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Electrochemical and Computational Study of Ion Association in the 3− Electroreduction of PW12O40 J

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Electrochemical and Computational Study of Ion Association in the 3− Electroreduction of PW12O40 J Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX pubs.acs.org/JPCC Electrochemical and Computational Study of Ion Association in the 3− Electroreduction of PW12O40 J. M. Gomez-Gil,́ † E. Laborda,† J. Gonzalez,† A. Molina,*,† and R. G. Compton‡ † Departamento de Química Física, Facultad de Química, Regional Campus of International Excellence “Campus Mare Nostrum”, Universidad de Murcia, 30100 Murcia, Spain ‡ Department of Chemistry, Physical & Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom *S Supporting Information ABSTRACT: Insights into ion pairing effects on the redox 3− properties of the Keggin-type polyoxotungstate PW12O40 are gained by combining electrochemical experiments and density functional theory (DFT) calculations. Such effects have been reported to affect the performance of these species as molecular electrocatalysts. Experimental square wave voltam- 3− metry (SWV) of the two-electron reduction of PW12O40 in 4− acetonitrile evidences that the reduced forms PW12O40 and 5− fi PW12O40 can be signi cantly stabilized by ion association. The strength and stoichiometry of the corresponding aggregates are estimated as a function of the nature of the cation (lithium, sodium, and tetramethylammonium) and the oxidation state of the polyoxometalate. The results obtained in combination with DFT enable us to examine the roles of the cation solvation and the charge number and distribution of the polyanions. 1. INTRODUCTION electrochemical and quantum-chemical approach. As will be discussed, the “apparent” relative stability in solution of PW3− Polyoxometalates (POMs) are anionic metal-oxide clusters that 4− 5− show particular electronic properties of great interest in a large and the reduced forms PW and PW can be elucidated from − number of areas,1 5 specifically in electrocatalysis (water voltammetric measurements in an aprotic medium (acetoni- oxidation,6,7 epoxidation of alkenes,8 bromate reduction,9 trile). For this, the use of square wave voltammetry (SWV) in hydrogen evolution reaction,10 etc.).11,12 POMs can undergo combination with microelectrodes provides important advan- 1,13,14 tages for accurate quantitative analyses, mainly well-defined, multiple electron transfers where the stability and 34,35 ff fi peak-shaped signals with reduced ohmic drop and reactivity of the di erent oxidation states are de ned by their 36−38 intrinsic electronic properties15 (structure16,17 and elemental capacitive distortions. As will be discussed, the variation 18−20 “ ” 21 of the position of the experimental SWV voltammograms composition )andalsoby environmental factors 3− 22−24 corresponding to the first two electroreductions of PW upon (solvation, protonation, ... ). Among the latter, ion pairing ff hasbeenreportedtoaffect the efficiency of molecular the addition of di erent monovalent cations (lithium, sodium, ff and tetramethylammonium) clearly reflects the occurrence of electrocatalysis of di erent systems even in aqueous fi media25,26 by decreasing the reactivity of the catalyst upon ion association, which depends signi cantly on the nature of the ion association;15 also, the catalytic pathway and turnover the cation and on the oxidation state of the polyoxometalate. 27 ff Through the theory developed in a recent work for frequency can be a ected as a result of the change in the 39 “apparent” formal potential9 and the electron density multielectron transfers coupled with chemical equilibria and ff with the assistance of density functional theory (DFT) distribution (see below). For the investigation of these e ects, 40−46 electrochemical methods are very valuable, since they enable calculations, a consistent picture is gained about the ion fi direct access to the redox behavior of species under operational pairing of polyoxometalates, including the identi cation of the conditions, either dissolved in solution or surface-immobilized. chief physicochemical factors (ion size, charge number and POMs have been reported to undergo ion association distribution, solvation, and steric hindrance), the determination − ffi processes28 30 such that their redox behavior and electro- of the anion:cation stoichiometries (which can be a di cult catalytic activity will be affected by the ionic composition of the − medium.31 33 In this work, ion pairing effects on the Received: July 18, 2017 electrochemical properties of the Keggin-type polyoxotungstate Revised: November 2, 2017 3− 3− PW12O40 (PW ) will be investigated in detail via a joint Published: November 2, 2017 © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.7b07073 J. Phys. Chem. C XXXX, XXX, XXX−XXX The Journal of Physical Chemistry