Isotopic switch

Jacobs, G., Crawford, A.C., and Davis, B.H. 2005. Water-gas shift Steady state isotope switching study of the water-gas shift reaction over Pt/ceria using in-situ DRIFTS. Catal. Lett. 100 147-52. 

Fig. 6.1 Immediate hypersensitivity reaction. These reactions are the result of the production of IgE antibody in response to an allergen. IgE binds to the mast cells via Fc receptors and its reexposure to the allergen causes degranulation and secretion of endogenous mediators. In allergic responses, TH2 cells are important in recognizing allergens in the context of MHC molecules and secrete IL-4, IL-5 and IL-13. IL-4 induces isotope switching from IgG to IgE, and IL-5 is involved in eosinophil recruitment. IL-8 serves as a chemical signal to attract neutrophils at the site of inflammation. The collective effects of endogenous mediators include rashes, inflammation, smooth-muscle contraction, bronchospasm, asthma and severe anaphylactic shock, which may even cause death (see Color Insert)...

Due to experimental limitations, it was impossible to record transients during water treatment. The vapor feed was turned off after 16 h and new isotope switches were carried out after 2-3 hours of FTS under dry conditions. 

Fig. 3 DRIFT spectra (A) and normalised responses (B) of hydroxyl groups (OH/OD) and gas phase methane (CH/CD) after the isotopic switch CRi/He - CD4/He at 1013 K over 3Mo/HZSM-...

Fig. 6 Normalised experimental (symbols) and simulated (solid lines) responses of a) argon tracer, b) gas phase methane (CD bands) and c) hydroxyl groups (OD bands) after the isotopic switch CHi/He -> CDjnt at 1013 K over 3Mo/HZSM-5.

The assumption made of constant rates of production of B, constant rate constants, and specific coverages of intermediates are reasonable provided that there is no kinetic isotope effect such as for the switch from H2 to D2. In practice, the isotope switch used in FTS studies is mainly that from CO/H2 to CO/H2. The kinetic isotope effect for the combination, because of the small mass difference between the two isotopes, is sufficiently small and it can be neglected in most, if not all, transient studies. 

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. 

Mims and McCandlish utilized a different approach to study the FTS over 23 wt% Co/Si02 and precipitated iron catalyst containing Si02 plus Cu and K promoters. As in other studies, an isotope switch was made from CO/H2 to CO/H2 and the rate of incorporation in CH4, and C3 to Cg olefins was followed. However, a distinguishing feature of this study was that accumulated products at different time periods during the transient were also subjected to an NMR analysis to obtain the fraction of in the various positions of the olefin products. The results indicated that incorporation of in the C3 to Cg olefin products was the same at the various carbon positions of the olefins. Further, the fraction of in the total olefin content was the same for C3 to Cg olefins. 

Fig. 4. Typical normalized isotopic transient responses in product species P following an isotopic switch in reactant R— R. An inert tracer, I, is introduced to determine the gas-phase holdup of the reactor system.

The transient isotope-switching apparatus has been described previously. The reactor was a plug-flow, fixed-bed quartz microreactor, 8 mm in diameter, with a total volume of 0.8 cc. Approximately 0.4 g of catalyst was supported in the middle of the reactor on a fused quartz frit. Temperature in the catalyst bed was measured with a chromel-alumel thermocouple shielded in a quartz jacket. The reactant gas stream was controlled by a mass flow control switching system capable of switching one or more of the reactants to its isotopically labelled counterpart in approximately 1 s. The partial pressures and flow rates of the reactants are not altered by this switch so that the steady-state concentrations of reactants and products in the gas phase, and on the catalyst surface, are not disturbed. 

