Interactions Between the Room-Temperature Ionic Liquid [C2C1Im

Article pubs.acs.org/JPCC

Interactions Between the Room-Temperature Ionic Liquid [C2C1Im][OTf] and Pd(111), Well-Ordered Al2O3, and Supported Pd Model Catalysts from IR Spectroscopy Stefan Schernich,† Dmytro Kostyshyn,† Valentin Wagner,‡ Nicola Taccardi,‡ Mathias Laurin,*,† Peter Wasserscheid,‡,§ and Jörg Libuda†,§ †

Lehrstuhl für Physikalische Chemie II, ‡Lehrstuhl für Chemische Reaktionstechnik, and §Erlangen Catalysis Resource Center, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Egerlandstraße 3, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: The interactions between ionic liquids and their supports determine many of their applications. The adsorption of the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C2C1Im][OTf] on Pd(111), ordered Al2O3/NiAl(110), and Pd nanoparticles supported on Al2O3/NiAl(110) was investigated under ultrahigh vacuum (UHV) conditions using time-resolved infrared reflection absorption spectroscopy (TR-IRAS). On Pd, the [OTf]− anion stands up with its CF3 group directed toward the vacuum, whereas the anion is less clearly oriented on the oxide. We also find that strong interactions of the IL with the Pd result in migration of the IL from the oxide to the metal nanoparticles.

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INTRODUCTION

EXPERIMENTAL AND THEORETICAL METHODS IRAS Apparatus. The IRAS experiments were performed in a UHV system described elsewhere (base pressure around 2 × 10−10 mbar).18 Briefly, the system allows up to four effusive beams and one supersonic beam to be superimposed on the sample surface. For the present experiments, a home-built IL evaporator replaced one effusive beam source.12 The system is equipped with a Fourier-transform infrared (FTIR) spectrometer (Bruker IFS66/v), two quadruple mass spectrometers, a vacuum transfer system, and all necessary preparation tools. For the preparation of the model systems, the NiAl(110) and Pd(111) surfaces were cleaned by several cycles of Ar+ sputtering and annealing in a vacuum. To prepare the Al2O3 film, the NiAl(110) surface was further treated by two or three cycles of oxidation in 10−6 mbar O2 at 550 K and annealing in UHV at 1135 K. Details about the procedure may be found elsewhere.19,20 The quality of the film was checked by low energy electron diffraction (LEED). Pd (Alfa Aesar, >99.9%) was deposited from a wire using a commercial electron-beam evaporator (Focus EFM3). IR spectra were acquired in timeresolved mode during the exposure of the sample to IL. The spectral resolution was 2 cm−1 with a typical acquisition time of 1 min/spectrum. Synthesis of [1-Ethyl-3-methylimidazolium][trifluoromethanesulfonate] [C2C1Im][OTf]. The ionic liquid was synthesized via an alkylation carried out under dry argon conditions using standard Schlenk techniques. A flask was charged with freshly distilled 1-ethylimidazole (7.38 g, 7.69 mmol) and 50 mL of dry dichloromethane. Commercial methyl

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The unique properties of ionic liquids (ILs) have made them a subject of intensive research in the field of catalysis. Besides many applications in homogeneous two-phase transition-metal catalysis, more recent research has demonstrated novel catalysis concepts such as the so-called solid catalyst with ionic liquid layer (SCILL).3 In a SCILL catalyst, a thin IL film is deposited onto a conventional heterogeneous catalyst that usually consists of noble metal particles dispersed on an oxide support. In this case, the IL can alter the reactivity of the noble metal component by a ligand-like interaction4 or modify diffusion processes due to the different solubilities of the reactants or possible intermediates within the IL film. The SCILL concept is, however, still in an early stage of development. Up to now, most studies addressing SCILL catalysts have focused on hydrogenation reactions.3,5−10 Even though an increase in selectivity has been observed in many of them, the total turnover rates were usually lower compared to nonmodified systems because of new diffusion barriers introduced by the IL film and partial poisoning of catalytic sites. In spite of these intriguing results, there are only a few studies that have dealt with the underlying principles of SCILL catalysts on an atomic level,4,11−16 probably in part because the required surface scientific studies are experimentally involved. Yet, the interactions at the IL/solid interface are crucial for the performance of the catalyst since they have a major influence on the aforementioned reactivity and diffusion behavior.17 Investigation of these interactions by surface scientific methods is therefore key to a knowledge-driven design of new SCILL catalysts.11 © XXXX American Chemical Society

