Study on Novel SCR Catalysts for Denitration of High Concentrated Nitrogen Oxides and Their Reaction Mechanisms

Abstract

With the rapid development of industrialization, the emission of nitrogen oxides (NO_x) has become a global environmental issue. Uranium is the primary fuel used in nuclear power generation. However, the production of uranium, typically based on the uranyl nitrate method, usually generates large amounts of nitrogen oxides, particularly NO₂, with concentrations in the exhaust gas exceeding 10,000 ppm. High concentrations of nitrogen dioxide are also produced during silver electrolysis processing and the treatment of waste electrolyte solutions. Traditional V-W/TiO₂ NH₃-SCR catalysts typically exhibit high catalytic activity at temperatures ranging from 300 to 400 °C, under conditions of low NO_x concentrations and high gas hourly space velocity. However, their performance is not satisfying when reducing high concentrations of NO₂. This study aims to optimize the traditional V-W/TiO₂ catalysts to enhance their catalytic activity under conditions of high NO₂ concentrations (10,000 ppm) and a wide temperature range (200–400 °C). On the basis of 3 wt% Mo/TiO₂, various loadings of V₂O₅ were selected, and their catalytic activities were tested. Subsequently, the optimal ratios of active component vanadium and additive molybdenum were explored. Simultaneously, doping with WO₃ for modification was selected in the V-Mo/TiO₂ catalyst, followed by activity testing under the same conditions. The results show that: the NO_x conversion rates of all five catalysts increase with temperature at range of 200–400 °C. Excessive loading of MoO₃ decreased the catalytic performance, with 5 wt% being the optimal loading. The addition of WO₃ significantly enhanced the low-temperature activity of the catalysts. When the loadings of WO₃ and MoO₃ were both 3 wt%, the catalyst exhibited the best denitrification performance, achieving a NO_x conversion rate of 98.8% at 250 °C. This catalyst demonstrates excellent catalytic activity in reducing very high concentration (10,000 ppm) NO₂, at a wider temperature range, expanding the temperature range by 50% compared to conventional SCR catalysts. Characterization techniques including BET, XRD, XPS, H₂-TPR, and NH₃-TPD were employed to further study the evolution of the catalyst, and the promotional mechanisms are explored. The results revealed that the proportion of chemisorbed oxygen (O_α) increased in the WO₃-modified catalyst, exhibiting lower V reduction temperatures, which are favorable for low-temperature denitrification activity. NH₃-TPD experiments showed that compared to MoO_x species, surface WO_x species could provide more acidic sites, resulting in stronger surface acidity of the catalyst.

1. Introduction

Nitrogen oxides (NO_x) are primarily produced through industrial processes and combustion, including high concentrations of NO_x generated during metal processing. Uranium oxide plays a key role in the production of fuel [1]. The production of uranium trioxide from uranyl nitrate is illustrated in Equation (1) [2], a process with low flue gas temperatures and high NO₂ concentrations. Furthermore, during the manufacture of silver electrolytes, a vigorous reaction occurs between silver powder and nitric acid, resulting in the generation of a large amount of nitrogen oxide gases. Additionally, when treating waste electrolytes from silver electrolysis and silver powder washing solution, significant amounts of nitrogen oxides are also produced during the heating process of mixing these two solutions, with concentrations reaching up to 40,000 mg/m³ [3]. Nitrogen oxides contribute to acid rain, photochemical smog, ozone depletion, and greenhouse effects. When these gases react with other compounds in the atmosphere, they form harmful substances such as ozone and fine particulate matter, which directly impact air quality and have significant implications for human health and climate change [4,5,6].

UO₂(NO₃)₂ → UO₃ + xN₂O + yNO₂ + O₂

(1)

The conventional methods for treating nitrogen oxides (NO_x) in flue gas include adsorption, wet scrubbing, selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), electron beam methods, plasma methods, etc. [7]. Due to their high efficiency, selectivity, and economic viability, selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are considered two effective approaches for reducing NO_x emissions [8,9]. In order to reduce the emissions of nitrogen oxides resulting from industrial production, the selective catalytic reduction (SCR) technique is commonly employed, where NH₃ is used as a reducing agent [10,11,12]. This technique utilizes the catalytic effect of a catalyst to selectively reduce nitrogen oxides into N₂ and H₂O, thereby meeting emission standards. The reaction equation is as follows:

