IRREVERSIBLE ELECTROCATALYIC 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. P-31 NMR spectra demonstrated that PMo11VO404- and PMo10V2O405- were the dominant P-containing species under electrocatalytic conditions. The incorporation of V(v) into the polyoxoanion led to a shift in potential from E-degrees(VO2+(aq)/VO2+(aq)) = +0.80 V vs Ag/AgCl for free V(V)/V(IV) to E-degrees' = +0.55 V vs Ag/AgCl for V(V)/V(IV) bound in the heteropolyoxometalate (PMo11VO404-). This shift in formal potential corresponded to an equilibrium Constant of 1.7 x 10(4) M-1 for preferential binding of V(V) over V(VI) 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 V(IV) with free V(V) in 1.0 M H2SO4(aq), resulted in the irreversible electrocatalytic reduction Of VO2+(aq) to VO2+(aq).