Solid catalysts with ionic liquid layers (SCILLs) have recently attracted a lot of attention, as the ionic liquid (IL) coating can give rise to drastically improved selectivity. Here, we studied the interaction of the IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)-imide [C 4 C 1 Pyr]-[NTf 2 ] with Pt(111) and Pt nanoparticles (NPs) on highly oriented pyrolytic graphite under ultrahigh vacuum conditions. The IL film on Pt(111) and on the Pt NPs consists of a strongly bound monolayer and a weakly bound bulk phase. In the monolayer, [NTf 2 ] − adopts cis conformation and binds via the SO 2 groups. Adsorption of [NTf 2 ] − at Pt defect sites is preferred to adsorption at terraces, whereas preadsorbed CO blocks the adsorption at defects. Further, IL coadsorption leads to desorption and displacement of on-top CO on terraces, whereas CO resides in the bridging position. IL multilayers desorb at 380 K, whereas the strongly adsorbed monolayer on Pt resides and gradually desorbs and decomposes between 400 and 500 K. Finally, we studied the permeability of IL layers for CO by pressure modulation experiments in combination with in situ infrared reflection absorption spectroscopy. We show that the IL multilayer completely blocks CO adsorption, whereas CO easily penetrates the IL monolayer film and forms a mixed adsorbate phase. It is noteworthy that dynamic CO adsorption is much more facile on Pt NPs than on Pt(111). Our results suggest that strongly adsorbed IL monolayers may play an important role in real SCILLs.
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.
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