Influence of Sulfur-Containing Sodium Salt Poisoned V₂O₅–WO₃/TiO₂ Catalysts on SO₂–SO₃ Conversion and NO Removal

Abstract

A series of poisoned catalysts with various forms and contents of sodium salts (Na₂SO₄ and Na₂S₂O₇) were prepared using the wet impregnation method. The influence of sodium salts poisoned catalysts on SO₂ oxidation and NO reduction was investigated. The chemical and physical features of the catalysts were characterized via NH₃-temperature programmed desorption (NH₃-TPD), H₂-temperature programmed reduction (H₂-TPR), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FT-IR). The results showed that sodium salts poisoned catalysts led to a decrease in the denitration efficiency. The 3.6% Na₂SO₄ poisoned catalyst was the most severely deactivated with denitration efficiency of only 50.97% at 350 °C. The introduction of SO₄²⁻ and S₂O₇²⁻ created new Brønsted acid sites, which facilitated the adsorption of NH₃ and NO reduction. The sodium salts poisoned catalysts significantly increased the conversion of SO₂–SO₃. 3.6%Na₂S₂O₇ poisoned catalyst had the strongest effect on SO₂ oxidation and the catalyst achieved a maximum SO₂–SO₃-conversion of 1.44% at 410 °C. Characterization results showed sodium salts poisoned catalysts consumed the active ingredient and lowered the V⁴⁺/V⁵⁺ ratio, which suppressed catalytic performance. However, they increased the content of chemically adsorbed oxygen and the strength of V⁵⁺=O bonds, which promoted SO₂ oxidation.

1. Introduction

Nitrogen oxides (NO_x) are recognized as a major air pollutant. They destroy the ozone layer, form acid rain, affect the ecological environment, and endanger human health. The main source of NO_x in China is thermal power plants [1,2]. NO_x are listed as a binding assessment indicator for total air pollution control. Consequently, selective catalytic reduction (SCR) flue gas denitration equipment is being used on a large scale in China’s thermal power plants. Catalysts are the heart of SCR flue gas denitration technology. The most extensively used commercial catalyst is V₂O₅–WO₃/TiO₂ [3]. Zhundong coal enriches a large amount of sodium (the total content is higher than 2%) because of the special coal-forming environment and the effect of groundwater [4]. The sodium in the coal is not completely stable in the furnace after burning [5]. The presence of large amounts of fly ash and alkali metals in the flue gas can cause catalyst clogging and poisoning. The former is generally reversible and belongs to physical function. The latter belongs to chemical action. In recent years, the toxic effect of alkali metals on the catalyst has been extensively investigated [6,7]. The mechanism of toxicity can be summarized as follows: (1) The presence of alkali metal causes V–O–H to be replaced by V–O–M and decreases the strength and number of Brønsted acid sites. This leads to the reduction of denitration efficiency [8]. (2) Alkali metal can weaken the intensity of V⁵⁺=O bonds, decreasing the oxidation ability of the catalysts. Moreover, alkali metals interact with the active ingredient on the catalyst surface. This causes the chemical valence of the elements and the concentration of the active ingredient to change [9,10].

Peng et al. [11,12] studied the mechanism of alkali metals poisoning catalysts. They concluded that after the alkali metals were added they would interact with the V species, causing a reduction in the surface acidity and inhibition of the adsorption of NH₃. This is thought to have resulted in the decreased activity of the catalysts. According to a series of alkali metal bromide poisoning results obtained by Chang et al. [13], the addition of alkali metal compounds decreased the intensity of the V=O bonds and the content of chemically adsorbed oxygen on the catalyst surface. Consequently, the redox ability of the vanadium-based catalysts was weakened. When considering catalysts poisoning, most researchers have focused on the toxic effects of different forms of alkali metals on the catalysts. However, the flue gas contains a large amount of SO₂. Therefore, it will react with gas phase NaCl to form substances such as Na₂SO₄ and Na₂S₂O₇ [14,15,16]. The reaction formulas are as follows:

$$2\mathrm{NaCl}+2{\mathrm{SO}}_2+{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_7+2\mathrm{HCl}$$

(1)

$$4\mathrm{NaCl}+2{\mathrm{SO}}_2+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{Na}}_2{\mathrm{SO}}_4+4\mathrm{HCl}$$

