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Transition temperature, phase molybdate

Modem day bismuth molybdate catalysts, in addition to A, —3.V i2.v hio04 phases, contain other compounds such as Bi2Mo06 and Bi2M02O9 as well as small amounts transition metal molybdates that not only increase conversion rates and selectivity but also increase catalyst hfetime and allow operation at lower temperatures. ... [Pg.3435]

Abbattista et al. (26) found that phosphorus addition prevents crystallization of the y-alumina phase and the transformation from y- to a-alumina in the system AI2O3 —AIPO4 (Fig. 23). More precisely, Morterra et al. (77) reported that phosphates do not affect the phase transition from low-temperature spinel alumina (y-alumina) to high-temperature spinel aluminas 8 and 6 phases) but delay the transition of 8 and 9 to a-alumina (corundum). Stanislaus et al 46) also reported that phosphorus significantly improves the thermal stabihty of the y-alumina phase in P/Al catalysts. However, the same authors found that the positive effect of phosphorus seems to be canceled in the presence of molybdenum due to the formation of aluminum molybdate. Thermal treatments of MoP/Al catalysts at temperatures >700°C result in a considerable reduction of SSA and mechanical strength. The presence of phosphorus does not prevent the reaction between the molybdenum oxo-species and alumina since the interaction between molybdates and phosphates is weak. The presence of nickel does not obviously affect the positive effect of phosphorus in terms of thermal stability 46). On the other hand, Hopkins and Meyers 78) reported that the thermal stability of commercial CoMo/Al and NiMo/Al catalysts is improved by the addition of phosphorus. [Pg.456]

S. Krivovichev, T. Armbruster, D. Chernyshov, P. Bums, E. Nazarchuk, and W. Depmeier, Chiral Open-framework Uranyl Molybdates. 3. Synthesis, Structure and the C2221 to P212121 Fow-temperature Phase Transition of [C6H16N]2[(U02)6(Mo04)7(H20)2](H20)2. Microporous Mesoporous Mater., 2005, 78, 225-234. [Pg.262]

Catalysts based on transition metal molybdates, typically bismuth, cobalt and nickel molybdates [2-6], have received recent attention. Of the transition metal molybdates, those based on nickel, and in particular the stoichiometric NiMo04, have attracted the greatest interest. NiMo04 presents two polymorphic phases at atmospheric pressure a low temperature a phase, and a high temperature P phase [2,7]. Both phases are monoclinic with space group dim. These phases differ primarily in the coordination of molybdenum which is distorted octahedral in the a phase and distorted tetrahedral in the P phase. The P phase has been shown to be almost twice more selective in propene formation than the a phase for comparable conversion at the same temp>erature [2]. A similar effect has been noted for oxidative dehydrogenation of butane, with the P phase being approximately three times more selective in butene formation than the a phase [8]. The reason for the difference in selectivities is unknown, but the properties of the phases are known to be dependent on the precursors from which they are derived. Typically, nickel molybdates are prepared by calcination of precipitated precursors. [Pg.368]

The experimental study of the initial stages of M02C electrocrystallization from tungstate-molybdate-carbonate melts with electrodes prepared from various materials over a wide temperature range allows us to put forward the following concepts of nucleation. Thus, using inert substrates at r< 1073-1173 K, we observed considerable crystallization hindrances associated with the formation of three-dimensional nuclei. An increase in the electrolysis temperature facilitates the diffusion of atoms of the components into the substrate, which results in a decrease of crystallization overvoltage. Simultaneously, a transition from three- to two-dimensional nucleation is observed and, in some instances, to depolarization phenomena due to solid-phase saturation of the boundary layers of the electrode with the components (molybdenum and carbon) and the formation of an alloy with the material of the electrode. [Pg.316]


See other pages where Transition temperature, phase molybdate is mentioned: [Pg.202]    [Pg.393]    [Pg.157]    [Pg.162]    [Pg.263]    [Pg.108]    [Pg.277]    [Pg.617]    [Pg.618]    [Pg.749]    [Pg.614]   
See also in sourсe #XX -- [ Pg.110 ]




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