Mechanisms and reaction pathways for simultaneous oxidation of NOx and SO₂ by ozone determined by in situ IR measurements

Lab- and pilot-scale wet scrubber study on the redox-mediated simultaneous removal of NOx and SO2 using a CaCO3-based slurry with KI as a redox catalyst

This study presents a novel approach that integrates ozone-driven chemical oxidation to convert NO into soluble NO2, followed by the simultaneous absorption of NO2 and SO2 into a CaCO3-based slurry using the redox catalyst potassium iodide (KI). Using cyclic voltammetry, we demonstrate the redox properties of the I2/2I- couple, which facilitates NO2 reduction into soluble NO2- and catalyst regeneration through sulfite (SO32-)-driven reduction, thus establishing a closed catalytic cycle within the components of flue gas. In lab-scale wet-scrubbing tests, we explore the effect of various operational parameters (i.e., KI concentration, pH, and SO2 concentration), with a 15 h stability test demonstrating >60% NOx and >99% SO2 removal efficiency when the pH is controlled between 7.5 and 8.5. A successful pilot-scale implementation conducted at an inlet flow rate of 1000 m3 h-1 further confirmed the reproducibility of the proposed redox-catalytic cycle. Our study offers a cost-effective, sustainable, and scalable solution for effectively mitigating NOx and SO2 emissions at low temperatures.

Enhanced NOx absorption in flue gas by wet oxidation of red mud and phosphorus sludge

The environmental problem caused by industrial emissions of NOx has been studied in the past dacades. In this study, red mud coupling with phosphorus sludge were used to enhance the solution to absorb NOx from the flue gas. Firstly, red mud reacted with the binder silicic acid in the phosphorus sludge, destroying the emulsion structure of the phosphorus sludge. Then, the P4 in the phosphorus sludge is completely released, and the P4 reacted with O2 in the flue gas to produce O3 and O. NO and NO2 contained in the flue gas reacted with the active O and O3 to produce high-valent NOx, such as NO3, N2O5. At last, the mixed slurry of red mud and phosphorus sludge absorbed the high-valent NOx, resulting in the formation of Ca5(PO4)3F along with HNO3. Using phosphorus sludge to produce O3 in the reaction process can reduce the production cost of O3 and achieve waste utilization. Meanwhile, the interaction between red mud and phosphorus sludge can promote phosphorus sludge to produce O3 and remove F- from phosphorus sludge, as well as avoid the problem of secondary pollution. This study should be helpful for red mud and phosphorus sludge utilization and flue gas denitration.

NO Selective Catalytic Reduction over Atom-Pair Active Sites Accelerated via In Situ NO Oxidation

Selective catalytic reduction (SCR) of NO_x with NH₃ is the most efficient technology for NO_x emissions control, but the activity of catalysts decreases exponentially with the decrease in reaction temperature, hindering the application of the technology in low-temperature SCR to treat industrial stack gases. Here, we present an industrially practicable technology to significantly enhance the SCR activity at low temperatures (<250 °C). By introducing an appropriate amount of O₃ into the simulated stack gas, we find that O₃ can stoichiometrically oxidize NO to generate NO₂, which enables NO reduction to follow the fast SCR mechanism so as to accelerate SCR at low temperatures, and, in particular, an increase in SCR rate by more than four times is observed over atom-pair V₁-W₁ active sites supported on TiO₂(001) at 200 °C. Using operando SCR tests and in situ diffuse reflectance infrared Fourier transform spectra, we reveal that the introduction of O₃ allows SCR to proceed along a NH₄NO₃-mediated Langmuir-Hinshelwood model, in which the adsorbed nitrate species speed up the re-oxidation of the catalytic sites that is the rate-limiting step of SCR, thus leading to the enhancement of activity at low temperatures. This technology could be applicable in the real stack gas conditions because O₃ exclusively oxidizes NO even in the co-presence of SO₂ and H₂O, which provides a general strategy to improve low-temperature SCR efficacy from another perspective beyond designing catalysts.

