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Sergey Vasenkov

Publications and source records attributed to Sergey Vasenkov.

9 recordsLinked to original sources
PubMed2006

Internal concentration gradients of guest molecules in nanoporous host materials: measurement and microscopic analysis.

Evolution of internal concentration profiles of methanol in 2-D pore structure of ferrierite crystal was measured in the pressure range of 0 to 80 mbar with the help of the recently developed interference microscopy technique. The measured profiles showed that both a surface barrier and internal diffusion controlled the kinetics of adsorption/desorption. Furthermore, they indicated that in the main part of the crystal, the z-directional 10-ring channels were not accessible to methanol and that the transport of methanol mainly occurred via 8-ring y-directional channels. The roof-like part of the crystal was almost instantaneously filled/emptied during adsorption/desorption, indicating accessible 10-ring channels in this section. The measured profiles were analyzed microscopically with the direct application of Fick's second law, and the transport diffusivity of methanol in ferrierite was determined as a function of adsorbed phase concentration. The transport diffusivity varied by more than 2 orders of magnitude over the investigated pressure range. Transport diffusivities, calculated from measured profiles from small and large pressure step changes, were all found to be consistent. Simulated concentration profiles obtained from the solution of Fick's second law with the calculated functional dependence of diffusivities on concentration compared very well with the measured concentration profiles, indicating validity and consistency of the measured data and the calculated diffusivities. The results indicate the importance of measuring the evolution of concentration profiles as this information is vital in determining (1) the direction of internal transport, (2) the presence of internal structural defects, and (3) surface/internal transport barriers. Such detailed information is available neither from common macroscopic methods since, they measure changes in macroscopic properties and use model assumptions to predict the concentration profiles inside, nor from microscopic methods, since they only provide information on average displacement of diffusing molecules.

Journal Article↗
PubMed2006

Single-file diffusion near channel boundaries.

Molecular transport under the conditions of single-file diffusion was investigated near the channel boundaries by using dynamic Monte Carlo and molecular dynamics simulations of tracer exchange between single-file channels and their surroundings. The boundary effect reported in our recent papers (Vasenkov S.; Kärger, J. Phys. Rev. E 2002, 66, 052601. Schüring, A.; Vasenkov S.; Fritzsche, S. J. Phys. Chem. B 2005, 109, 16711) was studied in detail. This boundary effect is characterized by deviations of the intrachannel concentration profiles of tracer molecules observed in the case of single-file diffusion near the channel boundaries from the corresponding profiles typical for normal diffusion. It has been shown in our previous studies that these deviations occur under the conditions when the potential-energy difference inside and outside of single-file channels was both comparable and much larger than the activation energy of intrachannel diffusion. Here, we report a quantitative model describing the boundary effect. According to this model, an occurrence of the boundary effect is related to a complex character of diffusion in finite single-file systems. Such diffusion can be described by the following two types of movements occurring in parallel: (i) correlated displacements of all molecules in any particular channel and (ii) fast displacements of single molecules, which are uncorrelated with the displacements of all other molecules in the same channel. The latter displacements are restricted to a certain length interval that depends on the channel length and the channel occupancy. This length interval is shown to determine the extensions of the channel margins where the boundary effect is observed.

Diffusion↗
PubMed2005

Influence of boundaries of nanoporous crystals on molecular exchange under the conditions of single-file diffusion.

We study the tracer exchange of molecules between the phase adsorbed in one-dimensional channels and the surrounding gas phase by molecular dynamics simulations. Under the conditions of single-file diffusion, a novel boundary effect is observed. The shape of the tracer-exchange concentration profiles deviates from those obtained under the conditions of normal diffusion. Compared to the profiles for normal diffusion, which correspond to the same degree of exchange, the equilibrium concentration is reached faster at the boundaries and slower in the middle part of the channel in the case of single-file diffusion. This boundary effect is observed for the system neopentane in AlPO4-5 (which was chosen as a reference system), as well as for modified systems. The effect can be understood considering two diffusion mechanisms which occur in parallel. First, the diffusion of the whole chain of particles, that is, the center-of-mass diffusion, obeying the laws of normal diffusion. Second, the individual movement of the particles relative to the center of mass of the chain. The second mechanism admits additional displacements which, on average, lead to an accelerated exchange of the marginal particles.

Journal Article↗
PubMed2005

Sticking probability on zeolites.

The sticking coefficient, i.e., the probability that, on hitting the surface of a nanoporous particle (zeolite), a molecule shall be able to enter the intracrystalline space, is a key quantity for the application of such materials in heterogeneous catalysis and molecular sieving. On the basis of pulsed field gradient NMR diffusion measurements and molecular dynamics simulations, typical values of this probability are found to be close to one. They exceed previous estimates on the basis of IR uptake measurements by many orders of magnitude.

Journal Article↗
PubMed2005

Long-range diffusion in beds of nanoporous particles: pitfalls and potentials.

Owing to the recent progress in the area of hardware and software of the pulsed field gradient NMR technique, molecular transport in real-life zeolite systems, such as zeolite beds and particles of formulated fluid catalytic cracking (FCC) catalysts, can be investigated in detail. These studies have revealed a number of important features of molecular transport in zeolites, which are reviewed in the present paper. In particular, the anomalous character of intracrystalline diffusion in MFI-type zeolites, dependence of the tortuosity factor in zeolite beds on diffusion regime and the role of various modes of diffusion in transport limitations arising for catalytic reactions in FCC catalysts will be discussed.

Diffusion↗
PubMed2002

Different time regimes of tracer exchange in single-file systems.

A complete description of the time dependence of tracer exchange in single-file systems is given. It is based on the combined application of dynamic Monte Carlo simulations and analytical calculations. Depending on the time elapsed, the tracer exchange is found to have different time scaling related to transport modes of either normal diffusion or single-file diffusion. The mode of single-file diffusion is reproduced by recent tracer exchange experiments conducted by laser-polarized 129Xe NMR spectroscopy.

Journal Article↗
PubMed2002

Regular intergrowth in the AFI-type crystals: influence on the intracrystalline adsorbate distribution as observed by interference and FTIR-microscopy.

Interference microscopy and FTIR microscopy are applied to study intracrystalline concentration profiles of methanol in CrAPO-5 zeolite crystals. By using both techniques, the high spatial resolution of interference microscopy is complemented by the ability of FTIR spectroscopy to pinpoint adsorbates by their characteristic IR bands. For the first time two-dimensional concentration profiles of an unprecedented quality are reported which show a nonhomogeneous distribution of adsorbate in zeolite crystal under equilibrium with the adsorbate vapor. These nonhomogeneous profiles are attributed to regular intergrowth effects in CrAPO-5. A possible internal structure of CrAPO-5 crystals is suggested.

Journal Article↗
PubMedDate not supplied

Structure-mobility relations of molecular diffusion in nanoporous materials.

Depending on the measuring conditions, pulsed field gradient (PFG) NMR measurements of molecular diffusion in beds of nanoporous particles may provide information about the propagation rate of guest molecules in both the intra- and interparticle spaces, as well as through the interface between them. Recent progress in both PFG NMR instrumentation and computational techniques have initiated studies of novel aspects in each of these areas, which are reviewed in this communication. They concern the possibility of multicomponent diffusion measurements with ultra-high pulsed field gradients, the peculiarities of molecular diffusion in channel networks, the determination of the surface-to-volume ratio of nanoporous particles and the dependence of the tortuosity factor of long-range diffusion on the diffusion mode in the intercrystalline space.

Adsorption↗