Weijia Ren1,2, Qi Xin1,2, Zhesheng Hua1,2, Zhong Zheng1,2, Lifeng Xiao1,2, Shaojun Liu1,2, Chenghang Zheng This email address is being protected from spambots. You need JavaScript enabled to view it.1,2, Yang Yang This email address is being protected from spambots. You need JavaScript enabled to view it.1,2 

1 State Key Lab of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
2 State Environmental Protection Center for Coal-Fired Air Pollution Control, Zhejiang University, Hangzhou 310027, China


Received: February 20, 2023
Revised: July 18, 2023
Accepted: August 8, 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.


Download Citation: ||https://doi.org/10.4209/aaqr.230032  


Cite this article:

Ren, W., Xin, Q., Hua, Z., Zheng, Z., Xiao, L., Liu, S., Zheng, C., Yang, Y. (2023). Research on Performance and Mechanism of the NH3-SCR Reaction over Fly Ash-derived Mn-Ce/Zeolite X. Aerosol Air Qual. Res. 23, 230032. https://doi.org/10.4209/aaqr.230032


HIGHLIGHTS

  • Zeolite X was synthesized from coal fly ash in the optimum reaction conditions.
  • Mn-Ce-FX showed similar performance with commercial zeolite X catalyst.
  • The mechanism of Mn-Ce-FX on NH3-SCR activity was studied via DRIFTS.
  • Two SCR mechanisms occurred simultaneously on the surface of Mn-Ce-FX.
 

ABSTRACT


Zeolite X was synthesized from fly ash under optimized conditions, which was supported with Mn, Ce, and Mn-Ce respectively. Fly ash-derived zeolite X catalysts exhibited similar catalytic performance as compared with the commercial zeolite X catalyst. XRD, N2 adsorption, SEM, XPS, and H2-TPR were used to investigate the relationship among the structure, physicochemical properties, and catalytic activities of the catalysts. Finally, the mechanism of NH3-SCR reaction on Mn-Ce/zeolite X was systematically performed by using diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The characterization results showed that the amorphous structure and good dispersion between Mn and Ce on the surface of the zeolite changed the electronic properties of the active components, improved its low-temperature catalytic activity, brought it characteristics of Mn and Ce at the same time, and broaden the reaction temperature range. The NO conversion rate of Mn-Ce-FX catalyst remained above 80% at 200°C–300°C. From DRIFTS we suggest that Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism are simultaneously carried out on the catalyst surface. According to the FTIR results, the Eley-Rideal mechanism has a great influence on the reaction below 250°C, and the Brønsted acid sites adsorb a large amount of NH3, resulting in an excellent low-temperature activity. When above 250°C, Langmuir-Hinshelwood mechanism plays a dominant role. The nitrates on the surface gradually convert to bidentate nitrates, impeding the SCR reaction, could be one of the reasons for reducing the high-temperature activity.


Keywords: Fly ash, Catalyst, Emission controls


1 INTRODUCTION


Coal fly ash (CFA) is a by-product of coal combustion, and the recycling for other applications is rather low, resulting in a large amount of CFA accumulation (Luo et al., 2021). CFA contains toxic and harmful substances such as polycyclic aromatic hydrocarbons (PAHs), heavy metals, and radioactive elements (232Th, 226Ra, 40K, etc.), which will cause damage to soil condition and polluted water bodies, besides, spread dust pollution to the air (Gollakota et al., 2019; Khan and Umar, 2019; Ren et al., 2020). A case can be made that this problem affects people from all walks of life (Borm, 1997; Silva et al., 2012; Wang et al., 2020). Instead, CFA could be transformed into a raw material to remove gas pollutants produced by coal combustion, which would not only alleviate the pressure on the environment but also help to reduce gas pollution and improve its utilizing value.

