Influence of Sulfur-Containing Sodium Salt Poisoned V 2 O 5 – WO 3 / TiO 2 Catalysts on SO 2 – SO 3 Conversion and NO Removal

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 SO4 and S2O7 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.


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 V 2 O 5 -WO 3 /TiO 2 [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 V 5+ =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 NH 3 .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 SO 2 .Therefore, it will react with gas phase NaCl to form substances such as Na 2 SO 4 and Na 2 S 2 O 7 [14][15][16].The reaction formulas are as follows: 4NaCl V 2 O 5 -WO 3 /TiO 2 catalysts also have a catalytic effect on the conversion of SO 2 to SO 3 .SO 2 conversion is also the main index in evaluating the denitration performance of an SCR catalyst.SO 3 will cause the corrosion of gas pipes.NH 3 will react with SO 3 to produce (NH 4 ) 2 SO 4 and NH 4 HSO 4 .These aforementioned compounds can plug the air preheater and cause great harm.Li et al. [17] found that the presence of SO 2 decreased the catalytic activity of K poisoned catalysts because of the generation of K 2 S 2 O 7 .They suggested that K 2 S 2 O 7 inhibited the adsorption of NH 3 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 SO 4 2− produced new Brønsted acid sites and promoted the adsorption of NH 3 .Hu et al. [20] and Chen et al. [21] specifically studied the role of SO 4 2− in catalyst deactivation.Their research found the addition of SO 4 2− can create more acid sites on the catalyst surface.These acted as Brønsted acid sites and adsorbed more NH 3 .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 SO 2 oxidation and the effect of SO 2 on denitration efficiency have rarely been studied.To address this, a series of different concentrations of Na 2 SO 4 and Na 2 S 2 O 7 poisoned catalysts were prepared via the wet impregnation method.The influence of sulfur-containing sodium salts poisoned catalysts on SO 2 oxidation and NO removal was investigated experimentally.The results indicate that the SO 2 -SO 3 -conversion of different catalysts increases gradually with increasing temperature.The SO 2 -SO 3 -conversion of the pure catalyst increases from 0.52% at 290 • C to 0.83% at 410 • C. The concentration of SO 3 increases from 9.33 ppm to 14.97 ppm.The most significant increase is for the 3.6% Na 2 S 2 O 7 poisoned catalyst.The SO 2 -SO 3 -conversion increases from 0.83% to 1.44% and the concentration of SO 3 increases from 15 ppm to 26 ppm.The overall variation of 3.6% Na 2 SO 4 poisoned catalyst is lower than that of 1.2% Na 2 S 2 O 7 .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.

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].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 SO 3 generation [25].Zhang et al. [26] and Ma et al. [27] suggested that the addition of sodium salts (in the presence of SO 2 and O 2 ) caused the generation of VOSO 4 .With an increase in temperature, VOSO 4 was reoxidized to SO 3 and V 2 O 5 .Hence, SO 2 -SO 3 -conversion increased.For Na 2 S 2 O 7 poisoned catalysts, Alvarez et al. [18] and Wang et al. [28] considered that the addition of Na 2 S 2 O 7 inhibited the formation of VOSO 4 .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 SO 2 -SO 3 -conversion of the pyrosulfates poisoned catalysts.

Effect of SO 2 on SO 3 Generation
Figure 2 shows the effect of SO 2 concentration on both SO 2 -SO 3 -conversion and SO 3 concentration for different catalysts.It shows that the SO 3 concentration generated with all catalysts used increases gradually.However, it does not increase linearly and a turning point can be identified.SO 2 -SO 3 -conversion decreases with increasing SO 2 concentration.When the SO 2 concentration is 3000 ppm, the SO 3 generation concentration for 3.6%-Na 2 S 2 O 7 -SCR increases to 23 ppm.However, the SO 2 -SO 3 -conversion is only 0.77%.Therefore, the concentration of SO 3 should also be considered.In high concentration SO 2 flue gas, the diffusion rate of SO 2 is much larger than the reaction rate.Consequently, the main factors in determining the SO 2 -SO 3 -conversion are the SO 2 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 SO 2 is greater than the reacted amount of SO 2 .This results in the decrease of SO 2 -SO 3 -conversion.However, increasing SO 2 can also inhibit the decomposition of SO 3 which can result in the growth of SO 3 concentration [29].The addition of Na 2 S 2 O 7 causes less reduction in the catalyst activity than addition of Na 2 SO 4 .Hence, the SO 3 concentration of the Na 2 S 2 O 7 poisoned catalyst is higher than Na 2 SO 4 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 SO 2 .This results in low SO 2 -SO 3 -conversion at high SO 2 concentrations [30].

