Ligand Effects at Ionic Liquid-Modified Interfaces: Coadsorption of

Article pubs.acs.org/JPCC

Ligand Effects at Ionic Liquid-Modified Interfaces: Coadsorption of [C2C1Im][OTf] and CO on Pd(111) Tanja Bauer,† Sascha Mehl,† Olaf Brummel,† Kaija Pohako-Esko,‡ Peter Wasserscheid,‡,§ and Jörg Libuda*,†,§ †

Lehrstuhl für Physikalische Chemie II, ‡Lehrstuhl für Chemische Reaktionstechnik, and §Erlangen Catalysis Resource Center and Interdisciplinary Center Interface-Controlled Processes, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, D-91058 Erlangen, Germany

ABSTRACT: Thin films of ionic liquids (ILs) can be used to tune the activity and selectivity of heterogeneous catalysts and electrocatalysts (solid catalysts with IL layer, SCILL). In several cases it has been found that these IL layers have a strong beneficial effect on the selectivity. To explore the molecular origin of this phenomenon, we have performed a model study on ultrahigh-vacuum conditions. We have investigated the coadsorption of CO and the room-temperature IL [C2C1Im][OTf] (1ethyl-3-methylimidazolium trifluoromethanesulfonate) on Pd(111) by time-resolved infrared reflection−absorption spectroscopy, temperature-programmed reflection absorption spectroscopy, and temperature-programmed desorption. We find that the [OTf]− anion adsorbs specifically to the Pd(111) surface via the SO3− group, thereby adopting a well-defined orientation with the molecular axis oriented perpendicular to the surface. At higher IL coverage, unspecific but oriented adsorption occurs, before the orientation is successively lost in the multilayer region. Upon coadsorption of [C2C1Im][OTf] on a CO-saturated Pd(111) surface at 300 K (θ = 0.5) a well-defined coadsorption layer is formed without any loss of adsorbed CO and with very similar CO site occupation. In the coadsorption layer [OTf]− is specifically adsorbed between the CO with a molecular orientation perpendicular to the surface. Thus, a dense and homogeneous coadsorption layer is formed in which Pd surface atoms are simultaneously coordinated to both CO and [OTf]− ions. From this compressed layer, CO desorbs with peak temperature at 410 K (heating rate, 3.3 K/s). Above this temperature, a low-coverage coadsorption phase of CO and surface-adsorbed IL resides, with little influence of the IL on the CO desorption temperature (peak temperature, 470 K). Coadsorption of the IL gives rise to a pronounced red shift of the CO stretching frequency in the order of 50 cm−1. The effect originates from the electrostatic interfacial field (Stark effect) generated by the coadsorbed IL and, at high coverage, possibly from additional short-range interactions. The results show that ILs form dense and well-defined mixed phases with strongly adsorbing reactants such as CO, in which a specifically adsorbed carpet of IL anions directly modifies the active surface sites by ligand-like effects.

1. INTRODUCTION

physicochemical properties according to the requirements of the application. In catalysis the most straightforward concept is the so-called supported ionic liquid phase (SILP) in which a homogeneous catalyst is immobilized in a thin IL film on a porous support.1−3 A more recent and possibly more surprising application is the solid catalyst with ionic liquid layer (SCILL), in which a

Recently the use of ionic liquid (IL) thin films as modifying layers has received much attention, bridging the fields of homogeneous catalysis,1−3 heterogeneous catalysis,4−9 and even electrocatalysis.10−12 The great potential of ILs in these applications arises from their unusual properties, specifically their low volatility, their tunable solvation and miscibility behavior, their high thermal stability, and their large electrochemical window.13,14 In addition, ILs offer a huge structural diversity with a virtually unlimited number of anion−cation combinations, which opens up the possibility of tailoring the © 2016 American Chemical Society

Received: January 13, 2016 Revised: February 3, 2016 Published: February 3, 2016 4453

DOI: 10.1021/acs.jpcc.6b00351 J. Phys. Chem. C 2016, 120, 4453−4465

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The Journal of Physical Chemistry C

2. EXPERIMENTAL DETAILS The infrared refelction−absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD) experiments were performed in a UHV system with a base pressure of 2 × 10−10 mbar. Briefly, the system allows the simultaneous deposition of several metals and ILs and the exposure to reactant gases while IR spectra are acquired in a time-resolved fashion. The system is equipped with all necessary preparation tools, i.e., ion gun, gas dosers, and low-energy electron diffraction (LEED)/Auger optics, etc. IR spectra were acquired with a vacuum Fourier transform infrared (FTIR) spectrometer (Bruker Vertex 80v). The Pd(111) crystal was cleaned by Ar+-sputtering (p ≈ 5 × 10−5 mbar; E = 1 keV; I = 10 μA) and subsequently annealed in 1 × 10−8 mbar O2 (Linde, 5.0) for 10 min to >873 K. After the sample was allowed to cool to room temperature (RT) in oxygen atmosphere, it was flashed to 900 K and cooled again to RT in UHV (see, e.g., refs 39 and 40 and references therein). To remove residual CO on the crystal, it was in a third step flashed to 500 K immediately before starting the experiments. It turned out that this last step additionally improved the cleanliness of the sample. The cleaning procedure was checked via IRAS using CO as a probe molecule and repeated until the result was satisfying. For the deposition of [C1C2Im][OTf] a home-built thermal IL evaporator was used. The deposition rate can be estimated from the time-resolved IR spectra which allow differentiating between monolayer and multilayer adsorption. In the experiments presented the deposition rate was approximately 0.1 monolayer/min. Here we define a monolayer qualitatively as the IL thickness for which we find the spectroscopic signatures of an interaction with the surface. Based on the molar volume (1.88 × 10−4 m3/mol), a monolayer would correspond to a layer thickness of 6.8 Å. All CO (Westfalen, 3.7) adsorption experiments were performed in a remote-controlled mode (National Instruments (NI) and LabVIEW, NI, interface) by dosing pulses of CO via a computer-controlled gas doser equipped with an electromagnetic valve and subsequent acquisition of the IR spectra. All IR spectra were recorded with a spectral resolution of 2 cm−1. The acquisition time for the adsorption spectra was 2 min/spectrum, and for the temperature-programmed (TP) IRAS it was 1 min/spectrum. The heating rate during the TP-IRAS experiments was 2 K/min (0.033 K/s). The TPD spectra were taken with a quadrupole mass spectrometer (Hiden Analytical) equipped with a Feulner cup.41 The entrance of the nozzle was placed approximately 0.5 mm in front of the sample surface so that only gases desorbing from the Pd crystal could enter the QMS. The heating rate for the TPD experiments was 3.3 K/s. Synthesis of [1-Ethyl-3-methylimidazolium][trifluoromethanesulfonate] ([C2C1Im][OTf]). A 74.720 g (0.419 mol, 1.01 equiv) amount of ethyl trifluoromethanesulfonate (Aldrich) was dropwise added to 34.195 g (0.417 mol) of freshly distilled N-methylimidazole (Aldrich) under argon atmosphere at 0 °C. After addition the reaction mixture was allowed to warm to room temperature and stirred overnight. The reaction was monitored by 1H NMR spectroscopy (Jeol ECX, 400 MHz). The product was obtained in quantitative yield as a clear colorless liquid. The product was dried by vacuum at about 10−2 mbar and 60 °C for 16 h. The water content after drying was determined by Karl Fischer titration

