*Article* **Physicochemical Features and NH3-SCR Catalytic Performance of Natural Zeolite Modified with Iron—The Effect of Fe Loading**

**Magdalena Saramok 1,\* , Marek Inger 1,\*, Katarzyna Antoniak-Jurak <sup>1</sup> , Agnieszka Szymaszek-Wawryca <sup>2</sup> , Bogdan Samojeden <sup>2</sup> and Monika Motak <sup>2</sup>**


**Abstract:** In modern dual-pressure nitric acid plants, the tail gas temperature usually exceeds 300 ◦C. The NH<sup>3</sup> -SCR catalyst used in this temperature range must be resistant to thermal deactivation, so commercial vanadium-based systems, such as V2O<sup>5</sup> -WO<sup>3</sup> (MoO<sup>3</sup> )-TiO<sup>2</sup> , are most commonly used. However, selectivity of this material significantly decreases above 350 ◦C due to the increase in the rate of side reactions, such as oxidation of ammonia to NO and formation of N2O. Moreover, vanadium compounds are toxic for the environment. Thus, management of the used catalyst is complicated. One of the alternatives to commercial V2O<sup>5</sup> -TiO<sup>2</sup> catalysts are natural zeolites. These materials are abundant in the environment and are thus relatively cheap and easily accessible. Therefore, the aim of the study was to design a novel iron-modified zeolite catalyst for the reduction of NO<sup>x</sup> emission from dual-pressure nitric acid plants via NH<sup>3</sup> -SCR. The aim of the study was to determine the influence of iron loading in the natural zeolite-supported catalyst on its catalytic performance in NOx conversion. The investigated support was firstly formed into pellets and then impregnated with various contents of Fe precursor. Physicochemical characteristics of the catalyst were determined by XRF, XRD, low-temperature N<sup>2</sup> sorption, FT-IR, and UV–Vis. The catalytic performance of the catalyst formed into pellets was tested on a laboratory scale within the range of 250–450 ◦C using tail gases from a pilot nitric acid plant. The results of this study indicated that the presence of various iron species, including natural isolated Fe3+ and the introduced FexOy oligomers, contributed to efficient NO<sup>x</sup> reduction, especially in the high-temperature range, where the NO<sup>x</sup> conversion rate exceeded 90%.

**Keywords:** nitric acid plant; selective catalytic reduction; clinoptilolite; iron-modified zeolite catalyst

### **1. Introduction**

NO<sup>x</sup> emitted from stationary (power plants, nitric acid, or adipic acid production) and mobile sources are treated as a serious environmental problem. They contribute to the formation of acid rain and photochemical smog and cause deterioration of water and soil quality [1]. Therefore, it is highly necessary to reduce industrial NO<sup>x</sup> emissions. The method of NO<sup>x</sup> abatement is usually correlated with the emission origin. In the plants, which produce nitric acid, high-efficiency absorption, non-selective catalytic reduction (NSCR), selective catalytic reduction (SCR), and absorption in sodium hydroxide solution can be used [2]. Among them, selective catalytic reduction with ammonia (NH3-SCR) is

**Citation:** Saramok, M.; Inger, M.; Antoniak-Jurak, K.; Szymaszek-Wawryca, A.; Samojeden, B.; Motak, M. Physicochemical Features and NH3-SCR Catalytic Performance of Natural Zeolite Modified with Iron—The Effect of Fe Loading. *Catalysts* **2022**, *12*, 731. https:// doi.org/10.3390/catal12070731

Academic Editor: Anker Degn Jensen

Received: 18 May 2022 Accepted: 28 June 2022 Published: 1 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the most efficient. The process involves selective reduction of NO<sup>x</sup> with NH<sup>3</sup> to form N<sup>2</sup> and H2O, as presented by Equations (1)–(3):

$$4\text{ NO} + 4\text{ NH}\_3 + \text{ O}\_2 \rightarrow 4\text{ N}\_2 + 6\text{ H}\_2\text{O} \tag{1}$$

$$2\text{ NO}\_2 + 4\text{ NH}\_3 + \text{O}\_2 \rightarrow 3\text{ N}\_2 + 6\text{ H}\_2\text{O} \tag{2}$$

$$2\text{ NO}\_2 + \text{NO} + 2\text{ NH}\_3 \rightarrow 2\text{ N}\_2 + 3\text{ H}\_2\text{O} \tag{3}$$

Ammonia, used as the reducing agent, is easily available in nitric acid plants since it is a substrate in the production of HNO3. Typically, the SCR reactor is installed at the end of the technological line and does not significantly affect the production of acid. Therefore, NH3-SCR can be used in most existing nitric acid plants. The catalyst used in the NH3-SCR process is required to exhibit high activity in low- and high-temperature regions, satisfactory selectivity to N2, and good thermal stability. In fact, these requirements are met by metal oxide systems, such as the commercial catalyst V2O5-WO<sup>3</sup> (MoO3)-TiO<sup>2</sup> [3]. However, the material is not free from some important drawbacks, such as the toxicity of vanadium compounds. Moreover, selectivity of the catalysts above 350 ◦C is limited by the side reactions described by Equations (4) and (5):

$$2\text{ NH}\_3 + 2\text{ O}\_2 \rightarrow \text{ N}\_2\text{O} + 3\text{ H}\_2\text{O} \tag{4}$$

$$4\text{ NH}\_3 + 5\text{ O}\_2 \rightarrow 4\text{ NO} + 6\text{ H}\_2\text{O}\tag{5}$$

Due to the above-mentioned problems, a number of materials have been investigated as the alternative catalysts of NH3-SCR [4–7]. According to the study reported by Kobayashi et al. [8], application of TiO<sup>2</sup> does not provide sufficient dispersion of the active phase, surface acidity, and thermal stability of the catalyst. Therefore, further research has shifted to alternative supports of the novel catalyst. Among them, natural zeolites were found to be very promising precursors of the novel catalysts [9,10]. The great advantage of these materials is their abundance in the environment and thus their relatively low price, which is very beneficial for industrial applications. The representative of natural zeolites is clinoptilolite, belonging to the heulandite (HEU) family [11]. The material shows strongly acidic character, determined by its Si/Al molar ratio of ca. 4, its well-developed pore system, and its thermal stability. According to the Eley–Rideal mechanism, NH3-SCR assumes simultaneous adsorption of alkaline NH<sup>3</sup> and neutral NO and their interaction on the catalyst surface [12]. Therefore, high concentration of acid centers delivered by clinoptilolite improves ammonia adsorption capacity and NH3-SCR reaction rate. Moreover, clinoptilolite provides good ion exchange capacity, and as a consequence, its acidic character can be easily elevated by acid pretreatment [13]. Additionally, the presence of micro- and mesopores in the zeolitic structure facilitates the diffusion of gas molecules through the catalyst's pores and easy access to active centers. Lastly, clinoptilolite belongs to the residual materials, usually stored on heaps. Therefore, its recycling is in agreement with the assumptions of circular economy. All in all, the above-mentioned properties make clinoptilolite a promising candidate for the precursor of a new catalyst of NH3-SCR [13–15]. To date, research on the application of clinoptilolite was mostly limited to SCR with hydrocarbons as reducing agents. Ghasemian et al. [16] proved that protonated clinoptilolite is a promising precursor of a new catalyst of SCR with methane as a reducing agent. Another study conducted by the authors [15,17] concerned clinoptilolite as a possible support for the catalyst of SCR with propane. However, only few studies have explored zeolite as a support for SCR with ammonia [13,18].

