Hybrid Cu-Fe/ZSM-5 Catalyst Prepared by Liquid Ion-Exchange for NOx Removal by NH3-SCR Process

Doan, Tuan; Dang, Anh; Nguyen, Dat; Vuong, Thanh Huyen; Le, Minh Thang; Thanh, Huyen Pham

doi:https://doi.org/10.1155/2021/5552187

Journal of Chemistry

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Abstract Introduction Experimental Results and Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2021 | Article ID 5552187 | https://doi.org/10.1155/2021/5552187

Hybrid Cu-Fe/ZSM-5 Catalyst Prepared by Liquid Ion-Exchange for NO_x Removal by NH₃-SCR Process

Tuan Doan,¹Anh Dang,¹Dat Nguyen,¹Thanh Huyen Vuong,¹Minh Thang Le,¹and Huyen Pham Thanh¹

Academic Editor: Jamal Rafique

Received22 Jan 2021

Revised21 May 2021

Accepted21 Jun 2021

Published29 Jun 2021

Abstract

A series of Cu/ZSM-5, Fe/ZSM-5, and Cu-Fe/ZSM-5 catalysts (Si/Al in ZSM-5 = 25) were prepared by different metal loadings using the liquid ion-exchange method. Several characterization methods were conducted to explore the effects of metals on the physical and chemical properties of catalysts. Meanwhile, the electron paramagnetic resonance method is also used to assess the copper and/or iron elements’ coordination and valence state at intersections or in channels of ZSM-5. The metal-loading effects of all catalysts on the catalytic activities were studied for the removal of NO_x in a fixed-bed flow reactor using selective catalytic reduction with ammonia (NH₃-SCR). The results showed that the iron’s addition could markedly broaden the operation temperature range of the Cu/ZSM-5 catalyst for NH₃-SCR between 200 and 550°C due to the presence of more isolated Cu²⁺ ions as well as additional oligomeric Fe³⁺ active sites and Fe_xO_y oligomeric species. This paper gives a facile and straightforward way to synthesize the practical-promising catalyst applied in NH₃-SCR technology to control NO_x emissions.

1. Introduction

Under industrialization with the industry’s rapid growth, environmental problems are also accumulating and increasing swiftly, making the community concerned. Specifically, pollutants in the atmosphere have become significant perils to human health. Therefore, air pollution control is one of the urgent issues for industrial countries and all over the world. The scientific demonstration of the polluting properties of several chemical compounds has paved the way for more rigorous regulations to control their emissions. There are lots of causes for this problem, but the contribution of motorized traffic plays an important role. Based on each chemical compound’s polluting properties, different laws are in place to control their emissions. In particular, the exhaust from the internal combustion engine contributes mainly to this problem. The vital pollutant compounds are incombustible hydrocarbon (HC), carbon monoxide (CO), particulate matter (PM), and nitrogen oxide (NO_x) emissions, which are primarily due to spontaneous oxidation of NO [1]. Accordingly, one of the most notable air pollutants is NO_x, which is obtained mainly from emissions in stationary and mobile sources of high-temperature combustion and from electrical discharge during thunderstorms [2]. The prominent harmful effects of NO_x are acid rain formation, photochemical smog, ozone reduction, and fog.

Recently, with continuous efforts to satisfy more rigorous regulations on NO_x emissions in both mobile and stationary sources, several innovative technologies have been exploited in parallel with the application of disposable methods as the three-way catalyst, NO_x storage and reduction (NSR), and selective catalytic reduction (SCR) [3]. In particular, one of the most effective NO_x reduction technologies is NH₃-SCR, which is using NH₃ or aqueous urea as a reducing agent to convert NO_x to nitrogen [4]. This technology uses catalysts to assist in stimulating the reaction between NH₃ and NO_x to produce more quickly harmless N₂ and H₂O productions as the following reaction [5]:

