CO Permeability and Wetting Behavior of Ionic Liquids on Pt(111): An IRAS and PM-IRAS Study from Ultrahigh Vacuum to Ambient Pressure

Abstract

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.

Adsorption Motifs and Molecular Orientation at the Ionic Liquid/Noble Metal Interface: [C2C1Im][NTf2] on Pt(111)

“…Despite of the subtle differences between the ATR IR and IRA spectrum of the thick film, we assume that the multilayer has bulklike properties. From the literature it is known that IL molecules adopt a random orientation in the bulklike multilayer. ,,,− For this reason, we assume nearly random orientation of the [C 2 C 1 Im] + and [NTf 2 ] − ions in the multilayer. The minor influence of the Pt(111) support at multilayer coverage allows us to compare the experimental IRAS data to DFT gas phase calculations.…”

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

“…37 After desorption of the IL multilayer at 370 K, two negative CO bands appear at 2024 and 1790 cm −1 . These bands indicate dynamic CO adsorption through the IL wetting layer when the surface is exposed to the CO pulse (see our previous work for details 37,38 ).…”

“…Note also that the desorption temperatures in a background pressure of CO are also higher than in a conventional desorption experiment in UHV. 38 The dynamical changes of the CO and IL coverage during the temperature-programmed experiment are illustrated by the differential spectra shown in Figure 2c,d. The differential spectra were obtained as a difference of two subsequent spectra recorded with CO on and off during the temperature ramp (see Figure 1 and Experimental Section).…”

“…13,21−28 The application of powerful surface science methods, such as X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRAS), etc., opens the possibility to obtain quite detailed insights into the surface chemistry of IL/ catalyst interfaces. 13,21−31 Some of the most intensively studied systems include ILs, often with imidazolium or pyrrolidinium cations, on metal single crystals such as Au(111), 32,33 Pd(111), 34−36 Pt(111), 37,38 and Ag(111). 31,39 In many of these studies, it was found that the IL can form a wetting layer (WL) on metal surfaces, above which more randomly oriented multilayers start to grow.…”

“…Previously, we have shown that IRAS is a versatile tool to characterize the adsorption, orientation, growth, and stability of ILs on metal and metal oxide surfaces. 23,[34][35][36][37][38]41,42 Compared to conventional temperature-programmed desorption (TPD), temperature-programmed IRAS (TP-IRAS) can both determine the desorption kinetics of adsorbates and, importantly, provide the chemical sensitivity to differentiate different adsorbates, such as on-top CO and bridging CO. 35,37,38 In addition, we have studied the coadsorption of CO with ILs on metal surfaces and found that the CO bands are highly sensitive to the presence of coadsorbed ILs. 35,37,38 Typically, the local electric field induced by coadsorbed IL leads to a red-shift of the CO stretching band (vibrational Stark shift).…”