of Fe₂O₃ were not visible at 350°C and 400°C whereas they were present at 450°C. It should be noted when the calcination temperature raised to 500°C, the diffraction peaks of Fe₂O₃ were more obvious. A negative correlation was observed between the crystalline phase strength of Fe₂O₃ on the surface of the catalysts and their catalytic activity. Based on the previous reports, the low crystallization or amorphous phase for composite oxides catalysts is responsible for SCR activity due to short-range ordered structure in the location of active sites (Li et al., 2012; Xin et al., 2018). Thus, a reasonable explanation can be drawn that as the calcination temperature increased, finer Fe₂O₃ phases were precipitated on the catalysts, which decreased the amount of solid solution formed by Fe and Sm components and weakened the interaction between samarium oxide and iron oxide, reducing the activity of the catalysts.

Fig. 8. Effect of calcination temperatures on NH₃-SCR activity of SmFe mixed oxide catalysts.

 
3.5.5 Effect of SO₂ on SCR activity

A tiny quantity of SO₂ is typically present in the flue gas during industrial application, which causes the catalyst to become inactive. So, at 200°C, the impact of SO₂ on the SCR activity of Sm_0.025Fe_0.975 was examined. As shown in Fig. 9, when 0.05% SO₂ was added to the reaction gases, the NO_x conversion slowly decreased and then stabilized. The NO_x conversion decreased from 99% of the initial value to about 86% then recovered to about 94% after SO₂ was removed about 1 h, which was slightly lower than the initial value, indicating that the catalyst was partially deactivated. According to the literature (Wang et al., 2020b), this might be a result of the catalyst developing ammonium sulfate or ammonium bisulfate on its surface, which clogged the pores and took up the catalyst's active sites, decreasing its activity.

Fig. 9. Effect of SO₂ on the SCR activity of Sm_0.025Fe_0.975 catalyst.

 
4 CONCLUSIONS

1. The doping of rare earth oxides significantly improved the iron-based oxides with similar NO_x removal efficiency.
   
   
2. With the increase of rare earth Sm, the deNO_x efficiency of the catalyst first increased and then decreased. Sm_0.075Fe_0.925O_x catalyst exhibited the highest activity, resulting from the improved structural properties and component interaction.
   
   
3. The catalyst activity was significantly impacted by the calcination temperature. The order of catalytic activity is 350°C ≈ 400°C > 450°C > 500°C. This is due to the decreased solid solution formed by Fe and Sm components and weakened interaction between samarium oxide and iron oxide, reducing the activity of the catalysts.
   
   
4. The catalyst prepared by using NH₃·H₂O as the precipitating agent exhibited better activity than that of the catalyst prepared by (NH₄)₂CO₃ and NaOH.
   
   
5. When adding 0.05% SO₂, the deNO_x efficiency was decreased to 86% from 98% and recovered to 93% after SO₂ was removed, indicating the Sm_0.075Fe_0.925 catalyst can be basically regenerated.

 
CONFLICTS OF INTEREST

The authors declare the following competing financial interest(s): Rui Wang and Ying Wei declare a financial interest. A Chinese patent related to this research has been authorized (ZL201410195455.3).

 
ACKNOWLEDGEMENTS

This work was supported by the Key Research and Development Program of Shandong Province, China [2017GSF217006] and the Shandong Provincial Independent Innovative Project for Important Key Technology Development [2014GJJS0501].