Experimental study on integrated desulfurization and denitrification of low-temperature flue gas by oxidation method

In this paper, TiO2 catalysts doped with different Fe contents (Fe-TiO2 catalysts) were prepared by coprecipitation method and the Fe loading capacity was optimized, and then the integrated pollutant removal experiment was conducted, in which TiO2 doped with Fe as catalyst and H2O2 as oxidant. The results show that under the condition of constant H2O2/(SO2 + NO) molar ratio, low concentration of SO2 can promote the oxidation and removal efficiency of NO, while high concentration of SO2 can inhibit the removal of NOx. The pollutant removal efficiency is proportional to the amount of catalyst, liquid–gas ratio and pH value of the absorbing solution. The optimal experimental conditions are H2O2/(SO2 + NO) molar ratio 1.5, space velocity ratio 10,000 h−1, H2O2 mass fraction 10 wt%, liquid gas ratio 10, pH 10. Correspondingly, NO oxidation efficiency reaches 88%, NOx removal efficiency 85.6%, and SO2 is almost completely removed. The microstructure of the catalyst before and after the reaction was characterized, and the crystal structure did not change obviously. However, with the deepening of the reaction, the specific surface area of the catalyst decreases, and the catalytic effect decreases slightly.

In the context of the objectives of "carbon peak" and "carbon neutrality," the installed capacity of new energy generation, represented by solar and wind power, continues to grow 1 .Thermal power generation actively engages in facilitating deep load adjustments to accommodate the integration of new energy generation, thereby mitigating the inevitable impact of new energy integration on the power grid.However, when the units are low-load operation, the flue gas volume reduces, and the flue gas temperature reduces below the optimal catalytic temperature for Selective Catalytic Reduction (SCR), then the denitration efficiency is greatly reduced.Moreover, if the temperature remains below the low-temperature ammonia injection threshold for an extended period, ammonia can react with SO 3 to produce ammonium hydrogen sulfate.This compound has a tendency to capture fly ash from the flue gas, leading to adhesion on heated surfaces and the catalyst, thereby causing blockages in surface voids, reduction in overall specific surface area, and a decrease in active catalytic sites.Consequently, the catalyst experiences diminished lifespan and decreased activity 2 .
During low-load operation, the flue gas flow rate decreases, resulting in reduced flow velocity as the flue gas passes through the pores of the catalyst.This scenario increases the risk of pore clogging due to ash accumulation within the catalyst pores 3 .Existing deep load adjustment units have implemented a series of strategies to mitigate the impact of low inlet flue gas temperatures on denitrification efficiency without altering the catalyst configuration.These methods include placing a portion of the reheater heating surface downstream of the SCR device to reduce upstream heat absorption and thereby elevate the inlet temperature to the SCR 4 .Additionally, a bypass arrangement in the water side of the reheater modifies the inlet water flow rate, effectively regulating the heat absorption by the flue gas 5 .However, most of the aforementioned measures are often implemented at the cost of sacrificing boiler thermal efficiency.Consequently, it becomes necessary to explore an alternative to traditional denitrification methods as a complementary solution for low-load operation of existing SCR systems.This novel

Experiment system
As depicted in Fig. 1, the simulated flue gas experimental system is primarily designed for catalyst screening under laboratory conditions.This experimental setup comprises a gas distribution system, a vertical tube furnace, a quartz glass reactor, an H 2 O 2 gasification system, an exhaust gas treatment device, and a flue gas analyzer.The fundamental procedure of the experiment involves pre-fabricating simulated flue gas composition through the gas distribution system.The gas components are thoroughly mixed using a gas control device.Subsequently, an H 2 O 2 solution is pumped into the gasification device by a peristaltic pump.In the quartz reactor, the gasified mixture undergoes catalytic oxidation, facilitated by the catalyst, which converts pollutants such as NO into other forms, including NO 2 .This process is a crucial step in altering the chemical composition of the pollutants, leading to their subsequent removal or conversion into less harmful substances.Finally, the treated flue gas is www.nature.com/scientificreports/absorbed by NaOH solution and dried with anhydrous calcium chloride before being directed into the flue gas analyzer for analysis.This study has developed an industrial-scale integrated pollutant removal experimental system, as illustrated in Fig. 2. The experimental setup comprises a 350 kW hot water boiler, a gas distribution system, an integrated tower, an oxidant atomizing spray device, a flue gas analysis system, a slurry pump, and a fan.The fundamental experimental procedure is as follows: high-temperature flue gas is generated by the hot water boiler and controlled to the desired temperature using a heat exchanger before entering the integrated tower.Inside the integrated tower, the flue gas is mixed with the atomized oxidant, ensuring thorough counter-flow mixing.The catalysts arranged within the tower catalytically oxidize pollutants, particularly nitrogen oxides, converting them into high-valence compounds.Subsequently, the absorbent slurry is introduced into the tower through spray pipes via a slurry pump, achieving extensive mixing with the oxidized flue gas and enabling efficient washing and removal of pollutants.The flue gas analysis system is employed to measure the composition and concentration of pollutants in the post-purification flue gas.Ultimately, the purified flue gas is discharged using a fan.

