Kinetic of hydride formation

Applicability of Surface Compositional Analysis Techniques for the Study of the Kinetics of Hydride Formation... 

When we deal with the kinetics of hydride reactions we have to be aware that hydride thermodynamics cannot be properly formulated without taking into account the (relative) immobility of the metal component. This immobility can sometimes render the interpretation of the experimental reaction kinetics ambiguous. With this difficulty in mind, let us outline concepts which describe the kinetics of hydride formation and decomposition. An extensive account, including a first order phenomenological treatment, has been given by [P.S. Rudman (1983)]. The conceptual framework for a more rigorous discussion is found in, for example, [G. B. Stephenson (1988)]. 

NMR spectroscopy has been used to detect hydrides on various oxide-supported metals in the presence of H2 and on La203-supported Ir4, in the absence of H2 [37]. The kinetics of chemisorption of H2 supports the inference of hydride formation by dissociative adsorption of H2 [38]. 

ICR mass spectrometry has been employed to study the kinetics and equilibria of the generation of silylenium ions by hydride transfer from silanes to carbenium ions (43,47). The kinetics of the formation of silylenium ions from fluoromethylsilanes [(CH3) SiF4 , n = 1-3] have also been investigated by this technique (48). Fluoride transfer was the dominant reaction in this system. 

The range of experimental methods employed has not so far been very wide further work on the spectroscopic, thermochemical, and magnetic properties and on the kinetics of the formation and substitution reactions can be expected to be illuminating. Most work has been done on the trimeric chloride, the lowest stable member of the series, and the most abundant. As in the boron hydride series, significant advances are likely to result from investigations of the higher polymers. 

The usefulness of this procedure for speciation analysis, however, is restricted by either the thermodynamic inability of hydride formation for some species or considerable kinetic limitation to hydride formation. Nevertheless, the technique is still essential for some classes of compounds [1]. 

Ytterbium, again, showed a significantly different behavior. The intensity ratio of the signals from oxide oxygen and surface OH groups was the same as in the other heavy rare earths indicating the same depth of the oxide islands but the uptake kinetics following water exposure was linear (see fig. 7). This difference was tentatively attributed by Padalia et al. (1976) to the lack of hydride formation in the reaction of Yb with water. 

Kinetic studies of hydride formation are somewhat limited for both the rare earths and the actinides. Early work for the rare-earth elements is reviewed by Libowitz and Maeland (1979) work for the actinides is described in a recent review by Haschke (1991). In comparison to work on the acinides, kinetic studies on rare-earth hydriding are at present a dormant area. Only the U + H reaction is extensively investigated to... 

Viallard, R., 1960, Kinetic Studies of Hydride Formation of the Lanthanides, in Proceedings 4th International Symposium on Reactivity of Solids, Amsterdam, p. 168. 

Moreover, in the case of hydride intervention, still a further factor, namely the kinetics of hydrogen diffusion into the metal, influences also the overall kinetics by removing a reactant from a reaction zone. In order to compare the velocity of reaction of hydrogen, catalyzed by palladium, with the velocity of the same reaction proceeding on the palladium hydride catalyst, it might be necessary to conduct the kinetic investigations under conditions when no hydride formation is possible and also when a specially prepared hydride is present in the system from the very beginning. 

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