Sebastian Kunz

Adsorptive separation of isobutene and isobutane on Cu3(BTC)2.

The metal organic framework material Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate) has been synthesized using different routes: under solvothermal conditions in an autoclave, under atmospheric pressure and reflux, and by electrochemical reaction. Although the compounds display similar structural properties as evident from the powder X-ray diffraction (XRD) patterns, they differ largely in specific surface area and total pore volume. Thermogravimetric and chemical analysis support the assumption that pore blocking due to trimesic acid and/or methyltributylammoniummethylsulfate (MTBS) which has been captured in the pore system during reaction is a major problem for the electrochemically synthesized samples. Isobutane and isobutene adsorption has been studied for all samples at different temperatures in order to check the potential of Cu3(BTC)2 for the separation of small hydrocarbons. While the isobutene adsorption isotherms are of type I according to the IUPAC classification, the shape of the isobutane isotherm is markedly different and closer to type V. Adsorption experiments at different temperatures show that a somewhat higher amount of isobutene is adsorbed as compared to isobutane. Nevertheless, the differential enthalpies of adsorption are only different by about 5 kJ/mol, indicating that a strong interaction between the copper centers and isobutene does not drive the observed differences in adsorption capacity. The calculated breakthrough curves of isobutene and isobutane reveal that a low pressure separation is preferred due to the peculiar shape of the isobutane adsorption isotherms. This has been confirmed by preliminary breakthrough experiments using an equimolar mixture of isobutane and isobutene.

The effect of particle proximity on the oxygen reduction rate of size-selected platinum clusters

The diminished surface-area-normalized catalytic activity of highly dispersed Pt nanoparticles compared with bulk Pt is particularly intricate, and not yet understood. Here we report on the oxygen reduction reaction (ORR) activity of well-defined, size-selected Pt nanoclusters; a unique approach that allows precise control of both the cluster size and coverage, independently. Our investigations reveal that size-selected Pt nanoclusters can reach extraordinarily high ORR activities, especially in terms of mass-normalized activity, if deposited at high coverage on a glassy carbon substrate. It is observed that the Pt cluster coverage, and hence the interparticle distance, decisively influence the observed catalytic activity and that closely packed assemblies of Pt clusters approach the surface activity of bulk Pt. Our results open up new strategies for the design of catalyst materials that circumvent the detrimental dispersion effect, and may eventually allow the full electrocatalytic potential of Pt nanoclusters to be realized.

Control and Manipulation of Gold Nanocatalysis: Effects of Metal Oxide Support Thickness and Composition

Control and tunability of the catalytic oxidation of CO by gold clusters deposited on MgO surfaces grown on molybdenum, Mo(100), to various thicknesses are explored through temperature-programmed reaction measurements on mass-selected 20-atom gold clusters and via first-principles density functional theory calculations. Au(20) was chosen because in the gas phase it is characterized as an extraordinarily stable tetrahedral-pyramidal structure. Dependencies of the catalytic activities and microscopic reaction mechanisms on the thickness and stoichiometry of the MgO films and on the dimensionalities and structures of the adsorbed gold clusters are demonstrated and elucidated. Langmuir-Hinshelwood mechanisms and reaction barriers corresponding to observed low- and high-temperature CO oxidation reactions are calculated and analyzed. These reactions involve adsorbed O(2) molecules that are activated to a superoxo- or peroxo-like state through partial occupation of the antibonding orbitals. In some cases, we find activated, dissociative adsorption of O(2) molecules, adsorbing at the cluster peripheral interface with the MgO surface. The reactant CO molecules either adsorb on the MgO surface in the cluster proximity or bind directly to the gold cluster. Along with the oxidation reactions on stoichiometric ultrathin MgO films, we also study reactions catalyzed by Au(20) nanoclusters adsorbed on relatively thick defect-poor MgO films supported on Mo and on defect-rich thick MgO surfaces containing oxygen vacancy defects.

Oxidation State and Symmetry of Magnesia supported Pd13Ox Nanocatalysts influence Activation Barriers of CO Oxidation

Combining temperature-programmed reaction measurements, isotopic labeling experiments, and first-principles spin density functional theory, the dependence of the reaction temperature of catalyzed carbon monoxide oxidation on the oxidation state of Pd(13) clusters deposited on MgO surfaces grown on Mo(100) is explored. It is shown that molecular oxygen dissociates easily on the supported Pd(13) cluster, leading to facile partial oxidation to form Pd(13)O(4) clusters with C(4v) symmetry. Increasing the oxidation temperature to 370 K results in nonsymmetric Pd(13)O(6) clusters. The higher symmetry, partially oxidized cluster is characterized by a relatively high activation energy for catalyzed combustion of the first CO molecule via a reaction of an adsorbed CO molecule with one of the oxygen atoms of the Pd(13)O(4) cluster. Subsequent reactions on the resulting lower-symmetry Pd(13)O(x) (x < 4) clusters entail lower activation energies. The nonsymmetric Pd(13)O(6) clusters show lower temperature-catalyzed CO combustion, already starting at cryogenic temperature.

Microkinetic simulations of the oxidation of CO on Pd based nanocatalysis: a model including co-dependent support interactions

The catalysed oxidation of CO using mass-selected Pd(13) clusters supported on thin MgO films was modelled using a microkinetic simulation of the reaction. The model of the system includes reverse spill-over calculations which were intrinsically incorporated into the formulation of the kinetics. The spill-over model is based on a capture-zone approach including a co-dependence on the variables of the kinetic equations. The experimental values were determined using dual pulsed-molecular beam measurements and recorded at a range of temperatures. The experiment allowed the turn-over frequency and reaction probability to be determined as a function of mole fraction. Comparison of the kinetic model with the experimental data gives excellent agreement and strongly highlights the importance of substrate effects. In particular, the origin of the low temperature catalysis of the Pd clusters is elucidated. The model allows the mole fraction and temperature dependent values such as the sticking coefficients for these clusters to be predicted.

