Biomedical subjects

Bjerne S Clausen

Publications and source records attributed to Bjerne S Clausen.

5 recordsLinked to original sources

PubMed2006

Cluster-support interactions and morphology of MoS2 nanoclusters in a graphite-supported hydrotreating model catalyst.

Supported MoS(2) nanoparticles constitute the active component of the important hydrotreating catalysts used for industrial upgrading and purification of the oil feedstock for the production of fossil fuels with a low environmental load. We have synthesized and studied a model system of the hydrotreating catalyst consisting of MoS(2) nanoclusters supported on a graphite surface in order to resolve a number of very fundamental questions related to the atomic-scale structure and morphology of the active clusters and in particular the effect of a substrate used in some types of hydrotreating catalysts. Scanning tunneling microscopy (STM) is used to image the atomic-scale structure of graphite-supported MoS(2) nanoclusters in real space. It is found that the pristine graphite (0001) surface does not support a high dispersion of MoS(2), but by introducing a small density of defects in the surface, highly dispersed MoS(2) nanoclusters could be synthesized on the graphite. From high-resolution STM images it is found that MoS(2) nanoclusters synthesized at low temperature in a sulfiding atmosphere preferentially grow as single-layer clusters, whereas clusters synthesized at 1200 K grow as multilayer slabs oriented with the MoS(2)(0001) basal plane parallel to the graphite surface. The morphology of both single-layer and multilayer MoS(2) nanoclusters is found to be preferentially hexagonal, and atom-resolved images of the top facet of the clusters provide new atomic-scale information on the MoS(2)-HOPG bonding. The structure of the two types of catalytically interesting edges terminating the hexagonal MoS(2) nanoclusters is also resolved in atomic detail in STM images, and from these images it is possible to reveal the atomic structure of both edges and the location and coverage of sulfur and hydrogen adsorbates.

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PubMed2005

Controlling the catalytic bond-breaking selectivity of Ni surfaces by step blocking.

The reactivity of catalytic surfaces is often dominated by very reactive low-coordinated atoms such as step-edge sites. However, very little knowledge exists concerning the influence of step edges on the selectivity in reactions involving multiple reaction pathways. Such detailed information could be very valuable in rational design of new catalysts with improved selectivity. Here we show, from an interplay between scanning tunnelling microscopy experiments and density functional theory calculations, that the activation of ethylene on Ni(111) follows the trend of higher reactivity for decomposition at step edges as compared with the higher-coordinated terrace sites. The step-edge effect is considerably more pronounced for the C-C bond breaking than for the C-H bond breaking, and thus steps play an important role in the bond-breaking selectivity. Furthermore, we demonstrate how the number of reactive step sites can be controlled by blocking the steps with Ag. This approach to nanoscale design of catalysts is exploited in the synthesis of a new high-surface-area AgNi alloy catalyst, which is tested in hydrogenolysis experiments.

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PubMed2004

Atomic-scale imaging of carbon nanofibre growth.

The synthesis of carbon nanotubes with predefined structure and functionality plays a central role in the field of nanotechnology, whereas the inhibition of carbon growth is needed to prevent a breakdown of industrial catalysts for hydrogen and synthesis gas production. The growth of carbon nanotubes and nanofibres has therefore been widely studied. Recent advances in in situ techniques now open up the possibility of studying gas-solid interactions at the atomic level. Here we present time-resolved, high-resolution in situ transmission electron microscope observations of the formation of carbon nanofibres from methane decomposition over supported nickel nanocrystals. Carbon nanofibres are observed to develop through a reaction-induced reshaping of the nickel nanocrystals. Specifically, the nucleation and growth of graphene layers are found to be assisted by a dynamic formation and restructuring of mono-atomic step edges at the nickel surface. Density-functional theory calculations indicate that the observations are consistent with a growth mechanism involving surface diffusion of carbon and nickel atoms. The finding that metallic step edges act as spatiotemporal dynamic growth sites may be important for understanding other types of catalytic reactions and nanomaterial syntheses.

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PubMed2002

Silicon drift detectors as a tool for time-resolved fluorescence XAFS on low-concentrated samples in catalysis.

A silicon drift detector (SDD) was used for ex situ and time-resolved in situ fluorescence X-ray absorption fine structure (XAFS) on low-concentrated catalyst samples. For a single-element and a seven-element SDD the energy resolution and the peak-to-background ratio were verified at high count rates, sufficient for fluorescence XAFS. An experimental set-up including the seven-element SDD without any cooling and an in situ cell with gas supply and on-line gas analysis was developed. With this set-up the reduction and oxidation of a zeolite supported catalyst containing 0.3 wt% platinum was followed by fluorescence near-edge scans with a time resolution of 10 min each. From ex situ experiments on low-concentrated platinum- and gold-based catalysts fluorescence XAFS scans could be obtained with sufficient statistical quality for a quantitative analysis. Structural information on the gold and platinum particles could be extracted by both the Fourier transforms and the near-edge region of the XAFS spectra. Moreover, it was found that with the seven-element SDD concentrations of the element of interest as low as 100 ppm can be examined by fluorescence XAFS.

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PubMed2002

Atom-resolved imaging of dynamic shape changes in supported copper nanocrystals.

In situ transmission electron microscopy is used to obtain atom-resolved images of copper nanocrystals on different supports. These are catalysts for methanol synthesis and hydrocarbon conversion processes for fuel cells. The nanocrystals undergo dynamic reversible shape changes in response to changes in the gaseous environment. For zinc oxide-supported samples, the changes are caused both by adsorbate-induced changes in surface energies and by changes in the interfacial energy. For copper nanocrystals supported on silica, the support has negligible influence on the structure. Nanoparticle dynamics must be included in the description of catalytic and other properties of nanomaterials. In situ microscopy offers possibilities for obtaining the relevant atomic-scale insight.

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