Molybdenum In Aqueous Solution Research Articles

Molybdenum Oxide/Dopamine-Derived Carbon Electrodes with Enhanced Electrochemical Activity in Energy Storage Systems

Herein, a versatile sol-gel reaction produces new electrode materials consisting of tightly integrated molybdenum oxide and carbon derived from chemically incorporated dopamine molecules, and the materials show enhanced electrochemical activity in nonaqueous Li-ion and aqueous Zn-ion energy storage systems. The novel synthesis entails the oxidation of molybdenum in aqueous solutions of excess and limited dopamine (Dopa) hydrochloride via a hydrogen peroxide-initiated process.The transformation of molybdenum under the condition of Dopa excess (Mo:Dopa molar ratio of 1:1) resulted in the formation of a metastable precipitate of polydopamine (PDopa) spheres encapsulated by Dopa-preintercalated molybdenum oxide, (Dopa)xMoOy@PDopa. Hydrothermal treatment (HT) of (Dopa)xMoOy@PDopa precursor was concomitant with concurrent Dopa carbonization and molybdenum reduction processes, resulting in a formation of spherical matrices of Dopa-derived carbon decorated by MoO2 nanoplatelets (HT-MoO2/C), as determined through FTIR spectroscopy, Raman spectroscopy, and SEM imaging. Annealing (An) of HT-MoO2/C at 600°C under argon atmosphere (AnHT-MoO2/C) led not only to improvements in MoO2 crystallinity, but also to an increased oxidation state of molybdenum and a facilitated interaction between molybdenum-based and Dopa-derived components, resulting in an intimate MoO2/C heterointerface. Consequently, while both HT-MoO2/C and AnHT-MoO2/C showed reversible intercalation-type behavior when evaluated as electrodes versus Li/Li+ in nonaqueous lithium-ion cells, AnHT-MoO2/C demonstrated higher capacities, enhanced capacity retention, better rate capability, and lower charge transfer resistance. The AnHT-MoO2/C electrode showed an initial specific capacity of 260 mAh/g and 67% capacity retention after 50 cycles at 10 mA/g, compared to an initial specific capacity of 235 mAh/g and 47% capacity retention shown by HT-MoO2/C at the same current density. Furthermore, in rate capability experiments, HT-MoO2/C and AnHT-MoO2/C delivered specific capacities of 93 mAh/g and 120 mAh/g respectively at 100 mA/g.When molybdenum was transformed in the presence of Dopa deficit (Mo:Dopa molar ratio of 5:1), a (Dopa)xMoOy powder precursor was isolated, and subsequent hydrothermal treatment of this precursor produced an MoO3 material with carbonized Dopa molecules, HT-MoO3/C. Reference α-MoO3 electrodes (α-MoO3-ref) were synthesized similarly but in the absence of Dopa molecules in the initial sol-gel reaction. The appearance of characteristic D and G bands in the Raman spectra and distinct vibrational modes in the FTIR spectra of HT-MoO3/C confirmed the presence of carbon in its structure. SEM images showed a uniform nanobelt morphology with fragmentation due to interactions between interlayer Dopa and MoO3 layers under the conditions of hydrothermal treatment. HT-MoO3/C delivered a second-cycle capacitance of 141.4 F/g when cycled at 2 mV/s in a -0.25–0.70 V versus Ag/AgCl potential window in 5M ZnCl2 electrolyte, while α-MoO3-ref delivered a nearly two-fold smaller second-cycle capacitance of 76.1 F/g under the same conditions. HT-MoO3/C also showed increased capacitance compared to α-MoO3-ref when cycled at increasing sweep rates up to 20 mV/s. The superior performance of HT-MoO3/C prompted a study of the electrode in an expanded potential window, based on previous reports in which MoO3 showed electrochemical activity at negative potentials versus Ag/AgCl. The HT-MoO3/C electrode exhibited a capacitance of 347.6 F/g on the second cycle when cycled between -0.85–1.00V versus Ag/AgCl at 2 mV/s in 5M ZnCl2 electrolyte.This work demonstrates a new strategy to improve the electrochemical performance of transition metal oxide electrodes for next-generation energy storage systems. Integration of oxides with carbon through the wet chemistry synthesis approaches that involve carbonization of organic molecules can be used to control oxide crystal phase and heterointerfaces leading to improved charge transfer and energy storage properties.

