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Characteristics of the Metal–Metal Oxide Reaction Matrix
Published in Anthony Peter Gordon Shaw, Thermitic Thermodynamics, 2020
High-temperature systems that exclusively produce solid products are rare. This is because many metals and metal oxides melt at temperatures below 2000 K. There are several strategies for obtaining solid products at extremely high temperatures. Some examples are given in Table 2.4. One approach is to select systems that produce refractory oxides and metals. However, many of these systems are quite exothermic, and the resulting reaction temperatures may be sufficient to melt a portion or all of the products nonetheless, as in entries 1 and 2. In entry 3, the Mn/WO3 system produces solids, but the adiabatic temperature is on the low side. Tantalum pentoxide is not as strong an oxidizer as tungsten trioxide. Accordingly, the metal/Ta2O5 systems in entries 4 and 5 are more likely to produce solid products. In entry 5, the temperature is limited by the ZrO2(s-l) transition.
Pyrometallurgy
Published in C. K. Gupta, Extractive Metallurgy of Molybdenum, 2017
The standard free energy change of the reaction should be the most negative possible. This is achieved when the reducing agent M’ has an affinity for oxygen which is much greater than that of the metal, M. For the purpose of illustration, the standard free energies of formation of some metal oxides are given in Table 7. Metals whose free energies of formation are lower in the list are able to reduce those above, and the larger the difference in the free energies of formation, the larger is the reducibility of one by the other. If, for example, aluminum is used to reduce tungsten trioxide, then any element above aluminum present in the oxide form in tungsten oxide will become reduced and join the reduced tungsten. Silicon, sodium, and potassium oxides are the typical ones to be reduced and enter the metal phase, whereas calcium oxide, which is not reducible by aluminum, joins the slag phase. In this way, a reference to the values of the free energies of formation in the first instance can clearly distinguish between the harmful associations from those less harmful in a metallothermic oxide reduction process under consideration. In the case of tungsten oxide reduction with aluminum, the presence of lime along with the oxide is acceptable or tolerated. The presence of silica and alkali metal oxides, however, cannot be so viewed. The temperature should be as low as possible, subject to condition that the metal and the slag should be sufficiently fluid to separate efficiently.
Advances in Fabrication of Functionally Graded Materials
Published in T. S. Srivatsan, T. S. Sudarshan, K. Manigandan, Manufacturing Techniques for Materials, 2018
Jedamzik et al. (2000) fabricated W/Cu gradient materials using electrochemical deposition. In the study, tungsten preforms with an open porosity were produced by partially sintering tungsten powders of specific grain size (4 or 10 μm containing about 1% nickel). The preforms were graded in an electrochemical cell using the setup shown in Figure 12.8. For the gradation of tungsten, the anodic dissolution reaction of tungsten in alkaline solution has been used. The reaction consists of the oxidation of tungsten to tungsten trioxide and subsequent dissolution of the tungsten trioxide in the alkaline electrolyte. This method is widely used in the industries because it produces fully dense gradient materials.
Photocatalytic NO removal by WO3 samples prepared via a ball milling treatment under different parameters
Published in Inorganic and Nano-Metal Chemistry, 2022
Yuqing Wang, Fei Chang, Zhixun Wei, Cheng Yang, Deng-guo Liu, Tianyi Yan, Qingyun Pang, Shengwen Chen
Tungsten trioxide (WO3) is a non-toxic and harmless semiconductor with a moderate band-gap energy, possessing good optical and electrical properties.[9–11] It is widely employed to sparkle photocatalytic processes, such as hydrogen evolution,[12–14] degradation, and transformation of organic or inorganic pollutants.[15,16] However, due to the serious recombination of charge carriers, photocatalytic performance of WO3 is quite weak.[17,18] General modification strategies include morphological modulation,[19] composites construction,[20] photosensitization,[21] ion doping,[22] and noble metals deposition.[23] Morphological modulation is a common and effective modification manner, by which the migration distance of charge carriers can be shortened, thus prolonging lifetime of charge carriers and further enhancing photocatalytic performance.[24]
Reclamation of tungsten from spent HDS catalyst: a detailed study
Published in Indian Chemical Engineer, 2022
Surjeet Mahalik, A. R. Sheik, Barsha Dash, C. K. Sarangi, K. Sanjay
The as obtained tungstic acid was calcined at 700°C to get tungsten trioxide. The XRD pattern of the tungsten trioxide is presented in Figure 12. The W content in the as obtained tungsten trioxide is 69.79%. Theoretical W content in WO3 is 79.31%. According to that the purity of the as obtained WO3 at 700°C is 88%. The SEM images of the as obtained WO3 is presented in Figure S4 (Supplimentary Material). The trigonal bi pyramidal particle of tungstic acid acquired polygonal shape which are closer to spherical structure clearly showing the changeover transition.
Photocatalytic reduction of Cr(VI) using Mg-doped WO3 nanoparticles
Published in Environmental Technology, 2020
Tungsten trioxide exists in different crystal phases. These are monoclinic II which exists at temperatures less than −50°C, triclinic (−50°C to 17°C), monoclinic I (17°C to 330°C), orthorhombic (330°C to 740°C) and tetragonal (> 740°C) [34]. The evolution of the crystal structure of WO3 is normally accompanied by a reduction on band gap energy (Eg), and whose change is in the following order monoclinic > orthorhombic > tetragonal [35]. The most stable phase is monoclinic, which in this context, shall be referred to as m-WO3.