Methanol Selective Oxidation to Formaldehyde over Iron-Molybdate Catalysts

This review deals with the important industrial reaction of formaldehyde manufacture by methanol oxidation over iron molybdate catalysts. Detailed reference is made to the used catalyst, preparation techniques (coprecipitation, sol-gel like, mechanical mixing, etc.) including unsupported and supported catalysts, promoters and characterization methods. The controversial active phase assignment (stoichiometric versus Mo rich iron molybdate) is discussed. The proposed reaction mechanisms and kinetic laws for the main and side reactions are examined. The catalyst deactivation processes are reviewed and the role of Mo excess on these processes is underlined. Finally conclusions and perspectives are presented.     

Selective conversion of propane to propene by the catalytic oxidative dehydrogenation over cobalt and magnesium molybdates

Catalytic performances of various metal molybdates were tested in the oxidative dehydrogenation of propane to propene with molecular oxygen under an atmospheric pressure. Most of the molybdates tested promoted the selective oxidative conversion of propane to propene and among them cobalt and magnesium molybdates were found highest in the activity and selectivity. It was also found that their catalytic activities were highly sensitive to the catalyst composition, and it turned out that Co_0.95MoO _x and Mg_0.95MoO _x catalysts which have slightly excess molybdenum showed the highest activity in the oxidative dehydrogenation of propane. Under the optimized reaction conditions, higher reaction temperatures and lower partial pressures of oxygen, these catalysts gave 60% selectivity to propene at 20% conversion of propane. Since the molybdates having the surface enriched with molybdenum oxide tended to show high activity for the propane oxidation, surface molybdenum oxide clusters supported on metal molybdate matrix seem to be the active sites for the selective oxidative dehydrogenation of propane.     

The formation of active species for oxidative dehydrogenation of propane on magnesium molybdates

Pure and mixed magnesium molybdate phases (MoO₃, MgMoO₄, and MgMo₂O₇) have been examined for the oxidative dehydrogenation reaction of propane. The results are very sensitive to the stoichiometry and method of preparation. The catalysts exhibiting superior activity and selectivity are characterized by a unique temperature-programmed reduction peak that is not present for the poorly active or selective catalysts. Mixtures of MgMoO₄ and MoO₃ or MgMoO₄ and MgMo₂O₇, materials that perform poorly by themselves, show significant improvements in performance upon heating. The solid-state interactions leading to these improvements correspond to the appearance of the characteristic reduction peak. The results suggest that the beneficial synergistic effects seen with mixtures of inactive phases are due to formation of a new phase or species, rather than remote communication between phases (e.g., oxygen spillover). This revised version was published online in November 2006 with corrections to the Cover Date.     

A potential red-emitting phosphors scheelite-like triple molybdates LiKGd_2(MoO_4)_4:Eu~(3+) for white light emitting diode applications

A series of novel red-emitting phosphors scheelite-like triple molybdates LiKGd2-xEux(MoO4)4(0.1 ≤ x ≤ 1.9) were synthesized by solid state reaction method and their photoluminescence properties were investigated. The photoluminescence results show that all samples can be excited efficiently by UV (396 nm) light and blue (466 nm) light and emit red (615 nm) light with line spectra,which are coupled well with the characteristic emission from UV-LED and blue LED,respectively. The experimental results and thei...     

Quantitative determination of the number of surface active sites and the turnover frequency for methanol oxidation over bulk metal vanadates

The present work investigates the number and nature of the surface active sites, selectivity and turnover frequency towards methanol selective oxidation of a series of bulk metal vanadates. The catalysts were synthesized through an organic route and characterized by laser Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and specific surface area analysis (BET). The number of surface active sites (N_s) was determined by measuring the concentration of surface methoxy species adsorbed on the catalysts exposed to an atmosphere of 2000 ppm of methanol in helium at 100 °C. The specific activity values (TOFs) were calculated by normalizing the methanol oxidation reaction rate by the number of surface active sites probed by methanol chemisorption. The comparison of the methanol oxidation products distribution from bulk metal vanadates, pure V₂O₅ and corresponding metal oxides (NiO, MnO, etc.) strongly suggests that the metal vanadate catalysts consist of only surface vanadium oxide sites. The comparison of the TOF values demonstrated that bulk metal vanadates possess similar activity to monolayer vanadium oxide supported catalysts and are more active than bulk metal molybdates for methanol selective oxidation. Moreover, bulk metal vanadates are as active and selective as the commercial MoO₃/Fe₂(MoO₄)₃ catalysts at high methanol conversion.     

