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

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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.