Thermal Decomposition of Sulfur Hexafluoride

Question

What is the thermal decomposition temperature for $\ce{SF6}?$ I've heard it decomposes into $\ce{F2}$ and $\ce{SF4}$ only to decompose further into $\ce{S2}$ and $\ce{F2},$ but I haven't found much on the topic.

I became curious since I was considering using it to flush out oxygen for some reactions I was doing, due to its density and stability (plus we have a bunch lying around), but was concerned about it decomposing into more toxic compounds if the reaction or apparatus were to get too hot or if a spark were to occur.

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I do not plan to actually do this. It was a silly idea I had, and I realized soon after posting this that $\ce{SF6}$ is a bad idea for these purposes since it is a greenhouse gas.

Answer 1

From an IEEE paper¹:

$\ce{SF6}$ begins to decompose at $\pu{300 °C}$ and the main decomposition components contain $\ce{CO2 , SO2F2 , SOF2 , SOF4 , SO2}$ and $\ce{H2S}$; [...] $\ce{H2S}$ is the special component only appears when thermal fault proceeds to some degree (above $\pu{360°C}$).

For more information about the decomposition mechanism and product yiels, see here.

References

1. F. Zeng et al., "Decomposition characteristics of SF6 under thermal fault for temperatures below 400°C," in IEEE Transactions on Dielectrics and Electrical Insulation, vol. 21, no. 3, pp. 995-1004, June 2014, doi: 10.1109/TDEI.2014.6832242.

Answer 2

As already noted the thermal decomposition s commence at $\pu{300 °C}$ and includes $\ce{CO2 , SO2F2 , SOF2 , SOF4 , SO2}$ and possibly $\ce{H2S}$.

Not mentioned relating to its stability per an Atomistry reference is:

Mixed with hydrogen it withstands a high temperature, but under the influence of powerful electric sparks formation of hydrogen sulphide and hydrogen fluoride can be effected. At a red heat copper and silver are without action on the gas, although magnesium and sodium effect its decomposition; at lower temperatures, however, it resists even these two metals and sodium can be melted unchanged in an atmosphere of the gas, as also can the hydroxides of the alkali metals. Hydrogen chloride and ammonia are unaffected by sulphur hexafluoride, but hydrogen sulphide reacts rapidly according to the equation:

$\ce{SF6 + 3 H2S = 6 HF + 4 S }$

So, with heating and also with hydrogen in the presence of electric sparks (atomic hydrogen), some hydrogen sulfide formation also. Note, electric arc generated atomic hydrogen is more powerful than the hydrogen radical ($\ce{.H}$), albeit the latter is more ubiquitously created, and I would avoid scenarios involving its potential introduction in the possible event, it also attacks SF6, speculatively (see relatedly, Eq (5) here) as follows (which could involve the extremely problematic formation of a deadly chemical weapon, disulfur decafluoride):

$\ce{SF6 + .H ->? .SF5 + HF}$

$\ce{.SF5 + .SF5 -> S2F10}$ (Source)

More definitely per above, however, H2S subsequently can further react with SF6 rapidly liberating problematic HF.

So, on the question, "concerned about it decomposing into more toxic compounds if the reaction or apparatus were to get too hot or if a spark were to occur", a definite yes from me.

Also, as a side note, SF6 reacts vigorously with Lithium metal (used to power the United States Navy's Mark 50 torpedo closed Rankine-cycle propulsion system per this source).