Selective catalytic reduction (SCR) is a commonly used and effective technology to remove NOx from coal combustion (Carja et al., 2007; Boningari et al., 2018). V2O5/TiO2 mixed with WO3 or MoO3 is one of the most widely used catalysts for NH3-SCR at present (Liu et al., 2014; Xu et al., 2018; Nuguid et al., 2019; Xu et al., 2019; Kuma et al., 2020). However, the optimum operating temperature (350–400°C) is relatively narrow, in which case the catalyst performance is vulnerable to damage when working upstream of the exhaust system (Zhang et al., 2015; Jiang et al., 2019; Li et al., 2020; Zheng et al., 2020). The researchers had demonstrated that Mn-containing catalysts exhibited good low-temperature activities in SCR reactions, and when doped with Ce, higher catalytic activities and a wider operating temperature range will be obtained. Research in this direction has become one of the hotspots in active components of SCR catalysts (Chen et al., 2017; Nam et al., 2017; You et al., 2017; Li et al., 2018b; Yang et al., 2020; Lei et al., 2021).

Many research studies have shown that support has a strong effect on the activity of catalyst (Corma et al., 2002; Panagiotopoulou and Kondarides, 2006). The combination of zeolite microstructure and surface-loaded active components can form complex structures with special structural features and specific acid-base characteristics (Chen et al., 2000; Pérez-Ramírez et al., 2005; Carja et al., 2007; Klukowski et al., 2009). In theory, zeolite with low Si/Al has more acid sites and better Redox capacity (Xu et al., 2006). Compared with commercial zeolite, CFA synthesized zeolite has a lower cost, and its special texture and surface characteristics can provide more possibilities for the improvement of catalyst activity. Therefore, the development of CFA-derived catalysts with the same catalytic activities as commercial catalysts is of great significance for the large-scale application of zeolite-derived catalysts (Carja and Delahay, 2004; Frey et al., 2009; Li et al., 2018a). Whereas, few studies investigated the application of CFA-derived zeolite catalysts in the field of SCR.

During this work, Mn, Ce, and Mn-Ce active components were loaded on the optimized zeolite prepared by CFA. We conducted catalytic activity tests and catalytic characterization experiments to study the relationship between the structure, physicochemical properties, and catalytic activities of the catalyst. Finally, the mechanism of NH3-SCR reaction was studied by in-situ infrared characterization technology. The objective of the present paper was to obtain a high value-added utilization of CFA to meet the application requirement for low-cost and low-temperature NOx removal.

 
2 EXPERIMENTAL


 
2.1 Materials and Chemicals

The CFA was provided by Datang International Power Generation Co., Ltd. (Zhejiang Province, China). NaAlO2 (98%), NaOH (96%), HCl (36.0%–38.0%), Na2SiO3·9H2O (98%) Mn(OAc)2 and Ce(NO3)3 were purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial zeolite X was obtained from the Nankai University Catalyst Factory (Tianjin, China). Deionized water was used in all solution preparations.

 
2.2 Optimal Synthesis of CFA-derived Zeolite X

20 g CFA was treated with 400 mL 10wt% HCl at 85°C for 8 h, 5 g of which is added with 6 g NaOH and calcined at 550°C for 2 h to get raw material of CFA. 5.225 g NaOH, 1.025 g NaAlO2, 8.5666 g Na2SiO3·9H2O were mixed with 22.5 mL water and reacted at 65°C for 1 h to prepare seed guide liquid. The raw material of CFA was added with a certain amount of Na2SiO3·9H2O and deionized water to control the Si/Al at 2.4. After adding 3 mL seed guide liquid and aging at room temperature for 24 h, the mixed liquid was reacted in an autoclave at 90°C for 18 h. The product was centrifuged and washed, then put into the oven at 110°C to dry overnight. At the end of the work, the CFA-derived zeolite X sample was ground and stored for later use.

All the zeolite products were ion-exchanged with 1M NH4Cl two times at 85°C for 2 h, then calcined at 550°C for 5 h to convert into H-form. The results of the optimal synthesis of CFA-derived zeolite X can be found in Fig. S1. And Table S1 shows the elemental composition during the synthesis of CFA-derived zeolite X compared with the commercial one.

 
2.3 Catalyst Preparation

Three kinds of zeolite-derived SCR catalysts were prepared via incipient wetness impregnation method using Mn(OAc)2, Ce(NO3)3, and their mixture. The active groups of SCR catalysts were Mn and Ce, with a total load of 14wt.%. The zeolite powder was impregnation at room temperature for 2 h, and dried in a water bath at 85°C. The impregnated solids were dried overnight at 105°C and then calcined in a muffle oven at 550°C for 5 h. Finally, the samples were crushed and sieved to 40–60 mesh to get the catalyst sample, which was denoted as, according to the different active components and carriers, zeolite base of CFA: Ce/FX, Mn/FX, and Mn-Ce/FX; Commercial zeolite base: Mn-Ce/CX.

