Homoexchange

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

For a thorough interpretation of these measurements, several parameters may be calculated. From homoexchange experiments, the initial rate of equilibration, used to compare the activity of the metal in the activation of oxygen, is systematically given by ... 

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 ... 

O/ O homoexchange reaction (or equilibration reaction) on supported metal catalysts. In fact. Of O homoexchange is an overall measurement of the adsorption/desoiption rate. Recent results from our Group are presented hereafter. 

Homoexchange proceeds as a result of dihydrogen dissociation and allows to get information on the intensity of the latter. 

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

The time dependences of these parameters can be written as linear differential equations. In a closed system with constant a we obtain for the deuterium homoexchange of the molecular hydrogen a rate equation... 

The homoexchange, denoted as isotopic equilibration , is when only steps 1 and -1 occur at a significant rate. Isotopic equilibration gives useful information on the nature and surface area of metallic particles. Experiments are generally carried out with equimolar mixtm es of X2 and X2-... 

When adsorption/desorption is the rate-determining step, the rate of exchange becomes very close to the rate of equilibration (homoexchange) between X2 and X2 on the metal. In this case, no information relative to the support can be obtained. 

In most cases, this reaction is much more rapid on the metal than on the support. Those metals which cannot catalyse equilibration are poor X donors for exchange with the support. In what follows, we will consider the case where homoexchange occurs at a significant rate only on the metal. 

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. 

When homoexchange obeys one of the first two mechanisms (zero or one adsorbed atoms), the general rate equation is given byt ... 

P2xx P x the partial pressures of XX at t = 0 and at equilibrium respectively. During homoexchange, Pq and Og remain constant and P x is given by... 

When homoexchange obeys the third mechanism (two adsorbed atoms), the rate equation is more complex and a second term must be added to the right-hand part of Eq. (40).I l As Schwab s mechanism is rarely encountered, distinction should be made between the mechanisms involving one or two adsorbed atoms. A simple method for discriminating these two mechanisms was proposed by Klier et It is based on the variation of the... 

The homoexchange is generally performed with 50%-50% mixtures of X2 and X2- An example of experiment is shown on Fig. 9 for a Rh/Al20s catalyst. The initial slope of the curve P xx versus time allows the calculation of the rate of equilibration rq by Eq. (38). 

Several authors concluded that the high-affinity uptake system might represent a homoexchange process (Levi and Raiteri, 1974 Simon et al., 1974 Raiteri et al., 1975 Levi et al., 1976). Synaptosome preparations preloaded with [ H]-GABA or [ H]-glycine released them in a dose-dependent manner when... 

Roberts P J (1976) Gamma-aminobutyric acid homoexchange in sensory ganglia Brain Res 113, 206-209 Roskoski R (1978) Net uptake of L-glutamate and GABA by high-affinity synaptosomal transport systems / Neurochem 31, 493-498 Rubin R P (1970) The role of calcium in the release of neurotransmitter substances and hormones Pharmacol Rev 22, 389-498. 

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