A multifunctional integrated carbon nanotubes/polyphenylene sulfide composite: preparation, properties and applications

Although great progress has been achieved in polyphenylene sulfide (PPS) composites by the use of carbon nanotubes (CNTs), the development of cost-efficient, well dispersive and multifunctional integrated PPS composites has yet to be achieved because of the strong solvent resistance of PPS. In this work, a CNTs-PPS/PVA composite material has been prepared by mucus dispersion-annealing, which employed polyvinyl alcohol (PVA) to disperse PPS particles and CNTs at room temperature. Dispersion and scanning electron microscopy observations revealed that PVA mucus can uniformly suspend and disperse micron-sized PPS particles, promoting the interpenetration of the micro-nano scale between PPS and CNTs. During the annealing process, PPS particles deformed and then crosslinked with CNTs and PVA to form a CNTs-PPS/PVA composite. The as-prepared CNTs-PPS/PVA composite possesses outstanding versatility, including excellent heat stability with resistant temperatures up to 350 °C, corrosion resistance against strong acids and alkalis for up to 30 days, and distinguished electrical conductivity with 2941 S m-1. Besides, a well-dispersed CNTs-PPS/PVA suspension could be used to 3D print microcircuits. Hence, such multifunctional integrated composites will be highly promising in the future of new materials. This research also develops a simple and meaningful method to construct composites for solvent resistant polymers.

Purification Technologies for NOx Removal from Flue Gas: A Review

Nitrogen oxide (NOx) is a major gaseous pollutant in flue gases from power plants, industrial processes, and waste incineration that can have adverse impacts on the environment and human health. Many denitrification (de-NOx) technologies have been developed to reduce NOx emissions in the past several decades. This paper provides a review of the recent literature on NOx post-combustion purification methods with different reagents. From the perspective of changes in the valence of nitrogen (N), purification technologies against NOx in flue gas are classified into three approaches: oxidation, reduction, and adsorption/absorption. The removal processes, mechanisms, and influencing factors of each method are systematically reviewed. In addition, the main challenges and potential breakthroughs of each method are discussed in detail and possible directions for future research activities are proposed. This review provides a fundamental and systematic understanding of the mechanisms of denitrification from flue gas and can help researchers select high-performance and cost-effective methods.

Simultaneous Removal of SO2 and NO x Using Steel Slag Slurry Combined with Ozone Oxidation

In this work, steel slag slurry was used in combination with O3 oxidation for the simultaneous removal of SO2 and NO x in a laboratory-scale wet flue gas desulfurization process. The effects of the oxidation temperature, steel slag concentration, initial SO2 concentration, and pH value on the desulfurization and denitrification efficiencies were studied. The results showed that the highest NO x removal efficiency occurred at an oxidation temperature of 90 °C. With an increase of the oxidation temperature above 90 °C, the denitrification efficiency decreased due to the decomposition of N2O5. The effect of the SO2 concentration on denitrification was complicated. When the concentration of SO2 was 500 ppm, generation of SO3 2- promoted the absorption of NO2. However, higher SO2 concentrations strengthened the competitive absorption of SO2 and NO x . In the pH range of 8.5-4.5, the denitrification efficiency was maintained at about 96%. The component analyses of the aqueous solution and the solid residue were conducted to investigate the compositions of the absorption products. The results showed that NO3 - and SO4 2- were the major anions in the aqueous solution. The nitrogen balance was analyzed to be 95.8%, clearly illustrating the migration and transformation path of nitrogen. In the solid residue, most alkaline substances were consumed, and the final products were mainly CaSO4 and FeO. Accordingly, the reaction mechanism of simultaneous desulfurization and denitrification using steel slag combined with ozone oxidation was proposed.

O3 oxidation combined with semi-dry method for simultaneous desulfurization and denitrification of sintering/pelletizing flue gas

With the vigorous development of China's iron and steel industry and the introduction of ultra-low emission policies, the emission of pollutants such as SO₂ and NO_x has received unprecedented attention. Considering the increase of the proportion of semi-dry desulfurization technology in the desulfurization process, several semi-dry desulphurization technologies such as flue gas circulating fluidized bed (CFB), dense flow absorber (DFA) and spray drying absorption (SDA) are briefly summarized. Moreover, a method for simultaneous treatment of SO₂ and NO_x in sintering/pelletizing flue gas by O₃ oxidation combined with semi-dry method is introduced. Meantime, the effects of key parameters such as O₃/NO molar ratio, CaSO₃, SO₂, reaction temperature, Ca/(S+2N) molar ratio, droplet size and approach to adiabatic saturation temperature (AAST) on denitrification and desulfurization are analyzed. Furthermore, the reaction mechanism of denitrification and desulfurization is further elucidated. Finally, the advantages and development prospects of the new technology are proposed.