4NH₃ + 4NO + O₂ → 4N₂ + 6H₂O

(2)

2NH₃ + NO + NO₂ → 2N₂ + 3H₂O

(3)

4NH₃ + 2NO₂ + O₂ → 3N₂ + 6H₂O

(4)

Reaction (2) is referred to as the “conventional SCR reaction”, while reaction (3) is known as the “fast SCR reaction”. The fast SCR reaction, compared to the conventional SCR reaction, can occur at lower temperatures and does not require the involvement of oxygen. Due to the rapid oxidation process, the reaction can be completed in a shorter time [13,14,15].

The V₂O₅/TiO₂ catalyst possesses excellent resistance to SO₂ and high selectivity towards N₂, making it widely applied in industrial processes [5,16]. However, the V₂O₅/TiO₂ catalyst exhibits a narrow effective temperature range (300–400 °C). When the flue gas temperature falls below 300 °C, the catalytic activity of conventional denitrification catalysts decreases, leading to reduced denitrification efficiency and increased ammonia escape rate. The key to SCR technology lies in the development and utilization of catalysts [17,18]: Catalysts with high specific surface area and unique crystal structures signify increased contact area and a greater number of vacancies and defects [19,20,21]. The increased number of acidic sites on the catalyst enhances the adsorption of NH₃ [22,23,24]. The reduction in the reduction temperature of metal ions leads to a higher redox capability [25,26,27].

The activity of the catalyst can be increased by the addition of active components. Liu et al. [28] investigated the influence of manganese on the performance of the V₂O₅/TiO₂ catalyst, the results demonstrated that, below 400 °C, the addition of manganese significantly enhanced the activity of the V₂O₅/TiO₂ catalyst in NH₃-SCR, the existence of Mn can promote the formation of reactive intermediates. Jihene Arfaoui et al. [29] reported on cerium and sulfate co-modified V₂O₅/TiO₂ catalysts, and characterization revealed that the catalyst possessed excellent features, including high specific surface area, large pore volume, and good thermal stability, the cerium was introduced with the aim of enhancing the NO-SCR activity at low temperature, while, sulfate groups were added in order to improve the N₂ selectivity at high temperature. Min et.al. [30] investigated the effects of incorporating Sb, La, Ce, and Mo additives into V₂O₅-WO₃/TiO₂ catalysts with high vanadium content for NH₃-SCR, the addition of Sb, La, or Ce to the V-site of the V4/W5/G5 catalyst led to improved reaction activity, which was attributed to an increase in the nonstoichiometric V⁴⁺⁽³⁺⁾ ratio and enhanced surface adsorbed oxygen species. Su et.al. [31] investigated the Sb/Si-doped TiO₂ prepared using the sol–gel method, which was used as support of V₂O₅/TiO₂ SCR catalysts, the result indicates that the introduction of Sb or Si in TiO₂ support indeed affected the physicochemical properties of the prepared V₂O₅/TiO₂ catalysts, simultaneous enhanced surface acidity and redox capacity led to the better SCR activity for V₂O₅/SbTiO₂.

Some scholars have improved the low-temperature activity of catalysts through catalyst modification. Dong et al. [32] employed antimony-doped V-based catalysts, and when V was loaded onto the synthesized Sb/TiO₂, 2V/2Sb/TiO₂ prepared after antimony was added to TiO₂ showed the best denitrification efficiency at temperatures below 250 °C it exhibited excellent performance at low temperatures, the formation of VSbO₄ structure increased redox property, which was important in the SCR reaction, making the valence state of vanadium unstable. Wang et.al. [33] found that the doping of Cu on the V/TiO₂ catalyst can enhance the low-temperature SCR activity, the enhancement of redox ability and surface acidity, and the increase in reduced vanadium species and active oxygen species all contribute to the good performance of V-Cu/TiO_2. Wang et al. [34] regulated the pH value and doping cobalt based on a V₂O₅-MoO₃/TiO₂ catalyst to develop novel low-temperature SCR catalysts, when the pH was 10 and the molar fraction of Co doping was 1 wt%, the catalyst exhibited optimal denitrification activity, the regulation of pH can increase the number of acid sites. Jeongtak Kim et al. [35] reported that the effects of Fe doping on the performance of low-temperature NH₃-SCR using V/W/TiO₂ catalysts and V/W/TiO₂ exhibited an excellent De-NOx efficiency at 180 °C when the loading of Fe is 0.5%, the denitrification rate is decreasing with increasing Fe content, because of the formation of Fe³⁺, nonstoichiometric V⁴⁺, and W⁵⁺ species, the redox properties of the V/W-0.5Fe/TiO₂ catalyst were significantly improved by the addition of Fe.

