Hydrides formation kinetics

It is important to note that the calculations by Lynch and Trevena for H diffusion were based on an approximate value for H diffusivity of lCT cm /s. This was based on hydride formation kinetics in Mg-2Ce since values for H diffusivity in pure Mg were unavailable. It was stated that for the given crack velocity insignificant H diffusion would occur for D < 2 x 10 cm /s. However, Makar et al. [113] postulated that the H diffusivity might be as large as 10 cm /s. 

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. 

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]. 

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)]. 

The kinetics of this reaction have been studied in detail and a hydroxy-carbonyl is specifically proposed as an intermediate consistent with the kinetic data. Decomposition of this intermediate hydroxycarbonyl may proceed by -elimination of the platinum hydride product since the hydroxycarbonyl is a 16-electron coordinatively unsaturated complex. Another well-known example of metal hydride formation from CO and H20 is the reaction of iron carbonyl in aqueous alkali (55) (36). 

Nevertheless, because the disproportionation reaction requires rearrangement of metal atoms vhile in the hydride reaction the motion of atoms is minimal, the ternary hydride formation is kinetically favored at lo v temperature [35[. Ho vever, at high temperature (around 573 K [73]) the disproportionation reaction is more likely to... 

Gerard, N. and Ono, S. (1992) Hydride formation and decomposition kinetics, in Hydrogen in Intermetallic Compounds II, vol. 67 (ed. L. Schlapbach), Springer-Verlag, Berlin, Chapter 4. 

The formation of a carboalkoxy ligand, although kinetically favored, is reversible, whereas ligand dissociation and hydride formation are irreversible. 

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]. 

Kinetic studies provide a useful probe into the nature of the transition state during metal hydride formation. Activation parameters are available for some cases of Hj oxidative addition that give isolable dihydrides (particularly with Rh and Ir centers values are small (up to 60 kJ mol ), and AS values always negative (- 155 to - 60 J mol K" ). Parameters measured within these ranges for net heteroiytic processes giving monohydrides thus offer indirect support for the two-step process of equation (k). 

Kinetic data have been reported for cyclohexene reduction with a 1 6 Cr(acac)3- Bu3Al catalyst in heptane at 30 C, which showed a first-order dependence on catalyst and H2. Hydrogenation rates generally decrease with increasing substitution of the alkene substrate. Similar kinetic results were independently obtained for the Cr(acac)3- Bu3Al catalyst. A proposed mechanism involves alkylation of the metal-halide [equation (a)], hydride formation [equation (b)], followed by reversible insertion of the olefin substrate into the metal-hydride bond [equation (c)], and hydrogenolysis of the resulting metal-alkyl bond [equation (d)]. ... 

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... 

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