Copper(I) and Copper(II) Coordination Structure under Hydrothermal Conditions at 325 °C: An X-ray Absorption Fine Structure and Molecular Dynamics Study

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 its 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 coo...