Another important issue in the design of a novel NH3-SCR catalyst is the active phase. Over recent years, the focus of researchers has shifted to systems with transition metals, especially iron [5,12,19,20]. The choice of Fe was motivated by its environmentally benign characteristics, low price, and prominent thermal stability. Additionally, iron catalysts exhibit excellent medium- and high-temperature activity and satisfactory selectivity to N2. Moreover, the facile redox equilibrium, Fe3+ <sup>↔</sup> Fe3O<sup>4</sup> <sup>↔</sup> Fe2+, contributes to high oxygen storage capacity, which is very beneficial for SCR catalysts.

Highly satisfactory activity of iron-modified clinoptilolite in SCR with ammonia was confirmed in the previous study [18]. It was found that raw clinoptilolite in a form of fine grain showed 30% of NO conversion in the range of 350–450 ◦C. The high efficiency of the material in NH3-SCR with the gas mixture reflecting the industrial composition was also confirmed. It was observed that at 400 ◦C, NO<sup>x</sup> conversion for Fe-clinoptilolite exceeded 80%, and high selectivity to N<sup>2</sup> was preserved in the entire temperature range. Additionally, 82% NO conversion was obtained for the previously shaped iron-modified clinoptilolite. In conditions similar to industrial ones, the highest catalytic activity was obtained above 400 ◦C, and these temperatures also maintained very favorable selectivity towards N2. Importantly, no formation of N2O was observed during the catalytic reaction.

In this work, the aim was to investigate the influence of iron loading on the lowand high-temperature catalytic performance of Fe-modified clinoptilolite formed into pellets. In this research, iron was considered the active phase since this transition metal exhibits outstanding redox properties and, at the same time, neutrality to the environment. Therefore, the experiments will contribute to the development of more ecologically friendly catalysts of the NH3-SCR process. Moreover, in the experiments, a real tail gas mixture, which normally enters SCR reactors in nitric acid plants, was used. To the best of our knowledge, no one so far has investigated the catalytic performance of such material under near-industrial conditions. Thus, this work makes a significant contribution to the field of low-price and nontoxic industrial catalysts of NH3-SCR.

#### **2. Results and Discussion**

#### *2.1. Physicochemical Properties of the Materials*

2.1.1. Chemical Composition, Crystal Structure, and Morphology of the Materials

The chemical compositions of raw (Clin), protonated (H-Clin), and Fe-modified clinoptilolite (Fe-Clin-1, 2, or 3) are presented in Table 1. The crystalline structure of the materials was analyzed using XRD, and the obtained patterns are shown in Figure 1.


**Table 1.** Chemical composition (in wt.%) of the analyzed materials determined by XRF.

As presented in Table 1, the raw clinoptilolite consisted mainly of SiO<sup>2</sup> and Al2O<sup>3</sup> and contained some additives of other alkaline metal oxides. Additionally, the analysis provided strong evidence of the presence of iron oxide in the natural zeolite. After protonation, the percentage contribution of SiO<sup>2</sup> increased with a simultaneous slight decrease of Al2O<sup>3</sup> content. This result is in line with that obtained by Burris and Juenger [21], who ascribed the decrease in aluminum content to partial dealumination of the material or its dissolution in acidic medium. However, since the XRD pattern of H-Clin corresponded to that of Clin, the degrading influence of the acid can be excluded. After the deposition of iron, the detected content of Fe2O<sup>3</sup> significantly increased, proving efficient incorporation of various iron species into the zeolite structure. However, it can be also observed that after the third impregnation, the amount of Fe2O<sup>3</sup> was very close to that obtained after the second impregnation. Additionally, the catalysts contained considerable amounts of SO<sup>3</sup> as the result of using FeSO<sup>4</sup> as the precursor of iron. Therefore, the applied calcination temperature was probably insufficient to provide effective decomposition of the salt deposited on the zeolite matrix.

lite.

disappeared, indicating decreased crystallinity of the catalysts compared to the raw zeo-

phous substitution or deposition of bulky species of Fe2O3 on its surface.

**Sample Fe2O3 (%) SiO2 (%) Al2O3 (%) Na2O (%) MgO (%) SO3 (%) K2O (%) CaO (%) TiO (%) MnO (%)**  Clin 2.1 74.7 12.1 0.8 0.8 0.04 3.2 3.7 0.2 0.07 H-Clin 2.0 80.5 11.4 0.1 0.7 0.03 3.0 1.7 0.2 0.03 Fe-Clin-1 8.6 71.0 10.1 0.1 0.6 5.00 2.6 1.5 0.2 0.01 Fe-Clin-2 11.1 69.2 10.0 0.1 0.5 4.59 2.5 1.5 0.2 0.01 Fe-Clin-3 11.9 66.7 9.4 0.1 0.5 7.41 2.3 1.2 0.2 0.01

**Table 1.** Chemical composition (in wt.%) of the analyzed materials determined by XRF.

After modification with iron, the positions of diffraction maxima characteristic of the clinoptilolite phase remained unchanged. Thus, deposition of the active phase did not cause any significant damage to the structure. However, the intensity of the reflections was the lowest for the material with the highest concentration of iron. Larger aggregates of Fe2O3 on the zeolite surface can potentially be present at *2θ* of 42.0, 45.8, 60.2, and 68.2° [23]. However, apart from bigger particles, iron also isomorphously substituted for aluminum in the zeolite framework and thus was impossible to be detected by XRD technique. The replacement of Al by Fe can be also confirmed by lower intensities of the structural diffraction maxima of clinoptilolite. A similar effect was obtained by Kessouri et al. [24] after the deposition of iron into an MFI framework. Hence, the noticeably decreased intensity of the reflections for Fe-Clin-3 can be explained by the highest rate of isomor-

**Figure 1.** XRD patterns obtained for raw clinoptilolite (Clin), protonated clinoptilolite (H-Clin), and clinoptilolite modified with iron (Fe-Clin-1, Fe-Clin-2, Fe-Clin-3).

