X-ray absorption spectroscopy study of Cu(II) coordination in the interlayer of montmorillonite

Abstract

Cu(II) coordination in the interlayer of an expandable clay mineral montmorillonite is studied using X-ray absorption spectroscopy (XAS) along with electron paramagnetic resonance (EPR) and X-ray diffraction (XRD). Ab initio calculations are performed using FEFF code to reproduce the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) portion of the experimentally measured XAS spectra. Cu(II) coordination changes depending on the Cu(II) loading and hydration state of the interlayer. XRD shows that the Cu-saturated montmorillonite has d001-spacing values corresponding to the interlayer thickness of four and one water layer(s) for the hydrated and dehydrated interlayers, respectively. EPR shows that Cu(II) in the fully hydrated, unsaturated interlayer behaves similarly to free Cu(II) ion in a bulk aqueous solution, while Cu(II) forms a square planar complex in the dehydrated interlayer. Cu(II) in the fully hydrated, Cu-saturated montmorillonite has a characteristic singlet 1st derivative XANES spectrum. FEFF calculations show that this singlet feature originates from a quasi-regular octahedral coordination of water molecules around the interlayer Cu(II) atom. All other samples and model compounds including the dry Cu-saturated montmorillonite, wet and dry unsaturated montmorillonite, aqueous Cu(II), cupric nitrate salt (Cu(NO3)2·4.5H2O), and Cu(II) hydroxide precipitates have doublet 1st derivative XANES spectra. FEFF calculations suggest that the doublet features arise from an axially elongated octahedral coordination under the Jahn–Teller effect or square planar coordination. FEFF calculations of the EXAFS spectra as a function of the axial oxygen bond length demonstrate that a destructive interference between backscattering from equatorial oxygen (Oeq) and that from axial oxygen (Oax) atoms leads to an apparent coordination number (CN) less than six expected for the tetragonal coordination, with the farther, loosely bound axial oxygen atoms making a minor, yet negative contribution to the CN determined by the EXAFS analysis. This study shows that Cu(II) has interchangeable octahedral, tetragonal, and square planar coordinations in the interlayer of montmorillonite, depending on Cu(II) loading and degree of hydration. The quasi-regular octahedral coordination of the interlayer Cu(II) in montmorillonite is a new finding of this study.