In Situ Vanadium K-Edge and Tungsten L III -Edge X-ray Absorption Fine Structure of Vanadium-Substituted Heteropolytungstates Immobilized in a High-Area Carbon Electrode in Acid Aqueous Electrolytes D. E. Clinton, † D. A. Tryk, † I. T. Bae, † F. L. Urbach,* ,† M. R. Antonio, ‡ and D. A. Scherson* ,† Ernest B. Yeager Center for Electrochemical Sciences and the Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106-7078, and Chemistry DiVision, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439-4831 ReceiVed: June 17, 1996 X 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 K 4 PVW 11 O 40 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 Cs 6 PV 3 W 9 O 40 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 WL 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 H 3 PW 12 O 40 adsorbed on XC-72 under otherwise identical conditions. Introduction The search for highly active and highly specific homogeneous and heterogeneous redox catalysts has prompted the synthesis and characterization of species capable of undergoing multiple, reversible electron transfer reactions. 1-4 Attention in the electrochemical area has focused recently on heteropolyoxo- metalates, a unique class of compounds found to promote the rates of a growing number of important processes, 5 including the oxidation of small organics 3 and the reduction of nitrite and nitric oxide. 6 Interest in this laboratory has centered on the ability of a variety of heteropolytungstates to act as cocatalysts (with Pt) for the electrochemical oxidation of methanol in acid solutions when immobilized in high-area carbon-supported Pt electrodes. The confinement of redox catalysts on electrode surfaces can facilitate heterogeneous redox reactions involving multiple electron transfer by allowing a direct delivery of electrons from the electrode to the electrocatalyst within a time scale short enough for the substrate to remain within the interfacial region. Essential to the further understanding of the electronic and structural factors underlying the electrocatalytic activity of immobilized materials is the acquisition of spectroscopic information in situ, i.e. in an environment that closely ap- proximates that found in a practical electrode, under potential control. This paper presents vanadium K-edge and tungsten L III -edge X-ray absorption near-edge structure (XANES) of Keggin-type mono- and trisubstituted vanadium heteropolytungstates, K 4 - PVW 11 O 40 and Cs 6 PV 3 W 9 O 40 , immobilized in a high-area carbon electrode. As has been shown in the literature, the replacement of W VI (and also Mo VI ) with V V makes it possible to modify the overall redox properties of the heteropolyoxo- metalate. 3,7 It is therefore of interest to identify unambiguously the nature of the redox processes associated with these versatile V-substituted Keggin ion derivatives. In situ XANES were recorded in aqueous acid electrolytes in an electrochemical cell as a function of the applied potential using techniques and procedures similar to those developed in this research group for studies involving iron porphyrins irreversibly adsorbed on high-area carbon. 8 Measurements were performed for the immobilized heteropolyoxometalates in three stable redox states in 0.1 M H 2 SO 4 solutions, as determined by cyclic voltammetry. The results obtained have provided strong evidence that the second reduction step of both the mono- and tri-vanadium-substituted heteropolytungstates in this medium yields a V III center in a nearly octahedral environment. Experimental Section Data Acquisition and Data Analysis. All of the XANES measurements were carried out at the Stanford Synchrotron Radiation Laboratory (beamline 4-1) at a ring energy of 3.0 GeV and ring currents in the range 50-100 mA. The radiation was monochromatized using two Si(111) crystals. Harmonic rejection was achieved by detuning the primary beam to 50% of its original intensity. The V K-edge XANES and W L III - edge XANES were recorded in the fluorescence mode in steps of 0.5 eV using an Ar-purged Lytle-type ionization detector with three absorption lengths Zn or Ti filters for W or V, respectively. Because of the small amounts of V involved in the case of the mono-vanadium compound, it was necessary to average four scans to obtain a reasonable signal-to-noise ratio. In situ WL III -edge XANES were acquired using third-har- monic radiation to increase the energy resolution of the beam. For these measurements, the fundamental beam at energies in the range 3000-3500 keV was eliminated by placing the cell about 50 cm away from the beam exit to the hutch. This distance is sufficient to decrease the fundamental beam intensity † Case Western Reserve University. ‡ Argonne National Laboratory. X Abstract published in AdVance ACS Abstracts, November 1, 1996. 18511 J. Phys. Chem. 1996, 100, 18511-18514 S0022-3654(96)01781-9 CCC: $12.00 © 1996 American Chemical Society