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J. Phys. Chem. 1996, 100, 18511-18514 18511 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 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, 1996X 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. 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 heteropolyoxometalates, a unique class of compounds found to promote the rates of a growing number of important processes,5 including the oxidation of small organics3 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 approximates that found in a practical electrode, under potential control. This paper presents vanadium K-edge and tungsten LIII-edge X-ray absorption near-edge structure (XANES) of Keggin-type mono- and trisubstituted vanadium heteropolytungstates, K4PVW11O40 and Cs6PV3W9O40, immobilized in a high-area carbon electrode. As has been shown in the literature, the replacement of WVI (and also MoVI) with VV makes it possible to modify the overall redox properties of the heteropolyoxo† Case Western Reserve University. Argonne National Laboratory. X Abstract published in AdVance ACS Abstracts, November 1, 1996. ‡ S0022-3654(96)01781-9 CCC: $12.00 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 H2SO4 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 VIII 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 LIIIedge 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 W LIII-edge XANES were acquired using third-harmonic 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 © 1996 American Chemical Society 18512 J. Phys. Chem., Vol. 100, No. 47, 1996 by 3 orders of magnitude. Contributions to the beam due to the fifth harmonic were rejected by detuning the beam by 50%. Polyoxometalate Synthesis. K4PVW11O40 and Cs6PV3W9O40, and H3PW12O40 were prepared and purified according to the procedures outlined in refs 9 and 10, respectively, and subsequently characterized by IR and UV-visible spectroscopy and cyclic voltammetry. Electrode Preparation. a. In situ XAFS. For these experiments, the polyoxometalates, i.e. K4PVW11O40, Cs6PV3W9O40, and H3PW12O40, were immobilized within the structure of a Teflon-bonded, Nafion-containing high-area carbon electrode prepared by mixing 20 mg of Vulcan XC-72 (Cabot Corp. Billerica, MA), 0.25 mL of a 20 mg/mL Teflon T30B aqueous suspension diluted from the as-received 60 wt % emulsion (DuPont, Wilmington, DE) with distilled water, and 0.1 mL of 5% Nafion (equiv wt ca. 900) in a low molecular weight alcohol solution (Solution Technology, Mendenhall, PA). These constituents were mixed together into a paste, to which 0.1 mL of aqueous solutions of K4PVW11O40 (20 mM) or Cs6PV3W9O40 or H3PW12O40 (10 mM) was added. The wet paste was airdried at room temperature to remove excess liquid, then handpressed onto a Pt gauze disk (52 mesh, Aesar, 99.9%), and again air-dried at ca. 80 °C. The final thickness of the electrode was approximately 1 mm. Just prior to the XAFS measurements, the electrode was pressed onto a 5 × 5 cm piece of Kapton tape (50 µm thick), which was then attached to the open end of the Kel-F electrochemical cell, forming the X-ray transparent window. A saturated calomel electrode (SCE) and a Pt wire were used as reference and counter electrodes, respectively. Potentials are reported throughout versus a reversible hydrogen electrode (RHE) in the same media. b. Cyclic Voltammetry in the Immobilized State. Electrochemical characterization of the immobilized polyoxometalates was carried out by pressing a layer of a paste prepared by mixing 4 mg of XC-72 carbon and 50 µL of the dilute Teflon suspension onto a 5 mm diameter ordinary pyrolytic graphite (OPG) disk electrode. This electrode was then immersed into 10-20 mM solutions of the phosphovanadotungstates in 0.1 M H2SO4 in a conventional electrochemical cell using a RHE (pH2 ) 1 atm) as a reference and a Pt wire counter electrode. Incorporation of the polyoxometalates into the electrode structure was achieved by redox cycling, typically, 3 cycles between 1.0 and -0.5 V at 2 mV/s. The electrode was removed from the solution and placed in a cell containing neat 0.1 M H2SO4 in order to obtain cyclic voltammograms of the immobilized polyoxometalate. c. Solution-Phase Voltammetry. The