 
2.4 Catalytic Activity Tests

The activity of the catalyst was tested in a fixed bed quartz reactor with a dosage of 0.2 g, in which the total gas flow was 200 mL min1, composed of 500 ppm NH3, 500 ppm NO, and 30000 ppm O2. N2 was used as a balance gas, and the outlet gas concentration was detected by the infrared Spectrometer. At each temperature point, the experiment was stable for at least 60 min until the data were stable. The conversion rate (X) of NO was calculated by the following formula:

 

[NO]in and [NO]out represent NO concentration at the inlet and outlet respectively.

 
2.5 Characterization Techniques

The crystal phase and structure of the samples were analyzed by X-ray diffraction spectrometer (XRD, Rigaku, D/max-2200, Japan) with Cu-Kaα1 radiation (λ = 1.5406A), and the scanning range was 5–90° with a step size of 0.02°.

The specific surface area and pore structure of the samples were measured at 77K using Micromeritics ASAP 2460 analyzer (Mack Corporation, USA). In all cases, the sample was evacuated and pretreated at 250°C for 8 h prior to the measurement. The Brumauer-Emmett-Teller (BET) method was used to calculate the specific surface areas. The pore size distribution and pore volume were calculated by Barrett-Joyner-Halenda (BJH) formula.

The chemical composition of fly ash samples and its derivatives were determined by the X-ray fluorescence (XRF) technique using an ADVANT’X 4200 spectrometer (Thermo Fisher, USA).

Scanning electron microscope (SEM, Hitachi, S-4800) was used to identify the surface morphologies of the samples under 15–20 kV operating voltage.

X-ray photoelectron spectroscopy (XPS, Thermo Escalab 250XI, Thermo Fisher, USA) was used to obtain the chemical states of constituent elements, using monochromatic Kα ray (H = 1486.6 eV, power 150 W) of Al target as the detection source and 500 µm beam spot size.

The H2 Temperature-programmed Reduction (H2-TPR) is carried out using a chemisorption machine (AutoChem II 2920, Micromeritics Inc.). Before H2-TPR, a 50 mg sample was placed in a micro-quartz reactor, and the helium (pretreated gas) was purged at 300°C (the programmed heating rate was 10°C min1) for 1 h and then reduced to 30°C. Following this step, H2 was adsorbed at a flow rate of 30°C min1 for 30 min, followed by helium purge at 30°C for 1 h. After the baseline stabilized, the temperature rose to 900°C at a rate of 10°C min1. The oxygen consumption in the oxidation process of catalyst was determined by analyzing TCD spectrum.


2.6 DRIFTS Studies

The formation and reaction of surface functional groups were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet, Nexus 6700). Before each experiment, all samples were purged in the in-situ reaction cell by nitrogen with a flow rate of 100 mL min1 at 350°C for 1 hour, after which background spectra were collected at different temperatures. In order to explain the SCR reaction path, three kinds of experiments were designed to study the reaction mechanism. Reactant adsorption experiments were carried out to investigate the adsorption properties and stability of NH3 and NO at temperature of 100, 150, 200, 250, 300, and 350°C. Transient reaction experiments were carried out to investigate the reaction moment of NH3-SCR with NO (or NH3) at 250°C after the saturation of NH3 (or NO, N2 as an equilibrium gas, 100 mL min1). When a mixture of 100 mL gas (NH3 500 ppm, NO 500 ppm, O2 3%, N2 as equilibrium gas) was introduced, steady-state reaction experiments were carried out to study the steady state reaction of NH3-SCR at the temperature of 100, 150, 200, 250, 350°C and reacted steadily for 60 min.

Instrumental resolutions for spectral recording ranged from 400 to 4000 cm1. During the measurements, the N2 purge spectrometer and optical devices were used in the measurement to avoid interference caused by diffusing air. A highly sensitive MCT-A detector which made of CdTe semiconductor and semi-metallic HgTe was used in the experiment to acquire spectral data. All the scanning times were set as 32 times, with a resolution of 4 cm1.

