Search Results

You are looking at 11 - 20 of 59 items for :

  • "heterogeneous catalysis" x
  • Refine by Access: All Content x
Clear All

Abstract  

Polyacrylic hydrogels were14C- and3H-labeled in three main steps. In the first one, the monomer as sodium salt was14C-labeled at the carboxyl group via the carbonation of vinylmagnesium bromide and [1-14C]acrylic acid was obtained, followed by labeling with tritium via isotopic exchange in heterogeneous catalysis. The monomer was purified and its radiochemical purity was analyzed using TLC. In the last step, the hydrogel was obtained through radiopolymerization of the doubly labeled monomer using a60Co source. The oligomers and unpolymerized monomers were eliminated from the hydrogel by swelling in water followed by dehydration with methanol.

Restricted access

Abstract  

Fundamentals and fields of application of time- and temperature-resolved diffraction methods are presented. X-ray diffraction and neutron diffraction will be considered. Dynamic diffraction methods are increasingly applied in different fields like solid state reactions, heterogeneous catalysis and biological sciences. New methods like synchrotron radiation and position-sensitive detectors permit a considerable expansion of potential research areas. The dynamic diffraction methods are compared with the classical thermoanalytical methods thermogravimetry and DSC.

Restricted access
Journal of Flow Chemistry
Authors:
Viktor Misuk
,
Andreas Mai
,
Konstantinos Giannopoulos
,
Dominik Karl
,
Julian Heinrich
,
Daniel Rauber
, and
Holger Löwe

The method of combining the concept of fluorous biphasic catalysis (FCB) with micro multiple emulsions benefits from the advantages of homogeneous as well as from heterogeneous catalysis in continuous micro flow. In this particular case, three immiscible fluid phases in continuous micro segmented flow were used to perform palladium-catalyzed Heck cross-coupling reactions of styrene with aryl halides. A capillary tube-in-tube coaxial flow setup in combination with a glass micro reactor was used to produce monodisperse aqueous phase/organic phase/perfluorinated phase double emulsions. The resulting emulsions had a core–shell droplet structure composed of a perfluorcarbon fluid in which a palladium catalyst with fluorinated phosphine ligands was dissolved, an organic phase consisting of a solvent and two reagents, and an alkaline aqueous solution. The fluorous and organic phases of the double emulsion form a thermomorphous system which can be converted into one phase by an increase of temperature above 150 °C, and the catalytic reaction is performed temporarily. By decreasing the temperature, a phase separation takes place; after that, the organic phase contains the product and the catalyst is located in the fluorous phase. The separated catalyst solution was reused several times without a noticeable loss of activity. The main advantage of this method is to use temporarily very high catalyst concentrations in each droplet, while employing only small amounts of the catalyst for the overall reaction volume.

Restricted access

Abstract

The effect of carbon nanotubes’ (CNT) crystal structure on chemical reactivity has been studied in much detail in the liquid phase using CNT suspension. This type of information is pertinent for developing CNT separation strategies. However, few experimental studies are available providing data for gas–CNT interactions utilizing ultra-high vacuum (UHV) surface science techniques. Structure–activity relationships (SAR) for gas–surface interactions are important for sensor designs and heterogeneous catalysis exploring, for example, CNT’s potential as a support for fuel cell catalysts. We report on UHV kinetics experiments with single-wall metallic, semiconducting, and mixed CNTs in order to provide the experimental basis to correlate CNT’s crystal structure and chemical activity. Thermal desorption spectroscopy (TDS), a simple temperature ramping technique, has been used to determine the binding energies of a number of probe molecules including alkanes, alcohols, thiophene, benzene, and water on CNTs at UHV conditions. TDS allows for the identification of adsorption sites of probe molecules in CNT bundles, using gold foil or silica as a support for the drop-and-dry technique. A weak and probe molecule dependent SAR is present for adsorption inside the CNTs but not for the population of external sites by the probe molecules. The experimental data are in part consistent with current theoretical predictions by other groups. In addition, the effect of different solvents (methanol, SDS, and NMP) and cleaning procedures will briefly be discussed using results of spectroscopic (Auger electron spectroscopy) and kinetic techniques. Furthermore, molecular beam scattering techniques were utilized to characterize the adsorption dynamics, i.e., the gas-to-surface energy transfer processes of alkanes on CNTs. For example, opening the CNT tube ends by high temperature annealing, increases the so-called initial adsorption probability, that is, the probability for adsorption in the limit of zero surface concentration (coverage). This result directly illustrates the effect of large surface areas of CNTs, using internal and external surfaces, for gas adsorption.

Restricted access

: applications in heterogeneous catalysis ” J. Cabrera , R. Padilla , M. Bru , R. Lindner , T. Kageyama , K. Wilckens , S. L. Balof , H.-J. Schanz , R. Dehn , J. H. Teles , S. Deuerlein , K. Müller , F. Rominger , M. Limbach* Chemistry — A European

Restricted access

. 1979 Spectroscopy in Heterogeneous Catalysis Academic Press New York 1 . 5 Bergaya , F Lagaly , G 2001 Appl. Clay Sci

Restricted access

Introduction The chemistry of niobium is not widely known. Over the years, many studies about its materials have been increasing the interest in heterogeneous catalysis, where they are pure used, like promoters and supports

Restricted access

.1016/S0920-5861(99)00019-X . 7. Ertl , G , Knözinger , H 1997 Handbook of heterogeneous catalysis Wiley Weinheim 10.1002/9783527619474 . 8

Restricted access
Reaction Kinetics, Mechanisms and Catalysis
Authors:
Viorel Chihaia
,
Karl Sohlberg
,
Karl Sohlberg
, and
Niculae I. Ionescu

interested in solid–gas interactions and heterogeneous catalysis, and her multiple activities in scientific research and teaching with her PhD students have been widely recognized and appreciated. Dr. Caldararu was born in Bucharest on May 12, 1946. She

Restricted access

. 2. (a) Farnetti , E. ; Di Monte , R. ; Kaspar , J. Homogeneous and Heterogeneous Catalysis in Inorganic and Bio-inorganic Chemistry ; Bertin , I. Eds

Restricted access