Hydrides, decompositions

Hydride phase Hydride partly decomposed Metal catalyst after hydride decomposition ... 

Fig. 11. X-ray diffraction pattern of a Ni99Cul alloy partially transformed into its (3-hydride (0 NiCuH) before (a) and after (b) hydride decomposition. Arrows point to the diffraction peaks representing the rich in copper alloy phsae desegregated from the initial alloy after a multiple hydrogen absorption-desorption treatment. After Palczew-ska and Majchrzak (48).

As far as hydride decomposition is concerned, the relations are reversed. The larger the metal crystals are the slower their hydride decomposes (62). Moreover some deposits situated on the exit points of dislocations, for example on the surface of a nickel hydride crystal, inhibit hydrogen desorption and result in prolonging the hydride existence in the crystal (87). 

Shore and coworkers—nuclearity studies over Ru carbonyl catalysts. Shore et al.64 studied reactions of K[DRu3(CO)n] + CO + H20 <- HD + Ru3(CO)i2 + KOH. They found that, at room temperature and 1 atm pressure of Pco, HD is rapidly evolved. In the absence of CO, however, the HD was formed only in trace quantities. They proposed two possible mechanisms to account for this behavior, (a) a concerted mechanism where CO promotes hydride decomposition (Scheme 28), or (b) an associative mechanism involving a complex-CO adduct, which decomposes with H20 (Scheme 29). 

Since we are primarily concerned with kinetics, we do not intend to discuss all the proposed mechanisms or the numerous boron hydrides formed during the pyrolyses. The literature available on this field up to 1964 has been reviewed by Adams81 and by Lipscomb85. Rapid progress is being made in studies of the kinetics of boron hydride decomposition so that this discussion must be regarded as preliminary. 

The plateau region on the PCX cnrves in Fig. 2.43a is very flat, showing only a minimal slope (Sect. 1.4.1). However, each absorption-desorption pair of the PCX curves clearly exhibits a pressure hysteresis. This means that the pressure needed for absorption (hydride formation), is always greater than that of hydride decomposition, p. The cause of pressure hysteresis in metal hydride systems is not fnlly nnderstood. A number of models that attempted to explain this phenomenon... 

Neutral formyl complexes which contain ligating CO often decompose by decarbonylation however, several exceptions exist. For instance, the osmium formyl hydride Os(H)(CO)2(PPh3)2(CHO) evolves H2(54). Although the data are preliminary, the cationic iridium formyl hydride 49 [Eq. (14)] may also decompose by H2 evolution (67). These reactions have some precedent in earlier studies by Norton (87), who obtained evidence for rapid alkane elimination from osmium acyl hydride intermediates Os(H)(CO)3(L)(COR) [L = PPh3, P(C2H5)3], Additional neutral formyls which do not give detectable metal hydride decomposition products have been noted (57, 65) however, in certain cases this can be attributed to the instability of the anticipated hydride under the reaction conditions (H2 loss or reaction with halogenated solvents). 

Effluent from the hydrogenation reactor is depressured to about 400 psig. This level of hydrogen is required to prevent the reverse reaction, diethylaluminum hydride decomposition, which results in plating of aluminum on the process equipment. Product diethylaluminum hydride, unreacted aluminum, and solvent are charged to the ethylation reactor. Ethylene is introduced and undergoes a rapid, exothermic reaction to form triethylaluminum. A tubular reactor with high heat transfer capabilities is required to control this reaction (12). 

Suppose that hydride decomposition is fast enough to maintain the equilibrium concentration in the solution c(f) = c Desorption outflow is compensated by hydride decomposition. Desorption is bulk. This means that the material is sufficiently porous, so J = be2. In general case Q = (Q t). c=c(t)- We can suppose than Q and c are constant within the thin peak of TDS-spectra for the... 

In the beginning (p0=L0) potential hydride decomposition flux exceeds desorption flux / = k T)Q > b(T)c1 Hydride decomposes with the rate necessary to maintain the equilibrium c in a phase. When at p —> 0 this will be impossible,... 

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