Behaviour of molybdate-passivated zinc coated steel exposed to corrosive chloride environments

The behaviour of molybdate conversion coatings on zinc coated mild steel in corrosive chloride environments was investigated using electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and neutral salt fog exposure. It was found that the presence of a simple molybdate coating initially increased the rate of corrosion of zinc. Molybdenum species were initially present in the conversion coating in either the +V or +VI oxidation states. Exposure to neutral salt fog reduced molybdenum to either Mo(IV) or Mo(III). This reduction of molybdenum, an additional cathodic process, may result in the activation of zinc observed in these studies. For molybdate-passivated surfaces in the early stages of exposure to neutral salt fog, corrosion products were found to be less voluminous than those observed on untreated surfaces. This may be due to the presence of inhibiting Mo(IV) or Mo(III) species in the corrosion product layers. However, after 24 h exposure to salt fog, no molybdenum could be detected. This implies that the lower oxidation state molybdenum species formed are soluble. However, surfaces passivated from molybdate solutions appear to forestall the onset of red rust, during immersion in chloride solutions and exposure to salt fog, by approximately 12 to 24 h. This behaviour may be attributable to corrosion inhibition by Mo(III) and Mo(IV) species while they are present at the surface.     

Speciation of Molybdenum (VI) In Aqueous and Organic Phases of Selected Extraction Systems

Abstract

The extraction study of molybdenum (VI) by 30% tri-n-butyl phosphate in n-dodecane and 0.2 M octyl (phenyl)-N,N-di-isobutylcarbamoylmethylphosphine oxide in 30% tri-n-butyl phosphate extraction systems was performed from aqueous solution containing HCl, HNO₃ and acetohydroxamic acid. Depending on extraction conditions, acetohydroxamic acid can significantly affect the speciation of molybdenum and can increase or decrease its distribution ratio. Our investigation confirmed the strong ability of the acetohydroxamic acid to form complexes with Mo even in highly acidic solutions. UV absorption spectra confirmed that a fraction of the Mo(VI)-AHA species can be present in the organic phase after extraction.     

Mechanism of anodic oxidation of molybdenum in nearly-neutral electrolytes studied by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy

Anodic oxidation of molybdenum in weakly acidic, nearly neutral and weakly alkaline electrolytes was studied by voltammetric and electrochemical impedance spectroscopic measurements in a wide potential and pH range. Current vs. potential curves were found to exhibit two pseudo-Tafel regions suggesting two parallel pathways of the dissolution process. Electrochemical impedance spectra indicated the presence of at least two reaction intermediates. X-ray photoelectron spectroscopic (XPS) results pointed to the formation of an oxide containing Mo(IV), Mo(V) and Mo(VI), the exact ratio between different valence states depending on potential and pH of the solution. A physico-chemical model of the processes is proposed and a set of kinetic equations for the steady-state current vs. potential curve and the impedance response are derived. The model is found to reproduce quantitatively the current vs. potential curves and impedance spectra at a range of potentials and pH and to agree qualitatively with the XPS results. Subject to further improvement, the model could serve as a starting point for the optimization of the electrochemical fabrication of functional molybdenum oxide coatings.     

Inclusion Compound of Poly(vinyl alcohol) with Multivalent Molybdenum Coordination Compound

An inclusion compound of Mo(VI,V,IV) complexes and poly(vinyl alcohol) (PVA) that contains some carbonyl groups was prepared by a photoelectron transfer reaction between PVA and 12-molybdophosphoric acid (PMo₁₂). A dark-blue film was obtained when the aqueous solution of PMo₁₂ and PVA was irradiated with UV light. The film exhibited the characteristic electron spin resonance (ESR) signal of Mo(V). The existence of Mo(VI,V,IV) complexes was supported by XPS analysis of the film. Furthermore, the infrared (IR) and ultraviolet–visible (UV–Vis) spectra of the film showed weak absorption bands at 1710 cm^–1 and 300–314 nm that are consistent with carbonyl groups, which presumably are a result of partial oxidation of secondary hydroxyl groups on PVA. Differential scanning calorimetry (DSC) of the dark-blue film exhibited a peak at 220.3°C that is different from PMo₁₂ and PVA. Except for the IR absorption at 1710 cm^–1, the wavelengths and shapes of the other IR bands were similar to those of PVA. The UV–Vis spectrum of the film showed a new absorption band at 742–770 nm. In addition, the XRD spectrum of the film, the diffraction angles, and the crystalline sites were different from that in PMo₁₂ or PVA. However, the peak shape was similar to that of PVA. In summary, the DSC, IR, UV–Vis, and XRD data support a Mo(VI,V,IV) complex that is included into the spiral tube structure of PVA, which contains some carbonyl groups.     

