Identification and electroanalytical characterization of redox transitions in solid-state Keggin type phosphomolybdic acid

The tetragonal single crystal, H3PMo12O40 center dot 29H(2)O, is considered here as a model system to describe multielectron redox reactions of Keggin type phosphododecamolybdate. Electrochemical experiments have been performed in solid-state, i.e. in the absence of contact with liquid electrolyte phase, and they have utilized a gold wire ultramicrodisk (diameter: 10 mu m) working electrode, a silver disk quasireference electrode, and a glassy carbon counter electrode within all solid cell. The results of voltammetric measurements are explained and verified against the simulated data. The reversible redox reactions of phosphododecamolybdic acid (H3PMo12O40 center dot 29H(2)O) single crystal, that appear at most positive potentials, can be understood in terms of two overlapping one-electron redox processes (separated by only approximate to 50 mV), rather than a single two-electron redox transition, followed by another one-electron process. Comparison is made to the conventional electrochemical behavior of phosphododecamolybdate in solution (0.5 mol dm(-3) H2SO4) as well as to the characteristic of its adsorbate on the electrode surface. The following parameters have been determined from the combination of potential step experiments performed in two limiting (radial and linear) diffusional regimes: the concentration of heteropolymolybdate redox centers, 1.4 mol dm(-3), and the apparent diffusion coefficient for charge propagation, 3.6 x 10(-7) cm(2) s(-1). The result supports the concept of the kinetic control of charge transport by electron hopping (self-exchange) between mixed-valence (Mo-VI/Mo-V) sites.