The Decomposition Products of Sulfur Hexafluoride (SF₆) with Metals Dissolved in Liquid Ammonia

Abstract

Sulfur hexafluoride is a highly chemically inert gas with several important industrial applications. It is stable against fused alkali, oxygen and ammonia, even at several hundred degrees Celsius. In this work, the reactions between metals (Li–Cs, Sr, Ba, Eu, Yb) dissolved in liquid ammonia and SF₆ are reported, leading to mono- or bivalent fluorides and sulfides. To this end, SF₆ was passed into a cooled solution of the respective metal in liquid ammonia. The identity of the products was confirmed by powder X-ray diffraction and IR spectroscopy. The reactions could lead to a cheap and effective disposal method of the present amounts of stored SF₆, for possible generation of H₂S and HF.

1. Introduction

Sulfur hexafluoride, SF₆, is a colorless, nonflammable and highly inert gas with an extremely high global warming potential (GWP) [1,2]. Because of its excellent electric characteristics and high chemical stability, it is used as a dielectric medium in high-voltage circuit breakers, transformers, capacitors and many more industrial applications [3]. Since the search for a replacement material that is less harmful to the environment or that is easier to dispose of has not yet been successful, the decomposition of sulfur hexafluoride remains a current field of research [4,5,6].

The activation of SF₆ was first shown in the literature in 1900 by Moissan and Lebeau [7]. They described the reaction between sulfur hexafluoride and hydrogen sulfide to sulfur and hydrogen fluoride. Furthermore, reactions with elemental sodium at 200 °C and several catalytic activations with nickel- or rhodium-complexes of SF₆ have been published [4,5,8].

The reaction between SF₆ and sodium, dissolved in liquid ammonia, was reported in 1964 by Demitras and MacDiarmid; however, they were not able to identify the products [9]. The reaction between SF₆ and caesium in liquid ammonia at −61 °C gave several good hints as to the formation of CsF and Cs₂S [10].

8 Cs + SF₆ → 6 CsF + Cs₂S

(1)

During the reaction of ⅛ mol SF₆ per 1 mol of Cs, they observed that the blue colour of the solution of Cs in NH₃ disappeared. The weight of the products compared to the weight of the reactants showed a ratio close to 1.14. This would be the expected ratio for the postulated reaction according to Equation (1). Dissolving the white-appearing products in water and adding some HCl leads to the formation of H₂S, which also represents an indication for the postulated reaction.

In the present work, the reaction between metals that dissolve in liquid ammonia at −60 °C and internal pressure and SF₆ were investigated in detail.

2. Results and Discussion

The reactions between the alkali metals dissolved in liquid ammonia and SF₆ lead to the formation of the respective products (Equation (2)) that Demitras and MacDiarmid predicted for the reaction of Cs dissolved in NH₃ with SF₆.

$$8M^{\mathrm{I}}+{\mathrm{SF}}_6\stackrel{-60°\mathrm{C},{\mathrm{NH}}_3}{\to }{M}_2\mathrm{S}+6M^{\mathrm{I}}\mathrm{F}\left(M=\mathrm{Li}-\mathrm{Cs}\right)$$

(2)

$$4M^{\mathrm{I}}+{\mathrm{SF}}_6\stackrel{-60°\mathrm{C},{\mathrm{NH}}_3}{\to }{M}^{\mathrm{II}}\mathrm{S}+3M{\mathrm{F}}_2\left(M=\mathrm{Sr},\mathrm{Ba},\mathrm{Eu}\right)$$

(3)

The mono- or bivalent fluorides and sulfides could be obtained at −50 °C at reaction times ranging from nearly one to two and a half hours. The results are presented in the form of selected examples.

2.1. Alkali Metals (Li–Cs)

The conversion of SF₆ into alkali metal fluorides and the respective sulfides of the form M^IF and M^I₂S took between 50 min for caesium (4.9 mmol) and 135 min for sodium (38 mmol). The reaction times and turnover rates for the reaction are shown in

Table 1. Reaction times and turnover rates for the reaction of alkali metals with SF₆.

	Li	Na	K	Rb	Cs
Reaction time [min]	130	135	90	75	50
Turnover rate [mmol/h]	16.6	17.2	6.80	6.66	5.85

. During the reaction the blue to bronze colors of the solutions changed slowly to a light brownish white. The reaction rates did not correspond to the trend of rising reactivity of the alkali metals from Li to Cs, and the influence of the liquid ammonia solvent is also not clear. A precise control of the SF₆ gas flow by a flowmeter was not carried out.