conventional supported catalyst material is modified by loading it with a thin layer of an IL.4−9 In many cases it has been found that SCILLs exhibit substantially enhanced selectivities, an effect that has been convincingly demonstrated in selective catalytic hydrogenation for several catalysts, substrates, and ILs.4,5,7,9,15 More recently, superior performance of SCILL-type materials has been demonstrated also in electrocatalysis for example for the oxygen reduction reaction.10−12 Different explanations have been invoked to explain the beneficial effect of the ILs on selectivity.9 For the case of thick IL films or large IL loading so-called “solubility effects” have been discussed which may help to accumulate reactants (or deplete products) at the active site as a result of different gas solubilities. In several cases it was found, however, that the selectivity enhancement occurs very specifically for certain ions, often with a strong dependence on the anion.8,16 In addition, it is often found that ultrathin films or very small amounts of ILs are sufficient to observe strong changes in reactivity.10 These observations suggest that the ILs directly interact with the catalytic site, similar to a ligand. This so-called “ligand effect”, first identified experimentally by Arras et al.,8,16,17 may either lead to deactivation of specific sites (i.e., ideally of those which form the undesired product and thus lead to low selectivity) or the IL ligand may be present as a coadsorbate modifying both competing reaction pathways.18 To understand the origins of IL-induced effects on reactivity, we need to explore the interaction of the IL with the catalyst surface at the molecular level. Specifically, this implies coadsorption studies which provide insight into interaction of the IL with the reactant at the catalyst surface. Such studies are possible using a surface science and model catalysis approach.18−25 In “IL surface science”, thin IL layers are prepared on atomically well-defined surfaces under ultrahighvacuum (UHV) conditions.19 Following this approach it could be shown that ILs indeed show specific interaction with different surface sites,18,25,26 and form different,27−29 sometimes even ordered structures30−36 on single crystal surfaces. In thicker films structure formation is found to extend over several molecular layers. Here we study, to the best of our knowledge for the first time, the influence of an IL on a molecular species adsorbed on an atomically clean metal surface in UHV. We explore the influence of the room-temperature IL 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C1C2Im][OTf] on the adsorption of CO on a Pd(111) single crystal surface. We choose this system because [C1C2Im][OTf] is an interesting modifier for catalytic reactions and which, at the same time, is ideally suited for vibrational spectroscopy.25,37 Pd(111) is the most stable crystal facet of Pd nanoparticles, which catalyze both hydrogenation and oxidation reactions.38 In our previous studies we have shown that, in [C1C2Im][OTf] films on Pd(111), the [OTf]− adsorbs specifically at the Pd(111) surface.25 Our results show that dense and homogeneous mixed adsorbate layers are formed in which the CO is directly surrounded by the coadsorbed IL. This implies (i) that the IL film at the surface controls the access of reactants and (ii) that the active Pd sites are directly modified by the IL coadsorbed in the immediate environment. The latter result underlines the importance of ligand effect on IL-modified catalysts. 4454


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Figure 1. IR spectra taken during deposition of [C2C1Im][OTf] onto Pd(111) at a sample temperature of 300 K. The acquisition time was 2 min/ spectrum; the total deposition time was 40 min.

(Metrohm, 756 KF Coulometer) and was found to be below 500 ppm. 1 H NMR (DMSO-d6, 400 MHz): δ 1.41 (t, 3H, J = 7.2 Hz), 3.84 (s, 3H), 4.18 (q, 2H, J = 7.2), 7.67 (s, 1H), 7.76 (s, 1H), 9.08 (s, 1H).

Two important conclusions are derived from these spectra: First, we observe that the symmetric modes strongly dominate, in spite of the fact that in liquid phase the νas(SO3−) band is the prominent feature.37 Up to deposition times of approximately 20 min (∼1 monolayer) the antisymmetric modes remain practically absent. The reason for this behavior can be found in the metal surface selection rule (MSSR),43 which states that only the component of the dynamic dipole perpendicular to the surface gives rise to IR absorption on a metal surface. Taking into consideration that the symmetric modes of [OTf]− are polarized along the molecular axis and the antisymmetric modes are polarized perpendicular to the molecular axis, we conclude that [OTf]− adopts a well-defined orientation with the molecular axis perpendicular to the Pd(111) surface. The second observation is related to the splitting and shifts of the symmetric modes. For low coverage the νs(SO3−) band shows a strong red shift to 1006 cm−1. At deposition times of 12 min (∼0.6 monolayer) a second band appears at 1035 cm−1, which, in contrast to the band at 1006 cm−1, shows nearly no shift as a function of coverage. We attribute the band at 1006 cm−1 to specifically adsorbed [OTf]− anions which bind to the surface via the SO3− groups. The second band at 1035 cm−1 is attributed to additional [OTf]− anions which are still aligned with respect to the surface but adsorb between the previous ones in a weaker fashion. The preferential orientation follows from the observation that the symmetric bands still dominate the spectrum, in sharp contrast to the bulk phase where the asymmetric bands dominate (compare ref 25). A similar behavior of the bands is observed in the region of the νas(CF3) mode, but with a smaller spectral shift between the two types of

3. RESULTS AND DISCUSSION 3.1. Adsorption and Desorption of [C1C2Im][OTf] on Pd(111) Studied by Temperature-Resolved IRAS (TRIRAS) and and Temperature-Programmed IRAS (TPIRAS). We start by reconsidering the IR spectra of [C1C2Im][OTf] on Pd(111) that were briefly discussed in our previous publication.25 Here, we present new higher quality data in the monolayer region, both for adsorption and desorption. The IR spectra were acquired during deposition of [C1C2Im][OTf] onto Pd(111) at a deposition rate of approximately 0.05 monolayer/min and with a total deposition time of 40 min. The corresponding data are shown in Figure 1. We focus on the anion region ([OTf]−) between 1000 and 1300 cm−1, where the IR bands are most prominent. Four main bands are identified, the assignment of which has been discussed in previous publications.25 The two symmetric stretching vibrations of the SO3− group (νs(SO3−)) and of the CF3 group (νs(CF3)) appear around 1006−1038 cm−1 and at 1216−1228 cm−1, respectively. The two antisymmetric stretching vibrations of the SO3− group (νas(SO3−)) and of the CF3 group (νas(CF3)) are visible at around 1276 cm−1 and at 1174 cm−1, respectively. For a detailed discussion of the full vibrational spectrum of the molecule, we refer to the literature.25,42 4455


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Figure 2. (a) Temperature-programmed IR spectra taken while heating the [C2C1Im][OTf] film deposited onto Pd(111) at 300 K. The reference spectrum was taken after IL deposition. The heating rate was 2 K/min. (b) Integral intensity of the νs(SO3−) band as a function of temperature.

