Electrochemistry and spectroscopy of UO2+2 in acidic AlCl3-EMIC

The redox behavior of uranyl (UO22+) has been examined in acidic AlCl3-1-ethyl-3-methyl-imidazolium chloride (> 50 mol% AlCl3), a room temperature ionic liquid. Uranyl(VI) was reduced in bulk solution by a chloraluminate species (AlCl4-, Al2Cl7, or Al3Cl10-) to produce U(V) chloride (UCl6-). The kinetics of the reduction was investigated spectroscopically by monitoring the growth of spectral bands of U(V) chloride in the visible and near-IR regions of the spectrum over a five day period. Cyclic voltammetry of uranyl(VI) in acidic AlCl3-EMIC at a scan rate of 100 mV/second indicated three reduction reactions occurred at the electrode surface corresponding to the U(VI)/U(V), U(V)/U(IV) and U(IV)/U(III) redox couples. At slower scan speeds (5 mV/second) only two reductive processes were observed which are assigned to the U(V)/U(IV) and U(IV)/U(III) redox reactions. Bulk coulometry of UO2Cl2 in acidic AlCl3-EMIC indicated an average 1.7 electron reduction producing a solution which was verified spectroscopically to be U(IV). A possible mechanism for the behavior of uranyl(VI) in acidic AlCl3-EMIC involves removal of the oxygens from UO22+ in reactions with the solvent prior to the reduction, followed by reduction with AlCl4-. Uranyl(VI) is not a stable species in acidic room temperature ionic liquids. (orig.)