Исследование модифицированной модели сальтаторного проведения возбуждения

In the article it is considered the Nerve Impulse Saltatore Conduction model in which the Impulse Neuron model modification is used for describing Ranvier Interruptions dynamics. The model contains ordinary differential equations and delay differential equations. The system is investigated by the asymptotic step-by-step integration method. All results were obtained analytically.В работе рассматривается модель сальтаторного проведения возбуждения, в которой для описания динамики перехватов Ранвье используется модификация импульсного нейрона. Модель содержит обыкновенные дифференциальные уравнения и дифференциальные уравнения с запаздыванием. Система уравнений исследуется методом пошагового асимптотического интегрирования. Все результаты получены аналитически

Analytic approximations for the broadening of the spectral lines of hydrogen-like ions

Broadband approximate expressions for calculating the broadening of the spectral lines of hydrogenlike ions in a multicomponent plasma are derived taking into account both the influence of the interaction between plasma particles on the distribution function of the plasma microfield and the effect of the microfield dynamics on the broadening of the central component of the spectral line. With the approximate expressions proposed, the calculation of the shape of a given spectral line of a certain ion in a plasma with a given ion composition requires only a few seconds of computer time. The approximate expressions provide a good computational accuracy not only for the central component of the spectral line but also for the spectral line wings

The Wings of an Experiment

The Maiak Clothing Association in Gorky and the Bol'shevichka Clothing Association in Moscow have been functioning for nearly a year on the basis of direct contractual ties with trade organizations. A good deal, almost entirely positive, has been written and said about this experiment. Today everyone admits that direct contractual ties between the supplying enterprise and the store definitely constitute something that offers good prospects and is economically beneficial.

An Investigation of the Modified Nerve Impulse Saltatore Conduction Model

In the article it is considered the Nerve Impulse Saltatore Conduction model in which the Impulse Neuron model modification is used for describing Ranvier Interruptions dynamics. The model contains ordinary differential equations and delay differential equations. The system is investigated by the asymptotic step-by-step integration method. All results were obtained analytically

Model of Nerve Impulse Saltatore Conduction through the Forked Nerve Fibre

We propose a model which demonstrates the process of nerve impulse conduction through the forked myeline axon

Модель сальтаторного проведения возбуждения по разветвляющемуся нервному волокну

We propose a model which demonstrates the process of nerve impulse conduction through the forked myeline axon.Предложена модель, описывающая процесс распространения волны возбуждения по разветвляющемуся миелинизированному аксону

Distortion of the tetrahedral coordination of Fe(III) ions stabilized in ZSM-5 zeolite framework

A simple qualitative method to analyze d-d-electronic transitions in cations of the transition elements in oxide matrices is proposed. In the particular case, all the excited states of interest differ only in the electronic configuration of d-orbitals, and the energies of transitions can be computed via the configuration interaction (CI) method restricted by the active space of five cation d-orbitals. An ordinary cluster model that takes into account the first coordination sphere of transition metal ion consisting of the framework of oxygen ions is sufficient for this purpose. The systematic overestimation error of transition energies can be corrected through the empirical factor calculated to fit experimental UV-VIS spectra. The physical meaning of the scaling factor proposed is the dynamic part of electron correlation that remains unaccounted for in the chosen active CI space. The observed d-d-transitions of Fe3+ ions in MFI zeolites are analyzed in detail. It is suggested that the specifics of the observed electronic spectra are caused by the distortion of the tetrahedron of oxygen atoms around Fe3+. The latter can be easily taken into account when selecting an appropriate Fe3+ cluster model in the framework. It is shown that the occurrence of the weak low-frequency band below 21,000 cm-1 indicates the distortion of the tetrahedral environment around Fe3+

Distortion of the tetrahedral coordination of Fe(III) ions stabilized in ZSM-5 zeolite framework