 
2.7 Characterization of Catalysts

Nitrogen adsorption/desorption isotherm were carried out as shown in Fig. 1 and Table 1. It can be found that the method of loading active components increases the specific BET surface area and pore volume, but the pore diameter decreases. For catalysts supported by a single component, The specific surface area of Mn-FX catalyst is higher than that of Ce-FX catalyst, while for catalysts supported by two components, the specific surface area is larger than that of catalysts supported by a single component. Therefore, more sites can be provided, which is more conducive to the SCR reaction. There is little difference between the pore volumes of the two-component catalyst and the single-component catalyst, but the average pore size decreases, due to the blockage caused by highly dispersed Mn and Ce. At the same time, the pore structure of the catalyst was destroyed during the synthesis process, resulting in a decrease in the average pore size. In addition, the specific surface area of the catalyst with CFA-derived zeolite X as the support is higher than that with commercial zeolite X catalyst, which may be affected by impurities and promote the dispersion of active components on the support surface (Wang et al., 2013). The SEM micrographs in Fig. S2 also shows a phenomenon, that CFA-derived zeolite X has a similar structure to commercial zeolite X which is more closely packed.

Fig. 1. Nitrogen sorption isotherms of CFA-derived zeolite X, Mn-FX, Ce-FX, Mn-Ce-FX, and Mn-Ce-CX catalysts.Fig. 1. Nitrogen sorption isotherms of CFA-derived zeolite X, Mn-FX, Ce-FX, Mn-Ce-FX, and Mn-Ce-CX catalysts.

Table 1. Textural properties of CFA-derived zeolite X, Mn-FX, Ce-FX, Mn-Ce-FX, and Mn-Ce-CX catalysts.

The XRD measurement was performed to verify the crystal structure of the catalyst, and the results are shown in Fig. 2. On the whole, the active substance has a strong peak strength, which covers some characteristic peaks of the carrier. From the point of view of single component catalysts, Mn-FX and Ce-FX show all the diffraction peaks of CeO2 and MnOx, which may be caused by the large load or the small pore structure of CFA–derived zeolite X. Similar spectra can be obtained for Mn-Ce-FX and Mn-Ce-CX catalysts supported in two components. However, it can be seen from the figure that the diffraction peak intensity of CeO2 and MnOx is weakened, which means that the surface of the carrier forms an amorphous structure. Furthermore, the characteristic peaks of zeolite X are also partially displayed, indicating that the two components disperse well on the surface of the catalyst, thus proving the better catalytic activity of two-component supported catalysts as shown in Fig. 5.

Fig. 2. XRD patterns of Mn-FX, Ce-FX, Mn-Ce-FX, and Mn-Ce-CX catalysts.Fig. 2. XRD patterns of Mn-FX, Ce-FX, Mn-Ce-FX, and Mn-Ce-CX catalysts.

 
2.8 Redox Properties of Catalysts

XPS spectra were determined to elucidate the effect of Mn and Ce valence states on catalytic activities. Fig. 3 indicates that Mn2P3/2 and Mn2P1 appear in the figure, which is a typical characteristic of the coexistence of Mn4+ and Mn3+ (Zhang et al., 2007). By resolving the peaks of Mn2P3/2, the proportion of various Mn valence states on the surface was determined. As can be seen from the figure, the sample of Mn-FX catalyst contains Mn4+, Mn3+, and Mn2+ three characteristic peaks (Zhang et al., 2014), of which the peak area of Mn3+ is larger than that of Mn4+, that is, the concentration of Mn3+ on the surface of the catalyst is greater than that of Mn4+. However, the introduction of Ce affects the Mn valence states, that the peak area of Mn4+ and Mn3+ becomes larger. The increase of Mn4+ can provide more Lewis acid sites and improve the adsorption of NH3, while the increase of Mn3+ can also make it easier for the reaction species to adsorb on the Brønsted acid sites, thus improving the catalytic activities in Fig. 5.

Fig. 3. XPS spectra of Mn-FX, Ce-FX, and Mn-Ce-FX catalysts.Fig. 3. XPS spectra of Mn-FX, Ce-FX, and Mn-Ce-FX catalysts.

