Electrocatalytic studies on imidazolium based ionic liquids: defining experimental conditions

Influence of interfacial water and cations on the oxidation of CO at the platinum/ionic liquid interface

CO oxidation is fundamental to the development of new catalyst materials for fuel cells and key for complete oxidation of small alcohols like methanol or ethanol on Pt catalysts. So far, room-temperature ionic liquids (RTIL) have been used to modify the selectivity and activity in electrocatalysis. In order to understand the mechanism of CO oxidation in RTIL in more detail we have investigated this reaction at the Pt(111)/1-butyl-3-methylimidazolium trifluorosulfonylimide [BMIM][NTf₂] electrode/electrolyte interface as a function of H₂O concentration and electrode potential with in situ sum-frequency generation (SFG) spectroscopy and infrared absorption spectroscopy (IRAS). Using SFG spectroscopy, we address the changes of linearly bonded CO molecules on Pt(111), while we monitor the changes in the bulk electrolyte with IRAS through vibrational bands from H₂O, CO₂ and CO. The presence of water in [BMIM][NTf₂] shifts the onset potential for CO oxidation by more than 200 mV when the water concentration is increased from 0.01 to 1.5 M, which we relate to the incorporation and the availability of water at the electrode/electrolyte interface. The nature of the RTIL cation has also a large effect on the surface excess of H₂O since RTILs like [BMMIM][NTf₂] and [BMPyrr][NTf₂] which are prone to form closed-packed structures, can block the incorporation of water and lead to more sluggish CO oxidation with larger overpotentials and oxidation in a much wider potential range for which we provide evidence by additional SFG measurements. These results clearly show that the choice of the RTIL is important for CO oxidation on Pt(111) electrode surfaces - an observation that is likely highly relevant also to other catalysts and catalytic reactions that require the presence of interfacial water.

Electrocatalytic Conversion of CO2 to Formate at Low Overpotential by Electrolyte Engineering in Model Molecular Catalysis

An electrolyte engineering strategy was developed for CO₂ reduction into formate with a model molecular catalyst, [Rh(bpy)(Cp*)Cl]Cl, by modifying the solvent (organic or aqueous), the proton source (H₂ O or acetic acid), and the electrode/solution interface with imidazolium- and pyrrolidinium-based ionic liquids (ILs). Experimental and theoretical density functional theory investigations suggested that π⁺ -π interactions between the imidazolium-based IL cation and the reduced bipyridine ligand of the catalyst improved the efficiency of the CO₂ reduction reaction (CO₂ RR) by lowering the overpotential, while granting partial suppression of the hydrogen evolution reaction. This allowed tuning the selectivity towards formate, reaching for this catalyst an unprecedented faradaic efficiency (FE_HCOO -) ≥90 % and energy efficiency of 66 % in acetonitrile solution. For the first time, relevant CO₂ conversion to formic acid/formate was reached at low overpotential (0.28 V) using a homogeneous catalyst in acidic aqueous solution (pH=3.8). These results open up a new strategy based on electrolyte engineering for enhancing carbon balance in CO₂ RR.

Acid Catalysis with Alkane/Water Microdroplets in Ionic Liquids

Ionic liquids are composed of an organic cation and a highly delocalized perfluorinated anion, which remain tight to each other and neutral across the extended liquid framework. Here we show that n-alkanes in millimolar amounts enable a sufficient ion charge separation to release the innate acidity of the ionic liquid and catalyze the industrially relevant alkylation of phenol, after generating homogeneous, self-stabilized, and surfactant-free microdroplets (1-5 μm). This extremely mild and simple protocol circumvents any external additive or potential ionic liquid degradation and can be extended to water, which spontaneously generates microdroplets (ca. 3 μm) and catalyzes Brönsted rather than Lewis acid reactions. These results open new avenues not only in the use of ionic liquids as acid catalysts/solvents but also in the preparation of surfactant-free, well-defined ionic liquid microemulsions.