Birte Jürgens

New gold and silver-gold catalysts in the shape of sponges and sieves

Gold with a nanoporous sponge-like morphology, generated by leaching of AuAg alloys is presented as a new unsupported material system for catalytic applications. The role of residual silver for catalytic activity towards CO oxidation in the temperature range from −20 to 50°C has been investigated by comparison with Au and Au/Ag zeolite catalysts. As revealed by a systematic variation of the silver content in the zeolite catalysts, bimetallic systems exhibit a significantly higher activity than pure gold, probably due to activation/dissociation of molecular oxygen by silver. By STEM tomography we can unambiguously prove that at least some of the particles form inside the zeolite lattice.

UHV studies of methanol decomposition on mono- and bimetallic CoPd nanoparticles supported on thin alumina films

Bimetallic nanoparticles often turn out to be superior to the corresponding monometallic systems with respect to their catalytic properties. To study such effects for the methanol decomposition reaction, model catalysts were prepared by physical vapor deposition of Pd and Co under ultrahigh-vacuum (UHV) conditions. Monometallic Pd and Co particles as well as CoPd core-shell particles were generated on an epitaxial alumina film grown on NiAl(110). The interaction with methanol is examined by temperature-programmed desorption of methanol and carbon monoxide and by X-ray photoelectron spectroscopy. The decomposition of methanol proceeds in two reaction pathways independent of the particle composition: complete dehydrogenation towards carbon monoxide and hydrogen, and C--O bond scission yielding carbon deposits. Pd is the most active material studied here. The relative importance of the two channels varies for the different particle systems: on Pd dehydrogenation is preferred, whereas the C--O bond cleavage is more pronounced on Co. The bimetallic clusters show a moderate performance for both pathways. Carbon deposition poisons the model catalysts by blocking the adsorption sites for methoxide, which is the first intermediate product during methanol decomposition. In particular on Co, large amounts of carbon deposits can also be caused by dissociation of the final product of the dehydrogenation pathway, carbon monoxide. A comparison with the results of methanol decomposition on Co, Pd, and CoPd catalysts in continuous-flow reactors demonstrates that the findings of the present UHV study are relevant for catalytic performance under high-pressure conditions.

Ligand Exchange with Thiols: Effects on Composition and Morphology of Colloidal CoPt Nanoparticles

Among the multitude of colloidal nanoparticles, bimetallic systems have been increasingly attracting interest due their tunable physical (e.g. magnetism) and chemical (e.g. catalytic) properties. Oxidation-resistant magnetic bimetallic particles can be self-assembled into ordered layers to become the building blocks for a future generation of data storage devices or enable the retrieval and recycling of catalytically active organic molecules in a chemical reactor. On the other hand, the composition of such nanoparticles can be chosen in a way that the particles themselves show an increased activity and selectivity for catalytic processes due to their bimetallic nature. Potential applications also include the growing field of biotechnology where functionalized magnetic nanoparticles could enable the field-guided localized delivery of pharmaceutical agents or the hyperthermal treatment of tumors or provide means for an efficient and highly sensitive separation of biomolecules. 6] Many characteristics of the particles, such as solubility, biocompatibility, adhesive properties and chemical functionality, which are crucial for most of these applications, are determined by the shell of ligand molecules bound to their surface during preparation. To selectively tune the chemical properties, ligand exchange strategies have been developed in the recent years allowing the addition of new functionalities to the nanoparticles. As compared to monometallic particles, ligand exchange on bimetallic systems is more challenging as complications can arise due to different binding affinities between the ligand molecules and the two metals, possibly not only resulting in changes in particle diameter and size-distribution but also in stoichiometry and shape by selective dissolution of one metal component. Previous studies report only small changes in morphology and composition when for example, exchanging nonpolar ligands to polar ligands. Little attention, however, has been paid to a systematic investigation of all possible effects taking place during the exchange process. Focusing on thiol ligands, which are particularly versatile and widespread in colloidal chemistry, we show for cobalt–platinum nanoparticles that the thiol ligands interact predominantly with one component resulting in composition and morphology changes after long exposure times. At the same time, we can show that the original amine ligands are not completely removed. The alloy particles chosen for the study represent a wellcharacterized system with a narrow selectable size distribution in which the magnetic cobalt component is passivated against oxidation through the addition of platinum, resulting in particles with potential use in magnetic and catalytic applications. In their as-prepared state, these particles are enveloped by a binary ligand system of 1-adamantanecarboxylic acid (1-ACA) and hexadecylamine (HDA). Due to their strong tendency for binding to cobalt, platinum and their alloys, thiols were selected to replace these original ligands, acting as a base for further functionalisation. Of the various thiols of different structure and endgroups, four examples are discussed herein: mercaptosuccinic acid (MSA), 6-mercaptohexan-1-ol (MHL), 11-mercaptoundecan-1-ol (MUL) and dodecanthiol (DDT). A system of ligand-stabilized nanoparticles in colloidal suspension is always in a state of dynamic equilibrium with a portion of ligand molecules constantly and statistically detaching from and reattaching to the surfaces. By adding an excess of suitable target ligand with a strong affinity to the particle materials, a practically complete replacement of the original ligand sphere can be obtained. In the present study, the solubility behavior as well as X-ray photoelectron spectroscopic data clearly revealed that this is also the case for the CoPt particles. As a first sign of the successful exchange procedure with MSA, MHL and MUL, the particles were no longer soluble in non-polar toluene but could be readily redissolved in ethanol. To elucidate the interaction of the thiol ligands with the CoPt nanoparticles in more detail, X-ray photoelectron spectroscopy (XPS) of particle layers spin-coated onto alumina substrates was employed. In all spectra of the S2p binding energy region, a sulfur signal is detected (Figure 1), which is split up into two peaks: a signal around 163 eV from S-atoms in a thiol-like environment and a broad peak between 168 and 169 eV due to oxidized sulfur in sulfonates. The oxidation of attached thiol ligands to sulfonates is not surprising and has been reported previously. Interestingly, the peak is more pronounced for MHL. According to the literature, an increased conversion to sulfonates is to be expected for shorter chain mercapto alcohols due to the fact that their thiolheadgroups are more prone to oxidation than those protected by longer hydrocarbon chains or branched molecules like MSA. As far as the thiol signals at lower binding energy are concerned, they can be fitted with two subcomponents each. While the peak at higher binding energy ( 163.5 eV) is assign[a] B. Gehl, M. Erbacher, B. J rgens, Prof. Dr. M. B umer Institut f r Angewandte und Physikalische Chemie Universit t Bremen, Leobener Str. NW2, 28359 Bremen (Germany) [b] Dr. V. Aleksandrovic, Dr. A. Kornowski, Prof. Dr. H. Weller Institut f r Physikalische Chemie Universit t Hamburg, Grindelallee 117, 20146 Hamburg (Germany) [c] Dr. M. Sch renberg Bruker Daltonik GmbH Fahrenheitstr. 4, 28399 Bremen (Germany)