Molybdenum Oxide Phase Research Articles

Qualitative bonding models for some molybdenum oxide phases

Qualitative bonding models for some molybdenum oxide phases

Lithium Insertion in Several Molybdenum(IV) Oxide Phases at Room Temperature

The electrochemical insertion of lithium into several Li‐Mo(IV)‐O ternary phases, as well as itself, is described. The standard molar Gibbs' free energies of formation of the various insertion products have been measured and are compared with those of the parent materials. Further, lithium chemical diffusion coefficients are reported for several compositions.

Direct observation by controlled atmosphere electron microscopy of the changes in morphology of molybdenum oxide and sulfide supported on alumina and graphite

The changes in morphology of monolayer quantities of molybdenum oxide and sulfide phases supported on nonporous alumina and graphite thin films were studied by controlled atmosphere electron microscopy in oxygen and in H 2S H 2 gas mixtures. When molybdenum oxide was supported on alumina and heated in oxygen at temperatures up to 825 K, a strong oxide-support interaction was observed, resulting in a highly dispersed molybdenum phase. When supported on graphite, the support interaction was weaker and bulk molybdenum oxide formed that became mobile on the surface at temperatures higher than 930 K. On mixed supports containing both alumina and graphite regions, the mobile molybdenum oxide particles on graphite at 930 K were observed to disappear when they contacted alumina edges due to rapid spreading of the molybdenum oxide over the alumina surface. This behavior can be explained by the relatively low surface energies for MoO 3 and graphite compared to the high surface energy for Al 2O 3. The molybdenum oxide-support interaction on alumina was broken by sulfiding in 5% H 2S H 2 at ca. 750 K, with the formation of crystallites of MoS 2 located preferentially at grain boundaries on alumina. Larger particles were obtained at higher sulfidation temperatures. Reoxidation at temperatures higher than 645 K redispersed the crystallites due to spreading of molybdenum oxide over alumina. On the graphite support, a MoS 2 “rag phase” was formed from the MoO 3 crystallites upon treatment in 5% H 2S H 2 at 475 K, and molybdenum species did not spread over the graphite surface during reoxidation at temperatures up to 1100 K. Finally, sulfidation and reoxidation of a cobalt-promoted molybdenum/ alumina specimen showed behavior similar to that observed for the unpromoted samples.

A molybdenum oxide with a WO 3-type structure obtained by oxidation of (orthorhombic) Mo 4O 11

The molybdenum oxide phase called λ, formed by oxidation of Mo 4O 11 at 450–503 K, has been characterized by X-ray powder diffraction, electron diffraction and high-resolution electron microscopy. The λ-phase is of WO 3-type, with a unit cell four times the size of the WO 3-type subcell. Thermogravimetric oxygen determination indicated a composition close to MoO 3. The λ-phase is transformed exothermically to MoO 3 on heating. It is probably identical with the recently reported “β-MoO 3” modification. A reaction mechanism is suggested for the oxidation of Mo 4O 11 to the λ-phase.

A comparative study on the dispersion and carrier-catalyst interaction of molybdenum oxides supported on various oxides by electron spectroscopy for chemical analysis

A comparative electron spectroscopy for chemical analysis (ESCA) study on the nature of molybdenum oxide phase dispersed on four different supports is reported. From the fwhm and Mo/sub 3d/sub 5/2// BE data it is concluded that Mo on ..gamma..-Al/sub 2/O/sub 3/ is highly dispersed and that at low Mo loadings there is some electron transfer from the support to Mo; however, this effect is not detectable at higher loadings. SiO/sub 2/ shows no electronic effect at all, and TiO/sub 2/ and ZrO/sub 2/ show only mild effects. In general the dispersion, as reflected by Mo to support ESCA peak intensity ratio, tends to decrease at high loadings for all supports Co is found to augment the dispersion of Mo and to transfer electrons to Mo supported on Al/sub 2/O/sub 3/.

Studies of the structure of molybdenum oxide and sulfide supported on thin films of alumina

The morphology of monolayer quantities of molybdenum oxide and sulfide phases supported on thin films of nonporous alumina was studied by X-ray photoelectron spectroscopy and conventional and scanning transmission electron microscopy. Following oxidation at 770 K, a strong oxide-support interaction was observed, resulting in a highly dispersed molybdenum phase for loadings below ca. 5 Mo atoms nm−2. The molybdenum oxide-support interaction was broken by sulfiding in H2SH2 at ca. 770 K, with the formation of faceted single crystallites of MoS2. The MoS2 crystallites were present as hexagonally shaped slabs with one truncated edge and they were bonded with their high-energy edge planes to the alumina surface. The edge planes contacting the gas phase were the 1010 set of planes, while the truncated edge bonded to the surface of alumina was the 2110 plane. After 910 K sulfidation, the particles became larger, and many were oriented with their basal planes parallel to the support. Reoxidation at 770 K redispersed the crystallites due to spreading of molybdenum oxide over alumina. These results can be explained by the formation of MoOAl linkages during oxidation and the breaking of these linkages during sulfiding. As these linkages that anchor the MoS2 crystallites to the support are broken, the MoS2 basal planes become oriented at angles close to 90 ° to the support surface. Upon more extensive sulfidation, the MoS2 crystallites became oriented with their basal planes parallel to the support when the MoOAl linkages at the edge plane are finally broken.

Structure and Chemistry of Oxide Films Thermally Grown on Molybdenum Silicides

The oxides grown during thermal oxidation of molybdenum silicides were studied using electron microscopy, infrared absorption, and thermogravimetric techniques. Analysis of the oxidation mechanism based on thermochemical and diffusion rate data was correlated with experimental results. Because of the higher oxidation potential of silicon, it may be preferentially depleted from the silicide substrate to form a single-phase silica layer. Whether this occurs depends also on the relative diffusion rates in the oxide and substrate layers. Alternately, a duplex oxide layer containing both silica and molybdenum oxide phases will form. The molybdenum oxide phase is a necessary condition for the silicide pest, a low-temperature failure mechanism in which oxidation results in a nonadherent powder rather than in a protective film.