Metal Molybdates Research Articles

Mechanochemical synthesis of nanocrystalline nickel-molybdenum compounds and their morphology and application in catalysis: II. Effect of mechanochemical activation parameters—process power density and exposure time—on the composition and structure of nickel-molybdenum compounds

The effect of mechanochemical activation parameters-process power density and the time of treatment of a mixture of nickel hydroxocarbonate NiCO3 · 2Ni(OH)2 · nH2O and ammonium paramolybdate (NH4)6Mo7O24 · 4H2O -on the phase composition, specific surface area, and morphology of mechanically activated products at different Ni: Mo atomic ratios was studied. It was found that, at a mechanochemical activation process power density of 6.2 W/g, layered transition metal molybdates (LTM compounds) with different amounts of crystal water were the constituents of the products, whereas the products mechanically activated at a power density of 28.0 W/g contained a mixture of LTM compounds and heteropoly compounds with the Anderson structure regardless of activation time.

Aqueous sol–gel synthesis and thermoanalytical study of the alkaline earth molybdate precursors

The preparation and characterization of the M′–Mo–O nitrate–tartrate (M′ = Mg, Ca, Sr, and Ba) gels, which were produced by the simple aqueous sol–gel method and calcined at 500, 600, 700, 800, 900, and 1,000 °C temperatures are reported. The crystalline alkaline earth metal molybdates (MgMoO4, CaMoO4, SrMoO4, and BaMoO4) and as-prepared M′–Mo–O nitrate–tartrate gels investigated by thermal analysis (TG/DSC), X-ray diffraction (XRD), and scanning electron microscopy (SEM). TG/DSC analysis showed the possible decomposition mechanism of synthesized gels. XRD studies allowed the identification of main types of crystalline structures in the MgMoO4, CaMoO4, SrMoO4, and BaMoO4 systems. Moreover, SEM analysis revealed the changes of surface morphology of the final compounds depending on annealing temperatures.

Hydrothermal Synthesis of Zn2(MoO4)(SeO3) and Photocatalytic Applications

A novel mixed metal molybdates, Zn2(MoO4)(SeO3) 1 has been synthesized by a simple hydrothermal process. Samples obtained are characterized using powder and single crystal X-ray diffraction (XRD), transmission electron microscopy (TEM). Furthermore, the photocatalytic performances of samples obtained are investigated for degradation of RhB under visible light irradiation. photocayalysis experimental result has illustrated that the compound exhibits good photocatalysis.

Porous Iron Molybdate Nanorods: In situ Diffusion Synthesis and Low-Temperature H2S Gas Sensing

In the paper, we developed an in situ diffusion growth method to fabricate porous Fe2(MoO4)3 nanorods. The average diameter and the length of the porous nanorods were 200 nm and 1.2-4 μm, respectively. Moreover, many micropores existed along axial direction of the Fe2(MoO4)3 nanorods. In terms of nitrogen adsorption-desorption isotherms, calculated pore size was in the range of 4-115 nm, agreeing well with the transmission electron microscope observations. Because of the uniquely porous characteristics and catalytic ability at low temperatures, the porous Fe2(MoO4)3 nanorods exhibited very good H2S sensing properties, including high sensitivity at a low working temperature (80 °C), relatively fast response and recovery times, good selectivity, and long-term stability. Thus, the porous Fe2(MoO4)3 nanorods are very promising for the fabrication of high-performance H2S gas sensors. Furthermore, the strategy presented here could be expended as a general method to synthesize other hollow/porous-type transition metal molybdate nanostructures by rational designation in nanoscale.

A Series of Metal Molybdates as Flame-Retardants and Smoke Suppressants for Flexible PVC

A series of metal molybdates were synthesized as flame-retardants and smoke suppressants for flexible poly (vinyl chloride). The powder of molybdates was characterized by the x-ray diffraction. The flame retardancy, smoke suppression and thermal behavior of PVC treated with a series of molybdates were studied by means of limiting oxygen index (LOI), smoke density rating (SDR), thermogravimetry (TG), differential thermogravimetry (DTG), differential thermal analysis (DTA) and scanning electron microscopy (SEM). The results shown that, the flame retardancy and smoke suppression performance of PVC samples depend on the strength of the Lewis acid which generated in the first degradation stage and the stability of residue char in the second stage. While the strong Lewis acid can also function as cracking catalysts in the second stage, and cracking of the principally hydrocarbon char can promote the generation of volatile organic compounds, which act as efficient fuels. This situation results in less smoke, but increased flame.

