Marek Sobota

Toward Ionic-Liquid-Based Model Catalysis: Growth, Orientation, Conformation, and Interaction Mechanism of the [Tf2N]− Anion in [BMIM][Tf2N] Thin Films on a Well-Ordered Alumina Surface

Aiming at a better understanding of the interaction of ionic liquid (IL) thin films with oxide supports, we have performed a model study under ultrahigh vacuum (UHV) conditions. We apply infrared reflection absorption spectroscopy (IRAS) in combination with density functional theory (DFT). Thin films of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][Tf(2)N] are grown on an atomically flat, well-ordered alumina film on NiAl(110) using a novel UHV-compatible evaporator. Time-resolved IRAS measured during the growth and subsequent thermal desorption points toward reversible molecular adsorption and desorption. There was no indication of decomposition. The vibrational bands are assigned with the help of DFT calculations. Strong relative intensity changes in individual [Tf(2)N](-) bands are observed in the monolayer region. This indicates pronounced orientation effects for the anion. The adsorption geometry of [Tf(2)N](-) is determined on the basis of a detailed comparison with DFT. The results suggest that [Tf(2)N](-) anions adopt a cis conformation in the submonolayer region. They adsorb in a slightly tilted orientation with respect to the surface, mainly interacting with the support via the sulfonyl groups.

Dehydrogenation of Dodecahydro-N-ethylcarbazole on Pd/Al2O3 Model Catalysts

To elucidate the dehydrogenation mechanism of dodecahydro-N-ethylcarbazole (H(12)-NEC) on supported Pd catalysts, we have performed a model study under ultra high vacuum (UHV) conditions. H(12)-NEC and its final dehydrogenation product, N-ethylcarbazole (NEC), were deposited by physical vapor deposition (PVD) at temperatures between 120 K and 520 K onto a supported model catalyst, which consisted of Pd nanoparticles grown on a well-ordered alumina film on NiAl(110). Adsorption and thermally induced surface reactions were followed by infrared reflection absorption spectroscopy (IRAS) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) in combination with density functional theory (DFT) calculations. It was shown that, at 120 K, H(12)-NEC adsorbs molecularly both on the Al(2)O(3)/NiAl(110) support and on the Pd particles. Initial activation of the molecule occurs through C-H bond scission at the 8a- and 9a-positions of the carbazole skeleton at temperatures above 170 K. Dehydrogenation successively proceeds with increasing temperature. Around 350 K, breakage of one C-N bond occurs accompanied by further dehydrogenation of the carbon skeleton. The decomposition intermediates reside on the surface up to 500 K. At higher temperatures, further decay to small fragments and atomic species is observed. These species block most of the absorption sites on the Pd particles, but can be oxidatively removed by heating in oxygen at 600 K, fully restoring the original adsorption properties of the model catalyst.

Ligand effects in SCILL model systems: site-specific interactions with Pt and Pd nanoparticles.

N The intriguing properties of ionic liquids (ILs) [ 1 , 2 ] have led to the development of novel concepts in heterogeneous catalysis, such as “supported ionic liquid phase (SILP)” [ 3–5 ] catalyst materials or the “solid catalysts with an ionic liquid layer (SCILL)” technology. [ 6 , 7 ] SCILL systems involve the modifi cation of conventional supported catalysts by thin IL fi lms, taking advantage of the tunability of the physico-chemical properties of the ILs and their distinct potential to chemically interact with supported catalytic nanoparticles. Due to their low vapor pressure, the IL fi lms reside on the catalyst surface under reactions conditions. Enhanced selectivity has been demonstrated for SCILL systems in hydrogenation catalysis, [ 6 , 7 ] but the microscopic origins of such effects are still unclear. [ 8 ] On the one hand, the concentration of reactants available at the catalytic center may be tuned via the reactant solubility in the IL. [ 1 ] On the other hand, the IL may act as a ligand directly interacting with the catalytically active nanoparticle. Such interactions may even lead to decomposition of the IL under reaction conditions, with the co-adsorbed decomposition products further modifying the catalytic properties. In this communication, we report evidence that a typical ionic liquid like 1-butyl-3-methylimidazolium bis(trifl uoromethylsulfonyl)imide ([BMIM][Tf 2 N]) indeed develops strong ligand-like interactions with supported Pt and Pd nanoparticles. Even tightly bound adsorbates such as CO are partially replaced by the IL from the surface of nanoparticles. Interestingly, these interactions are specifi c to the particle sites and materials, i.e., different surface sites are selectively emptied on Pt or Pd nanoparticles. The co-adsorbed

