Rare earth vanadates

Similar improvements of activity and selectivity were reported in the oxidative dehydrogenation by C02 of ethane over Ga2C>3 (18.6% ethylene yield and 94.5% selectivity)391 and that of propane over rare earth vanadates.392 Cr203 shows medium activity in the oxidative dehydrogenation of ethane, but support on Si02 enhances the catalytic performance (55.5% ethylene yield at 61% conversion at 650°C).393... 

The solution chemistry of the rare earth vanadates has received considerable attention [359,360]. It has been found that when ammonium metavanadate solution is added to rare earth nitrate solutions, an increase in acidity results with the formation of the orthovanadates (eq. 35). However, when the solutions are mixed in the opposite order the pH of the medium remains unchanged and precipitates of metavanadates the (eq. 36) are produced. 

Among numerous rare earth vanadate compounds, YV04 Eu has been investigated most for its value in practical use. Its bulk polycrystals could reach a quantum yield (QY) of ca. 70% as commercial red phosphor. Huignard et al. obtained 7-YV04 Eu " NPs with sizes around 15-30 nm... 

Hydrothermal routes Under ambient conditions, the low reaction temperature and fast precipitation rate have deleterious effect on the crystallization and optical performance of rare earth vanadate nanomaterials. Referring to traditional solid-state reactions, bulk YV04 Eu phosphors require a calcinations temperature above 1300 K, but it is too high for the preparation of nanomaterials. Alternatively, hydrothermal routes could provide the adequate energy for solution phase reactions, which have been widely described in preparation of ceramic powders. The high pressure and temperature largely promote the dissolution-reprecipitation process, so as to decrease the lattice defects of NCs. With fine modulation, this method is also efficient to produce nano-sized crystals. 

Sulfur competition has been the Achilles heel of other technologies used to trap vanadium. While RV4+ technology picks up some sulfur, it does not appear to hinder its performance. In fact, its propensity to pick up sulfur diminishes rapidly as its ability to capture vanadium increases, suggesting that the rare earth vanadates formed are more stable than rare earth sulfates. This can be seen in Figure 10. Also evident in Figure 10 is the high amount of vanadium on RV4+, approximately 11,000 ppm. The highest level achieved was in excess of... 

The DRS spectrum of the EXV catalyst after reduction showed the presence of vanadium in V " oxidation state. Based on thermodynamics redox results, it is expected that when vanadium and cerium present some interaction, the last one should present an easier reduction. The reduction of cerium is favored because of its higher potential (1.64 eV), that should maintain vanadium in the oxidation state [16]. Therefore, the formation of rare earth vanadate is favored. On the IMPV catalyst, where probably cerium is dispersed over the zeolite, the reduction process would be preserved. On the other hand, for the EXV catalyst, the reduction of cerium exchanged in the presence of vanadium leads to an easier reduction of both components. But it is not possible here to distinguish and to quantify the formation of and cerium in a Ce" oxidation state. 

Eigermann W, Mtiller-Vogt G, Wendl W (1978) Solubility curves in high-temperature melts for the growth of single ciystals of rare earth vanadates and phosphates. Phys Stat Sol (a) 49 145-148 Ewing RC (1975) The ciystal chemistry of complex niobium and tantalum oxides. IV. The metamict state. Am Mineral 60 728-733... 

Smith SH and Wanklyn BM (1974) Flux growth of rare earth vanadates and phosphates. J Ciyst Growth 21 23-28... 

Different from other Bi -doped other, we noticed that Bi -doped rare-earth vanadates are promising phosphor candidates for NUV-WLED [35-37]. Here are the examples. 

In more complex solids such as rare earth vanadates (investigated in transmission by Tonomura et al. (1978)) or LaCoOs as examined in reflection loss by Richter et al. (1980), the plasmon energy is controlled by the electronic structure of the group accompanying the rare earth ion, but the 5p resonance loss of the rare earth ion continues to be prominent. In LajSj and SmjSj (Balabanova et al. 1983) the bulk plasmon positions as observed in transmission loss are consistent with values inferred from UV reflectance data (Zhuze et al. 1980), but the energy values (l5.9eV and 17.3 eV respectively) are well above what is expected from a free electron plasmon model. 

Rare earth vanadates having vanadium in lower oxidation states 248... 

Ternary rare earth vanadates are formed with alkali and ammonium ions. At least the following compositions are definitely known M3R(V04)2 and MRV2O7. In both compounds, vanadium is at the pentavalent state. 

The physical properties of the rare earth vanadates have been widely investigated. They show interesting magnetic as well as optical properties. Their most important technical significance is in the use of Eu YVO4 red phosphor in high pressure mercury lamps (Palilla et al., 1965 Luscher and Datta, 1970). 

Thermoanalytical studies on RVOj have shown that the oxidation to orthovanadate occurs at temperatures below 400°C and the product is monoclinic RVO4. Oxidation of vanadites at temperatures above 400°C produces a tetragonal form (Bazuev et al., 1975). Stubican and Roy (1963) achieved a reconstructive transformation from xenotime to scheelite structure in most of the rare earth vanadates (Pr Lu) at high pressure and temperatures up to 600 C. 

The precipitated polymeric rare earth vanadates are amorphous or microcrystalline. Except for the decavanadates the crystal structures are uncertain. The existence of VjO, VO3, and V2O17 radicals has been proposed and orthorhombic and monoclinic structures have been suggested for the hexavanadates. 

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