Until now, NH₃-SCR technology has been applied with a wide variety of catalysts such as precious metals, metal oxides, and zeolite-based catalysts [1]. The catalysts used in this technology are of the utmost importance with lots of particular requirements. For instance, to accommodate exhaust emissions from both light and heavy diesel engines and to increase the N₂ selectivity of the NO_x reduction reaction, the catalyst must have a wide operating temperature range (i.e., from 100 to 600°C). Besides, it also needs to be stabilized in water vapor and sulfur oxides generated from fuel combustion [3]. Consequently, the improvement of catalyst properties to enhance their efficiency in the SCR method has been increasingly studied. In particular, the most promising alternative to the currently common-use commercial catalyst as vanadium-based catalysts is the combination of zeolite-based catalysts and different transition metals such as Fe, Cu, Mo, and Cr [3]. The well-recognized CHA-structural zeolites for a deNOx application might be called SSZ-13 in particular, but the synthesis with a costly prototype is the key constraint on process commercialization of the SSZ-13 (N, N, N-trimethyl-l-adamantammonium hydroxide) [6, 7]. This disadvantage, on the other hand, could be overcome by using ZSM-5, whose synthesis is more economical and convenient. Specifically, the highly active iron and copper-denatured ZSM-5 catalyst has received more and more attention recently [8–10]. ZSM-5 is a nanometer-sized pore-structured molecular sieve material with strong acid sites that can adsorb NH₃ onto the surface and converts them to NH₄⁺. Also, the individual characteristic activity of this catalyst in the SCR reaction is shown through a miniature “nanoreactor” that exists in the internal space, providing a favorable reaction space and an electronic environment to create highly dispersed transition metal oxide clusters [11]. The advantage of forming oxide clusters and nanoparticles can be explained by the ultrasonic cavitation effect. It can support the dispersion of the active component inside micropores of ZSM-5 and diminish the relative supersaturation of solution leading to the inhibition of secondary nucleation of precursors and extra development of the crystal nucleus. Thus, the catalyst based on ZSM-5 shows outstanding catalytic activity at the low-temperature range due to the creation of more active centers through the impact of oxide clusters and nanoparticles.

It has been reported that Cu-exchanged to zeolites exhibited more activity below 300°C; when the temperature is above 350°C, the NO conversion is decreased due to the oxidation of ammonia. Meanwhile, Fe-doped zeolite gave better activity for NH₃-SCR at high temperatures [4]. However, there is still an open discussion concerning the nuclearity and identity of Fe and Cu active sites for NH₃-SCR reaction. Chronologically, the Cu-based zeolite is the first metal loading on zeolite discovered active in the SCR reaction. It is generally admitted that both the copper oxo cations and Cu²⁺ ions, which are active sites in this catalyst, play an important role in the deNOx process [1, 12]. In opposition to the Cu-zeolite system, the Fe-zeolite system submitted multiple active sites such as intrazeolite binuclear Fe-oxo, isolated Fe²⁺ and Fe³⁺ ions, small Fe_xO_y clusters, or oxygen bridged binuclear iron species [HO–Fe–O–Fe–OH]²⁺, groups of a wide range of nuclearity and large sets of oxides [13–15]. Nowadays, bimetal catalysts based on zeolites have attracted much attention, especially codoping of Cu and Fe supported on zeolite, which became a promising catalyst for the NH₃-SCR reaction. Recent research has indicated that such materials show higher NO_x removal efficiency over a more extended temperature range than with individual Fe- or Cu-based zeolite catalysts [16–18]. Iron and copper’s simultaneous appearance promotes more acid sites, an alteration of electronic properties, and augmentation of redox ability than for a single metal catalyst.

This study aims to clarify the application of copper-iron bimetallic-supported commercial ZSM-5 for the NH₃-SCR catalyst. Additionally, Cu-Fe/ZSM-5 samples were compared with single metal on ZSM-5 (Cu/ZSM-5 and Fe/ZSM-5), thereby clarifying supplementary advantages, reciprocal effects of both copper and iron to catalytic properties, and the ability to widen the operating temperature range for deNOx with NH₃ reductant. In this work, the aqueous ion-exchange method was applied for preparing all the metal catalysts. The Cu-Fe/ZSM-5, Cu/ZSM-5, and Fe/ZSM-5 catalysts were characterized by various techniques to determine the physicochemical properties. The main objective of this study focuses on the following contents:(i)The effects of Cu, Fe, and codoping Cu-Fe supported on ZSM-5 zeolite to their catalyst properties were clarified(ii)Also, the NH₃-SCR activity and the hydrothermal durability of catalysts were conducted

2. Experimental

2.1. Catalyst Preparation

Metals (Cu and/or Fe) were added to ZSM-5 (DNH-Z, SiO₂/Al₂O₃ = 25 of Tosoh Corporation) through a two-step liquid ion-exchange process. In the first step, ZSM-5 was changed into ammonium form by exchanging with ammonium acetate (CH₃COONH₄, Sigma-Aldrich) solution two times. The first exchange period was the continuous agitation of ZSM-5 zeolite with solid ammonium acetate and purified water at 80°C for 5 hours; then the substantial part was gathered by centrifugation. After that, the same process was performed one more time, but with a discrepancy in exchange time (18 hours) and stirring at room temperature. The centrifuged solid portion was subsequently dried at 120°C to accomplish NH₄⁺/ZSM-5 powders. In the second step, the ammonium form of ZSM-5 was ion-exchanged with metals, in which Cu and/or Fe were doped using Cu(CH₃COO)₂.H₂O (Sigma-Aldrich) and/or Fe(CH₃COO)₂.4H₂O (Sigma-Aldrich), respectively. The solution was mixed continuously at room temperature within 24 hours and then centrifuged and washed with distilled water to reach a solid component. The collected part was then dried at 120°C for the night and calcined at 550°C for 5 hours in the flow of 20% O₂/Ar. The ZSM-5 catalyst samples were denoted as xCu/ZSM-5, yFe/ZSM-5, and xCu-yFe/ZSM-5 in which x and y are the weight percentage of Cu and/or Fe used in synthesis, respectively.

