Showing papers in "Journal of The Less Common Metals in 1974"

Hydrogen absorption in LaNi5 and related compounds: Experimental observations and their explanation

Abstract: Results are given of a study of the change in hydrogen sorption properties caused by partly replacing La or Ni in LaNi5 by other metals. Desorption isotherms at 40 deg C were measured for or Cu, and for La0.8R0.2Ni5, where represents Nd, Gd, Y, and Er, also Th and Zr. Equilibrium hydrogen pressures in the two-phase region can be explained qualitatively by assuming that the more stable the original RNi5 compound, the weaker is the tendency to form a RNi5--hydride, i.e., the higher the equilibrium pressure obtained for the hydride. This assumption which connects the stability of ternary hydrides with the heat of formation of the original compounds can also be formulated in a more quantitative way. This leads to a fair description of the experimental observations for the compounds having the CaCu5 type of structure.

The role of cobalt and molybdenum sulphides in hydrodesulphurisation catalysts: A review☆

Abstract: Although originally in the form of cobalt and molybdenum oxides, the usual hydrodesulphurisation catalysts undergo extensive reduction and sulphurisation before and during the hydrodesulphurisation process. Many researches have been devoted to the study of the relationship between the properties of the oxides and the catalytic activity. Much less work has been concerned with the role of the sulphides. The purpose of the present review is to summarise the information which has been gained concerning the role of cobalt and molybdenum sulphides in hydrodesulphurisation and hydrotreating catalysts. We first briefly review the structure of the various cobalt or molybdenum sulphides and of the only compound sulphide, CoMo 2 S 4 , at present known. A second part is devoted to the results concerning the adsorption properties and the catalytic activity of the simple sulphides. Cobalt sulphides exhibit no significant activity. The discussion is centered on MoS 2 . A third part is concerned with the roie of mixed or compound sulphides in catalysis. The possible action of oxides or of oxygen containing phases present with the sulphides is first discussed. Some results suggest that the sulphides might be the only active phases in catalytic conditions. Catalysis by the sulphides should be explained by some promoting effect of cobalt on MoS 2 . The possible mechanisms for this promoting effect are reviewed. An extensive study of e.p.r. signals of hydrodesul-phurisation catalysts has been made. E.p.r. signals of cobalt-molybdenum sulphides are briefly discussed in this connection.

Rare-earth hydrides and rare-earth oxides in and from thin films of rare-earth metals

Abstract: Thin films of rare-earth metals always getter hydrogen very easily under ordinary conditions of preparation. It is shown from X-ray and electron diffraction experiments that the f.c.c. hydrides appear as single-crystal inclusions especially in the case of the yttric series (except for ytterbium). The hydride inclusions yield conspicuous f.c.c. diffraction patterns which should not be mistaken for a rare-earth monoxide, LnO, or for an f.c.c. phase of the metal. Oxidation of the “metallic” thin films yields sesquioxide thin films. The electron diffraction pattern of C-type sesquioxides for large crystals in thin films exhibits “forbidden” diffraction spots which apparently do not agree with the recognized T7h space group. It is shown that these forbidden spots appear because of multiple diffraction effects.

Cation exchange reactions and layer solvate complexes of ternary phases MxMoS2

Abstract: Layer intercalation compounds of molybdenum disulphide with alkali and alkali-earth metals show cation exchange behaviour after decomposition with water or aqueous ammonia. Mono- and divalent inorganic and organic cations can be taken into exchange positions in the interlayer space of the MoS 2 crystals. The interlayer distance depends on the type of cation exchanged and on cation hydration. Intracrystalline swelling of the compounds is observed with a whole series of polar inorganic and organic molecules. The cation exchange phenomena are explained in terms of the reduction of a part of the Mo(IV) atoms in the sulphide layers by alkali metal. Oxidation and heating restore the original MoS 2 lattice.