NO oxidation efficiency
where No in is the inlet NO concentration, mg m −3 ; NO out is the outlet NO concentration, mg m −3 ; η NO is the NO oxidation efficiency, %.

NO x removal efficiency
In the given equations: "NO in "-inlet NO concentration, mg m −3 ; NO out -outlet NO concentration, mg m −3 ; NO 2,out -outlet NO concentration, mg m −3 ; η NOx -NO x oxidation efficiency, %.

Catalyst preparation and characterization methods
Due to factors such as the tetravalent state of titanium in TiO 2 and the stability of Ti-O bonds, the enhancement of TiO 2 's catalytic performance in reactions is inherently limited.This limitation often leads to TiO 2 being selected as a catalyst carrier.However, doping TiO 2 with other metals can alter its lattice structure and introduce active sites for catalysis.Iron, as a doping metal, demonstrates an effective ability to substitute Ti, creating active sites on the TiO 2 surface that promote the decomposition of H 2 O 2 into •OH radicals.The catalyst used in this experiment was prepared using the co-precipitation method to produce iron-loaded titanium dioxide, which was subsequently investigated for its catalytic oxidation denitrification performance at different loading ) were precisely weighed according to the required Fe/Ti molar ratio using an electronic balance.A specific quantity of deionized water at 0 °C was used to dissolve and homogeneously mix the two powders.The mixture was continuously stirred at a constant temperature in a water bath for 1 h.Subsequently, 25 wt% ammonia solution was gradually added dropwise while stirring until the pH of the solution reached 10.The stirring process was continued for another 1 h to ensure complete precipitation.The resulting precipitate was then subjected to thorough washing with deionized water using a vacuum filtration device until the pH of the washout solution was in the range of 7 to 8. Afterward, the washed precipitate was dried in a 105 °C drying oven for 12 h to achieve complete dehydration.Upon cooling, the dried material was preliminarily ground into a powdered form, which was subsequently placed in a high-temperature muffle furnace (JZ-4-1200, Shanghai Jingzhao Machinery Equipment Co., Ltd., China) for calcination at 400 °C for 6 h to enhance the catalyst's activity.Finally, the calcined sample was finely ground and sieved to obtain the catalyst sample with the desired loading ratio.Catalyst samples were prepared with Fe/Ti molar ratios of 0%, 0.5%, 1%, 2%, and 3% using the aforementioned method.
After screening the catalysts prepared using the co-precipitation method through the simulated flue gas experimental system, a catalyst formulation with outstanding performance was identified.Based on this formulation, a honeycomb-like cordierite (Ruilan Environmental Technology Co., Ltd., China) structure with a pore size of 3Fmm (100-200CPSI), commonly used in industrial catalysis, was selected as the carrier.The preparation involved a coating process, in which the Fe/TiO 2 catalyst powder was loaded onto the honeycomb-like cordierite carrier.The Fe/TiO 2 catalyst powder is shown in Fig. 3a, and the honeycomb catalyst supported by Fe/TiO 2 is shown in Fig. 3b.
In this study, the catalyst's material composition and internal atomic or molecular structure were investigated using an XRD-6100 X-ray diffractometer manufactured by Shimadzu Corporation, Japan.The catalyst's morphology and microscale dimensions were examined with an SU8230 scanning electron microscope produced by Hitachi, Japan.Furthermore, the catalyst's specific surface area, pore size, pore size distribution, and nitrogen adsorption-desorption isotherms before and after the reaction were analyzed using a JW-TB440 instrument, designed for characterizing surface properties and pore structures of micro-nano materials, provided by Beijing Jingwei Gaobo Technology Co., Ltd.These advanced instruments were employed to comprehensively explore the microstructural changes of the catalyst before and after the reaction.

