A Molecular View of the Ionic Liquid Catalyst Interface of SCILLs: Coverage-Dependent Adsorption Motifs of [C4C1Pyr][NTf2] on Pd Single Crystals and Nanoparticles

Solid catalysts with ionic liquid layers (SCILLs) are heterogeneous catalysts coated with a thin layer of an ionic liquid (IL) to improve their selectivity. In this study, we investigated the interplay of the room-temperature IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [C(4)C(1)Pyr][NTf2] with transition metal surfaces of both Pd(111) single crystals and Pd nanoparticles (NPs) supported on highly oriented pyrolytic graphite (HOPG). To this end, we combine theoretical insights obtained by density functional theory (DFT) and molecular dynamics (MD) calculations with experimental data acquired via time-resolved infrared reflection absorption spectroscopy (TR-IRAS) performed under ultrahigh vacuum (UHV) conditions. IL monolayer films formed on Pd(111) and Pd NPs are strongly bound to the surface via the SO2 moiety of the [NTf2](-) anion. By combination of IRAS, DFT, and MD, we identify the most common adsorption motifs. Most importantly, the binding motif of the anion changes as a function of the IL coverage. On supported Pd NPs, additional effects arise from the particle morphology and from the diffusion of carbon from the support to the Pd NPs. Our results suggest that the effect of the IL film on the catalytic activity should strongly depend on the local coverage and the morphology of the catalyst NPs.

Automated summary

This study investigated the interaction and adsorption motifs of IL coverage on Pd(111) and Pd nanoparticles, finding that the effect of IL film on catalytic activity depends strongly on coverage and particle morphology.