Intermetallic compound decomposition

Hydrocarbon Synthesis Using Catalysts Formed by Intermetallic Compound Decomposition... 

Catalysts formed by intermetallic compound decomposition show impressive resistance to H2S. Results obtained are shown in Figure 2. It is to be noted that decomposed ThNis is more resistant to poisoning than decomposed ZrNis or the commercial supported nickel catalyst. It is not clear at this time what factors produce these diflFerences. Perhaps the metallic area was smaller for the kieselguhr-supported material it was not determined. The metallic areas of the two decomposed inter-metallics were established and were comparable. 

Figure 3.32 shows XRD patterns of (MgH -i-LiAlH ) composites after DSC testing up to 500°C. The primary phases present are Mg and Al. Peaks of MgO and (LiOH) HjO arise from the exposure of Mg and Li (or possibly even some retained LiH) to the environment during XRD tests. Apparently, XRD phase analysis indicates that a nearly full decomposition of original MgH and LiAlH hydride phases has occurred to the elements during a DSC experiment. In addition, no diffraction peaks of any intermetallic compound are observed in those XRD patterns. That means that no intermetallic compound was formed upon thermal decomposition of composites in DSC. Therefore, the mechanism of destabilization through the formation of an intermediate intermetallic phases proposed by Vajo et al. [196-198] and discussed in the beginning of this section seems to be ruled out of hand. 

Other methods for the preparation of elemental Am, mostly variations on the above two, have been studied (1, 23, 63). Thermal decomposition of the intermetallic compound PtjAm has also been used to prepare Am metal (36, 82,110). 

Two other methods have been used successfully to prepare very pure Cm metal. A rather unique one is thermal decomposition of the intermetallic compound PtjCm produced by hydrogen reduction of curium oxide in the presence of Pt (36, 82). The second method, the method of choice for gram-scale preparations of very pure Cm metal, involves reduction of curium oxide with Th metal (8, 83) in an apparatus... 

Reaction 5.45 is at least partly hypothetical. Evidence that the Cl does react with the Na component of the alanate to form NaCl was found by means of X-ray diffraction (XRD), but the final form of the Ti catalyst is not clear [68]. Ti is probably metallic in the form of an alloy or intermetallic compound (e.g. with Al) rather than elemental. Another possibility is that the transition metal dopant (e.g. Ti) actually does not act as a classic surface catalyst on NaAlH4, but rather enters the entire Na sublattice as a variable valence species to produce vacancies and lattice distortions, thus aiding the necessary short-range diffusion of Na and Al atoms [69]. Ti, derived from the decomposition of TiCU during ball-milling, seems to also promote the decomposition of LiAlH4 and the release of H2 [70]. In order to understand the role of the catalyst, Sandrock et al. performed detailed desorption kinetics studies (forward reactions, both steps, of the reaction) as a function of temperature and catalyst level [71] (Figure 5.39). 

In some cases, absorption of hydrogen by intermetallic compounds causes decomposition as indicated by Reaction 3. Decomposition also can lead to the formation of a new intermetallic compound, as is observed when Mg2Cu reacts with hydrogen (6) in Reaction 4. Reactions 3 and 4 take place at elevated temperatures where diffusion of the metal atoms becomes possible. 

It is crucial to discover the relationship between chemical compositions and hydrogen decomposition pressures of the AB5 compounds. Intermetallic compounds of lanthanide and transition metals form an interesting class of structures. The AB5 series crystallize in the hexagonal CaCus (P6/mmm) structure (see Figure 1). Generally, radius ratios (raAb) greater than 1.30 form the CaCus-type... 

The intermetallic compound Sm2(Fe0 7Co0.3)i7C0.3 has a Th2Zn)7 structure and possesses uniaxial anisotropy along the c-axis. It can be sintered at around 1300 K in Ar without decomposition. 

In the work of CNF synthesis by decomposition of hydrocarbon gases over intermetallic compounds of La and Ni, made by us earlier, it was shown, that the particles of metal nickel can be formed directly during pyrolysis reaction [9]. 

In pure titanium, the crystal structure is dose-packed hexagonal (a) up to 882°C and body-centered cubic (p) to the melting point. The addition of alloying dements alters the a—p transformation temperature. Elements that raise the transformation temperature are called a-stabilizers those that depress the transformation temperature, p-stabilizers the latter are divided into p-isomorphous and p-eutectoid types. The p-isomorphous elements have limited a-solubility and increasing additions of these dements progressively depresses the transformation temperature. The p-eutectoid elements have restricted p-solubility and form intermetallic compounds by eutectoid decomposition of the p-phase. The binary phase diagram illustrating these three types of alloy... 