2.2. Characterizations

All characterization methods have been effectuated for as-calcined samples. Powder X-ray diffractometer Bruker D8 with Cu-Kα radiation (λ = 1.5418 Å) at a scanning speed of 1°/min within 2θ = 5–50° was used to achieve the X-ray diffraction (XRD) patterns of catalysts. The field emission scanning electron microscopy (FE-SEM) was performed on Hitachi S-4800 (Japan) to explore the samples’ different morphology. Simultaneously, the chemical compositions of the catalysts were also determined by the Hitachi S-4800 (Japan) machine equipped with an energy-dispersive X-ray spectrometer (EDS). Measurements of nitrogen adsorption-desorption were performed at −196°C using a Micromeritics Analyzers ASAP 2020. The samples were degassed under a vacuum at 300°C before the examination. The overall surface area was evaluated based on the Brunauer–Emmette–Teller (BET) theory and t-plot for external surface and surface and volume. UV-Vis diffuse reflectance spectra (UV-Vis DRS) were performed on an Avantes Avaspec-ULS2048XL-EVO spectrometer equipped with an integrating sphere coated with BaSO₄, from 200 to 1000 nm, and the program AvaSoft 8 was used. The different copper and iron species were quantified by the area ratios of the respective subbands relative to each other [19]. Temperature-programmed desorption of ammonia (NH₃-TPD) experiments for the surface acidity determination were performed on a Micromeritics Auto Chem 2920 instrument. The TPD measurements were conducted in He flow (30 mL/min), and the temperature was increased from 100 to 600°C with a heating rate of 10°C/min. Besides, the temperature-programmed reduction with H₂ (H₂-TPR) was also carried out on Micromeritics Auto Chem 2920 instrument. In the initial, 0.1 g of the catalyst was placed into the quartz U-shaped microreactor and pretreated in He at 300°C for 1 hour, followed by cooling down to 50°C before the acquisition. Afterward, the ramping program was processed by the flow of 10 vol% H₂/He from 50 to 900°C (30 mL/min), with a heating rate of 10°C/min. A detector continuously collected the signals during this process. Additionally, the reactor’s cooling trap was positioned next to the experiment to prevent the byproducts of experiments (i.e., H₂O, CO₂) from seeping into the detector. Electron paramagnetic resonance (EPR) spectra were performed on the Bruker EMX-Micro-EPR spectrometer, with a continuous wave X-band, using a microwave power of 6.3 mW, a modulation frequency of 100 kHz, an amplitude of 0.5 mT, and a magnetic field of 1000–5000 G.

2.3. Catalytic Performance

The catalytic activity test of all catalysts for the NH₃-SCR reaction was carried out in a fixed-bed quartz reactor working under a steady flow mode with ABB Gas Analyzer AO2020 Limas21 (UV sensor) spectrometer in the condition of excess oxygen (Figure 1). For each activity test, 100 mg catalyst (40–60 mesh) and the total flow rate of 200 mL/min were used, leading to the gaseous hourly space velocity of 120 000 h⁻¹. Moreover, to minimize condensation in downstream tubing, all the gas lines were heated and kept at approximately 100°C. In this reaction system, the used reactant gas has a composition of 1000 ppm NO, 1000 ppm NH_3, 8 vol% O₂, and Ar as balance. The temperature range of the reaction was from 200°C to 550°C. Firstly, the catalyst was activated in a flow of 20% O₂/Ar at 550°C for 1 hour. At each temperature, the expected time to obtain a steady state was about 1 hour. A similar condition was followed for the NH₃ oxidation reaction, where NO was not added to the gas mixture. In the investigation of hydrothermal stability, the catalysts were aged in a quartz tube reactor at 650°C or 750°C in 10% H₂O/air under a total flow rate of 1 L/min for 24 hours before the activity test. When the system was already stable, calculation of the NO and NH₃ conversions was performed through the inlet and outlet gas contents by the following equations:

