Research on the removal mechanism of antimony on α-MnO₂ nanorod in aqueous solution: DFT + U method

Although previous papers have reported the desorption process of antimony (Sb) ions adsorbed on α-MnO₂ nanomaterials, some trace Sb(OH)₄^- molecular observed in experiments have not been understood clearly. Using two models as popular bulk surface and new microfacet, several parameters, such as adsorption energy, bond length, total density of state (TDOS) and activation energy, were calculated to research and analyze the catalytic reaction of Sb oxides on α-MnO₂. The results show that the bulk surface model has the "mirror effect" in revealing the catalytic property of α-MnO₂ nanorods. Using MnO₂[(100 × 110)] microfacet model, a new molecular Sb(OH)₄- molecular appears in the reaction process of Sb(OH)₃ + H₂O → Sb(OH)₄^- + H⁺. Further comparing the geometric morphology and TDOS of Sb(OH)₄- with Sb(OH)₆^- molecular, it is found that their bonding length, dihedral and energy orbital of bonding peaks are too close to set the Sb(OH)₄- as the precursor product of Sb(OH)₆^- molecular. Then the desorption process of Sb ions on α-MnO₂ nanorods is virtually transformed into Sb(OH)₃ → Sb(OH)₄^-  → Sb(OH)₆^- way in aqueous solution. Thus, our findings open an avenue for detailed and comprehensive theoretical studies of catalytic reaction by nanomaterials.