Studies have shown that Ce-containing catalysts can store and release free oxygen through the Ce4+ and Ce3+ in the catalyst. Thus, the conversion of NO to NO2 at low temperature is promoted and the catalytic efficiency at low temperature is improved (Xiong et al., 2015). It can be found in the figure that both the XPS spectra of Ce-FX and Mn-Ce-FX catalysts have characteristic peaks of Ce. From the 3D peak pattern of Ce given in the figure, we obtained characteristic peaks of u, u'', u''', v, v'', and v''' which belong to the Ce4+. Meanwhile, there are also characteristic peaks of u’ and v' in the spectrum, which can be attributed to the Ce3+ (Fang et al., 2013; Shen et al., 2014). By comparing their contents, the v' and u' peak areas of Ce3+ were much smaller than those of Ce4+, and the peak areas of Ce3+ further decreased with the addition of Mn. It can be seen that the proportion of Mn4+ in Mn increased after the addition of Ce, and the catalytic activities increased accordingly. However, after the addition of Mn, the proportion of Ce3+ in Ce decreased.

Furthermore, H2-TPR experiments were also conducted to further study the Redox characteristics. As shown in Fig. 4, Ce-containing catalysts such as Ce-FX and Mn-Ce-FX catalysts have reduction peaks before and after 700°C respectively. For Mn-Ce-CX catalyst, all reduction peaks are before 700°C. In general, the reduction peak of surface coordination unsaturated oxides is located at the lower H2-TPR reduction peak, while the reduction peak of bulk oxides is located at the higher temperature peak (Trovarelli, 1996). In the catalytic reaction, the surface of coordination unsaturated oxides is more important than the bulk oxides. There is a narrow reduction peak of Mn-FX catalyst sample at 375°C, which can be considered as the reduction peak of Mn4+ (De Lucas et al., 2005). Comparing the two-component catalyst with the single-component catalyst, it can be found that the two-component catalyst has the hydrogen reduction peak with lower reduction temperature, which should be due to the strong interaction between MnOx and CeO2, thus changing the electronic properties of the surface-active component and improves its reduction ability (Fig. 5).

Fig. 4. H2-TPR profiles of Mn-FX, Ce-FX, Mn-Ce-FX and Mn-Ce-CX catalysts.Fig. 4. H2-TPR profiles of Mn-FX, Ce-FX, Mn-Ce-FX and Mn-Ce-CX catalysts.

Fig. 5. SCR activity of Mn-FX, Ce-FX, Mn-Ce-FX, and Mn-Ce-CX catalysts. Reaction conditions: 500 ppm NO, 500 ppm NH3, 3% O2, 60000 mL g−1 h−1.Fig. 5. SCR activity of Mn-FX, Ce-FX, Mn-Ce-FX, and Mn-Ce-CX catalysts. Reaction conditions: 500 ppm NO, 500 ppm NH3, 3% O2, 60000 mL g−1 h−1.

Combined with the analysis of XPS and H2-TPR results, the interaction between MnOx and CeO2 changed the electronic properties and transferred electrons. It would improve the catalytic reduction ability, and then enhanced the catalytic activity. The same effect was also observed in the increase of Mn4+ after the addition of Ce, which also explains that the Mn-Ce composite catalyst had a lower hydrogen reduction temperature than the single-component catalyst.

 
2.9 Results and Discussions on SCR Performance of Mn and/or Ce/CFA-derived Zeolite X and Commercial Zeolite X

Activities of four types of SCR catalysts in the temperature range of 100–400°C are shown in Fig. 5. MnOx and CeO2 were active species, loaded onto fly-ash based zeolite X (FX) and commercial zeolite X (CX). For catalysts with single component support (Mn-FX and Ce-FX), the catalytic activity of Mn-FX was higher than that of Ce-FX at a lower temperature (< 250°C). However, when the temperature was higher than 250°C, the activity of Mn-FX catalyst decreased significantly, while Ce-FX catalyst was maintained. As for the two-component supported catalysts, the catalytic activity was improved at low temperature with the addition of Ce, and the NO conversion rate remained above 80% in the range of 200–300°C. Meanwhile, the catalytic activity at high temperature was significantly improved compared with Mn-FX catalyst. Finally, we compared the activity of the SCR catalyst supported by CFA-derived zeolite X (Mn-Ce-FX) and the commercial zeolite X catalyst (Mn-Ce-CX). It can be found that the NO conversion trend is consistent with the temperature change, and the catalytic activity is similar at different temperatures. Nevertheless, Mn-Ce-CX catalyst was superior to Mn-Ce-FX catalyst at low temperature and reached a maximum value of 95% at 200°C. Generally, with the addition of Ce, the catalytic activity of the CFA-derived zeolite SCR catalyst was greatly optimized, and the catalytic performance of Mn-Ce-FX catalyst is comparable to that of Mn-Ce-CX catalyst.

