Abstract

The primary objective of this study is to determine the effect of substrate type on the coordination environments of Cu2+ adsorbed on amorphous SiO2, γ-Al2O3, and anatase at a surface coverage of approximately 1 μmol/m2. We also collected X-ray absorption fine structure (XAFS) data for several Cu2+-containing model compounds, including tenorite (VICuO), spertiniite [VICu(OH)2], dioptase (VICuSiO2·H2O), shattuckite [VICu5(SiO3)4(OH)2], chrysocolla [VI(Cu,Al)2H2Si2O5 (OH)4·nH2O], and Cu2+ acetate monohydrate [VICu(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 Å) 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 Å, 3.30 Å, and 5.72 Å, 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)2 precipitate has formed in the Cu2+/amorphous SiO2 system. Cu2+ sorbed on γ-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 Å. 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 γ-Al2O3 or anatase, indicating that the substrate has an important effect on the type of Cu2+ sorption complex or precipitates formed.

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