XAFS study of Cu model compounds and Cu2+ sorption products on amorphous SiO2, γ-Al2O3, and anatase

The primary objective of this study is to determine the effect of substrate type on the coordination environments of Cu2+ adsorbed on amorphous SiO2, gamma-Al2O,, and anatase at a surface coverage of approximately 1 mu mol/m(2). We also collected X-ray absorption fine structure (XAFS) data for several Cu2+-containing model compounds, including tenorite ((CuO)-Cu-VI), spertiniite [Cu-VI(OH)(2)], dioptase ((CuSiO2)-Cu-VI. H2O), shattuckite [Cu-VI(5)(SiO3)(4)(OH)(2)], chrysocolla [(VI)(Cu,Al)(2)H2Si2O5 (OH)(4). nH(2)O], and Cu2+ acetate monohydrate [Cu-VI(CH3CO2)(2). H2O], for comparison with the sorption sample data. Detailed analysis of these model compounds indicates that the bonding of second neighbors surrounding a central Cu absorber determines whether these second neighbors can be detected by XAFS. The XAFS results of Cu2+ sorption samples are consistent with the presence of Jahn-Teller distorted Cu2+(O,OH)(6) octahedra, with four equatorial Cu-O bonds (1.95 Angstrom) and two longer axial bonds; the axial Cu-O bonds are difficult to characterize quantitatively by XAFS spectroscopy. Cu2+ sorbed on amorphous SiO2 was found to have Cu second and third neighbors at 2.95 Angstrom, 3.30 Angstrom, and 5.72 Angstrom, but no Cu-Si correlation was detected for these sorption products associated with amorphous SiO2. Based on XAFS and wet chemical results, it seems Likely that a Cu(OH), precipitate has formed in the Cu2+/amorphous SiO2 system. Cu2+ sorbed on gamma-Al2O3 is present as a mixture of monomeric, dimeric, and perhaps a small number of oligomeric hydroxo-bridged Cu(O,OH)(6) species with a Cu-Cu distance of approximately 2.95 K. Sorbed Cu2+ on anatase is present predominantly as hydroxo-bridged Cu dimers. At similar sorption densities, Cu2+ cluster sizes on amorphous SiO2 are significantly larger than those on gamma-Al2O, or anatase, indicating that the substrate has an important effect on the type of Cu2+ sorption complex or precipitates formed.