Wells–Dawson-derived tetrameric complexes {K28H8[P2W15Ti3O60.5]4} electrochemical behaviour and electrocatalytic reduction of nitrite and of nitric oxide

At low pH, trivacant polyoxometalate α-[P2W15O56]12- reacts with titanyl and forms tetrameric {K28H8[P2W15Ti3O60.5]4} (1). The purity of [P2W15Ti3O60.5]436− has been confirmed by IR spectroscopy, elemental analysis, and 31P NMR spectrometry. The electrochemical behaviour of the tetrameric complexes in aqueous solution has been investigated at various pHs. [P2W15Ti3O60.5]436− exhibits successive reduction processes of the W atoms in a negative potential range. All waves are pH dependent at pH < 4. The anion showed an electrocatalytic effect on the reduction of HNO2, but this effect is greatly influenced by pH. At low pH (pH < 3.5) the cathodic peak current of the first WVI/V couple is enhanced while its anodic couterpart disappears. When pH > 3.5 no catalytic effect is observed. On the contrary, upon saturation of the POM solution with NO, even at pH > 3.5, electrocatalytic reduction of NO was observed. The tetrameric tri-TiIV-substituted Wells–Dawson polyoxometalate [P2W15Ti3O60.5]436− was not degraded by repeated cycling between its oxidized and reduced states.