Carbon dioxide binding at dry FeOOH mineral surfaces: evidence for structure-controlled speciation

Interactions between CO2(g) and mineral surfaces are important to atmospheric and terrestrial settings. This study provides detailed evidence on how differences in mineral surface structure impact carbonate speciation resulting from CO2(g) adsorption reactions. It was achieved by resolving the identity of adsorption sites and geometries of (bi)carbonate species at surfaces of nanosized goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) particles. Fourier transform infrared spectroscopy was used to obtain this information on particles contacted with atmospheres of CO2(g). Vibrational modes of surface hydroxo groups covering these particles were first monitored. These showed that only one type of the surface groups that are singly coordinated to Fe atoms (-OH) are involved in the formation of (bi)carbonate species. Those of higher coordination numbers (μ-OH, μ3-OH) do not participate. Adsorption geometries were then resolved by investigating the C-O stretching region, assisted by density functional theoretical calculations. These efforts provided indications leaning toward a predominance of monodentate mononuclear species, -O-CO2Hx=[0,1]. In contrast, monodentate binuclear species of (-O)2-COHx=[0,1], are expected to form at particle terminations and surface defects. Finally, calculations suggested that bicarbonate is the dominant species on goethite, while a mixture of bicarbonate and carbonate species is present on lepidocrocite, a result stemming from different hydrogen bonding patterns at these mineral surfaces.