Eu3+ Activated Molybdate and Tungstate Based Red Phosphors with Charge Transfer Band in Blue Region

Alkaline earth metal and rare earth tungstate and molybdate compounds are promising candidates as host materials for high efficiency narrow spectral emission red phosphors when activated by trivalent europium (Eu3+). These phosphors emit in the range of 610–620 nm with a full width at half maximum (FWHM) of less than 10 nm, which makes them suitable for use in high color rendering index phosphor-converted white light emitting diodes (pc-WLEDs). However these phosphors suffer from poor excitation using the currently available GaInN based high efficiency blue LEDs in the 400–470 nm range. This paper discusses the role of various crystallographic structures on the excitation characteristics of the phosphor. Using appropriate crystal structures, it has been shown that Mo6+-O2− charge transfer band edge shifts from far-UV (280–350 nm) to near-UV and Blue (380–430 nm) region as the coordination number of the transition metal ion is increased from 4 to 6 making these phosphors suitable for excitation with near-UV or Blue LEDs. Furthermore it has been demonstrated that phosphors crystallized from flux exhibit significantly higher emission intensity compared to powders of the same composition synthesized using solid state reactions.

Narrow spectral emission CaMoO4: Eu3+, Dy3+, Tb3+ phosphor crystals for white light emitting diodes

Alkaline earth metal molybdates are promising candidates as a host material for high efficiency narrow spectral emission phosphors. These phosphors could potentially be used for the fabrication of phosphor-converted light emitting diodes (pc-LEDs). Phosphor crystals of calcium molybdate doped with rare earth dopant Ln3+(Ln=Eu, Dy, Tb) grown using flux growth method have been shown to exhibit higher excitation efficiency than the powders synthesized by solid-state reaction process. Molybdenum (VI) oxide has been found to be a suitable flux for growing large size optically transparent high quality crystals at a temperature around 1100°C. Using the excitation wavelengths of 465nm, 454nm and 489nm for CaMoO4: Eu3+, CaMoO4: Dy3+ and CaMoO4: Tb3+, respectively, intense emission lines at wavelengths of 615nm, 575nm and 550nm were observed. The optimized doping concentrations of 12%, 2% and 5% for Eu3+, Dy3+ and Tb3+, respectively, provided the highest luminescence intensity.

Analysis of solid solutions stability in scheelite-type molybdates and tungstates

Mutual solubility of bivalent metal molybdates and tungstates with scheelite structure was theoretically estimated by calculating formation enthalpies and the maximal decomposition temperatures of solid solutions at different temperatures. The theoretical stability of continuous solid solutions in binary systems of bivalent metal molybdates and tungstates was found to be higher than reported literature data. After cooling down continuous substitution solid solution should remain in following systems: CaMoО 4–CdMoО 4, SrMoО 4– MMoО 4 ( M=Ba, Pb), BaMoО 4–PbMoО 4, SrWO 4– MWO 4 ( M=Ca, Pb), and BaWO 4–PbWO 4. There is a probability that at room temperature in systems CaMoО 4–SrMoО 4, CaWO 4–PbWO 4, and BaWO 4–SrWO 4 the single homogeneity region may decompose to limited solid solutions. It was shown experimentally that a continuous series of scheelite-structure solid solutions M 1− x M I x ТO 4 can be formed via citrate synthesis at temperatures below 500°С.