[OTf]− ions. This observation supports a picture that the [OTf]− anions bind to the surface via the SO3− group. In the next step we follow the desorption of [C1C2Im][OTf] from Pd(111) by TP-IRAS. We start from the situation after deposition of the IL (Figure 1) and record the reference spectrum at 300 K. Subsequently, the sample is heated with a rate of 2 K/min while continuously acquiring IR spectra with an acquisition time of 1 min/spectrum. The corresponding data are shown in Figure 2 (only every second spectrum is shown for clarity). The integral intensity in the region of the νs(SO3−) mode is given in Figure 2b (note that the band intensity does not directly reflect the IL coverage due to changes in the orientation and the dynamic dipole coupling). Immediately after heating we observe positive peaks at 1035 cm−1 and at 1228 cm−1. Note that the reference spectrum was taken after IL deposition. Therefore, positive peaks indicate a decrease of band intensity (desorption of the respective species) and negative bands indicate increase in band intensity (formation of the respective species). The development of the band intensities above 300 K indicates slow desorption of [C1C2Im][OTf], possibly accompanied by reorganization (e.g., droplet formation). The s-shape of the band is the result of a slight red shift with increasing temperature. A prominent change occurs at a temperature of around 380 K, where the νs(SO3−) bands show a sudden decrease in intensity (largest slope of the peak intensity; see Figure 2b). We attribute this change to desorption of the IL multilayer. Simultaneously with

multilayer desorption, an adsorption band at 1013 cm−1 appears. As discussed previously, this feature is attributed to specific adsorption of [OTf]− in the first monolayer. Upon multilayer deposition, this band is slightly damped (as can be seen in Figure 1), and the damping is lifted again upon multilayer desorption. As indicated by the temperature dependence of this band, the specifically adsorbed [OTf]− species in the first monolayer reside on the Pd(111) surface up to temperatures of 500 K and above. 3.2. Adsorption of CO on Pd(111) Studies by TR-IRAS. As a reference for the coadsorption experiment, we studied the adsorption of CO on Pd(111) in a time-resolved IRAS experiment. CO adsorption on Pd(111) is rather complex and has been investigated repeatedly over the most recent decades.40,43−48 In order to obtain coverage-dependent IRAS data of sufficiently high quality, we reinvestigated the system. Briefly, the Pd(111) crystal was exposed at a temperature of 300 K to 120 CO pulses from a remote-controlled dosing device with an IR spectrum taken after each pulse. A single pulse corresponded to a CO dose of 0.045 langmuir (1 langmuir = 10−6 Torr·s); the total CO dose in the experiment is 5.4 langmuirs. The spectra in the CO stretching frequency region are displayed in Figure 3a, together with the integral band intensity (see Figure 3b), the mean band position (see Figure 3c), and the band variance as a measure for the peak width (see Figure 3d). 4456


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Figure 3. (a) IR spectra of the ν(CO) region taken during stepwise exposure of Pd(111) to small doses of CO at a sample temperature of 300 K. Each CO pulse corresponds to a CO dose of 0.045 L. The total CO dose after 120 pulses was approximately 5.4 L (100%), and the acquisition time was 1 min/spectrum. (b) Integral intensity as a function of exposure. (c) Mean band position as a function of exposure. (d) Band variance as a function of exposure.

originating from coupling between the two CO molecules in the unit cell. Quite recently, the system was reinvestigated by Grönbeck and co-workers, who suggested that in the c(4 × 2)2CO structure CO occupies fcc and bridge sites.40 At 300 K and θ = 0.5 the LEED pattern is more diffuse indicating a less well defined structure.47 Indeed, Rose et al. found different coexisting CO domains by scanning tunneling microscopy (STM), in some of which the CO occupies bridge and in some it occupies hollow sites.48 Surnev et al. investigated the adsorption of CO at 300 K by high-resolution core-level spectrosocopy and showed that above θ = 0.3 bridge sites are occupied in addition to hollow sites, until at θ = 0.5 bridge and hollow sites are occupied with similar probability.47 This implies that, apart from the higher degree of disorder at 300 K, the local structure and site occupation is rather similar at θ = 0.5 irrespective of the adsorption temperature and features a combination of fcc hollow and bridge sites. In the following we use the recent model by Martin et al. for further discussion.40

The behavior of the bands can be understood on the basis of the literature.40,43−48 At low coverage CO occupies fcc hollow sites, giving rise to the band at 1812 cm−1 in the limit of low coverage. With increasing coverage, the band blue shifts to 1840 cm−1 while the surface reaches a coverage of θ = 0.33 and a (√3× √3)R30° superstructure is formed. At larger exposure, the behavior is more complex. The saturation coverage at 300 K has been shown to be θ = 0.5.43,44 The IR spectra (Figure 3) show a phase transformation during which a new peak evolves at around 1930 cm−1. At low temperature and a coverage of θ = 0.5 a c(4 × 2)2CO superstructure is formed. Originally, Bradshaw and Hoffmann attributed the corresponding IR band at around 1930 cm−1 to bridge-bonded CO.43,44 The assignment was questioned on the basis of photoelectron diffraction studies by Gießel at al.45 and by density function theory.46 In the latter study, Sautet and coworkers suggested a structure in which the CO adsorbs on fcc and hcp hollow sites with the high vibrational frequency 4457


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Figure 4. (a) Time-resolved IR spectra of the ν(CO) region taken during deposition of [C2C1Im][OTf] onto CO/Pd(111) at 300 K (2 min/ spectrum). The Pd was exposed to CO until saturation before the experiment. [C2C1Im][OTf] deposition started after 20 min (red spectrum), and the total deposition time was 70 min. (b) Integral intensity as a function of exposure. (c) Mean band position as a function of exposure. (d) Band variance as a function of exposure.

with a single phase (θ < 0.33 and θ ∼ 0.5) and larger in the transition region where the two phases coexist. We will come back to this point in the next section. 3.3. Coadsorption of [C1C2Im][OTf] and CO on Pd(111) Studies by TR-IRAS and TPD. In the next step we study the coadsorption of [C1C2Im][OTf] and CO on Pd(111). To this aim, we first saturated the Pd(111) surface with CO at 300 K and recorded a reference spectrum. Next we recorded IR spectra with an acquisition time of 2 min/spectrum, first without deposition of the IL (for 20 min) and then during IL deposition (deposition rate, 0.05 monolayer/min; for 70 min). The corresponding data are displayed in Figure 4 (CO stretching frequency region) and Figure 5 (spectral region of the [OTf]− anion). We discuss the CO stretching frequency region first. The point in time where the IL deposition was started is marked in Figure 4. Before IL deposition, the ν(CO) band undergoes a slow red shift from 1935 to 1926 cm−1 as a function of time. We associate this effect to slow desorption of CO already at

Noteworthy, this model is perfectly consistent with the appearance of the IR spectra which show an intense band around 1920−1930 cm−1 and a low-frequency shoulder. The high-frequency peak may be associated with the bridge-bonded CO and the low-frequency tail with the hollow CO (which loses intensity due to dynamic dipole coupling with the bridgebonded species). Several additional points should be mentioned with respect to the spectra in Figure 3. First, the intensity is strongly nonlinear as a function of the coverage. Figure 3b shows that the integral intensity increases only weakly in the high-coverage region. This effect is due to dynamic dipole coupling and is well-described in the literature.43 The mean band position (Figure 3c) may be a better indicator for the coverage, but it does not reflect the phase transitions on the surface. Some information on the degree of order may be derived from the band variance plotted in Figure 3d (because of the irregular band shapes and strong asymmetries, the variance is used instead of the bandwidth). The variance is small in the regions 4458


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Figure 5. Time-resolved IR spectra of the [OTf]− stretching region taken during deposition of [C2C1Im][OTf] onto CO/Pd(111) at 300 K (2 min/ spectrum). The Pd was exposed to CO until saturation before the experiment. The total deposition time for [C2C1Im][OTf] was 70 min.

