In SituVanadium K-Edge and Tungsten LIII-Edge X-ray Absorption Fine Structure of Vanadium-Substituted Heteropolytungstates Immobilized in a High-Area Carbon Electrode in Acid Aqueous Electrolytes

Abstract

Electronic and structural aspects of vanadium-substituted heteropolytungstates immobilized in a high-area carbon (XC-72) as a function of oxidation state have been examined by in situ X-ray absorption near-edge structure (XANES) in an acidic electrolyte. The results obtained for K4PVW11O40 revealed a sizable shift in the V K-edge XANES region, which is characterized by a prominent pre-edge peak, following a one-electron reduction. Such behavior has been attributed to the transfer of an electron to an orbital localized mainly on vanadium. Injection of a second electron gives rise to the near disappearance of the pre-edge peak without major shifts in the position of edge jump, a phenomenon ascribed to an increase in the symmetry of the vanadium site upon reduction to yield a nearly octahedral environment. Similar behavior is observed for Cs6PV3W9O40 when the electrode is polarized in the potential regions where the vanadium ions are reduced to VIV and VIII, respectively. No changes in the in situ W LIII-edge XANES could be discerned for these vanadium-substituted heteropolytungstates in their various oxidation states, for which the spectral features were the same as those of H3PW12O40 adsorbed on XC-72 under otherwise identical conditions.