(2)

V₂O₅–WO₃/TiO₂ catalysts also have a catalytic effect on the conversion of SO₂ to SO₃. SO₂ conversion is also the main index in evaluating the denitration performance of an SCR catalyst. SO₃ will cause the corrosion of gas pipes. NH₃ will react with SO₃ to produce (NH₄)₂SO₄ and NH₄HSO₄. These aforementioned compounds can plug the air preheater and cause great harm. Li et al. [17] found that the presence of SO₂ decreased the catalytic activity of K poisoned catalysts because of the generation of K₂S₂O₇. They suggested that K₂S₂O₇ inhibited the adsorption of NH₃ and weakened the oxidation ability of the catalysts. However, some researchers concluded that the presence of pyrosulfates would maintain the high oxidizability of V species to a certain degree [18]. Tian et al. [19] studied the influence of different Na salts on the deactivation of SCR catalysts. They concluded that the V–OH bonds were replaced by V–O–Na, reducing the amount of Brønsted acid sites. However, the presence of SO₄²⁻ produced new Brønsted acid sites and promoted the adsorption of NH₃. Hu et al. [20] and Chen et al. [21] specifically studied the role of SO₄²⁻ in catalyst deactivation. Their research found the addition of SO₄²⁻ can create more acid sites on the catalyst surface. These acted as Brønsted acid sites and adsorbed more NH₃. Consequently, the performance of the catalysts was enhanced. Dahlin et al. [22] studied the toxic effects of K, Na, P, S, and other poisons on the catalyst. They concluded that Na and K had the greatest toxic effect on the catalyst. However, sulfates were formed to prevent the alkali metal from interacting with the active sites of the catalyst when Na and S existed simultaneously. Thus, the poisoning effect of the alkali metal decreased.

Most researchers focus on the denitration performance and deactivation of the catalysts [23,24]. The catalytic effect of sulfur-containing sodium salts poisoned catalysts on SO₂ oxidation and the effect of SO₂ on denitration efficiency have rarely been studied. To address this, a series of different concentrations of Na₂SO₄ and Na₂S₂O₇ poisoned catalysts were prepared via the wet impregnation method. The influence of sulfur-containing sodium salts poisoned catalysts on SO₂ oxidation and NO removal was investigated experimentally.

2. Results and Discussion

2.1. Effect of Different Catalysts on SO₃ Generation

2.1.1. Effect of Temperature on SO₃ Generation

SO₂–SO₃-conversion for different catalysts at different reaction temperatures are shown in Figure 1. The results indicate that the SO₂–SO₃-conversion of different catalysts increases gradually with increasing temperature. The SO₂–SO₃-conversion of the pure catalyst increases from 0.52% at 290 °C to 0.83% at 410 °C. The concentration of SO₃ increases from 9.33 ppm to 14.97 ppm. The most significant increase is for the 3.6% Na₂S₂O₇ poisoned catalyst. The SO₂–SO₃-conversion increases from 0.83% to 1.44% and the concentration of SO₃ increases from 15 ppm to 26 ppm. The overall variation of 3.6% Na₂SO₄ poisoned catalyst is lower than that of 1.2% Na₂S₂O₇.

The sulfur-containing sodium salts poisoned catalysts leads to an increase in the amount of V–O–S bonds. The presence of V–O–S bonds promotes SO₃ generation [25]. Zhang et al. [26] and Ma et al. [27] suggested that the addition of sodium salts (in the presence of SO₂ and O₂) caused the generation of VOSO₄. With an increase in temperature, VOSO₄ was reoxidized to SO₃ and V₂O₅. Hence, SO₂–SO₃-conversion increased. For Na₂S₂O₇ poisoned catalysts, Alvarez et al. [18] and Wang et al. [28] considered that the addition of Na₂S₂O₇ inhibited the formation of VOSO₄. This would keep the V species in 5+ oxidation state below 400 °C, and ensure the catalytic effect. Meanwhile, the acid strength of the catalyst surface can be enhanced by the induction of the S=O group. The pyrosulfate substance provided stronger acidic sites than the sulfate substance. This resulted in an obvious increase in the SO₂–SO₃-conversion of the pyrosulfates poisoned catalysts.

