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

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 monoor 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.


Introduction
Sulfur hexafluoride, SF 6 , 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 SF 6 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 SF 6 have been published [4,5,8].
The reaction between SF 6 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 SF 6 and caesium in liquid ammonia at −61 • C gave several good hints as to the formation of CsF and Cs 2 S [10].
8 Cs + SF 6 → 6 CsF + Cs 2 S (1) During the reaction of 1/8 mol SF 6 per 1 mol of Cs, they observed that the blue colour of the solution of Cs in NH 3 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 H 2 S, 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 SF 6 were investigated in detail.

Results and Discussion
The reactions between the alkali metals dissolved in liquid ammonia and SF 6 lead to the formation of the respective products (Equation ( 2)) that Demitras and MacDiarmid predicted for the reaction of Cs dissolved in NH 3 with SF 6 .
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.

Alkali Metals (Li-Cs)
The conversion of SF 6 into alkali metal fluorides and the respective sulfides of the form M I F and M I 2 S 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.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 SF 6 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 SF 6 and sodium is shown in Figure 1, and indicates a nano powder of NaF and Na 2 S.

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.
( ) 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.

Alkali Metals (Li-Cs)
The conversion of SF6 into alkali metal fluorides and the respective sulfides of the form M I F and M I 2S 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.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 I F. For sodium, potassium and rubidium, the characteristic reflections for the alkali metal sulfide M I 2 S 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 SF 6 were sealed in a tantalum tube.A sublimation of elemental sulfur could not be observed.Another proof was the release of H 2 S when immersed in diluted HCl, as shown by the blackening of moist Pb(CH 3 COO) 2 paper.
Inorganics 2017, 5, 68 3 of 7 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 I F. For sodium, potassium and rubidium, the characteristic reflections for the alkali metal sulfide M I 2S 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)Å 3 for RbF and a = 7.7533(2) Å, V = 466.080(8)Å 3 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.As should be expected, the ATR IR spectrum of the mixture of RbF and Rb 2 S (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.

Alkaline Earth and Rare Earth Metals (Sr, Ba, Eu, Yb)
The conversion of SF6 into the fluorides and sulfides of the form M II F2 and M II S 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.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 II F2 and sulfides M II S. 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.

Sr Ba Eu Yb
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)Å 3 for SrF2 and of a = 6.0157(7)Å, V = 217.642(3)Å 3 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).

Alkaline Earth and Rare Earth Metals (Sr, Ba, Eu, Yb)
The conversion of SF 6 into the fluorides and sulfides of the form M II F 2 and M II S with alkaline earth and rare earth metals took place for Ba, Sr and Eu, according to Equation (3).In the case of Yb, YbF 3 could also be obtained.The reaction times and turnover rates are shown in Table 2.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 SF 6 , 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 II F 2 and sulfides M II S. 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 YbF 3 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 H 2 S.
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).

Conclusions
SF 6 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 I F, M I 2 S; M II F 2 , M II S) 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 SF 6 , 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 SF 6 -stocks.

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.About 20-25 mL of the predried liquid ammonia was condensed at −78 • C (Isopropanol/CO 2 -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 SF 6 , 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.

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].

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].

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.

Supplementary Materials:
The following are available online at www.mdpi.com/2304-6740/5/4/68/s1, Figure S1: IR spectrum of RbF and Rb 2 S measured on an ATR diamond module, Figure S2: IR spectrum of SrF 2 and SrS measured on an ATR diamond module.

Figure 1 .
Figure 1.SEM image of the dried products of the reaction between SF6 and Na.

Figure 1 .
Figure 1.SEM image of the dried products of the reaction between SF 6 and Na.

Figure 2 .
Figure 2. Rietfeld refinement of the X-ray powder diffraction pattern of RbF and Rb2S.The calculated reflection positions are represented by strokes below the observed and calculated powder pattern.The difference curves are plotted underneath.Profile R factors: Rp = 3.95%, wRp = 5.22%, GOF = 3.95.

Figure 2 .
Figure 2. Rietfeld refinement of the X-ray powder diffraction pattern of RbF and Rb 2 S. The calculated reflection positions are represented by strokes below the observed and calculated powder pattern.The difference curves are plotted underneath.Profile R factors: R p = 3.95%, wR p = 5.22%, GOF = 3.95.

Figure 3 .
Figure 3. Le Bail fitting of the X-ray powder diffraction pattern of SrF2 and SrS.The calculated reflection positions are represented by strokes below the observed and calculated powder pattern.The difference curves are plotted underneath.Profile R factors: Rp = 3.16%, wRp = 4.40%, GOF = 1.72.

Figure 3 .
Figure 3. Le Bail fitting of the X-ray powder diffraction pattern of SrF 2 and SrS.The calculated reflection positions are represented by strokes below the observed and calculated powder pattern.The difference curves are plotted underneath.Profile R factors: R p = 3.16%, wR p = 4.40%, GOF = 1.72.

Table 1 .
Reaction times and turnover rates for the reaction of alkali metals with SF 6 .

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

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

Table 2 .
Reaction times and turnover rates for the reaction of Sr, Ba, Eu and Yb with SF 6 .

Table 3 .
Amounts of alkali, alkaline earth and rare earth metals used for reaction with SF 6 in liquid ammonia.