Showing papers on "Palladium published in 1987"

A membrane reactor using palladium

Abstract: La deshydrogenation du cyclohexane en benzene et hydrogene est une reaction reversible; pour obtenir une reaction complete, on utilise une membrane de palladium pour separer l'hydrogene au fur et a mesure de sa formation

Palladium- or nickel-catalyzed reactions of alkenylmetals with unsaturated organic halides as a selective route to arylated alkenes and conjugated dienes: scope, limitations, and mechanism

Abstract: Stereo- and regiodefined alkenylmetals containing Al, Zr, and Zn react with aryl and alkenyl iodides and bromides in the presence of catalytic amounts of Pd or Ni complexes containing phosphine ligands, such as PPh/sub 3/, to give the corresponding cross-coupled products. Palladium catalysts permit nearly 100% stereospecificity in both alkenyl-aryl and alkenyl-alkenyl coupling reactions, whereas nickel catalysts lead to partial stereochemical scrambling in the alkenyl-alkenyl coupling. Although many other metals including Li, Mg, Cd, Hg, B, Si, Sn, Ti, and Ce were also used, the results were inferior to those obtained with Al, Zr, and Zn under the conditions used in the present study. The turnover numbers for the palladium-catalyzed reactions of PhI with (E)-1-octenylmetals containing Al, Zr, and Zn were 2, 3, and > 2000 mmol of (E)-1-octenylbenzene (8) per mmol of Pd(PPh/sub 3/)/sub 4/ per hour at room temperature, respectively. The stoichiometric reaction of PhPd(PPh/sub 3/)/sub 2/I (6) with 1.2 equiv of (E)-1-octenylzinc chloride (7) in a 2:1 mixture of CD/sub 2/Cl/sub 2/ and THF was examined in detail. The reaction follows second-order kinetics (k/sub 2/ = 2.9 L/(mol.min) at 0/sup 0/C) to give 8 without the buildup of any intermediate.

Synthesis of functionalized styrenes via palladium-catalyzed coupling of aryl bromides with vinyl tin reagents

Abstract: Copulation entre le (tributylvinyl) stannane et des bromoethers, bromoaldehydes, bromocetones et des composes dibromes benzeniques

Extending surface-enhanced Raman spectroscopy to transition-metal surfaces: carbon monoxide adsorption and electrooxidation on platinum- and palladium-coated gold electrodes

Abstract: Thin (ca. one to three monolayers) films of platinum and palladium electrodeposited on electrochemically roughened gold are observed to yield surface-enhanced Raman (SER) spectra for adsorbed carbon monoxide. The major vibrational band(s) on these surfaces are diagnosed from their frequencies as arising from C-O stretching vibrations, nu/sub CO/ bound to the transition-metal overlayers rather than to residual gold sites. The observed SFR nu/sub CO/ frequencies are closely similar to (within ca. 10 cm/sup -1/ of) those obtained for these systems from potential-difference infrared (PDIR) spectra. The major SERS and PDIR nu/sub CO/ features for the platinum and palladium surfaces appear at 2060-2090 and 1965-1985 cm/sup -1/, respectively, consistent with the presence of terminal and bridging CO on these two electrodes. The infrared as well as electrochemical properties of these systems are closely similar to those for the corresponding polycrystalline bulk electrodes. A difference between the SER- and IR-active adsorbed CO, however, is that the former undergoes electrooxidation on both surfaces at 0.2-0.3 V higher overpotentials than the latter form. Examination of the potential-dependent SERS bands for metal oxide vibrations, nu/sub PtO/, on the platinum surface shows that the electrooxidation potential for the SERS-active adsorbed CO coincides with that for themore » appearance of the nu/sub PtO/ band. Some broader implications to the utilization of SERS for examining transition-metal surfaces are pointed out.« less

Process for the oxidation of di-, tri-, oligo- and polysaccharides into polyhydroxycarboxylic acids, the catalyst used and the products so obtained

Abstract: Process for the selective oxidation of di-, tri-, oligo- and polysaccharides comprising a reducing terminal function of the aldose type into polyhydroxycarboxylic acids, wherein the oxidation is carried out in an alkaline medium by means of an oxygen-containing gas, in the presence of a noble metal based catalyst selected from the group constituted by palladium, platinum, rhodium and osmium and fixed on an inert support, said catalyst being "doped" with one or several metals, or promoters, of Groups IV, V or VI of the Periodic Table.

Laser direct‐write metallization in thin palladium acetate films

Abstract: Micron‐scale palladium lines have been produced by a photothermal laser direct‐write process in thin palladium acetate films. The range of observed structures relates to the complex thermal profiles generated by coupling of the incident laser radiation with the exothermic heat of reaction. Surprisingly, the chemical composition of these features does not vary significantly as a function of laser power and scan speed. Rather, deviation of the electrical resistivities of these features from that of pure palladium results from porosity in the lines.

