In situ characterization of the coordination sphere of Cu-II complexes supported on silica during the preparation of Cu/SiO2 catalysts by cation exchange

In situ diffuse reflectance UV/visible/near-IR and EPR spectroscopies, and also XAFS, were used to study the changes in the coordination sphere of cop per complexes during the different steps of preparation by cation exchange of supported Cu/SiO2 catalysts. Three different Cu-II precursors were used: [Cu(NH3)(4)(H2O)(2)](2+), [Cu(en)(1)(H2O)(4)](2+) and [Cu(en)(2)(H2O)(2)]( 2+). Before drying, the coordination sphere of the different Cu-II complexe s remains the same as in solution, and the complexes are in electrostatic i nteraction with the silica support. Depending on the strength of the ligand s, the Cu-II complexes are grafted onto the silica support as (3=SiO)(2)Cu( NH3)(2)(H2O)(4) or (=SiO)(2)Cu(en)(1)(H2O)(2), or remain in electrostatic i nteraction with the latter as [Cu(en)(2)(H2O)(2)](2+) when the samples are dried at 25 degrees C. Extended drying at 100 degrees C and re-exposure to air induces weak loss of NH3 and en ligands, and some water ligands are rev ersibly eliminated. When the samples are calcined, the NH3 and en ligands a re completely eliminated, and the same three Cu-II species are identified o n the three samples: (=SiO)(2)Cu-II(H2O)(4-x), (=SiO)(3)Cu-II and pairs of octahedral Cu-II species. On exposure to air, the three species transform i nto a single species: (=SiO)(2)Cu-II(H2O)(4).