voltammetric properties of the two phosphovanadotungstates and H3PW12O40 in solution phase were examined using smooth disk electrodes of either OPG or glassy carbon in a conventional glass cell in 0.1 M H2SO4 solutions using an RHE and a Pt wire as reference and counter electrodes, respectively. Results and Discussion Electrochemistry. The cyclic voltammetry of solution-phase K4PVW11O40 in 0.1 M H2SO4 (see panel A, Figure 1) yielded values of E°′ in agreement with voltammetric results reported previously in the literature.11,12 The small redox peaks around 0.85 V vs RHE have been assigned to the VV/VIV couple,11 whereas the two sets of features found at more negative potentials are attributed to W-based reductions.11 Voltammetric studies of the closely related species, SiVW11O405-, by Herve et al.12 locate the VV/VIV couple at approximately 0.78 V vs RHE and reveal an additional pH-sensitive couple in the potential region just positive of the W-based reductions, which they attribute to the VIV/VIII couple in the pH range below 2.3. Clinton et al. Figure 1. Cyclic voltammograms for K4PVW11O40 in 0.1 M H2SO4 in solution phase (panel A) (4 mM solution; electrode material, ordinary pyrolytic graphite (OPG); area ) 0.2 cm2; scan rate ν ) 50 mV/s) and immobilized on high-area carbon supported on OPG (panel B, ν ) 10 mV/s) and on a Pt gauze (panel C, scan rate ) 2 mV/s). Our cyclic voltammetric measurements of K4PVW11O40 immobilized in high-area carbon in 0.1 M H2SO4 using a pyrolytic graphite electrode as a support yielded three clearly defined redox peaks centered at about 0.75, 0.2, and and -0.2 V vs RHE (see panel B, Figure 1), assigned formally to the VV/VIV and VIV/VIII couples and to a two-electron reduction of WVI, respectively. Experiments in which this specific electrode was polarized at -0.3 V yielded, in the subsequent linear scan in the positive direction, features other than those observed in a regular cyclic scan. This indicates that the material in its most reduced state is not stable for the rather long times required to acquire in situ XAFS data. Two of these peaks at nearly the same potentials were also found for the immobilized carbonsupported material in the Pt gauze in the same cell where the in situ XAFS measurements were carried out (see panel C, Figure 1). The large current observed at potentials more negative than about -0.05 V vs RHE is due to hydrogen evolution on the Pt gauze. Integration of the charge under the voltammetric peak centered at ca. 0.2 V vs RHE yielded values on the order of 10-7 mol. Of this amount, approximately 10-8 mol was estimated to have been probed by the X-ray beam. For comparison, the voltammetric curves obtained with H3PW12O40 in 0.1 H2SO4 both in solution (panel A) and adsorbed on XC-72 carbon (panel B) are shown in Figure 2. V K-Edge XAFS. The V K-edge XANES of [PVW11O40]nin the three oxidation states examined (n ) 4, 5, 6) yielded distinctly different features. In situ XANES recorded by polarizing the electrode at potentials sufficiently positive to effect a stepwise reoxidation of the complex were nearly identical to those observed prior to the initial stepwise reduction. This behavior indicates that, within the elapsed time of these experiments and the sensitivity of the spectral measurements, the overall structure of the polyoxometalate in the three different oxidation states examined remains intact and that the electrochemical process is chemically reversible in the potential range positive of 0.0 vs RHE. As shown in Figure 3, both the original (n ) 4, solid line) and the one-electron-reduced (dashed line) material exhibit a prominent pre-edge peak associated with the 1s f 3d electronic transition.13 The occurrence of this formally dipole-forbidden band is associated with distortions in the octahedral coordination environment about vanadium in [PVW11O40]4-/5-. In a centrosymmetric ligand field, such as perfectly octahedral VO6 Vanadium-Substituted Heteropolytungstates J. Phys. Chem., Vol. 100, No. 47, 1996 18513 Figure 4. V K-edge XANES of Cs6PV3W9O40 immobilized on higharea carbon supported on a Pt gauze in 0.1 M H2SO4 obtained with the electrode polarized at 0.87 V (s), 0.20 V (- - -), and -0.02 V (‚‚‚) vs RHE (see text for