There are few studies on catalyst modification under high concentration, wide temperature range, and low gas hourly space velocity conditions, and the denitrification temperature range of catalysts should be further relaxed. In this study, on the basis of the previous laboratory research on industrial catalysts, the industrial catalysts have the most excellent low-temperature denitrification performance when the Mo loading concentration is 3 wt%, and the effect of V concentration on the activity of MoO₃-V₂O₅/TiO₂ catalysts in the wide-temperature domain (200–400 °C) was investigated. On the basis of this, by doping WO₃ to modify the V₂O₅-MoO₃/TiO₂ catalyst, optimizing the catalysts and analyzing mechanisms by characterization, so that it has a very good denitrification performance under the conditions of high concentration of NO₂, wide temperature range, and low gas hourly space velocity.

2. Results and Discussion

2.1. Catalyst Activity Testing

Figure 1 illustrates the performance trend of V₁Mo₃/TiO₂, V₂Mo₃/TiO₂, V₃Mo₃/TiO₂, V₄Mo₃/TiO₂, and V₅Mo₃/TiO₂ catalysts over the temperature range of 200–400 °C. The following conclusions can be drawn:

(1)

Within the temperature range of 200–400 °C, with increasing vanadium loading, there is little variation observed in the catalytic performance of the catalysts, the NO_x conversion rates of the five catalysts are similar.

(2)

As the temperature rises, the catalytic denitrification performance improves, indicating that higher temperatures lead to enhanced catalyst activity.

(3)

The catalytic effect of all five catalysts was above 80% at 200 °C, showing good low-temperature catalytic activity and the catalytic effect was close to 100% at 400 °C, and the high temperature did not affect the catalytic activity.

Figure 1. NO conversions of NH₃-SCR over catalysts with different V₂O₅ doping.

Figure 1. NO conversions of NH₃-SCR over catalysts with different V₂O₅ doping.

2.2. Catalyst Modification

Since varying the loading of V₂O₅ did not enhance the low-temperature denitrification activity of the catalyst, considering industrial applications, the loading of V₂O₅ was set to 3 wt%. Additionally, catalysts were prepared by introducing two additional variables based on a 3 wt% MoO₃ loading: 5 wt% MoO₃ and 10 wt% MoO₃. These catalysts were subjected to denitrification reaction activity tests. Furthermore, to investigate the denitrification performance of catalysts co-loaded with MoO₃ and WO₃ as promoters, as well as the interaction of V, W, and Mo species on the catalyst surface, V₃W₃Mo₃/TiO₂ and V₃W₅Mo₅/TiO₂ catalysts were synthesized and subjected to denitrification reaction activity testing. Figure 2 illustrates the investigation of different concentrations of MoO₃ and WO₃ loading on the basis of 3% V. The following conclusions can be drawn:

(1)

The increase in MoO₃ loading significantly enhances the denitrification performance of vanadium-based catalysts. Within the temperature range of 200–350 °C, the NO_x conversion rates of the V₃Mo₅/TiO₂ catalyst and the V₃Mo₁₀/TiO₂ catalyst are higher overall compared to the V₃Mo₃/TiO₂ catalyst. The V₃Mo₅/TiO₂ catalyst exhibits the optimal low-temperature denitrification performance, achieving a NO_x conversion rate of 93.4% at 200 °C, while the V₃Mo₁₀/TiO₂ catalyst and the V₃Mo₃/TiO₂ catalyst achieve rates of 91.7% and 86.5%, respectively. The order of catalytic activity among different catalysts is as follows: V₃Mo₅/TiO₂ > V₃Mo₁₀/TiO₂ > V₃Mo₃/TiO₂. It can be observed that when the MoO₃ loading is further increased to 10 wt%, the catalytic activity of the catalyst decreases instead, indicating an optimal loading ratio for the promoter MoO₃.