Received: January 20, 2014 Revised: January 24, 2014

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dx.doi.org/10.1021/jp5006692 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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the literature.25,26 Briefly, the Pd particles mainly expose (111) facets and a smaller proportion of (100) facets. Overview of the Measurements and Comparison of Multilayer Spectra. Figures 1a, 2a, and 3a display the IRAS spectra recorded during the deposition of the IL onto the three model systems at 300 K. The five main absorption peaks for the high coverage spectra are assigned as follows: The four peaks below 1300 cm−1 correspond to the anion.27−29 The absorption bands at 1035 cm−1 and 1286 cm−1 correspond to νs(SO3) and νas(SO3), respectively. The absorption bands at 1227 cm−1 and 1173 cm−1 correspond to νs(CF3) and νas(CF3), respectively. The absorption band at 1575 cm−1 corresponds to a vibration of the aromatic imidazolium ring in the cation. Because of the low intensity of the cation absorptions (including at wavenumbers different from the ones shown here), we limit our discussion to the anion. The multilayer spectra are not only similar for each system but also resemble transmission infrared (TIR) spectra of the IL diluted in KBr (see Figure S1, Supporting Information). This suggests that the anions have a random orientation in the multilayer. Similar observations have been made before with this and other ILs.12,29,30 Single Crystalline Metallic Sample: Pd(111). While the multilayer spectra of the three model systems are comparable, the monolayer spectra show significant differences. Figure 1b compares multilayer and monolayer spectra on Pd(111). It reveals that νs(SO3) is red-shifted by 27 cm−1 and νs(CF3) by 12 cm−1 relative to their values in the multilayer. The fact that the symmetric vibrations (oriented strictly along the molecular axis) are affected indicates an orientation of the anion normal to the surface. This assumption is reinforced by an analysis of the peak intensities. Taking advantage of the metal surface selection rule (MSSR),31,32 the orientation of the anion in the monolayer can be determined. Our approach to obtain the orientation of the anion on the surface is based on trigonometric considerations similar to those used in nearedge X-ray absorption fine structure (NEXAFS) spectrosco-

trifluoro-methanesulfonate (Sigma-Aldrich) was slowly added (9.1 mL, 80.7 mmol, 1.05 equiv) and was heated until reflux for five days. The product was obtained after drying the reaction mixture under reduced pressure (0.01 mbar, 40 °C) as a clear to pale yellow, low viscosity liquid in an 86% yield (17.2 g). 1 H NMR, DMSO-d6: δ 1.37 (t, J = 7.4 Hz 3, CH3); 3.80 (s, 3, CH3); 4.14 (q, J = 7.4 Hz, 2, CH2); 7.65 (t, J = 1.7 Hz 1, CH); 7.74 (t, J = 1.7 Hz 1, CH); 9.06 (s, 1, CH). 13C-{1H}: δ 15.7; 36.4; 44.5; 122.4; 124.1; 136.7. 19F: δ −77.8.

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RESULTS AND DISCUSSION In this work, we employ a rigorous surface science approach to study the adsorption of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate [C2C1Im][OTf] (Scheme 1) on a Pd(111) Scheme 1. Structure of [C2C1Im][OTf]

single crystal, an ordered alumina film grown on a NiAl(110) single crystal, and alumina-supported Pd nanoparticles by timeresolved infrared reflection absorption spectroscopy (TRIRAS).4,12,21−24 This particular IL was chosen as a model IL because the anion exhibits strong absorption bands in IR and the cation absorbs only weakly so that the analysis is simpler. Our group also extensively studied ILs with the [Tf2N]− anion that also contains SOn and CF3 groups so that a part of our expertise can be transferred to [OTf]− in a straightforward manner. All surfaces are prepared exclusively under ultrahigh vacuum (UHV) conditions. The IL is deposited onto the substrates by physical vapor deposition (PVD) from a home-built IL evaporator.12 A detailed characterization of the alumina film19,20 and the supported Pd nanoparticles can be found in

