Deep Oxidation Removal of Nitric Oxide and SO₂ via Singlet Oxygen and Hydroxyl Radicals in the LaCoO_3‑δ/H₂O₂ System

Abstract

This study proposes a novel LaCoO_3‑δ/H₂O₂ Fenton-like process for the efficient generation of nonradical singlet oxygen (¹O₂) and hydroxyl radicals (^•OH), enabling the deep oxidation of nitric oxide (NO), as well as simultaneous denitrification and desulfurization. A LaCoO_3‑δ perovskite material enriched with Co³⁺ ions and oxygen vacancies (OV) was successfully synthesized by using a one-step citric acid sol–gel method. OV enhances electron mobility and expedites the redox cycling of surface Co²⁺/Co³⁺ pairs, thereby promoting the generation of ^•OH and ^•O₂ ^–. The high Co³⁺ content further facilitates the transformation of ^•O₂ ^– into ¹O₂. Moreover, electron-rich centers and highly reactive lattice oxygen (O^2–) induced by abundant OV also contribute to the activation of H₂O₂, enhancing the production of ^•OH and ¹O₂. When combined with a comprehensive Na₂SO₃/NaOH absorption system, simultaneous removal efficiencies of 91.2% for NO and 100% for SO₂ were achieved under optimal experimental conditions while ensuring good operational stability. This innovative noniron-based Fenton-like system not only rivals the performance of typical iron-based Fenton-like systems but also provides new insights into the development of multipollutant deep oxidation removal technologies for application in coal-fired power plants.

1. Introduction

Excessive emissions of nitric oxide (NO) and sulfur dioxide (SO₂) from coal-fired flue gas can cause severe environmental pollution problems. − Conventional NH₃-selective catalytic reduction (NH₃–SCR) denitrification and limestone-based wet flue gas (Ca-WFGD) desulfurization , technologies are often unsuitable for green and low-energy development owing to high operational costs and potential secondary environmental contamination. − In recent years, heterogeneous Fenton-like oxidation combined with absorption processes for simultaneously removing multiair pollutants has attracted attention owing to its low cost and minimal environmental impact. − In this process, cost-effective and environmentally friendly hydrogen peroxide (H₂O₂) is activated by iron-based heterogeneous catalysts to generate hydroxyl radicals (^•OH) and superoxide radicals (^•O₂ ^–). , These reactive oxygen species (ROS) can oxidize water-insoluble NO into absorbable, higher-valence nitrogen-containing compounds. Subsequently, these compounds, along with SO₂, are simultaneously absorbed by the absorption device. However, the ROS generation process is typically restricted due to the relatively slow kinetics of the Fe²⁺/Fe³⁺ redox couple, the scarcity of active sites, and self-quenching effects of ROS, leading to a low NO oxidation efficiency and excessive consumption of H₂O₂. , Therefore, it is highly significant yet challenging to achieve the deep oxidation of NO by promoting the efficient generation and utilization of ROS in a heterogeneous Fenton-like system.

To enhance H₂O₂ utilization and ROS yield, a series of Fe-based materials, including Fe⁰, Fe₂O₃, Fe–Al, Fe/TiO₂, La_1–x Ca _x FeO₃, Fe₂(SO₄)₃, Fe₂(MoO₄)₃, Fe/ZSM5, and Fe₃O₄/Fe⁰/Fe₃C, have been comprehensively designed and developed. These studies demonstrated the efficient oxidation ability of ^•OH by accelerating the circulation of surface Fe²⁺/Fe³⁺ and increasing the number of active sites for H₂O₂ activation. Nevertheless, ^•OH can inevitably be consumed by certain undesirable side reactions, leading to the formation of H₂O₂ or ^•O₂ ^–. , On the one hand, the contribution of H₂O₂/^•O₂ ^– to NO oxidation is less significant compared to that of ^•OH. On the other hand, ^•OH can further react with ^•O₂ ^– to generate H₂O and O₂. Consequently, the excessive consumption of ^•OH and the negative effects caused by ^•O₂ ^– can result in an inevitable inefficiency in H₂O₂ utilization and deep removal of NO.

Research indicates that ^•O₂ ^– can be converted into nonradical singlet oxygen (¹O₂) through electron transfer during the activation of peroxymonosulfate (PMS). , Moreover, ¹O₂ has been demonstrated to play a crucial role in the degradation of persistent organic pollutants in water , and in the photocatalytic oxidation of NO to NO₂, which is attributed to its relatively long lifetime (3.5 μs), excellent oxidation capacity, and high selectivity. Consequently, it may be feasible to improve the utilization rate of H₂O₂ and promote the deep oxidation of NO by transforming ^•O₂ ^– into ¹O₂ within a Fenton-like process. Studies have shown that high-valent metal catalysts can provide an effective pathway for converting ^•O₂ ^– into ¹O₂. Qian et al. successfully synthesized U2.5-LaFeO₃ perovskite materials with high-valent iron (Fe⁴⁺) using an ammonia-mediated method, which significantly boosted the production of ¹O₂ through electron transfer from ^•O₂ ^– to Fe⁴⁺, indicating perovskite catalysts demonstrate significant potential for the generation of ¹O₂ in a Fenton-like system. Besides iron-based materials, compounds containing Co⁴⁺ (CoS_2‑x) and Mo⁶⁺ also facilitate the conversion of ^•O₂ ^– to ¹O₂, providing notable advantages for the selective elimination of water pollutants. However, in these catalytic systems, the generation of ^•OH is restricted to maximize ¹O₂ production, which is not beneficial for NO oxidation. Therefore, it is essential to investigate new catalytic materials or strategies to achieve balanced regulation of ¹O₂ and ^•OH and to develop a catalytic oxidation system cooperated by nonradical (¹O₂) and radical (^•OH) species.