2.1.2. Effect of SO₂ on SO₃ Generation

Figure 2 shows the effect of SO₂ concentration on both SO₂–SO₃-conversion and SO₃ concentration for different catalysts. It shows that the SO₃ concentration generated with all catalysts used increases gradually. However, it does not increase linearly and a turning point can be identified. SO₂–SO₃-conversion decreases with increasing SO₂ concentration. When the SO₂ concentration is 3000 ppm, the SO₃ generation concentration for 3.6%-Na₂S₂O₇-SCR increases to 23 ppm. However, the SO₂–SO₃-conversion is only 0.77%. Therefore, the concentration of SO₃ should also be considered. In high concentration SO₂ flue gas, the diffusion rate of SO₂ is much larger than the reaction rate. Consequently, the main factors in determining the SO₂–SO₃-conversion are the SO₂ oxidation process and the active sites of the catalysts. According to the Le Chatelier’s principle, the concentration of the reactant increases. This results in a shift of the equilibrium to the positive reaction direction in the reversible reaction. Hence, the increased reactant further reduces. But the reactant cannot be completely converted. Consequently, the increase of SO₂ is greater than the reacted amount of SO₂. This results in the decrease of SO₂–SO₃-conversion. However, increasing SO₂ can also inhibit the decomposition of SO₃ which can result in the growth of SO₃ concentration [29]. The addition of Na₂S₂O₇ causes less reduction in the catalyst activity than addition of Na₂SO₄. Hence, the SO₃ concentration of the Na₂S₂O₇ poisoned catalyst is higher than Na₂SO₄ poisoned catalyst at the same temperature. As far as the catalyst is concerned, the limited active sites on the catalyst surface restrict the adsorption of SO₂. This results in low SO₂–SO₃-conversion at high SO₂ concentrations [30].

2.2. Performance of Different Catalysts

2.2.1. Effect of Temperature on NO Conversion

The denitration efficiency curves for different catalysts at different temperatures are shown in Figure 3. The pure catalyst has a wide range of activity temperature and exhibits favorable denitration performance. The efficiency remains above 95% over the entire temperature range considered and reaches 99.33% at 350 °C. The addition of sodium salts results in the deactivation of catalysts. The activity sequence is as follows: SCR > 1.2%-Na₂S₂O₇-SCR > 1.2%-Na₂SO₄-SCR > 3.6%-Na₂S₂O₇-SCR > 3.6%-Na₂SO₄-SCR. The denitration efficiency decreases with the increase of sodium salt loading. It initially increases but then decreases with increasing temperature. For 1.2%-Na₂SO₄-SCR and 1.2%-Na₂S₂O₇-SCR, the denitration efficiency decreases slightly but remains above 80% (reaching 87.86% and 91.49% at 350 °C, respectively). When the loading increases to 3.6%, the denitration efficiencies of Na₂S₂O₇ and Na₂SO₄ poisoned catalysts drop to 75.57% and 50.97% at 350 °C, respectively. Overall, the degree of poisoning of Na₂SO₄ is higher than Na₂S₂O₇ under equal loading. Therefore, the degree of poisoning of catalysts is also related to the form of sodium salts used. For instance, S₂O₇²⁻ can provide more Brønsted acid sites and has stronger oxidizability than SO₄²⁻. This results in more NH₃ being adsorbed which promotes NO reduction [31].