After slowly removing the liquid ammonia under vacuum, a dark white to light brown powder remained in the Schlenk vessel. The powders were dried under vacuum at room temperature, and a powder X-ray diffraction pattern was recorded. The patterns showed rather broad reflexes over a large 2θ-range. A possible explanation for the broad reflexes are the small particle sizes. Recording an image with a scanning electron microscope (SEM) showed small particles with a diameter around or below 100 nm. One of these images of the product of the reaction between SF₆ and sodium is shown in Figure 1, and indicates a nano powder of NaF and Na₂S.

For a better crystallization of the samples, a few hundred milligrams of the reaction mixture were sealed in a borosilicate tube and annealed under vacuum at 450 °C for five days. After crystallization, all X-ray powder diffraction patterns clearly showed the formation of the respective alkali metal fluoride M^IF. For sodium, potassium and rubidium, the characteristic reflections for the alkali metal sulfide M^I₂S were also observable. In some cases, the formation of the alkali metal amides as a byproduct was able to be shown by powder X-ray diffraction. For the reactions with lithium and caesium, the diffraction pattern only showed the formation of the respective fluoride and remnants of lithium metal. To prevent the formation of elemental sulfur, the powder was sealed into a borosilicate glass tube under vacuum (1.5 × 10⁻³ mbar) at a temperature gradient from 450 °C to room temperature. Because of its reactivity with glass, the products of the reaction between Li and SF₆ were sealed in a tantalum tube. A sublimation of elemental sulfur could not be observed. Another proof was the release of H₂S when immersed in diluted HCl, as shown by the blackening of moist Pb(CH₃COO)₂ paper.

Figure 2 shows the X-ray powder diffraction pattern of the reaction between Rb and SF₆. The Rietfeld refinement of the products, RbF and Rb₂S, shows a relation of RbF:Rb₂S of 88.0(1):12.0(1), which corresponds to the predicted value of 86:14. The lattice parameters are with a = 5.6586(1) Å, V = 181.187(4) ų for RbF and a = 7.7533(2) Å, V = 466.080(8) ų for Rb₂S in accordance with the results reported in the literature [11,12].

As should be expected, the ATR IR spectrum of the mixture of RbF and Rb₂S (Figure S1) shows no bands in the range from 4000 to 400 cm⁻¹. An IR band of RbF should be observed at 376 cm⁻¹, so there is no IR activity expected in the measured region [13]. The measured intensities between 2200 and 1900 cm⁻¹ are characteristic for the ATR diamond module.

2.2. Alkaline Earth and Rare Earth Metals (Sr, Ba, Eu, Yb)

The conversion of SF₆ into the fluorides and sulfides of the form M^IIF₂ and M^IIS with alkaline earth and rare earth metals took place for Ba, Sr and Eu, according to Equation (3). In the case of Yb, YbF₃ could also be obtained. The reaction times and turnover rates are shown in

Table 2. Reaction times and turnover rates for the reaction of Sr, Ba, Eu and Yb with SF₆.

	Sr	Ba	Eu	Yb
Reaction time [min]	70	75	75	70
Turnover rate [mmol/h]	5.7	4.0	4.0	4.4

.

As well as the products of the reaction with the alkali metals, the dried products of the reaction between alkaline earth and rare earth metals with SF₆, were sealed in borosilicate glass tubes for the crystallization at 450 °C over five days in a single zone tube furnace. In the case of Sr, Ba and Eu, the X-ray powder diffraction patterns show the bivalent fluorides M^IIF₂ and sulfides M^IIS. Additionally, byproducts, such as unreacted metal (Ba), amide that converts to nitride during the crystallization process (EuN), or silicon from the glass tube, could be obtained in some reactions. For ytterbium, the trivalent fluoride YbF₃ could be observed next to YbN as a byproduct after crystallization, which suggests the formation of Yb(NH)₂ as an intermediate. In all cases, it was not possible to detect elemental sulfur and the addition of diluted HCl lead to the formation of H₂S.

Figure 3 shows the X-ray powder diffraction pattern of the reaction between Sr and SF₆. The lattice parameters of a = 5.8004(6) Å, V = 195.156(3) ų for SrF₂ and of a = 6.0157(7) Å, V = 217.642(3) ų for SrS are in accordance with the results shown in the literature [14,15].

The ATR IR spectrum (Figure S2) shows no bands in the range of 4000 to 400 cm⁻¹. An IR band of SrS should be observed at 185 cm⁻¹ and 280 cm⁻¹, so there is no IR activity expected in the measured region [16]. The measured intensities between 2200 and 1900 cm⁻¹ are signals from the ATR diamond module (Figure S2).