Results and discussion
Under the low-load operation conditions of the deep load-following power generation unit, the reduction in boiler fuel quantity leads to a decrease in the furnace outlet temperature.Without appropriate measures, the conventional Selective Catalytic Reduction (SCR) denitrification efficiency would be significantly reduced, and the SCR system might even fail to operate properly.Hydrogen Peroxide (H 2 O 2 ) oxidation denitrification can effectively overcome this drawback and can serve as one of the alternative technologies for pollutant removal under low-load conditions.To further investigate the integrated pollutant removal performance of H 2 O 2 in the presence of Fe/TiO 2 catalyst, an experimental system for integrated pollutant removal was established using an integrated tower.This study explores the influence of flue gas temperature, catalyst space velocity, H 2 O 2 / (SO 2 + NO) molar ratio, initial concentrations of NO and SO 2 , H 2 O content, liquid-to-gas ratio, and absorption solution pH on NO oxidation efficiency, NO x removal efficiency, and SO 2 removal efficiency.

Catalyst screening experiment
The experimental conditions involved a total flue gas flow rate of 1000 mL min −1 , a space velocity of 30000 h concentration of NO was 335 mg m −3 , the initial concentration of SO 2 was 714 mg m −3 , and N 2 was used as the balance gas.The absorbent solution used was 600 mL of 0.2 mol L −1 NaOH.Figures 4 and 5 illustrate the variations in NO oxidation efficiency, NO x removal efficiency, and SO 2 removal efficiency with increasing temperature for different loading ratios.From Fig. 4, it is evident that the addition of catalyst markedly enhances the NO oxidation efficiency.Taking the temperature of 200 °C as an example, compared to the scenario without catalyst, the inclusion of TiO 2 catalyst results in an increase in NO oxidation efficiency from 60.6 to 81.5%.Moreover, the efficiency is further augmented with the incorporation of catalyst loaded with Fe, reaching approximately 92% at a loading ratio of 3%.
From Fig. 5, it is evident that, at the same temperature, the NO x removal efficiency does not exhibit a significant improvement with increasing Fe loading ratio.The underlying reason for this can be attributed to the fact that the loading of Fe ions can indeed stimulate more oxygen vacancies, thereby enhancing the catalytic activity of the catalyst.However, an excessive loading of Fe may lead to pore blockage in the catalyst, resulting in a decrease in catalytic performance.As for the removal of SO 2 , it is almost completely removed both with and without the catalyst, indicating that NaOH is highly effective in absorbing SO 2 .Even when considering the maximum instrument error, the SO 2 removal efficiency exceeds 98%.At a loading ratio of 2% Fe, the NO oxidation efficiency ranges from 90.4 to 92.6%, and the NO x removal efficiency varies between 85.5 and 90%.With the increase in reaction temperature, there is a slight decrease in both NO oxidation and removal efficiency.This can be mainly attributed to the increased ineffective decomposition of H 2 O 2 in the system at  higher temperatures, which, to some extent, reduces the catalytic oxidation effect and subsequently leads to a decline in removal efficiency.However, within the temperature range of 140 to 260 °C, the changes in the catalytic oxidation and removal efficiency are relatively minor, indicating the catalyst's capability for pollutant removal under low-temperature conditions.Based on these observations, the Fe/TiO 2 catalyst with a loading ratio of 2% Fe is selected as the catalyst type for subsequent experiments in this study.

Effect of temperature on pollutant removal efficiency
The flue gas conditions were maintained consistent with the actual low-load operation conditions, except for particulate matter and flow rate.The remaining experimental conditions are consistent with those detailed in "Catalyst screening experiment" section.The variations of NO oxidation efficiency, NO x removal efficiency, and SO 2 removal efficiency with temperature are depicted in Fig. 6.
It is evident that as the flue gas temperature increases, almost complete removal of SO 2 is achieved, indicating that the variation in temperature due to changes in the inlet conditions has negligible impact on SO 2 removal efficiency in the catalytic section.Conversely, both NO oxidation efficiency and NO x removal efficiency exhibit an increasing trend with rising temperature.For instance, when the flue gas temperature rises from 140 to 200 °C, the NO oxidation efficiency increases from 71.8 to 80.5%, while the NO x removal efficiency increases from 67 to 78.2%.The primary reasons for these enhancements are as follows: first, the degree of H 2 O 2 gasification gradually improves with increasing temperature; second, the chemical reaction rate accelerates, leading to more generation of •OH on the catalyst surface, which contributes to an enhanced oxidation efficiency of the system.However, the temperature increase also results in the ineffective decomposition of H 2 O 2 , generating O 2 and H 2 O, thereby reducing the concentration of H 2 O 2 in the system.In summary, beyond 200 °C, the NO oxidation efficiency and NO x removal efficiency remain relatively stable, reaching approximately 82% and 79%, respectively.

