Promotional Corrugated-Type Catalyst of Nb-Modified V-Based Catalyst for NH₃-SCR over a Wide Temperature Range with Low SO₂/SO₃ Conversion

Abstract

This study investigates the catalytic activity and surface behaviors of V₂O₅ catalysts modified with promoter components to improve low-temperature activity and suppress SO₂-to-SO₃ oxidation. Moreover, we fabricated corrugated-type catalysts using glass fiber sheets as substrates, because powdered catalysts revealed limitations for practical applications. The modified catalysts were prepared via a slurry mixing method using vanadium precursor with different promoters such as W, Nb, Zr, Mo, Ce, Fe supported on TiO₂. The catalysts were fabricated into slurry coating type catalysts using glass fiber sheets and the catalytic activity, specific surface area, and acid sites were investigated. The performance of corrugated-type catalyst and oxidation of SO₂ to SO₃ over a wide temperature range were evaluated using a Micro-Reactor. Our results showed that adding a promoter improves the catalytic performance of VW/Ti catalysts by enhancing surface acidity. The results varied depending on the catalyst loading and promoter components over a wide temperature range. Among them, VWNb/Ti catalysts exhibited the highest NOx conversion of 80.9% at 350 °C despite the high gaseous velocity (AV of 51 m·h⁻¹) and the large flow rate of 20 L·min⁻¹, with the lowest Ea of 13.7 kJ·mol⁻¹. Among the evaluated promoters, Nb exhibited the most favorable balance of activity and SO₂-to-SO₃ oxidation. These results suggest that the Nb modification strategy can be extended to commercial SCR catalysts, providing a practical approach for improving catalytic performance.

1. Introduction

Nitrogen oxides (NOx), one of major air pollutants, are generated as by-products of combustion processes like those in thermoelectric power plants and diesel engines [1], and are a major source of particulate matter (PM) [2,3,4,5,6,7]. They cause various environmental and health problems [8,9], such as greenhouse effect, photochemical smog, acid rain, and respiratory disease. Selective catalytic reduction (SCR) is recognized as the most effective method to control NOx emissions. Industrial V₂O₅–WO₃/TiO₂ catalysts exhibit high NOx conversion and exceptional resistance to sulfur (SO₂) and H₂O. However, these catalysts also exhibit several disadvantages, including a high and narrow effective temperature range (300–400 °C) and a propensity to oxidize SO₂ to SO₃ [10,11,12,13].

SO₃ reacts with NH₃ to form ammonium sulfate (NH₄)₂SO₄ or bisulfate (NH₄HSO₄), which shortens the lifetime of the catalyst [5,14]. The formation of (NH₄)₂SO₄ or NH₄HSO₄ occurs within the operating range (250–350 °C) of the catalyst. The reaction of SO₃ with water vapor results in the formation of H₂SO₄, a corrosive mist that has the potential to damage the catalyst system and lead to equipment failure. Consequently, the utilization of SCR catalysts with high NOx conversion efficiencies at reduced operating temperatures is required to ensure economic viability and to minimize energy expenditure. This catalyst must exhibit the capacity to function at conventional low temperatures (200–250 °C), demonstrate enhanced catalyst properties, such as elevated NOx conversion coupled with diminished SO₂-SO₃ oxidation, and possess a prolonged lifespan.

In recent studies, various metal oxides as promoter species have been utilized to enhance the activity of vanadium-based catalysts in NH₃-SCR [15,16,17,18,19,20]. A substantial body of research has been dedicated to investigating the impact of promoters on the dispersion of catalyst components, with the objective of enhancing catalyst activity [21,22]. For instance, molybdenum (Mo) has been shown to enhance the durability of SO₂ and increase the surface acid site density, thereby promoting the activity of SCR catalysts [23]. Iron (Fe) has been reported to exhibit excellent SCR properties by enhancing the adsorption and activation of NH₃ [24,25]. Cerium (Ce) is renowned for its high oxygen storage and transport capacity and is extensively utilized to enhance catalytic activity [26,27]. Zirconium (Zr) has been reported to enhance thermal stability and augment the efficiency of catalysts by increasing the specific surface area [28,29]. The presence of niobium (Nb) has a positive effect on the performance of the catalyst, enhancing its effectiveness in the conversion process [17,30,31]. However, most previous studies have focused on powdered catalysts, and studies evaluating structured catalysts under practical conditions remain limited. In addition to their elevated specific surface areas, glass fiber sheets possess a low density, are readily formable, and exhibit optimal supporting ratios for the catalyst materials. In this context, the effect of metal oxides as promoter species (Nb, Mo, Ce, Fe, and Zr) on V₂O₅/TiO₂ catalysts was systematically investigated, and their catalytic performance was evaluated in comparison with commercial V₂O₅–WO₃/TiO₂ SCR catalysts. Unlike previous studies that focused on powdered catalysts or individual promoter effects, this study demonstrates the synergistic role of Nb addition in a fabricated corrugated-type catalyst under practical operating conditions, simultaneously achieving enhanced low-temperature activity and suppressed SO₂-to-SO₃ conversion.

