Ionic Liquids in High Vacuum

SPECTROSCOPY

Chemists at the University of Nottingham, in England, have used an ultra-high-vacuum (UHV) technique—X-ray photoelectron spectroscopy (XPS)—to investigate pure room-temperature ionic liquids and solutions of catalysts in ionic liquids (Chem. Commun., published online Oct. 17, dx.doi.org/10.1039/b512311a).

Team leader Peter Licence believes this is the first time any liquid sample has been investigated by any UHV spectroscopy. Prior to our work, all samples studied by these techniques were either solids or frozen samples. Normal liquids simply evaporate under UHV. Ionic liquids, on the other hand, have almost zero vapor pressure because they are composed entirely of ions. They therefore do not evaporate under UHV, he says.

XPS provides information about the electronic states of different kinds of atoms in a compound and has sufficient sensitivity to distinguish between atoms of the same element situated in chemically distinct environments in a compound, Licence explains.

Our preliminary experiments have demonstrated that it is possible to obtain high-quality XPS data on pure ionic liquids as well as on ionic liquids doped with simple metal salts, he says. We are therefore able to carry out in situ monitoring of catalysis in ionic liquids and obtain information about dissolved catalytic species that was never before available. In principle, we could use this information to explain catalytic performance or selectivity.

The Nottingham researchers obtained high-resolution X-ray photoelectron spectra of 1-ethyl-3-methylimidazolium ethylsulfate, a commercial ionic liquid. They also used the technique to monitor the reduction of Pd²⁺ ions in a solution of a palladium catalyst in the liquid.

Potential applications of XPS of ionic liquids are wide-ranging, according to Licence. They include the investigation of the dynamics of ionic liquid surfaces and interactions with solutes and the electrochemistry of ionic liquids in vacuo, he says.