Intermetallic compounds LaNis

Guenee et al. [169] synthesized by ingot metallurgy ternary intermetallic compounds LaNi Mg and NdNi Mg having the cubic MgCu Sn crystal structure. They can absorb reversibly up to four hydrogen atoms per formula unit at 7-8 bar and 50°C. The hydrides are stable at room temperature but desorb quite rapidly at 80°C in vacuum. In air, they decompose by catalytic water formation. 

There is sensitive La in the alloys based on intermetallic compound LaNi 5. If discussing production of large quantities of the said materials by metal component... 

Applying both technologies we ve produced large industrial lots of material on the basis of intermetallic compound LaNi,-. 

Imamura and Wallace (1980) used various intermetallic compounds (LaNis, CeNis, ThNis, TI1C05) as precursors. After decomposition under oxygen at 350°C they obtained catalysts which were clearly more active than the corresponding classical Ni catalysts. They suggested that geometrical effects were responsible for such a behavior. 

The intermetallic compound LaNi is well-known as a suitable material for hydrogen storage. Pseudobinary compounds with the same CaCu -type structure can be easily formed by partial substitution of nickel. Depending upon the rate and the nature of substitution, the hydriding properties (stability, maximum H-content, interstitial sites occupied by hydrogen. ..) are modified (1). 

Fig. 29. Synergetic effects with intermetallic compounds. Hydrogen evolution in 1 M KOH at 30 °C on (1) Ni, (2) La, and (3) LaNis. (MOE = mercury oxide electrode). Adapted from ref. 226, by permission of Elsevier Sequoia.

In most cases, intermetallic compounds are built by alloying a metal which easily forms stable hydrides (A) and another element which does not form stable hydrides (B). The intermetallics thus formed could then be grouped according to their stoichiometry such as AB5 (LaNis, CaNis), AB2 (ZrMn2, ZrV2), AB (TiFe) and A2B (Mg2Ni). 

Before closing this section it should be pointed out that the formation of most hydrides of intermetallic compounds is metastable vith respect to disproportionation [35]. Again, taking LaNis as an example, the disproportionation reaction ... 

This subject will be treated in details elsewhere in this issue and Lagarde and Dexpert have published an excellent review about this subject The wish here is to mention an EXAFS characterization of intermetallic compounds before and after various chemical reactions carbon oxide hydrogenation, ethane hydrogenolysis and propane hydrogenation LaNij, LaNi Fe and LaNijMn have been measured by comparison with pure Ni. The Fourier transforms for these compounds are given in Fig. 17. 

Several intermetallic compounds were studied LaNi5, LaNi, LaCo5, SmCo5, Sm2Co7 and SmCo2- It turned out that LaNi and SmCo2 were by far the most convenient... 

Transformed rare earth and actinide intermetallic compounds are shown to be very active as catalysts for the synthesis of hydrocarbons from CO2 and hydrogen. Transformed LaNis and ThNis the most active of the materials studied they have a turnover number for CH formation of 2.7 and 4.7 X 10 sec at 205°C, respectively, compared with I X 10 sec for commercial silica-supported nickel catalysts. Nickel intermetallics and CeFe2 show high selectivity for CHj formation. ThFcs shows substantial formation of C2H6 (15%) as well as CHi,. The catalysts are transformed extensively during the experiment into transition metal supported on rare earth or actinide oxide. Those mixtures are much more active than supported catalysts formed by conventional wet chemical means. 

Multicomponent metallic hydrogenation catalysts, based on intermetallic compounds (IMC) of rare-earth elements with nickel, copper, cobalt, and other bimetallic systems. Most studies were devoted to two structural systems LnMs and LnMs, where Ln = La, Sm, Gd, Ce, Pr, and Nd and M = Ni (see Klabunovskii, Konenko s group 183,251,252 Compaiison of LnNis catalysts with Ni catalysts supported on oxides of Ln, show higher activities of the IMC s and their hydrides in hydrogenation of propene (100°C, 1 bar), where LaNis proved to be the most active catalyst... 

Lanthanide metals are interesting as starting materials to prepare various divalent or trivalent lanthanide derivatives by reaction with or-ganic compounds, this point will not be detailed here and information can be found in ref. (10). It is well known that lanthanide metals can give intermetallic compounds with various metals (Co, Ni, Mg,..) which are able to absorb large quantities of hydrogen. Recently some of these hydride alloys were found to be able to reduce alkynes, mono and di-sub-stituted alkenes to saturated hydrocarbons (12). Some reactions performed with LaNi H at 0 C and then room temperature in THF-MeOH (2 3) are listed below ... 

