Hydride formation

Magnesium hydride formation RMgR + heat MgH2 + alkene... 

Embrittlement by internal hydride formation (e.g., Ti, Zr, Nb, Ta) [These metals are important as valve metals for impressed eurrent anodes (Seetion 7.1) and as materials in ehemieal plants (Seetion 21.4.3).]... 

Water as an impurity is known to promote the breakaway corrosion of a number of metals in addition to iron in CO2 the effect has been reported for magnesium (hydrocarbons have more effect on the oxidation of this metal), beryllium, zirconium and sodium. In the latter case water is known to convert the oxide to deliquescent NaOH but acceleration of beryllium oxidation probably results from hydride formation and mechanical damage to the oxide. 

As early as 1923 Hinshelwood and Topley (27) noted the exceptionally erratic behavior of palladium foil catalyst in the formic acid decomposition reaction within 140-200°C. The initially very high catalytic activity decreased 102 times during the exposure of palladium to hydrogen, which is a product of the reaction. Though the interpretation does not concern the /3-hydride formation, the authors observation deserves mentioning. 

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. 

Fig. 7. Changes of the coefficient of recombination, y, of H atoms on the surface of Pd-Au alloy foil catalysts at room temperature. O, Initial values of log y, final values representing catalytic activity of Pd and its alloys containing absorbed hydrogen. Broken line denotes the alloy Pd40Au60 which represents the upper limit of gold content in Pd-Au alloys closing the region of Pd-Au hydride formation. After Dickens et al. (86).

In order to follow further the effect that hydride formation has on the catalytic activity of palladium and its alloys it would be of interest to investigate a group of reactions involving the addition of hydrogen to a double or triple bond. Palladium itself has found a well-known wide application in such reactions. Nevertheless even where /3-hydride formation is very probable it is still relatively rare to find considerations of this possibility in most publications. 

Many other authors studied the catalytic activity of palladium in more complicated hydrogenation reactions because of being coupled with isomerization, hydrogenolysis, and dehydrogenation. In some cases the temperatures at which such reactions were investigated exceeded the critical temperature for coexistence of the (a + /3)-phases in the other case the hydrogen pressure was too low. Thus no hydride formation was possible and consequently no loss of catalytic activity due to this effect was observed. 

Quite recently Yasumori el al. (43) have reported the results of their studies on the effect that adsorbed acetylene had on the reaction of ethylene hydrogenation on a palladium catalyst. The catalyst was in the form of foil, and the reaction was carried out at 0°C with a hydrogen pressure of 10 mm Hg. The velocity of the reaction studied was high and no poisoning effect was observed, though under the conditions of the experiment the hydride formation could not be excluded. The obstacles for this reaction to proceed could be particularly great, especially where the catalyst is a metal present in a massive form (as foil, wire etc.). The internal strains... 

As mentioned previously in the introduction to the present review the ability to form the hydride phase is not characteristic solely of palladium or nickel. It would be of interest, therefore, to verify the results on the poisoning effect of hydride formation in the case of nickel or palladium by comparing with the other transition 3d, 4d, and 5d metals and the rare earth (4f) metals. 

The direct proof of hydride formation in situ in a reaction vessel is in principle possible. One can follow changes of resistance (of a film, a wire, etc.) or of magnetic susceptibility of a catalyst. Hydride identification by means of the X-ray diffraction method requires a catalyst sample to be taken out from a reaction vessel, and eventually frozen in order to avoid a rapid decomposition of the hydride under ambient conditions (67). 

The theoretical hmit of 5.4% (NaAlH4+2 mol% TiN) for the two subsequent decomposition reactions is in both cases only observed in the first cycle. The reason for the decrease in capacity is stiU unknown and litde is known about the mechanism of alanate activation via titanium dopants in the sohd state. Certainly, the ease of titanium hydride formation and decomposition plays a key role in this process, but whether titanium substitution in the alanate or the formation of a titanium aluminum alloys, i.e., finely dispersed titanium species in the decomposition products is crucial, is stiU under debate [41]. 

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]. 

The variation of the substituent pattern of the introduced silane provides further insight into the reaction mechanism of the CO activation process of scheme 2 (Table 1) The yield of ju-carbyne-complex (O-attack of the silane) compared to silyl hydride formation (Mn-attack of the silane) is a function of the Lewis-acidity of the silane. However, even with the strongly acidic HSiCl3 as reagent, the product ratio 12/13 is still 1 9. 

Previous investigations of the photolysis reactions of 5 in the presence of various silanes only showed the products of Si-H activation [20], Metal silyl hydride formation, however, becomes the main reaction path with sterically less hindered silanes [21]. 

Figure 3.14 The integrated oxide formation charge, relative to the charge under the hydride formation region, ( o/Gh as a function of anodic potential, for the cyclic voltammogram in Figure 3.1. From Angerslein-Kozlnwska et ai (1973)...

Low heat of formation to minimize the energy necessary for hydrogen release and low heat dissipation during the exothermic hydride formation... 

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