Hydrogen production by hydrothermal oxidation of FeO under acidic conditions

Abstract

International audienceHe production of H2 by oxidation of FeO, taken here as model compound for steel slags, has been investigated both in pure water and under acidic aqueous conditions in the 373–573 K temperature range. Whereas after 65 h, H2 yield was negligible in pure water at 423 K, the reaction 3 FeO(s) + H2O(l) → Fe3O4(s) + H2(aq) reached near completion at the same temperature within 10 h in a solution containing 0.05 mol/l acetic acid. Increasing acetic acid concentration by one order of magnitude did not yield significantly more H2. At identical initial pH, acetic acid was found to be more efficient than oxalic acid and hydrochloric acid at enhancing H2 production. Acidic conditions increased FeO dissolution kinetics and, consequently, improved H2 yield. The specific efficiency of acetic acid resides in its thermal stability as well as in the potential of ligand-promoted Fe(II) dissolution. We show that the positive kinetics effect of mild acetic acid solutions over H2 yield evidenced on FeO does not apply directly to steel slags which buffer the pH to high values due to the presence of large amounts of CaO

Hydrothermal Valorization of Steel Slags—Part I: Coupled H2 Production and CO2 Mineral Sequestration

“…Hydrothermal slag oxidation was conducted in a third 500 mL Hastelloy ® autoclave (Crouzet et al, 2017) under 500 rpm stirring. Four grams of steel slag were inserted in 200 mL of deionized water.…”

“…Potentially, pH can be dragged to acidic values (pH < 6) through the dissolution of CO2 and the formation of carbonates. A kinetics enhancement is then expected (Crouzet et al, 2017).…”

“…In order to better characterize this RedOx reaction, the pure FeO-water system has been investigated by Crouzet et al (2017) under hydrothermal conditions; FeO oxidation and H2 production rates were found to be strongly enhanced by replacing pure water by dilute organic acids. For example, Crouzet et al (2017) showed that FeO is fully oxidized into Fe3O4 within less than 10 h at a temperature as low as 150°C using a 0.05 M acetic acid solution (pH of ca.…”

“…For example, Crouzet et al (2017) showed that FeO is fully oxidized into Fe3O4 within less than 10 h at a temperature as low as 150°C using a 0.05 M acetic acid solution (pH of ca. 4.7).…”

“…However, the kinetics effect of dilute acid obtained on pure FeO cannot be directly transposed to steel slags. Indeed, due to their high Ca content, steel slags will tend to neutralize mild acids and buffer the pH to high values (Crouzet et al, 2017). Therefore, in the perspective of H2 production from steel slags at an industrial scale, large volumes and/or high concentrations of acid would be required in order to reach significant kinetics enhancement.…”