Cobalt-based heterogeneous catalysts exhibit considerable potential in Fenton-like catalysis, with the rapid cycling of Co²⁺/Co³⁺ being crucial for the activation of H₂O₂ to generate ^•OH and ^•O₂ ^–. , Notably, the standard reduction potential of cobalt (Co³⁺/Co²⁺ (E ^θ = 1.82 V)) surpasses that of iron (Fe³⁺/Fe²⁺ (E ^θ = 0.77 V)), indicating that Co³⁺ has a greater tendency to accept electron from ^•O₂ ^– compared to Fe³⁺. Meanwhile, the electron-accepting capability of Co³⁺ is comparatively lower than that of Co⁴⁺. Consequently, it is speculated that Co³⁺ with a moderate electron-accepting ability (Fe³⁺ < Co³⁺ < Co⁴⁺) may effectively convert ^•O₂ ^– into ¹O₂ without significantly affecting the generation of ^•OH. In addition, previous studies have suggested that oxygen vacancies (OV) not only facilitate the mobility of lattice oxygen but also create electron-rich centers that enhance electron transfer capabilities, consequently improving the catalytic performance of catalysts. , Therefore, materials with a high abundance of Co³⁺ and OV could serve as effective candidates for converting H₂O₂ into ¹O₂ and ^•OH. In Lanthanum-based perovskite materials (LaBO₃), lanthanum predominantly exists in its trivalent state (La³⁺), and OV can be easily formed and controlled. , Incorporating Co into the B-site to form a OV-rich LaCoO₃ perovskite may yield a promising material capable of effectively catalyzing H₂O₂ to generate abundant ¹O₂ and ^•OH. Recently, LaCoO₃ perovskite has been reported to activate peracetic acid (PAA) and H₂O₂ to facilitate the generation of ROS for the degradation of organic pollutants in liquid-phase systems. This further indicates that the LaCoO_3‑δ/H₂O₂ process holds potential for gas-phase applications such as deep oxidation of NO and efficient simultaneous denitrification and desulfurization. In this study, LaCoO_3‑δ materials rich in Co³⁺ and OV were successfully prepared by using a one-step citric acid (CA) sol–gel method. Comprehensive characterization techniques, including X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), inductively coupled plasma-optimal emission spectroscopy (ICP-OES), energy-dispersive spectroscopy (EDS), elemental analysis (EA), electron paramagnetic resonance (EPR), Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), were utilized to systematically investigate the physicochemical properties of LaCoO_3‑δ. Activity assessments and SO₂ resistance tests were performed to evaluate the catalytic performance and anti-SO₂ capability of LaCoO_3‑δ. Moreover, systematic experiments on sodium-based absorbent selection and operational parameter optimization were conducted to enhance the LaCoO_3‑δ/H₂O₂ oxidation and absorption process to achieve deep oxidation removal of NO and SO₂. Subsequently, the stability of this process was assessed through cyclic and continuous operation tests. Finally, the potential mechanisms of ROS generation and the removal pathways of NO and SO₂ were elucidated based on ROS determination, quenching experiments, and product analyses.

2. Experimental Section

2.1. Materials Synthesis

The LaCoO_3‑δ sample was synthesized by using the CA sol–gel method (Figure a). The detailed synthesis process of catalysts in this study is described as follows: (a) Preparation of the precursor solution: a molar ratio of 1:1:2 La­(NO₃)₃·6H₂O (Guaranteed Reagent, Macklin Reagent Company), Co­(NO₃)₂·6H₂O (Guaranteed Reagent, Macklin Reagent Company), and CA (C₆H₈O₇·H₂O, Analytical Reagent, Macklin Reagent Company) was dissolved in 80 mL of high-purity water to prepare a mixed precursor solution; (b) wet sol formation process: the precursor solution was placed in an open beaker and subjected to magnetic stirring at 300 rpm under water bath heating at 90 °C to evaporate the majority of the solvent water, resulting in the formation of a viscous wet sol; and (c) dry gel formation process: the wet sol was placed in an air-drying oven at 110 °C for 12 h to remove the residual water, resulting in the formation of the dry gel (note: the material was exposed to air in this process); (d) calcination process of dry gel: the dry gel was placed in an open crucible within a conventional muffle furnace at a heating rate of 5 °C/min, followed by calcination at 700 °C for 6 h (note: the material was exposed to static air in this process); and (e) sample grinding and sieving: following calcination, the material was ground using a ceramic mortar and subsequently sieved through a 200-mesh screen. Additionally, two other samples (La₂O₃ and Co₃O₄) were prepared by using La­(NO₃)₃·6H₂O or Co­(NO₃)₂·6H₂O as precursors through the same procedure.

1.

(a) Schematic diagram for the synthesis of the LaCoO_3‑δ catalyst and (b) flow diagram of the experimental apparatus. 1N₂, 2O₂, 3NO, 4SO₂, 5CO₂, 6pressure relief valve, 7mass flow controller, 8buffer bottle, 9evaporation device, 10thermal control electric heater, 11peristaltic pump, 12H₂O₂ solution, 13electrical furnace, 14catalytic reactor, 15midget impinger, 16spiral condenser, 17thermostat water bath, 18Na₂SO₃/NaOH absorption solution, 19dry tower, and 20flue gas analyzer.

2.2. Characterization Methods

XRD (Bruker D8 Advance) was employed to analyze the crystal structure of the dry gels prior to calcination and as-prepared samples within a scan range of 10°–80° (2θ) at a scan rate of 5°/min. FTIR spectroscopy (Nicolet iZ10) was performed to identify and distinguish the characteristic species present in LaCoO_3‑δ and La–Co–CA dry gel. EA (Vario EL Cube) and ICP-OES (Agilent 700 Series) analyses were conducted to determine the elemental composition and quantitative content of LaCoO_3‑δ. EPR spectroscopy using a Bruker A-300 instrument was employed to investigate OV in the materials at 150 °C, as well as to identify ROS generated during the catalytic process at room temperature. Specifically, 5,5-Dimethyl-1-pyrroline (DMPO) served as the spin-trapping agent for detecting ^•OH and ^•O₂ ^–, whereas ¹O₂ was identified using 2,2,6,6-tetramethyl-4-piperidone (TEMP). In quenching experiments, furfuryl alcohol (FFA) was used to scavenge both ^•OH (k^• _OH/FFA = (1.8 ± 0.2)×10¹⁰ M^–1 s^–1, pH 1–11) and ¹O₂ (k¹ _O2/FFA = (9.4 ± 0.1)×10⁷ M^–1 s^–1, pH 3–12), tert-butanol (TBA) was employed to quench ^•OH (k^• _OH/TBA = (4.8 ± 0.3)×10⁸ M^–1 s^–1, pH 3–11), and p-benzoquinone (BQ) was used to quench ^•O₂ ^– (k^• _O2 -_/FFA = 3–5 × 10⁸ M^–1 s^–1, pH < 7). SEM (Hitachi S-4800) and TEM (JEM-2100F, JEM (Japan Electron Optics Laboratory electron microscope)) were conducted to examine the morphology and microstructure of LaCoO_3‑δ. The BET (ASAP2020) method was applied to evaluate the specific surface area and pore structure of the LaCoO_3‑δ sample. EDS (Bruker quantax400) and EDS mapping were employed to analyze the distribution of the elements. XPS (ESCALAB 250Xi) was performed to determine the chemical states and relative contents of the specific elements. Additionally, ion chromatography (IC) (Thermo ICS-5000+) was used to analyze the sulfur- and nitrogen-containing species in the spiral condenser and absorbent solution.