decreases later on with continuing deposition. After 20 min of IL deposition (∼1 monolayer) the bandwidth corresponds to that of the IL-free CO layer and decreases even further upon additional deposition. We start by discussing the pronounced red shift induced by the coadsorbed IL. Such shifts may originate from two contributions. The first contribution is an electronic ligand effect induced by coadsorbates. We may differentiate between two types of coadsorbates, electron-withdrawing and electrondonating. According to the Blyholder model for the adsorption of CO, electron-withdrawing coadsorbates typically lead to a blue shift of the CO stretching frequency, as a result of reduced back-donation to the CO 2π* orbital.49 Typical examples of electronegative coadsorbates are atomic O or N species which give rise to blue shifts in the order of several 10 cm−1 (see, e.g., ref 50). Electropositive coadsorbates, on the other hand, give rise to red shifts as a result of enhanced back-donation. Typical examples of electron-donating species are alkali atoms which become partially positively charged upon adsorption. For the case of coadsorbed alkali ions the red shift may become very large and even exceed 100 cm−1.51 It is important to note, however, that the electronic ligand effect is very short ranged

room temperature. It is likely that the CO coverage temporarily exceeds the saturation coverage of θ = 0.5 during CO exposure and, subsequently, the excess CO desorbs slowly. Once the band reaches a frequency of 1926 cm−1, the red shift becomes very small on the time scale of our experiment and we consider the CO adsorbate layer as stable. Upon IL deposition (red line in Figure 4) we observe a strong red shift of the ν(CO) band, accompanied by a decrease in intensity and a change in the band shape. These effects are further analyzed in the insets in Figure 4. In Figure 4b the integral band intensity is plotted as a function of deposition time. We observe that the integral intensity decreases rapidly during the first minutes of deposition and subsequently levels off at 71 ± 4% of its original intensity. A very prominent change is observed in the band position. The numerically calculated mean band position (see Figure 4c) decreases rapidly during the first 10 min of deposition and more slowly thereafter. After about 50 min of deposition (∼2.5 monolayers) the red shift levels off at a value of Δν = −48 cm−1 (±2 cm−1). A very peculiar behavior is observed in Figure 4d showing the band variance (as a measure for the bandwidth). During the first minutes of deposition, the width increases steeply but then 4459


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layer is stable and the effects are entirely due to coadsorbed IL, or the effects are due to a partial loss of CO induced by the codeposited IL. In order to test whether the CO is partly replaced by the IL, we have performed TPD experiments of CO/ Pd(111) with and without the coadsorbed IL layer under otherwise identical conditions. The result is displayed in Figure 6.

and will only affect the Pd atoms in the direct vicinity of the of coadsorbate.51 The second and more generally observed contribution to the vibrational band shifts are due to the Stark effect, i.e., the electric field generated by the coadsorbate. This electric field (both static and dynamic) interacts differently with the adsorbate in the vibrational ground state and the excited state. The influence of the Stark effect has been investigated intensively. In solution chemistry solvatochromic effects in vibrational chromophores have been systematically analyzed to differentiate between continuum field descriptions and specific solute−solvent interaction.52 Also, the vibrational Stark effect has been studied intensively at electrochemical interfaces at single crystal surfaces53 and by theoretical modeling.54−58 In a systematic DFT study, Wasileski et al. analyzed the Stark tuning rates and changes in binding energy as a function of the field for a number of prototype adsorbates.55,56 Of special interest are the UHV coadsorption studies performed by Weaver and coworkers with the intention of modeling electrochemical interfaces from a surface science perspective.59−65 The authors showed that for adsorbed CO on Pt(111), ν(CO) undergoes a pronounced red shift upon co-deposition of 1−2 monolayers of the solvent.65 Because of the volatility of most solvents the experiments were performed at 100 K. The authors tested solvents with different polarity and found red shifts between 15 and 50 cm−1. In a second series of experiments the authors correlated the frequency shift to work function changes and concluded that the shifts are primarily due to the Stark effect, i.e., due to changes in the electric field at the interface upon addition of the solvent.63 Finally, the authors additionally coadsorbed K, which forms K+ at low coverages and thereby modifies the electrostatic interfacial field.60 They could differentiate convincingly between the short-range ligand effect of coadsorbed K+ and electrostatic effects observed upon addition of water and solvation of the K+. Comparing to the aforementioned work, the similar magnitude and coverage dependence suggests that the ILinduced spectral shift mainly arises from the interfacial electric field, i.e., the Stark effect. Later we will show, however, that IL and CO form a dense and mixed coadsorbate phase in which direct electronic ligand effects are also possible. With this in mind we investigate the IR spectra of the IL (see Figure 5) which provide additional information on the interaction of the IL with the Pd surface. In the limit of low IL exposure, we observe only symmetric [OTf]− modes (νs(SO3−) and νs(CF3)). Similar to that in the absence of CO, this observation shows that the [OTf]− adsorbs with the molecular axis perpendicular to the surface. Noteworthy, we observe two distinct signals in the νs(SO3−) region. The band at 1037 cm−1, which appears first, we attribute to specific adsorption of [OTf]− at the Pd surface, which is shifted with respect to the CO-free case due to the interaction of the coadsorbates. Note that the band intensities indicate an upright standing [OTf]− and the νs(CF3) band shows no shift, suggesting that the interaction occurs specifically via the SO3− group. The second signal at 1033 cm−1 slowly transforms into the multilayer band and is attributed to less specifically bound [OTf]− in the first layer. At deposition times exceeding 10 min (red spectrum in Figure 5) the asymmetric modes become visible, indicating multilayer adsorption with less well defined orientation. Next we return to the red shift and the decrease in intensity in the CO spectra upon IL coadsorption. There are two possible explanations for these phenomena: Either, the CO

Figure 6. TPD spectra taken after saturation of Pd(111) with CO at 300 K (blue) and after saturation of Pd(111) with CO and subsequent deposition of approximately 2 monolayers of [C2C1Im][OTf] at 300 K.

We observe that after IL deposition more CO desorbs at lower temperature, but the total amount of adsorbed CO at 300 K remains perfectly constant (the integral area of the CO desorption peak decreases by 2% only which is within the experimental error). This finding has a very important consequence: It implies that coadsorption of the IL occurs without any loss of CO and the specifically adsorbed [OTf]− ions adsorb between the CO molecules. This leads to a compression of the CO layer which is supported by the observation that the average desorption temperature decreases. To obtain further information on the structure and homogeneity of the coadsorption layer, we investigate the peak shape of the CO spectra in more detail. As pointed out before, the CO peak shape is very sensitive to the site occupation and the homogeneity of the layer. In Figure 6, we compare the peak shapes before and after IL co-deposition. Compensating for the red shift and the decrease in the intensity, we find that the peak shapes are very similar before and after IL deposition. This observation suggests that upon IL coadsorption there is little change regarding the local arrangement of CO, occupying a mixture of bridge and hollow sites. Consequently, the red shift is attributed to the Stark effect (and, possibly, to additional short-range electronic interactions) but not to a loss of CO. The decrease in peak intensity is attributed to a decrease in the dynamic dipole moment of CO, possibly due to dynamic screening by the coadsorbed IL. Our hypothesis of the IL-CO adlayer forming a dense and homogeneous coadsorption layer is supported by the development of the CO bandwidth as a function of IL exposure (Figure 4c). Whereas the bandwidth increases upon deposition of very small amounts of IL (due to the fact that the CO molecules experience different local environments), the bandwidth decreases again after deposition of 0.5 monolayer of the IL (∼10 min). In fact the overall CO band shape in the 4460