2. Experimental

2.1. Catalyst Preparation

Catalysts with different promoter components were prepared in the following five steps. The catalyst slurry was prepared by simple mixing of TiO₂, binders, main catalyst (ammonium metavanadate, NH₄VO₃, AMV, 99%, DAEJUNG, Siheung-si, Republic of Korea and ammonium metatungstate, (NH₄)₆H₂W₁₂O₄₀, AMT, 99.9%, Sigma-Aldrich, St. Louis, MI, USA), and precursor of promoter species (Nb; niobium oxide (V) oxalate hydrate, C₁₀H₅NbO₂₀·xH₂O, 99.9%, Thermo Fisher Scientific, Waltham, MA, USA, Ce; cerium (IV) diammonium nitrate, (NH₄)₂Ce(NO₃)₆; CeH₈N₈O₁₈, 97%, DAEJUNG, Zr; zirconium (IV) oxynitrate hydrate, ZrO(NO₃)₂ · xH₂O, 99%, Sigma-Aldrich, Fe; ferric nitrate nonahydrate, Fe(NO₃)₃ · 9H₂O, 99%, DAEJUNG, Mo; hexaammonium heptamolybdate tetrahydrate, (NH4)6Mo7O24 · 4H₂O; H₂₄Mo₇N₆O₂₄ · 4H₂O, 98%, DAEJUNG) in silica sol. The catalyst slurry was coated onto a glass fiber support sheet and fabricated. (1) AMV was mixed with the silica sol for 30 min using a magnetic stirrer. (2) TiO₂ and silica sol were mixed with precursor of promoter species (Nb, Ce, Zr, Mo, and Fe) for 1 h. (3) Vanadate/silica sol prepared in step (1) was added to the previously synthesized slurry with precursor of promoter species, obtained in step (2), and mixed for 30 min. (4) Clay (an inorganic binder) and polyvinyl alcohol (PVA an organic binder) were added to the catalyst slurry obtained in step (3). The slurry was mixed using a homogenizing stirrer (HS-30E, DAIHAN Scientific, Wonju, Republic of Korea), followed by ball milling for 2 h. (5) The resultant slurry was coated on a glass fiber sheet to form a corrugated catalyst. The obtained SCR catalyst was dried at 80 °C for 12 h, and then calcined in air at 500 °C for 3 h. For comparison, V₂O₅–WO₃/TiO₂ (V-W/Ti) catalysts containing 1 wt% V₂O₅ and 4 wt% WO₃ were prepared. V₂O₅–WO₃–MOx/TiO₂ (V-W-M/Ti) catalysts were prepared using the same procedure described above, except that a metal oxides as promoter species (i.e., Nb, Mo, Ce, Zr, Fe) was added.

2.2. Characterization

X-ray diffraction (XRD, PANalytical, X’Pert PRO High Resolution X-Ray Diffractometer, Almelo, The Netherlands) was conducted using Cu Kα (40 kV, 40 mA) radiation in a continuous scan mode at 20° ≤ 2θ ≤ 80°. The specific surface areas and average particle size of the composites were determined (ASAP 2020, Micromeritics, Norcross, GA, USA) using liquid N₂ at 77 K. The specific surface area (S_BET) was calculated using the Brunauer–Emmett–Teller (BET) method. Temperature programmed desorption of NH₃ was performed using a chemisorption analyzer (NH₃-TPD, Autochem II 2920 chemisorption analyzer, Micromeritics, Norcross, GA, USA). Samples were preheated at 300 °C with He for 1 h followed by NH₃ adsorption to saturation and He purging at 100 °C. Further, temperature programmed desorption of O₂ was performed using the chemisorption analyzer (O₂-TPD, Autochem II 2920, Micromeritics, Norcross, GA, USA), and the samples were preheated to 300 °C using 10% O₂/Ar for 1 h and cooled down to 50 °C. Then, the samples were saturated with 1% O₂/Ar for 1 h, and physically adsorbed O₂ was removed through Ar purging. The gas temperature increased from 100 to 800 °C. Desorption was performed from 100 to 800 °C at a heating rate of 10 °C/min. H₂-temperature-programmed reduction (H₂-TPR) was carried out using a chemisorption analyzer (Autochem II 2920, Micromeritics, Norcross, GA, USA). H₂-TPR was investigated during the temperature increase from 100 to 800 °C under a flow of 10% H₂/Ar gas. In situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFT) spectra was conducted using Vertex 70v (Bruker, Billerica, MA, USA). The gas flow, which was constantly controlled through a mass flow controller (MFC), consisted of 1000 ppm of NO, NH₃, 5 vol% of O₂ and balance with N₂ to maintain the overall flow at 300 mL∙min⁻¹. The in situ DRIFTS sequence was conducted in two steps; one was the gas flow of [NO + O₂] after [NH₃], and the other was that [NH₃] after [NO + O₂]. Prior to each step, the catalyst samples were pretreated with N₂ flow at 200 °C for 2 h.