details). Figure 2. Cyclic voltammograms for H3PW12O40 in 0.1 H2SO4 in solution phase (4 mM solution, panel A; electrode material, OPG; area ) 0.2 cm2; ν ) 50 mV/s) and adsorbed on XC-72 carbon (panel B, ν ) 2 mV/s). Figure 3. V K-edge XANES of PVW11O404- immobilized on higharea carbon supported on a Pt gauze in 0.1 M H2SO4 obtained with the electrode polarized at 0.90 V (s), 0.50 V (- - -), and +0.05 V (‚‚‚) vs RHE (see text for details). coordination, the pre-edge peak is either absent or very weak (see below).14 In contrast, strong pre-edges are observed in V K-edge XANES of complexes containing the vanadyl VdO group. The prominence of the two pre-edge peaks of Figure 3 is consistent with the presence of vanadyl groups [VVdO]3+ and [VIVdO]2+ in the original (n ) 4) and the one-electronreduced (n ) 5) anions, respectively. The observed shift of the pre-edge peak from 5470 eV in the original anion (n ) 4, solid line) to ca. 5468 eV in the one-electron-reduced anion (n ) 5, dashed line) is consistent with the reduction of VV to VIV. This observation is in harmony with the reported ESR and magnetic data for the one-electron-reduced forms of R-1,2[SiV2W10O40]6- and R-1,2,3-[SiV3W9O40]6-.15 The unpaired electrons of these electrochemically reduced di- and trisubstituted vanadium derivatives of [SiW12O40]4- are trapped on the vanadium atoms. The second electron reduction of the immobilized [PVW11O40]4- electrode produces a striking change in the V K-edge XANES. As shown in Figure 3 (dotted line), polarization of the electrode at +0.05 V results in the virtually total disappearance of the pre-edge peak. As reported for other complexes involving transition metals of the first row, e.g. iron, the intensity of the 1s f 3d electronic transition decreases as the symmetry of the metal site increases.13,14,16 On the basis of the behavior found for other vanadium complexes, the near absence of the 1s f 3d electronic resonance for [PVW11O40]6- (Figure 3, dotted line) is consistent with the absence of a VdO group. Rather, these XANES data indicate that the V atom is formally trivalent and in an undistorted octahedral coordination environment. There are several examples reported in the literature for VIII compounds displaying XANES with either very weak or no discernible preedge features. These include K3V(cat)3,14a VIII in H2SO4 and HCl solutions,14a,b and NaV(EDTA)‚H2O and Na3[V(NTA)2}.14c On this basis, it is reasonable to conclude that the second oneelectron reduction of PVW11O404- produces a VIII center in a nearly octahedral environment. An increase in the symmetry would be expected, since the VdO double bond involving the terminal oxygen would be replaced by the significantly longer V-OH2 single bond upon reduction from VVI to VIII.12 A nearly complete disappearance of the pre-edge peak was also observed in the case of the tri-vanadium-substituted polytungstate, PV3W9O406-, upon polarizing the electrode at sufficiently negative potentials (see Figure 4). Unfortunately, the voltammetric curves for this species were not as well-defined as for the singly vanadium substituted analog, and therefore, it is not clear whether the potentials selected for the measurements are defined precisely enough to render the material in a single oxidation state. Nevertheless, the results obtained do suggest that the vanadium sites for this more complex ion display nearly ideal octahedral symmetry in their VIII state. Clear evidence for the existence of octahedral sites of various degrees of distortion surrounding substitutional metals in this type of oxometalate compounds may be found in recent studies of the crystal structures of MIII-substituted dimeric Keggin species. In particular, Wassermann et al.17 reported an “almost ideal octahedral environment” for M ) Cr, whereas Yamase et al.18 found a strongly distorted octahedral environment for M ) Ti. Attempts have also been made to infer the symmetry of the substitutional metal site from the intensity of the visible region d-d transitions.19 Structural assignments made on that basis, however, are often complicated due to the occurrence of partially overlapping bands of higher intensity in that same spectral region and, therefore, may not be deemed reliable. The data shown in