FT-IR, XPS AND DENSITY FUNCTIONAL THEORY STUDY OF ADSORPTION MECHANISM OF SODIUM FORMATE ONTO GOETHITE OR HEMATITE

The adsorption of sodium formate on goethite or hematite surface was investigated by FT-IR and XPS, respectively. Periodic plane-wave density functional theory (DFT) calculations were performed on models of carboxylate surface complexes on the goethite (101), (100) surfaces or hematite (001) surface. The photoemission core-level shifts (CLS) of the adsorbed surface iron sites calculated with periodic interfacial structures were compared with the experimental XPS. FTIR results reveal that the interfacial structures may be assigned to bidentate mononuclear (BM) complexes formed via bonding of one surface iron atom of goethite or hematite to both oxygen atoms of carboxylate group. In agreement with the adsorption energies determined by DFT calculations, the calculated results show that only the calculated Fe2p CLS of the adsorbed surface iron sites on goethite (100) and hematite (001) surfaces are correspondingly in accordance with the experimental observed Fe2p CLS, which confirms that the BM structure occurs upon adsorption.