Dehydrogenation Mechanism of Liquid Organic Hydrogen Carriers: Dodecahydro‐N‐ethylcarbazole on Pd(111)

Dodecahydro-N-ethylcarbazole (H12-NEC) has been proposed as a potential liquid organic hydrogen carrier (LOHC) for chemical energy storage, as it combines both favourable physicochemical and thermodynamic properties. The design of optimised dehydrogenation catalysts for LOHC technology requires a detailed understanding of the reaction pathways and the microkinetics. Here, we investigate the dehydrogenation mechanism of H12-NEC on Pd(111) by using a surface-science approach under ultrahigh vacuum conditions. By combining infrared reflection-absorption spectroscopy, density functional theory calculations and X-ray photoelectron spectroscopy, surface intermediates and their stability are identified. We show that H12-NEC adsorbs molecularly up to 173 K. Above this temperature (223 K), activation of C-H bonds is observed within the five-membered ring. Rapid dehydrogenation occurs to octahydro-N-ethylcarbazole (H8-NEC), which is identified as a stable surface intermediate at 223 K. Above 273 K, further dehydrogenation of H8-NEC proceeds within the six-membered rings. Starting from clean Pd(111), C-N bond scission, an undesired side reaction, is observed above 350 K. By complementing surface spectroscopy, we present a temperature-programmed molecular beam experiment, which permits direct observation of dehydrogenation products in the gas phase during continuous dosing of the LOHC. We identify H8-NEC as the main product desorbing from Pd(111). The onset temperature for H8-NEC desorption is 330 K, the maximum reaction rate is reached around 550 K. The fact that preferential desorption of H8-NEC is observed even above the temperature threshold for H8-NEC dehydrogenation on the clean surface is attributed to the presence of surface dehydrogenation and decomposition products during continuous reactant exposure.

