Heteroexchange

Moreover, the isotopic heteroexchange of oxygen is a very sensitive photoassisted gas-solid reaction. For example, it is much more affected by the texture (33) or by the doping (35) of Ti02 samples than the oxidation of different organic compounds in gas (33, 35), liquid or aqueous solutions (35). It can therefore be used to evaluate more precisely the effect of various treatments. 

Figure 6. Initial rates vs Pt contents of the Pt/TiC>2 specimens for liquid methanol (A) or 1-propanol. (B) dehydrogenation at 298 K cyclopentane-deuterium exchange in gas phase at 263 K (C) oxygen isotope heteroexchange at 298 K over non-preoxidized (D) or preoxidized (E) samples.

Because of the weakness of the Bronsted acidity of the surface hydroxyl groups, one can suppose that the synchronous mechanism is relatively efficient for these substitution reactions. The simplest reaction of this type is H/D isotopic heteroexchange with D-containing molecules. The heteroexchange with water is known to proceed rapidly at a temperature as low as 20°C. With molecular deuterium, such exchange occurs at 350-400°C with an activation energy of 15-20 kcal/mol (55). 

As a result, two types of reactions may be studied homoexchange and heteroexchange and two different levels of information are accessed. 

Nevertheless the rate equation depends on the mechanism of homoexchange. Different methods were in fact developed for the purpose of more fundamental and mechanistic studies [52,53,55,56,64-66]. On the other hand, the rate of isotopic heteroexchange may be accessed from the mass balance in O atoms during the experiments. Neglecting the O accumulation in metal particles, we get ... 

The heteroexchanges ATP o for ADP,j j, and ADP m) for ATP,j ), were also studied, but under conditions where both substrates were present in equimolar concentration on both sides of the membrane. The rate of entry of radiolabelled ADP under these conditions was studied at saturating substrate levels at variable Aif/ from -Hl20 to —180 mV with respect to the external side of the membrane. The ratio of the rate of ADP entry to ATP entry was plotted as a function of Atp and the ratio varied from 5 to 0.2 as a linear function of Aip from -1-120 to —90 mV. 

Furthermore, in the intact striatum dihydrokainic acid increased the extracellular level of both Asp and Glu during K+ depolarization (Herrera-Marschitzet al., 1996). If heteroexchange was responsible for the depolarization-induced increase in extracellular Asp one would expect the Asp levels to be decreased by dihydrokainic acid (but see Fallgren and Paulsen, 1996). 

Heteroexchange occurring between moleciilar hydrogen (dihydrogen) and the other components of the reaction system (hydrocarbons) would mean that not all dehydrogenation steps were irreversible. 

Heteroexchange alters the tracer content a. Homoexchange lowers the deviation y of the isotopic molecules concentrations from their related equilibrium distribution values. 

The heteroexchange, denoted as isotopic exchange , is when steps 1-4... 

Isotopic heteroexchange on supported metal catalysts — general parameters... 

The mechanisms of homoexchange are similar to those of heteroexchange, where the atoms of the solid in the latter case are being replaced by-adsorbed atoms in homoexchange. Three mechanisms can occur according to the number of these adsorbed atoms involved in the reaction zero, one or two atoms. 

H2 heteroexchange with species of alumina in the presence of metais... 

---