3. Conclusions

SF₆ reacts readily at −50 °C with dissolved metals (Li–Cs, Sr, Ba, Eu) in liquid ammonia under normal pressure. The reaction leads to mono- or bivalent fluorides and sulfides (M^IF, M^I₂S; M^IIF₂, M^IIS) and, in some cases, byproducts like amides are formed. For Yb, the trivalent fluoride could also be obtained. The method described here represents an elegant and easy path for the decomposition of SF₆, which can be realized for small and, conceivably, for industrial batch sizes. The reactions could offer an opportunity for an effective removal of the existing SF₆-stocks.

4. Materials and Methods

All work was carried out excluding moisture and air in an atmosphere of dried and purified argon (5.0, Praxair, Düsseldorf, Germany) using high-vacuum glass lines and a glovebox (MBraun, Garching, Germany). Liquid ammonia (Gerling Holz & Co., 99.98%, Hamburg, Germany) was dried and stored over sodium (VWR, Germany) in a special high-vacuum glass line with a Hg pressure relief valve. Sulfur hexafluoride (Solvay, 99.9%, Hannover, Germany) was directly passed into the reaction vessel without any further treatment.

The alkali metals Li, Na, K (Merck, >99%, Darmstadt, Germany; Lab Chem Röttinger, >99%, Dinslaken, Germany; Merck 99%) and the alkali earth metals Sr and Ba (Onyxmet, 99%; Onyxmet 99.8%, Olsztyn, Poland) were used as supplied. Rb and Cs were distilled according to Hackspill in a borosilicate glass distillery, which was treated with hot aqua regia several times und flame-dried under vacuum before the distillation [17]. Eu and Yb were extracted several times over a fritt with liquid ammonia in a special Schlenk vessel [18]. All metals were stored in the glove box under argon in closed flasks.

All glass vessels were made of borosilicate glass, and were flame-dried several times under vacuum before use.

For the reactions, the Schlenk vessels were charged with an amount of metal shown in

Table 3. Amounts of alkali, alkaline earth and rare earth metals used for reaction with SF₆ in liquid ammonia.

Metal	m/mg	n/mmol
Li	250	36.0
Na	890	38.7
K	400	10.2
Rb	711	8.3
Cs	647	4.9
Sr	589	6.7
Ba	689	5.0
Eu	753	5.0
Yb	890	5.1

. About 20–25 mL of the predried liquid ammonia was condensed at −78 °C (Isopropanol/CO₂-bath) onto the metal. An intense blue to metallic bronze color was obtained for the solution. Subsequently, because of the sublimation point of −63.8 °C for SF₆, the mixture was warmed to −50 °C by cooling the vessel with isopropanol and an Ultra-Low Refrigerated-Heating Circulator (Julabo FP 90, Seelbach, Germany). After 50–135 min, the blue colour of the solution disappeared and the liquid ammonia was removed slowly under vacuum.

4.1. Powder X-ray Diffraction

The powder X-ray diffraction patterns were recorded at room temperature with a STOE Stadi MP powder diffractometer (STOE, Darmstadt, Germany). The diffractometer used Cu Kα1 radiation, a Ge monochromator and a Mythen1K detector. The samples were powdered in a glovebox under argon atmosphere in agate mortars and sealed into borosilicate capillaries with a diameter of 0.3 mm, which were flame-dried several times under vacuum before utilization. All samples were charged in the glove box in a flame-dried ampoule (8 mm outer diameter, 1.5 mm wall thickness). A stopcock attached to the ground joint allowed transfer of the ampoule to a Schlenk line. The bottom of the ampoule was cooled with liquid nitrogen, and the ampoules were flame sealed under vacuum (1 × 10⁻³ mbar). These ampoules were annealed in a tube furnace for 5 days at 450 °C for better crystallization.

The evaluation of the powder X-ray patterns was carried out with the WinXPOW software package and the ICDD powder diffraction file database [19,20]. Le Bail fitting and Rietveld refinement were done with JANA 2006 [21].

4.2. IR Spectroscopy

The IR spectra were recorded on a Bruker alpha FT-IR-spectrometer (Bruker Optik GmbH, Rosenheim, Germany) using the ATR Diamond module with a resolution of 4 cm⁻¹. The spectrometer was located inside a glovebox under argon atmosphere. The spectra were processed with the OPUS software package [22].

4.3. SEM Imaging

The scanning electron microscopy images were recorded on a JEOL JIB 4610F (Jeol, Freising, Germany), dual beam FIB/SEM with a Schottky thermal files emission gun (acceleration voltage 0.2–30 kV, resolution 1.2 nm at 30 kV, 3.0 nm at 1 kV), a Gallion ion source (acceleration voltage 1–30 kV, resolution 5 nm at kV and a backscattered and secondary electron detector.