Effect of catalyst space velocity ratio on pollutant removal efficiency
The flue gas temperature was set at 230 °C, with all other experimental conditions remaining consistent with those described in "Catalyst screening experiment" section.NO x removal efficiency, and SO 2 removal efficiency with different H 2 O 2 /(SO 2 + NO) molar ratios at various space velocities are illustrated in Figs.7 and 8.
From Fig. 8, it is evident that SO 2 is almost completely removed.Both the space velocity and H 2 O 2 /(SO 2 + NO) molar ratio significantly influence NO oxidation and removal efficiency.Under the same space velocity conditions, increasing the H 2 O 2 /(SO 2 + NO) molar ratio from 0.5 to 2 results in a noticeable enhancement in NO oxidation efficiency and removal efficiency.This effect can be attributed to the fact that, at lower molar ratios, the system contains a lower amount of H 2 O 2 , thus increasing H 2 O 2 leads to a more pronounced improvement in catalytic oxidation efficiency.However, when the molar ratio exceeds 2, the increase in NO oxidation and NO x removal efficiency becomes gradual and tends to stabilize.Under the same molar ratio conditions, a smaller space velocity of the catalyst corresponds to higher NO oxidation and removal efficiency.This is due to the fact that a lower space velocity implies more active sites on the catalyst, which enhances the ability of H 2 O 2 to decompose into •OH.As a result, more NO and NO 2 are oxidized into higher-valence oxides.Additionally, a smaller space velocity corresponds to a higher proportion of active components on the catalyst.The excess active sites facilitate the reduction of NO 2 back to NO during the catalytic oxidation process, as represented by Eqs. ( 4) and ( 5).This somewhat attenuates the enhancement in oxidation and removal efficiency resulting from reducing the space velocity.Considering the overall economic efficiency and NO catalytic oxidation removal efficiency, the subsequent experiments will utilize a catalyst with a space velocity of 10,000 h −1 and an H 2 O 2 /(SO 2 + NO) molar ratio of 1.5.Under these conditions, the NO oxidation efficiency and NO x removal efficiency are approximately 88% and 84%, respectively, while SO 2 is almost completely removed.It is evident from the results that the SO 2 removal efficiency is hardly affected by the water content in the system.The differences in NO oxidation and NO x removal efficiency under different water content levels show only minor fluctuations within a narrow range, indicating no significant deviations in efficiency.Generally, the influence of water (H 2 O) on the catalytic oxidation effect of H 2 O 2 mainly manifests in the competition between H 2 O and H 2 O 2 for some active sites on the catalyst surface, leading to a reduction in the utilization efficiency of H 2 O 2 .Additionally, water vapor can diffuse into the catalyst pores and condense there through capillary action,  www.nature.com/scientificreports/further hindering the contact of H 2 O 2 on the catalyst surface.However, based on the results of this experiment, it can be concluded that the water content has not exerted a significant impact on the pollutant removal efficiency, indicating that the selected catalyst in this study possesses certain water tolerance.Consequently, the subsequent experiments will continue using the 10 wt% H 2 O 2 solution to further investigate the pollutant removal behavior.
With the same H 2 O 2 concentration, a significant increase in both NO oxidation efficiency and NO x removal efficiency is observed as the H 2 O 2 /(SO 2 + NO) molar ratio increases from 0.5 to 1.5.Taking 10 wt% H 2 O 2 as an example, the NO oxidation efficiency and NO x removal efficiency increase from 64.5 and 61.7% to 88 and 86.6%, respectively.When the molar ratio exceeds 1.5, the efficiencies remain relatively constant.This behavior can be attributed to the gradual excess of H 2 O 2 with an increase in the H 2 O 2 /(SO 2 + NO) molar ratio, which leads to the occurrence of self-consumption reactions among ·OH, ·OOH, and H 2 O 2 , as described in Eqs. ( 6) to (9) below.This phenomenon results in a decreased utilization efficiency of ·OH, thereby limiting further enhancement of NO catalytic oxidation efficiency with an increasing dosage of H 2 O 2 .

