Redox behavior of europium in the Preyssler heteropolyanion [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12-}

In aqueous, mineral-acid electrolytes, the cyclic voltammetry of the europium- exchanged Preyssler heteropolyanion, [Eu{sup III}P{sub 5}W{sub 30}O{sub 110}]{sup 12-}, is unique among all the other trivalent-lanthanide-exchanged anions, [Ln{sup III}P{sub 5}W{sub 30}O{sub 110}]{sup 12-} for Ln {triple_bond} Ce-Lu. All [LnP{sub 5}W{sub 30}O{sub 110}]{sup 12-}, including Eu, form heteropoly blues upon reduction. In order to obtain insights about this issue, we conducted in situ Eu L{sub 3}-edge XANES (X-ray absorption near edge structure) spectroelectrochemical experiments on an aqueous solution of [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12-} (5.5 mM) in a supporting electrolyte of 1 M H{sub 2}SO{sub 4} at two extreme potentials. The results demonstrate that the Eu{sup III} ion in the colorless Preyssler anion solution at open circuit potential (+0.21 V vs Ag/AgCl) is electroactive and is reduced to Eu{sup II} in the resulting dark blue solution from constant-potential bulk electrolysis at -0.55 V vs Ag/AgCl. This unusual redox behavior of [EuP{sub 5}W{sub 30}O{sub 110}]{sup 12-} may be of technological importance in the area of oxidation catalysis