2.2.2. Effect of SO₂ on NO Conversion

The influence of SO₂ concentration on the denitration efficiency of different catalysts is shown in Figure 4. The denitration efficiency of pure catalysts remains approximately constant after initially decreasing from 99.33% to 88%. The 3.6% Na₂SO₄ and Na₂S₂O₇ poisoned catalysts initially increase before decreasing and then plateauing. The denitration efficiencies (when SO₂ concentration is 1800 ppm) of catalysts poisoned with 3.6% Na₂SO₄ and 3.6% Na₂S₂O₇ reach maximum values of 57.03% and 82.95%, respectively. Wu et al. [32] examined pure catalysts after the introduction of SO₂ and found that a large amount of Lewis acid sites on the catalyst surface were covered by ammonium sulfate. This weakened the adsorption of NH₃ and NO by the catalyst. Anstrom et al. [33] and Zhu et al. [34] suggested that due to the addition of SO₂, NO, and SO₂ competed for adsorption on the catalyst surface. This would explain why the adsorption amount of NO was reduced and the denitration efficiency was lowered. For catalysts poisoned with Na salts, Hu et al. [20] concluded that the anions from the sodium salt provided more acidic sites and V–O–S bonds. This was found to promote the catalytic oxidation of SO₂ and enhance the adsorption capacity of NH₃. Consequently, the denitration efficiency improved. The amount of acid sites on the catalyst surface began to decrease when the concentration of SO₂ in the flue gas was increased to a certain extent. The adsorption of SO₂ on V₂O₅ resulted in the formation of an intermediate structure, namely VOSO₄. This then reduced the V⁵⁺ concentration for the SCR reaction, which caused the denitration efficiency to decrease.

2.3. Catalyst Characterization

2.3.1. NH₃-TPD and H₂-TPR Analysis

The adsorption capacity of NH₃ on the acid sites indicates the activity of the catalyst. Therefore, the NH₃-TPD experiment was conducted. The spectrum obtained can indicate the strength and acidity of the acid center. The larger the area of the desorption peak, the higher the corresponding acid concentration. The higher the peak temperature is, the greater the corresponding acid strength will be. Figure 5 shows the NH₃-TPD patterns of different catalysts. It is generally believed that the desorption peak below 200 °C corresponds to the desorption of physisorbed NH₃, the desorption peak in the range of 200 to 350 °C corresponds to the weakly chemisorbed NH₃, and the desorption peak between 350 and 500 °C corresponds to the strongly chemisorbed NH₃ [35,36]. For 1.2% and 3.6% Na₂SO₄ poisoned catalysts, NH₃ adsorption decreased most noticeably. For 1.2% and 3.6% Na₂S₂O₇ poisoned catalysts, the amount of physisorbed NH₃ reduces between 100 and 200 °C. In contrast, the amount of strongly chemisorbed NH₃ significantly increased between 350 to 500 °C. According to Zheng et al. [37], this increase can be attributed to the catalyst surface being sulfated through the addition of Na₂S₂O₇. The traces of these compounds remaining on the surface of catalysts can provide strong acid sites, which is beneficial to the adsorption of NH₃. Chang et al. [13] have shown that Na preferentially coordinated on V–OH bonds and V–O–H was replaced by V–O–Na after addition of alkali metals. This resulted in a reduced number of acid sites and lower catalytic activity. The reaction formulas are as follows:

$$–\mathrm{V}–\mathrm{OH}+{\mathrm{Na}}_2{\mathrm{SO}}_4\to –\mathrm{V}–\mathrm{O}–\mathrm{Na}+\mathrm{NaHS}{\mathrm{O}}_4$$

(3)

$$–\mathrm{V}–\mathrm{OH}+\mathrm{NaHS}{\mathrm{O}}_4\to –\mathrm{V}–\mathrm{O}–\mathrm{Na}+{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$$

(4)

$$–\mathrm{V}–\mathrm{OH}+{\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_7\to –\mathrm{V}–\mathrm{O}–\mathrm{Na}+\mathrm{NaH}{\mathrm{S}}_2{\mathrm{O}}_7$$

(5)

$$–\mathrm{V}–\mathrm{OH}+\mathrm{NaH}{\mathrm{S}}_2{\mathrm{O}}_7\to –\mathrm{V}–\mathrm{O}–\mathrm{Na}+{\mathrm{H}}_2{\mathrm{S}}_2{\mathrm{O}}_7$$

(6)

Figure 6 shows the NH₃-TPD curves of different catalysts when the SO₂ concentration is 0 and 1800 ppm. The desorption of the physisorbed and weakly chemisorbed NH₃ on the poisoned catalyst surfaces increased significantly after the introduction of SO₂. This improved the denitration performance of the catalyst to some extent. Giakoumelou et al. [38] noted that the introduction of SO₂ led to the formation of more SO₄²⁻ on the poisoned catalyst surface. SO₄²⁻ can adsorb more NH₃ in the form of NH₄⁺ on the catalyst surface to react with NO in the flue gas. For the pure catalyst, the desorption of physisorbed NH₃ increased slightly and the peak area (ranging from 250 °C to 450 °C) clearly decreased. This was unfavorable to denitration performance. After the introduction of SO₂, the denitration efficiency of the pure catalyst is mainly attributed to the competitive adsorption between SO₂ and NO [39]. This agrees with the experimental results.