Effect of SO 2 initial concentration on pollutant removal efficiency
Under typical conditions, when the load of the peak load regulation unit is below 30%, the boiler outlet SO 2 concentration usually falls within the range of 500 to 1000 mg m −3 .Hence, the selected initial SO 2 concentration range is set from 360 to 1570 mg m −3 .The remaining experimental conditions were chosen based on the optimal conditions established earlier in the text.the variations of NO oxidation efficiency and pollutant removal efficiency with respect to the SO 2 concentration are depicted in Fig. 11.
The results demonstrate that as the initial SO 2 concentration increases from 360 to 560 mg m −3 , both the NO oxidation efficiency and NO x removal efficiency exhibit an increment from 87.4 and 84% to 89.2 and 85.4%, respectively, after which they tend to stabilize 12 .However, when the SO 2 concentration exceeds 860 mg m −3 , the NO x removal efficiency gradually decreases, reaching 80.7% at an SO 2 concentration of 1570 mg m −3 , while the NO oxidation efficiency remains relatively constant at 84%.At lower SO 2 concentrations, the promotion of NO oxidation is attributed to the partial reduction of Fe 3+ on the catalyst surface to Fe 2+ by SO 2 , creating active sites that enhance the generation of ·OH and subsequently increase both the NO oxidation and NOx removal efficiency 13 .Conversely, there is a competition between SO 2 and NO for H 2 O 2 utilization.As the SO 2 concentration gradually increases, it competes with NO for H 2 O 2 , leading to a depletion of available H 2 O 2 .Additionally, the presence of SO 2 can partially block the oxygen vacancies on the catalyst surface, thereby reducing the efficiency of NO oxidation and removal at higher SO 2 concentrations.It is noteworthy that when the H 2 O 2 content in the system remains constant, the SO 2 removal efficiency decreases beyond an initial SO 2 concentration of 860 mg m −314 .

Effect of initial concentration of NO on pollutant removal efficiency
Typically, for in-depth peak load regulating units, the NO concentration at the boiler outlet ranges from 300 to 600 mg m −3 when operating at a load below 30%.Therefore, in this experiment, the initial NO concentration was selected within the range of 140 to 650 mg m −3 .The remaining experimental conditions were similarly selected based on the optimal conditions derived from the previous sections.The variations of NO oxidation efficiency and pollutant removal efficiency with respect to the NO concentration are depicted in Fig. 12.