Review on the NO removal from flue gas by oxidation methods

Due to the increasingly strict emission standards of NOx on various industries, many traditional flue gas treatment methods have been gradually improved. Except for selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) methods to remove NOx from flue gas, theoxidation method is paying more attention to NOx removal now because of the potential to simultaneously remove multiple pollutants from flue gas. This paper summarizes the efficiency, reaction conditions, effect factors, and reaction mechanism of NO oxidation from the aspects of liquid-phase oxidation, gas-phase oxidation, plasma technology, and catalytic oxidation. The effects of free radicals and active components of catalysts on NO oxidation and the combination of various oxidation methods are discussed in detail. The advantages and disadvantages of different oxidation methods are summarized, and the suggestions for future research on NO oxidation are put forward at the end. The review on the NO removal by oxidation methods can provide new ideas for future studies on the NO removal from flue gas.

Scale-up experiments of SO2 removal and the promoting behavior of NO in moving beds at medium temperatures

The dry flue gas desulfurization (FGD) method was studied, which is a part of the integrated removal of multi-pollutants at medium temperatures. Although dry flue gas treatment is a simple and effective method, it is still a highly empirical-led application technology. A superior desulfurization adsorbent, fine powder of NaHCO₃ (hereinafter called fine NaHCO₃), was selected by scale-up experiments. A deep understanding of the reaction process and mechanism is then explored, which helps the further optimization of dry desulfurization. Based on the multi-factor experiments for NaHCO₃, the effect mechanism of NO on desulfurization using NaHCO₃ is also proposed. The conversion of SO₃²⁻ → SO₄²⁻ is promoted by the existence of NO. Therefore, a slight decline can be found. According to the influences of the SO₂ concentration and the residence time, it is concluded that the diffusion of SO₂ into the channel of NaHCO₃ is the rate-limiting step. Impressively, the reaction process of reactants was clearly studied by in situ FTIR spectroscopy to determine the whole process. Moreover, the recycling of NaHCO₃ is the main direction for reducing adsorbent consumption in the next step. The predictable insights are beneficial for profoundly understanding the gas composition synergetic interaction for the SO₂ removal by the dry treatment using NaHCO₃.

A superior desulfurizer, fine NaHCO₃ was selected by scale-up experiments. A deep understanding of the reaction process and mechanism was explored. The effect mechanism of NO on desulfurization using NaHCO₃ was proposed by in situ FTIR results.

Simultaneous removal of SO2 and NOx with OH from the catalytic decomposition of H2O2 over Fe-Mo mixed oxides

In this paper, the simultaneous removal of SO₂ and NO_x catalyzed by Fe-Mo mixed oxides at varying Mo/Fe atomic ratios was reported for the first time with the aim of reducing H₂O₂ consumption and elucidating the roles of Fe and Mo species in the catalytic process. Fe-Mo mixed oxides with varying Mo/Fe atomic ratios were synthesized and the catalytic performances were systematically studied. The catalyst with Mo/Fe atomic ratio of 2.0 exhibited the highest activity, with which removal efficiencies of 89.4 % for NO_x and 100 % for SO₂ can be attained at extremely low H₂O₂ dosage. Products analysis revealed that SO₂ was mainly removed via wet scrubber, while the adequate oxidation resulting from OH radicals was the prerequisite for NO_x removal. The redox pair of Fe²⁺/Fe³⁺ played a significant role in decomposing H₂O₂, while Mo species had double effect on catalytic activity. Higher Mo content resulted in abundant oxygen vacancies and stronger surface acidity, which favored OH formation. However, the excessive Mo content involved severe surface Mo enrichment and remarkably reduced the active sites of Fe species. The H₂O₂/Fe-Mo catalyst system showed excellent stability and had a promising prospect for simultaneously removing SO₂ and NO_x in coal-fired flue gas.

