Cu(II) EPR: A Complementary Method for the Thermodynamic Description of Surface Complexation

Abstract

Static EPR parameters of adsorbed Cu²⁺_aq ions onto hydrous oxide surfaces indicate the formation of inner sphere type complexes with surface functional groups. The formed complexes are suggested to be of the type (≡MeO)_4–nCu(H₂O)_n+2, n = 2 for δ-Al₂O₃, one type of SiO₂, (≡MeO)₄Cu(H₂O) for TiO₂(Anatase), and one type of SiO₂. An empirical relationship has been found between thermodynamic stability constants of the complex and the EPR-parameter g_‖. This method is applied in a series of ternary complexes on δ-Al₂O, of the type (≡AlO)₂Cu L₁L₂(H₂O)₂(L₁L₂ = bidentate ligand) to yield information about speciation of Cu(II) and the corresponding stability constants. The copper hyperfine data are interpreted in terms of an LCAO-MO – approach. New possibilities arise for the interpretation of the behaviour of Cu(II) in biological systems. An alternative description is presented to the current picture of cation adsorption in layer silicates and systems of interest in catalysis (e.g. zeolites). ENDOR spectroscopy of copper(II) complexes on δ-alumina confirms the conclusions reached at EPR stage.