L. Schlapbach

Surface effects and the formation of metal hydrides

Abstract Hydrogen is stored atomically in metal hydrides. Dissociative chemisorption and associative desorption are therefore important steps in the hydrogen absorption and desorption processes. We summarize the theoretical and experimental knowledge of hydrogen chemisorption on clean and precovered metal surfaces and correlate it with the techniques for preparing metal hydrides. At the surface of hydride-forming intermetallics, precipitates of d metals and a metallic subsurface are produced by surface segregation and decomposition. The subsurface and the precipitates are able to dissociate H2. Our recent work on the surface analysis of LaNi5, FeTi, Mg2Ni and ErFe2 is reviewed.

Magnetic properties of LaNi5, FeTi, Mg2Ni and their hydrides☆

We measured the magnetic susceptibility x as a function of field and temperature for the intermetallic compounds LaNi5, FeTi and Mg2Ni in the form of bulk material and of activated powder in the hydrogenated and dehydrogenated states. LaNi5, FeTi and Mg2Ni are Pauli paramagnets with bulk susceptibilities of 4.6 × 10−6, 3.4 × 10−6 and 0.9 × 10−6 emu g−1 respectively. The three compounds show segregation of lanthanum, titanium and magnesium respectively in a surface layer and the formation of ferromagnetic particles of nickel or iron. Because the bulk is only weakly magnetic and the specific surface area of the powders is large, the surface precipitates of nickel and iron dominate the magnetism of the powders. We checked these surface decomposition effects by measuring the Curie temperature of the precipitates and found the Curie temperatures of nickel in LaNi5 and Mg2Ni powders and that of iron in FeTi powder. The nickel precipitates are superparamagnetic at room temperature and ferromagnetic at 4.2 K. By fitting the magnetization curve with the Langevin function we calculated the size of the superparamagnetic nickel and iron precipitates (6000 nickel atoms in LaNi5, 2000 iron atoms in FeTi). Hydrogenation reduces the bulk susceptibility of LaNi5, FeTi and Mg2Ni.

Surface analysis of Mg2NiMg, Mg2Ni and Mg2Cu

Abstract The formation of MgH2 directly from magnesium and molecular hydrogen is extremely slow; however, reasonable hydrogenation kinetics have been reported in compounds and alloys of magnesium with nickel, copper and rare earth metals. Using X-ray photoemission spectroscopy and Auger electron spectros-copy, we analysed the chemical states and the concentrations of magnesium, nickel and copper at the surface of the eutectic alloy MgMg2Ni and of the intermetallic compounds Mg2Ni and Mg2Cu. Air-exposed samples are greatly magnesium enriched on their surface, the magnesium being oxidized. Surfaces cleaned in ultrahigh vacuum show segregation of magnesium under the influence of oxygen even at room temperature. The segregated magnesium oxidizes and keeps the nickel and copper in the metallic state. Molecular hydrogen can be dissociated at the metallic nickel or copper particles or at the metallic subsurface. The catalytic activity of Mg2Ni and Mg2Cu in the hydrogenation of magnesium observed by Reilly and Wiswall is therefore due to a surface decomposition which continues the supply of atomic hydrogen.

Deuterium storage in FeTi. Measurement of desorption isotherms and structural studies by means of neutron diffraction

Abstract Activation of pure stoichiometric FeTi turned out to be difficult and deactivation processes due to gas impurities limit the hydrogen storage capability. Isotherms of deuterium desorption are reported for the temperature range T = (25÷70)°C. At room temperature the crystal structures of FeTiD x (x = 0÷1.74) and FeTiH 0.78 D 0.22 were investigated by means of neutron diffraction at D 2 -pressures up to 84 bar. Deuterium absorption in FeTi yields ternary deuterides and is associated with symmetry reduction corresponding to considerable distortions of the CsCl-type metal structure. In FeTiD deuterium occupies octahedral sites with two iron and four titanium atoms as nearest neighbour coordination (space group P222 1 or P 2 c ). Hydrogenation does not change essentially the order parameter of FeTi.