The redox performance of the catalyst accounts for another important factor in catalytic reduction. The lower the temperature corresponding to the reduction peak is, the easier the catalytic redox reaction of the catalyst will be. Peak area represents H₂ consumption. To test the impact of sulfur-containing sodium salts poisoned catalysts on the redox performance, the H₂-TPR was carried out and the experimental results are shown in Figure 7. The reduction peak temperature of the pure catalysts appeared at 556 °C, which can be attributed to the reduction of V⁵⁺ to V³⁺. After the addition of sodium salts, the reduction peak temperature of the catalysts increased. This indicates that sodium salts poisoned catalysts decreased the oxidizability. Chen et al. [40] suggested that the interaction of sodium salts with V species hindered the release of lattice oxygen in the catalyst, making V species more difficult to be reduced. The temperature shifted towards higher values with the increased loading. However, the reduction peak temperatures of the Na₂SO₄ and Na₂S₂O₇ poisoned catalysts with the same loading were very similar. The H₂ consumptions were 0.251, 0.897, 1.614, 2.690, and 4.698 mmol/g for pure catalyst, 1.2% Na₂SO₄, 1.2% Na₂S₂O₇, 3.6% Na₂SO₄, and 3.6% Na₂S₂O₇ poisoned catalyst, respectively. The H₂ consumption significantly increases for sodium salts poisoned catalysts. In particular, the Na₂S₂O₇ poisoned catalyst is 1.8 times higher than Na₂SO₄ poisoned catalyst with the same loading. This is because S₂O₇²⁻ has stronger oxidizability than SO₄²⁻.

Figure 8 shows the H₂-TPR curves of 3.6% Na₂SO₄ and Na₂S₂O₇ poisoned catalysts when the SO₂ concentration is 0 and 1800 ppm. The reduction peak temperature shifted towards the higher end after addition of SO₂, which decreased the denitration performance of the catalyst. However, the H₂ consumption also increased to 3.120 and 5.129 mmol/g, respectively. Yu et al. [41] considered that the introduction of SO₂ made the surface of the poisoned catalyst sulfated, increasing the catalytic performance of the poisoned catalyst. However, this is not the only influence factor for catalytic performance based on catalytic performance results.

2.3.2. XPS Analysis

The chemical species and surface atomic concentration of several major elements were analyzed using X-ray photoelectron spectroscopy (XPS).

Table 1. The surface atomic concentrations for these samples.

Samples	Surface Atomic Concentration (%)
	O	Na	Ti	V	W	S
Pure catalyst	65.04	0.07	16.27	0.60	3.50	0.05
1.2%-Na2SO4-SCR	60.24	4.42	14.32	0.40	2.61	1.13
3.6%-Na2SO4-SCR	59.88	5.58	13.01	0.35	2.14	3.54
1.2%-Na2S2O7-SCR	61.93	4.17	15.19	0.44	2.77	2.32
3.6%-Na2S2O7-SCR	61.08	5.25	14.77	0.38	2.69	5.12

shows the atomic concentrations on the catalyst surface. Figure 9 shows the spectra of O_1S and V_2P, respectively.

Table 2. The states of O and V on the catalyst surfaces.

Samples	Surface Atomic Concentration (%)				Surface Atomic Ratio
	Oα/(Oα+Oβ)	Oβ/(Oα+Oβ)	V4+	V5+	V4+/V5+
Pure catalyst	44.3	55.7	43.6	56.4	0.77
1.2%-Na2SO4-SCR	51.0	49.0	41.4	58.6	0.71
3.6%-Na2SO4-SCR	53.1	46.9	40.2	59.8	0.67
1.2%-Na2S2O7-SCR	51.8	48.2	40.6	59.4	0.69
3.6%-Na2S2O7-SCR	55.3	44.7	39.8	60.2	0.66

shows the states of O and V on these catalyst surfaces. Table 1 shows that with increasing loading of sodium salts, the atomic concentration of the active component on the catalyst surface significantly decreased. This result is consistent with the XRD measurement, indicating that sodium salts interacted with the active component of the catalyst.