Laser Raman spectroscopy (LRS) and time differential perturbed angular correlation (TDPAC) study of surface species on Mo/SiO2 and Mo,Na/SiO2. Their role in the partial oxidation of methane

Silica-supported molybdenum (1.6 and 5.0 wt%) and molybdenum (5 wt%)-sodium (0.4 wt%) catalysts have been characterized by laser Raman spectroscopy (LRS), time differential perturbed angular correlation (TDPAC), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The presence of different molybdenum species was correlated with activity and selectivity to formaldehyde during the methane partial oxidation reaction. The main species identified on the Mo(5.0 wt%) /SiO₂ surface were MoO₃ and monomeric species with a single Mo=O terminal bond. The pre-impregnation of the silica support with sodium strongly diminishes the Mo=O concentration due to the formation of Na₂Mo₂O₇ species and tetrahedral monomers with a high degree of symmetry. As a result of these modifications, both methane conversion and formaldehyde formation are strongly inhibited. The combination of LRS and TDPAC techniques resulted in a powerful tool for the identification and quantification of the molybdenum species present on the surface of a silica support. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electrochemical and spectroscopic studies of molybdenum(VI) complexes with aminopolycarboxylic acids

The complexes of molybdenum(VI) with iminodiacetic acid (IDA), nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA) were investigated by means of polarographic and spectroscopic methods. The composition of the complexes was determined by the Job's method as following: 1Mo(VI)/1IDA; 1Mo(VI)/1NTA and 2Mo(VI)/1EDTA. The polarographic wave height was fairly dependent on the nature of the complexes. The overall reaction rate for the reduction of Mo(VI)-IDA complex was limited by diffusion. On the other hand, the polarographic reduction of Mo(VI)-NTA and - EDTA complexes were kinetically controlled by the preceding chemical reaction which was composed of protonation, dimerization and substitution of ligands, however, the rate-determining step was the substitution of ligands. The mechanism for the electroreduction of each complex was discussed.     

New crosslinked N‐vinylpyrrolidone copolymers: Synthesis and sorptive properties

The crosslinked copolymers of N‐vinylpyrrolidone with 1,4‐(N,N′‐bismaleimido)benzol and with trimethoxyvinylsilane were synthesized by free radical copolymerization. The copolymers were characterized by FTIR spectroscopy and thermal analysis. The values of specific surface area and porosity of the copolymers were determined with use of low‐temperature adsorption. Sorption capacity of the copolymers toward Re (VII), Mo (VI), and W(VI) ions was investigated and was found to depend strongly on the pH. A possibility to separate Re(VII) and Mo(VI) ions with use of the copolymers under investigation in their combined presence in neutral and alkaline media was shown. Moreover, in the conjoined presence of Mo(VI) and W(VI) ions at pH 5–14, tungsten(VI) can be separated from molybdenum(VI) with the copolymer of N‐vinylpyrrolidone with trimethoxyvinylsilane. POLYM. ENG. SCI., 56:1303–1312, 2016. © 2016 Society of Plastics Engineers     

Selective Adsorption of Mo(VI) From Re(VII) Containing Effluent on Orange Waste Gel Modified by Ethylenediamine,Diethylamine, and Triethylamine

