John R. Günter

Topotactic dehydration of molybdenum trioxide-hydrates

Abstract It is shown that thermal dehydration of molybdenum trioxide-dihydrate proceeds in two steps, both of which are strongly topotactic. The topotactic relations have been established and been used to propose a structure model for the intermediate product, molybdenum trioxide-monohydrate, the unit cell of which has been determined. The guiding elements of the two topotactic reactions are planes of corner-sharing octahedra. The conservation of these planes throughout the reactions is confirmed by scanning electron micrographs in addition to the results of X ray diffraction.

Activation of unsupported CoMo catalysts in thiophene hydrodesulfurization

Abstract A series of unsupported oxidic CoMo catalysts with different mole fractions r = Co (Co + Mo) was prepared by coprecipitation of solutions of (NH 4 ) 6 Mo 7 O 24 and Co(NO 3 ) 2 . The calcined catalysts contain a-CoMoO 4 ( r = 0.5) mixed with MoO 3 ( r 3 O 4 ( r > 0.5) according to X-ray diffraction (XRD) and electron diffraction. The a-CoMoO 4 transforms partially into b-CoMoO 4 upon grinding. The higher the cobalt ( r ) and b-CoMoO 4 contents, the higher are the surface area increase and the degree of reduction of calcined catalysts during hydrogen treatment at 673 K. Electron microscopy (EM) data agree well with the surface area increase observed after reduction. X-ray photoelectron spectroscopy (XPS) shows the reduction of molybdenum rather than that of cobalt. Reduced crystalline phases cannot be identified by these techniques. Sulfidation with a mixture of H 2 /thiophene following reduction caused a drastic drop in surface area but the particle size seen by EM does not increase. Weak oxythiomolybdate XRD bands appeared after slight sulfidation, most XRD signals disappeared after massive sulfidation of samples with r = 0.5. Cobalt promotes sulfidation of molybdenum in the bulk, but the maximum sulfidation degree was about half of the stoichiometric value. XPS shows surface cobalt enrichment, XRD and EM traces of Co 9 S 8 in samples with r = 0.38 and 0.50. A pronounced maximum was observed in initial hydrodesulfurization (HDS) activity and hydrogenation (HYD) selectivity at medium Co content. On used catalysts, this synergism disappeared. We attribute the highest HDS activity of short-living cobalt-oxythiomolybdate(s) formed initially during sulfidation. HYD was promoted by sulfided molybdenum and by less surface Cobalt.

Topotactic decomposition and crystal structure of white molybdenum trioxide-monohydrate: Prediction of structure by topotaxy

Abstract Single crystals of the white MoO3 · H2O modification (“α-molybdic acid”) were transformed by heating to 160°C into perfect pseudomorphs built up from oriented MoO3 crystallites of known structure. From the mutual orientation relationship of the unit cells of both phases involved in this topotactic reaction, as determined by X-ray photographs, a model for the so far unknown crystal structure of white MoO3 · H2O could be deduced. Independently, this structure was determined by X-ray diffractometer data then: space group P 1 , a = 7.388, b = 3.700, c = 6.673 A, α = 107.8, β = 113.6, γ = 91.2°, Z = 2. The structure was solved from the Patterson function and refined until R = 0.088. It is built up from isolated double chains of strongly distorted [MoO5(H2O)]-octahedra sharing two common edges with each other. This result agrees well with the model derived from topotaxy, and it becomes evident how the MoO3 lattice is formed through corner linking of the isolated double chains after the water molecules are removed. The study of topotactic phenomena seems rather generally applicable to deduce the main features of structures involved and for better understanding of structural relationships.

Direct synthesis and single crystal structure determination of cubic pyrochlore-type tungsten trioxide hemihydrate, WO3 · 0.5H2O

Cubic pyrochlore-type WO3 · 0.5H2O has been synthesized hydrothermally directly from solution and its crystal structure determined by X-ray diffraction of a single crystal: space group Fd3m, a = 10.305(3) A, Z = 16. The structure contains corner-sharing WO6-octahedra, leaving tunnels along the [110]-directions, in which the water molecules are located. Dehydration yields cubic pyrochlore-type WO3, which transforms into the monoclinic modification at higher temperature.

Sintering of platinum-black in hydrogen: morphology and catalytic activity

Abstract Pt-black (Pt-473 and Pt-633) reduced in aqueous solution with HCHO was sintered at 473 and 633 K in hydrogen. Electron micrographs show a considerable sintering, especially at 633 K. High-resolution EM indicates the presence of agglomerates of rounded crystals; lattice fringes are also seen. Samples sintered at different temperatures show identical catalytic activity in ring opening and aromatization of alkylcyclopentanes; their activity was also similar in bond-shift isomerization of heptane isomers. On the other hand, aromatization, hydrogenolysis, and C 5 -cyclic isomerization were more rapid on Pt-473, while C 5 -cyclization was more rapid on Pt-633. These effects are discussed in terms of various active sites for these skeletal reactions and their abundance on two samples which have different morphology, hence different hydrogen retention ability; also the chemical state of their carbon impurity may be different.

