Molybdenum Oxide Phase Research Articles

Molybdenum oxides synthesized by hydrothermal treatment of A2MoO4 (A=Li, Na, K) and electrochemical lithium intercalation into the oxides

Molybdenum oxides were synthesized by a hydrothermal reaction at 120–180 °C for 1 day in mixture aqueous solutions of A2MoO4 (A=Li, Na, K) and HCl at various ratios of (H in HCl)/(A in A2MoO4). The crystal system of resultant products depended on the kinds of A cations because of their different ionic radii. In case of A=Na, molybdenum oxide phases with hexagonal and orthorhombic cells were formed. In case of A=K, we obtained three phases of triclinic, hexagonal, and orthorhombic MoO3-based oxides and their mixtures. When Li2MoO4 was used as the starting material, the existence of ε-MoO3 and orthorhombic phases was confirmed in the products. They contained some water molecules and A ions in the structure, and their compositions depended on the starting H/A ratio of the hydrothermal solution. The orthorhombic x(Li2O)·MoO3·y(H2O) electrode which was hydrothermally formed from Li2MoO4 system underwent electrochemical lithium intercalation up to Li/Mo=∼1.6 [>300 mA h (g oxide)−1] on electroreduction until 1.3 V vs. Li metal.

Raman spectroscopy of molybdenum oxides

A special sample was prepared by controlled oxidation of MoO2, which contained MoO2, Mo4O11 and MoO3, in order to extend the knowledge about the resonance Raman effect in reduced molybdenum oxides from those close to MoO3 to those close to MoO2. This knowledge is of paramount importance because technical partial oxidation catalysts often contain intermediate Mo oxides of the Magneli type, e.g. Mo4O11, or Mo5O14. The Raman spectra of orthorhombic Mo4O11 and MoO2 have been identified in a Raman microspectroscopic image of 100 single spectra recorded of a mixture of MoO3, MoO2 and Mo4O11. A resonance Raman effect was proven to be responsible for the detection of the molybdenum oxide phases Mo4O11 and MoO2 in dilution with BN when excited at a laser wavelength of 632.8 nm by comparison with Raman microspectroscopic images of the identical sample when excited at 532 nm. The resonance Raman detection of reduced molybdenum oxide phases is discussed in the above mentioned context of their active role in catalytic partial oxidation reactions.

Raman spectroscopy of molybdenum oxides

A special sample was prepared by controlled oxidation of MoO2, which contained MoO2, Mo4O11 and MoO3, in order to extend the knowledge about the resonance Raman effect in reduced molybdenum oxides from those close to MoO3 to those close to MoO2. This knowledge is of paramount importance because technical partial oxidation catalysts often contain intermediate Mo oxides of the Magneli type, e.g. Mo4O11, or Mo5O14. The Raman spectra of orthorhombic Mo4O11 and MoO2 have been identified in a Raman microspectroscopic image of 100 single spectra recorded of a mixture of MoO3, MoO2 and Mo4O11. A resonance Raman effect was proven to be responsible for the detection of the molybdenum oxide phases Mo4O11 and MoO2 in dilution with BN when excited at a laser wavelength of 632.8 nm by comparison with Raman microspectroscopic images of the identical sample when excited at 532 nm. The resonance Raman detection of reduced molybdenum oxide phases is discussed in the above mentioned context of their active role in catalytic partial oxidation reactions.

Synthesis and characterization of novel nanoscopic molybdenum oxide fibers

A molybdenum oxide with high aspect ratio has been synthesized by a template-directed process. The reaction of molybdic acid and primary amines with long alkyl chains, followed by hydrothermal treatment, led to a lamellar molybdenum oxide–amine composite. The subsequent removal of the intercalated amines with nitric acid yielded a pure molybdenum oxide phase with a fibrous morphology. According to X-ray powder diffraction and chemical analysis, these fibers crystallize with the structure of α-MoO3·H2O. Electron microscopy investigations have revealed that the fibers are up to 15 µm long and that their diameters generally range from 50 to 150 nm. The presented synthesis approach uses a cheap and air-stable precursor and, thus, provides an advantageous access to gram quantities of this novel anisotropic nanomaterial.

Ligand influences of the structures of molybdenum oxide networks.