A kinetic study of the reduction of molybdenum trioxide by hydrogen

Abstract: A study has been made of the kinetics of the reduction of molybdenum trioxide to molybdenum dioxide by hydrogen in the temperature range 480–600 °C, and the subsequent reduction of the dioxide to molybdenum metal in the range 650–800 °C. The activation energies, E , and frequency factors, A , found for a sample of sublimed molybdenum trioxide were: MoO 3 + H 2 → MoO 2 + H 2 O E = 113 kJ mol −1 , A = 4.2 × 10 −1 s −1 , MoO 2 + 2H 2 → Mo + 2H 2 O E = 98.6 kJ mol −1 , A = 2.2 × 10 −2 s −1 , The reduction of the trioxide follows a linear rate equation and an explanation of this somewhat unusual result is presented. However, reduction of the dioxide to the metal followed the more usual deceleratory path characteristic of the contracting sphere model. The results obtained in the present work are compared with the results of previous investigations.

Alloying behavior of zirconium, hafnium and the actinides in several series of isostructural compounds

Abstract: The crystal structure of intermetallic compounds in ternary systems at the equiatomic composition has been studied. Structural data are presented for 39 compounds (MgZn 2 , Fe 2 P and MgAgAs-types) of TT′X alloys in which T is Ti, Zr, Hf, Th, U, or Pu; T′ is Mn, Fe, Co, Rh, or Pt; and X is Al, Ga, Sn, or Sb. It is shown that the alloying behavior of Th, U, and Pu resembles that of Zr and Hf more closely than that of the lanthanides. The transition from MgZn 2 to Fe 2 P to MgAgAs-type structures is related to an increase in electron concentration.

MoO3Fe2(MoO4)3 catalysts for methanol oxidation

Abstract: The present state of research on the FeMo oxide catalyst is summarised. The preparation of the catalyst is described and the nature of the precipitated amorphous, hydrous iron molybdate is discussed. After thermal dehydration and crystallisation a Fedefective iron molybdate is obtained, as shown by X-ray diffraction studies. The preparation of other trivalent metal molybdates is also discussed. The results of our kinetic studies are summarised and compared with those of other research groups. A reaction mechanism is proposed that satisfactorily accounts for the kinetic data. Surface acidity is emphasised as a most important property of the FeMo oxide catalyst. A correlation was found between surface acidity and catalytic activity. I.r. studies showed this acidity to be connected with Lewis sites at the usual reaction temperatures. Such sites, where methanol chemisorption should take place, can be described as anionic vacancies produced by dehydroxylation. Evidence is reported that suggests that these acid sites are connected with Mo6+ ions. Finally the role of Fe3+ and Mo6+ ions in this oxidation is discussed. It seems that the function of Mo6+ ions, in octahedral coordination, should be that of providing methanol chemisorption sites able to direct the subsequent oxidation towards formaldehyde, while the presence of Fe3+ ions should make the dehydroxylation of the catalyst surface easier, thus increasing the concentration of methanol adsorption sites in stationary conditions. E.p.r. studies showed that Fe3+ ions are reduced to Fe2+ by methanol; however the reoxidation of Fe2+ ions is slower, so that the redox process should be carried out by the Mo ions, Fe2+ ions probably being present at the surface.

The high-temperature hardness of ZrB2 and HfB2

Abstract: The variation of hardness with temperature has been investigated for ZrB2 and HfB2 in the temperature range 25–550°C, using a high temperature microscope equipped with a microhardness tester. A plot of log hardness versus temperature displays straight-line behaviour. Hardness anisotropy is demonstrated for both diborides. The room-temperature hardness and cell dimensions have also been determined for solid solutions of zirconium and hafnium in β-rhombohedral boron. The room-temperature hardness of ZrB2 is 2940 × 107 N/m2, and that of HfB2 is 3150 × 107 N/m2.

The preparation and crystal structure of gold monochloride, AuCl

Abstract: Large single crystals of AuCl were prepared by a vapour-transport method. AuCl is tetragonal, a = 6.734, c = 8.674 A , space group 141/amd, Z = 8. The structure consists of zig-zag chains of Au and Cl; Au is linearly coordinated by Cl at distances of 2.36 A, the bond angles Au-Cl-Au are 92 °.

The lattice parameters of La, Ce, Pr, Nd, Sm, Eu and Yb

Abstract: Lattice parameters of La, Ce (β and γ), Pr, Nd, Sm, Eu and Yb were determined on well-characterized, high-purity metals in the form of small-grained wires. The effect of sample preparation on the lattice parameters of these metals is discussed. A method to obtain crystallographically-pure polycrystalline d.h.c.p. lanthanum is presented.