Comparison of MoVTaTeO and MoVNbTeO <Emphasis Type="Italic">M1</Emphasis> crystal chemistry

The structure of a new catalytically active MoVTaTeO variant of the M1-phase propane (amm)oxidation catalyst has been refined. This Ta-containing variant is isostructural with the well-known MoVNbTeO form of the M1 phase, in this case with a = 21.1484(9) ?, b = 26.6472(11) ?, c = 4.00332(1) ? with Z = 4. The formula unit of can be written as (TeO)_0.43 Mo_4.08 V_0.70Ta_0.22O₁₄ to be compared to the previously determined composition for the Nb-containing phase: (TeO)_0.47Mo_3.98V_0.59Nb_0.43O₁₄. The Ta-containing variant is somewhat vanadium-rich compared to its Nb counterpart, but the Ta solubility is only about half of the corresponding Nb solubility. Debate over the true location and role of Nb in the MoVNbTeO M1 phase may be better understood by comparison with the refined structure of this chemically similar Ta analog. In this case Ta is clearly distinguishable from each of the other metals from scattering experiments, unlike Nb.     

Novel FexCr2−x(MoO4)3 electrocatalysts for oxygen evolution reaction

Transition metal mixed oxides of Fe, Cr and Mo with nominal compositional formula, Fe_xCr_2−x(MoO₄)₃ (x = 0, 0.25, 0.50 and 0.75) have been obtained by a co-precipitation method and investigated for their structural and electrocatalytic properties by XRD, TEM, XPS, BET, electrochemical impedance spectroscopy and anodic Tafel polarization. Results show that introduction of Fe for Cr from 0.25 to 0.75 mol into the Cr₂(MoO₄)₃ matrix improved the electrocatalytic activity toward the O₂ evolution reaction (OER) in 1 M KOH considerably; the magnitude of improvement being maximum with 0.5 mol Fe. Values of the Tafel slope were close to 35 mV at low and 2.303RT/F at high overpotentials on Fe-substituted oxides. The OER follows nearly second order kinetics in OH⁻ concentration at low overpotentials.     

Hollow Molybdate Microspheres as Catalytic Hosts for Enhancing the Electrochemical Performance of Sulfur Cathode under High Sulfur Loading and Lean Electrolyte

Lithium–sulfur battery possesses a high energy density; however, its application is severely blocked by several bottlenecks, including the serious shuttling behavior and sluggish redox kinetics of sulfur cathode, especially under the condition of high sulfur loading and lean electrolyte. Herein, hollow molybdate (CoMoO₄, NiMoO₄, and MnMoO₄) microspheres are introduced as catalytic hosts to address these issues. The molybdates present a high intrinsic electrocatalytic activity for the conversion of soluble lithium polysulfides, and the unique hollow spherical structure could provide abundant sites and spatial confinement for electrocatalysis and inhibiting shuttling, respectively. Meanwhile, it is demonstrated that the unique adsorption of molybdates toward polysulfides exhibits a “volcano-type” feature with the catalytic performance following the Sabatier principle. The NiMoO₄ hollow microspheres with moderate adsorption show the highest electrocatalytic activity, which is favorable for enhancing the electrochemical performance of sulfur cathode. Especially, the S/NiMoO₄ composite could achieve a high areal capacity of 7.41 mAh cm⁻² (906.2 mAh g⁻¹) under high sulfur loading (8.18 mg cm⁻²) and low electrolyte/sulfur ratio (E/S, 4 µL mg⁻¹). This work offers a new perspective on searching accurate rules for selecting and designing effective host materials in the lithium–sulfur battery.     

钼的同质异象变体及其应用

综述了钼化合物（二硫化钼、钼酸铵、氧化钼）同质异象变体及其应用。     

电场碱分解工业MoO3的碱浸流程研究

用交变电场碱分解的碱浸流程取代酸洗流程和传统的碱浸流程,这一改进流程既不同于酸洗流程,也不同于传统的碱浸流程。其主要优点是：钼的回收率高,生产成本低,能生产各种纯度的钼酸盐产品,无污染,有利于环境保护和可持续发展。