A simple qualitative method to analyze d-d-electronic transitions in cations of the transition elements in oxide matrices is proposed. In the particular case, all the excited states of interest differ only in the electronic configuration of d-orbitals, and the energies of transitions can be computed via the configuration interaction (CI) method restricted by the active space of five cation d-orbitals. An ordinary cluster model that takes into account the first coordination sphere of transition metal ion consisting of the framework of oxygen ions is sufficient for this purpose. The systematic overestimation error of transition energies can be corrected through the empirical factor calculated to fit experimental UV-VIS spectra. The physical meaning of the scaling factor proposed is the dynamic part of electron correlation that remains unaccounted for in the chosen active CI space. The observed d-d-transitions of Fe3+ ions in MFI zeolites are analyzed in detail. It is suggested that the specifics of the observed electronic spectra are caused by the distortion of the tetrahedron of oxygen atoms around Fe3+. The latter can be easily taken into account when selecting an appropriate Fe3+ cluster model in the framework. It is shown that the occurrence of the weak low-frequency band below 21,000 cm-1 indicates the distortion of the tetrahedral environment around Fe3+

The Genesis of a single Phase (MoVW)₅O₁₄-Type Solid Precursor Formation in aqueous solution

Motivation Molybdenum, vanadium and tungsten containing suboxides are widely used as partial oxidation catalysts [1-3] e.g. the partial oxidation of acrolein to acrylic acid is performed on such a system [4, 5]. Recently (MoVW)5O14 was synthesised as a single phase. Compounds with identical XRD patterns however, show different catalytic activity. This is due to different “real structures” of the active catalyst. The aim of this work is to suggest a reaction mechanism in solution leading to a precursor. Due to his molecular structure this precursor is vital to give the Mo5O14 type structure single phase after calcination [6]. Further, the generation of defects in the solid, which might be important for the catalytic process could be monitored and finally controlled. Experimental A detailed calcination procedure is described in the literature [6]. Respective amounts of AHM, AMT and vanadyl oxalate are dissolved. mixed and spray-dried. This is followed by the calcination procedure. The structural changes in solution are followed by UV/Vis, Raman and ESR spectroscopy. Results A significant increase in the absorption is monitored in the region between 30000 cm-1 and 50000 cm-1 when the pH is lowered from pH = 5.5 to pH = 3. This change in the metal ligand charge transfer region is caused by a protonation of oxygen atoms. The addition of AMT enhances this effect. Additionally, a band in the region between 18000 cm-1 and 19000 cm-1 emerges when vanadyl oxalate is added stepwise. The blue shift of this band at higher vanadyl concentrations can be explained by an increasing degree of polymerisation. The band position speaks for a intervalence charge transfer transition [7]. This result is corroborated by EPR experiments. The higher the vanadyl concentration the less isolated vanadyl species are present. Further, EPR shows that oligomeric or polymeric units are present. Discussion Reaction pathways of transition metal compounds such as molybdenum, vanadium or tungsten have recently been reviewed by Cruywagen [8]. Depending on the pH Mo7O246-, [HMo7O24]5-, and Mo8O264- species prevail. In strongly acidic media [Mo36O112(H2O)16] is found. The replacement of molybdenum by tungsten in hepta molybdenum species has been reported as well [9]. These species can be interlinked by vanadyl bridges and form a polymeric network. Comparison with the literature shows that in the respective pH range [HMo7O24]5- and Mo8O264- species are very likely. A good reason for the [Mo36O112(H2O)16] species would be the pentagonal bipyramide as a structural element which also appears in Mo5O14