THe corrosion of chromium, iron, and stainless steel in liquid sodium

Abstract: The corrosion products NaCrO/sub 2/ and Na/sub 4/FeO/sub 3/ were observed on the surfaces of chromium, iron, and stainless steel after exposure to liquid sodium at 600 deg C and above. The products were identified by x-ray diffraction without the removal of the covering sodium layer. (8 figures) (auth)

Gradual reduction of molybdosilicates and related compounds

Abstract: For SiMo 12 O 40 4− and related compounds, the products obtained by the gradual reduction of Mo(VI) atoms form redox-reversible, very mobile systems. Their redox and chemical behaviour (stability and disproportionation reactions) and the relations between α and β isomers have been studied. For higher reduction stages, the reduced derivatives form a new series with different properties. The members of each series are in redox-reversible equilibrium with each other, but not with members of the other series.

The europium-palladium system

Abstract: The binary system europium-palladium has been investigated by differential thermal analysis, X-ray diffraction, metallography and magnetic susceptibility measurements. Six intermetallic phases have been found. Four of these, viz., Eu5Pd2, Eu3Pd2, EuPd, and EuPd2, contain divalent europium; in the remaining two, EuPd3 and EuPd5, the europium behaves as a trivalent rare-earth metal. To allow comparison with EuPd, the structures of CaPd, SrPd and BaPd have been determined.

Bismuth molybdates as oxidation catalysts

Abstract: The catalytic action of bismuth molybdates in general and of Bi 2 MoO 6 in particular, is discussed in its relation to the solid structure, the method of preparation, the kinetics of the reaction catalysed and the adsorption of reactants and products. Mechanisms for the oxidation of olefins and of the ammoxidation of C 3 H 6 and NH 3 are proposed in which the active site is essentially bifunctional in combining two operations, viz ., dissociation of H and oxygen donation + electron abstraction which occur on a set of neighbouring but different adsorption sites. The adsorption sites are connected with anion vacancies over Mo- and Bi-cations. It is found that the density of active sites on the surface is remarkably low, indicating that there exists a superstructure on the surface with interdistances considerably greater than unit cell dimensions in the solid. Re-oxidation of a reduced catalyst seems to start on a surface site different from that on which the reaction of the hydrocarbon occurs.

The crystal structure and lattice constants of RE3Pd4, Y3Pd4 and Th3Pd4 compounds

Abstract: Alloys of the rare earths (R.E.), yttrium, thorium, hafnium and uranium with palladium of the M3Pd4 composition have been studied. For this composition, with the exception of Eu, U and Hf, single phases are formed, crystallizing with the Pu3Pd4-type of structure. The stability of these compounds appears to be restricted to the radius ratio ( R M R Pd ) range between 1.20 and 1.37.

Molybdenite-leaching: Use of a high-temperature microbe☆

Abstract: A high-temperature, chemoautotrophic microorganism was studied to establish its capability for leaching molybdenite and to ascertain its response to soluble molybdenum. Experimental results confirm that the organism can tolerate 2000 ppm hexavalent molybdenum and can grow when the concentration of soluble molybdenum is 750 ppm. A molybdenite concentrate is leached at a maximum rate of 6.6 mg Mo solubilised/1./day; this rate is maximised by supplementing the medium with 0.02% yeast extract and 1% iron(II) sulphate. Batch reactor-leaching suppresses the rate of molybdenum solubilisation; the original leaching rate can be re-established by periodic replacement of the suspending medium. Some organic compounds enhance the leaching rate, but combinations of these compounds are not additive. Sulphur is inhibitory to molybdenite leaching; pyrite suppresses molybdenum solubilisation if the concentration of pyrite exceeds 50% of the total solid substrate. Molybdenite ore is successfully leached. Molybdenite waste and tailings contain acid-consuming gangue and lime, respectively, which raise the pH well above optimum for biogenic leaching. Hence, biogenic leaching of waste and tailings is not successful by the technique employed herein. Direct observations of the high-temperature microorganism on molybdenite fines illustrate colonial growth and developmental characteristics in a microbial—mineral interaction. The organism responds to hexavalent molybdenum by reducing this metal to the pentavalent state in aerobic and nitrogen gas atmospheres.