**Figure 1.** XRD patterns obtained for raw clinoptilolite (Clin), protonated clinoptilolite (H-Clin), and

clinoptilolite modified with iron (Fe-Clin-1, Fe-Clin-2, Fe-Clin-3). 2.1.2. Textural Properties of the Materials Low-temperature N2 adsorption–desorption isotherms obtained for raw clinoptilolite (Clin), protonated clinoptilolite (H-Clin), and iron-modified zeolite (Fe-Clin-1, Fe-Clin-2, Fe-Clin-3) are presented in Figure 2. Furthermore, pore volume distribution is shown in Figure 3, while the textural and structural parameters of the samples are summarized in Table 2. Raw clinoptilolite demonstrated IV(a) type isotherm with the According to the results of XRD, the analyzed sample consisted mainly of heulandite/clinoptilolite, confirmed by the diffraction maxima at 2*θ* of 9.8, 11.4, 12.9, 16.8, 17.4, 20.7, 22.6, 30.0, 32.0, 32.9, 35.5, 36.7, and 50.3◦ . The reflection at 2*θ* of 26.6◦ corresponds to SiO2, while those at 21.9 and 28.1◦ are due to the presence of cristobalite impurities in the solid [13]. The observed diffraction maxima are in good agreement with those reported in the literature [18,22]. The comparative analysis of the materials showed that protonation by acid treatment did not result in any noticeable structural changes. However, some of the diffraction maxima exhibited lower intensity or completely disappeared, indicating decreased crystallinity of the catalysts compared to the raw zeolite.

After modification with iron, the positions of diffraction maxima characteristic of the clinoptilolite phase remained unchanged. Thus, deposition of the active phase did not cause any significant damage to the structure. However, the intensity of the reflections was the lowest for the material with the highest concentration of iron. Larger aggregates of Fe2O<sup>3</sup> on the zeolite surface can potentially be present at 2*θ* of 42.0, 45.8, 60.2, and 68.2◦ [23]. However, apart from bigger particles, iron also isomorphously substituted for aluminum in the zeolite framework and thus was impossible to be detected by XRD technique. The replacement of Al by Fe can be also confirmed by lower intensities of the structural diffraction maxima of clinoptilolite. A similar effect was obtained by Kessouri et al. [24] after the deposition of iron into an MFI framework. Hence, the noticeably decreased intensity of the reflections for Fe-Clin-3 can be explained by the highest rate of isomorphous substitution or deposition of bulky species of Fe2O<sup>3</sup> on its surface.

#### 2.1.2. Textural Properties of the Materials

Low-temperature N<sup>2</sup> adsorption–desorption isotherms obtained for raw clinoptilolite (Clin), protonated clinoptilolite (H-Clin), and iron-modified zeolite (Fe-Clin-1, Fe-Clin-2, Fe-Clin-3) are presented in Figure 2. Furthermore, pore volume distribution is shown in Figure 3, while the textural and structural parameters of the samples are summarized in Table 2. Raw clinoptilolite demonstrated IV(a) type isotherm with the hysteresis loop H3, according to the IUPAC classification [25]. This isotherm is characteristic of materials with wedge-shaped mesopores and nonrigid aggregates of platelike particles [25,26]. The specific surface area of the nonmodified clinoptilolite is in range of 16–30 m<sup>2</sup> ·g −1 , which

is typical for clinoptilolite [27]. Vassileva and Voikova [26] reported that the relatively low values of specific surface area and pore volume exhibited by nonmodified clinoptilolite are caused by the limited access of N<sup>2</sup> molecules to the internal structure of the zeolite. As a result, the adsorbate was deposited mainly on the external surface of the material. The results of the experiment performed after NH3-SCR tests showed that the specific surface area was preserved, even in the case of the catalytic reaction being conducted under severe conditions. After the dealumination procedure, the volume of mesopores in clinoptilolite significantly increased, suggesting the formation of a mesopore system. The isotherms obtained for iron-modified clinoptilolite are characterized by the isotherms of type IV(a), confirming their mesoporous nature. However, the introduction of iron resulted in a change in the shape of the hysteresis loop from H3 to H4 [28]. This result suggests the transformation of wedge-shaped mesopores into slit-shaped ones. Additionally, as presented in Table 2, after modification with iron, the specific surface area, the volume of mesopores, and the average pore diameter decreased due to pore blockage probably caused by the deposition of iron oxide species [29]. Catalysts of Fe-Clin-X series characteristically possess similar pore distribution (Figure 3). For Fe-Clin-X series, a wide bimodal pore distribution—pores with diameters ranging from 10 to 1000 Å and pores with diameters from 1500 to 7000 Å—was observed. However, the porous structure was definitely dominated by pores with diameters ranging from 10 to 1000 Å (mesopores with Dmeso in the range of 208–223 Å). Interestingly, multiple impregnations with FeSO<sup>4</sup> did not result in considerable differences between the Dmeso values. Nevertheless, the gradual decline of Vmeso with the increasing iron content suggested that iron species were effectively deposited in the inner structure of clinoptilolite. the material. The results of the experiment performed after NH3-SCR tests showed that the specific surface area was preserved, even in the case of the catalytic reaction being conducted under severe conditions. After the dealumination procedure, the volume of mesopores in clinoptilolite significantly increased, suggesting the formation of a mesopore system. The isotherms obtained for iron-modified clinoptilolite are characterized by the isotherms of type IV(a), confirming their mesoporous nature. However, the introduction of iron resulted in a change in the shape of the hysteresis loop from H3 to H4 [28]. This result suggests the transformation of wedge-shaped mesopores into slit-shaped ones. Additionally, as presented in Table 2, after modification with iron, the specific surface area, the volume of mesopores, and the average pore diameter decreased due to pore blockage probably caused by the deposition of iron oxide species [29]. Catalysts of Fe-Clin-X series characteristically possess similar pore distribution (Figure 3). For Fe-Clin-X series, a wide bimodal pore distribution—pores with diameters ranging from 10 to 1000 Å and pores with diameters from 1500 to 7000 Å—was observed. However, the porous structure was definitely dominated by pores with diameters ranging from 10 to 1000 Å (mesopores with Dmeso in the range of 208–223 Å). Interestingly, multiple impregnations with FeSO4 did not result in considerable differences between the Dmeso values. Nevertheless, the gradual decline of Vmeso with the increasing iron content suggested that iron species were effectively deposited in the inner structure of clinoptilolite.

hysteresis loop H3, according to the IUPAC classification [25]. This isotherm is characteristic of materials with wedge-shaped mesopores and nonrigid aggregates of platelike particles [25,26]. The specific surface area of the nonmodified clinoptilolite is in range of 16– 30 m2·g−1, which is typical for clinoptilolite [27]. Vassileva and Voikova [26] reported that the relatively low values of specific surface area and pore volume exhibited by nonmodified clinoptilolite are caused by the limited access of N2 molecules to the internal structure of the zeolite. As a result, the adsorbate was deposited mainly on the external surface of

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 5 of 16

**Figure 2.** Low-temperature N2 adsorption–desorption isotherms obtained for raw clinoptilolite (Clin) and the investigated catalysts (for better visibility, the isotherms were shifted by the values given in the figure). **Figure 2.** Low-temperature N<sup>2</sup> adsorption–desorption isotherms obtained for raw clinoptilolite (Clin) and the investigated catalysts (for better visibility, the isotherms were shifted by the values given in the figure).