2.3. Experimental Procedure and Activity Tests

As illustrated in Figure b, activity tests of the as-prepared catalysts were conducted in a fixed-bed apparatus. The total flow rate of the simulated flue gas was set at 1.5 L/min and was precisely controlled using a mass flowmeter. The simulated flue gas compositions included N₂, NO (100–1100 mg/m³), SO₂ (500–3000 mg/m³), O₂ (0–15%, v/v), and CO₂ (0–20%, v/v). A peristaltic pump delivered the hydrogen peroxide solution (pH = 3–8) at a constant rate of 4.5 mL/h into a custom-designed vaporization device. Subsequently, the H₂O₂ vapor was thoroughly mixed with the simulated flue gas and directed to the catalyst bed (catalyst dosage: 0–0.25 g, catalytic reaction temperature: 110–210 °C), where the catalytic oxidation reaction occurred within a quartz tube measuring 150 mm in length and 10 mm in diameter. Quartz wool was employed to ensure a uniform distribution of the catalyst throughout the tube, thereby maximizing the contact between the simulated flue gas containing H₂O₂ vapor and the catalyst. The oxidation products were collected using an ice-bath condenser and an absorbent solution (0.1 mol/L Na₂SO₃ + 0.1 mol/L NaOH, 500 mL) located at the tail end of the system. Finally, the concentrations of pollutants (NO, NO₂, and SO₂) in the flue gas were measured by using a flue gas analyzer (Ecom-J2KN, Germany), and the removal efficiencies of NO and SO₂ were calculated using the following equations:

$$\mathrm{N}\mathrm{O}\mathrm{conversion}\mathrm{efficiency}(\%)=[(C_{[\mathrm{Inlet}\mathrm{N}\mathrm{O}]}-C_{[\mathrm{Outlet}\mathrm{N}\mathrm{O}]})/C_{[\mathrm{Inlet}\mathrm{N}\mathrm{O}]}]\times 100\%$$ 1

$$\mathrm{N}\mathrm{O}\mathrm{removal}\mathrm{efficiency}(\%)=[(C_{[\mathrm{Inlet}\mathrm{N}\mathrm{O}]}-C_{[\mathrm{Outlet}(\mathrm{N}\mathrm{O}+\mathrm{N}{\mathrm{O}}_2)]})/C_{[\mathrm{Inlet}\mathrm{N}\mathrm{O}]}]\times 100\%$$ 2

$$S{\mathrm{O}}_2\mathrm{removal}\mathrm{efficiency}(\%)=[(C_{[\mathrm{Inlet}\mathrm{S}{\mathrm{O}}_2]}-C_{[\mathrm{Outlet}\mathrm{S}{\mathrm{O}}_2]})/C_{[\mathrm{Inlet}\mathrm{S}{\mathrm{O}}_2]}]\times 100\%$$ 3

where “C” denotes the pollutant concentration, mg/m³.

3. Results and Discussion

3.1. Characterization of the LaCoO_3‑δ Sample

The crystal structures of the three presynthesized materials were characterized by XRD, and the results are presented in Figure a. The diffraction peaks observed at 2θ = 23.2°, 32.9°, 33.3°, 39.0°, 40.7°, and 79.5° correspond to the characteristic peaks of LaCoO₃ (JCPDS PDF#84-0848), which exhibits a rhombohedral crystal structure. Furthermore, the use of lanthanum nitrate as the sole precursor resulted in the formation of La₂O₃ (JCPDS PDF#74-2430), whereas the use of cobalt nitrate as the sole precursor led to the formation of Co₃O₄ (JCPDS PDF#74-2120). However, the three precursor dry gels showed no distinct XRD characteristic peaks (Figure S1) and exhibited an amorphous structure. The average crystallite sizes of the three catalysts calculated using the Scherrer equation were 38.0 nm (LaCoO_3‑δ), 18.8 nm (La₂O₃), and 28.6 nm (Co₃O₄), respectively (Table S2). In the XRD pattern of LaCoO₃, the absence of characteristic peaks corresponding to La₂O₃ and cobalt oxides confirms the successful synthesis of high-purity LaCoO₃ samples via the CA sol–gel method, which is further supported by the FTIR spectra of LaCoO₃ and its precursor dry gel (Figure S2).

2.

(a) XRD patterns of LaCoO₃, Co₃O₄, and La₂O₃ and (b) EPR spectra of the detected OV in LaCoO_3‑δ, Co₃O₄, and La₂O₃.

CNH/O element analysis and ICP-OES tests were performed on the LaCoO₃ sample to further determine the atomic ratios of each element (C, N, H, O, La, and Co), and the results are summarized in Table . The absence of C, N, and H elements indicates that the precursors (CA and NO₃ ^–) have been completely calcined in an air atmosphere, with no other species remaining. The La/Co atomic ratio was close to the stoichiometric ratio of 1 (1.01–1.03). However, the O/La­(Co) atomic ratio (2.28–2.40) was lower than the theoretical value of 3. This ratio was denoted as 3-δ (0.60 ≤ δ ≤ 0.72), and the LaCoO₃ compound was denoted as LaCoO_3‑δ. These findings suggest the presence of oxygen defects in the LaCoO_3‑δ material. Consequently, EPR measurements were carried out on LaCoO_3‑δ to quantify the OV. As depicted in Figure b, the results confirm that the LaCoO_3‑δ samples exhibit a substantial number of OV, which is consistent with the results of element analysis. Additionally, EPR tests were conducted on the La₂O₃ and Co₃O₄ samples, and the oxygen vacancy concentrations in these two samples were significantly lower than those observed in LaCoO_3‑δ. This difference may be attributed to the lattice defects formed during the crystallization of the perovskite structure, as a portion of the lattice oxygen in LaCoO_3‑δ is derived from atmosphere oxygen (with another part originates from metal nitrate) that enters the La–Co perovskite lattice structure via bulk diffusion during the air calcination process. , Assuming that the catalyst without OV possesses an ideal chemical formula of LaCoO₃, the content of OV of three samples was 24.0%, 22.3%, and 20% (relative error: 8.6%, 0.9%, 9.5%), respectively, suggesting a relatively stable OV concentration, which demonstrates that the incorporation of Co into the B-site of the LaBO₃ perovskite structure enables the stable synthesis of the OV-rich LaCoO_3‑δ material via the CA sol–gel method in the present study.

