In Situ Vanadium 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

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 V^IV and V^III, respectively. No changes in the in situ W L_III-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.