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Thermodynamics of Gases at Low Pressures
Published in Igor Bello, Vacuum and Ultravacuum, 2017
Molybdenum trioxide has melting point of only 795 °C, which contrasts very much with the melting point of metallic Mo (2623 °C). As a very good example can serve etching of diamond in hydrogen, hydrogen/argon, and hydrogen/argon/3% O2 using a hot filament plasma reactor. Etching in an H2/Ar gas mixture activated by tungsten filaments with temperature of 2100 °C and a substrate bias clearly indicates etching process at which the flat faceted surfaces of diamond polycrystallites turn to the surface with evident etching pits. When oxygen is added in an amount of 3% to a hydrogen/argon gas mixture, the surface morphology is drastically changed.
Atomic Layer Deposition of Two-Dimensional Semiconductors
Published in Mohammad Karbalaei Akbari, Serge Zhuiykov, Ultrathin Two-Dimensional Semiconductors for Novel Electronic Applications, 2020
Mohammad Karbalaei Akbari, Serge Zhuiykov
Molybdenum trioxide (MoO3) exhibits unique catalytic, electrical and optical properties and also several promising functionalities originated from excellent characteristics of MoO3. The bis(tert-butylimido) bis(dimethylamido) molybdenum ((tBuN)2(NMe2)2Mo) is the most common precursor for MoO3 deposition, since it provides good volatility and thermal stability [60–62]. The temperature window in ALD process of molybdenum oxide with O2 plasma and ((tBuN)2(NMe2)2Mo) precursor is in the range of 50–350°C. The as-deposited films at temperature ranges below 250°C are amorphous; thus, a postdeposition annealing process is required in order to transition from an amorphous to crystalline state. While the as-deposited films are substoichiometric, the stoichiometry can be adjusted by the modulation of plasma parameters [63]. Mo(CO)6 is another chemical precursor mostly employed with O2 plasma to deposit MoO3 films in the narrow ALD window of 152–172°C [64,65]. It has been observed that the employment of combination of various oxygen precursors resulted in the linear growth rate of 0.75 Å/cycle. The as-deposited amorphous films were highly uniform with the low level of surface roughness. The oxygen deficiency was detected at the hetero-interfaces between substrate and MoO3 film. The improvement of crystallinity was achieved after postannealing at temperature above 500–600°C. In another strategy, the ozone plasma was used on the surface of ALD MoO3 film deposited on a 300 mm Si wafer [66]. Mo film was deposited by using Si2H6 and MoF6 precursors at 200°C.
Properties and Applications of Molybdenum
Published in C. K. Gupta, Extractive Metallurgy of Molybdenum, 2017
Molybdenum also has an important role to play in the form of chemicals107 in diverse applications such as paint pigments, corrosion inhibitors, catalysts, smoke and flame retardants, and as previously mentioned, high duty lubricants. Molybdates, unless combined with a colored cation or mixed with a colored compound, are white and so have the potential for use as white pigment. Two compounds, calcium molybdate and zinc molybdate, have so far been widely used for this purpose. These pigments are nontoxic and excellent corrosion inhibitors. They have replaced in some applications the lead- and chromium-based corrosion inhibitive pigments. Molybdenum in various forms has long been used as a catalyst. Molybdenum-containing catalysts are prepared from solutions of molybdenum trioxide in ammonia or from various ammonium molybdates. For catalysts two general aspects of molybdenum chemistry are relevant. The first pertains to the molybdenum oxidation states; VI and V especially are very easily interconvertible. The second aspect relates to the change of coordination number. These two features are of considerable importance in providing low activation energy paths for electron transfer and for the creation of anionic vacancies at molybdenum sites. At these vacant sites, catalyst-substrate interaction occurs. For catalyzed oxidation, substrates react with oxide ions of the catalyst lattice and not directly with molecular oxygen. The replenishment of lattice oxide takes place by the reaction of the lattice with oxygen. Thus the metal ions in the catalyst take part in redox reactions involving oxide ion transfer. A special feature of molybdenum is its ability to transfer oxide ions. Selective catalysts are those which provide oxygen via the lattice. Molybdenum is a micronutrient that is essential for nearly all plant life and probably for animals too. In plants, molybdenum is essential to the processes of nitrogen fixation and nitrate reduction. Some soils, especially acid soils, require supplemental molybdenum for healthy plant life. When molybdenum is not available in the soil in amounts adequate for optimum growth, farmers use supplemental molybdenum through fertilizers, foliar sprays, and seed treatments.