The influence of organonitrogen ligands on the network structure of molybdenum oxides was examined by preparing three new molybdenum oxide phases [MoO3(4,4'-bpy)0.5] (MOXI-8), [HxMoO3(4,4'-bpy)0.5] (MOXI-9), and [MoO3(triazole)0.5] (MOXI-32). The structure of [MoO3(4,4'-bpy)0.5) consists of layers of corner-sharing MoO5N octahedra, buttressed by bridging 4,4'-bipyridyl ligands into a three-dimensional covalently bonded organic-inorganic composite material. Partial reduction of [MoO3(4,4'-bpy)0.5] yields the mixed-valence material [HxMoO3(4,4'-bpy)0.5] (x approximately 0.5). The most apparent structural change upon reduction is found in the Mo-ligand bond lengths of the MoO5N octahedra, which exhibit the usual (2 + 2 + 2) pattern in [MoO3(4,4'-bpy)0.5] and a more regular (5 + 1) pattern in [HxMoO3(4,4'-bpy)0.5]. Substitution of triazole for 4,4'-bipyridine yields [MoO3(triazole)0.5], which retains the layer motif of corner-sharing MoO5N octahedra but with distinct sinusoidal ruffling in contrast to planar layers of [MoO3(4,4'-bpy)0.5] and [HxMoO3(4,4'-bpy)0.5]. The folding reflects the ligand constraints imposed by the triazole ligand that bridges adjacent Mo sites within a layer. MOXI-8, C5H4NMoO3: monoclinic P2(1)/c, a = 7.5727(6) A, b = 7.3675(7) A, c = 22.433(3) A, beta = 90.396(8) degrees, Z = 8. MOXI-9, C5H4.5NMoO3: monoclinic I2/m, a = 5.2644(4) A, b = 5.2642(4) A, c = 22.730(2) A, beta = 90.035(1) degrees, Z = 4. MOXI-32, C2H3N3Mo2O6: orthorhombic Pbcm, a = 3.9289(5) A, b = 13.850(2) A, c = 13.366(2) A, Z = 4.

Partial oxidation of methane to formaldehyde over Mo/HZSM-5 catalysts

Two Mo/HZSM-5 catalyst series have been tested in the partial oxidation of methane to formaldehyde at atmospheric pressure. Conventional aqueous impregnation with ammonium heptamolybdate was used for preparing catalysts with Mo loading ranging from 0.3 to 3.3 wt.%. Two different molybdenum species were detected in these catalysts: tetrahedral monomeric Mo(VI) species and octahedral coordinated polymolybdate species. For molybdenum loading higher than 1.3 wt.%, polymolybdate species are predominant causing a decrease in the dispersion and a partial blocking up of the HZSM-5 zeolite channels. A second catalyst series were prepared from HZSM-5 zeolite by aqueous impregnation and subsequent washing with an aqueous ammonia solution, yielding Mo/HZSM-5 catalysts with Mo contents ranging from 0.08 to 0.45 wt.%. A well-dispersed molybdenum oxide phase mainly constituted of tetrahedral monomeric species joined to the zeolite surface was characterized. Both catalyst series show different activity in methane conversion. Polymolybdate species of impregnated Mo/HZSM-5 catalysts are more reactive in methane activation than acid sites of HZSM-5 but lead to low HCOH selectivity values. In contrast, treated catalysts show lower methane conversion but higher HCOH selectivity. These results demonstrate the specificity of the MoO sites of monomeric species for HCOH production.

Formation of buried oxide layers in molybdenum by high-fluence oxygen ion implantation

High-energy oxygen ions were implanted with fluences from 2.5×1017 up to 1.5×1018 O+-ions/cm2 into molybdenum at temperatures between −60°C and room temperature. Subsequent annealing processes were carried out at 500°C up to 60 min. The chemical composition was investigated using Rutherford backscattering spectrometry. The concentration distribution of the implanted oxygen is characterized by two kinds of profiles: gaussian profiles are observed, when the fluence is lower than the critical fluence required to form a buried molybdenum oxide layer, and a concentration plateau, when the fluence exceeds this critical concentration. Annealing reinforces the tendency to form a flat-topped distribution. Electron and X-ray diffraction studies show that the molybdenum oxide phase δ-MoO2 is formed during annealing. Cross-sectional TEM studies of the as-implanted and annealed oxygen implanted molybdenum are presented. After implantation a buried amorphous layer between strongly damaged material could be found in the near surface region. With increasing annealing time different oxide precipitate morphologies and a buried molybdenum oxide layer covered by an amorphous layer is formed.

