Tailoring the Surface Enrichment of a Pt Catalyst in Ionic Liquid Solutions by Choice of the Solvent

The so-called buoy-effect, that is, the targeted surface enrichment of a Pt catalyst dissolved in ionic liquids (ILs), is achieved by attaching perfluorinated alkyl chains to the ligand system, which drags the metal complex toward the interface. Using angle-resolved X-ray photoelectron spectroscopy, it is demonstrated how this surface enrichment can be tailored by variation of the solvent IL. In [C(n)C(1)Im][PF6] ILs (n = 2, 4, 8), the surface is fully saturated with the complex at 10%(mol) bulk content, while in [C(4)C(1)Im][Tf2N] only at 20%(mol) saturation is observed. At low catalyst concentrations of 1%(mol), where saturation is not yet reached, the enrichment increases with decreasing length of the IL alkyl chain. As a general rule, the degree of surface enrichment decreases with the decrease in surface tension of the solvent IL, that is, in the order [C(2)C(1)Im][PF6] > [C(4)C(1)Im][PF6] > [C(8)C(1)Im][PF6] > [C(4)C(1)Im][Tf2N]. In ILs with very low surface tension, enrichment is even suppressed. These results reveal the surface tension of the solvent IL as rational parameter for tailoring the interfacial structure of IL-based catalyst systems, such as supported ionic liquid phase (SILP) catalysis, where the nature of the IL/gas interface is expected to strongly influence the performance of the process.