Volkmar Zielasek

Nanoporous Gold Catalysts for Selective Gas-Phase Oxidative Coupling of Methanol at Low Temperature

Methanol Coupling Catalyzed with Gold Gold surfaces can be effective catalysts for partial oxidation reactions, in part because lower interaction strengths of molecules absorbed on gold allow products to desorb before further unwanted oxidations occur. One challenge in these reactions is the low rate of formation of reactive atomic surface oxygen. Wittstock et al. (p. 319; see the Perspective by Christensen and Nørskov) created high–surface area gold catalysts by leaching silver from gold-silver alloys. This material proved to be an effective catalyst for partial oxidative coupling of methanol, yielding methyl formate. Residual silver appears to play a key role in activating the dissociation of molecular oxygen. Leaching of gold-silver alloys creates a highly active catalyst for partial oxidation reactions. Gold (Au) is an interesting catalytic material because of its ability to catalyze reactions, such as partial oxidations, with high selectivities at low temperatures; but limitations arise from the low O2 dissociation probability on Au. This problem can be overcome by using Au nanoparticles supported on suitable oxides which, however, are prone to sintering. Nanoporous Au, prepared by the dealloying of AuAg alloys, is a new catalyst with a stable structure that is active without any support. It catalyzes the selective oxidative coupling of methanol to methyl formate with selectivities above 97% and high turnover frequencies at temperatures below 80°C. Because the overall catalytic characteristics of nanoporous Au are in agreement with studies on Au single crystals, we deduced that the selective surface chemistry of Au is unaltered but that O2 can be readily activated with this material. Residual silver is shown to regulate the availability of reactive oxygen.

Surface-chemistry-driven actuation in nanoporous gold

Although actuation in biological systems is exclusively powered by chemical energy, this concept has not been realized in man-made actuator technologies, as these rely on generating heat or electricity first. Here, we demonstrate that surface-chemistry-driven actuation can be realized in high-surface-area materials such as nanoporous gold. For example, we achieve reversible strain amplitudes of the order of a few tenths of a per cent by alternating exposure of nanoporous Au to ozone and carbon monoxide. The effect can be explained by adsorbate-induced changes of the surface stress, and can be used to convert chemical energy directly into a mechanical response, thus opening the door to surface-chemistry-driven actuator and sensor technologies.

Colloidal synthesis and structural control of PtSn bimetallic nanoparticles.

PtSn bimetallic nanoparticles with different particle sizes (1-9 nm), metal compositions (Sn content of 10-80 mol %), and organic capping agents (e.g., amine, thiol, carboxylic acid and polymer) were synthesized by colloidal chemistry methods. Transmission electron microscopy (TEM) measurements show that, depending on the particle size, the as-prepared bimetallic nanocrystals have quasi-spherical or faceted shapes. Energy-dispersive X-ray (EDX) analyses indicate that for all samples the signals of both Pt and Sn can be detected from single nanoparticles, confirming that the products are actually bimetallic but not only a physical mixture of pure Pt and Sn metal nanoparticles. X-ray diffraction (XRD) measurements were also conducted on the bimetallic particle systems. When compared with the diffraction patterns of monometallic Pt nanoparticles, the bimetallic samples show distinct shifts of the Bragg reflections to lower degrees, which gives clear proof of the alloying of Pt with Sn. However, a quantitative analysis of the lattice parameter shifts indicates that only part of the Sn atoms are incorporated into the alloy nanocrystals. This is consistent with X-ray photoelectron spectroscopy (XPS) measurements that reveal the segregation of Sn at the surfaces of the nanocrystals. Moreover, short PtSn bimetallic nanowires were synthesized by a seed-mediated growth method with amine-capped bimetallic particles as precursors. The resulting nanowires have an average width of 2.3 nm and lengths ranging from 5 to 20 nm.

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.

Photoemission study of praseodymia in its highest oxidation state: The necessity of in situ plasma treatment

A cold radio frequency oxygen plasma treatment is demonstrated as a successful route to prepare clean, well-ordered, and stoichiometric PrO(2) layers on silicon. High structural quality of these layers is shown by x-ray diffraction. So far unobserved spectral characteristics in Pr 3d x-ray photoelectron (XP) spectra of PrO(2) are presented as a fingerprint for praseodymia in its highest oxidized state. They provide insight in the electronic ground state and the special role of praseodymia among the rare earth oxides. They also reveal that former XP studies suffered from a significant reduction at the surface.