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, S2O7 2− can provide more Brønsted acid sites and has stronger oxidizability than SO4 2− .This results in more NH3 being adsorbed which promotes NO reduction [31].

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%-Na 2 S 2 O 7 -SCR > 1.2%-Na 2 SO 4 -SCR > 3.6%-Na 2 S 2 O 7 -SCR > 3.6%-Na 2 SO 4 -SCR.The denitration efficiency decreases with the increase of sodium salt loading.It initially increases but then decreases with increasing temperature.For 1.2%-Na 2 SO 4 -SCR and 1.2%-Na 2 S 2 O 7 -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 Na 2 S 2 O 7 and Na 2 SO 4 poisoned catalysts drop to 75.57% and 50.97% at 350 • C, respectively.Overall, the degree of poisoning of Na 2 SO 4 is higher than Na 2 S 2 O 7 under equal loading.Therefore, the degree of poisoning of catalysts is also related to the form of sodium salts used.For instance, S 2 O 7 2− can provide more Brønsted acid sites and has stronger oxidizability than SO 4 2− .
This results in more NH 3 being adsorbed which promotes NO reduction [31].

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, S2O7 2− can provide more Brønsted acid sites and has stronger oxidizability than SO4 2− .This results in more NH3 being adsorbed which promotes NO reduction [31].

Effect of SO 2 on NO Conversion
The influence of SO 2 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% Na 2 SO 4 and Na 2 S 2 O 7 poisoned catalysts initially increase before decreasing and then plateauing.The denitration efficiencies (when SO 2 concentration is 1800 ppm) of catalysts poisoned with 3.6% Na 2 SO 4 and 3.6% Na 2 S 2 O 7 reach maximum values of 57.03% and 82.95%, respectively.Wu et al. [32] examined pure catalysts after the introduction of SO 2 and found that a large amount of Lewis acid sites on the catalyst surface were covered by ammonium sulfate.This weakened the adsorption of NH 3 and NO by the catalyst.Anstrom et al. [33] and Zhu et al. [34] suggested that due to the addition of SO 2 , NO, and SO 2 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 SO 2 and enhance the adsorption capacity of NH 3 .Consequently, the denitration efficiency improved.The amount of acid sites on the catalyst surface began to decrease when the concentration of SO 2 in the flue gas was increased to a certain extent.The adsorption of SO 2 on V 2 O 5 resulted in the formation of an intermediate structure, namely VOSO 4 .This then reduced the V 5+ concentration for the SCR reaction, which caused the denitration efficiency to decrease.

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 V 5+ concentration for the SCR reaction, which caused the denitration efficiency to decrease.

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

NH 3 -TPD and H 2 -TPR Analysis
The adsorption capacity of NH 3 on the acid sites indicates the activity of the catalyst.Therefore, the NH 3 -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 NH 3 -TPD patterns of different catalysts.It is generally believed that the desorption peak below 200 • C corresponds to the desorption of physisorbed NH 3 , the desorption peak in the range of 200 to 350 • C corresponds to the weakly chemisorbed NH 3 , and the desorption peak between 350 and 500 • C corresponds to the strongly chemisorbed NH 3 [35,36].For 1.2% and 3.6% Na 2 SO 4 poisoned catalysts, NH 3 adsorption decreased most noticeably.For 1.2% and 3.6% Na 2 S 2 O 7 poisoned catalysts, the amount of physisorbed NH 3 reduces between 100 and 200 • C. In contrast, the amount of strongly chemisorbed NH 3 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 Na 2 S 2 O 7 .The traces of these compounds remaining on the surface of catalysts can provide strong acid sites, which is beneficial to the adsorption of NH 3 .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: 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: 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 SO4 2− on the poisoned catalyst surface.SO4 2− 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.Figure 6 shows the NH 3 -TPD curves of different catalysts when the SO 2 concentration is 0 and 1800 ppm.The desorption of the physisorbed and weakly chemisorbed NH 3 on the poisoned catalyst surfaces increased significantly after the introduction of SO 2 .This improved the denitration performance of the catalyst to some extent.Giakoumelou et al. [38] noted that the introduction of SO 2 led to the formation of more SO 4 2− on the poisoned catalyst surface.SO 4 2− can adsorb more NH 3 in the form of NH 4 + on the catalyst surface to react with NO in the flue gas.For the pure catalyst, the desorption of physisorbed NH 3 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 SO 2 , the denitration efficiency of the pure catalyst is mainly attributed to the competitive adsorption between SO 2 and NO [39].This agrees with the experimental results.
Catalysts 2018, 8, x FOR PEER REVIEW 6 of 16 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: 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 SO4 2− on the poisoned catalyst surface.SO4 2− 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 H 2 consumption.To test the impact of sulfur-containing sodium salts poisoned catalysts on the redox performance, the H 2 -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 V 5+ to V 3+ .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 Na 2 SO 4 and Na 2 S 2 O 7 poisoned catalysts with the same loading were very similar.The H 2 consumptions were 0.251, 0.897, 1.614, 2.690, and 4.698 mmol/g for pure catalyst, 1.2% Na 2 SO 4 , 1.2% Na 2 S 2 O 7 , 3.6% Na 2 SO 4 , and 3.6% Na 2 S 2 O 7 poisoned catalyst, respectively.The H 2 consumption significantly increases for sodium salts poisoned catalysts.In particular, the Na 2 S 2 O 7 poisoned catalyst is 1.8 times higher than Na 2 SO 4 poisoned catalyst with the same loading.This is because S 2 O 7 2− has stronger oxidizability than SO 4 2− .
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 V 5+ to V 3+ .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 S2O7 2− has stronger oxidizability than SO4 2− .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.Figure 8 shows the H 2 -TPR curves of 3.6% Na 2 SO 4 and Na 2 S 2 O 7 poisoned catalysts when the SO 2 concentration is 0 and 1800 ppm.The reduction peak temperature shifted towards the higher end after addition of SO 2 , which decreased the denitration performance of the catalyst.However, the H 2 consumption also increased to 3.120 and 5.129 mmol/g, respectively.Yu et al. [41] considered that the introduction of SO 2 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.