1. Analysis of the Chemical Elemental Composition of the LaCoO_3‑δ Material.

LaCoO3-δ sample	La (wt %)	Co (wt %)	O (wt %)	C/N/H (wt %)	atomic ratios (La/Co/O)	chemical composition	OV content (%)
1	58.2	25.3	15.3	not detected	1:1.03:2.28	LaCo1.03O2.28	24.0
2	58.6	25.0	15.8	not detected	1:1.01:2.33	LaCo1.01O2.33	22.3
3	58.0	24.8	16.0	not detected	1:1.02:2.40	LaCo1.02O2.40	20.0
average	58.3	25.0	15.7	not detected	1:1.02:2.34	LaCo1.02O2.34	22.1

^aData obtained from ICP-OES tests.

^bData obtained from CNH/O EA.

Figure a–c presents the SEM and low-resolution TEM images of the LaCoO_3‑δ sample, respectively. It is clearly observed that LaCoO_3‑δ exhibits a uniformly spherical morphology. The average grain size was determined to be 87.0 nm by using Nano Measure 1.2 software, which is larger than the average crystallite size (38.5 nm), indicating that the LaCoO_3‑δ catalyst exists as a polycrystalline aggregate material. Additionally, a slight degree of particle agglomeration may be attributed to intergranular electrostatic attraction. The BET test of LaCoO_3‑δ revealed a specific surface area of 7.1 m²/g. The relatively low specific surface area was primarily attributed to the high-temperature sintering process during synthesis. The adsorption–desorption isotherms are presented in Figure S3, the average pore diameter was 16.4 nm, and the total pore volume was 0.03 cm³/g. These mesoporous and macroporous structures were formed due to the substantial release of CO₂ and NO₂ during the pyrolysis of CA and nitrates, which are expected to positively influence gas–solid heterogeneous catalysis reactions.

3.

(a,b) SEM images of LaCoO_3‑δ; (c) low-resolution TEM image of LaCoO_3‑δ; (d) high-resolution TEM images of LaCoO_3‑δ; and (e) EDS elemental mapping images of LaCoO_3‑δ.

Figure d shows the high-resolution TEM image of LaCoO_3‑δ, where the crystalline interplanar spacings of 0.27 and 0.22 nm correspond to the (−1 1 0) and (0 2 0) planes of rhombohedral LaCoO_3‑δ, respectively. These results are consistent with the XRD data and further confirm the successful synthesis of LaCoO_3‑δ. EDS mapping was performed to investigate the elemental distribution across the LaCoO_3‑δ sample, as shown in Figure e. The results revealed that La, Co, and O were uniformly dispersed on the surface of the sample, which facilitated the exposure and dispersion of active sites and significantly enhanced the catalytic performance.

3.2. Oxidation Removal Performances of NO and SO₂

3.2.1. Evaluation of Catalytic Activity and SO₂ Resistance

The catalytic activities of the three samples were evaluated using the NO conversion efficiency as an indicator. The results are shown in Figure a. The catalytic activity followed the order LaCoO_3‑δ > Co₃O₄ > La₂O₃. Notably, La₂O₃ exhibited almost negligible catalytic activity, while LaCoO_3‑δ demonstrated the highest catalytic performance. This suggests that the LaCoO_3‑δ/H₂O₂ system generated the largest amount of ROS, thereby achieving the highest NO conversion efficiency. Subsequently, 1000 mg/m³ of SO₂ was introduced into the catalytic reaction system to evaluate its impact on the catalytic performances of Co₃O₄ and LaCoO_3‑δ. The experimental results are presented in Figure b. Prior to the addition of SO₂, both systems maintained stable NO conversion rates (91.0% for LaCoO_3‑δ and 74.0% for Co₃O₄) and NO₂ production levels (287 mg/m³ for LaCoO_3‑δ and 114 mg/m³ for Co₃O₄), with a higher NO conversion rate and NO₂ production observed in the LaCoO_3‑δ system compared to the Co₃O₄ system. Upon the introduction of SO₂ (t = 30 min), the NO conversion rate of the LaCoO_3‑δ system only decreased by 2.0%, from 91.0% to 89.0%, but subsequently stabilized. In contrast, the NO conversion rate in the Co₃O₄ system continuously declined by 23%, from 74.0% to 51.0%, between 30 and 210 min before stabilizing. Moreover, the variation trend of the NO₂ production was consistent with that of the NO conversion rate in both systems. These results indicate that the LaCoO_3‑δ system exhibits strong resistance to SO₂ interference, whereas the Co₃O₄ system demonstrates relatively weak tolerance to SO₂. When the supply of SO₂ was terminated (t = 270 min), the NO conversion rate of the LaCoO_3‑δ system returned to its initial value, whereas that of the Co₃O₄ system increased by approximately 1.5% over the subsequent 3 h, further suggesting that LaCoO_3‑δ exhibits greater stability than Co₃O₄ under SO₂ exposure conditions.

4.

(a) Catalytic activity tests of LaCoO_3‑δ, Co₃O₄, and La₂O₃ and (b) SO₂ tolerance tests of LaCoO_3‑δ and Co₃O₄.

3.2.2. Evaluation of Sodium-Based Absorbent

To achieve more efficient oxidation of NO and simultaneous absorption of NO₂ and SO₂, the performance of sodium-based absorbents (Na₂SO₃ and NaOH), which have demonstrated excellent effectiveness in previous studies, , was evaluated in combination with the LaCoO_3‑δ/H₂O₂ system, and the results are shown in Figure . Using Na₂SO₃ alone exhibited relatively high absorption efficiency for NO₂ and SO₂; however, the desulfurization efficiency of this system gradually decreased over time, indicating limited stability. Using NaOH alone achieved 100% desulfurization efficiency but demonstrated inferior absorption performance for NO₂ compared to that for Na₂SO₃. In contrast, the combination of Na₂SO₃ and NaOH maintained a relatively high level of simultaneous desulfurization and denitrification efficiency over an extended period, suggesting the superior absorption capacity and stability of the combined absorbent system.