Bimetallic AuAg Nanoparticles: Enhancing the Catalytic Activity of Au for Reduction Reactions in the Liquid Phase by Addition of Ag

Au-based catalysts were introduced to the scientific community by the groundbreaking work of Haruta and Hutchings in the 1980s. Since then, nano-particulate gold catalysts have been intensively investigated, leading to new fields of application and fostering the understanding of the catalytic activity of gold on the nanoscale. Gold can be used as catalyst either for reactions in the gas phase—for example the aerobic oxidation of alcohols —or in the liquid phase for the aldol reaction, addition of alcohols in alkynes, C C bond formation, and isomerization of w-alkynylfurans, just to name a few examples. Although it is mainly used as an oxidation catalyst, several reduction reactions, such as hydrogenation of alkenes, and imines as well as the reduction of nitrocompounds can also be catalyzed. 3d] The reduction of nitrophenols, to name a specific class of compounds, is important for the pharmaceutical industry and finds wide-spread use as corrosion inhibitor and dyeing agent. Nevertheless, neither for this example nor for the other reactions gold nanoparticulate catalysts have been employed in industrial applications so far partly because the knowledge about the factors determining the activity and options to maximize it are still fragmentary. In an effort to broaden the knowledge and explore possibilities to tune the catalytic properties, it was only a matter of time until bimetallic combinations of Au with other metals also moved into the focus of attention. Some of the bimetallics show indeed improved catalytic activity in comparison to pure gold catalyst. For instance, Hutchings and coworkers have successfully prepared bimetallic Au–Pd nanocatalysts and extensively investigated their catalytic properties for different reactions. 7] The addition of a second metal often changes the electronic structure of the catalyst (ligand effect) and may also change possible adsorption geometries for the reactants (ensemble effect) so that adsorption energies and activation barriers can be tailored for a specific reaction. To understand such changes and effects for a specific bimetallic catalyst, the synthesis of a series of compositions and their investigation with a suitable model reaction is an established strategy. In the field of bimetallic gold catalysts the reduction of 4-nitrophenol is one of the most frequently used model reactions: AuPd, AuPt, and AuCu have already been investigated as well as AuAg. For the latter combination, a higher activity was occasionally reported in the literature in comparison to pure gold catalysts and proposed to be a consequence of the different work functions of the two metals. This difference should lead to electron enrichment in the gold near the interface, which facilitates the reduction of the reactant on the gold surface. Yet, in contrast to these studies, other groups did not observe such an effect when using AuAg catalysts for 4-nitrophenol reduction 12] . When trying to unravel this contradictory picture, it turns out that clear-cut conclusions are difficult, since the compared systems exhibited partly different morphologies. In one study, for example, hollow AuAg nanoparticles (NPs) were compared to spherical monometallic Au and Ag NPs. In another study, AuAg dendrite structures prepared by a galvanic replacement reaction were just compared to monometallic Ag dendrites (the starting-material). In addition, no structural information about the catalysts after the reaction was provided, rendering it impossible to judge whether structural changes occurred during the reaction. To clarify the situation, it is of course important to compare catalysts with identical particle sizes to ensure that observed differences are not related to size effects. To this end, we prepared Au, Ag, and AuAg nanoparticles by using block copolymer micelles as nanoreactors, following our former work which was inspired by the pioneering work of Antonietti and Mçller. As we demonstrated earlier, the AuAg particles obtained by the applied synthesis are bimetallic alloy NPs with narrow size distribution and fine control over their size. We now show that these nanoparticles prepared in toluene and protected by their nanoreactors versus Ostwald ripening can be released simply by changing the solvent from toluene to ethanol, resulting in catalytically active particles of the same size and shape. Using these particles as catalysts for 4-nitrophenol reduction in ethanol, our approach enabled us to systematically study the effect of Ag on the catalytic properties and to exclude particle size effects as a possible reason for differences in the reduction of 4-nitrophenol catalyzed by Au, Ag and AuAg nanoparticles. [a] Dr. W. G. Menezes, B. Neumann, Dr. V. Zielasek, Prof. Dr. M. B umer University of Bremen Institute of Applied and Physical Chemistry Leobener Str. NW2, 28359 Bremen (Germany) E-mail : mbaeumer@uni-bremen.de [b] Dr. W. G. Menezes CNPq, Conselho Nacional de Desenvolvimento Cient fico e Tecnol gico, 71605-001, Bras lia (Brazil) [c] Dr. K. Thiel Fraunhofer Institute for Manufacturing Technology and Advanced Materials, Wiener Straße 12 28359 Bremen (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201201100.