New adsorption gels were prepared by chemically immobilizing functional groups of ethylenediamine, diethylamine and/or triethylamine on orange waste, named OW-en, OW-DEA, and OW-TEA, respectively. By comparing with the adsorption of other coexisting metals, such as Re(VII), Pb(II), Fe(III), Zn(II), Mn(VII), Ca(II), and Cu(II), the novel gels exhibited selectivity only for Mo(VI) and the adsorption behavior obeys the Langmuir model. The maximum adsorption capacity for molybdenum was in the order, OW-en (2.17 mol/kg) > OW-TEA (1.26 mol/kg) > OW-DEA (0.88 mol/kg). A kinetic study for the adsorption of molybdenum at various temperatures confirmed that the endothermic adsorption process followed pseudo-second order kinetics. In addition, its excellent adsorption characteristics for Mo(VI) were confirmed by the adsorption and elution tests using a column packed with the OW-en gel, especially by separation of Mo(VI) from Mo-Re containing industrial effluent.     

Separation of Molybdenum(VI) and Tungsten(VI) through a Supported Liquid Membrane Impregnated with Trioctylmethylammonium Chloride

Abstract

Transport separation of molybdenum(VI) through a supported liquid membrane (SLM) was investigated by employing trioctylmethylammonium chloride (TOMAC) as a mobile carrier. The transport behavior of Mo(VI) and W(VI) was greatly dependent on hydrochloric acid and chloride ion concentrations in the feed solution. Molybdenum(VI) was effectively transported together with W(VI) from dilute HCl solutions in the presence of 0.1 M tartaric acid into NaOH stripping solutions. Molybdenum(VI) in high HCl concentration was transported in preference to W(VI). Addition of NaCl to the feed solution reduced the W(VI) transport, and this enhanced the separation of Mo(VI) from W(VI). Lower TOMAC concentration in SLM was favorable for the separation of the two metal ions, and a separation factor (α_Mo/W) of 46 was obtained with 0.1 M TOMAC-SLM.     

A kinetic study on the corrodability of anodic oxide films formed on molybdenum in NaOH solutions

The corrodability of anodic oxide films formed on molybdenum in NaOH solutions was studied using impedance and potential measurements. The corrosion rate was found to increase with increase of alkali concentration, film thickness and temperature and was nearly independent of the rate of oxide formation. The dissolution process was found to involve a valency change from Mo(IV) to Mo(VI) where it seemed, from cathodic polarization, that no electron transfer through the oxide film to/from the metal surface was involved during the dissolution process. In concentrated NaOH solutions ([OH^–]9 M), the dissolution process appeared to follow zero-order kinetics.     

Behavior of Molybdenum (VI) in {DMDOHEMA–HDEHP/nitric acid} Liquid–Liquid Extraction Systems

The EXAm (extraction of americium) process was developed for americium recycling in future nuclear fuel cycles. In this solvent extraction system, a combination of two extractants, N,N′-dimethyl-N,N′-dioctyl-hexylethoxy malonamide (DMDOHEMA) and di-2-ethylhexyl phosphoric acid (HDEHP), in TPH (hydrogenated tetrapropylene) is used to extract americium in the first step of the process at high acidity (HNO₃ 5–6 M). Americium is co-extracted with light lanthanides and other fission products like molybdenum, iron, ruthenium, etc.. Molybdenum is selectively scrubbed during the second step at low acidity using citric or glycolic acid as a buffer and complexing agent. The speciation of Mo(VI) in aqueous solutions is highly dependent on acidity and Mo concentration. In this article, a simple thermodynamical model is proposed for Mo(VI) scrubbing based on batch extraction experiments (with pH and cation concentration variations) and stoichiometries of complexes formed in the organic phase according to electrospray ionization mass spectrometry (ESI-MS) experiments and published data on Mo(VI) speciation. At high acidity ([HNO₃] > 1 M), the MoO₂²⁺ species is strongly extracted by the solvent DMDOHEMA–HDEHP according to a solvate mechanism. At lower acidity ([HNO₃] < 1 M), cation exchange mechanisms become predominant and DMDOHEMA does not participate to the extraction Mo(VI) anymore. During Mo scrubbing at pH higher than 1, the extraction of Mo as neutral species (like MoO₃) and anionic species (like MoO₄^2–) has to be taken into account in the model to predict the “bell-shape” of Mo distribution ratio evolution as a function of pH. This model was then implemented in the PAREX simulation code developed by the CEA to build the flowsheet for the “Mo scrubbing” section of EXAm process and predict Mo concentrations profiles in batteries of mixer-settlers during pilot-scale tests.     