Supported, Ligand-Functionalized Nanoparticles: An Attempt to Rationalize the Application and Potential of Ligands in Heterogeneous Catalysis

The binding of molecules to the surface of nanoparticles (NPs) for the use as ligands to manipulate the catalytic properties of NPs is an emerging research area. Various studies with interesting results have been reported in the past few years, but it seems not clear how these findings could be merged into some kind of unified picture, describing the mechanism of action of ligands in heterogeneous catalysis. The aim of this article is to summarize some of the recent achievements in this field with focus on discussing these results using concepts from heterogeneous and homogeneous catalysis. By this it is attempted to separate the influence of ligands into (i) changing the surface properties and (ii) acting as a function above or perpendicular to the surface. The first aspect can be rationalized by the knowledge from bimetallic catalysis. In contrast, the second proposes the relevance of ligand–reactant interactions, as known from homogeneous catalysis, in order to manipulate adsorption, activation, and conversion of reactants. As the application of ligands in heterogeneous catalysis is still a young research field and the full potential of the approach still unknown, this article does not claim to give a complete summery of all results gained within this field. Instead, the author aims to present a picture that may give some guidance for future studies in this area, based on established knowledge from homo- and heterogeneous catalysis.

1-Naphthylamine functionalized Pt nanoparticles: electrochemical activity and redox chemistry occurring on one surface

We present the preparation and electrochemical application of Pt nanoparticles (Pt NPs) functionalized with 1-naphthylamine. Under electrochemical conditions, Pt surface bound 1-naphthylamine (NA) can be reversibly switched (oxidized and reduced), while simultaneously electrocatalytic reactions (e.g. CO oxidation) can proceed on the Pt surface. While the redox activity of the ligand is established immediately after functionalization, the functionalized NPs have to be stored as a colloidal dispersion in tetrahydrofuran (THF) prior to deposition onto the support material in order to induce their catalytic activity. We interpret this catalytic activation due to partial desorption of ligands from the particle surface induced by storing the particles in THF. However, the experimental results do not indicate a loss of ligands from the ligand shell, but evidence that the ligands form oligomers when kept as colloids in THF. As a result the catalytic surface becomes partially available while the redox activity of the ligands is maintained.

Nanoparticles in a box: a concept to isolate, store and re-use colloidal surfactant-free precious metal nanoparticles

A concept is introduced that allows for the isolation, storage and re-use of surfactant-free precious metal nanoparticles (NPs) of catalytic relevance (Pt and Ru). “Surfactant-free NPs” well-defined in size (1–2 nm) are prepared in alkaline ethylene glycol. After synthesis these NPs are stabilized by surface bound CO, formed during synthesis by solvent oxidation, and OH−, added to the reaction mixture. We present a protocol that allows switching reversibly the stabilization between a “CO-protected” and “OH−-protected state”. Most importantly, “OH−-protected” Pt and Ru NPs exhibit remarkable resistance against sintering. These NPs can be isolated as solids, stored and “put into boxes” to be shipped. Thereafter they can be redispersed without changes in particle size or loss in catalytic activity. These results are expected to be of scientific and industrial relevance, as a methodology is introduced to handle “surfactant-free” catalytic nanoparticles like a normal solid chemical.

Temperature Modulation of a Catalytic Gas Sensor

The use of catalytic gas sensors usually offers low selectivity, only based on their different sensitivities for various gases due to their different heats of reaction. Furthermore, the identification of the gas present is not possible, which leads to possible misinterpretation of the sensor signals. The use of micro-machined catalytic gas sensors offers great advantages regarding the response time, which allows advanced analysis of the sensor response. By using temperature modulation, additional information about the gas characteristics can be measured and drift effects caused by material shifting or environmental temperature changes can be avoided. In this work a miniaturized catalytic gas sensor which offers a very short response time (<150 ms) was developed. Operation with modulated temperature allows analysis of the signal spectrum with advanced information content, based on the Arrhenius approach. Therefore, a high-precise electronic device was developed, since theory shows that harmonics induced by the electronics must be avoided to generate a comprehensible signal.

Novel nanoparticle catalysts for catalytic gas sensing

The state-of-the-art approach to stabilize nanoparticles (NPs) for applications in heterogeneous catalysis is to support them on inert inorganic material, which is limited to low loadings of the catalytic component. However, applications such as catalytic gas sensing require a high density of catalytically active sites at a low total heat capacity. To offer an alternative to the supporting of NPs, the stabilization of catalytic NPs with organic ligands in solid state is presented. Therefore, the preparation strategy, consisting of NP synthesis and subsequent functionalization with mono- and bifunctional ligands, is introduced. The molecular linkage of Pt NPs with bifunctional amine ligands (ligand-linking) results in three-dimensional porous networks with ligand-free surface sites. The catalytic properties of ligand-stabilized NPs were investigated in a thermoelectric hydrogen sensor. In addition to an enhanced activity, the stability of the NPs can be significantly improved by ligand-linking. One reason may be that the bifunctional ligand is anchored on the NPs by two head groups. The criteria for ligand structures to enable a successful NP stabilization were identified. para-Phenylenediamine (PDA) combines the criteria and, consequently, by linking of Pt NPs with PDA a constant catalytic activity over more than 20 h on stream was achieved. Thus, ligand-stabilized NPs are presented as a novel catalyst for catalytic gas sensing.