5.

Oxidation–absorption properties of NO/SO₂/NO₂ in the LaCoO_3‑δ/H₂O₂ + sodium-based absorbents (A): 0.2 mol/L Na₂SO₃ solution, (B): 0.2 mol/L NaOH solution, and (C): 0.1 mol/L NaOH + 0.1 mol/L Na₂SO₃ mixed solution systems.

Based on the above experimental results, it can be concluded that the LaCoO_3‑δ/H₂O₂ catalytic oxidation system effectively promotes the oxidation of NO, while the Na₂SO₃–NaOH absorption system achieves highly efficient simultaneous absorption of NO₂ and SO₂. Therefore, the deep oxidation-removal of NO and SO₂ was accomplished through the synergistic action of the combined LaCoO_3‑δ/H₂O₂ and Na₂SO₃–NaOH system.

3.2.3. Effects of Experimental Parameters

Given the outstanding catalytic performance of the LaCoO_3‑δ catalyst and its potential for simultaneous desulfurization and denitrification, various experimental conditions, including H₂O₂ concentration (molar ratio of H₂O₂ to NO), catalyst dosage (gas hourly space velocity, GHSV), catalytic temperature, pH of the H₂O₂ solution, and gas compositions (NO, SO₂, O₂, and CO₂), were systematically investigated, and the results are shown in Figure .

6.

(a) Effect of H₂O₂ concentration (molar ratio of H₂O₂ to NO) on simultaneous removal of NO and SO₂ in the LaCoO_3‑δ/H₂O₂ system; (b) effects of LaCoO_3‑δ dosage and GHSV on simultaneous removal of NO and SO₂; (c) effect of reaction temperature on simultaneous removal of NO and SO₂; (d) effect of H₂O₂ solution pH on simultaneous removal of NO and SO₂; (e) effect of NO concentration on simultaneous removal of NO and SO₂; (f) effect of SO₂ concentration on simultaneous removal of NO and SO₂; (g) effect of O₂ concentration on simultaneous removal of NO and SO₂; (h) effect of CO₂ concentration on simultaneous removal of NO and SO₂; and (i) cyclic and continuous running experiments for the simultaneous removal of NO and SO₂.

It is evident that under all operating conditions, SO₂ was entirely removed by the absorption liquid, and its removal efficiency showed minimal sensitivity to variations in the experimental conditions. This clearly highlights the exceptional desulfurization performance of this system. In contrast, the NO removal efficiency was considerably influenced by the various experimental parameters. Notably, the concentration of H₂O₂ played a critical role in NO removal (Figure a). As the concentration of H₂O₂ increased from 0.2 to 1.0 mol/L and the molar ratio rise from 0.6 to 3, the NO removal efficiency progressively improved. This improvement can be attributed to the increased generation of ROS resulting from the reaction between LaCoO_3‑δ and higher concentrations of H₂O₂, which enhances the oxidation rate of NO and, consequently, boosts its removal efficiency. However, as the concentration of H₂O₂ continued to increase beyond this range, the rate of improvement in the NO removal efficiency gradually diminished. This phenomenon was primarily due to the saturation of the active sites of the catalyst and radical self-quenching. , Therefore, taking into account both removal efficiency and economic feasibility, as indicated by the red circular annotations, we have determined 1 mol/L H₂O₂ (with a molar ratio of H₂O₂ to NO of 3) as the optimal experimental condition. By increasing the dosage of LaCoO_3‑δ (Figure b), the number of active sites for the catalytic reaction was directly enhanced, and the GHSV of the system decreased. This prolongs the contact time between H₂O₂ and LaCoO_3‑δ, thereby promoting the complete reaction of H₂O₂ with the catalyst and facilitating ROS generation. Considering overall cost and treatment capacity, an optimal LaCoO_3‑δ dosage of 0.15 g (corresponding to a GHSV of 150,000 h^–1) was determined and highlighted by red circles in Figure b. As shown in Figure c, as the temperature increased, the NO removal efficiency initially exhibited a significant enhancement, followed by a slight decline. On the one hand, an increase in temperature enhanced the reaction rate constant and facilitated the atomization and diffusion of H₂O₂, thereby accelerating the catalytic process. On the other hand, H₂O₂ tends to decompose into H₂O and O₂ at excessively high temperatures, leading to a reduction in the formation of ROS, which negatively influences the removal efficiency of NO. , Consequently, as indicated by the red circle in Figure c, 150 °C represents the optimal temperature condition for the experiment. In Figure d, under acidic conditions, the removal efficiency of NO was maintained at a high and stable level. In contrast, in neutral or alkaline environments, the ineffective decomposition of H₂O₂ leads to a marked decrease in the NO removal efficiency. As the pH of the 1 mol/L H₂O₂ solution was 5.3, no additional pH adjustment of the solution was required, and pH 5.3 was the optimal parameter in this study, as highlighted by the red circle in Figure d.

It is important to examine the effects of coexisting gases on NO removal efficiency because the load of industrial boilers typically influences the concentration of pollutants and the composition of gas components; the results are presented in Figure e–h. As shown in Figure e, as the NO concentration increased, the removal efficiency of NO decreased owing to the limited availability of ROS for oxidizing excessive NO. In Figure f, the NO removal efficiency exhibited an initial increase, followed by a decrease as the SO₂ concentration increased. An appropriate level of SO₂ can enhance the conversion and absorption of NO and NO₂; however, excessive SO₂ competes with NO for the limited oxidant, leading to a reduction in NO removal efficiency. As shown in Figure g, the presence of O₂ accelerates the absorption of NO and NO₂, improving the NO removal efficiency to some extent. Furthermore, CO₂ had a relatively minor impact on the NO removal efficiency (Figure h).