3. Results and Discussion

3.1. Structure and Texture of Catalysts

The catalysts’ structure investigated by the XRD method is presented in Figure 2. In JCPDS data cards (JCPDS 42-0024), which show peak diffractions at 7.92°, 8.80°, 14.78°, 23.10°, 23.90°, and 24.40°, the prevalent diffraction peaks for commercial ZSM-5 can be simple to index into the standard parameters for MFI topology. The XRD patterns of all catalysts have included all characteristic peaks corresponding to ZSM-5, suggesting that ZSM-5 does not destroy its original form during the process of aqueous ion-exchange and calcination [20]. However, in contrast to those of the parent sample, the intensities of Fe/ZSM-5, Cu/ZSM-5, and Cu-Fe/ZSM-5 decrease due to the higher absorption coefficient of metal compounds for X-ray radiation or treatment with neutral aqueous solutions [21].

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We can claim safely from Table 1, in conjunction with the chemical composition, that the synthesized materials are Fe- and Cu-containing ZSM-5-zeolites. No diffraction peaks were found at either metal and/or to metal oxide clusters, indicating that the copper and/or iron formed species are well distributed and within the range of nanometers [22]. Interestingly, the Cu-Fe/ZSM-5 synthesized does not have a peak of diffraction like α-Fe₂O₃ and CuO_x. In contrast, its peak amplitude and corresponding crystallinity are slightly more significant than those of Cu/ZSM-5, as shown in Figure 2. This result indicates that incorporating Cu and Fe in ZSM-5 simultaneously could decrease the effects of a single metal onto ZSM-5. Furthermore, in all samples, the Na concentrations are below 0.09 wt.% (Table 1), which does not impact catalyst NH₃-SCR performance of the catalysts [23].

FE-SEM observed the morphological properties of the synthesized zeolites and the relevant photographs in Figure 3. It is apparent that the ZSM-5 parent primarily consists of bright polycrystalline aggregates, irregularly localized, and distinct edged in the typical geometry [24]. Moreover, after doping metal(s), the primary morphology for all catalysts remains almost unchanged. However, depending on the metal loading, small amounts of aggregates composed of nanosized particles, particularly in the high amount of metal loading that can be created during calcination, emerge from the primary morphology of Fe/ZSM-5, Cu/ZSM-5, and Cu-Fe/ZSM-5. Or Doan et al. note that the combined Cu_xO or α–Fe₂O₃ nanoparticles can be ascribed to particular layers of suspended matter [21]. Alongside the EDS results (Table 1) and the XRD patterns (Figure 2), this can be due to the fact that some large metal comprising agglomerations are well-distributed crystal metal oxide particles onto ZSM-5 composition. For Cu-Fe bimetal catalysts based on ZSM-5, which have a metal content of about three percent, the catalysts are smoother than the other two individual metal-fueled zeolites with a metal load of over two percent.

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Nitrogen adsorption-desorption isotherms for all catalysts are shown in Figure 4. The ZSM-5, Fe/ZSM-5, Cu/ZSM-5, and Cu-Fe/ZSM-5 isotherms are categorized as category I, characteristic of the micropores materials according to IUPAC classification. The nitrogen intake at about P/P₀ = 0.01 of all catalysts is close to that of the parent ZSM-5, indicating that the microporous is not greatly blocked during the preparation. Table 2 displays the significant textural properties of all nanocatalysts. The calculation of N₂ physisorption is a function of zeolites’ topology, including S_BET, V_pore, and average pore diameter. It suggests that the ZSM-5 carrier has large surface areas and the active components could scatter well on the carrier through the liquid ion-exchange technique. As seen in Table 2, metal species introduction had little effect on the textural properties of ZSM-5. Overall, a slight decrease in the BET surface area typically depends on the content and form of metal. However, V_pore is a little higher for all nanocatalysts than the ZSM-5. This is why urea alkaline environment-induced ZSM-5 particles split into smaller particles and thus increased pore volume [25]. Furthermore, a minor increase in pore diameter of ZSM-5 also resulted from metal loading. The explanation may be due to the dissociation of some boundaries in ZSM-5 because of the urea alkaline environment, and consequently, the pore diameter of all catalysts increased [25].

