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Ultramicroscopy ; 182: 233-242, 2017 11.
Artigo em Inglês | MEDLINE | ID: mdl-28734230


A combined X-ray and scanning tunneling microscopy (STM) instrument is presented that enables the local detection of X-ray absorption on surfaces in a gas environment. To suppress the collection of ion currents generated in the gas phase, coaxially shielded STM tips were used. The conductive outer shield of the coaxial tips can be biased to deflect ions away from the tip core. When tunneling, the X-ray-induced current is separated from the regular, 'topographic' tunneling current using a novel high-speed separation scheme. We demonstrate the capabilities of the instrument by measuring the local X-ray-induced current on Au(1 1 1) in 800 mbar Ar.

J Phys Chem C Nanomater Interfaces ; 121(21): 11407-11415, 2017 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-28603579


Using a home-built reflectometer, we have investigated the changes in the optical reflectivity of a Pd(100) model catalyst during CO oxidation under high-pressure, high-temperature conditions. We observe changes in optical contrast when exposing the surface to CO oxidation conditions at 200 mbar from room temperature up to 400 °C. These changes in reflectivity are a result both of the formation of a surface oxide layer and of a change in surface roughness because of gas exposure. However, the reflectivity is more sensitive to the presence of a thin, flat oxide layer than to surface roughness. CO oxidation plays an important role in the decrease of the reflectivity. Since adding a reducing agent to the gas mixture renders it unlikely that the oxide thickness increases, we conclude that the observed decrease in reflectivity is dominated by increased surface roughness because of the catalytic reaction. We contribute this observed surface roughening to a Mars-van Krevelen-type reaction mechanism.

Rev Sci Instrum ; 88(2): 023704, 2017 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-28249468


We have developed an experimental setup for optically monitoring a catalytically active surface under reaction conditions. A flow reactor with optical access allows us to image the behavior of an active catalyst surface down to the millimeter length scale. We use reflectance difference measurements with 625 nm light to investigate CO oxidation on Pd(100) at 300 mbar and 320 °C. We conclude that the changes in visible contrast result from the formation of an oxide layer after surface oxidation.

Rev Sci Instrum ; 87(11): 113705, 2016 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-27910601


We have developed a new instrument combining a scanning probe microscope (SPM) and an X-ray scattering platform for ambient-pressure catalysis studies. The two instruments are integrated with a flow reactor and an ultra-high vacuum system that can be mounted easily on the diffractometer at a synchrotron end station. This makes it possible to perform SPM and X-ray scattering experiments in the same instrument under identical conditions that are relevant for catalysis.

Biochim Biophys Acta ; 1798(3): 637-45, 2010 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-20036635


Photosynthetic membranes comprise a network of light harvesting and reaction center pigment-protein complexes responsible for the primary photoconversion reactions: light absorption, energy transfer and electron cycling. The structural organization of membranes of the purple bacterial species Rb. sphaeroides has been elucidated in most detail by means of polarized light spectroscopy and atomic force microscopy. Here we report a functional characterization of native and untreated membranes of the same species adsorbed onto a gold surface. Employing fluorescence confocal spectroscopy and light-induced electrochemistry we show that adsorbed membranes maintain their energy and electron transferring functionality. Gold-adsorbed membranes are shown to generate a steady high photocurrent of 10 microA/cm(2) for several minutes and to maintain activity for up to three days while continuously illuminated. The surface-adsorbed membranes exhibit a remarkable functionality under aerobic conditions, even when exposed to light intensities well above that of direct solar irradiation. The component at the interface of light harvesting and electron cycling, the LH1 complex, displays exceptional stability, likely contributing to the robustness of the membranes. Peripheral light harvesting LH2 complexes show a light intensity dependent decoupling from photoconversion. LH2 can act as a reversible switch at low-light, an increased emitter at medium light and photobleaches at high light.

Membrana Celular/efeitos da radiação , Transferência de Energia/efeitos da radiação , Ouro/química , Luz , Fotossíntese/fisiologia , Rhodobacter sphaeroides/citologia , Adsorção/efeitos da radiação , Membrana Celular/ultraestrutura , Eletrodos , Transporte de Elétrons/efeitos da radiação , Complexos de Proteínas Captadores de Luz/metabolismo , Microscopia de Força Atômica , Rhodobacter sphaeroides/efeitos da radiação , Soluções , Espectrometria de Fluorescência , Propriedades de Superfície/efeitos da radiação