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cm−1. The latter was found to be characteristic for the specific adsorption of [OTf]− on the CO-free surface (see section 3.1). Therefore, we assign this signal to [OTf]− adsorbed to ensembles of Pd atoms, which are not coordinated to CO. 3.5. Discussion. Finally, we discuss the implications of our findings in terms of the structure of the coadsorption layers and their effect on reactivity Following our discussion in section 3.2, we use the c(4 × 2)2CO superstructure at θ = 0.5 together with the adsorption structure suggested by Martin et al. as a representative model of the local CO arrangement at 300 K. In this structure, one CO per unit cell occupies a bridge site (CObr) and one CO occupies a hollow site (COh; see Figure 9a). Our experimental results show that coadsorption of the IL neither changes the CO coverage nor does it change substantially the distribution between the local sites. The homogeneity of the coadsorption structure, as evidenced by IRAS, implies that the [C2C1Im][OTf] adsorb between the CO with some [OTf]− anions in direct contact with the Pd. A schematic representation of a possible film structure is depicted in Figure 9b, at an IL coverage that corresponds to one [OTf]− per two (4 × 2) unit cells (namely, θ[C1C2Im][OTf] = 0.125). The molecular entities are shown with their true van der Waals radii. After local rearrangement of the CO (keeping the hollow/ bridge ratio constant) the [OTf]− can be accommodated between the CO with little compression of the adsorbate layer only. To illustrate the behavior at lower coverage, we choose the (√3× √3)R30° CO phase at θ = 0.33 (see Figure 9b). This represents the upper coverage limit of a structure with CO occupying hollow sites only. After rearrangement the same number of [OTf]− anions are accommodated, with the difference being that now part of the [OTf]− ions occupy Pd ensembles which are free of CO ligands. Experimentally we could differentiate between the [OTf]− species with and without CO neighbors (via the frequency of the νs(SO3−) band; see Figure 9b). Only at coverages well below θ = 0.33 (see Figure 9c) the [OTf]− exclusively adsorbs at Pd atoms which are not coordinated to any CO. In the latter case we do not expect any electronic short-range interaction between CO and the [OTf]−. As expected the corresponding IR spectra show a νs(SO3−) stretching frequency identical to the CO-free Pd surface. In addition the high-temperature peak in the TPD (Figure 6) indicates a CO binding energy similar to the IL-free surface. The aforementioned considerations have important implications for the role of ILs as reactivity-modifying agents in SCILL-type catalysts. First, we find that CO and IL form welldefined and well-mixed coadsorbate layers in which the [OTf]− anion specifically adsorbs directly next to the CO. The present experiments do not allow one to determine the position of the [C2C1Im]+ cation, but we assume that it is located in a less well defined (nonspecific) position next to the anion. This arrangement, as schematically depicted in Figure 10, helps to rationalize the origin of the large electrostatic interfacial field (Stark shift) experienced by the CO. This field effect can modify binding and reactivity of the adsorbed CO. A second contribution arises from the fact that the coadsorption layer is so dense that CO and [OTf]− ions share common Pd surface atoms; i.e., the active site is directly affected by coadsorption of [OTf]−. This implies that ILinduced short-ranged electronic interactions are possible even on extended and well-ordered Pd(111) facets. A final

coadsorbate phase is very similar to the CO band shape in a fully saturated, pristine CO adsorbate layer on Pd as shown in Figure 7.

Figure 7. Comparison of the IR spectra in the ν(CO) region for pristine Pd(111) after saturation with CO at 300 K (red line) and after saturation with CO and subsequent deposition of ∼2 monolayers of [C2C1Im][OTf] at 300 K (open circles). After scaling and shifting the two spectra are nearly identical, indicating that both spectra have nearly the same band shape (see text for further discussion).

This observation indicates that the CO molecules experience a similar local environment thus excluding the formation of large domains. We investigated the coadsorption layers by LEED, but could not observe any LEED pattern after IL deposition. Not surprisingly, this shows that the degree of longrange order in the coadsorption layer is poor in spite of the well-defined local CO environment. 3.4. Desorption of [C1C2Im][OTf] and CO from Pd(111) Studies by TP-IRAS. Finally, we investigate desorption from the coadsorbed CO and IL by TP-IRAS. The experimental results are shown in Figure 8. Consistently with the TPD experiments we observe that CO desorption already starts at temperatures slightly above 300 K. We observe a positive (desorption) peak at around 1880 cm−1, which is accompanied by a negative (formation) peak undergoing a red shift from about 1848 to 1765 cm−1 with increasing temperature. The origin of this peak can be understood based on the coverage dependence of the CO IR spectra (see Figure 3). Note that the spectra were taken as difference spectra with the CO-saturated surface used as a reference. At low temperature the negative peaks arise from the coverage-dependent red shift of ν(CO) upon CO desorption; at higher temperature (350−400 K) the band reflects the CO adsorbed in fcc hollow positions. It is interesting to compare the red shift of the CO at hollow sites in the presence of IL (1765 cm−1) and without IL (1812 cm−1). This spectral shift (47 cm−1) is very similar to the shift observed for the on-top CO (48 cm−1). At about 410 K, desorption of CO in the TPIRAS experiment is practically complete. Note that this temperature is lower than the desorption temperature observed in the TPD experiment. The difference is largely attributed to the heating rate which is higher by a factor of 100 for the TPD experiment (TP-IRAS, 0.033 K s1−; TPD, 3.3 K s1−). Of special interest is the behavior in the νs(SO3−) region. Together with appearance of the CO sites in hollow positions (1765−1785 cm−1; green line, Figure 8) we observe also the appearance of the red-shifted νs(SO3−) signal at 1000−1010 4461


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Figure 8. Temperature-programmed IR spectra taken while heating the [C2C1Im][OTf] film deposited onto CO/Pd(111). The reference spectrum was taken after IL deposition. The heating rate was 2 K/min.

be most effective at high IL loading, i.e., in the presence of a liquid IL bulk phase.

implication is related to the stability of the coadsorption structures: Their stability implies that the IL layer remains permeable for strongly adsorbing reactants such as CO; i.e., the CO will be able to access the catalyst surface even in the presence of the IL. We expect, however, that the dense IL film with the specifically adsorbed “mesh” of [OTf]− anions will affect the access of reactants, possibly reducing the reactant coverage and blocking more weakly adsorbing reactants. Relating these findings to real SCILL catalysts, several additional effects have to be taken into account such as the dependence on the IL loading and on the particle structure. Regarding the latter, we expect that the interaction strength of the anion will depend on the local Pd site. Indeed we could show in our previous work with [C1C2Im][NTf2] that lowcoordinated sites on the nanoparticles are preferentially blocked.18 It remains to be explored to what extent this effect depends on the choice of the IL, in specific, on the coordination strength of the anion. Regarding the IL loading we could recently show that on a supported model catalyst the IL preferentially migrates from the alumina support to the metal nanoparticles.25 This observation implies that the ligand effects identified in this work will play an important role even at very low IL loading, whereas solubility effects are expected to