**Table 2.** Textural and structural parameters of raw clinoptilolite and the Fe-clinoptilolite catalysts.

**Vmeso <sup>c</sup> (cm3·g−1)**  **Dmeso <sup>c</sup> (nm)** 

**SExt <sup>b</sup> (m2·g−1)** 

Clin 16 7 0.025 17.0

**(m2·g−1)** 

**Sample SBET <sup>a</sup>**

the t-plot method; c

H-Clin 30 9 0.281 28.2 Fe-Clin-1 15 10 0.277 20.8 Fe-Clin-2 12 13 0.262 20.8 Fe-Clin-3 10 10 0.224 22.3 a Specific surface area determined using the BET method; b external surface area determined using

average mesopore volume and diameter determined using the BJH method.

**Figure 3.** Pore volume obtained for Fe-clinoptilolite catalysts. **Figure 3.** Pore volume obtained for Fe-clinoptilolite catalysts.



Clin-3, it was not as sharp as for the other materials; thus, multiple repetition of Fe deposition resulted in the removal of OH groups bonded to the zeolitic structure. The band at <sup>a</sup> Specific surface area determined using the BET method; <sup>b</sup> external surface area determined using the t-plot method; <sup>c</sup> average mesopore volume and diameter determined using the BJH method. tion with iron, these peaks were noticeably separated. The new small peak at 780 cm−1, formed through this division, raised from the stretching vibrations of [SiO4] tetrahedra

#### 3400 cm−1, broad for raw and protonated clinoptilolite and sharper for Fe-modified zeolite, corresponded to the vibrations of O-H⋯O bonds [30]. 2.1.3. Characteristic Chemical Groups in the Materials the detection of structural peaks of the materials. Another difference in the spectra of iron-

from the zeolitic framework. Therefore, removal of the amorphous silica could enhance

In the second region of the spectra, 1700–700 cm−1, the peak at 1650 cm−1, attributed to deformation vibrations of physisorbed water molecules, showed a similar shape for all materials [31]. However, the characteristic bands at 1200 cm−1 and 1050 cm−1 were almost absent for clinoptilolite modified with Fe. Both of the peaks were related to Al-O or Si-O asymmetric stretching vibrations; thus, incorporation of iron resulted in structural interruptions, such as the removal of charge-balancing Ca2+ and Mg2+. A similar effect was ob-The FT-IR spectra obtained for the raw and protonated clinoptilolite and the zeolites with various loadings of iron are presented in Figure 4. The characteristic peaks can be divided into three regions: (1) O-H stretching vibrations (3800–3400 cm−<sup>1</sup> ); (2) Si-O stretching vibrations, Al-Me-OH stretching vibrations, and O-H bending vibrations from H2O (1700–700 cm−<sup>1</sup> ); and (3) pseudo lattice vibrations (700–450 cm−<sup>1</sup> ) [13]. modified clinoptilolite compared to the raw or protonated form was the presence of lowintense peaks at 585 cm−1, which corresponded to the symmetric stretching vibrations of [AlO4] tetrahedra [34]. Two bands at 600 cm−1 and 475 cm−1 were related to O-Al-O or O-Si-O bending vibrations and Si-O stretching vibrations, respectively [35].

**Figure 4.** FT-IR spectra of raw clinoptilolite (Clin), protonated clinoptilolite (H-Clin), and clinoptilo-**Figure 4.** FT-IR spectra of raw clinoptilolite (Clin), protonated clinoptilolite (H-Clin), and clinoptilolite modified with different Fe contents (Fe-Clin-1, Fe-Clin-2, Fe-Clin-3).

In general, for iron-modified zeolites, three main regions in UV–Vis spectra are expected: (1) bands below 300 nm, corresponding to the oxygen-to-metal charge transfer (CT), assigned to isolated framework and extra framework pseudotetrahedral Fe3+ species; (2) bands in the range of 300–500 nm, related to oligomeric FexOy or Fe2O3 nanoparticles; and (3) bands detected above 500 nm, assigned to Fe2O3 clusters on the external surface of

The speciation of iron in the analyzed materials was strongly correlated with the metal loading. As presented in Figure 5, all the investigated samples, including nonmodified clinoptilolite, showed absorption bands at 230 and 260 nm. This result confirmed that iron was originally present (Clin) or isomorphously deposited (Fe-Clin-1, 2, and 3) in the zeolite structure in the form of extraframework cations with octahedral coordination [36]. Furthermore, the band at 350 nm, detected only for the zeolite modified with iron, corresponded to small, oligonuclear clusters of iron oxide [37]. The bands at 475 nm, characteristic of bigger particles of Fe2O3, were observed for the samples with increased iron content

lite modified with different Fe contents (Fe-Clin-1, Fe-Clin-2, Fe-Clin-3).

2.1.4. Speciation of the Active Phase

the support [19].

ious contents of iron are presented in Figure 5.

In the first of the above-mentioned regions, 3800–3400 cm−<sup>1</sup> , the shape of the spectra was similar for all of the materials except Fe-Clin-3. The peak at 3650 cm−<sup>1</sup> suggested the presence of Brönsted sites provided by the acidic hydroxyl Si-O(H)-Al. In the case of Fe-Clin-3, it was not as sharp as for the other materials; thus, multiple repetition of Fe deposition resulted in the removal of OH groups bonded to the zeolitic structure. The band at 3400 cm−<sup>1</sup> , broad for raw and protonated clinoptilolite and sharper for Fe-modified zeolite, corresponded to the vibrations of O-H· · · O bonds [30].

In the second region of the spectra, 1700–700 cm−<sup>1</sup> , the peak at 1650 cm−<sup>1</sup> , attributed to deformation vibrations of physisorbed water molecules, showed a similar shape for all materials [31]. However, the characteristic bands at 1200 cm−<sup>1</sup> and 1050 cm−<sup>1</sup> were almost absent for clinoptilolite modified with Fe. Both of the peaks were related to Al-O or Si-O asymmetric stretching vibrations; thus, incorporation of iron resulted in structural interruptions, such as the removal of charge-balancing Ca2+ and Mg2+. A similar effect was observed by Cobzaru et al. [32] after the modification of natural clinoptilolite with nitric acid. Since FeSO<sup>4</sup> is regarded as a strongly acidic medium, our results are in line with this research. Moreover, the band at 1150 cm−<sup>1</sup> , intense for Clin and H-Clin, partially disappeared after the introduction of iron. Since the peak corresponds to three-dimensional networks of amorphous Si-O-Si units, the modification procedure could partially remove this phase from natural and protonated zeolite. Additionally, a small peak at 1385 cm−<sup>1</sup> , appearing only for Fe-Clin-3 and thus with the highest concentration of iron species, was probably ascribed to sulfate groups bonded to iron ions deposited in the zeolitic structure [33].

