Irreversible electrocatalytic reduction of V(V) to V(IV) using phosphomolybdic acid
Although VO₂⁺(aq) reduction is kinetically slow at glassy carbon and Pt electrodes, phosphomolybdic acid is shown to catalyze the electrochemical reduction of VO₂⁺(aq) to VO²⁺(aq) in 1.0 M H₂SO₄(aq). A second-order rate constant of 33 M⁻¹ s⁻¹ was observed for this process. ³¹P NMR spectra demonstrated that PMo₁₁VO₄⁴⁻ and PMo₁₀V₂O₄⁵⁻ were the dominant P-containing species under electrocatalytic conditions. The incorporation of Vⱽ into the polyoxoanion led to a shift in potential from E⁰(VO₂⁺(aq)/VO²⁺(aq)) = +0.80 V vs Ag/AgCl for free Vⱽ/Vᴵⱽ to E°' = +0.55 V vs Ag/AgCl for Vⱽ/Vᴵⱽ bound in the heteropolyoxometalate (PM₁₁VO₄₀⁻). This shift in formal potential corresponded to an equilibrium constant of 1.7 X 10⁴ M⁻¹ for preferential binding of Vⱽ over Vᴵⱽ by the heteropolyoxoanion. This negative shift in redox potential, combined with the slow electrochemical kinetics of free VO₂⁺(aq) reduction and with the facile reaction of bound Vᴵⱽ with free Vⱽ in 1.0 M H₂SO₄(aq), resulted in the irreversible electrocatalytic reduction of VO₂⁺(aq) to VO²⁺(aq).
© 1994 American Chemical Society. Received January 28, 1993. We acknowledge ARPA for support of this work and thank Professors F. C. Anson and J. E. Bercaw and Dr. J. Labinger of Caltech for helpful discussions and Dr. D. A. Keire for help with ^(31)P NMR. We also acknowledge Dr. J. Grate for generously supplying a preprint of ref 3d.