Carbon-hydrogen bond activation by electrophilic transition-metal compounds. Palladium(II)-mediated oxidation of arenes and alkanes including methane

Abstract: Oxydation du p-Xylene, du methane et de l'adamantane en ester de methyl-4benzyle, methyle et adamantyle-1 de l'acide trifluoroacetique

Thin‐film palladium and silver alloys and layers for metal‐insulator‐semiconductor sensors

Abstract: The addition of Ag to Pd in the gate metal of a metal‐insulator‐semiconductor gas sensing diode can improve the performance and change the selectivity of the sensors for a variety of reactions. Data on the response of diodes with 12 different ratios of Ag to Pd in alloys and layers of Pd and Ag to hydrogen and other gases are reported. Diodes with as much as 32% Ag respond very well to H2 gas and the films are much more durable to high hydrogen exposure than pure Pd films. Improvements in the rate of response and aging behavior are found for certain Ag combinations; others give poorer performance. The presence of Ag on the surface changes the catalytic activity in some cases and examples of H2 mixed with O2 and/or NO2, propylene oxide, ethylene, and formic acid are given. Such selectivity forms the basis for miniature chemical sensor arrays which could analyze complex gas mixtures.

Palladium-catalyzed decarboxylation-allylation of allylic esters of α-substituted β-keto carboxylic, malonic, cyanoacetic, and nitroacetic acids

Abstract: La decarboxylation-allylation conduit a des cetones homoallyliques pour des β-cetoesters, a des esters allyliques pour des diesters, a des nitriles homoallyliques pour des cyanoesters

Investigations of a reduced palladium chemical modifier for graphite furnace atomic absorption spectrometry

Abstract: Palladium is a very useful chemical modifier for graphite furnace atomic absorption spectrometry. It can be used to stabilise many elements several hundred degrees higher than is possible with current methods. Its performance as a modifier is strongly affected by the sample matrix. The addition of a reducing agent provides for more consistent performance. The main purpose of the reducing agent is to “modify” the form of the palladium, guaranteeing that palladium is reduced to the metal early in the temperature programme. Differences were seen in the behaviour of the palladium modifier depending on the reduction method used. A comparison of palladium modifier methods in spike recovery studies of thallium was carried out to investigate differences between the reduction methods. It was suspected that differences in the physical form of palladium contributed to the variability in results. Scanning electron micrographs were used to investigate the physical form of palladium on the graphite surface. A method using the palladium modifier for the determination of selenium was compared with a commonly used procedure in spike recovery studies.

Palladium Catalyzed Hydrostannylation and Hydrogermylation of Acetylenes

Abstract: Treatment of acetylenes with Ph3SnH or Ph3GeH in the presence of a catalytic amount of Pd(PPh3)4 provides the corresponding alkenyltriphenylstannanes or alkenyltriphenylgermanes in good yields.

New method for the classification of nucleophiles in the palladium-catalyzed substitution of allylic acetates

Abstract: Les reactions avec divers nucleophiles ont lieu avec les deux composes suivants: l'acetate du cyclopentene-2ol et l'acetate du methano-4,7 indenol-1 (hexahydro-«3a,4,5,6,7,7a»)

Preparative and 1H NMR spectroscopic studies on palladium(II) and platinum(II) quinoline-8-carbaldehyde (1) complexes. X-ray structures of the cyclometalated acyl complex PdCl(C(O)C9H6N)(PPh3)•PPh3 and trans-PtCl2(1)(PEt3)

Abstract: Les deux composes du titre cristallisent dans le systeme monoclinique, groupe d'espace P2 1 /c et leur structure est affinee jusqu'a R=0,058 pour le complexe de palladium et jusqu'a R=0,045 pour le complexe de platine

Vibrational electron energy loss spectroscopic study of benzene, toluene, and pyridine adsorbed on Pd(111) at 180 K

Abstract: The vibrational spectra of benzene, toluene, and pyridine adsorbed on Pd(111) have been measured by using the technique of electron energy loss spectroscopy (EELS). These three aromatics adsorb associatively at T = 180 K on this surface. At this temperature these molecules weakly bond to the surface with their ring planes oriented parallel to the surface plane. Only small shifts from liquid-phase frequencies are observed. At 310 K the bonding of pyridine to the surface appears to be predominately through the nitrogen lone pair. In addition, the authors compare the adsorption of these molecules on palladium to that on platinum, where there is a greater adsorbate-metal interaction.