Etching Studies of Mo in Aqueous Solutions

Molybdenum (Mo) has been a choice of substrate metal for chalcopyrite or kesterite solar cells because of its low thermal expansion coefficient, high melting point, good thermal and electrical conductivity, and high wear resistance. Mo surface, however, is almost always covered with molybdenum oxides, unless kinetic limitations prevail. Previously, we reported the mild electrochemical conditions of forming molybdenum blue as well as molybdenum oxides colored in yellow and red in aqueous solutions. Figure 1 shows the representative changes in resonance frequency and current observed when the potential was applied to the molybdenum/quartz in aqueous solutions. The starting potential was –0.7 V, where the resonance frequency was set to be at the base line. As the potential was scanned up to –0.1 V, apparently the anodic current flew and simultaneously the resonance frequency increased. Near –0.1 V, the sharp increase in current as well as resonance frequency was observed, suggesting the significant dissolution of molybdenum into the solution as molybdenum ions. In this work we wish to report the results of electrochemical quartz crystal microbalance studies of etching of molybdenum in aqueous solutions. The present results may be important in relation to cleaning or polishing of molybdenum, which has been traditionally performed in strong acidic solutions such as mixtures of phosphoric acid and sulfuric acid. Details will be presented at the meeting. Acknowledgements Acknowledgements are made for SACHEM’s supply of some samples used in this work. Helpful discussions with Drs. Craig Allen and Paul Vermeulen are greatfully acknowledged. References. 1. Sang-Min Lee, Young-Ho Kim, Mi-Kyung Oh, Suk-In Hong, Hang-Ju Ko, and Chi-Woo Lee, J. Korean Electrochem. Soc. 13, 89-95 (2010). 2. Sang-Min Lee and Chi-Woo Lee, Bull. Korean Chem. Soc. 33, 1443 (2012) 3. Viswanathan S. Saji and Chi-Woo Lee, Electrochem. Acta, 137, 647-653 (2014). 4. Viswanathan S. Saji, Chan-Yong Jung and Chi-Woo Lee, J. Electrochem. Soc. 162, D465-D479, (2015). Figure 1

Separation of rhenium from electric-oxidation leaching solution of molybdenite

The separation of rhenium from molybdenum in aqueous solution has always been a problem in hydrometallurgy. The separation of rhenium from the electro-oxidation leachate of molybdenite and its mechanism were investigated. The results show that pH of the leachate significantly affects adsorption rate compared with other experimental parameters. When temperature is 30 °C, pH=8, and adsorbing time is 1 h, adsorption rates of rhenium and molybdenum are 93.46% and 3.57%, respectively, and separation factor of D301 resin for rhenium and molybdenum is 169.56. In addition, the separation factor is higher when the initial molybdenum concentration in model solution is increased. The saturated adsorption capacity of D301 resin for molybdenum and rhenium calculated based on simulated results are 4.263 3 mmol/g and 4.235 5 mmol/g, respectively. D301 resin is an effective separation material of rhenium from electric-oxidation leachate of molybdenite. The adsorption kinetics results also show that the adsorption of rhenium is easier than that of molybdenum, and the adsorption process of D301 for rhenium and molybdenum may be controlled by liquid film diffusion.