The characteristic peaks detected in the third analyzed region, 700–450 cm−<sup>1</sup> , evidenced partial removal of amorphous silica. This effect can be confirmed by the presence of the sharp peaks at 800 cm−<sup>1</sup> in the spectra of all the materials. However, after modification with iron, these peaks were noticeably separated. The new small peak at 780 cm−<sup>1</sup> , formed through this division, raised from the stretching vibrations of [SiO4] tetrahedra from the zeolitic framework. Therefore, removal of the amorphous silica could enhance the detection of structural peaks of the materials. Another difference in the spectra of iron-modified clinoptilolite compared to the raw or protonated form was the presence of low-intense peaks at 585 cm−<sup>1</sup> , which corresponded to the symmetric stretching vibrations of [AlO4] tetrahedra [34]. Two bands at 600 cm−<sup>1</sup> and 475 cm−<sup>1</sup> were related to O-Al-O or O-Si-O bending vibrations and Si-O stretching vibrations, respectively [35].

#### 2.1.4. Speciation of the Active Phase

The comparative UV–Vis spectra of the raw clinoptilolite and the catalysts with various contents of iron are presented in Figure 5.

In general, for iron-modified zeolites, three main regions in UV–Vis spectra are expected: (1) bands below 300 nm, corresponding to the oxygen-to-metal charge transfer (CT), assigned to isolated framework and extra framework pseudotetrahedral Fe3+ species; (2) bands in the range of 300–500 nm, related to oligomeric FexO<sup>y</sup> or Fe2O<sup>3</sup> nanoparticles; and (3) bands detected above 500 nm, assigned to Fe2O<sup>3</sup> clusters on the external surface of the support [19].

The speciation of iron in the analyzed materials was strongly correlated with the metal loading. As presented in Figure 5, all the investigated samples, including nonmodified clinoptilolite, showed absorption bands at 230 and 260 nm. This result confirmed that iron was originally present (Clin) or isomorphously deposited (Fe-Clin-1, 2, and 3) in the zeolite structure in the form of extraframework cations with octahedral coordination [36]. Furthermore, the band at 350 nm, detected only for the zeolite modified with iron, corresponded to small, oligonuclear clusters of iron oxide [37]. The bands at 475 nm, characteristic of bigger particles of Fe2O3, were observed for the samples with increased iron content (Fe-Clin-2, Fe-Clin-3). Therefore, the extraframework phase of Fe2O<sup>3</sup> became dominant as a result of the increase in iron loading due to the agglomeration of the species into bigger particles.

particles.

(Fe-Clin-2, Fe-Clin-3). Therefore, the extraframework phase of Fe2O3 became dominant as a result of the increase in iron loading due to the agglomeration of the species into bigger

**Figure 5.** UV–Vis spectra of raw clinoptilolite (Clin) and clinoptilolite modified with different Fe **Figure 5.** UV–Vis spectra of raw clinoptilolite (Clin) and clinoptilolite modified with different Fe contents (Fe-Clin-1, Fe-Clin-2, Fe-Clin-3).

#### contents (Fe-Clin-1, Fe-Clin-2, Fe-Clin-3). *2.2. NH3-SCR Catalytic Tests Performed with Industrial Gas Mixture*

*2.2. NH3-SCR Catalytic Tests Performed with Industrial Gas Mixture*  NH3-SCR catalytic tests over protonated clinoptilolite were conducted under the conditions reflecting that of industrial nitric acid plant (regarding catalytic bed loading and temperature range). The obtained results are presented in Figure 6. The tests were carried out at two catalytic bed loads. It was observed that in both cases, NOx conversion exceeded 50% in the entire temperature range. The maximum conversion of more than 90%, was reached above 400 °C, and higher NOx conversion was achieved for the lower catalytic bed loading. In the case of GHSV = 4500 h−1 (tail gas flow 0.15 Nm3·h−1), 93% of NOx con-NH3-SCR catalytic tests over protonated clinoptilolite were conducted under the conditions reflecting that of industrial nitric acid plant (regarding catalytic bed loading and temperature range). The obtained results are presented in Figure 6. The tests were carried out at two catalytic bed loads. It was observed that in both cases, NO<sup>x</sup> conversion exceeded 50% in the entire temperature range. The maximum conversion of more than 90%, was reached above 400 ◦C, and higher NO<sup>x</sup> conversion was achieved for the lower catalytic bed loading. In the case of GHSV = 4500 h−<sup>1</sup> (tail gas flow 0.15 Nm<sup>3</sup> ·h −1 ), 93% of NO<sup>x</sup> conversion was obtained at 400 ◦C. On the other hand, for GHSV = 9000 h−<sup>1</sup> (tail gas flow 0.3 Nm<sup>3</sup> ·h −1 ), the material exhibited 80% of NO<sup>x</sup> conversion at 450 ◦C. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 9 of 16

**Figure 6.** NOx conversion as a function of temperature and catalyst load for H-Clin.

**Figure 6.** NOx conversion as a function of temperature and catalyst load for H-Clin. During the test, N2O concentration upstream and downstream of the catalytic bed was measured as well. The dash line in Figure 7 represents the ratio of the N2O concentration downstream to the inlet concentration of N2O. It was clearly indicated that higher loading of the catalytic bed resulted in lower N2O concentration downstream of the bed compared to

> GHSV = 4500 h<sup>−</sup><sup>1</sup> GHSV = 9000 h<sup>−</sup><sup>1</sup>

250 300 350 400 450

Figure 8 shows the results of the catalytic tests obtained for iron-modified samples and the protonated clinoptilolite, while the selectivity of the materials to N2 is listed in Table 3. In all cases, NOx conversion of iron-modified zeolite was higher than that of H-Clin. Above 350 °C, regardless of the iron content in the sample, NOx conversion of over 90% was achieved. The highest activity in the entire temperature range was exhibited by Fe-Clin-2. Additionally, selectivity of Fe-clinoptilolite catalysts to N2 was in the range of 93–100%, confirming the negligible contribution of the side reactions to the whole mech-

Temperature (°C)

**Figure 7.** N2O/N2O(in) ratio as a function of temperature and catalyst load for H-Clin.

0.4

anism of NH3-SCR performed on the materials.

0.6

0.8

N O/N 2

O (in) (-)

2

1.0

1.2

1.4

1.6

NOx conversion (%)

the inlet concentration of N2O. This effect was observed over almost the entire investigated temperature range. Moreover, regardless of the catalytic bed load, the highest selectivity was observed at 450 ◦C. **Figure 6.** NOx conversion as a function of temperature and catalyst load for H-Clin.