3.2. Chemisorption Results

The catalyst’s surface acidity always makes a significant contribution to the NH₃-SCR reaction for the removal of NO_x [26]. This research also used the NH₃-TPD method to measure the strength and the number of acid sites in the prepared catalysts. The corresponding profiles are shown in Figure 5 and Table 3. Typically, NH₃ adsorption on weak, medium, and strong acid sites answers to the three distinct peaks at low (120–220°C), medium-high (250–380°C), and high (380–500°C) temperatures, respectively. Ammonia desorption at a lower temperature is related to weakly physisorbed NH₃ at Lewis acid sites. Oppositely, the high-temperature desorption peak is attributed to the adsorbed strongly NH₃ molecules on Brønsted acid sites [27, 28]. Although there are still many controversies, the assignment of the medium-high desorption peak is somehow attributed to NH₃ weakly adsorbed on Brønsted acid sites or associated with extraframework Al [28]. These Brønsted and Lewis sites can be attributed to acidic protons and to extraframework Al atoms, which compensate framework Al atoms charge, respectively [29]. Consequently, it appears that the presence of Al atoms in ZSM-5 zeolite is in charge of the formation of centers for ammonia chemisorption. According to Table 3 and Figure 5, the doping of Cu and/or Fe into ZSM-5 decreases the acidity of ZSM-5. In comparison with ZSM-5, sites for NH₃ chemisorption are less generated over metal introduction into the catalyst due to the replacement of Brønsted acid protons by metal [30]. Compared to individual Fe-doped ZSM-5, both strength and acid amounts of the strong acid sites are slightly decreased when codoping Cu and Fe together. In this case, it is assigned to the incorporation of Fe into the framework of Cu-Fe/ZSM-5 [31]. It was reported that the strength and acid amount of the strong acid sites is an essential factor for the adsorption of NH₃. Therefore, it influences the NH₃-SCR activity significantly [32]. With a suitable number of acid sites on Cu-Fe/ZSM-5, the catalytic activity may be further enhanced, and thus, it is more beneficial for the SCR reaction [33].

The H₂-TPR profiles of Fe/ZSM-5, Cu/ZSM-5, and Cu-Fe/ZSM-5 catalysts are shown in Figure 6 to understand the redox proprieties of the catalysts better. There can be three H₂ consumption zones observed in the TPR pattern of the Cu/ZSM-5 catalyst: the first peak at 180°C is corresponding to the reduction of external surface nanosized CuO to Cu⁺ [34], the second sharp peak at about 250°C is related to the reduction of isolated Cu²⁺ ions to Cu⁺ in ion-exchange positions of zeolites, and another one located at 370°C as a trace, can be assigned to the reduction of CuO particles to Cu⁰ and Cu ⁺ to Cu⁰ [35]. According to the high copper content, the excess amount of copper is detected as CuO when the exchange sites have been involved. One primary peak of the Fe/ZSM-5 catalyst at around 470°C ascribes to the reduction of Fe³⁺ to Fe²⁺ at zeolite ion-exchange positions [36, 37] as well as the reduction of Fe₂O₃ to Fe₃O₄ [38]. Also, a small and broad peak attended at approximately 630°C, which can refer to the reduction of Fe₃O₄ to FeO [39] and a residual fraction of framework Fe³⁺ that is hardly reduced [40]. Krishna et al. [41] reported that if there have been extended FeO_x phases, they commonly point to reduction peaks around 600°C. Ariel Guzmán-Vargas et al. [39] based on different temperature-programmed reduction studies of Fe/ZSM-5 arranged that the peak for hydrogen consumption met at around 430°C can be associated with the reduction of Fe³⁺ to Fe²⁺ in Fe species by the framework-charge compensation. When Cu and Fe are doped simultaneously into ZSM-5, the results showed a transition of the first reduction peak to a higher temperature than Cu/ZSM-5. This change is due to the strong interaction between copper and iron metallic components and the integration in Cu-Fe/ZSM-5, making them harder to reduce. Moreover, the peak centered at 420°C relates to the reduction of Fe species [17], leading to the appearance of the iron reduction peaks at high temperatures which become more self-evident, and the copper peaks become weaker. The broader peaks present after Fe loading demonstrates that Fe species had various forms. Thus, improving the redox proficiency of such Fe species can enhance the NO_x reduction effectiveness at high temperatures.

