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.     

Synthesis and evaluation of novel pyridine based PLG tripeptidomimetics

Analogues of the pyridine based PLG (Pro-Leu-Gly-NH(2)) peptidomimetic were synthesized and evaluated as dopamine modulating agents. Modifications in the position corresponding to the leucine side chain in PLG afforded derivatives , and , substituted with H, Me and Bn instead of the isobutyl group, respectively. Changes in the proline residue produced derivative , substituted with a symmetrical piperidine ring instead of the pyrrolidine ring and , in which the pyrrolidine ring is connected to the pyridine ring via a hydroxymethyl group instead of a keto function. The peptidomimetics were tested for their ability to enhance the maximal effect of N-propylapomorphine (NPA) at dopamine D2 receptors in the functional cell-based R-SAT assay. Compounds , , and , produced a statistically significant increase in the maximal NPA response at 10 nM (117 +/- 6%, 118 +/- 6%, and 116 +/- 3%, respectively), which is similar to the effect of PLG in this assay, whereas was able to potentiate the response to a similar extent at 1 nM concentration (115 +/- 5%). All derivatives produced a bell-shaped dose-response curve and none of the compounds were active at the D2 receptor alone, which indicates that the mechanism behind the activity of both the pyridine based mimetics and PLG is the same. Interestingly, l-Pro-d-Leu-Gly-NH(2) was found to be more potent than PLG and produced a 119 +/- 1% increase in the NPA response at 1 nM.     

Critical exponents for line tension and dipole density difference from lipid monolayer domain boundary fluctuations

The potential physiological relevance of liquid-liquid phase separation in lipid membranes to the formation and stability of "lipid rafts" in cellular plasma membranes has prompted extensive investigation of the physical chemistry underlying these phenomena. In this contribution, the line tension (gamma) and dipole density differences ( micro) between demixed fluid phases of monolayers comprised of dimyristoylphosphatidylcholine (DMPC) and dihydrocholesterol (DChol) were investigated by measuring the two-dimensional thermal fluctuations of domain boundaries visualized by the inclusion of a fluorescent tracer lipid. These parameters are essential determinants of domain stability, and their quantification will yield an increased understanding of the physical processes responsible for aspects of lateral phase separation. Employing an extensive data set, the surface pressure dependence of gamma and mu was determined at three different monolayer compositions (30%, 35%, and 40% DChol). Both parameters were found to decrease with a power law dependence as the surface pressure approached the phase transition pressure (pi t), in agreement with previous measurements. Additionally, photobleaching effects and domain size influence were quantified and found to be small in our system. We suggest that the method of flicker spectroscopy can be helpful in identifying line-active compounds.     

Length-dependent optical effects in single walled carbon nanotubes

Recently, Heller et al. reported length-dependent effects on the relative photoluminescence (PL) quantum yield of single walled carbon nanotubes (SWNTs) [Heller et al J. Am. Chem. Soc. 2004, 126, 14567-14573]. We propose a simple model involving thermal diffusion of excitons along the nanotube axis and quenching at the ends, to explain the observed trend in their data. By fitting to our model, we extract a diffusion coefficient of 6 cm(2)/s for excitons in SWNTs. Assuming a mono exponential decay of exciton PL, we also predict that effective length-dependent PL lifetimes for these excitons lie in the range of 1-27 ps. Experimental observations are shown to be consistent with stochastic rather than wavepacket-like exciton migration, which is in agreement with ultrafast excitonic dephasing. Edge effects seem to limit the use of short SWNTs in imaging and optical sensing applications.     

High-throughput synthesis and characterization of bulk ceramics from dry powders

A high-throughput experimental (HTE) setup using conventional powders as raw materials was developed to synthesize and characterize ceramic bulk samples avoiding the customary liquid- or vapor-phase synthesis routes. Its functionality was verified using the well-known binary material system ZrO2-Y2O3. Libraries of 0.5 mol % yttria resolution were prepared using varying boundary systems as well as a number of liquid mixing aids. Automated powder X-ray diffraction (XRD) was applied to check the monoclinic phase content retained after heat-treatment on various positions for each sample. This phase information was used for comparison with a reference library comprising conventionally prepared samples by means of a regression analysis and mean deviation of monoclinic phase percentage. Out of the eight HTE libraries, three showed a significant comparability to the reference library.     

Encapsulating of single quantum dots into polymer particles

Single semiconductor quantum dots were embedded into polymer particles with diameters below 0.1 μm by an emulsion polymerization procedure or via a secondary dispersion approach. The photoluminescence properties of the nanocrystals are retained upon encapsulation, as demonstrated by fluorescence confocal microscopy.

Crystalline inclusion compounds of lower rim propyl substituted calix[4]arenes featuring different number and positions of the modifying groups

Three lower rim n-propyl substituted calix[4]arenes (1–3) with varied number and position of the modifying groups have been prepared. Inclusion compounds (five species) involving different kinds of guest solvents have been isolated. Their X-ray crystal structures were determined and comparatively discussed using isostructurality calculations. Two of the inclusion compounds obtained (1a and 1b) are polymorphs containing the same host and guest molecules in equal stoichiometric ratio but different Z′ values caused by a phase transition around 140 K. The inclusion compounds 2a and 2b refer to the interesting case of a mixed solvent complex while 3a allows studying the effect of full lower rim n-propyl substitution.     

Dipodal ferrocene-based adsorbate molecules for self-assembled monolayers on gold

1,1'-Difunctionalised ferrocene derivatives have been studied, which contain groups suitable for chemisorption on gold substrates, namely -NC, -PR(2) as well as a range of sulfur-containing units like -NCS, -SR, and thienyl. Thin films on gold have been fabricated from solution with most of these adsorbate species. Film thickness, composition and structure were investigated primarily by X-ray photoelectron and near-edge X-ray absorption fine-structure spectroscopy. The quality of self-assembled monolayers fabricated from 1,1'-diisocyanoferrocene (1) and 1,1'-diisothiocyanatoferrocene (2) turned out to be superior to that of films based on the other adsorbate species investigated. In addition to the surface coordination behaviour of 1 towards gold substrates, relevant aspects of the molecular coordination chemistry of 1 have also been addressed, including the synthesis and characterisation of [(mu-1){Cr(CO)(5)}(2)], [Ag(2)(mu-1)(2)](NO(3))(2) x H(2)O and [(mu-1)(AuCl)(2)]. The crystal structure of the gold complex is governed by aurophilic interactions and can be taken as a model for the arrangement of 1 in self-assembled monolayers on gold.