2.3. Catalytic Performance Test

The SCR activity was measured in a Micro-Reactor (MR, NANO Co., Sangju-si, Republic of Korea) as shown in Figure S1. A slurry coated catalyst was prepared with dimensions of 3.5 cm × 5.5 cm and a thickness of about 1 mm. The corrugated catalyst sample inserted into the MR had a diameter of approximately 50 mm and length of 1 m. The test conditions were as follows: 300 ppm NO, 300 ppm NH₃, 300 ppm SO₂, 5 vol% O₂, 10 vol% H₂O, and N₂ as the balance gas. Total flow through the reactor was 20 L·min⁻¹, area velocity (AV) was 51 m·h⁻¹, and the operating temperatures were 200 to 450 °C. The SO₂-to-SO₃ conversion was measured after 48 h of aging. The aging conditions were as follows; 300 ppm NO, 300 ppm SO₂, 5 vol% O₂, 10 vol% H₂O, and the balance gas was N₂. The total flow through the reactor was 20 L·min⁻¹, the AV was 51 m·h⁻¹ (i.e., GHSV of ~20,000 h⁻¹) and operating temperatures were between 200 and 450 °C. SO₂-to-SO₃ conversion analysis was performed using an Ion chromatography (IC, Dionex ICS-5000, ThermoFisher, Sunnyvale, CA, USA) to measure the SO₄²⁻ formed when SO₃ in the outlet gas was dissolved in water. The performance of NH₃-SCR catalyst was assessed by NOx conversion, NH₃ conversion, N₂ selectivity, turnover frequency (TOF), and SO₂-to-SO₃ conversion, which were determined using the following equations:

$${\mathrm{N}\mathrm{O}}_{\mathrm{X}} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{\%}\right)={\displaystyle \frac{{\left({\mathrm{N}\mathrm{O}}_{\mathrm{X}}\right)}_{\mathrm{i}\mathrm{n}}-{\left({\mathrm{N}\mathrm{O}}_{\mathrm{X}}\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{\left({\mathrm{N}\mathrm{O}}_{\mathrm{X}}\right)}_{\mathrm{i}\mathrm{n}}}}\times 100$$

(1)

$${\mathrm{N}\mathrm{H}}_3\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{\%}\right)={\displaystyle \frac{{\left({\mathrm{N}\mathrm{H}}_3\right)}_{\mathrm{i}\mathrm{n}}-{\left({\mathrm{N}\mathrm{H}}_3\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{\left({\mathrm{N}\mathrm{H}}_3\right)}_{\mathrm{i}\mathrm{n}}}}\times 100$$

(2)

$${\mathrm{N}}_2 \mathrm{s}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}\left(\mathrm{\%}\right)={\displaystyle \frac{(\mathrm{N}\mathrm{O}{)}_{\mathrm{i}\mathrm{n}}+({\mathrm{N}\mathrm{H}}_3{)}_{\mathrm{i}\mathrm{n}}-(\mathrm{N}\mathrm{O}{)}_{\mathrm{o}\mathrm{u}\mathrm{t}}-({\mathrm{N}\mathrm{H}}_3{)}_{\mathrm{o}\mathrm{u}\mathrm{t}}-({\mathrm{N}}_2\mathrm{O}{)}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{(\mathrm{N}\mathrm{O}{)}_{\mathrm{i}\mathrm{n}}+({\mathrm{N}\mathrm{H}}_3{)}_{\mathrm{i}\mathrm{n}}-(\mathrm{N}\mathrm{O}{)}_{\mathrm{o}\mathrm{u}\mathrm{t}}-({\mathrm{N}\mathrm{H}}_3{)}_{\mathrm{o}\mathrm{u}\mathrm{t}}}}\times 100$$

(3)

$$\mathrm{T}\mathrm{O}\mathrm{F}({\mathrm{h}}^{-1})={\displaystyle \frac{\left({\mathrm{F}}_{\mathrm{N}\mathrm{O}}\right)\times \left(\mathrm{N}\mathrm{O} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\right)\times ({\mathrm{N}}_2 \mathrm{s}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y})}{(\mathrm{R})\times (\mathrm{T})\times ({\mathrm{m}\mathrm{o}\mathrm{l}}_{\mathrm{V}\mathrm{W}})}}\times 100$$

(4)

$${\mathrm{S}\mathrm{O}}_2–\mathrm{t}\mathrm{o}–{\mathrm{S}\mathrm{O}}_3 \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\left(\mathrm{\%}\right)={\displaystyle \frac{{\left({\mathrm{S}\mathrm{O}}_3\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}-{\left({\mathrm{S}\mathrm{O}}_3\right)}_{\mathrm{i}\mathrm{n}}}{{\left({\mathrm{S}\mathrm{O}}_2\right)}_{\mathrm{i}\mathrm{n}}}}\times 100$$

(5)

3. Results and Discussion

3.1. Catalytic Performances

The catalyst was fabricated as a coated sheet type with dimensions of 3.5 cm × 5.5 cm, as illustrated in Figure 1a. The composition of the manufactured catalyst was confirmed via XRF analysis (Figure 1b and Table S1). The weight percentage of VO_X was approximately 1 wt%, while WO_X was ~4 wt%. The weight percentage of MO_X (M = Nb, Zr, Mo, Ce, and Fe) was ~6 wt%. The presence of catalytic active materials (i.e., V, W, M = Nb, Ce, Zr, Mo, and Fe) was confirmed. The structural properties of the catalysts were deduced from the X-ray diffraction patterns (Figure 1c), and the observed peaks correspond to the anatase phase of TiO₂ (ICSD No. 98-017-2914). However, other peaks for catalytic active materials were not detected. The absence of diffraction patterns indicates that the metal oxides were either non-crystalline or well dispersed on the support surface with a crystal particle size of diameter of <4 nm. They were too small to be detected by XRD [32]. Prepared catalysts exhibited comparable textural properties, with a specific surface area (S_BET) of 104.6–118.8 m²·g⁻¹, as indicated in Table S2. The higher S_BET and pore volume will improve reaction activity by facilitating interaction with the flue gas [33]. However, given the presence of other contributing factors, the observed effects were not found to be absolute. This indicates that differences in catalytic performance are not solely governed by surface area, but also influenced by other properties such as surface acidity and redox behavior.