3.3. Cu and Fe Species onto ZSM-5

The UV-Vis DRS was also used to analyze the nature and coordination of copper and iron species in the catalysts and the respective spectra are compared in Figure 7. The spectra of Fe/ZSM-5, Cu/ZSM-5, and Cu-Fe/ZSM-5 are strikingly different, showing that the Fe and Cu species exist in distinct forms. The absorption peaks of around 270 nm are due to ZSM-5 zeolite [16]. In the Cu/ZSM-5 sample’s spectra, the strong band at about 230 nm is assigned to the d − d transition of the charge-transfer band, which relates to the transition O ⟶ Cu from lattice oxygen to isolated Cu²⁺ ions [6]. In addition, the broad but less intense band between 650 and 850 nm in Cu/ZSM-5 and Cu-Fe/ZSM-5 relates to the d − d transition of Cu²⁺ ions in an octahedral environment due to the distributed CuO particles [42]. With the Fe species, the band below 300 nm confirms the existence of isolated iron ions in the structure of tetrahedrons or octahedrons. Besides, the signal between 300 and 400 nm refers to the Fe-oxo oligomers, and the peak higher than 400 nm is due to iron oxide aggregates [43]. In this research, the Fe/ZSM-5 pattern shows a band at 214 nm which is ascribed to the Fe intrusion instead of Fe³⁺ ions coordinated with the tetrahedra within the framework of the zeolites [44], and a band located at 390 nm relative to the oligo nuclear Fe_xO_y cluster [45, 46]. In the meantime, both Fe/ZSM-5 and Cu-Fe/ZSM-5 display an aggressive absorption band at 250 nm due to the charge transfer of oxygen to iron, explaining the presence of isolated Fe species entered into the zeolite framework in the tetrahedral coordination [47]. Moreover, the d-d transition of the quite-inactive factor of the NH₃-SCR as α-Fe₂O₃ also gives a band centered at 500 nm [48].

In order to analyze the state of iron and copper ions in our catalysts, EPR was conducted due to its powerful technique [49]. The obtained EPR spectra at −173°C are shown in Figure 8. In the EPR spectrum of copper samples, isolated Cu²⁺ presents a parallel signal with four adsorption peaks and a sharp peak in the vertical region [50]. On the other hand, Cu²⁺ species in dimeric species and Cu ⁺ ions do not interact with this technique [50, 51]. This phenomenon is due to the fact that isolated Cu²⁺ ions owned a nuclear spin (I = 3/2) and the 3d unpaired electrons coupled with each other. The EPR spectra of the samples were simulated using the software package EasySpin implemented in MATLAB [52]. In the results shown in Figure 8(a), all Cu/ZSM-5 samples exhibit an axial EPR spectrum of isolated Cu²⁺ ions with resolved hyperfine structure with _|| = 2.389, A_|| = 400 MHz, _┴ = 2.095, and A_┴ = 57 MHz. This has been assigned to copper species close to the channel intersection in several zeolites, for example, hydrated Cu-BEA and Cu-MFI [53]. Increasing the amount of doped Cu from 1 wt.% to 2 wt.% leads to a significant increase in the axial signal’s intensity, indicating the formation of more isolated Cu²⁺ ions (25.5% in the sample 2Cu/ZSM-5 compared to 21.5% in the sample 1Cu/ZSM-5). However, with higher Cu content of 3 wt.%, the amount of isolated Cu²⁺ species decreases to 17.5%. Besides, the isotropic signal at = 2.17 in the sample 3Cu/ZSM-5 is also broader. This broad signal is related to the magnetically interacting Cu²⁺ species in dimeric species and such Cu sites located in close vicinity or CuO clusters.

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Figure 8(b) illustrates the EPR spectra of Fe/ZSM-5 samples recorded at −173°C. It is clear to see that iron species give three different EPR signals, containing a low-field shoulder at = 4.23 and two broader signals at = 2.29 and = 2.05. To be more specific, the signal at = 4.23 is attributed to strong rhombic, highly distorted isolated Fe³⁺ species and higher coordination numbers. This could be due to the isolated iron monomers located paramagnetically at ion-exchange positions inside the zeolite micropores [54]. The weak low-field line = 2.29 and 2.05 signal might be an axial stack of distorted Fe³⁺ monomers and/or for interacting octahedrally with residual Fe₂O₃ phase [54]. Another observation made from the spectrum is that the intensity of EPR signals was raised significantly when increasing the content of Fe doping, especially with samples containing more than 1 wt.% of Fe loading. In other words, the higher intensity of the signals at = 2.29 and = 2.05 in these samples suggests the presence of more clustered Fe oxides.

As shown in Figure 8(c), compared to the monometal catalysts, EPR signals of bimetallic catalysts exhibit dominant axial signals from isolated Cu²⁺ sites. Compared to the same amount of copper loading in monocopper catalysts, the intensity of isolated Cu²⁺ in 2Cu-1Fe/ZSM-5 was increased while that of 2Cu-2Fe/ZSM-5 was lower. This behavior suggests that a suitable content of iron could lead to the formation of a higher amount of isolated Cu²⁺ ions. Moreover, increasing the amount of Fe loading in bimetallic catalysts could reveal the signals of isolated Fe³⁺ in tetrahedral coordination at = 4.23. Meanwhile, the appearance of Fe_xO_y clusters cannot be shown given that their signals at = 2.29 and 2.05 might be covered by the isolated active Cu²⁺ signal.