Effect of liquid-gas ratio on pollutant removal efficiency
Keeping all other experimental conditions constant, the experiments were conducted by adjusting the liquidto-gas ratio.the variations of NO oxidation efficiency, NO x removal efficiency, and SO 2 removal efficiency with different liquid-to-gas ratios are presented in Figs. 13 and 14 15,16 .From Fig. 13, it can be observed that the NO oxidation efficiency remains relatively constant under different liquid-to-gas ratio conditions.This behavior is attributed to the direct influence of the liquid-to-gas ratio on the washing water quantity of the integrated tower's absorption side, which subsequently affects the removal efficiency of NO x and SO 2 .On the other hand, Fig. 14 demonstrates that the SO 2 removal efficiency is not significantly affected by the liquid-to-gas ratio.When the H 2 O 2 /(SO 2 + NO) molar ratio increases from 0.5 to 1, the three different liquid-to-gas ratios exhibit relatively consistent removal efficiencies.However, with further increases in the H 2 O 2 /(SO 2 + NO) molar ratio, beyond 1.5, the liquid-to-gas ratios of 10 and 15 demonstrate higher NO x removal efficiencies compared to the liquid-to-gas ratio of 5.The underlying reason for this trend lies in the depth of the catalytic oxidation reaction.A larger liquid-to-gas ratio corresponds to a higher amount of absorption solution sprayed per unit time, leading to a more pronounced washing and removal effect.Ultimately, the NO x removal efficiency stabilizes at around 88%, and the SO 2 is almost completely removed.Moreover, it is believed that due to the lower temperature on the absorption side, some components such as HNO 2 , HNO 3 , and H 2 SO 4 will undergo condensation after oxidation, further contributing to the removal effect.After comprehensive analysis, a liquid-to-gas ratio of 10 is chosen, which corresponds to a molar ratio of 1.5, resulting in an NO x removal efficiency of 85.6% 17,18 .Figure 15 shows that under the same H 2 O 2 /(SO 2 + NO) molar ratio conditions, the absorption solution pH has minimal impact on the NO oxidation efficiency.Instead, the absorption solution pH primarily influences the pollutant removal efficiency on the absorption side of the integrated tower.From Fig. 16, it can be observed that when the H 2 O 2 /(SO 2 + NO) molar ratio is greater than 1.5, the absorption solution pH significantly affects the NO x and SO 2 removal efficiencie .As the absorption solution pH increases, the corresponding NO x removal efficiency under the same molar ratio conditions also increases.This can be attributed to the higher OHconcentration in the absorption solution at higher pH levels, which enhances the ability to absorb and remove acidic oxidants, such as HNO 2 , HNO 3 , and H 2 SO 4 , from the flue gas.Moreover, the spraying effect on the absorption side allows for the combination and condensation of some acidic oxidants in the flue gas with water vapor, as well as the condensation removal of HNO 2 , HNO 3 , and H 2 SO 4 .These processes contribute to the overall pollutant removal efficiency, leading to only minor changes in efficiency with varying pH levels.Considering the economic viability, stability, and continuity of the removal process, it is recommended to choose an absorption solution pH of 10.This pH value strikes a balance between effective pollutant removal and the cost-effectiveness of adding alkaline solution 19 .NO x (liquid-gas ratio5) NO x (liquid-gas ratio10) NO x (liquid-gas ratio15) SO 2 (liquid-gas ratio5) SO 2 (liquid-gas ratio10) SO 2 (liquid-gas ratio15)  It is evident that as the catalyst usage time increases, both the NO oxidation efficiency and NO x removal efficiency show a decreasing trend, while the SO 2 removal efficiency exhibits relatively minor fluctuations.Notably, the NO oxidation efficiency decreased significantly from 88.2% in the second test to 84.9% compared to the first test.However, in the subsequent three tests, there was only a marginal variation, stabilizing at approximately 84%.On the other hand, the NO x removal efficiency remained consistently above 80% throughout all four tests.In order to understand the underlying reasons for the observed changes in NO oxidation efficiency and NO x removal efficiency, we conducted thorough microscopic characterization and analysis of the catalyst at each stage 20,21  From Fig. 18, it is evident that the structure of the fresh catalyst appears relatively porous, and the particles exhibit a relatively uniform distribution.Figure 19 reveals that, in comparison to the catalyst before the reaction, there is no significant alteration in the overall surface morphology.However, as the catalyst usage time increases, the particles become more tightly packed, and their volume undergoes expansion.This phenomenon is particularly pronounced in the SEM images of the catalyst after the fourth performance test.The primary reason behind this transformation is the occurrence of slight particle agglomeration during the catalyst's operational life.Specifically, during the reaction, SO 2 on the catalyst surface reacts with H 2 O 2 and•OH to form sulfate or sulfate salt precipitates.These precipitates continuously agglomerate and adsorb on the existing active surface, leading to partial blockage of some micropores and a reduction in catalytic efficiency 22 .To gain further insights into this phenomenon, X-ray fluorescence (XRF) analysis was conducted on the catalyst samples, with a specific focus on the sulfur (S) element content, as shown in Table 1.The results indicate a gradual increase in the S element content with the prolonged usage time of the catalyst.
Overall, the SEM and XRF analyses provide valuable insights into the changes in the catalyst's microstructure and elemental composition over multiple testing cycles, offering significant information on the reasons behind the variations in catalytic performance observed during the tests.

XRD characterization
Figure 20 illustrates the X-ray diffraction (XRD) patterns of the Fe/TiO 2 catalyst.It is evident that both the pre-reaction and post-reaction samples exhibit the main crystal phase corresponding to the rutile phase, as identified by the PDF standard card PDF#21-1272.No peaks corresponding to anatase or hematite structures are observed, and there are no prominent diffraction peaks of Fe ions, indicating that the loaded Fe did not alter the crystal phase of the catalyst and was successfully incorporated into the TiO 2 structure.A comparison between the XRD patterns of the fresh catalyst and the catalyst after four performance tests reveals that the positions of the corresponding diffraction peaks remain unchanged, and no new diffraction peaks are observed.This finding suggests that the overall crystal structure of the catalyst remains largely unaffected after the reaction.The XRD results indicate that the Fe/TiO 2 catalyst can maintain its original crystal structure after the reaction, thereby enhancing its resistance to deactivation 23,24 .Therefore, the XRD analysis demonstrates that the Fe/TiO 2 catalyst retains its crystalline structure after the reaction, which contributes to its improved resistance against deactivation, ultimately enhancing its catalytic performance.