 
2.10 SCR Mechanism on Mn-Ce-FX Catalyst

NH3 was used as the probe molecule to obtain the adsorption spectra of NH3 at different temperatures, so as to study the distribution of acid sites on the catalyst surface. The result is depicted in Fig. 6. As we know, the N-H stretching vibration region located at 3400–3000 cm–1 is the coordination adsorption of NH3 and oxygen atoms (Cheshkova and Stoilova, 2001). The peak located at 1590 cm–1 is classified as the antisymmetric stretching vibration adsorption peak of NH3 (Larrubia et al., 2000). The symmetric bending vibration mode of NH3 adsorbed on the Lewis acid site is located at the peak at 1244 cm–1 (Chen et al., 2010), and the peaks located at 1432–1456 cm–1 are classified as the asymmetric bending vibration mode of NH4+ adsorbed on the Brønsted acid site (Shan et al., 2012). According to the changes in peak intensity and position in the light spectrum, it is obvious that the intensity of all vibration peaks related to NH3 decreases significantly with the temperature rise. Contrary, the intensity of the absorption peak at 1590 cm–1 remains essentially unchanged. The above results, combined with the characteristics of NH3 easy partial oxidation at high temperature, proved that the absorption peaks also represented the oxides of NH3 on the catalyst surface. Additionally, the peak strength of NH4+ adsorbed on the Brønsted and NH3 adsorbed on Lewis acid sites at different temperatures substantiate the weaker thermal stability of all species on the Brønsted acid site.

Fig. 6. In-situ FTIR spectra for NH3 adsorption on Mn-Ce-FX catalyst at different temperatures. Fig. 6. In-situ FTIR spectra for NH3 adsorption on Mn-Ce-FX catalyst at different temperatures.

Nitrites and nitrates were also obtained in the adsorption experiments of NO + O2 at different temperatures (Fig. 7). A series of peaks were formed on the surface of the catalyst, which were located at 1600, 1569, 1400, 1290, and 1240 cm–1 respectively. Among them, 1600 cm–1 is similar to the asymmetric vibration of NO2 (Long and Yang, 2000; Nam et al., 2017). The peak at 1400 cm–1 belongs to -NO2 (Hadjiivanov, 2000). As a result of investigations, it is inferred that as the temperature increases from 50°C to 250°C, the peak of 1569 cm–1, which is close to the bidentate nitrates (Hadjiivanov, 2000; Yang et al., 2013), gradually becomes stronger, indicating that bidentate nitrates are generated on the catalyst surface. However, the peak at 1290 cm–1 belonging to monodentate nitrates remains unchanged (Trovarelli, 1996; Underwood et al., 1999; Chi and Chuang, 2000; Kantcheva and Cayirtepe, 2006; Liu et al., 2009), while the peak at 1240 cm–1 represents bridge nitrates gradually increasing (Qi and Yang, 2004). These results indicate that from 50°C to 250°C, with the rise of temperature, the nitrates adsorption capacity of Mn-Ce-FX catalyst increases, and the active monodentate nitrates will be transformed into relatively stable bridged nitrates or chelate bidentate nitrates. As the temperature continues rising, the adsorption strength of NOx on the catalyst surface gradually weakens.

Fig. 7. In-situ FTIR spectra of NO + O2 adsorption on Mn-Ce-FX catalyst at different temperatures.Fig. 7. In-situ FTIR spectra of NO + O2 adsorption on Mn-Ce-FX catalyst at different temperatures.