Ionic Equilibria and Ion Exchange of Molybdenum(VI) from Strong Acid Solution

was changed from 1 to 9M. From the data on the complex formation of molybdenum in aqueous solution, a new distribution diagramof Mo(VI) was constructed in the pH range from zero to 10. AG 1 X-8, an anion exchange resin, and Diphonix,a cation exchange resin were used in the loading experiments. Ion exchange results indicate that anioniccomplexes of Mo(VI) begins to form from 3 M HCl and 1 M H

Molybdenum (VI) binded to humic and nitrohumic acid models in aqueous solutions. Salicylic, 3-nitrosalicylic, 5-nitrosalicylic and 3,5 dinitrosalicylic acids: part 2

In this work electrochemical and Ultraviolet-Visible studies were performed in solutions of salicylic acid models of humic and nitrohumic acids, a laboratory artifact, and molybdenum in order to determine the affinity of these models towards the metal ion. Molybdenum, which plays a very important role in the soil chemistry, and together with humic substances, impart fertility to soil and water and is a key element in the activity of nitrogenase. The obtained results showed that at least one complexed species is present at the pH range of 6.3 to 8.0, even for the less basic chosen models, the nitrosalicylic acids. Previous study showed that phthalic and nitrophthalic, also humic and nitrohumic acids model compounds, presented complexed species with molybdenum only till pH 6.5. The calculated formation constants showed that the substitution of the nitro group in the orto position was less favoured than in the para substitution, probably due to a steric hindrance in the former, which was clearly seen in the double substituted salicylic nitro derivative. The cyclic voltammetry as well as the Ultraviolet-Visible obtained spectra were able to show that the chemistry of molybdenum in aqueous solutions as the pH is increased is very complex, and the molybdate stops acting as an anion only after pH around 4, when it finally becomes a cation MoO2(2+) (M).

Study on the reaction of 2,3-dihydroxybenzoic acid with molybdenum in aqueous solution. I. Synthesis and characterization of the oligomeric complexes formed

Dimeric or oligomeric oxo-complexes of Mo(VI) with 2,3-dihydroxybenzoic acid were prepared in aqueous solutions in the presence or not of K 2S 2O 5 (acting as a reducing agent) in various conditions. The complexes were found to contain the cis - ( Mo 2 O 5 ) 2 + core and the ligands in the catecholate, semiquinonate or mixed valence oxidation form, depending on the reaction conditions and especially on the presence or not of the reductant. The isolated complexes in the presence or absence of reductant and the oxidation products in solution in the presence of air were studied via elemental, thermogravimetric and electrochemical analysis, Infrared, Raman, NMR and ESR spectroscopies and Electrospray Mass Spectra. The general molecular formula for the complexes is {[(PPh 4) 2(Mo 2O 5L 2X 2] · xH 2O)} n , where the coordinated ligand’s L oxidation form varies and X involves coordinated water or hydroxyl group depending on the ligand oxidation state.

Corrosion and passivation behaviors of molybdenum in aqueous solutions of different pH

The corrosion and passivation behaviors of molybdenum in aqueous solutions was investigated. The effect of oxygen on the open-circuit potential behavior of the metal was examined. The steady state electrode potential of molybdenum was found to be a linear function of the solution pH. In naturally aerated solutions, a slope of ≈0.048 V/decade for the E ss vs. pH relation in the whole pH range was obtained. It means that the mechanically polished Mo-electrode behaves in aqueous solutions at constant oxygen concentration like a pH-indicator electrode. In oxygen containing solutions, the steady state potential gets more positive and the value of E ss is controlled by both the oxygen concentration and the solution pH. The metal surface is always covered by a passive film. The electrochemical impedance spectroscopy (EIS) and polarization measurements have shown that the naturally occurring passive film is more stable in acidic solutions. In basic solutions, the stability of the passive film decreases due to the formation of soluble species (HMoO 4 − and MoO 4 2−). The dissolution parameters of the passive film were calculated from both EIS and polarization measurements and the activation energy of the dissolution process in the different solutions was calculated. It turned out that the activation energy of oxide film dissolution is lower in basic solutions (≈28.3 kJ mol −1) than in neutral (≈52.5 kJ mol −1) or acidic solutions (≈62.4 kJ mol −1) which reflects the relative instability of the passive film in basic solutions.

Influence of the drying rate on the distribution of active metals and on the activity of heavy crude hydrotreating catalyst

It has been shown that rapid dryinq of an impregnated catalyst containing molybdenum in aqueous solution in its pores produced a heterogeneous distribution of metals in the particle. A transfer of solution from meso-macropores to macropores by capillarity was responsible for this heterogeneity. In Mo/Al 2O 3 catalyst, micro- and macroheterogeneity produced tower activity for hydrodesulfurization and hydrodemetallization reactions of large diameter molecules controlled by interparticle diffusion.

