Degradation of sulfuryl fluoride by dielectric barrier discharge synergistically with reactive gas Open Access

Sulfuryl fluoride (SO₂F₂) is a synthetic, inorganic, colorless, and odorless compound that is widely used as a fumigant as well as an insecticide and has a trend to gradually replace traditional fumigants such as aluminum phosphide, methyl bromide, and phosphine because of its ability to effectively kill insects and rodents, low boiling point, high vapor pressure, and stable physical and chemical properties.^1,2 However, SO₂F₂ is a toxic gas, where itself and its decomposition products are harmful to the human nervous and respiratory systems.^3,4 On the other hand, SO₂F₂ is a greenhouse gas with a global warming potential (GWP) 4780 times greater than that of carbon dioxide (CO₂).⁵ The global emission of SO₂F₂ has increased over the last 40 years, from 500 tons/year in 1980 to 2900 tons/year in 2019, and it is still increasing every year, which poses a huge potential threat to the global atmosphere.⁶ The abatement of SO₂F₂ should be taken into consideration in today’s urgent situation of global climate change. However, SO₂F₂ can only be purged after use to avoid harm to humans, and the effective emission reduction methods are in shortage.⁷ In recent years, methods based on physical adsorption, chemical absorption, and plasma discharge degradation were studied for SO₂F₂ harmless treatment. Archer et al. found that SO₂F₂ could be absorbed by physical adsorbents such as Al₂O₃ and nitrated St-DVB porous microspheres,^8,9 but the complex preparation process and small adsorption capacity limit the application. Nie et al. used NaOH solution to chemically absorb SO₂F₂ and found that SO₂F₂ can be removed when the inlet SO₂F₂ volume concentration, spray density, gas flow rate, and NaOH concentration were 0.50%, 12 m³/(m²h), 0.12 m³/h, and 0.68 mol/l, respectively.¹⁰ 

Non-thermal plasma (NTP) is now a research hotpot in eco-friendly pollution control areas.¹¹ There are many chemically reactive species, such as radicals, ions, and excited states, in NTP systems, which are not available in conventional chemical reactors.¹² Under the electric field, ionization, dissociation, and Penning dissociation would be excited by high energy particles colliding with each other or ground state molecules and bombarded by electrons. All of these physical and chemical reactions can be realized in NTP at a low temperature.¹³ NTP can be generated in various ways, such as microwave discharge, corona discharge, gliding arc discharge, dielectric barrier discharge (DBD), etc.^14–19 Compared with other methods, DBD technology has the advantages of simple configuration, stable discharge, and cheap power supply, which is widely used in VOC gas treatment,¹⁶ CO₂ conversion,¹⁷ and SO₂F₂ degradation.²⁰ SO₂F₂ is also one of the main decomposition products in SF₆ degradation, which is a major problem in the application of harmless SF₆ degradation due to its stable physicochemical properties.²¹ 

Research proved that NTP technology can be used in the degradation of SO₂F₂. Nie et al. studied the effects of energy density, initial SO₂F₂ concentration, gas residence time, and packing material on the destruction and removal efficiency (DRE) of SO₂F₂ in a packed-bed DBD system. They found that SO₂F₂ could be removed completely when the initial volume concentration, energy density, and residence time were 0.5%, 33.9 kJ/l, and 5.1 s, respectively. The major decomposition products were SO₂, SiF₄, and solid S.²² However, the initial concentration of SO₂F₂ in Ref. 22 was relatively low, ranging from 0.4% to 0.9%, and the degradation products were not quantified and analyzed. Considering that higher concentrations in the actual application and treatment of the degradation products are required, relevant studies are necessary. Research about DBD synergistic with reactive gas shows that gases can effectively promote degradation and regulate the degradation products.^13,23,24 However, now, the effect of reactive gases on the degradation of SO₂F₂ in the NTP system is still unclear.

Furthermore, the addition of additional gas can effectively change the decomposition path. This research group explored the degradation effect of additional H₂O and NH₃ on SF₆ and found that the addition of H₂O can initiate the formation of SO₂.²⁵ The addition of NH₃ produces a solid product.²¹ The addition of external gas can participate in the decomposition reaction, combine with the decomposition intermediates, effectively change the decomposition path of the waste gas, can make the waste gas efficient and have a harmless degradation rate, and can produce some available products. It is necessary to explore the decomposition effect of external gas on SO₂F₂ and analyze the decomposition path. It provides a theoretical basis for the degradation of SO₂F₂.

