Atomic Hydrogen Adsorption and Water Formation on Fe₃O₄ (011): Unveiling the Onset of Iron Oxide Reduction

The reduction of Fe3O4 by hydrogen is a process of significant industrial relevance, particularly in catalysis and hydrogen-based metallurgical applications. To gain atom-level insights into the underlying mechanisms, we perform density functional theory calculations to investigate hydrogen adsorption and water formation on both perfect and oxygen-deficient Fe3O4(011) surfaces. Our results reveal a consistent site preference for hydrogen adsorption at 2-fold-coordinated oxygen sites over 3-fold-coordinated oxygen sites, driven by electronic structure differences as revealed by projected density of states analysis. We identify the most stable adsorption configurations across a range of hydrogen coverages, including configurations that lead to surface-bound H2O formation. Comparison of H2O desorption energies indicates that H2O is more readily released from O-deficient surfaces, highlighting the promoting role of oxygen vacancies in oxide reduction. Thermodynamic modeling incorporating the Gibbs free energy of adsorption reveals that 2-fold-coordinated oxygen vacancies suppress H incorporation, while 3-fold-coordinated oxygen vacancies enhance H uptake across a broad range of temperature and H2 pressure. Surface free energy analyses further show that under reducing conditions, surfaces containing 3-fold-coordinated oxygen vacancies become more thermodynamically stable than the defect-free surface. These findings underscore the environment-dependent reactivity of Fe3O4(011) and demonstrate how the type and coordination of oxygen vacancies govern H adsorption and surface reducibility. The results have practical implications for tailoring the redox behavior of iron oxides in hydrogen-rich environments.