Phase transitions molybdates

Although Wolfs indicated that the catalyst particles are covered by a skin of bismuth molybdate, Batist (112) recently found bismuth, molybdenum, and iron in the surface layers of multicomponent catalysts. Additional data are needed to determine if multicomponent catalysts gain their activity as a result of the formation of compounds such as bismuth iron molybdate, or by surface enhancement of an active component such as 7-phase bismuth molybdate, or by creation of low-energy electronic transitions. Of course, due to their complexity, all of these factors may be important. 

Such an effect might be expected when boehmite supported cobalt is being calcined, viz. during the phase transition AIO(OH) - y-Al203. Figure 7 shows spectra of pyridine, adsorbed on the sample CoMo-124 B, which has been prepared in this way. Spectra for MoCo-122, -123 and -124, containing 2, 3 and 4 wt% CoO resp. are shown for comparison. All these catalysts have had a final calcination of 650°C. Comparison of the spectra of CoMo-124 B and MoCo-124 indicates that the intensity of the 1612 cm l band, which is introduced by the interaction of the cobalt ions and the molybdate layer, is lower for CoMo-124 B than for MoCo-124. The spectrum for CoMo-124 B resembles that of CoMo-123, indicating that a part of the cobalt ions does not participate in this interaction. 

Rare earth molybdates and timgstates share a general formula of A2B2O9, where A represents rare earth, and B represents molybdenum or tungsten. Usually, they show reversible phase transition from a monoclinic... 

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. 

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. 

Palai, R. Choudhary, RN.R. Sharma, S. (2001). Ferroelectric phase transition in Ni-doped lead molybdate ceramics.ferroeZec/rics, 256., 1., (April 2006) 33-45, ISSN 0015-0193. 

Lanthanum molybdate, LaiMoiOg, has been reported to exhibit fast oxide ion conducting properties comparable with the conventional zirconia and ceria compositions. This compound presents a different crystal structure from all known oxide electrolytes, and consists of isolated [Mo04] units in a three-dimensional matrix of [LaiO]" " . La2Mo209 undergoes a reversible phase transition from the non-conductive monoclinic a-form to the highly conductive cubic -form at approximately 580 °C. Powder X-ray diffraction (XRD) of the phases a and fl are practically identical, because the structural phase transition a is actually a transition from a static to dynamic distribution of the oxygen defects... 

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. ... 

Bulk Mixed Oxide Catalysts. - Raman spectroscopy of bulk transition metal oxides encompasses a vast and well-established area of knowledge. Hie fundamental vibrational modes for many of the transitional metal oxide complexes have already been assigned and tabulated for systems in the solid and solution phases. Perhaps the most well-known and established of the metal oxides are the tungsten and molybdenum oxides because of their excellent Raman signals and applications in hydrotreating and oxidation catalysis. Examples of these two very important metal-oxide systems are presented below for bulk bismuth molybdate catalysts, in this section, and surface (two-dimensional) tungstate species in a later section. 

Characteristic features of vanadium containing heteropoly catalysts for the selective oxidation of hydrocarbons have been described. MAA yield ftom isobutyric acid was successfully enhanced by the stabilization of the vanadium-substituted heteropolyanions by forming cesium salts. As for lower alkane oxidation by using vanadium containing heteropoly catalysts, it was found that the surface of (V0)2P207 was reversibly oxidized to the Xi (8) phase under the reaction conditions of n-butane oxidation. The catalytic properties of cesium salts of 12-heteropolyacids were controlled by the substitution with vanadium, the Cs salt formation, and the addition of transition metal ions. By this way, the yield of MAA from isobutane reached 9.0%. Furthermore, vanadium-substituted 12-molybdates in solution showed 93% conversion on H2O2 basis in hydroxylation of benzene to phenol with 100% selectivity on benzene basis. 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

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. 

The Raman spectra of thin films of transition metal molybdates are shown in Fig. 2. The spectrum of bare Si (100) wafer is also given as reference. The intense and sharp peak located at 520 cm and a broad band extending from approximately 930 to 1000 cm attributed to the vibrational mode of the silicon substrate [11], are observed in all studied systems. All thin films are free of microcrystalline M0O3 since no signal at 997 cm was observed. Moreover, for Ni-Mo-0 samples a weak band at 830 and a shoulder at 960 cm" were observed. These confirm the presence of a-NiMo04 phase, which has Mo-0 stretching modes in the Raman spectrum at 706, 830 (weak), 914 and 960 cm" ... 

Thin films of transition metal molybdates, with general formula MM0O4 where M = Co, Ni Cu and Fe, were prepared by the citrate method and deposited by spin-coating on a Si (100) substrate. All characterization data indicated that Co, Ni, and Cu-systems behaved as expected and pure phases P-C0M0O4, a-NIMo04 and a-CuMo04, respectively, were obtained without any... 

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. 

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