Showing papers on "Palladium published in 1991"

Kelvin probe force microscopy

Abstract: Measurements of the contact potential difference between different materials have been performed for the first time using scanning force microscopy. The instrument has a high resolution for both the contact potential difference (better than 0.1 mV) and the lateral dimension (<50 nm) and allows the simultaneous imaging of topography and contact potential difference. Images of gold, platinum, and palladium surfaces, taken in air, show a large contrast in the contact potential difference and demonstrate the basic concept.

Catalytic palladium‐based membrane reactors: A review

Abstract: This paper is an extensive review of the literature dealing with the class of catalytic membrane reactors which involves hydrogen permeable membranes made of palladium and palladium alloys. The fundamental factors which affect hydrogen permeability are first discussed. A classification of the many reactions which have been conducted in such reactors at both laboratory and commercial scales is then presented. The various techniques for the preparation of palladium- based membranes are described and the literature on modeling and design of these reactors is also reviewed.

Low-temperature, palladium(II)-catalyzed, solution-phase oxidation of methane to a methanol derivative

Abstract: On propose un mecanisme electrophile plutot qu'un mecanisme radicalaire pour l'oxydation etudiee

Theoretical study of the activation of alkane C-H and C-C bonds by different transition metals

Abstract: The activation of C-H and C-C bonds by different transition metal atoms has been studied using quantum chemical methods including electron correlation. The metals studied are iron, cobalt, nickel, rhodium, and palladium. A general result for all these metals is that the barrier for C-C insertion is found to be 14-20 kcal/mol higher than the barrier for C-H insertion. This can be explained by the difference in directionality between bonds to methyl groups and to hydrogen atoms. The size of the activation barrier is similar among transition metals in the same row but is considerably lower for the second-row metals than for the first-row metals studied here. This latter result follows from the more efficient sd-hybridization obtained for second-row metals, which in turn follows from the more similar size of the nd and (n + 1)s orbitals for these atoms. The differences in the atomic spectra between first- and second-row metals also play a part in making the barrier for second-row metals lower.

Regioselective, palladium-catalyzed hetero- and carboannulation of 1,2-dienes using functionally substituted aryl halides

Abstract: Aryl halides bearing heteroatom- or potential carbanion-containing functionality in the ortho position react regioselectively with 1,2-dienes in the presence of a palladium catalyst and a carbonate base to afford five- and six-membered ring hetero- and carbocycles in high yield

X-ray photoemission and Auger energy shifts in some gold-palladium alloys.

Abstract: La comparaison des resultats avec les previsions de la version de l'atome excite du modele quasiatomique, montre que les corrections de transfert de charge au deplacement de l'energie cinetique d'Auger de l'or sont petites. Les resultats sont aussi coherents avec l'interpretation du deplacement du parametre d'Auger de l'or et avec l'analyse de la contribution de l'electron de valence a l'energie de liaison de cœur de l'or

Low temperature adsorption of ethylene and butadiene on platinum and palladium surfaces: A theoretical study of the diσ/π competition

Abstract: The different low temperature coordination modes of ethylene and butadiene on a platinum (111) face, (110) face and on a palladium (111) face are compared on the basis of extended Huckel calculations. The nature of the chemical interaction between the olefin and the surface is detailed and the electronic factors that govern the coordination mode of the hydrocarbon are underlined. The different surfaces are modelled by a 49 or a 44 atoms cluster. A correction is applied in the calculation in order to minimize the artefact introduced by this cluster representation of an extended surface. For the adsorption, the respective importance of two electrons interactions and four electrons repulsions is the key point for the determination of the preferred mode. The di-σ coordination is more stable on platinum (111) but on the platinum (110) face the π coordination yields the same adsorption energy than the di-σ one. This is roughly the same result for the palladium (111) face. The π mode is there favored by a decrease of the four electrons repulsions caused either by a smaller number of metal neighbours for the surface atom (Pt(110)) or by a reduced radial expansion of the metal orbitals (Pd(111)). This π coordination is associated with a smaller hybridization of the ethylene molecule. The results are extended to the adsorption of butadiene and this allows a qualitative explanation of the better selectivity for butadiene partial hydrogenation on palladium compared with platinum.

Palladium(II) acetate-tert-alkyl isocyanide as a highly efficient catalyst for the inter- and intramolecular bis-silylation of carbon-carbon triple bonds

Abstract: A new catalyst, palladium (II) acetate-tert-alkyl isocyanide, permitted the intermolecular bis-silylation of alkynes by otherwise unreactive disilanes, such as hexamethyldisilane and 1,2-diphenyl-1,1,2,2-tetramethyldisilane, to give bis-silylated alkenes in yields up to 98%. Extension of the reaction of the intramolecular bis-silylation of carbon-carbon triple bonds led to the regioselective formation of cyclic organosilicon compounds in good yield

In situ electrochemical scanning tunneling microscopy of single‐crystal surfaces of Pt(111), Rh(111), and Pd(111) in aqueous sulfuric acid solution

Abstract: In situ electrochemical scanning tunneling microscopy (STM) was applied to single‐crystal platinum(111), rhodium(111), and palladium(111) surfaces in an aqueous sulfuric acid solution. Atomically flat surfaces of Pt(111) are roughened in solutions by the electrochemical oxidation–reduction cycle. It is shown that a single potential cycle causes the formation of many adatoms and very small clusters on the Pt(111) terrace. A steady‐state surface structure can be clearly observed after applying a few potential cycles. The STM image is composed of regularly arrayed islands whose diameter and height are in the ranges of 2–3 and 0.5–0.75 nm, respectively. Atomically flat terrace‐step structures can be also observed on Rh(111) and Pd(111) surfaces. The effect of oxidation–reduction cycle on these surfaces is also discussed.

4.3 – Vinyl Substitutions with Organopalladium Intermediates

Abstract: Organopalladium halides, acetates, triflates and similar derivatives lacking β sp3-bonded hydrogens generally react easily with unhindered alkenes to form new alkenes in which an original, vinyl hydrogen is replaced by the organic group of the palladium reactant. The reactions occur in solution. Other Group VIII metals are ineffective in bringing about this transformation in acceptable yields. In the palladium reaction the organopalladium(II) reactant is reduced to the zero valent state while the halide, acetate, triflate or other, initial anionic ligand is converted into the corresponding acid (equation 1). If alkyl groups are attached directly to the double-bond carbons, double-bond rearrangement may occur and allylically substituted products can be produced. Normally, substitution occurs before any double-bond rearrangement occurs in the reactant alkene and the substituent is placed on one of the initial double-bond carbons (equation 2). Rearrangement of the double bond along a hydrocarbon chain to positions more distant from the substituent than the allylic position is occasionally observed, most often when X is a halogen and the halogen acid produced by the reaction is not neutralized as it is formed by inclusion of a base.

Palladium cross-coupling reactions of aryl fluorosulfonates: an alternative to triflate chemistry

Abstract: A new and efficient electrophilic partner for palladium(0)-catalyzed cross coupling is reported. Aryl fluorosulfonates are readily prepared in high yield by treatment of the appropriate phenol with fluorosulfonate anhydride. The palladium catalyzed coupling reactions of these fluorosulfonates with vinyl- and aryltin reagents, as well as organozinc chlorides, takes place under mild conditions in a regio- and stereoselective manner