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International audience; Unlike the conversion of monosaccharides to the corresponding polyols, the production of maltitol by hydrogenation of maltose has been seldom investigated in the literature, despite its industrial importance. Monometallic Ni catalysts are known for their lack of stability, and the objective of the present paper is to determine through a kinetic study, to what extent a Ni-Fe/SiO 2 bimetallic catalyst would outperform a Ni/SiO 2 catalyst in the aqueous phase hydrogenation of maltose, as they have been reported to do for monosaccharides. The effect of reaction parameters (T = 80-150 °C, P H2 = 20-40 bar, maltose mass fraction in water = 4.4-17.5 wt.%) on activity, selectivity, and stability was examined. In all cases, maltitol was the major product, with a carbon balance higher than 98%, but maltose hydrolysis to glucose occurred in the upper range of temperature. In order to preserve both the catalyst selectivity and stability, a temperature of 80 °C was selected for the kinetic study. A first order model including an inhibiting term based on maltose concentration could fit the evolution of the conversion of maltose as a function of time. The adsorption constant of maltose and the apparent hydrogenation rate constant for the Ni-Fe catalyst were both larger by a factor 2 to 3 compared with the Ni catalyst, indicating a stronger interaction of maltose with the Ni-Fe surface. Another major difference was a reaction order of 0.5 with respect to the hydrogen pressure on Ni-Fe/SiO 2 compared with a near zero-order on Ni/SiO 2 , stressing significant differences in coverage of the bimetallic surface. The activity of the Ni-Fe catalyst remained constant for three runs of reaction without major structural changes, while the Ni catalyst deactivated by transforming to a phyllosilicate phase. As far as activity, selectivity and stability are concerned; Ni-Fe/SiO 2 appeared as a better suited catalyst than Ni/SiO 2 for the aqueous phase hydrogenation of maltose at 80 °C, with a more pronounced benefit than formerly reported for xylose on the same catalysts.

Ni (15 wt%)/Al 2 O 3 catalysts prepared by decomposition of supported Ni(II)–ethylenediamine complexes in inert atmosphere, followed by post-treatment in hydrogen, are able to catalyze carbon monoxide hydrogenation at temperatures lower than for a catalyst prepared from nickel nitrate and containing a fraction of surface nickel aluminate. Unlike the latter, the catalyst prepared from Ni(II)–en complexes exhibits mostly surface metallic sites and is more stable under reaction despite the absence of nickel aluminate. Deactivation occurs during the reaction, probably by carbon deposition, but less strongly than for an ex-nickel nitrate catalyst.

Nanoparticles of Co–Ni alloys were prepared by thermal decomposition of layered double hydroxides (LDH) containing co-intercalated complexes of two metals. The precursor compounds [LiAl2(OH)6]2{Ni1−xCox(EDTA)} · 4H2O (where EDTA is ethylenediaminetetraacetate) obtained by anion exchange from [LiAl2(OH)6]Cl · 1.5H2O were treated at 450 °C in vacuum. The microstructure of the thermal products was studied using local (TEM, SAED, EDX) as well as integral (XRD, SAXS) methods. It was found that Ni–Co alloy nanoparticles are formed uniformly dispersed in the carbonized matrix of lamellar mixed oxides, and the increase of the relative cobalt content in Ni1−xCox from x = 0 to 0.86 leads to gradual enlargement of the average alloy particle size from 9 to 16 nm. According to magnetic measurements by SQUID magnetometry and FMR, all alloys, except Ni0.14Co0.86, are superparamagnetic at room temperature. The results on particle size obtained by different techniques are in good agreement. The variation of nanoparticle size via chemical composition in alloys and the use of LDH intercalation matrices is a promising way to synthesize nanocomposites with target properties.

Hydration of gamma-Al2O3 is often reported to occur via the superficial transformation of the alumina surface into aluminum hydroxide-like layers. However, very little evidence has been given so far to support this hypothesis. It is demonstrated here by X-ray diffraction, TEM, electron diffraction, and solubility studies that a second process of hydration takes place that involves the dissolution of alumina and subsequent precipitation of well-shaped Al(OH)3 particles from supersaturated alumina aqueous solution. This process can be observed on a macroscopic scale (XRD, TEM) for any pH5, provided that the contact time between alumina and water exceeds 10 h. The least thermodynamically stable phase of aluminum hydroxide, bayerite, becomes favored compared with gibbsite when the pH of the solution is increased. It is assumed that the rate of formation of bayerite germs is greater than that of gibbsite due to variations in aluminum speciation in solution as a function of pH.

The decomposition in flowing argon of the neutral complex [Ni II (glycinate) 2 (H 2 O) 2 ] leads to a mixture of face-centered cubic (fcc) and hexagonal close-packed (hcp) metallic nickel. The latter is the main phase when the Ni(II) complex is supported on alumina. Unlike most hexagonal Ni phases described earlier, and similar to hexagonal Ni 3 C, the unit cell parameters ( a = 0.2493 and c = 0.4084 nm ) lead to Ni–Ni distances equal to those encountered in fcc Ni. TEM shows that the nanoparticles are protected by graphite layers, whose elimination by heating in hydrogen results in transformation to the fcc phase and crystal growth. Magnetic measurements provide evidence of the coexistence of superparamagnetic and ferromagnetic nanoparticles. This result is in line with the broad size distribution observed by TEM and is interpreted on the basis of the metallic character of hcp Ni particles.