Hydroxylation effects on the DFT-modeled adsorption of benzene and cyclohexane on hematite

The adsorption of benzene and cyclohexane on the Fe-terminated and a fully hydroxylated (0001) surface of hematite (α-Fe₂O₃) was investigated using a van der Waals-corrected DFT approach. Our calculations reveal that, on the Fe-terminated surface, both adsorbates adopt parallel orientations, whereas on the hydroxylated surface they can interact in both parallel and perpendicular configurations. Adsorption is stronger on the Fe-terminated surface. In the case of benzene, π-electron polarizability leads to a displacement of the π-cloud toward a surface Fe ion, resulting in a relatively strong attraction. For cyclohexane, the interaction is mainly driven by surface-induced polarization, which leads to a weaker interaction with the surface. On the fully hydroxylated surface, the bonding strength for both molecules is reduced due to the diminished inductive effect of the hydroxyl groups. Interestingly, despite their differing behavior on the Fe-terminated surface, benzene and cyclohexane exhibit comparable adsorption strengths on the hydroxylated surface. Vibrational analysis shows that surface hydroxylation reduces high-frequency C-H stretching perturbations, while its influence is mainly manifested in low-frequency ring modes. Partial hydroxylation of the Fe-terminated surface leads to a marked stabilization of benzene adsorption. Correlations between energetic and geometric parameters are also discussed.