Kristallstruktur von Pd5Ga2

Abstract: Zusammenfassung Das Phasendiagramm in der Nahe der Phase Pd 5 Ga 2 wird festgelegt; es enthalt die neuen Phasen Pd 13 Ga 5 , Pd 5 Ga 2 , Pd 7 Ga 3 . Die Kristallstruktur von Pd 5 Ga 2 wird analysiert; sie hat eine orthorhombisch verzerrte kubisch dichteste Kugelpackung bei der die Komponenten in bestimmter Weise geordnet sind. Die ubrigen genannten Phasen haben ahnliche Strukturen. Die Phase Pd 55 Ga 45 (h) hat eine W-Struktur. Die Strukturverwandtschaft von Pd 5 Ga 2 zu Pd 2 Ga und Pd 5 Ga 3 wird erlautert und es werden Argumente fur die Struktur aus Annahmen uber die Ortskorrelation der Elektronen hergeleitet.

The rare-earth arsenides: Non-stoichiometry in the rocksalt phases

Abstract: The rare-earth arsenides with the rocksalt structure are non-stoichiometric compounds. Both anion and cation lattice sites are incompletely occupied. A range of arsenic solubility is observed in all cases, and in samarium arsenide at 700 °C, it extends from Sm 0.98 As 0.81 to Sm 0.98 As 0.98 . Over part of the solubility range at the arsenic-rich end, variations in arsenic content can occur with no detectable change in lattice parameter. Further decrease in arsenic content can occur at 700 °C in all but Y, Ho, Er and Yb arsenides, and is accompanied by a reduction in the lattice parameter. A range of metal solubility exists at high temperatures for all phases and results in considerable reduction of lattice parameter at the lower metal concentrations. There is no evidence for vacancy ordering.

On the eutectoid decomposition of CaCu5-type rare earth-cobalt phases

Abstract: The decomposition of CaCu5-type rare earth-cobalt compounds has been studied at various stages. It is shown that the occurrence of one of the phases, after decomposition in the form of closely-spaced parallel lamellae, corresponds to an intermediate stage of the decomposition process in which the initial precipitates have not yet coalesced into larger regions. The influence of various impurities on the metastable character of the RCo5 compounds has been investigated. It is shown that impurities can shift the decomposition temperature over a range of several hundred degrees Celsius.

Synthesis and electrochemical properties of heteropolymolybdates

Abstract: The beta form of 12-molybdosilicic acid was prepared and isolated as a yellow hygroscopic solid from a 1:1 mixture of water-dioxane. The identity of the beta- isomer was established polarographically and spectrophotometrically. The range of stability of the beta-isomer as a function of hydrogen ion concentration was determined. In addition, techniques were developed for preparing, in pure form, several heteropoly compounds used for polarographic and conductivity measurements. The solvolytic stability, electrolyte strength and oxidation-reduction behaviour of several heteropoly compounds were determined by pH, conductivity, and polarographic measurements. Heteropolymolybdate oxidation—reduction behaviour and stability in solution were examined by direct current and alternating current polarography and cyclic voltammetry in aqueous and water-dioxane solutions. The reduction of the anions proceeds in several steps for which the half-wave potentials were determined. Conductivity and pH measurements on the heteropoly acids H4 [PMo11V040]and H5 [PMo10V2O40]at 25 °C in aqueous solution, and in mixed oxygen-containing solvents, show that the vanadium compounds exhibit behaviour typical of unsymmetrical strong electrolytes in oxygen-containing solvents and are stable in these media.

Statistical model for the non-stoichiometric Cr2N phase

Abstract: In order to obtain some information on the bonding in the non-stoichiometric Cr 2 N 1 − x phase, a statistical thermodynamic approach has been made The interaction energy between the nitrogen atom and the chromium atom, E N-Cr , was estimated to be −344 kcal/mole, and that between chromium atoms, E Cr-Cr , to be −134 kcal/mole The interaction energy between nitrogen atoms, E N-N , was found to have positive values, varying with temperature These results show that both the metal-metal and metal-nonmetal bonds play an important role in the Cr 2 N phase The atomic partition functions, which have been conventionally put equal to unity in almost all the statistical thermodynamic considerations, were also evaluated for the nitrogen atom and the chromium atom in the Cr 2 N lattice From the evaluated values for these atomic partition functions, the importance of the contribution of the electronic structures to the atomic partition functions may be appreciated