XPS Analysis
The chemical species and surface atomic concentration of several major elements were analyzed using X-ray photoelectron spectroscopy (XPS).Table 1 shows the atomic concentrations on the catalyst surface.Figure 9 shows the spectra of O1S and V2P, respectively.Table 2 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.Intensity (a.u.) Binding Energy (eV)

XPS Analysis
The chemical species and surface atomic concentration of several major elements were analyzed using X-ray photoelectron spectroscopy (XPS).Table 1 shows the atomic concentrations on the catalyst surface.Figure 9 shows the spectra of O 1S and V 2P , respectively.Table 2 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.

XPS Analysis
The chemical species and surface atomic concentration of several major elements were analyzed using X-ray photoelectron spectroscopy (XPS).Table 1 shows the atomic concentrations on the catalyst surface.Figure 9 shows the spectra of O1S and V2P, respectively.Table 2 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.Intensity (a.u.) Binding Energy (eV)   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% Na 2 SO 4 poisoned catalyst the surface oxygen concentration was lower than for other catalysts (see Table 1).Therefore, the chemical adsorption oxygen content of the Na 2 SO 4 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 SO 2 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 V 5+ .The peak corresponding to 514.0-514.6 eV belonged to V 4+ [45].Table 2 shows that the V 4+ ratio was 43.6% for pure catalyst.It also shows the V 4+ content of other sodium salt poisoned catalysts were reduced and the corresponding V 4+ /V 5+ ratios decreased.Economidis et al. [46] confirmed experimentally that in the process of synthesizing the vanadium-based catalyst, V 5+ partially transformed into V 4+ .Equivalently, the reduction of VO 2 + to VO 2+ occurred.
The ratio of V 4+ /V 5+ 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.

BET and XRD Analysis
The structural characteristics of different catalysts were determined by N 2 adsorption-desorption experiments.The specific surface area and pore structure also affected the denitration performance of the catalyst to an extent.Table 3 describes the specific surface area and pore characteristics of different catalysts.For example, the specific surface area, pore volume and pore diameter of the pure catalyst were found to 95 m 2 •g −1 , 0.21 cm 3 •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.Na 2 S 2 O 7 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 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 V 4+ O5 pyramids linked by two bridging V 5+ O4 tetrahedra.This structure had strong magnetic exchange between the V 4+ along the chain, which was beneficial to catalytic performance.Intensity (a.u.)