Ordering and Phase Transitions in Ionic Liquid‐Crystalline Films

Ionic liquids are low melting salts that combine extremely low vapor pressures with a large number of interesting chemical and physicochemical properties depending on the nature of their cation/anion combination. Apart from bulk ionic-liquid applications, a concept known as “supported ionic liquid phase (SILP)” technology has attracted a lot of attention in the recent five years. In SILP materials, a very thin film of ionic liquid (typically a couple of nanometers thick) is confined on the surface of a solid by physisorption, tethering, or covalent anchoring of ionic liquid fragments. In this way, a material is obtained, the surface of which is modified by a molecular defined liquid with extremely low vapor pressure. Surface properties that can be realized in this manner depend on the chemical nature of the ionic liquid (e.g. acidic or complexing properties) or on the functionalities of compounds (e.g. catalyst complexes or molecular carriers) dissolved therein. The concept is actually investigated in detail for the design of new catalytic materials, new adsorber systems or new supported liquid membranes. Ionic liquid crystals (ILCs) are formed by one or two anisotropically shaped ions involving, typically, imidazolium ions with long N-alkyl substituents (i.e. , CnH2n + 1 alkyl substituents with n 12). Compared to “traditional”, neutral liquid crystals, ILCs offer the additional feature of displaying ionic conductivity. In addition, they form uncommon ordering of their liquid-crystalline states due to strong coulombic and van der Waals interactions. Interestingly, their ability to form smectic mesophases in easily accessible temperature ranges (in most cases below 100 8C) leads to temperature-switchable structures and properties. Confining ILCs on solids in a SILP-type fashion is a highly interesting, yet fully unexplored, concept to obtain new SILP materials with attractive thermomorphic surface properties. SILP catalysis in a temperature-switchable anisotropic environment or temperature-switchable selectivities of ILC-SILP membranes are only two fascinating potential applications of such new materials. Herein, we provide for the first time spectroscopic insights into the behavior of ILC films physisorbed in a SILP-type fashion on a planar support. By describing in detail the structural changes induced in supported ILCs by temperature variation we create a fundamental basis to explore these options in great detail in the near future. In spite of the intriguing idea to use supported LC thin films as temperature-switchable functional surfaces for, for example, catalysis or separation technologies, experimental verification of the concept turns out to be demanding. The reason is that there are hardly any experimental probes which would allow us to monitor the local structure and ordering of LC thin films when dispersed on porous catalyst supports. Typical methods such as polarized optical microscopy (POM) or differential scanning calorimetry (DSC) are not applicable, at least in the limit of low loadings. However, direct information on the structure of the ILC phase is crucial, as both the dissolved catalyst and the reactants may have an appreciable influence on the structure of the LC phase. Thus, the degree of ordering in the LC phase under reaction conditions may substantially differ from the pure ILC phase. Also, the support may impose structural effects on the LC phase. Interactions with the support surface or the confinement in small nanometer-sized pores may shift or suppress phase transitions. It will not be possible to monitor such phenomena with conventional methods such as DSC or POM. Here, the application of vibrational spectroscopy may be helpful to provide the desired information. Fourier-transform infrared (FTIR) spectroscopy has been used as a tool to monitor the local structure in organic thin-film systems including Langmuir–Blodgett (LB) films and self-assembled monolayers (SAM) . From band intensities and positions of the CH2 antisymmetric stretching, symmetric stretching, scissoring, and rocking modes, information can be extracted on the molecular orientation, the degree of ordering, the concentration of gauche defects and even on the subcell packing (see e.g. ref. [32] and references therein for details). FTIR spectroscopy can be straightforwardly applied both to powders using diffuse reflectance IR FT spectroscopy (DRIFTS) and to planar surfaces using IR reflection absorption spectroscopy (IRAS), the latter with submonolayer sensitivity. Although IRAS has been extensively used to investigate molecular orientation at interfaces, to the best [a] M. Sobota, M. Happel, Dr. M. Laurin, Prof. Dr. J. Libuda Lehrstuhl f r Physikalische Chemie II Friedrich-Alexander-Universit t Erlangen-N rnberg Egerlandstrasse 3, 91058 Erlangen (Germany) Fax: (+ 49) 9131-8528867 E-mail : mathias.laurin@chemie.uni-erlangen.de libuda@chemie.uni-erlangen.de [b] Prof. Dr. K. Meyer, Prof. Dr. P. Wasserscheid, Prof. Dr. J. Libuda Erlangen Catalysis Resource Center Friedrich-Alexander-Universit t Erlangen-N rnberg Egerlandstrasse 3, 91058 Erlangen (Germany) [c] X. Wang, Prof. Dr. K. Meyer Lehrstuhl f r Anorganische Chemie Friedrich-Alexander-Universit t Erlangen-N rnberg Egerlandstrasse 1, 91058 Erlangen (Germany) [d] Dr. M. Fekete, Prof. Dr. P. Wasserscheid Lehrstuhl f r Chemische Reaktionstechnik Friedrich-Alexander-Universit t Erlangen-N rnberg Egerlandstrasse 3, 91058 Erlangen (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201000144.

Controlling the adsorption kinetics via nanostructuring: Pd nanoparticles on TiO2 nanotubes.