As shown in Figure 9a, the spectra of O_1S were divided into two peaks. The peak corresponding to 531.0–531.6 eV was attributed to surface chemical adsorption oxygen (defined as O_α). The peak corresponding to 528.4–528.7 eV was attributed to lattice oxygen (defined as O_β) [42,43]. The concentration ratio of chemical adsorption oxygen (O_α/(O_α+O_β)) was computed using peaking software, which is shown in Table 2. The sodium salts poisoned catalysts increased chemical adsorption oxygen content on the catalyst surface. However, for 1.2% and 3.6% Na₂SO₄ poisoned catalyst the surface oxygen concentration was lower than for other catalysts (see Table 1). Therefore, the chemical adsorption oxygen content of the Na₂SO₄ poisoned catalyst was low. The chemical adsorption oxygen is very active and is indispensable for oxidation reactions. There is a positive correlation between the oxidative properties of the catalyst and the surface chemically adsorbed oxygen content [44]. Therefore, the relatively high chemically adsorbed oxygen content in the sodium salt poisoned catalysts are beneficial for SO₂ oxidation.

Figure 9b shows the spectra of V_2P. The spectra were also divided into two peaks. The peak corresponding to 515.3–516 eV belonged to V⁵⁺. The peak corresponding to 514.0–514.6 eV belonged to V⁴⁺ [45]. Table 2 shows that the V⁴⁺ ratio was 43.6% for pure catalyst. It also shows the V⁴⁺ content of other sodium salt poisoned catalysts were reduced and the corresponding V⁴⁺/V⁵⁺ ratios decreased. Economidis et al. [46] confirmed experimentally that in the process of synthesizing the vanadium-based catalyst, V⁵⁺ partially transformed into V⁴⁺. Equivalently, the reduction of VO₂⁺ to VO²⁺ occurred. The ratio of V⁴⁺/V⁵⁺ plays a pivotal role in the redox reaction of the catalyst. The denitration efficiency could be improved by appropriately increasing the ratio. However, the addition of sodium salts decreased the ratio (causing the decrease in denitration performance). This is consistent with the denitration performance test.

2.3.3. BET and XRD Analysis

The structural characteristics of different catalysts were determined by N₂ adsorption-desorption experiments. The specific surface area and pore structure also affected the denitration performance of the catalyst to an extent.

Table 3. Structural characteristics of different Na salts loadings.

Samples	SBET (m2·g−1)	Vtotal (cm3·g−1)	Dp (nm)
Pure catalyst	95	0.21	9.3
1.2%-Na2SO4-SCR	75	0.18	10.5
1.2%-Na2S2O7-SCR	69	0.17	10.5
3.6%-Na2SO4-SCR	63	0.16	11.3
3.6%-Na2S2O7-SCR	44	0.12	11.1

describes the specific surface area and pore structure characteristics of different catalysts. For example, the specific surface area, pore volume and pore diameter of the pure catalyst were found to 95 m²·g⁻¹, 0.21 cm³·g⁻¹, and 9.3 nm, respectively. After loading with sodium salts, the specific surface area and pore volume of the catalyst significantly reduced. With increased loading, the degree of reduction increased. Na₂S₂O₇ poisoned catalyst has a greater influence on the specific surface area and pore volume. Xiao et al. [47] suggested that sodium pyrosulfate had larger molecular size, which led to the catalyst blockage. Conversely, the pore size increased. This is because the addition of sodium salts caused the microporous blockage and the proportion of macropores consequently increased.

Figure 10 shows the Barret–Joyner–Halenda (BJH) pore size curves of different catalysts. The catalyst surface mainly contains mesopores (2–50 nm), as shown in Figure 10. The sodium salts poisoned catalyst increased the pore size compared with the pure catalyst.