(2)

When V₂O₅ loading is 3 wt%, MoO₃ loading is 3 wt%, and WO₃ loading is 3 wt%, the V₃W₃Mo₃/TiO₂ catalyst exhibits superior denitrification performance. It achieves slightly higher NO_x conversion rates at 200 °C and 225 °C compared to the optimal V-Mo ratio V₃Mo₅/TiO₂ catalyst. Moreover, at 250 °C, the denitrification conversion rate reaches 98.8%, whereas the V₃Mo₅/TiO₂ catalyst without WO₃ only achieves 94.5%.

(3)

When both MoO₃ and WO₃ loading are increased to 5 wt%, the denitrification performance of the V₃W₅Mo₅/TiO₂ catalyst decreases within the temperature range of 200–300 °C, indicating an optimal loading amount for MoO₃ and WO₃ when co-doped. Considering the NO_x conversion rates of catalysts with different compositions.

The order of catalytic activity is as follows: V₃W₃Mo₃/TiO₂ > V₃Mo₅/TiO₂ > V₃W₅Mo₅/TiO₂ > V₃Mo₁₀/TiO₂. Whether adding only the promoter MoO₃ or co-loading MoO₃ and WO₃, the denitrification performance of the catalysts exhibits a trend of initially increasing and then decreasing with increasing loading amounts. To investigate the effects of different MoO₃ loading amounts and the addition of WO₃ on catalyst activity, characterization analysis was performed on the four catalysts: V₃W₃Mo₃/TiO₂, V₃Mo₅/TiO₂, V₃W₅Mo₅/TiO₂, and V₃Mo₁₀/TiO₂.

2.3. Catalyst Reaction Mechanism Investigation

2.3.1. Catalyst Physical Characterization

BET tests were conducted on the four catalysts, and the calculated specific surface areas obtained using the BET theory are presented in

Table 1. BET test results of the four catalysts.

Sample	BET (m2/g)
V3Mo3W3/TiO2	56.7
V3W5Mo5/TiO2	56.2
V3Mo10/TiO2	66.7
V3Mo5/TiO2	54.4

. It is observed from Table 1 that the specific surface area of V₃Mo₁₀/TiO₂ is the largest, approximately 66.7 m²/g, while that of V₃Mo₅/TiO₂ is the smallest, approximately 54.4 m²/g. However, based on the results of the catalyst activity tests, it is found that V₃Mo₁₀/TiO₂ exhibits relatively low activity despite having the largest specific surface area, whereas V₃Mo₃W₃/TiO₂ demonstrates the best activity despite having a relatively smaller specific surface area. This suggests that the influence of specific surface area on catalyst activity is minimal, and changes in physical structural parameters have a weaker effect on catalytic performance. The alteration in activity is predominantly attributed to chemical interactions. According to the N₂ adsorption-desorption isotherms shown in Figure 3a, both catalysts exhibit type IV isotherms with H₂ hysteresis loops, indicating the presence of significant mesoporous characteristics, consistent with the N₂ adsorption-desorption isotherms. Figure 3b presents the pore size distribution of V₃Mo₃W₃/TiO₂ and V₃Mo₅/TiO₂. It can be observed that both catalysts primarily consist of mesopores. The peak of the pore size distribution curve of the V₃Mo₃W₃/TiO₂ catalyst is lower than that of the V₃Mo₅/TiO₂ catalyst, indicating that the most probable pore size of the V₃Mo₃W₃/TiO₂ catalyst is smaller than that of the V₃Mo₅/TiO₂ catalyst.

2.3.2. Catalyst Crystal Structure Analysis

Further analysis of the catalysts’ crystal structure was conducted using X-ray diffraction (XRD). As shown in Figure 4, the XRD spectra of the four catalysts are nearly identical, with diffraction peaks appearing at similar positions such as 25.24°,37.8°,48.06°, and 62.72°. The predominant substance identified is anatase TiO₂. No other crystalline phases were detected by XRD, indicating that V₂O₅, MoO₃, and WO₃ are highly dispersed on the catalyst surface or may exist in an amorphous state [36]. The intensity of XRD peaks among the four catalysts follows the order: V₃Mo₅/TiO₂ > V₃W₃Mo₃/TiO₂ > V₃Mo₁₀/TiO₂ > V₃W₅Mo₅/TiO₂. It is observed that with increasing loading amounts of MoO₃ and WO₃, the intensity of XRD diffraction peaks decreases. This phenomenon may be attributed to the smaller grain size of the TiO₂ support, leading to a more pronounced surface effect, a decrease in internal crystallinity, and a deterioration in crystallinity, along with the presence of a certain number of amorphous components. The XRD diffraction peaks of the V₃Mo₅/TiO₂ and V₃W₃Mo₃/TiO₂ catalyst samples are stronger, indicating better crystallinity, and larger grain size of the TiO₂ support.

