Vibrational properties of CO₂ adsorbed on the Fe₃O₄ (111) surface: Insights gained from DFT

By virtue of density functional theory calculations, this work discusses several carbonate, carboxylate, and bicarbonate species on two thermodynamically relevant metal terminations of the (111) surface of magnetite, Fe₃O₄. We present adsorption energies and vibrational wavenumbers and conclude in assigning the observed infrared reflection-absorption spectroscopy bands. CO₂ prefers to adsorb molecularly on the Fe_tet1 terminated Fe₃O₄(111) surface, a finding consistent with observation. Calculations compared with the experiment lead to interpreting results in favor of the Fe_tet1 (single metal) terminated Fe₃O₄(111) surface as the regular surface termination. Formation of carbonate and bicarbonate requires metal impurities on that surface. Such impurities exist, for instance, on the Fe_oct2 (double metal) termination, which can thus be used as a model for "metal-rich terminations" of more complex surfaces.