Microwave synthesis of NaLa(MoO4)2 microcrystals and their near-infrared luminescent properties with lanthanide ion doping (Er3+, Nd3+, Yb3+)

Abstract The doubled metal molybdate NaLa(MoO 4 ) 2 microcrystals with various morphologies have been synthesized using microwave assisted hydrothermal method without any organic additives or surfactants. Its morphology and size can be tuned by the initial concentrations of the reagents and the reaction temperatures. Remarkably, the morphologies were the rod, the highly branched dendrite, the ellipsoid-like, when the initial concentration of the precursors was increased from 0.01 to 0.05 M at 180 °C; the morphology can be changed from the octahedral bipyramid to the dendrite when the reaction temperature was increased from 100 to 180 °C at 0.02 M of the reagents. The luminescent properties of the lanthanide ion (Nd 3+ , Er 3+ , Yb 3+ ) doped NaLa(MoO 4 ) 2 microcrystals were studied, and all of them exhibited strong near-infrared luminescence emissions.

Solid-solution red phosphors for white LEDs

Solid solutions of Eu 3+-doped metal tungstate and molybdate phosphors were synthesized by solid-state reactions. The crystal structure of the solid-solution phosphors was determined to be tetragonal with a space group of I4 1/c. The red-emitting solid-solution phosphors exhibited a broad absorption band in the range 220–340 nm and sharp excitation peaks in the near UV to green region. The emission intensities of the solid-solution phosphors were enhanced due to the stiff lattices, as a result of the ordered distribution of cations and anions according to differences in ionic size. The increase in the energy transfer is discussed in terms of metal–ligand distances, ionization potential of Mo and the relatively large overlap between the excitation and emission spectra. Such solid-solution phosphors with a bright red emission intensity, relatively short decay time and appropriate color chromaticity have potential for use as red-emitting materials for white LEDs.

On the sol–gel preparation of different tungstates and molybdates

The preparation and characterization of the M′–M′′–O nitrate–tartrate (M′ = Ca, Ba, Gd and M′ = W, Mo) precursor gels synthesized by simple, inexpensive, and environmentally benign aqueous sol–gel method is reported. The obtained gels were studied by thermal (TG/DSC) analysis. TG/DSC measurements revealed the possible decomposition pathway of synthesized M′–M′′–O nitrate–tartrate gels. For the synthesis of different metal tungstates and molybdates, the precursor gels were calcined at different temperatures (650, 800, and 900 °C). According to the X-ray diffraction (XRD) analysis data, the crystalline compounds CaMo1-x W x O4 doped with Ce3+ ions, BaMo1-x W x O4 doped with Eu3+ ions and Gd2Mo3O12 were obtained from nitrate–tartrate gels annealed at 650–900 °C temperatures. The XRD data confirmed that the fully crystalline single-phase powellite, scheelite, or Gd2(MoO4)3 structures were formed already at 650 °C. Therefore, the suggested sol–gel method based on the complexation of metal ions with tartaric acid is suitable for the preparation of mixed tungstates–molybdates at relatively low temperature in comparison with solid-state synthesis.

Investigation on the thermal decomposition and flame retardancy of wood treated with a series of molybdates by TG–MS

A flame-retardant wood was prepared using a series of insoluble molybdates through the double bath technique. The flame retardancy of the wood samples was studied with the limiting oxygen index (LOI) method. The relationships between the flame-retardant performance and the thermal property of wood were studied by the thermogravimetry (TG), derivative thermogravimetry (DTG), differential thermal analysis (DTA), scanning electron microscopy (SEM), and the thermogravimetry–mass spectrometry (TG–MS) analysis methods. The results showed that the insoluble molybdates, which were precipitated into the wood by the double bath technique, can obviously improve the flame retardancy of wood. Similarly, the transition metal molybdates showed higher flame-retardant efficiency than the main group metal molybdates do, which probably due to the thermal barrier effect that Fe2(MoO4)3 acts during the combustion of the samples. At the same time, Fe2(MoO4)3 catalyzed the dehydration and carbonization reactions of wood, and caused an increase in the amount of char produced, and an improvement of the stability of the char residue. Moreover, the mass spectrometry results indicated that the excess transition metal ions speed up the deep decomposition of the char residue, and resulting in the smoldering of wood.

