Palladium catalytic activity

Nickel and palladium catalytic activities were tested on ethylene hydrogenation into ethane in an isothermal differential reactor. 

Reductive elimination—to yield the coupling product 3 and regeneration of the catalytically active palladium-(O) complex 5. 

Acetone is the best solvent for NBR hydrogenation in the presence of palladium carboxylates. No hydrogenation is achieved when chloroform or chlorobenzene are the solvents. Since it is understood that palladium is reduced to colloidal metal in the presence of hydrogen, attempts have also been made to reduce the palladium by hydrazine [76], methylaluminoxane [84], and trialky] aluminum [85] to improve the catalytic activity. 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

In sliding electrical contact applications, palladium plating has been criticised on the basis of a tendency due to its catalytic activity to cause polymerisation of organic vapours from adjacent equipment with the formation of insulating films on the surface. This effect is important in certain circumstances, but is not serious in many practical applications... 

Another useful class of palladium-catalyzed cycloisomerizations is based on the general mechanistic pathway shown in Scheme 13. In this chemistry, a hydridopalladium acetate complex is regarded as the catalytically active species.27b-29 According to this pathway, coordination of a generic enyne such as 59 to the palladium metal center facilitates a hydropalladation reaction to give intermediate 60. With a pendant alkene, 60 can then participate in a ring-form-... 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

III. Catalytic Activity of Hydride Phases of Palladium and Its Alloys with... 

The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations which are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. 

A short survey of information on formation, structure, and some properties of palladium and nickel hydrides (including the alloys with group IB metals) is necessary before proceeding to the discussion of the catalytic behavior of these hydrides in various reactions of hydrogen on their surface. Knowledge of these metal-hydrogen systems is certainly helpful in the appreciation, whether the effective catalyst studied is a hydride rather than a metal, and in consequence is to be treated in a different way in a discussion of its catalytic activity. 

The screened proton model of nickel or palladium hydrides and Switendick s concept of the electronic structure do not constitute a single approach sufficient to explain the observed facts. In this review, however, such a model will be used as the basis for further discussions. It allows for the explanation and general interpretation of the observed change of catalytic activity of the metals, when transformed into their respective hydrides. 

However this was not always the case. It is possible to demonstrate, on the basis of selected examples from the literature representing the experimental evidence and the authors original interpretation, that the catalytic activity of palladium or its alloys changes sometimes dramatically, when there is a possibility of their being converted into the corresponding hydrides. 

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. 

When studying the kinetics of diffusion of hydrogen through palladium, Farkas (28) noticed the difference in catalytic activity of both sides of the palladium disks or tubes for the parahydrogen conversion the energy of activation was greater on the inlet side than on the outlet side, where due to extensive desorption of the hydrogen its concentration could be lower. 

The catalytic activity of the pure /3-palladium hydride has been studied under the appropriate temperature and pressure conditions. The palladium sample was converted into the hydride in a manner which bypassed the area of coexistence of the phases. This was achieved by suitably saturating the metal with hydrogen at 35 atm above the critical temperature and then subsequently cooling the sample to the required temperature and reducing the hydrogen pressure. This method of sample prepare tion allowed one to avoid cracking the palladium crystallites, which would... 

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. 

Rennard and Kokes (39) in their paper stated directly that their purpose was just to study the catalytic activity of palladium hydride in the hydrogenation of olefins, in this case ethylene and propylene. Kokes (39a) in his article recently published in Catalysis Reviews summarizes the results of studies on such catalytic systems. 

The catalytic system studied by Rennard and Kokes was in fact very complex. It can be expected that the satisfactory prolongation of the reaction should, however, result in a deviation from the formulated kinetics. Unfortunately no investigation comparable to that of Scholten and Kon-valinka has been done in the case of olefin hydrogenation. Such a study of the catalytic activity of the pure /3-phase of palladium hydride in comparison with the a- or (a + /3)-phases would supplement our knowledge concerning catalytic hydrogenation on palladium. 

Fig. 9. Decrease of the catalytic activity of palladium on pumice with time. A— catalytic activity of Pd in initial measurements at 30°C B—catalytic activity of Pd at 30°C after mercury vapor is frozen out C—catalytic activity of Pd at 118°C after removing mercury vapor. (r0)i and (r0) are the initial reaction rates for the first and nth reactions (mm Hg/min). After Mann and Lien (41)-...

A similar reaction was studied by Kowaka Jfi) who investigated the catalytic activity of palladium and its alloys with silver in the hydrogenation of ethylene. The author alluded to the poisoning effect of hydrogen pretreatment of the palladium catalyst. 

The results used for a subsequent comparison of catalytic activity of all group VIII metals are related by Mann and Lien to palladium studied at a temperature of 148°C. At this temperature the appearance of the hydride phase and of the poisoning effect due to it would require a hydrogen pressure of at least 1 atm. Although the respective direct experimental data are lacking, one can assume rather that the authors did not perform their experiments under such a high pressure (the sum of the partial pressures of both substrates would be equal to 2 atm). It can thus be assumed that their comparison of catalytic activities involves the a-phase of the Pd-H system instead of palladium itself, but not in the least the hydride. 

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. 

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