Mechanistic analysis of the electrocatalytic properties of dissolved alpha and beta isomers of [SiW12O40]4- and solid [Ru(bipy)3]2[alpha-SiW12O40] on the reduction of nitrite in acidic aqueous media

Voltammetric studies on the reduction of alpha and beta isomers of the Keggin polyoxometalate anion SiW12O40]4- reveal a series of electrochemically reversible processes in acidic aqueous media. In the presence of NO2-, catalytic current is detected in the potential region of the SiW12O40]4-/5- process. Electronic spectroscopy and simulation of voltammetric data undertaken at variable NO2-] and H+] allow the following mechanism to be postulated, SiW12O40]4- + e- <-->SiW12O40]5-, H+ + HNO2 <--> NO+ + H2O, NO+ + SiW12O40]5- --> NO + SiW12O40]4-. The second-order rate constant for the rate-determining step is faster for the alpha isomer than for the beta one. This may be attributed to the different reversible potentials of -0.144 V (alpha isomer) and -0.036 V vs Ag/AgCl (beta isomer) and, hence, smaller driving force for an assumed outer sphere electron-transfer reaction in the case of beta isomer. A stable, water-insoluble, thin-film Ru(bipy)3]2alpha-SiW12O40] chemically modified electrode was generated electrochemically via ion-exchange of Ru(bipy)3]2+ with Bu4N+ in the Bu4N]4alpha-SiW12O40] solid. The first reduction process with this modified electrode gives rise to the reaction Ru(bipy)3]2alpha-SiW12O40](solid) + H+(soln) + e- <--> HRu(bipy)3]2alpha-SiW12O40](solid). The need to transfer a proton from the solution to the solid phase for charge neutralization purposes introduces a hydrogen-ion concentration dependence into this reaction, which is not found in the solution-phase study. Nevertheless, the voltammetric catalytic activity with respect to nitrite reduction is retained with the chemically modified electrode. However, nitrite catalysis with the Ru(bipy)3]2alpha-SiW12O40]-modified electrode is now independent of concentration of H+, rather than exhibiting a first-order dependence, and full mechanistic details for this process are unknown.