 GHSV = 4500 h<sup>−</sup><sup>1</sup> GHSV = 9000 h<sup>−</sup><sup>1</sup>

250 300 350 400 450

Temperature (°C)

*Catalysts* **2022**, *12*, x FOR PEER REVIEW 9 of 16

**Figure 7.** N2O/N2O(in) ratio as a function of temperature and catalyst load for H-Clin.

**Figure 7.** N2O/N2O(in) ratio as a function of temperature and catalyst load for H-Clin. Figure 8 shows the results of the catalytic tests obtained for iron-modified samples and the protonated clinoptilolite, while the selectivity of the materials to N2 is listed in Table 3. In all cases, NOx conversion of iron-modified zeolite was higher than that of H-Clin. Above 350 °C, regardless of the iron content in the sample, NOx conversion of over 90% was achieved. The highest activity in the entire temperature range was exhibited by Fe-Clin-2. Additionally, selectivity of Fe-clinoptilolite catalysts to N2 was in the range of Figure 8 shows the results of the catalytic tests obtained for iron-modified samples and the protonated clinoptilolite, while the selectivity of the materials to N<sup>2</sup> is listed in Table 3. In all cases, NO<sup>x</sup> conversion of iron-modified zeolite was higher than that of H-Clin. Above 350 ◦C, regardless of the iron content in the sample, NO<sup>x</sup> conversion of over 90% was achieved. The highest activity in the entire temperature range was exhibited by Fe-Clin-2. Additionally, selectivity of Fe-clinoptilolite catalysts to N<sup>2</sup> was in the range of 93–100%, confirming the negligible contribution of the side reactions to the whole mechanism of NH3-SCR performed on the materials. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 10 of 16

**Figure 8.** NOx conversion as a function of temperature for protonated clinoptilolite (H-Clin) and Fe-**Figure 8.** NO<sup>x</sup> conversion as a function of temperature for protonated clinoptilolite (H-Clin) and Fe-clinoptilolite catalysts (Fe-Clin-1, Fe-Clin-2, Fe-Clin-3) at GHSV = 9000 h−<sup>1</sup> .

**Sample 250 °C 300 °C 350 °C 400 °C 450 °C**  Fe-Clin-1 99.2 98.2 96.7 96.9 100.0 Fe-Clin-2 99.6 98.6 97.7 94.6 100.0 Fe-Clin-3 98.3 - 97.1 93.1 100.0

The N2O concentrations measured during the experiments are shown in Figure 9. In the case of protonated clinoptilolite, the N2O concentration increased above the inlet value only at 350 °C. For iron-modified samples, the courses of the curves are similar to each other. Up to the temperature of 400 °C, the concentration of N2O behind the bed slightly increased in relation to the initial concentration (N2O/N2O(in) > 1), and then, a sharp decrease in the concentration of N2O at the temperature of 450 °C was noted. The greatest decrease was obtained for the Fe-Clin-1 and Fe-Clin-2 samples. Overall, satisfactory catalytic performance exhibited by the investigated catalysts confirmed that one or two iron impregnations of clinoptilolite are sufficient to obtain an effective NH3-SCR catalyst.

clinoptilolite catalysts (Fe-Clin-1, Fe-Clin-2, Fe-Clin-3) at GHSV = 9000 h<sup>−</sup>1.

**Table 3.** N2 selectivity of Fe-clinoptilolite catalysts.


**Table 3.** N<sup>2</sup> selectivity of Fe-clinoptilolite catalysts.

The N2O concentrations measured during the experiments are shown in Figure 9. In the case of protonated clinoptilolite, the N2O concentration increased above the inlet value only at 350 ◦C. For iron-modified samples, the courses of the curves are similar to each other. Up to the temperature of 400 ◦C, the concentration of N2O behind the bed slightly increased in relation to the initial concentration (N2O/N2O(in) > 1), and then, a sharp decrease in the concentration of N2O at the temperature of 450 ◦C was noted. The greatest decrease was obtained for the Fe-Clin-1 and Fe-Clin-2 samples. Overall, satisfactory catalytic performance exhibited by the investigated catalysts confirmed that one or two iron impregnations of clinoptilolite are sufficient to obtain an effective NH3-SCR catalyst. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 11 of 16

**Figure 9.** The ratio of N2O concentration downstream of the catalytic bed and N2O inlet concentration as a function of the temperature for H-Clin, Fe-Clin-1, Fe-Clin-2, Fe-Clin-3 at GHSV = 9000 h−<sup>1</sup> .

#### **Figure 9.** The ratio of N2O concentration downstream of the catalytic bed and N2O inlet concentration as a function of the temperature for H-Clin, Fe-Clin-1, Fe-Clin-2, Fe-Clin-3 at GHSV = 9000 h<sup>−</sup>1. **3. Materials and Methods**

#### *3.1. Catalysts Preparation*

**3. Materials and Methods**  *3.1. Catalysts Preparation*  The precursor of the investigated catalysts was raw zeolite with a high content of clinoptilolite phase. Firstly, the material was dealuminated using 5% HNO3 solution. The operation was repeated three times in order to increase the dealumination rate. After each dealumination step, the precursor was washed with demineralized water until pH was <6 and dried at 105–110 °C. Subsequently, the zeolite was fractioned into 0.3–0.8 mm grains and formed into pellets of 5.0 × 4.8 mm dimensions, illustrated in Figure 10. Afterwards, the materials were calcined at 450 °C for 2 h. Iron-modified materials were prepared using the wet impregnation method using an aqueous solution of 1 M FeSO4 as Fe precursor. The samples were left in contact with the solution at 50 °C for 1 h, then dried at 105–110 °C and calcined at 500 °C for 2 h. The impregnation procedure was performed one, two, The precursor of the investigated catalysts was raw zeolite with a high content of clinoptilolite phase. Firstly, the material was dealuminated using 5% HNO<sup>3</sup> solution. The operation was repeated three times in order to increase the dealumination rate. After each dealumination step, the precursor was washed with demineralized water until pH was <6 and dried at 105–110 ◦C. Subsequently, the zeolite was fractioned into 0.3–0.8 mm grains and formed into pellets of 5.0 × 4.8 mm dimensions, illustrated in Figure 10. Afterwards, the materials were calcined at 450 ◦C for 2 h. Iron-modified materials were prepared using the wet impregnation method using an aqueous solution of 1 M FeSO<sup>4</sup> as Fe precursor. The samples were left in contact with the solution at 50 ◦C for 1 h, then dried at 105–110 ◦C and calcined at 500 ◦C for 2 h. The impregnation procedure was performed one, two, or three times in order to obtained catalysts with various Fe loadings The precursors were dried and calcined before each impregnation treatment. The preparation procedure is schematically illustrated in Figure 10.

**Figure 10.** Schematic preparation procedure of Fe-clinoptilolite catalysts.

samples with the corresponding descriptions are listed in Table 4.

or three times in order to obtained catalysts with various Fe loadings The precursors were dried and calcined before each impregnation treatment. The preparation procedure is

The formed samples of protonated clinoptilolite and Fe-clinoptilolite catalysts, prepared on a laboratory scale, are presented in Figure 11A,B, respectively. The codes of the

250 300 350 400 450

**Figure 9.** The ratio of N2O concentration downstream of the catalytic bed and N2O inlet concentration as a function of the temperature for H-Clin, Fe-Clin-1, Fe-Clin-2, Fe-Clin-3 at GHSV = 9000 h<sup>−</sup>1.