The Structural Role of Metal-Organonitrogen Subunits in the Molecular Manipulation of Molybdenum Oxides

Although solid state metal oxides are of both fundamental and practical interest, the designed synthesis of such materials remains an elusive goal. However, valuable synthetic guidelines may be derived from a consideration of the plethora of Nature's remarkable materials which contain composites of molecules or microstructures in which inorganic components coexist with organic components. The presence of organic subunits can profoundly influence the crystallization of the inorganic microstructure, offering a powerful tool for the design of novel materials. In the specific case of molybdenum oxide phases, metal-organoamine molecules, fragments, an even polymeric coordination complex cations may be exploited in the self-assembly of complex hierarchical materials.

Sol–gel bismuth–molybdenum–titanium mixed oxides I. Preparation and structural properties

Bismuth molybdenum titanium oxides, potential catalysts for the partial oxidation of olefins, were prepared via the sol–gel route. Either Bi(NO 3) 3·5H 2O and (NH 4) 6Mo 7O 24·4H 2O, or BiCl 3 and MoOCl 4 were used as precursors together with Ti-isopropoxide. One sample was dried by semicontinuous extraction with supercritical CO 2, affording an aerogel, the others were heated in vacuo resulting in xerogels. For comparison, two bismuth molybdenum oxides on a titania support, and a bismuth molybdate were prepared. The resulting materials were characterized by ICP-AES, N 2 physisorption, XRD, XPS, FT Raman, and UV–vis spectroscopies. The sol–gel derived materials were X-ray amorphous and possessed a high surface area after drying, and a marked mesoporosity in case of the aerogel. The morphology changed upon calcination in O 2 at 773 K, resulting in diminished surface area and a loss of micropores. The bismuth molybdenum oxide phases of the materials containing more than 50 wt% titania remained X-ray amorphous after calcination. The Bi- and Mo-content, the type of Bi- and Mo-precursors and their prehydrolysis, as well as the drying method had a major influence on the structural properties, and the surface and bulk composition of the aerogel and xerogels. Application of Bi-chloride and Mo-chloride precursors can result in significant deviation from the desired bulk composition, likely due to incomplete hydrolysis and evaporation during the subsequent drying and calcination steps.

Effect of Acid Treatment on the Performance of the CuO−MoO3/Al2O3 Catalyst for the Destructive Oxidation of (CH3)2S2

Our previous work confirmed the excellent activity of CuO−MoO3/Al2O3 for the destructive oxidation of (CH3)2S2. This study further investigates the effect of acid treatment on the performance of this catalyst. Four different kinds of inorganic acids (HCl, H2SO4, HNO3, H3PO4) are used to treat the γ-Al2O3 support before impregnating it with copper and molybdenum metal salts. The prepared catalysts are then used for the destructive oxidation of (CH3)2S2. Experimental results indicate that HCl-treated and H2SO4-treated catalysts exhibit a higher activity than those without acid treatment. In addition, the H2SO4-treated catalyst (CuO−MoO3/Al2O3−SO42-) displays the best stability. As BET surface area analysis demonstrates, the surface area and average pore diameter of the H2SO4-treated catalysts are larger than those without treatment; however, the increase in surface area is not the main factor in promoting the activity. XRD analysis reveals that the CuO−MoO3/Al2O3−SO42- catalyst also has the crystalline structure of Cu3Mo2O9 and CuMoO4 which have been found in CuO−MoO3/Al2O3 and which might play a prominent role in the catalyst activity. XPS and TGA analyses suggest that it might have the functional group of SO42- which exists on the surface of the CuO−MoO3/Al2O3−SO42- catalyst. The NH3-TPD pattern shows that the acidity is remarkably enhanced when the catalyst is treated with an H2SO4 solution. We believe that the substantial increase in activity is attributed primarily to the presence of the SO42- group and the formation of a copper molybdenum oxide phase on the catalyst surface.

