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Review date: July 24, 2003
PNNL-SA-27883

Copper(I) and Copper(II) Coordination Structure under Hydrothermal Condition at 325°C: An XAFS, and MD Study

Fulton, J. L., Hoffmann, M. M., Darab, J. G., Palmer, B. J., Stern, E. A., J. Phys. Chem. A, (2000), In press.

Abstract
X-ray absorption fine structure (XAFS) spectroscopy was used to measure the coordination structure about Cu²⁺, Cu¹⁺ and Br^- in water at temperatures up to 325°C. The hexaaqua Cu²⁺ species maintains it's distorted octahedral structure up to 325°C whereas at higher temperatures, dehydration reactions occur producing CuO. Under reducing conditions, the dibromo Cu¹⁺ species, [CuBr2]^-, is predominant at 200°C and above for systems having excess Br^-. Even for a very high salt concentration of 2.0 m NaBr, only the dibromo Cu¹⁺ species, [CuBr2]^-, is observed with no evidence of higher Br^- coordination. For this dibromo-species there are no tightly-bound hydration waters in the first shell. In the absence of excess Br^-, a monoaqua monobromo Cu¹⁺ species, [Cu(H2O)Br] is observed.

For certain systems, both Cu and Br XAFS were acquired and a global model was used to fit the two independent sets of XAFS data. Thus, the results represent a complete picture of the coordination structure about Cu¹⁺ including the coordination numbers, distances for the ion-ion and water-ion associations and also a high-quality measurement of the binding strength and amount of disorder (Debye-Waller factor and the anharmonicity) of the Cu¹⁺/Br^- association.

Molecular dynamics (MD) simulations were used to further explore the structure and the binding forces for the [CuBr2]^- species under hydrothermal conditions. We found quantitative agreement for the Cu-Br interactions but the simulation has difficulty predicting the experimental Cu-H2O interaction. In particular, the amount of scattering from the water in the dibromo Cu¹⁺ complex was highly over-predicted so that it is clear that simple intermolecular potential models do not adequately capture this structural feature.