4. CONCLUSIONS We have studied the coadsorption of CO and the roomtemperature IL [C2C1Im][OTf] on a Pd(111) single crystal surface under UHV conditions by TR-IRAS, TP-IRAS, and TPD. The results provide detailed insights into the molecular origins of the altered reactivity and selectivity that are observed upon modification of supported catalysts by thin IL layers (SCILL-type systems). Specifically, we find the following: (1) Upon deposition of sub-monolayer amounts of [C2C1Im][OTf] onto a Pd(111) at 300 K, the [OTf]− anions adsorb specifically via the SO3− group with the molecular axis oriented perpendicular to the Pd surface. At higher IL coverage, less specific adsorption occurs, yet preserving the preferential orientation of [OTf]− perpendicular to the surface. In the multilayer region, the preferential orientation is successively lost. Upon heating, the IL multilayer desorbs from the surface at temperatures around 380 K (heating rate, 0.033 K/s). The adsorbed monolayer remains strongly bound on the Pd(111) and resides on the surface up to temperatures of 500 K and above. 4462


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layer the desorption maximum is shifted to lower temperature and appears at 410 K (heating rate, 3.3 K/s). At lower CO coverages, [OTf]− binds to Pd sites which are not influenced by CO and vice versa. In this regime the IL has no influence on the CO adsorption energy. (4) Coadsorption of the IL gives rise to pronounced red shifts of the CO stretching frequency in the order of 50 cm−1. The effect is rationalized on the basis of the electrostatic interfacial field (Stark effect) generated by the coadsorbed IL and, at high coverage, by short-range electronic interactions. The results show that, on IL-modified catalysts, strongly adsorbing reactants and ILs can form dense and well-defined mixed phases in which the reactant and the IL are in the direct neighborhood; i.e., they share common surface atoms. In other words, the IL is directly coordinated to the active surface site. This implies that IL film can remain permeable for strongly adsorbing reactants such as CO which can bind between the specifically adsorbed anions. Second, the active sites will be directly modified by a ligand-like effect of the IL. In addition, a strong electrostatic interfacial field is created by the adsorbed IL which, together with the steric effects of the adsorbed “anion mesh”, will generate diffusion barriers and modify the access of reactants.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. FAX: +49 9131 8528867. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG) within the framework of SPP 1708 “Material Synthesis near Room Temperature”. We acknowledge financial support by the DFG within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative and by the Clariant AG. Further financial support by the European Commission (“chipCAT”, FP7-NMP-2012SMALL-6, Grant Agreement No. 310191) and travel support by the DAAD by COST Action CM1104 “Reducible oxide chemistry, structure and functions” are gratefully acknowledged. We also acknowledge intense discussions at the 3rd Catalysis Summit (with Hans-Peter Steinrück, Jeroen von Bokhoven, Karin Föttinger, Julia Kunze, Gerhard Mestl, Richard Fischer, and Sebastian Günther) which have helped to greatly improve this manuscript.

Figure 9. Schematic representation of CO adsobate structures on Pd(111) (a) at θ = 0.5, (b) at θ = 0.33, and (c) at low coverage in the absence and in the presence of [C1C2Im][OTf]. The unspecifically adsorbed [C1C2Im]+ cations are omitted for clarity; the vibrational frequencies are those observed in the experiments (light blue, bridging CO; dark blue, hollow CO; red/yellow, [OTf]−).

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REFERENCES

(1) Riisager, A.; Fehrmann, R.; Haumann, M.; Wasserscheid, P. Supported Ionic Liquids: Versatile Reaction and Separation Media. Top. Catal. 2006, 40, 91−102. (2) Riisager, A.; Fehrmann, R.; Haumann, M.; Wasserscheid, P. Supported Ionic Liquid Phase (Silp) Catalysis: An Innovative Concept for Homogeneous Catalysis in Continuous Fixed-Bed Reactors. Eur. J. Inorg. Chem. 2006, 2006, 695−706. (3) Riisager, A.; Fehrmann, R.; Flicker, S.; van Hal, R.; Haumann, M.; Wasserscheid, P. Very Stable and Highly Regioselective Supported Ionic-Liquid-Phase (SILP) Catalysis: Continuous Flow Fixed-Bed Hydroformylation of Propene. Angew. Chem., Int. Ed. 2005, 44, 815− 819. (4) Mangartz, T.; Hacker, L.; Korth, W.; Kern, C.; Jess, A. Selective Hydrogenation of 1,3-Butadiene: Improving the Selectivity to Butene Using a Solid Pd-Catalyst with an Ionic Liquid Layer (SCILL). Oil Gas Eur. Mag. 2014, 40, 84−90.

Figure 10. Schematic representation of the interaction between coadsorbed CO and[C2C1Im][OTf] on Pd(111).

(2) Upon coadsorption of [C2C1Im][OTf] onto a COsaturated Pd(111) surface at 300 K (θ = 0.5) a dense and homogeneous coadsorption layer is formed without any loss of CO. Also the site occupation ratio of CO in the coadsorbate structure is hardly changed by the IL. In the coadsorption structure, some [OTf]− ions are specifically adsorbed at the Pd surface and compress the CO layer. This implies that Pd surface atoms are simultaneously coordinated to both CO and [OTf]−. (3) The CO desorption temperature is influenced by the coadsorbed [C2C1Im][OTf]. Due to compression of the CO 4463