As illustrated in Figure 2a–c, the enhancement of catalytic activity was not solely dependent on high specific surface areas [33,34]. In the examined temperature range (200–450 °C), the V/Ti catalyst with the highest S_BET of 118.8 m²·g⁻¹ exhibited the lowest NOx conversion. Conversely, the VWNb/Ti catalyst with the S_BET of 115.4 m²·g⁻¹ exhibited the highest NOx conversion. The performance of the NH₃-SCR was evaluated in terms of NOx conversion (Figure 2a), NH₃ conversion (Figure 2b), and N₂ selectivity (Figure 2c), thereby providing a comprehensive assessment of the system’s functionality. The catalysts were tested to determine NOx conversion and NH₃ slip within a temperature range of 200 ≤ T_OP ≤ 450 °C. The majority of the catalysts exhibited elevated activity across a substantial temperature range following the incorporation of metal oxides as promoter species. The NOx efficiencies of the majority of catalysts exhibited an increase until the temperature reached 350 °C, followed by a decrease from 400 °C. Furthermore, the NH₃ slip of the catalysts decreased with increasing NOx conversion. Given the theoretical equivalence of NOx and NH₃ at a ratio of 1:1, instances of NH₃ slip can occur in practical applications due to NH₃ overspray. NH₃ slip is a poisoning factor in the catalyst because the slipped NH₃ can form ammonium sulfate by reacting with SO₂ in the flue gas. NH₃ was observed to form ammonium salts at relatively low temperatures. At elevated temperatures, ammonium salts undergo decomposition and oxidation to N₂O and NO₂, thereby accelerating NOx generation.

Among all catalysts, V/Ti and VWFe/Ti exhibited the lowest NOx conversion over the temperature range of 200–450 °C. Generally, vanadium-based catalysts demonstrate high NOx conversion efficiencies at elevated temperatures (>300 °C); however, efficiencies of 40–50% are observed at relatively low temperatures. The incorporation of the metal oxides as promoter species enhanced NOx conversion at low temperatures. The process of NOx conversion is contingent upon the characteristics of the catalyst, including acidity, specific surface area, the bond between the catalyst and the support material, and its dispersion. Furthermore, the impact of flue gas must be considered. The presence or absence of SO₂ and H₂O in the flue gas has been reported to directly affect catalytic activity [11,35,36,37]. This is due to the competition between NH₃ and the aforementioned gases for catalytic acid sites. SO₂ and H₂O have been observed to react with acid sites, thereby prohibiting them through a process of inhibition and salt formation. This, in turn, results in the deactivation of the catalyst [38,39,40]. The catalysts were examined in the presence of H₂O to consider the aforementioned factors. The NOx conversion efficiencies of the majority of the catalysts exhibited an increase in the high-temperature range (i.e., ≥300 °C). The NOx conversion efficiencies at temperatures below 200 °C were ranked in the following sequence: VWNb/Ti ≥ VW/Ti ≥ VWCe/Ti > VWZr/Ti > VWMo/Ti > VWFe ≥ V/Ti. The incorporation of metal oxides as promoter species into V/Ti led to an enhancement in its catalytic activity across a broad temperature range. This behavior indicates that Nb incorporation contributes to enhanced catalytic activity and stability over a broad temperature range. The results indicate that the composition of metal oxides as promoter species influences the operating temperature range. VWM/Ti at 200–450 °C demonstrated enhanced catalytic activity and resistance to H₂O and SO₂ in comparison to V/Ti [41]. The incorporation of metal oxides, such as Nb or Ce, into a VW/Ti composition led to enhanced NOx conversion efficiencies across a substantial temperature range. The VWFe/Ti exhibited low activity, which sharply decreased at temperatures above 400 °C. The VWCe/Ti also demonstrated elevated activity across a broad temperature spectrum. Among the catalysts, the VWNb/Ti exhibited the highest performance, exhibiting an almost ~80% NOx conversion across a broad temperature range. The incorporation of Nb into the catalyst composition resulted in the enhancement of vanadium dispersion over the TiO₂ surface, thereby generating optimal acid sites and facilitating reaction with flue gases. This behavior was considered beneficial for NOx conversion across a broad temperature range.

It is important to consider both the NO conversion rate and the catalyst’s N₂ selectivity, given the necessity to convert both NO and NH₃ to N₂ and H₂O. This phenomenon can be attributed to the toxicity of N₂O and NH₃, which are produced as a consequence of the NH₃-SCR reaction. Depending on the added co-catalyst, the N₂ selectivity according to the N₂O concentration also varied. The catalyst turnover frequency (TOF) is calculated using these factors and the amount of catalyst (Figure 2d). Among these, the catalyst demonstrating superior TOF compared to VW/Ti was the Nb-added catalyst, which was the only one across the entire temperature range. Furthermore, the activation energy was calculated using the Arrhenius equation, and the Nb catalyst exhibited the lowest value at 13.7 kJ mol⁻¹ (Figure 2e), consistent with the TOF results. These results further confirm that Nb addition enhances catalytic activity.

