Solid catalysts with ionic liquid
layers (SCILLs) show improved
performance as compared to ionic liquid (IL)-free catalysts. To realize
the beneficial IL-induced modification, the IL layer should be stable
under reaction conditions but also permeable for gaseous reactants
entering through the IL phase. Herein, we applied (polarization modulation-)
infrared reflection absorption spectroscopy ((PM-)IRAS) to investigate
the CO permeability of model SCILL systems. We investigated three
different IL model systems prepared by physical vapor deposition (PVD)
in ultrahigh vacuum (UHV) on atomically clean Pt(111), namely, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4C1Pyr][NTf2]), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2C1Im][NTf2]), and 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate
([C4C1Pyr][OTf]). The adsorption geometries
of the anions depend on the surface structure, IL coverage, and precoverage
of CO. At room temperature, IL multilayers of randomly oriented species
grow on top of strongly adsorbed wetting layers. Upon heating, a partial
wetting transition induces the coexistence of an IL wetting monolayer
film with three-dimensional droplets. Gas-phase CO does not permeate
through IL multilayers, while it penetrates the IL wetting monolayer
leading to mixed IL/CO films. The partial dewetting transition and
the permeability differ drastically with the temperature and IL. Consequently,
the morphology of the IL films could be a factor that determines the
catalytic behavior of SCILLs to a significant extent.
In a solid catalyst with ionic liquid layer (SCILL), ionic liquid (IL) coatings are used to improve the selectivity of noble metal catalysts. To understand the origins of this selectivity control, we performed model studies by surface science methods in ultrahigh vacuum (UHV). We investigated the growth and thermal stability of ultrathin IL films by infrared reflection absorption spectroscopy (IRAS). We combined these experiments with scanning tunneling microscopy (STM) to obtain information on the orientation of the ions, the interactions with the surface, the intermolecular interactions, and the structure formation. Additionally, we performed DFT calculations and molecular dynamics (MD) simulations to interpret the experimental data. We studied the IL 1‐ethyl‐3‐methylimidazolium trifluoromethanesulfonate [C2C1Im][OTf] on Au(111) surfaces. We observe a weakly bound multilayer of [C2C1Im][OTf], which is stable up to 390 K, while the monolayer desorbs at ∼450 K. [C2C1Im][OTf] preferentially adsorbs at the step edges and elbows of the herringbone reconstruction of Au(111). The anion adsorbs via the SO3 group with the molecular axis perpendicular to the surface. At low coverage, the [C2C1Im][OTf] crystallizes in a glass‐like 2D phase with short‐range order. At higher coverage, we observe a phase transition to a 6‐membered ring structure with long‐range order.
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][NTf 2 ] 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 SO 2 moiety of the [NTf 2 ] − 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.
Among other N-heterocycles, indole and its substituted derivatives, such as methylindoles, are considered promising Liquid Organic Hydrogen Carriers (LOHCs) for the storage of renewable energy. We used X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and density-functional theory (DFT) to investigate the low temperature adsorption and consecutive dehydrogenation reaction during heating of 2-methylindole, 2-methylindoline, and 2-methyloctahydroindole on Pt(111) and their viability as the LOHC system. In the photoemission experiments, for all Hx-2-methylindoles, we find deprotonation at the NH bond starting between 240 and 300 K, resulting in a 2-methylindolide species. Simultaneously or before this reaction step, the dehydrogenation of 2-methyloctahydroindole via 2-methylindoline and 2-methylindole intermediates is observed. For 2-methyloctahydroindole, we also find π-allyl intermediates above 230 K. Starting at ∼390 K, decomposition of the remaining 2-methylindolide species takes place under the conditions of our surface science experiments. DFT calculations give insight into the relative energies of the various species, reaction intermediates, and their isomers both in the gas phase and on the Pt(111) surface.
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