Showing papers on "Nickel boride published in 1997"

Microcalorimetric studies of CO and H2 adsorption on nickel, nickel-boride, and nickel-phosphide catalysts

Abstract: Nickel powder, and nickel boride (Ni-B) and nickel phosphide (Ni-P) alloy particles prepared by aqueous chemical reduction, were studied by microcalorimetry. Treatment of Ni powder, Ni-B, and Ni-P in H2 at 623 K was sufficient to produce reduced nickel surfaces for chemisorption. After treatment, these samples were shown by X-ray diffraction to be crystalline. The presence of B and P increased the BET surface areas to values of 8.4 and 19.7 m2/g, respectively, compared to the value of 1.7 m2/g for Ni powder. Microcalorimetric measurements of CO and H2 adsorption performed at 308 K on reduced Ni powder exhibited initial heats of 120 and 85 kJ/mol, respectively. The presence of B decreased the initial heats of CO and H2 adsorption by 20 and 10 kJ/mol, respectively and decreased the saturation uptakes of these molecules per unit surface area. The presence of P decreased the initial heats of CO and H2 adsorption by 30 and 20 kJ/mol, respectively, and also decreased the saturation uptakes of these molecules pe...

A New Coupling Reaction of Alkyl Iodides with Electron Deficient Alkenes Using Nickel Boride (cat.)-Borohydride Exchange Resin in Methanol

Abstract: The radical addition reaction of alkyl iodides with alpha,beta-unsaturated esters, nitriles, and ketones proceeds in moderate to excellent yields (50-95%) using Ni(OAc)(2) (0.05-0.2 equiv)-BER (3-5 equiv) in methanol in 1-9 h at room temperature or at 65 degrees C. Nickel boride on borohydride exchange resin (BER) is a good alternative reagent to tributyltin hydride for the coupling of alkyl iodides with the electron deficient alkenes in methanol. Compared with tributyltin hydride method, this method has an advantage of simple workup, since nickel boride-BER can be removed readily by filtration.

F430 model chemistry. an investigation of nickel complexes as catalysts for the reduction of alkyl halides and methyl coenzyme-m by sodium borohydride

Abstract: The ability of nickel(II) macrocycle and coordination complexes 1−8 to mediate the reductive dehalogenation of cyclohexyl bromide and the CH3−S bond cleavage of methyl CoM by sodium borohydride in diglyme/alcohol, DMF/alcohol, or acetonitrile/alcohol was investigated. Methyl CoM, or CH3SCH2CH2SO3-, is the cofactor that carries the methyl group in the final step of methanogenesis in methanogenic bacteria. Because of the potential for production of the heterogeneous catalyst nickel boride during these reactions, the activities of several nickel salts that afford nickel boride when reduced with borohydride were examined for purposes of comparison. Complexes 1−8 homogeneously catalyze the dehalogenation of cyclohexyl bromide by sodium borohydride. The facility of the reaction varies markedly with the structure of the ligands and the solvent composition. Nickel boride is a moderately active heterogeneous catalyst for the dehalogenation of cyclohexyl bromide and produces small yields of methane from methyl CoM ...

Decomposition of B2H6 on Ni(100)

Abstract: The decomposition of diborane (B2H6) has been studied on the Ni(100) surface by means of thermal desorption spectrometry (TDS), x-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At low gas exposures, decomposed B2H6 on the Ni(100) surface at low temperatures produces several peaks in the H2 TDS spectra. At high gas exposures, B4H10 was observed desorbing from the surface. The TDS spectra and XPS results for low gas exposures indicate that heating to 500 K results in complete decomposition of B2H6, leaving B bonded to the Ni(100) surface. Heating a thick boron film to 700 K results in a significant shift in the B [1s] and Ni [2p3/2] XPS peak positions, indicating that adsorbed B reacts with Ni(100) to form a nickel boride phase. This nickel boride phase is stable on the surface at temperatures up to 900 K. Nickel boride standards (NiB, Ni2B, Ni3B) were used for comparison with the surface nickel boride phase. XPS analysis of the standards led to the determination that the phase forming on the surface at 700 K was Ni2B. As the temperature is increased above 900 K, XPS results indicate that the boron diffuses rapidly into the bulk. It has also been found that high-temperature annealing in the presence of B2H6 results in the formation of a sharp (3×3) LEED pattern produced by 2/9 ML of B on the Ni(100) surface.

Evolution of nickel borides in Ni-Si-B brazed joints of nickel during post-braze heat treatment

Abstract: In brazes of pure nickel with Ni-Si-B filler metals, nickel boride forms in the central region of the brazed gap by solidification, and in the pure nickel base metal along bothjointfaying surfaces by solid state diffusion and precipitation. During subsequent heat treatment, the gap boride gradually decomposes and dissolves, while the lining boride along the joint faying surfaces increases both in size and quantity. This phenomenon is the result of an uphill diffusion of boron from the brazed gap, which remains very much a ternary Ni-Si-B system, into the base metal, which is largely a binary Ni-B system owing to the rapid diffusion of boron ahead of silicon into the base metal. The driving force for such diffusion is the high chemical potential of boron in the gap owing to the presence of silicon, which substantially lowers the solubility of boron in y-Ni. On the other hand, the separation between the opposite lining boride layers increases with brazing time and is controlled by the diffusion of s...

Desulfurisation of phosphonylated thioamides: a new way to functionalised aminophosphonates

Abstract: A new way to functionalised (aminomethyl)- or (aminoethyl)-phosphonates via the desulfurisation by nickel boride of phosphono substituted thioamides [(thiocarbamoyl)- and (thiocarbamoylmethyl)-hosphonates], readily available from phosphono-dithioesters and functionalised amines, is described.

Two-Carbon Homologation of Carboxylic Acids Using Acrylamide as a Radical Trap.

Abstract: Photolysis of O-acyl derivatives of N-hydroxy-2-thiopyridone in the presence of acrylamide in dichloromethane furnished crystalline 2-(pyridine-2-thiyl)-carboxamides. Desulfurization of the latter by nickel boride afforded primary amides in excellent yields. © 1997 Elsevier Science Ltd.

A New Coupling Reaction of Alkyl Iodides with Electron Deficient Alkenes Using Nickel Boride (cat.)-Borohydride Exchange Resin in Methanol.

Abstract: The radical addition reaction of alkyl iodides with alpha,beta-unsaturated esters, nitriles, and ketones proceeds in moderate to excellent yields (50-95%) using Ni(OAc)(2) (0.05-0.2 equiv)-BER (3-5 equiv) in methanol in 1-9 h at room temperature or at 65 degrees C. Nickel boride on borohydride exchange resin (BER) is a good alternative reagent to tributyltin hydride for the coupling of alkyl iodides with the electron deficient alkenes in methanol. Compared with tributyltin hydride method, this method has an advantage of simple workup, since nickel boride-BER can be removed readily by filtration.