Mechanistic studies of the electro-oxidation pathway of ammonia in several room-temperature ionic liquids

A mechanistic study of the direct oxidation of ammonia has been reported in several room-temperature ionic liquids (RTILs), namely, [C₄mim] [BF₄], [C₄mim][OTf], [C₂IrUm][NTf₂], [C₄mim][NTf₂], and [C₄mim][PF₆], on a 10 μm. diameter Pt microdisk electrode. In four of the RTILs studied, the cyclic voltammetric analysis suggests that ammonia is initially oxidized to nitrogen, N₂, and protons, which are transferred to an ammonia molecule, forming NH₄ ⁺ via the protonation of the anion(s) (A^-). In contrast, NH₄ ⁺ is formed first in [C_4-mim] [PF₆], followed by the protonated anion(s), HA. In all five RTILs, both HA and NH₄ ⁺ are reduced at the electrode surface, forming hydrogen gas, which is then oxidized. The effect of changing the RTIL anion is discussed, and this may have implications in the defining of pk_a in RTIL media. This work also has implications in the possible amperometric sensing of ammonia gas.