In order to study the changes of species on the catalyst surface, in situ diffuse reflection infrared transient experiments were carried out. Fig. 8 shows the characterization results of NO + O2 after NH3 adsorption at 250°C. The peak at 1570 cm–1 is the antisymmetric stretching vibration adsorption peak of NH3. The peak at 1230 cm–1 can represent the symmetric bending vibration mode of NH3 adsorbed on the Lewis acid site (Chen et al., 2010), due to similar to the peak of bridge nitrate, it may interfere with each other. Therefore, another independent peak needs to be found. At this point, the peak located at 1644 cm–1 can represent the antisymmetric stretching vibration absorption peak of NH3 at the Lewis acid site, and an asymmetric bending vibration mode peak of NH4+ adsorbed on the Brønsted acid site was found at 1440 cm–1 (Shan et al., 2012). It was found that the adsorption of NH3 on Lewis and Brønsted acid sites decreased with the passage of NO and O2, and the conversion at Brønsted acid site was the dominant one. Among the enhanced peaks, the peaks located at 1570 cm–1, 1272 cm–1, and 1230 cm–1 are similar to the peaks of chelate bidentate nitrates (Hadjiivanov, 2000; Yang et al., 2013), monodentate nitrates (Kantcheva and Cayirtepe, 2006) and bridging nitrates (Qi and Yang, 2004), respectively. According to the above spectral results, with the introduction of NO, monodentate nitrates were mainly generated in the first 10 minutes of the reaction, while only a small amount of bidentate nitrates were generated. Over time, a large number of bidentate nitrates began to be transformed on the catalyst surface. The NH3 adsorption can react with the gaseous phase NO + O2, and the bidentate nitrates should be generated in large amounts along with the reaction.

Fig. 8. In-situ FTIR spectra for NO + O2 adsorption on Mn-Ce-FX catalyst after adsorbing NH3 at 250°C.Fig. 8. In-situ FTIR spectra for NO + O2 adsorption on Mn-Ce-FX catalyst after adsorbing NH3 at 250°C.
 

Transient in-situ diffuse reflectance infrared examination of NO + O2 adsorbed at 250°C followed by NH3 was also performed (Fig. 9). Among the weakened peaks, the absorption peaks at 1570 cm–1, 1272 cm–1, and 1230 cm–1 correspond to chelate bidentate nitrates (Hadjiivanov, 2000; Yang et al., 2013), monodentate nitrates (Kantcheva and Cayirtepe, 2006) and bridge nitrates, respectively. With the entry of NH3, all the adsorption peaks related to NOx began to weaken gradually. Corresponding to the enhanced peaks, the peak of 3400–3000 cm–1 is the N-H stretching vibration region, which is the coordination adsorption between NH3 and oxygen atoms on the surface of the catalyst. The antisymmetric stretching vibration absorption peak of NH3 at the Lewis acid position is located at 1614 cm–1, and the symmetrical bending vibration mode is located at 1250 cm–1 (Chen et al., 2010). The peaks located at 1700 cm–1 and 1440 cm–1 are classified as the asymmetric bending vibration mode of NH4+ adsorbed on the Brønsted acid site (Shan et al., 2012). In conclusion, all NOx-related species on the surface of the catalyst were transformed and gradually reduced by NH3 entry, while the number of NH3 adsorption on the Lewis and Brønsted acid sites increased gradually, and the N-H vibration peak was also enhanced.

Fig. 9. In-situ FTIR spectra for NH3 adsorption on Mn-Ce-FX catalyst after adsorbing NO + O2 at 250°C.Fig. 9. In-situ FTIR spectra for NH3 adsorption on Mn-Ce-FX catalyst after adsorbing NO + O2 at 250°C.
 

Based on the reactant adsorption and transient experiments, we further carried out the steady-state experiments to study the species changes on the catalyst surface at different temperatures (Fig. 10). N-H stretching vibration region located at 3400–3000 cm–1 is the coordination adsorption of NH3 and oxygen atoms. 1243 and 1625 cm–1 represent NH3 at Lewis acid site, and the peaks at 1440–1434 cm–1 belong to the species adsorbed on the Brønsted acid site. According to the infrared spectrum results, NH3 of the Lewis and Brønsted acid sites were mainly presented in the low-temperature section of the steady-state reaction and are gradually consumed with the increase of temperature. In addition, the peak of 1569 cm–1 is similar to that of bidentate nitrates (Hadjiivanov, 2000; Yang et al., 2013). Although it may be covered by the NH3 adsorption peak at low temperature, the bidentate nitrates peak still exists at 350°C, and the NH3 adsorption nearly disappears. Therefore, NH3 is more easily activated on the surface of the catalyst.

