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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
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