The precursor of the investigated catalysts was raw zeolite with a high content of clinoptilolite phase. Firstly, the material was dealuminated using 5% HNO3 solution. The operation was repeated three times in order to increase the dealumination rate. After each dealumination step, the precursor was washed with demineralized water until pH was <6 and dried at 105–110 °C. Subsequently, the zeolite was fractioned into 0.3–0.8 mm grains and formed into pellets of 5.0 × 4.8 mm dimensions, illustrated in Figure 10. Afterwards, the materials were calcined at 450 °C for 2 h. Iron-modified materials were prepared using the wet impregnation method using an aqueous solution of 1 M FeSO4 as Fe precursor. The samples were left in contact with the solution at 50 °C for 1 h, then dried at 105–110 °C and calcined at 500 °C for 2 h. The impregnation procedure was performed one, two, or three times in order to obtained catalysts with various Fe loadings The precursors were dried and calcined before each impregnation treatment. The preparation procedure is

Temperature **(**°C**)**

**Figure 10.** Schematic preparation procedure of Fe-clinoptilolite catalysts. **Figure 10.** Schematic preparation procedure of Fe-clinoptilolite catalysts. H-Clin Protonated clinoptilolite

schematically illustrated in Figure 10.

0.4

**3. Materials and Methods**  *3.1. Catalysts Preparation* 

0.6

0.8

1.0

N O/N 2

O (in) (-)

2

1.2

1.4

1.6

 H-Clin Fe-Clin-1 Fe-Clin-2 Fe-Clin-3

The formed samples of protonated clinoptilolite and Fe-clinoptilolite catalysts, prepared on a laboratory scale, are presented in Figure 11A,B, respectively. The codes of the samples with the corresponding descriptions are listed in Table 4. The formed samples of protonated clinoptilolite and Fe-clinoptilolite catalysts, prepared on a laboratory scale, are presented in Figure 11A,B, respectively. The codes of the samples with the corresponding descriptions are listed in Table 4. Fe-Clin-1 The catalyst obtained by single impregnation with Fe precursor Fe-Clin-2 The catalyst obtained by dual impregnation with Fe precursor Fe-Clin-3 The catalyst obtained by triple impregnation with Fe precursor

**Figure 11.** Protonated clinoptilolite (**A**) and Fe-clinoptilolite catalyst formed into pellets (**B**). **Figure 11.** Protonated clinoptilolite (**A**) and Fe-clinoptilolite catalyst formed into pellets (**B**).

*3.2. Catalysts Characterization*  **Table 4.** The list of the samples with their codes and descriptions.


#### scans of 0.02° and the counting time of 1 s per step). The specific surface area, total pore volume, and mesopore volume were determined using an ASAP® 2050 Xtended Pressure *3.2. Catalysts Characterization*

sorption analyzer (Micromeritics Instrument Co., Norcross, GA, USA) based on N2 adsorption–desorption isotherms at −196 °C using the BET adsorption model (Brunauer– Emmett–Teller) and the BJH transformation (Barret–Joyner–Halenda). Fourier transform infrared spectroscopy studies (FT-IR) were conducted using a Perkin Elmer Frontier FT-IR spectrometer. The spectra were obtained in the wavelength range of 4000–400 cm−1 with a resolution of 4 cm−1. Before each measurement, the sample was mixed with KBr in a ratio of 1: 100 and pressed into a disk. Coordination and aggregation of iron species were determined by UV–Vis spectroscopy at a wavelength range of 200–900 nm with a resolution of 1 nm using a Perkin Elmer Lambda 35 UV–Vis spectrophotometer. *3.3. Catalytic Tests in Real Gas Conditions*  The activity and selectivity of the catalysts in the NH3-SCR process were tested in the laboratory installation in the flow of the tail gases stream derived from the pilot ammonia oxidation plant. The laboratory installation consisted of a reactor (R) with a diameter of X-ray fluorescence (XRF) was used to determine the chemical composition of the samples using Energy Dispersive X-ray Fluorescence EDXRF Spectrometer, Epsilon 3XLE PANalytical Company. The crystalline structure of the samples was analyzed using an X-ray diffraction (XRD) technique. X-ray diffraction patterns were obtained using an Empyrean diffractometer (Panalytical) equipped with a copper-based anode (Cu-Kα LFF HR, λ = 0.154059 nm). The measurement was conducted in the 2*θ* range of 2.0–70.0◦ (2*θ* step scans of 0.02◦ and the counting time of 1 s per step). The specific surface area, total pore volume, and mesopore volume were determined using an ASAP® 2050 Xtended Pressure sorption analyzer (Micromeritics Instrument Co., Norcross, GA, USA) based on N<sup>2</sup> adsorption–desorption isotherms at −196 ◦C using the BET adsorption model (Brunauer– Emmett–Teller) and the BJH transformation (Barret–Joyner–Halenda). Fourier transform infrared spectroscopy studies (FT-IR) were conducted using a Perkin Elmer Frontier FT-IR spectrometer. The spectra were obtained in the wavelength range of 4000–400 cm−<sup>1</sup> with a resolution of 4 cm−<sup>1</sup> . Before each measurement, the sample was mixed with KBr in a ratio of 1:100 and pressed into a disk. Coordination and aggregation of iron species were

25 mm and heat exchangers (HEx and HExNH3) used for preheating tail gases and ammonia. In the tests, the height of the catalyst layer was 70 mm. The installation is schemat-

into the catalytic bed. The composition of the tail gases was similar to the tail gases emitted from industrial nitric acid plants; consisted of NO, NO2, N2O, O2, N2, and H2O; and contained approximately 900–1100 ppm of NOx, (NO/NO2 = 2–2.6) 400–600 ppm of N2O, 2–3 vol.%. of O2, and 0.3–0.5 vol.% of H2O. The amount of NH3 used in the reaction was determined by UV–Vis spectroscopy at a wavelength range of 200–900 nm with a resolution of 1 nm using a Perkin Elmer Lambda 35 UV–Vis spectrophotometer.

#### *3.3. Catalytic Tests in Real Gas Conditions*

The activity and selectivity of the catalysts in the NH3-SCR process were tested in the laboratory installation in the flow of the tail gases stream derived from the pilot ammonia oxidation plant. The laboratory installation consisted of a reactor (R) with a diameter of 25 mm and heat exchangers (HEx and HExNH3) used for preheating tail gases and ammonia. In the tests, the height of the catalyst layer was 70 mm. The installation is schematically presented in Figure 12. The heated tail gases were mixed with ammonia and turned into the catalytic bed. The composition of the tail gases was similar to the tail gases emitted from industrial nitric acid plants; consisted of NO, NO2, N2O, O2, N2, and H2O; and contained approximately 900–1100 ppm of NOx, (NO/NO<sup>2</sup> = 2–2.6) 400–600 ppm of N2O, 2–3 vol.%. of O2, and 0.3–0.5 vol.% of H2O. The amount of NH<sup>3</sup> used in the reaction was increased and optimized to the level providing maximum NO<sup>x</sup> conversion with minimal NH<sup>3</sup> slip (less than 10 ppm). Thus, the NH<sup>3</sup> concentration was maintained at 0.14–0.15 vol.%, depending on the inlet NO<sup>x</sup> concentration. *Catalysts* **2022**, *12*, x FOR PEER REVIEW 13 of 16 increased and optimized to the level providing maximum NOx conversion with minimal NH3 slip (less than 10 ppm). Thus, the NH3 concentration was maintained at 0.14–0.15 vol.%, depending on the inlet NOx concentration.

