Oxygen isotopic exchange techniques

Oxygen isotopic exchange techniques are used to measure the oxygen exchange rate, which is usually decomposed into two parts the surface exchange rate and the rate of oxygen diffusion in the bulk. Different techniques are used. 

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In this chapter, we present an example of a study following the second line of approach. The catalytic activity of a highly divided stoichiometric nickel oxide, one of the best catalysts in oxidation reactions (18), has been studied, for several years, first at the Faculty des Sciences of Lyon and, then, at the Institut de Recherches sur la Catalyse, Villeurbanne, France, in carbon monoxide oxidation and related reactions (oxygen isotopic exchange, nitrous oxide decomposition) with the help of different experimental techniques. It is fortunate that the same type of investigations on the same material were also conducted at the Institute of Physical-Chemistry in Prague, Czechoslovakia. This allowed many comparisons and checks of experimental results and interpretations. 

Here we intended to study the DC conductivity, oxygen isotope exchange, andTPD techniques which give the active site information at the high temperature [10]. Two kinds of magnesia catalysts, i.e., MgO and Li/MgO were studi. The defect notations follow after Kroger [11]. 

For predominant electronic conductors like, for example, the perovskites Lai xSrxCoi.yFey03 is determined by the ratio of the oxygen self diffusivity and surface exchange coefficient. Both parameters can be measured simultaneously by using isotopic exchange techniques. Calculations show that... 

Oxygen self-diffusion was studied in alumina single crystals at 1520 to 1750C by using the gas-solid isotope exchange technique. After diffusion annealing, 1 0 profiles were determined using secondary ion mass spectrometry. The results indicated that 2 diffusion mechanisms were involved a bulk diffusion mechanism and a sub-boundary one. The bulk diffusion was described by ... 

The tracer diffusion coefficient of oxide ions in single crystals was measured by using the gas-solid isotopic exchange technique with 1 0 tracer. Least-squares analysis of tracer diffusion coefficients under an oxygen pressure of 4.5 x lO Pa gave ... 

Bouwmeester, H.J.M., Song, C., Zhu, J., Yi, J., van Sint Annaland, M., and Boukamp, BA. (2009) A novel pulse isotopic exchange technique for rapid determination of the oxygen surface exchange rate of oxide ion conductors. Phys. Chem. Chem. Phys., 11 (42), 9641U9643. 

Some more recent dynamic studies of material transport may be noted. Selfdiffusion studies of Ce02-x at high temperatures (850-1150°C) utilizing the isotope exchange technique (Steele and Floyd, 1971) have confirmed oxygen diffusion by a vacancy mechanism. At a composition of CeO. 2 the diffusion coefficient may be written... 

To study water exchange on aqua metal ions with very slow exchange of water molecules, an isotopic labeling technique using oxygen-17 can be used. A necessary condition for the applicability of this technique is that the life time, tm, of a water molecule in the first coordination shell of the ion is much longer that the time needed to acquire the 0-NMR spectrum. With modern NMR spectrometers and using enrichments up to 40% in the acquisition time can be as short as 1 s. 

Positional isotope exchange ( PIX ) is a very valuable technique in determining enzyme mechanisms, particularly those utilizing ATP, GTP, or another NTP substrate -. For example, the nucleotide substrate can be labeled with in bridging (i.e., P—O—P or phos-phoanhydride oxygen) and/or its nonbridging positions. 

With the improvement of analytical techniques, it is now possible to measure oxygen isotopic profiles in minerals. By analogy to the concepts of bulk mineral closure temperature versus closure temperature at every point in the interior of a mineral for the case of thermochronology, future theoretical development will need to clarify the concepts and meanings of Tc and Tae based on interior isotopic or elemental compositions of two contacting minerals. Furthermore, the principles of the above discussion may also be applied to elemental exchange reactions, such as Fe-Mg exchange between minerals. 

Ultraviolet or y irradiation of a number of oxides in the presence of excess oxygen leads to the adsorption of oxygen and the formation of Oj and in some cases OJ ions. A variety of techniques have been used, such as thermal desorption, isotopic exchange, and conductivity measurements, but the principal evidence comes from EPR studies. Both the formation and stability of these species are discussed in the sections dealing with the appropriate oxide, but the overall picture is summarized below. 

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. 

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