A. Percheron-Guégan

Hydrogen adsorption in the nanoporous metal-benzenedicarboxylate M(OH)(O2C–C6H4–CO2)(M = Al3+, Cr3+), MIL-53

Hydrogen adsorption has been studied in the nanoporous metal-benzenedicarboxylate M(OH)(O2C-C6H4-CO2) (M = Al3+, Cr3+); these solids show a hydrogen storage capacity of 3.8 and 3.1 wt.% respectively when loaded at 77 K under 1.6 MPa.

Neutron and X-ray diffraction profile analyses and structure of LaNi5, LaNi5−xAlx and LaNi5−xMnx intermetallics and their hydrides (deuterides)☆

Abstract LaNi 5 , LaNi 5−x Al x , LaNi 5−x Mn x and the deutendes of these intermetallics were investigated by neutron and X-ray diffraction. Nickel is replaced by manganese on both the 2c and 3g sites but by aluminium only on the 3g site. Contrary to previous structure refinements, the deuterides are described by a five-site structural model with the space group P6 mmm . The presence of aluminium prohibits occupation of three of the five interstices found to be occupied in LaNi 5 D 6.6 . Anisotropie line broadening of the activated compounds, which is attributed to particle size and microstrain effects, is reduced by both aluminium and manganese.

Thermodynamic and structural properties of LaNi5−yAly compounds and their related hydrides

Abstract Among the intermetallic compounds able to absorb reversibly large amounts of hydrogen in convenient conditions, LaNi5 presents large possibility of substitution involving a change of the plateau pressure of the related hydrides in a wide range of pressure. The results presented concern the metallurgical, structural and thermodynamic properties of the compounds LaNi5−yAly and their related hydrides: The limit of existence of pseudobinary intermetallic compounds with hexagonal structure P6/mmm is determined (YA1 = 1.25). The lattice parameters and cell volume of the intermetallic compounds are determined in terms of YA1. The enthalpy of formation of LaNi5 and LaNi4Al are measured by a calorimetric method. The lattice parameters and cell volume of the hydride are determined by X-ray diffraction. The study of the absorption and desorption hydrogen isotherms show a decrease of the plateau pressures with the increasing rate of aluminium and decrease of the capacity. The enthalpy and entropy of formation of hydrides are deduced from the study of the change of their equilibrium pressures as a function of temperature. Correlations are established between structural and thermodynamical properties of intermetallic compound of this series and their related hydrides.

Thermodynamic and structural properties of LaNi5−xMnx compounds and their related hydrides

Abstract Metallurgical, structural and thermodynamic properties of LaNi5−xMnx compounds and their related hydrides were determined. The limit of existence of pseudo-binary intermetallic compounds with the hexagonal structure P6/mmm is x = 2.2 at 800 °C. The lattice parameters and the cell volume increase with increasing manganese content, the change being linear up to x = 1.5. The hydrogen absorption and desorption isotherms were determined by an incremental method between 25 and 200 °C. The partial substitution of nickel by manganese involves a large decrease of the plateau pressure which is proportional to the manganese content. Neutron diffraction measurements show that manganese atoms mainly occupy the 3g sites in the LaNi5 structure and that the deutendes retain the same structure in which, although deuterium atoms are widely distributed on interstitial sites, there is a preference for the 6m and 12n sites.

Anisotropic diffraction peak broadening and dislocation substructure in hydrogen-cycled LaNi5 and substitutional derivatives

An erroneous equation and some consequently underestimated values of dislocation densities in the paper by Cerný et al. [J. Appl. Cryst. (2000), 33, 997–1005] are corrected.