2.3.3. Catalyst Surface Acidity Analysis

Figure 5 shows the NH₃-TPD profiles of the V₃Mo₅/TiO₂ and V₃W₃Mo₃/TiO₂ catalyst samples. The first NH₃ desorption peaks of V₃Mo₅/TiO₂ and V₃W₃Mo₃/TiO₂ catalysts occur at 120 °C and 130 °C, respectively, with similar NH₃ desorption amounts. This suggests that the number of weak acidic sites on both catalysts is comparable. The desorption peaks observed in the low-temperature range are primarily influenced by VO_x species, corresponding to the desorption of ammonium ions NH⁴⁺ adsorbed on weak Brønsted acidic sites generated by the addition of V₂O₅. The second NH₃ desorption peak of the V₃Mo₅/TiO₂ catalyst appears at 239 °C, while for the V₃W₃Mo₃/TiO₂ catalyst, the second NH₃ desorption peak is observed at 273 °C, with a larger NH₃ desorption amount. This indicates that the WO₃-modified catalyst possesses a higher quantity of medium-strong acidic sites, mainly associated with WO_x and MoO_x species. These sites primarily consist of weakly stable Lewis acidic sites adsorbing ammonia NH₃, with WO_x species providing a greater number of acidic sites, resulting in a stronger surface acidity of the catalyst. The third NH₃ desorption peak is observed at 598 °C for the V₃W₃Mo₃/TiO₂ catalyst, indicating that the addition of WO₃ introduces new strong acidic sites to the catalyst, corresponding to the desorption of ammonia NH₃ adsorbed on strong Lewis acidic sites.

2.3.4. Catalyst Reductivity Analysis

Figure 6 depicts the H₂-TPR of the V₃Mo₅/TiO₂ and V₃W₃Mo₃/TiO₂ catalyst samples. In the case of the V₃Mo₅/TiO₂ catalyst, the peak observed at 421 °C corresponds to the first reduction step of polymeric octahedral Mo species (weakly bound to the support) from Mo⁶⁺ to Mo⁴⁺ and the reduction in V species. For the V₃W₃Mo₃/TiO₂ catalyst, the first peak appears at 410 °C, corresponding to the first reduction step of Mo⁶⁺ to Mo⁴⁺, W⁶⁺ to W⁴⁺, and the reduction in V species. The first reduction peak of the V₃W₃Mo₃/TiO₂ catalyst shifts towards lower temperatures and exhibits a higher hydrogen consumption, indicating better low-temperature reduction capability. This suggests strong interactions between V and W, affecting the properties, dispersion, or aggregation of V, W, and Mo species in the co-loaded catalyst, thereby influencing the reducibility of the catalyst. The peaks observed at 538 °C for V₃W₃Mo₃/TiO₂ and at 540 °C for V₃Mo₅/TiO₂ are attributed to the reduction in polymeric octahedral Mo species from Mo⁶⁺ to Mo⁴⁺ with different degrees of aggregation [37]. The peak observed at 746 °C for V₃W₃Mo₃/TiO₂ corresponds to the reduction of amorphous tetrahedral Mo species and W species strongly interacting with the support (Mo⁶⁺ to Mo⁴⁺, W⁶⁺ to W⁴⁺), as well as the further reduction in Mo and W species (Mo⁴⁺ to Mo⁰, W⁴⁺ to W⁰) [38]. A similar peak is observed at 719 °C for V₃Mo₅/TiO₂, corresponding to the reduction in Mo species. The shift of the third reduction peak towards higher temperatures for V₃W₃Mo₃/TiO₂ indicates stronger interactions between WO₃ and the support. In summary, the stronger low-temperature redox capability of the V₃W₃Mo₃/TiO₂ catalyst is one of the key factors contributing to its optimal low-temperature catalytic activity.

