Periodic DFT Study of Radical Species on Crystalline Silica Surfaces

Periodic DFT (BLYP, B3LYP, and BHandHLYP) calculations have been used to study the properties of SiO• radical defect on quartz, cristobalite, tridymite, and amorphous surface models. Crystalline orbitals are constructed by an expansion of Gaussian type orbitals in which all atoms are represented with a double-ζ plus polarization quality basis set. Starting from fully hydroxylated 2D slab models, the radical defect is constructed by removing a hydrogen atom from a silanol of the surface while conserving the other features of the crystalline polymorph. Among the different functionals used, the hybrid BHandHLYP is the one that better compares to the experimental EPR data and provides reaction energies in better agreement with CCSD(T). The GGA BLYP functional, however, tends to delocalize the spin density, which can have important consequences on the H-bonding at the surface, especially when it exhibits geminal silanols. Comparison between the hydroxylated and the radical slab shows that the radical defect at the surface does not induce significant tension at the crystalline structure, the Si−O bond distance associated to the radical defect being the only geometrical parameter that varies significantly (0.04 Å). The spin density analysis shows that, regardless of the surface, the unpaired electron is mainly localized on the radical defect and, thus, does not provide a clue to understanding the different behavior between crystalline and amorphous silica on the ≡SiO• + H2O → ≡SiOH + OH• reaction. Instead, the ability of the surfaces to establish new H-bonds with the recovered silanol appears to be a relevant feature on the process that triggers OH• formation from SiO•.