Based on the above experimental results, the optimal reaction parameters for the catalytic system are determined as follows: H₂O₂ concentration: 1 mol/L (molar ratio of H₂O₂/NO: 3), LaCoO_3‑δ dosage: 0.15 g, reaction temperature: 150 °C, pH of H₂O₂ solution: 5.3, NO concentration: 500 mg/m³, SO₂ concentration: 2000 mg/m³, O₂ concentration: 6% (v/v), and CO₂ concentration 8% (v/v). Subsequently, a systematic test of the stability of LaCoO_3‑δ was conducted under optimal experimental conditions, and the experimental results are presented in Figure i. The optimal simultaneous removal efficiencies of NO and SO₂ were 91.2% and 100%, respectively. After undergoing ten experimental cycles, the catalytic performance of the catalyst did not show a significant decline. Furthermore, the removal efficiency of NO decreased by only 0.9% after continuous operation for 18 h. Additionally, no new diffraction peaks were observed in the XRD spectrum of the spent LaCoO_3‑δ sample (Figure S4), which indicates the excellent stability of LaCoO_3‑δ. The energy consumption rates in this study were calculated to be 7.61 × 10^–3 kWh/g (NO) and 1.74 × 10^–3 kWh/g (SO₂) (calculation process provided in the Supporting Information), suggesting good potential for industrial application.

3.3. ROS Generation Mechanisms

EPR spectroscopy was employed to investigate the generation of ROS (¹O₂, ^•OH, and ^•O₂ ^–) in the LaCoO_3‑δ/H₂O₂ and Co₃O₄/H₂O₂ systems. In Figure a–c, it is observed that the signal intensities of the three ROS in the H₂O₂ system were nearly negligible, indicating that H₂O₂ scarcely generated ROS in the absence of a catalyst. In the LaCoO_3‑δ/H₂O₂ system, the relative signal intensities followed the order ¹O₂ (1.1× 10¹⁷ spins/mL) > ^•OH (6.1× 10¹⁶ spins/mL) ≈ ^•O₂ ^– (6.0 × 10¹⁶ spins/mL), whereas in the Co₃O₄/H₂O₂ system, the order was ^•O₂ ^– (8.9× 10¹⁶ spins/mL) > ^•OH (5.7 × 10¹⁶ spins/mL) > ¹O₂ (5.2 × 10¹⁷ spins/mL). As shown in Figure c, although the generation of ^•O₂ ^– in the Co₃O₄/H₂O₂ system exceeded that in the LaCoO_3‑δ/H₂O₂ system, the catalytic activity of Co₃O₄ was nonetheless lower than that of LaCoO_3‑δ, indicating that ^•O₂ ^– played a relatively minor role in the NO oxidation process. Additionally, the LaCoO_3‑δ/H₂O₂ system exhibited a higher production of ¹O₂ than the Co₃O₄/H₂O₂ system (Figure a), while both systems demonstrated similar levels of ^•OH generation (Figure b). These findings confirm that ¹O₂ is primarily responsible for the superior catalytic performance of LaCoO_3‑δ over Co₃O₄. To further investigate the contributions of various ROS to the NO oxidation process, quenching experiments were conducted in the LaCoO_3‑δ/H₂O₂ system, and the results are presented in Figure d. In the absence of quenchers, the NO removal efficiency was 90.3%. Upon the addition of FFA, TBA, and BQ, the NO removal efficiency decreased by 39.8% (20.9% for ¹O₂), 18.9% (^•OH), and 15.7% (^•O₂ ^–), respectively. The contribution of ^•O₂ ^– (15.7%) was deemed unreliable owing to its relatively low oxidation capacity, and it was hypothesized that the formation of ¹O₂ was restricted because ^•O₂ ^–, acting as an intermediate in the generation of ¹O₂, being quenched by BQ. Consequently, an EPR test was conducted to measure ¹O₂ in the LaCoO_3‑δ/H₂O₂/BQ system to further verify this hypothesis. As shown in Figure e, the signal intensity of ¹O₂ diminished upon the introduction of BQ, indicating that the quenching of ^•O₂ ^– resulted in a reduction in the generation of ¹O₂, thereby confirming that ^•O₂ ^– served as an intermediate in the formation of ¹O₂. Therefore, the cooperation of ¹O₂ and ^•OH in the LaCoO_3‑δ/H₂O₂ system played a significant role in the oxidation of NO. Furthermore, it is important to investigate the SO₂ oxidation process. Radical quenching experiments during SO₂ oxidation in the LaCoO_3‑δ/H₂O₂ system were conducted under both NO-containing and NO-free conditions. As shown in Figure f, under NO-free conditions, the addition of FFA and TBA led to a reduction in the SO₂ oxidation efficiency by 9.9% and 9.2%, respectively. Under NO-containing conditions, the corresponding reductions were 6.4% and 6.7%, respectively. In addition, the introduction of BQ exerts a negligible influence on the oxidation rate of SO₂. These results indicate that ¹O₂ and ^•O₂ ^– play a negligible role in the oxidation of SO₂, whereas ^•OH and H₂O₂ are identified as the predominant oxidative species responsible for SO₂ oxidation.

7.

(a) EPR spectra of the detected TEMP-¹O₂ in H₂O₂, Co₃O₄/H₂O₂, and LaCoO_3‑δ/H₂O₂ systems; (b) EPR spectra of the detected DMPO–OH in H₂O₂, Co₃O₄/H₂O₂, and LaCoO_3‑δ/H₂O₂ systems; (c) EPR spectra of the detected DMPO-O₂ ^– in H₂O₂, Co₃O₄/H₂O₂, and LaCoO_3‑δ/H₂O₂ systems; (d) radical quenching experiments during NO removal for the LaCoO_3‑δ/H₂O₂ system; (e) EPR spectra of the detected TEMP-¹O₂ in H₂O₂, LaCoO_3‑δ/H₂O₂, and LaCoO_3‑δ/H₂O₂/BQ systems; (f) radical quenching experiments during SO₂ oxidation for the LaCoO_3‑δ/H₂O₂ system; (g) the La 3d XPS spectra of fresh and spent LaCoO_3‑δ; (h) the Co 2p XPS spectra of fresh and spent LaCoO_3‑δ; and (i) the O 1s XPS spectra of fresh and spent LaCoO_3‑δ.

