1023-1935/04/4005- © 2004 åÄIä “Nauka /Interperiodica” 0500 Russian Journal of Electrochemistry, Vol. 40, No. 5, 2004, pp. 500–509. Translated from Elektrokhimiya, Vol. 40, No. 5, 2004, pp. 565–575. Original Russian Text Copyright © 2004 by Zagrebin, Borzenko, Vasil’ev, Tsirlina. INTRODUCTION Heteropoly anions (HPA) with central atoms in high oxidation degrees can be considered as model systems for studying processes with long-range electron trans- fer [1]. Indeed, if the central ion is reduced at less neg- ative potentials as compared with oxometalate ligands, then, irrespective of the reactant location near the inter- face, the transfer distance appears to be no less (and, in the general case, higher) than the characteristic size of the oxometalate ligand, i.e. exceeds 0.5 nm. In the absence of the specific adsorption (when the reactant is localized in the diffuse part of the electric double layer), the distance of the electron transfer to the central ion is about 1 nm, which is close to characteristic transfer dis- tances in metaloproteins and other complex molecules, which are usually considered as alternative model sys- tems. The apparent advantages of polyoxometalate reactants under discussion are their extremely high sta- bility constants, relatively simple geometry, highly symmetric species, as well as ligands inert with respect to charge conjugation, which allows one to expect a sufficiently simple elementary act mechanism (the absence of bridge, resonance, and other complicating phenomena). Few HPA of this kind studied earlier do not entirely satisfy the requirements to model reactants for the problem formulated above. Complexes of different transition metals (Mn, Co, Ni, etc.) with polymolybdate ligands make it possible to reduce the central ion with- out simultaneously reducing the ligands and (or) oxi- dizing the electrode material only in a narrow potential range. In contrast, heteropoly tungstates that permit the ligand reduction only at potentials hundreds millivolt more negative as compared with equilibrium potentials of redox transitions of transition-metal heteroatoms appear to be more suitable. So far, the kinetics of cen- tral atom reduction in polytungstates was studied only for Na 2 K 6 [MnW 6 O 24 ] [1]. This reactant has demon- strated specific complications, namely, the two-elec- tron reduction of Mn(IV) in a narrow potential range and the destruction of the formed complex. A study of this kind for the solutions of a salt of CeW 10 (CeW 10 ) anion appears to be more promis- ing [2–4]. This Ce(IV)-based heteropoly tungstate allows one to expect the one-electron reduction to give a stable product, namely, an isostructural Ce(III) com- plex [4–6]. According to x-ray diffraction (XRD) data [3], the octacoordinated cerium ion in decatungstate is surrounded by two W 5 O 18 fragments, which form a square antiprism around it 1 (Fig. 1) with the total sym- metry D 4d . By the oscillation spectroscopy [6] and 1 The CeW 10 anion is a structural analog of the isopoly tungstate ion W 10 , which also comprises two W 5 O 18 fragments with four common oxygen atoms. O 36 8– O 32 4– Kinetics of Central Ion Electroreduction in Cerium (IV)-Decatungstate P. A. Zagrebin, M. I. Borzenko z , S. Yu. Vasil’ev, and G. A. Tsirlina Moscow State University, Faculty of Chemistry, Vorob’evy Gory 1, Moscow, 119992 Russia Received March 14, 2003 Abstract—A general characteristics is given to the kinetics of electroreduction of cerium-decatugstate anions on mercury, polycrystalline gold, and the pyrolytic-graphite basis plane in acetate buffer solutions (pH 3.5–6.0). Based on the analysis of UV absorption spectra, the ratio of two reactant forms differing in the protonation degree in solutions under study is estimated. At a negatively charged mercury electrode, the reduction of the Ce(IV) central ion is shown to proceed as an outer-sphere process with classical manifestations of the psi-prime effect (polarization curves reveal a current minimum which deepens with supporting electrolyte dilution and an increase in the reactant negative charge). On the positively charged surface, the current is observed to increase with an increase in the supporting electrolyte concentration, which is interpreted in terms of the strong adsorp- tion of the reactant and its coadsorption with cations. The gold electrode demonstrates pronounced effects of strong chemisorption. Adsorption complications observed on pyrolytic graphite are shown to become strongther for surfaces with more pronounced nonideal behavior. For low concentrations of atomar steps (apparently, for low coverages of pyrolytic-graphite surface with the adsorbed reactant), the quasireversible electron transfer with the rate increasing with increasing pH is observed. Key words: cerium(IV)-decatungstate, dropping mercury electrode, pyrolytic graphite, polycrystalline gold z Corresponding author, e-mail: borzenko@elch.chem.msu.ru