C Article task30), and the value of the association constants. The results concentrations of 0.05−5 mM with the Fuoss−Hsia− can assist the optimization of operating conditions for the use Fernandez−Prini equation51,52 (see section S.2 of the of polyoxometalates as electrocatalysts. Supporting Information). The experimental values obtained for the association constant and the limiting molar conductivity 2. EXPERIMENTAL SECTION Λ are given in Table 1; note that in all cases the 0-value agrees 2.1. Chemical Reagents. Anhydrous acetonitrile (MeCN, satisfactorily with the data reported in the literature. Sigma-Aldrich, 99.8%), ferrocene (Fe(C5H5)2, Aldrich, 97%), K tungstophosphoric acid sodium salt (Na3[PW12O40], Riedel-de- Table 1. Values of the Ion Association Constant ( A) and Haen,̈ analytical reagent grade), tetrahexylammonium hexa- Limiting Molar Conductivity (Λ ) between the Ions of the fl 0 uorophosphate (THAPF6, Sigma-Aldrich, 97%), tetramethy- Supporting Electrolytes Obtained via Conductivity with the fl 51,52 a lammonium hexa uorophosphate (TMAPF6, Sigma-Aldrich, Fuoss−Hsia−Fernandez−Prini Equation 98%), sodium hexafluorophosphate (NaPF , Sigma-Aldrich, 6 c Λ Λ 48,53−55 fl KA (this work) 0 (this work) 0 98%), and lithium hexa uorophosphate (LiPF6, Sigma-Aldrich, −1 2 −1 2 −1 fi electrolyte (M ) (S cm mol ) (S cm mol ) 98%) were all used as received without further puri cation. ± ± ± 2.2. Instrumentation. LiPF6 21 4 169.4 0.2 169.75 0.05 All electrochemical measurements ± ± ± NaPF6 29 3 174.3 0.2 178.6 0.02 were performed with a home-built potentiostat. A Pt wire was ± ± used as the counter electrode, a silver wire as the quasi- TMAPF6 33 4 204.9 0.2 196.75 fi aT = 298 ± 2 K. Error bars correspond to the standard deviation reference electrode, and a carbon ber (CF) microdisc ff electrode of 33 μm diameter (ALS Co.) or a glassy carbon obtained from three di erent sets of measurements. (GC) disc of 3 mm diameter (CH Instruments) as the working electrode. The electrodes were polished prior to the experi- ments using 1.0, 0.3, and 0.05 μm alumina−water slurry on soft 2.5. Computational Details. The Gaussian 09, revision lapping pads (Buehler, Illinois), and the electrode radius was 56 calibrated via chronoamperometry.37,38,47 D.01, package program was employed in all of the quantum- The conductivity measurements were performed with a chemical computations performed. All of the density functional theory (DFT) calculations were carried out with the B3LYP conductimeter BASIC 30 (Crison) with built-in temperature 57,58 correction. functional and the 6-31+G(d) basis set. In the case of the PW species, quasi-relativistic pseudopotentials of the W atoms 2.3. Electrochemical Measurements. The study of the 59 3− 3− proposed by Hay and Wadt were employed and the electroreduction of [PW12O40] (PW ) was performed at different concentrations of hexafluorophosphate salts of the LANL2DZ basis sets associated with the pseudopotential were adopted. For the rest of the elements, 6-31+G(d) was cations under study: LiPF6, NaPF6, or TMAPF6. Acetonitrile solutions were deaerated prior to experiments, and a nitrogen employed as the basis set. For optimizations, the SCF convergence criteria was set to atmosphere was maintained in the cell meanwhile. A silver wire −7 fi was employed to avoid any water contamination and 10 . An ultra ne integration grid was considered for the density functional theory (DFT) calculations and a fine one to uncertainties related to junction potentials, with the ferro- − cene−ferrocenium (Fc/Fc+) redox couple as the internal solve the coupled perturbed Hartree Fock (CPHF) equations. − reference.48 50 Frequency calculations were performed at the same level of In order to fix the ionic strength in all solutions at 0.1 M, theory as the geometry optimizations to characterize the fl stationary points as local minima (equilibrium structures). No tetrahexylammonium hexa uorophosphate (THAPF6)was ff employed, which can be expected to be fully dissociated scaling procedures were considered. Also, the e ect of the + solvent was taken into account by using the CPCM solvation given the large size of the THA cation. For the same reason, 60−62 ion association between the PW anions and THA+ can be model (conductor-like polarizable continuum model). disregarded (see sections S.3 and S.4 of the Supporting Information). This was further verified with SWV experiments 3. RESULTS AND DISCUSSION under different concentrations of THA+ where the experimental 3.1. Theoretical Treatment of the Electrochemical SWV curves did not show any dependence on the THA+ SWV Response. Given that species PW3− and the reduced concentration beyond that associated with the variation of the forms PW4− and PW5− are bulky and highly charged, the ionic strength. possibility of association with multiple cations can be envisaged. 2.4. Supporting Electrolyte Ion Pairing.
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