Electrochemical properties of organo-titanium substituted heteropolytungstate and its electrocatalytic reduction of nitrite

Electrochemical and electrocatalytic behaviors of Keggin type organic–inorganic hybrid organo-titanium substituted heteropolytungstate, β-(Bu4N)7SiW9O37(CpTi)3 (Cp = –C5H5), were investigated in acetonitrile solution and in acidic aqueous solutions using cyclic voltammetry (CV), normal pulse voltammetry (NPV) and amperometry. In acetonitrile solution, by NPV the heteropolytungstate exhibits two reduction waves, and by CV it does two couples of redox peaks. It was proved that the first one corresponds to three overlapping Ti(IV/III) single-electron transfers and the more negative one belongs to four overlapping W(VI/V) single-electron transfers of a β-(Bu4N)7SiW9O37(CpTi)3 molecule, respectively. In acidic aqueous solutions, the heteropolytungstate doped in silica gel and then modified onto a glassy carbon electrode exhibits the similar electron-transfer processes as it in acetonitrile solution and possesses an excellent catalytic property in the electrochemical reduction of NO2-. It is the reduced Ti(III) centers but the reduced W(V) centers that act as a catalyst to catalyze the electrochemical reduction of NO2- to N2. The high sensitivity and good stability serve the modified electrode as a sensor for the determination of NO2-. Its detection limit is 1.2 × 10−6 M (Signal:Noise = 3:1).