To investigate the generation mechanisms of ROS and the catalytic reactions occurring in the LaCoO_3‑δ/H₂O₂ system, XPS analysis was performed on the La 3d, Co 2p, and O 1s peaks for both fresh and spent LaCoO_3‑δ samples. In Figure g, La 3d_5/2 and La 3d_3/2 peaks were observed at binding energies of 834.0–838.3 eV and 850.8–855.1 eV, respectively. The spin–orbit splitting of the La 3d level was measured at 16.8 eV, which is consistent with the values of pure La₂O₃, demonstrating the presence of stable La³⁺ in the fresh and spent LaCoO_3‑δ samples, indicating La³⁺ was not involved in the catalytic reaction. As shown in Figure h, the Co 2p spectrum of fresh LaCoO_3‑δ exhibits two cobalt surface cobalt species. The binding energies of peaks at 779.8 and 795.0 eV were corresponded to Co³⁺, while those at 782.0 and 797.2 eV were assigned to Co²⁺. Quantitative analysis revealed that the molar ratio of surface Co³⁺/Co²⁺ was 67.6%/32.4%, suggesting a relatively high Co³⁺ content in the fresh LaCoO_3‑δ samples. The formation of Co²⁺ in the perovskite structure is primarily attributed to the generation of OV to ensure charge balance. , Following the reaction between LaCoO_3‑δ and H₂O₂, the Co³⁺ content decreased, while the Co²⁺ content increased, as evidenced by the binding energy of the Co³⁺ peak shifting to a higher value (from 779.8 to 780.3 eV for Co 2p3/2) resulting from a decrease of electron cloud density on the Co atom. These findings indicate that the Co³⁺/Co²⁺ redox pair plays a critical role in the catalytic activation of H₂O₂. , In Figure i, the O 1s spectra of both fresh and spent LaCoO_3‑δ samples displayed three predominant oxygen species: lattice oxygen O_lat (O^2–, 528.9 or 529.3 eV), absorbed H₂O or surface hydroxyl (O_I, 532.9 eV), and surface-chemisorbed oxygen (O_II, 531.2 eV) originating from OV. ,, In fresh LaCoO_3‑δ, the relatively low content of O_I (8%) indicated a limited presence of water molecules or surface hydroxyl groups, whereas the high level of O_II (51%) suggested an abundance of electron-donating centers attributed to substantial OV. After the reaction, the contents of O_II and O_lat decreased by 13% and 7%, respectively. Moreover, the binding energy of O_lat shifted to a higher value, suggesting a decrease in electron density on the O atom. In contrast, the O_I content increased by 20%, indicating that electron-rich structures (originating from OV) and lattice oxygen (O^2–) are involved in the catalytic decomposition of H₂O₂ and form surface hydroxyl groups or water molecules.

Thus, the potential ROS generation mechanisms are proposed as follows (Figure a): the cyclic redox pair of surface Co²⁺/Co³⁺ can catalyze H₂O₂ to generate ^•OH (eq ) and ^•O₂ ^– (eq ). ,, The abundant surface Co³⁺ species can further react with intermediate ^•O₂ ^– to form ¹O₂ (eq ). The presence of electron-rich OV (OV­(e^–)) enhanced the electron transfer capability and facilitated the transformation between surface Co³⁺ and Co²⁺ (eq ). , Additionally, OV reduces the stability of O^2– and promote its reactivity, enabling it to react with ^•O₂ ^– to generate ¹O₂ (eq ). Moreover, the electron-rich centers on the surface can donate electrons to H₂O₂, thereby generating more ^•OH (eq ).

$${\mathrm{Co}}_{\mathrm{surface}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Co}}_{\mathrm{surface}}^{3+}+{}^{•}\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}$$ 4

$${\mathrm{Co}}_{\mathrm{surface}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Co}}_{\mathrm{surface}}^{2+}+{}^{•}{\mathrm{O}}_2^{-}+2{\mathrm{H}}^{+}$$ 5

$${\mathrm{Co}}_{\mathrm{surface}}^{3+}+{}^{•}{\mathrm{O}}_2^{-}\to {\mathrm{Co}}_{\mathrm{surface}}^{2+}+{}^1{\mathrm{O}}_2$$ 6

$$\mathrm{OV}({\mathrm{e}}^{-})+{\mathrm{Co}}_{\mathrm{surface}}^{3+}\rightleftharpoons \mathrm{OV}+{\mathrm{Co}}_{\mathrm{surface}}^{2+}$$ 7

$${\mathrm{O}}^{2-}+\mathrm{OV}+{}^{•}{\mathrm{O}}_2^{-}\to \mathrm{OV}({}^{*}{\mathrm{O}}_2^{-})+{}^{•}{\mathrm{O}}_2^{-}\to {\mathrm{O}}^{2-}+\mathrm{OV}({\mathrm{e}}^{-}){+}^1{\mathrm{O}}_2$$ 8

$$\mathrm{OV}({\mathrm{e}}^{-})+{\mathrm{H}}_2{\mathrm{O}}_2\to \mathrm{OV}(\mathrm{O}\mathrm{H})+{}^{•}\mathrm{O}\mathrm{H}$$ 9

8.

(a) Potential ROS generation mechanism in the LaCoO_3‑δ/H₂O₂ system and (b) products analysis and N/S balances.

3.4. Deep Oxidation Removal Pathways of NO and SO₂

To investigate the reaction pathways of NO and SO₂ in both the oxidation and absorption systems, the oxidation products from the LaCoO_3‑δ/H₂O₂ catalytic system and the absorption products from the Na₂SO₃/NaOH absorption system were analyzed by using exhaust gas detection and IC technology, respectively. Additionally, mass balance analysis was performed for N- and S-containing species, as presented in Figure b. The results demonstrated that the primary oxidation products of NO were NO₂ (44.7%), NO₂ ^– (12.2%), and NO₃ ^– (43.1%), demonstrating a higher ratio of (NO₂ + NO₃ ^–) compared to previous studies which reported that ^•OH was the predominant ROS in the Fenton-like system, , indicating that the synergistic effect of ¹O₂ and ^•OH can lead to a more extensive oxidation of NO compared to ^•OH acting alone. Following the absorption treatment, the predominant absorption products were NO₃ ^– (62.3%) and NO₂ ^– (37.7%). The relative errors in the N balance during the oxidation and absorption processes were 9.8% and 6.8%, respectively. Regarding SO₂, approximately 17.6% was oxidized to SO₄ ^2– during the oxidation process, while the remaining 83.4% was fully removed and converted to SO₄ ^2– during the absorption process. The relative errors in the S-element balance for the two processes were 7.7% and 6.6%, respectively. These findings suggest that within this oxidation–absorption reaction system, NO and SO₂ are effectively removed with the efficient transformation of their respective products. Based on the above analysis and relevant refs , , , – , and , this study proposes the following possible reaction pathways for the simultaneous removal of NO and SO₂. During the oxidation stage, the majority of NO was efficiently oxidized to NO₂, NO₂ ^–, and NO₃ ^– via reactions with H₂O₂, ^•OH, and ¹O₂. A minor fraction of SO₂ competes with NO for active oxidative species, leading to its oxidation to SO₄ ^2–. In the absorption stage, residuals of NO₂ and SO₂ were absorbed by Na₂SO₃ and NaOH, thereby generating NO₂ ^– and SO₃ ^2–. Finally, NO₂ ^– and SO₃ ^2– can be oxidized by the O₂ to generate NO₃ ^– and SO₄ ^2–.