Article


[BMIm][Tf2N] Thin Films on a Well-Ordered Alumina Surface. Langmuir 2010, 26, 7199−7207. (25) Schernich, S.; Kostyshyn, D.; Wagner, V.; Taccardi, N.; Laurin, M.; Wasserscheid, P.; Libuda, J. Interactions between the RoomTemperature Ionic Liquid [C2C1Im][OtF] and Pd(111), WellOrdered Al2O3, and Supported Pd Model Catalysts from IR Spectroscopy. J. Phys. Chem. C 2014, 118, 3188−3193. (26) Schernich, S.; Laurin; Lykhach, Y.; Steinrück, H.-P.; Tsud, N.; Skála, T.; Prince, K. C.; Taccardi, N.; Matolín, V.; Wasserscheid, P.; et al. Functionalization of Oxide Surfaces through Reaction with 1,3Dialkylimidazolium Ionic Liquids. J. Phys. Chem. Lett. 2013, 4, 30−35. (27) Uhl, B.; Cremer, T.; Roos, M.; Maier, F.; Steinrück, H. P.; Behm, J. At the Ionic Liquid|Metal Interface: Structure Formation and Temperature Dependent Behavior of an Ionic Liquid Adlayer on Au(111). Phys. Chem. Chem. Phys. 2013, 15, 17295−17302. (28) Cremer, T.; Wibmer, L.; Calderon, S. K.; Deyko, A.; Maier, F.; Steinrück, H. P. Interfaces of Ionic Liquids and Transition Metal Surfaces-Adsorption, Growth, and Thermal Reactions of Ultrathin [C1C1Im][Tf2N] Films on Metallic and Oxidised Ni(111) Surfaces. Phys. Chem. Chem. Phys. 2012, 14, 5153−5163. (29) Cremer, T.; Stark, M.; Deyko, A.; Steinrück, H. P.; Maier, F. Liquid/Solid Interface of Ultrathin Ionic Liquid Films: [C1C1Im][Tf2N] and [C8C1Im][Tf2N) on Au(111). Langmuir 2011, 27, 3662− 3671. (30) Wen, R.; Rahn, B.; Magnussen, O. M. Potential-Dependent Adlayer Structure and Dynamics at the Ionic Liquid/Au(111) Interface: A Molecular-Scale in Situ Video-STM Study. Angew. Chem., Int. Ed. 2015, 54, 6062−6066. (31) Hayes, R.; Warr, G. G.; Atkin, R. Structure and Nanostructure in Ionic Liquids. Chem. Rev. 2015, 115, 6357−6426. (32) Carstens, T.; Hayes, R.; El Abedin, S. Z.; Corr, B.; Webber, G. B.; Borisenko, N.; Atkin, R.; Endres, F. In Situ STM, AFM and DTC Study of the Interface 1-Hexyl-3-Methylimidazolium Tris(Pentafluoroethyl)Trifluorophosphate/Au(111). Electrochim. Acta 2012, 82, 48−59. (33) Endres, F.; Borisenko, N.; El Abedin, S. Z.; Hayes, R.; Atkin, R. The Interface Ionic Liquid/Electrode: In Situ STM and AFM Measurements. Faraday Discuss. 2012, 154, 221−233. (34) Hayes, R.; Borisenko, N.; Tam, M. K.; Howlett, P. C.; Endres, F.; Atkin, R. Double Layer Structure of Ionic Liquids at the Au(111) Electrode Interface: An Atomic Force Microscopy Investigation. J. Phys. Chem. C 2011, 115, 6855−6863. (35) Atkin, R.; Borisenko, N.; Druschler, M.; El Abedin, S. Z.; Endres, F.; Hayes, R.; Huber, B.; Roling, B. An in Situ STM/AFM and Impedance Spectroscopy Study of the Extremely Pure 1-Butyl-1Methylpyrrolidinium Tris(Pentafluoroethyl) Trifluorophosphate/ Au(111) Interface: Potential Dependent Solvation Layers and the Herringbone Reconstruction. Phys. Chem. Chem. Phys. 2011, 13, 6849−6857. (36) Atkin, R.; El Abedin, S. Z.; Hayes, R.; Gasparotto, L. H. S.; Borisenko, N.; Endres, F. AFM and STM Studies on the Surface Interaction of [BMP]TFSA and [EMIm]TFSA Ionic Liquids with Au(111). J. Phys. Chem. C 2009, 113, 13266−13272. (37) Schernich, S.; Wagner, V.; Taccardi, N.; Wasserscheid, P.; Laurin, M.; Libuda, J. Interface Controls Spontaneous Crystallization in Thin Films of the Ionic Liquid [C2C1Im][OTf] on Atomically Clean Pd(111). Langmuir 2014, 30, 6846−6851. (38) Henry, C. R. Morphology of Supported Nanoparticles. Prog. Surf. Sci. 2005, 80, 92−116. (39) Mitsui, T.; Rose, M. K.; Fomin, E.; Ogletree, D. F.; Salmeron, M. Water Diffusion and Clustering on Pd(111). Science 2002, 297, 1850−1852. (40) Martin, N. M.; Van den Bossche, M.; Grönbeck, H.; Hakanoglu, C.; Zhang, F.; Li, T.; Gustafson, J.; Weaver, J. F.; Lundgren, E. CO Adsorption on Clean and Oxidized Pd(111). J. Phys. Chem. C 2014, 118, 1118−1128. (41) Feulner, P.; Menzel, D. Simple Ways to Improve Flash Desorption Measurements from Single-Crystal Surfaces. J. Vac. Sci. Technol. 1980, 17, 662−663.

(5) Schwab, F.; Weidler, N.; Lucas, M.; Claus, P. Highly Cis-Selective and Lead-Free Hydrogenation of 2-Hexyne by a Supported Pd Catalyst with an Ionic-Liquid Layer. Chem. Commun. 2014, 50, 10406−10408. (6) Meyer, C.; Hager, V.; Schwieger, W.; Wasserscheid, P. Enhanced Activity and Selectivity in N-Octane Isomerization Using a Bifunctional SCILL Catalyst. J. Catal. 2012, 292, 157−165. (7) Jess, A.; Kern, C.; Korth, W. Solid Catalyst with Ionic Liquid Layer (SCILL) - a Concept to Improve the Selectivity of Selective Liquid Gas and Gas Phase Hydrogenations. Oil Gas Eur. Mag. 2012, 128, 38−45. (8) Arras, J.; Paki, E.; Roth, C.; Radnik, J.; Lucas, M.; Claus, P. How a Supported Metal Is Influenced by an Ionic Liquid: In-Depth Characterization of SCILL-Type Palladium Catalysts and Their Hydrogen Adsorption. J. Phys. Chem. C 2010, 114, 10520−10526. (9) Kernchen, U.; Etzold, B.; Korth, W.; Jess, A. Solid Catalyst with Ionic Liquid Layer (SCILL) - a New Concept to Improve Selectivity Illustrated by Hydrogenation of Cyclooctadiene. Chem. Eng. Technol. 2007, 30, 985−994. (10) Zhang, G. R.; Munoz, M.; Etzold, B. J. M. Boosting Performance of Low Temperature Fuel Cell Catalysts by Subtle Ionic Liquid Modification. ACS Appl. Mater. Interfaces 2015, 7, 3562−3570. (11) Snyder, J.; Livi, K.; Erlebacher, J. Oxygen Reduction Reaction Performance of [MTBD][BETI]-Encapsulated Nanoporous NiPt Alloy Nanoparticles. Adv. Funct. Mater. 2013, 23, 5494−5501. (12) Snyder, J.; Fujita, T.; Chen, M. W.; Erlebacher, J. Oxygen Reduction in Nanoporous Metal-Ionic Liquid Composite Electrocatalysts. Nat. Mater. 2010, 9, 904−907. (13) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; WileyVCH: Weinheim, Germany, 2007. (14) Wasserscheid, P., Stark, A., Eds. Handbook of Green Chemistry. Green Solvents. Vol. 6: Ionic Liquids; Wiley-VCH: Weinheim, Germany, 2013. (15) Herrmann, T.; Rossmann, L.; Lucas, M.; Claus, P. HighPerformance Supported Catalysts with an Ionic Liquid Layer for the Selective Hydrogenation of Acetylene. Chem. Commun. 2011, 47, 12310−12312. (16) Arras, J.; Steffan, M.; Shayeghi, Y.; Claus, P. The Promoting Effect of a Dicyanamide Based Ionic Liquid in the Selective Hydrogenation of Citral. Chem. Commun. 2008, 4058−4060. (17) Arras, J.; Steffan, M.; Shayeghi, Y.; Ruppert, D.; Claus, P. Regioselective Catalytic Hydrogenation of Citral with Ionic Liquids as Reaction Modifiers. Green Chem. 2009, 11, 716−723. (18) Sobota, M.; Happel, M.; Amende, M.; Paape, N.; Wasserscheid, P.; Laurin, M.; Libuda, J. Ligand Effects in SCILL Model Systems: Site-Specific Interactions with Pt and Pd Nanoparticles. Adv. Mater. 2011, 23, 2617−2621. (19) Steinrück, H. P.; Libuda, J.; Wasserscheid, P.; Cremer, T.; Kolbeck, C.; Laurin, M.; Maier, F.; Sobota, M.; Schulz, P. S.; Stark, M. Surface Science and Model Catalysis with Ionic Liquid-Modified Materials. Adv. Mater. 2011, 23, 2571−2587. (20) Steinrück, H. P.; Wasserscheid, P. Ionic Liquids in Catalysis. Catal. Lett. 2015, 145, 380−397. (21) Kolbeck, C.; Niedermaier, I.; Taccardi, N.; Schulz, P. S.; Maier, F.; Wasserscheid, P.; Steinrück, H. P. Monitoring of Liquid-Phase Organic Reactions by Photoelectron Spectroscopy. Angew. Chem., Int. Ed. 2012, 51, 2610−2613. (22) Steinrück, H. P. Surface Science Goes Liquid! Surf. Sci. 2010, 604, 481−484. (23) Maier, F.; Gottfried, J. M.; Rossa, J.; Gerhard, D.; Schulz, P. S.; Schwieger, W.; Wasserscheid, P.; Steinrück, H. P. Surface Enrichment and Depletion Effects of Ions Dissolved in an Ionic Liquid: An X-Ray Photoelectron Spectroscopy Study. Angew. Chem., Int. Ed. 2006, 45, 7778−7780. (24) Sobota, M.; Nikiforidis, I.; Hieringer, W.; Paape, N.; Happel, M.; Steinrück, H. P.; Görling, A.; Wasserscheid, P.; Laurin, M.; Libuda, J. Toward Ionic-Liquid-Based Model Catalysis: Growth, Orientation, Conformation, and Interaction Mechanism of the [Tf2N]− Anion in 4464