Thickness dependence of structure in thin films of low-temperature silver selenide

Abstract The growth and structure of thin films of low-temperature (LT) silver selenide (Ag 2 Se) were studied by HREM and electron diffraction. Samples were produced by the reaction of vacuum-deposited epitaxial silver films (1–30 nm thick) on NaCl substrates with equivalent amounts of selenium vapour. The structure of thin Ag 2 Se crystals is initially found to be monoclinic, pseudo-tetragonal, while it turns to the known orthorhombic LT phase at higher thicknesses (⩾ 20 nm). It is noteworthy that the difference of phases observed is only an effect of crystal thickness. Comparisons were made between such thin crystals with those grown on continuous 30 nm thick single-crystalline silver layers. A reversible phase transition between the orthorhombic LT and cubic high-temperature (HT) phases could be induced by changing the electron beam intensity during TEM investigation (75 kV). The following orientation relationships between the body-centred cubic ( C , a = 0.498 nm ) HT phase, the monoclinic, pseudo-tetragonal ( M , a = b = 0.706 nm , c = 0.498 nm , β = 90°) and orthorhombic ( O , a = 0.433, b = 0.706, c = 0.776 nm ) LT phases were determined: [100] C ‖[001] M and [110] M , [110] C ‖[111] M and [100] M ; [20 1 ] M ‖[100] O , [010] M ‖[010] O and [101] M ‖[001] O ; [ 2 22] C ‖[100] O , [110] C ‖[010] O and [1 1 2] C ‖[001] O . No triclinic or monoclinic (different from the determined pseudo-tetragonal) phases could be observed, although numerous references (listed in this paper) about such phases exist.

Intermetallic phase formation in aluminium and iron thin film systems

Abstract It was demonstrated that vapour-deposited thin film systems of aluminium and iron are very suitable for studying the formation of different intermetallic phases. Morphology, crystallography and composition of the phases developed as a result of heat treatments were analysed by electron microscopic techniques. F.c.c. aluminium and b.c.c. iron were present in the as-deposited films prepared by sequential deposition of the constituents at room temperature. During heat treatment of multilayer systems the icosahedral phase developed first above 250°C, simultaneously with metastable Al 6 Fe. In AlFe bilayer systems Al 6 Fe started to develop above 250°C; no icosahedral phase was detected by selected area electron diffraction. Above 500°C, mainly the stable η-Al 5 Fe 2 was observed in bilayer systems, while Θ-Al 3 Fe formed in multilayer samples. At a substrate temperature of 300°C, the icosahedral phase developed directly by sequential deposition of aluminium and iron.

High-resolution electron microscopy of cobalt-molybdenum catalysts: structural changes and activity

Abstract Unsupported cobalt—molybdenum oxide catalyst (atomic ratio of Co: Mo = 1:1) was calcined in air at 723 K, reduced in hydrogen at 673 K and sulphided at 673 K with hydrogen—thiophene mixtures for 10 and 70 min and with hydrogen—hydrogen sulphide for 120 min. High-resolution electron micrographs of the calcined sample show the presence of CoMoO4; this is also the main component detected after reduction, but its crystallinity is poor in the latter instance. MoS2 layers appear after even brief sulphidation. Sulphidation with hydrogen sulphide results in a catalyst covered almost entirely with MoS2, with traces of Co9S8 among its layers. Prolonged sulphidation with hydrogen—thiophene results in segregation of MoS2 and Co9S8 into separate crystallites; also, elemental sulphur appears. The catalytic activity at these stages has been evaluated.

Metastable cubic and tetragonal zirconium dioxide, prepared by thermal oxidation of the dichalcogenides

The reactions of zirconium disulfide and zirconium diselenide with air or oxygen have been studied. They proceed topotactically and yield nanocrystalline, nanoporous metastable cubic and tetragonal zirconium dioxide at temperatures considerably below their stability range. This synthetic passway may therefore be considered to belong to the field of soft chemistry.

Insertion of bivalent cations into monoclinic NbS3 prepared under high pressure and their secondary batteries

Abstract A study on solid electrochemistry of insertion of bivalent cations such as Mg 2+ , Zn 2+ into monoclinic NbS 3 prepared under high pressure is reported. The magnesium and zinc solid state secondary batteries in which monoclinic NbS 3 prepared under high pressure was used as cathode were investigated. Under constant current cycling carried out between 1.82 and 1.0 V the magnesium solid state secondary battery had quite a large discharge capacity. The zinc battery had a lower discharge capacity. Both were rechargeable.