Biosorption of hexavalent chromium and molybdenum ions using extremophilic cyanobacterial mats: efficiency, isothermal, and kinetic studies
Published in International Journal of Phytoremediation, 2023
Khairia M. Al-Qahtani, Mohamad S. Abd Elkarim, Foziah F. Al-Fawzan, Afify D. G. Al-Afify, Mohamed H. H. Ali
Molybdenum (Mo) is one of the most biologically active transition elements and is considered significant for plants and animals (Tu et al.2016). Global molybdenum (as Mo metal) mine production was estimated at 281,000 tons in 2014 (Polyak 2016). Half of this amount was converted into molybdenum trioxide (Christensen et al. 2015), and both are slightly soluble in water. Molybdenum concentration becomes toxic when it exceeds 5 mg/L in water, and the molybdate compounds’ toxicity is less than chromate toxicity (Lian et al.2020). Notably, the large effluents of Mo from industrial tailings, such as alloying with steel, printing inks, paints, and ceramics without any treatment, cause serious pollution problems in surface water (Halmi et al.2014). For a long time, workers exposed to Mo(VI) may experience general weakness, irritability, loss of appetite, muscle and joint pain, weight loss, and headache (Brion-Roby et al.2018). There is sufficient evidence for the carcinogenicity of molybdenum trioxide in experimental animals, possibly carcinogenic to humans (IARC 2018). Hexavalent molybdenum, in the form of MoO42 and MoO3, is highly toxic; thus, Mo(VI) removal from aqueous solutions has received considerable attention in recent years (Wu et al.2021).
Efficient preparation of phosphazene chitosan derivatives and its applications for the adsorption of molybdenum from spent hydrodesulfurization catalyst
Published in Journal of Dispersion Science and Technology, 2022
Hala. A. Ibrahium, Bahig M. Atia, Nasser. S. Awwad, A. A. Nayl, Hend A. Radwan, Mohamed A. Gado
Adsorption and complete recovery of Mo(VI) from the leach liquor was performed through the optimum conditions, which included 250 ml leach liquor, 1 g of PZEN/Chi, pH 3.5, 40 min at room temperature. After the adsorption process has been completed, PZEN/Chi was separated and the Mo(VI) content was recovered using 1.5 M HCl for 30 min. The solution containing Mo(VI) was then subjected to precipitation with ammonia, which neutralized the solution around pH 11 To precipitate ammonium molybdate, HCl was used to lower the pH of the solution to 2 and then heated to 90 °C, allowing the creation of the precipitate.,[73] The precipitate of ammonium molybdate was filtered and neutralized with deionized water. Finally, molybdenum trioxide was obtained by calcining the product at 450 °C. The MoO3 product was found chemically to be 97.30% pure, with 2.39% Al, 0.003% P, 0.29% Si, 0.01% Ni, and 0.002% V. the obtained product was confirmed using different techniques EDAX, SEM, and XRD, Figure 5.
Kinetics and mechanism of formation of MoO2 by solid state reaction between MoS2 and MoO3
Published in Canadian Metallurgical Quarterly, 2020
Commercially, molybdenite is the only important sulphide, and is produced from molybdenite ores as a by-product of the copper industry. Molybdenite concentrates, with over 45% Mo as MoS2, are oxidised (roasted) in air at 873–893 K to molybdenum trioxide in multiple-hearth furnaces. The main oxidation reactions can be written as follows: