Jens Kullmann

Microimaging of Transient Concentration Profiles of Reactant and Product Molecules during Catalytic Conversion in Nanoporous Materials

Microimaging by IR microscopy is applied to the recording of the evolution of the concentration profiles of reactant and product molecules during catalytic reaction, notably during the hydrogenation of benzene to cyclohexane by nickel dispersed within a nanoporous glass. Being defined as the ratio between the reaction rate in the presence of and without diffusion limitation, the effectiveness factors of catalytic reactions were previously determined by deliberately varying the extent of transport limitation by changing a suitably chosen system parameter, such as the particle size and by comparison of the respective reaction rates. With the novel options of microimaging, effectiveness factors become accessible in a single measurement by simply monitoring the distribution of the reactant molecules over the catalyst particles.

Nanoporous Glass as a Model System for a Consistency Check of the Different Techniques of Diffusion Measurement

The remarkable differences in the guest diffusivities in nanoporous materials commonly found with the application of different measuring techniques are usually ascribed to the existence of a hierarchy of transport resistances in addition to the diffusional resistance of the pore system and their differing influence due to the differing diffusion path lengths covered by the different measuring techniques. We report diffusion measurements with nanoporous glasses where the existence of such resistances could be avoided. Molecular propagation over diffusion path lengths from hundreds of nanometers up to millimeters was thus found to be controlled by a uniform mechanism, appearing in coinciding results of microscopic and macroscopic diffusion measurement.

Porous glass membranes with an aligned pore system via stretch forming in combination with thermally induced phase separation

In this study, we report about a stretching process coupled with the phase separation of alkali-borosilicate glasses to produce porous glasses with different degrees of pore alignment. For this purpose two different initial glasses with the composition (wt %): 62.5SiO2 · 30.5B2O3 · 7Na2O and respectively 70SiO2 · 23B2O3 · 7Na2O were molten at 1500°C for 3 h and solidified in a casting mold. The resulting glass blocks were cut into thin plates with appropriate dimensions. These slides were clamped into a glass-stretching apparatus in which deformation and phase separation could occur simultaneously. The glass plates were stretched until breaking, thus having different elongation periods, i. e. phase separation periods. Different temperatures and tensile loads have been applied, thus having two independent parameters to influence the time until fracture. The elongated parts of the samples were cut off and underwent an acid and an alkaline treatment to remove the soluble components. The pore size distribution, the specific surface area and the microstructure of selected samples have been determined by mercury intrusion, nitrogen sorption and environmental scanning electron microscopy, respectively. Mercury intrusion porosimetry showed porosities up to 60% with pore sizes in the 100 nm range. The results were compared to those obtained for undeformed samples treated at comparable temperatures and times.