Hydrogen cycling induced degradation in LaNi5-type materials

Defect generation in hydrogen cycled LaNi5 and substituted derivatives was studied in previous work by analysis of the X-ray line broadening. In the present work, the pulverization in the same samples is analyzed by granulometric measurements and scanning electron microscopy. Both phenomena correspond to irreversible degradation of the initial intermetallic compounds, as confirmed by first cycle hysteresis presence in pressure composition isotherms. They are analyzed in terms of the lattice expansion occurring at the discrete phase transition between α and β phases and measured by X-ray diffraction. This study shows that, in addition to this latter parameter, the limit of elasticity and the resistance to rupture must be considered.

Electrochemical study of cobalt-free AB5-type hydrogen storage alloys

Abstract Cobalt-free AB5-type hydrogen storage alloys have been examined for the purpose of lowering MH raw material costs. The electrochemical measurements showed that the cobalt-containing LaNi3.55Mn0.4Al0.3Co0.75 alloy had a maximum capacity of 300 mA h/g at a rate of C/6. The capacity decrease was calculated by 5% after 20 cycles at C/6 rate with 100% depth of discharge (DOD). The cobalt-free LaNi3.55Mn0.4Al0.3Fe0.75 alloy showed a very good cycling stability, although the initial capacity ( 250 mA h/g at C/6 rate) was lower than that of the cobalt containing alloy. The capacity decrease of the cobalt-free alloy amounted to 1% after 20 cycles under the same rate and DOD. The mechanism of hydriding/dehydriding reaction on the alloy electrodes was also investigated using a constant potential discharge (CPD) and cyclic voltammetry (CV) techniques. The obtained values of the hydrogen diffusion coefficient DH, were, 2×10−8 and 1.1×10 −10 cm 2 s −1 for LaNi3.55Mn0.4Al0.3Co0.75 and 8×10−9 and 1.0×10 −10 cm 2 s −1 for LaNi3.55Mn0.4Al0.3Fe0.75, by CV and by CPD, respectively.

Improvement of the electrochemical activity of ZrNiCr Laves phase hydride electrodes by secondary phase precipitation

Abstract Within the framework of research on intermetallic compounds for nickel-hydride batteries, we have studied ternary ZrNiCr alloys. In that system, we have measured both the hydride thermodynamic properties by solid-gas reaction and the electrochemical capacities for ternary Laves phases and binary ZrNi intermetallic compounds. A knowledge of the ternary phase diagram has allowed the synthesis of alloys where these two kinds of phase are in equilibrium, at predetermined rates. Study of the electrochemical capacities of these two-phase alloys has shown that the surface modification induced by the precipitation of ZrNi binaries considerably enhances the electrochemical capacity discharged by the Laves phase.

Influence of the support on the catalytic properties of nickel/ceria in carbon monoxide and benzene hydrogenation

Abstract In this work the catalytic properties of nickel supported on various supports (Al 2 O 3 , SiO 2 , CeO 2 ) in syngas conversion are compared. The influence of the temperature of reduction pretreatment was studied. The characterization of the catalysts was performed by temperature programmed reduction, isothermal reduction, CO and H 2 chemisorption, X-ray diffraction, X-ray absorption spectroscopy, magnetization and X-ray photoelectron spectroscopy. The modification of the catalytic properties of Ni/CeO 2 catalysts with reduction pretreatment is correlated to the transformation of the CeO 2 support and to strong interactions between these species and metal particles.

Hydrogenation of carbon monoxide on carbon-supported cobalt rare earth catalysts

A study of carbon monoxide hydrogenation over cobalt—lanthanum and cobalt—cerium catalysts supported on carbon was carried out for the purpose of identifying the effects of rare earth elements. It was observed that these promoters give rise to a 100-fold increase in specific activity and turnover frequency, which is not a result of the inhibition of carbon monoxide or hydrogen adsorption. Moreover, the selectivity for the C2C4 fraction increased from 4% to about 40% and this fraction is essentially olefinic. Such variations in catalytic properties, especially activity, are a strong indication of the formation of new sites. In comparison with the results of recent work, one may conclude that carbon monoxide and hydrogen hydrocondensation needs redox centres that could easily be formed with metal-rare earth catalysts.