Figure 1. (a) TR-IRAS spectra recorded during the deposition of [C2C1Im][OTf] onto Pd(111) at 300 K, (b) comparison between the spectra of an IL monolayer and multilayer, and (c) suggested orientation of the anion in the first monolayer. B



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Figure 2. (a) TR-IRAS spectra recorded during the deposition of [C2C1Im][OTf] onto Al2O3/NiAl(110) at 300 K, (b) comparison between the spectra of an IL monolayer and multilayer, and (c) suggested orientation of the anion in the first monolayer.

Figure 3. (a) TR-IRAS spectra recorded during the deposition of [C2C1Im][OTf] onto Pd/Al2O3/NiAl(110) at 300 K and (inset) comparison between low coverage spectra of the IL on Pd(111), Al2O3/NiAl(110), and Pd/Al2O3/NiAl(110) in the range of the symmetric SO3 stretch. (b) Comparison between the spectra of an IL monolayer and multilayer and (c) suggested orientation and migration of the anion within the first monolayer.

py.33 We define θ as the angle between the molecular axis of the anion and the surface normal, μs and μas are the dipole moments associated with νs and νas, the symmetric and asymmetric stretching vibrations of the terminal groups SO3 or CF3 of the anion. It is noteworthy that μs is oriented along the molecular axis while μas is oriented perpendicular to it (see inset in Figure 4b). The MSSR31,32 of IRAS further states that only dynamic dipole moments with a component normal to a metallic surface will give rise to peaks in the absorption spectrum. The MSSR also holds true for thin oxide films on metallic substrates. The intensity of the peak, I, therefore depends on the angle θ between the surface normal and the

molecular axis with Is = μ2s cos2(θ) and Ias = 2 μ2as sin2(θ). The factor 2 in Ias is due to the degeneracy of the asymmetric vibrations. The intensity ratio at thickness h, rh, is thus rh =

μ2 cos2(θ ) μs2 Is = s2 2 = Ias 2μas sin (θ ) 2μas2 tan 2(θ )

where Is and Ias are obtained by integrating the respective features in the measured spectra. Figure 4a plots this function with our experimental data. Then, random orientation in the bulk phase gives the magic angle θ = 54.7°. Therefore, assuming that the multilayer spectra C



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lifted by the formation of hydrogen bonds between ethanol and two of the oxygen or fluorine atoms of the anion. The extent of the splitting provides information about which of the groups is directly involved in the interaction. They also noted similarities between the spectra of the IL diluted in ethanol and deposited on Al2O3 nanoparticles. They concluded that the [OTf]− anion interacts with the surface via the SO3 group. Previous works within our group showed similar results for the [Tf2N]− anion on Al2O3/NiAl(110).12 The results of this work also seem to be in line with these findings: In contrast to Pd(111), there is no shift of the symmetric vibrations but νas(SO3) is red-shifted in the monolayer spectrum (Figure 2b). These observations point to an adsorption geometry in which the anion interacts less strongly with the surface via the SO3 group and has its molecular axis tilted from the surface normal. Employing the same approach as above, we find an angle of 30 ± 10° between the molecular axis of the anion and the surface normal (see Figure 4b). An illustration summarizing these results is depicted in Figure 2c. Pd Nanoparticles on Alumina. The final part of our discussion deals with Pd nanoparticles supported on Al2O3/ NiAl(110). As expected, the monolayer spectrum of this model catalyst (Figure 3c) shows features from the spectra on Pd(111) and pristine Al2O3. Yet, while νs(SO3) and νs(CF3) are clearly visible as a peak at 1010 cm−1 and a shoulder at 1215 cm−1, the asymmetric stretches are barely visible. This suggests that the IL preferentially adsorbs onto the Pd nanoparticles, while the alumina support remains largely uncovered. The inset of Figure 3a compares the νs(SO3) region at low coverage on the three systems confirming that the spectrum acquired on the Pd nanoparticles is more similar to the Pd(111) spectra. Note that the effect is possibly even underestimated, as the surface area of the oxide should be larger than the one of the nanoparticles. In an earlier study, we could show that the interaction between another IL and supported Pd nanoparticles is able to replace even strongly bound adsorbates such as CO.4 We therefore assume that, at low coverage, the IL diffuses on the substrate and preferentially binds to the Pd nanoparticles. A scheme depicting the binding situation of the anion on Pd/ Al2O3 is given in Figure 3c.