Hydrodesulfurization of dibenzothiophenes over molybdenum catalyst supported on TiO2–Al2O3

Composite types of TiO₂–Al₂O₃ supports, which are γ-aluminas coated by titania, have been prepared by chemical vapor deposition (CVD), using TiCl₄ as a precursor. Then supported molybdenum catalysts have been prepared by an impregnation method. As supports, we employed γ-alumina, anatase types of titania, and composite types of TiO₂–Al₂O₃ with different loadings of TiO₂. We studied the conversion of Mo from oxidic to sulfidic state through sulfurization by X-ray photoelectron spectroscopy (XPS). The obtained spectra unambiguously revealed the higher reducibility from oxidic to sulfidic molybdenum species on the TiO₂ and TiO₂–Al₂O₃ supports compared to that on the Al₂O₃ support. Higher TiO₂ loadings of the TiO₂–Al₂O₃ composite support led to higher reducibility for molybdenum species. Furthermore, the catalytic behavior of supported molybdenum catalysts has been investigated for hydrodesulfurization (HDS) of dibenzothiophene (DBT) and methyl-substituted DBT derivatives. The conversion over the TiO₂–Al₂O₃ supported Mo catalysts, in particular for the 4,6-dimethyl-DBT, is much higher than that obtained over Al₂O₃ supported Mo catalyst. The ratio of the corresponding cyclohexylbenzene (CHB)/biphenyl (BP) derivatives is increased over the Mo/TiO₂–Al₂O₃. This indicates that the prehydrogenation of an aromatic ring plays an important role in the HDS of DBT derivatives over TiO₂–Al₂O₃ supported catalysts.     

Adsorption and separation of rhenium(VII) using N‐methylimidazolium functionalized strong basic anion exchange resin

BACKGROUND: Rhenium is a rare and valuable metal coexisting with molybdenum in molybdenite and sulfide copper ores and is hard to separate. Methods such as solvent extraction, ion exchange, adsorption, precipitation, etc. have been used to separate and purify Re(VII), and resin ion exchange has been widely investigated and used owing to its convenient operation and relatively low‐cost. The main challenge comes from the preparation of resins having high performance and stability. In an earlier study, an N‐methylimidazolium functionalized strong basic anion exchange resin was successfully synthesized and used for Cr(IV) extraction. This paper, reports on further synthesis and application of this resin for adsorption and separation of Re(VII) and Mo(VI), especially from copper arsenic filter cake. RESULTS: At pH 6.25, Re(VII) and Mo(VI) can be effectively separated with the N‐methylimidazolium functionalized strong basic anion exchange resin, giving a Re(VII) recovery of up to 93.3%, but only 5.1% for Mo(VI). When using the copper arsenic filter cake sample 89.1% Re(VII) recovery was achieved. CONCLUSION: The N‐methylimidazolium functionalized strong basic anion exchange resin exhibited high selectivity and recovery for Re(VII) in a mixed system. © 2012 Society of Chemical Industry     

Extraction and separation of molybdenum(VI) from acidic media by fatty hydrazides

BACKGROUND: The increasing demand for molybdenum has encouraged the development of low‐cost and environmentally friendly extractants to recycle and recover this metal. In the present study, solvent extraction of Mo(VI) from acidic media using a mixture of fatty hydrazides synthesised from palm olein as the extractant was carried out. The effects of various parameters such as acid, diluent, contact time, extractant concentration, metal ion concentration and stripping agent and the separation of Mo(VI) from other metal ions such as Co(II), Ni(II), Al(III) and Mn(II) were investigated. RESULTS: It was found that the extraction of Mo(VI) into the organic phase involved the formation of 1:3 complexes. Mo(VI) was successfully separated from commonly associated metal ions such as Ni(II), Co(II), Al(III) and Mn(II). Mo(VI) stripping from the loaded organic phase was studied using different acidic and alkaline solutions and was found to be optimal with ammonium hydroxide solution. CONCLUSION: These results are useful for the development of a method to recover Mo(VI) from acidic media utilising fatty hydrazides as the extractant. Copyright © 2008 Society of Chemical Industry     