Figure 3 reveal striking differences between the shifts in the position of the edge jump (Eedge), defined as the energy at which the normalized absorption or fluorescence is one-half, following the first one-electron reduction (compare solid and dotted curves, ca. 3.0 eV) and second one-electron reduction (compare dotted and dashed lines, less than 0.5 eV). Although shifts in Eedge of about 3.0 eV per electron transferred are not unusual for simple inorganic transition metal oxides of the first row,20 it is not possible to 18514 J. Phys. Chem., Vol. 100, No. 47, 1996 Clinton et al. of the W LIII-edge XANES of [PVW11O40]n- for all n and [PW12O40]3- is not at all surprising. Acknowledgment. Support for this work was provided by a grant from ARPA. The research was carried out at the Stanford Synchrotron Radiation Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. M.R.A. is supported by the U.S. DOE, Basic Energy Sciences-Chemical Sciences, under Contract No. W-31-109-ENG-38. References and Notes Figure 5. W LIII-edge XANES recorded using third-harmonic radiation to increase resolution of [PW12O40]3- immobilized on high-area carbon supported on a Pt gauze in 0.1 M H2SO4 obtained with the electrode polarized at 0.50 V (s), 0.03 V (- - -), and -0.25 V (‚‚‚) vs RHE (see text for details). assign oxidation states based purely on the edge position. Specifically, as stated by Bianconi,21 there exists no linear relationship between the oxidation state and Eedge, since Eedge is determined by the threshold of dipole-allowed transitions and, hence, by the mixing of unoccupied d and sp orbitals in the final states. Furthermore, the energy of multiple scattering resonances depends markedly on the interatomic distance giving rise to a much larger energy shift compared to that of the core exciton (pre-edge peak). For example, Frank et al.14a report that the position of the rising edge for the VIII-tris(catecholate) complex occurs about 2 eV lower in energy than that for VIII in sulfuric acid. In addition, Furenlid et al.22 have examined the XANES of a series of nickel tetraazamacrocycles and observed shifts in the values of Eedge between NiI and NiII-type compounds ranging from -3.0 to -0.5 eV, depending on the nature of the macrocyclic ligand and the Ni axial coordination. Further support for the conclusions presented above could, in principle, be gained from V K-edge EXAFS. The replacement of the terminal VdO bond with a V-OH2 bond upon a two-electron reduction would result in a V-O bond length change sufficiently large to be resolved in the EXAFS data. In fact, the terminal WdO (1.66 Å) and bridging W-O (1.96 Å) bonds are resolved in the Fourier transform data of the W LIIIedge EXAFS for [PW12O40]3-.23,24 Unfortunately, the signal obtained in V K-edge in situ XAFS experiments collected either with a Lytle detector or with an energy-resolved detector developed at Lawrence Berkeley Laboratory was not of sufficient quality to carry out such an EXAFS analysis. W LIII-Edge XAS. No discernible changes in the W LIII edge XAS, including the XANES and EXAFS regions (see the Experimental Section), could be detected for [PVW11O40]n- in the three oxidation states examined (not shown in this work). In fact, experiments involving H3PW12O40 in which the in situ W LIII edge XANES were recorded using third-harmonic radiation (see the Experimental Section) yielded identical results for the species in the three oxidation states (see Figure 5). On its own, this observation does not imply that both electron transfer processes involve orbitals without tungsten character. This is because any modification of the W 5d orbital occupation in [PVW11O40]n- with cluster charge (n ) 4, 5, 6) may be too small (i.e., 1 or 2 electrons among 11 W atoms) to be detectable with the overall energy resolution of these XANES measurements. This is especially true if the W orbitals are involved in the lowest unoccupied molecular orbital, which may be delocalized over the whole polyoxometalate framework. Nevertheless, in light of the V K-edge XANES results (Vide supra), which indicate that the added electrons are localized at V, the similarity (1) Kozhevnikov, I. V.; Matveev, K. I. Russ. Chem. 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