BET characterization
Figure 21 depicts the N 2 adsorption-desorption isotherms and BJH pore size distribution of the Fe/TiO 2 catalyst before and after the reaction.A comparison of the isotherms obtained during the four performance tests reveals the occurrence of hysteresis loops within the range of relative pressure P/P 0 = 0.43.According to the classification standards, the isotherms of the catalyst before and after the reaction belong to Type IV adsorption-desorption isotherms, with H3 hysteresis loops observed in both cases.The pore size distribution curves before and after the reaction show that the proportion of pores around 5 nm significantly decreases with increasing reaction time, while the number of pores with a size of around 10 nm increases.This phenomenon can be attributed to partial agglomeration occurring on the catalyst surface during the catalytic process, leading to the blockage of some mesopores, which is supported by the SEM and XRF results 25 .The specific surface area of the catalysts before and after the reaction, as indicated by BET analysis, is presented in Table 2.With increasing reaction time, the catalyst's specific surface area experiences a certain degree of reduction.Although the specific surface area alone cannot directly determine the catalyst's activity, a larger specific surface area promotes the generation of •OH radicals, which can contribute to the catalytic process.Therefore, the decrease in specific surface area after the reaction is one of the factors contributing to the reduction in denitrification efficiency.In summary, the N 2 adsorption-desorption isotherms and pore size distribution analysis reveal changes in the mesoporous structure of the Fe/TiO 2 catalyst during the reaction, leading to reduced specific surface area and potentially affecting its catalytic performance 26,27 .

Figure 1 .
Figure 1.Schematic diagram of simulated flue gas experimental system.

Figure 5 .
Figure 5.Effect of different Fe loading ratios on pollutant removal efficiency (a) and magnified section image (b).

Figure 6 .
Figure 6.Effect of temperature on NO oxidation and pollutant removal efficiency.

Figure 7 .Figure 8 .
Figure 7. Effect of catalyst space velocity ratio on NO oxidation efficiency.

Figure 11 .
Figure 11.Effect of SO 2 on NO oxidation and pollutant removal efficiency.

Figure 12 .Figure 13 .
Figure 12.Effect of NO concentration on oxidation efficiency and pollutant removal efficiency.

Figure 19 .
Figure 19.SEM images of catalysts in various performance testing stages.

2 O 2 on pollutant removal efficiency
Under the conditions of the same H 2 O 2 /(SO 2 + NO) molar ratio and initial concentrations of NO and SO 2 , H 2 O 2 solutions with concentrations of 10 wt%, 20 wt%, and 30 wt% were utilized.Due to the variations in H 2 O 2 concentrations, the flow rates were different, resulting in different amounts of H 2 O 2 participating in the reaction.This investigation aims to explore the impact of H 2 O 2 concentration on pollutant removal efficiency.The remaining experimental conditions were selected based on the optimal pollutant removal conditions identified in "Effect of catalyst space velocity ratio on pollutant removal efficiency" section.The results depicting the NO oxidation efficiency, NO x removal efficiency, and SO 2 removal efficiency under different H 2 O 2 concentrations and H 2 O Vol.:(0123456789) Scientific Reports | (2024) 14:3527 | https://doi.org/10.1038/s41598-024-53765-ywww.nature.com/scientificreports/Effect of H 2 / (SO 2 + NO) molar ratios are presented in Figs. 9 and 10.
. Effect of pH value of absorption solution on pollutant removal efficiency.SEM characterizationTo investigate the microscopic morphological changes of the Fe/TiO 2 catalyst at various stages, we performed scanning electron microscopy (SEM) analysis on the fresh catalyst and samples taken at different testing stages after the reaction.The results are presented in Figs.18 and 19.

Table 1 .
Sulfur element content of catalysts at each performance testing stage.
Figure 20.XRD patterns of catalysts in various performance test stages.