Perspective on europium activated fine-grained metal molybdate phosphors for solid state illumination

An attempt has been made to synthesize sulfur free Eu3+-activated M1−xMoO4:Eux3+ (M = Zn, Mg, Ca, Ni and Co; x = 0.2) phosphors by an oxalate sol–gel process. The process involves gel formation from solutions of metal nitrates in ethanol, ammonium heptamolybdate tetrahydrate in de-ionized water and oxalic acid in ethanol, digestion for 12 h, drying at 100 °C for 24 h, decomposition in air at 600 °C or above to yield fine grained powder exhibiting different morphologies, viz. distorted spherical agglomerates, dumbbell shaped particles, rectangular bars, faceted particles, ellipsoids or parallelepipeds. The optical absorption bands have been attributed to (i) ligand-to-metal charge transfer (LMCT), i.e., oxygen to molybdenum in the MoO42−group and (ii) f–f electron transitions (viz., 7F0 → 5L6 at 395 nm and 7F0 → 5D2 at 465 nm) of Eu3+ ions upon excitation with a wavelength of ∼395 nm, which occupy non-centrosymmetric metal (M) sites. Eu3+ ions exhibit a strong electric dipole 5D0 → 7F2 transition emitting red light at ∼612 nm with quantum efficiency in a 1.0 : 1.7 : 3.2 ratio for M0.8MoO4:Eu0.23+ (M = Mg, Zn and Ca) phosphors, respectively. Further, their CIE coordinates are (x = 0.68 and y = 0.32) close to known values (x = 0.665 and y = 0.331) of Y2O2S:Eu0.053+ red phosphor. Hence, M0.8MoO4:Eu0.23+ (M = Mg, Zn and Ca) phosphor can possibly be a potential alternative to Y2O2S:Eu0.053+ for fabrication of near UV InGaN based light emitting diodes. Yet, another phosphor of composition Co0.8MoO4:Eu0.23+ is shown to have promise for white light emission with its CIE coordinates as x = 0.279 and y = 0.245.

Study of the selectivity in FCC naphtha hydrotreating by modifying the acid–base balance of CoMo/γ-Al 2O 3 catalysts

CoMo catalysts supported on a commercial γ-Al 2O 3 modified with B, Na or K were prepared in order to analyze the effect of acid–base characteristics on the FCC naphtha hydrotreatment. Catalysts were characterized by NH 3 TPD, IR of pre-adsorbed pyridine and CO 2, XPS, Raman, and XRD. In general, with either boron or alkaline metal modification the HDS/HYDO selectivity did not significantly improve. However, it is remarkable that when the alumina was modified to be either more acidic (B) or more basic (K, Na), improvements in the ratio between the HDS and the conversion of the internal branched olefin as well as in the ratio double-bond isomerization to HYD of linear olefins were observed. For both modifications, this fact was related to the promotion of the double-bond isomerization reaction from external to internal positions. For alkaline-doped catalysts, it was found that the double-bond isomerization was related to the existence of basic sites. For the boron, there is a range of boria content (2–3 wt.%) where the double-bond isomerization was promoted selectively over the cracking and alkylation reactions because only weak Brönsted acid sites are present. Additionally, both modifications (Boron or alkaline metals) of alumina led to a decrease in the HDS activity, which was found to be related to changes in the distribution of Co and Mo species in the oxide state. Alkaline metal introduction led to the formation of alkaline metal molybdates, whereas boron introduction increased the proportion of octahedral polymeric Mo species, consequently, decreasing the dispersion of the Mo phase.

New cadmium and rare-earth metal molybdates with scheelite-type structure

New cadmium and rare-earth metal molybdates with the formula Cd 0.25 RE 0.50MoO 4 ( RE = Pr, Nd, Sm–Dy) were synthesized by the solid-state reaction of CdMoO 4 with corresponding RE 2(MoO 4) 3. The obtained compounds crystallize in the scheelite-type structure. They were characterized here by XRD, DTA-TG, IR and EPR methods. The Cd 0.25 RE 0.50MoO 4 compounds showed solubility in CdMoO 4 forming the Cd x RE 2−2 x (MoO 4) 3−2 x (0.50 < x < 1.00) solid solutions. It can be found from EPR measurements of the samples with volatile Gd 3+ ions content, that Gd 3+ ions can be located at sites of octahedral symmetry, and both temperature and gadolinium content have an influence on local magnetic interaction and relaxation processes of Gd 3+ ions.

