Isotopic transient experiment

Various improvements have broadened the research in the held of zeoUte membranes and films, such as the development of new synthesis procedures, the use of new supports with speciUc characteristics (monoliths, foams, etc.) or the use of modified supports by means of masking or grafting techniques, the appUcation of new analytical techniques (isotopic-transient experiments, permporometry, etc.), the control of the orientation of the crystals (by means of covalent Unkages, synthesis conditions, etc.) and of the thickness of the membranes, and the preparation of new zeolites as membranes or new zeoUte related-materials. In addition, a variety of zeoUtes can now be prepared as coUoidal systems with particle dimensions ranging from tens to a few hundred nanometers. 

XPS (x-ray photoelectron spectroscopy) utilizes photoionization and energy-disperse analysis of the emitted photoelectrons to study the composition and electronic state of a region of the surface of a zeolite. However, aU these techniques are destructive ones, and for that reason other methods such as isotopic-transient experiments or reflectance [16] and fluorescence [17] imaging can be used to estimate the effective membrane thickness. 

There are macroscopic (uptake measurements, liquid chromatography, isotopic-transient experiments, and frequency response techniques), and microscopic techniques (nuclear magnetic resonance, NMR and quasielastic neutron spectrometry, QENS) to measure the gas diffusivities through zeolites. The macroscopic methods are characterized by the fact that diffusion occurs as the result of an applied concentration gradient on the other hand, the microscopic methods render self-diffusion of gases in the absence of a concentration gradient [67]. 

An early study showing the presence of two intermediates was that of Soong et al. who studied the methanation of CO/H2 over Raney Nickel catalyst at 210 °C and at a H2/CO ratio of two. Two types of isotopic transient experiments were carried out. In the first type, the catalyst was exposed to CO/H2 (or CO/H2) until the reaction reached steady state after which the isotopic switch was made to CO/H2 (or CO/H2). In the second type of experiment, the catalyst was first exposed to CO/H2 until steady state was attained it was then exposed for a short time (2 min) to CO/H2 and then exposed again to C0/H2. 

The isotopic transient experiment begins with a standard reaction similar to the one above using a reactant with a particular isotopic label, typically... 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

The following isotopic labeling experiment was performed in order to quantify the contribution of the direct and indirect reaction routes to CO formation After steady-state reaction with CH4/02/He was achieved, an abrupt switch of the feed from CH4/02/He to an isotopic mixture of CH4/1 02/ C 02/He was made, in which the partial pressures of CH4 and 62 were kept exactly the same as in the ordinary CH4/02/He mixture, so as not to disturb the steady-state condition. However, C 02 was added to the isotopic mixture in an amount corresponding to approximately 10-15% of the CO2 produced during reaction of the mixture. The purpose was to measure the production of C 0 due to reforming of CH4 with C 02 only (indirect reaction scheme) under steady-state conditions of the working catalyst surface. Figure 3 shows the transient responses of and C O... 

After approximately 3 min on stream in the isotopic mixture of CH4/02/C 02/He, a steady-state value in the rate of formation is obtained (Fig. 3). This value is used to estimate the relative contribution of the CO2-reforming route to the overall production of CO (direct + indirect routes). Proper analysis of this result, taking into consideration scrambling of isotopic oxygen Figure 3. Transient isotopic labelling experiment atoms between the CO2 and O2 molecules... 

Another reactor equation useful in reaction kinetics analyses represents the transient CSTR. This situation is encountered in temperature-programmed desorption and isotope tracer experiments. The material balance for the transient CSTR operating at constant total pressure for an ideal gas is... 

The composition and reactivity of the carbon laid down during the initial stages of the propane dehydrogenation reaction was examined by transient isotope labelling experiments using [2-]3C]-C3HgandC3JHs as tracers in a series of reactions in a pulsed flow microcatalytic reactor. In these experiments alternate series of labelled and unlabelled propane pulses were passed over the catalyst sample and the products analysed by glc and mass spectrometry. 

In the present communication we report on the influence of water on the FT synthesis studied by SSITKA and conventional kinetic experiments. Steady-state isotopic transient kinetic analysis (SSITKA) has proved to be a powerful technique for this work. The technique involves switching between CO and " CO in the feed gas and analyzing the transients with respect to the formation of products containing C and C. This technique allows the determination of the true turnover frequency of the active site, decoupled from site coverage. Applied to the FTS over metal promoted cobalt catalysts SSITKA has shown that the true turnover frequency of cobalt always remains the same, regardless of the second metal [6-8]. 

Since the isotopic transient technique involves the number and type of intermediates on the catalyst surface, independent transient experiments (with or without the use of isotopes) have also been used to determine these parameters. The simplest reaction for analysis by the isotopic transient kinetic technique for the conversion of syngas is the production of methane. Studies of methanation provide a background to the isotopic transient kinetic studies and independent justification for the number and type of adsorbed species involved in FTS. Furthermore, the production of methane is undesirable for FTS and an understanding of the mode of its production will aid in FTS catalyst and process design. 

In principle, the dependence of on temperature can be determined by conducting the same isotope switching experiment at different temperatures. In practice, this approach is limited by the narrow temperature range that can be used. As the temperature increases, the FWHM of the desorption transient rapidly becomes comparable to or shorter... 

The readsorption of olefins is an important reaction in the Fischer-Tropsch synthesis that reverses the overall termination and increases the chain growth probability. Thus, readsorption results in heavier products. A larger time delay of the transient for the isotopic responses of olefin as compared with corresponding paraffin with increase in residence time (see Figure 51.10 for an example data on the ethane-ethene pair) is due to olefin readsorption. Transient experiments indicated that 1-olefins are the major candidates for readsorption on the catalyst surface, while the internal and iso-olefins readsorbed to a much less extent. 

Pulse intensities in vacuum experiments range from 10 to 10 molecules per pulse with a pulse width of 250 ps and a pulse frequency of between 0.1 and 50 pulses per second. Such a spectrum of time resolution is unique among kinetic methods. Possible experiments include high-speed pulsing, both single-pulse and multipulse response, steady-state isotopic transient kinetic analysis (SSITKA), temperature-programmed desorption (TPD), and temperature-programmed reaction (TPR). 

For some systems, there is no resonance Raman or SERS effect to be utilized, and the sensitivity becomes the main problem. In this case, a potential difference method will be of great help [11], Here, a spectrum is acquired at potentials where there is no or only a weak surface signal which is subtracted from that at the potential of interest. In addition, a change in the composition of the electrolyte or an isotopic labeling experiment may be considered to identify the surface species and verify its orientation and structure. For temporally resolved studies, electrochemical transient techniques are helpful to understand the surface dynamics and the reconstruction processes of surfaces. For nonuniform surfaces, spatially resolved measurements provide more reliable and complete information on the surface. This is also useful for electrode surfaces that change either chemically or topographically in a microzone upon variation of potential. 

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