Oxygen isotopic exchange experiments

Figure 36.1 Isotopic oxygen exchange experiments (symbols) and model curves (lines), at different partial pressures of oxygen at 973 K for LSCF [33], given as the atomic fraction of (og) and the fraction of (f ).

Not only is the uranyl ion thermodynamically robust, it is also kinetically inert. Experiments designed to measure the rate of isotopic oxygen exchange between the oxo atoms and water at room temperature, establish that the exchange half-life is greater than 40,000 hours [19]. This overall chemical stability accounts for an extensive coordination chemistry which is exploited, for example, in the solvent extraction separation processes used in the nuclear fuel cycle [20]. 

Oxygen exchange experiments were performed between single crystals of sanidine feldspar and oxygen gas enriched in 1°0, at 869 to 1053C, under a total pressure of latm. The O isotope diffusion profiles in a direction perpendicular to (001) were determined using an ion microprobe. The experimental data obeyed the Arrhenius relationship ... 

The question raised by Anderson (1970,1971) and Anderson et al (1973) as to whether anion point defects are eliminated completely by the creation of extended CS plane defects, is a very important one. This is because anion point defects can be hardly eliminated totally because apart from statistical thermodynamics considerations they must be involved in diffusion process. Oxygen isotope exchange experiments indeed suggest that oxygen diffuses readily by vacancy mechanism. In many oxides it is difficult to compare small anion deficiency with the extent of extended defects and in doped complex oxides there is a very real discrepancy between the area of CS plane present which defines the number of oxygen sites eliminated and the oxygen deficit in the sample (Anderson 1970, Anderson et al 1973). We attempt to address these issues and elucidate the role of anion point defects in oxides in oxidation catalysis (chapter 3). 

The isotopic 180 labelling experiments for the incorporation of oxygen into the epoxide, support mainly an Ag-O complex as the reactive species, because the incorporation of 180 labelled oxygen from H2180 into the epoxide is consistent with an independent Ag-O intermediate which undergoes isotopic exchange with 180 enriched water (ref. 10)... 

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. 

Observation of reactive oxygen species on oxides is significantly more difficult. Figure 3 shows an isotope exchange experiment with the molecular oxide H4PVM011O40 14 H2O, a heteropoly acid (HPA) [26,27] and oxygen 18 in the gas... 

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