Interaction between Ionic Liquids and a Pt(111) Surface Probed by Coadsorbed CO as a Test Molecule

We used temperature-programmed infrared reflection absorption spectroscopy (TP-IRAS) to study the desorption behavior of CO on Pt(111) coadsorbed with four kinds of ionic liquids (ILs), namely 1-butyl-1-methyl-pyrrolidinium-bis(trifluoromethylsulfonyl)imide ([C₄C₁Pyr][NTf₂]), 1-ethyl-3-methyl-imidazolium-bis(trifluoromethylsulfonyl)imide ([C₂C₁Im][NTf₂]), 1-butyl-1-methyl-pyrrolidinium-trifluoro-methanesulfonate ([C₄C₁Pyr][OTf]), and 1-butyl-1-methyl-pyrrolidinium-hexafluorophosphate ([C₄C₁Pyr][PF₆]). We found that CO desorbs earlier from a Pt(111) surface with coadsorbed ILs than without. In addition, the CO desorption temperature varies between different types of coadsorbed ILs, which follows the order: [C₄C₁Pyr][PF₆] (365 K) > [C₄C₁Pyr][NTf₂] (362 K) > [C₂C₁Im][NTf₂] (352 K) > [C₄C₁Pyr][OTf] (348 K). We ascribe the difference in CO desorption temperature to the different interaction strength between ILs and the Pt(111) surface. A stronger IL-Pt(111) interaction leads to a lower CO desorption temperature. We suggest that TP-IRAS experiments of CO coadsorbed with ILs can be a useful method to aid the characterization of the interaction strength between ILs and metal surfaces such as Pt(111).