The SO₂ in the flue gas is oxidized to SO₃ via side reaction of the V-based catalyst. SO₃ reacts with water present in flue gas and ammonia, forming sulfuric acid (H₂SO₄), ammonium sulfate ((NH₄)₂SO₄), and ammonium bisulfate (NH₄HSO₄). These by-products are formed within the catalyst pores, resulting in catalyst deactivation. The presence of SO₂ and NH₃ in flue gas has been reported to induce catalyst deactivation, even at low ppm levels. Ammonium sulfate deposits have been observed to accumulate on the surface of the air preheater (APH) and downstream of the reactor. This accumulation has been shown to affect the gas flow rate, thereby causing serious problems for the selective catalytic reduction (SCR) system. The SO₂-to-SO₃ conversion has been observed to be associated with the concentrations of SO₂, NH₃, and O₂. This conversion was generally increased at temperatures above 300 °C. The SO₃ produced reacts with unreacted NH₃, resulting in equipment corrosion and catalyst deactivation. Therefore, controlling SO₃ formation is important. The impact of the promoter and the broad temperature range on the oxidation of SO₂ to SO₃ was investigated. The outcomes are presented in Figure 2f. The catalysts showed the following trend in SO₂-to-SO₃ conversion: VWNb/Ti < VWZr/Ti < VWMo/Ti < VWCe/Ti < VW/Ti < VWFe/Ti < V/Ti. The catalysts that have been augmented with promoters demonstrated a reduced propensity for SO₂-to-SO₃ oxidation in comparison to those that have not been supplemented with promoters. Niobium (Nb) and zirconium (Zr) have been shown to exhibit high durability in the presence of sulfur (SO₂), exhibiting a comparable performance to tungsten (W). The presence of Nb and Zr-oxides as promoter species has been observed to result in a non-exponential increase in SO₂-to-SO₃ conversion with temperature, demonstrating stable suppression even at temperatures above 400 °C. Consequently, the Nb and Zr-oxides demonstrated thermal stability and significant resistance to SO₂ and H₂O. A comparison of the data from VWCe/Ti and VWMo/Ti reveals a high degree of similarity. In general, Ce-oxides exhibits a high degree of resistance to SO₂ [39,42,43,44]. The reaction of VWFe/Ti with SO₂ was attributed to the oxidation of Fe at elevated temperatures, particularly above 350 °C [41]. This reaction led to a rapid conversion of SO₂ to SO₃ upon the addition of Fe to the catalyst. Previous studies have reported that SO₂-to-SO₃ oxidation is associated with the oxidation of SO₂ by V = O, which results in the sulfation and/or oxidation of vanadia sites [37]. It is suggested that the incorporation of promoter elements modifies the surface vanadium species and alters the oxygen environment, which may reduce the active sites responsible for SO₂-to-SO₃ oxidation. In consideration of the aforementioned hypothesis and the findings of our investigation, the following conclusions can be drawn. Initially, the oxidation of SO₂-to-SO₃ was influenced by an elevation in the catalyst operating temperature. Secondly, a high SO₂ adsorption capacity is equivalent to a low rate of oxidation of SO₂-to-SO₃ [45]. These results indicate that the promoter is the primary factor influencing SO₂-to-SO₃ conversion. Moreover, while the corrugated structure enables practical operation under realistic flow conditions, the observed performance differences among catalysts are primarily governed by the promoter composition, particularly Nb incorporation. Thus, the promoter composition is a key factor in determining SO₂-to-SO₃ conversion behavior.