A Novel Method for Simultaneous Removal of NO and SO2 from Marine Exhaust Gas via In-Site Combination of Ozone Oxidation and Wet Scrubbing Absorption

The stringent international regulations on marine emission abatement have exerted a huge push on the development of marine desulfurization and denitrification technologies. However, for the traditional vessels driven by large two-stroke diesel engines, simultaneous removal of NOx and SO2 is still a big challenge at present. Here, a one-stage ozone oxidation combined with in-situ wet scrubbing for simultaneous removal of NO and SO2 is proposed. A series of experiments were performed based on a bench-scale reaction system. The results showed that in-situ wet scrubbing could effectively decrease flue gas temperature, and then suppress the thermal decomposition of ozone, which was beneficial for improve oxidant utilization. Meanwhile, the in-situ combination of ozone injection and wet scrubbing was in favor of improving the selectivity oxidation of NO over SO2 by ozone, which was possibly due to the high aqueous solubility of SO2 in water. Aiming to reduce the electric power consumption by an ozone generating system, O3/NO molar ratio was kept as low as possible. A complete removal of SO2 and a high NOx removal efficiency could be achieved through the introduction of other oxidative additives in scrubbing solution. This integrated system designed for marine application was of great significance.

Direct conversion of NO and SO2 in flue gas into fertilizer using ammonia and ozone

This study focused on the simultaneous removal of NO and SO₂ from an industrial flue gas stream. To evaluate the removal efficiency of NO and SO₂ using O₃ and NH₃, the consumption of two reactants (O₃ and NH₃) in line with the conversion of NO and SO₂ was quantified experimentally. In addition, NO and SO₂ were converted to valuable fertilizers, NH₄NO₃ and (NH₄)₂SO₄. To identify a principle strategy to enhance the generation of fertilizer, Fourier transform infrared spectroscopy was used to examine the reaction mechanisms for the formation of NH₄NO₃ and (NH₄)₂SO₄. Acceleration of SO₂ oxidation could be achieved effectively by adding NO to a gas mixture of SO₂, NH₃, and O₃. The formation of HNO₃ might be enhanced by the simultaneous feeding of NO and SO₂. Particle generation was also 10 times higher for NH₃/(NO + SO₂) than for NH₃/NO and for NH₃/SO₂, which is a prominent feature of this study. Moreover, the introduction of steam had a positive influence on particle generation. This method offers dual applications for NO and SO₂ removal from a flue gas stream and direct fertilizer generation.

Sensitivity of different parameters for selection of higher plants in urban afforestation: Exposure of Guabiroba (Campomanesia xanthocarpa O. Berg.) to diesel engine exhaust

Urban afforestation can mitigate the effects of air pollution by acting as a sink for atmospheric emissions, but these emissions (e.g., combustion gases from diesel engines) can be a precursor of structural and physiological changes in higher plant species, which could compromise the success of afforestation projects. In this study, Guabiroba (Campomanesia xanthocarpa O. Berg.) plants were exposed in greenhouses to combustion gases emitted by a diesel engine over 120 days, with daily intermittent gas exposure. Every 30 days, leaf injury (chlorosis and necrosis), plant biomass and physiological/biochemical parameters (proteins, chlorophyll and peroxidase enzyme activity) were evaluated. The data obtained were used to construct a hierarchy of the sensitivity (and inversely, of the resistance or tolerance) of this higher plant species to the diesel oil combustion gases: peroxidase > biomass ≈ chlorophyll > protein > leaf injury. Variations in these parameters could be used for the early diagnosis of plant stress or as a marker for stress tolerance in trees. In the first case, a sensitive species could be used for the phytomonitoring of air quality and in the second case the lack of significant variations in these parameters would indicator tolerance of the plant species to air pollution. The results showed that Guabiroba, a plant native to the Atlantic forest, is sensitive to air pollution and could therefore be used for air quality monitoring, since all parameters analyzed were affected by the polluted air.