Article

The Journal of Physical Chemistry C (42) Gejji, S. P.; Hermansson, K.; Lindgren, J. Abinitio Vibrational Frequencies of the Triflate Ion, (CF3SO3)−. J. Phys. Chem. 1993, 97, 3712−3715. (43) Hoffmann, F. M. Infrared Reflection-Absorption Spectroscopy of Adsorbed Molecules. Surf. Sci. Rep. 1983, 3, 107. (44) Bradshaw, A. M.; Hoffmann, F. M. Chemisorption of CarbonMonoxide on Palladium Single-Crystal Surfaces - IR Spectroscopic Evidence for Localized Site Adsorption. Surf. Sci. 1978, 72, 513−535. (45) Gießel, T.; Schaff, O.; Hirschmugl, C. J.; Fernandez, V.; Schindler, K.-M.; Theobald, A.; Bao, S.; Lindsay, R.; Berndt, W.; Bradshaw.; et al. A Photoelectron Diffraction Study of Ordered Structures in the Chemisorption System Pd{111}-CO. Surf. Sci. 1998, 406, 90−102. (46) Loffreda, D.; Simon, D.; Sautet, P. Dependence of Stretching Frequency on Surface Coverage and Adsorbate-Adsorbate Interactions: A Density-Functional Theory Approach of CO on Pd (111). Surf. Sci. 1999, 425, 68−80. (47) Surnev, S.; Sock, M.; Ramsey, M. G.; Netzer, F. P.; Wiklund, M.; Borg, M.; Andersen, J. N. CO Adsorption on Pd(111): A HighResolution Core Level Photoemission and Electron Energy Loss Spectroscopy Study. Surf. Sci. 2000, 470, 171−185. (48) Rose, M. K.; Mitsui, T.; Dunphy, J.; Borg, A.; Ogletree, D. F.; Salmeron, M.; Sautet, P. Ordered Structures of CO on Pd(111) Studied by STM. Surf. Sci. 2002, 512, 48−60. (49) Blyholder, G. Molecular Orbital View of Chemisorbed Carbon Monoxide. J. Phys. Chem. 1964, 68, 2772. (50) Kim, C. M.; Yi, C.-W.; Goodman, D. W. CO-NO and CO-O2 Interactions on Cu(100) between 25 and 200 K Studied with Infrared Reflection Absorption Spectroscopy. J. Phys. Chem. B 2005, 109, 1891. (51) Bonzel, H. P. Alkali-Metal-Affected Adsorption of Molecules on Metal Surfaces. Surf. Sci. Rep. 1988, 8, 43−125. (52) Levinson, N. M.; Fried, S. D.; Boxer, S. G. Solvent-Induced Infrared Frequency Shifts in Aromatic Nitriles Are Quantitatively Described by the Vibrational Stark Effect. J. Phys. Chem. B 2012, 116, 10470−10476. (53) Chang, S. C.; Weaver, M. J. Coverage-Dependent Dipole Coupling for Carbon-Monoxide Adsorbed at Ordered Platinum(111)Aqueous Interfaces - Structural and Electrochemical Implications. J. Chem. Phys. 1990, 92, 4582−4594. (54) Bagus, P. S.; Nelin, C. J.; Müller, W.; Philpott, M. R.; Seki, H. Field-Induced Vibrational Frequency-Shifts of CO and CN Chemisorbed on Cu(100). Phys. Rev. Lett. 1987, 58, 559−562. (55) Wasileski, S. A.; Koper, M. T. M.; Weaver, M. J. FieldDependent Electrode-Chemisorbate Bonding: Sensitivity of Vibrational Stark Effect and Binding Energetics to Nature of Surface Coordination. J. Am. Chem. Soc. 2002, 124, 2796−2805. (56) Wasileski, S. A.; Koper, M. T. M.; Weaver, M. J. FieldDependent Chemisorption of Carbon Monoxide on Platinum-Group (111) Surfaces: Relationships between Binding Energetics, Geometries, and Vibrational Properties as Assessed by Density Functional Theory. J. Phys. Chem. B 2001, 105, 3518−3530. (57) Ferre, D. C.; Niemantsverdriet, J. W. Vibrational Stark Tuning Rates from Periodic DFT Calculations: CO/Pt(1 1 1). Electrochim. Acta 2008, 53, 2897−2906. (58) Uddin, J.; Anderson, A. B. Trends with Coverage and pH in Stark Tuning Rates for CO on Pt(111) Electrodes. Electrochim. Acta 2013, 108, 398−403. (59) Villegas, I.; Weaver, M. J. Modeling Electrochemical Interfaces in Ultrahigh Vacuum: Molecular Roles of Solvation in Double-Layer Phenomena. J. Phys. Chem. B 1997, 101, 10166−10177. (60) Villegas, I.; Weaver, M. J. Infrared Spectroscopy of Model Electrochemical Interfaces in Ultrahigh Vacuum: Roles of Solvation in the Vibrational Stark Effect. J. Phys. Chem. B 1997, 101, 5842−5852. (61) Villegas, I.; Weaver, M. J. Infrared Spectroscopy of Model Electrochemical Interfaces in Ultrahigh Vacuum: Interfacial Cation Solvation by Ammonia on Pt(111). Surf. Sci. 1996, 367, 162−176. (62) Villegas, I.; Weaver, M. J. Progressive Cation Solvation at Pt(111) Model Electrochemical Interfaces in Ultrahigh Vacuum as

Probed by Infrared Spectroscopy and Work-Function Measurements. Electrochim. Acta 1996, 41, 661−673. (63) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Model Electrochemical Interfaces in Ultra-High-Vacuum - Solvent-Induced Surface-Potential Profiles on Pt(111) from Work-Function Measurements and Infrared Stark Effects. Surf. Sci. 1995, 336, 37−54. (64) Villegas, I.; Kizhakevariam, N.; Weaver, M. J. InfraredSpectroscopy of Model Electrochemical Interfaces in UltrahighVacuum - Some Implications for Ionic and Chemisorbate Solvation at Electrode Surfaces. Surf. Sci. 1995, 335, 300−314. (65) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Model Electrochemical Interfaces in Ultrahigh-Vacuum - Solvent Effects Upon Coverage-Dependent Infrared-Spectra of Carbon-Monoxide on Platinum(111). Langmuir 1995, 11, 2777−2786.

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