**Figure 12.** The scheme of the laboratory installation for NH3-SCR catalytic tests. **Figure 12.** The scheme of the laboratory installation for NH<sup>3</sup> -SCR catalytic tests.

In each test, 30 g of the catalyst in a form of pellet (*d* = 5.0 × 4.8 mm) was placed into the reactor. The activity studies were performed at 250, 300, 350, 400, and 450 °C. The temperature inside the reactor was controlled by a thermocouple installed at the gas outlet from the bed. The research was conducted in GHSV = 4500 and 9000 h−1 (tail gas flow 0.15 and 0.3 Nm3·h−1, respectively). Temperature, GHSV, and shape of catalyst were selected to be as close as possible to conditions prevailing in industrial plants. The measurements of the inlet and outlet concentrations of NO, NO2, N2O, and NH3 were conducted at each temperature after stabilizing the equilibrium conditions and operating parameters. The concentrations of unreacted NO, NO2, and N2O were analyzed downstream of the reactor by a GASMET FT-IR analyzer (Vantaa, Finland). NOx reduction was important in this study; thus, NO and NO2 concentrations were not considered separately. NOx conversion In each test, 30 g of the catalyst in a form of pellet (*d* = 5.0 × 4.8 mm) was placed into the reactor. The activity studies were performed at 250, 300, 350, 400, and 450 ◦C. The temperature inside the reactor was controlled by a thermocouple installed at the gas outlet from the bed. The research was conducted in GHSV = 4500 and 9000 h−<sup>1</sup> (tail gas flow 0.15 and 0.3 Nm<sup>3</sup> ·h −1 , respectively). Temperature, GHSV, and shape of catalyst were selected to be as close as possible to conditions prevailing in industrial plants. The measurements of the inlet and outlet concentrations of NO, NO2, N2O, and NH<sup>3</sup> were conducted at each temperature after stabilizing the equilibrium conditions and operating parameters. The concentrations of unreacted NO, NO2, and N2O were analyzed downstream of the reactor by a GASMET FT-IR analyzer (Vantaa, Finland). NO<sup>x</sup> reduction was important in this study; thus, NO and NO<sup>2</sup> concentrations were not considered separately. NO<sup>x</sup> conversion was calculated according to Equation (6):

where X౮—NOx conversion, NO୶ሺinሻ—inlet concentration of NOx, while NO୶—NOx

This paper has demonstrated the catalytic potential of protonated or protonated and iron-modified clinoptilolite in the form of pellets in NH3-SCR within the range of 250–450 °C. Pretreatment with HNO3 and deposition of iron changed the shape of mesopores and resulted in the formation of secondary porosity. Additionally, deposition of iron caused some interruptions in the order of the zeolite framework. Nevertheless, the crystallinity was not affected by the performed modifications. Catalytic tests were conducted using a gas mixture which reflected industrial conditions. For H-Clin, the maximum conversion of NOx of over 90% was achieved above 400 °C and GHSV = 4500 h−1. At the load of 9000 h−1, the conversion of NOx reached more than 60% in the entire temperature range. Satisfactory results obtained for the protonated zeolite without the addition of the active phase can be explained by the natural presence of iron species in the clinoptilolite structure. Regardless of iron loading, NOx conversion obtained for the catalysts was higher than that

$$\text{X}\_{\text{NO}\_{\text{x}}} = \frac{\text{NO}\_{\text{x}}(\text{in}) - \text{NO}\_{\text{x}}}{\text{NO}\_{\text{x}}(\text{in})} \cdot 100\% \tag{6}$$

concentration in the gas after catalytic reaction.

**4. Conclusions** 

where XNOx—NO<sup>x</sup> conversion, NOx(in)—inlet concentration of NOx, while NOx—NO<sup>x</sup> concentration in the gas after catalytic reaction.

#### **4. Conclusions**

This paper has demonstrated the catalytic potential of protonated or protonated and iron-modified clinoptilolite in the form of pellets in NH3-SCR within the range of 250–450 ◦C. Pretreatment with HNO<sup>3</sup> and deposition of iron changed the shape of mesopores and resulted in the formation of secondary porosity. Additionally, deposition of iron caused some interruptions in the order of the zeolite framework. Nevertheless, the crystallinity was not affected by the performed modifications. Catalytic tests were conducted using a gas mixture which reflected industrial conditions. For H-Clin, the maximum conversion of NO<sup>x</sup> of over 90% was achieved above 400 ◦C and GHSV = 4500 h−<sup>1</sup> . At the load of 9000 h−<sup>1</sup> , the conversion of NO<sup>x</sup> reached more than 60% in the entire temperature range. Satisfactory results obtained for the protonated zeolite without the addition of the active phase can be explained by the natural presence of iron species in the clinoptilolite structure. Regardless of iron loading, NO<sup>x</sup> conversion obtained for the catalysts was higher than that of the H-Clin. In the case of Fe-Clin-1 and Fe-Clin-2, NO<sup>x</sup> conversion exceeded 90% above 350 ◦C. Slightly lower NO<sup>x</sup> reduction was recorded for Fe-Clin-3. In summary, it was demonstrated that even a single impregnation of natural zeolite (Fe-Clin-1) resulted in the satisfactory catalytic performance, since more than 90% of NO<sup>x</sup> conversion was achieved between 350–450 ◦C. Additionally, it was noted that N2O concentration decreased by 20% compared to the initial concentration. The strength and significance of our work lies especially in the minimization of the catalyst preparation steps, which is highly beneficial from technological and economical points of view. In summary, it was demonstrated that Fe-clinoptilolite catalysts are advantageous, low-cost, and easy-to-prepare materials that exhibit satisfactory features in the NH3-SCR process.

**Author Contributions:** Conceptualization, M.S., M.M. and M.I.; methodology, M.S. and M.I.; resources, K.A.-J.; investigations, M.S. and A.S.-W.; writing—original draft preparation, M.S. and A.S.-W.; writing—review and editing, M.I. and B.S.; supervision, M.I., M.M. and B.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** M.S. acknowledges the financial support to the program of the Ministry of Science and Higher Education entitled "Implementation Doctorate".

**Acknowledgments:** A.S.-W. gratefully acknowledges financial support from the National Science Centre Grant Preludium 19 (no. 2020/37/N/ST5/00186) The research results presented by M.M. have been developed with the use of equipment financed from the funds of the "Excellence Initiative-Research University" program at AGH University of Science and Technology. B.S.'s research project was partly supported by the program "Excellence initiative—research university" for the AGH University of Science and Technology".

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

#### **References**

