Mathias Laurin

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.

Particle size dependent adsorption and reaction kinetics on reduced and partially oxidized Pd nanoparticles

Combining scanning tunneling microscopy (STM), IR reflection absorption spectroscopy (IRAS) and molecular beam (MB) techniques, we have investigated particle size effects on a Pd/Fe(3)O(4) model catalyst. We focus on the particle size dependence of (i) CO adsorption, (ii) oxygen adsorption and (iii) Pd nanoparticle oxidation/reduction. The model system, which is based on Pd nanoparticles supported on an ordered Fe(3)O(4) film on Pt(111), is characterized in detail with respect to particle morphology, nucleation, growth and coalescence behavior of the Pd particles. Morphological changes upon stabilization by thermal treatment in oxygen atmosphere are also considered. The size of the Pd particles can be varied roughly between 1 and 100 nm. The growth and morphology of the Pd particles on the Fe(3)O(4)/Pt(111) film were characterized by STM and IRAS of adsorbed CO as a probe molecule. It was found that very small Pd particles on Fe(3)O(4) show a strongly modified adsorption behavior, characterized by atypically weak CO adsorption and a characteristic CO stretching frequency around 2130 cm(-1). This modification is attributed to a strong interaction with the support. Additionally, the kinetics of CO adsorption was studied by sticking coefficient experiments as a function of particle size. For small particles it is shown that the CO adsorption rate is significantly enhanced by the capture zone effect. The absolute size of the capture zone was quantified on the basis of the STM and sticking coefficient data. Finally, oxygen adsorption was studied by means of MB CO titration experiments. Pure chemisorption of oxygen is observed at 400 K, whereas at 500 K partial oxidation of the particles occurs. The oxidation behavior reveals strong kinetic hindrances to oxidation for larger particles, whereas facile oxidation and reduction are observed for smaller particles. For the latter, estimates point to the formation of oxide layers which, on average, are thicker than the surface oxides on corresponding single crystal surfaces.

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

Model Catalytic Studies of Liquid Organic Hydrogen Carriers: Dehydrogenation and Decomposition Mechanisms of Dodecahydro-N-ethylcarbazole on Pt(111)

Liquid organic hydrogen carriers (LOHC) are compounds that enable chemical energy storage through reversible hydrogenation. They are considered a promising technology to decouple energy production and consumption by combining high-energy densities with easy handling. A prominent LOHC is N-ethylcarbazole (NEC), which is reversibly hydrogenated to dodecahydro-N-ethylcarbazole (H12-NEC). We studied the reaction of H12-NEC on Pt(111) under ultrahigh vacuum (UHV) conditions by applying infrared reflection–absorption spectroscopy, synchrotron radiation-based high resolution X-ray photoelectron spectroscopy, and temperature-programmed molecular beam methods. We show that molecular adsorption of H12-NEC on Pt(111) occurs at temperatures between 173 and 223 K, followed by initial C–H bond activation in direct proximity to the N atom. As the first stable dehydrogenation product, we identify octahydro-N-ethylcarbazole (H8-NEC). Dehydrogenation to H8-NEC occurs slowly between 223 and 273 K and much faster above 273 K. Stepwise dehydrogenation to NEC proceeds while heating to 380 K. An undesired side reaction, C–N bond scission, was observed above 390 K. H8-NEC and H8-carbazole are the dominant products desorbing from the surface. Desorption occurs at higher temperatures than H8-NEC formation. We show that desorption and dehydrogenation activity are directly linked to the number of adsorption sites being blocked by reaction intermediates.

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.

Functionalization of Oxide Surfaces through Reaction with 1,3-Dialkylimidazolium Ionic Liquids.

Practical applications of ionic liquids (ILs) often involve IL/oxide interfaces, but little is known regarding their interfacial chemistry. The unusual physicochemical properties of ILs, including their exceptionally low vapor pressure, provide access to such interfaces using a surface science approach in ultrahigh vacuum (UHV). We have applied synchrotron radiation photoelectron spectroscopy (SR-PES) to the study of a thin film of the ionic liquid [C6C1Im][Tf2N] prepared in situ in UHV on ordered stoichiometric CeO2(111) and partially reduced CeO2-x. On the partially reduced surface, we mostly observe decomposition of the anion. On the stoichiometric CeO2(111) surface, however, a layer of surface-anchored organic products with high thermal stability is formed upon reaction of the cation. The suggested acid-base reaction pathway may provide well-defined functionalized IL/solid interfaces on basic oxides.

Enhanced Activity and Selectivity in Catalytic Methanol Steam Reforming by Basic Alkali Metal Salt Coatings

Steam reforming is the method of choice if hydrogen has to be produced from methanol in high yields.[1] Under ideal conditions, the reaction converts methanol and water into carbon dioxide and three moles of hydrogen in a moderately endothermic transformation, as shown in Equation (1).

Local reaction rates and surface diffusion on nanolithographically prepared model catalysts: Experiments and simulations

Combining molecular beam methods and angular resolved mass spectrometry, we have studied the angular distribution of desorbing products during CO oxidation on a planar Pd/silica supported model catalyst. The model catalyst was prepared by means of electron beam lithography, allowing individual control of particle size, position, and aspect ratio, and was characterized by atomic force microscopy and scanning electron microscopy before and after reaction. In the experiment, both oxygen and CO rich regimes were investigated using separate molecular beams for the two reactants. This allows exploration of diffusion effects of reactants on the particles and of shadowing and backscattering phenomena. A reaction-diffusion model was developed in order to extract information about local reaction rates on the surface of the catalyst nanoparticles. The model takes into account the structural parameters of the catalyst as well as the backscattering of the reactants and products from the support. It allows a quantitative description of the experimental data and provides a detailed understanding of temperature and reactant flux dependent effects. Moreover, information on the surface mobility of oxygen under steady-state reaction conditions could be obtained by comparison with the experimental results.

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.