3.2. Catalytic Characterization

H₂-temperature-programmed-reduction (H₂-TPR) profiles were investigated to assess the reducibility of the catalysts, and the outcomes are delineated in Figure 3a and Table S3. The results indicated a correlation between NOx conversion and the aforementioned parameters. Redox properties are of paramount importance in the context of SCR catalysts. To this end, H₂-TPR analysis was conducted to ascertain the mobility and transfer capacity of oxygen. The participation of oxygen in the SCR reaction is understood to occur in the form of oxygen supplied to the gas phase, lattice oxygen species present in the catalyst, and changes in the oxidation form of the catalyst [46]. Therefore, these results indicate that the enhanced participation of the surface lattice oxygen led to an improvement in the catalytic redox properties. In the SCR mechanism, adsorbed NH₃ is activated by V⁵⁺ = O, which results in the generation of V-ONH₃-V⁴⁺-OH surface species. The V-ONH₃-V⁴⁺-OH surface species reacted with NO, yielding N₂ and H₂O. In this process, the generation of V⁴⁺-OH is observed, which subsequently decomposes into V⁵⁺ = O and V³⁺ during the reaction. Reduced V³⁺ is re-oxidized to V⁵⁺ = O by gaseous oxygen, the catalytic redox cycle is facilitated. The re-oxidation rate of this V⁵⁺ = O species increases, resulting in enhanced catalytic activity. The two primary reduction peaks were commonly observed in all catalyst samples, with the exception of those containing Fe and Ce components. A broad peak was detected between 450 and 600 °C. These results were associated with a reduction in active lattice oxygen, oxygen species, Ti⁴⁺, and Ti³⁺ on the catalyst sample surface. In essence, the vanadium and vanadium-tungsten-based catalyst is associated with monomeric surface vanadia species V⁵⁺ to V³⁺ at temperatures ranging from 400 to 500 °C. The temperature range of 500 to 600 °C was indicated by the W⁶⁺ to W⁴⁺ of WO₃ on the surface, as well as the polymeric vanadia species V⁵⁺ to V³⁺. These results were confirmed by co-reduction. The temperature region around 580 °C was identified as crystalline VO_X and W⁶⁺ to W⁴⁺, and the temperature range of 700–800 °C was identified as W⁴⁺ to W⁰. The catalysts can be broadly categorized into two groups based on their TPR profiles. Typical reduction patterns of V and/or W appeared; those showing distinctly modified profiles due to Ce or Fe addition. The reduction peaks of V/Ti appear at approximately 519 °C, while the reduction peak of VW/Ti is observed at 529 °C. The reduction peaks exhibit variations in accordance with the incorporation of each promoter. Furthermore, analogous peak areas were observed, with the exception of Ce and Fe components. In the contrast, the VWCe/Ti exhibited three reduction peaks at 460, 582, and 768 °C. The 460 to 590 °C peak was identified as the Ce⁴⁺ to Ce³⁺ transition. It is hypothesized that the presence of high temperatures, reaching a maximum of 768 °C, may be attributable to the reduction in W species in interaction with the TiO₂ support. In addition, as indicated in other literature on the subject, the reduction peak at 450–600 °C in the catalyst is found to be coincident with the vanadium-ceria reduction peak. Furthermore, the reduction peak near 770 °C is found to be coincident with the tungsten-ceria reduction peak. The 460 °C is attributed to the surface-capping oxygen of the Ce⁴⁺-O-Ce⁴⁺ type stoichiometric ceria. Overall, Ce exhibited a propensity to attain a higher reduction peak temperature in comparison to the other catalysts, a finding that is consistent with its elevated NOx conversion capacity. Similarly, in the case of the VWFe/Ti, the reduction peaks exhibit a shift towards a low-temperature range, with peaks observed at 462, 495, and 679 °C. This outcome suggests that the reduction reaction becomes more favorable when a greater quantity of reactive oxygen species is present. The broad peaks at 350–800 °C can be seen as overlapping reductions from Fe³⁺ to Fe²⁺, V⁵⁺ to V⁴⁺ and/or V³⁺. In addition, the 628 and 679 °C peak has been observed to exhibit a possible reduction of Fe²⁺ to Fe⁰. In other words, Fe-loaded catalysts exhibited higher levels of reactive oxygen species compared to other catalysts. The H₂-TPR profile of VWMo/Ti exhibits a peak at the 440–520 °C range, indicative of a reduction in Mo⁶⁺ to Mo⁴⁺ and a concomitant increase in temperature, suggesting the formation of metallic Mo [47,48,49]. Furthermore, the V and Mo reduction peaks exhibit partial overlap. The VWZr/Ti catalyst demonstrated a reduction peak at 557 °C. Pure ZrO₂ exhibits no redox properties; therefore, this peak may be attributable to the reduction of surface V⁵⁺ to V³⁺. In summary, Zr-oxides as promoter species has been reported to expand the reduction area, thereby facilitating alterations in surface properties and oxygen mobility [50,51]. The addition of Nb to the VWNb/Ti catalyst has been shown to enhance surface reactive oxygen species and H₂-consumption, thereby improving the catalytic oxidation characteristics [30,31,52]. The incorporation of Nb resulted in the extension of the reduction peak to its maximum extent (531 °C) and the broadening of the low-temperature range [17]. Consequently, the Nb-loaded catalyst exhibited enhanced reducibility, which was attributed to the interaction between the V-Nb species. This interaction was found to correlate with improved SCR performance [17,30], with enhanced reducibility and oxygen mobility contributing to this behavior.

Catalyst adsorption of NH₃ and surface acidity have been identified as critical factors in SCR reactions [41,53]. A quantitative analysis of the number of surface acid sites and the adsorption strength was conducted using a NH₃-temperature-programmed-desorption (NH₃-TPD) profiles. The high adsorption capacities indicate that NH₃ reacts with more surface acid sites on the catalyst, which in turn helps improve the NOx conversion [54]. Therefore, the present section is concerned with an analysis of the influence of the promoter component on the strength of the catalytic acid sites. The NH₃-TPD profiles were investigated within a temperature range of 100–800 °C, and the profiles were categorized based on the presence of three primary peaks (Figure 3b). The initial peak, which occurred at 107–250 °C, was attributed to the presence of weakly adsorbed NH₃ on acid sites. The second peak, situated at 300–480 °C, was attributed to medium-strong acid sites. In general, the NH₃ desorption at the relatively low temperature region of 100–240 °C is attributed to partially adsorbed ionic NH₄⁺. Peaks at the elevated temperature range of 250–450 °C are attributed to the desorption of NH₄⁺ on Brønsted acid sites, while coordinated NH₃ on Lewis acid sites is also a contributing factor [22,30,54,55,56]. Lastly, the ammonia pyrolysis peak, which occurs above 600 °C, is difficult to analyze in terms of its properties. Consequently, a greater number of ammonia species were desorbed from Brønsted acid sites than Lewis at low temperature. Brønsted acid sites contributed to the catalytic activity in the low-temperature range, whereas the Lewis sites contributed to the catalytic activity in the high-temperature range. The observed outcomes suggest that the variations in the distribution of acid sites, attributable to alterations in the chemical and physical adsorption characteristics with respect to the catalyst temperature, effectively imply that the activation energy for NH₃ desorption is contingent on temperature.

