Hydride formation palladium

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. 

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. 

Figure 12.2. Palladium hydride formation from divalent palladium and CO and ethene in the presence of water or methanol as initiation reaction (P=PPh2)...

The examples in Table III, show that the hydrogen atoms occupy tetrahedral holes at the beginning of the transition series. As we move along the transition series, we observe the interstitial hydride shift toward octahedral holes and the hydrides of the heavier elements become progressively unstable. Palladium is exceptional since it is the only heavy element of group VIII that gives a simple hydride. Hydride formation is accompanied in most cases by a change in metallic lattice type and in all cases by a considerable increase in metal-metal distances. 

We now turn to the formation of some of these hydride structures. The majority of them are based on a fee array of metal atoms, as shown by the open circles in Figure 2. The dihydride structure comes from filling the tetrahedral interstice (large solid circles) in the lattice with hydrogens and gives the well known CaF2 or calcite structure. Similarly, if one fills the octahedral interstice (small solid circles), one gets the NaCl or rocksalt structure found in nickel hydride and palladium hydride, which we will discuss near the end of this chapter. 

We focus on dissociative hydrogen adsorption, hydrogen dissolution, and palladium hydride formation (68). Figure 28 provides a comparison of H2-TDS spectra... 

The coadsorption of CO and hydrogen on Al203-supported palladium nanoparticles was found to be quite different from that on the single crystal. As shown in the preceding section on palladium hydride formation (Section IV.C.4), H subsur-face/bulk dissolution occurs more easily in palladium nanoparticles. Consequently, preadsorbed H can be replaced from the palladium particle surface even at 100 K, leading to a CO-saturated surface (cf. Fig. 31c) thus, in contrast to the observations... 

Some authors have observed an activity decrease during hydroaenolysis reactions on Pd (ref. 1). This has been attributed to a detrimental effect of palladium hydride formation which under given conditions could spontaneously form during the reaction. 

Pd hydride formation was routinely determined by chemisorption and calorimetric measurements and heats of formation of the g-phase hydride were consistently near 10 kcal/mole H2 absorbed, independent of support and crystallite size, which is in good agreement with literature values for bulk palladium. 

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