Nanometer effect promoting arsenic removal on α-MnO2 nano-surface in aqueous solution: DFT+U research.

The nanometer effect in the process of arsenic ions removal on α-MnO2 nano-surface is studied by the first-principle method through microfacet models. Several parameters, such as adhesion energy, electrostatic potential, and Mulliken population were calculated to illuminate the internal mechanism. The results show that the adsorption energies of As(OH)3 molecules on MnO2[(100×110)] nanostructure are smaller than that on the bulk surface with the same concentration, which means the nanometer effect is beneficial to enhance the adsorption ability of MnO2 nano-surface. In an aqueous solution, there exist two possible removal ways of As ions. One is the direct reaction of As(OH)3→As(OH)6-, which occurs both in bulk surface and nano-surface. However, to nanomaterials, there exists another removal way of As(OH)3→As(OH)4→As(OH)6- through an intermediate As(OH)4 molecule produced by nanometer effect. Furthermore the smaller electrostatic potential of As ions on [(100×110)] nano-surface is beneficial to enhance the removal capability of As ions. Then the reason why MnO2 nanomaterials have better catalytic activity than the bulk materials is originated from its much less adhesion energy, much more removal ways, and much smaller electrostatic potential. So this research provides a detailed understanding of the removal capability of toxic ions influenced by a nanometer effect.

Insight into the surface activity of defect structure in α-MnO2 nanorod: first-principles research.

The contribution of defect structure to the catalytic property of α-MnO2 nanorod still keeps mysterious right now. Using microfacet models representing defect structure and bulk models with high Miller index, several parameters, such as cohesive energy, surface energy, density of state, electrostatic potential, et al., have been used to investigate the internal mechanism of their chemical activities by first-principles calculation. The results show that the trend in surface energies of microfacet models follows as Esurface[(112 × 211)] > Esurface[(110 × 211)] > Esurface[(100 × 211)] > Esurface[(111 × 211)] > Esurface[(112 × 112)] > Esurface[(111 × 112)], wherein all of them are larger than that of bulk models. So the chemical activity of defect structure is much more powerful than that of bulk surface. Deep researches on electronic structure show that the excellent chemical activity of microfacet structure has larger value in dipole moments and electrostatic potential than that of bulk surface layer. And the microfacet models possess much more peaks of valent electrons in deformantion electronic density and molecular orbital. Density of state indicates that the excellent chemical activity of defect structure comes from their proper hybridization in p and d orbitals.

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.