Electrochimica Acta, Vol.44, No.13, 2271-2279, 1999
Electrochemical reduction of HNO2 or oxidation of benzyl alcohol by electrocatalyst coatings consisting of alternating layers of [P2Mo18O62](6-) anions and Os(II)- or Ru(II)-polypyridine cations
A previously described procedure for coating electrodes with multiple layers of heteropolyanions and charge-balancing dipositive cations [1] was extended to incorporate catalytically active reactants in the coatings. Clear electrocatalytic activity was demonstrated for both anionic ([P2Mo18O62](6-)) and cationic ([cis-Ru(dcbpy)(2)(OH2)(2)](2+) (dcbpy = 6,6'-dichloro-2,2'-bipyridine)) components of coatings on roughly polished edge plane pyrolytic graphite electrodes. However, the stacking of multilayers on top of the first layer of [P2Mo18O62](6-) adsorbed on the electrode produced almost no increase in catalytic currents for the reduction of HNO2 to N2O and the activity of [cis-Ru(dcbpy)(2)(OH2)2](2+) in coatings toward the catalytic oxidation of benzyl alcohol was short-lived. The behavior observed provided insight into the features that must be controlled to obtain useful coatings consisting of multilayers of electrocatalysts. The kinetics of the reduction of HNO2 by [H2P2Mo18O62](6-) with both reactants present in solution were also examined. The reaction produces N2O and a rate constant for the process of 3.5 x 10(2) M-1 s(-1) was estimated from rotating disk voltammetric measurements. O-2 does not react with the reduced complex. [H2Mo18O62](6-), but during its reaction with HNO2 any O-2 also present undergoes catalytic reduction. The intermediate proposed to be involved in both catalytic reactions is an adduct formed by the reaction of HNO2 with [H2P2Mo18O62](6-).