Fig. 10. In-situ FTIR spectra of SCR steady-state reactions on Mn-Ce-FX catalysts at different temperatures.Fig. 10. In-situ FTIR spectra of SCR steady-state reactions on Mn-Ce-FX catalysts at different temperatures.

The reactant adsorption and transient experiments above indicated that NH3 is adsorbed and transformed into several species when NH3-SCR reaction occurs on the catalyst of CFA-derived zeolite X loaded with Mn-Ce. The peak strength of NH4+ adsorbed on the Brønsted acid site is stronger than that on the Lewis acid site, but the thermal stability is on the opposite. Hence, the NH4+ at the Brønsted acid site decreases rapidly with increasing temperature. At 250°C, under the condition of NO and O2 being passed through the NH3-saturated catalyst, all the species adsorbed on acid sites were reduced, illustrating that NH3 adsorbed on the catalyst surface can react with NO + O2 in the gaseous phase.

When the temperature rises, the amount of NO2 and nitrates formed by NOx increases, and the unstable monodentate nitrates will be converted into more stable bridged nitrates or chelate bidentate nitrates. With the further increase in temperature, the species of NOx remained unchanged, but the adsorption strength gradually decreased. After NH3 was added, all the adsorption peaks related to NOx began to weaken, but no reduction of bidentate nitrates was found. This result may explain why the NOx-adsorbed species on the catalyst surface can react with NH3, whereas the resulting bidentate nitrates might block the active site at that time, thus impeding the SCR reaction on the catalyst.

It can be speculated that the NH3 adsorbed on the Brønsted and Lewis acid sites is the main activated NH3, while NO is oxidized to NO2 or converted to nitrate. In the lower temperature section (< 250°C), the nitrate can react with the NH3 adsorbed on the acid site of the catalyst surface, obeying Langmuir-Hinshelwood mechanism. Furthermore, when the temperature exceeds 250°C, the nitrates gradually transform into bidentate nitrates, which are difficult to react with NH3 (Qu et al., 2016), and may be one of the reasons for the reduced catalytic activity. At the same time, NH3 adsorbed on the acid site became the main active species on the surface of the catalyst. They could directly react with the gaseous NO to generate intermediates, and further generate N2 and H2O, which follow the Eley-Rideal mechanism.

 
3 CONCLUSION


We have optimized the preparation method of CFA-derived zeolite X, which was obtained at these conditions, NaOH/CFA = 1.2, Si/Al = 2.4, and synthesized at 90°C for 18 h. The synthesized zeolite has a single crystal phase, showing a relatively perfect octahedral structure, comparatively complete crystallization, and uniform particle distribution. Its crystal morphology is the same as the commercial zeolite X. For the SCR reaction, Mn and Ce have an obvious synergistic effect on CFA-derived zeolite X, forming an amorphous structure on the surface of the catalyst, which changes the electronic properties of the surface active component. In the temperature range of 200°C–300°C, Mn-Ce-FX catalyst, its NO conversion rate remained above 80%, had a better low-temperature catalytic activity than Ce-HX catalyst, while its high-temperature catalytic activity was significantly improved than Mn-HX catalyst. Moreover, on the surface of Mn-Ce-FX catalyst, Eley-Rideal mechanism and Langmuir-Hinshelwood mechanism are simultaneously carried out in SCR reaction. In the low-temperature section, the Eley-Rideal mechanism has a great influence on the reaction, in which the Brønsted acid site plays a major role. This leads to a large amount of NH3 adsorption, so the catalyst has good low-temperature activity. By contrast, in the high-temperature section, Langmuir-Hinshelwood mechanism plays a major role, and the chelate bidentate nitrates and bridge nitrate will occupy the active site and hinder the catalytic reaction, thus making the high temperature activity weakened. In practice, the composition of industrial waste gas is complex (including water and SOx, etc.). The focus should be on the adaptability of catalysts to various flue gas conditions, which is of great significance for large-scale applications.


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