Vanadium thermal properties

The sequence distribution of EPM and EPDM samples prepared using vanadium/aluminium chloride and metallocene catalysts was studied by carbon-13 NMR and Fourier transform IR spectroscopy. The influence of sequence distribution on thermal properties was also investigated. 13 refs. 

The properties of the zeolite play a significant role in the overall performance of the catalyst. Understanding these properties increases our ability to predict catalyst response to changes in unit operation. From its inception in the catalyst plant, the zeolite must retain its catalytic properties under the hostile conditions of the FCC operation. The reaclor/regenerator environment can cause significant changes in chemical and structural composition of the zeolite. In the regenerator, for instance, the zeolite is subjected to thermal and hydrothermal treatments. In the reactor, it is exposed to feedstock contaminants such as vanadium and sodium. 

Loop Tests Loop test installations vary widely in size and complexity, but they may be divided into two major categories (c) thermal-convection loops and (b) forced-convection loops. In both types, the liquid medium flows through a continuous loop or harp mounted vertically, one leg being heated whilst the other is cooled to maintain a constant temperature across the system. In the former type, flow is induced by thermal convection, and the flow rate is dependent on the relative heights of the heated and cooled sections, on the temperature gradient and on the physical properties of the liquid. The principle of the thermal convective loop is illustrated in Fig. 19.26. This method was used by De Van and Sessions to study mass transfer of niobium-based alloys in flowing lithium, and by De Van and Jansen to determine the transport rates of nitrogen and carbon between vanadium alloys and stainless steels in liquid sodium. 

The fabrication process of vanadium oxide (VO2) has also been studied using RBS/C. Since optieal and electrical properties of VO2 are dramatically changed at 68°C due to phase transition, VO2 is regarded as one of the candidates for thermally activated electronic or optical switching devices for optieal fibers or sensors. To obtain the desired properties, the development of the fabrication process for very thin films, without crystalline defects on various substrates, is required. Single-crystalline VO2 thin films on (0001) plane of a sapphire substrate have been synthesized by a laser ablation method. The quality of VO2 was examined by X-ray diffraction and RBS/C method. The eleetrieal resistanee and the optical transmittance of the VO2 film were measured under inereasing and deereasing temperatures. At a temperature of 68 °C, an abrupt transition of resistanee from metal to... 

The several oxides of vanadium have already been referred to in the section describing the general properties of vanadium compounds (see p. 80). They are set out in the table on p. 38. The thermal changes involved in their formation are discussed collectively on p. 32. 

The carbides and nitrides of vanadium and titanium crystallize in the same face centered cubic (fee) system, and because of the closeness of their cell parameters (Table 15.1) form solid solutions. These ceramic materials exhibit interesting mechanical, thermal, chemical and conductive properties.1,2 Their high melting point, hardness and wide range of composition have therefore attracted considerable attention in the last decade. Moreover, their good abrasion resistance and low friction also make these ceramics attractive for protective coating applications.3-5 Chemical vapor deposition (CVD) is a commonly used technique for the production of such materials. In the conventional thermally activated process, a mixture of gases is used.6-9 In the case of TiC, TiN, VC and VN, this mixture is... 

In addition to the aluminosilicates, some obvious synthetic analogues, such as borosilicates, gallosilicates and ferrisilicates, have also been explored. The boro-silicates are more difficult to prepare, probably because the much smaller boron, with its tendency towards planar, three-fold coordination, is not an obvious substitute for the role of aluminum. Gallium and iron(III) materials, though, are well known and usually have properties similar to those of the aluminosilicates. There is evidence, however, for some unique behavior with gallosilicates, as in their ability to catalyze cyclization reactions and activate methane [30]. Other silicate-based systems include the titanosilicates, mentioned earlier, and some recently discovered vanadium silicates that show excellent thermal stability and can potentially be activated for catalysis [31]. 

The temperature regulation in the desorption stage can result in its essential intensification, as shown by the study of reaction of vanadium oxychloride with silica [48]. But it is necessary to take into account the properties of surface groups, in particular, their thermal stability. The latter, as it is known from the papers [49,50], can essentially influence the structural conversions of solid matrix. 

Redox properties of the catalysts can be determined, on the contrary, by submitting them to TPR (Thermal Programmed Reduction) with hydrogen (see for example Fig.l). Reduced catalysts can be reoxidized with pulses of oxygen so determining also the catalyst dispersion. These determinations have been made for vanadium based catalysts. It is very interesting to observe, for example, that clusters of vanadia of different sizes, corresponding to different dispersion indexes, can show very different redox properties, as it can be seen in Table 4 and Fig. 1. 

All the vanadium silicate samples have been characterized to investigate their thermal stability, the location, co-ordination number and oxidation state of vanadium in different atmospheres by a variety of techniques. Some of the physical properties of representative vanadium silicates are given in Table 1. The oxidation reactions reported here are confined to calcined fin air) samples of VS-2 and V-NCL-1 with SiA = 79 and 250, respectively. Absence of ESR signals due to paramagnetic vanadium species indicated that all vanadium is in pentavalent oxidation state in these samples. For comparison, data on their respective silica polymorphs viz., silicalite-2 (S-2) and Si-NCL-1 are also included in Table 1. These samples as well as those with different Si/V ratios, (Figs. 1 and 2) probably contain vanadium in the silicalite framework as may be inferred from the following observations - 1) The unit cell parameters of V-samples increase linearly with vanadium content, as shown in Rg. 1... 

The catalytic properties of vanadium containing compounds (expanded layered inateriats where unudaie us the interlayer anion) were investigated for photo oxidation of isopropyl alcohol to acetone and oxidation of o-xylenc to o-tolualdehydc [42]. The samples were active when thermally decomposed in between 620-670K. possibly due to better access of vanadium sites in the interlayer. At this temperature, the framework is partially destroyed with the collapse of the layered structure but without any significant reaction between the vanadate and the framework species. 

Optical Crystallographic Properties. As prepared by the thermal decomposition of ammonium hexafluorovanadate-(III), vanadium(III) fluoride has too fine a crystal size for the determination of optical properties with a polarizing microscope. Heating to 1350° in a platinum tube forms twinned crystals that are easily studied. According to Weaver, these crystals are biaxial (y=-) with optic angle (2V) of 5 to 10°. The a- and y-refractive indexes are, respectively, 1.536 and 1.544. 

Most commonly, the battery will be configured with a stack of bipolar cells (10 -100 cells per stack) to give a useful output voltage and with parallel flows for the electrolytes to each of the cells in the stack. Hence, the electrodes will be bipolar with a solid core from carbon, graphite, or a carbon/polymer composite and the three-dimensional elements bonded or pressed onto either side of the solid core. The composites are a blend of a chemically stable polymer and a micron-scaled carbon powder, most commonly an activated carbon Radford et al. [127] have considered the influence of the source of the carbon and the chemical and thermal treatments on the properties of such activated carbons, especially the pore size and distribution [126]. Even though reticulated vitreous carbon has been used for the three-dimensional elements [117], the predominant materials are graphite cloths or felts with a thickness of up to 5 mm, and it is clear that such layers are essential to scale the current density and thereby achieve an acceptable power density. Details of electrode performance in the more developed flow batteries are not available but, for example, Skyllas-Kazacos et al. [124] have tabulated an overview of the development of the all vanadium redox flow battery that includes the electrode materials and the chemical and thermal treatments used to enhance activity and stability. 

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