Modified sulfur-iodine process with Haber-Bosch step?
The sulfur-iodine cycle is the lowest-temperature thermochemical water splitting cycle by a wide margin. While most processes require temperatures
>1200 C, the sulfur-iodine cycle tops out at just 850 C, which is a much easier temperature to achieve in a solar concentrator:
2 H2SO4 >> 2 H2O + 2 SO2 + O2 [850 C)
SO2 + I2 + excess H2O >> HI (aq) + H2SO4 (aq) [250 C, reverses if too concentrated]
2 HI >> H2 + I2 [450 C]
However, the last step, hydrogen and iodine separation, is very difficult because the products are both gases with low boiling points, iodine is
expensive and must be retained effectively, and the reaction reverses at low temperatures.
However, hydrogen storage is very difficult, and conversion to ammonia uses only ~15% of the stored energy and is considered a promising candidate.
The Haber-Bosch process also occurs at around 400 - 500 C. As such we might consider a modification:
8 HI + N2 >> 2 NH4I + 3 I2 (exothermic, ~500 C)
The product ammonium iodide is a solid. While ammonia infamously reacts with iodine to generate explosive NI3*NH3, the primary decomposition products
of the latter are in fact NH4I and I2, so we should expect these to remain as stated. NH4I sublimes at 550 C, well above the boiling point of iodine,
and so can be collected as a solid.
Then we can do something like:
NH4I + H3PO4 >> NH4PO4H2 + HI (150 C-ish?)
NH4PO4H2 >> NH3 + H3PO4 (250 C)
I'm not sure about the last two steps. It may be necessary to use a stronger acid? Possibly H4P2O7?
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