Nickel precursor

Ni(III) complexes, 8, 118—119 preparation, 8, 63 via transmetallation, 8, 69—70 Nickel-porphyrin complexes, with platinum(II), 8, 477 (j-Nickel precursors... 

Table 2. Deposition Conditions that Have Been Used for Different Nickel Precursors... 

Some a-Al203-based filter discs (Schumacher, Germany) were vacuum impregnated with a solution containing appropriate amounts of nickel nitrate and urea. After the excess solution was drained off, the discs were placed in a closed vessel and kept at 90 °C for a certain period, resulting in precipitation of nickel precursor by the slow hydrolysis of urea in the pores of the discs. After reaction, the filter discs were dried at 110 °C for a few hours and calcined at 450 °C for 4 h. Then the nickel-modified ceramic filter discs were obtained. For the conventional impregnation method, other steps and experimental conditions were identical to the preparation procedure with the urea method except for the absence of urea in the impregnation solution. 

In the deposition-precipitation method with urea, experimental parameters such as the reaction temperature, the reaction time and the concentrations of urea and Ni in the impregnation solution determine the fixation degree of nickel, which is the amount of nickel precursor fixed by precipitation on the support before drying relative to the maximum amount that can be deposited. At present, there is no general agreement in the literature [9, 13-14] about the choice of these experimental parameters. So, it is necessary to investigate the urea method first, to determine adequate values of reaction time and urea/nickel molar ratio for the impregnation of the ceramic substrates. 

The study of the influence of the reaction time for urea decomposition and the urea concentration in the urea method was performed on a-Al203 powder rather than a ceramic filter disc since the supply of the ceramic discs was limited. Experimental details have been reported elsewhere [11], To investigate the role of the reaction time, the urea/nickel molar ratio was fixed at a value of 1.3. The results are shown in Fig. 1. The precipitation amount of nickel precursor and the suspension pH both increased with reaction time. The amount of nickel precipitate sharply increased during the first 6 h reaction time and increased slowly after 6 h. This suggested that at least 6 h reaction time was needed to fix a reasonable amount of precursor on the support during the wet stage of the urea method. The nickel precipitate was identified By analysis as Ni3(N03)2(0H)4 in all cases. 

Fig. 1. Precipitation amount of the nickel precursor and the suspension pH as a function of reaction time (urea/nickel molar ratio 1.3)...

Despite the linear relation between the composition of the feedstock and that of the deposited flhns, it can be noted that the concentration of nickel doping in the deposited films was lower than that of the liquid feedstock. This effect might be justified either by a selective depletion of the nickel precursor during the transport or by its lower sticking coefficient on the surface of the substrate at the deposition temperature. The IR analyses of the deposited NixCo3.x04 (0 < x < 1) films, not shown, revealed their spinel structure, and a shift of the characteristic vibration bands indicated the incorporation of nickel into the spinel lattice. Further evidence about the controlled incorporation of nickel into the lattice of cobalt oxide can be drawn from conductivity measurements. Fig. 5b shows a substantial increase in conductivity as a function of the concentration of nickel incorporated into the film. The formation of a secondary phase should be expressed by a discontinuity, which is clearly absent in the investigated range of conditions. 

These reactions are likely to occur by oxidative addition through a radical mechanism, as evidenced by the loss of stereochemistry of the starting alkyl halide during the coupling process (Equation 19.14b). Despite the radical mechanism, some reactions of benzylic electrophiles have been conducted enantioselectively (Equation 19.14c). Even reactions of alkylboron reagents witti secondary alkyl halides catalyzed by nickel complexes have now been reported. These reactions were conducted with nickel precursors in combination with trflns-l,2-cyclohexanediamine (Equation 19.14d). ... 

Supported nickel nanoparticles with well-defined morphologies (eombination of icosahedra, rods, cubes, polyhedra) were obtained through optimized syntheses in presence of a nickel precursor, a surfactant, a strong reducer and the alununa support. Prepared catalysts are active and selective in the hydrogenation reaction of a styrene/ isoprene mixture. The modification of the exposed actives sites due to the well defined morphologies induces a modification of the adsorption properties of the reactants, thus leading to an improvement of the selectivity when compared to a conventional catalyst. 

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