Therefore, in the current work, the SO₂F₂ degradation process with different additional reactive gases was studied. The SO₂F₂ degradation rate and energy yield, degradation product types, and concentrations were analyzed under three conditions with no additional reactive gas, additional H₂O, and additional H₂. The results can provide experimental and theoretical support for the harmless treatment of SO₂F₂ gas by DBD methods.

The DBD experimental platform consists of a gas distribution section, a reaction section, and a chemical detection section, as shown in Fig. 1.

The gas distribution section mainly consists of 10% SO₂F₂, 99.99% Ar, 3.99% H₂ standard gases (Newradar Gas Co. Ltd., China) and a gas distributor. Ar was chosen as the background gas for SO₂F₂ degradation because it can exhibit higher average electron energy and electron density in the DBD discharge system than He, N₂, and air,²⁶ and the prices of Ar, compressed air, and N₂ are relatively low. SO₂F₂ gas was prepared by a gas distributor (GC500, Tunkon Electrical Technology Co. Ltd., China) with a maximum ratio of 300:1 and a precision of ±1% F.S. The gas distributor was operated in one atmosphere. The SO₂F₂ mixed gas carries H₂O into the reactor through the precision water-gas generator (FD-WG, Friend Experimental Equipment Co. Ltd., China), and the volume fraction of H₂O mixed in was detected with a mirror dew point meter. In addition, the gas path was connected to a Teflon pipe containing several strong corrosive gases produced by SO₂F₂ degradation, such as SO₂, SOF₂, and HF.

The reaction section mainly consists of a DBD reactor and AC plasma power. The reactor is a double-layer quartz dielectric coaxial cylindrical reactor. The outer quartz tube has an outer diameter of 25 mm and a thickness of 2.5 mm, while the inner tube has an outer diameter of 8 mm and a thickness of 2 mm, which is wrapped by the inner electrode. The inner electrode is a copper rod, and the outer electrode is a stainless-steel mesh wrapped around the outer surface of the outer tube, creating a discharge area of ∼20 mm in length and 52 cm³ in volume. The AC plasma power supply (CTP-2000K, Suman Co. Ltd., China) can output voltages of up to 30 kV with a frequency modulation range of 1–100 kHz. The power supply panel is able to display the power supplied to the overall circuitry, which is described as input power in the current work. The discharge power is the power consumed by the reactor of SO₂F₂ degradation, as calculated by the Lissajous figure.

The detection section mainly includes a Gas Chromatograph (GC), optical emission spectrometer (OES), Fourier transform infrared spectrometer (FTIR), and Gas Chromatography-Mass Spectrometer (GC/MS). The volume fraction of SO₂F₂ before and after the degradation reaction is measured by the GC (GC500, Tunkon Co. Ltd., China). The active particle species in the plasma discharge region during the degradation were detected by the OES (MX2500+, Ocean Optics Co. Ltd., USA), which can assist in inferring the reaction process. The SO₂F₂ degradation products were detected qualitatively by the FTIR (iS50, Thermo Co. Ltd., USA) with a detection range of 400–1600 cm⁻¹, a detection accuracy of 1 mm, and a light range length of 10 cm for the selected gas light range cell. The quantification of SO₂F₂ decomposition products was detected by the GC/MS (QP2020-NX, SHIMADZU Co. Ltd., China).

Figure 2 shows the DRE and EY of different SO₂F₂ concentrations without additional reactive gases. The DRE decreased significantly from 81.2% to 40.48% as the initial SO₂F₂ concentration increases from 1% to 3%. The reason mainly was that as the SO₂F₂ concentration increases, too many SO₂F₂ molecules enter the discharge system but could not be degraded under the limited degradation capacity.

The EY first increases with the concentration of SO₂F₂ and reaches a maximum value of 10.78 g/kW h when the concentration is 2%. However, as the concentration increases further, it decreased slowly to 10.28 g/kW h, which may be due to the strong electronegativity of the F atoms produced by the decomposition of SO₂F₂. When the concentration of SO₂F₂ was low, the inhibiting effect on the DBD discharge was not obvious. However, when the SO₂F₂ concentration is 1%–2%, a relatively larger number of SO₂F₂ molecules decomposed in the discharge region, resulting in a significant increase in energy yield. As the SO₂F₂ concentration increased, the inhibition of the discharge by SO₂F₂ molecules increased, resulting in a decrease in EY. The phenomena proved that the concentration of SO₂F₂ is an important factor in the degradation effect.