3.4. Catalyst Performance

The catalytic performance of the prepared catalysts has been evaluated in SCR of NO with NH₃ and is shown in Figure 9 in a microcatalytic flow reactor. The conversion of NOx with Cu/ZSM-5 samples in Figure 9(a) dramatically improves from 250°C to 300°C and stays above 80% between 300°C and 450°C, particularly in the 300–400°C range when the yield achieves over 90%. Later, the conversion of NO_x decreases steadily above 400°C and only exceeds 58% at 550°C. Among copper-loaded samples, the sample with 2 wt.% Cu shows the highest activity. Fe/ZSM-5 samples’ catalytic efficiency reveals, as seen in Figure 9(b), that the addition of iron in ZSM-5 increases the SCR reaction slightly over the whole temperature range. However, a decent range of temperatures from 400 to 500°C could only be obtained, which is typical of the Fe-zeolite mechanism with maximal operation at high temperatures [2]. Higher Fe content (up to 3 wt.%) NO_x conversion tends to a slight decrease at high temperatures. These findings suggest that higher metal loading could have a detrimental impact on conversions of NO_x. Therefore, selection for the content of the doped metal is necessary to enhance the removal of NO_x. In summary, the results of Figures 9(a) and 9(b) show that sufficient metal content might support the conversion of NO_x while excessive Cu or Fe loads may block the zeolite channel. This could be investigated in Figure 4 and Table 2 by the result of N₂ adsorption-desorption.

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When incorporating both Cu and Fe into ZSM-5, 2Cu-1Fe/ZSM-5 catalyst significantly enhances the NO_x conversion in the broader temperature range of 350–550°C, while 2Cu-2Fe/ZSM-5 sample only presents a temperature window of 450–550°C as seen in Figure 9(c) (defined as NO conversion more than 2Cu/ZSM-5 sample). Compared to both of these experiments, the findings in Figure 9(c) reveal that the 2Cu-1Fe/ZSM-5 sample shows the most potent catalyst. In specific, relative to 2Cu/ZSM-5, the conversion of NO_x over 2Cu-1Fe/ZSM-5 catalyst decreases marginally at low temperatures (below 250°C), accompanied by a rise in the range of 350–550°C. Notably, the NO_x removal yields still reached 75% at 550°C, suggesting that Cu-Fe/ZSM-5 catalysts have expanded the operating temperature window.

It is well known that NO_x conversion decrease with the reduction in N₂ selectivity at high temperatures is due to oxidation in NH₃ to unwanted N₂O formation. The NH₃ oxidation activity was also carried out for chosen catalysts. As seen in Figure 10, 2Cu/ZSM-5 and 2Cu-1Fe/ZSM-5 samples give the same curve shapes of NH₃ conversion. The reaction to ammonia oxidation begins at 225°C but happens at a low rate in the temperature range below 300°C. The NH₃ conversion is rising dramatically above this temperature. A singular copper catalyst shows the highest activity of NH₃ oxidation, while bimetal samples demonstrate the lower action. The Cu/ZSM-5 conversion was almost 100% at 550°C, while it was about 90% in the codoping Cu-Fe. Figure 9 also shows the catalytic behavior of Fe/ZSM-5 decomposing NH₃ catalysts that display an initial reaction temperature of 400°C and achieve just 35% with 550°C.

The reactions on the catalyst involve mainly the standard SCR and NH₃ oxidation reactions expressing

As shown in Figures 9 and 10, the sample efficiency of the NH₃-SCR reaction has been slightly enhanced over the NH₃ oxidation reaction below 300°C, indicating a slight effect on catalytic activity in this temperature range due to the NH₃ oxidation reaction. A strong ammonia inhibition effect of iron heads to a limited NH₃-SCR performance below 350°C for the 2Cu-1Fe/ZSM-5 samples [13]. However, this catalyst’s catalytic activity displayed an improvement in the range of 250–450°C, explained by the rise in the quantity of isolated Cu²⁺ ions identified as active sites for SCR reactions (Figures 7 and 8) [50]. Compared to Cu/ZSM-5, the 2Cu-1Fe/ZSM-5 sample could not increase its NH₃ oxidation activity. This may be explained by the decrease in the number of bulk CuO species that are expected to be extremely active for NH₃ oxidation in the bimetal study [55]. A reduction in the number of bulk CuO species on the surface with Fe and Cu prevents the NH₃ oxidation reaction. Consequently, the decrease in the NH₃ oxidation potential and increase in the sample catalytic activity of 2Cu-1Fe/ZSM-5 contributes by the small amount of oligomeric Fe_xO_y species and isolated Cu²⁺.

