Irreversible electrocatalytic reduction of V(V) to V(IV) using phosphomolybdic acid

Abstract

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).

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