For metals promoting other metals, an interesting case was studied by Hurst and Rideal.2 In the combustion of mixtures of hydrogen and carbon monoxide, using copper as the basic catalyst the ratio of the gases burnt depends on the temperature, and also on the amount of small additions of palladium made to the copper. The proportion of carbon monoxide burnt is increased by addition of palladium, a maximum proportion of carbon monoxide being burnt when 0-2 per cent, of palladium is. present. With further amounts of palladium, the ratio CO H2 burnt falls off slowly until, with 5 per cent, palladium, it is nearly the same as with pure copper. This effect of palladium is ascribed to the introduction of a new type of surface, the line of contact between palladium and copper, though the proof that this is the cause of promotion is perhaps not complete. Mit-tasch and others,3 in elaborate studies of the promotion of various metal catalysts, particularly molybdenum, for the synthesis or decomposition of ammonia, concluded that the formation of intermetallic compounds... 

Gerard, N. and Ono, S. (1992) Hydride formation and decomposition kinetics, in Hydrogen in Intermetallic Compounds II, vol. 67 (ed. L. Schlapbach), Springer-Verlag, Berlin, Chapter 4. 

The crystal structures of all complex aluminum hydrides are built up by [AlH4] tetrahedra or [AlHg] octahedral units. These building units can be either isolated, as for example in NaAlH4, or they can form more complex structures such as chainlike structures, as for CaAlHs. The decomposition of alkaline earth aluminum hydrides proceeds via hydrides to intermetallic compounds whereas alkali metal alanates decompose via an intermediate hexahydride structure to the corresponding hydride. Table 5.2 summarizes the physical data of selected complex aluminum hydrides. 

Figure 3 also shows conversion and selectivity to C2H4 with Sn added to the Pt catalyst[15]. Both the alkane conversion and the selectivity to olefins increase significantly with added Sn. X-ray diffraction and XPS of the Pt-Sn catalyst indicate intermetallic compound formation rather than fee metal, and this surface evidently increases the alkane conversion and reduces the decomposition of olefins. 

In the ternary A1 - Ti - Cr system a large compositional region has been established experimentally, in which the eutectic transformation of a melt into two solid phases is realized L <=> Ll2 + p. This transformation is univariant and occurs in a narrow temperature interval [29], The alloys formed by two cubic phases Ll2 and P possess a more attractive combination of strength and deformation before fracture (see Fig.5) than alloys, in which the decomposition of the P-phase produces the less symmetric intermetallic compounds AlCr2 or TiAlCr. 

For other reactions like methane, ammonia or methanol synthesis the intermetallic compounds are used as precursors of new active catalysts. In this case a bulk decomposition of the starting compounds is required. This can be performed either under a synthesis gas reactant or by applying either an oxidation pretreatment under air, O2, CO2, N2O or by hydrogenation at high temperature. 

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. 

Since Netzer and Bertel wrote their chapter in volume 5 of this Handbook (1982), further studies have shown that the best activities are related to more extensive decomposition. This was, for example, the case with a series of Ni-based intermetallic compounds (Paul-Boncour et al. 1986). For LaNi4X (X = Ni, Cr, Al and Cu) Ando et al. (1995) observed that only LaNi5 and LaNLtCr had significant activity in the methanation reaction. XRD analyses revealed their decomposition into metallic nickel... 

The best catalyst for the synthesis of methanol from CO + H2 mixtures is copper/zinc oxide/alumina. Intermetallic compounds of rare earth and copper can be used as precursors for low-temperature methanol synthesis as first reported by Wallace et al. (1982) for RCu2 compounds (R = La, Ce, Pr, Ho and Th). The catalytic reaction was performed under 50 bar of CO + H2 at 300°C, and XRD analyses revealed the decomposition of the intermetallic into lanthanide oxide, 20-30 nm copper particles and copper oxide. Owen et al. (1987) compared the catalytic activity of RCux compounds, where R stands mainly for cerium in various amounts, but La, Pr, Nd, Gd, Dy and even Ti and Zr were also studied (table 4). The intermetallic compounds were inactive and activation involved oxidation of the alloys using the synthesis gas itself. It started at low pressures (a few bars) and low temperatures (from 353 K upwards). Methane was first produced, then methanol was formed and it is believed that the activation on, for example, CeCu2, involved the following reaction, as already proposed for ThCu2 (Baglin et al. 1981) ... 

Apart from these specific hydrogen absorption and desorption properties, it is clear that the intermetallic compounds are more often used as precursors of new and more active catalysts. The decomposition of intermetallic compounds into transition-metal particles and rare-earth hydride, oxide or hydroxide in the course of the catalytic reaction was evidenced in a large number of catalytic reactions, especially those involving CO dissociation, which produces oxidizing species. The rise of catalytic activity was... 

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