Figure 3 shows the DRE and EY of different SO₂F₂ gas flow rates. As the SO₂F₂ gas flow rate increased, the DRE gradually decreases, and the EY gradually increases. The reason is that the gas flow rate mainly affects the residence time of SO₂F₂ molecules in the discharge system. When the gas flow rate was too fast, the gas residence time was too short; in addition, some SO₂F₂ molecules left the discharge area directly without degradation while the EY gradually increased, indicating that more SO₂F₂ molecules were degraded overall.

As shown in Fig. 3(b), the EY of 3% SO₂F₂ is slightly higher than that of 2% under a low gas flow rate and slightly lower than that of 2% when the gas flow rate is high. The reason is that when the gas flow rate is low, the degradation of SO₂F₂ molecules makes the concentration of SO₂F₂ in the discharge system decreased, and the inhibition effect on the discharge of the DBD system was not obvious. Compared to the 2% SO₂F₂/98% Ar system, more SO₂F₂ molecules degrade in the 3% SO₂F₂/97% Ar system, showing a higher EY. However, under higher gas flow rates, the gas residence time was significantly short, the inhibition of effect of the 3% SO₂F₂/97% Ar system on the DBD system was slightly stronger than that of the 2% SO₂F₂/98% Ar system, and the EY is less.

Figure 6 shows the optimal EY for different SO₂F₂ mixtures and the comparison with the study by Nie et al.^20,22 Compared with Refs. 20 and 22, the EY of SO₂F₂ degradation was significantly higher in this work. This indicates that the stronger coordination between the input power and the reactor in this experiment was able to create a stronger discharge area in the reactor; the reason may be that the volume of the discharge area in this work is 52 cm³, which is much smaller than the 285 cm³ in the study by Nie. As shown in Fig. 6, with the addition of H₂O and H₂, the type and number of reactive particles increase significantly, which facilitated the degradation process of SO₂F₂, resulting in a further increase in EY.

Figure 7 shows the comparison of the emission spectrum of the 2% SO₂F₂/1% H₂O/97% Ar system and the 2% SO₂F₂/1% H₂/97% Ar system. The characteristic peaks in both systems are mainly distributed between 300 and 400 nm and 600–800 nm, and the intensity of the 600–800 nm characteristic peak ix significantly higher than that of the 300–400 nm characteristic peak. Referring to the NIST spectral line database,²⁹ the spectral peaks of the two systems are shown in Figs. 7(a) and 7(b), respectively, and the relatively high intensity of the spectral lines is mainly concentrated in Ar and F elements. Therefore, the 2% SO₂F₂/1% H₂O/97% Ar system had a higher electron energy density and a greater contribution than the 2% SO₂F₂/1% H₂/97% Ar system, resulting in a higher DRE.

As shown in Fig. 7, OH and H radicals are observed between 300 and 400 nm for both systems. For the 2% SO₂F₂/1% H₂O/97% Ar system, the OH and H radicals mainly come from the ionization of the H₂O molecule. In the 2% SO₂F₂/1% H₂/97% Ar system, OH radicals are generated by the combination of H radicals and O radicals that are produced by the bond breaking of SO₂F₂ and the decomposition of H₂O molecules produced by reactions (10)–(12).

When the concentration of H₂ is low, the energy generated by the plasma source could support the bond breakage of H₂ and SO₂F₂ molecules as well as the ionization of a certain amount of H₂O molecules, which could provide OH radicals for the reaction system and promote the degradation of SO₂F₂. When the concentration of H₂ reaches 1%, the decomposition of H₂ and H₂O molecules and degradation of SO₂F₂ reached a relative equilibrium and occupied all the energy generated by the plasma source; thus, the DRE and EY are at their maximum. However, as the concentration of H₂ increased, the concentration of H₂O generated in the system may be excessive, and the decomposition of H₂ and H₂O consumed too many high-energy electrons, which hindered the degradation of SO₂F₂, thus leading to a decrease in DRE.