2.3.5. Catalyst Surface Elemental Analysis

Figure 7a presents the XPS spectra of the O1s orbital for the catalyst samples. The characteristic peaks observed around 530.2 eV correspond to lattice oxygen (O²⁻, denoted as O_β), while those around 531.5 eV correspond to chemisorbed oxygen (O, denoted as O_α). The positions and intensities of the O1s peaks are quite similar among the four catalyst samples. According to the data in

Table 2. Peak areas of V, Mo, and elements on the surface of catalysts and the proportions of different valence states.

Sample	Peak Area		Atomic Ratio %
	V	Mo	V4+/(V5+ + V4+)	Mo6+/(Mo6+ + Mo5+)	Oα/(Oα + Oβ)
V3W3Mo3/TiO2	6242.5	12,949.0	85.8	71.0	35.2
V3W5Mo5/TiO2	5412.5	15,172.2	85.1	76.1	29.1
V3Mo5/TiO2	5180.2	20,918.6	87.9	63.3	31.2
V3Mo10/TiO2	3684.6	24,661.4	72.9	74.7	29.4

, the ratios of surface O_α/(O_α + O_β) for the V₃W₃Mo₃/TiO₂, V₃W₅Mo₅/TiO₂, V₃Mo₅/TiO₂, and V₃Mo₁₀/TiO₂ catalysts are 35.2%, 29.1%, 31.2%, and 29.4%, respectively. This indicates that excessive introduction of MoO₃ and WO₃ additives reduces the amount of surface chemisorbed oxygen, thereby lowering the denitrification activity of the catalyst. The V₃W₃Mo₃/TiO₂ catalyst exhibits the highest proportion of surface chemisorbed oxygen, thus demonstrating the best denitrification activity.

The XPS spectra of the V2p orbitals for catalysts with different MoO₃ and WO₃ loading amounts are presented in Figure 7b. Table 2 provides the peak areas and relative content of V, Mo, and O elements in different catalysts. As shown in Figure 7b, the V2p spectra for all four catalyst samples exhibit similar shapes, displaying distinct double-peak structures. Each peak can be fitted to represent two peaks corresponding to V⁴⁺ and V⁵⁺ ions. The peaks around 517.2 eV and 525.3 eV represent V⁵⁺ 2p3/2 and V⁵⁺ 2p1/2, respectively, while those around 516.3 eV and 523.6 eV represent V⁴⁺ 2p3/2 and V⁴⁺ 2p1/2, respectively. Although the V₂O₅ loading amounts are the same for all four catalysts, the peak areas differ. According to XPS analysis, the peak areas for the V₃W₃Mo₃/TiO₂, V₃W₅Mo₅/TiO₂, V₃Mo₅/TiO₂, and V₃Mo₁₀/TiO₂ catalysts are 6245.5, 5412.5, 5180.2, and 3684.6, respectively, indicating a decrease in vanadium content with increasing additive amounts. Catalysts containing only MoO₃ show a greater decrease in surface vanadium content with increasing MoO₃ loading, suggesting that excessive additive introduction leads to the aggregation of surface atoms, blocking catalyst pores. However, the addition of WO₃ enables better dispersion of active components, thus increasing the surface vanadium atomic concentration. According to Table 2, the relative content ratios of surface V⁴⁺/(V⁴⁺ + V⁵⁺) for the V₃W₃Mo₃/TiO₂, V₃W₅Mo₅/TiO₂, V₃Mo₅/TiO₂, and V₃Mo₁₀/TiO₂ catalysts are 85.8%, 85.1%, 87.9%, and 72.9%, respectively. MoO₃ promotes the generation of V⁴⁺ due to its higher work function. However, the V⁴⁺/(V⁴⁺ + V⁵⁺) ratio for the V₃Mo₁₀/TiO₂ catalyst is the smallest, as further increases in loading lead to the formation of crystalline MoO₃ and V₂O₅, resulting in spatial hindrance due to changes in spatial configuration, thereby increasing the V⁵⁺ content [39]. The absence of spatial hindrance effects in the V₃W₃Mo₃/TiO₂ and V₃W₅Mo₅/TiO₂ catalysts suggests that simultaneous increases in WO₃ loading stabilize V⁴⁺. The ratio of V⁴⁺/(V⁴⁺ + V⁵⁺) is a crucial factor affecting the catalytic activity of SCR. The presence of non-stoichiometric vanadium species (Vⁿ⁺, n ≤ 4) facilitates electron transfer, and under certain conditions, the V⁴⁺/(V⁴⁺ + V⁵⁺) ratio is positively correlated with the catalytic activity of SCR [40]. The combined influence of surface vanadium atomic concentration and V⁴⁺ content leads to higher SCR activity in the V₃Mo₅/TiO₂ and V₃W₃Mo₃/TiO₂ catalysts compared to the V₃Mo₁₀/TiO₂ and V₃W₅Mo₅/TiO₂ catalysts.