Crystal engineering of inorganic/organic composite solids: the structure-directing role of aromatic ammonium cations in the synthesis of the ‘step’-layered molybdenum oxide phase [4,4′-H2bpy][Mo7O22]·H 2O

P. J. Zapf, R. C. Haushalter and J. Zubieta, Chem. Commun., 1997, 321 DOI: 10.1039/A607022A

Simultaneous two energies anomalous scattering for the study of the dynamics of structural transformation

In this work the wiggler of VEPP-3 is used for the generation of two beams of synchrotron radiation. After the focusing monochromator, both beams have an energy near the Ni K-edge; they differ by 20 eV. At the sample position the two beams are separated by 2.3 mm. The simultaneous registration of two X-ray diffraction patterns at different energies was made by two position sensitive detectors OD-2. A comparison of the two diffraction patterns, registered at different energies, clearly shows the anomalous scattering effect for the test sample NiO. This method was used for the investigation of the behavior of the Ni atom during the solid state chemical reaction NiO + MoO 3 → NiMoO 4 at T = 600–800°C. This method gives an anomalous scattering signal when the Ni atoms appear in the molybdenum oxide phase. The kinetics of formation of β-NiMoO 4 was obtained. At different temperatures the Ni atoms occupied different positions in the β-NiMoO 4 structure, at 650°C they were at the sites of the crystal lattice, at 690°C they were in the interstitial positions.

Molecular Structures and Reactivity of Supported Molybdenum Oxide Catalysts

Supported molybdenum oxide catalysts were prepared by an equilibrium adsorption method. The molecular structures of the molybdenum oxide overlayers on different oxide supports (Al 2O 3, TiO 2, ZrO 2, SiO 2, and MgO), under in situ conditions, were investigated by Raman spectroscopy. The molybdenum oxide species on TiO 2, ZrO 2, and Al 2O 3 possess a highly distorted, octahedrally coordinated surface molybdenum oxide species with one short MoO bond regardless of the molybdenum oxide content. The MoO 3/SiO 2, catalysts primarily contain crystalline MoO 3 because of the lower density and reactivity of the silica surface OH groups. The MoO 3/MgO catalysts possess MgMoO 4 and CaMoO 4 compounds due to the high aqueous solubility of MgO and CaO (an impurity in the MgO support) and the strong acid-base interaction between molybdenum oxide and MgO/CaO. The methanol oxidation reaction studies revealed that the MoO 3/TiO 2 (anatase and rutile) and MoO 3/ZrO 2 catalysts are the most active catalysts and that their activities (TOFs) are at least 1-2 orders of magnitude higher than those of the MoO 3/Al 2O 3, MoO 3SiO 2, and MoO 3/MgO catalysts. It was also found that the catalytic activities correlate with the reducibility of the surface molybdenum oxide species on the oxide supports as well as their surface morphology (e.g., molybdenum oxide dispersion and molybdate compound formation). These studies demonstrate that the specific oxide support controls the reactivity of the supported molybdenum oxide phases.

Mo-USY zeolites for hydrodesulphurization. I.: Structure and distribution of molybdenum oxide phase

Ultrastable (USHY) zeolite has been used as a support material for a series of molybdenum catalysts prepared using various precursors and activation procedures. Studies using atomic absorption spectroscopy. X-ray photoelectron spectroscopy, Fourier transform IR of the mid-infrared range and of adsorbed nitric oxide and pyridine, and sorption capacity of water indicate that the distribution of molybdena in the zeolite is strongly influenced by the preparation procedures. Catalysts calcined by a non-conventional procedure, involving decomposition of the precursor by heating under vacuum over extended periods of time, show a migration of the molybdena from the external surface into the lattice cavities.

Local site symmetry of dispersed molybdenum oxide catalysts: XANES at the Mo L2,3-edges

The local site symmetry of nanodispersed molybdenum oxide phases have been determined for the first time using X-ray absorption near edge spectroscopy (XANES) at the Mo L[sub 2,3]-edges. Local site symmetries varying between pure tetrahedral coordination through mixed symmetries to pure octahedral coordination were determined for species present at concentrations above and below a submonolayer. X-ray absorption near-edge spectra at the Mo L[sub 2,3]-edges were obtained for a series of supported molybdate catalysts and Mo[sup 6+] reference materials with a range of oxygen coordination types. Basic ligand field concepts can be used to interpret the spectra. Tetrahedral molybdates display d-orbital splitting in the 1.8-2.0-eV range, whereas octahedrally coordinated molybdates display split peaks in the 3.0-4.5-eV range. The direct comparison between a complete set of molybdenum reference compounds and dispersed magnesium oxide-supported molybdate catalysts has been used to determine the detailed symmetries of the dispersed molybdenum phases that are present as amorphous, crystalline, and mixed phases. 26 refs., 5 figs., 1 tab.