Hydrogen storage in FeTi: Surface segregation and its catalytic effect on hydrogenation and structural studies by means of neutron diffraction

Abstract Surface studies by means of AES and XPS show that during the activation process of FeTi titanium diffuses to the surface and metallic iron clusters are formed. They catalyze the hydrogen absorption and are the reason for the irreversible change of the magnetic properties of FeTi upon hydrogenation. At room temperature the surface segregation is not effective enough to restore the catalytic active surface so that FeTi is deactivated easily in air. Structural studies of FeTi, FeTiD and FeTiD1.74 were performed. The best agreement between observed and calculated intensities of FeTiD is obtained with space groups P2221 (Pmc21) and P2/c. The lattice parameters of the orthorhombic unit cell are a = 2.966 A , b = 4.522 A and c = 4.370 A .

Neutron and X-ray diffraction investigation of deuterium storage in La7Ni3

Abstract Quantitative phase analysis by neutron powder diffractometry yields conclusive evidence for the irreversible decomposition of La7Ni3 into LaD3 and LaNi5 during deuteration at room temperature. Refinement of the LaD3 structure confirms that the D atoms are distributed over the tetrahedral and octahedral interstices of the f.c.c. metal atom arrangement. The D atoms occupying the octahedral interstices are not exactly at the centre but appear to be shifted away from it along different directions. It is suggested that this delocalization results from large anharmonic thermal displacements of the D atoms mainly along 〈111〉 directions. The crystal structure of La7Ni3 was refined both by single-crystal X-ray analysis and by neutron powder diffractometry. The results are compared and discussed in relation to other Th7Fe3-type structures.

Structural phase transitions of FeTi-deuterides

Absorption and desorption isotherms of FeTiDx (0≤x≤1.9) were measured at room temperature. They indicate isotope effects. The dependence of the crystal structure of FeTiDx on D2-pressure (≤153 bar) was investigated at room temperature by means of neutron diffraction on powder samples both in absorption and desorption. The results show that the distribution of the deuterium atoms in the orthorhombic β-phase change from 0.880.12 for the DI/DII sites at x=1.0 (1) to 0.920.45 at x=1.4. This can be interpreted as a transition from a β1-phase of composition FeTiD to a β2-phase of approximate composition (FeTi)3D4. For the last composition Lebsanft (2) has reported a 0.700.70 distribution over the two D-sites. In agreement with (3) the structure of γ-FeTiD (x>1.9) is found to be monoclinic (space group P2m). Associated with considerable lattice expansions deuterium atoms occupy both in β- and γ-phases octahedral sites.

Hydrides of LaNi and CeNi intermetallic compounds

Abstract The hydriding characteristics of La7Ni3, LaNi and Ce7Ni3 have been investigated. These compounds form the stable hydrides La7Ni3H19.3, LaNiH3.85 and Ce7Ni3H19.2. In structural and magnetic investigations La7Ni3is found to decompose into LaH3 and LaNi5 on rapid hydrogenation. On desorption at elevated temperatures the original La7Ni3 structure is reformed. An analogous decomposition is found in Ce7Ni3. The enthalpies of formation ΔH have been measured calorimetrically for the hydrides of LaNi compounds. ΔH varies linearly with the LaNi composition, independently of the various compound structures and of structural transformations on hydrogenation. The experimental values of ΔH are described by a phenomenological model. Despite their high hydrogen content, these hydrides are not suitable for use as hydrogen storage materials because of their high stability.

The formation of superparamagnetic metallic Ni and Fe particles at the surface of intermetallics by surface segregation

By means of magnetic investigations and surface analysis (XPS) of LaNi5, TiFe, Mg2Ni and ErFe2 we show that many intermetallic compounds disproportionate in the near surface region by chemisorption induced surface segregation. Superparamagnetic metallic particles of Ni and Fe are formed on the metallic subsurface. They have excellent catalytic properties for hydrogenation reactions.

Magnetism and hydrogen storage in LaNi5, FeTi and Mg2Ni

Abstract The intermetallic compounds LaNi5, FeTi and Mg2Ni are Pauli paramagnets with the susceptibilities 4.6, 3 to 4, and 0.9 × 10-6 emu/g, resp. Hydrogen absorption reduces the bulk susceptibility of LaNi5 and Mg2Ni. All 3 compounds decompose at the surface forming superparamagnetic Ni and Fe particles which dominate the magnetism of powders of these compounds.