Corrosion and electrochemical behavior of certain alloys of uranium with zirconium, niobium, and molybdenum in aqueous solutions

Corrosion and electrochemical behavior of certain alloys of uranium with zirconium, niobium, and molybdenum in aqueous solutions

The electroreduction of hexavalent molybdenum in aqueous solution

Polarographic, voltammetric and rotating ring-disk studies have been used to study the electroreduction of hexavalent molybdenum in aqueous solutions of various pH. In all cases investigated, the formation of adsorbed surface films plays a predominant role in the overall electrochemical behavior. Correlations between the voltammetric and the rotating ring-disk studies on the one hand with the polarographic behavior on the other suggest that Mo(V) and Mo(III) species are the sole electrode products and that the appearance of more than two polarographic waves is most probably the result of surface film formation and does not thefefore reflect the formation of Mo(IV) or Mo(II) species as suggested previously.

The electroreduction of hexavalent molybdenum in aqueous solution

Summary Polarographic, voltammetric and rotating ring-disk studies have been used to study the electroreduction of hexavalent molybdenum in aqueous solutions of various pH. In all cases investigated, the formation of adsorbed surface films plays a predominant role in the overall electrochemical behavior. Correlations between the voltammetric and the rotating ring-disk studies on the one hand with the polarographic behavior on the other suggest that Mo(V) and Mo(III) species are the sole electrode products and that the appearance of more than two polarographic waves is most probably the result of surface film formation and does not thefefore reflect the formation of Mo(IV) or Mo(II) species as suggested previously.

The state and adsorption behaviour of traces of molybdenum(VI) in aqueous solutions

The state of hexavalent molybdenum in aqueous solution has been studied by dialysis, electromigration and centrifugation. It was found that molybdenum forms neither true colloids nor pseudocolloids in the pH region investigated and that in the region 1–2·5 and 5 in the form of anion HMoO 4 − present in the form of nondissociated molecules, at pH between 2·5 and 5 in the form of anion HMoO 4 −. In very acid solutions ( c HClO 4 > 6 M the cationic form MoO 2 2+ prevails. The adsorption of traces of molybdenum on ion exchangers, glass, plexiglass, polyethylene and paper has been measured as a function of pH, ionic strength of the solution and molybdenum concentration and probable mechanisms of sorption have been proposed.

Molybdän(VI)-hydroxylamin-komplexe II. Ternäre komplexe von molybdän(VI) mit hydroxylamin und chelonen sowie vergleich mit binären molybdän (V)-und molybdän(vi)-chelonaten

It has been shown from u.v. absorption spectra that hexavalent molybdenum reacts with hydroxylamine with the formation of complexes. The chemical behaviour of these complexes and measurements of their i.r. spectra show that in them the hydroxylamine behaves as a bidentate ligand and is co-ordinated with the molybdenum both through the nitrogen and the oxygen. Spectral measurements in the u.v. and in the visible, i.r. spectra, sorption experiments with a chelate-forming exchange resin, and mass-analysis studies, show that the binary molybdenum(VI) hydroxylamine complex forms ternary complexes with ethylenediaminetetraacetic acid, diaminocyclohexanetetraacetic acid and nitrilotriaceticic acid. The ternary complex with EDTA contains molybdenum, hydroxylamine and EDTA in a molar ratio of 1:1:1. The infrared spectra show that all four carboxyl groups of the EDTA are bound in the same way to the molybdenum(VI), which indicates that six co-ordination sites of the central atom are occupied by the EDTA. The additional occupation of two co-ordination sites by the hydroxylamine provides evidence of an eight-fold co-ordination of hexavalent molybdenum in aqueous solution. Reference is made to the possibility of chelometric titration of hexavalent molybdenum with EDTA, DCTA or with NTA ∗ ∗ See list of abbreviations given on p. 163. . The advantages of this procedure are pointed out.