Efficient SO2 Removal and Highly Synergistic H2O2 Production Based on a Novel Dual-Function Photoelectrocatalytic System

The direct conversion of SO₂ to SO₃ is rather difficult for flue gas desulfurization due to its inert dynamic with high reaction activation energy, and the absorption by wet limestone-gypsum also needs the forced oxidation of O₂ to oxidize sulfite to sulfate, which is necessary for additional aeration. Here, we propose a method to remove SO₂ with highly synergistic H₂O₂ production based on a novel dual-function photoelectrocatalytic (PEC) system in which the jointed spontaneous reaction of desulfurization and H₂O₂ production was integrated instead of nonspontaneous reaction of O₂ to H₂O₂. SO₂ was absorbed by alkali liquor then oxidized quickly into SO₄^2- by a nanorod α-Fe₂O₃ photoanode, which possessed high alkali corrosion resistance and electron transport properties. H₂O₂ was produced simultaneously in the cathode chamber on a gas diffusion electrode and was remarkably boosted by the conversion reaction of SO₃^2- to SO₄^2- in the anode chamber in which the released chemical energy was effectively used to increase H₂O₂. The photocurrent density increased by 40% up to 1.2 mA·cm^-2, and the H₂O₂ evolution rate achieved 58.8 μmol·L^-1·h^-1·cm^-2 with the synergistic treatment of SO₂, which is about five times than that without SO₂. This proposed PEC cell system offers a cost-effective and environmental-benign approach for dual purpose of flue gas desulfurization and simultaneous high-valued H₂O₂ production.

Simultaneous removal of SO2, NO and Hg0 using an enhanced gas phase UV-AOP method

The core for simultaneous removal of SO₂, NO and Hg⁰ is the oxidation of NO and Hg⁰. Radical induced oxidation of NO and Hg⁰ is considered to be the most efficient method. We develop a novel gas phase advanced oxidation process (AOP) of UV-Heat/H₂O₂-NaClO₂ to simultaneously remove SO₂, NO and Hg⁰ due to a great synergism between H₂O₂ and NaClO₂ under thermal and ultraviolet (UV) co-catalysis. The results indicated that the SO₂ removal was always good, while the removal of NO and Hg⁰ was affected by NaClO₂ and UV. Higher catalytic temperature and longer flue gas residence time favored the removal of NO and Hg⁰. The presence of SO₂ and NO facilitated Hg⁰ removal. Kinetics analyses were conducted to provide the reaction rate of removal of NO and Hg⁰ under different conditions. X-ray photoelectron spectroscopy (XPS) revealed the product composition as Cl^-, Hg²⁺, NO₃^- and SO₄^2-. Electron spin resonance (ESR) tests confirmed the generation of HO. Cost analyses demonstrated the better cost performance of the proposed method compared to SCR-ACI combined method. HO and ClO₂ were proved to be the main oxidant. The reaction mechanism for removal of NO and Hg⁰ by using UV-Heat/H₂O₂-NaClO₂ were proposed finally.

Simultaneous Removal of SO2 and NO by O3 Oxidation Combined with Wet Absorption

The effects of ozone concentration, NaOH concentration, type and concentration of additives, initial pH, temperature, and NO and SO₂ concentration on simultaneous removal of NO and SO₂ were studied through ozone oxidation combined with wet absorption. Results indicated that ozone concentration and the type and concentration of additives had the most significant effect on NO removal. The optimal ozone concentration was 250 ppm (NO/NO₂ = 1), and the best additive was KMnO₄. The removal efficiency of NO _x was as high as 97.86% when NO/NO₂ = 1, and the concentration of KMnO₄ was 0.025 mol/L. Considering economic and other factors, the KMnO₄ concentration was selected to be 0.006 mol/L. At this time, the removal efficiencies of NO _x and SO₂ were 81.35 and 100%, respectively. This method has potential application prospects for simultaneous removal of SO₂ and NO in the industrial flue gas.

Radical-induced oxidation removal of multi-air-pollutant: A critical review

Sulfur dioxide (SO₂), nitric oxide (NO) and elemental mercury (Hg⁰) are three common air pollutants in flue gas. SO₂ and NO are the main precursors for chemical smog and Hg⁰ is a bio-toxicant for human. Cooperative removal of multi-air-pollutant in flue gas using radical-induced oxidation reaction is considered as one of the most promising methods due to the high removal efficiency, low cost and less secondary environmental impact. The common radicals used in air pollution control can be classified into four types: (1) hydroxyl radical (OH), (2) sulfate radical (SO₄^-), (3) chlorine-containing radicals (Cl, ClO₂, ClO, HOCl^-, etc.) and (4) ozone. This review summarizes the generation methods and mechanism of the four kinds of radicals, as well as their applications in the removal of multi-air-pollutant in flue gas. The reactivity, selectivity and reaction mechanism of the four kinds of radicals in multi-air-pollutant removal were comprehensively described. Finally, some future research suggestions on the development of new technique for cooperative removal of multi-air-pollutant in flue gas were provided.