Impact of Organic Ligands on the Structure and Hydrogenation Performance of Colloidally Prepared Bimetallic PtSn Nanoparticles

The catalytic performance of ligand‐capped monometallic Pt and bimetallic PtSn nanoparticles supported on SiO2 toward the selective hydrogenation of acetylene to ethylene in ethylene‐rich streams was investigated. To evaluate the effect of organic ligands on the hydrogenation performance, dodecylamine (DDA)‐capped monometallic and bimetallic nanoparticles were compared with ligand‐free nanoparticles. Both ligands and Sn promote the selectivity of the catalysts. The most selective catalyst was found to be the DDA‐capped PtSn catalyst, with a nominal metal atom ratio of 1:1. An increase in Pt content resulted in a decrease in the selectivity, whereas an increase in Sn content caused a decrease in the activity.

A versatile sol–gel coating for mixed oxides on nanoporous gold and their application in the water gas shift reaction

Based on a sol–gel coating method, a series of nanoporous gold (npAu) catalysts functionalized with titania–ceria mixed oxides were prepared. Metal-oxides with different composition were formed inside the mesoporous material (ligaments and pores ∼45 nm) after thermal treatment at over 200 °C for 2 h. The water-gas shift (WGS) reaction (H2O + CO → H2 + CO2) was studied in a continuous flow reactor at ambient pressure using these Ce–TiOx/npAu catalytic materials. Formation of CO2 was observed at temperatures between 200 °C and 450 °C. The addition of CeO2 to TiO2 resulted in an strongly increased activity; the sample (with the molar ratio of Ce : Ti = 1 : 2 abbreviated as Ce1Ti2Ox/npAu) shows the highest activity which was nearly twice as high as the activity of all other samples at 300 °C. The loss of activity after 2 catalytic runs was only about 10% at 450 °C for the Ce1Ti2Ox/npAu sample and no coarsening was observed. Raman spectroscopic characterization of the materials indicates a dynamic correlation between the crystallization (oxygen storage) of the metal-oxides under oxidizing and reducing conditions.

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.

Ethylene diamine-assisted synthesis of iron oxide nanoparticles in high-boiling polyolys.

The decomposition of iron(III) acetylacetonate in high-boiling polyols such as diethylene glycole is an efficient way to produce water-soluble iron oxide nanoparticles (IONPs) with small sizes. We present an extension of this method by introducing ethylene diamine (EDA) or diethylene triamine (DTA) as a structure-directing agent and adding polyvinylpyrrolidone (PVP) as a stabilizing agent. The synthesis was studied with respect to effects of the chain length of the polyol used as solvent, the chain length of the structure-directing agent, the presence of PVP, the heating rate, and the nature of the precursor. By varying these parameters, we were able to show, that probably an interplay of the structure-directing agent and the polyol plays an important role for the stabilization and growth of the different facets of the IONP crystal. The chain length of the polyol used as solvent alters the influence of EDA or DTA as stabilizer of {111} facets, leading to IONPs with spherical, tetrahedral, or nanoplate morphology and mean diameters ranging from 4 nm up to 25 nm. PVP in the reaction medium narrows down particle size and shape distributions and promotes the formation of very stable, water-based colloidal solutions. The saturation magnetization of the particles was determined by a superconducting quantum interference device (SQUID) and their ability to act as a T2-contrast agent was tested by magnetic resonance imaging (MRI).