Markus Happel

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.

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

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.

Interaction of NO2 with Model NSR Catalysts: Metal–Oxide Interaction Controls Initial NOx Storage Mechanism

Using scanning tunneling microscopy (STM), molecular-beam (MB) methods and time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we investigate the mechanism of initial NO(x) uptake on a model nitrogen storage and reduction (NSR) catalyst. The model system is prepared by co-deposition of Pd metal particles and Ba-containing oxide particles onto an ordered alumina film on NiAl(110). We show that the metal-oxide interaction between the active noble metal particles and the NO(x) storage compound in NSR model catalysts plays an important role in the reaction mechanism. We suggest that strong interaction facilitates reverse spillover of activated oxygen species from the NO(x) storage compound to the metal. This process leads to partial oxidation of the metal nanoparticles and simultaneous stabilization of the surface nitrite intermediate.

Nitrite and nitrate formation on model NOx storage materials: on the influence of particle size and composition

A well-defined model-catalyst approach has been utilized to study the formation and decomposition of nitrite and nitrate species on a model NO(x) storage material. The model system comprises BaAl(2x)O(1+3x) particles of different size and stoichiometry, prepared under ultrahigh-vacuum (UHV) conditions on Al(2)O(3)/NiAl(110). Adsorption and reaction of NO(2) has been investigated by molecular beam (MB) methods and time-resolved IR reflection absorption spectroscopy (TR-IRAS) in combination with structural characterization by scanning tunneling microscopy (STM). The growth behavior and chemical composition of the BaAl(2x)O(1+3x) particles has been investigated previously. In this work we focus on the effect of particle size and stoichiometry on the reaction with NO(2). Particles of different size and of different Ba(2+) : Al(3+) surface ion ratio are prepared by varying the preparation conditions. It is shown that at 300 K the reaction mechanism is independent of particle size and composition, involving initial nitrite formation and subsequent transformation of nitrites into surface nitrates. The coordination geometry of the surface nitrates, however, changes characteristically with particle size. For small BaAl(2x)O(1+3x) particles high temperature (800 K) oxygen treatment gives rise to particle ripening, which has a minor effect on the NO(2) uptake behavior, however. STM shows that the morphology of the particle system is largely conserved during NO(2) exposure at 300 K. The reaction is limited to the formation of surface nitrites and nitrates, which are characterized by low thermal stability and completely decompose below 500 K. As no further sintering occurs before decomposition, NO(2) uptake and release is a fully reversible process. For large BaAl(2x)O(1+3x) particles, aggregates with different Ba(2+) : Al(3+) surface ion ratio were prepared. It was shown that the stoichiometry has a major effect on the kinetics of NO(2) uptake. For barium-aluminate-like particles with high Al(3+) concentration, the formation of nitrites and nitrates on the BaAl(2x)O(1+3x) particles at 300 K is slow, and kinetically restricted to the formation of surface species. Only at elevated temperature (500 K) are surface nitrates converted into well-defined bulk Ba(NO(3))(2). This bulk Ba(NO(3))(2) exhibits substantially higher thermal stability and undergoes restructuring and sintering before it decomposes at 700 K. For Ba(2+)-rich BaAl(2x)O(1+3x) particles, on the other hand, nitrate formation occurs at a much higher rate than for the barium-aluminate-like particles. Furthermore, nitrate formation is not limited to the surface, but NO(2) exposure gives rise to the formation of amorphous bulk Ba(NO(3))(2) particles even at 300 K.

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.