Figure 11 exhibits the XRD diffractograms of different catalysts. The XRD patterns of all catalyst samples are mostly similar, which demonstrates that the addition of sodium salts did not change the basic structure of the support. However, for the 1.2% and 3.6% Na₂SO₄ poisoned catalyst, the diffraction peak of Na₂SO₄ could be detected. For the pure catalyst and 1.2% Na₂S₂O₇ poisoned catalyst, there was no peak of V₂O₅ and no appearance of Na₂S₂O₇. But for the 3.6% Na₂S₂O₇ poisoned catalyst, the diffraction peak of Na₄V₃O₉ could be detected. This was because some molten sodium pyrosulfate interacted with V₂O₅ during the preparation process. The appearance of the new peaks indicates that the accumulation of sodium salts on the catalyst surface blocked the catalyst pores and reduced the specific surface area. This was not conducive to the catalytic reaction. However, Shpanchenko et al. [48] considered that the new vanadium oxide complex Na₄V₃O₉ had special structural and magnetic properties. It was made from isolated chains of square V⁴⁺O₅ pyramids linked by two bridging V⁵⁺O₄ tetrahedra. This structure had strong magnetic exchange between the V⁴⁺ along the chain, which was beneficial to catalytic performance.

2.3.4. FT-IR Analysis

The characteristic peaks of different functional groups on different catalyst surfaces were obtained via FT-IR measurements. The results are shown in Figure 12. These catalysts exhibit two key peaks located at 1015 cm⁻¹ and 1627 cm⁻¹. The peak at 1015 cm⁻¹ is attributed to the terminal vanadium oxy group (V⁵⁺=O) [49]. The peak at 1627 cm⁻¹ is attributed to the Lewis acid sites [50]. According to the experimental results, the sodium salts poisoned catalyst was found to have reduced the Lewis acid sites. With increased loading, the degree of reduction increased. This indicates that the weak acidic strength of the catalyst surface was lowered. This is consistent with the results of NH₃-TPD. The peak intensity at 1015 cm⁻¹ increased and Na₂S₂O₇ poisoned catalyst had the most significant effect. The increase of V⁵⁺=O bond strength is beneficial for promoting the conversion of SO₂ to SO₃ [51]. Alvarez et al. [18] suggested that the presence of pyrosulfates would maintain the high oxidizability of V species to a certain degree. This result is consistent with the results of SO₂ oxidation experiments.

3. Experimental

3.1. Sample Preparation

The commercial SCR catalysts were obtained from Beijing Nation Power Group Co., Ltd., Beijing, China. The poisoned catalysts were prepared using the wet impregnation method. A certain amount of sodium sulfate or sodium pyrosulfate was weighed according to the mass percentage of Na in the active ingredient of catalysts. After being formulated into solution, they were mixed with catalysts and ultrasonically shaken for 4 h. They were then dried in the blast drying oven at 110 °C for 12 h. Finally, the catalysts were calcined in the muffle furnace at 350 °C for 5 h and ground to 40–60 mesh to obtain x-Na₂SO₄/Na₂S₂O₇-SCR poisoned catalysts. Here, x means the mass percentage of sodium element (1.2% or 3.6%).

3.2. Catalyst Characterization

The experiment was performed using the model tp-5080 temperature programmed adsorption instrument (manufactured by Tianjin Xianquan Company, Tianjin, China) for NH₃-TPD and H₂-TPR of different catalysts. For a NH₃-TPD test, 0.1 g samples were prepared and then pretreated for 1 h at 250 °C. Next, they were cooled to ambient temperature in a pure N₂ atmosphere (30 mL/min). Then 10%NH₃ (N₂ as balance gas) was passed over the samples for 1 h. After NH₃ was cut off, the catalysts were warmed to 60 °C and purged in pure N₂ for 20 min. Finally, they were heated to 800 °C (at a rate of 10 °C/min) and maintained at 800 °C for 5 min. The consumption of NH₃ was recorded. For a H₂-TPR test, 0.05 g samples were prepared and then pretreated for 1 h at 250 °C. Next, they were cooled to ambient temperature in pure N₂ atmosphere (30 mL/min). Then 5%H₂ (N₂ as balance gas) was passed over the samples as a reducing agent. Finally, the catalysts were heated to 800 °C (at a rate of 10 °C/min) and maintained at 800 °C for 5 min. The consumption of H₂ was recorded.

The pore structural parameters of the samples were determined via the specific surface area and pore size analyzer (ASAP 2010, Micromeritics Instrument Corporation, Norcross, GA, USA). The specific surface area of the catalyst was obtained by linear regression via the Brunauer–Emmett–Teller (BET) equation. The pore size was calculated using the BJH model.

