Of palladium metal

In the spring of 1989, it was announced that electrochemists at the University of Utah had produced a sustained nuclear fusion reaction at room temperature, using simple equipment available in any high school laboratory. The process, referred to as cold fusion, consists of loading deuterium into pieces of palladium metal by electrolysis of heavy water, E)20, thereby developing a sufficiently large density of deuterium nuclei in the metal lattice to cause fusion between these nuclei to occur. These results have proven extremely difficult to confirm (20,21). Neutrons usually have not been detected in cold fusion experiments, so that the D-D fusion reaction familiar to nuclear physicists does not seem to be the explanation for the experimental results, which typically involve the release of heat and sometimes gamma rays. 

The various palladium species can be subject to decomposition and deposition of palladium metal, which generally leads to catalyst inactivation. Apart from their effect on the catalyst activity, the ligands and bases also affect catalyst longevity. 

Typically, the Pd species for Heck couplings are homogeneous catalysts, stabilized by air-sensitive ligands. They present economic and environmental problems regarding separation, regeneration and reuse. These difficulties can be diminished with heterogeneous catalysts that are more easily recoverable from the reaction mixture. As mentioned in Sect. 2.6, a catalyst consisting of palladium metal deposited on por-... 

The catalyst reported by Drent [48] was generated in situ by mixing a palladium source with the ligand. A palladium source is broadly defined as a complex or any form of palladium metal whereby upon mixing with the ligand an active catalyst is formed. Many palladium sources are possible, but the sources exemplified by Drent aretris(dibenzylideneacetone)dipalladium(0)(Pd2(dba)3),bis(dibenzylideneacetone) palladium(O) (Pd(dba)2), or palladium(II) acetate. 

In none of the above cases has a reaction been performed whilst taking the EXAFS data. Hamill et al. [50] have investigated catalysis of the Heck reaction by palladium salts and complexes in room-temperature ionic liquids. On dissolution of palladium ethanoate in [BMIMj and N-butylpyridinium ([BP] ) hexafluorophos-phate and tetrafluoroborate ionic Hquids, and triethyl-hexyl ammonium bis(trifluo-romethanesulfonyl)imide, a gradual change from ethanoate coordination to the formation of palladium metal was observed in the Pd K-edge EXAFS, as shown in Figure 4.1-13. 

The properties of metallic hydrides depend on their composition, which is a function of the partial pressure of H2 gas in the surroundings. For example, PdH behaves as a metallic conductor for small values of x but becomes a semiconductor when x reaches about 0.5. (Semiconductors are discussed in Section 21.5.) The H atoms in PdH are highly mobile, and H2 can pass through a membrane of palladium metal. The process probably involves dissociation of H2 into H atoms on one surface of the membrane, diffusion of H atoms through the membrane as they jump from one interstice to another, and recombination to form H2 on the opposite surface of the membrane. Because other gases don t penetrate palladium, this process can be used to separate H2 or D2 from other components of gas mixtures. 

Some interesting chemistry has appeared relating to the ability of the isocyanide ligand to stabilize unusual oxidation states. A series of palladium metal - metal bonded complexes has been synthesized by redox reactions involving two metal complexes in different formal oxidation states (33 -35). Similar ruthenium(I) and osmium(I) dimers have been prepared by an unusual homolytic fission of a ruthenium-carbon bond (36) or by singleelectron oxidation of Os(CNXylyl)5 (18). 

Recently, attempts have been made to reduce the cost of palladium metal membranes by preparing composite membranes. In these membranes a thin selective palladium layer is deposited onto a microporous ceramic, polymer or base metal layer [19-21], The palladium layer is applied by electrolysis coating, vacuum sputtering or chemical vapor deposition. This work is still at the bench scale. 

Traces of palladium metal can cause a precipitate to appear greenish. Palladium is removed on recrystallization. 

Essentially the same route as that of Ruff has been followed by us, except that two new reducing agents whose chemistry is under study in these laboratories, namely sulphur and selenium tetrafluorides, have been used. Attention has already been drawn to the usefulness of selenium tetrafiuoride as a mild reducing agent in fluorine chemistry while sulphur tetrafiuoride also gives evidence of possessing similar properties. In the first series of experiments, palladium trifluoride, formed by the reaction between palladous iodide and elementary fluorine at room temperature, was treated with a stream of sulphur tetrafluoride at 250-300°. The violet-coloured powder which resulted was proved, by Af-ray examination, to contain palladium difluoride in admixture with about 10% of palladium metal. 

PdF, was prepared by the reduction of palladia fluoride with seleniiun tetrafluoride and the sample was heated in the tetrafluoride vapour at 280 °C. for 30 minutes. This temperature is somewhat higher than that used for the preparation of pure samples of the difluoride (which were imfortunately poorly crystalline) and the resulting material contained a small quantity of palladium metal. Since palladous fluoride is hydrolysed in moist air, thin walled X-ray specimen capillaries, 0-5 nun. diameter (supplied by Pantak Ltd, Slough) were filled and sealed off in a dry box. 

All observed X-ray reflexions, were with the exception of the faint lines of palladium metal, indexed on a tetragonal cell of the rutile type. Since only one of the palladium reflexions overlapped with a difluoride reflexion the presence of metal did not interfere with the structure determination. The dimensions of the bi-molecular imit cell are compared below with those given by Ebert. 

The wider utility of palladium metal catalysts and hydride cleavage of allylic systems has more or less replaced interest in using platinum-based catalysts such as Adams catalyst. Where this has been used for conjugation-stabilized allylic centers good yields have been achieved." The use of platinum for de-benzylation at low H2 pressure is effective." ... 

During the reaction, palladium metal precipitation was observed as would be expected in the vinylation reaction (Reaction 1). During product isolation, water is added to the reaction system. In most of the reactions run at 25°C. this addition resulted in further precipitation of palladium metal from the brown solution, probably owing to decomposition of the trace of 7r-olefin complex of palladium (II) present. However, the acetic acid solutions of products obtained from 100°C. reactions containing chloride were bright yellow and did not precipitate more palladium when water was added. This color is typical of 7r-allylpalladium chloride complexes and indeed di-/ix-chloro-di-7r-(methyl-3-ethylallyl) dipalladium (II) could be isolated from the reaction mixture. Formation of these complexes, 7r-olefin or 7r-allyl would, of course, result in decreased yields of vinylation products. 

Interaction of the dibenzylideneacetone complex of palladium with a high surface area carbon gave a supported complex which on heating produced a Pd/C catalyst having a uniform distribution of palladium metal particle sizes. The rhodium carbonyls, Rh4(CO) 2 and Rh6(CO) 6, were adsorbed on silica to give... 

Quite a surprising reaction has recently been reported [74]. With a catalyst of palladium metal on carbon in aqueous phase, propene is oxidized with oxygen to give acrylic acid, probably via allyl alcohol in a allylic-type oxidation (for allylic oxidation see Section 3.3.14). In the presence of chloride or oxidants the normal Wacker-type reaction product acetone arises. 

Inspection of PdSA after reaction disclosed a different feature of the dispersion of palladium. The electron microscopic examination detected many crystals of palladium metal, Pd, in the catalyst which contained more than 0.03 meq palladium per gm of SA, but failed to detect amorphous palladium metal, Pd , in the catalyst which contained less than 0.03 meq palladium per gm of SA. 

Donohue D. L. and Petek M. (1991) Isotopic measurements of palladium metal containing protium and deuterium by glow discharge mass spectrometry, Anal Chem 63 740-744. 

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