The electrochemical oxidation and reduction of nitrate ions in the room temperature ionic liquid [C2mim][NTf2]; the latter behaves as a ‘melt’ rather than an ‘organic solvent’

Abstract

The electrochemical oxidation of 1-butyl-3-methylimidazolium nitrate [C₄mim][NO₃] was studied by cyclic voltammetry in the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₂mim][NTf₂]. A sharp peak was observed on a Pt microelectrode (d = 10 μm), and a diffusion coefficient at infinite dilution of ca. 2.0 × 10⁻¹¹ m² s⁻¹ was obtained. Next, the cyclic voltammetry of sodium nitrate (NaNO₃) and potassium nitrate (KNO₃) was studied, by dissolving small amounts of solid into the RTIL [C₂mim][NTf₂]. Similar oxidation peaks were observed, revealing diffusion coefficients of ca. 8.8 and 9.0 × 10⁻¹² m² s⁻¹ and solubilities of 11.9 and 10.8 mM for NaNO₃ and KNO₃, respectively. The smaller diffusion coefficients for NaNO₃ and KNO₃ (compared to [C₄mim][NO₃]) may indicate that NO₃⁻ is ion-paired with Na⁺ or K⁺. This work may have applications in the electroanalytical determination of nitrate in RTIL solutions. Furthermore, a reduction feature was observed for both NaNO₃ and KNO₃, with additional anodic peaks indicating the formation of oxides, peroxides, superoxides and nitrites. This behaviour is surprisingly similar to that obtained from melts of NaNO₃ and KNO₃ at high temperatures (ca. 350–500 °C), and this observation could significantly simplify experimental conditions required to investigate these compounds. We then used X-ray photoelectron spectroscopy (XPS) to suggest that disodium(I) oxide (Na₂O), which has found use as a storage compound for hydrogen, was deposited on a Pt electrode surface following the reduction of NaNO₃.