This paper aims to clarify the opportunities and benefits of a carbon dioxide based mild hydrothermal
process for the selective production of hydrogen gas H2 from water via the oxidation of pure metallic iron
powder Fe0 by focusing on (i) the effects of different process parameters such as temperature, initial carbon
dioxide pressure and grain size of the metallic iron powder and (ii) the occurring reaction mechanisms.
With the aid of process parameter optimisation, highly pure hydrogen gas (>99 mol%) with a
percentage yield of approximately 80% could be experimentally obtained by treating metallic iron powder
with a grain size of 5 lm under mild hydrothermal conditions. The hydrogen formation reaction took
place in a stirred 100 mL stainless steel autoclave, filled for 40 mL with a 1 M aqueous potassium hydroxide
solution, at 160 C for 16 h and made use of an absolute initial carbon dioxide pressure of 6 bar (6 bar
CO2, 25 C, taking place in a volume of 60 mL: 15 mmol CO2). Hence, this hydrothermal process operates
at milder reaction conditions compared to previously reported similar processes, which operate at 300 C.
This paper shows that specifically under mild hydrothermal conditions, by complete dissolution of the
introduced carbon dioxide gas in the 1 M potassium hydroxide solution, in situ formed carbonate ions
CO3 2 play a key role in assisting the hydrogen gas formation by acting as catalysts. More specifically,