Among the evaluated promoters, Nb and Ce most effectively modified the distribution of Brønsted and Lewis acid sites, contributing to their NOx conversion. The amount of NH₃ desorption followed the trend: V/Ti > VWNb/Ti = VW/Ti = VWCe/Ti = VWZr/Ti > VWFe/Ti > VWMo/Ti. These results confirmed that the addition of promoters influences surface acidity; however, the NOx conversion did not directly correlate with the NH₃-TPD results. The majority of the catalyst samples exhibited analogous trends; a significant amount of acid sites was observed in the range of 150–400 °C, and the peaks were substantial, suggesting elevated adsorption. It was confirmed that V/Ti and VW/Ti possessed sufficient surface acid sites and other promoters, with the exception of Mo and Fe. In general, the number of catalytic acid sites increases when Mo is added, thereby exhibiting great catalytic efficiency at low temperatures [14,47,49]. However, the results of this study indicated contradictory findings. Furthermore, while Fe exhibited lower amounts of acid sites, its peak point exhibited an expansion in comparison to other catalysts. The incorporation of Ce into the vanadium-based catalyst has been demonstrated to enhance both acid sites (i.e., Brønsted and Lewis acid sites) and NOx conversion. The findings provided a quantitative assessment of the observed effects. The results of the NH₃-TPD experiment demonstrate that the addition of promoters to vanadium-based systems has the effect of altering the NH₃ desorption peak over a considerable temperature range. Among these promoters, it was confirmed that the addition of Ce and Nb expanded and altered the area of Brønsted and Lewis acid sites [12,30,57]. Consequently, Nb enhanced surface acidity, thereby augmenting catalytic activity in comparison to alternative catalysts across a broad temperature range. We confirmed that ammonia adsorption varied with the promoter and temperature ranges. Thus, the desorption temperature underwent a shift in response to the incorporation of the promoter. However, a direct connection between the NH₃-TPD results and the NOx conversion results was not observed. Therefore, catalytic performance is not determined solely by surface acidity.

The O₂-temperature-programmed-desorption (O₂-TPD) experiment was conducted to ascertain the oxidation characteristics, oxygen species, and oxygen mobility (Figure 3c). Therefore, the objective of the O₂-TPD measurement was to ascertain the NOx conversion effect on the surface oxygen species of the catalyst. Improved oxygen mobility has been reported to promote adsorption and activation [49,58]. The presence and behavior of oxygen have been identified as significant factors in the performance of catalysts. This investigation focused on the effect of each promoter. The catalysts exhibited broad desorption peaks within the temperature range of 200 to 500 °C and 700 °C. These peaks were attributed to the desorption of multiple oxygen species. As demonstrated in the result, the vanadium-based catalyst has been shown to be indicative of a relationship between desorption and release of O₂ on the surface of catalysts. The third to fifth desorption peaks were identified for each promoter. Specifically, for all catalysts corresponding to adsorbed oxygen (O_ads) and lattice oxygen (O_latt), a weak desorption peak below 400 °C and a strong desorption peak above 400 °C was observed [47,58,59].

As is the case with V-based catalysts in general, the trend is analogous. The flow of the total desorption amount is displayed in Figure 3d and Table S3. A comparative analysis of the V-based catalysts revealed that they exhibited analogous trends; however, when a promoter was introduced, the reaction occurred at a reduced temperature. The results of the study are as follows: VWNb/Ti = VWCe/Ti > VW/Ti > VWZr/Ti > VWFe/Ti > V/Ti > VWMo/Ti. With the exception of V/Ti and VWMo/Ti, the peak of the other promoters added sample commenced at low temperatures of approximately 200 °C. These catalysts have been shown to broaden the oxygen mobility range. The aforementioned trends are associated with the NOx conversion results. The conversion process involves the oxidation of NO to NO₂, a process that occurs in conjunction with adsorbed surface oxygen species and vacancies. The NO can be oxidized to NO₂, which further forms nitrate species (NO₃⁻), which subsequently adsorb on the surface of the catalyst. Conversely, NO₂ and NO₃⁻ have been observed to react with adsorbed NH₃ and/or NH₄⁺, yielding NH₄NO₂ and NH₄NO₃ intermediates. These intermediates are further decomposed to produce N₂ and H₂O [60]. In summary, the expansion to a low-temperature range was associated with an increase in catalytic activity at low temperatures. The incorporation of the promoter resulted in the augmentation of surface adsorbed oxygen species and the enhancement of NO adsorption capacity. The addition of the promoter enhanced the reducibility of the V/Ti and VW/Ti catalysts. This indicates that improved oxygen mobility and adsorption capacity contribute significantly to enhanced NOx conversion.

