Reductive Dissolution Mechanisms at the Hematite-Electrolyte Interface Probed by in Situ X‑ray Scattering

The electron-catalyzed dissolution and reprecipitation of iron (oxyhydr)­oxides constitute critical steps in natural geochemical iron cycling. However, the atomic-scale mechanisms of reductive dissolution and oxidative precipitation of Fe2+ remain poorly understood because they are difficult to directly experimentally probe. Using in situ synchrotron X-ray scattering and a novel electrochemical cell, we interrogate the interfacial structure between the hematite (α-Fe2O3) (11̅02) surface and acidic aqueous solution (5 mM Na2SO4, pH 4.0) under controlled electrochemical potential from the open circuit condition to cathodic bias as the reductive dissolution potential is approached and then exceeded. The crystalline order of the surface improves under mild reducing conditions, and the surface Fe stoichiometry changes with cathodic bias. After significant reductive dissolution occurs and cathodic bias is removed, dissolved Fe is redeposited, forming a disordered interface. Unlike at circumneutral pH, water layers at the hematite interface with acidic solution are poorly ordered, likely due to the adsorption of sulfate from the electrolyte. These results provide a novel atomic-scale view into the behavior of reducible transition metal oxide surfaces under fluctuating (electro)­chemical conditions.