In this work, the SO₂F₂ degradation products were detected qualitatively by the FTIR. Figure 8 shows the FTIR spectrum of the SO₂F₂ degradation products for the 2% SO₂F₂/1% H₂O/97% Ar system and the 2% SO₂F₂/1% H₂/97% Ar system; the peak information was obtained from the US NIST database³⁰ and the study by Kurte R et al.³¹ The mainly gas products of both H₂O and H₂ systems are SOF₂, SO₂, HF, and SiF₄, which are similar to the results of the study by Nie et al. (SO₂, HF, and SiF₄),²² indicating that the addition of H₂O or H₂ would not affect the main gas degradation product species of SO₂F₂. In addition, the production of Si in SiF₄ indicates that HF from the decomposition of SO₂F₂ caused an etching reaction on the reactor tube wall. In addition, the absorbance of the SiF₄ peak was significantly higher in the H₂O system than in the H₂ system, which means that the etching was more intense in the H₂O system.

In addition to the gaseous products from the degradation of SO₂F₂, a yellowish solid could be observed on the inner wall of the reaction tube during the H₂ system, as shown in Fig. 9. As the H₂ concentration increased, the yellowish solid becomes more apparent. The yellowish solid was examined by x-ray photoelectron spectroscopy (XPS). According to the results, its main element composition is the S element, which indicates that a part of the SO₂F₂ molecule was completely decomposed in the 2% SO₂F₂/1% H₂/97% Ar system and reduced to form the monomer S solid.

In order to investigate the formation of SO₂F₂ decomposition products, the content of SOF₂, SO₂, and H₂S was quantified using the GC/MS in this paper. Figure 10 shows the gas product proportions of SO₂F₂ degradation with the input power when there is no additional reactive gas. The main 2% SO₂F₂/98% Ar system degradation products are SOF₂ and SO₂. The proportion of SO₂ increases with the input power, while the proportion of SOF₂ gradually decreased. On the other hand, the SOF₂ proportion decreases with the SO₂F₂ DRE, proving that the enhanced discharge process promotes the decomposition of SO₂F₂ molecules.

Figure 11 shows the proportion of SOF₂, SO₂, and H₂S gas products of different additional gas concentrations. As shown in Fig. 11(a), with the increase in H₂O concentration, the proportion of SOF₂ decreased from 25.31% to 2.68%, and the proportion of SO₂ increased from 74.68% to 97.32%. There is no H₂S, which means that the increase in H₂O concentration could promote the production of SO₂ and inhibit the production of SOF₂. As shown in Fig. 11(b), the proportion of SOF₂ decreased significantly with the H₂ concentration, and the proportion of SO₂ showed an increasing trend first and then a decreasing trend. When the concentration of H₂ is higher, the degradation of SO₂F₂ would produce a small amount of H₂S. Meanwhile, the proportion of SO₂ in the overall gas product of the H₂ system is slightly higher than that of the H₂O system, and the highest proportion is 98.29%.

By analyzing and summarizing the results of this work, the reaction equations of the generation of SO₂F₂ degradation products and their reaction heats are shown in Table I. When there is no additional reactive gas, the degradation of SO₂F₂ relies mainly on high-energy electron collisions, resulting in bond breaking reactions (3) and (4). Both of the reactions are heat-absorbing, so the reactivity is slowed down, and the reformation reactions of SO₂F₂ become violent, resulting in a lower DRE.

For the SO₂F₂/H₂O/Ar system, the H and OH radicals shown in Fig. 7(a) could demonstrate that the H₂O molecules collided with high-energy electrons as shown in reaction (5) and generate H and OH radicals, which promotes reactions (6) and (7) and makes the degradation of SO₂F₂ to tend to produce SO₂, as well as inhibit the production of SOF₄. On the other hand, reactions (6) and (7) also promote the production of HF, and SiF₄ is produced by etching of the reactor’s interior through reaction (18). However, as reaction (5) is a heat-absorbing reaction, the high concentration of H₂O absorbs a large number of high-energy electrons, causing a weakening of reactions (3) and (4) and a decrease in DRE.