The technique of transient isotope-switching can provide fundamental information about the kinetics of catalytic reactions that is difficult or impossible to obtain with other methods. Following a gas-phase isotope switch at steady state, transients of products labeled with the original isotope result from reactions of intermediates that have been left behind on/in the catalyst after the gas-phase switch. The area under the transients provides a direct measure of the steady-state concentrations of intermediates on/in the catalyst at the time of the switch. The temporal shape of the transients provides information on the reaction kinetics of the intermediates. In the present study, we compliment the transient techniques with steady-state measurements to obtain information on the rates of the various... 

Kinetic Analysis of Oxygen Isotope Switching Results... 

Eq. 10 shows that immediately following the isotope switch, the rate of production i 0 60 increases rapidly, reaches a maximum when the surface concentrations of 0 and 60 are equal at t = 0.35(kd nOo)", and then declines more slowly as the concentration of 60 at the surface falls off exponentially. The FWHM of the peak is equal to 0.88(kcurve using Eq. 10. The value for kd obtained from fitting the... 

Fig. 3. Results from a model that describes the kinetics involved in oxygen isotope switching from 20% to 20% 02. The surface coverages of and 0, and the desorption rates of and 0 are plotted as a function of time.

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... 

To gather information for the role of O2 in the H2-SCR mechanism, similar SSITKA experiments with the use of 02 were conducted at 140 °C. Figure 26.8 presents the transient response curves of N2 0, N2 0, and Ar obtained on Pt/Lao.sCeo.sMnOs (Figure 26.8a) and Pt/Si02 (Figure 26.8b) catalysts after the switch N0/H2/ 02/Ar/He N0/H2/ 02/He was made at 140 °C. As seen in Figure 26.8, the concentration of N2 0 produced by both catalysts is reduced after the isotopic switch, whereas the continuous evolution of N2 0 is noticed. The sum of the steady-state concentrations of N2 0 and N2 0 formed under H2-SCR in the isotopic gas mixture is the same as the steady-state concentration... 

Figure 26.8 Dimensionless transient response curves of N2 0, N2 0, and Ar obtained following the isotopic switch NO/H2/ 02/Ar/He N0/H2/ 02/Ar/He at 140 °C over...

In the absence of detailed kinetic parameters for individual reaction steps, the relative propagation of the aromatic- to olefin-based catalytic cycles can be assessed by determining the rate and selectivity of termination products of these cycles that are predominantly unreactive in subsequent MTH reactions. One possibility is to use the ethylene/isobutane selectivity as a measure to describe the relative rates of propagation for the aromatic- and olefin-based cycles [62], Kinetic studies show that ethylene methylation is at least an order of magnitude slower than other olefin methylation reactions, and isotopic switching studies show that the C incorporation of ethylene matches that of aromatics on various zeolites therefore, ethylene can be considered to be a termination product of the aromatics-based cycle. Isotopic switching studies on H-ZSM-5 also show that the incorporation of atoms in isobutene matches that of other olefins, showing that on H-ZSM-5, isobutene is... 

The basic principle of SSITKA is the following. Under steady-state isothermal and isobaric operating conditions, an isotopic transient is introduced by an isotope switch or, in other words, a sudden replacement of one labeled compound with its isotope. Analysis is done by GC and MS. The amount of adsorbed intermediates Np converted in products is defined as... 

A typical example of such a transient simulation following a CO isotope switch, from which residence halftimes of CO and methane can be determined, is shown in Figure 16.23a. 

Analysis of transient isotope switch experiments [52] provides the possibility to decompose the overall catalyst kinetics into differences between rates of elementary reactions. Especially, the group of Holmen [44, 53-55] has recently produced important transient kinetics data for cobalt and mthenium catalysts as a function... 

The case of methane formation is interesting. Its rate independence of the isotope switch implies that its rate is controlled by the activation of CO. It agrees with the need for a fast rate of CH removal and relatively low rate of CO dissociation to have a high selectivity to methane. This situation may occur on dense transition-metal surfaces or low-reactivity metals. The experimental condition for the different inverse H/D isotope exchange effect for methane formation and Cj formation is that of excess production of methane. Then methane is produced on different sites than in the FT reaction. 

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