3.3. Reaction Between Pre-Adsorbed NO + O₂ and NH₃

An investigation was conducted into the adsorption–desorption behaviors of gases on catalysts. Gaseous adsorption–desorption behaviors on catalysts during SCR reaction were investigated by collecting in situ diffuse reflection infrared Fourier transform (DRIFT) spectra. First, all catalysts were adsorbed by flowing NH₃ of 1000 ppm with balanced N₂ for 20 min. Thereafter, gases with concentrations of 1000 ppm NO and 5 vol% O₂ were simultaneously flowed for 20 min after 3 min of N₂ purge (Figure 4). In situ DRIFTS was utilized to conduct this investigation. Initially, NH₃ was flowed over the catalysts. This was followed by NO and O₂. In general, the disappearance of peaks assigned to NH₃ species by the introduction of NO and O₂ indicates that the adsorbed NH₃ species reacted with gases of NO and O₂ through the SCR reaction. The adsorbed ammonia species on Lewis acid sites were denoted as L-NH_3,ads, while the adsorbed ammonia species on Brønsted acid sites were denoted as B-NH₄⁺. Upon the injection of NH₃, the peaks for adsorbed-NH₃ on V/Ti immediately appeared (Figure S2). A peak assigned to L-NH_3,ads appeared at 1607 cm⁻¹, and the peak for NH₂ species at 1583 cm⁻¹ corresponded to the active form of L-NH_3,ads [61,62]. In particular, broad peaks over a wavelength range of 2000–1600 cm⁻¹ were identified as gaseous NH₃ (denoted as NH₃(g)) on the catalyst [63]. These peaks disappeared during the 3 min N₂ purge. The injection of NO and O₂ resulted in the disappearance of the peak for L-NH_3,ads after 18 min, concurrent with the emergence of new peaks for adsorbed NO species.

3.4. Reaction Between Pre-Adsorbed NH₃ and NO + O₂

Additional in situ DRIFTS spectra were collected to confirm the influences of the other promoter added into VW/Ti (Figure 5). Gaseous behaviors over Zr added catalyst were investigated (Figure 5a), and physically adsorbed NH₃ species appeared over a range from 1900 to 1700 cm⁻¹. In a case of NH₃ species adsorbed on acid sites, appeared B-NH₄⁺ at 1675 cm⁻¹ remained even after flowing gases of NO and O₂ for 20 min, and L-NH_3,ads at 1607 cm⁻¹ disappeared after 16 min [51]. In a DRIFT result of VWMo/Ti (Figure 5b), peaks which appeared at 1676 cm⁻¹ and on 1607 cm⁻¹, were respectively corresponded to B-NH₄⁺ and L-NH_3,ads [23,49]. By the introduction of NO and O₂, former disappeared after 10 min, whereas the latter disappeared after 14 min. However, a peak at 1434 cm⁻¹ (assigned to B-NH₄⁺) remained even after 20 min. For VWCe/Ti (Figure 5c), B-NH₄⁺ species appeared at 1680, 1450, and 1434 cm⁻¹, and L-NH_3,ads appeared at 1600 cm⁻¹ [64,65,66]. After flow of NO and O₂, B-NH₄⁺ disappeared after 2 min, and L-NH_3,ads did after 10 min. In a case of VWFe/Ti (Figure 5d), peaks at 1675 and 1440 cm⁻¹ were attributed to B-NH₄⁺, and a peak for L-NH_3,ads appeared at 1607 cm⁻¹ [67]. After flowing the gases of NO and O₂, the appeared adsorbed NH₃ peaks disappeared after 12 min, completely. Although Ce could make the reaction rate fast, NH₃ was still physically adsorbed over VWCe/Ti. In summary, the addition of Nb as promoter could help to increase the SCR reaction rate by advantageously changing the adsorption form of NH₃ for SCR reaction

4. Conclusions

The corrugated type SCR catalysts were fabricated using glass fiber sheets and modified by the addition of a promoter. The corrugated type V-W-M/Ti catalyst was prepared via a simple mixing process. The NOx conversion was found to be the highest in the VWNb/Ti catalyst at 200–450 °C, and the SO₂-to-SO₃ oxidation was less than 1% at 350 °C. The incorporation of the promoter resulted in alterations to the acid sites (Brønsted and Lewis acid sites) present on the catalyst surface, thereby significantly affecting its catalytic activity. The primary factors contributing to the enhanced NOx conversion and reduced oxidation of SO₂-to-SO₃ are attributed to promoter-support interaction and modified surface properties induced by the incorporated promoter. Consequently, the addition of Nb enhanced the activity and properties of the VW/Ti catalyst, comparable to commercial catalysts. Furthermore, it demonstrated improved catalytic activity over a broad temperature range. These results demonstrate the applicability of corrugated Nb-modified catalysts for practical SCR applications. Therefore, the developed structured catalyst has strong potential for practical NH₃-SCR applications in industrial flue gas treatment systems. However, these results do not reflect the optimization of the catalyst content. Further optimization of catalyst composition and loading is expected to improve catalytic performance.