Oxidation process

$$2\mathrm{N}\mathrm{O}+3{\mathrm{H}}_2{\mathrm{O}}_2\to {2\mathrm{H}\mathrm{N}\mathrm{O}}_3+{2\mathrm{H}}_2\mathrm{O}$$ 10

$$\mathrm{N}\mathrm{O}+2{}^{·}\mathrm{O}\mathrm{H}\to {\mathrm{N}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$ 11

$$\mathrm{N}\mathrm{O}+{}^{·}\mathrm{O}\mathrm{H}\to {\mathrm{H}\mathrm{N}\mathrm{O}}_2$$ 12

$${2\mathrm{N}\mathrm{O}+}^1{\mathrm{O}}_2\to 2{\mathrm{N}\mathrm{O}}_2$$ 13

$${\mathrm{N}\mathrm{O}}_2+{}^{·}\mathrm{O}\mathrm{H}\to {\mathrm{H}\mathrm{N}\mathrm{O}}_3$$ 14

$$\mathrm{S}{\mathrm{O}}_2+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$$ 15

$$\mathrm{S}{\mathrm{O}}_2+2{}^{•}\mathrm{O}\mathrm{H}\to {\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$$ 16

Absorption process

$$\mathrm{N}{\mathrm{O}}_2+{\mathrm{S}{\mathrm{O}}_3}^{2-}+{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{S}{\mathrm{O}}_4}^{2-}+{\mathrm{N}{\mathrm{O}}_2}^{-}+{\mathrm{H}}_2\mathrm{O}$$ 17

$$\mathrm{N}{\mathrm{O}}_2+{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{N}{\mathrm{O}}_2}^{-}+{\mathrm{H}}_2\mathrm{O}$$ 18

$$\mathrm{S}{\mathrm{O}}_2+2{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{S}{\mathrm{O}}_3}^{2-}+{\mathrm{H}}_2\mathrm{O}$$ 19

$${\mathrm{O}}_2+{\mathrm{S}{\mathrm{O}}_3}^{2-}+{\mathrm{N}\mathrm{O}}_2^{-}\to {\mathrm{S}{\mathrm{O}}_4}^{2-}+{\mathrm{N}{\mathrm{O}}_3}^{-}$$ 20

4. Conclusions

An innovative LaCoO_3‑δ/H₂O₂ Fenton-like process was systematically investigated for the simultaneous removal of NO and SO₂ in the present study. Based on the experimental results and characterization analysis, the following conclusions were drawn:

• (1)Under the optimum operating conditions (molar ratio of H₂O₂ to NO, 3; LaCoO_3‑δ dosage, 0.15 g; GHSV, 150,000 h^–1; catalytic reaction temperature: 150 °C; and H₂O₂ pH, 5.3), 91.2% of NO and 100% of SO₂ can be simultaneously removed, with energy consumption rates of 7.61 × 10^–3 kWh/g (NO) and 1.74 × 10^–3 kWh/g (SO₂), showing a promising prospect for industrial application prospect. After undergoing ten cycles and 18 h of continuous operation, the denitrification efficiency of the system decreased by 0.7% and 0.9%, respectively, which demonstrates the exceptional stability of LaCoO_3‑δ.
• (2)The surface Co³⁺/Co²⁺ redox pairs and abundant OV in the LaCoO_3‑δ catalyst serve as the primary active sites for activating H₂O₂ to generate ROS (^•OH, ^•O₂ ^–, and ¹O₂). Among these, ^•OH and ¹O₂ are the predominant ROS responsible for the deep oxidation of NO.
• (3)NO₂ (44.7%) and NO₃ ^– (43.1%) were identified as the primary oxidation products, whereas NO₂ ^– (37.7%), NO₃ ^– (62.3%), and SO₄ ^2– (100%) were the main absorption products. Furthermore, a good N/S balance suggests that both the LaCoO_3‑δ/H₂O₂ advanced oxidation system and the Na₂SO₃/NaOH absorption system exhibit high efficiency and stability.
• (4)This study proposes a novel strategy for the effective utilization of H₂O₂ in Fenton-like advanced oxidation processes by the co-operation of ¹O₂ and ^•OH and offers sustainable implications for the development of simultaneous denitrification and desulfurization technologies.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c04716.

• XRD analysis of dry gels prior to calcination; FTIR spectroscopy of LaCoO_3‑δ and its dry gel precursor; adsorption/desorption isotherms of LaCoO_3‑δ samples; XRD patterns of fresh and spent LaCoO_3‑δ samples; pH measurement of precursor solution; average crystallite size of LaCoO_3‑δ, La₂O₃, and Co₃O₄ from XRD; semiquantitative comparison results of EPR signals in the LaCoO_3‑δ/H₂O₂ and Co₃O₄/H₂O₂ systems; calculation of energy consumption rates for simultaneous removal of NO and SO₂; experimental conditions in this study; and comparative analysis of the present work and representative Fe-based Fenton-like systems (PDF)

Xingzhou Mao: Methodology, validation, formal analysis, and writingoriginal draft. Yujia Wang: Methodology, validation, and investigation. Zhipeng Ma: Methodology and investigation. Runlong Hao: Conceptualization, writingreview and editing, and supervision. Dong Fu: Conceptualization and supervision. Bo Yuan: Conceptualization, resources, writingreview and editing, and project administration. Yi Zhao: Conceptualization, resources, writingreview and editing, supervision, and project administration.

Natural Science Foundation of Hebei Province (No. E2024502082 and E2021502002) and Hebei Province Youth Talent Support Program (BJK2023089)

The authors declare no competing financial interest.