Isothermal isotope exchange

To estimate the oxygen diffusion coefficient from the results of oxygen isotope exchange experiments for powders, the isothermal isotope exchange curves were fitted using a complete system of isotope-kinetic and diffusion equations (30-34). In modeling, the values of diffusion coefficients (coefficients of diffusion relaxation td = D/h ) and the share of the... 

Fig. 4.18 shows the result of Cd2+ adsorption on illite in presence of Ca2+ (Comans, 1987). The data are fitted by Freundlich isotherms after an equilibration time of 54 days. It was shown in the experiments leading to these isotherms that adsorption approaches equilibrium faster than desorption. Comans has also used 109Cd to assess the isotope exchange he showed that at equilibrium (7-8 weeks equilibration time) the isotopic exchangeabilities are approximately 100 % i.e., all adsorbed Cd2+ is apparently in kinetic equilibrium with the solution. The available data do not allow a definite conclusion on the specific sorption mechanism. 

In a similar study (Comans et al., 1990), the reversibility of Cs+ sorption on illite was studied by examining the hysteresis between adsorption and desorption isotherms and the isotopic exchangeability of sorbed Cs+. Apparent reversibility was found to be influenced by slow sorption kinetics and by the nature of the competing cation. Cs+ migrates slowly to energetically favorable interlayer sites from which it is not easily released. 

The area is an important surface parameter for catalytic studies. It is needed to evaluate the rate constant of the surface reaction from the kinetics as well as to allow a fair comparison to be made of the effectiveness of different catalysts. Areas are commonly determined by nitrogen or krypton gas adsorption interpreted by the Brunauer-Emmett Teller (BET) isotherm [30, 32], A number of other methods has been proposed and utilised including microscopy, isotopic exchange, chromatography, gas permeability, adsorption from solution, and negative adsorption (desorption) of co-ions [30, 33]. 

Comparison of the computerized results obtained for the kinetic curves (Fig. 2a) reveals a very interesting feature of the IE process under discussion. Kinetic curves F (Fo) for variants I and III, resolved when a favorable isotherm of the B ion and the relation D >Dg apply, are described by the kinetic equation of diffusion into a spherical bead with constant diffiisivity D. In other words the kinetic curves F(Fo) coincide with the isotope exchange kinetic curves if a sharp profile appears in the bead and do not coincide if the exchange zone is greatly spread. The remaining kinetic curves in Fig. 2 formally correspond to the exchange process where varies in value. This is especially evident when comparing kinetic curves II and I.e (Fig. 2a). 

Studies of the homomolecular and heterolytic exchange processes are generally in the form of the measurement of rates under isothermal conditions. However, studies have also been made of temperature programmed isotopic exchange, in which the oxide is subjected to a temperature ramp under the reaction atmosphere, and the partial pressures of various isotopic oxygen species is determined as a function of temperature (e.g., Refs. 20-21). The photoactivation of oxygen exchange has also been reported in a number of studies which have been performed under UV irradiation (e.g.. Refs. 18, 22, 23). 

Isotopic exchange between zeolites CaY and aqueous solutions of the cations appeared to be controlled by solid diffusion. Following isothermal investigation at 0 to 65C, it was found that this process took place in two stages, the second one being rather slow, and this provided a means for calculating the activation energies. The data were described by ... 

Reaction or exchange with stable isotopic tracers and quantitative identification of all products by mass spectrometry provides indications for molecular interactions on the surface. Reactions can be studied at steady state or by following the transient distribution of isotopic products. Langer and co-workers (25,26) presented the first steady-state mechanistic analysis for the electrocatalytic hydrogenation of ethylene on Pt in deuterated electrolytes. Proton abstraction in electroorganic synthesis has also been verified using deuterated solvents (374, 375). On-line mass spectrometry permitted indirect identification of adsorbed radicals in benzene and propylene fuel cell reactions (755,795,194). Isotopic radiotracers provided some notion on adsorption isotherms (376, 377) and surface species on electrocatalysts (208, 378, 379). 

In the case of highly nonlinear isotherm systems, such as chemical adsorption (chemisorption) systems, application of the theory developed for linear systems may seem unfeasible. However nonlinearity of the isotherm relation of chemisorption of hydrogen on nickel catalyst extends to a very low concentration. In such a case, it may be useful if hydrogen exchange rates on catalyst surface can be understood at moderate concentration levels. Employment of isotope technique may provide an answer to this type of need. 

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