Nitrite Electroreduction Enhanced by Hybrid Compounds of Keggin Polyoxometalates and 1-Butyl-3-Vinylimidazolium

Abstract

We describe here an immobilization method of four Keggin-type polyoxometalates (POMs) ([H₂W₁₂O₄₀]⁶⁻, [BW₁₂O₄₀]⁵⁻ [SiW₁₂O₄₀]⁴⁻, [PW₁₂O₄₀]³⁻) by using the reaction with an ionic liquid, 1-butyl-3-vinylimidazolium (BVIM) bromide. The reaction yields a hybrid material (BVIM-POM) as a water-insoluble salt. The chemical structure of both compounds is preserved, as indicated by infrared spectroscopy (FT-IR), although with a reduced crystallinity (shown by X-ray diffraction analysis) due to a decrease of water content (shown by thermogravimetric analysis). Cross polarization ¹H-³¹P NMR evidenced the presence of BVIM in the structure of (BVIM)₃[PW₁₂O₄₀]. The salt is mixed with carbon powder and Nafion to prepare an ink and casted on glassy carbon electrodes. The electrochemical behavior of immobilized POMs material is preserved. The electrochemical activity for nitrite reduction is measured by cyclic voltammetry and differential electrochemical mass spectrometry (DEMS). It was observed that the reduction current of 10 mM HNO₂ at pH 1 in 0.5 M Na₂SO₄ is enhanced in the presence of these hybrid materials. DEMS has evidenced the formation of nitrous oxide (N₂O) at potentials more positive compared to the use of parent POMs in solution.