The XPS spectra of the Mo3d orbitals for the catalysts are shown in Figure 7c. From the figure, it can be observed that the Mo3d spectra of the catalysts exhibit two peaks, Mo3d5/2 (around 236.0 eV) and Mo3d3/2 (around 232.8 eV), and the peak areas increase with higher MoO₃ loading. According to Table 2, the ratios of surface Mo⁶⁺/ (Mo⁵⁺ + Mo⁶⁺) for the V₃W₃Mo₃/TiO₂, V₃W₅Mo₅/TiO₂, V₃Mo₅/TiO₂, and V₃Mo₁₀/TiO₂ catalysts are 71.0%, 76.1%, 63.3%, and 74.7%, respectively. The Mo⁶⁺ content also increases with increasing MoO₃ loading, but the denitrification activity of the catalysts decreases instead. This indicates that surface Mo species exhibit low or even no activity for the SCR reaction. The reactivity of surface V species is several orders of magnitude higher than that of W and Mo species. W and Mo species promote the reactivity of surface V species through oligomerization reactions rather than electron effects [41].

3. Experiment

3.1. Catalyst Preparation

This study employed the incipient wetness impregnation method to prepare catalysts with varying loadings of active components and promoters. The experimental reagents are shown in

Table 3. Experimental reagents.

Number	Chemical Formula	Specifications	Manufacturer
1	TiO2	Nano grade	Macklin (Shanghai, China)
2	(NH4)10H2(W2O7)6	Analytical grade	Macklin (Shanghai, China)
3	NH4VO3	Analytical grade	Macklin (Shanghai, China)
4	(NH4)6Mo7O24	Analytical grade	Macklin (Shanghai, China)
5	C2H2O4·2H2O	Analytical grade	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)

. The preparation procedures are as follows: Initially, the mass of active components and promoters (i.e., V₂O₅, WO₃, MoO₃) required for catalyst loading was calculated. Ammonium metavanadate and other necessary chemicals were weighed according to a 1:1 molar ratio of elements. These chemicals were then placed in a beaker and dissolved in an appropriate volume of deionized water. To ensure thorough dissolution of ammonium metavanadate powder, oxalic acid was added to the solution at a molar ratio of 1:2. The solution was stirred at 80 °C until complete dissolution was achieved. Subsequently, the desired amount of TiO₂ support was added to the solution and stirred magnetically at 80 °C until a paste-like consistency was attained. Ultrasonication for 20 min was performed to ensure uniform dispersion of the active components. The resulting paste was then dried in an oven at 105 °C for 12 h, followed by calcination in a muffle furnace at 500 °C for 3 h. The resulting material was ground to a particle size of 45–80 mesh and stored for the following study. The prepared catalysts are denoted in

Table 4. Chemical composition and nomenclature of catalysts.

Chemical Composition	Named
V: 1%, Mo: 3%, TiO2:96%	V1Mo3/TiO2
V: 2%, Mo: 3%, TiO2:95%	V2Mo3/TiO2
V: 3%, Mo: 3%, TiO2:94%	V3Mo3/TiO2
V: 4%, Mo: 3%, TiO2:93%	V4Mo3/TiO2
V: 5%, Mo: 3%, TiO2:92%	V5Mo3/TiO2
V: 3%, Mo: 5%, TiO2:92%	V3Mo5/TiO2
V: 3%, Mo: 10%, TiO2:87%	V3Mo10/TiO2
V: 3%, W: 3%, Mo: 3%, TiO2:91%	V3W3Mo3/TiO2
V: 3%, W: 5%, Mo: 5%, TiO2:87%	V3W5Mo5/TiO2

.