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 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% Na 2 SO 4 poisoned catalyst, the diffraction peak of Na 2 SO 4 could be detected.For the pure catalyst and 1.2% Na 2 S 2 O 7 poisoned catalyst, there was no peak of V 2 O 5 and no appearance of Na 2 S 2 O 7 .But for the 3.6% Na 2 S 2 O 7 poisoned catalyst, the diffraction peak of Na 4 V 3 O 9 could be detected.This was because some molten sodium pyrosulfate interacted with V 2 O 5 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 Na 4 V 3 O 9 had special structural and magnetic properties.It was made from isolated chains of square V 4+ O 5 pyramids linked by two bridging V 5+ O 4 tetrahedra.This structure had strong magnetic exchange between the V 4+ along the chain, which was beneficial to catalytic performance.
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 V 4+ O5 pyramids linked by two bridging V 5+ O4 tetrahedra.This structure had strong magnetic exchange between the V 4+ along the chain, which was beneficial to catalytic performance.Intensity (a.u.)

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

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 (V 5+ =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 NH 3 -TPD.The peak intensity at 1015 cm −1 increased and Na 2 S 2 O 7 poisoned catalyst had the most significant effect.The increase of V 5+ =O bond strength is beneficial for promoting the conversion of SO 2 to SO 3 [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 SO 2 oxidation experiments.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 (V 5+ =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 V 5+ =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.

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%).

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.

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-Na 2 SO 4 /Na 2 S 2 O 7 -SCR poisoned catalysts.Here, x means the mass percentage of sodium element (1.2% or 3.6%).

Catalyst Characterization
The experiment was performed using the model tp-5080 temperature programmed adsorption instrument (manufactured by Tianjin Xianquan Company, Tianjin, China) for NH 3 -TPD and H 2 -TPR of different catalysts.For a NH 3 -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 N 2 atmosphere (30 mL/min).Then 10%NH 3 (N 2 as balance gas) was passed over the samples for 1 h.After NH 3 was cut off, the catalysts were warmed to 60 • C and purged in pure N 2 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 NH 3 was recorded.For a H 2 -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 N 2 atmosphere (30 mL/min).Then 5%H 2 (N 2 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 H 2 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.

Test. Setup
Catalytic performance and SO 2 -SO 3 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 NH 3 , 500 ppm NO, 5% O 2 , 2% H 2 O, and the SO 2 concentration ranged from 0 to 3000 ppm (N 2 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 (O 2 , SO 2 , NO) concentrations were monitored using the Testo 350 flue gas analyzer (Testo AG, Lenzkirch, Germany).SO 3 was gathered by the Graham Condenser, which was placed in a constant temperature 80 • C water bath.The gathered SO 3 was converted to SO 4 2− using an 80% isopropanol solution.Then the SO 4 2− content in the solution was measured using an ion chromatograph to determine the SO 3 content, which was averaged over multiple measurements.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.

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 SO4 2− using an 80% isopropanol solution.Then the SO4 2− 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: The catalytic performance of the catalyst is represented by the conversion of NO, which is defined as: The SO 2 -SO 3 conversion is indirectly expressed by the SO 2 oxidation, which is defined as:

Conclusions
The influence of different sulfur-containing sodium salts (Na 2 SO 4 and Na 2 S 2 O 7 ) poisoned catalysts on SO 2 oxidation and NO reduction was investigated.Sodium salts poisoned catalysts led to a decrease in the denitration efficiency, while significantly improving the SO 2 -SO 3 -conversion.The degree of change is related to the loading and form of the sodium salts poisoned catalysts.The degree of poisoning of Na 2 S 2 O 7 poisoned catalyst was weaker because S 2 O 7 2− can create more Brønsted acid sites and has stronger oxidizability than SO 4 2− .The introduction of SO 2 clearly increased the number of surface acid sites.However, it had little effect on the redox capacity of the catalyst.Hence, SO 2 slightly enhanced the denitration efficiency of the sodium salts poisoned catalysts.According to analysis of NH 3 -TPD, FT-IR, and H 2 -TPR results, sodium salts poisoned catalysts reduced the Lewis acid sites and redox They also increased Brønsted acid sites and V 5+ =O bonds strength.The presence of the V 5+ =O bonds facilitates SO 2 -SO 3 conversion.XPS results showed that sodium salts poisoned catalysts increased the chemically adsorbed oxygen content and promoted SO 2 oxidation.However, sodium salts poisoned catalysts reduced the V 4+ /V 5+ ratio and inhibited denitration performance.

2. 1 .
Effect of Different Catalysts on SO 3 Generation 2.1.1.Effect of Temperature on SO 3 Generation SO 2 -SO 3 -conversion for different catalysts at different reaction temperatures are shown in Figure 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.

Table 1 .
The surface atomic concentrations for these samples.

Table 1 .
The surface atomic concentrations for these samples.

Table 1 .
The surface atomic concentrations for these samples.

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

Table 3 .
Structural characteristics of different Na salts loadings.