For the SO₂F₂/H₂/Ar system, the H₂ molecules could collide with high-energy electrons and produce H radicals as shown in reaction (8). These H radicals could react with the SO₂F₂ molecules as shown in reactions (9) and (10) and produce SO₂ and SOF₂, which could further react with H radicals and produce solid S monomers through reactions (11) and (12). In this case, reactions (9) and (12) were also accompanied by the production of HF, which would corrode the inner walls of the reactor and produce SiF₄ as shown in reaction (18). The OH radicals could be detected in the emission spectrum of the H₂ system as shown in Fig. 7(b), which was mainly due to the production and decomposition of the H₂O produced from reactions (10)–(12).

With the increase in the H₂ concentration from 1%, reactions (10)–(12) are promoted significantly, and the production of H₂O increased. The decomposition of both H₂ and H₂O molecules would collide with and consume a large number of high-energy electrons, leading to a weakening of reactions (3) and (4). The electronic affinity of SO₂ and SOF₂ is higher than that of SO₂F₂, which made reactions (11) and (12) more likely to react than reactions (9) and (10), both of which made the DRE decrease even if there were more H and OH radicals. The promotion of reactions (11) and (12) made more SO₂ and SOF₂ molecules convert to S monomers. When the H₂ concentration reaches 2%, the S monomer is further converted to H₂S as shown in reaction (19). From a product handling perspective, the reduction in solid-phase products and the increase in gas-phase products are not conducive to SO₂F₂ degradation product handling.

Figure 12 shows the degradation pathway of SO₂F₂ under H₂ and H₂O production. The products of SO₂F₂ degradation such as SOF₂, SO₂, H₂S, and HF are toxic gases, and therefore, secondary treatment is required. SOF₂ is insoluble in water but soluble in lye, SO₂ is soluble in water to form H₂SO₃ and can be absorbed by lye, and H₂S is soluble in water. In chemical industries such as coal and coke ovens, SO₂ and H₂S are produced as by-products and often absorbed using metal base pairs such as NaOH, Na₂CO₃ and Ca(OH)₂ solutions.^32,33 In the current work, for SO₂F₂ with an initial concentration of 2% and a gas flow rate of 150 ml/min, when the additional H₂O concentration was 1%, the DRE could reach 86.26% and the EY could reach 13.55 g/kW h. The SO₂F₂ degradation products can be absorbed fully by saturated Ca(OH)₂ solution. The addition of H₂ can regulate the type of SO₂F₂ degradation products by reducing to solid monomer S, which is not only convenient for treatment but can also be dried and processed for industrial application such as sulfur with certain economic value.

In the current work, the effect of addition of reactive H₂O and H₂ was investigated, the DRE and EY of SO₂F₂, emission spectrum, and degradation products were analyzed, and the SO₂F₂ degradation reaction pathway and its matrix reaction data were speculated based on the results of experimental work.

The results indicate that the additional reactive gas can provide more reactive particles to the reaction system; on the one hand, these reactive particles can participate in the decomposition path of SO₂F₂, making the degradation of SO₂F₂ to not rely solely on the high-energy electrons. On the other hand, the reactive particles can hinder the reformation of SO₂F₂ molecules. At 80 W input power, the 2% SO₂F₂/1% H₂O/97% Ar system achieved 86.26% DRE and 13.55 g/kW h, and the 2% SO₂F₂/1% H₂/97% Ar system achieved 80.29% DRE and 12.61g/kW h EY, which are 133.80% and 124.54% higher than those of the SO₂F₂/Ar system, respectively. As the concentration of H₂O and H₂ increases further, the additional reactive gas would absorb and consume high-energy electrons and inhibit the decomposition of SO₂F₂, resulting in a lower DRE and EY. The addition of reactive gases also affects the product formation pattern by significantly promoting the formation of SO₂. The H₂ systems can produce solid S monomers, which are more pronounced at higher H₂ concentrations, making the products more easily recoverable.

The above-mentioned results can provide experimental support for the efficient and harmless degradation of SO₂F₂ in industry.

This work was funded by Guizhou Province (Central) [Grant/Award No. Qian Ke He Zhi Cheng (2022) General 207] and China Southern Power Grid Company Limited (Project No. GZKJXM20220049).

The authors have no conflicts to disclose.

Ying Zhang: Writing – original draft (equal). Mingwei Wang: Writing – original draft (equal). Chang Zhou: Writing – review & editing (equal). Yalong Li: Writing – review & editing (equal). Zhaodi Yang: Writing – review & editing (equal). Xiaoxing Zhang: Project administration (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.