Methods for preparation and characterisation of heterogeneous catalysts

Whilst much research effort was spent on optimisation of catalyst performance, less attention has been drawn to problems concerning catalyst preparation. It is commonly known that catalyst research is facing two problems: the Materials gap and the Pressure gap. The “Materials gap” describes the discrepancy between commercial catalyst material that is often too complex to be successfully characterised and (single crystal) model catalysts that are often not able to achieve good product rates. The “Pressure gap” addresses the problem that surface investigations are commonly performed under UHV conditions whereas commercial processes are carried out at ambient or high pressure. As a consequence information of reaction mechanisms or the “real structure” under reaction conditions is very limited. Although catalysis experiments on single crystals led to new information about catalyst behaviour, it is now commonly believed that the main catalytic processes happen on centres with a high- but often unidentified- number of defects. A major task for catalyst preparation is therefore to produce highly defective metastable material. New syntheses have to be developed that fulfil many more requirements such as to ensure high reproducibility and products easy to characterise. Whilst the former can be achieved by monitoring each reaction step in-situ, the latter is taken care of by preparing thin films on a substrate. These new preparation methods will be demonstrated on the example of MoVW supported catalysts, which are used in industry for the synthesis of acrylic acid [1-5]. Despite this industrial importance, there is still a lack of information concerning structure formation during synthesis and the atomic arrangements with respect to different preparation routes and element ratios. Earlier work [6-9] showed a significant increase in selectivity for partial oxidation products in the presence of a Mo5O14 type structure. This structure, which was first identified by Kihlborg et.al. [10], is built up by pentagonal bipyramids and octahedrally coordinated metal centres [Figure 1]. It is metastable until crystallisation and oxidative decomposition into binary oxide phases occurs under high oxygen partial pressure (air and above). The element ratio is (Mo0.68V0.23W0.09)5O14. At the same time binary molybdenum based oxides doped with different elements such as Nb, W and Ta have been synthesised and their structure was identified as that of the Mo5O14-type [11, 12]. These phases were found to be stable at a wide temperature range. For the synthesis of this oxide, solutions of ammonium heptamolybdate, ammonium metatungstate, and vanadyl oxalate were spray-dried and subsequently calcined in air and helium. The Mo5O14 structure is an idealised endpoint that is formed under reduced oxygen partial pressure during the organisation process of a mixture of oligo anions, which are generated in solution. It is therefore necessary to characterise not only the structure itself but also the full preparation process with all intermediates. It seems plausible that different thermal treatments of the precursor solutions affect a) the composition of the usually mixed phase catalysts and b) the crystallite sizes of the different constituting phases. Thus, the understanding of the aqueous precursor chemistry is required to control the preparation of such mixed oxide catalysts. Furthermore, subsequent drying and activation procedures from the liquid precursor to the active and selective catalyst are of paramount importance for the development of the optimal catalytic performance. A preparation that is based on understanding of the system would allow precise control of the phase composition of the mixed oxide catalyst, the crystallite size, the crystallinity, and the morphology of the active phase. A developed synthesis routine thus could lead to defined crystallite sizes or even nano-crystalline (MoVW)5O14 mixed oxide catalysts. Moreover, it offers a versatile path to control its elementary composition. Effects of crystallite size / morphology and elemental composition could be studied separately on the catalytic performance. To this end, some steps of the developed aqueous preparation procedure are characterised by in situ micro Raman spectroscopy. The important, subsequent drying process as well as further activation and formation procedures are investigated by in situ Raman spectroscopy, HREM and XRD. Comparison with Raman spectra of well defined, single-crystalline reference oxides [13] can be used to assign the obtained spectra during these catalyst preparation routes to certain oxides, such as MoO2, Mo4O11, Mo8O23, MoO3, or Mo5O14. A different approach is currently carried out to synthesize the MoVW oxide by a Sol gel method. The Sol-gel chemistry is widely used to synthesize metal oxides by inorganic polymerisation of molecular precursors in organic media (alcohols, hydrocarbons). The low synthesis temperatures often lead to the formation of oxides with amorphous or metastable phases, which are not observed using other synthesis routes. The sol-gel synthesis of molybdenum oxides has received little attention, especially in comparison with transition metal oxides such as TiO2, V2O5 and WO3. The overall aim of this work is the rational preparation of molybdenum-based oxides via sol-gel synthesis of alkoxide precursors. The work concentrates on the mechanisms of solid formation from solution by in-situ measurements (Raman and UV-vis) in order to find new synthesis methods for high surface molybdenum oxides