Figure 4. (a) Plot of rh, the intensity ratio between the symmetric and the asymmetric stretching vibrations of the SO3 (●) and CF3 (▲) groups as a function of the deposited thickness h, on the Pd(111) (black) and Al2O3 (red) surfaces and (b) θ, the corresponding molecular angles. An illustration of the binding situation of the [OTf]− anion on a solid substrate is shown as an inset. μ indicates the dynamic dipole moments corresponding to stretching vibrations. The dashed line identifies the magic angle; other lines are guides to the eye.

behave like the bulk, the intensity ratio at large exposure becomes r∞ =

μ2 Is = s2 Ias 4μas

θ can now be obtained at any coverage by solving rh/r∞ = 2tan−2(θ), which gives θ = arctan

2r∞ rh

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CONCLUSION In conclusion, we studied the adsorption and orientation of the triflate anion of the IL [C2C1Im][OTf] on Pd(111), Al2O3/ NiAl(110), and Pd nanoparticles supported on Al2O3/NiAl(110) at 300 K using TR-IRAS. Characteristic differences can be observed in the monolayer spectra, while the multilayer spectra of the three systems are comparable and suggest a random orientation of the anion in the bulk. On Pd(111), the symmetric vibrations of the anion are red-shifted, whereas on Al2O3 the asymmetric vibrations are shifted. The anion interacts with both surfaces via the SO3 group. The anion seems to stand up on the Pd(111) surface while it appears to be tilted on Al2O3. Finally, our results suggest that the interactions between the IL and the Pd nanoparticles are stronger than with the Al2O3 support leading to a preferential adsorption of the IL on the Pd nanoparticles and migration of the IL to the metal particles.

On Pd(111), we obtain an angle of 10 ± 10° (Figure 4b) with an estimated error that accounts for uncertainties in baseline correction and peak integration. This value is in good agreement with the results of earlier works on the adsorption of (bi)sulfate on Pt(111) by means of vibrational spectroscopy,34−37 scanning tunneling microscopy (STM),38−40 and density functional theory (DFT).41−43 The fact that νs(SO3) is shifted more than twice as much as νs(CF3) further suggests that the anion is interacting with the surface via the SO3 group, while the CF3 group points toward the vacuum.29 Similar observations have been made for the [Tf2N]− anion on Au(111).13 Figure 1c summarizes these findings. Ordered Oxide: Alumina. The situation is very different on Al2O3/NiAl(110). In their recently published work, Andanson and Baiker studied the interaction of this IL with various organic solvents and Al2O3 nanoparticles using a combination of attenuated total reflection infrared (ATR-IR) spectroscopy and DFT calculations.29 They found that the type of interactions between the solvent molecules and the anion determines the shape of the IR spectra. Indeed, they show that the 2-fold degeneracy of the asymmetric vibrations may be

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ASSOCIATED CONTENT

S Supporting Information *

Transmission infrared spectra of [C2C1Im][OTf]. This material is available free of charge via the Internet at http://pubs.acs.org. D



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AUTHOR INFORMATION

Corresponding Author

*Fax: +49 9131 8528867. Tel: +49 9131 8527310. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by the “Deutsche Forschungsgemeinschaft” (DFG) within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative. S.S. gratefully acknowledges financial support from the “Fonds der Chemischen Industrie”.

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Interactions Between the Room-Temperature Ionic Liquid [C2C1Im

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