In the 2Fe-2Cu/ZSM-5 sample, as discussed in EPR results, the oligomeric Fe³⁺ ions have replaced a small number of isolated Cu²⁺ ions, whereas Cu²⁺ isolated ions are readily combined with CuO_x species [6]. This means that the decrease in the isolated Cu²⁺ count (Figure 8(c)), surface area decreases, and ZSM5 pores volume (Table 2) by the aggregated Fe₂O₃ particles decreases 2Fe-2Cu/ZSM-5 sample SCR operation at lower and medium temperatures at below 400°C [56]. In the meantime, the increase in SCR performance above 400°C might affect an increased amount of oligomeric Fe_xO_y and Fe₂O₃ clusters in the sample.

The hydrothermal stability of catalysts is an essential problem for SCRs since the generation of heat during the process of regeneration of DPF must be dealt with [57]. To investigate this property, the 2Cu/ZSM-5 and 2Cu-1Fe/ZSM-5 catalysts were treated in 10% H₂O/air under a total flow rate of 1 L/min at 650°C and 750°C for 24 hours. Figure 11(a) depicts the SCR activity differences between fresh and hydrothermal aged samples, observing that the NO_x conversion of hydrothermally processed samples reduces the whole temperature range. In particular, both 2Cu-1Fe/ZSM-5_650°C and 2Cu-1Fe/ZSM-5_750°C achieved more than 60% NO_x conversion from 300 to 500°C. Both 2Cu-1Fe/ZSM-5 samples with two hydrothermal aging conditions have considerably higher NO_x conversion than 2Cu/ZSM-5_650°C, meaning that Cu-Fe/ZSM-5 is more stable and resistant to relatively harsh hydrothermal treatment than Cu/ZSM-5. To be more specific, XRD patterns of the fresh and hydrothermal aging catalysts are shown in Figure 11(b). The result deconvoluted that all the other catalysts have the MFI zeolite structure apart from the 2Cu/ZSM-5_750°C sample. This singular copper exchanged ZSM-5 catalyst appeared to be more sensitive to hydrothermal treatment since the collapse of the MFI structure occurred at 750°C. This framework damage could be probably due to the aggregation of copper species, which could affect negatively the stability of the structure and lead to the deterioration in catalytic activity on NH₃-SCR [18]. On the opposite, the hydrothermal treatment even at harsh conditions just had a negligible effect on the structure of 2Cu-1Fe/ZSM-5 samples. This insignificant effect on the bimetallic catalyst suggests that the addition of iron can help to stabilize the copper dispersion during aging; therefore, the increase in dispersing copper attends to be the primary explanation for the fact that the Cu-Fe/ZSM-5 catalysts improve hydrothermal stability.

(a)

(b)

4. Conclusion

In conclusion, hybrid Cu-Fe/ZSM-5 zeolites have been successfully synthesized using a liquid ion-exchange technique. Both catalysts systematically characterized the physicochemical properties by different techniques and were compared with mono Fe/ZSM-5, Cu/ZSM-5 zeolites developed using the same process. The XRD, FE-SEM, and N₂ adsorption-desorption also demonstrated that most of the physical structure and crystal of ZSM-5 remained while Cu and Fe are codoped. The H₂-TPR findings show that Cu-Fe/ZSM-5, owing to high-temperature redox capacities and abundance types of iron materials, has strong redox capability even over a broader temperature spectrum than Cu/ZSM-5. The UV-Vis DRS detected multiple active locations after Fe loading, namely, Fe³⁺ and Fe_xO_y, and a close relationship between Fe and Cu. The EPR results also confirm this. Compared to the Cu/ZSM-5 catalyst having the same amount of copper loading, the quantity of Cu²⁺ isolated was increased when the required amount of Fe was included in the Cu-Fe/ZSM-5 catalyst. Besides, the increased number of Fe_xO_y oligomeric in this experiment profits from the NH₃-SCR reaction at high temperatures. Due to these characteristics, the Cu-Fe/ZSM-5 synthesized hybrid exhibits superior catalytic efficiency in the NH₃-SCR reaction., i.e., high NO conversion in a wide temperature window (350–550°C) and increase in N₂ selectivity. Our research offers an alternate NH₃-SCR catalyst for potential applications shortly.

Data Availability

X-ray diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, nitrogen adsorption/desorption, UV-Vis diffuse reflectance spectroscopy, temperature-programmed desorption of ammonia, temperature-programmed reduction of hydrogen, and electron paramagnetic resonance methods used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant no. 104.05–2018.306 and Vingroup Innovation Foundation (VINIF) under grant no. VINIF.2020.TS.124. This work has also been supported by the RoHan Project funded by the German Academic Exchange Service (DAAD, No. 57315854) and the Federal Ministry for Economic Cooperation and Development (BMZ) inside the framework “SDG Bilateral Graduate School Programme”.

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