Two types of metal hydrides electrodes, comprising the AB, and AB2 classes of intermetallic compounds, are currently of interest. The AB, alloys have the hexagonal CaCu, structure where the A component comprises one or more rare earth elements and B consists of Ni, or another transition metal, or a transition metal combined with other metals, The paradigm compound of this class is LaNi, which has been well investigated because of its utility in conventional hydrogen-storage applications. Unfortunately LaNi, is too costly, too unstable, and too corrosion— sensitive for use as a battery electrode. Thus commercial AB, electrodes use mischmetal, a low-cost combination of rare earth elements, as a substitute for La. The B, component remains primarily Ni but is substituted in part with Co, Mn, Al, etc. The partial substitution of Ni increases the thermodynamic stability of the hydride phase fl2] and corrosion resistance. Such an alloy is commonly written as MmB, where Mm represents the mischmetal component. The compositions of normal and cerium-free mischmetal are given in Table 2. 

A surface segregation model (Schlapbach et al, 1980) based on the analysis of surface properties by means of photoelectron spectroscopy and magnetic susceptibility measurements, very successfully explains the great reactivity of hydride-forming intermetallic compounds AB (e.g, LaNi ). Selective oxidation and lower surface energy of the electropositive component A (La) induces a surface segregation (Fig.12). 

Very few results on the adsorption of H on UHV clean surfaces of intermetallic compounds and ordered or disordered alloys are known. The surface electronic structure of e,g, FeTi and LaNi are essentially those of a d-transition metal with a high density of states at E, Accordingly,... 

Here AH , d/ffj represent the monovacancy energies of R and M atoms in the pure metals R and M, and Fr and are the atomic volumes of these metals. The quantities/m and/r are the atomic fractions of M and R atoms surrounding a given M site in RM . All these quantities are listed by Miedema for practically all metals, so that A Hu can be calculated for given metal combinations RM as a function of concentration n. In fig. 5 the vacancy energies at Ni sites in various LaNi intermetallics have been calculated and plotted as a function of Ni concentration. Note that d/ffj in La-rich compounds is considerably less than in pure Ni metal. The initial concentration independence of results from the fact that in intermetallic compounds the Ni atoms try to surround themselves with an optimal number of the larger La atoms. The broken line represents the results for a solid solution, realized more or less in amorphous La-Ni alloys. 

Hysteresis effects are extremely pronounced in hydrides based on intermetallic compounds containing Ce (van Vucht et al., 1970 Huang et al., 1978 Dayan et al., 1980). Here one has to take account of the fact that the valence state of the Ce ions in the hydride can be different from that in the starting material. A reduction in hysteresis was found upon substitution of elements of group 3 or 4 of the periodic table for Ni in LaNis (Mendelsohn et al., 1979). 

Guenee and Yvon [217] synthesized LaNi MOj intermetallic with a YNi Alj-type crystal structure. However, its hydrogen storage properties are no better than those of LaNij-type because it could barely absorb -1.4 wt.%Hj forming LaNi MnjH and its equilibrium temperature at 1 bar H, is about 60°C. No attempt of nanostructuring the compound by ball milling was made. 

The Aoki group [218] has been developing intermetallic alloys based on a CaSi compound that is alloyed with Si, Al, Ge, Mg, and Sr. However, the alloys cannot compete with the LaNi -type or even TiFe because they absorb only slightly more than 2 wt.%H at 100°C and desorb at 200°C. 

In 1970 van Vucht et al. [41], in the Netherlands, discovered practical hydrides such as LaNis, and in 1974 Reilly and Wiswall, in the United States, discovered Fe-Ti hydrides [42]. In LaNis and other rare earth compounds such as YNis, NdNis and others, the plateau pressures are affected by the A-atom (example La atom) substitution. This led to numerous studies around the world and development of practical hydride work really started. There have been several excellent reviews over the years, among them notably references [2, 26,43-68]. Other notable papers on intermetallics in general, are those of Anderson and Maeland [63], Lou et al. [65, 66, 69], Bowman et al. [70], Cerney et al. [71], Percheron-Guegan and co-workers [29, 72], and Latroche et al. [73]. More recently an internet database has been set by Sandrock and Thomas (lEA/DOE/Sandia National Laboratory) [23], Jai-Young Lee and co-workers [74, 75], Uchida et al. [76], Yvon and Fichner (structural properties) [77], Gupta and Schlapbach (electronic properties) [78], and Flanagan and Oates (thermodynamic properties) [79]. 

---