Kinetic studies with a sulfur-tolerant water gas shift catalyst

Experimental measurements have demonstrated high water gas shift (WGS) activity for molybdenum sulfide-based catalyst in the presence of hydrogen sulfide. The defect structure of the catalyst, controlled by the sulfur activity in the gas phase, has a dominant effect on the reaction rate. The reaction kinetics are interpreted in terms of Langmuir-Hinshelwood surface reaction. The mechanism is shown to involve a redox cycle with Mo(V) as an important species in WGS catalysis. XPS measurements provide information on the reductive sulfidation of the catalysts from the Mo(VI)-oxide state.     

Optical Spectroscopic Investigation of Hexavalent Actinide Ions in n-Dodecane Solutions of Tri-butyl Phosphate

ABSTRACT

The extraction of hexavalent actinides An(VI) by tri-butyl phosphate (TBP) was investigated by electronic absorption and vibrational spectroscopies. Through a series of spectral subtractions, vibrational spectra associated with TBP, TBP–HNO₃ adducts, and An(VI)–TBP complexes could be isolated. Investigation of U(VI) extracts indicated spectral features consistent with the formation of the expected [UO₂(NO₃)₂(TBP)₂] complex, but spectral features of other species were clearly evident. Likewise, multiple species were evident in the electronic absorption and vibrational spectra of TBP phases generated by extraction of Pu(VI). Although definitive characterization of the additional species formed could not be achieved in this work, it is hypothesized that they contain 3:1 TBP-to-An(VI) stoichiometry.     

Application of crosslinked chitosan in the analysis of ultratrace Mo(VI)

In this work, the adsorption properties of crosslinked chitosan for Mo(VI) were studied. The adsorption rate of Mo(VI) by CCTS was 97% at pH 3.0. Adsorption balance time, elution conditions, the effect of coexisting elements, and the adsorption mechanism were investigated. A novel method of ultratrace Mo(VI) preconcentration with CCTS and determination by graphite furnace atomic absorption spectrometry was found. The detection limit (3σ, n = 10) of the method was 0.040 μg L^?1, and the relative standard deviation was 2.98% at 1.00 μg L^?1. The method has been applied to the determination of Mo(VI) in environmental water samples, with satisfactory results. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 432–435, 2006     

The electroreduction of molybdenum(VI) complex with tartrate ion in a neutral solution

Hexavalent molybdenum was reduced electrochemically in a neutral solution containing tartrate ion. The formation of a complex of Mo(VI) with tartrate ion was confirmed by means of uv spectroscopy and potentiometry. The polarographic wave was kinetic at higher pH of lower concentration of tartrate ion, and the electrochemical reaction was proceeded by the protonation reaction in which the electro-active species MoO₄ (Htar)^4?₂ was produced.     

Synthesis of Higher Alcohols from Syngas over Ultrafine Mo—Co—K Catalysts

Ultrafine Mo–Co–K catalysts were prepared and tested for higher alcohol synthesis. The catalysts exhibited high catalytic activity. The effect of the mole ratio of cobalt and molybdenum in the catalysts upon the catalytic performance of higher alcohol synthesis was investigated. Among the ultrafine Mo–Co–K catalysts, the best one corresponded to the Co/Mo mole ratio of 1:7. The XPS spectra revealed that molybdenum was present in two species: Mo⁶⁺ and Mo⁴⁺ on the surface of reduced catalysts, and the Mo⁴⁺ species content depended strongly on the Co/Mo mole ratio. The selectivity towards higher alcohols was found to be related to the Mo⁴⁺ species content. A linear relation between the selectivity and Mo⁴⁺ species content led to the conclusion that the Mo⁴⁺ species was the main active species for higher alcohol synthesis over the ultrafine Mo–Co–K catalysts.