Controllable synthesis and luminescent properties of novel erythrocyte-like CaMoO4hierarchical nanostructuresvia a simple surfactant-free hydrothermal route

Synthesis of metal molybdates (XMoO(4), X = Ca, Sr, Ba) have received much attention recently because of their interesting structural and luminescent properties. Here novel erythrocyte-like CaMoO(4) hierarchical nanostructures are synthesized via a simple surfactant-free hydrothermal route. The formation of the calcium molybdate erythrocytes is controllable through adjusting the fundamental experimental parameters including reaction time, temperature and DMAc to H(2)O ratio. The as-synthesized products are characterized using X-ray powder diffraction, scanning electron microscopy, Brunauer-Emmett-Teller and transmission electron microscopy. The results show that the nucleation and growth of the novel erythrocyte-like CaMoO(4) hierarchical nanostructures are governed by an oriented attachment growth mechanism. The luminescent properties of the CaMoO(4) erythrocytes are then studied using a spectrophotometer and the erythrocyte-like CaMoO(4) nanostructures display a strong blue emission. This study provides an easy surfactant-free synthetic route for the controllable construction of inorganic materials with high hierarchy in the absence of any surfactants.

Comparative study of the synthesis of layered transition metal molybdates

Mixed metal oxides (MMOs) prepared by the mild thermal decomposition of layered double hydroxides (LDHs) differ in their reactivity on exposure to aqueous molybdate containing solutions. In this study, we investigate the reactivity of some T–Al containing MMOs ( T=Co, Ni, Cu or Zn) towards the formation of layered transition metal molybdates (LTMs) possessing the general formula AT 2(OH)(MoO 4) 2·H 2O, where A=NH 4 +, Na + or K +. The phase selectivity of the reaction was studied with respect to the source of molybdate, the ratio of T to Mo and the reaction pH. LTMs were obtained on reaction of Cu–Al and Zn–Al containing MMOs with aqueous solutions of ammonium heptamolybdate. Rehydration of these oxides in the presence of sodium or potassium molybdate yielded a rehydrated LDH phase as the only crystalline product. The LTM products obtained by the rehydration of MMO precursors were compared with LTMs prepared by direct precipitation from the metal salts in order to study the influence of preparative route on their chemical and physical properties. Differences were noted in the composition, morphology and thermal properties of the resulting products.

X-ray diffraction studies of transition metal molybdates

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Tungstate and Molybdate Scintillators to Search for Dark Matter and Double Beta Decay

Results are presented on our latest research, aimed at the development and study of oxide scintillation crystals (ZnWO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , ZnMoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , PbWO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , PbMoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , and MgWO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) with high scintillation yield and low intrinsic radioactivity. We report on the improvement of these properties for conventional scintillators, as well as on new promising crystals based on metal tungstates and molybdates. The results are discussed in view of applying these materials in cryogenic experiments searching for dark matter and/or neutrinoless double beta decay.

Linking Simultaneous In Situ WAXS/SAXS/Raman with Raman/ATR/UV–vis Spectroscopy: Comprehensive Insight into the Synthesis of Molybdate Catalyst Precursors

A new setup is presented which enables simultaneous wide- and small-angle X-ray scattering (WAXS/SAXS) and Raman spectroscopic experiments during the synthesis of Mo-based mixed oxide catalyst precursors at the μ-spot beamline at the Berliner Elektronenspeicherring-Gesellschaft fur Synchrotronstrahlung (BESSY). Furthermore, we report about the separate monitoring of the same reactions under comparable conditions by simultaneous combined ATR/UV–vis/Raman spectroscopic measurements. For testing the performance of both experimental setups two syntheses were described comprising the precipitation of metal molybdates by mixing solutions of metal nitrates and ammonium heptamolybdate. Additionally, the effect of H3PO4 admixture on precipitation was investigated. The combined evaluation of spectroscopic and WAXS/SAXS data allows the discrimination between different molybdate species appearing in solution and precipitate. Furthermore, these molybdate species could be assigned to separate phases of different crystallinity.