Unexpected Complexity in the Electro-Oxidation of Iodide on Gold in the Ionic Liquid 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide

摘要

The electro-oxidation of iodide on a gold electrode in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide has been investigated using transient cyclic voltammetry, linear-sweep semi-integral voltammetry, an electrochemical quartz crystal microbalance technique, and coulometry/electrogravimetry. Two oxidation processes are observed, with an electron stoichiometry of 1:1, compared with the well-known 2:1 electron stoichiometry observed on other commonly used electrode materials, such as platinum, glassy carbon, and boron-doped diamond, under identical conditions. Detailed mechanistic information, obtained in situ using an electrochemical quartz crystal microbalance, reveals that this unusual observation can be attributed to the dissolution of the gold electrode in the presence of iodide. Coulometric/electrogravimetric analysis suggests that the oxidation state of the soluble gold species is +1 and that diiodoaurate, [AuI_2]~-, is the likely intermediate. A proportionally smaller amount of triiodide intermediate is also detected by means of UV-vis spectroscopy. On this basis, it is proposed that iodide oxidation on gold occurs via two parallel pathways: predominantly via a diiodoaurate intermediate 2I~- + Au <=> [AuI_2]~- + e~- and [AuI_2]~- <=> I_2 + Au + e~- and to a lesser extent via a triiodide intermediate 3I~- <=> I_3~- + 2e~- and I_3~- <=> 3/2I_2 + e~-. This proposed mechanism was further supported by voltammetric investigations with an authentic sample of the anionic [AuI_2]~- complex.