A study of ZrCo and related ternary phases represented by the general formula, Zr50Co50 − xNix

Abstract: As an essential preliminary to the study of the mechanical behaviour and physical properties of some ternary intermediate phases represented by the general formula, Zr 50 Co 50 − x Ni x (where x represents the atomic percent, and varies from 0 to 50), the structure and constitution of these alloys have been investigated. Metallographic examination of the binary ZrCo phase indicates that it melts congruently and there is also evidence of a Widmanstatten-type precipitate and a eutectoid decomposition in the ZrCo phase after certain mechanical and heat treatments. The effect of substituting cobalt with nickel in the ternary alloys is to produce a room-temperature, martensitic structure at approximately the 14% Ni ★ composition. At certain temperatures, appreciable ductility is observed in the range 6–14% Ni and the marked temperature dependence of the mechanical properties is thought to be related to the transformation characteristics of the ternary alloys. The electrical resistivity measurements in the temperature range 77–300 K indicate a possible low-temperature phase transition at the 10 and 12% Ni compositions.

Thermal expansion, Debye temperature and Gruneisen constant of carbides and nitrides

Abstract: The experimental data on the thermal expansion of the carbides and nitrides of the early transition metals (with B 1 structure) have been collected and evaluated, and average curves of α against temperature and composition have been determined. Some of the difficulties encountered in making good thermal expansion measurements peculiar to this class of compounds are briefly discussed. The resultant values of α, together with the experimental specific heat and elastic constants, are used to determine the temperature dependence on the Debye temperature and the Gruneisen constant.

Determination of the phase diagram of the MoFe system using diffusion couples

Abstract: The phase diagram of the MoFe system between 800 and 1300 °C has been determined using diffusion couples consisting of the pure metals or their alloys. The boundary concentrations were measured with an electron probe microanalyser. The μ-(Fe7Mo6) phase has a homogeneity range from 57 to 61.5 at.% Fe; above 1250 °C the Fe-rich boundary shifts towards the Mo-rich side. The R-phase is stable above 1200 °C. Its Fe-rich boundary lies at 65 at.% Fe and is vertical, but the Mo-rich one shifts from 65 at.% Fe at 1200 °C to 62 at.% Fe at 1300 °C. The σ-phase is stable above about 1235 °C and contains about 45 at.% Fe. α-Fe can dissolve 17.9 at.% Mo and Mo 8 at.% Fe at 1300 °C.

Solid solubilities of oxygen, carbon and nitrogen in yttrium

Abstract: The solid solubilities of oxygen, carbon and nitrogen in yttrium metal have been investigated and a partial diagram is proposed for the Y-YN system. The temperature dependence of the solubility of each solute in α yttrium was fitted empirically to Arrhenius-type equations, and the heats of solution for the co-existing oxide, carbide or nitride phase were found to be 6.8 ± 0.4, 8.0 ± 1.0 and 11.6 ± 1.3 kcal/mole, respectively.

Die ternären systeme (Titan, Zirkonium, Hafnium)-kupfer-Silizium

Abstract: Zusammenfassung Bei der Untersuchung der Dreistoffe {Ti, Zr, Hf}-CuSi wurde eine Reihe von neuen ternaren Phasen aufgefunden. Es handelt sich neben den bereits beschriebenen E-Phasen TiCuSi, ZrCuSi und HfCuSi sowie Zr3Cu4Si2 und Hf3Cu4Si2 vom Fe2P-Typ um die Phasen Zr3Cu4Si4 und Hf3Cu4Si4 vom Gd6Cu8Ge8-Typ und die TiAl3-verwandten Verbindungen Zr2CuSi4, ZrCuSi2, Zr2Cu3Si4, Hf2CuSi4 und HfCuSi2. Die isothermen Schnitte der ternaren Phasendiagramme wurden fur eine Temperatur von 800 °C bestimmt. Die Bauprinzipien der neu aufgefundenen Phasen werden im Zusammenhang mit verwandten Strukturen in den homologen Systemen (mit Fe, Co, Ni, Al statt Cu) diskutiert und die Existenzkriterien untersucht. Die wesentliche Bedingung fur das Auftreten bestimmter Phasen ist die Anderung des Wertes der Valenzelek-tronenkonzentration durch den gegenseitigen Austausch von Silizium durch Kupfer und umgekehrt.

