Mechanistic analysis of the electrocatalytic properties of dissolved α and β isomers of [SiW12O40]4- and solid [Ru(bipy)3]2[α-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 reversible arrow 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 one. This may be attributed to the different reversible potentials of -0.144 V (alpha isomer) and -0.036 V vs Ag/AgCl (P isomer) and, hence, smaller driving force for an assumed outer sphere electron-transfer reaction in the case of P isomer. A stable, water-insoluble, thin-film [Ru(bipy)(3)](2)[(alpha-SiW12O40] chemically modified electrode was generated electrochemically via ion-exchange of [Ru(bipy)(3)](2+) with Bu4N+ in the [Bu4N](4)[(alpha-SiW12O40] solid. The first reduction process with this modified electrode gives rise to the reaction [Ru(bipy)(3)](2)[(alpha-SiW12O40](solid) + H+(soln) + e- reversible arrow H[Ru(bipy)(3)](2)[alpha-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)](2)[(alpha-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.