Isotope oxygen exchange method

Results are similar for films deposited on YSZ however, there appears to be a difference between films deposited on ceria vs YSZ in terms of interfacial electrochemical resistance. As shown previously in Figure 6c, LSC films on YSZ often exhibit a second high-frequency impedance associated with oxygen-ion exchange across the electrode/electrolyte interface.That this difference is associated with the solid—solid interface has been confirmed by Mims and co-workers using isotope-exchange methods. As discussed in greater detail in sections 6.1—6.3, this interfacial resistance appears to result from a reaction between the electrode and electrolyte, sometimes detected as a secondary phase at the interface. 

However, to date there does not appear to be much evidence that mixed conduction in the electrolyte plays the dominant role in the enhancements mentioned above. First, it should be emphasized that a finite rate of oxygen exchange at the electrolyte surface (as measured at equilibrium by isotope methods) is a necessary but insufficient criterion for finite rates of oxygen reduction for there to be a net production of at the electrolyte surface, electrons... 

The oxygen atoms in the silicalite framework can be classified into approximately three kinds by the isotope exchange method proposed by Endoh et al. [12]. On the exchange reaction of in the zeolite framework with C 02 in the gas phase at 773 K, the most reactive oxygen atoms were in the terminal SiOH... 

We synthesized nine silicalites which had different concentrations of defect sites in the zeolite framework determined by isotope exchange method. These silicalites were treated with aluminium trichloride vapor under the same reaction conditions 923 K temperature, 1 h time, 11 kPa aluminium trichloride vapor pressure. Figure 1 shows the plots of the amount of aluminium atoms introduced into the framework against the amount of oxygen atoms on the defect sites. A... 

Most of the classical physicochemical methods for the study of hydration of ions fail to distinguish between water in the first coordination sphere and water more remote from the central ion which also comes under the influence of its charge. Some of the methods more recently applied have provided a clearer picture. The oxygen isotope exchange method, where applicable, not only can define the composition of the first coordination sphere but also can be used to measure the lability of the aquo ion. Thus it has served to establisli Cr(0H2)e (64) (NHs)6Co (0H2)+++ H4) > and (NHs)4Co(OH2)2 " + (.105) as well-defined species in solution and also to fix the half-time for exchange of... 

However, a problem remains how to relate the observations (at equilibrium) from isotopic exchange to the conditions met during membrane operation. In chemical relaxation experiments, the oxide is studied after perturbation of the equilibrium state. These methods are thus complementary and probably their combined application, whenever possible together with spectroscopic techniques, such as FT-IR, UV and EPR, has a great capacity to elucidate the kinetics of surface oxygen exchange. 

Investigation of Oxygen Mobility in Synthetic Zeolites by Isotopic Exchange Method... 

The diffusion of oxygen in PUO2 has quite recently been measured by Deaton and Wiedenheft (313) using the 0 isotope exchange method in the temperature range 700 1100°C, resulting in a diffusion coefficient of D = 1.19 X 10 exp( - 42,200/RT). 

Another approach to the nature of the phosphate esterification reaction has been made with the use of isotope exchange methods. Boyer et al. and Swanson have shown that an exchange reaction between ATP and inorganic P is catalyzed by anaerobic mitochondria, and suggest that the exchange represents a reversible step in the process of oxidative phosphorylation. Similar conclusions were reached by Cohn and Drys-dale, who found in experiments with mitochondria that the oxygen of... 

Chemical exchange methods for the separation of isotopes utilise the small differences of concentration that exist at equilibrium in a reversible reaction involving two chemical substances. As a method of enriching carbon-13, it was discovered that the formation of salt-type carbamates (2) when carbon dioxide combines with amines (1) in non-aqueous solutions (Equation 2) fulfilled the desired conditions [30—32]. Oxygen-18 enrichment occurred in the gaseous carbon dioxide and carbon-13 in the carbamate (2). The emiched carbamate readily... 

The isotope exchange method of studying selfdiffusion was extended to the structural prototype of the homologous series Pr70i2 which is isomorphous with the oxides of similar composition in the ceria and terbia systems. These measurements were made on powdered samples by Weber and Eyring (1971) and on polycrystalline specimens and on single crystals by Lau et al. (1976). The dependence of the diffusion coefficient on pressure is shown in fig. 27.18. The results of studies on these three physical forms were remarkably consistent. The temperature dependence of the selfdiffusion coefficient at 106 Torr oxygen, for example, may be expressed as... 

By itself, the isotope-exchange method does not allow to elucidate the physical nature of the participating oxygen forms. However, it allows one to make certain conclusions about their amount, atomic composition, bonding strength with the surface, mobility, types of elementary steps and relation between their rates. To elucidate the nature of the active oxygen forms, the isotope exchange method should be applied in combination with other experimental (O2 TPD, TPR) and theoretical techniques (vide infra). 

The use of larger particles in the cyclotron, for example carbon, nitrogen or oxygen ions, enabled elements of several units of atomic number beyond uranium to be synthesised. Einsteinium and fermium were obtained by this method and separated by ion-exchange. and indeed first identified by the appearance of their concentration peaks on the elution graph at the places expected for atomic numbers 99 and 100. The concentrations available when this was done were measured not in gcm but in atoms cm. The same elements became available in greater quantity when the first hydrogen bomb was exploded, when they were found in the fission products. Element 101, mendelevium, was made by a-particle bombardment of einsteinium, and nobelium (102) by fusion of curium and the carbon-13 isotope. 

In principle, the three isotope method may be widely applied to new isotope systems such as Mg, Ca, Cr, Fe, Zn, Se, and Mo. Unlike isotopic analysis of purified oxygen, however, isotopic analysis of metals that have been separated from complex matrices commonly involves measurement of several isotopic ratios to monitor potential isobars, evaluate the internal consistency of the data through comparison with mass-dependent fractionation relations (e.g., Eqn. 8 above), or use in double-spike corrections for instrumental mass bias (Chapter 4 Albarede and Beard 2004). For experimental data that reflect partial isotopic exchange, their isotopic compositions will not lie along a mass-dependent fractionation line, but will instead lie along a line at high angle to a mass-dependent relation (Fig. 10), which will limit the use of multiple isotopic ratios for isobar corrections, data quality checks, and double-spike corrections. 

Finally, another possibility often discussed in the literature is that cation dopants from the electrode may enhance the electronic conductivity of the gas-exposed surface of the electrolyte in the vicinity of the TPB, thereby extending the reduction zone along the electrolyte surface via mixed conduction. The surface exchange rate of oxygen on both YSZ- and rare-earth-doped ceria (as measured by isotope methods) is only about 1 order of magnitude lower than on LSM at 700 Thus, if there were sufficient... 

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