The XRD patterns of samples were obtained using the Bruker D8 (Bruker AXS Company, Karlsruhe, Germany) Advance to determine the crystallinity and dispersion of the surface material of the samples. The measurement was performed using a Cu Kα irradiation source with a scan range of 10–80°. XPS was performed using the AXIS ULTRADLD (Kratos Company, Shimadzu, Kyoto, Japan). An X-ray source was used as a monochromatic Al target and C1s (284.8 eV) was used for correction when fitting the peak. FT-IR spectra were recorded in a Nicolet Nexus 670 FT-IR (Nicolet Company, Madison, WI, USA). The catalysts were ground and then blended with KBr powder at the mass ratio of 1:100 with a resolution of 4 cm⁻¹. The recorded spectral range was 600–4000 cm⁻¹, and the number of scans was 32.

3.3. Test. Setup

Catalytic performance and SO₂–SO₃ conversion test apparatus is presented in Figure 13. Each catalyst (2 mL, 40–60 mesh) was laid in a quartz tube reactor. The simulated flue gas used in the experiment included 500 ppm NH₃, 500 ppm NO, 5% O₂, 2% H₂O, and the SO₂ concentration ranged from 0 to 3000 ppm (N₂ acted as a balance gas). The total gas flow rate was 1.5 L/min, resulting in a GHSV of 45,000 h⁻¹. The experimental test temperature was 250–410 °C. The inlet and outlet flue gas (O₂, SO₂, NO) concentrations were monitored using the Testo 350 flue gas analyzer (Testo AG, Lenzkirch, Germany). SO₃ was gathered by the Graham Condenser, which was placed in a constant temperature 80 °C water bath. The gathered SO₃ was converted to SO₄²⁻ using an 80% isopropanol solution. Then the SO₄²⁻ content in the solution was measured using an ion chromatograph to determine the SO₃ content, which was averaged over multiple measurements.

The catalytic performance of the catalyst is represented by the conversion of NO, which is defined as:

$${\mathsf{\eta }}_{\mathrm{NO}}(\%)=\left(\frac{{\left[\mathrm{NO}\right]}_{\mathrm{inlet}}-{\left[\mathrm{NO}\right]}_{\mathrm{outlet}}}{{\left[\mathrm{NO}\right]}_{\mathrm{inlet}}}\right)\times 100\%$$

(7)

The SO₂–SO₃ conversion is indirectly expressed by the SO₂ oxidation, which is defined as:

$${\mathrm{SO}}_2–{\mathrm{SO}}_3-\mathrm{conversation}(\%)=\left(\frac{{\left[{\mathrm{SO}}_3\right]}_{\mathrm{outlet}}}{{\left[{\mathrm{SO}}_2\right]}_{\mathrm{inlet}}}\right)\times 100\%$$

(8)

4. Conclusions

The influence of different sulfur-containing sodium salts (Na₂SO₄ and Na₂S₂O₇) poisoned catalysts on SO₂ oxidation and NO reduction was investigated. Sodium salts poisoned catalysts led to a decrease in the denitration efficiency, while significantly improving the SO₂–SO₃-conversion. The degree of change is related to the loading and form of the sodium salts poisoned catalysts. The degree of poisoning of Na₂S₂O₇ poisoned catalyst was weaker because S₂O₇²⁻ can create more Brønsted acid sites and has stronger oxidizability than SO₄²⁻. The introduction of SO₂ clearly increased the number of surface acid sites. However, it had little effect on the redox capacity of the catalyst. Hence, SO₂ slightly enhanced the denitration efficiency of the sodium salts poisoned catalysts. According to analysis of NH₃-TPD, FT-IR, and H₂-TPR results, sodium salts poisoned catalysts reduced the Lewis acid sites and redox capacity. They also increased Brønsted acid sites and V⁵⁺=O bonds strength. The presence of the V⁵⁺=O bonds facilitates SO₂–SO₃ conversion. XPS results showed that sodium salts poisoned catalysts increased the chemically adsorbed oxygen content and promoted SO₂ oxidation. However, sodium salts poisoned catalysts reduced the V⁴⁺/V⁵⁺ ratio and inhibited denitration performance.