3.2. Catalyst Characterization

BET characterization was carried out using the ASAP 2460 multi-station fully automated specific surface and pore size analyzer from Micromeritic Instruments, Inc. (Norcross, GA, USA) to test and analyze the catalysts using the low-temperature N₂ physical adsorption method. XRD was carried out using a fully automated polycrystalline X-ray diffractometer, ULTIMA IV, Rigaku, Japan, with a test range of 2θ = 10–80°and a step size of 0.2°, and the data were obtained and then analyzed using Jade software for graphical analysis (Jade 2022.0.03. (SP2)); The NH₃-TPD test was carried out by using a Mack AutoChem Ⅱ 2920 chemisorbent analyzer (Norcross, GA, USA). The H₂-TPR test was carried out using a Mack AutoChem Ⅱ 2920 chemisorbent analyzer (Norcross, GA, USA). XPS spectroscopy was carried out on a Thermo Scientific Escalab 250Xi X-ray photoelectron spectrometer (Norcross, GA, USA), with the radiation source chosen to be Al Kα, and the charge correction was carried out by using C1s (284.8 eV).

3.3. Activity Test

The catalyst activity testing equipment depicted in Figure 8 consists of a quartz tube. The inside diameter of the quartz tube reactor is 19 mm. The total gas flow rate entering the quartz tube is 120 mL/min. The catalyst quantity is 4 mL, with gas hourly space velocity of 1800 h⁻¹. The inlet concentrations of the simulated flue gas components are [NO₂] = [NH₃] = 10,000 ppm and [O₂] = 13%, with the remaining N₂ serving as the balance gas. Initially, under a nitrogen atmosphere, the catalyst is preheated to 200 °C. Subsequently, the mixed gas is introduced for the SCR reaction. The exhaust gas is then connected to a flue gas analyzer. Data is recorded once the readings stabilize. The procedure is repeated at increments of 25 °C until reaching 400 °C. The catalytic efficiency of the catalyst can be calculated using the following formula:

$$X=\left(1-\frac{{NO}_{Xout}}{{NO}_{Xin}}\right)\times 100\mathrm{\%}$$

4. Conclusions

In this study, the conventional V-based catalysts were optimized for the high concentration of NO₂, low space velocity, and low-temperature flue gas generated in the production process such as nuclear industry, and the following conclusions were obtained by optimizing the ratio of V-W-Mo to obtain the catalysts with good activity in a wide range of temperature range (200–400 °C), high concentration of NO₂ (10,000 ppm), and low space velocity (1800 h⁻¹):

(1) The NO_x conversion rates of catalysts with different V₂O₅ loading concentrations generally increased with increasing temperature and within the temperature range of 200 °C–400 °C, the NO_x conversion rates of the five catalysts were nearly identical. Excessive loading of MoO₃ can decrease the denitrification performance of the catalyst, with an optimal loading amount observed at 5 wt%; the addition of WO₃ significantly enhances the low-temperature activity of the catalyst. When both WO₃ and MoO₃ are loaded at 3 wt%, the catalyst exhibits the best denitrification performance, achieving a conversion rate of 98.8% at 250 °C.

(2) Increasing the loading of MoO₃ to 10 wt% results in an increase in the catalyst’s specific surface area, but a deterioration in surface crystallinity and a decrease in the proportion of V⁴⁺ species on the surface. The reaction activity of surface V species, especially the reduced V⁴⁺, is significantly higher than that of Mo species. Therefore, the denitrification performance of the V₃Mo₁₀/TiO₂ catalyst decreases.

(3) The low-temperature activity of the catalyst was significantly enhanced by WO₃ modification. XRD and N₂ adsorption-desorption experiments indicated that there was almost no change in the physical structure and morphology of the catalyst before and after modification, suggesting that this was not the main reason for the increased activity. XPS and H₂-TPR results revealed that the proportion of chemisorbed oxygen (O_α) increased in the WO₃-modified catalyst, exhibiting lower V reduction temperatures, which are favorable for low-temperature denitrification activity. NH₃-TPD experiments showed that compared to MoO_x species, surface WO_x species could provide more acidic sites, resulting in stronger surface acidity of the catalyst.