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

Abstract

The primary objective of this study is to determine the effect of substrate type on the coordination environments of Cu{sup 2+} adsorbed on amorphous SiO{sub 2}, {gamma}-Al{sub 2}O{sub 3}, and anatase at a surface coverage of approximately 1 {mu}mol/m{sup 2}. The authors also collected X-ray absorption fine structure (SAFS) data for several Cu{sup 2+}-containing model compounds, including tenorite, spertinite, dioptase, shattuckite, chrysocolla, and Cu{sup 2+} acetate monohydrate, 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 Cu{sup 2+} sorption samples are consistent with the presence of Jahn-Teller distorted Cu{sup 2+}(O,OH){sub 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. Cu{sup 2+} sorbed on amorphous SiO{sub 2} was found to have Cu second and third neighbors at 2.95 {angstrom}m 3,39 {angstrom}, and 5.72 {angstrom}, but no Cu-Si correlation was detected for these sorption products associated with amorphous SiO{sub 2}. Based on XAFS and wet chemical results, it seems likely that a Cu(OH){sub 2} precipitatemore » has formed in the Cu{sup 2+}/amorphous SiO{sub 2} system. Cu{sup 2+} sorbed on {gamma}-Al{sub 2}O{sub 3} is present as a mixture of monomeric, dimeric, and perhaps a small number of oligometric hydroxo-bridged Cu(O,OH){sub 6} species with a Cu-Cu distance of approximately 2.95 {angstrom}. Sorbed Cu{sup 2+} on anatase is present predominantly as hydroxo-bridged Cu dimers. At similar sorption densities, Cu{sup 2+} cluster sizes on amorphous SiO{sub 2} are significantly larger than those on {gamma}-Al{sub 2}O{sub 3} or anatase, indicating that the substrate has an important effect on the type of Cu{sup 2+} sorption complex or precipitates formed.« less