Creep resistance and embrittlement of a Ti-6%Al-5%Zr-1%W-0.4% Si alloy

Abstract: Precipitation of titanium and zirconium silicides occurs in the alloy Ti-6Al-5Zr-1W-0.4Si during extended heating in the 450–550 °C temperature range. When the alloy is quenched from the β-field, i.e., when the structure is martensitic, some medium-sized silcides are aligned along the martensite plates and are responsible for the loss of tensile ductility after extended heating. Other suicides, which are thinner, precipitate on dislocations within the plates and increase the creep resistance, in addition to the increase due to solute silicon atoms; the early nucleation of the orientated silicides implies a strong relationship between the improved creep resistance and embrittlement in silicon-containing titanium alloys. When the alloy is cooled from the (α + β) field, large silicides precipitate during creep and the creep resistance is not then improved by the silicon addition. An ordering reaction is also observed during exposure in the same temperature range, whatever the initial structure. It reaches a maximum near 560 °C and leads to the precipitation of α2(Ti3Al) particles on some dislocations, but does not induce any embrittlement. The tests have also shown that embrittlement by ω-phase, by surface contamination, or by hydrogen are not significant in this case.

Intermetallic compounds in the cobalt-rich part of the R-cobalt systems (R = Ce, La, CeLa)

Abstract: The tentative equilibrium phase diagrams of the binary CeCo and LaCo systems on the Co-rich side are given. Two isothermal sections of the ternary CeLaCo system (one at 850°C and the other at 1000°C) on the Co-rich side have been determined. The intermetallic compounds CeCo5 and Ce2Co17 are found to be congruently melting, contrary to the existing literature, and to possess a large homogeneity region. The extent of La solubility in CeCo5 over the whole homogeneity range has been studied. It is shown that the structure of the hyper-stoichiometric ★ AB5 compounds (A = rare earths, B = 3d transition metals) is different from that of the CaCu5 type with nearly the same lattice constants. How the formation of these hyper-stoichiometric RCo5 intermetallic compounds can lead to a drastic drop in the magnitude of the coercive fields of permanent magnets based on them is discussed.

Molybdenum in hydrodesulphurisation catalysts

Abstract: The sulphides of the Group VI metals are good hydrogenation catalysts with the added advantage of stability in the presence of organosulphur compounds and H2S. Molybdenum is widely used in hydrodesulphurisation reactions, usually in conjunction with promoters, such as Co or Ni, on a carrier, such as γ-alumina. In practice, the metal oxides on the support are pretreated before going on stream so as to convert them, at least partially, to the sulphides. Using thiophene as a model compound it has been shown, at least at atmospheric pressure, that C—S bond fission takes precedence over ring hydrogenation, yielding butadiene as an initial product. There is evidence that at least two types of catalytic site are operative, one in the hydrogenolysis reaction, the other in olefin hydrogenation. However, it is not necessarily valid to extrapolate these results to higher molecular weight compounds, or to the actual desulphurisation of petroleum distillates which is carried out at higher pressures and in the presence of a variety of other substances. There is beginning to be some agreement among different workers that the catalytic site for the hydrogenolysis reaction is an anion vacancy in the sulphide lattice. However, there is less certainty concerning the role.of the promoter in that it is thought of as either conferring structural stability to the catalyst surface, or truly promoting sites of high activity. These functions, on the other hand, need not be mutually exclusive.

X-ray photoelectron (ESCA) spectra of some molybdenum-containing catalysts

Abstract: We report core-electron binding energies for molybdenum(VI), cobalt(II), and iron(III) in ammonium di- and hepta-molybdates, cobalt(II) and iron(III) molybdates and fresh and sulphided cobalt—molybdenum—alumina (CMA) hydrodesulphurisation catalysts. The information which could be obtained from the ESCA spectra was limited because the peaks were weak and broad especially for the CMA catalysts. There were no features of the spectra which could be attributed to molybdenum and cobalt in different structural environments. Structural changes occurring when iron(III) molybdate is calcined were not revealed in the ESCA spectra. Molybdenum binding energies were exceptionally high in CMA catalysts, showing that the molybdenum has a high effective positive charge and so should be a particularly good substrate acceptor. Cobalt and molybdenum binding energies were lowered after sulphiding CMA catalysts.