Efficient and stable catalyst of α-FeOOH for NO oxidation from coke oven flue gas by the catalytic decomposition of gaseous H2O2

A catalyst α-FeOOH possesses excellent catalytic activity for NO oxidation, and N₂O₅ is first found in NO oxidation products.

Simultaneous removal of NO and SO2 using vacuum ultraviolet light (VUV)/heat/peroxymonosulfate (PMS)

Simultaneous removal process of SO₂ and NO from flue gas using vacuum ultraviolet light (VUV)/heat/peroxymonosulfate (PMS) in a VUV spraying reactor was proposed. The key influencing factors, active species, reaction products and mechanism of SO₂ and NO simultaneous removal were investigated. The results show that vacuum ultraviolet light (185 nm) achieves the highest NO removal efficiency and yield of and under the same test conditions. NO removal is enhanced at higher PMS concentration, light intensity and oxygen concentration, and is inhibited at higher NO concentration, SO₂ concentration and solution pH. Solution temperature has a double impact on NO removal. CO₂ concentration has no obvious effect on NO removal. and produced from VUV-activation of PMS play a leading role in NO removal. O₃ and ·O produced from VUV-activation of O₂ also play an important role in NO removal. SO₂ achieves complete removal under all experimental conditions due to its very high solubility in water and good reactivity. The highest simultaneous removal efficiency of SO₂ and NO reaches 100% and 91.3%, respectively.

Selective denitrification of flue gas by O3 and ethanol mixtures in a duct: Investigation of processes and mechanisms

A novel selective denitrification process, referred as O3-ethanol oxidation method, was developed by injecting O3 and ethanol mixtures into the simulated flue gas duct. The organic radicals, generated through the ethanol oxidation by O3, can oxidize NO into NO2, and finally into important industrial raw, namely, nitrate organics or aqueous nitrate acids. The residual ethanol in the tail can be recycled. The CO3(2-), HCO3(-) and SO2 in the flue gas hardly exhibit any effect on the NOX removal. Compared to the conventional O3 oxidation method, the present method shows higher selective oxidation of NO, higher NO(X) removal and less O3 consumption as well as proves lower initial investment and operating costs with more compact equipment.

Numerical evaluation of the effectiveness of NO2 and N2O5 generation during the NO ozonation process

Wet scrubbing combined with ozone oxidation has become a promising technology for simultaneous removal of SO2 and NOx in exhaust gas. In this paper, a new 20-species, 76-step detailed kinetic mechanism was proposed between O3 and NOx. The concentration of N2O5 was measured using an in-situ IR spectrometer. The numerical evaluation results kept good pace with both the public experiment results and our experiment results. Key reaction parameters for the generation of NO2 and N2O5 during the NO ozonation process were investigated by a numerical simulation method. The effect of temperature on producing NO2 was found to be negligible. To produce NO2, the optimal residence time was 1.25sec and the molar ratio of O3/NO about 1. For the generation of N2O5, the residence time should be about 8sec while the temperature of the exhaust gas should be strictly controlled and the molar ratio of O3/NO about 1.75. This study provided detailed investigations on the reaction parameters of ozonation of NOx by a numerical simulation method, and the results obtained should be helpful for the design and optimization of ozone oxidation combined with the wet flue gas desulfurization methods (WFGD) method for the removal of NOx.

Numerical study of the simultaneous oxidation of NO and SO2 by ozone

This study used two kinetic mechanisms to evaluate the oxidation processes of NO and SO₂ by ozone. The performance of the two models was assessed by comparisons with experimental results from previous studies. The first kinetic mechanism was a combined model developed by the author that consisted of 50 species and 172 reactions. The second mechanism consisted of 23 species and 63 reactions. Simulation results of both of the two models show under predictions compared with experimental data. The results showed that the optimized reaction temperature for NO with O₃ ranged from 100~200 °C. At higher temperatures, O₃ decomposed to O₂ and O, which resulted in a decrease of the NO conversion rate. When the mole ratio of O₃/NO was greater than 1, products with a higher oxidation state (such as NO₃, N₂O₅) were formed. The reactions between O₃ and SO₂ were weak; as such, it was difficult for O₃ to oxidize SO₂.