Remarkable spreading behavior of molybdena on silica catalysts. Anin situ EXAFS-Raman study

In contrast to the frequently reported lack of interaction between hexavalent molybdenum and SiO2 and the tendency of silica-supported MoO3 to coalescence, it has been found that on dehydration small molybdenum oxide clusters spread over a silica support. A combined Raman spectroscopy-X-ray absorption study shows a significantly altered structure of the molybdenum oxide phase after dehydration. In EXAFS the total Mo-Mo coordination number drops from 3.27 to 0.20 after anin situ thermal treatment at 673 K. The increase of the peak in the XANES region (Is -→ 4d) indicates that the coordination sphere of the molybdenum atoms strongly alters after dehydration. The Raman spectra reflect the change of the structure through a shift of the position of the terminal Mo=O bond from 944 to 986 cm−1 and the disappearance of the bridged Mo-O-Mo vibration at 880 cm−1. It is concluded that dehydration produces almost isolated molybdenum sites in this highly dispersed sample. Water ligands stabilize the oligomeric clusters under ambient conditions; the removal of water causes spreading of these clusters.

Morphology and activity of MoS 2 on various supports: Genesis of the active phase

The morphology and hydrodesulfurization (of thiophene) activity of unpromoted MoS 2 supported on alumina, silica, titania, and zirconia have been examined. In the oxidic catalyst, no surface molybdenum phases could be identified by transmission electron microscopy. For all supports except titania, an additional bulk molybdenum oxide phase appeared above a particular Mo loading. In the case of the alumina support, bulk Al 2(MoO 4) 3 also appeared. In the sulfided catalysts, the bulk species retained their original morphology and were surrounded by a skin of MoS 2 several layers thick. The core of the bulk MoO 3 was reduced to Mo metal. On the surface of the SiO 2 and Al 2O 3, the MoS 2 was present as flakes up to five layers thick, sitting vertically on the support. For TiO 2 and ZrO 2 the MoS 2 was present as “islands,” which expanded with increased Mo loading, eventually encapsulating the support particles. The reactivity behavior also fell into two groups, corresponding to the two forms of MoS 2 morphology. The specific activity of MoS 2 supported on TiO 2 and ZrO 2 was higher than that for the case of Al 2O 3 and SiO 2 supports.

Dispersity and reducibility of the molybdenum oxide phase in SiO2, SiO2−Al2O3 and γ-Al2O3 supported molybdena catalysts

XRD, isothermal and temperature-programmed reduction (TPR) experiments were carried out with SiO2, SiO2−Al2O3 and γ-Al2O3 supported catalysts. Molybdena is in a more disperse state on supports containing more alumina and it is more reducible on SiO2−Al2O3 than on SiO2 or γ-Al2O3. TPR curves were shown to reflect connections between reduction kinetics and dispersity.

Qualitative bonding models for some molybdenum oxide phases

Both the well-known layered and recently characterised “cubic” forms of MoO 3, as well as the molybdic acids, MoO 3· nH 2O ( n=1,2), react with dissociated hydrogen at room temperature to form hydrogen insertion phases. These reactions are characterised by the occurrence of only minimal overall structural rearrangement of the parent phases on reduction. However, significant structural differences in the molybdenum-oxygen coordination environments in the parent oxide phases are found. In this work, qualitative bonding models and associated energy level schemes are constructed for these oxide phases. The changes in both the electronic structures and molybdenum-oxygen coordination environments caused by the addition of further electrons (on reduction with hydrogen) are then examined.

Electron microscopy of some molybdenum oxide phases after use as catalysts in oxidative ammonolysis and ammoxidation of toluene

Five molybdenum oxide samples, subjected to conditions of oxidative ammonolysis and ammoxidation of toluene at 450 and 460°C, respectively, have been characterized using X-ray diffraction, scanning and transmission electron microscopy, and specific surface area measurements. In oxidative ammonolysis, the relatively large, freshly prepared MoO 3 crystals are reduced to smaller MoO 2 crystals with a crystallite size of 5–30 nm. This process gives a perfectly pseudomorphous product with pores less than 5 nm. The specific surface area increases from <0.1 m 2/g to almost 40 m 2/g. In subsequent ammoxidation, MoO 2 transforms first into orthorhombic Mo 4O 11 and finally into MoO 3. The crystals of Mo 4O 11 are about 1 μm in diameter, and their formation leads to a decrease of specific surface area. The original MoO 3 morphology is retained even after the sequence of transformation as follows: MoO 3 → MoO 2 → Mo 4O 11 (→MoO 3). In some cases, the new generation of MoO 3 crystals grows parallel to the original MoO 3 crystals.