Activity and selectivity of supported catalysts critically depend on transport and adsorption properties. Combining self-organized porous oxide films with different metal deposition techniques, we have prepared novel Pd/TiO(2) catalysts with a new level of structural control. It is shown that these systems make it possible to tune adsorption kinetics via their nanostructure. Self-organized TiO(2) nanotubular arrays (TiNTs) prepared by electrochemical methods are used as a support, on which Pd particles are deposited. Whereas physical vapor deposition (PVD) in ultrahigh vacuum (UHV) allows us to selectively grow Pd particles at the tube orifice, Pd/TiNT systems with homogeneously distributed Pd aggregates inside the tubes are available by particle precipitation (PP) from solution. Both methods also provide control over particle size and loading. Using in-situ infrared reflection absorption spectroscopy (IRAS) and molecular beam (MB) methods, we illustrate the relation between the nanostructure of the Pd/TiNT systems and their adsorption kinetics. Control over the metal nanoparticle distribution in the nanotubes leads to drastic differences in adsorption probability and saturation behavior. These differences are rationalized based on differences in surface and gas phase transport resulting from their nanostructure. The results suggest that using carefully designed metal/TiNT systems it may be possible to tailor transport processes in catalytically active materials.

Functional nickel complexes of N-heterocyclic carbene ligands in pre-organized and supported thin film materials

Nickel(II) complexes with double alkyl chain functionalized N-heterocyclic carbene (NHC) ligands, [NiCl2(C12MIM)2] and [NiCl2(C12C12IM)2], where C12MIM = 1-dodecyl-3-methylimidazolin-2-ylidene (1) and C12C12IM = 1,3-didodecylimidazolin-2-ylidene (2), have been prepared and fully characterized by 1H NMR, 13C NMR, and CHN elemental analyses. Furthermore, we have developed a system, in which double long alkyl chain derivatized Ni–NHC complexes are dissolved in the related ionic liquid crystalline 1,3-didodecylimidazolium tetrafluoroborate, [C12C12IM][BF4], to form pre-organized structures for enhanced reactivity. Remarkably, differential scanning calorimetry, polarized optical microscopy, and temperature-programmed IR reflection absorption spectroscopy performed on a mixture of 10 wt% Ni complexes in [C12C12IM][BF4] demonstrate that this system retains an ionic liquid crystalline phase; even after immobilization onto a silica-100 support with pore filling α = 1.

Interactions of Imidazolium-Based Ionic Liquids with Oxide Surfaces Controlled by Alkyl Chain Functionalization

From a different angle: Thin films of functionalized ionic liquids are deposited on cerium oxides following a surface science approach. The functionalization of the alkyl chain changes its orientation with respect to the surface plane from normal to parallel. This then leads to a different surface chemistry at higher temperatures.

Model NOx Storage Materials at Realistic NO2 Pressures

The interaction of NO2 with single‐crystal‐based model NOx storage materials, consisting of barium aluminate nanoparticles on Al2O3/NiAl(110), are investigated by time‐resolved infrared reflection absorption spectroscopy (TR‐IRAS) at realistic NO2 partial pressures up to 1.75 mbar. The data is compared to spectra obtained under ultrahigh vacuum (UHV) conditions on the same model system. At 300 K, the NO2 uptake at pressures around 1 mbar proceeds through rapid initial formation of surface nitrites and nitrates, similar to that under UHV conditions. The vibrational spectra of the surface species formed at realistic NO2 pressures are comparable to those for species formed under UHV conditions. Beyond the formation of surface species, the formation of bulk nitrates occurs, but is kinetically strongly hindered. At a very low rate, the formation of a disordered barium bulk nitrate is detected. At 500 K, this kinetic hindrance is overcome and the available Ba2+ is quantitatively converted to bulk Ba(NO3)2. The IRAS spectrum of these Ba(NO3)2 particles differs characteristically from those obtained for nitrate multilayers formed upon incomplete conversion under UHV conditions. In addition to the formation of bulk Ba(NO3)2, a more weakly bound disordered nitrate species is formed. This species gives rise to a dynamic NO2 uptake and release well below the decomposition temperature of bulk Ba(NO3)2. The experiments show that model studies under UHV conditions mainly provide information on the initial reaction mechanism, whereas the observation of actual bulk NOx storage phases requires experiments at realistic temperatures and pressures.