Site-Selective Reduction and Surface Restructuring of α-Fe2O3(0001) upon Hydrogen Adsorption

Abstract

The reduction of iron oxides using hydrogen as a reducing agent represents an alternative route toward the production of green steel. Based on density functional theory (DFT) calculations with a Hubbard U term, we explore the initial adsorption and incorporation of hydrogen in the near-surface region of hematite (0001). Starting with the Fe–O3–Fe termination, which is stable over a wide range of oxygen chemical potentials, we vary the H concentration and distribution in the surface layers. Adsorption of H on the surface or in the near-surface region is energetically favored, in contrast to incorporation in bulk hematite. H binds to an oxygen ion, while simultaneously a Fe3+ in the vicinity is reduced to Fe2+ with a distinct orbital polarization of the sixth electron at Fe2+ depending on the position and concentration of H: a1g in the surface vs a dxy orbital in the subsurface layer. Upon adsorption of 3H at the Fe–O3–Fe termination, a complete rearrangement of the top layers takes place with subsurface oxygen moving to the surface and forming hydroxyl groups, resulting in a 3OH/3Fe/stacking. A key finding is that H2 adsorption leading to H2O formation and desorption is an efficient way for removing lattice oxygen at a much lower energy cost than the direct formation of oxygen vacancies, thus highlighting a further mechanism toward reduction of the surface. Our results demonstrate migration of both anions and cations upon hydrogen adsorption and the emergence of lower coordinated Fe sites that can be regarded as a nucleus for the transformation from hematite to other reduced iron oxide phases such as magnetite.